Synergistic nitrogen and endohedral MoCl_5 doping for ultrahigh-conductivity carbon nanotube fibers

Domain: chemistry
Published: 2026-02-24
Editorial outcome: minor_revision

Abstract
Lightweight and highly conductive carbon nanotube fibers (CNTFs) are attractive for flexible electronics, yet their performance remains constrained by inefficient charge transport. Here we report a synergistic doping strategy that integrates in-plane nitrogen doping with endohedral molybdenum pentachloride (MoCl5) incorporation to produce CNTFs with exceptional electrical properties and environmental durability. Nitrogen doping creates sidewall defect sites that promote MoCl5 encapsulation, yielding a strong charge-transfer effect and markedly increased carrier density. The resulting fibers achieve a high specific electrical conductivity of 14166 Sm2 kg−1 and a current carrying capacity of 1241 A mm−2, surpassing copper by 115% and 28%, respectively. The CNTFs also exhibit high flexibility and environmental stability, retaining performance under thermal, mechanical, and solvent stresses. When woven into textiles, they deliver an electromagnetic shielding effectiveness of 92.7 dB (8.2-12.4 GHz). This work establishes a scalable doping approach for fabricating ultrahigh-conductivity CNTFs for advanced flexible electronics.

Introduction
Fibers with high flexibility, electrical conductivity, and current-carrying capacity are highly desired for the development of portable electronics and smart textiles1–6. Conventional metallic wires, such as copper (Cu) and aluminum (Al), have long been used as electrical conductors due to their excellent electrical conductivity. However, their high density, limited flexibility, and susceptibility to oxidation and corrosion in harsh environments significantly constrain their applicability in emerging lightweight and flexible electronic technologies. These limitations highlight the urgent need for alternative conductive materials that combine high electrical performance with mechanical robustness and environmental stability. Carbon nanotube fibers (CNTFs), formed by assembling individual carbon nanotubes (CNTs) into uniaxially aligned architectures, have garnered significant attention as a lightweight and mechanically robust alternative to conventional metallic wires7–12. These fibers are expected to translate the extraordinary properties of their nanoscale building blocks to the macroscale, offering low density, high mechanical strength and flexibility, excellent electrical and thermal conductivities, and robust environmental stability. However, despite these advantages, the electrical conductivity and specific electrical conductivity of CNTFs remain inferior to those of Cu (~60 MS m−1 and 6590 Sm2 kg−1)13 and are significantly lower than those of individual CNTs (100 MS m−1)14. This performance gap arises from structural limitations in fiber assembly and the inherently low carrier density. As a result, charge transport within CNTFs remains inefficient, leading to increased Joule heating, greater energy dissipation, and reduced current-carrying capacity, thereby limiting their widespread utility in high-performance electrical and energy applications.

To address these challenges, various strategies have been explored to enhance the electrical conductivity of CNTFs through modifying their hierarchical structures and optimizing their compositions. Posttreatment approaches, such as solution-assisted wet-stretching15–18 and mechanical densification19–24, have been used to reorganize the hierarchical structures of CNTFs produced via direct aerogel spinning or vertically aligned array spinning. These approaches result in significant improvements in nanotube alignment, packing density, and intertube interactions, thereby reducing contact resistance between individual CNTs and enhancing electrical performance. However, despite these advancements, the electrical conductivity of the resulting fibers remains an order of magnitude lower than that of Cu. In contrast, CNTFs fabricated through liquid crystal spinning exhibit well-aligned and densely packed structures, achieving electrical conductivities in the range of 1–12 MS m⁻1 25–33. Additionally, an alternative approach involves metallizing CNTFs to fabricate CNT/metal core-shell fibers, which integrate the lightweight properties and high current-carrying capacity of CNTFs with the remarkable electrical conductivity of metals34–39. However, weak interfacial interaction between the CNTFs and the metal layer, coupled with limited long-term stability, poses significant challenges to the performance and applications of these hybrid fibers. Doping carbon-based fibers using acids40,41, halogens25,42–44, or metal halides45, has emerged as a promising strategy to increase the carrier density and effectively enhance the fiber conductivity. Doped fibers exhibit the electrical and specific electrical conductivities that rival those of metals, while maintaining the low density of pristine CNTFs (PCNTFs). However, there is still plenty of room for improving the electrical properties of CNTFs, and challenges such as the instability of dopants and associated performance degradation also continue to hinder the practical applications of these fibers46. Therefore, addressing these limitations is pivotal to the development of CNTFs with high electrical conductivity, ultrahigh specific conductivity, and remarkable performance stability, paving the way for their integration into advanced technological applications.

In this work, we develop a doping engineering strategy to fabricate CNTFs co-doped with in-plane nitrogen and endohedral molybdenum pentachloride (MoCl5), achieving significant enhancements in electrical performance. In-plane nitrogen doping introduces sidewall defects on the nanotubes, modestly improving electrical conductivity and creating sites for MoCl5 adsorption and encapsulation within the nanotube inner cavities. This synergistic approach yields CNTFs with a high specific electrical conductivity of 14166±48 Sm2 kg−1 and an excellent current-carrying capacity of 1241.38±94.34 A mm−2, exhibiting 115% and 28% improvements compared to Cu, respectively. The doped fibers also exhibit high tensile strength, flexibility, and robust environmental stability, maintaining their performance under diverse operational conditions. These properties make them particularly well-suited for lightweight conductive wires and high-performance electromagnetic interference shielding textiles. This work highlights an approach to fabricate highly conductive CNTFs, offering a versatile and scalable pathway for their integration into next-generation electronic and textile applications.

Results
As illustrated in Fig. 1a, we developed a doping engineering strategy that integrated in-plane nitrogen doping with endohedral MoCl5 doping to enhance the electrical performance and stability of CNTFs. PCNTFs, produced via wet spinning, were sequentially treated with inductively coupled plasma (ICP) in the oxygen/argon and nitrogen/argon atmospheres to create sidewall defects and enable nitrogen doping, resulting in nitrogen-doped CNTFs (N-CNTFs). N-CNTFs were then subjected to a two-zone vapor transport process to achieve nitrogen and MoCl5 co-doping CNTFs (N/MoCl5-CNTFs). In this process, N-CNTFs and dopants were placed at the two opposite ends of a vacuum-sealed glass tube. Upon heating, MoCl5 sublimated and diffused toward the fiber zone, where it was absorbed and encapsulated within the nanotube inner cavities. The successful incorporation of MoCl5 into N/MoCl5-CNTFs was demonstrated through radial cross-section analysis using transmission electron microscopy (TEM) and high-angle annular dark-field scanning TEM (HAADF-STEM). TEM imaging (Fig. 1b) reveals that dark MoCl5 particles distribute within the nanotubes. This observation is further supported by Z-contrast HAADF-STEM imaging (Fig. 1c), which shows that particles with brighter contrast are clearly observed within the nanotube inner cavities. This brighter contrast is attributed to MoCl5’s higher atomic number (Z) compared to carbon47–49. Energy dispersive X-ray spectroscopy (EDS) mappings confirm these findings, revealing a uniform spatial distribution of nitrogen (N) throughout the fiber, indicative of effective in-plane nitrogen doping. Simultaneously, preferentially localized molybdenum (Mo) and chlorine (Cl) within the nanotube channels corroborate the endohedral incorporation of MoCl5. As a result, the resulting N/MoCl5-CNTFs exhibit excellent electrical performance, with a high electrical conductivity of 27.34 ± 0.09 MS m−1, a significant specific electrical conductivity of 14166 ± 48 Sm2 kg−1, and a current-carrying capacity of 1241.38 ± 94.34 A mm−2. These values represent increases of 220, 202, and 262% in electrical conductivity, specific electrical conductivity, and current-carrying capacity, respectively, compared to PCNTFs (Fig. 1d), underscoring the effectiveness of this doping engineering strategy in significantly enhancing the electrical properties of CNTFs.

We continuously produced monofilament and multifilament PCNTFs via wet spinning from a liquid crystalline solution of CNTs in chlorosulfonic acid (CSA) (Supplementary Fig. 1)27. The as-received CNTs were purified to reduce impurities and improve crystallinity. The purified CNTs exhibited a low oxygen content, and the average aspect ratio was subsequently estimated to be approximately 4300 using a capillary thinning rheometric method50,51 (Supplementary Fig. 2). The high-purity and high-crystallinity CNTs enable their dispersion in CSA, forming well-defined liquid crystals. PCNTFs were then obtained by extruding the spin dope through 1- or 20-hole nozzles into an acetone coagulation bath, followed by collection onto a rotating drum. The as-spun PCNTFs exhibit smooth surfaces and highly aligned, densely packed structures, as characterized by surface and cross-sectional images (Supplementary Fig. 3a–c). Sequential treatments with oxygen/argon plasma and nitrogen/argon plasma were applied to introduce sidewall defects, enabling the in-plane nitrogen doping. Optimization of nitrogen/argon plasma exposure reveals that N-CNTFs retain a well-ordered structure after plasma treatments (Supplementary Fig. 3d–f), and a 5-min treatment results in the highest tensile strength and electrical conductivity (Supplementary Fig. 3g, h). In parallel, MoCl5-doped CNTFs (MoCl5-CNTFs) were prepared by exposing PCNTFs to a two-zone vapor transport process. A treatment duration of 48 h is found to optimize electrical conductivity without altering the structural integrity of fibers (Supplementary Fig. 4). Finally, N-CNTFs were further processed via the above two-zone vapor transport process to produce N/MoCl5-CNTFs, combining in-plane nitrogen doping with endohedral MoCl5 incorporation (Fig. 2a, b). We further reversed the doping sequence, where MoCl5 doping was performed initially, followed by nitrogen plasma treatment, producing CNTFs with opposite doping sequences (MoCl5/N-CNTFs). As shown in Supplementary Fig. 5, these fibers exhibit a significantly reduced electrical conductivity compared to both MoCl5-CNTFs and N/MoCl5-CNTFs. This reduction is likely due to the thermal effects associated with the plasma treatment, which may cause partial dopant desorption52. These results confirm the critical importance of treatment sequence and demonstrate that nitrogen functionalization prior to MoCl5 doping is essential for achieving effective endohedral incorporation and enhancing electrical conductivity.

To assess the microstructure of these fibers, wide-angle X-ray scattering (WAXS) measurements were performed to evaluate the nanotube orientation within fibers. The nearly unchanged azimuthal scan curves and Herman’s orientation factor values derived from WAXS patterns for PCNTFs, N-CNTFs, MoCl5-CNTFs, and N/MoCl5-CNTFs confirm their highly ordered structures (Fig. 2c and Supplementary Fig. 6), indicating that the doping processes do not compromise the structural integrity of these fibers. We performed radial cross-sectional elemental analysis, X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy to confirm the successful doping of the fibers. Elemental mappings demonstrate the homogeneous distribution of N and Mo throughout the fibers (Fig. 2d–f), originating from nitrogen plasma and MoCl5 doping, respectively. XPS was employed to investigate the chemical composition and bonding states of the fibers (Fig. 2g). For N-CNTFs, the N 1s spectrum confirms the successful incorporation of N, with pyridinic N identified as the predominant bonding configuration (Supplementary Fig. 7a and Table 1), known to induce p-type doping in CNTFs53. For N/MoCl5-CNTFs, the XPS spectrum shows the characteristic peaks corresponding to C 1s, Mo 3d, Cl 2p, N 1s, and O 1s. High-resolution Mo 3d spectrum of N/MoCl5-CNTFs exhibits four peaks at 231.8, 233.1, 234.9, and 236.3 eV, assigned to Mo(IV) 3d5/2, Mo(IV) 3d3/2, Mo(V) 3d5/2, and Mo(V) 3d3/2 states, respectively (Supplementary Fig. 7b)54. The Mo4+ state likely arises from partial Cl liberation during the MoCl5 intercalation process55. Additionally, the C 1s spectrum of PCNTFs, centered at 284.5 eV, shifts by 0.4 eV toward lower binding energy upon doping (Supplementary Fig. 7c). This shift reveals electronic charge transfer from CNTs to the encapsulated material56–59. Raman spectroscopy using a 532 nm excitation wavelength provided further structural and electronic insights. The IG/ID value of N-CNTFs (46.51 ± 0.30), where IG and ID represent the D-band and G-band intensities of CNTFs, respectively, shows a slight decrease compared to that of PCNTFs (49.75 ± 0.39) (Supplementary Fig. 7d, and Table 2), consistent with the introduction of minor sidewall defects during nitrogen plasma treatment60. Similarly, MoCl5-CNTFs and N/MoCl5-CNTFs show IG/ID values of 48.54 ± 0.43 and 45.25 ± 0.33, respectively, indicating that the structural integrity of the nanotube lattice is largely preserved following both doping processes61, thereby supporting efficient charge transport performance. Additionally, Raman spectra of N-CNTFs, MoCl5-CNTFs, and N/MoCl5-CNTFs reveal progressive blueshifts of the G-band compared to PCNTFs, with the largest shift of about 19 cm−1 observed in N/MoCl5-CNTFs (Fig. 2h and Supplementary Table 3). These shifts are attributed to doping-induced charge transfer, wherein electron donation from CNTs to N and MoCl5 results in strong p-type doping and corresponding modifications to the CNTs electronic structure62,63.

To further investigate the transition in electrical behavior, we employed Kelvin probe force microscopy (KPFM) to measure the surface potentials of the fibers (Supplementary Fig. 8). The surface potentials of N-CNTFs, MoCl5-CNTFs, and N/MoCl5-CNTFs are progressively higher than that of PCNTFs, corresponding to an increase in work function from 4.45 ± 0.08 eV to 4.56 ± 0.03, 4.63 ± 0.02, and 4.87 ± 0.03 eV, respectively (Fig. 2i). This increase in the work function is attributed to p-type doping effect of N and MoCl5, which induces a downshift in the Fermi level (EF) of CNTs57. These observations are further supported by density functional theory (DFT) calculations. Simulations reveal that MoCl5 acts as a strong electron acceptor, with Bader charge analysis showing electron transfer of 0.85 e− per encapsulated molecule and 0.55 e− per adsorbed molecule (Supplementary Fig. 9a, b). Band structure analysis furthers hows that nitrogen doping lowers the EF, indicative of hole doping64. Subsequent MoCl5 doping leads to a further downshift in the EF (Supplementary Fig. 9c–f), reinforcing the increasingly p-type character of the system. These electronic modulations are consistent with both our spectroscopic and KPFM measurements (Fig. 2h, i). The relationship between hole density (p) and the EF is described as follows:

p = ni exp[(Ei − EF) = kT] (1)

where ni, k, and T represent the intrinsic carrier density, Boltzmann constant, and absolute temperature, respectively. This exponential dependence highlights how progressive EF shifts lead to substantial increases in carrier density40,58,65. We further performed Hall effect measurements using a van der Pauw configuration on CNT films fabricated under the same doping conditions as the fibers. Although the van der Pauw method is not ideally suited for highly anisotropic materials, it still offers qualitative insights into intrinsic transport trends45, rather than quantitative values representative of the high-conductivity fibers. As expected, carrier mobility decreases upon doping due to enhanced carrier scattering in N-CNT and N/MoCl5-CNT films compared to PCNT films. In contrast, carrier densities for N-CNT, MoCl5-CNT, and N/MoCl5-CNT films increase substantially relative to PCNT films (Supplementary Table 4), consistent with the doping-induced enhancement in carrier density. In accordance with the Drude model, which defines that electrical conductivity is proportional to the product of mobility and carrier density, these results explain how even with a modest decrease in mobility, the marked rise in carrier density results in significantly improved electrical conductivity. Taken together, the high electrical conductivity of the fibers results from the synergistic effects of preserved structural integrity supporting adequate carrier mobility, and significantly elevated carrier density enabled by effective doping.

The ordered structures and increased carrier density significantly enhance the electrical conductivity and current-carrying capacity of the fibers. Electrical conductivity (σ) was measured using a four-point probe method over a fixed fiber length of 80 mm (Supplementary Fig. 10). To ensure reliable electrical contact, conductive silver paste was applied at both the current and voltage electrodes. A constant current was supplied through the outer electrodes, and the resulting voltage drop was measured across the inner electrodes. Electrical conductivity was then calculated using the relation,

σ = L / RS (2)

where R is the measured resistance, L is the gauge length between the voltage probes, and S is the cross-sectional area of the fiber. The average diameter of each fiber, obtained from the lateral SEM measurements (Supplementary Fig. 11), was used to calculate the fiber cross-sectional area applied in both mechanical and electrical property analyses. For each fiber type, five independent measurements were performed using multiple samples to determine the corresponding mechanical and electrical properties (Supplementary Tables 5–8). Notably, N/MoCl5-CNTFs obtained with a doping duration of 48 h exhibit an optimal composition, delivering an exceptional electrical conductivity of 27.34 ± 0.09 MS m−1, which is 3.20, 2.73, and 1.58 times that of PCNTFs, N-CNTFs, and MoCl5-CNTFs, respectively (Fig. 3a, Supplementary Fig. 12a, b, and Table 9). To evaluate the influence of fiber length on electrical conductivity, we further measured the raw resistance of N/MoCl5-CNTFs over extended lengths (100–200 mm). A strong linear relationship between resistance and fiber length is observed (Supplementary Fig. 12c, d), confirming the structural uniformity of the fibers. To evaluate the effectiveness of MoCl5 relative to other dopants, we prepared N/I2-CNTFs, N/CuI-CNTFs, and N/FeCl3-CNTFs using I2, CuI, and FeCl3 as dopants, respectively, via a similar two-zone vapor transport method. The electrical conductivity of N/I2-CNTFs reaches 16.89 ± 0.06 MS m−1, while N/CuI-CNTFs and N/FeCl3-CNTFs exhibit slightly lower conductivities of 25.97 ± 0.09 and 26.87 ± 0.08 MS m−1, respectively, compared to N/MoCl5-CNTFs (Supplementary Fig. 13). This trend suggests MoCl5 induces stronger p-type doping and more significant modulation of the CNTs electronic structure through charge transfer interactions, resulting in the highest electrical conductivity. These results highlight MoCl5 as the most effective dopant among those tested. N/MoCl5-CNTFs achieve a notable specific electrical conductivity of 14166 ± 48 Sm2 kg−1 (or 13746 ± 47 Sm2 kg−1 based on the linear density method; see Supplementary Table 9), exceeding those of gold, zinc, silver, and Cu by factors of 4.60, 4.49, 1.19, and 1.15, respectively, and is comparable to that of aluminum66 (Fig. 3b and Supplementary Table 10). Additionally, N/MoCl5-CNTFs exhibit a high tensile strength of 2.49 ± 0.14 GPa and a specific tensile strength of 1.26 ± 0.07 N tex−1, significantly surpassing those of conventional metals. In comparison with the previously reported CNTFs and metals, N/MoCl5-CNTFs exhibit the unique combination of high specific electrical conductivity and specific tensile strength (Fig. 3c and Supplementary Fig. 14a, b).

The current-carrying capacity is defined as the maximum current density sustained by a freestanding fiber under ambient air conditions. It is calculated by dividing the maximum current measured prior to failure by the fiber’s cross-sectional area. As shown in Supplementary Fig. 14c, the current density initially increases linearly with voltage until a sharp drop occurs, indicating fiber failure due to Joule heating30,36. Based on infrared imaging, the estimated failure temperature is approximately 630 °C (Supplementary Fig. 14d, e). N/MoCl5-CNTFs show a high current-carrying capacity of 1241.38 ± 94.34 A mm−2, yielding increases of 262, 217, and 63% compared with PCNTFs, N-CNTFs, and MoCl5-CNTFs, respectively (Fig. 3d). The specific current-carrying capacity of N/MoCl5-CNTFs is 6.43 × 104 A cm g−1, which is 5.87 times higher than that of commercial Cu wires67. A comparative analysis of the current-carrying capacity and electrical conductivity of N/MoCl5-CNTFs with those of previously reported CNTFs, buckypapers, metal/CNT composites, and Cu wires highlights the extraordinary combination of high current-carrying capacity and excellent electrical conductivity (Fig. 3e and Supplementary Table 11). We further investigated the temperature-dependent relative resistance of CNTFs and commercial Cu wires (Fig. 3f and Supplementary Fig. 15a). The electrical resistance of fibers at 300 K (R0) and other temperatures (RT) was compared, revealing that the minimum resistance of CNTFs appears near 33 K. Below this temperature, the resistance of CNTFs increases with decreasing temperature, and the plot of ln R versus T−1/4 shows a high linearity (Supplementary Fig. 15b), consistent with Mott’s variable-range hopping transport41. Above this temperature, the resistance of CNTFs increases with increasing temperature due to the phonon scattering (Supplementary Fig. 15c), showing metallic behavior. Additionally, the temperature coefficient of resistance (TCR) increases with the degree of doping. In contrast, Cu wires exhibit a continuous decrease in resistance as the temperature decreases. The TCR values for N/MoCl5-CNTFs and Cu wires within the 100–400 K range show that N/MoCl5-CNTFs possess a lower value than Cu.

We further compared the environmental stability of the doped fibers. While MoCl5-CNTFs exhibit a notable decline in the electrical conductivity over time, N/I2-CNTFs, N/CuI-CNTFs, N/FeCl3-CNTFs, and N/MoCl5-CNTFs maintain more significantly stable electrical conductivities over 50 days under ambient conditions (Fig. 4a and Supplementary Fig. 16a). Among these, N/MoCl5-CNTFs demonstrate the smallest change, indicating superior long-term stability. Additionally, N/MoCl5-CNTFs exhibit minimal changes in electrical conductivity after prolonged exposure to various solvents, in contrast to the marked decrease observed in MoCl5-CNTFs (Supplementary Fig. 16b). We also assessed the performance of N/MoCl5-CNTFs under cyclic thermal and mechanical stresses. The electrical conductivity of N/MoCl5-CNTFs decreases by only 21% upon heating to 200 °C and exhibits a minimal resistance increase of 1.2% following repeated thermal cycling (Fig. 4b). Moreover, the electrical resistance of N/MoCl5-CNTFs remains nearly unchanged, with only a 1% variation after 2000 bending cycles (Fig. 4c), underscoring the robust flexibility and durability of the fibers. To investigate the origin of the exceptional environmental stability observed in N/MoCl5-CNTFs, we performed additional HAADF-STEM and EDS analyses on the radial cross-sections of MoCl5-CNTFs and N/MoCl5-CNTFs. In MoCl5-CNTFs, bright contrast associated with MoCl5 predominantly locates in interstitial spaces between adjacent CNTs (Fig. 4d). Corresponding EDS mapping further confirms Mo and Cl signals mainly localize outside the nanotube interiors (Fig. 4e–h), in sharp contrast to the N/MoCl5-CNTFs, where MoCl5 is confined within the inner cavities of the nanotubes (Fig. 1c and Supplementary Fig. 17). This confirms that nitrogen doping promotes the endohedral incorporation of MoCl5. This internal confinement is believed to shield the dopants from external perturbations, effectively mitigating the impact of solvents and thermal cycling on the electrical performance of N/MoCl5-CNTFs.

The exceptional electrical performance and environmental stability of N/MoCl5-CNTFs make them highly suitable for applications as lightweight conductive wires and electrothermal fibers. Multifilament N/MoCl5-CNTFs act as a lightweight conductor capable of illuminating four bulbs (Fig. 5a). Additionally, the N/MoCl5-CNT tow with a thickness of about 71 μm exhibits excellent electrothermal properties, making them promising candidates for high-performance fibrous heaters. To evaluate their electrothermal performance, an infrared thermal camera was used to monitor the temperature distribution and measure the saturated temperature under different applied voltages. At applied voltages of 0.5, 1.5, and 3 V, the saturated temperatures of fibrous heaters increase from 39, 211, to 397 °C, respectively (Fig. 5b, c), demonstrating high electrothermal efficiency. The fibrous heater also exhibits a rapid electrothermal response, with a heating rate of 1254 °C s−1 at a low voltage of 3 V, which is comparable with the cooling rate and outperforms most existing fibrous heaters68,69. N/MoCl5-CNTFs-based fibrous heaters show low operation voltage, rapid electrothermal response, and high saturated temperature, highlighting their potential for high-performance electrothermal applications.

The substantial electrical conductivity of N/MoCl5-CNTFs makes them promising candidates for electromagnetic interference shielding effectiveness (EMISE). Leveraging the excellent mechanical flexibility and high tensile strength of as-obtained fibers, plain-woven textiles were successfully fabricated using a machine loom, with polyester fibers as the warp and PCNTFs or N/MoCl5-CNTFs as the weft (Supplementary Fig. 18). As shown in Fig. 5d, the EMISE performance of PCNTFs- and N/MoCl5-CNTFs-based textiles was measured across the frequency range of 8.2–12.4 GHz (X-band). The EMISE of N/MoCl5-CNTFs-based textiles is ~81.1 dB, significantly higher than that of PCNTFs-based textiles (50.9 dB). Furthermore, the EMI SE performance of double-layer N/MoCl5-CNTFs-based textiles reaches ~92.7 dB. Figure 5e compares the total EMISE (SET), absorption EMISE (SEA), and reflection EMISE (SER) of different textiles. It is evident that SER and SEA significantly contribute to the overall SET, and increases in the electrical conductivity and textile thickness result in an improvement in SET70. To further elucidate the EMI shielding mechanism, we compared the power efficiency of absorption (A), reflection (R), and transmission (T) of the textiles. Notably, the power efficiency of R of all textiles is significantly higher than A and T at the frequency of 8.2–12.4 GHz (Supplementary Fig. 19). In particular, the power efficiency of R for N/MoCl5-CNTFs-based textiles is 0.966, higher than those of A (0.034) and T (8.04 × 10–7), suggesting the EMISE performance is primarily attributed to reflection rather than absorption71,72. Figure 5f illustrates the EMI SE mechanism of textiles. Most incident electromagnetic waves are immediately reflected due to the impedance mismatch between the highly conductive N/MoCl5-CNTFs and free space. The residual electromagnetic waves are then efficiently attenuated by conductance loss and multiple internal reflections. Finally, after the multiple back-and-forth reflections within the dense fiber structure, the remaining electromagnetic waves are fully absorbed, followed by the minimal transmission73,74. Owing to the high EMI SE performance, N/MoCl5-CNTFs-based textiles can serve as an effective shielding barrier, capable of disabling the wireless phone charging (Fig. 5g–i). As a result, the woven N/MoCl5-CNTFs-based textiles show great potential for use in personal protection for EMI shielding fields.

Methods
Materials and chemicals
Molybdenum pentachloride (MoCl5), molybdenum oxide (MoO3), chlorosulfuric acid (CSA, 99.5%), and acetone (>99.5%) were purchased from Sinopharm Chemical Reagent Co., Ltd. All chemicals were used as received without further purification unless otherwise specified.

Purification of CNTs
The CNTs used in this study were purchased from Cnano Technology Co., Ltd. (Jiangsu, China) and subsequently subjected to a purification process. The as-received CNTs were first dispersed in 30% H2O2 (1 mg mL−1) and stirred for 24 h, followed by vacuum filtration. The recovered CNTs were then dispersed in 12 M HCl (5 mg mL−1) and stirred for 12 h to remove residual metal catalysts. After neutralization with deionized water, the CNTs were frozen in liquid nitrogen and dried overnight via freeze-drying.

Preparation of wet-spun PCNTFs
PCNTFs were prepared via a wet spinning process based on our previously reported protocol27, with a key modification to improve alignment and fiber quality. Briefly, a liquid crystal spinning dope containing 0.8–1.5 wt% CNTs was prepared by dispersing CNTs into CSA. The dope was extruded through 1-, 20-, or 100-hole nozzles with an inner diameter of 0.1–0.31 mm into an acetone coagulation bath. To enhance the initial alignment of CNTs during extrusion, we designed the nozzles with a narrower convergent angle of approximately 10°. This adjustment improved flow stability and promoted more uniform orientation of CNTs along the fiber axis during coagulation and subsequent stretching. The spinning rates (0.01–0.2 mL min–1) and winding rates (2–3 m min–1) were carefully controlled to achieve a draw ratio above 1.6, facilitating the formation of as-spun fibers. The resulting fibers were further washed with distilled water and dried at 170 °C for 24 h.

Preparation of N-CNTFs
PCNTFs were placed inside a quartz reaction tube, positioned approximately 4 cm from the induction coil of a radio-frequency power supply (VERG-1000, Zhongshan K-Mate General Electronics Co., Ltd.). The fibers were dried at 100 °C under vacuum (~1 mbar) for 24 h to eliminate residual moisture. The dried fibers were then subjected to ICP treatment at a frequency of 13.56 MHz and a power of 50 W for 2 min in an O2/Ar (50/150 sccm) atmosphere. This treatment generated sidewall defects on the CNTs, enhancing the availability of nitrogen doping sites. Subsequently, the fibers were exposed to an N2/Ar plasma (50/150 sccm) under a background pressure of ~1 mbar, using the frequency of 13.56 MHz and power of 50 W, for 2–15 min to introduce nitrogen atoms into the nanotube structure, yielding the N-CNTFs.

Preparation of MoCl5-CNTFs
As shown in Supplementary Fig. 20, PCNTFs and a mixture of MoCl5 and MoO3 in a mass ratio of 1:0.6 were placed separately inside a glass tube. The tube was dried at 100 °C under vacuum (~1 mbar) for 24 h to remove residual moisture, then flame-sealed, and annealed at 300 °C for 9–72 h in a furnace with a heating/cooling rate of 10 °C min−1. After cooling to room temperature, the fibers were rinsed with acetone to remove residual dopants and dried at 60 °C to obtain the final MoCl5-CNTFs.

Preparation of N/MoCl5-CNTFs
N/MoCl5-CNTFs were prepared using the same two-zone vapor transport method. N-CNTFs and a mixture of MoCl5 and MoO3 in a mass ratio of 1:0.6 were loaded separately inside a glass tube. Following vacuum drying at 100 °C for 24 h (~1 mbar), the tube was flame-sealed and annealed at 300 °C for 9–72 h in a furnace. The resulting fibers were then washed with acetone and dried at 60 °C to yield N/MoCl5-CNTFs.

Preparation of N/I2-CNTFs, N/CuI-CNTFs, and N/FeCl3-CNTFs
For comparison, N-CNTFs and each dopant (I2, CuI, and FeCl3) were placed separately inside glass tubes, and dried at 100 °C under vacuum (~1 mbar) for 24 h to remove residual moisture. Subsequently, the flame-sealed glass tubes were annealed in a furnace at 200 °C for I2, 750 °C for CuI, and 360 °C for FeCl3, respectively. After cooling to room temperature, the fibers were removed, rinsed with acetone to eliminate adsorbed dopants, and dried at 60 °C to obtain N/I2-CNTFs, N/CuI-CNTFs, and N/FeCl3-CNTFs.

Measurement of the diameter of the fibers
The average diameter of each fiber was obtained from the lateral dimensions of the fibers’ SEM (FEI Quattro S, Thermo Fisher Scientific, operated at an acceleration voltage of 10 kV) images, based on thirty measurements for each sample. SEM images confirm that the fiber diameter is uniform along the axial direction. To further verify measurement consistency, the effective diameters were determined from cross-sectional area measurements. Cross-sections were prepared using FIB (Helios G4 UX, Thermo Fisher Scientific) cutting and subsequently characterized by SEM. The fibers exhibit an approximately circular cross-sectional morphology. Cross-sectional areas were quantified from the SEM images using ImageJ software (v1.54g). For each condition, at least three independently prepared fibers were analyzed to ensure statistical reliability. The effective diameters obtained from cross-sectional analysis agree well with those derived from the lateral SEM measurements under the assumption of circular geometry (Supplementary Table 9), confirming the reproducibility and low variability of the results. Accordingly, the average diameter of each fiber obtained from the lateral SEM measurements was used to calculate the cross-sectional area employed in both electrical conductivity and tensile strength analyses.

Other methods
A comprehensive description of other methods, including average aspect ratio measurements of the CNTs, Raman spectroscopy, mechanical and electrical property tests, Hall effect, KPFM, WAXS, EMI performance, DFT calculations, and other characterizations, can be found in Supplementary Information.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This paper demonstrates a major advancement in CNT fiber properties. The previously published record was around 10–12\nMS/m, whereas this work reports a conductivity of 27.34 MS/m. For specific conductivity, the previous record stood at 5640\nSm²/kg, and here the new maximum value is 14166 Sm²/kg—nearly a threefold increase in both metrics. Notably, this\nbreakthrough does not stem from incremental improvements in CNT alignment or aspect ratio, but rather from a specific\nprocessing step that introduces defects and places dopants within the CNTs. Furthermore, the enhanced conductivity\nremains stable over time and under various environmental conditions. Consequently, this paper is highly significant for a\nwide materials science audience, as it demonstrates that there is substantial room for further development of CNT fiber\nproperties.\nHowever, the approach of adding defects during the doping process would ordinarily be expected to reduce mobility, so the\nmagnitude of the reported conductivity increase is surprising and warrants strong supporting evidence. Because the findings\nare so important, it is essential that the paper provide enough detail both to validate the significant conductivity\nimprovements and to enable replication of the methodology. The following major points should be addressed before\npublication:\nMajor Points\nScrutiny of conductivity measurement and error analysis\na. Please clarify how you calculated the measurement errors and indicate the size of your datasets. You list error values for\nconductivity, but how were these derived? Are they from single measurements, or are there multiple measurements on\nmultiple fibers (how can you support that this is not a one-off)? How do you calculate each fiber’s cross-sectional area and\nincorporate that into the conductivity calculation, particularly for irregularly shaped cross sections? (I see references to this in\nthe supplemental mechanical section, but it is not clear if the same area calculation applies for conductivity.) Over what\nlength is conductivity measured? Seeing raw resistance (in ohms) over relatively long lengths (>10 cm) would be helpful to\nsee how deviations could affect conductivity values.\nb. How is specific conductivity calculated? You mention measuring linear density, but how many measurements are taken at\na time, and are you working near the sensitivity limits of your balance? Other papers have published erroneously high\nspecific conductivities due to the large relative error in weighing small fibers, even with specialized microbalances. How\nhave you mitigated this issue?\nc. Given the critical importance of the high conductivity values, I recommend placing all relevant details on conductivity and\nits measurement in the main text rather than in the supplemental material (though I defer to the authors and editor).\nEnvironmental conditions for maximum current density\nUnder what conditions was the maximum current density measured? Was it in vacuum, under argon, or on/off a substrate?\nHow was the maximum value established—by fiber failure? Additionally, what were the temperatures involved?\nInitial CNT properties\nThe CNTs in their as-synthesized state appear to be relatively highly conductive, so it would be helpful to include more\ninformation about them. Some details might be proprietary, but additional data would help others evaluate and replicate your\nresults. A critical question for CNT conductivity is the aspect ratio. If it can be measured or even estimated, it would help\ncontextualize these findings in the broader literature. If that is not possible, perhaps more details on the CNT growth process\nand purification steps (e.g., confirming zero iron content via TGA in the supplement) could be provided.\nEvidence that dopant resides within the CNTs\nThe fact that conductivity remains stable over time and even after solvent washing is a strong testament-- however, how do\nyou confirm that MoCl₅ is actually entering the CNT molecule? The EDS shown in the main paper does not make it entirely\nclear whether MoCl₅ is between the CNTs or within them. Could you provide EDS data of MoCl₅ fibers without N-plasma\ntreatment to highlight any differences? For the direct TEM imaging, it is hard to be certain that the dark spots represent the\ndopant. What if you reversed the sequence: doping first, then N-plasma treatment? If the conductivity still improved\nsignificantly, that might support or refute the conclusion that MoCl₅ is entering the CNTs.\nChoice of MoCl₅ as dopant\nIt would be interesting to know what motivated the selection of MoCl₅ as the dopant. Would other dopants work similarly\nwell? More details on the doping process, such as the background gas, temperature, and setup configuration, would be\nhelpful. Including a diagram or photograph of the setup could also provide valuable clarity.", "reviewer_2": "This article reports nitrogen and MoCl5 doping co-effect on high-conductivity carbon nanotube fibers. This is important and\nvery interesting for developing light-weight electrical conductors to replace Cu wires. The manuscript seems to be interesting\nand the results showed very high performance. I would recommend this article for publication in Nature Comm with major\nrevision.\n1. In this article, the N-doping seems to be critical for improving electrical conductivity of MoCl5 doped CNTF. Explain the\nreason and show any theoretical evidence.\n2. In line 189,\nBy what mechanism does the dopant (MoCl5) inside the CNT contribute to the improvement of conductivity? Since the\nelectrons will move along the CNT surface in the flow of electrons, wouldn't the dopants outside contribute to the\nimprovement of conductivity more effectively?\n3. In line 204~205\nThe current flowing through the fiber is proportional to fiber’s thickness. Therefore, temperature changes due to changes in\nvoltage are important, but information about thickness must be included in application part or Fig5\n4. In line 245,\nEven excluding the effect of doping, the strength and electrical conductivity of pristine CNTF are excellent. As mentioned in\nthe Method part, CNTF was manufactured according to the results of previous research. However, the properties of pristine\nCNTF were significantly improved compared to the previous research (ref. 28). If there are any additional material or process\nchanges, it is recommended to mention them.\n5. The specific electrical conductivity and ampacity of N/MoCl5 CNTF are surprisingly excellent. Based on the XPS results\nin this fiber, what is the weight ratio of N to MoCl5? Has the doping content been optimized?\n6. I would think this process for doping with N and MoCl seems to be batch process, not continuous process. What is the\nmaximum length of N/MoCl5 doped CNTF? Is it possible to produce N- and MoCl5 doped continuous fibers?\n7. Compared to previous reports in references, what could be benefit of this materials and process?", "reviewer_3": "The manuscript describes a strategy to enhance the electrical conductivity of carbon nanotube fibers (CNTFs) by combining\nnitrogen (N) doping and vacuum-assisted endohedral MoCl₅ doping. The authors claim that N doping introduces sidewall\ndefects on CNTs, which reportedly facilitates the adsorption and encapsulation of MoCl₅, thereby significantly improving the\nfiber’s electrical conductivity. While electrical conductivity of wet-spun CNTF including specific value is somewhat\nsurprising, the techniques described (N doping and endohedral/chemical doping with MoCl₅) are already well-established in\nthe literature.\nNitrogen doping of CNTs is a long-standing, extensively used strategy to tune electronic properties and create defect sites.\nMetal halide doping under vacuum or other inert conditions is also a widely employed method. Simply combining these two\nknown techniques, without in-depth mechanistic exploration or demonstrating a markedly improved performance over the\nstate of the art, is insufficient for publication in a high-impact journal. In addition, the manuscript posits that nitrogen-induced\ndefects help trap MoCl₅ particles in the CNT interior. However, there is no robust, quantitative analysis (such as d-spacing\nand SAED pattern by HR TEM or in situ spectroscopic studies) proving that the synergy of N doping and MoCl₅\nencapsulation distinctly outperforms single-doping routes. Moreover, it is unclear why MoCl₅ was specifically chosen,\ncompared to other dopants (I₂, Br₂, FeCl₃, etc.), and how this choice leads to superior or unique structural/electronic\noutcomes.\nMinor comments: While the manuscript includes some electron microscopy and spectroscopic characterizations, additional\ndata or deeper analysis (e.g., quantitative defect density measurements, doping efficiency, or doping stability over time) would be necessary to convince readers of the robustness of the approach and the reproducibility of the results.\nGiven the widely known nature of both N doping and vacuum-assisted metal halide doping approaches, and the insufficient\njustification for their specific combination, the manuscript in its current form does not meet the criteria for publication.", "reviewer_4": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature\nCommunications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career\nResearchers who co-review manuscripts."}, "author_response": "Point-by-point Responses to Reviewers’ Comments\nReviewer #1:\nThis paper demonstrates a major advancement in CNT fiber properties. The previously published record was\naround 10–12 MS/m, whereas this work reports a conductivity of 27.34 MS/m. For specific conductivity, the\nprevious record stood at 5640 S m2/kg, and here the new maximum value is 14166 S m2/kg—nearly a threefold\nincrease in both metrics. Notably, this breakthrough does not stem from incremental improvements in CNT\nalignment or aspect ratio, but rather from a specific processing step that introduces defects and places dopants\nwithin the CNTs. Furthermore, the enhanced conductivity remains stable over time and under various\nenvironmental conditions. Consequently, this paper is highly significant for a wide materials science audience,\nas it demonstrates that there is substantial room for further development of CNT fiber properties.\nHowever, the approach of adding defects during the doping process would ordinarily be expected to reduce\nmobility, so the magnitude of the reported conductivity increase is surprising and warrants strong supporting\nevidence. Because the findings are so important, it is essential that the paper provide enough detail both to\nvalidate the significant conductivity improvements and to enable replication of the methodology. The\nfollowing major points should be addressed before publication.\nReply: We sincerely thank you for recognizing the significance of our performance advancement and for the\nconstructive suggestions to enhance the clarity and robustness of the manuscript. In response, we have\nprovided detailed point-by-point replies (in blue) and thoroughly revised the text.\nTo address the concern regarding potential mobility reduction due to the approach of adding defects during\nthe doping process, we conducted comprehensive structural and electrical characterizations. Raman\nspectroscopy was used to quantify defect density (n ) via the I /I value, where I and I are the intensities\nD G D D G\nof D-band and G-band of CNTFs, respectively. Compared to PCNTFs, N-CNTFs show a slight decrease in\nthe I /I value from 49.75 ± 0.39 to 46.51 ± 0.30, consistent with minor defect introduction through nitrogen\nG D\nplasma under carefully controlled conditions (Supplementary Fig. 6d and Table 2) (Carbon, 2020, 167,\n881). Similarly, MoCl -CNTFs and N/MoCl -CNTFs show I /I values of 48.54 ± 0.43 and 45.25 ± 0.33,\n5 5 G D\nrespectively, indicating that only limited structural disorder was introduced by the doping processes (Nature,\n1997, 388, 257; Microelectron. Eng., 2022, 252, 111666). All doped fibers maintain high I /I values (> 45),\nG D\nindicating that the nanotube lattice remains largely intact. The corresponding defect densities, calculated using\nan established Raman-based method (Nano Lett., 2011, 11, 3190), are (4.829 ± 0.032) × 109 cm−2 for N-\nCNTFs and (4.956 ± 0.035) × 109 cm−2 for N/MoCl -CNTFs, representing only modest increases of 5.8% and\n5\n8.6%, respectively, compared to PCNTFs [(4.563 ± 0.036) × 109 cm−2]. These results confirm that the\nstructural integrity is well-preserved within acceptable limits that supports efficient charge transport.\nTo further evaluate the electrical transport behavior, we performed Hall effect measurements using a van\nder Pauw configuration on CNT films (1 cm × 1 cm) fabricated under the same doping conditions as the fibers.\nAlthough the van der Pauw method is not directly applicable to fibers, it is well-established for thin films and\noffers meaningful insight into intrinsic transport trends (Adv. Mater., 2016, 28, 7941). As expected, carrier\nmobility decreases upon doping due to enhanced carrier scattering, with reductions of ~55.3% for N-CNT\nfilms and ~49.1% for N/MoCl -CNT films compared to PCNT films (Supplementary Table 3). However,\n5\ncarrier density shows substantial increases, and values of N-CNT, MoCl -CNT, and N/MoCl -CNT films\n5 5\nreach 3.49, 4.44, and 10.67 × 1020 cm−3, representing 2.05-, 2.61-, and 6.27-fold increases compared to PCNT\n1\nfilms, respectively. In accordance with the Drude model, which defines that electrical conductivity is\nproportional to the product of mobility and carrier density, these results explain how even with a modest\ndecrease in mobility, the marked rise in carrier density results in significantly improved electrical conductivity.\nTo complement our experimental analysis, we further employed density functional theory (DFT)\ncalculations to elucidate the electronic effects of nitrogen and MoCl doping. Simulations reveal that MoCl\n5 5\nacts as a strong electron acceptor, with Bader charge analysis showing electron transfer of 0.85 e− per\nencapsulated molecule and 0.55 e− per adsorbed molecule (Supplementary Figs. 8a, b). Band structure\nanalysis further shows that nitrogen doping lowers the Fermi level (E ) by ~0.75 eV, indicative of hole doping\nF\n(ACS Nano, 2010, 4, 4165). Subsequent MoCl doping leads to a further downshift in the E (Supplementary\nFigs. 8c-f), reinforcing the increasingly p-type character of the system. These electronic modulations are\nconsistent with both our Raman spectra and KPFM measurements (Figs. 2h, i). The relationship between hole\ndensity (p) and the E is described as follows,\nF\np =n exp  ( E −E )/kT )  (1)\ni i F\nwhere n, k, and T represent the intrinsic carrier density, Boltzmann constant, and absolute temperature,\ni\nrespectively. Obviously, the exponential dependence of carrier density on E position underscores how these\nF\nshifts translate directly into the observed increases in electrical conductivity (Acc. Chem. Res., 2016, 49, 390).\nTaken together, the high electrical conductivity of the fibers results from the synergistic effects of preserved\nstructural integrity supporting adequate carrier mobility, and significantly elevated carrier density enabled by\nproposed doping strategy.\nFinally, to ensure that our findings can be independently validated and reproduced, we have substantially\nprovided the experimental details in the revised manuscript. These include detailed resistance data over\nrelatively long lengths of fibers, error analysis for electrical conductivity, environmental conditions for\nmaximum current density, initial CNT properties, evidence of dopant incorporation. These additions are noted\nin the following point-by-point responses and clearly marked the revised manuscript, and we believe they\nensure reproducibility and transparency of the proposed methodology.\nRevision:\nIn the main text,\nLine 116 in Page 6, the description of comparison of I /I values and defect densities of the fibers was added.\nG D\nThe I /I value of N-CNTFs (46.51 ± 0.30), where I and I represent the D-band and G-band intensities\nG D D G\nof CNTFs respectively, shows a slightly decrease compared to that of PCNTFs (49.75 ± 0.39)\n(Supplementary Fig. 6d and Table 2), consistent with the introduction of minor sidewall defects during\nnitrogen plasma treatment59. Similarly, MoCl -CNTFs and N/MoCl -CNTFs show I /I values of 48.54 ± 0.43\n5 5 G D\nand 45.25 ± 0.33, respectively, indicating that the structural integrity of the nanotube lattice is largely\npreserved following both doping processes. The corresponding defect densities, based on the established\nRaman-based method60, reveal only modest increases of 5.8% for N-CNTFs and 8.6% for N/MoCl -CNTFs\n5\ncompared to PCNTFs (Supplementary Table 2), confirming that the structural integrity is well-preserved\nwithin acceptable limits that supports efficient charge transport performance.\nLine 135 in Page 7, the description of density functional theory (DFT) calculations was added.\nreveal that MoCl acts as a strong electron acceptor, with Bader charge analysis showing electron transfer of\n5\n0.85 e− per encapsulated molecule and 0.55 e− per adsorbed molecule (Supplementary Figs. 8a, b). Band\nstructure analysis further shows that nitrogen doping lowers the E , indicative of hole doping63. Subsequent\nF\nMoCl doping leads to a further downshift in the E (Supplementary Figs. 8c-f), reinforcing the increasingly\np-type character of the system. These electronic modulations are consistent with both our spectroscopic and\nKPFM measurements (Figs. 2h, i). The relationship between hole density (p) and the E is described as follows,\nF\np =n exp\n\n( E −E )/kT )\n\n\ni i F\nwhere n, k, and T represent the intrinsic carrier density, Boltzmann constant, and absolute temperature,\ni\nrespectively. This exponential dependence highlights how progressive E shifts lead to substantial increases\nF\nin carrier density40,57,64.\nLine 146 in Page 7, the description of the Hall effect measurements for assessing the intrinsic transport trends\nwas added.\nWe further performed Hall effect measurements using a van der Pauw configuration on CNT films\nfabricated under the same doping conditions as the fibers. Although the van der Pauw method is not directly\napplicable to fibers, it is well-established for thin films and offers meaningful insight into intrinsic transport\ntrends45. As expected, carrier mobility decreases upon doping due to enhanced carrier scattering, with\nreductions of ~50% for N-CNT and N/MoCl -CNT films compared to pristine controls (Supplementary\n5 5\nCNT films representing 2.05-, 2.61-, and 6.27-fold increases compared to PCNT films, respectively. In\naccordance with the Drude model, which defines that electrical conductivity is proportional to the product of\nmobility and carrier density, these results explain how even with a modest decrease in mobility, the marked\nrise in carrier density results in significantly improved electrical conductivity. Taken together, the high\nelectrical conductivity of the fibers results from the synergistic effects of preserved structural integrity\nsupporting adequate carrier mobility, and significantly elevated carrier density enabled by effective doping.\nIn the Supplementary Information,\nLine 76 in Page 3, the description of the defect density measurement was added.\nRaman spectra and defect density tests of the CNTs and fibers\nRaman spectra of the CNTs and fibers were collected by a Horiba LabRAM HR 800 with a 532 nm laser\nwavelength. The defect density (n ) in the CNTFs was estimated using the I /I value from Raman\nD G D\nspectroscopy, which reflects the level of disorder introduced by doping. This approach is well-established, as\nthe D-band arises from intervalley scattering associated with defects, while the G-band corresponds to the in-\nplane vibration of sp2 carbon atoms. The defect density was calculated using the following empirical relation3,\n1.8×1022 I\nn (cm−2)= ( D) (1)\nD λ4 I\nL G\nWhere λ (nm) represents the wavelength used in the Raman spectroscopy measurement.\nL\nG D\nSupplementary Fig. 6 | XPS spectra, I /I value and defect density of different CNTFs.\nG D\nd Comparison of I /I value and defect density of different CNTFs.\nG D\nPage 25, the comparison of defect density of different CNTFs was added.\nSupplementary Table 2 | Comparison of defect density of different CNTFs.\nDefect density\nSample I /I\nG D\n(n , × 109 cm−2)\nD\nPCNTFs 49.75 ± 0.39 4.563 ± 0.036\nN-CNTFs 46.51 ± 0.30 4.829 ± 0.032\nMoCl -CNTFs 48.54 ± 0.43 4.639 ± 0.038\nThe CNT films were prepared using a blade coating method11-13, and subsequently cut into 1 cm × 1 cm\nsquares for Hall effect measurements. The measurement used a Hall effect test system (8404, Lake shore\nCryotronics, inc.) under a fixed current of 10 mA and a magnetic field of 5 kG.\nPage 26, the description of Hall effect measurement results of CNT films were added.\nSupplementary Table 3 | Hall effect measurements of carrier density and mobility of PCNT, N-CNT,\nMoCl -CNT and N/MoCl -CNT films at room temperature.\n5 5\nCarrier density Carrier mobility\nSample Carrier type\n(× 1020 cm−3) (cm2 V−1 s−1)\nPCNT films 1.70 6.78 hole\nN-CNT films 3.49 3.03 hole\nMoCl -CNT films 4.44 6.18 hole\n5\nDensity functional theory (DFT) calculations\nAll the calculations were carried out within the framework of DFT using the Vienna Ab initio Simulation\nPackage (VASP)7. The exchange-correlation effects were treated using the Perdew-Burke-Ernzerhof (PBE)\nfunctional within the generalized gradient approximation (GGA), combined with the projector augmented\nwave (PAW) method to describe ion-electron interactions8,9. A plane-wave basis set with an energy cutoff of\n450 eV was employed to ensure the convergence of total energy and force calculations. Structural\noptimizations were performed by allowing full relaxation of both atomic positions and electronic states until\nthe convergence criteria of 10−4 eV for total energy and 0.02 eV Å−1 for atomic forces were met. To eliminate\nspurious interactions between periodic images, a vacuum region exceeding 15 Å was introduced perpendicular\nto the periodic directions. Long-range dispersion forces were incorporated using Grimme’s DFT-D3\nsemiempirical correction scheme10, which improves the accuracy of van der Waals interactions critical to\ninterfacial systems.\nPage 15, the description of DFT calculation results was added.\nSupplementary Fig. 8 | Charge density differences and electronic band structures of doped CNTs.\na, b Comparison of charge density for encapsulation MoCl (a) and MoCl adsorbed on the outer wall (b) per\n5 5\nsimulation unit. c-f Comparison of electronic band structures of pristine CNTs (c), N-CNTs (d), MoCl -CNTs\n5\n(e) and N/MoCl -CNTs (f).\n5\nScrutiny of conductivity measurement and error analysis\na. Please clarify how you calculated the measurement errors and indicate the size of your datasets. You list\nerror values for conductivity, but how were these derived? Are they from single measurements, or are there\nmultiple measurements on multiple fibers (how can you support that this is not a one-off)? How do you\ncalculate each fiber’s cross-sectional area and incorporate that into the conductivity calculation, particularly\nfor irregularly shaped cross sections? (I see references to this in the supplemental mechanical section, but it is\nnot clear if the same area calculation applies for conductivity.) Over what length is conductivity measured?\nSeeing raw resistance (in ohms) over relatively long lengths (>10 cm) would be helpful to see how deviations\ncould affect conductivity values.\nReply: Thank you for your thoughtful and constructive comments. We are pleased to clarify the\nmethodologies used for conductivity measurements, error analysis, and cross-sectional area determination.\nThe reported conductivity values and their associated errors are based on five independent measurements\nconducted on multiple individual fibers of each type. The error bars represent the standard deviation across\nthese replicate measurements. To ensure transparency and reproducibility, we have now included the full set\nof raw mechanical and electrical data in the Supplementary Information (Supplementary Tables 4-7).\nRegarding the determination of each fiber’s cross-sectional area, we adopted a dual-validation approach.\nFirst, longitudinal SEM images (Figs. R1a-d) confirm the uniformity of fiber diameters along the axial\ndirection. The measured average diameters are 11.57 ± 0.08 μm (PCNTFs), 11.42 ± 0.02 μm (N-CNTFs),\n11.34 ± 0.04 μm (MoCl -CNTFs), and 11.30 ± 0.02 μm (N/MoCl -CNTFs). To verify the reliability of the\n5 5\ndiameter-based area estimation, we also performed direct area measurements on focused ion beam (FIB)-cut\ncross-sectional SEM images (Figs. R1e-h), analyzed using ImageJ software. The cross-sectional areas derived\nfrom image analysis are consistent with those calculated from the corresponding average diameters under the\nassumption of circular geometry. Accordingly, we used the average diameter of each fiber to calculate its\ncross-sectional area, which was subsequently employed in both electrical conductivity and tensile strength\ncalculations. These values are now provided in Supplementary Table 8.\nFig. R1 | Longitudinal and cross-sectional SEM images of CNTFs.\na-d Longitudinal SEM images of (a) PCNTFs (average diameter: 11.57 ± 0.08 μm), (b) N-CNTFs (average\ndiameter: 11.42 ± 0.02 μm), (c) MoCl -CNTFs (average diameter: 11.34 ± 0.04 μm), and (d) N/MoCl -CNTFs\n5 5\n(average diameter: 11.30 ± 0.02 μm), all showing uniform diameters along the axial direction. e-f Cross-\nsectional SEM images of (e) CNTFs, (f) N-CNTFs, (g) MoCl -CNTFs, and (h) N/MoCl -CNTFs, prepared by\n5 5\na FIB.\nmm (Supplementary Fig. 9a), which was calculated using the equation,\nL\nσ=\nRS\nwhere R, L, and S represent fiber resistance, gauge length between the voltage probes, and cross-sectional area\nof the fibers, respectively. For example, N/MoCl -CNTFs exhibit a resistance of 29.15 ± 0.08 Ω over 80 mm,\n5\ncorresponding to a conductivity of 27.34 MS m−1. To evaluate the robustness of our conductivity\nmeasurements across different lengths, we extended the resistance measurement to fibers of 100, 120, 140,\n160, 180, and 200 mm. The corresponding resistance values are 37.18 ± 0.08, 44.03 ± 0.11, 51.96 ± 0.10,\n58.35 ± 0.09, 64.34 ± 0.07, and 73.32 ± 0.09 Ω, respectively. The resulting electrical conductivity values\n(26.74 ± 0.06, 27.10 ± 0.07, 26.79 ± 0.05, 27.27 ± 0.04, 27.82 ± 0.03, 27.12 ± 0.03 MS m−1) remain consistent\nacross these lengths (Supplementary Fig. 9d). A strong linear correlation between resistance and length\nfurther confirms the reliability of these measurements, with an 2 value of 0.998 (Supplementary Fig. 9e),\nconfirming the reliability and scalability of our conductivity measurements. These results demonstrate that the\n𝑅𝑅\nreported electrical conductivity values are not isolated or anomalous, but instead represent reproducible,\nlength-independent properties of the fibers under the tested conditions.\nFinally, we have rigorously re-examined all datasets in response to your comments and confirm the\nconsistency and validity of our results. We believe the added experimental details and supplementary data\nsignificantly strengthen the manuscript and address your concerns regarding accuracy and reproducibility.\nRevision:\nIn the main text,\nLine 174 in Page 8, the descriptions of sampling and raw resistance over extended fiber lengths were added.\nTo evaluate the influence of fiber length on electrical conductivity, we further measured the raw resistance\nof N/MoCl -CNTFs over extended lengths (100 ~ 200 mm). A strong linear relationship between resistance\n5\nand fiber length is observed (Supplementary Figs. 9d, e), confirming the structural uniformity of the fibers\nand indicating that length variation has a negligible effect on the electrical conductivity.\nIn the Supplementary Information,\nPage 27, the raw mechanical and electrical properties of PCNTFs were added.\nSupplementary Table 4 | Raw mechanical and electrical properties of PCNTFs.\nSpecific tensile Electrical Specific electrical\nTensile strength\nPCNTFs strength conductivity conductivity\n(GPa)\n(N tex−1) (MS m−1) (S m2 kg−1)\n1 2.73 1.51 8.53 4686.26\n2 2.61 1.44 8.56 4700.93\n3 2.52 1.39 8.51 4678.13\n4 2.65 1.46 8.58 4716.64\n5 2.53 1.40 8.53 4684.66\nAverage value 2.61 1.42 8.54 4693.32\nError value 0.09 0.05 0.03 15.47\nSupplementary Table 5 | Raw mechanical and electrical properties of N-CNTFs.\nSpecific tensile Electrical Specific electrical\nTensile strength\nN-CNTFs strength conductivity conductivity\n(GPa)\n(N tex−1) (MS m−1) (S m2 kg−1)\n1 2.08 1.18 9.98 5672.73\n2 2.06 1.17 10.11 5745.80\n3 2.10 1.20 9.98 5668.76\n4 2.08 1.18 9.97 5663.74\n5 2.05 1.16 9.98 5668.21\nAverage value 2.07 1.18 10.00 5683.85\nError value 0.02 0.01 0.06 34.78\nPage 29, the raw mechanical and electrical properties of MoCl -CNTFs were added.\nTensile strength\nMoCl -CNTFs strength conductivity conductivity\n5\n(GPa)\n(N tex−1) (MS m−1) (S m2 kg−1)\n1 2.46 1.18 17.39 8740.31\n2 2.33 1.12 17.15 8618.02\n3 2.37 1.14 17.24 8665.20\n4 2.38 1.14 17.46 8774.06\n5 2.44 1.17 17.33 8710.24\nAverage value 2.38 1.14 17.32 8701.57\nError value 0.05 0.03 0.12 61.51\nPage 30, the raw mechanical and electrical properties of N/MoCl -CNTFs were added.\nTensile strength\nN/MoCl -CNTFs strength conductivity conductivity\n5\n(GPa)\n(N tex−1) (MS m−1) (S m2 kg−1)\n1 2.39 1.20 27.29 14141.07\n2 2.68 1.35 27.21 14100.67\n3 2.60 1.31 27.46 14220.60\n4 2.34 1.18 27.41 14203.38\n5 2.44 1.23 27.32 14152.88\nAverage value 2.49 1.26 27.34 14166.48\nError value 0.15 0.07 0.09 48.51\nWe adopted a dual-validation approach to determine each fiber’s cross-sectional area. First, longitudinal\nSEM images confirmed the uniformity of fiber diameters along the axial direction. The measured average\ndiameters are 11.57 ± 0.08 μm (PCNTFs), 11.42 ± 0.02 μm (N-CNTFs), 11.34 ± 0.04 μm (MoCl -CNTFs),\n5\nand 11.30 ± 0.02 μm (N/MoCl -CNTFs). To verify the reliability of the diameter-based area estimation, we\n5\nalso performed direct area measurements on focused ion beam (FIB)-cut cross-sectional SEM images,\nanalyzed using ImageJ software. The cross-sectional areas derived from image analysis are consistent with\nthose calculated from the corresponding average diameters under the assumption of circular geometry.\nAccordingly, we used the average diameter of each fiber to calculate its cross-sectional area, which was\nsubsequently employed in both conductivity and tensile strength calculations.\nPage 31, we added the average diameters of CNTFs and re-examined their mechanical and electrical properties.\nSupplementary Table 8 | Mechanical and electrical properties of CNTFs.\nN/MoCl -\nDiameter (μm) 11.57 ± 0.08 11.42 ± 0.02 11.34 ± 0.04 11.30 ± 0.02\nLinear density (tex) 0.19 ± 0.03 0.18 ± 0.05 0.21 ± 0.04 0.20 ± 0.02\nDensity (g cm−3） 1.82 1.76 1.99 1.93\nSpecific tensile strength\n1.42 ± 0.05 1.18 ± 0.01 1.14 ± 0.03 1.26 ± 0.07\n(N tex−1)\nTensile strength (GPa) 2.61 ± 0.09 2.07 ± 0.02 2.38 ± 0.05 2.49 ± 0.15\nYoung’s modulus (GPa) 165.76 ± 12.31 130.64 ± 4.46 157.80 ± 25.27 147.98 ± 9.86\nSpecific electrical\nconductivity 4693 ± 15 5684 ± 35 8702 ± 62 14166 ± 48\n(S m2 kg−1)\nElectrical conductivity\n8.54 ± 0.03 10.00 ± 0.06 17.32 ± 0.12 27.34 ± 0.09\n(MS m−1)\nCurrent carrying capacity\n342.80 ± 62.20 391.81 ± 59.23 763.48 ± 67.32 1241.38 ± 94.34\n(A mm−2)\nPage 16, we added the results of electrical conductivity and raw resistance over different fiber lengths.\nCNTFs.\na Schematic illustration of the four-probe method used to measure the electrical conductivity of CNTFs. d\nElectrical conductivity of N/MoCl -CNTFs as a function of test length. e Raw resistance of N/MoCl -CNTFs\n5 5\nplotted against test length.\nb. How is specific conductivity calculated? You mention measuring linear density, but how many\nmeasurements are taken at a time, and are you working near the sensitivity limits of your balance? Other\npapers have published erroneously high specific conductivities due to the large relative error in weighing small\nfibers, even with specialized microbalances. How have you mitigated this issue?\nReply: Thank you for your insightful comment. The specific electrical conductivity was calculated as the\nratio of electrical conductivity to fiber density. To accurately determine the density of CNTFs, we employed\nthe density gradient column method (Chem. Rev., 1963, 63, 257; Carbon, 2024, 226, 119180), using a two-\nsolvent system composed of carbon tetrachloride and dibromoethane to establish the density gradient. The\nfibers were introduced into the columns and allowed to equilibrate for 24 hours, and their position within the\ncolumn was recorded to determine density. To validate and cross-check these values, we performed\nindependent measurements based on the linear density and cross-sectional area of the fibers. Linear density\n10\nwas measured using a high-precision microbalance with a sensitivity of 1 μg, and we used 10-meter fiber\nlengths per sample to mitigate relative error. Measurements were conducted on at least three independently\nprepared fibers for each condition, ensuring statistical robustness and reproducibility. Cross-sectional areas\nwere determined from SEM images, as described previously, and used in the electrical conductivity and\ndensity calculations. The corresponding fiber densities derived from these methods were consistent, with\nvalues of 1.82, 1.76, 1.99, and 1.93 g cm−3 for PCNTFs, N-CNTFs, MoCl -CNTFs, and N/MoCl -CNTFs,\n5 5\nrespectively (Supplementary Table 8). These combined strategies allow us to ensure the accuracy and\nreproducibility of our specific conductivity calculations, while avoiding the overestimation errors previously\nreported in the literatures. We added these results and descriptions into the revised manuscript.\nRevision:\nIn the Supplementary Information,\nLine 84 in Page 3, the description of the linear density and fiber density was added.\nLinear density and density tests of the fibers\nThe linear density of the fiber was measured using a high-precision microbalance with a sensitivity of 1 μg\n(MYA 2.5Y, RADWAG Wagi Elektroniczne). To ensure reliable mass readings well above the instrument’s\ndetection threshold, we used fiber lengths of 10 meters per sample. Measurements were performed on at least\nthree independently prepared fibers for each condition. Linear density was calculated by\nW\nD= (2)\nL\nwhere D (tex), W (g), and L (m) are the linear density, weight, and length of the fibers, respectively. The\ndensity of each fiber type was then determined by combining the linear density with the cross-sectional area\nobtained from scanning electron microscopy (SEM) imaging. To validate and cross-check these values, the\nfiber density was also determined using the density gradient column method4,5. A two-solvent system\ncomposed of carbon tetrachloride and dibromoethane was used to establish the density gradient. The fibers\nwere introduced into the columns and allowed to equilibrate for 24 hours, and their position within the column\nwas recorded to determine density.\nc. Given the critical importance of the high conductivity values, I recommend placing all relevant details on\nconductivity and its measurement in the main text rather than in the supplemental material (though I defer to\nthe authors and editor).\nReply: Thank you for your valuable suggestion. In response, we have moved all relevant details on\nconductivity and its measurement from the Supplementary Information into the main text for enhanced\ntransparency and emphasis.\nRevision:\nIn the main text,\nLine 162 in Page 8, the description of the relevant details on conductivity measurement was added to the main\ntext.\nElectrical conductivity (σ) was measured using a four-point probe method over a fixed fiber length of 80\nmm (Supplementary Fig. 9a). To ensure reliable electrical contact, conductive silver paste was applied at\n11\nboth the current and voltage electrodes. A constant current was supplied through the outer electrodes, and the\nresulting voltage drop was measured across the inner electrodes. Electrical conductivity was then calculated\nusing the relation,\nL\nσ=\nRS\nwhere R is the measured resistance, L is the gauge length between the voltage probes, and S is the cross-\nsectional area of fiber. The same cross-sectional areas were used in both mechanical and electrical property\nanalyses. Five independent measurements were performed using multiple samples for each fiber type\n(Supplementary Tables 4-7).\nIn the Supplementary Information,\nLine 118 in Page 4, the description of electrical conductivity measurement of the fibers was also added.\nElectrical resistance of the fibers was measured using a four-point probe method over fiber lengths ranging\nfrom 80 to 200 mm, using a Keithley 2450 Source Meter and a Keithley 4200A-SCS parameter analyzer.\nConductive silver paste was applied at the electrode interfaces to ensure contact reliability. Conductivity was\ncalculated based on measured resistance, electrode spacing, and cross-sectional area derived from SEM.\nFive independent measurements per fiber type were used to obtain average values and standard deviations\n(Supplementary Tables 4-7). Specific electrical conductivity was further calculated as the ratio of electrical\nconductivity to fiber density.\nEnvironmental conditions for maximum current density\nUnder what conditions was the maximum current density measured? Was it in vacuum, under argon, or on/off\na substrate? How was the maximum value established—by fiber failure? Additionally, what were the\ntemperatures involved?\nReply: Thank you for your useful comment. We clarify that the current carrying capacity is defined as the\nmaximum current density sustained by a freestanding fiber (i.e., off-substrate) under ambient air conditions.\nIt is calculated by dividing the maximum current measured prior to failure by the fiber’s cross-sectional area.\nTo measure this, the test was conducted entirely in ambient air without thermal insulation or inert atmosphere.\nThe fiber was connected to the electrodes of a programmable power supply, and a voltage sweep was applied.\nThe corresponding current was continuously monitored. As shown in Supplementary Fig. 11c, the current\ndensity initially increases linearly with voltage until a sharp drop occurs, indicating electrical failure of the\nfiber due to Joule heating (ACS Nano, 2023, 17, 9245; Adv. Funct. Mater., 2024, 34, 2404538). Based on\ninfrared imaging, the estimated failure temperature is approximately 630°C (Supplementary Figs. 11d, e).\nThese results and relevant discussions were added into the revised manuscript.\nRevision:\nIn the main text,\nLine 194 in Page 9, the description of the current-carrying capacity of fibers was added.\nThe current carrying capacity is defined as the maximum current density sustained by a freestanding fiber\nunder ambient air conditions. It is calculated by dividing the maximum current measured prior to failure by\nthe fiber’s cross-sectional area. As shown in Supplementary Fig. 11c, the current density initially increases\n12\nlinearly with voltage until a sharp drop occurs, indicating fiber failure due to Joule heating30,36. Based on\ninfrared imaging, the estimated failure temperature is approximately 630°C (Supplementary Figs. 11d, e).\nIn the Supplementary Information,\nLine 128 in Page 5, the description of the current carrying capacity measurements was added.\nCurrent carrying capacity tests of the fibers\nA 50 mm-long freestanding fiber was connected to the electrodes of a programmable power supply\n(Keithley 2200-60-2) using conductive silver paste. A continuous voltage sweep was applied under ambient\nair conditions, and the corresponding current was recorded in real time. As the voltage increased, current\ndensity rose linearly until a sudden drop occurred, indicating fiber failure due to Joule heating. The\ncorresponding maximum current density was current carrying capacity of the fiber. The failure temperature\nwas estimated via infrared thermal imaging (Fluke Ti10).\nPage 18, the temperature evolution as the function of applied voltages and the infrared thermal image just\nprior to fiber failure were added.\nSupplementary Fig. 11 | Mechanical and electrical properties of CNTFs.\nd Temperature evolution as the function of applied voltages under ambient air conditions. e Infrared thermal\nimage of the fiber just prior to failure under ambient air conditions.\nInitial CNT properties\nThe CNTs in their as-synthesized state appear to be relatively highly conductive, so it would be helpful to\ninclude more information about them. Some details might be proprietary, but additional data would help others\nevaluate and replicate your results. A critical question for CNT conductivity is the aspect ratio. If it can be\nmeasured or even estimated, it would help contextualize these findings in the broader literature. If that is not\npossible, perhaps more details on the CNT growth process and purification steps (e.g., confirming zero iron\ncontent via TGA in the supplement) could be provided.\nReply: Thank you for your insightful comment. In response, we have provided additional details regarding\nthe properties of the CNTs, their purification steps, and characterizations to support reproducibility and\ncontextualize our results within the broader literature.\nThe CNTs used in this study were purchased from Cnano Technology Co., Ltd. (Jiangsu, China) and\n13\nsubsequently subjected to a purification process. The as-received CNTs were first dispersed in 30% H O (1\n2 2\nmg mL−1) and stirred for 24 hours, followed by vacuum filtration. The recovered CNTs were then dispersed\nin 12 M HCl (5 mg mL−1) and stirred for 12 hours to remove residual metal catalysts. After neutralization with\ndeionized water, the CNTs were frozen in liquid nitrogen and dried overnight via freeze-drying. This treatment\nreduced the impurity content from 3.03 wt% to 0.84 wt%, as confirmed by thermogravimetric analysis\n(Supplementary Fig. 2b). It also improved the crystallinity, evidenced by an increase in the I /I value from\nG D\n44.5 to 49.3 (Supplementary Fig. 2c). Additionally, XPS analysis further reveals a low oxygen content of\n2.9 at% (Supplementary Fig. 2d). These improvements promote efficient protonation in chlorosulfonic acid\n(CSA), promoting liquid crystalline phase formation that is essential for producing well-aligned CNT\nassemblies and achieving high-performance fibers (ACS Nano, 2010, 4, 3969; ACS Appl. Mater. Interfaces,\n2017, 9, 36189; Sci. Adv., 2022, 8, eabn0939).\nTo estimate the average aspect ratio of the CNTs, we employed a capillary thinning rheometric method\nbased on dripping-onto-substrate extensional rheometry of CNT/CSA solutions, following established\nprotocols (Macromolecules, 2007, 40, 4043; Macromolecules, 2016, 49, 681; Carbon, 2022, 196, 59). In the\nelastocapillary regime, the filament diameter decreases exponentially with time, and the extensional viscosity\nis extracted from the mid-filament thinning profile (Supplementary Figs. 2g, h). Using theoretical models\nthat relate extensional viscosity to aspect ratio in semi-dilute rigid-rod systems, we determined the average\naspect ratio of our CNTs to be approximately 4300.\nTo contextualize this result, we compared the electrical conductivity of our fibers with those of wet-spun\nCNTFs reported in the literature that were fabricated from CNTs with known aspect ratios (Fig. R2). A strong\npositive correlation is observed between CNT aspect ratio and fiber conductivity, and our data align well with\nthis established trend. These results support the conclusion that the high performance of our as-spun fibers is\nenabled by the use of high-purity and high-aspect-ratio CNTs.\nFig. R2 | A comparison of electrical conductivities of wet-spun CNTFs as a function of the average CNT\naspect ratios. The results show a clear positive correlation, indicating that higher CNT aspect ratio contributes\nto improved fiber conductivity. The data from this work aligns well with previously reported trends in the\nliterature.\nIn the main text,\nLine 81 in Page 5, the description of additional information on CNTs was added.\nThe as-received CNTs were purified to reduce impurities and improve crystallinity. The purified CNTs\nexhibited a low oxygen content, and the average aspect ratio was subsequently estimated to be approximately\n4300 using a capillary thinning rheometric method50,51 (Supplementary Fig. 2).\nLine 303 in Page 12, the description of purification steps of CNTs was added.\nPurification of CNTs. The CNTs used in this study were purchased from Cnano Technology Co., Ltd.\n(Jiangsu, China) and subsequently subjected to a purification process. The as-received CNTs were first\ndispersed in 30% H O (1 mg mL−1) and stirred for 24 hours, followed by vacuum filtration. The recovered\n2 2\nCNTs were then dispersed in 12 M HCl (5 mg mL−1) and stirred for 12 hours to remove residual metal catalysts.\nAfter neutralization with deionized water, the CNTs were frozen in liquid nitrogen and dried overnight via\nfreeze-drying.\nIn the Supplementary Information,\nLine 66 in Page 3, the description of the measurement of CNT aspect ratio was added.\nAverage aspect ratio measurements of the CNTs\nThe average aspect ratio of CNTs was determined using a capillary thinning rheometric method based on\ndripping-onto-substrate extensional rheometry of CNT/chlorosulfonic acid (CSA) solution at a concentration\nof 200 ppm. The measurements were performed under axial extension using a custom-built rheometer\nequipped with two Teflon pistons (2 mm in diameter), operated at a controlled stretching rate of 25 mm s−1. A\nhigh-speed camera was used to capture the filament thinning process in real time. The recorded videos were\nanalyzed to extract the filament diameter as a function of time. The extensional viscosity was then calculated\nbased on the elastocapillary thinning regime, and the average CNT aspect ratio was derived using theoretical\nmodels developed for semi-dilute rigid-rod systems1,2.\nPage 9, the additional information on CNTs was added.\na SEM image of CNTs. b Thermogravimetric analysis of CNTs. c Raman spectrum of CNTs. d XPS spectrum\nof the purified CNTs, showing a low oxygen content. e, f TEM images of CNTs. The inset image shows\nstatistical analysis of CNT diameters. g Filament profiles captured during capillary thinning experiments of\nCNT/CSA solution under axial extension. h Evolution of filament diameter as a function of time from\nextensional viscosity measurements. The linear fit in the elastocapillary regime is used to extract the intrinsic\nextensional viscosity. The corresponding average aspect ratio of the CNTs is estimated to be approximately\n4300.\nEvidence that dopant resides within the CNTs\nThe fact that conductivity remains stable over time and even after solvent washing is a strong testament--\nhowever, how do you confirm that MoCl is actually entering the CNT molecule? The EDS shown in the main\n5\npaper does not make it entirely clear whether MoCl is between the CNTs or within them. Could you provide\n5\nimaging, it is hard to be certain that the dark spots represent the dopant. What if you reversed the sequence:\ndoping first, then N-plasma treatment? If the conductivity still improved significantly, that might support or\nrefute the conclusion that MoCl is entering the CNTs.\nfollowing experiments and analyses, described in detail below.\n5\nangle annular dark-field scanning transmission electron microscopy (HAADF-STEM) on radial cross-sections\nof the fibers. As shown in Figs. 1c and 4i, bright contrast spots are clearly observed within the nanotube inner\ncavities. This contrast is consistent with the higher atomic number of Mo compared to carbon (Nature, 2010,\n464, 571; Nat. Commun., 2021, 12, 1578; Nat. Commun., 2025, 16, 6300). Corresponding energy-dispersive\nX-ray spectroscopy (EDS) mapping further confirms the co-localization of Mo and Cl signals within the\nnanotube channels (Figs. 4j-m), indicating successful endohedral incorporation of MoCl .\nanalyses on MoCl -CNTFs. As shown in Figs. 4d-h, bright contrast associated with MoCl predominantly\n5 5\nlocates in interstitial spaces between adjacent CNTs, rather than within the nanotube inner cavities.\nThis structural difference correlates with reduced conductivity stability in MoCl -CNTFs compared to that of\n5\nwashing. These findings indicate that nitrogen doping plays a key role in facilitating and stabilizing MoCl\n5\nincorporation into nanotube inner cavities.\nAs you suggested, we conducted a control experiment in which the doping sequence was reversed: MoCl\n5\ndoping was performed first, followed by nitrogen plasma treatment, producing MoCl /N-CNTFs. As shown\n5\nin Supplementary Fig. 13a, these fibers exhibit a significantly reduced electrical conductivity of 12.32 ± 0.07\nMS m−1 compared to both MoCl -CNTFs and N/MoCl -CNTFs. X-ray photoelectron spectroscopy (XPS)\n5 5\nanalysis reveals a corresponding decrease in Mo content, from 13.54 at% in N/MoCl -CNTFs to 7.35 at% in\n5\nwith the plasma treatment, which may cause partial dopant desorption (Sci. Rep., 2017, 7, 10175). These\nresults confirm the critical importance of treatment sequence and demonstrate that nitrogen functionalization\nprior to MoCl doping is essential for achieving effective endohedral incorporation and synergistically\n5\nenhancing both electrical conductivity and environmental stability.\nRevision:\nIn the main text,\nLine 67 in Page 4, the description of MoCl incorporation within the nanotube inner cavities was added.\nparticles with brighter contrast are clearly observed within the nanotube inner cavities. This brighter contrast\nis attributed to MoCl ’s higher atomic number (Z) compared to carbon47-49.\nc HAADF-STEM image of N/MoCl -CNTFs and corresponding elemental mappings of N, Mo, and Cl.\n5\n5\nstability in N/MoCl -CNTFs was added.\n5\nperformed additional HAADF-STEM and EDS analyses on the radial cross-sections of MoCl -CNTFs and\n5 5 5\ninterstitial spaces between adjacent CNTs (Fig. 4d). Corresponding EDS mapping further confirms Mo and\nCl signals mainly localize outside the nanotube interiors (Figs. 4e-h). In contrast, N/MoCl -CNTFs exhibit\n5\npredominant bright contrast spots within the inner cavities of the nanotubes (Fig. 4i), with EDS mapping\nconfirming co-localized Mo and Cl signals (Figs. 4j-m). This confirms that nitrogen doping promotes the\nendohedral incorporation of MoCl . This internal confinement is believed to shield the dopants from external\n5\nperturbations, effectively mitigating the impact of solvents and thermal cycling on the electrical performance\nof N/MoCl -CNTFs. We further reversed the doping sequence, where MoCl doping was performed first,\n5 5\nfollowed by nitrogen plasma treatment, producing MoCl /N-CNTFs. As shown in Supplementary Fig. 13a,\n5\nthese fibers exhibit a significantly reduced electrical conductivity of 12.32 ± 0.07 MS m−1 compared to both\nMoCl -CNTFs and N/MoCl -CNTFs. XPS analysis reveals a corresponding decrease in Mo content,\n5 5\nfrom 13.54 at% in N/MoCl -CNTFs to 7.35 at% in MoCl /N-CNTFs. This reduction is likely due to the\n5 5\nthermal effects associated with the plasma treatment, which may cause partial dopant desorption67. These\nresults confirm the critical importance of treatment sequence and demonstrate that nitrogen functionalization\nprior to MoCl doping is essential for achieving effective endohedral incorporation and synergistically\n5\nenhancing both electrical conductivity and environmental stability.\nPage 22, the comparison of HAADF-STEM images and corresponding elemental mappings for MoCl -CNTFs\n5\nand N/MoCl -CNTFs was added.\nd-h HAADF-STEM image (d) of MoCl -CNTFs and corresponding elemental mappings of C (e), N (f), Mo\n5\n(g), and Cl (h). i-m HAADF-STEM image (i) of N/MoCl -CNTFs and corresponding elemental mappings of\nIn the Supplementary Information,\nPage 20, the description of electrical conductivity and Mo content of MoCl /N-CNTFs was added.\n5\na Electrical conductivity of MoCl -CNTFs, N/MoCl -CNTFs and MoCl /N-CNTFs, indicating a significant\n5 5 5\nreduction in electrical conductivity for MoCl /N-CNTFs. b XPS spectra comparing N/MoCl -CNTFs and\n5 5\nMoCl /N-CNTFs, showing a marked decrease in Mo content for MoCl /N-CNTFs.\n5 5\nChoice of MoCl as dopant\n5\nwork similarly well? More details on the doping process, such as the background gas, temperature, and setup\nconfiguration, would be helpful. Including a diagram or photograph of the setup could also provide valuable\nclarity.\nReply: Thank you for your insightful comment. In response, we have conducted a comparative analysis with\nalternative dopants and provided additional experimental details on the doping process, including temperature,\nbackground conditions, and the setup configuration.\nTo evaluate the effectiveness of MoCl relative to other dopants, we prepared N/I -CNTFs, N/CuI-CNTFs,\n5 2\nand N/FeCl -CNTFs using I , CuI, and FeCl as dopants, respectively, via a similar two-zone vapor transport\n3 2 3\nmethod. The electrical conductivity of N/I -CNTFs reaches 16.89 ± 0.06 MS m−1, comparable to that of\n5 3\nconductivities of 25.97 ± 0.09 and 26.87 ± 0.08 MS m−1, respectively, compared to N/MoCl -CNTFs (27.34\n5\n± 0.09 MS m−1) (Supplementary Fig. 10a). Raman spectra of all doped fibers show progressive blue shifts in the G-band relative to PCNTFs, with the largest shift observed in N/MoCl -CNTFs (Supplementary Fig.\n5\n10b). This trend suggests MoCl induces stronger p-type doping and more significant modulation of the CNTs\n5\nelectronic structure through charge transfer interactions. We also evaluated the long-term environmental\nstability of these doped fibers. Among all candidates, N/MoCl -CNTFs demonstrate the smallest change in\n5\nelectrical conductivity over 50 days under ambient conditions (Supplementary Fig. 12a). These results\ncollectively indicate MoCl as the most effective dopant among those tested, offering superior performance in\n5\nboth electrical performance and environmental stability.\nAdditionally, as suggested, we have provided a detailed description of the doping process, as well as a\nschematic of the setup (Supplementary Fig. 16). In a typical process, the dopant (a mixture of MoCl and\n3\ndried at 100°C under vacuum (~1 mbar) for 24 hours to remove residual moisture. It was then flame-sealed\nand annealed in a tube furnace at 300°C for 9 ~ 72 hours in a furnace with a heating/cooling rate of 10 °C\n19\nmin−1. After cooling to room temperature, the fibers were rinsed with acetone and dried at 60°C to obtain the\nfinal N/MoCl -CNTFs. Detailed doping conditions for the other dopants have also been included in the revised\n5\nmanuscript.\nRevision:\nIn the main text,\nLine 178 in Page 8, the description of comparing the performance of different doped CNTFs was added.\nTo evaluate the effectiveness of MoCl relative to other dopants, we prepared N/I -CNTFs, N/CuI-CNTFs,\n5 2\nand N/FeCl -CNTFs using I , CuI, and FeCl as dopants, respectively, via a similar two-zone vapor transport\n3 2 3\nmethod. The electrical conductivity of N/I -CNTFs reaches 16.89 ± 0.06 MS m−1, while N/CuI-CNTFs and\n3\ncompared to N/MoCl -CNTFs (Supplementary Fig. 10a). Raman spectra of all doped fibers show\n5\nprogressive blue shifts in the G-band relative to PCNTFs, with the largest shift observed in N/MoCl -CNTFs\n5\n(Supplementary Fig. 10b). This trend suggests MoCl induces stronger p-type doping and more significant\n5\nmodulation of the CNTs electronic structure through charge transfer interactions, resulting in the highest\nelectrical conductivity. These results highlight MoCl as the most effective dopant among those tested.\nadded.\nWe further compared the environmental stability of the doped fibers. While MoCl -CNTFs exhibit a notable\n5\ndecline in the electrical conductivity over time, N/I -CNTFs, N/CuI-CNTFs, N/FeCl -CNTFs, and N/MoCl -\n2 3 5\nCNTFs maintain more significantly stable electrical conductivities over 50 days under ambient conditions\n(Fig. 4a and Supplementary Fig. 12a). Among these, N/MoCl -CNTFs demonstrate the smallest change,\n5\nindicating superior long-term stability.\nLine 319 in Page 12, the detailed description of the doping processes was added.\nPreparation of N-CNTFs. PCNTFs were first placed inside a quartz reaction tube and dried at 100°C under\nvacuum (~1 mbar) for 24 hours to eliminate residual moisture. The dried fibers were then subjected to\ninductively coupled plasma treatment at a frequency of 13.56 MHz and a power of 50 W for 2 minutes in an\nO /Ar (50/150 sccm) atmosphere. This treatment generated sidewall defects on the CNTs, enhancing the\n2\navailability of nitrogen doping sites. Subsequently, the fibers were exposed to a N /Ar plasma (50/150 sccm)\n2\nfor 2 ~ 15 minutes to introduce nitrogen atoms into the nanotube structure, yielding the N-CNTFs.\nPreparation of MoCl -CNTFs. As shown in Supplementary Fig. 16, PCNTFs and a mixture of MoCl and\n5 5\nMoO in a mass ratio of 1:0.6 were placed separately inside a glass tube. The tube was first dried at 100°C\n3\nunder vacuum (~1 mbar) for 24 hours to remove residual moisture, then flame-sealed, and annealed at 300°C\nfor 9 ~ 72 hours in a furnace with a heating/cooling rate of 10 °C min−1. After cooling to room temperature,\nthe fibers were rinsed with acetone to remove residual dopants and dried at 60°C to obtain the final MoCl -\nPreparation of N/MoCl -CNTFs. N/MoCl -CNTFs were prepared using the same two-zone vapor transport\n5 5\nmethod. N-CNTFs and a mixture of MoCl and MoO in a mass ratio of 1:0.6 were first loaded separately\n5 3\n20\ninside a glass tube. Following vacuum drying at 100°C for 24 hours (~1 mbar), the tube was flame-sealed and\nannealed at 300°C for 9 ~ 72 hours in a furnace. The resulting fibers were then washed with acetone and dried\nat 60°C to yield N/MoCl -CNTFs.\n2 3\ndopant (I , CuI, and FeCl ) were placed separately inside glass tubes, and dried at 100°C under vacuum (~1\n2 3\nmbar) for 24 hours to remove residual moisture. Subsequently, the flame-sealed glass tubes were annealed in\na furnace at 200°C for I , 750°C for CuI, and 360°C for FeCl , respectively. After cooling to room temperature,\n2 3\nthe fibers were removed, rinsed with acetone to eliminate adsorbed dopants, and dried at 60°C to obtain N/I -\nPage 17, the comparison of electrical conductivity and Raman spectra of different doped CNTFs was added.\nSupplementary Fig. 10 | Electrical conductivity and Raman spectra of different CNTFs.\na Comparison of electrical conductivity among N-CNTFs doped with I , CuI, FeCl , and MoCl , showing the\n2 3 5\nhighest conductivity for N/MoCl -CNTFs. b Raman spectra of N/I -CNTFs, N/CuI-CNTFs, N/FeCl -CNTFs,\n5 2 3\nand N/MoCl -CNTFs, showing progressive blue shifts in the G-band relative to PCNTFs.\nSupplementary Fig. 12 | Comparison of environmental stability of different doped CNTFs.\na Comparison of electrical conductivity of N/I -CNTFs, N/CuI-CNTFs, N/FeCl -CNTFs, and N/MoCl -\n2 3 5\nCNTFs over 50 days under ambient conditions. b Relative resistance changes of MoCl -CNTFs and N/MoCl -\n5 5\nCNTFs after immersion in different solvents for 24 hours. All error bars show mean ± SD.\nSupplementary Fig. 16 | Schematic diagram of the two-zone vapor transport method for doped CNTFs.\nCNTFs and the selected dopant were placed separately inside a glass tube and vacuum-dried. The tube was\nthen flame-sealed and annealed in a furnace at the desired temperature and duration. After cooling to room\ntemperature, the fibers were removed, rinsed with acetone to remove residual dopants, and dried to obtain the\ndoped CNTFs.\nThis article reports nitrogen and MoCl doping co-effect on high-conductivity carbon nanotube fibers. This is\n5\nimportant and very interesting for developing light-weight electrical conductors to replace Cu wires. The\nmanuscript seems to be interesting and the results showed very high performance. I would recommend this\narticle for publication in Nature Comm with major revision.\nReply: Thank you for your positive evaluation and recognition of the significance of our work. To further\nenhance the quality of the manuscript, we have carefully addressed all your comments and have revised the\nmanuscript accordingly. Detailed point-by-point responses are provided below.\n1. In this article, the N-doping seems to be critical for improving electrical conductivity of MoCl doped CNTF.\nReply: Thank you for this important observation and valuable comment. In response, we have conducted a\ncomprehensive set of structural, electronic, and theoretical analyses to clarify the synergistic role of N-doping\nin enhancing the electrical conductivity of doped CNTFs. Our results reveal that N-doping facilitates MoCl\n5\nencapsulation, modulates the electronic structure, and increases carrier density, all contributing to improved\nelectrical performance.\nTo evaluate structural changes induced by N-doping, we analyzed the Raman I /I value and calculated\nG D\ndefect density. Compared to PCNTFs, N-CNTFs show a slight decrease in the I /I value from 49.75 ± 0.39\nG D\nto 46.51 ± 0.30, consistent with minor defect introduction through nitrogen plasma under carefully controlled\nconditions (Supplementary Fig. 6d and Table 2) (Carbon, 2020, 167, 881). Similarly, MoCl -CNTFs and\nstructural disorder introduced by the doping processes (Microelectron. Eng., 2022, 252, 111666; Nature, 1997,\n388, 257). All doped fibers maintain high I /I values (> 45), indicating that the nanotube lattice remains\nG D\nlargely intact. The corresponding defect densities, calculated using an established Raman-based method (Nano\nLett., 2011, 11, 3190), are (4.829 ± 0.032) × 109 cm−2 for N-CNTFs and (4.956 ± 0.035) × 109 cm−2 for\nN/MoCl -CNTFs, representing only modest increases of 5.8% and 8.6%, respectively, compared to PCNTFs\n5\n[(4.563 ± 0.036) × 109 cm−2]. These results confirm that the structural integrity is well-preserved within\nacceptable limits that supports efficient charge transport.\nTo investigate the influence of N-doping on MoCl incorporation, we conducted high-angle annular dark-\n5\nfield scanning transmission electron microscopy (HAADF-STEM) imaging and energy-dispersive X-ray\nspectroscopy (EDS) mapping on MoCl -CNTFs and N/MoCl -CNTFs. In MoCl -CNTFs, bright contrast spots\n5 5 5\npredominantly locate in interstitial spaces between adjacent CNTs (Fig. 4d). This contrast is consistent with\nthe higher atomic number of Mo compared to carbon (Nature, 2010, 464, 571; Nat. Commun., 2021, 12, 1578;\nNat. Commun., 2025, 16, 6300). Corresponding EDS mapping further confirms Mo and Cl signals mainly\nlocalize outside the nanotube interiors (Figs. 4e-h). In contrast, in MoCl -CNTFs, bright contrast associated\n5\nwith MoCl are clearly observed within the nanotube inner cavities (Figs. 1c and 4i), with EDS mapping\n5\nconfirming co-localized Mo and Cl signals (Figs. 4j-m). This confirms that N-doping promotes the endohedral\nincorporation of MoCl . This structural difference correlates with reduced conductivity stability in MoCl -\n5 5\nCNTFs compared to that of N/MoCl -CNTFs, which exhibit high electrical conductivity even after prolonged\n5\nexposure and solvent washing (Fig. 4a and Supplementary Fig. 12b). These findings indicate that N-doping\n23\nplays a key role in facilitating and stabilizing MoCl incorporation into nanotube inner cavities.\nspectroscopy (XPS), Raman, Kelvin probe force microscopy (KPFM), and Hall effect measurements. First,\nXPS was employed to investigate the chemical composition and bonding states of the fibers. For N-CNTFs,\nthe N 1s spectrum confirms the successful incorporation of N, with pyridinic N identified as the predominant\nbonding configuration (Supplementary Fig. 6a and Table 1), known to induce p-type doping in CNTs (Adv.\nMater., 2017, 29, 1702268). Second, Raman spectroscopy using a 532 nm excitation wavelength provided\nfurther electronic insights. Raman spectra of N-CNTFs, MoCl -CNTFs, and N/MoCl -CNTFs reveal\n5 5\nprogressive blue shifts of the G-band compared to PCNTFs, with the largest shift of 19 cm−1 observed in\nN/MoCl -CNTFs (Fig. 2h). These shifts are attributed to doping-induced charge transfer, wherein electron\n5\ndonation from CNTs to N and MoCl results in p-type doping and corresponding modifications to the CNTs\n5\nelectronic structure (Nature, 1997, 388, 257; J. Am. Chem. Soc., 2011, 133, 5941). Third, KPFM was used to\nmeasure the surface potentials of the fibers to further investigate the transition in electrical behavior. The\nsurface potentials of N-CNTFs, MoCl -CNTFs, and N/MoCl -CNTFs are progressively higher than that of\n5 5\nPCNTFs, corresponding to an increase in work function from 4.45 ± 0.08 eV to 4.56 ± 0.03, 4.63 ± 0.02, and\n4.87 ± 0.03 eV, respectively (Fig. 2i). This increase in the work function is attributed to p-type doping effect\nof N and MoCl , which induces a downshift in the E of CNTs. Finally, we performed Hall effect\nmeasurements using a van der Pauw configuration on CNT films (1 cm × 1 cm) fabricated under the same\ndoping conditions as the fibers. Although the van der Pauw method is not directly applicable to fibers, it is\nwell-established for thin films and offers meaningful insight into intrinsic transport trends (Adv. Mater., 2016,\n28, 7941). As expected, carrier mobility decreases upon doping due to enhanced carrier scattering, with\nreductions of ~55.3% for N-CNT films and ~49.1% for N/MoCl -CNT films compared to pristine controls\n5\n(Supplementary Table 3). However, carrier density shows substantial increases, and values of N-CNT,\nMoCl -CNT, and N/MoCl -CNT films reach 3.49, 4.44, and 10.67 × 1020 cm−3, representing 2.05-, 2.61-, and\n5 5\n6.27-fold increases compared to PCNT films, respectively. In accordance with the Drude model, which defines\nthat electrical conductivity is proportional to the product of mobility and carrier density, these results explain\nhow even with a modest decrease in mobility, the marked rise in carrier density results in significantly\nimproved electrical conductivity.\nTo complement our experimental analysis, we further employed density functional theory (DFT)\ncalculations to elucidate the electronic effects of N- and MoCl doping. Simulations reveal that MoCl acts as\n5 5\na strong electron acceptor, with Bader charge analysis showing electron transfer of 0.85 e− per encapsulated\nmolecule and 0.55 e− per adsorbed molecule (Supplementary Figs. 8a, b). Band structure analysis further\nshows that N-doping lowers the Fermi level (E ) by ~0.75 eV, indicative of hole doping (ACS Nano, 2010, 4,\nF\n4165). Subsequent MoCl doping leads to a further downshift in the E (Supplementary Figs. 8c-f),\nreinforcing the increasingly p-type character of the system. These electronic modulations are consistent with\nboth our spectroscopic and KPFM measurements (Figs. 2h, i). The relationship between hole density (p) and\nthe E is described as follows,\nF\np =n exp  ( E −E )/kT )  (1)\ni i F\nwhere n, k, and T represent the intrinsic carrier density, Boltzmann constant, and absolute temperature,\ni\nrespectively. Obviously, the exponential dependence of carrier density on E position underscores how these\nF\n24\nshifts translate directly into the observed increases in electrical conductivity (Acc. Chem. Res., 2016, 49, 390).\nTaken together, the high electrical conductivity of the fibers results from the synergistic effects of preserved\nstructural integrity supporting adequate carrier mobility, and significantly elevated carrier density enabled by\neffective doping.\nIn conclusion, our results demonstrate that N-doping is essential to the performance enhancement of doped\nCNTFs. N-doping maintains structural integrity, facilitates stable endohedral MoCl incorporation, and\n5\nsynergistically enhances carrier density through cumulative p-type doping effects. These combined\ncontributions account for the observed high electrical conductivity and environmental stability of N/MoCl -\nRevision:\nIn the main text,\nLine 67 in Page 4, the description of MoCl incorporation within the nanotube inner cavities was added.\nparticles with brighter contrast are clearly observed within the nanotube inner cavities. This brighter contrast\nis attributed to MoCl ’s higher atomic number (Z) compared to carbon47-49.\nc HAADF-STEM image of N/MoCl -CNTFs and corresponding elemental mappings of N, Mo, and Cl.\nFor N-CNTFs, the N 1s spectrum confirms the successful incorporation of N, with pyridinic N identified\nas the predominant bonding configuration (Supplementary Fig. 6a and Table 1), known to induce p-type\ndoping in CNTFs52.\nLine 116 in Page 6, the description of comparison of I /I values and defect densities of the fibers was added.\nG D\nThe I /I value of N-CNTFs (46.51 ± 0.30), where I and I represent the D-band and G-band intensities\nG D D G\nof CNTFs respectively, shows a slightly decrease compared to that of PCNTFs (49.75 ± 0.39)\n(Supplementary Fig. 6d and Table 2), consistent with the introduction of minor sidewall defects during\nnitrogen plasma treatment59. Similarly, MoCl -CNTFs and N/MoCl -CNTFs show I /I values of 48.54 ± 0.43\n5 5 G D\nand 45.25 ± 0.33, respectively, indicating that the structural integrity of the nanotube lattice is largely\npreserved following both doping processes. The corresponding defect densities, based on the established\nRaman-based method60, reveal only modest increases of 5.8% for N-CNTFs and 8.6% for N/MoCl -CNTFs\n5\ncompared to PCNTFs (Supplementary Table 2), confirming that the structural integrity is well-preserved\n25\nwithin acceptable limits that supports efficient charge transport performance.\nLine 228 in Page 10, the description of MoCl endohedral incorporation-enabled exceptional environmental\n5\nstability in N/MoCl -CNTFs was added.\n5\nperformed additional HAADF-STEM and EDS analyses on the radial cross-sections of MoCl -CNTFs and\n5 5 5\ninterstitial spaces between adjacent CNTs (Fig. 4d). Corresponding EDS mapping further confirms Mo and\nCl signals mainly localize outside the nanotube interiors (Figs. 4e-h). In contrast, N/MoCl -CNTFs exhibit\n5\npredominant bright contrast spots within the inner cavities of the nanotubes (Fig. 4i), with EDS mapping\nconfirming co-localized Mo and Cl signals (Figs. 4j-m). This confirms that nitrogen doping promotes the\nendohedral incorporation of MoCl . This internal confinement is believed to shield the dopants from external\n5\nperturbations, effectively mitigating the impact of solvents and thermal cycling on the electrical performance\nof N/MoCl -CNTFs.\n5\nand N/MoCl -CNTFs was added.\nd-h HAADF-STEM image (d) of MoCl -CNTFs and corresponding elemental mappings of C (e), N (f), Mo\n5\n(g), and Cl (h). i-m HAADF-STEM image (i) of N/MoCl -CNTFs and corresponding elemental mappings of\nLine 135 in Page 7, the description of density functional theory (DFT) calculations was added.\nThese observations are further supported by density functional theory (DFT) calculations. Simulations\nreveal that MoCl acts as a strong electron acceptor, with Bader charge analysis showing electron transfer of\n5\n0.85 e− per encapsulated molecule and 0.55 e− per adsorbed molecule (Supplementary Figs. 8a, b). Band\nstructure analysis further shows that nitrogen doping lowers the E , indicative of hole doping63. Subsequent\nF\nMoCl doping leads to a further downshift in the E (Supplementary Figs. 8c-f), reinforcing the increasingly\np-type character of the system. These electronic modulations are consistent with both our spectroscopic and\nKPFM measurements (Figs. 2h, i). The relationship between hole density (p) and the E is described as follows,\nF\n26\np =n exp\n\n( E −E )/kT )\n\ni i F\nhere n, k, and T represent the intrinsic carrier density, Boltzmann constant, and absolute temperature,\ni\nrespectively. This exponential dependence highlights how progressive E shifts lead to substantial increases\nF\nin carrier density40,57,64.\nLine 146 in Page 7, the description of the Hall effect measurements for assessing the intrinsic transport trends\nwas added.\nWe further performed Hall effect measurements using a van der Pauw configuration on CNT films\nfabricated under the same doping conditions as the fibers. Although the van der Pauw method is not directly\napplicable to fibers, it is well-established for thin films and offers meaningful insight into intrinsic transport\ntrends45. As expected, carrier mobility decreases upon doping due to enhanced carrier scattering, with\nreductions of ~50% for N-CNT and N/MoCl -CNT films compared to PCNT films (Supplementary Table\n5\n3). In contrast, carrier concentrations show substantial increases, with N-CNT, MoCl -CNT, and N/MoCl -\n5 5\nCNT films representing 2.05-, 2.61-, and 6.27-fold increases compared to PCNT films, respectively. In\naccordance with the Drude model, which defines that electrical conductivity is proportional to the product of\nmobility and carrier density, these results explain how even with a modest decrease in mobility, the marked\nrise in carrier density results in significantly improved electrical conductivity. Taken together, the high\nelectrical conductivity of the fibers results from the synergistic effects of preserved structural integrity\nsupporting adequate carrier mobility, and significantly elevated carrier density enabled by effective doping.\nIn the Supplementary Information,\nLine 76 in Page 3, the description of the defect density measurement was added.\nRaman spectra and defect density tests of the CNTs and fibers\nRaman spectra of the CNTs and fibers were collected by a Horiba LabRAM HR 800 with a 532 nm laser\nwavelength. The defect density (n ) in the CNTFs was estimated using the I /I value from Raman\nD G D\nspectroscopy, which reflects the level of disorder introduced by doping. This approach is well-established, as\nthe D-band arises from intervalley scattering associated with defects, while the G-band corresponds to the in-\nplane vibration of sp2 carbon atoms. The defect density was calculated using the following empirical relation3,\n1.8×1022 I\nn (cm−2)= ( D) (1)\nD λ4 I\nL G\nWhere λ (nm) represents the wavelength used in the Raman spectroscopy measurement.\nL\nPage 13, the XPS spectrum of N-CNTFs in the N 1s and comparison of I /I values and defect density of the\nG D\nfibers was added.\nG D\na XPS spectrum of N-CNTFs in the N 1s. d Comparison of I /I value and defect density of different CNTFs.\nG D\nPage 25, the comparison of defect density of different CNTFs was added.\nSupplementary Table 2 | Comparison of defect density of different CNTFs.\nDefect density\nSample I /I\nG D\n(n , × 109 cm−2)\nD\nPCNTFs 49.75 ± 0.39 4.563 ± 0.036\nN-CNTFs 46.51 ± 0.30 4.829 ± 0.032\nMoCl -CNTFs 48.54 ± 0.43 4.639 ± 0.038\nThe CNT films were prepared using a blade coating method11-13, and subsequently cut into 1 cm × 1 cm\nsquares for Hall effect measurements. The measurement used a Hall effect test system (8404, Lake shore\nCryotronics, inc.) under a fixed current of 10 mA and a magnetic field of 5 kG.\nPage 26, the description of Hall effect measurement results of CNT films were added.\nSupplementary Table 3 | Hall effect measurements of carrier density and mobility of PCNT, N-CNT,\nMoCl -CNT and N/MoCl -CNT films at room temperature.\n5 5\nCarrier density Carrier mobility\nSample Carrier type\n(× 1020 cm−3) (cm2 V−1 s−1)\nPCNT films 1.70 6.78 hole\nN-CNT films 3.49 3.03 hole\nMoCl -CNT films 4.44 6.18 hole\n5\nDensity functional theory (DFT) calculations\nAll the calculations were carried out within the framework of DFT using the Vienna Ab initio Simulation\nPackage (VASP)7. The exchange-correlation effects were treated using the Perdew-Burke-Ernzerhof (PBE)\nfunctional within the generalized gradient approximation (GGA), combined with the projector augmented\nwave (PAW) method to describe ion-electron interactions8,9. A plane-wave basis set with an energy cutoff of\n450 eV was employed to ensure the convergence of total energy and force calculations. Structural\noptimizations were performed by allowing full relaxation of both atomic positions and electronic states until\nthe convergence criteria of 10−4 eV for total energy and 0.02 eV Å−1 for atomic forces were met. To eliminate\nspurious interactions between periodic images, a vacuum region exceeding 15 Å was introduced perpendicular\nto the periodic directions. Long-range dispersion forces were incorporated using Grimme’s DFT-D3\nsemiempirical correction scheme10, which improves the accuracy of van der Waals interactions critical to\ninterfacial systems.\nPage 15, the description of DFT calculation results was added.\nSupplementary Fig. 8 | Charge density differences and electronic band structures of doped CNTs.\na, b Comparison of charge density for encapsulation of MoCl (a) and MoCl adsorbed on the outer wall (b)\n5 5\nper simulation unit. c-f Comparison of electronic band structures of pristine CNTs (c), N-CNTs (d), MoCl -\n5\n29\n2. In line 189, By what mechanism does the dopant (MoCl ) inside the CNT contribute to the improvement of\n5\nconductivity? Since the electrons will move along the CNT surface in the flow of electrons, wouldn't the\ndopants outside contribute to the improvement of conductivity more effectively?\nReply: Thank you for this valuable and insightful comment. According to the Drude model, electrical\nconductivity is proportional to the product of mobility and carrier density. As detailed in our response to\nComment 1, the structural integrity of the CNTFs remains well-preserved following the doping processes,\nwhich ensures that carrier mobility is not significantly compromised. More importantly, encapsulated MoCl\n5\nmolecules interact with the inner carbon surface of the nanotubes, allowing for stronger and more uniform\ncharge-transfer interactions compared to surface-adsorbed dopants. This leads to a pronounced downshift in\nthe E , reinforcing the p-type character of the CNTFs and significantly increasing carrier density (Angew.\nF\nChem. Int. Ed., 2025, 137, e202501669; Adv. Funct. Mater., 2025, 35, 2424565; Adv. Sci., 2025, 12, 2500933).\nThese conclusions are supported by our experimental and theoretical results. Raman and KPFM analyses show\nprogressive G-band blue shifts and increases work function upon MoCl doping, consistent with effective\n5\ncharge transfer and E modulation. Hall effect measurements reveal that N/MoCl -CNT films exhibit a ~6.27-\nF 5\nfold increase in carrier density compared to PCNT films, resulting in a net increase in conductivity despite a\nmodest reduction in mobility. DFT calculations further confirm that encapsulated MoCl acts as a stronger\n5\nelectron acceptor than adsorbed ones, with Bader charge analysis showing electron transfer of 0.85 e− per\nencapsulated molecule and 0.55 e− per adsorbed molecule (Supplementary Figs. 8a, b). This indicates a\nthermodynamic preference for encapsulation and stronger electronic coupling within the confined\nenvironment of the CNTs. In summary, while surface-adsorbed dopants can contribute to electronic\nmodulation, MoCl encapsulation provides a more effective and stable doping strategy. The improvement in\n5\nelectrical conductivity arises from increased carrier density driven by internal charge-transfer interactions,\nrather than direct electron transport along the nanotube inner wall. This conclusion is robustly supported by\nour combined spectroscopic, electrical, and theoretical analyses.\n3. In line 204~205, The current flowing through the fiber is proportional to fiber’s thickness. Therefore,\ntemperature changes due to changes in voltage are important, but information about thickness must be included\nin application part or Fig5.\nReply: Thank you for your useful suggestion. In response, we have prepared radial cross-section of the CNTF\ntows using a FIB and measured the fiber dimensions. The CNTFs exhibit a thickness of approximately 71 μm\n(Fig. R3). These results and relevant discussions were added into the revised manuscript.\nFig. R3 | SEM image showing the radial cross-section of CNTF tows.\nIn the main text,\nLine 250 in Page 11, the description of thickness of CNTF tows was added\nAdditionally, the N/MoCl -CNTF tow with a thickness of about 71 μm exhibit excellent electrothermal\n5\nproperties, making them promising candidates for high-performance fibrous heaters.\nIn the Supplementary Information,\nLine 136 in Page 5, the description of electrothermal property tests of the fibers was added.\nElectrothermal property tests of the fibers\nThe electrothermal performance of the CNTF tow with a thickness of about 71 μm was evaluated under\nambient conditions. The CNTF tow was connected to the electrodes of a programmable DC power supply\n(Keithley 2200-60-2) using conductive silver paste to ensure stable electrical contact. A constant voltage was\nthen applied, and the temperature evolution of the tow was monitored in real time using an infrared thermal\nimaging camera (Fluke Ti10).\n4. In line 245, Even excluding the effect of doping, the strength and electrical conductivity of pristine CNTF\nare excellent. As mentioned in the Method part, CNTF was manufactured according to the results of previous\nresearch. However, the properties of pristine CNTF were significantly improved compared to the previous\nresearch (ref. 28). If there are any additional material or process changes, it is recommended to mention them.\nReply: Thank you for your useful comment. In response, we have revised the manuscript to clarify the material\ndifferences and processing improvements relative to ref. 28. These include the purification protocol,\ncharacterization of the CNT raw materials, and optimization of spinning conditions, all of which contribute to\nthe enhanced performance of our PCNTFs.\nSpecifically, we employed an effective CNT purification protocol that reduces residual impurities and\nenhances CNT crystallinity. The CNTs used in this study were purchased from Cnano Technology Co., Ltd.\n(Jiangsu, China) and subsequently subjected to a purification process. The as-received CNTs were first\ndispersed in 30% H O (1 mg mL−1) and stirred for 24 hours, followed by vacuum filtration. The recovered\n2 2\nCNTs were then dispersed in 12 M HCl (5 mg mL−1) and stirred for 12 hours to remove residual metal catalysts.\nAfter neutralization with deionized water, the CNTs were frozen in liquid nitrogen and dried overnight via\nfreeze-drying. This treatment reduces the impurity content from 3.03 wt% to 0.84 wt%, as confirmed by\nthermogravimetric analysis (Supplementary Fig. 2b). It also improves the crystallinity, evidenced by an\nincrease in the I /I value from 44.5 to 49.3 (Supplementary Fig. 2c). Additionally, the average CNT aspect\nG D\nratio is approximately 4300, using a capillary thinning rheometric method based on dripping-onto-substrate\nextensional rheometry of CNT/CSA solutions (Supplementary Figs. 2g, h) (Macromolecules, 2007, 40, 4043;\nMacromolecules, 2016, 49, 681; Carbon, 2022, 196, 59). XPS analysis further reveals a low oxygen content\nof 2.9 at% (Supplementary Fig. 2d), facilitating efficient protonation in CSA and promoting liquid crystal\nphase formation critical for fiber alignment (ACS Nano, 2010, 4, 3969; ACS Appl. Mater. Interfaces, 2017, 9,\n36189; Sci. Adv., 2022, 8, eabn0939). In contrast, the nitric acid purification at 60°C used in ref. 28 introduced\nmore oxygen-containing groups, resulting in a higher oxygen content of 6.6 at% (Fig. R4), potentially\ndisrupting π-conjugation and solubility in CSA and thus adversely affecting alignment and fiber performance\n(Carbon, 2024, 228, 119317).\n31\n[REDACTED]\nFig. R4 | XPS spectrum of the purified CNTs in ref. 28, showing an oxygen content of 6.6 at%.\nIn addition to raw material improvements, we refined the spinning process by designing nozzles with a\nnarrower converging angle (~10°), which improves flow stability and facilitates initial CNT alignment during\nwet spinning (Chem. Eng. J., 2021, 412, 128650; Carbon, 2022, 196, 59). This process optimization resulted\nin a high orientation factor of 0.962 for our PCNTFs. For comparison, the CNTFs in ref. 28 had an orientation\nfactor of 0.94, which further decreased to 0.92 upon PBIA incorporation.\nIn summary, the excellent strength and electrical conductivity of our PCNTFs arises from the synergistic\neffect of high-purity, high-crystallinity CNTs with a low oxygen content, and optimized spinning geometry\nthat promotes uniform alignment and fiber assembly. These improvements have been described in the revised\nmanuscript and Supplementary Information for clarity.\nRevision:\nIn the main text,\nLine 81 in Page 5, the description of additional information on CNTs was added.\nThe as-received CNTs were purified to reduce impurities and improve crystallinity. The purified CNTs\nexhibited a low oxygen content, and the average aspect ratio was subsequently estimated to be approximately\n4300 using a capillary thinning rheometric method50,51 (Supplementary Fig. 2).\nLine 303 in Page 12, the description of purification steps of CNTs and preparation of wet-spun PCNTFs was\nadded.\nPurification of CNTs. The CNTs used in this study were purchased from Cnano Technology Co., Ltd.\n(Jiangsu, China) and subsequently subjected to a purification process. The as-received CNTs were first\ndispersed in 30% H O (1 mg mL−1) and stirred for 24 hours, followed by vacuum filtration. The recovered\n2 2\nCNTs were then dispersed in 12 M HCl (5 mg mL−1) and stirred for 12 hours to remove residual metal catalysts.\nAfter neutralization with deionized water, the CNTs were frozen in liquid nitrogen and dried overnight via\nfreeze-drying.\nPreparation of wet-spun PCNTFs. PCNTFs were prepared via a wet spinning process based on our\npreviously reported protocol28, with a key modification to improve alignment and fiber quality. Briefly, a\nliquid crystal spinning dope containing 0.8 ~ 1.5 wt% CNTs was prepared by dispersing CNTs into CSA.\nThe dope was extruded through 1-, 20-, or 100-hole nozzles with an inner diameter of 0.1 ~ 0.31 mm into an\nacetone coagulation bath. To enhance the initial alignment of CNTs during extrusion, we designed the nozzles\n32\nwith a narrower convergent angle of approximately 10°. This adjustment improved flow stability and\npromoted more uniform orientation of CNTs along the fiber axis during coagulation and subsequent stretching.\nIn the Supplementary Information,\nLine 66 in Page 3, the description of the measurement of CNT aspect ratio was added.\nAverage aspect ratio measurements of the CNTs\nThe average aspect ratio of CNTs was determined using a capillary thinning rheometric method based on\ndripping-onto-substrate extensional rheometry of CNT/chlorosulfonic acid (CSA) solution at a concentration\nof 200 ppm. The measurements were performed under axial extension using a custom-built rheometer\nequipped with two Teflon pistons (2 mm in diameter), operated at a controlled stretching rate of 25 mm s−1. A\nhigh-speed camera was used to capture the filament thinning process in real time. The recorded videos were\nanalyzed to extract the filament diameter as a function of time. The extensional viscosity was then calculated\nbased on the elastocapillary thinning regime, and the average CNT aspect ratio was derived using theoretical\nmodels developed for semi-dilute rigid-rod systems1,2.\nPage 9, the additional information on CNTs was added.\nSupplementary Fig. 2 | Characterization of CNTs.\na SEM image of CNTs. b Thermogravimetric analysis of CNTs. c Raman spectrum of CNTs. d XPS spectrum\nof the purified CNTs, showing a low oxygen content. e, f TEM images of CNTs. The inset image shows\n33\nstatistical analysis of CNT diameters. g Filament profiles captured during capillary thinning experiments of\nCNT/CSA solution under axial extension. h Evolution of filament diameter as a function of time from\nextensional viscosity measurements. The linear fit in the elastocapillary regime is used to extract the intrinsic\nextensional viscosity. The corresponding average aspect ratio of the CNTs is estimated to be approximately\n4300.\n5. The specific electrical conductivity and ampacity of N/MoCl CNTF are surprisingly excellent. Based on\n5\nthe XPS results in this fiber, what is the weight ratio of N to MoCl ? Has the doping content been optimized?\nof the doping duration to determine the optimal weight ratio of N to MoCl that yields the highest electrical\n5\nperformance in the N/MoCl -CNTFs.\n5\nhours) to obtain a series of fibers designated as N/MoCl -CNTFs-9h through N/MoCl -CNTFs-72h. XPS\n5 5\nresults reveal a progressive increase in Mo content with increasing doping durations (Supplementary Fig.\n9b). The weight ratio of N to MoCl was calculated based on their atomic concentrations and relative atomic\n5\nweights using the following equation:\nm c ×A\nW = N = N r(N) ×100%\n（N:MoCl 5 ） m c ×A\nMoCl Mo r(MoCl )\n5 5\nWhere c and c represent the atomic percentages (at%) of N and Mo, and A and A are the relative\nN Mo r(N) r(MoCl5)\natomic masses of N and MoCl , respectively. The calculated weight ratios of N to MoCl are 271% (N/MoCl -\n5 5 5\nCNTFs-9h), 13.88% (N/MoCl -CNTFs-12h), 6.84% (N/MoCl -CNTFs-24h), 4.98% (N/MoCl -CNTFs-48h),\n5 5 5\nand 3.58% (N/MoCl -CNTFs-72h), respectively. The corresponding electrical conductivities are 21.77 ± 0.77,\n5\n22.06 ± 0.06, 23.44 ± 0.08, 27.34 ± 0.09, and 21.44 ± 0.10 MS m−1 (Supplementary Fig. 9c), respectively.\nAmong these, the sample treated for 48 hours (N/MoCl -CNTFs-48h) with a weight ratio of 4.98%, exhibit\n5\nthe highest electrical conductivity of 27.34 ± 0.09 MS m−1, and an ultrahigh specific electrical conductivity of\n14166 ± 48 S m2 kg−1. These results demonstrate that the electrical performance is highly sensitive to the\nweight ratios of N to MoCl , and that a doping duration of 48 hours yields the optimal composition for\n5\nachieving maximum electrical performance.\nRevision:\nIn the main text,\nLine 171 in Page 8, the description of the optimization of doping durations was added.\nNotably, N/MoCl -CNTFs obtained with a doping duration of 48 hours exhibit an optimal composition,\n5\ndelivering an exceptional electrical conductivity of 27.34 ± 0.09 MS m−1, which is 3.20, 2.73, and 1.58 times\nthat of PCNTFs, N-CNTFs, and MoCl -CNTFs, respectively (Fig. 3a, Supplementary Figs. 9b, c, and Table\n5\n8).\nLine 331 in Page 13, the description of preparation of N/MoCl -CNTFs was added.\n5 5\nmethod. N-CNTFs and a mixture of MoCl and MoO in a mass ratio of 1:0.6 were first loaded separately\n5 3\n34\ninside a glass tube. Following vacuum drying at 100°C for 24 hours (~1 mbar), the tube was flame-sealed and\nannealed at 300°C for 9 ~ 72 hours in a furnace. The resulting fibers were then washed with acetone and dried\nat 60°C to yield N/MoCl -CNTFs.\nPage 16, the results of optimized doping durations of N/MoCl -CNTFs was added.\nCNTFs.\nb XPS spectra of N/MoCl -CNTFs with varying MoCl doping durations (9 ~ 72 hours). c Electrical\n5 5\nconductivity of different N/MoCl -CNTFs.\n5\n6. I would think this process for doping with N and MoCl seems to be batch process, not continuous process.\n5 5\ncontinuous fibers?\nReply: Thank you for your thoughtful comment. Currently, this doping process for fabricating N/MoCl -\nproduced per batch is approximately 50 meters, which is limited by the size of the glass tubes and furnace\nused during the MoCl doping step. We believe that continuous production of N/MoCl -CNTFs is feasible\n5 5\nwith further process optimization. In particular, N-doping can already be implemented in a continuous manner\ndue to its short treatment time (~5 minutes). However, MoCl doping, which currently requires 48 hours for\n5\noptimal performance, presents a bottleneck. To enable continuous processing, the MoCl doping step must be\n5\nsignificantly shortened. Potential strategies include increasing the dopant concentration, raising the doping\ntemperature, or pre-doping the N-doped CNTs with MoCl prior to fiber spinning. The latter approach may\n5\nallow for direct fabrication of doped fibers from already functionalized CNTs. We are actively exploring these\ndirections to enable the scalable and continuous production of high-performance N/MoCl -CNTFs.\n5\n7. Compared to previous reports in references, what could be benefit of this materials and process?\nReply: Thank you for your useful comment. The key benefits of our work lie in the significant enhancement\nof fiber performance and generality and scalability of the doping strategy, which together represent a\nsubstantial advancement over previous reports.\nFirst, N/MoCl -CNTFs developed in this study exhibit ultrahigh electrical conductivity, exceptional\n5\n35\nspecific electrical conductivity, and robust environmental stability. Previous state-of-the-art CNTFs achieved\nelectrical conductivities of up to 11.2 MS m−1, and specific electrical conductivity of 5685 S m2 kg−1 through\nhierarchical structuring and optimized wet-spinning techniques (Carbon, 2022, 196, 59; Fig. R5 and\nSupplementary Table 9). The incorporation of conductive polymers such as polyaniline further improved\nthese values to 11.9 MS m−1 and 6200 S m2 kg−1 (Carbon, 2023, 213, 118308). However, these values remain\nfar below the intrinsic electrical properties of individual CNTs. In contrast, our doping engineering strategy,\ninvolving in-plane nitrogen and endohedral MoCl , enables a much higher electrical conductivity of 27.34 MS\n5\nm−1, surpassing even that of K doped graphene fibers (22.4 MS m−1, Adv. Mater., 2016, 28, 7941) and\ncomparable to high-conductivity graphitic intercalation compounds (Solid State Commun., 1977, 24, 371;\nSynth. Met., 1987, 18, 503; Adv. Phys., 2002, 51, 1; Adv. Mater., 2021, 33, 2008432). Importantly, owing to\nthe lightweight nature of CNTFs, this corresponds to a specific electrical conductivity of 14166 S m2 kg−1,\nwhich exceeds that of copper by 115%, and a current-carrying capacity of 1241.38 A mm−2, exceeding that of\ncopper by 28%. Furthermore, the endohedral encapsulation of MoCl within CNT inner cavities shields the\n5\ndopants from external perturbations, significantly enhancing resistance to solvents, mechanical and thermal\ncycling, and ambient degradation, and thus enabling outstanding environmental stability. As a result, the\ndoped fibers are well suited for lightweight conductive wires and high-performance EMI shielding textiles.\nFig. R5 | Development of electrical conductivity and specific electrical conductivity of CNTFs (raw data\nin Supplementary Table 9).\nsame two-zone vapor transport method, we successfully prepared N/I -CNTFs, N/CuI-CNTFs, and N/FeCl -\n2 3\nCNTFs by replacing MoCl with I , CuI, and FeCl , respectively. The resulting fibers show the electrical\n5 2 3\nconductivities of 16.89 ± 0.06 (N/I -CNTFs), 25.97 ± 0.09 (N/CuI-CNTFs) and 26.87 ± 0.08 MS m−1\n2\n(N/FeCl -CNTFs), all comparable to or approaching that of N/MoCl -CNTFs (Supplementary Fig. 10a).\n3 5\nRaman spectra of all doped fibers show progressive blue shifts in the G-band relative to PCNTFs\n(Supplementary Fig. 10b), indicating strong p-type doping and effective electronic structure modulation.\nAdditionally, all doped fibers demonstrate minimal conductivity degradation over 50 days (Supplementary\nFig. 12a), further confirming their excellent environmental stability.\nIn summary, our work presents a generalizable, tunable, and potentially scalable doping platform that\nsimultaneously achieves record-high electrical performance, long-term environmental robustness, and broad\ndopant compatibility. These features make the developed CNTFs highly suitable for next-generation\napplications in flexible electronics, lightweight power transmission, and wearable textiles. In response, we\nadded these results and discussions in the revised manuscript.\nRevision:\nIn the main text,\nLine 178 in Page 8, the description of comparing the performance of different doped CNTFs was added.\nTo evaluate the effectiveness of MoCl relative to other dopants, we prepared N/I -CNTFs, N/CuI-CNTFs,\n5 2\nand N/FeCl -CNTFs using I , CuI, and FeCl as dopants, respectively, via a similar two-zone vapor transport\n3 2 3\nmethod. The electrical conductivity of N/I -CNTFs reaches 16.89 ± 0.06 MS m−1, while N/CuI-CNTFs and\n3\ncompared to N/MoCl -CNTFs (Supplementary Fig. 10a). Raman spectra of all doped fibers show\n5\nprogressive blue shifts in the G-band relative to PCNTFs, with the largest shift observed in N/MoCl -CNTFs\n5\n(Supplementary Fig. 10b). This trend suggests MoCl induces stronger p-type doping and more significant\n5\nmodulation of the CNTs electronic structure through charge transfer interactions, resulting in the highest\nelectrical conductivity. These results highlight MoCl as the most effective dopant among those tested.\nadded.\nWe further compared the environmental stability of the doped fibers. While MoCl -CNTFs exhibit a notable\n5\ndecline in the electrical conductivity over time, N/I -CNTFs, N/CuI-CNTFs, N/FeCl -CNTFs, and N/MoCl -\n2 3 5\nCNTFs maintain more significantly stable electrical conductivities over 50 days under ambient conditions\n(Fig. 4a and Supplementary Fig. 12a). Among these, N/MoCl -CNTFs demonstrate the smallest change,\n5\nindicating superior long-term stability.\nLine 293 in Page 12, the description of key benefits of the materials and process was added.\nAdditionally, the strategy demonstrates broad generality and dopant compatibility, as evidenced by similarly\nhigh electrical conductivities and robust environmental stability achieved with I , CuI, and FeCl dopants.\n2 3\nThis strategy provides a versatile and scalable platform for producing lightweight and highly conductive\nLine 336 in Page 12, the description of preparation of different doped CNTFs was added.\nPreparation of N/I -CNTFs, N/CuI-CNTFs, and N/FeCl -CNTFs. For comparison, N-CNTFs and each\n2 3\ndopant (I , CuI, and FeCl ) were placed separately inside glass tubes, and dried at 100°C under vacuum (~1\n2 3\nmbar) for 24 hours to remove residual moisture. Subsequently, the flame-sealed glass tubes were annealed in\na furnace at 200°C for I , 750°C for CuI, and 360°C for FeCl , respectively. After cooling to room temperature,\n2 3\nthe fibers were removed, rinsed with acetone to eliminate adsorbed dopants, and dried at 60°C to obtain N/I -\nFig. 3 | Electrical properties of fibers.\na Electrical conductivity of PCNTFs, N-CNTFs, MoCl -CNTFs, and N/MoCl -CNTFs. b Comparison of the\n5 5\nspecific electrical conductivity of fibers in this work and commonly used metallic conductors. c Comparison\nof the specific tensile strength as a function of specific electrical conductivity of N/MoCl -CNTFs with\n5\npreviously reported CNTFs and metals. d Current carrying capacity of different fibers. e Comparison of the\ncurrent carrying capacity as a function of electrical conductivity of N/MoCl -CNTFs with various materials.\n5\nf Temperature dependence of relative resistance of N/MoCl -CNTFs and commercial Cu wires. The resistance\n5\nat 300 K and different temperatures is defined as R and R , respectively. All error bars show mean ± SD.\nIn the Supplementary Information,\nPage 17, the comparison of electrical conductivities and Raman spectra of different doped CNTFs was added.\na Comparison of electrical conductivity among N-CNTFs doped with I , CuI, FeCl , and MoCl , showing the\n2 3 5\nhighest conductivity for N/MoCl -CNTFs. b Raman spectra of N/I -CNTFs, N/CuI-CNTFs, N/FeCl -CNTFs,\n5 2 3\nand N/MoCl -CNTFs, showing progressive blue shifts in the G-band relative to PCNTFs.\nSupplementary Fig. 12 | Comparison of environmental stability of different doped CNTFs.\na Comparison of electrical conductivity of N/I -CNTFs, N/CuI-CNTFs, N/FeCl -CNTFs, and N/MoCl -\n2 3 5\nCNTFs over 50 days under ambient conditions. b Relative resistance changes of MoCl -CNTFs and N/MoCl -\n5 5\nCNTFs after immersion in different solvents for 24 hours. All error bars show mean ± SD.\nPage 32, the detailed comparison of electrical performance of different fibers was added.\nSupplementary Table 9 | Comparison of mechanical and electrical performance between CNTFs and\nmetals.\nSpecific\nTensile Electrical Specific electrical\ntensile\nstrength conductivity conductivity Ref.\nstrength\n(GPa) (MS m−1) (S m2 kg−1)\n(N tex−1)\n1.0 0.78 5.0 3500 14\nCNTFs\nspun from\n2.43 1.62 7.7 4870 15\n39\nsolution 3.94 2.05 10.4 5400 16\n2.4 1.54 8.5 5500 17\n4.2 2.1 10.9 5640 18\n5.0 2.55 11.2 5685 19\n5.05 2.63 11.9 6200 20\n1.65 0.84 11.01 5620 21\n4.48 4.08 2.50 2270 22\nCNTFs\nspun from 4.34 3.3 2.27 1750 23\naerogels\n- 2.6 - 1870 24\n1.4 0.73 0.41 213.54 25\nGFs spun\nfrom 0.113 0.28 0.106 42.4 26\nsolution\n2.3 1.5 0.77 411.76 27\nFeCl doped\n3\n0.12 0.075 7.7 4740\nGFs\nBr doped\n2\n0.11 0.069 15 9240 13\nGFs\nK doped\n0.13 0.081 22.4 13800\nGFs\nCu 0.209 0.02 ~ 60 6590 28\nZn 0.115 0.14 18.4 2579.3\nAg 0.125 0.01 68.0 6478.9\nAu 0.11 0.06 48.8 2527.8\nN/MoCl -\n5\n2.49 ± 0.14 1.26 ± 0.07 27.34 ± 0.09 14166 ± 48 This work\nCNTFs\nNote: GFs refer to graphene fibers.\nThe manuscript describes a strategy to enhance the electrical conductivity of carbon nanotube fibers (CNTFs)\nby combining nitrogen (N) doping and vacuum-assisted endohedral MoCl doping. The authors claim that N\n5\ndoping introduces sidewall defects on CNTs, which reportedly facilitates the adsorption and encapsulation of\nMoCl , thereby significantly improving the fiber’s electrical conductivity. While electrical conductivity of\n5\nwet-spun CNTF including specific value is somewhat surprising, the techniques described (N doping and\nendohedral/chemical doping with MoCl ) are already well-established in the literature. Nitrogen doping of\nMetal halide doping under vacuum or other inert conditions is also a widely employed method. Simply\ncombining these two known techniques, without in-depth mechanistic exploration or demonstrating a markedly\nimproved performance over the state of the art, is insufficient for publication in a high-impact journal. In\naddition, the manuscript posits that nitrogen-induced defects help trap MoCl particles in the CNT interior. However,\n5\nthere is no robust, quantitative analysis (such as d-spacing and SAED pattern by HR TEM or in situ\nspectroscopic studies) proving that the synergy of N doping and MoCl encapsulation distinctly outperforms\n5\nsingle-doping routes. Moreover, it is unclear why MoCl was specifically chosen, compared to other dopants (I ,\n5 2\nBr , FeCl , etc.), and how this choice leads to superior or unique structural/electronic outcomes.\n2 3\nReply: Thank you for these valuable and constructive comments. In the revised manuscript, we have clarified\nthe novelty of our strategy, strengthened the mechanistic evidence, and expanded the comparative analysis\nwith alternative dopants.\nFirst, we would like to clarify our strategy innovations, mechanism insights, and performance\nbreakthroughs. We acknowledge that N doping and metal halide doping (including MoCl ) are individually\n5\nwell-established. However, their application to CNTFs has so far been constrained by modest conductivity\ngains and poor environmental stability, particularly in metal halide-doped fibers. To overcome these\nlimitations, we develop an effective strategy involving in-plane N doping with endohedral MoCl doping.\n5\ninner cavities. This internal confinement not only synergistically amplifies the p-type doping effect and thus\nsignificantly improve electrical performance, but also shield dopants from ambient, solvent, mechanical, and\nthermal degradation, thereby ensuring long-term performance retention. The resulting N/MoCl -CNTFs\n5\nexhibit an exceptional electrical conductivity of 27.34 ± 0.09 MS m−1, an ultrahigh specific electrical\nconductivity of 14166 ± 48 S m2 kg−1, and a high current carrying capacity of 1241.38 ± 94.34 A mm−2. To\nthe best of our knowledge, the electrical conductivity and specific electrical conductivity of our fibers exceeds\nt exceeds\nthe previously reported CNTFs and doped graphene fibers (Adv. Mater., 2016, 28, 7941), which is a milestone\nprogression in the performance improvement of CNTFs (Fig. R5). Additionally, the specific electrical\nconductivity of our fibers even surpasses some metals, such as gold, zinc, silver, and copper. Experimental\nevidence and density functional theory (DFT) calculations attribute this milestone performance to cooperative\nelectronic structure modulation and dopant stabilization enabled by endohedral confinement. Moreover, the\nstrategy demonstrates broad generality and dopant compatibility, as evidenced by similarly high electrical\nconductivities and robust environmental stability achieved with I , CuI, and FeCl dopants. We believe these\n2 3\nmechanistic insights offer a broadly applicable approach for producing lightweight and high-conductivity\nCNTFs with unprecedented environmental robustness.\nin Supplementary Table 9).\nSecond, to directly demonstrate that N doping and MoCl encapsulation act synergistically and outperform\n5\neither doping alone, we conducted complementary structural and electronic characterizations.\nSequence control experiments show that reversing the doping order (MoCl doping first, then N doping)\n5\nyields MoCl /N-CNTFs with a significantly lower conductivity (12.32 ± 0.07 MS m−1) than both MoCl -\n5 5\nCNTFs and N/MoCl -CNTFs (Supplementary Fig. 13). This confirms that nitrogen functionalization prior\n5\nto MoCl is essential for effective endohedral incorporation.\nenergy-dispersive X-ray spectroscopy (EDS) mapping reveal predominant MoCl in interstitial spaces\n5\nbetween adjacent CNTs for MoCl -CNTFs (Figs. 4d-h), but clearly confined within nanotube inner cavities\n5\nin N/MoCl -CNTFs, with co-localized Mo and Cl signals (Figs. 1c and 4i-m). This endohedral confinement\n5\ncorrelates with the superior conductivity stability of N/MoCl -CNTFs after prolonged ambient exposure and\n5\nsolvent washing, highlighting the role of N doping in facilitating and stabilizing MoCl incorporation.\nforce microscopy (KPFM), and Hall effect measurements, further supports the cooperative effect. XPS reveals\npredominant pyridinic N incorporation, inducing p-type doping (Supplementary Fig. 6a). Raman G-band\n42\nshifts increase progressively from N-CNTFs to MoCl -CNTFs, reaching a maximum of 19 cm−1 in N/MoCl -\n5 5\nCNTFs, consistent with enhanced charge transfer (Fig. 2h). KPFM measurements show the largest work\nfunction increase for N/MoCl -CNTFs, indicating the greatest Fermi level (E ) downshift (Fig. 2i). Hall effect\nmeasurements on equivalently doped CNT films reveal modest mobility reductions but dramatic carrier\ndensity increases, representing 2.05-, 2.61-, and 6.27-fold increases for N-CNT, MoCl -CNT, and N/MoCl -\n5 5\nCNT films compared to PCNT films (Supplementary Table 3), respectively. In accordance with the Drude\nmodel, which defines that electrical conductivity is proportional to the product of mobility and carrier density,\nthese results explain how even with a modest decrease in mobility, the marked rise in carrier density results\nin significantly improved electrical conductivity.\nDFT calculations corroborate these findings. MoCl acts as a strong electron acceptor, with Bader charge\n5\nanalysis showing electron transfer of 0.85 e− per encapsulated molecule and 0.55 e− per adsorbed molecule\n(Supplementary Figs. 8a, b). This is attributed to encapsulated MoCl molecules interact with the inner\n5\ncarbon surface of the nanotubes, allowing for stronger and more uniform charge-transfer interactions. Band\nstructure analysis further shows that N doping lowers the E by ~0.75 eV, with subsequent MoCl doping\nF 5\nleads to a further downshift in the E (Supplementary Figs. 8c-f), exponentially increasing carrier density.\nF\nTaken together, these results confirm that N-doping facilitates stable endohedral MoCl incorporation, and\n5\nthat their combined p-type doping effects yield both record-high electrical conductivity and exceptional\nenvironmental stability in CNTFs.\nThird, to evaluate the effectiveness of MoCl relative to other dopants, we have conducted a comparative\n5\nanalysis with alternative dopants and provided additional experimental details on the doping process.\nSpecifically, we prepared N/I -CNTFs, N/CuI-CNTFs, and N/FeCl -CNTFs using I , CuI, and FeCl as\n2 3 2 3\ndopants, respectively, via a similar two-zone vapor transport method. The electrical conductivity of N/I -\n3\nrespectively, compared to N/MoCl -CNTFs (27.34 ± 0.09 MS m−1) (Supplementary Fig. 10a). Raman spectra\n5\nof all doped fibers show progressive blue shifts in the G-band relative to PCNTFs, with the largest shift\nobserved in N/MoCl -CNTFs (Supplementary Fig. 10b). This trend suggests MoCl induces stronger p-type\n5 5\ndoping and more significant modulation of the CNTs electronic structure through charge transfer interactions.\nWe also evaluated the long-term environmental stability of these doped fibers. Among all candidates,\nN/MoCl -CNTFs demonstrate the smallest change in electrical conductivity over 50 days under ambient\n5\nconditions (Supplementary Fig. 12a). These results collectively indicate MoCl as the most effective dopant\n5\namong those tested, offering superior performance in both electrical performance and environmental stability.\nIn conclusion, our study provides a synergistic nitrogen and endohedral MoCl doping strategy that achieves\n5\nrecord-high electrical performance in CNTFs while ensuring exceptional environmental stability. Supported\nby complementary experimental evidence and DFT analysis, we demonstrate that this approach combines\nstrategy innovations, mechanistic insights, and performance breakthroughs. The method is broadly applicable\nto other dopants, offering a versatile, scalable platform for producing lightweight and high-conductivity\nCNTFs. Such fibers hold strong potential for next-generation conductive wires and advanced functional\ntextiles. All relevant data, analyses, and discussions have been incorporated into the revised manuscript.\nIn the main text,\nLine 67 in Page 4, the description of MoCl incorporation within the nanotube inner cavities was added.\nparticles with brighter contrast are clearly observed within the nanotube inner cavities. This brighter contrast\nis attributed to MoCl ’s higher atomic number (Z) compared to carbon47-49.\nc HAADF-STEM image of N/MoCl -CNTFs and corresponding elemental mappings of N, Mo, and Cl.\n5\nstability in N/MoCl -CNTFs was added.\n5\nperformed additional HAADF-STEM and EDS analyses on the radial cross-sections of MoCl -CNTFs and\n5 5 5\ninterstitial spaces between adjacent CNTs (Fig. 4d). Corresponding EDS mapping further confirms Mo and\nCl signals mainly localize outside the nanotube interiors (Figs. 4e-h). In contrast, N/MoCl -CNTFs exhibit\n5\npredominant bright contrast spots within the inner cavities of the nanotubes (Fig. 4i), with EDS mapping\nconfirming co-localized Mo and Cl signals (Figs. 4j-m). This confirms that nitrogen doping promotes the\nendohedral incorporation of MoCl . This internal confinement is believed to shield the dopants from external\n5\nperturbations, effectively mitigating the impact of solvents and thermal cycling on the electrical performance\nof N/MoCl -CNTFs.\n5\nand N/MoCl -CNTFs was added.\n5\nd-h HAADF-STEM image (d) of MoCl -CNTFs and corresponding elemental mappings of C (e), N (f), Mo\n5\n(g), and Cl (h). i-m HAADF-STEM image (i) of N/MoCl -CNTFs and corresponding elemental mappings of\nLine 237 in Page 10, the description of doping sequence control experiments was added.\nWe further reversed the doping sequence, where MoCl doping was performed first, followed by nitrogen\n5\nplasma treatment, producing MoCl /N-CNTFs. As shown in Supplementary Fig. 13a, these fibers exhibit a\n5\nsignificantly reduced electrical conductivity of 12.32 ± 0.07 MS m−1 compared to both MoCl -CNTFs and\n5 5\nCNTFs to 7.35 at% in MoCl /N-CNTFs (Supplementary Fig. 13b). This reduction is likely due to the thermal\n5\neffects associated with the plasma treatment, which may cause partial dopant desorption67. These results\nconfirm the critical importance of treatment sequence and demonstrate that nitrogen functionalization prior to\nMoCl doping is essential for achieving effective endohedral incorporation and synergistically enhancing both\n5\nelectrical conductivity and environmental stability.\nLine 106 in Page 5, the description of N-bond types in N-CNTFs was added.\nFor N-CNTFs, the N 1s spectrum confirms the successful incorporation of N, with pyridinic N identified\nas the predominant bonding configuration (Supplementary Fig. 6a and Table 1), known to induce p-type\ndoping in CNTFs52.\nLine 146 in Page 7, the description of the Hall effect measurements for assessing the intrinsic transport trends\nwas added.\nWe further performed Hall effect measurements using a van der Pauw configuration on CNT films\nfabricated under the same doping conditions as the fibers. Although the van der Pauw method is not directly\napplicable to fibers, it is well-established for thin films and offers meaningful insight into intrinsic transport\ntrends45. As expected, carrier mobility decreases upon doping due to enhanced carrier scattering, with\nreductions of ~50% for N-CNT and N/MoCl -CNT films compared to PCNT films (Supplementary Table\n5\n3). In contrast, carrier density shows substantial increases, with N-CNT, MoCl -CNT, and N/MoCl -\n5 5\nCNT films representing 2.05-, 2.61-, and 6.27-fold increases compared to PCNT films, respectively. In\n45\naccordance with the Drude model, which defines that electrical conductivity is proportional to the product of\nmobility and carrier density, these results explain how even with a modest decrease in mobility, the marked\nrise in carrier density results in significantly improved electrical conductivity. Taken together, the high\nelectrical conductivity of the fibers results from the synergistic effects of preserved structural integrity\nsupporting adequate carrier mobility, and significantly elevated carrier density enabled by effective doping.\nLine 135 in Page 7, the description of DFT calculations was added.\nThese observations are further supported by density functional theory (DFT) calculations. Simulations\nreveal that MoCl acts as a strong electron acceptor, with Bader charge analysis showing electron transfer of\n5\n0.85 e− per encapsulated molecule and 0.55 e− per adsorbed molecule (Supplementary Figs. 8a, b). Band\nstructure analysis further shows that nitrogen doping lowers the E , indicative of hole doping63. Subsequent\nF\nMoCl doping leads to a further downshift in the E (Supplementary Figs. 8c-f), reinforcing the increasingly\np-type character of the system. These electronic modulations are consistent with both our spectroscopic and\nKPFM measurements (Figs. 2h, i). The relationship between hole density (p) and the E is described as follows,\nF\np =n exp\n\n( E −E )/kT )\n\ni i F\nwhere n, k, and T represent the intrinsic carrier density, Boltzmann constant, and absolute temperature,\ni\nrespectively. This exponential dependence highlights how progressive E shifts lead to substantial increases\nF\nin carrier density40,57,64.\nLine 178 in Page 8, the description of comparing the performance of different doped CNTFs was added.\nTo evaluate the effectiveness of MoCl relative to other dopants, we prepared N/I -CNTFs, N/CuI-CNTFs,\n5 2\nand N/FeCl -CNTFs using I , CuI, and FeCl as dopants, respectively, via a similar two-zone vapor transport\n3 2 3\nmethod. The electrical conductivity of N/I -CNTFs reaches 16.89 ± 0.06 MS m−1, while N/CuI-CNTFs and\n3\ncompared to N/MoCl -CNTFs (Supplementary Fig. 10a). Raman spectra of all doped fibers show\n5\nprogressive blue shifts in the G-band relative to PCNTFs, with the largest shift observed in N/MoCl -CNTFs\n5\n(Supplementary Fig. 10b). This trend suggests MoCl induces stronger p-type doping and more significant\n5\nmodulation of the CNTs electronic structure through charge transfer interactions, resulting in the highest\nelectrical conductivity. These results highlight MoCl as the most effective dopant among those tested.\nadded.\nWe further compared the environmental stability of the doped fibers. While MoCl -CNTFs exhibit a notable\n5\ndecline in the electrical conductivity over time, N/I -CNTFs, N/CuI-CNTFs, N/FeCl -CNTFs, and N/MoCl -\n2 3 5\nCNTFs maintain more significantly stable electrical conductivities over 50 days under ambient conditions\n(Fig. 4a and Supplementary Fig. 12a). Among these, N/MoCl -CNTFs demonstrate the smallest change,\n5\nindicating superior long-term stability.\nLine 336 in Page 13, the detailed description of the doping process was added.\nPreparation of N/I -CNTFs, N/CuI-CNTFs, and N/FeCl -CNTFs. For comparison, N-CNTFs and each\n2 3\n46\ndopant (I , CuI, and FeCl ) were placed separately inside glass tubes, and dried at 100°C under vacuum (~1\n2 3\nmbar) for 24 hours to remove residual moisture. Subsequently, the flame-sealed glass tubes were annealed in\na furnace at 200°C for I , 750°C for CuI, and 360°C for FeCl , respectively. After cooling to room temperature,\n2 3\nthe fibers were removed, rinsed with acetone to eliminate adsorbed dopants, and dried at 60°C to obtain N/I -\nPage 20, the description of electrical conductivity and Mo content of MoCl /N-CNTFs was added.\na Electrical conductivity of MoCl -CNTFs, N/MoCl -CNTFs and MoCl /N-CNTFs, indicating a significant\n5 5 5\nreduction in electrical conductivity for MoCl /N-CNTFs. b XPS spectra comparing N/MoCl -CNTFs and\n5 5\nMoCl /N-CNTFs, showing a marked decrease in Mo content for MoCl /N-CNTFs.\n5 5\nPage 13, the description of XPS spectrum of N-CNTFs in the N 1s was added.\nSupplementary Fig. 6 | XPS spectra, I /I value and defect density of different CNTFs.\nG D\na XPS spectrum of N-CNTFs in the N 1s.\nLine 196 in Page 7, the description of Hall effect measurement of CNT films was added.\nThe CNT films were prepared using a blade coating method11-13, and subsequently cut into 1 cm × 1 cm\nsquares for Hall effect measurements. The measurement used a Hall effect test system (8404, Lake shore\nPage 26, the description of Hall effect measurement results of CNT films were added.\nSupplementary Table 3 | Hall effect measurements of carrier density and mobility of PCNT, N-CNT,\nMoCl -CNT and N/MoCl -CNT films at room temperature.\n5 5\nCarrier density Carrier mobility\nSample Carrier type\n(× 1020 cm−3) (cm2 V−1 s−1)\nPCNT films 1.70 6.78 hole\nN-CNT films 3.49 3.03 hole\nMoCl -CNT films 4.44 6.18 hole\nDensity functional theory (DFT) calculations\nAll the calculations were carried out within the framework of DFT using the Vienna Ab initio Simulation\nPackage (VASP)7. The exchange-correlation effects were treated using the Perdew-Burke-Ernzerhof (PBE)\nfunctional within the generalized gradient approximation (GGA), combined with the projector augmented\nwave (PAW) method to describe ion-electron interactions8,9. A plane-wave basis set with an energy cutoff of\n450 eV was employed to ensure the convergence of total energy and force calculations. Structural\noptimizations were performed by allowing full relaxation of both atomic positions and electronic states until\nthe convergence criteria of 10−4 eV for total energy and 0.02 eV Å−1 for atomic forces were met. To eliminate\nspurious interactions between periodic images, a vacuum region exceeding 15 Å was introduced perpendicular\nto the periodic directions. Long-range dispersion forces were incorporated using Grimme’s DFT-D3\nsemiempirical correction scheme10, which improves the accuracy of van der Waals interactions critical to\ninterfacial systems.\nPage 15, the description of DFT calculation results was added.\na, b Comparison of charge density for encapsulation of MoCl (a) and MoCl adsorbed on the outer wall (b)\n5 5\nper simulation unit. c-f Comparison of electronic band structures of pristine CNTs (c), N-CNTs (d), MoCl -\nSupplementary Fig. 10 | Electrical conductivity and Raman spectra of different CNTFs.\na Comparison of electrical conductivity among N-CNTFs doped with I , CuI, FeCl , and MoCl , showing the\n2 3 5\nhighest conductivity for N/MoCl -CNTFs. b Raman spectra of N/I -CNTFs, N/CuI-CNTFs, N/FeCl -CNTFs,\n5 2 3\nand N/MoCl -CNTFs, showing progressive blue shifts in the G-band relative to PCNTFs.\n5\nSupplementary Fig. 12 | Comparison of environmental stability of different doped CNTFs.\na Comparison of electrical conductivity of N/I -CNTFs, N/CuI-CNTFs, N/FeCl -CNTFs, and N/MoCl -\n2 3 5\nCNTFs over 50 days under ambient conditions. b Relative resistance changes of MoCl -CNTFs and N/MoCl -\n5 5\nCNTFs after immersion in different solvents for 24 hours. All error bars show mean ± SD.\n2. Minor comments: While the manuscript includes some electron microscopy and spectroscopic\ncharacterizations, additional data or deeper analysis (e.g., quantitative defect density measurements, doping\nefficiency, or doping stability over time) would be necessary to convince readers of the robustness of the\napproach and the reproducibility of the results. Given the widely known nature of both N doping and vacuum-\nassisted metal halide doping approaches, and the insufficient justification for their specific combination, the\nmanuscript in its current form does not meet the criteria for publication.\nReply: Thank you for this constructive comment. In response, we added new analyses of defect density\nmeasurements, doping efficiency, and doping stability over time, to further demonstrate the robustness and\nreproducibility of our approach.\nTo determine the defect densities of doped fibers, we conducted comprehensive structural analysis based\non their Raman spectra. Raman spectroscopy was used to quantify defect density (n ) via the I /I value,\nD G D\nwhere I and I are the intensities of D-band and G-band of CNTFs, respectively. Compared to PCNTFs, N-\nD G\nCNTFs show a slight decrease in the I /I value from 49.75 ± 0.39 to 46.51 ± 0.30, consistent with minor\nG D\ndefect introduction through nitrogen plasma under carefully controlled conditions (Supplementary Fig. 6d\nand Table 2) (Carbon, 2020, 167, 881). Similarly, MoCl -CNTFs and N/MoCl -CNTFs show I /I values of\n5 5 G D\n48.54 ± 0.43 and 45.25 ± 0.33, respectively, indicating that only limited structural disorder was introduced by\nthe doping processes (Nature, 1997, 388, 257; Microelectron. Eng., 2022, 252, 111666). All doped fibers\nmaintain high I /I values (> 45), indicating that the nanotube lattice remains largely intact. The corresponding\nG D\ndefect densities, calculated using an established Raman-based method (Nano Lett., 2011, 11, 3190), are (4.829\n± 0.032) × 109 cm−2 for N-CNTFs and (4.956 ± 0.035) × 109 cm−2 for N/MoCl -CNTFs, representing only\n5\nmodest increases of 5.8% and 8.6%, respectively, compared to PCNTFs [(4.563 ± 0.036) × 109 cm−2]. These\nresults confirm that the structural integrity is well-preserved within acceptable limits that supports efficient\ncharge transport.\nTo compare their doping efficiency, we characterized the detailed compositions of doped fibers. XPS\nspectrum of N-CNTFs reveals a N content of 4.87 ± 0.76 at%, in which pyridinic N is identified as the\n50\npredominant bonding configuration (Supplementary Fig. 6a and Table 1). N-CNTFs were treated with the\nMoCl dopant for varying durations (9, 12, 24, 48, and 72 hours) to obtain a series of fibers designated as\n5 5\nranging from 0.24 to 18.16 at% with increasing doping durations (Supplementary Fig. 9b). The weight ratio\nof N to MoCl was calculated based on their atomic concentrations and relative atomic weights, which is used\n5\nto assess the doping efficiency. The calculated weight ratios of N to MoCl are 271% (N/MoCl -CNTFs-9h),\n5 5\n13.88% (N/MoCl -CNTFs-12h), 6.84% (N/MoCl -CNTFs-24h), 4.98% (N/MoCl -CNTFs-48h), and 3.58%\n5 5 5\n(N/MoCl -CNTFs-72h), respectively. The corresponding electrical conductivities are 21.77 ± 0.77, 22.06 ±\n5\n0.06, 23.44 ± 0.08, 27.34 ± 0.09, and 21.44 ± 0.10 MS m−1 (Supplementary Fig. 9c), respectively. Among\nthese, the sample treated for 48 hours (N/MoCl -CNTFs-48h) with a weight ratio of 4.98%, exhibit the highest\n5\nelectrical conductivity of 27.34 ± 0.09 MS m−1. These results demonstrate that the electrical performance is\nhighly sensitive to the weight ratios of N and MoCl , and that a doping duration of 48 hours yields the optimal\n5\ncomposition for achieving maximum electrical performance.\nFurthermore, we evaluated long-term stability under ambient air, solvent washing, and mechanical/thermal\ncycling. While MoCl -CNTFs exhibit a notable decline in the electrical conductivity over time, N/I -CNTFs,\n5 2\nN/CuI-CNTFs, N/FeCl -CNTFs, and N/MoCl -CNTFs maintain more significantly stable electrical\n3 5\nconductivities over 50 days under ambient conditions (Fig. 4a and Supplementary Fig. 12a). Among these,\nN/MoCl -CNTFs demonstrate the smallest change, indicating superior long-term stability. Additionally,\n5\nsolvents, in contrast to the marked decrease observed in MoCl -CNTFs (Supplementary Fig. 12b). We also\n5\nassessed the performance of N/MoCl -CNTFs under cyclic thermal and mechanical stresses. The electrical\n5\nconductivity of N/MoCl -CNTFs decreases by only 21% upon heating to 200°C and exhibits a minimal\n5\nresistance increase of 1.2% following repeated thermal cycling (Fig. 4b). Moreover, the electrical resistance\nof N/MoCl -CNTFs remains nearly unchanged, with only a 1% variation after 2000 bending cycles (Fig. 4c),\n5\nunderscoring the exceptional flexibility and durability of the fibers.\nIn summary, these additional analyses confirm that our strategy introduces only minimal structural defects,\nachieves optimal doping efficiency at controlled weight ratios of N to MoCl , and provides superior stability\n5\nunder environmental, mechanical, and thermal stresses, thereby reinforcing both the robustness and\nreproducibility of our approach. These results and relevant discussions have been added into the revised\nmanuscript.\nRevision:\nIn the main text,\nLine 116 in Page 6, the description of comparison of I /I values and defect densities of the fibers was added.\nG D\nThe I /I value of N-CNTFs (46.51 ± 0.30), where I and I represent the D-band and G-band intensities\nG D D G\nof CNTFs respectively, shows a slightly decrease compared to that of PCNTFs (49.75 ± 0.39)\n(Supplementary Fig. 6d and Table 2), consistent with the introduction of minor sidewall defects during\nnitrogen plasma treatment59. Similarly, MoCl -CNTFs and N/MoCl -CNTFs show I /I values of 48.54 ± 0.43\n5 5 G D\nand 45.25 ± 0.33, respectively, indicating that the structural integrity of the nanotube lattice is largely\npreserved following both doping processes. The corresponding defect densities, based on the established\n5\ncompared to PCNTFs (Supplementary Table 2), confirming that the structural integrity is well-preserved\nwithin acceptable limits that supports efficient charge transport performance.\nLine 171 in Page 8, the description of the optimization of doping durations was added.\nNotably, N/MoCl -CNTFs obtained with a doping duration of 48 hours exhibit an optimal composition,\n5\ndelivering an exceptional electrical conductivity of 27.34 ± 0.09 MS m−1, which is 3.20, 2.73, and 1.58 times\nthat of PCNTFs, N-CNTFs, and MoCl -CNTFs, respectively (Fig. 3a, Supplementary Figs. 9b, c, and Table\n5\n8).\nLine 331 in Page 13, the description of preparation of N/MoCl -CNTFs was added.\n5 5\nmethod. N-CNTFs and a mixture of MoCl and MoO in a mass ratio of 1:0.6 were first loaded separately\n5 3\ninside a glass tube. Following vacuum drying at 100°C for 24 hours (~1 mbar), the tube was flame-sealed and\nannealed at 300°C for 9 ~ 72 hours in a furnace. The resulting fibers were then washed with acetone and dried\nat 60°C to yield N/MoCl -CNTFs.\nLine 76 in Page 7, the description of the defect density measurement was added.\nRaman spectra and defect density tests of the CNTs and fibers\nRaman spectra of the CNTs and fibers were collected by a Horiba LabRAM HR 800 with a 532 nm laser\nwavelength. The defect density (n ) in the CNTFs was estimated using the I /I value from Raman\nD G D\nspectroscopy, which reflects the level of disorder introduced by doping. This approach is well-established, as\nthe D-band arises from intervalley scattering associated with defects, while the G-band corresponds to the in-\nplane vibration of sp2 carbon atoms. The defect density was calculated using the following empirical relation3,\n1.8×1022 I\nn (cm−2)= ( D) (1)\nD λ4 I\nL G\nWhere λ (nm) represents the wavelength used in the Raman spectroscopy measurement.\nL\nPage 25, the comparison of defect density of CNTFs was added.\nSupplementary Table 2 | Comparison of defect density of CNTFs.\nDefect density\nSample I /I\nG D\n(n , × 109 cm−2)\nD\nPCNTFs 49.75 ± 0.39 4.563 ± 0.036\nN-CNTFs 46.51 ± 0.30 4.829 ± 0.032\nMoCl -CNTFs 48.54 ± 0.43 4.639 ± 0.038\nG D\n52\nthe fibers was added.\nSupplementary Fig. 6 | XPS spectra, I /I value and defect density of different CNTFs.\nG D\na XPS spectrum of N-CNTFs in the N 1s. d Comparison of I /I value and defect density of different CNTFs.\nG D\nPage 24, the description of elemental analysis of different CNTFs was added.\nSupplementary Table 1 | Elemental analysis of different CNTFs.\nPyrrolic N Pyridinic N Graphitic N\nSample N (at%) Mo (at%)\n(at%) (at%) (at%)\nN-CNTFs 4.87 ± 0.76 - 29.55 ± 8.11 56.77 ± 3.89 13.68 ± 5.86\nMoCl -CNTFs - 13.54 ± 1.64 -\n5\n4.45 ± 0.34 13.03 ± 0.83 29.55 ± 1.38 60.74 ± 3.22 9.71 ± 2.29\nCNTFs\nPage 16, the results of optimized doping durations of N/MoCl -CNTFs was added.\nCNTFs.\nb XPS spectra of N/MoCl -CNTFs with varying MoCl doping durations (9 ~ 72 hours). c Electrical\n5 5\nconductivity of different N/MoCl -CNTFs.\n5\nI co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the\nNature Communications initiative to facilitate training in peer review and to provide appropriate recognition\nfor Early Career Researchers who co-review manuscripts.\nReply: Thank you for your time, thoughtful feedback, and contribution to the review process.\n54"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors demonstrated that high-quality solution spun CNT fibers followed by novel endohedral doping leads to record breaking conductivities with robust environmental stability. Endohedral doping is demonstrated with HAADF-STEM, property preservation after environmental cycling, and showing outcomes with/without initial defect incorporation. Density and record-breaking specific conductivity are measured statistically with different methodologies that do not exclusively rely on a potentially variable diameter measurement. Record-breaking conductivity is also measured statistically with diameter measured two different ways, where homogeneity along the fiber was demonstrated. Their exhaustive experimentation with multiple techniques demonstrated high conductivity correlates with carrier density, particularly with large shifts in Raman spectra. This important paper with exhaustive, well-organized supplemental should be published without delay.", "reviewer_2": "In the revised manuscript, the authors have convincingly addressed the earlier comments and concerns. The revisions have enhanced both the precision of the arguments and the overall readability of the paper. I consider the work to be scientifically robust, clearly articulated, and of sufficient originality and relevance to merit publication.\nOn this basis, I recommend acceptance of the manuscript in its present form, with no further changes required.", "reviewer_3": "The authors have provided thorough and convincing responses to my previous comments. Additional experiments and analyses (HAADF-STEM/EDS mapping, stability tests, and dopant comparisons) adequately support their conclusions and have been clearly reflected in the revised manuscript. I am satisfied that my concerns have been addressed, and I recommend acceptance after minor editorial revisions."}, "author_response": "Point-by-point Responses to Reviewers’ Comments\nReviewer #1:\nThe authors demonstrated that high-quality solution spun CNT fibers followed by novel endohedral doping\nleads to record breaking conductivities with robust environmental stability. Endohedral doping is\ndemonstrated with HAADF-STEM, property preservation after environmental cycling, and showing outcomes\nwith/without initial defect incorporation. Density and record-breaking specific conductivity are measured\nstatistically with different methodologies that do not exclusively rely on a potentially variable diameter\nmeasurement. Record-breaking conductivity is also measured statistically with diameter measured two\ndifferent ways, where homogeneity along the fiber was demonstrated. Their exhaustive experimentation with\nmultiple techniques demonstrated high conductivity correlates with carrier density, particularly with large\nshifts in Raman spectra. This important paper with exhaustive, well-organized supplemental should be\npublished without delay.\nReply: We sincerely thank the reviewer for the careful reevaluation and positive assessment of our revised\nmanuscript. We greatly appreciate your recognition of the additional experiments and analyses."}]

Decision letter