Unlocking limited electric double-layer capacity via electrochemically-driven continuous partial desolvations in carbon nanopores

Domain: physics
Published: 2026-01-09
Editorial outcome: major_revision

Abstract
Electrochemical capacitors are primarily limited by the low electric double-layer storage capacity and narrow operating window that avoids the formation of solid electrolyte interface layers. Herein, we demonstrate that electric double-layer adsorption of solvated Na+ in carbon nanopores is achievable under a large offset potential of −2.95 V vs. potential of zero charge in diethylene-glycol-dimethyl-ether electrolyte, even accompanying with the as-formed solid electrolyte interface layers. The largely enlarged offset potential in situ drives the continued partial desolvations in carbon nanopores, which largely reduces the average solvation numbers from 2.1 to 0.6, leading to a high electric double-layer capacitance of 172 F g−1, a high capacity of 508 C g−1 and high initial coulombic efficiency of 92.2% at 0.1 A g−1 (0.5 mA cm−2), together with high-rate capability and long-term cycling stability. Thereby, such increased electric double-layer charge storage enables the redesign and assembly of sodium-ion capacitor pouch cells that display a high specific density of 40 Wh kg−1 (on cell level) and 30,000 cycles at a fast (dis)charging rate of 51 C (20 mA cm−2). Such sodium-ion capacitors are assembled without any pretreatments that are beneficial for scale-up fabrications in industry.

Introduction
Electrochemical capacitors deliver high power density and tens of thousands of cycles, which are suitable for power smoothing and phase regulation in grid storage, energy harvesting and recovery in transportation, and other applications require fast charging or high-power delivery in minutes or seconds 1–3. Commercial electric double-layer capacitors (EDLCs) use activated carbon as both positive and negative electrodes in organic electrolytes through electric double-layer (EDL) adsorption/desorption of ions in nanopores 4–8. Despite their attractive properties, EDLCs face limitations in terms of low gravimetric/volumetric energy densities owing to their limited EDL capacitance and capacity 9,10.

The gravimetric EDL capacitance (C, F g−1) is determined as follows:

C = ε0 εr SA / d (1)

where, ε0 is the dielectric constant of vacuum, εr is the electrolyte dielectric constant, SA is the accessible surface area (m2 g−1, usually measured by gas adsorption) of the porous carbon, and d is the approaching distance between the adsorbed ions and carbon surface 11. Route ① (Fig. 1a) indicates the enlargement of C by increasing the SA of carbon materials. Unfortunately, such an enlargement is not as expected 12. Liu et al. 2 evaluated nanoporous carbons in standard tetraethylammonium tetrafluoroborate (TEABF4) in acetonitrile (ACN) and demonstrated that SA did not linearly affect the C (ref. 2). Instead of SA, disordered carbons with smaller graphene-like domains are more significant in determining C, whereas stronger interactions between ions and carbon atoms lead to more efficient storage of ions. Compared with the other ordered carbons, the highly disordered carbon (ACS-PC) displayed a high capacitance of 138 F g−1, but leading to poorer cycling stability owing to the rich defects 2. Route ② (Fig. 1a) represents reducing d through matching the sizes between nanopores and adsorbed ions, resulting in an increased capacitance of ~140 F g−1 13,14. In situ nuclear magnetic resonance (NMR) and electrochemical quartz crystal microbalance (EQCM) techniques demonstrated that such increased C is attributed to the partial desolvation of ions entering the nanopores (the pore size is smaller than the size of fully solvated ions), resulting in ions close to carbon wall with reduced d, identified as nanoconfinement effect 15–17. However, the fine tuning of the porous structure, disordering and SA of carbon materials is very complex and costly, and the accurate guidance is still lacking 2,4. There is a strong desire to break the limited EDL capacitance and capacity through an efficient mechanism, yet this remains largely unexplored.

The EDL charge storage (Q, C g−1 or mAh g−1) essentially relies on the balance between surface adsorption of the opposite ions from the electrolyte and the applied potential on the electrodes 4:

Q = C Δφ (2)

where Δφ = φ − φPZC represent the electrochemical polarization away from the potential of zero charge (PZC) 18. Route ③ (Fig. 1a) shows the ability to increase the Q of a negative electrode by expanding Δφ to lower potentials 19. The operating voltage of commercial EDLCs using TEABF4 in ACN is limited to 2.7 V (rarely up to 3 V) owing to the decomposition of solvents beyond its stable potential window 20,21. However, organic solvents and slats are reduced to form solid electrolyte interface (SEI) layers on negative electrodes at low potentials 22,23. One well-known function of SEI layers (formed in ethylene carbonate (EC) based electrolytes) is to sieve solvation shells and allow the intercalation of net Li+ into graphite, resulting in a theoretical capacity of 372 mAh g−1 by forming LiC6 (0.167 e− per carbon) 24. This finding significantly promoted the commercialization of lithium-ion batteries since 1991 23. Conversely, the high-SA activated carbon anode cycling in the EC-based electrolyte displayed very low initial coulombic efficiencies (ICEs), owing to the severe decomposition of electrolyte and formation of thick EC-SEI layers 25,26. This conventional view on the function of SEI layers blocks the development of unitizing the EDL adsorption of solvated ions in nanopores under a largely expanded Δφ 11.

Challenging above-mentioned concepts, in this work, we demonstrate that the highly efficient EDL capacitive charge storage in nanopores is achievable even Δφ is expended to −2.95 V (corresponding to a very low potential of 0.05 V vs. Na+/Na away from the PZC) in carbon nanopores by using Na+-based diethylene glycol dimethyl ether (DGDE) electrolyte. The as-formed DGDE-SEI layers allow the flux of solvated Na+ into nanopores. Unlike tuning the pore size and disordering of carbon materials 2,27–29 or using different sizes of cations/anions 30, we find that expanding Δφ is able to electrochemically drive the continuous partial desolvation, as demonstrated by in situ EQCM and NMR techniques. The enhanced desolvation degree, but not complete, leads to an in situ reduction in d and thus a significant increase in capacitance and capacity of EDL capacitive charge storage. Under a large Δφ of −2.95 V, commercial activated carbon (YP50F) delivers a high EDL capacitance of 172 F g−1 and a high capacity of up to 508 C g−1 (refers to 0.063 e− per carbon). Moreover, it delivers a high ICE of 92.2%, a total capacitive-dominated contribution of 99.1%, high-rate capabilities, and long-term cycling stability. This finding enables us to redesign and assemble the hybrid sodium-ion capacitor (SIC) pouch cells without any pretreatments, which display improved specific energy, high power ability, and long cycle life.

Results
DGDE-SEI layers enable the EDL adsorption of solvated ions in nanopores

YP50F has a high Brunauer‒Emmett‒Teller (BET) SA of ~1700 m2 g–1 and an average pore size of 0.9 nm (Supplementary Fig. 1) 2,17. In the electrolyte of 1 M NaPF6 in EC: diethylenecarbonate (DMC), the YP50F electrode displays box-like cyclic voltammetry (CV) curves in 3−1.0 V vs. Na+/Na at 1 mV s–1 (Fig. 1b), which is typical of EDL capacitive behavior. Nevertheless, the CV curves become distorted and the box-like EDL capacitive behavior completely disappears as soon as the cut-off potential below 1.0 V vs. Na+/Na along with the formation of EC-SEI layers. When directly cycled in 3−0.05 V vs. Na+/Na, the CV curves (Supplementary Fig. 2a) and galvanostatic (dis)charge profiles at 0.1 A g–1 (0.5 mA cm–2) (Supplementary Fig. 3a) show the irreversible electrolyte decomposition at ~0.8 V vs. Na+/Na and a very low ICE of 20.9% (Supplementary Fig. 3b,c). The thick SEI layers covering on YP50F particles are observed by ex situ scanning electron microscopy (SEM) and transmission electron microscopy (TEM) (Supplementary Fig. 4), which are composed of rich Na2CO3 and R−C=O composites analysed by ex situ X-ray photoelectron spectroscopy (XPS) (Supplementary Fig. 5) 31,32. After the formation of the EC-SEI layers, the characteristic microporous-type adsorption isotherms entirely disappeared, and SA decreased to only 6.5 m2 g−1 (Supplementary Fig. 6). Therefore, the YP50F electrode in EC:DMC in 3−0.05 V vs. Na+/Na displays the loss of characteristic capacitor-like CV curves (Fig. 1b and Supplementary Fig. 2a) and reaction kinetics (a low capacitive contribution of 47.6% at 0.2 mV s−1, Supplementary Fig. 7) 33. The above results support the function of EC-based SEI layers (Fig. 1c) 23,24,31, which sieve solvation shells and allow net Na+ adsorption.

Conversely, the YP50F electrode in the DGDE electrolyte continues to display butterfly-shaped CV curves during the expanding Δφ, until the cut-off potential low to 0.05 V vs. Na+/Na (Fig. 1d). When directly cycled in 3−0.05 V vs. Na+/Na, the well-overlapping CV curves indicate the minimal electrolyte decomposition (Supplementary Fig. 8). After cycles, the YP50F particles have clean surface with the partial distributions of NaF nanograins (~3 nm, Supplementary Fig. 9) 34. The ex situ XPS results (Supplementary Fig. 10) reveal a small amount of reduced products, including inorganic NaF, Na2CO3 and organic polyether (R−C=O and R−C−O), from the thin DGDE-SEI layers 35. Importantly, after cycles, the YP50F electrode with DGDE-SEI layers maintains microporous adsorption isotherms and a high SA of 961 m2 g−1 (Supplementary Fig. 11), indicating that the as-formed DGDE-SEI layers do not block the micropores for the flux of solvated ions. The YP50F electrode in the DGDE electrolyte displays a total capacitive-dominant contribution of 99.1% at 0.2 mV s−1 (Fig. 1e and Supplementary Fig. 12). The functions of the EC-SEI and DGDE-SEI layers are further demonstrated by the exchange of the applied electrolyte systems (Supplementary Fig. 13). Above all, the DGDE-SEI layers on YP50F allow flux of solvation Na+ ions (Fig. 1f) and efficient EDL capacitive behavior in nanopores under Δφ = −2.95 V (in the protentional window of 3−0.05 V vs. Na+/Na).

Electrochemically-driven continuous partial desolvation of EDL capacitive mechanism in nanopores

In situ EQCM and NMR techniques were employed to track the fluxes of ions into nanopores in real time and the corresponding chemical environmental changes 17,36. In situ EQCM was operated via an Al-coated quartz crystal resonator (Supplementary Fig. 14) 17. First, CV was performed simultaneously with the in situ EQCM response in the DGDE electrolyte in 2.0−4.0 V vs. Na+/Na (Fig. 2a). The φPZC of ~3.02 V vs. Na+/Na is close to the open-circuit voltage (OCV) of 3.03 V. Thus, a cut-off potential of 3.0 V is applied for convenience. For the potential window of 3−4 V vs. Na+/Na, which is above φPZC, the fitted mass change per electron (Δm=dq) of 155 g mol−1 indicates an anion adsorption dominated mechanism (the molar mass of PF6− is 145 g mol−1) 37.

In situ EQCM tests in the potential window of 3−0.05 V vs. Na+/Na were subsequently performed. In the DGDE electrolyte, highly reversible mass changes are exhibited (Supplementary Fig. 15), accompanied by slight electrolyte decomposition. Dissipation factors (D) of the pristine and cycled YP50F@crystal disks were further analysed (Supplementary Fig. 16), while the small ΔD/Δn values were on the order of 10−6, indicating that the stiff films were valid for the Sauerbrey equation 38–41. The EQCM data at the 5th cycle (Fig. 2b) were further analysed when stable DGDE-SEI layers formed. The linear fits of Δm=dq are divided into three regions (Fig. 2c), with values increasing and then decreasing during expanding ΔV. The low Δm=dq of 80 g mol−1 in region I of 3−1.5 V vs. Na+/Na refers to an anion exchange process between [Na-2:1DGDE]+ and PF6− (Supplementary Fig. 17a), which is further confirmed by ex situ 19F NMR spectra (Supplementary Fig. 18). The calculated solvent number of ~2.1 is consistent with the stable solvation of Na+ coordinating with two DGDE molecules 42,43. In region II of 1.5−0.25 V vs. Na+/Na, the increased Δm=dq value of 174 g mol−1 suggests a transition into the counter-ion adsorption dominated process, whereas the average solvent number is ~1.1 (Supplementary Fig. 17b), corresponding to a partially desolvated [Na-1:1DGDE]+ flux into the nanopores. Furthermore, in region III of 0.25−0.05 V vs. Na+/Na, the Δm=dq of 108 g mol−1 corresponds to an average value of 0.6, indicating a reduced partial desolvation of [Na-0:6DGDE]+ flux into the nanopores. For the positive scan, the trends of multistep partial desolvation processes are also observed (Supplementary Fig. 19). By calculating the overall cations maintained in the nanopores during expanding Δφ (Supplementary Fig. 20), the total average coordinated solvation number is reduced from 2.1 (at 1.5 V vs. Na+/Na) to ~1.3 (at 0.25 V vs. Na+/Na) and then to ~1.1 (at 1.5 V vs. Na+/Na), leading to dense packing of solvated [Na-xDGDE]+ ions in nanopores.

Compared to the EQCM results in DGDE electrolyte, the mass changes in the EC:DMC electrolyte are highly irreversible (Supplementary Fig. 21) because of the formation of thick EC-SEI layers. The EC-SEI layers are able to sieve the solvation shells 23,24,31. The Δm=dq of 30.5 g mol−1 is slightly higher than 23 g mol−1 for net Na+ (Supplementary Fig. 22), which might be owing to the contributions of electrolyte decomposition or the adsorption of solvated ions on the surface of EC-SEI layers. However, the completely desolvated Na+ ions should cross through the thick EC-SEI layers, resulting in poor rate capability and large voltage hysteresis (Supplementary Fig. 3c−f).

Owing to “ring current effects” originating from delocalized π-electrons in the carbon rings, 23Na NMR spectra (Supplementary Fig. 23) display resonances at more negative chemical shifts for “in-pore” ions (adsorbed in nanopores) compared to “ex-pore” ions (located outside the pores) 2,17. Accordingly, in situ 23Na NMR spectroscopy was employed to track the changes in the population and microstructure of “in-pore” cations 2,44. An almost reversible change in the chemical shift range of 1000 to −1000 ppm indicates the EDL process during cycling in DGDE electrolyte (Supplementary Fig. 24), which differs from the formation of Na0 clusters in hard carbon anodes 45,46. A clearer understanding is achieved by analysing in situ 23Na NMR spectra from the 2nd cycle (Fig. 2d), which eliminates the influence associated with SEI formation. The in situ 23Na NMR spectra (Fig. 2d) and difference spectra (Supplementary Fig. 25) revealed a notable broadening of signals at lower potentials during the expanding Δφ. The consistent trend with upfield shifting and broadening in-pore signals was also observed by ex situ 23Na NMR spectra (Supplementary Fig. 26). This broadening indicates a stronger interaction between solvated Na+ and the carbon surface, attributed to a decrease in the d (Eq. 1). Following this, spectral deconvolution was performed on the basis of the spectra (Supplementary Fig. 27 and Supplementary Table 1). The observed two-step increases in the deconvoluted areas of the “in-pore” signals vs. capacity (Fig. 2e) correlate well with the ion exchange and counter-ion adsorption processes confirmed by the EQCM results. Notably, as the electrode discharges from 1.4 to 1.2 V, the half width of the “in-pore” signals increases appreciably from ~23 to 40 ppm (Fig. 2f). This is followed by an early linear increase in the half width and a corresponding linear decrease in the chemical shift to the low potential of 0.05 V vs. Na+/Na, at which point the half width of the “in-pore” signal remains constant. These enhanced solvated Na+ ion‒carbon interactions observed via in situ NMR closely align with the EQCM results. The large electrochemical polarization (Δφ = −2.95 V) electrochemically drives continuous partial desolvation from [Na-2:1DGDE]+ to [Na-1:1DGDE]+ and then to [Na-0:6DGDE]+ (Fig. 2g), leading to gradually reducing d and thus the increase in capacitance. In situ 23Na NMR in EC:DMC (Supplementary Fig. 28) shows the irreversible formation of EC-SEI single during discharging to 0.05 V and recharging to 3 V, which is very different from the reversible changes of spectra cycled in DGDE electrolyte (Supplementary Fig. 24).

Boosted EDL capacitive charge storage of YP50F electrode in DGDE under large Δφ

The YP50F electrode shows linear charge and discharge profiles during the initial 5 cycles in DGDE in 3−0.05 V vs. Na+/Na (a large Δφ of −2.95 V), corresponding to the EDL capacitive adsorption of solvated [Na-xDGDE]+ in nanopores (Fig. 3a). Remarkably, the ICE reaches to 92.2% and subsequent CEs quickly reach to ~100% (Supplementary Fig. 29), suggesting minimal electrolyte decomposition even using high-SA carbon materials. At the low specific currents of 0.01 and 0.05 A g−1, the YP50F electrodes also deliver very high ICEs (Supplementary Fig. 30), indicating that the low current has no influence on the partial desolvation steps of [Na-xDGDE]+ in nanopores. In contrast, the YP50F electrode in EC:DMC displays a highly curved shape owing to the adsorption of net Na+ ions, which require the total stripping of solvation shells and then crossing through thick EC-SEI layers. Figure 3b shows the relationship between the capacitance and capacity with increasing Δφ (Supplementary Fig. 31). The capacitance gradually increases from 85 to 172 F g−1 as Δφ increases from −1.5 to −2.95 V owing to the continuous desolvation of [Na-xDGDE]+ with decreasing d. Compared with the controls of pore size, surface doping or increasing the ordering of carbon materials 2,27–29, the large increases in capacitance caused by in situ increases in Δφ are highly effective, convenient, and cost-saving. Importantly, owing to the simultaneously increased capacitance and Δφ, the charge storage capacity of the YP50F electrode reaches a high value of 508 C g−1 (141 mAh g−1, corresponding to 0.063 e− per carbon).

The EDL performances of various reported porous carbons in aqueous, organic, and ionic-liquid electrolytes are collected (Supplementary Table 2). There is no apparent correlation between the capacity and SA of porous carbon or electrolyte systems (Supplementary Fig. 32) 2. Nevertheless, the relationship between gravimetric capacity and capacitance is dependent on the operating potential window of different electrolytes (Fig. 3c). Owing to the EDL capacitive charge storage under a large Δφ of −2.95 V in the DGDE electrolyte, the commercial YP50F is able to achieve the high capacity among the state-of-the-art results (Fig. 3c, d and Supplementary Table 2). The normalized capacitance of 10.1 μF cm−2 (Fig. 3d) still falls within the range of 5–20 μF cm−2 for EDL capacitive adsorption in porous carbon 4. Remarkably, YP50F under the Δφ of −2.95 V achieves the high normalized capacity of 30.0 μC cm−2.

The storage capacity and rate performance of the EDL adsorption of [Na-xDGDE]+ are further compared with those of the Faradaic reactions in carbon materials at the consistent mass loading of 5.0 ± 0.1 mg cm−2 (Fig. 3e and Supplementary Figs. 33 and 34). The capacity of 508 C g−1 for [Na-xDGDE]+ adsorption under Δφ = −2.95 V is significantly higher than that of 135 C g−1 for commercial EDLCs (1 M TEABF4 in ACN) and getting close to 1005 C g−1 for Na+ intercalation in hard carbon and 1218 C g−1 for Li+ intercalation in graphite 47,48. However, when the current density is over 5 mA cm−2 (the same charging and discharging currents), these intercalation-type materials show rapidly reduced capacity. But, importantly, the EDL capacitive adsorption of [Na-xDGDE]+ shows high rate capabilities in the current densities of 5–50 mA cm−2. The YP50F electrode displays negligible capacity loss after 10,000 cycles (Fig. 3f) in the DGDE electrolyte in 3.0−0.05 V vs. Na+/Na under half-cell tests. These improved EDL capacitive charge storages support the key concept that increased Δφ facilitates denser ion packing in nanopores, leading to simultaneously increased capacitances and capacities.

Redesign and assembly of hybrid sodium ion capacitor pouch cells

The hybrid capacitor comprises an EDL electrode and a battery-type electrode 49. Commercial hybrid LICs are composed of pre-lithiated graphite as the negative electrode and activated carbon as the positive electrode (Fig. 4a). The LICs necessarily require a pre-lithiation process for device operation, which is complex and requires extra costs in industry 50. Additionally, sluggish Li+ intercalation into graphite limits the high-power capability and increases the risk of dendritic growth under fast charging 22,51. Recently, hybrid SICs composed of hard carbon negative electrode and activated carbon positive electrode were proposed, accompanying the development of pre-sodiaiton methods 52–54.

Differently, we redesign and assemble hybrid SIC pouch cells using YP50F as the negative electrode, matching with battery-type material as positive electrode (Fig. 4b). Na3V2(PO4)3 (NVP) was selected owing to its high-rate capabilities (Supplementary Fig. 35a–d) 55, which displayed well-matched reaction kinetics to the YP50F negative electrode (Supplementary Fig. 35e). The three-electrode device demonstrates that the assembled hybrid SIC follows a sodium-ion-transport mechanism (Fig. 4c), that is, the extraction of Na+ from NVP and adsorption of solvated Na+ into the YP50F. The discharging process is inverse. In particular, in this design system, the YP50F negative electrode reversibly operates low to 0.05 V vs. Na+/Na in the DGDE electrolyte (Fig. 4c), which enables the hybrid SIC to work up to 3.4 V.

The YP50F | 1 M NaPF6 in DGDE | NVP hybrid SIC pouch cells (rated 545 F / 515 mAh, inset of Fig. 4d and Supplementary Fig. 36) are assembled without any presodiation treatments, which delivers a high ICE of 96.1% (Supplementary Fig. 37). This assembly technology is cost-efficient and easily matched with the production processes of the battery or supercapacitor industry. The self-discharge ratio of the SIC pouch cell is ~13.0% after setting for 120 h, which is lower than that of ~38.0% for an EDLC device (Supplementary Fig. 38). The SIC pouch cell shows a capacity retention of 71.4% after 30,000 cycles at 20 mA cm−2, corresponding to a fast (dis)charging time of 70 s that is ~51 C (Fig. 4d and Supplementary Fig. 39), demonstrating the long-term cycling stability.

Moreover, the SIC pouch cell displays triangular charge-discharge curves at various current densities ranging from 0.5 to 40 mA cm−2 (Fig. 4e). At 10, 20 and 40 mA cm−2, it delivers capacities of 396, 310 and 211 mAh, corresponding to fast charging abilities with state of charge (SoC) of 76.9%, 60.6%, and 41.0% finished in 196, 70 and 26 s, respectively. The energy efficiencies of the SIC pouch cell are 93.3%, 80.4%, and 61.0% for the (dis)charge times of 5035, 196, and 26 s, respectively, which are comparable to those of the EDLC and LIB (Supplementary Fig. 40). Additionally, when the charge current was set at 4 mA cm−2, the pouch cell could output large discharge currents of up to 100 mA cm−2 with a high capacity of 196 mAh in 10 s (Supplementary Fig. 41), indicating the ability of high power delivery. The Ragone plots (Fig. 4f) show the SIC pouch cell combined with both high energy and high power densities. It delivers a high specific energy of 40 Wh kg−1 (calculated based on the total weight of pouch cell), which is comparable with state-of-the-art commercial LICs (Supplementary Table 3) and EDLCs (Supplementary Table 4) 9,56,57.

Methods
Electrode and electrolyte preparation

The slurry comprised YP50F (Kuraray Co.), ketjen black (EC-600JD, Nouryon Co.), carboxymethyl cellulose (CMC, >99.5%, Canrd New Energy Technology Co.), and styrene butadiene rubber (SBR, >99.5%, Canrd New Energy Technology Co.) at a ratio of 92:3:3:2 by weight. The slurry with deionized water as the dissolving solvent was homogeneously coated onto Al foil (16 μm in thickness, >99.5%, Shenzhen Kejing Star Technology Co., Ltd.) and dried at 100°C for 12 h in a vacuum oven. The thick-film YP50F electrode has a single-sided thickness of 100 μm, corresponding to a mass loading of 5.0 mg cm−2.

The graphite anode (mass loading of 5.0 mg cm−2) on Cu foils (25 μm in thickness, Shenzhen Kejing Star Technology Co. Ltd.) and hard carbon (mass loading of 5.0 mg cm−2) on Al foil were prepared as the same process and component as the YP50F electrode. The Na3V2(PO4)3@C (NVP@C, Hubei Energy Technology Co. Ltd.) electrode was prepared by mixing NVP@C powder, ketjen black, and polyvinylidene fluoride (PVDF, Battery grade, Shenzhen Kejing Star Technology Co. Ltd.) at a ratio of 90:5:5 by weight. The slurry with N-methylpyrrolidine (NMP, >99%, Acmec) was homogeneously coated onto Al foil and dried at 120°C for 12 h in a vacuum oven. The single-sided mass loading of the NVP electrode was controlled at 7.5 mg cm−2. The above electrodes were cut into 14 mm in diameter for coin cell testing. The electrolytes of 1 M NaPF6 in diethylene-glycol-dimethyl-ether (DGDE, >99.8%) and 1 M NaPF6 in ethylene carbonate (EC): diethylenecarbonate (DMC) (1:1 by weight, >99.8%) were obtained from DuoDuo Chemical Technology Co. The solvents of DGDE (>99.8%) and DMC (>99.8%) were obtained from DuoDuo Chemical Technology Co.

Electrochemical measurements

The half-cells were assembled in an Ar-filled glove box (H2O < 0.1 ppm and O2 < 0.1 ppm). The CR2025 coin cells were composed of a sodium foil (>99.9%, 0.6 mm in thickness, 15.8 mm in diameter, Sinopharm Chemical Reagent Co.) as the counter and reference electrode, a Celgard-2325 separator, and a YP50F electrode as the working electrode. The volume of ~60 μL electrolyte is applied for each coin cell. Galvanostatic charge-discharge profiles were measured via a NEWARE battery test system. Three or more cells were tested for a single electrochemical experiment. All the electrochemical results were measured at 25°C (MSK-TE906-150P-70-5, Shenzhen Kejing Star Technology Co. Ltd.).

Characterization techniques

Powder X-ray diffraction (XRD) was performed on Bruker-Axs X-ray diffractometer (Cu Kα radiation, λ=1.5406 Å) over a diffraction angle range of 10°−60°, at a speed size of 5° min−1. The Brunauer–Emmett–Teller (BET) specific surface area was measured via nitrogen sorption isotherms (Micromeritics Tristar 3020) at 77 K. To prepare cycled YP50F powders for nitrogen sorption isotherms, a pouch cell using YP50F electrodes and sodium foil in the DGDE (or EC:DMC) electrolyte was assembled. After 5 cycles, the cell was disassembled in a glove box and washed with DGDE (or DMC) to remove residual electrolytes. All powders were scraped from the current collector and dried for BET analysis. Scanning electron microscopy (SEM) images were recorded by using SU-70, conducted at 20 keV. The cross-sectional SEM image of the electrodes was obtained with a Leica EM TIC 3X. Transmission electron microscopy (TEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images and EDS maps were obtained through aberration-corrected transmission electron microscopy (Titan Cubed Themis G2 300). To prepare the sample for ex situ TEM analysis, the cycled YP50F electrodes were disassembled in a glove box and washed with DGDE or DMC solvent to remove residual electrolytes. Afterwards, the powders were scraped from Al foil and then subjected to ultrasonic treatment in alcohol, and deposited onto a grid for TEM characterization 58. Throughout the transfer procedure, the samples were kept in a sealed loading chamber under an Ar atmosphere to prevent oxidation. Ex situ XPS survey scans were performed on a Kratos Axis Supra system equipped with a monochromatic Al Kα X-ray source (1486.7 eV) 58. Coin cells were disassembled inside the glove box, and the YP50F electrodes were rinsed with either DGDE or DMC solvent. After drying, the anodes were introduced into the XPS chamber for measurement. To avoid air contact, the XPS chamber was directly linked to an Ar-filled glove box for sample preparation and transfer. All spectra were calibrated for charge referencing based on the C 1s peak (at 284.8 eV).

In situ EQCM measurements and analyses

The EQCM measurements were conducted on a QCD-300 single-channel quartz crystal microbalance (Bio-Logic), lined with a potentiostat (Bio-Logic SP-200). The slurry comprising YP50F, carbon nanotubes, and CMC at a ratio of 90:5:5 by weight was dispersed in deionized water through strong ultrasonic treatment. Then, the slurry was sprayed onto an Al-coated quartz crystal disk (5 MHz). The crystal disk was placed onto a hot plate at 80°C, while a steel mask was covered on the disk with the exposure of the Al-coated surface for the spray of slurry. Compressed air was used to control the spray pressure. The carbon deposited crystal disks were dried in a vacuum oven for 4 h at 100°C. The mass loading was controlled at ~50 μg cm−2. The dissipation factors (D) at different overtones of the pristine YP50F@crystal disks and the electrochemically cycled disks at the fully discharged state were measured by the Q-Sense Explorer (Biolin Scientific).

Electrochemical measurements were conducted with simultaneous recording of the quartz resonance frequency. According to the Sauerbrey equation (Eq. 3), the frequency shift (Δf) of the quartz resonator is effectively converted into the corresponding mass change (Δm) of the electrodes and quartz crystal 16,17:

Δm = − (C f × Δf) (3)

where ρq means the density of quartz, μq means the shear modulus of quartz, f0 means the fundamental resonance frequency of the quartz, and Cf means the calibration constant (or sensitivity factor). The calculation of Cf was based on the method provided by Griffin et al 17. In detail, a sliver deposition experiment with 0.01 M silver nitrate and 0.1 M tetraethylammonium tetrafluoroborate in DGDE was applied (Supplementary Fig. 14). Thus, the calculated Cf was 21.02 ng Hz−1.

In situ and ex situ NMR testing

In situ 23Na NMR measurements were performed on a AVANCE III 500 MHz spectrometer. Self-designed in situ probe and operando cells were applied (Supplementary Fig. 24a). A single pulse sequence with a π/12 angle of 1.5 μs and a recycle delay of 0.02 s was used to acquire the 23Na NMR spectra with an interval time of 30 min, and every single spectrum required ~10 min.

Ex situ NMR spectroscopy was performed on a Bruker AVANCE NEO 300 MHz NMR spectrometer. The coin cells were operated to the target potential and disassembled in an Ar-filled glove box immediately. Then materials were scraped from the Al foils and packed into 3.2 rotors for NMR tests. 23Na NMR spectra were acquired using a π/12 pulse of 0.6 μs and a recycle delay of 0.02 s with magic angle spinning (MAS) frequency of 8 kHz, referenced to NaCl powders (7.2 ppm). 19F NMR spectra were acquired using a π/4 pulse of 1.5 μs and a recycle delay of 10 s with MAS frequency of 3 kHz, referenced to LiF (−204 ppm).

Ex situ nitrogen adsorption-desorption isotherms

The Brunauer–Emmett–Teller (BET) specific surface area was measured via nitrogen adsorption-desorption isotherms (Micromeritics Tristar 3020) at 77 K. The YP50F powders (~100 mg) were placed into a glass bulb gas cell and degassed at 150°C in vacuum for 16 h before the measurement 2. To prepare the cycled YP50F electrode for nitrogen adsorption-desorption isotherms, a pouch cell using YP50F electrodes and sodium foil in the DGDE (or EC:DMC) electrolyte was assembled. After 5 cycles, the cells were disassembled in a glove box and washed with DGDE (or DMC) to remove residual electrolytes. All powders were scraped from the current collector and dried at 120°C in the glove box. Then, the powders (~100 mg) were placed into a glass bulb gas cell and degassed at 150°C in vacuum for 16 h before measurement.

Fabrication of hybrid SIC pouch cells

The hybrid sodium-ion capacitor pouch cells were assembled by repeatedly stacking 16 layers of YP50F negative electrodes (double-side coating with the mass loading of 10 mg cm−2), Celgard-2325 separators, and 15 layers of NVP positive electrodes (double-side coating with the mass loading of 15 mg cm−2). The YP50F and NVP electrodes were cut into 43 mm × 56 mm in size. The pouch cells were assembled in Ar-filled glove box, injected with the electrolyte (~13 mL of 1 M NaPF6 in DGDE), and vacuum sealed at 200°C. The pouch cells were firstly tested at 0.2 mA cm−2 for initial cycle in the voltage window of 0–3.4 V and then for other electrochemical performance measurements, under the external pressure of ~100 kPa. All the electrochemical results of pouch cells were measured at 25°C.

The specific capacity (Qm, C g−1 or mAh g−1), specific capacitance (Cm, F g−1), specific energy (Em, Wh kg−1), and specific power (Pm, W kg−1) are calculated via Eq. 4−7.

Qm = (I × t) / m (4)

Cm = (I × t) / (ΔV × m) (5)

Em = (1 / (3.6 × m)) × ∫ IV dt (6)

Pm = Em / t (7)

where t = t2 − t1 refers to the time for a discharging process and the discharge voltage profiles are applied for the calculation of specific energy. For SIC pouch cells, the m represents total weight of cell.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The manuscript titled “Unlocking limited electric double-layer capacity via electrochemically-driven continuous partial desolvations in carbon nanopores” authored by Fan et el. aims to investigate the solvation environment of inserted Na+ into YP50F activated carbon. However, there are some major concerns on data collection and interpretation, as described below:\n• My major concern is on the EQCM data interpretation, which is a very important part of discussion in the manuscript. The authors did not explain how the mass was calculated. It seems the authors applied Sauerbrey equation to convert the frequency change to mass change. However, the frequency change measured by EQCM can be contributed by mass change, viscoelastic property changes (the density, viscosity, and elasticity of the film electrode material) or sample roughness change [Ref: Acc. Chem. Res. 2018, 51, 69−79; J Polym Sci. 2021;1–18; Chem. Rev. 1992, 92, 1355]. The Sauerbrey Equation is only valid when the film remains stiff, uniform, flat, and no dissipation during cycling. With the way the authors prepared the electrolyte (dripcasting tens or hundreds µm size of YP50F particles on the quartz), the electrode roughness is already high. In addition, the electrolyte is more viscous than aqueous electrolyte, which further increased the dissipation of the resonance of the quartz. This will cause an extremely broadened frequency peak, which might lead to inaccurate reading of the resonance peak. Furthermore, during cycling, there is SEI formation and potential morphological electrode changes caused by the ion insertion. Both phenomena can also contribute to frequency change. Therefore, the authors will need to provide dissipation information (or Q factor, internal resistance, etc) during cycling to justify the mass change and solvation number discussion. Without dissipation information, the EQCM results are highly questionable. This obviously impact the validity of the discussion on solvation number.\n• Another major point is that the way the authors described their observation is confusing and misleading. The term “electrical double layer (EDL)” described ion forming on at the electrode/electrolyte interface. The EDL naturally form on any immersed surface, whether the electrode is charged or at open circuit potential. When the YP50F is immersed in the electrolyte, all the accessible pores will be filled with ions already. Higher overpotential even reaching SEI formation, will drive more ions and/or solvents into the pore and also decompose the electrolyte to form SEI, but there is always EDL layer on the surface. The term the authors should use is “double layer capacitive behavior”. The authors described the charging process it as “When cycled towards lower potential, the EDL will disappear”. In fact, the EDL layer is always present, just the redox related current kicked in and becomes the main capacity contribution.\n• In the experimental section for the BET measurement, there is no details given regarding the drying process of the sample. If the post-cycle sample is conducted without thorough drying, the pores will be packed with adsorbed liquid, which will also give an extremely low SA.\nBased on the above-mentioned reasons, this work is not qualified to be published in Nature Communications.", "reviewer_2": "The authors present a novel and systematic study aimed at enhancing the energy storage capacity of a commercial porous carbon electrode (YP50F) by extending the operating voltage window (Δφ) to 2.95 V. The ion desolvation and SEI plays a critical role in enabling stable and reversible Na⁺ ion adsorption/desorption in this high-voltage regime. The work thoroughly investigates the interplay between electrolyte composition and SEI formation on Na⁺ storage mechanisms. A high specific capacity of 141 mAh g⁻¹ was achieved, surpassing previously reported values in both organic and ionic liquid electrolytes. Unlike intercalation-based mechanisms, the charge storage here is governed by electric double-layer (EDL)-type Na⁺ ion adsorption, which is favorable for high-power applications. This is well-supported by the pouch cell demonstration provided in the manuscript. The authors also conducted comprehensive characterizations of the SEI composition and its role in the charge storage process. Notably, partial desolvation of Na⁺ facilitated by the SEI is proposed as a key factor driving performance improvement, which aligns well with recent studies in the field. The manuscript is well-organized and presents significant findings. I recommend its publication in Nature Communications after minor revisions.\nSpecific Comments:\n1. Sodium adsorption mechanism\na) In Fig1b, I agree that the CV get distorted below 1V because of the irreversible charge storage process. However, I disagree with the full desolvation mechanism (of Na ions) as proposed in Fig1c. Generally speaking, a full desolvation typically leads to higher capacity, since the ion get closer to the electrode, as the authors also noted in this manuscript. Also, the stronger interaction between ions/host materials also enhances charge transfer, which also increase the capacity (Nature Communications, 2025, 16(1): 3813). However, in the present data, the difference of sodium insertion capacity below 1V is relatively small (0.5 A/g Fig1b vs 0.4 A/g, Fig1c. A rough estimation based on complete Na⁺ occupation in the pores of YP50F (0.7 cm³/g pore volume, Na atomic radius ≈ 0.19 nm) yields a theoretical capacity of ~1154 mAh/g, suggesting that Na ions are far away from full desolvation. The authors are encouraged to elaborate more on the mechanism in the EC:DMC electrolyte.\nb) The butterfly-shaped CV deviates from the classical rectangular profile of pure EDL behavior, indicating potential-dependent ion–electrode interactions. This behavior is often influenced by specific ion effect (specific ion-electrode interactions, REF28). Using different cations rather than Na+ may be beyond the scope of this work, but a brief discussion of how Na⁺-YP50F specific interactions contribute to the CV shape would enrich the interpretation. The effects of surface functionalities, graphitization degree, and average pore size (confinement) would be interesting for future works.\n2. Question about EQCM measurements\na) There appears to be a large irreversible current at low potentials (Supplementary Figure 16), likely due to electrolyte decomposition. This complicates the use of EQCM for accurate Na⁺ desolvation analysis, as side reactions being emphasized on the gold-coated quartz surface at slower scan rates. Thus, molecular weight calculations based on irreversible charge may not reliably reflect the solvation state.\nb) The integrated charge values from CV curves at 5 mV/s (Fig. 2b) are noticeably lower than those shown in Fig. 2c. For better clarity and reproducibility, it is strongly recommended that the authors ensure both figures reflect data from the same sample and experimental conditions.\nc) In Fig2c, similar to my previous comments, the same applies here that the molecular weight calculation based on slope in region iii (0.25-0.05 V) is a bit challenging. There is also irreversible current (side reaction) at the negative scan (Na adsorption), alternatively, the reverse scan (positive scan) may provide more reliable insights into the solvation state, considering that the EDL adsorption/desorption process are highly reversible in coin cells (Figure 1d, 1e).\n3. Minor corrections:\na) Page 3, Line 49: Remove “with” for grammatical clarity.\nb) Page 3, Line 52: Standardize the citation format for “ref 2”.\nc) Page 6, Line 118: Replace “box-like CV” with “butterfly-shaped CV” or “symmetric CV” will be more accurate.\nd) Page 9, Line 206: Avoid the term “EDL reaction,” which may be confused with faradic reactions.", "reviewer_3": "Authors provided interesting report on sodium-ion capacitors (SICs) development and their approach on avoiding so-called pre-sodiation step. I find the results noteworthy and deserving publication, however, certain aspects require further comments.\n1. Please comment on the EQCM experiment, especially the aspect of energy dissipation. The electrodes were fabricated with CMC and CNTs, thus, it might perturb the experiment results. What was the resistance change recorded?\n2. Please calculate the energy efficiency for the full cell and comment on that.\n3. Please comment on the impact of scan rate or current load for desolvation step. Usually, SEI formation is very sensitive to these parameters and the performance of the device could change when the deep polarisation is performed at various scan rates. Did the recent data were optimized in that context?\n4. The electrochemical characterization is performed with due care and follows the best practices, however, the leakage currents or self-discharge properties should be at least commented.\nFinally, the work might be of high significance for the community and the recent literature is well discussed, nevertheless, some other concepts such as sacrificial materials for pre-sodiation or redox-active electrolytes could be mentioned in order to provide a comprehensive discussion.\nI have no concerns regarding the methodology and the details presented in the manuscript and supplementary information are enough to reproduce the work.\nAfter addressing the points raised above, I would be happy to support publication of this manuscript.", "reviewer_4": "In this work, the authors study the sodium storage mechanism in a supercapacitor with an ether-based electrolyte under increased potential window by EQCM and NMR.\nThe NMR section effectively demonstrates, consistent with previous studies, that Na⁺ ions located inside and outside the pores can be clearly distinguished. The in situ experiments reveal reversible sodium insertion into the pores without the Knight shift typically associated with the formation of Na⁰ clusters or “pore filling” seen in hard carbon. Additionally, broader in-pore signals are observed, indicating stronger interactions between Na⁺ ions and the pores, as well as a decrease in the distance between the Na⁺ ions and the carbon surface. These changes correlate well with the partial desodiation results obtained from EQCM measurements, showing good alignment between the two techniques.\nMinor points to consider include correcting the spelling of “shifts” in the caption of Figure S21 and potentially adjusting the colour scheme in Figure S22, as some voltage ranges (0.05–3.0 V and 2.5–3.0 V) are represented by similar colours, which may reduce clarity.\nOverall, I am satisfied with this section and have no major concerns.", "reviewer_5": "Electrochemical capacitors are characterised by their high-power density but are limited by their gravimetric capacitance and capacity. Typically, they operate in a narrow potential window before the formation of an SEI. In this manuscript, the authors present a carbon nanosphere-based supercapacitor with a diethylene-glycol-dimethyl-ether (DGDE) electrolyte, which can operate under a large potential difference of -2.95 V between the TOC and discharged state. This leads to a high specific capacitance of 172 F/g and capacity of 508 C/g.\nThe authors introduce Na+-based DGDE as an electrolyte which forms an SEI layer that allows the migration of solvated Na+ ions into nanospheres, in contrast to the SEI formed by ethylene carbonate that strips the solvation shell during Na ion migration. The authors claim that expanding the potential range can electrochemically drive a partial desolvation, reducing the distance between the carbonaceous surface and the sodium ions, and therefore increasing the capacitance. SEM and TEM combined with CV measurements showed a thick SEI on the carbon particles when using an EC/DMC-based electrolyte, which was accompanied by a significant decrease in the specific surface area. In contrast to this, the same YP50F electrode material showed a box-like CV during an expanding potential range when using the DGDE electrolyte.\nEQCM was used to analyse the flow into nanopores. Using EC/DMC-based electrolytes, this resulted in mass changes per electron that the authors assign to a PF6- anion adsorption mechanism in the potential window between 3-4 V, and an adsorption of desolvated Na+ ions in the potential window 3-0.05 V vs. Na/Na+. In addition, the mass change differs significantly between the cycles. In contrast to this, the DGDE electrolyte showed more reversible mass changes and regions of different mass changes per transferred electron. The authors attribute these different mass changes to ion exchange, counter-ion adsorption, and the flux of partially desolvated Na+ Ions into the nanopores. The authors claim to support this idea with in situ NMR measurements and the broadening of the in-pore peak with increasing potential. The capacity and capacitance were further verified in pouch cells using NVP as a counter electrode that does not require an additional pre-sodiation process.\nGeneral comments and questions:\n1) Is there supporting evidence for the assumptions of the different stages of ion flux at the potential ranges when using DGDE as an electrolyte? As the mass changes only approximately match the masses of the assigned ions, the uncertainty seems to be very high. Could the authors comment on the discrepancies between the observed mass changes and those that they are proposing? The authors state several times that the observed mass changes are “close to” the mass changes they are proposing, but they can differ by over 25%.\n2) The correct term in l. 162 should be “ring current effects” instead of “ring effects”\n3) Is there evidence for the assumption that the 23Na NMR signal at -10.5 ppm belongs to the electrolyte in the porous electrode? Were spectra recorded at variable temperature to rule out the possibility of it being an exchange peak between the free electrolyte and the in-pore electrolyte?\n4) The signal-to-noise ratio of the 23Na NMR signal assigned to the fraction of Na in nanopores is very weak. Mixing of this signal with random noise might lead to high uncertainties in the peak position, and all information about the peak shape is lost. Is there a possibility to conduct measurements with more scans to verify the peak position, and to analyse if the peak shoulder is real or an artefact from random noise?\n5) The description of Supp. Figure 21 includes the phrase “reversible shits” instead of “reversible shifts”\n6) The fits of the NMR spectra in Supp. Fig. 23 do not always match the experimental spectra. Especially the ones recorded at higher potentials might be fitted better with broader in-pore peaks, which would contradict the author’s hypothesis.\n7) Were any ex situ experiments done under MAS conditions? This would improve the resolution and allow for more definitive peak assignment.\n8) Could the authors comment on the use of 23Na for the in situ experiments? Better resolution would be afforded by 19F NMR. 19F NMR would also provide insight into whether the mechanism proposed in Figure S.18 is valid.\n9) Please check Figure S.13. The captions for part c) and f) are the same and the schematics show that the SEIs formed in both solvents sieve the solvents, which contradicts what the authors say.\n10) Could the authors comment on the supposed broadening of the “in-pore” during discharge? What is the source of the broadening? Why does the peak width remain constant from 0.25 V?\n11) All other characterization techniques were performed on cells using DGDE and on cells using EC:DMC, except for NMR. Have the authors performed in situ NMR experiments on cells using EC:DMC electrolyte? If not, these measurements should be done to confirm the interpretation of the 23Na NMR spectra of the DGDE system.\nIn conclusion, the manuscript presents an interesting finding of an electrolyte that forms a supercapacitor-compatible SEI and therefore works well at low potentials. Nevertheless, the supporting evidence for the assumptions about the mechanisms is not very strong. If the authors could provide more supporting information in the form of the additional experiments listed above, then it will be suitable for publication in Nature Communications."}, "author_response": "Response Letter\nManuscript ID: NCOMMS-25-27796\nTitle: Unlocking limited electric double-layer capacity via electrochemically-driven continuous partial\ndesolvations in carbon nanopores\nResponse to Reviewer #1\nThe manuscript titled “Unlocking limited electric double-layer capacity via electrochemically-driven\ncontinuous partial desolvations in carbon nanopores” authored by Fan et el. aims to investigate the\nsolvation environment of inserted Na+ into YP50F activated carbon. However, there are some major\nconcerns on data collection and interpretation, as described below:\nResponse: We appreciate your valuable comments and suggestions. We carefully revised the\nmanuscript based on these valuable suggestions. The point-to-point responses are listed below.\n1. My major concern is on the EQCM data interpretation, which is a very important part of discussion\nin the manuscript. The authors did not explain how the mass was calculated. It seems the authors\napplied Sauerbrey equation to convert the frequency change to mass change. However, the frequency\nchange measured by EQCM can be contributed by mass change, viscoelastic property changes (the\ndensity, viscosity, and elasticity of the film electrode material) or sample roughness change [Ref: Acc.\nChem. Res. 2018, 51, 69−79; J Polym Sci. 2021, 1–18; Chem. Rev. 1992, 92, 1355]. The Sauerbrey\nEquation is only valid when the film remains stiff, uniform, flat, and no dissipation during cycling.\nWith the way the authors prepared the electrolyte (dripcasting tens or hundreds µm size of YP50F\nparticles on the quartz), the electrode roughness is already high. In addition, the electrolyte is more\nviscous than aqueous electrolyte, which further increased the dissipation of the resonance of the quartz.\nThis will cause an extremely broadened frequency peak, which might lead to inaccurate reading of the\nresonance peak. Furthermore, during cycling, there is SEI formation and potential morphological\nelectrode changes caused by the ion insertion. Both phenomena can also contribute to frequency\nchange. Therefore, the authors will need to provide dissipation information (or Q factor, internal\nresistance, etc) during cycling to justify the mass change and solvation number discussion. Without\ndissipation information, the EQCM results are highly questionable. This obviously impact the validity\nof the discussion on solvation number.\nResponse: We thank you for the valuable comments. The EQCM experiments and analysis were\nperformed according to previous work [ref 15. Nat. Mater. 2015, 14, 812; ref. 16. J. Am. Chem. Soc.\n2014, 136, 8722; ref. 37 J. Am. Chem. Soc. 2010, 132, 13220]. Here, we applied the identical spray\ncoating method to prepare the activated carbon coated crystal disks, which enable their high-quality\nsuitable for EQCM analysis. The detailed calculations for deriving EQCM results were originally\npresented in Supplementary Note 2. To enhance the clarity and align with your suggestion, we now\nhave integrated the detailed calculation method into the Methods section of main text.\nAccording to your suggestion, we measured the overtones of the pristine YP50F@crystal disks\n(Figure R1-1a,b) and the electrochemically cycled one at the fully discharged state (Figure R1-1c,d)\nvia Q-Sense Explorer (Biolin Scientific). The small ΔD/∆n values are on the order of 10−6, indicating\nthat the stiff films of the pristine and cycled one (Figure R1-1b and d) that were adoptable for the\nSauerbrey equation. Additionally, the reversible frequency changes and low signal-to-noise ratios\n(Supplementary Figure 17) indicated the reversible EDL capacitive adsorption of solvated ions into\nthe nanopores of YP50F in the DGDE electrolyte. Furthermore, our EQCM results (Figure 2a) show\nthe electrochemically measured CV curves and mass changes in the DGDE electrolyte in 2−4 V vs.\nNa+/Na, which are as good quality as those in Figure 2 from ref. 37 J. Am. Chem. Soc. 2010, 132,\n13220, which demonstrated the reliability of our EQCM experiments. The above results indicate the\nstability of the YP50F film on quartz for EQCM testing. We thank the reviewer for providing three\nvaluable references, which have been added in the revised manuscript.\nAdditionally, in this work, we performed in situ 23Na NMR (Figure 2d-f) to investigate the EDL\ncapacitive mechanism. It displayed the nearly linear increase in the half width and a corresponding\nlinear decrease in the chemical shift until the potential reached 0.05 V. These enhanced solvated Na+‒\ncarbon interactions were also observed via in situ NMR, which is consistent with the EQCM results.\nThe large electrochemical polarization (Δφ=−2.95 V) electrochemically drives continuous partial\ndesolvation of [Na-xDGDE]+ shells, leading to gradually reducing d and thus the increase in\ncapacitance. Overall, the investigations of the charge storage mechanism were supported together with\nthe in situ EQCM, in situ NMR, ex situ XPS, ex situ N isothermal adsorption, and electrochemical\n2\nkinetics analyses and systematically discussed with recent literatures. We believe that these interesting\nfindings will promote the development of electrochemical capacitors.\nFigure R1-1. (a) Normalized shifts of frequency 𝛥𝑓/𝑛 (negative) and resonance width 𝛥𝑊/𝑛 (positive)\nand (b) 𝛥D/𝑛 at different overtones as a function of time for the pristine YP50F@crystal film in air\nand in DGDE electrolyte. (c) Normalized 𝛥𝑓/𝑛 and 𝛥𝑊/𝑛 and (d) 𝛥D/𝑛 at different overtones as a\nfunction of time for the cycled YP50F@crystal film (at the fully discharged state) in air and in DGDE\nelectrolyte.\nThe following descriptions were added on Pages 6-7 in revised manuscript and supplementary file:\n“Dissipation factors (D) of the pristine and cycled YP50F@crystal disks were further analysed\n(Supplementary Fig. 16), while the small ΔD/∆n values were on the order of 10−6, indicating that the\nstiff films were valid for the Sauerbrey equation38-41.”\nThe following descriptions were added on Page 14 in revised manuscript and supplementary file:\n“The EQCM measurements were conducted on a QCD-300 single-channel quartz crystal microbalance\n(Bio-Logic) lined with a potentiostat (Bio-Logic SP-200). The slurry comprising YP50F, carbon\nnanotubes and CMC at a ratio of 90:5:5 by weight was dispersed in deionized water through strong\nultrasonic treatment. Then, the slurry was sprayed onto an Al-coated quartz crystal disk (5 MHz). The\ncrystal disk was placed onto a hot plate at 80°C, while a steel mask was covered on the disk with the\nexposure of the Al-coated surface for the spray of slurry. Compressed air was used to control the spray\npressure. The carbon deposited crystal disks were dried in a vacuum oven for 4 h at 100°C. The mass\nloading was controlled at ~50 μg cm−2. The dissipation factors (D) at different overtones of the pristine\nYP50F@crystal disks and the electrochemically cycled disks at the fully discharged state were\nmeasured by the Q-Sense Explorer (Biolin Scientific).\nElectrochemical measurements were conducted with simultaneous recording of the quartz\nresonance frequency. According to the Sauerbrey equation (Eq. 3), the frequency shift (Δf) of the quartz\nresonator is effectively converted into the corresponding mass change (Δm) of the electrodes and\nquartz crystal.\n√p\nq\nu\nq\nΔm = − Δf = −C Δf (3)\n2f f\n0\nwhere ρ is the density of quartz, μ is the shear modulus of quartz, 𝑓 is the fundamental resonance\nq q 0\nfrequency of the quartz, and 𝐶 is the calibration constant (or sensitivity factor). The calculation of\n𝑓\n𝐶 was based on the method provided by Griffin et al17. In detail, a sliver deposition experiment with\n𝑓\n0.01 M silver nitrate and 0.1 M tetraethylammonium tetrafluoroborate in DGDE was applied\n(Supplementary Figure 14). Thus, the calculated 𝐶 was 21.02 ng Hz−1.”\n𝑓\n• Another major point is that the way the authors described their observation is confusing and\nmisleading. The term “electrical double layer (EDL)” described ion forming on at the\nelectrode/electrolyte interface. The EDL naturally form on any immersed surface, whether the\nelectrode is charged or at open circuit potential. When the YP50F is immersed in the electrolyte, all\nthe accessible pores will be filled with ions already. Higher overpotential even reaching SEI formation,\nwill drive more ions and/or solvents into the pore and also decompose the electrolyte to form SEI, but\nthere is always EDL layer on the surface. The term the authors should use is “double layer capacitive\nbehavior”. The authors described the charging process it as “When cycled towards lower potential, the\nEDL will disappear”. In fact, the EDL layer is always present, just the redox related current kicked in\nand becomes the main capacity contribution.\nResponse: We thank you for this valuable suggestion. The porous carbon electrodes store charges\nthrough electric double-layer (EDL) capacitive adsorption/desorption of ions in nanopores. This work\nproves that the EDL capacitive adsorption of solvated Na+ in carbon nanopores is achievable under a\nlarge offset potential (Δφ) of −2.95 V vs. PZC in DGDE electrolyte. Previously, the “box-like EDL\nbehavior completely disappears” and “the YP50F electrode displays the loss of characteristic\ncapacitor-type CV curves” were used. According to reviewer’s suggestion, the “EDL capacitive\nbehavior in nanopores” was used to describe the storage process in the revised manuscript and figures.\nAll the changes are highlighted in red colour in the revised manuscript.\n• In the experimental section for the BET measurement, there is no details given regarding the drying\nprocess of the sample. If the post-cycle sample is conducted without thorough drying, the pores will\nbe packed with adsorbed liquid, which will also give an extremely low SA.\nResponse: We thank you for this comment. The degassing treatment was the same method provided\nby [ref. 2 Science 2024, 384, 321-325]. In detail, the YP50F powder (Figure S1a) was placed into a\nglass bulb gas cell and degassed at 150℃ in vacuum for 16 h before the measurement. The YP50F\npower has a high BET SA of 1700 m2 g−1, in consistent with the value of 1694 m2 g−1 by ref. 2.\nFor the YP50F electrode without cycles (Figure S11a,b), the powders were scraped from the\ncurrent collector and placed into a glass bulb gas cell and degassed at 150℃ in vacuum for 16 h before\nthe measurement. The measured BET SA decreased to 1170 m2 g−1, owing to the addition of binders\nand conductive carbons.\nFurthermore, the ex situ nitrogen sorption isotherms of the YP50F electrode cycled in DGDE\n(Figure S11c,d) and EC:DMC (Figure 6a,b) were measured through the same processes. In detail, a\npouch cell using YP50F as the working electrode and sodium foil as the counter and reference electrode\nin the DGDE (or EC:DMC) electrolyte was assembled. After 5 cycles, the cell was disassembled in a\nglovebox and washed with DGDE (or DMC) to remove residual electrolytes. Then, the cycled YP50F\npowders were scraped from the current collector and dried at 120°C in the glove box. Then, the cycled\nYP50F powders (~100 mg) were placed into a glass bulb gas cell and degassed at 150℃ in vacuum\nfor 16 h to remove any liquids before the nitrogen sorption isotherms. The same degassed treatment of\nthe cycled YP50F electrode led to two distinguished values: the high SA of 961 m2 g−1 for YP50F\ncycled in the DGDE electrolyte and the low SA of 6.5 m2 g−1 for YP50F cycled in EC:DMC, which\nare largely related to their formed SEI layers.\nAccording to reviewer’s suggestion, the detailed experimental information is provided on page 15\nof the Methods section in the revised manuscript.\n“Ex situ nitrogen adsorption-desorption isotherms\nThe Brunauer–Emmett–Teller (BET) specific surface area was measured via nitrogen adsorption-\ndesorption isotherms (Micromeritics Tristar 3020) at 77 K. The YP50F powders (~100 mg) were\nplaced into a glass bulb gas cell and degassed at 150 ℃ in vacuum for 16 h before the measurement2.\nTo prepare the cycled YP50F electrode for nitrogen adsorption-desorption isotherms, a pouch cell\nusing YP50F electrodes and sodium foil in the DGDE (or EC:DMC) electrolyte was assembled. After\n5 cycles, the cells were disassembled in a glovebox and washed with DGDE (or DMC) to remove\nresidual electrolytes. All powders were scraped from the current collector and dried at 120°C in the\nglove box. Then, the powders (~100 mg) were placed into a glass bulb gas cell and degassed at 150℃\nin vacuum for 16 h before measurement.”\nBased on the above-mentioned reasons, this work is not qualified to be published in Nature\nCommunications.\nResponse: Here, we want to strengthen the important findings in our work, which are highly evaluated\nby the other reviewers. The EDL capacitive adsorption of solvated Na+ in carbon nanopores was\nachievable at a large ∆V of −2.95 V vs. PZC in DGDE electrolyte, which was demonstrated by the\ncombination of in situ EQCM, in situ NMR, ex situ XPS, ex situ TEM, ex situ nitrogen adsorption-\ndesorption isotherms and systematic electrochemical kinetic analysis. Importantly, the\nelectrochemically driven continuous partial desolvation of [Na-xDGDE]+ into nanopores with boosted\nion‒carbon interactions leads to increased storage capacitance and capacity, which is different from\nthe previously reported method of matching the pore size and adsorbed ions to realize partial\ndesolvation. Remarkably, we realize an ultrahigh capacity of up to 508 C g−1 (that is 0.063 e− per\ncarbon), together with ultrahigh-rate capability, long-term cycling stability and high initial coulombic\nefficiency of 92.2% for the activated carbon (YP50F). Additionally, this EDL reaction is based on the\nsolvated Na+ ions as charge carriers, which allows the development of hybrid YP50F|NVP SIC pouch\ncells with higher energy density than commercial EDLCs and LICs. The assembly process waives pre-\nsodiation treatments. This work sheds light on overcoming the limited EDL capacitive charge storage\ncapacity and energy density of capacitors.\nAll the changes are highlighted in red colour in the revised manuscript.\nResponse to Reviewer #2\nThe authors present a novel and systematic study aimed at enhancing the energy storage capacity of a\ncommercial porous carbon electrode (YP50F) by extending the operating voltage window (Δφ) to 2.95\nV. The ion desolvation and SEI plays a critical role in enabling stable and reversible Na⁺ ion\nadsorption/desorption in this high-voltage regime. The work thoroughly investigates the interplay\nbetween electrolyte composition and SEI formation on Na⁺ storage mechanisms. A high specific\ncapacity of 141 mAh g⁻¹ was achieved, surpassing previously reported values in both organic and ionic\nliquid electrolytes. Unlike intercalation-based mechanisms, the charge storage here is governed by\nelectric double-layer (EDL)-type Na⁺ ion adsorption, which is favorable for high-power applications.\nThis is well-supported by the pouch cell demonstration provided in the manuscript. The authors also\nconducted comprehensive characterizations of the SEI composition and its role in the charge storage\nprocess. Notably, partial desolvation of Na⁺ facilitated by the SEI is proposed as a key factor driving\nperformance improvement, which aligns well with recent studies in the field. The manuscript is well-\norganized and presents significant findings. I recommend its publication in Nature Communications\nafter minor revisions.\nResponse: We appreciate you for the high recommendation of our manuscript. We carefully revised\nthe manuscript on the basis of your valuable suggestions. The point-to-point responses are listed below.\nSpecific Comments:\n1. Sodium adsorption mechanism\na) In Fig1b, I agree that the CV get distorted below 1V because of the irreversible charge storage process.\nHowever, I disagree with the full desolvation mechanism (of Na ions) as proposed in Fig1c.\nGenerally speaking, a full desolvation typically leads to higher capacity, since the ion get closer to the\nelectrode, as the authors also noted in this manuscript. Also, the stronger interaction between ions/host\nmaterials also enhances charge transfer, which also increase the capacity (Nature Communications,\n2025, 16(1): 3813). However, in the present data, the difference of sodium insertion capacity below\n1V is relatively small (0.5 A/g Fig1b vs 0.4 A/g, Fig1c). A rough estimation based on complete Na⁺\noccupation in the pores of YP50F (0.7 cm³/g pore volume, Na atomic radius ≈0.19 nm) yields a\ntheoretical capacity of ~1154 mAh/g, suggesting that Na ions are far away from full desolvation. The\nauthors are encouraged to elaborate more on the mechanism in the EC:DMC electrolyte.\nResponse: We thank you for your valuable suggestion. The assumed high capacity of 1154 mAh/g\nmight be based on the “pore-filling” mechanism with the formation of metallic Na0 clusters that were\nproposed in the closed pores of hard carbon materials, which displayed a long plateau at ~0 V vs.\nNa+/Na.\nComparing the reversible charge and discharge profiles of the YP50F electrodes in the DGDE and\nEC:DMC electrolytes (Figure R2-1a), they did not show the typical long plateau at ~0 V vs. Na+/Na\nattributing to the formation of quasi-metallic Na0 clusters. The missing quasi-metallic Na cluster signal\nlocated at 20~1100 ppm range in the ex situ 23Na NMR spectrum of YP50F electrode at 0.05 V vs.\nNa+/Na cycled in EC:DMC (Figure R2-1b) further identifies the consistent results [ref. National\nScience Review, 2022, 9(8), nwac084]. In addition, it is well known that EC-SEI is able to sieve\nsolvation shells and allows only net Na+ storage pass through the EC-SEI layers. Therefore, according\nto the well-established sieving function of the EC-SEI layers, in situ EQCM, the distinct charge storage\nbehavior in EC:DMC and DGDE electrolytes (Figure R2-1 and Supplementary Figure 13), and the\ndistinguished charge storage behavior of the EC:DMC in 3−1 V and 3−0.05 V vs. Na+/Na\n(Supplementary Figure 2), a full desolvation mechanism for YP50F in EC:DMC is proposed. However,\nthe as-formed thick EC-SEI layers affected the overall reaction kinetics, leading to the limited capacity\nof YP50F in EC:DMC electrolyte (Figure R2-1a).\nFigure R2-1. (a) Charge-discharge profiles of the YP50F electrode in DGDE and EC:DMC electrolytes\nat 0.1 A g−1 at the 5th cycle. (b) Ex situ 23Na NMR spectra of the YP50F electrode at 0.05 V vs. Na+/Na\ncycled in EC:DMC electrolyte.\nb) The butterfly-shaped CV deviates from the classical rectangular profile of pure EDL behavior,\nindicating potential-dependent ion–electrode interactions. This behavior is often influenced by specific\nion effect (specific ion-electrode interactions, REF28). Using different cations rather than Na+ may be\nbeyond the scope of this work, but a brief discussion of how Na⁺-YP50F specific interactions\ncontribute to the CV shape would enrich the interpretation. The effects of surface functionalities,\ngraphitization degree, and average pore size (confinement) would be interesting for future works.\nResponse: We thank you for this valuable suggestion. The use of “butterfly-shaped CV” to describe\nthe phenomenon of increased EDL capacitive behavior is highly appropriate. In this work, the\nenhanced solvated Na+ ion‒carbon interactions is observed through in situ NMR (Figure 2d-f). The\nlarge electrochemical polarization (Δφ=−2.95 V) electrochemically drives continuous partial\ndesolvation (Fig. 2g), leading to a gradual decrease in d and thus an increase in capacitance.\nWe agree that it would be very interesting if other cations (e.g., Li+) could have similarly enhanced\nsolvated cation‒carbon interactions. According to your suggestion, we measured the charge and\ndischarge profiles of the YP50F electrode in LiBF in DGDE (Figure R2-2a). A boosted EDL\n4\ncapacitive behavior was also observed. For different pore sizes, we measured commercial mesoporous\nCMK-3 (Figure R2-2b), and the similar “butterfly-shaped CV” in 3−0.05 V vs. Na+/Na in NaPF -\npotential and solubility of the Li+, Na+ and K+ systems should be investigated in the future.\nFigure R2-2. (a) The charge‒discharge curves of the YP50F electrode in LiBF -DGDE electrolyte,\n4\nunder different cut-off potentials at 0.5 A g−1. Each interval is 0.1 V. (b) CV curves of the CMK-3\nelectrode with expanding ∆φ in the NaPF -DGDE electrolyte at 1 mV s−1.\n6\n2. Question about EQCM measurements\na) There appears to be a large irreversible current at low potentials (Supplementary Figure 16), likely\ndue to electrolyte decomposition. This complicates the use of EQCM for accurate Na⁺ desolvation\nanalysis, as side reactions being emphasized on the gold-coated quartz surface at slower scan rates.\nThus, molecular weight calculations based on irreversible charge may not reliably reflect the solvation\nstate.\nResponse: We thank you for this valuable suggestion. Supplementary Figure 16 shows the EQCM-\nCV results of YP50F electrode in EC:DMC electrolyte with large irreversible current at low potentials,\nwhich is consistent with the results measured in coin cells (Figure 1b). Supplementary Figure 3c and\nFigure 15 show the low CEs during the initial cycles, further supporting the conclusion that electrolyte\ndecomposition led to an irreversible current at low potentials. To minimize the influence of SEI\nformation, we used Al-coated quartz, which showed less irreversible currents than that of Au-coated\nquartz. Furthermore, the linear fitting is based on the 5th cycle, in which the weight changes became\nmore stable. The fitted ∆𝑚/𝑑𝑞 of 30.5 g mol−1 is slightly higher than that of 23 g mol−1 for net Na+,\nwhich might be owing to the contributions of electrolyte decomposition or the adsorption of solvated\nions outside the EC-SEI layers. In addition, Supplementary Figure 2 and Supplementary Figure 13\ndemonstrate the loss of reversible EDL capacitive charge storage in nanopores when EC-SEI layers\nare formed. Therefore, combining the electrochemical behavior, EQCM results and well-known\nfunctions of the EC-SEI layers, the desolvation mechanism of YP50F in EC:DMC in 3−0.05 V vs.\nNa+/Na was proposed.\nb) The integrated charge values from CV curves at 5 mV/s (Fig. 2b) are noticeably lower than those\nshown in Fig. 2c. For better clarity and reproducibility, it is strongly recommended that the authors\nensure both figures reflect data from the same sample and experimental conditions.\nResponse: We thank the reviewer for this valuable comment. Supplementary Figures 15 and 17 show\nthe cycling time, indicating a scan rate of 2 mV/s. We apologize for the mistakes. We double-checked\nand revised the text in the manuscript.\nc) In Fig2c, similar to my previous comments, the same applies here that the molecular weight\ncalculation based on slope in region iii (0.25-0.05 V) is a bit challenging. There is also irreversible\ncurrent (side reaction) at the negative scan (Na adsorption), alternatively, the reverse scan (positive\nscan) may provide more reliable insights into the solvation state, considering that the EDL\nadsorption/desorption process are highly reversible in coin cells (Figure 1d, 1e).\nResponse: We thank you for this valuable suggestion. The CV curves measured by the EQCM cell\nmight have larger inner resistances and overpotentials than those measured by the coin cells. According\nto your suggestion, we analysed the positive scan of the EQCM results (Figure R2-3). A small ∆𝑚/𝑑𝑞\nof 123 g mol−1 in the low potential range of 0.05−0.35 V and a large ∆𝑚/𝑑𝑞 of 145 g mol−1 in the\nrange of 0.35−1.1 V and 269 g mol−1 in the range of 1.1−1.9 V and a low ∆𝑚/𝑑𝑞 of 56 g mol−1 in the\nrange of 1.9−3 V, that is the continuous partial desolvation process in consistent with the trend for the\nnegative scan.\nFigure R2-3. Linear fitting of ∆𝑚/𝑑𝑞 during the positive CV scan in DGDE electrolyte, showing the\ntrends of multistep partial desolvation processes.\nThe following descriptions were added on Page 7 in revised manuscript and supplementary file:\n“For the positive scan, the trends of multistep partial desolvation processes are also observed\n(Supplementary Fig. 19).”\n3. Minor corrections:\na) Page 3, Line 49: Remove “with” for grammatical clarity.\nb) Page 3, Line 52: Standardize the citation format for “ref 2”.\nc) Page 6, Line 118: Replace “box-like CV” with “butterfly-shaped CV” or “symmetric CV” will be\nmore accurate.\nd) Page 9, Line 206: Avoid the term “EDL reaction,” which may be confused with faradic reactions.\nResponse: We thank the reviewer for these valuable suggestions. We have corrected these errors and\nrevised the descriptions.\nAll the changes are highlighted in red colour in the revised manuscript.\nReviewer #3 (Remarks to the Author)\nAuthors provided interesting report on sodium-ion capacitors (SICs) development and their approach\non avoiding so-called pre-sodiation step. I find the results noteworthy and deserving publication,\nhowever, certain aspects require further comments.\nResponse: We appreciate the positive recommendation of our manuscript. We have carefully revised\nthe manuscript based on your valuable suggestions. The point-to-point responses are listed below.\n1. Please comment on the EQCM experiment, especially the aspect of energy dissipation. The\nelectrodes were fabricated with CMC and CNTs, thus, it might perturb the experiment results. What\nwas the resistance change recorded?\nResponse: We thank you for the valuable suggestion. The CMC binder was used to provide the\nadherence of YP50F on the crystal disks. For the EQCM measurements, various of experiments\nwithout CNTs or with other carbon conductive additives (e.g., SP carbon and ketjen black) have been\ntried, whereas displaying distorted CV curves with large overpotentials during the in situ EQCM tests.\nWhen CNTs were used, the CNT networks covered on the YP50F particles (Figure R3-1a), which\nincreased the conductivities of the composite films. Additionally, the CNTs delivered a capacity of\nonly 80 mAh g−1 (Figure R3-1a), which was much lower than that of 141 mAh g−1 for YP50F in the\nDGDE electrolyte. The low content of 5 wt.% CNTs had a small influence on the experimental results.\nFigure R3-1. (a) SEM images of the YP50F:CNTs:CMC (90:5:5 in weight) composites. (b) Charge\nand discharge profiles of the pure CNTs electrode in DGDE electrode at 0.1 A g−1.\nAccording to your suggestion, the energy dissipation is further discussed. We measured the\novertones of the pristine YP50F@crystal disks (Figure R3-2a,b) and the electrochemically cycled disks\nat the fully discharged state (Figure R3-2c,d) via the Q-Sense Explorer (Biolin Scientific). The small\nΔD/∆n values are on the order of 10−6, indicating the stiff films of the pristine and cycled ones (Figure\nR3-2b and d) were suitable for the Sauerbrey equation.\nFigure R1-1. (a) Normalized shifts of frequency 𝛥𝑓/𝑛 (negative) and resonance width 𝛥𝑊/𝑛 (positive)\nand (b) 𝛥D/𝑛 at different overtones as a function of time for the pristine YP50F@crystal film in air\nand in DGDE electrolyte. (c) Normalized 𝛥𝑓/𝑛 and 𝛥𝑊/𝑛 and (d) 𝛥D/𝑛 at different overtones as a\nfunction of time for the cycled YP50F@crystal film (at the fully discharged state) in air and in DGDE\nelectrolyte.\nThe following descriptions were added on Page 7 in revised manuscript and supplementary file:\n“Dissipation factors (D) of the pristine and cycled YP50F@crystal disks were further analysed\n(Supplementary Fig. 16), while the small ΔD/∆n values were on the order of 10−6, indicating that the\nstiff films were valid for the Sauerbrey equation38-41.”\n2. Please calculate the energy efficiency for the full cell and comment on that.\nResponse: We thank you for the valuable suggestions. According to your suggestion, we calculated\nthe energy efficiency of the SIC pouch cell, as shown in Figure R3-3. To better comment on the values\nof energy efficiency, the commercial EDLC and LIB (using LiCoO cathode) were measured at\n2\ndifferent currents (I =I ). The values are compared on the basis of their (dis)charging times.\ncharge discharge\nThe energy efficiency of the SIC pouch cell lies between those of the EDLC and LIB devices. At low\nrates, the energy efficiencies of the SIC pouch cell, LIB and EDLC are higher than 93%. Under fast\ncharging and discharging rates, the SIC and EDLC devices presented higher energy efficiency than\nthat of LIB devices. Compared with commercial EDLC devices, our lab-made SIC pouch cell still has\nroom for further improvement.\nFigure R3-3. The calculated energy efficiency of the SIC pouch cell, commercial EDLC device and\ncommercial LIB device (LiCoO as the cathode).\n“The energy efficiencies of the SIC pouch cell are 93.3%, 80.4% and 61.0% for the (dis)charge times\nof 5035, 196 and 26 s, respectively, which are comparable to those of the EDLC and LIB\n(Supplementary Fig. 40).”\n3. Please comment on the impact of scan rate or current load for desolvation step. Usually, SEI\nformation is very sensitive to these parameters and the performance of the device could change when\nthe deep polarisation is performed at various scan rates. Did the recent data were optimized in that\ncontext?\nResponse: We thank you for the valuable suggestion. According to your suggestion, we measured the\ncharge and discharge performance of YP50F electrodes at 0.01 and 0.05 A g−1 in DGDE electrolytes\n(Figure R3-4). The initial cycles of YP50F electrodes in DGDE at low specific currents of 0.01 and\n0.05 A g−1 also displayed high reversibility, with high ICEs of 94.8% and 92.8%, respectively, which\nis consistent with the high ICE of 92.2% at 0.1 A g−1. The reversible capacities are 527, 510 and 508\nC g−1 at 0.01, 0.05 and 0.1 A g−1, respectively. Therefore, the high ICEs and high reversible capacity\nof the YP50F electrode in DGDE in 3−0.05 V vs. Na+/Na depend on its reversible EDL capacitive\nstorage mechanism. Therefore, the slow current load has no influence on the desolvation steps.\nFigure R3-4. Initial charge-discharge profiles of the YP50F electrode at (a) 0.01 A g−1 and (b) 0.05 A\ng−1 in DGDE in 3−0.05 V vs. Na+/Na.\nThe following descriptions were added on Page 9 in revised manuscript and supplementary file:\n“At the low specific currents of 0.01 and 0.05 A g−1, the YP50F electrodes also deliver very high ICEs\n(Supplementary Fig. 30), indicating that the low current load has no influence on the partial desolvation\nsteps of [Na-xDGDE]+ in nanopores.”\n4. The electrochemical characterization is performed with due care and follows the best practices,\nhowever, the leakage currents or self-discharge properties should be at least commented.\nResponse: We thank you for the valuable suggestion. The self-discharge ratio of the SIC pouch cell is\n~13.0% after setting for 120 h, which is lower than that of ~38.0% for an EDLC device (Figure R3-5).\nFigure R3-5. The self discharge performance of the SIC pouch cell. The charge and discharge profiles\nof the SIC pouch cell and EDLC devcie with a setting time of 120 h. (b) Calculated self-discharge ratio\nof SIC pouch cell and commerial EDLC after setting for 120 h.\nThe following descriptions were added on Page 11 in revised manuscript and supplementary file:\n“The self-discharge ratio of the SIC pouch cell is ~13.0% after setting for 120 h, which is lower than\nthat of ~38.0% for an EDLC device (Supplementary Fig. 38).”\nFinally, the work might be of high significance for the community and the recent literature is well\ndiscussed, nevertheless, some other concepts such as sacrificial materials for pre-sodiation or redox-\nactive electrolytes could be mentioned in order to provide a comprehensive discussion.\nResponse: We thank you for the high praise of our work. According to your valuable suggestions, the\nsacrificial materials for pre-sodiation and redox-active electrolytes are briefly discussed.\nThe sacrificial materials for pre-sodiation are mainly categorized into inorganic and organic\nsodium salts [ACS Nano 2025, 19(18), 17062–17086]. Inorganic sodium salts such as Na NiO and\n2 2\nNaCrO have a low sodium replenishment capacity, and they tend to persist at the positive electrode,\n2\nforming a “dead mass” after decomposing. NaN , Na P, Na O, and Na S are unstable in air and require\n3 3 2 2\nthe preparation of electrodes in a glovebox, a cumbersome process that limits their large-scale\napplication. Organic sodium salts, such as Na C O and NaAC, undergo electrochemically irreversible\n2 6 6\ndecomposition to release sodium ions. However, the byproduct of organic sacrificial materials is gas,\nwhich requires secondary treatment of the packages. Additionally, these methods often suffer from\nincomplete decomposition at excessively high potentials and result in detrimental residues. The\nproduction of hybrid ion capacitors without any pretreatment is beneficial for scale-up fabrication in\nindustry.\nRedox-active electrolytes are very interesting. Recently, Chen et al. reported sodium thiocyanate\n(NaSCN) as an electrolyte additive for presodiation in sodium-ion battery cells [Adv. Mater. 2025, 37,\n2502251]. It will be very interesting to develop appropriate redox-active electrolytes to realize the pre-\nsodiation treatments for hybrid ion capacitors in the future.\nThe following descriptions were added on Page 10 in revised manuscript and supplementary file:\n“Recently, hybrid SICs composed of hard carbon negative electrode and activated carbon positive\nelectrode was proposed, accompanying the development of pre-sodiaiton methods52-54.”\nI have no concerns regarding the methodology and the details presented in the manuscript and\nsupplementary information are enough to reproduce the work.\nAfter addressing the points raised above, I would be happy to support publication of this manuscript.\nResponse: We appreciate the positive recommendation of our manuscript. We carefully revised the\nmanuscript on the basis of your valuable suggestions. The point-to-point responses are listed below.\nAll the changes are highlighted in red colour in the revised manuscript.\nReviewer #4 (Remarks to the Author):\nIn this work, the authors study the sodium storage mechanism in a supercapacitor with an ether-based\nelectrolyte under increased potential window by EQCM and NMR.\nThe NMR section effectively demonstrates, consistent with previous studies, that Na⁺ ions located\ninside and outside the pores can be clearly distinguished. The in situ experiments reveal reversible\nsodium insertion into the pores without the Knight shift typically associated with the formation of Na⁰\nclusters or “pore filling” seen in hard carbon. Additionally, broader in-pore signals are observed,\nindicating stronger interactions between Na⁺ ions and the pores, as well as a decrease in the distance\nbetween the Na⁺ ions and the carbon surface. These changes correlate well with the partial desodiation\nresults obtained from EQCM measurements, showing good alignment between the two techniques.\nResponse: We appreciate the high recommendation of our manuscript. We have carefully revised the\nmanuscript on the basis of your valuable suggestions. The point-to-point responses are listed below.\nMinor points to consider include correcting the spelling of “shifts” in the caption of Figure S21 and\npotentially adjusting the colour scheme in Figure S22, as some voltage ranges (0.05–3.0 V and 2.5–\n3.0 V) are represented by similar colours, which may reduce clarity.\nResponse: We thank you for these valuable suggestions and reminders. We have revised the typo of\nthe caption of Figure S21 (Figure S21 was reordered as Figure S24 in the revised supplementary file).\nAdditionally, we have revised the colour scheme in Figure S22 (Figure S22 was reordered as\nFigure S25 in the revised supplementary file) for a better presentation.\nSupplementary Figure 25 | Difference spectra of in situ 23Na NMR spectra during the 2nd discharging\nprocess.\nOverall, I am satisfied with this section and have no major concerns.\nResponse: We thank you for the positive recommendation of our manuscript.\nAll the changes are highlighted in red colour in the revised manuscript.\nReviewer #5 (Remarks to the Author):\nElectrochemical capacitors are characterized by their high-power density but are limited by their\ngravimetric capacitance and capacity. Typically, they operate in a narrow potential window before the\nformation of an SEI. In this manuscript, the authors present a carbon nanosphere-based supercapacitor\nwith a diethylene-glycol-dimethyl-ether (DGDE) electrolyte, which can operate under a large potential\ndifference of -2.95 V between the TOC and discharged state. This leads to a high specific capacitance\nof 172 F/g and capacity of 508 C/g.\nThe authors introduce Na+-based DGDE as an electrolyte which forms an SEI layer that allows\nthe migration of solvated Na+ ions into nanospheres, in contrast to the SEI formed by ethylene\ncarbonate that strips the solvation shell during Na ion migration. The authors claim that expanding the\npotential range can electrochemically drive a partial desolvation, reducing the distance between the\ncarbonaceous surface and the sodium ions, and therefore increasing the capacitance. SEM and TEM\ncombined with CV measurements showed a thick SEI on the carbon particles when using an EC/DMC-\nbased electrolyte, which was accompanied by a significant decrease in the specific surface area. In\ncontrast to this, the same YP50F electrode material showed a box-like CV during an expanding\npotential range when using the DGDE electrolyte. EQCM was used to analyse the flow into nanopores.\nUsing EC/DMC-based electrolytes, this resulted in mass changes per electron that the authors assign\nto a PF - anion adsorption mechanism in the potential window between 3-4 V, and an adsorption of\n6\ndesolvated Na+ ions in the potential window 3-0.05 V vs. Na/Na+. In addition, the mass change differs\nsignificantly between the cycles. In contrast to this, the DGDE electrolyte showed more reversible\nmass changes and regions of different mass changes per transferred electron. The authors attribute\nthese different mass changes to ion exchange, counter-ion adsorption, and the flux of partially\ndesolvated Na+ Ions into the nanopores. The authors claim to support this idea with in situ NMR\nmeasurements and the broadening of the in-pore peak with increasing potential. The capacity and\ncapacitance were further verified in pouch cells using NVP as a counter electrode that does not require\nan additional pre-sodiation process.\nResponse: We appreciate the positive recommendation of our manuscript. We have carefully revised\nthe manuscript on the basis of your valuable suggestions. The point-to-point responses are listed below.\nGeneral comments and questions:\n1) Is there supporting evidence for the assumptions of the different stages of ion flux at the potential\nranges when using DGDE as an electrolyte? As the mass changes only approximately match the masses\nof the assigned ions, the uncertainty seems to be very high. Could the authors comment on the\ndiscrepancies between the observed mass changes and those that they are proposing? The authors state\nseveral times that the observed mass changes are “close to” the mass changes they are proposing, but\nthey can differ by over 25%.\nResponse: We thank you for the valuable suggestion. First, we measured the EQCM results in the\npotential range of 2−4 V vs. Na+/Na, according to [ref. 37 J. Am. Chem. Soc. 2010, 132, 13220]\nreported the EQCM results of activated carbon in the electrolyte of LiBF in PC. In our results, the\n4\nφ is ~3.02 V vs. Na+/Na. In the potential window of 3−4 V vs. Na+/Na (which is above φ ), the\nPZC PZC\nfitted mass change per electron (∆𝑚/𝑑𝑞) of 155 g mol−1 indicates an anion adsorption mechanism (the\nmolar mass of PF − is 145 g mol−1)37. The anion adsorption might be accompanied by some solvent\n6\nmolecules, leading to slightly higher values than that of 145 g mol−1 for PF −, which is comprehensible.\ntesting in 2−4 V in DGDE electrolyte. (c) Linear fitting of ∆𝑚/𝑑𝑞 in 3−0.05 V in DGDE electrolyte.\nFurthermore, in this work, we devote to investigate the abnormal phenomenon of YP50F in Na+-\nDGDE electrolyte, which shows promising EDL capacitive behavior without the influence of DGDE-\nSEI layers. The fitting ∆𝑚/𝑑𝑞 results in Figs. 2a and c are applied to investigate the flux of solvent\nions into nanopores. The detailed calculation steps are provided in Supplementary Figure 18. Briefly,\nwhen the applied potential is below φ , the EDL reaction might include the co-ion desorption of\nPZC\nanions, ion exchange between cations and anions, or counter-ion adsorption of cations9. On the basis\nof the linear fit of the ∆𝑚/𝑑𝑞 results of 80, 174 and 108 g mol−1 e−1 (Fig. 2c), there are three regions\nin the potential window of 3.0−0.05 V vs. Na+/Na. The three ∆𝑚/𝑑𝑞 values are above 0, which\nexcludes the mechanism of co-ion desorption of anions. If only the counter-ion adsorption of cations\noccurs, the corresponding average solvation number x in [Na-xDGDE]+ is 0.43 in 3.0−1.5 V vs. Na+/Na,\nto 1.13 in 1.5−0.25 V vs. Na+/Na and then 0.64 in 0.25−0.05 V vs. Na+/Na. Usually, the low x indicates\nsmall sizes of solvated cations, which contrasts with the continuous EDL adsorption of cations in\nnanopores with increased response currents and increased capacitance during expanding ∆φ (Fig. 3b).\nTherefore, ion exchange between cations and anions occurs in Region I (in 3.0−1.5 V vs. Na+/Na), and\ncounter-ion adsorption of cations occurs in Regions II (in 1.5−0.25 V vs. Na+/Na) and Region III (in\n0.25−0.05 V vs. Na+/Na). The average solvation number of x in [Na-xDGDE]+ was calculated via eq.\nS13.\nAdditionally, according to your valuable suggestions, we have further collected the 19F NMR\nspectra (Figure R5-1), which displays a decrease in the “in-pore” signal of 19F (from PF −) at 1.5 V,\n6\nindicating the ion exchange process in 3−1.5 V vs. Na+/Na. These EQCM, NMR and electrochemical\nresults collectively conclude that the electrochemically driven continuous partial desolvation leads to\nreduced d and increased capacitance.\nFigure R5-1. Ex situ 19F NMR spectra of the YP50F electrode in DGDE electrolyte at several selected\nstates.\nIn this work, the EQCM results in EC:DMC were also measured. Compared with the ∆𝑚/𝑑𝑞 of\nYP50F in DGDE, the fitted ∆𝑚/𝑑𝑞 of 30.5 g mol−1 in EC:DMC was much smaller (Supplementary\nFigure 16). As known, the EC-SEI layers are able to sieve solvation shells and allow the intercalation\nof net Na+ ions passing through thick EC-SEI layers. To minimize the influence of EC-SEI formation,\nthe linear fitting was based on the 5th cycle, in which the weight changes became more stable. The\nfitted ∆𝑚/𝑑𝑞 of 30.5 g mol−1 is slightly higher than that of 23 g mol−1 for Na+, which might be owing\nto the contributions of electrolyte decomposition or the adsorption of solvated ions outside the EC-SEI\nlayers. Therefore, combining the electrochemical behavior, EQCM results and well-known functions\nof the EC-SEI layers, the desolvation mechanism of YP50F in EC:DMC in 3−0.05 V vs. Na+/Na was\nconcluded.\nThe following descriptions were added on pages 7 in the revised manuscript and supplementary\nfile.\n“The ∆𝑚/𝑑𝑞 of 30.5 g mol−1 is slightly higher than 23 g mol−1 for net Na+ (Supplementary Fig. 22),\nwhich might be owing to the contributions of electrolyte decomposition or the adsorption of solvated\nions on the surface of EC-SEI layers. However, the completely desolvated Na+ ions should cross\nthrough the thick EC-SEI layers, resulting in poor rate capability and large voltage hysteresis\n(Supplementary Fig. 3c-f).”\n2) The correct term in l. 162 should be “ring current effects” instead of “ring effects”.\nResponse: We thank the reviewer for the kind reminder. We have revised the descriptions in the\nmanuscript.\n3) Is there evidence for the assumption that the 23Na NMR signal at -10.5 ppm belongs to the electrolyte\nin the porous electrode? Were spectra recorded at variable temperature to rule out the possibility of it\nbeing an exchange peak between the free electrolyte and the in-pore electrolyte?\nResponse: We thank you very much for bringing this to our attention. We have performed the 23Na\nNMR spectroscopy of YP50F with different electrolyte loadings (Fig. R5-2). For a low electrolyte\nloading (2.5 mg of YP50F:1 µL of electrolyte) indicating full adsorption of the electrolyte in nanopores,\na peak appears at −15.5 ppm in the 23Na spectrum, which is attributed to “in-pore” Na+ in the pristine\npowders. As the electrolyte proportion increased (2.5 mg of YP50F:2 µL or 3 µL electrolyte), there is\na discernible shift towards positive chemical shifts at −10.5 ppm and −8.4 ppm for the 23Na spectra.\nThe 23Na resonance of the electrolyte is −8.4 ppm. Thus, the signal at −10.5 ppm likely belongs to the\nelectrolyte in the porous electrode, which suffers the bulk magnetic susceptibility effect.\nFigure R5-2. 23Na NMR spectra of YP50F with different amounts of DGDE electrolyte.\nAccording to your suggestions, we have further obtained the variable-temperature 23Na NMR\nspectra of YP50F with the electrolyte (24 mg YP50F: 36 µL electrolyte) and the pure electrolyte\n(Figure R5-3). The spectra show that the dominant 23Na signal (overlapping resonances of the free\nelectrolyte at −8.4 ppm and another signal at ~−10.5 ppm) shifts upfield with increasing temperature,\nwhereas both the chemical shift of the in-pore electrolyte and their intensities almost keep constant.\nThe almost unchanged 23Na shift of in-pore signal and its intensity demonstrate a low ion exchange\nrate between the free electrolyte and in-pore electrolyte in this system and the temperature range. This\nalmost identical shift of the dominant 23Na signal in YP50F with the electrolyte and pure electrolyte\ndemonstrated that the primary contribution of this shift to the change of solvation structure in the\nelectrolyte and enhanced exchange between free Na+ and solvated Na+.\nFigure R5-3. 23Na NMR spectra of YP50F with 1 M NaPF in DGDE electrolyte (24 mg : 36 μL) and\n6\npure DGDE electrolyte with increasing temperature from 298 to 318 K. The spectra were obtained\nusing 4 mm magic angle spinning (MAS) probe at static condition.\nThese data were added to Supplementary Figure 23 in the revised Supplementary file.\n4) The signal-to-noise ratio of the 23Na NMR signal assigned to the fraction of Na in nanopores is very\nweak. Mixing of this signal with random noise might lead to high uncertainties in the peak position,\nand all information about the peak shape is lost. Is there a possibility to conduct measurements with\nmore scans to verify the peak position, and to analyse if the peak shoulder is real or an artefact from\nrandom noise?\nResponse: We thank you for noticing this. To ensure the accurate analyses of the in situ spectra,\nwe applied several critical processing criteria:\n1) The in situ 23Na NMR spectra (Figure R5-4a) and difference spectra (Figure R5-4b) reveal a\npronounced signal broadening at lower potentials during the expanding ∆φ. These difference spectra\nprovide essential peak pattern information of the enhanced in-pore signals, enabling a rational spectral\ndeconvolution.\n2) To identify peak patterns originating from free electrolytes, SEI and background signals, the in situ\n23Na NMR spectrum of cycled in situ cell was carefully fitted. These peak positions and line-widths\nwere fixed, ensuring consistent and reliable deconvolution results.\n3) Additionally, ex situ 23Na NMR spectra were obtained following your suggestion (Figure R5-5c),\nconsistently demonstrating an analogous trend with upfield shifting and increasing line-width of in-\npore signals during discharge to 0.05 V vs. Na+/Na. These data validate the authenticity of in situ\nresults, ruling out potential experimental artifacts.\nFigure R5-4. (a) In situ NMR spectra of 23Na during discharging to 0.05 V vs. Na+/Na in DGDE\nelectrolyte. (b) Difference spectra of in situ 23Na NMR spectra. (c) Ex situ 23Na NMR spectra of the\nYP50F electrode in the DGDE electrolyte at several selected states.\nFor the in situ NMR tests, we had performed several times which required the collection of\nreversible electrochemical cycles and enough NMR spectra simultaneously. The critical point is the\nvolume of electrolyte in the in situ NMR cell. As a certain volume of electrolyte is necessary for\nelectrochemical cycling operation, it exhibits long free induction decay (FID) signal compared to the\nin-pore electrolyte. This requires a significantly larger domain time to acquire the free electrolyte signal,\nwhereas impacting the signal-to-noise (SN) of in-pore signals with much short domain time. Our\nexperiments revealed an optimal electrolyte volume of 10 μL in our in situ NMR test (Supplementary\nFigure 24), ensuring reliable cycling performance. Meanwhile, the spectra acquisition requires\nconstrained time intervals for avoiding significant state of charge (SoC) drift during testing, thereby\nstabilizing the electrochemical and confined electric double layer information for accurate\ncharacterization. Here, we employed an acquisition time of 20 min (accounting for ~3 mAh g-1) and\npulse length of /12 in each spectrum to balance the spectral quality and SoC variations.\nFigure R5-5 showed an example of in situ NMR result when an electrolyte volume of 12 μL was\napplied. These spectra exhibit much lower SN of in-pore signals due to the strong free electrolyte.\nWhen the 8−9 μL electrolyte was used in the in situ NMR cells, the electrochemical cycling is failed\nto operating.\nFigure R5-5. (a,b) In situ NMR spectra of 23Na cycled in DGDE with a volume of 12 μL.\n5) The description of Supp. Figure 21 includes the phrase “reversible shits” instead of “reversible shifts”.\nResponse: We thank the reviewer for this kind reminder. We have revised the typo of the caption of\nSupplementary Figure S21.\n6) The fits of the NMR spectra in Supp. Fig. 23 do not always match the experimental spectra.\nEspecially the ones recorded at higher potentials might be fitted better with broader in-pore peaks,\nwhich would contradict the author’s hypothesis.\nResponse: We thank the reviewer for this valuable suggestion. According to your suggestion, we have\nperformed the refined deconvolution analysis to increase the fitting accuracy (Figure R5-6). The fitting\nresults together with the difference spectroscopy (Figure R5-4b) and the ex situ 23Na test results (Figure\nR5-4c) unambiguously reveal an upfield shift and line-width broadening upon discharge. These\nobservations, reinforced by EQCM and electrochemical analysis, corroborate our proposed mechanism\nthat the enhanced Na+-carbon interaction and denser confined EDL in nanopores under Δφ of −2.95 V\nvs. PZC.\nFigure R5-6. Representative deconvolutions of 23Na in situ NMR spectra at different states.\nFigure R5-4. (a) In situ NMR spectra of 23Na during discharging to 0.05 V vs. Na+/Na in DGDE\nelectrolyte. (b) Difference spectra of in situ 23Na NMR spectra. (c) Ex situ 23Na NMR spectra of the\nYP50F electrode in the DGDE electrolyte at several selected states.\nThese data were replaced to Supplementary Figure 27 in the revised Supplementary file and the\nrelated fitting results in Figure 2d,f in the revised manuscript.\n7) Were any ex situ experiments done under MAS conditions? This would improve the resolution and\nallow for more definitive peak assignment.\nResponse: We thank you for this constructive suggestion. We have acquired the ex situ 23Na NMR\nspectra (Figure R5-7) under magic angle spinning (MAS) frequencies of 8 kHz for several selected\nstates. The coin cells were operated to the designated potentials and were immediately dissembled in\na glove box. The YP50F powders were quickly scraped from the current collectors without any other\nprocessing, then packed into 3.2 mm rotors. It is noted that even this rapid processing within 5 min\nminimizes the effect of post-mortem characterization for the as-operated confined EDL structure,\nensuring the consistent condition of this confined EDL structure at such low potentials for the post-\nmortem and in situ tests is critically challenging. Despite that, these ex situ 23Na NMR spectra\ndemonstrate a consistent trend with upfield shifting and line-width increasing of the in-pore signals\nduring discharging to 0.05 V vs. Na+/Na. Notably, obtaining mass-normalized spectra for ex situ NMR\nanalysis, analogous to the in situ method (based on the mass of YP50F), remains problematic due to\nthe uncontrollable residual electrolytes in the YP50F electrode material. These residues introduce\nintrinsic variations and quantitative errors across samples.\nFigure R5-7. Ex situ 23Na NMR spectra of the YP50F electrode in the DGDE electrolyte at several\nselected states.\n8) Could the authors comment on the use of 23Na for the in situ experiments? Better resolution would\nbe afforded by 19F NMR. 19F NMR would also provide insight into whether the mechanism proposed\nin Figure S.18 is valid.\nResponse: We thank you for this constructive suggestion. We investigated the EDL capacitive\nbehavior of YP50F in Na+-DGDE electrolyte in 3−0.05 V vs. Na+/Na, which is below PZC and related\nto the counter-ion adsorption (that is solvated Na+ ions). Thus, the in situ 23Na NMR experiment was\nperformed.\nAccording to your valuable suggestions, the 19F NMR spectra were further collected (Fig. R5-1),\nwhich displayed the decrease of in-pore 19F signal (from PF −) at 1.5 V, indicating the ion exchange\n6\nprocess in 3−1.5 V vs. Na+/Na. Therefore, these EQCM, NMR and electrochemical analysis results\ncollectively conclude that the electrochemically driven continuous partial desolvation leads to reduced\nd and increased capacitance.\nFigure R5-1. Ex situ 19F NMR spectra of the YP50F electrode in DGDE electrolyte at several selected\nstates.\n9) Please check Figure S.13. The captions for part c) and f) are the same and the schematics show that\nthe SEIs formed in both solvents sieve the solvents, which contradicts what the authors say.\nResponse: We thank you for the kind reminder. In Supplementary Figure 13, we show the changes in\nthe electrolyte for different cycled YP50F electrodes. These results support the point when EC-SEI\nlayers formed, the loss of EDL capacitive behavior in nanopores was observed. For better presentation,\nwe revised the statements and schematics in the revised Supplementary materials.\nSupplementary Figure 13 | Exchanging the applied electrolyte systems of cycled YP50F electrode.\n(a) The CV curves of YP50F electrode with EC-SEI layers then cycled in DGDE electrolyte for another\n10 scans. (b) The CV curves of YP50F in EC:DMC electrolyte and YP50F with EC-SEI layers cycled\nin DGDE electrolyte, at 1.0 mV s−1. (c) Schematic of YP50F electrode with EC-SEI layers then cycled\nin DGDE electrolyte. (d) The CV curves of YP50F electrode with DGDE-SEI layers then cycled in\nEC:DMC electrolyte for another 10 scans. (e) The CV curves of YP50F in DGDE electrolyte and\nYP50F with DGDE-SEI layers in EC:DMC electrolyte, at 1.0 mV s−1. (f) Schematic of the YP50F\nelectrode with EC-SEI layers then cycled in DGDE electrolyte.\nThe YP50F electrode with EC-SEI layers shows the distorted CV curves when cycled in DGDE\nelectrolyte. However, the DGDE-SEI layers on YP50F seem “transparent” and allow EDL adsorption\nwhen cycled in 3.0−1.0 V vs. Na+/Na in EC:DMC. Then, the newly formed EC-SEI layer on DGDE-\nSEI layer blocks the EDL capacitive behavior in nanopores again when cycled in 3.0−0.05 V vs.\nNa+/Na (Supplementary Figure 13e).\n10) Could the authors comment on the supposed broadening of the “in-pore” during discharge? What\nis the source of the broadening? Why does the peak width remain constant from 0.25 V?\nResponse: We thank you for the valuable suggestion. This work shows that the Δφ is able to be\nexpanded to −2.95 V vs. PZC while maintaining EDL capacitive behavior in nanopores in Na+-DGDE\nelectrolyte, which is much larger than the |Δφ| of 1.35 V for the 2.7 V EDLC (the electrolyte of TEABF\n4\nin ACN). A large Δφ refers to the strong electric field of the double layers between the carbon atoms\nand solvated Na+ ions. The strong electric field could drive the enhanced Na+-carbon interaction that\nleads to broadening of the in-pore signal. Based on the EQCM results (Figure 2c), there are three steps\nincluding ion exchange between [Na-2̅̅.̅1̅DGDE]+ and PF − in Region I (in 3.0−1.5 V vs. Na+/Na), and\n6\ncounter-ion adsorption of [Na-1̅̅.̅1̅DGDE]+ in Regions II (in 1.5−0.25 V vs. Na+/Na) and counter-ion\nadsorption of [Na-0̅̅.̅6̅DGDE]+ in Region III (in 0.25−0.05 V vs. Na+/Na). The large electrochemical\npolarization (Δφ=−2.95 V) electrochemically drives continuous partial desolvation from [Na-\n2̅̅.̅1̅DGDE]+ to [Na-1̅̅.̅1̅DGDE]+ and then to [Na-0̅̅.̅6̅DGDE]+, leading to gradually reducing d and thus\nthe increase in capacitance. This dense-packed cation structure, formed by 0.25 V and largely retain\nthereafter, likely explains the nearly constant 23Na line-width observed in both in situ and ex situ 23Na\nNMR spectra.\n11) All other characterization techniques were performed on cells using DGDE and on cells using\nEC:DMC, except for NMR. Have the authors performed in situ NMR experiments on cells using\nEC:DMC electrolyte? If not, these measurements should be done to confirm the interpretation of the\n23Na NMR spectra of the DGDE system.\nResponse: We thank you for this valuable suggestion. According to your suggestions, the in situ 23Na\nNMR in EC:DMC electrolyte was measured (Figure R5-8). It shows the increase of SEI single during\ndischarging to 0.05 V (Figure R5-8b) and charging to 3 V (Figure R5-8c). The overlapping spectra of\ndischarging to 0.05 V and recharging to 3 V indicate the irreversible formation of the EC-SEI layers,\nwhich is very different from the reversible changes of spectra cycled in the DGDE electrolyte (Figure\nR5-9).\nFigure R5-8. In situ NMR spectra of 23Na in EC:DMC electrolyte. (a) The corresponding galvanostatic\ncharge and discharge profiles during in situ NMR testing. (b,c) In situ NMR spectra of 23Na during (b)\ndischarging and (c) charging processes.\nFigure R5-9. In situ NMR spectra of 23Na during (a) discharging and (b) charging processes in DGDE\nelectrolyte.\nThe following descriptions were added on Page 8 in the revised manuscript.\n“In situ 23Na NMR in EC:DMC (Supplementary Fig. 28) shows the irreversible formation of EC-SEI\nsingle during discharging to 0.05 V and recharging to 3 V, which is very different from the reversible\nchanges of spectra cycled in DGDE electrolyte (Supplementary Fig. 24).”\nIn conclusion, the manuscript presents an interesting finding of an electrolyte that forms a\nsupercapacitor-compatible SEI and therefore works well at low potentials. Nevertheless, the\nsupporting evidence for the assumptions about the mechanisms is not very strong. If the authors could\nprovide more supporting information in the form of the additional experiments listed above, then it\nwill be suitable for publication in Nature Communications.\nResponse: We appreciate you for the positive recommendation of our manuscript. We have carefully\nperformed the experiments according to your valuable suggestions. We believe the quality of the\nmanuscript has significantly improved.\nResponse Letter\nManuscript ID: NCOMMS-25-27796A\nTitle: Unlocking limited electric double-layer capacity via electrochemically-driven\ncontinuous partial desolvations in carbon nanopores\nReviewer #1 (Remarks to the Author):\nThe authors has significantly improved the manuscript with more technical details based on the\nreviewers' suggestions. Specifically, they provided evidence regarding the EQCM results that\nproved the frequency changed observed is mainly originated from the mass change of the\nelectrode. The interpretation on EQCM is sound. The revised manuscript can be accepted for\npublishing in Nature Communications.\nResponse: We appreciate that you are satisfied with our responses and reversions.\nReviewer #2 (Remarks to the Author):\nThe authors have thoroughly addressed the previous concerns in their revised manuscript. Their\nrebuttal provides clear and convincing modifications, particularly in clarifying the sodium storage\nmechanisms, which significantly improves the paper’s scientific rigor.\nOverall, this revision successfully resolves all outstanding issues, and the manuscript now meets\nthe high publication standards of Nature Communications. I strongly recommend acceptance in\nits current form.\nResponse: We appreciate that you are satisfied with our responses and reversions.\nReviewer #3 (Remarks to the Author):\nAuthors provided detailed and convincing explanations together with revised manuscript. Thus, I\nfind it now acceptable for publishing.\nResponse: We appreciate that you are satisfied with our responses and reversions.\nReviewer #5 (Remarks to the Author):\nResponse: We appreciate that you are satisfy with the most of our responses. We are glad to\nfurther address the comments/suggestions and improve the quality of our manuscript.\n1) I will let the other reviewers who are much more experienced with EQCM respond the issues\nwith the data. It does seem strange that the number of solvent molecules can be calculated so\naccurately, yet there is ambiguity in the mass of PF -. Regarding the 19F NMR spectra, was the\n6\ndecrease in the “in-pore” peak gradual? The authors only show spectra for 3.0 and 1.5 V, which\nshow the presence and absence of “in-pore” peaks, respectively, but nothing in between. Could\nthe authors also comment on the additional broad, low-intensity signals around -70 ppm in some\nof the spectra?\nResponse: We sincerely appreciate your valuable suggestions. According to your suggestion, we\nfurther collected the additional ex situ 19F NMR spectra at the potential of 2.5 and 2.0 V vs. Na+/Na,\nas shown in the revised Supplementary Figure 18. The signal of the “in-pore” peaks is gradually\nreduced, further indicating the ion-exchange process in the potential window of 3−1.5 V vs.\nNa+/Na.\nSupplementary Figure 18 | Ex situ 19F NMR spectra of the YP50F electrode in DGDE electrolyte\nat several selected states. The decrease of in-pore signal of 19F (from the PF −) at 1.5 V vs. Na+/Na\n6\nindicates the ion exchange process in the potential window of 3−1.5 V vs. Na+/Na.\nThe in-pore signal progressively shifted upfield as the potential decreased to 1.5 V vs.\nNa+/Na. Similar trends were observed in literatures. However, at the states with lower potentials,\na peak emerged at around −70 ppm. This peak signal is weak and broad as such low potential. The\nobserved chemical shifts largely deviated from the previously reported value at downfield and\nexhibited broader line-width compared to the bulk electrolyte signals. This peak might be linked\nto the different surficial environments under such strong interfacial electric fields (largely\nincreased Δφ away from PZC).\n2) Thank you for correcting the term.\nResponse: We appreciate that you are satisfied with the responses.\n3) The variable temperature experiments satisfy this question.\nResponse: We appreciate that you are satisfied with the responses.\n4) Thank you for the additional experiments. Are they authors able to share any details about the\nin situ NMR cell that they used? Would it not be possible to simply use bigger/thicker YP50\nelectrodes to increase the amount of carbon in the cell? I think the lack of resolution and poor\nsignal-to-noise ratios are the main drawback to this study.\nResponse: We thank you for the helpful comments and suggestions regarding our in situ NMR\nmethodology. Our in situ NMR test relies a manually built in situ NMR probe with a saddle coil\n(ID: 10 mm) and a specific threaded-style cylindrical cells (OD: 10 mm; ID: 8 mm), as shown in\nFigure R5-1. This in situ NMR cell design has been validated in our previous studies [ref. Nat.\nNanotech. 15, 883–890 (2020); Sci. Adv. 7, eabj3423 (2021)]. The photo of in situ NMR cell is\nadded to the revised Supplementary Figure 24a for better reading.\nFigure R5-1. Photo of the NMR probe and the in situ NMR cells.\nThank you for the valuable suggestion of bigger/thicker YP50 electrodes. The ~7 mm\ndiameter of the electrode is limited by the in situ NMR cell. The YP50F electrode was prepared\nvia the wet-coating method using CMC:SBR as the binder. However, owing to the low compaction\ndensity and rich porosity of activated carbon (YP50F), the thickness is limited to ~100 μm (single-\nsided coating) by our wet-coating method. We attempted to increase the thickness, but the thick\nfilm was separated from the current collector. Further exploration with advanced in situ NMR\ncells and high-resolution techniques to enhance the signal of in situ spectra is suggested. Inspired\nby the recent reference [Science 384, 321-325 (2024)], the use of different porous carbons might\nbe helpful for increasing the “in-pore” signal, which is worth investigating in the future.\n5) Thank you for correcting the term.\nResponse: We appreciate that you are satisfied with the responses.\n6) The fits of the spectra at higher potentials do appear better. Could the authors comment on what\nthe refined deconvolution procedure involved?\nResponse: We thank you for this valuable suggestion. To ensure accurate analyses of the in situ\nspectra, we applied several critical processing criteria. First, the difference spectra\n(Supplementary Figure 25) reveal pronounced signal broadening at lower potentials during the\nexpanding ∆φ. These difference spectra provide essential peak pattern information of the\nenhanced in-pore signals, enabling rational spectral deconvolution. Then, ex situ 23Na NMR\nspectra were obtained following your suggestion using broader peaks (Supplementary Figure 27\nand Supplementary Table 1), consistently demonstrating an analogous trend with upfield shifting\nand increasing line width of in-pore signals during discharge to 0.05 V vs. Na+/Na. The detailed\ndeconvolution process is provided in the legend of Supplementary Figure 27, as shown below.\nSupplementary Figure 27. Representative deconvolutions of in situ 23Na NMR spectra\nrecorded for cells. The NMR data were acquired via Topspin software, exported via DMFIT\nsoftware, and finally fitted via Origin software. The experimental lines are depicted in gray,\nwhereas the sum of the individual fitted components is shown in blue. The deconvolution process\nwas conducted via Lorentz lines to delineate the various features present in the spectra. First,\nthe difference spectra were obtained to provide critical information for the chemical shifts and the\ncorresponding linewidths of the “in-pore” signals. In each case, only the minimum number of\ncomponents required to model the spectra was employed. Indeed, some spectra require several\nline shapes to describe the free electrolyte and “in-pore” resonance. This phenomenon can be\nattributed to bulk magnetic susceptibility (BMS) effects, leading to a diverse range of local fields\nand consequent shifts for identical chemical species.\n7) Addressed. 8) Addressed. 9) Addressed. 10) Addressed.\nResponse: We appreciate that you are satisfied with the responses.\n11) Could the authors please show the complete data set for the in situ experiments (i.e., the charge\nand discharge, similar to Supplementary Figure S21?\nResponse: We thank you for this valuable suggestion. According to your suggestions, we redrew\nthe revised Supplementary Figure 28 for the in situ 23Na NMR spectra in the EC:DMC electrolyte.\nSupplementary Figure 28 | In situ 23Na NMR spectra in EC:DMC electrolyte. (a) The\ncorresponding galvanostatic charge and discharge profiles during in situ NMR testing. 3D plots\nof in situ 23Na NMR spectra (b) between 1000 and −1000 ppm and (c) between 100 and −100\nppm. In situ 23Na NMR spectra during (d) discharging and (e) charging processes in EC:DMC\nelectrolyte."}, {"round": 2, "reviewer_comments": {"reviewer_5": "I am happy with the changes made to the manuscript and agree that it can be published."}, "author_response": ""}]

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