Smart-flame-retarding layered composite Li negative electrode for safe Li metal battery

Domain: neuroscience
Published: 2026-03-26
Editorial outcome: major_revision

Abstract
The high flammability problem of Li metal negative electrode has persistently impeded the safe application of energy-dense Li metal batteries. Conventional flame retardant mixing strategy also fails in addressing this challenge because of the severe function-destroying corrosion reaction between flame retardant and Li metal. In this work, we propose a layered graphene oxide/flame retardant/lithiophilic ZnO layer/Li metal negative electrode for eliminating the corrosion between Li metal and flame retardant, while smart releasing the thermal-triggered flame-retarding gases for covering and bonding with Li metal against combustion. This smart-flame-retard layered Li negative electrode afford 2677.78% prolonged battery cycle life than pure Li negative electrode after 600℃ ignition, and 94.68% suppressed side reaction than conventional flame retardant-Li mixture during regular electrode operation. This resolution on the flammability challenge of Li metal negative electrode presents a meaningful advancement for safety Li metal battery construction.

Introduction
With the rapid development of electric vehicle, portable electronic devices and clean energy consumption market, scientists are in eager persuit of new battery technologies with both high energy density and high safety characteristics 1-3. Among the negative electrode candidate materials for high-energy-density Li battery technology, Li metal stands out due to the highest theoretical specific capacity (3860 mAh g-1) and lowest electrochemical potential (-3.04 V versus standard hydrogen electrode), rendering it the ultimate negative electrode choice for building energy-dese Li batteries 4,5. However, Li metal batteries (LMBs) face serious safety concerns due to the inherent high flammability of Li metal negative electrode 6,7. As one of the most reductive metal, metallic Li can be very easily ignited when exposed to heat, humidity or air, generating a ~1200℃ intense combustion that cannot be extinguished by regular strategy (Fig. 1a). This will result in high thermal runaway and catastrophic accidents that far exceeds conventional Li-ion battery 8, posing significant challenge on the safe application of Li metal battery.

Despite this well-known severe combustion problem of Li metal negative electrode, current research efforts on safe Li battery construction are mainly focused on developing non-flammable liquid or solid electrolytes 9-12, high thermal-stable separators 13,14, intelligent response separators 15,16 and battery management systems 17. Other strategies such as artificial protection layer are also developed to improve the air-stability of Li metal to some extent 18-20. However, all these above strategies fail to provide sufficient flame-retardant effects to extinguish the ignition of Li metal negative electrode, because of the high flammability and reductivity of Li metal and failure of delivering fire-extinguishing function inside the Li metal. This safety issue remains as one of the most critical unsolved challenges since the invention of Li battery, impeding the large-scale practical application of LMBs for decades.

To solve the high flammability challenge of Li metal negative electrode, the necessary solution is incorporating flame-retardant materials into the negative electrode (Fig. 1b). However, this conventional mixing strategy is ineffective due to the severe side reaction between highly reductive Li metal and flame-retardant materials. During Li electrode fabrication processes such as molten Li infusion or mechanical mixing (Fig. 1c), these phosphorus or halogen element-based flame retardants will severely react with Li metal, resulting in the simultaneous loss of the flame-extinguishing effect of flame retardant and negative electrode function of Li metal negative electrode. Furthermore, if the flame retardant-containing Li metal electrode is assembled into the battery as an negative electrode, the flame retardant will be dissolved into the electrolyte, corrode the Li metal active materials and generate unstable solid electrolyte interphase (SEI), which greatly deteriorates the battery performance (Fig. 1d) 21,22. This incompatibility between Li metal and flame retardants presents significant technical bottlenecks for achieving the effective flame-retardant modification of Li metal negative electrode. Therefore, no strategy has ever been proposed to tackle the high flammability problem o f Li metal negative electrode, making it a well-known unsolved problem that has impeded the application of LMB technology, since its invention decades ago.

Here, we develop a layered host strategy that layer-by-layered assembly Li metal, lithiophilic zinc oxide (ZnO) layer and fire retardant onto three-dimensional graphene oxide foam, successfully eliminating the incompatibility between Li metal and flame retardants and affording smart-flame-retarding function for addressing the flammability challenge of Li metal negative electrode. We discover that this smart-flame-retarding ability should be attributed to three key material design concepts: (1) the high thermal insulation/dissipation property of three-dimensional aerogel structure against heating; (2) effective blocking of side reaction between Li and flame retardant during both electrode fabrication and battery operation from the ZnO interlayer (Fig. 1e and f); (3) thermal-triggered, smart in-situ release of flame-retarding gases from inside the composite negative electrode to dilute air and bond with Li towards self-flame-retarding under ignition condition (Fig. 1g). From these merits, our smart-flame-retarding layered composite Li negative electrode successfully decreases the self-extinguishing time of Li metal from 979.27 to 0 s g-1 even if immersed in open fire for 10 seconds, affording 85.81% electrode capacity reservation and 93.57% full-cell capacity reservation after 250 cycles even if the Li negative electrode was pre-immersed in an open flame. Fabricated pouch cell using smart-flame-retarding layered composite Li negative electrode maintains consistent power output under the condition of direct ignition on Li negative electrode, demonstrating the successful resolution of Li metal battery safety hazard. Meanwhile, the severe side reaction between Li metal and flame retardant during regular electrode operation is also eliminated by flame retardant-sealing layered structure design, delivering 301.61% prolonged cycle life than conventional flame retardant-Li mixture. This smart-flame-retarding layered composite host design provides ideas for addressing the long-existing high flammability challenges of Li metal negative electrode for safe and durable Li metal battery application.

Results
Fabrication of smart-flame-retarding layered composite Li negative electrode

The composite Li metal negative electrode was fabricated by flame-retarding host preparation, layered structure construction and molten Li infusion steps (Fig. 2a). Initially, a mixed graphene oxide (GO) and triphenyl phosphate (TPP flame retardant) aqueous solution was coated onto glass substrate and freeze-dried to form a porous foam, which was then roll-to-roll calendared to a thickness of 80 μm. Scanning electron microscope (SEM) images and X-ray diffraction (XRD) spectra demonstrate that TPP was uniformly coated onto the 40-nm-thick GO layers, generating a 260-nm-thick TPP-on-GO, bilayer-structured foam (TPP/GO) (Fig. 2b and c, Supplementary Fig. 1a to c and Supplementary Fig. 2). Subsequently, diethylzinc and water vapors were alternatively pulsed into the TPP/GO foam, absorbed on the TPP/GO layer and purged out using an industrial-level atomic layer deposition (ALD) fabrication procedure, converting into a uniform ZnO film that covers the TPP/GO aerogel into ZnO/TPP/GO host (Fig. 2d and Supplementary Fig. 1d). This ZnO layer can provide good lithiophilicity for molten Li infusion, while shielding the inner TPP layer from contacting Li metal. Therefore, the tetra layered composite negative electrode (Li/ZnO/TPP/GO) can be fabricated by edge-contacting molten Li absorption into the host 23 (Fig. 2e, Supplementary Fig. 1e and Supplementary Fig. 3). Absence of TPP decomposition-derived smoke was witnessed during this molten Li infusion into ZnO/TPP/GO host (Supplementary Movie 1). In contrast, large amount of TPP-Li side reaction-generated smoke and failure of molten Li infusion were observed during contacting TPP/GO foam (without ZnO layer) with molten Li (Supplementary Movie 2). This proves the shielding effect of ZnO layer and layered structure for terminating the corrosion reaction between TPP and Li metal, so we also fabricate the Li@TPP/GO control sample by attaching regular Li foil to TPP/GO host for comparison (Supplementary Fig. 4).

Cross-sectional focused ion beam (FIB)-SEM image on Li/ZnO/TPP/GO electrode shows that the Li metal was filled into the micropores of the ZnO/TPP/GO host (Fig. 2f), while magnified EDS mapping images show a clear layered distribution of characteristic elements: carbon-containing GO and TPP layer inside, zinc and oxygen-containing ZnO layer in the middle, and carbon/oxygen-free metallic Li layer outside (Fig. 2g and Supplementary Fig. 5). XRD spectra show that TPP, ZnO and Li were stepwise incorporated into the host during fabrication (Fig. 2h), converting the ZnO into LiZn without deteriorating TPP inside. These characterizations prove the successful fabrication of tetralayer Li/ZnO/TPP/GO electrode, which confines TPP inside the ZnO layer and separate TPP from metallic Li during electrode preparation and battery operation. This enables the compatibility between flame retardant and Li metal, which is the key to achieving the flame retardancy for the Li metal negative electrode while ensuring the electrochemical performance of LMBs. In addition, this composite negative electrode has great potential for large-scale manufacturing and practical applications, as the preparation steps are elaborately designed based on commercially available raw materials (GO and TPP solution) 24 and mature industrial production techniques (ALD and composite Li fabrications procedures) 25,26, enabling decimeter-scale electrodes sufficient for pouch cell fabrication even in laboratories (Supplementary Fig. 6). Detailed calculations in Supplementary Table 1 and Note 1 prove that our composite Li negative electrode design have minor effect on reducing the specific energy andenergy density of the lithium metal battery.

Mechanism and effect of the flame retardancy from layered host design

To elucidate the flame-retarding effect of our layered composite host design, we conducted a self-extinguishing time (SET) test by recording the burning/extinguishing time span of different kinds of electrode (using both digital camera and infrared thermal camera (ITC)), after 3 times of 2-second ignitions in 600℃ flame (Fig. 3a) 9. After testing the SET of Li/ZnO/TPP/GO electrodes with different TPP loading, we discovered that the layered Li/ZnO/TPP/GO electrode with TPP and GO mass percentage of 12.4% and 6.2% can illustrate a complete suppression of ignition, good electrode integration and constant temperature maintenance below 400℃ (Supplementary Fig. 7, Fig. 3b and Supplementary Movie 3). Even after a further sustained 10 seconds of constant ignition, Li/ZnO/TPP/GO electrode still illustrates an SET of 0 s g-1, defining it as a non-combustible material 9 (Supplementary Fig. 8 and Movies 4-8). In comparison, the conventional pure Li electrode shows a long-duration 1219.4℃ flame after only 0.9 seconds of ignition (Fig. 3c and Supplementary Movie 9), illustrating a SET of 979.27 s g-1 as highly flammable materials. The Li@TPP/GO control sample with same TPP/GO loading (but absence of ZnO protection or layered structure) also illustrate a severe combustion at 1127.4℃ with high SET of 744.31 s g-1 after only 0.5 seconds of ignition (Fig. 3d and Supplementary Movie 10). The SEM images of the three electrodes after ignition indicate that the layered composite electrode structure of Li/ZnO/TPP/GO was well retained after ignition, while the structure of pure Li and Li@TPP/GO electrodes were deteriorated and converted into powders due to the severe combustion (Supplementary Fig. 9).

Air-atmosphere differential scanning calorimetry (DSC) characterizations show that pure Li electrode exhibited a large exothermic peak at 475.8℃ with heat generation of 2498.61 J g−1, while the Li@TPP/GO electrode displayed a similar heat-releasing peak at 437.9℃ with heat generation of 1392.93 J g−1, which corresponded to the intense combustion reaction of Li metal. In comparison, the Li/ZnO/TPP/GO electrode illustrated a significantly decreased exothermic peak (732.06 J g−1 at 407.0℃), suggesting the heat-releasing reaction during metallic Li combustion in air was greatly suppressed owing to our fire-retardant electrode layered structure design (Fig. 3e). XRD spectra show that the metallic Li in pure Li and Li@TPP/GO electrode was fully converted to the combustion product of Li2O, Li2CO3 and LiOH after ignition, while the Li/ZnO/TPP/GO electrode well remained its Li and LiZn-dominant component (Fig. 3f). These comparisons between Li/ZnO/TPP/GO and Li@TPP/GO show that the good flame retardancy can be attributed to the ZnO interlayered structure we designed in host, because they have the identical TPP content.

To elucidate the mechanisms of the fire retardancy of Li/ZnO/TPP/GO electrode, we conducted multiple simulations and characterizations on these electrodes from three perspectives. The first mechanism at the initial stage of ignition is the low thermal conduction and high thermal dissipation-induced flame-retardant effect of the porous aerogel structure 27, as evidenced by the infrared thermal mapping showing relatively lower temperature of most parts in the electrode than ignition site after several seconds of ignition (Supplementary Figs. 10 and 11). The freeze-dried porous GO aerogel structure can deliver good fire-retardancy and heat dissipation after ignition, while the dense GO film structure with exactly same composition were rapidly burned out after 1 second of ignition, suggesting that this three-dimensional porous structure can improve the flame-retardant property of the electrode (Supplementary Fig. 12, Movies 11 and 12). COMSOL simulations further illustrate that the thermal conductivity of foam-structured porous Li (1.3327 W m-1 K-1) is around 1/63 of that of dense Li (84 W m-1 K-1), while the thermal dissipation speed of Li foam is also much faster. Therefore, under a constant heat source at 600℃ on one side, dense Li presents a uniform temperature distribution of high 600℃ across the whole electrode after only 2 seconds, while foamed Li maintains a distinct and dynamically stable temperature gradient that shows <300℃ at most of the electrode even after 10 seconds (Fig. 3g and Supplementary Movies 13 and 14). This greatly promoted heat dissipation effect can greatly delay the heat conduction and the melting of Li metal within the electrode, thereby promoting the flame retardancy of three-dimensional porous Li electrode, which cannot be realized by dense Li foil or the Li foil-TPP/GO host attaching structure in Li@TPP/GO electrode (Supplementary Fig. 13). As the electrode temperature rises, the second mechanism involves the releasing of TPP decomposition-derived flame-retardant gases around Li metal to isolate it from oxygen gas, as evidenced by the observed flame-retardant gas release (Supplementary Figs. 14 and 15). Two control samples containing three-dimensional structure or flame-retardant composition only illustrate slightly modified yet insufficient flame-retarding effect (Supplementary Fig. 16, Movie 15 and Note 2), demonstrating the respective contributions and synergetic effect of three-dimensional structures and TPP flame retardant for safe Li metal negative electrode. Thermal-gravimetric analysis (TGA) curves (Fig. 3h) indicate that after ZnO layer coating, the decomposition-releasing temperature of TPP in the Li/ZnO/TPP/GO electrode is increased from ≤300℃ to gradual decomposition within the temperature range of 200 to 1200℃, which should be attributed to the low thermal conduction and high thermal dissipation effect of the porous electrode. This higher temperature, gradual decomposition and release of TPP in layered Li/ZnO/TPP/GO structure design can guarantee its stability during molten Li infusion fabrication step, and consistently deliver flame-retardant gases under ignition condition, which cannot be achieved by the attaching structure of Li@TPP/GO electrode (Supplementary Fig. 17). Numerous micro-scale holes generated on the surface of Li/ZnO/TPP/GO electrode confirm the successful flame-retardant gases release during ignition 28 (Fig. 3i and Supplementary Fig. 18). COMSOL simulations demonstrate that high density flame-retarding gas can be in-situ, uniformly released from the Li/ZnO/TPP/GO and accumulate on the upper surface of electrode based on the gravity effect, while some flame-retarding gas can be adsorbed on the lower surface (Fig. 3j and Supplementary Movie 16), which is consistent with the observed flame-retardant gas diffusion behavior in realistic ignition condition (Supplementary Fig. 15). This gas-covering effect isolates the electrode from O2 atmosphere, effectively preventing the Li-O2 combustion reaction that cannot be realized by simply attaching Li foil to TPP/GO foam.

Last but not least, density functional theory (DFT) simulations illustrate the binding energy between Li atoms and PO· radicals (-3.15 eV, derived from TPP thermal decomposition 29) is much higher than that with O· radicals (-2.48 eV), supporting the prominent free Li atoms-capturing ability of PO· for interrupting the combustion chain reaction between Li and O2, thereby achieving the flame-retardant effect (Fig. 3k, Supplementary Fig. 19, Supplementary Data 1 and 2) 28-33. XRD pattern of the combustion products of the mixture of TPP and metallic Li and the X-ray photoelectron spectroscopy (XPS) depth profiles of the Li/ZnO/TPP/GO electrode after ignition both display characteristic peaks of Li3PO4 (55.6 eV in Li 1s spectra, 133.6 eV in P 2p spectra and 531.4 eV in O 1s spectra 34,35), which is the reaction product of POS· with Li atom, supporting this flame-retardant mechanism (Supplementary Figs. 20 and 21). In contrast, the XPS profiles of Li@TPP/GO and pure Li after ignition exhibit no Li3PO4-related peaks, because the TPP-derived phosphate composite (formed by the low decomposing temperature of flame retardant during the initial ignition) was carried away by thehot gas generated from the intense combustion of Li metal that last for several tens of seconds. In general, the smart-flame-retarding layered Li/ZnO/TPP/GO electrode design achieves good flame retardancy through the combined mechanisms of high thermal insulation/dissipation property of three-dimensional porous structure, the oxygen isolation effect of the in-situ released flame-retardant gas and the strong free Li atom-capturing ability of TPP-derived PO·. This provides strategies and mechanisms for solving the high combustion problem of Li metal negative electrode.

Battery performances during and after ignition condition

The flame-retarding property of the Li/ZnO/TPP/GO electrode not only enables good resistance to Li metal combustion under ignition or heating conditions, but also enables the retention of Li negative electrode function after ignition. Before ignition, the pure Li negative electrode exhibits a specific capacity of 3591.81 mAh g-1 that is close to its theoretical capacity, while Li@TPP/GO illustrates a significantly decreased specific capacity of 1101.08 mAh g-1 that suggests severe corrosion reactions between metallic Li and TPP to deteriorate negative electrode fuction (Fig. 4a). After ignition, pure Li and Li@TPP/GO exhibit 0 remaining negative electrode capacity, proving all metallic Li was burned out toward full loss of the electrode function. In comparison, the Li/ZnO/TPP/GO electrode only delivers a slightly decreased capacity from 2866.65 to 2459.99 mAh g-1 (85.81% electrode capacity reservation), demonstrating most of the Li negative electrode function was reserved after experiencing external ignition. Because of these significant differences in electrode function reservation, the symmetric Li||Li cell using after-burning Li/ZnO/TPP/GO electrodes can support stable Li plating/stripping cycling for 1000 hours with an overpotential of ~100 mV in LP40FV electrolyte 36, while the after-burning pure Li and Li@TPP/GO electrode exceeded protection voltage at the beginning of the first charging process (Fig. 4b and Supplementary Fig. 22a). This suggests that Li/ZnO/TPP/GO electrodes maintain stable Li stripping/plating negative electrode function even after ignition, which was also confirmed by the lower and stable charge transfer resistance of Li/ZnO/TPP/GO electrode (7.774 Ω to 12.20 Ω through 100 to 300 cycles) than pure Li and Li@TPP/GO electrode (Fig. 4c and Supplementary Fig. 22b). In addition, the after-burning Li/ZnO/TPP/GO electrodes also exhibit good rate performance at varying current densities (Supplementary Fig. 23). Top-view SEM images illustrate a uniform Li deposition morphology without any dendrites after 100 complete Li plating/stripping cycles in after-burning Li/ZnO/TPP/GO electrode (Supplementary Fig. 24). These results suggest that the Li/ZnO/TPP/GO electrode well retained its Li metal negative electrode function after ignition.

Benefiting from the good flame retardancy of Li/ZnO/TPP/GO negative electrode, the Li metal negative electrode-based full-cell can afford stable battery performance even during or after ignition. As shown in Fig. 4d, the Li||LiNi0.8Co0.1Mn0.1O2 (NCM811) full-cell using after-burning Li/ZnO/TPP/GO negative electrode exhibits 93.72% capacity retention after 250 cycles at a specific current of 94 mA g-1 in LHCETTE electrolyte 37, with stable voltage profiles, coulombic efficiency and charge-transfer resistance (Fig. 4e and Supplementary Figs. 25 and 26). At the same time, this cell exhibits good rate performance of 88.26% capacity retention at 376 mA g-1 and 63.22% capacity retention at 940 mA g-1 (Supplementary Fig. 27). In comparison, the full-cell with after-burning pure Li negative electrode exhibits a rapid capacity decay to 36.03% after only 80 cycles, while the full cell using ignited Li@TPP/GO negative electrode shows a lower 35.08% capacity retention after only 50 cycles (Fig. 4f), along with significantly degraded rate performance. These comparisons prove the good flame-resistance and capacity retention ability when the electrode was ignited.

Owing to the good fire retardancy of our Li/ZnO/TPP/GO negative electrode, the Li metal pouch cell can afford a self-fire-extinguishing ability and supply energy output even when the battery core was exposed to fire. As shown in Fig. 4g, we evaluate the safety property by recording the light-emitting diode (LED)-powering function of a 100 mAh Li metal battery using non-flammable electrolyte (1 M LiPF6 in a mixture of fluoroethylene carbonate /3,3,3-fluoroethylmethyl carbonate/1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether (FEC:FEMC:HFE, 2:6:2 by weight)) 7 when the Li metal negative electrode in the battery core was exposed to external fire through the damage site in package (Supplementary Fig. 28 and Movie 17). Fig. 4h shows that the conventional Li metal pouch cell using pure Li negative electrode shows a severe combustion after only 2 seconds of ignition, rapidly spreading through the whole battery and lasting for more than 20 seconds to fully destroy the battery (Supplementary Movie 18). This represents an intense combustion issue to be more violent than conventional Li-ion battery (Supplementary Fig. 29 and Movie 19). Debris analysis in Supplementary Fig. 30 proves that the Li metal negative electrode, separator and package was all burned out, while the NCM811 positive electrode maintained the original morphology, suggesting the high safety issue of LMB should be attributed to the Li metal negative electrode. In comparison, the Li metal pouch cell employing Li/ZnO/TPP/GO negative electrode successfully keeps powering the LED without any combustion during three times of 2-second ignitions (Fig. 4i), or even a 20-second-lasting ignition (Supplementary Movies 20-23 and Supplementary Fig. 31). This good flame-retardant effect of the Li metal pouch cell should be attributed to our electrode design against lithium oxidation reaction, rather than the non-flammable electrolyte (Supplementary Table 2, Supplementary Figs. 33, 33 and Movies 24, 25) 38,39. Nail penetration test and accelerating rate calorimetry (ARC) measurement are conducted to further evaluate the battery's safety under realistic abuse conditions. During the nail penetration test, Li/ZnO/TPP/GO||NCM811 pouch cell exhibited neither visible smoke nor open flame that overwhelms the exploded pure Li metal negative electrode-based pouch cell (Supplementary Figs. 34 and 35). In ARC measurement, Li/ZnO/TPP/GO||NCM811 pouch cell demonstrated a high self-heating onset temperature (T1) of 176.37°C (versus 160.84°C for the pure Li system) and a reduced peak temperature (T3) of 352.97°C (versus 500.12°C for the pure Li system), and consistently lower temperature increasing rate (dT/dt ≤ 1.96℃ min-1) than the pure Li system (T2 = 295.67°C when dT/dt ≥ 100℃ min-1) (Supplementary Fig. 36 and Movies 26, 27) 40,41. These results indicate that our smart-flame-retard layered composite Li electrode design does successfully address the high combustibility problem of Li metal negative electrode, affording a fire-resistant, high safety Li metal battery with great significance for safe energy storage applications.

Electrochemical performances during regular battery operation

In addition to the good flame retardancy, the ZnO interlayer from layered host design also enables complete shielding of TPP dissolution-induced Li/TPP corrosion reaction, affording significantly improved Li negative electrode performance. As shown in Fig. 5a, the Li@TPP/GO electrode-based symmetric cell exhibits a greatly shortened cycle life (<100 hours) with unstable voltage profiles than pure Li negative electrode (800 hours), and the averaged 82.32% Li plating/stripping coulombic efficiency (CE) in TPP/GO host is also much lower than regular copper (98.81%, Fig. 5b and Supplementary Fig. 37). This should be attributed to the dissolution of unprotected TPP into electrolyte and severe corrosion reaction between Li negative electrode and TPP. In contrast, the layered Li/ZnO/TPP/GO electrode-based symmetric cell exhibits stable voltage profiles with minimal hysteresis (1200 h with an overpotential of 97.5 mV). In the Li||host half-cell testing, the ZnO/TPP/GO host (after complete Li stripping from the Li/ZnO/TPP/GO) also illustrates a greatly improved Li plating/stripping CE of 99.06% and stable overpotentials ranging from 124.5 to 175.6 mV between 100 and 400 Li plating/stripping cycles. This improvement can also be observed in Aurbach's CE testing (Supplementary Fig. 38). These comparisons prove that our layered electrode design can successfully terminate the TPP dissolution and side reaction-induced Li negative electrode degradation issue, as further demonstrated by electrochemical impedance spectroscopy (EIS) spectra and high-current, high-capacity symmetric cell cycling results (Supplementary Figs. 39 to 41). In addition, the high lithiophilic ZnO-converted LiZn coating layer 42 and three-dimensional structure 43 of Li/ZnO/TPP/GO can further improve the reversibility of Li negative electrode reaction, which is responsible for the higher performance than pure Li negative electrode. Based on these advantages, the Li metal||NCM811 full-cell using 4 mAh cm-2 Li/ZnO/TPP/GO negative electrode and 1.63 mAh cm-2 NCM811 positive electrode can deliver a 79.09% capacity retention after 500 cycles with stable voltage profiles at a specific current of 94 mA g-1, outperforming those of pure Li||NCM811 full-cell (79% retention after 233 cycles) and Li@TPP/GO||NCM811 full-cell (79% retention after only 125 cycles, Fig. 5c and Supplementary Figs. 42 and 43). The same negative: positive electrode capacity ratio (RN/P) of the three full cells was realized by completely stripping Li from Li/ZnO/TPP/GO negative electrode in half-cell, depositing 4 mAh cm-2 Li into negative electrode, dissembling from half-cell and assembling into the full-cells (Supplementary Figs. 44 and 45). EIS spectra and rate performance also illustrate significantly improved negative electrode kinetic from the layered host design (Supplementary Figs. 46 and 47). In the pouch cell with higher energy density, the Li/ZnO/TPP/GO negative electrode still exhibits good cycle stability (Supplementary Figs. 48 and 49). These results prove that the our ZnO interlayer and layered host design can successfully solve the incompatibility and side reaction issue between flame retardant and Li metal negative electrode, constructing Li metal batteries with high safety and cycle stability.

The mechanisms of layered host design for suppressing Li-flame retardant corrosion reaction and improving negative electrode performance were revealed by characterizing SEI composition and deposited Li morphology. XPS depth profiles of Li@TPP/GO electrode shows high Li3PO4-related peaks in P 2p (133.6 eV) and Li 1s spectra (55.6 eV) 34,35, suggesting large amount of Li3PO4 species existed in the SEI of Li@TPP/GO negative electrode (Fig. 5d and e). Considering the absence of phosphorus-related elements in LHCETTE electrolyte, this result proves that TPP in the unprotected TPP/GO host was dissolved into electrolyte and corrode the Li metal negative electrode, while the generated Li3PO4-containing SEI is unfavorable for uniform Li deposition and reversible Li negative electrode operation 11,44. In comparison, the XPS depth profiles of Li/ZnO/TPP/GO negative electrode illustrates full absence of Li3PO4-peaks (Fig. 5f and g), demonstrating that the ZnO interlayer design can completely seal the TPP component inside the interlayer of Li/ZnO/TPP/GO negative electrode during regular negative electrode cycling, preventing TPP from Li metal corrosion reactions and allowing the formation of a favorable LiF-dominant SEI 45-47 (Supplementary Figs. 50 to 52). In addition, our layered composite negative electrode design also avoids the flame-retardant dissolution-derived phosphors-containing positive electrode-electrolyte interphase, which is also beneficial for positive electrode cycling stability (Supplementary Figs. 53-55). Owing to the advantages of high-surface-area three-dimensional host structure 43 and the lithiophilic properties of ZnO-converted LiZn interlayer 42,48, lower nucleation and migration barriers are realized in the Li/ZnO/TPP/GO electrode. After 100 complete Li plating/stripping cycles, it illustrates the uniform, averaged 7.3 μm-diameter grain-sized Li deposition morphology (Fig. 5h and Supplementary Fig. 56), overwhelming the highly dendritic deposition morphology in pure Li (Fig. 5i) and Li@TPP/GO (Fig. 5j). These comparisons prove that the layered host design can successfully address the incompatibility and corrosion reaction challenge between Li metal and flame retardants, affording simultaneous achievement of high safety and long cycle life in Li metal battery.

Methods
Preparation of the smart-flame-retarding layered host (ZnO/TPP/GO)

The TPP (Beijing InnoChem Science & Technology Co., Ltd., the purity is 98%, used without any treatment) ethanol solution (Ordered from https://shop507945882. taobao.com/, the purity is 99%, used without any treatment) (0.125 g ml-1) and commercially available GO solution (Hangzhou Gaoxi Technology, 10 mg g-1, dispersed in water) were uniformly mixed according to the mass ratio of TPP to GO of 2:1. The mixed solution was coated on the plastic film (Ordered from https://shop36317712. taobao.com/, 24 cm length, 16 cm width) with a thickness of 750 μm using a doctor blade (Modern Instruments, GB-SZQ-1000), and dried at -20 ± 5℃ in a freeze-dryer (Henan Fujing Instrument and Equipment Co., Ltd., FK-04) for 24 h to form a porous TPP/GO foam. The foam was stripped from the film and sandwiched between two layers of polypropylene films (purchased from Canrd Co., Ltd., MA-EN-SE-002901 16 um), and calendared to 80 μm (for Li||host coin cells and Li||NCM811 coin cells) or 125 μm (for symmetric coin cells) with a roller press (Shenzhen KeJing Star Technology Co., Ltd., GMSK-2150) in an air atmosphere. The calendared TPP/GO foam was cut into circular foams with a diameter of 11.5 mm for symmetric coin cells, circular foams with a diameter of 15 mm for Li||host coin cells and Li||NCM811 coin cells using puncher (Ordered from https://shop 3S0350276. taobao.com/, 11.5 and 15mm diameter) and rectangular foam with dimensions of 5.7 cm × 4.5 cm using scissor in an air atmosphere. ZnO nanolayer was deposited into the foam using the atomic layer deposition machine (Xiamen Yunmao Technology Co., Ltd., Mini AL D system) at 150°C with diethyl zinc (DEZn, Nanjing ai mou yuan Scientific equipment Co., Ltd., 99.9999%, liquid) and deionized water (0.1 multiply 10^6 Ω*cm) as precursors. The source MFC carrier gas flow rate of DEZn and water were 60 SCCM and the source in times were 85 ms and 50 ms, with 3-seconds static transition and 20-seconds transition purge (using nitrogen gas (Jinan Deyang Special Gas CO., Ltd., purity is 99.999%)) to remove excess precursors after each pulse. Approximately 500 ALD cycles were needed to prepare the sample.

Preparation of the smart-flame-retarding layered composite Li electrode (Li/ZnO/TPP/GO)

In an argon-atmosphere glove box (Mikrouna (Shanghai) Ind. Int. Tech. Co., Ltd., H₂O and O₂ content < 0.01 ppm), approximately 0.1 grams of Li metal foil (Shandong Zhongshan Photoelectric Materials Co., Ltd., the purity is 99.95%) was heated on a 350 ± 10℃ hotplate (purchased from DLAB Scientific Co., Ltd., MS7-H550-Pro) for 5 minutes. This Li metal foil was quickly melted into a molten Li droplet on the hotplate. After fully molten of the Li metal, the previously obtained ZnO/TPP/GO host was transferred onto the hotplate. By gently bringing the edge of the host into contact with the molten Li, rapid infiltration of the liquid metal throughout the host structure was achieved within 10 seconds. Measured by the balance (Shanghai Shunyu Hengping Scientific Instrument Co., Ltd. FA2004, the measuring range is 200 g, the accuracy is 0.1 mg.) in the glovebox, about 2.72 mg cm-2 Li was calculated to be infused into the host, detailed mass ratios of each component are shown in Supplementary Table 1. The obtained electrodes were naturally cooled to 25 ± 5℃ for battery assembly inside the glovebox.

Preparation of the Li@TPP/GO control electrode

The control electrode (Li@TPP/GO) was fabricated in an argon-atmosphere glovebox (Mikrouna (Shanghai) Ind. Int. Tech. Co., Ltd., H₂O and O₂ content < 0.01 ppm). A piece of pure Li foil was affixed to the pre-formed TPP/GO foam disc to construct the electrode. The Li foils with two kinds of areal capacities/sizes were used (purchased from Shandong Zhongshan Photoelectric Materials Co., Ltd., the purity is 99.95%): 20 mAh cm⁻² (11.5 mm diameter) for symmetric cells and 4 mAh cm-² (15 mm diameter) for Li||NCM811 cells. The TPP/GO foam was correspondingly prepared as 11.5 mm or 15 mm discs as well. The finished electrode was stored within the glovebox at 25 °C ± 5 °C until battery assembly.

NCM811 positive electrode fabrication

The NCM811 positive electrode was fabricated by mixing 92% NCM811 powder (purchased from Nanjing Mojiesi Company, purity is 99%), 4% super carbon 65 (purchased from Canrd Co., Ltd., MA-EN-CO-0004, purity is 99.52%, average particle size is 150 nm), and 4% polyvinylidene difluoride binder (purchased from Canrd Co., Ltd., MA-EN-BI-0008, purity is 99.5%, molecular weight: ≥ 1.5 million) into N-methyl pyrrolidone (Beijing InnoChem Science & Technology Co., Ltd., purity is 99.5%, water content≤50 ppm) with automatic planetary centrifugal vacuum stirring defoaming machine (Shenzhen Zhidi Technology Co., Ltd., ZD-T600C) for one hour at atmospheric environment with a temperature of 25 ± 5°C to prepare a slurry. The slurry was then coated onto both sides of carbon coated aluminum foil (purchased from Canrd Co., Ltd., MA-EN-CU-0003. The thickness and purity of carbon-coated layers were 1 μm and 99.5% respectively. The thickness and purity of the uncoated Al foil were 15 μm and 99.6% respectively.) at atmospheric environment with a temperature of 25 ± 5°C, and dried in a vacuum oven (Shanghai Yiheng Scientific Instrument Co., Ltd., DZF-6021, 133 Pa) at 60 ± 1℃ for 24 hours. The calculated theoretical areal capacity of the NCM811 electrode was around 4.29 mAh cm-2 per surface (about 250 μm thick). The electrodes were cut into 4.5 × 5.7 cm2 and stored in the glove box at 25 ± 5°C (Mikrouna (Shanghai) Ind. Int. Tech. Co., Ltd., water and O content <0.01 ppm) for pouch cell long cycle, nail penetration and accelerating rate calorimetry (ARC) test.

Construction and electrochemical characterization of coin cells.

All the coin cells (CR2032, 304 stainless steel, Canrd Co., Ltd.), comprising the case (CR2032, Canrd Co., Ltd.) spacer (15.8 mm × 1.0 mm, Canrd Co., Ltd.) and funnel spacer (15.4 mm × 1.1 mm, Canrd Co., Ltd.), were assembled in an argon-filled glovebox (Mikrouna (Shanghai) Ind. Int. Tech. Co., Ltd., H₂O and O₂ content < 0.01 ppm) using Celgard 2325 separators (Canrd Co., Ltd., MA-EN-SE-000201 , porosity: 39%, avg. pore size: 0.028 μm, thickness: 25 μm, diameter: 19 cm). After assembly, the cells were allowed to rest for 24 hours at 25 ± 5°C prior to testing. All the subsequent electrochemical tests were conducted using a LAND battery test system (Wuhan Land Electronics Co., Ltd., CT3002A, 5 V, 1 mA & 10 mA 8C1U). During testing, the cells were placed in a constant-temperature environmental chamber (Wuhan Yueda Intelligent Technology Co., Ltd., model YDC-300HT) maintained at 25.0 ± 0.2°C.

Symmetric coin cells were fabricated using either conventional or after-burning pure Li (20 mAh cm-², purchased from Shandong Zhongshan Photoelectric Materials Co., Ltd., the purity is 99.95%), Li@TPP/GO or Li/ZnO/TPP/GO electrodes (11.5 mm diameter). Two types of electrolytes were employed: LP40FV (1M LiPF₆ in EC/DEC with 10% FEC and 1% VC) and LHCETTE (LiFSI:DME:TTE=1:1.2:3). The LP40FV electrolyte was formulated by combining 89 wt% of a 1 M LiPF₆ solution in EC/DEC (purchased from Canrd Co., Ltd., MA-EN-ET-0202; H₂O content ≤ 20 ppm, used without further purification or treatment) with 10 wt% fluoroethylene carbonate (FEC, purchased from Canrd Co., Ltd., MA-EN-ET-0893; purity is 99.95%, H₂O content≤ 50 ppm, used without further purification or treatment) and 1 wt% vinylene carbonate (VC, purchased from Canrd Co., Ltd., MA-EN-ET-0892; purity is 99.9%, H₂O content ≤ 30 ppm, used without further purification or treatment). The LHCETTE electrolyte was prepared by mixing lithium bis(fluorosulfonyl)imide (LiFSI, purchased from Canrd Co., Ltd., MA-EN-ET-0832, purity is 99.5%, H₂O content ≤ 100 ppm, used without further purification or treatment), 1,2-dimethoxyethane (DME, purchased from Beijing InnoChem Science & Technology Co., Ltd., purity is 99.5%, H₂O content ≤50 ppm, used without further purification or treatment), and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE, purchased from Canrd Co., Ltd., purity is 97%, pre-dried with 500 μm Li foil for 7 days until Li color stabilized). The components were mixed in a molar ratio of LiFSI:DME:TTE = 1:1.2:3. The Li plating/stripping stability of these symmetric cells was evaluated via galvanostatic cycling at current densities of 1.0 or 3.0 mA cm-² with capacity densities of 1.0 or 3.0 mAh cm-². Rate capability was tested by applying a sequence of current densities (0.5, 1, 2, 3, 4, 5, 6, and back to 0.5 mA cm-²), holding each for 10 cycles.

Li||host coin cells were fabricated with a working electrode consisting of either copper foil (Canrd Co., Ltd., MA-EN-CU-000202; purity: 99.99%, thickness: 9 μm, diameter: 16 mm), TPP/GO foam (thickness: 80 μm, diameter: 15 mm), or a ZnO/TPP/GO host. The ZnO/TPP/GO host was obtained by electrochemically stripping Li completely from the pre-obtained Li/ZnO/TPP/GO electrode, using a charging current of 0.2 mA until a voltage of 0.2 V was reached. A pure Li foil (Shandong Zhongshan Photoelectric Materials Co., Ltd.; purity: 99.95%, thickness: 500 μm, diameter: 11.5 mm) served as counter electrode, and 60 μL of LHCETTE electrolyte was used in each cell. The Coulombic efficiency (CE) of Li plating/stripping in the Li||host coin cells was determined by the ratio of discharging capacity to charging capacity. Furthermore, the long-term cycling stability of these cells was evaluated at a current density of 1 mA cm-², with a Li plating areal capacity set at 1 mAh cm-² and a Li stripping cut-off potential set at 150 mV. Additionally, Aurbach CE tests were conducted according to the following sequence: Deposite Li into the host (copper foil, TPP/GO foam or ZnO/TPP/GO host) at an area capacity of 5 mAh cm-2; Strip Li from it to a cut-off potential of 0.15 V; Deposite Li at 5 mAh cm-2 (capacity denoted as QA) again; Perform 10 cycles at a Li plating/stripping current density of 0.5 mA cm-2 and an areal capacity density of 0.5 mAh cm-2 (capacity denoted as QB); Strip Li to a cut-off potential of 0.15 V again (capacity denoted as QC). The average CE in Aurbach CE test was calculated using the formula: CE_average = (10QB + QC) / (10QB + QA)

Li||NCM811 coin cells were fabricated with the following components: NCM811 positive electrode (1.63 mAh cm-², purchased from Canrd Technology Co Ltd., SY33XX, punched into 12 mm discs using a manual slicer (Shenzhen Kejing Star Technology Co., Ltd., MSK-T10)), 4 mAh cm-² pure Li (Shandong Zhongshan Photoelectric Materials Co., Ltd., 99.95%), Li@TPP/GO or Li/ZnO/TPP/GO negative electrode, and 90 μL of ELHCETTE electrolyte. The pre-stripping/plating process was conducted at a constant current of 0.2 mA for both stripping and plating, with the stripping process terminated upon reaching a cut-off potential of 0.2 V. For initial activation, the full-cells were charged to 4.35 V and then discharged to 3.0 V at a specific current of 9.4 mA g-¹ for 2 cycles. Following activation, long-term cycling tests were carried out at a charge/discharge specific current of 94 mA g-¹. Rate capability was evaluated by varying the specific current in a sequence (37.6 - 94 - 188 - 376 - 564 - 940 - 564 - 376 - 188 - 94 - 37.6 mA g-¹), holding each for 5 cycles.

Electrochemical impedance spectroscopy (EIS) measurements were conducted with a CHI 760E electrochemical workstation. All tests were executed under potentiostatic conditions with an applied AC amplitude of 5 mV, scanning across a frequency spectrum of 100 kHz to 0.1 Hz. To ensure a stable baseline, the system was held at the open-circuit voltage for 10 minutes prior to each measurement.

Fabrication of Li||NCM811 pouch cells for long cycle test

The Li||NCM811 pouch cells used for long cycle test were assembled using 4.5 × 5.7 cm2 NCM811 positive electrode (4.29 mAh cm-2 (one side)) and 10 mAh cm-2 pure Li (Shandong Zhongshan Photoelectric Materials Co., Ltd., 99.95%) or Li/ZnO/TPP/GO in an argon-filled glovebox (Mikrouna (Shanghai) Ind. Int. Tech. Co., Ltd., water and O content <0.01 ppm) at 25 ± 5°C. The separator was 16 μm microporous polyethylene (PE) membrane (purchased from Canrd Technology Co Ltd.). The cell core was sealed in an aluminum plastic packing film (purchased from Canrd Co., Ltd., MA-EN-FC-001103), and LHCETTE electrolyte was injected into the pouch cell (by pipette) according to 2 g Ah-1, followed by sealing and gas releasing of the pouch cell using vacuum pre-sealing machine (Shenzhen Kejing Star Technology Company, MSK-115A-III). These cells were rested for 24 hours before 37.6/56.4 mA g-1 charging/discharging cycling between 3.0 V and 4.5 V at 25 ± 0.2°C. No external pressure is applied during the battery cycling process.

Fabrication of pouch cells for ignition test

The Li||NCM811 and graphite||NCM811 pouch cells used for the ignition test were assembled using 4.5 × 5.7 cm 2 NCM811 positive electrode (3.82 mAh cm-2, purchased from Canrd Technology Co Ltd.) and 4.5 × 5.7 cm2 pure Li (Shandong Zhongshan Photoelectric Materials Co., Ltd., 99.95%), Li/ZnO/TPP/GO or graphite (purchased from Canrd Technology Co Ltd.) in an argon-filled glovebox (Mikrouna (Shanghai) Ind. Int. Tech. Co., Ltd., water and O content <0.01 ppm). The separator was 12 μm microporous polyethylene (PE) membrane (purchased from Canrd Technology Co Ltd., MA-EN-SE-0007). The cell core was sealed in an aluminum plastic packing film (purchased from Canrd Co., Ltd., MA-EN-FC-001103), and 0.25 mL all-fluorinated electrolyte (1 M LiPF6 (purchased from Canrd Co., Ltd., MA-EN-ET-0840, purity is 99.95%, water content ≤50 ppm, used without any treatment)in a mixture of fluoroethylene carbonate /3,3,3-fluoroethyl methyl carbonate/1,1,2,2-tetrafluoro ethyl-2,2,2-trifluoro ethyl ether (FEC (purchased from Canrd Co., Ltd., MA-EN-ET-0893, purity is 99.95%, water content ≤50 ppm, used without any treatment):FEMC (purchased from Canrd Co., Ltd., MA-EN-ET-0894, purity is 99.95%, water content ≤20 ppm, used without any treatment):HFE (purchased from Canrd Co., Ltd., MA-EN-ET-0874, purity is 99.9%, water content ≤20 ppm, used without any treatment), 2:6:2 by weight)) electrolyte was injected into the pouch cell (by pipette), followed by sealing and gas releasing of the pouch cell using vacuum pre-sealing machine (Shenzhen Kejing Star Technology Company, MSK-115A-III). Before the ignition tests, these pouch cells were charged to 4.35 V at a specific current of 9.4 mA g-1 at 25 ± 0.2°C. No external pressure is applied during the battery charging process.

Safety performance test

The nail penetration test was carried out in a commercial nail penetration system (BE Nail Penetration Tester, BE-6047C-20T). A 5 mm diameter steel nail was driven at a speed of 25 mm s−1 through the Li metal pouch cells with 100% stage of charge (SOC). The thermal behavior of the Li metal pouch cells was characterized by an accelerating rate calorimeter (ARC HEL BTC-130) in a standard heat–wait–seek procedure.

Characterization

SEM images were taken with a scanning electron microscope (SEM, ZEISS EVD-18). Transmission electron microscopy (TEM) are collected with JEOL JEM-2100F. FIB-SEM images were taken through cross-sectioning sample with a Ga+ ion beam and observing with the electron beam on a focus ion beam-SEM (FIB-SEEM, Helios 5 CX). The composition of the samples was examined by X-ray diffraction (XRD, Bruker AXS D2 Phaser X-ray powder diffractometer), Cu Kα radiation (λ = 1.5418 Å). The XPS spectra of the SEI were measured by X-ray photoelectron spectroscopy (XPS, Thermo escalab xi+). The binding energies were calibrated with respect to the C 1s peak at 284.6 eV. All the cycled electrodes were rinsed with pure anhydrous DME solvent (purchased from Beijing InnoChem Science & Technology Co., Ltd., purity is 99.5%, water content ≤50 ppm without any treatment) to remove residual LiFSI, dried, and then sealed in the glovebox at 25 ± 5°C before being transferred using a vacuum transfer vessel for SEM, TEM and XPS characterizations. The infrared thermal imaging videos was shot by Huajingkang K26E25 online infrared thermal camera at atmospheric environment with a temperature of 25 ± 5°C.

Computational method

COMSOL simulation: The heat transfer behavior of dense Li metal and foam Li metal under continuous flame was simulated by finite element method in a three-dimensional model. Specifically, the solid fluid heat transfer module in COMSOL was used for calculation. The heat transfer inside the Li metal and between the Li metal and the surrounding gas followed the Fourier heat transfer law. The gas around the Li metal was set as air. The solution process used the Hex-Dominant hexahedron grid method, and the relative tolerance was set to 1 e-6 to ensure the accuracy of the solution results.

The gas diffusion behavior in three-dimensional space around the smart-flame-retarding layered Li metal (Li/ZnO/TPP/GO) under flame was simulated by finite element method. Specifically, Ansys Fluent was used for parameter setting and flow field solving. The continuity equation, momentum equation and energy equation were followed in the fluid mechanics calculation process. The VOF model based on Euler-Euler method was used for the multiphase flow model between gas mixtures, and k-ω SST model was used for turbulence model. The inlet was set as a polyphase inflow port, defined as a mass inflow port. The outlet was a pressure outlet, and the outlet gauge pressure was 0 Pa. The solution process used the Hex-Dominant hexahedron grid method, and the relative tolerance was set to 1 e-6 to ensure the accuracy of the solution results.

Density functional theory: Based on Density functional theory (DFT) and the calculation method of quantum mechanics, the DMol module in Materials Studio software was used to calculate the binding energy. The single point energy of Li, O2, PO·, Li2O after Li combining with O2 and Li3PO4 after Li combining with PO· were calculated respectively, and the differences were obtained to obtain binding energies. The formula is as follows:
ΔE1 = ELi2O - (ELi + EO2) ; ΔE2 = ELi3PO4 - (ELi + EPO·)

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "In this work, the authors report a layered composite Li anode (Li||ZnO||TPP||GO) designed to mitigate the extreme\nflammability of Li metal while maintaining its electrochemical functionality. They demonstrate that a ZnO interlayer prevents\nside reactions between Li and the flame retardant (TPP) under normal operation, whereas upon ignition, thermally triggered\ndecomposition of TPP releases gases that act as a flame retardant, thereby ensuring complete non-combustibility and\nimproved cycling stability. However, several critical scientific aspects have not been sufficiently addressed. First, although\nTPP is a well-known flame retardant and, it does not, by itself, provide sufficient originality. Moreover, the study does not\nsystematically examine how varying the TPP content relative to the electrolyte affects flame-retardant behavior, making it\ndifficult to assess the practical extinguishing effect in real cells. Second, real-cell combustion arises from both Li metal and\nthe flammable liquid electrolyte, but the pouch-cell experiments show no ignition at all, which weakens the credibility of the\nresults. Third, the explanation of the claimed transport properties and underlying mechanism is incomplete and requires\nadditional clarification. If TPP is truly sealed under ZnO during normal operation, rapid gas release on heating would\nrequire a defined permeation pathway or a thermally induced opening in that layer. The manuscript presents this only through\nsimulation, with insufficient explanation of the actual experimental evidence. Fourth, Li metal has an intrinsic melting point\nnear 180 ℃. Yet the data show no melting or flow behavior, and this complete absence is not convincingly explained by the\npresence of TPP, ZnO, or GO. Finally, the scalability and practical relevance of the ALD-based ZnO interlayer and the multi-\nlayer host architecture are not addressed at a level commensurate with the strength of the safety claims.\nIn summary, although the idea of integrating a sealed flame-retardant layer with a lithiophilic interlayer within a porous host\nis interesting and the dataset is extensive, the materials advance is limited, the key mechanism is insufficiently supported,\nand several headline claims appear overstated. Therefore, this manuscript is not suitable for publication in Nature\nCommunications. Suggestions for improvement are provided as follows:\n1. Cell ignition stems not only from the lithium-metal anode but also from the flammable electrolyte. However, the developed\nanode does not specify the amount of TPP relative to the electrolyte. Please quantify the TPP loading versus electrolyte\nvolume/mass and explain how varying TPP content influences the observed flame-retardant effect.\n2. The anode is claimed to provide both structural flame-retardant behavior and chemical extinguishing via TPP, but the\ncurrent explanation does not clearly decouple these effects. Please clarify the respective contributions and operating\nwindows of the structural design and the TPP-based chemical mechanism.\n3. While the flame-retardant mechanism of TPP is broadly known, the description of how TPP functions in the presence of\nlithium metal is insufficient. Please provide additional rationale or evidence supporting the proposed mechanism under Li-\nmetal conditions.\n4. Under normal conditions, TPP is described as residing between the ZnO interlayer and Li to suppress side reactions.\nHowever, it remains unclear how TPP is released upon heating and how that release contributes to flame retardancy.\nPlease include additional experimental results and mechanistic explanation.\n5. In general, TPP requires substantial chemical transformation and gas release before delivering a flame-retardant effect.\nHowever, in this study, ignition was absent or extinguished almost immediately. Because TPP decomposition typically\nneeds time and elevated temperature, please explain why, in this design, TPP can act nearly instantaneously.\n6. Lithium metal melts at ~180 ℃, a fundamental property that should persist regardless of structure. Yet no melting behavior\nis reported here. Please provide the proposed mechanism and observations with Li’s intrinsic melting point.\n7. ZnO and GO are designed to suppress electrochemical side reactions between TPP and Li. If so, TPP-related changes\nshould be consistently detectable after combustion without regard to ZnO or GO. However, post-ignition XPS for\nLi@TPP||GO shows no phosphorus signal, unlike Li||ZnO||TPP||GO. Please explain in more detail how these post-\ncombustion analyses support the claimed flame-retardant action.\n8. A pouch cell contains Li metal, a cathode, and a liquid electrolyte, and flammable electrolytes are major factor driver of\nthermal runaway. Despite this, the pouch-cell tests showed no ignition. Please explain how electrolyte combustion was\nprevented and comment on the robustness and reliability of the pouch-cell experiments.\n9. Please discuss whether the proposed mechanism and results are realistically reproducible at large-format cell or pack\nscales. Also elaborate on the design’s sufficiency for scale-up and integration into practical cells.\n10. Given dataset limitations and the absence of certain key comparisons, please provide additional explanation to ensure\nthat safety and performance claims are not overstated.", "reviewer_2": "This work by Qi et al. proposes a smart-flame-retarding layered host design for safe Li metal battery. This host achieves self-\nfire-extinguishing function of Li metal anode even under extreme ignition conditions and completely seals the flame retardant\nfrom Li metal corrosion for durable battery operation. Their design does solve the extreme flammability challenge of Li metal\nanode that has hindered the practical application of lithium metal batteries for decades. Their anode flame-retardant effect,\nespecially for pouch cells which remained unignited after being continuously exposed to fire for 20 seconds, is very\nimpressive. Based on my research experience, there has never been any study that can address the flammability issue of Li\nmetal anode-everyone in this research field knows this serious concern, but nobody has ever solved it. This work is the first-\never demonstration, so I believe this work will become a meaningful breakthrough in the whole Li metal battery field, giving\nother scientists new ideas on how to address the safety issue of Li metal anode. Therefore, I strongly recommend\nconsidering publishing this manuscript in Nature Communications, after addressing following comments:\n1. From Supplementary Figure 8, the Li||ZnO||TPP||GO electrode still illustrates an excellent SET of 0 s/g even after a\nsustained 10 seconds of constant ignition. This result can be more convincing if the authors provide infrared thermal images\nof multiple repeating samples into supplementary information to further confirm this excellent effect.\n2. As shown in Figure 4d, the Li||LiNi0.8Co0.1Mn0.1O2 (NCM811) full-cell using after-burning Li||ZnO||TPP||GO anode\nexhibits a significant improvement in capacity retention and cycle life. It would be better to present the CE comparison of the\nthree kinds of full-cells to further support this advantage.\n3. In the full cells test under regular operating conditions, stripping Li completely from Li||ZnO||TPP||GO anode in half-cell is\nthe first step towards achieving the same RN/P of the three full cells. It would be better to conduct XRD tests on the host at\nthe fully stripped state for confirming the complete stripping of Li, thereby demonstrating the strict control of the RN/P.\n4. Supplementary Figure 15 exhibits the uniform Li deposition morphology of after-burning Li||ZnO||TPP||GO electrode. But\nSEM characterizations on the dendrites are known to be highly erratic. It would be better to add more reproducing SEM\ncharacterization results for better supporting the authors‘ declaration.\n5. The authors demonstrated that their design effectively promotes the uniform deposition of Li through the absence of\ndendrites in Figure 5h. To confirm this effect, it would be best to conduct more reproducible SEM characterization and add\nthem into Supplementary Information.\n6. The authors declare that the lithiophilic properties of ZnO-converted LiZn interlayer can facilitate uniform and dendrite-free\nLi deposition. It would be better to cite more references and add some discussions to support this claim.\n7. For a manuscript that primarily focuses on safety, it is necessary to include validation through tests such as pouch cell nail\npenetration and accelerating rate calorimetry (ARC) measurements, in addition to ignition experiments.\n8. Figure S16 should provide the equivalent circuit diagram used for impedance fitting, and Figure S16b should be\nappropriately enlarged to improve the clarity of the manuscript.\n9. The annotations within Figure S21 are incorrect (0.5 mA, 0.5 mAh). The author should revise them clearly or remove the\nannotations altogether.\n10. This manuscript focuses on anode protection, with the cathode playing a secondary role. However, comprehensive\ncharacterization is essential for a high-level publication; therefore, appropriate cathode characterization should be included.\n11. The protection of the lithium anode in this manuscript requires the introduction of a large number of inactive materials\nsuch as GO, ZnO, and TPP. A major concern is whether the addition of these materials will significantly reduce the\ngravimetric and volumetric energy densities of the lithium anode. The authors should provide detailed calculations to\naddress this issue.\n12. The manuscript only presents combustion safety tests on pouch cells, but no performance evaluation. The authors\nshould supplement the work with long-cycle performance tests of high-energy-density pouch cells to enhance the impact of\nthe article."}, "author_response": "The authors are grateful to the reviewers for their constructive comments on the\noriginal manuscript. All reviewers’ comments were carefully considered and\nimplemented/responded to in the revised manuscript. A detailed response is given for\neach comment. We italicized the reviewers’ comments and suggestions and highlight\nour responses in purple for clarity.\nReviewer #1 (Remarks to the Author):\nComments:\nIn this work, the authors report a layered composite Li anode (Li||ZnO||TPP||GO)\ndesigned to mitigate the extreme flammability of Li metal while maintaining its\nelectrochemical functionality. They demonstrate that a ZnO interlayer prevents side\nreactions between Li and the flame retardant (TPP) under normal operation, whereas\nupon ignition, thermally triggered decomposition of TPP releases gases that act as a\nflame retardant, thereby ensuring complete non-combustibility and improved cycling\nstability. However, several critical scientific aspects have not been sufficiently\naddressed. First, although TPP is a well-known flame retardant and, it does not, by\nitself, provide sufficient originality. Moreover, the study does not systematically\nexamine how varying the TPP content relative to the electrolyte affects flame-retardant\nbehavior, making it difficult to assess the practical extinguishing effect in real cells.\nSecond, real-cell combustion arises from both Li metal and the flammable liquid\nelectrolyte, but the pouch-cell experiments show no ignition at all, which weakens the\ncredibility of the results. Third, the explanation of the claimed transport properties and\nunderlying mechanism is incomplete and requires additional clarification. If TPP is\ntruly sealed under ZnO during normal operation, rapid gas release on heating would\nrequire a defined permeation pathway or a thermally induced opening in that layer. The\nmanuscript presents this only through simulation, with insufficient explanation of the\nactual experimental evidence. Fourth, Li metal has an intrinsic melting point near\n180 ℃. Yet the data show no melting or flow behavior, and this complete absence is not\nconvincingly explained by the presence of TPP, ZnO, or GO. Finally, the scalability and\npractical relevance of the ALD-based ZnO interlayer and the multi-layer host\narchitecture are not addressed at a level commensurate with the strength of the safety\nclaims.\nIn summary, although the idea of integrating a sealed flame-retardant layer with a\nlithiophilic interlayer within a porous host is interesting and the dataset is extensive,\nthe materials advance is limited, the key mechanism is insufficiently supported, and\nseveral headline claims appear overstated. Therefore, this manuscript is not suitable\nfor publication in Nature Communications. Suggestions for improvement are provided\nas follows:\nResponse:\nWe thank you for the comments. We also thank the reviewer for many valuable\nand constructive suggestions to improve our paper. In the following, we provide our\npoint-by-point responses to each of the comments.\nRegarding your general comments, here we also provide several point-to-point\nexplanations for your comments as following:\n(1) Reply to “although TPP is a well-known flame retardant and, it does not, by\nitself, provide sufficient originality. Moreover, the study does not systematically\nexamine how varying the TPP content relative to the electrolyte affects flame-retardant\nbehavior, making it difficult to assess the practical extinguishing effect in real cells.”:\nRegarding the reviewer’s originality concern, here we want to express that the\nmost important originality and novelty is the achievement of compatibility between the\nflame retardant and metal Li in the electrode preparation and the battery cycling process,\nas well as the smart flame-retardant effect of the Li metal anode, rather than the type of\nthe flame retardant. The originality of our works lies in the first-ever report of a self-\nfire-extinguishing composite Li metal anode that withstands tens seconds of ignition by\nour layered composite electrode design. Therefore, choosing TPP as the flame retardant\ndoes not affect the originality of our work. In addition, the systematical study on how\nvarying the TPP content relative to the electrolyte affects flame-retardant behavior is\nalso added in this revision, as detailed discussed in the response to Comment 1.\n(2) Reply to “real-cell combustion arises from both Li metal and the flammable\nliquid electrolyte, but the pouch-cell experiments show no ignition at all, which weakens\nthe credibility of the results”:\nRegarding this question, we would like to apologize that we do not emphasize\nenough in our manuscript that the electrolyte we used in the pouch cells has already\nbeen non-flammable (1 M LiPF in a mixture of fluoroethylene carbonate /3,3,3-\n6\nfluoroethylmethyl carbonate/1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether\n(FEC:FEMC:HFE, 2:6:2 by weight), proposed by Fan et al. in Nature Nanotech, 2018,\n13, 715–722, in both pure Li metal anode-based pouch cell and composite Li metal\nanode-based pouch cell). This non-flammable electrolyte has a self-extinguishing time\nof 0 s g-1 and exhibits excellent combustion suppression properties in lithium-ion\nbatteries. Therefore, the possibility of the electrolyte flammability is excluded, which\nconfirms the robustness and reliability of the pouch-cell experiments. Detailed\ndiscussions are also added in the response to Comments 1 and 8.\n(3) Reply to “the explanation of the claimed transport properties and underlying\nmechanism is incomplete and requires additional clarification. If TPP is truly sealed\nunder ZnO during normal operation, rapid gas release on heating would require a\ndefined permeation pathway or a thermally induced opening in that layer. The\nmanuscript presents this only through simulation, with insufficient explanation of the\nactual experimental evidence”:\nRegarding this question, we add image capture of the digital videos into the\nmanuscript when the composite Li anode during annealing as strong evidence. These\nimages clearly capture releases of gas from the composite electrode upon heating. We\nalso add the SEM images and XPS characterizations of the ignited Li||ZnO||TPP||GO\nelectrode as strong evidences, showing newly appeared micrometer-scale pores and\nflame retardant-derived phosphate-containing surface that support the mechanism of\nflame-retardant release from inside the electrode to the surface. Detailed discussions\nare also added in the response to Comment 4. We believe these new experimental results\nand discussions can prove strong evidence for our claim.\nSupplementary Figure 15. Digital photos of the flame-retarding gas released during\ncontinuous ignition of Li||ZnO||TPP||GO electrodes (a-c) and Li||ZnO||TPP||GO ||\nNCM811 pouch cells (d-f).\n(4) Reply to “Li metal has an intrinsic melting point near 180 ℃. Yet the data\nshow no melting or flow behavior, and this complete absence is not convincingly\nexplained by the presence of TPP, ZnO, or GO”:\nRegarding this question, we provide explanations from two aspects: Firstly, the\nthree-dimensional aerogel structure of the host with high thermal dissipation effect can\nreduce the thermal conductivity of the electrode, thereby delaying the temperature\nincreasement inside the electrode compared to the ignition site and the melting of the\ninternal Li metal. Secondly, the three-dimensional structure of the host with high\nlithiophilicity can adhere the locally melted Li metal against liquid flow, thereby the\nphenomenon of flowing liquid-state molten Li metal cannot be observed. We also add\nan infrared temperature distribution mapping image of the electrode after being ignited\nfor 10 seconds and a magnified plot of the DSC curve of the electrode to prove the\ndelay of temperature increasement and the actual occurrence of Li melting. Detailed\ndiscussions are also added in the response to Comment 6.\n(5) Reply to “the scalability and practical relevance of the ALD-based ZnO\ninterlayer and the multi-layer host architecture are not addressed at a level\ncommensurate with the strength of the safety claims”:\nRegarding this question, as there has been almost no previous research on flame-\nretardant designs targeting Li metal anodes and no corresponding mechanism\nexploration, our work focuses on proposing the anode flame-retardant mechanism and\ndemonstrating the flame-retardant effect of laboratory-scale Li metal anode and Li\nmetal batteries. Furthermore, the thermal insulation of aerogel structure and Li atom\ncapture mechanisms we proposed are theoretically independent of the electrode size.\nTherefore, we speculate that these mechanisms and results are also applicable in large-\nscale batteries. Additionally, because the raw materials used in our anode preparation\nprocess are highly commercialized, and the industrial production technologies are\nmature, this design has great potential for large-scale manufacturing and practical\napplication. We also demonstrate more large-scale electrodes to prove the scalability\nand practical relevance of our design, which is already feasible for pouch cell\nfabrication. Detailed discussions are also added in the response to Comment 9.\nComments: 1) Cell ignition stems not only from the lithium-metal anode but also from\nthe flammable electrolyte. However, the developed anode does not specify the amount\nof TPP relative to the electrolyte. Please quantify the TPP loading versus electrolyte\nvolume/mass and explain how varying TPP content influences the observed flame-\nretardant effect.\nResponse:\nWe thank the reviewer for raising this suggestion. Firstly, we would like to\napologize that we do not emphasize enough in our manuscript that the electrolyte we\nused in the pouch cells (both pure Li metal anode and composite Li metal anode) has\nalready been non-flammable electrolyte (1 M LiPF in a mixture of fluoroethylene\n6\ncarbonate /3,3,3-fluoroethylmethyl carbonate/1,1,2,2-tetrafluoroethyl-2,2,2-trifluoro\nethyl ether (FEC:FEMC:HFE, 2:6:2 by weight), Nature Nanotech. 2018, 13, 715–722)\nin Figure 4h and i of the main text. We are grateful for this suggestion from the reviewer,\nwhich has reminded us to clarify this point. We also add the ignition test on this non-\nflammable electrolyte into Supplementary Fig. 27 and Movie 17 to prove its excellent\nnon-flammability.\nRegarding the reviewer’s concerns on “Cell ignition stems not only from the\nlithium-metal anode but also from the flammable electrolyte.” Here we add new\nliterature citations from Nat. energy, 2025, 10, 1382–1390 and Adv. Energy Mater. 2023,\n13, 2203144., which discover that the lithium oxidation reaction is main cause for\nbattery thermal runaway, even under conditions of solid electrolytes with cracked\nseparators and non-flammable liquid electrolytes. This emphasizes the importance of\nour composite Li anode design for controlling lithium oxidation reaction and Li metal\nbattery safety.\nFollowing the reviewer’s suggestion, we calculate the mass and volume ratios of\nTPP and the non-flammable electrolyte in the pouch cells used for the ignition test, the\nspecific data are shown in Supplementary Table 2. As for the influences of the varying\nTPP content on the flame-retardant effect, we have demonstrated self-extinguishing\ntime of electrodes with different TPP loading in Supplementary Fig. 7. In order to study\nits effect on the flame retardancy of the pouch cells, we reduce the content of TPP in\nthe anode by half, and conducted a new burning test on the Li metal pouch cell using\nthis anode. We find that the anode and pouch cell began to burn after being continuously\nignited for 8 seconds, which confirmed the optimality of the TPP content in our design\n(Supplementary Fig. 31 and Movie 24).\nTo further confirm that it is our composite Li anode design (rather than non-\nflammable electrolyte) that plays a decisive role in the flame-retardant effect of the\npouch cell, we assemble a new pouch cell using pure Li anode and doubled the amount\nof the non-flammable electrolyte (500 mL, 7.8 g Ah-1) for ignition test. As shown in\nSupplementary Fig. 32 and Movie 25, this cell began to burn after being ignited for 2\nseconds, and the flame lasts for tens of seconds that is similar to previous Li metal\npouch cell, proving the minor effect of non-flammable electrolyte for controlling\nlithium oxidation reaction and battery combustion that is well consistent with the results\nof severe combustion in Li metal battery using non-flammable liquid and solid\nelectrolyte in Nat. Energy, 2025, 10, 1382–1390 and Adv. Energy Mater. 2023, 13,\n2203144.\nChange to the Supplementary Information:\nSupplementary Figure 27. Digital photos of glass fibers soaked in non-flammable\n(1 M LiPF in a mixture of fluoroethylene carbonate /3,3,3-fluoroethylmethyl\n6\ncarbonate/1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether (FEC:FEMC:HFE, 2:6:2\nby weight)) electrolyte under constant ignition.\nTPP (12.4 wt% in Electrolyte Ratio\nanode) (TPP: electrolyte)\nMass (g) 0.02322 0.39 0.0595:1\nVolume (cm3) 0.01836 0.25 0.0734:1\nSupplementary Table 2. The content of TPP and electrolyte in the pouch cell used for\nthe ignition test.\nSupplementary Figure 7. Self-extinguishing time of electrodes with different TPP\nmass loading.\nSupplementary Figure 31. Digital photos of pouch full-cell using Li||ZnO||TPP||GO\nanode with same electrolyte amount (250 mL, 3.9 g Ah-1) but half TPP content (6.6%\nin anode) under constant ignition. After reducing the TPP content, the pouch cell could\nnot achieve the original excellent flame-retardant effect, proving the optimality of the\nTPP content in our design.\nSupplementary Figure 32. Digital photos of pouch full-cell using pure Li anode with\ndouble amount of non-flammable electrolyte (500 mL, 7.8 g Ah-1) under constant\nignition. Even if the content of the non-flammable electrolyte is increased, the pouch\nbattery with pure Li as the anode still cannot achieve the flame-retardant effect, proving\nthat it is our anode design that plays a decisive role in the flame-retardant effect of the\npouch cell.\nChange to the Manuscript:\nPage 15, line 16:\nAs shown in Fig. 4g, we evaluate the safety property by recording the light-emitting\ndiode (LED)-powering function of a 100 mAh Li metal battery using non-flammable\nelectrolyte (1 M LiPF in a mixture of fluoroethylene carbonate /3,3,3-\n6\nfluoroethylmethyl carbonate/1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether\n(FEC:FEMC:HFE, 2:6:2 by weight)) 7 when the Li metal anode in the battery core was\nexposed to external fire through the damage site in package (Supplementary Fig 27 and\nMovie 17).\nPage 16, line 4:\nThis excellent flame-retardant effect of the Li metal pouch cell should be attributed to\nour electrode design against lithium oxidation reaction, rather than the non-flammable\nelectrolyte (Supplementary Table 2, Supplementary Figs. 31, 32 and Movies 24, 25)\n36,37.\nComments: 2) The anode is claimed to provide both structural flame-retardant\nbehavior and chemical extinguishing via TPP, but the current explanation does not\nclearly decouple these effects. Please clarify the respective contributions and operating\nwindows of the structural design and the TPP-based chemical mechanism.\nResponse:\nWe thank the reviewer for raising this suggestion. Following the reviewer’s\nsuggestion, we clarify the respective operating windows and contributions of the\nstructural design and the TPP-based chemical mechanism. At the beginning of the\nignition test (around 5 seconds), only the thermal insulation effect of the three-\ndimensional structure was effective to prevent the thermal conduction to the whole\nelectrode, which is evidenced by the relatively lower temperature of most parts in the\nelectrode (except the ignition site) (Supplementary Fig. 10). After several seconds of\nignition, as the temperature gradually reached the decomposition temperature of TPP\n(at about 200 ℃) at the ignition site, the thermal decomposition and release of TPP-\nbased flame retarding reaction start to take effect, and work together with three-\ndimensional structure to exert the flame-retardant effect.\nTo clarify the respective contributions of the structural design and the TPP-based\nchemical mechanism, we conducted ignition tests on the electrodes that only have a\nthree-dimensional structure but do not contain TPP (Li||ZnO||GO in Supplementary Fig.\n7), as well as those that contain TPP but do not have a three-dimensional structure\n(Li@TPP||GO in Supplementary Fig. 7). As clearly shown and compared in the\nSupplementary Movies, the pure Li start to burn after 0.9 seconds of ignition (Fig. 3c\nand Supplementary Movie 9), while the three-dimensional Li||ZnO||GO electrode start\nto burn after 5.5 seconds of ignition due to the low thermal conduction and high thermal\ndissipation properties (Supplementary Fig. 16 and Movie 15), while the TPP-containing\nLi@TPP||GO electrode starts to burn after 0.5 seconds of ignition with clear observation\nof TPP-derived smoke (Fig. 3d and Movie 10). In terms of self-extinguishing time\n(SET), the average SET values of the two kinds of control electrodes are 691.76 and\n744.31 s g-1 respectively, which are also lower than pure Li metal (979.27 s g-1). Due to\nthe absence of the TPP-based non-flammable gas release/free radical capture functions\nin the former electrode and the absence of heat transfer delay effect provided by the\nthree-dimensional structure in the later electrode, neither of these two control electrodes\ncan achieve a satisfactory fire-retardant effect. These results demonstrate the respective\ncontributions and synergetic effect of three-dimensional structures and TPP flame\nretardant for safe Li metal anode.\nChange to the Supplementary Information:\nSupplementary Figure 10. Infrared thermal image of Li||ZnO||TPP||GO electrode after\na sustained 5 seconds of constant ignition.\nSupplementary Figure 16. Digital photos of Li||ZnO||GO electrode (only have a\nthree-dimensional structure but do not contain TPP) under ignition conditions.\nSupplementary Figure 7. Self-extinguishing time of electrodes with different TPP\nmass loading.\nSupplementary Note 2: The respective flame-retardant contributions of the\nstructural design and the TPP-based chemical mechanism\nTo clarify the respective contributions of the structural design and the TPP-based\nchemical mechanism, we conducted ignition tests on the electrodes that only have a\nthree-dimensional structure but do not contain TPP (Li||ZnO||GO in Supplementary Fig.\n7), as well as those that contain TPP but do not have a three-dimensional structure\n(Li@TPP||GO in Supplementary Fig. 7). As clearly shown and compared in the\nSupplementary Movies, the pure Li start to burn after 0.9 seconds of ignition (Fig. 3c\nand Supplementary Movie 9), while the three-dimensional Li||ZnO||GO electrode start\nto burn after 5.5 seconds of ignition due to the low thermal conduction and high thermal\ndissipation properties (Supplementary Fig. 16 and Movie 15), while the TPP-containing\nLi@TPP||GO electrode starts to burn after 0.5 seconds of ignition with clear observation\nof TPP-derived smoke (Fig. 3d and Supplementary Movie 10). In terms of self-\nextinguishing time (SET), the average SET values of the two kinds of control electrodes\nare 691.76 and 744.31 s g-1 respectively, which are also lower than pure Li metal\n(979.27 s g-1). Due to the absence of the TPP-based non-flammable gas release/free\nradical capture functions in the former electrode and the absence of heat transfer delay\neffect provided by the three-dimensional structure in the later electrode, neither of these\ntwo control electrodes can achieve a satisfactory fire-retardant effect. These results\ndemonstrate the respective contributions and synergetic effect of three-dimensional\nstructures and TPP flame retardant for safe Li metal anode.\nChange to the Manuscript:\nFigure 3c. Infrared thermal images and digital images of pure Li electrode under\nignition conditions.\nFigure 3d. Infrared thermal images and digital images of Li@TPP||GO electrode under\nignition conditions.\nPage 10, line 5:\nThe first mechanism at the initial stage of ignition is the low thermal conduction and\nhigh thermal dissipation-induced flame-retardant effect of the porous aerogel structure\n27, as evidenced by the infrared thermal mapping showing relatively lower temperature\nof most parts in the electrode than ignition site after several seconds of ignition\n(Supplementary Figs. 10 and 11).\nPage 10, line 23:\nAs the electrode temperature rises, the second mechanism involves the releasing of TPP\ndecomposition-derived flame-retardant gases around Li metal to isolate it from oxygen\ngas, as evidenced by the observed flame-retardant gas release (Supplementary Figs. 14\nand 15). Two control samples containing three-dimensional structure or flame-retardant\ncomposition only illustrate slightly modified yet insufficient flame-retarding effect\n(Supplementary Fig. 16, Movie 15 and Note 2), demonstrating the respective\ncontributions and synergetic effect of three-dimensional structures and TPP flame\nretardant for safe Li metal anode.\nComments: 3) While the flame-retardant mechanism of TPP is broadly known, the\ndescription of how TPP functions in the presence of lithium metal is insufficient. Please\nprovide additional rationale or evidence supporting the proposed mechanism under Li-\nmetal conditions.\nResponse:\nWe thank the reviewer for raising this great suggestion. Following the reviewer’s\nsuggestion, we refer to more literature to study how TPP functions in the presence of\nlithium metal, and propose that it has a similar flame-retardant mechanism as in the\npolymer combustion (Polym. Degrad. Stab., 2024, 223, 110735; Powder Technol., 2019,\n354, 71-81; Soft Sci. 2022, 2, 21; J. Therm. Anal. Calorim., 2018, 132, 263–273;\nCombust. and Flame, 2024, 266, 113534; Compos. Part B: Eng., 2025, 304, 112641):\nThe TPP is annealed and decomposes in the flame to produce PO·. It has a higher ability\nto capture Li atoms than O·, and can capture the free Li atoms in the combustion zone\n(similar to the capture of H·), interrupting the combustion chain reaction between Li\nand O , thereby achieving the effect of flame retardancy (Supplementary Fig. 19). We\n2\nalso physically mixed the TPP and metallic Li through repeated rolling-folding\nprocesses, and conducted XRD tests on the combustion products of the mixture.\nThe appearance of the Li PO characteristic peak proved the capture of Li atoms by\n3 4\nPO· (Supplementary Fig. 20).\nChange to the Supplementary Information:\nSupplementary Figure 19. Schematic diagram of the proposed free radical capture\nmechanism of the TPP against combustion of lithium metal.\nSupplementary Figure 20. XRD pattern of the combustion products of the mixture of\nTPP and metallic Li.\nChange to the Manuscript:\nPage 11, line 20:\nLast but not least, density functional theory (DFT) simulations illustrate the binding\nenergy between Li atoms and PO· radicals (-3.15 eV, derived from TPP thermal\ndecomposition 29) is much higher than that with O· radicals (-2.48 eV), supporting the\nprominent free Li atoms-capturing ability of PO· for interrupting the combustion chain\nreaction between Li and O , thereby achieving the flame-retardant effect (Fig. 3k and\nof TPP and metallic Li and the X-ray photoelectron spectroscopy (XPS) depth profiles\nof the Li||ZnO||TPP||GO electrode after ignition both display characteristic peaks of\nLi PO (55.6 eV in Li 1s spectra, 133.6 eV in P 2p spectra and 531.4 eV in O 1s spectra\n3 4\n34,35), which is the reaction product of PO· with Li atom, supporting this flame-retardant\nmechanism (Supplementary Figs. 20 and 21).\nComments: 4) Under normal conditions, TPP is described as residing between the ZnO\ninterlayer and Li to suppress side reactions. However, it remains unclear how TPP is\nreleased upon heating and how that release contributes to flame retardancy.\nPlease include additional experimental results and mechanistic explanation.\nResponse:\nWe thank the reviewer for raising this suggestion. Following the reviewer’s\nsuggestion, we provide a detailed mechanism explanation of the release process of TPP\nand add corresponding experimental results into supplementary information:\n(1) At room temperature, TPP is encapsulated within the ZnO layer, thereby\npreventing its side reactions with Li metal.\n(2) When heated beyond 200 ℃, TPP will gradually decompose into PO· vapors.\nThese gas release break the outer ZnO and Li metal layers, occupying the space on the\nelectrode surface and thereby isolating it from O , and the PO· vapors can capture Li\n2\natoms to interrupt the combustion chain reaction between O and Li metal, achieving\n2\nthe flame-retardant effect (Supplementary Fig. 14).\n(3) This gas release phenomenon can be directly observed by the sinking gas\nproduced during the electrode ignition process (Supplementary Fig. 15), and\ndemonstrated by the small micrometer-scale holes on the surface of the electrode after\nignition (Supplementary Fig. 18 and Fig. 3i) (J. Therm. Anal. Calorim., 2018, 132, 263–\n273). The Li atom capture mechanism can be also proved by the appearance of Li PO\n3 4\non the electrode surface after ignition (Supplementary Fig. 21).\nChange to the Supplementary Information:\nSupplementary Figure 14. Schematic diagram of the release and decomposition\nprocess of TPP in the electrode.\nSupplementary Figure 15. Digital photos of the flame-retarding gas released during\ncontinuous ignition of Li||ZnO||TPP||GO electrodes (a-c) and Li||ZnO||TPP||GO ||\nNCM811 pouch cells (d-f).\nSupplementary Figure 18. Top-view SEM images of Li||ZnO||TPP||GO electrode after\nignition, showing abundant newly generated pores and channels that prove the thermal-\ntriggered release of flame-retarding vapors from inside the electrode.\nSupplementary Figure 21. The composition of Li||ZnO||TPP||GO electrode after\nignition. a XPS depth profile analysis on the composition of the Li||ZnO||TPP||GO after\nignition. b Reconstructed chemical composition and distribution of the\nLi||ZnO||TPP||GO after ignition from the XPS depth profiles.\nChange to the Manuscript:\nPage 10, line 23:\nAs the electrode temperature rises, the second mechanism involves the releasing of TPP\ndecomposition-derived flame-retardant gases around Li metal to isolate it from oxygen\ngas, as evidenced by the observed flame-retardant gas release (Supplementary Figs. 14\nand 15).\nPage 11, line 11:\nNumerous micro-scale holes generated on the surface of Li||ZnO||TPP||GO electrode\nconfirm the successful flame-retardant gases release during ignition 28 (Fig. 3i and\nSupplementary Fig. 18).\nComments: 5) In general, TPP requires substantial chemical transformation and gas\nrelease before delivering a flame-retardant effect. However, in this study, ignition was\nabsent or extinguished almost immediately. Because TPP decomposition typically\nneeds time and elevated temperature, please explain why, in this design, TPP can act\nnearly instantaneously.\nResponse:\nWe thank the reviewer for raising this great suggestion. Following the reviewer’s\nsuggestion, we provide the following explanations:\nFirstly, due to the low heat conduction and high thermal dissipation effect of the\nthree-dimensional structural design, the time required for the electrode to reach the\ntemperature of TPP decomposition and Li metal combustion is prolonged during\nignition, as illustrated by the infrared thermal imaging in Supplementary Fig. 10.\nTherefore, TPP does not act instantaneously at the beginning of the ignition test, but\nrather begins to function only after the temperature gradually rises to its decomposition\ntemperature, which can be supported by the TGA result in Figure 3h.\nSecondly, even if no combustion occurs after initial seconds, there will still be\nTPP-related chemical transformation and gas release as the temperature rises after\nseveral seconds of ignition. The chemical transformation can be verified by XPS depth\nprofile analysis of the after-burning electrode (Supplementary Fig. 21). The gas release\ncan be directly observed through the images during the ignition process\n(Supplementary Fig. 15), and it can also be proved by the small holes on the after-\nburning electrode (Fig. 3i and Supplementary Fig. 18).\nChange to the Supplementary Information:\nSupplementary Figure 10. Infrared thermal image of Li||ZnO||TPP||GO electrode after\na sustained 5 seconds of constant ignition.\nSupplementary Figure 21. The composition of Li||ZnO||TPP||GO electrode after\nignition. a XPS depth profile analysis on the composition of the Li||ZnO||TPP||GO after\nignition. b Reconstructed chemical composition and distribution of the\nLi||ZnO||TPP||GO after ignition from the XPS depth profiles.\nSupplementary Figure 15. Digital photos of the flame-retarding gas released during\ncontinuous ignition of Li||ZnO||TPP||GO electrodes (a-c) and Li||ZnO||TPP||GO ||\nNCM811 pouch cells (d-f).\nSupplementary Figure 18. Top-view SEM images of Li||ZnO||TPP||GO electrode after\nignition.\nChange to the Manuscript:\nPage 10, line 5:\nThe first mechanism at the initial stage of ignition is the low thermal conduction and\nhigh thermal dissipation-induced flame-retardant effect of the porous aerogel structure\n27, as evidenced by the infrared thermal mapping showing relatively lower temperature\nof most parts in the electrode than the ignition site after 5 seconds of ignition\n(Supplementary Figs. 10 and 11).\nPage 10, line 23:\nAs the electrode temperature rises, the second mechanism involves the releasing of TPP\ndecomposition-derived flame-retardant gases around Li metal to isolate it from oxygen\ngas, as evidenced by the observed flame-retardant gas release (Supplementary Figs. 14\nand 15).\nPage 11, line 11:\nNumerous micro-scale holes generated on the surface of Li||ZnO||TPP||GO electrode\nconfirm the successful flame-retardant gases release during ignition 28 (Fig. 3i and\nSupplementary Fig. 18).\nComments: 6) Lithium metal melts at ~180 ℃, a fundamental property that should\npersist regardless of structure. Yet no melting behavior is reported here. Please provide\nthe proposed mechanism and observations with Li’s intrinsic melting point.\nResponse:\nWe thank the reviewer for raising this suggestion. Following the reviewer’s\nsuggestion, we provide the following explanations:\nFirstly, the three-dimensional aerogel structure of the host can reduce the thermal\nconductivity of the electrode with high thermal dissipation effect, resulting in a delay\nof the temperature increasement in the interior of the electrode compared to the ignition\nsite. Consequently, it delays the melting of the internal Li metal. The infrared\ntemperature distribution mapping image of the electrode after being ignited by an open\nflame for 10 seconds can prove that most of the electrode do not reach the melting\ntemperature of Li metal (except for the local area in direct contact with the flame,\nSupplementary Fig. 11).\nSecondly, as the temperature gradually rises, the Li metal at the ignition site does\nindeed melt. However, the three-dimensional structure of the host with high\nlithiophilicity can adhere the locally melted Li metal against liquid flow, thereby the\nphenomenon of flowing liquid-state molten Li metal cannot be observed in the image.\nThirdly, we also add a magnified plot on the DSC curve of composite electrode\ninto Supplementary Fig. 13, which illustrate a clear endothermic peak that prove the\nactual happening of Li metal melting.\nChange to the Supplementary Information:\nSupplementary Figure 11. Infrared thermal image of Li||ZnO||TPP||GO electrode after\na sustained 10 seconds of constant ignition.\nSupplementary Figure 13. Magnified DSC curves of Li||ZnO||TPP||GO electrode in\nair atmosphere.\nChange to the Manuscript:\nPage 10, line 20:\nThis greatly promoted heat dissipation effect can greatly delay the heat conduction and\nthe melting of Li metal within the electrode, thereby promoting the flame retardancy of\nthree-dimensional porous Li electrode, which cannot be realized by dense Li foil or the\nLi foil-TPP||GO host attaching structure in Li@TPP||GO electrode (Supplementary Fig.\n13).\nComments: 7) ZnO and GO are designed to suppress electrochemical side reactions\nbetween TPP and Li. If so, TPP-related changes should be consistently detectable after\ncombustion without regard to ZnO or GO. However, post-ignition XPS for\nLi@TPP||GO shows no phosphorus signal, unlike Li||ZnO||TPP||GO. Please explain\nin more detail how these post-combustion analyses support the claimed flame-retardant\naction.\nResponse:\nWe thank the reviewer for raising this suggestion. Following the reviewer’s\nsuggestion, we provide the following explanations. The Li@TPP||GO electrode is\nfabricated by attaching regular Li foil to TPP||GO host (Supplementary Fig. 4). As a\nresult, the flame retardant does not mix uniformly with Li metal inside the electrode,\nthus unable to achieve in-situ release within the electrode to give an excellent flame-\nretardant effect. As a consequence, the phosphorus-containing layer (formed by the low\ndecomposing temperature flame retardant during the initial ignition) is carried away by\nthe extremely hot gas generated from the intense combustion of Li metal that last for\nseveral tens of seconds, hence no phosphorus signal could be observed on the surface.\nIn contrast, due to the in-situ release of TPP and the absence of any intense combustion\nphenomenon in the Li||ZnO||TPP||GO electrode, the phosphorus-containing layer can\nbe remained and detected on the surface of the post-combustion electrode.\nChange to the Supplementary Information:\nSupplementary Figure 4. The digital photo and Schematic diagram of Li@TPP||GO\ncontrol sample.\nChange to the Manuscript:\nPage 12, line 2:\nIn contrast, the XPS profiles of Li@TPP||GO and pure Li after ignition exhibit no\nLi PO -related peaks, because the TPP-derived phosphate composite (formed by the\n3 4\nlow decomposing temperature of flame retardant during the initial ignition) was carried\naway by the extremely hot gas generated from the intense combustion of Li metal that\nlast for several tens of seconds.\nComments: 8) A pouch cell contains Li metal, a cathode, and a liquid electrolyte, and\nflammable electrolytes are major factor driver of thermal runaway. Despite this, the\npouch-cell tests showed no ignition. Please explain how electrolyte combustion was\nprevented and comment on the robustness and reliability of the pouch-cell experiments.\nResponse:\nWe thank the reviewer for raising this great suggestion. Following the reviewer’s\nsuggestion, we provide the following explanations:\nFirstly, we would like to apologize that we do not emphasize enough in our\nmanuscript that the electrolyte we used in the pouch cells (both pure Li metal anode\nand composite Li metal anode) has already been non-flammable electrolyte (1 M LiPF\n6\nin a mixture of fluoroethylene carbonate /3,3,3-fluoroethylmethyl carbonate/1,1,2,2-\ntetrafluoroethyl-2,2,2-trifluoroethyl ether (FEC:FEMC:HFE, 2:6:2 by weight), Nature\nNanotech. 2018, 13, 715–722) in Figure 4h and i. This non-flammable electrolyte has\na self-extinguishing time of 0 s g-1 and exhibits excellent combustion suppression\nproperties in lithium-ion batteries (Supplementary Figs. 27 and 28). Therefore, the\npossibility of the electrolyte flammability is excluded, which confirms the robustness\nand reliability of the pouch-cell experiments.\nSecondly, as disclosed in recent literatures in Nat. Energy, 2025, 10, 1382–1390\nand Adv. Energy Mater. 2023, 13, 2203144., the lithium oxidation reaction is the\nprimary factor causing thermal runaway in Li metal battery, rather than electrolyte. We\nuse this type of non-flammable electrolyte, which is highly effective in lithium-ion\nbatteries but insufficient in lithium-metal batteries, in order to demonstrate our\ncomposite material design is necessary for solving the safety problems associated with\nthe Li metal anode.\nChange to the Supplementary Information:\nSupplementary Figure 27. Digital photos of glass fibers soaked in non-flammable\n(1 M LiPF in a mixture of fluoroethylene carbonate /3,3,3-fluoroethylmethyl\n6\ncarbonate/1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether (FEC:FEMC:HFE, 2:6:2\nby weight)) electrolyte under constant ignition.\nSupplementary Figure 28. Digital photos of graphite||NCM811 pouch full-cell using\nnon-flammable electrolyte (1 M LiPF in a mixture of fluoroethylene carbonate /3,3,3-\n6\nfluoroethylmethyl carbonate/1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether\n(FEC:FEMC:HFE, 2:6:2 by weight)) under constant ignition. The graphite||NCM811\npouch cell remains non-combustible even after being exposed to a 20-second open\nflame, confirming the excellent flame-retardant properties of this electrolyte in lithium-\nion batteries.\nChange to the Manuscript:\nPage 15, line 16:\nAs shown in Fig. 4g, we evaluate the safety property by recording the light-emitting\ndiode (LED)-powering function of a 100 mAh Li metal battery using non-flammable\nelectrolyte (1 M LiPF in a mixture of fluoroethylene carbonate /3,3,3-\n6\nfluoroethylmethyl carbonate/1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether\n(FEC:FEMC:HFE, 2:6:2 by weight)) 7 when the Li metal anode in the battery core was\nexposed to external fire through the damage site in package (Supplementary Fig. 27\nand Movie 17).\nPage 15, line 24:\nThis represents an intense combustion issue to be more violent than conventional Li-\nion battery (Supplementary Fig. 28 and Movie 19).\nComments: 9) Please discuss whether the proposed mechanism and results are\nrealistically reproducible at large-format cell or pack scales. Also elaborate on the\ndesign’s sufficiency for scale-up and integration into practical cells.\nResponse:\nWe thank the reviewer for raising this suggestion. Following the reviewer’s\nsuggestion, we provide the following explanations:\nFirstly, as there had been almost no previous research on flame-retardant designs\nspecifically targeting Li metal anodes and no corresponding mechanism exploration,\nour work focused on proposing the electrode flame-retardant mechanism and\ndemonstrating the flame-retardant effect of laboratory-scale Li metal anode and Li\nmetal batteries.\nSecondly, because the raw materials (graphene oxide and TPP flame retardant)\nused in our anode preparation process are highly commercialized and the industrial\nproduction technologies (freeze-drying and atomic layer deposition) are mature (J. of\nMater. Res. and Technol., 2020, 9, 11587-1610; Joule, 2018, 2, 2583-2604; Chem. Soc.\nRev., 2021, 50, 3889-3956), this design has great potential for large-scale\nmanufacturing and practical application.\nThirdly, in order to verify the feasibility and reproducibility of the preparation\nprocess, we add more digital photos of 4 pieces of decimeter-scale Li||ZnO||TPP||GO\nanodes into Supplementary Fig. 6, which is already feasible for pouch cell fabrication.\nFurthermore, the thermal insulation of aerogel structure and Li atom capture\nmechanisms we proposed are theoretically independent of the electrode size. Therefore,\nwe speculate that these mechanisms and results are also applicable in large-scale\nbatteries. In the future researches, we will verify the effectiveness of anode flame-\nretardant design in large-format cell. Once again, we would like to express our gratitude\nto the reviewer for this valuable suggestion, which will be of great help in improving\nour work.\nChange to the Supplementary Information:\nSupplementary Figure 6. Digital photos of the large size TPP||GO aerogel (a) and 4\npieces of Li||ZnO||TPP||GO composite anode (b-e), showing the prospect of large-scale\npreparation.\nChange to the Manuscript:\nPage 7, line 14:\nIn addition, this composite anode has great potential for large-scale manufacturing and\npractical applications, as the preparation steps are elaborately designed based on\ncommercially available raw materials (GO and TPP solution) 24 and mature industrial\nproduction techniques (ALD and composite Li fabrications procedures) 25,26, enabling\ndecimeter-scale electrodes sufficient for pouch cell fabrication even in laboratories\n(Supplementary Fig. 6).\nComments: 10) Given dataset limitations and the absence of certain key comparisons,\nplease provide additional explanation to ensure that safety and performance claims are\nnot overstated.\nResponse:\nWe thank the reviewer for raising this great suggestion. Following the reviewer’s\nsuggestion, we add more repetitive electrode and pouch cell ignition tests to prove that\nthe flame-retardant effect has not been overstated, and have removed all the description\nthat have the possibility to be considered as overstatement.\nChange to the Supplementary Information:\nSupplementary Figure 8. Infrared thermal images and digital photos of 5 pieces of\nrepetitive Li||ZnO||TPP||GO electrodes during a sustained 10 seconds of constant\nignition.\nSupplementary Figure 30. Digital photos of 3 pouch full-cells with Li||ZnO||TPP||GO\nanodes under constant ignition.\nChange to the Manuscript:\nPage 9, line 6:\nEven after a further sustained 10 seconds of constant ignition, Li||ZnO||TPP||GO\nelectrode still illustrates an excellent SET of 0 s g-1, defining it as a non-combustible\nmaterial 9 (Supplementary Fig. 8 and Movies 4-8).\nPage 16, line 1:\nIn comparison, the Li metal pouch cell employing Li||ZnO||TPP||GO anode successfully\nkeeps powering the LED without any combustion during three times of 2-second\nignitions (Fig. 4i), or even a 20-second-lasting ignition (Supplementary Movies 20-23\nand Supplementary Fig. 30).\nPage 4, line 11:\nHere, we develop a layered host strategy that layer-by-layered assembly Li metal,\nlithiophilic zinc oxide (ZnO) layer and fire retardant onto three-dimensional graphene\noxide foam, successfully eliminating the incompatibility between Li metal and flame\nretardants and affording unprecedented smart-flame-retarding function for addressing\nthe flammability challenge of Li metal anode. We discover that this first-ever smart-\nflame-retarding ability should be attributed to three key material design concepts:\nPage 5, line 3:\nThis smart-flame-retarding layered composite host design provides ideas opens a new\navenue for addressing the long-existing extreme flammability challenges of Li metal\nanode for safe and durable Li metal battery application.\nPage 12, line 10:\nThis provides new strategies and mechanisms for solving the high combustion problem\nof Li metal anode.\nPage 15, line 14:\nthe Li metal pouch cell can afford an unprecedented self-fire-extinguishing ability and\nsupply energy output even when the battery core was exposed to fire.\nPage 16, line 17:\nThese results indicate that our smart-flame-retard layered composite Li electrode design\ndoes successfully address the high combustibility problem of Li metal anode for the\nfirst time, affording an unprecedented fire-resistant, high safety Li metal battery with\ngreat significance for safe energy storage applications.\nPage 22, line 12:\nIn summary, we develop a smart-flame-retarding layered host design for unprecedently\nand successfully addressing the high flammability problem of dangerous Li metal\nanode, realizing the first-ever self-fire-extinguishing and safe Li metal battery.\nReviewer #2 (Remarks to the Author):\nComments:\nThis work by Qi et al. proposes a smart-flame-retarding layered host design for safe Li\nmetal battery. This host achieves self-fire-extinguishing function of Li metal anode even\nunder extreme ignition conditions and completely seals the flame retardant from Li\nmetal corrosion for durable battery operation. Their design does solve the extreme\nflammability challenge of Li metal anode that has hindered the practical application of\nlithium metal batteries for decades. Their anode flame-retardant effect, especially for\npouch cells which remained unignited after being continuously exposed to fire for 20\nseconds, is very impressive. Based on my research experience, there has never been any\nstudy that can address the flammability issue of Li metal anode-everyone in this\nresearch field knows this serious concern, but nobody has ever solved it. This work is\nthe first-ever demonstration, so I believe this work will become a meaningful\nbreakthrough in the whole Li metal battery field, giving other scientists new ideas on\nhow to address the safety issue of Li metal anode. Therefore, I strongly recommend\nconsidering publishing this manuscript in Nature Communications, after addressing\nfollowing comments:\nResponse:\nWe thank the reviewer for the positive comments. We also thank the reviewer for\nmany valuable and constructive suggestions to improve our paper. In the following, we\nprovide our point-by-point responses to each of the comments.\nComments: 1) From Supplementary Figure 8, the Li||ZnO||TPP||GO electrode still\nillustrates an excellent SET of 0 s/g even after a sustained 10 seconds of constant\nignition. This result can be more convincing if the authors provide infrared thermal\nimages of multiple repeating samples into supplementary information to further\nconfirm this excellent effect.\nResponse:\nWe thank the reviewer for raising this question. We add more infrared thermal\nimages and digital photos of 5 repeating Li||ZnO||TPP||GO electrodes during a sustained\n10 seconds of constant ignition into Supplementary Fig. 8.\nChange to the Supplementary Information:\nSupplementary Figure 8. Infrared thermal images and digital photos of five repeating\nLi||ZnO||TPP||GO electrodes during a sustained 10 seconds of constant ignition.\nChange to the Manuscript:\nPage 9, line 6:\nEven after a further sustained 10 seconds of constant ignition, Li||ZnO||TPP||GO\nelectrode still illustrates an excellent SET of 0 s g-1, defining it as a non-combustible\nmaterial 9 (Supplementary Fig. 8 and Movies 4-8).\nComments: 2) As shown in Figure 4d, the Li//LiNi Co Mn O (NCM811) full-cell\n0.8 0.1 0.1 2\nusing after-burning Li//ZnO//TPP//GO anode exhibits a significant improvement in\ncapacity retention and cycle life. It would be better to present the CE comparison of the\nthree kinds of full-cells to further support this advantage.\nResponse:\nWe thank the reviewer for raising this suggestion. Following the reviewer’s\nsuggestion, we add the CE comparison of the three kinds of full cells into Figure 4d\nshowing stable cycling of the full cell.\nChange to the Manuscript:\nFigure 4d. Cycling performance of full-cells using 1.63 mAh cm-2 NCM811 cathode\nand after-burning pure Li, Li@TPP||GO and Li||ZnO||TPP||GO anode with LHCETTE\nelectrolyte at 0.5C cycling current.\nPage 15, line 2:\nAs shown in Fig. 4d, the Li||LiNi Co Mn O (NCM811) full-cell using after-\n0.8 0.1 0.1 2\nburning Li||ZnO||TPP||GO anode exhibits 93.72% capacity retention after 250 cycles,\nwith stable voltage profiles, coulombic efficiency and charge-transfer resistance (Fig.\n4e and Supplementary Fig. 25).\nComments: 3) In the full cells test under regular operating conditions, stripping Li\ncompletely from Li//ZnO//TPP//GO anode in half-cell is the first step towards achieving\nthe same R of the three full cells. It would be better to conduct XRD tests on the host\nN/P\nat the fully stripped state for confirming the complete stripping of Li, thereby\ndemonstrating the strict control of the R .\nN/P\nResponse:\nWe thank the reviewer for raising this suggestion. Following the reviewer’s\nsuggestion, we add the XRD test of the Li||ZnO||TPP||GO composite electrode after\nelectrochemical stripping of Li metal in Supplementary Fig. 43 The absence of metallic\nLi-related peaks proves the effectiveness of the stripping procedure to control the R .\nN/P\nChange to the Supplementary Information:\nSupplementary Figure 43. XRD pattern of the Li||ZnO||TPP||GO composite electrode\nafter electrochemical stripping of Li metal in half-cell.\nChange to the Manuscript:\nPage 19, line 11:\nThe same negative: positive electrode capacity ratio (R ) of the three full cells was\nN/P\nrealized by completely stripping Li from Li||ZnO||TPP||GO anode in half-cell,\ndepositing 4 mAh cm-2 Li into anode, dissembling from half-cell and assembling into\nthe full-cells (Supplementary Figs. 42 and 43).\nComments: 4) Supplementary Figure 15 exhibits the uniform Li deposition morphology\nof after-burning Li//ZnO//TPP//GO electrode. But SEM characterizations on the\ndendrites are known to be highly erratic. It would be better to add more reproducing\nSEM characterization results for better supporting the authors‘ declaration.\nResponse:\nWe thank the reviewer for raising this suggestion. Following the reviewer’s\nsuggestion, we add more top-view SEM images of cycled after-burning\nLi||ZnO||TPP||GO electrodes into Supplementary Fig. 24 to support our claim.\nChange to the Supplementary Information:\nSupplementary Figure 24. Top-view SEM images of deposited Li on after-burning\nLi||ZnO||TPP||GO in symmetric cell with LP40FV electrolyte (a) and LHCETTE\nelectrolyte (b) at a cycling current density of 1 mA cm−2 with a capacity density of 1\nmAh cm−2 after 100 cycles.\nChange to the Manuscript:\nPage 14, line 24:\nTop-view SEM images illustrate a uniform Li deposition morphology without any\ndendrites after 100 cycles in after-burning Li||ZnO||TPP||GO electrode (Supplementary\nFig. 24).\nComments: 5) The authors demonstrated that their design effectively promotes the\nuniform deposition of Li through the absence of dendrites in Figure 5h. To confirm this\neffect, it would be best to conduct more reproducible SEM characterization and add\nthem into Supplementary Information.\nResponse:\nWe thank the reviewer for raising this suggestion. Following the reviewer’s\nsuggestion, we add more top-view SEM images of deposited Li on Li||ZnO||TPP||GO\nelectrodes into Supplementary Fig. 54 to support our claim.\nChange to the Supplementary Information:\nSupplementary Figure 54. Top-view SEM images of deposited Li on four\nLi||ZnO||TPP||GO electrodes in repeating symmetric cell with LHCETTE electrolyte at\na cycling current density of 1 mA cm−2 with a capacity density of 1 mAh cm−2 after 100\ncycles.\nChange to the Manuscript:\nPage 20, line 11:\nAfter 100 cycles, it illustrates the uniform, averaged 7.3 μm-diameter grain-sized Li\ndeposition morphology (Fig. 5h and Supplementary Fig. 54)\nComments: 6) The authors declare that the lithiophilic properties of ZnO-converted\nLiZn interlayer can facilitate uniform and dendrite-free Li deposition. It would be better\nto cite more references and add some discussions to support this claim.\nResponse:\nWe thank the reviewer for raising this suggestion. Following the reviewer’s\nsuggestion, we cite more references and add some discussions in the manuscript (Nat\nCommun, 2025, 16, 5462).\nChange to the Manuscript:\nPage 20, line 8:\nOwing to the advantages of high-surface-area three-dimensional host structure 43 and\nthe lithiophilic properties of ZnO-converted LiZn interlayer 42,48, lower nucleation and\nmigration barriers are realized in the Li||ZnO||TPP||GO electrode.\nComments: 7) For a manuscript that primarily focuses on safety, it is necessary to\ninclude validation through tests such as pouch cell nail penetration and accelerating\nrate calorimetry (ARC) measurements, in addition to ignition experiments.\nResponse:\nWe thank the reviewer for raising this suggestion. Following the reviewer’s\nsuggestion, we conduct pouch cell nail penetration and accelerating rate calorimetry\n(ARC) measurements and add the results into Supplementary Fig. 33-35 and Movie 26-\n27. These results do well prove the improved safety properties from our composite Li\nmetal anode design.\nChange to the Supplementary Information:\nSupplementary Figure 33. The nail penetration test of pure Li || NCM811 (a) and\nLi||ZnO||TPP||GO || NCM811 (b) pouch cells with non-flammable electrolyte.\nSupplementary Figure 34. The temperature and voltage plots of Li||ZnO||TPP||GO ||\nNCM811 pouch cells with non-flammable electrolyte during the nail penetration test.\nSupplementary Figure 35. Thermal runaway temperature (a) and temperature rise rate\n(b) plots of pure Li || NCM811 and Li||ZnO||TPP||GO || NCM811 pouch cells with non-\nflammable electrolyte during the ARC test. T is the temperature at which self-heating\n1\nonset for the first time. After the equipment stops heating for a period of time, the\nbattery's self-heating stops, then the equipment resumes heating until the self-heating\nis completely triggered. T ’ is the temperature at which self-heating was started again\n1\nand no longer stop. T is the thermal runaway onset temperature (when dT/dt reaches\n2\n≥ 100℃ min-1). T is the peak temperature.\nPage 16, line 7:\nNail penetration test and accelerating rate calorimetry (ARC) measurement are\nconducted to further evaluate the battery's safety under realistic abuse conditions.\nDuring the nail penetration test, Li||ZnO||TPP||GO || NCM811 pouch cell exhibited\nneither visible smoke nor open flame that overwhelms the exploded pure Li metal\nanode-based pouch cell (Supplementary Figs. 33 and 34). In ARC measurement,\nLi||ZnO||TPP||GO || NCM811 pouch cell demonstrated a high self-heating onset\ntemperature (T ‘) of 176.37°C (versus 160.84°C for the pure Li system) and a reduced\n1\npeak temperature (T ) of 352.97°C (versus 500.12°C for the pure Li system), and\n3\nconsistently lower temperature increasing rate (dT/dt ≤ 1.96℃ min-1) than the pure Li\nsystem (T = 295.67°C when dT/dt ≥ 100℃ min-1) (Supplementary Fig. 35 and Movies\n2\n26, 27) 38,39.\nComments: 8) Figure S16 should provide the equivalent circuit diagram used for\nimpedance fitting, and Figure S16b should be appropriately enlarged to improve the\nclarity of the manuscript.\nResponse:\nWe thank the reviewer for raising this suggestion. Following the reviewer’s\nsuggestion, we add the equivalent circuit diagrams in Figure S25a and Figure S25b and\nan enlarged electrochemical impedance spectroscopy in Figure S25b.\nChange to the Supplementary Information:\nSupplementary Figure 25. Electrochemical impedance spectroscopy of full-cells\nusing 1.63 mAh cm-2 NCM811 cathode and different after-burning anodes with\nLHCETTE electrolyte before (a) and after (b) 50 cycles at a cycling current density of\n0.5 C.\nChange to the Manuscript:\nPage 15, line 2:\nAs shown in Fig. 4d, the Li||LiNi Co Mn O (NCM811) full-cell using after-\n0.8 0.1 0.1 2\nburning Li||ZnO||TPP||GO anode exhibits 93.72% capacity retention after 250 cycles,\nwith stable voltage profiles, coulombic efficiency and charge-transfer resistance (Fig.\n4e and Supplementary Fig. 25).\nComments: 9) The annotations within Figure S21 are incorrect (0.5 mA, 0.5 mAh). The\nauthor should revise them clearly or remove the annotations altogether.\nResponse:\nWe thank the reviewer for raising this great suggestion. Following the reviewer’s\nsuggestion, we remove the annotations and fix the statements in Figure S37.\nChange to the Supplementary Information:\nSupplementary Figure 37. Aurbach CE test of Li || Cu, Li || TPP||GO and Li ||\nZnO||TPP||GO host half-cells in LHCETTE electrolyte (discharging 5mAh cm-2,\ncharging to 0.15V, discharging 5mAh cm-2, cycle at a cycling current density of 0.5 mA\ncm-2 and an areal capacity density of 0.5 mAh cm-2 for 10 times, then charging to 0.15V\nagain).\nChange to the Manuscript:\nPage 18, line 26:\nThis improvement can also be observed in Aurbach's CE testing (Supplementary Fig.\n37).\nComments: 10) This manuscript focuses on anode protection, with the cathode playing\na secondary role. However, comprehensive characterization is essential for a high-level\npublication; therefore, appropriate cathode characterization should be included.\nResponse:\nWe thank the reviewer for raising this great suggestion. Following the reviewer’s\nsuggestion, we add cathode characterizations into Supplementary Fig. 51-53, such as\nSEM morphology characterizations, TEM characterizations and XPS analysis. These\nresults prove our layered composite anode design successfully avoids the flame\nretardant dissolution-derived phosphors-containing cathode-electrolyte interphase,\nwhich is also beneficial for cathode cycling stability.\nChange to the Supplementary Information:\nSupplementary Figure 51. SEM morphology characterizations on NCM811 cathodes\nafter 100 cycles in LHCETTE electrolyte using Li||ZnO||TPP||GO (a) or Li@TPP||GO\n(b) anodes.\nSupplementary Figure 52. TEM characterizations on NCM811 cathodes after 100\ncycles in LHCETTE electrolyte using Li||ZnO||TPP||GO (a) or Li@TPP||GO (b) anodes.\nSupplementary Figure 53. P 2p spectra XPS analysis on the composition of the\nNCM811 cathodes after 100 cycles in LHCETTE electrolyte using Li||ZnO||TPP||GO\nor Li@TPP||GO anodes.\nChange to the Manuscript:\nPage 20, line 5:\nIn addition, our layered composite anode design also avoids the flame-retardant\ndissolution-derived phosphors-containing cathode-electrolyte interphase, which is also\nbeneficial for cathode cycling stability (Supplementary Figs. 51-53).\nComments: 11) The protection of the lithium anode in this manuscript requires the\nintroduction of a large number of inactive materials such as GO, ZnO, and TPP. A\nmajor concern is whether the addition of these materials will significantly reduce the\ngravimetric and volumetric energy densities of the lithium anode. The authors should\nprovide detailed calculations to address this issue.\nResponse:\nWe thank the reviewer for raising this suggestion. Following the reviewer’s\nsuggestion, we calculate the mass and volume proportions of each component of the\nelectrode (Supplementary Table 1). From these data, we confirm the mass of inactive\nmaterial accounts for 25.58% in the Li metal anode. In addition, the inactive volume\naccounts for 36.42% in the Li metal anode.\nTo accurately quantify the impact of our composite anode design on the\ngravimetric and volumetric energy densities of Li metal battery, we extract the detailed\nmass and volume proportion of Li metal anode in a high-performing 5.2 Ah-level, 618.2\nWh kg-1 and 918.4 Wh L-1 Li metal pouch battery prepared by Huang et al (Nature,\n2025, 644, 660–667) (Table R1 and Figure R1). In their pouch battery, the mass and\nvolume proportions of their 9 layers of 50 μm-thick lithium metal anode are 2.62% and\n7.29% respectively. Therefore, when our composite Li anode is utilized in their Li metal\npouch cell, the weight of the pouch cell will be increased to 100.9%, and the volume of\nthe pouch cell will be increased to 104.18%. Thus, the mass energy density and volume\nenergy density will only decrease by 0.89% (from 618.2 Wh kg-1 to 612.68 Wh kg-1)\nand 4.01% (from 918.4 Wh L-1 to 881.61 Wh L-1) respectively. These results prove that\nour composite Li anode design has minor effect on reducing the gravimetric and\nvolumetric energy densities of the lithium anode.\nTable R1. The specifications of Ni90||Li pouch battery prepared by Huang et al (618.2\nWh kg−1 in 5.2 Ah).\nFigure R1. Cycling performance and digital photo of 5.2 Ah Ni90||Li pouch battery\nprepared by Huang et al.\nChange to the Supplementary Information:\nSupplementary Table 1. The content and proportion of each component in the\ncomposite Li anode.\nGO TPP ZnO Li Pores and electrode\nchannels\nMass (mg) 0.4 0.8 0.45 4.8 0 6.45\nMass 6.20 12.4 6.98 74.42 0 100\nproportion\n(%)\nDensity 1.8 1.265 5.61 0.534 0.656\n(g cm-3)\nVolume 0.2222 0.6325 0.0802 8.9888 4.2135 14.1372\n(mm-3)\nVolume 1.57 4.47 0.57 63.58 29.81 100\nproportion\n(%)\nSupplementary Note 1: The influence of the introduction of inactive materials in\nthe composite anode on the gravimetric and volumetric energy density of Li metal\nbatteries\nWe calculate the mass and volume proportions of each component of the electrode\n(Supplementary Table 1). From these data, we confirm the mass of inactive material\naccounts for 25.58% in the Li metal anode. In addition, the inactive volume accounts\nfor 36.42% in the Li metal anode.\nTo accurately quantify the impact of our composite anode design on the\ngravimetric and volumetric energy densities of Li metal battery, we extract the detailed\nmass and volume proportion of Li metal anode in a high-performing 5.2 Ah-level, 618.2\nWh kg-1 and 918.4 Wh L-1 Li metal pouch battery prepared by Huang et al 1. In their\npouch battery, the mass and volume proportions of their 9 layers o f 50 μm-thick lithium\nmetal anode are 2.62% and 7.29% respectively. Therefore, when our composite Li\nanode is utilized in their Li metal pouch cell, the weight of the pouch cell will be\nincreased to 100.9%, and the volume of the pouch cell will be increased to 104.18%.\nThus, the mass energy density and volume energy density will only decrease by 0.89%\n(from 618.2 Wh kg-1 to 612.68 Wh kg-1) and 4.01% (from 918.4 Wh L-1 to 881.61 Wh\nL-1) respectively. These results prove that our composite Li anode design has minor\neffect on reducing the gravimetric and volumetric energy densities of the lithium anode.\nChange to the Manuscript:\nPage 7, line 19:\nDetailed calculations in Supplementary Table 1 and Note 1 prove that our composite\nLi anode design have minor effect on reducing the gravimetric and volumetric energy\ndensities of the lithium metal battery.\nComments: 12) The manuscript only presents combustion safety tests on pouch cells,\nbut no performance evaluation. The authors should supplement the work with long-\ncycle performance tests of high-energy-density pouch cells to enhance the impact of the\narticle.\nResponse:\nWe thank the reviewer for raising this suggestion. Following the reviewer’s\nsuggestion, we assemble a single-layer 220 mAh Li||ZnO||TPP||GO || NCM811 pouch\ncell and add its cycling performance and photograph to Supplementary Fig. 46 and 47.\nWhen excluding the weight of the package (cathode, anode, electrolyte, separator,\ncurrent collector and tab still included), this pouch cell exhibits an energy density of\n505.16 Wh kg-1 (capacity :217.8 mAh; average discharge voltage: 3.766 V; battery core\nweight: 3.7445 (the whole battery)-2.1208 (package)=1.6237 gram; energy\ndensity=217.8×3.766/1.6237=505.16 Wh kg-1). We believe that future Li metal pouch\ncell with more layers of stacking electrode will have a similar energy density of ≥\n500Wh kg-1 and high safety that favors practical application.\nChange to the Supplementary Information:\nSupplementary Figure 46. a The sizing photo of 220 mAh Li||ZnO||TPP||GO || NCM811\npouch battery. b The weighing photo of 220 mAh Li||ZnO||TPP||GO || NCM811 pouch\nbattery. c The sizing photo of the battery package. d The weighing photo of the battery\npackage.\nSupplementary Figure 47. a Cycling performance of pouch cells using NCM811\ncathode and Li||ZnO||TPP||GO anode with LHCETTE electrolyte under a\ncharge/discharge rate of 0.2/0.3 C. b Capacity-voltage profiles of pouch cells using\nNCM811 cathode and Li||ZnO||TPP||GO anode with LHCETTE electrolyte after\ndifferent cycles. (capacity :217.8 mAh; average discharge voltage: 3.766 V; battery core\nweight: 3.7445 (the whole battery)-2.1208 (package)=1.6237 gram; energy\ndensity=217.8×3.766/1.6237=505.16 Wh kg-1)\nChange to the Manuscript:\nPage 19, line 16:\nIn the pouch cell with higher energy density, the Li||ZnO||TPP||GO anode still exhibits\ngood cycle stability (Supplementary Figs. 46 and 47)."}, {"round": 2, "reviewer_comments": {"reviewer_1": "I have carefully reviewed the revised manuscript. The authors have provided clear and satisfactory responses to all reviewer comments and concerns, and the quality of the manuscript has been significantly improved. Therefore, I recommend acceptance of this work for publication in Nature Communications.", "reviewer_2": "The authors have made a very good revision that well addresses my concerns. I believe their work will become an important milestone to the field of safe lithium-metal batteries. I recommend acceptance of this paper in Nature Communications, with the following minor revisions mainly in formats:\n(1) In page 4, line 90, a unit “s g-1” should be added after the 0.\n(2) The authors name their precursor as aerogel, while some places appear as “foam”. The authors should revise them to be consistent.\n(3) In Figure 3j, the title of the scale, \"O2 volume fraction\", should be followed by the unit \"%\".\n(4) In Figure 4d, additional annotations should be added into the Figure to explain the cathode areal capacity, the electrolyte category, and the current density of the full-cell.\n(5) In the annotations of Supplementary Figures 27, 28, 31, 32 and 33, spaces should be added between the numbers and the time unit \"s\" or \"h\".\n(6) It would be better to add more reproducing SEM characterization results of NCM811 cathode after 100 cycles using Li||ZnO||TPP||GO anode for better supporting the positive effect of the author's anode design on the cathode's cycling stability."}, "author_response": "Authors’ Response to Reviewers’ Comments\nThe authors are grateful to the reviewers for their constructive comments on the\nmanuscript. All reviewers’ comments were carefully considered and\nimplemented/responded to in the revised manuscript. A detailed response is given for\neach comment. We italicized the reviewers’ comments and suggestions and highlight\nour responses in purple for clarity.\nReviewer #1 (Comments to the Author):\nComments:\nI have carefully reviewed the revised manuscript. The authors have provided clear\nand satisfactory responses to all reviewer comments and concerns, and the quality of\nthe manuscript has been significantly improved. Therefore, I recommend acceptance of\nthis work for publication in Nature Communications.\nResponse:\nWe are very grateful for the reviewer #1’s high evaluation of our work. We also\nthank the reviewer for many valuable and constructive suggestions to improve our paper.\nReviewer #2 (Comments to the Author):\nComments:\nThe authors have made a very good revision that well addresses my concerns. I believe\ntheir work will become an important milestone to the field of safe lithium-metal\nbatteries. I recommend acceptance of this paper in Nature Communications, with the\nfollowing minor revisions mainly in formats:\nResponse:\nWe thank the reviewer for the positive comments. We also thank the reviewer for\nmany valuable and constructive suggestions to improve our paper. In the following, we\nprovide our point-by-point responses to each of the comments.\nComments: 1) In page 4, line 90, a unit “s g-1” should be added after the 0.\nResponse:\nWe thank the reviewer for raising this question. We have corrected this error in the\nmanuscript.\nChange to the Manuscript:\nPage 4, line 23:\nFrom these merits, our smart-flame-retarding layered composite Li negative electrode\nsuccessfully decreases the self-extinguishing time of Li metal from 979.27 to 0 s g-1\neven if immersed in open fire for 10 seconds, affording 85.81% electrode capacity\nreservation and 93.57% full-cell capacity reservation after 250 cycles even if the Li\nnegative electrode was pre-immersed in an open flame.\nComments: 2) The authors name their precursor as aerogel, while some places appear\nas “foam”. The authors should revise them to be consistent.\nResponse:\nWe thank the reviewer for raising this suggestion. We have corrected all\noccurrences of \"aerogel\" in the descriptions of the TPP||GO precursor to \"foam\" to\nensure the consistency of the precursor’s name.\nChange to the Manuscript:\nPage 5, line 12:\nInitially, a mixed graphene oxide (GO) and triphenyl phosphate (TPP flame retardant)\naqueous solution was coated onto glass substrate and freeze-dried to form a porous\nfoam, which was then roll-to-roll calendared to a thickness of 80 μm. Scanning electron\nmicroscope (SEM) images and X-ray diffraction (XRD) spectra demonstrate that TPP\nwas uniformly coated onto the 40-nm-thick GO layers, generating a 260-nm-thick TPP-\non-GO, bilayer-structured foam (TPP/GO) (Fig. 2b and c, Supplementary Fig. 1a to c\nand Supplementary Fig. 2). Subsequently, diethylzinc and water vapors were\nalternatively pulsed into the TPP/GO foam, absorbed on the TPP/GO layer and purged\nout using an industrial-level atomic layer deposition (ALD) fabrication procedure,\nconverting into a uniform ZnO film that covers the TPP/GO aerogel into ZnO/TPP/GO\nhost\nPage 5, line 27:\nIn contrast, large amount of TPP-Li side reaction-generated smoke and failure of\nmolten Li infusion were observed during contacting TPP/GO foam (without ZnO\nlayer) with molten Li\nPage 9, line 17:\nThis gas-covering effect isolates the electrode from O atmosphere, effectively\n2\npreventing the Li-O combustion reaction that cannot be realized by simply attaching\nComments: 3) In Figure 3j, the title of the scale, \"O volume fraction\", should be\n2\nfollowed by the unit \"%\".\nResponse:\nWe thank the reviewer for raising this question. We have corrected this error in\nFigure 3j.\nChange to the Manuscript:\nFigure 3j. COMSOL simulations of gas release/diffusion behavior in the three-\ndimensional space around Li||ZnO||TPP||GO under flame.\nComments: 4) In Figure 4d, additional annotations should be added into the Figure to\nexplain the cathode areal capacity, the electrolyte category, and the current density of\nthe full-cell.\nResponse:\nWe thank the reviewer for raising this question. We have added the corresponding\ndescriptions in Figure 4d.\nChange to the Manuscript:\nFigure 4d. Cycling performance of full-cells using 1.63 mAh cm-2 NCM811 positive\nelectrode and after-burning pure Li, Li@TPP/GO and Li/ZnO/TPP/GO negative\nelectrode with LHCETTE electrolyte at a specific current of 94 mA g-1.\nComments: 5) In the annotations of Supplementary Figures 27, 28, 31, 32 and 33,\nspaces should be added between the numbers and the time unit \"s\" or \"h\".\nResponse:\nWe thank the reviewer for raising this suggestion. We have corrected these errors\nin Supplementary Information.\nChange to the Supplementary Information:\nSupplementary Figure 28. Digital photos of glass fibers soaked in non-flammable\n(1 M LiPF in a mixture of fluoroethylene carbonate /3,3,3-fluoroethylmethyl\n6\ncarbonate/1,1,2,2-tetrafluoroethyl-2′,2′,2′-trifluoroethyl ether (FEC:FEMC:HFE, 2:6:2\nby weight)) electrolyte under constant ignition.\nSupplementary Figure 29. Digital photos of graphite||NCM811 pouch full-cell using\nnon-flammable electrolyte (1 M LiPF in a mixture of fluoroethylene carbonate /3,3,3-\n6\nfluoroethyl methyl carbonate/1,1,2,2 – tetrafluoroethyl - 2′,2′,2′ - trifluoroethyl ether\n(FEC:FEMC:HFE, 2:6:2 by weight)) under constant ignition. The graphite||NCM811\npouch cell remains non-combustible even after being exposed to a 20-second open\nflame, confirming the good flame-retardant properties of this electrolyte in lithium-ion\nbatteries.\nSupplementary Figure 32. Digital photos of pouch full-cell using Li/ZnO/TPP/GO\nnegative electrode with same electrolyte amount (250 mL, 3.9 g Ah-1) but half TPP\ncontent (6.6% in negative electrode) under constant ignition. After reducing the TPP\ncontent, the pouch cell could not achieve the original flame-retardant effect, proving\nthe optimality of the TPP content in our design.\nSupplementary Figure 33. Digital photos of pouch full-cell using pure Li negative\nelectrode with double non-flammable electrolyte (500 mL, 7.8 g Ah-1) content under\nconstant ignition. Even if the content of the non-flammable electrolyte is increased, the\npouch battery with pure Li as the negative electrode still cannot achieve the flame-\nretardant effect, proving that it is our negative electrode design that plays a decisive\nrole in the flame-retardant effect of the pouch cell.\nSupplementary Figure 34. The nail penetration test of pure Li||NCM811 (a) and\nLi/ZnO/TPP/GO||NCM811 (b) pouch cells with non-flammable electrolyte.\nComments: 6) It would be better to add more reproducing SEM characterization results\nof NCM811 cathode after 100 cycles using Li//ZnO//TPP//GO anode for better\nsupporting the positive effect of the author's anode design on the cathode's cycling\nstability.\nResponse:\nWe thank the reviewer for raising this suggestion. Following the reviewer’s\nsuggestion, we add more top-view SEM images of NCM811 positive electrode after\n100 cycles using Li/ZnO/TPP/GO negative electrode in Supplementary Fig. 53.\nChange to the Supplementary Information:\nSupplementary Figure 53. SEM morphology characterizations on 1.63 mAh cm-2\nNCM811 positive electrodes after 100 cycles at a specific current of 94 mA g-1 (cycled\nbetween 3.0 V and 4.5 V at 25 ± 0.2°C, screened at discharged state) in LHCETTE\nelectrolyte using Li/ZnO/TPP/GO (a-g) or Li@TPP/GO (h) negative electrodes."}]

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