Polybenzimidazole functionalized electrolyte with Li‐wetting and self‐fluorination functionalities for practical Li metal batteries

National Key Laboratory of Science and Technology on Advanced Composites in Special Environments, and Center for Composite Materials and Structures, Harbin Institute of Technology, Harbin, China School of Physics, University of Electronic Science and Technology of China, Chengdu, China School of Materials and Energy, University of Electronic Science and Technology of China, Chengdu, China Institute of Atomic and Molecular Physics, Sichuan University, Chengdu, Sichuan, China


| INTRODUCTION
The surging demand for high-energy-density batteries urges the utilization of lithium (Li) metal anode due to its high theoretical capacity (3860 mAh g À1 ), favorable gravimetric density (0.534 g cm À3 ), and negative electrochemical potential (À3.04 V vs normal hydrogen electrode). 1 Nevertheless, highly reactive Li metal and electrolyte decomposition at high voltage cause safety concerns due to notorious dendrite growth and intense heat released from the exothermic reaction between Li anode and liquid electrolytes, 2 leading to two major research efforts in the field of Li metal batteries (LMBs), namely suppression on Li dendrite formation and enhancement on physicochemical properties of electrolytes.
Extensive research efforts have been centered on improving the applicability of the Li anode in liquid LMBs. For instance, Li anode architecture design, including constructing 2D/3D Li anode skeleton 3 and utilizing aprotic bimetallic Li-Na alloy, 4 effectively suppresses Li protuberance by accommodating charge distribution. Liquid electrolyte engineering based on organic trisaminocyclopropenium cations 5 and N,N-dimethylacetamide additives 6 can inhibit dendrite growth through electrostatic shielding effect and reconstruction of the Li + solvation structure. Constructing an artificial solid electrolyte interface (SEI) with thermoplastic polyurethane, 7 tetraethyl orthosilicate, 8 and hydrophobic SiO 2 nanoparticles coupled with CF 3 SO 3 À functional group, 9 has been proposed to stabilize Li interface without dendrite growth. Compositing monolayer separators with organosulfur compounds to build a resilient SEI is also proposed to attenuate ionic concentration gradient and eliminate dendrite growth. 10 In addition, protective coating layers 11 and multilayer functional separators 12 have been realized to avoid the perturbation in the Li deposition process. Nonetheless, all these aforementioned strategies are based on the utilization of volatile, inflammable liquid electrolytes, still posing potential threats to safe operation of LMBs.
Solid polymer electrolytes (SPEs) with increased thermostability have been proposed as alternatives to liquid electrolytes. 13,14 Polyethylene oxide (PEO) polymer enables Li + transport through segmental motion and is frequently exploited due to low cost and lightweight. 15 However, highly crystalline PEO requires a high operating temperature (~60 C) to agitate chain motion which sacrifices its mechanical strength. 16 Poly(vinylidene fluoride) (PVDF), a polar and semicrystalline polymer with small van der Waals radius (1.32 Å) fluorine atoms and robust C F bond (485 kJ mol À1 ), owns low surface energy, high dielectric constant and inertness to oxidation, all of which are essential for SPEs. 17,18 Based on this conception, PEO and PVDF polymer blend with increased amorphous region was studied to enhance room-temperature conductivity. 19 Nevertheless, the low melting point of PEO owing to feeble intermolecular van der Waals forces prevents it from operation above 90 C. 20 High melting point polymers, such as poly(methylmetacrylate) (PMMA) 21 and poly(acrylonitrile) (PAN), 22 own improved thermostability to resist dimensional deformation, but those polymer additives are flammable due to the easy cleavage of C C bonds. 23 In this report, we design a flame-retardant SPE by incorporating fire-resistant polybenzimidazole (PBI) to reinforce α-phase PVDF matrix. The multiple proton donor sites ( NH ) of PBI form dense hydrogen bonds (À0.26 eV, 5.4 Â 10 À3 mol g À1 ) with PVDF, decreasing its dimensional shrinkage by 40% at 300 C. Nuclear magnetic resonance (NMR) and x-ray photoelectron spectroscopy (XPS) of the SPE combustion residues and emissions confirm that partial CH 2 CF 2 structure in PVDF is converted to nonflammable CF 2 CF 2 segments of polytetrafluoroethylene (PTFE) under the catalysis of imidazole rings of PBI in the burning process. PBI also forms a high-graphitization residual carbon layer on the SPE surface to block heat and O 2 for extinguishing. Density functional theory (DFT) calculations verify that negatively charged PBI owns a large adsorption energy with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and shares an overlapping electron density with the Li anode, giving rise to expedited Li + conduction at room temperature and a homogeneous electric field to yield a dendrite-free Li anode. A LiFePO 4 cell with the PBI/PVDF/ LiTFSI SPE delivers a 127 mAh g À1 discharge capacity at 25 C with an 87.5% capacity retention after 500 cycles at 0.5 C (0.15 mA cm À2 ).

| Design concepts and characterizations of PBI electrolyte
A PBI molecular monomer owns a five-membered aromatic heterocycle with two intermediate nitrogen atoms (imidazole) and a benzene ring in the polymer backbone ( Figure 1C). The thermostability of a polymer depends primarily on the strength of chemical bonds to endure chemical structure change at extreme temperatures. 24 The bond energy of delocalized π-bond (5.37 eV), C C (6.59 eV), and C N (6.37 eV) bonds in the benzimidazole ring is above that of the C C bond (3.69 eV) in the PVDF backbone, leading to better thermal stability as compared with PVDF. Electrostatic potential analysis illustrates that the benzimidazole chain owns a negatively charged surface to coordinate with Li + and improve Li + dissociation, 25 while the negative charges in PVDF reside on fluorine atoms ( Figure 1B-D). Both the proton acceptor ( N ) and donor ( NH ) groups in the imidazole ring contribute to robust intramolecular hydrogen bonds and  Figure 1E). 26 The interaction between PBI and PVDF is further confirmed with DFT calculations, as shown in Figure S1. The adsorption energy between PBI and PVDF is À0.26 eV, which is comparable to the hydrogen bond.
The thickness of the SPEs is 50 μm (Figure S2B,C) and homogeneous distribution of PBI and lithium salts is demonstrated with N and S elements mapping in Figure  1G,H. As shown in Figure S2A, the images of scanning electron microscopy (SEM) show surficial cavities at the PVDF/LiTFSI SPE interface, which induces uneven lithium-ion distribution and dendrite growth at the SPE/Li anode interface ( Figure 1A). In contrast, Figure  1F shows a compact and dense surface of PBI/PVDF/ LiTFSI SPE due to the dense hydrogen bonds between PBI and PVDF (5.4 Â 10 À3 mol g À1 , calculation details in molecular dynamics simulations). Fourier transform infrared (FT-IR) spectroscopy of PBI confirms the characteristic peaks of in-plane C N, in-plane C H, and N H wagging vibration at 1480, 1227, and 1184 cm À1 ( Figure  S3), respectively. 27,28 For PVDF, the peaks at 488, 766, and 851 cm À1 are attributed to α-phase PVDF, 29 and the peaks at 766 and 851 cm À1 correspond to the bending and out-of-plane deformation of CF 2 and C H bonds, respectively. The composite of the PBI and PVDF verifies all characteristic bonds of both. As shown in Figure 1I, Raman spectra show the C C and C N vibrations of the benzimidazole ring at 1546 and 1616 cm À1 after the addition of PBI. 30 The peak intensity of the CH 2 group at 2980 cm À1 in PVDF significantly decreases and those of CH (1432 cm À1 ) and C C (1132 cm À1 ) increase with PBI incorporation. This is because imidazole rings act as Lewis bases to donate lone pair electrons and induce partial deprotonation of CH 2 in PVDF to form C C bonds. 31 Figure S4 shows that the key performance of PBI/PVDF/LiTFSI, including flame-retardant capability, operational temperature range, cell capacity, cycle life, and dendrite suppression, outperforms the state-of-theart composite polymer/polymer solid electrolytes. 32,33

| Binary PBI/PVDF SPE for highperformance batteries
The electrochemical window of PBI is investigated by calculating the energy levels of the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO). Stable electrochemical operation requires a LUMO value above À3.96 eV and a HOMO value below À5.36 eV (calculation details in Experiments section). As shown in Figure S5A, the HOMO and LUMO values are À5.44 and À1.82 eV, respectively, suggesting that PBI is electrochemical stable in the redox reactions, which is consistent with the linear sweep voltammetry experimental results ( Figure S5B). Figure S5C shows that the reduction process commences at 2.1 V for PVDF/LiTFSI, while it is extended to 1.8 V for PBI/PVDF/LiTFSI, indicating that the addition of PBI effectively improves the electrochemical stability of PVDF/LiTFSI in low potential. As shown in Figure 2A, cyclic voltammetry (CV) shows much higher redox peaks for PBI/PVDF/LiTFSI as compared with those of PVDF/ LiTFSI, indicating improved kinetics in electrochemical reactions. No redox peak shift is observed within the first three cycles for the cell with PBI/PVDF/LiTFSI SPE (Figure 2B), indicating a short cell stabilization time due to fast Li + transfer. As shown in Figure 2C, electrochemical impedance spectroscopy (EIS) shows that the PBI/PVDF/ LiTFSI cell owns a much smaller semicircle diameter (R ct ) at the medium and high frequency region as compared with PVDF/LiTFSI cell, which is attributed to the lower charge transfer resistance at the PBI/Li metal interface. The Li + transference number ( Figures 2D and S6) of PBI/PVDF/LiTFSI is 0.49, higher than that of PVDF/ LiTFSI (0.37), which is attributed to improved Li + dissociation from LiTFSI and impeded movement of TFSI À by the electron-enriched benzimidazole chains featured with negatively charged surfaces. By contrast, the dipoles (C H and C F) of α-PVDF, the most thermally stable PVDF at room temperature, are antiparalleled with TGTG' (T-trans, G-gauche + , G'-gauche À ) dihedral conformation, leading to a nonpolar molecular structure, which is ineffective in improving the Li + transference number. 34 Symmetric Li/Li cells are tested at 0.025-0.2 mA cm À2 to evaluate dendrite suppression effects of various SPEs. Figure 2E shows a stable voltage profile for the PBI/PVDF/LiTFSI cell over 260 h. In contrast, the voltage polarization of the PVDF/LiTFSI cell is higher at all currents. Dramatical voltage fluctuation starts as the current density increases to 0.2 mA cm À2 and a short circuit occurs after 220 h. Since transient nonuniform protrusions could occur at a high current density due to fast Li + plating, the voltage fluctuation of the PBI/PVDF/LiTFSI cell is possibly due to lithium cracks, pulverization, and peeling off from the protrusions. [35][36][37] As shown in Figure  S7, a long-term (110 h) stable voltage profile is demonstrated with PBI/PVDF/LiTFSI at 0.2 mA cm À2 , two times longer than that of PVDF/LiTFSI (40 h). Figure 2F shows a higher ionic conductivity for the PBI/PVDF/ LiTFSI SPE as compared with the PVDF/LiTFSI SPE. We further conduct high-temperature cycling to evaluate the thermal stability of the PBI-reinforced SPE. As shown in Figure 2G, the PVDF/LiTFSI cell shows a rapid capacity decrease as the temperature exceeds 75 C and stops working at 90 C. In comparison, the PBI/PVDF/LiTFSI cell delivers a capacity of~85 mAh g À1 even at 90 C, demonstrating the much-enhanced thermal stability after the incorporation of PBI. As shown in Figure 2H, the PBI/PVDF/LiTFSI cell delivers discharge capacities of 143 mAh g À1 at 0.1 C and 93 mAh g À1 2 C, while the PVDF/LiTFSI cell 138 mAh g À1 at 0.1 C and 64 mAh g À1 at 2 C, indicating significantly improved rate performances. Figure 2I shows that the voltage hysteresis of the PBI/PVDF/LiTFSI cell (101.6 mV) is below that of the PVDF/LiTFSI cell (245.6 mV). As shown in Figure 2J, the PBI/PVDF/LiTFSI cell delivers a high discharge capacity of 127 mAh g À1 at 0.5 C (0.15 mA cm À2 ) with an 87.5% capacity retention after 500 cycles (111.2 mAh g À1 ). For comparison, the capacities of the PVDF/LiTFSI cell significantly decrease after~300 cycles due to the large interfacial resistance and inferior thermal stability. Such advantageous performance of PBI/PVDF/LiTFSI SPE is also demonstrated with high-loading cathodes (Figure S8).
The effects of the benzimidazole functional groups with various mass fractions in the composite solid electrolyte are investigated. Prolonged cycling life and improved capacity are observed for the PBI/PVDF/LiTFSI cell as compared with the PVDF/LiTFSI cell ( Figure S9). The cell capacity increases with increasing PBI content from 5% to 8% since the hydrogen bonds between PBI and PVDF reinforce the structure, stabilize the SPE/Li anode interphase, and accelerate Li-ion transport. However, excessive PBI (8%-10%) leads to capacity decay because the rigid PBI chains impair the motion of PVDF chains. We further fabricate a PBI/LiTFSI SPE without PVDF. Robust intramolecular hydrogen bonds between proton acceptor ( N ) and donor ( NH ) groups in the imidazole ring form a rigid chain structure in which C C bonds on the polymer backbone fail to rotate and oscillate to transport Li + . In addition, high glass phase transition temperature of PBI (425 C) prevents the chain from swinging effectively below 425 C. Consequently, the cell assembled with PBI/LiTFSI fails to run, demonstrating that PBI alone cannot be used as a SPE.

| Inhibition mechanisms of Li dendrite growth
DFT calculations are carried out to understand the Li transfer and plating behaviors. Figure S10 shows that the adsorption energy between LiTFSI and PBI (À1.49 eV) is far beyond that between LiTFSI and PVDF (À0.86 eV),  Figure S11, the adsorption energy of Li + on PBI is À2.8 eV, which is significantly higher than that on PVDF (À1.98 eV) and the Li (0 0 1) surface (À1.37 eV), showing that Li + ions are preferentially adsorbed on PBI rather than the Li metal surface and PVDF. As shown in Figure 3A, the rigid chain structure of PBI prevents it from rotating and thus shows a planar conformation on the Li (0 0 1) surface, both sharing uniform electron distribution, which uniformizes the Li + concentration at the interface ( Figure S12A). The higher adsorption energy of Li (0 0 1)/PBI (À2.22 eV) compared with Li (0 0 1)/PVDF (À0.42 eV) verifies increased affinity between the PBI/PVDF SPE and Li metal. The robust adsorption of PBI with both LiTFSI and Li metal expedites Li + conduction, accounting for the as-obtained improved Li + conductivity at room temperature. As PVDF is on the Li (0 0 1) surface, the electronegativity difference between H (2.20) in C H dipole and F (3.98) in C F dipole causes the C C bond on the backbone to rotate where C F bond is more prominent and closer to the surface of Li metal. The partially overlapping charge density between PVDF and Li (0 0 1) indicates that Li + ions tend to accumulate to form dendrites at the Li metal surface due to the uneven electric field on the Li anode surface (Figures 3B and S12B). During the following Li plating/striping processes, a solid electrolyte interphase (SEI) consisting of LiF is formed on Li metal surface due to the decomposition of LiTFSI, 38 as confirmed with XPS and FT-IR ( Figure S13A,B). 39,40 As shown in Figure S13C, the uniform electron distribution between LiF (1 0 0) and PBI indicates that PBI with planar configuration also favors homogeneous Li deposition at SEI, delivering a long-term stable voltage profile over 2000 h at a current density of 0.1 mA cm À2 ( Figure  3C), which is over three times longer than that of the PVDF/LiTFSI SPE (620 h).
Phase-field simulations are employed to numerically study the electric-field distribution at the SPEs/Li metal interface, as shown in Figure 3D,E. Parameter ξ = 0 (blue region) denotes the solid electrolyte phase, and ξ = 1 (red region) denotes the lithium anode phase. The region between the blue and red zones represents their interface. After Li electroplating (Figure 3D), the PVDF/LiTFSI model shows needle-like protrusions at the SPE/Li metal interface. The current density at the top of the protuberance is up to 0.2 mA cm À2 , but only 0.05 mA cm À2 at the bottom. Based on the electrostatic potential equation (equation (S3) in the Supporting Information S1), the nonuniform protrusions lower the electric potential and accumulate Li ions at the top of the protrusions, aggravating the unevenness of ion distribution and leading to the growth of large Li dendrites. For PBI/PVDF/LiTFSI, uniform interfacial current density (0.05 mA cm À2 ) is observed on the Li metal surface. The uniform spatialelectric-field distribution demonstrates that the lithiophilic benzimidazole chain effectively suppresses lithium dendrite growth.
The surface morphologies of various SPEs after cycling are characterized to illustrate the theoretical simulations. As shown in Figure S14A, mossy-like dendrites are observed on the PVDF/LiTFSI SPE. In comparison, PBI/PVDF/LiTFSI SPE shows smooth and flat morphology ( Figure S14B), further revealing that the incorporation of PBI in the solid electrolyte effectively stabilizes Li metal anode for uniform Li deposition ( Figure 1B). The dendrite formation kinetics is further visualized with phase-field simulations. Li metal with PVDF/LiTFSI SPE exhibits obvious dendrite growth over time ( Figure 3F and Video S1), which is consistent with the SEM image after Li deposition, showing rough surface with rough protrusions ( Figure 3G). For comparison, the Li plating morphology in the PBI/PVDF/LiTFSI cell owns a smooth surface from both theoretical and experimental results ( Figure 3H,I and Video S2), as attributed to the homogeneous spatial-electric-field distribution at the PBI/Li anode interphase. As shown in Figure S14C-I, the interface morphologies of PBI/LiTFSI SPE, PBI/PVDF, and Li anode show no difference after assembled in cells for 24 h, indicating that both PBI and PVDF are stable with Li metal.

| Superior thermostability of PBI/ PVDF SPE with flame-retardant properties
To gain further insights into intermolecular interaction between PBI and PVDF, molecular dynamics (MD) simulations are performed. Figure 4A shows that the linear chains of pure PVDF disentangle and orient to the stretching direction under uniaxial tension, accounting for the weak van der Waals force in neighboring chains. A microscale void forms initially and propagates rapidly into a large crack, leading to a final rupture without pulling chains. With PBI ( Figure 4B), intermolecular and intramolecular hydrogen bonds form between N H (donors) and electronegative atoms N and F (acceptors). PBI serves as an "anchor" to twine the PVDF chains through physical crosslinks, dissipates energy by relaxing the stretched polymer chains, and, thus, effectively prevents the PVDF chain from sliding and unwinding even under an increased strain. The experimental strain-stress curves show that the PBI/PVDF/LiTFSI SPE owns better mechanical properties (5.3 MPa) in comparison with PVDF/LiTFSI SPE (4.8 MPa), which is consistent with theoretical MD simulations, as shown in Figure S15. We further construct a mathematical model to estimate the strength of solid electrolytes. The solid electrolyte is assumed to be elastic and isotropic. The interatomic force and potential energy associated with the separation distance of the atoms are schematically shown in Figure 4C,D, respectively.
Based on the dependence of the stress upon the atomic displacement, we assume that the relationship between stress σ s and the atomic separation distance from the equilibrium position x approximately follows a sine function: where σ max is the maximum stress (theoretical strength) and λ is the wavelength of sine function. The Hooke's law gives the stress between two atoms: where E is the Young's modulus of the electrolyte and r 0 is the equilibrium distance between pairs of atoms. Based on the Griffith's theory, the thermodynamics requires that the work to break bonds should be above the work to create a new surface, yielding: where G be is the bond energy. The integral leads to: Combining Equations (1)-(4) yields: where σ max and r 0 are the theoretical strength and equilibrium distance between pairs of atoms, respectively. G be and E are the bond energy and the Young's modulus of the solid electrolyte obtained from DFT calculations. We then analyze the failure modes of solid electrolytes by correlating strength and bond energy based on Equation (5). According to the estimated bond strength (Table. S1), the strength of hydrogen bond is~4 times stronger than that of the van der Waals force, suggesting that introducing hydrogen bonds between the two polymer molecules is effective for enhancing the mechanical strength of PVDF/LiTFSI SPE. Forward-looking infrared radiometer (FLIR) is employed to analyze the thermal stability of the SPEs. The conformation of PVDF/LiTFSI experiences severe deformation due to the accumulation of heat as the temperature is above 150 C ( Figure 4E). For comparison, PBI/PVDF/LiTFSI exhibits a uniform thermal distribution as temperature increases from 30 C to 300 C. The thermal shrinkage of the PBI/PVDF/LiTFSI SPE is 29% at 300 C, which is much smaller as compared with PVDF/LiTFSI (49%, Figure 4F). The 40% reduction in thermal shrinkage demonstrates that heat transfers rapidly from the substrate to the PBI reinforced SPE surface without local heat accumulation. The flame-retardant properties of the PBI/PVDF/LiTFSI SPE are investigated with igniting tests. Thermogravimetry analysis shows the inevitable existence of combustible plasticizer 1-methyl-2-pyrrolidinone in the PVDF matrix ( Figure S16). As shown in Figure 4G and Video S3, the PVDF/LiTFSI SPE shows immediate shrinkage and complete combustion as exposed to fire, which carbonizes PVDF with the release of a huge amount of heat. Although PBI/PVDF/LiTFSI SPE is ignited at the initial stage, the flame is significantly weaker than that of PVDF/LiTFSI SPE and shows self-extinguishing properties, demonstrating the superior fire retarding capability ( Figure 4H and Video S4).

| Evolution of binary PBI/PVDF SPE in fire
We simulate the PBI/PVDF architecture evolution in a high-temperature environment through Ab initio molecular dynamic (AIMD) calculations to evaluate the flameretardant mechanism, as shown in Figure 5A. PVDF is pyrolyzed to a low-molecular-weight fluoropolymer at 200 fs (Equation (6)), accompanied with the formation of hydrogen radicals (H˙) and fluorine radicals (F˙) due to the C H and C F bonds breakage (Equations (7) and (8)). Figure S17, the energy barrier for the C H and C F bond breakages in PVDF chain is 3.62 and 2.94 eV, respectively. After the addition of PBI, the energy barriers remarkably decrease to 2.55 and 1.65 eV. Since defluorination and formation of H˙are energetically favorable in the alkaline environment, 41 we propose that the imidazole ring, acting as a Lewis base, catalyzes the dissociation of H and F atoms from the PVDF chain in fire, leading to formation of more H˙and F˙. Figure S18 shows that H˙reacts with oxygen (O 2 ) to form H 2 O through the intermediate state of oxyhydrogen radical (˙OH) in fire (Equations (9) and (10)). 42 In addition, PVDF chain also readily forms HF molecules through dehydrofluorination (500 fs), 43 leading to the formation of C C bond (Equation (11) and process 1 of Figure 5C). We hypothesize that the activated C C bonds in the fluoropolymer chain react with free F˙through nucleophilic addition reaction, giving rise to the PVDF structure variation from CF CH CF 2 CH 2 to CF 2 CHF CF 2 CH 2 (Equation (12) and process 2 of Figure 5C).

As shown in
As the combustion continues, residual hydrogen atoms in the modified-PVDF structure are continuously replaced by fluorine atoms in repeated processes 1-2 (Equations (11)-(13) and Figure 5C), giving rise to the formation of the CF 2 CF 2 structure.
As shown in Figure 5B, F˙and modified PVDF chain without H atoms are taken to simulate fluorination addition reaction with AIMD calculations. F˙is attached to carbon atom in the polymer chain with the formation of C F bond (80 fs), and CF 2 CF 2 structure is observed at 190 fs, confirming the formation of PTFE segments. Because high fluorine contents enable dramatically enhanced thermostability, 17,18 the modified PVDF polymer with the CF 2 CF 2 structure owns fire selfextinguishing properties.
NMR is employed to verify experimentally the PVDF molecular structure variation by investigating the composition of the PBI/PVDF/LiTFSI residues after combustion. As shown in Figure 5D, the peaks located at À125.7, À49.2, and À122 ppm correspond to CF 2 CF 3 and CF 2 CF 2 CF 2 fluorated groups, 44 which is consistent with the results of the PTFE powders except the peak intensity, confirming the formation of CF 2 CF 2 segments in the modified PVDF molecular structure. Particularly, unsaturated C C bond is observed at À109.2 ppm, 45 which supports our hypothesis that activated C C bonds react with free F˙through nucleophilic addition reaction to form high fluorine contents segments ( CF 2 CF 2 ) (process 2 of Figure 5C), confirming the self-extinguishing properties of PBI/PVDF/LiTFSI in fire. For comparison, no CF 2 CF 2 signal is detected for the residues of PVDF/LiTFSI, as shown in Figure S19. The dominant peaks at À91.4 and À78.7 ppm correspond to crystalline domain of PVDF ( Figure S19), and the double peaks at À114.9 and À113.7 ppm are attributed to head-to-head PVDF units. 46 We further substitute PBI with polyimide (PI) that has a heterocyclic structure containing nitrogen atom to compound PVDF and investigate the residue composition after combustion. The NMR spectrum of PI/PVDF/LiTFSI shows no CF 2 CF 2 signal, confirming that imidazole ring of PBI catalyzes the dehydrogenation and fluorination of PVDF to form CF 2 CF 2 structure. Figure S19 shows that no CF 2 CF 2 signal is observed for the pristine PBI/PVDF/LiTFSI SPE before combustion, suggesting that the catalytic effect of PBI on PVDF to form PTFE segments is insignificant without fire. We further employ XPS to analyze the combustion products at various burning states. As shown in Figure 5E, at the initial burning state of PVDF/LiTFSI, the Ni F signal (~685 eV) in F 1s spectra is attributed to the interaction between small resultant fluorochemical particulates, such as LiF (~684 eV), and Ni foam substrate during the combustion. 47 The peak at~688.4 eV corresponds to CF CH group derived from CF 2 CH 2 in the PVDF chain after dehydrofluorination process, 48 which is consistent with the process 1 in the PTFE formation mechanism ( Figure 5C). For PBI/PVDF/LiTFSI ( Figure 5F), the CF 2 CF 2 signals protrude at 689.2 and 691.5 eV in F 1s spectra, 49 leading to superior heat-resistant and flameretardant capability of PBI/PVDF/LiTFSI. The combustion product of PBI/PVDF polymer without LiTFSI is then studied. As shown in Figure 5G, although the various C F bonds in F 1s spectra shift to lower binding energy, the CF 2 signals of PFTE are prominent at 688.7 and 690.9 eV, 50,51 confirming that partial PVDF is converted to fire-resistant PTFE segments through dehydrogenation and fluorination under the catalysis of imidazole rings in fire. In addition, we characterize the carbon residues of the SPEs after the burning test with Raman spectroscopy. As shown in Figure S20, the two dominant peaks at 1315 and 1585 cm À1 correspond to D band (amorphous carbon) and G band (graphitized carbon) of carbon, and the intensity ratio between D band and G band (I G /I D ) represents the graphitization. For PBI/PVDF/LiTFSI, the value of I G /I D is 7.43, higher than that of PVDF/LiTFSI (4.87), indicating that the benzimidazole chain decomposes to highly graphitized carbon layer on PVDF to block heat and O 2 .

| CONCLUSION
In summary, a PBI-reinforced solid polymeric electrolyte is developed. The binary SPE owns superior self-extinguishing properties by converting partial PVDF into fire-resistant PTFE segments in fire with PBI-catalyzed dehydrogenation and fluorination. The highly graphitized residual carbon layer on the SPE surface blocks heat and O 2 to further enhance the fire-extinguishing capability of the SPE. Theoretical simulations corroborate strong adsorption of lithiophilic PBI on both LiTFSI and the Li anode, which expedites Li + conduction at room temperature and uniformizes the interfacial electric field for homogenous Li deposition, yielding a dendrite-free Li metal. Based on the proposed architecture, the all-solid-state Li metal batteries with PBI/PVDF/LiTFSI SPE delivers a high capacity of 111.2 mAh g À1 after 500 cycles at 0.5 C (0.15 mA cm À2 at 25 C).

SUPPORTING INFORMATION
Additional supporting information may be found in the online version of the article at the publisher's website.