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

Rough Li plating, low ionic conductivity, and low thermal stability of conventional electrolytes post-primary challenges for achieving reliable high-capacity rechargeable lithium batteries, for which lithium metal is frequently proposed as the most promising anode material. Conventional low-polarity commercial polypropylene/polyethylene separators fail to support the application of high-energy-density Li anodes due to their rigid physicochemical properties and the high reactivity of Li metal, leading to fatal dendrite formation and vigorous exothermic reaction with electrolytes. Herein, we develop a Li-wetting, flame-retardant binary polymer electrolyte by functionalizing poly(vinylidene fluoride) (PVDF) separators with nonflammable polybenzimidazole (PBI) to build safe room-temperature solid-state electrolyte membranes. A dendrite-free LiFePO4 cell with the solid polymer electrolyte (SPE) delivers a discharge capacity of 127 mAh g(-1) at 25 degrees C with a capacity retention of 87.5% after 500 cycles at 0.5 degrees C (0.15 mA cm(-2)). Phase-field simulations and density functional theory calculations demonstrate that the negatively charged benzimidazole chains of PBI own superior affinity to lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and shares overlapping electron density with Li anode, giving rise to accelerated Li+ conduction at room temperature and uniform Li electrodeposition at the electrolyte/Li metal interface. The SPE is also flame-retardant since heat-resistant polytetrafluoroethylene and a dense, heat-blocking graphitized carbon layer are formed in intense heat through dehydrogenation/fluorination of PVDF under the catalysis of Lewis base imidazole rings and the decomposition of benzimidazole rings in PBI. No such fire-resistant mechanism is ever reported in conventional electrolytes.

Automated summary

In this study, a Li-wetting, flame-retardant binary polymer electrolyte was developed by functionalizing PVDF separators with PBI, creating a safe room-temperature solid-state electrolyte membrane that can enhance Li+ conduction rate and uniform Li electrodeposition. This unique solid polymer electrolyte also exhibits flame-retardant properties, which is not reported in conventional electrolytes.