Gel electrolyte design for nonflammable lithium-ion batteries with high-rate and high-voltage characteristics

High-energy, high-power, and high-safety are the main pursuits for developing lithium-ion batteries. Previous approaches have focused on individual problems but rarely proposed holistic solutions. The present study develops a gel polymer electrolyte (GPE) that simultaneously achieves high power/voltage capability, long cycle life, and enhanced safety, through a polymer–solvent combination of poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP), known for high polarity and stability, with ethoxy(pentafluoro)cyclophosphazene (PFPN), a flame-retardant solvent. The interaction between the electron-rich P–N=P of PFPN and the electron-deficient –CH₂– of PVdF-HFP effectively creates networked polymer, conferring gelling and flame-retardant properties to the electrolyte. The PFPN, together with low-viscosity and F-containing solvents and cathode-reinforcing anions, in the Li⁺-solvation sheath attracts the PVdF-HFP chains, thereby forming solid-electrolyte and cathode-electrolyte interphases (SEI and CEI, respectively) containing disrupted PVdF-HFP fragments for interphase reinforcement and Li⁺-transfer promotion. The developed GPE exhibits high ionic conductivity and the resulting LiNi_0.8Mn_0.1Co_0.1O₂||Graphite cell has thin and dense SEI and CEI and exhibits long-term cycling (to 1000 cycles) and stable operation at high rates (to 12 mA cm⁻², for a single-layer cathode) and high voltages (to 4.6 V). A nail penetration test on a high-energy (2.5 Ah) pouch cell at 4.4 V demonstrates the superior flame-retardant ability of the GPE.