A flame-retardant and weakly solvated gel electrolyte for high-performance and high-safety Ah class sodium-ion batteries

Abstract

Phosphate-based electrolytes can effectively improve flame retardancy of sodium-ion batteries (SIBs), but there is a problem of incompatibility with carbon based electrodes. Here, a flame-retardant gel electrolyte (FRGE) strategy can solve the above problems simultaneously by adopting perfluorinated electrolyte and high content of phosphate ester, and cross-linked gel polymer matrix. The electrolyte solvation structure is optimized by the electrostatic adsorption action of the gel, which prompts the entry of anions into the first solvation layer, forming more ion aggregates (AGGs) and contact ion pairs (CIPs). The FRGE enhances the interaction between Na-ion and the PF₆⁻ anion, promoting the decomposition of the anion at the interface to form a stable dense fluorinated SEI film, while reducing the possibility of phosphate ester co-embedding in the carbon-based anode. Remarkably, the designed 2.4 Ah Na_0.9[Cu_0.22Fe_0.3Mn_0.48]O₂/HC pouch cell incorporating FRGE demonstrates an impressive low-capacity degradation rate of 0.0035 % per cycle over 1000 cycles, attesting to a high energy density of 124.6 Wh kg⁻¹. The composite gel electrolyte not only enhances flame retardancy but also boosts electrochemical properties, furthering the practical application of SIBs.