All-Cyclic-Solvent Electrolyte Design Enables Ultra-Low-Temperature and Fast-Charging Sodium-Ion Batteries.

Conventional sodium-ion battery (SIB) electrolytes are typically composed of mixed cyclic and linear carbonates, which face critical challenges at low temperatures, including solvent crystallization and sluggish Na⁺ transport. In this work, we developed a low-cost, all-cyclic-solvent electrolyte in which cyclic ethers, tetrahydrofuran (THF) and cyclopentyl methyl ether (CPME), are gradually introduced into the high-polarity cyclic carbonates, propylene carbonate (PC) and ethylene carbonate (EC), systematically reconstructing the Na⁺ solvation structure. Competitive coordination between the high-polarity cyclic carbonates and low-polarity cyclic ethers generates an anion-rich solvation environment, simultaneously suppressing electrolyte crystallization and lowering Na⁺ desolvation energy. The optimized electrolyte exhibits high ionic conductivity, an ultralow freezing point (< -130°C), and excellent interfacial stability, enabling outstanding performance, including ultra-fast charging (89.9 mAh g⁻ ¹ at 50 C), extreme low-temperature operation (57% capacity retention at -70°C), and ultra-long cycling stability (>10000 cycles). Its practical applicability was further validated using 26700 cylindrical cells, which demonstrated stable cycling for over 2000 cycles at room temperature and maintained stable performance at -40°C. The cells exhibited high safety under abuse conditions, including thermal abuse and nail penetration. This design strategy can be generalized to other all-cyclic-solvent systems, providing a universal approach for low-cost, fast-charging, ultralow-temperature SIBs.