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

ABSTRACT 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.