Designing Low‐Cost High‐Conductivity and Nonflammable Phosphate Electrolytes Toward High‐Energy Sodium‐Ion Batteries

Abstract Safety hazard induced by flammable electrolytes are troubling the advancement of practical sodium‐ion batteries (SIBs). Non‐flammable phosphate electrolytes are competent to address this issue, yet current designs struggle to balance the interfacial chemistry and high ionic conductivity due to the untamed interaction between Na + and anion. Herein, we evaluate the effect of dielectric constant (DC) and binding energy ( E a ) of solvents on this interaction, revealing a systematic approach to achieve desired designs. It is shown that incorporating solvents with high DC and moderate E a forms anion‐rich solvation structure for stabilizing the anode interface and achieving necessary salt dissociation for high ionic conductivity. Specifically, propylene carbonate is incorporated as the eligible solvent into triethyl phosphate, affording non‐flammable electrolytes with low cost and high ionic conductivity of 6.22 mS cm −1 . Consequently, 1.25 Ah hard carbon (HC)|NaNi 1/3 Fe 1/3 Mn 1/3 O 2 (NFM) pouch cell delivers a high‐power discharge of 5C rate and maintains an impressive capacity retention of 87.1% after 700 cycles at 2C rate. Moreover, 6.61 Ah HC|NFM pouch cell with 152.3 Wh kg −1 avoids smoke production and volume expansion during nail penetration. This work discloses an advanced strategy for deigning practical phosphate electrolytes, paving the way for developing high‐performance SIBs.