Electrolyte Engineering with Tamed Electrode Interphases for High‐Voltage Sodium‐Ion Batteries

Abstract Sodium‐ion batteries (SIBs) hold great promise for next‐generation grid‐scale energy storage. However, the highly instable electrolyte/electrode interphases threaten the long‐term cycling of high‐energy SIBs. In particular, the instable cathode electrolyte interphase (CEI) at high voltage causes persistent electrolyte decomposition, transition metal dissolution, and fast capacity fade. Here, this work proposes a balanced principle for the molecular design of SIB electrolytes that enables an ultra‐thin, homogeneous, and robust CEI layer by coupling an intrinsically oxidation‐stable succinonitrile solvent with moderately solvating carbonates. The proposed electrolyte not only shows limited anodic decomposition thus leading to a thin CEI, but also suppresses dissolution of CEI components at high voltage. Consequently, the tamed electrolyte/electrode interphases enable extremely stable cycling of Na 3 V 2 O 2 (PO 4 ) 2 F (NVOPF) cathodes with outstanding capacity retention (>90%) over 3000 cycles (8 months) at 1 C with a high charging voltage of 4.3 V. Further, the NVOPF||hard carbon full cell shows stable cycling over 500 cycles at 1 C with a high average Coulombic efficiency (CE) of 99.6%. The electrolyte also endows high‐voltage operation of SIBs with great temperature adaptability from −25 to 60 °C, shedding light on the essence of fundamental electrolyte design for SIBs operating under harsh conditions.