Stabilizing Interfacial Reactions for Stable Cycling of High‐Voltage Sodium Batteries

Abstract Developing advanced electrolytes is critical for stabilizing electrode/electrolyte interfacial reactions and thus extending cycling stability of sodium (Na) batteries, especially when a high‐voltage cathode (such as NaNi 0.68 Mn 0.22 Co 0.10 O 2 (NaNMC)) is used to achieve high energy density in batteries. Here, an advanced electrolyte based on sodium bis(fluorosulfonyl)imide (NaFSI)–triethyl phosphate, which is highly stable against a high‐voltage cathode, enabling long‐term cycling of sodium batteries, is reported. Na||NaNMC cells with this electrolyte demonstrate 89% capacity retention after 500 cycles with a cutoff voltage of 4.2 V versus Na/Na + . A full cell of hard carbon||NaNMC also exhibits good capacity retention of 83.5% after 200 cycles. Postmortem analyses on the cycled electrodes reveal that stabilization of the high‐voltage cathode can be attributed to the formation of a stable electrode/electrolyte interphase layer. The interphase is generated mainly by salt decomposition, which suppresses transition metal dissolution and surface reconstruction on the cathode. The optimized electrolyte can minimize solid electrolyte interphase dissolution to avoid capacity loss. This study offers a feasible pathway to achieve extended cycling of high‐voltage sodium batteries and guide further improvements in cycling performance of batteries by manipulating the chemistry of the electrolytes and interphase properties.