Tuning the solvation structure with salts for stable sodium-metal batteries

Sodium-metal batteries are an appealing, sustainable, low-cost alternative to lithium metal batteries due to the high abundance and theoretical specific capacity (1,165 mA h g⁻¹) of sodium. However, the poor compatibility of the electrolyte with the cathode and anode leads to unstable electrode–electrolyte interphases. Here we introduce the concept of using a salt as a diluent, which enables the use of a single non-flammable solvent, such as trimethyl phosphate. By using sodium nitrate (NaNO₃) salt as a model diluent, we report a 1.1 M NaFSI–NaNO₃–trimethyl phosphate electrolyte that forms a stable interface with sodium-metal anode. In addition, the formation of robust cathode–electrolyte interphases on Na(Ni_0.3Fe_0.4Mn_0.3)O₂ cathode facilitates smooth phase transitions, thus leading to stable cycle life with a capacity retention of 80% over 500 cycles at C/5 rate in Na||Na(Ni_0.3Fe_0.4Mn_0.3)O₂ cells. The work demonstrates a promising approach towards the development of safe, low-cost, sustainable high-performance sodium-metal batteries.