LiF/NaF-rich interphase enables low N/P ratio sodium metal batteries with wide operating temperature range

Sodium metal batteries (SMBs) are regarded as compelling energy storage systems owing to their high theoretical capacity and the abundance of sodium (Na). However, SMBs with cycling stability under extreme climatic conditions remain highly challenging, due to the instability of the solid electrolyte interphase (SEI) and dendrite growth, which is further aggravated when operating under low negative-to-positive capacity ratio (N/P) conditions. Herein, a high-energy-density and wide-temperature SMB is realized using a hetero-salt-regulated electrolyte strategy. Specifically, lithium difluorophosphate (LiDFP) is introduced as a hetero-salt additive into a dual-salt ether electrolyte to stabilize the Na metal anodes. Li⁺ competitively coordinates with solvent molecules, causing the Na⁺ solvation structure to shift toward an anion-rich configuration. The formed anion-dominated solvation shell promotes the formation of a mechanically robust and electronically insulating LiF/NaF-rich SEI, which in turn suppresses interfacial side reactions and effectively mitigates dendrite growth. Consequently, the assembled Na||Na₃V₂(PO₄)₂F₃ (NVPF) SMBs (N/P = 4:1) achieve a high energy density of 335.90 Wh kg^-1 (with power density of 26,650.00 W kg^-1) at 25 °C (based on total mass of the active materials on anode and cathode) and maintain stable operation across a wide temperature range of -40 to 45 °C. This work expedites the development of SMBs for energy-intensive applications in extreme environments.