Lithium metal batteries (LMBs) show great promise for achieving energy densities over 400 Wh·kg⁻¹. However, highly flammable organic electrolytes are a long-lasting problem that triggers safety hazards and hinders the commercial application of LMBs. Here, a nonflammable diluted highly concentrated electrolyte (DHCE) with ethoxy(pentafluoro)cyclotriphosphazene (PFPN) as a diluent is developed to simultaneously achieve high safety and cycling stability of high-voltage LMBs. The optimal DHCE not only ensures reversible Li deposition/dissolution behavior with a superior average Coulombic efficiency (CE) over 99.1% on lithium metal anode (LMA), but also suppresses side reactions and stress crack on the LiCoO₂ (LCO) under high cut-off voltage. The newly developed DHCE exhibits high thermal stability, showing complete nonflammability and reduced heat generation between the electrolyte and delithiated LCO/cycled LMA. This work offers an opportunity for rational designing nonflammable electrolytes toward high-voltage and safe LMBs.

Sodium metal is a promising anode for sodium batteries due to its high theoretical capacity and low cost. However, the serious Na dendrite growth and low Coulombic efficiency, especially at high current densities/cycling capacities, severely limit the application of sodium metal anodes. Herein, trifluoromethylfullerene, C₆₀(CF₃)₆, is designed as an electrolyte additive to enable the high-rate cycling of sodium metal anodes with high Coulombic efficiency. The CF₃ groups contribute to the formation of stable NaF-rich solid electrolyte interface layer, while C₆₀ cages induce the uniform distribution of sodium ions and promote the formation of smooth and compact morphology. Thus, Na||Cu cell with C₆₀(CF₃)₆ can be cycled at 2 mA·cm⁻² and 10 mAh·cm⁻² over 180 cycles with an average Coulombic efficiency of 99.9%, and Na||Na cell can be cycled at 10 mA·cm⁻² over 600 cycles. Furthermore, Na||NaV₂(PO₄)₃@C full cell exhibits high capacity retention of 84% over 2,000 cycles at 20 C (~ 3 mA·cm⁻²).