High Capacity and Cycle-Stable Hard Carbon Anode for Nonflammable Sodium-Ion Batteries.

Nonflammable phosphate electrolytes are in principle able to build intrinsically safe Na-ion batteries, but their electrochemical incompatibility with anodic materials, especially hard carbon anode, restricts their battery applications. Here, we propose a new strategy to enable high-capacity utilization and cycle stability of hard carbon anodes in the nonflammable phosphate electrolyte by using low-cost Na+ salt with a high molar ratio of salt/solvent combined with an solid electrolyte interphase film-forming additive. As a result, the carbon anode in the trimethyl phosphate (TMP) electrolyte with a high molar ratio of [NaClO4]/[TMP] and 5% fluoroethylene carbonate additive demonstrates a high reversible capacity of 238 mAh g-1, considerable rate capability, and long-term cycling life with 84% capacity retention over 1500 cycles. More significantly, this work provides a promising route to build intrinsically safe and low-cost sodium-ion batteries for large-scale energy storage applications.