A fully flame-retardant electrolyte with laminated SEI for exceptionally safe, long-life, and high-voltage lithium metal batteries

Designing an electrolyte that exhibits intrinsic nonflammability, superior compatibility with lithium metal anodes, and excellent tolerance to high-voltage cathodes is a pivotal, yet highly challenging task for the development of high-energy lithium metal batteries. Herein, these three desirable features are simultaneously achieved by incorporating a fire-retardant diluent, ethoxy(pentafluoro)cyclotriphosphazene, together with a trace additive trioxane into triethylphosphate-based electrolytes. Ethoxy(pentafluoro)cyclotriphosphazene and trioxane both compete against triethylphosphate for the coordination of Li +, inducing the formation of a unique laminated solid-electrolyte interphase (SEI) for reversible Li plating/stripping reactions. This SEI exhibits an outer layer with interwoven components of trioxane-derived polymers and N/P/F-rich inorganics and a deeper region enriched with LiF. It is shown that this trioxane-triggered laminated SEI is essential for effectively inhibiting the unwanted TEP decomposition at the anode, resulting in a record-high Coulombic Efficiency of 99.7% in Li||Cu cells. The Li||NCM811 full cells can be cycled steadily at up to 4.8 V, showing outstanding capacity retention of 92% @300th cycle. Importantly, the designed electrolyte is intrinsically nonflammable, and the smoke it releases upon heating can even extinguish open flames. The resulting 1.1 Ah Li||NCM811 pouch cells show no signs of thermal runaway upon heating to 250 °C, demonstrating exceptional safety.