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.