Phosphazene Additives for High-Performance Lithium Metal Batteries with Ether-Based Gel Polymer Electrolyte, Enabling 4.5v Long Cycling

Abstract

Ether-based electrolytes are favored for their robust reduction stability but face challenges such as limited oxidative stability and compatibility issues with NCM cathodes under high-voltage conditions. Here, phosphazene (TH) additives with inherent bulky steric hindrance effect and strong electron-withdrawing properties are prepared, which are then applied to fabricate fluorine-free ether-based electrolyte for high-voltage lithium metal batteries (LMBs). The presence of only 2% TH in the electrolyte achieves stable lithium plating-stripping with a high coulombic efficiency of 99.69%. Additionally, TH effectively addresses the oxidation issues of ether-based electrolytes under high-voltage cycling. Moreover, the prevalent lithium salt LiPF6 reacts with trace water to generate HF, significantly compromising battery performance. TH effectively adsorbs PF6−, reducing HF generation while simultaneously lowering the HOMO energy level of PF6−. This enables the formation of a stable CEI (Cathode-Electrolyte Interphase) enriched with P, N, and F elements during cycling. When cycled in NCM811//Li cells using TH@DOL at a cutoff voltage of 4.5V, the battery retains 76.21% of its capacity after 300 cycles. Furthermore, the high concentration of flame-retardant elements in TH significantly enhances the electrolyte's flame resistance at elevated temperatures, thereby improving both the fire safety and mechanical durability of pouch cells.

Keywords: DOL, high voltage, NCM811, Flame retardancy, Fluorine-free