High Polymerization Conversion and Stable High-Voltage Chemistry Underpinning an In Situ Formed Solid Electrolyte

In situ polymerization system can provide a compact and compatible interface with minimum polymer electrolyte, which is imperative to address the bottleneck of notorious solid–solid interface issues for high-energy-density solid-state batteries. However, the existing in situ formed solid-state electrolyte still faces many problems, such as low polymerization conversion and inferior high-voltage stability, prohibiting its applications in practical high-voltage lithium-metal batteries. Herein, we present a deep eutectic solvent (DES)-based in situ polymerized solid electrolyte, which is facile and well matched with the commercially available lithium-ion battery technology. The DES precursor is made from a molten mixture of solid powders, containing a synthesized monomer named (2-(((2-oxo-1,3-dioxolan-4-yl) methoxy) carbonylamino))-ethyl methacrylate (CUMA), a succinonitrile (SN) plastic crystal, and two kinds of lithium salts. After in situ ploymerization triggered by free radical, the liquid again turns into a solid composite electrolyte (PDES-CPE) with a superior polymerization conversion of 99.8%. It delivers a promising lithium-ion conductivity (1.07 × 10–3 S/cm with a high lithium-ion transference number of 0.62 at 30 °C) and prominent high-voltage stability (100 cycles with 82.4% capacity retention coupled with 4.6 V LiCoO2 cathode). Through in situ Fourier transform infrared (FTIR) spectroscopy, we reveal a robust interface chemistry with thermodynamically improved high-voltage stability (compared to polyether-based electrolyte). This as-presented strategy makes a big leap to address the interface issues and boost the development of high-energy-density solid-state lithium-metal batteries.