A Polymerized‐Ionic‐Liquid‐Based Polymer Electrolyte with High Oxidative Stability for 4 and 5 V Class Solid‐State Lithium Metal Batteries

Abstract Polymer solid electrolytes for solid‐state batteries typically suffer from low ionic conductivity and low oxidative stability. Herein, a polymer electrolyte based on a polymerized ionic liquid and an ionic liquid plasticizer offering simultaneously a high room‐temperature ionic conductivity of 0.8 mS cm −1 and a high oxidative stability >5.0 V versus Li + /Li, is reported. The electrolyte is compatible with lithium metal and non‐flammable upon direct flame exposure. In symmetric lithium metal cells, the electrolyte enables stable lithium plating and stripping at 0.1 mA cm −2 with a capacity of 1 mAh cm −2 per half cycle for over 1700 h at 25 °C. Cells with a lithium metal anode cycled against a state‐of‐the‐art high‐energy uncoated layered LiNi 0.8 Mn 0.1 Co 0.1 O 2 cathode achieve an initial capacity of 162 mA h g −1 and a capacity retention of 72% after 600 cycles to an upper cut‐off voltage of 4.4 V at 25 °C. Cells with a high‐voltage spinel LiMn 1.5 Ni 0.5 O 4 cathode reach an initial capacity of 132 mAh g −1 and a capacity retention of 76% after 300 cycles to an upper cut‐off voltage of 5 V at 25 °C, confirming the high oxidative stability and technological potential of this electrolyte for high‐voltage solid‐state lithium metal batteries.