Designing Isocyanate‐Containing Elastomeric Electrolytes for Antioxidative Interphases in 4.7 V Solid‐State Lithium Metal Batteries

Abstract To facilitate the use of solid polymer electrolytes (SPEs) with high‐nickel (Ni) cathodes in high‐voltage lithium (Li) metal batteries (LMBs), it is crucial to address the challenges of low oxidative stability and the formation of vulnerable interphases. In this study, isocyanate groups (−N═C═O) are incorporated to develop an SPE with a bi‐continuous structure, consisting of elastomeric and plastic crystal phases. This rationally designed SPE exhibits high ionic conductivity (0.9 × 10 −3 S cm −1 at 25 °C), excellent elasticity (elongation at break of 330%), and enhanced oxidative stability (over 4.8 V vs. Li/Li⁺). A full cell, incorporating this SPE with a thin Li foil of 40 µm, and a high‐Ni LiNi 0.8 Co 0.1 Mn 0.1 O 2 (NCM811) cathode operating at 4.7 V vs. Li/Li⁺, demonstrates excellent cyclability, retaining 70% of its initial capacity after 200 cycles under a high C‐rate of 1C at 25 °C. The extended cycling of isocyanate‐containing SPE at 4.7 V vs. Li/Li⁺ is attributed to robust and compact inorganic‐rich interphases enabled by antioxidative −N−C═O components, as well as uniform Li deposition attributed to the bi‐continuous structured SPE. This study suggests that the isocyanate‐containing SPE system is a promising candidate for high‐voltage solid‐state LMBs by constructing stable interphases.