Review on Cathode‐Electrolyte Interphase for Stabilizing Interfaces in Solid‐State Lithium Batteries

The cathode‐electrolyte interphase in solid‐state batteries faces new issues due to the solid nature of the electrolyte, including poor self‐repair of CEI, insufficient interface passivation, physical contact loss, and inferior lithium ion transport kinetics. These issues is examined from a broader perspective, promoting the proposal of a generalized CEI to solid‐state batteries. Abstract Solid‐state lithium batteries (SSLBs) hold great promise for improving battery safety and achieving higher energy density. Nevertheless, the matching diversity between solid‐state electrolytes (SEs) and the paired high‐voltage cathodes complicate to understand the formation and failure mechanisms of cathode‐electrolyte interphase (CEI). Furthermore, the structural degradation of cathodes and the electrochemical decomposition of SEs during cycling continuously reconfigure the CEI and alter the CEI composition and structure, adding further complexity to its analysis. The limited understanding of these processes hinders the development of effective strategies to address persistent cathode‐electrolyte interfacial issues (e.g., high interfacial impedance and contact loss) that shorten SSLB lifetime. A prerequisite for overcoming these challenges is to establish a comprehensive knowledge of CEI properties, which remains elusive and is often underestimated. Therefore, this review provides a comprehensive overview of the mechanisms governing CEI formation and failure, elucidating the complex nature of the interphase. Key challenges related to interfacial stability are discussed in detail, along with corresponding strategies for CEI regulation. Innovatively, an expanded conceptual CEI framework is proposed based on recent advances, facilitating a deeper understanding of CEI properties. Finally, forward‐looking perspectives are provided for designing stable CEIs and further advancing SSLB technology. The cathode-electrolyte interphase in solid-state batteries faces new issues due to the solid nature of the electrolyte, including poor self-repair of CEI, insufficient interface passivation, physical contact loss, and inferior lithium ion transport kinetics. These issues is examined from a broader perspective, promoting the proposal of a generalized CEI to solid-state batteries. Abstract Solid-state lithium batteries (SSLBs) hold great promise for improving battery safety and achieving higher energy density. Nevertheless, the matching diversity between solid-state electrolytes (SEs) and the paired high-voltage cathodes complicate to understand the formation and failure mechanisms of cathode-electrolyte interphase (CEI). Furthermore, the structural degradation of cathodes and the electrochemical decomposition of SEs during cycling continuously reconfigure the CEI and alter the CEI composition and structure, adding further complexity to its analysis. The limited understanding of these processes hinders the development of effective strategies to address persistent cathode-electrolyte interfacial issues (e.g., high interfacial impedance and contact loss) that shorten SSLB lifetime. A prerequisite for overcoming these challenges is to establish a comprehensive knowledge of CEI properties, which remains elusive and is often underestimated. Therefore, this review provides a comprehensive overview of the mechanisms governing CEI formation and failure, elucidating the complex nature of the interphase. Key challenges related to interfacial stability are discussed in detail, along with corresponding strategies for CEI regulation. Innovatively, an expanded conceptual CEI framework is proposed based on recent advances, facilitating a deeper understanding of CEI properties. Finally, forward-looking perspectives are provided for designing stable CEIs and further advancing SSLB technology. Advanced Science, Volume 12, Issue 48, December 29, 2025.