Rational design of robust cathode-electrolyte interphases: linking interfacial chemistry to functional stability in sodium-ion batteries

Sodium-ion batteries (SIBs) are considered promising candidates for sustainable energy storage; however, their practical advancement is fundamentally limited by interfacial instability at high-voltage cathodes. Unlike the well-established understanding of the solid-electrolyte interphase (SEI), the cathode-electrolyte interphase (CEI) in SIBs remains poorly defined in terms of its formation thermodynamics, kinetic evolution, and structure–function relationships. Given that CEI governs interfacial redox reactions, ion transport, and surface reconstruction under high potentials, a mechanistic understanding of its dynamic behavior is essential. This review provides a comprehensive analysis of CEI chemistry in SIBs, focusing on its formation mechanisms, compositional architecture, degradation pathways, and functional implications. Strategies including electrolyte and additive engineering, as well as cathode surface regulation, are discussed within a unified framework linking interfacial chemistry to electrochemical stability. Advanced in-situ and operando characterization approaches are highlighted for elucidating CEI evolution across temporal and spatial scales. By integrating mechanistic insights with rational design principles, this review aims to establish a coherent foundation for engineering robust CEI systems that enable stable and efficient high-voltage sodium-ion interphases.