Toward Unified Interphase Engineering: The Solid-Electrolyte Interphase in Batteries and Supercapacitors

Abstract

The development of next-generation electrochemical energy storage requires devices that synergistically combine the high energy density of batteries with the exceptional power capability and cycle life of supercacitors, yet fundamental understanding of the interfacial phenomena governing performance across these platforms remains fragmented. While the solid-electrolyte interphase (SEI), a nanometer-scale passivation layer formed by electrolyte decomposition, has been extensively characterized in battery systems, analogous interfacial films in supercapacitors have received limited systematic investigation despite mounting ex-differing only in the final equilibrium thickness and the rate of dynamic reconstruction. 57,58 By recognizing this mechanistic continuity, we can systematically transfer knowledge, experimental techniques, and engineering strategies between these traditionally separate research domains. 59,60 This review synthesizes fundamental principles, experimental characterization, computational modeling, and engineering strategies for SEI control across batteries and supercapacitors, progressing systematically from atomic-scale mechanisms to device-level performance optimization.This review synthesizes knowledge from multiple research communities (electrochemistry, materials science, battery engineering, and supercacitor technology) to construct a unified understanding of SEI phenomena. Our literature search encompassed peer-reviewed journal articles, conference proceedings, and authoritative reviews published between 1979 (Peled's seminal SEI discovery) and 2025. Primary databases included Web of Science, Scopus, PubMed, Google Scholar, and specialized electrochemical databases. Search terms combined "solid electrolyte interphase," "SEI," "supercapacitor," "electrochemical capacitor," "EDLC," "pseudocapacitor," "interfacial film," "passivation layer," and "electrolyte decomposition" with Boolean operators to capture both battery-focused and capacitor-focused literature.Inclusion criteria prioritized: (i) experimental studies employing advanced characterization techniques (XPS, cryo-TEM, operando spectroscopy) to probe interfacial chemistry;(ii) computational investigations using ab-initio methods, molecular dynamics, or machine learning to elucidate SEI mechanisms; (iii) engineering studies demonstrating electrolyte additive effects, surface modifications, or artificial interphase strategies; and (iv) review articles synthesizing battery SEI knowledge with potential applicability to supercapacitors.We excluded purely device-performance studies lacking mechanistic insight into interfacial processes and preliminary conference abstracts without peer-reviewed follow-up.Cross-referencing was performed to identify seminal works cited across both battery and