Nanoconfinement as an electrolyte-state selector in hard carbon for sodium storage

Sodium-ion batteries are promising candidates for large-scale energy storage, but hard-carbon anodes still suffer from irreversible sodium loss and impedance growth arising from early electrolyte reduction and unstable interphase formation. Here, we make explicit a central but often underdeveloped idea: nanoconfinement in hard carbon does not merely restrict transport but selects the local electrolyte state within accessible pores. This confined coordination regime, defined by the balance among solvent-separated ion pairs, contact ion pairs, and larger aggregates relative to the bulk electrolyte, governs desolvation, reduction pathways, and the resulting interphase trajectory. This viewpoint complements established sodium-storage mechanisms, including interlayer insertion, defect adsorption, and closed-pore filling, by focusing on how local electrolyte states regulate access, reduction, and interphase stability. Framed in this way, apparently inconsistent results across hard carbons and electrolyte families become more interpretable on a common basis. We argue that interphase survival, rather than SEI thickness alone, is the decisive criterion under confinement because transport stability depends on preserving electrochemically accessible pore pathways during cycling. Viewing nanoconfinement as an electrolyte-state selector therefore provides a testable conceptual framework for understanding and comparing sodium-storage behavior in porous hard carbons.