Beyond the lithium template: a solvation-first framework for nanoscale sodium-ion interphases, cross-talk, and...

Sodium-ion (Na-ion) batteries are moving from validation to deployment, yet durable full-cell operation is now limited by the chemistry, structure, and mechanics of nanoscale electrode-electrolyte interphases rather than by bulk-material discovery. The solid (SEI) and cathode (CEI) electrolyte interphases in Na-ion cells cannot be read as scaled lithium analogs. They are nanoscale, mosaic, dynamically reorganizing layers shaped by a Na⁺ solvation sheath, electrical double layer, and chemo-mechanical environment that differ markedly from Li⁺ analogs. This article reframes Na-ion interphase design around three linked ideas: (i) the primary Na⁺ solvation sheath, not the bulk electrolyte-stability window, selects first-layer chemistry at the nanoscale, through adsorption, desolvation and absorption at the electrode surface; (ii) under lean electrolyte and practical negative-to-positive (N/P) ratios, both interphases co-evolve through cross-talk rather than as independent passivation layers; and (iii) credible mechanistic claims require an explicit evidence-tier hierarchy and minimum reporting set. We anchor the discussion in deployment-relevant markers, including a representative hard-carbon full-cell ICE near 91.4%, a reported optimal N/P of about 0.9 in a layered-oxide/hard-carbon pairing, and emerging 1.5 and 3.5 A h cylindrical Na-ion formats, and close with a co-design logic and benchmarking checklist for Na-ion full-cell interphase studies.