SEI and CEI

An organic electrolyte must contact both a high-potential cathode and a low-potential anode, so it cannot remain perfectly stable across an unlimited voltage window. During first charge the hard-carbon potential falls and selected solvent, salt, or additive species are reduced; cathode potential rises and other species may oxidize. Reaction products deposit on particle surfaces as compositionally heterogeneous layers from nanometres toward micrometres. The existence of SEI and CEI does not simply mean that the electrolyte has failed. It is part of establishing reversible operation in a high-voltage non-aqueous cell. The key question is whether side reactions converge quickly during formation.

A useful interphase requires apparently conflicting properties. It should be electronically insulating enough to stop continuing electrolyte reduction, yet sufficiently sodium-ion conductive to avoid excessive charge-transfer resistance. It must adhere to particle surfaces through volume changes, temperature cycles, and long electrolyte exposure. Inorganic species such as NaF, Na2CO3, and salt-derived products may improve mechanical or chemical stability, while polymeric and organic species can support flexibility and coverage. A real SEI is commonly layered, mosaic-like, or compositionally graded rather than the uniform straight film shown in a simple textbook diagram.

Many sodium interphase salts are more soluble in common organic electrolytes than their lithium analogues, and the resulting films can be less compact. Dissolved products leave the surface, fresh electrolyte meets active sites, and the cell continues to consume sodium and electrolyte. Hard-carbon dimensional changes, particle cracks, and elevated-temperature storage can accelerate rupture and reformation. Research therefore aims not to form more film, but to form a thinner, less soluble, faster sodium-ion-conducting film. Low-solvation electrolytes, high-concentration or localized-high-concentration formulations, film-forming additives, and surface treatments all pursue this objective.

Greater first-cycle consumption leaves less sodium for later cycling and limits full-cell capacity. Excessive interfacial resistance raises overpotential during fast charge and at low temperature, potentially bringing the anode closer to sodium deposition. Continued interphase growth adds heat and narrows usable voltage. An unstable cathode interphase can sustain electrolyte oxidation, gas generation, and transition-metal dissolution at high potential. Initial efficiency, rate, cold performance, storage, and cycle fade are therefore not independent metrics; the interphases connect them. A successful electrolyte and formation strategy must work at both cathode and anode rather than optimizing one side in isolation.

Capacity fade or impedance rise may arise from interphases, but also from particle cracking, loss of conductive contact, collector corrosion, or changing test temperature. Studies often combine first-cycle differential capacity, cyclic voltammetry, impedance, XPS, solid-state NMR, electron microscopy, and gas analysis, with controlled changes in electrolyte or electrode surface. Post-mortem samples are altered by air, moisture, washing, and vacuum, so the measured composition may not be identical to the operating interface. A reliable diagnosis connects time evolution, spatial distribution, and electrochemical behaviour rather than assigning cycle life to one NaF signal or one spectral peak.