Meaning
Electrochemical impedance measurements and microstructural characterization track the continuous growth of a solid passivation layer on anode active material surfaces over time. Diagnostic tools evaluate surface layer evolution against baseline film thickness to quantify rate capability decay and cell aging. Progressive sei thickening consumes active lithium ions and solvent molecules from the liquid electrolyte, forming an increasingly resistive barrier at the electrode interface.
Higher storage temperatures and elevated operating voltages accelerate solvent breakdown, driving continuous film growth. Uncontrolled layer growth leads to lithium plating risk and internal impedance spikes during fast charging.
Passivation Kinetics
Initial formation cycles create a protective solid-electrolyte interface layer that passivates graphite anode surfaces against further electrolyte decomposition. Continuous trace reactions cause slow, ongoing decomposition that increases film thickness over months of operation. Sustained sei thickening depends on electron tunneling mechanisms and solvent diffusion through the existing passivation film.
Additives like fluoroethylene carbonate help stabilize initial layer structure.
Resistance Growth
Thickened interface films impede the diffusion of solvated lithium ions into graphite intercalation sites. Dynamic electrochemical impedance spectroscopy reveals growing high-frequency resistance arcs corresponding to film transport resistance. Continuous sei thickening elevates cell internal resistance, reducing power output during high-current discharge pulses.
Operating at elevated temperatures accelerates resistance growth.
Capacity Loss
Active lithium ions become permanently trapped inside insoluble organic and inorganic reaction products within the growing passivation layer. Irreversible capacity loss correlates directly with the quantity of charge consumed by secondary interface reactions. Continuous sei thickening serves as the primary driver of calendar aging in commercial lithium ion batteries.
