Isolation and quantification of charge- and discharge-induced reversible capacity loss in commercial lithium-ion and sodium-ion batteries under coupled temperature–C-rate conditions

The accessible capacity of rechargeable batteries depends strongly on operating temperature and the charge/discharge current rate (C-rate). A central but under-explored phenomenon arising from these factors is reversible capacity loss (RCL), the temporary reduction in accessible capacity caused by transport and interfacial limitations rather than permanent material degradation. Most prior studies have examined temperature or C-rate separately, have focused on a single lithium-ion chemistry, and have rarely addressed asymmetric charge/discharge conditions; sodium-ion cells remain insufficiently characterised. This work investigates four commercial 18,650-format cells: NCA, NMC, LFP, and a sodium-ion cell. A two-phase protocol decouples discharge-induced RCL (Phase I: charge at 25 °C reference, discharge at 10–50 °C, 0.25–1C) from charge-induced RCL (Phase II: charge at 10–45 °C, 0.25–1C, discharge at 25 °C reference). Electrochemical impedance spectroscopy (EIS) is recorded at every condition, and an equivalent-circuit model fitted to extract chemistry-dependent ohmic, charge-transfer, and diffusion parameters. Because these spectra are acquired at zero DC bias, they serve as a qualitative mechanistic indicator of the temperature-activated kinetics rather than a quantitative decomposition of the coupled effect. A closed-form cubic Response-Surface Methodology (RSM) model then maps temperature and C-rate to predicted RCL. Calibrated under constant-current full charge/discharge conditions, it is proposed as an offline static reference for battery-management power de-rating and thermal-management triggering, and as a correction layer for range estimation under quasi-steady operation, at negligible computational cost. The results provide quantitative cross-chemistry benchmarks under identical protocols for electric-vehicle, aerospace and grid-scale storage applications.