Protocol-aware standardization of relaxation-induced kinetic masking in incremental capacity fingerprints of lithium-ion batteries

Abstract

Incremental capacity analysis (ICA) is widely used to probe lithium-ion battery degradation because it preserves material-related information in the voltage response. Its mechanistic value, however, depends on whether the measured voltage trajectory provides a comparable projection of electrode phase responses, polarization, and transport limitations. Here, we show that insufficient pre-charge relaxation introduces a relaxation-induced kinetic masking that can distort ICA fingerprints before material-related interpretation. A controlled-aging dataset of 12 commercial NMC-type 21700 cells and 166 reference performance tests was used, in which four ICA-related charges with 60, 30, 10, and 0 min of pre-charge relaxation were repeatedly conducted under otherwise identical conditions within each test. Short relaxation first shifts the charging-voltage trajectory and then amplifies this deviation in the dQ/dU domain, producing peak displacement, centroid drift, and area redistribution within reproducible fingerprint windows. The distortion depends systematically on charge rate, depth of discharge, state of health, and voltage window, indicating that it is a structured non-equilibrium projection. To mitigate this protocol-induced masking, we developed a voltage-domain relaxation lens based on an effective voltage-dependent relaxation timescale, τ(U), a compact, effective descriptor of the transferable relaxation-induced voltage bias in the voltage domain. The framework does not aim to recover an ideal equilibrium curve; instead, it moves short-relaxation charges toward the corresponding 60 min long-rest reference before recalculating ICA. Under strict cross-cell validation, the framework improves the comparability of the P2–P4 fingerprint windows within clear boundaries: 40% depth of discharge is a stable success region, 70% is condition-dependent, and 100% remains the most challenging scenario. A temperature-aware extension provides only localized additional benefit, mainly in the hardest cases. Overall, these results show that part of the apparent evolution of ICA fingerprints under practical testing can arise from relaxation-induced kinetic masking rather than irreversible material change, and highlight relaxation standardization as a protocol-aware route toward more reliable ICA fingerprint interpretation from cycler-accessible signals. The standardized fingerprints should therefore be interpreted as long-rest-referenced responses with improved comparability.