Capacity and degradation mode estimation for lithium-ion batteries based on partial charging curves at different current rates

Abstract

The open circuit voltage (OCV) curve of a lithium-ion cell can be described as the difference between the half-cell open circuit potential curves of both electrodes. Fitting a reconstructed OCV curve to the OCV curve of an aged cell allows identification of degradation modes. In this study, we show that this method can also be applied to partial charging curves of a commercial cell with silicon–graphite and NMC-811 as electrode materials. Both the degradation modes and the remaining cell capacity can be determined from the reconstructed OCV curve. For the investigated cell, accurate OCV reconstruction and degradation mode estimation can be obtained from C/30 partial charging curves if the state of charge (SOC) window between 20% and 70% is available. We show that the method is also applicable to charging curves at higher current rates if the additional overpotential is considered by subtracting a constant voltage offset. Capacity estimation with an accuracy of 2% of the nominal capacity is possible for current rates up to approximately C/4 if partial charging curves between 10% and 80% SOC are used, while a maximum current rate of C/15 should be used for accurate estimation of the degradation modes.