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Differential voltage analysis (DVA) is a conventional approach for estimating capacity degradation in batteries. During charging, a graphite electrode goes through several phase transitions observed as plateaus in the voltage response. The transitions between these plateaus emerge as observable peaks in the differential voltage. The DVA method utilizes these peaks for estimating cell degradation. Unfortunately, at higher C-rates (above C/2) the peaks flatten and become unobservable. In this work, we show that, unlike the differential voltage, the peaks in the 2nd derivative of the expansion with respect to capacity remain observable up to 1C and thus make possible diagnostic algorithms at these charging rates. To understand why that is the case, we have developed an electrochemical and expansion model suitable for model-based estimation. In particular, we demonstrate that the single particle modeling methodology is not able to capture the peak smoothing effect, therefore a multi-particle approach for the graphite electrode is needed. Additionally, model parameters are identified using experimental data from a graphite/NMC pouch cell. The proposed model produces an excellent fit for the observed electric and mechanical swelling response of the cells and could enable physics-based data-driven degradation studies at practical charging rates.

Estimation of electrode state of health (eSOH) is essential to understanding battery degradation status in detail. This is accomplished by considering electrode capacity and a utilization range as eSOH parameters. In this paper, we propose a novel combination of two eSOH estimation approaches (i.e. voltage fitting and differential voltage analysis). By utilizing peak information in the differential voltage curve, the proposed method can separate individual electrode's contributions from the full-cell voltage. This separation allows the proposed method to identify changes in the positive electrode half-cell potential curve and to calibrate the aged half-cell potential function by refitting the coefficient of the basis functions. A comparison between the conventional voltage fitting method and the proposed method for an aged NMC/graphite cell shows that the proposed method can significantly reduce the voltage fit error while matching the location of distinct peaks in the differential voltage curve that the conventional method fails to do.