Formulation of Capacity Fading Caused by SOC Imbalance in Lithium-Ion Batteries Based on Side Reaction Currents

One of the most critical remaining challenges for extending the lifetime of lithium-ion batteries (LIBs) is the suppression of capacity fading caused by the state-of-charge (SOC) imbalance between the electrodes.Although it has been recognized that side reactions occurring at both the positive and negative electrodes play a crucial role in this degradation process, few theoretical studies have quantitatively related these reactions to battery lifetime.In this work, we formulate the relationship between the rate of side reactions, referred to as the side reaction current (ISR), and the rate of capacity fading induced by SOC imbalance in LIBs.We first demonstrate that the presence of side reactions establishes local cells within the electrodes, thereby inducing SOC shifts in both the positive and negative electrodes.The types of side reactions responsible for SOC imbalance are then classified and discussed.Furthermore, we describe experimental methods for measuring ISRs, both those intrinsic to electrode materials and those occurring within LIB cells.Finally, we show that the rate of capacity loss originating from SOC imbalance is governed by the ISR values of the electrodes and clarify its correlation with conventional lifetime indicators such as Coulombic efficiency and capacity retention.The theoretical framework and equations presented here provide a kinetic basis for analyzing capacity fading caused by the SOC imbalance, offering a unified foundation for evaluating and comparing lifetime improvement strategies in LIBs.