Mixed Potential Behavior in Thick Li‐Ion Battery Electrodes Under Fast Charging

For lithium‐ion batteries (LIBs) with high energy and power densities, the demand for fast charging with thick‐film electrodes is growing; however, fast charging can induce a nonuniform state of charge (SoC) within the electrodes. This work investigates the effects of nonuniform SoC within thick electrodes using a pseudo‐two‐dimensional model of LIBs. During fast (dis)charging along a sloping voltage profile, nonuniform Li‐ion insertion produced a gradient in electrode potential along the electrode depth, and the resulting mixed potential acted as an additional source of overpotential. In contrast, along a voltage plateau, the gradient of Li‐ion insertion was largely preserved throughout the rest period, and the unavoidable nonuniform SoC induced by fast charging did not produce appreciable overpotential, which became more pronounced with increasing electrode thickness and C‐rate. The mixed potential thus plays a dual role in thick‐electrode LIBs: in sloping open‐circuit voltage regions it homogenizes the electrode at the cost of additional overpotential, while in plateau regions it avoids the overpotential penalty but leaves through‐thickness gradients unrelaxed, potentially driving mechanical stress. These findings indicate that two‐phase electrodes such as LiFePO 4 may circumvent mixed‐potential overpotential during fast charging of thick electrodes, although the persistent gradients warrant careful consideration of the associated mechanical consequences.