Analyzing Internal Resistance in Lithium Nickel Cobalt Oxide (LiNiCoO) Vehicle Batteries for Enhanced SOC and SOH Prediction

Internal resistance of a battery reflects its distinct characteristics, including factors such as state of health (SOH), state of charge (SOC), reversibility, thermal runaway, etc. Variation of the internal resistance with the testing currents (i.e. C-rate) suggests a relationship between internal resistance and the number of charges stored in the electrode materials (i.e. the SOC of the battery). Additionally, internal resistance is influenced by the homogeneity of the charge distribution under various testing currents (i.e. whether the charges are locally distributed near the surface area due to diffusion limitations or homogeneously stored in the electrode materials). The determination of internal resistance and the examination of the correlation between internal resistance and deliverable capacity can be utilized to predict the SOH and SOC of the battery, and even to understand its aging mechanisms. The paper introduces a simple approach based on modified intermittent current interruption (ICI) methods to characterize the internal resistance at controlled voltage (or SOC). The paper elucidates that the lifetime of a battery can be divided into four stages - initial stages, transition states, stable states, and aged stages, wherein each stage exhibits a unique relationship between internal resistance and SOC. The stable stage of the battery is characterized by the lowest internal resistance among the four stages. Both the stable stages and transition stages exhibit a distinctive decreasing trend in internal resistance while increasing SOC. The paper also identifies a robust correlation between the usable capacity of the battery and internal resistance, in which the deliverable capacity experiences a significant drop when internal resistance increases, particularly in aged batteries and batteries tested under large testing current. An analytical model is proposed to correlate the rise in internal resistance with changes in deliverable capacity, SOC, and SOH under various testing currents. Furthermore, when integrated with the Electrochemical Impedance Spectroscopy (EIS) methods, the correlation of internal resistance becomes a valuable tool for predicting SOC and SOH and comprehending the battery's aging mechanisms.