Probing depth-dependent inhomogeneous lithium concentration in thick LiNi0.88Co0.09Al0.03O2 cathodes for lithium-ion batteries

A high energy density is an essential requirement for commercial lithium-ion batteries (LIBs) in electric vehicles (EVs) because it directly affects driving range. In addition, the fast charging performance of LIBs, which determines the charging time, is critical in the EV industry. A common approach to enhance the energy density of LIBs is to increase the active material loading by increasing the electrode thickness. However, there is limited scope for increasing the thickness of LIB electrodes in fast charging operations because the power density characteristics of LIB cells are degraded with increasing electrode thickness. Here, we estimate the lithium concentration distribution within thick LIB cathodes after charging/discharging by studying the work function distribution within the thick electrodes using Kelvin probe force microscopy (KPFM). First, to determine the relationship between the work function and lithium content in LIB cathodes, we measure the work function of thin LiNi0.88Co0.09Al0.03O2 (NCA) electrodes under various state-of-charge (SOC) conditions. From these measurements, it is demonstrated that the work function of the NCA electrodes is proportional to the SOC. The results suggest that the work function measurements obtained using KPFM can be used to estimate the Li concentration of NCA electrodes. Next, we perform depth-dependent KPFM measurements in thick NCA samples, which can provide depth profiles of the work function depending on the C-rate conditions. The work function shows a monotonic increase along the depth direction of the thick NCA cathodes, implying a depth-dependent inhomogeneous distribution of lithium concentrations.