Nanoscale Ion Diffusion and Electric Charging-Discharging in Oriented Textured LiCoO₂ Thin Films

LiCoO₂ (LCO) is the first commercialized and still a widely used cathode material for lithium-ion batteries found in a range of modern applications. Even with decades of research, there is a lack of understanding of the nanoscale function and characteristics of LCO and other state-of-the-art cathode materials in lithium-ion batteries. This in turn limits opportunities to enhance battery performance. A key challenge in understanding and developing better electrode materials in lithium-ion batteries is the surface, in particular the evolution of the surface during use. The difficulty is compounded by the combination of limited analytical techniques that can probe the surface and their inherent condition requirements and the variability on the electrode surface depending on electrode processing steps used. Here, Li ion transport behavior in LCO thin films with differently oriented grains is studied by employing Kelvin probe force microscopy (KPFM), conductive atomic force microscopy (c-AFM), and sequential excitation electrochemical strain microscopy (SE-ESM) to study the conductance and surface potential of LCO that are closely related to the local Li ion movement. Varying electric polarities lead to distinct relaxation times due to the attraction or repulsion between Li ions and injected charges, which are experimentally visualized. Notably, a localized charge-discharge behavior can be simulated when conducting c-AFM measurements, which illustrates the impact of these processes on the surface morphology. This study offers insight into the nanoscale surface-focused properties, which can now be tuned by changing the microstructure, and this may lead to improved lithium-ion battery performance.