Nanoscale Investigation of LiCoO2 Thin Films via Scanning Probe Microscopy

Scanning probe microscopy (SPM) is utilized to characterize and manipulate the materials’ properties at nanoscale. This high-resolution insight can uncover the underlying mechanisms that drive macroscopic material behavior. Concurrently, thin film Lithium batteries are extensively utilized for the good energy efficiency and light weight. LiCoO2 (LCO), a widely used cathode material, remains a subject of research due to limited understanding of its nanoscale behavior. This study investigates the nanoscale properties of LCO thin films through multiple SPM techniques, such as conductive-AFM (C-AFM), Kelvin Probe Force Microscopy (KPFM), I-V spectroscopy, and Electrochemical Strain Microscopy (ESM). Being a layered material with a 2D lithium diffusion pathway, the anisotropic properties of polycrystalline LCO significantly affect its performance. Here, KPFM reveals a surface potential anisotropy, showing that surface potential decreases similarly with increased temperature across grains of different orientations. Conductive-AFM and I-V curve tests confirm that conductivity in LCO is influenced by crystal orientation, while ESM results suggest that the activation energy of Li-ions is isotropic. This uniformity across grains is vital to the overall stable electrochemical performance of batteries. Furthermore, the behavior of cathode materials can be strongly affected by the application of electric bias. Notably, distinct conductivity relaxation times as a result of poling with different bias polarities are observed by C-AFM measurements, which reveal the attracting/repelling effect of injected charges on Li-ions. Sequential Excitation-ESM demonstrates the easier lithium ionic diffusion at LCO grain boundaries and the nanoscale localized charge-discharge behavior is also investigated. The electrical-mechanical property coupling of LCO is studied using Force-Distance mapping, revealing a softening effect as lithium ions are extracted. A local nano-hardening effect is realized in layered LCO thin film by high-force tip pressing, causing c-axis compression and Li-ion expulsion. This mechanical modification reduced surface potential and conductivity, as shown by C-AFM and KPFM. Overall, this thesis suggests that SPM techniques allow the systematic characterization of nanoscale properties of a battery thin film cathode material LCO. This study offers insights that could lead to enhanced Li-ion battery performance.