J. Ociepa, M. Adam, B. Checinski
OCI Vacuum Microengineering Inc.,
Keywords: Li-based battery anode and cathode, Li diffusion, battery interface, battery degradation
Summary:“Natural” Li diffusion is defined as the process of Li atom/ions migration under a concentration gradient and activated by thermal energy from atomic vibrations of the host structure at room temperature. This process of passive Li diffusion is important for a better understanding of the active diffusion processes that are happening in lithium-ion batteries (LIBs), where external energy component such as electrical potential is applied. It is expected that materials that exhibit good “natural” Li diffusion properties will perform much better under the external electrical potential. This approach offers a unique opportunity to observe the free movement of lithium atoms/ions into the solid structure and simplify the understanding of diffusion processes especially if single-crystal structures are used. The single-crystal structures are free from grain boundaries and the lithium diffusion process is limited to lattice diffusions such as interstitial, vacancies, and dislocations. This approach allows for categorizing materials that are attractive to lithium diffusion based on the pure lattice component. The characterizing techniques are Auger electron spectroscopy (AES) for tracing Lithium concentration on the surface (Li-KVV peak at 52 eV) and Low Energy Electron Diffraction (LEED) for surface crystallography changes. The lithium concentration gradient is created on the surface of the host material by the evaporation of ultra-thin film of lithium with an effective thickness of 10 Angstrom under ultra-high vacuum conditions. The data obtained from these experiments are showing different lithium diffusion behavior on the selected materials and there is an indication of three categories of the studied materials. 1-Rapid lattice diffusion of Li into HOPG and no change in the surface crystalline structure. 2-Moderate lattice diffusion of Li into CVD Diamond, SiC-6H, LiNbO3, and TiO2 and some change in the surface crystalline structure. 3-No lattice diffusion of Li into Si single crystals, Ga2O3 and SrTiO3, and no long-range order in the surface crystalline structure. Most likely rapid diffusion occurs only in graphite but there are several materials with moderate diffusion properties, and these are potential for novel LIBs electrodes or chemically stable interfaces with enhanced performance. The materials with passive no-diffusion properties present challenges in application to LIBs as there is a special need to create active diffusion paths because there is no lattice diffusion contribution. In addition, this method of tracing lithium passive diffusion using AES is suitable for comparing fabricated polycrystalline LIBs electrodes as a metrology tool for quality control. This method of monitoring passive Li diffusion will be also applicable for failure analysis of disassembled electrodes such as a capacity fade after cycling.