Lithium Diffusion in Li_4/3Ti_5/3O₄ Particles during Insertion and Extraction

Lithium diffusion in a small Li_4/3Ti_5/3O₄ (LTO) particle was investigated from kinetic viewpoints of two-phase transition process based on a core-shell model by means of galvanostatic and potentiostatic measurements of thin LTO composite electrodes. High-rate galvanostatic charge (insertion) – discharge (extraction) properties of the thin LTO composite electrode showed that the insertion into the LTO particle was significantly slower than the extraction. An apparent chemical diffusion coefficient (D_app ) of lithium in the LTO particle during the insertion and extraction was evaluated from the results of potential step chronoamperometry (PSCA) with a spherical finite diffusion model. The phase-boundary movements between the two phases in the cathodic and the anodic potential steps for a long-time region were controlled by lithium diffusion through Li_7/3Ti_5/3O₄ rock-salt (LTO-rock-salt) and Li_4/3Ti_5/3O₄ spinel (LTO-spinel) shell, respectively. D_app in the LTO-rock-salt and the LTO-spinel phase were estimated to be approximately 1 × 10₋₁₂ cm₂/s and 1.6 × 10₋₁₁ cm₂/s, respectively. The slower insertion was mainly due to a D_app value one order of magnitude smaller in the LTO-rock-salt than that in the LTO-spinel phase. Electrochemical kinetic properties of the LTO particle with the core-shell structure were interpreted by lithium diffusion through the LTO-rock-salt shell during the insertion and the LTO-spinel shell with the low electron conductivity during the extraction.