Lithium-Ion Diffusion in Near-Stoichiometric Polycrystalline and Monocrystalline LiCoO2

Abstract

Lithium-metal-oxide-based cathode materials like LiCoO2 are an essential part of lithium-ion batteries, which are intensively researched and continuously improved. For a basic understanding of kinetic processes that control lithium incorporation and removal into/from electrodes, the lithium-ion transport in the cathode material is of high relevance. This concerns lithium diffusivities, as well as defect structures, transport mechanisms, and confined diffusion paths, such as grain boundaries. In the present study, lithium tracer self-diffusion is investigated by means of isotope exchange and secondary ion mass spectrometry in polycrystalline sintered bulk samples of stoichiometric LiCoO2 with an average grain size of about 70 nm and in single crystalline LiCoO2 in the ab-plane and c-axis in the temperature range between 200 and 700 °C. For the polycrystals, we found an activation enthalpy of ΔH = 0.75 eV. In the single crystal, the lithium-ion diffusivities along the ab-plane are identical to the diffusivities in polycrystalline LiCoO2. This indicates that diffusion along grain boundaries is similar to bulk diffusion and does not play a dominating role for the overall lithium-ion migration. Along the c-axis, diffusivities are some orders of magnitude lower, but only a slightly higher activation energy of 0.94 eV is found. This provides the experimental evidence of the often-claimed sluggish lithium diffusion along the c-axis. We suggest that lithium diffusion along the c-axis is most likely determined by fast diffusion in the ab-plane and a slow transfer of lithium ions across the CoO2 layers.