Enhanced Lithium Diffusivity in Reduced Cerium Oxides: A First-Principles Study

Cerium oxide is considered as a promising protective coating material for the high-energy cathode of a Li-ion battery (LIB) against surface structural degradation. However, detailed knowledge on how Li migrates in cerium oxide is still limited, which is important for further development of cerium oxide as an LIB high-energy cathode coating material. Herein, using first-principles density functional theory calculations, we investigate Li insertion and diffusion mechanisms in pristine CeO2, reduced CeO2 with oxygen vacancy, and Ce2O3 as a representative material for highly reduced cerium oxide. For two different concentrations (1:8 and 1:27 Li:Ce), we find that Li can occupy empty octahedral sites of CeO2 in either the neutral or ionized state. Li diffusion in fluorite CeO2 is isotropic to the ⟨110⟩ direction, and the energetic barrier is significantly high. An addition of O vacancy in the CeO2 fluorite structure breaks the isotropy of Li diffusion; the barrier is decreased if the vacancy is located unidirectional with the ⟨110⟩ direction, and the barrier is increased if it is in the opposite of the ⟨110⟩ direction. In the highly reduced CeO2, i.e., Ce2O3, we observe that Li can only intercalate in the ionized Li+ state. The diffusion barrier of Li+ in this structure is significantly lower compared to the previous pristine and reduced CeO2 in two different concentrations, i.e., 1:2 and 1:16 Li:Ce. This indicates that the degree of reduction highly correlates to Li diffusivity in cerium oxide. Therefore, applying a highly reduced and O-poor CeO2 coating is suggested to allow fast and nonobstructive Li diffusion.