Hybrid functional study on diffusion of silicate cathode material Li2NiSiO4

The orthosilicate systems Li2MSiO4 (M = Fe, Mn, Co and Ni) are recently believed to be a promising alternative to the olivine phosphates. In this paper, we present an interpretation of the diffusion mechanism for polaron-Li vacancy complexes in Li2NiSiO4 based on the hybrid functional method HSE06. A comparison between the results obtained by GGA+U and HSE06 methods is carried out. The results confirm that the HSE06 method succeeds in describing the polaron localization in Li2NiSiO4. A polaron-Li vacancy complex model (V. A. Dinh et al., Appl. Phys. Express, 5, 045801 (2012)) is used to handle the explanation of the diffusion mechanism in this material. Four elementary diffusion processes for the polaron-Li vacancy complex are investigated and the preferable diffusion pathways are constructed by combining the possible elementary processes. It is found that the Li diffusion may proceed along the two preferable pathways in the [100] and [001] directions with the activation barriers of 1.17 and 0.96 eV, respectively. Furthermore, the accompanied migration of polaron can enhance the diffusion rate of Li ion in Li2NiSiO4.