(Invited) Grain Boundary and Surface Atomic Structures and Diffusion in Li-Ion Battery Related Crystals

Abstract

The properties of lithium ion battery cathodes strongly depend on the diffusion of lithium ions during charge/discharge process. Then, direct visualization of lithium site is required to understand the mechanism of the diffusion of lithium ions. In this study, aberration (Cs) corrected STEM were applied to directly observe the {010} surface, which corresponded to perpendicular to the 1-D diffusion orientation, of the olivine LixFePO4. The morphology of the interface between Li-rich and Li-poor phases of LixFePO4 after chemical delithiation were observed with atomic resolution at fit intervals during half a year. It was found that orientation of boundary layers at the FePO4/Li2/3FePO4 interface gradually changed from lower index planes to higher index planes. This indicates that intermediate phase plays an important role in healing crystal cracking by allowing the interface to remain coherent so that Li ions can diffuse back into regions depleted during delithiation. The mechanism of the lithiation/delithiation from and to the surface will be discussed based on the observation results.Lithium lanthanum titanate (LLTO) is expected to apply for an electrolyte in the all-solid-state Li-ion battery because of its high Li-ion conductivity in the bulk. However, it has been reported that Li-ion conductivity is strongly suppressed at the grain boundaries (GBs) in polycrystalline materials. It is therefore needed to understand the atomistic mechanism of the reduction of Li-ion conductivity at individual GBs in order to design suitable LLTO polycrystals. In this study, two different GBs, Σ5 and Σ13, were prepared by fabricating the bicrystals, and their structures were observed by Cs corrected STEM and atomic force microscopy (AFM). Then, the charge states, Li-ion conductivities, atomic and electronic structures at the respective Σ5 and Σ13 GBs of LLTO were systematically and quantitatively investigated. It was found that the Σ5 GB has no significant influence on Li-ion conductivity, but the Σ13 GB shows the significant reduction of the Li-ion conductivity. We further found that Σ13 GB is positively charged by the formation of large amount of oxygen vacancies at the GB. Li-ion depletion layers is considered to be formed at the Σ13 GB, which causes the significant reduction of Li-ion conductivity, to compensate such positive charge at the GB.References Kobayashi, C.A.J.Fisher, T.Kato,Y.Ukyo,T.Hirayama and Y.Ikuhara, Nano Lett. 16, 5409 (2016).Kobayashi, A. Kuwabara, C. A.J. Fisher, Y. Ukyo and Y. Ikuhara,Nat. Commun., 9, 2863 (2018)Sasano, R. Ishikawa, I. Sugiyama, T. Higashi, T. Kimura, Y. H. Ikuhara, N. Shibata and Y. Ikuhara, Applied Physics Express, 10, 061102 (2017)Sasano, R. Ishikawa, K. Kawahara, T. Kimura, Y. H. Ikuhara, N. Shibata and Y. Ikuhara, Applied Physics Letters, 116, 043901 (2020)