Yuichi Ikuhara1 2

1, The University of Tokyo, Tokyo, , Japan
2, Japan Fine Ceramics Center, Tokyo, , Japan

The properties of lithium ion battery (LIB) cathodes strongly depend on the lithium ions diffusion across the interfaces and from the surfaces during charge/discharge process. Since this behavior determines the stability, lifetime and reliability, direct visualization of lithium site is required to understand the mechanism of the lithium ions diffusion. Aberration corrected STEM is very powerful imaging technique to directly observe the atomic columns inside a crystal. In this study, aberration corrected HAADF and ABF STEM are applied to directly observe the interface and the surface of the olivine LixFePO4 and delithiated olivine (FePO4) [1,2], and the mechanism of the lithiation/delithiation will be discussed based on the observation results.
For the cathode of LiFePO4, previous studies showed that the lithiation/delithiation in LiFePO4 is basically the two-phase process, that is, LiFePO4/FePO4 interfaces propagate through the bulk region with inserting/extracting lithium. TEM observations showed that the average particle size is of the order of µm, and most particles have core-shell structures. HAADF STEM revealed that the phase interface is parallel to the {100} plane, and the lattice variation widths are highly orientation-dependent. The a-axis lattice length (la) shows a narrow variation width, less than 10 nm, whereas the change of b-axis lattice length (lb) is gradual and extends over 30 nm. The two phase interface is thus found to have very complicated feature, which is related to the lithiation/delithiation mechanism.
In order to understand the surface reconstruction during lithiation/delithiation, the (010) LiFePO4 surfaces was directly observed by STEM [4]. Commercially available LiFePO4 singe crystals (Oxide Co., Japan) were used for all experiments. Crystals were cut perpendicular to their (010) axis and polished, and the structures of (010) surfaces before and after chemical delithiation were characterized by STEM. It was found that P and Fe atom columns undergo comparatively large displacements near the surface, which was consistent with the results from first-principles calculations. The magnitudes of the P and Fe displacements were also found to depend on the location of the outmost Li sites.
A part of this work was supported by the Research & Development Initiative for Scientific Innovation of New Generation Batteries II (RISING II).
References [1] S. D. Findlay et al., Microscopy, 66 (2017) 3, [2] R.Huang and Y.Ikuhara, Curr. Opin.Sol.Stat. & Mat.Sci.,16 (2012)31, [3] A. Nakamura, et al., Chem. Mater., 26 (2014) 6178, [4] S. Kobayashi et al., Nano Lett. 16 (2016) 5409