Li Distribution in a LiNi1/3Mn1/3Co1/3O2 Positive Electrode Analyzed Using Reflection Electron Energy-Loss Spectroscopy

Abstract

Li-ion batteries are commonly used as energy-storage devices, where it is important to examine the spatial distribution and chemical state of Li with high spatial resolution to understand various phenomena pertaining to their use. To this end, analytical techniques using electron beam sources are advantageous due to their high spatial resolution. Observations of the Li distribution on the nanometer scale can be successfully obtained using electron energy-loss spectroscopy (EELS) with transmission electron microscopy (TEM) or scanning TEM (STEM) [1, 2]. However, TEM/STEM have inherent requirements in terms of sample preparation such as size or thickness, which limits its application range. Scanning electron microscopy (SEM) is much more commonly used and has a high spatial resolution on the nanometer scale, and it can observe larger particles or multiple samples in one sample holder. Ishida et al. reported the Li distribution of carbon and Li metal by using scanning auger electron spectroscopy (AES) [3]. Lithium-transition metal oxides with low electron conductivity are frequently used as electrode materials. However, the Li KLL signal in AES often overlaps with AES signals from other transition metal elements. The charge-up phenomena of a sample can cause problems during AES analysis, which complicates the discussions around chemical shifts. Despite this, the technique is applicable in analyzing light elements in materials with high spatial resolution. We performed Li analysis by reflection EELS (REELS) with SEM, and found that this technique is effective in obtaining the K-edge core-loss spectrum of Li. We identified the differences in chemical state in Li K-edge spectra for each sample, where the features of these REELS spectra were similar to those observed by TEM-EELS. As such, we proposed REELS as a method for lithium analysis in Li-metal oxides, and reported the successful detection of different chemical states of Li in various Li compounds. Using a single crystal Li4Ti5O12 substrate, elemental Li mapping was successfully carried out before and after electrochemical Li insertion and extraction [4]. In the present study, we expand the application of REELS to the analysis of positive electrode materials. The positive electrode material in Li-ion batteries is typically a Li-transition metal oxide of micron particle size. We applied REELS to determine the Li distribution in a LiNi1/3Mn1/3Co1/3O2 positive electrode after a charging/discharging cycle. Combined with scanning spreading resistance microscopy (SSRM), the changes in electrical conductivity of the Li-poor particles inside the electrode were visualized. [1] J. Kikkawa, T. Akita, M. Tabuchi, M. Shikano, K. Tatsumi, and M. Kohyama, Electrochem. Solid-State Lett. 11 183 (2008).[2] N. Taguchi, T. Akita, H. Sakaebe, K. Tatsumi, and Z. Ogumi, J. Electrochem. Soc. 160 2293 (2013).[3] N. Ishida, D. Fujita, J. Electron Spectrosc. Relat. Phenom. 186 39 (2013).[4] N. Taguchi, M. Kitta, H. Sakaebe and T. Akita, J. Electron Spectrosc. Relat. Phenom. 203 40 (2015).