Relaxation Analysis of Li-Co-O Cathode Material for Li Ion Secondary Batteries

Abstract

Li-ion secondary batteries (LIBs) have received much attention as one of the next generation battery system due to its high energy density and operation power, and the research study to improve its performance have been proceeding actively with the recent drastic increase of demands for portable electronic devices and electric vehicle. The research on the crystal phase/ structure of electrode materials is essential to understand the electrode behavior at the lithium insertion/extraction process and to design the advanced high efficiency electrode. The relaxation analysis is important to make the transition of electrode material from kinetic state to equilibrium state clear. We have reported on the relaxation phenomenon of changing their crystal phase/structure after the lithium insertion/extraction process for various electrode materials such as γ-Fe2O3, 1‐3 LiMn2O4, 4‐6 LiFePO4 7 and LiNi1/3Co1/3Mn1/3O2, 8 by using powder X-ray diffraction (XRD) measurement and the Rietveld analysis method. In the previous study on relaxation analysis of γ-Fe2O3 electrode material, it was found that the iron occupancy of 8a sites decreased and that of 16c sites increased with lithium insertion, then the iron occupancy of 8a sites increased and that of 16c sites gradually decreased at the relaxation process after the lithium insertion. It is considered that the lithium preferentially favored the 8a site kinetically, whereas the 16c site is occupied thermodynamically. 1‐3 Recently, we investigated the relaxation phenomenon of LiMn2O4 with the coexistence state of two phases, the Li-rich phase and the Lilean phase, after the lithium insertion by using the Rietveld method, and found that the mole fraction of the Li-rich phase increased and that of the Li-lean phase decreased with the relaxation time. It is considered that, at the lithium insertion process, the Li-lean phase has more vacancies than the Li-rich phase due to lithium deficiency and that the structure of the Li-lean phase was maintained even containing excess lithium in order to promote lithium diffusion. It is considered that the Li-lean phase containing excess lithium formed during the lithium insertion process separated into the Li-lean phase without excess lithium and the Li-rich phase during the relaxation process. 4‐6 LiCoO2 with a layered rock-salt structure is the most widely used cathode material for LIBs due to its excellent reversibility and a high electrochemical potential at the lithium contents x from 1 to 0.5 in terms of LixCoO2. 9‐12 Reimers et al. reported that the phase of LixCoO2 changes as single hexagonal phase for x = 1.00‐0.93, two hexagonal phases for x = 0.93‐0.75, single hexagonal phase for x = 0.75 - near 0.5. 12 In this study, we prepared LixCoO2 samples (x = 0.90, 0.85 and 0.80) at the region of two hexagonal phases coexistence by charging/discharging the cell at the various C rate, then investigated the phase change of the two hexagonal phases with the elapsed time after the lithium extraction/insertion process by using XRD measurement and the Rietveld analysis method.