Solid-State Diffusion Coefficient of Li Ions in Li[Ni1/2Mn3/2]O4 Determined By the Diluted Electrode Method

Abstract

The input- and output-power of lithium-ion batteries strongly depend on the rate capability of the lithium insertion electrode used as positive and negative electrodes. In our previous studies, the rate capability of the electrodes have revealed that the diffusion of Li ions in the electrolyte inside the electrode is a rate-determining step [1]. Recently, the use of a dilute electrode in which a part of the active material is replaced with a spectator material makes the Li ion transport process in the solid phase rate-determining. In other words, the diluted electrode method allows to measure the rate capability of the material and to determine the solid-state Li-ion diffusion coefficient in an active material.In this study, we investigate the dependence of the rate capability of materials on the particle size. The material used was Li[Ni1/2Mn3/2]O4 (LiNiMO), which is a 5V class positive electrode material whose rate capability have been variously changed depending on the crystal structure, particle size, and reaction mechanism. LiNiMO with different particle size was synthesized by changing the synthesis temperature from 800 ℃ to 1100 ℃. The particle size of LiNiMO increased with rising the synthesis temperature. Dilute electrodes containing 10 wt% of LiNiMO active material was prepared and the rate capability tests were carried out. The lower the synthesis temperature, the better the rate capability. The sample with the smallest particle size (> 1㎛) synthesized at 800 ℃ showed 4 times higher rate capability compared to the sample with the largest particle size (7 ㎛) synthesized at 1100 ℃. This result suggests that the smaller the particle size of the material, the shorter the diffusion distance of lithium ions in the solid phase, and thus the superior rate capability was exhibited.In the presentation, based on the results of these rate tests, the solid-state diffusion coefficient of Li ions in the LiNiMO was determined, and the relationship between physicochemical properties such as particle size, crystal structure, and reaction mechanism of LiNiMO and the diffusion coefficient was investigated.[1] K. Ariyoshi et al., J. Electrochem. Soc., 165, A3965-A3970 (2018). Figure 1