Differences between the Kinetically Preferred States of LiFePO₄during Charging and Discharging Observed Using In Situ X-ray Diffraction Measurements

Abstract

In-situ X-ray diffraction measurements were performed on LiFePO 4 during charging and discharging using synchrotron radiation to elucidate the origin of the hysteresis in the kinetic mechanisms of the charging and discharging processes. The experimental results clearly showed that fewer intermediate phases were formed during the charging process than the discharging process, in keeping with previous studies. Further, during the discharging process, a Li-containing Li-lean phase and a Li-defective Li-rich phase with large lattice spacing were generated kinetically in the initial stage. Based on these results, we propose a new model for the hysteresis between the charging and discharging processes. Lithium iron phosphate [Li 1-x FePO 4 (0 ≤ x ≤ 1)] is a promising cathode material for Li-ion batteries. The charge/discharge processes of this material are well known to involve a two-phase Li insertion/extraction reaction between the structurally very similar LiFePO 4 and FePO 4 phases, both of which belong to the Pnma space group.1-3 Previously, material engineering approaches to improving the LiFePO 4 performances have been proposed, resulting in significant improvements. [4][5][6] There have been several studies focusing on the nonequilibrium intermediate phase. [7][8][9][10] We believe that elucidating this kinetic-state transition will further improve the electrochemical performance of LiFePO 4 and lead to the development of batteries based on LiFePO 4 cathodes, because the kinetically preferred states determine battery performance. Understanding the hysteretic phenomena between charging and discharging are especially important for using, controlling, and designing LiFePO 4 -based lithium ion batteries.Recently, Takahashi et al. clearly showed that the metastable intermediate phase is formed only during the discharge process based on the results of in-situ X-ray diffraction (XRD) measurements.11 However, the kinetics of the formation process of the intermediate phase remain unclear; the discussion did not focus on the formation process of this intermediate phase. Michael et al. also performed in-situ XRD measurements to determine the asymmetry of the constituent intermediate phases and firstly cleared the differences between the kinetic formation processes that occur during charging and discharging. 12Their data are of great value for understanding the differences. Phase transformation in the lateral direction of the electrode from outside the X-ray beam spot to the center of the spot dominated the reaction in the paper, as they explained. The average electrode voltage changes due to the progression of charging or discharging. It is considered about their cell structure that the reaction at the beam spot progresses depends on the potential difference as the driving-force. Even when the operation rate was very small (0.1 C), the reaction proceeded rapidly in the last stage of charging process. This indicates that their study focused on a system dominated kinetically not only by the materials' inherent kinetic stat...

Hysteretic Phenomena between Charging and Discharging in LiNi0.5Mn1.5O4-σ

“…The solid solutions have sufficient lattice spacings coincident with lithium defects to facilitate lithium diffusion, as observed for the LiFePO 4 two-phase reaction with a certain degree of overpotential. 17 While the hysteretic phenomena between charging and discharging in the plural-phase region are similar to those of LiFePO 4 , 17-19 the kinetically preferred states in the low SOC single reaction region (0.20 ≤ x ≤ 0.40) with large overpotential are remarkably characteristic in d-LNMO. The discharging process may increases the local structural gap between the reacted and unreacted regions in the 1 st phase, making it difficult for the lithium insertion reaction to progress.…”