Imaging XAFS Study on Reaction Distribution of Composite Electrode Used As Commercial Lithium-Ion Battery

Demands for large-scale lithium-ion batteries for electric vehicles and grids have increased in order to reduce CO₂ emissions in consideration of the global environment. To improve the reliability for practical large-scale batteries, further improvements in safety, rate capability, and cycle life are required. One of the unique behaviors taking place in lithium-ion battery composite electrodes is an inhomogeneous reaction of active materials.¹ Since the heterogeneous reaction behavior induces overcharge and degradation of local active materials, the homogeneous charge-discharge reaction should be required. Ouvrard et al. reported that an inhomogeneous electrode reaction was caused during the charge-discharge process in LiFePO₄ lithium-ion battery cathode, which was detected by using X-ray absorption spectroscopy.² The study concluded that the ionic and electronic connectivity influenced by applied pressure is a crucial parameter for reaction distribution in composite electrodes.

In this study, we examine the commercially utilized composite electrodes where enough electronic conduction path is formed. We compare the electrochemical characteristics using the commercial electrodes with self-made composite electrodes prepared in the laboratory and then measured the reaction distribution by imaging XAS.

Two types of composite electrodes using LiFePO₄ as the active material were used, and their thickness and mass loading were almost similar. Two electrode cells were assembled using 1 M LiFP₆ in a solvent mixture of ethylene carbonate (EC) and ethyl metal carbonate (EMC) at a volume ratio of 3:7, and lithium counter electrode. Charge-discharge measurements were performed with an upper limit voltage of 3.6 V and a lower limit voltage of 2.0 V at rates of 0.1, 0.2, 0.5, 1.0, 2.0, 5.0, 10, 20, 30, 40 and 50 C. Imaging XAS measurements were performed at beamline BL-4 in Ritsumeikan University SR Center. Fe K-edge XAS was measured in a transmission mode using a two-dimensional detector.

As a result of the discharge rate capability test, the capacity retention of 5 C was 92% for the commercial electrode and 5.4% for the self-made electrode, compared with the discharge capacity at 0.1 C, and the self-made electrode showed a remarkable decrease in capacity at high rates. The imaging XAS of the commercial electrodes implies a lack of inhomogeneous reaction after discharging at 5C. The previous reports have shown that the reaction in the planar direction of LiFePO₄ proceeds from island-shaped points.³ We assume that the commercial composite electrodes are developed to be improved ionic and electronic connectivity.

Reference:

1. J. Liu, M. Kunz, K. Chen, N. Tamura, J. Richardson, J. Phys. Chem. Lett., 1, 2120-2123 (2010).

2. G. Ouvrard, M. Zerrouki, P. Soudan, B. Lestrez, C. Masquelier, M. Morcrette, S. Hamelet, S. Belin, A. M. Flank, F. Baudelet, J. Power Sources, 229, 16-21 (2013).

3. M. Katayama, K. Sumiwaka, R. Miyahara, H. Yamashige, H. Arai, Y. Uchimoto, T. Ohta, Y. Inada, Z. Ogumi, J. Power Sources, 269, 994-999 (2014).