X-Ray CT and XAS Analysis of NMC811 Cycled at Various Rates

LiNi0.8Mn0.1Co0.1O2 (NMC811) is attracting attention as high capacity cathode materials for lithium-ion batteries, which potentially increases specific capacity and reduces cost by replacing the rare metal cobalt with nickel. However, NMC811 has an issue with cycle capability compared to the conventionally used LiCoO2 (LCO). Various factors contribute the degradation of NMC811. Although the crystal structure evolution, such as cation mixing and oxygen loss1, surface reconstruction2 and intraparticle crack formation induced by charge-discharge cycles were proposed3, previous studies tend to focus on the specific factors. In addition, the effect from high rate charge-discharge has not been analyzed. In this study, the cycle capability of NMC811 was examined at various rates. The particle morphology and the surface chemical state after charge-discharge cycle were examined X-ray computed tomography (CT) and soft X-ray absorption spectroscopy (XAS), respectively. Composite electrodes were prepared by mixing cathode active material (NMC811 or LCO), conductive additive (acetylene black) and binder (polyvinylidene fluoride) in a mass ratio of 8:1:1. Current collector was an aluminum foil. 2032-type coin cells were assembled, using 1 M LiPF6 in a mixture of ethylene carbonate and ethyl methyl carbonate in a 3:7 v/v as electrolyte, and lithium metal foil as the counter electrode. Galvanostatic charge-discharge measurements were conducted at cut-off potential 3.2-4.3 V at a rate of 0.1C and 2C. After the cycle test, the coin cells were disassembled, and the electrode sheets were washed with dimethyl carbonate and then vacuum dried. X-ray CT measurements of the disassembled electrodes were performed at the beamline BL20XU of SPring-8. O K-edge XAS measurements were performed at the beamline BL-2 of the Ritsumeikan University SR Center. When comparing the results of charge-discharge cycle tests over 50 cycles at 0.1C and 2C rates using NMC811 and LCO, a significant degradation was observed for NMC811 cycled at 2C rate. X-ray CT analysis compared the initial state of the particles with the state after 50 charge-discharge cycles. Only surface cracks were observed in the LCO, whereas cracks extending to the interior were observed in the NMC811 after 50 cycles. In addition, an increase in microcracks on the particle surface was observed at the electrode after 2C cycles for NMC811. O K-edge XAS obtained from the total electron yield mode reflects the chemical state of the particle surface. In the electrode of NMC811 after 50 cycles at 2C, the peak derived from active materials disappeared and the peak from surface film increased. These findings indicate that higher cycle rates in NMC811 induce mechanical stress on the particles, leading to cracking and the formation of a resistive layer, which ultimately cause significant capacity degradation. References: 1: A. Gao, X. Li, F. Meng, S. Guo, X. Lu, D. Su, X. Wang, Q. Zhang, L. Gu, Small Methods, 2000730 (2021) 2: F. Lin, I. Markus, D. Nordlund, T. Weng, M. Asta, H. Xin, M. Doeff, Nat. Commun., 3529 (2014) 3:Q. Lin, W. Guan, J. Zhou, J. Meng, W. Huang, T. Chen, Q. Gao, X. Wei, Y. Zeng. J. Li, Z. Zhang, Nano Energy, 105021 (2020).