Investigation of Li+ Insertion in NMC622 Cathode Material upon Deep Lithiation Via Electrochemistry and x-Ray Absorption Spectroscopy

Abstract

Nickel-based cathode materials have become an exciting alternative to the widely used LixCoO2 (LCO) for consumer electronics. The main advantages of this type of cathode materials over the LCO electrodes are a higher practical capacity and excellent safety characteristics.[1] Additionally, the tendency to increase the nickel content of these electrodes proves to be cost effective due to their lower cobalt content.[2] One known drawback of nickel-containing layered cathodes is their poor first cycle efficiency of 87-88.6% when cycled in the standard potential window of 2.8 V to 4.4V, whereas LCO cathodes reach a first cycle efficiency of 98%.[3]In this study, we focus on the electrochemical properties of LixNi0.6Mn0.2Co0.2O2 (NMC622) cathode materials. We start by demonstrating that it is possible to recover the capacity “lost” by the NMC622 electrode in the first cycle by increasing the applied overpotential, as was previously shown for other nickel-based cathodes, including LiNiO2, LiNi1/3Mn1/3Co1/3O2, and LiNi0.8Co0.10Al0.05O2.[3-4] We then establish through electrochemical measurements that it is possible to overlithiate the NMC622 cathode material by as much as 300 mAh/g relative to the pristine electrode. Finally, we complement our electrochemical characterization with ex-situ X-ray diffraction as well as soft and hard X-ray absorption spectroscopy experiments, to elucidate how the structure of the material and the oxidation states of the transition metals (nickel, cobalt, manganese) change throughout the overlithiation process.[1] Andre, D. et al. Future generations of cathode materials: An automotive industry perspective. J. Mater. Chem. A 3, 6709–6732 (2015).[2] Xu, J., Lin, F., Doeff, M. M. & Tong, W. A review of Ni-based layered oxides for rechargeable Li-ion batteries. J. Mater. Chem. A 5, 874–901 (2017).[3] Kang, S. H., Yoon, W. S., Nam, K. W., Yang, X. Q. & Abraham, D. P. Investigating the first-cycle irreversibility of lithium metal oxide cathodes for Li batteries. J. Mater. Sci. 43, 4701–4706 (2008).[4] Dahn, J. R., von Sacken, U. & Michal, C. A. Structure and electrochemistry of Li1+/-xNiOz and a new Li2Ni02 phase with the Ni (OH) 2 structure. Solid State Ionics 44, 87–97 (1990).