In Situ ATR-FTIR Spectroscopic Analysis of NMC622 Cathode: Correlating Specific Chemistries within the Cathode-Electrolyte Interface to FTIR Signatures

In the last decade, cathodes based on lithium nickel manganese cobalt oxide, LiNi1-x-yMnxCoyO2, have garnered much attention due to their high theoretical capacity, up to 280mAhg-1.1 In general, increasing the Ni content in these materials boosts the capacity, however this usually comes at the cost of decreased stability.2 Thus, stabilization of cathode surfaces is essential if high-voltage and low-Co (due to scarcity and fluctuating price of Co) NMC-based cathodes (NMC811 or NMC9055) are to be widely-adopted in application.3 To understand cell failure mechanisms and develop mitigation strategies enabling high-stability electrodes, much effort has been devoted to spectroscopic investigations of batteries and their constituents. Recently, we have developed in situ Raman and FTIR techniques that can monitor the surface chemistry and evolution of the electrode-electrolyte interphase, providing a better understanding of the electrochemical performance of next-generation lithium-ion batteries (LiBs).4 ,5 Here, we present an in situ study of the chemistry and evolution within the NMC622 model cathode—electrolyte interphase (CEI) during galvanostatic cycling using ATR-FTIR to accurately investigate the voltage-dependent interactions between the cathode and electrolyte (1.2M LiPF6 in EC:EMC, 3:7 wt%). Our newly developed in situ cell design provided both reliable and reproducible electrochemical performance and strong FTIR vibrational absorption signals near the cathode surface. Our study reveals three important aspects of the cell evolution: 1) solvation of the Li+ ions by the solvent molecules near the cathode interface is related to the extent of electrode polarization and the extraction (or insertion) of Li+ ions from (into) the NMC622 cathode during charging (discharging) of the battery, 2) NMC622 lattice vibrational modes are correlated to structural changes of the crystals based on the redox of constituent transition metal(s), and 3) formation and evolution of a CEI surface layer due to electrolyte degradation can be monitored, which is important for evaluating the Coulombic efficiency and cycle lifetime of the cell and nature of the cathode surface passivation. This useful approach can be extended to the evaluation of novel electrode materials and newly developed electrolyte formulations with and without additives. References: Nitta, N.; Wu, F.; Lee, J. T.; Yushin, G., Li-ion battery materials: present and future. Materials Today 2015, 18 (5), 252-264. Schipper, F.; Erickson, E. M.; Erk, C.; Shin, J.-Y.; Chesneau, F. F.; Aurbach, D., Review—Recent Advances and Remaining Challenges for Lithium Ion Battery Cathodes: I. Nickel-Rich, LiNixCoyMnzO2. Journal of The Electrochemical Society 2017, 164 (1), A6220-A6228. Croy, J. R.; Abouimrane, A.; Zhang, Z., Next-generation lithium-ion batteries: The promise of near-term advancements. MRS bulletin 2014, 39 (5), 407-415. Ha, Y.; Tremolet de Villers, B. J.; Li, Z.; Xu, Y.; Stradins, P.; Zakutayev, A.; Burrell, A.; Han, S. D., Probing the Evolution of Surface Chemistry at the Silicon-Electrolyte Interphase via In Situ Surface-Enhanced Raman Spectroscopy. The Journal of Physical Chemistry Letters 2020, 11 (1), 286-291. Tremolet de Villers, B. J.; Yang, J.; Bak, S. M.; Han, S. D., In-situ ATR-FTIR Study of NMC622 Cathode Reveals Relationship Between Electrolyte Solution Structure and Cathode Transition Metal Redox Chemistry, in preparation