Understanding the Expansion and Electrochemistry of Nickel-Rich NMC - Graphite Full Cells Using in-Situ Dilatometry

Abstract

The higher energy density and lower cost of Ni-rich lithium nickel manganese cobalt (NMC) have made them one of the best cathode materials for Li-Ion batteries (LIBs). Understanding the mechanical behaviour, particularly the volume changes during charging and discharging, is important for the cell design. So far, this has mainly been studied using X-ray diffraction (XRD), which is powerful in understanding the nanoscale crystal level of the material volume change. However, recent papers have shown that XRD is insufficient to capture macroscopic cell expansion [1]. It is also getting more important to consider the synergistic effect of cathodes and anodes in the context of full cells. It is therefore valuable to have in-situ dilatometry filling the gap and unravelling how nano and macro electrochemical and mechanical behaviour differs.Here, we coupled electrochemical analysis with in-situ electrochemical dilatometry to observe macroscopic dimensional changes of NMC811−graphite full cells during galvanostatic cycling under different pressure conditions, voltages, and cathode particle morphologies. Our setup uses a non-contact dilation sensor and independent control over the stack pressure to achieve highly accurate measurements of the electrode expansion, with a linearity of ≤ ±90 nm, resolution of ≤ 0.5 nm, and a drift of ≤ 100 nm/month.The initial investigations of graphite v.s. NMC811 pouch cells (220 mAh) have offered clear evidence of the strong correlation between dilation and electrochemistry, where reversible dilation features in every charge-discharge cycle can be assigned to cathode and anode phase transitions. In addition, our system can effectively isolate the reversible thickness change from the irreversible thickness change known to be potentially correlated to solid electrolyte interphase (SEI),[2] particle rearrangement,[3] and electrode delamination.[4] With different experimental conditions (C-rates, pressure and voltage windows), the full cells demonstrated different electrochemical performances alongside varied dilation behaviours. The observed in-situ dimensional changes improve our understanding of the physical and chemical processes happening while cycling NMC-Graphite full cells.[1] F. B. Spingler, S. Kücher, R. Phillips, E. Moyassari, and A. Jossen, “Electrochemically Stable In Situ Dilatometry of NMC, NCA and Graphite Electrodes for Lithium-Ion Cells Compared to XRD Measurements,” J. Electrochem. Soc., vol. 168, no. 4, p. 040515, 2021, doi: 10.1149/1945-7111/abf262.[2] A. J. Louli, L. D. Ellis, and J. R. Dahn, “Operando Pressure Measurements Reveal Solid Electrolyte Interphase Growth to Rank Li-Ion Cell Performance,” Joule, vol. 3, no. 3, pp. 745–761, 2019, doi: 10.1016/j.joule.2018.12.009.[3] D. Y. W. Yu, M. Zhao, and H. E. Hoster, “Suppressing Vertical Displacement of Lithiated Silicon Particles in High Volumetric Capacity Battery Electrodes,” ChemElectroChem, vol. 2, no. 8, pp. 1090–1095, 2015, doi: 10.1002/celc.201500133.[4] H. Michael et al., “A Dilatometric Study of Graphite Electrodes during Cycling with X-ray Computed Tomography,” J. Electrochem. Soc., vol. 168, no. 1, p. 010507, 2021, doi: 10.1149/1945-7111/abd648.