Interfacial Degradation in NMC811-Graphite Batteries during Extended Cycling

Abstract

To improve the energy density in Li-ion batteries, development of cathode materials with higher capacities are required, where much of the current focus is on Ni-rich lithium nickel manganese oxides (NMC). However, higher Nickel content is typically accompanied by accelerated degradation and thus poor cycle life, with undesired reactions at the electrode-electrolyte interfaces strongly implicated. There have been numerous attempts to mitigate this, using various methods including surface coatings, structural doping and advanced synthesis techniques such as core-shell or concentration gradient particles with much lower nickel concentrations at their surface. Designing improved solutions, requires a deeper mechanistic understanding of the underlying interfacial processes involved in degradation and their relative contributions. In this study these reactions are investigated in LiNi0.8Mn0.1Co0.1O2–graphite cells cycled using LP57 electrolyte (1 M LiPF6 in EC:EMC, 3:7) for different number of cycles, providing information about how interfacial degradation proceeds. Electrochemical measurements are combined with post mortem techniques including surface sensitive soft X-ray absorption spectroscopy, and photoelectron spectroscopy measured with different excitation energies. Based on fitted differential voltage analysis (DVA) data from the cells it is shown how the initial capacity fading during the first 300 cycles is almost fully attributable to excessive electrolyte reduction exhibited as SEI thickening, whereas further cycling leads to a larger contribution to capacity fading from loss of active NMC material. The initial capacity fading due to electrolyte reduction can be correlated with more plating of manganese, as compared to nickel, onto the graphite electrode, see figure. However, after 600 cycles both nickel and manganese are found in similar concentrations within the SEI. The changing ratio between the amount of plated nickel and manganese with cycle number indicates that manganese on the NMC surface initially dissolves more easily than the nickel, but as the unstable manganese ions are dissolved more and more nickel will contribute to the transition metal dissolution. The nickel ions in the SEI are also found to change their chemical surrounding as the cycling proceeded, showing that they do not form stable species and are thus likely to contribute to continued breakdown of the SEI. We compare the observed evolution of the SEI that results from cross talk between the NMC and graphite electrodes, to the case where EC-free electrolytes are used and graphite is replaced with Li4Ti5O12 (LTO) such that contributions from electrolyte reduction at the anode are expected to be minimised [1,2].In addition to the side reactions at the graphite other important side reactions take place on the NMC, some of which are potentially initiated by the graphite electrode [3]. For instance it was found that the Li2CO3 impurities present on the NMC surface decompose throughout the cells’ cycle life rather than just during the initial cycles. Previous studies have shown how Li2CO3 coatings severely decrease the cycle life, and therefore the observations here show that decomposition related to Li2CO3 is a problem that does not disappear during the formation cycling. Instead minimizing the initial Li2CO3 formation is critical. The insights obtained here on the evolution of surface layers coupled with the individual electrochemical degradation of the electrodes will advance the understanding of the Ni-rich NMC’s capacity retention behaviour and inform future efforts to achieve longer cycle lifetimes.[1] Björklund, E., Göttlinger, M., Edström, K., Younesi, R. & Brandell, D. Sulfolane‐Based Ethylene Carbonate‐Free Electrolytes for LiNi0.6Mn0.2Co0.2O2−Li4Ti5O12 Batteries. Batter. Supercaps 3, 201– 207 (2020).[2] Björklund, E., Göttlinger, M., Edström, K., Brandell, D. & Younesi, R. Investigation of Dimethyl Carbonate and Propylene Carbonate Mixtures for LiNi0.6 Mn0.2Co0.2O2-Li4 Ti5 O12 Cells. ChemElectroChem 6, 3429–3436 (2019).[3] Björklund, E., Brandell, D., Hahlin, M., Edström, K. & Younesi, R. How the Negative Electrode Influences Interfacial and Electrochemical Properties of LiNi1/3Co1/3Mn1/3O2 Cathodes in Li-Ion Batteries. J. Electrochem. Soc. 164, A3054–A3059 (2017). Figure 1