EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Increasing the cycle life and the energy storage density of Li-ion batteries are key challenges in making rechargeable batteries a more sustainable option. This has led to the development of Ni-rich LiNixMnyCo1-x-yO2 (NMC) materials with offer higher energy densities, but for commercial applications their rapid capacity fade must be addressed. We report here out detailed ex-situ studies of NMC vs. Graphite full cells that links electrochemical signatures of cell degradation with surface chemistry changes taking place on the electrodes using Hard X-ray Photoelectron Spectroscopy (HaXPES). From the cycling data it is seen that electrolyte reduction is the main cause of the capacity fading during the first few hundred cycles, followed by an increasing contribution from loss of active NMC material. It is well established that transition metals in the solid electrolyte interphase (SEI) can cause ongoing electrolyte reduction. Here we reveal that the relative rates of plating for the different transition metals change as cycling proceeds. Mn is found to be plated more rapidly in the beginning, whereas Ni becomes a more significant contribution at higher cycle numbers. From x-ray absorption spectroscopy (XAS) changes are apparent in the Ni spectra of the graphite electrodes, indicating chemical changes to the plated Ni-ions with cycle number. For the NMC electrodes, Li2CO3 impurities are found on their surface which decompose over the course of extended cycling, rather than just in the initial cycles as previously assumed.

Despite the insights obtained, ex situ measurements suffer from potential ambiguity due to changes to the electrode surfaces occurring during glovebox disassembly, and cannot capture intermediate species involved in interface degradation. We will therefore also report our initial results using operando electrochemical cells that we have recently developed that accommodate a thin silicon nitride window through which soft XAS can be measured during cell cycling.