Studying the Role of Additives in Carbonate Electrolytes for Graphite||NMC811 Batteries with Interface-Sensitive X-Ray Spectroscopies

Li-ion rechargeable batteries are increasingly used as power sources for portable consumer products and electric vehicles. The formation of stable electrode-electrolyte interphases is critical to long-term performance, however, these typically degrade during extended cycling due to ongoing side reactions and electrode cross-talk. Over the past few years, various electrolyte additives have been reported as being able to improve the properties of the interphases formed on electrode surfaces and suppress cell degradation. However, these are often tuned for a specific electrode (e.g. cathode), and may negatively impact on the other electrode (e.g. anode). A key experimental challenge is therefore to reveal how additives affect the interfacial layers formed at both anode and cathode in full cells, their resulting influence on the cross-talk, and on the long-term stability of each electrode. Soft X-ray absorption spectroscopy (soft-XAS) and X-ray photoelectron spectroscopy (XPS) are powerful techniques to investigate the evolution of the short-range local structure and chemical species in complex systems. These properties make soft-XAS and XPS a suitable technique to study phase evolution, chemical species, local structure and stability of the interface formed on the electrodes. Here, we have studied carbonate-based electrolytes with various additives (such as Vinylene Carbonate, Lithium difluorophosphate, and fluoroethylene carbonate) in Graphite||NMC811 full cell. Soft-XAS and XPS have been performed to understand how additives in the electrolyte alter the chemical nature of interface formed on both anode and cathode in early cycles and their influence on extended longer-term degradation related to cross-talk, such as transition metal cross-over. Further, we identified which additives are beneficial for which electrodes and which cross-talk reactions they help to mitigate, this informs the development of electrolyte formulations where different additives are combined to improve battery lifetime and performance. Comparing the chemical information obtained with electrochemical cycling data we determined the connection between the chemical degradation of the electrodes and the reduction in electrochemical performance.