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

Nickel-rich cathode materials such as LiNi$_{0.8}$Mn$_{0.1}$Co$_{0.1}$O$_2$ (NMC811) are promising next generation materials for lithium-ion batteries due to their high theoretical capacity and low cobalt content. However, NMC811 suffers from rapid capacity fading and poor cycle life. The solid electrolyte interphase (SEI) layer forms as a result of electrolyte decomposition during electrochemical cycling at low potentials. Transition metal (TM) dissolution, particularly Mn dissolution has been assumed to behave as an (electro)catalyst, consuming active lithium promoting further growth of the SEI layer. The mechanisms by which this occurs are poorly understood partly due to the wide adoption of post-mortem characterisation methods.
Adoption of Operando characterisation techniques (i.e. measurements performed during electrochemical cycling) are beginning to be utilised by the battery community. Techniques such as Raman and x-ray spectroscopies can provide information on the chemical and structural evolution of battery materials during cycling. However, these techniques often require a specialised and high cost cell design which may not be representative of standard cells for battery cycling.
Herein, we demonstrate a novel operando cell design which incorporates an x-ray transparent SiN$_x$ membrane window and allows soft X-ray absorption spectroscopy (XAS) during cycling of realistic battery electrode materials. XAS measurements in total electron yield (TEY) mode reveal chemical changes to the electrode-electrolyte interface related to electrolyte decomposition and electrode degradation processes. We will show how this approach can elucidate the precise chemical changes occurring at specific cell potentials enabling greater understanding of the impact of key battery degradation processes, such as TM dissolution. This understanding is expected to inform the selection of electrode and electrolyte materials as well as the design of mitigation strategies that inhibit the observed degradation pathways.