Operando Raman analysis of electrolyte changes in Li-ion batteries with hollow-core optical fibre sensors

Abstract New methods are urgently required to identify degradation and failure mechanisms in high energy density energy storage materials such as Ni-rich LiNi0.8Mn0.1Co0.1O2 cathodes (NMC811) for Li-ion batteries. Understanding and ultimately avoiding these mechanisms requires in-situ tracking of the complex electrochemical processes that occur in different parts of battery cells. Here we demonstrate a new operando spectroscopy method that enables the tracking of electrolyte chemistry, applied here for high energy density Li-ion batteries with a NMC811 cathode, during electrochemical cycling. This is achieved by embedding a novel hollow-core optical fibre probe inside the battery to monitor the evolution of electrolyte species by background-free Raman spectroscopy. Our data reveals changes in the ratio of carbonate solvents and electrolyte additives as a function of the cell voltage, as well as changes in the lithium-ion solvation dynamics. This advanced operando methodology delivers a new way to study battery degradation mechanisms, and the understanding it develops should contribute to extending the lifetime of next-generation batteries.