Towards understanding electrolyte-dependant dynamics and kinetics of lithium deposition and stripping by operando neutron imaging

Rechargeable lithium (Li) metal batteries, which use Li metal as the negative-electrode, promise energy densities that are two times higher than those achievable with conventional Li-ion batteries. However, the practical application of Li-metal batteries is currently constrained, and where the central challenge is dendritic Li growth, leading to cell failure. To overcome these limitations, electrolytes compatible with Li metal are required, where various electrolyte formulations have been proposed, but a mechanistic understanding of their effects to the kinetics and dynamics of Li deposition remains incomplete. Here, we introduce operando neutron imaging as a versatile modality for observing electrolyte-dependent Li-deposit nucleation, Li-metal plating and stripping behaviour with high temporal and spatial resolution. We tested three different carbonate electrolytes with varying concentrations of fluoroethylene carbonate (FEC) and found that low levels of the FEC additive contribute to better Li cycling reversibility, while higher concentrations lead to adverse effects, revealing an unexpected and critical limitation of FEC-additive application for improving Li-metal cycling. Our imaging methodology can be a starting point for much broader operando studies of the plating and stripping behaviour of Li metal in any electrolyte, potentially making it a key tool for future electrolyte developments.