Abstract

The demand for an extended range of electric vehicles has created a renaissance of interest in replacing the common lithium-ion with a higher energy-density metal anode (e.g., Li or Na). However, alkali metal cells suffer from capacity fading and potential safety issues. The uneven metal electrodeposition often results in dendrite formation and potentially hazardous situations such as cell short-circuiting and thermal runaway.

Our recent studies of lithium plating on copper for anode-free batteries showed that battery electrolytes break down spontaneously on the copper current collector, forming a new interface that triggers dendritic lithium plating and potential safety issues. We demonstrated lithium loss in anode-free cells occurs due to solid electrolyte interphase (SEI) breakdown and repair, formation of ‘dead lithium’ and Galvanic corrosion of lithium on copper [1].

Though lithium plating has been studied widely, a better understanding of the short-circuiting mechanisms and metal battery failure is required. Moreover, a considerable performance gap exists between symmetric metal cells and realistic metal batteries.

“Soft shorts” are small localised electrical connections between two electrodes that allow the co-existence of direct electron transfer and interfacial reaction. Although soft shorts were identified as a significant safety issue in the early nineties, their detection and prevention were not widely studied. Therefore, a fundamental understanding of SEI -forming metals’ plating and a reliable testing method for soft short circuits is critical for realising metal batteries, such as Li-Air, Li-S, and anode-free batteries.

Here, we compared short circuit formation mechanisms and degradation in lithium symmetric cells using coupled galvanostatic impedance spectroscopy (GEIS) and in-situ nuclear magnetic resonance spectroscopy (NMR) for the first time. In-situ NMR and EIS coupling allows the observation of metal batteries’ electrochemical and chemical dynamics and degradation in real time without affecting the cells’ operation.

We demonstrated that lithium short circuit formation mechanisms fundamentally differ and strongly depend on electrolyte composition and SEI stability. This new understanding of the metal plating mechanism is crucial to developing the next generation of rechargeable batteries with high energy density, prolonged cycling life and improved sustainability [2].