Transient Short Circuit-Based Degradation of Lithium and Sodium Anode-Free Batteries

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.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 major safety issue in the early nineties, their detection and prevention were not widely studied. Therefore, a fundamental understanding of passivated 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 symmetric cells and anode-free lithium and sodium batteries using coupled galvanostatic impedance spectroscopy (GEIS) and in-situ nuclear magnetic resonance spectroscopy (NMR) for the first time.The coupling of in-situ NMR and EIS allows the observation of metal batteries' electrochemical and chemical dynamics and degradation in real time without affecting the cells' operation.We demonstrated that lithium and sodium 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 develop the next generation of rechargeable batteries with high energy density, prolonged cycling life and improved sustainability.