The demand for extended-range electric vehicles has created a renaissance of interest in replacing the common metal-ion with higher energy-density metal-anode batteries, i.e. lithium, sodium and zinc. However, metal cells suffer from capacity fading and potential safety issues due to uneven metal electrodeposition.

"Anode-free" (AF) batteries comprise a metal-ion cathode and a current collector, the cathode consisting of the metal source. The metal is then plated on the current collector during charging. These batteries present a significant advantage due to their higher energy density, superior safety, and ease of production. However, realising metal and AF batteries requires a better understanding of alkali and multivalent metal plating, short-circuiting mechanisms, and metal battery degradation.

A considerable performance gap between lithium symmetric cells and practical lithium batteries motivated us to explore the correlation between the shape of voltage traces and degradation. The coupling of operando nuclear magnetic resonance (NMR) and galvanostatic electrochemical impedance spectroscopy (GEIS) allowed us to observe metal batteries' electrochemical and chemical dynamics and degradation in real-time without affecting the electrochemical reactions in the cell. 

"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 potential safety issue for lithium-ion batteries in the early nineties, their detection and prevention were not widely studied. Using coupled EIS and operando NMR, we showed that transient (i.e., soft) shorts form in realistic conditions for battery cycling.

The typical rectangular-shaped voltage trace, widely considered ideal, was proven, under the conditions studied here, to be a result of soft shorts. Recoverable soft-shorted cells were demonstrated during a symmetric cell polarisation experiment, defining a new type of critical current density: the current density at which the soft shorts are not reversible. We showed that soft shorts are predictive towards the formation of hard shorts, demonstrating the potential use of EIS as a relatively low-cost and non-destructive method for early detection of catastrophic shorts and battery failure while demonstrating the strength of operando NMR as a research tool for metal plating in metal batteries. 

While today's lithium-ion battery chemistries are ubiquitous, powering mobile devices and increasingly electric vehicles, they contain critical minerals such as cobalt and minerals with supply concerns, mainly if these batteries are used at the scale needed to meet the 2050 climate goals. To address this challenge, "beyond-lithium" technologies are investigated. These battery chemistries allow earth-abundant, safer, and low-cost energy storage.

Sodium metal anodes have been studied broadly, assuming sodium and lithium metal anodes respond similarly to cycling. We established that lithium and sodium short circuit formation mechanisms fundamentally differ and strongly depend on electrolyte composition and SEI stability.

Zinc metal anodes have gained increasing interest due to the sustainability of the aqueous electrolytes; however, a greater insight into their unique plating, hydrogen evolution, and corrosion mechanisms is critical. In our SECM study of zinc plating, we compared the effects of various electrolyte compositions and plating conditions on SEI heterogeneity and zinc metal morphology. We showed that the heterogeneity of the surface reactions on zinc electrodes has a more significant effect on zinc anode cyclability than electrolyte 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. A fundamental understanding and reliable testing methods for soft short circuits are critical for commercialising metal (e.g., metal-air, metal-sulphur) and AF batteries.