Electrochemical Formation of Li Metal Anodes to Enable Li-Free Manufacturing of Solid-State Batteries

Abstract

Solid-state batteries could not only present a safer alternative to conventional Li-ion batteries, but also the possibility for drastic increases in energy density. Replacing flammable liquid electrolytes with a non-flammable solid electrolyte could improve safety and using metallic Li instead of state-of-the-art graphite would double the energy density. While solid electrolytes could enable advanced cathode chemistries, currently it is believed that state-of-the-art cathodes (e.g. NMC, NCA, spinel, olivine) are the most viable to achieve >1000Wh L-1, high cycle life, and cost targets. However, given that state-of-the-art cathodes are fully lithiated, any pre-deposited Li will add extraneous volume, negating the energy density benefits of the Li metal. Furthermore, due to the difficulty and cost of handling free-standing Li foils, incorporating metallic Li anodes with representative thicknesses, low interfacial resistance, and using scalable processes remains a major manufacturing challenge. In lithium phosphorus oxynitride (LiPON) solid-state systems, “Li-free” configurations have been feasibly demonstrated, where the Li metal anode is formed in operando upon charging, using the Li contained within the cathode. However, with bulk-processed electrolytes like the garnet Li7La3Zr2O12 (LLZO), in operando formation has not been feasibly demonstrated. This work evaluates the potential for “Li-free” battery manufacturing using LLZO electrolytes. It is demonstrated that Li metal anodes with thicknesses >20μm (>5mAh cm-2) can be formed in operando without any evidence of Li filament propagation, as characterized using electrochemical and materials characterization methods. Furthermore, the mechanics of Li electrodeposition at the solid-solid current-collector/electrolyte interface are investigated. Finally, the preliminary cycling performance of in operando formed Li anodes is evaluated. The results not only provide insight into the mechanisms of electrodeposition at solid-solid interfaces, but also demonstrates the feasibility of “Li-free” manufacturing of Li metal solid-state batteries.