Purity of lithium metal electrode and its impact on lithium stripping in solid-state batteries

Recent studies emphasize that incorporating lithium metal electrodes can increase the energy density of next generation batteries. However, the production of lithium metal with high purity requires multi-stage purification steps due to its high reactivity. Furthermore, subsequent handling under inert conditions is required to prevent degradation. To circumvent handling of lithium metal and further improve energy density, researchers are exploring reservoir-free cells often referred to as “anode-free” cells. Reservoir-free cells are assembled without using lithium metal. Instead, lithium is electrodeposited at the interface between a current collector and a solid electrolyte from positive electrode materials during the first charge. Despite the potential of reservoir-free cells, there is limited understanding of the purity of electrodeposited lithium metal and how impurities might affect the electrochemical kinetics. This study examines first the purity of electrodeposited lithium at the steel|Li6PS5Cl interface. Then, it shows how impurities in lithium electrodes affect stripping capacity when using commercial lithium metal foils with both Li6PS5Cl and Li6.25Al0.25La3Zr2O12 as solid electrolytes. By using time-of-flight secondary mass spectrometry and X-ray photoelectron spectrometry, we reveal that a lithium layer with high purity is electrodeposited at the negative electrode in reservoir-free cells and that common impurities in lithium metal (reservoir-type) electrodes like e.g. sodium negatively influence the accessible lithium capacity during discharge.