Diffusion Limitation of Lithium Metal and Li–Mg Alloy Anodes on LLZO Type Solid Electrolytes as a Function of Temperature and Pressure

The morphological instability of the lithium metal anode is the key factor restricting the rate capability of lithium metal solid state batteries. During lithium stripping, pore formation takes place at the interface due to the slow diffusion kinetics of vacancies in the lithium metal. The resulting current focusing increases the internal cell resistance and promotes fast lithium penetration. In this work, galvanostatic electrochemical impedance spectroscopy is used to investigate operando the morphological changes at the interface by analysis of the interface capacitances. Therewith, the effect of temperature, stack pressure, and chemical modification is investigated. The work demonstrates that introducing 10 at% Mg into the lithium metal anode can effectively prevent contact loss. Nevertheless, a fundamental kinetic limitation is also observed for the Li‐rich alloy, namely the diffusion controlled decrease of the lithium metal concentration at the interface. An analytical diffusion model is used to describe the temperature‐dependent delithiation kinetics of Li–Mg alloys. Overall, it is shown that different electrode design concepts should be considered. Mg alloying can increase lithium utilization, when no external pressure is applied while pure lithium metal is superior for setups that allow stack pressures in the MPa range.