The Fast Charge Transfer Kinetics of the Lithium Metal Anode on the Garnet-Type Solid Electrolyte Li6.25Al0.25La3Zr2O12

Abstract

The charge transfer kinetics between a lithium metal electrode and an inorganic solid electrolyte is of key interest to assess the rate capability of future lithium metal solid state batteries. In an in situ microelectrode study run in a scanning electron microscope, it is demonstrated that—contrary to the prevailing opinion—the intrinsic charge transfer resistance of the Li|Li6.25Al0.25La3Zr2O12 (LLZO) interface is in the order of 10−1 Ω cm2 and thus negligibly small. The corresponding high exchange current density in combination with the single ion transport mechanism (t+ ≈ 1) of the inorganic solid electrolyte enables extremely fast plating kinetics without the occurrence of transport limitations. Local plating rates in the range of several A cm−2 are demonstrated at defect free and chemically clean Li|LLZO interfaces. Practically achievable current densities are limited by lateral growth of lithium along the surface as well as electro-chemo-mechanical-induced fracture of the solid electrolyte. In combination with the lithium vacancy diffusion limitation during electrodissolution, these morphological instabilities are identified as the key fundamental limitations of the lithium metal electrode for solid-state batteries with inorganic solid electrolytes.