The kinetics and thermodynamics of void formation are investigated in all-solid-state lithium metal batteries and found to be controlled by interfacial lithium concentration and diffusivity. Diffusivity can be increased using a grain-refining approach, by introducing a Mg–1 wt% La inoculant in the melt, enabling a critical current density of 2.2 mA cm−2 at 1.1 mAh cm−2.