Lithium-metal solid-state batteries offer advantages of high energy density and improved safety compared with lithium-ion batteries1,2. However, solid-state batteries fail through short-circuiting even at low charging rates (less than 1 mA cm−2) due to lithium dendrite initiation and propagation3–5. The location of dendrite initiation is under debate, particularly regarding whether initiation occurs within the interior of the solid electrolyte6–9 or at the surface10–14. Here we develop an in-plane biaxial compression method that provides direct evidence that dendrite initiation occurs within the interior of garnet Li6.6La3Zr1.6Ta0.4O12 solid electrolytes during long-term cycling when the surface initiation mechanisms are rendered ineffective in shorting the cell. The biaxial compression deflects dendrite propagation so that it is perpendicular to the electric field direction, leading to the generation of an unprecedentedly high density of dendrites without short-circuiting, even at an extreme fast-charging rate of 100 mA cm−2. After long-term cycling, dendrites eventually appeared throughout the entire thickness of the solid electrolyte. Under extreme cycling conditions, isolated lithium deposits are observed at grain-boundary junctions and pores, and these act as the dendrite initiation sites. This work reconciles the surface and interior initiation mechanisms in garnet solid electrolytes and demonstrates that in-plane biaxial compressive stress can prevent both from short-circuiting the cell.
