Charging rates, cycling performance and safety of solid-state batteries using metal negative electrodes are often limited by dendrites1–3, the growth of which depends on coupling between electrochemical and mechanical driving forces. Previously, it has been assumed that dendrites propagate when plating-induced stresses reach the fracture stress of the solid electrolyte. Here we show that dendrites can propagate at far lower stresses. Using operando birefringence microscopy4, we directly measure stresses around growing dendrites in garnet Li6.6La3Zr1.6Ta0.4O12, a highly stable solid electrolyte5–7. Plating-induced stresses are present throughout growth and approach the mechanical fracture stress for the slowest-growing dendrites. As current densities and dendrite velocities increase, the stresses accompanying dendrite growth surprisingly decrease, with dendrite propagation occurring at stresses up to 75% lower than under mechanical load alone. Cryogenic scanning transmission electron microscopy (STEM) of dendrites propagated at high current reveals electrolyte decomposition to new phases, associated with which is a net molar volume contraction. The electrochemically induced mode of embrittlement may be mitigated through understanding and control of the nature of phase transitions accompanying instability.
