Electrochemical corrosion accompanies dendrite growth in solid electrolytes

Charging rates, cycling performance and safety of solid-state batteries using metal negative electrodes are often limited by dendrites^1-3, 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 Li_6.6La₃Zr_1.6Ta_0.4O₁₂, a highly stable solid electrolyte^5-7. 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.