Direct Observations of Dendrite Growth in Ceramic Electrolytes

Although solid-state batteries with metal anodes promise to enable safer, higher energy density batteries, metal protrusions (dendrites) grow when charging faster than a critical current density. It is generally believed that dendrites grow when plating-induced stresses exceed that required for fracture of the solid-electrolyte. It is commonly assumed that the threshold stress for failure depends on the electrolyte's fracture toughness—commonly taken as a material constant. However, because the dendrite-electrolyte interface is buried, characterization of dendrite growth has proved challenging. Here, we study plan-view solid-state cells with solid electrolytes thinned to the point of translucency, allowing us to analyze dendrites growing through the electrolyte plane. We develop <i>operando</i> birefringence microscopy to directly measure dendrite-induced stresses. During propagation, dendrite-induced stresses appear to evolve with time in a fashion that depends on the current density or dendrite velocity. We find that increasing current densities increase the dendrite velocity. At all times, the measured stress associated with dendrite growth is below the critical stress expected for fracture of the electrolyte—dendrite propagation occurs under subcritical conditions. Cryogenic Scanning Transmission Electron Microscopy (Cryo-STEM) reveals decomposed electrolyte phases at the dendrite tip. This decomposition is associated with a volume contraction. All experiments were conducted on the most electrochemically stable Li-ion conducting solid electrolyte (tantalum-doped lithium lanthanum zirconium oxide). Together, these experiments allow separate study of electrochemical and mechanical phenomena underlying dendrite growth in ceramic electrolytes.<br/><br/>Acknowledgements:<br/>Funding is gratefully acknowledged from Mechano-Chemical Understanding of Solid Ion Conductors, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Science, contract DE-SC0023438C.