Ceramic electrolytes hold considerable promise for next-generation batteries because their elastic moduli can in principle suppress dendrite nucleation from morphological instability. Lithium-ion-conductive garnet oxides based on Li7La3Zr2O12 have room-temperature conductivity approaching 1 mS/cm; certain dopants extend the window of voltage stability to a range where lithium metal is stable. Through the crystal lattice, cations move with near-unit transference. Given these favorable properties, LLZO surprisingly still exhibits a ‘critical current’, above which lithium dendrites form.
Our group has put significant effort into the development of consistent transport theories to describe solid electrolytes of various types, including ionomer gels, glasses, and ceramics. We have extended multicomponent transport theory to account for excluded-volume effects, which arise from the thermodynamics of material volume, and have developed transport constitutive laws that describe space charging at the interfaces of ceramics.
This talk will summarize our recent progress toward developing a theoretical model that can be used to rationalize the critical current of LLZO in electromechanical terms. We describe a variety of new measurements that help to characterize elastic solid electrolytes, lay out the modifications of familiar transport laws that are needed to account rigorously for the energetic impact of electrolyte elasticity, and examine how electrochemical/mechanical coupling affects practical data such as impedance spectra. Interfaces are found to affect critical currents by changing the balance of bulk ohmic loss and capacitive surface charging, the latter of which leads to a buildup of stresses within the material. Our theory produces scaling laws that agree well with experiments, predicting how the critical current varies with temperature and interfacial properties.