Abstract

Ceramic electrolytes are of interest for next-generation batteries because they suppress morphological instability when lithium plates at the lithium-metal/electrolyte interface. Lithium-ion-conductive garnet oxides based on Li7La3Zr2O12 (LLZO) have room-temperature conductivity approaching 1 mS/cm; various dopants ensure thermodynamic stability against lithium metal. Cations move through the crystal lattice of LLZO with near-unit transference. The shear modulus of LLZO is of the order of 50 GPa, well above the ~8 GPa needed for morphologically stable deposition. Despite these favorable properties, LLZO surprisingly still exhibits a ‘critical current’, above which lithium dendrites form.

Our group has put significant effort into developing consistent electrochemical models to describe electrolytes of various types, including liquids, ionomer gels, glasses, and ceramics. We have extended multicomponent transport theory to account for excluded-volume effects, which arise from considering the thermodynamics of a material’s density, and have used principles of irreversible thermodynamics to produce transport constitutive laws whose application can illustrate the mechanical consequences of viscous drag in liquid electrolytes and space charging at the interfaces of ceramics.

This talk will summarize our recent progress toward a theory that rationalizes 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 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.