Operando analysis of the molten Li|LLZO interface: Understanding how the physical properties of Li affect the critical current density

Solid-state electrolytes are promising for enabling high-energy-density Li metal batteries. However, despite significant progress in recent years, shorting due to Li penetration of the solid electrolyte at high current densities hinders further adoption of solid-state batteries. In this study, we use operando optical and scanning electron microscopy to probe mechanisms behind Li penetration in molten Li|LLZO model systems. A step increase in critical current density (CCD) is observed across the melting point of Li, reaching a CCD of 530 mA/cm2 at 195°C for LLZO. Postmortem microscopy and analysis of the CCD across the melting point of Li are used to support theories explaining how LLZO fractures and Li penetration occurs. A mechanical model is developed describing the transition from the viscoplastic behavior of solid Li to the viscous behavior of molten Li and reveals the critical role of mechanical properties of Li metal in determining CCD.