Electro-Chemo-Mechanical Evolution of Sulfide Solid Electrolyte-LiMg Alloy Interfaces—Effect of Current, Temperature and Stacking Pressure

All-solid-state batteries (ASSBs) with sulfide electrolytes have gathered attention for the potential to achieve high energy density. While incorporating Li metal anode in sulfide-based ASSBs boosts energy density, Li metal anode still exhibits mechanical instability at high current density -- void formation during stripping, inhomogeneous Li growth in the electrolyte during plating, and eventually result in cell failure. β-phase Li alloy has a similar electrochemical potential as Li and is considered a replacement for Li metal anode without sacrificing cell-level energy density in ASSBs. β-phase Li alloy is known for its rapid internal Li transport capability and morphological stability with oxide-based solid electrolytes. Yet, the mechanical properties of β-phase Li alloy and its contributions to the cycling stability of sulfide-based ASSBs are not well understood. Understanding the electro-chemo-mechanical evolution of β-phase Li alloy during Li stripping/plating would help boost the performance. Here, we demonstrate a higher critical current density of sulfide-based ASSBs enabled by β-phase Li-Mg alloy anode. The mechanical properties of Li-Mg and Li are characterized via tensile tests and nanoindentation. The mechanical stability of metal anode against sulfide electrolyte is evaluated electrochemically and morphologically under various stacking pressure. We then studied the electro-chemo-mechanical evolution of Li-Mg anode induced by plating or stripping as a function of current density. Cross-sectional scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) on cryo-polished samples reveal the morphology and elemental distribution of Li-Mg anode after the addition of Li. The interfacial stability of Li-Mg and SE during stripping is evaluated via <i>in situ</i> electrochemical impedance spectroscopy (EIS). Post-mortem analysis of Li-Mg anode after prolonged and high-current cycling further supports the model we propose to describe the interface between SE and β-phase Li alloy. Last, we demonstrate the capability of Li-Mg anode to enable higher critical current density (CCD) and in symmetric and full cell cycling.<br/>Acknowledgement: This work was supported by the U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy (EERE) under the Vehicle Technologies Program under Contract DE-EE0008864.