Li-ion batteries that include all solid-state components show promise as an emerging energy storage technology, as the solid electrolyte can improve safety and enable longer cycle life than current batteries based on flammable and reactive liquid electrolytes. Mechanical compatibility and durability of the solid components during large intercalation-induced volume changes are key concerns. For example, deformation and fracture of the solid electrolyte during cycling is expected to slow Li-ion conduction and create pathways for dendritic growth of metallic Li that can short-circuit the device. Identification and evaluation of key mechanical parameters of solid electrolytes is critical to predicting designs mitigating mechanical degradation.

With Li conductivity in excess of 10^-4 S/cm, 70/30 mol% Li₂S-P₂S₅ (LPS) is among the most promising solid electrolytes to date. Here we fabricated LPS and verified its amorphous structure and high ionic conductivity by X-ray diffraction and electrochemical impedance spectroscopy, respectively. We evaluated mechanical properties including Young’s elastic modulus, hardness, and fracture toughness via instrumented indentation as 18.5 +/- 0.9 GPa, 1.9 +/- 0.2 GPa, and 0.23 +/- 0.04 MPa m^1/2, respectively. These measurements were enabled by immersion of the sample in nonreactive fluids that stabilized composition of this moisture-sensitive compound.

We found Young’s modulus to be in agreement with prior published values derived with alternative measurement techniques, and report the first measurements of hardness and fracture toughness. While the stiffness and hardness of this solid electrolyte material were much lower than that of active materials within battery electrodes, the fracture toughness was also surprisingly low relative to typical glass and ceramic materials. Thus, the mechanical behavior of this candidate solid electrolyte is compliant (low resistance to reversible deformation) and brittle (low resistance to fracture). We will discuss the relationship between these observed mechanical properties and integration of these materials as solid electrolytes in all-solid-state batteries.