Micro-Mechanical Testing of Highly Air-Sensitive Argyrodite and Argyrodite-Zirconia Composites by In Situ Nanoindentation and Micro-Cantilever Bending

All-solid-state-batteries (ASSBs) pave the way for safe use of lithium-metal electrodes due to their use of a solid electrolyte (SE). Short circuit failure caused by the penetration of lithium dendrites, resulting in the fracture of SEs, remains one of the largest challenges facing ASSBs; it is therefore essential to gain a better understanding of the mechanical properties of SEs. Due to the highly air-sensitive nature of SEs conventional mechanical testing techniques are not possible and as a result very little is known about their mechanical properties.<br/><br/>In this work, the mechanical properties of Argyrodite, an air-sensitive sulphide, was investigated using in-situ nanoindentation inside an enclosed Argon glovebox system. Both Berkovich and cube-corner nanoindentation testing procedures were used to measure elastic, plastic and fracture properties. A modulus of 20.4 GPa, hardness of 960 MPa and a fracture toughness of 0.76 MPa m^1/2 was measured. It has been shown that lithium dendrites are able to plate intergranularly through SEs, along the grain boundaries, leading to transgranular fracture of the grains. This led to the question as to what is the grain boundary strength of these SEs? Using an innovative technique, the grain boundary strength was measured to be 91.1 MPa. This was achieved by preparing a pentagonal microcantilever containing a single grain boundary at its fixed end and by bending the free end using cube-corner nanoindentation. This value demonstrates the pressure which the Li metal must exceed to be able to ingress into the SE along the grain boundary.<br/><br/>Our initial results show that to further advance the performance of SEs, their fracture properties must be improved. To facilitate this, Argyrodite-Zirconia composites were made to increase the fracture toughness. Using the same nanoindentation testing procedures, the mechanical properties of these composites were investigated. As the proportion of Zirconia was increased, so did the modulus and hardness values, as predicted by the Voigt and Reuss models. The modulus increased to 29.6 GPa and the hardness to 1.35 GPa; as further Zirconia was added, both modulus and hardness decreased. This trend was also observed for fracture toughness with an improvement to 0.96 MPa m^1/2 being achieved. These improvements in mechanical properties are also accompanied by improvements in our initial electrochemical test results, which show that increasing the proportion of Zirconia raises the current density needed to induce lithium dendrites.