Methodology for Mechanical Pillar Array Indentation of Alkali Metals

Alkali metal foils (lithium, sodium, and potassium) are desired anode candidates for next generation solid-state batteries (SSB). To achieve safe implementation, new mechanical characterization techniques are required to examine the deformation behavior of alkali metal foils under the mechanical conditions found in realistic batteries. The current repertoire of mechanical interrogation techniques for alkali metals usually investigates the metals under idealized conditions. The presence of surface films and contact inhomogeneity are two parameters that are usually excluded from analysis. The extent of contact between alkali metals and solid-state electrolytes is usually variable, with contact inhomogeneity being the norm in laboratory-scale tests. Rough solid-solid interfaces reduce the applicability of single-pillar indentation or compression tests and simulations that hold parallel uniform contact as an assumption. Furthermore, alkali metals have low yield strength (~1 MPa or lower) and are sensitive to creep, which has important implications for behavior in solid-state batteries; such deformation behavior needs to be understood under realistic multi-contact conditions. Here, we demonstrate a new constant loading indentation mechanical deformation technique that employs indenter arrays of different sizes and spacings that probe the average material response to an array of contacting points, which is directly applicable to the conditions realized in bench-scale SSB production. The arrays are etched into a stainless-steel ram rod by a femtosecond laser with patterns ranging from 180 mm to 30 mm in diameter and spaced apart by a ratio of 4 times. The ram rod is released from a 1 mm drop to introduce contact with the alkali foils which will allow quantification of the initial deformation of the alkali foils by the impact of the indenter array. Subsequently, time dependent constant load deformation is recorded with a non-contact voltage displacement sensor. Importantly, this platform investigates displacement of the stainless-steel indenter array’s penetration over a representative area of the alkali foils with sub-micron resolution. This high-resolution monitoring enables a wide range of loading rates to be examined on this length scale above and below the theoretical yield strength of lithium, sodium, and potassium. This indentation array technique will provide important knowledge for analyzing realistic deformation behavior of lithium and will provide insight into the action of creep in “healing” voids at the alkali anode/SSE interface.