The Effect of Aspect Ratio on Creep Behavior of Lithium Metal in Relevant Solid-State Battery Configuration

Enabling Li metal anodes for use in Li metal solid-state batteries (SSBs) is one approach to dramatically exceed the energy density of state-of-the-art Li-ion batteries. However, commercialization of Li metal SSBs requires a greater understanding of the Li metal/solid-state electrolyte interface during cycling. For example, contact must be maintained between Li metal and the solid-state electrolyte during discharge. This requires that the electrochemical flux of Li stripped from the interface and the mechanical flux of Li towards the interface must be balanced. While the electrochemical behavior of Li has been well-studied, the mechanical properties of Li have only recently been investigated. Preliminary studies of Li metal in compression have shown that the mechanical properties are strongly dependent on the dimensions or aspect ratio (height/diameter) of the Li specimen. Mechanical compression tests generally aim to eliminate friction between the specimen and testing apparatus, while the behavior of Li metal under applied stack pressure in a SSB will be influenced by frictional effects from both the solid-state electrolyte and the current collector. These frictional forces may change the mechanical deformation behavior of Li in SSBs where the anode must be thin (&lt;25 µm) to achieve high energy density.<br/>This work studies the mechanical behavior of Li metal in compression coupled with a relevant model solid-state electrolyte and current collector as a function of Li foil thickness (100-700 µm) and applied stress (1-10 MPa). It is shown that under 1 MPa compressive stress, the strain rate of all Li foil thicknesses was almost four orders of magnitude lower (negligible) than predicted from tensile creep tests. While steady-state creep behavior was observed for Li foil ≥350 µm at 5 MPa and ≥250 µm at 10 MPa, sustained creep deformation was not observed for 100 and 150 µm Li foils at any applied stress. It is hypothesized that for thin Li foil, the deformation of Li is controlled by friction at the Li/LLZO and Li/Ni interfaces, generating hydrostatic stress in Li metal. The predominately hydrostatic stress state of thin Li anodes will alter the mechanical behavior, and therefore charge/discharge behavior, in Li metal SSBs compared to laboratory protype cells using thick Li metal foils.