Silicon-based Solid-State Batteries: Electrochemistry and Mechanics to Guide Design and Operation

Solid-state batteries are promising alternatives to the incumbent lithium-ion technology however, they face a unique set of challenges that must be overcome to enable their widespread adoption. These challenges include solid-solid interfaces that are highly resistive, with slow kinetics, and a tendency to form interfacial voids leading to delamination which results in diminished cycle life. This modelling study probes the evolution of stresses at the solid electrolyte (SE) solid-solid interfaces, by linking the chemical and mechanical material properties to their electrochemical response, which can be used as a guide to optimise the design and manufacture of silicon (Si) based SSBs. A thin-film solid-state battery consisting of an amorphous Si negative electrode (NE) is studied, which exerts compressive stress on the SE, caused by the lithiation-induced expansion of the Si. By using a 2D chemo-mechanical model, continuum scale simulations are used to probe the effect of applied pressure and C-rate on the stress-strain response of the cell and their impacts on the overall cell capacity. A complex concentration gradient is generated within the Si electrode due to slow diffusion of Li through Si which leads to localised strains. To reduce the interfacial stress and strain at 100% SOC, operation at moderate C-rates with low applied pressure are desirable. Alternatively, the mechanical properties of the SE could be tailored to optimise cell performance. To reduce Si stress, a SE with a moderate Young’s modulus similar to that of lithium phosphorous oxynitride (~ 77 GPa) with a low yield strength comparable to sulfides (~ 0.67 GPa) should be selected. However, if the reduction in SE stress is of greater concern, then a compliant Young’s modulus (~ 29 GPa) with a moderate yield strength (1-3 GPa) should be targeted. This study emphasises the need for SE material selection and to consider other cell components in order to optimise the performance of thin film solid-state batteries.