Critical Roles of Mechanical Properties of Solid Electrolyte Interphase for Potassium Metal Anodes

The mechanical properties of the solid electrolyte interphase (SEI) have attracted increasing attention, but their importance in guiding electrolyte design remains ambiguous. Here it is revealed that, despite a decrease in ionic conductivity for both electrolyte and SEI, exceptional cycling performance of K-metal batteries is achieved in a low concentration carbonate electrolyte by optimizing the mechanical stability of the SEI. The SEI formed in the studied carbonate electrolytes is predominantly organic. Its inorganic content increases with increasing electrolyte concentration and corresponds to an increase in Young's modulus (E) and ionic conductivity of SEI and a decrease in elastic strain limit (ε_Y). The maximum elastic deformation energy combines effects of E and ε_Y, achieving a maximum in 0.5 m electrolyte. Finite element simulations indicate that SEI with low either E or ε_Y inevitably triggers dendrite growth. These findings foreshadow an increased focus on the mechanical properties of the SEI, where low concentrations of carbonate electrolytes display merit.