First-principles evaluation of the elastic properties of crystalline Li-ion conductors

Abstract

All-solid-state batteries have emerged as alternative rechargeable batteries offering high energy density and enhanced safety. However, suppressing their mechanical degradation is challenging. In particular, inorganic solid electrolytes must form mechanically stable solid–solid interfaces with electrode active materials, making the examination of their elastic properties essential for creating robust interfaces. Pugh's ratio (B/G) serves as a key parameter for estimating ductility, with a desirable value exceeding 1.75—a criterion originally proposed for polycrystalline metals. In this study, the elastic properties of Li-ion-conducting crystalline electrolytes were comprehensively evaluated via first-principles calculations. The calculated mechanical properties of their crystal structures were classified based on their anion elements. The elastic moduli of sulfide and halide crystals were relatively lower than those of oxide and nitride materials. The Pugh ratios of sulfide crystals were generally higher than 1.75, while those of oxide crystals clustered around 1.75 and nitride crystals typically fell below this threshold. Additionally, a nonlinear correlation between mean atomic volume and elastic constants was observed. Among the various electrolytes, Li₂SO₄ exhibited exceptional elastic properties: α-Li₂SO₄ demonstrated a significantly high B/G value of 4.28, indicating distinctive ductility.