Computational Characterization of β-Li₃PS₄ Solid Electrolyte: From Bulk and Surfaces to Nanocrystals

The all-solid-state lithium-ion battery is a new class of batteries being developed following today's demand for renewable energy storage, especially for electric cars. The key component of such batteries is the solid-state electrolyte, a technology that promises increased safety and energy density with respect to the traditional liquid electrolytes. In this view, β-Li₃PS₄ is emerging as a good solid-state electrolyte candidate due to its stability and ionic conductivity. Despite the number of recent studies on this material, there is still much to understand about its atomic structure, and in particular its surface, a topic that becomes of key relevance for ionic diffusion and chemical stability in grain borders and contact with the other device components. In this study, we performed a density functional study of the structural and electronic properties of β-Li₃PS₄ surfaces. Starting from the bulk, we first verified that the thermodynamically stable structure featured slight distortion to the structure. Then, the surfaces were cut along different crystallographic planes and compared with each other. The (100) surface is confirmed as the most stable at T = 298 K, closely followed by (011), (010), and (210). Finally, from the computed surface energies, the Wulff nanocrystals were obtained and it was verified that the growth along the (100) and (011) directions reasonably reproduces the shape of the experimentally observed nanocrystal. With this study, we demonstrate that there are other surfaces besides (100) that are stable and can form interfaces with other components of the battery as well as facilitate the Li-migration according to their porous structures.