Computational Investigation of Known and Predicted Li Boracites Li4B7O12Cl and Li4B7S12Cl as Solid Electrolytes Having Possible Applications for Developing All Solid-state Li Ion Batteries

Abstract

The lithium chloroboracite Li4B7O12Cl with fractionally occupied Li sites located in diffusion channels formed by a rigid B7O12 framework possesses essential features of an ideal Li-ion conductor. Early experimental work in 1977 [1] identified three structural modifications related to a face-centered cubic pattern. The room-temperature α form was suggested to have a rhombohedral distortion, but was not fully characterized. Our first-principles calculations find the optimized structure of the α phase to have a face centered rhombohedral structure with space group R3c (No. 161) and structurally very similar to the higher temperature cubic structures of the ꞵ and ɣ forms apart from the fractional occupancy patterns of the Li configurations. Using the convex hull approach, we estimate α-Li4B7O12Cl to be thermodynamically stable within the composition space of Li2O, B2O3, and LiCl. Our analysis of Li ion mechanisms indicates that a concerted process, involving two host Li sites and a neighboring unoccupied site based on the face-centered cubic structure, provides the most efficient ion transport in Li4B7O12Cl. The room-temperature ionic conductivity calculated from molecular dynamic (MD) simulations is on the order of 10-4 S/cm, showing good agreement with the recent experimental measurement for pure polycrystalline samples of Li4B7O12Cl [2]. The temperature-dependent occupancies of Li sites, as the main structural distinction of the three reported modifications of Li4B7O12Cl, are also observed in the MD simulations performed at various temperatures. We also predict that the Li ion conductivity can be enhanced by substituting S for O to form Li4B7S12Cl, which has the same R3c ground state structure with an expanded lattice. Our preliminary studies show that Li4B7S12Cl has better Li conducting performance than Li4B7O12Cl and therefore also shows promise as a solid-state electrolyte for all-solid-state Li-ion batteries.------------------------------------------[1] W. Jeitschko, T. A. Bither, and P. E. Bierstedt. Crystal structure and ionic conductivity of Li boracites. Acta Crystallographica Section B, 33(9): 2767-2775, 1977.[2] D. Tan, F. Wang, T. Pietsch, M. A. Grasser, T. Doert, and M. Ruck. Low-Temperature Ionothermal Synthesis of Li-Ion Conductive Li4B7O12Cl Solid-State Electrolyte. ACS Applied Energy Materials, 2(7): 5140-5145, 2019.------------------------------------------Acknowledgements -- This work was supported by NSF grant DMR-1940324. Computations were performed on the Wake Forest University DEAC cluster, a centrally managed resource with support provided in part by the university.