Ultrafast Lithium-Ion Diffusion in Oxyhalides
Grace Wei, University of California, Berkeley
The global transition to renewable energy and increasing energy demands on the electric grid necessitate the development of novel lithium-ion batteries with improved performance and safety. All solid-state batteries are a promising technology due to their employment of inorganic solid electrolytes (SEs), which are safer compared to their flammable, liquid counterparts. Recently, halide SEs have gained significant attention due to their relatively high ionic conductivity (greater than 1 mS/cm), wide electrochemical stability window, and compatibility with various electrode materials. However, prior to 2023, no known halide materials could reach the conductivities of the state-of-the-art sulfide SEs (greater than 10 mS/cm). This can be attributed to the close-packed anion arrangement of existing halide conductors, which confines lithium ions to narrow and high-energy conduction pathways.
Recently, a new Li oxyhalide superionic conductor LiMOCl4 (M=Nb/Ta) was reported with extremely high ionic conductivities greater than 10 mS/cm. Unlike previously found conductors, LiMOCl4 has a non-close-packed anion framework that lends to a large conduction pathway. However, due to the novelty of the material and its crystal structure, the diffusion mechanism in LMOCl4 is not directly known. Understanding the cause underlying the high ionic conductivity of LMOCl4 is crucial to improving its performance and will provide essential design criteria for constructing new Li halide superionic conductors with conductivity greater than 10 mS/cm. In this work, we employ first-principles calculations to investigate the stability, electrochemical window, diffusivity, and chemical substitutions for LiMOCl4. By conducting ab initio molecular dynamics simulations spanning tens of ns, and subsequent computational analysis, we identify the diffusion mechanisms in the material and use our findings to inform future work on halide materials discovery.
Abstract Author(s): Grace Wei, KyuJung Jun, Gerbrand Ceder