Atomistic Scale Analysis of Motion and Dynamics of Li-Ion in Li-Zn-Zr-S Compound Electrolyte

Solid-state electrolytes are considered as key materials for overcoming the limitations of the liquid-electrolyte-based Li-ion batteries in terms of energy capacity, cyclability, and safety. Among the many possible solid-state electrolyte materials, inorganic sulfides are of intensive research interest mainly because the larger and more polarizable sulfur anions substantially enhance the mobility of cationic Li species. In particular, Li-P-S electrolytes, such as Li10GeP2S12and Li7P3S11, have been reported to have room-temperature Li-ion conductivity of greater than 10-2 S/cm, which is comparable to that of conventional liquid electrolytes. Despite the facile Li-ion transport, the Li-P-S electrolytes tend to undergo irreversible structural degradation within typical operating conditions of the battery. This lack of electrochemical stability leads to the formation of high impedance intermediate transition layers at the electrode/electrolyte interfaces, hindering the practical application of the Li-P-S electrolytes. Partial replacement of the element P with other elements within a Li-P-S structural family can improve the lithium-ion conductivity, but notable improvement in their electrochemical stability has yet to be achieved. Li-M-S (M = transition metals, such as Cr, Zr, Nb, Mo, and Sn) families can be possible candidates for electrochemically stabile solid-state electrolytes, while having excellent Li-ion transport properties of the inorganic sulfides. Like the Li-P-S structure, numerous Li-M-S structural derivatives have already been synthesized by varying the element M or by replacing a portion of the element M with another element, and the Li-ion conductivity and electrochemical stability are expected to be heavily dependent on the element composition and crystalline structure. In this study, we investigate the motion and dynamics of Li-ions in Li-Zr-S and Li-Zn-Zr-S compound electrolyte by performing a series of first-principles calculations. To this end, we generate multiple Li-Zn-Zr-S compound model structures with varying Zr-to-Zn ratios. Then, we evaluate Li-ion conductivities by using velocity auto correlation scheme combined ab initio molecular dynamics (AIMD) simulations. More specifically, we calculate the diffusivity and ionic conductivity of the stoichiometric structure and at a range of defect concentrations on Li2+2xZnxZr1-xS3, from x = 0 to x = 0.5. Our calculation results reveal how the introduction of Zn into the Li-Zr-S compound affects the Li-ion mobilities. The theoretical insight obtained in this study well corroborates with recently reported experimental results for varying ionic conductivities in Li-Zn-Zr-S electrolytes depending on the Zr-to-Zn ratio. This fundamental understanding can be an important theoretical basis for developing practically applicable Li-M-S electrolytes. Figure 1