This research addresses the long-standing dilemma in solid-state batteries, the low ionic conductivity across solid-solid interfaces and through the solid electrolyte. Understanding the mechanism behind lithium ion diffusion in solid electrolytes can provide significant insights in developing new battery materials. This study focuses on tuning the conductivity of lithium-oxyhalide anti-perovskites by optimizing the Cl to Br ratio, increasing Li vacancies, and engineering the crystallinity. To explain the variations in conductivity, we simulated Li₃OCl and Li₃OCl_1-xBr_x with varying vacancy concentrations, including both Li and Cl vacancies. The hypothesis that correlated motion affects conductivity is quantified with a novel analysis.

Methods

First principles Density Functional Theory – Molecular Dynamics was performed on 4x4x4 supercells using VASP. The size of the supercell (320 atoms) allows for a higher percentage Br substituted doping, comprising of 22% and 30% Br:Cl. The simulations each ran for at least 20 ps. The supercells had at least one and up to four Li⁺ and Cl^- vacancies, as the conduction mechanism is vacancy mediated. Analysis was performed to determine how polarization of anions affects Li⁺ ion diffusion. Maximally Localized Wannier Functions (MLWF) are used to quantify the covalent character of the Li-anion bonds, in particular their dynamic behavior. As is standard, the diffusion coefficient for each compound was determined using the mean squared displacement.

Results

We identified how the number of Cl and Br bonds and changes in covalency during Li⁺ jumping affects the diffusion pathway. The bonding characters that were determined from the MLWF analysis show that the bromine has more covalent bonds with Li⁺ than the chlorine. We quantified how the concentration of Li+ vacancies and fluctuations in polarization of anions affects the diffusion mechanism. The polarizability of the anions was calculated using MLWF and showed that the Li⁺ diffusion affects the polarization of the second nearest neighboring anion by increasing its polarization. We found that correlated motion between Li⁺ vacancies increased diffusion; the more vacancies, the more pronounced the effect of correlation on diffusion.

References

• Adelstein, N. and Wood, B. Li+ conductivity in a superionic electrolyte driven by dynamically frustrated bond disorder. Journal of Materials Chemistry (2016).
• Zhang, Y., Zhao, Y., & Chen, C. (2013). Ab initio study of the stabilities of and mechanism of superionic transport in lithium-rich anti-perovskite. Rev. B Physical Review B, 87(13).