Alloying of anions is a promising engineering strategy for tuning ionic conductivity in halide-based inorganic solid electrolytes. We explain the alloying effects in Li3InBr6−xClx, in terms of strain, chemistry, and microstructure, using first-principles molecular dynamics simulations and electronic structure analysis. We find that strain and bond chemistry can be tuned through alloying and affect the activation energy and maximum diffusivity coefficient. The similar conductivities of the x = 3 and x = 6 compositions can be understood by assuming that the alloy separates into Br-rich and Cl-rich regions. Phase-separation increases diffusivity at the interface and in the expanded Cl-region, suggesting microstructure effects are critical. Similarities with other halide superionic conductors are highlighted.

Introduction The search for solid electrolytes with fast Li+ diffusion is essential to the development of safer and higher energy-density all-solid state batteries. The presented simulations aim to understand the effect of the dynamic electronic structure on Li+ diffusion by not only identifying the atomic scale Li+ diffusion process, but also the effect of anion substitution on the interaction of the Li+ and the surrounding lattice. In this report, the diffusion processes of Li+ in Li3InCl­6 and Li3InBr6-xClx are probed using molecular dynamics simulation. Methodology Ab-initio molecular dynamics using Quantum ESPRESSO is used to simulate Li+ diffusion in Li3InCl­6 electrolytes. A 2x2x1 supercell structure containing 80 atoms is sufficient to capture the local and mid-range environment around the diffusing Li+. Computational experiments examined the effect of different changing temperature, volume, and the ratio of Br to Cl. The activation energies of Li+ diffusion in the electrolytes are determined by plotting the mean squared displacement versus time and temperatures. Maximally localized Wannier functions (MLWFs) are used for further electronic structure analysis of bond breaking and forming and polarization, thus allowing for an innovative analysis of the correlation of the electronic and ionic dynamics1. Results and Discussion Mixed lithium indium halides have unusual non-monotonic conductivity2 that cannot be explained by a simple picture of ionic interaction between Li+ and the anions sub-lattice. Li3InBr6 has the highest conductivity; if the decrease in conductivity is due to the smaller radius of Cl and thus a smaller volume, then the conductivity should decrease monotonically as Cl is introduced. Instead the Li3InBr3Cl3 alloy has a similar conductivity to the pure Br electrolyte, while other ratios have significantly lower conductivity. This trend can only be understood with first principles simulations of the Li+ diffusion pathways in the lithium indium halides. Molecular dynamics simulations of Li+ (at different temperatures) allows for the estimation of the activation energy and comparison to experiment. The simulations that vary temperature and volume also test the hypothesis posed in previous work1 that the driving force for Li+ mobility in the electrolyte arises from an internal frustration between the electronic (bonding) and structural (geometric) lattice. Mobility of Li+, a “jumping” ion, reveals a conduction pathway that involves transitions between octahedral sites through a tetrahedral interstitial site. The presence of a Cl­- anion can aid or disrupt the frustration that affects Li+ diffusion through correlation of Li+ diffusion events. The Cl- anions change the nature of the Li+ - halide polar-covalent bond, which can be uniquely analyzed with our novel dynamical methods. Therefore, in comparison with Br-, Cl- tips the scale of the bonding character of Li3InCl­6 just enough to disrupt the electronic-geometrical frustration. References 1. Adelstein, N. and Wood, B. Li+ conductivity in a superionic solid electrolyte driven by dynamically frustrated bond disorder. Journal of Materials Chemistry (2016) submitted. 2. Tomita, Y. Substitution effect of ionic conductivity in lithium ion conductor, Li3InBr6-xClx. Solid State Ionics. (2008) 179, 867-870.

Our simulations address a long-standing problem in the design of solid-state batteries – the low ionic conductivity across solid-solid interfaces and through the solid electrolyte. Significant insights in developing new battery materials can be achieved through understanding of Li+ ion diffusion, using first-principles molecular dynamics simulations. Previous research from our group has determined the mechanism for the lithium ion diffusion pathway through the Li3InBr6 crystal1. Our results expanded on these findings by simulating diffusion in Li3InCl6 and Li3InBr6-xClx. In particular, we focus on determining the effects of high temperature and electronic structure, in order to explain variation in the Li+ conductivity in these lithium halides. We explore the hypothesis that covalent-like bonds between Li+ and the halide anions help drive conductivity through correlated motion of the Li+ ions. Our novel approach dynamically tracks covalent bonds, especially correlated bond breaking and forming. Thus, the electronic structure must be simulated and to do this we use ab-initio DFT simulations, within the Quantum Espresso implementation. In our previous research, we noted that the conductivity of Li3InBr6 is non-Arrhenius at high temperatures. The diffusion mechanism does not change with temperature, so the effect was hypothesized to be from changes in the electronic structure, such as the weakening of bonds. The first step of our investigation was to determine the simulated melting temperature of Li3InBr6, as our simulations at 900K may be near the simulated melting temperature, which could explain the weakening of bonds. The melting temperature is simulated starting with an amorphous, 3 x 3 x 3 supercell containing 520 atoms. Then using the two-phase approach (TPA) with 1040 atoms, we determined the melting temperature. Second, we simulated Li+ diffusion along with the breaking and forming of covalent-like bonds using first principle dynamics and Wannier analysis. Exchange of Br anions with Cl anions changes the nature of covalent type bonds, but does not affect ionic bonds. To further explore nature of the interaction between the Li+ and the anion sub-lattice, the Br concentration in Li3InBr6-xClx was varied and bond breaking and forming is tracked. We established that lithium ion diffusion, near the melting temperature, is impeded compared to lower temperatures owing to a degradation of covalent-like bonds. The Li+ diffusion mechanism, from octahedral to tetrahedral back to octahedral sites, is propelled by the fluctuations of neighboring covalent-like bonds. Decreasing the Br concentration in Li3InBr6-xClx shows a non-monotonic change in conductivity, which is unexplainable in a purely ionic view of Li+ diffusion. The disorder caused by different bonds strength between Li+ and Cl- versus Br- leads to a relatively high conductivity when the ratio of Cl to Br is 1:1 but a marked decrease with other ratios2. Our novel first-principles study of the dynamic electronic structure is the only way to understand these unusual trends in conductivity and potentially predict conductivity in many solid state electrolytes. 1) Adelstein, N. and Wood, B. Li+ conductivity in a superionic solid electrolyte driven by dynamically frustrated bond disorder. Journal of Materials Chemistry (2016) submitted. 2. Tomita, Y. Substitution effect of ionic conductivity in lithium ion conductor, Li3InBr6-xClx. Solid State Ionics. (2008) 179, 867-870.