Mixed Lithium Indium Halides As Solid-Electrolytes: Computational Experiments on Drivers of Li+ Diffusion
Authors: Alves Thomaz Coelho , Zevgolis Alysia , Adelstein Nicole 
Published: February 2020

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.