A first-principle investigation of the Li diffusion mechanism in the super-ionic conductor lithium orthothioborate Li3BS3 structure

The research of superior ionic conductors is a very active field. Identifying such materials would allow for the design of improved solid-state Li-ion batteries, solving the safety hazard posed by the liquid electrolytes and improving the electrochemical stability and thus the energy efficiency. In this work, we study lithium orthothioborate Li3BS3 by means of first-principle atomistic calculations based on density functional theory. This material is a very promising super ion conductor candidate, as it is a layered compound based on a deformed body-centred structure of the anion sublattice. The minimum energy paths for the diffusion of the lithium ions are identified, and the activation energy are evaluated using the nudged elastic band method. A very fast 1D diffusive channel is found, contained in a Li-rich layer, with an activation energy below 0.1 eV. The other paths connecting these layers and extending the mobility of lithium to the whole structure are found to have activation barriers of 0.25 eV or lower. This bottleneck corresponds to a diffusion coefficient of the order of 10-6 cm2 s−1, thus characterising lithium orthothioborate as an excellent ionic conductor.