Theoretical Insights into Li Ion Diffusion in β—Li3PS4 solid Electrolyte

The attractive safety and long-term stability of all solid-state batteries has added a new impetus to the discovery and development of solid electrolytes for lithium batteries. Sulfide-based compounds are promising in terms of conductivity and cyclability, and in the chemical systems Li₂S—P₂S₅/GeS₂/SnS₂, crystalline compounds like Li₃PS₄,[1] Li₁₀GeP₂S₁₂ (LGPS)[2] and Li₁₀SnP₂S₁₂[3] have garnered attention because the achievable Li ion conductivity is comparable to liquid electrolytes.

Li₃PS₄ is one of the stable compositions in the Li₂S—P₂S₅ system. It exists in two phases: the room temperature phase, γ—Li₃PS₄ and the high temperature phase, β—Li₃PS₄. Liu et al reported ionic conductivity on the order of 1.6*10^-4 S/cm in nanosized Li₃PS₄, whereas a very low ionic conductivity on the order of 10^-7 S/cm was reported in the bulk phases of Li₃PS₄. [1, 4] The increase in conductivity was rationalized by the formation of nanoscale crystallites and the stabilization of the disordered high conductivity β phase at the nanoscale. The distinct features of β phase (Pmna, SG:62) are the higher symmetry and partially occupied Li-ion sites compared to γ phase (Pmn2₁, SG:31). We focus on β—Li₃PS₄with emphasis on the role of Li-ion site disorder and the local chemical environment responsible for the observed high ionic conductivity.

We used a series of ab initio molecular dynamics simulations to understand the Li-ion diffusivity in β—Li₃PS₄. We determined the Li-ion diffusivity, the temperature dependence diffusivity, and analyzed the trajectories to determine the mechanisms and structural origins of the high Li diffusivity. We find that Lithium ion diffusion is facilitated by the vibrational modes related to P—S bond stretch, and the diffusion is facile with an activation barrier on the order of 0.35 eV. Lithium ions exhibit pseudo-liquid like behavior at elevated temperatures (~600 K) while the PS₄^3- anions vibrate about their lattice positions.  Our results provide further insights into the arrangement of Li ion sub-lattice in the crystal structure of β—Li₃PS₄, role of lithium-ion disorder on ionic conductivity and the local chemical environment responsible for Li-ion sub-lattice melting and consequent improvement in ionic conductivity.

This research was conducted at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. Computing resources provided by the National Energy Research Scientific Computing Center, supported by US-DOE Office of Science under Contract DE-AC02-05CH11231, were used and gratefully acknowledged.

1.         Liu, Z., Fu, W., Payzant, E. A., Yu, X., Wu, Z., Dudney, N. J., Kiggans, J., Hong, K., Rondinone, A. J., and Liang, C., Anomalous High Ionic Conductivity of Nanoporous β-Li₃PS₄. Journal of the American Chemical Society, 2013. 135(3): p. 975-978.

2.         Kamaya, N., Homma, K., Yamakawa, Y., Hirayama, M., Kanno, R., Yonemura, M., Kamiyama, T., Kato, Y., Hama, S., Kawamoto, K., and Mitsui, A., A lithium superionic conductor. Nat Mater, 2011. 10(9): p. 682-686.

3.         Bron, P., Johansson, S., Zick, K., Schmedt auf der Günne, J., Dehnen, S., and Roling, B., Li₁₀SnP₂S₁₂: An Affordable Lithium Superionic Conductor. Journal of the American Chemical Society, 2013. 135(42): p. 15694-15697.

4.         Tachez, M., Malugani, J.-P., Mercier, R., and Robert, G., Ionic conductivity of and phase transition in lithium thiophosphate Li₃PS₄. Solid State Ionics, 1984. 14(3): p. 181-185.