Role of Li Distribution in High Li Conductivity of Li x (Ge,P)3S12

Abstract

Li x (GeP)2S12 (LGPS) is a lithium superionic conductor with the obvious potential application as an electrolyte for all-solid-state batteries that could be safer and have higher power compared to conventional batteries using liquid electrolytes. Li10GeP2S12 shows an exceptionally high conductivity of 12 mS/cm at room temperature [1], and doubling of the conductivity to 25 mS/cm was attained in the related compound Li9.54Si1.74P1.44S11.7Cl0.3 [2]. LGPS has a complicated structure. The backbone is a framework of GeS4 and PS4 tetrahedra where there are (Ge,P) shared sites and P-only sites. Li is four-folded coordinated with S and occupy four types of sites, Li1 to Li4. (Li1)S4 and (Li3)S4 form a one-dimensional chain and this is considered to be the primary Li diffusion route. The original article [1] claimed that the Li1 site is clearly double-split while the other Li sites are not split. However, in our presentation we show that the Li3 site is actually double-split at room temperature and triple-split at 10K. Li splitting means that there are more local minima for Li, which in turn results in increased complexity in Li migration paths. Understanding and controlling of Li splitting is important to further enhance Li conductivity. Both Li1 and Li3 sites, which are found to split, take part in the primary Li conduction path and therefore the intriguing high Li conductivity of LGPS should be strongly influenced by fine details of the Li potential. Our first principles calculations show that that neither Li1 nor Li3 sites splits in Li12Ge3S12 where all Li sites are occupied, and reducing the Li concentration, or introducing Li vacancies, was found to be necessary for splitting of these Li sites. Kamaya, N.; Homma, K.; Yamakawa, Y.; Hirayama, M.; Kanno, R.; Yonemura, M.; Kamiyama, T.; Kato, Y.; Hama, S.; Kawamoto, K.; Mitsui, A., A lithium superionic conductor. Nat. Mater. 2011, 10 (9), 682.Kato, Y.; Hori, S.; Saito, T.; Suzuki, K.; Hirayama, M.; Mitsui, A.; Yonemura, M.; Iba, H.; Kanno, R., High-power all-solid-stae batteries using sulfide superionic conductors. Nature Energy 2016, 1 (4), 16030.