Structural Insights and 3D Diffusion Pathways within the Lithium Superionic Conductor Li₁₀GeP₂S₁₂

Abstract

Inspired by the ongoing debate about the ion dynamics in the lithium superionic conductor Li 10 GeP 2 S 12 (LGPS), we present neutron powder diffraction data in combination with analyses of differential bond valence and nuclear density maps to elucidate the underlying diffusion pathways in Li 10 GeP 2 S 12 . LGPS exhibits quasi-isotropic three-dimensional lithium diffusion pathways, which is a combination of one-dimensional diffusion channels crossing two diffusion planes. Furthermore, ultrasonic speeds of sound measurements are used to understand the lattice dynamics and obtain the Debye temperature of LGPS. Temperature dependent X-ray diffraction is performed in order to understand the local temperature-dependent behavior of the prevalent structural backbone, as well as the thermal stability of the material. At elevated temperatures, the superionic conducting Li 10 GeP 2 S 12 phase partially decomposes into Li 4 P 2 S 6 , explaining the deterioration of the ionic conductivity upon heating.

Ion dynamics in solid electrolytes for lithium batteries

“…Over the last couple of years some breakthroughs have, however, been reported. As an example, in 2011 Kanno and co-workers reported on extremely high ionic conductivity σ ion of 1.7 × 10 −2 S cm −1 for Li 10 GeP 2 S 12 [35] which exhibits quasi-isotropic three-dimensional lithium diffusion pathways [60]; for Li 7 P 3 S 11 , on the other hand, Seino et al [52] showed that in heat-treated and highly dense Li 7 P 3 S 11 σ ion can reach values up to 1.7 × 10 −2 S cm −1 at room temperature, too. The same holds for Li 7 P 3 S 11 that was prepared through spark plasma sintering [61].…”

Relevance of solid electrolytes for lithium-based batteries: A realistic view

“…Li4 connects the Li3 − [Li1 − Li1] − Li3 diffusion channels along c and opens up the possibility for Li diffusion also in the a-b plane. Indeed, Janek, Zeier and co-workers [71] confirmed the significant contribution of the in-plane Li diffusion (along Li3 − [Li2 − Li2] − Li3 and Li4 − [Li1 − Li1] − Li4) pathways by neutron powder diffraction combined with a maximum entropy analysis. Hence, they demonstrated that already at room temperature a quasi 3D diffusion mechanism is operative in LGPS, with the 3D character of Li diffusion further increasing at higher T [71,72] (Fig.…”

“…In particular, the nominal stability window may be different (likely smaller) for different solid solution members than currently available experimental data suggest [90]. Possible complications include the transformation of LGPS into orthorhombic thio-LISICON phases with different stoichiometry and the concomitant formation of side phases, as already observed for LGPS by the formation of Li 4 P 2 S 6 between 300 and 400°C [71]. As the authors of that study point out, however, it is well conceivable that the formation of decomposition products commences already near room temperature, which is particularly critical for the long-term operation of all-solid-state batteries based on LGPS.…”

“…This apparent thermal stability however needs to be put into perspective: As suggested by a T-dependent X-ray diffraction study [71] and theoretical calculations [77], LGPS may not be intrinsically stable across the entire relevant temperature range between RT and 300°C. In particular, the nominal stability window may be different (likely smaller) for different solid solution members than currently available experimental data suggest [90].…”

Metal-organic chemical vapor deposition enabling all-solid-state Li-ion microbatteries: A short review

“…(iv) Lithium dendrites formation can be restrained, which allows the use of metallic lithium as anode due to the mechanically more robust nature of solid-state electrolytes. [23][24][25], and Li 10 GeP 2 S 12 [26][27][28], polymeric Li-ion conductors [12,29], (nitrogen doped) lithium phosphate (Li 3 PO 4 and LiPON) [30][31][32] and MeO-based oxides (LiNbO 3 [33], LiTaO 3 [34] and Garnets-type Li 7 La 3 Zr 2 O 12 [35][36][37]). Generally, lithium thiophosphates and complex sulfides have a moisture sensitive nature.…”