Dynamics Study of Lithium Ion Diffusion in Super Lithium Ion Conductors, Li10GeP2S12 (LGPS)
Authors: Yonemura Masao , Ohmin Kwon , Hirayama Masaaki , Hori Satoshi , Kanno Ryoji , Mori Kazuhiro , Shibata Kaoru , Yamada Takeshi , Kawakita Yukinobu 
Published: February 2020

Fast ionic conduction in solid electrolytes plays a key role in feasibility of the all-solid-state battery system. Among the lithium ion conductors, the Li10GeP2S12 (LGPS) system shows the conductivity comparable to organic liquid electrolytes [1]. The recent study focused on the synthesis of solid solutions of this material, which might introduce either lithium vacancy or interstitial lithium ions and might affect its ionic conductivity and electrochemical stability. The crystal structures of the solid solution were studied using neutron diffraction technique. Their conduction pathway was estimated using Maximum Entropy Method (MEM) based on the structure information. These results indicate the lithium conduction pathway in LGPS is one-dimensional pathway along c axis at room temperature, and three-dimensional one at higher temperature [2]. However, the results of MEM provided only the space area of possible lithium ion conduction. Then, the study of lithium diffusive behaviors is necessary to understand lithium ionic conductivity in more details. The dynamics of lithium ion diffusions can be obtained directly by using the Quasielastic Neutron Scattering (QENS) technique because the quasielastic scattering spectrum is a broadening of elastic peak caused by the diffusion of atoms or ions within a material. When ions cause diffusion motions on a fixed sublattice, the quasielastic scattering spectra can exhibit a Q-dependence, which provides information on the dynamical structure of the ions diffusion. In this study, the QENS measurements were performed using the high-resolution Si crystal analyzer TOF type near-backscattering spectrometer, DNA, at MLF/J-PARC, Tokai, Ibaraki, Japan, at the energy resolution of 3.6 µeV [3]. S(Q,ω) spectra from 150 to 640 K were collected. After the analyzed those using jump-diffuse model [4,5], self-diffusion constants, jump length and mean residence time of conduction lithium ions in LGPS system were determined. The jump length was 1.73 Å at 473K and 2.72 Å at 640 K respectively. The jump length at 473 K was consistent with conduction along caxis. The jump length at 640 K also corresponded to the three dimensional conduction model. In this presentation, the mechanism of lithium ion diffusion in LGPS system will be discussed.

References

[1] N. Kamaya, K. Homma, Y. Yamakawa, M. Hirayama, R. Kanno, M. Yonemura, T. Kamiyama, Y. Kato, S. Hama, K. Kawamoto and A. Mitsui, Nature Materials, 10, 5 (2011).
[2] O. Kwon, M. Hirayama, K. Suzuki, Y. Kato, T. Saito, M. Yonemura, T. Kamiyama and R. Kanno, J. Mater. Chem. A, 3, 9 (2014).
[3] K. Shibata, N. Takahashi, Y. Kawakita, M. Matsuura, T. Yamada, T. Tominaga, W. Kambara, M. Kobayashi, Y. Inamura, T. Nakatani, K. Nakajima, and M. Arai, JPS Conf. Proc. 8, 036022 (2015).
[4] C.T. Chudley and R.J. Elliott, Proc. Phys. Soc. 77, 353 (1967)
[5] P.A. Egelstaff, B.C. Haywood and F.J. Webb, Proc. Phys. Soc. 90, 681 (1967)