Editor’s choice Li conduction pathways in solid-state electrolytes: Insights from dynamics and polarizability Tsukasa Takahashi a , Koki Nagagiri a , Yasuhiko Iwadate a , Futoshi Utsuno b , Hiroshi Yamaguchi b , Takahiro Ohkubo a,⇑ a Graduate School of Engineering, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan b Advanced Technology Research Laboratories, Idemitsu Kosan Co. Ltd., 1280 Kami-izumi, Sodegaura, Chiba 299-0293, Japan article info Article history: Received 24 January 2018 In final form 8 March 2018 Available online 11 March 2018 abstract We investigated the dynamical and polarizable properties of Li 7 P 3 S 11 , which is a fast Li-conducting mate- rial, by performing ab initio molecular dynamics simulations. A zone analysis based on Li migration high- lighted the effective path along which Li diffuses in the crystal. The effective Li diffusion was analyzed in terms of the dynamics and polarizability of the sulfur surrounding the Li migration path. High flexibility and large anisotropic polarizability were the characteristics identified as necessary for the formation of an effective Li migration path. These findings provide principles for understanding Li conduction in solid-state electrolytes. Ó 2018 Elsevier B.V. All rights reserved. 1. Introduction New all-solid-state batteries could provide a new form of energy storage. However, the replacement of currently used organic liquid electrolytes with inorganic solids is a remarkable challenge [1–4]. A major advantage of these solid-state batteries is that they avoid the potential risks associated with flammable organic liquids and they have large electrochemical windows. Pre- vious efforts to develop these batteries included studying a series of lithium sulfide ionic conductors. Room-temperature ionic con- ductivities greater than 10 mS/cm have been reported for these conductors [5,6], implying that they could be competitive with tra- ditional organic liquid electrolytes. The Li 7 P 3 S 11 and Li 10 GeP 2 S 12 material families are considered candidates for practical applica- tions. The advantage of Li 7 P 3 S 11 material [7], which was developed first, is its cost benefit and easy synthesis. The anomalously high conductivity of this material has attracted much interest, and var- ious structural studies have been carried out based on diffraction [8–10], spectroscopic [11,12], and computational approaches [13–15]. Yamane and Onodera performed synchrotron X-ray and neutron powder diffraction measurements, and revealed the crys- tal structure by using the Rietveld method [8,9]. The crystal struc- ture is composed of seven Li, two PS 4 , and two P 2 S 7 units with triclinic P 1 symmetry. Using density functional theory (DFT) meth- ods, Lepley and Holzwarth reported that the Li migration barrier along the b-axis is lower than those along the other two axes [15]. A DFT study showed that Li 7 P 3 S 11 was entropically stabilized at temperatures above 630 K and preferential diffusion along the b- axis [14]. Xiong et al. also carried out a nudged elastic band (NEB) analysis to investigate the energy change along the b-axis, and their findings suggested that there are many interstitial Li ion sites between PS 4 and P 2 S 7 [13]. Although these computational studies suggested a Li diffusion path, our current investigation further attempts to use ab initio molecular dynamics (AIMD) to explain the origin of the atomic and electronic structures in Li 7 P 3 S 11 , and their relation to an effective Li diffusion path in view of the Li dynamics. 2. Experimental All ab initio calculations were performed using the Quantum Espresso package [16]. First, a structural optimization (cell con- stants and atomic coordinations) of the experimental Li 7 P 3 S 11 structure [8] with a 1  2  1 super-cell (Li(28), P(12), and S(44)) was carried out, and the fully relaxed structure without symmetry was used in the subsequent AIMD simulations. We used a 1  1  1 Monkhorst-Pack grid and a plane-wave basis up to an energy cutoff (ecutwfc = 80 Ry) for GGA-PBE [17] calculations using the projector augmented-wave (PAW) potentials taken from PS library 0.3.1 [18]. Energy convergence was confirmed within 0.001 Ry for the ab initio calculation. The 1  2  1 super-cell was the same as in a previous AIMD study of Li 7 P 3 S 11 [14]. We performed the AIMD simulations for 300 ps with a time step of 2.0 fs at 300 K. The nuclei were treated classically, and their equations of motion were integrated using the Verlet algorithm https://doi.org/10.1016/j.cplett.2018.03.014 0009-2614/Ó 2018 Elsevier B.V. All rights reserved. ⇑ Corresponding author. E-mail address: ohkubo.takahiro@faculty.chiba-u.jp (T. Ohkubo). Chemical Physics Letters 698 (2018) 234–239 Contents lists available at ScienceDirect Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett