Synthesis and Electrochemical Property of I^-4 Type Li_1+2xZn_1-XPS₄ Solid Electrolyte

Accompanying the recent technological advance, sulfide-based all-solid-state Li ion battery (ASSB) is anticipated to reach to the stage of commercialization within 10 years. In particular, significant progress has been made within the last five years with the discoveries of several sulfide-based compounds whose conductivity is greater than 10 mS/cm, such as Li 10 GeP 2 S 12 [1], Li 7 P 3 S 11 [2], and Li 9.54 Si 1.74 P 1.44 S 11.7 Cl 0.3 [3]. Usually such new Li-ion conductive materials have been discovered through engineering manners, such as changing the composition of a known superionic compound, or adjusting the synthesis conditions. Recently, however, there have been other attempts to predict new SE materials by means of a computational approach using first-principles calculations. W. Richards et al. [4] predicted that Na 10 SnP 2 S 12 , in which Li is substituted to Na in Li 10 SnP 2 S 12 , would be a good Na ion conductor. They also synthesized Na 10 SnP 2 S 12 in practice and confirmed that the experimentally measured ionic conductivity and the activation energy of Na 10 SnP 2 S 12 agreed well with those of the predicted values. In this study, we examined I - 4 type Li 1+2x Zn 1-x PS 4 — a new Li ionic conductor computationally predicted by Richards et al. [5-6]. They predicted that the stoichiometric LiZnPS 4 is almost insulative, but non-stoichiometric compound of Li 1+2x Zn 1-x PS 4 can show superionic conductivity with the predicted room temperature ionic conductivity (σ 25 ℃ ) as large as 50 mS/cm at x=0.5. Li 1+2x Zn 1-x PS 4 was synthesized by using solid state reaction. We ball-milled the starting materials and heated it at elevated temperature under vacuum. We measured the XRD patterns and the Raman spectra for Li 1+2x Zn 1-x PS 4 at various x and confirmed that the samples with x ≤ 0.625 have I - 4 structure with little impurities. Figure 1 shows the ionic conductivity at 25℃ (σ 25 ℃ ) and the activation energy (E a ) for Li 1+2x Zn 1-x PS 4 at various x. Li 1+2x Zn 1-x PS 4 shows negligible conductivity when x ≤ 0.2. At x = 0.375, σ 25 ℃ starts increasing and E a starts decreasing. The highest σ 25 ℃ achieved was 5.7 x 10 -4 S/cm at x = 0.625. This tendency qualitatively agrees with the theoretical predictions [6], but there remains a considerable amount of discrepancy. When x ≥ 0.75, σ 25 ℃ decreases, which likely corresponds to low phase purity and poor crystallinity of the material. We also examined the feasibility of Li 2.25 Zn 0.375 PS 4 (x=0.625) in a practical ASSB with NCM cathode and various anodes (graphite, indium, LTO), and found that this material was not stable with graphite and indium, but can be paired with NCM and LTO. 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 (2011), 682. [2] Y. Seino, T. Ota, K. Takada, A. Hayashi and M. Tatsumisago, Energy & Environmental Sciences, 7 (2014) 627. [3] Y. Kato, S. Hori, T. Saito, K. Suzuki, M. Hirayama, A. Mitsui, M. Yonemura, H. Iba, R. Kanno, Nature Energy, 1 (2016) 16030. [4] W. Richards, T. Tsujimura, L. Miara, Y. Wang, J. Kim, S. Ong, I. Uechi, N. Suzuki and G. Ceder, Nature Communications, 7 (2016) 11009. [5] Y. Wang, W. D. Richards, S. P. Ong, L. J. Miara, J. C. Kim, Y. Mo and G. Ceder, Nature Materials, (2015), 14, 1026. [6] W. Richards, Y. Wang, L. Miara, J. Kim and G. Ceder, Energy and Environmental Sciences, 9 (2016) 3272. Figure 1. σ 25℃ and E a for Li 1+2x Zn 1-x PS 4 .