Synthesis and Electrochemical Property of I-4 Type Li1+2xZn1-XPS4 Solid Electrolyte
Authors: Suzuki Naoki , Richards William Davidson , Wang Yan Eric , Miara Lincoln J. , Kim Jae Chul , Jung In-Sun , Tsujimura Tomoyuki , Ceder Gerbrand 
Published: February 2020

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 Li10GeP2S12 [1], Li7P3S11 [2], and Li9.54Si1.74P1.44S11.7Cl0.3[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 Na10SnP2S12, in which Li is substituted to Na in Li10SnP2S12, would be a good Na ion conductor. They also synthesized Na10SnP2S12 in practice and confirmed that the experimentally measured ionic conductivity and the activation energy of Na10SnP2S12 agreed well with those of the predicted values.
In this study, we examined I-4 type Li1+2xZn1-xPS4 — a new Li ionic conductor computationally predicted by Richards et al. [5-6]. They predicted that the stoichiometric LiZnPS4 is almost insulative, but non-stoichiometric compound of Li1+2xZn1-xPS4 can show superionic conductivity with the predicted room temperature ionic conductivity (σ25
℃) as large as 50 mS/cm at x=0.5.
Li1+2xZn1-xPS4 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 Li1+2xZn1-xPS4 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℃ (σ25
℃) and the activation energy (Ea) for Li1+2xZn1-xPS4 at various x. Li1+2xZn1-xPS4 shows negligible conductivity when x ≤ 0.2. At x = 0.375, σ25
℃ starts increasing and Ea starts decreasing. The highest σ25
℃ 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, σ25
℃ decreases, which likely corresponds to low phase purity and poor crystallinity of the material.
We also examined the feasibility of Li2.25Zn0.375PS4(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
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Figure 1. σ25℃ and Ea for Li1+2xZn1-xPS4.