Synthesis, Structure and Property Analysis of Argyrodite-Type Li5.5PS4.5-XBr1.5Ox solid Electrolytes

Introduction The development of solid electrolytes is important for the practical application of all solid-state batteries (ASSBs). Sulfide-based solid electrolytes have the disadvantage of low chemical stability against air and moisture, and the improvement of this stability is an essential issue in the practical application of all-solid-state batteries. Several examples have been reported in which elemental replacement of S with O in sulfide solid electrolytes improves properties such as ionic conductivity and chemical stability.[1,2] However, the results vary depending on the material composition, and the details are not yet known. In this study, we focused on argyrodite-type Li5.5PS4.5Br1.5,[3] which has high ionic conductivity among sulfides, in order to improve each performance by elemental replacement of S with O in this composition. Experimental The raw materials Li2S, P2S5, LiBr, and Li2O were weighed in appropriate molar ratios under an inert atmosphere, mixed and grounded at 500 rpm for 15 hours using a planetary ball mill, and then vacuum sintered at 673 K for 10 hours. The resulting sampleLi5.5PS4.5-xBr1.5Ox (x=0, 0.1, 0.3, 0.5) was subjected to X-ray diffusion (XRD) and energy dispersive X-ray spectroscopy (SEM-EDX) to identify the sample phase. Ionic conductivity was evaluated in the temperature range of 298-373 K by AC impedance measurement using a compact sample (380 MPa). For battery evaluation, all-solid-state batteries consisting of a cathode composite (LCO-LGPS)/solid electrolyte Li5.5PS4.5-xBr1.5Ox (x=0, 0.1)/Li-In alloy anode were fabricated and charge-discharge tests were performed at 0.1 C (57 μA). Results and Discussion Fig. 1(a) shows the XRD patterns of the synthesized Li5.5PS4.5-xBr1.5Ox (x=0, 0.1, 0.3, 0.5). Peaks corresponding to the Li6PS5Br structure and trace impurity LiBr were observed in all samples. No peak shift to the low/high angle side due to oxygen substitution was observed. The ionic conductivity of the synthesized Li5.5PS4.5-xBr1.5Ox (x=0, 0.1, 0.3, 0.5) solid electrolytes was investigated by temperature dependent AC impedance. As shown in Fig.1(b), the ionic conductivities of Li5.5PS4.4Br1.5O0.1, Li5.5PS4.2Br1.5O0.3, and Li5.5PS4Br1.5O0.5 are 4.00, 3.82, and 3.19 mS cm-1, respectively, which was lower than that of Li5.5PS4.5Br1.5. This result indicates that oxygen doping decreases the Li+ conductivity of the Li5.5PS4.5Br1.5 electrolyte. Finally, The Initial charge-discharge curve is shown in Fig. 1(c). The initial capacities of the all-solid-state batteries with solid electrolytes Li5.5PS4.5Br1.5 and Li5.5PS4.4Br1.5O0.1 were 114.3 mAh g-1 and 126.8 mAh g-1, respectively. The initial capacity was improved by oxygen doping. These results indicate the great potential of oxygen-containing argyrodite solid electrolytes. References [1] H, Ren, et al., Rare Met., 41, 106-114 (2022). [2] L, Peng, et al., ACS. Appl. Mater. Interface., 14, 4179-4185 (2022). [3] C. Yu, et al., Energy Storage Materials., 30, 238–249 (2020). Figure 1