Synthesis, Structure and Property Analysis of Argyrodite-Type Li 5.5 PS 4.5-X Br 1.5 O x 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 Li 5.5 PS 4.5 Br 1.5 , [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 Li 2 S, P 2 S 5 , LiBr, and Li 2 O 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 sampleLi 5.5 PS 4.5-x Br 1.5 O x (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 Li 5.5 PS 4.5-x Br 1.5 O x (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 Li 5.5 PS 4.5-x Br 1.5 O x (x=0, 0.1, 0.3, 0.5). Peaks corresponding to the Li 6 PS 5 Br 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 Li 5.5 PS 4.5-x Br 1.5 O x (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 Li 5.5 PS 4.4 Br 1.5 O 0.1 , Li 5.5 PS 4.2 Br 1.5 O 0.3 , and Li 5.5 PS 4 Br 1.5 O 0.5 are 4.00, 3.82, and 3.19 mS cm -1 , respectively, which was lower than that of Li 5.5 PS 4.5 Br 1.5 . This result indicates that oxygen doping decreases the Li + conductivity of the Li 5.5 PS 4.5 Br 1.5 elec