Li₂s-PI₃ Cathode for High-Energy-Density All-Solid-State Lithium-Sulfur Batteries

Abstract

1.Introduction All-solid-state lithium-sulfur batteries (ASSLSBs) are one of the most promising next-generation large scale energy storage devices due to their remarkably high theoretical energy density, which is generated by high capacity of both cathodes, like element S or Li 2 S and Li metal anode. However, due to insufficient electronic and/or ionic conductivities of element S and Li 2 S, the low sulfur utilization and correspondingly unsatisfied practical energy density hinder the application of ASSLSBs largely. By heterovalent doping, vacancies in Li 2 S could be created, leading to higher ionic conductivity [1-2] . In this study, we developed different proportion of PI 3 -doping Li 2 S as active material to improve the sulfur utilization. Mixing with CNovel as electronic additive, Li 2 S-PI 3 -CNovel as cathode material shows good electrochemical performance. In order to clarify the effect of doping PI 3 , a detailed analysis of the point defect structure was performed using pair distribution function analysis using high-energy X-ray diffraction, synchrotron X-ray diffraction measurements, and X-ray absorption measurements to correlate with the electrochemical properties. 2. Experimental Li 2 S-PI 3 as active material was synthesized by mixing Li 2 S and PI 3 as raw materials with ball milling method and the proportion of raw materials has been optimized. The characterization of the prepared Li 2 S-PI 3 was conducted by Synchrotron XRD coupled with pair Distribution Function analysis. The ionic conductivities were measured by AC impedance. The cathode materials were prepared based on active materials (90 wt%) and CNovel (10 wt%) as electronic conductor. The batteries composed of Li 2 S-PI 3 -CNovel as cathode, Li 3 PS 4 as SSE and Li-In alloy as anode were assembled in the glove box filled with Ar atmosphere. The charge-discharge curves were obtained by galvanostatic measurement. The electronic structure and morphology of the Li 2 S-PI 3 -CNovel after electrochemical measurements were examined by X-ray Absorption Spectroscopy (XAS) for S K -edge, P K -edge, and I K -edge. 3.Results and Discussion Pair distribution function (PDF) analysis revealed that with PI 3 doping, iodine atoms would occupy sulfur sites and phosphorous atoms would occupy lithium sites, creating lithium vacancies in the antifluorite structure. Figure 1 shows the charge/discharge curves of the Li 2 S-PI 3 -CNovel as cathode. The Li 2 S-PI 3 -CNovel cathode shows a charge capacity of 440 mAh g -1 and a discharge capacity of 494 mAh g -1 in the first cycle. And the capacities increase to 538 mAh g -1 during the subsequent cycles, with an average voltage of 1.81 V (vs Li/Li + ). Even at 1 C (1 C=626 mAh g -1 ), the cathode without solid-state electrolyte could deliver 207 mAh g -1 , corresponding to 38 % capacity retention of that at 0.05 C, which demonstrates good rate performance (Figure 2). The charge compensation is attributed to S, suggesting the conversion reaction between Li 2 S and element S, which was confirmed by XAS for S K -edge and P K -edge. References H Gamo. et al ., Adv . 2022 , 3 , 2488-2494. NHH Phuc. et al ., Energy Res ., 2021 , 8 , 606023. Acknowledgement This research was financially supported by JST ALCA-SPRING Project. Figure 1