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 Li2S and Li metal anode. However, due to insufficient electronic and/or ionic conductivities of element S and Li2S, the low sulfur utilization and correspondingly unsatisfied practical energy density hinder the application of ASSLSBs largely. By heterovalent doping, vacancies in Li2S could be created, leading to higher ionic conductivity [1-2]. In this study, we developed different proportion of PI3-doping Li2S as active material to improve the sulfur utilization. Mixing with CNovel as electronic additive, Li2S-PI3-CNovel as cathode material shows good electrochemical performance. In order to clarify the effect of doping PI3, 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 Li2S-PI3 as active material was synthesized by mixing Li2S and PI3 as raw materials with ball milling method and the proportion of raw materials has been optimized. The characterization of the prepared Li2S-PI3 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 Li2S-PI3-CNovel as cathode, Li3PS4 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 Li2S-PI3-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 PI3 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 Li2S-PI3-CNovel as cathode. The Li2S-PI3-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 Li2S 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