All-solid-state Li/S batteries with inorganic solid electrolytes have been studied because of their advantages of large energy density, good reversibility, and low cost. Preparation of sulfur composite electrodes using solid electrolyte and carbon additives is important for enhancing the utilization of sulfur and Li₂S as an active material [1,2]. However, increasing sulfur or Li₂S content in the composite electrodes tends to decrease reversible capacity because of their insulating nature. A melt diffusion process of sulfur into carbon pores is widely used for forming good sulfur-carbon contacts and inhibiting dissolution of polysulfide moieties into liquid electrolytes. We have prepared sulfur composite electrodes including 50 wt% sulfur by mechanical milling at 155^oC, which is over the melting point of sulfur, and all-solid-state Li/S cells with the composite electrode showed the reversible capacity of over 1000 mAh g^-1 at room temperature [3]. Alternative strategy to increase utilization of active materials is enhancing their lithium ion conductivity. Partial replacement of sulfide anions (S^2-) in crystalline Li₂S with iodide anions (I^-) introduces lithium defects to Li₂S and increases its lattice constant, and the structural modification will bring about conductivity enhancement.

In this study, we have prepared Li₂S-LiI solid solutions by mechanical milling [4]. The starting materials of crystalline Li₂S and LiI were milled using a planetary ball mill apparatus. X-ray diffraction (XRD) for the (100-x)Li₂S∙xLiI (mol%, 0 ≤ x ≤ 20) milled samples indicated that XRD peaks attributable to crystalline LiI disappeared and those attributable to Li₂S shifted to the lower angle side with an increase in the LiI content. The lattice constant of Li₂S increased with an increase in the LiI content, suggesting that solid solutions in the system Li₂S-LiI were prepared by mechanical milling. The solid solution at the composition of x = 20 showed the highest Li⁺ ion conductivity of 2.2×10^-6 S cm^-1, which is two orders of magnitude higher than that of Li₂S. An all-solid-state cell (Li-In/Li₃PS₄/Li₂S-LiI) operated as a rechargeable battery at room temperature. The cell with the x=20 solid solution as an active material exhibited a stable reversible capacity of 930 mAh per gram of Li₂S (80% utilization) for 50 cycles at 0.07C. Further conductivity enhancement was achieved in the ternary system Li₂S-LiI-LiBr, and the highest conductivity was over 10^-5 S cm^-1at the composition with 40 mol% lithium halides. The concept of increasing ionic conductivity of sulfur-based active materials is effective in increasing their utilization and reversible capacity for Li/S cells.

Acknowledgement

This research was financially supported by ALCA-SPRING project.

References

[1] M. Nagao et al., Electrochim. Acta, 56(2011) 6055.

[2] M. Nagao et al., J. Mater. Chem., 22(2012) 10015.

[3] M. Nagao et al., Energy Technol., 1(2013) 186. .

[4] T. Hakari et al., Chem. Lett., 44 (2015) 1664.