Liquid-phase synthesis of sulfide solid electrolytes for all solid-state lithium-ion batteries

Abstract

Solid electrolytes are one of the key components for all –solid-state lithium-ion batteries (ASLBs) for large-scale applications. The performance of solid electrolytes (SEs) with high ionic conductivity and electrochemical stability should be achieved for ASLBs. However, sulfide solid electrolytes which have been mostly studied as SEs have the low electrochemical stability and are oxidized over ~3 V (vs. Li+/Li) forming resistive layer between active materials and SEs In addition, the reaction between conventional transition metal oxide active materials and SEs is one of the most important issues. To solve this problem, oxy-sulfide solid electrolytes with a substitution of small amounts of sulfur to oxygen were reported. Even though the electrochemical stability of the oxy-sulfide SEs increased, the substitution lead a decrease of ionic conductivities due to lattice contraction and the impurity phases. However, there is few research about electrochemical stability of SEs by a compositional tuning of SEs. Furthermore, developing synthesis protocols for solid electrolytes with high yield, efficiency in reaction such as shorter time and lower heat-treatment temperature is one of the most important issues. In the conventional ball-milling methods, the reaction proceeds in a pot with high-energy balls and usually takes long time to obtain final products. In this regard, liquid-phase synthetic methods have been explored as an alternative . The liquid-phase synthesis method has advantages of mild reaction conditions and provides possibilities to find new SEs in terms of composition and structure. In previous reports, several SEs, such as Li3PS4 and Li7P2S8I, were prepared using organic solvents such as tetrahydrofuran (THF) and acetonitrile (AN). In this study, we report Li-deficient β-Li3PS4, Li3-xPS4 (0≤x≤0.3) prepared by a co-solvent using THF and ο-xylene. The excellent solubility of elemental sulfur to o-xylene allows for sulfur to fully participate in the liquid-phase reaction with Li2S and P2S5, resulting. From the complementary analyses of x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), Raman spectroscopy and particle size analyzer, the evolution of bridging sulfur and better homogeneity for Li3-xPS4 derived using the co-solvent are confirmed, compared with that obtained using a single solvent (THF). The better electrochemical performances of NCM/Li-In cells employing Li3-xPS4 using co-solvent than those for using single solvent are shown. Furthermore, the as-synthesized Li3-xPS4 (0≤x≤0.3) likely to show an improved oxidation stability than pristine Li3PS4.