Hybrid Cathode for All-Solid-State Li-S Batteries

Abstract

Lithium-sulfur (Li-S) battery is one of the most promising energy storage candidates to satisfy the rising demands in multi-functional personal electronics, and the development of electric vehicles due to its high energy density and low cost. However, the commercialization of conventional Li-S batteries is subject to technical challenges, in which polysulfide shuttle is the most predominant as it often results in low columbic efficiency and poor capacity retention. The replacement of conventional liquid electrolyte with a solid-state electrolyte is the most effective solution to eliminate the polysulfide shuttling – albeit development of complex cathode host to retain polysulfide – since these lithium polysulfides cannot dissolve in the solid electrolyte. The all-state-state Li-S system may be the ultimate solution for commercializing Li-S cell technology. In this thesis, research was carried out to find new cathode host materials for solid-state Li-S batteries. Vanadium disulfide (VS2) in particular features excellent electronic conductivity, enabling the material to have great potential to replace the traditional carbon black additives in the cathode composite. Therefore, metal sulfides were studied as cathode host materials in a solid-state Li-S cell. The performance of metal sulfide cathodes using different fabrication methods and morphologies was also investigated. This thesis encompasses two projects. The first project of this thesis is an intercalation-conversion hybrid cathode that combines intercalation-type VS2 with conversion-type sulfur to construct high performance solid-state lithium-sulfur batteries. The layered VS2 nanomaterial features Li-ion transport channels, metallic conductivity, and active capacity contribution, all of which provide an ideal platform for the solid-state S/Li2S redox couple to unlock its high gravimetric capacity. The S/VS2 hybrid cathode composite was prepared by a facile, low-cost, and low-energy mechanical blending process. The all-solid-state cell using S/VS2 hybrid cathode exhibited sulfur utilization of ~85%, with a coulombic efficiency of close to 100 %. High areal capacity up to 7.8 mA·h·cm-2 with an active material loading (S/VS2) as high as 15.5 mg·cm-2 was achieved. In the second project, I reported a Li2S/LiVS2 core shell composite synthesized from lithium sulfide reacting with metal halide for cathode active materials in all solid-state Li-S battery. Compared with VS2-S composite, Li2S/LiVS2 core shell composite has an advantage of accommodating the volume change by confining Li2S in metal sulfide shell during cycling.