Exploring Scalable Material Strategies Towards the Practical Metal-Sulfur Batteries

Abstract

Lithium-ion batteries (LIBs) dominate the market of portable electronic devices and currently serve as the stand-alone power source for electric vehicles. LiBs are approaching their theoretical energy density limit, necessitating the development of alternative battery chemistries. Metal-sulfur batteries are a crucial class of high-energy-density batteries within the realm of post-Li-ion battery systems. This thesis proposes strategies for the development of practical metal-sulfur batteries, with a focus on monovalent (Li) and bivalent (Zn) sulfur battery systems. The practical implementation of metal-sulfur batteries is hindered by several critical challenges, including the polysulfide shuttle effect, sluggish redox kinetics, volume expansion, and overoxidation of metal sulfide in aqueous electrolytes. We have added commercially available carbon modified with a polar transition metal oxide to the sulfur host to effectively immobilize and mediate the conversion of lithium polysulfide to lithium sulfide. We systematically investigated the catalytic activity of titanium diselenide (TiSe2) towards the sulfur redox kinetics, namely, the reduction reaction (SRR) and sulfur evolution reaction (SER). Subsequently, we employed TiSe2 as an electrocatalyst additive in the sulfur cathode. These cathodes were evaluated under practically relevant conditions in coin and pouch cell formats, including high sulfur loadings and lean electrolyte environments. In aqueous electrolytes, the free protons lead to the hydrogen evolution reaction and the oxidation of ZnS to zinc sulfate (ZnSO4). To suppress the water activity, we have developed a hybrid electrolyte. The hybrid electrolyte enables the reversible deposition and dissolution of Zn-ions. We characterized the electrochemical conversion of S to ZnS using in-situ spectroscopy/diffraction methods. The performance of the Zn-S battery was assessed in both coin and pouch cell configurations. The volume expansion of the S-cathode and the challenges associated with the Zn-metal anode are addressed by developing an anode-free battery, which utilizes carbon-coated ZnS nanoparticles as the cathode material. Furthermore, we investigate the role of ZnI2 in facilitating the reversible deposition and dissolution of Zn2+ ions on the Cu current collector.