Metal-Sulfur Batteries Based On Solid-State Reaction
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Sulfur/Polyacrylonitrile composites provide a promising route towards cathode materials that overcome multiple, stubborn technical barriers to high-energy, rechargeable lithium-sulfur (Li-S) cells. Using a facile thermal synthesis procedure in which sulfur and polyacrylonitrile (PAN) are the only reactants, we create a family of sulfur/PAN (SPAN) nanocomposites in which sulfur is maintained as S3/S2 during all stages of the redox process. By entrapping these smaller molecular sulfur species in the cathode through covalent bonding to and physical confinement in a conductive host, these materials obviate polysulfide dissolution and shuttling between lithium anode and sulfur cathode. We show that in the absence of any of the usual salt additives required to stabilize the anode in traditional Li-S cells, Li-SPAN cells cycle trouble free and at high Coulombic efficiencies in simple carbonate electrolytes. Electrochemical and spectroscopic analysis of the SPAN cathodes at various stages of charge and discharge further show a full and reversible reduction and oxidation between elemental sulfur and Li-ions in the electrolyte to repeatably produce Li2S as the only discharge product over hundreds of cycles of charge and discharge at fixed current densities. The SPAN concepts have further extended to sodium-sulfur batteries, since research efforts have encountered more severed challenges, such as cathode material dissociation and dissolution, instable sodium deposition and insufficient cycle life with rapid capacity decay. We present a stable cycling room temperature Na-S battery that uses a sodium metal anode, a microporous carbon-sulfur composite, and a carbonate electrolyte with a hybrid ionic liquid silica nanoparticle (SiO2-IL-ClO4) as additive. The cell can stably cycle for over 100 cycles at 0.5C (1C = 1675 mAh/g) with 600 mAh/g reversible capacity and nearly 100 percent Coulombic efficiency iii achieved. Spectroscopic and electrochemical analysis indicates a solid-state reaction probably happens only inside micropore on the cathode side, which contributes to the high stability and reversibility of the cell. iv