Metal–Sulfur Battery Cathodes Based on PAN–Sulfur Composites

Sulfur/polyacrylonitrile composites provide a promising route toward 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 are shown to completely eliminate polysulfide dissolution and shuttling between lithium anode and sulfur cathode. We also 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 produce Li₂S as the only discharge product over hundreds of cycles of charge and discharge at fixed current densities

Highly Conductive, Sulfonated, UV-Cross-Linked Separators for Li–S Batteries

Metal (based on Li, Na, Mg, or Al)–sulfur batteries are promising candidates for rechargeable electrochemical energy storage devices capable of high charge storage. However, the success of metal–sulfur battery technology calls for solutions of fundamental problems associated with the inherently complex solution chemistry and interfacial reactivity of sulfur and polysulfide species in commonly used electrolytes. It is understood that the dissolution and shuttling of polysulfides induce rapid capacity degradation, poor cycling stability, and low efficiency of these cells. Herein, we report on the synthesis and transport properties of membranes containing sulfonate groups that are able to rectify transport of polysulfide species in liquid electrolytes. Composed of a cross-linked polyethylene glycol (PEG) framework containing pendant SO₃^2– groups, the membranes facilitate electrolyte wetting and Li⁺ ion transport, but are highly selective in preventing migration of negatively charged sulfur species (S_<i>n</i>^2–) dissolved in liquid electrolytes. We argue that the ion rectifying properties originate from two sources, the small tortuous pores originating from cross-linking small PEG molecules and from repulsive electrostatic interactions between the pendant SO₃^2– groups and large migrating S_<i>n</i>^2– species. Here we demonstrate the effectiveness of these membranes in Li–S batteries and we find that the materials enable high-efficiency (>98%) cycling in LiNO₃ additive-free electrolytes. Such membranes are also attractive in other electrochemical cell designs where they serve to decouple transport of positive and negative charged ions in the electrolyte to minimize polarization

Stabilizing Protic and Aprotic Liquid Electrolytes at High-Bandgap Oxide Interphases

Approaches for regulating electrochemical stability of liquid electrolytes in contact with solid-state electrodes are a requirement for efficient and reversible electrical energy storage in batteries. Such methods are particularly needed in electrochemical cells in which the working potentials of the electrodes lie outside the thermodynamic stability limits of the liquid electrolyte. Here we study electrochemical stability of liquids at electrolyte/electrode interfaces protected by nanometer thick, high electrical bandgap ceramic phases. We report that well-designed ceramic <i>interphases</i> extend the oxidative stability limits for both protic and aprotic liquid electrolytes, in some cases by as much as 1.5 V. It is shown further that such interphases facilitate stable electrodeposition of reactive metals such as lithium at high Coulombic efficiency and in electrochemical cells subject to extended galvanostatic cycling at a current density of 3 mA cm^–2 and at capacities as high as 3 mAh cm^–2. High-resolution cryo-FIB-SEM characterization reveals that solid/compact Li electrodeposits anchored by the ceramic interphase are the source of the enhanced Li deposition stability. The results enable a proof-of-concept “anode-free” Li metal rechargeable battery in which Li initially provided in the cathode is the only source of lithium in the cell