Since the entry of lithium-ion rechargeable batteries into the market, considerable improvements have been made in their gravimetric and volumetric energy densities, especially compared to aqueous systems such as Pb-acid, Ni-Cd, and Ni-MH. Sulfur cathodes have been well studied for over three decades, due to many of their attractive features such as high specific energy, abundance, and low cost. A successful sulfur-based battery is, however, still to emerge, especially for ambient temperature applications.

For stationary applications, the state-of-the-art energy storage systems are inadequate to meet the requirements of large-scale (>1 MW) energy systems, which include long lifetimes, safety, and low cost. The conventional rechargeable systems are unsuitable for this application due to their high cost and safety issues. Ideally, a rechargeable system with liquid reactants, similar to a vanadium redox battery, will be beneficial, since the reactants can be stored external to the battery to make the system attractive from energy and cost perspectives.

Sulfur is an attractive cathode material when combined with a lithium anode. Despite its considerable advantages, the Li-S cell is plagued with problems that have hindered its widespread practical realization. The major problem associated with the use of high-energy and low-cost sulfur cathode is related to the solubility of discharge intermediates, lithium polysulfides, in most of the known organic electrolytes. It has been a challenge to find a suitable electrolyte system wherein the polysulfide is insoluble, and which is also compatible with lithium anode. To circumvent this problem, sulfur cathode is used in a “liquid” form, i.e., in soluble polysulfide form. Also, flow-battery architectures, suitable for large-scale energy systems, are used. In such configurations, the stored energy can be scaled independently of power such that the system-level energy density can be vastly improved.

There are several unique attributes of this flow cell, which is similar to the nonaqueous, high-voltage lithium flow battery: (1) the reaction involves the shuttling of lithium ions from the anolyte to catholyte, much like with traditional Li-ion cells; (2) the reactions involved at both electrodes are mostly chemical, with the oxidized or reduced lithium reacting with the liquid active materials; and (3) the electrodes are only for current collection purposes, which precludes any morphological or interfacial changes at the electrode. All these features will, in principle, contribute to a long cycle life, calendar life, and safety with low self-discharge rates.

Another significant novelty here is the use of low-cost and high-energy sulfur cathode in flow-cell architecture. Such sulfur cathode-based flow batteries containing Li anode or Li naphthalide are expected to provide several-fold improvements in energy density, cost, and safety compared to the state-of-the-art flow batteries for stationary energy storage technologies.

This work was done by Ratnakumar V. Bugga, William C. West, and Marshall C. Smart of Caltech for NASA’s Jet Propulsion Laboratory. For more information, contact This email address is being protected from spambots. You need JavaScript enabled to view it..

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