Arumugam Manthiram
Materials Science and Engineering Program
The University of Texas at Austin

Energy, power, cycle life, safety, and cost are important criteria in employing rechargeable battery technologies for transportation and stationary storage of electricity produced by renewable sources like solar and wind. The energy density of current lithium-ion batteries is limited by the cathode capacity of < 200 mAh/g at ~ 4 V. Interestingly, sulfur cathodes offer an order of magnitude higher capacity (theoretical capacity: 1,675 mAh/g) at an operating voltage of ~ 2.1 V. While the high capacity can significantly enhance the energy density of the batteries, the lower operating voltage can also offer better safety. However, the commercialization of rechargeable lithium-sulfur batteries is impeded by two major challenges: (i) poor cycle life due to the dissolution of the polysulfide intermediates (Li2S8, Li2S6, and Li2S4) formed during the charge-discharge process and (ii) low electrochemical utilization of sulfur cathodes due to the high insulating nature of sulfur and the discharge product Li2S. To overcome these difficulties, this presentation will focus first on a series of composite cathodes with unique nanostructures that improve the electrical conductivity and utilization of active materials. For example, sulfur-carbon nanocomposites synthesized by a scalable in situ sulfur deposition route exhibit much better electrochemical performance than pristine sulfur. Similarly, sulfur-polypyrrole composites in which the sulfur particles are coated by a nanolayer of polypyrrole show improved capacity and cyclability. The presentation will then focus on the fabrication of a scalable, binder/current collector-free, nanostructured sulfur-carbon nanotube (S-CNT) composite cathode without employing toxic solvents during electrode processing, which exhibits excellent capacity retention at high rates, e.g., >1,000 mAh/g capacity after 50 cycles at 1C rate. Finally, a novel lithium-sulfur battery structure with an interlayer between the separator and the cathode to capture the polysulfide ions exhibits significantly improved energy and power. These materials and strategies are scalable and can enable packaged cells with an anticipated energy density of > 600 Wh/kg and power density of > 1000 W/kg, which are three times higher than those of current lithium-ion batteries.

Elton Cairns
Environmental Energy Technologies Division
Lawrence Berkeley National Lab

Current lithium ion cells are reaching their maximum energy storage capability (~200 Wh/kg) and are still not able to provide a safe, low-cost battery of sufficient energy storage capability for electric vehicles of more than 100-mile range.  A new generation of battery with a specific energy of at least 400 Wh/kg, low cost (<$200/kWh), good safety, and low environmental impact is urgently needed.  The next generation of rechargeable cells must have a theoretical specific energy well above 1000 Wh/kg (as compared to ~550 Wh/kg for Li-ion cells).  The Li/S cell is probably the most attractive candidate for the next cell beyond the Lithium Ion cell.  It has a theoretical specific energy of 2600 Wh/kg, and an estimated practical specific energy of about 600 Wh/kg.

The commercial development of the Li/S cell has been prevented by the short cycle life of the sulfur electrode caused by the rapid capacity loss.  This capacity loss is attributed to: 1) the solubility of lithium polysulfides in the (organic solvent) electrolyte, 2) possible loss of contact between the (non-conductive) sulfur and the current collector, 3) the large volume change of the sulfur as it is charged and discharged (76%).  Various approaches have been taken to solve these problems, with some success.  A selection of attempts to improve the performance and cycle life of the Li/S cell will be reviewed and discussed, along with recent results from our laboratory.