In a paper in the RSC journal Energy & Environmental Science , the researchers report that such cathodes with a final loading of 65% sulfur can operate at a high rate of 2C (a 1C rate corresponds to a complete charge or discharge in 1 hour) for more than 500 cycles with nearly 100% coulombic efficiency.

A team at Toyota Research Institute of North America (TRINA) ( earlier post ) has developed a nanostructured sulfur cathode with a truffle-like architecture which comprises a sulfur particle embedded with hollow carbon nanospheres and encapsulated with an ion-selective, flexible layer-by-layer (LBL) nanomembrane decorated with conductive carbon.



A three-dimensional view of the concept depicting the carbon-infused sulfur core, the hollow carbon nanoparticles and multilayer selective polymer membrane decorated with functionalized carbon. Click to enlarge.

Ordered, supramolecular structures which aim at tailoring the surface properties of materials have been greatly influenced by the advent of the layer-by-layer (LBL) self-assembly techniques. Any material with bonding abilities (ionic or hydrogen bonds) and which is accessible by a solvent can be transformed by assembly into a multilayer by the LBL approach. … These results indicate that this type of nanointerfacial engineering could also be a promising solution for the low electrical conductivity of other battery cathodes. —Bucur et al.



Capacity fade over the first 600 cycles for optimized “truffle” cathodes (7 layers PVP/PAA/PEO). Batteries consist of 2032 coin cells with a sulfur loading of ~1 mg/cm2 and ~50% sulfur loading by mass in the finished cathode. Battery cycling was performed at 0.5C (green), 2C (blue) and 5C (garnet). Bucur et al. Click to enlarge.

Sulfur is the very attractive cathode for the next-generation of advanced batteries due to its high theoretical capacity of 1672 mAh/g. Practical application is still challenged by issues such as high resistance, low loading of active material and dissolution of the intermediate polysulfide into the electrolyte during charge and discharge. These cause low columbic efficiency, fast capacity fade and self-discharge.

Various research teams have been exploring the use of polymer electrolytes, coatings, and membranes to enhance to performance of the Li-S battery by impeding the diffusion of polysulfides.

In a review published earlier this year (Muldoon et al.) on the use of polymers in Li-air and Li-S batteries, members of the TRINA team noted that while polymer-based electrolytes have been widely investigated to impede polysulfide dissolution, their conductivities are significantly lower than traditional, liquid based electrolytes—i.e., high discharge rates cannot be achieved.

However, the use of polymers as interface modifiers, composites, or coatings have improved the cyclability of sulfur cathodes. In addition, polymers can provide an elastic framework for sulfur which can accommodate the ~20% volume expansion reported between the charged and discharged species.

The use of polymers as encapsulating membranes for sulfur particles holds the promise of reducing the polymer contents seen in sulfur/polymer composites and thus increasing the overall capacities based on total cathode mass. Ideally, the encapsulating membrane does not impede electronic or ionic conduction of the lithium salts while providing a barrier to the bulkier polysulfides. The membrane should also be conformal to the encapsulating substrate and thin, such that the sulfur content is maximized and flexible to accommodate the volume changes between charge and discharge. —Muldoon et al.

To suppress the dissolution of the intermediate polysulfides and minimize the addition of conductive carbon, and also to address the limiting issues noted above, the TRINA team created the controlled nanoarchitecture template with the layer-by-layer nanomembrane.

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