The challenges with an increased energy density and reduced cost of secondary batteries can only be tackled by new battery chemistries. Among the most promising chemistries is the one based on the electrochemical reaction of lithium and sulfur, the Li-sulfur battery. The interest in this technology is spurred by the high theoretical capacity (1675 mAh/g) and energy density (2600 Wh/kg). Furthermore, the natural abundance and the low cost of the active material, i.e. sulfur, are expected to reduce the costs of Li-sulfur based energy storage systems compared to current Li-ion batteries. However, in practice there are several problems which prevent the technology from being implemented. These arise from the fact that the intermediate products, polysulfides, formed during the electrochemical reaction, going from S to Li₂S, are soluble in common electrolytes used, leading to a low cycle life.

To mitigate those problems large efforts have been spent in the last decade to find new electrolytes as well as to design advanced composite cathode materials that limit the dissolution of polysulfides. … In this paper, we report on a powerful strategy to stabilize a high-loading (i.e. 70% S) sulfur composite cathode using a polyvinylidene fluoride-based (PVdF) gel polymer electrolyte (GPE). The S-C composite is synthesized by a straightforward route using high energy mechanical milling (HEMM) technique, i.e. a process very suitable for scaling-up. With a standard liquid electrolyte this kind of electrode would be quickly dissolved leading to formation of polysulfides and capacity fading. We demonstrated that the GPE, prepared by solvent casting and swollen with a liquid electrolyte solution, mitigates sulfur dissolution thus stabilizing the delivered capacity to a value close to the theoretical limit for the Li-S process (1675 mAh g^-1).

—Agostini et al.

The GPE was prepared by solvent casting providing a porous inner-structure in order to be able to trap a liquid electrolyte phase. The membrane has a low surface area of 3.5 m²/g and micro-pores with the size of about 10 nm in average—a low value compared to that of a commercial Whatman separator (i.e. 0.1 μm). Both surface area and pores size of the GPE membrane were tailored to reduce the electrolyte up-take during the swelling process in the liquid electrolyte solution.

The researchers compared the liquid electrolyte uptake of the GPE membrane and of a commercial Whatman separator. They found a huge difference in the amount of electrolyte taken up, 640% for the Whatman separator and about 40% for the GPE membrane, confirming that the difference in porosity and pore size reduces the volume available in the GPE membrane. Even though the uptake of the liquid electrolyte is lower the conductivity of the swollen GPE membranes is good.

They then tested the GPE in a Li-S cell utilizing a sulfur-carbon composite with high sulfur loading (70%) and compared the performance to cells using a standard Whatman separator.

Discharge capacity upon cycle number comparison between the cells using the GPE (green) and the Whatman membrane (red). Agostini et al. Click to enlarge.

Resources

• Agostini, M., Lim, D. H., Sadd, M., Chiara, F., Navarra, M. A., Panero, S., Brutti, S., Matic, A. and Scrosati, B. (2017) “Stabilizing the performance of high-capacity sulfur composite electrodes by a novel gel polymer electrolyte configuration.” ChemSusChem doi: 10.1002/cssc.201700977