One of the major limitations for automobile application for lithium-ion batteries is their inability to power electric cars for long distances due to their low cycle life, low specific capacity and low energy efficiency. Consequently, tremendous research efforts have been made to improve the energy and power density of rechargeable lithium batteries for possible use in electrical cars. Researchers at Stanford University, led by H. Dai and Y. Cui have developed a new type of lithium battery by synthesizing a graphene-wrapped sulfur-based composite material that shows high and stable specific capacities of up to 600 mAh/g over more than 100 cycles. The researchers suggest that this type of material shows great promise as a high-performance cathode material for Li-S batteries.
As reported in the July issue of Nano Letters (DOI: 10.1021/n1200658a; p. 2644), the team fabricated this material by wrapping poly(ethylene glycol) (PEG) coated sub-micrometer-sized sulfur particles with mildly oxidized graphene oxide sheets decorated with carbon black nanoparticles. The PEG and graphene coating layers are important for accommodating volume expansion of the coated sulfur particles during discharge, thereby trapping soluble polysulfide intermediates and rendering the sulfur particles electrically conducting. The team found that the average size of the sulfur particles was less than 1 µm and energy dispersive spectroscopy was used to confirm the structure and composition of the graphene-sulfur composite. Colorimetric titration experiments determined that the graphene-sulfur composition contained 70 wt% of sulphur (with 15% of mildly oxidized graphene sheets and 8% carbon black).
The researchers fabricated coin cells to test the electrochemical performance of the graphene-sulfur composite material with a Li foil as the anode in an electrolyte of 1.0M lithium bis-trifluoromethane sulfonylyimide in 1,3 dioxilane and 1,2 dimethyoxyethane. The results demonstrated that the PEG-coated graphene-sulfur composite can maintain a specific capacity of ~600 mAh/g of more than 100 cycles. The capacity fading for more than 100 cycles was 10-15%.
Further work is now required to understand the large performance variability observed in the lithium-sulphur batteries tested in this study. For example, about 30-50% of the Li-system for this study showed a declining cycle performance of 20-25% decay over 100 cycles.
Nevertheless, according to the researchers, the high specific capacity and good cycling stability obtained from the graphene-sulfur composite is an initial step in creating a promising potential material for future lithium-ion batteries with high energy density.