The specific capacities of the anode materials in lithium batteries are 370 mAh/g for graphite and 4200 mAh/g for silicon. By contrast, the cathode specific capacities are 170 mAh/g for LiFePO4 and only 150mAh/g for layered oxides. Researchers have made a cathode that retains a specific capacity of more than 600 mAh/g over 100 charging cycles. Even though the material maintains a high specific capacity over 100 cycles, Wang and co say the capacity drops by 15 per cent in the process.
We report the synthesis of a graphene-sulfur composite material by wrapping polyethyleneglycol (PEG) coated submicron sulfur particles with mildly oxidized graphene oxide sheets decorated by carbon black nanoparticles. The PEG and graphene coating layers are important to accommodating volume expansion of the coated sulfur particles during discharge, trapping soluble polysulfide intermediates and rendering the sulfur particles electrically conducting. The resulting graphene-sulfur composite showed high and stable specific capacities up to ~600mAh/g over more than 100 cycles, representing a promising cathode material for rechargeable lithium batteries with high energy density.
Chemists have known for many years that sulphur has potential: it has a theoretical specific capacity of 1672 mAh/g. But it also has a number of disadvantages, not least of these is the fact that sulphur is a poor conductor. On top of this, polysulphides tend to dissolve and wash away in many electrolytes while sulphur tends to swell during the discharge cycle causing it to crumble.
But Wang and co say they’ve largely overcome these problems using a few clever nanoengineering techniques to improve the performance. Their trick is to create submicron sulphur particles and coat them in a kind of plastic called polyethyleneglycol or PEG. This traps polysulphides and prevents them from washing away.
Next, Wang and co wrap the coated sulphur particles in a graphene cage. The interaction between carbon and sulphur renders the particles electrically conducting and also supports the particles as they swell and shrink during each charging cycle.