Materials Science Materials Science
By Tyler Irving
Posted September 2011
Although still in the experimental stages, lithium-sulphur (Li-S)batteries could offer three to five times more energy density than lithium-ion, the current industry standard for personal electronics and electric vehicles. Now, a discovery at the University of Waterloo has brought Li-S batteries one step closer to commercial viability.
Li-S batteries depend on a reversible reaction that converts elemental lithium and sulphur to Li2S. The problem is that the reaction has a number of intermediate species — a series of polysulphide ions — which are soluble in the electrolyte. If these species dissolve and are lost from the positive electrode, they can no longer do useful work, reducing the capacity of the battery.
In 2009, Linda Nazar and her team at the University of Waterloo came partway toward a solution by embedding the sulphur in a polymer-coated mesoporous conductive carbon matrix. This material creates an environment that delivers the electrons required for the electrochemical reaction, but also holds in most of the soluble polysulphides. Now, Nazar’s team has gone one step further by adding silica particles with a very high surface area and pore volume into the same carbon matrix. The silica particles have a weak affinity for the soluble polysulphides, which physically absorb within their pores instead of leaving the matrix. Although the polysulphides can’t undergo further electrochemical reaction within the silica, the weak binding means that they are gradually released later in the charge cycle to form insoluble Li2S and complete the reaction. Importantly, this process is reversible.
The ability to hold the polysulphides in silica particles allows the carbon matrix itself to have larger pores. This increases the rate of discharge up to 10-fold while maintaining energy density. “We’re hanging on to 82 per cent of the polysulphides over 30 cycles,but you can tune the pore size and the surface area of this additive to really improve upon that process,” says Nazar. The work was published this past May in Nature Communications.
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