Lithium batteries: Between the sheets

Trapping tin oxide between graphene nanosheets produces higher performance lithium ion batteries.

Growing demand for electronic devices requires lithium-ion batteries with longer lifetimes and more charge. Scientists seeking to boost these properties need anode materials that can absorb and desorb large amounts of lithium ions. Now, Itaru Honma and colleagues¹ have developed a three-dimensionally nanostructured anode with a reversible charge capacity that greatly exceeds typical batteries.

Fig. 1: Cross-sectional TEM image and schematic diagram for GNS/SnO2. White arrows in the figure indicate graphene nanosheets (GNS) which are homogeneously distributed between tin oxide nanoparticles.

Graphite is commonly used as an anode in lithium-ion batteries because of its large surface area and reversible behaviour. The interior of graphite consists of thin crystal sheets of carbon atoms—known as graphene—stacked upon one another. Honma and colleagues found that a means of increasing the surface area, and hence capacity, is to separate these graphene sheets.

“When graphite is strongly oxidized and sonicated in a water solution, it forms a graphene oxide dispersion. This solution contains graphene oxide sheets of individual monolayers,” says Honma.

To further increase the battery’s capacity, Honma’s team incorporated tin oxide (SnO₂) nanoparticles between the graphene nanosheets. The strong chemical attraction between SnO₂ and lithium ions produces a very large storage capacity—but unfortunately, it comes at a price.

“In the charging process, a large number of lithium ions are inserted into the SnO₂ particles, inducing a volume change of about 300%,” says Honma. When the anode discharges, large gaps are left in the SnO2, casing the material to crumble into pieces after a few uses.

However, by confining the SnO₂ between graphene nanosheets, the researchers accomplished a clever feat. Transmission electron microscopy revealed that SnO₂ packed evenly and loosely between thin, flexible layers of graphene. This porous nanostructure allowed SnO₂ swelling to occur – but only to a point. The graphene sheets limited the volume expansion, and the SnO₂ nanoparticle retained its structure.

The combination of SnO₂ chemistry and an accessible graphene surface produced an anode with large reversible charge capacities – nearly double those of the typical graphite anodes.

Graphene nanosheets may find applications in other energy technologies, such as hydrogen storage and fuel cells. “I believe a lot of interesting results will come in this area,” says Honma.