|Fig. 1: Graphene is a single sheet of carbons atoms bonded hexagonally. (Source: Wikimedia Commons)|
As energy producers begin to prioritize portability, efficiency, and environmental impact, improvements in energy storage are highly sought after. Graphene was only first isolated in 2004 after years of research and speculation, and its potential applications in the contexts of electronics and energy storage have been the subject of further research since its relatively recent discovery.  One of the most explored applications is in lithium-ion (Li-ion) storage. The literature strongly suggests that a hybrid solution utilizing graphene in conjunction with another technology, method, or material results in the most favorable outcomes.
Graphene, which is pure carbon, presents as a transparent, thin sheet. Despite its seemingly diminutive appearance, its properties are impressive. Graphene is the strongest material ever measured, as well as one of the thinnest at just one atom thick.  Graphene's properties make it an intriguing prospect as an alternative electrode material. One essential characteristic is surface area. Graphene's theoretical surface area has been reported to be as great as ~2630 m2/g, far superior to that of both graphite and SWCNTs, which are around 1315 m2/g and 10 m2/g.  Additionally, graphene's electrical conductivity far exceeds that of SWCNTs and is not highly susceptible to temperatures changes. This allows for highly mobile electrons, with electrons in grapheme having been measured to be ~200 times more mobile than electrons in silicon. This is significant as it translates into significant improvements in charge carrier speeds. Furthermore, graphene sheets (GNSs) are highly flexible when compared to the rigidity of graphite, and graphene can therefore be useful in flexible electronic devices. 
|Fig. 2: Intercalation (Source: Wikimedia Commons)|
Lithium-ion batteries are prized for their lightweight nature and specific energy. It follows that there has been a electrode materials possessing characteristics such as rapid Li- ion diffusion and high electron mobility.  As mentioned above, graphene has tremendous physical properties - especially mechanical and thermal ones - which would be expected to produce superior battery performance when graphene-based anode materials are used. However, graphene sheets tend towards agglomeration due to van der Waals forces. These van der Waals forces may form graphite, thus inhibiting lithium intercalation.  To prevent this from happening, many studies have explored using metal oxides (in the form of nanowires/particles grown on graphene sheets).  For instance, one study in particular compared the performance of a graphene anode to a zinc-oxide nanowire graphene hybrid anode. The results (see Fig. 1 at right) demonstrate the positive synergistic effects of the zinc-oxide nanowires evenly distributed throughout the graphene sheet, thus preventing the agglomeration that inhibited Li-ion diffusion.
At present, graphite is the material of choice for Li-ion batteries due to its Li storage capability via Li intercalation between layers of graphite. However, this kind of battery's low capacity renders it unsuitable for use in instances such as the powering of electric vehicles.  Given the expected increase in the global electric vehicle market alone, further exploration of graphene-based Li-ion batteries warrants investment of time and resources.
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