Since the announcement of the 2010 Nobel Prize in physics, graphene has received considerable attention from researchers worldwide. In 2004, Dr. Andre Geim at the University of Manchester successfully used micromechanical techniques to isolate single sheets of graphene from highly ordered pyrolytic graphene. Graphene’s unique properties have made it a highly attractive topic of research.
What is graphene, anyway?
Graphene is a hexagonal, two-dimensional allotrope of carbon. It can be formed into zero-dimensional fullerines, one-dimensional carbon nanotubes, and stacked to form three-dimensional graphite. As such, graphene is a basic component for other graphite materials. Graphene possesses many special properties. It has a tensile strength of 130GPa, 100 times stronger than steel. Its thermal conductivity is 500W·m-1K-1, three times that of diamond. Its electron mobility is 15000cm2·V-1s-1, over ten times greater than commercial silicon. At 2630m2·g-1, it has an extremely high specific surface area. It is conductive at room temperature, and functions much more quickly than present-day conductors.
Graphine itself is only one carbon atom wide. It is theorized to have a large specific surface area, high conductivity, and a honeycomb-like structure, which is what gives it potential for use in lithium-ion batteries. The proposed applications include direct use as an anode; use in tin-based, silicon-based, or transition metal anode composites; use as a composite in lithium iron phosphate cathodes; or use as a conductive additive.
(1) Use of graphene as an anode in lithium-ion batteries
Because graphene is composed of a single atomic layer of carbon, lithium ions can be placed between two layers of graphene to create Li2C6, a superior electrode material (with an energy density of 744mAh·g-1) compared to traditional carbon anodes. The lithium ions are stored in the spaces between the graphene sheets. It is this morphology and structure that determine the effectiveness of graphene as an anode material.
Because pure graphene has a low coulombic efficiency, a high charge-discharge platform, and low cycle stability, graphene in itself is unlikely to replace existing carbon-based commercial materials currently used in lithium-ion battery anodes. Moreover, graphene sheets stacked together lose the advantage of a large surface area to store lithium ions. However, graphene makes an excellent composite material for electrodes.
(2) Graphene-composite anodes
Graphene is highly conductive, demonstrates high mechanical strength, flexibility, and stability, and possesses a high specific surface area. In particular, chemically transformed graphene has a high number of functional groups, making it useful as a substrate for composite electrodes.
Graphene composites with tin, silicon, and transition metals have already been researched in depth. Tin and silicon-based electrodes, when doped with graphene, can maximize synergies between the two materials. Graphene can reduce the size of the active material, prevent the agglomeration of nanoparticles, improve electrical and ionic transmission, and improve mechanical stability. These improvements lead to better capacity and rate performance, as well as longer lifecycles.
The preparation of graphene-composite electrode materials requires the uniform distribution of nanoparticles on one or multiple layers of graphene. The effectiveness of the composite is determined by the interaction between the two materials. Presently, graphene composite manufacturing is developing to a stage where the morphology of composites can be well-controlled through in-situ and interfacial reactions. Nonetheless, easier and more effective preparation methods are crucial for the future application of graphene in lithium-ion batteries.
(3) Applications of graphene as cathode material
Conductivity of cathodes is a major limit to the effectiveness of a battery. Many cathode materials – particularly in cases of large electrical discharge – demonstrate lower capacity than should theoretically be the case. As a result, researchers hope that graphene’s unique surface area and conductive properties will improve the conductivity of cathode materials and increase lithium ion transmission. Adding graphene into the cathode mix reduces interfacial resistance between the electrolyte and active cathode material, and improves Li+ transmission. At the same time, graphene placed on the surface of the cathode prevents metal oxides from dissolving or transforming, thereby maintaining structural stability.
Graphene is used most commonly with lithium iron phosphate cathodes. In these composites, graphene functions as a current collector coating and conductive additive. Graphene’s two-dimensional conductive surface provides a highly active and conductive electrode, thereby improving the battery’s conductivity and rate performance.
(4) Graphene as a conductive additive
In order to improve conductivity, conductive additives such as graphite, acetylene black, and Super P are added to battery electrodes. As a carbon material, graphene is also very effective as a conductive additive for lithium-ion batteries.
On carbon-based anodes, graphene provides more consistent conductivity throughout many discharge cycles regardless of the presence of active substances which would otherwise interrupt conduction, as compared to acetylene black. This allows for better cycling and rate performance.
On lithium iron phosphate cathodes, Super P uses small particles to fill gaps in the cathode, thereby improving conductivity. By comparison, graphene’s flexible surfaces achieve greater conductivity with fewer materials.
The biggest barrier to adoption of graphene conductive additives is that its high-rate performance is yet lacking. Current research has been limited to improving cycling and capacity under low rate conditions.
Since graphene was first isolated, it has shown usefulness in many applications –electrochemical batteries, optoelectronics, catalysts, and more. There is further potential for applications in hydrogen storage, supercapacitors, lithium-sulfur batteries, lithium-air batteries, and other technologies. Looking forward, graphene will benefit from increasing scale, lower costs, better manufacturing methods, and improved functionality. CNESA is keeping a close eye on further developments and future applications for graphene.