Rechargeable batteries such as Li-ion batteries, Li-S batteries, and Na-ion batteries hold a great place in the development of renewable energy systems. The existent drawbacks of these batteries require novel solutions in order to meet expectations. Graphene is considered to be a promising material for these developments. The high conductivity, high surface areas, and mechanical flexibility of graphene are greatly utilized in rechargeable batteries.
The ever increasing energy requirements of the modern world have pushed scientists to develop novel solutions to the problems on the way to necessary improvements. Renewable energy sources, clean substitutes to important polluters such as cars, and efficient energy storage systems have been the center of attention. Energy storage applications take a bridging role between novel energy systems and energy-efficient machines such as electric cars. So far Lithium-ion batteries have been the most prevalent commercially available batteries in the market. However, their limited theoretical capacities, high cost, and scarcity of the Li element compel the science community to look for different options. Enhancing the capacity of Li-ion batteries, reviving the ancient Li-S batteries, and developing new technologies such as sodium-ion batteries (SIBs) are promising options under investigation.
Lately, graphene has become the star player of these improvements. Graphene is actually a one-atom thick sheet of carbon, arranged in an sp2-bonded hexagonal network. This seemingly simple structure possesses various excellent physical and chemical properties. Graphene provides atomic thickness, unique electronic properties, high mechanical strength, high thermal conductivity and large surface area (2630 m2 g−1).
The unique properties of graphene are utilized in developing efficient electrodes for novel energy storage applications. The high electron mobility (2.5 × 105 cm2 V−1 s−1) and high surface area of graphene facilitate the improvement of charge capacity for Li-ion, Li-S, and Na-ion batteries. The flexible nature of graphene buffers the volume fluctuations during the charging/discharging cycles.
Conventional lithium-ion battery (LIB) technology includes graphite anodes and LiCoO2 cathode. Despite the fact that these batteries show significant energy density and charge/discharge capacity, they fall behind in satisfying the energy demands of recent technological developments. Therefore, the development of different anode materials with better charge storage capacity and promising cyclic stability is required. Amongst the variety of carbon materials; graphene, graphene oxide (GO) and their derivatives have become the center of attention because of their highly conductive nature, high surface area (>2000 m2/g), and good charge carrier mobility. These materials promote Li-ion mobility and electron transfer while the high diffusivity of Li on graphene enhances the rate capability. The theoretical capacity of graphene (764 mAh/g) is found to be twice that of the traditional graphite electrodes. The electrical properties of GOs are easily tuned by changing the concentration of oxygen-containing groups in their structure. Another advantage of GO is the increased number of active sites for anchoring electroactive materials. While graphene and GO can be directly used as electrodes it is also possible to modify these structures through incorporating metal oxides, other carbon-based materials such as CNTs or creating defects in their structures through different reduction strategies, heteroatom doping, and co-atom doping. All of these methods aim to increase the charge capacity and cycling stability of LIBs. Even though the majority of studies have focused on graphene-based anodes, graphene-based cathodes have also recently been attracting attention in the science community.
For anode materials graphene can be used in different forms such as graphene nanosheets GNSs), carbon nanotubes (CNTs), carbon nanoribbons (CNRs) or a combination of these structures. Disordered graphene structures with defects and edges are also utilized in LIB anodes increasing the reversible capacity to 794–1054 mAh g−1. GOs are also promising materials with the first charge capacity of 1400 mAh/g. However, they have poor cycling stabilities. In order to enhance charge capacity and cycle stability of graphene-based anodes, several different materials are incorporated into graphene and GO structures. The most common materials are metals, metal oxides, and sulfide nanoparticles. Different studies have found that ZnO, MoS2, Fe3O4, and several materials with similar nature have the potential to enhance Li-ion batteries significantly. In addition to the above methods, doped graphene/GO structures are also investigated for the LIB anodes. N-doped graphene/GO structures with metal oxides have provided high capacities and cycling stabilities.
Although graphene-based LIB cathodes are not as widespread as graphene-based LIB anodes, researchers have recently started to investigate this potential. Some of the most commonly studied cathode materials for lithium ion batteries are LiCoO2, LiMn2O4, and LiFePO4. The relatively low electrical conductivity of these materials is often compensated by incorporating some additives such as carbon black. However, they still require improvements. Graphene is an excellent cathode material due to its high conductivity, high surface area, and wide electrochemical potential window. High surface area facilitates ion mobility and provides a substrate for the growth of metal oxide/mixed metal oxides with improved rate performance. Graphene composites with other carbon materials increase the ion transfer rate and Li+ diffusivity. The major drawbacks of graphene-based LIB cathodes are high volume and SEI formation. Because of these drawbacks, the improvement of these systems is necessary.
As the theoretical capacities of LIBs are incapable of satisfying the ever-increasing energy demand our world different types of batteries have attracted attention. Moreover, LIBs are still due to their high Li content. Lithium-sulfur (Li-S) batteries are considered to be one of the most promising alternatives to LIBs due to their extremely high theoretical capacities and abundance of sulfur. Li-S batteries have been around for a while. So far commercial Li–S battery offers specific energy of over 350 Wh kg−1which significantly higher than commercial LIBs (150–200 Wh kg−1). Over 600 Wh kg−1 in specific energy is believed to achieve in the foreseeable future.However, their improvement has been hindered by its quick capacity decay and short lifespan because of the insulating nature of sulfur/Li2S and the high solubility of lithium polysulphides. The high solubility of lithium polysulphides creates a “shuttle effect” and deteriorates both anode and cathode. Moreover, the conversions between sulfur and Li2S result in a 70% volume change creating cracks in the structure. Graphene and its derivatives have the potential to overcome these problems. Graphene and GO can be used in cathodes, anodes, and the interlayer of Li-S batteries. High conductivity of graphene facilitates the electron transfer and compensates the insulating nature of sulfur while its flexibility and mechanical robustness buffer the large volume changes during charge/discharge cycles. The porous structure and high surface area of graphene provide a suitable platform for sulfur loading. GO contains various different functional groups that have the ability to capture polysulphides and increase the efficiency of Li-S batteries. The polysulphide capturing properties of graphene, GO, and their derivatives can be enhanced by doping or functionalizing with different functional groups or heteroatoms. N-doping, incorporation of other carbon materials such as CNTs, and graphene-polymer composites are commonly studied methods for the enhancement of Li-S batteries.
Another alternative to LIBs is considered to be sodium-ion batteries (SIBs). Especially the low cost and abundance of Na are attractive compared to LIBs. They will be suitable for most kinds of energy storage applications, but particularly the so-called “large format” applications, including stationary energy storage such as grid storage, renewable energy storage as wind and solar power, backup systems as uninterrupted power supplies, and automotive. Various suitable cathode options such as sodium manganese hexacyanoferrate, Na3V2(PO4)/carbon composite, and Na3V2(PO4)/G composite have been suggested as cathode materials for SIBs. However, the development of anode materials is still in progress. Graphene and graphene-based materials are found to be suitable for SIB cathodes. Graphene can act as a support for electroactive nanomaterials, and hinder their re-stacking by lowering the van der Waals forces among the layers. Moreover, the extensive, elastic and highly conductive graphene improves the electrical conductivity of the composite and buffers volume expansion of electrode materials during cycling. Similar to LIBs and Li-S batteries, the functionalization and doping of graphene further enhances the capacity and cycling stability of SIBs. rGO nanosheets are also found to be excellent host material for Na ions providing reversible capacity as high as 174.3 mAh g−1 at a current density of 40 mAg−1.
What is the Significance of Graphene in Rechargeable Batteries?
The urgent need for improvements in the energy industry encourages scientists to develop new methods. Without a doubt, rechargeable batteries are an integral part of ever-growing energy systems. The most important rechargeable batteries in these systems are Li-ion batteries, Li-S batteries, and Na-ion batteries. Scientists have been utilizing graphene for the development of these batteries. The 3D structure of graphene provides an electron conducting network due to its high conductive nature and high surface area. Increased electron conductivity consequently improves rate capability and cycle stability of the batteries.