The use of graphene electrodes may lead to new means of storing energy for electric vehicle, renewable energy and smart grid applications. Researchers from the two US American companies Nanotek Instruments and Angstrom Materials propose in a recent article published in prestigious Nano Letters an approach aimed on combining the advantages of batteries and electrochemical double-layer capacitors (supercapacitors) offering both high-power and high-energy densities.
Scenarios of the mobility and energy power supply of the future strictly relate the success to the excistence of suitable energy storage devices. Currently, the main strategies are seen in supercapacitors, in lithium-ion batteries or their combination. Both solutions come along with challenging disadvantages. While lithium-ion batteries offer high-energy densities with low power densities, supercapacitors provide high-power densities, but with low energy densities. However, it would be wishful to have at hand simultaneously high-power and high-energy densities. Hence, strong research activities are present for the improvement of the single approaches, but also new approaches are under investigation.
Supercapacitors, despite conventional capacitors do not rely on a dielectric. The capacitor’s electrolyte, due to the electrical double layer effect, leads to an effective separation of charge even though the physical separation of the layers is vanishingly. Generally, supercapacitors offer advantages such as high-power densities, long life times, simple charging circuits, high degree of safety, and low costs. However, disadvantages include low amounts of energy stored per unit weight, high self-discharge, and low maximum voltage.
Lithium-ion batteries can be generally described by their three primary functional components, namely the anode, the cathode and the electrolyte. The non-aqueous electrolyte is most commonly a mixture of organic carbonates and contains lithium ions. The cathode is based on a metal oxide and the most common anode material is graphite. These batteries require the diffusion of lithium ions between the anode and the cathode and the possibility of these ions to migrate from or to the anode and cathode material. Hereby, the low solid-surface diffusion rates limit the maximum power-density. Research activities have been focused on applying the nanotechnologies in order to increase the power-density characteristics of lithium-ion batteries. However, progress is slow.
Hence, Jan an collaborators have chosen a new approach relying on nanostructured graphene as the electrode material. The porous nanostructured graphene is attached both to the anode and the cathode and separated by a porous separator. In addition, there are immersed in the electrolyte. The flow of currents is based on the exchange of lithium between the surface of the two nanostructure graphene electrodes. The graphene surfaces can rapidly and reversibly capture lithium ions through surface adsorption and/or surface redox reaction.
The authors have experimented using varying graphene structures comprising graphene from chemically reduced graphene oxide nanosheets, mesophase carbon-derived graphite, or carbon fibers. The results are promising and the so far published data reveals that the proposed approach allowed storing an energy density up to 160 Wh/kg per unit cell. This means a 30 times higher energy density with respect to conventional supercapacitors and only slightly lower than that of lithium ion batteries. In addition, the achieved power densities of almost 100 kW/kg per unit cell were about 10 times higher than that of conventional supercapacitors and around 100 times higher than that of conventional lithium-ion batteries. The researchers declare that the materials and the structures are yet to be optimized and the underlying functional principles still need to be further elucidated and understood. While these results seem promising only further research activities will allow to learn whether these types of lithium cells may be applied to industrial applications in the future.
Jang et al.
Graphene Surface-Enabled Lithium Ion-Exchanging Cells: Next-Generation High-Power Energy Storage Devices
Nano Lett. 2011, 11, 3785–3791