The battery is a common power source for many household and industrial applications. Many things we use every day contain them, notably computers and mobile phones. Unfortunately, as computers run much faster and smart phones become much smarter, the battery life seems to keep getting shorter and shorter. Battery advancement cannot keep pace with Moore's law in the computer industry. So scientists and engineers never stop seeking more powerful and lighter batteries, just as they have done since Alessandro Volta invented the voltaic pile 1800. Many new technologies, such as Ni-MH and Li-ion, enormously increased battery performance in the late 20th century.
"There's plenty of room at the bottom." - Richard Feynman. Nanotechnology has become the new driving force of battery advancement in the 21st century. A great variety of new battery designs based on nanostructures have recently been proposed. In this report, I will discuss several nano-battery examples.
Li is the most electropositive (3.04 V against the standard hydrogen electrode) and the lightest (equivalent weight 46.94 g/mol, specific gravity 0.53 g/cm3) metal, which makes it in some sense an optimal battery cathode material. [1] Silicon is an attractive anode material for lithium batteries because it has a low discharge potential and the highest known theoretical charge capacity (4,200 mAh/g). [2] Although this is more than ten times higher than commonly used graphite anodes, silicon anodes have few applications because silicon's volume changes by 400% upon insertion and extraction of lithium which results in pulverization and capacity fading. [2,3] However, by using silicon nanowires as anode material, facile strain relaxation in the nanowires allows them to increase in diameter and length without breaking during cycling. [4] In addition, silicon nanowires also provide shorter distance for Li diffusion and continuous electron pathways. [4] Thus high power batteries can be made with silcon nanowire anodes. [4] C. K. Chan et al. achieved the theoretical charge capacity for silicon anodes (4,200 mAh/g, that is 10× higher than a traditional graphite anode 370mAh/g) and maintained a discharge capacity close to 75% of this maximum, with little fading during cycling. [4]
Current cathode materials, such as those based on transition metal oxides and phosphates, have an inherent theoretical capacity limit of 300mAh/g. A maximum practically usable capacity of only 210mAh/g has been reported. [5] The theoretical capacity of sulfur is 1672 mAh/g. The corresponding theoretical specific energy of the lithium-sulfur batter is 2600 Wh/kg. [6] However, sulfur-based cathodes present a variety of problems, including low electronic conductivity, significant structural and volumetric changes during reaction, and dissolution of lithium polysulfides in the electrolyte. [7] Highly ordered mesoporous carbon exhibits a uniform pore diameter, very high pore volume and interconnected porous structure, so it can exhibit high conductivity. By heating up the mixture of highly ordered mesoporous carbon and sulphur the melt is absorbed into the channels by capillary action, whereupon it solidifies and shrinks to form sulphur nanofibres that are in intimate contact with the conductive carbon walls. [8] The carbon framework not only acts as an electronic conduit to the active mass within, it also serves as a mini-electrochemical reaction chamber. Hence, this lithium-sulphur battery with nanostructure carbon framework can overcome current limitations. The composite materials reported by X. L. Ji et al. can supply up to nearly 80% of the theoretical capacity of sulphur (1,320 mAh/g), representing more than three times the energy density of lithium transition-metal oxide cathodes, at reasonable rates with good cycling stability. [8]
There are all kinds of nano-batteries nowdays. For instance, A. Kiebele and G. Gruner have successfully developed a "nanotube ink" for manufacturing flexible batteries using printed electronics techniques. [9] Many nano-batteries have been commercialized. A123 system, Altair, Altairnano, Toshiba, U.S. Photonics and many other companies have their own nano-battery products. By using different nano-materials, better performance in various aspects can be achieved. Some nanotechnology can increase the available power from a battery and decrease the time required to recharge a battery. Other nanomaterials can increase the shelf life of a battery or reduce the possibility of batteries catching fire. Nano-batteries are the future of batteries.
© Shuang Li. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] J. M. Tarascon and M. Armand, "Issues and Challenges Facing Rechargeable Lithium Batteries," Nature 414, 359 (2001).
[2] B. A. Boukamp, G. C. Lesh and R. A. Huggins, "All-Solid Lithium Electrodes With Mixed-Conductor Matrix," J. Electrochem. Soc. 128, 725 (1981).
[3] U. Kasavajjula, C. Wang, and A. J. Appleby, "Nano- and Bulk-Silicon-Based Insertion Anodes for Lithium-Ion Secondary Cells," J. Power Sources 163, 1003 (2007).
[4] C. K. Chan et al., "High-Performance Lithium Battery Anodes Using Silicon Nanowires," Nature Nanotechnology 3, 31 (2008).
[5] M. S. Whittingham, "Lithium Batteries and Cathode Materials," Chem. Rev. 104, 4271 (2004).
[6] Y. V. Mikhaylik and J. R. Akridge, "Polysulfide Shuttle Study in the Li/S Battery System," J. Electrochem. Soc. 151, No. 11, A1969 (2004).
[7] Y. Yang et al., "New Nanostructured Li2S/Silicon Rechargeable Battery with High Specific Energy," Nano Lett. 10, 1486 (2010).
[8] X. L. Ji, K. T. Lee and L. F. Nazar, "A Highly Ordered Nanostructured Carbon-Sulphur Cathode for Lithium-Sulphur Batteries," Nature Materials 8, 500 (2009).
[9] A. Kiebele and G. Gruner, "Carbon Nanotube Based Battery Architecture," Appl. Phys. Lett. 91, 144104 (2007).