ABSTRACT
Lithium-ion batteries are the state-of-the-art rechargeable power sources and have been widely used not only for portable equipment such as cellular phones, personal computers, and digital cameras, but also power tools and industrial applications like robot and electric vehicles. With the improvement of material and battery technologies, the battery capacity has doubled since its commercialization in the early 1990s while its basic concept and chemistry has remained unchanged. Recently we have seen this improvement slow down, as it is approaching the theoretical limit of the system. A new battery system is necessary to meet the rigorous demands of energy and power increments for the progress of multi-functional mobile equipment and to secure sufficient mileage of an electric vehicle per charge. Under these circumstances, alloy anode materials are expected to be promising candidates to remarkably increase the
capacity of lithium-ion batteries. In this chapter, the potential and benefits of these alloy anode materials and recent breakthroughs that have been achieved are introduced. 4.1 IntroductionLithium metal forms a highly attracting anode that makes a high-energy density battery system due to its low potential and intrinsically high gravimetric electrochemical capacity.1 However, lithium metal electrode has poor electrochemical cycle efficiency, resulting in lower cycle performance, and it also been facing serious battery safety problem, as lithium metal tends to form dendrite during the charging process and, therefore, its practical use seems to be difficult in current technologies. Some elements and alloys have been known to react with lithium.2,3 Much effort and different strategies have been put into lithium alloys in order to overcome these problems of lithium metal anode and deal with the volume changes that occur during cycling.4-21 Despite much effort, none of them has succeeded in obtaining the electrode that has sufficient cycle number without sacrificing high capacities of alloy electrodes. In the meantime, it was demonstrated that lithium can be reversibly inserted into graphite at room temperature in an organic electrolyte in 1983.22 Lithium-ion battery was first commercialized with this carbon-based anode in 1991. Graphite has a capacity of 372 mAh/g, corresponding to the intercalation of one lithium atom per six carbon atoms.23-27 Though carbon has rather lower capacity than lithium metal and lithium alloy anodes, volume change was small and represented longer cycle performance. After this commercialization, many researchers and companies have put their effort on new carbonaceous anodes. The advantage of lithium-ion battery is their high energy density. They can store more energy than conventional nickel/cadmium and nickel/metal hydride batteries. Lithium-ion batteries have clearly established their position as a rechargeable battery technology of choice for cellular phones and notebook computers. And now they are also taking over the world of power tools. Most representative commercialized lithium-ion batteries use a layered compound, such as lithium cobalt oxide, as the positive electrode and graphite as the negative electrode. The battery energy density has doubled so far
with the improvement of material and battery technologies without changing its basic concept and chemistry and has been approaching the theoretical limit of the system. Further energy and power improvement is, however, necessary to meet the rigorous demand of the state-of-the-art functional equipment for future mobile society. Besides, global attempts and development have been energetic to use lithium-ion batteries for hybrid, plug-in hybrid, and electric vehicles toward future extinction of oil resources. For hybrid and plug-in hybrid vehicle market, higher output/input characteristic is necessary for the battery system to assist the engine and accept the recovering energy in a short time in a wide range of temperatures. Especially for hybrid electric vehicle application, graphite electrode has a potential safety hazard for lithium deposition due to its kinetic limitation of lithium acceptability at lower temperatures and the lower potential that is too close to that of elemental lithium. For electric vehicle application, immediate representative chemistry is the lithium manganese oxide spinel cathode/natural carbon anode to ensure the battery power and lower the cost of materials. However, superior new system is intrinsically reputed in securing the sufficient mileage of an electric vehicle per charge and power at wide range of temperatures. Thus, the applications do not stop at current existing technologies and remarkable improvement of energy and power densities for lithium-ion batteries is an important theme on which the market can develop. From the energy increment point of view, it is important and desirable to improve both cathode and anode materials. There are a few cathode materials that have higher capacity than lithium cobalt oxide, but their expected capacity progress is in the range of tens of percents at most. On the other hand, there are some candidates for anode materials to have very high specific capacities. Consequently research interest is being focused on alloy anode system and considerable research and development works have been conducted on lithium alloy systems over the last decades. After 2000, nanotechnology has been in the limelight in global technologies, and nano-lithium or nano-alloy anodes have also been energetic. In this chapter, the issues of lithium and the potential of alloy anode materials are briefly touched upon along with recent breakthroughs that have been achieved. Future development trend and its challenge have also been stated.
