Development of metal alloy anodes

Abstract

Since the beginning of the 1990s we have witnessed the development of lithium-ion batteries (LIB), which currently are the power source of choice for most portable electronic devices, such as cell phones, digital cameras, and laptop computers. Although these batteries offer the highest specific energy density, the demand for greater electronic performance is continuing to place increasing pressure on their storage capabilities. Recently, many ongoing research activities have focused on the development of second-generation cathode and anode materials with enhanced capacity or cyclic performance. Although the specific capacity (in mA h g-1) of the cathode material is often lower (about half that of anode material), it is unlikely that significant advancement can be made in the near future. On the contrary, materials with theoretical capacities many times higher than that of carbonaceous anodes exist. Advanced LIB therefore can be designed using high-capacity anode material to accommodate for low-capacity cathode material in the limited volume of a battery interior. The advantage of using higher-capacity anode material can be easily quantified. Considering the capacity (CC) of a given cathode material, this may vary from ca. 140 (LiCoO2, spinels) to ca. 200 mA h g-1 (LiMnO2 and its derivatives); there is a simple relationship between the total capacity of the LIB material as a function of anode specific capacity (CA). Qualitative expression of total specific capacity with the variation in CA can be expressed as (Equation presented) Equation (11.1) shows that for a fixed cathode-specific CC, the total specific capacity does not increase linearly with the linear increase in CA. A simulation based on (Graph presented) (11.1) is plotted in Fig. 11.1 ; total capacity increases quickly with an initial increase in CA then a plateau-like region appears. This behavior can be further confirmed by exploring the limit of CA → ∞ (Equation presented) The rate at which the total capacity increases will depend on the value of CC. As seen in the figure, there are two patterns of increase: A comparatively fast increase in total capacity with increasing C A from 300-1,200 mA h g-1 followed by a "tail" with a smaller slope when CA exceeds ca. 1,200 mA h g-1. Therefore, the most noticeable improvement in LIB will be seen if the presently used carbonaceous anode is replaced with one having a capacity in the order of 1,000 mA h g-1. The driving force of such studies is based on the fact that the potential of the anode vs. Li+ should be close to 0 V. Hence, the electrochemical reaction at the anode site should not necessarily be based on an intercalation type of reaction. Alloying of Li+ with certain metals is very attractive, because the Li:M mole ratio in the Li x M alloy at the end of charge might be much higher than in the case of intercalation hosts which generally cannot accommodate and release large amounts of Li+ in order to maintain a stable crystal structure over the cycles. Numerous metals - Si, Al, Sn, Sb, Ge, Pb, Ag, etc - are able to alloy electrochemically with lithium when polarized to a sufficiently negative potential in a Lix -containing liquid organic electrolyte. The respective solid phases LixM that form during this electrochemical alloying exhibit high theoretical capacity. From a practical point of view tin, silicon, and their alloys are the most attractive candidates, because they not only offer high theoretical capacity, but also are abundant and environmentally friendly. Unfortunately, most high-capacity anodes based on the aforementioned elements are intrinsically unstable during cycling and their advantages often are undermined by their short cycle life. The lack of cycling performance is due primarily to mechanical stress induced in the material during repetitive charge and discharge operations where large changes in the specific volume of the material occur. The goal of this chapter is to give an overview on silicon-based anode materials for ambient temperature rechargeable LIB. A brief chronological survey of lithium alloying anodes will introduce the principle concepts to overcome the problems with the dimensional instability of the metallic host materials. Recent work on promising (composite and thin film) silicon-based lithium-alloying electrodes will be highlighted. © 2009 Springer Science+Business Media, LLC.