ABSTRACT
A key challenge in the development of a new generation of lithium batteries for electric and hybrid electric vehicles is to increase energy and power density while maintaining an overall safe and low-cost system. One method being explored is to replace the conventional carbon anode with a host material that can accommodate a much greater lithium density (e.g., Si or Ge). However, these high-capacity electrodes were generally considered unsuitable for rechargeable lithium batteries since they exhibit extremely low cycling rates at room temperature and undergo large structural changes during cycling. Many of these problems can be mitigated through the use of nanoscale and nanocomposite electrode materials. Rapid lithium diffusion and lattice relaxation in nanoscale electrodes is believed to mitigate microstructural damage that is a common problem in bulk lithium alloy electrodes during cycling. In addition,
nanoscale materials tend to circumvent conventional mechanisms of mechanical deterioration. These nanoscale electrodes offer enhanced kinetics, material stability (reversibility), and gravimetric capacity with respect to their bulk counterparts. Lithium alloy anodes such as Li-Si have a real potential to replace the conventional carbon anode, but this development will require a better understanding of how nanostructure affects electro-chemical performance and new methods to tailor or tune these materials on the nanoscale. In this chapter, we present an overview of our research on the electrochemical reaction of lithium with nanoscale Si and Ge electrodes, degradation mechanisms, routes to improve performance, along with an update of recent develop-ments in this area. 2.1 IntroductionWidespread commercialization of plug-in hybrid-electric and all-electric vehicles is dependent upon the development of safe electrical energy storage systems with high energy and power densities. The lithium battery appears to be the best candidate to replace nickel-metal hydride batteries in these vehicles, but a number of challenges remain. A key requirement is the development of new, stable, nontoxic electrode materials that can accommodate greater lithium densities. For the anode, the simplest and greatest energy densities are achieved with pure lithium metal. However, issues of safety resulting from internal shorts that can form during cycling have confined lithium anodes to primary and small rechargeable batteries. Larger, rechargeable lithium batteries require a framework material to host the lithium during cycling. The most common lithium anode is graphite, which is stable upon lithium intercalation up to LiC6 (372 mAh/g). Lithium alloy electrodes (e.g., M + xLi LixM) and lithium conversion electrodes (e.g., MOx + xLi (x/2) Li2O + M) are currently of interest as “next generation” lithium anodes due to their extremely high energy densities (3-10 times greater than that of LiC6) and low operating voltage versus lithium (0-1 V). The average voltage and theoretical capacity of lithium metal, LiC6, and a number of alternative lithium anodes are shown in Table 2.1.
