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Nanoscale Materials for Lithium-Ion Batteries

Charles R. Sides, Naichao Li, Charles J. Patrissi, Bruno Scrosati, and Charles R. Martin Abstract Template synthesis is a versatile nanomaterial fabrication method used to make monodisperse nanoparticles of a variety of materials including metals, semiconductors, carbons, and polymers. We have used the template method to prepare nanostructured lithium-ion battery electrodes in which nanofibers or nanotubes of the electrode material protrude from an underlying current-collector surface like the bristles of a brush. Nanostructured electrodes of this type composed of carbon, LiMn2O4, V2O5, tin, TiO2, and TiS2 have been prepared. In all cases, the nanostructured electrode showed dramatically improved rate capabilities relative to thin-film control electrodes composed of the same material. The rate capabilities are improved because the distance that Li must diffuse in the solid state (the current- and power-limiting step in Li-ion battery electrodes) is significantly smaller in the nanostructured electrode. For example, in a nanofiber-based electrode, the distance that Li must diffuse is restricted to the radius of the fiber, which may be as small as 50 nm. Recent developments in template-prepared nanostructured electrodes are reviewed.

nanotubes for anode applications,37,38 while vanadium oxide nanotubes have been considered as cathode materials.39,40 Finally, aerogels, highly porous structures created by removing water under supercritical conditions, have emerged as a promising class of nanostructured battery cathode materials.41–45 Nanoparticle electrodes mitigate the problem of slow diffusion because the distance that Li must diffuse in the solid state is limited to the radius of the nanoparticle. We have investigated electrodes of this type prepared by the template method.22–34 This method entails synthesis of the desired material within the cylindrical and monodisperse pores of a microporous membrane or other solid (Figure 1). We have used microporous polycarbonate filters, prepared by means of the track-etch method46 (Figure 1a), and microporous aluminas, prepared electrochemically from Al foil47 (Figure 1b), as our template materials. Cylindrical nanostructures with monodisperse diameters and lengths are obtained, and depending on the membrane and synthetic method used, these may be solid nanofibers or hollow nanotubes.

Keywords: energy storage, nanofibers, rechargeable lithium batteries, template synthesis.

Introduction Since their introduction by Sony in 1991, lithium-ion batteries have rapidly taken over the high-performance rechargeablebattery market.1–4 These batteries can store two to three times more energy per unit weight and volume than lead-acid or Ni-Cd batteries, have long cycle lives (
1000 cycles), low self-discharge, and long shelf life. In spite of this success in the commercial marketplace, there is currently an enormous international research effort aimed at improving Li-ion battery technology. This effort focuses on all aspects of the batter