Li-ion batteries have become the power source of choice for consumer electronic devices such as cell phones and laptop computers.[1±4] This is because these batteries have good rechargeability (1000+ cycles) and offer higher energy density (stored charge per unit volume or mass of the battery) than competing battery technologies. [1,2] However, it is well documented that Li-ion batteries show poor low-temperature performance.[5±9] Specifically, the amount of charge delivered from the battery at temperatures below 0 C is substantially lower than the amount of charge delivered at room temperature. [5,7,8] This precludes the utilization of these batteries in a number of defense, space, and even terrestrial applications.[10]We have been investigating the application of nanotechnology to Li-ion battery electrode design.[11±13] Based on these studies it seemed likely that Li-ion battery electrodes composed of nanoscopic particles of the electrode material could mitigate this low-temperature performance problem. We prove this case here by showing that nanofibers (diameter = 70 nm) of the electrode material V 2 O 5 deliver dramatically higher specific discharge capacities at low temperatures than V 2 O 5 fibers with micrometer-sized diameters. While there is controversy in the scientific literature, [5,7±9] the most likely causes of the poor low-temperature performance are either: 1) diminution in the rates of these electrochemical charge/discharge reactions at low temperatures; or 2) diminution in the rate at which Li + diffuses within the particles that make up the electrode at low temperature. We hypothesized that in either case, an electrode composed of nanoscopic particles (diameters less than 100 nm) would provide better low temperature performance than the~10 lm sized particles [14] used in commercial battery electrodes. This is because an electrode composed of nanoscopic particles would, in general (vide infra), have a higher surface area than an electrode composed of large particles, and this would mitigate the slow electrochemical kinetics problem. Furthermore, the distance that Li + must diffuse within the particle would be decreased for nanoscopic particles, and this would mitigate the slow solid-state diffusion problem.To prove this point we have used the template-synthesis method [15] to prepare cathodes composed of monodisperse V 2 O 5 nanofibers (diameter = 70 nm) that protrude from a current-collector surface like the bristles of a brush (Fig. 1a). We compare the low-temperature charge/discharge performance of these nanofiber cathodes with cathodes composed of V 2 O 5 fibers with diameters of 0.8 lm (Fig. 1b), as well as with cathodes composed of 0.45 lm diameter fibers (Fig. 1c). If our hypothesis is correct, the low-temperature performance of the

We describe here a new type of template-prepared nanostructured LiFePO 4 electrode, a nanocomposite consisting of monodispersed nanofibers of the LiFePO 4 electrode material mixed with an electronically conductive carbon matrix. This unique nanocomposite morphology allows these electrodes to deliver high capacity, even when discharged at the extreme rates necessary for many pulse-power applications. For example, this nanocomposite electrode delivers almost 100% of its theoretical discharge capacity at the high discharge rate of 3 C, and 36% of its theoretical capacity at the enormous discharge rate of 65 C. This new nanocomposite electrode shows such excellent rate capabilities because the nanofiber morphology mitigates the problem of slow Li + -transport in the solid state, and the conductive carbon matrix overcomes the inherently poor electronic conductivity of LiFePO 4 .Lithium-ion batteries are the power source of choice for portable electronics, a multibillion dollar market. 1 This outstanding commercial success has spawned great international interest in applying this technology to systems that demand higher power, such as the electric component of hybrid vehicles. 2 This would require new electrode materials that are less expensive, more energetic, and more environmentally friendly than the present ones. Of particular interest is the olivine-structured LiFePO 4 cathode developed by Goodenough and co-workers, 3 which offers several appealing features, such as a high, flat voltage profile and relatively high theoretical specific capacity ͑168 mAh g −1 ͒, combined with low cost and low toxicity. However, the current designs of cells based on LiFePO 4 technology have not shown the ability to deliver high specific capacity at high discharge rates. For this reason, LiFePO 4 is currently not a promising electrode material for high-rate and pulse-power applications.The discharge reaction for LiFePO 4 ͑Eq. 1͒ entails intercalation of Li + ͑from the contacting electrolyte phase͒ along with an equivalent number of electrons into the electrode materialThe rate capabilities of LiFePO 4 are limited primarily by its intrinsically poor electronic conductivity and by the low rate of Li + transport within the micrometer-sized particles used to prepare the battery electrode. A number of approaches have been proposed to improve this material's inherent poor electronic conductivity, including carbon coating 4 nanofibril textures, 5 optimized synthesis procedures, 6 and foreign metal doping. 7 We describe here a new approach for preparing high ratecapability LiFePO 4 electrodes. This approach builds on the application of the template synthesis method for preparing nanofiber Li-ion battery electrodes. 8-12 However, the method was modified such that the template-prepared LiFePO 4 nanofibers are mixed with carbon particles, and coated by thin carbon films, to yield a nanocomposite LiFePO 4 /carbon matrix. As we have shown previously, 10-12 the nanofiber morphology mitigates the slow Li + -transport problem, because the distance Li...

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.

Two structurally related perfluorinated ionomer materials, one a conventional sulfonic-acid-based ionomer ͑Nafion͒ and the other an experimental bis͓͑perfluoroalkyl͒sulfonyl͔imide-based ionomer in which the sulfonic acid group has been replaced by a sulfonyl imide acid group, were studied in parallel to evaluate their relative utility as membrane materials for use in polymer electrolyte membrane ͑PEM͒ fuel cells. Studies focused on membrane ionic conductivity and water content under varying conditions of relative humidity, and on device-level fuel-cell tests using membrane-electrode assemblies ͑MEAs͒ fabricated from membranes of the two ionomers. The overall finding is that the two ionomer materials behave similarly with respect to their electrochemical properties and performance in PEM fuel-cell devices. In one instance, a sulfonyl-imide-based MEA exhibited substantially improved performance relative to a comparable Nafion-based MEA in fuel-cell tests. The improvement is probably attributable to a combination of favorable materials properties and membrane thickness effects.Perfluorinated ionomers such as DuPont's Nafion and other closely related perfluorosulfonic acid ͑PFSA͒ ionomers are leading candidates for use as membrane materials in polymer electrolyte membrane ͑PEM͒ fuel cells. They possess many of the desirable qualities required for a successful PEM fuel cell, namely, high protonic conductivity, good mechanical properties, and excellent longterm chemical stability. Despite the desirable qualities of these materials, however, they also possess certain limitations, among which is a tendency toward diminished protonic conductivity under conditions of low water availability. 1 Several members of a related class of ionomers based on the bis͓͑perfluoroalkyl͒sulfonyl͔imide acid group have been synthesized and characterized by DesMarteau and co-workers. 2-4 The perfluorosulfonyl imide group is known to possess stronger gas-phase acidity 5 and improved thermal stability relative to the perfluorosulfonic acid moiety. Additionally, enhanced oxygen reduction kinetics have been reported in phosphoric acid fuel cells ͑PAFCs͒ when a small amount of a monomeric bis͓͑perfluoroalkyl͒sulfo-nyl͔imide species was used as an electrolyte additive. 6 Recent work by Sumner and co-workers has established that sulfonyl imide-based ionomers ͑Fig. 1b͒ exhibit ionic conductivities that are at least as high as that of Nafion ͑Fig. 1a͒ ionomers of similar structure and equivalent weight. 4,7 Thus, there is reasonable expectation that ionomer materials based on the perfluorosulfonyl imide moiety are good candidates for use as membranes in PEM fuel cells.Work presented in this paper explores the similarities and differences between perfluorosulfonylimide ionomers and perfluorosulfonic acid ionomers as membrane materials for use in PEM fuel cells. Membrane-electrode assemblies ͑MEAs͒ were fabricated from Nafion and the experimental sulfonylimide ionomer membranes using the thin-film decal transfer method. 8 Performance of the MEAs was te...