High Performance Nanofiber Electrodes for Li-Ion Batteries Using Particle/Polymer Electrospinning

Abstract

The demand for rechargeable batteries with high energy and power densities has never been greater. The functionality of many portable electronic devices is limited by battery size and lifetime. Similarly, electric vehicle propulsion systems require significant improvements to satisfy consumer expectations for long drive distances and short recharge times. To meet the demands of energy-hungry consumers, new battery electrodes with high capacities and long cycle life at fast charging rates must be developed.[1-2] These requirements have motivated intense research efforts to develop next-generation Li-ion battery materials, in particular replacement designs for conventional slurry cast electrodes. One approach is the use electrospun electrodes containing engineered void spaces to enhance Li+ transport rates for high capacities at fast charge/discharge rates. Battery researchers largely focus on preparing new electrodes with high gravimetric capacity, but this metric alone is insufficient to assess a material’s applicability for consumer devices. An electrode’s volumetric capacity is another important measure for applications, such as electric vehicle propulsion, where battery space is limited. Likewise, electrodes must have high areal capacities to limit the amount of inactive components (e.g., current collectors and separators) that contribute to the overall battery size and weight. Because gravimetric, areal, and volumetric capacities are important and complementary measures of battery performance, all three should be examined to fully assess the strengths and weaknesses of new electrode materials and designs. The present work is the latest installment by Pintauro and co-workers on the use of polymer/particle electrospun nanofiber mats for fuel cell and Li-ion battery electrodes.[1-5] While previously reported electrospun battery electrodes have shown impressive performance, these materials were prepared by carbonizing polymer fiber precursors at high temperature. In particle/polymer electrospinning, there is no fiber pyrolysis; the electrospinning and post-processing are performed at room temperature to preserve the beneficial characteristics of the polymer binder that holds electrode particles together. Due to the these characteristics, high gravimetric, areal, and volumetric storage capacities can be achieved at fast charge/discharge rates through the use of thick, densely packed particle/polymer nanofiber mats. Scanning electron microscopy images of as-spun and highly compacted carbon/poly(vinylidene fluoride) (C/PVDF) nanofiber anodes are shown in Figures 1a and 1b. For comparison, a slurry cast anode of the same composition has a very dense structure with essentially no visible voids (see Figure 1c). The presence of intra- and interfiber voids in the electrospun mat is important for battery operation; electrolyte intrusion throughout these voids ensures rapid Li+ transport between the electrolyte and the active material nanoparticles in the radial fiber direction. In addition to C/PVDF anodes, particle/polymer electrospinning can be used to prepare nanofiber electrodes from new active materials and binders as they are developed. In this presentation, recent results will be presented on the fabrication and use of several electrospun particle/polymer nanofiber anodes (with titania, carbon, and silicon powders) and cathodes (with LiCoO2 powder). Charge/discharge properties of half and full cells will be shown as a function of C-rate. Electrode capacities for different fiber mat thicknesses and mat porosities will also be shown and discussed. Overall, the results will demonstrate that proper micron-scale design of electrodes can enhance Li+ transport rates and thus improve Li-ion battery performance. Acknowledgements This work was funded in part by the National Science Foundation (NSF EPS-1004083) through the TN-SCORE program under Thrust 2.