New Si Nanofiber Mat Designs for Li-Ion Battery Anodes

Abstract

Electrospinning is a convenient technique for the fabrication of non-woven mats of micron/sub-micron diameter fibers. Electrospun mats of nanoparticle/polymer fibers have a broad range of applications, including their use as fuel cell and battery electrodes.1,2 Electrospun fiber electrodes for Li-ion batteries exhibit characteristics superior to those of conventional slurry cast electrodes, including: (i) a high interfacial electrode/electrolyte area with short Li+ ion diffusion paths in the radial-fiber direction and (ii) an adjustable inter-fiber void space (porosity) which enhances electrolyte infiltration and allows for the fabrication and use of thick electrodes. Pintauro and co-workers reported on several types of electrospun electrodes for Li-ion batteries: anodes3-5 with TiO2/C, Si/C or carbon powder and a cathode with LiCoO2/C particles1, where the polymer binder was either poly(acrylic acid) (PAA) or poly(vinylidene fluoride). The present work extends the above studies by focusing on dual-fiber systems where separate Si/PAA and C/polyacrylonitrile (PAN) fibers are co-electrospun to form a porous mixed fiber electrode. The dual interpenetrating fiber network morphology is expected to have increased robustness due to interlocking of the dimensionally variable Si/PAA fibers within an electrically conductive cage of C/PAN fibers. Preliminary results are very encouraging. For example, an electrode with 57 wt.% Si-PAA fibers (40 wt.% PAA) and 43 wt.% C-PAN fibers (39 wt.% PAN) cycled in a half cell at 0.1C showed a terminal discharge capacity of 1000 mAh/g (gravimetric) at 2.4 mAh/cm2. The terminal discharge capacity at 1C was 611 mAh/g (gravimetric) and 1.5 mAh/cm2 (areal). During this talk, details will be given on the fabrication methods of these new composite anodes, along with their electrochemical performance and capacity during charge/discharge cycling at various C-rates. The effects of fiber composition (particle/binder weight ratio and the relative weights of Si and C fibers in a dual fiber anode) on gravimetric, volumetric, and areal capacities will be discussed. Acknowledgements: This work was funded by the United States Department of Energy, Vehicle Technologies Office, grant No. DE-EE0007215.