Effects of Vapor Grown Carbon Fiber Substitution for Conductive Carbon in Anode Systems for LiB Applications
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Effects of Vapor Grown Carbon Fiber Substitution for Conductive Carbon in Anode Systems for LiB Applications Yu Kambe1, Alberto J. Fernandes,1 Lynden A. Archer2 1
Cornell U. Department of Materials Science and Engineering, Ithaca NY, 14853, U.S.A.
2
Cornell U. Department of Chemical and Biomolecular Engineering, Ithaca NY, 14853, U.S.A.
ABSTRACT Isotropic and anisotropic conductive carbon particles, carbon black (CB) and vapor grown carbon fiber (VGCF), were incorporated into a Lithium Titanate (LTO) battery anode material composition, and their effect on conductivity and electrochemical properties investigated. Nanocomposite electrodes comprised of LTO, polyvinyldine floride (PVDF) and as little as 5 wt% VGCF are reported to manifest more than one order of magnitude enhancement in conductivity over their CB counterparts. VGCF-based anodes are also found to exhibit more stable voltage discharge profiles and as much as 20% improvement in capacity retention during extended electrochemical cycling at charge/discharge rates as high as 2.625 A/g (15 C). Remarkably, we find that the benefits of VGCF relative to CB conductivity aids diminish at higher particle loadings and that a LTO anode formulation containing 5 wt% CB | 5 wt% VGCF yields optimal capacity retention. At 5C, this composite system outperformed both the 10 wt% VGCF and 10 wt% CB electrode systems by delivering 20% higher capacity during extended charge/discharge cycling. We explain this finding in terms of two synergetic effects: enhanced electrode conductivity facilitated by incorporation of a percolated network of anisotropic VGCF particles; and shorter transport distances between the insulative LTO and high surface area CB. INTRODUCTION Although conductive carbons particles in a range of sizes, shapes, and aspect ratios are now available, Carbon Black (CB) remains one of the most popular conductivity aides for Lithium ion battery electrode applications. However, the low aspect ratio of CB demands high concentrations (~10 wt%) to achieve sufficient levels of conductivity in the electrode; its low gravimetric density relative to typical active electrode materials comes at a significant cost in terms of electrode volume and volumetric storage capacity of a battery. Significant research efforts are underway to improve the quality of the conductive carbon and to reduce the amounts required for reliable battery operation [1 - 6]. In particular, previous studies by Zheng et al. replaced the carbon black in a LTO nanocomposite anode with VGCF and determined the effect on rate performance. These studies show that substitution of CB by VGCF can produce notable improvements in rate capability of LTO anodes, but little fundamental understanding of the effect or of how to optimize it are available. Towards these goals, we herein report on LTO nanocomposite electrodes containing varying compositions of VGCF, CB, and their mixtures [7].
EXPERIMENT Vapor grown carbon fibers (VGCF) were obtained from Showa Denko USA. The carbon black (CB), PVdF polymer binder, and L
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