The Development of nanomaterials for high performance lithium ion battery anodes

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Title: The Development of nanomaterials for high performance lithium ion battery anodes
Author: DiLeo, Roberta
Abstract: Many industries spanning macro to micro applications need advanced energy storage capabilities and Li<super>+</super> batteries are the prevalent technology to meet those demands. High Li<super>+</super> capacity semiconductor materials (e.g. Si, Ge) in concert with carbon nanotubes (CNTs) have been investigated as alternative materials for Li<super>+</super> batteries. Nanomaterials offer many advantages to high performance batteries by increasing storage capacities, Li<super>+</super> diffusion, and more adequately accommodating volumetric expansion that occurs in cycling. Silicon and Ge are known to have very high Li<super>+</super> storage capacities of 4200 and 1600 mAh/g, respectively, and can be used in combination with CNTs to form free-standing anodes. The proper incorporation of semiconductor materials onto and throughout a CNT network through thin film, solution processing, and gas-phase processing techniques, has been studied to develop ultra-high capacity free-standing electrodes. Given the free-standing nature, the removal of binders and metal foil current collectors contributes to an increased electrode energy density over conventional composites on metal substrates. The CNT and semiconductor materials have been characterized in coin and pouch cells upon identifying the synthesis parameters and processing steps to be optimized for several of the incorporation techniques. Anodes fabricated through PVD techniques realize capacities over 800 mAh/g and a predicted >50% increase in energy density over conventional graphite anodes. The use of thin film Ti contacts on high energy Ge-SWCNT anodes demonstrates a 5-fold improvement in Li<super>+</super> capacity at 1C extraction rates, a drastic improvement in the anode power capabilities. Pairing these electrodes with a high power cathode LiFePO<sub>4</sub> can lead to a 60% improvement in power and energy density. A 3-dimensional network of Ge nanoparticles (Ge-NPs), Si and CNTs demonstrates capacities of 1000 and 2000 mAh/g with modest cycling capabilities up to 1C and first cycle coulombic efficiencies of 88%. The performance of these novel anodes in full cells with commercially available cathodes realized electrode-pair energy densities over 300 Wh/kg, while current technologies have energy densities of 150 - 200 Wh/kg. This work demonstrates the combination of high Li<super>+</super> capacity Si and Ge with highly conductive CNTs in a balanced high energy and high power anode for lithium ion batteries with a 2x improvement in cell energy density.
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Date: 2012-05

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