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Nano-Dimentionally Controlled Graphite for Fast Li+ Intercalation/De-Intercalation

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Title
Nano-Dimentionally Controlled Graphite for Fast Li+ Intercalation/De-Intercalation
Author
Park, Jeong-Seok
Advisor
Song, Hyun-Kon
Issue Date
2013-02
Publisher
Graduate School of UNIST
Abstract
As high rate charge and discharge characteristics of energy storage devices become more important with the market of electric vehicles intensively growing, the kinetics of lithiation or delithiation of electrode materials for lithium ion batteries are required to be enhanced. Graphites, the most widely used anode materials, have a limited power density at high discharge rates while their alternatives such as silicon and transition metal oxides show even inferior rate capability. This work consist of two strategies. The first is edge-exfoliated graphite that was motivated from an idea of what if the edge opening of graphite was zipped more open to lithium ions in electrolyte. By edge-selective functionalization, the peripheral d-spacing of graphite (d0) was locally controlled. Larger values of d0 led to higher capacity especially at high discharge rates. Around two-fold enhancement of capacity or energy density was achieved at 50C discharge rate from 110 mAh g-1 to 190 mAh g-1 by exfoliating graphite locally in its edge region. Also, the d0 dependency of delithiation kinetics confirmed that the electrochemical step of Li+ influx into or efflux out of interlayer space of graphite is possibly the rate determining step of lithiation or delithiation. The second strategy is diffusion length (L) controlled graphite. To prove the effects of length as the kinetics-controlling parameter, three different graphitic carbons were selected: Herring bone carbon nanotubes (HBCNT, L = 20 nm), small natural graphite (sNG, L = 8um) and natural graphite (NG, L = 20 um). Evident differences originating from the different dimension of L were obtained with the three model graphitized carbons, showing that the smaller crystallite dimension leads to the faster kinetics. The improvement of kinetics was more pronounced during charge or lithiation rather than during discharge or delithiation. More than 70 % of intrinsic capacity was charged at 5 C-rate with L = 20 nm while less than 20 % was charged at the same rate with L = 20 um. Therefore, we believe that nano-dimensionally optimized graphite is beyond conventional graphites in terms of kinetics, satisfying the requirements of lithium ion batteries for electric vehicles.
Description
Battery Science & Technology
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