Chiwon Kang1‡, Indranil Lahiri1‡, Rangasamy Baskaran2, Won-Gi Kim2, Yang-Kook Sun2, Wonbong Choi1, 3*
1Nanomaterials and Device Laboratory, Department of Mechanical and Materials Engineering, Florida International University; 10555 West Flagler Street, Miami, FL 33174, USA 2Department of Energy Engineering, Hanyang University; 17 Haengdang-dong, Seongdong-gu, Seoul 133-791, Korea 3Department of Materials Science and Engineering, University of North Texas; North Texas Discovery Park 3940 North Elm St. Suite E-132, Denton, TX 76207, USA
‡These authors contributed equally.
Carbon nanotubes, in different forms and architectures, have demonstrated good promise as electrode material for Li-ion batteries, owing to large surface area, shorter Li-conduction distance and high electrical conductivity. However, practical application of such Li-ion batteries demands higher volumetric capacity, which is otherwise low for most nanomaterials, used as electrodes. In order to address this urgent issue, we have developed a novel 3-dimensional (3D) anode, based on multiwall carbon nanotubes (MWCNTs), for Li-ion batteries. The unique 3D design of the electrode allowed much higher solid loading of active anode material, MWCNTs in this case and resulted in more amount of Li+ ion intake in comparison to those of conventional 2D Cu current collector. Though one such 3D anode was demonstrated to offer 50% higher capacity, compared to its 2D counterpart, its ability to deliver much higher capacity, by geometrical modification, is presented. Furthermore, deposition of amorphous Si (a-Si) layer on the 3D electrode (a-Si/MWCNTs hybrid structure) offered enhancement in electrochemical response. Correlation between electrochemical performances and structural properties of the 3D anodes highlights the possible charge transfer mechanism.
Li-ion batteries, carbon nanotubes, 3D Cu current collector, anode materials, amorphous Si, a-Si/MWCNTs composite
Li-ion batteries (LIB) has been widely used as one of the most important energy storage devices in diverse applications such as green electric vehicles (EV), portable electronics and power tools, since it is commercialized by Sony in 1991 . The commercial cell is assembled by carbonaceous anode, separator and a Li containing layered structure cathode (e.g. LiCoO2). In terms of carbonaceous anodes, graphite and soft or poorly ordered carbons (e.g. mesocarbon microbeads or spherical graphite, microcarbon fiber) have been employed. The reasons behind their commercial prominence contain the relatively low cost of carbon, the excellent mechanical sustainability for lithium insertion and desertion (having minimum volume change ) and their formation of a protective surface film with many electrolytes [2-4]. Nevertheless, fully intercalated highly crystalline graphites have relatively lower specific capacity (372 mAhg-1, the stoichiometric formulae of LiC6) and cannot meet the demands of next generation LIB with respect to high specific capacity and volumetric capacity. To address these issues, other elemental compounds have been explored such as Al, Si, Ge and Sn . Among those elements, Si is known to have highest theoretical specific capacity (4,200 mAhg-1), however huge volume expansion/contraction (300–400%) during lithiation/delithiation brings about pulverization, resulting in capacity fading in a high number of cycles. To overcome such inherit limitations of bulk electrode materials, worldwide research groups have intensively focused on novel and suitable nanomaterials such as silicon nanotubes , silicon nanowires , nano sized transitional metal oxides [8-10], graphene  and carbon nanotubes...