3-Dimensional Carbon Nanotube for Li-Ion Battery Anode

3 Dimensional Carbon Nanotube for Li-Ion Battery Anode (Journal of Power Sources 219 (2012) 364-370) Chiwon Kang1†¡, Indranil Lahiri1†¡, Rangasamy Baskaran2, Won-Gi Kim2, Yang-Kook Sun2, Wonbong Choi1, 3* Nanomaterials 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, USACorresponding Author *Email: [email  protected] edu Author Contributions †¡These authors contributed equally. Abstract 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 conductiv ity. 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. Graphical abstract Keywords Li-ion batteries, carbon nanotubes, 3D Cu current collector, anode materials, amorphous Si, a-Si/MWCNTs composite 1. IntroductionLi-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 [1]. 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 elec trolytes [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 [5].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 [6], silicon nanowires [7], nano sized transitional metal oxides [8-10], graphene [11] and carbon nanotubes 12-14]. Out of the many nanomaterials available, carbon nanotubes (CNTs) have attracted great attention for anode materials due to their high surface area, short diffusion length of Li+ ions and high electrical conductivity [15]. Past researches including from our group have demonstrated outstanding performance of MWCNT based binder-free anodes in terms of high specific capacity, excellent rate capability and extremely or nil capacity degradation during long cycle operation [16-17].However, carbon nano materials are known as low-density materials, which results in low volumetric capacity and low volumetric energy/power density. Therefore higher solid loading of MWCNTs as active materials is one of the most significant issues to be realized in practice. Very recently, it argued that nanotube based active materials have a critical shortcoming in terms of their very low weight per unit electrode area [18].Thus, their gravimetric energy density may not give a realistic picture to commercial application. The critical limitation may lead to s cale-up issues for their potential application in the development of EV. To counter this issue, we propose a new geometry of 3D Cu current collectors, which can play a crucial role in creating higher surface area to accommodate more solid loading of MWCNTs on the uniformly arrayed patterns in the 3D structure, leading to higher specific capacity and C-rate capability.Until now, efforts have been dedicated to employ a number of 3D structured current collectors including carbon papers [19], a self-assembled 3D bicontinuous nanoarchitecture [20], aluminum nanorods [21], and nanoporous nickel [22]. The previous research proved that a self-assembled 3D bicontinuous nanoarchitecture could be one of the ideal electrode architectures in order to realize not only high volume fraction of nanostructured electrolytically active materials (NiOOH/Nickel and MnO2 cathodes) but also their efficient ion and electron transport [20].In addition, ALD coated TiO2 anodes on 3D aluminum nanorod current co llectors showed the 10 times increase in their theoretical area and total capacity (0. 0112 mAhcm-2), compared to those resulted from the same anodes on 2D flat aluminum plate and high rate capability (the capacity ratios at 10 C/0. 5 C and 20 C/0. 5 C of the 3D anode were 0. 4 and 0. 35, respectively. ) [21]. Currently, the diverse types of hybrid anode structures have been designed and synthesized in order to expect the synergetic combination of two different types of nanomaterials for the igher electrochemical performances. As one of the most preferable combinations, MWCNTs/Si hybrid structure can be chosen due to the better mechanical accommodation of MWCNTs of the large volume expansion/constriction of Si during lithiation/delithiation process and the higher bonding strength between MWCNTs and Si. There were some selected reports on MWCNTs/Si composite structures, employing either SiH4 CVD method [23] or sputter deposition [24]. In this study, we present a novel concept 3D anod e system, comprising of MWCNTs directly grown on 3D Cu mesh using catalytic thermal CVD method [25].Electrochemical performances of this 3D anode structure are compared with those of MWCNTs directly grown on 2D Cu foil. Furthermore, enhanced electrochemical properties of a-Si/MWCNTs hybrid structure, synthesized on 3D Cu mesh using a two step process of CVD and sputtering deposition, are presented. Morphology and structure of as-grown MWCNTs and a-Si/MWCNTs hybrid anode structures and their role in the electrochemical performance are discussed. 2. Experimental A Cu mesh (TWP Inc. with average dimensions of 50  µm thickness and 65  µm hole size was prepared. In parallel, a 50  µm thick pure Cu foil (Nimrod Hall Copper, 99. 9% purity) was also employed. Both types of samples were used as substrates for depositing Ti (underlayer)/Ni (catalyst) thin film through a RF and DC magnetron sputtering system. These Ti/Ni thin film deposited samples were cut to 14 mm diameter disc shape f or 2032 button cell assembly, before inserting into a thermal CVD system for direct MWCNT growth.During CVD, samples were heated very rapidly, under an inert Ar gas environment, to the growth temperature of 750 °C, and MWCNT growth began with flow of a mixture of ethylene (C2H4) and hydrogen (H2) gas (1:2 volume ratio) in the chamber. After 50 minutes of growth, the samples were cooled to room temperature within the furnace under an Ar gas envelope. Amorphous Si (a-Si) was deposited further on the as-grown MWCNT samples using the sputtering system with the incorporation of Ti adhesion layer in order to enhance bonding strength between a-Si and MWCNTs.Weights of samples were measured before and after CVD growth to exactly identify weights of the active materials (i. e. MWCNTs and a-Si thin layer). Morphology and structural properties of the prepared anode structures were carefully investigated using field emission scanning electron microscopes (FESEM) (JEOL, JSM-7000F), an energy d ispersive spectroscope (EDS) (Thermo Electron Corporation, NORAN System SIX), a Raman spectrometer (Ar+ laser with ? = 514 nm, 33 mW power) and a field emission transmission electron microscope (FETEM) (FEI, TECHNAI F20).Electrochemical performance for these anodic materials was conducted in a typical coin cell (half cell). The cells were assembled in a CR2032 press. The complete cell assembly was carried out in an argon glovebox under extremely low levels of oxygen and humidity (both individually