CNT@TiO2 nanohybrids for high-performance anode of lithium-ion batteries
© Wen et al.; licensee Springer. 2013
Received: 12 October 2013
Accepted: 8 November 2013
Published: 22 November 2013
This work describes a potential anode material for lithium-ion batteries (LIBs), namely, anatase TiO2 nanoparticle-decorated carbon nanotubes (CNTs@TiO2). The electrochemical properties of CNTs@TiO2 were thoroughly investigated using various electrochemical techniques, including cyclic voltammetry, electrochemical impedance spectroscopy, galvanostatic cycling, and rate experiments. It was revealed that compared with pure TiO2 nanoparticles and CNTs alone, the CNT@TiO2 nanohybrids offered superior rate capability and achieved better cycling performance when used as anodes of LIBs. The CNT@TiO2 nanohybrids exhibited a cycling stability with high reversible capacity of about 190 mAh g-1 after 120 cycles at a current density of 100 mA g-1 and an excellent rate capability (up to 100 mAh g-1 at a current density of 1,000 mA g-1).
The use of limited fossil fuel resources and their negative impact on the environment are significant challenges facing world economies today, creating an urgent demand for new technologies that enable high efficiencies in energy harvesting, conversion, and storage devices[1, 2]. Various technologies, including fuel cells, batteries, solar cells, and capacitors, show great promise to significantly reduce carbon footprints, decrease reliance on fossil fuels, and develop new driving forces for economic growth[3, 4]. Lithium-ion batteries (LIBs) have been regarded as one of the most promising energy storage technologies for various portable electronics devices, and one of the key goals in developing LIBs systems is to design and fabricate functional electrode materials that can lower costs, increase capacity, and improve rate capability and cycle performance[6–9].
It has been extensively reported that TiO2 is a promising candidate to compete with commercial graphite anode for LIBs due to its multiple advantages of high abundance, low cost, high Li-insertion potential (1.5 to 1.8 V vs. Li+/Li), structural stability, and excellent safety during cycling. Practical applications of TiO2 in LIBs, however, face significant challenges of poor electrical conductivity and low chemical diffusivity of Li, which are two key factors for the lithium insertion-deinsertion reaction. Therefore, it is highly desirable to develop reliable strategies to advance electrical conductivity and Li+ diffusivity in TiO2[11, 12]. In fact, continued breakthroughs have been made in the preparation and modification of TiO2-based nanomaterials for high performance energy conversion and storage devices[13, 14].
It is generally acknowledged that there are three routes available to tune the properties of TiO2 for its corresponding applications: (1) preparation of TiO2-based nanostructures with specific morphology. For example, TiO2-based nanorods were reported to show enhanced rate capability and improved stability as electrodes in LIBs due to their one-dimensional (1D) structure and high surface area[15, 16]. (2) Synthesis of TiO2 nanocrystals with specific crystal surface orientations. It was reported that TiO2-based nanocubes dominated by (001) planes had much higher catalytic activity for photo-degradation of organic dyes than the conventional TiO2 with mixed crystallographic facets[18, 19]. (3) Fabricating TiO2-based nanohybrids with other functional materials. Carbon nanostructures, such as carbon nanotubes (CNTs) and graphene, are the most appealing functional materials for improving the performance of TiO2 nanostructures due to their unique structure, excellent electrical conductivity, high stability, and great mechanical properties[20, 21].
We recently developed a convenient procedure to synthesize TiO2 nanoparticle-decorated CNT hybrid structures (CNTs@TiO2) through annealing treatment of carbonaceous polymer-modified CNTs with adsorbed Ti4+. The as-prepared CNT@TiO2 nanocomposites exhibit multiple favorable features, such as excellent electrical conductivity and considerable high surface area, which make them to be potentially used for promising electrode material of electrochemical energy storage and conversion devices. We systematically investigated the electrochemical properties of CNT@TiO2 nanohybrids as anodes of LIBs, and demonstrated that the unique properties of both CNTs and TiO2 can merge well in the CNT@TiO2 nanohybrids with synergetic effects. In this way, the CNTs@TiO2 can potentially address the intrinsic issues associated with TiO2 anodes in LIBs, namely poor electrical conductivity and low chemical diffusivity of Li ions, and thus significantly improve performance in term of capacity, cycle performance, and rate capability.
Materials and synthesis
All chemicals were purchased from Sigma-Aldrich (St. Louis, MO, USA) and used without further purification, except CNTs (200 nm in diameter) which were purchased from Carbon Nanotechnologies, Inc. (Sunnyvale, CA, USA). CNTs@TiO2 were prepared through a modified route reported previously. Typically, 0.15-g CNTs were completely mixed with a 60-ml glucose solution (0.5 mg/ml) under sonication. The mixed turbid liquid was then placed in a 100-ml Teflon-lined stainless steel autoclave and heated at 180°C for 5 h. Next, 0.2 g of the product after centrifuging and drying, namely carbonaceous polymer-modified CNTs (CNTs@Cpolymer), was then dispersed in 15 ml ethanol with the addition of 1 ml of titanium isopropoxide (TIP, 97%) under vigorous agitation. After centrifuging and drying, the solid products were then calcined at 400°C and exposed in an air atmosphere to evolve into CNTs@TiO2. Powder X-ray diffraction (XRD) was conducted on a Scintag XDS 2000 X-ray powder diffractometer (Scintag Inc., Santa Clara, CA, USA) using monochromatized CuKα as radiation (λ = 1.5418 Å); the data were collected by scanning angles (2θ) from 20° to 60°. N2 adsorption-desorption experiments were tested at 77 K by a Quantachrome autosorb gas-sorption system (Boynton Beach, FL, USA). The morphologies of the as-prepared samples were observed using a Hitachi (H 9000 NAR, Tokyo, Japan) transmission electron microscope (TEM) and a Hitachi S-4800 scan electron microscope (SEM).
The working electrode of LIB was prepared by compressing a mixture of active materials (80%), acetylene black (10%), and polyvinylidene fluoride (10%) as a binder dissolved in 1-methyl-2-pyrrolidinone solution onto a copper foil. The pellet was dried in vacuum at 120°C for 10 h and then assembled into a coin cell in an Ar-protected glove box. The electrolyte solution was 1 M LiPF6 dissolved in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC), with a volume ratio of EC/DMC = 4:6. Galvanostatic cycling experiments were conducted to measure the electrode activities using a Maccor battery tester system (Tulsa, OK, USA) at room temperature. Cyclic voltammograms (CVs) were carried out with three-electrode cells and recorded from 3.0 to 1.0 V at a scan rate of 0.1 mV s-1 using a CHI 600 electrochemical station (CHI Inc., Austin, TX, USA). Discharge–charge curves were recorded at fixed voltage limits between 3.0 and 1.0 V at various current densities. The specific capacity was calculated based on the total mass of the active materials. Electrochemical impedance spectroscopy (EIS) measurements were carried out at the open-circuit voltage state of fresh cells using a CHI600 (Austin, TX, USA) electrochemical workstation. The impedance spectra were recorded potentiostatically by applying an AC voltage of 5-mV amplitude over a frequency range from 100 kHz to 5 mHz.
Results and discussion
There is an observable decrease of cathodic current in the second CV compared with the first CV for the TiO2 electrode, which agrees with the previous report on TiO2 anode materials and can be attributed to the irreversible lithium insertion-deinsertion reaction, indicating a large capacity loss during the first two cycles. The CNTs@TiO2, however, only display a small change during the initial two CVs, suggesting a small capacity loss in the initial two cycles.
Figure 3b schematically illustrates the Li+ insertion/deinsertion in CNT@TiO2 nanohybrids and demonstrates advantages of the high electrical conductivity and facile transport of Li+ in CNT@TiO2 nanohybrids. The improved electrochemical performance in CNT@TiO2 nanohybrids can be attributed to the following factors: above all, the TiO2 nanoparticles were uniformly decorated on the surface of one-dimensional CNTs, which offered excellent flexibility and enough space for alleviating the effects of electrode degradation and volume change upon cycling. Additionally, the large surface area (109.9 m2 g-1) and suitable pore size (11.5 nm) in CNTs@TiO2 can facilitate the transport of electrolytes and Li+ on the interface of electrodes, leading to good rate capability. Furthermore, the electrical conductivity, thanks to the CNT's core, is expected to be greatly enhanced, which can significantly decrease the capacity loss from Ohmic resistance.
In summary, we demonstrated the electrochemical properties of the nanohybrids of TiO2 nanoparticle-decorated CNTs as an anode of lithium-ion batteries. The CNT@TiO2 hybrids showed better electrochemical performance than the pure TiO2 nanoparticles with regard to specific capacity (except the initial cycle), rate capability, and cycling stability. The improved electrochemical performance can be ascribed to the synergetic effects of combined properties, including the one-dimensional structure, high-strength with flexibility, excellent electrical conductivity, and large surface area.
ZHW obtained his Ph.D. from the Chinese Academy of Sciences in 2008. After working as a Humboldt postdoctoral research scholar at the Max-Planck Institute for Polymer Research in Germany. He started his postdoctoral research at the University of Wisconsin-Milwaukee (UWM). His research is primarily focused on electrochemical or photocatalytic energy storage and conversion. SQC worked as a lecturer at Nanchang Hangkong University in China after receiving her Ph.D. in Biochemical Engineering from the Institute of Process Engineering, Chinese Academy of Sciences. Currently, she is a postdoctoral researcher at the University of Wisconsin-Milwaukee and working on electrochemical analysis and electrocatalysis. SM received his Ph.D. in Mechanical Engineering from UWM in 2010 for the study of hybrid nanomaterials for biosensing applications. After graduation, he worked as a project director at NanoAffix Science, LLC for a hydrogen sensor project. He is currently a postdoctoral fellow at UWM. His research is focused on hybrid nanostructures (i.e., graphene/CNT with nanocrystals) for energy and environmental applications. SMC received his Ph.D. in Mechanical Engineering from UWM in 2013 and is currently a postdoctoral fellow at UWM. His research interests include synthesis of nanoparticles, synthesis of nanohybrids combining nanocarbons (graphene and carbon nanotubes) with nanoparticles, and developing environment and energy applications using nanomaterials. ZH is an associate professor of the Department of Civil and Environmental Engineering at Virginia Polytechnic Institute and State University. He received his B.E. degree from Tongji University, M.Sc. degree from the Technical University of Denmark, and Ph.D. from Washington University in St. Louis. He completed his postdoctoral training at the Mork Family Department of Chemical Engineering and Materials Science and the Department of Earth Sciences at the University of Southern California. Before joining VT, he was an assistant professor of civil engineering at UWM. His research focuses on the fundamental understanding of engineered systems for bioenergy production from wastes and development of bioelectrochemical systems for water and wastewater treatment. JHC received his B.E. degree in thermal Engineering from Tongji University, Shanghai, China, in 1995 and M.S. and Ph.D. degrees in Mechanical Engineering from the University of Minnesota, Minneapolis, MN, in 2000 and 2002, respectively. From 2002 to 2003, he was a postdoctoral scholar in Chemical Engineering at California Institute of Technology. He is currently a full Professor in the Department of Mechanical Engineering at UWM. His current research interests include carbon nanotube- and graphene-based hybrid nanomaterials, plasma reacting flows, and nanotechnology for sustainable energy and environment.
This work was financially supported by the US National Science Foundation (ECCS-1001039 and CBET-1033505) and the US Department of Energy (DE-EE0003208). The SEM imaging was conducted at the UWM Bioscience Electron Microscope Facility, and the TEM analyses were conducted in the UWM Physics HRTEM Laboratory.
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