ARTICLE pubs.acs.org/JPCC
Rapid Synthesis of Li4Ti5O12 Microspheres as Anode Materials and Its Binder Effect for Lithium-Ion Battery Shu-Lei Chou,*,†,‡ Jia-Zhao Wang,*,†,‡ Hua-Kun Liu,†,‡ and Shi-Xue Dou† †
Institute for Superconducting and Electronic Materials and ‡ARC Centre of Excellence for Electromaterials Science Australia, University of Wollongong, Wollongong, NSW 2522 Australia ABSTRACT:
Li4Ti5O12 microspheres composed of nanoflakes were synthesized within 1 h by a combination of a microwave-assisted hydrothermal method and a microwave postannealing process. The Li4Ti5O12 microspheres were characterized by X-ray diffraction, BrunauerEmmettTeller N2 adsorption, scanning electron microscopy, Raman spectroscopy, and transmission electron microscopy. Sodium carboxymethyl cellulose (CMC) was also investigated as a low-cost green binder. The electrochemical tests, including constant current chargedischarge, cyclic voltammetry, and electrochemical impedance spectroscopy, demonstrated that the electrode using CMC as binder had better high-rate capability than the one with polyvinylidene fluoride (PVDF) binder. The electrode using CMC and PVDF as binder had the same lithium diffusion coefficient. The electrode using CMC as binder showed much lower charge transfer resistance, lower apparent activation energy, and lower apparent diffusion activation energy than for the electrode using PVDF as the binder. Apparent activation energies of Li4Ti5O12 microsphere electrodes using CMC and PVDF as binder were calculated to be 26.8 and 33.6 kJ mol1, respectively.
1. INTRODUCTION Transport is one of the largest sources of greenhouse gas emissions and fossil-fuel consumption. For reduction of emissions of carbon dioxide and conquer the greater and greater scarcity of fossil fuels, one of the most effective ways is to use electrical vehicles (EVs) or hybrid electrical vehicles (HEVs). Lithium-ion batteries have now shown that they have a promising future in the coming era of EVs/HEVs. However, the current lithium-ion battery is handicapped by several critical disadvantages for EV/HEV applications, including short cycling life, low power density, and safety hazards. Spinel lithium titanate, Li4Ti5O12, has attracted great interest as anode material for rechargeable Li-ion batteries because it can offer a great improvement in safety because of its high and flat Li insertion voltage at ∼1.55 V versus Li/Li+, which prevents the growth of lithium dendrites and the decomposition of electrolyte as well as providing long cycle life and high rate capability.16 High rate capability can be achieved via reducing the particle size of Li4Ti5O12. Kavan and Gr€atzel reported excellent high rate performance in a nanocrystalline thin-film Li4Ti5O12 electrode, even at a charging rate as high as 250C.5 This indicates that r 2011 American Chemical Society
Li4Ti5O12 nanomaterials with high surface area can significantly improve the rate capability. However, the large surface area also gives a low tap density, which will reduce the volumetric energy density. Three-dimensional architecture seems to be an effective way to achieve high tap density. Recently, Sorensen et al.7 demonstrated that 3D ordered macroporous Li4Ti5O12 electrode could deliver excellent high rate capacity. Amine et al.8 reported that their Li4Ti5O12 sample, with a morphology of microsize secondary particles, which were composed of nanosize primary particles, had a nanoporous structure that allowed for fast lithium diffusion because of the short lithium pathways within the nanoparticles and could achieve outstanding rate capability. Zhou et al.9 reported that a 3D TiO2P2O5 (crystalline-glass) mesoporous nanocomposite structure with a lithium-ion pathway through the nanochannels and an electronic pathway through glass-phase network is able to undergo a rapid chargedischarge process. Received: April 28, 2011 Revised: July 6, 2011 Published: July 07, 2011 16220
dx.doi.org/10.1021/jp2039256 | J. Phys. Chem. C 2011, 115, 16220–16227
The Journal of Physical Chemistry C
ARTICLE
Table 1. Experimental Parameters for Preparing Lithium Titanium Oxide Precursor Microspheres Using the Microwave-Assisted Hydrothermal Method sample
LiOH (mmol)
30% H2O2 (ml)
Ti-butoxide (mmol)
TB-120
8
1
TB-150 TB-180
8 8
1 1
TP-120
8
1
TP-150
8
1
TP-180
8
1
temperature (°C)
time (min)
2
120
15
2 2
150 180
15 15
2
120
15
2
150
15
2
180
15
Furthermore, in most of previous works, Li4Ti5O12 powders were fabricated via either high-temperature (8001000 °C) or great time-consuming (1224 h) methods.15 These methods require large energy consumption. Recently, Tarascon’s group reported the synthesis of nanocrystalline Li4Ti5O12 by the solution-combustion method in
