Lithium storage study on MoO3-grafted TiO2 nanotube arrays
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Titanium dioxide nanotube arrays (TNAs) were fabricated via anodic ionization. Porous MoO3 was grafted on TNAs with the help of hydrothermal method. Scanning electron microscopy and X-ray powder diffraction was utilized for the confirmation of one dimensional morphology and phase identification. The porous MoO3 nanoflake-grafted TNAs (MoO3/TNAs) electrode was used as anode material in lithium ion battery (LIB) and it was found that the areal specific capacity of MoO3/TNAs (~797 µAh cm−2) was three times higher than those of anatase TNAs (~287 µAh cm−2) and porous MoO3 (~234 µAh cm−2) at 50 µA cm−2.
KeywordsMolybdenum oxide Titanium dioxide nanotube arrays (TNAs) Anode Lithium-ion batteries (LIBs)
Lithium ion battery (LIB) is one of the most reliable power sources for portable electronic devices. The improved performance of microbatteries is highly necessary for modern microelectronic devices such as PC memory, microelectromechanical systems (MEMS), medical implants, hearing aids, “smart” cards, RF-ID tags, remote sensors and energy harvesters, etc. (Kyeremateng 2014; Matiko et al. 2014; Patil et al. 2008; Pikul et al. 2013). The requirement of high-performance LIBs encourages scientists to develop new anode materials with capacity higher than graphite (Reddy et al. 2013; Wu et al. 2012a; Wu and Hong 2014; Xiong et al. 2014). TiO2 is a promising material for lithium storage due to its low volume expansion, environmental benignity and widespread availability. Amongst the various nanostructures of TiO2 (Armstrong et al. 2006; Cao et al. 2010; Liu et al. 2012; Qiu et al. 2010; Ren et al. 2012; Wang et al. 2011), titanium dioxide nanotube arrays (TNAs) (Guo et al. 2012) are favorable due to their high specific surface area, high porosity, vertical orientation which accommodate volume expansion and also provide short lithium ion diffusion path (Wu et al. 2012b). However, the areal specific capacity, even for the optimized TNAs, is found to be low (Tauseef Anwar et al. 2015). Three different methods have been proposed to enhance the specific capacity: (1) doping TNAs with metal or nonmetal elements (Kyeremateng et al. 2013b; Liu et al. 2008, 2009); (2) coating TNAs with conductive reagents (Guan and Wang 2013; Kim et al. 2010; Zhang et al. 2009); (3) modify TNAs with oxide materials that have larger capacities [SnO2 (Meng et al. 2013), Co3O4 (Fan et al. 2013; Kyeremateng et al. 2013a), Nb2O5 (Yang et al. 2013) and Fe2O3 (Yu et al. 2013)] to yield hybrid or composite structures.
MoO3 is an anode material candidate due to its high theoretical capacity (1117 mAh g−1). The orthorhombic phase layered structure of α-MoO3 hosts Li+ by insertion and deinsertion reaction. However, the electrochemical properties of TNAs could be further enhanced with the extra porous hybrid material such as MoO3 (Fan et al. 2013; Guan et al. 2014a, b; Kyeremateng et al. 2013a; Meng et al. 2013; Wang et al. 2013; Xue et al. 2011; Zhu et al. 2015). Considering low electronic conductivity and high volume expansion, Yu et al. (2014) synthesized porous MoO3 thin films and elucidated better performance as compared to bulk MoO3. Zhao et al. (2013) synthesized porous MoO3 thin films via electro-deposition which exhibit a high capacity of 650 mAh g−1 at high current density of 3 A g−1. Yu et al. synthesized porous MoO3 nanosheets by hydrothermal method at Ti substrate and the nanosheets showed specific capacity of 750 mAh g−1 at 1C-rate. There are rare reports on the MoO3/TNAs as anode material in LIBs. However, different fabrication of coating MoO3 on TNAs led difference in their electrochemical properties. The hydrothermal synthesis for the grafting of MoO3 nanoflakes at TNAs was used first time. The fabrication method and porosity would play important role for future electrochemical properties of material.
Synthesis of MoO3/TNAs
Prior to anodic oxidation, titanium foil (0.125-mm-thick foil, 99.7 % purity, Sigma Aldrich) was degreased by sonication in acetone, ethanol and deionized water in turn, then dried in air. The electrochemical cell for anodization was a two-electrode cell, consisting of Ti foil as working electrode and platinum foil as counter electrode. Electrochemical anodization experiments were conducted at a constant potential with a DC power supply (DH1722A-2 110V/3A). The electrolyte was 0.3 wt% NH4F and 2 vol.% water in ethylene glycol (99.8 %, anhydrous). All the tests were performed at room temperature. The TNAs were synthesized at the voltage of 50 V for 2 h. The as-prepared TNAs were annealed at 450 °C to transform its phase.
The porous MoO3 were deposited by hydrothermal method reported elsewhere (Yu et al. 2014). The TNAs containing Ti substrate was placed against the wall of Teflon liner with interested surface downwards. The prepared 30 mL solution of (NH4)6Mo7O24·4H2O (1 mmol) and thiourea (0.484 g) was transferred gently in Teflon-lined stainless steel autoclaves. Hereafter, the autoclave was sealed and maintained at 180 °C for different reaction time (2, 4, 6, 8, 10 h) and cooled down to room temperature spontaneously. The samples were collected and rinsed with distilled water for several times to remove the residual reactant and dried in vacuum oven at 80 °C for 30 min. Now MoS2-deposited TNAs were obtained and annealed at 400 °C for 2 h to convert MoS2 into MoO3. For comparison, porous MoO3 were grown in the similar way at titanium substrate.
The surface and cross-sectional morphologies of the TNAs and MoO3/TNAs were characterized using field emission scanning electron microscopy (FE-SEM LEO 1530). The phase structure of the TNAs, porous MoO3 and MoO3/TNAs were characterized by X-ray powder diffraction (XRD). The Cu Kα radiation (λ = 0.15 nm) were used for XRD analysis. The electrochemical kinetics were studied by cyclic voltammetry (CV) test measured at a scan rate of 5 mV s−1 at a potential between 0 and 3 V.
The lithium storage performances of electrode were evaluated using Li| MoO3/TNAs half-cells. The cells were 2032 coin cell and assembled in an argon-filled glove box. The cathode was MoO3/TNAs without additives, the anode was lithium foil, and the separator is celgard 2300. The electrolyte is 1 M LiPF6 dissolved in the mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) with volume ratio of 1:1. The cells were galvanostatically charged and discharged between 0.005 and 3 V (vs. Li/Li+) at the current of 0.01 mA for the initial two cycles and then at 0.05 mA for the following cycles. The electrodes for comparison were configured and analyzed at same parameters.
Results and discussion
The synthesis process of porous MoO3/TNAs is schematically illustrated in Fig. 1. Well oriented TNAs are synthesized at the nanostructured substrate via electrochemical anodization of Ti foil at room temperature (Fig. 1 I and II). MoS2 is coated on the top of these vertical TNAs via hydrothermal reaction (Fig. 1 III) and oxygen annealing converts MoS2 in MoO3 (Fig. 1 IV).
The annealed TNAs and MoO3/TNAs are characterized by SEM, as shown in supporting information Fig. S1 and S2, respectively. It is found that the TNAs were compact and uniform, without secondary nanostructures on the top or side surface (Fig. S1). The average inner diameter of the TNAs is around 60 nm and the length was about 3–5 µm. For MoO3/TNAs sample, MoO3 nanoflakes are grafted on the top of TNAs and the thickness of MoO3 is controlled via deposition time (Fig. S2). The MoO3 precipitate coat the surface partially when hydrothermal reaction was continued for 2 h (Fig. S2a) and the side of TNAs was almost as neat as pristine TNAs (inset Fig. S2b). As the hydrothermal duration increases to 4 h, both the top surface and side surface of TNAs are fully covered with MoO3 nanoflakes and nanochannels were blocked (Fig. S2c, d). The MoO3 nanoflakes with various lateral dimensions ranging in nanometers are grafted on the top, inner and outer surface of TNAs. The coating layer thickness increased linearly as reaction time increased as shown in Fig. S2. The TNAs might incorporate MoO3 for stacking and control volumetric changes on lithiation. MoO3 nanoflakes due to their 2D structure and high surface area can facilitate the transport of ions/electrons thus improve the response of system and recovery kinetics (Alsaif et al. 2014) (Fig. 2).
The reaction kinetics is studied with the help of CV measured at a scan rate of 5 mV s−1 at 0–3 V, as shown in Fig. 5a–c. The cathodic peaks are observed at 0.6 and 1.45 V for the anatase TNAs, while 1.94 V for porous MoO3 (Li et al. 2006; Ryu et al. 2012). The cathodic peaks for MoO3/TNAs are observed at 0.66 V corresponding to the reduction of electrolyte solution and formation of solid electrolyte interface (SEI) layer on the surface of working electrode. The anodic peaks observed for anatase TNAs at 2.37 V, for porous MoO3 at 1.52 V while for MoO3/TNAs at 2.34 and 1.51 V were due to the delithiation from oxides (Li et al. 2006; Ryu et al. 2012).
Li ions insert into anatase TNAs or react with electrolyte as potential drops below 0.5 V. The lithiation in MoO3/TNAs appears in a different way. At lower voltage regions Li ions react with the solid solution (LixMoO3) to form Mo metal and Li2O oxides which are irreversible, but the nano-textured synthesis induces a reversible reaction of Li2O during charging (Guan et al. 2014a, b).
The rate performance of the three samples is measured with different current densities (50, 100, 150, 200, 250 and again 50 µA cm−2). The rate capability of MoO3/TNAs is the highest compared to anatase TNAs and porous MoO3. After the current density switched back to 50 µA cm−2, the capacity for MoO3/TNAs is higher enough and more stable. These results reveal that the incorporation of anatase TiO2 into MoO3 nanostructures can greatly enhance the electrochemical performance for lithium storage.
Comparison of Guan et al. MoO3/TNAs and our synthesized MoO3/TNAs
MoO3/TNAs (our work)
Titanium dioxide nanotube arrays length (µm)
1st discharge capacity (µAh cm−2)
1340 at 800 µA cm−2
(Guan et al. 2014a)
947 at 10 µA cm−2
3rd discharge capacity (µAh cm−2)
154.9 at 50 µA cm−2
(Guan et al. 2014b)
797 at 50 µA cm−2
TNAs were grown via anodic oxidation method and MoO3 nanoflakes were grafted at TNAs via hydrothermal method for the first time. The optimal electrochemical properties of MoO3/TNAs were obtained for 3 h deposition of MoO3 nanoflakes. The specific capacity (~797 µAh cm−2) of MoO3/TNAs was three times higher than anatase TNAs (~287 µAh cm−2) and porous MoO3 (~234 µAh cm−2). The rate performance and efficiency of LIB (in which MoO3/TNAs used as anode material) were also enhanced. The anatase TiO2 incorporates MoO3 nanostructures and enhances the electrochemical performance, hence MoO3/TNAs electrode might be a useful anode material for lithium ion micro-batteries. The carbon-free conducting nanocoated electrodes will be able to open new opportunities in the development of high-performance next-generation lithium-ion micro-batteries.
This work is supported by the National Natural Science Foundation of China (Grant no. 21271114); Tsinghua University independent research and development fund (20111080982) and Program for Changjiang Scholars and Innovative Research Team in University (IRT13026).
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