Critical dual roles of carbon coating in H2Ti12O25 for cylindrical hybrid supercapacitors
Introduction
Around the world, studies on energy storage devices as alternatives to fossil fuels and on high efficiency in energy usage are being actively conducted. Among them, the lithium ion secondary batteries and supercapacitors are currently being used in electric vehicles (EVs), hybrid electric vehicles (HEVs), and energy storage systems (ESS), and uninterruptible power supplies (UPS) [1], [2], [3]. The lithium ion secondary batteries and supercapacitors have merits of high energy density and high power density, respectively, due to their different principles of capacity implementation [4]. The hybrid supercapacitors were designed as an energy storage device that combines the advantages of the lithium ion secondary batteries and the supercapacitors.
The spinel Li4Ti5O12 is one of the promising materials being investigated as a negative electrode material for energy storage devices because of its excellent Li-ion insertion/extraction reversibility and high structural stability. However, Li4Ti5O12 has a limitation when applying to an actual product due to its low theoretical specific capacity of 175 mAh g−1 [5], [6], [7], [8]. Recently, Akimoto et al. [9] reported that H2Ti12O25 as a negative electrode has a specific capacity of 229 mAh g−1 with a comparable cycle performance to that of Li4Ti5O12 [10]. These properties can be explained by the tunnel structure of material [11].
In order to improve the conductivity of the metal oxide-based electrode, several methods have been presented, such as reducing particle size [12], [13], doping with metal ions [14], [15], and coating with conductive materials [16], [17]. Of these various methods, we selected the carbon coating method for the H2Ti12O25 negative electrode in order to consider not only improving the conductivity related rate capability but also the swollen phenomenon related cycle performance. In this paper, we fabricated hybrid supercapacitors using carbon-coated H2Ti12O25 as the negative electrode and activated carbon (AC) as the positive electrode and investigated the effects of the carbon coating on the electrochemical performances of the hybrid supercapacitors.
Section snippets
Experimental
The carbon-coated H2Ti12O25 negative electrode material was first prepared by mixing Na2CO3 (99.5%) and TiO2 at a molar ratio of 1:3. This mixture, Na2Ti3O7, was synthesized at 800 °C for 20 h in air. The H2Ti3O7 sample was fabricated from Na2Ti3O7 via Na+/H+ ion exchange reaction using a 1 M HCl solution for 3 days at 60 °C. The prepared H2Ti3O7 was washed with deionized water until a pH of 7 was reached, and then dried at 100 °C for 24 h. The H2Ti12O25 was obtained by heating the H2Ti3O7
Result and discussion
Fig. 1 shows the x-ray diffraction patterns of the synthesized pristine and carbon-coated H2Ti12O25. All peaks corresponded to the H2Ti12O25 phase and no impurity phase was observed, as previously reported [18]. It can thus be inferred that carbon does not affect the crystallinity of H2Ti12O25.
The microstructures of pristine and carbon-coated H2Ti12O25 particles are shown in Fig. 2. All samples show almost similar morphologies and sizes regardless of different amounts of carbon coating. The
Conclusion
As a new energy storage device, the cylindrical hybrid supercapacitors were fabricated using pristine and carbon-coated H2Ti12O25/activated carbon. Among them, the 4.5 wt% carbon-coated H2Ti12O25 with 3 nm thickness was confirmed as the optimum condition. The 4.5 wt% carbon-coated H2Ti12O25 shows not only the electrochemical performance but also suppresses of the swollen phenomenon. The 4.5 wt% carbon-coated H2Ti12O25 shows the energy and power densities ranged from 5.71 to 38.8 Wh kg−1 and
Acknowledgement
The authors would like to thank the over 100 sponsors of the Center for Advanced Life Cycle Engineering (CALCE) of University of Maryland and the Hong Kong Polytechnic University for supporting research activities from the National Natural Science Foundation of China (Grant No. 61473242).
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