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High surface area TiO2 nanospheres as a high-rate anode material for aqueous aluminium-ion batteries
Solid State Ionics 300 (2017) 32–37
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Solid State Ionics journal homepage: www.elsevier.com/locate/ssi
High surface area TiO2 nanospheres as a high-rate anode material for aqueous aluminium-ion batteries Mahdi Kazazi ⁎, Pedram Abdollahi, Mahdi Mirzaei-Moghadam Department of Materials Engineering, Faculty of Engineering, Malayer University, Malayer, Iran
a r t i c l e
i n f o
Article history: Received 28 September 2016 Received in revised form 18 November 2016 Accepted 21 November 2016 Available online xxxx Keywords: Aluminium-ion batteries Titanium dioxide High-rate performance Anode material Sol-gel
a b s t r a c t Aluminium-ion batteries are promising electrochemical energy storage systems in replacement of Li-ion batteries owing to the low cost, non-toxicity, safety and three-electron transfer nature of Al-intercalation process. Anatase titanium dioxide has been suggested as a potential anode material for aqueous Al-ion batteries. In this work, TiO2 nanospheres (TiO2-NSs) with highly mesoporous morphology and large specific surface area (179.9 m2 g−1) were synthesized via hydrolysis of titanium glycolate precursor and subsequent hydrothermal treatment and calcination in an argon atmosphere. The structure and morphology of the prepared TiO2-NSs were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, transmission electron microscopy and nitrogen adsorption-desorption. The high surface area TiO2-NSs provide large electrode/electrolyte interface and good electrolyte-holding characteristic, leading to shorten the diffusion path of electrons and ions to the active material, which results in the better high-rate performance. The TiO2-NSs anode exhibited a high initial discharge capacity of 183 mAh g−1 at a current rate of 0.15 C and a superior rate performance with a capacity of 108 mAh g−1 even at a discharge current rate of 6.0 C, which are much higher than those of TiO2-P25 electrode. © 2016 Elsevier B.V. All rights reserved.
1. Introduction With the rapid growth of portable electronic devices, it is necessary to develop secondary batteries with a combination of high energy density, high power, relatively low cost, reliability, and safety [1,2]. Among the various types of commercial rechargeable batteries, the Li-ion batteries are the most common energy storage technology owing to their high power and energy density and long cycling stability [3,4]. However, there are some serious problems for Li-ion batteries, including high cost, toxicity, and safety concerns [5,6]. Furthermore, due to the growing demand for Li-ion batteries, there is a possible shortage of lithium resources [7,8]. Therefore, development of alternative battery systems based on earth abundant active materials is critical for next-generation secondary batteries. Among the various rechargeable batteries, aluminium-ion battery is attractive owing to the advantages of natural abundance, low cost and environmental friendliness of aluminium active material [9–13]. In addition, the intercalation/deintercalation process of Al3+ ion in aluminium-ion battery involves three electron transfers, as compared with the single-electron transfers during the redox reactions in Li-ion and Na-ion batteries [14,15]. Based on this understanding, Al-ion batteries can achieve higher specific capacity and energy density than Li-ion and Na-ion batteries [16,17]. ⁎ Corresponding author. E-mail addresses: [email protected], [email protected] (M. Kazazi).
http://dx.doi.org/10.1016/j.ssi.2016.11.028 0167-2738/© 2016 Elsevier B.V. All rights reserved.
Common rechargeable batteries contain organic-based solvents as the electrolyte, which are flammable and toxic and thus can cause safety hazards [18,19]. One of the most effective approaches is the use of nontoxic aqueous electrolytes instead of organic ones. From this point of view, green aqueous electrolytes are normally preferred to organic electrolytes. Most recently, it has been shown that trivalent aluminium ions can be reversibly intercalated in titanium dioxide (TiO2) in AlCl3 aqueous electrolyte [18,20,21], showing TiO2 can be considered as an anode material for such aqueous Al-ion batteries. This can be probably due to the Furthermore, vanadium oxides [16,22], graphite [23] and copper hexacyanoferrate [24] have been investigated as the cathode materials for Al-ion batteries. As the electrode active material, TiO2 exhibits advantages of good chemical stability and nontoxicity [25,26]. However, high surface area TiO2 is desired to improve the active material utilization and rate capability. It is well-known that the accessibility of electrons and electrolyte ions to the intercalation electrode materials can be improved if the electrode active particles are in nano-dimensions with large surface area and high porosity [27]. These features can shorten the diffusion path of electrons and ions into the electrode materials, leading to the increase in the electrochemical utilization of active materials and good power density. In the present study, highly porous TiO2 nanospheres with large specific surface area were successfully prepared via hydrolysis of titanium glycolate precursor, followed by subsequent hydrothermal process and calcination in an argon atmosphere for increasing the crystallinity of the TiO2 nanospheres. Titanium glycolate precursor was
M. Kazazi et al. / Solid State Ionics 300 (2017) 32–37
33
out between −0.5 and −1.2 V (vs. Ag/AgCl) on a three-electrode battery test system (BTS, Iran) at various current rates ranging from 0.15 to 6.0 C (1 C = 335 mA g−1). All the electrochemical tests were conducted at ambient temperature. The specific capacities were calculated on the basis of the weight of TiO2 active material in the electrodes.
prepared using a sol-gel method. In addition, the characterization and electrochemical performances of TiO2 nanospheres as the anode material of the aqueous Al-ion batteries were investigated at different rates in detail. The prepared TiO2 nanospheres due to their high surface area and fine particle size facilitates fast Al3 + ion intercalation/ deintercalation process, leading to high discharge capacity and highrate performance.
3. Results and discussion
2. Experimental
3.1. Characterization of TiO2-NSs
2.1. Preparation and characterization of TiO2 nanospheres
To investigate the structure of the prepared TiO2-NSs, XRD analysis was conducted as shown in Fig. 1a. The main seven diffraction peaks at 25.3°, 37.9°, 48.0°, 54.1°, 62.5°, 69.9° and 75.1° correspond to the (101), (004), (200), (105), (204), (220) and (215) crystal planes of pure anatase phase, respectively, which are in good agreement with standard diffraction patterns of anatase TiO2 with a tetragonal structure (JCPDS No: 21–1272) [30]. Moreover, the mean crystallite size of the polycrystalline TiO2-NSs was estimated by the Scherrer equation [31]:
TiO2 nanospheres (TiO2-NSs) with high specific surface area were prepared with a template-free hydrothermal process using titanium glycolate (TG) as a precursor [28]. Firstly, titanium glycolate precursor was prepared using a sol-gel process, as described in the literature with little modifications [29]. In a typical procedure, tetrabutyl titanate (5 ml; Merck) was added dropwise to ethylene glycol (100 ml; Merck) and stirred at room temperature overnight. Then, the mixture was added into acetone (350 ml; Merck) containing 1 ml distilled water and stirred for another 2 h. Finally, the white precipitates of TG were filtrated and washed several times with distilled water and absolute ethanol and dried in an oven at 60 °C for 12 h. Subsequently, to obtain TiO2-NSs, 1 g of the as-prepared TG was added into 200 ml distilled water and then the mixture was refluxed at 90 °C for 2 h. After filtrating and washing of the products with distilled water and absolute ethanol, the wet powders were added into 40 ml ethanol-distilled water solution (2:1, v/v), and then the obtained suspension was transferred to a 50 ml teflon-lined autoclave and heated at 180 °C for 24 h. After the autoclave was naturally cooled to room temperature, the precipitates were collected by centrifugation and washed and dried at 60 °C for 12 h. Finally, the highly crystalline TiO2-NSs were obtained by the calcinations of the products at 400 °C for 4 h under argon atmosphere. The crystal structure of the prepared TiO2-NSs was analyzed using Xray diffraction (XRD, Unisantis XMD-300) with Cu Kα radiation source (λ = 0.154 nm). The composition of the product was examined using Fourier Transform Infrared Spectroscopy (FTIR, Perkinelmer, 400). The surface morphology of the TiO2-NSs was observed using a transmission electron microscope (TEM, CM200FEG-Philips). The specific surface area and pore volume of the as-prepared TiO2-NSs were obtained at 77 K using a BELSORP-mini instrument (BEL, Japan Inc.) with the Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) measurements, respectively. Before N2 adsorption–desorption measurements, the sample was degassed at 150 °C for 2 h.
d¼
0:94λ w cosθ
where d is the mean crystallite size, λ is the Cu Kα wavelength of X-ray (0.154 nm), θ is the Bragg angle and w is the full width at half maximum of the XRD peak, in radians. From this estimation, the mean crystallite size of the anatase TiO2-NSs was calculated to be 6.6 nm. The chemical structure of the prepared TiO2-NSs was further investigated by FTIR in the range of 400–4000 cm− 1, as depicted in Fig. 1b. The band at 1627 cm− 1 and the broad band around 3326 cm− 1 in the spectrum are attributed to the bending vibrations of the surface-adsorbed water molecules and stretching vibration of hydroxyl group, respectively [32, 33]. The peaks below 800 cm−1 are ascribed to the stretching and bending vibrations of Ti-O and Ti-O-Ti groups [34–36].
2.2. Electrode preparation and electrochemical measurements To prepare the working electrodes, the TiO2-NSs (80 wt%) were mixed with acetylene black conducting agent (10 wt%) and polyvinylidene fluoride binder (10 wt%), dissolved in NMP (N-methyl2-pyrrolidone) using an ultrasonic bath to form a homogeneous slurry. The slurry was coated onto 10 mm diameter nickel disks. After organic solvent evaporation, the prepared electrodes were dried in an oven at 80 °C overnight. The TiO2-P25 (Degussa) working electrode was prepared by the same way for comparison. The TiO2 content was the same in all the electrodes. All the electrochemical performances of the TiO2-NSs and TiO2-P25 electrodes were investigated in a conventional three-electrode cell containing 1.0 M AlCl3 aqueous solution as the electrolyte. A silver/silver chloride electrode and platinum electrode were used as the reference electrode and the counter electrode, respectively. Cyclic voltammetry (CV) measurements were performed between − 0.4 to − 1.5 V (vs. Ag/AgCl) at a scan rate of 10 mV s−1 on an Ivium electrochemical workstation. AC impedance of the cells before discharge was measured over the frequency range from 100 KHz to 10 mHz with the amplitude of ± 10 mV. Also, the galvanostatic charge–discharge tests were carried
ð1Þ
Fig. 1. (a) XRD pattern and (b) FTIR spectrum of as-prepared TiO2-NSs.
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M. Kazazi et al. / Solid State Ionics 300 (2017) 32–37
The morphological characterization of the as-prepared TiO2 was examined by TEM, as given in Fig. 2 at different magnifications. From the low-magnification image in Fig. 2a, it can be seen that the TiO2-NSs have successfully synthesized with a uniform size distribution. Moreover, Fig. 2b at higher magnification indicates that the sample consists of semi-spherical nanoparticles, which are orderly attached to each other, forming a three-dimensional porous structure. The TEM image reveals that the nanospheres offer an average particle size of about 15 nm. It is expected that the prepared TiO2-NSs with porous morphology and very fine particle size yield a high specific surface area, which could facilitate the transport of Al3+ ions from the electrolyte to the active TiO2, resulting in a better rate capability. Also, the fine particle size can cause to improve the accessibility of electrons and aluminium ions to the active TiO2, leading to an appropriate electrochemical utilization of active material and hence a high specific capacity. Considering that the anode capacity and the high-rate performance are highly related to the specific surface area and pore size of the active material, surface area and porosity measurements were performed using BET surface area analysis and corresponding pore size distribution, as presented in Fig. 3. As seen in Fig. 3a, the prepared TiO2 sample has a type V isotherm with H1-type hysteresis loop according to the IUPAC (International Union of Pure and Applied Chemistry) classification, which is characteristic of mesoporous materials [37,38], as further confirmed from the Barrett-Joyner-Halenda (BJH) pore size distribution curve in Fig. 3b. According to BET plot of nitrogen adsorption isotherm, the specific surface area of the TiO2-NSs was found to be 179.9 m2 g−1, which is much higher than that of the used TiO2-P25 (50 m2 g−1).
Fig. 3. (a) Nitrogen adsorption-desorption isotherms and (b) BJH desorption pore size distribution plot of the TiO2-NSs.
Moreover, the total pore volume was estimated to be 0.48 cm3 g−1. As can be seen from Fig. 3b, the TiO2-NSs have a uniform pore size distribution mainly centered at 4.0 nm. The mesoporous structure with high specific surface area of TiO2-NSs can provide fast and efficient transport for electrons and Al3+ ions in the anode structure as well as large contact area between TiO2 active materials and electrolyte, leading to enhance the electrochemical performance. To study the electrochemical characteristics of the prepared TiO2NSs, cyclic voltammetry (CV) was performed on the TiO2-NSs and TiO2-P25 anode materials at a scan rate of 10 mV s− 1 in the 1.0 M AlCl3 aqueous solution, as shown in Fig. 4. In both electrodes, a couple of well-defined reversible redox peaks can be clearly observed at
Fig. 2. TEM images of as-prepared TiO2-NSs at different magnifications.
Fig. 4. Cyclic voltammograms of the TiO2-NSs and TiO2-P25 electrodes at a scan rate of 10 mV s−1.
M. Kazazi et al. / Solid State Ionics 300 (2017) 32–37
around − 1.25 and − 0.87 V (vs. Ag/AgCl), which are assigned to the electrochemical intercalation/deintercalation process of Al3 + ion in TiO2, as has been reported previously [18,20,39]. Compared with the TiO2-P25 electrode, the voltage difference between reduction and oxidation peaks for the TiO2-NSs anode is slightly smaller, indicating the higher reversibility with less polarization of the TiO2-NSs anode. Furthermore, the intensity of the redox peaks and area under the CV curve for the TiO2-NSs anode has greatly increased, suggesting that the TiO2-NSs anode exhibits better active material utilization and hence indicates higher specific capacity. In the cathodic scan, it can be seen that hydrogen evolution process occurs after Al3+ intercalation, indicating the aluminium ion intercalation is preferential to the hydrogen evolution. This is probably due to the strong solvation of ions in aqueous electrolytes, resulting to increase the hydrogen evolution overpotential [40,41]. To investigate the Al3+ storage capacity and high-rate performance of the TiO2-NSs anode, initial galvanostatic charge-discharge measurements were performed at various rates (0.15, 1.5 and 6.0 C) on the TiO2-NSs and TiO2-P25 anodes. As seen in panels a and b of Fig. 5, there are clear charge and discharge voltage plateaus for both anode materials, which are well consistent with the redox peaks in the CV curves of the electrodes (Fig. 4). Compared to the TiO2-P25 anode, the TiO2-NSs anode shows a slightly more negative discharge voltage
Fig. 5. Initial galvanostatic charge-discharge curves of (a) TiO2-NSs and (b) TiO2-P25 electrodes at different current rates.
35
plateau, which can be attributed to the facile electron and aluminium ion transfer in the TiO2-NSs electrode structure mainly due to its porous structure with high surface area. Moreover, it is obvious that the discharge capacities of the TiO2-NSs anode are higher than those of the TiO2-P25 anode at the same rate. The TiO2-NSs anode exhibits discharge capacities of 183, 126 and 108 mAh g−1, respectively, at the discharge rates of 0.15, 1.5 and 6.0 C. It means that the TiO2-NSs anode presents a discharge capacity loss of 41.0% as current rate increases from 0.15 to 6.0 C, which is lower than that for the TiO2-P25 anode (73.8%), implying the better rate capability of the TiO2-NSs electrode. The TiO2-NSs active material can adsorb and infiltrate the electrolyte in its porous structure, enabling a facile access of Al3 + ion to the active material. Also, the TiO2-NSs anode provides high electrode/electrolyte contact area, leading to reduce the contact resistance and improve the charge transfer [27]. The maximum discharge capacity of 183 mAh g−1 for the TiO2-NSs anode corresponds to Al0.18TiO2. With monovalent lithium storage in TiO2, the same discharge capacity would be obtained with threefold increase of the ion insertion (corresponding to Li0.54TiO2). With this regard and taking into account the smaller ionic radius of Al3+ (53.5 pm) compared with that of the Li+ (76 pm), Al intercalation results in less volume expansion of the host TiO2, which is highly effective on the cycle stability and battery performance. The rate performance and cycle stability of the TiO2-NSs anode were further investigated by cycling the material at current range of 0.15– 6.0 C. As shown in Fig. 6, the TiO2-NSs electrode delivers reversible capacities of 180, 158, 127, 109, and 96 mAh g−1 at current rates of 0.15, 0.6, 1.5, 3.0 and 6.0 C, respectively. When the discharge rate is turned back to 0.15 C, the capacity is resumed to 174 mAh g−1 after 25 cycles at various rates, showing promising rate capability and good cycle stability. The excellent cycle stability of the TiO2-NSs at high current rates is due to the stable structure during cycling and the short diffusion pathway provided by the porous morphology. The interfacial charge transfer resistance and Al3+ ion diffusion in the host TiO2 structure were more precisely investigated by EIS measurements on the both TiO2 electrodes. Fig. 7 shows the Nyquist spectra of the fresh TiO2-P25 and TiO2-NSs anodes. As seen, the both impedance spectra are composed of a depressed semicircle in the high-medium frequency region corresponding to the charge transfer process and a straight line at lower frequencies corresponding to the Warburg behavior, which is associated with aluminium ion diffusion in the electrode structure. The intercept on real axis in the high frequency region corresponds to the resistance of electrolyte. The impedance spectra were analyzed with the equivalent circuit displayed in inset of Fig. 7, where Re, Cdl, Rch and W are the electrolyte resistance, the constant phase element (CPE) arising from double-layer capacitance, the charge-transfer
Fig. 6. Rate performance of TiO2-NSs and TiO2-P25 electrodes at different current rates.
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M. Kazazi et al. / Solid State Ionics 300 (2017) 32–37
4. Conclusion In conclusion, the anatase TiO2 nanospheres (TiO2-NSs) anode material for aqueous aluminium-ion batteries were successfully prepared using hydrolysis of titanium glycolate precursor, followed by subsequent hydrothermal process and calcination in an argon atmosphere. The prepared active material exhibited a mesoporous structure with a high surface area, confirmed by XRD, FTIR, TEM, BET and BJH techniques. The insertion/desertion process of Al3 + ion in the TiO2-NSs was investigated by cyclic voltammetry, galvanostatic charge-discharge at various rates and EIS measurements. Compared with the TiO2-P25 anode, the TiO2-NSs anode exhibited a higher discharge capacity (183 mAh g−1 at a current rate of 0.15 C) and an excellent rate capability (108 mAh g−1 even at 6.0 C). These can be attributed to the shorter transfer pathways for both Al3+ ions and electrons provided by mesoporous structure of TiO2-NSs active material with large electrode/electrolyte interface.
Fig. 7. Nyquist spectra of fresh TiO2-NSs and TiO2-P25 electrodes.
resistance and the Warburg impedance, respectively [1]. The chargetransfer resistance of the TiO2-NSs and TiO2-P25 electrodes was calculated to be 36.4 and 82.7 Ω, respectively. Moreover, it is clear that the TiO2-NSs anode has a steeper and shorter Warburg line in the low frequency region, implying a higher Al3+ ion diffusion rate in the electrode structure [27,42]. Clearly, the diffusion coefficients of Al3+ (D) were calculated for both anodes by the following equation [43]:
D ¼ 0:5
RT 2 An2 F σ
!2 wC
ð2Þ
where R is the gas constant, T is the absolute temperature, A is the area of the electrode surface, n is the valence state of the diffusion species and here is 3 for Al3+, F is the Faraday's constant, C is the molar concentration of Al3+ ions and σw is the Warburg coefficient, which has relationship with impedance real part Zre in the low frequency region (ω = 2πf): Zre ¼ RD þ RL þ σ w ω−0:5
ð3Þ
where ω, RD and RL are angular frequency in the low frequency range, diffusive resistance and liquid resistance, respectively. The relationship between Zre and reciprocal root square of the lower angular frequencies (ω-1/2) for both anode materials are shown in Fig. 8. The slope of the fitted line is the σw. The aluminium ion diffusion coefficients of the TiO2-NSs and TiO2-P25 electrodes were calculated to be 5.33 × 10−12 and 4.73 × 10−13 cm2 s−1, respectively. The fast Al3+ diffusion, along with the small charge-transfer resistance, contributes to the superior high-rate performance of the TiO2-NSs electrode.
Fig. 8. The relationship between Zre and ω-1/2 at low frequencies for TiO2-P25 and TiO2-NSs electrodes.
Acknowledgement This work was financially supported by Malayer University (No. 84/ 5-1-601). Dr. Shahab Moghadas and Mrs. Saremi are gratefully acknowledged for their assistance in sample preparation and electrochemical measurements, respectively. References [1] M. Kazazi, M.R. Vaezi, A. Kazemzadeh, Ionics 20 (2014) 635–643. [2] M. Kazazi, F. Ghadami, M.R. Dadfar, M. Sobhani, A.H. Mohammadi, Solid State Ionics 290 (2016) 40–46. [3] H. Sun, W. Wang, Z. Yu, Y. Yuan, S. Wang, S. Jiao, Chem. Commun. 51 (2015) 11892–11895. [4] Y. Wang, R. Chen, T. Chen, H. Lv, G. Zhu, L. Ma, C. Wang, Z. Jin, J. Liu, Energy Storage Mater. 4 (2016) 103–129. [5] M.D. Slater, D. Kim, E. Lee, C.S. Johnson, Adv. Funct. Mater. 23 (2013) 947–958. [6] M.S. Park, J.G. Kim, Y.J. Kim, N.S. Choi, J.S. Kim, Isr. J. Chem. 55 (2015) 570–585. [7] S.W. Kim, D.H. Seo, X.H. Ma, G. Ceder, K. Kang, Adv. Energy Mater. 2 (2012) 710–721. [8] F. Risacher, B. Fritz, Aquat. Geochem. 15 (2009) 123–157. [9] N. Jayaprakash, S.K. Das, L.A. Archer, Chem. Commun. 47 (2011) 12610–12612. [10] S.C. Jung, Y.J. Kang, D.J. Yoo, J.W. Choi, Y.K. Han, J. Phys. Chem. C 120 (2016) 13384–13389. [11] L. Geng, G. Lv, X. Xing, J. Guo, Chem. Mater. 27 (2015) 4926–4929. [12] J.V. Rani, V. Kanakaiah, T. Dadmal, M.S. Rao, S. Bhavanarushi, J. Electrochem. Soc. 160 (2013) A1781–A1784. [13] M. Chiku, H. Takeda, S. Matsumura, E. Higuchi, H. Inoue, ACS Appl. Mater. Interfaces 7 (2015) 24385–24389. [14] W. Wang, B. Jiang, W. Xiong, H. Sun, Z. Lin, L. Hu, J. Tu, J. Hou, H. Zhu, S. Jiao, Sci. Rep. 3 (2013) 3383–3388. [15] Q. Li, N.J. Bjerrum, J. Power Sources 110 (2002) 1–10. [16] H. Wang, Y. Bai, S. Chen, X. Luo, C. Wu, F. Wu, J. Lu, K. Amine, ACS Appl. Mater. Interfaces 7 (2015) 80–84. [17] F. Wu, C. Wu, Chin. Sci. Bull. 59 (2014) 3369–3376. [18] Y.J. He, J.F. Peng, W. Chu, Y.Z. Li, D.G. Tong, J. Mater. Chem. A 2 (2014) 1721–1731. [19] M. Illbeigi, A. Fazlali, M. Kazazi, A.H. Mohammadi, Solid State Ionics 289 (2016) 180–187. [20] S. Liu, J.J. Hu, N.F. Yan, G.L. Pan, G.R. Li, X.P. Gao, Energy Environ. Sci. 5 (2012) 9743–9746. [21] W. Zhong, S. Sang, Y. Liu, Q. Wu, K. Liu, H. Liu, J. Power Sources 294 (2015) 216–222. [22] L.D. Reed, E. Menke, J. Electrochem. Soc. 160 (2013) A915–A917. [23] M.C. Lin, M. Gong, B. Lu, Y. Wu, D.Y. Wang, M. Guan, M. Angell, C. Chen, J. Yang, B.J. Hwang, H. Dai1, Nature 520 (2015) 324–328. [24] S. Liu, G.L. Pan, G.R. Li, X.P. Gao, J. Mater. Chem. A 3 (2015) 959–962. [25] X. Shi, Z. Zhang, K. Du, Y. Lai, J. Fang, J. Li, J. Power Sources 330 (2016) 1–6. [26] O. Rhee, G. Lee, J. Choi, ACS Appl. Mater. Interfaces 8 (2016) 14558–14563. [27] W. Wen, J.M. Wu, Y.Z. Jiang, S.L. Yu, J.Q. Bai, M.H. Cao, J. Cui, Sci. Rep. 5 (2015) 11804. [28] Z. Xiu, X. Hao, Y. Wu, Q. Lu, S. Liu, J. Power Sources 287 (2015) 334–340. [29] X. Zhang, P.S. Kumar, V. Aravindan, H.H. Liu, J. Sundaramurthy, S.G. Mhaisalkar, H.M. Duong, S. Ramakrishna, S. Madhavi, J. Phys. Chem. C 116 (2012) 14780–14788. [30] V. Augustyn, E.R. White, J. Ko, G. Grüner, B.C. Regan, B. Dunn, Mater. Horiz. 1 (2014) 219–223. [31] Y. Yeo, J.W. Jung, K. Park, I.D. Kim, Sci. Rep. 5 (2015) 13862. [32] N. Venkatachalam, M. Palanichamy, V. Murugesan, Mater. Chem. Phys. 104 (2007) 454–459. [33] G. Soler-Illia, A. Louis, C. Sanchez, Chem. Mater. 14 (2002) 750–759. [34] J.C. Yu, L.Z. Zhang, Z. Zheng, J.C. Zhao, Chem. Mater. 15 (2003) 2280–2286. [35] J. Zhao, X.X. Zou, J. Su, P.P. Wang, L.J. Zhou, G.D. Li, Dalton Trans. 42 (2013) 4365–4368.
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Solid State Ionics journal homepage: www.elsevier.com/locate/ssi
High surface area TiO2 nanospheres as a high-rate anode material for aqueous aluminium-ion batteries Mahdi Kazazi ⁎, Pedram Abdollahi, Mahdi Mirzaei-Moghadam Department of Materials Engineering, Faculty of Engineering, Malayer University, Malayer, Iran
a r t i c l e
i n f o
Article history: Received 28 September 2016 Received in revised form 18 November 2016 Accepted 21 November 2016 Available online xxxx Keywords: Aluminium-ion batteries Titanium dioxide High-rate performance Anode material Sol-gel
a b s t r a c t Aluminium-ion batteries are promising electrochemical energy storage systems in replacement of Li-ion batteries owing to the low cost, non-toxicity, safety and three-electron transfer nature of Al-intercalation process. Anatase titanium dioxide has been suggested as a potential anode material for aqueous Al-ion batteries. In this work, TiO2 nanospheres (TiO2-NSs) with highly mesoporous morphology and large specific surface area (179.9 m2 g−1) were synthesized via hydrolysis of titanium glycolate precursor and subsequent hydrothermal treatment and calcination in an argon atmosphere. The structure and morphology of the prepared TiO2-NSs were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, transmission electron microscopy and nitrogen adsorption-desorption. The high surface area TiO2-NSs provide large electrode/electrolyte interface and good electrolyte-holding characteristic, leading to shorten the diffusion path of electrons and ions to the active material, which results in the better high-rate performance. The TiO2-NSs anode exhibited a high initial discharge capacity of 183 mAh g−1 at a current rate of 0.15 C and a superior rate performance with a capacity of 108 mAh g−1 even at a discharge current rate of 6.0 C, which are much higher than those of TiO2-P25 electrode. © 2016 Elsevier B.V. All rights reserved.
1. Introduction With the rapid growth of portable electronic devices, it is necessary to develop secondary batteries with a combination of high energy density, high power, relatively low cost, reliability, and safety [1,2]. Among the various types of commercial rechargeable batteries, the Li-ion batteries are the most common energy storage technology owing to their high power and energy density and long cycling stability [3,4]. However, there are some serious problems for Li-ion batteries, including high cost, toxicity, and safety concerns [5,6]. Furthermore, due to the growing demand for Li-ion batteries, there is a possible shortage of lithium resources [7,8]. Therefore, development of alternative battery systems based on earth abundant active materials is critical for next-generation secondary batteries. Among the various rechargeable batteries, aluminium-ion battery is attractive owing to the advantages of natural abundance, low cost and environmental friendliness of aluminium active material [9–13]. In addition, the intercalation/deintercalation process of Al3+ ion in aluminium-ion battery involves three electron transfers, as compared with the single-electron transfers during the redox reactions in Li-ion and Na-ion batteries [14,15]. Based on this understanding, Al-ion batteries can achieve higher specific capacity and energy density than Li-ion and Na-ion batteries [16,17]. ⁎ Corresponding author. E-mail addresses: [email protected], [email protected] (M. Kazazi).
http://dx.doi.org/10.1016/j.ssi.2016.11.028 0167-2738/© 2016 Elsevier B.V. All rights reserved.
Common rechargeable batteries contain organic-based solvents as the electrolyte, which are flammable and toxic and thus can cause safety hazards [18,19]. One of the most effective approaches is the use of nontoxic aqueous electrolytes instead of organic ones. From this point of view, green aqueous electrolytes are normally preferred to organic electrolytes. Most recently, it has been shown that trivalent aluminium ions can be reversibly intercalated in titanium dioxide (TiO2) in AlCl3 aqueous electrolyte [18,20,21], showing TiO2 can be considered as an anode material for such aqueous Al-ion batteries. This can be probably due to the Furthermore, vanadium oxides [16,22], graphite [23] and copper hexacyanoferrate [24] have been investigated as the cathode materials for Al-ion batteries. As the electrode active material, TiO2 exhibits advantages of good chemical stability and nontoxicity [25,26]. However, high surface area TiO2 is desired to improve the active material utilization and rate capability. It is well-known that the accessibility of electrons and electrolyte ions to the intercalation electrode materials can be improved if the electrode active particles are in nano-dimensions with large surface area and high porosity [27]. These features can shorten the diffusion path of electrons and ions into the electrode materials, leading to the increase in the electrochemical utilization of active materials and good power density. In the present study, highly porous TiO2 nanospheres with large specific surface area were successfully prepared via hydrolysis of titanium glycolate precursor, followed by subsequent hydrothermal process and calcination in an argon atmosphere for increasing the crystallinity of the TiO2 nanospheres. Titanium glycolate precursor was
M. Kazazi et al. / Solid State Ionics 300 (2017) 32–37
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out between −0.5 and −1.2 V (vs. Ag/AgCl) on a three-electrode battery test system (BTS, Iran) at various current rates ranging from 0.15 to 6.0 C (1 C = 335 mA g−1). All the electrochemical tests were conducted at ambient temperature. The specific capacities were calculated on the basis of the weight of TiO2 active material in the electrodes.
prepared using a sol-gel method. In addition, the characterization and electrochemical performances of TiO2 nanospheres as the anode material of the aqueous Al-ion batteries were investigated at different rates in detail. The prepared TiO2 nanospheres due to their high surface area and fine particle size facilitates fast Al3 + ion intercalation/ deintercalation process, leading to high discharge capacity and highrate performance.
3. Results and discussion
2. Experimental
3.1. Characterization of TiO2-NSs
2.1. Preparation and characterization of TiO2 nanospheres
To investigate the structure of the prepared TiO2-NSs, XRD analysis was conducted as shown in Fig. 1a. The main seven diffraction peaks at 25.3°, 37.9°, 48.0°, 54.1°, 62.5°, 69.9° and 75.1° correspond to the (101), (004), (200), (105), (204), (220) and (215) crystal planes of pure anatase phase, respectively, which are in good agreement with standard diffraction patterns of anatase TiO2 with a tetragonal structure (JCPDS No: 21–1272) [30]. Moreover, the mean crystallite size of the polycrystalline TiO2-NSs was estimated by the Scherrer equation [31]:
TiO2 nanospheres (TiO2-NSs) with high specific surface area were prepared with a template-free hydrothermal process using titanium glycolate (TG) as a precursor [28]. Firstly, titanium glycolate precursor was prepared using a sol-gel process, as described in the literature with little modifications [29]. In a typical procedure, tetrabutyl titanate (5 ml; Merck) was added dropwise to ethylene glycol (100 ml; Merck) and stirred at room temperature overnight. Then, the mixture was added into acetone (350 ml; Merck) containing 1 ml distilled water and stirred for another 2 h. Finally, the white precipitates of TG were filtrated and washed several times with distilled water and absolute ethanol and dried in an oven at 60 °C for 12 h. Subsequently, to obtain TiO2-NSs, 1 g of the as-prepared TG was added into 200 ml distilled water and then the mixture was refluxed at 90 °C for 2 h. After filtrating and washing of the products with distilled water and absolute ethanol, the wet powders were added into 40 ml ethanol-distilled water solution (2:1, v/v), and then the obtained suspension was transferred to a 50 ml teflon-lined autoclave and heated at 180 °C for 24 h. After the autoclave was naturally cooled to room temperature, the precipitates were collected by centrifugation and washed and dried at 60 °C for 12 h. Finally, the highly crystalline TiO2-NSs were obtained by the calcinations of the products at 400 °C for 4 h under argon atmosphere. The crystal structure of the prepared TiO2-NSs was analyzed using Xray diffraction (XRD, Unisantis XMD-300) with Cu Kα radiation source (λ = 0.154 nm). The composition of the product was examined using Fourier Transform Infrared Spectroscopy (FTIR, Perkinelmer, 400). The surface morphology of the TiO2-NSs was observed using a transmission electron microscope (TEM, CM200FEG-Philips). The specific surface area and pore volume of the as-prepared TiO2-NSs were obtained at 77 K using a BELSORP-mini instrument (BEL, Japan Inc.) with the Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) measurements, respectively. Before N2 adsorption–desorption measurements, the sample was degassed at 150 °C for 2 h.
d¼
0:94λ w cosθ
where d is the mean crystallite size, λ is the Cu Kα wavelength of X-ray (0.154 nm), θ is the Bragg angle and w is the full width at half maximum of the XRD peak, in radians. From this estimation, the mean crystallite size of the anatase TiO2-NSs was calculated to be 6.6 nm. The chemical structure of the prepared TiO2-NSs was further investigated by FTIR in the range of 400–4000 cm− 1, as depicted in Fig. 1b. The band at 1627 cm− 1 and the broad band around 3326 cm− 1 in the spectrum are attributed to the bending vibrations of the surface-adsorbed water molecules and stretching vibration of hydroxyl group, respectively [32, 33]. The peaks below 800 cm−1 are ascribed to the stretching and bending vibrations of Ti-O and Ti-O-Ti groups [34–36].
2.2. Electrode preparation and electrochemical measurements To prepare the working electrodes, the TiO2-NSs (80 wt%) were mixed with acetylene black conducting agent (10 wt%) and polyvinylidene fluoride binder (10 wt%), dissolved in NMP (N-methyl2-pyrrolidone) using an ultrasonic bath to form a homogeneous slurry. The slurry was coated onto 10 mm diameter nickel disks. After organic solvent evaporation, the prepared electrodes were dried in an oven at 80 °C overnight. The TiO2-P25 (Degussa) working electrode was prepared by the same way for comparison. The TiO2 content was the same in all the electrodes. All the electrochemical performances of the TiO2-NSs and TiO2-P25 electrodes were investigated in a conventional three-electrode cell containing 1.0 M AlCl3 aqueous solution as the electrolyte. A silver/silver chloride electrode and platinum electrode were used as the reference electrode and the counter electrode, respectively. Cyclic voltammetry (CV) measurements were performed between − 0.4 to − 1.5 V (vs. Ag/AgCl) at a scan rate of 10 mV s−1 on an Ivium electrochemical workstation. AC impedance of the cells before discharge was measured over the frequency range from 100 KHz to 10 mHz with the amplitude of ± 10 mV. Also, the galvanostatic charge–discharge tests were carried
ð1Þ
Fig. 1. (a) XRD pattern and (b) FTIR spectrum of as-prepared TiO2-NSs.
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The morphological characterization of the as-prepared TiO2 was examined by TEM, as given in Fig. 2 at different magnifications. From the low-magnification image in Fig. 2a, it can be seen that the TiO2-NSs have successfully synthesized with a uniform size distribution. Moreover, Fig. 2b at higher magnification indicates that the sample consists of semi-spherical nanoparticles, which are orderly attached to each other, forming a three-dimensional porous structure. The TEM image reveals that the nanospheres offer an average particle size of about 15 nm. It is expected that the prepared TiO2-NSs with porous morphology and very fine particle size yield a high specific surface area, which could facilitate the transport of Al3+ ions from the electrolyte to the active TiO2, resulting in a better rate capability. Also, the fine particle size can cause to improve the accessibility of electrons and aluminium ions to the active TiO2, leading to an appropriate electrochemical utilization of active material and hence a high specific capacity. Considering that the anode capacity and the high-rate performance are highly related to the specific surface area and pore size of the active material, surface area and porosity measurements were performed using BET surface area analysis and corresponding pore size distribution, as presented in Fig. 3. As seen in Fig. 3a, the prepared TiO2 sample has a type V isotherm with H1-type hysteresis loop according to the IUPAC (International Union of Pure and Applied Chemistry) classification, which is characteristic of mesoporous materials [37,38], as further confirmed from the Barrett-Joyner-Halenda (BJH) pore size distribution curve in Fig. 3b. According to BET plot of nitrogen adsorption isotherm, the specific surface area of the TiO2-NSs was found to be 179.9 m2 g−1, which is much higher than that of the used TiO2-P25 (50 m2 g−1).
Fig. 3. (a) Nitrogen adsorption-desorption isotherms and (b) BJH desorption pore size distribution plot of the TiO2-NSs.
Moreover, the total pore volume was estimated to be 0.48 cm3 g−1. As can be seen from Fig. 3b, the TiO2-NSs have a uniform pore size distribution mainly centered at 4.0 nm. The mesoporous structure with high specific surface area of TiO2-NSs can provide fast and efficient transport for electrons and Al3+ ions in the anode structure as well as large contact area between TiO2 active materials and electrolyte, leading to enhance the electrochemical performance. To study the electrochemical characteristics of the prepared TiO2NSs, cyclic voltammetry (CV) was performed on the TiO2-NSs and TiO2-P25 anode materials at a scan rate of 10 mV s− 1 in the 1.0 M AlCl3 aqueous solution, as shown in Fig. 4. In both electrodes, a couple of well-defined reversible redox peaks can be clearly observed at
Fig. 2. TEM images of as-prepared TiO2-NSs at different magnifications.
Fig. 4. Cyclic voltammograms of the TiO2-NSs and TiO2-P25 electrodes at a scan rate of 10 mV s−1.
M. Kazazi et al. / Solid State Ionics 300 (2017) 32–37
around − 1.25 and − 0.87 V (vs. Ag/AgCl), which are assigned to the electrochemical intercalation/deintercalation process of Al3 + ion in TiO2, as has been reported previously [18,20,39]. Compared with the TiO2-P25 electrode, the voltage difference between reduction and oxidation peaks for the TiO2-NSs anode is slightly smaller, indicating the higher reversibility with less polarization of the TiO2-NSs anode. Furthermore, the intensity of the redox peaks and area under the CV curve for the TiO2-NSs anode has greatly increased, suggesting that the TiO2-NSs anode exhibits better active material utilization and hence indicates higher specific capacity. In the cathodic scan, it can be seen that hydrogen evolution process occurs after Al3+ intercalation, indicating the aluminium ion intercalation is preferential to the hydrogen evolution. This is probably due to the strong solvation of ions in aqueous electrolytes, resulting to increase the hydrogen evolution overpotential [40,41]. To investigate the Al3+ storage capacity and high-rate performance of the TiO2-NSs anode, initial galvanostatic charge-discharge measurements were performed at various rates (0.15, 1.5 and 6.0 C) on the TiO2-NSs and TiO2-P25 anodes. As seen in panels a and b of Fig. 5, there are clear charge and discharge voltage plateaus for both anode materials, which are well consistent with the redox peaks in the CV curves of the electrodes (Fig. 4). Compared to the TiO2-P25 anode, the TiO2-NSs anode shows a slightly more negative discharge voltage
Fig. 5. Initial galvanostatic charge-discharge curves of (a) TiO2-NSs and (b) TiO2-P25 electrodes at different current rates.
35
plateau, which can be attributed to the facile electron and aluminium ion transfer in the TiO2-NSs electrode structure mainly due to its porous structure with high surface area. Moreover, it is obvious that the discharge capacities of the TiO2-NSs anode are higher than those of the TiO2-P25 anode at the same rate. The TiO2-NSs anode exhibits discharge capacities of 183, 126 and 108 mAh g−1, respectively, at the discharge rates of 0.15, 1.5 and 6.0 C. It means that the TiO2-NSs anode presents a discharge capacity loss of 41.0% as current rate increases from 0.15 to 6.0 C, which is lower than that for the TiO2-P25 anode (73.8%), implying the better rate capability of the TiO2-NSs electrode. The TiO2-NSs active material can adsorb and infiltrate the electrolyte in its porous structure, enabling a facile access of Al3 + ion to the active material. Also, the TiO2-NSs anode provides high electrode/electrolyte contact area, leading to reduce the contact resistance and improve the charge transfer [27]. The maximum discharge capacity of 183 mAh g−1 for the TiO2-NSs anode corresponds to Al0.18TiO2. With monovalent lithium storage in TiO2, the same discharge capacity would be obtained with threefold increase of the ion insertion (corresponding to Li0.54TiO2). With this regard and taking into account the smaller ionic radius of Al3+ (53.5 pm) compared with that of the Li+ (76 pm), Al intercalation results in less volume expansion of the host TiO2, which is highly effective on the cycle stability and battery performance. The rate performance and cycle stability of the TiO2-NSs anode were further investigated by cycling the material at current range of 0.15– 6.0 C. As shown in Fig. 6, the TiO2-NSs electrode delivers reversible capacities of 180, 158, 127, 109, and 96 mAh g−1 at current rates of 0.15, 0.6, 1.5, 3.0 and 6.0 C, respectively. When the discharge rate is turned back to 0.15 C, the capacity is resumed to 174 mAh g−1 after 25 cycles at various rates, showing promising rate capability and good cycle stability. The excellent cycle stability of the TiO2-NSs at high current rates is due to the stable structure during cycling and the short diffusion pathway provided by the porous morphology. The interfacial charge transfer resistance and Al3+ ion diffusion in the host TiO2 structure were more precisely investigated by EIS measurements on the both TiO2 electrodes. Fig. 7 shows the Nyquist spectra of the fresh TiO2-P25 and TiO2-NSs anodes. As seen, the both impedance spectra are composed of a depressed semicircle in the high-medium frequency region corresponding to the charge transfer process and a straight line at lower frequencies corresponding to the Warburg behavior, which is associated with aluminium ion diffusion in the electrode structure. The intercept on real axis in the high frequency region corresponds to the resistance of electrolyte. The impedance spectra were analyzed with the equivalent circuit displayed in inset of Fig. 7, where Re, Cdl, Rch and W are the electrolyte resistance, the constant phase element (CPE) arising from double-layer capacitance, the charge-transfer
Fig. 6. Rate performance of TiO2-NSs and TiO2-P25 electrodes at different current rates.
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4. Conclusion In conclusion, the anatase TiO2 nanospheres (TiO2-NSs) anode material for aqueous aluminium-ion batteries were successfully prepared using hydrolysis of titanium glycolate precursor, followed by subsequent hydrothermal process and calcination in an argon atmosphere. The prepared active material exhibited a mesoporous structure with a high surface area, confirmed by XRD, FTIR, TEM, BET and BJH techniques. The insertion/desertion process of Al3 + ion in the TiO2-NSs was investigated by cyclic voltammetry, galvanostatic charge-discharge at various rates and EIS measurements. Compared with the TiO2-P25 anode, the TiO2-NSs anode exhibited a higher discharge capacity (183 mAh g−1 at a current rate of 0.15 C) and an excellent rate capability (108 mAh g−1 even at 6.0 C). These can be attributed to the shorter transfer pathways for both Al3+ ions and electrons provided by mesoporous structure of TiO2-NSs active material with large electrode/electrolyte interface.
Fig. 7. Nyquist spectra of fresh TiO2-NSs and TiO2-P25 electrodes.
resistance and the Warburg impedance, respectively [1]. The chargetransfer resistance of the TiO2-NSs and TiO2-P25 electrodes was calculated to be 36.4 and 82.7 Ω, respectively. Moreover, it is clear that the TiO2-NSs anode has a steeper and shorter Warburg line in the low frequency region, implying a higher Al3+ ion diffusion rate in the electrode structure [27,42]. Clearly, the diffusion coefficients of Al3+ (D) were calculated for both anodes by the following equation [43]:
D ¼ 0:5
RT 2 An2 F σ
!2 wC
ð2Þ
where R is the gas constant, T is the absolute temperature, A is the area of the electrode surface, n is the valence state of the diffusion species and here is 3 for Al3+, F is the Faraday's constant, C is the molar concentration of Al3+ ions and σw is the Warburg coefficient, which has relationship with impedance real part Zre in the low frequency region (ω = 2πf): Zre ¼ RD þ RL þ σ w ω−0:5
ð3Þ
where ω, RD and RL are angular frequency in the low frequency range, diffusive resistance and liquid resistance, respectively. The relationship between Zre and reciprocal root square of the lower angular frequencies (ω-1/2) for both anode materials are shown in Fig. 8. The slope of the fitted line is the σw. The aluminium ion diffusion coefficients of the TiO2-NSs and TiO2-P25 electrodes were calculated to be 5.33 × 10−12 and 4.73 × 10−13 cm2 s−1, respectively. The fast Al3+ diffusion, along with the small charge-transfer resistance, contributes to the superior high-rate performance of the TiO2-NSs electrode.
Fig. 8. The relationship between Zre and ω-1/2 at low frequencies for TiO2-P25 and TiO2-NSs electrodes.
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