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Enhanced electrochemical performance of TiO2 by Ti3+ doping using a facile solvothermal method as anode materials for lithium-ion batteries
Electrochimica Acta 138 (2014) 41–47
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Enhanced electrochemical performance of TiO2 by Ti3+ doping using a facile solvothermal method as anode materials for lithium-ion batteries Yaqi Ren, Jianpeng Li, Jie Yu ∗ Shenzhen Engineering Lab of Flexible Transparent Conductive Films, Shenzhen Key Laboratory for Advanced Materials, Department Material Science and Engineering, Shenzhen Graduate School, Harbin Institute of Technology, University Town, Shenzhen, China
a r t i c l e
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Article history: Received 21 January 2014 Received in revised form 8 June 2014 Accepted 12 June 2014 Available online 19 June 2014 Keywords: titanium dioxide Ti3+ doping solvothermal method lithium-ion batteries
a b s t r a c t A facile method has been developed to synthesize Ti3+ -doped TiO2 nanoparticles as high-performance anode materials for Li-ion batteries. After reducing some Ti4+ ions to Ti3+ ions with zinc powders in TiCl4 solution, Ti3+ -doped TiO2 was synthesized by solvothermal methode. The obtained Ti3+ -doped TiO2 nanoparticles are relatively uniform and better dispersed with an average size of 30 nm. Great improvement of the electrochemical performance was obtained by Ti3+ -doping comparing with the pure TiO2 . The Ti3+ -doped TiO2 nanoparticles prepared at the Zn:Ti molar ratio of 4% are able to deliver a reversible capacity of 202.1 mAh g−1 at a current density of 100 mA g−1 and exhibit superior high-rate discharge/charge capability and cycling stability at the current density up to 3000 mA g−1 in a half cell configuration. The improved reversible capacity and rate capability could be ascribed to the presence of Ti3+ , which improves the electrical conductivity and reduces the charge transfer resistance of the TiO2 electrode. © 2014 Elsevier Ltd. All rights reserved.
1. Introduction Nanostructured TiO2 with an anatase structure has been spotlighted as one of the most attractive anode materials for lithium rechargeable batteries due to its chemical and electrochemical stability, environmental benignity, low cost, and structural integrity over many discharge/charge cycles [1–4]. However, the low electrical conductivity and ion diffusivity of TiO2 degrade its electrochemical performance greatly. Therefore, many measures such as coating with carbon [5,6], compositing with graphene [7], and doping have been taken to increase its conductivity. Among the different measures, doping with appropriate ions or atoms is advantageous since this method can improve the intrinsic nature of TiO2 by adjusting its electronic structure rather than only introducing conductive additives between the particles [8]. The reported dopants include C-N [9], Li+ [10], Fe3+ [11], N [12], and so on, all of which show beneficial effect in increasing the electrical conductivity more or less. However, doping with alien ions may cause thermal instability of TiO2 [13]. Because of this concern, doping with Ti3+ is considered suitable for overcoming the problem of thermal instability
∗ Corresponding author. E-mail address: [email protected] (J. Yu). http://dx.doi.org/10.1016/j.electacta.2014.06.068 0013-4686/© 2014 Elsevier Ltd. All rights reserved.
[14]. Theoretically, the presence of Ti3+ in the TiO2 structure can provide conduction band electrons, which undoubtedly improves its conductivity. However, the exact lattice site and atomic environment of the Ti3+ ions in the TiO2 structure are not well described in the open literatures now. It is considered in most literatures that the Ti3+ in the TiO2 structure exists in the form of oxygen vacancy [Ti(III)-Ov ] [15], where the Ti3+ is still at the center position of the original octahedron units but one or more vertex oxygen atoms shared by two or more octahedrons lose. The doping with Ti3+ for improving the conductivity of TiO2 has been reported by several groups [3,8,16], which has been demonstrated to be effective in increasing the reversible capacity. For example, Liu et al. [16] synthesized Ti3+ -doped TiO2 nanotube arrays arising from annealing in CO. Such TiO2 nanotube arrays exhibit excellent lithium ion intercalation performance with an initial discharge capacity of 101 mAh g−1 at a high current density of 10 A g−1 when measured using a standard three-electrode system. In most reports the doping of TiO2 with Ti3+ was carried out by annealing at high temperature in various reducing atmosphere [3,8,16–18]. But the simpler method such as low temperature solvothermal method has rarely been reported. Meanwhile, the doped Ti3+ in the TiO2 structure should be kept at an appropriately low concentration since too much Ti3+ may change the crystal structure of TiO2 and thus reduce the capacity. Although the electrochemical performance of the Ti3+ -doped TiO2 is greatly dependent on the doping
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amount of Ti3+ [3], the research on this issue has rarely been reported. Herein, we present a simple and controllable methodology to prepare the Ti3+ -doped TiO2 . This was achieved by a solvothermal process at lower temperature using zinc powders as the reducing agent. Ti3+ -doped TiO2 prepared by this low temperature solvothermal method possesses anatase structure and relatively uniform nanostructured granular morphology. The present method can generate uniform doping structure due to the in situ doping process rather than additional treatment after the formation of TiO2 phase. Furthermore, for this method the doping amount can be easily controlled by changing the Zn/Ti molar ratio in the reaction process. Excellent lithium ion intercalation/deintercalation performance was observed for the obtained samples. 2. Experimental Ti3+ -doped
TiO2 was synthesized by a facile solvothermal method. In a typical process, TiCl4 (2 mL) was slowly added into anhydrous ethanol (50 mL) at room temperature under vigorous stirring to form a uniform and transparent solution. After that, zinc powders were added into the solutions with the Zn:Ti molar ratios of 2%, 4%, and 6%, respectively, followed by stirring for 10 min. During the step, the transparent solution turned from light yellow to blue. Pure TiO2 was also prepared without adding zinc for comparison. For this case we call the adding ratio of Zn:Ti as 0% for convenience. Subsequently, the obtained solutions were transferred into a Teflon-lined stainless-steel autoclave with a capacity of 100 mL, and then kept at 180 ◦ C for 24 h. After cooling to room temperature, the liquid supernatant was poured and then an appropriate amount of 1 M HCl aqueous solution was added to the remaining liquid followed by vigorous stirring for 1 h to remove the residual Zn2+ . Finally, the desired precipitate was obtained by pumping filtration, washing with distilled water, and drying in vacuum at 60 ◦ C. Scanning electron microscopy (SEM, HITACHI S-4700) and X-ray diffraction (XRD, Rigaku D/Max 2500/PC) were used to characterize the structure of the Ti3+ -doped TiO2 . X-ray photoelectron spectroscopy (XPS, Physical Electronics PHI-5600) was used to detect the bonding states and composition of the samples. Electron paramagnetic resonance (EPR, Bruker ER200D-SRC) was used to investigate the oxygen vacancies or unpaired spins of Ti3+ 3d1 in the samples. The electrochemical measurements were carried out using a two-electrode coin-type test cell with Li foil as both the counter and the reference electrodes at room temperature. The working electrode was prepared by mixing the active materials, acetylene black, and polyvinylidene fluoride (PVDF) according to the weight percentage of 80%, 10%, and 10%, respectively. The mass of the active materials was controlled at 1 mg cm−2 . The applied electrolyte was 1 M LiPF6 solution in ethylene carbonate and dimethyl carbonate (1:3 by volume). The coin cells were cycled under different current densities within the voltage range of 1.0-3.0 V using a battery measuring system (CT2001A, Wuhan Land Electronics Co. Ltd). Electrochemical impedance spectra (EIS) were measured using an electrochemical workstation (CHI760C, Shanghai Chenhua Instrument Co. Ltd, China). The frequency range and potential amplitude for the EIS measurements were 100 kHz–10 mHz and 5 mV, respectively. All cells were cycled for fifteen times at the current density of 100 mA g−1 before the EIS measurements.
Fig. 1. Photographs of TiCl4 solutions in ethanol with different Zn:Ti molar ratio. (From left to right: 0, 2%, 4%, 6%. To recognize the colors of different solutions, the reader should refer to the web version of the article.).
is the optical photos showing the color changes of the TiCl4 solution after adding zinc powders. It can be seen that the solution color is strongly dependent on the adding amount of the zinc powders. With increasing the Zn:Ti molar ratio the solution changed from yellow for the pure TiCl4 solution to light blue for the solution with the Zn:Ti molar ratio of 6%. The color change arises from the conversion of Ti4+ to Ti3+ due to reduction by the added zinc powders. To clarify the influence of the Ti3+ doping on the morphology of TiO2 samples, their SEM images were measured, as shown in Fig. 2. It is observed that all the samples are composed of many very small nanoparticles with an average size of about 30 nm. These nanoparticles are relatively uniform in size and better dispersed. The samples with different doping amount show about similar morphology, indicating that the Ti3+ doping does not cause marked change of the morphology. The small size and better dipersion of the obtained Ti3+ -doped TiO2 samples are beneficial to the electrolyte access and ion intercalation/deintercalation. XRD was used to examine the crystal structure of the Ti3+ -doped TiO2 powders, as shown in Fig. 3. All the XRD patterns exhibit similar diffraction peaks, which appear at 25.3, 37.0, 37.8, 48.0, 53.9, and 55.1◦ , respectively. By comparing with the standard pattern (JCPDS No. 21-1272) the prepared samples can be identified as anatase TiO2 . The above perks correspond to the crystal planes of (101), (103), (004), (200), (105), and (211), respectively. Meanwhile, no
3. Results and discussion The Ti3+ -doped TiO2 nanoparticles were prepared at different adding ratio of zinc for obtaining different doping amount. Fig. 1
Fig. 2. SEM images of the samples prepared at different Zn:Ti molar raitio: (a) 0%, (b) 2%, (c) 4%, (d) 6%.
Y. Ren et al. / Electrochimica Acta 138 (2014) 41–47
Fig. 3. XRD patterns of Ti3+ -doped TiO2 samples prepared at different Zn:Ti molar ratio.
Table 1 Lattice parameters of samples prepared at different Zn:Ti molar ratio. samples
0%
2%
4%
6%
a(nm) c(nm)
0.379 0.952
0.380 0.953
0.380 0.953
0.380 0.953
any peaks related to zinc appear in the XRD patterns. The lattice parameters for the samples prepared at different Zn:Ti ratio were calculated by Bragg equation and shown in Table 1. It can be seen that the lattice constants keep almost constant with the Ti3+ doping [15,19]. To detect the elements and their valence states of TiO2 samples, the XPS and EPR spectra are shown in Fig. 4 and Fig. 5. From Fig. 4(a) we did not find the presence of Zn in the TiO2 samples within the detect limit of XPS, indicating that Zn was removed by acid washing. Fig. 4(b) shows the Cl 2p XPS spectra collected from the pure TiO2 and Ti3+ -doped TiO2 prepared at the Zn:Ti molar ratios of 4%. Both samples show similar spectra and have the similar peak positon at both 198.1 eV and 199.2 eV, corresponding to Cl− [20]. Besides, they show about similar Cl− concentrations, which are 1.19% and 1.25%, respectively. Therefore, the electrochemical performance improvement of the doped samples is most probably caused by the Ti3+ doping.
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Fig. 5(a) shows the Ti 2p XPS spectra measured on the surface of pure TiO2 and Ti3+ -doped TiO2 prepared at the Zn:Ti molar ratio of 4%. Each sample has two peaks at binding energy of 458.8 eV and 464.6 eV, corresponding to the typical Ti 2p3/2 and 2p1/2 peaks of anatase TiO2 , respectively [21]. No Ti3+ signals were observed, which may be due to the oxidation of Ti3+ to Ti4+ on the surface [8,14,15,18]. To confirm the presence of Ti3+ in the doped TiO2 samples, high resolution spectra of Ti 2p of the samples prepared at the Zn:Ti molar ratios of 0%, 2%, 4%, and 6% were measured after 60 seconds of argon ion sputtering (about 3 nm in sputtering depth) and shown in Fig. 5(b-e), respectively. All the spectra reveal a major peak at 458.8 eV with a shoulder at 456.8 eV, corresponding to the Ti 2p3/2 signals. The Ti 2p1/2 peak can also be deconvoluted into two components with binding energies at 464.5 and 463.2 eV, respectively. The peaks at 458.8 and 464.5 eV are assigned to the 2p3/2 and 2p1/2 core levels of Ti4+ , whereas the peaks at 456.8 and 463.2 eV are attributed to the 2p3/2 and 2p1/2 core levels of Ti3+ , respectively [22]. It can be seen from the Fig. 5(b) that Ti3+ can be detected in pure TiO2 because argon ion bombardment can lead to the reduction of Ti4+ to Ti3+ [23,24]. The calculated concentration of Ti3+ for the samples prepared at the Zn:Ti molar ratios of 0%, 2%, 4%, and 6% are 15.6%, 16.9%, 19.6%, and 22.8%, respectively. It is noted that the Ti3+ concentration of the undoped sample is 15.6%, which is formed during the Ar ion bombardment. We thus speculate that the real Ti3+ concentrations for the doped samples are much smaller that the above values. Considering the stable XRD patterns at different Zn:Ti ratio the doping concentrations of Ti3+ should be at a tiny level. Anyway, it is found that the Ti3+ concentration of the samples increases with the increasing Zn:Ti molar ratio, revealing the effects of the present doping method. Consequently, the increase of the electrical conductivity for the Ti3+ -doped TiO2 samples described below can be ascribed to the presence of Ti3+ [8]. To confirm the presence of Ti3+ , EPR spectrum was recorded and the result is shown in Fig. 5(f). The Ti3+ -doped TiO2 sample prepared at the Zn:Ti molar ratios of 4% gives a EPR signal at g = 1.999 (inset of Fig. 5(f)), which cannot be detected for the pure TiO2 [14], indicating the existense of Ti3+ [17,18,21,25,26]. The discharge and charge performances of the pure TiO2 and Ti3+ -doped TiO2 were measured using lithium metal as the anode and shown in Fig. 6. In spite of the structure similarity for the four samples, different reversible capacity was obtained, which should be caused by the difference in doping state and concentration. The discharge/charge curves show very flat discharge and charge plateaus, corresponding to the phase transition between the tetragonal and orthorhombic phases upon Li intercalation/deintercalation process [8,27]. As shown in Fig. 6(a), the reversible specific capacities at the current density of 100 mA g−1
Fig. 4. (a) XPS spectrum in the binding energy range of Zn for the Ti3+ -doped TiO2 sample prepared at the Zn:Ti ratio of 4% after acid washing, (b) Cl 2p XPS spectra of the samples prepared at different Zn:Ti molar ratio.
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Fig. 5. (a) Ti 2p XPS spectra measured on the surface of the samples prepared at different Zn:Ti molar ratio, (b-e) Ti 2p XPS spectra of Ti3+ -doped TiO2 samples prepared at different Zn:Ti molar ratio after argon ion sputtering for 60 seconds: (b) 0%, (c) 2%, (d) 4%, (e) 6%, (f) EPR spectrum of Ti3+ -doped TiO2 sample.
for the samples prepared at the Zn:Ti molar ratios of 0%, 2%, 4%, and 6% are 179.1, 188.3, 202.1, and 174.6 mAh g−1 , respectively. Fig. 6(b) compares the charge capacity of the first fifteen cycles at a current density of 100 mA g−1 for the different samples. The charge capacities increase in the initial 2 cycles and then almost keep stable for the samples prepared at the Zn:Ti molar ratios of 4% and 6% while the charge capacities decrease with the cycles for samples at the molar ratios of 0% and 2%. Calculating from the second cycle, the capacity retention of the samples prepared at the Zn:Ti molar ratios of 0%, 2%, 4%, and 6% are 93.6%, 92.1%, 96%, and 98.8%, respectively, which indicates that increasing the doping amount can efficiently improve the cycling stability. It is indicated from Fig. 6 that the sample at the Zn:Ti ratio of 4% has much enhanced specific capacity, demonstrating that appropriate doping with Ti3+ can improve the electrochemical performance of TiO2 effectively. The specific capacity is strongly dependent on the doping amount and the optimized doping amount occurs at the Zn:Ti molar ratio of
4%. The increase of the reversible capacity by the Ti3+ doping results from the increase of the electrical conductivity, which increases the transportation rate of electrons required for the lithium ion intercalation/deintercalation [3]. The effects of Ti3+ doping on the electrochemical property of TiO2 were also investigated by measuring the EIS of the different samples. Fig. 7 shows the Nyquist plots of the obtained samples after 15 cycles at the current density of 100 mA g−1 . All the plots comprise two semicircles followed by an inclined linear tail. The very high frequency intercept on the Z’ axis is related to the ohmic resistance (Rs ). The diameters of the first and second semicircles correspond to the resistances caused by the solid electrolyte interface film (Rf ) and the charge transfer resistance (Rct ) [28]. The tail in the low frequency region represents the diffusion process of ion in the electrode materials. After fitting with the equivalent circuit [29,30], the Rs , Rf , and Rct were determined and listed in Table 2. It is clearly observed from Fig. 7 and Table 2 that Rct constitutes the
Y. Ren et al. / Electrochimica Acta 138 (2014) 41–47
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Table 2 Calculated Rs , Rf , and Rct for the samples prepared at different Zn:Ti molar ratio. samples
0%
2%
4%
6%
Rs () Rf () Rct ()
3.79 6.25 37.38
4.00 6.41 31.94
4.58 5.73 27.3
4.32 6.59 24.59
main part of the whole resistance for the cells. The Rct decreases obviously with increasing the doping amount, which may be caused by the improving effects of Ti3+ doping on the electrical conductivity of TiO2 . Ti3+ doping in the TiO2 lattice will increase the carrier concentration, and thus facilitate the electron transportation, enhancing the electrochemical activity of TiO2. Since the increase of Ti3+ in TiO2 leads to the decrease of Ti4+ amount and thus the decrease of intercalated Li+ , too much Ti3+ doping will reduce the capacity of TiO2 anode, as observed for the sample prepared at the Zn:Ti ratio of 6%. Resultantly, the optimal doping amount appears at the Zn:Ti ratio of 4%, where the highest capacity was obtained. The effect of Ti3+ doping on the cycle stability of TiO2 nanoparticles was probed by continuous charging/discharging for 50 cycles at the current densities of 200 mA g−1 and 1000 mA g−1 , respectively, which are shown in Fig. 8. As shown in Fig. 8a, the sample prepared at the Zn:Ti ratio of 4% shows an initial capacity of 175.2 mAh g−1 at the current density of 200 mA g−1 , which is much higher than that for the pure TiO2 (77.6 mAh g−1 ). After 50 cycles the reversible capacity for the above two samples changes to 145.8 and 84.8 mAh
Fig. 6. Electrochemical performances of Ti3+ -doped TiO2 samples prepared at different Zn:Ti molar ratio at the current density of 100 mA g−1 : (a) discharge/charge curves of the second cycle, (b) charge (deintercalation) capacity versus cycle number.
Fig. 7. Nyquist plots for the samples prepared at different Zn:Ti molar ratio after 15 cycles. (The lines are the fitted results according to the data points and the equivalent circuit is shown in the inset.).
Fig. 8. Cycling performance of the samples prepared at the Zn:Ti molar ratio of 0% and 4% at different current density: (a) 200 mA g−1 , (b) 1000 mA g−1 .
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enhanced electrochemical performance in such as reversible capacity, rate performance, and stability comparing with the pure TiO2 . The reversible capacity of 202.1 mAh g−1 at the current density of 100 mA g−1 and 79.1 mAh g−1 at 3000 mA g−1 were achieved for the optimal sample, showing good rate performance. Meanwhile, this sample also gives a high retention of 96% after 50 cycles at the current densities of 1000 mA g−1 , indicating excellent stability. The present method to prepare the Ti3+ -doped TiO2 is promising due to the production simplicity and efficiency in improving the material performance.
Acknowledgements This work is supported by the National Basic Research Program of China (2012CB933003), National Natural Science Foundation of China (No. 51272057), and Shenzhen Basic Research Program (JCYJ20130329150737027). Fig. 9. Rate capability of the samples prepared at the Zn:Ti molar ratio of 0% and 4% at different current density (50-3000 mA g−1 ). The hollow and solid symbols show the charge and discharge capacity, respectively.
g−1 , respectively. Noting that the capacity of the pure TiO2 increases rapidly to 149.3 mAh g−1 after 2 cycles due to the activation process, if calculating from the 3th cycle the retention is 56.8% for the pure TiO2 and that of the sample prepared at the Zn:Ti ratio of 4% is 86.9%. Fig. 8b shows the cycling performance of the above two samples at a high current density of 1000 mA g−1 . The coin cells were activated at a current density of 50 mA g−1 for two cycles before test. The sample doped at the Zn:Ti ratio of 4% gives an initial capacity of 133.2 mAh g−1 and a remained capacity of 128.2 mAh g−1 , corresponding to a high retention of 96%. In contrast, the reversible capacity decreases to 81.2 mAh g−1 from the initial value of 126.7 mAh g−1 after 50 cycles for the pure TiO2 sample, showing a much lower retention of 64%. Comparing with the pure TiO2 , Ti3+ -doped TiO2 has greatly improved cycling stability. Fig. 9 compares the rate performance of the pure and Ti3+ -doped TiO2 samples prepared at the Zn:Ti molar ratio of 0% and 4%, respectively. Both of the samples were tested from 50 mA g−1 to 3000 mA g−1 with identical discharge and charge current density, where 5 discharge/charge cycles were conducted at each current density. At a relatively low current density of 50 mA g−1 , the Ti3+ -doped TiO2 exhibits a reversible capacity of 253 mAh g−1 , which is much higher than that for the pure TiO2 (199.6 mAh g−1 ). The reversible capacities of the pure TiO2 at the current density of 100, 200, 500, 1000, 2000 and 3000 mA g−1 are 184.9, 166.6, 132.7, 104.3, 65.5 and 46.7 mAh g−1 and those for the Ti3+ -doped TiO2 are 197, 176.7, 152.4, 129.8, 99.4 and 79.1 mAh g−1 , respectively. It is indicated that the capacity of the doped TiO2 is higher than that of the pure one at every current density. After the high rate cycles, the cells were remeasured at a relatively low current density of 50 mA g−1 once more. It is observed that the remeasured capacity of Ti3+ -doped TiO2 remains 227.3 mAh g−1 and still improves slightly in the next cycles. However, the capacity for the pure TiO2 is 195.6 mAh g−1 and decreases in the following cycles. This indicates that the rate performance and cycling stability of TiO2 can be greatly improved by the Ti3+ doping due to the increase of the electrical conductivity. 4. Conclusion In summary, Ti3+ -doped TiO2 nanoparticles were synthesized by a simple solvothermal method using zinc powders as the reducing reagents. The electrochemical performance of the Ti3+ -doped TiO2 nanoparticles is dependent on the Zn/Ti molar ratio and the optimal one is obtained at the Zn:Ti molar ratio of 4%. The Ti3+ -doped TiO2 nanoparticles prepared under the optimal condition show much
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Enhanced electrochemical performance of TiO2 by Ti3+ doping using a facile solvothermal method as anode materials for lithium-ion batteries Yaqi Ren, Jianpeng Li, Jie Yu ∗ Shenzhen Engineering Lab of Flexible Transparent Conductive Films, Shenzhen Key Laboratory for Advanced Materials, Department Material Science and Engineering, Shenzhen Graduate School, Harbin Institute of Technology, University Town, Shenzhen, China
a r t i c l e
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Article history: Received 21 January 2014 Received in revised form 8 June 2014 Accepted 12 June 2014 Available online 19 June 2014 Keywords: titanium dioxide Ti3+ doping solvothermal method lithium-ion batteries
a b s t r a c t A facile method has been developed to synthesize Ti3+ -doped TiO2 nanoparticles as high-performance anode materials for Li-ion batteries. After reducing some Ti4+ ions to Ti3+ ions with zinc powders in TiCl4 solution, Ti3+ -doped TiO2 was synthesized by solvothermal methode. The obtained Ti3+ -doped TiO2 nanoparticles are relatively uniform and better dispersed with an average size of 30 nm. Great improvement of the electrochemical performance was obtained by Ti3+ -doping comparing with the pure TiO2 . The Ti3+ -doped TiO2 nanoparticles prepared at the Zn:Ti molar ratio of 4% are able to deliver a reversible capacity of 202.1 mAh g−1 at a current density of 100 mA g−1 and exhibit superior high-rate discharge/charge capability and cycling stability at the current density up to 3000 mA g−1 in a half cell configuration. The improved reversible capacity and rate capability could be ascribed to the presence of Ti3+ , which improves the electrical conductivity and reduces the charge transfer resistance of the TiO2 electrode. © 2014 Elsevier Ltd. All rights reserved.
1. Introduction Nanostructured TiO2 with an anatase structure has been spotlighted as one of the most attractive anode materials for lithium rechargeable batteries due to its chemical and electrochemical stability, environmental benignity, low cost, and structural integrity over many discharge/charge cycles [1–4]. However, the low electrical conductivity and ion diffusivity of TiO2 degrade its electrochemical performance greatly. Therefore, many measures such as coating with carbon [5,6], compositing with graphene [7], and doping have been taken to increase its conductivity. Among the different measures, doping with appropriate ions or atoms is advantageous since this method can improve the intrinsic nature of TiO2 by adjusting its electronic structure rather than only introducing conductive additives between the particles [8]. The reported dopants include C-N [9], Li+ [10], Fe3+ [11], N [12], and so on, all of which show beneficial effect in increasing the electrical conductivity more or less. However, doping with alien ions may cause thermal instability of TiO2 [13]. Because of this concern, doping with Ti3+ is considered suitable for overcoming the problem of thermal instability
∗ Corresponding author. E-mail address: [email protected] (J. Yu). http://dx.doi.org/10.1016/j.electacta.2014.06.068 0013-4686/© 2014 Elsevier Ltd. All rights reserved.
[14]. Theoretically, the presence of Ti3+ in the TiO2 structure can provide conduction band electrons, which undoubtedly improves its conductivity. However, the exact lattice site and atomic environment of the Ti3+ ions in the TiO2 structure are not well described in the open literatures now. It is considered in most literatures that the Ti3+ in the TiO2 structure exists in the form of oxygen vacancy [Ti(III)-Ov ] [15], where the Ti3+ is still at the center position of the original octahedron units but one or more vertex oxygen atoms shared by two or more octahedrons lose. The doping with Ti3+ for improving the conductivity of TiO2 has been reported by several groups [3,8,16], which has been demonstrated to be effective in increasing the reversible capacity. For example, Liu et al. [16] synthesized Ti3+ -doped TiO2 nanotube arrays arising from annealing in CO. Such TiO2 nanotube arrays exhibit excellent lithium ion intercalation performance with an initial discharge capacity of 101 mAh g−1 at a high current density of 10 A g−1 when measured using a standard three-electrode system. In most reports the doping of TiO2 with Ti3+ was carried out by annealing at high temperature in various reducing atmosphere [3,8,16–18]. But the simpler method such as low temperature solvothermal method has rarely been reported. Meanwhile, the doped Ti3+ in the TiO2 structure should be kept at an appropriately low concentration since too much Ti3+ may change the crystal structure of TiO2 and thus reduce the capacity. Although the electrochemical performance of the Ti3+ -doped TiO2 is greatly dependent on the doping
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amount of Ti3+ [3], the research on this issue has rarely been reported. Herein, we present a simple and controllable methodology to prepare the Ti3+ -doped TiO2 . This was achieved by a solvothermal process at lower temperature using zinc powders as the reducing agent. Ti3+ -doped TiO2 prepared by this low temperature solvothermal method possesses anatase structure and relatively uniform nanostructured granular morphology. The present method can generate uniform doping structure due to the in situ doping process rather than additional treatment after the formation of TiO2 phase. Furthermore, for this method the doping amount can be easily controlled by changing the Zn/Ti molar ratio in the reaction process. Excellent lithium ion intercalation/deintercalation performance was observed for the obtained samples. 2. Experimental Ti3+ -doped
TiO2 was synthesized by a facile solvothermal method. In a typical process, TiCl4 (2 mL) was slowly added into anhydrous ethanol (50 mL) at room temperature under vigorous stirring to form a uniform and transparent solution. After that, zinc powders were added into the solutions with the Zn:Ti molar ratios of 2%, 4%, and 6%, respectively, followed by stirring for 10 min. During the step, the transparent solution turned from light yellow to blue. Pure TiO2 was also prepared without adding zinc for comparison. For this case we call the adding ratio of Zn:Ti as 0% for convenience. Subsequently, the obtained solutions were transferred into a Teflon-lined stainless-steel autoclave with a capacity of 100 mL, and then kept at 180 ◦ C for 24 h. After cooling to room temperature, the liquid supernatant was poured and then an appropriate amount of 1 M HCl aqueous solution was added to the remaining liquid followed by vigorous stirring for 1 h to remove the residual Zn2+ . Finally, the desired precipitate was obtained by pumping filtration, washing with distilled water, and drying in vacuum at 60 ◦ C. Scanning electron microscopy (SEM, HITACHI S-4700) and X-ray diffraction (XRD, Rigaku D/Max 2500/PC) were used to characterize the structure of the Ti3+ -doped TiO2 . X-ray photoelectron spectroscopy (XPS, Physical Electronics PHI-5600) was used to detect the bonding states and composition of the samples. Electron paramagnetic resonance (EPR, Bruker ER200D-SRC) was used to investigate the oxygen vacancies or unpaired spins of Ti3+ 3d1 in the samples. The electrochemical measurements were carried out using a two-electrode coin-type test cell with Li foil as both the counter and the reference electrodes at room temperature. The working electrode was prepared by mixing the active materials, acetylene black, and polyvinylidene fluoride (PVDF) according to the weight percentage of 80%, 10%, and 10%, respectively. The mass of the active materials was controlled at 1 mg cm−2 . The applied electrolyte was 1 M LiPF6 solution in ethylene carbonate and dimethyl carbonate (1:3 by volume). The coin cells were cycled under different current densities within the voltage range of 1.0-3.0 V using a battery measuring system (CT2001A, Wuhan Land Electronics Co. Ltd). Electrochemical impedance spectra (EIS) were measured using an electrochemical workstation (CHI760C, Shanghai Chenhua Instrument Co. Ltd, China). The frequency range and potential amplitude for the EIS measurements were 100 kHz–10 mHz and 5 mV, respectively. All cells were cycled for fifteen times at the current density of 100 mA g−1 before the EIS measurements.
Fig. 1. Photographs of TiCl4 solutions in ethanol with different Zn:Ti molar ratio. (From left to right: 0, 2%, 4%, 6%. To recognize the colors of different solutions, the reader should refer to the web version of the article.).
is the optical photos showing the color changes of the TiCl4 solution after adding zinc powders. It can be seen that the solution color is strongly dependent on the adding amount of the zinc powders. With increasing the Zn:Ti molar ratio the solution changed from yellow for the pure TiCl4 solution to light blue for the solution with the Zn:Ti molar ratio of 6%. The color change arises from the conversion of Ti4+ to Ti3+ due to reduction by the added zinc powders. To clarify the influence of the Ti3+ doping on the morphology of TiO2 samples, their SEM images were measured, as shown in Fig. 2. It is observed that all the samples are composed of many very small nanoparticles with an average size of about 30 nm. These nanoparticles are relatively uniform in size and better dispersed. The samples with different doping amount show about similar morphology, indicating that the Ti3+ doping does not cause marked change of the morphology. The small size and better dipersion of the obtained Ti3+ -doped TiO2 samples are beneficial to the electrolyte access and ion intercalation/deintercalation. XRD was used to examine the crystal structure of the Ti3+ -doped TiO2 powders, as shown in Fig. 3. All the XRD patterns exhibit similar diffraction peaks, which appear at 25.3, 37.0, 37.8, 48.0, 53.9, and 55.1◦ , respectively. By comparing with the standard pattern (JCPDS No. 21-1272) the prepared samples can be identified as anatase TiO2 . The above perks correspond to the crystal planes of (101), (103), (004), (200), (105), and (211), respectively. Meanwhile, no
3. Results and discussion The Ti3+ -doped TiO2 nanoparticles were prepared at different adding ratio of zinc for obtaining different doping amount. Fig. 1
Fig. 2. SEM images of the samples prepared at different Zn:Ti molar raitio: (a) 0%, (b) 2%, (c) 4%, (d) 6%.
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Fig. 3. XRD patterns of Ti3+ -doped TiO2 samples prepared at different Zn:Ti molar ratio.
Table 1 Lattice parameters of samples prepared at different Zn:Ti molar ratio. samples
0%
2%
4%
6%
a(nm) c(nm)
0.379 0.952
0.380 0.953
0.380 0.953
0.380 0.953
any peaks related to zinc appear in the XRD patterns. The lattice parameters for the samples prepared at different Zn:Ti ratio were calculated by Bragg equation and shown in Table 1. It can be seen that the lattice constants keep almost constant with the Ti3+ doping [15,19]. To detect the elements and their valence states of TiO2 samples, the XPS and EPR spectra are shown in Fig. 4 and Fig. 5. From Fig. 4(a) we did not find the presence of Zn in the TiO2 samples within the detect limit of XPS, indicating that Zn was removed by acid washing. Fig. 4(b) shows the Cl 2p XPS spectra collected from the pure TiO2 and Ti3+ -doped TiO2 prepared at the Zn:Ti molar ratios of 4%. Both samples show similar spectra and have the similar peak positon at both 198.1 eV and 199.2 eV, corresponding to Cl− [20]. Besides, they show about similar Cl− concentrations, which are 1.19% and 1.25%, respectively. Therefore, the electrochemical performance improvement of the doped samples is most probably caused by the Ti3+ doping.
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Fig. 5(a) shows the Ti 2p XPS spectra measured on the surface of pure TiO2 and Ti3+ -doped TiO2 prepared at the Zn:Ti molar ratio of 4%. Each sample has two peaks at binding energy of 458.8 eV and 464.6 eV, corresponding to the typical Ti 2p3/2 and 2p1/2 peaks of anatase TiO2 , respectively [21]. No Ti3+ signals were observed, which may be due to the oxidation of Ti3+ to Ti4+ on the surface [8,14,15,18]. To confirm the presence of Ti3+ in the doped TiO2 samples, high resolution spectra of Ti 2p of the samples prepared at the Zn:Ti molar ratios of 0%, 2%, 4%, and 6% were measured after 60 seconds of argon ion sputtering (about 3 nm in sputtering depth) and shown in Fig. 5(b-e), respectively. All the spectra reveal a major peak at 458.8 eV with a shoulder at 456.8 eV, corresponding to the Ti 2p3/2 signals. The Ti 2p1/2 peak can also be deconvoluted into two components with binding energies at 464.5 and 463.2 eV, respectively. The peaks at 458.8 and 464.5 eV are assigned to the 2p3/2 and 2p1/2 core levels of Ti4+ , whereas the peaks at 456.8 and 463.2 eV are attributed to the 2p3/2 and 2p1/2 core levels of Ti3+ , respectively [22]. It can be seen from the Fig. 5(b) that Ti3+ can be detected in pure TiO2 because argon ion bombardment can lead to the reduction of Ti4+ to Ti3+ [23,24]. The calculated concentration of Ti3+ for the samples prepared at the Zn:Ti molar ratios of 0%, 2%, 4%, and 6% are 15.6%, 16.9%, 19.6%, and 22.8%, respectively. It is noted that the Ti3+ concentration of the undoped sample is 15.6%, which is formed during the Ar ion bombardment. We thus speculate that the real Ti3+ concentrations for the doped samples are much smaller that the above values. Considering the stable XRD patterns at different Zn:Ti ratio the doping concentrations of Ti3+ should be at a tiny level. Anyway, it is found that the Ti3+ concentration of the samples increases with the increasing Zn:Ti molar ratio, revealing the effects of the present doping method. Consequently, the increase of the electrical conductivity for the Ti3+ -doped TiO2 samples described below can be ascribed to the presence of Ti3+ [8]. To confirm the presence of Ti3+ , EPR spectrum was recorded and the result is shown in Fig. 5(f). The Ti3+ -doped TiO2 sample prepared at the Zn:Ti molar ratios of 4% gives a EPR signal at g = 1.999 (inset of Fig. 5(f)), which cannot be detected for the pure TiO2 [14], indicating the existense of Ti3+ [17,18,21,25,26]. The discharge and charge performances of the pure TiO2 and Ti3+ -doped TiO2 were measured using lithium metal as the anode and shown in Fig. 6. In spite of the structure similarity for the four samples, different reversible capacity was obtained, which should be caused by the difference in doping state and concentration. The discharge/charge curves show very flat discharge and charge plateaus, corresponding to the phase transition between the tetragonal and orthorhombic phases upon Li intercalation/deintercalation process [8,27]. As shown in Fig. 6(a), the reversible specific capacities at the current density of 100 mA g−1
Fig. 4. (a) XPS spectrum in the binding energy range of Zn for the Ti3+ -doped TiO2 sample prepared at the Zn:Ti ratio of 4% after acid washing, (b) Cl 2p XPS spectra of the samples prepared at different Zn:Ti molar ratio.
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Fig. 5. (a) Ti 2p XPS spectra measured on the surface of the samples prepared at different Zn:Ti molar ratio, (b-e) Ti 2p XPS spectra of Ti3+ -doped TiO2 samples prepared at different Zn:Ti molar ratio after argon ion sputtering for 60 seconds: (b) 0%, (c) 2%, (d) 4%, (e) 6%, (f) EPR spectrum of Ti3+ -doped TiO2 sample.
for the samples prepared at the Zn:Ti molar ratios of 0%, 2%, 4%, and 6% are 179.1, 188.3, 202.1, and 174.6 mAh g−1 , respectively. Fig. 6(b) compares the charge capacity of the first fifteen cycles at a current density of 100 mA g−1 for the different samples. The charge capacities increase in the initial 2 cycles and then almost keep stable for the samples prepared at the Zn:Ti molar ratios of 4% and 6% while the charge capacities decrease with the cycles for samples at the molar ratios of 0% and 2%. Calculating from the second cycle, the capacity retention of the samples prepared at the Zn:Ti molar ratios of 0%, 2%, 4%, and 6% are 93.6%, 92.1%, 96%, and 98.8%, respectively, which indicates that increasing the doping amount can efficiently improve the cycling stability. It is indicated from Fig. 6 that the sample at the Zn:Ti ratio of 4% has much enhanced specific capacity, demonstrating that appropriate doping with Ti3+ can improve the electrochemical performance of TiO2 effectively. The specific capacity is strongly dependent on the doping amount and the optimized doping amount occurs at the Zn:Ti molar ratio of
4%. The increase of the reversible capacity by the Ti3+ doping results from the increase of the electrical conductivity, which increases the transportation rate of electrons required for the lithium ion intercalation/deintercalation [3]. The effects of Ti3+ doping on the electrochemical property of TiO2 were also investigated by measuring the EIS of the different samples. Fig. 7 shows the Nyquist plots of the obtained samples after 15 cycles at the current density of 100 mA g−1 . All the plots comprise two semicircles followed by an inclined linear tail. The very high frequency intercept on the Z’ axis is related to the ohmic resistance (Rs ). The diameters of the first and second semicircles correspond to the resistances caused by the solid electrolyte interface film (Rf ) and the charge transfer resistance (Rct ) [28]. The tail in the low frequency region represents the diffusion process of ion in the electrode materials. After fitting with the equivalent circuit [29,30], the Rs , Rf , and Rct were determined and listed in Table 2. It is clearly observed from Fig. 7 and Table 2 that Rct constitutes the
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Table 2 Calculated Rs , Rf , and Rct for the samples prepared at different Zn:Ti molar ratio. samples
0%
2%
4%
6%
Rs () Rf () Rct ()
3.79 6.25 37.38
4.00 6.41 31.94
4.58 5.73 27.3
4.32 6.59 24.59
main part of the whole resistance for the cells. The Rct decreases obviously with increasing the doping amount, which may be caused by the improving effects of Ti3+ doping on the electrical conductivity of TiO2 . Ti3+ doping in the TiO2 lattice will increase the carrier concentration, and thus facilitate the electron transportation, enhancing the electrochemical activity of TiO2. Since the increase of Ti3+ in TiO2 leads to the decrease of Ti4+ amount and thus the decrease of intercalated Li+ , too much Ti3+ doping will reduce the capacity of TiO2 anode, as observed for the sample prepared at the Zn:Ti ratio of 6%. Resultantly, the optimal doping amount appears at the Zn:Ti ratio of 4%, where the highest capacity was obtained. The effect of Ti3+ doping on the cycle stability of TiO2 nanoparticles was probed by continuous charging/discharging for 50 cycles at the current densities of 200 mA g−1 and 1000 mA g−1 , respectively, which are shown in Fig. 8. As shown in Fig. 8a, the sample prepared at the Zn:Ti ratio of 4% shows an initial capacity of 175.2 mAh g−1 at the current density of 200 mA g−1 , which is much higher than that for the pure TiO2 (77.6 mAh g−1 ). After 50 cycles the reversible capacity for the above two samples changes to 145.8 and 84.8 mAh
Fig. 6. Electrochemical performances of Ti3+ -doped TiO2 samples prepared at different Zn:Ti molar ratio at the current density of 100 mA g−1 : (a) discharge/charge curves of the second cycle, (b) charge (deintercalation) capacity versus cycle number.
Fig. 7. Nyquist plots for the samples prepared at different Zn:Ti molar ratio after 15 cycles. (The lines are the fitted results according to the data points and the equivalent circuit is shown in the inset.).
Fig. 8. Cycling performance of the samples prepared at the Zn:Ti molar ratio of 0% and 4% at different current density: (a) 200 mA g−1 , (b) 1000 mA g−1 .
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enhanced electrochemical performance in such as reversible capacity, rate performance, and stability comparing with the pure TiO2 . The reversible capacity of 202.1 mAh g−1 at the current density of 100 mA g−1 and 79.1 mAh g−1 at 3000 mA g−1 were achieved for the optimal sample, showing good rate performance. Meanwhile, this sample also gives a high retention of 96% after 50 cycles at the current densities of 1000 mA g−1 , indicating excellent stability. The present method to prepare the Ti3+ -doped TiO2 is promising due to the production simplicity and efficiency in improving the material performance.
Acknowledgements This work is supported by the National Basic Research Program of China (2012CB933003), National Natural Science Foundation of China (No. 51272057), and Shenzhen Basic Research Program (JCYJ20130329150737027). Fig. 9. Rate capability of the samples prepared at the Zn:Ti molar ratio of 0% and 4% at different current density (50-3000 mA g−1 ). The hollow and solid symbols show the charge and discharge capacity, respectively.
g−1 , respectively. Noting that the capacity of the pure TiO2 increases rapidly to 149.3 mAh g−1 after 2 cycles due to the activation process, if calculating from the 3th cycle the retention is 56.8% for the pure TiO2 and that of the sample prepared at the Zn:Ti ratio of 4% is 86.9%. Fig. 8b shows the cycling performance of the above two samples at a high current density of 1000 mA g−1 . The coin cells were activated at a current density of 50 mA g−1 for two cycles before test. The sample doped at the Zn:Ti ratio of 4% gives an initial capacity of 133.2 mAh g−1 and a remained capacity of 128.2 mAh g−1 , corresponding to a high retention of 96%. In contrast, the reversible capacity decreases to 81.2 mAh g−1 from the initial value of 126.7 mAh g−1 after 50 cycles for the pure TiO2 sample, showing a much lower retention of 64%. Comparing with the pure TiO2 , Ti3+ -doped TiO2 has greatly improved cycling stability. Fig. 9 compares the rate performance of the pure and Ti3+ -doped TiO2 samples prepared at the Zn:Ti molar ratio of 0% and 4%, respectively. Both of the samples were tested from 50 mA g−1 to 3000 mA g−1 with identical discharge and charge current density, where 5 discharge/charge cycles were conducted at each current density. At a relatively low current density of 50 mA g−1 , the Ti3+ -doped TiO2 exhibits a reversible capacity of 253 mAh g−1 , which is much higher than that for the pure TiO2 (199.6 mAh g−1 ). The reversible capacities of the pure TiO2 at the current density of 100, 200, 500, 1000, 2000 and 3000 mA g−1 are 184.9, 166.6, 132.7, 104.3, 65.5 and 46.7 mAh g−1 and those for the Ti3+ -doped TiO2 are 197, 176.7, 152.4, 129.8, 99.4 and 79.1 mAh g−1 , respectively. It is indicated that the capacity of the doped TiO2 is higher than that of the pure one at every current density. After the high rate cycles, the cells were remeasured at a relatively low current density of 50 mA g−1 once more. It is observed that the remeasured capacity of Ti3+ -doped TiO2 remains 227.3 mAh g−1 and still improves slightly in the next cycles. However, the capacity for the pure TiO2 is 195.6 mAh g−1 and decreases in the following cycles. This indicates that the rate performance and cycling stability of TiO2 can be greatly improved by the Ti3+ doping due to the increase of the electrical conductivity. 4. Conclusion In summary, Ti3+ -doped TiO2 nanoparticles were synthesized by a simple solvothermal method using zinc powders as the reducing reagents. The electrochemical performance of the Ti3+ -doped TiO2 nanoparticles is dependent on the Zn/Ti molar ratio and the optimal one is obtained at the Zn:Ti molar ratio of 4%. The Ti3+ -doped TiO2 nanoparticles prepared under the optimal condition show much
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