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Hierarchically structural TiO2 nanorods composed of rutile core and anatase shell as a durable anode material for lithium-ion intercalation
Journal of Electroanalytical Chemistry 804 (2017) 87–91
Contents lists available at ScienceDirect
Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem
Hierarchically structural TiO2 nanorods composed of rutile core and anatase shell as a durable anode material for lithium-ion intercalation
MARK
Yubin Liua,b, Tianli Dinga,b, Deli Shena,b, Jie Doua,b, Mingdeng Weia,b,⁎ a b
State Key Laboratory of Photocatalysis on Energy and Environment, Fuzhou University, Xueyuan Road 2, Fuzhou, Fujian 350116, China Institute of Advanced Energy Materials, Fuzhou University, Xueyuan Road 2, Fuzhou, Fujian 350116, China
A R T I C L E I N F O
A B S T R A C T
Keywords: TiO2 Core-shell structure Dual-phase Lithium-ion battery Electrochemical property
In the present work, hierarchically structural TiO2 nanorods composed of rutile core and anatase shell were successfully synthesized via a one-step hydrothermal route for the first time, and then were characterized by Xray diffraction (XRD), Raman spectroscopy and scanning and transmission electron microscopy (SEM/TEM) measurements. It was found that the crystal phases of the same can be easily adjusted by changing the amount of the reactant during the reaction process. Furthermore, the sample was used as an anode for lithium-ion intercalation and exhibited high capacity and long-term cycling stability due to the synergistic effects of dual-phase TiO2. For instance, the capacities of 102.7 and 84.7 mA h g− 1 can be kept at 10 C for 1000 cycles and 20 C for 5000 cycles, respectively.
1. Introduction Lithium-ion batteries (LIBs) are considered to be one of the most promising types of efficient energy storage devices [1–3]. Up to now, a large number of Ti-based materials have been investigated as alternative anode materials for LIBs due to their environmental benign, low cost, enhanced safety as well as some performance advantages [4–5], such as Li4Ti5O12 [6–8], TiO2 [9–14], H2Ti3O7 [15] and Li2ZnTi3O8 [16]. In comparison with pure phase materials, Ti-based oxides with a dual-phase are believed to combine their respective advantages and show great superior electrochemical properties. Recently, there are increasing reports on dual-phase Ti-based oxides and their application in LIBs. For example, Guo et al. [17] reported that rutile TiO2 terminated Li4Ti5O12 nanosheets showed much improved rate capability and specific capacity compared with pure phase Li4Ti5O12. Similarly, Jiang et al. [18] synthesized dual-phase Li4Ti5O12-anatase TiO2 nanocrystallines, which exhibited a good electrochemical performance over individual pure phase Li4Ti5O12. Fan et al. [19] prepared unique anatase/ TiO2-B composite nanosheets, in which anatase as an electron-accepting phase and TiO2-B as a Li+-accepting phase; this material demonstrated higher specific capacity and good rate capability. Among Ti-based materials, nanostructured anatase and rutile TiO2 are easily obtained by using some simple methods. To date, a large number of experiments and theoretical calculations have been per-
⁎
formed to investigate pure phase rutile or anatase TiO2 [20–25]. It is believed that pure phase anatase TiO2 is a more facile host for Li-insertion/extraction and can deliver a high reversible capacity [4], while pure phase rutile TiO2 appears more stable cycling performances due to the existence of the mesostructure rocksalt lithium titanate during the initial discharge process [26,27]. To the best of our knowledge, however, there are few reports on the application of nanostructural TiO2 with dual-phase of rutile and anatase in LIBs. For instance, Wang et al. [28] reported that anatase/rutile TiO2 microspheres with a hierarchically porous structure showed a capacity of about 103 mA h g− 1 after 100 cycles at 5100 mA g− 1. Xu et al. [29] adopted a two-step route to prepare a composite of rutile/anatase TiO2 nanotubes arrays, which displayed a capacity of about 185 mA h g− 1 after 100 cycles at 33.5 mA g− 1. Obviously, these results both of capacity and cycling stability are needed to be enhanced. Therefore, to develop a newly facile route for synthesizing unique nanostructural dual-phase TiO2 with an excellent electrochemical performance is still a challenge. In the present work, a one-step synthetic route was designed for synthesizing hierarchically structural TiO2 nanorods composed of rutile core and anatase shell for the first time. It was found that the crystal phases of the products were easily controlled by varying the amount of the reactant during the reaction process. Furthermore, this material was used as an anode for LIBs and exhibited durable electrochemical performances.
Corresponding author at: State Key Laboratory of Photocatalysis on Energy and Environment, Fuzhou University, Xueyuan Road 2, Fuzhou, Fujian 350116, China. E-mail address: [email protected] (M. Wei).
http://dx.doi.org/10.1016/j.jelechem.2017.09.051 Received 25 April 2017; Received in revised form 22 September 2017; Accepted 25 September 2017 Available online 27 September 2017 1572-6657/ © 2017 Elsevier B.V. All rights reserved.
Journal of Electroanalytical Chemistry 804 (2017) 87–91
Y. Liu et al.
2. Experimental 2.1. Preparation and characterization of samples Hierarchically structural TiO2 nanorods composed of rutile core and anatase shell (hierarchical rutile@anatase TiO2 nanorods) were obtained as follows: 0.1 g of p-phthalic acid (PTA) was dispersed in a mixed solution of 25 mL H2O and 5 mL sodium lactate. And then, 2 mL of titanium butoxide (TBOT) was added in the above solution under vigorous stirring. After stirring for several hours, the resulting suspension was transferred into a Teflon lined stainless steel autoclave with a capacity of 50 mL. The autoclave was kept at 160 °C for 18 h and then naturally cooled to room temperature. The white precipitate was harvested via centrifugation, washed thoroughly with deionized water and ethyl alcohol for several times, and then dried at 70 °C for 12 h. 2.2. Characterizations of the samples X-ray diffraction (XRD) patterns of samples were recorded on a Rigaku Ultima IV X-ray diffractomator by using CuKα radiation. Scanning electron microscopy (SEM, Hitachi S4800 instrument) and transmission electron microscopy (TEM, FEI F20 S-TWIN instrument) were applied for the determination of samples morphology and composition. The Raman spectra were recorded in a Renishaw inVai Raman microscope with a 532 nm laser. N2 adsorption-desorption was performed on a Micromeritics ASAP 2020 instrument (Micromeritics, Norcross, GA, USA).
Fig. 1. (a) XRD patterns and (b) Raman spectrum of hierarchical rutile@anatase TiO2 nanorods.
2.3. Electrochemical measurements anatase TiO2. SEM image in Fig. 2a reveals that the synthesized sample consisted of nanorods with uniform morphology. The length and width of these nanorods were about 200 and 50 nm, respectively. As shown in Fig. 2b–c, it is found that the surface of the hierarchical rutile@anatase TiO2 nanorods was very rough. Fig. 2d displays TEM images of a single nanorod, in which rutile TiO2 nanorod was covered by a large amount of anatase TiO2 nanoparticles, forming a core-shell structure. Fig. 2e exhibits the HRTEM image taken from region I in Fig. 2d (marked by a white rectangle). Obviously, a large amount of nanoparticles with several nanometers were embedded on the surface of the nanorod. On the other hand, these nanoparticles had identical lattice fringe of 0.35 nm, corresponding to the d101 spacing in the XRD pattern, indicating that the crystal phase of these nanoparticles was anatase TiO2. In comparison to the lattice fringe of 0.35 nm with random orientations, the lattice fringe of rutile TiO2 nanorod appeared in the same direction, and was approximately 0.32 nm. Fig. 2f is a high magnification TEM taken from the side edge of the rutile@anatase TiO2 nanorods (region II in Fig. 2d). Similarly, two kinds of lattice fringes of 0.35 and 0.32 nm were clearly observed, indicating that the rutile TiO2 were almost completely covered with anatase TiO2 naoparticles. At the same time, it was found that p-phthalic acid (PTA) plays a key role in formation of hierarchical rutile@anatase TiO2 nanorods during the reaction process. Namely, PTA is particularly effective in producing TiO2 with different crystal phases and different morphologies. The samples were obtained in the presence of 0 and 0.2 g of PTA, and denoted as rutile TiO2 and rutile/anatase TiO2, respectively. As shown in Figs. S1–2, the pure rutile TiO2 can be obtained in the absence of PTA. With the increasing PTA, the content of rutile TiO2 in composites decreased gradually. This might be ascribed to a steric block effect of PTA on the arrangement of the TiO6 octahedra, resulted in the formation of anatase-type TiO2 nuclei, which is similar to SO42 − [32]. On the other hand, the pure rutile TiO2 displayed an irregular cuboid-like morphology as shown in Fig. S3. When the content of PTA was increased to 0.2 g, the obtained rutile/anatase TiO2 presented a large amount of anatase TiO2 nanoparticles besides the partial rutile@anatase TiO2 nanorods. At the same time, it was also found that these anatase TiO2 nanoparticles were aggregated severely (in Fig. S4).
The electrochemical performance of all samples was performed using 2025-type coin cells with two-electrodes. First, the resulting samples were admixed with polyvinylidene fluoride (PVDF) binder and acetylene black carbon in a weight ratio of 7:2:1 to form a slurry which was pressed on copper foil circular flakes and dried at 110 °C in a vacuum overnight. Copper foils coated active materials were used the work electrodes and Lithium foils were used as the counter electrodes. The mass loading of the active materials was about 0.8 mg cm2. The electrolyte was 1 M LiPF6 in a 1:1:1 (volume ratio) mixture of ethylene carbonate (EC), ethylene methyl carbonate (EMC) and dimethyl carbonate (DMC). Celgard2400 (America) microporous polypropylene membrane was used as the separator. Cell assembly was carried out in a glove box filled with highly pure argon gas (O2 and H2O levels < 1 ppm). Cyclic voltammetry (CV) and charge-discharge tests of all electrodes were performed using an electrochemical workstation (CHI 600C) and Land automatic batteries tester (Land, CT 2001A, Wuhan, China), respectively. Both rate and cycling testing were performed in the working voltage range of 1.0–3.0 V (vs. Li+/Li). 3. Results and discussion The crystalline phases of the samples were confirmed by XRD and Raman and the results are depicted in Fig. 1. Fig. 1a shows the XRD patterns of the hierarchical rutile@anatase TiO2 obtained in the presence of 0.1 g PTA. All of the diffraction peaks can be indexed to the mixed phases of anatase TiO2 (JCPDS 89-4921) and rutile TiO2 (JCPDS 77-0442), in which the peaks of anatase TiO2 were broad while rutile TiO2 were sharp, indicating that the size of anatase TiO2 was small and rutile TiO2 was relatively large. Raman spectrum in Fig. 1b further reveals that the samples were composed of anatase and rutile TiO2. Strong bands at 145, 198, 401, 515 and 640 cm− 1 were observed clearly. All of them can be assigned to the anatase TiO2 with the symmetries of Eg, Eg, B1g, A1g and Eg, respectively [30]. Other three bands at 235, 448 and 610 cm− 1 were attributed to the A1g, two-phonon scattering and Eg modes of rutile TiO2 [31]. Fig. 2 shows SEM and TEM images of the hierarchical rutile@ 88
Journal of Electroanalytical Chemistry 804 (2017) 87–91
Y. Liu et al.
Fig. 2. (a–b) SEM, (c–d) TEM and (e–f) HRTEM images of hierarchical rutile@anatase TiO2 nanorods.
results are depicted in Fig. 3d–f. At a current rate of 0.5 C, an initial discharge capacity of 308.3 mA h g− 1 and subsequent charge capacity of 183.5 mA h g− 1 can be achieved, and a Coulombic efficiency of 64.6% was also reached. After 100 cycles, a reversible capacity of 152.9 mA h g− 1 was retained. To evaluate the capability at a high rate, the discharge-charge current rates were increased to 10 and 20 C. As shown in Fig. 3e–f, the discharge capacities of 102.7 and 84.9 mA h g− 1 can be obtained after 1000 and 5000 cycles, respectively. In addition, the Coulombic efficiency rapidly increased to nearly 100% after initial several cycles. The outstanding cycling stability and extraordinary rate capability reveal the excellent durability of the electrode composed of the hierarchically structural rutile@anatase TiO2 nanorods. The electrochemical properties of electrodes for LIBs usually are related to their physical and chemical properties. Therefore, the electrochemical properties of rutile TiO2 and rutile/anatase TiO2 were also investigated (Figs. S5–S7). The previous work reported that rutile TiO2 was irreversibly transformed into rocksalt-type Li0.9TiO2 during the first discharge process and the mesostructure of Li0.9TiO2 remained stable during subsequent discharge-charge processes, leading a stable cycling performance [27]. Similarly, in the present work, pure rutile TiO2 also showed a stable cycling performance, however, along with a relative low capacity of 52.7 mA h g− 1 after 1000 cycles at a current rate of 10 C (Fig. S7). On the other hand, the rutile/anatase TiO2 displayed high capacities after the initial several cycles and dramatically fade to a capacity of 45.8 mA h g− 1 after 1000 cycles (Fig. S7). In the present work, the hierarchically structural rutile@anatase TiO2 nanorods are
The electrochemical properties of the hierarchical rutile@anatase TiO2 nanorods were investigated in detail and the results are shown in Fig. 3. Fig. 3a shows the CV curves of the sample at a scanning rate of 0.5 mV s− 1 between 1.0 and 3.0 V. It shows well-defined cathodic peaks at about 1.75 and anodic peaks at appropriately 2.0 V, which were assigned to the insertion/extraction of the Li-ion into/from the anatase TiO2, respectively [33–35]. In the initial scan, two cathodic peaks at about 1.4 and 1.1 V were clearly observed, which were attributed to the Li-ion insertion into the rutile TiO2 [27,36,37]. The former can be identified as the phase transition of rutile TiO2 to spineltype Li0.44TiO2, while the latter was attributed to a second phase transition to rocksalt-type Li0.9TiO2. Interestingly, the two cathodic peaks disappeared in the subsequent scans, suggesting an irreversible process in the rutile TiO2. Fig. 3b shows the charge-discharge profiles of the hierarchical rutile@anatase TiO2 nanorods at a current rate of 0.5 C. During the initial discharge process, three voltage plateaus at appropriately 1.7, 1.4 and 1.1 V can be observed. Subsequently, the second discharge profile exhibited only voltage plateau at about 1.75 V. These results are generally in agreement with the CV analysis results. Fig. 3c displays the rate capability of the hierarchical rutile@anatase TiO2 nanorods at various current rates. The capacities of 158.8, 147.7, 130.2, 109.6, 93.8 and 81.2 mA h g− 1 were achieved at 0.5, 1, 2, 5, 10 and 20 C, respectively. When the current rate was backed to 1 C, the capacity of 144.3 mA h g− 1 was retained after 50 cycles. The results reveal that such a dual-phase material can deliver an excellent rate capability. At the same time, the cycling performance of the hierarchical rutile@anatase TiO2 nanorods was also investigated and the 89
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Fig. 3. The electrochemical properties of hierarchical rutile@anatase TiO2 nanorods: (a) CV curves with a scan rate of 0.5 mV s− 1; (b) charge-discharge profiles at a current rate of 0.5 C; (c) rate capability; cycling performances at different current rates of (d) 0.5, (e) 10 and (f) 20 C. (1 C = 168 mA g− 1).
to the synergistic effects (Scheme 1): (i) nanosized anatase TiO2 embedded on the surface of rutile TiO2 nanorods offer more storage sites for Li-ion, thus promoting fast lithium insertion/extraction into/from electrode and enhanced capacities; (ii) the core of rutile TiO2 nanorods can deliver a stable cycling performance and buffer aggregation of anatase TiO2 nanoparticles as a support; (iii) the hierarchically structural rutile@anatase TiO2 nanorods had a relative high BET surface area of 114.2 m2 g− 1 (Fig. S8) and enhanced the contact between the electrode and the electrolyte, resulting in improved electrochemical properties.; (vi) the core-shell structure can provide an optimal pathway for electron transfer through boundaries of core and shell, due to the highest conductivity direction being parallel to the [001] direction of rutile TiO2 nanorod [38]. 4. Conclusions In summary, a one-step synthetic route was developed for synthesizing the hierarchically structural rutile@anatase TiO2 nanorods with a core-shell structure for the first time. At the same time, it was found that PTA is a particular key in producing TiO2 with different crystal phases and morphologies. A cell made of the hierarchically structural rutile@anatase TiO2 nanorods for Li-ion intercalation exhibited a capacity of 102.7 mA h g− 1 after 1000 cycles at a current rate of 10 C and a capacity of 84.7 mA h g− 1 after 5000 cycles at a current rate of 20 C. The excellent electrochemical properties might be contributed to the
Scheme 1. Schematic of a tentative transport pathway of Li+ ions and electrons in hierarchical rutile@anatase TiO2 nanorods.
composed of rutile and anatase TiO2 in the form of core-shell structure. In comparison with pure rutile TiO2 or rutile/anatase TiO2, this sample exhibited a superior cycling performance with a high capacity. The excellent electrochemical properties of this material can be attributed 90
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synergistic effects of rutile and anatase TiO2 in a dual-phase material system. We believe that such a unique hierarchical TiO2 with a coreshell structure has a great potential application in field of photocatalysis and Na-ion batteries. Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jelechem.2017.09.051. Acknowledgment This work was financially supported by National Natural Science Foundation of China (NSFC U1505241; 91433104). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
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Contents lists available at ScienceDirect
Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem
Hierarchically structural TiO2 nanorods composed of rutile core and anatase shell as a durable anode material for lithium-ion intercalation
MARK
Yubin Liua,b, Tianli Dinga,b, Deli Shena,b, Jie Doua,b, Mingdeng Weia,b,⁎ a b
State Key Laboratory of Photocatalysis on Energy and Environment, Fuzhou University, Xueyuan Road 2, Fuzhou, Fujian 350116, China Institute of Advanced Energy Materials, Fuzhou University, Xueyuan Road 2, Fuzhou, Fujian 350116, China
A R T I C L E I N F O
A B S T R A C T
Keywords: TiO2 Core-shell structure Dual-phase Lithium-ion battery Electrochemical property
In the present work, hierarchically structural TiO2 nanorods composed of rutile core and anatase shell were successfully synthesized via a one-step hydrothermal route for the first time, and then were characterized by Xray diffraction (XRD), Raman spectroscopy and scanning and transmission electron microscopy (SEM/TEM) measurements. It was found that the crystal phases of the same can be easily adjusted by changing the amount of the reactant during the reaction process. Furthermore, the sample was used as an anode for lithium-ion intercalation and exhibited high capacity and long-term cycling stability due to the synergistic effects of dual-phase TiO2. For instance, the capacities of 102.7 and 84.7 mA h g− 1 can be kept at 10 C for 1000 cycles and 20 C for 5000 cycles, respectively.
1. Introduction Lithium-ion batteries (LIBs) are considered to be one of the most promising types of efficient energy storage devices [1–3]. Up to now, a large number of Ti-based materials have been investigated as alternative anode materials for LIBs due to their environmental benign, low cost, enhanced safety as well as some performance advantages [4–5], such as Li4Ti5O12 [6–8], TiO2 [9–14], H2Ti3O7 [15] and Li2ZnTi3O8 [16]. In comparison with pure phase materials, Ti-based oxides with a dual-phase are believed to combine their respective advantages and show great superior electrochemical properties. Recently, there are increasing reports on dual-phase Ti-based oxides and their application in LIBs. For example, Guo et al. [17] reported that rutile TiO2 terminated Li4Ti5O12 nanosheets showed much improved rate capability and specific capacity compared with pure phase Li4Ti5O12. Similarly, Jiang et al. [18] synthesized dual-phase Li4Ti5O12-anatase TiO2 nanocrystallines, which exhibited a good electrochemical performance over individual pure phase Li4Ti5O12. Fan et al. [19] prepared unique anatase/ TiO2-B composite nanosheets, in which anatase as an electron-accepting phase and TiO2-B as a Li+-accepting phase; this material demonstrated higher specific capacity and good rate capability. Among Ti-based materials, nanostructured anatase and rutile TiO2 are easily obtained by using some simple methods. To date, a large number of experiments and theoretical calculations have been per-
⁎
formed to investigate pure phase rutile or anatase TiO2 [20–25]. It is believed that pure phase anatase TiO2 is a more facile host for Li-insertion/extraction and can deliver a high reversible capacity [4], while pure phase rutile TiO2 appears more stable cycling performances due to the existence of the mesostructure rocksalt lithium titanate during the initial discharge process [26,27]. To the best of our knowledge, however, there are few reports on the application of nanostructural TiO2 with dual-phase of rutile and anatase in LIBs. For instance, Wang et al. [28] reported that anatase/rutile TiO2 microspheres with a hierarchically porous structure showed a capacity of about 103 mA h g− 1 after 100 cycles at 5100 mA g− 1. Xu et al. [29] adopted a two-step route to prepare a composite of rutile/anatase TiO2 nanotubes arrays, which displayed a capacity of about 185 mA h g− 1 after 100 cycles at 33.5 mA g− 1. Obviously, these results both of capacity and cycling stability are needed to be enhanced. Therefore, to develop a newly facile route for synthesizing unique nanostructural dual-phase TiO2 with an excellent electrochemical performance is still a challenge. In the present work, a one-step synthetic route was designed for synthesizing hierarchically structural TiO2 nanorods composed of rutile core and anatase shell for the first time. It was found that the crystal phases of the products were easily controlled by varying the amount of the reactant during the reaction process. Furthermore, this material was used as an anode for LIBs and exhibited durable electrochemical performances.
Corresponding author at: State Key Laboratory of Photocatalysis on Energy and Environment, Fuzhou University, Xueyuan Road 2, Fuzhou, Fujian 350116, China. E-mail address: [email protected] (M. Wei).
http://dx.doi.org/10.1016/j.jelechem.2017.09.051 Received 25 April 2017; Received in revised form 22 September 2017; Accepted 25 September 2017 Available online 27 September 2017 1572-6657/ © 2017 Elsevier B.V. All rights reserved.
Journal of Electroanalytical Chemistry 804 (2017) 87–91
Y. Liu et al.
2. Experimental 2.1. Preparation and characterization of samples Hierarchically structural TiO2 nanorods composed of rutile core and anatase shell (hierarchical rutile@anatase TiO2 nanorods) were obtained as follows: 0.1 g of p-phthalic acid (PTA) was dispersed in a mixed solution of 25 mL H2O and 5 mL sodium lactate. And then, 2 mL of titanium butoxide (TBOT) was added in the above solution under vigorous stirring. After stirring for several hours, the resulting suspension was transferred into a Teflon lined stainless steel autoclave with a capacity of 50 mL. The autoclave was kept at 160 °C for 18 h and then naturally cooled to room temperature. The white precipitate was harvested via centrifugation, washed thoroughly with deionized water and ethyl alcohol for several times, and then dried at 70 °C for 12 h. 2.2. Characterizations of the samples X-ray diffraction (XRD) patterns of samples were recorded on a Rigaku Ultima IV X-ray diffractomator by using CuKα radiation. Scanning electron microscopy (SEM, Hitachi S4800 instrument) and transmission electron microscopy (TEM, FEI F20 S-TWIN instrument) were applied for the determination of samples morphology and composition. The Raman spectra were recorded in a Renishaw inVai Raman microscope with a 532 nm laser. N2 adsorption-desorption was performed on a Micromeritics ASAP 2020 instrument (Micromeritics, Norcross, GA, USA).
Fig. 1. (a) XRD patterns and (b) Raman spectrum of hierarchical rutile@anatase TiO2 nanorods.
2.3. Electrochemical measurements anatase TiO2. SEM image in Fig. 2a reveals that the synthesized sample consisted of nanorods with uniform morphology. The length and width of these nanorods were about 200 and 50 nm, respectively. As shown in Fig. 2b–c, it is found that the surface of the hierarchical rutile@anatase TiO2 nanorods was very rough. Fig. 2d displays TEM images of a single nanorod, in which rutile TiO2 nanorod was covered by a large amount of anatase TiO2 nanoparticles, forming a core-shell structure. Fig. 2e exhibits the HRTEM image taken from region I in Fig. 2d (marked by a white rectangle). Obviously, a large amount of nanoparticles with several nanometers were embedded on the surface of the nanorod. On the other hand, these nanoparticles had identical lattice fringe of 0.35 nm, corresponding to the d101 spacing in the XRD pattern, indicating that the crystal phase of these nanoparticles was anatase TiO2. In comparison to the lattice fringe of 0.35 nm with random orientations, the lattice fringe of rutile TiO2 nanorod appeared in the same direction, and was approximately 0.32 nm. Fig. 2f is a high magnification TEM taken from the side edge of the rutile@anatase TiO2 nanorods (region II in Fig. 2d). Similarly, two kinds of lattice fringes of 0.35 and 0.32 nm were clearly observed, indicating that the rutile TiO2 were almost completely covered with anatase TiO2 naoparticles. At the same time, it was found that p-phthalic acid (PTA) plays a key role in formation of hierarchical rutile@anatase TiO2 nanorods during the reaction process. Namely, PTA is particularly effective in producing TiO2 with different crystal phases and different morphologies. The samples were obtained in the presence of 0 and 0.2 g of PTA, and denoted as rutile TiO2 and rutile/anatase TiO2, respectively. As shown in Figs. S1–2, the pure rutile TiO2 can be obtained in the absence of PTA. With the increasing PTA, the content of rutile TiO2 in composites decreased gradually. This might be ascribed to a steric block effect of PTA on the arrangement of the TiO6 octahedra, resulted in the formation of anatase-type TiO2 nuclei, which is similar to SO42 − [32]. On the other hand, the pure rutile TiO2 displayed an irregular cuboid-like morphology as shown in Fig. S3. When the content of PTA was increased to 0.2 g, the obtained rutile/anatase TiO2 presented a large amount of anatase TiO2 nanoparticles besides the partial rutile@anatase TiO2 nanorods. At the same time, it was also found that these anatase TiO2 nanoparticles were aggregated severely (in Fig. S4).
The electrochemical performance of all samples was performed using 2025-type coin cells with two-electrodes. First, the resulting samples were admixed with polyvinylidene fluoride (PVDF) binder and acetylene black carbon in a weight ratio of 7:2:1 to form a slurry which was pressed on copper foil circular flakes and dried at 110 °C in a vacuum overnight. Copper foils coated active materials were used the work electrodes and Lithium foils were used as the counter electrodes. The mass loading of the active materials was about 0.8 mg cm2. The electrolyte was 1 M LiPF6 in a 1:1:1 (volume ratio) mixture of ethylene carbonate (EC), ethylene methyl carbonate (EMC) and dimethyl carbonate (DMC). Celgard2400 (America) microporous polypropylene membrane was used as the separator. Cell assembly was carried out in a glove box filled with highly pure argon gas (O2 and H2O levels < 1 ppm). Cyclic voltammetry (CV) and charge-discharge tests of all electrodes were performed using an electrochemical workstation (CHI 600C) and Land automatic batteries tester (Land, CT 2001A, Wuhan, China), respectively. Both rate and cycling testing were performed in the working voltage range of 1.0–3.0 V (vs. Li+/Li). 3. Results and discussion The crystalline phases of the samples were confirmed by XRD and Raman and the results are depicted in Fig. 1. Fig. 1a shows the XRD patterns of the hierarchical rutile@anatase TiO2 obtained in the presence of 0.1 g PTA. All of the diffraction peaks can be indexed to the mixed phases of anatase TiO2 (JCPDS 89-4921) and rutile TiO2 (JCPDS 77-0442), in which the peaks of anatase TiO2 were broad while rutile TiO2 were sharp, indicating that the size of anatase TiO2 was small and rutile TiO2 was relatively large. Raman spectrum in Fig. 1b further reveals that the samples were composed of anatase and rutile TiO2. Strong bands at 145, 198, 401, 515 and 640 cm− 1 were observed clearly. All of them can be assigned to the anatase TiO2 with the symmetries of Eg, Eg, B1g, A1g and Eg, respectively [30]. Other three bands at 235, 448 and 610 cm− 1 were attributed to the A1g, two-phonon scattering and Eg modes of rutile TiO2 [31]. Fig. 2 shows SEM and TEM images of the hierarchical rutile@ 88
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Fig. 2. (a–b) SEM, (c–d) TEM and (e–f) HRTEM images of hierarchical rutile@anatase TiO2 nanorods.
results are depicted in Fig. 3d–f. At a current rate of 0.5 C, an initial discharge capacity of 308.3 mA h g− 1 and subsequent charge capacity of 183.5 mA h g− 1 can be achieved, and a Coulombic efficiency of 64.6% was also reached. After 100 cycles, a reversible capacity of 152.9 mA h g− 1 was retained. To evaluate the capability at a high rate, the discharge-charge current rates were increased to 10 and 20 C. As shown in Fig. 3e–f, the discharge capacities of 102.7 and 84.9 mA h g− 1 can be obtained after 1000 and 5000 cycles, respectively. In addition, the Coulombic efficiency rapidly increased to nearly 100% after initial several cycles. The outstanding cycling stability and extraordinary rate capability reveal the excellent durability of the electrode composed of the hierarchically structural rutile@anatase TiO2 nanorods. The electrochemical properties of electrodes for LIBs usually are related to their physical and chemical properties. Therefore, the electrochemical properties of rutile TiO2 and rutile/anatase TiO2 were also investigated (Figs. S5–S7). The previous work reported that rutile TiO2 was irreversibly transformed into rocksalt-type Li0.9TiO2 during the first discharge process and the mesostructure of Li0.9TiO2 remained stable during subsequent discharge-charge processes, leading a stable cycling performance [27]. Similarly, in the present work, pure rutile TiO2 also showed a stable cycling performance, however, along with a relative low capacity of 52.7 mA h g− 1 after 1000 cycles at a current rate of 10 C (Fig. S7). On the other hand, the rutile/anatase TiO2 displayed high capacities after the initial several cycles and dramatically fade to a capacity of 45.8 mA h g− 1 after 1000 cycles (Fig. S7). In the present work, the hierarchically structural rutile@anatase TiO2 nanorods are
The electrochemical properties of the hierarchical rutile@anatase TiO2 nanorods were investigated in detail and the results are shown in Fig. 3. Fig. 3a shows the CV curves of the sample at a scanning rate of 0.5 mV s− 1 between 1.0 and 3.0 V. It shows well-defined cathodic peaks at about 1.75 and anodic peaks at appropriately 2.0 V, which were assigned to the insertion/extraction of the Li-ion into/from the anatase TiO2, respectively [33–35]. In the initial scan, two cathodic peaks at about 1.4 and 1.1 V were clearly observed, which were attributed to the Li-ion insertion into the rutile TiO2 [27,36,37]. The former can be identified as the phase transition of rutile TiO2 to spineltype Li0.44TiO2, while the latter was attributed to a second phase transition to rocksalt-type Li0.9TiO2. Interestingly, the two cathodic peaks disappeared in the subsequent scans, suggesting an irreversible process in the rutile TiO2. Fig. 3b shows the charge-discharge profiles of the hierarchical rutile@anatase TiO2 nanorods at a current rate of 0.5 C. During the initial discharge process, three voltage plateaus at appropriately 1.7, 1.4 and 1.1 V can be observed. Subsequently, the second discharge profile exhibited only voltage plateau at about 1.75 V. These results are generally in agreement with the CV analysis results. Fig. 3c displays the rate capability of the hierarchical rutile@anatase TiO2 nanorods at various current rates. The capacities of 158.8, 147.7, 130.2, 109.6, 93.8 and 81.2 mA h g− 1 were achieved at 0.5, 1, 2, 5, 10 and 20 C, respectively. When the current rate was backed to 1 C, the capacity of 144.3 mA h g− 1 was retained after 50 cycles. The results reveal that such a dual-phase material can deliver an excellent rate capability. At the same time, the cycling performance of the hierarchical rutile@anatase TiO2 nanorods was also investigated and the 89
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Fig. 3. The electrochemical properties of hierarchical rutile@anatase TiO2 nanorods: (a) CV curves with a scan rate of 0.5 mV s− 1; (b) charge-discharge profiles at a current rate of 0.5 C; (c) rate capability; cycling performances at different current rates of (d) 0.5, (e) 10 and (f) 20 C. (1 C = 168 mA g− 1).
to the synergistic effects (Scheme 1): (i) nanosized anatase TiO2 embedded on the surface of rutile TiO2 nanorods offer more storage sites for Li-ion, thus promoting fast lithium insertion/extraction into/from electrode and enhanced capacities; (ii) the core of rutile TiO2 nanorods can deliver a stable cycling performance and buffer aggregation of anatase TiO2 nanoparticles as a support; (iii) the hierarchically structural rutile@anatase TiO2 nanorods had a relative high BET surface area of 114.2 m2 g− 1 (Fig. S8) and enhanced the contact between the electrode and the electrolyte, resulting in improved electrochemical properties.; (vi) the core-shell structure can provide an optimal pathway for electron transfer through boundaries of core and shell, due to the highest conductivity direction being parallel to the [001] direction of rutile TiO2 nanorod [38]. 4. Conclusions In summary, a one-step synthetic route was developed for synthesizing the hierarchically structural rutile@anatase TiO2 nanorods with a core-shell structure for the first time. At the same time, it was found that PTA is a particular key in producing TiO2 with different crystal phases and morphologies. A cell made of the hierarchically structural rutile@anatase TiO2 nanorods for Li-ion intercalation exhibited a capacity of 102.7 mA h g− 1 after 1000 cycles at a current rate of 10 C and a capacity of 84.7 mA h g− 1 after 5000 cycles at a current rate of 20 C. The excellent electrochemical properties might be contributed to the
Scheme 1. Schematic of a tentative transport pathway of Li+ ions and electrons in hierarchical rutile@anatase TiO2 nanorods.
composed of rutile and anatase TiO2 in the form of core-shell structure. In comparison with pure rutile TiO2 or rutile/anatase TiO2, this sample exhibited a superior cycling performance with a high capacity. The excellent electrochemical properties of this material can be attributed 90
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