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anatase microparticles: Improved anodes for lithium ion batteries
CJCHE-00162; No of Pages 7 Chinese Journal of Chemical Engineering xxx (2015) xxx–xxx
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Chinese Journal of Chemical Engineering journal homepage: www.elsevier.com/locate/CJCHE
Energy, Resources and Environmental Technology
In-situ synthesized mesoporous TiO2-B/anatase microparticles: Improved anodes for lithium ion batteries☆ Wei Zhuang 1,2, Linghong Lu 1,⁎, Wei Li 1, Rong An 1, Xin Feng 1, Xinbing Wu 1, Yudan Zhu 1, Xiaohua Lu 1,⁎ 1 2
State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing Tech University, Nanjing 210009, China National Engineering Technique Research Center for Biotechnology, College of Biotechnology and Pharmaceutical Engineering, Nanjing Tech University, Nanjing 211816, China
a r t i c l e
i n f o
Article history: Received 25 June 2013 Received in revised form 27 November 2013 Accepted 21 January 2014 Available online xxxx Keywords: Titania Lithium ion battery Microparticles Mesoporous TiO2-B
a b s t r a c t Mesoporous TiO2-B/anatase microparticles have been in-situ synthesized from K2Ti2O5 without template. The TiO2-B phase around the particle surface accelerates the diffusion of charges through the interface, while the anatase phase in the core maintains the capacity stability. The heterojunction interface between the main polymorph of anatase and the trace of TiO2-B exhibits promising lithium ion battery performance. This trace of 5% (by mass) TiO2-B determined by Raman spectra brings the first discharge capacity of this material to 247 mA·h·g−1, giving 20% improvement compared to the anatase counterpart. Stability testing at 1 C reveals that the capacity maintains at 171 mA·h·g−1, which is better than 162 mA·h·g−1 for single phase anatase or 159 mA·h·g−1 for TiO2-B. The mesoporous TiO2-B/anatase microparticles also show superior rate performance with 100 mA·h·g−1 at 40 C, increased by nearly 25% as compared to pure anatase. This opens a possibility of a general design route, which can be applied to other metal oxide electrode materials for rechargeable batteries and supercapacitors. © 2015 The Chemical Industry and Engineering Society of China, and Chemical Industry Press. All rights reserved.
1. Introduction Lithium ion batteries have attracted increasing attentions because of their important role in the stationary energy storage systems for smart grids and electric vehicle batteries [1,2]. However, the current lithium ion batteries employing carbonaceous materials as anodes are inadequate to meet the requirements of high power, large energy capacity and safety because of material-related issues [3,4]. Various nanomaterials such as carbon nanotubes, graphene, carbon nanofibres and titanates have been developed to improve the performance of batteries [5,6]. Titanates have a higher lithium intercalation/deintercalation potential at approximately 1.5 V versus Li+/Li, which can effectively avoid safety problems [6,7]. TiO2, which has multiple polymorphs, including anatase, rutile, brookite, and TiO2-B, has succeeded as one of the most fascinating materials in the modern era [8,9]. In these four phases, anatase and TiO2-B provide a nearly theoretical specific capacity of 335 mA·h·g−1 (LiTiO2) as lithium insertion hosts [10–12]. Because of its high insertion voltage and low toxicity, anatase is suggested as an
☆ Supported by the Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT 0732), the National Natural Science Foundation of China (21136004, 20736002, 21176113, 20876073), NSFC-RGC (20731160614), China Postdoctoral Science Foundation (20110491407) and the National Basic Research Program of China (2009CB623407, 2009CB219902 and 2009CB226103). ⁎ Corresponding authors. E-mail addresses: [email protected] (L. Lu), [email protected] (X. Lu).
alternative anode material for lithium ion batteries and shows improved capacity retention and safe operation [13]. Different morphologies of anatase have been prepared, such as nanotube, nanofiber, nanoparticle and mesoporous structure [7,14]. They all perform equivalently well as anode materials in the secondary rechargeable lithium ion batteries [15,16]. However, anatase suffers from relatively poor rate performance, probably due to its dense crystal structure [17]. Recent researches have showed that the lithium ion storage capacity of TiO2-B is larger than that of anatase (335 mA·h·g−1 corresponding to the insertion of one Li per TiO2 inducing a complete reduction Ti4+ → Ti3+) and it is safe against overcharging [18,19]. However, TiO2-B is a metastable crystalline phase, which can gradually convert to anatase, degrading both structure stability and lithium intercalation stability of TiO2-B [10,20,21]. Therefore, developing novel low-cost TiO2 anode materials with long term cycling stability, large capacity, and excellent rate performance is still a challenge. Excellent rate capability and stability are contributed by high lithium ion diffusion rate in both bulk and interface of materials [22]. A small particle size and a large surface area can significantly reduce the diffusion distance of lithium ions in the bulk phase, increasing charge and discharge rates and decreasing volumetric changes and lattice stresses. Another similar strategy is to fabricate mesoporous materials for improving the kinetics of storage reactions [14,23]. The transfer of electron can be effectively improved through interface modification, such as surface carbon coating [24], metal coating [25] and non-metallic element lattice doping [26]. However, it is difficult to improve lithium ion diffusion performance of anode materials [27,28]. Recently, heterojunction
http://dx.doi.org/10.1016/j.cjche.2014.11.020 1004-9541/© 2015 The Chemical Industry and Engineering Society of China, and Chemical Industry Press. All rights reserved.
Please cite this article as: W. Zhuang, et al., In-situ synthesized mesoporous TiO2-B/anatase microparticles: Improved anodes for lithium ion batteries, Chin. J. Chem. Eng. (2015), http://dx.doi.org/10.1016/j.cjche.2014.11.020
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W. Zhuang et al. / Chinese Journal of Chemical Engineering xxx (2015) xxx–xxx
composites have been proposed to enhance lithium ion interface transfer, especially those with lattice matched heterojunction interface modification [29,30]. In this work, we start a controllable synthesis method for preparing mesoporous nanostructured TiO2 materials with TiO2-B/anatase heterojunction interface. Such TiO2 nanocomposite electrode material may combine the advantages of both anatase and TiO2-B polymorphs. TiO2-B phase accelerates the diffusion of charges through the interface, while the anatase phase maintains the capacity stability. The rate capability and cycling performance are also characterized.
After natural cooling, anatase, TiO2-B and TiO2-B/anatase composite were synthesized, described by reactions (2) and (3). 810 ∘ C
TiO2 nH2 O þ K2 CO3 → K2 Ti2 O5 þ nH2 O þ CO2 þ
K2 Ti2 O5 þ H2 O þ H
8 100 ∘ C 500 ∘ C > <→ H2 Ti2 O5 þ H2 Ti5 O11 → anatase=TiO2 −B ðSample 1Þ > :
170 ∘ C
500 ∘ C
ð2Þ
→ H2 Ti5 O11 → TiO2 −B ðSample 2Þ
K2 Ti2 O5 þ H2 O ∘
2. Experimental
ð1Þ
þ 25 C
þ H → H2 Ti2 O5
8 <
800 ∘ C
→ anatase ðnonporousÞ ðSample 3Þ ∘
500 C :→ anatase ðmesoporousÞ ðSample 4Þ
ð3Þ
2.1. Sample preparation Titania with various structures has been successfully synthesized by the template-free and soft chemical method [31,32]. The summary of the preparation method is as follows. (I) Preparation of K2Ti2O5: a TiO2/K2O mixture with molar ratio of 1.9 was prepared by uniformly adding K2CO3 (reagent grade) to TiO2·nH2O and then sintered at 810 °C for 2 h, described by reaction (1). (II) Hydration and ion exchange of K2Ti2O5: by controlling hydration temperatures at 25 °C, 100 °C and 170 °C, three types of intermediates can be obtained after ion exchange, H2Ti2O5, H2Ti5O11/H2Ti2O5 and H2Ti5O11 materials, respectively, as shown by reactions (2) and (3). It needs to be noted that the phase transition under certain conditions for preparation of K2Ti2O5 is a non-uniform phase transition in the solid–liquid interface. Hydrated K2Ti2O5 was suspended in 100 ml of vigorously stirred 0.1 mol·L−1 HCl solution until K+ ions were completely exchanged. The product was filtered and washed with distilled water and dried in a desiccator at 60 °C under vacuum. Residual of K+ b 0.2% (by mass) in the final product, detected by inductively coupled plasma-mass spectrometry (ICP-MS). (III) Thermal treatment of hydrated titanate: the dried samples were calcinated in a muffle oven at 500 °C in air for 2 h.
2.2. Characterization The material crystal phase was determined by powder X-ray diffraction (XRD, Bruker D8, CuK α radiation) and Raman spectra (Super LabRam). The textural properties were studied by N 2 adsorption–desorption measurements (ASAP 2020M) at a liquid nitrogen temperature. The sample morphology and microstructure were observed by scanning electron microscopy (SEM, Hitachi S-4800) and transmission electron microscopy (TEM, Philips Tecnai G2 20) at 200 kV. 2.3. Electrochemical performance All electrochemical measurements were done in Swagelok type cells with pure lithium foil as the counter and reference electrode and Whatman glass fiber as the separator. For preparation of working electrode, a slurry was prepared by mixing the active material, Super P carbon (Alfa Aesar) and PVdF (Kynarflex) in a mass ratio of 85:8:7 in Nmethyl-pyrrolidone. The slurry was cast on a copper foil (Alfa Aesar,
Fig. 1. (a) FE-SEM image of Sample 1 with microparticle morphology; (b) magnified FE-SEM image with mesoporous structure (inset: N2 adsorption–desorption curves of Sample 1); (c) TEM image of Sample 1 constituted with nanocrystals (inset: selected area electron diffraction result); (d) HRTEM image of Sample 1 with anatase nanocrystals packaged with TiO2-B thin layer and conformed heterojunction structure [inset: schematic diagram of distributed TiO2-B with cross-sectional view (left) and surface view (right)].
Please cite this article as: W. Zhuang, et al., In-situ synthesized mesoporous TiO2-B/anatase microparticles: Improved anodes for lithium ion batteries, Chin. J. Chem. Eng. (2015), http://dx.doi.org/10.1016/j.cjche.2014.11.020
W. Zhuang et al. / Chinese Journal of Chemical Engineering xxx (2015) xxx–xxx
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Table 1 Specific surface area, pore volume and most probable pore size of different samples Samples
Specific surface area/m2·g−1
Pore volume/cm3·g−1
Most probable pore size/nm
Sample 1 Sample 2 Sample 3 Sample 4
77.3 77.8 10.3 74.5
0.181 0.179 / 0.175
7.05 4.04 / 6.11
thickness 0.05 mm) and was dried in vacuum at 100 °C for 24 h. The electrolyte used for the above cell configuration was 1 mol·L−1 LiPF6 in a mixture of ethylene carbonate (Aldrich) and dimethyl carbonate (Aldrich) (mass ratio of 1:1). The galvanostatic charge–discharge characteristics of the cells were recorded over the voltage range between 1.0 V and 3.0 V using a NEWARE BTS-5 V50 mA computercontrolled battery test station at different rates of 1–40 C at room temperature (25 °C). 1 C is equivalent to 330 mA·g−1 for titania. Cyclic voltammetry was performed in the voltage range of 1.0–3.0 V using an advanced electrochemical system of Autolab 302 N at a scanning rate of 0.1 mV·s−1. 3. Results and Discussion 3.1. Morphology and structural properties Mesoporous TiO2-B/anatase nanocomposite particles (Sample 1) with a high surface area were synthesized by the template-free soft chemistry method. Reference samples were prepared with similar methods, including mesoporous TiO2-B particles (Sample 2), nonporous anatase particles (Sample 3) and mesoporous anatase particles (Sample 4). The microstructure of samples was characterized by FE-SEM and N2 adsorption/desorption isotherms. As shown in Fig. 1(a), Sample 1 exhibits microparticle morphology with densely distributed mesopores. In Fig. 1(b), Sample 1 presents loosely aligned about 30 nm nanocrystals with around 10 nm mesopores. N2 adsorption–desorption isotherms
[the inset of Fig. 1(b)] of Sample 1 indicate the presence of welldeveloped mesoporosity. The textural properties of other samples calculated by BET and BJH method are summarized in Table 1. All the three mesoporous samples have a similar surface area and a pore structure. It allows us to exclude the electrochemical performance differences caused by the surface area and pore structure of these samples. TEM result of Fig. 1(c) shows that Sample 1 is combined by connecting nanocrystals, with anatase and TiO2-B coexisting. HRTEM image [Fig. 1(d)] gives the evidence of bi-crystalline structure of Sample 1 with typical TiO2-B distributed on anatase crystal lattice surface and conforms to the heterojunction structure. The crystalline phase of the samples was investigated by powder Xray diffraction (XRD) analyses, as shown in Fig. 2(a). Five distinct diffraction peaks of Samples 1, 3 and 4 can be indexed to anatase (JCPDF 21-1272). Peaks corresponding to the TiO2-B phase (JCPDF 35-0088) are not observed for these three samples because the amount is very low and the distribution is uniform. The diffraction peaks of Sample 2 are mainly ascribed to the TiO2-B phase. Raman spectra as shown in Fig. 2(b) reveal that TiO2-B and anatase coexist in Sample 1, while Samples 3 and 4 are pure anatase phase and Sample 2 is pure TiO2-B phase. In Fig. 2(c), a low peak belonging to TiO2-B phase is observed on the partial magnified XRD patterns of Sample 1 with TiO2-B and anatase phases coexisting, confirming the low amount of TiO2-B. However, no peak in this area is seen for other samples. UV Raman spectroscopy is found to be more sensitive to the surface phase of a solid sample that absorbs UV light [33]. We can see in Fig. 2(d) that the characteristic peaks of
Fig. 2. (a) XRD patterns of samples and standard peak spectra of anatase and TiO2-B; (b) Raman spectra (incident light laser: 514 nm) of different TiO2 samples; (c) partial XRD patterns of samples; (d) Raman spectra of Sample 1 at different incident light lasers.
Please cite this article as: W. Zhuang, et al., In-situ synthesized mesoporous TiO2-B/anatase microparticles: Improved anodes for lithium ion batteries, Chin. J. Chem. Eng. (2015), http://dx.doi.org/10.1016/j.cjche.2014.11.020
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W. Zhuang et al. / Chinese Journal of Chemical Engineering xxx (2015) xxx–xxx
(a)
1.0
IA/(IA+IB)
0.8
Calibrated Literature Sample
0.6
We correlate the Raman areas of anatase and TiO2-B main peaks to the relative proportions of both polytypes, as shown in Fig. 3(a). The standard curve plotted by Eq. (4) characterized with a 514 nm laser as the incident light agrees well with reported results in literature. y ¼ kR x=½1 þ ðkR − 1ÞxðkR ¼ 6:1Þ
ð4Þ
0.4
CTiO -B=5% 2
0.2 0.0 0.0
0.2
0.4
0.6
0.8
1.0
m A /(m A +m B )
(b)
Anatase
intensity
Sample 1
TiO2-B
80
120
160
200
Raman shift/cm
240
-1
Fig. 3. Calibrated curves of the intensity of Raman spectra and mass percent (a) and content analysis (b) of TiO2-B in Sample 1.
TiO2-B detected by UV Raman spectroscopy are stronger than visible Raman spectroscopy. These confirm that Sample 1 is TiO2-B surrounded anatase.
The ratios IA/(IA + IB), where IA and IB are the Raman peak areas of 144 and 123 cm− 1 of anatase and TiO2-B, respectively, are plotted against the mass anatase percentage in Fig. 1(c). The content of TiO2-B calculated from the standard curve is only about 5%, as shown in Fig. 3(a) (star). The analysis process of TiO2-B in Sample 1 is shown in Fig. 3(b). 3.2. Electrochemical performance The electrochemical performance of these materials was evaluated in lithium half-cells. The activity materials were coated on Cu foil, with thickness about 15 μm and loading amount about 2 mg·cm−2. Fig. 4(a) shows the discharge capacities of different samples at the current rate of 130 mA·g−1 within a cut-off window of 1.0–3.0 V. The capacity is divided into three parts: homogeneous bulk lithium insertion, bulk intercalation storage and pseudocapacitive (interfacial) storage [34]. Samples 3 and 4 present a close bulk intercalation capacity of up to 140 mA·h·g−1. As the heat treatment temperature increases, nanoparticle size grows, so the mesoporous structure disappears and the surface area decreases. Therefore, the surface lithium capacity stored in the form of pseudocapacitance in Sample 3 is lower than that in Sample 4. Comparison between Samples 4 and 2 clearly shows the superiority in lithium capacity of TiO2-B material as compared to anatase, which is consistent with previous reports [35,36]. As shown in Fig. A1, TiO2-B/anatase nanocomposite of Sample 1 presents the capacity between Samples 2 and 4, indicating the enhanced capacity due to the TiO2-B phase. As discussed above, the 5% (by mass) TiO2-B
Fig. 4. Electrochemical properties of TiO2 samples. (a) Discharge curves at 130 mA·g−1 (A: homogeneous Li insertion into the bulk; B: bulk intercalation reaction; C: pseudo-capacitive (interfacial) storage); (b) cyclic voltammograms of Samples 1 and 4 at the scan rate of 0.1 mV·s−1 between 1.0 and 3.0 V; (c) rate performance at the rates of 1 C to 40 C of Samples 1 and 4; (d) cycling performance of Sample 1 compared to other samples at the current rate of 1 C (1 C = 330 mA·g−1).
Please cite this article as: W. Zhuang, et al., In-situ synthesized mesoporous TiO2-B/anatase microparticles: Improved anodes for lithium ion batteries, Chin. J. Chem. Eng. (2015), http://dx.doi.org/10.1016/j.cjche.2014.11.020
W. Zhuang et al. / Chinese Journal of Chemical Engineering xxx (2015) xxx–xxx
contributes 20% improvement compared to the anatase counterpart of Sample 4. The cyclic voltammogram [CVs, Fig. 4(b)] profiles of Samples 1 and 4 show pairs of cathodic/anodic peaks at 1.70 V/2.05 V, attributing to the reversible transition of lithium-ion in anatase phase. In addition, two additional pairs of peaks with formal potentials in the range of 1.5–1.6 V appear in the CVs of Sample 1. This can be assigned to the pseudocapacitive behavior of lithium storage in TiO2-B phase lattice attributing to the oxidation/reduction of Ti3+/Ti4+ along with the lithium insertion/extraction [1]. By analyzing the differential capacity increment curve of the charge branch (Fig. A2), the contribution of TiO2-B to the total lithium storage is 20%, which is much higher than its mass content of 5%. This indicates that the TiO2-B phase in Sample 1 plays key roles for maintaining the capacity by the fast pseudocapacitive process of lithium storage and synergistically enhancing the lithium capacity because of its uniform distribution and specific TiO2-B/anatase heterojunction interfaces [37]. Fig. 4(c) gives clear results for rate performance of samples. All the anatase containing samples show good capacity retentions higher than 90% attributed to the structure stability of anatase polymorph, while that of single phase TiO2-B (Sample 2) is less than 85%. The results suggest that Sample 1 has high capacity and excellent stability of lithium ion intercalation and de-intercalation. Interestingly, the irreversible capacity of Sample 1 accounts for 17.5%, less than 18.7% of Sample 2, i.e. Sample 1 gives better stability than Sample 2. The capacity and stability are further investigated by cycling charge–discharge performance at different current rates (1 C = 330 mA·g−1), as shown in Fig. 4(d). The results reveal that the lithium-half cells assembled (Sample 1) exhibit high Li+ storage capacity (208 mA·h·g−1 at 1 C) and superior rate performance, which is much better than those of anatase counterpart Sample 4 as well as other anatase nanomaterials reported in literature, such as 154 mA·h·g−1 for mesoporous nanospheres [38], 165 mA·h·g− 1 for anatase nanoporous mesocrystals, and 115 mA·h·g−1 for anatase nanoparticles [16]. Such TiO2 nanocomposite electrode material combines the advantages of both anatase and TiO 2 -B polymorphs. Surprisingly, this trace of 5% (by mass) TiO2-B determined by Raman spectra brings the first discharge capacity of this material to 247 mA·h·g − 1 (vs. 205 mA·h·g− 1 of the anatase counterpart) at the current rate of 130 mA·g− 1. Stability testing at 1 C reveals that the capacity maintains at 171 mA·h·g− 1 , which is better than 162 mA·h·g− 1 for
5
single phase anatase or 159 mA·h·g− 1 for TiO2-B. The rate capability is significantly higher than pure anatase material, from 80 mA·h·g− 1 to 100 mA·h·g− 1 at 40 C, increased by 25%. The improved performance may be attributed to the combination effects of mesoporous anatase structures, open lattice channel of TiO2-B and TiO2-B/anatase heterojunction interfaces, as shown in Fig. 5. The primary crystal phase of anatase provides the capacity stability owing to the structural stability. The existence of mesoporous structures shortens the diffusion distance of lithium ions, increases the contact area with electrolyte, and significantly improves the capacity. The structure of the surface layer of TiO2-B (010) crystal face with a larger lattice channel is beneficial to the transfer of lithium ions. Regarding the lithium-ion adsorption of TiO2-B (010), quantitative calculation results show that the surface of crystal channels plays an important role in lithium ion penetration into the materials. The structure accelerates the diffusion of charges within the interface and bulk of materials. At the TiO2B/anatase heterojunction interfaces, lattice and band-gap matched structures provide an ideal connection of the two individual components in the composite, rendering synergistically enhanced performance. The structure speeds up the diffusion of lithium ions and electrons within the interface and materials, and structural stability of the anatase core increases the capacity stability.
4. Conclusions TiO2-B/anatase microparticles were synthesized through a templatefree and soft chemistry method. 5% (by mass) of TiO2-B gives 20% improvement in the first discharge capacity compared to the anatase counterpart. Stability testing reveals that the nanocomposite is more stable than the TiO2-B counterpart. This nanocomposite also shows superior rate performance, with 100 mA·h·g−1 at 40 C, increased by nearly 25% compared to pure anatase. The superior lithium storage performance is attributed to the mesoporous structure offering a large electrode–electrolyte contact area and a specific TiO2-B/anatase interface facilitating electronic transport along and across the interface. Thus the developed novel microparticle electrode material processes the advantages of both anatase and TiO2-B polymorphs with a mesoporous nanostructure and a heterojunction interface, meeting the demands of electric vehicle batteries and other stationary energy storage applications.
Fig. 5. Schematic illustration of the relationship between structure and performance of mesoporous TiO2-B/anatase microparticles composed of nanocrystals. (I) Geometric factors: TiO2-B shell with large Li+ diffusion channel enhancing the diffusion rate; (II) electronic factors: the heterojunction structure between TiO2-B and anatase promoting electronic conductivity.
Please cite this article as: W. Zhuang, et al., In-situ synthesized mesoporous TiO2-B/anatase microparticles: Improved anodes for lithium ion batteries, Chin. J. Chem. Eng. (2015), http://dx.doi.org/10.1016/j.cjche.2014.11.020
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Appendix
Fig. A1. Discharge–charge curves of the samples at the current rate of 130 mA·g−1.
Fig. A2. Cyclic voltammograms (CV) at the scan rate of 0.1 mV·s−1 between 1.0 and 3.0 V. (a) Sample 1; (b) Sample 2; (c) Sample 3; (d) Sample 4 and (e) the content analysis of TiO2-B in Sample 1 by CV.
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Contents lists available at ScienceDirect
Chinese Journal of Chemical Engineering journal homepage: www.elsevier.com/locate/CJCHE
Energy, Resources and Environmental Technology
In-situ synthesized mesoporous TiO2-B/anatase microparticles: Improved anodes for lithium ion batteries☆ Wei Zhuang 1,2, Linghong Lu 1,⁎, Wei Li 1, Rong An 1, Xin Feng 1, Xinbing Wu 1, Yudan Zhu 1, Xiaohua Lu 1,⁎ 1 2
State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing Tech University, Nanjing 210009, China National Engineering Technique Research Center for Biotechnology, College of Biotechnology and Pharmaceutical Engineering, Nanjing Tech University, Nanjing 211816, China
a r t i c l e
i n f o
Article history: Received 25 June 2013 Received in revised form 27 November 2013 Accepted 21 January 2014 Available online xxxx Keywords: Titania Lithium ion battery Microparticles Mesoporous TiO2-B
a b s t r a c t Mesoporous TiO2-B/anatase microparticles have been in-situ synthesized from K2Ti2O5 without template. The TiO2-B phase around the particle surface accelerates the diffusion of charges through the interface, while the anatase phase in the core maintains the capacity stability. The heterojunction interface between the main polymorph of anatase and the trace of TiO2-B exhibits promising lithium ion battery performance. This trace of 5% (by mass) TiO2-B determined by Raman spectra brings the first discharge capacity of this material to 247 mA·h·g−1, giving 20% improvement compared to the anatase counterpart. Stability testing at 1 C reveals that the capacity maintains at 171 mA·h·g−1, which is better than 162 mA·h·g−1 for single phase anatase or 159 mA·h·g−1 for TiO2-B. The mesoporous TiO2-B/anatase microparticles also show superior rate performance with 100 mA·h·g−1 at 40 C, increased by nearly 25% as compared to pure anatase. This opens a possibility of a general design route, which can be applied to other metal oxide electrode materials for rechargeable batteries and supercapacitors. © 2015 The Chemical Industry and Engineering Society of China, and Chemical Industry Press. All rights reserved.
1. Introduction Lithium ion batteries have attracted increasing attentions because of their important role in the stationary energy storage systems for smart grids and electric vehicle batteries [1,2]. However, the current lithium ion batteries employing carbonaceous materials as anodes are inadequate to meet the requirements of high power, large energy capacity and safety because of material-related issues [3,4]. Various nanomaterials such as carbon nanotubes, graphene, carbon nanofibres and titanates have been developed to improve the performance of batteries [5,6]. Titanates have a higher lithium intercalation/deintercalation potential at approximately 1.5 V versus Li+/Li, which can effectively avoid safety problems [6,7]. TiO2, which has multiple polymorphs, including anatase, rutile, brookite, and TiO2-B, has succeeded as one of the most fascinating materials in the modern era [8,9]. In these four phases, anatase and TiO2-B provide a nearly theoretical specific capacity of 335 mA·h·g−1 (LiTiO2) as lithium insertion hosts [10–12]. Because of its high insertion voltage and low toxicity, anatase is suggested as an
☆ Supported by the Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT 0732), the National Natural Science Foundation of China (21136004, 20736002, 21176113, 20876073), NSFC-RGC (20731160614), China Postdoctoral Science Foundation (20110491407) and the National Basic Research Program of China (2009CB623407, 2009CB219902 and 2009CB226103). ⁎ Corresponding authors. E-mail addresses: [email protected] (L. Lu), [email protected] (X. Lu).
alternative anode material for lithium ion batteries and shows improved capacity retention and safe operation [13]. Different morphologies of anatase have been prepared, such as nanotube, nanofiber, nanoparticle and mesoporous structure [7,14]. They all perform equivalently well as anode materials in the secondary rechargeable lithium ion batteries [15,16]. However, anatase suffers from relatively poor rate performance, probably due to its dense crystal structure [17]. Recent researches have showed that the lithium ion storage capacity of TiO2-B is larger than that of anatase (335 mA·h·g−1 corresponding to the insertion of one Li per TiO2 inducing a complete reduction Ti4+ → Ti3+) and it is safe against overcharging [18,19]. However, TiO2-B is a metastable crystalline phase, which can gradually convert to anatase, degrading both structure stability and lithium intercalation stability of TiO2-B [10,20,21]. Therefore, developing novel low-cost TiO2 anode materials with long term cycling stability, large capacity, and excellent rate performance is still a challenge. Excellent rate capability and stability are contributed by high lithium ion diffusion rate in both bulk and interface of materials [22]. A small particle size and a large surface area can significantly reduce the diffusion distance of lithium ions in the bulk phase, increasing charge and discharge rates and decreasing volumetric changes and lattice stresses. Another similar strategy is to fabricate mesoporous materials for improving the kinetics of storage reactions [14,23]. The transfer of electron can be effectively improved through interface modification, such as surface carbon coating [24], metal coating [25] and non-metallic element lattice doping [26]. However, it is difficult to improve lithium ion diffusion performance of anode materials [27,28]. Recently, heterojunction
http://dx.doi.org/10.1016/j.cjche.2014.11.020 1004-9541/© 2015 The Chemical Industry and Engineering Society of China, and Chemical Industry Press. All rights reserved.
Please cite this article as: W. Zhuang, et al., In-situ synthesized mesoporous TiO2-B/anatase microparticles: Improved anodes for lithium ion batteries, Chin. J. Chem. Eng. (2015), http://dx.doi.org/10.1016/j.cjche.2014.11.020
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W. Zhuang et al. / Chinese Journal of Chemical Engineering xxx (2015) xxx–xxx
composites have been proposed to enhance lithium ion interface transfer, especially those with lattice matched heterojunction interface modification [29,30]. In this work, we start a controllable synthesis method for preparing mesoporous nanostructured TiO2 materials with TiO2-B/anatase heterojunction interface. Such TiO2 nanocomposite electrode material may combine the advantages of both anatase and TiO2-B polymorphs. TiO2-B phase accelerates the diffusion of charges through the interface, while the anatase phase maintains the capacity stability. The rate capability and cycling performance are also characterized.
After natural cooling, anatase, TiO2-B and TiO2-B/anatase composite were synthesized, described by reactions (2) and (3). 810 ∘ C
TiO2 nH2 O þ K2 CO3 → K2 Ti2 O5 þ nH2 O þ CO2 þ
K2 Ti2 O5 þ H2 O þ H
8 100 ∘ C 500 ∘ C > <→ H2 Ti2 O5 þ H2 Ti5 O11 → anatase=TiO2 −B ðSample 1Þ > :
170 ∘ C
500 ∘ C
ð2Þ
→ H2 Ti5 O11 → TiO2 −B ðSample 2Þ
K2 Ti2 O5 þ H2 O ∘
2. Experimental
ð1Þ
þ 25 C
þ H → H2 Ti2 O5
8 <
800 ∘ C
→ anatase ðnonporousÞ ðSample 3Þ ∘
500 C :→ anatase ðmesoporousÞ ðSample 4Þ
ð3Þ
2.1. Sample preparation Titania with various structures has been successfully synthesized by the template-free and soft chemical method [31,32]. The summary of the preparation method is as follows. (I) Preparation of K2Ti2O5: a TiO2/K2O mixture with molar ratio of 1.9 was prepared by uniformly adding K2CO3 (reagent grade) to TiO2·nH2O and then sintered at 810 °C for 2 h, described by reaction (1). (II) Hydration and ion exchange of K2Ti2O5: by controlling hydration temperatures at 25 °C, 100 °C and 170 °C, three types of intermediates can be obtained after ion exchange, H2Ti2O5, H2Ti5O11/H2Ti2O5 and H2Ti5O11 materials, respectively, as shown by reactions (2) and (3). It needs to be noted that the phase transition under certain conditions for preparation of K2Ti2O5 is a non-uniform phase transition in the solid–liquid interface. Hydrated K2Ti2O5 was suspended in 100 ml of vigorously stirred 0.1 mol·L−1 HCl solution until K+ ions were completely exchanged. The product was filtered and washed with distilled water and dried in a desiccator at 60 °C under vacuum. Residual of K+ b 0.2% (by mass) in the final product, detected by inductively coupled plasma-mass spectrometry (ICP-MS). (III) Thermal treatment of hydrated titanate: the dried samples were calcinated in a muffle oven at 500 °C in air for 2 h.
2.2. Characterization The material crystal phase was determined by powder X-ray diffraction (XRD, Bruker D8, CuK α radiation) and Raman spectra (Super LabRam). The textural properties were studied by N 2 adsorption–desorption measurements (ASAP 2020M) at a liquid nitrogen temperature. The sample morphology and microstructure were observed by scanning electron microscopy (SEM, Hitachi S-4800) and transmission electron microscopy (TEM, Philips Tecnai G2 20) at 200 kV. 2.3. Electrochemical performance All electrochemical measurements were done in Swagelok type cells with pure lithium foil as the counter and reference electrode and Whatman glass fiber as the separator. For preparation of working electrode, a slurry was prepared by mixing the active material, Super P carbon (Alfa Aesar) and PVdF (Kynarflex) in a mass ratio of 85:8:7 in Nmethyl-pyrrolidone. The slurry was cast on a copper foil (Alfa Aesar,
Fig. 1. (a) FE-SEM image of Sample 1 with microparticle morphology; (b) magnified FE-SEM image with mesoporous structure (inset: N2 adsorption–desorption curves of Sample 1); (c) TEM image of Sample 1 constituted with nanocrystals (inset: selected area electron diffraction result); (d) HRTEM image of Sample 1 with anatase nanocrystals packaged with TiO2-B thin layer and conformed heterojunction structure [inset: schematic diagram of distributed TiO2-B with cross-sectional view (left) and surface view (right)].
Please cite this article as: W. Zhuang, et al., In-situ synthesized mesoporous TiO2-B/anatase microparticles: Improved anodes for lithium ion batteries, Chin. J. Chem. Eng. (2015), http://dx.doi.org/10.1016/j.cjche.2014.11.020
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Table 1 Specific surface area, pore volume and most probable pore size of different samples Samples
Specific surface area/m2·g−1
Pore volume/cm3·g−1
Most probable pore size/nm
Sample 1 Sample 2 Sample 3 Sample 4
77.3 77.8 10.3 74.5
0.181 0.179 / 0.175
7.05 4.04 / 6.11
thickness 0.05 mm) and was dried in vacuum at 100 °C for 24 h. The electrolyte used for the above cell configuration was 1 mol·L−1 LiPF6 in a mixture of ethylene carbonate (Aldrich) and dimethyl carbonate (Aldrich) (mass ratio of 1:1). The galvanostatic charge–discharge characteristics of the cells were recorded over the voltage range between 1.0 V and 3.0 V using a NEWARE BTS-5 V50 mA computercontrolled battery test station at different rates of 1–40 C at room temperature (25 °C). 1 C is equivalent to 330 mA·g−1 for titania. Cyclic voltammetry was performed in the voltage range of 1.0–3.0 V using an advanced electrochemical system of Autolab 302 N at a scanning rate of 0.1 mV·s−1. 3. Results and Discussion 3.1. Morphology and structural properties Mesoporous TiO2-B/anatase nanocomposite particles (Sample 1) with a high surface area were synthesized by the template-free soft chemistry method. Reference samples were prepared with similar methods, including mesoporous TiO2-B particles (Sample 2), nonporous anatase particles (Sample 3) and mesoporous anatase particles (Sample 4). The microstructure of samples was characterized by FE-SEM and N2 adsorption/desorption isotherms. As shown in Fig. 1(a), Sample 1 exhibits microparticle morphology with densely distributed mesopores. In Fig. 1(b), Sample 1 presents loosely aligned about 30 nm nanocrystals with around 10 nm mesopores. N2 adsorption–desorption isotherms
[the inset of Fig. 1(b)] of Sample 1 indicate the presence of welldeveloped mesoporosity. The textural properties of other samples calculated by BET and BJH method are summarized in Table 1. All the three mesoporous samples have a similar surface area and a pore structure. It allows us to exclude the electrochemical performance differences caused by the surface area and pore structure of these samples. TEM result of Fig. 1(c) shows that Sample 1 is combined by connecting nanocrystals, with anatase and TiO2-B coexisting. HRTEM image [Fig. 1(d)] gives the evidence of bi-crystalline structure of Sample 1 with typical TiO2-B distributed on anatase crystal lattice surface and conforms to the heterojunction structure. The crystalline phase of the samples was investigated by powder Xray diffraction (XRD) analyses, as shown in Fig. 2(a). Five distinct diffraction peaks of Samples 1, 3 and 4 can be indexed to anatase (JCPDF 21-1272). Peaks corresponding to the TiO2-B phase (JCPDF 35-0088) are not observed for these three samples because the amount is very low and the distribution is uniform. The diffraction peaks of Sample 2 are mainly ascribed to the TiO2-B phase. Raman spectra as shown in Fig. 2(b) reveal that TiO2-B and anatase coexist in Sample 1, while Samples 3 and 4 are pure anatase phase and Sample 2 is pure TiO2-B phase. In Fig. 2(c), a low peak belonging to TiO2-B phase is observed on the partial magnified XRD patterns of Sample 1 with TiO2-B and anatase phases coexisting, confirming the low amount of TiO2-B. However, no peak in this area is seen for other samples. UV Raman spectroscopy is found to be more sensitive to the surface phase of a solid sample that absorbs UV light [33]. We can see in Fig. 2(d) that the characteristic peaks of
Fig. 2. (a) XRD patterns of samples and standard peak spectra of anatase and TiO2-B; (b) Raman spectra (incident light laser: 514 nm) of different TiO2 samples; (c) partial XRD patterns of samples; (d) Raman spectra of Sample 1 at different incident light lasers.
Please cite this article as: W. Zhuang, et al., In-situ synthesized mesoporous TiO2-B/anatase microparticles: Improved anodes for lithium ion batteries, Chin. J. Chem. Eng. (2015), http://dx.doi.org/10.1016/j.cjche.2014.11.020
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(a)
1.0
IA/(IA+IB)
0.8
Calibrated Literature Sample
0.6
We correlate the Raman areas of anatase and TiO2-B main peaks to the relative proportions of both polytypes, as shown in Fig. 3(a). The standard curve plotted by Eq. (4) characterized with a 514 nm laser as the incident light agrees well with reported results in literature. y ¼ kR x=½1 þ ðkR − 1ÞxðkR ¼ 6:1Þ
ð4Þ
0.4
CTiO -B=5% 2
0.2 0.0 0.0
0.2
0.4
0.6
0.8
1.0
m A /(m A +m B )
(b)
Anatase
intensity
Sample 1
TiO2-B
80
120
160
200
Raman shift/cm
240
-1
Fig. 3. Calibrated curves of the intensity of Raman spectra and mass percent (a) and content analysis (b) of TiO2-B in Sample 1.
TiO2-B detected by UV Raman spectroscopy are stronger than visible Raman spectroscopy. These confirm that Sample 1 is TiO2-B surrounded anatase.
The ratios IA/(IA + IB), where IA and IB are the Raman peak areas of 144 and 123 cm− 1 of anatase and TiO2-B, respectively, are plotted against the mass anatase percentage in Fig. 1(c). The content of TiO2-B calculated from the standard curve is only about 5%, as shown in Fig. 3(a) (star). The analysis process of TiO2-B in Sample 1 is shown in Fig. 3(b). 3.2. Electrochemical performance The electrochemical performance of these materials was evaluated in lithium half-cells. The activity materials were coated on Cu foil, with thickness about 15 μm and loading amount about 2 mg·cm−2. Fig. 4(a) shows the discharge capacities of different samples at the current rate of 130 mA·g−1 within a cut-off window of 1.0–3.0 V. The capacity is divided into three parts: homogeneous bulk lithium insertion, bulk intercalation storage and pseudocapacitive (interfacial) storage [34]. Samples 3 and 4 present a close bulk intercalation capacity of up to 140 mA·h·g−1. As the heat treatment temperature increases, nanoparticle size grows, so the mesoporous structure disappears and the surface area decreases. Therefore, the surface lithium capacity stored in the form of pseudocapacitance in Sample 3 is lower than that in Sample 4. Comparison between Samples 4 and 2 clearly shows the superiority in lithium capacity of TiO2-B material as compared to anatase, which is consistent with previous reports [35,36]. As shown in Fig. A1, TiO2-B/anatase nanocomposite of Sample 1 presents the capacity between Samples 2 and 4, indicating the enhanced capacity due to the TiO2-B phase. As discussed above, the 5% (by mass) TiO2-B
Fig. 4. Electrochemical properties of TiO2 samples. (a) Discharge curves at 130 mA·g−1 (A: homogeneous Li insertion into the bulk; B: bulk intercalation reaction; C: pseudo-capacitive (interfacial) storage); (b) cyclic voltammograms of Samples 1 and 4 at the scan rate of 0.1 mV·s−1 between 1.0 and 3.0 V; (c) rate performance at the rates of 1 C to 40 C of Samples 1 and 4; (d) cycling performance of Sample 1 compared to other samples at the current rate of 1 C (1 C = 330 mA·g−1).
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contributes 20% improvement compared to the anatase counterpart of Sample 4. The cyclic voltammogram [CVs, Fig. 4(b)] profiles of Samples 1 and 4 show pairs of cathodic/anodic peaks at 1.70 V/2.05 V, attributing to the reversible transition of lithium-ion in anatase phase. In addition, two additional pairs of peaks with formal potentials in the range of 1.5–1.6 V appear in the CVs of Sample 1. This can be assigned to the pseudocapacitive behavior of lithium storage in TiO2-B phase lattice attributing to the oxidation/reduction of Ti3+/Ti4+ along with the lithium insertion/extraction [1]. By analyzing the differential capacity increment curve of the charge branch (Fig. A2), the contribution of TiO2-B to the total lithium storage is 20%, which is much higher than its mass content of 5%. This indicates that the TiO2-B phase in Sample 1 plays key roles for maintaining the capacity by the fast pseudocapacitive process of lithium storage and synergistically enhancing the lithium capacity because of its uniform distribution and specific TiO2-B/anatase heterojunction interfaces [37]. Fig. 4(c) gives clear results for rate performance of samples. All the anatase containing samples show good capacity retentions higher than 90% attributed to the structure stability of anatase polymorph, while that of single phase TiO2-B (Sample 2) is less than 85%. The results suggest that Sample 1 has high capacity and excellent stability of lithium ion intercalation and de-intercalation. Interestingly, the irreversible capacity of Sample 1 accounts for 17.5%, less than 18.7% of Sample 2, i.e. Sample 1 gives better stability than Sample 2. The capacity and stability are further investigated by cycling charge–discharge performance at different current rates (1 C = 330 mA·g−1), as shown in Fig. 4(d). The results reveal that the lithium-half cells assembled (Sample 1) exhibit high Li+ storage capacity (208 mA·h·g−1 at 1 C) and superior rate performance, which is much better than those of anatase counterpart Sample 4 as well as other anatase nanomaterials reported in literature, such as 154 mA·h·g−1 for mesoporous nanospheres [38], 165 mA·h·g− 1 for anatase nanoporous mesocrystals, and 115 mA·h·g−1 for anatase nanoparticles [16]. Such TiO2 nanocomposite electrode material combines the advantages of both anatase and TiO 2 -B polymorphs. Surprisingly, this trace of 5% (by mass) TiO2-B determined by Raman spectra brings the first discharge capacity of this material to 247 mA·h·g − 1 (vs. 205 mA·h·g− 1 of the anatase counterpart) at the current rate of 130 mA·g− 1. Stability testing at 1 C reveals that the capacity maintains at 171 mA·h·g− 1 , which is better than 162 mA·h·g− 1 for
5
single phase anatase or 159 mA·h·g− 1 for TiO2-B. The rate capability is significantly higher than pure anatase material, from 80 mA·h·g− 1 to 100 mA·h·g− 1 at 40 C, increased by 25%. The improved performance may be attributed to the combination effects of mesoporous anatase structures, open lattice channel of TiO2-B and TiO2-B/anatase heterojunction interfaces, as shown in Fig. 5. The primary crystal phase of anatase provides the capacity stability owing to the structural stability. The existence of mesoporous structures shortens the diffusion distance of lithium ions, increases the contact area with electrolyte, and significantly improves the capacity. The structure of the surface layer of TiO2-B (010) crystal face with a larger lattice channel is beneficial to the transfer of lithium ions. Regarding the lithium-ion adsorption of TiO2-B (010), quantitative calculation results show that the surface of crystal channels plays an important role in lithium ion penetration into the materials. The structure accelerates the diffusion of charges within the interface and bulk of materials. At the TiO2B/anatase heterojunction interfaces, lattice and band-gap matched structures provide an ideal connection of the two individual components in the composite, rendering synergistically enhanced performance. The structure speeds up the diffusion of lithium ions and electrons within the interface and materials, and structural stability of the anatase core increases the capacity stability.
4. Conclusions TiO2-B/anatase microparticles were synthesized through a templatefree and soft chemistry method. 5% (by mass) of TiO2-B gives 20% improvement in the first discharge capacity compared to the anatase counterpart. Stability testing reveals that the nanocomposite is more stable than the TiO2-B counterpart. This nanocomposite also shows superior rate performance, with 100 mA·h·g−1 at 40 C, increased by nearly 25% compared to pure anatase. The superior lithium storage performance is attributed to the mesoporous structure offering a large electrode–electrolyte contact area and a specific TiO2-B/anatase interface facilitating electronic transport along and across the interface. Thus the developed novel microparticle electrode material processes the advantages of both anatase and TiO2-B polymorphs with a mesoporous nanostructure and a heterojunction interface, meeting the demands of electric vehicle batteries and other stationary energy storage applications.
Fig. 5. Schematic illustration of the relationship between structure and performance of mesoporous TiO2-B/anatase microparticles composed of nanocrystals. (I) Geometric factors: TiO2-B shell with large Li+ diffusion channel enhancing the diffusion rate; (II) electronic factors: the heterojunction structure between TiO2-B and anatase promoting electronic conductivity.
Please cite this article as: W. Zhuang, et al., In-situ synthesized mesoporous TiO2-B/anatase microparticles: Improved anodes for lithium ion batteries, Chin. J. Chem. Eng. (2015), http://dx.doi.org/10.1016/j.cjche.2014.11.020
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Appendix
Fig. A1. Discharge–charge curves of the samples at the current rate of 130 mA·g−1.
Fig. A2. Cyclic voltammograms (CV) at the scan rate of 0.1 mV·s−1 between 1.0 and 3.0 V. (a) Sample 1; (b) Sample 2; (c) Sample 3; (d) Sample 4 and (e) the content analysis of TiO2-B in Sample 1 by CV.
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