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C with a novel lower voltage plateau for rechargeable sodium-ion batteries
Journal of Colloid and Interface Science 474 (2016) 88–92
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science journal homepage: www.elsevier.com/locate/jcis
Short Communication
Pyro-synthesis of a nanostructured NaTi2(PO4)3/C with a novel lower voltage plateau for rechargeable sodium-ion batteries Yubin Niu a,b, Maowen Xu a,b,⇑, Chunxian Guo a,b, Chang Ming Li a,b,c,⇑ a
Institute for Clean Energy & Advanced Materials, Faculty of Materials and Energy, Southwest University, Chongqing 400715, PR China Chongqing Key Laboratory for Advanced Materials and Technologies of Clean Energies, Chongqing 400715, PR China c Institute of Materials Science and Devices, Suzhou University of Science and Technology, Suzhou 215011, PR China b
g r a p h i c a l a b s t r a c t Carbon coated NaTi2(PO4)3 composite was synthesized by a pyro-synthetic reaction and it delivers a pair of novel reversible oxidation/reduction peaks at around 0.44 V.
a r t i c l e
i n f o
Article history: Received 8 March 2016 Revised 14 April 2016 Accepted 14 April 2016 Available online 16 April 2016
a b s t r a c t A pair of novel reversible oxidation/reduction peaks at around 0.44 V is discovered during the deep sodiation of NaTi2(PO4)3/C obtained by the pyro-synthetic approach. This novel low-voltage plateau doubles the charge/discharge capacity of NaTi2(PO4)3, thus turning it into a more promising anode for Na-ion batteries. Ó 2016 Elsevier Inc. All rights reserved.
Keywords: NaTi2(PO4)3 Pyro-synthetic Sodium-ion batteries Nanoparticles
1. Introduction ⇑ Corresponding authors at: Institute for Clean Energy & Advanced Materials, Faculty of Materials and Energy, Southwest University, Chongqing 400715, PR China. E-mail addresses: [email protected] (M. Xu), [email protected] (C.M. Li). http://dx.doi.org/10.1016/j.jcis.2016.04.021 0021-9797/Ó 2016 Elsevier Inc. All rights reserved.
Nowadays, as the demand for electrochemical energy storage system increases, the challenges faced by lithium-ion batteries (LIBs) are becoming more and more serious. This is mainly due
Y. Niu et al. / Journal of Colloid and Interface Science 474 (2016) 88–92
to the scarcity and rising prices of lithium resources. As one of the post-lithium ion batteries, sodium-ion batteries (SIBs) have been favored by many chemists because of its vast resources, low cost, safety, sustainability and similar electrochemical energy storage mechanism with LIBs [1,2]. So far, numerous innovative materials, such as biomass carbon [3], alloys [4,5], metal oxides [6], metal selenides [7], sulphides [8], and polyanion structure materials [9,10], were exploited as electrodes for SIBs. Of them, polyanionic compounds have more stable host frameworks, to a certain extent, which result in a longer cycle life and better safety [11,12]. NaTi2(PO4)3 crystallizes in the rhombohedral NASICON (Na super ionic conductor), which has been reported as aqueous [13,14] and non-aqueous [15–18] SIBs anode materials. For the latter, it possess a high theoretical capacity of 132.8 mA h g 1 accompanied by a sodiation voltage plateau at 2.1 V in the voltage range of 1.2–2.8 V (vs. Na+/Na) [15–18]. The plateau corresponds to the insertion of two Na ions to form Na3Ti2(PO4)3. However, as an anode electrode material, the voltage plateau is too high, resulting in the overall energy density of the battery is relatively low. Excitedly, we inadvertently found that the material exhibits another pair of primary redox peak at a voltage as low as ca. 0.44 V, which theoretically can contribute the specific capacity of 119.2 mA h g 1, thus turning NaTi2(PO4)3/C electrode into a good candidate as a SIB anode material combined with practical application.
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6300F). The thermal decomposition of the as-prepared product was investigated using thermogravimetric analyser (TGA, TA Instruments, USA) at a heating rate of 5 °C min 1 under air flow from room temperature to 700 °C. 2.3. Electrochemical measurements Electrochemical properties of the sample was investigated using coin cells (type 2032). The working electrode was prepared by adding the NaTi2(PO4)3/C powder, acetylene black, and polyvinylidene fluoride in the weight ratio of 70:20:10 to the Nmethyl-2-pyrrolidone solvent to form a homogeneous slurry. The resultant slurry was uniformly cast onto Cu foil by using a doctor blade and dried in a vacuum oven overnight at 120 °C. The cells were assembled in a high-purity Ar-filled glove box with both the moisture and oxygen content below 1 ppm. The counter and reference electrode was metallic sodium foil and the electrolyte was 1 M NaClO4 in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC; 1:1 v/v) with 3 wt% fluoroethylene carbonate (FEC) as an additive. Galvanostatic charge-discharge and cycling performance tests were performed in the voltage range of 0.01–2.8 V (vs. Na+/Na) at various current densities (herein, 1 C = 120 mA g 1) by using a LAND battery tester (CT2001A) at room temperature. Cyclic voltammetry (CV) measurements were carried out at a scan rate of 0.05 mV s 1 using an Arbin Instruments testing system.
2. Experimental 3. Results and discussion 2.1. Materials preparation The synthesis of NaTi2(PO4)3/C powder was carried out by a pyro-synthetic reaction (Scheme 1). In a typical synthesis, sodium acetate and phosphoric acid were dissolved in ethylene glycol in a molar ratio 1:3 at room temperature with constantly stirring. Stoichiometry of tetrabutyl titanate was then added into the above solution. The obtained homogenous milky solution was poured onto a hot-plate and ignited. Subsequently, the as-prepared precursor was annealed at 800 °C for 5 h under Ar atmosphere to obtain the NaTi2(PO4)3/C powder. 2.2. Morphology and structure characterization The structure of the material was characterized by powder Xray diffraction (PXRD) employing Cu-Ka radiation (k = 1.5416 Å) at a scanning rate of 0.4° min 1 in the 2h range of 10–80°. Morphology and microstructure of the as-prepared sample were examined by field-emission scanning electron microscopy (FESEM; JEOL-
Fig. 1a schematically shows that the framework of NaTi2(PO4)3 is built on an open 3D framework consisting of TiO6 octahedra and PO4 tetrahedra. In the structure, two TiO6 octahedra are separated by three PO4 tetrahedra and they share all their corner oxygens [18,19]. The phase composition of the as-synthesized product was characterized by PXRD. As shown in Fig. 1b, the positions and relative intensities of all diffraction peaks can be perfectly indexed as rhombohedral structured NaTi2(PO4)3 (PDF No.85–2265) with lattice parameters of a = 8.48088 Å, c = 21.79546 Å, a = 90.0, c = 120.0 and Volume = 1357.6 Å3. There is no additional peaks from impurities are detected, indicating the high purity of the asprepared NaTi2(PO4)3/C sample. The morphology of NaTi2(PO4)3/C was examined by FESEM at different magnifications (Fig. 2). The images reveal that the size distribution of NaTi2(PO4)3/C is wide, from 200 nm to several tens of nanometers. From Fig. S1a it can be seen that particles are relatively small in addition to an identified a carbon-coating with 3 nm thickness along the particle boundary. The lattice spacing of 0.369 nm and 0.423 nm are recog-
Scheme 1. Schematic illustration of the pyro-synthesis reaction.
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Y. Niu et al. / Journal of Colloid and Interface Science 474 (2016) 88–92
Fig. 1. (a) Schematic illustration of the crystal structure of NaTi2(PO4)3 along with different directions; (b) XRD pattern of the as-prepared NaTi2(PO4)3/C sample.
nized in corresponding to the inter-planar distances of (1 1 3) and (1 1 0) of the rhombohedral NaTi2(PO4)3 in Fig. S1b and c, respectively. In addition, the carbon content was determined by TG analysis under air atmosphere (Fig. S2). The result shows that the carbon content is approximately 5.6 wt% in the NaTi2(PO4)3/C sample. CV measurements were firstly employed to understand the electrochemical sodiation/desodiation processes in the NaTi2(PO4)3/C sample. Fig. 3a shows the CV curves for the first six cycles at a scan rate 0.05 mV s 1 in the potential window of 0.01–2.8 V (vs. Na+/Na). Obviously, there are one set of minor oxidation/reduction peaks near 0.09/0.01 V (Region 1) and two sets of primary oxidation/reduction peaks at around 0.52/0.35 V (Region 2) and 2.25/2.02 V (Region 3), respectively. Note that the minor redox
peaks in the Region 1 are most likely due to the sodiation/desodiation of the carbon layer and acetylene black. In the 1st CV cycle, there is a cathodic wide peak at 0.53 V (violet ellipse), however, and anodic peak is absent, which is attributed to the formation of the solid electrolyte interphase (SEI) and the decomposition of the electrolyte, which is expected at such low anode potentials below the stability limit of the electrolyte. After two cycles, the CV curves overlap well, indicating the reversible insertionextraction reaction of Na+ in the NaTi2(PO4)3 lattice for the reaction. For the Region 3, the redox peak can be attributed to the phase transition between NaTi2(PO4)3 and Na3Ti2(PO4)3 whose equilibrium potential is 2.14 V. Accordingly, the valence state of titanium will change from +4 to +3, which results in the high theoretical specific capacity of 132.8 mA h g 1. Unexpectedly, with further decreasing potential to the Region 2, an additional redox peak is observed, to a great extent, which can be attributed to the formation of Na5Ti2(PO4)3 with a titanium oxidation state of +2 whose equilibrium potential is 0.44 V. According to the above inference that the pair of redox peaks will generate the specific capacity of 119.2 mA h g 1, that is, as the anode electrode material, the theoretical capacity of NaTi2(PO4)3 up to 252 mA h g 1, and which is currently under further in-depth investigation in our laboratory. The rate capability of the NaTi2(PO4)3 electrode is summarized in Fig. 3b, which has deducted the capacity contribution of acetylene black (Fig. S3). The cell was cycled at different current densities of 0.2, 0.5, 1, 2, 3, 4, 5 and back to 0.5 C in a voltage window of 0.01–2.8 V. Notably, while the current rate increases from 0.2 to 5 C, the discharge capacity of NaTi2(PO4)3/C electrode slightly decreases from 225.3 (5th cycle) to 180.9 mA h g 1 (35th cycle) gradually, indicating its excellent rate performance, and the charge capacity at 0.2C is lower than the other rate capacities may be due to the activation of the materials and the formation of the SEI film. Most importantly, even after long cycles with different rates, when the rate is reverted to 0.5 C, the full capacity retention as that of its original capacity is observed. In addition, the charge-discharge curves at various current densities are shown in Fig. 3c. It is evident that the charge-discharge plateaus of NaTi2(PO4)3/C electrode at the current rate of 5 C still generate a distinct voltage plateau and the plateau voltages shift subtly as the charge/discharge currents increase from 0.2 to 5 C, implying the electrode owns exceptional sodium ions storage capability. Also, the cycle performance and Coulombic efficiency of the NaTi2(PO4)3/C anode at the current densities of 0.5 and 5 C are shown in Fig. S4 and Fig. 3d. The cycle performance reveals that the capacity of the electrode is 316.4 (1st) and 246.4 (2nd) mA h g 1 at 0.5 C (Fig. S4) and the capacity
Fig. 2. SEM images of the as-prepared NaTi2(PO4)3/C sample under different magnification.
Y. Niu et al. / Journal of Colloid and Interface Science 474 (2016) 88–92
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Fig. 3. Electrochemical performance of the NaTi2(PO4)3/C sample. (a) CV curves at a scan rate of 0.05 mV s 1 showing the first six cycles at a voltage window of 0.01–3 V; (b) Rate capability at rates of 0.2, 0.5, 1, 2, 3, 4, 5 and back to 0.5 C; (c) Typical charge-charge-discharge curves at different rates; (d) Cycling performance and Coulombic efficiency at 5 C after rate cycle.
of 179.4 mA h g 1 can be achieved after 70 cycles. After 250 charge-discharge cycles at 5 C (Fig. 3d), the capacity is 150 mA h g 1 with the Coulombic efficiency of ca. 100%, proving its well tolerance of fast sodium ions insertion and extraction for long-life SIBs. To further study the rate capability of NaTi2(PO4)3/C, the charge-discharge curves were differentiated to get the differential capacity (dQ/dV) vs voltage curves and the corresponding change of the difference DE between oxidation and reduction peaks and the intensity of cathodic peak in the Region 2 are shown in Fig. 4a and b, respectively. Obviously, as the current increases, the electrochemical polarization increases and the intensity of cathodic peak decreases; when the current reaches the maximum
value, the polarization is most serious and the peak strength of the cathode reaches a minimum, however, when the current back to 0.5 C, the polarization drastically reduce and the peak strength sharp increase, further indicating that the electrode material has excellent electrochemical recovery capability, which should be ascribed to the NASICON structure of highly covalent 3D framework with large interstitial spaces for Na ion diffusion.
4. Conclusions In summary, NaTi2(PO4)3/C powder was prepared by the pyrosynthetic approach and its 0.44 V low-voltage plateau in the Na half cells was reported. NaTi2(PO4)3/C can deliver a reversible
Fig. 4. (a) The corresponding differential capacity vs voltage (dQ/dV) curves of Fig. 3c; (b) The corresponding change of the electrochemical polarization DE and the intensities of cathodic peaks in the region 2 of (a).
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Y. Niu et al. / Journal of Colloid and Interface Science 474 (2016) 88–92
capacity of 179 mA h g 1 between 0.01 and 2.8 V at 0.5 C with good capacity retention. Moreover, NaTi2(PO4)3/C electrode also show a remarkable rate capability (reversible capacity reaches 180.9 mA h g 1 at the current density of 5 C) and long-term cyclability. Hence, the NaTi2(PO4)3/C powder is expected to be a promising anode material for SIBs. Acknowledgements This work is financially supported by Chongqing Key Laboratory for Advanced Materials and Technologies of Clean Energies under cstc2011pt-sy90001, Start-up grant under SWU111071 from Southwest University and the Basic and frontier research project of Chongqing (cstc2015jcyjA50031). M.W. Xu would like to thank the support from the Specialized Research fund for the Doctoral program of Higher Education (RFDP) (Grant No. 20130182120026) and Y.B. Niu would like to thank the support from the Graduate student research innovation project in Chongqing (CYB2015053). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2016.04.021. References [1] X. Xiang, K. Zhang, J. Chen, Recent advances and prospects of cathode materials for sodium-ion batteries, Adv. Mater. 27 (2015) 5343–5364. [2] M. Sawicki, L.L. Shaw, Advances and challenges of sodium ion batteries as post lithium ion batteries, RSC Adv. 5 (2015) 53129–53154. [3] E.M. Lotfabad, J. Ding, K. Cui, A. Kohandehghan, W.P. Kalisvaart, M. Hazelton, D. Mitlin, High-density sodium and lithium ion battery anodes from banana peels, ACS Nano 8 (2014) 7115–7129. [4] C. Yue, Y. Yu, S. Sun, X. He, B. Chen, W. Lin, B. Xu, M. Zheng, S. Wu, J. Li, J. Kang, L. Lin, High performance 3D Si/Ge nanorods array anode buffered by TiN/Ti interlayer for sodium-ion batteries, Adv. Funct. Mater. 25 (2015) 1386–1392.
[5] B. Farbod, K. Cui, W.P. Kalisvaart, M. Kupsta, B. Zahiri, A. Kohandehghan, E.M. Lotfabad, Z. Li, E.J. Luber, D. Mitlin, Anodes for sodium ion batteries based on tin–germanium–antimony alloys, ACS Nano 8 (2014) 4415–4429. [6] N. Zhang, X. Han, Y. Liu, X. Hu, Q. Zhao, J. Chen, 3D porous c-Fe2O3@C nanocomposite as high-performance anode material of Na-ion batteries, Adv. Energy Mater. 5 (2015). [7] K. Zhang, Z. Hu, X. Liu, Z. Tao, J. Chen, FeSe2 microspheres as a highperformance anode material for Na-ion batteries, Adv. Mater. 27 (2015) 3305– 3309. [8] X. Xie, Z. Ao, D. Su, J. Zhang, G. Wang, MoS2/graphene composite anodes with enhanced performance for sodium-ion batteries: the role of the twodimensional heterointerface, Adv. Funct. Mater. 25 (2015) 1393–1403. [9] D. Wang, N. Chen, M. Li, C. Wang, H. Ehrenberg, X. Bie, Y. Wei, G. Chen, F. Du, Na3V2(PO4)3/C composite as the intercalation-type anode material for sodiumion batteries with superior rate capability and long-cycle life, J. Mater. Chem. A 3 (2015) 8636–8642. [10] Z. Jian, Y. Sun, X. Ji, A new low-voltage plateau of Na3V2(PO4)3 as an anode for Na-ion batteries, Chem. Commun. 51 (2015) 6381–6383. [11] Y. Niu, M. Xu, C. Cheng, S. Bao, J. Hou, S. Liu, F. Yi, H. He, C.M. Li, Na3.12Fe2.44(P2O7)2/multi-walled carbon nanotube composite as a cathode material for sodium-ion batteries, J. Mater. Chem. A 3 (2015) 17224–17229. [12] Y. Niu, M. Xu, S.-J. Bao, C.M. Li, Porous graphene to encapsulate Na6.24Fe4.88(P2O7)4 as composite cathode materials for Na-ion batteries, Chem. Commun. 51 (2015) 13120–13122. [13] M. Vujkovic´, M. Mitric´, S. Mentus, High-rate intercalation capability of NaTi2(PO4)3/C composite in aqueous lithium and sodium nitrate solutions, J. Power Sources 288 (2015) 176–186. [14] B. Zhao, B. Lin, S. Zhang, C. Deng, A frogspawn-inspired hierarchical porous NaTi2(PO4)3-C array for high-rate and long-life aqueous rechargeable sodium batteries, Nanoscale 7 (2015) 18552–18560. [15] J. Yang, H. Wang, P. Hu, J. Qi, L. Guo, L. Wang, A high-rate and ultralong-life sodium-ion battery based on NaTi2(PO4)3 nanocubes with synergistic coating of carbon and rutile TiO2, Small 11 (2015) 3744–3749. [16] G. Yang, H. Song, M. Wu, C. Wang, Porous NaTi2(PO4)3 nanocubes: a high-rate nonaqueous sodium anode material with more than 10 000 cycle life, J. Mater. Chem. A 3 (2015) 18718–18726. [17] C. Wu, P. Kopold, Y.-L. Ding, P.A. van Aken, J. Maier, Y. Yu, Synthesizing porous NaTi2(PO4)3 nanoparticles embedded in 3D graphene networks for high-rate and long cycle-life sodium electrodes, ACS Nano 9 (2015) 6610–6618. [18] G. Pang, P. Nie, C. Yuan, L. Shen, X. Zhang, H. Li, C. Zhang, Mesoporous NaTi2(PO4)3/CMK-3 nanohybrid as anode for long-life Na-ion batteries, J. Mater. Chem. A 2 (2014) 20659–20666. [19] G. Pang, C. Yuan, P. Nie, B. Ding, J. Zhu, X. Zhang, Synthesis of NASICON-type structured NaTi2(PO4)3-graphene nanocomposite as an anode for aqueous rechargeable Na-ion batteries, Nanoscale 6 (2014) 6328–6334.
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science journal homepage: www.elsevier.com/locate/jcis
Short Communication
Pyro-synthesis of a nanostructured NaTi2(PO4)3/C with a novel lower voltage plateau for rechargeable sodium-ion batteries Yubin Niu a,b, Maowen Xu a,b,⇑, Chunxian Guo a,b, Chang Ming Li a,b,c,⇑ a
Institute for Clean Energy & Advanced Materials, Faculty of Materials and Energy, Southwest University, Chongqing 400715, PR China Chongqing Key Laboratory for Advanced Materials and Technologies of Clean Energies, Chongqing 400715, PR China c Institute of Materials Science and Devices, Suzhou University of Science and Technology, Suzhou 215011, PR China b
g r a p h i c a l a b s t r a c t Carbon coated NaTi2(PO4)3 composite was synthesized by a pyro-synthetic reaction and it delivers a pair of novel reversible oxidation/reduction peaks at around 0.44 V.
a r t i c l e
i n f o
Article history: Received 8 March 2016 Revised 14 April 2016 Accepted 14 April 2016 Available online 16 April 2016
a b s t r a c t A pair of novel reversible oxidation/reduction peaks at around 0.44 V is discovered during the deep sodiation of NaTi2(PO4)3/C obtained by the pyro-synthetic approach. This novel low-voltage plateau doubles the charge/discharge capacity of NaTi2(PO4)3, thus turning it into a more promising anode for Na-ion batteries. Ó 2016 Elsevier Inc. All rights reserved.
Keywords: NaTi2(PO4)3 Pyro-synthetic Sodium-ion batteries Nanoparticles
1. Introduction ⇑ Corresponding authors at: Institute for Clean Energy & Advanced Materials, Faculty of Materials and Energy, Southwest University, Chongqing 400715, PR China. E-mail addresses: [email protected] (M. Xu), [email protected] (C.M. Li). http://dx.doi.org/10.1016/j.jcis.2016.04.021 0021-9797/Ó 2016 Elsevier Inc. All rights reserved.
Nowadays, as the demand for electrochemical energy storage system increases, the challenges faced by lithium-ion batteries (LIBs) are becoming more and more serious. This is mainly due
Y. Niu et al. / Journal of Colloid and Interface Science 474 (2016) 88–92
to the scarcity and rising prices of lithium resources. As one of the post-lithium ion batteries, sodium-ion batteries (SIBs) have been favored by many chemists because of its vast resources, low cost, safety, sustainability and similar electrochemical energy storage mechanism with LIBs [1,2]. So far, numerous innovative materials, such as biomass carbon [3], alloys [4,5], metal oxides [6], metal selenides [7], sulphides [8], and polyanion structure materials [9,10], were exploited as electrodes for SIBs. Of them, polyanionic compounds have more stable host frameworks, to a certain extent, which result in a longer cycle life and better safety [11,12]. NaTi2(PO4)3 crystallizes in the rhombohedral NASICON (Na super ionic conductor), which has been reported as aqueous [13,14] and non-aqueous [15–18] SIBs anode materials. For the latter, it possess a high theoretical capacity of 132.8 mA h g 1 accompanied by a sodiation voltage plateau at 2.1 V in the voltage range of 1.2–2.8 V (vs. Na+/Na) [15–18]. The plateau corresponds to the insertion of two Na ions to form Na3Ti2(PO4)3. However, as an anode electrode material, the voltage plateau is too high, resulting in the overall energy density of the battery is relatively low. Excitedly, we inadvertently found that the material exhibits another pair of primary redox peak at a voltage as low as ca. 0.44 V, which theoretically can contribute the specific capacity of 119.2 mA h g 1, thus turning NaTi2(PO4)3/C electrode into a good candidate as a SIB anode material combined with practical application.
89
6300F). The thermal decomposition of the as-prepared product was investigated using thermogravimetric analyser (TGA, TA Instruments, USA) at a heating rate of 5 °C min 1 under air flow from room temperature to 700 °C. 2.3. Electrochemical measurements Electrochemical properties of the sample was investigated using coin cells (type 2032). The working electrode was prepared by adding the NaTi2(PO4)3/C powder, acetylene black, and polyvinylidene fluoride in the weight ratio of 70:20:10 to the Nmethyl-2-pyrrolidone solvent to form a homogeneous slurry. The resultant slurry was uniformly cast onto Cu foil by using a doctor blade and dried in a vacuum oven overnight at 120 °C. The cells were assembled in a high-purity Ar-filled glove box with both the moisture and oxygen content below 1 ppm. The counter and reference electrode was metallic sodium foil and the electrolyte was 1 M NaClO4 in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC; 1:1 v/v) with 3 wt% fluoroethylene carbonate (FEC) as an additive. Galvanostatic charge-discharge and cycling performance tests were performed in the voltage range of 0.01–2.8 V (vs. Na+/Na) at various current densities (herein, 1 C = 120 mA g 1) by using a LAND battery tester (CT2001A) at room temperature. Cyclic voltammetry (CV) measurements were carried out at a scan rate of 0.05 mV s 1 using an Arbin Instruments testing system.
2. Experimental 3. Results and discussion 2.1. Materials preparation The synthesis of NaTi2(PO4)3/C powder was carried out by a pyro-synthetic reaction (Scheme 1). In a typical synthesis, sodium acetate and phosphoric acid were dissolved in ethylene glycol in a molar ratio 1:3 at room temperature with constantly stirring. Stoichiometry of tetrabutyl titanate was then added into the above solution. The obtained homogenous milky solution was poured onto a hot-plate and ignited. Subsequently, the as-prepared precursor was annealed at 800 °C for 5 h under Ar atmosphere to obtain the NaTi2(PO4)3/C powder. 2.2. Morphology and structure characterization The structure of the material was characterized by powder Xray diffraction (PXRD) employing Cu-Ka radiation (k = 1.5416 Å) at a scanning rate of 0.4° min 1 in the 2h range of 10–80°. Morphology and microstructure of the as-prepared sample were examined by field-emission scanning electron microscopy (FESEM; JEOL-
Fig. 1a schematically shows that the framework of NaTi2(PO4)3 is built on an open 3D framework consisting of TiO6 octahedra and PO4 tetrahedra. In the structure, two TiO6 octahedra are separated by three PO4 tetrahedra and they share all their corner oxygens [18,19]. The phase composition of the as-synthesized product was characterized by PXRD. As shown in Fig. 1b, the positions and relative intensities of all diffraction peaks can be perfectly indexed as rhombohedral structured NaTi2(PO4)3 (PDF No.85–2265) with lattice parameters of a = 8.48088 Å, c = 21.79546 Å, a = 90.0, c = 120.0 and Volume = 1357.6 Å3. There is no additional peaks from impurities are detected, indicating the high purity of the asprepared NaTi2(PO4)3/C sample. The morphology of NaTi2(PO4)3/C was examined by FESEM at different magnifications (Fig. 2). The images reveal that the size distribution of NaTi2(PO4)3/C is wide, from 200 nm to several tens of nanometers. From Fig. S1a it can be seen that particles are relatively small in addition to an identified a carbon-coating with 3 nm thickness along the particle boundary. The lattice spacing of 0.369 nm and 0.423 nm are recog-
Scheme 1. Schematic illustration of the pyro-synthesis reaction.
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Y. Niu et al. / Journal of Colloid and Interface Science 474 (2016) 88–92
Fig. 1. (a) Schematic illustration of the crystal structure of NaTi2(PO4)3 along with different directions; (b) XRD pattern of the as-prepared NaTi2(PO4)3/C sample.
nized in corresponding to the inter-planar distances of (1 1 3) and (1 1 0) of the rhombohedral NaTi2(PO4)3 in Fig. S1b and c, respectively. In addition, the carbon content was determined by TG analysis under air atmosphere (Fig. S2). The result shows that the carbon content is approximately 5.6 wt% in the NaTi2(PO4)3/C sample. CV measurements were firstly employed to understand the electrochemical sodiation/desodiation processes in the NaTi2(PO4)3/C sample. Fig. 3a shows the CV curves for the first six cycles at a scan rate 0.05 mV s 1 in the potential window of 0.01–2.8 V (vs. Na+/Na). Obviously, there are one set of minor oxidation/reduction peaks near 0.09/0.01 V (Region 1) and two sets of primary oxidation/reduction peaks at around 0.52/0.35 V (Region 2) and 2.25/2.02 V (Region 3), respectively. Note that the minor redox
peaks in the Region 1 are most likely due to the sodiation/desodiation of the carbon layer and acetylene black. In the 1st CV cycle, there is a cathodic wide peak at 0.53 V (violet ellipse), however, and anodic peak is absent, which is attributed to the formation of the solid electrolyte interphase (SEI) and the decomposition of the electrolyte, which is expected at such low anode potentials below the stability limit of the electrolyte. After two cycles, the CV curves overlap well, indicating the reversible insertionextraction reaction of Na+ in the NaTi2(PO4)3 lattice for the reaction. For the Region 3, the redox peak can be attributed to the phase transition between NaTi2(PO4)3 and Na3Ti2(PO4)3 whose equilibrium potential is 2.14 V. Accordingly, the valence state of titanium will change from +4 to +3, which results in the high theoretical specific capacity of 132.8 mA h g 1. Unexpectedly, with further decreasing potential to the Region 2, an additional redox peak is observed, to a great extent, which can be attributed to the formation of Na5Ti2(PO4)3 with a titanium oxidation state of +2 whose equilibrium potential is 0.44 V. According to the above inference that the pair of redox peaks will generate the specific capacity of 119.2 mA h g 1, that is, as the anode electrode material, the theoretical capacity of NaTi2(PO4)3 up to 252 mA h g 1, and which is currently under further in-depth investigation in our laboratory. The rate capability of the NaTi2(PO4)3 electrode is summarized in Fig. 3b, which has deducted the capacity contribution of acetylene black (Fig. S3). The cell was cycled at different current densities of 0.2, 0.5, 1, 2, 3, 4, 5 and back to 0.5 C in a voltage window of 0.01–2.8 V. Notably, while the current rate increases from 0.2 to 5 C, the discharge capacity of NaTi2(PO4)3/C electrode slightly decreases from 225.3 (5th cycle) to 180.9 mA h g 1 (35th cycle) gradually, indicating its excellent rate performance, and the charge capacity at 0.2C is lower than the other rate capacities may be due to the activation of the materials and the formation of the SEI film. Most importantly, even after long cycles with different rates, when the rate is reverted to 0.5 C, the full capacity retention as that of its original capacity is observed. In addition, the charge-discharge curves at various current densities are shown in Fig. 3c. It is evident that the charge-discharge plateaus of NaTi2(PO4)3/C electrode at the current rate of 5 C still generate a distinct voltage plateau and the plateau voltages shift subtly as the charge/discharge currents increase from 0.2 to 5 C, implying the electrode owns exceptional sodium ions storage capability. Also, the cycle performance and Coulombic efficiency of the NaTi2(PO4)3/C anode at the current densities of 0.5 and 5 C are shown in Fig. S4 and Fig. 3d. The cycle performance reveals that the capacity of the electrode is 316.4 (1st) and 246.4 (2nd) mA h g 1 at 0.5 C (Fig. S4) and the capacity
Fig. 2. SEM images of the as-prepared NaTi2(PO4)3/C sample under different magnification.
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Fig. 3. Electrochemical performance of the NaTi2(PO4)3/C sample. (a) CV curves at a scan rate of 0.05 mV s 1 showing the first six cycles at a voltage window of 0.01–3 V; (b) Rate capability at rates of 0.2, 0.5, 1, 2, 3, 4, 5 and back to 0.5 C; (c) Typical charge-charge-discharge curves at different rates; (d) Cycling performance and Coulombic efficiency at 5 C after rate cycle.
of 179.4 mA h g 1 can be achieved after 70 cycles. After 250 charge-discharge cycles at 5 C (Fig. 3d), the capacity is 150 mA h g 1 with the Coulombic efficiency of ca. 100%, proving its well tolerance of fast sodium ions insertion and extraction for long-life SIBs. To further study the rate capability of NaTi2(PO4)3/C, the charge-discharge curves were differentiated to get the differential capacity (dQ/dV) vs voltage curves and the corresponding change of the difference DE between oxidation and reduction peaks and the intensity of cathodic peak in the Region 2 are shown in Fig. 4a and b, respectively. Obviously, as the current increases, the electrochemical polarization increases and the intensity of cathodic peak decreases; when the current reaches the maximum
value, the polarization is most serious and the peak strength of the cathode reaches a minimum, however, when the current back to 0.5 C, the polarization drastically reduce and the peak strength sharp increase, further indicating that the electrode material has excellent electrochemical recovery capability, which should be ascribed to the NASICON structure of highly covalent 3D framework with large interstitial spaces for Na ion diffusion.
4. Conclusions In summary, NaTi2(PO4)3/C powder was prepared by the pyrosynthetic approach and its 0.44 V low-voltage plateau in the Na half cells was reported. NaTi2(PO4)3/C can deliver a reversible
Fig. 4. (a) The corresponding differential capacity vs voltage (dQ/dV) curves of Fig. 3c; (b) The corresponding change of the electrochemical polarization DE and the intensities of cathodic peaks in the region 2 of (a).
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capacity of 179 mA h g 1 between 0.01 and 2.8 V at 0.5 C with good capacity retention. Moreover, NaTi2(PO4)3/C electrode also show a remarkable rate capability (reversible capacity reaches 180.9 mA h g 1 at the current density of 5 C) and long-term cyclability. Hence, the NaTi2(PO4)3/C powder is expected to be a promising anode material for SIBs. Acknowledgements This work is financially supported by Chongqing Key Laboratory for Advanced Materials and Technologies of Clean Energies under cstc2011pt-sy90001, Start-up grant under SWU111071 from Southwest University and the Basic and frontier research project of Chongqing (cstc2015jcyjA50031). M.W. Xu would like to thank the support from the Specialized Research fund for the Doctoral program of Higher Education (RFDP) (Grant No. 20130182120026) and Y.B. Niu would like to thank the support from the Graduate student research innovation project in Chongqing (CYB2015053). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2016.04.021. References [1] X. Xiang, K. Zhang, J. Chen, Recent advances and prospects of cathode materials for sodium-ion batteries, Adv. Mater. 27 (2015) 5343–5364. [2] M. Sawicki, L.L. Shaw, Advances and challenges of sodium ion batteries as post lithium ion batteries, RSC Adv. 5 (2015) 53129–53154. [3] E.M. Lotfabad, J. Ding, K. Cui, A. Kohandehghan, W.P. Kalisvaart, M. Hazelton, D. Mitlin, High-density sodium and lithium ion battery anodes from banana peels, ACS Nano 8 (2014) 7115–7129. [4] C. Yue, Y. Yu, S. Sun, X. He, B. Chen, W. Lin, B. Xu, M. Zheng, S. Wu, J. Li, J. Kang, L. Lin, High performance 3D Si/Ge nanorods array anode buffered by TiN/Ti interlayer for sodium-ion batteries, Adv. Funct. Mater. 25 (2015) 1386–1392.
[5] B. Farbod, K. Cui, W.P. Kalisvaart, M. Kupsta, B. Zahiri, A. Kohandehghan, E.M. Lotfabad, Z. Li, E.J. Luber, D. Mitlin, Anodes for sodium ion batteries based on tin–germanium–antimony alloys, ACS Nano 8 (2014) 4415–4429. [6] N. Zhang, X. Han, Y. Liu, X. Hu, Q. Zhao, J. Chen, 3D porous c-Fe2O3@C nanocomposite as high-performance anode material of Na-ion batteries, Adv. Energy Mater. 5 (2015). [7] K. Zhang, Z. Hu, X. Liu, Z. Tao, J. Chen, FeSe2 microspheres as a highperformance anode material for Na-ion batteries, Adv. Mater. 27 (2015) 3305– 3309. [8] X. Xie, Z. Ao, D. Su, J. Zhang, G. Wang, MoS2/graphene composite anodes with enhanced performance for sodium-ion batteries: the role of the twodimensional heterointerface, Adv. Funct. Mater. 25 (2015) 1393–1403. [9] D. Wang, N. Chen, M. Li, C. Wang, H. Ehrenberg, X. Bie, Y. Wei, G. Chen, F. Du, Na3V2(PO4)3/C composite as the intercalation-type anode material for sodiumion batteries with superior rate capability and long-cycle life, J. Mater. Chem. A 3 (2015) 8636–8642. [10] Z. Jian, Y. Sun, X. Ji, A new low-voltage plateau of Na3V2(PO4)3 as an anode for Na-ion batteries, Chem. Commun. 51 (2015) 6381–6383. [11] Y. Niu, M. Xu, C. Cheng, S. Bao, J. Hou, S. Liu, F. Yi, H. He, C.M. Li, Na3.12Fe2.44(P2O7)2/multi-walled carbon nanotube composite as a cathode material for sodium-ion batteries, J. Mater. Chem. A 3 (2015) 17224–17229. [12] Y. Niu, M. Xu, S.-J. Bao, C.M. Li, Porous graphene to encapsulate Na6.24Fe4.88(P2O7)4 as composite cathode materials for Na-ion batteries, Chem. Commun. 51 (2015) 13120–13122. [13] M. Vujkovic´, M. Mitric´, S. Mentus, High-rate intercalation capability of NaTi2(PO4)3/C composite in aqueous lithium and sodium nitrate solutions, J. Power Sources 288 (2015) 176–186. [14] B. Zhao, B. Lin, S. Zhang, C. Deng, A frogspawn-inspired hierarchical porous NaTi2(PO4)3-C array for high-rate and long-life aqueous rechargeable sodium batteries, Nanoscale 7 (2015) 18552–18560. [15] J. Yang, H. Wang, P. Hu, J. Qi, L. Guo, L. Wang, A high-rate and ultralong-life sodium-ion battery based on NaTi2(PO4)3 nanocubes with synergistic coating of carbon and rutile TiO2, Small 11 (2015) 3744–3749. [16] G. Yang, H. Song, M. Wu, C. Wang, Porous NaTi2(PO4)3 nanocubes: a high-rate nonaqueous sodium anode material with more than 10 000 cycle life, J. Mater. Chem. A 3 (2015) 18718–18726. [17] C. Wu, P. Kopold, Y.-L. Ding, P.A. van Aken, J. Maier, Y. Yu, Synthesizing porous NaTi2(PO4)3 nanoparticles embedded in 3D graphene networks for high-rate and long cycle-life sodium electrodes, ACS Nano 9 (2015) 6610–6618. [18] G. Pang, P. Nie, C. Yuan, L. Shen, X. Zhang, H. Li, C. Zhang, Mesoporous NaTi2(PO4)3/CMK-3 nanohybrid as anode for long-life Na-ion batteries, J. Mater. Chem. A 2 (2014) 20659–20666. [19] G. Pang, C. Yuan, P. Nie, B. Ding, J. Zhu, X. Zhang, Synthesis of NASICON-type structured NaTi2(PO4)3-graphene nanocomposite as an anode for aqueous rechargeable Na-ion batteries, Nanoscale 6 (2014) 6328–6334.