Hydrothermal Synthesis of Sodium Titanium

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Oct 10, 2016 - aforementioned electrodes, Celgard 3501 separator, and 1 M Na2SO4 aqueous electrolyte were fabricated under ambient conditions. For the.
Research Article pubs.acs.org/journal/ascecg

Hydrothermal Synthesis of Sodium Titanium Phosphate Nanoparticles as Efficient Anode Materials for Aqueous Sodium-Ion Batteries Tai-Feng Hung,*,† Wei-Hsuan Lan,‡ Yu-Wen Yeh,† Wen-Sheng Chang,† Chang-Chung Yang,† and Jing-Chie Lin‡ †

New Energy Technology Division, Energy & Environment Research Laboratories, Industrial Technology Research Institute, Hsinchu 31040, Taiwan ‡ Institute of Materials Science and Engineering, National Central University, Taoyuan 32001, Taiwan S Supporting Information *

ABSTRACT: Sodium titanium phosphate (NaTi2(PO4)3, NTP) with a sodium superionic conductor structure is considered as an efficient anode material for aqueous sodium-ion batteries because of its moderate potential range and high structural stability. In this study, a series of NTP nanoparticles (NPs) were synthesized using a facile and costeffective hydrothermal method without further calcination to explore the influence of reaction time on their crystalline structures and morphologies. The NTP NPs hydrothermally synthesized for 5 h were subsequently subjected to a carboncoating procedure, and the resulting carbon-coated NTP NPs exhibited remarkable reversible capacities, rate capabilities, and cycling performances. These features were attributable to the nanotailoring of the NTP NPs, which reduced both the ionic and electronic transporting paths, and continuous carbon layers coated on the NTP surfaces to promote their electronic conductivities. KEYWORDS: Sodium titanium phosphate, Aqueous rechargeable sodium-ion batteries, Anode material, Hydrothermal synthesis, Carbon coating



INTRODUCTION In recent years, sodium (Na)-based electrochemical energy storage (EES) technologies (e.g., Na−S, Na-NiCl2, Na−O2, and Na-ion batteries (NIBs)) have received considerable attention because Na exhibits similar physicochemical properties with lithium and material abundances.1 Among them, developing rechargeable NIBs as alternatives to lithium-ion batteries has rapidly increased owing to the adequate understanding and development of lithium-based electrochemical systems, which are suitable for effectively developing NIBs.2−5 Compared with NIBs using organic electrolytes, aqueous NIBs are more attractive because of their low cost and high safety in addition to them being environmentally benign. Furthermore, aqueousbased electrolytes provide not only higher ionic transportation abilities but also more improved kinetics than those of organic systems.6,7 Therefore, aqueous NIBs would be promising candidates for use in comprehensive EES applications. Considering its moderate potential range and high structural stability, sodium titanium phosphate (NaTi2(PO4)3, NTP) with a sodium superionic conductor (NASICON) structure was first demonstrated as an appropriate anode material for aqueous NIBs in 2011.8 However, the low electronic conductivity intrinsically restricts its practical applicability in high-performance NIBs, although the open three-dimensional frameworks © 2016 American Chemical Society

existing in NASICON structures effectively ensure fast Na-ion diffusions. Several technologies, involving Pechini,8,9 microwave,10 solid-state,11,12 solvothermal,13−15 sol−gel,16−22 and gel combustion23 processes, that entail combining carbon coating or conductive additives with a NTP to produce effective conductive networks, have been reported to enhance the electronic conductivities of the resulting NTP nanocomposites. In addition to the traditional carbon coating8−12,16,18,19,21−23 or constructing the NTP-graphene hybrid structure,13,15,17 the hierarchical carbon-decorated wafer-like porous NTP (NTP/CW) was also proposed by a self-assembled method.24 The hierarchical carbon not only provided a bicontinuous conductive network for fast electron transport, but also constructed a highly porous structure for highly efficient electrolyte percolation. Both advantages were favorable for a fast charge transfer reaction. Moreover, the hierarchical carbon also acted as a buffering protective shell, which stabilized the crystal upon cycling. Therefore, the prepared NTP/C-W showed superior electrochemical performances than the simple carbon-coated NTP. Received: August 16, 2016 Published: October 10, 2016 7074

DOI: 10.1021/acssuschemeng.6b01962 ACS Sustainable Chem. Eng. 2016, 4, 7074−7079

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ACS Sustainable Chemistry & Engineering

room temperature to 900 °C at a rate of 10 °C min−1 and under oxygen flow of 100 mL min−1. Electrochemical Measurements. To evaluate the reversible capacities in addition to the rate capabilities and cyclabilities, homogeneous mixtures composed of NTP-5h/C, Super P, and PVDF at a weight ratio of 75:15:10 were hot-pressed at 200 °C for 50 s to form anodes with a diameter of 1.2 cm. A hot-pressing procedure was also employed to prepare the cathodes by using 86 wt % sodium manganese oxide (Na0.44MnO2), which was synthesized according to a previously reported method,26 as the active material, 7 wt % super P, and 7 wt % PVDF. Coin-type full cells combining the aforementioned electrodes, Celgard 3501 separator, and 1 M Na2SO4 aqueous electrolyte were fabricated under ambient conditions. For the rate capability test, the mass loadings of the active NTP and Na0.44MnO2 materials on the electrodes were about 48 mg and 168 mg. As for the cyclability measurement, the mass loadings of the active NTP and Na0.44MnO2 materials on the electrodes were about 68 mg and 164 mg. Accordingly, the weight ratio of the Na0.44MnO2 cathode and the NTP anode was about 3.5 for the rate capability test and 2.4 for the cyclability measurement, respectively. The assembled cells were connected to a computer-controlled test system (Series 4000, Maccor, Inc.) and galvanostatically tested at ambient temperature. The rate capability and cyclability were tested at voltage ranges of 0.1−1.3 V and 0.7−1.3 V, respectively. The capacity values reported throughout this study were calculated based on the NTP mass. An electrochemical impedance spectroscopy (EIS) analysis was conducted using a multichannel electrochemical workstation (VMP3, Bio-Logic) at the 100% depth-of-discharge, and a low AC perturbation of 10 mV was applied with a frequency sweep ranging from 1 MHz to 10 mHz.

In contrast to the aforementioned techniques, the current study presents a hydrothermal method for synthesizing NTP nanoparticles (NPs) without further calcination due to its simplicity and the environmentally friendly conditions of this method. Powder X-ray diffractometry (PXRD) and fieldemission scanning electron microscopy (FE-SEM) were utilized to explore the influence of reaction time on the crystalline structures and morphologies of the NTP NPs, respectively. In addition, the NTP NPs hydrothermally synthesized for 5 h (NTP-5h) were further dispersed in a glucose solution and subjected to a heating process in argon atmosphere to form a carbon-coated sample. The electrochemical properties of the resulting NTP-5h/C, including reversible capacities, rate capabilities, and cyclabilities, were evaluated using a galvanostatic charge−discharge procedure and aqueous sodium sulfate solution as the electrolyte. To the best of our knowledge, this is the first report utilizing a facile and cost-effective hydrothermal method for synthesizing NTP NPs with great crystallinity and high phase-purity on aqueous NIBs. The proposed approach may also offer new possibilities on the synthesis of various sodium metal phosphates for extensive applications.



EXPERIMENTAL SECTION

Chemicals. All reagents, titanium(IV) tetrachloride (TiCl4, ≥ 99%, Merck), ammonium hydroxide solution (NH4OH, approximately 25% NH3 basis, Sigma-Aldrich), sodium hydroxide (NaOH, A.C.S. Reagent, J.T. Baker), phosphoric acid (H3PO4, ≥85%, Sigma-Aldrich), glucose (C6H12O6, A.C.S. Reagent, J.T. Baker), carbon black (Super P, Timcal Ltd.), poly(vinylidenedifluoride) (PVDF, MW: approximately 534,000, Sigma-Aldrich), and sodium sulfate (Na2SO4, 99%, SigmaAldrich), were used as received. Deionized (DI) water produced from a Milli-Q SP ultrapure-water purification system (Nihon Millipore Ltd.) was adopted throughout the experiments. Synthesis. Before the sodium titanium phosphate (NaTi2(PO4)3, NTP) NPs were synthesized, titanium hydroxide (Ti(OH)4) was prepared by adding 4 M NH4OH aqueous solution dropwise to 1 M TiCl4 aqueous solution in an ice−water bath with continuous stirring. The resulting precipitates were obtained after centrifugation, repeated rinsing with DI water, and oven drying. Subsequently, NaOH and asprepared Ti(OH)4 were added to 30 mL of H3PO4 solution and stirred at ambient temperature to form the precursor solution. The molar ratios of NaOH, Ti(OH)4, and H3PO4 were 16:1:48. The resulting mixture was carefully transferred into a Teflon-lined stainless autoclave, which was sealed and its temperature was maintained at 250 °C for 3, 5, and 12 h. The residues were collected using the same procedures as those for Ti(OH)4 and were ground in an agate mortar to yield fine NTP NPs. The synthesized NTP NPs were denoted as NTP-3h, NTP-5h, and NTP-12h, respectively. Regarding the carbon coating procedure, the desired amounts of NTP-5h powders and C6H12O6 dispersed thoroughly in DI water were dried at 80 °C. The mixtures were heat-treated under an argon atmosphere at 800 °C for 3 h to yield the carbon-coated NTP-5h (NTP-5h/C). Material Characterizations. The crystalline structures of the NTP NPs hydrothermally synthesized at various reaction times were identified using a powder X-ray diffractometer (PXRD, D2 PHASER, Bruker AXS Inc.) with a Cu target (λ = 1.541 Å) that was excited at 30 kV and 10 mA. The corresponding PXRD pattern recorded in the range of 2θ from 10° to 100° was refined via Rietveld analysis using TOPAS 4.2 software.25 A field-emission scanning electron microscope (FE-SEM, JSM-7000F, JEOL Ltd.) and transmission electron microscope (TEM, JEM-2100, JEOL Ltd.) operated at an accelerating voltage of 200 kV were adopted for morphological observations. Raman spectra of pristine NTP and NTP-5h/C were collected using a Thermo Scientific DXR Raman microscope equipped with a 532 nm solid-state laser. The amounts of carbon presented in the NTP-5h/C were determined using a thermogravimetric analyzer (TGA, SDT Q600, TA Instruments) by varying the heating temperature from



RESULTS AND DISCUSSION To explore the influence of reaction time on the crystalline structures and morphologies of as-synthesized NTP compounds, their Rietveld refined PXRD patterns and FE-SEM micrographs are presented in Figure 1. As can be seen, the

Figure 1. Rietveld refined PXRD patterns and FE-SEM micrographs of the NTP-3h (a−b), NTP-5h (c−d), and NTP-12h (e−f): scale bar, 100 nm for (b), (d), and (f). 7075

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ACS Sustainable Chemistry & Engineering observed patterns (red color) shown in Figure 1a, 1c, and 1e matched well with the calculated profiles (black color, R-3C, rhombohedral, No. 167) with relatively low Rexp, Rwp, and Rp values as inset. Their corresponding refined results and cell parameters are summarized in Tables S1 and S2, revealing that the atomic positions, a-axis, c-axis, and cell volume of these three compounds were not significantly varied with the reaction time. This implies that the NASICON-type NTP compounds with great crystallinity and high purity would be successfully obtained using a facile and cost-effective hydrothermal process at 250 °C without further calcination. However, the obvious agglomerates composed of micro- and nanosized particles were noticed from the NTP-3h (Figure 1b). Such microsized particles may result from the incomplete reaction of the Ti(OH)4 precursors (Figure S1) during the short reaction period. In contrast to the PXRD pattern shown in Figure 1a, no diffraction peaks assigned to the Ti(OH)4 could be ascribed to its amorphous nature.27 In comparison with the NTP-3h, it can be seen that more uniform nanoparticles were observed from the NTP-5h (Figure 1d). Likewise, there were no evident differences in the morphology and particle size between the NTP-5h and NTP-12h (Figure 1f), suggesting that the nucleation and growth of the nanosized NTP were sufficient during 5 h. The high-magnification TEM micrograph, lattice-fringe image, and Raman spectrum of the NTP-5h are shown in Figure 2. The particle size observed from Figure 2a was nearly

Table 1. Raman Band Positions and Corresponding Assignments of the NTP-5h Band positions This study

ref 16

Assignments

1095 1067 1008 984 970 543 430 345 335 303 270 220 191 154

1098 1070 1009 985 970 547 432 350 337 305 273 222 195 155

v3: A1g + 3Eg (antisymmetric stretching vibration)

v1: A1g + Eg (symmetric stretching vibration) v4: A1g + 3Eg (antisymmetric bending vibration) v2: 2A1g + 2Eg (symmetric bending vibration) (Ti−O) vibration mode Vibration and translation of phosphorus ions

T1 and R1 (PO)4

linity and phase-purity were efficiently synthesized at 250 °C for 5 h without further calcination, showing the benefit of less energy consumption. Considering the practical applicability of the nanosized NTP as anode materials in aqueous Na-ion batteries (NIBs), the NTP-5h were sequentially coated with thin carbon layers to provide effective conductive networks. Figure 3 shows the

Figure 3. (a) Raman spectrum, (b) TGA thermogram, (c) and (d) TEM micrographs of the NTP-5h/C: scale bar, 20 nm for (c) and 5 nm for (d).

Figure 2. (a) High-magnification TEM micrograph, (b) lattice-fringe image, and (c) Raman spectrum of the NTP-5h: scale bar, 50 nm for (a) and 5 nm for (b).

Raman spectrum, TGA thermogram, and TEM micrographs of the carbon-coated NTP-5h (NTP-5h/C). As expected, the Raman bands reflected from the NTP-5h/C (Figure 3a) were essentially similar to those of the pristine NTP-5h shown in Figure 2c. Nevertheless, the bands at 970, 984, and 1008 cm−1 merged and appeared at 988 cm−1, which was also observed for the carbon-coated LiTi2(PO4)3.28−30 This phenomenon is attributable to the trace reduction of Ti4+ to Ti3+ and oxygen vacancy created during the heat treatment process in inert atmosphere with the presence of a pitch.31 In addition to the mentioned bands, the NTP-5h/C clearly revealed the existence of carbon with two characteristic bands at 1333 and 1586 cm−1.

100 nm, which was similar to the result displayed in Figure 1d. The lattice-fringe image presented in Figure 2b reveals that the distance measured between adjacent planes was ∼0.61 nm, corresponding to the (012) plane of rhombohedral NTP. The Raman spectrum illustrated in Figure 2c and band assignments given in Table 1 exhibit that not only the band position but also the intensity were highly consistent with those reported for the NTP.16 On the basis of the evidence mentioned above, the NASICON-type NTP nanoparticles possessing great crystal7076

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Figure 4. (a) Capacity profiles, (b) rate capabilities, (c) cycling performances, and (d) electrochemical impedance spectra of the NTP-5h/C. The Crate used for (a) and (c) was 0.2 and 1 C, respectively. Inset of (d) illustrates the equivalent circuit model used for the parameter-fitting.34

of our NTP-5h/C delivered at 2 C were similar to that of an NTP-graphene nanocomposite and self-assembled wafer-like porous NTP decorated with hierarchical carbon.13,24 It would be attributed to the thinner carbon layers being continuously coated onto the NTP surface as demonstrated in Figure 3d, resulting in the comparable rate performance. However, the capacity suddenly decreased from ∼103 to ∼68 mAh g−1 as the C-rate was further increased from 2 to 5 C. The unexpected decrease in capacity may be because a thicker electrode was used for measurement, enlarging the distances on Na-ion diffusions.11 Notably, the capacity returned to nearly 111 mAh g−1 at 0.5 C (i.e., ∼97% of the initial value recorded at the same rate), demonstrating the remarkable rate capabilities of the NTP-5h/C. Regarding the cycling performance, an approximately 75% capacity retention and >99.5% Coulombic efficiency were revealed in Figure 4c after 300 cycles at 1C when the cell was tested at voltages ranging between 0.7 and 1.3 V. The electrochemical impedance spectra recorded after the 10th, 30th, and 50th cycles of 100% depth-of-discharge with an amplitude of 10 mV over the frequency range of 1 MHz to 10 mHz are plotted in Figure 4d. The resulting spectra were fit using the equivalent circuit model that was inset in Figure 4d, where Rs represents the solution resistance, RCT is the chargetransfer resistance, W is the Warburg impedance, and CPE is the constant phase element.34 It is seen that the spectra are composed of a semicircle (interfacial resistance between the electrode and the electrolyte) in the high-frequency region and a straight line (Warburg impedance) in the low-frequency region. In comparison with the fitted RCT values (i.e., [Rs + RCT] − [Rs]), it can be noticed that the RCT value of the NTP5h/C slightly decreased upon cycling, i.e., 2.3 Ω at the 10th cycle and 2.1 Ω at the 50th cycle. Moreover, the NTP-5h/C exhibited superior cyclability to the NTP-5h coated with 3 wt % carbon (Figure S2a), which exhibited a capacity retention of ∼63% after 200 cycles (Figure S2b). This was attributable to an inadequate carbon coating, leading to high RCT value (4.3 Ω) as

The former is related to the disordered carbon, whereas the later corresponds to the G band with an optically allowed E2g vibration of the graphitic structure.32 The peak intensity ratio between the D and G bands (ID/IG) generally serves as a useful index for comparing the degree of crystallinity of various carbon materials (i.e., a lower ID/IG ratio indicates a higher degree of ordering in the carbon material).33 In the current study, the ID/ IG ratio of the NTP-5h/C was approximately 1.06, indicating that the carbon formed is fairly ordered. The amount of carbon coated on the NTP-5h/C was estimated using the TGA in oxygen atmosphere by varying the temperature from room temperature to 900 °C. According to its TGA thermogram plotted in Figure 3b, the weight loss (∼0.3 wt %) at temperatures lower than 100 °C was induced by the evaporation of water, while the weight loss detected in the temperature range from 100 to 900 °C was mainly contributed to the carbon burned in oxygen atmosphere. The TGA results indicated that the amount of carbon coated on the NTP-5h/C was 5.7 wt %. Consequently, the ordered and continuous carbon layer (∼5 nm in thickness, shown in Figure 3d) coated onto the surface of NTP can be beneficial for achieving higher electronic conduction between adjacent NTP nanoparticles. For the electrochemical evaluations, coin-type full cells comprising the NTP-5h/C anode, Na0.44MnO2 cathode, and 1 M Na2SO4 aqueous electrolyte were assembled under ambient conditions. As plotted in Figure 4a, the initial charge and discharge capacities based on the NTP mass were 131 and 121 mAh g−1, respectively, corresponding to a Coulombic efficiency of ∼92%. The high reversibility was attributed to their open three-dimensional frameworks within the NTP structures and improved kinetics of the aqueous electrolyte, facilitating the swift transportation of Na ions.11,18 Figure 4b further compares the discharge capacities that were recorded at various C-rates. With increasing the rate to 2 C, it is found that the discharge capacity decreased to approximately 103 mAh g−1, which was ∼85% of the value recorded at 0.2 C. The reversible capacities 7077

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high-rate performance sodium-ion batteries. ChemSusChem 2015, 8, 2948−2955. (4) Zheng, Y.; Zhou, T.; Zhang, C.; Mao, J.; Liu, H.; Guo, Z. Boosted charge transfer in SnS/SnO2 heterostructures: toward high rate capability for sodium-ion batteries. Angew. Chem., Int. Ed. 2016, 55, 3408−3413. (5) Lin, B.; Zhang, S.; Deng, C. Understanding the effect of depressing surface moisture sensitivity on promoting sodium intercalation in coral-like Na3.12Fe2.44(P2O7)2/C synthesized via a flash-combustion strategy. J. Mater. Chem. A 2016, 4, 2550−2559. (6) Kim, D. J.; Jung, Y. H.; Bharathi, K. K.; Je, S. H.; Kim, D. K.; Coskun, A.; Choi, J. W. An aqueous sodium ion hybrid battery incorporating an organic compound and a Prussian Blue derivative. Adv. Energy Mater. 2014, 4, 1400133. (7) Hung, T. F.; Chou, H. L.; Yeh, Y. W.; Chang, W. S.; Yang, C. C. Combined experimental and computational studies of potentialtuneable Na2Ni1‑xCuxFe(CN)6 cathode for aqueous rechargeable sodium-ion batteries. Chem. - Eur. J. 2015, 21, 15686−15691. (8) Park, S. I.; Gocheva, I.; Okada, S.; Yamaki, J. Electrochemical properties of NaTi2(PO4)3 anode for rechargeable aqueous sodium-ion batteries. J. Electrochem. Soc. 2011, 158, A1067−A1070. (9) Fernández-Ropero, A. J.; Saurel, D.; Acebedo, B.; Rojo, T.; CasasCabanas, M. Electrochemical characterization of NaFePO4 as positive electrode in aqueous sodium-ion batteries. J. Power Sources 2015, 291, 40−45. (10) Wu, W.; Mohamed, A.; Whitacre, J. F. Microwave synthesized NaTi2(PO4)3 as an aqueous sodium-ion negative electrode. J. Electrochem. Soc. 2013, 160, A497−A504. (11) Li, Z.; Young, D.; Xiang, K.; Carter, W. C.; Chiang, Y. M. Towards high power high energy aqueous sodium-ion batteries: the NaTi2(PO4)3/Na0.44MnO2 system. Adv. Energy Mater. 2013, 3, 290− 294. (12) Chen, L.; Zhang, L.; Zhou, X.; Liu, Z. Aqueous batteries based on mixed monovalence metal ions: a new battery family. ChemSusChem 2014, 7, 2295−2302. (13) Pang, G.; Yuan, C.; Nie, P.; Ding, B.; Zhu, J.; Zhang, X. Synthesis of NASICON-type structured NaTi2(PO4)3-graphene nanocomposite as an anode for aqueous rechargeable Na-ion batteries. Nanoscale 2014, 6, 6328−6334. (14) Pang, G.; Nie, P.; Yuan, C.; Shen, L.; Zhang, X.; Li, H.; Zhang, C. Mesoporous NaTi2(PO4)3/CMK-3 nanohybrid as anode for longlife Na-ion batteries. J. Mater. Chem. A 2014, 2, 20659−20666. (15) Wu, C.; Kopold, P.; Ding, Y. L.; van Aken, P. A.; Maier, J.; Yu, Y. Synthesizing porous NaTi2(PO4)3 nanoparticles embedded in 3D graphene networks for high-rate and long cycle-life sodium electrodes. ACS Nano 2015, 9, 6610−6618. (16) Velchuri, R.; Kumar, B. V.; Rama Devi, V.; Seok, S. I.; Vithal, M. Low temperature preparation of NaTi2(PO4)3 by sol-gel method. Int. J. Nanotechnol. 2010, 7, 1077−1086. (17) Li, X.; Zhu, X.; Liang, J.; Hou, Z.; Wang, Y.; Lin, N.; Zhu, Y.; Qian, Y. Graphene-Supported NaTi2(PO4)3 as a high rate anode material for aqueous sodium ion batteries. J. Electrochem. Soc. 2014, 161, A1181−A1187. (18) Wu, X.; Sun, M.; Shen, Y.; Qian, J.; Cao, Y.; Ai, X.; Yang, H. Energetic aqueous rechargeable sodium-ion battery based on Na2CuFe(CN)6-NaTi2(PO4)3 intercalation chemistry. ChemSusChem 2014, 7, 407−411. (19) Hou, Z.; Li, X.; Liang, J.; Zhu, Y.; Qian, Y. An aqueous rechargeable sodium ion battery based on a NaMnO2-NaTi2(PO4)3 hybrid system for stationary energy storage. J. Mater. Chem. A 2015, 3, 1400−1404. (20) Zhao, B.; Lin, B.; Zhang, S.; Deng, C. A frogspawn-inspired hierarchical porous NaTi2(PO4)3-C array for high-rate and long-life aqueous rechargeable sodium batteries. Nanoscale 2015, 7, 18552− 18560. (21) Chen, L.; Liu, J.; Guo, Z.; Wang, Y.; Wang, C.; Xia, Y. Electrochemical profile of LiTi2(PO4)3 and NaTi2(PO4)3 in lithium, sodium or mixed ion aqueous solutions. J. Electrochem. Soc. 2016, 163, A904−A910.

shown in Figure S2c. These electrochemical results clearly demonstrate that the NTP coated with 5.7 wt % carbon can be potentially applied as efficient anode materials for aqueous NIBs.



CONCLUSIONS In summary, this study reports the effective synthesis of nanosized NTP by introducing a facile and cost-effective hydrothermal route at 250 °C for 5 h without further calcination. The Rietveld refined PXRD pattern and Raman spectrum revealed that the NTP-5h possessed a normal NASICON structure, great crystallinity, and high phase-purity. The resulting NTP-5h with ∼100 nm of particle size were clearly observed from FE-SEM and TEM micrographs. Considering the practical application of the nanosized NTP as anode materials on aqueous NIBs, nearly 5.7 wt % and 5 nm of the ordered and continuous carbon layers were coated onto the surface NTP as verified by TGA, Raman, and highmagnification TEM characterizations. The NTP-5h/C delivered a high reversible capacity (∼121 mAh g−1 at 0.2 C) in addition to comparable rate capability (∼103 mAh g−1 at 2 C) and cycling performance (capacity retention: ∼75% at 300 cycles at 1 C). Given by their distinctive NASICON structure and lower charge-transfer resistance determined from the EIS analysis, the NTP-5h/C could be promising as efficient anode materials with favorable sodium storage abilities for aqueous NIBs.



ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b01962. FE-SEM micrograph of Ti(OH)4 precursors; TGA thermogram and cycling performance of the NTP-5h coated with 3 wt % carbon; electrochemical impedance spectra of the NTP-5h coated with different carbon contents; Rietveld refinement results and refined cell parameters of the NTP compounds hydrothermally synthesized at 250 °C for 3, 5, and 12 h (PDF)



AUTHOR INFORMATION

Corresponding Author

*Tai-Feng Hung. E-mail: [email protected]; Fax: +886-3-5915372; Tel: +886-3-582-0030. Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS We gratefully appreciate the financial support from the Bureau of Energy (BOE), Ministry of Economy Affair (MOEA), Taiwan, and facilities from National Central University (NCU).



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ACS Sustainable Chemistry & Engineering (22) Wang, D.; Liu, Q.; Chen, C.; Li, M.; Meng, X.; Bie, X.; Wei, Y.; Huang, Y.; Du, F.; Wang, C.; Chen, G. NASICON-structured NaTi2(PO4)3@C nanocomposite as the low operation-voltage anode material for high-performance sodium-ion batteries. ACS Appl. Mater. Interfaces 2016, 8, 2238−2246. (23) Vujković, M.; Mitrić, M.; Mentus, S. High-rate intercalation capability of NaTi2(PO4)3/C composite in aqueous lithium and sodium nitrate solutions. J. Power Sources 2015, 288, 176−186. (24) Zhao, B.; Wang, Q.; Zhang, S.; Deng, C. Self-assembled waferlike porous NaTi2(PO4)3 decorated with hierarchical carbon as a highrate anode for aqueous rechargeable sodium batteries. J. Mater. Chem. A 2015, 3, 12089−12096. (25) Hung, T. F.; Cheng, W. J.; Chang, W. S.; Yang, C. C.; Shen, C. C.; Kuo, Y. L. Ascorbic acid-assisted synthesis of mesoporous sodium vanadium phosphate nanoparticles with highly sp2-coordinated carbon coatings as efficient cathode materials for rechargeable sodium-ion batteries. Chem. - Eur. J. 2016, 22, 10620−10626. (26) Wang, C. H.; Yeh, Y. W.; Wongittharom, N.; Wang, Y. C.; Tseng, C. J.; Lee, S. W.; Chang, W. S.; Chang, J. K. Rechargeable Na/ Na0.44MnO2 cells with ionic liquid electrolytes containing various sodium solutes. J. Power Sources 2015, 274, 1016−1023. (27) Legrand, V.; Merdrignac-Conanec, O.; Paulus, W.; Hansen, T. Study of the thermal nitridation of nanocrystalline Ti(OH)4 by X-ray and in-situ neutron powder diffraction. J. Phys. Chem. A 2012, 116, 9561−9567. (28) Aravindan, V.; Chuiling, W.; Reddy, M. V.; Subba Rao, G. V.; Chowdari, B. V. R.; Madhavi, S. Carbon coated nano-LiTi2(PO4)3 electrodes for non-aqueous hybrid supercapacitors. Phys. Chem. Chem. Phys. 2012, 14, 5808−5814. (29) Aravindan, V.; Chuiling, W.; Madhavi, S. Electrochemical performance of NASICON type carbon coated LiTi2(PO4)3 with a spinel LiMn2O4 cathode. RSC Adv. 2012, 2, 7534−7539. (30) Liu, Z.; Qin, X.; Xu, H.; Chen, G. One-pot synthesis of carboncoated nanosized LiTi2(PO4)3 as anode materials for aqueous lithium ion batteries. J. Power Sources 2015, 293, 562−569. (31) Luo, J. Y.; Chen, L. J.; Zhao, Y. J.; He, P.; Xia, Y. Y. The effect of oxygen vacancies on the structure and electrochemistry of LiTi2(PO4)3 for lithium-ion batteries: a combined experimental and theoretical study. J. Power Sources 2009, 194, 1075−1080. (32) Ferrari, A. C.; Robertson, J. Interpretation of Raman spectra of disordered and amorphous carbon. Phys. Rev. B: Condens. Matter Mater. Phys. 2000, 61, 14095−14107. (33) Muraliganth, T.; Vadivel Murugan, A.; Manthiram, A. Facile synthesis of carbon-decorated single-crystalline Fe3O4 nanowires and their application as high performance anode in lithium ion batteries. Chem. Commun. 2009, 7360−7362. (34) Lu, Y.; Zhang, S.; Li, Y.; Xue, L.; Xu, G.; Zhang, X. Preparation and characterization of carbon-coated NaVPO4F as cathode material for rechargeable sodium-ion batteries. J. Power Sources 2014, 247, 770−777.

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Electronic supplementary information

Hydrothermal Synthesis of Sodium Titanium Phosphate Nanoparticles as Efficient Anode Materials for Aqueous Sodium-Ion Batteries

Tai-Feng Hung,†,* Wei-Hsuan Lan,‡ Yu-Wen Yeh,† Wen-Sheng Chang,† Chang-Chung Yang† and Jing-Chie Lin‡



New Energy Technology Division, Energy & Environment Research Laboratories, Industrial Technology Research Institute, Hsinchu 31040, Taiwan.



Institute of Materials Science and Engineering, National Central University, Taoyuan 32001, Taiwan.

*

Corresponding authors.

E-mail addresses: [email protected] (T.F. Hung); Fax: +886-3-591-5372; Tel: +886-3-582-0030. S1

Figure S1 FE-SEM micrograph of Ti(OH)4 precursors, scale bar: 1 µm.

S2

Figure S2 (a) TGA thermogram and (b) cycling performance of the NTP-5h coated with 3 wt. % carbon. (c) Electrochemical impedance spectra of the NTP-5h coated with different carbon contents that were recorded after 10th cycles of 100 % depth-of-discharge.

S3

Table S1 Rietveld refinement results of the NaTi2(PO4)3 compounds hydrothermally synthesized at 250℃ for 3, 5 and 12 h.

Wyckoff site

x

y

z

Occ.

Beq.

Na

6b

0

0

0

1

2.69(19)

Ti

12c

0

0

0.14508(10)

1

1.462(59)

P

18e

0.28653(30)

0

0.25

1

0.721(68)

O1

36f

0.17682(44)

-0.02262(52)

0.19316(17)

1

0.92(11)

O2

36f

0.19525(40)

0.16469(41)

0.08670(20)

1

1.38(10)

Na

6b

0

0

0

1

2.05(15)

Ti

12c

0

0

0.145504(81)

1

0.657(46)

P

18e

0.28614(26)

0

0.25

1

0.706(58)

O1

36f

0.17365(40)

-0.02362(45)

0.19058(14)

1

0.699(92)

O2

36f

0.19169(34)

0.16348(36)

0.08590(17)

1

1.106(86)

Na

6b

0

0

0

1

2.05(27)

Ti

12c

0

0

0.14612(14)

1

0.274(79)

P

18e

0.28393(47)

0

0.25

1

0.58(11)

O1

36f

0.16940(75)

-0.03219(82)

0.18912(24)

1

0.38(17)

O2

36f

0.19611(62)

0.16662(67)

0.08488(29)

1

1.82(17)

Sample Atom

NTP-3h

NTP-5h

NTP-12h

S4

Table S2 The refined cell parameters of the NaTi2(PO4)3 compounds hydrothermally synthesized at 250℃ for 3, 5 and 12h. Sample

a-axis (Å)

c-axis (Å)

cell volume (Å3)

NTP-3h

8.51466(20)

21.72047(68)

1363.748(77)

NTP-5h

8.50012(17)

21.74843(58)

1360.846(66)

NTP-12h

8.50042(33)

21.7499(11)

1361.03(13)

S5