Hydrothermal Synthesis of Sodium Titanium Phosphate Nanoparticles

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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 Lin Chie ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b01962 • Publication Date (Web): 10 Oct 2016 Downloaded from http://pubs.acs.org on October 12, 2016

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ACS Sustainable Chemistry & Engineering

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.

1

ACS Paragon Plus Environment


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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 cost-effective hydrothermal method without further calcination to explore the influence of reaction time on their crystalline structures and morphologies. The NTP NPs hydrothermally synthesized for 5h were subsequently subjected to a carbon-coating 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

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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, aqueous-based 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 existed 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 their electronic conductivities of the resulting NTP nanocomposites. In addition to the traditional 3



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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/C-W) 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. 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 environment friendly conditions of this method. Powder X-ray diffractometer (PXRD) and field-emission scanning electron microscope (FE-SEM) were utilized to explore the influence of reaction time on their crystalline structures and morphologies of the NTP NPs, respectively. In addition, the NTP NPs hydrothermally synthesized for 5h (NTP-5h) were further dispersed in a glucose solution and subjected to a heating process in argon atmosphere to form 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 4


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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%, Sigma-Aldrich), 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 as-prepared Ti(OH)4 were added to 30 mL 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 maintained at 250℃ for 3, 5 and 12h. 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 5



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desired amounts of NTP-5h powders and C6H12O6 dispersed thoroughly in DI water were dried at 80℃. The mixtures were heat-treated under an argon atmosphere at 800 ℃ for 3h 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 ScientificTM 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 instrument) by varying the heating temperature from room temperature to 900℃ at a rate of 10℃ 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℃ for 50 sec 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 6


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synthesized according to a previously reported method,26 as the active material, 7 wt. % super P®, and 7 wt. % PVDF. Coin-type full cells combined the aforementioned electrodes, Celgard® 3501 separator, and 1 M Na2SO4 aqueous electrolyte were fabricated under ambient conditions. For 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 rate capability test and 2.4 for 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.

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 observed patterns (red color) showed in Figure 1a, 1c and 1e matched well with the calculated profiles (black color, R-3C, rhombohedral, No. 167) with a relatively low 7



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Rexp, Rwp and Rp values as inset. Their corresponding refined results and cell parameters are summarized in Table 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℃ without further calcination. However, the obvious agglomerates composed of micro and nano-sized particles were noticed from the NTP-3h (Figure 1b). Such micro-sized particles may result from the incomplete reaction of the Ti(OH)4 precursors (Figure S1) during the short reaction period. In contrast to PXRD pattern showed 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 nano-sized NTP were sufficient during 5h.

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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).

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 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. Raman spectrum illustrated Figure 2c and band assignments given in Table 1 exhibit that not only the band position but also the intensity were 9



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highly consistent with those reported for the NTP.16 On the basis of the evidences mentioned above, the NASICON-type NTP nanoparticles possessed of great crystallinity and phase-purity were efficiently synthesized at 250℃ for 5h without further calcination, showing the benefit of less energy consumption.

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).

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Table 1 Raman band positions and corresponding assignments of the NTP-5h. Band positions This study Ref. 16 1095 1098 1067 1070 1008 1009 984 985 970 970 543 547 430 432 345 350 335 337 303 305 270 273 220 222 191 195 154 155

Assignments 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 phosphorous ions

T1 and R1 (PO)4

Considering the practical applicability of the nano-sized 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 show the 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. The former is related to the disordered carbon, whereas the later corresponds to the G band with an optically allowed E2g vibrations of the graphitic structure.32 The peak intensity ratio between 11



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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℃. According to its TGA thermogram plotted in Figure 3b, the weight loss (~0.3 wt. %) at temperatures lower than 100℃ was induced by the evaporation of water, while the weight loss detected in the temperature range from 100℃ to 900℃ 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.

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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).

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 13



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0.2 C. The reversible capacities 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 were 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 C 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 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 charge-transfer resistance, W is the Warburg impedance and CPE is the constant phase element.34 It is seen that the spectra 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 NTP-5h/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 14


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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 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.

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

CONCLUSIONS In summary, this study reports the effective synthesis of nano-sized NTP by introducing a facile and cost-effective hydrothermal route at 250℃ for 5h without 15



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further calcination. The Rietveld refined PXRD pattern and Raman spectrum revealed that the NTP-5h possessed of 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 nano-sized 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 high-magnification 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 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℃ for 3, 5 and 12h. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION Corresponding Author *

Tai-Feng

Hung.

E-mail:

[email protected];

Fax:

+886-3-591-5372;

Tel:

+886-3-582-0030. Notes The authors declare no competing financial interest.

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

Synopsis: Carbon-coated NaTi2(PO4)3 synthesized by a cost-effective hydrothermal method delivered the remarkable electrochemical performances for aqueous sodium-ion batteries.

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Hydrothermal Synthesis of Sodium Titanium Phosphate Nanoparticles

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