Highly Reversible and Ultrafast Sodium Storage in NaTi2

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Highly Reversible and Ultra-fast Sodium Storage in NaTi2(PO4)3 Nanoparticles Embedded in Nanocarbon Networks Yu Jiang, Jinan Shi, Min Wang, Linchao Zeng, Lin Gu, and Yan Yu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b09811 • Publication Date (Web): 14 Dec 2015 Downloaded from http://pubs.acs.org on December 19, 2015

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ACS Applied Materials & Interfaces

Highly Reversible and Ultra-fast Sodium Storage in NaTi2(PO4)3 Nanoparticles Embedded in Nanocarbon Networks

Yu Jiang,a Jinan Shi,c Min Wang,a a

Linchao Zeng,a Lin Guc and Yan Yu*,a,b

Key Laboratory of Materials for Energy Conversion, Chinese Academy of Sciences

(CAS), Department of Materials Science and Engineering, University of Science and Technology of China, Hefei, Anhui, 230026, China. E-mail:[email protected] b

State Key Laboratory of Fire Science, University of Science and Technology of

China, Hefei, Anhui, 230026, China c

Beijing Laboratory for Electron Microscopy, Institute of Physics, Chinese Academy

of Sciences (CAS), Beijing, 100190, China. Abstract Sodium ion batteries (NIBs) have been considered as an alternative for Li-ion batteries (LIBs). NaTi2(PO4)3 (denoted as NTP) is a superior anode material for NIBs. However, the poor electrochemical performance of NTP resulting from the low electronic conductivity prevents its application. Here, NTP nanoparticles embedded in carbon network (denoted as NTP/C) were fabricated using a simple soft-template method. This anode material exhibits superior electrochemical performance when used as anode electrodes for NIBs, including highly reversible capacity (108 mAhg-1 at 100 C) for excellent rate performance and long cycle life (83 mAhg-1 at 50 C after 6000 cycles). The excellent sodium storage property can be resulted from the synergistic effects of nanosized NTP, thinner carbon shell, and the interconnected carbon network, leading to the low charge transfer resistance, the large surface area for electrolyte to soak in and enough void to buffer the volume variation during the repeated cycle. Keywords: NaTi2(PO4)3, soft-template method, carbon network, superior rate capability, sodium ion batteries.

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1. Introduction Lithium-ion batteries (LIBs) are identified as one prime candidate to the power sources for a variety of energy application systems, such as electric products and power storage.1-3 However, the concerns about the cost and the lithium resources limit the large-scale application of the LIBs.4,

5

As the high availability of the sodium

resources and the low price, the sodium-ion batteries (NIBs) are considered as the promising substitutes for LIBs.6-8 However, resulting from the larger sodium ionic radius and the huge volume expansion during the charge /discharge process, the NIBs shows the poor long cycle life and the high rate capability.5, 6, 9 Therefore, it seems very necessary and desirable for developing suitable electrode materials with the excellent rate performance and ultralong cycle life for NIBs in the future.6,

10

Nowadays, many electrode materials with superior electrochemical property for NIBs have been researched by several groups, such as Na2Ti3O711, NaxCoO212, Na4Fe(CN)6/C6,

13

, hard carbon14 and MoS215. Obviously, among them, the

phosphate-based Na hosts are best choices as electrode materials due to the strong P-O covalent that offers the remarkable structure and thermal stability.16 Therefore, the NaTi2(PO4)3 (denoted as NTP) is one excellent electrode material in large scale application, due to the open 3D framework with the large interstitial spaces, fast Na+ diffusion, and small volume expansion during cycle.17, 18

19

Unfortunately,

the NASICON-type NTP shows poor intrinsic electric conductivity, which limits its superior rate performance and cycle performance for high-performance NIBs.6, 10, 17, 20-23

Much efforts have been made to overcome these challenges. Among such efforts, downsizing the particle size to nanoscale and designing the reasonable morphology for the NTP particles are considered to be effective.24-26, Here, we reported a simple method. The scatter of carbon-coated NTP particles were embedded into the porous substrates which can limit the volume expansion of the NTP during charge/discharge process, resulting in the superior Na+/e- conductivity, leading to the improved cyclability and rate capability. However, too much carbon will definitely decrease the


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energy density of batteries.25 With the aim of further improving the cyclability and energy densities of NTP anodes, we should design the NTP particles with small particle size, thinner carbon coating and porous structure. Herein, we prepared NTP nanoparticles with electrical conductive carbon coating layers (denoted as NTP/C) by using the cationic surfactant (CTAB) as the carbon source. The as-obtained NTP/C shows superior sodium storage performance when used as anode for NIBs, attributed to the special structure of the NTP/C composite combining a variety of advantages: i) the carbon shell provides a rapid electron pathway to NTP,10 ii) the carbon shell can prevent the NTP grain growth20 and iii) the NTP/C with porous structure facilitates the electrolyte to access the active materials.27 The obtained NTP/C exhibits the reversible capacity of 108 mAhg-1 at 100 C (discharge or charge time of 36 seconds) and the discharge capacity of 83 mAhg-1 with the capacity fading of 0.52% per cycles at 50 C for 6000 cycles.

2. Results and discussion Scheme 1 illustrates the fabrication process and the obtained porous electrode architectures. First, Titanium (IV) acetylacetonate was dissolved in the ethanol. After dissolved completely, cetyltrimethylammonium bromide (CTAB), sodium acetate， ammonium dihydrogen phosphate and de-ionized water were put into the above solution with continuous vigorous stirring for several hours. Then, the solution was dried at 70 oC to get the white powder. Finally, NTP/C was obtained after removing the organic template and heat treatment in Ar/H2 at 700oC. As shown in the Scheme 1, the 3D connected carbon layer functions as an electron conductor, NTP functions as Na storage material, and the open pores offer easy electrolyte access. For comparison, we also prepared pure NTP particles using similar process except that CTAB was not added. Figure 1a shows the XRD patterns of both samples. The two samples have same diffraction angles without any impurities, in good agreement with previous results.25, 26

In addition, the NTP/C has no graphite diffraction angles, showing the amorphous

nature of carbon coating layer. Figure 1b shows a typical Raman spectrum of the



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carbon in NTP/C. Obviously, two strong bands were found at 1356 cm-1 and 1560 cm-1, representing the disordered carbon (D-band) and graphitic carbon (G-band), respectively. The intensity ratio of D to G band value (ID/IG) is about 0.98, confirming a relatively low degree of graphitization.17, 28 The carbon content of NTP and NTP/C were calculated by the thermogravimetric method (Figure S1). The carbon content is 7.01% in NTP/C. By contrast, the carbon content in NTP is negligible (Figure S1). Figure 2 shows the Brunauer-Emmet-Teller (BET) surface area and the pore size distribution of both samples. Obviously, the NTP/C has a larger BET surface area of 30.3 m2g−1, much higher than that of the NTP (3.5 m2g−1) (Figure 2& Table S1). This relatively high BET surface area for NTP/C may result from the small particle size and the existence of the some pores. Figure 2b shows the pore size distributions of the both samples. The major pore size of the NTP/C is about 10.6 nm. In addition, the pore volume for NTP/C is about 0.18 m2g−1 (Table S1). Both of these pores and the large surface area have a great influence on the superior circularity and high capability for the electrode materials, resulting from the excellent impregnation of the electrode and the superior Na+ transfer in the active electrodes. Figure 3a shows the particle size of the NTP ranging from hundred nanometers to several micrometers. No carbon layer in the control sample is observed in the Figure S2. Figure 3b&inset of Figure 3b shows that NTP/C nanocomposites are composed of microsized clusters with nano-particles aggregation with a small particle size of 50 nm. The morphology difference is attributed to the carbon layer that can availably prevent the NTP particles growth in the process of sintering. The microscopic features of NTP/C can be further studied by using TEM, HR-TEM (Figure 3c&3d) and Scanning Transmission Electron Microscope (STEM) (Figure 4). Figure 3c displays that the NTP particles are made up with fifty-nano-sized primary particles, which is in good agreement of the SEM images. In addition, there are some pores between the primary particles. The clear lattice fringes of NTP were displayed in the HRTEM image (Figure 3d). Moreover, the lattice spacings with around 0.204 nm and 0.305 nm were observed, corresponding to the (223) and (024) planes. It is in good agreement with the XRD results (Figure 1a). Furthermore, the three nano-thichness


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carbon layer is observed on surface of the NTP nanocrystal. The STEM and the elemental mapping (Figure 4) demonstrate that the NTP particles are uniform distributed in the carbon network. To further confirm the carbon network, the NTP is removed from NTP/C nanocomposites by using hydrofluoric acid. This porous carbon matrix can be clearly observed in Figure S3. Such a thin carbon with interconnected morphology also shows an advantage for materials to be applied in batteries, since the carbon network facilitates the Na +/e- during sodiation/desodiation process. The NTP/C anode electrodes are demonstrated to show the outstanding electrochemical performance in sodium storage. Figure 5a displays the first cyclic voltammetry (CV) curves for both samples for NIBs at the voltage window between 1.5 and 2.8 V. The pair of the Ti4+/Ti3+ redox peaks of each sample have been found. It is

consistent with the phase transition during sodium ion extraction/insertion

processes. Comparing with NTP, the NTP/C shows the better-defined peaks and smaller potential interval between anodic peak and cathodic peak, which demonstrates the lower electrochemical polarization and better reversibility of the NTP/C electrode.29 Figure 5b shows the first charge-discharge profiles of both samples at 1C (1 C =130 mAhg-1). The NTP/C shows an ideal discharge/charge profiles with plain plateaus comparing with the pure NTP. Here, the capacity deriving from carbon in NTP/C can be ignored.10, 18

The NTP/C delivers a capacity of 132 mAhg-1 for 1st

discharge, approaching to the theoretical capacity (133 mAhg-1). Comparing with NTP, the NTP/C also shows lower polarization and a higher initial Coulombic efficiency (~ 98%), indicating the efficient usage of the NTP/C particles and the highly repeated sodiation/desodiation process. Figure 5c compares the cycling performances of both simples at 1C. The NTP/C electrode shows a reversible discharge capacity of 119 mAhg-1, with the capacity retention of 90 % after 800 cycles. However, after 30 cycles, the capacity of NTP drops rapidly to 34 mAhg-1, which results from the poor intrinsic electric conductivity and the large particle size of the pure NTP (Figure 2a). Figure 5d shows the long cycling stability of the NTP/C at a high current density (5 C). The initial capacity of 126 mAhg-1 could be obtained and 90% is retained after



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2000 cycles and the Coulombic efficiency maintains approximately 100% during cycling (Figure 5d). Figure 6a shows the charge-discharge profiles of the NTP/C at 1−100 C. The NTP/C displays a plain plateaus around 2.1 V and the specific capacity is 132 mAhg-1 at 1C. As the current density increase from 5 to 10, 20, 40, 60 and 80 C, the specific capacities for NTP/C are 125, 124, 123, 120, 114 and 109 mAhg-1. Moreover, the NTP/C shows excellent specific capacity of 108 mAhg-1 at 100 C (discharge or charge time of 36 seconds), displaying the superior rate performance for NTP/C due to the excellent electric conductivity. The electric conductivity of the simples was measured by the four point probe method at room temperature, as shown in Table S2. The value of electric conductivity for the NTP/C (9.77×10-3S m-1) is much larger than that of NTP (1.07×10-4S m-1), demonstrating the significant improved electric conductivity of the NTP/C due to the carbon network. The rate capability for NTP/C is displayed in Figure 6b. Clearly, the discharge specific capacity for NTP/C anode electrode is 128 mAhg-1 after 40 cycles at higher rates. As shown in Figure S4, the rate performance of NTP/C electrode is much better than the results in the published literatures. Figure 6c&6d displays the long-term charge–discharge cycling performance of NTP/C at 10 C and 20 C, respectively. Impressively, the NTP/C delivers a specific capacity of 122 mAhg-1 at the first cycle. After 3000 cycles at 10 C, the discharge specific capacity of NTP/C is 118 mAhg-1. It means that the capacity-decay rates of the NTP/C anode is only ~ 1.3% per cycle at 10 C. After 1500 cycles, the specific capacity of NTP/C is 118 mAhg-1 at 20 C (Figure 6d). To further demonstrating its superior rate performance, the NTP/C anode material is further investigated at 50 C (Figure 7). After 6000 cycles the discharge capacity of the NTP/C is 83 mAhg-1 with the capacity fading of 0.52% per cycles. Over the whole cycle process, the Coulombic efficiency can achieve ~100%, inidicating the super stabile structure of the NTP/C electrode material during repeated extraction/insertion of sodium ion processes at high current densities. In order to better understand the electrochemical performance for NTP/C and NTP electrodes, the electrochemical impendance spectroscopy (EIS) is used. Figure 8a


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displays the Nyquist plots of both simples after three cycles at the current densities of 1 C with equivalent circuit. The semicircle of the curves is attributed to Na+ transfer via the interfacial between active materials and the electrolyte. Obviously, the charge transfer resistance (Rct) of the NTP/C electrode is 349 Ω. However, the Rct of the pure NTP is 568 Ω. This result shows that the special structure of the NTP/C could improve the electrochemical performance for NTP/C electrodes.27 The straight line is attributed to Na+ transfer in the active materials. The general equation (D= 0.5 (RT/AF2Cσ)2 , R , T, A, n, F, C and σ, corresponding to gas constant, absolute temperature, the surface area of the electrode, the number of electrons per molecule during oxidization, the Faraday constant, the concentration of sodium ion and the Warburg factor) was used for calculating the diffusion coefficient of the Na+ in the batteries.10, 20 The relationship between Zˊ and reciprocal square root of frequency was displayed in Figure 8b. Notably, the calculated diffusion coefficient value of NTP/C (1.91×10-11 cm2s-1) is much larger than that of the NTP (4.56×10-12 cm2s-1), demonstrating the carbon matrix of the NTP/C could significantly improve the ion and electron conductivity, resulting in the outstanding rate capability and cycle stability for the electrode in the sodium storage. The excellent sodium storage performance of NTP/C can be ascribed to the advantages of their favorable structure: (1) The nanosize of NTP is beneficial to the ionic and electronic transport. (2) The interconnected carbon shell can improve the electrical conductivity of NTP significantly; (3) The porous nanostructure offers the large contact area between the electrodes and electrolyte. (4) The structure of the NTP can remain stable due to the carbon layer, limiting the volume change during sodiation/desodiation process. 3. Conclusion In summary, NTP nanoparticles with nanocarbon network are prepared by a simple soft-template method. The NTP is encapsulated in 3D interconnected thin carbon network, thus leading to an improved electrical conductivity. Also, the carbon network can limit the volume expansion even at the fast and repeated Na+ insertion/extraction. Therefore, the NTP/C shows the superior electrochemical performance in NIBs. The



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NTP/C electrode delivers the discharge capacity of 108 mAhg-1 at the high current densities of 100 C (discharge or charge time of 36 seconds) and the discharge capacity of 83 mAhg-1 with the capacity fading of 0.52% per cycles at 50 C for 6000 cycles. It is believe that this simple method could be further stretched to produce others anode and cathode materials for both LIBs and NIBs. Due to the high voltage plateau of the NTP anode, when assemble a full battery, it requires select some cathodes with high discharge potential to match and realize high power release (e.g., Na3V2(PO4)2F3 ).

4. Experimental Sections 4.1 Synthesis The porous carbon-coated NaTi2(PO4)3 (NTP/C) was made via a practical means. Firstly, the Titanium (IV) acetylacetonate (2mmol, C10H14O5Ti) was poured in the ethanol (250mL, C2H5OH). After dissolved completely, cetyltrimethylammonium bromide (1.5mmol, CTAB), sodium acetate (1mmol, CH3COONa), ammonium dihydrogen phosphate (3mmol, NH4H2PO4) and de-ionized water (150mL, H2O) were added into the above solution with continuous vigorous stirring for 12 h. Then, the white powder was obtained by drying at 70 oC. Finally, the obtained powder was sintered at 700 oC for 6 hours in the Ar/H2 (95:5) to get the NTP/C. 4.2 Materical characterization The crystallographic information was carried on the X-ray powder diffraction. The character of the carbon for the NTP/C was studied by the Raman spectroscopy. The TG was performed to determine the carbon contents in samples. The morphologies and the structural properties of the simples were researched by the Filed-Emission Scanning electron microscopy, transmission electron microscopy (TEM) and the high-resolution TEM (HRTEM). The Brunauer-Emmet-Teller (BET) surface area and the pore size of both simples were investigated by An ASAP 2020 Accelerated Surface Area and Porosimetry instrument. 4.3 Electrochemical characterization The electrochemical performance of simples was tested in the CR2032 cells. The electrode slurry contains 70% active material, 20% carbon black and 10% PVDF and


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the N-methyl-2-pyrrolidone. The working electrode slurry was coated on the surface of the Cu foil. Then the obtained electrode was dried in a vacuum chamber at 70 oC. Sodium metal was used as the counter electrode. The 1 M NaClO4 in ethylene carbonate (EC) and dimethyl carbonate (DMC) in the volume ratio 1:1 were used as the electrolyte. The separator for the cell is the glass fiber. The typical loading mass of active material is about 0.6 mg/cm2. The cycle performance and the rate performance tests were performed by a battery test system (Neware BTS-610) at a voltage window of 1.5-2.8 V. The capacity was computed based on the active material except the carbon content. The CV dates for the electrode were obtained by the electrochemical workstation. EIS tests for the working electrode were carried by a CHI 660D electrochemical workstation. The electric conductivity of the samples were measured by the four-probe method.

Acknowledgements This work was financially supported by the National Natural Science Foundation of China (no. 21171015, no. 21373195), the Recruitment Program of Global Experts, the program for New Century Excellent Talents in University (NCET), the Fundamental Research Funds for the Central Universities (WK2060140014, WK2060140016), the Collaborative Innovation Center of Suzhou Nano Science and Technology.

Supporting Information Available: Supporting Information Available: TG patterns of NTP and NTP/C, surface parameters of the NTP and NTP/C, TEM image of NTP, SEM image and the EDX element of the carbon network after the corrosion of the partly NTP particles in the NTP/C, the electrode parameters of the NTP and NTP/C, the comparison of the NTP/C electrode with the results in the published literatures about NTP.

This material is available free of charge via the internet at

http://pubs.acs.org.



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2009, 54, 2253-2258.

Figures and Captions

Scheme 1. Schematic of experimental process to fabricate NTP/C nanocomposites. The 3D connected carbon layer functions as an electron conductor, NTP functions as Na storage material, and the open pores offer easy electrolyte access.

Figure 1. (a) XRD patterns of the pure NTP and NTP/C nanocomposites and (b)



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Raman spectrum of the NTP/C nanocomposites.

Figure 2. (a) N2 absorption-desorption isotherm of the NTP and NTP/C nanocomposites and (b) the pore-size distribution curves of the NTP and NTP/C nanocomposites.


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Figure 3. SEM images of NTP (a) and NTP/C (b); TEM (c) and HRTEM (d) images of NTP/C.



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Figure 4. Images of Scanning Transmission Electron Microscope (STEM) (a) and corresponding elemental mapping of the NTP/C showing the C element uniform distribute in the surface of the NTP particles (b-f).

Figure 5. (a) The cyclic voltammogram curves of the first cycle for both NTP and NTP/C at 0.1 mVs-1. (b) Discharge-charge curves of NTP and NTP/C at 1 C for the first cycles. (c) The cycle performance of NTP and NTP/C at 1 C for 800 cycles and the Columbic efficiency of the NTP/C for 800 cycle at 1C. (d) Specific capacity of


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NTP/C at a rate of 5 C for 2000 cycles.

Figure 6. (a) The discharge-charge curves of NTP/C at 1, 5, 10, 20, 40, 60, 80 and 100 C. (b) Rate performance of NTP/C at different current densities (from 1 C to 100 C). (c) The cycle performance of NTP/C at 10 C for 3000 cycles. (d) Specific capacity of NTP/C at a rate of 20 C for 1000 cycles.

Figure 7. The ultra-long cycle life of the NTP/C electrode for 6000 cycles at 50 C.



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Figure 8. (a) The Nyquist plots of both NTP and NTP/C electrodes measured in thrice cycled test cells at 1 C with the equivalent circuit inset. (b) The relationship of Z＇and ω-1/2 of the NTP/C electrode in the low frequency region.


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TOC


Highly Reversible and Ultrafast Sodium Storage in NaTi2

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