Iso-Oriented NaTi2(PO4)3 Mesocrystals as Anode Material for High

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Iso-Oriented NaTi2(PO4)3 Mesocrystals as Anode Material for High-Energy and Long-Durability Sodium Ion Capacitor Tongye Wei, Gongzheng Yang, and Chengxin Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08778 • Publication Date (Web): 25 Aug 2017 Downloaded from http://pubs.acs.org on August 25, 2017

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ACS Applied Materials & Interfaces

Iso-Oriented NaTi2(PO4)3 Mesocrystals as Anode Material for High-Energy and Long-Durability Sodium Ion Capacitor

Tongye Wei1, Gongzheng Yang1*, Chengxin Wang1, 2*

1

State Key Laboratory of Optoelectronic Materials and Technologies, School of Physics

Science and Engineering, Sun Yat-sen (Zhongshan) University, Guangzhou 510275, People’s Republic of China 2

The Key Laboratory of Low-carbon Chemistry & Energy Conservation of Guangdong

Province, Sun Yat-sen (Zhongshan) University, Guangzhou 510275, People’s Republic of China

* Correspondence and requests for materials should be addressed to C. X. Wang. Tel & Fax: +86-20-84113901 E-mail: [email protected]; [email protected]

Keywords: Sodium ion capacitor; Anode; Energy storage; NaTi2(PO4)3; Mesocrystal; Iso-oriented growth

ACS Paragon Plus Environment


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Abstract Sodium-ion capacitors (SIC) combine the merits of both high-energy batteries and high-power electrochemical capacitors, as well as the low cost and high safety. But they are also known suffering from the severe deficiency of suitable electrode materials with high initial Coulombic efficiency (ICE) and kinetic balance between both electrodes. Herein, we report a facile solvothermal synthesis of NaTi2(PO4)3 nanocages constructed by iso-oriented tiny nanocrystals with a mesoporous architecture. It is notable that the NaTi2(PO4)3 mesocrystals exhibit a large ICE of 94%, outstanding rate capability (98 mA h g–1 at 10 C), and long cycling life (over 77% capacity retention after 10000 cycles) in half cells, all of which are in favor to be utilized into a full cell. When assembled with commercial activated carbon to a SIC, the system delivers an energy density of 56 Wh kg–1 at a power density of 39 W kg–1. Even at a high current rate of 5 A g–1 (corresponds to finish a full charge/discharge process in 2 minutes), the SIC still works well after 20000 cycles without obvious capacity degradation. With the merits of impressive energy/power densities and longevity, the obtained hybrid capacitor should be a promising device for highly efficient energy storage systems.


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Introduction Nowadays, exploiting an alternative sustainable energy-storage system of low-stability and high-cost lithium ion batteries (LIB) is a crucial issue to satisfy the growing demands for electric vehicles and efficient utilization of renewable and clean energies.

1-3

In the past decade, lithium-ion capacitor (LIC), which is consist of a

carbonaceous electrode by surface adsorption reactions and a lithium insertion-type counter electrode material, is regarded as a possible solution and has attracted a lot of attentions since their complementary features of both high-power supercapacitors and high-energy rechargeable LIBs with high safety and long-term cycling stability.

4 - 11

But the rising prices of lithium sources inhibit its practical applications and arouse innovative researchers worldwide to develop substitutable charged species beyond lithium ions. Thereafter sodium ions-based energy storage devices that possess the similar physical-chemical properties of lithium ions and particularly their abundant reserves have naturally launched into a research focus. 12 - 14 However, it is known that sodium-storage technologies suffer from a severe deficiency of suitable negative materials that simultaneously manifest long durability, large initial coulombic efficiency (ICE), and high-rate performance, which is primarily because of the structural collapse of active materials upon the intercalation of heavy sodium ions.

15

Searching for electrode materials with small volume change during sodium ions insertion/extraction, less interfacial reactions of electrode/electrolyte and fast ions/electrons transfer paths is an important step to meet these requirements. Among various available negative materials, titanium-based compounds, such as anatase TiO2 16 - 20 and layered Na2Ti3O7 21 - 23, show the better cycling stabilities than that of many electrode materials with alloy-type (Sn,24, 25 Sb,26, 27 Ge,28 P

29, 30

) and

conversion-type mechanism (MoS2,31 Fe2O3 32) due to their intrinsic structural stability. A variety of titanium-based negative materials applying in sodium ion batteries (SIB) with excellent features in terms of capacity and voltage has been developed. For instance, P. Senguttuvan et al. firstly reported that layered Na2Ti3O7 can work as an effective low-voltage (0.3 V) insertion sodium host with an ability to reversibly uptake two sodium ions per formula unit. 21 Soon after ordered three-dimensional (3D)



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Na2Ti3O7 nanoarrays were fabricated on a flexible Ti substrate which successfully achieved a high reversible capacity (227 mA h g–1) and a superior long cycling stability (10000 cycles).

33

The battery performances of TiO2 in SIBs were firstly

examined by Xiong and co-workers, while they revealed that the sodium-storage of TiO2 was based on the Ti4+/Ti3+ redox couple.

16

To overcome the sluggish sodium

reaction kinetics of wide-band-gap TiO2, nanostructured TiO2 and TiO2-conductive carbon hybrids have been intensively investigated as potential anode materials for SIBs.

19, 20, 33

In particular, an intercalation pseudocapative charge storage

phenomenon was observed in TiO2/graphene nanocomposites, which enabled a specific capacity of ~ 90 mA h g–1 at an extremely high current rate of 12 A g–1. Benefiting from above merits, Na2Ti3O7

35

34

and TiO2 36 were employed as negative

materials for sodium ion capacitors (SIC). It is notable that a full cell, which was consist of mesoporous single-crystal-like anatase TiO2 anode coupling with a carbon-based cathode, had demonstrated a high energy density of 64.2 Wh kg–1 at 56.3 W kg–1 that was far higher than that of electric double-layer capacitors (typically 10 Wh kg–1) using carbons as electrodes. 37 However, despite of these inspiring results, Na2Ti3O7- and TiO2-based SICs universally suffer from the low ICEs which will severely restrict their further applications. Sodium super ionic conductor (NASICON)-structured NaTi2(PO4)3 (NTP), which possesses a stable 3D framework made up of corner- and edge-connected [PO4] tetrahedra and [TiO6] octahedra that allows two sodium ions intercalation (corresponding to a theoretical capacity of 133 mA h g–1), exhibits great potential for long-life SIBs.

38 - 40

Specially, almost all the

reported NTP-based SIBs extensively show high ICEs (over 90%) that are conducted to be assembled into a full cell and should be a good choice for hybrid capacitor application. 38 - 42 Very recently, a novel NTP-based SIC with NTP grown on graphene nanosheets as anode and two-dimensional graphene nanosheets as cathode was firstly presented by Thangavel and co-workers.

43

This new SIC delivered a high energy

density of ~80 Wh kg–1 at a specific power density of ~8 W kg–1 that surpassed most of previously documented hybrid capacitors. Herein, we describe a facile solvothermal method to prepare quasi-cubic


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mesoporous NaTi2(PO4)3 nanocages and their application as high-performance anode in sodium-storage hybrid capacitors. It is notable that the obtained NTP nanocages are built by bottom-up assemblies of tiny nanocrystals with a mutual growth orientation, resulting to a uniform cube-like structure filling with 3D quasi-cubic nanopores. The tiny nanostructure and abundant pores can provide more active sites and shorten the ion transport path to promote the sodium-ion transport kinetics. The as-obtained NTP nanocages show excellent battery performance in SIB half-cell with a high reversible capacity (120 mA h g–1 at 0.2 C), long cycling life (74% capacity retention after 10000 at 5 C) and high rate capability (97 mA h g–1 at 10 C). Thereafter we assemble a full cell with activated carbon (AC) as positive electrode, NTP nanocages as anode material for SIC, and achieve capacity of 42.8 mA h g–1 (calculated on the total mass of AC and NTP) with a maximum energy density of ~56 Wh kg–1 which excesses most of that obtained in AC-based LICs/SICs. Moreover, our NTP/AC full cell can realize rapid charging in 10 seconds and the following fast/slow discharging with very little performance degradation.

Results and discussion Figure 1a shows a panoramic SEM image of the as-synthesized NTP nanocages, in which one can see that all the products display cube-like morphologies ranging in 20 - 50 nm. The stark contrast between the edge and inner regions of the nanocrystals (Figure 1b) implies the hollow nature of the samples. The sharp diffraction peaks in the XRD patterns (Figure 1c) suggest the highly crystalline of the products, while all the peaks match well with NASICON-structured NaTi2(PO4)3 (JCPDF no. 33-1296). TEM analyses were employed to further study the microstructure of NTP nanocages. Shown in Figure 1d, the products demonstrate a relatively uniform quasi-cubic architecture. Magnified TEM image in Figure 1e confirms their hollow structures. Interestingly, by careful observations it can see that all the nanocrystals are not standard cubes, while the two adjacent edges of any nanocrystal include an angle of 91°. Moreover, even the angles between the edges of the hollow pores are 91° (Figure 1f - h), indicating an unusual growth of the NTP nanocages. N2 adsorption-desorption



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measurements were introduced to examine the surface areas and pore size distribution (PSD) of the products. As depicted in Figure 1i and j, the N2 sorption isotherm exhibits a characteristic of type IV with a surface area of 67.4 m2 g–1 and the PSD of the NTP nanocages centers at around 16 nm, which is in good agreement with the TEM results. Figure 2a displays a TEM image of an individual NTP nanocage, while the energy-dispersive X-ray (EDX) spectrum collected from this area (inset in Figure 2a) testifies the high purity of the products. A locally enlarged high-resolution TEM (HRTEM) image in Figure 2b shows clear lattice fringes. Two identical widths of 0.61 nm between neighboring lattice fringes can be ascribed to (012) and (–102) planes of NTP, respectively. The corresponding selected-area electron diffraction (SAED) pattern (Figure 2c) indicates that the nanocage is high single-crystalline structure. It is worth noting that the angle between (012) and (–102) planes is also 91° (Figure 2b) that is equal to the above measured values (Figure 1f - h). Figure 2d exhibits a low-magnification TEM image where three nanocages closely surround together with a mutual nanocage. The HRTEM image of the shared nanocrystal is illustrated in Figure 2e, in which the lattice spacings and Fast Fourier Transform (FFT) patterns are same with the nanocage shown in Figure 2a and the characterized plane is exposed with (2–21) plane. As zooming in the connecting grain boundaries, it is intriguing that the lattice fringes of the peripheral three nanocages are parallelly and uniformly extended from the shared nanocrystal, despite of small lattice dislocations (Figure 1f h). Nearly the identical FFT patterns (Figure S1, supporting information) further suggest the inherent relation during the growth of the four nanocages. In view of the structural symmetry of hexagonal system, both the (012) and (–102) planes can be attributed to {012} crystal plane group, namely, the surrounding nanocages may be formed from the homoepitaxial growth of the shared nanocage along the {012} directions. Besides, there are also numerous edge-bridging conjoined NTP nanocrystals existing in the products. Similar NTP multi-nanocrystals with varying number and different packing arrangements of subunits can be easily observed, as shown in Figure S2.


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To clarify the growth processes of these interesting architectures, we performed a series of time-dependent experiments and corresponding results are presented in Figure S3. Many intermediates, such as tiny quantum dots, aggregating nanoparticles, growing and finishing nanocages with multi-layers, are obtained at different stages. One possible growth mechanism is proposed to elucidate the formation of the NTP nanocages (Figure S4): Firstly, as achieving a proper supersaturation and temperature, NTP nuclei will homogeneously spring up and quickly develop to stable primary crystals. Upon the following transport of crystal seeds to the surface terrace, smooth ledge or kink site, newly generated crystals will rapidly grow up over the primary crystals to create a hierarchical structure (Figure S3a). In this system, we speculate that the {012} planes may possess the higher surface activity and energy than other crystal planes of NTP, which will lead to an iso-oriented growth along the {012} planes and the final elimination of high-energetic {012} planes (Figure S3b). Precisely because the angle of adjacent {012} planes (e.g. (012) and (–102)) is 91°, this interesting accumulation of basic blocks ultimately brings out the resulting dispersed quasi-cubic particles. For the inner nanoparticles, the migration and adsorption of the growth units onto their surface become more difficult since the shielding effect of outside facets, leading to their slow growth and the subsequent formation of hollow quasi-cubic pores in each nanocage and the edge-sharing conjoined nanocrystals. Thus, it can conclude that such a highly ordered mesoporous material is constructed from the bottom-up aggregation of NTP nanocrystals in a same orientation. The electrochemical performance of the quasi-cubic mesoporous NTP nanocages were evaluated using a half cell with sodium metal as both counter and reference electrode. Figure 3a shows the cyclic voltammogram (CV) of NTP electrode collected at a scanning rate of 0.1 mV s–1 within the potential window of 1.5 - 2.8 V (vs. Na/Na+). The reduction and oxidation peaks at around 2.1 and 2.2 V were observed during the beginning three cycles, which correspond to the redox couple Ti4+/Ti3+ that is in good agreement with previous studies.

44

Figure 3b gives the representative

galvanostatic charge/discharge profiles, in which it can be seen that the NTP electrode



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delivers an initial charge and discharge capacity of 117 and 122 mAh g–1, corresponding to an ICE of ~94% that is much higher than that of various carbons and metal alloy-based anode materials.

24 - 29, 45, 46

The rate performance of the NTP

electrode was investigated by cycling at various current densities as show in Figure 3c. With the current rates increasing from 0.2 to 0.5, 1, 2, 5, and 10 C (1 C = 133 mA g–1), the discharge capacities decreased slightly from 122 to 115, 111, 107, 102, and 98 mA h g–1, respectively. A specific capacity of 117 mA h g–1 was recovered when the current rate reduced back to 0.2 C after over 30 cycles at higher rates. Furthermore, the NTP nanocages electrode also exhibits long stable cycling performance with 77% retention of its initial capacity cycled at 5 C after 10000 cycles and a high average coulombic efficiency of 99.8%, where both parameters are crucial for the practical applications. Figure S5 presents the voltage capacity profiles associated with charge-discharge at different current rates and cycle numbers, respectively. The sodium-storage properties of AC in SIB half-cell was analyzed in a potential window of 2.5 - 4.5 V. The CV curves of AC (Figure S6a) demonstrates a quasi-rectangular shape, suggesting its capacitive behaviors based on surface desorption/adsorption of ClO4- anions. The typical charge/discharge profiles of AC electrodes with a triangular shape are displayed in Figure S6b that equals to an average discharge capacity of 90 mA h g–1 at 0.2 A g–1. In addition, the AC electrodes also have demonstrated acceptable rate-performance and long cycling life (Figure S6c and d). Taking the superior electrochemical performances of the mesoporous NTP into account, we fabricated a hybrid capacitor using NTP as anode material, commercial AC as cathode material, and 1 M NaClO4 (in PC) as electrolyte, while the mass ratio of anode and cathode was set to 1:1.5 and the battery performance of the SIC was tested between 0.01 to 2.5 V. Figure 4a illustrates the working mechanism of the SIC. Upon charging, Na+ ions are intercalated into NTP by a redox reaction, creating a charge imbalance in the system. To achieve the charge balance, ClO4– anions are quickly adsorbed on the surface of AC to form a double layer and stabilize the system. During discharging, Na+ ions are extracted from NTP, and meanwhile, ClO4– anions are desorbed from surface of AC electrode. Such a system combines the advantages of


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the batteries and supercapacitors, ensuring a relative high capacitance, outstanding rate performance and long cycle stability. As shown in Figure 4b and Figure S7 the obvious humps between 0.9 - 1.6V in CV curves can be attributed to the rapid insertion/extraction

of

Na+

from

the

NTP

accompanying

with

the

adsorption/desorption of ClO4–. Apparently, the CV curves are well overlapped since the 2nd cycle, indicating a good cyclability. Figure 4c presents that two NTP/AC SICs can be readily integrated in serial to power a logo consisting of 35 light-emitting diodes (LEDs). Figure 4d demonstrates the stable charge-discharge profiles which match well with the CV results. To clearly demonstrate the high-power performance of the NTP/AC full cell, CV tests at sweep rate of 10 mV s–1 to an extremely high rate of 1000 mV s–1 (corresponding to finish a full discharge/charge process in 5 seconds) were recorded. It can be seen from Figure 5a that the peak currents appear in direct proportion to the increasing sweep rates and the well-defined sharp redox peaks are still maintained, implying an excellent rate-performance of this device. Figure 5b shows the galvanostatic charge/discharge measurements at different current densities. The average discharge capacities are 42, 38, 35, 33, 31, 28, and 27 mA h g–1 at current rates of 0.2, 0.5, 1, 2, 5, and 10 A g–1, respectively (the current densities were set up according to the mass of anode materials; charge-discharge curves at various currents please see in Figure S8, supporting information). Thereafter, the long-term performance of the SIC was evaluated at a high current density of 5 A g–1 (Figure 5d). The cell runs well after the first charge process and retains nearly 100% of its initial value after 20000 cycles, demonstrating the splendid reversibility of the SIC. On account of the galvanostatic charge/discharge measurements, both the energy and power densities of the NTP/AC full cell based on the mass of active materials (namely NTP and AC) can be calculated. A high energy density of 56 Wh kg–1 can be achieved at a power density of 39 W kg–1; even at a high power density of 4096 W kg–1, this cell can still deliver an energy density of 31 Wh kg–1. By comparison, our SIC performance is higher than most of that reported batteries and capacitors that employ intercalation-based

electrodes

and

capacitor


systems,

such

as


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Na2CuFe(CN)6/NaTi2(PO4)3,

NaMnO2/NaTi2(PO4)3,

NaTi2(PO4)3/Na2NiFe(CN)6,

NTP@rGO/NVP/C, LiCoPO4F/C, MgO-MWCNT/AC, NVP/AHD, MWTOG/AC. 47 51

The specific energy/power densities of the as-assembled SIC obtained herein are

compared to the above systems in a Ragone plot (Figure S9). To understand the reasons of the excellent rate capability of the SIC, electrochemical impedance spectroscopy (EIS) measurements of the pristine cell and the cycled battery after different cycles at 1 A g–1 were carried out. From Figure 5c, it is apparently seen that the Nyquist plots are composed of two independent semicircles at high and medium frequency regions, and an inclined line at low frequency regions, except for the pristine cell in which there is only one semicircle in the high frequency region. In general, the semicircle is usually ascribed to summation of the contact, interface capacitance of electrode/electrolyte, and charge-transfer resistance (Rct), while the straight slopping line to the ions-diffusion processes into the host materials. As observed in the Nyquist plots, the Rct value of pristine cell is merely ~1.6 Ω that is quite similar to that of many reported carbon-based aqueous electric double-layer capacitor systems.

52

For the emerging two semicircles in the following cycles, we

suggest that it might derive from the co-existence of charge-transfer resistance and related double-layer capacitance in both electrolyte/anode and electrolyte/cathode since the SIC was operating in a relatively high current rate (1 A g–1). Nevertheless, the Rct values of the subsequent cycles are still very small, implying that the diffusion of Na+ in the NTP is quickly enough, the surface of AC is highly active for the adsorption/desorption of ClO4–, and accordingly the ultrafast sodium-storage properties. Furthermore, the morphologies and structural stabilities of the mesoporous NTP nanocages after long-term cycles were in detail examined by TEM and XRD characterizations. Encouragingly, as presented in Figure 6a, c, one can see that the architectures of the NTP nanocages have been perfectly preserved. The clear lattice fringes in the HRTEM images (Figure 6d, e) attaching with the distinct SAED patterns demonstrate the high-crystallinity of the electrodes. The XRD patterns of the electrodes after cycling in SIC are shown in Figure 6b. All the diffraction peaks can still be well indexed to NASICON-type NTP, which demonstrates the minor


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volumetric variation and further verifies the structural stability of the NTP upon repeated sodium intercalations that are similar to many other NASICON-type compounds with excellent cycling stability. 53 - 55 Taking these remarkable features into account, we think that the SIC can realize an ultrafast charging in several seconds followed with a long-time steady power supply to an electrical appliance. At first the rapid-charging/slow-discharging properties of the SIC were carefully studied and corresponding results in Figure 7 confirm this feasibility. As shown in Figure 7a, the SIC can operate well in more than 1 hour after full of electricity in about 1 min. Besides, the cell not only illustrates a 100% utilization of the stored energy but also a long cycling life (Figure 7b - c). For demonstrating the practical applications of the SIC, we assembled a full cell to power a time-meter. Interestingly, after being charged in about 10 seconds, the time-meter had continued to run well in more than 18 hours (Figure 7e). All of these results conclusively imply the promising application of the NTP/AC battery in advanced energy storage systems.

Conclusion In summary, we have reported a facile preparation of a novel NTP nanostructure with a mesoporous and quasi-cubic morphology and revealed its possible iso-oriented growth mechanism. When tested in SIB half-cell, the NTP nanocages exhibit an acceptable specific capacity and rate-performance. Subsequently, when packed with commercial AC to a full cell, the NTP/AC system delivers a high energy density of 56 Wh kg–1 at a power density of 39 W kg–1. Moreover, the SIC can work well for 20000 cycles at a high rate of 5 A g–1, that corresponds to finish a charge-discharge process in ~2 min, but without no obvious capacity degradation. Significantly, we have successfully demonstrated the practicability of SIC in rapid charging applications. Such superior performances can be attributed to the intrinsic characteristics of the mesocages constructed by bottom-up assembly of crystallographically oriented NTP nanocrystals, accompanied by a large surface area and uniform mesoporous nature. The hollow nanostructure provides ample space for the infiltration of electrolyte that



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is in favor of lowing the transport paths of sodium ions and enhancing the structural stability upon sodium ions insertion. Despite these inspiring results, the adoption of commercial AC is a shortcoming in this system. Exploiting a suitable carbon-based cathode with higher capacities instead of AC will greatly improve the energy-storage performances and this work is still an ongoing process.

Experimental section Synthesis. A facile solvothermal method was introduced to fabricate the NTP nanomaterials. In a typical synthesis, 0.8 mL acetic acid (CH3COOH) and 0.83 mL phosphoric acid (H3PO4, 85 wt%) were successively added into 22.4 mL isopropyl alcohol (C3H8O). Then, 590 mg sodium acetate (CH3COONa) was dispersed into the mixed solution. After severe stirring for about 30 min, 0.834 mL tetrabutyl titanate (C16H36O4Ti) was dropped into the turbid solution. The resulting precursor suspension was transferred into a sealed 40 mL Teflon-lined autoclave and heated to 200 °C for 12 hours. As natural cooling to room temperature, the precipitates in the autoclave were washed with deionized water followed by absolute ethanol three times and dried at 80 ℃ for 12 hours to obtain the final products. Characterization. Field-emission scanning electron microscopy (FE-SEM, Carl Zeiss) and transmission electron microscopy (TEM, FEI Tecnai G2 F30 at 300 kV) were used to investigate the morphology of the as-prepared samples. XRD analysis was performed on a D/Max 2500 (Rigaku co., japan) using Cu Kα radiation at a generator voltage of 40 kV and a generator current of 30 mA with a scanning sped of 2 °min–1. Nitrogen sorption measurement was carried out with an ASAP 2020 Surface Area Analyzer (Micromeritics Co., USA). The specific surface area was obtained by Brunauer-Emmett-Teller (BET) method. Electrochemical Characterization. The half-cell performance was measured by using either NTP nanocrystals or AC as working electrode and metallic sodium foil as counter electrode. The working electrode was fabricated by mixing the active material, a conductive (super-P, Sigma-Aldrich), and sodium alginate (Sigma-Aldrich) in a


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weight ratio of 7：2：1. Next, the mixture was coated on an aluminium foil with a thickness of ca. 100 µm. The total mass of the electrode materials is ca. 2.2 mg, measured by an ultramicro analytical balance (Mettle Toledo XP2U, 0.1 mg resolution), while the mass of the active material is ca. 1.5 mg. After drying in air at 90 °C for 6h, the electrodes were assembled into coin-like cells (CR2032) inside an argon-filled glove box with a moisture content of

Iso-Oriented NaTi2(PO4)3 Mesocrystals as Anode Material for High

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