Constructing Flexible and Binder-Free NaTi2(PO4)3 Film Electrode

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Constructing Flexible and Binder-Free NaTi2(PO4)3 Film Electrode with a Sandwich Structure by a Two-Step Graphene Hybridizing Strategy as an Ultrastable Anode for Long-Life Sodium-Ion Batteries Donglei Guo, Jinwen Qin, Chaozhen Zhang, and Minhua Cao Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01549 • Publication Date (Web): 03 May 2018 Downloaded from http://pubs.acs.org on May 4, 2018

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Crystal Growth & Design

Constructing Flexible and Binder-Free NaTi2(PO4)3 Film Electrode with a Sandwich Structure by a Two-Step Graphene Hybridizing Strategy as an Ultrastable Anode for Long-Life Sodium-Ion Batteries

Donglei Guo, Jinwen Qin, Chaozhen Zhang, and Minhua Cao*

Key Laboratory of Cluster Science, Ministry of Education of China, Beijing Key Laboratory of Photoelectronic/Electrophotonic Conversion Materials, School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing 100081, P. R. China.

1 ACS Paragon Plus Environment

Crystal Growth & Design 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ABSTRACT: Flexible energy storage devices show promising advantages in next-generation bendable, wearable and implantable electronic systems and therefore they attract considerable interest for researchers to fulfill requirements of future energy storage devices. Here, we report a facile two-step strategy to construct flexible and binder-free NaTi2(PO4)3/graphene film with a sandwich structure (GN/NaTi2(PO4)3/GN). Such a design makes the resulting film with an excellent flexibility, which can be used for a binder-free anode for flexible sodium-ion batteries (SIBs). GN nanosheet supported NaTi2(PO4)3 nanocrystals (NaTi2(PO4)3/GN) are first prepared by facile hydrothermal route. Then, the resultant NaTi2(PO4)3/GN hybrid is embedded homogenously in interconnected GN nanosheet framework to form three dimensional flexible GN/NaTi2(PO4)3/GN film. The flexible film exhibits excellent cycling stability (91% capacity retention over 1000 cycles at 500 mA g-1) for sodium half cells. Furthermore, it can be assembled into flexible full SIBs (Na0.44MnO2 as the cathode) for practical application. The flexible full battery shows good cycling stability both under flat and bent states, maintaining 92.9% retention after the first 30 cycles under a flat state compared to the original capacity and 86% retention after another 40 cycles under a bent state. This synthesis route offers a general method for constructing other flexible films for SIBs. KEYWORDS: flexible and binder-free; GN/NaTi2(PO4)3/GN film; sandwich structure; interconnected framework; sodium-ion batteries


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1. INTRODUCTION The development of bendable, implantable and wearable electronics has promoted increasing demand for the power sources with high energy/power density, cost-effectiveness, lightweight, as well as ultrathin and high flexible features.1-4 In order to meet the flexible and thin requirements of devices, the design and fabrication of flexible, binder-free thin-film electrodes are indispensable and up to now various thin and flexible electrode materials have been reported.5,6 These flexible and thin electrodes generally require not only electrode materials with excellent electrochemical performances, but also high mechanical integrity upon bending or folding, compact design and light weight.7-11 To fulfill the demand of these flexible energystorage devices in practical application, tremendous effort has been made in recent years. Graphene (GN), a new two-dimensional (2D) carbon material, has been considered to be a potential material for use on energy conversion and storage because of its high conductivity, large surface area, ultrathin layer structure and mechanical stability.12-14 GN paper (assembled by GN sheets) with high in-plane electrical conductivity, have been applied as high-performance flexible electrodes for rechargeable batteries.15-17 A direct growth or loading of active materials on GN sheets could lead to strongly coupled composite materials with a chemical and electrical coupling. GN sheets could replace traditional metal foil current collectors, which in turn offer great flexibility in novel geometric designs.18,19 Recently, varieties of GN-based hybrid thin films have been reported, such as CoO/GN,20 Si/reduced graphene oxide (rGO),21 Co3O4/GN/carbon nanotubes (CNTs),22 MoS2/GN,23 and NiCo2O4/GN24 have been demonstrated to exhibit superior electrochemical lithium-storage performances as bendable electrodes for lithium-ion batteries (LIBs), which may benefit from the short transporting pathways for charge carriers and lithium ions, and interconnected flexible networks of GN sheets. Recently, sodium-ion batteries (SIBs)



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have received more attention than LIBs because of their high energy density and easy availability, and the cost-effectiveness of sodium.25-27 However, we found that only a few flexible electrode materials have been used for SIBs. So it can be predicted that flexible SIBs would provide huge possibilities for advanced next-generation flexible energy-storage devices. Developing suitable high-performance electrode materials for SIBs is a huge challenge due to the larger ionic radius of sodium ions and their strong binding in the rigid inorganic lattices. This has motivated many researchers to study promising anode materials, such as carbonaceous materials,28-30 phosphide,31,32 metallic alloys,33-36 titanates,37,38 and 2D metal carbides (MXenes)39,40 for application in SIBs. Recently, NASICON-type NaTi2(PO4)3 has been considered as an promising anode material for SIBs because of its high Na+ conductivity and good thermal stability.41-44 NaTi2(PO4)3 possesses an open three-dimensional (3D) framework, in which TiO6 octahedra share corners with PO4 tetrahedra with the Na+ ions occupying the interstitial sites, and such a structure is particularly beneficial for Na+ transport. However, NaTi2(PO4)3 still suffers from poor electronic conductivity, thus degrading its electrochemical performance. As a consequence, extensive effort has been devoted to improving the electrochemical performance of NaTi2(PO4)3, such as tailoring the NaTi2(PO4)3 particle size to shorten both the electronica and ionic transporting pathways and enhancing its electronic conductivity by hybridizing highly conductive carbonaceous materials.45-48 However, to the best our knowledge, most of works mainly focus on the synthesis of NaTi2(PO4)3-based composite powder material as an anode material for conventional SIBs and flexible NaTi2(PO4)3-based film electrode with excellent electrochemical performance has not been reported. Herein, we report a simple yet facile two-step GN hybridizing strategy to prepare a flexible GN/NaTi2(PO4)3/GN film with a sandwich structure, which can be directly used as the anode for


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high rate and cycling performance SIBs. In this film structure, NaTi2(PO4)3 ultrafine nanocrystals are homogenously embedded in 3D interconnected framework of GN sheets to form a flexible GN/NaTi2(PO4)3/GN film. The excellent 3D conductive network and homogenously distributed NaTi2(PO4)3 ultrafine nancrystals could afford short Na+ diffusion paths, good electronic conductivity and volume flexibility, all of which are responsible for the superior electrochemical performance. Specifically, the GN/NaTi2(PO4)3/GN film electrode deliveres high specific capacities of 136.7, 108, 99, and 93 mA h g-1 at 100, 300, 500, and 1000 mA g-1, respectively, and even 91% of the original capaicty can be maintained after 1000 cycles at 500 mA g-1. Moreover, to investigate its practical application, a full cell was also assembled by coupling the GN/NaTi2(PO4)3/GN film anode with the Na0.44MnO2 cathode. The resultant full cell displays an initial discharge capacity of 114 mA h g-1 based on the anode mass, with an initial coulombic efficiency (CE) of 82.3%. This work provides a simple yet facile two-step GN hybridizing strategy for preparing 3D flexible electrode materials for advanced next-generation flexible energy storage devices. 2. EXPERIMENTAL SECTION 2.1 Synthesis of graphene oxide (GO): All chemicals used in this work were analytical grade without further purification. The GO was prepared according to a modified Hummers method.49,50 In a typical synthesis, 1 g of expandable graphite and 0.75 g of NaNO3 (0.75 g) were placed in a 1000 mL of beaker. 75 mL of sulfuric acid (H2SO4, 95-98 wt%) was then added in an ice-water bath with magnetic stirring and then 4.5 g of potassium permanganate (KMnO4) was slowly added in the mxied solution within 1 h. After 2 h of magnetic tirring, the beaker was then transferred into an oil bath and maintained at 40 °C for 5 days with vigorous electric stirring. At last, 10 mL of hydrogen peroxide (H2O2, 30 wt%) was added under the magnetic stirring at room



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temperature for another 2 h. The mixture was then washed by a mixed aqueous solution of 3 wt% H2SO4/0.5 wt% H2O2 and deionized water. Finally, the resultant homogeneous GO was dispersed in deionized water (the concentration is 8 mg/mL) and storaged in a refrigerator. 2.2 Synthesis of NaTi2(PO4)3/GN powder: In a typical synthesis, 10 mL of the resultant GO solution was dispersed into 30 mL of ethylene glycol (EG) to form a mixture. Then, 5 mL of NH3·H2O (25 wt%) and 3.4 mL of tetrabutyl titanate (TBOT) were added to the mixture under constant magnetic stirring. After stirring for 1 h, 15 mmol of ammonium dihydrogen phosphate (NH4H2PO4), 2.5 mmol of sodium carbonate (Na2CO3) and 15 mL of deionized water were added. The obtained brown gel was then transferred into an 80 mL of Teflon-lined stainless steel autoclave and kept at 150 °C for 24 h. After cooling down to room temperature naturally, the black product was washed with deionized water and ethanol for more than five times and then dried at 60 °C for 12 h. Subsequently, the dried sample was annealed at 700 °C for 2 h at a heating rate of 2 °C min-1 in a nitrogen atmosphere to obtain NaTi2(PO4)3/GN powder. For comparison, pristine NaTi2(PO4)3 was prepared in a manner similar to that of NaTi2(PO4)3/GN, except for the absence of GO in the synthesis process. 2.3 Synthesis of GN/NaTi2(PO4)3/GN film: First, the obtained GO dispersion (5 mL) was dispersed in 35 mL of deionized water with ultrasonicating to form a 1 mg/L GO dispersion. Then a certain amount of NaTi2(PO4)3/GN powder was well dispersed in the GO dispersion under stirring and ultrasonication conditions. Finally, the mixed NaTi2(PO4)3/GN/GO dispersion was transferred into a glass culture dish, followed by drying in a vacuum oven at 50 °C. Then NaTi2(PO4)3/GN/GO film was first obtained. In order to get the final GN/NaTi2(PO4)3/GN film, the obtained NaTi2(PO4)3/GN/GO film was treated by 55 wt% HI aqueous solution and then cleaned with ethanol and deionized water several times , followed by drying in a vacuum oven at


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50 °C. For comparison, GN film was prepared in a manner similar to that of GN/NaTi2(PO4)3/GN film, except for the absence of NaTi2(PO4)3/GN in the synthesis process. 2.4 Synthesis of Na0.44MnO2: First, 4.9 g of manganese acetate [Mn(Ac)2·4H2O] was dissolved in 20 mL of deionized water, and then 0.3872 g of sodium hydroxide (NaOH) in 20 mL deionized water was added with magnetic stirring to form a mixture. After freeze-drying for 48 h, the mixture powder was annealed in a program-controlled furnace at 450 °C for 5 h and then at 900 °C for 15 h under an air atmosphere. Finally, Na0.44MnO2 was obtained. 2.5 Materials characterizations: The morphology and microstructure features of the asprepared samples were characterized by using a field-emission scanning electron microscope (FE-SEM, Hitachi S-4800), transmission electron microscopy (TEM) and high-resolution transmission electron microscope (HRTEM). Energy dispersive X-ray spectra (EDS) and element mapping of the samples were taken on a JEOL S-4800 FESEM instrument. X-ray diffraction (XRD) patterns were recorded to confirm the components of the samples on a Bruker D8 X-ray diffractometer at a voltage of 40 kV. X-ray photoelectron spectra (XPS) were carried out on a Thermo Scientific ESCALAB250 spectrometer with Al Ka radiation as the excitation source. Raman spectra was recorded on an Invia Raman spectrometer with an excitation laser wavelength of 633 nm. The thermogravimetric analysis (TGA) was performed from 25 to 700 °C in air using a DTG-60AH instrument. The element contents of the samples were carried out by using inductively coupled plasma mass spectrometry (ICP-MS) (ICAP-9000). 2.6 Electrochemical measurements: The electrochemical performances of the as-prepared samples were examined by using CR2025 coin cells with sodium metal foil as the counter electrode. The flexible and binder-free GN/NaTi2(PO4)3/GN and GN film were directly used as the working electrode. The separator of coin cell was a glass fiber membrane (Whatman GF/D).



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The electrolyte was 1 M NaClO4 dissolved in a 1:1 volume mixture solvent of ethylene carbonate/dimethyl carbonate (EC/DMC) to which 5 wt% fluoroethylene carbonate (FEC) was added. Galvanostatic charge-discharge cycling of the as-prepared samples was evaluated on a LAND-CT2001A (Wuhan, china) battery tester at different current densities in the potential range 1-3.0 V for the half-cell. Cyclic voltammetry (CV) was performed on a CHI-760E electrochemical workstation (Shanghai Chenhua instrument company, China) under different scan rates. The electrochemical impedance spectroscopy (EIS) of the cell was investigated by applying a sine wave with amplitude of 5 mV over the frequency range from 100 kHz to 0.01 Hz. To test the full sodium-ion cells, the GN/NaTi2(PO4)3/GN film was used as the anode and the as-prepared Na0.44MnO2 as the cathode. For the preparation of Na0.44MnO2 cathode film, the Na0.44MnO2 active material, conductive carbon black and polyvinylidene fluoride (PVDF) binders were dissolved in N-methylpyrrolidone (NMP) solution with a weight ratio of 80:10:10 to make a homogeneous slurry. The as-resultant slurry was uniformly pasted on the Al foil and finally dried at 70°C for 12 h and then 120 °C for another 12 h in vacuum oven. The GN/NaTi2(PO4)3/GN film was directly used as the anode film. The capacities of the GN/NaTi2(PO4)3/GN anode and the Na0.44MnO2 cathode should be matched at 1.2:1 rate. The assembled full cells were tested in the voltage range of 2-4 V. As for the aluminium laminated soft-packed full cells, the GN/NaTi2(PO4)3/GN film was cut into 2 cm × 3 cm rectangle slice for rectangle cell anode. The corresponding shaped glass fiber membrane and Na0.44MnO2 film were used as the separator and the cathode, respectively. When bending or folding occurs to the full flexible cells, the electrolyte held by electrodes and glass fiber membrane tends to be squeezed aside, leading to relatively poor electrolyte wetness and increased capacity loss. So, the


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electrolyte needs to be excess and meanwhile the electrodess were protected by silica gel plate film. 3. RESULT AND DISCUSSION The GO nanosheets used in this work were prepared according to a modified Hummers method by Seung-Min Paek et.al.49,50 Figure 1a shows the schematically illustrates illustrat the typical formation process for the GN/NaTi NaTi2(PO4)3/GN film (details details see Experiment Section), Section which mainly involves three steps. Briefly, GN supported NaTi2(PO4)3 nanocrystals (NaTi NaTi2(PO4)3/GN powder) were prepared via a simple hydrothermal route,, followed by annealing under N2 atmosphere (Figure S1,, Supporting Information Information). Then, the as-prepared NaTi2(PO4)3/GN powder was mixed homogenously with GO dispersion possessing the interconnected framework to form a flexible NaTi2(PO4)3/GN/GO film by sonicating and drying in Petri dish.. Subsequently, the

Figure 1. (a) Schematic illustration for the formation of GN/NaTi2(PO4)3/GN film electrode. (b) Digital photographs of as-fabricated fabricated GN/NaTi2(PO4)3/GN film and corresponding electrode disks.



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NaTi2(PO4)3/GN/GO film was reduced by 55 wt% HI aqueous solution to form the flexible GN/NaTi2(PO4)3/GN film. Figure 1b shows the digital photographs of the as-fabricated GN/NaTi2(PO4)3/GN film. It can be clearly seen that the GN/NaTi2(PO4)3/GN flexible film synthesized by our method shows high flexibility and can be directly cut into the disks of ≈ 1.5 cm2 as the working electrodes for SIBs. Figure 2a shows X-ray diffraction (XRD) patterns of GN/NaTi2(PO4)3/GN film and NaTi2(PO4)3 powder. For the GN/NaTi2(PO4)3/GN film, all the diffraction peaks in the XRD pattern (red curve) could be assigned to standard NaTi2(PO4)3 phase (JCPDS card no. 01-0852265) without detecting any impurity phases. This result confirms the existence of pure NASICON-type NaTi2(PO4)3 phase in the resultant flexible film. However, the typical diffraction peaks belonging to GN (2θ = 26°, (002) planes) are not observed, which may be due to the low content of GN and the high crystallinity of NaTi2(PO4)3 in this hybrid film. The asprepared pristine NaTi2(PO4)3 powder has also been confirmed to belong to pure NaTi2(PO4)3 phase (JCPDS card no. 01-085-2265) (black curve in Figure. 2a). The presence of GN in GN/NaTi2(PO4)3/GN film is confirmed by its Raman spectrum (Figure 2b). Specifically, two obvious peaks at 1330 and 1590 cm-1 can be assigned to typical D and G bands of GN in this hybrid, respectively. Meanwhile, the intensity ratio (ID/IG) between the D- and G-bands is around 0.9, suggesting that the GO is partly reduced by the HI solution used in our experiments.51 This result also indicates that GN in GN/NaTi2(PO4)3/GN film has a relatively high degree of graphitization after the reduction by the HI solution, thus ensuring high conductivity of GN/NaTi2(PO4)3/GN film. Besides, the peaks in the range of 100-800 cm-1 can be attributed to the characteristic scattering peaks of NaTi2(PO4)3. Also, the XRD pattern of NaTi2(PO4)3/GN powder is shown in Figure S1 (Supporting Information) and its crystalline phase is confirmed to


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still be NaTi2(PO4)3. Figure 2c shows thermogravimetric analysis (TGA) curve of GN/NaTi2(PO4)3/GN film carried out in the temperatures ranging from 25 to 700 °C at an air atmosphere. The XRD pattern (Figure S2, Supporting Information) of the obtained power after the TGA test shows that all the diffraction peaks could be assigned to NaTi2(PO4)3 phase (JCPDS card no. 01-085-2265) without detecting any impurity phases, indicating that the NaTi2(PO4)3 phase keeps a fairly stable state during the TGA test. As shown in Figure 2c, the weight loss at the temperature below 100 oC corresponds to the water evaporation, while the following weight loss of 22.7 wt% between 100 and 550 °C can be attributed to the decomposition of GN. No weight loss is found when the temperature is above 550 oC, indicating that GN can be completely decomposed at the temperature higher than 550 oC and NaTi2(PO4)3 has a good thermal stability at the high temperature. Hence, the contents of GN and NaTi2(PO4)3

Instensity / a.u.

(300)

100

NaTi2(PO4)3

D G

Weight / %

(113)

(b)

GN/NaTi2(PO4)3/GN film (202) (024) (116)

(104) (110)

Intensity / a.u.

(012)

(a)

30

40

2q / degree

50

(d)

60

(e) P 2p

1064 1071 1078126 132 138 144 Binding energy / eV

Intensity / a.u.

Na 1s

400

800 1200 1600-1 2000 2400 Raman shift / cm

(f)

Ti 2p3/2 Ti 2p1/2

455

60 40

460 465 470 Binding energy / eV

0

100 200 300 400o 500 600 700 Temperature / C

(g)

O 1s

C 1s

Intensity / a.u.

20

22.7%

80

Intensity / a.u.

10

(c)

20

JCPDS card no. 01-085-2265

Intensity / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


528 532 536 Binding energy / eV

540 280

285 290 Binding energy / eV

295

Figure 2. (a) XRD patterns of GN/NaTi2(PO4)3/GN film and pristine NaTi2(PO4)3. (b) Raman spectrum of GN/NaTi2(PO4)3/GN film. (c) TGA curve of GN/NaTi2(PO4)3/GN. (d) Highresolution XPS spectra of GN/NaTi2(PO4)3/GN film: (d) Na 1s and P 2p, (e) Ti 2p, (f) O 1s and (g) C 1s. The green, blue, cyan and magenta lines represent fitted curve, the red lines represent base curve and the black dots represent original curve.



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in the GN/NaTi2(PO4)3/GN film are calculated to be 22.7 wt% and 77.3 wt%, respectively. The TGA of NaTi2(PO4)3/GN powder was also tested to calculated the content of GN in NaTi2(PO4)3/GN (Figure S3, Supporting Information). As can be seen, the content of GN in NaTi2(PO4)3/GN powder is 4.2 wt%. To further determine the surface chemical composition of GN/NaTi2(PO4)3/GN film, X-ray photoelectron spectroscopy (XPS) measurements were performed. The survey spectrum (Figure S4, Supporting Information) discloses that the asprepared flexible film was composed of Na, Ti, P, O and C species. Figure 2d shows the highresolution Na 1s and P 2p spectra, the binding energy at 1072.1 eV can be assigned to Na+ and the binding energy at 133.95 eV can be assigned to P5+ of PO43-. Figure 2e shows the highresolution Ti 2p spectrum of the GN/NaTi2(PO4)3/GN film, in which can exhibit two peaks with the binding energies at 459.1 and 465.2 eV belong to Ti 2p3/2 and Ti 2p1/2 of Ti4+ in an octahedral environment,45,52 respectively. Besides, for the high-resolution O 1s XPS spectrum (Figure 2f), the binding energy at 531.5 eV can be assigned to the typical lattice oxygen bonds (Ti-O) of NaTi2(PO4)3, while the binding energy at 533.2 eV corresponds to C-O bond. The peak of the C 1s XPS spectrum can be fitted to four peaks (Figure 2g). The main peak with the binding energy located at 284.1 eV can be assigned to the C-C bond of graphitic C, whereas the other three binding energies at 284.8, 285.6 and 288.8 eV are assigned to the oxygen-containing functional groups of C-O, C=O, and O-C=O, respectively.45,53 Furthermore, it can be seen that the XPS spectra of Na 1s and P 2p scatter more seriously, which can be due to the relatively low contents of Na (4.41 wt%) and P (17.8 wt%) compared to those of C 1s (22.7 wt%) and O 1s (36.7 wt%). The atomic ratio of Na and Ti elements is calculated to be 1:1.97 in the GN/NaTi2(PO4)3/GN film by inductively coupled plasma-mass spectrometry (ICP-MS) measurements, which is very close to the stoichiometry of the standard NaTi2(PO4)3 phase.


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The morphology and microstructure features of the GN/NaTi2(PO4)3/GN film were first characterized by field emission scanning electron microscopy (FESEM). Figure 3a displays a photograph of the GN/NaTi2(PO4)3/GN film treated by 55% HI solution. It can be clearly seen that this film has a macroscopic size and can be bent into arbitrary shape, demonstrating its well integrity and excellent flexibility. High-magnification FESEM images clearly present the ripples and wrinkles of the GN/NaTi2(PO4)3/GN film without any particles attached on the film surface (Figure 3b, c). This result also demonstrates that the NaTi2(PO4)3 nanocrystals are firmly enwrapped into the GN sheets, thus ensuring that the NaTi2(PO4)3 nanocrystals cannot break away the GN sheets during repeated cycling and keeping the film with well intact state. Figure 3d shows low magnification FESEM image of GN/NaTi2(PO4)3/GN film of ≈ 10 μm in thickness. Obviously, a relatively oriented assembly of GN sheets is observed. High-magnification FESEM image (Figure 3e) presents the existence of some macropores between two adjacent layers. The

Figure 3. (a) Digital photograph of the as-fabricated flexible GN/NaTi2(PO4)3/GN film. (b,c) FESEM images of the as-fabricated GN/NaTi2(PO4)3/GN film with different magnifications. (d-f) FESEM images of the cross section of the as-fabricated GN/NaTi2(PO4)3/GN film at different magnifications and positions.



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GN sheets in 3D network are highly interconnected together to prevent them from restacking and to maintain a highly porous framework. This kind of structure is particularly beneficial for rapid ion transport and good access between the electrolyte and the electrode for rechargeable batteries. FESEM image (Figure 3f) recorded on the GN sheets with rough surface discloses that ultrafine NaTi2(PO4)3 nanocrystals are embedded in the GN sheets, which is similar to the microstructure features of NaTi2(PO4)3/GN powder (Figure S5b, Supporting Information). Therefore, this flexible film results from following process, i.e. the NaTi2(PO4)3/GN hybrid is embedded in the interconnected 3D framework of the GN nanosheets to form the flexible GN/NaTi2(PO4)3/GN film. The FESEM image of pristine NaTi2(PO4)3 is also provided in Figure S5a (Supporting Information), which shows that pristine NaTi2(PO4)3 sample is composed of large aggregates, comparable to bulk NaTi2(PO4)3. To examine the detailed microstructure of the as-fabricated flexible GN/NaTi2(PO4)3/GN film, transmission electron microscopy (TEM) was further performed. Figure 4a,b show TEM images of the GN/NaTi2(PO4)3/GN film, which clearly reveal the presence of the GN sheets. The high-resolution TEM image (HRTEM) further discloses that the ultrafine NaTi2(PO4)3 nanocrystals with an average size of around 7 nm are homogeneously and firmly anchored on the GN sheets (Figure 4c). As we know, such a small size for any particles tends to aggregation if no any capping agent is surrounding their surface. Therefore, the intimate contact between the NaTi2(PO4)3 nanocrystals and the GN sheets plays an important role for preventing the aggregation of the ultrasmall NaTi2(PO4)3 nanocrystals, thus avoiding degradation of electrochemical performance of the GN/NaTi2(PO4)3/GN film. The marked lattice fringes in a typical HRTEM image are separated by ≈0.365 nm, which can be indexed to the (113) plane of NaTi2(PO4)3 (Figure 4d). The corresponding selected-area electron diffraction (SAED) pattern (the inset in Figure 4b) reveals


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Figure 4. (a,b)) TEM and (c,d) HRTEM images of GN/ GN/NaTi2(PO4)3/GN film.. The inset in Fig Figure 4bb is corresponding SEAD pattern. (e) EDS spectrum of GN/NaTi2(PO4)3/GN film. film (f) Element mapping images of GN/NaTi2(PO4)3/GN film. diffraction rings of NASICON-type type structure of NaTi2(PO4)3, which is also consistent with the XRD result. Moreover, the energy dispersive spectrometer (EDS) measurement further confirms the co-existence of Na, Ti, P, O and C elements in the GN/NaTi2(PO4)3/GN film (Figure 4e) and the surface-scanning element mapping mappings further reveal that these elements are homogeneously distributed, as shown in Figure 4f. The electrochemical performance of the as-fabricated flexible GN/NaTi2(PO4)3/GN film, as a flexible and binder-free anode in SIBs SIBs, was evaluated by using CR2025 coin half cells with sodium metal foil as the counter. The GN/NaTi2(PO4)3/GN film can be directly used as the working electrodes without any further manufacturing processes processes. Figure 5aa shows the voltage v profiles of GN/NaTi2(PO4)3/GN film electrode at different current densities in the voltage range of 1-3 V vs. Na+/Na. The GN/NaTi2(PO4)3/GN film electrode delivers high specific capacities of 136.7, 108.0, 99.0, and 93.0 mA h g-1 at 100, 300, 500, and 1000 mA g-1 respectively. respectively The



capacity of GN/NaTi2(PO4)3/GN film here is based on the mass of NaTi2(PO4)3. Figure 5b shows the rate performances of GN/NaTi2(PO4)3/GN, NaTi2(PO4)3/GN and NaTi2(PO4)3. The electrodes were tested at step-wise current densities from 100 to 1000 mA g-1 and then being reversed to 100 mA g-1. Obviously, the GN/NaTi2(PO4)3/GN film exhibits much higher specific capacity compared with NaTi2(PO4)3 and NaTi2(PO4)3/GN at each rate. And even after cycling at a higher 3.0

160 Discharge capacity / mAh g

Voltage / V

100

100 mA g

120

2.0 -1

100 mA g -1 300 mA g -1 500 mA g -1 1000 mA g

1.5 1.0

(b)

-1

(a)

2.5

0

30 60 90 120 -1 Specific capacity / mAh g

150

300

500

-1

1000

80 GN/NaTi2(PO4)3/GN film NaTi2(PO4)3/GN

40 0

NaTi2(PO4)3 0

20

40 60 Cycle number / n

80

100

2 Voltage / V

GN film

1 2 NaTi2(PO4)3

3 2

GN/NaTi2(PO4)3/GN film

-1

1 0

20 40 60 80 100 -1 Specific capacity / mAh g

120

140

150

(d)

100 GN/NaTi2(PO4)3/GN film

120

80 NaTi2(PO4)3/GN

90 60

40

30 0

20

G film

0

20


(e)

120

60

NaTi2(PO4)3

80

0 100

100

100 80

-1

500 mA g

80

60

60

40

40 20 0

20

GN/(NaTi2(PO4)3/GN film 0

200


800

0 1000

Coulombic efficiency / %

Discharge capacity / mAh g

-1

(c)


100th, 50th, 10th, 1st 3


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 35

Figure 5. (a) Voltage profiles of GN/NaTi2(PO4)3/GN film electrode at different current densities in the voltage range of 1-3 V vs. Na+/Na. (b) The rate performances of GN/NaTi2(PO4)3/GN film, NaTi2(PO4)3/GN and NaTi2(PO4)3 electrodes. (c) Voltage profiles of GN film, NaTi2(PO4)3 and GN/NaTi2(PO4)3/GN film electrodes at a current density of 100 mA g1 for the 1st, 10th, 50th and 100th . (d) The cycling performances of GN/NaTi2(PO4)3/GN film, NaTi2(PO4)3/GN, pristine NaTi2(PO4)3 and GN film electrodes. (e) The long-term cycling stability of GN/NaTi2(PO4)3/GN film electrode at 500 mA g-1.


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current density of 1000 mA g-1, a constant capacity of around 120.0 mA h g-1 is restored when the current density is reversed to 100 mA g−1, which is only a little lower than the original capacity at 100 mA g-1, indicative of remarkable electrochemical reversibility and robustness for GN/NaTi2(PO4)3/GN film. To highlight the excellent sodium storage performance of the GN/NaTi2(PO4)3/GN film, we also performed corresponding tests for pristine NaTi2(PO4)3 and pure GN film. Figure 5c shows the voltage profiles curves of the GN film, pristine NaTi2(PO4)3 and GN/NaTi2(PO4)3/GN film at a current density of 100 mA g-1 for the 1st, 10th, 50th and 100th cycles. Clearly, the 1st discharge and charge capacities of the GN/NaTi2(PO4)3/GN film electrode can reach 136.7 and 117.6 mA h g-1, respectively, and after 100 cycles, 88.7% of the initial discharge capacity and 99% of the initial charge capacity can be retained. In contrast, the pristine NaTi2(PO4)3 and GN film both have shorter discharge plateaus and lower specific capacity. Especially for the pristine NaTi2(PO4)3 electrode, it suffers very serious capacity fading, which is due to the fact that the large particles are easy to pulverize and agglomerae during the repeated Na+ insertion/extraction. The cycling performances of GN/NaTi2(PO4)3/GN film, NaTi2(PO4)3/GN, pristine NaTi2(PO4)3 and GN film electrodes at a current density of 100 mA g-1 are shown in Figure 5d. Obviously, compared to NaTi2(PO4)3/GN, pristine NaTi2(PO4)3 and GN film electrodes, GN/NaTi2(PO4)3/GN film electrode shows the best cycling stability and highest capacity. Specifically, GN/NaTi2(PO4)3/GN film electrode presents an initial discharge capacity of 136.7 mA h g-1, and 121.3 mA h g-1 can be retained after 100 cycles. However, the NaTi2(PO4)3 electrode shows 111.7 mA h g-1 of the initial discharge capacity and can maintain only 52.1% of the initial capacity after 100 cycles, whereas the GN film shows only 32.9 mAh g1

of the initial discharge capacity. For NaTi2(PO4)3/GN, the GN can suppress the pulverization

and agglomeration of NaTi2(PO4)3 particles to some extent, so NaTi2(PO4)3/GN presents better



cycling

stability

than

pristine

NaTi2(PO4)3.

The

Page 18 of 35

long-term

cycling

stability

of

GN/NaTi2(PO4)3/GN film electrode at 500 mA g-1 is shown in Figure 5e. The capacity retention rate is as high as 91% after 1000 cycles, exhibiting excellent cycling stability. The excellent rate performance and cycling stability of GN/NaTi2(PO4)3/GN film electrode for sodium-ion storage can attribute to our assembled technique. Specifically, the ultrafine NaTi2(PO4)3 nanocrystals were coated on the GN nanosheets to form the NaTi2(PO4)3/GN composites, and then embedded in the interconnected framework of the GN nanosheets to form the flexible GN/NaTi2(PO4)3/GN film with 3D conducting networks. The ultrafine NaTi2(PO4)3 nanocrystals coated on the GN nanosheets can reduce both the ionic and electronic transporting pathways and the excellent electrical conductivity of the GN nanosheets affords good electrical contact between the current collector and the active material, and low charge transfer resistance. Moreover, the NaTi2(PO4)3/GN composites embedded in the interconnected GN framework to form the flexible GN/NaTi2(PO4)3/GN film with 3D conducting networks facilitate electron transport. The flexible GN/NaTi2(PO4)3/GN film, as a promising free-standing and binder-free anode for SIBs, can not only construct efficient ionic and electronic transfer pathways, but also overcome the kinetic limitations. Therefore, the GN/NaTi2(PO4)3/GN film electrode delivers high reversible capacities and exhibits promising rate performance and excellent cycling stability for SIBs. To further evaluate the sodium storage ability of GN/NaTi2(PO4)3/GN film, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were measured by using the CR2025 coin half cells. Figure 6a shows the comparison of CV curves of GN/NaTi2(PO4)3/GN film and pristine NaTi2(PO4)3 electrodes at a scan rate of 0.5 mV s-1. It can be clearly seen that a pair of well-defined redox peaks appear on both electrodes, corresponding to the reversible transformation of Ti4+/Ti3+.42,48 The separation of the peak potentials for


Page 19 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


GN/NaTi2(PO4)3/GN film is 284 mV, obviously lower than that of pristine NaTi2(PO4)3 (333 mV), indicating that the GN/NaTi2(PO4)3/GN film has lower electrochemical polarization and higher reversibility due to the high conductivity of GN.54 Figure S6 (Supporting Information) shows the CV curves of GN/NaTi2(PO4)3/GN film electrode at a scan rate of 0.1 mV s-1. As can be seen, nearly all of the CV profiles overlap with each other, suggesting excellent electrochemical reversibility and superior charge balance between anodic and cathodic charge. The CV curves of GN/NaTi2(PO4)3/G film and pristine NaTi2(PO4)3 electrodes at different scan rates from 1 to 9 mV s-1 are shown in Figure 6b and Figure S7 (Supporting Information) to further evaluate the electrochemical sodiation/desodiation kinetics of NaTi2(PO4)3. The intensities of current peaks gradually increase with the increasing of the scan rate, and the relationship between peak current (Ip) and the square root of the scan rate (v1/2) for GN/NaTi2(PO4)3/GN film and pristine NaTi2(PO4)3 are also presented in Figure 6c. The linear relationship of Ip and v1/2 indicates the reaction of sodiation/desodiation is diffusion-controlled. Hence, the diffusion of Na+ kinetics of GN/NaTi2(PO4)3/GN film and pristine NaTi2(PO4)3 can be interpreted on the basis of classical Randles-Sevchik equation.55,56 Ip = 2.69×105×n3/2×A×DNa1/2×C0×ν1/2

(1)

Where Ip is the peak current, n is the charge-transfer number of the reaction, A is the electrode area, DNa is the diffusion coefficient of Na+, C0 is the concentration of Na+ in the electrode, and ν is the scanning rate. In our work, the electrodes of GN/NaTi2(PO4)3/GN film and pristine NaTi2(PO4)3 are selected with the same mass of active materials. So, the diffusion coefficient of DNa is in direct ratio with the Ip/ν1/2. It can be seen that the diffusion coeffcient DNa is determined by the slope in Figure 6c because the n, A, and Co values are constant for GN/NaTi2(PO4)3/GN



0.004 0.003

(a)

0.010 Epa

Current / A

Current / A

0.002 0.001 0.000

-0.001

-1

0.5 mV s

-0.002

(b)

0.005 0.000 -1

1 mV s -1 3 mV s -1 5 mV s -1 7 mV s -1 9 mV s

-0.005

Epc GN/NaTi2(PO4)3/GN film

-0.003

-0.010

NaTi2(PO4)3

-0.004 1.0

1.5 2.0 2.5+ Potential / V vs Na / Na

3.0

Z''/ O cm

-2

NaTi2(PO4)3

0.006

RL(QR)(CR)WC

80


60

NaTi2(PO4)3

40

Zw

Rct

20 0.002 1.5 1/2 2.0 -1 1/22.5 v / (mV s )

3.0

0

Rct

Rs

0.004

1.0

3.0

100


0.008

1.5 2.0 2.5+ Potential / V vs Na / Na

(d)

120

(c) 0.010


1.0

0.012

Ip/ A

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 35

Zw

0

40

o

o

45

45

80 120 -2 Z'/ O cm

160

Figure 6. (a) Comparison of CV curves of GN/NaTi2(PO4)3/GN film and NaTi2(PO4)3 electrodes at a scan rate of 0.5 mV s-1. The inset is corresponding data. (b) CV curves of GN/NaTi2(PO4)3/GN film at different scan rates. (c) Linear relationship of peak current (Ip) and the square root of the scan rate (v1/2) for GN/NaTi2(PO4)3/GN film and pristine NaTi2(PO4)3. (d) Electrochemical impedance spectra of GN/NaTi2(PO4)3/GN film and pristine NaTi2(PO4)3. film and pristine NaTi2(PO4)3. As expected, GN/NaTi2(PO4)3/GN film electrode shows larger diffusion coefficient of Na+ and the DNa values can be calculated to be 1.91×10-10 and 1.48×10-10 cm2 s-1 for GN/NaTi2(PO4)3/GN film and pristine NaTi2(PO4)3, respectively. These calculated values are similar to those of NaTi2(PO4)3@C,57 Na3V2(PO4)3/C58 and Li3V2(PO4)3/C59 hybrid materials due to the nature of the fast ionic conductor. The accelerated Na+ diffusion behavior can improve the sodiation/desodiation reaction kinetics and consequently lead to a higher rate performance

of

GN/NaTi2(PO4)3/GN

film.55

Figure

6d

shows

EIS

spectra

of

GN/NaTi2(PO4)3/GN film and pristine NaTi2(PO4)3. The EIS can provide the dynamic


Page 21 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


information of the electrode materials. The high frequency intercept at real axis (Z') refers to the equivalent series resistance (Rs), which mainly represents the resistance of the electrolyte solution. The semicircle in the middle frequency range indicates the charge transfer resistance (Rct). In the low frequency, a straight line with the slope of nearly 45° is clearly observed in Figure 6d, corresponding to the Warburg impedance (Zw) that represents the diffusion rate of sodium ion into the channel of electrode materials.60,61 Clearly, the semicircle of GN/NaTi2(PO4)3/GN film is much smaller than that of pristine NaTi2(PO4)3. From the fitted impedance parameters from Nyquist plots by ZSimplWin software, the Rs values are 1.09 W cm2

for GN/NaTi2(PO4)3/GN film and 2.43 W cm-2 for pristine NaTi2(PO4)3. The lower Rs for

GN/NaTi2(PO4)3/GN film can be attribute to the presence of the GN nanosheets that can afford excellent electrical contact between the active electrode materials and current collector. The Rct value of GN/NaTi2(PO4)3/GN film is 43.36 W cm-2, which is far smaller than the Rct of pristine NaTi2(PO4)3 (113.2 W cm-2), indicating the GN/NaTi2(PO4)3/GN film electrode has faster charge transfer. The Zw values are 10.1 W cm-2 for GN/NaTi2(PO4)3/GN film and 12.76 W cm-2 for pristine NaTi2(PO4)3, indicating a faster Na+ diffusion for GN/NaTi2(PO4)3/GN film electrode, which is in consistent with the higher Na+ diffusion coefficient calculated by Randles-Sevchik equation. Moreover, the analysis results of CV and EIS for GN/NaTi2(PO4)3/GN film and pristine NaTi2(PO4)3 are also in good agreement with the electrochemical performance analysis. To demonstrate the potential practical application of as-fabrication GN/NaTi2(PO4)3/GN film for SIBs anode, the full cell was assembled by coupling the GN/NaTi2(PO4)3/GN film anode with Na0.44MnO2 electrode as the cathode. The Na0.44MnO2 nanorods were synthesized by a freeze-dried method (see Experimental Section). Figure 7a shows XRD pattern of homemade Na0.44MnO2 and the inset is its FESEM image. As can be clearly seen, the as- prepared



-1

120 100

30 40 2q / degree

50

40 20 0 0

20

40 60 80 Cycle number / n

30 60 90 -1 120 Specific capacity / mAh g

100 120

(d) 100

120

1 st 2 nd 10 th 50 th 100 th

2.4 2.0 0

60

160


(c)

3.6

2.8

60 -1

20

80

80 -1

100 mA g

80

60 40

40 0 0

20 20


80


Na0.44MnO2

10

3.2

(b)

140

JCPDS card no.27-0750

4.0

160


Intensity / a.u.

(a)

Voltage / V

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 35

0 100

Figure 7. (a) XRD pattern of homemade Na0.44MnO2 and the inset is its FESEM image. (b) Cycling performance of the homemade Na0.44MnO2 at 100 mA g-1. (c) Voltage profiles of GN/NaTi2(PO4)3/GN film//Na0.44MnO2 electrode for the 1 st, 2 nd 10 th, 50 th and 100 th at a current density of 100 mA g-1 in the voltage range of 2-4 V; (d) Cycling performance of the full battery at 100 mA g-1. Na0.44MnO2 crystallizes well in the orthorhombic Na4Mn9O18 phase (JCPDS card no.27-0750). The FESEM image (the inset in Figure 7a) indicates that Na0.44MnO2 is composed of nanorod bundles and the nanorods have sizes of ~ 100 nm in diameter. The cycling performance of the homemade Na0.44MnO2 at 100 mA g-1 is shown in Figure 7b, which exhibits good cycling performance. According to Chiang’s report62 on assembling a full battery, the 1.2:1 rate of the cell capacity balance of GN/NaTi2(PO4)3/GN film anode and Na0.44MnO2 cathode is the optimization of the cell itself. Figure 7c shows the voltage profiles of GN/NaTi2(PO4)3/GN film//Na0.44MnO2 full sodium ion battery with the voltage window in the range of 2-4 V at 100


Page 23 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


mA g-1 after 1, 2, 10, 50 and 100 cycles. The initial discharge capacity is 114 mA h g-1 based on the anode mass, with an initial Coulombic oulombic efficiency (CE) as high as 82.3%, which is better than many reported studies.63-65 Figure 7d shows the cycling performance and CE of the as as-assembled full cell. It is evident that GN/NaTi NaTi2(PO4)3/GN film//Na0.44MnO2 full cell holds a good capacity retention of 82.8% after 100 cycles, and the CE increases to over 98% and maintains for the measured 100 cycles. From these results, we can see that the GN/ GN/NaTi NaTi2(PO4)3/GN film//Na0.44MnO2 full cell exhibits a better perform performance ance in terms of specific capacity, cycling performance and CE. Furthermore, to investigate the potential practical application of the asfabricated flexible GN/NaTi2(PO4)3/GN film for use as flexible electrodes, a flexible cell was assembled by coupling GN/NaTi2(PO4)3/GN film as the anode and Na0.44MnO2 film as the cathode, as illustrated in Figure 8a.. The full cell was tested to obtain the effect of bending on the

Figure 8. (a) Schematic illustration for flexible GN/NaTi2(PO4)3/GN film//Na0.44MnO2 full cell. (b) Digital picture for the flexible GN/NaTi2(PO4)3/GN film//Na0.44MnO2 full cell tested at flat and bent states, and the GN/NaTi NaTi2(PO4)3/GN film after cycling 100 cycles. (c) Cycling performance of the full battery under flat and bent states at 50 mA g-1. (d) Digital picture of full cell that lights a LED for the flatting and bending state.



Page 24 of 35

electrochemical performance (Figure 8c) and the bending angle is 180°. It is obvious that the capacity of the battery being bent at 180° has a little change compared with that of the original flat battery. Moreover, the flexible battery shows a good cycling stability under both flat and bent states. The cell can maintain 92.9% retention after the first 30 cycles under a flat state compared to the original capacity and 86% retention after another 40 cycles under a bent state. More importantly, when it recovers to the flat state, it still can keep an excellent stability. Moreover, the as-fabricated flexible GN/NaTi2(PO4)3/GN film can still keep in a well flexible state after cycling for 100 cycles shown in Figure 8b. Such a facile and practical fabrication process of GN/NaTi2(PO4)3/GN film might be an efficient route to design and fabricate other high-performance flexible anode materials for SIBs. To further prove its potential application in ﬂexible electronics, the as-fabricated GN/NaTi2(PO4)3/GN film//Na0.44MnO2 flexible full cell was also used to power a commercial red light-emitting diode (LED). Figure 8d shows the digital picture of the flexible GN/NaTi2(PO4)3/GN film//Na0.44MnO2 full cell tested at flat and bent states. It can be seen that a LED (2 V, 10 mW) can be easily lighted whether the battery is bended or not. These results further confirm the potentiality of the as-fabricated flexible GN/NaTi2(PO4)3/GN film as a promising flexible anode material for flexible SIBs. 4. CONCLUSION In summary, we have reported a simple yet facile method to prepare a flexible and binder-free electrode of GN/NaTi2(PO4)3/GN film with a sandwich structure for SIBs. In this structure, the NaTi2(PO4)3 ultrafine nanocrystals firmly embedded in the GN nanosheets can reduce both the ionic and electronic transporting pathways and at the same time the GN nanosheets act as good current collector with low charge transfer resistance. Moreover, the flexible GN/NaTi2(PO4)3/GN


Page 25 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


film resulting from the NaTi2(PO4)3/GN hybrid embedded homogenously in the interconnected framework of the GN nanosheets can offer 3D conducting networks that facilitate the electron transport. The flexible GN/NaTi2(PO4)3/GN film can be directly used as the working electrodes without any further manufacturing processes. The GN/NaTi2(PO4)3/GN film electrode synthesized by our method shows excellent sodium storage performance and that it can be assembled in flexible full sodium ion battery for practical application. Moreover, this strategy developed in our research can offer a general route for preparing other graphene-based flexible films. ASSOCIATED CONTENT Supporting Information XRD

patterns

of

NaTi2(PO4)3/GN

and

NaTi2(PO4)3

powder.

XPS

spectrum

of

GN/NaTi2(PO4)3/GN film. FESEM images of (a) pristine NaTi2(PO4)3 and (b) NaTi2(PO4)3/GN powder. TGA curves of CV curves of NaTi2(PO4)3/GN powder. Pristine NaTi2(PO4)3 electrode at different scan rates. The Supporting Information is available free of charge on the ACS. AUTHOR INFORMATION Corresponding Author *Fax: +86-10-81381360. Tel: +86-10-81381360. E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (21471016 and 21601014) and the 111 Project (B07012).



Page 26 of 35

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[9] Ko, H.; Kapadia, R.; Takei, K.; Takahashi, T.; Zhang, X. B.; Javey, A. Multifunctional, Flexible

Electronic

Systems

Based

on

Engineered

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For Table of Contents Use Only

Constructing Flexible and Binder Binder-Free NaTi2(PO4)3 Film Electrode with a Sandwich Structure by a Two-Step Graphene Hybridizing Strategy S as an Ultrastable Anode for Long-Life Long Sodium-Ion Ion Batteries Batter

Donglei Guo, Jinwen Qin, Chaozhen Zhang, and Minhua Cao*

NaTi2(PO4)3/GN film with a sandwich structure can be assembled The as-fabricated flexible GN/NaTi into flexible full sodium ion battery, which shows a good cycling stability both under flat and bent states.


Constructing Flexible and Binder-Free NaTi2(PO4)3 Film Electrode

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