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Nature Inspired Na2Ti3O7 Nanosheets Formed ThreeDimensional Micro-Flowers Architecture as a High-Performance Anode Material for Rechargeable Sodium Ion Batteries Shoaib Anwer, Yongxin Huang, Jia Liu, Jiajia Liu, Meng Xu, Ziheng Wang, Renjie Chen, Jiatao Zhang, and Feng Wu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b01519 • Publication Date (Web): 16 Mar 2017 Downloaded from http://pubs.acs.org on March 19, 2017

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ACS Applied Materials & Interfaces

Nature Inspired Na2Ti3O7 Nanosheets Formed Three-Dimensional Micro-Flowers Architecture as a High-Performance Anode Material for Rechargeable Sodium Ion Batteries Shoaib Anwer †, ‡, Yongxin Huang§, ‡, Jia liu†, Jiajia Liu†, Meng Xu†, Ziheng Wang§, Renjie Chen*,§,║, Jiatao Zhang*,†, Feng Wu§, ║ †

Beijing Key Laboratory of Construction-Tailorable Advanced Functional Materials and Green

Applications, School of Materials Science & Engineering, Beijing Institute of Technology, Beijing, 100081, China §

School of Materials Science & Engineering, Beijing Key Laboratory of Environmental Science

and Engineering, Beijing Institute of Technology, Beijing 100081, China ║

Collaborative Innovation Center of Electric Vehicles in Beijing, Beijing 100081, China

KEYWORDS: Sodium titanate; 2D nanosheets; 3D micro-flowers architecture; Sodium-ion batteries; Sodium ion battery anode

ABSTRACT: Low cycling stability and poor rate performance are two of the distinctive drawbacks of most electrode materials for sodium-ion batteries (SIBs). Here, inspired by natural flower structures, we take advantage of the 3D hierarchical flower-like stable microstructures formed by two-dimensional (2D) nanosheets to solve these problems. By precise control of the hydrothermal synthesis conditions, a novel three-dimensional (3D) flower-like architecture

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consisting of 2D Na2Ti3O7 nanosheets (Na-TNSs) has been successfully synthesized. The arbitrarily arranged but closely interlinked thin nanosheets in carnation-shaped 3D Na2Ti3O7 micro-flowers (Na-TMFs) originate a good network of electrically conductive paths in an electrode. Thus, Na-TMFs can get electrons from all directions and be fully utilized for sodium ion insertion and extraction reactions, which can improve sodium storage properties with enhanced rate capability and super cycling performance. Furthermore, the large specific surface area provides a high capacity, which can be ascribed to the pseudo-capacitance effect. The wettability of the electrolyte was also improved by the porous and crumpled structure. The remarkably improved cycling performance and rate capability of Na-TMFs make a captivating case for its development as an advanced anode material for sodium ion batteries.

1. INTRODUCTION

The growing interest of energy storage has endorsed the great attainment of lithium-ion batteries (LIBs) in portable electric devices and dispersed energy storage systems, which require higher charge/discharge rates and larger (scale) with a greater emphasis on the price, rate capacity, and safety of the batteries.1-2 However, the growing demand for LIBs drives an increase in the price of lithium due to its inadequate resources. For this reason, room temperature SIBs have attracted increasing attention for large-scale energy storage applications because of the natural abundance and low cost of sodium resources.3-4 Recently, extensive attention has been paid to exploring SIBs electrode materials.5-7 Many layered transition metal oxides have been suggested as cathode materials for SIBs, which has provoked wide interest.8-11 In addition, NASICON type Na3V2(PO4)3 electrode also exhibits efficient electrochemical properties as a superior cathode.12 However, only limited anode materials are available for SIBs,13-16 which has


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been considered to be one of the key challenges for the widespread application of SIBs. The large volume deformation during the insertion/extraction process of sodium ion leads to a poor cycling stability. Organic Na2C8H4O4 was also reported with a noticeable sodium storage capacity of ca. 250 mAh g−1, but with poor initial coulombic efficiency and electron conductivity.17 The TiO2-based nanomaterials have triggered great excitement due to its interesting structural characteristics and potential applications in energy conversion and storage.18-23 Since the leading report of titanate nanostructures via a wet chemical method.24 Titanium (Ti)-based compounds have been expansively explored and considered as promising anode materials for Na-ion batteries due to their low cost, wide abundance, and low toxicity.25-30 Of the finest studied systems, Layered Na2Ti3O7 was reported as the well-known for its lowest operating potential (about 0.3 V vs. Na+/Na) as an oxide anode but suffers from poor cycling performance.31-34 Nevertheless, the reported works have not been able to provide a high rate performance and long cycle life since the larger ionic radius of sodium ions makes their diffusion in these materials much more difficult compared to lithium ions. Therefore, to accommodate the sodium ions and allow reversible and rapid ion insertion and extraction for high rate performance are in high demand. 35 In particular, inspiration from nature is to be considered as a key model to designed new nanostructures, to overcome the limitation of current processes.36-38 The Na2Ti3O7 structure consists of zigzag layers of titanium oxygen octahedrons with sodium ions in the interlayer space that are easily exchanged. A combination of advantages of the layered and high surface area 3D microstructures is expected to enhance the structural stability and rate performance of the titanates. In this work, 3D Na-TMFs architecture, consisting of thin 2D Na-TNSs, has been synthesized by using a hydrothermal approach. These 3D microstructures with the large specific


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surface area and volume, with effective pore size, are beneficial for the fast Na-ion diffusion, resulting in superior rate and cycle performances. This as-prepared conduction additive-free electrode showed the excellent cycling stability and rate performance with negligible capacity fading. The enhanced surface-to-volume ratio and reduced scale of transport length for both mass and charge transport made this 3D Na-TMFs architecture a promising electrodes system for SIBs. 2. EXPERIMENTAL SECTION Analytical grade commercially available reagents were used as received without further purification. 2.1 Synthesis of Na-TMFs Na-TMFs used in this work were synthesized by using a solvothermal approach. In a typical synthesis, 1 mL of titanium precursor tetra butyl titanate was added in 10mL NaOH aqueous solution (0.3 M). After stirring at room temperature for 1 h, the resulting milky suspension was transferred into a Teflon-lined autoclave and heated at 180 °C for 24 h. Finally, the solid product was filtered, washed with water, and dried at 80 °C overnight to give the as-prepared Na-TMFs. The obtained white fluffy powder was then calcined from room temperature to 200 °C at a heating rate of 2 °C·min−1 for 4 h in an Ar atmosphere under a 30 mL·min−1 flow rate in a tube furnace to yield the final product. 2.2 Structural and physical characterization The detailed microstructures and surface morphologies of the 3D Na-TMFs were studied using a low-resolution transmission electron microscopy (TEM; JEOL JEM 1200EX working at


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100 kV) and high-resolution TEM (HRTEM; FEI Tecnai G2 F20 S-Twin working at 200 kV). Scanning electron microscopy (SEM) images were obtained using a Hitachi FE-SEM 4800. The crystallinity and phases of the product were determined by X-ray diffraction (XRD) in a Bruker D8 Discover diffractometer equipped with Cu Kα radiation (λ = 1.5418 Å). Raman spectra were recorded on a microscopic confocal Raman spectrometer (Horiba Jobin Yvon, LabRAM HR) with an excitation of 532 nm laser light. X-ray photoelectron spectroscopy (XPS) was carried on Perkin–Elmer PHI 5300 ESCA system (Mg Ka) at 250W under a vacuum better than 10-6 Pa. The binding energies of the samples were calibrated by taking the carbon 1s peak as a reference (284.6 eV). Nitrogen adsorption-desorption isotherms were measured with a Quanta Chrome Adsorption Instrument. The surface area was calculated by using the Brunauer–Emmett–Teller (BET) equation. The pore-size distribution (pore diameter, pore volume, and micropore surface area of the samples) was determined by the Barrett–Joyner–Halenda (BJH) method. 2.3 Electrochemical characterizations The electrochemical measurements of all prepared samples were carried out using CR2032type coin-cell assembled in an Ar-filled glove box. The working electrodes were prepared by mixing 80 wt% as-prepared active material Na2Ti3O7, 10 wt% conductive agents (acetylene black) and 10 wt% binder (poly (vinylidene fluoride) (PVDF) dissolved in N-methyl-2pyrrolidone (NMP) with 5 wt%). The resultant slurry was pasted onto the copper foil using a slicker and then dried in a vacuum oven at 80°C overnight. The active material loading amount of pole piece is approximately to 2 mg cm-2. Sodium metal pieces were used as counter and reference electrode. The glass microfiber (Whatman) saturated with electrolyte as the separator obstructs the electron transport between positive and negative electrodes. The electrolyte solution was 1 M NaClO4 dissolved in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC)


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with a volume ratio of 1:1, in which 5 vol% fluoroethylene carbonate (FEC) was added as an electrolyte additive. Cyclic voltammetry (CV) was carried out on a CHI 660 D electrochemical workstation between 0.01 and 2.5 V vs. Na+/Na with a scan rate of 0.1 mV s-1. For the electrochemical impedance spectroscopy (EIS), the frequency range was selected from 1×105 Hz to 0.01 Hz. The galvanostatic charge–discharge measurements were performed at different current densities in the voltage range from 0.01 to 2.5 V vs. Na+/Na. 3. RESULTS AND DISCUSSION 3.1 Morphology and structural Characterization of Na-TMFs Figure 1 provides scanning electron microscopy (SEM) images of the prepared sample. The samples were composed of uniform carnation-shaped flower-like particles (Figure S4) with diameters that range from 1.0 to 2 µm (Figure 1a). Figure 1b indicates that the well-defined NaTMFs present three-dimensional (3D) microstructures with large interface areas. These microflower structures are assembled with many sparsely and randomly arranged thin nanopetals (Figure 1c). A magnified image (Figure 1d) reveals that these nanopetals are actually 5 nm thin Na-TNSs, which is also confirmed by the HRTEM image of unitary flake nanosheet in Figure S1. These 5 nm thin Na-TNSs have a tendency to aggregate into large spherical architecture, it can be considered that there is a self-assembly of the Na-TNSs which is exhibiting a carnationflower like morphology. In general, the Na-TNSs growth process must experience nucleation and growth. The chemistry behind the specimen synthesis is that there are Ti-O-Na bonds in the NaOH treated sample. The hydrolysis of tetra butyl titanate in the presence of NaOH solution under stirring formed Na-TNSs and then further growth of Na-TNSs crystallites to microflower assembly are later swayed by the hydrothermal heat treatment. TEM image in Figure S2 shows


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uniform Na-TMFs structure at the large scale. These 3D microstructures are attractive because the internal void space can accommodate the volume variation during the charge-discharge processes and the thin nanosheets facilitate fast transport of sodium ion/electrons owing to the short diffusion distance. TEM image in Figure 2a confirms that the prepared Na-TMFs structures consist assembly of curved thin nanosheets to from the flower shape. The energy dispersive spectroscopy (EDS) under scanning TEM (STEM) mode was used to examine the composition of single curled Na-TNS. The EDS mapping images (Figure 2b) proves the homogeneous distribution of O, Na and Ti elements in the nanosheet. Moreover, the EDS spectrum of a single Na-TMF reinforces the existence of oxygen, sodium, and titanium elements (Figure S3). The bright-field HRTEM images in Figure 2c and Figure 2d clearly demonstrates the index lattice spacing of 0.20 nm and 0.82 nm, which are in good agreement with the lattice spacing of (020) and (001) planes of Na2Ti3O7 respectively. 39-40 The XRD pattern and the refinement results of the as-prepared Na-TMFs sample are described in Figure 2e, which is well indexed to the standard XRD trace of Na2Ti3O7 (JCPDS no. 31-1329).41-43 The characteristic peaks situated in the vicinity of 2θ=11°, 26°, and 30° correspond to the (001), (011) and (300) planes of a monoclinic phase formation with a space group P121. There are two different Na+ ions storage sites arranged in the edge-shared triple TiO6 octahedrons as shown in the inset image. One of them is Na-O bond in a nine-fold coordination. The other is located in a seven-fold coordination formed by trigonal prisms of oxygen atoms. Owing to credible low factors (Rp=9.42% and Rwp=11.86%) based on Na2Ti3O7 model, Rietveld refinement indicated that well-defined zigzag layers of (Ti3O7)2- provide a broad and stable sodium ions transport trajectory. The fitted lattice parameters are calculated as a=9.133 Å, b=3.806 Å, c=8.566 Å, α= 90.00°, β=101.57°, γ=90°, while the detailed information of crystal structure is listed in Table S1.


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N2 adsorption-desorption isotherm measurement was performed to investigate the surface and 3D microporous structures of the prepared sample. As shown in Figure 2f, N2 adsorptiondesorption isotherms of Na-TMFs can be identified as type IV isotherm curves. The isotherms present a hysteresis loop within a relative pressure P/P0 range of 0.41-1, suggesting the welldeveloped 3D porous structure. This conclusion is fully supported by the Barrett Joyner Halenda (BJH) pore size distribution curve (inset image in Figure 2f) obtained from the N2 isotherms, which shows pore sizes is mainly distributed in the range of 2-6 nm (micropores). According to the result, the specific Brunauer-Emmett-Teller (BET) surface area of Na-TMFs was found to be 109.7 m2g-1. Moreover, the total pore volume for Na-TMFs is 0.22 cm3g-1. The large surface area and porous structure not only guarantee adequately contact between electrode materials and electrolyte but also facilitate fast transport of sodium ion.44-45 The Raman spectrum collected from the Na-TMFs displays the peaks at about 120, 195, 278, 374, 468, 597, 708, 781, and 915 cm-1, as shown in Figure 3a. The Raman peaks at 195 and 278 results from the Na-O-Ti stretching vibrations and the Raman peak at 915 cm-1 are assigned to the stretching vibration of terminal short Ti-O bonds involving non-bridging oxygen atoms that are coordinated with Na ions. 46-47 The exact assignment of the bands in the Raman spectrum is very difficult for Na2Ti3O7 due to the lack of reports relating directly the Raman peaks to specific active modes of layered titanates. The elemental composition of Na-TMFs and the oxidation state of the elements were further assessed by XPS. Figure 3b shows the wide spectrum and Figure 3c shows the highresolution spectra of Ti (at 458.2 eV and 462.45 eV). The Ti2P3/2 peak is at around 458.2 eV, which is close to the Ti4+ peak in TiO2, suggesting that Ti is present predominantly as Ti4+. The peak observed at 1069.95 eV corresponds to Na 1s (Figure 3d), which confirm the presence of sodium in the lattice in a valence state of +1.48


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3.2 Electrochemical Performances The Figure 4a exhibits the superior electrochemical properties of Na-TMFs electrode measured at a low current density of 100 mA g-1. The specific discharge capacity can be maintained at 108.7 mAh g-1 after 500 cycles, and the coulombic efficiency is raised from 31.1% in the initial cycle to ~100% after just 20 cycles. It can be clearly seen that the capacity fading mainly observed in the first 100 cycles which may be caused by the gradual disappearance of irreversible reactions and structure optimization. The first charge and discharge process were presented in Figure 4b, which can be divided into three parts. The part I during the voltage range of 2.25 – 1.25 V delivered a specific capacity of 62.6 mAh g-1, which can be attributed to the pseudo-capacitance effect due to a large specific surface of 3D flower-like structure. This component of capacity gradually disappeared along with the increasing cycle number, indicating that the deposition of active materials in the electrode is partially reversible. The major irreversible capacity focused on part II, in the voltage range of 1.25 – 1.0 V, corresponding to the formation of solid electrolyte interphase (SEI) film on the interfaces between electrode and electrolyte. This part delivered a high capacity about 135.3 mAh g-1, which cannot be observed at the subsequent cycles (see Figure 4c). In the voltage range of 1.0 – 0.01 V, the discharge curve in part III shows an oblique line, indicating that Na+ ions inserted into the host structure of the asprepared Na2Ti3O7 electrode forming Na4Ti3O7. However, part III delivered a capacity of 452.7 mAh g-1, illustrating that both of pseudo-capacitance sodium ions storage and side reaction with electrolyte existed in this part. It needs to be emphasized that the irreversible capacity was caused by the two reasons, containing of electrolyte decomposition forming passivation layer and slight crystal structure change to adapt Na+ ions insertion. According to the charge-discharge curves for subsequent cycles, as shown in Figure 4c, it can be inferred that the insertion process


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of Na+ ions occurs below the voltage of 1.0 V, exhibiting a reversible capacity approximately to 120 mAh g-1. The polarization voltage between the charge and discharge voltage plateaus exhibited a constant value about 200 mV during the 500 cycles, which is propitious to the cycling stability. Figure S5a showed that the morphology of the Na-TMFs remained unchanged after the first cycle, while Figure S5b displayed the related HRTEM image. To reveal the morphology change in Na-TMFs the TEM (Figure S5c) and HRTEM (Figure S5d) images were also obtained after 100 cycles and it was observed that the whole particle morphology was slightly changed but crystallinity still well retained there as before. Furthermore, benefiting from the special 3D structure with a large specific surface area of 109.7 m2g-1 and reserved volume expansion space, the Na-TMFs electrode exhibited a good rate performance to various current densities (see Figure 4d). The as-prepared electrodes delivered discharge capacities of about 173.4, 121.3, 99.7, 84.6 and 73.8 mAh g-1 with current densities of 50, 100, 200, 400 and 800 mA g-1, respectively corresponding to 0.28 C (the theoretical capacity of Na2Ti3O7 is 177 mAh g1

corresponding to the two Na+ ions transfer), 0.56 C, 1.1 C, 2.2 C and 4.5 C. When the current

recovered to 50 mA g-1, the discharge capacity can still be kept at 148.8 mAh g-1 in the following cycles. Surprisingly, the coulombic efficiency of electrode remained at about 100 % during the process of current density increasing. There is a certain gap between the initial capacity and 110th capacity at the same current density of 50 mA g-1, which may be caused by the polarization voltage rise (see Figure 4e). Actually, the potential polarization became more and larger with the increasing rate. At a rate of 4.5 C, the starting values of the discharge and charge voltage profiles were 0.39 and 0.88 V, which are an obvious deviation from the set values (0.1 and 3.0 V). Nevertheless, the rate performance of the 3D Na-TMFs electrode is very prominent compared to the other Na2Ti3O7 nanostructure materials.40 In order to further demonstrate the excellent


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cycling stability and rate performance of the as-prepared Na2Ti3O7 electrode, the long cycle testing was carried out at a large current density of 400 mA g-1 (see Figure 4f). The electrode can remain at about 85 mAhg-1 after 1100 cycles, and the coulombic efficiency sustained at 100 % over the whole cycles range. Which is outstanding compared to the reported performance for the similar materials as indicated in Table. S2. Figure 5 shows the reasonable mechanism for the outstanding capability of bearing a fast sodiation and desodiation. The 3D flower-like microstructures formed by thin 2D nanosheets provide a large space for sodium ions adsorption and abundant channels for sodium ions insertion/extraction. Among them, the surface adsorption resulting in a pseudo-capacitance effect, which is similar to the operational principle of a supercapacitor, leads to a fast Na+ ions storage capability. Furthermore, the large specific surface area provides more pathways for Na+ ions to transfer into the particles. Therefore, the rich-interfaces between electrode and electrolyte show excellent kinetic performance. The further investigation of the electrochemical process of the Na-TMFs electrode was performed by the cyclic voltammetry curves as shown in Figure 6a. The test conditions were set to a voltage of 0.01 – 2.5 V at a scan rate of 0.1 mV s-1 for 5 cycles. In the first cathodic sweep, there are three reductive peaks located at 1.44, 0.98 and 0.11 V, respectively, and among them, only the second one disappeared in the following cycles. This irreversible peak corresponds to the formation of the SEI film caused by side reaction between the electrode materials and electrolyte. It can be speculated that the SEI film completes their growth in the initial cycle. The cathodic peak in the vicinity of 1.44 V has a symmetrical oxidation peak at 1.46 V. They belong to the pseudo-capacitance sodium ions storage process. However, they gradually disappear during the subsequent cycles, which can be ascribed to the surface passivation film effect. The cathodic peak at the low potential (0.11 V) is assigned to the


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intercalation reaction of the Na+ ions with the valence change of Ti4+/3+ couple, which decreases the sodium-ion coordination number from 9 and 7 in the pristine state to 6 after full intercalation with two Na+ ions.49 There is a corresponding oxidation peak at 0.57 V, which shows a reversible process of Na+ ions insertion/extraction. Afterward, this pair of redox peaks approached each other, the reductive peak at 0.25 V and the oxidative peak at 0.51 V, resulting in a lower polarization voltage compared to the first cycle. This phenomenon can be contacted to the kinetic activation and passivation film stabilization process. The good overlap between the second and fifth loops illustrates the highly reversible electrode reaction. The dynamical properties of the Na-TMFs electrode were explored by the typical Nyquist plots (see Figure 6b). The recorded data were interpreted using the equivalent circuit as depicted in the inset. The plot has two main parts, including a depressed semicircle in the high-frequency region and a sloping line in the low-frequency region. The former corresponds to the chargetransfer process (Rct) and the later corresponds to the semi-infinite Warburg diffusion process, respectively. Interestingly, the initial resistance before the cycle is 425.3 Ω, which is far greater than the resistance of the fifth cycle (~183.4 Ω). This dynamic activation process is consistent with the previous test results.31-32 It can be inferred that the higher surface energy and large surface area of the initial sample generated larger interface impedance. After the SEI film covering the surface, the surface energy of Na+ ions adsorption might have been reduced to a small value. In order to evaluate the Na+ ions insertion/extraction kinetics of Na-TMF electrode, the CV curves were carried out at different scan rates of 0.1, 0.2, 0.5, 1, 2 and 5 mV s-1 (Figure S6a). The symmetrical shape of CV curves was maintained with the increasing scan rates from 0.1 to 5 mV s-1. This phenomenon illustrated the Na-TMF electrode exhibited fast Na+ ions insertion/extraction kinetics, which can be ascribed to the rich interfaces and nanoscale size. The


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linear relation between logarithms of scan rates and peak currents can be observed in Figure S6b. In terms of experience, the slope approaches 0.5, indicating a diffusion-controlled process, while the slope is close to 1, indicating a surface capacitance-dominated process. Therefore, the high slope values for both the cathodic (0.893) and anodic (0.835) illustrated a favorable capacitive kinetics of Na-TMF.

4. CONCLUSIONS In summary, inspired by nature flowers structures, carnation-shaped Na-TMFs architecture consisting of 2D nanosheets has been magnificently synthesized by a hydrothermal process. The appropriate microscopy characterizations and BET measurements have confirmed the 3D flowerlike structure and large specific surface area of Na2Ti3O7 particles. The open structure and nanoarchitecture of the Na-TMFs with 3D morphology provide more channels via thin nanosheets and shorter paths for easy diffusion of the Na ions and good wettability of the electrolyte, ensuring its outstanding rate performance. Furthermore, the microflowers structure also increased the specific surface area and the contact area with the electrolyte. Such novel 3D architecture greatly facilitates the electron/ion transport kinetics of Na2Ti3O7 and guarantees the electrode structure integrity, leading to an excellent electrochemical performance in terms of high rate capability and long cycle-life. Electrochemical tests showed that at a large current density of 400 mA g-1 the discharge capacity of Na-TMFs electrode still can remain at about 85 mAh g-1 after 1100 cycles with 80% retention of the initial charge capacity, and the coulombic efficiency sustained at 100 % over the whole cycles range, outperforming the cycling stability of the reported Na2Ti3O7 anodes. A significant result is that, even at a high current density of 800 mA g-1, the discharge capacity is still maintained at 73.8 mAh g-1. All of the results disclose that


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the as-prepared Na-TMFs with unique morphology and novel architecture have the potential to be used as anode materials in large-scale applications for rechargeable SIBs at low cost while maintaining excellent performance. ASSOCIATED CONTENT Supporting Information. HRTEM image highlighting the nanosheet thickness of the Na-TMF, TEM image at large scale, and EDS spectrum of the Na-TMF. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected], [email protected] Author Contributions All authors have given approval to the final version of the manuscript. ‡These authors contributed equally to this work. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Grant No. 21322105, 51631001, 91323301, 51372025, 51501010), the National Key Research and Development Program (2016YFB0901501), Major achievements Transformation Project for


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Central

University

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Beijing

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Beijing

Science

and

Technology

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(D151100003015001).

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Figure 1. (a and b) Low magnification SEM images of as-prepared carnation-shaped Na-TMFs; (c and d) Higher magnification SEM images highlighting the 3D architecture and unitary nanosheet thickness of the Na-TMFs.


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(e)

Figure 2. (a) TEM image of Na-TMFs; (b) Scanning transmission electron microscopy (STEM) highangle annular dark field (HAADF) image of as-prepared curled Na-TNS with EDS mapping of O, Na, and Ti elements, implying uniformly elemental distribution; (c and d) HRTEM images of Na-TNS with lattice spacing; (e) XRD pattern with refinement results of as-prepared Na-TMFs sample; (red dots) observed; (black curve) calculated; (blue curve) difference plot; (green bars) Bragg positions. Inset is the crystal structure of Na2Ti3O7 sample; (f) N2 adsorption-desorption isotherms of Na-TMFs.


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Figure 3. (a) Raman spectrum; (b) XPS full scan survey spectrum; (c) high-resolution spectrum of Ti 2p; (d) high-resolution spectrum of Na 1s regions of as-prepared Na-TMFs.


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Figure 4. (a) Specific capacity and coulombic efficiency of Na-TMFs during the chargedischarge process; (b) first charge and discharge curves and (c) continuous discharge and charge curves of Na-TMFs measured at a current density of 100 mA g-1. Specific capacity and coulombic efficiency (d) and charge-discharge curves (e) of Na-TMFs measured at different current densities; (f) the long cycle performance of Na-TMFs sample measured at a current density of 400 mA g-1.


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Figure 5. Schematic illustration of Na+ ions storage process in the Na-TMFs electrode.

Figure 6. (a) CV curves of Na-TMFs conducted at a scan rate of 0.1 mV s-1 between voltage window of 0.01 – 2.5 V; (b) Nyquist impedance plots and corresponding fitting curves.


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