Hollow bismuth ferrite combined graphene as advanced anode material for sodium-ion batteries

Xuli Ding,Yi Liu

a Department of Physics,School of Science,Jiangsu University of Science and Technology,Zhenjiang,Jiangsu,212003,China

b Shanghai Synchrotron Radiation Facility,Shanghai Institute of Applied Physics,Chinese Academy of Science,239 Zhangheng Road,Pudong New Area Shanghai,201204,China

ABSTRACT Hollow bismuth ferrite nanofibers were fabricated via simple electrospinning process,and the storage properties of Na-ions were investigated by atmospheric pressure X-ray photoelectron spectrum (APXPS) and Synchrotron Radiation.The results show that the hollow bismuth ferrite nanotubes demonstrate impressive sodium storage properties and good cycling stability that the specific capacity can exceed 500 mA h g-1 and the cycle number reaches several hundred cycles.The graphene-coated bismuth ferrite nanotubes exhibit a specific capacity of 600 mAhg-1and a 89% capacity retention after 200 cycles.The present strategy can be a significant step to fabricate hollow perovskite oxides and serve as sodium ion batteries.

Keywords:Bismuth ferrite Perovskite Hollow Sodium-ion battery Anode

1.Introduction

Hollow nano-structured perovskite oxides can be promising for various applications,such as sodium ion batteries due to their great potential in energy conversion and storage systems [1-5].New electrode materials with high energy densities are expected to offer opportunities for both fundamental nature studies and commercial practical outcome.Bismuth ferrite (BiFeO3,BFO) with a perovskitetypeABO3structure has a simple component but rich physical properties,such as ferroelectric [6,7],piezoelectric [8,9],thermoelectric[10,11],and photoelectric [12-15],is a potential candidate of energy devices for applications.However,compared with its multiferroic properties,BFO has few reports on its energy storage properties,especially in sodium-ion batteries.Inspired by the capability to store 770 mA h g-1Li for lithium-ion batteries [16],as well as the similar electrochemical behavior between Na and Li,it is deserved to explore the BFO as anode materials for NIBs.Additionally,a more clear physical and chemical picture about the electrochemical Na ions storage mechanism in BFO is not achievable despite Li storage mechanism in some oxides has been proposed in the past decades [17-20].

A deep understanding of the electrochemical properties of perovskite oxides is of great importance for both fundamental science of energy storage and technological applications.The oxidation and reduction of oxides with Na,the alloying reaction of Bi-Na also generate large volume change,in most cases resulting in the cracks and capacity decay of active species.Since hollow nanostructure with high aspect ratio can shorten the charges shift distance in active materials [21,22],here BFO with hollow nano-sized tube-shape structure was synthesized using the coaxial electrospining method.The electrochemical behavior and the mechanism for sodium ions store in BFO as anode for SIBs were investigated.The microstructure and phase transformation at various discharge and charge voltage plateau were revealed using in situ synchrotron x-ray diffraction and atmospheric pressure X-ray photoelectron spectrum (APXPS).The present work also clarify the revolution of microstructure and the variation of bonding structure for BFO during Na ions insertion and desertion process.

2.Experimental

2.1.Synthesis of BiFeO3 nanotube

Bismuth Ferrite nanotubes were obtained by the electrospinning during which materials of 2.425 g bismuth nitrate(Bi(NO3)3∙5H2O,Alfa Aesar) and 2.02 giron nitrate (Fe(NO3)3∙9H2O,Alfa Aesar) were weighed in 1:1 M ratio and dissolved in 10 ml N,N-Dimethyl propionamid (DMF,Sigma Aldich) to obtain suspension solution.After the solution became transparent,1.0 g PVP(99+%,Mv=130,000,Sigma Aldich) was added into the 10 ml DMF,with continuous stirring until the suspended solution returned transparent and used as precursor for the following electrospinning.Paralleled,the PMMA (Sigma Aldich)solution (concentration 20-30% in DMF) was prepared for the formation of inner core during spinning.The mixture solution was pumped at a speed of 0.6 ml h-1with diplopore needle at a voltage of 18kVand a distance of 15 cm.All the electrospining were conducted at room temperature under a relative humidity of<30%.

The obtained nanofibers were preheated at 100 °C for 2 h to stabilize the fiber-shaped structure,and then heated up to 350 °C in a heating rate of 1°C/min,after holding 30 min at 350 °C,and then calcinations at 550°C for 3 h with a heating rate of 5°C/min under air atmosphere.During the calcinations process,the PVP and PMMA were burned offin the air; and the hollow BFO nanotubes were obtained.In order to obtain the BFO/Graphene (BFO/Gra) composite,the synthesized BFO was mixed using ultrasound with graphene with 10:1 mass ratio,and then centrifugation and dry to obtain the final BFO/Gra product for the following electrode slurry preparation.

Fig.3a specifies a schematic fabrication process for the hollow BFO nanofibers,and the BFO crystal cell structure is shown in Fig.3b.The synthesis details are illustrated in the experimental section as descript previously.Firstly,a coaxial BFO nanofiber is fabricated through the electrospinning method,secondly,a hollow tube structure is generated via the calcinations process to evaporate the inner PMMA under the air environment,finally the BFO/Gra composite is produced via the ultrasonic mixture following centrifugation and drying.

2.2.Characterization

Morphologies were characterized using transmission electron microscopy(TEM) and electron diffraction spectrum (EDS) performed on field-emission JEOL 2011 at an acceleration voltage of 200 KV.The phase evolution was identified via the in situ XRD at wavelength of 1.2398 Å (Shanghai Synchrotron Radiation Facility).The in situ XRD data were collected and analyzed based on LaB6calibration.A cell with 2 mm widow on both shell sides can allow the X-rays to penetrate through the active materials.For comparison,the ex situ XRD was also captured using the knocked-down cells at different discharge-charge platforms.The photoelectron spectroscopy near atmospheric pressure was performed via the soft X-ray at BL02B (Shanghai Synchrotron Radiation).Raman spectrum was measured via the FRA 106/S at the excitation wavelength of 632.8 nm.

2.3.Electrochemical measurement

Half-cell (CR2032) tests were carried out using two-electrode coin cells with Na metal as the counter electrode in an argon-filled glove box.The processing parameters were current densities in range of 50-500 mA g-1,the voltage range of 0.01-3.0V vs Na/Na+.The electrolyte was 1 mol l-1NaPF6solution,in mixed solvent of ethylene carbonate (EC) and dimethyl carbonate (DMC)(EC:DMC=1:1) with 5%fluoroethylene carbonate (FEC)additive.The separator was Whatman GF/F,and the current collector was copper foil (Aladdin).

The LANHE (Wuhan,China) measurement instrument was adopted to carry out the electrochemical characterization.The electrode was constitute of active materials:Super P:binder=8:1:1,the mass loading on the copper collector was around 0.5 mg cm-2.Electrochemical cycling of BFO electrode was executed using same procedure,at different current densities varied from 30 to 500 mA g-1.Electrochemical impedance spectroscopy (EIS) measurements were conducted at an AC voltage of 10 mV amplitude and DC open circuit voltage in the frequency range of 0.01Hz-1.0 MHz at room temperature using the DH7000 workstation.The specific capacity of the composite is calculated based on the mass of BFO and BFO/Gra,respectively.

3.Results and discussion

Fig.1a-b shown the TEM images for the synthesized BFO nanotube,as shown,vimineous and continuous hollow tubes are produced without the inner cores (PMMA) during the calcinations process.Pores are also generated together with the volatilization of PMMA.The diameter of the inner core is about 30 nm,and the overall diameter is 90 ± 5 nm with a shell 60 ± 5 nm (BFO).

The high-magnified TEM image in Fig.1c shows the distinguishable lattice stripes corresponding to (100),(111),and (311) planes of the obtained BFO.These planes in the selected area electrons diffusion(SAED) patterns have been provided in the inset of Fig.1c,as shown bright spots with regular arrange.The lattice stripes with the SAED diffraction patterns reveal the crystal nature of the obtained samples.Fig.1d-f is the SEM-EDS element mapping results for the Bi,Fe and O,respectively.These elements distribute uniformly in the selected area.The TEM images in Fig.2afor the BFO/Gra composite shows that graphene is adhered on the surface of BFO,and BFO is packaged by graphene at the edges,as also indicated in the HRTEM image (Fig.2b).

The XRD patterns of the synthesized BFO nanofibers are displayed in Fig.2c.All XRD diffraction peaks are consistent with the standard JCPDS card data ofICSD#20372.The peaks centered sharply at 22°,32°,39.5°,and the typical characteristic peaks of BFO located at 46°,52°,and 57ocorresponding to the planes of(100),(110),(111),(200),(210)and (211),respectively.

The Raman spectra in the frequency region between 50 and 2000 cm-1is displayed in.

Fig.2d and the inset provides the wave number range from 100 to 600 cm-1.As indicated,four intense peaks centered at 135,160,210 and 470 cm-1,These strong peaks can because by the longitudinal-optical (LO) photos mode in the ABO3-type perovskite.The weak peaks centered at 252 and 370 cm-1are attributed to the transverse-optical(TO) photos [23-26].It can infers that the covalent character impact more than the ionic oxides nature in the BFO nonmaterial according to the Born effective charge theory through the different photos modes[26],which facilitates the ions exchange and speeds the charge transfer during charge-discharge process.

Fig.4a shows the charge-discharge curves of the as-prepared BFO/Gra in a range of 0.01-3.0V at 50 mA g-1.The composite delivers an initial discharge capacity of 600 mA h g-1for the initial cycles,and then descends to 562 mA h g-1after 200 cycles.Stable voltage plateaus located at 0.6V and 0.4V during the discharge process,while the charge plateaus are appeared at 0.8V and 0.6V.

At different cycle numbers of 1st,5th,50th and 100th cycles in Fig.4b,both discharge and charge curves show similar features of voltage plateaus.Additionally,similar discharge-charge plateau variation is also observed at different current density varied from 50 to 500 mA h g-1that indicates good stability of the BFO/Gra anode even under increased current.The rate capacity of the BFO/Gra is exhibited in Fig.4c,as the current density is increased from 50,100,150,200,300,400,and 500 mA g-1,the specific capacity of fabricated anode is 600,520,480,420,370,350,and 320 mA h g-1,respectively.When the current density is adjusted to 150 mA g-1again,the specific capacity recovered to 450 mAh g-1as well,this indicates good reversibility of the fabricated electrode.

In addition,the rate capacity for the bare BFO anode is compared,as indicated in Fig.4c,the rate capacity of the bare BFO is inferior to the BFO/Gra.In the cycle measurement,the specific capacity of the BFO/Gra is 597 mAh g-1in the first cycle,and the capacity decreases to 532 mAh g-1after 200 cycles,while the bare BFO shows an initial discharge specific capacity of 580 mA h g-1,and the capacity is decayed to 460 mA h g-1after 200 cycles (compared in Fig.4d).

Fig.5a shows the cyclic voltammetry (CV) curves from the first to the fifth cycles of the BFO electrode.In the first discharge process,there are one broad band from 1.3V to 0.7V corresponding to the electrolyte decompose and one peak at 0.3V corresponding to Na-Bi alloy formation.In the following cycles,two pairs of redox peaks appear on each cycle,which indicates two steps redox reactions during the chargedischarge process.From the second to fifth cycles,when the voltage varies from 3.0V to 0.6V and even to 0.3V,NaBi phase appears firstly and then some of the phase is transformed into Na3Bi phase.When the anode is completely discharged to 0.05 V,the Na3Bi phase is the main phase existed in the active material.In the charging process,the Na3Bi phase is transformed gradually and vanishes in the end,while the NaBi phase reappeared at 0.7V and a small peak at 0.75V for the Bi phase also arise.As a consequence,the reaction mechanism can be speculated that as Na is.

Initially inserted into the BFO active species,Bi elements will be first reduced from the BFO with the formation of Na2O,when the voltage is further decreased,two steps of alloying process between Na and Bi occur,and the detailed content will be further discussed later by in situ and ex situ XRD measurement from Shanghai synchrotron radiation.

The impedance spectroscopy is a relatively powerful method to examine electrochemical behavior and electrical properties of electrode,to characterize the interface properties between active species and electrolyte,and to investigate the dynamics of mobile charges in the bulk and interfacial region in the solid,solid-liquid interface.Fig.5b,the Nyquist plot consists of a suppressed semicircle in the high frequencies and a diagonal line in the low frequencies region.The former is related to the interface process caused by the ions conduction and charge transfer between electrolyte and active materials [27-30].The latter is related to the diffusion processes of ions in active materials[28,29,31].It is observed that the semicircle radius of BFO/Gra is reduced compared with the bare BFO sample,indicating the improved electric conduction due to the presence of Gra.While in the low frequency region,the straight-line slope is decreased with the increased cycles,which is mainly ascribed to the SEI interface formation and active species phase change with the Na ions insertion and desertion.

The XPS samples were fixed by double-sided adhesive tape and transferred to the vacuum chamber.The spectra of the Bi4f,C1s,O1s,Fe2f,and Na1slevels were recorded (Fig.6a).The peaks of Bi4f5/2and Bi4f7/2are shifted toward higher binding energy as the discharge voltage is decreased from 0.6V to 0.4V,while in the charge process all the Bi4f(Bi4f5/2,Bi4f7/2) peaks are returned or close to the initial discharge stages in Fig.6b.The detailed information for peaks position and shift at different discharge and charge voltage stages are compared in Fig.6c,d,and .f,respectively.The binding energy of the Bi+3and Bi+5are 159.75eV and 164.57eV separately,and varied at different charge-discharge states,which indicates that Bi species carry different binding energy at varied discharge-charge levels.

From the fitting results as shown in Fig.6c-f,the different binding energy for BFO at different discharge-charge status are clearly visible,as the discharge voltage is decreased from 0.6V to 0.4V,the main peaks position are shifted towards to lower binding energy accompanied by the advent of side peaks,indicating new phases appear with the sodiation process.In the charge process,the main peak centered at 159.5 eV is returned as charge voltage reverses to 0.6V again,and side peaks are appeared as the voltage is increased to 3.0V,more research works are still needed for in-depth exploration.

Fig.7 shows the in-situ measurement results at diffraction cycles for the BFO samples at discharging stages of 0.3 and 0.5V in the wake of Na ions desertion.The diffraction peaks shift towards higher diffraction angles when anode is discharged from 0.5V to 0.3V.It can also be deduced that the crystal lattice constant will slight decreased companying the desertion of Na ions;new Bi phase appears in situ XRD patterns and the diffraction circles.The BFO perovskite structure is not changed since the insertion of Na ions only replace the Bi element,and Bi atoms will remove outside the structure cell,so the perovskite structure is kept without destroying as Na ions insertion and desertion in BFO.

Fig.8a gives the change of discharge-charge curve of the BFO relative to the time evolution,where discharge platform centered at 0.6V,0.4V and charge platform at 0.6V,0.8V.The XRD patterns at these discharge-charge stages corresponding to Fig.8a are shown in Fig.8b.the different diffraction peaks are appeared in the different dischargecharge states,indicting the different phases are formed at different sodiation states.The peaks at 27°,38°,and 39° for Bi phase arise when the anode is discharged to 0.6V,despite the NaBi phase appears at peak position of 18°.At further discharge to 0.4V,the peaks of Na3Bi appear and a new peak is found at 44° corresponded to the Fe element.In contrast to the discharge state,as charged to 0.4V,the peak intensity of Na3Bi is apparent decreased compared to the original discharged state.While charged up to 3.0V,the patterns returned nearly to the original state without obvious change.All the diffraction peaks at different voltage platforms correspond to phase of NaBi,Na3Bi,and Bi,but the phases for Fe2O3and Bi2O3are not detected.

These findings demonstrate that,Bi metals will be first restored from the unit cell and the Na3FeO3is formed when Na is initially inserted in to the BFO species,.Based on the above ex-situ XRD results and the CV plots(Fig.5a),the reaction involved during the discharge process is expressed as following:

The two peaks in the CV curve centered at 0.7V and 0.2V are corresponding to the two steps of alloying process between Na and Bi.As a consequence,the reaction corresponding to the two steps alloying process can be expressed as follows:

Beyond the XRD patterns after electrochemical test,it is inferred that the perovskite structure has not been destroyed after Na insertion and extraction and the anodes still maintain the original perovskite structure that allows the Bi element to sodiation and resodiation reversibly.More importantly,the Bi elements play an important role for the charges transfer in the sodium ions storage process,while the Fe elements responsible for stabilizing the BFO lattice structure.Different from the previous reports [32,33],the BFO has different storage properties compared to other perovskite oxides and the recent report fluoride perovskites [34],since the coaxial core-shell nanofibers outperform and provide superiority to achieving unique properties that are hard to obtain from the common substance.In most cases,there are multiple diffusion paths in polycrystalline ceramic materials (inside body DL,grain boundary DBand free surface DS),but the Ds dominate largely relative to the other two in the hollow nanostructure.According to the first law of Adolf Fick[35],the ions diffusion flux is proportional to the diffusion coefficient,especially in hollow circular body; ions diffusion can be expressed by the following equation:

in which,J is the diffusion flux,Ds is the free surface diffusion coeffi-cient,andis the mass concentration gradient.In the hollow nanotubes structure,the free surface diffusion coefficient Ds is larger than that of solid core under the same mass gradient.As a consequence,the fabricated hollow BFO nanofibers have good charges diffusion coeffi-cient compared to the non-hollow solid structure; correspondingly,sodium ions in the fabricated anodes can have good ions diffusion velocity,which stimulate the electrochemical activity of the active materials.In terms of the good electrochemical performance,the hollow BFO nanofibers can be a potential candidate for the next generation high specific energy low-cost SIBs.

4.Conclusions

Hollow bismuth ferrite nanofibers have been fabricated via a simple electrospinning method are evidenced by morphology,structural and electrochemical investigations.The entire phase evolution of BFO as a function of the voltage variation during the discharge-charge process is investigated with the help of ex-situ XRD measurement.The results demonstrate that the hollow bismuth ferrite has good sodium storage properties for sodium ion batteries.Used as anode materials,the BFO/Gracan deliver a～600 mA h g-1specific capacity and stable 200 cycles with 89%capacity retention.The phase transition during the dischargecharge process and the Na ions storage mechanism are revealed via insitu and ex-situ synchrotron XRD patterns.The present work can contribute to the synthesis of hollow nanostructure oxides,and the application of perovskite oxides as electrodes in sodium ion batteries.

Declaration of competing interest

There are no conflicts of interest to declare.

Acknowledgments

The work was supported by the National Natural Science Foundation of China (Grants No.11874282 and 11604245),and Jiangsu Province Six Big Talent Peak -High-level Talents (XYN-074)and the project of Scientific Research Start Project from Jiangsu University of Science and Technology(Grant No.1052931707).

Appendix A.Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.pnsc.2020.01.016.E