简介概要

Co₂B₂O₅ as an anode material with high capacity for sodium ion batteries

来源期刊：Rare Metals2020年第9期

论文作者：Han Chen Bei-Bei Xu Qiu-Shi Ping Bao-Zhu Wu Xi-Kai Wu Qiang-Qiang Zhuang Hao-Li Wang Bao-Feng Wang

文章页码：1045 - 1052

摘要：In this work,a flake-structured Co₂B₂O₅ material was obtained by a simple sol-gel method and researched for use in sodium ion batteries firstly.When serving as anode material for sodium ion batteries,it exhibits the high initial reversible capacity of 466 mAh·g^-1 at a current density of 100 mA·g^-1.Through the recombination of carbon nanotubes（CNTs）,the composite Co₂B₂O₅/CNTs delivers the initial reversible capacity of 464 mAh·g^-1,and324 mAh·g^-1 is obtained after 60 cycles under the current density of 100 mA·g^-1.When under the current density of 1000 mA·g^-1,a capacity of 236 mAh·g^-1 is obtained for Co₂B₂O₅/CNTs while 160 mAh·g^-1 for Co₂B₂O₅.Moreover,the sodium storage behavior of Co₂B₂O₅ is identified by kinetic analysis.The higher Na+capacitive contribution of Co₂B₂O₅/CNTs could account for the enhanced rate performance.The results indicate that Co₂B₂O₅ is a promising anode material for sodium ion batteries.

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稀有金属(英文版) 2020,39(09),1045-1052

Co₂B₂O₅ as an anode material with high capacity for sodium ion batteries

Han Chen Bei-Bei Xu Qiu-Shi Ping Bao-Zhu Wu Xi-Kai Wu Qiang-Qiang Zhuang Hao-Li Wang Bao-Feng Wang

Shanghai Key Laboratory of Materials Protection and Advanced Materials in Electric Power,Shanghai University of Electric Power

作者简介：*Bao-Feng Wang e-mail:wangbaofeng@shiep.edu.cn;

收稿日期：17 November 2019

基金：financially supported by the National Natural Science Foundation of China (No.21673136);the Science and Technology Commission of Shanghai Municipality (No.19DZ2271100);

Co₂B₂O₅ as an anode material with high capacity for sodium ion batteries

Han Chen Bei-Bei Xu Qiu-Shi Ping Bao-Zhu Wu Xi-Kai Wu Qiang-Qiang Zhuang Hao-Li Wang Bao-Feng Wang

Shanghai Key Laboratory of Materials Protection and Advanced Materials in Electric Power,Shanghai University of Electric Power

Abstract：

In this work,a flake-structured Co₂B₂O₅ material was obtained by a simple sol-gel method and researched for use in sodium ion batteries firstly.When serving as anode material for sodium ion batteries,it exhibits the high initial reversible capacity of 466 mAh·g^-1 at a current density of 100 mA·g^-1.Through the recombination of carbon nanotubes(CNTs),the composite Co₂B₂O₅/CNTs delivers the initial reversible capacity of 464 mAh·g^-1,and324 mAh·g^-1 is obtained after 60 cycles under the current density of 100 mA·g^-1.When under the current density of 1000 mA·g^-1,a capacity of 236 mAh·g^-1 is obtained for Co₂B₂O₅/CNTs while 160 mAh·g^-1 for Co₂B₂O₅.Moreover,the sodium storage behavior of Co₂B₂O₅ is identified by kinetic analysis.The higher Na+capacitive contribution of Co₂B₂O₅/CNTs could account for the enhanced rate performance.The results indicate that Co₂B₂O₅ is a promising anode material for sodium ion batteries.

Keyword：

Co₂B₂O₅; Carbon nanotubes; Anode material; Sodium ion batteries;

Received： 17 November 2019

1 Introduction

A series of problems caused by the widespread use of fossil fuels,such as resource depletion,environmental pollution and global warming,have received increasing attention.It is urgent to develop cleaner energy sources such as solar,wind and nuclear energy [ 1, 2, 3, 4] .However,these energy sources are intermittent,and large-scale energy storage system (ESS) is essential.Among various energy storage systems,electrochemical energy storage device is regarded as a promising method for its flexibility,high energy density and pollution-free operation [ 5, 6, 7, 8] .Rechargeable lithium ion batteries (LIBs),featured with high energy densities,low self-discharge,environmental benignity and no memory effect,have been studied as the main candidates for ESS [ 9, 10, 11, 12] .The high cost and limited lithium resources in the earth,however,hamper the application of them in the market.Owing to the unlimited distribution of sodium resources and similar intercalation mechanism to LIBs,sodium ion batteries (SIBs) are increasingly studied as the next generation of secondary batteries [ 13, 14, 15, 16] .The graphite,which has been successfully used to anode material for LIB s,is not suitable for the insertion of sodium ions because of its large sodium ion radius.Therefore,it is significant to search for suitable novel anode materials for SIBs [ 17, 18] .

Polyanion-type inorganic compounds attract much interest due to their tunable operating voltage [ 19, 20] .Among many poly anion-type inorganic compounds,borate anode materials have been studied for LIBs because of their lightweight and environmental friendliness [ 21] .To date,various borate materials such as Co₂B₂O₅ [ 22] ,Ni₃(B0₃)2 [ 23] ,Fe₃B0₆ [ 24] ,FeBO₃ [ 25] ,VBO₃ [ 26] and LiZnBO₃ [ 27] have been reported as anode materials for rechargeable LIBs.Similarly,there have been some reports on the using of borate anode materials for SIBs in recent years.In 2017,Fe₃B0₆ material was reported for anode of SIBs for the first time,and it shows the high reversible capacity of 504.2 mAh·g^-1 at 100 mA·g^-1 and outstanding rate performance (249 mAh·g^-1 at 8000 mA·g^-1) [ 28] .Ni₃(BO₃)2 was first reported for SIBs anode material by Xu et al. [ 29] ,and it delivers the initial reversible capacity of428.9 mAh·g^-1,and 366.4 mAh·g^-1 could be maintained after 50 cycles.The above-mentioned borate anode materials have high initial capacity and excellent rate performance.However,their cycle stability needs to be improved.Within the methods studied,carbon coating is an effective way for enhancing the electrochemical performance,especially cycling stability.Mesh-like LiZnBO₃/C anode material was reported for LIBs.It demonstrates high initial capacity of 860 mAh·g^-1 and good cycle stability.The good cycling stability could be ascribed to mesh-like morphology of carbon and good conductivity of composite [ 27] .In 2018,N-C@Zn₃B₂O₆ was reported as the potential anode material for sodium ion batteries,and it delivers a high reversible charge capacity of 446.2 mAh·g^-1.When it was used in full cells,the discharge capacity of 98.4mAh·g-1 is obtained at 1000 mA·g^-1 and the retained discharge capacity is 43.6 mAh·g^-1 after 300 cycles.The increased sodium storage abilities of N-C@Zn₃B₂O₆ could be put down to the flake morphologies and the addition of N-doped carbon [ 30] .Co₂B₂O₅ anode material has shown the high capacity of 1048 mAh·g^-1 and suitable platform for LIBs [ 22] ,but has rarely been reported in SIBs.

In this paper,Co₂B₂O₅ anode material was synthesized via sol-gel method and applied to SIBs for the first time.The high initial discharge capacity of 715 mAh·g^-1 was obtained for Co₂B₂O₅.Through compositing the carbon nanotubes (CNTs),cycle stability and rate performance of Co₂B₂O₅ are significantly enhanced.CNTs could be used as the conductive network of materials due to their good electrical conductivity.Moreover,the cause of enhanced rate performance was discussed by kinetic analysis.

2 Experimental

2.1 Materials synthesis

2.1.1 Synthesis of Co2B2O5

Co₂B₂O₅ material was first prepared via sol-gel method.Co(CH₃COO)2·4H₂O and H₃BO₃ were dissolved in deionized water in a molar ratio of 2:3 (50%of boron being in excess) under the magnetic stirring for 20 min at 90℃.Next,citric acid (5:1 molar ratio of citric acid to Co(CH₃COO)2·4H₂O) was added to the solution while stirring until the gel was formed.The gel was carbonized at250℃for 3 h in an oven to obtain the fluffy foam precursor.Then,the precursor was ground for several minutes and sintered at 850℃for 4 h under the air.Finally,excess boron was removed by washing to obtain pure Co₂B₂O₅.

2.1.2 Synthesis of Co2B2O5/CNTs

In a typical synthesis,50 mg of CNTs was treated with acid and dispersion dispersed into 80 ml of deionized water with ultrasonic processing for 30 min.Then,1.0 g of prepared Co₂B₂O₅ material was added into the solution under magnetic stirring and at room temperature for 12 h.Asobtained sediment was washed with deionized water and subsequently dried in an oven at 80℃overnight.

2.2 Materials characterization

The structure of Co₂B₂O₅ material was measured by X-ray diffraction (XRD,RINT 2200,Rigaku) with Cu Kαradiation ranging from 10°to 70°.X-ray photoelectron spectroscopy (XPS) spectrum was recorded by Thermo ESCALAB 250XI.The morphology,size and structure of the Co₂B₂O₅ material were characterized by field-emission scanning electron microscope (FESEM,JEM-7800F),transmission electron microscope (TEM,JEM-21 00F,JEOL) and high-resolution TEM (HRTEM,JEM-2100F,JEOL).

Fig.1 a XRD patterns and b crystal structure of Co₂B₂O₅

2.3 Electrochemical measurements

The electrochemical behaviors of Co₂B₂O₅ and Co₂B₂O₅/CNTs materials were examined by using CR2016-type coin cells.The Co₂B₂O₅ active material was mixed with acetylene black and sodium carboxyl methyl cellulose (CMC)binder in a weight ratio of 80:10:10 to form slurry,and then the slurry was coated on copper foil uniformly.The electrolyte was 1 mol·L^-1 NaClO₄ dissolved in a mixture of ethylene carbonate and diethyl carbonate (EC/DEC,1:1 in volume) and 5 vol%fluorinated ethylene carbonate (FEC).Galvanostatic discharge and charge test was performed on a Land CT2001A battery tester,with cutoff potentials of0.01 V for discharge and 3.00 V for charge.Cyclic voltammetry (CV) measurements and electrochemical impedance spectroscopy (EIS) were carried out on an Autolab PGSTAT302.

3 Results and discussion

The crystal phase of Co₂B₂O₅ was further measured by XRD.As shown in Fig.1a,all the diffraction peaks of Co₂B₂O₅ could be indexed to the triclinic structure of Co₂B₂O₅ which belongs to P-1 (2) space group (JCPDS No.73-1772).The crystal structure diagram of Co₂B₂O₅ is described in Fig.1b.It reveals that one B atom is coordinated by three O atoms and forms the BO₃ triangle.The average space available per oxygen atom,given by the quotient between the volume of the unit cell and the number per cell of oxygen atoms,is 0.0163 nm³ [ 31] .In addition,the resulting Co0₆ octahedral structure is consisted of one Co atom and six oxygen atoms.

In order to study elementary composition and chemical states of the elements in Co₂B₂O₅ material,especially Co element,XPS analysis was performed for Co₂B₂O₅material.The wide-scan XPS spectrum shows the presence of Co,B,O and C in Co₂B₂O₅ (Fig.2a).Figure 2b presents XPS spectrum of Co 2p,and it can be fitted into four main peaks at 781.2,785.9,797.1 and 802.9 eV,which are attributed to the palent cobalt 2p1/2 and 2p3/2 spin orbits and their corresponding satellite peaks [ 32, 33, 34] .The peak located at 191.6 eV corresponds to B-O in B 1s spectrum(Fig.2c).The O 1s spectrum of Co₂B₂O₅ material is shown in Fig.2d,and the peaks located at 532.0,and 530.9 eV correspond to B-O and Co-O,respectively [ 35, 36] .

Fig.2 a Wide-scan XPS spectrum of Co₂B₂O₅;XPS spectra of b Co 2p,c B 1s and d O 1s of Co₂B₂O₅

Fig.3 a,b FESEM images of Co₂B₂O₅;c TEM and d HRTEM images of Co₂B₂O₅;e STEM image of Co₂B₂O₅ and EDS mappings of f Co,g B and h O

The morphology and structure of Co₂B₂O₅ material were characterized by FESEM and TEM.As shown in Fig.3a,b,the prepared Co₂B₂O₅ material has a flake structure with a size of 1-3μm and a thickness of 100 nm.This flake structure could also be confirmed by TEM image(Fig.3c).To accurately investigate the microstructure of the Co₂B₂O₅ material,HRTEM image (Fig.3d) exhibits the lattice fringes with the spacing of 0.41 nm,which corresponds to (011) plane of Co₂B₂O_5.Furthermore,the elementary composition of Co₂B₂O₅ is evidenced through EDS mappings,as shown in Fig.3e-h;Co,B and O are uniformly distributed in micron flake,and the resolution of B mapping image is not very high due to the lightweight of B.

Figure 4a shows the initial three cycles CV curves of the as-prepared Co₂B₂O₅ material at 0.2 mV·s^-1.During the first reduction process,there are two obvious peaks positioned at 0.01 and 0.30 V,and this can be assigned to structural transformation and the formation of SEI layer [ 29, 37] .In the oxidation process,the peak is positioned at1.8 V.In the following cycles,the reduction peak is located at a relatively high potential of 0.80 V,and in oxidation process,a weak peak is located at 1.90 V.The first three galvanostatic charge/discharge curves of Co₂B₂O₅ are displayed in Fig.4b.There are also two platforms in the first cycle discharge curve,which are located at 0.50 and0.25 V,respectively,while the discharge plateau of following cycles is about 1.00 V.Those results are similar to the CV results.

Figure 4c displays the cycling performance of Co₂B₂O₅and Co₂B₂O₅/CNTs at 100 mA·g^-1.Co₂B₂O₅ shows the initial charge capacity of 466 mAh·g^-1 and the coulombic efficiency of the first cycle of 62.9%.The huge capacity loss could be assigned to the structural transformation,and the specific reasons are still needed to be studied.After 60cycles,the charge capacity is only about 170 mAh·g^-1 with the capacity retention ratio of 37.4%.Through the addition of CNTs,the cycle stability of Co₂B₂O₅ is improved.It delivers initial charge capacity of 464 mAh·g^-1 with the capacity retention ratio of 70.1%after 60 cycles.Figure 4d shows the rate capability of Co₂B₂O₅ and Co₂B₂O₅/CNTs with current densities from 100 to 1000 mA·g^-1.The Co₂B₂O₅ material acquires reversible capacities of 435,372,308,246 and 160 mAh·g^-1 at 100,200,400,800 and1000 mA·g^-1,while those of Co₂B₂O₅/CNTs are 446,376,309,248 and 236 mAh·g^-1 at 100,200,400,800 and1000 mA·g^-1,respectively.When the current density returns to 200 mA·g^-1,the charge capacity of Co₂B₂O₅/CNTs still reaches 328 mA·h·g-1 compared to277 mA·h·g^-1 of Co₂B₂O₅.Obviously,the rate performance of Co₂B₂O₅/CNTs material is much better than that of Co₂B₂O₅.

Fig.4 a CV curves of Co₂B₂O₅;b galvanostatic charge-discharge curves of Co₂B₂O₅ in initial three cycles;c cycle performance of Co₂B₂O₅and Co₂B₂O₅/CNTs;d rate capability of Co₂B₂O₅ and Co₂B₂O₅/CNTs at different current densities

Fig.5 Electrochemical impedance spectra of Co₂B₂O₅ and Co₂B₂O₅/CNTs (inset being equivalent circuit)

To explore the kinetic behavior of the Co₂B₂O₅ material,the EIS (0.01 Hz-100 kHz) curves of Co₂B₂O₅ and Co₂B₂O₅/CNTs electrodes are shown in Fig.5,where Z'is the real part of the impedance Z and Z"is the imaginary part.The Nyquist profiles of two materials are similar,which both consist of a semicircle in medium-/high-frequency region and the straight line in the low-frequency region.In the equivalent circuit diagram as fitted,R_s,R_ct,Zo and CPE represent the solution resistance,charge transfer resistance,cotangent hyperbolic and constant phase angle element,respectively [ 38, 39, 40] .According to the fitting data,the R_ct values of Co₂B₂O₅/CNTs electrode(244Ω) are lower than those of Co₂B₂O₅ electrode (514Q).It reveals that the charge transfer resistance of material is reduced due to the addition of CNTs,and the CNTs endow the Co₂B₂O₅/CNTs composite with improved cycle and rate performance.

Fig.6 CV curves of a Co₂B₂O₅ and b Co₂B₂O₅/CNTs at various scan rates;logarithmn peak current versus logarithm scan rate plots of c Co₂B₂O₅ and d Co₂B₂O₅/CNTs at different potentials;CV curves of e Co₂B₂O₅ and f Co₂B₂O₅/CNTs with capacitive fraction shown by shaded area at a scan rate of 1.0 mV·s^-1;ratio of capacitive contribution of g Co₂B₂O₅ and h Co₂B₂O₅/CNTs at different scan rates

Kinetic analysis based on CV at different scan rates from 0.2 to 1.0 mV·s^-1 was used to explain the enhanced rate performance of Co₂B₂O₅/CNTs.In Fig.6a,b,every CV curve in each cycle presents similar shape with four redox peaks and that the current density increases with the increase in scan rates.The relationship between the peak current (i) and sweep rate (v) of the CV curves is dominated by the equation below [ 29, 36, 41, 42, 43] :

wnere a ana b are aajustable parameters,respectively.ror the value of b,if b=0.5,the charge/discharge process is dominated by typical diffusion-controlled process [ 43, 44] .If b=1.0,the charge/discharge process shows capacitivecontrolled process [ 29, 45] .Figure 6c,d shows the plots of lgi versus lgv for 1-4 peaks of Co₂B₂O₅ and Co₂B₂O₅/CNTs,and the values of b are 0.58,0.85,0.61 and 0.64 for Co₂B₂O₅,0.60,0.90,0.75 and 0.94 for Co₂B₂O₅/CNTs.It indicates that Co₂B₂O₅/CNTs electrode shows a more favored capacitive kinetics than Co₂B₂O₅.Therefore,in order to calculate the ratios of capacitive contribution more accurately,the equation can be rewritten as [ 41, 46] :

where k₁v,k₂v^1/2 represent the surface capacitive effects and Na⁺diffusion-controlled processes,respectively.Eq.(2) could also be converted into:

The values of k₁ (slope) and k₂ (intercept) could be calculated through plotting lgi versus lgv at different potentials,and the ratio of the capacitive contribution at different scan rates could be determined.From Fig.6e,f,the contribution from capacitive Na⁺storage in Co₂B₂O₅/CNTs electrode is 78.6%at 1.0 mV·s^-1,which is higher than that of Co₂B₂O₅ (68.5%).In addition,with the increases in scan rates from 0.2 to 1.0 mV·s^-1,the capacitive contribution of Co₂B₂O₅ gradually rises from47.3%to 68.5%(Fig.6g),and that of Co₂B₂O₅/CNTs rises from 58.8%to 78.6%(Fig.6h).

4 Conclusion

In summary,a flake-structured Co₂B₂O₅ material was successfully prepared via sol-gel method and firstly used as anode material for SIBs,showing the high initial discharge capacity of 715 mAh·g^-1 and reversible charge capacity of466 mAh·g^-1.The rate performance and cycling stability of Co₂B₂O₅ can be improved by compounding with CNTs.The capacity retention ratio of Co₂B₂O₅/CNTs is 70.1%after 60 cycles at 100 mA·g^-1,which is much higher than that (37.4%) of Co₂B₂O₅.The reversible capacity of Co₂B₂O₅/CNTs is 236 mAh·g^-1 at high current density of1000 mA·g^-1,while it is only 160 mAh·g^-1 for Co₂B₂O₅.The EIS data indicate that Co₂B₂O₅/CNTs composite has lower charge transfer resistance,which is the reason for the improvement of electrochemical performance of Co₂B₂O₅.Kinetic analysis reveals that the capacitance is identified as a major energy storage mechanism for Co₂B₂O₅,and it also indicates the better rate performance of Co₂B₂O₅/CNTs.The favorable capacity and potential utility indicate that Co₂B₂O₅ has a great prospect in sodium ion batteries as a novel anode material.

Acknowledgements This work was financially supported by the National Natural Science Foundation of China (No.21673136) and the Science and Technology Commission of Shanghai Municipality(No.19DZ2271100).