一种新的流变相法制备锂离子电池纳米_LiVOPO_4正极材料_英文_

熊利芝1,2

何则强1,2,*

(1吉首大学生物资源与环境科学学院,湖南吉首

416000;

2

中南大学化学化工学院,长沙410083)

摘要：采用新型流变相法制备锂离子电池正极材料纳米-LiVOPO 4.采用X 射线衍射、扫描电子显微镜以及电化学测试等手段对LiVOPO 4的微观结构、表面形貌和电化学性能进行了表征.结果表明,采用流变相法制备的LiVOPO 4由粒径大约在10-60nm 的小颗粒组成.首次放电容量,首次充电容量以及库仑效率分别为135.7mAh

·g -1,145.8mAh ·g -1和93.0%.0.1C (1C =160mA ·g -1)放电时,60次循环后,放电容量保持在134.2mAh ·g -1,为首次放电容量的98.9%,平均每次循环的容量损失仅为0.018%.而1.0C 和2.0C 放电时的放电容量达到0.1C 放电容量的96.5%和91.6%.随着放电次数的增加,电荷转移阻抗增加,而锂离子在电极中的扩散系数达到10-11cm 2·s -1数量级.实验结果显示采用流变相法制备的LiVOPO 4是一种容量高、循环性能好、倍率性能好的锂离子电池正极材料.关键词：锂离子电池;流变相法;

LiVOPO 4;

倍率性能;

扩散系数

中图分类号：O6

A New Rheological Phase Route to Synthesize Nano -LiVOPO 4Cathode Material for Lithium Ion Batteries

XIONG Li -Zhi 1,2

HE Ze -Qiang 1,2,*

(1College of Biology and Environmental Sciences,Jishou University,Jishou 416000,Hunan Province,P.R.China ;2

College of Chemistry and Chemical Engineering,Central South University,Changsha 410083,P.R.China )Abstract ：A novel lithium -ion battery cathode material,nano -LiVOPO 4,was synthesized by a new rheological phase method.The microstructure,surface morphology,and electrochemical properties were characterized by various electrochemical methods in combination with X -ray diffraction (XRD)and scanning electron microscopy (SEM).Results show that the orthorhombic LiVOPO 4,obtained by this rheological phase method,is made up of 10-60nm particles.The first discharge capacity,charge capacity,and columbic efficiency of LiVOPO 4were found to be 135.7mAh ·g -1,

145.8mAh

·g -1,and 93.0%,respectively.After 60cycles,the discharge capacity remained 134.2mAh ·g -1,at 98.9%of the first discharge capacity,and the capacity loss per cycle was only 0.018%at 0.1C (1C =160mA ·g -1).More than 96.5%and 91.6%of the discharge capacity at 0.1C were maintained at 1.0C and 2.0C ,respectively.The charge transfer resistance increased with the increase of the cycle number and the diffusion coefficient of lithium ion in the nano -LiVOPO 4was in the order of 10-11cm 2·s -1.Experimental results suggest that the rheological phase method is a good route for the synthesis of LiVOPO 4cathode material of high capacity,good cycling performance,and good current rate capability for lithium ion batteries.Key Words ：Lithium ion battery;

Rheological phase method;

LiVOPO 4;

Current rate capability;

Diffusion

coefficient

[Article]

www.whxb.pku.edu.cn

物理化学学报(Wuli Huaxue Xuebao )

Acta Phys.-Chim.Sin .,2010,26(3)：573-577

March Received:October 18,2009;Revised:December 16,2009;Published on Web:January 13,2010.

*

Corresponding author.Email:csuhzq@163.com;Tel:+86-743-8563911.

The project was supported by the National Natural Science Foundation of China (20873054),Natural Science Foundation of Hunan Province,China (07JJ3014),Postdoctoral Science Foundation of China (2005037700),Scientific Research Fund of Hunan Provincial Education Department,China (07A058),and Postdoctoral Science Foundation of Central South University,China (2004107).国家自然科学基金(20873054)、湖南省自然科学基金(07JJ3014)、中国博士后科学基金(2005037700)、湖南省教育厅科研项目(07A058)和中南大学博士后科学基金(2004107)资助

鬁Editorial office of Acta Physico -Chimica Sinica

573

Recently,performance of mobile electronic devices,such as mobile phone or laptop computer,is drastically improving and so,the demands for battery become more severe.Due to its large power density and cycle stability,lithium ion battery is now widely used for the electric source of mobile equipment.The current most important requirement for lithium ion rechargeable battery is to decrease cost and increase the power density.In the current battery,LiCoO2and graphitic carbon are commonly used for cathode and anode,respectively.However,natural abun-dance of Co is limited and this element is expensive.Therefore, development of cathode material without containing Co is strongly required.At present,great attentions are paid for tansi-tion metal phosphates,such as LiMPO4(M=Fe,Mn,Co)[1-4], Li3V2(PO4)3[5-10],and LiVPO4F[11-12],as a new class of cathode ma-terials for lithium ion batteries.These materials contain both mo-bile lithium ions and redox-active transition metals within a rigid phosphate network,and display remarkable electrochemical,and thermal stabilities as well as comparable energy density.Among these materials,LiFePO4is of great interest for the replacement of LiCoO2in Li ion batteries due to its low cost,nontoxicity and good electrochemical properties since1997[1,13-17].However,com-pared with LiFePO4,LiVOPO4has an advantage of higher poten-tial(4.0and3.7V(versus Li/Li+))for charge and discharge, and this phosphate is highly interesting from the viewpoint of the alternative cathode[18-21].Kerr et al.[22]presented that the tri-c linic phase LiVOPO4synthesized fromε-VOPO4showed the capacity of100mAh·g-1up to100cycles at C/10of current rate. Azmi et al.[19,23]reported that orthorhombic phase of LiVOPO4 could be synthesized by impregnation method and exhibited fairly good cycle stability for Li de-intercalation and intercala-tion.

For all functional materials,their properties were greatly in-fluenced by the synthesis methods.Many preparation methods have been investigated with an aim to achieve high capacity LiVOPO4,however,the capacity of the products ever reported is usually unsatisfactory in particular when discharged at a high current rate.To meet high power demands of lithium ion batteries in new applications,the rate capability of LiVOPO4has to be sig-nificantly improved.There are two main frequently employed strategies:one is to increase the intrinsic electronic conductivity by microstructure controlling,the other is to enhance lithium ion transport by reducing the bulk diffusion length,which can be achieved by utilization of nanostructured materials.

The rheological phase method is the process of preparing compounds or materials from a solid-liquid rheological mixture. That is,the solid reactants are fully mixed in a proper molar ratio, and made up by a proper amount of water or other solvents to form a Bingham body in which the solid particles and liquid substance are uniformly distributed,so that the product can be obtained under suitable experiment conditions[24].Because of its low temperature,short calcination time,and products with small particle with uniform distribution,rheological method has been used to synthesize cathode and anode materials for lithium ion batteries[25-26].In the present study,rheological technique is used to synthesize nano-LiVOPO4.The microstructure and electro-chemical properties of LiVOPO4as cathode material for lithium ion batteries were studied.

1Experimental

Analytical grade powders of LiOH·2H2O(AR),NH4VO3(AR), (NH4)2HPO4(AR)and citric acid(AR)with equal amount of substance were mixed uniformly to get a mixture.Then1.5mL distilled water per gram mixture was added to the mixture under magnetic force stirring to obtain a mash.The mash was dried in vacuum at80℃for4h to form the precursor.The precursor was calcined in Ar atmosphere at650℃for6h to obtain blue LiVOPO4powders.

Phase identification and surface morphology studies of the samples were carried out by an X-ray diffractometer(XRD;D/ MAX-gA,Rigaku Corporation,Japan)with Cu Kαradiation and scanning electron microscope(SEM;JSM5600LV,JEOL Ltd., Japan,accelerating voltage of20kV).Elemental analyses for lithium,vanadium,and phosphorus were determined by atomic absorption spectroscopy(AAS;SP-3530AA)and inductively cou-pled plasma-atomic emission spectrometer(ICP;TY9900).

A slurry containing80%(mass fraction,similarly hereinafter) LiVOPO4,10%acetylene black,and10%PVDF(polyvinyli-dene fluoride)was made using N-methylprrolidinone(NMP)as the solvent.The electrodes with an area of1cm2were prepared by coating the slurry(about100μm in thickness)on alumin-um foils followed by drying in vacuum at60℃for12h.Elec-trochemical tests were performed using a conventional cointype cell,employing lithium foil as a counter electrode and1.0mol·L-1LiPF6in ethylene carbonate/dimethyl carbonate(EC/DMC) (with EC and DMC volume ratio of1∶1)as the electrolyte.The assembly was carried out in an Ar-filled glove box.The electr-ochemical tests were carried out with an electrochemical work station(CHI660B,CHI Instruments Inc.,Shanghai,China).

2Results and discussion

Fig.1shows the XRD pattern of LiVOPO4derived from rheo-logical phase method.As shown in Fig.1(a),All the reflections from the LiVOPO4could be indexed reliably using a standard structural refinement program.XRD peaks in Fig.1agree well with those of the standard JCPDS card No.72-2253.The LiVOPO4 compound possesses an orthorhombic symmetry,space group Pnma,characterized by the unit cell parameters a=0.7446(4) nm,b=0.6278(4)nm,and c=0.7165(4)nm.Except for peaks corresponding to LiVOPO4,no other peaks can be found,sug-gesting that the rheologically synthsized LiVOPO4is very pure. The LiVOPO4framework structure is closely related to that found in VOPO4and comprises infinite chains of corner-shared VO6 octahedra,cross-linked by corner-sharing PO4tetrahedron[27-28]. The cell parameters for the rheologically prepared material com-pare favorably with literature values reported by Lii et al.[28]for a

574No.3XIONG Li-Zhi et al.：A New Rheological Phase Route to Synthesize Nano-LiVOPO4Cathode Material

Fig.1XRD pattern(a)and SEM image(b)of LiVOPO4

hydrothermally prepared sample,i.e.,a=0.7446(3)nm,b=0.6292(2) nm,and c=0.7177(2)nm.Elemental analysis results confirmed the expected stoichiometry of LiVOPO4.

As seen from Fig.1(b),the scanning electron microscopy(SEM) examination indicated that the rheologically synthsized LiVOPO4 consists of particles with average primary size in the range of 10-60nm,which agrees well with the average crystal size of around35nm calculated from the XRD profile.They also showed the presence of considerable material agglomeration. The agglomerates averaged around50nm in size.

The lithium extraction/insertion behavior for the LiVOPO4 active material relies on the reversibility of the V4+/V5+redox couple:

LiVOPO4圳VOPO4+Li++e-

Fig.2shows the initial charge-discharge curve of the rheologi-cally synthesized LiVOPO4material.These data were collected at25℃at an approximate0.1C(16mA·g-1)rate using voltage limits of3.0and4.3V(vs Li/Li+).As shown in Fig.2,at low current density,orthorhombic LiVOPO4prepared by rheological phase method is highly attractive as the alternative cathode for lithium ion rechargeable battery.This is because LiVOPO4ex-hibits high discharge potential of3.85V and reasonably large capacity.The initial oxidation process equates to a material spe-cific capacity of145.8mAh·g-1during this lithium extraction. Based on a theoretical specific capacity for LiVOPO4of166 mAh·g-1[20]and assuming no side reactions,the fully charged material corresponds to Li0.12VOPO4.Excursions to higher oxi-dation potentials(ultimately up to5.0V(vs Li/Li+))resulted in the increased irreversibility as well as active material degrada-tion evidenced by electrolyte discoloration.The reinsertion pro-cess amounts to135.7mAh·g-1,indicating a higher first-cycle charge reversibility of93%than the literature value(85%)repor-ted by Barker et al.[29].

The cycling performance was tested at0.1C(16mA·g-1)in the range of3.0-4.3V as shown in the insert figure in Fig.2.Af-ter cycling60times,the discharge capacity of LiVOPO4is sus-tained at134.2mAh·g-1,which is98.9%of the initial capacity,and the capacity loss per cycle is only0.018%,suggesting LiVOPO4 derived by rheological phase method is promising as alternative cathode material for lithium ion batteries with high capacity and good cycling performance.

Fig.3shows the discharge capacity of LiVOPO4as a function of current rate.As shown in Fig.3,discharge capacity of LiVOPO4drastically decreased with increasing current rate due to the increase of the polarization of electrode.The discharge capacity of LiVOPO4at0.1C(16mA·g-1),1.0C(160mA·g-1),

Fig.2Electrochemical performance data for a typical Li/ LiVOPO4cell cycled between3.0and4.3V at approximate

0.1C(16mA·g-1)rate for charge and discharge

The inset figure in Fig.2is the cycling performance curve.Fig.3Discharge capacity of LiVOPO4as a function of

current rate

potential window:3.0-4.3V(vs Li/Li+);1C=160mA·g -1

575Acta Phys.-Chim.Sin.,2010Vol.26

Fig.4i-t(a)and i-t-1/2(b)curves of nano-LiVOPO4electrode

Fig.5Electrochemical impedance spectroscopy of nano-LiVOPO4electrode at various cycling times

In the equivalent circuit,R e is the electrolyte resistance,R ct is the charge-transfer resistance,C dl is the double layer capacitance,Z w is the Warburg impedance,and

C L is the intercalation capacitance.

and2.0C(320mA·g-1)is135.7,130.9,and124.3mAh·g-1,resp-ectively.More than96.5%and91.6%of the discharge capacity at0.1C are sustained at1.0C and2.0C,respectively.This result is better than that of the LiVOPO4reported by Azmi et al.[19],in-dicating good current rate capability of LiVOPO4synthesized by rheological phase method.The good current rate capability may result mainly from the small particle size and large surface area of LiVOPO4nanoparticles.The smaller the particle size,the larger the surface area and the lower the current density,which results in less polarization of electrode and better current rate capability of LiVOPO4.Further work is underway to find out if there are any other reasons leading to good current rate capabili-ty of LiVOPO4.

The chemical diffusion coefficient was measured with the po-tential step technique.In this method,the current generated due to an applied voltage step,is measured as a function of time. The measured current decays as the lithium ion diffuses through

the electrode.The step ends when the current becomes less than 1%of the maximum current at the onset of the applied potential. The i-t and i-t-1/2curves for the two powders at the applied po-tential step of0.1V(vs Li/Li+)(3.94→4.04V)are shown in Fig.

4.By assuming that the semi-finite diffusion of lithium ion in the electrode is the rate-determining procedure,the diffusion coefficient(D)of lithium ion in the electrode can be determined by the following Cottrell equation[30]:

i=nFD1/2c0π-1/2t-1/2

where,n is the number of the redox reactions,F is the Faraday constant,and c0is the lithium ion concentration in the solid electrode,which can be calculated from the open circuit voltage. According to Fig.4and Cottrell equation,the diffusion coeffi-cient of lithium ion in the electrode can be calculated to be 5.52×10-11cm2·s-1,which is as same magnitude again as the value(2.79×10-11cm2·s-1)reported by Ren et al.[20].The experi-ment results show that the current rate capability of LiVOPO4by rheological phase method is better than that reported by Azmi et al.[19],while the diffusion coefficient of lithium ion in the elec-trode is in the same order.This may be due to the difference in preparation methods of materials and testing means of diffusion coefficient.

The electrochemical impedance spectroscopy of nano-LiVOPO4and the equivalent circuit are displayed in Fig.5.

All the spectra show a semicircle in the high frequency range and an inclined line in the low frequency range.The semicircle in the high frequency range is associated with the“charge trans-fer reactions”at the interface of electrolyte/oxide electrode,which corresponds to the charge transfer resistance.The inclined line in the low frequency range is attributable to“Warburg impedan-ce”that is associated with lithium ion diffusion through the oxide electrode.The semicircle increases with the increase of cycle number.This indicates that the“charge transfer”resistance be-comes larger with the increase of cycle number.The figure also shows that the slope of the inclined line varies with the cycle number.The slope of the inclined line at the first cycle is the biggest and after cycling10times it gets smaller.However, when the cycle number reaches60,the slope of the inclined line becomes stable.

3Conclusions

(1)Orthorhombic nano-LiVOPO4with particle size in the range of10-60nm was synthesized by a new rheological phase method.

(2)The first discharge of LiVOPO4is135.7mAh·g-1and 98.9%of that is kept after60cycles.More than96.5%and

576No.3XIONG Li-Zhi et al.：A New Rheological Phase Route to Synthesize Nano-LiVOPO4Cathode Material

91.6%of the discharge capacity at0.1C are sustained at1.0C and2.0C,respectively.The chemical diffusion coefficient of lithium ion in the nano-LiVOPO4was measured with the potential step technique and the value is in the order of10-11 cm2·s-1.

(3)Rheological phase method is a good route to synthesize LiVOPO4cathode material with high capacity,good cycling performance,and good current rate capability for lithium ion batteries.

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