采用基于乙醇體系的一步草酸共沉淀法制備層狀富鋰錳基正極材料

　寇建文　王昭　包麗穎,,*　蘇岳鋒,,*　胡宇　陳來(lái)　徐少禹　陳芬陳人杰,　孫逢春　吳鋒,

(1北京理工大學(xué)材料學(xué)院，北京100081；2北京電動(dòng)車輛協(xié)同創(chuàng)新中心，北京100081；3中國(guó)北方車輛研究所，北京100072)

采用基于乙醇體系的一步草酸共沉淀法制備層狀富鋰錳基正極材料

寇建文1王昭1包麗穎1,2,*蘇岳鋒1,2,*胡宇1陳來(lái)1徐少禹3陳芬3陳人杰1,2孫逢春2吳鋒1,2

(1北京理工大學(xué)材料學(xué)院，北京100081；2北京電動(dòng)車輛協(xié)同創(chuàng)新中心，北京100081；3中國(guó)北方車輛研究所，北京100072)

首次報(bào)道了一種新穎的基于乙醇溶液的一步草酸共沉淀法合成富鋰錳基正極材料的方法。在這種方法中，包括鋰元素在內(nèi)的所有元素均能在共沉淀反應(yīng)步驟發(fā)生沉淀反應(yīng)，以此實(shí)現(xiàn)更為均勻的元素混合。此外，相比傳統(tǒng)的草酸銨共沉淀法，該法省略了前驅(qū)體初燒的步驟，節(jié)約了合成的時(shí)間和成本。通過(guò)X射線衍射(XRD)、掃描電子顯微鏡(SEM)和電化學(xué)測(cè)試等檢測(cè)手段表征了所得樣品的晶體結(jié)構(gòu)與電化學(xué)性能，研究了兩種方法所制備的富鋰錳基正極材料的結(jié)構(gòu)、形貌與電化學(xué)性能。結(jié)果表明，一步草酸共沉淀法合成的富鋰材料，擁有更好的結(jié)晶性、更大的層間距；材料的顆粒更為均勻和細(xì)小。這些晶體結(jié)構(gòu)與形貌上的優(yōu)勢(shì)，使得該法制備的富鋰材料展現(xiàn)出了更高的放電比容量、更好的循環(huán)性能和倍率性能。這些結(jié)果均展示了我們所提出的一步草酸共沉淀法的可行性與優(yōu)勢(shì)。這種新穎而簡(jiǎn)便的共沉淀法，可推廣于其他層狀材料的合成與設(shè)計(jì)。

鋰離子電池；正極材料；Li2MnO3；乙醇；草酸共沉淀法；電化學(xué)性能

1 Introduction

Transportable devices driven by Li-ion cells such as cell phones, laptop computers,and uninterrupted power supplies(UPS)have become indispensable parts in our daily life for the pastdecades. To fulfill the requests of these applications,efficient cathode materials are required to fabricate high performance Li-ion batteries w ith low mass density,reliable safety,and high energy density1.Recently,layered lithium-rich(LR)cathodematerials, Li2MnO3-LiMO2(M=Mn,Ni,Co,etc.)have been extensively studied as a new generation of prom ising cathodematerials for advanced Li-ion batteries thanks to theirhigher reversible capacity (ca 250mAh?g－1),lower cost,and lessenvironmental impactor2－7.

The electrochemical performance of thesematerials strongly dependsupon the selected syntheticmethod,which determines the uniform and homogeneousdegree of finalproducts8.In previous researches,hydroxide coprecipitation instead of solid state reaction is w idely used to produce uniform and homogeneous transition metal precursors9－12.However,this method is less competitive when producing precursors with high manganese contentbecause Mn2+can be easily oxidized to Mn3+athigh pH value,formingmanganese oxyhydroxide(MnOOH)and leading to a deteriorative performance13.Carbonate coprecipitation,asan alternativemethod,could keep the stateof the cationsas+2 forall transitionmetals14－18.But the samples synthesized by thismethod usually have a low er tap density than those by hydroxide coprecipitation.More recently,Zheng et al.19reported the direct correlation between voltage fadeand differentsyntheticmethods,and they found that thematerialsprepared by the coprecipitation and sol-gelmethodsexhibita faster voltage fade and poorer cycling stability than thematerials prepared by the hydrothermal assisted method.This strongly implied that the electrochemical characteristics of LRmaterialsare prone to be affected by preparation condition.

In our previous research,ammonium oxalate was applied to serve as the precipitator to replace the hydroxide and carbonate in aweak-acidic aqueoussolution20.Theexperimental conditionsof thismethod aremuch less harsh,and the oxidation state of the cations can be easily keptat+2 for all transitionmetals.However, such an ammonium oxalate coprecipitationmethod,like other coprecipitationmethods,stillneed to prepare the precursors firstly, and thenmixed with lithium salts followed by calcination treatment.Such complicated process is costive,time consuming,and may further cause a propensity of nonuniform ly distribution of transitionmetals.Herein,we presentanovelethanol-based onestep oxalate coprecipitationmethod for precipitating all theelements including Liduring the coprecipitation reaction,to realize a more homogeneous particle morphology and give a better electrochemistry performance.Li1.2Mn0.54Ni0.13Co0.13O2is used here asan example to verify the feasibility of thenew method.

2 Materials and methods

Ourapproach isshown in Fig.1.All the raw reagents employed here w ere of analytical purity grade.The ammonium oxalate coprecipitationmethod is the same as reported in our previous research20.In this ammonium oxalate coprecipitation method, Ni(NO3)2(Aladdin),Co(NO3)2(Aladdin),and Mn(NO3)2(Aladdin, 50%aqueoussolution)with amole ratio of 1:1:4.15and stoichiometric(NH4)2C2O4were dissolved in deionized water,respectively.Both of the solutionswere dropped into a strongly stirred beaker.Precipitate w as separated from the solution by vacuum filtration and dried at80°C for 12 h.A ftermixingw ith a stoichiometric amountof lithium hydroxide,the dried powder waspresintered at450°C for5h,and ground after cooling.Finally,the decomposedm ixturewas pressed into pelletsand calcined at900°C for12 h to obtain Li1.2Mn0.54Ni0.13Co0.13O2(hereafter named assampleA).While in the one-step oxalate coprecipitation method,Li(NO3)2(A laddin)w as simultaneously added into the transition solution,and H2C2O4was chosen as the precipitator. Both of these two solutionswere dissolved in ethanoland dropped into a strongly stirred beaker to synthesize the precursors.After filtration and drying,the obtained pow derwas directly pressed into pellets and calcined at900°C for 12 h to obtain the final product(hereafternamed assample B).

A structural analysis was carried out using X-ray diffraction (XRD,Rigaku Ultima IV-185)with a Cu Kαradiation source.The sampleswere scanned from 2θ=20°to 2θ=80°ata scan rateof 2(°)?m in－1.To analyze the samplemorphology,FEIQUANTA 250field em ission scanning electronm icroscope(FESEM)w as used.Electrochem icalperformancesof the sampleswere examined with CR2025coin type cells.The positive electrodeswere made of 80%(w,mass fraction)as-prepared materials pow der, 10%(w)acetylene black,and 10%(w)polyvinylidene fluoride. The electrolytewas 1mol?L－1LiPF6dissolved in amixture of ethylene carbonate and ethylmethyl carbonate(1:1,volume ratio).The coin-type half cells,which chose lithium metalas the negativeelectrode and Celgard 2400membraneasseparator,were assembled in aglove box filled with argon.Electrochemical tests were performed by CT2001A Land instrument(Wuhan,China)atpotential range of 2.0－4.8 V.The currentdensity of 250m A?g－1wasdefined as 1C rate during the test.Electrochemical impedance spectra(EIS)of the cellswerealso conducted using the CHI660electrochemicalworkstation at frequencies from 105Hz to0.01Hz w ith an AC perturbation signal of 5mV.And the resultswere analyzed using ZSimpW in softw are.

Fig.1 Schematic diagram for synthetic process

3 Results and discussion

The X-ray powder diffraction(XRD)patternsof samplesA and B,aswellas their lattice parameters are shown in Fig.2.Clearly, XRD patternsof both the samplesA and B arewell indexedwith α-NaFeO2layered structure(R3ˉm space group)except for some weak superlattices reflectionsaround 2θ=20°－25°caused by Li+/ Mn4+cation ordering in the transitionmetal layers5,21.The ratio of c/a of both samples A and B is higher greater than 4.9,which indicated the formation of ahighly ordered lamellar structure22. While the(003)and(104)peak intensity ratio of I(003)/I(104)(bigger than 1.2)means that the materials have less cation disorder23. Besides,for sample A,themain XRD peaks corresponding to the LiMnO3-like super latticesarea slightly weaker andw ider than that thoseof sample B.In conclusion,theabsenceof presintered at 450°C in one-step oxalate coprecipitation method will not influence the crystallinity of sample B.On the contrary,sample B showsahigher crystallinity from the ratio of c/a and I(003)/I(104).

Fig.3 compares SEMimagesof the Li[Li0.2Mn0.54Ni0.13Co0.13]O2particles synthesized by the two coprecipitationmethods.Itcan be clearly seen from the SEMimages thatbothof thematerialsare quite homogeneous in particle sizes with inerratic bulk morphology.However,particles in sample B areof smaller sizes.The significantly reduced dimensionsof smaller particlesof sample B may largely increase the rate of lithium insertion/removal;and their higher surface area could perm it a higher contactareaw ith the electrolyte and hence a high lithium-ion flux across the interface24.

Fig.2 XRD patterns and lattice param etersof sam p les A and B

Fig.4shows electrochem ical performancesof samples A and B. Fig.4a shows the initial and 80th charge-discharge curves at25mA?g－1(0.1C rate)of the samplesA and B.Both of them present a typicalpotentialplateau of LRmaterials atabout4.5V regions at the initial cycle,which is originated from the electrochemical activation of Li2MnO3phase25－27.This plateau atabout4.5V of sample B is longer than that of sample A,indicating amore completeactivation of Li2MnO3phase,whichmay benefit from its smaller particle sizes.For the same reason,sample B also shows a lowerelectrode polarization than sample A.Naturally,sample B deliversamuch higherdischarge capacity(286.1mAh?g－1)than sample A(242.5mAh?g－1).Both of them suffer from obvious capacity drop and voltagedecay after80cycles,whichmay be due to the increased interface reaction impedanceand layered-to-spineltransformation.However,theattenuation trend in sample B is less obvious.The corresponding discharge differential capacity(d Q/ d V)plotsof Fig.4a are illustrated in Fig.4b.The reduction peaks atabout3.75V isgenerally considered as the reduction of Ni4+or Co4+ions in the rhombohedralphase,while the peaksatabout3.3 V areassociatedwith reduction ofMn4+from LiMO2phase28.It is clearer in Fig.4b thatsample B presents lessened voltage decay.

Fig.3 SEMimagesof the Li[Li0.2Mn0.54Ni0.13Co0.13]O2powdersof samp le A(a)and sam ple B(b)

Fig.4Electrochem icalper formancesof sam plesA and B (a)the initialand 80th charge-discharge curvesat0.1C rate in avoltage rangeof 2.0－4.8V;(b)corresponding d Q/d V plots; (c)cycleperformanceat0.1C rate;(d)discharge capacity atvarious ratesasa function of cyclenumber

Cycling performance is shown in Fig.4c.A fter 80cycles,a reversible capacity of 231.3m Ah?g－1of sam ple B could still be maintained,w hich ismuch higher than that of samp le A(186.1 mAh?g－1).Herewe choose3.5V asademarcation line to separate the discharge capacity into two parts for comparison:one is the capacity from the reduction of Ni4+or Co4+above3.5V,and the other is from the reduction of Mn4+below 3.5V29,30.Due to the layered-to-spinel transformation,the discharge capacity of both the samplesabove3.5V decreasesmonotonously during cycling as shown in Fig.4c.Before about50cycles,this decrease isaccompanied by an increase of discharge capacity below 3.5V for samples A and B due to increased formation of the spinel-like phase.While after 50cycles,sample A exhibits continuous capacity decay below 3.5V,imp lying a deteriorated structure for lithium ion storage.On the contrary,sample B exhibits a capacity increase in the region below 3.5V,indicating excellentcapacityretention.These advances in discharge capacity and cycling performance of sample B shows the advantages of the novel method we presented here.Fig.4d compares the rate performances of the two samples.Itcan be seen that the discharge capacity of the sample B atany rates isalwayshigher than thatof the sample A.Combinedw ith the lattice parameters in Fig.2 and results of Fig.3,thebetter rate performanceof sample Bmay partly own to its higher c value and smaller sizes,which indicates thatsam ples synthesized by one-step oxalate coprecipitation method could providemoreopen and faster Li+transportchannels.

To gain a better understanding of factors leading to the differences in rate capabilitiesof the two samples,EISmeasurements have been carried out after 3 cyclesas shown in Fig.5.A possible equivalent circuit is also proposed,and the fitting data of the sampleA and the sample B are shown in Table 1.Before the EIS measurements,all the sampleshavebeen charged to 50%stateof charge(SOC)to reach an identicalstatus.The Nyquistplots for the electrodes in Fig.5shows sim ilar shapes.They are com posed of two semicircles in thehigh frequency,which isassociatedwith the resistance for lithium ion diffusion in solid electrolyte interface (SEI)layer(Rs)and the charge transfer resistance(Rct)31,32,and a quasi-straight line in the low frequency corresponding toWarburg im pedance(Zw).As seen in Table 1,the Rsof the sam ple B is slightly larger than thatof the sampleA.However,the sample B showsamuch smaller Rctcompared to the sampleA.The lower surface charge-transfer resistance of samp le B should also be responsible for itsbetterhigh-rate performance.

Fig.5EISspectra of cells of samp lesA and B

Tab le 1 EIS data of sam plesA and B

4Conclusions

Layered lithium-rich cathodematerialsnamed as samplesA and B were successfully synthesized by conventional ammonium oxalate coprecipitationmethod and anovelone-step oxalate coprecipitationmethod,respectively.XRD results shows thatsample B owns better crystallinity and larger interlayer spacing.SEMimages shows thatparticlesof sample B aremorehomogeneous and smaller.Electrochemical tests reveal that samples synthesized by one-step oxalate coprecipitationmethod exhibitmuch higher discharge capacity,better cycle performance,better rate performance,and smaller Rct.And these enhancements in electrochem ical performances indicate the advantagesof one-step oxalate coprecipitation method,which may realize amore uniform ity cation distribution in sample B.This novel and convenient coprecipitation method we developed here introduces a possible strategy to realize amore uniform transitionmetal distribution in layered materials,which may open a great opportunity form itigating voltage fade in lithium-rich cathodematerials.

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Layered Lithium-Rich Cathode Materials Syn thesized by an Ethanol-Based One-Step Oxalate Coprecipitation Method

KOU Jian-We
n1WANG Zhao1BAO Li-Ying1,2,*SU Yue-Feng1,2,*HU Yu1CHEN Lai1XU Shao-Yu3CHEN Fen3CHEN Ren-Jie1,2SUN Feng-Chun2WU Feng1,2
(1School ofMaterials Science and Engineering,Beijing Institute ofTechnology,Beijing 100081,P.R.China;
2Collaborative Innovation CenterofElectric Vehicles in Beijing,Beijing 100081,P.R.China;3China North Vehicle Research Institute,Beijing 100072,P.R.China)

We synthesized layered lithium-rich cathodematerials by a novelethanol-based one-step oxalate coprecipitationmethod.Using thismethod,all the elements including lithium could be coprecipitated during the coprecipitation reaction p rocess to realize a homogeneousm ixture of lithium and transitionmetalelements.In add ition,com pared w ith the conventionalammonium oxalate coprecipitationmethod,the precursor preheating p rocess was elim inated,which should decrease reaction time and cost.X-ray diffraction(XRD),scanning electronm icroscopy(SEM),and electrochem icalmeasurementswere used to investigate the differences in the crysta lstructure,morphology and electrochem ica lperformance of sam ples synthesized using the above two methods.Com pared w ith the samp les synthesized by the conventional ammonium oxalate cop recipitation method,sam p les prepared by our novelone-step oxalate coprecipitationmethod exhibit higher crystallinity w ithlarger interlayer spacing,and sma ller,more homogeneous particles.Such crysta lstructure andmorpho logy endow the sam p les prepa red by the oxala te coprecipita tion m ethod w ith better d ischarge capacity,cycle performance and rate performance than those synthesized by the conventionalmethod.The sim ple,e fficient coprecipitationmethod developed heremay provide a new app roach to fabricate layeredmateria ls forhighperformance lithium-ion batteries.

November10,2015;Revised:December24,2015;Published onWeb:December30,2015.

Lithium-ion batte ry;Cathodemateria l;Li2MnO3;Ethanol;Oxalate coprecipitationmethod; Electrochem ica lperformance

O646

10.3866/PKU.WHXB201512301

*Corresponding authors.SU Yue-Feng,Email:suyuefeng@bit.edu.cn.BAO Li-Ying,Email:baoliying@bit.edu.cn;Tel:+86-10-68918099.

The projectwas supported by theNational Key Basic Research Program of China(973)(2015CB251100),NationalNatural Science Foundation of China(51472032,51202083),Program for New Century ExcellentTalents in University,China(NCET-13-0044),Special Fund of Beijing Co-Construction Project,China(20150939013),BIT Scientific and Technological Innovation Project,China(2013CX 01003).

國(guó)家重點(diǎn)基礎(chǔ)研究發(fā)展規(guī)劃項(xiàng)目(973)(2015CB251100),國(guó)家自然科學(xué)基金(51472032,51202083),新世紀(jì)優(yōu)秀人才支持計(jì)劃(NCET-13-0044),北京市共建項(xiàng)目(20150939013)和北京理工大學(xué)重大項(xiàng)目培育專項(xiàng)計(jì)劃項(xiàng)目(2013CX01003)資助?Editorialofficeof Acta Physico-Chim ica Sinica