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On the Performance of LiNi1Õ3Mn1Õ3Co1Õ3O2 Nanoparticlesas a Cathode Material for Lithium-Ion BatteriesHadar Sclar,a Daniela Kovacheva,b Ekaterina Zhecheva,b Radostina Stoyanova,b

Ronit Lavi,a Giora Kimmel,c Judith Grinblat,a Olga Girshevitz,a

Francis Amalraj,a Ortal Haik,a Ella Zinigrad,a Boris Markovsky,a,* andDoron Aurbacha,**,z

aDepartment of Chemistry, Bar-Ilan University, Ramat-Gan 52900, IsraelbInstitute of General and Inorganic Chemistry, Academy of Sciences, Sofia 1113, BulgariacBen-Gurion University of the Negev, Beer-Sheva 84105, Israel

We report on the behavior of nanometric LiMn1/3Ni1/3Co1/3O2 �LiMNC� as a cathode material for Li-ion batteries in comparisonwith the same material with submicrometric particles. The LiMNC material was produced by a self-combustion reaction, and theparticle size was controlled by the temperature and duration of the follow-up calcination step. X-ray diffraction, scanning electronmicroscopy, X-ray photoelectron spectroscopy, Fourier transform infrared, Raman spectroscopy, electron paramagnetic resonance,inductively coupled plasma, and atomic force microscopy were used in conjunction with standard electrochemical techniques�cyclic voltammetry, chronopotentiometry, and electrochemical impedance spectroscopy� for characterizing the electrode materi-als. The effect of cycling and aging at 60°C was also explored. Nanomaterials are much more reactive in standard electrolytesolutions than LiMNC with a submicrometric particle. They develop surface films that impede their electrochemical response,while their bulk structure remains stable during aging and cycling at elevated temperatures. The use of nanomaterials in Li-ionbatteries is discussed.© 2009 The Electrochemical Society. �DOI: 10.1149/1.3212850� All rights reserved.

Manuscript submitted June 15, 2009; revised manuscript received August 4, 2009. Published October 1, 2009.

0013-4651/2009/156�11�/A938/11/$25.00 © The Electrochemical Society

A major challenge in the field of Li-ion batteries is to replace thecommonly used LiCoO2 cathode with more advanced materials of ahigher voltage and capacity, better safety features, and lower prices�cobalt is an expensive metal and is environmentally unfriendly aswell�.1 Natural substitutes are LixMnO2

2 and LixNiO2.3 However,both materials are too problematic. The most stable form ofLixMnO2 is the LiMn2O4 spinel whose capacity and high tempera-ture performance are limited.4 LiNiO2 is too reactive with ethylenecarbonate �EC�-based LiPF6 electrolyte solutions and suffers fromsafety and stability problems. Intensive work on lithiated transition-metal oxide cathode materials in recent years led to the developmentof several cathode materials based on lithiated oxides of a mixtureof transition metals �instead of compounds with single transitionmetals�.5-7 Li�NiCo�O2 is a better cathode material than bothLiNiO2 and LiCoO2, and recently, a further modification producedLiNi0.8Co0.15Al0.05O2 as an excellent cathode material for Li-ionbatteries, with reasonable stability, cyclability, safety features, andan initial capacity �190 mAh/g.8,9 In parallel, extensive work onmanganese compounds resulted in a description of LiMn0.5Ni0.5O2with a layered structure as a promising high capacity cathode inwhich the redox activity of Ni2+ � Ni3+ � Ni4+ may provide ca-pacities up to 200 mAh/g.10,11 This material develops a unique sur-face chemistry that allows the good passivation of the active mass,enabling its prolonged cycling at elevated temperatures.12 The ratecapability of cathodes comprising LiMn0.5Ni0.5O2 is questionable.There is an interesting publication on this material produced by ionexchange �from sodium compounds�,13 which demonstrated fastrates of Li insertion. However, from most of the publications de-voted to this compound, one can conclude that the material is slowin Li charge–discharge reactions mostly due to the mismatch of ionsin the lattice �Li+ vs Ni2+ and Mn4+ ions�, which impedes the fastbulk transport of Li ions in LixNi0.5Mn0.5O2 particles.14,15 As a fur-ther improvement, LiMn1/3Ni1/3Co1/3O2 �LiMNC� was suggested asa promising cathode material for Li-ion batteries.16-20 The replace-ment of part of Mn and Ni by Co in the layered structure seems to

* Electrochemical Society Active Member.** Electrochemical Society Fellow.

z E-mail: aurbach@mail.biu.ac.il

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have a stabilizing effect, which prevents mixing between Li andtransition-metal ions in the lattice, thus allowing a smooth and fastLi-ion bulk transport in this material.21,22

Our recent comparative work proved that, in general, LiMNC isthe fastest compared to layered compounds, such as LiCoO2,LiNiO2, Li�NiCo�O2, Li�NiCoAl�O2, and Li�MnNi�O2, on the ac-count of specific capacities �160–170 mAh/g initially�, which arelower than those of the latter two systems.23 Since the publicationson LiMNC about eight years ago,24,25 hundreds of papers have beenpublished on different synthetic modes of this material, on possiblesurface modification, and on the correlation among synthesis, mor-phology, and electrochemical performance.18,26-29

An important progress in this field was demonstrated by the re-cent work on Li–Mn–Ni–Co–O “integral” compounds, which can beconsidered as solid mixtures of Li2MnO3 and layeredLiMn0.5Ni0.5O2 or LiMn1/3Ni1/3Co1/3O2. Some of these systems pro-vide very high capacities up to 250 mAh/g �reversible� in the poten-tial range of 4.8–3 V �vs Li/Li+�.30-32 There is no question that thisfamily of cathode materials is very promising, especially for appli-cations such as electric vehicles, which require high energydensity.33 In any event, much work is still needed to demonstratestability in prolonged cycling, high rate capability, and suitablesafety features for these integral cathode materials. Therefore, thereis room for further studies on improved manganese-nickel-cobalt�MNC�- and nickel-cobalt-aluminum �NCA�-type cathode materials.In parallel to the intensive work on lithiated transition-metal oxidecathode materials, there are rigorous works underway on olivine-type cathode materials.34,35 Many hundreds of papers have beenpublished to date on LiFePO4 �3.5 V vs Li/Li+, 170 mAh/gtheoretically�.36,37 Recently, LiMnPO4

38 and LiCoPO4,39 whose re-dox potentials are higher �4.1 and 4.8 V vs Li/Li+, respectively�,have been the focus of attention. The use of these LiMPO4 olivinesas cathode materials opens highly important issues related to the useof nanomaterials in Li batteries. Intrinsically, LiMPO4 compoundssuffer from very low ionic �Li+� and electronic conductivities.40

Only their use as nanoparticles along with surface coating by elec-tronically conductive materials, e.g., carbon, enables the elaborationof practical cathodes comprising nano-LiMPO4 as the activemass.41-43

Apparently, the use of nanomaterials as cathode materials for Libatteries in general should be considered favorable in achieving highrate capability because solid-state Li-ion transport in the bulk mate-

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rials may be the rate-determining step for the entire cathodic pro-cesses. Hence, the use of nanoparticles means reducing the diffusionlength for Li-ion transport to a minimum. The cathodes comprisingnanoparticles of LiMn0.5Ni0.5O2

12 and LiMn1.5Ni0.5O444 demon-

strate faster kinetics than electrodes based on micrometric particles.However, the use of nanoparticles in these composite electrodes mayinterfere badly with their electrical integrity and the interparticleelectrical contact. Moreover, electrode materials, such as LiMO2,may be reactive with components of the standard electrolyte solu-tions based on alkyl carbonate solvents and LiPF6 �which unavoid-ably contain detrimental contaminants such as HF, trace water, PF5,and POF3�.

45 Alkyl carbonate molecules, which can be consideredelectrophilic, can be attacked nucleophilically by the negativelycharged oxygen atoms of LiMnO3 compounds.46 Hence, nanopar-ticles with a relatively high surface area that may be very reactivewith solution species may develop detrimental passivation phenom-ena and high impedance in solutions. Consequently, the possible useof nanomaterials in electrodes for Li-ion batteries should be studiedrigorously and specially for each electrode material. It is impossibleto generalize the balance between the pros and cons of using elec-trodes comprising nanoparticles in batteries. This paper is aimed atexploring the possible use of nanoparticles of LiMn1/3Ni1/3Co1/3O2as a cathode material for Li-ion batteries.

Despite the intensive work devoted so far to this material,16-29 itseems that this subject has not been explored in depth. Nano-LiMNC was synthesized in this work by a self-combustion reaction�SCR�.47 This synthetic mode produces nanoparticles, whose furthercalcination under air at high temperatures produces submicrometricand micrometric particles.48 Hence, it is possible in the work pre-sented in this paper to compare electrodes comprising nano- andsubmicrometric LiMNC produced by the same synthesis, which en-abled us to concentrate on the size effect of this active mass. X-raydiffraction �XRD�, high resolution electron microscopy, electronparamagnetic resonance �EPR�, X-ray photoelectron spectroscopy�XPS�, Fourier transform infrared �FTIR�, Raman spectroscopy, andatomic force microscopy �AFM� �several modes of operation� wereused in conjunction with electrochemical techniques to understandthe effect of aging, cycling, and elevated temperatures on the behav-ior of these LiMNC electrodes. The thermal behavior of these cath-ode materials with EC–dimethyl carbonate �DMC�/LiPF6 solutionswas explored by differential scanning calorimetry �DSC� measure-ments. The study of nano-LiMNC materials, with which surfacephenomena may be intensive, enables a better understanding of thesurface phenomena of LiMNC submicrometric and micrometric ac-tive masses, whose surface studies are more difficult.

Experimental

A nanocrystalline LiMn1/3Ni1/3Co1/3O2 material was prepared byan SCR among lithium nitrate, nickel�II� nitrate, manganese�II� ni-trate, and cobalt�II� nitrate �all from Aldrich�, which acted as theoxidants, and sucrose, which acted as the fuel. The amount of su-crose was chosen for the oxidant/fuel ratio = 1:1.6.49 The modelreaction was

LiNO3 + 1/3Ni�NO3�2·6H2O + 1/3Co�NO3�2·6H2O

+ 1/3Mn�NO3�2·4H2O + 0.5C12H22O11 + 2.5O2

→ LiMn1/3Ni1/3Co1/3O2 + 6CO2 + 10.83H2O + 1.5N2

The exact composition of gases evolved during this reaction wasstudied by the thermogravimetry-mass spectrometry �TG-MS� tech-nique and was described in Ref. 48.

We used two modes of preparation of nano-LiMn1/3Ni1/3Co1/3O2particles, as described in previous papers.12,47-49 In the first mode,the reaction mixture was slowly evaporated at 120°C and graduallytransformed to a syrup and then to a foamy mass that after heating to200°C burned upon ignition. The product consisted of black spon-gelike flakes comprising amorphous LiMn1/3Ni1/3Co1/3O2 nanopar-ticles. Using the second mode, the reagent solution was rigorouslystirred and sprayed with relatively large droplets into the internal

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surface of a heated 200–250°C quartz tube. The combustion reac-tion took place within the small volume of the individual droplets,and a very fine powder of the oxide material was obtained. Theas-prepared samples, nanoparticles of LiMn1/3Ni1/3Co1/3O2, wereamorphous. Nanocrystalline particles were further obtained by an-nealing at 700°C for 1 h in air. The size of the particles of thismaterial could be controlled by further annealing to higher tempera-tures. We also examined materials annealed at 700, 800, 900, and1000°C for 22 h, which were analyzed by XRD and electron mi-croscopy.

For electrochemical measurements, two- and three-electrodecells in a coin-type 2325 configuration were used with a Celgard,Inc. polypropylene separator. Parts for these cells were purchasedfrom National Research Council, Canada. The working electrodeswere prepared in the form of thin films of a mixture ofLiMn1/3Ni1/3Co1/3O2, carbon black, and a poly�vinylidene fluoride��PVDF� binder �75:15:10 by weight� on aluminum foil, as describedin our previous reports.12,44 We also studied electrodes in which theLiNi1/3Mn1/3Co1/3O2 powder was mechanically attached to bothsides of aluminum foil ��99.9%, Strem Chemicals Inc.� by wipingthe foil covered by the powder with delicate task wipers �Kimberly-Clark Corporation�. Using this method, uniform and homogeneousthin layers of the LiMn1/3Ni1/3Co1/3O2 material on Al foils wereobtained, free of the PVDF binder and carbon black additives. Dueto the softness of the aluminum foil, the LiMn1/3Ni1/3Co1/3O2 par-ticles, which are sufficiently hard, could be easily embedded �bywiping them�, thus remaining in very good electrical and mechanicalcontact with the foil. After the preparation, the electrodes were driedin vacuum overnight. These additive-free electrodes are ideal andpreferable for fine electrochemical and spectroscopic measurementsof the active mass, as well as for aging studies. The amount of theelectrodes’ active mass was known by rigorous weighing. The finalworking electrodes were usually disks whose active mass and geo-metric surface area, exposed to the electrolyte solution, were 2–4mg and 1.54 cm2, respectively. A lithium disk and a lithium chipserved as counter and reference electrodes. Electrochemical cellswere assembled in a glove box filled with highly pure argon �VAC,Inc.�. We used standard electrolyte solutions �high purity, Li batterygrade� comprising DMC and EC �weight ratio of 2:1� and 1.5 MLiPF6 �Tomiyama, Japan, could be used as received�. The contentsof HF and water in this solution were not more than 100 and 20ppm, respectively. After assembling, the electrochemical cells werestored at room temperature for 12–24 h to ensure a complete im-pregnation of the electrodes and the separator with the electrolytesolution. The electrochemical measurements �impedance spectros-copy, cyclic voltammetry, and chronopotentiometry� were carriedout using a battery test unit model 1470 coupled with an FRA model1255 from Solartron, Inc. �driven by CorrWare and ZPlot softwarefrom Scribner Associates Inc.� and a multichannel battery testerfrom Maccor, Inc., model 2000. Two-electrode cells were testedusing galvanostatic and constant current–constant voltage �CC-CV�modes at a C/5 rate. In the CC-CV mode, a final potentiostatic stepat E = 4.60 V was added. The duration of potentiostatic steps wasdetermined according to the currents applied at the galvanostaticcharging step, and they varied from 5 h at C/10 to 10 min at 3C. Thealternating voltage amplitude in impedance measurements was 3mV, and the frequency range was from 50 kHz to 5 mHz. All thepotentials in this paper are given vs Li/Li+. The accuracy of thecalculations of the electrodes’ capacity in all the graphs presented isaround 95%. The electrochemical measurements were performed at30 and 60°C. For aging tests, LiNi1/3Mn1/3Co1/3O2 powder �200mg� was stored in polyethylene vials filled with 10 mL of an EC–DMC �1:2�/1.5 M LiPF6 solution �stirred� at 30 and 60°C underargon atmosphere. Periodically, after 1–8 weeks of aging,LiMn1/3Ni1/3Co1/3O2 powders were separated from solutions bycentrifugation, rinsed with DMC in a glove box, and dried invacuum overnight. The aged LiMn1/3Ni1/3Co1/3O2 powder was mea-sured using FTIR and Raman spectroscopies. For EPR measure-ments of pristine and aged particles, a Bruker ESR 100d X-band

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spectrometer was used. The solid sample was inserted into a quartztube, and the spectra were recorded with the following parameters:frequency, 9.42 GHz; microwave power, 2 mW; scan width, 4000 G;resolution, 1024 points; receiver gain, 2 � 102; conversion time, 82ms; time constant, 0.64 ms; and sweep time, 84 s. To analyze theEPR spectra of both pristine and aged samples, EPR standards ofLi�Li�1−2x�/3NixMn�5−x�/3�O4, 0 � x � 0.5, were used. These stan-dard samples were prepared by a solid-state reaction between copre-cipitated Li–Ni–Mn hydroxides and LiOH at 900°C.50 AgedLiNi1/3Mn1/3Co1/3O2 powder was measured as an active mass inelectrochemical measurements using additive-free electrodes �par-ticles embedded in an Al foil current collector�, as described above.The electrolyte solutions remaining after the aging experiments wereanalyzed using the inductively coupled plasma �ICP� technique�Ultima-2 spectrometer from HORIBA Jobin Yvon Inc.� to followthe possible dissolution of metal ions from these materials duringaging. Samples of electrolyte solutions were dissolved in concen-trated aqueous H2SO4 and HNO3 �1:1� to get rid of the organicsolvents. After the completion of the redox process, the residualproduct was diluted with highly pure water for the ICP measure-ments.23

FTIR measurements of pristine and aged powders were carriedout in a diffuse reflectance mode using a Magna 860 spectrometerfrom Nicolet, placed in a home-made glove box, the atmosphere ofwhich was kept free of carbon dioxide and water �the dew point wasaround −65°C, Balston Inc. condensed air purifier�. The Ramanspectra of pristine and aged powders were collected ex situ in abackscattered configuration using micro-Raman spectrometerHR800 �HORIBA Jobin Yvon Inc.�, holographic grating 1800grooves/mm, and a He–Ne laser �excitation line of 632.8 nm andmeasured power of 0.3 mW�. The spectra were measured from atleast three to five different locations in a single LiNi1/3Mn1/3Co1/3O2powder sample, and Raman plots were smoothed using the LOESSmodeling method �SigmaPlot software�. XRD measurements wereperformed using an AXS D8 Advance diffractometer �reflection �-�geometry, Cu K� radiation, receiving slit of 0.2 mm, scintillationcounter, 40 mA, and 40 kV� from Bruker, Inc. �Germany�. Thefitting of the XRD patterns for the structural analysis was carried outwith the PowderCell program, and the lattice parameters were de-termined using WIN-METRIC and CelRef programs. High reso-lution transmission electron microscopy �HRTEM� and high reso-lution scanning electron microscopy �HRSEM� images ofLiNi1/3Mn1/3Co1/3O2 particles were obtained by JEOL-JEM-2011�200 kV� and JEOL-JSM-7000F electron microscopes, respectively,equipped with an energy-dispersive X-ray microanalysis systems.The specific surface areas of nanosized LiMn1/3Ni1/3Co1/3O2 par-ticles �annealed at 700°C for 1 h� and submicrometric particles �an-nealed at 900°C for 22 h� measured by the Brunauer, Emmett, andTeller method �Gemini 2375, Micromeritics, multipoint mode� were7.80 and 2.8 m2/g, respectively. A relatively low surface area wasmeasured for the nanoparticles because they tend to agglomerate�confirmed by scanning electron microscopy �SEM� measurements�.XPS measurements were carried out using HX axis systems fromKratos, Inc. with monochromic Al K� �1486.6 eV� X-ray beamradiation, as described previously.23,44 AFM measurements and im-aging were carried out using a Nanoscope V Multimode scanningprobe microscope �Digital Instruments, Santa Barbara, CA�. To col-lect height, phase shift, stiffness, and adhesion data simultaneously,we used a HarmoniX AFM in air at room temperature. All imageswere obtained using the tapping mode with a single HMX siliconprobe �force constant of 4 N/m, Digital Instruments, Santa Barbara,CA�. The torsional resonance frequency of this cantilever was ap-proximately 1110 kHz. The ratio of torsional to vertical resonancefrequencies was 17.9. The scan angle was maintained at 90°, and theimages were captured in the retrace direction with a scan rate of 0.5Hz. For each experimental condition, two silicon sheets were pre-pared, and at least three images were obtained for each silicon-sheetsample. The suspension of the LiNi1/3Mn1/3Co1/3O2 particles in an-hydrous ethanol was prepared by sonication for 20 min, and a drop

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of it was attached onto freshly cleaved silicon sheets and dried undernitrogen. The dimensions of the LiMn1/3Ni1/3Co1/3O2 particles �pris-tine and aged in solutions� were determined by an analysis of thephase and height in the AFM images collected simultaneously usingthe Nanoscope Software Version 7.3. Before the measurements, the“zoom” and “planefit” functions were applied to each particle in theimages. AFM width and height measurements were obtained, to-gether with section analyses. Differential calorimetric measurementsof LiMNC powders and solutions were carried out using the DSCsystem model 822e from Mettler Toledo, Inc. in hermetically sealedcrucibles from the same company �sealed with the materials in aglove box�. Usually, the temperature was scanned at 1°C/min fromroom temperature to 300°C.

Results and Discussion

Characterization of LiMn1/3Ni1/3Co1/3O2 particles produced bySCR.— The SCR of Li, Mn, Ni, and Co nitrates and sucrose pro-duces amorphous nanoparticles with an excellent mixing of the me-tallic and oxygen ions.49 Further calcination at elevated tempera-tures is required to prepare crystalline layered LiMn1/3Ni1/3Co1/3O2nanoparticles. We found that heating the product of this SCR to700°C for 1 h is sufficient to produce crystalline nanoparticles. Fig-ure 1 presents the XRD patterns of this product, demonstrating itspurity as a single-phase layered material. The diffraction lines in thisfigure can be indexed on the basis of a hexagonal lattice structure of�-NaFeO2 with space group R3m, 166. The lattice parameters ofthis LiMn Ni Co O nanomaterial were a = 2.867 Å and c

2 theta / degrees15 20 25 30 35 40 45 50 55 60 65 70

003

101

104

015 107

018 110

113

006/012

9000C/22 h

C/1 h0700

50 nm

20 – 30 nm003

Mn

NiCo

006/012018

110

015

keV2 4 10 16

a

b

Figure 1. �Color online� XRD patterns of the LiMn1/3Ni1/3Co1/3O2 particlesobtained by SCRs �between metal nitrates and sucrose� and annealed furtherin air at 700°C for 1 h and at 900°C for 22 h, as indicated. Miller indexes ofmain reflections are indicated. Insert �a� demonstrates an HRTEM image ofnanoparticles obtained by annealing at 700°C for 1 h. The scale bar is 50 nm.Several nanoparticles of 20–30 nm are shown in an oval of this image. Insert�b� shows EDS spectra of the LiMn1/3Ni1/3Co1/3O2 particles annealed at900°C for 22 h. Peaks of Mn at 1–6 keV and of Co,Ni at 6–8 keV areindicated.

1/3 1/3 1/3 2

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= 14.259 Å. Insert �a� in Fig. 1 shows a typical HRTEM image ofthe nanoparticles thus produced. Their size was estimated to bearound 20–50 nm, and the specific surface area was 7.8 m2/g. For acomparison, Fig. 1 shows the XRD patterns of the submicrometricLiNMC material produced by heating the product of the SCR at900°C for 22 h. This material is characterized by lattice parameters

a = 2.864 Å and c = 14.261 Å, particle sizes of around 200–450nm, and a specific surface area of 2.8 m2/g. A comparison in Fig. 1demonstrates the chemical and structural identities of both materials.From the elemental chemical analysis �by the ICP technique� andenergy-dispersive spectroscopy �EDS� studies, an atomic ratio of1:1:1 was measured for Mn, Ni, and Co of theseLiMn1/3Ni1/3Co1/3O2 materials. Highly resolved splitting of the 006/012 and 018/110 peaks of the LiMn1/3Ni1/3Co1/3O2 material �an-

8000C

9000C 10000C

7000C

100 nm 100 nm

100 nm 1 µm

Figure 2. HRSEM images of the LiMn1/3Ni1/3Co1/3O2 particles obtained bySCRs and annealed further in air for 22 h at 700, 800, 900, and 1000°C, asindicated.

E / V vs. Li/Li+2.0 2.5 3.0 3.5 4.0 4.5 5.0

I/µA

-150

-100

-50

0

50

100

1503.90 V

3.55 VT=300C a2.0 2.5

I/µA

-150

-100

-50

0

50

100

150

T=600C

E / V vs. Li/Li+2.0 2.5 3.0 3.5 4.0 4.5

I/µA

-150

-100

-50

0

50

100

150

3.70 V

3.80 V

Charge 160 mAh/g

Disch. 150 mAh/g

Charge 16

T=600C

Disch. 155

Charge 160 mAh/g

Disch. 155 mAh/g

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nealed at 900°C for 22 h� and the ratio of the lattice parametersc/a = 4.98 provide evidence of a characteristic well-ordered layeredstructure of the compound. The ratios of the integrated intensities of003 and 104 peaks of the XRD patterns were around 1.10 and 1.17,respectively, for the above materials produced at 700°C for 1 h andat 900°C for 22 h, therefore indicating no cation mixing.10 Lowvalues of the R-factor R = �I012 + I006�/I101, around 0.41–0.42,which relate to the integrated intensities of the corresponding peaksof the LiMn1/3Ni1/3Co1/3O2 materials, also confirm their highenough hexagonal ordering.

By further heating of the as-prepared LiMn1/3Ni1/3Co1/3O2 ma-terial to 800–1000°C, submicrometric and micrometric particles canbe produced. Figure 2 presents SEM micrographs of LiMNC par-ticles produced at different calcination conditions �700–1000°C for22 h in air�, as indicated. The images of Fig. 2 clearly demonstratethe effect of temperature on the particle size, which increases byincreasing the annealing temperature of the product. The specificsurface areas were 14.2, 8.4, 2.8, and 1.4 m2/g for theLiMn1/3Ni1/3Co1/3O2 materials produced by annealing for 22 h at700, 800, 900, and 1000°C, respectively.

The electrochemical behavior of LiMn1/3Ni1/3Co1/3O2 electrodesand the effect of aging.— In this work, we compare the electro-chemical behavior of electrodes comprising nano- and submicromet-ric particles of LiMn1/3Ni1/3Co1/3O2 produced from SCR by calci-nation at 700°C for 1 h and at 900°C for 22 h, respectively. Figure3 shows charts of a slow scanning rate �60 �V/s�, repeated steady-state cyclic voltammograms �CVs� �in the standard solutions used�of electrodes comprising nano-LiMNC at 30 and 60°C �3a and b,respectively�, as well as a chart showing the slow scan rate CVresponse of electrodes comprising the submicrometric LiMNC at60°C �3c�. The relevant specific capacity obtained for these elec-trodes is indicated on each chart. In general, these experiments re-flect a steady behavior of the electrodes. Qualitatively speaking, thedifference in the CV response of the nano-LiNMC electrodes at 30and 60°C is surprisingly small. The broad peaks superimposed oncorresponding cathodic and anodic waves reflect insertion–deinsertion reactions that form solid solutions rather than separatedphases. These peaks indicate Ni2+/Ni3+/Ni4+ oxidation/reduction re-

V vs. Li/Li+3.5 4.0 4.5 5.0

3.90 V

3.60 V b

/g

Figure 3. Slow scan rate CVs measured at�a� 30 and �b� 60°C from composite elec-trodes comprising LiMn1/3Ni1/3Co1/3O2nanoparticles �annealed at 700°C for 1 h�,carbon black, and PVDF. �c� CVs of asimilar composite electrode comprisingLiMn1/3Ni1/3Co1/3O2 submicrometric par-ticles �annealed at 900°C for 22 h�. Therelevant specific capacity obtained forthese electrodes in charge and dischargeprocesses is indicated on each chart.Three-electrode coin-type cells; EC–DMC�1:2�/1.5 M LiPF6 solutions. Scan rate was60 �V/s.

E /

3.0

5.0

c

5 mAh

mAh/g

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actions followed by the cobalt Co3+/Co4+ redox processes, whichaccompany lithium deinsertion and insertion processes from/into thehost material.28 The minor effect of the 30°C difference in the tem-peratures, to which charts 3a and b relate, may indicate that at el-evated temperatures these electrodes are very reactive and develop

Capacity / mAhg-10 50 100 150 200

Potential/V

2.0

2.5

3.0

3.5

4.0

4.5

5.0

0 50

Potential/V

2.0

2.5

3.0

3.5

4.0

4.5

5.0

4C C/5

4C C/5

4C

Nano-LiMn1/3Ni1/3Co1/3O2 Submicr

a

Cycle number2 4 6 8 10 12 14 16 18 20

Dischargecapacity/mAhg-1

40

80

120

160C/5

C/2

1C

7000 C/1 h

9000 C/22 h

C/5

Cycle number2 4 6 8 10 12

Dischargecapacity/mAhg

-1

100

120

140

160

180Initial state

7 days14 days

21 days

T=600C

Time of discharge / h0 1 2 3 4 5 6

Dischargecapacity/mAhg

-1

80

100

120

140

160

180C/5C/21C4C

Rate of dicharge

7000C/1 h

9000C/22 h

ab

Figure 5. Cycling behavior at 30°C of electrodes comprisingLiMn1/3Ni1/3Co1/3O2 nanoparticles �700°C/1 h� and LiMn1/3Ni1/3Co1/3O2submicrometric particles �900°C/22 h� at various rates, as indicated. Two-electrode coin-type cells; EC–DMC �1:2�/1.5 M LiPF6 solutions. Cyclingmode was CC–CV. Inserts: �a� Discharge capacity of the electrode compris-ing LiMn1/3Ni1/3Co1/3O2 nanoparticles �700°C/1 h� measured at 60°C at itsinitial state and after cycling and aging the cell at 60°C for 7, 14, and 21days, as indicated. The capacities obtained are average values from cyclicvoltammetry measurements of three identical LiMn1/3Ni1/3Co1/3O2 elec-trodes. Scan rate was 50 �V/s; EC–DMC �1:2�/1.5 M LiPF6 solutions. �b�Rate capabilities �capacity vs Crates, namely, the amount of charge in units offull electrode capacity, discharged in 1 h�, measured at 30°C from electrodescomprising LiMn1/3Ni1/3Co1/3O2 nanoparticles �700°C/1 h� andLiMn1/3Ni1/3Co1/3O2 submicrometric particles �900°C/22 h�.

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high impedance due to the formation of surface films, which levelsoff the gains in the kinetics of transport phenomena, such as chargetransfer and solid-state diffusion due to the elevated temperature.The CV of the electrodes comprising submicrometric LiMNC mea-sured at 60°C �chart 3c� demonstrates sharper anodic �delithiation�and cathodic �lithiation� peaks. The difference in the potential be-tween both peaks �chart 3c� is only 100 mV, three times lower com-pared to the peak separation measured with the electrodes compris-ing nanomaterials. This response may indicate faster electro-chemical kinetics of electrodes prepared from submicrometricLiMNC.

Figure 4 compares the voltage profiles of the electrodes compris-ing the nanometric and submicrometric LiMNC materials �4a and b�at C/5 and 4C rates, as indicated. Figure 5 presents cycling data�galvanostatic operation� for 20 consecutive cycles of electrodescomprising the nano- and submicrometric LiMNC active masses,during which the rates were changed. In the first five cycles, theelectrodes were operated at C/5 rates, after which the rate was in-creased to C/2, to 1C, and then back to C/5, as indicated. The insertto Fig. 5 �chart 5b� shows a presentation of capacity vs rate for thetwo types of electrodes. The data in Fig. 3-6 show that the nano-LiMNC material has an initial slightly higher capacity than the sub-micrometric MNC �170 vs 160 mAh/g, respectively�. However, thesubmicrometric active mass is considerably faster in Li insertion/deinsertion reactions and demonstrates better rate capability. Thecharts in Fig. 4 and 5 are very spectacular in this respect and clearlydemonstrate this fact. Figure 6 relates to prolonged galvanostaticcycling experiments with electrodes comprising nano- and submi-crometric LiMNC. In some of these experiments, the rates werechanged during cycling, as indicated. These experiments demon-strated an impressive stability of the submicrometric LiMNC elec-trodes upon cycling in standard electrolyte solutions. The data inFig. 6 also demonstrate the inferior rate capability of the nanometricmaterial. The insert to Fig. 5 shows cycling data measured at 60°Cof electrodes comprising nanometric LiMNC. These experiments in-

city / mAhg-1100 150 200

C/5

C/5

iMn1/3Ni1/3Co1/3O2

b

Figure 4. Voltage profiles measured at30°C from electrodes comprising �a�LiMn1/3Ni1/3Co1/3O2 nanoparticles �an-nealed at 700°C for 1 h� and �b�LiMn1/3Ni1/3Co1/3O2 submicrometric par-ticles �annealed at 900°C for 22 h� at C/5and 4C rates, as indicated. Two-electrodecoin-type cells; EC–DMC �1:2�/1.5 MLiPF6 solutions.

Cycle number

Aging at 300C, 1 weekevery 100 cycles

300 400 500

3C

-micron-LiMn1/3Ni1/3Co1/3O29000C /22 h

cFigure 6. �a� Cycling behavior of theelectrode comprising LiMn1/3Ni1/3Co1/3O2nanoparticles �700°C/1 h� at various rates�as indicated� measured at 30°C. �b� Fur-ther cycling performance at a 2C rate ofthe same LiMn1/3Ni1/3Co1/3O2 electrode asin �a�. �c� Prolonged cycling behavior andaging for 1 week every 100 cycles of anelectrode comprising LiMn1/3Ni1/3Co1/3O2submicrometric particles �900°C/22 h� at3C rate. This electrode was cycled previ-ously at various rates �200 cycles�. Two-electrode coin-type cells; EC–DMC �1:2�/1.5 M LiPF6 solutions.

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cluded aging steps at 60°C. The electrodes were stored for periodsof 1 week at 60°C in a fully lithiated state, after which they under-went a few galvanostatic cycles at the C/5 rate, as indicated. Theelectrochemical data presented in Fig. 3-6 are very coherent. Theyshow that despite the smaller particle size and the higher surfacearea of the nanometric LiMNC cathode material �properties that arefavorable for a fast bulk solid-state diffusion of Li ions and for fastinterfacial charge-transfer processes, respectively�, it demonstrates alower rate capability than the submicrometric material, whose size ison average much bigger, and the specific surface area is smaller. Thenanometric active mass demonstrates a decrease in capacity uponaging in standard solutions at 60°C. Hence, it was important torigorously explore the aging and cycling of the nanometric LiMNCby additional spectroscopic and microscopic tools to understand thereason for the inferiority of this material, compared to the activemass of the same chemical and crystal structure whose particle sizeis bigger.

Figure 7 shows typical impedance spectra of the compositenano-LiMn1/3Ni1/3Co1/3O2 electrode in the standard electrolyte so-lution measured at three different potentials �3.8, 4.0, and 4.3 V�during the charge process �delithiation� at 60°C, then at 30°C, andfurther at 60°C. In these experiments, the electrode underwent sev-eral voltammetric cyclings at each temperature before the imped-ance spectroscopic measurements to bring the electrode to steadyconditions. These spectra show three main superimposed features: ahigh frequency semicircle, which seems to be a potential invariant, amedium frequency semicircle, which slightly depends on the poten-tial �i.e., the equilibrium potential on which the alternating voltagewas superimposed�, and a low frequency straight line. The interpre-tation of the impedance spectra of composite intercalation electrodesis very problematic, as discussed in papers from Levi andAurbach,51 Aurbach et al.,52 and Prosini et al.42 The compositestructure: Particulated active mass, carbon powder, and binder maydistort the electrochemical response of the active mass, which un-dergoes reversible ion insertion reactions. In the present case, theelectrodes were thin �a load of up to 2 mg/cm2�, and therefore it isassumed that the electrochemical response of the electrodes pre-sented herein represents mostly the active mass. Beyond the possibleimpact of the composite structure of these electrodes on their im-pedance response, there is a question as to what extent the electricalcontact between the aluminum current collector and the active mass

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influences the impedance spectra measured with Li insertionelectrodes.53 The electrochemical impedance spectroscopy �EIS� re-sponse presented in Fig. 7 is somewhat similar to the impedanceresponse that we measured with many composite electrodes com-prising lithiated transition-metal oxides �LiMn0.5Ni0.5O2,12

LiMn1.5Ni0.5O4,44 LiCoO2,54 etc�. We treat the EIS data on the quali-tative level as a probe for examining the electrodes’ passivation as afunction of their aging. Tentatively, we attribute the high frequencysemicircle to the impedance response of the surface films that areobviously formed on all LixMOy particles55 �resistance of Li-ionmigration coupled with film capacitance�. The medium semicircle isattributed to charge-transfer processes, and the low frequency fea-tures �usually a straight sloping line� are connected to the solid-statediffusion processes of Li ions in the bulk of the active mass.12,52,55

The EIS measurements of electrodes comprising LiMn0.5Ni0.5O2,LiMn1.5Ni0.5O4, and LiMn1/3Ni1/3Co1/3O2 nano- to microparticles asthe active mass all show the same trend. The impedance responsestabilizes to pictures, very similar to those presented in Fig. 7 withina few CV cycles or storage during a few days at constant potential.Increasing the temperature, e.g., from 30 to 60°C, does not decreasethe impedance considerably although the impedance response ofthese electrodes is activation controlled �e.g., Li-ion migrationthrough surface films, interfacial charge-transfer, and diffusion pro-cesses�. Hence, one would expect that the impedance of these elec-trodes should decrease accordingly. It is very logical to assume thatincreasing the temperature in these systems enhances the surfacereactions that make the surface films thicker, which increases theoverall electrode impedance and the levels of the above-expectedeffect of temperature. However, the electrodes’ surface �probably thesurface films that they develop� stabilizes when the electrodes areprocessed �cycled and aged� once at the elevated temperature. Thisis well reflected in Fig. 7. Panel A shows the impedance data of anelectrode that was stabilized at 60°C. The measurements of the sameelectrode at 30°C reflect the expected increase in the electrode’simpedance �panel B� because the surface films formed and stabilizedat 60°C behave as typical activation-controlled resistances. Furthercycling and aging of the same electrode at 60°C do not change itsimpedance response considerably, as clearly seen by comparing pan-els A and C in Fig. 7.

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V

5 10 15 20 25 30

V

5 10 15 20 25 30

C

200 mHz

200 mHz

200 mHz

5 mHz

5 mHz

12.6 mHz

Figure 7. Impedance spectra of an elec-trode comprising LiMn1/3Ni1/3Co1/3O2nanoparticles �annealed at 700°C/1 h� ob-tained first at 60°C �panel A� after theelectrode was cycled by several CVs�50 �V/s� from 2.5 to 4.5 V at 30°C�panel B�, and finally the electrode wascycled by several CVs �scan rate50 �V/s� from 2.5 to 4.5 V at 60°C andimpedance spectra were measured at thiselevated temperature �panel C�. Open-circuit potentials at which the impedancespectra were measured are indicated.Three-electrode coin-type cell containingEC–DMC �1:2�/1.5 M LiPF6 solution.

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Surface chemistry studies of LiMn1/3Ni1/3Co1/3O2 electrodes.—The surface chemistry of these electrodes was explored by FTIR andXPS measurements, comparing between pristine and cycled to agedelectrodes. In general, the surface chemistries developed on allLi�MnNi�O2 or Li�MnNiCo�O2 compounds in standard electrolytesolutions �alkyl carbonates with EC as a critical component andLiPF6� are similar, as was demonstrated and discussed in previouspapers.56 Therefore, we do not show here the FTIR or XPS datasimilar to those that we showed previously.12,44,57,58 In brief, thesolutions unavoidably contain HF, PF5, and PF3O, resulting from thepartial decomposition of LiPF6 and the reaction between PF5 andtrace water. These species react with LixMOy �acid–base reactions�to form surface LiF, MF, and M–O–POxFy compounds �well dem-onstrated by XPS measurements�. The oxygens of the LixMOy com-pounds are nucleophilic and attack the alkyl carbonate molecules,which are electrophilic. These reactions form surface ROCO2Li and�ROCO2�Mx compounds. However, ROCO2

− anions can further at-tack alkyl carbonate molecules, thus leading to polymerization thatforms surface polycarbonate species �well detected by FTIRmeasurements12�. Alkyl carbonates can also undergo polymerizationvia a cationic mechanism due to oxidation when the electrodes aredelithiated. Elevated temperatures accelerate all of these surface re-actions. A quantitative study of the surface films thus formed byFTIR and XPS is not trivial. However, on the qualitative level, it canbe claimed that nanoparticles that have a high specific surface areaand less smooth facets are much more surface reactive than micro-metric particles. Consequently, LixMOy nanoparticles may developthicker surface films than submicrometric or micrometric particles.This means a higher surface impedance that may balance the pos-sible advantages in kinetics due to the small particle size �shortdiffusion length� and high surface area �which facilitates interfacialcharge transfer�. However, it was important to explore how agingindeed influences the bulk structure, morphology, and surface prop-erties of the LiMn1/3Ni1/3Co1/3O2 nanoparticles and to find out if themain impact is on the bulk or the surface structure.

LiMn1/3Ni1/3Co1/3O2 has very typical Raman spectra, which aredemonstrated in Fig. 8 �peak assignment therein�. The Raman peaksat 595 and 480 cm−1 are attributed to the active A1g and Eg modes,respectively, and they mainly correspond to vibrations of oxygenatoms of LiMO2 compounds.14 As seen in Fig. 8, the aging ofLiMn1/3Ni1/3Co1/3O2 nanoparticles, even at 60°C, does not seem tochange their bulk response. Aged particles retain their structure, aswas confirmed by comparative XRD measurements �patterns are notshown here�. Further important information about aging these nano-particles was obtained by EPR measurements.

EPR studies of LiMn1/3Ni1/3Co1/3O2 nanoparticles.— Figure 9compares the EPR spectra of pristine LiMn1/3Ni1/3Co1/3O2 nanopar-ticles and particles aged for 5 and 8 weeks in EC–DMC �1:2�/1.5 MLiPF6 solutions at 30°C. The EPR spectrum of pristineLiMn1/3Ni1/3Co1/3O2 is dominated by a broad line with a Lorentzianshape and a linewidth of about 880 G. The g-factor decreases whendecreasing the registration temperature, reaching a g-value of 2.00 at130 K. A narrow Lorentzian shape superimposed on the broad signalis clearly resolved. The g-factor is higher than 2.00 and remainsconstant in the temperature range of 130–290 K: g = 2.148. Thelinewidth is about 60 G.

To assign broad and narrow EPR signals at pristineLiMn1/3Ni1/3Co1/3O2, X-band and high frequency EPR studies onLiNiyCo1−yO2 and LiMn1/3Ni1/3Co1/3O2 compositions have to betaken into account.50,58-64 The paramagnetic ions stabilized in lay-ered LiMn1/3Ni1/3Co1/3O2 are Ni2+ and Mn4+ ions, while a Ni3+ ionin a low spin state appears only for layered LiNiyCo1−yO2. For bothcompositions, Co3+ ions �adopting low spin configuration� are dia-magnetic. The EPR spectrum of LiMn1/3Ni1/3Co1/3O2 has beenshown to display a signal due to Mn4+ ions, Ni2+ ions beinginactive.50,60-64 The EPR linewidth and g-factor reflect the localmetal environment of Mn4+ in LiMn Ni Co O . The EPR spec-

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trum of LiNiyCo1−yO2 consists of a single Lorentzian line with g= 2.142 and linewidth increasing with Ni3+ content.

According to previous EPR studies on LiMn1/3Ni1/3Co1/3O2 andLiNiyCo1−yO2 compositions, the broad and narrow EPR signals ofpristine LiMn1/3Ni1/3Co1/3O2 can be attributed to Mn4+ and Ni3+

ions, respectively. The changes in the g-factor of Mn4+ with a reg-istration temperature are close to that established forLiMn1/3Ni1/3Co1/3O2 samples obtained by a coprecipitationmethod,60 supporting the identical origin of the EPR signals for bothcompositions. However, the EPR linewidth remains smaller, about90 and 120 mT for pristine LiMn1/3Ni1/3Co1/3O2 �produced by SCR�and for LiMn1/3Ni1/3Co1/3O2 samples obtained by the coprecipita-tion method, respectively. The narrower EPR signal observed forpristine LiMn1/3Ni1/3Co1/3O2 is related to the local Mn4+ environ-ment.

In an attempt to study this phenomenon, one has to take intoaccount50,61-64 the EPR parameters of Co-free analogsLiNi1/2Mn1/2O2, where the Ni-to-Mn ratio is kept constant, and thelayered Li�Li�1−2x�/3NixMn�2−x�/3�O2 series, where the Ni-to-Mn ratiovaried from 0 to 0.5. As in LiMn1/3Ni1/3Co1/3O2 samples, the EPRspectra of LiNi1/2Mn1/2O2 and Li�Li�1−2x�/3NixMn�2−x�/3�O2 seriescontain one broad Lorentzian line due to Mn4+ only. When the Ni-to-Mn ratio is kept constant, the EPR linewidth is slightly lower thanthat of the Co-free analogs LiNi1/2Mn1/2O2: 100–110 and 110–130mT for LiNi1/2Mn1/2O2 and LiMn1/3Ni1/3Co1/3O2 prepared by thecoprecipitation method, respectively.50,61-64 This is a simple conse-

Raman shift / cm-1

400 500 600 700 800

PristineLiNi1/3Mn1/3Co1/3O2nano-particles

Aged particles(in solution at 300C,

8 weeks)

Aged particles(in solution at 600C,

6 weeks)

A1g mode590 - 600 cm-1

Eg mode480 cm-1

Figure 8. �Color online� Raman spectra of the pristine LiMn1/3Ni1/3Co1/3O2nanoparticles �annealed at 700°C/1 h� and particles aged in EC–DMC �1:2�/1.5 M LiPF6 solutions at 30°C for 8 weeks and at 60°C for 6 weeks. Themain Raman-active peaks are indicated. Raman plots were smoothed usingthe LOESS modeling method �SigmaPlot software�. The dashed lines are aguide for the eyes only. Aging was carried out in polyethylene vials usingmagnetic stirring in a glove box �argon atmosphere�.

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quence of the magnetic dilution of Ni2+–Mn4+ spin systems by dia-magnetic Co3+ ions. The EPR linewidth undergoes a strong narrow-ing when the Ni-to-Mn ratio decreases. Based on this comparativeanalysis, the narrow EPR signal observed for pristineLiMn1/3Ni1/3Co1/3O2 indicates that the local Ni-to-Mn ratio isslightly lower than 1. This result means a nonhomogeneous Mn4+

distribution. �The composition inhomogeneity detected by EPR ismainly developed up to three metal neighbors and is not detected bythe XRD technique.50�

In regard to the second signal due to Ni3+ ions, its intensity ismore than 3 orders of magnitude lower than the intensity of theMn4+ signal. This shows that the Ni3+ ions have a much lowerconcentration than the Mn4+ ions. The EPR linewidth of Ni3+ ions inpristine LiMn1/3Ni1/3Co1/3O2 tends to the EPR linewidth of Ni3+

spin probes in LiCoO2, where the Ni3+ content is less than 0.005.58

The appearance of Ni3+ in a Co3+ environment, which is not dis-turbed by the presence of the main paramagnetic Mn4+ and Ni2+

ions, can be explained with the formation of LiCoO2-like domains inpristine LiMn1/3Ni1/3Co1/3O2. This is in accordance with the nonho-mogeneous cationic distribution.

The EPR spectrum consisting of signals due to Mn4+ and Ni3+

ions is preserved after the aging of oxides in electrolyte solutions�comprising EC, DMC, and LiPF6� at 30 and 60°C �Fig. 9a and b�.The EPR linewidth of Mn4+ ions remains constant after aging ofoxides at 30°C: about 885 G. On the contrary, the high temperatureaging yields a line broadening: 885, 964, and 1034 G for pristineLiNi1/3Co1/3Mn1/3O2, LiNi1/3Co1/3Mn1/3O2 aged for 2 weeks at60°C, and LiNi1/3Co1/3Mn1/3O2 aged for 5 weeks at 60°C, respec-tively. In the same sequence, the EPR linewidth of Ni3+ shows atendency to decrease, especially in when oxides are aged at 60°C:59 � 5, 47 � 4, and 52 � 4 G for pristine LiNi1/3Co1/3Mn1/3O2,LiNi1/3Co1/3Mn1/3O2 aged for 2 weeks at 60°C, andLiNi1/3Co1/3Mn1/3O2 aged for 5 weeks at 60°C, respectively. Theconstancy in the EPR parameters of both Mn4+ and Ni3+ ions illus-trates the stability of layered LiNi1/3Co1/3Mn1/3O2 in electrolyte so-lutions at 30°C. By increasing the temperature of electrolyte solu-tions, the observed EPR parameters of Mn4+ point to local changesin the layered structure. From the EPR point of view, the local

a)

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structural changes comprise the Ni-to-Mn ratio. Apparently, the lo-cal Ni-to-Mn ratio tends to increase after the aging of oxides at60°C.

A highly important aspect related to the use of lithiatedtransition-metal oxides as cathode materials in Li-ion batteries is thepossible dissolution of transition-metal ions into the standard elec-trolyte solutions. Such ions, when present in solutions, are reducedin the anode side to metallic clusters that very detrimentally affectthe passivation of graphite electrodes4,65 �the commonly used an-odes in Li-ion batteries�. Figure 10 shows the amount of transition-metal ions �in percent� that dissolved from nano-LiMNC powderinto a standard electrolyte solution �EC–DMC/LiPF6� at 30 and at60°C, measured by rigorous ICP measurements of the solutions. Theresults thus obtained are highly interesting and important. As ex-pected, the three transition-metal cations indeed dissolve from thismaterial to the solution phase. However, upon aging during several

c field / G

Pristine particles

Aged particles(2 weeks, 600C)

Aged particles(5 weeks, 600C)

Mn4+

Figure 9. �Color online� EPR spectra ofthe pristine LiMn1/3Ni1/3Co1/3O2 nanopar-ticles �annealed at 700°C/1 h� and par-ticles aged in EC–DMC �1:2�/1.5 M LiPF6solutions at 30°C during 5 and 8 weeks�a� and in the above solutions at 60°C dur-ing 2 and 5 weeks. �b� Experimentalcurves and Lorentzian lines of signals dueto Mn4+ and Ni3+ ions are shown. Agingof LiMn1/3Ni1/3Co1/3O2 nanoparticles wascarried out in polyethylene vials usingmagnetic stirring in a glove box �argon at-mosphere�.

0

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0.006

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0.014

0.016

0 1 2 3 4 5 6 7 8 9

Time of aging in solution / weeks

Dissolution/%

Mn

NiCo

Ni

Mn

Co

600C

300C

Figure 10. Dissolution of Mn, Ni, and Co from LiMn1/3Ni1/3Co1/3O2 nano-particles �annealed at 700°C/1 h� during aging in EC–DMC �1:2�/1.5 MLiPF6 solutions at 30°C �open symbols� and 60°C �filled symbols�. Agingwas carried out in polyethylene vials using magnetic stirring in a glove box�argon atmosphere�.

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weeks, initially the amount of cations dissolved increases with time,but then it levels off and reaches a steady value. Hence, dissolutionstops after a few weeks of aging. The dissolution rate of all threetransition metals at 60°C seems to be higher than that measured at30°C, as expected. It is interesting to see that the dissolution of Niions at 60°C is comparable to that measured at 30°C, and for Coand Mn ions the dissolution at 60°C �3–4 weeks aging� is almost 5times higher than that at 30°C. The dissolution of ions at 60°Cceases after about 3 weeks of storage, while at 30°C it takes morethan 6 weeks of aging to stabilize the active mass �no further iondissolution, Fig. 10�. These results can be explained by very pro-nounced passivation phenomena that occur at 60°C and avoid fur-ther interactions between the bulk active mass and the solutionphase. Hence, at 30°C, the completed and efficient passivation ofthe nano-LiMNC powder needs a much longer aging period.

HRTEM and AFM imaging of pristine and agednano-LiMn1/3Ni1/3Co1/3O2 particles: Formation of aging layers.—The surface films that passivate the nano-LiMNC cathode materialstudied herein �in standard electrolyte solutions� are clearly seen byHRTEM imaging, as demonstrated in Fig. 11. HRTEM images ofpristine nano-LiMn1/3Ni1/3Co1/3O2 powder �Fig. 11a� clearly showthe nanocrystalline particles with a very good reflection of theatomic planes at the right spacing �d = 4.72 Å�. In Fig. 11b, whichshows an image of aged powder �60°C in an EC–DMC �1:2�/1.5 MLiPF6, within 3 weeks�, it is easy to identify the nanocrystallineparticles, surrounded by an amorphous phase �marked in Fig. 11b�.The effect of the aging of nano-LiMNC powder in EC–DMC/LiPF6electrolyte solutions on the morphology and surface properties ofthe active mass was further explored by AFM �tapping mode�.

Figure 12a shows an AFM image of the pristinenano-LiMn1/3Ni1/3Co1/3O2 particles dispersed on a smooth siliconsurface. Typical sections with height profiles of the particles arepresented in the insert. This image demonstrates the dimension ofthe particles produced by SCR, followed by calcination at 700°C for1 h �most of the particles are of 20 and 40–50 nm, resulting from thestatistical measurements of several AFM images�. Figure 12b showsa typical AFM image of a powder �dispersed on a similar siliconsurface� after being aged for 3 weeks at 60°C in an EC–DMC �1:2�/1.5 M LiPF6 solution. Typical height profiles along sections in theimage �marked therein� are presented in the insert on the right-handcorner of the picture. It can clearly be seen that, in general, theaverage particle size increases due to the formation of thick surfacelayers during aging in solution. These surface layers may compriseLiF, Mn �Ni,Co�-fluorides, transition-metal oxides, M–O–POxFy,ROCO2Li, and �ROCO2�Mx compounds and polycarbonates due tothe aging of the LiMn1/3Ni1/3Co1/3O2 particles, as discussed above.The properties of these surface layers �formed by aging� are differ-ent from those of the bulk particles, as was demonstrated by

LiMn1/3Ni1/3Co1/3O2d=4.72 Å

LiMn1/3Ni1/3Co1/3O2d=4.72 Å

Aging Layer

LiMn1/3Ni1/3Co1/3O2d=4.72 Å

10 nm 10 nm

LiMn1/3Ni1/3Co1/3O2d=4.72 Å

a b

Figure 11. �Color online� HRTEM images of �a� pristineLiMn1/3Ni1/3Co1/3O2 nanoparticles annealed at 700°C/1 h and �b� particlesaged in an EC–DMC �1:2�/1.5 M LiPF6 solution at 60°C for 3 weeks. Agingwas carried out in polyethylene vials using magnetic stirring in a glove box�argon atmosphere�.

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HarmoniX-AFM studies. The formation of a thick amorphous layerwas also observed on the surface of the LiNi0.8Co0.2O2 cathode par-ticle �using HRTEM� by Amine et al.66 after a prolonged cycling testof lithium-ion cells at elevated temperatures.

2

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Figure 12. �Color online� �a� Typical HarmoniX-AFM topographic imagesof pristine LiMn1/3Ni1/3Co1/3O2 nanoparticles �annealed at 700°C/1 h� and�b� particles aged in EC–DMC �1:2�/1.5 M LiPF6 solutions at 60°C for 3weeks. The particles were dispersed on smooth silicon surfaces for the AFMmeasurements. Inserts represent section analysis and height profiles of theabove LiMn1/3Ni1/3Co1/3O2 particles. Aging was carried out in polyethylenevials using magnetic stirring in a glove box �argon atmosphere�.

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By controlling the force applied to the AFM tip and measuringthe tip’s response when it is lifted up from a particle by the tappingmode used, these AFM measurements can also provide informationabout the adhesion forces between the particles and the tip and thestiffness of the particles thus imaged. These properties relate directlyto the state of the surface of the measured particles.

Figure 13a and b shows the adhesion forces and the stiffness ofpristine nano-LiMn1/3Ni1/3Co1/3O2 particles, such particles after ag-ing at 30°C for 8 weeks, and such particles after aging at 60°C for3 weeks, as indicated. It is clearly demonstrated how aging increasespronouncedly the adhesion forces and reduces the particles’ stiff-ness. Hence, these measurements complete the microscopic studiesof the nano-LiMNC material, showing how aging leads to surfacefilm formation in standard solutions, which makes the particlesstickier �thus reflecting the formation of amorphous surface layer, asexpected�. All the measurements presented herein demonstrate thatat elevated temperatures, the surface film formation is accelerated,and hence, the surface properties are considerably changed.

These studies also included thermal studies by DSC. In brief, interms of specific heat evolution, there is no difference betweennano- or micrometric particles. Clearly, the entire active mass reactswith solution species, no matter what the particle size is. However,the onset of the thermal reaction is very dependent on the nature ofthe passivation layers developed on the LiMNC particles �whichdepends also on their size�. The connection among the aging history,the nature of the passivation layer, and the characteristics of thethermal reactions are complicated and deserve a separate publica-tion.

Conclusion

It seems that lithiated transition-metal oxides are very reactivewith standard EC–DMC/LiPF6 solutions. In fact, all Li�MnNi�O2,Li�MnNi�2O2 �spinel�, and Li�MnNiCo�O2 compounds develop avery similar, rich surface chemistry in electrolyte solutions based onalkyl carbonates and LiPF6, which include the acid–base reactions�LixMOy + HF, PF5, and POF3�, the nucleophilic reactions betweenthe negatively charged oxygens of the active mass, and the electro-philic alkyl carbonate molecules and polymerization steps initiatedby anionic or cationic surface species, in which EC molecules areprobably involved, to form polycarbonate surface species. In thiswork, LiMn1/3Ni1/3Co1/3O2 was studied, and the main theme was acomparison between nano- and submicrometric particles producedby the same basic reaction, a SCR between the metal nitrates andsucrose, followed by calcination at high temperatures. Heating to700°C for 1 h was enough to produce crystalline nanoparticleswhose morphology, properties, and behavior were compared tothose of particles obtained by calcination at 900°C for 22 h �submi-crometric particles�. For nanoparticles, there is a nonhomogeneousMn4+ distribution, together with a formation of LiCoO2-like do-mains. It was found that the nanoparticles are much more reactivewith solution species, especially at elevated temperatures. However,the reactivity relates to the surface and not to the bulk. Core-shellstructures are formed in which the shell comprises thick surface

(b)a)

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films that impede the electrochemical reactions of the active mass.At elevated temperatures, the surface reactions accelerate. However,due to effective passivation, the bulk active mass is slightly per-turbed with respect to the local Ni-to-Mn ratio. Consequently, theelectrochemical performance of electrodes comprising nanoparticlesof Li�MnNiCo�O2 is worse than that of electrodes comprising sub-micrometric particles in terms of rate capability.

Generally, it can be concluded that cathode materials comprisingLi–Mn–Ni–Co–O elements at any stoichiometry should be very re-active with standard electrolyte solutions, and thus they developsurface films and passivation phenomena. The smaller the particles,the higher the surface reactivity. Therefore, despite the apparent ki-netic advantage to the use of nanomaterials in electrochemical reac-tions due to short diffusion length and facile interfacial charge-transfer reactions, the use of nanoparticles in cathodes comprisingLi–Mn–Ni–Co–O compounds is not at all favorable due to theabove-described surface phenomena and their impact on propertiessuch as electrodes’ impedance. This is in contrast to the case ofLiMPO4 cathodes where the use of nanoparticles is critical for goodperformance. The use of LMPO4 as nanoparticles is possible be-cause these compounds are much less basic and nucleophilic com-pared to LixMOy. Hence, they are much less reactive with solutionspecies. The possibility to use nanoparticles of transition-metal ox-ide cathodes for high rate capability depends on the ability to controlthe pronounced surface reactions of these materials in standard elec-trolyte solutions. There are two directions: modifying the surfacechemistry by the use of active additives and coating the particles bythin surface films that can act as buffers against reactions with so-lution species �e.g., coating by basic species such as MgO that avoidreactions between the LixMOy cathode materials and trace HF insolutions�.

Acknowledgment

Partial support for this work was obtained from the Israeli Na-tional Science Foundation. E.Zh. and R.S. are grateful for the finan-cial support from the National Science Fund of Bulgaria �Ch 1701/2007�.

Bar-Ilan University assisted in meeting the publication costs of this ar-ticle.

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