Electrochimica Acta 49 (2004) 557–563

Synthesis and structural characterization of layeredLi[Ni 1/3Co1/3Mn1/3]O2 cathode materials by

ultrasonic spray pyrolysis method

S.H. Parka, C.S. Yoonb, S.G. Kangb, H.-S. Kimc, S.-I. Moonc, Y.-K. Suna,∗a Department of Chemical Engineering, Hanyang University, Seungdong-Gu, Seoul 133-791, South Korea

b Division of Materials Science and Engineering, Hanyang University, Seungdong-Gu, Seoul 133-791, South Koreac Battery Research Group, Korea Electrotechnology Research Institute, 28-1 Sungjoo-dong Changwon, Kyungnam 641-120, South Korea

Received 4 April 2003; received in revised form 25 August 2003; accepted 3 September 2003

Abstract

The layered Li[Ni1/3Co1/3Mn1/3]O2 materials were synthesized by a spray pyrolysis method using citric acid as a polymeric agent. TheLi[Ni 1/3Co1/3Mn1/3]O2 powders were characterized by means of X-ray diffraction (XRD), charge/discharge cycling, cyclic voltammetry, andhigh-resolution transmission electron microscopy (TEM). The discharge capacity increases linearly with the increase of the upper cut-offvoltage limit. TEM analysis showed that particles in the as-prepared powder possessed a polycrystalline structure. During cycling, the particlestructure is mostly preserved although some surface grains on the polycrystalline particle became separated and transformed to the spinelphase.© 2003 Elsevier Ltd. All rights reserved.

Keywords: Lithium-ion battery; Cathode materials; Layered structure; Spray pyrolysis; HRTEM

1. Introduction

Considerable effort has been expended in the last fewyears in the development of rechargeable batteries to meetdemands for the powering of portable electronic devices suchas cellular phones and laptop computers and more recentlyelectric vehicles. Lithium secondary batteries have satisfiedthis demand to a greater degree than other rechargeable bat-tery systems. Layer-structured LiCoO2 has been commer-cialized as active cathode material for lithium secondarybatteries. However, due to its high cost and toxicity, an in-tensive search for new cathode materials has been underwayin recent years. LiNiO2 and LiMnO2 have been extensivelystudied as possible alternatives to LiCoO2.

However, LiNiO2 and LiMnO2 have problems for prac-tical applications. LiNiO2, iso-structured with LiCoO2, hasreceived great interest as an alternative material due toits higher capacity of more than 150 mA h g−1 with high,

∗ Corresponding author. Tel.:+82-2-2290-0524;fax: +82-2-2282-7329.

E-mail address: yksun@hanyang.ac.kr (Y.-K. Sun).

smooth, and monotonous voltage profile[1]. However, thematerial could not be used due to the difficulty in synthesisof stoichiometric LiNiO2 and instability of Ni4+ ions athigh voltages, which transformed into NiO2 phase at thesurface of the particles during electrochemical cycling[2].LiMnO2 is not thermodynamically stable as the layeredstructure, but as an orthorhombic phaseo-LiMnO2 [3].Therefore, the LiMnO2 were observed to undergo a detri-mental phase transformation to a spinel-like phase throughminor atomic rearrangements during the first removal andsubsequent cycling of Li, leading to eventual degradationof electrode performance[4,5].

Recently, a concept of a one-to-one solid solution ofLiCoO2, LiNiO2, and LiMnO2, i.e., Li[Ni1/3Co1/3Mn1/3]O2,was adopted by Ohzuku and Makimura to overcomethe disadvantage of LiNiO2 and LiMnO2 [6]. TheLi[Ni 1/3Co1/3Mn1/3]O2 has the�-NaFeO2 structure withspace groupR3̄m, which is characteristic of the layeredLiCoO2 and LiNiO2 structures and showed larger capac-ity of more than 150 mA h g−1 in the voltage range of2.5–4.2 V with excellent cycleability and no transforma-tion to spinel phase during electrochemical cycling. The

0013-4686/$ – see front matter © 2003 Elsevier Ltd. All rights reserved.doi:10.1016/j.electacta.2003.09.009

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Li[Ni 1/3Co1/3Mn1/3]O2 powders were synthesized by themixed hydroxide method using LiOH·H2O and a nickelcobalt manganese hydroxide with a homogeneous cationdistribution.

Spray pyrolysis method is a useful method for the syn-thesis of high purity, narrow size distribution, homogenouscomposition oxide particles with a spherical morphology.Recently, Taniguchi et al. reported that spherical spinelLiMn2O4 powders could be synthesized using this methodby varying gas flow rates and temperature profiles in thereactor[7].

In this work, we report on the synthesis and electrochemi-cal properties of the Li[Ni1/3Co1/3Mn1/3]O2 powders usingultrasonic spray pyrolysis method.

2. Experimental

Li[Ni 1/3Co1/3Mn1/3]O2 was prepared as follows:[Ni1/3Co1/3Mn1/3]O2 precursor was first synthesized us-ing the spray pyrolysis method. Nickel nitrate hexahy-drate (Ni(NO3)·6H2O, Aldrich), cobalt nitrate hexahydrate(Co(NO3)2·6H2O, Aldrich), manganese nitrate tetra hydrate(Mn(NO3)2·4H2O, Sigma) salts were used as starting ma-terials for the synthesis of the [Ni1/3Co1/3Mn1/3]O2 pow-ders. A stoichiometric amount of Ni, Co, and Mn nitratesalts (cationic ratio of Ni:Co:Mn= 1:1:1) was dissolved indistilled water. The dissolved solution was added into a con-tinuously agitated aqueous citric acid solution. Citric acidwas used as a polymeric agent for the reaction. The molarratio of total metal ions to citric acid was fixed at 0.2. Thestarting solution was atomized using an ultrasonic nebulizerwith a resonant frequency of 1.7 MHz. The aerosol streamwas introduced into the vertical quartz reactor heated at500◦C. The inner diameter and length of the quartz reactorare 50 and 1200 mm, respectively. The flow rate of air usedas a carrier gas was 10 L min−1. The prepared powderswere mixed with excess amount of LiOH·H2O. After themixture were sufficiently ground, the mixture was heated to900◦C with a heating rate of 1◦C min−1.

The powder X-ray diffraction (XRD, Rint-2000, Rigaku,Japan) measurement using Cu K� radiation was employedto identify the crystalline phase of the synthesized material.Rietveld refinement of the collected data was made usingthe Fullprof Rietveld program[8]. Particle morphology ofthe powders after calcination was observed using a scan-ning electron microscope (SEM, JSM 6400, JEOL, Japan).Atomic absorption spectroscopy analysis (AAS Vario 6, An-alytikjenaAG, Germany) was investigated to the real chem-ical composition.

Galvanostatic charge/discharge cycling was performedin a 2032-type coin cell (Hohsen Co. Ltd., Japan).For the fabrication of the positive electrode, 20 mg ofLi[Ni 1/3Co1/3Mn1/3]O2 was mixed with 12 mg of conduc-tive binder (8 mg of teflonized acetylene black and 4 mg ofgraphite). The mixture was pressed onto 200 mm2 stainless

steel mesh used as the current collector and dried at 130◦Cfor 10 h in a vacuum oven. Lithium foil was used as thenegative electrode. The electrolyte solution was 1 M LiPF6in a mixture of ethylene carbonate (EC) and diethyl carbon-ate (DEC) in a 1:1 volume ratio. The cell was assembled inan argon-filled dry box and tested at room (30◦C) and high(55◦C) temperatures.

3. Results and discussion

The XRD pattern of the [Ni1/3Co1/3Mn1/3]Oy precursorobtained at 500◦C is shown inFig. 1(a). Although the XRDpattern of the precursor has a very lower crystallinity, itwas found that the precursor is composed of a mixture ofNiO (upper tick, JCPDS No. 47-1049), Co3O4 (middle tick,JCPDS No. 43-1003), and Mn3O4 (lower tick, JCPDS No.13-0162). From the AAS analysis, the chemical compositionwas determined to be Ni0.326Co0.332Mn0.335O2. This wouldbe ascribed to the homogeneous powder precursor, in whichcations such as Ni, Co and Mn are uniformly distributedin an atomic scale. The Rietveld refinement analysis of the

Fig. 1. (a) Powder X-ray diffraction pattern (XRD) of [Ni1/3Co1/3

Mn1/3]Oy precursors. (b) Rietveld refinement patterns of the Li[Ni1/3

Co1/3Mn1/3]O2 cell powders calcined at 900◦C.

S.H. Park et al. / Electrochimica Acta 49 (2004) 557–563 559

Fig. 2. (a) Charge/discharge curves of Li/Li[Ni1/3Co1/3Mn1/3]O2 cell at 30◦C. (b) Corresponding specific discharge capacity of Li/Li[Ni1/3Co1/3Mn1/3]O2

cell in various voltage ranges at a constant current density of 0.2 mA cm−2 (20 mA g−1).

Li[Ni 1/3Co1/3Mn1/3]O2 was shown inFig. 1(b). TheR3̄m

model has smaller difference between both intensities andBragg R-factor of 1.86% would show a successful refine-ment. Therefore, the refinement pattern would be identifiedas a hexagonal layered structure with a space group,R3̄m

(No. 166). The observed diffraction lines shows a large in-tegrated peak ratio (0 0 3) to (1 0 4) of 1.48 and a clear splitof the (1 0 8) and (1 1 0) peaks, which suggests a minimaldisorder in the host structure. The hexagonal lattice param-eters of Li[Ni1/3Co1/3Mn1/3]O2 were determined to bea =2.863(7) Å andc = 14.255(7) Å calculated by Rietveld re-finement from the X-ray diffraction data, which are consis-tent with the values published by Ohzuku and Makimura[6].

Fig. 2(a) shows charge/discharge curves of Li/Li[Ni1/3Co1/3Mn1/3]O2 cell cycled in various voltage ranges at aconstant current density of 0.2 mA cm−2 (20 mA g−1) and30◦C; corresponding discharge capacities are shown inFig. 2(b). The Li/Li[Ni 1/3Co1/3Mn1/3]O2 cell has a very

smooth and monotonous voltage profile, similar to the volt-age profiles reported by other researchers[6,9]. It is notedthat their charge/discharge curves do not change even up to4.6 V. The discharge capacities increase linearly with theincrease of the upper cut-off voltage limit. In the voltagerange of 2.8–4.3, 2.8–4.4, 2.8–4.5 and 2.8–4.6 V, the dis-charge capacities of Li[Ni1/3Co1/3Mn1/3]O2 electrode are163, 171, 181, 188 mAh g−1, respectively, with excellentcycleability at a high current density.

Fig. 3 shows specific discharge capacity vs. number ofcycle for Li/Li[Ni 1/3Co1/3Mn1/3]O2 cell at 30 and 55◦C inthe voltage range of 2.8–4.4 V at a constant current densityof 0.2 mA cm−2 (20 mA g−1). The Li[Ni1/3Co1/3Mn1/3]O2electrode delivers an initial discharge capacities of 170 and185 mAh g−1 at 30 and 55◦C, respectively. The dischargecapacities at 30 and 55◦C slowly decrease with cycling andremained at 163 and 173 mAh g−1 after 50 cycles, which are96 and 94% of initial capacities, respectively.

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Fig. 3. Specific discharge capacity vs. number of cycle for Li/Li[Ni1/3Co1/3Mn1/3]O2 cell at 30 and 55◦C in the voltage range of 2.8–4.4 V.

Fig. 4 shows the charge/discharge curves of Li/Li[Ni1/3Co1/3Mn1/3]O2 cell at various current density and 30◦C be-tween potential limits of 2.8–4.4 V. The cell was chargedusing a current density of 0.32 mA cm−2 (0.2C rate) be-fore each discharge test. The discharge capacity slowly de-creases with increasing current density and reach up to90 and 78% at 1.6 (1C rate) and 6.4 mA cm−2 (4C rate)compared with 0.32 mA cm−2 (0.2C rate), respectively. TheLi[Ni 1/3Co1/3Mn1/3]O2 electrode has an excellent rate ca-pability.

Fig. 5shows cyclic voltammogram of the first five cyclesof the Li/Li[Ni 1/3Co1/3Mn1/3]O2 cell between 2.5 and 4.6 Vat 30◦C. The important feature is the difference between thefirst and subsequent cycles. The first anodic scan has two ox-

Fig. 4. Charge/discharge curves of Li/Li[Ni1/3Co1/3Mn1/3]O2 cell at various current density between 2.8 and 4.4 V. The cell was charged using a currentdensity of 0.32 mA cm−2 (0.2C rate) before each discharge test.

idation peaks, a major peak centered at 3.93 V and a minorone at 4.5 V corresponding to irreversible capacity observedin the first charge profile (Fig. 2(a)). The peak 3.93 V in thefirst anodic scan is shifted by 0.07 V to lower voltage. Onthe subsequent cycle, the oxidation and reduction processesshow only one major peak centered at 3.86 and 3.66 V, re-spectively, and there are no redox peak changes after fivecycles. This behavior implies that structural degradation isnot expected during the lithium extraction/insertion processof Li[Ni 1/3Co1/3Mn1/3]O2 electrode unlike LiNiO2. It hasbeen well known that the muti-phase reactions resulting fromthree peaks in LiNiO2 lead to structural change, resulting ineventual degradation of electrode performance[10]. More-over, it is noted that no reduction peak near 3.2 V resulted

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Fig. 5. Cyclic voltammogram of Li/Li[Ni1/3Co1/3Mn1/3]O2 cell between 2.5 and 4.6 V at a scan rate of 100�V s−1.

Fig. 6. (a) Bright field TEM image of the as-prepared Li[Ni1/3Co1/3Mn1/3]O2. (b) TEM image of the cycled Li[Ni1/3Co1/3Mn1/3]O2 electrode cycled atroom temperature. The arrows indicate particles broken off the polycrystalline particle. (c) TEM image of the cycled Li[Ni1/3Co1/3Mn1/3]O2 electrodecomposed of both spinel and layered phases. Electron diffraction pattern on the top is in the 1 1 0 zone of the layered phase whereas one on the bottomshows the 1 0 0 zone of the spinel phase.

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Fig. 6. (Continued ).

from the reduction of Mn4+ to Mn3+ appearing in lithiumrich layered materials, Li[Li(1−2x)/3NixMn(2−x)/3]O2 phasewas observed during electrochemical cycling[11,12].

Microstructures of the as-prepared and cycled electrodewere examined using transmission electron microscopy(TEM). Shown inFig. 6(a)is the bright field image of theas-prepared Li[Ni1/3Co1/3Mn1/3]O2 powders. The powdersproduced using spray pyrolysis consisted of polycrystallineaggregates whose size averaged∼500 nm. Each polycrys-talline aggregate was made up with grains whose size rangedfrom 10 to 50 nm. The observed microstructure of theas-prepared powders were quite different from the previouspowders produced using conventional solid state sinteringor sol–gel method in which individual particles were typi-cally composed of single crystals[12,13]. Fig. 6(b) showsthe TEM image of the electrode cycled at room tempera-ture. The polycrystalline structure observed inFig. 6(a)wasmore or less maintained after 50 cycles; however, it was alsoobserved some of grains separated from the polycrystallineparticle as indicated inFig. 6(b). Using electron diffraction,crystallographic structure of these separated phases wasidentified in Fig. 6(c). Single crystal electron diffractionanalysis confirmed that the separated grains were a mixtureof spinel and layered hexagonal phases. Although the num-ber of particles examined using TEM was rather limited, itmay be speculated that a part of grains on the surface ofthe particle underwent a phase transformation to the spinelstructure and that the strain involved in the phase transfor-mation could have led to the break-up of the surface grains.

TEM analysis showed that the powder synthesized usingthe spray pyrolysis process possessed a unique microstruc-ture consisting of polycrystalline aggregates. It appears thatthe interior part of the polycrystalline aggregate well main-tained its structural integrity during cycling. Some of the

surface grains, however, became separated and transformedto the spinel structure.

4. Conclusion

Layered Li[Ni1/3Co1/3Mn1/3]O2 powders with high ho-mogeneity and high capacity was synthesized by spraypyrolysis. The Li[Ni1/3Co1/3Mn1/3]O2 electrode delivers ahigh discharge capacity of 188 mAh g−1 between 2.8 and4.6 V at a high current density of 0.2 mA cm−2 (20 mA g−1)with excellent cycleability. The charge/discharge and cyclicvoltammogram studies of the Li[Ni1/3Co1/3Mn1/3]O2 elec-trode showed only one redox peak, suggesting minimalstructural degradation during cycling. Close examinationof the particle using TEM also showed that the particlestructure is mostly preserved after cycling although somesurface grains on the polycrystalline particle appear to havetransformed to the spinel phase.

Acknowledgements

This work was performed by the partly financial supportof Center for Nanostructured Materials Technology’ under‘21st Century Frontier R&D Programs’ of the Ministry ofScience and Technology, and supported by Korea Scienceand Engineering Foundation via Research Center for EnergyConversion and Storage, Korea.

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