Electrochimica Acta 50 (2004) 435–441

Preparation and properties of spherical LiNi0.75Co0.25O2 asa cathode for lithium-ion batteries

Yao Chena,∗, G.X. Wanga, J.P. Tianb, K. Konstantinova, H.K. Liua

a Institute for Superconducting and Electronic Materials, University of Wollongong, Wollongong, NSW 2522, Australiab Changshun Chemical Co. Ltd., China

Received 2 June 2003; received in revised form 20 March 2004; accepted 20 March 2004Available online 29 July 2004

Abstract

Spherical LiNi0.75Co0.25O2 compounds were synthesized by sintering spherical Ni0.75Co0.25(OH)2 and LiOH·H2O precursors at varioustemperatures in an oxygen atmosphere. A pure phase LiNi0.75Co0.25O2 could be identified. SEM observation showed that the LiNi0.75Co0.25O2

particles are spherical in shape and are composed of many small crystals. Magnetic susceptibility measurements reveal that the sphericalLc and goodc©

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iNi 0.75Co0.25O2 compounds have a more ordered layered structure than that of non-spherical LiNi0.75Co0.25O2. The spherical LiNi0.75Co0.25O2

athodes demonstrated a stable electrochemical performance in lithium-ion cells with a high reversible capacity of 167 mAh/gyclability.2004 Elsevier Ltd. All rights reserved.

eywords: Lithium battery; Spherical; Magnetic susceptibility; Precursor

. Introduction

LiCoO2 compound has been widely used as a cathode ma-erial in commercial lithium-ion battery production becauset is reasonably easy to synthesize and shows a stable dis-harge capacity. However, due to its high cost and toxicity,any efforts have been made to replace LiCoO2. LiNiO2 isn attractive material because of its low cost and its possi-ility of a high charge/discharge capacity. However, LiNiO2lso has a few disadvantages compared to LiCoO2. Its majorisadvantages are the difficulty in preparation so as to achievetoichiometry and poor cycle life. In an attempt to overcomehe problems associated with LiNiO2, a great deal of effort haseen concentrated on dopant materials that may possess prop-rties superior to LiNiO2. Recently, LiNi1−xMxO2, where M

s one of the transition metals, has been studied extensivelyn order to solve these problems by stabilizing the struc-ure of LiNiO2 [1–5]. The results have shown that, in these

∗ Corresponding author. Fax: +61 2 4221 5731.E-mail address:yao chen@uow.edu.au (Y. Chen).

substituted compounds, cobalt substitution sharply decrthe non-stoichiometry of lithium nickel oxide and thus sbilizes the alternating layered structure. So LiCoxNi1−xO2has been identified as one of the most attractive mate[6,7].

Spherical nickel hydroxides are widely used as poselectrode materials in Ni–Cd and Ni–MH batteries. Thibecause spherical shaped powder particles have a highing density and a homogeneous micro-morphology. Wthose powders with spherical shaped particles are uselectrode materials, more active materials can be loadthe electrode. Therefore, the energy density of the baies can be significantly improved. Similar principle canapplied to lithium-ion batteries when using spherical etrode materials. Recently, spherical LiCoxNi1−xO2 has become much more attractive due to its high tap-densityhigh uniform morphology[8]. In this paper, the sphecal LiNi0.75Co0.25O2 compounds were synthesized bying spherical Ni0.75Co0.25(OH)2 and LiOH·H2O as precursors. The physical and electrochemical properties of sphLiNi 0.75Co0.25O2 cathodes were systematically tested.

013-4686/$ – see front matter © 2004 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2004.03.053

436 Y. Chen et al. / Electrochimica Acta 50 (2004) 435–441

2. Experimental

2.1. Materials preparation

The spherical Ni0.75Co0.25(OH)2 precursor powders weresynthesized by chemical co-precipitation and controlled crys-tallization. The reagents NiSO4·6H2O (99%, Aldrich) andCoSO4·7H2O (99.9%, Aldrich) were dissolved in de-ionizedwater and homogeneously mixed. A mixture of NaOH andNH4OH solution was added to precipitate the hydroxide. ThepH (10–11) value, temperature and pressure were strictlycontrolled. The precipitations were then aged in a pres-surized vessel at 60–80◦C for a few hours to allow thespherical crystals to grow. The co-precipitates were thor-oughly washed using de-ionized water and then dried in avacuum oven to yield spherical (Ni0.75Co0.25)(OH)2 pow-ders. The spherical (Ni0.75Co0.25)(OH)2 and LiOH·H2O pre-cursor powders were intimately mixed with a molar ra-tio of Ni0.75Co0.25:Li = 1:1.04 (due to evaporation of Liduring high temperature sintering) by mortar and pestle.The mixture was then sintered at high temperature between650 and 850◦C for 12 h under flowing oxygen to obtainspherical LiNi0.75Co0.25O2 compounds. The non-sphericalLiNi 0.75Co0.25O2 compounds were prepared by sintering theprecursors Li2CO3, NiO, and CoO at 750◦C under flowingo

2

C n( Kr st ani iC db ningeC tumD

2

eb bonb ind urry.T m).T ovena ssedt d thec elec-t stc aun,U ec-t -l 1 by

volume, provided by MERCK KgaA, Germany). The cellswere galvanostatically charged and discharged in the voltagerange of 3.0–4.3 V versus Li/Li+ at a constant current densityof 0.4 mA/cm2. Three-electrode cells were constructed forcyclic voltammetry (CV) and ac impedance measurements,in which lithium metal foils were used for both counter elec-trode and reference electrode. The CV were performed on anEG&G Scanning Potentiostat (Model 362) at a scanning rateof 0.1 mV/s and ac impedance measurements was carried outon an EG&G Electrochemical Impedance Analyzer (Model6310).

3. Results and discussion

3.1. Physical characterization of sphericalLiNi0.75Co0.25O2 compounds

Ni0.75Co0.25(OH)2 is a cobalt-doped derivative ofNi(OH)2. Fig. 1shows the X-ray diffraction pattern of spher-ical Ni0.75Co0.25(OH)2 powders. The Ni0.75Co0.25O2 com-pounds adopted the�-Ni(OH)2 phase. All diffraction lineswere indexed based on a hexagonal structure (SG:p3m1).No impurity phases were detected by X-ray diffraction.This clearly demonstrates that Co2+ can partially substi-t 2+ -a ri ea fewm

L 700,7 yd r-i em-p sw -plt ra-tp re-p ing,wTssstr wo -s , thes sm on ofL ichL s

xygen.

.2. Physical characterization

The structure of the Ni0.75Co0.25(OH)2 and LiNi0.75o0.25O2 were characterized by X-ray diffractio

MO3xHF22, MacScience Co. Ltd., Japan) using Cu�adiation. The lattice constants were calculated againnternal silicon standard. The morphologies of the N0.75o0.25(OH)2 and LiNi0.75Co0.25O2 powders were observey using SEM (Leica/Cambrdge Steroscan 440 scanlectron microscope). The susceptibility of the LiNi0.75o0.25O2 compounds was measured using a Quanesign Magnetometer (PPMS, USA).

.3. Electrochemical measurement

The spherical LiNi0.75Co0.25O2 electrodes were mady dispersing 80 wt.% active materials, 15 wt.% carlack and 5 wt.% polyvinylidene fluoride (PVDF) binderimethyl phthalate solvent to form a homogeneous slhe slurry was spread onto an aluminum foil disk (Ø 12 mhe coated disk-type electrodes were dried in a vacuumt 120◦C for 12 h. The dried electrodes were then pre

o enhance the contact between the active material anonductive carbon black particles. The thickness of therode was approximately 50–60�m after pressing. The teells were assembled in an argon filled glove-box (Mbrnilab, USA) using lithium metal foil as the counter el

rode. The electrolyte was 1 M LiPF6 in a mixture of ethyene carbonate (EC) and dimethyl carbonate (DMC) (1:

ute for Ni in the �-Ni(OH)2 structure. An SEM imge of the as-prepared Ni0.75Co0.25O2 precursor powde

s presented inFig. 2. The Ni0.75Co0.25O2 particles havspherical shape and an average particle size of aicrons.The precursor mixtures of spherical Ni0.75Co0.25O2 and

iOH·H2O were sintered at different temperatures, 650,50, 800, and 850◦C, respectively.Fig. 3 shows the X-raiffraction patterns of LiNi0.75Co0.25O2 obtained by sinte

ng at different temperatures. When sintered at low terature, e.g., 650, 700, and 750◦C, some impurity phaseere detected. However, when sintered above 800◦C, phaseure LiNi0.75Co0.25O2 was obtained. InFig. 3, all diffraction

ines were indexed based on a hexagonal�-NaFeO2 struc-ure (R3m). Fig. 3also shows a large integrated intensityio of I(0 0 3)/I(1 0 4) above 1.50 for LiNi0.75Co0.25O2 com-ound sintered at 800◦C. The intensity ratio has beenorted to be closely related to undesirable cation mixhich is reduced as the value of the ratio is increased[9].he lattice parametersa andc of LiNi 0.75Co0.25O2 synthe-ized at 800◦C are 2.859 and 14.164A, respectively.Fig. 4hows SEM images of as-prepared LiNi0.75Co0.25O2 powderintered at 800◦C. A general view of LiNi0.75Co0.25O2 par-icles is shown inFig. 4(a). The LiNi0.75Co0.25O2 particlesetain the spherical shape.Fig. 4(b) shows a magnified vief a single spherical LiNi0.75Co0.25O2 particle, which conists of many small crystals. During the sintering processpherical shape of the precursor Ni0.75Co0.25O2 particles waaintained. Therefore, we can deduce that the formatiiNi 0.75Co0.25O2 phase is a diffusion process during whi+ ions diffuse into spherical�-Ni(OH)2 precursor particle

Y. Chen et al. / Electrochimica Acta 50 (2004) 435–441 437

Fig. 1. X-ray diffraction pattern of�-Ni0.75Co0.25(OH)2.

at high temperature. The final product retains the sphericalshape of the precursor particles.

Temperature-dependent magnetic susceptibility measure-ments of the spherical LiNi0.75Co0.25O2 powders were per-formed in a dc field of 1000 G using a Quantum DesignMagnetometer (PPMS, USA).Fig. 5shows the temperaturedependence of the magnetic mole susceptibility of spheri-cal LiNi0.75Co0.25O2 powders. As a comparison, the molesusceptibility of non-spherical LiNi0.75Co0.25O2 is also pre-sented inFig. 5. The magnetic susceptibility of the spheri-cal and non-spherical LiNi0.75Co0.25O2 powders obeys the

pherica

Curie–Weiss law:χ = C/(T − θ), whereχ is the magneticsusceptibility,C is the Curie constant, andθ is the param-agnetic Curie temperature. The spherical LiNi0.75Co0.25O2powders have a negative paramagnetic Curie temperatureθ

= −25 K, indicating antiferromagnetic behavior belowTc,while non-spherical LiNi0.75Co0.25O2 powders have a pos-itive paramagnetic Curie temperatureθ = 19 K, indicatingferromagnetic interactions of the magnetic centers (Ni3+ andCo3+) [10–12].

LiNi 0.75Co0.25O2 compounds have a rhombohedral lay-ered structure (R3m), in which Li+, Ni3+/Co3+, and

Fig. 2. SEM image of the s

l Ni0.75Co0.25(OH)2 precursor.

438 Y. Chen et al. / Electrochimica Acta 50 (2004) 435–441

Fig. 3. X-ray diffraction patterns of LiNi0.75Co0.25O2 compounds sintered at different temperatures.

Fv

O2− ions occupy the 3a, 3b, and 6c sites, respectively. BothNi3+ (3d7, low spin) and Co3+ (3d6, low spin) are mag-netic ions. Non-magnetic Li+ layers alternate with mag-netic Ni3+/Co3+ layers. Therefore, magnetic correlations be-tween Ni3+/Co3+ ions in the ideal LiNi0.75Co0.25O2 crystalstructure are considered to be two-dimensional (Ni3+/Co3+(3b)–Ni3+/Co3+ (3b)). Once there are magnetic Ni3+/Co3+ions in 3a (Li+) sites, a three-dimensional magnetic in-teraction between Ni3+/Co3+ (3b)–Ni3+/Co3+ (3a) lay-ers will be created, which usually causes ferromagneticanomalies. The ferromagnetism observed for non-sphericalLiNi 0.75Co0.25O2 indicates cation disorder in its layeredstructure. In contrast, the spherical LiNi0.75Co0.25O2 pow-ders demonstrated strong antiferromagnetism, which couldresult from a frustrated antiferromagnetism of the trigonallyarranged Ni3+/Co3+ ions within one layer[13,14]. Therefore,the spherical LiNi0.75Co0.25O2 powders have a more orderedlayered structure than that of non-spherical LiNi0.75Co0.25O2compounds.

3.2. Electrochemical characteristics of sphericalLiNi0.75Co0.25O2 cathodes

Cyclic voltammetry measurements were performed ona spherical LiNi0.75Co0.25O2 electrode at a sweep rate of0e tionp ob-

ig. 4. SEM images of spherical LiNi0.75Co0.25O2 particles: (a) generaliew of spherical particles and (b) a single spherical particle.

s ateda cess,a aredi re-s theF as:C

.1 mV/s.Fig. 6shows the CV curve of the LiNi0.75Co0.25O2lectrode, in which a pair of well-defined redox reaceaks were observed. The main oxidation peak waserved at 4.17 V, and the main reduction peak was loct 3.84 V. During the anodic and cathodic scanning propair of additional oxidation and reduction peaks appe

n the CV profile, which are located at 4.25 and 4.18 V,pectively. The operating potential, which is related toermi energy of the electrons in the 3d orbital, variesoO2(3d5/3d6) > NiO2(3d6/3d7) [15]. If Ni 3+ cations in

Y. Chen et al. / Electrochimica Acta 50 (2004) 435–441 439

Fig. 5. The mole magnetic susceptibilityχ vs.T (K) for LiNi 0.75Co0.25O2 compounds.

the LiNi0.75Co0.25O2 cathode mainly contribute to the re-dox reaction during Li+ insertion/extraction, these additionalredox peaks observed at high potentials could be presentbecause of the participation of Co3+ in the redox reac-tion, since Co3+/Co4+ has a higher potential than that ofNi3+/Ni4+.

The first charge/discharge curve of the sphericalLiNi 0.75Co0.25O2 cathode is shown inFig. 7. The electrodewas cycled in the voltage range of 3.0–4.3 V at a constantcurrent density of 0.4 mA/cm2. In the first cycle, theLiNi 0.75Co0.25O2 cathode delivered a reversible dischargecapacity of 167.4 mAh/g, which is much higher than that of aLiCoO2 cathode. There is a 40.6 mAh/g irreversible capacityin the first cycle. This irreversible capacity could be utilizedto compensate for the lithium consumption on the surface of

f the sp

the carbon anode to form the SEI during the first chargingprocess in the commercial lithium-ion batteries. In order todetermine the cyclability of the spherical LiNi0.75Co0.25O2cathode materials, a LiNi0.75Co0.25O2 cathode was cycledfor 50 cycles.Fig. 8 shows the results of the cycling test.The spherical LiNi0.75Co0.25O2 cathode materials demon-strated a stable cyclability over charge/discharge cycling.In our synthesis process, the spherical�-Ni0.75Co0.25(OH)2precursor was prepared by a solution route, in which Ni2+and Co2+ ions were homogeneously mixed on the atomiclevel. When forming LiNi0.75Co0.25O2 compounds, Ni2+and Co2+ ions should distribute themselves uniformly in thecrystal structure. The results of the magnetic susceptibilitymeasurements confirmed that the spherical LiNi0.75Co0.25O2compounds have a highly ordered layered structure. These

Fig. 6. Cyclic voltammogram o

herical LiNi0.75Co0.25O2 cathode.

440 Y. Chen et al. / Electrochimica Acta 50 (2004) 435–441

Fig. 7. The first charge/discharge curves of the spherical LiNi0.75Co0.25O2 cathode.

Fig. 8. The discharge capacity of the spherical LiNi0.75Co0.25O2 cathode vs. cycle number.

physical characteristics could contribute to the good elec-trochemical performance of the spherical LiNi0.75Co0.25O2cathode materials.

4. Conclusions

Single-phase spherical LiNi0.75Co0.25O2 cathode materi-als were successfully synthesized by sintering LiOH·H2Oand spherical Ni0.75Co0.25(OH)2 precursors at high tempera-ture. The magnetic susceptibility measurements show thatthe spherical LiNi0.75Co0.25O2 compounds have a highlyordered layered structure. The spherical LiNi0.75Co0.25O2cathode materials demonstrated a high reversible capacity of167 mAh/g and stable cyclability. When these spherical cath-

ode materials are used as cathodes in lithium-ion batteries,high energy densities could be achieved.

Acknowledgements

The Australian Research Council (ARC), Sons of GwaliaLtd., OM Group Inc., and Changshun Chemical Co. Ltd. sup-port this work.

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