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Materials Science and Engineering B 133 (2006) 8–13

Phase transformation of LiNi0.75Co0.25O2�

Hui He, Xuan Cheng, Ying Zhang ∗Department of Materials Science and Engineering, State Key Laboratory for Physical Chemistry of Solid Surfaces,

Xiamen University, Fujian 361005, PR China

Received 17 February 2006; accepted 15 April 2006

bstract

In this article, the phase transformation of cathode material, LiNi0.75Co0.25O2, prepared by sol–gel pretreatment and solid-phase formation, wastudied in detail. The characteristic temperatures associated with the phase transformation of LiNi0.75Co0.25O2 were selected. Accordingly, twoeries of experiments at different temperatures were performed in situ and ex situ by XRD and SEM to examine the structure and morphology

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f the material during the phase transformation. It was found that at the temperature below 600 C a cubic structure was formed and the nickeletal in the material was presented as Ni2+. A hexangular layer structure started to form when the temperature reached or became higher

han 600 ◦C. The structure became better defined with the increase of temperature. The material was characteristic of R-3 m spatial structuret 725 ◦C.

2006 Published by Elsevier B.V.

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eywords: Lithium battery; Cathode material; Phase transformation

. Introduction

Lithium cobalt oxide, LiCoO2, has been widely used as aathode material for commercial secondary lithium-ion batter-es due to its easy preparation, high voltage, good reversibilitynd large theoretical specific capacity. Unfortunately, layeredithium cobalt oxide often suffers from structural instability andafety problems, especially when the lithium content is lowerhan 0.5 or the charge voltage exceeds 4.3 V [1]. In addition,ithium nickel oxide, LiNiO2, has been intensively studied ands considered as a promising cathode material for rechargeableithium batteries due to its low cost and high specific energy.owever, it is difficult to be synthesized with consistent qualityecause of its tendency for non-stoichiometry. Furthermore, aoor cycling performance, its structural instability upon cyclingesults in it has greater safety problems.

In order to improve the overall performance of cathode

aterials, cationic substitutions on the cobalt sites has been

ought extensively. Of these substituted compounds, the layerediNixCo1−xO2 (0 < x < 1) compounds have been widely studied

� Supported by the National Natural Science Foundation of China (No.0472098).∗ Corresponding author. Tel.: +86 592 2180999; fax: +86 592 218 3047.

E-mail address: yzh@xmu.edu.cn (Y. Zhang).

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921-5107/$ – see front matter © 2006 Published by Elsevier B.V.oi:10.1016/j.mseb.2006.04.010

s cathode materials for lithium batteries [5–8]. Several meth-ds, such as sol–gel [9,11,12,16,17], precipitation [10,14], solidhase formation [13], and spray-drying techniques [15], wereeported to prepare LiNixCo1−xO2. In sol–gel method, differenthelating agents can be used, for example, a mixture of citriccid and glycine [9], citric acid [11,12,16], maleic acid. [17]lthough the structure remains hexangular when x varies fromto 1, the electrochemical performance of materials preparedith different x values changed. When x = 0.5, the characteristicf charge–discharge curves tended to be resemble of LiNiO211], as no platform appeared in the curves. With the decreasef x values, the charge–discharge curves became smooth at theeginning of discharge and the ending of charge, implying themproved stability of the material structure [9–18]. The capa-ility loss also decreased with the decrease of x [9–18] byreated off the first cycle charge/discharge capability. The max-mum charge/discharge capability, 211 mA h/g, can be achievedt x = 0.8 with an acceptable capability loss of 29 mA h/g [18].

Hence, the electrochemical characteristics and structure ofathode materials depend strongly on the synthesis method andvalues. In general, the better defined structure can be formed atigher sinter temperature, however, at higher temperature more

ithium will be volatilized, in another word, the layered structurean not be formed when the sinter temperature is too low. Its, therefore, very important to prepare for LiNixCo1−xO2 at aight sinter temperature. During preparation, the material will

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H. He et al. / Materials Science

ndergo many changes in a wide range of temperatures; thenderstanding of phase transformation becomes a key factor inontrol of the structure, accordingly the performance.

This work was carried out to study the structure variation dur-ng the phase transformation. LiNi0.75Co0.25O2 was prepared byol–gel pretreatment and solid-phase formation. The character-stic temperatures near the phase transformation were selectednd two series of experiments were performed in situ and exitu at each selected temperature. Possible structures formednd species presented at different temperatures are discussed.

hen x value is fixed, the temperature becomes one of the mostritical parameters in synthesis process.

. Experimental

.1. Material preparation

LiNi0.75Co0.25O2 was prepared by sol–gel pretreatment usingitric acid as a chelating agent and solid-phase formation. A sto-chiometric amount of lithium nitrate (LiNO3), nickel nitrateNi(NO3)·6H2O) and cobalt nitrate (Co(NO3)·6H2O) was dis-olved in distilled water and mixed with aqueous solution ofitric acid. The resulting solution was stirred at 80 ◦C for morehan 12 h to obtain a clear viscous gel. The gel was dried inn oven at 120 ◦C for 12 h. LiNi0.75Co0.25O2 was calcined atonstant selected temperatures after precalcining the obtainedrecursor at 380 ◦C. During heating and cooling, the variationate of the temperature was fixed at 1 ◦C/min.

.2. Material characterization

.2.1. Thermogravimetric analysisThe precursors of dry gels were first examined by thermo-

ravimetric analysis (TGA) and differential thermal analysisDTA). The thermal analysis was carried out on a Netzsch STA

00 analyzer with 50 mL/min of flowing air and a heating rate of0 ◦C/min in the temperature range of 323–1273 K. The charac-eristic temperatures of cathode materials associated with theirhase transformation were selected based upon the TGA andTA results.

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Fig. 1. Typical XRD patterns of LiNi0.75Co0.25O2 at room temperature. (a) Ful

ngineering B 133 (2006) 8–13 9

.2.2. XRD analysisThe powder X-ray diffraction (XRD) test, using Philips Pan-

lytical X’pert diffractometer with Cu K� radiation operated at0 kV and 30 mA, was performed. Data were collected in theragg angle of 10–90◦ using a step size of 0.0167◦ and a count-

ng time of 10 s per step. Two series of temperature-dependenteasurements were performed in situ and ex situ. For the series

f in situ experiments, the precursors were raised in a heatinghamber of XRD in such a way that at each selected tempera-ure, the temperature was kept constant for 2 h, obtained XRDpectrum, the samples were heated up again to the next selectedemperature, and repeated XRD measurement. For the series ofx situ experiments, the precursors were first heated up to eachf the selected temperatures and kept constant in the oven forh, and then were analyzed by XRD at room temperature.

.2.3. SEM observationThe samples were coated by Au (non-conducting nature) to

btain surface morphology using a scanning electron micro-cope (LEO 1530 field emission SEM, Oxford Instruments),hich was operated at 15 kV. Environmental scanning electronicroscope (XL30) was also used to in situ observe the surfaceorphology of the samples at different temperatures.

. Results and discussion

Fig. 1 shows typical XRD patterns of finished product,iNi0.75Co0.25O2, with an enlarged illustration in the pair peaksf (0 0 6)/(1 0 2) and (0 1 8)/(1 1 0). A single phase of the �-aFeO2 type with a space group of R-3m was determined as

an be seen in Fig. 1a. The ordering of the structure can bevaluated from the XRD spectra in terms of the intensity ratiof (0 0 3)/(1 0 4) (I003/I104) and the degree in peak splitting ofither (0 1 8)/(1 1 0) or (0 0 6)/(1 0 2). The large value of I003/I1041.243) and well-splited peaks in (0 1 8)/(1 1 0) and (0 0 6)/(1 0 2)Fig. 1b) indicate a good layered structure of LiNi0.75Co0.25O2.

The TGA-DTA curves of the dry-gel precursor are given inig. 2. Four distinctive regions, labeled as a–d, were observed in

he temperature range of 30–1000 ◦C. Below 200 ◦C (in region), a small amount of weight loss was due to desorption of super-

l-range (b) an expanded view on peak of (0 0 6)/(0 1 2) and (0 1 8)/(1 1 0).

10 H. He et al. / Materials Science and Engineering B 133 (2006) 8–13

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Table 1Possible species presented at different Bragg angles from in situ XRD spectra

2θ Possible species

20◦–22◦ Li2NiO2.91

42◦–44◦ CoO, NiO, Li0.62CoO2

44◦–45◦ Co1.29Ni1.71O4, Li2NiO2, Li1.47Co3O4, Co3O4,Li0.62CoO2, LiNiO2, Co2NiO4,Ni2O3

51◦–52◦ Co2O3, Ni2O3

62◦–63◦ NiO, CoO75◦–77◦ Li2Ni8O10, Li1.47Co3O4, Co2NiO4, Li2NiO2

FF

Psfture was lower than 650 ◦C, various Co, Ni and Li oxides were

Fig. 2. TG-DTG curves of the dry-gel precursor of LiNi0.75Co0.25O2.

cial and structural water. In region b (200–350 ◦C), the weightoss was attributed to the decomposition of nitrate and/or theehydration of metal citrate to aconitate [21]. A sharp dropn region c corresponded to the combustion of aconitate andts complex with 70% of total weight loss. In the temperatureange of 400–600 ◦C (region d), a broad peak indicates the phaseransformation. The weight remained virtually unchanged at theemperature higher than 600 ◦C. The characteristic temperaturesf 350, 380, 400, 450, 500, 550, 600, 650 and 725 ◦C were, there-ore, selected based upon the TGA and DTA results.

In an effort to understand the structure formation, in situ andx situ XRD and SEM measurements were conducted at thebove selected temperatures. The precursors were first heated upo each of the selected temperatures and kept constant in the ovenor 2 h, and then analyzed ex situ by XRD at room temperature.he temperature-dependent XRD results are compared in Fig. 3

nd possible compounds are summarized in Table 1.

It is evident from Fig. 3 that the characteristic peaks of (0 0 3)nd (1 0 4), and the pair peaks of (0 0 6)/(1 0 2) and (0 1 8)/(1 1 0)ormed at the temperature above 650 ◦C as compared to Fig. 1.

ig. 3. XRD patters of LiNi0.75Co0.25O2 on synthesized at different temperatureor 2 h.

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ig. 4. The ratio of peak intensity of (0 0 3)/(1 0 4) for LiNi0.75Co0.25O2 fromig. 3.

eak (0 0 3) started to form at room temperature, and becametronger and sharper with the rise of temperature. Peak (1 0 4)ormed at much more higher temperature. When the tempera-

resented and stablized as can be seen in Fig. 3 (at 2θ angles near0◦–45◦) and Table 1. Similarly, two pair peaks, (0 0 6)/(1 0 2)

ig. 5. In situ XRD patterns of LiNi0.75Co0.25O2 prepared and after being keptt the temperature indicated for 2 h.

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H. He et al. / Materials Science

2θ near 35◦–40◦) and (0 1 8)/(1 1 0) (2θ near 60◦–65◦), formedt the temperature higher than 650 ◦C. The results presented inig. 3 provide strong evidence for the structure change, i.e., aell-defined larger hexagonal structure of LiNi0.75Co0.25O2 is

ormed at the temperature above 650 ◦C. Below 650 ◦C, a cubictructure is dominant.

The ordering of the structure was evaluated by plotting theatio in the intensities of (0 0 3)/(1 0 4) in Fig. 4. It is evidenthat there is a rapid increase in the ratio value of I003/I104 in theemperature range of 400–650 ◦C. Again, it confirms that thehase transformation from cubic to hexagonal occurred in theemperature range of 400–550 ◦C, which agrees well with theesults of TGA in Fig. 2 and XRD in Fig. 3.

In an effort to get more information on phase transformation,he in situ XRD test was performed in the temperature rangef 400–550 ◦C and the results are given in Fig. 5. As comparedith ex situ XRD results in Fig. 3, two well-defined peaks in 2θ

ngles of 40◦–50◦ were stable at higher temperatures (500 ◦C)han ex situ case. However, the two peaks joined and became aingle peak when the sample was kept at 500 ◦C for 2 h, and at50 ◦C. The single peak splited into two when the temperature

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Fig. 6. SEM micrographs of LiNi0.75Co0.25O2 at di

ngineering B 133 (2006) 8–13 11

ropped from 550 to 30 ◦C. This observation suggests that NiOxisted and became more stable when the sinter temperatureas raised to the selected point, the hexagonal structure formed

equired certain growth time, i.e., both the sinter temperaturend the time needed for the growth of crystalline are critical tohe formation of desirable layer structure. A small peak observedn 2θ of 51◦–52◦ at 350 ◦C, shown in Fig. 3, became strongert 550 ◦C or at 500 ◦C for 2 h, indicating the presence of Co2O3r Ni2O3 as impurities. This peak did not go away when theemperature was reduced directly from 550 to 30 ◦C, implieshat the valence of Co and Ni ions changed from +2 to +3, e.g.,rom NiO to Ni2O3.

Figs. 6 and 7 are micrographs of samples obtained ex situ andn situ, respectively. As can be seen in Fig. 6, fine crystallinearticles formed when synthesized at 450 ◦C for 2 h, the size ofarticles increased and the particles became better-defined as theemperature increased. The morphologies of particles at 650 and

25 ◦C in Fig. 6 did not change. In Fig. 7, the micrographs werebtained at each temperature indicated, the sample was compactt 400 ◦C, and became porous with the formation of crystallinearticles as the temperature increased, clearly showing the pro-

fferent sintering temperatures holding for 2 h.

12 H. He et al. / Materials Science and Engineering B 133 (2006) 8–13

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ess of phase transformation in the range of 400–580 ◦C, whichs in good agreement with TGA and XRD results.

To provide more evidence for the change of Ni ion valence,he density of state (DOS) for LixNi0.75Co0.25O2 was calculatedhen x = 1 and 0 using VASP and the results are shown in Fig. 8.uring the deintercalation of lithium (x = 0) the Fermi energy

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Fig. 8. Density of state for LixNi0.7

.25O2 at different sintering temperatures.

Ef) decreased, and the DOS curve at the energy lower than Efas more uniformly distributed as compared to the intercalation

f lithium (x = 1), implying the enhancement in the degree ofolarization on Ni–O and Co–O. In addition, the orbit of Ni–egt x = 0 moved to a higher lever of Fermi energy. This supportshat the valence of Ni ion has been changed.

5Co0.25O2 (a) x = 1, (b) x = 0.

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. Conclusion

The LiNi0.75Co0.25O2 was prepared by sol–gel pretreatmentnd solid-phase formation and was investigated in detail atifferent temperatures selected based on TGA results. Thehase transformation occurred in the temperature range of00–550 ◦C, and was closely examined by SEM and XRDoth ex situ and in situ. It was demonstrated that the sinteremperature and the time required for the crystalline growthre crucial in the control of structure. The sample underwentrom a cubic phase to a hexagonal phase from 400 to 600 ◦C,nd the valence of Ni ion changed from +2 to +3. In situRD and SEM techniques are useful in identification ofetal stable phase or impurities presented in phase trans-

ormation.

cknowledgement

This work was supported by the National Natural Scienceoundation of China under the Grant No. 10472098.

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