LiCoO2전극 물질 특성에 미치는 나노구조의 영향

화학공학의 이론과 응용 제8권 제1호 2002년

Theories and Applications of Chem. Eng., 2002, Vol. 8, No. 1

화학공학의 이론과 응용 제8권 1호 2002년

최청송

서강대학교 화학공학과

The Effect of Nanostructure on the Characteristics of LiCoO2 Electrode Materials

Cheong-Song Choi

Dept. of Chemical Engineering, Sogang University

1. Introduction

Intercalation compounds of the layered rocksalt structure such as LiCoO2 and LiNiO2, and spinels such as LiMn2O4, are widely studied and used as electrodes in advanced lithium batteries[1]. The composition, structure, and cation ordering of these compounds determine the intercalation voltage and theoretical charge capacity, whereas extrinsic factors such as particle size and microstructure have important influence on characteristics such as cycle life and power capacity. Of theses, the research on LiCoO2 has been more active than that on the other oxides because of the simplicity of preparing the material.

In addition, much attention has recently focused on LiCoO2 as the most alternative cathode material for molten carbonate fuel cell[2]. A wide variety of methods has been employed in the synthesis of intercalation oxides[3-5].

LixCoO2 is a typical layer compound formed by stacking sheets consisting of CoO6 octahedra. The octahedra in one layer share six edges each, resulting in a stoichiometry of CoO2 within one octahedron layer. The stacking sequence of the CoO2 layers is ABCABC, leading to a trigonal unit cell with space group R-3m.

In this investigation, a novel technique was introduced for the preparation of cathode materials for lithium-ion batteries, in which precipitation process was applied to obtain the precursor which is calcined to give superfine LiCoO2 powders.

2. Experimental

Co(OH)2 was precipitated by adding a 0.1M solution of Co(NO)3.6H2O(ALDRICH, 98%) in deionized water to a stirred aqueous solution of LiOH.H2O(SIGMA, 99%) in deionized water, continuously monitored and titrated by a solution of LiOH.H2O to maintain the pH at 11, near the minimum solubility point Co(OH)2[6]. After precipitation the suspension was left at room temperature without stirring for several hours and then settled by centrifugation, and rinsed with a buffer solution of LiOH.H2O to remove nitrate ions.

Finally, to obtain a highly homogeneous Co(OH)2 powders, the rinsed precipitate was freeze-dried by a freeze dryer(EYELA, FD-81). To produce fine LiOH powders, after LiOH.H2O particles were dehydrated by a freeze dryer.

After LiOH powders were completely dissolved in methyl alcohol(assay above 99.5%), Co(OH)2 powders were dispersed in LiOH alcoholic solution. To obtain a highly homogeneous physical mixture of LiOH and Co(OH)2, then, suspension was  evaporated on hot plate with stirring at 70℃. LiCoO2 was synthesized by calcining the mixture of precursors at various temperatures and for various times.

Powder X-ray diffraction(XRD, Rigaku, D/MAX-A) with CuK□ radiation was used to identify the crystalline phase of the material calcined at various conditions such as described above. Particle morphology of the material were examined by scanning electron microscopy(SEM, Hitachi, S-3500N). The specific surface area of the materials was determined by the BET method(Micromeritics, ASAP-2010) with nitrogen adsorption. Rietveld analysis was then performed on the X-ray diffraction data to obtain the lattice constants. The structure was investigated by X-ray diffraction data using the RIETAN program[7] with a pseudo-Voigt function. The structure of HT-LiCoO2 was simulated by the commercial software Creius2.

3. Results and Discussion

The LiCoO2 samples were characterized by Rietveld refinement of powder X-ray diffraction data, which was simulated by RIETAN.

The Rietveld method[8] is a technique for refining structure parameters( fractional coordinates, occupation factors, magnetic moments, isotropic/anisotropic thermal parameters,  etc. ) and lattice parameters directly from whole powder diffraction patterns. Variable parameter, x, in Rietveld analysis is much more than those in single-crystal structure refinement in spite of less structural information contained in powder diffraction data. Peak-shift, background, profile-shape, preferred-orientation, and lattice parameters must be refined in addition to scale factors and structure parameters in Rietveld analysis.

In order to test the electrochemical performance of the synthesized oxide materials, cathodes were fabricated using the mixture of precursors heat treated at 800℃ for 2hr according to the procedure described earlier. These powders were selected for the electrochemical characterization since X-ray analysis of these powders indicated strong peaks characteristic of the well crystallized high temperature phase of LiCoO2. The cell type was half cell and the electrolyte used was 1M LiPF6 in EC/DMC(1:1). The cyclic voltammograms were performed at a scan rate 0.2mV/s. The peak potentials of the LiCoO2 prepared with Li/Co=1.0 and 1.15 were 4.00V/3.83V and 4.03V/3.83V respectively. When the molar ratio of Li ions to Co ions is 1.15, no minor peaks were observed during either the anodic or the cathodic scan. The initial capacities are 132.64mAhg-1 and 133.2mAhg-1, respectively. The discharge capacity slowly decreased with cycling and remained at about 109mAhg-1 and 120mAhg-1 up to the 25th cycle, which are 82% and 90% of initial capacity.

From the results it was found that LiCoO2 prepared under the conditions of calculation at 800℃ for 2hr has an excellent electrochemical property. A comparison was made between LiCoO2 powders prepared by different conditions to know which would be more effective structure, which are summarized in Table 1 and Fig. 1.

They show that LiCoO2 powder prepared by a modified solid state reaction is comparable to the simulation ideal LiCoO2 in the optimized structure, seem to give a certain evidence that there is something in what Nuspl et al.[9] and suggested on the lithium energy minimum migration path in LixCoO2.

4. Conclusion

The experimental results support the simulations, showing the Li+ ion migration in LixCoO2 takes place via an interstitial mechanism. A modified solid state reaction suggested in this study will be able to consider as a valuable process for manufacturing the LiCoO2 with excellent electrochemical properties. In fact, this method can also be used to synthesize other cathode materials for lithium-ion batteries and has good prospects for commercialization.

5. References

[1] B.C.H. Steele, "in Fast Ion Transport in Solids", p.103, North Holland, Amsterdam(1973)

[2] L. Plomp, E.F. Sitters, C. Vessies and F.C. Eckes, J. Electochem. Soc., 138, 629(1991)

[3] G.G. Amatucci, J.M. Tarascon, and L.C. Klein, Solid State Ionics, 84, 169(1996)

[4] P. Barbous, J.M. Tarascon, and F.K. Shokoohi, J. Solid State Chem., 94, 185(1991)

[5] W. Liu, G.C. Farrington, F. Chaput, and B. Dunn, J. Electochem. Soc., 143, 879(1996)

[6] Jr.C.F. Baes and R.E. Mesner, The Hydrolysis of Cations, Jhon Wiley & Sons, New York, 1976

[7] F. Izumi, The Rietveld Method, R. A. Young, Ed., Ch.13 (Oxford University Press, 1993), p.236

[8] H.M. Rietveld, J. Appl. Crystallogr., 2, 65(1969)

[9] Gerhard Nuspl and Masataka Nagaoka, Bull. Chem. Soc. Jpn., 71, 2259(1998)

화학공학의 이론과 응용 제8권 제1호 2002년

화학공학의 이론과 응용 제8권 제1호 2002년

화학공학의 이론과 응용 제8권 제1호 2002년

Table 1. Comparison of structural terms

Synthesis condition	Li/Co ratio	Co-O bond length (□)	O-Co-O angle (Degree)
Theoretical		1.947	92.7
800℃, 2hr	1.0	1.945	92.6
800℃, 2hr	1.15	1.938	93.2

( a )                                  ( b )

Fig. 1. Comparison of the O2--O2- interlayer distances

화학공학의 이론과 응용 제8권 제1호 2002년

(a) 800℃, 2hr, Li/Co = 1.0

(b) 800℃, 2hr, Li/Co = 1.15

화학공학의 이론과 응용 제8권 제1호 2002년