졸-겔법을 이용한 리튬이차전지용 cathode 활물질 Li1.03Mn2O4 제조 및 싸이클 특성

선양국 , 강신이

삼성종합기술원, 화학중앙연구소

Synthesis and cycling behaviour of spinel Li1.03Mn2O4 cathode materials prepared by a sol-gel method for lithium secondary batteries

Y.-K. Sun, and S.E. Kang

Central Research Institute of Chemical Technology, Samsung Advanced      Institute Technology

Introduction

The spinel LiMn2O4 has been extensively studied for the most promising cathode materials for lithium secondary batteries with high energy density [1]. However, it has a problem that its capacity fading and cyclability in the 4 V region are inferior to those of layered oxides such as LiCoO2 and LiNiO2 with cycling. The quality of LiMn2O4 powders used for lithium secondary batteries strongly dependens on its synthetic method [2]. LiMn2O4 powders have usually been prepared by solid-state reaction. Recently, considerable improvements have been made by using solution method in obtaining high performance cathode active materials for lithium secondary batteries [3-6]. Among the solution methods, a sol-gel method can produce highly homogeneous powders having increased surface area and thus enhanced performance of electrode.

In this study, Li1.03Mn2O4 powders with uniform submicron-sized particles were synthesized by a sol-gel method using glycolic acid as a chelating agent at the considerably lower temperatures and shorter time. The electrochemical characteristic the Li1.03Mn2O4 powders were studied.

Experimental

A stoichiometric amount of Li and Mn acetate salts with the cationic ratio of Li :Mn=1.03:2 were dissolved in the distilled water and mixed well with an aqueous solution of glycolic acid. After adjusting a pH of the solution to 6.5-7.5 with ammonium hydroxide, the resultant solution was evaporated at 70-80℃ for several hours until a transparent sol and gel was obtained. The gel precursors obtained were decomposed at 300-800℃ for 10 hrs in air to obtain phase-pure polycrystalline Li1.03Mn2O4 powders.

Powder X-ray diffraction (Rint-2000, Rigaku) using CuK□ radiation was used to identify the crystalline phase of the materials calcined at various temperatures. The particle morphological changes of the materials were observed using a field emission scanning electron microscope (Hitachi Co., S-4100). The specific surface area of the material was determined by the BET method (Autosorb-1, Quantachrome) with nitrogen adsorption.

The electrochemical properties of Li1.03Mn2O4 powders were determined in Li/polymer electrolyte/Li1.03Mn2O4 cells. The polymer electrolyte was made from PAN, plasticized by a solution of LiClO4 in a 1 : 1 mixture of ethylene carbonate (EC) and propylene carbonate (PC). The composite cathode was prepared by mixing at 110℃ the as-synthesized Li1.03Mn2O4 powders, acetylene black, and the polymer electrolyte in a weight ratio of 59.5 : 9.3 : 31.2 and then coating the resultant paste on an Al foil current collector. The charge-discharge cycling was performed galvanostatically with cut off voltage of 3.0 to 4.3 V (vs. Li/Li+).

Result and discussion

Fig. 1 shows the X-ray diffraction (XRD) patterns for the gel-derived materials calcined at various temperatures for 10 hrs in air, where the molar ratio of glycolic acid to total metal ions was 1.5. The X-ray diffraction pattern for the material calcined at 250℃ represents no diffraction peaks, which indicate an amorphous. When a material calcined at 300℃, the crystalline Li1.03Mn2O4 spinel begin to appear. Impurity peaks, such as Li2CO3 and MnCO3 are not observed, which are often found in other low temperature techniques. There is a gradual increase in the peak intensities accompanied by sharpening of the peaks with increasing the calcination temperature, which indicates an increase of crystallinity. The precursor was crystallized into a phase-pure Li1.03Mn2O4 spinel powders without any development of minor phase throughout the calcination temperature range.

Fig. 2 shows the dependence of lattice constant a of the cubic unit cell of the same material as shown in Fig. 1 on the calcination temperature. The lattice constant a increased linearly with increasing calcination temperature. It was reported that the lattice constant a of Li1+xMn2-xO4 powders should always be minimized at fixed composition to have less oxygen vacancies [1]. It is speculated that the value of the manganese average oxidation state in the spinel phase is closely related to the lattice constant of the cubic unit cell. A lower calcination temperature result in a more oxidized manganese cation because manganese ions are stable preferentially as Mn4+ at  lower temperatures [7]. Guyomard et. al have reported that the lattice constant a of slowly-cooled samples (c.a. 10 ℃/hr) was significantly smaller and seldom exceeding about 8.23Å in nominal formulations with x in LixMn2O4 greater than 1.0, which show the more stable maintence of capacity [1].

Fig. 3 shows the dependence of specific surface area of the same material as shown in Fig. 1 on the calcination temperature. The specific surface area of the  Li1.03Mn2O4 powders decreases linearly with increasing calcination temperature, due to the growth of Li1.03Mn2O4 crystallites. The specific surface area of 32.3 m2/g and 3.8 m2/g of the material calcined at 300 and 800Å, respectively.

Fig. 4 shows the charge-discharge behaviour and discharge capacities with number of cycles for the Li/polymer electrolyte/Li1.03Mn2O4 cells using  Li1.03Mn2O4 powders calcined at 760℃, where the molar ratio of glycolic acid to metal ions of was 1.0. The cells were activated by 1st cycle and then were cycled between cut-off voltages of 3.0 and 4.3V at a constant current density of 0.1 mA/cm2. The Li/polymer electrolyte/Li1.03Mn2O4 cells initially delivered 134 mAh/g. The discharge capacity slowly decreased with cycling and remained about 124 mAh/g up to 190th cycles, which was 92% of initial capacity. The LiMn2O4 lattice change which result in considerable strain on these interface is believed to deteriorate the electrical contact between the surfaces of the insertion-material particles and hence to decrease the capacity of the cathode during repeated charge/discharge cycling [8]. This phenomena is severe to the LiMn2O4 powders with broade-particle size distribution. Therefore, it is inferred from that the excellent cyclability result from good contacts among the composite cathode interfaces (Li1.03Mn2O4/polymer electrolyte/carbon) due to a narrow particle-size distribution of our Li1.03Mn2O4 powders.

REFERENCES

1. Guyomard, D. and Tarascon, J. M., Solid State Ionics, 69, 222(1994).

2. Gao, Y. and Dahn, J. R., J. Electrochem. Soc., 143, 100 (1996) .

3. Barboux, P., Tarascon, J. M. and Shokoohi, F. K., J. of Solid State Chem.,

94, 185 (1991).

4. Sun, Y. -K. and Oh, I. -H., J. Mater. Sci. Lett., 16(1), 30 (1997).

5. Liu, W., Farrington, G. C., Chaput, F. and Dunn, B., J. Electrochem. Soc.,

143, 873 (1996).

6. Sun, Y. -K. and Oh, I. -H., J. Mater. Chem., accepted in publication.

7. Masquelier, C., Tabuchi, M., Ado, K., Kanno, R., Kobayashi, Y.,Maki, Y.,

Nakamura, O. and Goodenough, B., J. Solid State Chem., 123, 255 (1996).

8. Jiang, Z. and Abraham, K. M., J. Electrochem. Soc., 143, 1591 (1996).

Fig. 1 X-ray diffraction patterns of gel derived materials calcined

at various temperatures. The molar ratio of glycolic acid to

total metal ions was 1

Fig. 2 Dependence of the lattice constant for    Fig. 3 Dependence of the specific surface

Li1.03Mn2O4 powders on the calcination          area for Li1.03Mn2O4  powders  on

temperature.                                 calcination temperature.

Fig. 4 (a) Cycling charge/discharge curves and (b) variationof specific discharge capacity

with number of cycles for the  Li/polymer electrolyte/Li1.03Mn2O4  cells  using

Li1.03Mn2O4 powders calcined at 760℃. Cycling was carried out galvanostatically

at constant charge/discharge current density of 0.1mAcm-2 between 3.0 and 4.3V.