졸-겔법을 이용한 층상망간산화물의 구조와 전기화학적 특성 연구

선양국, 박상호

Structural and Electrochemical Characteristics of Layered

Manganese Oxide by Sol-gel Method

Yang-Kook Sun , Sang Ho Park

Department of Chemical Engineering, College of Engineering,

Hanyang University, Seongdong-Gu, Seoul, 133-791, Korea

1. Introduction

Lithiated transition metal oxides LiMO2 (M = Co, Ni, Mn) have been extensively studied as cathode materials for commercial rechargeable lithium ion batteries. The layered LiMnO2 are promising candidates as cathodes material because of their high theoretical capacity (285 mAh/g), low cost, abundance and nontoxic [1,2]. Unfortunately, solid-state reaction at high temperature to prepare layered LiMnO2 has been unsuccessful due to the non-layered structure such as spinel LiMn2O4, orthorhombic LiMnO2, or rock salt Li2MnO3 [3]. But layered manganese oxides are transform to the spinel phases upon electrochemical cycling and this problems are associated with the Jahn-teller distortion [4]

In this work, a sol-gel method was employed to prepare Na0.7MnO2 powders. Layered Li0.7MnO2 samples obtained by an ion-exchange process from Na0.7MnO2. The structural and electrochemical properties of the prepared materials were investigated using various analytical techniques and correlated to explain the electrochemical activity of the materials.

2. Experimental

Samples of Na0.7MnO2 powders were synthesized using a sol-gel method with glycolic acid as chelating agent. Sodium acetate (CH3CO2·Na, Aldrich), manganese acetate (Mn(CH3COO)2·4H2O) salts were used as starting materials. The dissolved solution was added into a continuously agitated aqueous solution of glycolic acid, the chelating agent for the reaction. The prepared solution was heated several times, the sol turns into a viscous transparent gel. The resulting gel precursors were pre-calcined at 400 ℃ and calcined at 600 ℃ for 12 hr in air. After calcinations, the cathode material Li0.7MnO2 powder was prepared by ion exchange of sodium for lithium with 10 folds LiBr in ethanol.

The thermal decomposition behavior of the gel precursors was examined by means of thermogravimetric analyzer (TGA). Powder X-ray diffraction (XRD, Rigaku, Rint-2000) using Cu-K□ radiation was employed to characterize the structural properties of the powders before and after cycling test.

The electrochemical properties were performed using CR2032 coin-type cells. The cathode was fabricated with 20mg of accurately weighed active material and 12mg of conductive binder (8mg of Teflonized acetylene balck (TAB) and 4mg of praphite). An excess amount of lithium foil was used as an anode. The electrolyte used was a mixture of 1 M LiPF6-EC/DMC (1:2 by volume) (Ube Chemical, Japan). The charge-discharge cycling was galvanostatically performed at a current density of 0.4 mA/cm2 (C/5 rate) with cutoff potentials of 2.4 to 4.5 V (vs. Li/Li+).

3. Results and Discussion

Figure 1 shows TGA and DTA patterns for Na0.7MnO2 gel precursors. The weight loss of the gel precursors is terminated at below 400 ℃ with three discrete regions of the weight loss occurred at 30-170 oC, 170-320 ℃ and 320-400 ℃, respectively. The weight loss in the temperature range of 30-170 ℃ is due to the evaporation of water. The second weight loss is due to the removal of glycolic acid in 170-320 ℃ range. But the weight loss in the temperature range of 315-400 ℃ is attributed to the decomposition of metal acetate. For the gel precursor of metal acetates and glycolic acid, participates in the synthetic reaction at a third. however, it is considered that glycolic acid weight loss region to accelerate the decomposition rate of metal acetates. Also, DTA analysis reveals the evolution of an sharp exothermic peak at about 370 ℃, indicating the rapid decomposition of metal acetate and chelating agents. The TGA curve shows that no decomposition is observed after 400 ℃, indicating that the crystallization starts and the single phase product is formed. Therefore, sol-gel method might synthesized materials at lower temperature than solid state reaction.

Figure 2 shows x-ray diffraction patterns of layered manganese oxides. Fig. 2(a) sample is Na0.7MnO2 calcined at 600 ℃ before ion exchange. Fig. 2(b) are Layered Li0.7MnO2 obtained by ion-exchanging Na0.7MnO2 with LiBr in ethanol. Since no peak at 16o remains from the initial sodium manganese layered in Fig. 2(a), the ion exchange has almost been completed. The ion-exchange of Na0.7MnO2 easily leads to single phase Li0.7MnO2. Otherwise, the diffraction pattern exhibits broad peaks indicating poor crystallinity and stacking faults. Dahn et al. reported that the degree of ion-exchange depends on composition and crystal structure [5]. The Li0.7MnO2 powders have a rhombohedral symmetry with a space group R_3m (a = 2.83 Å, c = 14.64 Å) by Rietveld refinement using the XRD data of Fig 2(b). Figure 2(c) shows the X-ray diffraction patterns for the Li0.7MnO2 electrodes after 50th cycled at room temperature. For the cycled layered electrodes, no structural changes were observed after cycles; the x-ray pattern of the material (Fig. 2(c)) was the same as that of the as-prepared powder (Fig. 2(b)). However, for the cycled electrode, 2 theta 63o and 66o peaks were merged.

Figure 3. shows the discharge capacity for the Li0.7MnO2 electrode. The corresponding charge-discharge curves are shown inset Fig. 3 at room temperaterue. The cycling was carried out at a constant current density of 0.4 mA/cm2 (C/5 rate) and between potential limits of 2.4-4.5 V. The Li0.7MnO2 material yields an initial discharge capacity of 185 mAh/g on the first cycle and discharge capacity continuously increased and reached about 200mAh/g after the 50th cycle. And the first discharge curve exhibit the plateau in the 3 V region. a plateau also develops in 4 V region within the few cycles.

To examination of the C.V. for the layered compound compare to LiMn2O4 spinel in Fig. 4. For the layered Li0.7MnO2, oxidation and reduction peaks show the presence of a two step electrochemical process for lithium intercalation and deintercalation. The C.V. data of Li0.7MnO2 are different from spinel LiMn2O4. The two pairs of redox peaks are characteristic of spinel LiMn2O4 in the region between 3.9 and 4.1 V. The other hands, layered Li0.7MnO2 shows only one redox peak. Many groups are reported that LixMnO2 O3 structure undergo a first order phase transformation to spinel during only a few cycling [2]. However, we have not found decade of discharge capacity on cycling. This results are indicated that Li0.7MnO2 electrode doesn’t convert to spinel during the cycling .

Layered Li0.7MnO2 powders were synthesized by a sol-gel method using glycolic acid as a chelating agent. The Li0.7MnO2 materials prepared at 600 ℃ has a rhombohedral R-3m symmetry with O3 type oxygen packing. This material shows that the initial capacity is 185 mAh/g and 200 mAh/g after 50th cycling in the 2.4-4.5 V range. XRD results are reveals that the structure of the electrode retains its original rhombohedral structure, even after cycling. Therefore, sol-gel method is effective to synthesize layered manganese oxide at low temperature compared with conventional solid state reaction

Reference

[1] F. Capitaine, P. Gravereau, C. Delmas, Solid State Ionics 89 (1996) 197.

[2] A.R. Armstrong, P.G. Bruce, Nature 381 (1996) 499.

[3] M. Tabuchi, K. Ado, H. Kobayashi, H. Kageyama, C. Masquelier, A. Kondo, and R. Kanno, J. Electrochem. Soc., 145, L49 (1998).

[4] A.R. Armstrong, A.D. Robertson, P.G. Bruce, Electrochimica Acta 45 (1999) 285-294.

[5] J.M. Paulsen, J.R. Dahn, Solid State Ionics 126 (1999) 3-24.

Figure

Fig. 3. Discharge capacity vs. number of cycle for Li0.7MnO2. The inset showed the voltage profiles

Fig. 4. Cyclic voltammograms of Li0.7MnO2 measured at 120□/s in the voltage range of 2.0-4.8V

Fig. 1. Thermogravimetric and differential thermal analyses for the Na0.7MnO2 gel precursors.

Fig. 2. XRD patterns of layered manganese. (a) Na0.7MnO2, (b) Li0.7MnO2 and (c) Li0.7MnO2 after 50th cycled electrode.