관형 반응기를 이용한 초임계수 상에서 LiCoO2 나노 입자의 연속합성

구성모, 이창하,* 임종성, 김재덕, 이윤우

한국과학기술연구원 국가지정 초임계유체연구실, 연세대학교 화학공학과*

Continuous Production of Nano-Particulate LiCoO2

in supercritical water in a tubular reactor

Seong-Mo Koo, Chang-Ha Lee*,Jong Sung Lim,

Jae-Duck Kim, Youn Yong Lee,Youn-Woo Lee

National Research Lab for Supercritical Fluid., KIST

Dept. of Chem. Eng., Yonsei university*

Introduction

Lithium cobalt oxide (LiCoO2) has received considerable interest as a cathode material for high voltage (4V) rechargeable Li - ion batteries. LiCoO2 has a layered rock - salt structure with trigonal symmetry R3m in which the Li+ and Co+3 ions are ordered on the alternate (III) planes. LiCoO2  fine powders have been generally prepared by solid-state reaction at high temperature [1-2]. Many advanced technique, such as hydrothermal synthesis, sol-gel methods,  ion - exchange techniques and supercritical water synthesis, has been attempted to prepare high - active materials of high purity and crystallinity [3-9]. Supercritical water synthesis (SCWS) is a kind of hydrothermal synthesis which was introduced recently[10]. In SCWS, morphology of produced particles can be controlled to some extent with pressure and temperature, and homogeneous reducing or oxidizing atmospheres can be provided by introducing gases (O2) or additional components (H2O2, KOH) into supercritical water [11]. The objective of this study was to develop a system to produce continuously crystalline LiCoO2 with a uniform crystalline size.

Experimental

Figure 1 shows a continuous hydrothermal reaction apparatus for synthesis of fine LiCoO2 particles in this study. LiOH·H2O (Junsei,95+%) and Co(NO3)2·6H2O (Junsei, 97+%) were used as precursors. These salts were dissolved in distilled water at various concentrations and pressurized at 300 bar and mixed  at room temperature at a mixing point MP1 just before the entrance of the electric furnace. These solutions were mixed at a mixing point MP2 with supercritical water produced by a preheater and a high-pressure metering pump. The temperature and pressure in the reactor were maintained at the supercritical state of water, respectively. Hydrogen peroxide (H2O2) was used as an oxidant. The volume of the tubular reactor was 24 cm3 and the residence time was maintained within 60 seconds. The effluent from the reactor was cooled to room temperature by using a heat exchanger. After cooling, the particles formed were collected by Tee-type filters (pore diameter:0.5□m, TF series, Nupro Co., U.S.A) disposed in front of the back-pressure regulator (P/N:26-1721-24, Tescom Co., MN, U.S.A).  Morphology and size of the particles obtained was observed using scanning electron microscope (SEM) and transmission electron microscope (TEM) analysis. The reaction products were analyzed by X-ray diffraction (XRD) with  Cu K□ radiation between 5° and 90° 2□, and one main product, LiCoO2  and two byproducts, Co3O4 and CoO, were confirmed depending on the preparation conditions.

Result and Discussion

We have produced LiCoO2 with an average particle size of 500nm or less. Fig 2 shows SEM, Dark Field (DF), Selected Area Diffraction (SAD) and Convergent Beam Electron Diffraction (CBED) images of LiCoO2 synthesized in supercritical water. It can be seen from SAD that the LiCoO2 particles appeared to be single crystal. In Fig 3, The X-ray diffraction of LiCoO2 prepared is shown. It is clear that only one single phase of LiCoO2 was produced at 400℃ and 300bar and the particles were highly crystalline LiCoO2 with a uniform particle size. They didn't contain any other byproducts such Co3O4, CoO and Co2O3 which are usually formed in subcritical state and/or the lower molar ratio of Li / Co. It is interesting to note that our products show the high temperature (HT) phase eventhough they are produced at relatively low temperature of 400℃ and without any further heat treatment. Our LiCoO2 has four distinct peaks at [006], [012], [108] and [110] and the intensity of [003]/[104] indicated that no defects in its crystal structure.[14]. Experimental conditions and results are seen to Table 1. The chemical reaction route for synthesis of lithium cobalt oxide can be expressed as follows:

In subcritical route, Co(NO3)2 become Co(OH)2 by hydrolysis and then this is turned into CoO by dehydration, since limited oxygen is soluble in liquid phase where Co(NO3)2 is dissolved. In supercritical route, Co(NO3)2 changes into Co(OH)3  by the direct oxidation of Co+2 to Co+3 with H2O2. And then Co(OH)3 becomes Co2O3 through consecutive dehydration via CoOOH in supercritical water over 400℃. It is thought that the oxidation of Co+2 takes place easily over the critical temperature of water (374.2℃) since both reactants of oxygen and aqueous intermediate exist in a homogeneous phase. Therefore, it is desirable that the reaction should take place in supercritical water with a large excess Li source in order to get single phase LiCoO2, otaherwise side reactions can produce undesirable oxides such as CoO, Co2O3 or Co3O4. Finally LiCoO2 is produced as a result of the reaction of Co(OH)3 or CoOOH with LiOH.

Conclusion

We are able to produce defect free single phased LiCoO2 particles ( < 500 nm) by supercritical water synthesis (SCWS) with a large excess of LiOH at a short residence time ( < 1 min). The optimized conditions provide a highly crystalline LiCoO2 without side products like CoO and Co3O4. Supercritical state provides the effective conditions to oxide Co+2 into Co+3 with O2, since oxygen and aqueous reactants exist in homogeneous phase. In liquid phase heterogeneous reaction the rate of oxidation of Co+2 is limited by the mass transfer rate of oxygen.

Acknowledgement

This work has been supported by the National Research Laboratory Program for Supercritical Fluid and the authors would like to the Ministry of Science and Technology.

References

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Table 1. Experimental Conditions and results

Run	LiOH	Co(No3)2	H2O2	T(℃)	Pressure(bar)	□(sec)	Product
1	0.02 M	0.02 M	0.1 M	400	300	50	Co3O4
2	0.1 M	0.02 M	0.1 M	400	300	54	LiCoO2, Co3O4
3	0.4 M	0.08 M	0.07 M	397	300	55	LiCoO2 Co3O4
4	0.4 M	0.02 M	0 M	400	300	40	LiCoO2, CoO
5	0.8 M	0.04 M	0.07 M	400	300	60	LiCoO2
6	0.4 M	0.02 M	0.02 M	400	300	40	LiCoO2
7	0.4 M	0.02 M	0.1 M	300	300	94	Co3O4
8	0.4 M	0.02 M	0.1 M	350	350	93	Co3O4

1. Reactor

2. Furnace

3. High-pressure Pumps

4. Pre-heaters

5. Cooler

6. In - Line Filters

7. Back-Pressure Regulator

8. Filtrate Reservoir

9. Cobalt precursor (Co(NO3)2)

10. Lithium precursor (LiOH)

11. Oxidant (H2O2)

Figure 1. Supercritical water synthesis flow apparatus

SEM image	The DF image 160kV ×100K	[001] SAD pattern	The CBED pattern from [001] zone-axis

Fig 2. SEM, TEM images of LiCoO2 nano-particles prepared.

P = 300bar 40sec < □ < 45sec LiOH=0.4M, Co(NO3)2=0.02M, H202=0.02M	P = 300bar 390℃ < T < 410 ℃ 40 sec < □ < 45 sec

Figure 3. The X-ray diffraction of LiCoO2 nano-particles prepared