초임계 수열합성법에 의한 Lithium cobalt oxide 나노입자의 연속 합성

구성모, 이창하,* 임종성, 김재덕, 이윤우

한국과학기술연구원 국가지정 초임계유체연구실, 연세대학교 화학공학과*

Production of LiCoO2 Nano-Particles by Hydrothermal Synthesis

in Supercrital Water with a Continuous Flow Reactor

Seong-Mo Koo, Chang-Ha Lee*,

Jong Sung Lim, Jae-Duck Kim, Youn-Woo Lee

National Research Lab for Supercritical Fluid., KIST

Dept. of Chem. Eng., Yonsei university*

Introduction

The increasing demand for portable and cordless electronic appliances is driving the development of compact batteries. [1] Lithium ion batteries in particular have attracted much attention [2] because of high output voltage, high specific energy, long cycle-life, and no memory effect.  Recently, LiMeO2 (Me = Co, Mn, Ni, Fe) have been well known and accepted as a cathode active material in lithium secondary battery. LiCoO2 powders have generally been prepared by solid-state reaction using carbonate such Li2CO3 and CoCO3. [3,4]

The solid state synthesis is a simple and inexpensive method for ceramic powder production : the precursors are ground and mixed together and the mixtures are calcined at a suitable temperature. However, it is difficult for this process to prepare the powders which have uniform particle morphology of submicro size with narrow size distribution. For example, the particles obtained were irregular morphology, agglomerated, larger than 10 □m in size and broader size distribution. For the good electrochemical performance, smaller particle size with high crystallinity, higher packing density and higher surface area are required for the LiCoO2.

Many advanced chemical processes, such as the sol-gel process, spray decomposition, precipitation and freeze-drying rotary evaporation, supercritical drying, mechanical alloying and Supercritical water synthesis, have been evolved to prepare high-active materials of high purity and crystallinity [5-11].

Supercritical water synthesis (SCWS) is hydrothermal crystallization in supercritical state. The specific features of SCWS are that : 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 or additional components (O2, H2O2).

In this paper, LiCoO2 was prepared using SCWS method, in order to obtain single crystal transition metal oxide fine particles as cathode materials for Li-ion batteries.

Experimental

Figure 1 shows a continuous hydrothermal reaction apparatus for synthesis of fine LiCoO2 particles in this study. LiOH·H2O (Aldrich,98+%) and Co(NO3)2·6H2O (Aldrich, 98+%) were used as precursors. These salts were dissolved in distilled water at various concentrations and were pressurized at 300 bar and were mixed at mixing point MP1 at room temperature just before the entrance of the electric furnace. These solutions were mixed at mixing point MP2 with supercritical water produced

Figure 1. Supercritical water synthesis               flow apparatus

by a preheating electric furnace and a high - pressure mini-pump (Milton Roy, USA). The temperature and pressure of the supercritical water in the reactor were maintained at 400℃ and 300 bar . H2O2 dissolved in the supercritical water was used as an oxygen source for LiCoO2 production. The volume of the main reactor was 22 cm3 and the residence time was 60 sec, which means that the total oxide preparation process was completed within 1 min. This residence time is much shorter than that for any other preparation methods. The residence time in the flow type reactor, □, was evaluated from

following;[12]

[eqn: □={V} over {{F}×{□_25℃}over{□_400℃}}]

where, V is reactor volume and F is the total flow rate; □25℃ and □400℃ are the densities of pure water at room temperature and 400℃, respectively.

The effluent from the flow type reactor was cooled to room temperature by a cooling tube. At the exit of the reactor, the fluid was cooled by suing an external water jacket. After cooling, the formed particles were collected by Tee-type filter (pore diameter:0.5mm, TF series, Nupre Co., U.S.A) disposed in front of the back-pressure regulator (P/N:26-1721-24, Tescom Co., MN, U.S.A). The synthesized product particles were collected by diverting the flow to a parallel Tee-type filter with a Tee tube. The pressure of the reactor was controlled by a back-pressure regulator. Morphology and size of the particles obtained was observed using scanning electron microscope (SEM) analysis and Scherrer eqn. The reaction products were analyzed by X-ray diffraction (XRD) with  Cu K□ radiation between 5°and 90°2□, and three main   products were confirmed by the XRD patterns: LiCoO2 ,Co3O4, and CoO, depending on the preparation conditions.

Result and Discussion

The chemical reaction formulas for synthesis of Lithium cobalt oxide can be expressed as follows:

At first step, Co(NO3)2 become Co(OH)2 by hydration and then this is turned into CoO by dehydration. This step take place usually in sub-critical water below 400℃.

Another step, Co(NO3)2 change into Co(OH)3 with reaction with  H2O2 and oxidation of Co+2, and then Co(OH)3 became Co2O3 through dehydration in critical water over 400℃. It is thought that the reaction which H2O2 oxidize Co(NO3)2 vigorously take place in higher temperature than critical water temperature(374.2℃).

Next step, Co(OH)3 which formed in second step make reaction with LiOH, so that we gain water and LiCoO2. It is desirable that the reaction should take place in critical water over 400℃ in order to get LiCoO2, otherwise competition between reactions can make our purpose undesirable. It is a factor that an large excess of LiOH drives the reaction not to the side reactions but to the main reaction. On top of that, if an excess of amount LiOH is supplied, it is hard that side reaction take place but easy that main reaction rise up but if temperature is low and there isn't H2O2, Co(OH)2 will be made so that CoO and Co3O4 is gained for undesirable side products.

In this research, the average particle size of LiCoO2 is 50 ~ 100 nm. The higher temperature reaction take place in, the smaller size of particle we get. It is thought that rapid reaction cause to make the supersaturation of product. But single phase of particle, single crystall, and particle size were affected by many factors. The formed particle over 400℃ was the best for single crystall and it appeared in single phase LiCoO2 and didn't contain other side products like Co3O4, CoO. Moreover, we get to know the fact that Co3O4 usually formed below critical pressure(218atm), and Co3O4 was gained under the condition that the mole ratio Li / Co was below 10. The optimized conditions are seen to Table below.

Runn	LiOH	flow rate (10-3mol/min)	Co(No3)2	flow rate (10-3mol/min)	H2O2	flow rate (10-3mol/min)	T(℃)	□	□(sec)	Mean particle size*
1	0.8 M	1.744	0.04 M	0.0872	0.07 M	0364	400	0.35743	60	52 nm
2	0.4 M	0882	0.01 M	0.022	0.07 M	0.35	400	0.35743	60	52 nm
3	0.4 M	0852	0.02 M	0.0426	0.07 M	0441	410	0.2512	55	56.6 nm
4	0.4 M	0.832	0.02 M	0.0416	0.07 M	0.49	405	0.295	51	52 nm
5	0.4 M	0.6	0.02 M	0.03	0.02 M	0.116	405	0.295	44	62.3 nm
6	0.4 M	1	0.02 M	0.05	0.02 M	0.136	400	0.35743	40	88.8 nm

* mean particle size was calculated by Scherrer eqn (=[eqn: { 0.9 lambda } over { beta ·cos theta }] ) at 300 bar

(a)                               (b)

Fig.2 SEM images of LiCoO2 nano-particles prepared at (a) Run 3 and (b) Run 4.

Fig.3 The X-ray diffraction of LiCoO2 nano-particles prepared at Run 6

Conclusion

Supercritical water synthesis (SCWS) of LiCoO2 from cobalt(II) nitrate and Lithium hydroxide aqueous solution was performed with a rapid-heating flow system. LiCoO2 particles could be selectively produced with a large excess of LiOH at a residence time of ~ 1 min. The optimized conditions provide a highly crystalline LiCoO2 without side products like CoO, Co3O4.

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