In recent years, LiCoO₂ has been the main material of the positive electrode of lithium ion battery [1]. It is a structure formed by Co₃O₄ as a support and Li⁺ distributed inside the support. Since Co₃O₄ is so important in the structure of LiCoO₂, in order to prepare high-performance LiCoO₂, it is necessary to strictly control the performance indexes of the raw material Co₃O₄. Experiments have shown that when the particle size is small, the distribution is uniform, the specific surface area is large and the shape is spheroidal, Co₃O₄ has a better electrochemical performance [2].

At present, the main technical criterion of the battery-grade Co₃O₄ products are: the median diameter is 2 - 25 μm, the phase is cubic Co₃O₄, the appearance is black powder, and the crystal morphology is spherical or spheroidal. In this context, the precipitation method is used to investigate the influence of different parameters on the particle size distribution of Co₃O₄. It is of certain significance to prepare Co₃O₄ powder suitable for lithium ion batteries [3].

A certain amount of CoCl₂・6H₂O was dissolved in deionized water to prepare a certain concentration of CoCl₂ aqueous solution (according to Co²⁺: 90 g/L, 10 mL of CoCl₂ aqueous solution required CoCl₂・6H₂O 3.65 g). NH₄HCO₃ was dissolved in 26.8 mL of deionized water to prepare an aqueous solution of NH₄HCO₃, and the molar ratio of NH₄HCO₃ to CoCl₂ was a certain number (1.8 - 5). The two solutions were then mixed in a beaker. Before starting the feeding, pre-add deionized water (5, 30, 55, 80, 105 mL) in the three-necked bottle, heat the water in the water bath to the temperature required for the reaction (40˚C, 50˚C, 55˚C, 60˚C, 70˚C), and pour the mixed solution into it, a certain stirring strength and temperature are maintained throughout the process. In the reaction, the pH value is continuously measured with a pH meter. After a certain period of time (6, 8, 9, 10, 12 h), the product is taken out, and then solid-liquid separation is performed by suction filtration. 1 L of normal temperature deionized water was used to wash the solid precipitate. The solid sample was dried for 2 h at 105˚C. The precursor powder product was grinded, and then calcined at 850˚C - 900˚C for 8 h to obtain the Co₃O₄ powder.

CoCO₃ is a poorly soluble compound with a small solubility product Ksp value, so it is easy to achieve supersaturation. According to the principle of crystallography, the CoCO₃ precipitates through two stages, namely the formation of crystal nuclei and the growth of crystals, which determine the size of the CoCO₃ particles [4]. The crystallization process of insoluble compounds in aqueous solution is characterized by easy nucleation and difficult to grow particles [5]. In this paper, an ammonia complexing agent is added to the reaction system (Co²⁺ first reacts with NH 4 + forming cobalt ammine complex ions, and then it reacts with CO 3 2 − to form CoCO₃), the other reaction conditions are regulated to control the supersaturation of CoCO₃ in the solution, so that the nucleation and growth rate of CoCO₃ crystals reach a suitable ratio.

When the reaction temperature was set at 55˚C, the reaction time was 8 hours, and the pre-filled water amount was 5 mL, the molar ratio of ammonium hydrogencarbonate to cobalt chloride was changed, and the effect on the particle size of the precursor was as shown in Figure 1. As the molar ratio increases, the precursor particle size first increases and then decreases. When the molar ratio of NH₄HCO₃ to CoCl₂ was 1.8 - 3, the precursor prepared was purple and the pH was changed from 6.7 to 7. When the molar ratio of NH₄HCO₃ to CoCl₂ is 4, 4.5 and 5, the precursor obtained is pink, and the pH is changed from about 7.1 to 7.6.

The particle size distribution results obtained at a molar ratio of 4.5:1 are shown in Figure 2. The D50 of precursor was 6.13 μm and the particle size distribution was narrow.

The precursors obtained at different molar ratios were subjected to XRD test, and the results obtained are shown in Figure 3. The molar ratio has an important

influence on the precursor phase. When the molar ratio ≤ 3, the precursor obtained is basic cobalt carbonate (Co(CO₃)_0.5(OH)・0.11(H₂O)); when the molar ratio is equal to 4.5, the precursor is a mixture of cobalt carbonate (CoCO₃) and basic ammonia cobalt carbonate ((NH₄)₂Co₈(CO₃)₆(OH)₆・4H₂O). When the molar ratio of NH₄HCO₃ to CoCl₂ is 4.5:1, it is closer to the requirement of battery grade Co₃O₄. Therefore, 4.5:1 was chosen as the appropriate molar ratio.

The above other experimental conditions are unchanged. When the molar ratio of NH₄HCO₃ to CoCl₂ is 4.5:1, the pre-filled water amount (concentration) is changed, and each group of variables is added with 30, 55, 80, 105 mL of pure water, respectively. The effect of pre-filled water on the particle size of the precursor is shown in Figure 4. With the increase of water volume, the particle size of the precursor gradually decreased. It was found that the D50 of the precursor was 5.08 and 3.36 μm when the water was added at 30 and 55 mL, respectively. The pH changes were 6.7 - 7.8 and 6.8 - 7.6, respectively. The particle size distribution when adding 30 mL of water is shown in Figure 5. However, in order

to ensure that the final product of Co₃O₄ has a particle size between 2 and 25 μm, a water supply of 30 mL is selected.

The above other experimental conditions are unchanged. When the pre-filled water volume is 30 mL, the temperature is changed. The temperature of each group is 40˚C, 50˚C, 60˚C, 70˚C, and the previous temperature is 55˚C. The effect of temperature on the particle size of the precursor is shown in Figure 6. As the temperature increases, the particle size of the precursor increases gradually. It is found that the particle size distribution of the precursor is best at 60˚C as shown in Figure 7. And the D50 was 7.30 μm, the pH varies from 7.1 to 8.1.

The above other experimental conditions are unchanged. When the reaction temperature is 60˚C, the variables of each group are 6, 9, 10, 12 h, and the previous reaction time is 8 h. The effect of time on the particle size of the precursor

is shown in Figure 8. With the increase of time, the particle size of the precursor increased. Finally, the particle size distribution with a reaction time of 10 hours is best as shown in Figure 9, the pH was changed within 7.4 - 8.2, and the D50 was 8 μm. 10 hours was chosen as the reaction time.

The final precursor was prepared under these conditions and calcined at 850˚C. The XRD patterns of the precursor and calcined product are shown in Figure 10 and Figure 11, respectively; the morphology obtained by scanning electron microscope is shown in Figure 12 and Figure 13, respectively. The particle size distribution calcined product is shown in Figure 14. The obtained precursor and Co₃O₄ phase are relatively single, with few heterophases, and their morphology is a mixture of spherical and fibrous. The calcined product has a median diameter of 9.08 μm and a narrow particle size distribution range.

The relative magnitude of the rate of nucleation and the growth directly determines the type of precipitated material and the size of the precipitated particles [6].

Increasing the reaction temperature generally reduces the supersaturation of the solution. Since the nucleation rate of CoCO₃ is relatively sensitive to the change of supersaturation, although the increase of reaction temperature may increase the speed of various processes, the increase of nucleation rate is correspondingly weakened by the decrease of supersaturation. Therefore, increasing the temperature is more conducive to the increase of the growth rate of the nucleus. On the other hand, if the temperature is too high, the kinetic energy of the reactant molecules is increased too fast and it is not conducive to the formation of a stable crystal nucleus. Increasing the temperature promotes the dissolution of the small particle crystals and redeposition on the surface of the large particles [7]. Therefore, low temperature precipitation is favorable for the formation of fine crystals, and high temperature precipitation is favorable for the formation of larger crystals, as the temperature increases, the particle size of the precursor increases (Figure 6).

Analysis of the formation process of CoCO₃ crystals shows that the formation and growth of crystal nuclei takes a certain time, and the crystallization conditions are different, and the time required is also different [8]. In this project, the longer the time, the larger the precursor grain growth and the larger the particle size (Figure 8).

・ The reactant molar ratio in the NH₄HCO₃ to CoCl₂ system is one of the most direct factors affecting the precursor phase. When the molar ratio of NH₄HCO₃ to CoCl₂ is ≤3, the precursor obtained is basic cobalt carbonate; When the molar ratio of NH₄HCO₃ to CoCl₂ is equal to 4.5, the precursor is a mixture of cobalt carbonate and basic cobalt carbonate.

・ The suitable process conditions for preparing cobalt oxide by NH₄HCO₃ to CoCl₂ system are as follows: the molar ratio is NH₄HCO₃ to CoCl₂ is 4.5:1, the CoCl₂ concentration is 13 g/L, and the reaction temperature is 60˚C, the reaction time is 10 hours. The medium diameter of precursor and Co₃O₄ prepared under the conditions is 8 μm and 9 μm, respectively.

The author declares no conflicts of interest regarding the publication of this paper.

Peng, Y. (2019) Preparation of Micron Co₃O₄ by Liquid-Phase Precipitation. Journal of Materials Science and Chemical Engineering, 7, 29-38. https://doi.org/10.4236/msce.2019.712004