KR101297385B1 - Preparation method of cathode active materials for lithium secondary battery - Google Patents

Abstract

PURPOSE: A manufacturing method of a positive electrode active material is provided to provide a positive electrode active material with improved electrochemical properties. CONSTITUTION: A manufacturing method of a positive electrode active material for lithium secondary batteries comprises a step of manufacturing a solution mixed of KMnO4 and ethanol with distilled water; a step of putting the mixed solution into an autoclave container, stirring the solution, and conducting a hydrothermal synthesis reaction; a step of washing the reaction products with distilled water and ethanol several times and drying the washed reaction products; a step of carrying out a mixing reaction by mixing MnO(OH) with LiOH·H2O; and a step of heat-treating the lamellar Li2MnO3, which is the reaction product of the mixing step, in the atmosphere.

Description

Preparation method of cathode active material for lithium secondary battery {Preparation method of cathode active materials for lithium secondary battery}

The present invention relates to a positive electrode active material of a lithium secondary battery and a method for manufacturing the same. The present invention relates to a layered Li ₂ MnO ₃ -MnO (OH) using layered Li ₂ MnO ₃ or MnO (OH) prepared by reacting MnO (OH) with LiOH as a buffer material. It relates to a composite and a method of manufacturing the same.

Oxide materials containing lithium and transition metals are mainly used as positive electrode active materials of lithium secondary batteries, and the electrochemical properties are due to a change in the oxidation number of the transition metal in the active material.

The oxidation number of transition metal ions changes when lithium ions are inserted or extracted (intercalation / deintercalation), and the average voltage depends on the oxidation / reduction of each element. The layered Li ₂ MnO ₃ has an oxidation number of manganese of +4 when discharged (when lithium ions are intercalated) and +5 when charged (when lithium ions are extracted), thereby being electrically inactive (nonpatent See literature 1,2).

However, recent studies have reported represents the acid treatment with high dose induced by the natural oxidation / reduction reaction, or removing the Li ₂ O from Li ₂ MnO ₃ the cathode material making a lack (deficiency) of the material is performed. Li ions present between the _{[Li 1 / 3Mn 2/3} ] layer is involved in the electrochemical reaction (see Non-Patent Literature 3-5).

Li ₂ MnO ₃ -LiMO ₂ (M = Mn, Co, Ni, Cr, etc.) composite electrodes have different polymorphs and are known to reduce structural changes during charge / discharge cycles (Non-Patent Document 6, 7). Li ₂ MnO ₃ is known as an electrically inert material, but Li ₂ MnO ₃ in a layered structure is partially converted to a spinel structure after a charge / discharge cycle, thereby enabling manganese redox reaction to become an electrically active material, thereby making it a lithium secondary battery. It is used as the positive electrode active material of.

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The present invention relates to a layered Li ₂ MnO ₃ or a layered Li ₂ MnO ₃ -MnO (OH) composite having a structure and structure as a cathode active material of a lithium secondary battery having improved electrochemical characteristics such as discharge capacity and cycleability. It is to provide a manufacturing method.

Li ₂ MnO ₃ (Lithium Manganese Oxide) is known as an electrically inert material, but the layered structure is partially changed into the spinel structure after the charge / discharge cycle, thereby enabling the oxidation / reduction reaction of manganese to be electrochemically activated.

The present invention provides a method for producing layered Li ₂ MnO ₃ , which is a positive electrode active material, in which a structure and a structure are changed by controlling a heat treatment temperature in order to improve discharge capacity and life characteristics of a positive electrode material of a lithium secondary battery. The method for producing a layered Li ₂ MnO ₃ includes the following steps a) to e).

a) preparing a solution of KMnO ₄ and ethanol mixed with distilled water

b) adding the mixed solution to an autoclave vessel and stirring to hydrothermally react;

c) washing the reaction product with distilled water and ethanol several times and drying

d) mixing MnO (OH) obtained after the drying step with LiOH.H ₂ O to perform a solid phase reaction.

e) heat-treating the layered Li ₂ MnO ₃ in the air after the solid phase reaction step

In addition, the present invention provides a method for producing a layered Li ₂ MnO ₃ -MnO (OH) composite, a positive electrode active material that improves the discharge capacity and life characteristics by mixing MnO (OH) in the form of amorphous powder as a buffer material to the layered Li ₂ MnO ₃ to provide. The method for preparing a layered Li ₂ MnO ₃ -MnO (OH) complex includes the following steps a) to d).

a) preparing a solution of KMnO ₄ and ethanol mixed with distilled water

b) adding the mixed solution to an autoclave vessel and stirring to hydrothermally react;

c) washing the reaction product with distilled water and ethanol several times and drying

d) after the drying step by mixing the MnO (OH) and the resultant layered Li ₂ MnO ₃ to prepare a layered Li ₂ MnO ₃ -MnO (OH) complex

According to the present invention, after preparing MnO (OH) by hydrothermal synthesis, reacting with LiOH in a solid phase, and controlling the heat treatment temperature to produce layered Li ₂ MnO ₃ , the layered phase having both high crystallinity and high discharge capacity suitable for a lithium secondary battery cathode active material. Li ₂ MnO ₃ can be obtained, and layered Li ₂ MnO ₃ -MnO (OH) composites are prepared using MnO (OH) as the buffer material to reduce the structure and structure change of the layered Li ₂ MnO ₃ when used as a cathode active material. The discharge capacity and lifespan characteristics of the positive electrode active material can be improved.

1 is an SEM image of MnO (OH) according to synthesis temperature
(a) 100 (b) 110 (c) 120 (d) 130 (e) 140 (f) 150
2 is XRD patterns of MnO (OH) synthesized at 110 ℃ -150 ℃
3 is a SEM image of the layered Li ₂ MnO ₃ by synthesis temperature
(a) 600 ° C (b) 700 ° C (c) 800 ° C (d) 900 ° C
4 is XRD patterns of layered Li ₂ MnO ₃ heat-treated at 600 ℃-900 ℃
5A shows the discharge capacity after 15 cycles of a layered Li ₂ MnO ₃ cell heat treated at 600 ° C.-900 ° C.
5B shows the discharge capacity after 54 cycles of a layered Li ₂ MnO ₃ cell heat-treated at 800 ° C.-900 ° C.
6 is XRD data for measuring the structural change after the cycle test of layered Li ₂ MnO ₃ by heat treatment temperature (a) 600 ℃ (b) 700 ℃ (c) 800 ℃ (d) 900 ℃
7 shows the 1st, 3rd, 5th, 8th, 12th cycle test.
Discharge Capacity of Layered Li ₂ MnO ₃ -700 ℃ Heat Treatment
8 is a charge / discharge performance of the layered Li ₂ MnO ₃ -MnO (OH) composite

(Test evaluation condition)

Physical-chemical properties of the positive electrode samples were measured by X-ray diffraction (XRD, Rkgaku DMAX-III diffractometer), scanning electron microscope (SEM, Hitachi S-4300, Japan), Fourier transform infrared spectroscopy (FT-IR, Nicolet 380).

The coin cell electrode was made of 10 mg of positive electrode material and 4 mg of TAB (Welcos) and isopropyl alcohol. Coin cell (CR2032 type) was assembled in an argon-filled glove box, and the cell was composed of Li ₂ MnO ₃ , lithium metal, separator (polypropylene, Welcos) and electrolyte (1M LiPF6 in 1: 1 volume ratio of dimethyl carbonate (DMC). ) And ethylene carbonate (EC) solution, TECHNO Semichem Co.).

The charge / discharge test was performed after aging the coin cell for 12 hours with WBCS3000 (WonA Tech, Korea). Coin cells tested to observe the structural change of the anode material after the charge / discharge test were decomposed in an argon-filled glove box, washed with DMC to remove LiPF ₆ electrolyte, dried at room temperature, and then measured by X-ray diffraction (XRD). .

a) A solution of KMnO ₄ and ethanol mixed with distilled water is prepared. The amount of ethanol and distilled water per 1 g of KMnO ₄ is 1 to 2 mL, and 40 to 160 mL of distilled water is used. More preferably, 1 mL of ethanol and 80 mL of distilled water are mixed per 1 g of KMnO ₄ .

b) 30 mL of the mixed solution was added to an autoclave and stirred at a temperature of 100 to 150 ° C. for 24 hours for hydrothermal synthesis. The reaction time may be selected differently as needed and may have a range of 10 to 40 hours, for example.

c) The reaction product was washed several times with distilled water and ethanol and dried for 60 hours at a temperature of 80-100 ℃. The drying time may be selected differently as needed and may have a range of 60 to 100 hours, for example.

d) MnO (OH) obtained after the drying step is mixed with LiOH.H ₂ O (Aldrich), but the mixing ratio of MnO (OH) to LiOH.H ₂ O is mixed so that Li: Mn is 2.1: 1 in molar ratio. React.

e) After the reaction mixture step, the layered Li ₂ MnO ₃ reaction product was heat-treated for 4 hours at a temperature of 600-900 ℃ in the air. The heat treatment time may be differently selected as necessary and may have a range of 1 to 10 hours, for example.

The layered Li ₂ MnO ₃ for a lithium secondary battery positive electrode active material was manufactured by the above manufacturing method.

a) A solution of KMnO ₄ and ethanol mixed with distilled water is prepared. The amount of ethanol and distilled water per 1 g of KMnO ₄ is 1 to 2 mL, and 40 to 160 mL of distilled water is used. More preferably, 1 mL of ethanol and 80 mL of distilled water are mixed per 1 g of KMnO ₄ .

b) The mixed solution is added to an autoclave vessel and stirred at a temperature of 100 to 150 ° C. for 24 hours for hydrothermal synthesis. The reaction time may be selected differently as needed and may have a range of 10 to 40 hours, for example.

c) The reaction product was washed several times with distilled water and ethanol and dried for 60 hours at a temperature of 80-100 ℃. The drying time may be selected differently as needed and may have a range of 60 to 100 hours, for example.

d) the MnO (OH) obtained after the drying step, the layer Li ₂ MnO ₃ with a molar ratio of 1: 1 was prepared by mixing a layered Li ₂ MnO ₃ -MnO (OH) complex.

A layered Li ₂ MnO ₃ -MnO (OH) composite for a lithium secondary battery positive electrode active material was manufactured by the same method as described above.

In Example 1, the MnO (OH) obtained in step d) is preferably formed into a nano-rod structure with a short axis of 50-200 nm and a long axis of 0.5 μm or more, more preferably of 100-150 nm and a long axis of 1 μm. It is formed of the nano-rod structure as described above.

In Example 2, the MnO (OH) obtained in step d) is preferably formed of an amorphous powder having a diameter of 50-200 nm, and more preferably formed of an amorphous powder having a diameter of 70-80 nm.

Example 1 e) and layered Li ₂ MnO ₃ and carrying out the steps of Example 2 d) layered Li ₂ MnO ₃ used in step a structure monoclinic (monoclinic), shorter than the long axis is 50-200nm and nano rod 0.5μm It is preferable that each layer is constituted, and more preferably, each layer is composed of nanorods having a short axis of 100-150 nm and a major axis of 1 μm or more.

MnO (OH) is manufactured by a hydrothermal synthesis method as a manganese precursor (refer nonpatent literature 8). 1 shows the structure of MnO (OH) synthesized at various temperatures. At 100-120 ° C. reaction, spherical nanoparticles are formed, and the particle size of MnO (OH) is about 70-80 nm. When the synthesis reaction temperature rises to 130-150 ° C, the structure of MnO (OH) changes as shown in Fig. 1 (d)-(f). MnO (OH) has a short axis of 100-150nm and a long axis of 1μm or more. Becomes Particle size and cohesion increase with increasing reaction temperature.

FIG. 2 is X-ray diffraction (XRD) patterns showing crystallinity of manganese precursors synthesized at various temperatures. The pattern of MnO (OH) synthesized at 100 ° C. has a wide diffraction distribution without diffraction peaks due to the formation of nanoscale amorphous particles. However, as the reaction temperature increases, the diffraction intensity increases and the diffraction peaks become sharp, indicating that the crystallinity of MnO (OH) is improved. As the SEM image of FIG. 1 shows, the structure of MnO (OH) has a large rod-shaped domain as the reaction temperature increases, and the X-ray diffraction (XRD) pattern has a tendency to be consistent with previous reports. (See Non-Patent Documents 8 and 9). Although the minor impurity Mn ₃ O ₄ appears at about 28 degrees and about 60 degrees, it is clear that the MnO (OH) samples are well crystallized.

Figure 3 shows the SEM image of Li ₂ MnO ₃ in accordance with the heat treatment temperature. MnO (OH) precursor was used in the form of nano bars synthesized at 140 ℃. The structure of Li ₂ MnO ₃ is in the form of nanorods in the same manner as MnO (OH), which is called a 'chimie-douce effect' (see Non-Patent Documents 10 and 11). The chimie-douce reaction is a topotactic reaction and is one of the methods of controlling the form of the compound in which the reactants and the product have an equivalent structure. The synthesized Li ₂ MnO _{3 has} a diameter of about 100-150 nm and its length is in the form of nanorods similar to the precursor MnO (OH).

4 is X-ray diffraction (XRD) data showing the crystallinity of Li ₂ MnO ₃ synthesized at 600-900 ° C., and all of the crystal structures of Li ₂ MnO ₃ are monoclinic. This monoclinic structure peak is due to the cation alignment of Li + and Mn ₄ + ions present in the transition metal layer of Li ₂ MnO ₃ . The charge difference induces a difference in cation distribution of Li ₂ MnO ₃ , and simultaneously lowers the symmetrical structure from the hexagonal structure R3m to the monoclinic structure C2 / m (see Non-Patent Documents 1-3).

The superlattice characteristics at about 21 degrees of Li 2 MnO 3 synthesized at 600 ° C. disappear completely due to lamination disorder due to low crystallinity (see Non-Patent Document 12). However, in Li ₂ MnO ₃ synthesized at 600 ° C., the peaks of 64.5 degrees and 65.5 degrees, which are characteristic diffraction peaks of the monoclinic structure, are clearly shown as (135) and (060) reflecting surfaces, respectively. As the heat treatment temperature increases, the peaks of 21 degrees, 64.5 degrees, and 65.5 degrees become stronger and sharper, because the crystallinity of Li ₂ MnO ₃ is improved as the heat treatment temperature is increased.

In order to measure the electrochemical properties of the prepared Li ₂ MnO ₃ , the charge / discharge test was performed at 0.2C-rate under 2.5-4.5V cut-off conditions, the results are shown in FIG. It can be seen that the discharge capacity increases proportionally as the number of cycles increases and then decreases at a certain maximum. This rapid change is known to be due to an irreversible phase transition (see Non-Patent Documents 13 and 14). The cells of Li ₂ MnO ₃ heat-treated at 600 ° C. exhibit relatively stable capacities, and the difference in minimum maximum discharge capacity is also the smallest of the samples. The cells of Li ₂ MnO ₃ heat treated at 700 ° C. show the best maximum discharge capacity, but the initial capacity is smaller than the cells of Li ₂ MnO ₃ heat treated at 600 ° C. On the other hand, Li ₂ MnO ₃ cells heat-treated at 800 ° C. and 900 ° C. showed very low initial discharge capacities, but the cycleability was improved compared to cells at 600 ° C. and 700 ° C. heat treatment.

In fact, the stoichiometric Li ₂ MnO ₃ is difficult to diffuse lithium ions into the Mn layer, there is no electrochemical reaction. Thus, the diffusion of lithium ions will be increased in less crystallized particles. Although the maximum discharge capacities of the samples at various heat treatment temperatures are different, the trend of the overall discharge capacity is the same. This means that even though the heat treatment temperature is different, the structural change of the activating material follows the same mechanism during the repeated intercalation / deintercalation process of lithium ions.

In order to observe the structural change after the cell test, the anode material of the tested cell was washed with DMC, dried and XRD analysis was performed. At about 21 degrees, the superlattice diffraction peak of the layered Li ₂ MnO ₃ disappeared completely, and the monoclinic characteristic diffraction peak of about 65.5 degrees was also drastically reduced. This means that Li ₂ MnO _{3, which} is a layer structure of rock salt, partially collapsed (see Non-Patent Document 3).

After the cell test, minor peaks other than the main peak appear at about 40 degrees and about 58 degrees, which are spinel LiMn ₂ O ₄ . XRD results of gradually changing Li ₂ MnO ₃ of the rock salt structure of the present invention to spinel LiMn ₂ O ₄ are consistent with previous reports (Non-Patent Document 15). Due to the influence of the LiMn ₂ O ₄ of the Mn + ₃ Mn + 3 ions / + 4 is capable of the transition between the oxidation number, by electrochemically inactive Li ₂ MnO ₃ is activated electrochemically with increased discharge capacity. Therefore, when Li ₂ MnO ₃ of low crystallinity is heat-treated at a low temperature, it has a high stacking defect rate (see Non-Patent Document 16), and it is due to easier structural change compared to Li ₂ MnO ₃ having high crystallinity which is heat-treated at high temperature. This results in a relatively high discharge capacity. In addition, Li ₂ MnO ₃ having excellent crystallinity heat-treated at 800 ° C. and 900 ° C. has a slow structural change, and thus, it takes longer to reach the state where manganese is activated and a redox reaction occurs.

The discharge capacity-voltage curve of Li ₂ MnO ₃ heat treated at 700 ° C. to clarify the transition of manganese ions between + 3 / + 4 oxides is shown in FIG. 7. The flat plateau at about 4.0V due to manganese + 3 / + 4 oxidation / reduction reactions in the first few cycles is very short. However, as the number of cycles increases, the flat section near 4.0V gradually increases and the total discharge capacity increases.

The layered Li ₂ MnO ₃ changes in structure as the charge / discharge cycle proceeds. MnO (OH) is mixed in layered Li ₂ MnO ₃ as a manganese precursor to reduce this change in structure and structure. MnO (OH) nanoparticles of spherical synthesized at 100 ℃ is on the layer Li ₂ having a particle size than MnO ₃ small to act as a buffer for preventing the Organization and Structure of the layered Li ₂ MnO _3, and lithium ion is MnO (OH) After intercalation, it converts into an active material.

MnO (OH) was synthesized at 100 ° C. and mixed with Li ₂ MnO ₃ in a molar ratio of 1: 1 to prepare a positive electrode active material. The charge / discharge test of the layered Li ₂ MnO ₃ -MnO (OH) composite is performed in FIG. 8.

The overall cycle performance results showed a volcano shape with respect to the maximum discharge capacity and the initial capacity gradually increased. However, the maximum discharge capacity was increased than that of Li ₂ MnO ₃ alone and appeared at about 40th cycle. This means that the structural change from the layered structure to the spinel structure proceeded slowly by adding MnO (OH) as a buffer material, and also a slow structure change may occur during the intercalation / deintercalation of lithium ions and increase the discharge capacity. Means.

Claims (5)

a) preparing a solution of KMnO ₄ and ethanol mixed with distilled water;
b) adding the mixed solution to an autoclave vessel and stirring to hydrothermally react;
c) washing the reaction product with distilled water and ethanol several times and drying;
d) mixing MnO (OH) obtained after the drying step with LiOH.H ₂ O and mixing the mixture;
e) heat treating the layered Li ₂ MnO ₃ , which is a reaction product, in the air after the mixing reaction step;
Method for manufacturing a cathode active material for a lithium secondary battery comprising a a) preparing a solution of KMnO ₄ and ethanol mixed with distilled water;
b) adding the mixed solution to an autoclave vessel and stirring to hydrothermally react;
c) washing the reaction product with distilled water and ethanol several times and drying;
d) after the drying step by mixing the MnO (OH) and the resultant layered Li ₂ MnO ₃ to prepare a layered Li ₂ MnO ₃ -MnO (OH) complex;
Method of manufacturing a positive electrode active material for lithium secondary batteries, including The method according to claim 1 or 2,
KMnO ₄ Method for producing a positive electrode active material for lithium secondary batteries having a mixing ratio of 1 ~ 2mL ethanol per 1g, 40 ~ 160mL distilled water The method of claim 1,
The MnO (OH) obtained in step d) is a nanorod having a short axis of 50-200 nm and a major axis of at least 0.5 m,
After the step e), the layered Li ₂ MnO ₃ has a monoclinic structure and has a short axis of 50-200 nm and a long axis of 0.5 m or more, each layer is composed of a cathode active material for a lithium secondary battery, characterized in that The method of claim 2,
The MnO (OH) obtained in step d) is an amorphous powder with a diameter of 50-200 nm,
The layered Li ₂ MnO ₃ of step d) has a monoclinic structure, and each layer is composed of nanorods having a short axis of 50-200 nm and a major axis of 0.5 m or more.

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