KR102229451B1 - Method for preparing maghemite - Google Patents

Abstract

The present invention relates to a method for producing maghemite, and more particularly, NaBH ₄ and Fe(NO ₃ ) ₃ ·9H ₂ O by controlling the reaction time and reaction temperature to control the reaction time and crystalline lepidocrocite (γ-FeOOH ), and then heat treatment under inert gas conditions to produce high-purity maghemite (γ-Fe ₂ O ₃ ), which can be applied to the positive electrode of a lithium-sulfur secondary battery to improve the discharge capacity. .

Description

Manufacturing method of maghemite {METHOD FOR PREPARING MAGHEMITE}

The present invention relates to a method of manufacturing maghemite that can be used as an electrode catalyst for a lithium-sulfur secondary battery.

Recently, as the trend of miniaturization of electronic devices has gradually diversified to mobile phones, notebook computers (PCs), portable personal information terminals (PDAs), etc., interest in secondary battery technology is increasing. Furthermore, as an electric vehicle or a hybrid electronic vehicle is put into practical use, research on a secondary battery having high capacity and output and excellent stability is being actively conducted.

The secondary battery is composed of a positive electrode, a negative electrode, a separator, and an electrolyte, and the cost of the positive electrode is the highest among the cost of various materials. The cathode material of a lithium secondary battery generally has a high energy density when charging and discharging, and the structure should not be destroyed by intercalation and desorption of reversible lithium ions, and has high electrical conductivity and chemical stability to the organic solvent used as an electrolyte. It should be high. Furthermore, it is preferable that it is a material that has a low manufacturing cost and minimizes environmental pollution problems.

Recently, sulfur is attracting attention as a positive electrode active material capable of expressing high energy capacity in lithium ion batteries. Sulfur has a high theoretical capacity of about 1675 mAh/g and a high energy density of 2600 W h/Kg, and the price is low. It is because it has a non-toxic property.

However, lithium-sulfur secondary batteries using sulfur have been reported in the following patents, but sulfur has low electrical conductivity, forms polysulfides during charge/discharge reactions, and forms a protective layer on the surface of lithium metal, resulting in improved lifespan characteristics. There are many problems that must be solved before commercialization because it can cause problems that fall out and decrease electrochemical activity.

In order to solve these problems, research on electrode materials to improve the capacity and life characteristics of lithium-sulfur secondary batteries is being actively conducted, and maghemite is used to improve the discharge capacity and life characteristics of lithium-sulfur secondary batteries. As it has been reported that there is an effect, interest in the technology for manufacturing maghemite is increasing, but studies on the manufacturing technology of maghemite in a form suitable as an electrode material for lithium-sulfur secondary batteries have been actively conducted. The situation is not losing.

Korean Patent Registration No. 10-0482279 (2005.03.31), "Iron oxide nanopowder and its manufacturing method"

As a result of conducting various studies to solve the above problems, the present inventors _{reacted NaBH 4} and Fe(NO ₃ ) ₃ ·9H ₂ O in an aqueous solution of an appropriate concentration, but by controlling the reaction time and reaction temperature, γ- _{After preparing FeOOH, it was confirmed that high purity maghemite (γ-Fe 2} O ₃ ) could be prepared by heat-treating it under an inert gas.

Accordingly, an object of the present invention is to provide a method for producing high purity maghemite through a simple process.

In order to achieve the above object, the present invention

(1) _{mixing and reacting Fe(NO 3} ) ₃ ·9H ₂ O and a reducing agent represented by the following formula (1);

(2) After the step (1), it provides a method for producing maghemite comprising the step of heat-treating in an inert gas atmosphere.

[Formula 1]

M ¹ (BH ₄ ) _X

In Formula 1, M ¹ is any one selected from Li, Na, Mg, K, and Ca, and X is 1 or 2.

At this time, the step (2) may be a state in which an inert gas is continuously introduced under an inert gas atmosphere.

In this case, the inert gas may be selected from the group consisting of nitrogen, argon, helium, and mixtures thereof.

In this case, the heat treatment may be performed at 250 to 600°C.

At this time, the heat treatment may be performed for 1 to 4 hours.

According to the present invention, _{high purity maghemite (γ-Fe 2} O ₃ ) can be prepared by a simple process including a step of reacting _{NaBH 4} and Fe(NO ₃ ) ₃ ·9H _{2 O and heat treatment.} .

When the NaBH ₄ and Fe(NO ₃ ) ₃ ·9H ₂ O are reacted, the shape and purity of the _{produced maghemite (γ-Fe 2} O ₃ ) can be controlled by simply controlling the reaction temperature and the reaction time.

In addition, when the prepared maghemite (γ-Fe ₂ O ₃ ) is applied as a cathode material for a lithium-sulfur secondary battery, the discharge capacity of the lithium-sulfur secondary battery can be increased.

1 is a scanning electron microscope (SEM) photograph of a lepidocrocite prepared in Preparation Example.
2 is a scanning electron microscope (SEM) photograph of maghemite prepared in Example.
3 is a graph showing the results of X-ray Diffraction Spectroscopy (XRD) analysis of repidocrosite and maghemite prepared in Preparation Examples and Examples, respectively.
4 is a graph showing discharge capacity test results of lithium-sulfur secondary batteries to which repidocrocite and maghemite each prepared in Preparation Examples and Examples are applied.

Hereinafter, the present invention will be described in more detail to aid understanding of the present invention.

The terms or words used in the specification and claims should not be construed as being limited to their usual or dictionary meanings, and the inventor may appropriately define the concept of terms in order to describe his own invention in the best way. It should be interpreted as a meaning and concept consistent with the technical idea of the present invention based on the principle that there is.

The present invention relates to a method for manufacturing maghemite, and relates to a method for manufacturing maghemite having a form and physical properties capable of improving discharge capacity by applying it as a cathode material of a lithium-sulfur secondary battery. will be.

The method for producing maghemite according to the present invention is

(1) _{mixing and reacting Fe(NO 3} ) ₃ ·9H ₂ O and a reducing agent represented by the following formula (1);

(2) After the step (1), it may include a step of heat treatment in an inert gas atmosphere.

[Formula 1]

M ¹ (BH ₄ ) _X

In Formula 1, M ¹ is any one selected from Li, Na, Mg, K, and Ca, and X is 1 or 2.

It will be described in detail for each step below.

Lepidocrocite Manufacturing Step: Step (1)

Both the Fe(NO ₃ ) ₃ ·9H ₂ O and the reducing agent represented by Formula 1 may be in the form of an aqueous solution, and the Fe(NO ₃ ) ₃ ·9H ₂ O aqueous solution is added to the reducing agent aqueous solution represented by Formula 1 By mixing and reacting.

If, on the contrary, mixing and reaction proceeds, the purity of the prepared repidocrocite may decrease. That is, when the Fe(NO ₃ ) ₃ ·9H ₂ O aqueous solution is mixed and reacted by adding the reducing agent aqueous solution represented by Chemical Formula 1, the purity of the prepared repidocrocite may be lowered.

The Fe(NO ₃ ) ₃ ·9H ₂ O aqueous solution may be 0.04 to 0.08 M, preferably 0.05 to 0.06 M, and if it is less than 0.04 M, the production yield of repidocrocite may be lowered, and if it exceeds 0.08 M, then The physical properties of the produced maghemite may not be suitable for application as a cathode material for a lithium-sulfur secondary battery.

The reducing agent aqueous solution represented by Formula 1 may be 0.2 to 0.5 M, preferably 0.3 to 0.4 M, and if it is less than 0.2 M, repidocrocite is not prepared, and if it is more than 0.5 M, the reaction may not proceed.

According to a preferred embodiment of the present invention, the reducing agent represented by Formula 1 may be _{NaBH 4.}

When the Fe(NO ₃ ) ₃ ·9H ₂ O aqueous solution and the NaBH ₄ aqueous solution are reacted, the repidocrocite can be naturally synthesized in the aqueous solution after the ^{Fe 3 + cation is converted to the Fe metal form.}

Mixing of the Fe(NO ₃ ) ₃ ·9H ₂ O and the reducing agent represented by Formula 1 may be performed within a short time, and may be performed within 10 to 120 seconds, preferably 50 to 80 seconds. If the mixing time is less than 10 seconds, the mixture is too fast and gas is generated at once, so the reaction may proceed unevenly, and if the mixing time is more than 120 seconds, the mixing speed is slow. The phase of the material may be different.

In addition, the reaction temperature may be 10 to 60 ℃, preferably 20 to 50 ℃, more preferably 20 to 25 ℃, the reaction may not proceed if the reaction temperature is less than 10 ℃, if it is more than 60 ℃ Physical properties of the prepared repidocrocite may be modified. In addition, it may be desirable to react while maintaining the reaction rate at 20 to 25° C. for controlling the reaction rate.

In addition, the reaction time may be 10 minutes to 20 hours, preferably, 40 minutes to 2 hours, if less than 10 minutes, repidocrocite may not be formed, and if it is more than 20 hours, maghe The shape of the mite may be a shape that is not suitable as a cathode material for a lithium-sulfur secondary battery, and in particular, when reacted for 40 minutes to 2 hours, the desired physical properties of the lepidocrocite can be maintained without losing.

On the other hand, _{after the step of reacting the Fe(NO 3} ) ₃ ·9H ₂ O aqueous solution and the reducing agent aqueous solution represented by Formula 1, filtering and drying may be further included.

The filtering may be performed by a filtration process commonly used in the art, and for example, filter paper may be used.

The drying may be performed at 70 to 90° C. for 6 to 12 hours.

If the drying temperature is less than 70°C or the drying time is less than 6 hours, it is not completely dried to obtain repidocrocite in the form of particles. If the drying temperature is more than 90°C or more than 12 hours, the remaining water boils and the Physical properties can be denatured.

The repidocrocite prepared by the method as described above may be γ-FeOOH, and specifically, may be crystalline γ-FeOOH.

Maghemite Manufacturing Step: Step (2)

Maghemite may be prepared by heat-treating the repidocrocite prepared in step (1) in an inert gas atmosphere, and may be generated through the following Reaction Formula 1.

[Scheme 1]

2FeOOH(s) → Fe ₂ O ₃ (s) + H ₂ O(g)

The inert gas atmosphere may be (i) under an inert gas atmosphere in which the gas inside the reactor is replaced with an inert gas, or (ii) in a state in which the inert gas is continuously introduced to continuously replace the gas inside the reactor. . In the case of (ii), for example, the flow rate of the inert gas may be 1 to 500 mL/min, specifically 10 to 200 mL/min, and more specifically 50 to 100 mL/min.

Here, the inert gas may be selected from the group consisting of nitrogen, argon, helium, and mixtures thereof.

The heat treatment according to the present invention may be performed at 250 to 600°C. If the temperature is less than the above range, the repidocrocite may not be converted to maghemite, and if the temperature exceeds the above range, the particle structure of maghemite may collapse. It may change into particles, and the repidocrocite is _{converted to ε-Fe 2} O ₃ or the like, so that the desired maghemite cannot be produced, so it is appropriately adjusted within the above range.

In addition, the heat treatment may be performed for 1 to 4 hours by setting the temperature rise rate in the range of 0.1° C. per minute to 10° C. per minute. If the heat treatment time is less than the above range, maghemite cannot be produced because the temperature for producing maghemite cannot be reached, and if the heat treatment time exceeds the above time, the heat treatment temperature increases too much and the particle structure of the maghemite May collapse, change into particles of an unwanted size due to the sintering phenomenon, _{and may generate ε-Fe 2} O ₃ and the like, not maghemite, so it is appropriately adjusted within the above range.

According to an embodiment of the present invention, the prepared repidocrocite may be converted to 80% or more maghemite through a heat treatment step in an inert gas atmosphere, and preferably 90% or more may be converted to maghemite. . Referring to FIG. 3, as a result of XRD analysis of maghemite, it can be seen that the XRD peak of lepidocrocite was not detected, and thus 90% or more was converted to maghemite.

The prepared maghemite may have a shape of secondary particles formed by agglomeration of plate-shaped maghemite primary particles, and the secondary particles may have a spherical shape.

The shape of the prepared maghemite can be adjusted as needed by controlling the reaction time, and all of these can be applied as a cathode material for a lithium-sulfur secondary battery.

The prepared plate-shaped primary particles may have a particle diameter of more than 1 and less than or equal to 1000 nm, preferably 50 to 500 nm. The secondary site formed by the agglomeration of the primary particles may have a particle diameter of 1 to 50 µm, and preferably 1 to 20 µm. As the particle diameter of the secondary particles decreases within the above range, it is suitable as a cathode material for a lithium-sulfur secondary battery, and when the particle diameter of the secondary particles exceeds the above range, the particle size is large, making it unsuitable as a cathode material for a lithium-sulfur secondary battery. .

When the maghemite prepared by the method for producing maghemite as described above, for example, crystalline γ-Fe ₂ O ₃ is applied to a lithium-sulfur secondary battery, it is eluted during charging and discharging of the lithium-sulfur secondary battery. Since polysulfide can be adsorbed, the performance of lithium-sulfur secondary batteries can be improved.

Hereinafter, preferred embodiments are presented to aid in the understanding of the present invention, but the following examples are only illustrative of the present invention, and it is obvious to those skilled in the art that various changes and modifications are possible within the scope and spirit of the present invention. It is natural that changes and modifications fall within the scope of the appended claims.

Manufacturing example : Lepidochrosite Produce

0.3 M NaBH ₄ A 0.05 M Fe(NO ₃ ) ₃ ·9H ₂ O aqueous solution was mixed with the aqueous solution for 50 seconds while stirring at 400 rpm. At this time, NaBH ₄ may have a purity of> 95% as a product of TCI, and Fe(NO ₃ ) ₃ ·9H ₂ O may have a purity of> 98% as a product of Aldrich.

After mixing, the mixture was reacted at 24° C. for 40 minutes, filtered using a filter paper, and dried at 80° C. for 8 hours to prepare lepidocrocite.

Example : Maghemite Produce

The repidocrosite powder prepared in Preparation Example 1 was heat-treated at 400° C. for 1 hour while flowing nitrogen gas having a flow rate of 100 mL/min. At this time, the heating rate for the heat treatment was 10°C per minute. Maghemite was prepared through the heat treatment.

Comparative example : Maghemite Manufacturing (Korean Patent Registration 10-0482278)

0.2 mL of iron pentacarbonyl (Fe(CO) ₅ , 1.52 mmol, Aldrich 80-90%) was injected and the temperature was slowly raised to reflux for 5 hours.

In the process of refluxing, the orange-colored solution turned colorless to blackish brown. The solution was cooled to room temperature, an excess of ethanol was added to separate the precipitated powder, and dried to prepare iron oxide nanopowder.

Experimental example One: SEM (scanning electron microscope) analysis

SEM analysis (S-4800 FE-SEM of Hitachi Corporation) was performed on the repidocrocite and maghemite prepared in Preparation Examples and Examples, respectively.

1 is a SEM photograph of the repidocrosite prepared in Preparation Example.

2 is a SEM photograph of the maghemite prepared in Example.

Referring to FIG. 1, as a result of performing SEM analysis with a magnification of 50k, a plate-shaped repidocrosite of several hundred nm was observed, and it was confirmed that maghemite prepared by heat treatment also showed a plate-shaped structure as it is.

Experimental example 2: XRD analysis

XRD analysis (Bruker's D4 Endeavor) was performed on the lepidocrocite and maghemite prepared in Preparation Examples and Examples, respectively.

3 is a graph showing the XRD analysis results for repidocrosite and maghemite prepared in Preparation Examples and Examples, respectively.

Referring to FIG. 3, the XRD peak of repidocrosite can be confirmed, from which it can be seen that the pure phase crystalline repidocrosite was prepared in the Preparation Example.

In addition, by checking the XRD peak of the example, it can be seen that a pure plate-shaped maghemite of several hundred nm was prepared.

Experimental example 3: Comparative experiment of discharging capacity of lithium-sulfur secondary battery

In order to test the discharge capacity of the lithium-sulfur secondary battery according to the type of the positive electrode material, the discharge capacity was measured after configuring the positive electrode and the negative electrode of the lithium-sulfur secondary battery as described in Table 1 below.

The positive electrode of Comparative Experimental Example was a sulfur-carbon composite, the positive electrode of Experimental Example (1) contained a sulfur-carbon composite and lepidocrocite of Preparation Example, and the positive electrode of Experimental Example (2) was a sulfur-carbon composite and Magheh of Example. It was supposed to contain mite. At this time, the measurement current was 0.1C and the voltage range was 1.8 ~ 2.5V, and the results are shown in Table 1 and FIG. 4.

Lithium-sulfur secondary battery Discharge capacity
(mAh/g) cathode anode Comparative Experimental Example Metal lithium Sulfur-carbon composite + conductive material + binder (90:5:5, weight ratio) 1,088 Experimental Example (1) Metal lithium Sulfur-carbon composite + conductive material + binder + lepidocrocite of Preparation Example (90:5:5:10, weight ratio) 1,189 Experimental Example (2) Metal lithium Sulfur-carbon composite + + conductive material + binder + maghemite of Example (10 parts by weight) (90:5:5:10, weight ratio) 1,187

As a result, as shown in Tables 1 and 4, it can be seen that the discharge capacity of Experimental Examples (1) and (2) increased by about 100mAh/g compared to Comparative Experimental Example.

From these results, it was confirmed that the discharge capacity effect was excellent when the maghemite prepared in Example was added to the positive electrode of a lithium-sulfur secondary battery.

In the above, although the present invention has been described by a limited embodiment and drawings, the present invention is not limited thereto, and the technical idea of the present invention and the following description by those of ordinary skill in the art to which the present invention pertains. It goes without saying that various modifications and variations are possible within the equivalent range of the claims to be made.

Claims (16)

(1) _{mixing and reacting Fe(NO 3} ) ₃ ·9H ₂ O and a reducing agent represented by the following formula (1); And
(2) After the step (1), the step of heat-treating for 1 to 4 hours in an inert gas atmosphere; a method for producing maghemite comprising.
[Formula 1]
M ¹ (BH ₄ ) _X
(In Formula 1, M ¹ is any one selected from Li, Na, Mg, K, and Ca, and X is 1 or 2)
The method of claim 1,
The step (2) is a method for producing maghemite, which is performed in an inert gas atmosphere or in a state in which the inert gas is continuously introduced.
The method of claim 1,
The inert gas is selected from the group consisting of nitrogen, argon, helium, and mixtures thereof.
The method of claim 1,
The heat treatment is a method of producing maghemite made at 250 to 600°C.
delete The method of claim 1,
The heat treatment is a method for producing maghemite in which the temperature increase rate is controlled in the range of 0.1°C per minute to 10°C per minute.
The method of claim 1,
The Fe(NO ₃ ) ₃ ·9H ₂ O is a method of producing maghemite mixed in the form of an aqueous solution of 0.04 to 0.08 M. The method of claim 1,
The method for producing maghemite, wherein the reducing agent represented by Formula 1 is mixed in the form of an aqueous solution of 0.2 to 0.5 M.
The method of claim 1,
The mixing is carried out for 10 to 120 seconds, a method for producing maghemite.
The method of claim 1,
The reaction temperature in step (1) is 20 to 25 ℃ method for producing maghemite.
The method of claim 1,
The reaction time of step (1) is 40 minutes to 12 hours, a method for producing maghemite.
The method of claim 1,
A method for producing maghemite further comprising filtration and drying steps after the step (1).
The method of claim 12,
The drying is carried out at 70 to 90 °C for 6 to 12 hours, a method for producing maghemite.
The method of claim 1,
The maghemite is γ-Fe ₂ O ₃ Method of producing maghemite.
The method of claim 1,
The maghemite is a method for producing a plate-shaped maghemite.
The method of claim 1,
The maghemite is a method for producing maghemite having a particle diameter of 50 to 500 nm.

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