WO2019093660A2 - Method for manufacture of maghemite - Google Patents

Abstract

The present invention relates to a method for manufacture of maghemite. More specifically, NaBH₄ is reacted with Fe(NO₃)₃·9H₂O under the control of a reaction time and reaction temperature to afford crystalline lepidocrocite (γ-FeOOH) which is then thermally treated in a non-active gas condition to produce high purity maghemite (γ-Fe₂O₃). The application of the maghemite to a cathode of a lithium-sulfur secondary battery can improve the discharge capacity.

Description

Manufacturing method of maghemite

The present application claims the benefit of priority based on Korean Patent Application No. 10-2017-0147143, filed on November 7, 2017, the entire contents of which are incorporated herein by reference.

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

2. Description of the Related Art [0002] As the tendency toward miniaturization of electronic devices has recently diversified into mobile phones, notebook computers (PCs), and personal digital assistants (PDAs), interest in secondary battery technology has been increasing. Further, as electronic vehicles or hybrid electronic vehicles are put into practical use, studies on secondary batteries having high capacity and high output and high stability have been actively conducted.

The secondary battery is composed of an anode, a cathode, a separator, and an electrolyte, and the cost of the anode is the highest in the cost of various materials. The cathode material of a lithium secondary battery generally has a high energy density at the time of charging and discharging. At the same time, the structure should not be destroyed by intercalation or deintercalation of reversible lithium ions, and the chemical stability of an organic solvent used as an electrolyte is high It should be high. Further, it is preferable that the material is low in manufacturing cost and minimizes environmental pollution problem.

In recent years, sulfur has been attracting attention as a cathode active material capable of exhibiting a high energy capacity in a lithium ion battery. Sulfur has a high theoretical capacity of about 1675 mAh / g and a high energy density of 2600 Wh / Kg, Because it has a nature without toxicity.

However, although lithium-sulfur secondary batteries using sulfur have been reported in the following patents, sulfur has low electrical conductivity, forms polysulfides during charging / discharging reaction, forms a protective layer on the surface of lithium metal, And the electrochemical activity may be lowered. Therefore, there are many problems to be solved until commercialization.

In order to solve such problems, researches on electrode materials for improving the capacity and lifetime characteristics of lithium-sulfur secondary batteries have been actively carried out. Maghemite has been used to improve the discharge capacity and lifetime characteristics of lithium- As a result, there has been a growing interest in the technology for producing maghemite, but research on the manufacturing technology of maghemite suitable as an electrode material of lithium-sulfur secondary battery has been actively carried out It is a fact that it does not support.

[Prior Art Literature]

[Patent Literature]

(Patent Document 1) Korean Registered Patent No. 10-0482279 (March 31, 2005), "Iron Oxide Nanopowder and Method for Producing the Same"

As a result of various studies to solve the above problems, the present inventors have found that when NaBH ₄ and Fe (NO ₃ ) ₃ .9H ₂ O are reacted in an aqueous solution of a proper concentration, the reaction time and reaction temperature are controlled, FeOOH, and then heat-treated in an inert gas to produce high purity maghemite (? -Fe ₂ O ₃ ).

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

In order to achieve the above object,

(1) mixing Fe (NO ₃ ) ₃ .9H ₂ O and a reducing agent represented by the following formula (1) to react;

(2) After the step (1), heat treatment in an inert gas atmosphere is provided.

[Chemical 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 atmosphere or an inert gas is continuously introduced.

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

At this time, 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, a high purity maghemite (? -Fe ₂ O ₃ ) can be produced by a simple process including a step of reacting NaBH ₄ with Fe (NO ₃ ) ₃ .9H ₂ O and a heat treatment .

The morphology and purity of the produced γ-Fe ₂ O ₃ can be controlled by controlling the reaction temperature and the reaction time in the reaction of NaBH ₄ and Fe (NO ₃ ) ₃ .9H ₂ O.

In addition, when the produced maghemite (γ-Fe ₂ O ₃ ) is applied to the cathode material of 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 the lepidocrocite prepared in Preparation Example.

2 is a scanning electron microscope (SEM) photograph of the maghemite produced in the example.

FIG. 3 is a graph showing the results of XRD (X-ray diffraction spectroscopy) analysis of lepidocrocite and maghemite prepared in Production Examples and Examples, respectively.

FIG. 4 is a graph showing discharge capacity test results of a lithium-sulfur secondary battery to which lepidocrocite and maghemite, which are respectively prepared in Production Examples and Examples, are applied.

Hereinafter, the present invention will be described in detail in order to facilitate understanding of the present invention.

The terms and words used in the present specification and claims should not be construed in an ordinary or dictionary sense and the inventor can properly define the concept of the term to describe its invention in the best possible way It should be construed as meaning and concept consistent with the technical idea of the present invention.

The present invention relates to a method for producing maghemite, and more particularly, to a method for manufacturing a maghemite having a shape and physical properties capable of improving discharge capacity by being applied to a cathode material of a lithium-sulfur secondary battery will be.

The process for producing the maghemite according to the present invention comprises

(1) mixing Fe (NO ₃ ) ₃ .9H ₂ O and a reducing agent represented by the following formula (1) to react;

(2) After the step (1), heat treatment may be performed in an inert gas atmosphere.

[Chemical 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.

Each step will be described in detail below.

Preparation step of lepidocrocite: Step (1)

The Fe (NO ₃ ) ₃ .9H ₂ O and the reducing agent represented by the formula (1) may all be in the form of an aqueous solution, and the Fe (NO ₃ ) ₃ .9H ₂ O aqueous solution is added to the aqueous solution of the reducing agent represented by the formula Followed by mixing and reacting.

If the mixing and the reaction are carried out on the contrary, the purity of the produced lepidocrocite may be lowered. That is, when the aqueous solution of the reducing agent represented by Formula 1 is added to the Fe (NO ₃ ) ₃ .9H ₂ O aqueous solution and mixed and reacted, the purity of the prepared red precipitant may be lowered.

The aqueous solution of Fe (NO ₃ ) ₃ .9H ₂ O 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 yield of lepidocrocite production may be lowered. The physical properties of the manufactured maghemite may not be suitable for application as a cathode material of a lithium-sulfur secondary battery.

The reducing agent aqueous solution represented by the formula 1 may be 0.2 to 0.5 M, preferably 0.3 to 0.4 M. If less than 0.2 M, lepidocrocite is not produced, 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 aqueous solution of Fe (NO ₃ ) ₃ .9H ₂ O is reacted with NaBH ₄ aqueous solution, lepidocrocite can be naturally synthesized in an aqueous solution after the conversion of the Fe ^{3 +} cation into the Fe metal form.

The mixing of the Fe (NO ₃ ) ₃ .9H ₂ O and the reducing agent represented by the above 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 mixing may occur excessively and the gas may be generated all at once, so that the reaction may proceed unevenly. If the mixing time is more than 120 seconds, the mixing speed is slow, The phase of the material may be different.

The reaction temperature may be 10 to 60 ° C, preferably 20 to 50 ° C, more preferably 20 to 25 ° C. If the reaction temperature is less than 10 ° C, the reaction may not proceed, and if it is more than 60 ° C The physical properties of the lepidocrocite to be produced may be denatured. Further, it may be preferable to conduct the reaction while maintaining the temperature at 20 to 25 DEG C for controlling the reaction rate.

The reaction time may be 10 minutes to 20 hours, preferably 40 minutes to 2 hours. When the reaction time is less than 10 minutes, the reedocrocite may not be formed. When the reaction time exceeds 20 hours, The shape of the mite may not be suitable for the cathode material of the lithium-sulfur secondary battery. In particular, when the reaction is performed for 40 minutes to 2 hours, the desired properties of the redeposited material can be maintained without losing.

Meanwhile, after the step of reacting the aqueous solution of Fe (NO ₃ ) ₃ .9H ₂ O with the aqueous solution of the reducing agent represented by the formula (1), filtration and drying may be further carried out.

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

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

If the drying temperature is less than 70 ° C or less than 6 hours, the particles may not be completely dried to obtain the granular form of lepidocrocite. If the drying temperature exceeds 90 ° C or exceeds 12 hours, the remaining water may boil, The physical properties may be denatured.

The lepidocrocite prepared by the above-mentioned method may be? -FeOOH, and specifically may be crystalline? -FeOOH.

Preparation step of the maghemite: Step (2)

The maghemite may be produced by heat treating the lepidocrocite prepared in the step (1) in an inert gas atmosphere, and may be produced through the following reaction formula (1).

[Reaction 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) continuously undergoes a continuous flow of inert gas 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, 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 lower than the above range, it may not be converted to marmerite from lepidocrocite, and if it exceeds the above range, the particle structure of the maghemite may collapse and the sintering may cause undesired Particles, and since lepidocrocite is converted into ε-Fe ₂ O ₃ or the like to produce desired maghemite, it is suitably adjusted within the above range.

Also, the heat treatment may be performed for 1 to 4 hours and at a temperature raising rate of 0.1 ° C per minute to 10 ° C per minute. If the heat treatment time is less than the above range, the temperature for producing the maghemite can not be reached, so that the maghemite can not be produced. If the time exceeds the above time, the heat treatment temperature rises too much, Can be collapsed, can be transformed into particles of undesired size due to sintering, and ε-Fe ₂ O ₃ other than maghemite can be generated, so that it is appropriately controlled within the above range.

According to one embodiment of the present invention, the prepared lepidocrocite can be converted to at least 80% of the hemihydrate, preferably at least 90% of the hemihydrate through the heat treatment step in an inert gas atmosphere . As can be seen from FIG. 3, the XRD analysis of the maghemite revealed that the XRD peak of lepidocrocite was not detected and that it was converted to 90% or more of the maghemite.

The prepared maghemite may be in the form of secondary particles formed by lumps of maghemite primary particles in the form of a plate, wherein the secondary particles may be spherical.

The shape of the prepared maghemite can be controlled as needed by controlling the reaction time, and they can be applied to the cathode material of a lithium-sulfur secondary battery.

The prepared primary particles of the plate-like type may have a particle diameter of more than 1 nm but not more than 1000 nm, preferably 50 to 500 nm. The secondary particle having the primary particles aggregated therein may have a particle diameter of 1 to 50 탆, preferably 1 to 20 탆. As the particle size of the secondary particles decreases within the above range, it is suitable as the cathode material of the lithium-sulfur secondary battery. When the particle size of the secondary particles exceeds the above range, the particle size is large and is not suitable as the cathode material of the lithium- .

When maghemite such as crystalline γ-Fe ₂ O ₃ produced by the above-described method for producing a maghemite is applied to a lithium-sulfur secondary battery, the lithium- The polysulfide can be adsorbed and the performance of the lithium-sulfur secondary battery can be improved.

It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope and spirit of the invention as disclosed in the accompanying claims. Changes and modifications may fall within the scope of the appended claims.

Preparation example: Preparation of lepidocrocite

0.3 M NaBH ₄ Aqueous solution of 0.05 M Fe (NO ₃ ) ₃ .9H ₂ O was mixed for 50 seconds while stirring at 400 rpm. At this time, NaBH ₄ is a product of TCI and has a purity of > 95%, and Fe (NO ₃ ) ₃ .9H ₂ O may be a product of Aldrich, having a purity of > 98%.

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

Example: Preparation of Maghemite

The lepidocrocite powder prepared in Preparation Example 1 was subjected to heat treatment at 400 DEG C for 1 hour by flowing nitrogen gas at a flow rate of 100 mL / min. The heating rate for the heat treatment was 10 ° C per minute. Magnesite was prepared through the heat treatment.

Comparative Example: Preparation of Maghemite (Korean Patent No. 10-0482278)

To a mixed solution of 10 mL of dioctyl ether (Aldrich, 99%) and 1.44 mL of oleic acid (Aldrich, 90%, 4.56 mmol) as a surface active agent maintained at 100 ° C while flowing nitrogen gas, 0.2 mL of iron pentacarbonyl (Fe (CO) ₅ , 1.52 mmol, Aldrich 80-90%) was added and slowly heated to reflux for 5 h.

During the refluxing process, the orange solution turned colorless and dark brown. The solution was cooled to room temperature, excess ethanol was added, and the precipitated powder was separated and dried to prepare an iron oxide nano powder.

Experimental Example 1: SEM (scanning electron microscope) analysis

SEM analysis (Hitachi S-4800 FE-SEM) was performed on the lepidocrocite and maghemite produced in the production examples and the examples, respectively.

1 is an SEM photograph of the lepidocrocite prepared in Production Example.

2 is a SEM photograph of the maghemite produced in the example.

Referring to FIG. 1, SEM analysis was carried out with a magnification of 50k. As a result, a plate-shaped Lepidocrocite of several hundreds of nm was observed, and it was confirmed that the maghemite produced by the heat treatment showed a plate-like structure.

Experimental Example 2: XRD analysis

XRD analysis (D4 Endeavor from Bruker) was performed on the lepidocrocite and the maghemite, respectively, prepared in the preparation examples and the examples.

3 is a graph showing the XRD analysis results of lepidocrocite and maghemite produced in Production Examples and Examples, respectively.

Referring to FIG. 3, it can be seen that the XRD peak of lepidocrocite can be confirmed. From this, it can be seen that crystalline lepidocrocite of pure phase is prepared in the production example.

In addition, it can be seen that X-ray diffraction peak of the example was confirmed and pure plate-like maghemite of several hundred nanometers was produced.

Experimental Example 3: Comparison test of discharging capacity of lithium-sulfur secondary battery

In order to test the discharge capacity of the lithium-sulfur secondary battery according to the kind of the anode material, the post-discharge capacity of the anode and the cathode of the lithium-sulfur secondary battery was measured as shown in Table 1 below.

Carbon composite, the anode of Experimental Example (1) includes a sulfur-carbon composite and the preparation of repidocrocite, and the anode of Experimental Example (2) contains a sulfur-carbon composite and a mag- Mite. At this time, the measurement current was 0.1 C and the voltage range was 1.8 to 2.5 V. The results are shown in Table 1 and FIG.

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

As a result, as shown in Table 1 and FIG. 4, it can be seen that the discharge capacities of Examples (1) and (2) were increased by about 100 mAh / g as compared with Comparative Experimental Examples.

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

While the present invention has been described with reference to the particular embodiments and drawings, it is to be understood that the present invention is not limited thereto and that various changes and modifications will be apparent to those skilled in the art. And various modifications and variations are possible within the scope of the appended claims.

Claims (16)

1. (1) mixing Fe (NO ₃ ) ₃ .9H ₂ O and a reducing agent represented by the following formula (1) to react;

   (2) After the step (1), heat treatment is performed in an inert gas atmosphere.

   [Chemical Formula 1]

   M ¹ (BH ₄ ) _X

   (Wherein M ¹ is any one selected from Li, Na, Mg, K and Ca, and X is 1 or 2)
2. The method according to claim 1,

   Wherein the step (2) is carried out under an inert gas atmosphere or in a state where an inert gas is continuously introduced.
3. The method according to claim 1,

   Wherein the inert gas is selected from the group consisting of nitrogen, argon, helium, and mixtures thereof.
4. The method according to claim 1,

   Wherein the heat treatment is performed at 250 to 600 캜.
5. The method according to claim 1,

   Wherein the heat treatment is performed for 1 to 4 hours.
6. The method according to claim 1,

   Wherein the heat treatment is carried out at a temperature raising rate between 0.1 [deg.] C per minute and 10 [deg.] C per minute.
7. The method according to claim 1,

   Wherein the Fe (NO ₃ ) ₃ .9H ₂ O is mixed in the form of an aqueous solution of 0.04 to 0.08 M.
8. The method according to claim 1,

   Wherein the reducing agent represented by Formula 1 is mixed in the form of an aqueous solution of 0.2 to 0.5 M.
9. The method according to claim 1,

   Wherein the mixing is carried out for 10 to 120 seconds.
10. The method according to claim 1,

    Wherein the reaction temperature in step (1) is 20 to 25 占 폚.
11. The method according to claim 1,

    Wherein the reaction time in the step (1) is 40 minutes to 12 hours.
12. The method according to claim 1,

    Further comprising a step of filtering and drying after the step (1).
13. 13. The method of claim 12,

    Wherein the drying is carried out at 70 to 90 占 폚 for 6 to 12 hours.
14. The method according to claim 1,

    Wherein the maghemite is gamma-Fe ₂ O ₃ .
15. The method according to claim 1,

    Wherein the maghemite is in the form of a plate.
16. The method according to claim 1,

    Wherein the maghemite has a particle diameter of 50 to 500 nm.

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