KR102229459B1 - Cathode additives for lithium secondary battery and method for preparing the same - Google Patents

Abstract

The present invention relates to a positive electrode additive for a lithium secondary battery, the manufacturing method
(1) Fe(NO ₃ ) ₃ ·9H ₂ O and a reducing agent represented by the following Formula 1; mixing and reacting to produce FeOOH;
(2) mixing the FeOOH produced in step (1) and the carbon nanostructure to produce a FeOOH-carbon composite;
It relates to a method for producing a positive electrode additive for a lithium secondary battery comprising a.
[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.

Description

Positive electrode additive for lithium secondary battery and its manufacturing method {CATHODE ADDITIVES FOR LITHIUM SECONDARY BATTERY AND METHOD FOR PREPARING THE SAME}

The present invention relates to a positive electrode additive for a lithium secondary battery and a method of manufacturing the same.

Recently, interest in energy storage technology is increasing. As the fields of application to mobile phones, camcorders, notebook PCs, and even electric vehicles are expanded, efforts for research and development of electrochemical devices are increasingly being materialized.

Electrochemical devices are the field that is receiving the most attention in this respect, and among them, the development of secondary batteries capable of charging and discharging has become the focus of interest, and recently, in the development of such batteries, in order to improve capacity density and energy efficiency. Research and development on the design of new electrodes and batteries is underway.

Among the currently applied secondary batteries, the lithium secondary battery developed in the early 1990s has the advantage of having a higher operating voltage and significantly higher energy density than conventional batteries such as Ni-MH, Ni-Cd, and sulfuric acid-lead batteries using aqueous electrolyte solutions. It is in the limelight.

In particular, a lithium-sulfur (Li-S) battery is a secondary battery that uses a sulfur-based material having an SS bond (Sulfur-sulfur bond) as a positive electrode active material and lithium metal as a negative electrode active material. Sulfur, the main material of the positive electrode active material, has the advantage of being very rich in resources, non-toxic, and low weight per atom. In addition, the theoretical discharge capacity of the lithium-sulfur battery is 1,675mAh/g-sulfur, and the theoretical energy density is 2,600Wh/kg, and the theoretical energy density of other battery systems currently being studied (Ni-MH battery: 450Wh/kg, Li-FeS battery: 480Wh/kg, Li-MnO ₂ battery: 1,000Wh/kg, Na-S battery: 800Wh/kg).

During the discharge reaction of a lithium-sulfur battery, an oxidation reaction of lithium occurs at the anode and a reduction reaction of sulfur occurs at the cathode. Sulfur before discharging has a cyclic S ₈ structure, and the oxidation number of S decreases as the SS bond breaks during the reduction reaction (discharge), and the oxidation number of S increases as the SS bond is re-formed during the oxidation reaction (charging). It stores and generates electrical energy using a reduction reaction. During this reaction, sulfur is converted into linear lithium polysulfide (Lithium polysulfide, Li ₂ S _x , x = 8, 6, 4, 2) _{by a reduction reaction from the cyclic S 8, and eventually, this lithium polysulfide is completely converted.} When reduced, lithium sulfide (Li ₂ S) is finally produced. The discharge behavior of a lithium-sulfur battery by the process of being reduced to each lithium polysulfide is characterized by a stepwise discharge voltage, unlike a general lithium ion battery.

However, during the discharging of the lithium-sulfur battery, lithium polysulfide (Li ₂ S _x , x = 2 to 8) is dissolved in the electrolyte and diffused to the negative electrode, causing various side reactions and reducing the capacity of sulfur participating in the electrochemical reaction. Let it. In addition, during the charging process, the lithium polysulfide causes a shuttle reaction, which significantly reduces charging and discharging efficiency.

In addition, since sulfur has an electrical conductivity of 5Х10 ^-30 S/cm, which is close to a non-conductor, it is difficult to move electrons generated by an electrochemical reaction. Therefore, it was necessary to use an electrically conductive material such as carbon that can provide a smooth electrochemical reaction site. In this case, when a conductive material and sulfur are simply mixed and used, sulfur is leaked into the electrolyte during the oxidation-reduction reaction, which deteriorates the battery life, and when an appropriate electrolyte is not selected, lithium polysulfide, which is a reducing material of sulfur, is used. There was a problem that it was eluted and no longer participated in the electrochemical reaction.

Research on electrode materials for solving such problems and improving the capacity and life characteristics of lithium secondary batteries is being actively conducted. Among them, there are reports that transition metal oxides have lithium polysulfide adsorption capacity, and research is underway to increase discharge capacity and improve lifespan characteristics by adding this to the positive electrode of a lithium secondary battery. However, since it is generally difficult to produce small particles applicable to a battery in the manufacture of transition metal oxides, and the conductivity of the transition metal oxide itself is very low, the technology for manufacturing transition metal oxides in a form suitable as an electrode material for secondary batteries. There is no active research on this.

Chinese Patent Laid-Open No. 101483236, "Method of manufacturing lithium iron phosphate/carbon composite material for positive electrode materials for lithium ion batteries" Japanese Patent Laid-Open Publication No. 2002-270172, "A positive electrode active material for a non-aqueous electrolyte lithium secondary battery and its manufacturing method"

Accordingly, in the present invention, after conducting various researches, iron hydroxide (FeOOH), which has an effect of adsorbing lithium polysulfide, was mixed with a carbon nanostructure to apply a new positive electrode additive for lithium secondary batteries to the battery. The present invention was completed by confirming that the discharge capacity, storage capacity, etc. of the lithium secondary battery are improved and the life characteristics are improved.

Accordingly, an object of the present invention is to provide a positive electrode additive for a lithium secondary battery and a method for manufacturing the same.

In addition, another object of the present invention is to provide a positive electrode for a lithium secondary battery comprising a positive electrode additive for a lithium secondary battery manufactured by the above manufacturing method.

In addition, another object of the present invention is to provide a lithium secondary battery including the positive electrode additive with improved discharge capacity and storage capacity and improved lifespan characteristics.

In order to achieve the above object, the present invention,

(1) Fe(NO ₃ ) ₃ ·9H ₂ O and a reducing agent represented by the following Formula 1; mixing and reacting to produce FeOOH;

(2) mixing the FeOOH produced in step (1) and the carbon nanostructure to produce a FeOOH-carbon composite;

It provides a method for producing a positive electrode additive for a lithium secondary battery comprising a.

[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.

In one embodiment of the present invention, the Fe(NO ₃ ) ₃ ·9H ₂ O of step (1) is used in the form of an aqueous solution of 0.04 to 0.08 M.

In one embodiment of the present invention, the reducing agent represented by Formula 1 in step (1) is used in the form of an aqueous solution of 0.2 to 0.5 M.

In one embodiment of the present invention, the mixing in step (1) is carried out for 10 to 120 seconds.

In one embodiment of the present invention, the reaction temperature in step (1) is 20 to 25°C.

In one embodiment of the present invention, the reaction time in step (1) is 40 minutes to 12 hours.

One embodiment of the present invention is to further include a filtering step after step (1).

One embodiment of the present invention is to further include a drying step carried out for 6 to 12 hours at 70 to 90 ℃ after the step (2).

In addition, the present invention,

As a positive electrode for a lithium secondary battery comprising an active material, a conductive material and a binder,

The positive electrode provides a positive electrode for a lithium secondary battery comprising a FeOOH-carbon composite.

In one embodiment of the present invention, the content of the FeOOH-carbon composite is 0.1 to 15 parts by weight based on 100 parts by weight of the base solid content contained in the lithium secondary battery positive electrode.

In one embodiment of the present invention, the FeOOH-carbon composite includes carbon nanotubes (CNT) or carbon nanofibers (CNF).

In one embodiment of the present invention, the diameter of the carbon nanotube is 5 to 50 nm, and the length is 100 nm to 1 μm.

In one embodiment of the present invention, the diameter of the carbon nanofibers is 50 to 100 nm, and the length is 1 to 5 μm.

In one embodiment of the present invention, the FeOOH of the FeOOH-carbon composite has an average particle diameter of 10 to 500 nm.

In one embodiment of the present invention, the FeOOH-carbon composite has an average particle diameter of 1 to 50 μm.

In addition, the present invention,

Including a positive electrode, a negative electrode, a separator and an electrolyte interposed therebetween,

The positive electrode provides a lithium secondary battery that is the positive electrode for the lithium secondary battery described above.

In one embodiment of the present invention, the lithium secondary battery is a lithium-sulfur battery.

The positive electrode additive for a lithium secondary battery according to the present invention is a composite of FeOOH and a carbon nanostructure having an effect of adsorbing lithium polysulfide, and according to the manufacturing method of the present invention, FeOOH particles of small and uniform size applicable to a lithium secondary battery are prepared. In this way, there is an advantage of manufacturing a lithium secondary battery in a form in which FeOOH is well dispersed in the carbon nanostructure. In addition, the lithium secondary battery including the additive may include carbon nanotubes or carbon nanofibers used as carbon nanostructures, thereby having electrical conductivity and dispersibility comparable to a conductive material.

The lithium secondary battery provided with the positive electrode including the positive electrode additive has the advantage of improving the discharge capacity and storage capacity of the lithium secondary battery and improving the life characteristics without an additional conductive material.

1 is a schematic diagram showing a method of manufacturing a positive electrode additive for a lithium secondary battery according to Examples and Comparative Examples of the present invention.
2 is a SEM photograph of a positive electrode additive for a lithium secondary battery according to an embodiment of the present invention.
3 is a SEM photograph of a positive electrode additive for a lithium secondary battery according to an embodiment of the present invention.
4 is a SEM photograph of a positive electrode additive for a lithium secondary battery according to a comparative example of the present invention.
5 is a graph showing discharge capacity of lithium secondary batteries according to Examples and Comparative Examples of the present invention.
6 is a graph showing life characteristics of lithium secondary batteries according to Examples and Comparative Examples of the present invention.

Hereinafter, it will be described in detail with reference to the accompanying drawings so that those of ordinary skill in the art can easily implement the present invention. However, the present invention may be implemented in various different forms, and is not limited to this specification.

In the drawings, parts irrelevant to the description are omitted in order to clearly describe the present invention, and similar reference numerals are used for similar parts throughout the specification. In addition, the size and relative size of the components indicated in the drawings are not related to the actual scale, and may be reduced or exaggerated for clarity of description.

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 term “composite” as used herein refers to a material that exhibits more effective functions while forming different phases physically and chemically by combining two or more materials.

The present invention relates to a positive electrode additive for a lithium secondary battery including a FeOOH-carbon composite and a method for manufacturing the same, wherein the discharge capacity by forming a composite of iron hydroxide and a carbon nanostructure having lithium polysulfide adsorption capacity and applying it to a lithium secondary battery The present invention relates to a positive electrode additive for a lithium secondary battery that can implement a lithium secondary battery with improved over-life characteristics, and a method of manufacturing the same.

Lithium secondary batteries are manufactured by using materials capable of intercalation / deintercalation of lithium ions as negative and positive electrodes, and by charging an organic electrolyte or polymer electrolyte between the negative and positive electrodes, and lithium ions are inserted from the positive and negative electrodes. And an electrochemical device that generates electrical energy through oxidation/reduction reactions when desorbed. According to an embodiment of the present invention, the lithium secondary battery includes lithium-sulfur as an electrode active material of a positive electrode. It can be a battery.

Manufacturing method of positive electrode additive for lithium secondary battery

The manufacturing method of the positive electrode additive for a lithium secondary battery of the present invention

(1) Fe(NO ₃ ) ₃ ·9H ₂ O and a reducing agent represented by the following Formula 1; mixing and reacting to produce FeOOH;

(2) mixing the FeOOH produced in step (1) and the carbon nanostructure to produce a FeOOH-carbon composite;

It may include.

[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.

First, a method of preparing a positive electrode additive for a lithium secondary battery _{includes a step (1) of mixing and reacting Fe(NO 3} ) ₃ ·9H ₂ O and a reducing agent represented by the following formula (1) to produce FeOOH.

At this time, the Fe(NO ₃ ) ₃ ·9H ₂ O and the reducing agent represented by Formula 1 may be used in the form of an aqueous solution, and the Fe(NO ₃ ) ₃ ·9H ₂ O in the reducing agent aqueous solution represented by Formula 1 It may be mixed and reacted by adding an aqueous solution.

If mixing and reaction are performed in the reverse order, the purity of FeOOH produced may be lowered. 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 FeOOH produced 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 FeOOH may be lowered, and if it exceeds 0.08 M, the produced FeOOH Physical properties may not be suitable for application as a cathode material for lithium secondary batteries.

The reducing agent aqueous solution represented by Formula 1 may be in the form of an aqueous solution of 0.2 to 0.5 M, preferably 0.3 to 0.4 M, and if it is less than 0.2 M, FeOOH 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 additive represented by Chemical 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, FeOOH 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 ℃ The physical properties of the produced FeOOH 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, FeOOH may not be formed, and if it exceeds 20 hours, the shape of FeOOH is the lithium secondary battery. It may have a shape that is not suitable as a cathode material, and in particular, when reacted for 40 minutes to 2 hours, the desired physical properties of FeOOH can be maintained without losing.

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

The FeOOH produced as a result of the step (1) may be in the form of a gel, and the filtering may be performed by a filtration process commonly used in the art, and for example, filter paper may be used.

Next, the present invention may include a step (2) of producing a FeOOH-carbon composite by mixing the gel-like FeOOH and carbon nanostructures produced in step (1).

At this time, the carbon nanostructure may be a carbon nanotube (CNT) or a carbon nanofiber (CNF), preferably a carbon nanotube.

The carbon nanotubes may have a diameter of 5 to 50 nm and a length of 100 nm to 1 μm. If the diameter of the carbon nanotubes is less than the above range, the cost of the raw material is expensive and the manufacturing cost of the battery may increase, which may not be economical. If the diameter of the carbon nanotubes exceeds the above range, the large diameter carbon nanotubes are included, resulting in a large number of pores in the electrode. It is generated and has an inhibitory factor in terms of energy density, so it is appropriately adjusted within the above range.

In addition, when the length of the carbon nanotubes is less than the above range, the contact resistance between the carbon nanotubes increases, and there is a concern of occurrence of an overvoltage of the battery, and when the length of the carbon nanotubes exceeds the above range, there may be a problem in flowability when preparing a slurry for manufacturing an electrode.

The carbon nanofibers may have a diameter of 50 to 100 nm, and a length of 1 to 5 μm. If the diameter of the carbon nanofiber is less than the above range, the cost of the raw material is expensive, so the manufacturing cost of the battery may increase, which may not be economical.If the diameter of the carbon nanofiber exceeds the above range, the specific surface area of the carbon nanofiber on which FeOOH is to be supported decreases. There may be a problem of deteriorating the performance, and if it exceeds the above range, there may be a problem in flowability when preparing a slurry for manufacturing an electrode.

In the case of the FeOOH-carbon composite produced in step (2), FeOOH contained when using carbon nanotubes may have an average particle diameter of 10 to 100 nm, preferably 10 to 50 nm. In addition, FeOOH contained in the case of using carbon nanofibers may have an average particle diameter of 50 to 500 nm, preferably 100 to 200 nm.

If the product of step (1), FeOOH in the form of a gel is filtered and then dried to prepare a positive electrode additive, FeOOH causes an auto-aggregation reaction to form large particles of a level of 100 nm to 1 µm. However, it is possible to manufacture FeOOH of a small and uniform size supported on the carbon nanostructure by further including step (2) according to the present invention.

The present invention may further include a drying step after step (2), and the drying step may be carried out 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 and thus FeOOH in the form of particles cannot be obtained. If the drying temperature is more than 90° C. or more than 12 hours, the remaining water boils and the physical properties of FeOOH may be denatured. .

FeOOH prepared by the method as described above may be repidocrosite (γ -FeOOH), and specifically, may be crystalline γ -FeOOH.

Anode for lithium secondary battery

The present invention provides a positive electrode for a lithium secondary battery comprising a positive electrode additive for a lithium secondary battery manufactured by the above manufacturing method. Specifically, the present invention provides a positive electrode for a lithium secondary battery comprising an active material, a conductive material, and a binder, wherein the positive electrode includes a FeOOH-carbon composite.

In this case, the positive electrode of the lithium secondary battery may include a current collector and an electrode active material layer formed on at least one surface of the current collector, and the electrode active material layer may include a base solid containing an active material, a conductive material, and a binder. .

As the current collector, it may be preferable to use aluminum, nickel, or the like having excellent conductivity.

In one embodiment, 0.1 to 15 parts by weight of the FeOOH-carbon composite may be included based on 100 parts by weight of the base solid content including the active material, the conductive material, and the binder, and specifically 1 to 15 parts by weight, preferably 5 It may contain to 10 parts by weight. If it is less than the lower limit of the numerical range, the adsorption effect of polysulfide may be insignificant, and if it exceeds the upper limit, the capacity of the electrode decreases, which is not preferable. The FeOOH-carbon composite may be a FeOOH-carbon composite prepared by the manufacturing method presented in the present invention.

In one embodiment, the FeOOH-carbon composite may include carbon nanotubes (CNT) or carbon nanofibers (CNF), preferably carbon nanotubes.

The carbon nanotubes may have a diameter of 5 to 50 nm and a length of 100 nm to 1 μm. If the diameter of the carbon nanotubes is less than the above range, the cost of the raw material is expensive and the manufacturing cost of the battery may increase, which may not be economical. If the diameter of the carbon nanotubes exceeds the above range, the large diameter carbon nanotubes are included, resulting in many pores in the electrode. It is generated and has an inhibitory factor in terms of energy density, so it is appropriately adjusted within the above range.

In addition, when the length of the carbon nanotubes is less than the above range, the contact resistance between the carbon nanotubes increases, and there is a concern of occurrence of an overvoltage of the battery, and when the length of the carbon nanotubes exceeds the above range, there may be a problem in flowability when preparing a slurry for manufacturing an electrode.

The carbon nanofibers may have a diameter of 50 to 100 nm, and a length of 1 to 5 μm. If the diameter of the carbon nanofiber is less than the above range, the cost of the raw material is expensive, so the manufacturing cost of the battery may increase, which may not be economical.If the diameter of the carbon nanofiber exceeds the above range, the specific surface area of the carbon nanofiber on which FeOOH is to be supported decreases. There may be a problem of deteriorating the performance, and if it exceeds the above range, there may be a problem in flowability when preparing a slurry for manufacturing an electrode.

In one embodiment, in the case of the FeOOH-carbon composite according to the present invention, the FeOOH contained when using a carbon nanotube may have an average particle diameter of 10 to 100 nm, preferably 10 to 50 nm. . In addition, FeOOH contained in the case of using carbon nanofibers may have an average particle diameter of 50 to 500 nm, preferably 100 to 200 nm.

In one embodiment, the average particle diameter of the FeOOH-carbon composite according to the present invention may be 1 to 50 μm, preferably 10 to 30 μm. If the average particle diameter of the composite is less than 1 μm, the contact resistance between the composite particles increases, which may cause overvoltage to the lithium secondary battery. If the average particle diameter of the composite exceeds 50 μm, the relatively large size of the composite particle roughens the electrode surface. Therefore, since there may be a problem that may damage the separator, it is preferable to properly control the battery in the above range.

Meanwhile, among the base solids constituting the positive electrode of the present invention, the active material may include elemental sulfur (S ₈ ), a sulfur-based compound, or a mixture thereof, and the sulfur-based compound is specifically, Li ₂ S _n (n=1), an organosulfur compound or a carbon-sulfur complex ((C ₂ S _x ) _n : x=2.5 to 50, n=2), and the like.

The positive electrode for a lithium secondary battery according to the present invention may preferably include an active material of a sulfur-carbon composite, and since the sulfur material alone does not have electrical conductivity, it may be used in combination with a conductive material. The addition of the FeOOH-carbon composite according to the present invention does not affect the maintenance of this sulfur-carbon composite structure.

The positive electrode can be manufactured by a conventional method known in the art. For example, after preparing a slurry by mixing and stirring a solvent, a binder, an additive, and a dispersant as necessary in the positive electrode active material, it can be applied (coated) to a current collector of a metal material, compressed, and dried to prepare a positive electrode. .

In one embodiment, the sulfur-carbon composite may have a sulfur content of 60 to 90 parts by weight based on 100 parts by weight of the sulfur-carbon composite, and preferably 70 to 75 parts by weight. If the content of sulfur is less than 60 parts by weight, the content of the carbon material of the sulfur-carbon composite is relatively increased, and the specific surface area increases as the content of carbon increases, so that the amount of binder added during slurry preparation should be increased. Increasing the amount of the binder added eventually increases the sheet resistance of the electrode and acts as an insulator to prevent electron pass, thereby deteriorating battery performance. If the content of sulfur exceeds 90 parts by weight, sulfur or sulfur compounds that cannot be bonded to the carbon material are aggregated with each other or re-elute to the surface of the carbon material, making it difficult to receive electrons, and thus it may be difficult to directly participate in the electrode reaction. Adjust accordingly.

The carbon of the sulfur-carbon composite according to the present invention may have a porous structure or a high specific surface area, so long as it is commonly used in the art. For example, as the porous carbon material, graphite; Graphene; Carbon blacks such as denka black, acetylene black, ketjen black, channel black, furnace black, lamp black, and summer black; Carbon nanotubes (CNT) such as single-walled carbon nanotubes (SWCNT) and multi-walled carbon nanotubes (MWCNT); Carbon fibers such as graphite nanofibers (GNF), carbon nanofibers (CNF), and activated carbon fibers (ACF); And it may be one or more selected from the group consisting of activated carbon, but is not limited thereto, and the shape thereof is spherical, rod-shaped, needle-shaped, plate-shaped, tube-shaped, or bulk-shaped, and may be used without limitation as long as it is commonly used for lithium secondary batteries.

The active material may preferably constitute 50 to 95 parts by weight of 100 parts by weight of the base solid content, and more preferably, about 70 parts by weight. If the active material is contained within the above range, it is difficult to sufficiently exhibit the reaction of the electrode, and even if it is contained beyond the above range, it is difficult to exhibit a sufficient electrode reaction due to relatively insufficient amounts of other conductive materials and binders. It is desirable to determine the appropriate content.

Among the base solids constituting the positive electrode of the present invention, the conductive material is a material that acts as a path for electrons to move from the current collector to sulfur by electrically connecting the electrolyte and the positive electrode active material, causing chemical changes in the battery. It is not particularly limited as long as it does not have porosity and conductivity. For example, graphite-based materials such as KS6; Carbon blacks such as Super-P, carbon black, denka black, acetylene black, ketjen black, channel black, furnace black, lamp black, and summer black; Carbon derivatives such as fullerene; Conductive fibers such as carbon fibers and metal fibers; Metal powders such as carbon fluoride, aluminum, and nickel powder; Alternatively, conductive polymers such as polyaniline, polythiophene, polyacetylene, and polypyrrole may be used alone or in combination.

The conductive material is preferably to constitute 1 to 10 parts by weight of 100 parts by weight of the base solid content, and may preferably be about 5 parts by weight. If the content of the conductive material contained in the electrode is less than the above range, the non-reacting portion of the sulfur in the electrode increases, resulting in a decrease in capacity, and if it exceeds the above range, the high efficiency discharge characteristics and charge/discharge cycle life are adversely affected. It is preferable to determine the appropriate content within the above-described range because it affects.

In general, a conductive material may be added to the positive electrode composition in order to impart additional conductivity to the positive electrode active material, but the positive electrode additive according to the present invention may serve as a conductive material including carbon nanostructures such as CNT or CNF. There is an advantage that the battery can be driven without including a conductive material.

In addition, in order to provide the positive electrode active material with adhesion to the current collector, a binder may be additionally included in the positive electrode composition. The binder must be well soluble in a solvent, and not only must well configure a conductive network between the positive electrode active material and the conductive material, but also must have adequate impregnation property of the electrolyte.

The binder applicable to the present invention may be any binder known in the art, and specifically, a fluororesin binder including polyvinylidene fluoride (PVdF) or polytetrafluoroethylene (PTFE). ; Rubber-based binders including styrene-butadiene rubber, acrylonitrile-butadiene rubber, and styrene-isoprene rubber; Cellulose-based binders including carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, and regenerated cellulose; Poly alcohol-based binder; Polyolefin-based binders including polyethylene and polypropylene; Polyimide-based binders, polyester-based binders, silane-based binders; one or two or more mixtures or copolymers selected from the group consisting of, but is not limited thereto.

The content of the binder resin may be 0.5 to 30% by weight based on the total weight of the positive electrode for the lithium secondary battery, but is not limited thereto. When the content of the binder resin is less than 0.5% by weight, the physical properties of the positive electrode are deteriorated, so that the positive electrode active material and the conductive material may fall off, and if it exceeds 30% by weight, the ratio of the active material and the conductive material in the positive electrode is relatively reduced. The dose can be reduced.

The solvent for preparing the positive electrode composition for a lithium secondary battery in a slurry state should be easy to dry and can dissolve the binder well, but it is most preferable that the positive electrode active material and the conductive material are not dissolved and can be maintained in a dispersed state. When the solvent dissolves the positive electrode active material, since the specific gravity of sulfur in the slurry (D = 2.07) is high, sulfur sinks in the slurry, and sulfur gathers in the current collector during coating, causing problems in the conductive network, causing problems in the operation of the battery. have.

The solvent according to the present invention may be water or an organic solvent, and the organic solvent is an organic solvent containing at least one selected from the group of dimethylformamide, isopropyl alcohol, acetonitrile, methanol, ethanol, and tetrahydrofuran. It is possible.

Mixing of the positive electrode composition may be stirred in a conventional manner using a conventional mixer, such as a rate mixer, a high-speed shear mixer, or a homo mixer.

The positive electrode composition may be applied to a current collector and vacuum dried to form a positive electrode for a lithium secondary battery. The slurry may be coated on the current collector with an appropriate thickness according to the viscosity of the slurry and the thickness of the positive electrode to be formed, and preferably may be appropriately selected within the range of 10 to 300 μm.

At this time, the method of coating the slurry is not limited. For example, doctor blade coating, dip coating, gravure coating, slit die coating, spin coating ( Spin coating), comma coating, bar coating, reverse roll coating, screen coating, cap coating method, etc. may be performed.

The positive electrode current collector is not particularly limited as long as it has high conductivity without causing chemical changes to the battery, and can be generally made in a thickness of 3 to 500 µm, and has high conductivity without causing chemical changes to the battery. Not particularly limited. For example, a conductive metal such as stainless steel, aluminum, copper, or titanium may be used, and preferably an aluminum current collector may be used. The positive electrode current collector may be in various forms, such as a film, sheet, foil, net, porous body, foam or nonwoven fabric.

Lithium secondary battery

As an embodiment of the present invention, a lithium secondary battery includes the above-described positive electrode for a lithium secondary battery; A negative electrode including lithium metal or a lithium alloy as a negative electrode active material; A separator interposed between the anode and the cathode; And an electrolyte impregnated in the negative electrode, the positive electrode, and the separator, and including a lithium salt and an organic solvent.

The negative electrode has a form in which a negative electrode active material is stacked on a negative electrode current collector. If necessary, the negative electrode current collector may be omitted.

At this time, the negative electrode current collector is not particularly limited as long as it has conductivity without causing chemical changes to the battery. For example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, carbon, nickel on the surface of copper or stainless steel , Titanium, silver, etc. surface-treated, aluminum-cadmium alloy, etc. may be used. In addition, various forms such as a film, sheet, foil, net, porous body, foam, nonwoven fabric, etc. with fine irregularities formed on the surface thereof may be used.

The negative electrode is a material capable ^{of reversibly intercalating or deintercalating lithium ions (Li +} ) as a negative electrode active material, and a material capable of reversibly forming a lithium-containing compound by reacting with lithium ions , Lithium metal or lithium alloys may be used. The material capable of reversibly intercalating or deintercalating lithium ions may be, for example, crystalline carbon, amorphous carbon, or a mixture thereof. A material capable of reversibly forming a lithium-containing compound by reacting with the lithium ions may be, for example, tin oxide, titanium nitrate, or silicon. The lithium alloy may be, for example, an alloy of lithium and a metal selected from the group consisting of Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Ba, Ra, Al, and Sn.

In addition, in the process of charging and discharging a lithium secondary battery, sulfur used as a positive electrode active material is converted into an inactive material, and may be attached to the surface of the lithium negative electrode. As described above, inactive sulfur refers to sulfur in a state in which sulfur can no longer participate in the electrochemical reaction of the positive electrode through various electrochemical or chemical reactions, and the inactive sulfur formed on the surface of the lithium negative electrode is a protective layer of the lithium negative electrode. It also has the advantage of acting as a ). Accordingly, lithium metal and inert sulfur formed on the lithium metal, for example, lithium sulfide, may be used as the negative electrode.

The negative electrode of the present invention may further include a pretreatment layer made of a lithium ion conductive material and a lithium metal protective layer formed on the pretreatment layer in addition to the negative electrode active material.

The separator interposed between the positive electrode and the negative electrode separates or insulates the positive electrode and the negative electrode from each other, and enables transport of lithium ions between the positive electrode and the negative electrode, and may be made of a porous non-conductive or insulating material. Such a separator may be an independent member such as a thin thin film or film as an insulator having high ion permeability and mechanical strength, or may be a coating layer added to the anode and/or the cathode. In addition, when a solid electrolyte such as a polymer is used as the electrolyte, the solid electrolyte may also serve as a separator.

The pore diameter of the separator is generally 0.01 to 10 µm, and the thickness is generally 5 to 300 µm, and as such a separator, a glass electrolyte, a polymer electrolyte, or a ceramic electrolyte may be used. For example, sheets made of olefin-based polymers such as polypropylene, glass fiber, or polyethylene having chemical resistance and hydrophobic properties, non-woven fabrics, kraft paper, and the like are used. Representative examples currently on the market include Celgard ^R 2400, 2300 Hoechest Celanese Corp. product, polypropylene separator (Ube Industries Ltd. product or Pall RAI company product), and polyethylene series (Tonen or Entek).

The solid electrolyte membrane may contain less than about 20% by weight of a non-aqueous organic solvent, and in this case, may further include an appropriate gel-forming compound (Gelling agent) to reduce the fluidity of the organic solvent. Representative examples of such gel-forming compounds include polyethylene oxide, polyvinylidene fluoride, and polyacrylonitrile.

The electrolyte impregnated in the negative electrode, the positive electrode, and the separator is a non-aqueous electrolyte containing a lithium salt, and is composed of a lithium salt and an electrolyte. As the electrolyte, a non-aqueous organic solvent, an organic solid electrolyte, an inorganic solid electrolyte, and the like are used.

The lithium salt of the present invention is a material that is easily soluble in a non-aqueous organic solvent, such as LiSCN, LiCl, LiBr, LiI, LiPF ₆ , LiBF ₄ , LiSbF ₆ , LiAsF ₆ , LiB ₁₀ Cl ₁₀ , LiCH ₃ SO ₃ , LiCF ₃ SO ₃ , LiCF ₃ CO ₂ , LiClO ₄ , LiAlCl ₄ , Li(Ph) ₄ , LiC(CF ₃ SO ₂ ) ₃ , LiN(FSO ₂ ) ₂ , LiN(CF ₃ SO ₂ ) ₂ , LiN(C ₂ Group _{consisting of F 5} SO ₂ ) ₂ , LiN(SFO ₂ ) ₂ , LiN(CF ₃ CF ₂ SO ₂ ) ₂ , lithium chloroborane, lithium lower aliphatic carboxylic acid, lithium 4 phenyl borate, lithium imide, and combinations thereof One or more can be included from.

The concentration of the lithium salt depends on several factors such as the exact composition of the electrolyte mixture, the solubility of the salt, the conductivity of the dissolved salt, the charging and discharging conditions of the battery, the working temperature and other factors known in the lithium battery field, from 0.2 to 2 M, Specifically, it may be 0.6 to 2 M, more specifically 0.7 to 1.7 M. If the amount is less than 0.2 M, the conductivity of the electrolyte may be lowered and the electrolyte performance may be deteriorated. If it is used in excess of 2 M, the viscosity of the electrolyte may increase and ^{the mobility of lithium ions (Li +} ) may decrease.

The non-aqueous organic solvent must dissolve the lithium salt well, and the non-aqueous organic solvent of the present invention includes, for example, N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, di Ethyl carbonate, ethylmethyl carbonate, gamma-butyrolactone, 1,2-dimethoxy ethane, 1,2-diethoxy ethane, tetrahydroxy franc, 2-methyl tetrahydrofuran, dimethyl sulfoxide, 1, 3-dioxolane, 4-methyl-1,3-dioxene, diethyl ether, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, phosphate tryster, trimer Aprotic, such as oxymethane, dioxolane derivative, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivative, tetrahydrofuran derivative, ether, methyl pyropionate, ethyl propionate, etc. An organic solvent may be used, and the organic solvent may be one or a mixture of two or more organic solvents.

Examples of the organic solid electrolyte include polyethylene derivatives, polyethylene oxide derivatives, polypropylene oxide derivatives, phosphoric acid ester polymers, poly etch lysine, polyester sulfide, polyvinyl alcohol, polyvinylidene fluoride, ionic dissociation. Group-containing polymers and the like can be used.

As the inorganic solid electrolyte, for example, Li ₃ N, LiI, Li ₅ NI ₂ , Li ₃ N-LiI-LiOH, LiSiO ₄ , LiSiO ₄ -LiI-LiOH, Li ₂ SiS ₃ , Li ₄ SiO ₄ , Li ₄ Nitrides, halides, and sulfates of Li such as SiO ₄ -LiI-LiOH and Li ₃ PO4-Li ₂ S-SiS _{2 may be used.}

The electrolyte of the present invention includes, for example, pyridine, triethylphosphite, triethanolamine, cyclic ether, ethylene diamine, n-glyme, hexaphosphate triamide, nitro Benzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxy ethanol, aluminum trichloride, etc. may be added. . In some cases, in order to impart non-flammability, a halogen-containing solvent such as carbon tetrachloride and ethylene trifluoride may be further included, and carbon dioxide gas may be further included to improve high-temperature storage characteristics. carbonate), PRS (Propene sultone), FPC (Fluoro-propylene carbonate), etc. may be further included.

The electrolyte may be used as a liquid electrolyte, or may be used in the form of a solid electrolyte separator. When used as a liquid electrolyte, a separator made of porous glass, plastic, ceramic, or polymer as a physical separator having a function of physically separating the electrode may be further included.

The shape of the lithium secondary battery as described above is not particularly limited, and may be, for example, a jelly-roll type, a stack type, a stack-folding type (including a stack-Z-folding type), or a lamination-stack type, and is preferably It may be a stack-folding type.

After manufacturing an electrode assembly in which the positive electrode, the separator, and the negative electrode are sequentially stacked, the electrode assembly is placed in a battery case, and then an electrolyte is injected into the upper part of the case and sealed with a cap plate and a gasket to prepare a lithium secondary battery. .

The lithium secondary battery may be classified into a cylindrical type, a square type, a coin type, a pouch type, etc. according to the shape, and may be divided into a bulk type and a thin film type according to the size. The structure and manufacturing method of these batteries are widely known in this field, and thus detailed descriptions thereof will be omitted.

Hereinafter, the present invention will be described in more detail through examples and the like, but the scope and contents of the present invention are reduced or limited by the following examples, and thus cannot be interpreted. In addition, if based on the disclosure of the present invention including the following examples, it is obvious that the present invention for which no specific experimental results are presented can be easily carried out by a person skilled in the art. It is natural to fall within the scope of the claims.

[Example]

Preparation of positive electrode additive for lithium secondary battery

[Example 1]

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 is a product of TCI and has a purity of 95% or more, and Fe(NO ₃ ) ₃ ·9H ₂ O is a product of Aldrich and may have a purity of 98% or more.

After the mixing, it was reacted at 24° C. for 40 minutes and filtered using a filter paper to obtain FeOOH in the form of a gel.

5 g of FeOOH in the gel form and 5 g of carbon nanotubes (CNT) as a carbon nanostructure were mixed by ball milling for 3 hours at 200 rpm with 100 _{g of ZrO 2 balls and 5 g of distilled water.} At this time, carbon nanotubes having a diameter of 10 to 15 nm and a length of 200 to 800 nm were used.

Thereafter, the mixture of FeOOH and carbon nanotubes was filtered _{to remove distilled water and ZrO 2} balls, and dried in an oven at 80° C. for 24 hours to prepare a FeOOH-carbon composite as a positive electrode additive for a lithium secondary battery.

[Example 2]

It was carried out in the same manner as in Example 1, except that 5 g of carbon nanofibers (CNF) having a diameter of 50 to 100 nm and a length of 2 to 3 μm were used instead of carbon nanotubes as a carbon nanostructure.

[Comparative Example 1]

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 FeOOH.

[Experimental Example 1] SEM analysis results

The positive electrode additives for lithium secondary batteries prepared in Examples 1 to 2 and Comparative Example 1 were photographed with a scanning electron microscope (SEM, S-4800, HITACHI), and are shown in FIGS. 2 to 4.

2 and 3, in the case of Example 1, the average particle diameter of FeOOH forming the synthesized FeOOH-carbon composite was 10 to 50 nm, and in the case of Example 2, the average particle diameter of FeOOH was 100 to 200 nm. It was confirmed that in the case of the positive electrode additive for a lithium secondary battery according to an embodiment of the present invention, FeOOH having a reduced particle size was prepared in a well-dispersed form in the carbon nanostructure.

In contrast, as shown in FIG. 4, FeOOH prepared in Comparative Example 1 confirmed that the average particle diameter of FeOOH in the composite was 500 nm to 1 µm. Could.

In addition, it was confirmed that FeOOH having a smaller particle diameter was generated in the case of Example 1 using carbon nanotubes as the carbon nanostructures than the case of Example 2 using carbon nanofibers. It was found that carbon nanotubes are more effective as nanostructures.

[Experimental Example 2] Evaluation of battery properties

Sulfur-carbon composite: Binder (LiPAA/PVA mixed at 6.5:0.5): After preparing a slurry by mixing the positive electrode additives prepared in Examples 1 to 2 at a weight ratio of 88:7:10, aluminum having a thickness of 20 μm An electrode was manufactured by coating the current collector of foil.

In contrast, sulfur-carbon composite: binder (LiPAA/PVA mixed at 6.5:0.5): conductive material (VGCF): FeOOH of Comparative Example 1 as an additive was mixed in a weight ratio of 88:7:5:5, and an additive Sulfur-carbon composite without containing: binder (LiPAA/PVA mixed at 6.5:0.5): conductive material (VGCF) was mixed in a weight ratio of 88:7:5 to prepare a slurry, and then aluminum having a thickness of 20 μm An electrode was manufactured by coating the current collector of foil. The above conditions are summarized and shown in Table 1 below.

Experimental example Sulfur-carbon complex Conductive material bookbinder additive One 88 0 7 10 (Example 1) 2 88 0 7 10 (Example 2) 3 88 5 7 5 (Comparative Example 1) 4 88 5 7 0 (Comparative Example 2)

(Mixing ratio is weight ratio) A coin cell was manufactured using the prepared electrode as a positive electrode and lithium metal as a negative electrode. At this time, the coin cell was used as an electrolyte prepared with 2-Me-THF/DOL/DME (1:1:1), LiN(CF ₃ SO ₂ ) ₂ (LiTFSI) 1M, and LiNO _{3 0.1M.} As for the 2-Me-THF/DOL/DME, 2-Methyltetrahydrofuran, Dioxolane, and Dimethyl ether were used as solvents, respectively.

With respect to the manufactured coin cell, the discharge capacity from 1.5 to 2.5V and the life of the battery were measured and shown in FIGS. 5 and 6.

As shown in FIGS. 5 and 6, it was found that the discharge capacity of the battery was improved when the additive of Example 1 was included as compared to Comparative Example 2 which did not contain the positive electrode additive for a lithium secondary battery.

In addition, in the case of the battery including the additive of Example 1, the discharge capacity of the battery was similar to that of the case where the additive of Comparative Example 1 was included, but it was found that overvoltage was improved.

In terms of the discharge capacity of the battery according to the type of carbon nanostructure, it was found that Example 1 using CNT as the carbon nanostructure was superior to Example 2 using CNF in terms of discharge capacity.

Therefore, it was confirmed that the use of the FeOOH-carbon composite, rather than the use of FeOOH alone, had an overvoltage improvement effect, and the discharge capacity and life characteristics of the lithium secondary battery were improved despite the absence of a conductive material. It was found that the case of the included battery was superior to Example 2 and Comparative Examples 1 and 2 in terms of discharge capacity and lifespan characteristics.

Claims (17)

(1) Fe(NO ₃ ) ₃ ·9H ₂ O and a reducing agent represented by the following Formula 1; mixing and reacting to produce FeOOH;
(2) mixing the FeOOH produced in step (1) and the carbon nanostructure to produce a FeOOH-carbon composite;
Method for producing a positive electrode additive for a lithium secondary battery comprising a.
[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,
_{Fe (NO 3} ) ₃ · 9H ₂ O of the step (1) is used in the form of an aqueous solution of 0.04 to 0.08 M, a method for producing a positive electrode additive for a lithium secondary battery. The method of claim 1,
The method of manufacturing a cathode additive for a lithium secondary battery, wherein the reducing agent represented by Formula 1 in step (1) is used in the form of an aqueous solution of 0.2 to 0.5 M. The method of claim 1,
The mixing in the step (1) is a method for producing a positive electrode additive for a lithium secondary battery is carried out for 10 to 120 seconds. The method of claim 1,
The reaction temperature in the step (1) is 20 to 25 ℃ method for producing a positive electrode additive for a lithium secondary battery. The method of claim 1,
The reaction time in step (1) is 40 minutes to 2 hours, a method of manufacturing a positive electrode additive for a lithium secondary battery. The method of claim 1,
A method for producing a positive electrode additive for a lithium secondary battery further comprising a filtering step after the step (1). The method of claim 1,
A method for producing a positive electrode additive for a lithium secondary battery further comprising a drying step performed at 70 to 90° C. for 6 to 12 hours after the step (2). As a positive electrode for a lithium secondary battery that includes an active material and a binder, but does not include a conductive material,
The positive electrode includes a FeOOH-carbon composite,
The content of the FeOOH-carbon composite is 0.1 to 15 parts by weight based on 100 parts by weight of the base solid content included in the positive electrode for a lithium secondary battery,
The base solid content is a positive electrode for a lithium secondary battery comprising an active material and a binder. delete The method of claim 9,
The FeOOH-carbon composite positive electrode for a lithium secondary battery, characterized in that it comprises carbon nanotubes (CNT) or carbon nanofibers (CNF). The method of claim 11,
The cathode for a lithium secondary battery, characterized in that the diameter of the carbon nanotube is 5 to 50 nm, the length is 100 nm to 1 ㎛. The method of claim 11,
The cathode for a lithium secondary battery, characterized in that the diameter of the carbon nanofiber is 50 to 100 nm, the length is 1 to 5 ㎛. The method of claim 9,
The FeOOH of the FeOOH-carbon composite has an average particle diameter of 10 to 500 nm. The method of claim 9,
The FeOOH-carbon composite positive electrode for a lithium secondary battery, characterized in that the average particle diameter of 1 to 50 ㎛. Including a positive electrode, a negative electrode, a separator and an electrolyte interposed therebetween,
The positive electrode is a lithium secondary battery that is the positive electrode for a lithium secondary battery according to any one of claims 9 and 11 to 15. The method of claim 16,
The lithium secondary battery is a lithium secondary battery, characterized in that the lithium-sulfur battery.

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