KR101150515B1 - Composition for preparing a positive electrode of a lithium secondary cell, the positive electrode prepared with the composition, and the lithium secondary cell compring the positive electrode - Google Patents

Abstract

In the composition for producing a lithium secondary battery positive electrode of the present invention, the specific surface area and the electrical conductivity of the positive electrode are replaced by replacing some or all of the existing conductive agent, dispersant and / or binder with carbon nanofibers having a specific diameter and length range. It greatly improves (reduces resistance), thereby maximizing the rate characteristic by minimizing the reduction of the efficiency of the positive electrode active material, especially the capacity of the positive electrode active material during high-speed charging and discharging.

Carbon nanofiber, lithium secondary battery, electrode, binder

Description

COMPOSITION FOR PREPARING A POSITIVE ELECTRODE OF A LITHIUM SECONDARY CELL, THE POSITIVE ELECTRODE PREPARED WITH THE COMPOSITION, AND THE LITHIUM SECONDARY CELL COMPRING THE POSITIVE ELECTRODE}

The present invention relates to a composition for producing a cathode of a lithium secondary battery, a cathode prepared from the composition, and a lithium secondary battery comprising the same.

Lithium cobalt oxide (LiCoO ₂ ) has been mainly used as a cathode material for lithium secondary batteries. However, the cobalt (Co) element, which is used as a cathode active material, is expensive and causes environmental problems. Techniques have been developed which have been replaced with (Ni) and / or manganese (Mn). In particular, LiMn ₂ O ₄ has been considered as a positive electrode active material of a lithium secondary battery that can be applied to a high output lithium secondary battery such as a hybrid vehicle. Because manganese is rich in abundance and does not cause environmental problems, LiMn ₂ O ₄ This is because a high speed discharge is possible.

However, even when LiMn ₂ O ₄ is used as the positive electrode active material, problems such as instability at high temperatures, poor start-up at low temperatures, and deterioration of charging characteristics at low temperatures are still known. In particular, the problem that the battery capacity of the lithium secondary battery using the cathode active material as described above rapidly decreases in a high speed charge and discharge environment is recognized as a problem to be solved urgently in the field of high power lithium secondary battery.

On the other hand, as one direction of the study for improving the effectiveness of the LiMn ₂ O ₄ active material, researches to optimize the material constituting the electrode has been actively conducted.

In general, a positive electrode used in a lithium ion secondary battery includes an active material, a conductive agent, and a binder, and includes about 80% of an active material (eg, LiMn ₂ O ₄ ), and about 10% of a conductive material (eg, Super-P). , And about 10% binder.

The binder (also called a binder) serves as a crosslinking role for bonding the active material constituting the electrode material and the conductive agent, and the types thereof include CMC (carboxy methyl cellulose), polyvinylpyrrolidone (PVP), and fluorine poly Tetrafluoroethylene (PTFE), polyvinylidene fluorine (PVdF) powder and emulsion, and rubber-based styrene butadiene rubber (SBR). These binders are mostly polymer-based binders that are not conductive.

Looking at the characteristics of each binder as described above, CMC serves to increase the binding force with the current collector while maintaining the viscosity of the slurry in a state similar to the pool. CMC has the advantage of increasing the binding force, but after the slurry is cast (cast) rather has the disadvantage of increasing the embrittlement of the electrode material layer (casting). However, without using the CMC, it is very difficult to obtain a binding force between the current collector and the slurry, and is currently used as an electrode material.

Polyvinylpyrrolidone (PVP) acts as a dispersant and helps to disperse the particles in the slurry. However, if there is a small amount of addition and there is another substance that can help dispersing, it can be replaced.

Polytetrafluoroethylene (PTFE) or polyvinylidene fluorine (PVdF) can be encapsulated inside the slurry in an emulsion state and melted above the melting point, causing the polymer to enclose the particles, like a spider web. However, the melting point is so high that it is very difficult to activate it by raising the temperature or by applying pressure. In addition, F (fluorine) is decomposed in the process of charging / discharging to combine with H (hydrogen) present in the electrode to form HF to significantly reduce the characteristics of the product. Nevertheless, the reason why it is used as an electrode material is that it is difficult to find a material capable of stably increasing the bonding force between particles as PTFE.

Rubber-based styrene butadiene rubber (SBR) serves to protect the surface by coating the surface of the particles. Although it has the disadvantage of increasing the resistance due to the drop in electrical conductivity, it is widely used as a general electrode material because of its advantages over the disadvantage.

In consideration of the above-described properties of each binder, the selected binder is mixed with the active material and the conductive agent to prepare an electrode to have desired electrode characteristics. However, when the electrode is composed of such a polymeric material, the resistance of the binder itself is high (e.g., PTFE conductivity = 10 ^-18 S / cm), which causes an increase in the resistance of the electrode, and also includes a large amount. When the amount of the active material is relatively reduced, the storage capacity is reduced. In addition, since the reaction to react with a material present in the electrode to increase the resistance or generate a gas (HF), it causes a problem of deteriorating the quality of the lithium secondary battery. Therefore, it is preferable to use a minimum amount of the binder to maintain the physical properties of the electrode.

However, reducing the content of the binder in the electrode may weaken the binding of the electrode material to the current collector and cause collapse of the electrode material layer, in which case there is a risk that the capacity of the battery is sharply reduced. Because it is not easy. Therefore, the development of a binder that improves the above disadvantages or the development of a substitute material that can replace a part or all of the binder content is very important for increasing the utility of the positive electrode active material, in particular, lithium 2 which requires high output It seems to be essential for the battery.

The present invention is to solve the above problems of the prior art,

First, some or all of the existing conductive agents, dispersants and / or binders are replaced by carbon nanofibers having a specific diameter and length range, thereby dramatically improving (reducing resistance) the specific surface area and electrical conductivity of the anode. Accordingly, it is an object of the present invention to provide a composition for producing a cathode, which improves the rate characteristic by minimizing the reduction of the efficiency of the cathode active material, particularly the capacity of the cathode active material during high-speed charging and discharging.

Second, by the above configuration, without a separate dispersant, the positive electrode active material and the conductive agent is well dispersed in the positive electrode and uniformly distributed, strongly bound to each other to provide a composition for producing a positive electrode capable of producing a positive electrode having excellent durability It is done.

Third, it is manufactured with the composition for producing a positive electrode, which is excellent in the efficiency of the positive electrode active material, in particular, the rate-rate property is possible, high-speed charging and discharging is possible, and the long specific lifespan is maintained by the large specific surface area during the charge and discharge cycle It is therefore an object of the present invention to provide a lithium secondary battery positive electrode that can be usefully used in the production of a lithium secondary battery and a lithium secondary battery containing such a positive electrode.

The present invention,

As a composition for producing a cathode of a lithium secondary battery comprising a carbon nanofiber prepared by the electrospinning method of the spinning solution containing a cathode active material, a conductive agent, a carbon fiber precursor, and a binder,

The carbon nanofibers have an average diameter of 150 nm to 700 nm and an average length of 5 μm to 17 μm to provide a composition for manufacturing a cathode of a lithium secondary battery.

In addition, the present invention,

Current collector; And

Comprising a positive electrode forming layer coated on the current collector,

The cathode forming layer is formed of a composition for producing a cathode of the present invention provides a cathode for a lithium secondary battery.

In addition, the present invention,

In the lithium secondary battery comprising a positive electrode, a negative electrode, an electrolyte, the lithium secondary battery positive electrode of the present invention provides a lithium secondary battery, characterized in that formed with the positive electrode.

 The composition for producing a cathode of the present invention is to replace the conventional conductive agent, dispersant and / or binder part or all of the carbon nanofibers having a specific diameter and length range, thereby significantly improving the specific surface area and electrical conductivity of the positive electrode (Reduced resistance), thereby minimizing the reduction of the efficiency of the positive electrode active material, in particular, the capacity of the positive electrode active material during high-speed charging and discharging provides the effect of maximizing the rate characteristic.

In addition, the composition for producing a positive electrode of the present invention, since the positive electrode active material and the conductive agent are well dispersed in the positive electrode and uniformly distributed without a separate dispersant, it is possible to produce a very large electrode evenly. Since it is strongly bound in three dimensions without applying pressure by a roller or the like, it provides an effect of making it possible to manufacture a positive electrode having excellent durability.

In addition, the positive electrode for a lithium secondary battery of the present invention is made of the composition for producing a positive electrode as described above is excellent in the efficiency of the positive electrode active material, particularly high speed rate characteristics that minimize the reduction in capacity during high-speed charge and discharge is possible to fast charge and discharge In addition, since the path between the active material particles is maintained by the large specific surface area during the charge and discharge cycle, and thus has a long life, it can be usefully used in the production of a lithium secondary battery.

The present invention is a composition for producing a cathode of a lithium secondary battery comprising a carbon nanofiber prepared by the electrospinning method of the spinning solution containing a cathode active material, a conductive agent, a carbon fiber precursor, and a binder,

The carbon nanofibers have a mean diameter of 150 nm to 700 nm, and an average length of 5 μm to 17 μm.

The carbon nanofibers are more preferable in the case where the average diameter is 200 nm to 400 nm and the average length is 8 μm to 15 μm.

When the average diameter of the carbon nanofibers are less than 150nm, the rate property (C-rate) is remarkably decreased, and even when the average diameter of the carbon nanofibers exceeds 700 nm, the rate property properties are greatly decreased. The reason for this result is that when the diameter of carbon nanofibers is too small, other components and carbon nanofibers do not mix well, and agglomeration occurs, and when the diameter is too large, mixing with other components is not easy. Not only, but also because the specific surface area decreases.

In addition, when the average length of the carbon nanofibers is less than 5μm, it is difficult to sufficiently perform the crosslinking role between the electrode materials, and the rate characteristic is also lowered.If the average length of the carbon nanofibers exceeds 17μm, the slurry manufacturing process is difficult, and the prepared slurry It is also difficult to control the thickness of the electrode when casting it into an electrode, and the deterioration of the rate characteristic is also remarkable.

In the composition of the present invention, as the positive electrode active material, those known in the art can be used without limitation. Examples of the positive electrode active material include LiMn ₂ O ₄ , Li [Ni _1/3 Co _1/3 Mn _1/3 ] O ₂ , _{LiNi 2 O 4, LiCoO 2,} LiNiO 2, Li 2 MnO 3, LiFePO 4, LiNi x Co y O 2 (0 <x <= 0.15, 0 <y <= 0.85), V 2 O 5, CuV 2 O 6 , NaMnO ₂ , NaFeO ₂ , and the like, and materials in which two or more of these materials are combined, such as LiMn ₂ O ₄ / Li [Ni _1/3 Co _1/3 Mn _1/3 ] O ₂ , Li ₂ MnO ₃ / LiMnO _2, Li ₂ MnO ₃ / LiNiO ₂ , and the like. However, among these, LiMn ₂ O ₄ and LiMn ₂ O ₄ / Li [Ni _1/3 Co _1/3 Mn _1/3 ] O ₂ are rich in manganese, do not cause environmental problems, and can provide high-speed discharge. Can be preferably used in the present invention. In the composition for producing a positive electrode, when the content of the positive electrode active material is too small, the capacity of the electrode is small, and when the content of the positive electrode active material is too large, binding capacity or conductivity of the positive electrode active material is lowered. Therefore, the positive electrode active material in the present invention is preferably included in an amount of 60 to 95% by weight based on the total weight of the composition.

Although commercially available LiMn ₂ O ₄ may be used, LiMn ₂ O ₄ may be manufactured to a nano size through electrospinning using a precursor of LiMn ₂ O ₄ , and may be used. Specifically, Li (CH ₃ COO)? H ₂ O as the acetate salt of lithium and Mn (CH ₃ COO) _{2 to} 4H ₂ O as the acetate salt of manganese are dissolved in distilled water at a ratio of 17% by weight and 83% by weight. It is also possible to prepare an electrospinning solution by mixing it with a polymer solution and electrospinning it to prepare and use a nano-sized LiMn ₂ O _4. Alternatively, LiNO ₃ and Mn (NO ₃ ) _{2 to} 4H ₂ O can be used. It can be used as a precursor for electrospinning or electrospinning by mixing in a weight ratio of 1: 1 or 1: 2 to make an aqueous solution of 1 mol and then mixed with a polymer solution.

In the composition of the present invention, as the conductive agent, those known in the art can be used without limitation. Examples of such a conductive agent include graphite such as natural graphite and artificial graphite; Carbon blacks such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, summer black, super-p, toka black and denka black. have. Such a conductive agent may be appropriately selected in consideration of physical properties of the composition for manufacturing a positive electrode. The content of the conductive agent in the positive electrode composition may be adjusted in consideration of the conductivity of the electrode and the content of the other components, it is preferably included in an amount of 3 to 20% by weight relative to the total weight of the composition.

In the composition of the present invention, the binder means a component that binds the positive electrode active material and the conductive agent and between the materials and the current collector, and those known in the art may be used without limitation. Examples of such binders include CMC (carboxy methyl cellulose), polyvinylpyrrolidone (PVP), fluorine polytetrafluoroethylene (PTFE), polyvinylidene fluorine (PVdF) powder or emulsion, and rubber styrene butadiene rubber ( SBR) etc. can be mentioned. These binders are mostly polymer-based binders that are not conductive.

In the positive electrode composition of the present invention, the content of the binder may be selected in consideration of the binding power between the components forming the electrode and the current collector, and considering the size and the binding capacity of the electrode resistance, 1 to 1 to the total weight of the composition. It may be included in an amount of 20% by weight. In particular, since the carbon nanofibers are included in the composition for producing a cathode of the present invention, it is preferable to include 1 to 8% by weight. More preferably, it may be included in 1 to 5% by weight.

In the compositions of the present invention, carbon nanofibers are very important as components which replace some or all of the conductive agent, dispersant and / or binder. Carbon nanofibers have a large specific surface area for the anode and have excellent electrical conductivity, which greatly reduces the resistance of the electrode, thereby improving the efficiency of the electrode active material, and in particular, minimizing the reduction of the capacity of the cathode active material during high-speed charging and discharging. By maximizing the rate characteristic. Therefore, when the lithium secondary battery is configured using the composition for producing a cathode, a lithium secondary battery capable of high speed charge and discharge may be manufactured.

In addition, the carbon nanofiber enables the positive electrode active material and the conductive agent to be well dispersed and uniformly distributed in the positive electrode even without a dispersing agent, thereby making it possible to manufacture a very large electrode evenly. In the case of manufacturing a sheet with a conventional composition for producing a positive electrode, the dispersion degree of the slurry during manufacturing the sheet is reduced, there is a problem that the first and the rear part of the sheet is not uniform, it has no choice but to limit the size of the sheet produced.

In addition, the carbon nanofibers also increase the binding force between the positive electrode active material and the conductive agent, thereby making it possible to manufacture a positive electrode having excellent durability by forming a strong three-dimensional bond without applying pressure by a roller or the like.

The carbon nanofibers may be included in an amount of 1 to 30% by weight based on the total weight of the composition. In particular, the carbon nanofibers may be included in an amount of 1 to 15% by weight, more preferably 1 to 7% by weight. If it is included in less than 1% by weight, a relatively large amount of existing polymer binders should be added, so the degree of contribution to improving the binding strength and electrical conductivity is insignificant, and it is difficult to exert the function of dispersing other components. In addition, when the content exceeds 30% by weight, the content of the positive electrode active material is relatively decreased, thereby reducing the capacity of the electrode.

Since the carbon nanofibers used in the composition for producing a cathode of the present invention are manufactured by an electrospinning method, the fiber surface state and density thereof are different from those produced by the vapor phase growth method, and in particular, may include pores controlled by heat treatment. Has the advantage that it can.

In the case of manufacturing the tanno nanofibers by the vapor phase growth method, methane is required and the temperature of the raw material input part is 700 ° C. or less, but there is a difficulty in performing heat treatment at a very high temperature of 1100 ° C. to 1500 ° C. On the other hand, the carbon nanofibers used in the present invention are manufactured by an electrospinning, stabilization, and carbonization process, and have a characteristic of being easy to manufacture because the maximum temperature during carbonization does not exceed 1100 ° C.

A method of manufacturing the carbon nanofibers will be described in detail below.

Carbon nanofibers in the present invention

(a) electrospinning a spinning solution containing a carbon fiber precursor to prepare a nanofiber web;

(b) oxidative stabilization of the nanofiber web prepared in step (a) in air;

(c) carbonizing the oxidatively stabilized nanofiber web prepared in step (b) in an inert gas or vacuum; And

(d) is prepared, including the step of pulverizing the carbon nanofiber obtained in step (c).

In the step (a), the spinning solution may be prepared by further comprising a thermally decomposable polymer in addition to the carbon fiber precursor. In this case, since the thermally decomposable polymer decomposes at a high temperature carbonization process, pores are formed in the carbon nanofibers, and these pores may be controlled by the content of the thermally decomposable polymer in the spinning solution preparation.

As the carbon fiber precursor in the present invention, any material known in the art may be used without limitation as long as the material can be electrospun. For example, polyacrylonitrile (PAN), phenol resin (phenol-resin), polybenzylimidazole (PBI), cellulose, phenol, phenol, pitch, and polyimide PI), and these may be used alone or in combination of two or more.

As the thermally decomposable polymer in the present invention, any material known in the art may be used without limitation. For example, polyurethane, polyetherurethane, polyurethane copolymer, cellulose acetate, cellulose acetate butylate, cellulose acetate propionate, polymethylmethacrylate (PMMA), polymethylacrylate (PMA), polyacryl copolymer, Polyvinylacetate (PVAc), polyvinylacetate copolymer, polyvinyl alcohol (PVA), polyperfuryl alcohol (PPFA), polystyrene, polystyrene copolymer, polyethylene oxide (PEO), polypropylene oxide (PPO), polyethylene oxide air Copolymer, polypropylene oxide copolymer, polycarbonate (PC), polyvinyl chloride (PVC), polycaprolactone, polyvinylpyrrolidone (PVP), polyvinyl fluoride, polyvinylidene fluoride copolymer , Polyamide, etc. may be used, and these may be used alone or in combination of two or more.

Electrospinning in the present invention by connecting the spinning solution prepared above to the electrospinning nozzle using a supply device, by forming a high field (high density, 10kV ~ 100kV) using a high voltage generator between the nozzle and the current collector Conduct. The magnitude of the electric field is related to the distance between the nozzle and the current collector, and in order to facilitate the electrospinning, a relationship between them is used in combination. In this case, as the electrospinning apparatus to be used, generally used ones may be used, and an electro-blowing method or a centrifugal electrospinning method may be used. Nanofibers prepared by the method described above are in the form of a nonwoven fabric having an average diameter of mostly less than 1 μm.

The thickness of the nanofiber web electrospun in the electrospinning step should be uniform, and if the thickness is nonuniform or partially thick, the exothermic reaction occurs in the relatively thick part in the stabilization step, resulting in an increase in enthalpy. The web may burn.

Oxidation stabilization in step (b) can be carried out by applying a method known in the art without limitation. For example, the prepared nanofiber web is placed in an electric furnace capable of controlling the temperature controller and the air flow rate, and the temperature is increased from 0.5 to 5 ° C. per minute from room temperature to the glass transition temperature or less to obtain incompatible fiber. If the amount of hydrogen is too high or the amount of oxygen is too small, it will increase the weight and cause an exothermic reaction.

Carbonization in step (c) may be carried out by applying a method known in the art without limitation. Oxidation stabilized fibers are treated in an inert atmosphere or in a vacuum at a temperature in the range of 500-1500 ° C. to obtain carbonized nanofiber webs. The diameter of the nanofibers constituting the carbonized nanofiber web thus obtained is in the range of approximately 100 nm to 1000 nm.

In addition, the carbonized nanofiber may be used by further performing an activation and / or graphitization treatment. The graphitization is to obtain the graphitized nanofiber web by treating the carbonized nanofiber web at a temperature of 3000 ° C. or lower using a graphitization furnace.

In step (c), the grinding of the carbon nanofiber web is performed using a ball mill or a chopper. In the case of using the ball mill, dry and / or wet grinding may be used, and the length of the carbon nanofibers obtained is decreased as the ball milling time increases. In general, when the energy during ball milling is high, a lot of fine powder is generated. In addition, when a chopper is used, a lot of fine powder does not generate | occur | produce, and the length of about 30 micrometers-100 micrometers is obtained initially, and 10 micrometers-50 micrometers with time, and 1 micrometer-when time passes further An 8 μm length can be obtained.

The present invention also provides

Current collector; And

Comprising a positive electrode forming layer coated on the current collector,

The positive electrode forming layer is formed of a composition for producing a positive electrode of the present invention relates to a lithium secondary battery positive electrode.

As described above, the lithium secondary battery positive electrode of the present invention has a very high efficiency of the positive electrode active material, and does not have a large decrease in the capacity of the positive electrode active material even at high charge and discharge, and thus may be very useful for a lithium secondary battery requiring high power.

The present invention also provides

In a lithium secondary battery comprising a positive electrode, a negative electrode, and an electrolyte, the lithium secondary battery positive electrode of the present invention relates to a lithium secondary battery, characterized in that formed with the positive electrode.

The lithium secondary battery as described above maintains a high capacity even during high-speed charging and discharging, and can be used for a hybrid car or the like because it exhibits high power during discharge.

The present invention also provides

(a) electrospinning a spinning solution containing a carbon fiber precursor to prepare a nanofiber web;

(b) oxidative stabilization of the nanofiber web prepared in step (a) in air;

(c) carbonizing the oxidatively stabilized nanofiber web prepared in step (b) in an inert gas or vacuum; And

(d) pulverizing the carbon nanofibers obtained in step (c); And

(e) providing a method for preparing a composition for producing a cathode, comprising the step of mixing the pulverized carbon nanofibers obtained in step (d) with components including a cathode active material, a conductive agent and a binder to form a slurry. .

It is also possible to further include additional solvent in order to obtain the slurry form in step (e).

In the step (a), the spinning solution may further include a thermally decomposable polymer in addition to the carbon fiber precursor.

The positive electrode for a lithium secondary battery of the present invention,

Coating the composition for producing a positive electrode in the form of a slurry of the present invention on a current collector can be prepared by forming a positive electrode forming layer on the current collector. In the method of manufacturing the lithium secondary battery positive electrode, the positive electrode forming layer is coated with a thickness of approximately 10 ~ 100 ㎛, dried in a high temperature environment of approximately 100 ~ 150 ℃ according to the use and cut to form a predetermined length. In the above, the coating of the composition for producing a positive electrode on the current collector may be performed on one surface, both surfaces, or the entire surface.

Hereinafter, the present invention will be described in more detail with reference to Preparation Examples and Examples. However, the following Preparation Examples and Examples are intended to illustrate the present invention more specifically, but the scope of the present invention is not limited by the following Preparation Examples and Examples. The following Preparation Examples and Examples may be appropriately modified and changed by those skilled in the art within the scope of the present invention.

Preparation Example 1 Preparation of Nanofiber Web

To the total weight of the spinning solution, 30 wt% (based on solids) of polyacrylonitrile (PAN, Mw = 180,000) as a carbon fiber precursor was dissolved in a DMF solvent to prepare a spinning solution. The spinning solution was connected to the spinneret, and electrospinning was carried out while discharging at an applied voltage of 50 kV, a distance of 25 cm between the spinneret and the current collector, and 0.1 to 1 cc / g per hole.

The electrospinning yielded a uniform PAN nanofiber web (thickness: 55.6 μm) having a thickness of 100 nm to 1.2 μm, respectively.

A pitch nanofiber web having a uniform thickness was obtained by the same method as described above, except that pitch was used instead of polyacrylonitrile (PAN).

Preparation Example 2 Preparation of Carbon Nanofiber Web

The polyacrylonitrile nanofiber web prepared in Preparation Example 1 was gradually heated up at a rate of 5 ^o C per minute from room temperature to 300 ^o C using a hot air circulation furnace, and isothermally treated at 300 ^o C for 1 hour. Stabilized. The stabilized nanofiber web is heated up at a rate of 5 ^o C per minute at room temperature to a temperature that can be carbonized to 700 to 900 ^o C, and then the final temperature (700 ^o C, 800 ^o C or 900 under nitrogen gas atmosphere). ^o ) Carbonization by isothermal heat treatment for 2 hours at C) to prepare nanofiber webs with average diameters of 100 nm, 400 nm, 600 nm and 800 nm, respectively.

In the carbonized nanofiber web, the average diameter of the nanofibers varies depending on the carbonization temperature, but was formed in the range of 45% to 65% of the average diameter before the stabilization.

Preparation Example 3 Preparation of Carbon Nanofiber Web

A spinning solution was prepared by dissolving 20 wt% of PAN (Mw = 180,000) (based on solids) and 10 wt% (based on solids) of PMMA in a DMF solvent based on the total weight of the spinning solution. The spinning solution thus prepared was connected to the spinneret, and electrospinning was performed while discharging at an applied voltage of 50 kV, a distance of 25 cm between the spinneret and the current collector, and 0.1 to 1 cc / g per hole. The carbon nanofiber web was prepared by stabilizing and carbonizing the electrospun PAN / PMMA composite nanofiber web as described above in the same manner as in Preparation Example 2.

The carbon nanofiber web includes many pores formed by completely decomposing the thermally decomposable polymer (PMMA) during the carbonization process.

Preparation Example 4 Crushing or Cutting of Carbon Nanofiber Web

The carbon nanofiber web prepared in Preparation Example 2 was cut with a ball mill or a chopper to prepare carbon nanofibers having an average length of 3 μm, 8 μm, 10 μm, and 20 μm, respectively. In the case of using a ball mill, dry and wet grinding were carried out alternately.

Examples 1-4 and Comparative Examples 1-5.

(1) Preparation of positive electrode composition and positive electrode

The components shown in Table 1 were mixed at a corresponding ratio to prepare a composition for preparing a positive electrode in the form of a slurry.

In addition, the composition in the form of the slurry was cast on one surface of the positive electrode current collector, and dried to prepare a positive electrode of a lithium secondary battery.

object Furtherance CNF average diameter CNF average length Example 1 LMO + NCM: Super-P: PVdF: CNF = 46.5 + 46.5: 2: 2: 3 200 nm 10 μm Example 2 LMO + NCM: Super-P: PVdF: CNF = 46.5 + 46.5: 2: 2: 3 400 nm 10 μm Example 3 LMO + NCM: Super-P: PVdF: CNF = 46.5 + 46.5: 2: 2: 3 600 nm 10 μm Example 4 LMO + NCM: Super-P: PVdF: CNF = 46.5 + 46.5: 2: 2: 3 200 nm 8 μm Comparative Example 1 LMO + NCM: Super-P: PVdF: CNF = 46.5 + 46.5: 2: 2: 3 100 nm 10 μm Comparative Example 2 LMO + NCM: Super-P: PVdF: CNF = 46.5 + 46.5: 2: 2: 3 800 nm 10 μm Comparative Example 3 LMO + NCM: Super-P: PVdF: CNF = 46.5 + 46.5: 2: 2: 3 200 nm 3 μm Comparative Example 4 LMO + NCM: Super-P: PVdF: CNF = 46.5 + 46.5: 2: 2: 3 200 nm 20 μm Comparative Example 5 LMO + NCM: Super-P: PVdF = 46.5 + 46.5: 5: 2 - -

Note) LMO: LiMn ₂ O _4, NCM: Li [Ni _1/3 Co _1/3 Mn _1/3 ] O ₂ , CNF: Carbon non-fiber

 (2) Check the appearance of anode surface

Scanning electron microscopy (SEM) was used to observe the anode surfaces prepared from the compositions of Examples 1-4 and Comparative Examples 1-5. As a result, it was confirmed that the positive electrode surface prepared in Examples 1 to 4 and the positive electrode active material and the conductive material were dispersed significantly better than the positive electrode prepared in the compositions of Comparative Examples 1 to 5, showing a very uniform form. In particular, it showed significantly more uniform dispersion than the composition of Comparative Example 5 (see FIGS. 4A (Example 1) and 4B (Comparative Example 5)).

(3) Physical property test of anode surface

Rubbing the surface of the positive electrode prepared above with a finger, scraping with a fingernail to confirm whether the positive electrode active material is smeared out, or whether scratching occurs.

As a result, the positive electrode prepared with the positive electrode composition of Examples 1 to 4 of the present invention did not adhere to the hand even when the roller was not used at the time of manufacture, and scratches did not occur even by nails.

On the other hand, the positive electrode of Comparative Example 5 prepared by the composition generally used in this field, when rubbed with a finger, the positive electrode active material was smeared on the electrode surface, the scratch occurred when scratched with a fingernail. However, the anodes of Comparative Examples 1 to 4, including CNF, did not adhere to the hand, and no scratch was generated by the nails.

Test Example 1 Performance Comparison According to Rate of Anode

The expression capacity of the capacity according to the rate of the positive electrode prepared in Examples 1 to 4 and the positive electrode prepared in Comparative Examples 1 to 5 was measured, and the results are shown in Table 3 below.

CNF average
diameter CNF average
Length Volume Rate capacity (mAh / g) 0.2C 1C 3C 5C Example 1 200 nm 10 μm mAh / g  142.87 134.31 120.67 110.35 Capacity maintenance (%) - 94 84.4 77.23 Example 2 400 nm 10 μm mAh / g 130.46 119.01 106.18  97.93 Capacity maintenance (%) - 91.22 81.38 75.06 Example 3 600 nm 10
μm mAh / g 145.24 126.56 113.13 103.51 Capacity maintenance (%) - 87.13 77.89 71.26 Example 4 200 nm 8
μm mAh / g 130.99 119.55 104.4 93.58 Capacity maintenance (%) - 91., 26 79.7 71.44 Comparative Example 1 100 nm 10 μm mAh / g 139.17 118.38 59.4 - Capacity maintenance (%) - 85.06 42.68 - Comparative Example 2 800 nm 10
μm mAh / g 130.58 109.8 96.03 85.92 Capacity maintenance (%) - 84.08 73.54 65.79 Comparative Example 3 200 nm 3
μm mAh / g 127.37 115.22 99.90 89.10 Capacity maintenance (%) - 90.4 78.43 69.9 Comparative Example 4 200 nm 20
μm mAh / g 129.2 116.36 99.12 87 Capacity maintenance (%) - 90.0 76.71 67.33 Comparative Example 5 - - mAh / g 124.9 107.56 88.31 73.70 Capacity maintenance (%) - 86.1 70.7 59.0

As confirmed in Table 3, it can be seen that at the rate of 0.2C, the positive electrodes of Examples 1 to 4 and the positive electrodes of Comparative Examples 1 to 5 have almost equivalent rate capacities. However, at the rate of 5C, the positive electrodes of Examples 1 to 4 and the positive electrodes of Comparative Examples 1 to 4 showed a large difference in the rate capacities. In particular, the positive electrode of Comparative Example 1 made of carbon nanofibers having an average diameter of 100 nm and the positive electrode prepared in Comparative Example 5 not containing CNF showed a large rate reduction in capacity.

Figure 1 is a brief comparison of the manufacturing method of carbon nanofibers by the electrospinning method and the vapor phase growth method of the carbon nanofibers manufacturing method of the present invention.

Figure 2a is a SEM image of the polyacrylonitrile nanofiber web prepared by electrospinning in Preparation Example 1.

Figure 2b is a SEM image of the pitch nanofiber web prepared by electrospinning in Preparation Example 1.

Figure 2c is a SEM image of the cross section of the polyacrylonitrile nanofiber web prepared by electrospinning in Preparation Example 1.

FIG. 3 is an SEM image of carbon nanofibers cut by a chopper prepared in Preparation Example 4. FIG.

4A is an SEM image of the surface of a lithium secondary battery positive electrode prepared in Example 1. FIG.

4B is an SEM image of the surface of the lithium secondary battery positive electrode prepared in Comparative Example 5. FIG.

5A to 5F are graphs illustrating rate-rate characteristics (C-rate) of the anodes prepared in Examples 1 to 4 and Comparative Examples 1 to 5.

Claims (8)

Positive electrode active material, Challenger, Carbon nanofibers prepared by an electrospinning method of a spinning solution containing a carbon fiber precursor, and As a composition for manufacturing a positive electrode of a lithium secondary battery containing a binder, The carbon nanofiber has an average diameter of 150nm to 700nm, the average length is 5μm ~ 17μm composition for producing a positive electrode of a lithium secondary battery. The composition according to claim 1, wherein the carbon nanofibers have an average diameter of 200 nm to 400 nm and an average length of 8 μm to 15 μm. The method of claim 1, wherein the composition for producing a positive electrode of the lithium secondary battery, 60 to 95% by weight of the positive electrode active material, 1 to 20% by weight of the conductive agent, 1 to 30% by weight of carbon nanofibers, and A composition for producing a positive electrode of a lithium secondary battery, comprising 1 to 20% by weight of a binder. The composition for manufacturing a positive electrode of a lithium secondary battery according to claim 3, wherein the binder is contained in an amount of 1 to 8% by weight based on the total weight of the composition. The method according to claim 1, wherein the carbon fiber precursor is polyacrylonitrile (PAN), phenol resin (phenol-resin), polybenzylimidazole (PBI), cellulose (cellulose), phenol (phenol), pitch , And polyimide (PI). A composition for manufacturing a cathode of a lithium secondary battery, characterized in that it is composed of one or more selected from the group consisting of. The positive electrode composition of claim 1, wherein the carbon nanofiber is prepared by stabilizing and carbonizing a spinning solution containing a carbon fiber precursor and a thermally decomposable polymer. Current collector; And Comprising a positive electrode forming layer coated on the current collector, The cathode forming layer is a lithium secondary battery cathode, characterized in that formed from the composition for producing a cathode of any one of claims 1 to 6. A lithium secondary battery comprising a positive electrode, a negative electrode, and an electrolyte, wherein the positive electrode comprises a lithium secondary battery positive electrode of claim 7.

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