KR20180101190A - Solvent for coating positive electrode material of secondary battery, positive electrode material slurry comprising the same and secondary battery prepared therewith - Google Patents

Abstract

The present invention relates to a solvent for coating an anode active material of a secondary battery, anode active material slurry containing the same, and a secondary battery manufactured therefrom. According to the present invention, the anode active material slurry loaded with a high content can be provided by dispersing the anode active material in a very stable manner, and the anode active material layer with a significantly reduced loading deviation can be formed using the slurry. Also, the secondary battery having significantly enhanced capacity characteristics and life characteristics can be provided.

Description

TECHNICAL FIELD [0001] The present invention relates to a solvent for coating a cathode active material, a cathode active material slurry containing the cathode active material slurry, and a secondary battery manufactured from the cathode active material slurry and a cathode active material slurry containing the cathode active material slurry. BACKGROUND ART < RTI ID = 0.0 >

The present invention relates to a solvent for coating a cathode active material of a secondary battery, a cathode active material slurry containing the cathode active material slurry, and a secondary battery produced therefrom.

In response to the rapid development of communication industry such as electronic industry and various kinds of information communication such as mobile communication, and in response to a demand for light and small size of electronic devices, portable electronic devices such as notebooks, netbooks, tablet PCs, mobile phones, smart phones, PDAs, digital cameras, Electronic products and communication terminal devices have been widely used, and development of a battery, which is a driving power source for these devices, is also getting more attention.

In addition, with the development of electric vehicles such as hydrogen electric vehicles, hybrid electric vehicles and fuel cell vehicles, great attention has been paid to the development of batteries having high performance, large capacity, high density, high output and high stability, The development of batteries is also a big issue.

With this trend, a lithium secondary battery having a high energy density and voltage, a long cycle life, and a low self-discharge rate has been widely used and widely used. In order to maximize the above-mentioned effect, studies on a lithium secondary battery are actively conducted .

The secondary battery includes a positive electrode, a negative electrode, a separator, and an electrolyte as basic components. The positive and negative electrodes have positive and negative potentials, respectively, for converting and storing energy such as oxidation and reduction. The separator is positioned between the anode and the cathode to maintain electrical insulation and provide a path for charge transport. In addition, the electrolyte acts as an intermediary for charge transfer.

In a lithium secondary battery, which is one example of a secondary battery, battery performance (e.g., capacity) is most affected by the used positive electrode active material. In order to improve the performance of such a lithium secondary battery, the cathode active material can be loaded at an appropriately high numerical value and must be formed as a uniform and stable thickness layer on the current collector. In order to solve such a problem, it is easy to control the solid content, viscosity and the like of the slurry of the cathode active material.

The positive electrode of the lithium secondary battery may be prepared by coating a positive electrode active material slurry containing a positive electrode active material on a positive electrode current collector and drying the positive electrode active material slurry. The positive electrode active material slurry may be prepared by mixing a binder and an organic solvent, ≪ / RTI >

A conventional cathode active material is a mixture of a large amount of an organic solvent (for example, N-methyl-2-pyrrolidone) such as a small particle size and a carbon-coated specific surface area so as to have a low solid concentration (a solid content of 45% (NMP) or the like) is added to adjust the viscosity appropriately to solve the above problems. However, when a large amount of the organic solvent is used, undried portions may be formed even after drying, which makes it impossible to carry out high-loading at a high content and increases a process speed such as a drying speed, . Further, in the case of the slurry of the cathode active material having a low solid content concentration, there is a problem that the thickness of the layer and the loading deviation may be caused when the cathode active material layer is formed due to the lowered dispersion stability. Particularly, there is a problem in that it is not environmentally friendly due to the harmfulness of the organic solvent.

Under the above circumstances, the inventors of the present invention have made intensive studies for improving the above-mentioned problems, and have found that by using a coating solvent containing N-ethylformamide, the dispersibility of the slurry containing the cathode active material It is possible to provide a positive electrode having improved capacity characteristics and life characteristics and a secondary battery including the positive electrode by stably forming the positive electrode active material layer with a uniform thickness, Completed.

KR 10-0436708 B1 KR 10-1764470 B1

It is an object of the present invention to provide a solvent for coating a cathode active material of a secondary battery and a cathode active material slurry containing the same, which exhibits a low viscosity in spite of a high solid content and is excellent in dispersion stability for a cathode active material.

Still another object of the present invention is to provide a positive electrode comprising a positive electrode active material layer formed from the positive electrode active material slurry and a secondary battery comprising the same.

In order to solve the above problems, the present invention provides a solvent for coating a cathode active material of a secondary battery comprising N-ethylformamide.

The present invention also provides a cathode active material slurry comprising a cathode active material, a binder and N-ethyl formamide.

In the cathode active material slurry according to an embodiment of the present invention, the cathode active material may be represented by the following general formula (1).

[Chemical Formula 1]

Li _{1 + x} [Ni _a Co _b Mn _c ] O ₂

A-b, c? 1, x + a + b + c = 1 in the formula (1)

In the cathode active material slurry according to an embodiment of the present invention, the binder may be selected from a fluorine-based binder and a rubber-based binder.

In the cathode active material slurry according to an embodiment of the present invention, the cathode active material slurry may have a solid content of 30 to 80% by weight based on the total weight of the cathode active material slurry.

In the cathode active material slurry according to an embodiment of the present invention, the cathode active material may be 55 to 99 wt% based on the total solid content of the cathode active material slurry.

In the cathode active material slurry according to an embodiment of the present invention, the cathode active material slurry may further include a conductive material.

In the cathode active material slurry according to an embodiment of the present invention, the conductive material may be selected from a carbon-based material.

The present invention also provides a positive electrode comprising a positive electrode active material layer formed from the positive electrode active material slurry.

Also, the present invention may include the anode, the cathode, and a separator between the anode and the cathode, wherein the anode includes a cathode active material layer having a thickness of 10 to 100 탆 on one surface of the cathode current collector.

In the secondary battery according to an embodiment of the present invention, the secondary battery may have an initial discharge capacity of 200 mAh / g or more and a capacity retention rate of 50% or more at 90% or more.

In the secondary battery according to an embodiment of the present invention, the secondary battery may have a C-rate efficiency of 90% or more as expressed by Equation 1 below.

[Formula 1]

C-rate (%) = [(2C discharge capacity) / (0.1C discharge capacity)] x 100

According to the present invention, it is possible to realize a high loading content for a cathode active material with a low viscosity property even at a high solid content, and excellent dispersibility upon mixing with a cathode active material, The image can be maintained.

Further, according to the present invention, a cathode active material and a cathode including the cathode active material can be provided in an environmentally friendly manner since an organic solvent (for example, NMP or the like) having a high environmental hazard is not used.

Further, according to the present invention, it is possible to provide a secondary battery having improved capacity characteristics and life characteristics. Particularly, according to the present invention, a high initial discharge capacity, as well as a high discharge capacity even after 50 cycles, show a remarkably improved capacity retention rate.

BRIEF DESCRIPTION OF THE DRAWINGS The advantages and features of the present invention, and the manner of achieving them, will be apparent from and elucidated with reference to the embodiments described hereinafter in conjunction with the accompanying drawings. It should be understood, however, that the invention is not limited to the disclosed embodiments, but is capable of many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, To fully disclose the scope of the invention to those skilled in the art, and the invention is only defined by the scope of the claims. Hereinafter, a solvent for coating a cathode active material of a secondary battery according to the present invention, a cathode active material slurry containing the cathode active material slurry, and a secondary battery produced therefrom will be described in detail.

The present invention provides a solvent for coating a cathode active material of a secondary battery comprising N-ethylformamide. The coating solvent according to the present invention is characterized by exhibiting improved dispersibility upon mixing with the cathode active material. Although the exact reason is unknown, it is expected that the N-ethyl formamide according to the present invention is due to the interaction with the cathode active material. These interactions are judged to have a temporary or long-term effect on hydrogen bonding, the nature of physical entanglement due to covalent and non-covalent interactions, including hydrophobic interactions.

This phenomenon is a remarkable synergistic effect due to the inclusion of N-ethylformamide, and it is confirmed at all in the case of a formamide compound having a structural characteristic similar to that of N-ethylformamide (for example, formamide, N-methylformamide, etc.) It does not mean that it does not.

In addition, since the solvent for coating the cathode active material of the secondary battery according to the present invention does not use a harmful organic solvent such as N-methyl-2-pyrrolidone (NMP) which is conventionally used, the cathode active material layer And an electrode including the same.

The present invention also provides a slurry for a cathode active material comprising N-ethylformamide.

Specifically, the cathode active material slurry may include a cathode active material, a binder, and N-ethyl formamide.

The slurry for a cathode active material according to an embodiment of the present invention may include a cathode active material represented by the following formula (1).

[Chemical Formula 1]

Li _{1 + x} [Ni _a Co _b Mn _c ] O ₂

A-b, c? 1, x + a + b + c = 1 in the formula (1)

The slurry of the cathode active material according to an embodiment of the present invention not only can effectively suppress the capacity reduction of the cathode active material but also increase the content of residual lithium and prevent problems such as cell swelling due to generation of gas, LiCoO ₂ , LiNiO ₂ , LiMnO _2, and the like.

The cathode active material slurry according to an embodiment of the present invention may further include a cathode active material. For example, additional cathode active material is _{LiFePO 4, LiFeMnPO 4, LiFeMgPO 4} , LiFeNiPO 4, LiFeAlPO 4 and LiFeCoNiMnPO ₄ Or the like, but is not limited thereto.

In the cathode active material slurry according to one embodiment of the present invention, the binder is a component that assists in bonding of the metal oxide coating layer formed on the surface of the cathode active material or assists in bonding with an additional conductive material or the like, The material to be used is not limited, but may be one or more selected from among fluorine-based binders and rubber-based binders.

For example, the fluorine-based binder may be selected from the group consisting of polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HEP), chlorotrifluoroethylene (CFTF), and polytetrafluoroethylene PTFE) and the like.

Examples of the rubber binder include styrene butadiene rubber (SBR), butadiene rubber (BR), nitrile butadiene rubber (NBR), and isoprene rubber (IR).

Also, in the cathode active material slurry according to an embodiment of the present invention, the binder may be an acrylic binder such as polyacrylonitrile, polymethyl methacrylate, or polyacrylic acid; Polyvinyl alcohol, polyvinylpyrrolidone, polyethylene, polypropylene, and olefinic binders; And cellulose-based binders such as carboxymethylcellulose, hydroxypropylmethylcellulose, and regenerated cellulose; And < RTI ID = 0.0 > and / or < / RTI >

The slurry of the cathode active material according to an embodiment of the present invention may have a solid content of 30 to 80% by weight based on the total weight of the slurry of the cathode active material. At this time, the balance is N-ethylformamide.

As described above, the positive electrode active material slurry according to the present invention employs N-ethylformamide as a dispersion medium, that is, a solvent for coating a positive electrode active material, thereby achieving a low viscosity and remarkably increasing its dispersion stability .

The slurry of the cathode active material according to an embodiment of the present invention may have a solid content of 30 to 70 wt%, more specifically 40 to 60 wt%, based on the total weight of the slurry of the cathode active material .

The slurry for a cathode active material according to an embodiment of the present invention may have a viscosity of 5,000 to 30,000 g / cm · s (measured at 20 rpm using a Brookfield rotary viscometer at 23 ° C), specifically 8,000 To 30,000 g / cm · s, more specifically 10,000 to 25,000 g / cm · s. The above-mentioned range of viscosity corresponds to a low viscosity of 30 to 60% as compared with the case of using a conventional NMP or the like as a solvent.

For example, a cathode active material slurry having a solid content of 45 wt% (cathode active material: binder = 98: 2 (wt: wt)) may have a viscosity of 11,000 to 20,000 g / cm · s. At this time, the viscosity change rate (viscosity / initial viscosity after storage at high temperature for one day × 100, 45 ° C for high temperature storage) of the slurry of the cathode active material ranges from 1 to 5%.

For example, the cathode active material slurry having a solid content of 50 wt% can be a cathode active material: binder = 98: 2 (wt: wt) having a viscosity of 15,000 to 23,000 g / cm · s. At this time, the rate of change in viscosity of the positive electrode active material slurry (viscosity / initial viscosity after 100 days of high temperature storage × 100) ranges from 1 to 5%.

In the cathode active material slurry according to an embodiment of the present invention, the cathode active material may include 55 to 99% by weight, specifically 60 to 99% by weight, based on the total solid content of the cathode active material slurry Specifically 65 to 99% by weight. At this time, the remaining amount may be a binder.

The positive electrode active material slurry according to one embodiment of the present invention has a stable dispersed phase even at a high solid content as described above, so that there is no fear of property change (e.g., viscosity) despite relatively low viscosity. As described above, the slurry of the cathode active material that forms a stable dispersion phase can maintain a very stable phase even during use or storage of organ, and can provide process advantages.

The cathode active material slurry according to an embodiment of the present invention may further include a conductive material. The conductive material is not particularly limited as long as it has a surface roughness of the cathode active material layer and is conductive and provides electrical conductivity without causing chemical changes in the battery. Examples thereof include graphite such as natural graphite and artificial graphite; Carbon black such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, and summer black; And carbon fibers; And the like.

The conductive material may further include an additional conductive material in the above-described carbon-based material. For example, metal-based materials such as copper, nickel, aluminum, silver, zinc, and titanium; A fibrous material of the metal; Conductive polymers such as polyaniline, polyacetylene, polypyrrole, and polythiophenone; But is not limited thereto.

At this time, the amount of the conductive material to be used is not limited, but may be 0.01 to 10% by weight based on the total solid content of the cathode active material slurry. At this time, in view of realizing improved capacity characteristics, it may include, but is not limited to, 0.1 to 8 wt%, more specifically 0.5 to 5 wt%.

The present invention provides a positive electrode comprising a positive electrode active material layer formed from the positive electrode active material slurry described above.

The positive electrode according to an embodiment of the present invention may include a positive electrode active material layer formed in a uniform composition from the positive electrode active material slurry. The cathode active material layer may be formed at a high density. That is, in the case of a secondary battery employing the positive electrode comprising the positive electrode active material layer according to the present invention, the mobility of lithium ions in the electrolyte can be remarkably improved with improved ionic conductivity. Thus, it is possible to realize an improved battery characteristic even when the amount of the additional conductive material is reduced.

The anode according to one embodiment of the present invention can be manufactured by the following method.

Specifically, a binder and a cathode active material are added to N-ethylformamide to prepare a cathode active material slurry; And applying and drying the cathode active material slurry to the cathode current collector.

The cathode current collector is not particularly limited as long as it has high conductivity without causing chemical change in the secondary battery. Examples of the cathode current collector include copper, stainless steel, aluminum, nickel, titanium, sintered carbon, aluminum, Carbon, nickel, titanium or silver may be used. At this time, the cathode current collector may have a thickness of 3 to 500 탆.

The application is not particularly limited and can be carried out by a method commonly known in the art.

For example, the application may be performed by spraying or distributing the slurry of the cathode active material on at least one surface of the cathode current collector, and then uniformly dispersing the slurry using a doctor blade or the like.

For example, the application may be performed by a method such as die casting, comma coating, screen printing, or the like.

The drying may be a step of forming a cathode active material layer and removing moisture remaining in the process. At this time, the drying may be performed in a temperature range to remove residual moisture, specifically, 50 to 200 ° C, more specifically, 80 to 200 ° C.

In this case, since the drying time is different depending on the temperature, it can not be uniformly determined, but it can be performed for 1 hour or more, preferably 5 to 24 hours, more preferably 5 to 12 hours .

The cathode active material layer produced according to the above-described method may be formed on at least one surface of the cathode current collector to have a thickness of 10 to 100 탆. Specifically, it may be formed to a thickness of 10 to 80 탆, more specifically 10 to 50 탆.

According to an embodiment of the present invention, a cathode including a cathode active material layer having a uniformly reduced loading deviation can be provided in a very simple process, as well as a high density cathode active material layer. Thus, the secondary battery employing the positive electrode according to the present invention significantly improves the mobility of lithium ions in the electrolyte with improved ionic conductivity, and can exhibit excellent charge / discharge characteristics and life characteristics.

The present invention also provides a secondary battery comprising the positive electrode.

Specifically, a separator between the anode and the cathode according to an embodiment of the present invention includes a cathode, a cathode, and a separator between the anode and the cathode, wherein the cathode has a cathode active material layer formed on one surface of the cathode current collector, Battery.

By employing the positive electrode according to the present invention, the lithium secondary battery imparts improved lithium ion mobility in the electrolyte and exhibits excellent charge / discharge characteristics and life characteristics.

The secondary battery according to an embodiment of the present invention may have an initial discharge capacity of 200 mAh / g or more. Specifically, the initial discharge capacity may be 200 to 300 mAh / g, more specifically 200 to 250 mAh / g.

Particularly, the secondary battery according to the embodiment of the present invention minimizes the change of the discharge capacity even after using 50 cycles (50 cycles). Specifically, the secondary battery may have a capacity retention rate of 90% or more at 50 cycles.

For example, the secondary battery may have an initial discharge capacity of 200 mAh / g or more and a capacity retention rate at 50 cycles of 90 to 99%.

For example, the secondary battery may have an initial discharge capacity of 200 mAh / g or more, and the secondary battery may have a capacity retention rate of 95 to 99% at 50 cycles.

In addition, the secondary battery according to an embodiment of the present invention may have a C-rate efficiency of 90% or more as represented by the following formula (1).

[Formula 1]

C-rate (%) = [(2C discharge capacity) / (0.1C discharge capacity)] x 100

For example, the secondary battery may have an initial discharge capacity of 200 mAh / g or more, a capacity retention rate of 90 to 99% at 50 cycles, and a C-rate efficiency of 90 to 99%.

For example, the secondary battery may have an initial discharge capacity of 200 mAh / g or more, the capacity of the secondary battery may be 95 to 99% at 50 cycles, and the C-rate efficiency may be 90 to 95%.

Hereinafter, a lithium secondary battery according to an embodiment of the present invention will be described.

The lithium secondary battery according to the present invention may include a cathode, a cathode, and a separator.

The positive electrode may include a positive electrode active material layer formed on at least one surface of the positive electrode current collector from the positive electrode active material slurry described above. At this time, the lithium secondary battery according to the present invention can exhibit excellent charge / discharge characteristics and life characteristics as described above by including the cathode active material layer.

The negative electrode may be manufactured by applying a negative electrode active material slurry solution containing a negative electrode active material on a negative electrode collector, followed by drying. At this time, the negative electrode active material slurry solution may contain the following components as needed. The negative electrode active material is not limited as long as it is a commonly used material, and examples thereof include carbon such as hard graphitized carbon and graphite carbon; _{Li x Fe 2 O 3 (0≤x≤1} ), Li x WO 2 (0≤x≤1), Sn x Me 1 - x Me 'y O z (Me: Mn, Fe, Pb, Ge; Me' : Metal complex oxides such as Al, B, P, Si, Group 1, Group 2, Group 3 elements of the periodic table, Halogen, 0 < x &lt; Lithium metal; Lithium alloy; Silicon-based alloys; Tin alloy; _{SnO, SnO 2, PbO, PbO} 2, Pb 2 O 3, Pb 3 O 4, Sb 2 O 3, Sb 2 O 4, Sb 2 O 5, GeO, GeO 2, Bi 2 O 3, Bi 2 O 4, and Bi ₂ O ₅ ; Conductive polymers such as polyacetylene; Li-Co-Ni-based metal complex; And the like. The negative electrode current collector is not particularly limited as long as it has high conductivity without causing chemical changes in the secondary battery. Examples of the current collector include copper, stainless steel, aluminum, nickel, titanium, sintered carbon, copper or stainless steel Surface-treated with carbon, nickel, titanium or silver, and aluminum-cadmium alloy. The negative electrode current collector may be formed in various forms such as a film, a sheet, a foil, a net, a porous body, a foam, a nonwoven fabric, or the like by forming minute irregularities on the surface in the same manner as the positive electrode current collector, Of course it is. At this time, the anode current collector may have a thickness of 3 to 500 탆.

The separation membrane is interposed between the anode and the cathode, and an insulating thin film having high ion permeability and mechanical strength is used. For example, the separation membrane may have a pore diameter of the separation membrane of 0.01 to 10 mu m and a thickness of 5 to 300 mu m. Such a separation membrane is not limited as long as it is a commonly used material, and for example, an olefin-based polymer such as polypropylene which is chemically resistant and hydrophobic; A sheet or nonwoven fabric made of glass fiber, polyethylene or the like; And the like. Further, when a solid electrolyte such as a polymer is used as the electrolyte, the solid electrolyte may also serve as a separation membrane.

The lithium secondary battery may further include an electrolyte.

The electrolytic solution may be a non-aqueous electrolytic solution containing a lithium salt. For example, the lithium salt-containing non-aqueous electrolyte may include a non-aqueous organic solvent and a lithium salt. Examples of the non-aqueous organic solvent include N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, But are not limited to, lactone, 1,2-dimethoxyethane, tetrahydroxyfuran, 2-methyltetrahydrofuran, dimethylsulfoxide, 1,3-dioxolane, formamide, dimethylformamide, Nitrile, nitromethane, methyl formate, methyl acetate, phosphoric acid triester, trimethoxymethane, dioxolane derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives , Tetrahydrofuran derivatives, ether, methyl pyrophosphate, ethyl propionate and the like. Here, the non-aqueous organic solvent may be one or two or more kinds of mixed solvents selected from the above-mentioned non-magnetic organic solvents.

The lithium salt is not limited as long as it is a substance that can be dissolved in the non-aqueous organic solvent. For example, LiCl, LiBr, LiI, LiClO ₄ , LiBF ₄ , LiB ₁₀ Cl ₁₀ , LiPF ₆ , LiCF ₃ SO ₃ , LiCF ₃ CO ₂ , LiAsF ₆ , LiSbF ₆ , LiAlCl ₄ , CH ₃ SO ₃ Li, CF ₃ SO ₃ Li, (CF ₃ SO ₂ ) ₂ NLi, chloroborane lithium, lower aliphatic carboxylate lithium, and lithium 4-phenylborate, and LiPF ₆ is the most preferable.

The electrolyte may be an organic solid electrolyte. For example, one or more ionic compounds selected from a polyethylene derivative, a polyethylene oxide derivative, a polypropylene oxide derivative, a phosphate ester polymer, an agitation lysine, a polyester sulfide, a polyvinyl alcohol, And a dissociation group.

The electrolyte solution may be an 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 SiO 4 -LiI-LiOH , Li ₃ PO ₄ -Li ₂ S-SiS _{2, and} other nitrides, halides and sulfates of Li.

The lithium secondary battery may further include additional additives.

As an example, for the purpose of improving charge and discharge characteristics, flame retardancy, and the like, there can be mentioned pyridine, triethylphosphite, triethanolamine, cyclic ether, ethylenediamine, glyme, hexaphosphoric triamide, , A quinone imine dyestuff, an N-substituted oxazolidinone, an N, N-substituted imidazolidine, an ethylene glycol dialkyl ether, an ammonium salt, pyrrole, 2-methoxyethanol, aluminum trichloride and the like can do.

For example, for the purpose of imparting nonflammability, it may further include one or more selected from halogen-containing solvents such as carbon tetrachloride and ethylene trifluoride.

For example, carbon dioxide gas or the like may be further included for the purpose of improving the high-temperature storage characteristics.

The lithium secondary battery according to the present invention can be used not only in a battery cell used as a power source of a small device but also as a unit cell in a middle- or large-sized battery module including a plurality of battery cells.

Further, the battery module according to the present invention provides a battery pack included as a power source of a middle- or large-sized device, and the middle- or large-sized device includes an electric vehicle (EV), a hybrid electric vehicle (HEV) An electric vehicle including a plug-in hybrid electric vehicle (PHEV), a power storage device, and the like, but the present invention is not limited thereto.

Hereinafter, the present invention will be described in more detail by way of examples. However, the following examples are intended to further illustrate the present invention, and the scope of the present invention is not limited by the following examples. It is to be understood that the following embodiments can be suitably modified and changed by those skilled in the art within the scope of the present invention.

Unless otherwise stated in the present invention, temperatures are all in ° C, and unless otherwise defined, the unit of usage of each component is g.

(Assessment Methods)

1. Evaluation of charging and discharging capacity

Charging and discharging (charging at 0.5 C and discharging at 1 C) were carried out at 25 ° C in the voltage range of 3 to 4.5 V using the secondary battery monocell (battery capacity 4.3 mAh) manufactured in the following examples and comparative examples Respectively.

2. C-rate efficiency evaluation

C-rate efficiency was measured in a high temperature environment (45 캜) using the secondary battery monocell (battery capacity 4.3 mAh) prepared in the following examples and comparative examples. The C-rate efficiency was defined as the ratio of the capacity when the secondary battery charged at 0.5 C to the capacity at 0.1 C and the capacity when discharged to 2 C as shown in the following Equation 1.

[Formula 1]

C-rate (%) = [(2C discharge capacity) / (0.1C discharge capacity)] x 100

3. Evaluation of life characteristics

Was defined as the ratio of the discharge capacity after the charge and discharge 50 cycles (50th discharge capacity) and the initial discharge capacity (1st discharge capacity) measured in the charge and discharge capacity evaluation (see the following formula 2).

[Formula 2]

Capacity retention rate (%) = [(discharge capacity after 50 cycles) / (initial discharge capacity)] x 100

(Example 1)

20 g of a surface-coated LiCoO 2 cathode active material, a conductive material (carbon black) and a binder (polyvinylidene fluoride, PVdF) were mixed at a weight ratio of 95: 3: 2 to prepare 20 g, An active material slurry (viscosity: 20,000 g / cm · s, viscosity change ratio: 1%) was prepared. The cathode active material slurry was applied to an aluminum (Al) thin film as a cathode current collector having a thickness of about 20 μm, dried at 130 ° C. for 2 hours, and then rolled to produce a cathode. Lithium metal foil was used as the cathode. LiPF 6 was added to a non-aqueous organic solvent prepared by mixing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) as an electrolyte at a volume ratio of 1: 2 to prepare a 1 M LiPF 6 nonaqueous electrolyte solution. A polymer cell type test rechargeable battery monocell was prepared by including a polyethylene separator (Dornensa, F2OBHE, thickness = 20 mu m) between the anode and the cathode and injecting the non-aqueous electrolyte.

The initial charge / discharge capacity, C-rate efficiency and lifetime characteristics (capacity retention rate) of the secondary battery mono-cells manufactured by the above method were evaluated and shown in Table 1 below.

(Comparative Example 1)

In preparing the positive electrode active material slurry, a positive electrode active material slurry (viscosity: 50,000 g / cm · s, viscosity change rate: 15%) was prepared by using N-methyl-2-pyrrolidone (NMP) . Thereafter, a polymer cell type test secondary cell monocell was prepared in the same manner as in Example 1.

The initial charge / discharge capacity, C-rate efficiency and lifetime characteristics (capacity retention rate) of the secondary battery mono-cells manufactured by the above method were evaluated and shown in Table 1 below.

The secondary battery according to the present invention not only exhibits remarkable charging capacity but also has an initial discharge capacity of 200 mAh / g or more and excellent capacity retention.

Specifically, the secondary battery according to the present invention showed a high initial discharge capacity (203.9 mAh / g) and a C-rate efficiency of 91%. In particular, it was confirmed that the secondary battery according to the present invention had a capacity retention rate of 97%. The high capacity retention rate of the secondary battery according to the present invention corresponds to a remarkable effect of more than 114% as compared with the comparative example.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it is to be understood that the invention is not limited to the disclosed exemplary embodiments, but, on the contrary, And it goes without saying that the invention belongs to the scope of the invention.

Claims (12)

A solvent for coating a cathode active material of a secondary battery, comprising N-ethyl formamide. A cathode active material slurry comprising a cathode active material, a binder and N-ethyl formamide. 3. The method of claim 2,
Wherein the cathode active material is represented by the following formula (1): &lt; EMI ID =
[Chemical Formula 1]
Li _{1 + x} [Ni _a Co _b Mn _c ] O ₂
A-b, c? 1, x + a + b + c = 1 in the formula (1) 3. The method of claim 2,
Wherein the binder is selected from a fluorine-based binder and a rubber-based binder. 3. The method of claim 2,
Based on the total weight of the cathode active material slurry,
Wherein the positive electrode active material slurry has a solid content of 30 to 80 wt%. 6. The method of claim 5,
Based on the total solids content of the cathode active material slurry,
Wherein the positive electrode active material is 55 to 99 wt%. 3. The method of claim 2,
Wherein the cathode active material slurry further comprises a conductive material. 8. The method of claim 7,
Wherein the conductive material is a carbonaceous material. A positive electrode comprising a positive electrode active material layer formed from a positive electrode active material slurry according to any one of claims 2 to 8. A separator between the anode and the cathode of claim 9,
Wherein the positive electrode has a positive electrode active material layer formed on one surface of the positive electrode collector to a thickness of 10 to 100 탆. 11. The method of claim 10,
Wherein the initial discharge capacity is 200 mAh / g or more, and the capacity retention rate in 50 cycles is 90% or more. 11. The method of claim 10,
Wherein the C-rate efficiency represented by the following formula (1) is 90% or more.
[Formula 1]
C-rate (%) = [(2C discharge capacity) / (0.1C discharge capacity)] x 100