KR101769186B1 - Method for preparing positive electrode of lithium secondary battery, positive electrode repared by using the same, and lithium secondary battery comprising the positive electrode - Google Patents

Abstract

The present invention relates to a method for producing a positive electrode for a lithium secondary battery, and a positive electrode and a lithium secondary battery for a lithium secondary battery manufactured using the same, and a method for manufacturing the positive electrode and a lithium secondary battery using the composition for forming a positive electrode active material layer, Wherein the drying step comprises a multistage drying step of performing drying at a heat treatment temperature of 100 占 폚 to 180 占 폚 at least twice, The post-drying step is carried out at a temperature 5 to 10% lower than the heat treatment temperature in the preceding drying step, thereby preventing contamination of the rolling roll by the composition for forming the positive electrode active material layer and preventing the occurrence of defects on the surface of the anode And at the same time, the surface characteristics and the adhesion to the electrode current collector can be increased, and the capacity and output of the battery can be reduced.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a positive electrode for a lithium secondary battery, a positive electrode manufactured using the same, and a lithium secondary battery comprising the positive electrode, and a lithium secondary battery comprising the positive electrode.

The present invention relates to a lithium iron phosphate based cathode active material which is capable of preventing contamination of the rolling roll and improving the capacity and output characteristics of the battery by increasing the surface characteristics of the anode and the adhesion to the anode current collector A method for manufacturing a positive electrode for a lithium secondary battery, a positive electrode for a lithium secondary battery manufactured using the same, and a lithium secondary battery including the positive electrode.

As technology development and demand for mobile devices increase, the demand for secondary batteries as energy sources is rapidly increasing. Among such secondary batteries, lithium secondary batteries having a high energy density and voltage, a long cycle life, and a low self-discharge rate are commercially available and widely used.

Lithium transition metal complex oxides are used as the cathode active material of lithium secondary batteries, and lithium cobalt composite metal oxides of LiCoO ₂ having high action voltage and excellent capacity characteristics are mainly used. However, since LiCoO ₂ has low stability and high cost, it is difficult to mass-produce a lithium secondary battery.

As a material for replacing LiCoO ₂ , lithium manganese composite metal oxide, lithium iron phosphate, and lithium nickel composite metal oxide have been developed. Among them, lithium iron phosphate (LiFePO ₄ ) having an olivinic structure has a high bulk density of 3.6 g / cm 3, generates a high potential of 3.4 V, and the theoretical capacity is as high as about 170 mAh / g. In the initial state, LiFePO ₄ contains one electrochemically undopable Li per each Fe atom, which is a promising material for a cathode active material for a lithium secondary battery. Furthermore, since LiFePO ₄ contains iron, which is rich in resources and low cost, LiFePO ₄ is less expensive than LiCoO ₂ , LiNiO ₂ , or LiMn ₂ O ₄ described above, and has low toxicity, so that the effect on the environment is also small. However, since LiFePO ₄ itself has a high moisture content, much moisture is generated in the heat treatment. Therefore, when producing a positive electrode by using the LiFePO ₄ as a cathode active material, it is important to reduce the moisture content of LiFePO _4.

On the other hand, a cathode active material layer is formed by applying a composition for forming a cathode active material layer on a cathode current collector and drying it to evaporate the solvent present in the composition for forming a cathode active material layer, Is influenced by the content of the solvent in the active material layer and the distribution of the constituent components in the positive electrode active material layer.

Specifically, when the drying rate is excessively high, the cathode active material contained in the composition for forming the cathode active material layer together with the solvent and the binder may move together and be non-uniformly distributed in the cathode active material layer. Particularly, in the case of the binder, it can be concentrated on the upper portion of the positive electrode active material layer without being distributed in a portion close to the positive electrode collector (an interface portion between the positive electrode collector and the positive electrode active material layer). In this case, the adhesion between the positive electrode current collector and the positive electrode active material layer is lowered, so that the separation of the positive electrode may occur. On the other hand, if the drying speed is too slow, the productivity is lowered, and the solvent in the active material layer may remain, which may contaminate the rolled roll surface during the subsequent rolling process. In addition, the active material layer forming components including the active material adhered to the roll surface are transferred again to the electrode surface to cause defective electrode surface. If such electrode surface defects are severe, battery capacity and output characteristics may be degraded.

Korean Patent No. 1003136 (registered on December 15, 2010)

Accordingly, a first problem to be solved by the present invention is to prevent the occurrence of contamination on the rolling roll surface during the electrode rolling process in the production of a positive electrode containing a lithium iron phosphide-based positive electrode active material, And to improve the capacity and output characteristics of the battery by improving the surface characteristics and the adhesion to the positive electrode current collector, and a method for manufacturing the positive electrode for a lithium secondary battery.

A second problem to be solved by the present invention is to provide a cathode which is produced by the above production method and has excellent surface characteristics.

A third object of the present invention is to provide a lithium secondary battery including the positive electrode, a battery module, and a battery pack.

According to an embodiment of the present invention, there is provided a method of forming a cathode active material layer, comprising the steps of: applying a composition for forming a cathode active material layer comprising a lithium iron phosphide-based cathode active material on at least one surface of a cathode current collector; Drying step includes a multistage drying step of performing drying at a heat treatment temperature of 100 占 폚 to 180 占 폚 at least twice, wherein the after-drying step in the multistage drying step is 5 to 10% lower than the heat treatment temperature in the preceding drying step The present invention also provides a process for producing a positive electrode for a lithium secondary battery, which is carried out at a temperature.

According to another embodiment of the present invention, there is provided a positive electrode for a lithium secondary battery produced by the above manufacturing method.

According to another embodiment of the present invention, there is provided a lithium secondary battery including the positive electrode.

Further, according to another embodiment of the present invention, there is provided a battery module including the lithium secondary battery as a unit cell and a battery pack including the same.

Other details of the embodiments of the present invention are included in the following detailed description.

According to the present invention, it is possible to prevent the composition for forming the positive electrode active material layer from being contaminated with the rolling roll at the time of production of the positive electrode containing the lithium iron phosphite-based positive electrode active material. In addition, it is possible to prevent the occurrence of defects on the surface of the anode and to improve the surface characteristics and the adhesion to the electrode current collector, thereby improving the capacity and output of the battery.

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

The terms and words used in the present specification and claims should not be construed as limited to ordinary or dictionary terms and the inventor may appropriately define the concept of the term in order to best describe its invention It should be construed as meaning and concept consistent with the technical idea of the present invention.

The present invention optimizes the drying process for the positive electrode active material layer during the production of the positive electrode containing the lithium iron phosphate positive electrode active material in a multistage manner to prevent the composition for forming the positive electrode active material layer from being contaminated with the rolling roll, It is possible to prevent the occurrence of defects on the surface of the positive electrode and to improve the surface characteristics and the adhesion to the electrode current collector to improve the capacity and output of the battery.

Specifically, a method for manufacturing a positive electrode for a lithium secondary battery according to an embodiment of the present invention includes: coating a composition for forming a positive electrode active material layer containing a lithium iron phosphate positive electrode active material on at least one surface of a positive electrode collector; Wherein the drying step includes a multistage drying step of performing drying at a heat treatment temperature of 100 ° C to 180 ° C at least twice, wherein the post-drying step in the multistage drying step is performed at a temperature of 5 Lt; RTI ID = 0.0 > 10% < / RTI > lower.

In the above production method, the composition for forming the cathode active material layer may be prepared by mixing or dispersing the cathode active material in a solvent.

The cathode active material may specifically include a lithium iron phosphate having a composition represented by the following Formula 1, and more specifically, LiFePO ₄ having an olivine crystal structure.

[Chemical Formula 1]

Li _{1 + a} Fe _{1 - x} M _x (PO _{4 -b} ) X _b

Wherein M includes at least one element selected from the group consisting of Al, Mg, Ni, Co, Mn, Ti, Ga, Cu, V, Nb, Zr, Ce, In, , X is any one or two or more elements selected from the group consisting of F, S and N, and -0.5? A? 0.5, 0? B? 0.1 and 0? X?

Examples of the solvent include dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), acetone ) Or water, and either one of them or a mixture of two or more of them may be used.

The composition for forming the cathode active material layer may further include a binder in addition to the cathode active material.

The binder serves to improve adhesion between the positive electrode active material particles and adhesion between the positive electrode active material and the positive electrode collector. Specific examples include polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, There may be mentioned polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrene-butadiene rubber, fluorine rubber or various copolymers thereof. Can be used. The binder may be contained in an amount of 1 to 30% by weight based on the total weight of the composition for forming a positive electrode active material layer.

The composition for forming the positive electrode active material layer may further include at least one additive such as a conductive material, a filler, or a dispersant.

The conductive material is used for imparting conductivity to the electrode, and can be used without particular limitation as long as it does not cause any chemical change in the battery and has electronic conductivity. Specific examples thereof include graphite such as natural graphite and artificial graphite; Carbon-based materials such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, summer black and carbon fiber; Metal powder or metal fibers such as copper, nickel, aluminum and silver; The conductive whiskers such as zinc oxide whiskers, calcium carbonate whiskers, titanium dioxide whiskers, silicon oxide whiskers, silicon carbide whiskers, aluminum borate whiskers, magnesium borate whiskers, potassium titanate whiskers, silicon nitride whiskers, silicon carbide whiskers, alumina whiskers, (Whisker); Conductive metal oxides such as titanium oxide; And polyphenylene derivatives. These may be used alone or in admixture of two or more. The conductive material may be contained in an amount of 1 to 30% by weight based on the total weight of the composition for forming a cathode active material layer.

The filler is a component that inhibits the expansion of the positive electrode. The filler can determine whether to use the battery if necessary, and is not particularly limited as long as it is a fibrous material without causing chemical changes in the battery. Specific examples of the filler include olefin polymers such as polyethylene polypropylene; Or fibrous materials such as glass fiber and carbon fiber. One of these materials or a mixture of two or more of these materials may be used. The filler may be included in an amount of 0.1 to 10% by weight based on the total weight of the composition for forming a cathode active material layer.

In the above production process, the cathode current collector is not particularly limited as long as it has electrical conductivity without causing a chemical change in the battery, and examples thereof include stainless steel, aluminum, nickel, titanium, sintered carbon, Steel surfaces may be surface treated with carbon, nickel, titanium, silver, or the like.

In addition, the cathode current collector may have a thickness of 3 to 500 탆, and fine irregularities may be formed on the surface of the cathode current collector to increase the adhesion to the cathode active material layer. For example, it can be used in various forms such as a film, a sheet, a foil, a net, a porous body, a foam, and a nonwoven fabric.

In addition, in the above-described production method, the above-described coating process of the positive electrode current collector of the composition for forming a positive electrode active material layer can be performed by a method commonly known in the art, but a doctor blade or the like is used Or even die casting. Comma coating, screen printing, and the like.

In the above manufacturing method, the drying step for the composition for forming the positive electrode active material layer applied to the positive electrode current collector may be carried out by a multistage drying step in which drying at a heat treatment temperature of 100 ° C to 180 ° C is carried out twice have.

The drying process may be performed according to a conventional drying method, and specifically, a heat treatment in the temperature range described above, or a heat treatment method such as hot air injection. If the heat treatment temperature is less than 100 ° C, the drying speed is slowed. As a result, the process time becomes long. Further, since the solvent remains in the cathode active material layer, contamination of the rolled roll surface may become serious during the subsequent rolling process. On the other hand, if the heat treatment temperature during drying exceeds 180 ° C, the drying rate becomes excessively high, so that the cathode active material contained in the composition for forming a cathode active material layer together with the solvent and the binder move to the surface of the cathode active material layer, As a result, the adhesion between the positive electrode collector and the positive electrode active material layer may be deteriorated, resulting in the desorption of the positive electrode.

In the above production method, the subsequent drying step in the multistage drying step may be performed at a temperature 5 to 10% lower than the heat treatment temperature in the preceding drying step.

If the post-drying process does not meet the condition of being performed at a temperature 5 to 10% lower than the heat treatment temperature in the preceding drying process, complete removal of the solvent in the cathode active material layer and reduction of the moisture content in the cathode active material of lithium iron phosphate It is difficult to satisfy the optimum drying rate condition for uniform distribution of constituent components constituting the positive electrode active material layer.

The first drying step in the multistage drying step may be performed at a heat treatment temperature of 160 to 180 ° C. By performing the first drying step at a higher temperature than the conventional one, the evaporation of the solvent in the composition for forming the cathode active material layer is accelerated and at the same time, the water content contained in the cathode active material of lithium iron phosphate is remarkably reduced, Can be prevented.

The multi-stage drying process may be carried out under an air speed condition of 1000 rpm to 2500 rpm. The wind speed in the drying process is a factor affecting the drying speed together with the heat treatment temperature. The faster the wind speed, the faster the drying speed. The slower the wind speed, the slower the drying rate. If the multistage drying process is carried out under the condition of the wind speed less than 1000rpm, the drying process takes a long time. If it exceeds 2500rpm, the uniform distribution of the constituents within the cathode active material layer may be difficult due to too fast drying.

In order to obtain an optimum drying rate in consideration of the effect of the heat treatment temperature and the wind speed on the drying speed, the first drying step in the multistage drying step is preferably performed at a temperature of 160 to 180 DEG C and a wind speed of 1500 rpm to 2500 rpm Under conditions.

In the above manufacturing method, more specifically, the multi-step drying step is a step of drying at least one surface of the composition for forming a cathode active material in a drying zone composed of at least two or more zones, or two to six zones, or two to four zones The positive electrode collector may be continuously applied while passing through the positive electrode collector.

The drying zone may have a length of 1 m to 50 m, a temperature range of 100 ° C to 180 ° C, and an air speed range of 1000 rpm to 2500 rpm, wherein the zones may be set to different conditions within the range. For example, as described above, in each of the drying zones, the drying conditions of the following drying zones are set so that drying can be performed at a heat treatment temperature of 5 to 10% lower than that of the preceding drying zone in the drying heat treatment temperature range .

Also, the passing speed of the drying zone can be adjusted from 3 m / min to 20 m / min, and the drying can be performed for a total of 1 minute to 10 minutes. That is, the drying zone passing speed can be appropriately adjusted so that drying can be performed within the time range according to the entire length of the drying zone.

Also, the first drying step in the first drying zone in the drying zone may be performed at a temperature of 160 ° C to 180 ° C.

For example, when the drying zone is comprised of a first zone, a second zone, a third zone, and a fourth zone, drying in the first zone is performed at a heat treatment temperature of 160 ° C to 180 ° C, Drying in zone 3, zone 3 and zone 4 can be carried out at a temperature which is 5 to 10% lower than the heat treatment temperature in the preceding drying zone, respectively. More specifically, the drying in the second zone is performed at a heat treatment temperature of 150 ° C to 170 ° C, the drying in the third zone is performed at a heat treatment temperature of 130 ° C to 150 ° C, The drying can be carried out at a heat treatment temperature of 120 ° C to 140 ° C.

The drying in the first zone is carried out under the wind speed condition of 1500 rpm to 2500 rpm and the drying in the second zone, the third zone and the fourth zone is carried out in the wind speed range of 1000 rpm to 2000 rpm, Can be carried out under the wind speed condition below the wind speed.

More specifically, the drying in the first zone is performed at a heat treatment temperature of 150 to 170 캜 under an air speed condition of 1500 rpm to 2500 rpm, and the drying in the second zone is performed at a heat treatment temperature of 150 캜 to 170 캜, Under the wind speed condition of 1700 rpm, the drying in the third zone is carried out under the condition of the wind speed of 1500 rpm to 1700 rpm at the heat treatment temperature of 130 캜 to 150 캜 and at the heat treatment temperature of 120 캜 to 140 캜 at the fourth zone, Lt; / RTI > to 1600 rpm.

The multi-stage drying process may be performed until the LEL (lower explosive level) value in the drying zone is 0%.

LEL is a numerical value representing the concentration of the gaseous solvent volatilized in the drying zone when the composition for forming the cathode active material layer is applied to the cathode collector and then the solvent is evaporated. When the LEL is 0%, there is no volatile gaseous solvent in the drying zone. This means that no residual solvent is present in the cathode active material layer.

The manufacturing method may further include a holding step at a high temperature after the multistage drying step.

The high-temperature holding step may be performed within a heat treatment temperature range in the final multistage drying step, and may be performed at 100 to 130 캜. Through the process of maintaining the high temperature in the temperature range as described above, the water content in the lithium iron phosphate can be minimized, the volatile components contained in the process can be sufficiently removed, and a side reaction And deterioration of battery characteristics can be prevented.

The positive electrode prepared by the above-described production method has excellent surface characteristics and can exhibit an improved adhesion to the positive electrode collector due to the uniform distribution of the constituent components in the positive electrode active material layer. As a result, it is possible to improve the capacity and output characteristics of the battery and the life characteristics thereof.

Accordingly, according to another embodiment of the present invention, there is provided a positive electrode manufactured from the above-mentioned production method.

The positive electrode specifically includes a positive electrode collector and a positive electrode active material layer formed on at least one surface of the positive electrode collector.

The anode has a uniform distribution of constituent components in the cathode active material layer due to its unique manufacturing process. In particular, when the cathode contains a binder, it can exhibit an excellent adhesion of 7 gf / 10 mm or more to the cathode current collector due to uniform distribution of the binder.

According to another embodiment of the present invention, there is provided a lithium secondary battery including the anode.

The lithium secondary battery includes a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and a nonaqueous electrolyte, and the positive electrode is as described above.

In the lithium secondary battery, the negative electrode may be prepared by preparing a composition for forming a negative electrode including, for example, a negative electrode active material and optionally additives such as a binder, a conductive material, a filler and a dispersant on the negative electrode current collector, And the like.

At this time, the negative electrode active material is not particularly limited, and a compound capable of reversibly intercalating and deintercalating lithium may be used. Specific examples thereof include carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber, amorphous carbon and highly crystalline carbon; Metal compounds capable of alloying with lithium such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloys, Sn alloys or Al alloys; Or a composite containing a metallic compound and a carbonaceous material, and the like. Examples of low crystalline carbon include soft carbon and hard carbon. Examples of the highly crystalline carbon include natural graphite, kish graphite, pyrolytic carbon, High temperature calcined carbon such as mesophase pitch based carbon fiber, meso-carbon microbeads, mesophase pitches and petroleum or coal tar pitch derived cokes . One or a mixture of two or more of them may be used, and a metal lithium thin film may be used as the negative active material.

The additives such as the binder, the conductive material, the filler, and the dispersant may be the same as those described above for the positive electrode.

The negative electrode current collector is not particularly limited as long as it has high conductivity without causing chemical changes in the battery. Examples of the negative electrode current collector include copper, stainless steel, aluminum, nickel, titanium, sintered carbon, copper or stainless steel Surface-treated with carbon, nickel, titanium, silver or the like, aluminum-cadmium alloy, or the like can be used.

The negative electrode current collector may have a thickness of 3 to 500 탆, and similarly to the positive electrode current collector, fine unevenness may be formed on the surface of the positive electrode current collector to enhance the binding force of the negative electrode active material. For example, it can be used in various forms such as a film, a sheet, a foil, a net, a porous body, a foam, and a nonwoven fabric.

Meanwhile, in the lithium secondary battery, the separator is not particularly limited as long as it is used as a separator in a lithium secondary battery. In particular, it is preferable that the separator is low in resistance against ion movement of the electrolyte and excellent in electrolyte wettability. Specifically, porous polymer films such as porous polymer films made of polyolefin-based polymers such as ethylene homopolymers, propylene homopolymers, ethylene / butene copolymers, ethylene / hexene copolymers and ethylene / methacrylate copolymers, May be used. Further, a nonwoven fabric made of a conventional porous nonwoven fabric, for example, glass fiber of high melting point, polyethylene terephthalate fiber, or the like may be used. The separator may be a porous thin film having a pore diameter of 0.01 to 10 μm and a thickness of 5 to 300 μm.

In addition, the electrolyte may include an organic solvent and a lithium salt commonly used in an electrolyte, and is not particularly limited.

The organic solvent may be used without limitation as long as it can act as a medium through which ions involved in the electrochemical reaction of the battery can move. Specifically, examples of the organic solvent include ester solvents such as methyl acetate, ethyl acetate,? -Butyrolactone and? -Caprolactone; Ether solvents such as dibutyl ether or tetrahydrofuran; Ketone solvents such as cyclohexanone; Aromatic hydrocarbon solvents such as benzene and fluorobenzene; Dimethyl carbonate (DMC), diethylcarbonate (DEC), methylethylcarbonate (MEC), ethylmethylcarbonate (EMC), ethylene carbonate (EC), propylene carbonate PC) and the like can be used.

Among these, a carbonate-based solvent is preferable, and a cyclic carbonate (for example, ethylene carbonate or propylene carbonate) having a high ionic conductivity and a high dielectric constant, for example, such as ethylene carbonate or propylene carbonate, For example, ethyl methyl carbonate, dimethyl carbonate or diethyl carbonate) is more preferable.

The lithium salt can be used without particular limitation as long as it is a compound capable of providing lithium ions used in a lithium secondary battery. Specifically, the lithium _{salt, LiPF 6, LiClO 4, LiAsF} 6, LiBF 4, LiSbF 6, LiAl0 4, LiAlCl 4, LiCF 3 SO 3, LiC 4 F 9 SO 3, LiN (C 2 F 5 SO 3) 2 _{, LiN (C 2 F 5 SO} 2) 2, LiN (CF 3 SO 2) 2. LiCl, LiI, or LiB (C ₂ O ₄ ) ₂ may be used. The lithium salt is preferably contained in the electrolyte at a concentration of about 0.6 mol% to 2 mol%.

The electrolytes include, for example, pyridine, triethylphosphite, triethanolamine, cyclic ether, ethylenediamine, n-propylamine, and the like for the purpose of improving lifetime characteristics of the battery, N, N-substituted imidazolidine, ethylene glycol dialkyl ethers, ammonium salts, pyrroles, 2-ethylhexyl glycols, - methoxyethanol or aluminum trichloride may be further included. The additive may be included in an amount of 0.1 to 5% by weight based on the total weight of the electrolyte.

The lithium secondary battery of the present invention can be manufactured by disposing a separator between an anode and a cathode to form an electrode assembly, inserting the electrode assembly into a cylindrical battery case or a prismatic battery case, and then injecting an electrolyte. Alternatively, the electrode assembly may be laminated, impregnated with the electrolyte, and the resultant may be sealed in a battery case.

The battery case may be of any type conventionally used in the art, and is not limited to the outer shape depending on the use of the battery. For example, a cylindrical shape, a square shape, a pouch shape, or a coin shape And the like.

Since the lithium secondary battery according to the present invention stably exhibits excellent discharge capacity, output characteristics, and capacity retention rate, the lithium rechargeable battery according to the present invention can be applied to portable devices such as mobile phones, notebook computers, digital cameras, and hybrid electric vehicles (HEV) It is useful in the field of electric vehicles.

According to another aspect of the present invention, there is provided a battery module including the lithium secondary battery as a unit cell and a battery pack including the same.

The battery module or the battery pack may include a power tool; An electric vehicle including an electric vehicle (EV), a hybrid electric vehicle, and a plug-in hybrid electric vehicle (PHEV); Or a power storage system, as shown in FIG.

Hereinafter, embodiments of the present invention will be described in detail so that those skilled in the art can easily carry out the present invention. The present invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein.

[ Example  : Preparation of positive electrode]

LiFPO ₄ The cathode active material, the carbon black conductive material and the PVdF binder were mixed in a N-methylpyrrolidone solvent in a weight ratio of 90: 5: 5 to prepare a composition for forming a positive electrode (viscosity: 1500 mPa.s) The aluminum thin film was coated uniformly so that the thickness of the cathode active material layer in the anode finally prepared was 5 占 퐉. Thereafter, it was passed through a drying zone composed of 6 zones and dried under the same conditions as in Table 1 below.

Treatment condition Zone One 2 3 4 5 6 Zone Length (m) 3 3 3 3 3 3 Temperature (℃) 170 160 140 130 120 120 Wind speed (rpm) 1800 1550 1550 1500 1500 1500 Passing speed (m / min) 6 6 6 6 6 6 LEL (%, top) 37 47 10 0 0 0 LEL (%, back) 31 46 14 0 0 0

As shown in Table 1, it can be seen from the LEL values measured in each drying zone that the drying process for the composition for forming the cathode active material layer in the production of the positive electrode according to the embodiment is completed in the third drying zone, In the drying zone after the third drying zone, the high temperature maintenance process was performed.

Thereafter, the dried cathode mixture was rolled through a rolling roll to prepare a cathode.

[ Comparative Example : Preparation of positive electrode]

Except that the drying process was performed under the conditions shown in Table 2 below. ≪ tb > < TABLE >

Treatment condition Zone One 2 3 4 5 6 Zone Length (m) 3 3 3 3 3 3 Temperature (℃) 100 100 100 100 100 100 Wind speed (rpm) 1300 1300 1300 1400 1400 1400 Passing speed (m / min) 6 6 6 6 6 6 LEL (%, top) 10 5 17 20 0 0 LEL (%, back) 8 3 18 21 0 0

As shown in Table 2, from the LEL values measured in each drying zone, it can be seen that the drying process for the composition for forming the cathode active material layer in the production of the positive electrode according to the comparative example was completed in the fourth drying zone, In the drying zone after the fourth drying zone, a high-temperature holding step was performed.

[ Experimental Example  1: Rolling roll contamination evaluation]

In the anode manufacturing process of the Examples and Comparative Examples, contamination of rolling rolls, especially the main roll, was observed during the rolling process.

In this case, since the degree of contamination of the main roll is difficult to be visually recognized, the colorimeter was measured in contact with the main roll contamination site (three portions of the left, middle and right sides of the main roll) The brightness, chroma, and hue of the surface were expressed by numerals, and the degree of contamination of the main roll was evaluated based on the brightness value indicating the degree of lightness and darkness among these values. At this time, the lower the number, the darker it means that the pollution degree of the mail roll is high. The results are shown in Table 3 below.

Color difference value Comparative Example Example Left side Central part Right side Left side Central part Right side The initial (om) 79.45 78.87 79.45 79.83 79.39 79.54 30m 71.33
(10.2% ↓) 72.18
(8.5% ↓) 71.27
(10.3% ↓) 78.05
(2.2% ↓) 78.47
(1.2% ↓) 77.99
(1.9% ↓) 60m 68.17
(14.2% ↓) 68.26
(13.5% ↓) 69.01
(13.1% ↓) 76.47
(4.2% ↓) 77.18
(2.8% ↓) 76.53
(3.8% ↓) 90m 64.19
(19.2% ↓) 63.52
(13.1% ↓) 64.71
(18.6% ↓) 77.09
(3.4% ↓) 78.07
(1.7% ↓) 77.49
(2.6% ↓) 120m - - - 77.77
(2.6% ↓) 76.75
(3.3% ↓) 76.88
(3.3% ↓) 145m - - - 78.04
(2.2% ↓) 76.91
(3.1% ↓) 77.03
(3.2% ↓)

("-" in Table 3 means not measured).

As a result of the experiment, contamination of the main roll occurred as the rolling process of both the positive electrode and the negative electrode of the comparative example and the working example proceeded. However, when compared at the same point in time, the example showed significantly reduced roll contamination compared to the comparative example, and the roll contamination degree was further reduced as the rolling process was longer than 90 m.

[ Experimental Example  2: Evaluation of adhesion strength]

The adhesion between the positive electrode active material layer and the positive electrode collector was compared with the positive electrode prepared in the above Examples and Comparative Examples.

The adhesive force was measured by punching the positive electrode prepared in the above examples and comparative examples by TOP / BACK, Lane, left / middle / right point in the same lane using an electrode rudder (15 cm x 1 cm) And adhered to a slide glass having a double-sided tape to prepare samples. For the next sample, the surface of the rubbed electrode was evenly adhered to the double-sided tape using a roller of 2 kg load 13 to 15 times. Thereafter, a sample prepared in the grip of Universal Testing Machine (UTM) (LF Plus, manufactured by LLOYD) was mounted as an electrode adhesive force measuring instrument, and then a load cell of 5 N (1 lbf) was applied The adhesive strength was measured. The results are shown in Table 4 below.

Comparative Example Example Adhesion (gf / 10mm) 6.1 7.4

As a result of the test, the positive electrode of the example exhibited a remarkably high adhesion force as compared with the comparative example. This is because, in the comparative example, since the binder moves to the surface of the positive electrode active material layer together with the solvent, the adhesive force between the positive electrode active material layer and the positive electrode current collector is lowered while the positive electrode of the embodiment is uniformly distributed in the positive electrode active material layer , And the content of the binder existing between the positive electrode active material layer and the positive electrode current collector is increased.

[ Manufacturing example : Lithium secondary battery of  Produce]

A lithium secondary battery was prepared using the positive electrode prepared in the above example.

Specifically, mesogens such as MCMB (mesocarbon microbead), carbon black conductive material and PVdF binder were mixed in a N-methylpyrrolidone solvent at a weight ratio of 85: 10: 5 as an anode active material to prepare an anode forming composition And this was applied to the entire copper collector to prepare a negative electrode.

A lithium secondary battery was prepared by preparing an electrode assembly between the positive electrode and the negative electrode prepared in the above example through a separator of porous polyethylene, placing the electrode assembly in the case, and injecting an electrolyte into the case. The electrolyte solution was prepared by dissolving lithium hexafluorophosphate (LiPF ₆ ) at a concentration of 1.15 M in an organic solvent composed of ethylene carbonate / dimethyl carbonate / ethyl methyl carbonate (mixed volume ratio of EC / EMC / DEC = 3/4/3) Respectively.

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, And falls within the scope of the invention.

Claims (20)

Applying a composition for forming a cathode active material layer containing a lithium iron phosphide-based cathode active material onto at least one surface of a cathode current collector, and drying the composition;
Wherein the drying includes a multistage drying step of performing drying at a heat treatment temperature of 100 占 폚 to 180 占 폚 at least twice,
The subsequent drying step in the multistage drying step is performed at a temperature 5 to 10% lower than the heat treatment temperature in the preceding drying step,
Wherein the air velocity in the subsequent drying step is equal to or lower than the air velocity in the preceding drying step.
The method according to claim 1,
Wherein the lithium iron phosphate is a compound represented by the following formula (1).
[Chemical Formula 1]
Li _{1 + a} Fe _{1 - x} M _x (PO _{4 -b} ) X _b
Wherein M includes at least one element selected from the group consisting of Al, Mg, Ni, Co, Mn, Ti, Ga, Cu, V, Nb, Zr, Ce, In, , X includes any one or two or more elements selected from the group consisting of F, S, and N, and a, b, and x each represent -0.5? A? 0.5, 0? B? 0.1, 0.5)
The method according to claim 1,
Wherein the lithium iron phosphate is LiFePO ₄ having an olivine crystal structure.
The method according to claim 1,
Wherein the composition for forming the positive electrode active material layer further comprises a binder.
The method according to claim 1,
Wherein the multi-stage drying step is performed under an air speed condition of 1000 rpm to 2500 rpm.
The method according to claim 1,
Wherein the first drying step in the multistage drying step is performed at a heat treatment temperature of 160 ° C to 180 ° C.
The method according to claim 1,
Wherein the first drying step in the multistage drying step is performed at a heat treatment temperature of 160 ° C to 180 ° C and an air speed of 1500rpm to 2500rpm.
The method according to claim 1,
The multi-stage drying process may be carried out continuously by passing the cathode current collector coated with at least one surface of the composition for forming a cathode active material layer through a drying zone composed of two or more zones, and the lower explosive level (LEL) And the drying process is terminated when the value is 0%.
9. The method of claim 8,
Wherein the drying zone comprises a first zone, a second zone, a third zone and a fourth zone,
Drying in the first zone is carried out at a heat treatment temperature of 160 ° C to 180 ° C,
Wherein the drying in the second zone, the third zone and the fourth zone is carried out at a temperature which is 5 to 10% lower than the heat treatment temperature in the preceding drying zone, respectively.
10. The method of claim 9,
Drying in the second zone is carried out at a heat treatment temperature of from 150 캜 to 170 캜,
Drying in the third zone is carried out at a heat treatment temperature of 130 ° C to 150 ° C,
Wherein the drying in the fourth zone is performed at a heat treatment temperature of 120 ° C to 140 ° C.
10. The method of claim 9,
The drying in the first zone is performed under the wind speed condition of 1500 rpm to 2500 rpm,
Wherein the drying in the second zone, the third zone and the fourth zone is carried out under the condition of an air velocity below the wind velocity in the preceding drying zone within the range of the wind velocity of 1000rpm to 2000rpm.
10. The method of claim 9,
The drying in the first zone is carried out under a wind speed condition of 1500 rpm to 2500 rpm at a heat treatment temperature of 150 캜 to 170 캜 while the drying in the second zone is carried out under a wind speed condition of 1500 rpm to 1700 rpm at a heat treatment temperature of 150 캜 to 170 캜, The drying in the third zone is carried out under the wind speed condition of 1500 rpm to 1700 rpm at the heat treatment temperature of 130 캜 to 150 캜 and at the heat treatment temperature of 120 캜 to 140 캜 at the wind speed condition of 1500 rpm to 1600 rpm Wherein the positive electrode is a lithium secondary battery.
10. The method of claim 9,
Further comprising a step of maintaining a high temperature at 100 캜 to 130 캜 after the drying step.
A positive electrode for a lithium secondary battery produced by the manufacturing method according to claim 1.
15. The method of claim 14,
Wherein the anode includes a cathode current collector and a cathode active material layer formed on at least one surface of the cathode current collector,
Wherein the adhesive force between the positive electrode collector and the positive electrode active material layer is 7 gf / 10 mm or more.
A lithium secondary battery comprising a positive electrode according to claim 14.
17. A battery module comprising the lithium secondary battery according to claim 16 as a unit cell.
A battery pack comprising the battery module according to claim 17.
19. The method of claim 18,
A battery pack that is used as a power source for mid- to large-sized devices.
20. The method of claim 19,
Wherein the middle- or large-sized device is selected from the group consisting of an electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle and a system for power storage.