KR20160014461A - Method for preparing positive-electrode comprising a step of drying by adding organic acid and positive-electrode prepared thereby - Google Patents

Abstract

The present invention relates to a method for preparing a positive electrode of a rechargeable lithium battery and the rechargeable lithium battery prepared by using the method. The method includes the steps of: mixing a positive electrode active material including moisture on the surface of particles, a binder, a conductive material, and an organic acid in a solvent to prepare a positive electrode mixture in a slurry form; rolling a positive electrode mixture coating layer after coating the positive electrode mixture on a positive electrode current collector and drying the coated positive electrode mixture (first drying step); and drying an electrode (second drying step) after rolling the coated positive electrode mixture. The method can completely remove the moisture chemically coupled to the surface of the positive electrode active material particles included in the positive electrode in order to prevent the generation of hydrogen fluoride (HF) in the rechargeable lithium battery including the positive electrode. The present invention can also prevent the damage of the positive electrode or an electrode (Al-foil) due to the generated HF, generation of organic substances due to decomposed LiPF_6 salt, an increase in battery resistance, and reduction of the charging and discharging capacity thereof.

Description

A method for preparing a positive electrode comprising the step of adding an organic acid to remove moisture and a positive electrode prepared by the method.

The present invention relates to a method of manufacturing a positive electrode including a step of removing moisture by adding an organic acid and a positive electrode produced thereby.

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.

The conventional lithium secondary battery uses lithium-containing cobalt oxide (LiCoO ₂ ) as a main component of the cathode active material. However, since the lithium-containing cobalt oxide has low stability and high cost, it has been difficult to mass-produce a lithium secondary battery.

In recent years, lithium iron phosphate (LiFePO ₄₎ , which has a ~ 3.5 V voltage versus lithium, a high bulk density of 3.6 g / cm ³ , a theoretical capacity of 170 mAh / g, , Hereinafter referred to as LFP) compound is illuminated as a cathode active material of a lithium secondary battery. However, when the lithium iron phosphate compound has a relatively low electronic conductivity, it has a problem of increasing the internal resistance of the battery when used as a cathode active material. Therefore, in order to improve the conductivity, a conductive material such as carbon (C) Thinly coat on the surface.

In the case of the carbon-coated lithium iron phosphate compound (hereinafter referred to as LFP / C), the conductivity is improved but the amount of water adsorption is increased due to the increase of the specific surface area (BET) Lithium hexafluorophosphate (hereinafter referred to as LiPF ₆ ), which is a salt of the electrolyte due to the adsorbed moisture, is decomposed to produce hydrogen fluoride (HF). The generated HF attacks an anode or an electrode (Al-foil) or attack another LiPF ₆ salt to produce an organic material, which is mainly deposited on a cathode, thereby causing a problem of resistance increase and reduction in charge / discharge capacity.

JP 2013-026149 A

In order to solve the above-described problems, the present invention provides a method for producing a positive electrode, which comprises adding an organic acid together in a slurry-type positive electrode mixture preparation step, And a cathode made by the method.

In one embodiment to achieve the object of the present invention, there is provided a method for producing a cathode active material, comprising the steps of: mixing a cathode active material containing moisture, a binder, a conductive material and an organic acid in a solvent; Coating the positive electrode mixture on the positive electrode collector, drying the first mixture (first drying), and then rolling the positive electrode mixture coating layer; And drying the electrode after the rolling (second drying). The present invention also provides a method of manufacturing a positive electrode of a lithium secondary battery.

In another embodiment of the present invention, a cathode of a lithium secondary battery having a water content of less than 10 ppm of the cathode active material in the anode produced by the above method is provided.

In still another embodiment of the present invention, there is provided a lithium secondary battery comprising a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and a nonaqueous electrolyte solution containing a lithium salt,

According to the present invention, it is possible to completely remove moisture chemically bound to the surface of the cathode active material particles contained in the anode, thereby preventing the generation of hydrogen fluoride (HF) in the lithium secondary battery including the anode.

By preventing the generation of HF, it is possible to prevent various problems such as damage to the anode or the electrode (Al-foil) due to HF in the battery, generation of organic matter by decomposition of LiPF ₆ salt, .

FIG. 1 shows a reaction in which a common organic acid added according to an embodiment of the present invention removes moisture bound to the surface of a cathode active material particle contained in a cathode.
FIG. 2 shows the reaction of removing maleic acid added to the surface of the positive electrode active material particles contained in the positive electrode according to an embodiment of the present invention.
3 is a graph showing moisture increase rate in the atmosphere of a lithium secondary battery according to an embodiment of the present invention.
4 is a graph illustrating a result of analysis of gas generation according to overcharging of a lithium secondary battery according to an embodiment of the present invention.
5 is a graph showing the measurement results of output characteristics (resistance generation) using the HPPC method according to the SOC region of a lithium secondary battery according to an embodiment of the present invention.

Hereinafter, the present invention will be described in more detail.

In one embodiment of the present invention, there is provided a method for producing a positive electrode active material, comprising the steps of: mixing a positive electrode active material, a binder, a conductive material, and an organic acid, Coating the positive electrode mixture on the positive electrode collector, drying the first mixture (first drying), and then rolling the positive electrode mixture coating layer; And drying the electrode after the rolling (second drying). The present invention also provides a method of manufacturing a positive electrode of a lithium secondary battery. The terms "first drying" and "second drying"

The cathode active material may be a cathode active material generally used for a lithium secondary battery, for example, a lithium iron phosphate compound (LFP) or a carbon-coated lithium iron phosphate compound (LFP / C). On the other hand, the cathode active material may contain moisture on the particle surface.

The cathode active material may include at least one selected from the group consisting of individual particles (hereinafter referred to as primary particles) and secondary particles formed by aggregating two or more of the primary particles. That is, the cathode active material may be composed of primary particles, or secondary particles or a mixture of primary particles and secondary particles, depending on the production method. The secondary particles may have a porous particulate form.

Meanwhile, the cathode active material may be coated with carbon or a conductive material on the surface of the particles.

The average particle diameter (D ₅₀ ) of the primary particles is not particularly limited, but may be, for example, 50 nm to 500 nm. The average particle diameter (D ₅₀ ) of the secondary particles is not particularly limited, 15 mu m.

The average particle diameter (D ₅₀₎ is intended to means the particle size at 50% based on the particle size distribution, average particle diameter of the particles in accordance with one embodiment of the present invention (D _50), for example, laser diffraction method (laser diffraction method) . ≪ / RTI > The laser diffraction method generally enables measurement of a particle diameter of several millimeters in a submicron region, resulting in high reproducibility and high degradability.

The cathode active material contains moisture on the surface of the particles, and the water (water molecule (H ₂ O)) may not be completely removed during the production of the cathode active material, or may be absorbed from the air. The water molecules may be attached to the particle surface by Van der Waals forces, electrostatic attraction, surface tension or other forces. When the cathode active material is coated with carbon or a conductive material, the conductivity is improved but the amount of water adsorption is increased due to an increase in specific surface area (BET).

When the cathode active material is a lithium iron phosphate compound, the lithium iron phosphate compound may be one prepared by a solid phase method or a liquid phase method such as a hydrothermal synthesis method and a supercritical system method. Since water is used in the liquid phase method, moisture may remain in the lithium iron phosphate compound particles even if it is subjected to drying and calcination in the manufacturing process. Thus, the cathode active material of the present invention may be, for example, a lithium iron phosphate compound produced by the hydrothermal synthesis method or the supercritical system.

The binder is not particularly limited as long as it is a component that assists in bonding between the active material and the conductive material and bonding to the current collector. Examples of the binder include vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP) Polyvinylidene fluoride, polyacrylonitrile, polymethylmethacrylate, polyvinylalcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinyl (S) selected from the group consisting of ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene butadiene rubber (SBR) Lt; / RTI >

The binder may typically comprise from 1% to 30% by weight, based on the total weight of the mixture comprising the cathode active material.

The conductive material is not particularly limited as long as it has electrical conductivity without causing chemical changes in the battery, for example, graphite such as natural graphite or artificial graphite; Carbon black such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, and summer black; Conductive fibers such as carbon fiber and metal fiber; Metal powders such as carbon fluoride, aluminum, and nickel powder; Conductive whiskey such as zinc oxide and potassium titanate; Conductive metal oxides such as titanium oxide; Conductive materials such as polyphenylene derivatives and the like can be used.

The conductive material may typically comprise from 1% to 30% by weight based on the total weight of the mixture including the cathode active material.

Meanwhile, in one embodiment of the present invention, the organic acid to be added together with the cathode active material, the binder and the conductive material is continuously present as an organic acid in a cathode mix prepared in the form of a slurry, and in the second drying step after rolling, So that water molecules contained in the surface of the positive electrode active material particles contained in the positive electrode can be removed.

In one embodiment of the present invention, the organic acid may be maleic acid, glycolic acid, acetic anhydride or phthalic acid.

By controlling the amount of the added organic acid, the viscosity of the slurry can be controlled. For example, when the viscosity is low, the viscosity can be lowered by adding a small amount of an anhydride-form organic acid such as acetic anhydride. On the contrary, when the viscosity is low, a small amount of an organic acid such as maleic acid or glycolic acid is added to increase viscosity .

The prepared positive electrode mixture is coated on the positive electrode current collector, dried (first dried), and then the positive electrode active material coating layer is rolled.

The method used in the coating and drying (first drying) process, the coating layer rolling step and the like is not particularly limited and can be carried out by a method commonly used in the art.

The positive electrode current collector is not particularly limited as long as it has conductivity without causing chemical changes in the battery. For example, carbon, nickel, titanium, , Silver or the like may be used.

On the other hand, the current collector may have a thickness of 3 to 500 탆, and fine unevenness may be formed on the surface of the current collector to increase the adhesion of the positive 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.

The electrode subjected to the rolling step is dried again (second drying). The second drying step may be, for example, vacuum drying for 3 to 4 hours at a temperature in the range of 200 ° C to 300 ° C. In the second drying step, moisture (H ₂ O) is removed from the organic acid.

In one embodiment of the present invention, the moisture of the organic acid is removed through the second drying step to become an anhydride. The anhydride can react with water molecules contained in the surface of the cathode active material particles in the anode as shown in FIG. 1 (see FIG. 1).

Referring to FIG. 1, in the second drying step, 2 moles of the organic acid added in the step of preparing the cathode mixture is removed from water to become an anhydride. The anhydride is reacted with 1 mole of water molecules contained in the cathode active material particle surface To form two moles of organic acid, thereby removing water on the surface of the positive electrode active material particles (see Fig. 1).

Meanwhile, in an embodiment of the present invention, when maleic acid is used as the organic acid, the maleic anhydride is converted into maleic anhydride in which maleic anhydride is removed through the second drying step, and the maleic anhydride is added to the surface of the cathode active material particle And reacts with water molecules contained therein to become maleic anhydride again, thereby removing water on the surface of the cathode active material particles (see FIG. 2).

In one embodiment of the present invention, the moisture content of the cathode active material contained in the anode produced through the second drying step as described above may be less than 10 ppm. At this time, the water content can be measured by the Karl Fischer method or may be measured by using WDS (Water Desorption Spectroscopy).

Meanwhile, in one embodiment of the present invention, a lithium secondary battery comprising the positive electrode and the negative electrode, the separator interposed between the positive electrode and the negative electrode, and the nonaqueous electrolyte solution containing a lithium salt may be provided.

The negative electrode is prepared, for example, by coating a negative electrode mixture containing a negative electrode active material on a negative electrode collector and then drying the mixture. The negative electrode mixture may contain a conductive material, a binder, a filler ≪ / RTI >

The negative electrode current collector is not particularly limited as long as it has high conductivity without causing chemical changes in the battery. For example, the negative electrode current collector may be formed on the surface of copper, stainless steel, aluminum, nickel, titanium, sintered carbon, Carbon, nickel, titanium, silver or the like, aluminum-cadmium alloy, or the like may be used.

On the other hand, the current collector may have a thickness of 3 to 500 탆, and fine unevenness may be formed on the surface of the current collector to enhance the binding force of the negative active material, as in the case of the positive electrode current collector. 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.

The separator is interposed between the anode and the cathode, and an insulating thin film having high ion permeability and mechanical strength can be used.

The pore diameter of the separator is generally 0.01 탆 to 10 탆, and the thickness may be generally 5 탆 to 300 탆.

The separator may be, 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 can be used.

When a solid electrolyte such as a polymer is used as the electrolyte, the solid electrolyte may also serve as a separator.

The lithium salt-containing nonaqueous electrolyte solution is composed of an electrolyte solution and a lithium salt. As the electrolyte solution, a nonaqueous organic solvent or an organic solid electrolyte may be used.

Examples of the non-aqueous organic solvent include N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, gamma -Butyrolactone, 1,2-dimethoxyethane, tetrahydroxyfuran, 2-methyltetrahydrofuran, dimethylsulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile , Nitromethane, methyl formate, methyl acetate, triester phosphate, trimethoxymethane, dioxolane derivatives, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, Tetrahydrofuran derivatives, ether, methyl pyrophosphate, ethyl propionate and the like can be used.

Examples of the organic solid electrolyte include a polymer electrolyte such as a polyethylene derivative, a polyethylene oxide derivative, a polypropylene oxide derivative, a phosphate ester polymer, an agitation lysine, a polyester sulfide, a polyvinyl alcohol, a polyvinylidene fluoride, A polymer containing an ionic dissociation group and the like may be used.

The lithium salt is a material that is readily soluble in the non-aqueous electrolyte, for example, LiCl, LiBr, LiI, LiClO 4, LiBF 4, LiB 10 C 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 carboxylic acid lithium, 4-phenyl lithium borate, and imide .

For the purpose of improving the charge / discharge characteristics and the flame retardancy, the electrolytic solution is preferably mixed with an organic solvent such as pyridine, triethylphosphite, triethanolamine, cyclic ether, ethylenediamine, glyme, Benzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N, N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, . In some cases, a halogen-containing solvent such as carbon tetrachloride or ethylene trifluoride may be further added to impart nonflammability, or a carbon dioxide gas may be further added to improve high-temperature storage characteristics.

Meanwhile, the external shape of the lithium secondary battery manufactured according to the present invention is not particularly limited, but may be cylindrical, square, pouch type, coin type, or the like using a can.

As described above, the method for producing the positive electrode of the present invention and the positive electrode thus produced can remove the moisture bound to the surface of the positive electrode active material, thereby suppressing the generation of HF in the battery. As a result, It is possible to prevent various problems such as damage of the anode or the electrode (Al-foil), generation of organic matter by decomposition of LiPF ₆ salt, increase of the battery resistance or reduction of the charge / discharge capacity.

On the other hand, the lithium secondary battery including the positive electrode manufactured according to the above method can have a capacity retention ratio of 90% or more as compared to 0.1C based on 2.0C. At this time, the capacity retention rate is obtained by coating a positive electrode mixture prepared by mixing a positive electrode active material: conductive material: binder in a weight ratio of 90: 5: 5 to a positive electrode current collector (for example, Al foil) and adjusting porosity to 35 to 45% Rolled to prepare an anode. The anode was made into a coin cell (Toyo 2032), impregnated with an electrolyte through waiting and formation (0.1 C) for 24 hours, and then applied with a current suitable for each loading.

Example

Hereinafter, the present invention will be described in detail by way of examples with reference to the following examples. However, the embodiments according to the present invention can be modified into various other forms, and the scope of the present invention should not be construed as being limited to the embodiments described below. The embodiments of the present invention are provided to enable those skilled in the art to more fully understand the present invention.

Example  One

Ammonia water was added to a mixture of 0.5 mol of iron sulfate (FeSO ₄ .H ₂ O), 0.55 mol of phosphoric acid (P ₂ O ₅ ) and 1.5 mol of an aqueous iron sulfate solution containing an antioxidant and a lithium aqueous solution (LiOH.H ₂ O) Of 5.5 to 7 was prepared.

The mixed solution was charged into a continuous process supercritical reactor at a constant rate and reacted for several seconds to prepare a LiFePO ₄ solution, which is a lithium iron phosphate compound. At this time, the reaction was carried out at a temperature of 383 ° C to 385 ° C and a pressure of 250 bar to 252 bar.

The prepared lithium iron phosphate compound solution was filtered to obtain primary lithium iron phosphate compound particles having an average particle diameter of 250 nm. At this time, the average particle diameter was measured by SEM (scanning electron microscope) analysis of the obtained primary particles of lithium iron phosphate compound.

The primary particles of the lithium iron phosphate compound were added to distilled water to perform reslurry and then mixed with Super P and Super C which were line dispersed through carboxymethylsilulose (CMC) Dried by using a spray drier, sieved, and sintered at about 725 ° C in an atmosphere of nitrogen (0.1 l / min) to obtain a cathode active material mixed with a conductive material and lithium secondary phosphate compound secondary particles. At this time, the drying using the spray dryer was carried out at a temperature of 110 ° C, a spraying speed of -20 mbar, and a spraying rate of 40 kg / hr. The lithium iron phosphate compound secondary particles in the obtained cathode active material were obtained by aggregating two or more primary particles having an average particle diameter (D ₅₀ ) of 250 nm and having a mean particle diameter (D ₅₀ ) of 15 μm and a core portion having a diameter of 10 μm , And the solids content (TSC) was 15%.

Polyvinylidene fluoride (PDVF) as a binder and Super-P as a conductive material were mixed in a weight ratio of 90: 5: 5 to the prepared positive electrode active material, and this was mixed with N-methyl-2-pyrrolidone ) At a solid content of 50% to prepare a positive electrode mixture. At this time, a small amount of maleic acid, which is an organic acid, was added to remove water.

The prepared cathode mix was coated on a 20 μm thick aluminum film at 10 mg / cm ² , dried at a temperature of 130 ° C., adjusted to have a porosity of 35 to 40%, rolled, and rinsed according to the shape of the battery to be applied. After sweating, it was vacuum-dried at 250 DEG C for 3 hours to prepare a positive electrode.

Example  2

The positive electrode prepared in Example 1 was prepared.

The negative electrode was prepared by mixing 95.8 wt% of carbon powder as the negative electrode active material, 1.0 wt% of super-p as a conductive material, 2.2 wt% and 1.0 wt% of styrene butadiene rubber (SBR) and carboxymethyl cellulose (CMC) To prepare a negative electrode mixture. The negative electrode mixture was coated on a copper (Cu) thin film as an anode current collector having a thickness of 20 탆 and dried to prepare a negative electrode, followed by a roll press to produce a negative electrode.

Next, LiPF ₆ was added to a nonaqueous electrolyte solvent prepared by mixing ethylene carbonate and diethyl carbonate as electrolytes in a volume ratio of 30:70, to prepare a 1M LiPF ₆ nonaqueous electrolyte solution.

A polymer type battery was fabricated by a conventional method after interposing a mixed separator of polyethylene and polypropylene on the positive and negative electrodes prepared as described above, and the produced non-aqueous electrolyte was injected to prepare a lithium secondary battery.

Comparative Example  One

The positive electrode was prepared in the same manner as in Example 1 except that maleic acid, which is an organic acid, was not added in the preparation of the positive electrode mixture (the weight ratio of the positive electrode active material, the binder and the conductive material was 90: 5: 5).

Comparative Example  2

A lithium secondary battery was produced in the same manner as in Example 2, except that the positive electrode used in Comparative Example 1 was used.

Experiment 1 (Measurement of water content of cathode active material)

The water content of the lithium iron phosphate compound as a cathode active material in the anode according to Example 1 and Comparative Example 1 was measured by the Karl-Fischer method. The results are shown in FIG.

The water content was measured by the Karl Fischer method. The water content of the lithium iron phosphate compound according to Example 1 was 230 ppm.

Further, as shown in FIG. 3, in the case of adding maleic acid (Example 1) in the production of the positive electrode and in the production of the positive electrode mixture as in the present invention, the moisture content of the lithium iron phosphate, which is the positive electrode active material contained in the produced positive electrode And the increase rate was decreased.

Experiment 2 (lithium secondary battery Charging and discharging  Electrical resistance measurement)

The amount of gas generated when the lithium secondary battery manufactured according to Example 2 and Comparative Example 2 was charged and discharged was measured. The results are shown in FIG. At this time, gas was collected from the secondary battery bulged by using a syringe while charging and discharging the secondary battery at a high temperature, and measured by gas chromatography.

As shown in FIG. 4, in the case of the lithium secondary battery of Comparative Example 2 including a positive electrode having a large moisture content, the amount of generated hydrogen was greatly increased as compared with the lithium secondary battery of Example 2 including a positive electrode having a low moisture content according to the present invention . This means that it is possible to reduce the amount of hydrogen generated in a lithium secondary battery including it by moisture control in the cathode active material.

In addition, the output characteristics (resistance generation) using a hybrid pulse power characterization (HPPC) method according to state-of-charge (SOC) were measured. The results are shown in FIG.

Each of the lithium secondary batteries of Example 2 and Comparative Example 2 was subjected to measurement of output characteristics from SOC 10 to SOC 90 at room temperature (25 ° C) using a 3.3 C unit cell HPPC.

As shown in FIG. 5, the lithium secondary battery of Example 2 according to the present invention exhibits a lower resistance value than the lithium secondary battery of Comparative Example 2 in the entire region of the SOC subjected to the measurement. Specifically, when comparing the middle SOC region (50%), the lithium secondary battery of Example 2 exhibited 2.09?, While the secondary lithium secondary battery of Comparative Example 2 showed 2.22 ?. Thus, the electrical resistance decreased by 6% or more .

Claims (8)

Mixing a cathode active material, a binder, a conductive material, and an organic acid containing water in a solvent in a solvent to prepare a slurry-type cathode mix;
Coating the positive electrode mixture on the positive electrode collector and drying (first drying) the positive electrode active material coating layer, and then rolling the positive electrode active material coating layer; And
And drying the electrode after the rolling (second drying).
The method according to claim 1,
Wherein the positive electrode active material is a carbon-coated lithium iron phosphate compound (LiFePO ₄ ).
The method according to claim 1,
Wherein the viscosity of the slurry is controlled by controlling the amount of the organic acid to be added when preparing the positive electrode mixture.
The method according to claim 1,
Wherein the organic acid is maleic acid, glycolic acid, acetic anhycride, or phthalic acid.
The method according to claim 1,
Wherein the second drying step is vacuum drying for 3 to 4 hours within a temperature range of 200 ° C to 300 ° C.
The positive electrode active material in a positive electrode prepared by the method according to claim 1 has a moisture content of less than 10 ppm.
A lithium secondary battery comprising a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and a nonaqueous electrolyte solution containing a lithium salt,
And the positive electrode is a positive electrode according to claim 6.
A lithium secondary battery according to claim 7, wherein the capacity retention ratio of the lithium secondary battery is 90% or more under a condition of 0.1C rate at 25 ° C.

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