KR101456963B1 - Method for Preparing Cathode Active Material - Google Patents

Abstract

(A) synthesizing a lithium transition metal oxide; (b) stirring the lithium transition metal oxide in an HF aqueous solution; And (c) drying and then heat-treating the cathode active material.

Description

[0001] The present invention relates to a method for preparing a cathode active material,

The present invention relates to a method for producing a cathode active material for a secondary battery, which comprises synthesizing a lithium transition metal oxide, stirring the lithium transition metal oxide in an HF aqueous solution, and heat treating the lithium transition metal oxide after drying, .

As technology development and demand for mobile devices have increased, there has been a rapid increase in demand for secondary batteries as energy sources. Among such secondary batteries, lithium secondary batteries, which exhibit high energy density and operational potential, long cycle life, Batteries have been commercialized and widely used.

In recent years, there has been a growing interest in environmental issues, and as a result, electric vehicles (EVs) and hybrid electric vehicles (HEVs), which can replace fossil-fueled vehicles such as gasoline vehicles and diesel vehicles, And the like. Although a nickel metal hydride (Ni-MH) secondary battery is mainly used as a power source for such an electric vehicle (EV) and a hybrid electric vehicle (HEV), a lithium secondary battery having a high energy density, a high discharge voltage, Research is actively underway, and some commercialization is in progress.

Particularly, a lithium secondary battery used in an electric automobile must have a high energy density and characteristics capable of exhibiting a large output in a short time, and can be used for more than 10 years under severe conditions in which charging and discharging by a large current are repeated in a short time. It is inevitably required to have safety and long-term life characteristics far superior to those of a small lithium secondary battery.

Lithium-containing cobalt oxide (LiCoO ₂ ) having a layered structure is mainly used as a cathode active material of a lithium ion secondary battery used in a conventional small-sized battery, and LiMnO _{2 having a} layered crystal structure, LiMn _{2 having a} spinel crystal structure ₂ O ₄ , and lithium-containing nickel oxide (LiNiO ₂ ) are being used in some cases.

Among the cathode active materials, LiCoO ₂ is most widely used because it has excellent lifetime characteristics and charge / discharge efficiency, but it has a disadvantage that its structural stability is poor and its cost competitiveness is limited due to resource limitations of cobalt used as a raw material .

Lithium manganese oxides such as LiMnO ₂ and LiMn ₂ O ₄ have an advantage of being excellent in thermal stability and being inexpensive, but have a problem of small capacity and poor high temperature characteristics.

The LiNiO ₂ cathode active material exhibits a high discharge capacity of the battery, but is difficult to synthesize due to the problem of cation mixing between Li and the transition metal, thereby causing a serious problem in rate characteristics.

A large amount of Li by-products are generated depending on the depth of the cationic mixture, and most of the Li by-products are composed of LiOH and Li ₂ CO ₃ , which causes gelation during the production of the anode paste, This causes gas generation due to the progress of the back-charging and discharging.

Accordingly, efforts to solve such a problem have been consistently made in the related field, and the present invention has been made based on this technical background.

SUMMARY OF THE INVENTION It is an object of the present invention to solve the above-described problems of the prior art and the technical problems required from the past.

Specifically, an object of the present invention is to provide a method for producing a cathode active material for a secondary battery, which is capable of preventing gelation of a cathode paste and suppressing gas generation by removing Li by-products, and a cathode active material using the cathode active material.

In order to achieve this object,

(a) synthesizing a lithium transition metal oxide;

(b) stirring the lithium transition metal oxide in an HF aqueous solution; And

(c) heat treating after drying;

The present invention also provides a method for producing a cathode active material for a secondary battery.

In the process of synthesizing the lithium transition metal oxide, LiOH, Li ₂ CO ₃ and the like are generated as lithium by-products. As described above, these impurities not only cause gelation in the process of manufacturing the anode paste, but also cause gas generation as the cells are charged and discharged, which may severely affect the performance and safety of the battery.

In order to remove these impurities, the present invention stirs the lithium transition metal oxide in the HF aqueous solution to induce the reaction with the lithium by-product. The reaction between the HF and the lithium by-product can be represented by, for example, the following formulas 1 and 2.

_{LiOH + HF → LiF + H 2} O (1)

Li ₂ CO ₃ + 2HF? 2LiF + H ₂ O + CO ₂ (2)

The LiF produced by the above reaction is present on the surface of the lithium transition metal oxide and can inhibit the erosion of the lithium transition metal oxide by HF and has the effect of preventing swelling during charging and discharging processes .

The synthesis of the lithium-transition metal oxide in the step (a) can be carried out by any method as long as it is generally practiced. Non-limiting examples thereof include a method selected from the group consisting of a solid phase method, coprecipitation method and supercritical hydrothermal method .

Since the solid phase method, coprecipitation method, and supercritical hydrothermal method are well known in the art, detailed description thereof will be omitted herein.

In the HF aqueous solution of step (b), if the concentration of HF is too high, the surface of the lithium transition metal oxide may be eroded, so that the content of HF is preferably 2% by weight or less based on the total weight of the aqueous solution. For the above reasons, the content of HF is more preferably in the range of 0.2 to 2% by weight based on the total weight of the aqueous solution.

The stirring of step (b) is preferably performed for 30 seconds to 600 seconds. As described above, since HF may erode the lithium transition metal oxide, if it is stirred for too long, the lithium transition metal oxide may be eroded, which is undesirable. On the other hand, when stirring for too short a period of time, the reaction between HF and the lithium by-product can not sufficiently take place and a desired effect can not be obtained. For the above reasons, it is more preferable that the agitation is performed for 30 seconds to 300 seconds.

The heat treatment in step (c) may be performed at a temperature ranging from 150 to 400 ° C. If the heat treatment temperature is less than 150 ° C, crystal water derived from hydrate or the like may remain as impurities in the active material without being removed, and if it is higher than 400 ° C, the intended effect disappears and the intended effect disappears. For the above reasons, it is more preferable that the temperature is in the range of 200 to 350 ° C.

The lithium transition metal oxide may be a composition containing Ni as a transition metal. The lithium-transition metal oxide including Ni has an advantage that it can have a high discharge capacity, but there is a problem that the ion size of Ni ²⁺ is similar to the ion size of Li ⁺ , so that the cation mixing is likely to occur. Furthermore, since a large amount of lithium by-product is produced in the course of the production of the lithium transition metal oxide, it is more preferable to prepare the cathode active material based on the Ni-containing lithium transition metal oxide by the method of the present invention.

The Ni-containing lithium transition metal oxide may have a composition in which the content of Ni is 0.3 or more in mole fraction with respect to the entire transition metal. Preferable examples of such a lithium transition metal oxide include the following chemical formula (3).

_{Li 1 + x (Ni 1 -a-} b M a M 'b) O 2 (3)

In the above formula, M and M 'are transition metals except Ni, -0.3? X? + 0.3, 0? A + b? 0.7.

It is apparent to those skilled in the art that M and / or M 'may be a material selected from the group consisting of Mn and Co, and that the transition metal sites may partially substitute metal or nonmetal elements that are stable at 6 coordination. It is also apparent to those skilled in the art that some substituents such as halogen, sulfur, nitrogen and chalcogenide compounds may be substituted for the oxygen (O) site.

The present invention also relates to a lithium-transition metal oxide containing Ni, wherein the content of Ni is 0.3 or more in mole fraction with respect to the total transition metal, and is produced by the reaction of Li by-products derived from the synthesis of lithium transition metal oxide and HF And LiF is locally applied to the surface of the particles.

As described above, since the LiF is applied to the surface of the cathode active material, erosion of the lithium transition metal oxide by HF can be suppressed as described above, and swelling can be prevented during the charge / have.

The amount of LiF is preferably 1% by weight or less based on the total weight of the lithium transition metal oxide. When the amount of LiF is more than 1% by weight based on the total weight of the lithium transition metal oxide, it may interfere with occlusion and desorption of lithium, which is not preferable. For the above reasons, it is more preferable that the amount of LiF is 0.5% by weight or less based on the total weight of the lithium transition metal oxide.

The cathode active material according to the present invention may be prepared by adding a conductive material and a binder to the cathode mixture.

The mixture may contain a predetermined solvent such as water or NMP to form a slurry. The slurry may be coated on the positive electrode current collector, followed by drying and rolling to produce the positive electrode. As described above, the cathode active material according to the present invention is characterized in that lithium by-products are removed and gelation can be prevented during the production of the slurry.

The positive electrode is prepared, for example, by applying a slurry of a mixture of a positive electrode active material, a conductive material and a binder according to the present invention on a positive electrode current collector, followed by drying. If necessary, the positive electrode active material, The mixture (electrode mixture) of the binder and the like may further contain at least one substance selected from the group consisting of a viscosity adjusting agent and a filler.

The cathode current collector generally has a thickness of 3 to 500 mu m. Such a positive electrode current collector is not particularly limited as long as it has high conductivity without causing chemical change in the battery, and may be formed of a material such as stainless steel, aluminum, nickel, titanium, sintered carbon, or a surface of aluminum or stainless steel Treated with carbon, nickel, titanium, silver or the like may be used. The current collector may have fine irregularities on the surface thereof to increase the adhesive force of the cathode active material, and various forms such as a film, a sheet, a foil, a net, a porous body, a foam, and a nonwoven fabric are possible.

The conductive material is a component for further improving the conductivity of the electrode active material and may be added in an amount of 0.01 to 30% by weight based on the total weight of the electrode mixture. Such a 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; Carbon fibers such as carbon nanotubes and fullerene; conductive fibers such as carbon fibers and metal fibers; 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 binder is added to the binder in an amount of 1 to 30% by weight, based on the total weight of the mixture containing the cathode active material, as a component that assists in bonding between the active material and the conductive agent and bonding to the current collector. Examples of such binders include polyvinylidene fluoride, polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene , Polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene butylene rubber, fluorine rubber, various copolymers and the like.

The viscosity adjusting agent may be added up to 30% by weight based on the total weight of the electrode mixture, so as to control the viscosity of the electrode mixture so that the mixing process of the electrode mixture and the coating process on the collector may be easy. Examples of such viscosity modifiers include carboxymethylcellulose, polyvinylidene fluoride and the like, but are not limited thereto. In some cases, the above-described solvent may play a role as a viscosity adjusting agent.

The filler is optionally used as an auxiliary component for suppressing the expansion of the electrode. The filler is not particularly limited as long as it is a fibrous material without causing chemical change in the battery, and examples thereof include olefin polymers such as polyethylene and polypropylene; Fibrous materials such as glass fibers and carbon fibers are used.

The thus prepared positive electrode can be used for manufacturing a lithium secondary battery together with a negative electrode, a separator, and a non-aqueous electrolyte containing a lithium salt.

The negative electrode is manufactured by coating a negative electrode material on a negative electrode collector and drying the same, and if necessary, may further include components such as a conductive material and a binder as described above.

The negative electrode collector is generally made to have a thickness of 3 to 500 mu m. Such an anode current collector is not particularly limited as long as it has conductivity without causing chemical change in the battery, and may be formed of a material such as copper, stainless steel, aluminum, nickel, titanium, fired carbon, surface of copper or stainless steel A surface treated with carbon, nickel, titanium, silver or the like, an aluminum-cadmium alloy, or the like can be used. In addition, like the positive electrode collector, fine unevenness can be formed on the surface to enhance the bonding force of the negative electrode active material, and it can be used in various forms such as films, sheets, foils, nets, porous bodies, foams and nonwoven fabrics.

Examples of the negative electrode active material include carbon and graphite materials such as natural graphite, artificial graphite, expanded graphite, carbon fiber, non-graphitizable carbon, carbon black, carbon nanotube, fullerene and activated carbon; Metals such as Al, Si, Sn, Ag, Bi, Mg, Zn, In, Ge, Pb, Pd, Pt and Ti which can be alloyed with lithium and compounds containing these elements; Complexes of metals and their compounds and carbon and graphite materials; Lithium-containing nitrides, and the like. Among them, a carbon-based active material, a silicon-based active material, a tin-based active material, or a silicon-carbon based active material is more preferable, and these may be used singly or in combination of two or more.

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. The pore diameter of the separator is generally 0.01 to 10 mu m and the thickness is generally 5 to 300 mu m. Such separation membranes include, for example, olefinic polymers such as polypropylene, which are chemically resistant and hydrophobic; A sheet or nonwoven fabric made of glass fiber, polyethylene or the like is used. When a solid electrolyte such as a polymer is used as an electrolyte, the solid electrolyte may also serve as a separation membrane.

The lithium salt-containing non-aqueous electrolyte is composed of a non-aqueous electrolyte and a lithium salt. As the non-aqueous electrolyte, a non-aqueous electrolyte, a solid electrolyte, an inorganic solid electrolyte and the like are used.

Examples of the nonaqueous electrolytic solution 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 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, Polymers containing ionic dissociation groups, and the like can be used.

Examples of the inorganic solid electrolyte include 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 , Nitrides, halides and sulfates of Li such as Li ₄ SiO ₄ -LiI-LiOH and Li ₃ PO ₄ -Li ₂ S-SiS ₂ can 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 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 carboxylic acid lithium, lithium tetraphenyl borate and imide have.

For the purpose of improving charge / discharge characteristics, flame retardancy, etc., non-aqueous electrolytes may be used in the form of, for example, pyridine, triethylphosphite, triethanolamine, cyclic ether, ethylenediamine, glyme, N, N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxyethanol, aluminum trichloride and the like are added It is possible. In some cases, halogen-containing solvents such as carbon tetrachloride and ethylene trifluoride may be further added to impart nonflammability. In order to improve the high-temperature storage characteristics, carbon dioxide gas may be further added. FEC (Fluoro-Ethylene carbonate, PRS (propene sultone), FPC (fluoro-propylene carbonate), and the like.

The present invention also provides a battery module using the lithium secondary battery as a unit cell, and a battery pack including the battery module.

Therefore, the secondary battery according to the present invention can be suitably used as a power source for a middle- or large-sized device which is required not only for a battery cell used as a power source for a small device but also for a high-temperature safety, a long cycle characteristic and a high rate characteristic.

Preferred examples of the above medium to large devices include a power tool that is powered by an electric motor and moves; An electric vehicle including an electric vehicle (EV), a hybrid electric vehicle (HEV), a plug-in hybrid electric vehicle (PHEV), and the like; An electric motorcycle including an electric bike (E-bike) and an electric scooter (E-scooter); An electric golf cart, and the like, but the present invention is not limited thereto.

As described above, the positive electrode active material for a secondary battery manufactured by the method for producing a positive electrode active material according to the present invention can reduce the content of impurities and prevent swelling of the secondary battery.

1 is a schematic diagram schematically showing a flow chart of a manufacturing method according to the present invention;
2 is a SEM photograph of a cathode active material according to Example 1 of the present invention;
3 is a SEM photograph of a cathode active material according to Comparative Example 1 of the present invention.

Hereinafter, the present invention will be described with reference to the drawings, but the scope of the present invention is not limited thereto.

Fig. 1 schematically shows a flowchart of a manufacturing method according to the present invention.

Referring to FIG. 1, a lithium transition metal oxide is synthesized in step (a). As a method of synthesizing such a lithium transition metal oxide, any method may be used as long as a lithium transition metal oxide capable of lithium intercalation / deintercalation can be produced. Examples of the method include a solid phase method, coprecipitation method, and supercritical hydrothermal method have.

In step (b), the lithium transition metal oxide prepared in step (a) is stirred in an HF aqueous solution. In this process, LiOH and Li ₂ CO ₃ , which are byproducts present in the lithium transition metal oxide, react with HF to produce a LiF layer on the surface of the lithium transition metal oxide. The LiF layer prevents erosion of lithium transition metal oxide by HF and suppresses swelling during charging and discharging processes of a battery.

In step (c), the lithium transition metal oxide which has been reacted in step (b) is dried and heat-treated to obtain a cathode active material. In the case of the lithium secondary battery, if water is present in the battery, safety may be caused by side reaction due to moisture, so the moisture in the step (b) must be completely dried and heat-treated.

Hereinafter, the present invention will be described in detail with reference to Examples, but the scope of the present invention is not limited thereto.

≪ Example 1 >

Li was added to a precursor of a three-component system containing Ni to prepare a cathode active material. The prepared three-component type active material was stirred in a 1 wt% HF aqueous solution for about 60 seconds. The agitated cathode active material was filtered to recover solid phase components, dried at 120 ° C for 5 hours in a drier, and heat treated at 300 ° C to remove impurities and coated with LiF on the surface.

≪ Comparative Example 1 &

The cathode active material was prepared in the same manner as in Example 1, except that the HF aqueous solution was not stirred.

<Experimental Example>

SEM photographs of the cathode active materials of Example 1 and Comparative Example 1 are shown in FIGS. 2 and 3, respectively.

Referring to FIGS. 2 and 3, it can be seen that impurities are present on the surface of the cathode active material of FIG. 3 showing the cathode active material of Comparative Example 1. FIG. On the contrary, it can be confirmed that the surface of the positive electrode active material of FIG. 2 showing the positive electrode active material of Example 1 subjected to the stirring treatment in the HF aqueous solution is free from such impurities and is clean.

The amounts of impurities (Li ₂ CO ₃ and LiOH) in the cathode active materials of Example 1 and Comparative Example 1 were measured to the third decimal place as a percentage by weight based on the total weight, and the results are shown in Table 1 below.

<Table 1>

Referring to Table 1, it can be seen that the impurity content of Comparative Example 1, which is not subjected to stirring in the HF aqueous solution, is much higher than the impurity content of Example 1 after stirring the HF aqueous solution.

Those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.

Claims (19)

A method for producing a cathode active material for a secondary battery,
(a) synthesizing a lithium transition metal oxide;
(b) stirring the lithium transition metal oxide in an HF aqueous solution; And
(c) heat treating after drying;
Wherein the positive electrode active material is a positive electrode active material. The method for producing a cathode active material for a secondary battery according to claim 1, wherein the synthesis of the lithium-transition metal oxide in the step (a) is carried out by a method selected from the group consisting of a solid phase method, coprecipitation method and supercritical hydrothermal method . The method of claim 1, wherein the HF aqueous solution of step (b) has a HF content of 2% by weight or less based on the total weight of the aqueous solution. [4] The method of claim 3, wherein the HF content is in the range of 0.2 to 2% by weight based on the total weight of the aqueous solution. The method of claim 1, wherein the stirring of step (b) is performed for 30 seconds to 600 seconds. The method for manufacturing a cathode active material for a secondary battery according to claim 1, wherein the heat treatment in step (c) is performed at a temperature ranging from 150 to 400 ° C. The method of claim 1, wherein the lithium transition metal oxide comprises Ni. 8. The method of claim 7, wherein the Ni-containing lithium transition metal oxide has a molar fraction of Ni of 0.3 or more with respect to the total transition metal. 1. A lithium transition metal oxide comprising Ni, wherein a content of Ni is 0.3 or more in mole fraction with respect to the entire transition metal, LiF produced by the reaction of Li by-products derived from the process of synthesizing lithium transition metal oxide with HF, Wherein the cathode active material is applied locally to the cathode active material. 10. The positive electrode active material according to claim 9, wherein the lithium transition metal oxide has a composition represented by the following formula (3)
_{Li 1 + x (Ni 1 -a-} b M a M 'b) O 2 (3)
In the above formula, M and M 'are transition metals except Ni, -0.3? X? + 0.3, 0? A + b? 0.7. The positive electrode active material according to claim 9, wherein the amount of LiF is 1% by weight or less based on the total weight of the lithium transition metal oxide. A positive electrode material mixture for a secondary battery, comprising the positive electrode active material according to claim 9. A positive electrode for a secondary battery according to claim 12, wherein the positive electrode mixture is applied on a current collector. A secondary battery comprising a positive electrode according to claim 13. 15. The secondary battery according to claim 14, wherein the battery is a lithium secondary battery. The battery module according to claim 15, wherein the lithium secondary battery is used as a unit battery. A battery pack comprising the battery module according to claim 16. The battery pack according to claim 17, wherein the battery pack is used as a power source for a middle- or large-sized device. 19. The battery pack of claim 18, wherein the middle- or large-sized device is an electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, or a power storage system.

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