KR101850983B1 - Method for Preparing Cathode Active Material and Cathode Active Material Using the Same - Google Patents

Abstract

The present invention relates to a method for producing a cathode active material for a secondary battery, comprising the steps of: (a) synthesizing a lithium transition metal oxide in which Li byproduct remains on the surface of a particle; (b) reacting the reaction precursor which forms a stable phase by reacting with the Li by-product, with a lithium transition metal oxide by a liquid phase reaction; (c) pulverizing the reaction product after filtration and heat treatment; And a positive electrode active material.
The lithium transition metal oxide prepared by using the lithium transition metal oxide exhibits excellent performance as a cathode active material and has a remarkably small amount of reaction by-products such as Li ₂ CO ₃ and LiOH, resulting in problems such as gelation of the slurry and deterioration of high- Can be solved.

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

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 it is very difficult to synthesize due to the cation mixing problem between Li and the transition metal, thereby causing a great 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.

Therefore, a need exists for a technique for solving such a problem.

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

Specifically, it is an object of the present invention to provide a method for producing a cathode active material for a secondary battery, which can prevent the gelation of the anode paste and the generation of gas in the battery by removing Li by-products.

In order to achieve the above object, the present invention provides a method for producing a lithium secondary battery, comprising the steps of: (a) synthesizing a lithium transition metal oxide in which Li byproduct remains on the particle surface; (b) reacting the reaction precursor which forms a stable phase by reacting with the Li by-product, with a lithium transition metal oxide by a liquid phase reaction; (c) filtering and heat-treating the reactant and pulverizing the reactant, and a method for producing the 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 Li by-products. As described above, these byproducts not only cause gelation in the process of producing the anode paste, but cause gas generation as the cells are charged and discharged, which may seriously affect the performance and safety of the battery.

Accordingly, in the manufacturing method according to the present invention, a stable phase is formed by liquid-phase reaction of Li by-products located in the lithium transition metal oxide with a predetermined reaction precursor to form a stable structural phase of the cathode active material including the lithium transition metal oxide The stability can be improved.

Specifically, the reaction between the reaction precursor and the Li by-product can be represented by, for example, the following reaction formulas (a) and (b).

_{xLiOH.H 2 O + y (PO 4} ) z → aLi 3 PO 4 + bH 2 O + cNH 3 ↑ (a)

xLi ₂ CO ₃ + y (PO ₄ ) z → aLi ₃ PO ₄ + bH ₂ O + cNH ₃ ↑ + dCO ₂ (b)

By this reaction, the byproducts on the surface of the lithium transition metal oxide are converted into a stable phase such as Li ₃ PO ₄ , thereby preventing swelling in the course of charging and discharging.

Specifically, the lithium transition metal oxide may be represented by the following formula (1).

Li _{1 + x} M _1-y Ma _y O _{2 -z} A _z (1)

In the above formula (1), M is a stable element in the 6-coordination structure, and is at least one element selected from 1-period and 2-period transition metals; Ma is a metal or non-metal element stable to a 6 coordination structure; A is at least one member selected from the group consisting of halogen, sulfur, chalcogenide compounds and nitrogen; -0.3? X? + 0.3; 0? Y <0.7; And 0? Z <0.1.

The lithium transition metal oxide may be a composition containing Ni as a transition metal. Although the lithium transition metal oxide including Ni has an advantage that it can have a high discharge capacity, the ion size of Ni ²⁺ is similar to the ion size of Li ⁺ , so cation mixing occurs well. In the process of manufacturing lithium transition metal oxide, Li There is a problem that a large amount of by-products are generated. Thus, the Ni-containing lithium transition metal oxide-based cathode active material produced by the present method can prevent the gelation of the anode paste and the generation of gas in the battery by removing Li by-products.

In one specific example, the lithium transition metal oxide may be a compound of the following formula (2).

Li _{1 + x} (Ni _{y '} M' _{1-y '} ) O ₂ (2)

M 'is one or two or more selected from the group consisting of Mn, Co, Cr, Fe, V and Zr, and -0.3? X? +0.3; And 0.4? Y? 0.9.

In the above formula (2), Ni may be contained in a high content, and M 'is a transition metal satisfying the above-mentioned conditions. Specifically, it may be a composition containing Co as an essential element and Mn as a selective element. For example, the lithium transition metal oxide may be a compound represented by the following formula (3).

Li _{1 + x} (Ni _a Co _b Mn _c ) O ₂ (3)

In the above formula (3), 0.3? X? +0.3, 0.5? A? 0.9, 0 <b <0.5, 0? C <0.5.

More specifically, the lithium transition metal oxide may be LiNi _0.8 Mn _0.1 Co _0.1 O ₂ .

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.

As described above, the Li by-product is formed by cationic mixing between Li and a transition metal in the process of synthesizing a lithium transition metal oxide. Specifically, the lithium by-product may include lithium carbonate (Li ₂ CO ₃ ) and lithium hydroxide (LiOH) More specifically, it may be lithium carbonate (Li ₂ CO ₃ ) and lithium hydroxide (LiOH).

The reaction precursor may be a substance containing PO ₄ ^3- as a functional group, as shown in the above Schemes 1 and 2. Specifically, the reactive precursors may be in the X (PO ₄₎ z, where X is as _{Fe, LiFe, (NH 4)} 2 H, (NH 4) H 2, (NH 4) selected from the group consisting of ₃ More than one, and 0? Z? 3. Specifically, it may be monoammonium phosphate ((NH ₄ ) H ₂ PO ₄ ), diammonium phosphate ((NH ₄ ) ₂ HPO ₄ ), or triammonium phosphate ((NH ₄ ) ₃ PO ₄ ) it may be a di-ammonium phosphate _{((NH 4) 2 HPO 4} ).

The liquid phase reaction of the step (b) may be, for example, a mixing and stirring reaction of a lithium transition metal oxide and a reaction precursor in a solvent of water or an alcohol, and the alcohol is specifically a C ₁ to C ₆ lower alcohol Can be used.

As another example, the liquid phase reaction of step (b) may be performed by mixing a lithium transition metal oxide with a wax phase molten reaction precursor.

If such a liquid phase reaction is not carried out, the internal resistance of the battery may be increased due to by-products present on the surface of the cathode active material, and the electrochemical performance of the battery containing the same may decrease.

The reaction precursor may be included in an amount of 0.01 to 10% by weight based on the total weight of the lithium transition metal oxide. If the amount of the substance containing PO ₄ ^3- as a functional group is excessively large, it may interfere with occlusion and desorption of lithium, and if it is too small, it is not possible to sufficiently remove Li by-products. Thus, in detail, the amount of the substance containing PO ₄ ^3- as a functional group may be 3 to 8% by weight, more specifically 5% by weight, based on the total weight of the lithium transition metal oxide.

The amount of Li by-products remaining after the liquid phase reaction in the step (b) is determined according to the amount of the reaction precursor which forms a stable phase by reacting with Li by-products, but is preferably 10 wt% , And in particular less than 5% by weight.

The heat treatment in step (c) may be performed in a temperature range of more than 100 ° C to less than 600 ° C. If the heat treatment temperature is less than 100 ° C, crystal water derived from hydrate or the like may not be removed and remain impurities in the active material. If the heat treatment temperature is higher than 600 ° C, the intended effect disappears. For the above reasons, it is more preferable that the temperature is in the range of 100 to 500 ° C.

The present invention includes a cathode active material prepared according to the above method, wherein the cathode active material can be prepared by adding a conductive material and a binder to a cathode mixture.

The positive electrode 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 a positive electrode.

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 manufacturing method according to the present invention includes a step of forming a safe phase by reacting the lithium byproduct generated in the process of synthesizing the lithium transition metal oxide with a predetermined reaction precursor in a liquid phase to form the lithium transition metal It is possible to reduce the content of impurities in the cathode active material including the oxide and prevent the gelation of the anode paste due to the impurities and the suppression of the generation of gas in the battery to prevent the swelling of the secondary battery, Safety can be improved.

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

&Lt; Example 1 >

Li hydroxide was added to a precursor composed of a three-component system including Ni to prepare LiNi _0.8 Mn _0.1 Co _0.1 O ₂ . Thereafter, the mixture was stirred for about 300 seconds in an aqueous 5 wt% diammonium phosphate solution based on the total weight of the lithium transition metal oxide. The agitated cathode active material was filtered to recover the solid phase components, and the impurities were removed by heat treatment at 500 ° C.

&Lt; Example 2 >

The cathode active material was prepared in the same manner as in Example 1, except that the mixture was stirred in diammonium phosphate dissolved in alcohol.

&Lt; Comparative Example 1 &

A cathode active material was prepared in the same manner as in Example 1, except that the mixture was stirred in water not containing diammonium phosphate.

&Lt; Comparative Example 2 &

A cathode active material was prepared in the same manner as in Example 1, except that diammonium phosphate was not stirred in an aqueous solution.

<Experimental Example>

The efficiency of charge and discharge of the battery was measured for each of the positive electrode active materials prepared in Examples 1 and 2 and Comparative Examples 1 and 2. The results are shown in Table 1 below.

Method 4.4-3.0V 1st. Charge 1st. Discharge 1st. Efficiency Rate capability
(2.0C / 0.1C) Example 1 234.3 212.7 90.8 86.7 Example 2 231.3 208.1 90.0 86.2 Comparative Example 1 229.3 204.4 89.2 80.0 Comparative Example 2 231.8 208.7 90.0 85.7

Referring to Table 1, in the case of the cathode active material of Comparative Example 2, resistance is increased due to byproducts existing on the surface of the cathode active material without washing with water, and the electrochemical performance of the battery containing the cathode active material decreases. On the other hand, since the cathode active materials of Examples 1 and 2 are subjected to a washing process in an aqueous solution and an alcohol, respectively, the amount of lithium by-products on the surface of the cathode active material is reduced,

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 (18)

(a) synthesizing a lithium transition metal oxide in which Li byproduct remains on the particle surface;
(b) reacting the reaction precursor which forms a stable phase by reacting with the Li by-product, with a lithium transition metal oxide by a liquid phase reaction;
(c) pulverizing the reaction product after filtration and heat treatment;
Wherein the positive electrode active material is a positive electrode active material.
The lithium transition metal oxide is a compound represented by the following general formula (3)
Li _{1 + x} (Ni _a Co _b Mn _c ) O ₂ (3)
In the above formula (3), -0.3? X? +0.3, 0.5? A? 0.9, 0 <b <0.5, 0? C <0.5. delete delete 2. The process according to claim 1, wherein the lithium transition metal oxide of step (a) is LiNi _0.8 Mn _0.1 Co _0.1 O ₂ . The method according to claim 1, wherein the Li by-product is lithium carbonate (Li ₂ CO ₃ ) and lithium hydroxide (LiOH). The method of claim 1, wherein the reaction precursor is a material comprising PO ₄ ^3- as a functional group. The method of claim 6, wherein the reactant precursor X (PO ₄₎ z (where X is _{Fe, LiFe, (NH 4)} H 2, (NH 4) 2 H, ( one is selected from the group consisting of NH _{4) 3} Or more, and 0 < z &lt; = 3). The method of claim 7, wherein said reaction precursor is mono-ammonium phosphate _{((NH 4) H 2 PO} 4), dimethyl ammonium phosphate _{((NH 4) 2 HPO 4} ), or tri-ammonium phosphate _{((NH 4) 3 PO 4} ) in &Lt; / RTI &gt; The process according to claim 7, wherein the reaction precursor is diammonium phosphate ((NH ₄ ) ₂ HPO ₄ ). The production method according to claim 1, wherein the liquid phase reaction of step (b) is carried out by mixing and stirring a lithium transition metal oxide and a reaction precursor in a solvent of water or alcohol. The method according to claim 1, wherein the liquid phase reaction of step (b) comprises mixing a lithium transition metal oxide with a wax phase molten reaction precursor and reacting. The process according to claim 1, wherein the reaction precursor in step (b) is from 0.01 to 10% by weight, based on the total weight of the lithium transition metal oxide. 13. The process of claim 12, wherein the reaction precursor in step (b) is from 3 to 8% by weight, based on the total weight of the lithium transition metal oxide. The process according to claim 1, wherein the amount of Li by-products remaining after the liquid phase reaction of step (b) is less than 10% by weight based on the total weight of the lithium transition metal oxide. The process according to claim 1, wherein the heat treatment in step (c) is carried out in a temperature range of more than 100 ° C to less than 600 ° C. A positive electrode material mixture for a lithium secondary battery, comprising a positive electrode active material prepared by the method according to claim 1. A positive electrode for a lithium secondary battery, comprising the positive electrode mixture of claim 16. A lithium secondary battery comprising the positive electrode of claim 17.