KR20190057952A - Preparing method of the positive electrode active material for lithium secondary battery - Google Patents

Abstract

The present invention provides a method for manufacturing a positive electrode active material, which comprises following steps of: preparing the positive electrode active material comprising 70 mol% or more of nickel relative to a total number of moles of transition metals except lithium; and washing the positive electrode active material with an aqueous solution containing an organic acid. According to the present invention, it is possible to manufacture the positive electrode active material having improved resistance characteristics and lifespan characteristics.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a positive electrode active material for a lithium secondary battery,

The present invention relates to a method for producing a cathode active material for a lithium secondary battery.

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 among them, lithium cobalt composite metal oxides such as LiCoO ₂ having high action voltage and excellent capacity characteristics are mainly used. However, LiCoO ₂ has very poor thermal properties due to the destabilization of the crystal structure due to the depolymerization. In addition, since LiCoO ₂ is expensive, it can not be used in large quantities as a power source for fields such as electric vehicles and the like.

Lithium manganese composite metal oxides (such as LiMnO ₂ or LiMn ₂ O ₄ ), lithium iron phosphate compounds (such as LiFePO ₄ ), or lithium nickel composite metal oxides (such as LiNiO ₂ ) have been developed as materials for replacing LiCoO ₂ . Among them, research and development on a lithium nickel composite metal oxide having a high reversible capacity of about 200 mAh / g and facilitating the realization of a large capacity battery has been actively researched. However, LiNiO ₂ has a lower thermal stability than LiCoO _2, and if an internal short circuit occurs due to external pressure or the like in a charged state, the cathode active material itself is decomposed to cause rupture and ignition of the battery. Accordingly, a lithium nickel cobalt metal oxide in which a part of Ni is substituted with Co and Mn or Al has been developed as a method for improving low thermal stability while maintaining excellent reversible capacity of LiNiO ₂ .

However, in the case of the lithium nickel cobalt metal oxide, when the content of nickel is increased to 70 mol% or more in order to improve the capacity characteristics, the nickel in the lithium nickel cobalt metal oxide is reduced to Ni ^{2 +} There is a problem that a large amount of lithium by-products such as LiOH and Li ₂ CO ₃ are produced on the surface thereof. When the lithium nickel cobalt metal oxide having a high content of lithium by-products on the surface is used, the lithium secondary battery reacts with the electrolyte injected into the lithium secondary battery, thereby causing a swelling phenomenon in the lithium secondary battery. The battery could not sufficiently exhibit the battery performance.

In order to solve such a problem, methods have been proposed for reducing the content of lithium by-products present on the surface of the lithium nickel cobalt metal oxide by conducting a water washing process after synthesis of the lithium nickel cobalt metal oxide. Conventionally, a method of washing lithium nickel cobalt metal oxide with distilled water has been performed. However, in this case, removal of Li ₂ CO ₃ from the lithium by-products present on the surface of the lithium-nickel-cobalt metal oxide is not easy, and thus the effect of reducing the content of lithium by-products present on the surface is not sufficiently satisfactory.

Therefore, development of a method for producing a cathode active material capable of reducing lithium by-products present on the surface of a lithium nickel cobalt metal oxide containing a high nickel content is desired.

Korean Patent Publication No. 2012-0117822

In order to solve the above-mentioned problems, a first technical object of the present invention is to provide a positive electrode active material capable of reducing lithium by-products present on the surface of a positive electrode active material without damaging the positive electrode active material through a washing process after manufacturing the positive electrode active material And a method for producing the same.

The present invention provides a method for preparing a positive electrode active material, comprising: preparing a positive electrode active material comprising 70 mol% or more of nickel relative to the total number of moles of transition metals except lithium; And washing the positive electrode active material with an aqueous solution containing an organic acid.

According to the present invention, a positive electrode active material containing nickel in a high content is prepared, and then the positive electrode active material is washed with an aqueous organic acid solution in which organic acid is mixed with distilled water at the time of flushing to remove only lithium by- Can be reduced. As described above, by using the organic acid aqueous solution to reduce the amount of lithium by-products present on the surface, the surface of the cathode active material can be further activated to produce a cathode active material having improved charge / discharge efficiency, resistance characteristics, and lifetime characteristics.

1 is a graph showing lifetime characteristics of a lithium secondary battery produced in Example 1 and Comparative Examples 1 and 2. FIG.
FIG. 2 is a graph showing resistance characteristics of the lithium secondary battery manufactured in Example 1 and Comparative Examples 1 and 2. FIG.

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

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.

Generally, when an excessive amount of lithium by-product is present on the surface of the cathode active material, the lithium impurity reacts with the electrolyte injected into the lithium secondary battery, causing a swelling phenomenon of the lithium secondary battery. In addition, when the positive electrode active material contains nickel in a high content for high capacity, such swelling problem becomes more serious.

Accordingly, the present inventors have found that, in the production of a positive electrode active material containing nickel in a high content, it is possible to selectively remove only lithium by-products present on the surface without damaging the positive electrode active material by adding an organic acid to the wash solution. Completed.

More specifically, the present invention provides a method for preparing a positive electrode active material, comprising: preparing a positive electrode active material containing 70 mol% or more of nickel relative to the total number of moles of transition metals except lithium; And washing the positive electrode active material with an aqueous solution containing an organic acid.

In order to produce the cathode active material according to the present invention, first, a cathode active material containing 70 mol% or more of nickel relative to the total number of moles of transition metals except lithium is prepared.

Specifically, the cathode active material may preferably be represented by the following Formula 1:

[Chemical Formula 1]

Li _{1 + a} (Ni _b Co _c M ¹ _d M ² _e ) _1-a O ₂

Wherein M ¹ is at least one or more selected from the group consisting of Mn and Al and M ² is at least one element selected from the group consisting of Zr, B, Al, Co, W, Mg, Ce, Hf, Ta, La, , Ce, F, P, S and Y, and 0? A? 0.2, 0.7? B? 1.0, 0? C? 0.3, 0? D? 0.3, 0? E? being.

The molar ratio (Li / Me molar ratio) of lithium to the transition metals contained in the lithium transition metal oxide may be 1.0 to 1.2, preferably 1.0 to 1.15. When the range is satisfied, the lithium transition metal oxide shows excellent capacity .

In addition, the positive electrode active material may further include a selectively doping element M ^2, as needed. Wherein the doping element M ² is at least one selected from the group consisting of Zr, B, Al, Co, W, Mg, Ce, Hf, Ta, La, Ti, Sr, Ba, Ce, F, P, And when the cathode active material further includes the doping element M ² , the structural stability of the cathode active material is improved, and the lifetime characteristics of the battery can be further improved.

As shown in Formula 1, when the positive electrode active material contains 70 mol% or more of nickel relative to the total number of moles of the transition metal oxide excluding lithium, a positive electrode active material exhibiting a high capacity characteristic can be provided. However, as the content of nickel increases, the content of lithium by-products present on the surface also increases.

Thus, when the positive electrode active material containing nickel in a high content is washed with an aqueous solution containing an organic acid as in the present invention, lithium by-products present on the surface of the positive electrode active material can be effectively reduced.

Specifically, in the aqueous solution containing the organic acid, the cathode active material is washed with water to control lithium by-products present on the surface of the cathode active material to 0.3% by weight or less based on the total weight of the cathode active material. For example, the lithium impurity may be at least one selected from the group consisting of LiOH and Li ₂ CO ₃ .

The aqueous solution containing the organic acid includes distilled water and organic acid. For example, when the cathode active material is washed with distilled water alone, LiOH is easily dissociated in the distilled water and is easily removed from the surface of the cathode active material, whereas Li ₂ CO ₃ is difficult to dissociate in the distilled water because it is difficult to dissociate. Therefore, not only LiOH but also Li ₂ CO ₃ can be easily removed by using an aqueous solution obtained by mixing an appropriate amount of organic acid with distilled water.

For example, when the cathode active material is a nickel-containing cathode active material having a low content of nickel of less than 70 mol% based on the total number of moles of transition metal oxides except lithium, the content of lithium by- As compared with the case where nickel is contained in a high content as in the present invention, the amount of nickel is much smaller than that in the case of containing nickel in a high content. Thus, when washed with an aqueous solution containing an organic acid, damage to the surface of the cathode active material as well as lithium by- However, as in the present invention, a high content of nickel-containing cathode active material containing 70 mol% or more of nickel relative to the total moles of transition metal oxides excluding lithium as the positive electrode active material contains organic acid due to a large amount of lithium by- It is possible to selectively remove only the lithium by-product without damaging the surface of the cathode active material.

The organic acid is preferably a weak acid which can dissociate into distilled water and can minimize damage to the cathode active material. More preferably, the organic acid is at least one selected from the group consisting of citric acid, malic acid, formic acid, acetic acid, oxalic acid and ascorbic acid Or more. Since the organic acid is weakly acidic than inorganic acid, only the lithium impurity present on the surface of the cathode active material can be selectively dissolved without damaging the cathode active material, and Li ₂ CO ₃ among the lithium by-products can be effectively dissolved.

The aqueous solution containing the organic acid may be added in an amount of 0.5 to 5 parts by weight, preferably 1 to 3 parts by weight based on 100 parts by weight of distilled water. For example, when the organic acid is contained in an amount of less than 0.5 parts by weight based on 100 parts by weight of distilled water, removal of the lithium by-product on the surface of the cathode active material may not be easy. If the organic acid exceeds 5 parts by weight based on 100 parts by weight of distilled water , The surface of the positive electrode active material may be damaged.

40 parts by weight to 60 parts by weight of the cathode active material may be mixed and stirred with respect to 100 parts by weight of the aqueous solution containing the organic acid. For example, if the amount of the cathode active material is less than 40 parts by weight based on 100 parts by weight of the aqueous solution containing the organic acid, the cathode active material may be damaged due to a small amount of the cathode active material. If the amount of the cathode active material is more than 60 parts by weight based on 100 parts by weight of the aqueous solution containing an organic acid, the water is not smoothly washed out due to the relatively small amount of the aqueous solution, Removal may not be easy.

The water-washing temperature in the washing step is preferably from 10 캜 to 15 캜. When the water-washing temperature is lower than the above range, the amount of LiOH dissociated in the distilled water is lowered, and consequently, the removal of LiOH may not be easy. The washing time in the washing step is not particularly limited, but may be, for example, 30 minutes to 600 minutes, preferably 60 minutes to 120 minutes.

In the cathode active material according to the present invention, the cathode active material may include a lithium transition metal oxide containing 70 mol% or more of nickel based on the total moles of transition metal oxides except lithium, And the content of lithium by-products present on the surface of the cathode active material is 0.3% by weight or less based on the total weight of the cathode active material.

In the cathode active material, lithium byproducts present on the surface of the cathode active material are controlled to 0.3 wt% or less based on the total weight of the cathode active material. Therefore, all of the above-described methods for producing the positive electrode active material can be applied to the positive electrode active material.

Also, there is provided a positive electrode for a lithium secondary battery comprising the positive electrode active material according to the present invention. Specifically, the cathode for a secondary battery includes a cathode current collector, a cathode active material layer formed on the cathode current collector, and the cathode active material layer includes the cathode active material according to the present invention.

Since the cathode active material is the same as that described above, a detailed description thereof will be omitted and only the remaining constitution will be specifically described below.

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. In addition, the cathode 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 adhesive force of the cathode 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 cathode active material layer may include a conductive material and, optionally, a binder optionally together with the cathode active material.

At this time, the cathode active material may be contained in an amount of 80 to 99% by weight, more specifically 85 to 98.5% by weight based on the total weight of the cathode active material layer. When included in the above content range, excellent capacity characteristics can be exhibited.

The conductive material is used for imparting conductivity to the electrode. The conductive material is not particularly limited as long as it has electron conductivity without causing chemical change. 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; Conductive whiskey such as zinc oxide and potassium titanate; 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 included in an amount of 0.1 to 15% by weight based on the total weight of the cathode active material layer.

The binder serves to improve the adhesion between the positive electrode active material particles and the adhesion between the positive electrode active material and the current collector. Specific examples include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylalcohol, polyacrylonitrile, Polypropylene, ethylene-propylene-diene polymer (EPDM), sulphonated-EPDM, polyvinylpyrrolidone, polyvinylpyrrolidone, polyvinylpyrrolidone, Styrene-butadiene rubber (SBR), fluorine rubber, and various copolymers thereof, and one kind or a mixture of two or more kinds of them may be used. The binder may be included in an amount of 0.1 to 15% by weight based on the total weight of the cathode active material layer.

The positive electrode may be manufactured according to a conventional positive electrode manufacturing method, except that the positive electrode active material described above is used. Specifically, the cathode active material and optionally the binder and the conductive material may be dissolved or dispersed in a solvent to prepare a composition for forming a cathode active material layer, which is then applied onto the cathode current collector, followed by drying and rolling.

Examples of the solvent include dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), acetone, and the like. Water and the like, and one kind or a mixture of two or more kinds can be used. The amount of the solvent to be used is sufficient to dissolve or disperse the cathode active material, the conductive material and the binder in consideration of the coating thickness of the slurry and the yield of the slurry, and then to have a viscosity capable of exhibiting excellent thickness uniformity Do.

Alternatively, the positive electrode may be produced by casting the composition for forming the positive electrode active material layer on a separate support, and then laminating a film obtained by peeling from the support onto the positive electrode collector.

In addition, the present invention can produce an electrochemical device including the positive electrode. The electrochemical device may be specifically a battery, a capacitor, or the like, and more specifically, it may be a lithium secondary battery.

Specifically, the lithium secondary battery includes a positive electrode, a negative electrode disposed opposite to the positive electrode, and a separation membrane and an electrolyte interposed between the positive electrode and the negative electrode. The positive electrode is the same as that described above, Only the remaining configuration will be described in detail below.

The lithium secondary battery may further include a battery container for housing the electrode assembly of the anode, the cathode, and the separator, and a sealing member for sealing the battery container.

In the lithium secondary battery, the negative electrode includes a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector.

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. In addition, the negative electrode collector may have a thickness of 3 to 500 탆, and similarly to the positive electrode collector, fine unevenness may be formed on the surface of the 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.

The anode active material layer optionally includes a binder and a conductive material together with the anode active material.

As the negative electrode active material, a compound capable of reversible intercalation and deintercalation of lithium may be used. Specific examples thereof include carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber and amorphous 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; Metal oxides capable of doping and dedoping lithium such as SiO? (0 <? <2), SnO ₂ , vanadium oxide, and lithium vanadium oxide; Or a composite containing the metallic compound and the carbonaceous material such as Si-C composite or Sn-C composite, and any one or a mixture of two or more thereof may be used. Also, a metal lithium thin film may be used as the negative electrode active material. The carbon material may be both low-crystalline carbon and high-crystallinity carbon. Examples of the low-crystalline carbon include soft carbon and hard carbon. Examples of the highly crystalline carbon include natural graphite, artificial graphite, artificial graphite or artificial graphite, Kish graphite graphite, pyrolytic carbon, mesophase pitch based carbon fiber, meso-carbon microbeads, mesophase pitches and petroleum or coal tar coke derived cokes).

The negative electrode active material may include 80% by weight to 99% by weight based on the total weight of the negative electrode active material layer.

The binder is a component for assisting the bonding between the conductive material, the active material and the current collector, and is usually added in an amount of 0.1% by weight to 10% by weight based on the total weight of the negative electrode active material layer. Examples of such binders include polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene Examples thereof include ethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrene-butadiene rubber, nitrile-butadiene rubber, fluorine rubber and various copolymers thereof.

The conductive material may be added in an amount of 10 wt% or less, preferably 5 wt% or less, based on the total weight of the negative electrode active material layer, as a component for further improving the conductivity of the negative electrode active material. 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 acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal 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.

For example, the negative electrode active material layer is prepared by applying and drying a composition for forming a negative electrode active material layer, which is prepared by dissolving or dispersing a negative electrode active material on a negative electrode current collector, and optionally a binder and a conductive material in a solvent, Casting a composition for forming an active material layer on a separate support, and then laminating a film obtained by peeling from the support onto a negative electrode current collector.

The negative electrode active material layer may be formed by applying and drying a composition for forming a negative electrode active material layer prepared by dissolving or dispersing a negative electrode active material on a negative electrode collector and optionally a binder and a conductive material in a solvent, Casting the composition on a separate support, and then peeling the support from the support to laminate a film on the negative electrode current collector.

Meanwhile, in the lithium secondary battery, the separation membrane separates the cathode and the anode and provides a passage for lithium ion. The separation membrane can be used without any particular limitation as long as it is used as a separation membrane in a lithium secondary battery. Particularly, It is preferable to have a low resistance and an excellent ability to impregnate the electrolyte. 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. In order to secure heat resistance or mechanical strength, a coated separator containing a ceramic component or a polymer material may be used, and the separator may be selectively used as a single layer or a multilayer structure.

Examples of the electrolyte used in the present invention include an organic liquid electrolyte, an inorganic liquid electrolyte, a solid polymer electrolyte, a gel-type polymer electrolyte, a solid inorganic electrolyte, and a molten inorganic electrolyte that can be used in the production of a lithium secondary battery. It is not.

Specifically, the electrolyte may include an organic solvent and a lithium salt.

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; Alcohol solvents such as ethyl alcohol and isopropyl alcohol; R-CN (R is a linear, branched or cyclic hydrocarbon group having 2 to 20 carbon atoms, which may contain a double bond aromatic ring or an ether bond); Amides such as dimethylformamide; Dioxolanes such as 1,3-dioxolane; Or sulfolane may 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. In this case, when the cyclic carbonate and the chain carbonate are mixed in a volume ratio of about 1: 1 to about 1: 9, the performance of the electrolytic solution may be excellent.

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 concentration of the lithium salt is preferably in the range of 0.1 to 2.0 M. When the concentration of the lithium salt is within the above range, the electrolyte has an appropriate conductivity and viscosity, so that it can exhibit excellent electrolyte performance and the lithium ion can effectively move.

In addition to the electrolyte components, the electrolyte may contain, for example, a haloalkylene carbonate-based compound such as difluoroethylene carbonate or the like, pyridine, triethanolamine, or the like for the purpose of improving lifetime characteristics of the battery, Ethyl phosphite, triethanol amine, cyclic ether, ethylenediamine, glyme, hexametriamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N, At least one additive such as benzyl alcohol, benzyl alcohol, benzyl alcohol, benzyl alcohol, benzyl alcohol, benzyl alcohol, benzyl alcohol, benzyl alcohol, benzyl alcohol, The additive may be included in an amount of 0.1 to 5% by weight based on the total weight of the electrolyte.

As described above, since the lithium secondary battery including the cathode active material according to the present invention stably exhibits excellent discharge capacity, output characteristics and life characteristics, it can be used in portable devices such as mobile phones, notebook computers, digital cameras, and hybrid electric vehicles hybrid electric vehicle (HEV)).

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.

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.

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

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

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

Example  One

Ni _{0 . 86} Co _{0 . 10} Mn _{0 . 02} Al _{0 .02} (OH) ₂ precursor and LiOH were mixed so that the ratio of Li / Me was 1.15 and then fired in an oxygen atmosphere at 790 ° C. for 10 hours to obtain Li _{1 .15} Ni _0.86 Co _0.10 Mn _0.02 Al _0.02 O ₂ cathode active material.

Subsequently, an aqueous organic acid solution prepared by mixing 1.5 parts by weight of citric acid with 100 parts by weight of distilled water was mixed with 100 parts by weight of the organic acid aqueous solution and 40 parts by weight of the prepared cathode active material, Followed by washing with water, followed by vacuum drying at 130 DEG C for 10 hours.

The cathode active material washed, the carbon black conductive material, and the PVdF binder were mixed at a weight ratio of 96: 2: 2 in N-methylpyrrolidone (NMP) solvent to prepare a cathode active material slurry. This was coated on an Al foil having a thickness of 20 탆, followed by drying and roll pressing to prepare a positive electrode.

On the other hand, lithium metal was used as a cathode.

The positive electrode and the negative electrode prepared above were assembled with a co-cell of CR2032 together with a polypropylene (PP) separator, and a mixed solvent of ethylene carbonate: dimethyl carbonate: diethyl carbonate in a volume ratio of 1: 2: 1 was mixed with 1M of LiPF ₆ Was injected into the reactor to prepare a lithium secondary battery.

Comparative Example  One

A positive electrode and a secondary battery including the positive electrode were prepared in the same manner as in Example 1, except that the positive electrode active material was washed with distilled water.

Comparative Example  2

A positive electrode and a secondary battery including the positive electrode active material were prepared in the same manner as in Example 1, except that the positive electrode active material was washed with a washing solution prepared by mixing 1.5 parts by weight of citric acid with 100 parts by weight of ethanol.

Comparative Example  3

Li _{1 . 15} Ni _{0 . 6} Co _{0 . 2} Mn _{0 . 2} O ₂ cathode active material was washed with an aqueous organic acid solution prepared by mixing 1.5 parts by weight of citric acid with 100 parts by weight of distilled water to prepare a positive electrode and a secondary battery comprising the same .

Experimental Example  1: Surface of positive electrode active material Li  Busan quantity measurement

PH titration was performed to measure the amount of Li by-products present on the surface of the cathode active material prepared in Example 1 and Comparative Examples 1 and 2. pH was measured using a Metrohm pH meter and titrated by 1 mL. Specifically, the amount of lithium by-product on the surface of the cathode active material was measured by titrating the pH with a 0.1 N concentration of HCl using a Metrohm pH meter, as shown in Table 1 below.

Li byproduct LiOH Li ₂ CO ₃ sum Example 1 0.15 0.15 0.3 Comparative Example 1 0.15 0.35 0.5 Comparative Example 2 0.4 0.4 0.8

As shown in Table 1, it was confirmed that the cathode active material prepared in Example 1 had lithium byproducts on the surface of about 0.3 wt% or less. This is because LiOH was removed by water in the washing water solution and Li ₂ CO ₃ was easily removed by the organic acid contained in the washing water solution. However, in the case of Comparative Example 1, LiOH was easily removed by rinsing the cathode active material using only distilled water, while Li ₂ CO ₃ was not washed away from the surface of the cathode active material, and was found to exist on the surface. In the case of Comparative Example 2, the positive electrode active material was washed with water in an aqueous solution containing an organic solvent and an organic acid, whereby the detergency was lowered and removal of LiOH and Li ₂ CO ₃ was not easy.

Experimental Example  2: Lithium secondary battery Charging and discharging  Efficiency evaluation

The charge / discharge efficiency of the lithium secondary batteries prepared in Example 1 and Comparative Examples 1 to 3 was evaluated. The results are shown in Table 1 below.

Specifically, the lithium secondary batteries prepared in Example 1 and Comparative Examples 1 to 3 were each charged to 4.25 V at a constant current of 0.2 C at room temperature (25 캜), and discharged to 2.5 V at a constant current of 0.2 C After that, charge and discharge characteristics were observed in the first cycle. The charging / discharging efficiency according to the C-rate was measured by varying the discharging conditions at and after 2.0 C, and the results are shown in Table 2 below.

Charging @ 0.2C
(mAh / g) Discharge @ 0.2C
(mAh / g) efficiency (%) 1.0 C (%) 2.0 C (%) Example 1 230.1 205.4 89.3 92.0 87.4 Comparative Example 1 231.2 205.5 88.9 91.7 87.2 Comparative Example 2 231.2 201.4 87.1 90.2 86.1 Comparative Example 3 194.2 168.3 86.7 88.8 84.2

As shown in Table 2, the lithium secondary battery manufactured in Example 1 exhibited excellent efficiency of 89% or more when charged at a 0.2C-rate, and superior efficiency at 1C-rate and 2C-rate as compared with Comparative Examples 1 to 3 . &Lt; / RTI &gt; Particularly, in the case of Comparative Example 3, it was confirmed that the use of a transition metal oxide having a smaller content of Ni than Example 1 was used to improve charge-discharge efficiency by surface damage of the positive electrode active material during water washing.

Experimental Example  3: Evaluation of life characteristics

The lifetime characteristics of each of the lithium secondary batteries prepared in Example 1 and Comparative Examples 1 and 2 were measured according to cycles. Specifically, the lithium secondary batteries prepared in Example 1 and Comparative Examples 1 and 2 were charged to 4.25 V at a constant current of 0.3 C, respectively. Thereafter, discharging was performed until the voltage reached 2.5 V at a constant current of 0.3 C. The charging and discharging behaviors were set as one cycle. After repeating this cycle 30 times, the capacity retention ratios according to Examples 1 and 2 and Comparative Examples 1 and 2 were measured and shown in Fig.

As shown in FIG. 1, it was confirmed that the lithium secondary battery of Example 1 exhibited a better capacity retention ratio than the lithium secondary batteries of Comparative Examples 1 and 2 during 30 cycles of 30 cycles.

Experimental Example 4 : Evaluation of resistance characteristics

The resistance characteristics of each of the lithium secondary batteries prepared in Example 1 and Comparative Examples 1 and 2 were measured according to cycles. Specifically, the lithium secondary batteries prepared in Example 1 and Comparative Examples 1 and 2 were charged to 4.25 V at a constant current of 0.3 C, respectively. Thereafter, discharging was performed until the voltage reached 2.5 V at a constant current of 0.3 C. At this time, the voltage corresponding to 60 seconds was recorded, and the resistance was calculated every cycle by dividing the difference from the initial voltage by the applied current. The charging and discharging behaviors were set as one cycle. After repeating this cycle 30 times, the resistance increase rate according to Example 1 and Comparative Examples 1 and 2 was measured and shown in FIG.

As shown in FIG. 2, the resistance of the lithium secondary battery manufactured in Example 1 increased about twice during 30 cycles, while the resistance of the lithium secondary batteries prepared in Comparative Examples 1 and 2 was 2.5 times and 2.8 times It was confirmed that the resistance increase rate of the lithium secondary battery manufactured in Example 1 was lower than that of the lithium secondary batteries prepared in Comparative Examples 1 and 2. [

Claims (8)

Preparing a cathode active material comprising 70 mol% or more of nickel relative to the total number of moles of transition metals except lithium; And
And washing the positive electrode active material with an aqueous solution containing an organic acid.
The method according to claim 1,
Wherein the cathode active material is washed with an aqueous solution containing the organic acid to control lithium byproduct present on the surface of the cathode active material to 0.3 wt% or less based on the total weight of the cathode active material.
3. The method of claim 2,
The lithium by-products A method of manufacturing a positive electrode active material at least one or more selected from the group consisting of LiOH and Li ₂ CO _3.
The method according to claim 1,
Wherein the organic acid comprises at least one selected from the group consisting of citric acid, malic acid, formic acid, acetic acid, oxalic acid, and ascorbic acid.
The method according to claim 1,
Wherein the aqueous solution containing the organic acid contains 0.5 to 5 parts by weight of organic acid per 100 parts by weight of distilled water.
The method according to claim 1,
Wherein the positive electrode active material is contained in an amount of 40 to 60 parts by weight based on 100 parts by weight of the aqueous solution containing organic acid in the washing step.
The method according to claim 1,
Deg.] C to 15 [deg.] C in the water washing step.
The method according to claim 1,
Wherein the cathode active material is represented by the following formula (1): &lt; EMI ID =
[Chemical Formula 1]
Li _{1 + a} (Ni _b Co _c M ¹ _d M ² _e ) _1-a O ₂
In Formula 1,
M ¹ is at least one or more selected from the group consisting of Mn and Al,
M ² is at least one or more selected from the group consisting of Zr, B, Al, Co, W, Mg, Ce, Hf, Ta, La, Ti, Sr, Ba, Ce, F, P,
0? A? 0.2, 0.7? B? 1.0, 0? C? 0.3, 0? D? 0.3, 0? E?

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