KR20170043902A - Surface coated positive electrode active particle and secondary batterty comprising the same - Google Patents

Abstract

The present invention relates to surface-coated positive electrode active particles and a secondary battery comprising the same. Provided are the surface-coated positive electrode active material particles, comprising: positive electrode active material particles; and a coating layer applied on the surfaces of the positive electrode active material particles; wherein the coating layer comprises polyimid having a weight mean molecular weight of 3,000-50,000, and wherein the polyimid has a weight% of 0.1-1 with respect to the total weight of the positive electrode active material particles. The surface-coated positive electrode active material particles according to the present invention comprise the coating layer including 0.1 to 1 weight% of polyimid having a weight mean molecular weight of 3,000-50,000 with respect to the positive electrode active material particles. Accordingly, direct contact between the positive electrode active material particles and the electrolyte can be prevented, and thus side reaction between the positive electrode active material particles and the electrolyte can be suppressed, thereby significantly improving the life characteristic of a secondary battery. In particular, the life and conductive characteristics of a secondary battery under high-temperature and high-voltage conditions can be improved.

Description

TECHNICAL FIELD [0001] The present invention relates to a surface-coated cathode active material particle and a secondary battery including the cathode active material particle. BACKGROUND ART [0002]

The present invention relates to surface-coated cathode active material particles and a secondary battery comprising the same.

Lithium rechargeable batteries have been widely used as power sources for portable devices since they appeared in 1991 as small-sized, lightweight, and large-capacity batteries. In recent years, with the rapid development of the electronics, communication, and computer industries, camcorders, mobile phones, and notebook PCs have been remarkably developed and demand for lithium secondary batteries as a power source for driving these portable electronic information communication devices is increasing day by day .

The lithium secondary battery has a problem in that its service life is rapidly deteriorated by repeated charging and discharging.

This degradation in lifetime is caused by a side reaction between the anode and the electrolyte, and this phenomenon may become more serious under high voltage and high temperature conditions.

Therefore, it is necessary to develop a secondary battery for a high voltage. For this purpose, it is very important to control the side reaction between the cathode active material and the electrolyte or the electrode interface reaction.

To solve these problems, a technique has been developed for coating a surface of a cathode active material with a metal oxide including Mg, Al, Co, K, Na, or Ca.

In particular, it is generally known that oxides such as Al ₂ O ₃ , ZrO ₂ , and AlPO ₄ can be coated on the surface of the cathode active material on the surfaces of these cathode active materials. It is also true that the coating layer improves the safety characteristics of the cathode active material.

However, in the case of surface coating using the oxide coating layer, the oxide coating layer is finely dispersed in the form of nano-sized particles rather than entirely covering the surface of the cathode active material.

As a result, the surface modification effect of the cathode active material by the oxide coating layer is limited. In addition, the oxide coating layer is a kind of ion-insulating layer which is difficult to move lithium ions, and may cause deterioration of ionic conductivity.

Korean Patent No. 10-1105342

Accordingly, the present invention has been made in order to solve the above problems.

The present invention provides a surface-coated cathode active material particle having excellent lifetime characteristics by preventing a side reaction between a cathode active material and an electrolyte by forming a coating layer on the surface of the cathode active material particle.

According to an aspect of the present invention, there is provided a positive active material composition comprising a positive electrode active material particle and a coating layer coated on the surface of the positive electrode active material particle, wherein the coating layer comprises a polyimide having a weight average molecular weight of 3,000 to 50,000, The surface-coated cathode active material particles are contained in an amount of 0.1 wt% to 1 wt% based on the total weight of the cathode active material particles.

The present invention also provides a battery module and a battery pack including a cathode including a positive electrode mixture coated with the surface-coated positive electrode active material particles, a negative electrode, and a separator interposed between the positive electrode and the negative electrode.

Since the surface-coated cathode active material particles according to the present invention include a coating layer containing a polyimide having a weight average molecular weight of 3,000 to 50,000 in an amount of 0.1% by weight to 1% by weight based on the cathode active material particle, By preventing direct contact, side reactions between the cathode active material particles and the electrolytic solution can be suppressed, and the lifetime characteristics of the secondary battery can be remarkably improved. Particularly, it is possible to improve the lifetime characteristics and the conductive characteristics of the secondary battery under high temperature and high voltage conditions.

BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, which are incorporated in and form a part of the specification, illustrate preferred embodiments of the invention and, together with the description of the invention given above, It should not be construed as limited.
FIG. 1 is an electron microscope (FE-SEM) photograph of the surface of a cathode active material surface-coated with a nanoclay comprising the polyimide prepared in Example 1 of the present invention.
2 is an electron microscope (FE-SEM) photograph of the surface of the non-surface-coated cathode active material prepared in Comparative Example 1. FIG.
3 is an electron microscopic (FE-SEM) photograph of the surface of the cathode active material surface-coated with the polyimide prepared in Comparative Example 2. FIG.

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

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

The terminology used herein is for the purpose of describing exemplary embodiments only and is not intended to be limiting of the invention. The singular expressions include plural expressions unless the context clearly dictates otherwise.

In this specification, the terms "comprising," "comprising," or "having ", and the like are intended to specify the presence of stated features, integers, But do not preclude the presence or addition of one or more other features, integers, steps, components, or combinations thereof.

The surface-coated cathode active material particle according to an embodiment of the present invention includes a cathode active material particle and a coating layer coated on the surface of the cathode active material particle, and the coating layer includes a polyimide having a weight average molecular weight of 3,000 to 50,000 And the polyimide may be contained in an amount of 0.1 wt% to 1 wt% with respect to the total weight of the cathode active material particles.

The coating layer included in the surface-coated cathode active material particles according to an embodiment of the present invention includes a polyimide capable of lithium ion migration, not an ionic insulation layer such as an inorganic oxide coating layer that has been conventionally coated on the surface can do. By coating the surface of the cathode active material particle with the coating layer, a side reaction between the cathode active material and the electrolyte can be prevented, thereby improving the lifetime characteristics of the secondary battery.

Specifically, the coating layer may be coated in a thin film form so as to cover the entire surface of the cathode active material particle. As a result, the surface-coated cathode active material particles can further improve not only general voltage conditions but also life characteristics and conductivity at high temperature and high voltage conditions. As a result, the polyimide contained in the coating layer may serve as a protective film for preventing direct contact of the positive electrode active material with the electrolyte.

In the surface-coated cathode active material according to an embodiment of the present invention, the polyimide may have a weight average molecular weight of 3,000 to 50,000, specifically 10,000 to 30,000.

If the weight average molecular weight of the polyimide is less than 3,000, side reactions of the electrolyte solution and the cathode active material can not be effectively prevented as compared with polyimide having a weight average molecular weight of more than 3,000, When the weight average molecular weight of the polyimide is more than 50,000, it is difficult for lithium ions to migrate into the cathode active material, resulting in a problem of low discharge capacity and C-rate characteristics.

The polyimide may be contained in an amount of 0.1 to 1% by weight, specifically 0.2 to 0.7% by weight based on the total weight of the cathode active material particles. If the polyimide is contained in an amount of less than 0.1% by weight based on the positive electrode active material particles, the polyimide does not completely cover the surface of the positive electrode active material particles even when the weight average molecular weight satisfies the above range, If it is contained in an amount of more than 1% by weight, the resistance of the lithium ion migration increases, which may cause a problem of poor discharge capacity and C-rate characteristics.

The weight average molecular weight of the polyimide can be measured by Gel Permeation Chromatography (GPC). Specifically, by measuring the weight average molecular weight of the polyamic acid by GPC, the weight average molecular weight of the polyimide produced from the imidization reaction of the polyamic acid can be determined. At this time, the polyimide is generated from the imidization reaction of the polyamic acid. Since the difference between the weight average molecular weights of the polyamic acid and the polyimide is about 3%, which is lower than the GPC measurement error, the weight average molecular weight of the polyamic acid is measured to determine the weight average molecular weight of the polyimide can do.

The thickness of the coating layer may be 1 to 200 nm, specifically 5 nm to 50 nm. If the thickness of the coating layer is less than 1 nm, the effect of preventing the side reaction between the cathode active material and the electrolyte due to the coating layer may be insignificant. In addition, when the thickness of the coating layer exceeds 200 nm, the resistance may increase due to an obstacle to the mobility of lithium ions.

According to an embodiment of the present invention, the cathode active material particles can be applied to a general voltage or a high voltage, and can be used without limitation as long as they are capable of reversibly intercalating / deintercalating lithium.

Specifically, the surface-coated cathode active material particles according to one embodiment of the present invention include a hexagonal layered rock salt structure, an olivine structure, a lithium transition metal oxide of spinel having a cubic structure, V ₂ O ₅ , TiS, MoS, and the like.

More specifically, it may include at least one oxide selected from the group consisting of oxides of the following general formulas (1) to (3) and V ₂ O ₅ , TiS and MoS;

≪ Formula 1 >

Li _{1 + x} [Ni _a Co _b Mn _c ] O ₂ (-0.5? X? 0.6, 0? A, b, c? 1, x + a + b + c = 1);

(2)

LiMn _{2 - x} M _x O ₄ (M = at least one element selected from the group consisting of Ni, Co, Fe, P, S, Zr, Ti and Al, 0? X? 2);

(3)

_{Li 1 + a Fe 1 - x} M x (PO 4-b) X b (M = Al, Mg, Ni, Co, Mn, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn , and Y X is at least one element selected from the group consisting of F, S and N, -0.5? A? +0.5, 0? X? 0.5, and 0? B? 0.1. .

More specifically, the cathode active material particles may be formed of LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , LiMn ₂ O ₄ , Li [Ni _a Co _b Mn _c ] O ₂ (0 <a, b, c ≤ 1, a + b + c = 1), and LiFePO ₄ .

Meanwhile, according to one embodiment of the present invention, a method of manufacturing the surface-coated cathode active material particles can be provided.

A method of manufacturing a cathode active material particle according to an embodiment of the present invention includes dispersing cathode active material particles in an organic solvent diluted with polyamic acid having a weight average molecular weight of 1,000 to 30,000 to form a coating layer comprising a polyamic acid on the cathode active material particle surface (Step 1); And imidizing the cathode active material particles including the coating layer (step 2).

Specifically, in the method of preparing the surface-coated cathode active material particles according to an embodiment of the present invention, the step 1 is a step of dispersing the cathode active material particles in an organic solvent diluted with polyamic acid having a weight average molecular weight of 1,000 to 30,000, Thereby forming a coating layer containing a polyamic acid on the surface of the positive electrode active material particles.

Polyimide is produced from the imidization reaction of a polyamic acid, and a polyimide having a weight average molecular weight of 1,000 to 30,000 can be used to produce a polyimide having a similar weight average molecular weight.

In the polyamic acid-diluted organic solvent, 0.1 to 1 part by weight of polyamic acid may be included in 100 parts by weight of the organic solvent. As the polyimide coating layer is produced at the diluted concentration, a polyimide coating layer containing 0.1 to 1% by weight based on the total weight of the cathode active material particles can be finally produced.

The polyamic acid can be prepared by using an aromatic anhydride and a diamine in a conventional manner used in the art.

More specifically, the polyamic acid can be prepared by reacting an aromatic anhydride with diamine in the same equivalent amount.

Examples of the aromatic anhydrides include phthalic anhydride, pyromellitic dianhydride, 3,3'4,4'-biphenyltetracarboxylic dianhydride, 4'4-oxydiphthalic anhydride, 3, 3'4,4'-benzophenone tetracarboxylic dianhydride, trimellitic ethylene glycol, 4,4 '- (4'4-isopropylbiphenoxy) biphthalic anhydride and trimellitic anhydride Hydride, or a mixture of two or more thereof.

Examples of the diamine include 4,4'-oxydianiline, p-phenyldiamine, 2,2-bis (4- (4-aminophenoxy) -phenyl) Propane, p-methylene dianiline, propyltetramethyldisiloxane, polyaromatic amine, 4,4'-diaminodiphenylsulfone, 2,2'-bis (trifluoromethyl) Biphenyl, and 3,5-diamino-1,2,4-triazole, or a mixture of two or more thereof.

According to one embodiment of the present invention, the polyamic acid may include a four-component polyamic acid, and the four-component polyamic acid may include pyromellitic dianhydride, biphenyl dianhydride, It is preferably a polyamic acid including phenylenediamine and oxydianiline.

The organic solvent is not particularly limited as long as it is a solvent capable of dissolving the polyamic acid, and is selected from the group consisting of cyclohexane, carbon tetrachloride, chloroform, methylene chloride, dimethylformamide, dimethylacetamide and N-methylpyrrolidone And may include one or more species.

In addition, in the method of preparing the surface-coated cathode active material particles according to an embodiment of the present invention, the step 1 may be performed by dispersing the cathode active material particles in an organic solvent in which polyamic acid is diluted to form a polyamic acid on the surface of the cathode active material particle To form a coating layer.

The dispersion of the cathode active material particles is preferably dispersed in the mixed solution for 1 hour or more using a high-speed stirrer after the cathode active material particles are uniformly dispersed in the mixed solution. When the solvent is removed by heating and concentration after confirming uniform dispersion of the cathode active material particles, a cathode active material particle having a surface coated with a coating layer containing a polyamic acid can be obtained.

In the method of preparing the surface-coated cathode active material particles according to an embodiment of the present invention, the step 2 is a step in which a polyamic acid having a coating layer formed on the surface of the cathode active material particles obtained in the step 1 is subjected to imidization to form a polyimide And converting the received signal.

In the imidization reaction, the cathode active material particles containing the coating layer obtained in the step 1 are heated at a rate of 3 ° C / minute from 50 ° C to 100 ° C at a temperature of about 300 ° C to 400 ° C, For 10 minutes to 120 minutes. After the temperature is raised at an interval of 50 to 100 캜, the temperature may be maintained for 10 to 120 minutes, for example. More specifically, the positive electrode active material particles containing the coating were heated at 60 ° C, 120 ° C, 200 ° C, 300 ° C and 400 ° C at a rate of 3 ° C / min, , Held at 200 占 폚 for 60 minutes, at 300 占 폚 for 60 minutes, and at 400 占 폚 for 10 minutes, the imidization reaction can proceed.

The surface of the cathode active material particle obtained in the above step 1 may form a coating layer containing polyimide on the surface of the cathode active material particle by the step 2.

According to one embodiment of the present invention, a cathode active material particle; And a coating layer comprising a polyimide having a weight average molecular weight of 3,000 to 50,000 on the surface of the positive electrode active material particles, suppresses the reaction of the positive electrode active material particles with a direct electrolyte, The lifetime characteristics can be improved, and the improvement in lifetime characteristics particularly at high temperature and high voltage conditions can be further enhanced.

As used herein, the term &quot; normal voltage &quot; means the case where the charging voltage of the lithium secondary battery is in the range of 3.0 V to less than 4.2 V, and the term &quot; high voltage &quot; means that the charging voltage is in the range of 4.2 V to 5.0 V , And the term &quot; high temperature &quot; may mean in the range of 45 ° C to 65 ° C.

The present invention also provides a positive electrode comprising the surface-coated positive electrode active material particles.

The anode may be prepared by a conventional method known in the art. For example, a slurry is prepared by mixing and stirring a solvent, a binder, a conductive agent, and a dispersant, if necessary, on the surface-coated cathode active material particles, applying the slurry to a current collector of a metal material, An anode can be produced.

The current collector of the metal material is a metal having high conductivity and can be easily adhered to the slurry of the cathode active material, and any material can be used as long as it is not reactive in the voltage range of the battery. Non-limiting examples of the positive electrode current collector include foil produced by aluminum, nickel, or a combination thereof.

Examples of the solvent for forming the positive electrode include organic solvents such as NMP (N-methylpyrrolidone), DMF (dimethylformamide), acetone, and dimethylacetamide, and water. These solvents may be used alone or in combination of two or more Can be mixed and used. The amount of the solvent used is sufficient to dissolve and disperse the cathode active material, the binder and the conductive agent in consideration of the coating thickness of the slurry and the production yield.

Examples of the binder include polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride, polyacrylonitrile, polymethylmethacrylate, Polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene-propylene-diene monomer (EPDM) Various kinds of binder polymers such as sulfonated EPDM, styrene butadiene rubber (SBR), fluorine rubber, poly acrylic acid and polymers substituted with Li, Na or Ca, or various copolymers thereof are used .

The conductive material is not particularly limited as long as it has electrical conductivity without causing chemical changes in the battery, and examples thereof include graphite such as natural graphite and artificial graphite; Carbon black such as carbon black, acetylene black, Ketjen black, channel black, panes black, lamp black, and thermal black; Conductive fibers such as carbon fiber and metal fiber; Conductive tubes such as carbon nanotubes; Metal powders such as fluorocarbon, aluminum and nickel powder; Conductive whiskers 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 dispersing agent may be an aqueous dispersing agent or an organic dispersing agent such as N-methyl-2-pyrrolidone.

The present invention also provides a secondary battery including the positive electrode, the negative electrode, and the separator interposed between the positive electrode and the negative electrode.

As the negative electrode active material used for the negative electrode according to an embodiment of the present invention, a carbon material, lithium metal, silicon, or tin which lithium ions can be occluded and released can be used. Preferably, carbon materials can be used, and carbon materials such as low-crystalline carbon and highly-crystalline carbon can be used. Examples of the low crystalline carbon include soft carbon and hard carbon. Examples of highly crystalline carbon include natural graphite, Kish graphite, pyrolytic carbon, liquid crystal pitch carbon fiber high temperature sintered carbon such as mesophase pitch based carbon fiber, meso-carbon microbeads, mesophase pitches and petroleum or coal tar pitch derived cokes.

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.

As the binder and the conductive agent used for the cathode, those which can be commonly used in the art can be used as the anode. The negative electrode may be prepared by mixing and stirring the negative electrode active material and the additives, and then applying the negative electrode active material slurry to the current collector and compressing the negative electrode active material slurry.

As the separator, a conventional porous polymer film conventionally used as a separator, for example, a polyolefin such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, and an ethylene / methacrylate copolymer A porous polymer film made of a high molecular weight polymer may be used alone or in a laminated manner, or a nonwoven fabric made of a conventional porous nonwoven fabric such as a glass fiber having a high melting point, a polyethylene terephthalate fiber or the like may be used. It is not.

The lithium salt that can be used as the electrolyte used in the present invention may be any of those conventionally used in an electrolyte for a lithium secondary battery. For example, the anion of the lithium salt may include F ^- , Cl ^- , Br ^- , I ^{- _{NO 3 -, N (CN)}} 2 -, BF 4 -, ClO 4 -, PF 6 -, (CF 3) 2 PF 4 -, (CF 3) 3 PF 3 -, (CF 3) 4 PF 2 -, ^{_{(CF 3) 5 PF -,}} (CF 3) 6 P -, CF 3 SO 3 -, CF 3 CF 2 SO 3 -, (CF 3 SO 2) 2 N -, (FSO 2) 2 N -, CF 3 _{CF 2 (CF 3) 2 CO} -, (CF 3 SO 2) 2 CH -, (SF 5) 3 C -, (CF 3 SO 2) 3 C -, CF 3 (CF 2) 7 SO 3 -, CF ₃ CO ₂ ^- , CH ₃ CO ₂ ^- , SCN ^-, and (CF ₃ CF ₂ SO ₂ ) ₂ N ^- .

Examples of the electrolyte used in the present invention include an organic-based 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. no.

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.

Preferable examples of the above medium and large-sized devices include, but are not limited to, electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and electric power storage systems.

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

&Lt; Example 1 >

Step 1: Poly Amic acid  Produce

5.36 g (0.02 mol) of phenylene diamine (PDA) was dissolved in 40 g of dimethylacetamide (DMAC) as a reaction solvent while nitrogen gas was slowly passed through a 250-mL reactor equipped with a stirrer and a nitrogen- , 2.16 g (0.02 mole) of biphenyl-tetracarboxylic acid dianhydride (BPDA) and 0.559 g (3.777 mol) of endcapper imphthalic anhydride (PA) were added while nitrogen gas was passed through Add 50 g of solvent. And polymerized at -0 [deg.] C for 12 hours to obtain polyamic acid (BPDA + PDA). The weight average molecular weight (M _w ) of the polyamic acid (PMDA + DDA) was 10,000 g / mol as measured by a GPC instrument at a concentration of 0.5 dL / g in a dimethylacetamide solution at 30 ° C.

Step 2: Poly Amic acid  And a mixed solution of an organic solvent

As the organic solvent, 0.5 part by weight of a polyamic acid having a weight average molecular weight of 10,000 g / mol prepared in Step 1 was diluted with 100 parts by weight of dimethylacetamide to prepare a mixed solution containing a polyamic acid at a concentration of 0.5% by weight .

Step 3: Step of forming a film on the surface of the positive electrode active material particle

20 g of LiCoO ₂ particles as a cathode active material was added to 20 g of the mixed solution obtained in the step 2, and the mixture was stirred for 1 hour using a high-speed stirrer. The temperature was raised to the boiling point of the solvent while stirring was continued to evaporate the solvent, thereby preparing a cathode active material coated with a coating film comprising a polyamic acid having a weight average molecular weight of 10,000 g / mol.

Step 4: Imidated  Reacted Poly  A process for producing a surface-coated cathode active material comprising a coating layer comprising an imide

The positive electrode active material coated on the surface of the coating layer containing the polyamic acid obtained in the step 3 was heated at a rate of 3 ° C / min at 60 ° C, 120 ° C, 200 ° C, 300 ° C and 400 ° C, Min, 120 캜 for 30 minutes, 200 캜 for 60 minutes, 300 캜 for 60 minutes, and 400 캜 for 10 minutes to carry out the imidization reaction. After the imidization reaction was completed, a surface-coated LiCoO ₂ cathode active material was prepared, which comprised a coating layer containing polyimide and magnesium ions.

Step 5: Preparation of lithium secondary battery

Anode manufacturing

The coated surfaces prepared as described in Step 4 LiCoO ₂ was used as the positive electrode active material particle. The cathode active material particles, carbon black as a conductive agent and polyvinylidene fluoride (PVdF) as a binder were mixed in a weight ratio of 95: 3: 2, and the mixture was dispersed in a solvent of N-methyl-2-pyrrolidone (NMP) To prepare a positive electrode mixture slurry. The positive electrode mixture slurry was applied to an aluminum (Al) thin film having a thickness of about 20 탆 and dried at 130 캜 for 2 hours to prepare a positive electrode, followed by roll pressing to produce a positive electrode Respectively.

Cathode manufacture

Lithium metal foil was used as the cathode.

Electrolytic solution manufacturing

LiPF ₆ was added to a nonaqueous electrolyte solvent prepared by mixing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) as electrolytes in a volume ratio of 1: 2 to prepare a 1M LiPF ₆ nonaqueous electrolyte solution.

Lithium secondary battery manufacturing

After the thus fabricated positive electrode and negative electrode were fabricated by using a polyethylene separator (Dornensa, F2OBHE, thickness = 20 占 퐉) and a mixed separator of electrolyte and polypropylene interposed therebetween by a conventional method, And an electrolyte solution was injected to prepare a coin cell type lithium secondary battery.

&Lt; Example 2 >

The procedure of Example 1 was repeated except that polyimide having a weight average molecular weight (M _w ) of 30,000 g / mol was prepared by controlling the content of phthalic anhydride as end capper in the step 1 of Example 1. And a lithium secondary battery was produced in the same manner.

&Lt; Example 3 >

In Step 2 of Example 1, 0.2 part by weight of a polyamic acid having a weight average molecular weight of 10,000 g / mol prepared in Step 1 was diluted with 100 parts by weight of dimethylacetamide as an organic solvent, Acid was prepared in the same manner as in Example 1, except that a mixed solution containing an acid was prepared.

<Example 4>

In Step 2 of Example 1, polyamic acid having a weight average molecular weight of 10,000 g / mol prepared in Step 1 was diluted with 0.7 part by weight of dimethylacetamide as an organic solvent, Acid was prepared in the same manner as in Example 1, except that a mixed solution containing an acid was prepared.

&Lt; Comparative Example 1 &

LiCoO ₂ not surface-coated with the cathode active material particles was used.

&Lt; Comparative Example 2 &

The procedure of Example 1 was repeated except that polyimide having a weight average molecular weight (M _w ) of 70,000 g / mol was prepared by controlling the content of phthalic anhydride as end capper in Step 1 of Example 1. And a lithium secondary battery was produced in the same manner.

&Lt; Comparative Example 3 &

The procedure of Example 1 was repeated except that polyimide having a weight average molecular weight (M _w ) of 2,000 g / mol was prepared by controlling the content of phthalic anhydride as end capper in Step 1 of Example 1 And a lithium secondary battery was produced in the same manner.

&Lt; Comparative Example 4 &

In Step 2 of Example 1, 0.05 part by weight of polyamic acid having a weight average molecular weight of 10,000 g / mol prepared in Step 1 was diluted with 100 parts by weight of dimethylacetamide as an organic solvent, Acid was prepared in the same manner as in Example 1, except that a mixed solution containing an acid was prepared.

&Lt; Comparative Example 5 &

In Step 2 of Example 1, 1.21 parts by weight of polyamic acid having a weight average molecular weight of 10,000 g / mol, prepared in Step 1, was diluted with 100 parts by weight of dimethylacetamide as an organic solvent, Acid was prepared in the same manner as in Example 1, except that a mixed solution containing an acid was prepared.

&Lt; Experimental Example 1 > Charging / discharging capacity and efficiency characteristics evaluation

The lithium secondary batteries of Examples 1 to 4 and Comparative Examples 1 to 5 were charged and discharged at 45 ° C at 0.5 C / 1 C in a voltage range of 3 to 4.5 V. C-rate is a ratio of a capacity when a battery packed with 0.5 C is discharged to 0.1 C and a capacity when discharged to 2 C as shown in the following Equation 1:

[Equation 1]

Polyamic acid
(% By weight) / polyimide for the positive electrode active material particles
Content (wt% Polyamic acid
Molecular Weight
(Polyimide
Molecular Weight) Early
Discharge capacity
(mAh / g) C-rate 50th cycle
Capacity retention (%) Example 1 0.5 / 0.5 10,000 195 96 97 Example 2 0.5 / 0.5 30,000 195 95.5 97 Example 3 0.2 / 0.2 10,000 195 96.5 98 Example 4 0.7 / 0.7 10,000 195 95.5 97 Comparative Example 1 - 196 97 80 Comparative Example 2 0.5 / 0.5 70,000 180 90 90 Comparative Example 3 0.5 / 0.5 2,000 195 97 82 Comparative Example 4 0.05 / 0.05 10,000 195 97 82 Comparative Example 5 1.2 / 1.2 10,000 180 90 85

As can be seen from the above Table 1, the lithium secondary battery of the Example shows a lifetime characteristic (50th cycle capacity retention amount) up to 22.5% higher than that of the secondary battery of the comparative example.

Specifically, it can be seen that in Examples 1 and 2 having a weight average molecular weight of 10,000 or 30,000, the lifetime characteristics are as high as 7.7 to 18% as compared with Comparative Example 2 or 3 having a weight average molecular weight of 70,000 or 2,000 .

Further, in the case of Comparative Examples 4 and 5 in which the weight average molecular weight is 10,000 but the polyimide content is 0.05 wt% or 1.2 wt%, the capacity retention amount is 82 to 85%, the polyimide content is 0.2 to 0.7 wt% , Examples 1 to 4 show a capacity retention ratio of 97% to 98%, and a maximum capacity reserve ratio difference of 19%.

As a result, it can be seen that the secondary battery using the surface-coated positive electrode active material containing the polyimide having a weight average molecular weight of 3,000 to 50,000 in an amount of 0.1 to 1% by weight based on the weight of the positive electrode active material has a high life characteristic.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it is to be understood that the invention is not limited to the disclosed exemplary embodiments, And falls within the scope of the invention.

Claims (10)

And a coating layer coated on the surface of the positive electrode active material particle, wherein the coating layer comprises a polyimide having a weight average molecular weight of 3,000 to 50,000, and the polyimide is 0.1 To &lt; RTI ID = 0.0 &gt; 1%. &Lt; / RTI &gt;
The method according to claim 1,
Wherein the polyimide has a weight average molecular weight of 10,000 to 30,000.
The method according to claim 1,
Wherein the polyimide is contained in an amount of 0.2 wt% to 0.7 wt% based on the total weight of the cathode active material particles.
The method according to claim 1,
Wherein the coating layer has a thickness of 1 to 200 nm.
The method of claim 1, wherein
Wherein the cathode active material particles are at least one oxide selected from the group consisting of oxides of the following general formulas 1 to 3 and V ₂ O ₅ , TiS, and MoS:
&Lt; Formula 1 >
Li _{1 + x} [Ni _a Co _b Mn _c ] O ₂ (-0.5? X? 0.6, 0? A, b, c? 1, x + a + b + c = 1);
(2)
LiMn _{2 - x} M _x O ₄ (M = at least one element selected from the group consisting of Ni, Co, Fe, P, S, Zr, Ti and Al, 0? X? 2);
(3)
_{Li 1 + a Fe 1 - x} M x (PO 4-b) X b (M = Al, Mg, Ni, Co, Mn, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn , and Y X is at least one element selected from the group consisting of F, S and N, -0.5? A? +0.5, 0? X? 0.5, and 0? B? 0.1. .
The method of claim 1, wherein
Wherein the positive electrode active material particles are LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , LiMn ₂ O ₄ , Li [Ni _a Co _b Mn _c ] O ₂ (0 <a, b, c ≤ 1, a + b + And LiFePO ₄ , wherein the surface-coated cathode active material particle is at least one selected from the group consisting of LiFePO ₄ and LiFePO ₄ .
A secondary battery comprising: a positive electrode coated with a positive electrode material mixture comprising the surface-coated positive electrode active material particles of claim 1; a negative electrode; and a separator interposed between the positive electrode and the negative electrode.
A battery module comprising the secondary battery of claim 7 as a unit cell.
A battery pack comprising the battery module of claim 8 and used as a power source for a middle- or large-sized device.
10. The method of claim 9,
Wherein the middle- or large-sized device is selected from the group consisting of an electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle and a system for power storage.

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