KR102170026B1 - Preparing methode for lithium secondary battery having improved low temperature property and lithium secondary battery - Google Patents

Abstract

The present invention comprises the steps of: (1) manufacturing an electrode assembly including an anode, a cathode, and a separator interposed between the anode and the cathode; (2) inserting the electrode assembly into a battery case; (3) manufacturing a lithium secondary battery by injecting a first electrolyte not containing propylene carbonate into the battery case into which the electrode assembly is inserted; (4) performing the formation of the lithium secondary battery of step (3); (5) removing the gas generated through the activation of step (4); And (6) injecting a second electrolyte containing propylene carbonate into the lithium secondary battery that has undergone step (5), and relates to a method of manufacturing a lithium secondary battery, wherein the lithium secondary battery according to the present invention The method improves the electrolyte injection process when manufacturing a lithium secondary battery, solves the problem of delamination of a carbon material used as an active material of a negative electrode, particularly graphite, and improves the electrochemical performance of the manufactured lithium secondary battery.

Description

Manufacturing method of lithium secondary battery with improved low-temperature characteristics and lithium secondary battery {PREPARING METHODE FOR LITHIUM SECONDARY BATTERY HAVING IMPROVED LOW TEMPERATURE PROPERTY AND LITHIUM SECONDARY BATTERY}

The present invention relates to a method of manufacturing a lithium secondary battery with improved low-temperature characteristics, and more particularly, by improving an electrolyte injection process when manufacturing a lithium secondary battery, the electrochemical performance of graphite used as an active material of a negative electrode can be improved. The present invention relates to a method of manufacturing a lithium secondary battery and a lithium secondary battery with improved electrochemical performance of a negative electrode manufactured by the method.

As technology development and demand for mobile devices increase, the demand for secondary batteries as an energy source is rapidly increasing, and among such secondary batteries, lithium secondary batteries exhibit high energy density and operating potential, long cycle life, and low self-discharge rate. Batteries are commercialized and widely used.

In addition, as interest in environmental issues has increased in recent years, electric vehicles (EV) and hybrid electric vehicles (HEV) that can replace vehicles that use fossil fuels such as gasoline vehicles and diesel vehicles, which are one of the main causes of air pollution. There is a lot of research on the back. Ni-MH secondary batteries are mainly used as power sources for electric vehicles (EV) and hybrid electric vehicles (HEV), but lithium secondary batteries with high energy density, high discharge voltage and output stability are used. Research is being actively conducted, and some have been commercialized.

On the other hand, metal oxides such as LiCoO ₂ , LiMnO ₂ , LiMn ₂ O ₄ or LiCrO ₂ are used as the positive electrode active material constituting the positive electrode of the lithium secondary battery. As the negative electrode active material constituting the negative electrode, metal lithium and graphite A carbon based meterial such as (graphite) or activated carbon, or a material such as silicon oxide (SiO _x ) is used. Among the negative electrode active materials, metal lithium was mainly used at the beginning, but as the charging and discharging cycle proceeds, lithium atoms grow on the surface of the metal lithium to damage the separator, thereby damaging the battery, and recently, carbon-based materials are mainly used.

In such a rechargeable lithium battery, charging and discharging are performed while repeating the process of intercalating and deintercalating lithium ions transferred from the lithium metal oxide of the positive electrode into the carbon-based electrode of the negative electrode. At this time, since lithium is highly reactive, it reacts with the carbon electrode to generate Li ₂ CO ₃ , LiO, LiOH, etc. to form a film on the surface of the negative electrode. Such a film is called a solid electrolyte (SEI) film, and the SEI film formed at the beginning of charging prevents the reaction between lithium ions and carbon negative electrodes or other materials during charging and discharging. It also acts as an ion tunnel, allowing only lithium ions to pass. This ion tunnel serves to prevent the collapse of the structure of the carbon negative electrode by solvating lithium ions and causing organic solvents of a high molecular weight electrolyte to move together with the carbon negative electrode.

Therefore, in order to improve the high-temperature cycle characteristics and low-temperature output of the lithium secondary battery, a solid SEI film must be formed on the negative electrode of the lithium secondary battery.

In general, a two/three component electrolyte based on ethylene carbonate (EC) is used as an electrolyte for a lithium ion battery. However, since EC has a high melting point, its use temperature is limited, and it may cause a significant decrease in battery performance at low temperatures. Therefore, propylene carbonate (PC) is mixed and used together to improve low-temperature performance, but when PC is used together as an electrolyte, carbon, especially graphite, is interlayered rather than forming a stable SEI film on the surface of the carbon anode. There is a problem of destruction by exfoliation.

Accordingly, it is necessary to develop a new technology capable of improving low-temperature performance by using PC as an electrolyte and preventing destruction of graphite due to addition of PC.

An object to be solved of the present invention is to provide a method of manufacturing a lithium secondary battery capable of improving the electrochemical performance of graphite used as an active material of a negative electrode by improving an electrolyte injection process when manufacturing a lithium secondary battery.

Another object to be solved of the present invention is to provide a lithium secondary battery manufactured according to the manufacturing method of the lithium secondary battery.

In order to solve the above problems, the present invention

(1) manufacturing an electrode assembly including an anode, a cathode, and a separator interposed between the anode and the cathode;

(2) inserting the electrode assembly into a battery case;

(3) manufacturing a lithium secondary battery by injecting a first electrolyte not containing propylene carbonate into the battery case into which the electrode assembly is inserted;

(4) performing the formation of the lithium secondary battery of step (3);

(5) removing the gas generated through the activation of step (4); And

(6) It provides a method of manufacturing a lithium secondary battery comprising the step of injecting a second electrolyte containing propylene carbonate into the lithium secondary battery that has been subjected to step (5).

In addition, the present invention in order to solve the above other problems,

It provides a lithium secondary battery manufactured according to the manufacturing method of the lithium secondary battery.

The method of manufacturing a lithium secondary battery according to the present invention improves the injection process of an electrolyte solution when manufacturing a lithium secondary battery, and solves the problem of delamination of a carbon material used as an active material of a negative electrode, especially graphite, It can improve the electrochemical performance.

Hereinafter, the present invention will be described in more detail to aid understanding of the present invention.

The terms or words used in this specification and claims should not be construed as being limited to their usual or dictionary meanings, and the inventor may appropriately define the concept of terms in order to describe his own invention in the best way. It should be interpreted as a meaning and concept consistent with the technical idea of the present invention based on the principle that there is.

The method of manufacturing a lithium secondary battery of the present invention includes the steps of: (1) preparing an electrode assembly including a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode; (2) inserting the electrode assembly into a battery case; (3) manufacturing a lithium secondary battery by injecting a first electrolyte not containing propylene carbonate into the battery case into which the electrode assembly is inserted; (4) performing the formation of the lithium secondary battery of step (3); (5) removing the gas generated through the activation of step (4); And (6) injecting a second electrolyte containing propylene carbonate into the lithium secondary battery that has been subjected to step (5).

(1) manufacturing an electrode assembly comprising an anode, a cathode, and a separator interposed between the anode and the cathode

The electrode assembly may be manufactured by a conventional method known in the art.

The positive electrode can be manufactured by a conventional method known in the art. For example, after preparing a slurry by mixing and stirring a solvent, a binder, a conductive material, and a dispersant as necessary in a positive electrode active material, it is applied (coated) to a current collector of a metal material, compressed, and dried to prepare a positive electrode. have.

The current collector of the metal material is a metal with high conductivity, and is a metal that can be easily adhered to the slurry of the positive electrode active material, and is particularly limited as long as it has high conductivity without causing chemical changes to the battery in the voltage range of the battery. It is not, for example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surface treated with carbon, nickel, titanium, silver or the like may be used. In addition, it is possible to increase the adhesion of the positive electrode active material by forming fine irregularities on the surface of the current collector. The current collector can be used in various forms such as a film, sheet, foil, net, porous body, foam, non-woven fabric, and may have a thickness of 3 to 500 μm.

The positive electrode active material is, for example, lithium cobalt oxide (LiCoO ₂ ); Lithium nickel oxide (LiNiO ₂ ); Li[Ni _a Co _b Mn _c M ¹ _d ]O ₂ (In the above formula, M ¹ is any one selected from the group consisting of Al, Ga, and In, or two or more elements of them, and 0.3≦a<1.0, 0 ≤b≤0.5, 0≤c≤0.5, 0≤d≤0.1, a+b+c+d=1); Li(Li _e M ² _fe-f' M ³ _f' ) O _{2 - g} A _g (in the above formula, 0≤e≤0.2, 0.6≤f≤1, 0≤f'≤0.2, 0≤g≤0.2, and , M ² includes at least one selected from the group consisting of Mn and Ni, Co, Fe, Cr, V, Cu, Zn and Ti, and M ³ is 1 selected from the group consisting of Al, Mg and B A layered compound such as at least one species, and A is at least one selected from the group consisting of P, F, S and N) or a compound substituted with one or more transition metals; _{Li 1 + h Mn 2 - h} O 4 ( wherein 0≤h≤0.33), LiMnO _3, the lithium manganese oxide such as LiMn ₂ O _3, LiMnO _2; Lithium copper oxide (Li ₂ CuO ₂ ); Vanadium oxides such as LiV ₃ O ₈ , V ₂ O ₅ , and Cu ₂ V ₂ O ₇ ; Ni site type lithium nickel oxide represented by the formula LiNi _{1 - i} M ⁴ _i O ₂ (wherein, M ⁴ = Co, Mn, Al, Cu, Fe, Mg, B or Ga, 0.01≦ _y ≦0.3); Formula ^{_{LiMn 2 - j M 5 j O}} 2 ( ^{wherein, M 5 = Co, Ni,} Fe, Cr, and Zn, or Ta, 0.01≤y≤0.1) or ^{_{Li 2 Mn 3 M 6 O 8}} ( wherein, M ⁶ = lithium manganese composite oxide represented by Fe, Co, Ni, Cu or Zn); LiMn ₂ O _{4 in} which part of Li in the formula is substituted with alkaline earth metal ions; Disulfide compounds; LiFe ₃ O ₄ , Fe ₂ (MoO ₄ ) ₃ and the like, but are not limited thereto.

As a solvent for forming the anode, there are organic solvents such as NMP (N-methyl pyrrolidone), DMF (dimethyl formamide), acetone, dimethyl acetamide, or water, and these solvents may be used alone or in two or more Can be used by mixing. The amount of the solvent used is sufficient as long as it can dissolve and disperse the positive electrode active material, the binder, and the conductive material in consideration of the coating thickness and production yield of the slurry.

As the binder, polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidenefluoride, polyacrylonitrile, polymethylmethacrylate, Polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene-propylene-diene monomer (EPDM), Sulfonated EPDM, styrene butadiene rubber (SBR), fluorine rubber, polyacrylic acid and polymers in which hydrogen is substituted with Li, Na, or Ca, or Various types of binder polymers such as various copolymers may be used.

The conductive material is not particularly limited as long as it has conductivity without causing chemical changes in the battery, and examples thereof include graphite such as natural graphite or artificial graphite; Carbon blacks such as acetylene black, Ketjen black, channel black, Parnes black, lamp black, and thermal black; Conductive fibers such as carbon fibers and metal fibers; 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 may be used. The conductive material may be used in an amount of 1% to 20% by weight based on the total weight of the positive electrode slurry.

The dispersant may be an aqueous dispersant or an organic dispersant such as N-methyl-2-pyrrolidone.

The negative electrode may be manufactured by a conventional method known in the art. For example, additives such as a binder, a conductive material, and a thickener are optionally mixed and stirred together with the negative electrode active material to prepare a negative active material slurry, and then the negative electrode It can be applied to a current collector, dried, and then compressed.

As the negative active material, a carbon negative active material such as crystalline carbon, amorphous carbon, or a carbon composite may be used alone or in combination of two or more, and specifically, may be crystalline carbon.

As the crystalline carbon, both low crystalline carbon and high crystalline carbon may be used. As low crystalline carbon, soft carbon and hard carbon are typical, and high crystalline carbon is natural graphite, kish graphite, pyrolytic carbon, liquid crystal pitch-based carbon fiber (mesophase pitch based carbon fiber), carbon microspheres (meso-carbon microbeads), liquid crystal pitches (mesophase pitches), and high-temperature calcined carbons such as petroleum or coal tar pitch derived cokes.

The binder may be used to hold the molded body by binding the negative electrode active material particles, and it is not particularly limited as long as it is a conventional binder used for preparing the slurry for the negative electrode active material, for example, non-aqueous binders such as polyvinyl alcohol, carboxymethylcellulose, hydroxy Propylene cellulose, diacetylene cellulose, polyvinyl chloride, polyvinylpyrrolidone, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), polyethylene or polypropylene, etc. can be used, and acrylic as an aqueous binder Any one selected from the group consisting of ronitryl-butadiene rubber, styrene-butadiene rubber, and acrylic rubber, or a mixture of two or more of them may be used. Aqueous binders are economical and eco-friendly compared to non-aqueous binders, are harmless to workers' health, and have excellent binding effect compared to non-aqueous binders, so that the ratio of active materials per volume can be increased, thereby increasing capacity. Preferably, styrene-butadiene rubber can be used.

The binder may be included in an amount of 10% by weight or less in the total weight of the slurry for an anode active material, and specifically, may be included in an amount of 0.1% to 10% by weight. If the content of the binder is less than 0.1% by weight, the effect of using the binder is not preferable, and if it exceeds 10% by weight, the capacity per volume may decrease due to a decrease in the relative content of the active material due to an increase in the binder content, which is not preferable. not.

The conductive material is not particularly limited as long as it has conductivity without causing chemical changes in the battery, and examples of the conductive material include graphite such as natural graphite or artificial graphite; Carbon blacks such as acetylene black, ketjen black, channel black, furnace black, lamp black, and summer black; Conductive fibers such as carbon fibers and metal fibers; Metal powders such as carbon fluoride, aluminum, and nickel powder; Conductive whiskers such as zinc oxide and potassium titanate; Conductive metal oxides such as titanium oxide; Alternatively, a conductive material such as a polyphenylene derivative may be used. The conductive material may be used in an amount of 1% to 9% by weight based on the total weight of the slurry for the negative active material.

The thickener may be any thickener conventionally used in a lithium secondary battery, and an example is carboxymethyl cellulose (CMC).

The negative electrode current collector used for the negative electrode according to an embodiment of the present invention may have a thickness of 3 μm to 500 μm. The negative electrode current collector is not particularly limited as long as it has conductivity without causing a chemical change in the battery, for example, copper, gold, stainless steel, aluminum, nickel, titanium, calcined carbon, copper, or stainless steel. Surface treatment with carbon, nickel, titanium, silver, etc., aluminum-cadmium alloy, and the like may be used. In addition, by forming fine irregularities on the surface, the bonding strength of the negative electrode active material may be strengthened, and may be used in various forms such as films, sheets, foils, nets, porous bodies, foams, and nonwoven fabrics.

In addition, the separator is a conventional porous polymer film used as a separator, for example, polyolefin-based polymers such as ethylene homopolymer, propylene homopolymer, ethylene-butene copolymer, ethylene-hexene copolymer, and ethylene-methacrylate copolymer. The porous polymer film prepared with may be used alone or by stacking them, or a conventional porous nonwoven fabric, such as a nonwoven fabric made of a high melting point glass fiber, polyethylene terephthalate fiber, or the like, may be used, but is not limited thereto.

(2) inserting the electrode assembly into the battery case

The electrode assembly manufactured through step (1) is inserted into the battery case.

The battery case may be a battery case having an outer shape of a cylindrical shape, a square shape, a pouch type, or a coin type, and there is no particular limitation on the outer shape.

(3) manufacturing a lithium secondary battery by injecting a first electrolyte not containing propylene carbonate into the battery case into which the electrode assembly is inserted

After inserting the electrode assembly into the battery case, an electrolyte solution is injected. The electrolyte contained in the lithium secondary battery of the present invention can be divided into a first electrolyte that does not contain propylene carbonate (PC) and a second electrolyte that contains propylene carbonate, and does not contain propylene carbonate through this step. The first electrolyte is preferentially injected into the lithium secondary battery.

The first electrolyte may contain a non-aqueous solvent and a lithium salt, and does not contain propylene carbonate, so before the propylene carbonate forms a stable SEI film on the carbon negative electrode surface, a carbon material used as a negative electrode active material, especially black It is possible to prevent destruction by exfoliation between the layers of the linked active material.

The non-aqueous solvent is ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (dimethyl carbonate, DMC), ethylmethyl carbonate (EMC), methylpropyl carbonate, dipropyl carbonate, dimethyl sulfur. It may be at least one selected from the group consisting of oxide, acetonitrile, dimethoxyethane, diethoxyethane, vinylene carbonate, sulfolane, gamma-butyrolactone, propylene sulfite, and tetrahydrofuran.

Ethylene carbonate, which is a cyclic carbonate among the non-aqueous solvents, is an organic solvent of high viscosity and has a high dielectric constant, so it can be preferably used because it dissociates well lithium salts in the electrolyte, and low viscosity such as dimethyl carbonate and diethyl carbonate, When a low dielectric constant linear carbonate is mixed in an appropriate ratio, an electrolyte solution having a high electrical conductivity can be prepared, and thus it can be used more preferably. Therefore, in one example of the present invention, the non-aqueous solvent may include dimethyl carbonate, diethyl carbonate, or a mixture thereof together with ethylene carbonate.

The lithium salt is contained as an electrolyte, can be used without limitation, those which are commonly used in the secondary battery, the electrolytic solution, for example the lithium salt is ^{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 ^- may have any one anion selected from the group consisting of.

(4) performing the formation of the lithium secondary battery of step (3)

A lithium secondary battery is manufactured by injecting the first electrolyte through step (3), and then an activation process is performed.

The formation may include charging/discharging and aging processes, and the battery structure is stabilized and usable through the activation process.

In the activation process, in the initial charging process of the battery, lithium ions from the positive electrode active material move and are inserted into the negative electrode active material. At this time, lithium ions are highly reactive and react with the electrolyte at the negative electrode to react with compounds such as Li₂CO3, LiO, LiOH, etc. And they form a SEI (Solid Electrolyte Interface) film on the surface of the cathode.

According to the method of manufacturing a lithium secondary battery of the present invention, when the activation process is performed, since propylene carbonate is not contained in the electrolyte (first electrolyte) included in the lithium secondary battery, the SEI film is formed smoothly, and the It is possible to prevent destruction of the negative active material by propylene carbonate.

The activation process includes a charge/discharge process such as a charge-discharge-charge process, and a battery that has undergone the charge-discharge process undergoes an aging process.

In the aging process, the battery after the charging/discharging process is left at room temperature for a certain period of time to stabilize the formation of the SEI film and to distribute the electrolyte evenly on the electrode.

(5) removing gas generated through activation of step (4)

In the next step, the gas generated during the activation process is removed.

In the activation process, a large amount of gas generated by a side reaction between the positive electrode active material and the positive electrode active material and the electrolyte is generated through charge/discharge and aging processes. Accordingly, it is necessary to remove the gas from the battery, and the step of removing the gas includes discharging the gas through an outlet formed in the battery case, and fusing the outlet from which the gas is discharged ( degassing) process.

If the gas generated inside the battery cell is not efficiently removed during the activation process, the gas occupies a certain space inside the battery cell, preventing uniform activation, and adversely affecting battery performance and battery life such as capacity and output. In addition, due to the residual gas inside the battery cell, the capacity of the battery rapidly decreases as the number of times of charging and discharging increases, and there is a problem that the battery cell swells. Accordingly, in a typical battery manufacturing method, a gas pocket or the like is connected to a pouch-type unit cell so that gas generated during the activation process moves to an area such as the gas pocket. However, such a method aims to discharge the gas inside the battery cell by natural movement due to the pressure difference between the gas pocket and the inside of the battery cell. There is a limit to effectively removing the gas occupying space.

Accordingly, the step of removing the gas may be performed while applying pressure to the battery case. In this case, the pressure may be greater than 0 kgf/cm ² to 20 kgf/cm ² .

(6) Injecting a second electrolyte solution containing propylene carbonate into the lithium secondary battery through step (5)

After the manufactured lithium secondary battery undergoes an activation step and a gas removal step, a second electrolyte solution including propylene carbonate is injected into the lithium secondary battery.

The second electrolyte containing the propylene carbonate is injected to improve the low-temperature characteristics of the lithium secondary battery. For example, in the case of a two/three-component electrolyte based on ethylene carbonate (EC), which is generally used as an electrolyte for lithium ion batteries, the use temperature is limited due to the high melting point of ethylene carbonate and low battery at low temperatures. There are disadvantages such as performance, and when propylene carbonate is used, the disadvantages of ethylene carbonate as described above can be compensated.

In the method of manufacturing a lithium secondary battery of the present invention, a second electrolyte containing propylene carbonate is injected into the lithium secondary battery after an activation step and a degassing step for the manufactured lithium secondary battery. After forming a stable SEI film on the surface of the carbon negative electrode, propylene carbonate is injected, which is a problem when using propylene carbonate in the electrolyte, before propylene carbonate forms a stable SEI film on the surface of the carbon negative electrode, a carbon material used as a negative electrode active material. In particular, it is possible to solve the problem of destruction by exfoliation between layers of graphite. Therefore, the method of manufacturing a lithium secondary battery of the present invention can improve the low-temperature performance of the lithium secondary battery by using propylene carbonate as an electrolyte, while preventing the destruction of the carbon material negative active material of the negative electrode by the addition of propylene carbonate. .

In addition, the second electrolyte may further include cis-butylene carbonate, and specifically, the second electrolyte may include propylene carbonate and cis-butylene carbonate.

When the second electrolyte further includes cis-butylene carbonate, the low-temperature performance of the lithium secondary battery including the same may be further improved. This is because of the lower melting point of cis-butylene carbonate (-50°C), the electrolyte may exhibit greater fluidity at low temperatures than when only propylene carbonate (melting point -48.8°C) is used, and different types of electrolyte may be used together. This is because the effect of lowering the melting point according to use can provide an additional effect of improving fluidity.

The propylene carbonate and cis-butylene carbonate may be included in a weight ratio of 99:1 to 75:25, specifically 98:2 to 80:20, and more specifically 95:5 to 90:10.

When 1 part by weight or more of cis-butylene carbonate is included with respect to 99 parts by weight of propylene carbonate, an additional low-temperature performance improvement effect may be exhibited according to the mixed use of the cis-butylene carbonate, and 75 parts by weight of the propylene carbonate When the cis-butylene carbonate is included in an amount of 25 parts by weight or less, the stability of the electrolyte solution is deteriorated due to the inclusion of an excessive amount of cis-butylene carbonate, thereby preventing a problem in which low-temperature performance may be deteriorated.

Meanwhile, the first electrolyte and the second electrolyte may be used in a weight ratio of 95:5 to 50:50, specifically 90:10 to 70:30, more specifically 85:15 to 75:25. have..

When the second electrolyte is 5 parts by weight or more with respect to 95 parts by weight of the first electrolyte, the low-temperature characteristics of the battery can be improved, and when the second electrolyte is 50 parts by weight or less with respect to 50 parts by weight of the first electrolyte, the layer of graphite is It is possible to alleviate the problem of exfoliation and collapse of the graphite structure or intensifying side reactions.

In the method of manufacturing a lithium secondary battery according to an exemplary embodiment of the present invention, after the step of injecting the second electrolyte in step (6), a grading process for classifying the lithium secondary battery may be performed.

A lithium secondary battery can be manufactured through the manufacturing method of a lithium secondary battery as described above, and thus the present invention provides a lithium secondary battery manufactured according to the manufacturing method.

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

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

Example

Hereinafter, examples and experimental examples will be described in more detail to describe the present invention in detail, but the present invention is not limited by these examples and experimental examples. The embodiments according to the present invention may be modified in various forms, and the scope of the present invention should not be construed as being limited to the embodiments described below. Embodiments of the present invention are provided to more completely describe the present invention to those of ordinary skill in the art.

Example 1

<Production of anode>

94% by weight of Li(Li _0.2 Co _0.1 Ni _0.1 Mn _0.6 )O ₂ as a cathode active material, 3% by weight of carbon black as a conductive material, and 3% by weight of PVDF as a binder N-methyl-2 pyrrolidone as a solvent (NMP) was added to prepare a positive electrode mixture slurry. The positive electrode mixture slurry was coated on an aluminum (Al) thin film of a positive electrode current collector having a thickness of about 20 μm to prepare a positive electrode through drying, and then roll press was performed.

<Production of cathode>

As a negative active material, a negative electrode mixture slurry was prepared by adding 96% by weight of natural graphite, 1% by weight of Denka black (conductive material), 2% by weight of SBR (binder), and 1% by weight of CMC (thickener) to water.

The prepared negative electrode mixture slurry was coated on one surface of a copper current collector to a thickness of 65 μm, dried and rolled, and then punched to a predetermined size to prepare a negative electrode.

<Manufacture of lithium secondary battery>

A porous membrane made of polyethylene having a thickness of 17 μm was interposed between the prepared positive electrode and the negative electrode, and then inserted into a cylindrical battery case, and then ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed in a volume ratio of 30:70. A lithium secondary battery was manufactured by injecting a first electrolyte in which 1M LiPF ₆ was dissolved in one solvent.

Charge the lithium secondary battery at 25°C with a constant current (CC) of 0.8C until it reaches 4.25 V, and then charge it with a constant voltage (CV) until the charging current reaches 0.005 C (cut-off current). Was charged. Then, it was allowed to stand for 20 minutes, and then discharged to 2.5 V at a constant current of 0.8 C (CC). After charging this to 4.3V again, a pressure of 5 kgf/cm2 was applied to the outer surface of the lithium secondary battery to remove the gas, and aging was performed by leaving it at 20°C for 4 weeks.

A lithium secondary battery was manufactured by injecting propylene carbonate (PC) as a second electrolyte into the aging lithium secondary battery in an amount of 50 parts by weight based on 100 parts by weight of the first electrolyte.

Example 2

A lithium secondary battery was manufactured in the same manner as in Example 1, but instead of using propylene carbonate (PC) alone as the second electrolyte when the electrolyte was injected, propylene carbonate (PC) and cis-butylene carbonate ( c-BC) was injected in an amount of 50 parts by weight based on 100 parts by weight of the first electrolyte to complete the manufacture of a lithium secondary battery.

Example 3

A lithium secondary battery was manufactured in the same manner as in Example 1, but instead of using propylene carbonate (PC) alone as the second electrolyte when the electrolyte was injected, propylene carbonate (PC) and cis-butylene carbonate ( c-BC) was injected in an amount of 50 parts by weight based on 100 parts by weight of the first electrolyte to complete the manufacture of a lithium secondary battery.

Comparative Example 1

Instead of not injecting the second electrolyte in Example 1, the first electrolyte was mixed with ethylene carbonate (EC), propylene carbonate (PC), and diethyl carbonate (DEC) in a volume ratio of 10:20:70. A lithium secondary battery was manufactured in the same manner as in Example 1, except that an electrolyte solution in which 1M LiPF ₆ was dissolved was used.

Comparative Example 2

A lithium secondary battery was manufactured in the same manner as in Example 1, except that the second electrolyte was not injected in Example 1 above.

Experimental Example 1: Cycle characteristic evaluation experiment

In order to confirm the cycle characteristics of the lithium secondary batteries each completed in Examples 1 to 3 and Comparative Examples 1 and 2, an electrochemical evaluation experiment was performed as follows.

Specifically, in the case of the lithium secondary battery of Examples 1 to 3, the activation efficiency was improved through the first charging and discharging process after the injection of the first electrolyte, before the injection of the second electrolyte, as described in Example 1 above. Measurement was carried out, and this was repeated with 100 cycles thereafter.

Meanwhile, in the case of the lithium secondary batteries of Comparative Examples 1 and 2, the lithium secondary batteries each completed in Comparative Examples 1 and 2 were charged at 25° C. with a constant current (CC) of 0.8 C until 4.25 V, and then a constant voltage (CV ), and the first charging was performed until the charging current reached 0.005 C (cut-off current). Then, it was allowed to stand for 20 minutes, and then discharged to 2.5 V at a constant current of 0.8 C (CC). This was repeated with 1 to 100 cycles.

Table 1 shows the results of measuring the capacity retention rate of the battery after 100 cycles (at room temperature, 25°C) along with the first efficiency as the activation efficiency. Further, the capacity retention rate of the battery after 100 cycles was measured in the same manner as above, except that the temperature was set to -20°C, and the results (low temperature, -20°C) are shown in Table 1 below.

Example 1 Example 2 Example 3 Comparative Example 1 Comparative Example 2 Activation efficiency (%) 94 94 94 83 94 Battery capacity retention rate after 100 cycles
(Room temperature, 25 ℃) 97 95 93 76 88 Battery capacity retention rate after 100 cycles
(Low temperature, -20 ℃) 71 75 63 55 52

Referring to Table 1, compared to Comparative Example 1 in which an electrolyte containing propylene carbonate was injected prior to activation, the lithium secondary batteries of Examples 1 to 3 and Comparative Example 2 in which an electrolyte containing no propylene carbonate was injected were activated. It can be seen that this is high, and from the above results, it can be seen that in Comparative Example 1 in which an electrolyte solution containing propylene carbonate was injected, the negative electrode active material, graphite, was relatively damaged. In proportion to this, as a result of measuring the capacity retention rate of the battery after 100 cycles at room temperature, the lithium secondary battery of Comparative Example 1 in which the destruction of graphite, which is a negative active material during activation, was relatively high showed a low value.

The lithium secondary batteries of Examples 1 to 3 containing propylene carbonate exhibited a higher capacity retention rate of the battery after 100 cycles at room temperature compared to the lithium secondary battery of Comparative Example 2 which does not contain this.

In addition, looking at the result of measuring the capacity retention rate of the battery after 100 cycles at low temperature, the lithium secondary battery of Comparative Example 1 in which the destruction of graphite, which is a negative active material during activation, was relatively large showed a lower value in proportion to this, and activation efficiency And it was confirmed that the lithium secondary battery of Comparative Example 2, which had relatively excellent capacity retention rate after 100 cycles at room temperature, exhibited low low-temperature stability. On the other hand, it was found that the lithium secondary batteries of Examples 1 to 3 according to an example of the manufacturing method of the present invention exhibited significantly improved low-temperature stability compared to Comparative Example 2 due to the inclusion of propylene carbonate.

On the other hand, when comparing only the lithium secondary batteries of Examples 1 to 3, the capacity retention rate of the battery after 100 cycles at room temperature was the best in Example 1, and the capacity retention rate of the battery after 100 cycles at low temperature was Example 2 was the best.

The reason that the lithium secondary battery of Example 2 containing propylene carbonate and cis-butylene carbonate as the second electrolyte in a 95:5 weight ratio exhibits the most improved cycle characteristics at low temperatures is that cis-butylene carbonate itself is compared to propylene carbonate. It is believed that this is because the fluidity at low temperatures is further improved due to the fact that the melting point has a higher fluidity due to the low melting point, and the melting point lowering effect of the use of different kinds of electrolyte.

However, the lithium secondary battery of Example 3 also used propylene carbonate and cis-butylene carbonate as the second electrolyte, but the low-temperature cycle characteristics were good compared to the lithium secondary battery of Example 1 in which cis-butylene carbonate was not used. However, this is considered to be because when cis-butylene carbonate is used in an appropriate amount or more, the stability of the electrolyte solution is deteriorated and the low-temperature cycle characteristics are lowered.

Claims (15)

(1) manufacturing an electrode assembly including an anode, a cathode, and a separator interposed between the anode and the cathode;
(2) inserting the electrode assembly into a battery case;
(3) manufacturing a lithium secondary battery by injecting a first electrolyte not containing propylene carbonate into the battery case into which the electrode assembly is inserted;
(4) performing the formation of the lithium secondary battery of step (3);
(5) removing the gas generated through the activation of step (4); And
(6) A method of manufacturing a lithium secondary battery comprising the step of injecting a second electrolyte containing propylene carbonate into the lithium secondary battery that has undergone step (5).
The method of claim 1,
The method of manufacturing a lithium secondary battery, wherein the negative electrode includes a crystalline carbon material as a negative electrode active material.
The method of claim 1,
The first electrolyte is a method of manufacturing a lithium secondary battery comprising a non-aqueous solvent and a lithium salt.
The method of claim 3,
The non-aqueous solvent is ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (dimethyl carbonate, DMC), ethylmethyl carbonate (EMC), methylpropyl carbonate, dipropyl carbonate, dimethyl sulfur. Oxide, acetonitrile, dimethoxyethane, diethoxyethane, vinylene carbonate, sulfolane, gamma-butyrolactone, propylene sulfite, and at least one selected from the group consisting of tetrahydrofuran, a method for producing a lithium secondary battery.
The method of claim 3,
The non-aqueous solvent comprises ethylene carbonate, dimethyl carbonate, diethyl carbonate, or a mixture thereof, a method of manufacturing a lithium secondary battery.
The method of claim 3,
The lithium salt is ^{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 - any of the anions selected from the group consisting of Having a method of manufacturing a lithium secondary battery.
The method of claim 1,
The activation (formation) includes a process of charging and discharging and aging, a method of manufacturing a lithium secondary battery.
The method of claim 1,
The step of removing the gas includes discharging the gas through an outlet formed in the battery case and fusing the outlet from which the gas has been discharged.
The method of claim 1,
The step of removing the gas is performed while applying pressure to the battery case, a method of manufacturing a lithium secondary battery.
The method of claim 9,
The pressure is greater than 0 kgf / cm ² to 20 kgf / cm ^{2 A} method of manufacturing a lithium secondary battery.
The method of claim 1,
The second electrolyte solution further comprises cis-butylene carbonate, a method of manufacturing a lithium secondary battery.
The method of claim 11,
The second electrolyte is a method for manufacturing a lithium secondary battery comprising propylene carbonate and cis-butylene carbonate in a weight ratio of 99:1 to 75:25.
The method of claim 1,
The weight ratio of the first electrolyte and the second electrolyte is 95:5 to 50:50, a method of manufacturing a lithium secondary battery.
The method of claim 1,
After passing through the step of injecting the second electrolyte in step (6), a method of manufacturing a lithium secondary battery further undergoes a grading process of classifying the lithium secondary battery.
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