KR20230087039A - Lithium secondary battery - Google Patents

Abstract

Lithium secondary batteries according to embodiments of the present invention include a case and an electrode assembly accommodated in the case. The electrode assembly includes positive electrodes including a first positive active material layer including lithium iron phosphate (LiFePO ₄ ) and a second positive active material layer including a lithium-transition metal composite oxide, and negative electrodes facing at least one of the positive electrodes. and the ratio of the number of positive electrodes including the first positive electrode active material layer to the total number of positive electrodes included in the electrode assembly is 30% or more. Lithium iron phosphate absorbs/blocks heat generated from the lithium-transition metal composite oxide to suppress thermal runaway and heat propagation.

Description

Lithium secondary battery {LITHIUM SECONDARY BATTERY}

The present invention relates to a lithium secondary battery. More specifically, it relates to a lithium secondary battery including an electrode assembly.

A secondary battery is a battery capable of repeating charging and discharging, and is widely used as a power source for portable electronic communication devices such as camcorders, mobile phones, and notebook PCs with the development of information communication and display industries. In addition, recently, a battery pack including a secondary battery has been developed and applied as a power source for eco-friendly vehicles such as hybrid vehicles.

Secondary batteries include, for example, lithium secondary batteries, nickel-cadmium batteries, nickel-hydrogen batteries, etc. Among them, lithium secondary batteries have high operating voltage and energy density per unit weight, and are advantageous in charging speed and weight reduction. It is being actively developed and applied in this regard.

For example, a lithium secondary battery may include an electrode assembly including a positive electrode, a negative electrode, and a separator (separator), and an electrolyte impregnating the electrode assembly. The lithium secondary battery may further include, for example, a pouch-shaped exterior material for accommodating the electrode assembly and the electrolyte.

For example, a lithium secondary battery may experience thermal runaway or thermal propagation due to an external impact or an internal defect. In this case, the lithium secondary battery is rapidly heated and may explode or ignite.

In addition, in order to implement a high-capacity, high-output lithium secondary battery, a plurality of battery cells may be connected in series or parallel to be modularized and manufactured. In this case, the above-described thermal runaway or thermal propagation speed may be increased, and driving stability may be deteriorated.

Therefore, there is a need to develop a lithium secondary battery capable of ensuring driving stability and reliability while implementing high capacity and high output characteristics within a limited space.

For example, Korean Patent Publication No. 2017-0099748 discloses an electrode assembly for a lithium secondary battery and a lithium secondary battery including the same.

Korean Patent Publication No. 10-2017-0099748

One object of the present invention is to provide a lithium secondary battery having excellent operational stability and reliability.

A lithium secondary battery according to exemplary embodiments of the present invention includes a case and an electrode assembly accommodated in the case. The electrode assembly includes positive electrodes including a first positive electrode active material layer including lithium iron phosphate (LiFePO ₄ ) and a second positive electrode active material layer including a lithium-transition metal composite oxide, and a surface facing at least one of the positive electrodes. Including negative electrodes, the ratio of the number of positive electrodes including the first positive electrode active material layer to the total number of positive electrodes included in the electrode assembly is 30% or more.

In some embodiments, the positive electrodes include a first positive electrode current collector, the first positive electrode active material layer formed on one surface of the first positive electrode current collector, and the other surface facing the one surface of the first positive electrode current collector. It may include a first cathode including the second cathode active material layer formed thereon.

In some embodiments, the positive electrodes include a first positive electrode current collector and the first positive electrode active material layer formed on one surface of the first positive electrode current collector and the other surface opposite to the first positive electrode current collector. It may contain an anode.

In some embodiments, the first cathode may further include the second cathode active material layer formed on the first cathode active material layer.

In some embodiments, the first cathode may further include a heat insulating film disposed between the first cathode active material layer and the second cathode active material layer.

In some embodiments, the insulating film may include at least one of polyurethane, polystyrene, polyethylene, and polyester.

In some embodiments, the outermost anode of the electrode assembly may be the first anode.

In some embodiments, 3 to 5 positive electrodes sequentially disposed from the outermost periphery of the electrode assembly among the positive electrodes may be the first positive electrode.

In some embodiments, the positive electrodes include a second positive electrode current collector and a second positive electrode active material layer formed on one surface of the second positive electrode current collector and the other surface opposite to the second positive electrode current collector. An anode may be further included.

In some embodiments, the first positive electrode and the second positive electrode may be alternately and repeatedly arranged in a thickness direction of the electrode assembly.

In some embodiments, the first positive electrode active material layer may include both the lithium iron phosphate and the lithium-transition metal composite oxide.

In some embodiments, the first positive electrode active material layer includes a concentration gradient region in which the concentration of the lithium iron phosphate and the concentration of the lithium-transition metal composite oxide gradually change from one surface to the other surface of the first positive electrode active material layer. can include

In some embodiments in which the first positive electrode active material layer includes both the lithium iron phosphate and the lithium-transition metal composite oxide, the content of the lithium iron phosphate is 50% by weight relative to the total weight of the first positive electrode active material layer. may be ideal

In some embodiments, the sum of the thicknesses of the first positive electrode active material layer to the sum of the thicknesses of the positive electrodes in the electrode assembly may be 30% or more.

In some embodiments, the content of the lithium iron phosphate relative to the total weight of the first positive electrode active material layer in the electrode assembly may be 50% by weight or more.

In example embodiments, at least a portion of the positive electrode may include lithium iron phosphate (LiFePO ₄ , LFP). In this case, the lithium iron phosphate can absorb/intercept heat generated from the lithium-transition metal composite oxide to suppress thermal runaway and heat propagation. Accordingly, driving stability and reliability of the lithium secondary battery may be improved.

In exemplary embodiments, the number of positive electrodes containing lithium iron phosphate relative to the total number of positive electrodes may be 30% or more, preferably 40 to 70%. In this case, the aforementioned thermal runaway or thermal propagation phenomenon can be sufficiently suppressed, and an appropriate amount of the lithium-transition metal composite oxide can be included in the positive electrode. Accordingly, it is possible to implement a lithium secondary battery having high energy density and capacity characteristics while improving thermal stability.

In some embodiments, an insulating film may be further disposed on the anode. In this case, the transfer of heat generated from the positive electrode including the lithium-transition metal composite oxide can be further suppressed. Also, the output characteristics of the secondary battery may be improved by increasing the electronic conductivity.

1 is a schematic plan view illustrating a lithium secondary battery according to example embodiments.
2 to 4 are schematic cross-sectional views illustrating a first anode according to example embodiments.
5 is a schematic cross-sectional view illustrating a lithium secondary battery according to example embodiments.
6 is a schematic cross-sectional view illustrating a first anode according to example embodiments.
7 is a schematic cross-sectional view illustrating a second anode according to example embodiments.
8 and 9 are schematic cross-sectional views illustrating secondary lithium batteries according to exemplary embodiments.

Embodiments of the present invention provide a lithium secondary battery including an electrode assembly including positive electrodes and negative electrodes and having improved driving stability.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. However, this is merely an example and the present invention is not limited to the specific embodiments described as examples.

Terms such as "top", "bottom", "top", "bottom" and "outermost" used in this application do not designate absolute locations, but are used to distinguish different components or to distinguish relative positions between components. .

1 is a schematic plan view illustrating a lithium secondary battery according to example embodiments. For convenience of description, the positive electrode and the negative electrode are omitted in FIG. 1 .

Referring to FIG. 1 , a lithium secondary battery may include a case 160 and an electrode assembly 150 accommodated in the case 160 .

The electrode assembly 150 may include positive electrodes and negative electrodes facing at least one of the positive electrodes. The electrode assembly 150 may be accommodated and impregnated with an electrolyte in the case 160, for example.

For example, the positive electrodes may include a first positive active material layer including lithium iron phosphate (LiFePO ₄ , LFP) and a second positive active material layer including a lithium-transition metal composite oxide.

For example, the first positive electrode active material layer may refer to a positive electrode active material layer including lithium iron phosphate as a positive electrode active material.

For example, the second cathode active material layer may refer to a cathode active material layer including a lithium-transition metal composite oxide as a cathode active material.

2 to 4 are schematic cross-sectional views illustrating a first anode according to example embodiments.

2 to 4 , the first positive electrode 110 includes a first positive electrode current collector 115 and a first positive electrode active material layer 112 formed on at least one surface of the first positive electrode current collector 115. can do.

For example, a lithium secondary battery may include a lithium-transition metal composite oxide as a cathode active material in consideration of energy density and capacity characteristics.

For example, the lithium-transition metal composite oxide includes nickel (Ni) and may further include at least one of cobalt (Co) and manganese (Mn).

For example, the lithium-transition metal composite oxide may be represented by Chemical Formula 1 below.

[Formula 1]

Li _a Ni _x M _1-x O _2+y

In Formula 1, it may be 0.9≤a≤1.2, 0.5≤x≤0.99, -0.1≤y≤0.1. M is Na, Mg, Ca, Y, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Co, Fe, Cu, Ag, Zn, B, Al, Ga, C, Si, At least one element selected from Sn, Ba or Zr can be represented.

In a preferred embodiment, the molar ratio or concentration x of Ni in Chemical Formula 1 may be greater than or equal to 0.8, more preferably greater than or equal to 0.8.

Ni may serve as a transition metal related to the output and capacity of a lithium secondary battery. Therefore, as described above, by employing the High-Ni composition in the lithium-transition metal composite oxide particle, a high-output cathode and a high-output lithium secondary battery can be provided.

As the content of Ni increases, long-term storage stability and lifetime stability of the positive electrode or secondary battery at high temperatures may be relatively deteriorated. However, according to exemplary embodiments, life stability and capacity retention characteristics may be improved through Mn while maintaining electrical conductivity by including Co.

However, a positive electrode including only a lithium-transition metal composite oxide may generate heat due to an external impact or an internal defect. In this case, continuous heat generation occurs due to thermal runaway or thermal propagation, and the battery may explode or ignite.

In example embodiments of the present invention, the first positive electrode active material layer 112 may include lithium iron phosphate (LiFePO ₄ , LFP). In this case, the lithium iron phosphate absorbs/blocks heat generated from the lithium-transition metal composite oxide, thereby suppressing the aforementioned thermal runaway and heat propagation phenomena. Accordingly, driving stability and reliability of the lithium secondary battery may be improved.

In some embodiments, as shown in FIG. 2 , a first positive electrode active material layer 112 including lithium iron phosphate is formed on one surface of the first positive electrode current collector 115, and the first positive electrode current collector 115 is formed. A second positive electrode active material layer 114 including a lithium-transition metal composite oxide may be formed on the other surface of the entire body 115 opposite to the one surface.

In some embodiments, as shown in FIG. 3 , the first positive electrode active material layer 112 may be formed on one surface of the first positive electrode current collector 115 and on the other surface opposite to the one surface. there is.

In this case, a second cathode active material layer 114 including a lithium-transition metal composite oxide may be further formed on the first cathode active material layer 112 . Accordingly, the lithium iron phosphate included in the first positive electrode active material layer 112 may suppress transfer of heat generated from the lithium-transition metal composite oxide to other electrodes.

In some embodiments, as shown in FIG. 4 , a heat insulating film 116 may be further disposed between the first positive active material layer 112 and the second positive active material layer 114 . In this case, the transfer of heat generated in the second positive electrode active material layer 114 can be further suppressed. In addition, adhesion between the first and second positive electrode active material layers 112 and 114 may be improved while electron conductivity is increased. Accordingly, output characteristics and structural stability of the lithium secondary battery may be improved.

The insulating film 116 may include, for example, at least one of polyurethane, polystyrene, polyethylene, and polyester.

In example embodiments, the number of positive electrodes including the first positive electrode active material layer 112 (eg, the first positive electrode 110) is 30% of the total number of positive electrodes included in the electrode assembly 150. may be ideal

When the number of first anodes 110 is less than 30% of the total number of anodes, heat transfer is not sufficiently suppressed, and thus thermal runaway or heat propagation cannot be prevented. Accordingly, stability of the lithium secondary battery may be deteriorated.

In some preferred embodiments, the number of positive electrodes (eg, the first positive electrode 110) including the first positive electrode active material layer 112 may be 40 to 60% of the total number of positive electrodes. In this case, the aforementioned thermal runaway or thermal propagation phenomenon can be sufficiently suppressed, and an appropriate amount of the lithium-transition metal composite oxide can be included in the positive electrode. Accordingly, it is possible to implement a lithium secondary battery having high energy density and capacity characteristics while improving thermal stability.

In some embodiments, the content of lithium iron phosphate based on the total weight of the first positive electrode active material layer 112 may be 50% by weight or more. In this case, thermal stability can be sufficiently improved.

In some embodiments, the sum of the thicknesses of the first positive electrode active material layer 112 to the sum of the thicknesses of the positive electrodes in the electrode assembly may be 30% or more. In this case, heat generated inside the secondary battery may be sufficiently absorbed by the first positive electrode active material layer 112 without being transferred to the outside.

5 is a schematic cross-sectional view illustrating a lithium secondary battery according to example embodiments.

Specifically, FIG. 5 is a lithium secondary battery in the case where the first positive electrode active material layer 112 is formed on one side of the first positive electrode current collector 115 and the second positive electrode active material layer 114 is formed on the other side. It is a schematic cross-sectional view showing

Referring to FIG. 5 , the first positive electrode 110 and the negative electrode 130 may be alternately and repeatedly stacked with the separator 140 therebetween to form the electrode assembly 150 . In this case, each first cathode 110 may include a first cathode active material layer 112 formed on one surface of the first cathode current collector 115 . Accordingly, heat generated from the lithium-transition metal composite oxide of the second cathode active material layer 114 may be blocked or cooled by the first cathode active material layer 112 .

6 is a schematic cross-sectional view illustrating a first anode according to exemplary embodiments, and FIG. 7 is a schematic cross-sectional view illustrating a second anode according to exemplary embodiments.

For example, the first positive electrode 110 may mean a positive electrode including at least one first positive electrode active material layer 112 .

For example, the second positive electrode 120 may refer to a positive electrode having a second positive electrode active material layer 114 formed on one surface of the second positive electrode current collector 125 and the other surface opposite to the one surface. .

Referring to FIG. 6 , the first positive electrode active material layer 112 may be formed on one surface of the first positive current collector 115 and the other surface opposite to the one surface. Accordingly, the driving stability of the secondary battery may be improved by suppressing heat generation of the first positive electrode 110 .

Referring to FIG. 7 , the plurality of positive electrodes may further include a second positive electrode 120 that does not contain lithium iron phosphate.

For example, the second positive electrode 120 is a second positive electrode current collector 125 and a second positive electrode formed on one surface of the second positive electrode current collector 125 and the other surface opposite to the one surface. An active material layer 114 may be included.

8 and 9 are schematic cross-sectional views illustrating secondary lithium batteries according to exemplary embodiments.

Referring to FIG. 8 , the outermost positive electrode of the electrode assembly 150 may be the first positive electrode 110 including lithium iron phosphate.

In some embodiments, 3 to 5 positive electrodes sequentially disposed from the outermost periphery of the electrode assembly 150 among the positive electrodes may be the first positive electrode 110 . In this case, the transfer of heat generated inside the electrode assembly 150 to the outside of the secondary battery can be suppressed. Accordingly, it is possible to suppress ignition of the lithium secondary battery and prevent heat from being transferred to battery peripheral devices.

Referring to FIG. 9 , the above-described first anode 110 and second anode 120 may be alternately and repeatedly arranged in the thickness direction of the electrode assembly 150 . In this case, the first electrode unit 140a including lithium iron phosphate and sandwiching the second electrode unit 140b absorbs heat generated in the second electrode unit 140b including the lithium-transition metal composite oxide. can Accordingly, thermal stability of the lithium secondary battery may be improved and thermal runaway or heat propagation may be suppressed.

In some embodiments, the electrode assembly 150 may further include a separator 140 .

For example, the first positive electrode 110 and the negative electrode 130 each have a first positive electrode active material layer 112 formed on one side and the other side opposite to the first positive electrode current collector 115. The first electrode unit 140a may be formed by being stacked with the separator 140 interposed therebetween.

For example, the second positive electrode 120 and the negative electrode 130 having the second positive electrode active material layer 114 formed on one side and the other side opposite to the one side of the second positive electrode current collector 125, respectively. The second electrode unit 140b may be formed by being stacked with the separator 140 interposed therebetween.

In some embodiments, the electrode assembly 150 may be formed by alternately and repeatedly stacking the first electrode unit 140a and the second electrode unit 140b.

In some embodiments, the first positive electrode active material layer 112 may include both lithium iron phosphate and lithium-transition metal composite oxide.

For example, the first positive electrode active material layer 112 includes a concentration gradient region in which the concentration of lithium iron phosphate and the concentration of lithium-transition metal composite oxide gradually change from the bottom surface to the top surface of the first positive electrode active material layer 112 . can do.

For example, after coating the first positive electrode current collector 115 with the first positive electrode slurry (including lithium iron phosphate) and the second positive electrode slurry (including lithium-transition metal composite oxide), which will be described later, the density difference between the slurries A concentration gradient region can be formed using

For example, within the concentration gradient region, the lithium iron phosphate concentration gradient and the lithium-transition metal composite oxide concentration gradient may be formed in opposite directions.

In one embodiment, the lithium iron phosphate concentration may decrease and the lithium-transition metal composite oxide concentration may increase from the bottom surface side of the first positive electrode active material layer 112 to the top surface side.

In one embodiment, the lithium iron phosphate concentration may increase and the lithium-transition metal composite oxide concentration may decrease from the bottom side of the first positive electrode active material layer 112 to the top side.

When lithium iron phosphate and lithium-transition metal composite oxide are both included in the first positive electrode active material layer 112 , the content of lithium iron phosphate relative to the total weight of the first positive electrode active material layer 112 may be 50% by weight or more. In this case, the thermal stability of the lithium secondary battery can be sufficiently improved.

Hereinafter, a method of manufacturing a lithium secondary battery according to the above-described exemplary embodiments will be described.

The cathode current collectors 115 and 125 may include, for example, stainless steel, nickel, aluminum, titanium, copper, or an alloy thereof, and may preferably include aluminum or an aluminum alloy.

For example, a first positive electrode slurry may be prepared by mixing and stirring a positive electrode active material including lithium iron phosphate with a binder, a conductive material, and/or a dispersant in a solvent. After the first positive electrode slurry is coated on the first positive electrode current collector 115 , the first positive electrode active material layer 112 may be formed by drying and compressing the first positive electrode slurry.

For example, a second positive electrode slurry may be prepared by mixing and stirring a positive electrode active material including a lithium-transition metal composite oxide with a binder, a conductive material, and/or a dispersant in a solvent. After the second cathode slurry is coated on the second cathode current collector 125 , the second cathode active material layer 114 may be formed by drying and compressing the slurry.

For example, after coating the first positive electrode slurry and the second positive electrode slurry together on the first positive electrode current collector 115, drying and compressing the first positive electrode active material layer including the above-described concentration gradient section ( 112) can be formed.

The binder is, for example, vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidenefluoride (PVDF), polyacrylonitrile, polymethylmethacryl An organic binder such as polymethylmethacrylate or an aqueous binder such as styrene-butadiene rubber (SBR) may be included, and may be used together with a thickener such as carboxymethyl cellulose (CMC).

For example, a PVDF-based binder may be used as a binder for forming an anode. In this case, the required amount of the binder may be reduced and the amount of positive active material particles may be relatively increased, thereby improving the output and capacity of the secondary battery.

The conductive material may be included to promote electron movement between active material particles. For example, the conductive material may be a carbon-based conductive material such as graphite, carbon black, graphene, or carbon nanotube and/or a perovskite material such as tin, tin oxide, titanium oxide, LaSrCoO ₃ , or LaSrMnO ₃ . It may include a metal-based conductive material including the like.

The negative electrode 130 may include a negative electrode current collector 135 and a negative electrode active material layer 132 formed by coating at least one surface of the negative electrode current collector 135 with the negative electrode active material.

As the negative electrode active material, those known in the art, capable of intercalating and deintercalating lithium ions, may be used without particular limitation. carbon-based materials such as crystalline carbon, amorphous carbon, carbon composites, and carbon fibers; lithium alloy; Silicon or tin or the like may be used. Examples of the amorphous carbon include hard carbon, coke, mesocarbon microbeads (MCMB) calcined at 1500 ° C or less, and mesophase pitch-based carbon fibers (MPCF). Examples of the crystalline carbon include graphite-based carbon such as natural graphite, graphitized coke, graphitized MCMB, and graphitized MPCF. Examples of elements included in the lithium alloy include aluminum, zinc, bismuth, cadmium, antimony, silicon, lead, tin, gallium, and indium.

The anode current collector 135 may include, for example, gold, stainless steel, nickel, aluminum, titanium, copper, or an alloy thereof, and preferably may include copper or a copper alloy.

In some embodiments, a slurry may be prepared by mixing and stirring the negative electrode active material with a binder, a conductive material, and/or a dispersant in a solvent. After the slurry is coated on the negative electrode current collector, the negative electrode 130 may be manufactured by drying and compressing the slurry.

Materials substantially the same as or similar to the above-mentioned materials may be used as the binder and the conductive material. In some embodiments, a binder for forming an anode may include, for example, an aqueous binder such as styrene-butadiene rubber (SBR) for compatibility with a carbon-based active material, and carboxymethyl cellulose (CMC). Can be used with thickeners.

In some embodiments, a separator 140 may be interposed between the first anode 110 and the cathode 130 or between the second anode 120 and the cathode 130 . The separator 140 may include a porous polymer film made of a polyolefin-based polymer such as ethylene homopolymer, propylene homopolymer, ethylene/butene copolymer, ethylene/hexene copolymer, or ethylene/methacrylate copolymer. The separator 140 may include a nonwoven fabric formed of high melting point glass fiber, polyethylene terephthalate fiber, or the like.

According to exemplary embodiments, an electrode cell is defined by the first anode 110 and/or the second anode 120, the cathode 130, and the separator 140, and a plurality of the electrode cells are stacked. For example, the electrode assembly 150 in the form of a jelly roll may be formed. For example, the electrode assembly 150 may be formed through winding, lamination, or folding of the separator 140 .

A lithium secondary battery may be defined by accommodating the electrode assembly together with an electrolyte in the external case 160 . According to exemplary embodiments, a non-aqueous electrolyte may be used as the electrolyte.

The non-aqueous electrolyte includes a lithium salt as an electrolyte and an organic solvent, and the lithium salt is expressed as, for example, Li ⁺ X ^- , and as an anion (X ^- ) of the lithium salt, 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 ^- can be exemplified.

As the organic solvent, for example, propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethylmethyl carbonate (EMC ), methylpropyl carbonate, dipropyl carbonate, dimethylsulfuroxide, acetonitrile, dimethoxyethane, diethoxyethane, vinylene carbonate, sulfolane, gamma-butyrolactone, propylene sulfite and tetrahydrofuran, etc. may be used. . These may be used alone or in combination of two or more.

As shown in FIG. 1, electrode tabs (anode tab and cathode tab) protrude from the first and second cathode current collectors 115 and 125 and the anode current collector 135 belonging to each electrode cell, respectively, and the outer case ( 160) may be extended to one side. The electrode tabs may be fused together with the one side of the exterior case 160 to form electrode leads (anode lead 107 and cathode lead 127) extending or exposed to the outside of the exterior case 160 .

The lithium secondary battery may be manufactured in a cylindrical shape, a prismatic shape, a pouch shape, or a coin shape using a can, for example.

Hereinafter, preferred embodiments are presented to aid understanding of the present invention, but these embodiments are only illustrative of the present invention and do not limit the scope of the appended claims, and embodiments within the scope and spirit of the present invention It is obvious to those skilled in the art that various changes and modifications to the are possible, and it is natural that these variations and modifications fall within the scope of the appended claims.

manufacturing example

(1) Preparation of the first positive electrode slurry

LiFePO ₄ as a cathode active material, carbon black as a conductive material, and polyvinylidene fluoride (PVDF) as a binder were added and mixed in NMP (n-methyl 2-pyrrolidone) at a mass ratio of 92:5:3 to prepare a first cathode slurry. manufactured.

(2) Preparation of the second positive electrode slurry

Add and mix LiNi _0.8 Co _0.1 Mn _0.1 O ₂ as a cathode active material, carbon black as a conductive material, and polyvinylidene fluoride (PVDF) as a binder in NMP (n-methyl 2-pyrrolidone) at a mass ratio of 92:5:3 Thus, a second positive electrode slurry was prepared.

Example 1

(1) Manufacture of anode - first anode

A first positive electrode active material layer (130 μm) was formed by applying the first positive electrode slurry prepared in Preparation Example on one surface of an aluminum substrate, followed by drying and pressing.

The second cathode active material layer (140 μm) was formed by applying the second cathode slurry prepared in Preparation Example on the other side of the aluminum substrate, followed by drying and rolling.

At this time, all anodes were made of the first anode.

(2) cathode manufacturing

92% by weight of artificial graphite as an anode active material, 2% by weight of styrene-butadiene rubber (SBR)-based binder, 1% by weight of CMC as a thickener, and 5% by weight of flake-type amorphous graphite as a conductive material. SBR and CMC were dissolved in water and mixed. Then, artificial graphite and water were added/mixed to prepare an anode slurry. This was coated on both sides of the copper substrate, dried, and pressed to form a negative electrode active material layer (140 μm).

(3) Manufacture of lithium secondary battery

The prepared positive and negative electrodes were disposed with a polyethylene (PE) separator (25 μm) interposed therebetween to form a first electrode unit, and the first electrode units were repeatedly laminated to form an electrode assembly.

The electrode assembly was accommodated in a pouch and electrode tab portions were fused together. Thereafter, an electrolyte solution was injected and sealed to prepare a lithium secondary battery.

As the electrolyte, a solution obtained by dissolving 1M LiPF ₆ in a mixed solvent of EC/EMC/DEC (25/45/30; volume ratio) was used.

Chemical charging and discharging was performed on the manufactured lithium secondary battery (charging condition CC-CV 0.1C 4.3V 0.005C CUT-OFF, discharging condition CC 0.1C 3V CUT-OFF).

Example 2

(1) Manufacture of anode - first anode

A first positive electrode active material layer (65 μm) was formed by applying the first positive electrode slurry prepared in Preparation Example on both sides of the aluminum substrate, followed by drying and pressing.

A second cathode active material layer (70 μm) was formed by applying a second cathode slurry on each of the formed first cathode active material layers, drying, and rolling.

Except for the above, a lithium secondary battery was manufactured in the same manner as in Example 1.

Example 3

A positive electrode and a lithium secondary battery were manufactured in the same manner as in Example 2, except that a polystyrene-based positive electrode interface deposition binder was interposed as an insulating film (10 μm) between the first positive electrode active material layer and the second positive electrode active material layer.

Example 4

(1) Manufacture of anode - first anode and second anode

1) Preparation of the first anode

The first positive electrode slurry prepared in Preparation Example was coated on both sides of the aluminum substrate, dried and pressed to form a first positive electrode including a first positive electrode active material layer (130 μm).

2) Preparation of the second anode

The second positive electrode slurry prepared in Preparation Example was coated on both sides of the aluminum substrate, dried and pressed to form a second positive electrode including a second positive electrode active material layer (140 μm).

(3) Manufacture of lithium secondary battery

A first electrode unit was formed using the manufactured first positive electrode, and a second electrode unit was formed using the manufactured second positive electrode.

An electrode assembly was formed by disposing the first electrode unit on each of the uppermost and lowermost three layers, and disposing the second electrode unit on the remaining layers.

At this time, the ratio of the number of the first positive electrodes to the total number of the plurality of positive electrodes was 40%.

Except for the above, a lithium secondary battery was manufactured in the same manner as in Example 1.

Example 5

Example, except that the electrode assembly is formed by alternately and repeatedly stacking the first electrode unit and the second electrode unit, and the ratio of the number of the first positive electrode to the total number of the plurality of positive electrodes is 50%. A lithium secondary battery was manufactured in the same manner as in 4.

Comparative Example 1

A lithium secondary battery was manufactured in the same manner as in Example 1, except that only the positive electrode prepared by applying, drying, and compressing the second positive electrode slurry on both sides of the aluminum current collector was used.

Comparative Example 2

A lithium secondary battery was manufactured in the same manner as in Example 4, except that the ratio of the number of first positive electrodes to the total number of positive electrodes was 25%.

Experimental example

Thermal runaway evaluation

After attaching a thermal pad on the upper surface of the lithium secondary battery according to the above-described Examples and Comparative Examples, heat was applied to increase the temperature at a rate of 5 °C/sec.

Then, whether thermal runaway occurred and the time until thermal runaway occurred were measured.

Whether or not thermal runaway occurred was evaluated by whether deformation, ignition, or explosion of the lithium secondary battery occurred within 60 minutes.

Ο: Thermal runaway occurs.

Χ: No thermal runaway.

The evaluation results are shown in Table 1 below.

division thermal runaway Thermal runaway time (min) Example 1 Χ - Example 2 Χ - Example 3 Χ - Example 4 Χ - Example 5 Χ - Comparative Example 1 Ο 4 Comparative Example 2 Ο 35

Referring to Table 1 above, in Examples 1 to 5, thermal runaway did not occur within 1 hour because the LFP cathode active material layer was sufficiently included.

In Comparative Example 1, only lithium-transition metal composite oxide was used as a positive electrode active material, and thermal runaway occurred within a short time after heat was applied.

In Comparative Example 2, the LFP cathode active material layer was insufficiently included, so the thermal runaway inhibitory effect was lowered compared to Examples.

110: first positive electrode 115: first positive electrode current collector
112: first positive electrode active material layer 114: second positive electrode active material layer
116: insulating film 120: second anode
125: second positive current collector 130: negative electrode
132: negative electrode active material layer 135: negative electrode current collector
140: separator 150: electrode assembly
160: case

Claims (15)

case; and
An electrode assembly accommodated in the case, wherein the electrode assembly,
cathodes including a first cathode active material layer containing lithium iron phosphate (LiFePO ₄ ) and a second cathode active material layer containing a lithium-transition metal composite oxide; and
Including cathodes facing at least one of the anodes,
The lithium secondary battery, wherein the ratio of the number of positive electrodes including the first positive electrode active material layer to the total number of positive electrodes included in the electrode assembly is 30% or more. The method according to claim 1, wherein the positive electrodes are formed on a first positive current collector, the first positive electrode active material layer formed on one surface of the first positive current collector, and the other surface opposite to the one surface of the first positive current collector. A lithium secondary battery comprising a first positive electrode including the formed second positive electrode active material layer. The method according to claim 1, wherein the positive electrodes include a first positive electrode including a first positive electrode current collector and the first positive electrode active material layer formed on one surface of the first positive electrode current collector and the other surface opposite to the first positive electrode current collector Including, a lithium secondary battery. The lithium secondary battery of claim 3 , wherein the first positive electrode further comprises the second positive electrode active material layer formed on the first positive electrode active material layer. The lithium secondary battery of claim 4 , wherein the first positive electrode further comprises a heat insulating film disposed between the first positive electrode active material layer and the second positive electrode active material layer. The lithium secondary battery of claim 5 , wherein the insulating film includes at least one of polyurethane, polystyrene, polyethylene, and polyester. The lithium secondary battery of claim 3, wherein the outermost positive electrode of the electrode assembly is the first positive electrode. The lithium secondary battery according to claim 3, wherein among the positive electrodes, 3 to 5 positive electrodes sequentially disposed from the outermost periphery of the electrode assembly are the first positive electrodes. The method of claim 3,
The positive electrodes further include a second positive electrode current collector, and a second positive electrode including the second positive electrode active material layer formed on one surface of the second positive electrode current collector and the other surface opposite to the one surface, lithium. secondary battery. The lithium secondary battery of claim 9, wherein the first positive electrode and the second positive electrode are alternately and repeatedly arranged in a thickness direction of the electrode assembly. The lithium secondary battery of claim 1 , wherein the first positive electrode active material layer includes both the lithium iron phosphate and the lithium-transition metal composite oxide. The method according to claim 11, wherein the first positive electrode active material layer comprises a concentration gradient region in which the concentration of the lithium iron phosphate and the concentration of the lithium-transition metal composite oxide gradually change from one surface to the other surface of the first positive electrode active material layer , lithium secondary battery. The method according to claim 11, wherein the content of the lithium iron phosphate relative to the total weight of the first positive electrode active material layer in the electrode assembly is 50% by weight or more, the lithium secondary battery. The lithium secondary battery of claim 1, wherein the sum of the thicknesses of the first cathode active material layer to the sum of the thicknesses of the cathodes in the electrode assembly is 30% or more. The method according to claim 1, wherein the content of the lithium iron phosphate relative to the total weight of the first positive electrode active material layer in the electrode assembly is 50% by weight or more, the lithium secondary battery.

Images