KR20220015222A - Anode for lithium secondary battery and lithium secondary battery including the same - Google Patents

Abstract

A negative electrode for a lithium secondary battery according to exemplary embodiments of the present invention includes: a negative electrode current collector; a first negative electrode active material layer disposed on the negative electrode current collector and including a first binder including an acrylate-styrene butadiene copolymer; and a second negative electrode active material layer disposed on the first negative electrode active material layer, including a second binder including an acrylate-styrene butadiene copolymer, and having porosity in a state swollen by the electrolyte greater than that of the first negative electrode active material layer. The negative electrode for a lithium secondary battery can improve energy density and output characteristics of a battery.

Description

Anode for lithium secondary battery and lithium secondary battery including same

The present invention relates to a negative electrode for a lithium secondary battery and a lithium secondary battery including the same.

A secondary battery is a battery that can be repeatedly charged and discharged, and is widely applied 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. Also, recently, a battery pack including a secondary battery has been developed and applied as a power source for an eco-friendly vehicle such as a hybrid vehicle.

Examples of the secondary battery include a lithium secondary battery, a nickel-cadmium battery, a nickel-hydrogen battery, and the like. In this regard, R&D is being actively carried out.

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-type casing accommodating the electrode assembly and the electrolyte.

For example, a lithium secondary battery may include a negative electrode such as a carbon material capable of occluding and releasing lithium ions, a positive electrode made of a lithium-containing oxide, etc., and a non-aqueous electrolyte in which a lithium salt is dissolved in an appropriate amount in a mixed organic solvent.

On the other hand, amorphous carbon or crystalline carbon is used as an anode active material for a negative electrode, and among them, crystalline carbon is mainly used because of its high capacity. Examples of such crystalline carbon include natural graphite, artificial graphite, and the like.

For example, Korean Patent Application Laid-Open No. 10-2005-0004930 discloses a technology related to an anode active material including artificial graphite, but there may be limitations in improving discharge capacity or output.

Korean Patent Publication No. 10-2005-0004930

An object of the present invention is to provide a negative electrode for a lithium secondary battery having excellent fast charging performance.

Another object of the present invention is to provide a lithium secondary battery including the negative electrode for a lithium secondary battery.

A negative electrode for a lithium secondary battery according to exemplary embodiments of the present invention includes a negative electrode current collector; and a first anode active material layer disposed on the anode current collector and a first binder including an acrylate-styrene butadiene copolymer and disposed on the first anode active material layer, the acrylate-styrene butadiene copolymer A second binder comprising a, and the porosity in a swollen state by the electrolyte may include a negative active material layer including a second negative active material layer greater than that of the first negative active material layer.

In some embodiments, the porosity of the second anode active material layer may be 3 to 8% greater than the porosity of the first anode active material layer.

In some embodiments, the content of the repeating unit derived from the acrylate monomer in the acrylate-styrene butadiene copolymer may be greater in the second binder than in the first binder.

In some embodiments, the content of the repeating unit derived from the acrylate monomer in the acrylate-styrene butadiene copolymer may be 5 to 25% by weight of the first binder, and 25 to 90% by weight of the second binder. .

In some embodiments, the acrylate monomer in the acrylate-styrene butadiene copolymer included in the second binder with respect to the content of the repeating unit derived from the acrylate monomer in the acrylate-styrene butadiene copolymer included in the first binder The ratio of the content of the derived repeating unit may be greater than 1 and less than or equal to 10.

In some embodiments, the thickness of the second negative active material layer may be 3 to 70% of the total thickness of the negative electrode for a lithium secondary battery.

In some embodiments, the negative electrode for a lithium secondary battery further includes a buffer layer disposed between the first negative active material layer and the second negative active material layer, and the porosity of the buffer layer in a state swollen by the electrolyte solution may have a value between the porosity of the first anode active material layer and the porosity of the second anode active material layer.

In some embodiments, the rolled electrode density of the negative active material layer may be 1.65 g/cm ³ or more and 2 g/cm ³ or less.

A lithium secondary battery according to exemplary embodiments of the present invention includes a negative electrode, a positive electrode, and a separator interposed between the negative electrode and the positive electrode for the lithium secondary battery, wherein the negative electrode includes a negative electrode current collector; and a first negative active material layer disposed on the negative electrode current collector and including a first binder including an acrylate-styrene butadiene copolymer, and disposed on the first negative electrode active material layer, acrylate-styrene butadiene air The second anode active material layer may include a second binder including a coalescing, and a porosity in a state swollen by the electrolyte is greater than that of the first anode active material layer.

The second anode active material layer included in the negative electrode for a lithium secondary battery according to embodiments of the present invention has a higher porosity in a state swollen by the electrolyte than the first negative active material layer including the first binder by the second binder. can be large The movement of lithium ions may be facilitated by the second anode active material layer having a high porosity, and overall electrode resistance may be reduced.

Since the first negative active material layer included in the negative electrode for a lithium secondary battery according to the embodiments of the present invention has a relatively smaller porosity than the second negative active material layer, the energy density of the negative electrode active material layer is effectively prevented can do.

Accordingly, the lithium secondary battery including the negative electrode for the lithium secondary battery improves the movement of lithium ions, so that the rapid charging characteristics of the lithium secondary battery can be improved due to reduced diffusion resistance, and at the same time, excellent energy density can have

1 is a schematic cross-sectional view showing the structure of an exemplary lithium secondary battery.
2 is a schematic cross-sectional view showing the structure of an exemplary negative electrode for a lithium secondary battery.

An electrode for a lithium secondary battery according to exemplary embodiments of the present invention includes an anode current collector, a first anode active material layer disposed on the anode current collector, and a first binder comprising an acrylate-styrene butadiene copolymer; A second anode disposed on the first anode active material layer, including a second binder including an acrylate-styrene butadiene copolymer, and having a porosity in a state swollen by an electrolyte solution greater than that of the first anode active material layer and an active material layer. Accordingly, it is possible to easily implement an anode for a lithium secondary battery having a high energy density while having excellent fast charging characteristics.

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

1 is a schematic cross-sectional view showing the structure of an exemplary lithium secondary battery.

1, the lithium secondary battery 100 of the present invention is a lithium secondary battery negative electrode 200, a positive electrode 300, and a separator 400 interposed between the negative electrode 200 and the positive electrode 300 for a lithium secondary battery. may include

The lithium secondary battery 100 of the present invention is manufactured as an electrode structure in which a separator 400 is interposed between the negative electrode 200 and the positive electrode 300 for a lithium secondary battery according to the present invention, and then stored in a battery case, and non-water It may be prepared by injecting an electrolyte.

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

For example, the negative electrode 200 for a lithium secondary battery may include a negative electrode current collector 210 and a negative electrode active material layer 220 .

Copper or a copper alloy may be used as the negative electrode current collector 210, but is not limited thereto, and carbon, nickel, titanium, or silver is applied to the surface of stainless steel, nickel, copper, titanium or alloys thereof, copper or stainless steel. Processed ones may be used.

For example, the anode active material layer 220 is prepared by mixing and stirring the anode active material, a binder, a solvent, a conductive material, a dispersing material, etc. as needed to prepare a slurry, then applying (coating) it to a current collector of a metal material and compressing it. can be manufactured.

As the solvent, a non-aqueous solvent may be generally used. The non-aqueous solvent may include, for example, N-methyl-2-pyrrolidone (NMP), dimethylformamide, dimethylacetamide, N,N-dimethylaminopropylamine, ethylene oxide, tetrahydrofuran, and the like. .

As the conductive material, a conventional conductive carbon material may be used without particular limitation.

The structure of the anode active material layer 220 and the physical properties of the anode active material and the binder included in the anode active material layer 220 will be described later with reference to FIG. 2 .

For example, the positive electrode 300 may include a positive electrode current collector 310 and a positive electrode active material 320 . For example, the positive electrode 300 may be manufactured by coating the positive electrode active material on the positive electrode current collector 310 .

As the positive electrode current collector 310, aluminum or an aluminum alloy may be used, but is not limited thereto, and carbon, nickel, titanium, or silver is applied to the surface of stainless steel, nickel, aluminum, titanium or alloys thereof, or aluminum or stainless steel. Processed ones may be used.

The positive electrode active material is not particularly limited, and a commonly used positive electrode active material may be used. As a specific example, at least one selected from cobalt, manganese, and nickel and at least one of lithium complex oxides are preferable, and as a representative example thereof, the lithium-containing compound described below may be preferably used.

Li _x Mn _1-y M _y A ₂

Li _x Mn _1-y M _y O _2-z X _z

Li _x Mn ₂ O _4-z X _z

Li _x Mn _2-y M _y M' _z A ₄

Li _x Co _1-y M _y A ₂

Li _x Co _1-y M _y O _2-z X _z

Li _x Ni _1-y M _y A ₂

Li _x Ni _1-y M _y O _2-z X _z

Li _x Ni _1-y Co _y O _2-z X _z

Li _x Ni _1-yz Co _y M _z A _α

Li _x Ni _1-yz Co _y M _z O _2-α X _α

Li _x Ni _1-yz Mn _y M _z A _α

Li _x Ni _1-yz Mn _y M _z O _2-α X

In the formula, 0.9≤x≤1.1, 0≤y≤0.5, 0≤z≤0.5, 0≤α≤2.2, M and M' are the same or different, Mg, Al, Co, K, Na, Ca, Si, Ti, Sn, V, Ge, Ga, B, As, Zr, Mn, Cr, Fe, Sr, V and selected from the group consisting of rare earth elements, A is selected from the group consisting of O, F, S and P selected, and X is selected from the group consisting of F, S and P.

A method of coating the positive electrode active material on the positive electrode current collector 310 is not particularly limited as long as it is a method commonly used in the art, and a method of coating after preparing a positive electrode slurry may be used.

Specifically, in the positive electrode 300 according to the present invention, a slurry is prepared by mixing and stirring a solvent, a binder, a conductive material, a dispersing material, etc., in the positive electrode active material, if necessary, and then applying (coating) it on the positive electrode current collector 310 It can be manufactured by compression and drying.

As the solvent, a non-aqueous solvent may be generally used. The non-aqueous solvent may include, for example, N-methyl-2-pyrrolidone (NMP), dimethylformamide, dimethylacetamide, N,N-dimethylaminopropylamine, ethylene oxide, tetrahydrofuran, and the like. there is.

The binder is vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride (polyvinylidenefluoride, PVDF), polyacrylonitrile (polyacrylonitrile), polymethyl methacrylate (polymethylmethacrylate), etc. of an organic binder or an aqueous binder such as styrene-butadiene rubber (SBR).

For example, the binder may be used together with a thickener such as carboxymethyl cellulose (CMC).

As the conductive material, a conventional conductive carbon material may be used without particular limitation.

For example, as the separator 400, a conventional porous polymer film conventionally used as the separator 400, for example, ethylene homopolymer, propylene homopolymer, ethylene/butene copolymer, ethylene/hexene copolymer, and ethylene/ A porous polymer film made of a polyolefin-based polymer such as a methacrylate copolymer may be used alone or by laminating them, or a conventional porous nonwoven fabric, for example, a high melting point glass fiber, a polyethylene terephthalate fiber, or the like. can be used, but is not limited thereto.

As a method of applying the separator 400 to a battery, in addition to the general method of winding, lamination, stacking, and folding of the separator and the electrode are possible.

For example, the non-aqueous electrolyte may include a lithium salt as an electrolyte and an organic solvent.

As the lithium salt, those commonly used in electrolytes for lithium secondary batteries may be used without limitation, and may be expressed as Li ⁺ X ⁻ .

The anion of the lithium salt is not particularly limited, but 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 ⁻ and the like can be exemplified.

The organic solvent may be used without limitation, those commonly used in electrolytes for lithium secondary batteries, representatively propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (diethyl carbonate, DEC), dimethyl carbonate (DMC), ethylmethyl carbonate (EMC), methylpropyl carbonate, dipropyl carbonate, fluoroethylene carbonate (FEC), dimethylsulfuroxide, acetonitrile, dimethoxyethane, diethoxyethane, vinylene carbonate , sulfolane, gamma-butyrolactone, any one selected from the group consisting of propylene sulfite and tetrahydrofuran, or a mixture of two or more of these may be used.

2 is a schematic cross-sectional view showing the structure of an exemplary negative electrode for a lithium secondary battery.

Referring to FIG. 2 , a negative electrode for a lithium secondary battery according to exemplary embodiments includes a negative electrode current collector 210 , a first negative active material layer 222 and a first negative active material layer disposed on the negative current collector 210 ( 222) may include an anode active material layer 220 including a second anode active material layer 224 disposed thereon.

For example, the porosity of the second anode active material layer 224 in a state swollen by the electrolyte may be greater than that of the first anode active material layer 222 . For example, the porosity of the second anode active material layer 224 before swelling by the electrolyte may be the same as the porosity of the first anode active material layer 222 .

In this case, the second anode active material layer 224 may have low diffusion resistance to lithium ions due to its relatively large porosity. In addition, the first anode active material layer 222 can easily implement a higher energy density due to a relatively small porosity.

For example, the porosity is measured in a state in which the anode active material layer 220 is swollen by the electrolyte, and may be calculated by Equation 1 below.

[Equation 1]

For example, the anode active material layer 220 including the first anode active material layer 222 and the second anode active material 224 has a low diffusion resistance and thus has excellent fast charging characteristics and at the same time has excellent energy density. there is.

For example, as the anode current collector, the first anode active material layer 222 and the second anode active material layer 224 are sequentially disposed, and the second anode active material layer 224 is disposed adjacent to the separator 400 , , the diffusion resistance reduction effect by the second anode active material layer 224 may be further improved.

According to some embodiments, the porosity of the second anode active material layer 224 may be 3 to 8% greater than the porosity of the first anode active material layer 222 . For example, when the porosity difference between the second anode active material layer 224 and the first anode active material layer 222 satisfies the above range, the diffusion resistance reduction effect by the second anode active material layer 224 and the first anode The effect of increasing the energy density by the active material layer 222 may be further improved.

For example, when the difference between the porosity of the second anode active material layer 224 and the porosity of the first anode active material layer 222 exceeds 8%, the second anode active material layer 224 and the first anode active material layer The porosity difference in the state in which 222 is swollen by the electrolyte is too large, so that the interface between the first negative active material layer 222 and the second negative active material layer 222 is unstable, and the capacity retention rate of the lithium secondary battery is lowered can be

For example, the first anode active material layer 222 may include a first anode active material and a first binder. For example, the second anode active material layer 224 may include a second anode active material and a second binder.

In this case, the peak intensity ratio (ID/IG) of the first negative active material according to the Raman spectroscopy method may be smaller than the peak intensity ratio (ID/IG) of the second negative electrode active material according to the Raman spectroscopy method.

The peak intensity ratio (ID/IG) according to the Raman spectroscopy method maps a plurality of points of the negative electrode 200 for a lithium secondary battery to the D band intensity (ID) and G band intensity (IG) of the negative active material for each point After measuring , it may mean a value obtained by averaging the ratio (ID/IG) of D band intensity (ID) to G band intensity (IG).

For example, the D band may mean a peak intensity in a region of 1,300 to 1,420 cm ⁻¹ in a spectrum measured at an excited wavelength of 532 nm using a Raman spectrometer. For example, the G band may mean a peak intensity at 1,540 to 1,620 cm ⁻¹ in a spectrum measured at an excited wavelength of 532 nm using a Raman spectrometer.

For example, as the peak intensity ratio (ID/IG) according to the Raman spectroscopy is smaller, it may mean that the basal plane of the negative active material is long. Conversely, as the peak intensity ratio (ID/IG) according to the Raman spectroscopy method increases, it may mean a case where the surface of the edge plane exposed in the negative electrode active material is wide.

In this case, the first anode active material layer 222 disposed on the anode current collector 210 includes an anode active material having a long basal plane, and a second anode active material layer disposed on the first anode active material layer 222 ( 224) may include an anode active material with a wide surface of the exposed edge plane.

For example, the anode active material having a wide surface of the exposed edge plane included in the second anode active material layer 224 may easily perform intercalation with lithium ions. Accordingly, the low resistance characteristic of the second anode active material layer 224 may be improved.

In addition, the anode active material having a long basal plane included in the first anode active material layer 222 may further improve adhesion properties between the first anode active material layer 222 and the anode current collector 210 .

For example, the negative active material may include graphite particles. The graphite particle indicates that at least a part of the particle surface is graphite. The graphite particles are coal-based, petroleum-based pure coke, casine coke, coke such as needle coke (Coke, Car Sign coke, needle coke), mesophase globules, bulk mesophase, etc. Artificial graphite obtained by graphitizing graphite precursors such as mesophase carbons at about 150° C. or higher, preferably about 2,800° C. to 3,200° C., natural graphite in flaky, lumpy form, and spheroidizing, pulverizing and granulating natural graphite It may contain spherical spherical natural graphite.

For example, the graphite particles may be prepared by chemical or physical treatment. The chemical or physical treatment method is not particularly limited, but, for example, pulverization, classification, granulation, lamination, compression, compounding, mixing, coating, oxidation, vapor deposition, mechanochemical treatment, chamfering, spheroidization, and curvature , heat treatment, and the like.

According to some exemplary embodiments, the first anode active material layer 222 includes a first binder including an acrylate-styrene butadiene copolymer, and the second anode active material layer 224 includes an acrylate-styrene butadiene copolymer. A second binder including a coalescence may be included.

For example, the content of the repeating unit derived from the acrylate monomer in the acrylate-styrene butadiene copolymer may be greater in the second binder than in the first binder.

For example, the second negative active material layer 224 including the second binder having a large content of repeating units derived from acrylate monomers in the acrylate-styrene butadiene copolymer is swollen by the electrolyte by the second binder. The porosity in the can be increased.

For example, the first negative active material layer 222 including the first binder having a small content of repeating units derived from acrylate monomers in the acrylate-styrene butadiene copolymer is swollen by the electrolyte by the first binder. The porosity in the can be reduced.

Accordingly, the porosity of the second anode active material layer 224 may be greater than the porosity of the first anode active material layer 222, so that a negative electrode for a lithium secondary battery having low diffusion resistance and excellent energy density is easier can be implemented

For example, the content of the repeating unit derived from the acrylate monomer in the acrylate-styrene butadiene copolymer may be about 5 to 25% by weight of the first binder, and about 25 to 90% by weight of the second binder.

For example, the first negative active material layer 222 disposed on the negative electrode current collector 210 includes an acrylate-styrene butadiene copolymer in which the content of the repeating unit derived from the acrylate monomer satisfies the above range. 1 By including the binder, the energy density of the anode active material layer 220 may be further improved. In addition, adhesion between the negative electrode current collector 210 and the first negative electrode active material 222 may be improved.

For example, the second anode active material layer 224 disposed on the first anode active material layer 222 may include an acrylate-styrene butadiene copolymer in which the content of the repeating unit derived from the acrylate monomer satisfies the above range. As the second binder is included, the diffusion resistance property of the anode active material layer 220 is further improved,

Accordingly, it is possible to easily implement the anode active material layer 220 having a low resistance characteristic, excellent fast charging characteristics, excellent energy density, and excellent adhesion to the anode current collector 210 at the same time.

According to some embodiments, the content of the first binder may be about 1 to 3% by weight based on the total weight of the first negative active material layer 222 . For example, when the above range is satisfied, as the content of the acrylate-styrene butadiene copolymer having a low content of acrylate monomer-derived repeating units included in the anode active material layer 220 is increased, the anode active material layer 220 is Energy density can be improved.

Meanwhile, the lower limit of the content of the first binder is not particularly limited, but may be sufficient as long as it can maintain the function of the electrode, for example, it may be about 0.1% by weight or more based on the total weight of the first negative active material layer 222 .

According to some embodiments, the content of the second binder may be 1.5% by weight or less based on the total weight of the second negative active material layer 224 . For example, when the content of the second binder in the second active material layer 224 satisfies the above range, the porosity difference between the second negative active material layer 220 and the first negative active material layer is appropriate, so that the rapid It is possible to easily implement a lithium secondary battery having excellent charging characteristics and excellent energy density at the same time.

According to some embodiments, the acrylate monomer in the acrylate-styrene butadiene copolymer included in the second binder with respect to the content of the repeating unit derived from the acrylate monomer in the acrylate-styrene butadiene copolymer included in the first binder The content ratio of the derived repeating unit may be greater than about 1 and less than or equal to 10.

For example, when the content ratio of the acrylate monomer-derived repeating unit of the second binder to the first binder satisfies the above range, the anode active material layer 220 having excellent energy density while satisfying low resistance characteristics can be more easily implemented. can

In some embodiments, the acrylate-styrene butadiene copolymer included in the first binder or the second binder may have a core-shell structure. In this case, the core of the acrylate-styrene butadiene copolymer may include styrene and/or butadiene, and the shell of the acrylate-styrene butadiene copolymer may include a plurality of acrylate functional groups. .

According to some embodiments, the first anode active material layer or the second anode active material layer may further include at least one selected from the group consisting of polyvinylidene fluoride and carboxymethyl cellulose as a sub-binder. may include For example, the adhesive property of the first anode active material layer may be further improved or the low resistance property of the second anode active material layer may be further improved by the sub-binder.

According to some embodiments, the thickness of the second anode active material layer 224 may be about 3 to 70% of the total thickness of the anode active material layer 220 . For example, when the thickness of the second anode active material layer 224 satisfies the above range, the anode active material layer 220 can more easily implement improved low-resistance characteristics, so that the fast charging characteristics of the negative electrode for lithium secondary batteries This can be further improved.

According to some embodiments, the negative electrode 200 for a lithium secondary battery may further include a buffer layer disposed between the first negative active material layer 222 and the second negative active material layer 224 . In this case, the porosity of the buffer layer in a state swollen by the electrolyte may have a value between the porosity of the first negative active material layer 222 and the porosity of the second negative active material layer 224 .

For example, in the process of forming the first anode active material layer 222 and the second anode active material layer 224, the buffer layer includes a composition for forming the first anode active material layer 222 and a second anode active material layer ( 224) may be formed by mixing some of the composition. Therefore, the buffer layer may have a value between the porosity of the first negative active material layer 222 and the porosity of the second negative active material layer 224 .

For example, the buffer layer is disposed between the first anode active material layer 222 and the second anode active material layer 224, and the bonding force between the first anode active material layer 222 and the second anode active material layer 224 ( Cohesion), it is possible to improve the mechanical durability of the negative electrode 200 for a lithium secondary battery. In addition, by utilizing the buffer layer, it is possible to more easily implement a lithium secondary battery having excellent energy density and excellent fast charging characteristics.

According to some embodiments, the loading level of the anode active material layer 210 may be 8 mg/cm ² or more and 20 mg/cm ² or less. For example, the loading level may mean the amount of the negative active material disposed on the negative electrode current collector 210 per unit area.

For example, when the loading level satisfies the above range, the energy density of the negative electrode for a lithium secondary battery may be further improved. Therefore, it is possible to more easily implement a lithium secondary battery having excellent fast charging characteristics and energy density at the same time.

According to some embodiments, the rolled electrode density of the anode active material layer 220 may be 1.65 g/cm ³ or more and 2 g/cm ³ or less. For example, the rolled electrode density may be a specific density of the negative electrode active material layer 220 after reducing the thickness of the electrode by applying pressure with a roll in the rolling mill.

For example, when the rolled electrode density satisfies the above range, the energy density of the negative electrode for a lithium secondary battery may be further improved. Therefore, it is possible to more easily implement a lithium secondary battery having excellent fast charging characteristics and energy density at the same time.

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

Examples and Comparative Examples

Example 1

<Cathode>

A first negative electrode slurry was prepared by mixing single-particle graphite as a first negative active material and a first binder including an acrylate-styrene butadiene copolymer having an acrylate monomer content of 9% and then dispersing it in distilled water from which ions have been removed, The first negative electrode slurry was applied to one surface of a Cu-foil current collector to form a first negative electrode active material layer. The content of the first binder was 2% by weight based on the total weight of the first negative active material layer.

A second negative electrode slurry is prepared by mixing granulated graphite as a second negative active material and a second binder containing an acrylate-styrene butadiene copolymer having an acrylate monomer content of 50% and then dispersing it in distilled water from which ions have been removed, The second negative electrode slurry was applied to one surface of the first negative electrode active material layer to form a second negative electrode active material layer. The content of the second binder was 1 wt% based on the total weight of the second anode active material layer.

Thereafter, a negative electrode was manufactured by drying and pressing to form a negative active material layer having a size of 10 cm × 10 cm × 50 μm.

The thickness of the first negative active material layer was 30% of the total thickness of the negative active material layer, and the thickness of the second negative active material layer was 70% of the total thickness of the negative active material layer. The loading level of the negative active material layer was 14.5 mg/cm ² , and the rolled electrode density was 1.70 g/cm ³ .

<Anode>

Li _1.0 Ni _0.6 Co _0.2 Mn _0.2 O ₂ as a cathode active material, Denka Black as a conductive material, PVDF as a binder, and N-methyl pyrrolidone as a solvent 46: 2.5: 1.5: After preparing a positive electrode slurry with each mass ratio composition of 50, it was coated, dried, and pressed on an aluminum substrate to prepare a positive electrode.

<Battery>

The prepared positive electrode and negative electrode are each stacked by notching to an appropriate size, and a separator (polyethylene, thickness 25㎛) is interposed between the positive electrode plate and the negative electrode plate to construct a battery, and the tab part of the positive electrode and the tab part of the negative electrode are welded, respectively. did The welded anode/separator/cathode combination was placed in a pouch, and the tab portion was included in the sealing area to seal three sides except for the electrolyte injection side. After injecting the electrolyte into the remaining part and sealing the remaining side, it was impregnated for more than 12 hours. The electrolyte is a 1M LiPF ₆ solution with a mixed solvent of ethylene carbonate (EC) / ethylmethyl carbonate (EMC) / diethylene carbonate (DEC) (25/45/30; volume ratio), and then vinylene carbonate (VC) 1.5 wt%, 0.5 wt% of 1,3-propensultone (PRS) was used.

Thereafter, pre-charging was performed for 60 minutes at a current corresponding to 0.25C. After 1 hour of degasing and aging for more than 24 hours, chemical charge and discharge were performed (charge condition CC-CV 0.2C 4.2V 0.05C CUT-OFF, discharge condition CC 0.2C 2.5V CUT-OFF). After that, standard charge/discharge was performed (charge condition CC-CV 1/3 C 4.2V 0.05C CUT-OFF, discharge condition CC 0.5C 2.5V CUT-OFF).

<Measurement of porosity>

The porosity of the first negative active material layer, the second negative active material layer, and the entire negative electrode active material layer swollen by the electrolyte was calculated according to Equation 1 below.

[Equation 1]

The porosity of the first anode active material layer was 30%, the porosity of the second anode active material layer was 34%, and the porosity of the entire anode active material layer was 32%.

Examples 2 to 4 and Comparative Example 1

The content of the repeating unit derived from the acrylate monomer in the acrylate-styrene butadiene copolymer of the first binder included in the first anode active material layer, the acrylate of the second binder included in the second anode active material layer-styrene butadiene A lithium secondary battery in the same manner as in Example 1, except that the content of the repeating unit derived from the acrylate monomer in the copolymer, the loading level of the negative active material layer, and the rolled electrode density of the negative active material layer are as shown in Table 1 below. was prepared.

In addition, the calculated porosity of the first negative active material layer, the porosity of the second negative active material layer, and the total porosity of the negative active material layer were calculated and are shown in Table 1 below.

Comparative Examples 2 and 3

Only the active material layer having the same characteristics as the first anode active material layer was formed on the Cu-foil current collector (Comparative Example 2), or only the active material layer having the same characteristics as the second anode active material layer was formed on the Cu-foil current collector. Except (Comparative Example 3), a lithium secondary battery was prepared in the same manner as in Example 1.

division Recurring unit content (%) derived from acrylate monomers in the first binder Content of repeating units derived from acrylate monomers in the second binder (%) Loading level of the anode active material layer
(mg/cm ² ) Rolled electrode density of negative electrode active material layer
(g/cm ³ ) Example 1 9 50 14.5 1.7 Example 2 40 25 14.5 1.7 Example 3 25 40 14.5 1.7 Example 4 4 95 14.5 1.7 Comparative Example 1 50 9 14.5 1.7 Comparative Example 2 9 - 14.5 1.7 Comparative Example 3 - 50 14.5 1.7 division Impregnation porosity of the first negative electrode active material layer (%) Impregnation porosity of the second anode active material layer (%) Total porosity of the anode active material layer (%) Example 1 30 34 32 Example 2 31 33 32 Example 3 29 35 32 Example 4 27 37 32 Comparative Example 1 34 30 32

Experimental Example Adhesive force analysis experiments on the negative electrodes prepared according to Examples and Comparative Examples, and fast charging characteristics experiments were performed on lithium secondary batteries prepared according to Examples and Comparative Examples.

<Quick Charge Characteristics>

Cells having a large capacity of 10 Ah or more were manufactured using the negative electrodes prepared according to Examples and Comparative Examples and the same positive electrode. In this case, Ref. A three-electrode cell was manufactured by inserting an electrode (reference electrode) between the positive electrode and the negative electrode, and through this, the potential of the negative electrode during charging was checked.

While CC charging the three-electrode cell with the reference electrode inserted to 4.2V at a C-rate in the range of 0.75 to 2.5C, find the SOC point at which the CCV value of the cathode at each C-rate becomes constant below 0V, and charge this point. By designating the limit, the step-charging protocol of Comparative Examples and Examples was constructed.

The charging times of Comparative Examples and Examples were calculated with a step charging protocol prepared using a three-electrode cell, and the capacity retention rate was calculated by repeating rapid charging-1/3C discharge for 500 cycles by applying each step charging protocol.

<Energy density of lithium secondary battery>

For the lithium secondary batteries according to Examples 1 to 7 and Comparative Examples 1 to 3, energy density was calculated from cell capacity X average voltage) / (cell area X shipment cell (SOC100) thickness). The calculation results are shown in Table 2.

Quick Charge Time (min) Fast charge cycle capacity retention rate (%)
(@500 ^th ) Energy density (based on SOC 100) Example 1 34.8 95.1 555 Example 2 40.8 91.2 554 Example 3 38.1 96.0 556 Example 4 34.5 92.1 556 Comparative Example 1 41.2 89.5 553 Comparative Example 2 45.5 88.2 560 Comparative Example 3 34.6 95.4 544

Referring to Table 2, the lithium secondary batteries according to the embodiments in which the porosity of the second anode active material layer is higher than that of the first anode active material layer have superior fast charging characteristics than those of Comparative Examples and at the same time In addition, in the case of Example 5, as the porosity difference between the first anode active material layer and the second anode active material layer increased, the interface between the first anode active material layer and the second anode active material layer This became unstable, and the capacity retention rate decreased to some extent.

100: lithium secondary battery 200: negative electrode for lithium secondary battery
210: negative electrode current collector 220: negative electrode active material layer
222: first anode active material layer 224: second anode active material layer
300: positive electrode 310: positive electrode current collector
320: positive active material layer 400: separator

Claims (9)

negative electrode current collector; and
a first negative active material layer disposed on the negative electrode current collector and including a first binder including an acrylate-styrene butadiene copolymer; and
A second anode disposed on the first anode active material layer, including a second binder including an acrylate-styrene butadiene copolymer, and having a porosity in a state swollen by an electrolyte solution greater than that of the first anode active material layer A negative electrode for a lithium secondary battery comprising; an active material layer.
The negative electrode for a lithium secondary battery according to claim 1, wherein the porosity of the second negative active material layer is 3 to 8% greater than the porosity of the first negative active material layer. -
The negative electrode for a lithium secondary battery according to claim 1, wherein the second binder has a greater content of repeating units derived from acrylate monomers in the acrylate-styrene butadiene copolymer than the first binder.
The method according to claim 3, The content of the acrylate monomer-derived repeating unit in the acrylate-styrene butadiene copolymer is 5 to 25% by weight of the first binder, and 25 to 90% by weight of the second binder, for a lithium secondary battery cathode.
The method according to claim 1, The content of the acrylate monomer-derived repeating unit in the acrylate-styrene butadiene copolymer included in the first binder relative to the content of the acrylate monomer-derived repeating unit in the acrylate-styrene butadiene copolymer included in the second binder The ratio of the content of the unit is more than 1 and 10 or less, a negative electrode for a lithium secondary battery.
The negative electrode for a lithium secondary battery according to claim 1, wherein the thickness of the second negative active material layer is 3 to 70% of the total thickness of the negative electrode for a lithium secondary battery.
The method according to claim 1, further comprising a buffer layer disposed between the first anode active material layer and the second anode active material layer,
The porosity of the buffer layer in a state swollen by the electrolyte has a value between the porosity of the first negative active material layer and the porosity of the second negative active material layer, A negative electrode for a lithium secondary battery.
The negative electrode for a lithium secondary battery according to claim 1, wherein the rolled electrode density of the negative electrode active material layer is 1.65 g/cm ³ or more and 2 g/cm ³ or less.
cathode;
anode; and
and a separator interposed between the negative electrode for the lithium secondary battery and the positive electrode,
The cathode is
negative electrode current collector; and
a first anode active material layer disposed on the anode current collector and comprising a first binder comprising an acrylate-styrene butadiene copolymer;
A second anode disposed on the first anode active material layer, including a second binder including an acrylate-styrene butadiene copolymer, and having a porosity in a state swollen by an electrolyte solution greater than that of the first anode active material layer A lithium secondary battery comprising an active material layer.

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