KR20070090502A - Electrode assembly and lithium rechargeable battery using the same and method of making electrode plate - Google Patents

Abstract

An electrode assembly for a lithium secondary battery, and a method for preparing an electrode plate are provided to improve elastic modulus and tensile strength and to inhibit the swelling of an electrode plate. An electrode assembly comprises a positive electrode plate provided with a positive electrode active material layer; and a negative electrode plate provided with a negative electrode active material layer, wherein at least one of the positive electrode active material layer and the negative electrode active material layer comprises a binder which comprises polyvinylidene fluoride(PVdF) and 20-80 wt% of polymethyl methacrylate(PMMA).

Description

Electrode assembly and Lithium rechargeable battery using the same and Method of making electrode plate

1 is a cross-sectional view of a lithium secondary battery according to an embodiment of the present invention.

The present invention relates to an electrode assembly and a method for manufacturing a lithium secondary battery and a lithium secondary battery using the same, and more particularly, polyvinylidene fluoride (PVdF) as a binder during the production of a positive electrode slurry and a negative electrode slurry. By using a mixture of polymethyl methacrylate (PMMA) and an electrode that can suppress the swelling of the electrode plate during charging and discharging by improving the elastic modulus and tensile strength An assembly and a method of manufacturing a lithium secondary battery and a lithium secondary battery using the same.

In general, as the light weight and high functionality of portable wireless devices such as a video camera, a portable telephone, a portable computer, and the like progress, a lot of researches have been conducted on secondary batteries used as driving power. Such secondary batteries include, for example, nickel-cadmium batteries, nickel-hydrogen batteries, nickel-zinc batteries, and lithium secondary batteries. Among them, lithium secondary batteries are rechargeable, compact, and large-capacity, and are widely used in advanced electronic devices because of their high operating voltage and high energy density per unit weight.

Lithium secondary batteries are divided into cylindrical, square and pouch types according to their shapes. The cylindrical and rectangular lithium secondary batteries are formed including an electrode assembly, a can containing the electrode assembly, and a cap assembly sealing the upper opening of the can. In addition, the pouch-type lithium secondary battery is formed including an electrode assembly and a case accommodating the electrode assembly. The electrode assembly is formed to include a positive electrode plate and a negative electrode plate and a separator interposed therebetween. A positive electrode plate and a negative electrode plate are manufactured by mixing and stirring an electrode active material and a binder, as needed, and preparing a slurry, apply | coating this slurry to an electrode collector, and drying.

The binder provides a binding force between the electrode current collector and the active material or between the active material and the active material in coating the active material on the surface of the electrode current collector. The properties required as a binder should have not only good adhesion but also chemical stability, electrical stability, nonflammability, good electrolyte impregnation, small plate expansion, high dispersibility and high crystallinity.

Polyvinylidene fluoride (PVdF) is generally used as a binder for lithium secondary batteries, and N-methyl-2-pyrrolidone (NMP), which can dissolve polyvinylidene fluoride, is mainly used as a dispersion medium. do. However, polyvinylidene fluoride has a problem in that elasticity and tensile strength are not prevented and thus the active material structure collapse is not prevented during charging and discharging of a battery. As a result, as charging and discharging are repeated, the swelling phenomenon of the electrode plate becomes more severe, there is a possibility that an internal short may occur, and there is a problem that the cycle characteristics are deteriorated.

The present invention has been made to solve the above problems, in particular, polyvinylidene fluoride (PVdF) and polymethyl methacrylate (PolyMethyl MetaAcrylate: PMMA) as a binder (binder) in the production of anode slurry and cathode slurry ), An electrode assembly that can improve the elastic modulus and tensile strength to suppress swelling of the plate during charging and discharging, and a lithium secondary battery and a lithium secondary battery using the same. The purpose is to provide a method of manufacturing.

In the electrode assembly of the lithium secondary battery of the present invention devised to solve the above problems in the electrode assembly of a lithium secondary battery comprising a positive electrode plate having a positive electrode active material layer, and a negative electrode plate having a negative electrode active material layer, At least one of the positive electrode active material layer and the negative electrode active material layer is characterized in that it comprises a binder containing a mixture of polyvinylidene fluoride (PVdF) and polymethyl methacrylate (PMMA).

In addition, the binder may be 20% to 80% by weight of the polymethyl methacrylate (PMMA), preferably the binder is 30% to 70% by weight of the polymethyl methacrylate (PMMA) It may consist of%.

In addition, the lithium secondary battery of the present invention comprises a positive electrode plate having a positive electrode active material layer, an electrode assembly comprising a negative electrode plate having a negative electrode active material layer, a case for accommodating the electrode assembly, the positive electrode At least one of the active material layer and the negative electrode active material layer is characterized in that it comprises a binder comprising a mixture of polyvinylidene fluoride (PVdF) and polymethyl methacrylate (PMMA).

In addition, the binder may be 20% to 80% by weight of the polymethyl methacrylate (PMMA), preferably the binder is 30% to 70% by weight of the polymethyl methacrylate (PMMA) It may be made of.

In addition, the electrode plate manufacturing method of the present invention is a mixture manufacturing step of producing a mixture by melting and stirring polyvinylidene difluoride (PVdF) and polymethyl methacrylate (PMMA); A binder solution preparing step of preparing a binder solution by adding a solvent to the mixture; A slurry manufacturing step of mixing at least one of a cathode active material and an anode active material into the binder solution; A slurry coating step of coating the slurry prepared by the slurry manufacturing step on at least one surface of a current collector; And it characterized in that it comprises a slurry drying step of drying the slurry coated through the slurry coating step.

In addition, a conductive material may be further added in the slurry manufacturing step.

In addition, the solvent of the binder solution preparation step is at least one of N-methyl-2-pyrrolidone (NMP), dimethylformamide, dimethylacetamide, N, N- dimethylaminopropylamine, ethylene oxide, tetrahydrofuran Can be used.

In addition, the mixture manufacturing step may be made at 175 ℃ to 200 ℃.

In addition, the rotation speed of the rotor in the mixture manufacturing step may be 40rpm to 80rpm.

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

In the present invention, a mixture of polyvinylidene fluoride (PVdF) and polymethyl methacrylate (PMMA) may be used as the binder material to improve tensile strength and elasticity of the active material layer coated on the electrode plate of the lithium secondary battery. The mixture may be used only for the positive electrode active material layer or the negative electrode active material layer, and may also be used for both the positive electrode active material layer and the negative electrode active material layer. Preferably, the mixture is used in both the positive electrode active material layer and the negative electrode active material layer, thereby obtaining a swelling prevention effect of the electrode plate. The modulus of elasticity (E: Young Modulus) of polyvinylidene fluoride alone is 1520 MPa, and the tensile strength is about 50 MPa. On the other hand, the modulus of elasticity of polymethylmethacrylate alone is 3330 MPa, and the tensile strength is about 84 MPa. In addition, conventionally, a mixture of polyvinylidenedifluoride and polytetrafluoroethylene (PTFE) or a mixture of styrene-butadiene rubber (SBR) and polytetrafluoroethylene (PTFE) has been used as a binder to improve the binding strength of the electrode plate. . The polytetrafluoroethylene can improve the binding force between the active material particles, the active material layer and the current collector, but there is a problem of reducing the stability by reacting with lithium. The polymethyl methacrylate has almost no reaction with lithium and has an advantage of higher stability than polytetrafluoroethylene. The weight ratio of the polymethyl methacrylate in the mixture of the polyvinylidene fluoride and polymethyl methacrylate may be 20% to 80%, preferably 30% to 70%. If the weight ratio of the polymethyl methacrylate is less than 20%, the degree of improvement of the modulus of elasticity and tensile strength is insignificant and the object of the present invention is difficult to be achieved. On the other hand, when the weight ratio of the polymethyl methacrylate exceeds 80%, there is a problem that the active material layer may be detached from the electrode plate by external deformation such as winding of the electrode plate.

The mixture of polyvinylidene fluoride and polymethyl methacrylate is mixed with a solvent to form a binder solution. As the solvent, a nonaqueous solvent or an aqueous solvent is used. Examples of the non-aqueous solvent include N-methyl-2-pyrrolidone (NMP), dimethylformamide, dimethylacetamide, N, N-dimethylaminopropylamine, ethylene oxide and tetrahydrofuran. However, the type of the solvent is not limited thereto.

The lithium secondary battery according to the present invention includes a positive electrode, a negative electrode, and a nonaqueous electrolyte. The positive electrode includes a positive electrode active material capable of inserting and detaching lithium ions. The said positive electrode is formed by apply | coating the positive electrode active material slurry obtained by mixing and disperse | distributing a positive electrode active material, a binder, etc. to a solvent, to a positive electrode plate, and drying and rolling it.

As the positive electrode active material, at least one selected from cobalt, manganese, nickel and at least one of a composite oxide with lithium is preferable, and a lithium-containing compound described below may be preferably used.

Li _x Mn _{1 - y} M _y A ₂ (1)

Li _x Mn _{1 - y} MyO _{2 - z} X _z (2)

Li _x Mn ₂ O _{4 - z} X _z (3)

Li _x Mn _{2 - y} M _y M ' _z A ₄ (4)

Li _x Co _{1 - y} M _y A ₂ (5)

Li _x Co _{1 - y} M _y O _{2 - z} X _z (6)

Li _x Ni _{1 - y} M _y A ₂ (7)

Li _x Ni _{1 - y} M _y O _{2 - z} X _z (8)

Li _x Ni _{1 - y} Co _y O _{2 - z} X _z (9)

Li _x Ni _{1 -y- z} Co _y M _z A _α (10)

Li _x Ni _{1 -y- z} Co _y M _z O _{2 -α} X _α (11)

Li _x Ni _{1 -y- z} Mn _y M _z A _α (12)

Li _x Ni _{1 -y- z} Mn _y M _z O _{2 -α} X _α (13)

(Wherein 0.9≤x≤1.1, 0≤y≤0.5, 0≤z≤0.5, 0≤α≤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 rare earth elements, A is selected from the group consisting of O, F, S and P And X is selected from the group consisting of F, S and P.)

As the positive electrode current collector, a metal such as aluminum, copper, nickel, silver, stainless steel, an alloy of these metals, or the like may be used, and aluminum or an aluminum alloy is usually used.

The said negative electrode is formed by apply | coating the negative electrode active material slurry obtained by mixing and disperse | distributing a negative electrode active material, a binder, etc. to a solvent to a negative electrode collector, and drying and rolling it. As the solvent, a non-aqueous solvent or an aqueous solvent is used, and as the non-aqueous solvent, N-methyl-2-pyrrolidone (NMP), dimethylformamide, dimethylacetamide, N, N-dimethylaminopropylamine, ethylene oxide, tetra Hydrofuran and the like may be used as mentioned above.

The negative electrode active material is preferably capable of reversibly occluding and desorbing lithium ions, and a carbon-based material may be exemplified as the negative electrode active material. In addition, a metal material capable of alloying with lithium, and a composite material of the metal material and the carbon-based material may also be exemplified as a negative electrode active material. Examples of the metal capable of alloying with lithium may include Al, Si, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, and Ge. These metals can be used alone or in combination or alloying. In addition, the metal surface capable of alloying with lithium may be coated with a metal conductive material such as Ni, Cu or Fe and / or a carbon conductive material.

Examples of the carbon-based material include natural graphite, artificial graphite, graphitized carbon fiber, graphitized mesocarbon microbead (MCMB), graphitized mesoface pitch-based carbon fiber (MPCFF), fullerene, amorphous carbon, and the like. Can be. Examples of the amorphous carbon include hard carbon, coke, MCMB and MPCF fired at 1500 ° C. or lower. The carbon-based material is preferably a material having an d002 interplanar distance of 3.35 to 3.38 kPa and an Lc (crystallite size) of at least 20 nm by X-ray diffraction.

The negative electrode active material layer may further include a conductive material to improve electronic conductivity. The conductive material is not particularly limited, but may be graphite based on artificial graphite or natural graphite, carbon black based on acetylene black, Ketjen black, denka black, thermal black, channel black, conductive fibers such as carbon fiber, metal fiber, Metal powders such as copper, nickel, aluminum and silver, conductive metal oxides such as titanium oxide, or conductive polymers such as polyaniline, polythiophene, polyacetylene and polypyrrole may be used alone or in combination thereof. 0.1-10 weight% is preferable with respect to a negative electrode active material, and, as for the addition amount of a electrically conductive material, it is more preferable that it is 1-5 weight%. When the content of the conductive material is less than 0.1% by weight, the electrochemical properties are lowered, and when the content of the conductive material is more than 10% by weight, the energy density per weight is lowered.

Examples of the negative electrode current collector may include a punching metal, an punching metal, a gold foil, a foamed metal, a sintered mesh fiber, nickel foil, and copper foil.

The non-aqueous electrolyte may include a lithium salt and a non-aqueous organic solvent, and may further include additives for improving charge and discharge characteristics, preventing overcharge, and the like. The lithium salt acts as a source of lithium ions in the battery to enable operation of the basic lithium battery, and the non-aqueous organic solvent serves as a medium through which ions involved in the electrochemical reaction of the battery can move.

The lithium salt may be LiPF ₆ , LiBF ₄ , LiSbF ₆ , LiAsF ₆ , LiClO ₄ , LiCF ₃ SO ₃ , LiN (SO ₂ CF ₃ ) ₂ , LiN (SO ₂ C ₂ F ₅ ) ₂ , LiC (SO ₂ CF ₃ ) ₃ , LiN (SO ₃ CF ₃ ) ₂ , LiC ₄ F ₉ SO ₃ , LiAlO ₄ , LiAlCl ₄ It may be used in combination of one or two or more selected from the group consisting of. The concentration of the lithium salt is preferably used in the range of 0.6 to 2.0M, more preferably in the range of 0.7 to 1.6M. If the concentration of the lithium salt is less than 0.6M, the conductivity of the electrolyte is lowered, the performance of the electrolyte is lowered, and if it exceeds 2.0M, the viscosity of the electrolyte is increased, there is a problem that the mobility of lithium ions decreases.

The lithium secondary battery may include a separator that prevents a short circuit between the positive electrode and the negative electrode and provides a movement path of lithium ions, and the separator may include a polyolefin-based polymer film such as polypropylene or polyethylene, or a multi-film or microporous film thereof. Known such as, woven and nonwoven fabrics can be used. In addition, a film coated with a resin having excellent stability may be used for the porous polyolefin film.

1 is a cross-sectional view of a rectangular type lithium secondary battery shown as one embodiment of the present invention.

Referring to FIG. 1, a lithium secondary battery stores an electrode assembly 12 including a positive electrode 13, a negative electrode 15, and a separator 14 in a can 10 together with an electrolyte, It is formed by sealing the upper end with the cap assembly 20. The cap assembly 20 includes a cap plate 40, an insulating plate 50, a terminal plate 60, and an electrode terminal 30. The cap assembly 20 is combined with the insulating case 70 to seal the can 10.

The electrode terminal 30 is inserted into the terminal through-hole 41 formed in the center of the cap plate 40. When the electrode terminal 30 is inserted into the terminal through-hole 41, the tubular gasket 46 is coupled to the outer surface of the electrode terminal 30 and inserted together to insulate the electrode terminal 30 and the cap plate 40. . After the cap assembly 20 is assembled to the upper end of the can 10, the electrolyte is injected through the electrolyte injection hole 42 and the electrolyte injection hole 42 is closed by a stopper 43.

The electrode terminal 30 is connected to the negative electrode tab 17 of the negative electrode 15 or the positive electrode tab 16 of the positive electrode 13 to act as a negative electrode terminal or a positive electrode terminal.

The lithium secondary battery of the present invention is not limited to the above shape, and any shape such as a cylindrical shape, a pouch, etc., including the negative electrode of the present invention and capable of operating as a battery, is possible.

Next, the manufacturing method of the electrode plate which concerns on this invention is demonstrated.

The electrode plate manufacturing method according to the present invention comprises a mixture manufacturing step, a binder solution manufacturing step, a slurry (slurry) manufacturing step, slurry coating step and slurry drying step. The electrode plate may correspond to any one of the positive electrode plate and the negative electrode plate, or both the positive electrode plate and the negative electrode plate.

The mixing step is to prepare a mixture by melting and stirring polyvinylidene difluoride and polymethyl methacrylate. The mixture manufacturing step is preferably made at 175 ℃ to 200 ℃, more preferably may be made at about 180 ℃. Since the melting point of the polyvinylidene fluoride is about 174 ° C, the materials may not be sufficiently melted when melt-stirred at too low a temperature, and safety may be a problem when the heating temperature is too high. In addition, the mixture preparation step may be made in a thermoplastic mixing chamber, the stirring speed of the rotor (rotator) is preferably 40rpm to 80rpm, more preferably set to 60rpm. If the stirring speed is too slow, the mixture may not be sufficiently mixed, and if the stirring speed is too fast, safety may be a problem. In addition, the agitation time is suitable for about 10 minutes in which the constituent materials are in a single dispersed state.

The binder solution preparing step is a step of preparing a binder solution by adding a solvent to the mixture. As the solvent, at least one of N-methyl-2-pyrrolidone (NMP), dimethylformamide, dimethylacetamide, N, N-dimethylaminopropylamine, ethylene oxide, and tetrahydrofuran may be used. However, the type of the solvent is not limited thereto.

The slurry manufacturing step is a step of preparing a positive electrode slurry or a negative electrode slurry by mixing at least one of a positive electrode active material and a negative electrode active material in the binder solution.

The slurry coating step is a step of coating the slurry prepared by the slurry manufacturing step on at least one surface of the current collector.

The slurry drying step is a step of drying the coated slurry through the slurry coating step.

In this way, when the positive electrode plate and the negative electrode plate is manufactured, the electrode assembly is formed by winding up with a winding machine through a separator.

Hereinafter, preferred examples and comparative examples of the present invention are described. However, the following examples are only preferred embodiments of the present invention and the present invention is not limited to the following examples.

Example  One

A binder solution was prepared by dispersing a mixture of polyvinylidene fluoride and polymethyl methacrylate in a weight ratio of 7: 3 in N-methyl-2-pyrrolidone (NMP). A slurry was prepared by dispersing LiCoO ₂ (average particle diameter: 10 μm) and a carbon conductive material (super P) as a cathode active material in the binder solution. The slurry was applied on an aluminum foil having a thickness of 20 µm, dried, and rolled by a roll press to prepare a positive electrode plate.

A binder solution was prepared by dispersing a mixture of polyvinylidene fluoride and polymethyl methacrylate in a weight ratio of 7: 3 in N-methyl-2-pyrrolidone (NMP). A slurry was prepared by dispersing carboxymethyl cellulose (CMC) as an artificial graphite and a thickener as a negative electrode active material in the binder solution, and applying the slurry on a copper foil having a thickness of 15 μm, drying, and rolling it by a roll press to prepare a negative electrode plate. .

A separator made of a polyethylene (PE) porous film (thickness: 25 μm) was inserted between the positive electrode plate and the negative electrode plate, wound, compressed, placed in a square can, and an electrolyte was injected to prepare a lithium secondary battery. In this case, an ethylene carbonate (EC) / diethyl carbonate (DEC) mixed solution (1: 1 volume ratio) in which LiPF ₆ was dissolved in 1.0 M was used.

Example  2

The preparation was carried out in the same manner as in Example 1, except that the weight ratio of polyvinylidene fluoride and polymethyl methacrylate used as a binder in the production of the anode slurry and the cathode slurry was 3: 7.

Comparative example  One

The same procedure as in Example 1 was carried out except that polyvinylidene fluoride alone was used as a binder in the production of the positive electrode slurry and the negative electrode slurry.

Comparative example  2

The preparation of the positive electrode slurry and the negative electrode slurry was carried out in the same manner as in Example 1 except that the weight ratio of polyvinylidene fluoride and polyvinylacetate (PVA) as a binder was 7: 3.

<Swelling characteristics>

The lithium secondary batteries of Examples 1 and 2 and Comparative Examples 1 and 2 were charged at a charging voltage of 0.5 C and 4.2 V under constant current-constant voltage (CC-CV) conditions at 25 ° C., and then placed in a high temperature chamber at 90 ° C. After leaving for 4 hours, the change in cell thickness was measured and the results are shown in Table 1. The thickness change rate in Table 1 shows the change rate at the time of the battery thickness before high temperature standing.

Thickness change rate after 4 hours at 90 ℃ Example 1 14.0% Example 2 14.2% Comparative Example 1 16.5% Comparative Example 2 19.3%

As shown in Table 1, it can be seen that the batteries of Examples 1 and 2 using a mixture of polyvinylidene fluoride and polymethyl methacrylate as a binder show excellent swelling inhibitory effect.

According to the electrode assembly, the lithium secondary battery and the electrode plate manufacturing method of the lithium secondary battery according to the present invention, by improving the elastic modulus and tensile strength (swelling) by suppressing the swelling of the electrode plate during charge and discharge It is effective to prevent internal short and improve safety.

Claims (11)

In the electrode assembly of a lithium secondary battery comprising a positive electrode plate having a positive electrode active material layer and a negative electrode plate having a negative electrode active material layer, At least one of the positive electrode active material layer and the negative electrode active material layer is a lithium secondary comprising a binder comprising a mixture of polyvinylidene fluoride (PVdF) and polymethyl methacrylate (PMMA) Electrode assembly of a battery. The method of claim 1, The binder is an electrode assembly of a lithium secondary battery, characterized in that the weight ratio of the polymethyl methacrylate (PMMA) is 20% to 80%. The method of claim 1, The binder is an electrode assembly of a lithium secondary battery, characterized in that the weight ratio of the polymethyl methacrylate (PMMA) is 30% to 70%. An electrode assembly comprising a positive electrode plate having a positive electrode active material layer, and a negative electrode plate having a negative electrode active material layer, In the lithium secondary battery comprising a case for accommodating the electrode assembly, At least one of the positive electrode active material layer and the negative electrode active material layer is a lithium secondary comprising a binder comprising a mixture of polyvinylidene fluoride (PVdF) and polymethyl methacrylate (PMMA) battery. The method of claim 4, wherein The binder is a lithium secondary battery, characterized in that the weight ratio of the polymethyl methacrylate (PMMA) is 20% to 80%. The method of claim 4, wherein The binder is a lithium secondary battery, characterized in that the weight ratio of the polymethyl methacrylate (PMMA) is 30% to 70%. A mixture preparation step of melting and stirring polyvinylidene difluoride (PVdF) and polymethylmethacrylate (PMMA) to prepare a mixture; A binder solution preparing step of preparing a binder solution by adding a solvent to the mixture; A slurry manufacturing step of mixing at least one of a cathode active material and an anode active material into the binder solution; A slurry coating step of coating the slurry prepared by the slurry manufacturing step on at least one surface of a current collector; And The electrode plate manufacturing method comprising a slurry drying step of drying the slurry coated through the slurry coating step. The method of claim 7, wherein In the slurry manufacturing step, a conductive plate is further characterized in that the conductive material is added. The method of claim 7, wherein At least one of N-methyl-2-pyrrolidone (NMP), dimethylformamide, dimethylacetamide, N, N-dimethylaminopropylamine, ethylene oxide, and tetrahydrofuran is used as the solvent of the binder solution preparation step. The electrode plate manufacturing method characterized in that. The method of claim 7, wherein The mixture manufacturing step is the electrode plate manufacturing method, characterized in that made at 150 ℃ to 200 ℃. The method according to claim 7 or 10, The manufacturing method of the electrode plate, characterized in that the rotation speed of the rotor in the mixture manufacturing step is 40rpm to 80rpm.

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