KR20060102745A - Lithium secondary battery - Google Patents

Abstract

The present invention relates to a lithium secondary battery, comprising: a positive electrode including a positive electrode current collector and a positive electrode active material formed on the positive electrode current collector; A negative electrode including a negative electrode current collector and a negative electrode active material formed on the negative electrode current collector; It includes a non-aqueous electrolyte solution, at least one of the positive electrode current collector and the negative electrode current collector provides a lithium secondary battery comprising a polymer layer and a conductive layer formed on the polymer layer.

Description

Lithium secondary battery

1 is a cross-sectional view schematically showing the structure of an electrode current collector of a lithium secondary battery according to an embodiment of the present invention.

2 is a cross-sectional view schematically showing the structure of an electrode current collector of a lithium secondary battery according to another embodiment of the present invention.

3 is a cross-sectional view schematically showing the structure of an electrode of a lithium secondary battery according to an embodiment of the present invention.

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

The present invention relates to a lithium secondary battery. More particularly, the present invention relates to a lithium secondary battery that eliminates battery characteristics deterioration due to elution of a current collector when a high voltage battery is applied, and has excellent safety during heat generation due to abnormal operation of the battery.

As miniaturization and light weight of portable electronic devices have recently advanced, the necessity for miniaturization and high capacity of batteries used as driving power sources thereof is increasing. In particular, the lithium secondary battery has an operating voltage of 3.6 V or more, which is three times higher than a nickel-cadmium battery or a nickel-hydrogen battery, which is widely used as a power source for portable electronic devices, and rapidly expands in terms of high energy density per unit weight. There is a trend.

The lithium secondary battery is a cathode active material containing lithium-containing metal oxides such as lithium cobalt oxide (LiCoO ₂ ), lithium nickel oxide (LiNiO ₂ ), and lithium manganese oxide (LiMnO ₂ ) capable of deintercalation and intercalation of lithium ions. And a carbon-based material having a chemical potential of lithium metal, a lithium-containing alloy, or an insertion / desorption of lithium ions that can reversibly accept or supply lithium ions while maintaining structural and electrical properties. It is composed of an electrolyte solution in which an appropriate amount of lithium salt is dissolved in a mixed organic solvent, and is classified into a lithium ion battery and a lithium polymer battery by the characteristics of the solvent used as the electrolyte.

The positive electrode and the negative electrode of the lithium secondary battery are prepared by mixing such an active material, a binder, and optionally a conductive agent to prepare a slurry type composition, and applying the composition to an electrode current collector. At this time, aluminum is mainly used for the positive electrode as the electrode current collector, and copper is mainly used for the negative electrode.

Although aluminum current collectors are safe in a relatively wide potential window (0.5 to 4.1 V), they react with trace impurities (H ₂ O, O _2, etc.) in the electrolyte to form interfacial materials. Lithium salts and organic solvents also have a constant potential It can react with an aluminum current collector in the range. In particular, local corrosion of the aluminum current collector exposed to the high potential may proceed due to the small amount of impurities present in the electrolyte. In addition, when the voltage reaches around 0V due to overdischarge, elution of copper, which is the current collector of the negative electrode, begins, and the battery capacity rapidly ages.

On the other hand, when the battery is overcharged, depending on the state of charge, lithium is excessively precipitated in the positive electrode, lithium is excessively inserted in the negative electrode, and the positive electrode and negative electrode are thermally unstable, resulting in a sudden exothermic reaction such as decomposition of the organic solvent in the electrolyte and thermal runaway. (thermal runway) occurs, which seriously affects the safety of the battery.

The present invention is to solve the above problems, an object of the present invention is to prevent battery characteristics due to the elution of the electrode current collector, excellent safety through the current blocking of the current collector material during heat generation due to abnormal operation of the battery It is to provide a lithium secondary battery that can secure.

The present invention to achieve the above object is a positive electrode including a positive electrode current collector and a positive electrode active material formed on the positive electrode current collector; A negative electrode including a negative electrode current collector and a negative electrode active material formed on the negative electrode current collector; It includes a non-aqueous electrolyte solution, at least one of the positive electrode current collector and the negative electrode current collector provides a lithium secondary battery comprising a polymer layer and a conductive layer formed on the polymer layer.

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

The present invention can change the electrode current collector used in the manufacturing of the electrode of the lithium secondary battery, it is possible to prevent the degradation of the battery characteristics due to elution of the electrode current collector and improved safety through the current blocking of the current collector during abnormal heat generation of the battery It relates to a lithium secondary battery.

The electrode current collector of the present invention comprises a polymer layer and a conductive layer formed on the polymer layer. As shown in FIG. 1, the electrode current collector 100 of the present invention includes a polymer layer 110 and conductive layers 120 and 120 ′ formed on both surfaces of the polymer layer 110. The polymer layer 110 may support the electrode active material and may be formed in a film form formed of a polymer that does not participate in a battery reaction. The polymer constituting the polymer layer 110 may be selected from polyesters, polyolefins and copolymer polymers thereof, and specifically, polyethylene terephthalate (PET), polypropylene (PP), and the like may be used. It is not. The polymer layer 110 may have a thickness of 1 to 100 μm, preferably 2 to 50 μm, and more preferably 3 to 30 μm. If the thickness of the polymer layer 110 is thinner than 1㎛ difficult to handle, if thicker than 100㎛ may cause a problem that the energy density of the battery is reduced.

The conductive layers 120 and 120 'include a conductive agent and an acrylic rubber binder. The conductive layers 120 and 120 ′ are formed by coating a slurry in which a conductive agent and an acrylic rubber binder are mixed on both surfaces of the polymer layer 110. The coating method may include gravure roll coating, spray coating, slot die coating, or the like. The conductive layers 122 and 122 ′ may have a thickness of 0.1 to 20 μm, preferably 0.1 to 10 μm. When the thickness of the conductive layers 122 and 122 'is thinner than 0.1 mu m, the role of the conductive layer is insufficient. If the thickness of the conductive layers 122 and 122' is larger than 20 mu m, the energy density of the battery is reduced, which is not preferable. Acetylene black may be used as the conductive agent, but is not limited thereto. The conductive agent may be used having a particle size of 30 to 100nm. The content of the acrylic rubber binder is 0.1 to 20% by weight, preferably 0.1 to 10% by weight based on the conductive agent. If the content of the binder is too small, the adhesion between the conductive particles is insufficient, and if the content of the binder is too large, the adhesion is good, but the content of the conductive agent decreases by that amount, resulting in poor conductivity.

Another embodiment of the electrode current collector 200 of the present invention, as shown in Figure 2, made of a polymer layer 210 and the conductive layer (220, 220 ') formed on both sides of the polymer layer 210, The conductive layers 220 and 220 'include a conductive agent, an acrylic rubber binder, and microcapsules 225. The microcapsule 225 refers to a spherical, elliptical or irregular shape fine particles containing a material 225a that decomposes a polymer into a core material or inhibits conductivity in a wall material made of a polymer material 225b, and is usually 50 nm. It has a magnitude | size of -2000 micrometers. The wall material may have a single layer or a multi-layered structure, and the core material may contain a single component or a multi component, and any material in solid, liquid, or gas may be applied. The polymer material 225b constituting the wall material of the microcapsules 225 is preferably a resin which is melted at 90 to 140 ° C. in view of a normal use temperature range and safety of a battery, and an acrylic polymer may be used. have. As a material 225a that decomposes the polymer contained in the microcapsules 225 or inhibits conductivity, a radical generating initiator, for example, an organic peroxide, may be used. When the battery temperature rises rapidly due to overcharging or penetration, the core material 225a of the microcapsule 225 is released as the polymer material 225b of the microcapsules 225 is melted or deformed, and the core material 225a is released. This interrupts the conductive network of the conductive material and blocks the current, thereby ensuring the safety of the battery.

Microcapsules can be prepared using chemical methods including interfacial polymerization, in-situ polymerization, physical and chemical methods such as phase separation, and physical and mechanical methods such as spray drying.

The amount of the microcapsules is preferably 0.1 to 10% by weight based on the conductive agent. If the content of the microcapsules is too small, the current blocking effect is insufficient. If the content of the microcapsules is too high, the content of the conductive agent decreases, which is undesirable in terms of conductivity.

The electrode current collector of the present invention can be used for either the positive electrode current collector and the negative electrode current collector, or for both the positive electrode current collector and the negative electrode current collector, regardless of the type of the electrode plate. When the electrode 300 is manufactured using the electrode current collector of the present invention, as shown in FIG. 3, the conductive layers 320 and 320 'are formed on both surfaces of the polymer layer 310 and the polymer layer 310. ) And electrode active material layers 330 and 330 'formed on at least one surface of the conductive layers 320 and 320'. The electrode active material layers 330 and 330 'may be manufactured by coating the electrode active material on the conductive layers 320 and 320' of the current collector or in the form of a green compact.

As the cathode active material of the lithium secondary battery, a compound capable of inserting and desorbing lithium ions is used, and as the cathode active material, at least one selected from cobalt, manganese, nickel, and at least one of a composite oxide with lithium is preferable. As the representative examples thereof, the lithium-containing compound described below can 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-yz Co _y M _z A _α (10)

Li _x Ni _1-yz Co _y M _z O _2-α X _α (11)

Li _x Ni _1-yz Mn _y M _z A _α (12)

Li _x Ni _1-yz 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 a negative electrode active material of a lithium secondary battery, a compound capable of inserting and desorbing lithium ions is used, and as the negative electrode active material, carbon materials such as crystalline carbon, amorphous carbon, carbon composite, carbon fiber, lithium metal, and lithium alloy are used. Can be. For example, amorphous carbon includes hard carbon, coke, mesocarbon microbeads (MCMB) calcined at 1500 ° C. or lower, mesophase pitch-based carbon fibers (MPCF), and the like. The crystalline carbon includes a graphite material, and specific examples thereof include natural graphite, graphitized coke, graphitized MCMB, graphitized MPCF, and the like. The carbonaceous 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. As the lithium alloy, an alloy of lithium with aluminum, zinc, bismuth, cadmium, antimony, silicon, lead, tin, gallium or indium may be used.

The non-aqueous electrolyte of the lithium secondary battery includes 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.

Examples of the lithium salt include LiPF ₆ , LiBF ₄ , LiSbF ₆ , LiAsF ₆ , LiClO ₄ , LiCF ₃ SO ₃ , Li (CF ₃ SO ₂ ) ₂ N, LiC ₄ F ₉ SO ₃ , LiAlO ₄ , LiAlCl ₄ , LiN ( C _x F _{2x + 1} SO ₂ ) (C _y F _{2y + 1} SO ₂ ) (where x and y are natural numbers), LiCl, and LiI, or a mixture of one or two or more selected from the group consisting of Can be. 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.

As the non-aqueous organic solvent, carbonate, ester, ether or ketone may be used alone or in combination. Organic solvents should be used to have high dielectric constant (polarity) and low viscosity to increase ion dissociation and smooth ion conduction. Generally, solvents having high dielectric constant, high viscosity and low dielectric constant and low viscosity are used. It is preferable to use two or more mixed solvents configured.

In the case of the carbonate-based solvent of the non-aqueous organic solvent, it is preferable to use a mixture of cyclic carbonate and chain carbonate. The cyclic carbonates include ethylene carbonate (EC), propylene carbonate (PC), 1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentylene carbonate, 2,3-pentylene carbonate, vinyl Ethylene carbonate (VC) and the like may be used. Ethylene carbonate and propylene carbonate having high dielectric constants are preferred, and ethylene carbonate is preferred when artificial graphite is used as the negative electrode active material. As the linear carbonate, dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methylpropyl carbonate (MPC), ethylmethyl carbonate (EMC), ethylpropyl carbonate (EPC), etc. may be used. . Low viscosity dimethyl carbonate, ethylmethyl carbonate and diethyl carbonate are preferred.

The ester is methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, γ-butyrolactone (GBL), γ-valerolactone, γ-caprolactone, δ-valerolactone, ε- Caprolactone, and the like, and tetraetherfuran, 2-methyltetrahydrofuran, dibutyl ether, and the like may be used. As the ketone, polymethylvinyl ketone may be used.

The lithium secondary battery of the present invention may include a separator that prevents a short circuit between the positive electrode and the negative electrode and provides a passage of lithium ions, and the separator may include polypropylene, polyethylene, polyethylene / polypropylene, polyethylene / polypropylene / Known ones such as polyolefin-based polymer membranes such as polyethylene, polypropylene / polyethylene / polypropylene, or multiple membranes thereof, microporous films, woven fabrics and nonwoven fabrics can be used.

4 is a cross-sectional view of a lithium secondary battery shown as an embodiment of the present invention.

In the lithium secondary battery, the positive electrode 413, the negative electrode 415, and the separator 414 are stacked therebetween, and then the electrode assembly 412 wound in a jelly roll form is stored in the can 410 together with the electrolyte solution. The upper end of the can 410 is formed by sealing the cap assembly 420. The current collector of at least one of the positive electrode 413 and the negative electrode 415 includes a polymer layer and a conductive layer formed on the high molecular layer. Since the structure of the current collector of the present invention has been described in detail above, the description thereof will be omitted. The cap assembly 420 includes a cap plate 440, an insulating plate 450, a terminal plate 460, and an electrode terminal 430. The cap assembly 420 is combined with the insulating case 470 to seal the can 410.

The electrode terminal 430 is inserted into the terminal through hole formed in the center of the cap plate 440. When the electrode terminal 430 is inserted into the terminal hole, the tubular gasket 446 is coupled to the outer surface of the electrode terminal 430 and inserted together to insulate the electrode terminal 430 and the cap plate 40. After the cap assembly 420 is assembled to the upper end of the can 410, the electrolyte is injected through the electrolyte injection hole 442, and the electrolyte injection hole 442 is sealed by the cap 443.

The electrode terminal 430 is connected to the negative electrode tab 417 of the negative electrode 415 or the positive electrode tab 416 of the positive electrode 413 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 or a pouch may be possible.

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

Comparative Example 1

A positive electrode active material slurry was prepared by dispersing a LiCoO ₂ positive electrode active material, a polyvinylidene fluoride binder, and a carbon conductive agent (Super P) in an N-methyl-2-pyrrolidone (NMP) solvent at a weight ratio of 92: 4: 4. It was. The positive electrode active material slurry was coated on an aluminum foil current collector having a thickness of 15 μm, dried, and rolled to prepare a positive electrode. Crystalline artificial graphite as a negative electrode active material and PVDF as a binder were mixed in a weight ratio of 92: 8, and then dispersed in N-methyl-2-pyrrolidone to prepare a negative electrode slurry. The slurry was coated on a copper foil current collector having a thickness of 10 μm, and then dried and rolled to prepare a negative electrode.

The prepared electrodes were wound and compressed using a polyethylene separator, placed in a rectangular can, and then injected with an electrolyte to prepare a lithium secondary battery. At this time, an ethylene carbonate and diethyl carbonate mixed solution (1: 1 volume ratio) in which LiPF ₆ was dissolved in 1.0 M was used.

Example 1

A negative electrode current collector was prepared by forming a conductive layer comprising a mixture of 96% by weight of acetylene black and 4% by weight of acrylic rubber on a 12 μm-thick polyethylene terephthalate film having a thickness of 6 μm on both sides. A negative electrode was prepared by forming a negative electrode active material layer as in Comparative Example 1 on the negative electrode current collector. Acrylic polymer microcapsules having a melting point around 100 ° C. of 2-6 μm in diameter containing 94 wt% of acetylene black, 3 wt% of acrylic rubber, and radical generating initiator (Irgacure-651 ™) on a 12 μm thick polyethylene terephthalate film The 3% by weight of the conductive agent layer was formed to a thickness of 6㎛ on both sides to prepare a positive electrode current collector. A positive electrode was prepared by forming a positive electrode active material layer as in Comparative Example 1 on the positive electrode current collector.

The prepared electrodes were wound and compressed using a polyethylene separator, placed in a rectangular can, and then injected with an electrolyte to prepare a lithium secondary battery. At this time, an ethylene carbonate and diethyl carbonate mixed solution (1: 1 volume ratio) in which LiPF ₆ was dissolved in 1.0 M was used.

<Overcharge Test>

Examples 1 to 11 and Comparative Examples 1 to 11 in the battery at room temperature (25 ℃) 1C (790mAh) / 12V from the state of charge for 20 hours each under a constant current / constant voltage for 2 hours and a half It was. The battery state was confirmed, and the results are shown in Table 1.

<Penetration>

The fully charged lithium secondary batteries of Comparative Examples 1 and 1 were penetrated completely through the center at a rate of 40 mm / sec or more perpendicular to the length axis of the battery with a nail of 5φ. The state of the battery was confirmed, and the results are shown in Table 1.

<High rate characteristic>

The lithium secondary batteries of Comparative Example 1 and Example 1 were charged with 1C / 4.2V constant current-constant voltage and 20mAh cut-off, followed by 0.2C and 2C / 3V cut-off discharge, respectively, and the discharge capacity ratios are shown in Table 1. It was. Discharge capacity ratio = (2C discharge capacity) / (0.2C discharge capacity) x 100%.

In Table 1 below, the number in front of L means the number of test cells, and the overcharge safety evaluation criteria are as follows.

L0: good, L1: leakage, L2: flash, L2: flame, L3: smoke, L4: fire, L5: burst

For example, 10 L0 means that all ten batteries tested were good.

1C-12V overcharge (no PTC) Penetrate High rate characteristic (2C capacity / 0.2C capacity) Comparative Example 1 10L5 10L5 40% Example 1 10L0 10L0 40%

As can be seen from Table 1, the battery of Comparative Example 1 using the existing metal current collector in the overcharge and penetration test led to an explosion, the battery of Example 1 using the current collector of the present invention has no change in appearance of the battery at all It was confirmed that the safety was improved without. In addition, it was found that the battery of Example 1 maintains excellent high rate discharge characteristics.

The lithium secondary battery of the present invention prevents battery characteristics from deteriorating due to elution of the electrode current collector, and ensures excellent safety through current blocking of the current collector material during heat generation due to abnormal operation of the battery such as overcharge, short circuit, and penetration. have.

Claims (13)

A positive electrode including a positive electrode current collector and a positive electrode active material formed on the positive electrode current collector; A negative electrode including a negative electrode current collector and a negative electrode active material formed on the negative electrode current collector; Includes a non-aqueous electrolyte, At least one current collector of the positive electrode current collector and the negative electrode current collector is a lithium secondary battery, characterized in that consisting of a polymer layer and a conductive layer formed on the polymer layer. According to claim 1, wherein the polymer layer is a lithium secondary battery, characterized in that selected from polyesters, polyolefins and copolymers thereof. The lithium secondary battery according to claim 1, wherein the polymer layer is made of polyethylene terephthalate or polypropylene. The lithium secondary battery of claim 1, wherein the conductive layer comprises a conductive agent and an acrylic rubber binder. The lithium secondary battery of claim 4, wherein the conductive agent is acetylene black. The lithium secondary battery of claim 4, wherein the conductive agent layer further comprises a microcapsule containing a substance that degrades a polymer or inhibits conductivity. The lithium secondary battery according to claim 6, wherein a radical generating initiator is contained in the microcapsules. The lithium secondary battery according to claim 6, wherein the wall material of the microcapsule is made of a resin that is melted at 90 to 140 ° C. The lithium secondary battery of claim 8, wherein the resin forming the wall material is an acrylic polymer. The lithium secondary battery of claim 1, wherein the polymer layer has a thickness of 1 μm to 100 μm. The lithium secondary battery according to claim 1, wherein the conductive layer has a thickness of 0.1 to 20 µm. The lithium secondary battery according to claim 1, wherein the cathode active material is a lithium compound selected from the group consisting of the following (1) to (13). 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-yz Co _y M _z A _α (10) Li _x Ni _1-yz Co _y M _z O _2-α X _α (11) Li _x Ni _1-yz Mn _y M _z A _α (12) Li _x Ni _1-yz 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.) The lithium secondary battery of claim 1, wherein the negative electrode active material is selected from the group consisting of crystalline carbon, amorphous carbon, carbon composite, carbon fiber, lithium metal, and lithium alloy.

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