KR101506452B1 - Cathode for Secondary Battery - Google Patents

Abstract

The present invention provides a positive electrode for a secondary battery in which a positive electrode mixture is coated on one or both surfaces of a positive electrode collector, wherein the positive electrode mixture layer has a porosity ranging from 10% to 60%.

Description

[0001] The present invention relates to a cathode for a secondary battery,

The present invention relates to a cathode for a secondary battery in which a positive electrode mixture is coated on one or both surfaces of a positive electrode collector, wherein the positive electrode mixture layer has a porosity ranging from 10% to 60%.

As technology development and demand for mobile devices have increased, there has been a rapid increase in demand for secondary batteries as energy sources. Among such secondary batteries, lithium secondary batteries, which exhibit high energy density and operational potential, long cycle life, Batteries have been commercialized and widely used.

In recent years, there has been a growing interest in environmental issues, and as a result, electric vehicles (EVs) and hybrid electric vehicles (HEVs), which can replace fossil-fueled vehicles such as gasoline vehicles and diesel vehicles, And the like. Although a nickel metal hydride (Ni-MH) secondary battery is mainly used as a power source for such an electric vehicle (EV) and a hybrid electric vehicle (HEV), a lithium secondary battery having a high energy density, a high discharge voltage, Research is being actively carried out, and some are commercialized.

The lithium secondary battery has a structure in which a non-aqueous electrolyte containing a lithium salt is impregnated in an electrode assembly having a porous separator interposed between a positive electrode and a negative electrode coated with an active material on an electrode current collector.

Generally, in order to manufacture a positive electrode for a secondary battery, a positive electrode current collector in the form of a slurry is applied to a positive electrode collector, followed by drying and pressing.

When the porosity of the positive electrode is high, it is excellent in impregnation with the electrolyte, and the ion conductivity can be excellent, but the capacity per unit volume is lowered and the electron conductivity is lowered. In addition, when the porosity of the anode is low, the electron conductivity may be excellent, but the impregnability of the electrolyte is decreased, and the anode may be broken due to insufficient escape of internal gas.

Therefore, there is a great need for a cathode for a secondary battery that can satisfy both ion conductivity and electron conductivity.

SUMMARY OF THE INVENTION It is an object of the present invention to solve the above-described problems of the prior art and the technical problems required from the past.

The inventors of the present application have conducted intensive research and various experiments and confirmed that the desired effect can be achieved when the positive electrode mixture layer has a specific porosity as described later, It came.

Accordingly, the present invention provides a positive electrode for a secondary battery in which a positive electrode mixture is coated on one or both surfaces of a positive electrode collector, wherein the positive electrode mixture layer has a porosity ranging from 10% to 60%.

As described above, when the porosity is too low, the electron conductivity may be excellent, but the impregnation with the electrolyte is poor and the ionic conductivity is lowered, which is not preferable. Conversely, when the porosity is too large, the electrolyte impregnation and ionic conductivity may be relatively excellent, but the electronic conductivity and capacity are undesirably low. Therefore, the above porosity range is preferable.

For the above reason, the porosity of the positive electrode material mixture layer may be in the range of 20% to 50%, and more particularly, in the range of 25% to 45% based on the total volume of the positive electrode material mixture layer.

The positive electrode for a secondary battery is prepared by applying a positive electrode mixture, which is a mixture of a positive electrode active material, a conductive material and a binder, on a positive electrode collector, followed by drying and pressing, and if necessary, a filler may be further added to the mixture.

The cathode current collector generally has a thickness of 3 to 500 mu m. Such a positive electrode current collector is not particularly limited as long as it has high conductivity without causing chemical changes in the battery. Examples of the positive electrode current collector include stainless steel, aluminum, nickel, titanium, sintered carbon, aluminum or stainless steel A surface treated with carbon, nickel, titanium, silver or the like may be used. The current collector may have fine irregularities on the surface thereof to increase the adhesive force of the cathode active material, and various forms such as a film, a sheet, a foil, a net, a porous body, a foam, and a nonwoven fabric are possible.

The cathode active material may be a layered compound such as lithium cobalt oxide (LiCoO ₂ ), lithium nickel oxide (LiNiO ₂ ), or a compound substituted with one or more transition metals; Lithium manganese oxides such as Li _{1 + x} Mn _{2 -x} O ₄ (where x is 0 to 0.33), LiMnO ₃ , LiMn ₂ O ₃ , LiMnO ₂ and the like; Lithium copper oxide (Li ₂ CuO ₂ ); Vanadium oxides such as LiV ₃ O ₈ , LiFe ₃ O ₄ , V ₂ O ₅ and Cu ₂ V ₂ O ₇ ; A Ni-site type lithium nickel oxide expressed by the formula LiNi _1-x M _x O ₂ (where M = Co, Mn, Al, Cu, Fe, Mg, B or Ga and x = 0.01 to 0.3); Formula _{LiMn 2-x M x O 2} ( where, M = Co, Ni, Fe , Cr, and Zn, or Ta, x = 0.01 ~ 0.1 Im) or Li ₂ Mn ₃ MO ₈ (where, M = Fe, Co, Ni, Cu, or Zn); A lithium manganese composite oxide having a spinel structure represented by LiNi _x Mn _2-x O ₄ ; LiMn ₂ O _{4 in} which a part of Li in the formula is substituted with an alkaline earth metal ion; Disulfide compounds; Fe ₂ (MoO ₄ ) ₃ , and the like. However, the present invention is not limited to these.

In one specific example, the cathode active material may be a lithium manganese composite oxide having a spinel structure which is a high-potential oxide represented by the general formula (1).

Li _x M _y Mn _{2 - y} O _{4 - z z} (1)

Wherein 0 < y < 2, 0 z < 0.2,

M is at least one element selected from the group consisting of Al, Mg, Ni, Co, Fe, Cr, V, Ti, Cu, B, Ca, Zn, Zr, Nb, Mo, Sr, Sb, W, ;

A is one or more of an anion of -1 or -2.

Specifically, the lithium manganese composite oxide may be a lithium nickel manganese composite oxide represented by the following formula (2), more specifically, LiNi _0.5 Mn _1.5 O ₄ or LiNi _0.4 Mn _1.6 O ₄ .

Li _x Ni _y Mn _2-y O ₄ (2)

In the above formula, 0.9? X? 1.2 and 0.4? Y? 0.5.

The conductive material is usually added in an amount of 1 to 50% by weight based on the total weight of the mixture including the cathode active material. Such a conductive material is not particularly limited as long as it has electrical conductivity without causing chemical changes in the battery, for example, graphite such as natural graphite or artificial graphite; Carbon black such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, and summer black; Conductive fibers such as carbon fiber and metal fiber; Metal powders such as carbon fluoride, aluminum, and nickel powder; Conductive whiskey such as zinc oxide and potassium titanate; Conductive metal oxides such as titanium oxide; Conductive materials such as polyphenylene derivatives and the like can be used.

The binder is a component that assists in bonding of the active material and the conductive material and bonding to the current collector, and is usually added in an amount of 1 to 50 wt% based on the total weight of the mixture containing the cathode active material. Examples of such binders include polyvinylidene fluoride, polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene , Polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene butylene rubber, fluorine rubber, various copolymers and the like.

The filler is optionally used as a component for suppressing the expansion of the anode, and is not particularly limited as long as it is a fibrous material without causing a chemical change in the battery. Examples of the filler include olefin polymers such as polyethylene and polypropylene; Fibrous materials such as glass fibers and carbon fibers are used.

The present invention provides a rechargeable battery including the positive electrode, specifically, a lithium secondary battery.

The lithium secondary battery has a structure in which a lithium salt-containing electrolyte is impregnated in an electrode assembly having a separator interposed between an anode and a cathode.

The negative electrode is prepared by applying, drying and pressing an anode active material on an anode current collector, and may optionally further include a conductive material, a binder, a filler, and the like as described above.

The negative electrode current collector is generally made to have a thickness of 3 to 500 mu m. Such an anode current collector is not particularly limited as long as it has electrical conductivity without causing chemical changes in the battery, and examples of the anode current collector include copper, stainless steel, aluminum, nickel, titanium, sintered carbon, a surface of copper or stainless steel A surface treated with carbon, nickel, titanium, silver or the like, an aluminum-cadmium alloy, or the like can be used. In addition, like the positive electrode collector, fine unevenness can be formed on the surface to enhance the bonding force of the negative electrode active material, and it can be used in various forms such as films, sheets, foils, nets, porous bodies, foams and nonwoven fabrics.

The negative electrode active material may include, for example, carbon such as non-graphitized carbon or graphite carbon; _{Li x Fe 2 O 3 (0≤x≤1} ), Li x WO 2 (0≤x≤1), Sn x Me 1-x Me 'y O z (Me: Mn, Fe, Pb, Ge; Me' : Metal complex oxides such as Al, B, P, Si, Group 1, Group 2, Group 3 elements of the periodic table, Halogen, 0 &lt; x &lt; Lithium metal; Lithium alloy; Silicon-based alloys; Tin alloy; _{SnO, SnO 2, PbO, PbO} 2, Pb 2 O 3, Pb 3 O 4, Sb 2 O 3, Sb 2 O 4, Sb 2 O 5, GeO, GeO 2, Bi 2 O 3, Bi 2 O 4, and Bi ₂ O ₅ ; Conductive polymers such as polyacetylene; Li-Co-Ni-based materials; Titanium oxide; Lithium titanium oxide and the like can be used.

In one specific example, the negative electrode active material may be a lithium metal oxide represented by the following formula (3).

Li _a M ' _b O _{4-ca c} (3)

In the above formula, M 'is at least one element selected from the group consisting of Ti, Sn, Cu, Pb, Sb, Zn, Fe, In, Al and Zr;

a and b are 0.1? a? 4; Is determined according to the oxidation number of M 'in the range of 0.2? B? 4;

c is determined according to the oxidation number in the range of 0? c <0.2;

A is one or more of an anion of -1 or -2.

Specifically, the lithium metal oxide represented by Formula 3 may be Lithium Titanium Oxide (LTO) represented by the following Formula 4, which is easy to charge at a high rate, specifically Li _0.8 Ti _2.2 O ₄ , Li _2.67 Ti _1.33 O ₄ , LiTi ₂ O ₄ , Li _1.33 Ti _1.67 O ₄ , Li _1.14 Ti _1.71 O ₄ and the like. However, there is no particular limitation on the composition and kind of the lithium ion insofar as it can absorb / emit lithium ions. Specifically, it may be a spinel structure Li _1.33 Ti _1.67 O ₄ or LiTi ₂ O ₄ having a small change in crystal structure during charge and discharge and excellent in reversibility.

Li _a Ti _b O ₄ (4)

In the above formula, 0.5? A? 3, 1? B? 2.5.

In this case, it is preferable to use the spinel lithium nickel manganese composite oxide of Li _x Ni _y Mn _2-y O ₄ represented by the above formula (2) having a relatively high potential due to the high potential of LTO as the cathode active material.

The separation membrane is interposed between the anode and the cathode, and an insulating thin film having high ion permeability and mechanical strength is used. The pore diameter of the separator is generally 0.01 to 10 mu m and the thickness is generally 5 to 300 mu m. Such separation membranes include, for example, olefinic polymers such as polypropylene, which are chemically resistant and hydrophobic; A sheet or nonwoven fabric made of glass fiber, polyethylene or the like is used. When a solid electrolyte such as a polymer is used as an electrolyte, the solid electrolyte may also serve as a separation membrane.

The electrolyte solution containing the lithium salt is composed of an electrolyte solution and a lithium salt. The electrolyte solution may be a non-aqueous organic solvent, an organic solid electrolyte, or an inorganic solid electrolyte, but is not limited thereto.

Examples of the non-aqueous organic solvent include N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, gamma -Butyrolactone, 1,2-dimethoxyethane, tetrahydroxyfuran, 2-methyltetrahydrofuran, dimethylsulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane , Acetonitrile, nitromethane, methyl formate, methyl acetate, triester phosphate, trimethoxymethane, dioxolane derivatives, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate Nonionic organic solvents such as tetrahydrofuran derivatives, ethers, methyl pyrophosphate, ethyl propionate and the like can be used.

Examples of the organic solid electrolyte include a polymer electrolyte such as a polyethylene derivative, a polyethylene oxide derivative, a polypropylene oxide derivative, a phosphate ester polymer, an agitation lysine, a polyester sulfide, a polyvinyl alcohol, a polyvinylidene fluoride, A polymer containing an ionic dissociation group and the like may be used.

Examples of the inorganic solid electrolyte include Li ₃ N, LiI, Li ₅ NI ₂ , Li ₃ N-LiI-LiOH, LiSiO ₄ , LiSiO ₄ -LiI-LiOH, Li ₂ SiS ₃ , Li ₄ SiO ₄ , Nitrides, halides and sulfates of Li such as Li ₄ SiO ₄ -LiI-LiOH and Li ₃ PO ₄ -Li ₂ S-SiS ₂ can be used.

The lithium salt is a material that is readily soluble in the non-aqueous electrolyte, for example, LiCl, LiBr, LiI, LiClO 4, LiBF 4, LiB 10 Cl 10, LiPF 6, LiCF 3 SO 3, LiCF 3 CO 2, LiAsF _{6, LiSbF 6, LiAlCl 4,} CH 3 SO 3 Li, (CF 3 SO 2) 2 NLi, chloroborane lithium, lower aliphatic carboxylic acid lithium, lithium tetraphenyl borate and imide.

For the purpose of improving the charge / discharge characteristics and the flame retardancy, the electrolytic solution is preferably mixed with an organic solvent such as pyridine, triethylphosphite, triethanolamine, cyclic ether, ethylenediamine, glyme, Benzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N, N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, . In some cases, halogen-containing solvents such as carbon tetrachloride and ethylene trifluoride may be further added to impart nonflammability. In order to improve the high-temperature storage characteristics, carbon dioxide gas may be further added. FEC (Fluoro-Ethylene Carbonate, PRS (Propene sultone), and the like.

In one specific _{example, LiPF 6, LiClO 4, LiBF} 4, LiN (SO 2 CF 3) 2 , such as a lithium salt, a highly dielectric solvent of DEC, DMC or EMC Fig solvent cyclic carbonate and a low viscosity of the EC or PC of In a mixed solvent of linear carbonate

The present invention also provides a battery module including the secondary battery as a unit cell, and a battery pack including the battery module.

The battery pack may be used as a power source for devices requiring high temperature stability, long cycle characteristics, and high rate characteristics.

Specific examples of the device include a power tool which is powered by an electric motor and moves; An electric vehicle including an electric vehicle (EV), a hybrid electric vehicle (HEV), a plug-in hybrid electric vehicle (PHEV), and the like; An electric motorcycle including an electric bike (E-bike) and an electric scooter (E-scooter); An electric golf cart; And a power storage system, but the present invention is not limited thereto.

As described above, the positive electrode for a secondary battery according to the present invention has a specific porosity, so that the electrolyte-impregnating property, the ionic conductivity, and the electronic conductivity are both excellent.

FIG. 1 is a graph showing a comparison of discharge resistance values over time according to Experimental Example 1. FIG.

Hereinafter, the present invention will be described in more detail with reference to embodiments thereof, but the scope of the present invention is not limited thereto.

&Lt; Example 1 >

Manufacture of anode

(Denka black) and a binder (PVdF) were mixed at a ratio of 90: 5 (weight ratio) using LiNi _0.5 Mn _1.5 O ₄ having an average particle diameter (D50) of 4.56 탆 and a specific surface area (BET) of 0.81 m ² / : 5, and the mixture was mixed to prepare a positive electrode mixture. The positive electrode mixture was coated on an aluminum foil having a thickness of 20 탆, rolled and dried to prepare a positive electrode. At this time, the pressing pressure was adjusted so that the cathode mixture layer had a porosity of 30%.

Manufacture of lithium secondary battery

The anode was prepared by mixing an anode active material (Li _1.33 Ti _1.67 O ₄ ), a conductive material (Denka black) and a binder (PVdF) in NMP at a weight ratio of 93.5: 2: 4.5 and mixing to prepare an anode mixture. Coated on a foil, rolled and dried.

After the electrode assembly was manufactured with the separator (thickness: 20 占 퐉) interposed between the negative electrode and the positive electrode thus prepared, the electrode assembly was housed in the pouch-shaped battery case, and a carbonate complex containing 1 M of LiPF ₆ The solution was injected into the electrolyte and then sealed to prepare a lithium secondary battery.

&Lt; Examples 2 and 3 >

In Example 1, a positive electrode and a lithium secondary battery were prepared in the same manner as in Example 1, except that the pressing pressure was controlled so that the positive electrode material mixture layer had a porosity of 25 and 35%.

<Experimental Example 1>

The secondary batteries according to Examples 1 to 3 were charged and discharged at 1 C in the range of 2 V to 3.35 V to measure the capacity and the discharging resistance for 10 seconds and 0.1 second respectively. The results are shown in the following Table 1 and FIG. 1 .

1C Capacity (mAh) Discharge Resistance (10s, mΩ) Discharge resistance (0.1s, mΩ) Example 1 8.44 2.37 1.06 Example 2 8.23 2.54 1.12 Example 3 8.18 3.07 1.45

* The discharge resistance value is a conversion resistance of the same area (13 bicell).

Referring to Table 1 and FIG. 1, in the case of having a porosity of 25% with more pressing on the basis of the porosity of 30%, the electronic conductivity of the electrode is similar, It is difficult to diffuse the ions in the electrode. Therefore, it can be seen that the difference of discharge resistance increases with time.

On the other hand, when the porosity is 35% based on the 30% porosity, when the porosity is 35%, the discharge resistance of 0.1 second increases as the electronic conductivity decreases. On the other hand, sufficient pores increase the ion conductivity, Is not significantly changed.

That is, it is very important that the electrode has an appropriate level of porosity in order to have improved battery performance in various aspects because the cell performance in some aspects is slightly lowered when the porosity of the specific range is out of the range. have.

The inventors of the present application have further found that when the porosity exceeds 60%, it is difficult to form a smooth contact path between the active material and the conductive material, so that the initial resistance is remarkably increased as the conductivity is lowered, On the contrary, when the press rate is reduced to a very low level of less than 10%, a large force generated at the press breaks the secondary particle shape of the active material and the contact with the conductive material deteriorates, However, it was confirmed that the ionic conductivity was extremely low and the discharge resistance was greatly increased with time.

It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.

Claims (12)

A positive electrode having a positive electrode current collector coated with a positive electrode mixture on one surface or both surfaces thereof and having a porosity of 25% to 30% of the positive electrode mixture layer; And
cathode;
The secondary battery comprising:
The positive electrode includes a lithium nickel manganese complex oxide (LNMO) having a spinel structure represented by the following formula (2) as a positive electrode active material,
Wherein the negative electrode comprises lithium-titanium oxide (LTO) represented by the following formula (4) as an anode active material.
Li _x Ni _y Mn _2-y O ₄ (2)
Wherein 0.9? X? 1.2 and 0.4? Y? 0.5;

Li _a Ti _b O ₄ (4)
In the above formula, 0.5? A? 3, 1? B? 2.5. delete delete delete delete The secondary battery according to claim 1, wherein the lithium nickel manganese composite oxide of Formula 2 is LiNi _0.5 Mn _1.5 O ₄ or LiNi _0.4 Mn _1.6 O ₄ . delete delete A battery module comprising the secondary battery according to claim 1 as a unit cell. A battery pack comprising the battery module according to claim 9. A device comprising a battery pack according to claim 10. 12. The device of claim 11, wherein the device is an electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, or a system for power storage.

Images