KR20130003592A - Electrode active agent, lithium secondary battery comprising the same - Google Patents

Abstract

PURPOSE: A positive electrode active material is provided to increase the output of a lithium secondary battery and energy density by improving electric conductivity. CONSTITUTION: A positive electrode active material comprises a metal coated layer formed on the surface of an active material particle. The metal coating layer is plated with a conductive metal. The conductive metal is one or more selected from Ni, Au, Ag, Cu, Zn, Cr, Al, Co, Sn, Pt and Pd. The thickness of the metal coating layer is 10-300 nm. The active material particle comprises an active material with an olivine structure(LiFePO4). A positive electrode is obtained by spreading an active material slurry on a current collector which comprises the electrode active material. The comprised amount of the electrode active material is 10-100 weight%. [Reference numerals] (AA) Embodiment; (BB) Comparative embodiment

Description

Positive electrode active material, lithium secondary battery comprising same {Electrode active agent, lithium secondary battery comprising the same}

The present invention relates to a cathode active material having improved electrical conductivity and a lithium secondary battery including the same.

As technology development and demand for mobile devices increase, the demand for secondary batteries as energy sources is rapidly increasing. In particular, as interest in environmental problems increases, researches on electric vehicles and hybrid electric vehicles, which can replace vehicles using fossil fuels such as gasoline and diesel vehicles, which are one of the main causes of air pollution, are being conducted. . As a power source of such electric vehicles and hybrid electric vehicles, nickel-metal hydride secondary batteries are mainly used, but researches using lithium secondary batteries with high energy density and discharge voltage have been actively conducted and some commercialization stages are in progress.

Lithium ion secondary batteries are widely used as power sources for electronic devices such as mobile phones, notebook PCs, and digital cameras because they can realize characteristics such as light weight, high voltage, and high capacity compared to nickel hydrogen batteries. Therefore, electric vehicles are increasing interest due to the miniaturization, light weight, and long time of use per charge to improve the convenience of use, and the problem of depletion of petroleum resources and environmental pollution. 'S development calls for greater energy density in secondary batteries, the energy storage devices used in these devices.

In such lithium secondary batteries, carbon materials are mainly used as negative electrode active materials, and use of lithium metals, sulfur compounds, and the like is also contemplated. In addition, lithium cobalt oxide (LiCoO ₂ ) is mainly used as the positive electrode active material, and in addition, LiMnO _{2 having a} layered crystal structure and LiMn ₂ O _{4 having a} spinel crystal structure. Lithium manganese oxides, lithium nickel oxides such as (LiNiO ₂₎ has a lithium transition metal oxide is used and the like.

Among them, LiCoO ₂ is an excellent material having stable charge and discharge characteristics, excellent electronic conductivity, high battery voltage, high stability, and flat discharge voltage characteristics. However, Co has low reserves, is expensive, and toxic to humans. Therefore, development of other anode materials is desired. In addition, there is a disadvantage in that the crystal structure is unstable due to de-lithography at the time of charging and the thermal characteristics are very poor.

In order to improve this, many attempts have been made to substitute a portion of nickel with a transition metal element to shift the exothermic start temperature to the high temperature side or to broaden the exothermic peak to prevent the rapid exotherm. But still no satisfactory results are obtained.

In addition, LiMn ₂ O ₄ having a spinel structure is partially commercialized as a low-cost product, but its theoretical capacity is smaller than that of other materials at about 148 mAh / g. In addition, since it has a three-dimensional tunnel structure, the diffusion resistance during insertion and desorption of lithium ions is large, and the diffusion coefficient is lower than that of LiCoO ₂ and LiNiO ₂ having a two-dimensional structure. The property is not good. In particular, the high temperature property at 55 ° C or higher is inferior to LiCoO ₂ and is not widely used in actual batteries.

The lithium transition metal oxide used as the positive electrode active material has a low electrical conductivity, and there is a problem that the charge and discharge rate characteristics are not sufficiently high due to the low ion conduction characteristics exhibited by using a non-aqueous electrolyte.

In general, the active material layer formed on the electrode is prepared in a slurry state by adding a binder and a conductive material in addition to the positive electrode active material, and coated on the electrode current collector. However, the conductive materials added to increase the conductivity of the active material have a high specific gravity in the active material slurry, thereby increasing the thickness of the electrode and increasing the binder content due to the increase in the content of the conductive material. have.

In order to solve this problem, some prior art proposes a technique for coating or surface treatment of the surface of the active material of the positive electrode with a predetermined material, but a positive electrode active material that exhibits sufficient battery characteristics has not been developed yet. In particular, when the surface of the positive electrode active material is coated using a carbon-based material, there is a problem in that the permeability of the electrolyte solution into the electrode is greatly reduced. Therefore, this causes a local reaction, in particular, the problem of precipitation of lithium on the surface of the negative electrode, convection of the electrolyte solution is deteriorated, heat is filled inside the electrode, and heat dissipation is deteriorated, which causes the safety of the battery.

As a result, many studies have been conducted in various approaches, but the results have not been satisfactory.

In the present invention to solve the problems of the prior art as described above, an object of the present invention is to provide a positive electrode active material excellent in electrical conductivity.

Another object of the present invention is to provide a positive electrode including the positive electrode active material.

Further, another object of the present invention is to provide a lithium secondary battery including the positive electrode.

The positive electrode active material according to an embodiment of the present invention for achieving the above object may include a metal coating layer on the surface of the active material particles.

The metal coating layer may be plated with a conductive metal.

The conductive metal may be at least one selected from the group consisting of Ni, Au, Ag, Cu, Zn, Cr, Al, Co, Sn, Pt, and Pd.

It is preferable that the thickness of the said metal coating layer is 10-300 nm.

As the active material particles, an active material having an olivine structure (LiFePO ₄ ) may be included.

In addition, the positive electrode according to an embodiment for achieving another object of the present invention may be a coating of an active material slurry comprising a positive electrode active material including a metal coating layer.

The positive electrode active material including the metal coating layer is preferably contained in 10 to 100% by weight of all the positive electrode active material.

The electrode active material may include two or more kinds of active materials having different electrical conductivity.

The positive electrode active material slurry may further include a conductive material and a binder.

The content of the conductive material included in the cathode active material slurry is preferably 0.5 to 15% by weight.

In addition, to provide a further secondary object of the present invention, a lithium secondary battery comprising the positive electrode is provided.

According to the present invention, by including the metal coating layer on the surface of the active material particles constituting the positive electrode active material, the electrical conductivity is improved, it is possible to increase the output and energy density.

In addition, by using the active material particles including the metal coating layer it is possible to reduce the content of the conductive material contained in the electrode can increase the energy density per unit volume. In addition, since heat generated at the electrode can be quickly transferred to the current collector, it has an effect of suppressing thermal runaway at the anode.

1 is a graph measuring the capacity according to the voltage of a lithium secondary battery according to an embodiment and a comparative example of the present invention.

Hereinafter, the present invention will be described in detail.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms "a", "an" and "the" may include the plural forms as well, unless the context clearly indicates otherwise. Also, as used herein, "comprise" and / or "comprising" specifies the presence of the mentioned shapes, numbers, steps, actions, members, elements and / or groups of these. It is not intended to exclude the presence or the addition of one or more other shapes, numbers, acts, members, elements and / or groups.

The positive electrode active material according to the present invention includes a conventional positive electrode active material and a positive electrode active material having improved electrical conductivity by coating a metal coating layer on the surface of the active material particles.

The positive electrode active material having improved electrical conductivity is a metal coating layer coated on the surface of the active material particles, the surface of the positive electrode active material may appear roughness due to the metal coating layer, thereby increasing the adhesion to the electrode current collector further increases There may be.

The metal coating layer applied to the surface of the active material particles is preferably plated using a conductive metal.

As the cathode active material particles, various types of cathode active materials including an active material of an olivine structure (LiFePO ₄ ) may be used, and the kind thereof is not particularly limited. However, since it is to improve the electrical conductivity of the active material by applying a metal coating layer, it can be said that using an active material having a low electrical conductivity is more effective.

The conductive metal used for the metal coating layer formed on the surface of the active material particles may be at least one selected from the group consisting of Ni, Au, Ag, Cu, Zn, Cr, Al, Co, Sn, Pt, and Pd.

The metal coating layer may be formed on the surface of the electrode active material particles by using an electroless plating method. Therefore, Pd particles, which serve as catalysts for the electroless plating reaction, are deposited on the surface of the active material particles. To this end, the cathode active material particles are precipitated in a mixed solution of PdCl ₂ , HCl, and water for several minutes, and the particles are separated by centrifugation to obtain particles having Pd particles deposited on the surface thereof.

The particles are then dispersed in water for electroless plating with the conductive metal on the surface of the particles having a Pd catalyst deposited thereon, and conductive metal salts (e.g. NiSO ₄ ), NaH ₂ PO ₂ , sodium citrate The plating solution containing the plating solution for several minutes at 50 ~ 100 ℃ and weakly acidic conditions and then the conductive metal is plated through centrifugation to recover the active material particles coated with a metal coating layer.

The plating solution is preferably a concentration of 2 ~ 10g (L).

The metal coating layer prepared by the above process preferably has a thickness of 10 nm to 300 nm. When the metal coating layer is less than 10 nm, there is a problem in conductivity because no coating layer is generated, and on the contrary, when the metal coating layer is manufactured to a thickness exceeding 300 nm, the energy density per weight is not preferable.

In addition, the present invention is characterized by providing a positive electrode coated with an active material slurry containing a positive electrode active material including a metal coating layer on the surface of the active material particles.

In the present invention, a positive electrode active material including the metal coating layer as described above is included in 10 to 100% by weight of the total positive electrode active material.

In addition, the remainder is a conventional positive electrode active material that does not include a metal coating layer, for example, LiCoO ₂ , LiNiO ₂ , LiClO ₄ , LiCF ₃ SO ₃ , LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiN (CF ₃ SO ₂ ) ₂ Or a lithium intercalation material such as a lithium manganese oxide such as LiMn ₂ O ₄ , a lithium cobalt oxide, a lithium nickel oxide, a lithium iron oxide, or a composite oxide formed by a combination thereof, but is not limited thereto. .

If the content of the positive electrode active material including the metal coating layer is less than 10% by weight, the electrical conductivity improvement effect is not preferable, which is not preferable.

In addition, the cathode active material of the present invention more preferably includes two or more kinds of active materials having different electrical conductivity.

In addition, the cathode active material slurry may additionally include a conductive material and a binder. The conductive material can be used as long as it is an electroconductive material that does not cause chemical change in the battery. For example, carbon black, such as acetylene black, Ketjen black, Farnes black, and thermal black; Graphite such as natural graphite and artificial graphite; Carbon derivatives such as carbon nanotubes and fullerenes, conductive fibers such as carbon fibers and metal fibers; Metal powders such as carbon fluoride, aluminum, and nickel powder; Conductive whiskeys 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.

In the present invention, since the metal coating layer is coated on the surface of the active material particles constituting the positive electrode active material, the content of the conductive material included in the positive electrode active material slurry can be reduced. Therefore, the content of the conductive material included in the cathode active material slurry of the present invention is preferably 0.5 to 15% by weight. Thus, by reducing the content of the conductive material in the active material layer has an effect that can increase the energy density per unit volume.

As the binder contained in the active material slurry, any one of a thermoplastic resin and a thermosetting resin may be used, or a combination thereof may be used. Of these, polyvinylidene fluoride (PVdF) or polytetrafluoroethylene (PTFE) is preferred, but is not limited thereto.

The present invention also provides a lithium secondary battery including the positive electrode. A lithium secondary battery is comprised with a positive electrode, a negative electrode, a separator, lithium salt containing nonaqueous electrolyte, etc., for example.

The positive electrode is prepared, for example, by applying a mixture of a positive electrode active material, a conductive material and a binder on a positive electrode current collector, followed by drying, and if necessary, a filler is further added. 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 change in the battery. For example, the surface of stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel Surface treated with carbon, nickel, titanium, silver, or the like can be used. The current collector may form fine concavities and convexities on its surface to reinforce the bonding force of the electrode active material, and may be used in various forms such as a film, a sheet, a foil, a net, a porous body, a foam, and a nonwoven fabric.

The positive electrode of the present invention, the surface of the particles constituting the active material in advance to form a metal coating layer, and then mixed with the conductive material and the binder to prepare a slurry and applied on the current collector.

The negative electrode of the present invention is also produced by applying and drying a negative electrode material on a negative electrode current collector. 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 conductivity without causing chemical change in the battery, and may be formed of a material such as copper, stainless steel, aluminum, nickel, titanium, fired carbon, 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. The current collector may form fine concavities and convexities on its surface to reinforce the bonding force of the electrode active material, and may be used in various forms such as a film, a sheet, a foil, a net, a porous body, a foam, and a nonwoven fabric.

Examples of the negative electrode active material include carbon and graphite materials such as natural graphite, artificial graphite, expanded graphite, carbon fiber, non-graphitizable carbon, carbon black, carbon nanotube, fullerene, and activated carbon; Metals such as Al, Si, Sn, Ag, Bi, Mg, Zn, In, Ge, Pb, Pd, Pt, Ti which can be alloyed with lithium, and compounds containing these elements; Complexes of metals and their compounds and carbon and graphite materials; Lithium-containing nitrides, and the like. Among them, a carbon-based active material, a silicon-based active material, a tin-based active material, or a silicon-carbon based active material is more preferable, and these may be used singly or in combination of two or more.

The separator is an insulating thin film interposed between the anode and the cathode and having high ion permeability and mechanical strength. The pore diameter of the separator is generally 0.01 to 10 mu m and the thickness is generally 5 to 300 mu m. As such a separation membrane, for example, a sheet or a nonwoven fabric made of an olefin-based polymer such as polypropylene which is chemically resistant and hydrophobic, 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.

Another example of the separator may include a polyolefin-based separator substrate and an oil-containing layer comprising an active layer coated with a mixture of inorganic particles and a binder polymer, wherein at least one region selected from the group consisting of a surface of the substrate and a part of pores present in the substrate is coated. Inorganic composite porous separators may also be used. In some cases, the inorganic particles may be coated on the electrode side.

As such inorganic particles, inorganic particles having a dielectric constant of 5 or more, inorganic particles having piezoelectricity, inorganic particles having lithium ion transfer ability, and the like may be used.

Examples of the binder include polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, Polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene butyrene rubber, fluorine rubber, various copolymers, high polymer high polyvinyl alcohol, and the like.

The electrolyte solution includes a lithium salt and a non-aqueous solvent, the lithium salt is dissolved in the non-aqueous solvent, and serves as a source of lithium ions in the battery to enable operation of the basic lithium secondary battery, between the positive electrode and the negative electrode It is a substance that plays a role in promoting the movement of lithium ions. The lithium salt may be LiCl, LiBr, LiI, LiClO ₄ , LiBF ₄ , LiB10Cl ₁₀ , LiPF ₆ , LiCF ₃ SO ₃ , LiCF ₃ CO ₃ , LiAsF ₆ , LiSbF ₆ , LiAlCl ₄ , CH ₃ SO ₃ Li, CF ₃ SO ₃ Li, (CF ₃ SO ₂ ) ₂ NLi, lithium chloroborane, lower aliphatic lithium carbonate, lithium tetraphenylborate and the like can be used.

In addition, the non-aqueous organic solvent serves as a medium to move the ions involved in the electrochemical reaction of the battery.

As the non-aqueous solvent, for example, N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, gamma-butyl Low lactone, 1,2-dimethoxy ethane, tetrahydroxyfranc, 2-methyl tetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolon, formamide, dimethylformamide, dioxorone, aceto Nitrile, nitromethane, methyl formate, methyl acetate, phosphate triester, trimethoxy methane, dioxorone derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivative Aprotic organic solvents such as tetrahydrofuran derivative, ether, methyl pyroionate, and ethyl propionate can be used.

The mixing ratio in the case of mixing one or more of the organic solvents can be appropriately adjusted according to the desired battery performance, which can be widely understood by those skilled in the art.

The organic solvent may be a dispersion medium used in the production of a slurry for the production of a positive electrode, for example, water; Alcohols such as methanol, ethanol, propanol, isopropanol, butanol, isobutanol, s-butanol, t-butanol, pentanol, isopentanol and hexanol; Ketones such as acetone, methyl ethyl ketone, methyl propyl ketone, ethyl propyl ketone, cyclopentanone, cyclohexanone and cycloheptanone; Methyl ethyl ether, diethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, diisobutyl ether, din-amyl ether, diisoamyl ether, methylpropyl ether, methyl isopropyl ether, methyl butyl ether, Ethers such as ethyl propyl ether, ethyl isobutyl ether, ethyl n-amyl ether, ethyl isoamyl ether and tetrahydrofuran; Lactones such as gamma-butyrolactone and delta-butyrolactone; Lactams such as beta-lactam; Cyclic aliphatic compounds such as cyclopentane, cyclohexane and cycloheptane; Aromatic hydrocarbons such as benzene, toluene, o-xylene, m-xylene, p-xylene, ethylbenzene, propylbenzene, isopropylbenzene, butylbenzene, isobutylbenzene, n-amylbenzene; Aliphatic hydrocarbons such as heptane, octane, nonane and decane; Linear and cyclic amides such as dimethylformamide and N-methylpyrrolidone; Esters such as methyl lactate, ethyl lactate, propyl lactate, butyl lactate, and methyl benzoate; Although the liquid substance which comprises the solvent of the electrolyte solution mentioned later is mentioned, It is not limited only to these, You may use it, mixing about 2-5 types of said dispersion mediums.

The lithium secondary battery according to the present invention can be produced by conventional methods known in the art. In addition, the structure of the positive electrode, the negative electrode, and the separator in the lithium secondary battery according to the present invention is not particularly limited, and, for example, each of these sheets in a winding type (winding type) or stacking (stacking type) cylindrical, rectangular Or it may be a form inserted into the case of the pouch type.

The lithium secondary battery according to the present invention includes, for example, a power tool moving by being driven by an electric motor; Electric vehicles including electric vehicles (EVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs); Electric two-wheeled vehicles including E-bikes and E-scooters; Electric golf carts, and the like, but are not limited thereto.

Hereinafter, the present invention will be described by way of examples. However, it should not be understood that the present invention is limited thereto.

Example

(1) Preparation of positive electrode active material to which metal coating layer was applied

The positive electrode active material LiFePO ₄ was Ni plated in the following manner.

50 g of a mixed solution of ₂ g of PdCl ₂ , 20 ml of HCl, and 1000 ml of water were added thereto, and reacted at room temperature for 5 minutes. After the reaction was completed, the particles were separated by centrifugation. The separated particles were again plated with nickel plating solution containing NiSO ₄ 5g / l, NaH ₂ PO ₂ 3g / l and sodium citrate 4g / l at 80 ° C., pH = 4.5 for 10 minutes, and then centrifuged to separate LiFePO. ₄ Particles with Ni metal plated on the surface were recovered.

(2) Preparation of the anode:

The Ni metal covering the LiFePO ₄ 30% by weight of _{Li [Ni 1/3 Co 1} /3 Mn 1/3] O 2 a positive electrode active material 95% by weight mixed with 70% by weight, 1% by weight of carbon black as a conductive material, a binder, Slurry was prepared by mixing 4 wt% PVDF as and NMP as solvent. A positive electrode was prepared by coating the electrode slurry on a 20 μm thick Al foil using a mattress coater and drying at 130 ° C. for 60 minutes.

(3) lithium secondary battery manufacturing

1 mole of lithium hexa containing a positive electrode prepared as described above, and inserting a porous separator composed of polypropylene / polyethylene / polypropylene (PP / PE / PP) between the two electrodes between the graphite-based negative electrode A polymer type lithium secondary battery was prepared by injecting an electrolyte solution of a mixed solution of ethylene carbonate / propylene carbonate / diethyl carbonate (EC / PC / DEC = 30/20/50% by weight) in which fluorophosphate (LiPF ₆ ) was dissolved. .

After the polymer type lithium secondary battery was formed to 4.2V, charge and discharge were performed between 4.2V and 2.5V (C-rate = 1C).

Comparative example

Except that there is no metal coating layer on the surface of LiFePO _{4 in} the above embodiment, a positive electrode active material was prepared in the same manner as in the above embodiment, and was coated on a positive electrode current collector to prepare a positive electrode.

Experimental Example

The charge and discharge capacity of the full cell lithium secondary battery prepared by the above Examples and Comparative Examples was measured in the voltage range of 4.2V-2.5V and is shown in FIG. 1. The data shown in FIG. 1 is just one example, and the detailed capacitance value according to the voltage will vary depending on the specification of the cell. Therefore, the trend of the graph is more important than the detailed value.

The above description is merely illustrative of the technical idea of the present invention, and those skilled in the art to which the present invention pertains may make various modifications and changes without departing from the essential characteristics of the present invention. Therefore, the embodiments disclosed in the present invention are not intended to limit the technical spirit of the present invention, but are intended to be described. The scope of the present invention should be interpreted by the following claims, and all technical spirits within the equivalent scope thereof are It should be interpreted as being included in the scope of the present invention.

Claims (13)

A positive electrode active material comprising a metal coating layer on the surface of the active material particles.
The cathode active material of claim 1, wherein the metal coating layer is plated with a conductive metal.
The cathode active material of claim 2, wherein the conductive metal is at least one selected from the group consisting of Ni, Au, Ag, Cu, Zn, Cr, Al, Co, Sn, Pt, and Pd.
The cathode active material according to claim 1, wherein the metal coating layer has a thickness of 10 nm to 300 nm.
The electrode active material of claim 1, wherein the active material particles include an active material having an olivine structure (LiFePO ₄ ). The positive electrode to which the active material slurry containing the electrode active material containing the metal coating layer of Claim 1 was apply | coated.
The positive electrode of claim 6, wherein the electrode active material including the metal coating layer is included in an amount of 10 to 100% by weight of the electrode active material.
The electrode active material of claim 6, wherein the electrode active material comprises two or more active materials having different electrical conductivity.
The cathode of claim 6, wherein the active material slurry further comprises a conductive material and a binder.
The cathode of claim 9, wherein the conductive material is included in an amount of 0.5 to 15 wt%.
A lithium secondary battery comprising the positive electrode according to claim 6.
The medium-large battery module or battery pack of claim 11, comprising two or more lithium secondary batteries electrically connected to each other.
The method of claim 12, wherein the medium-large battery module or battery pack is a power tool; An electric vehicle including an electric vehicle (EV), a hybrid electric vehicle (HEV), and a plug-in hybrid electric vehicle (PHEV); Electric two-wheeled vehicles including E-bikes and E-scooters; Electric golf carts; Electric trucks; Medium-large battery module or battery pack, characterized in that used as a power source for any one or more of the medium-to-large device of a commercial vehicle or power storage system.

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