KR20070020759A - Lithium Secondary Battery Containing Oxide of Inorganic Material - Google Patents

Abstract

The present invention provides a lithium secondary battery in which a small amount of an inorganic oxide having a nano particle diameter is included in a negative electrode mixture including a negative electrode active material and a binder. Such a lithium secondary battery has a problem occurring when the metal oxide of the prior art is added. While all solved, it is possible to improve the wettability of the electrolyte to the electrode with high efficiency and ultimately to improve the rate characteristics and storage capacity of the battery.

Description

Lithium Secondary Battery Containing Oxide of Inorganic Material

1 is a graph illustrating charge and discharge test results for the battery of Example 1 and the battery of Comparative Example 2;

FIG. 2 is an enlarged graph showing the degree of voltage change initially occurring in the discharge voltage curves of the battery of Example 1 and the batteries of Comparative Examples 1 and 2.

The present invention relates to a lithium secondary battery, and more particularly, to a negative electrode mixture including a negative electrode active material and a binder, a small amount of an inorganic oxide having a nano particle size based on the total weight of the mixture is added to wettability of the electrolyte solution with respect to the negative electrode material. The present invention relates to a lithium secondary battery having improved wettability. Such an inorganic oxide solves a decrease in rate characteristics and a decrease in capacity caused by insufficient electrolyte covering the surface of the active material when the negative electrode material has a high hydrophobicity or a high density of the electrode, so that the porosity is very low.

As the development and demand for mobile devices increases, the demand for secondary batteries as energy sources is increasing rapidly. Recently, the use of secondary batteries as a power source for electric vehicles (EVs) and hybrid electric vehicles (HEVs) has been realized. have. Accordingly, many studies have been conducted on secondary batteries capable of meeting various needs, and in particular, there is a high demand for lithium secondary batteries having high energy density, high discharge voltage, and output stability.

A lithium secondary battery is generally a lithium ion secondary battery composed of a carbonaceous anode, a lithium transition metal oxide anode, a porous separator, and a non-aqueous electrolyte, and an electrolyte solution is impregnated in a polymer matrix instead of a separator to intervene between a cathode or a solid. It is distinguished by a so-called lithium polymer battery using an electrolyte.

As battery capacity is required to increase, lithium secondary batteries having higher energy densities of electrodes have been developed. However, due to the improvement of the energy density, the porosity of the electrode is very low, thereby increasing the difficulty of evenly infiltrating the electrolyte into the electrode. If the electrolyte does not sufficiently wet the surface of the active material constituting the electrode, the transfer path of lithium ions is limited, causing problems such as a decrease in rate characteristics and a decrease in capacity.

Therefore, much effort has been made to improve the wettability of the electrolyte solution with respect to the electrode.

For example, in Korean Patent Application Publication No. 2000-56301, in order to improve the nonuniformity of the electrolyte distribution of the electrode in a lithium ion secondary battery and to suppress the local expansion of the negative electrode and the occurrence of unreacted sites over the course of the cycle, Disclosed is a technique of forming a metal oxide layer such as MgO, Al ₂ O ₃ , BaO, ZnO, etc. having excellent electrolyte absorption ability on the surface of a negative electrode active material, or adding the metal oxide to a negative electrode active material.

In Korean Patent Application Laid-Open Nos. 2000-15289 and 2002-15257, in order to improve the cycle characteristics during high rate discharge by improving the electrolyte holding capacity for the negative electrode in lithium secondary batteries and improving the diffusion rate of lithium ions, ₂ O _3, CeO _2, a technique of adding a metal oxide such as TiO ₂ is described.

Although the objects and effects of the invention differ from the invention and the foregoing prior arts, some techniques for adding metal oxides to electrodes are also known.

For example, Korean Patent Application Laid-Open Publication No. 1998-81570 discloses a high capacity in lithium ion polymer batteries or lithium polymer batteries mainly using lithium nickel-based transition metal oxides as positive electrode active materials, thereby increasing the utilization of lithium ions during initial charge and discharge. And a technique for adding a metal oxide such as TiO ₂ , Cr ₂ O ₃ , MnO _2, etc. to the cathode for the purpose of improving the cycle characteristics.

However, as the field of application of secondary batteries expands and the need for secondary batteries with higher rate characteristics and storage capacities increases, there is a continuous need for a technology capable of satisfying them. For example, the metal oxides used as additives in the prior art are additional components, so it may be desirable to add them in as small amounts as possible. However, such metal oxides have a possibility of working with battery components due to their characteristics (see Korean Patent Application Laid-Open No. 1998-81570). In addition, despite the content range described in the prior art, it is necessary to add a certain amount or more to obtain a predetermined effect. Thus, there is still a need for additives that can provide high efficiency in small amounts without directly affecting the operating mechanism of the cell.

Accordingly, an object of the present invention is to solve the problems of the prior art as described above and the technical problems that have been requested from the past.

The present inventors have carefully observed the problems of the prior art, and after many experiments and in-depth studies, when a predetermined amount of nano-size inorganic oxide particles are added to the negative electrode material, surprisingly, it occurs when the metal oxide of the prior art is added. While solving all the problems, it has been found that the wettability of the electrolyte with respect to the electrode can be improved with high efficiency and ultimately, the rate characteristic and storage capacity of the battery can be improved.

The present invention has been completed based on this finding.

The lithium secondary battery according to the present invention is composed of an inorganic oxide having a particle size of 50 nm or less in the negative electrode mixture including the negative electrode active material and the binder in an amount of 0.01 to 2% by weight based on the total weight of the mixture.

Therefore, the lithium secondary battery of the present invention is characterized in that an inorganic oxide is added to the negative electrode mixture, that the particle size of the additive is nanoscale, and that the content in the negative electrode mixture of the additive is very low. The lithium secondary battery having all of these requirements can be confirmed in the following examples, etc., that a better effect can be obtained than the lithium secondary battery containing the metal oxide, which is an unexpected result.

In the lithium secondary battery, the negative electrode is typically composed of a carbon-based material and a binder as an active material, and in some cases, is prepared by applying a negative electrode mixture to which a conductive agent such as carbon black is added to improve electrical conductivity.

However, such negative electrode materials have a high hydrophobicity or a high density of electrodes, and thus a very low porosity, and thus, lithium-containing non-aqueous electrolytes do not sufficiently cover the surface of the electrode as described above. In addition to the deterioration of the characteristics, the heterogeneous reaction leads to a faster capacity reduction of the battery.

On the other hand, the inventors of the present invention, by adding a small amount of nano-size inorganic oxide particles in the manufacturing of the electrode, thermally, chemically, and electrically stable in the environment of use of the lithium secondary battery, and relatively good affinity with the electrolyte, the wettability of the electrode The improvement was confirmed that the resistance of the electrode was reduced.

The inorganic oxide is particularly preferably SiO ₂ . While the present invention uses SiO ₂ as a preferred additive, in the prior art such as Korean Patent Application Laid-Open Nos. 2000-15289 and 2002-15257, a battery using an inorganic oxide such as SiO ₂ has been added with a metal oxide or It is explained that the capacity retention rate is worse than that of the battery without the addition of the same. Nevertheless, it is assumed that the lithium secondary battery of the present invention exhibits a more excellent effect than the lithium secondary battery of the prior art including a metal oxide due to its specific particle diameter and content in addition to the physical properties of SiO ₂ , which is an inorganic oxide. do. Therefore, in the present invention, the inorganic oxide exhibits a desired effect in the present invention by forming a complex relationship between physical properties, particle diameters and contents thereof. Looking at this in detail.

The particle size of the inorganic oxide in the present invention is 50 nm or less, preferably 2 to 30 nm, as defined above, the content thereof is 0.01 to 2% by weight, preferably 0.05 to 1% by weight based on the total weight of the negative electrode mixture %to be.

If the particle size of the additive is larger than the above range, the content must be increased to achieve the same wettability improvement, and this increase in content reduces the amount of the electrode active material used. Moreover, since the inorganic oxide is an insulator, increasing the content in the negative electrode mixture may lower the electrical conductivity and ultimately reduce the rate characteristic of the battery. On the other hand, if the content of the inorganic oxide is too small, it is difficult to obtain the effect of the addition.

In particular, SiO _2, which can be preferably used in the present invention, is easy to prepare with the nano particle diameters in the above range and commercially obtainable because many products of such nano particle sizes are commercially available, while metal oxides are prepared with the nano particle diameter. Is difficult and very expensive even if manufactured. Thus, there is another advantage of using SiO ₂ as the inorganic oxide.

As said negative electrode active material in this invention, For example, carbon, such as hardly graphitized carbon and graphite type 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 <x ≦ 1; 1 ≦ y ≦ 3; 1 ≦ z ≦ 8); Lithium metal; Lithium alloys; Silicon-based alloys; Tin-based alloys; 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 metal oxides such as Bi ₂ O ₅ ; Conductive polymers such as polyacetylene; Li-Co-Ni-based materials and the like can be used. Especially, a carbon type active material is especially preferable.

The binder is a component that assists in bonding the active material and the current collector, and is typically added in an amount of 1 to 50 wt% based on the total weight of the negative electrode mixture. Examples of such binders 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 butylene rubber, fluorine rubber, various copolymers, and the like.

In some cases, a conductive agent for increasing electrical conductivity may be added in an amount of 1 to 50 wt% based on the total weight of the negative electrode mixture. Such a conductive agent is not particularly limited as long as it has conductivity without causing chemical change in the battery. Examples of the conductive agent include graphite such as natural graphite and artificial graphite; Carbon blacks such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, and summer black; Conductive fibers such as carbon fibers and metal fibers; Metal powders such as carbon fluoride powder, aluminum powder 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. Specific examples of commercially available conducting agents include acetylene black Chevron Chemical Company, Denka Singapore Private Limited, Gulf Oil Company, Ketjenblack, EC series (Armak Company), Vulcan XC-72 (Cabot Company), and Super P (Timcal).

In addition, fillers may optionally be used as components to inhibit expansion of the negative electrode. Such a filler is not particularly limited as long as it is a fibrous material without causing chemical change in the battery. Examples of the filler include olefinic polymers such as polyethylene and polypropylene; Fibrous materials, such as glass fiber and carbon fiber, can be used.

The negative electrode is prepared by applying a paste prepared by adding a negative electrode mixture such as a negative electrode active material and a binder to a dispersion, applying a compress to a negative electrode current collector, compressing and drying, and cutting it to a certain size.

As the dispersion, isopropyl alcohol, N-methylpyrrolidone (NMP), acetone, and the like may be typically used.

The negative electrode current collector is generally made to a thickness of 3 to 500 ㎛. Such a negative electrode current collector is not particularly limited as long as it has conductivity without causing chemical change in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel Surface-treated with carbon, nickel, titanium, silver and the like on the surface, aluminum-cadmium alloy and the like can be used. In addition, like the positive electrode current collector, fine concavities and convexities may be formed on the surface to enhance the bonding strength of the negative 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.

Looking at the specific manufacturing process of the negative electrode according to the present invention as follows.

First, a negative electrode active material, a binder, and an inorganic oxide powder are added and stirred in an amount of 1 to 30% by weight based on the total dispersion to prepare a paste, which is then coated on a metal plate for current collector, compressed, and dried to form a laminate. Preference is given to a method for producing the negative electrode.

The method of evenly applying the paste of the electrode material to the metal material can be selected from known methods or performed by a new suitable method in consideration of the properties of the material. For example, it is preferable to disperse the paste onto the current collector and then to disperse the paste uniformly using a doctor blade or the like. In some cases, a method of distributing and dispersing in one process may be used. In addition, die casting, comma coating, screen printing, or the like may be used. Alternatively, a separate substrate may be formed and then joined with a current collector by pressing or lamination. You can also

Drying of the paste applied on the metal plate is preferably dried for 1 to 3 days in a vacuum oven of 50 to 200 ℃.

The lithium secondary battery of the present invention has a structure in which a lithium salt-containing non-aqueous electrolyte is impregnated in an electrode assembly in which the negative electrode faces the positive electrode with a separator interposed therebetween.

For example, a positive electrode mixture of a positive electrode active material, a conductive agent, and a binder is coated on the positive electrode current collector, and a filler may be further added to the positive electrode mixture in some cases.

The positive electrode current collector is generally made to a thickness of 3 to 500 μm. Such a positive electrode current collector is not particularly limited as long as it has high conductivity without causing chemical changes 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 irregularities on its surface to increase the adhesion of the positive electrode active material, and may be 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 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 , Cu ₂ V ₂ O ₇ and the like; Ni-site type lithium nickel oxide represented by the formula LiNi _1-x M _x O ₂ , wherein 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 (wherein M = Co, Ni, Fe, Cr, Zn or Ta and x = 0.01 to 0.1) or Li ₂ Mn ₃ MO ₈ (wherein M = Fe, Co, Lithium manganese composite oxide represented by Ni, Cu or Zn); LiMn ₂ O _{4 in} which a part of Li in the formula is substituted with alkaline earth metal ions; Disulfide compounds; Fe ₂ (MoO ₄ ) ₃ and the like, but are not limited to these.

The conductive agent, the binder, the filler, and the like are the same in the description of the negative electrode.

The separator serves to prevent an internal short circuit between the anode and the cathode and to impregnate the electrolyte, and an insulating thin film having high ion permeability and mechanical strength is used. The pore diameter of the separator is generally from 0.01 to 10 ㎛ ㎛, thickness is generally 5 ~ 300 ㎛. As such a separator, for example, olefin polymers such as chemical resistance and hydrophobic polypropylene; Sheets or non-woven fabrics made of glass fibers or polyethylene are used. Typical examples on the market include Celgard series (Celgard ^TM 2400, 2300 (manufactured by Hoechest Celanese Corp.), polypropylene membrane (manufactured by Ube Industries Ltd. or Pall RAI), and polyethylene series (Tonen or Entek)). In the case where a solid electrolyte such as a polymer is used as the electrolyte, the solid electrolyte may also serve as a separator, and a gel polymer electrolyte may be coated on the separator to increase the stability of the battery. Representative examples include polyethylene oxide, polyvinylidene fluoride, polyacrylonitrile, and the like.

The lithium salt-containing non-aqueous electrolyte is composed of an organic solvent and a lithium salt, and may be classified into a liquid electrolyte, a solid electrolyte, an inorganic solid electrolyte, and the like depending on the shape.

As said liquid electrolyte solution, N-methyl- 2-pyrrolidinone, a propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, gamma-butyl Low lactone, 1,2-dimethoxy ethane, tetrahydroxy franc, 2-methyl tetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolon, formamide, dimethylformamide, dioxolon, 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 derivatives, ethers, methyl pyroionate and ethyl propionate can be used.

Examples of the organic solid electrolytes include polyethylene derivatives, polyethylene oxide derivatives, polypropylene oxide derivatives, phosphate ester polymers, polyedgetion lysine, polyester sulfides, polyvinyl alcohols, polyvinylidene fluorides, Polymers containing ionic dissociating groups and the like can 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, sulfates and the like of Li, such as Li ₄ SiO ₄ -LiI-LiOH, Li ₃ PO ₄ -Li ₂ S-SiS ₂ , and the like, may be used.

The lithium salt is a good material to be dissolved in the organic solvent, 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 ₃ Li, (CF ₃ SO ₂ ) ₂ NLi, chloroborane lithium, lower aliphatic lithium carbonate, lithium phenyl borate, imide and the like can be used. .

In addition, for the purpose of improving charge / discharge characteristics, flame retardancy, etc., the non-aqueous electrolyte solution includes, for example, pyridine, triethyl phosphite, triethanolamine, cyclic ether, ethylene diamine, n-glyme, and hexaphosphate triamide. Nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N, N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrroles, 2-methoxy ethanol, aluminum trichloride, etc. It may be. In some cases, in order to impart nonflammability, halogen-containing solvents such as carbon tetrachloride and ethylene trifluoride may be further included, and carbon dioxide gas may be further included to improve high temperature storage characteristics.

Hereinafter, the present invention will be further described with reference to Examples, but the scope of the present invention is not limited thereto.

Example 1

P15BHG from Nippon carbon, an artificial graphite-based negative active material, was mixed with a binder polyvinylidene fluoride in a weight ratio of 93: 7, and then silica (SiO ₂ ) was added in an amount of 0.1 wt% based on the total weight. ) Was added, NMP (N-methyl pyrolidinone) was added, mixed, and applied to a copper current collector, followed by drying in a vacuum oven at 120 ° C. to prepare an electrode. The silica used A150 manufactured by Degussa.

The electrode thus prepared was punched into a circle having a diameter of 11 mm, and then a battery was manufactured using a 2016 size coin cell. In this case, the counter electrode used was lithium metal, and an electrolyte was used as a volume ratio 1: 1 mixture of EC (ethylene carbonate) and DMC (dimethyl carbonate) in which 1M LiPF ₆ was dissolved. The battery thus produced is referred to as a 'Example 1 battery'.

Comparative Example 1

A battery was manufactured in the same manner as in Example 1, except that alumina (Al ₂ O ₃ ) was used instead of silica, and this is called a “Comparative Example 1 battery”. Alumina was used as Aluminum Oxide C manufactured by Degussa.

Comparative Example 2

An electrode was manufactured in the same manner as in Example 1 except that silica was not added, which is referred to as a 'Comparative Example 2 Battery'.

Experimental Example 1

The battery of Example 1, Comparative Example 1, and Comparative Example 2 prepared above were charged to 0.005 V at a current of 0.1 C, respectively, and then charged at a potential of 0.005 V until the current was reduced to 0.005 C. The discharge was discharged at a constant current of 0.1 C up to 2.0V.

1 is a graph showing the above experimental results for the battery of Example 1 and the battery of Comparative Example 2. As shown in Figure 1, the battery of Example 1 according to the present invention can be seen that the capacity is increased compared to the battery of Comparative Example 2.

In addition, FIG. 2 is a graph showing the degree of voltage change that occurs initially in the discharge voltage curve for each battery. As shown in Figure 2, the battery of Example 1 according to the present invention can be seen that the resistance is significantly reduced compared to the batteries of Comparative Example 1 and Comparative Example 2. This is an effect that is caused by the electrolyte solution penetrated deeply inside the electrode and evenly applied to the surface of the negative electrode active material.

As described above, the lithium secondary battery according to the present invention solves all the problems caused by the addition of the metal oxide of the prior art, while improving the wettability of the electrolyte to the electrode with high efficiency and ultimately the rate characteristics and storage capacity of the battery Can improve.

Those skilled in the art to which the present invention pertains will be able to perform various applications and modifications within the scope of the present invention based on the above contents.

Claims (6)

A lithium secondary battery comprising an inorganic oxide having a particle size of 50 nm or less in an amount of 0.01 to 2 wt% based on the total weight of the mixture in an anode mixture including a negative electrode active material and a binder. The lithium secondary battery of claim 1, wherein the inorganic oxide is SiO ₂ . The lithium secondary battery according to claim 1, wherein the inorganic oxide has a particle diameter of 2 to 30 nm. The lithium secondary battery of claim 1, wherein the content of the inorganic oxide is 0.01 to 2 wt% based on the total weight of the negative electrode mixture. The lithium secondary battery of claim 1, wherein the negative electrode active material is a carbon-based active material. The method of claim 1, wherein the negative electrode mixture, a conductive material for increasing the electrical conductivity and / or a filler for inhibiting the expansion of the negative electrode is added to the lithium secondary, characterized in that 1 to 50% by weight based on the total weight of the negative electrode mixture battery.

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