KR20070023293A - Lithium Secondary Battery with Improved Safety - Google Patents

Abstract

The present invention is a secondary battery composed of a positive electrode, a negative electrode, a separator, and an electrolyte, when the temperature of the battery rises above a predetermined temperature, an expandable material that expands simultaneously with the generation of flame retardant gas by endothermic reaction, preferably Including the expandable carbon in the positive electrode and / or the electrolyte provides a lithium secondary battery that can improve the safety of the battery.

Description

Lithium Secondary Battery with Improved Safety

The present invention relates to a lithium secondary battery having improved safety, and more particularly, to an anode and / or an expandable material in which a volume expands simultaneously with the generation of a flame retardant gas by an endothermic reaction when the temperature of the battery rises above a predetermined temperature. It is included in the electrolyte to provide a lithium secondary battery that can improve the safety.

As the development and demand for mobile devices increases, the demand for secondary batteries as energy sources is increasing rapidly. Among them, many researches on lithium secondary batteries with high energy density and discharge voltage have been conducted and commercialized. It is widely used.

However, in the conventional lithium secondary battery, when a large current flows within a short time due to overheating, overcharge, external short circuit, nail penetration, local crush, etc., the battery is heated by IR heating. Exposure to fire / explosion hazards. Specifically, when the temperature of the battery rises, the reaction between the electrolyte and the electrode is promoted, and as a result, heat of reaction is generated, which further increases the temperature of the battery, which in turn accelerates the reaction between the electrolyte and the electrode. Thus, the temperature of the battery rises rapidly, which in turn accelerates the reaction between the electrolyte and the electrode. Due to this vicious cycle, a thermal runaway phenomenon in which the temperature of the battery rises rapidly occurs, and when the temperature rises to a certain level or more, the battery may ignite. In addition, as a result of the reaction between the electrolyte and the electrode, gas is generated to increase the battery internal pressure, and the lithium secondary battery explodes above a certain pressure. The risk of ignition / explosion can be said to be the most fatal drawback of lithium secondary batteries.

Therefore, essential considerations for the development of a lithium secondary battery are to ensure safety. Safety devices for preventing the ignition / explosion of the lithium secondary battery due to overcharging include a method of mounting a device outside the cell and using a material inside the cell. PTC devices, CID devices, temperature protection circuits, and safety vents using changes in the breakdown voltage of the battery are examples of the former. The latter is the addition of substances that can be changed chemically and electrochemically.

Devices mounted outside the cell use a temperature, voltage, and withstand voltage, which can lead to reliable blocking, but require an additional installation process and installation space, and CID devices can be applied only to cylindrical batteries. In addition, it is known that it does not properly protect against tests requiring fast response time such as internal short circuit, nail penetration, local crush, and the like.

One method of using a substance inside a cell is to add an additive that improves safety to an electrolyte or an electrode. For example, a method in which an electrochemical polymerization reaction is added to an electrolyte under conditions such as overcharging, and a polymerization product of such a material forms an antifreeze film on the electrode or solidifies the electrolytic solution during overcharging, thereby suppressing abnormal operation of the battery. have. Chemical safety devices have the advantage that they can be applied to all kinds of batteries do not require additional processes and space, but do not provide a reliable operation and there is a problem that the performance of the battery is degraded due to the addition of materials.

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, an anode and / or an electrolyte with an expandable material that expands in volume at the same time as the generation of a flame retardant gas due to an endothermic reaction above a predetermined temperature. By adding a battery to produce a battery, when the temperature of the battery rises sharply, the heat of reaction inside the battery is reduced by the endothermic reaction, and the combustion reaction is suppressed by the flame retardant gas generated, and the expandable material increases the resistance inside the battery. And it was found that by suppressing the reaction of the electrode and the electrolyte can ultimately improve the safety of the battery.

The present invention has been completed based on this finding.

Lithium secondary battery according to the present invention for achieving this purpose is composed of a negative electrode, a positive electrode, a separator and an electrolyte, when the temperature of the battery rises above a predetermined temperature when the flame retardant gas generated by the endothermic reaction and the volume expands at the same time Including an expandable material in the positive electrode and / or electrolyte to improve the safety of the battery.

The term "specific temperature" refers to a temperature at which the safety of the battery may be threatened, and refers to a temperature at which the expandable material begins to expand in volume at the same time as the endothermic reaction, and may vary depending on the design of the battery. Although it is possible, it becomes like this. Preferably it is the range of 80-200 degreeC, More preferably, it is the range of 150-190 degreeC.

In the present invention, the expandable material is a material in which an endothermic reaction occurs when the temperature of the battery rises above the predetermined temperature, a flame retardant gas is generated, and the volume expands, thereby inhibiting the movement of lithium ions in the positive electrode and / or the electrolyte. It suppresses the energization and the reaction between the electrode and the electrolyte, reduces the heat of reaction in the battery due to the endothermic reaction, and suppresses the combustion reaction due to the generation of flame retardant gas. In addition, the expandable material does not react below the predetermined temperature and does not affect the performance and operation of the battery.

Such expandable materials may vary, and among these, expandable carbon is particularly preferable in which the volume expands at the same time as the endotherm at 150 ° C. or higher.

The expandable carbon is a material in which an acidic substance is contained in a layered crystal structure of carbon, and as the acidic substance existing between layers is vaporized by an endothermic reaction, it expands rapidly to 300 to 400% of the initial volume. have. Moreover, since carbon is a conductive material in itself, it does not adversely affect the performance and operation of the battery when included in the battery. Thus, expandable carbon can be added to the battery's positive electrode and / or electrolyte to achieve the desired safety as in the present invention. The acidic material contained in the layered crystal structure is generally sulfuric acid, nitric acid, or a mixture thereof. Since sources and methods for producing such expandable carbon are known, further description thereof is omitted herein.

In general, commercially available expandable carbon has a particle size of 150 μm, whereas in the present invention, it is preferable to grind and add it to a particle size of 40 μm or less.

The expandable carbon may be added to the positive electrode and / or the electrolyte of the battery as described above, and preferably added to the positive electrode.

When the expandable carbon is added to the positive electrode, it may be added in an amount of 0.5 to 20% by weight based on the total weight of the positive electrode mixture such as the positive electrode active material, the conductive agent, and the binder, and for example, a solvent such as NMP may be added. It is possible to prepare a positive electrode containing expandable carbon by mixing together in a positive electrode mixture when the positive electrode slurry is used to apply a positive electrode current collector and then coated on the positive electrode current collector. When the expandable carbon is added to the electrolyte, it may be added at 0.5 to 20% by weight based on the total weight of the electrolyte. If the amount of the expandable carbon added in the positive electrode and / or the electrolyte is too small, it is difficult to exert the effect of the addition. On the contrary, if the amount of the expandable carbon is too small, the content of the active material or the content of the electrolyte is relatively low, which is not preferable. This fact can be confirmed through the following examples and comparative examples.

Hereinafter, other components of the lithium secondary battery according to the present invention will be described.

The lithium secondary battery of the present invention is composed of a positive electrode, a negative electrode, a separator, a lithium salt-containing nonaqueous electrolyte, and the like.

The positive electrode is produced by, for example, applying a mixture of a positive electrode active material, a conductive agent, and a binder onto a positive electrode current collector, followed by drying, and optionally, a filler is further added.

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 is typically added in an amount of 1 to 50% by weight based on the total weight of the positive 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 oxides such as titanium oxide; Conductive materials such as polyphenylene derivatives and the like can be used. In some cases, since the conductive second coating layer is added to the positive electrode active material, the addition of the conductive agent may be omitted.

The binder is a component that assists in bonding the active material and the conductive agent to the current collector, and is usually added in an amount of 1 to 50 wt% based on the total weight of the positive 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.

The filler is optionally used as a component for inhibiting expansion of the positive electrode, and 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, are used.

The negative electrode is manufactured by applying and drying a negative electrode material on the negative electrode current collector, and if necessary, the components as described above may be further included.

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, the surface of copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel Surface-treated with carbon, nickel, titanium, silver, and the like, 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.

The negative electrode material may be, for example, carbon such as hardly 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 <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 oxides such as Bi ₂ O ₅ ; Conductive polymers such as polyacetylene; Li-Co-Ni-based materials and the like can be used.

The separator is interposed between the cathode and the anode, 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. When a solid electrolyte such as a polymer is used as the electrolyte, the solid electrolyte may also serve as a separator.

The lithium salt-containing non-aqueous electrolyte is composed of an organic solvent for electrolyte and a lithium salt. As the solvent for the electrolyte, a nonaqueous electrolyte, an organic solid electrolyte, an inorganic solid electrolyte, and the like are used.

As said non-aqueous electrolyte, 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 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 3 Li, (CF 3 SO 2) 2 NLi, chloroborane lithium, lower aliphatic carboxylic acid lithium, lithium tetraphenyl borate and imide have.

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.

The lithium secondary battery according to the present invention can be produced by conventional methods known in the art. That is, it may be prepared by inserting a porous separator between the anode and the cathode and injecting an electrolyte therein.

The positive electrode may be manufactured by, for example, applying a slurry containing the lithium transition metal oxide active material, the conductive material, and the binder described above onto a current collector and then drying. Similarly, the negative electrode can be prepared by, for example, applying a slurry containing the above-described carbon active material, a conductive material and a binder onto a thin current collector and then drying it.

In the lithium secondary battery according to the present invention, the structure of the positive electrode, the negative electrode, and the separator is not particularly limited. For example, each of these sheets may be wound, wound, or stacked in a cylindrical, rectangular, or pouch type. It may be inserted into the case of the mold.

Hereinafter, the present invention will be described in detail by way of examples, but the scope of the present invention is not limited thereto.

Example 1

The negative electrode was mixed with PVDF, which is a binder, and carbon black, which is a conductive agent, in a composition ratio of 93: 6: 1, stirred with NMP for 2 hours, coated on Cu foil, which is a current collector, and then, at 120 ° C. Prepared by drying over time.

The positive electrode was mixed with PVDF, which is a binder, and carbon black, which is a conductive agent, in a composition ratio of 85: 5: 10, and 5 wt% of expandable graphite having a particle size of 10 μm was stirred with NMP, and then mixed with Cu. After coating, it was prepared by drying at 120 ℃ for 2 hours.

After separating and assembling a separator between the negative electrode and the positive electrode thus prepared, an electrolyte was injected to prepare a lithium secondary battery.

Example 2

A lithium secondary battery was manufactured in the same manner as in Example 1, except that 10 wt% of expanded graphite having a particle diameter was added to prepare a positive electrode.

Example 3

A lithium secondary battery was manufactured in the same manner as in Example 1, except that an anode was prepared by adding 5 wt% of expandable graphite having a particle diameter of 1 μm.

Comparative Example 1

A lithium secondary battery was manufactured in the same manner as in Example 1, except that expandable graphite was not added.

Experimental Example 1

The lithium secondary batteries of Examples 1 to 3 and Comparative Example 1 were continuously charged for 120 minutes or more under conditions of 1 & 5 C rate, and whether or not the batteries were ignited in such a process was observed. The experimental results are shown in Table 1 below.

As shown in Table 1, it can be confirmed that the batteries of Examples 1 to 3 have improved safety compared to Comparative Example 1.

As described above, the lithium secondary battery according to the present invention includes an expandable material that expands in volume at the same time as the endothermic reaction at a predetermined temperature, so that the expandable material increases the resistance inside the battery when the temperature of the battery rises rapidly. By suppressing the reaction between the electrode and the electrolyte has the effect of ultimately improving the safety of the battery.

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 (8)

A lithium secondary battery composed of a positive electrode, a negative electrode, a separator, and an electrolyte, wherein when a battery temperature rises above a predetermined temperature, an expandable material that expands in volume at the same time as generating a flame retardant gas by an endothermic reaction, is formed on the positive electrode and / or the electrolyte. Lithium secondary battery, including improving the safety of the battery. The lithium secondary battery according to claim 1, wherein the predetermined temperature is in a range of 80 to 200 ° C. The lithium secondary battery of claim 2, wherein the predetermined temperature is in a range of 140 to 180 ° C. 4. The lithium secondary battery of claim 1, wherein the expandable material is expandable carbon. The lithium secondary battery according to claim 4, wherein the expandable carbon has a particle diameter of 40 μm or less. The lithium secondary battery of claim 4, wherein the expandable carbon is included in a positive electrode. The lithium secondary battery of claim 6, wherein the expandable carbon is included in an amount of 0.5 to 20 wt% based on the total weight of the positive electrode mixture. The lithium secondary battery of claim 1, wherein the expandable carbon is included in the electrolyte in an amount of 0.5 to 20 wt% based on the total weight of the electrolyte.