KR20070008087A - Lithium secondary battery containing thermally degradable capsule containing additives for overcharge protection - Google Patents

Abstract

Provided is a lithium secondary battery comprising a thermally degradable capsule containing anti-overcharging agent which increases an inner resistance or impedance of battery by electrochemical polymerization at abnormal working condition of battery owing to overcharging and decreases dangers of firing/explosion of battery, without affecting a performance of the battery at normal working condition of the battery. The lithium secondary battery comprising a cathode, an anode, and a separator, wherein the battery comprises a thermally degradable capsule containing a material which can increase an inner resistance or impedance of battery by electrochemical polymerization at abnormal working condition of the battery owing to overcharging of the battery, in electrolyte and/or electrode of the battery. A voltage at which electrochemical polymerization of the anti-overcharging agent is affected, is more than 4.5 V. The anti-overcharging agent is one, two or more selected from the group consisting of biphenyl, terphenyl, and cyclohexyl phenyl.

Description

Lithium Secondary Battery Containing Thermally Degradable Capsule Containing Additives For Overcharge Protection}

The present invention relates to a lithium secondary battery comprising a thermally degradable capsule carrying an overcharge preventing agent, and more particularly, to a battery of overcharging without affecting the performance of the battery in a normal operating state of the battery. In an abnormal operating state, the present invention relates to a lithium secondary battery including a thermally degradable capsule carrying an overcharge preventing agent that increases the internal resistance or impedance of the battery by electrochemical polymerization, thereby reducing the risk of ignition / explosion of the battery.

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

However, the conventional lithium secondary battery has a risk of fire / explosion upon overcharging. When the battery is overcharged, the temperature of the battery continues to rise due to the accumulation of IR heat. In addition, the electrode becomes chemically unstable and the reaction with the electrolyte proceeds actively. As a result, the heat of reaction occurs, and a thermal runaway phenomenon occurs in which the temperature inside the battery is rapidly increased. Ignition of the battery may occur. In addition, as a result of the reaction between the electrolyte and the electrode, a gas is generated to increase the internal pressure of the battery, 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. Chemical safety devices do not require additional processes and space and have the advantage of being applicable to all kinds of batteries, but there is a problem that the performance of the battery is degraded due to the addition of the material. Such materials include a material that forms a passivation layer on the electrode during overcharging, a material that generates a gas to operate a CID device, and a redox shuttle.

Among them, a material that forms a passivation layer on an electrode is widely used. Examples of such materials include biphenyl, terphenyl, and cyclohexyl benzene. Japanese Patent Application Laid-Open No. 1997-106835 discloses the use of biphenyl, 3-chloro thiophene, franc, etc. as an additive for preventing overcharge, and Japanese Patent Application Laid-Open No. 2001-126765 discloses Disclosed is the use of phenyl and propane sultone together. They form polymer membranes through electrochemical polymerization at the anode during overcharging. The polymer film formed on the surface of the anode rapidly increases the impedance, and as a result, the battery is shut down, thereby suppressing the ignition / explosion accident.

However, these materials are known to degrade the lifespan characteristics of batteries. This is because some of these materials react even at normal charging and discharging rather than overcharging to generate irreversible capacity. For example, when biphenyl is used as a battery additive, battery performance after high temperature preservation has been found to decrease rapidly (J. Appl. Electrocehm, 33, 143 (2003)). In general, the lower the electrochemical polymerization initiation voltage of a material that prevents overcharging by electrochemical polymerization, the higher the overcharge protection effect is, but the degree of deterioration of the life characteristics and high temperature storage characteristics also increases. In addition, when propane sultone is used together with biphenyl to solve this problem, propane sultone is limited to use as an environmentally harmful substance.

Therefore, there is a need for a new method that can suppress the ignition / explosion of the battery effectively when overcharging without using environmentally harmful materials and also using various materials, and at the same time, does not degrade the life characteristics and high temperature storage characteristics. High situation.

The present invention aims to solve the problems of the prior art as described above and the technical problems that have been requested from the past.

After extensive research and various experiments, the inventors of the present invention carry a material capable of increasing the internal resistance or impedance of an electrolyte by electrochemical polymerization when the lithium secondary battery is in an overcharged state, as described later. A thermally degradable capsule was prepared, and the lithium secondary battery manufactured by including the capsule in an electrolyte solution and / or an electrode was used in an overcharge state without degrading the performance of the battery without using environmentally harmful materials and using various materials. The risk of ignition / explosion has been found to be significantly reduced, and the present invention has been completed.

The lithium secondary battery according to the present invention for achieving the above object is a lithium secondary battery including a positive electrode, a negative electrode and a separator, the internal resistance of the battery by electrochemical polymerization in an abnormal operating state of the battery by the overcharge of the battery Or it consists of including the thermally degradable capsule which carries the substance which can raise an impedance in electrolyte solution and / or electrode of a battery.

As described above, the "abnormal operating state of the battery due to overcharging" means that the charge is performed at a voltage higher than the rated voltage of the battery. As described above, the temperature of the battery is continuously increased and the electrode is chemically unstable and reacts with the electrolyte. This is an active state in which the thermal runaway phenomenon occurs actively.

The rated voltage of the battery varies depending on the type of battery, for example, in the case of a lithium cobalt secondary battery, the rated voltage is set in the range of approximately 3 to 4.2V. Therefore, the thermally degradable capsule according to the present invention acts in the abnormal operating state as described above as charging is performed at a voltage above this rated voltage, thereby suppressing ignition and explosion of the battery.

The "material which can raise the internal resistance or impedance of a battery by electrochemical polymerization" is a substance in which a polymerization reaction occurs by electrochemical reaction at the voltage at the time of overcharging, and the product of such a polymerization reaction acts inside the battery. Means a material that raises the internal resistance or impedance of the battery. Hereinafter, such materials are sometimes referred to as overcharge inhibitors.

Since the voltage at which the overcharge inhibitor performs the electrochemical polymerization depends on the kind of material, a material exhibiting a polymerization start threshold voltage of about 4.5 V or more, preferably 5 V or more may be used.

In the prior art in which the overcharge inhibitor is added directly to the electrolyte, the additive is defined as a substance which performs the polymerization reaction at the threshold voltage without affecting the operation of the battery in the normal operation state. Since it is added in a state supported in a capsule, it does not have the same limitation as above. Thus, any material that can be electrochemically polymerized at such a threshold voltage can be used as any type of material that is known in the art or in the chemical art or can be synthesized later as a material having such properties. Examples of known overcharge inhibitors include, but are not limited to, biphenyl, terphenyl, cyclohexyl benzene, and the like. In some cases, two or more materials may be supported together in one capsule, or two or more materials may be added to the electrolyte while supported in different capsules.

The "thermally degradable capsule" refers to an outer shell structure, that is, a cell (i.e., a cell structure) in which an overcharge inhibitor contained therein can be exposed to the outside by being decomposed, dissolved, melted or softened in an abnormal operating state of the battery as defined above. means a shell). As described above, the overcharging raises the temperature of the battery, and such a rise in temperature causes a vicious cycle that triggers the reaction between the electrode and the electrolyte and raises the temperature again, so that the overheating is inevitable. Thus, the thermally degradable capsule does not cause side reactions to the components of the cell under normal operating conditions, but in the above abnormal operating conditions, the substance decomposes or dissolves by chemical reaction, the material having a melting point, and the softened glass. It may be made of a material having a transition temperature (Tg) and the like.

In one preferred embodiment, the thermally degradable capsule is a material carrying an overcharge preventing agent therein and is not reactive with the electrolyte solution, consisting of a polymer or oligomer having a melting point or glass transition temperature (Tg) of 80 ~ 200 ℃, It may be a spherical material having a diameter smaller than the pores of the separator. The preferred diameter of such capsules is 5 μm or less.

The thickness of the capsule and the amount of the overcharge preventing agent carried in one capsule (unit capsule) are not particularly limited, and may be determined by various factors such as an operating temperature to be set, a substance to be used, and a part of a battery including the capsule.

Capsules according to the invention may be included in the electrolyte, may be included in the electrode (anode and / or cathode), may be included in both the electrolyte and the electrode. The case contained in the electrode includes both the case mixed with the inside of the electrode active material and the case adhering to the surface. Preferably, the capsule is included in the electrolyte solution.

In-cell content of the capsule according to the present invention is such that the internal resistance or impedance of the battery may be increased by electrochemical polymerization in an abnormal operating state of the battery due to overcharging, and does not affect the performance of the battery in a normal operating state. If so, it is not particularly limited.

In the present invention, it is not possible to clearly explain the reactor charge which raises the internal resistance or the impedance of the battery by the electrochemical polymerization of the overcharge inhibitor, but for example, biphenyl as the overcharge inhibitor may be used in an abnormal operating state as described above. It is known to form a passivation film on the positive electrode by electrochemical polymerization to rapidly increase the internal resistance of the battery.

The present invention also provides lithium secondary batteries of the following modified examples.

As a first variant, the electrolyte and / or the electrode contains an initiator that does not affect the operation mechanism of the battery, and the monomer that can be polymerized by the initiator is contained in the thermally degradable capsule. It provides a lithium secondary battery contained in. Other configurations of the lithium secondary battery are the same as described above.

As a second variant, it provides a lithium secondary battery in which two or more kinds of monomers capable of condensation reaction are contained in an electrolyte solution and / or an electrode, respectively supported in separate thermally degradable capsules. Other configurations of the lithium secondary battery are the same as described above.

As a third modified example, in order to minimize the effect on the operating mechanism of the battery to provide a lithium secondary battery that is contained in the electrolyte solution and / or electrode in a state in which the monomer and the polymerization initiator of the monomer are each supported in a separate capsule. Other configurations of the lithium secondary battery are the same as described above.

Capsules according to the present invention can be prepared by a variety of known methods capable of preparing microcapsules.

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, and a lithium salt-containing nonaqueous electrolyte, a thermally degradable capsule contained in the electrolyte and / or the electrode.

The positive electrode is prepared 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 to the mixture.

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 (where 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 wt% based on the total weight of the mixture including the positive electrode active material. 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 generally added in an amount of 1 to 50 wt% based on the total weight of the mixture including the positive electrode active material. 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 cathode, 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 consists of a nonaqueous electrolyte and lithium. As the nonaqueous electrolyte, a nonaqueous electrolyte, a 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., for example, pyridine, triethyl phosphite, triethanolamine, cyclic ether, ethylene diamine, n-glyme, hexaphosphate triamide, etc. 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, a halogen-containing solvent such as carbon tetrachloride and ethylene trifluoride may be further included, or 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.

As described above, the lithium secondary battery according to the present invention includes a thermally degradable capsule carrying an overcharge preventing agent in the electrolyte solution and / or the electrode, and does not affect the performance of the battery in a normal operating state of the battery. In an abnormal operating state of the battery due to overcharging, in particular, an abnormal operating state after high temperature preservation, the internal resistance or impedance of the battery may be sharply increased to significantly reduce the risk of fire / explosion of the battery. In addition, since the overcharge preventing agent is included in the electrolyte or the electrode supported on the capsule, various kinds of materials can be used, and there is an advantage that the desired effect can be achieved without using environmentally harmful substances.

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 (10)

A lithium secondary battery comprising a positive electrode, a negative electrode, and a separator, a thermally degradable capsule containing a material capable of increasing internal resistance or impedance of a battery by electrochemical polymerization in an abnormal operating state of the battery due to overcharging of the battery. The lithium secondary battery comprised by including in the electrolyte solution and / or the electrode of a battery. The lithium secondary battery according to claim 1, wherein the voltage at which the overcharge inhibitor performs electrochemical polymerization is 4.5 V or more. The lithium secondary battery of claim 1, wherein the voltage is 5 V or more. The lithium secondary battery according to claim 1, wherein the overcharge preventing agent is one or two or more selected from the group consisting of biphenyl, terphenyl, and cyclohexyl phenyl. The method according to claim 1, wherein the thermally degradable capsule is a material that does not contain an overcharge preventing agent therein and is not reactive with the electrolyte solution, made of a polymer or oligomer having a melting point or glass transition temperature (Tg) of 80 ~ 200 ℃, Lithium secondary battery, characterized in that the spherical material having a diameter smaller than the pores of the separator. The lithium secondary battery of claim 4, wherein a diameter of the capsule is 5 μm or less. The lithium secondary battery of claim 1, wherein the capsule is contained in an electrolyte. A lithium secondary battery comprising a positive electrode, a negative electrode, and a separator, in which an initiator that does not affect the operation mechanism of the battery is included in the electrolyte solution and / or the electrode, and monomers which may be polymerized by the initiator are in an abnormal operating state of the battery. Lithium secondary battery contained in the electrolyte and / or the electrode supported on the thermally degradable capsule to decompose. A lithium secondary battery comprising a positive electrode, a negative electrode, and a separator, in which two or more kinds of monomers capable of condensation reaction are respectively supported in separate thermally degradable capsules decomposed in an abnormal operating state of the battery. And / or a lithium secondary battery contained in the electrode. A lithium secondary battery comprising a positive electrode, a negative electrode, and a separator, wherein a lithium secondary battery is included in an electrolyte solution and / or an electrode in a state in which a monomer and a polymerization initiator of the monomer are respectively supported in separate capsules decomposed in an abnormal operating state of the battery. battery.