The present invention relates to a lithium secondary battery; and, in particular, the invention relates to a rechargeable lithium secondary battery, which is improved in safety by having a self imposed safety function, to an electrolyte for the lithium secondary battery, and to an electric apparatus using the same.
A lithium secondary battery has a high voltage, a high energy density, and superior storage performance and repeat charge-discharge characteristics. Therefore, the lithium secondary battery is being used widely for portable electric consumer products. Furthermore, research and development for utilizing lithium secondary batteries as power sources, such as for electric vehicles and home power storage devices which provide power during the night, by developing batteries of increased size is being performed intensely. The lithium secondary battery is a product which is expected to be used widely in daily life as a clean energy source, and which can be expected to have a significant advantage in preventing environmental pollution and the warming-up of the earth from the release of carbon dioxide.
However, a flammable organic solvent is currently used in the battery in view of its reactivity with lithium and a restriction of the potential window. Therefore, if the temperature of the battery is elevated by any means, such as overcharging or exterior heating, the electrolyte causes a thermal runaway reaction and generates a flammable gas causing an increase in the internal pressure of the battery. The gas is released to the outside the battery can and causes an ignition or, in the worst case, an explosion. Therefore, it can not be emphasized too much that how widely the battery is used in the above objects depends on the extent its safety can be ensured. A carbonate group is generally used for the lithium battery, which uses carbon material for its negative electrode, because the carbonate group exhibits preferable battery characteristics. In particular, five membered ring compounds, such as ethylene carbonate and 1,2-propylene carbonate, are employed as a main solvent and are utilized as an indispensable solvent, because these compounds have a high dielectric constant, and readily dissociate lithium salts. These compounds cause a degradation reaction indicated by the following chemical equation (Equation 1), and generate a combustible gas, when they are heated or overcharged. 
The internal pressure of the battery is increased by the combustible gas, the combustible gas is released from the battery can, and, in the worst case, an ignition and explosion are caused.
A method of preventing the ignition and explosion of the battery has been disclosed in JP-A-6-290793 (1994); wherein a solvent, which causes a polymerization reaction with LiPF6, i.e., a lithium salt, is mixed as an electrolyte solvent, in order to make sure that the electrolyte will cause no decomposition reaction, but will produce a polymerization reaction when the temperature of the battery is elevated. JPA-6-283206 (1994) and JP-A-9-45369 (1997) disclose methods for solidifying the electrolyte by providing microcapsules, which contain a polymerization initiator and polymerizable material therein, in the electrolyte, in a separator, and the like, whereby these materials are released from the microcapsules to cause a polymerization reaction when the temperature of the battery is elevated.
In accordance with JP-A-6-290793 (1994), the solvent, which causes a polymerization reaction with LiPF6 is restricted, and mixing one of the compounds in a cyclic ether group is indispensable. However, if the battery is composed of a system wherein the use of the compound in the cyclic ether group is not desirable in view of the battery characteristics, the compound in the cyclic ether group can not be used. A result of analyzing the heat generating behavior of an electrolyte solvent, made by mixing ethylene carbonate (EC) and ethylmethyl carbonate (EMC) in a 1:1 ratio, which is one of the carbonate group solvents exhibiting desirable battery characteristics with a carbon negative electrode, using differential scanning calorimetry (DSC), indicates that the solvent alone does not exhibit a large heat generation. However, a rapid reaction is indicated near 250xc2x0 C. for the electrolyte dissolving LIPF6 at one mol/liter, the carbonate solvent is decomposed, and a combustible gas is generated. As a result of analyzing an infrared spectrum of the specimens after the above test, it was found that an absorption based on a carbonyl radical of the carbonate molecule still remained at 1700 cmxe2x88x921 with the specimen of the solvent alone. On the contrary, the absorption disappeared with the specimen of the electrolyte dissolving LIPF6 at one mol/liter. That means that the reaction indicated previously by the equation 1 has proceeded, and generation of lithium carbonate and ethylene gas could be observed. Accordingly, LIPF6 can not be used effectively as the polymerization initiator in a system using a carbonate solvent as a main solvent.
In a case where microcapsules are used, as disclosed in JP-A-6-283206 (1994) and JP-A-9-45369 (1997), the temperature at which the polymerization initiator and the polymerizable material are released can be controlled based on the material forming the wall of the capsule. However, using a large amount of the microcapsules in a battery is difficult in view of the need to maintain desirable battery characteristics. It is difficult to interrupt propagation of the thermal runaway reaction with dispersed capsules, if the polymerization reaction does not proceed with a significantly rapid reaction rate, because the reaction will be generated locally with a microscale.
One of the objects of the present invention is to provide a lithium secondary battery, which is capable of terminating functions of the battery safely when any of an overcharge, an overdischarge, or an abnormal temperature rise condition occurs, without an accompanying rapid change in appearance, gas generation, or pressure change, and to provide its electrolyte and an electric apparatus using the same as a power source.
The present invention is characterized by the provision of a lithium secondary battery comprising a negative electrode which is capable of absorbing and desorbing lithium; a positive electrode which is capable of absorbing and desorbing lithium; and an aprotic organic electrolyte, wherein the aprotic organic electrolyte can be solidified by a thermal reaction at a designated temperature. The aprotic organic electrolyte comprises a lithium salt and a non-aqueous solvent; and, the non-aqueous solvent is provided in an amount sufficient to dissolve the lithium salt, and comprises a thermally polymerizable non-aqueous solvent. The content of the non-aqueous solvent, which can dissolve the lithium salt, is in the range of 50-95% by volume, desirably in the range of 65-90% by volume; and, the content of the thermally polymerizable solvent is in the range of 5-50% by volume, and, desirably, it is in the range of 10-35% by volume. The aprotic organic electrolyte can be solidified by a thermal reaction at a designated temperature.
The present invention relates to a lithium secondary battery comprising a negative electrode which is capable of absorbing and desorbing lithium; a positive electrode which is capable of absorbing and desorbing lithium; and an aprotic organic electrolyte, wherein its functions can be terminated safely in a non-returned condition without an accompanying rapid change in appearance, gas generation, or pressure change, particularly a pressure increase, when any of an overcharge, an overdischarge, or an abnormal temperature rise condition occurs.
The present invention also relates to an electrolyte for lithium secondary batteries, the electrolyte being characterized as comprising a lithium salt and a non-aqueous solvent, which pan dissolve the lithium salt, which electrolyte can be solidified by a thermal reaction at a designated temperature.
The present invention further relates to an electric apparatus, which is characterized in that the above described lithium secondary battery is used therein as an electric power source.
In accordance with the present invention, the electric apparatus using the lithium secondary battery as a power source can be free of a protecting means, such as a device for measuring the temperature and pressure of the battery to detect any of an overcharge, an overdischarge, or an abnormal temperature rise condition. The electric apparatus is characterized in that it has only a means for detecting the voltage or the current of the battery and a controlling means for switching the power source based on the above detected values; and, when any of the above abnormal conditions of the secondary battery itself occur, the functions of the battery can be terminated safely in a non-reversible manner without causing damage to the appearance of the battery.
The above described electric apparatus, to which the present invention is applicable, includes electric vehicles, electric power storage devices, and so on.
In accordance with the present invention, a carbonate solvent having superior battery characteristics can be used, such that most of the electrolyte is polymerized and solidified at 100xc2x0 C. or higher in order to make the battery inactive and safe just before causing a degradation of the solvent by thermal runaway with reactions with the positive electrode and the negative electrode. That is, in order to solidify the electrolyte solvent rapidly in a short time by heating, it is advantageous to maintain the reaction initiator in a condition to be dissolved in the electrolyte. In this case, the reaction initiator must be inactive with the electrolyte at room temperature, stable electrochemically in a designated range of operation voltages, and reactive with the solvent at a temperature lower than the temperature for causing reactions with the charged positive electrode and the charged negative electrode. That is, the problem can be solved by mixing a thermal reaction type solvent, which is usable in a dissolved condition, with the carbonate group solvent in a range, wherein a battery characteristics are not deteriorated.
The above object of the invention can be achieved by making an appropriate polymerization initiator coexist at approximately 100xc2x0 C. with a six membered ring carbonate, which can be polymerized by anion polymerization, or cation polymerization; or, the object of the invention can be achieved by making an appropriate polymerization initiator coexist with at least a seven membered ring sulfite, which is known to be capable of causing a polymerization without de-sulfur dioxide. Linear diphenylcarbonate derivatives also operate as polymerization initiators. That is, any one of diphenylcarbonate derivatives, at least six membered ring carbonate derivatives, and at least seven membered ring sulfite derivatives can be used by co-dissolving them with an electrolyte of the carbonate group solvent. The object of the invention can also be achieved by using the polymerization initiator in a dissolved condition.
As an aprotic organic electrolyte, organic solvents dissolving a lithium salt as an electrolyte and their derivatives can be used, particularly, five or less-membered cyclic compounds are desirable. That is, as for an organic solvent, most of them are thermally polymerizable, but the solvents which can generate combustible gases by a thermal decomposition with the addition of a lithium salt are desirable; practically, organic solvents, such as ethylene carbonate, propylene carbonate, butylene carbonate, pentylene carbonate, hexylene carbonate, heptalene carbonate, octalene carbonate, dimethyl carbonate, diethyl carbonate, dipropyl carbonate, dibutyl carbonate, dipentyl carbonate, dihexyl carbonate, diheptyl carbonate, dioctyl carbonate, methyethyl carbonate, methylpropyl carbonate, methylbutyl carbonate, methylpentyl carbonate, methylhexyl carbonate, methylheptyl carbonate, methyloctyl carbonate, ethylpropyl carbonate, ethylbutyl carbonate, ethylpentyl carbonate, ethylhexyl carbonate, ethylheptyl carbonate, ethyloctyl carbonate, propylbutyl carbonate, propylpentyl carbonate, propylhexyl carbonate, propylheptyl carbonate, propyloctyl carbonate, butylpentyl carbonate, butylhexyl carbonate, butylheptyl carbonate, butyloctyl carbonate, pentylhexyl carbonate, pentylheptyl carbonate, pentyloctyl carbonate, hexylheptyl carbonate, hexyloctyl carbonate, heptyloctyl carbonate, dioxolane, xcex3-butylolactone, tetrahydrofuran, 2-methyl tetrahydrofuran, and their halogenated derivatives, and lactone derivatives, lactam derivatives, phosphoric acid ester derivatives, phosphazene derivatives, and the like, may be used.
As for the thermal reactive solvents, or a thermally polymerizable non-aqueous solvent, six or more-membered cyclic organic compounds are desirable. Practically, a 6- to 10-membered cyclic carbonate, such as the following compounds can be used: 1,3-propylene carbonate, 1,3-butylene carbonate, 1,4-butylene carbonate, 1,5pentylene carbonate, 1,6-hexylene carbonate, 1,7-heptylene carbonate, 1,8-octylene carbonate, and their alkyl substituted derivatives, allyl substituted derivatives, aromatic substituted derivatives, nitro substituted derivatives, amino substituted derivatives, halogen substituted derivatives, diphenyl carbonate, di(nitrophenyl) carbonate, di(methylphenyl) carbonate, di(methoxyphenyl) carbonate, di(aminophenyl) carbonate.
Furthermore, 7 to 11-membered cyclic sulfate compounds such as the following compounds can be used: 1,4-butylene sulfate, 1,5-pentylene sulfate, 1,6-hexylene sulfate, 1,7-heptylene sulfate, 1,8-octylene sulfate, and their alkyl substituted derivatives, allyl substituted derivatives, aromatic substituted derivatives, nitro substituted derivatives, amino substituted derivatives, halogen substituted derivatives, and the like.
The thermal reactive organic solvent of the present invention can solidify the electrolyte by thermal polymerization of the organic solvent itself. However, the non-aqueous solvent itself, which can dissolve the lithium salt, can be thermally polymerized, and the whole body can be solidified.
The thermal reaction initiator for the non-aqueous solvent is an additive for decreasing the initiation temperature of the thermal polymerization; and, practically, the following can be used: iodine, lithium iodide, lithium fluoride, lithium bromide, lithium chloride, tetrakis(4-fluorophenyl) sodium borate, tetrakis(4-fluorophenyl) lithium borate, isoazobutylnitrile, 1,1xe2x80x2-azobis(cyclohexane-carbonitrile), 2,2xe2x80x2-azobis(2-methyl-N-(1,1-bis(hydroxymethyl)ethyl) propionamide, methyl iodide, benzene bromide, tetrabutyl ammonium iodide, trifluorodiethyl borate, triester phosphate, and others.
The kind and content of the reaction initiator is selected so that the polymerization and solidification reaction is initiated desirably at least at 120xc2x0 C. in accordance with the temperature rise caused by an overcharge, or an overdischarge, or a temperature rise based on an external environmental condition. Further, the temperature is desirably at least 100xc2x0 C., and preferably at least 80xc2x0 C. Particularly, the reaction initiator, which is solidified by heating at 150xc2x0 C. for 10 minutes and is not ignited in a safety test, is desirable. Therefore, as the electrolyte, a non-aqueous solvent which can be thermally polymerized at a temperature in the range of 100-150xc2x0 C. is desirable. The content of the reaction initiator is desirably in the range of 0.5-10% by weight to the total weight of the electrolyte, and more desirably it is in the range of 1-5% by weight.
In accordance with the present invention, at least one of the current collectors of the negative electrode and the positive electrode desirably has a metallic layer, such as nickel plating and the like, which is made of a harder metal than the base metal of the current collector, on a roughened surface of the current collector.
In accordance with the present invention, at least one of the negative electrode active material and the positive electrode active material has graphite, and the graphite is desirably composed of 20% by weight or less of rhombohedral crystal and 80% by weight or more of hexagonal crystal.
In accordance with the present invention, a lithium secondary battery comprising a negative electrode having a negative electrode active material, which absorbs or desorbs lithium ions during a charging or a discharging period, on a surface of the current collector made of a thin metallic plate; a positive electrode having a positive electrode active material on the surface of the current collector made of a thin metallic plate; and a lithium ion conductive aprotic organic electrolyte or a polymer electrolyte, is desirably treated by a process, wherein an oxide layer composed of oxide whiskers are formed on the surface of the current collector, and subsequently the oxide layer is reduced for roughening the surface, before forming the respective active material on the surface of the at least one of the current collectors of the negative electrode and the positive electrode.
The active material is desirably formed on the surface of the current collectors composed of a thin metallic plate of at least one of the negative electrode and the positive electrode, after manufacturing the current collector to a desired thickness by cold milling and roughening the manufactured surface by the method previously described.
In accordance with the present invention, a current collector having its surface roughened by the method described previously is desirably used. That is, deterioration of the battery characteristics caused by the condition of a negative electrode current collector made of copper is mainly based on a decrease in the adhesiveness of a negative electrode current collector with the negative electrode active material. Therefore, the battery characteristics can be improved by increasing the adhesiveness of the current collector. Accordingly, a positive electrode, current collector having the same surface as the negative electrode current collector is desirable.
Since the positive electrode active material and the negative electrode active material are generally particles of 100 xcexcm or less in diameter, the above object of the invention can be achieved by improving the adhesiveness of the particles with the materials of the current collector, such as aluminum or copper.
When particles are adhered to a metal, it is effective when the surface of the metal, whereon the particles are to be adhered, is previously treated with various processes, such as a process for forming an oxide on the surface of the metal; a process for reducing a part of or all of the above oxide by a chemical method or an electrical method; or further a process for nickel plating. The copper surface treated as indicated above is in a roughened condition in comparison with the condition before the treatment. The surface of the copper without treating it with the nickel plating does not have the metallic luster of copper, but has a color of dark brown or black due to optical scattering based on the roughened surface. As a method of causing the particles to adhere onto the copper surface which has been roughened, a method comprising the steps of applying a mixture of the particles and a resin onto the roughened surface of the copper, and pressure welding and heating can be used. As another method, a method comprising the steps of applying a slurry formed by mixing the particles with a solvent dissolving a resin onto the roughened surface, and pressure welding and heating can be used. In this case, the pressure welding and the heating can be performed sequentially or concurrently, but the advantages of the present invention can be achieved similarly in either case. The metal, the surface of which has been roughened, is improved in adhesiveness with the particles, but in particular, the ratio of effective surface area to apparent surface area is desirably at least 2. For instance, in a case of a metallic foil 100 mm square and 20 xcexcm in thickness, the apparent surface area of the two planes is 20,000 mm2. When both planes of the metallic foil having the apparent surface area of S (mm2) are treated for roughening by the above method, the apparent surface area still remains as S (mm2). The weight of the roughened metal foil is assumed to be M (g). A specific surface area of the roughened metal foil determined by a BET method is assumed to be xcfx81(mm2/g) Then, the effective surface area obtained from the specific surface area is expressed by xcfx81xc3x97M (mm2). Therefore, the ratio of the effective surface area/apparent surface area is expressed by (xcfx81xc3x97M)/S (mm2/g).
The negative electrode current collector relating to the present invention desirably has an effective surface area which is at least two times the apparent surface area, more desirably at least three times, and preferably at least four times the apparent surface area for obtaining stable characteristics. The upper limit is desirably 30 times, more desirably less than 20, and preferably less than 15. The thickness of the metallic foil of the current collector is desirably in the range of 5-30 xcexcm, and more desirably it is in the range of 8-20 xcexcm.
The metallic foil of the current collector is made of aluminum for the positive electrode and is made of copper for the negative electrode. The metallic foil for surface roughening according to the present invention is desirably manufactured by the steps of roughening the surface of the metallic foil as it is rolled, applying a positive active material or a negative active material onto the surface of the metallic foil in a condition of enhanced surface strength, and fabricating by pressing. Although annealing may be performed after the rolling, its surface hardness is desirably adjusted in connection with the annealing temperature using a method of fabrication by pressing.
The aprotic organic electrolyte secondary battery uses a metallic foil, the surface of which is treated using a method comprising at least the steps of forming an oxide on a surface of the metal, reducing a part of or all of the oxide by a chemical method or an electrical method, and more desirably performing a nickel plating. The positive electrode current collector or the negative electrode current collector, for purposes of improving the adhesiveness of the current collectors with positive electrode active material or negative electrode active material, has preferable charge-discharge cycle characteristics, because neither a falling out nor a break away of the positive electrode active material or the negative electrode active material occurs as a result of the charge-discharge operation.
In order to strengthen the surface of the base metal of the current collector, a metallic film is desirably formed on the surface of the base metal by plating after roughening the surface. The metallic film is desirably made of a metal such as cobalt, nickel, and the like, which is flexible and harder than the base metal. A metallic film is desirable from the point of view of increasing the adhesiveness in forming the positive electrode active material and the negative electrode active material by preventing flattening during the fabrication with pressing, and for purposes of increasing the corrosion resistance of the surface of the aluminum and copper. The thickness of the metallic film is desirably in the range of 0.01-1 xcexcm.
The negative electrode active material can be in the form of particles, which are capable of absorbing and desorbing lithium ions, and examples of such materials are as follows: graphite group, amorphous carbon group, pyrocarbon group, cokes group, carbon fiber, metallic lithium, lithium alloys.(Lixe2x80x94Al, Lixe2x80x94Pb, etc.), inorganic compounds (carbide, oxide, nitride, boride, halide, intermetallic compounds, etc.), and intermetallic particle compounds such as aluminum, tin, and the like.
The above materials, other than the metals, have desirably an average particle diameter in the range of 5-30 xcexcm, and in particular, are preferably in the range of 10-20 xcexcm. Because small particles are harmful to the characteristics, the minimum size of the particles is 5 xcexcm or more, and the maximum size is 50 xcexcm or less. The metallic powder is effective to increase the conductivity of the film, and its average diameter is desirably in the range of 0.1-100 xcexcm, and more desirably in the range of 1-50 xcexcm. The graphite desirably contains rhombohedral crystal of 20% or less by weight, and, in particular, a range of 5-15% by weight is desirable.
For positive electrode active material, a complex oxide such as, lithium.cobalt oxide (LixCoO2), lithium nickel oxide (LixNiO2), lithium manganese oxide (LixMn2O4, LixMnO3) lithium nickel cobalt oxide (LixNiy, Co(1xe2x88x92y)O2), and the like, is usable. The above materials have desirably an average particle diameter in the range of 5-30 xcexcm, and, in particular, the same size as the negative electrode active material, other than metals, is desirable.
For the separator, a microporous polymer resin film such as nylon, cellulose, nitrocellulose, polysulfone, polyacryronitrile, polyvinylidene fluoride, polypropylene, polyethylene, and polyolefin group can be used.
For the electrolyte, lithium hexafluorophosphate (LiPF6), LiBF4, LiClO4, and the like are used. The content of these materials is desirably in the range of 0.2-5 mol/l and more desirably in the range of 0.5-3 mol/l.
For a conducting material used as the negative electrode active material or the positive electrode active material, flaky graphite, massive amorphous carbon, and massive graphite are desirable. Their average particle diameter is desirably equal to or less than 10-30 xcexcm, and the specific surface area is desirably in the range of 2-300 m2/g, more desirably, it is in the range of 15-280 m2/g.
Short carbon fiber 5-10 xcexcm in diameter and 10-30 xcexcm in length can be used desirably. In particular, the massive graphite has a preferable adhesiveness.
The negative electrode active material or the positive electrode active material contains a resin of 2-20% by weight, and is combined to the surface of the current collector by the resin. For the resin, polyvinylidene fluoride is used.
The aprotic organic electrolyte secondary battery relating to the present invention has a negative electrode current collector, the surface of which is appropriately roughened, and the adhesion strength of the current collector with the negative electrode mixture containing the negative electrode active material and the resin can be increased, because an anchor effect of the roughened surface of the negative electrode current collector is larger in comparison with that of a negative electrode current collector having a smooth surface.
Therefore, falling out and break away of the negative electrode mixture which tend to occur during expansion and shrinkage of the negative electrode active material during a charge-discharge operation can be prevented, and charge-discharge cycle characteristics of the aprotic organic electrolyte secondary battery can be improved.
The lithium batteries relating to the present invention can be formed in various shapes, such as a cylindrical shape, a coin shape, a rectangular shape, and the like, and they can be used in various portable electronic apparatus having a rating from several watt-hours to hundreds of watt-hours. In particular, the lithium batteries can be used for notebook type personal computers, note type word processors, palm top (pocket) personal computers, portable telephones, PHSS, portable facsimiles, portable printers, headphone stereo players, video cameras, portable TVs, portable CDs, portable MDs, electric shavers, electronic note books, transceivers, electric tools, radios, tape recorders, digital cameras, portable copiers, and portable game machines. And further, the lithium batteries can be used in electric vehicles, hybrid vehicles, automatic vending machines, electric carts, energy storage systems for load levelling, energy storage devices for home appliances, dispersed type energy storage systems (built in installed electric apparatus), energy supply systems for emergency, and others.