JP5704409B2 - Sealed lithium secondary battery - Google Patents

Description

  The present invention relates to a sealed lithium secondary battery (typically a sealed lithium ion battery). Specifically, the present invention relates to a sealed lithium secondary battery provided with a current interrupting mechanism that operates by increasing internal pressure.

  Lithium ion batteries and other lithium secondary batteries are smaller, lighter and have a higher energy density than the existing batteries, and are excellent in output density. For this reason, in recent years, it is preferably used as a so-called portable power source for personal computers and portable terminals, and a power source for driving vehicles.

  One form of such a battery is a sealed lithium secondary battery. The battery is typically mounted with a lid after an electrode body composed of positive and negative electrodes having a composite layer containing an active material is housed in a battery case together with an electrolyte (typically, an electrolyte). It is constructed by being sealed (sealed). Sealed lithium secondary batteries are generally used in a state where the voltage is controlled so as to be within a predetermined region (for example, 3.0 V or more and 4.2 V or less), but a current higher than usual is supplied to the battery due to an erroneous operation or the like. As a result, overcharging may occur beyond a predetermined voltage.

  Even in such a case, as a technology that can ensure safety, a current interruption device (CID: Current Interrupt Device) that cuts off a charging current when the pressure in the battery case becomes a predetermined value or more is widely used. Generally, when the battery is overcharged, the nonaqueous solvent of the electrolyte is electrolyzed and gas is generated. The current interruption mechanism can prevent further overcharging by cutting the charging path of the battery based on the gas generation. As a method for operating the mechanism more quickly, for example, a method of adding an inorganic compound to the positive electrode mixture layer can be mentioned. As such prior art, Patent Documents 1 to 3 contain a carbonate (lithium carbonate) in advance in the positive electrode mixture layer, so that the carbonate is decomposed during overcharge, and a large amount of carbon dioxide gas is generated. The method of obtaining is described.

  As another technique, the electrolyte contains a compound having a lower oxidation potential (that is, a lower voltage at which the oxidative decomposition reaction starts) than the nonaqueous solvent of the electrolyte (hereinafter referred to as “overcharge inhibitor”). There are known techniques to keep them. Such an overcharge inhibitor can be rapidly oxidized and decomposed on the surface of the positive electrode when the battery is overcharged to generate gas. And since the pressure in a battery case rises with the generated gas, an electric current interruption mechanism can operate | move rapidly. As this type of prior art, for example, Patent Documents 2 and 3 describe a method of adding an aromatic compound (such as cyclohexylbenzene (CHB) or biphenyl (BP)) as an overcharge inhibitor.

JP 2003-308443 A JP 2009-277397 A JP 2008-277106 A Japanese Patent No. 3032338

  However, in such a lithium secondary battery, when the use environment and / or the storage environment become a high temperature (for example, 50 ° C. to 60 ° C.), the viscosity of the electrolyte decreases, and the redox shuttle reaction of the aromatic compound is preferential. To occur. For this reason, the decomposition reaction of the overcharge inhibitor is suppressed, and the amount of gas generated is reduced. In particular, a large-sized (or large-capacity) battery used for a vehicle driving power source or the like has a relatively large space volume in the battery, and therefore requires a large amount of gas to operate the current interrupting mechanism. However, under such a high temperature environment, the gas generation amount is insufficient, so that the pressure in the battery case does not rise rapidly, and the operation of the current interrupt mechanism may be delayed.

  The present invention has been made in view of the above points, and an object of the present invention is a sealed lithium secondary battery including a current interrupting mechanism that operates due to a pressure increase in a battery case, and has high battery performance. It is an object of the present invention to provide a battery that is maintained and improved in reliability (typically, safety under a wide temperature environment) as compared with the conventional one.

In order to achieve the above object, there is provided a sealed lithium secondary battery in which an electrode body including a positive electrode and a negative electrode and an electrolyte are accommodated in a predetermined battery case. Here, the electrolyte includes an aliphatic carboxylic acid ester capable of generating carbon dioxide gas and an aromatic compound capable of generating hydrogen gas when a predetermined battery voltage is exceeded, and the battery described above. The case includes a current interrupting mechanism that operates when the pressure in the battery case increases with the generation of the carbon dioxide gas and / or hydrogen gas.
The battery has two types of overcharge inhibitors in the electrolyte. And as described in the test examples described later, the aromatic compound is violently oxidized and decomposed mainly in a low temperature to medium temperature environment (for example, 0 ° C. to 40 ° C.) to generate a large amount of hydrogen gas. On the other hand, aliphatic carboxylic acid esters are severely decomposed, particularly in a high temperature environment (typically 40 ° C. to 70 ° C., for example 50 ° C. to 60 ° C.), and generate a large amount of carbon dioxide. Therefore, by providing both in the battery case, a desired gas amount can be generated in a wide temperature environment (for example, 0 ° C. to 70 ° C.). Since the generation of gas causes a pressure increase in the battery case, the current interruption mechanism can be stably operated.

As a preferred embodiment of the sealed lithium secondary battery disclosed herein, the addition amount of the aliphatic carboxylic acid ester is 2% by mass or more and 15% by mass or less with respect to 100% by mass of the electrolyte. It is done.
When the addition amount of the aliphatic carboxylic acid ester is within the above range, it is possible to achieve both excellent battery performance and the effects of the present invention (that is, improvement in safety under a wide temperature environment) at a high level.

As a preferred embodiment of the sealed lithium secondary battery disclosed herein, the aliphatic carboxylic acid ester is at least one selected from the group consisting of methyl propionate, ethyl propionate, methyl butyrate and ethyl butyrate. It may be included.
The aliphatic carboxylic acid ester has high affinity with a non-aqueous solvent used as an electrolytic solution for a lithium secondary battery, and can be uniformly mixed in the electrolytic solution. Furthermore, since it has a low viscosity and a low boiling point compared to other non-aqueous solvents used in the battery, a battery with high battery performance (for example, low temperature characteristics and rapid charge / discharge characteristics) and high reliability can be obtained. Can do.
As a conventional technique, Patent Document 4 describes that methyl propionate is used as a solvent for an electrolytic solution. However, this conventional technique aims at improving low temperature characteristics, and the content of the methyl propionate is 25% by volume or more and 75% by volume or less (relative to 100% by volume of the solvent component in the electrolytic solution). Is described. That is, the purpose of using methyl propionate and the content are different from the technique disclosed here.

As a preferred embodiment of the sealed lithium secondary battery disclosed herein, the positive electrode includes a positive electrode current collector, and a positive electrode mixture layer including a positive electrode active material formed on the current collector. The positive electrode active material includes a lithium nickel cobalt manganese composite oxide.
The lithium nickel cobalt manganese composite oxide has high ion conductivity, and a battery using the oxide can achieve a high energy density. Moreover, since it is excellent also in thermal stability, it can use preferably.

As a preferred embodiment of the sealed lithium secondary battery disclosed herein, the lithium nickel cobalt manganese composite oxide is such that at least a part of nickel element, cobalt element and manganese element is substituted with zirconium element. Is mentioned.
The lithium nickel cobalt manganese composite oxide in which a transition metal element (ie, Ni, Co and Mn) as a constituent element is partially substituted with a zirconium element (Zr) has excellent crystal stability and thermal stability. In a battery using as a positive electrode, high battery performance (for example, retention characteristics in a high temperature environment) can be exhibited.

As a preferable aspect of the sealed lithium secondary battery disclosed herein, the amount of the zirconium element is set to 0.000% with respect to a total of 100 mass% of the zirconium element, the nickel element, the cobalt element, and the manganese element. It is mentioned that they are 1 mass% or more and 1.0 mass% or less.
When the substitution amount of zirconium is in the above range, the crystal stability and thermal stability are further improved. Therefore, in a battery using the material for the positive electrode, excellent battery performance (for example, retention characteristics in a high temperature environment described later) and the effect of the present invention (that is, improvement in safety in a wide temperature environment) are obtained. It is possible to achieve both at a higher level.

As a preferable embodiment of the sealed lithium secondary battery disclosed herein, the aromatic compound includes cyclohexylbenzene or biphenyl.
Since cyclohexylbenzene and biphenyl have an oxidation potential of about 4.5 V to 4.6 V, for example, in a battery having an upper limit charging voltage of 4.1 V to 4.2 V, it is rapidly oxidized and decomposed when overcharged, and hydrogen gas is removed. Can occur. For this reason, a current interruption mechanism can be operated more rapidly.

As a preferred embodiment of the sealed lithium secondary battery disclosed herein, the amount of the compound capable of generating hydrogen gas is 0.5% by mass to 7% by mass with respect to 100% by mass of the electrolyte. There are some.
In the technology disclosed here, the reaction efficiency of the overcharge preventing agent is high, so that the amount of gas necessary for reliably operating the current interrupting mechanism even at high temperatures is reduced with a smaller amount of overcharge preventing agent than in the past. Can be obtained. Therefore, excellent battery performance and the effects of the present invention (that is, improvement in safety under a wide temperature environment) can be achieved at a higher level.

As a preferable aspect of the sealed lithium secondary battery disclosed herein, the electrode body includes a positive electrode mixture layer having a predetermined width on a long positive electrode current collector, and a longitudinal direction of the current collector. A long positive electrode and a long negative electrode current collector formed on a long negative electrode current collector along a longitudinal direction of the current collector. A negative electrode in the form of a laminated electrode, and the positive electrode and the negative electrode are laminated and wound in the longitudinal direction.
Since a battery including a wound electrode body has a high capacity among lithium secondary batteries, it is particularly important to improve reliability (for example, safety measures in the case of malfunction). According to the technology disclosed herein, the safety of such a battery (for example, safety when an overcharge or an internal short-circuit occurs) can be improved as compared with the related art.

A preferred embodiment of the sealed lithium secondary battery disclosed herein is that the upper limit charging voltage is 3.0V to 4.1V.
The aliphatic carboxylic acid ester used here has a relatively low oxidation potential and depends on the solvent used, but is oxidatively decomposed at about 4.2 V to produce carbon dioxide. Therefore, by setting the upper limit voltage (operation upper limit voltage) of the battery within the above range, oxidative decomposition (generation of carbon dioxide) during normal use is suppressed, and only when necessary (that is, during overcharge). Gas can be generated effectively.

Further, there is provided a vehicle including the sealed lithium secondary battery disclosed herein as a driving power source.
The above-described battery has improved reliability (typically safety under a wide range of temperature environments), so it is a large-capacity, high-output large-sized battery, and the use and / or storage environment is at a high temperature. It is especially effective when it can be. Therefore, for example, a power source (drive power source) for driving a motor mounted on a vehicle (typically, an electric motor such as a plug-in hybrid vehicle (PHV), a hybrid vehicle (HV), or an electric vehicle (EV)). Can be suitably used.

1 is a perspective view schematically showing an outer shape of a sealed lithium secondary battery according to an embodiment of the present invention. It is a figure which shows typically the cross-sectional structure in the II-II line | wire of the sealed lithium secondary battery of FIG. It is a schematic diagram which shows the structure of the winding electrode body of the sealed lithium secondary battery based on one Embodiment of this invention. It is a side view which shows the vehicle (automobile) provided with the sealed lithium secondary battery based on one Embodiment of this invention. It is a graph which shows the gas generation amount at the time of overcharge, (A) shows the result of Example 1, (B) shows the result of Example 10, respectively.

In the present specification, the “lithium secondary battery” refers to a secondary battery that uses lithium ions as electrolyte ions and is charged / discharged by movement of charges accompanying the lithium ions between the positive and negative electrodes. A secondary battery generally referred to as a lithium ion battery (or a lithium ion secondary battery), a lithium polymer battery, or the like is a typical example included in the lithium secondary battery in this specification. Further, in this specification, the “active material” refers to a substance (compound) involved in power storage on the positive electrode side or the negative electrode side. That is, it refers to a substance that is involved in the insertion and extraction of electrons during battery charge / discharge.
In the present specification, the overcharge state refers to a state in which a charge depth (SOC: State of Charge) exceeds 100%. The SOC is a charge state (that is, a fully charged state) in which an upper limit voltage is obtained in an operating voltage range that can be reversibly charged and discharged, and a charge state (that is, a lower limit voltage is obtained). , The state of not being charged) shows the state of charge when 0%.

  Hereinafter, preferred embodiments of the sealed lithium secondary battery disclosed herein will be described. Note that matters other than matters specifically mentioned in the present specification and necessary for implementation can be grasped as design matters of those skilled in the art based on the prior art in this field. The sealed lithium secondary battery having such a structure can be implemented based on the contents disclosed in the present specification and common technical knowledge in the field.

As the positive electrode of the lithium secondary battery disclosed herein, a positive electrode active material, a conductive material, a binder (binder), etc. are mixed in a suitable solvent to include a slurry (paste or ink). The composition (hereinafter referred to as “positive electrode mixture slurry”) was prepared, and the slurry was applied onto the positive electrode current collector to form a positive electrode mixture layer (also referred to as a positive electrode active material layer). Use the form.
As a method for preparing the positive electrode mixture slurry, the positive electrode active material, the conductive material, and the binder may be kneaded at a time, or may be kneaded stepwise in several steps. Although not particularly limited, the solid content concentration (NV) of the positive electrode mixture slurry is about 50% to 75% (preferably 55% to 65%, more preferably 55% to 60%). In addition, as a method of forming the positive electrode mixture layer, the positive electrode mixture slurry is applied to one or both surfaces of the positive electrode current collector with a conventionally known coating device (for example, a slit coater, a die coater, a comma coater, a gravure coater, etc.). A method of applying an appropriate amount using and drying can be preferably employed.

  As the material for the positive electrode current collector, a conductive member made of a metal having good conductivity is preferably used, as in the current collector used for the positive electrode of a conventional lithium ion secondary battery. For example, aluminum, nickel, titanium, stainless steel, or an alloy mainly containing them can be used. The shape of the current collector is not particularly limited because it can vary depending on the shape of the battery to be constructed, and a rod-like body, a plate-like body, a foil-like body, a net-like body, or the like can be used. In addition, in the battery provided with the winding electrode body mentioned later, a foil-like body is mainly used. The thickness of the foil-shaped current collector is not particularly limited, but about 5 μm to 50 μm (more preferably 8 μm to 30 μm) can be preferably used in consideration of the capacity density of the battery and the strength of the current collector.

As the positive electrode active material, one type or two or more types of materials conventionally used in lithium secondary batteries can be used without any particular limitation. For example, an oxide containing lithium and a transition metal element as constituent metal elements such as lithium nickel oxide (eg LiNiO ₂ ), lithium cobalt oxide (eg LiCoO ₂ ), lithium manganese oxide (eg LiMn ₂ O ₄ ) Examples thereof include lithium transition metal oxides), phosphates including lithium and transition metal elements such as lithium manganese phosphate (LiMnPO ₄ ) and lithium iron phosphate (LiFePO ₄ ) as constituent metal elements. Among them, a positive electrode active material (typically, a lithium nickel cobalt manganese composite oxide having a layered structure represented by the general formula: LiNiCoMnO ₂ (for example, LiNi _1/3 Co _1/3 Mn _1/3 O ₂ ) as a main component (typically Is a positive electrode active material substantially made of a lithium nickel cobalt manganese composite oxide), and has a high ion conductivity, and a battery using the oxide can realize a high energy density. Moreover, since it is excellent also in thermal stability, it can use preferably. Although not particularly limited, the proportion of the positive electrode active material in the entire positive electrode mixture layer is 50% by mass or more (typically 70% by mass or more and 100% by mass or less, for example, 80% by mass or more and 99% by mass or less). It is preferable that

The lithium nickel cobalt manganese composite oxide includes nickel, cobalt, and manganese as constituent metal elements other than lithium. In addition to these essential metal elements, at least one metal element (ie, Li, Ni, Co, Mn It is preferable that a metal element other than the above or a metalloid element is included. Examples of such elements include transitions of group 2 of the periodic table (alkaline earth metals such as magnesium (Mg), calcium (Ca), and strontium (Sr)), group 4 (titanium (Ti), zirconium (Zr), and the like. Metal), group 5 (transition metals such as vanadium (V) and niobium (Nb)), group 6 (transition metals such as chromium (Cr), molybdenum (Mo), tungsten (W)), and group 8 (iron (Fe )), Group 9 (transition metals such as rhodium (Rh)), Group 10 (transition metals such as palladium (Pb) and platinum (Pt)), Group 11 (transition metals such as copper (Cu)) , Group 12 (metal such as zinc (Zn)), Group 13 (metal element such as boron (B) which is a metalloid element, or aluminum (Al), gallium (Ga), indium (In)), Group 14 ( Metalloid elements such as tin (Sn)) And the rare earth may be one or more elements of any of the elements belonging to the (lanthanum (La), cerium (Ce) metal element, etc.). For example, it preferably includes Zr, Mg, Sr, Ti, V, Nb, Mo, W, B, and Al, and particularly preferably includes Zr.
The amount of the substitutional constituent element is not particularly limited. For example, it is 0.1% by mass or more (typically 0.2% by mass with respect to a total of 100% by mass of the substitution element, Ni, Co, and Mn. % Or more, for example 0.3% by mass or more), and 1.0% by mass or less (typically 0.8% by mass or less, for example 0.7% by mass or less). When the amount of substitutional constituent elements (for example, zirconium) is in the above range, the crystal stability and thermal stability are further improved. Therefore, in a battery using the material for the positive electrode, excellent battery performance (for example, retention characteristics in a high temperature environment described later) and the effect of the present invention (that is, improvement in safety in a wide temperature environment) are obtained. It is possible to achieve both at a higher level.

As such a lithium transition metal oxide (typically in particulate form), for example, a lithium transition metal oxide powder prepared by a conventionally known method can be used as it is. Although not particularly limited, for example, the D ₅₀ particle size is substantially constituted by secondary particles in the range of about 1 μm to 25 μm (preferably 2 μm to 10 μm, more preferably 6 μm to 10 μm, for example 6 μm to 8 μm). The lithium transition metal oxide powder thus prepared can be preferably used. In the present specification, “particle size” means a particle size corresponding to 50% cumulative from the fine particle side in a volume-based particle size distribution measured by particle size distribution measurement based on a general laser diffraction / light scattering method. D ₅₀ particle diameter, also referred to as median diameter).

As the conductive material, one kind or two or more kinds of substances conventionally used in lithium secondary batteries can be used without any particular limitation. For example, it may be one or more selected from various carbon blacks (for example, acetylene black, ketjen black), graphite powder (natural, artificial), carbon fibers (PAN-based, pitch-based) and the like. Or metal fibers (e.g. Al, SUS or the like), conductive metal powder (e.g. Ag, Ni, Cu, etc.), metal oxides (e.g. ZnO, SnO _2, etc.), may be used the surface-coated synthetic fiber such as a metal . Among these, a preferable carbon powder is acetylene black. Although not particularly limited, the proportion of the conductive material in the entire positive electrode mixture layer can be, for example, approximately 0.1% by mass to 15% by mass, and approximately 1% by mass to 10% by mass (more preferably Is preferably 2% by mass to 6% by mass).

  The binder is a compound that can be uniformly dissolved or dispersed in a solvent to be described later, and one or more kinds of substances conventionally used in lithium secondary batteries can be used without any particular limitation. For example, when forming a positive electrode mixture layer using a solvent-based liquid composition (solvent-based composition in which the main component of the dispersion medium is an organic solvent), a polymer material that is dispersed or dissolved in an organic solvent is preferably employed. Can do. Examples of the polymer material include polyvinylidene fluoride (PVdF), polyvinylidene chloride (PVdC), polyethylene oxide (PEO), and the like. Alternatively, when the positive electrode mixture layer is formed using an aqueous liquid composition, a polymer material that is dissolved or dispersed in water can be preferably employed. Examples of such polymer materials include cellulose polymers, fluorine resins, vinyl acetate copolymers, rubbers, and the like. More specifically, carboxymethylcellulose (CMC), hydroxypropylmethylcellulose (HPMC), polyvinyl alcohol (PVA), polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), styrene butadiene rubber (SBR), acrylic acid-modified SBR resin (SBR latex), and the like. Although it does not specifically limit, the usage-amount of the binder with respect to 100 mass% of positive electrode active materials can be 0.1 mass%-10 mass% (preferably 1 mass%-5 mass%), for example.

  As the solvent, one kind or two or more kinds of solvents conventionally used in lithium secondary batteries can be used without any particular limitation. Such a solvent is roughly classified into an aqueous solvent and an organic solvent. Examples of the organic solvent include amide, alcohol, ketone, ester, amine, ether, nitrile, cyclic ether, and aromatic hydrocarbon. More specifically, N-methyl-2-pyrrolidone (NMP), N, N-dimethylformamide (DMF), N, N-dimethylacetamide, 2-propanol, ethanol, methanol, acetone, methyl ethyl ketone, methyl propenoate, Cyclohexanone, methyl acetate, ethyl acetate, methyl acrylate, diethyl triamine, N, N-dimethylaminopropylamine, acetonitrile, ethylene oxide, tetrahydrofuran (THF), dioxane, benzene, toluene, ethylbenzene, xylene dimethyl sulfoxide (DMSO), dichloromethane, Examples thereof include trichloromethane and dichloroethane, and N-methyl-2-pyrrolidone (NMP) can be preferably used. In addition, the aqueous solvent is preferably water or a mixed solvent mainly composed of water. As the solvent other than water constituting the mixed solvent, one or more organic solvents (lower alcohol, lower ketone, etc.) that can be uniformly mixed with water can be appropriately selected and used. For example, it is preferable to use an aqueous solvent in which about 80% by mass or more (more preferably about 90% by mass or more, more preferably about 95% by mass or more) of the aqueous solvent is water. A particularly preferable example is an aqueous solvent (for example, water) substantially consisting of water.

  In addition, the positive electrode mixture slurry prepared here includes various additives (for example, a material that can function as a dispersant and an inorganic material that can generate a gas during overcharge, as long as the effects of the present invention are not significantly impaired. Compound) and the like can also be added. Examples of the dispersant include a polymer compound having a hydrophobic chain and a hydrophilic group (for example, an alkali salt, typically a sodium salt), an anionic compound having a sulfate, a sulfonate, a phosphate, and the like. And a cationic compound. More specifically, carboxymethyl cellulose (CMC), methyl cellulose, ethyl cellulose, hydroxypropyl cellulose, butyral, polyvinyl alcohol, modified polyvinyl alcohol, polyethylene oxide, polyvinyl pyrrolidone, polycarboxylic acid, oxidized starch, phosphate starch and the like are exemplified. For example, a water-soluble polymer material such as carboxymethyl cellulose is preferably used. In addition, examples of the inorganic compound that generates gas at the time of overcharge include carbonates, oxalates, and nitrates. For example, lithium carbonate and lithium oxalate are preferably used.

Thereafter, the positive electrode mixture layer is dried by an appropriate drying means, and the solvent contained in the positive electrode mixture slurry is removed. As such a method, natural drying, hot air, low-humidity air, vacuum, infrared rays, far-infrared rays, electron beams and the like can be used alone or in combination.
After drying of the positive electrode mixture slurry, the thickness of the positive electrode mixture layer and the like can be appropriately determined by applying press treatment (for example, various conventionally known press methods such as a roll press method and a flat plate press method). The density can be adjusted. When the density of the positive electrode mixture layer formed on the positive electrode current collector is extremely low (that is, the amount of the active material in the positive electrode mixture layer is small), the capacity per unit volume may decrease. Further, when the density of the positive electrode mixture layer is extremely high, the internal resistance tends to increase particularly during large current charge / discharge or charge / discharge at a low temperature. For this reason, the density of the positive electrode mixture layer is, for example, 2.0 g / cm ³ or more (typically 2.5 g / cm ³ or more) and 4.2 g / cm ³ or less (typically 4. g / cm ³ or more). 0 g / cm ³ or less).

The negative electrode of the sealed lithium secondary battery disclosed herein is a slurry-like composition (including paste-like and ink-like ones) obtained by mixing a negative electrode active material, a binder, and the like, similarly to the positive electrode. , Referred to as “negative electrode composite slurry”). A slurry in which the slurry is applied onto a negative electrode current collector to form a negative electrode mixture layer (also referred to as a negative electrode active material layer) is used.
As a method for forming the negative electrode mixture layer, as in the case of the positive electrode described above, a method in which an appropriate amount of the negative electrode mixture slurry is applied to one or both surfaces of the negative electrode current collector and dried can be preferably employed.

As a material for the negative electrode current collector, a conductive member made of a metal having good conductivity is preferably used as in the current collector used for the negative electrode of a conventional lithium ion secondary battery. For example, copper, nickel, titanium, stainless steel, or an alloy mainly containing them can be used.
The shape of the negative electrode current collector can be the same as the shape of the positive electrode current collector.

  As the negative electrode active material, one kind or two or more kinds of materials conventionally used in lithium secondary batteries can be used without any particular limitation. For example, particulate carbon powder (carbon particles) including a graphite structure (layered structure) at least partially, an oxide such as lithium titanate, an alloy of tin (Sn), silicon (Si) and lithium, and the like can be given. As the carbon powder, graphite (graphite), non-graphitizable carbon (hard carbon), graphitizable carbon (soft carbon), or a combination of these can be used. Among these, graphite (for example, natural graphite (graphite) taken from natural minerals and artificial graphite produced from petroleum or coal-based materials) can be preferably used. The proportion of the negative electrode active material in the entire negative electrode mixture layer is not particularly limited, but it is usually appropriately about 50% by mass or more, preferably about 90% by mass to 99% by mass (for example, about 95% by mass to 99% by mass).

  As the binder, an appropriate material can be appropriately selected from the polymer materials exemplified as the binder for the positive electrode mixture layer. Typically, polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), styrene butadiene rubber (SBR) and the like are exemplified. What is necessary is just to select the usage-amount of a binder with respect to 100 mass% of negative electrode active materials suitably according to the kind and quantity of a negative electrode active material, for example, it shall be 1 mass%-10 mass% (preferably 2 mass%-5 mass%). be able to. In addition, additives such as various polymer materials (for example, carboxymethyl cellulose (CMC)) that can function as the above-described dispersant, conductive materials, and the like can also be used.

After the drying of the negative electrode mixture slurry, similarly to the case of the positive electrode, the negative electrode is appropriately subjected to press treatment (for example, various conventionally known pressing methods such as a roll press method and a flat plate pressing method can be adopted). The thickness and density of the composite material layer can be adjusted. The density of the negative electrode mixture layer is, for example, 1.1 g / cm ³ or more (typically 1.2 g / cm ³ or more, for example, 1.3 g / cm ³ or more), and 1.5 g / cm ³ or less. (Typically 1.49 g / cm ³ or less).

  The positive electrode and the negative electrode are laminated to produce an electrode body. The shape of the electrode body is not particularly limited. For example, a long positive electrode mixture layer having a predetermined width is formed on a long positive electrode current collector along the longitudinal direction of the current collector. And a long negative electrode in which a negative electrode mixture layer having a predetermined width is formed along the longitudinal direction of the current collector on the long negative electrode current collector. A wound electrode body wound in the direction can be used. Since such an electrode body (typically, the central part of the electrode body) has relatively low heat dissipation, it causes an increase in internal pressure in the battery case in order to operate the current interruption mechanism during overcharging. For this reason, there is a risk that the generation of the gas necessary for the above-described purpose will be suppressed. However, since the battery disclosed herein can stably generate a desired amount of gas under a wide range of temperature environments, the current interruption mechanism can be more reliably operated even in such a case.

Along with the electrode body, the electrolyte, and two or more overcharge inhibitors (that is, an aliphatic carboxylic acid ester capable of generating carbon dioxide and an aromatic compound capable of generating hydrogen gas during overcharge). By being accommodated in an appropriate battery case, a sealed lithium secondary battery is constructed. The case is provided with a current interruption mechanism (a mechanism capable of interrupting the current in response to an increase in the case internal pressure caused by the generation of the gas when the battery is overcharged) as a safety mechanism.
Since the sealed lithium secondary battery disclosed herein has two types of overcharge inhibitors in the electrolyte, a desired gas amount can be generated during overcharge under a wide temperature environment. And since generation | occurrence | production of this gas raises a pressure in a battery case, an electric current interruption mechanism can be operated stably.

  In a typical configuration of the sealed lithium secondary battery disclosed herein, a separator is interposed between the positive electrode and the negative electrode. As the separator, various porous sheets similar to those conventionally used for lithium secondary batteries can be used. For example, the separator is made of a resin such as polyethylene (PE), polypropylene (PP), polyester, cellulose, or polyamide. Examples thereof include porous resin sheets (film, nonwoven fabric, etc.). Such a porous resin sheet may have a single-layer structure or a plurality of structures of two or more layers (for example, a three-layer structure in which a PP layer is laminated on both sides of a PE layer). Although the thickness of a separator is not specifically limited, For example, it is 5 micrometers-50 micrometers (typically 10 micrometers-40 micrometers, for example, 10 micrometers-30 micrometers). The properties of the separator (typically a porous resin sheet) are not particularly limited. For example, the pore diameter is about 0.01 μm to 20 μm (typically 0.1 μm to 10 μm), and the porosity is For example, a material having a (porosity) of about 20 volume% to 90 volume% (typically 30 volume% to 80 volume%) can be used. Further, in a sealed lithium secondary battery (lithium polymer battery) using a solid electrolyte, the electrolyte may also serve as a separator.

  As a battery case, the material and shape which are used for the conventional lithium secondary battery can be used. Examples of the material include relatively light metal materials such as aluminum and steel, and resin materials such as PPS and polyimide resin. Further, the shape (outer shape of the container) is not particularly limited, and may be, for example, a cylindrical shape, a rectangular shape, a rectangular parallelepiped shape, a coin shape, a bag shape, or the like.

As the electrolyte, one kind or two or more kinds similar to the non-aqueous electrolyte used in the conventional lithium secondary battery can be used without any particular limitation. Such a nonaqueous electrolyte typically has a composition in which a supporting salt (lithium salt) is contained in a suitable nonaqueous solvent.
As the non-aqueous solvent, one or more aprotic solvents such as carbonates, esters, ethers, nitriles, sulfones, lactones and the like are not particularly limited as long as the effects of the present invention are not significantly impaired. Can be used. For example, ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran, Examples include 2-methyltetrahydrofuran, dioxane, 1,3-dioxolane, diethylene glycol dimethyl ether, ethylene glycol dimethyl ether, acetonitrile, propionitrile, nitromethane, N, N-dimethylformamide, dimethyl sulfoxide, sulfolane, and γ-butyrolactone. Of these, nonaqueous solvents mainly composed of carbonates are preferably used. For example, the nonaqueous solvent contains one or more carbonates, and the total volume of these carbonates is 60% by volume or more (more preferably 75% by volume or more, and further preferably 90% by volume) of the total volume of the nonaqueous solvent. The non-aqueous electrolyte solution occupying the above and substantially 100% by volume) is preferably used. The non-aqueous solvent is typically prepared without using (as the non-aqueous solvent) an aliphatic carboxylic acid ester that can function as the overcharge inhibitor. However, such an aliphatic carboxylic acid ester may also be used. it can. In such a case, for example, the total of the addition amount as a non-aqueous solvent and the addition amount as an overcharge inhibitor capable of generating carbon dioxide gas is about 1% by mass or more (relative to 100% by mass of the electrolyte) (typically 2% by mass or more, for example 3% by mass or more, preferably 5% by mass or more) and 20% by mass or less (typically 15% by mass or less, for example 10% by mass or less, preferably 8% by mass). Or less).

Examples of the supporting salt include LiPF ₆ , LiBF ₄ , LiClO ₄ , LiN (SO ₂ CF ₃ ) ₂ , LiN (SO ₂ C ₂ F ₅ ) ₂ , LiCF ₃ SO ₃ , LiC ₄ F ₉ SO ₃ , LiC. (SO ₂ CF ₃ ) ₃ , LiClO _{4 and the} like are exemplified. Of these, LiPF ₆ is preferably used. The concentration of the electrolyte is not particularly limited, but if the concentration of the electrolyte is too low, the amount of lithium ions contained in the electrolyte is insufficient, and the ionic conductivity tends to decrease. On the other hand, if the concentration of the supporting electrolyte is too high, the viscosity of the non-aqueous electrolyte becomes too high, and the ionic conductivity tends to decrease. For this reason, a nonaqueous electrolyte containing an electrolyte at a concentration of about 0.1 mol / L to 2 mol / L (preferably about 0.8 mol / L to 1.5 mol / L) is preferably used.

The electrolyte of the sealed lithium secondary battery disclosed herein includes an aliphatic carboxylic acid ester capable of generating carbon dioxide gas and an aromatic compound capable of generating hydrogen gas when a predetermined battery voltage is exceeded. Contained. The aliphatic carboxylic acid ester is severely decomposed particularly in a high temperature environment (typically 40 ° C. to 70 ° C., for example, 50 ° C. to 60 ° C.) to generate a large amount of carbon dioxide. On the other hand, the aromatic compound is violently oxidized and decomposed mainly in a low to medium temperature environment (for example, 0 ° C. to 40 ° C.) to generate a large amount of hydrogen gas. Therefore, by providing both in the battery case, a desired gas amount can be generated under a wide temperature environment.
Such an overcharge inhibitor is a compound that can be uniformly dissolved or dispersed in the electrolyte, and has an oxidation potential equal to or higher than the operating voltage of the lithium secondary battery, and is decomposed in an overcharged state to generate gas. As long as it is a substance that can be obtained, one or more kinds of substances conventionally used in lithium secondary batteries can be used without any particular limitation. Specifically, for example, when a battery is used in an operating range of 3.0 V or more and 4.1 V or less, it preferably has an oxidation potential that is higher by 0.1 V or more than the above-described operating upper limit voltage.

Examples of the aliphatic carboxylic acid ester include a general formula: R ₁ —COO—R ₂ and a general formula: R ₁ —CO—CH ₃ —O—R ₂ (where R ₁ and R ₂ are hydrogen atoms independent of each other). Examples thereof include ester compounds represented by an atom or a linear or branched alkyl group having 1 to 10 carbon atoms (preferably 1 to 6 carbon atoms). Examples of the alkyl group having 1 to 6 carbon atoms represented by R in the above chemical formula include, for example, methyl group, ethyl group, n-propyl group, isopropyl group, n-butyl group, sec-butyl group, tert-butyl group, An n-pentyl group, an n-hexyl group, etc. are mentioned. More specifically, propionic acid such as methyl propionate (MP) and ethyl propionate (EP), and butyric acid such as methyl butyrate and ethyl butyrate are used. Among them, methyl propionate having excellent storage characteristics is preferably used. be able to. The aliphatic carboxylic acid ester has high affinity with a non-aqueous solvent used as an electrolytic solution for a lithium secondary battery, and can be uniformly mixed in the electrolytic solution. Furthermore, since it has a low viscosity and a low boiling point compared to other non-aqueous solvents used in the battery, a battery with high battery performance (for example, low temperature characteristics and rapid charge / discharge characteristics) and high reliability can be obtained. Can do.
As in the case of the aromatic compound, the amount of the aliphatic carboxylic acid ester added to the electrolyte is not particularly limited, but if an excessive amount is added, battery performance (for example, high-temperature storage characteristics) may be deteriorated. For this reason, the amount of the aliphatic carboxylic acid ester added to 100% by mass of the electrolyte is, for example, about 1% by mass or more (typically 2% by mass or more, for example, 3% by mass or more, preferably 5% by mass or more). 20 mass% or less (typically 15 mass% or less, for example, 10 mass% or less, preferably 8 mass% or less). When the addition amount is in the above range, excellent battery performance and the effect of the present invention (that is, improvement in safety under a wide temperature environment) can be achieved at a high level.

Examples of the aromatic compound include biphenyl compounds, alkylbiphenyl compounds, cycloalkylbenzene compounds, alkylbenzene compounds, organic phosphorus compounds, fluorine atom-substituted aromatic compounds, carbonate compounds, cyclic carbamate compounds, and alicyclic hydrocarbons. More specifically, biphenyl (BP), cyclohexylbenzene (CHB), trans-butylcyclohexylbenzene, cyclopentylbenzene, t-butylbenzene, t-aminobenzene, terphenyl, 2-fluorobiphenyl, 3-fluorobiphenyl, 4 -Fluorobiphenyl, 4,4'-difluorobiphenyl, o-cyclohexylfluorobenzene, p-cyclohexylfluorobenzene, tris- (t-butylphenyl) phosphate, phenyl fluoride, 4-fluorophenyl acetate, diphenyl carbonate, methylphenyl carbonate , Bicterary butyl phenyl carbonate, diphenyl ether, dibenzofuran and the like. Among them, aromatic compounds such as cyclohexylbenzene (CHB) and biphenyl (BP), whose oxidation potential is relatively low at about 4.5 to 4.6 V, have an upper limit charging voltage of 4.1 V to 4.2 V, for example. It can be preferably used in a battery.
The amount of the aromatic compound added to the electrolyte is not particularly limited. However, if the amount added is too small, the amount of gas generated during overcharge decreases, and the current interrupt mechanism may not operate normally. Moreover, if an excessive amount that places importance on safety is added, battery performance may be reduced (for example, increase in battery resistance or deterioration in cycle characteristics). Therefore, the amount of the aromatic compound added to 100% by mass of the electrolyte is, for example, about 0.1% by mass or more (typically 0.5% by mass or more, for example, 1% by mass or more), and 10% by mass or less ( Typically, it can be 7% by mass or less, for example, 5% by mass or less, preferably 3% by mass or less, and more preferably 2% by mass or less. Since the battery disclosed here contains two types of overcharge inhibitors that can generate a large amount of gas under different temperature environments, the current interruption mechanism operates under a wide range of temperature environments even with a small amount of addition compared to conventional batteries. The amount of gas required to make it possible can be obtained.

  The current interrupting mechanism is not particularly limited as long as the current can be interrupted according to the increase in pressure in the battery case (that is, using the increase in internal pressure as a trigger for operation), and the current provided in this type of battery is not limited. A mechanism similar to any conventionally known blocking mechanism can be appropriately employed. As an example, for example, a configuration shown in FIG. 2 described later can be preferably used. In such a configuration, when the internal pressure of the battery case rises, the member constituting the conductive path from the electrode terminal to the electrode body is deformed, and the conductive path is cut by being separated from the other.

  The operating voltage range of the battery constructed as described above is not particularly limited because it varies depending on constituent members (for example, active material, non-aqueous solvent, etc.) provided in the battery. However, the aliphatic carboxylic acid ester used here has a relatively low oxidation potential and is oxidatively decomposed at about 4.2 V to generate carbon dioxide. For this reason, the charging upper limit voltage (operation upper limit voltage) of the battery is 3.0 V or higher (typically 3.5 V or higher, for example, 4.0 V or higher), and less than 4.2 V (typically 4.V). 1V or less). In such a case, oxidative decomposition (generation of carbon dioxide) during normal use is suppressed, and gas can be effectively generated only when necessary (ie, during overcharge).

  Although not intended to be particularly limited, as a schematic configuration of a sealed lithium secondary battery according to an embodiment of the present invention, a flatly wound electrode body (winded electrode body) and a non-aqueous electrolyte As an example, a sealed lithium secondary battery (single cell) in a form in which is stored in a flat rectangular parallelepiped (box) container is shown in FIGS. In the following drawings, members / parts having the same action are denoted by the same reference numerals, and redundant description may be omitted or simplified. The dimensional relationship (length, width, thickness, etc.) in each figure does not reflect the actual dimensional relationship.

FIG. 1 is a perspective view schematically showing the outer shape of a sealed lithium secondary battery 100 according to an embodiment of the present invention. FIG. 2 is a diagram schematically showing a cross-sectional structure taken along line II-II of the sealed lithium secondary battery shown in FIG.
As shown in FIGS. 1 and 2, the sealed lithium secondary battery 100 according to the present embodiment includes a wound electrode body 80 and a hard case (outer container) 50. The hard case 50 includes a flat cuboid (box-shaped) hard case main body 52 having an open upper end, and a lid 54 that closes the opening. On the upper surface of the hard case 50 (that is, the lid 54), a positive electrode terminal 70 that is electrically connected to the positive electrode sheet of the wound electrode body 80 and a negative electrode terminal 72 that is electrically connected to the negative electrode sheet of the electrode body are provided. ing. In addition, the lid 54 is provided with a safety valve 55 for discharging gas generated inside the battery case to the outside of the case, similarly to the hard case of the conventional sealed lithium secondary battery. The safety valve 55 is typically set to be opened at a pressure higher than the pressure at which the current interrupt mechanism 30 operates.

  Inside the battery case 50, an electrode body (a long positive electrode sheet 10 and a long negative electrode sheet 20 are wound flatly via long separators 40 </ b> A and 40 </ b> B) A wound electrode body 80 is accommodated together with a non-aqueous electrolyte (not shown). Further, the positive electrode sheet 10 is formed such that the positive electrode mixture layer 14 is not provided (or removed) at one end along the longitudinal direction, and the positive electrode current collector 12 is exposed. Similarly, the wound negative electrode sheet 20 is not provided with (or removed from) the negative electrode mixture layer 24 at one end along the longitudinal direction, so that the negative electrode current collector 22 is exposed. Is formed. A positive electrode current collector plate 74 is attached to the exposed end of the positive electrode current collector 12, and a negative electrode current collector plate 76 is attached to the exposed end of the negative electrode current collector 22. Each is electrically connected to the negative terminal 72.

  In addition, a current interrupt mechanism 30 that operates when the internal pressure of the battery case increases is provided inside the battery case 50. The current interruption mechanism 30 only needs to be configured to cut a conductive path (for example, a charging path) from at least one electrode terminal to the electrode body 80 when the internal pressure of the battery case 50 increases, and has a specific shape. It is not limited to. In this embodiment, the current interruption mechanism 30 is provided between the positive electrode terminal 70 fixed to the lid body 54 and the electrode body 80, and reaches from the positive electrode terminal 70 to the electrode body 80 when the internal pressure of the battery case 50 rises. The conductive path is configured to be cut.

  More specifically, the current interrupt mechanism 30 may include a first member 32 and a second member 34, for example. When the internal pressure of the battery case 50 rises, at least one of the first member 32 and the second member 34 is deformed and separated from the other, thereby cutting the conductive path. In this embodiment, the first member 32 is a deformed metal plate, and the second member 34 is a connection metal plate joined to the deformed metal plate 32. The deformed metal plate (first member) 32 has an arch shape in which a central portion is curved downward, and a peripheral portion thereof is connected to the lower surface of the positive electrode terminal 70 via a current collecting lead terminal 35. Further, the tip of the curved portion 33 of the deformed metal plate 32 is joined to the upper surface of the connection metal plate 34. A positive current collector plate 74 is joined to the lower surface (back surface) of the connection metal plate 34, and the positive current collector plate 74 is connected to the positive electrode 10 of the electrode body 80. In this way, a conductive path from the positive electrode terminal 70 to the electrode body 80 is formed.

  The current interrupt mechanism 30 includes an insulating case 38 made of plastic or the like. The insulating case 38 is provided so as to surround the deformed metal plate 32, and hermetically seals the upper surface of the deformed metal plate 32. The internal pressure of the battery case 50 does not act on the upper surface of the hermetically sealed curved portion 33. The insulating case 38 has an opening into which the curved portion 33 of the deformed metal plate 32 is fitted, and the lower surface of the curved portion 33 is exposed from the opening to the inside of the battery case 50. The internal pressure of the battery case 50 acts on the lower surface of the curved portion 33 exposed inside the battery case 50. In the current interrupt mechanism 30 having such a configuration, when the internal pressure of the battery case 50 increases, the internal pressure acts on the lower surface of the curved portion 33 of the deformed metal plate 32, and the curved portion 33 curved downward is pushed upward. The upward push of the curved portion 33 increases as the internal pressure of the battery case 50 increases. When the internal pressure of the battery case 50 exceeds the set pressure, the curved portion 33 is turned upside down and deformed so as to bend upward. Due to the deformation of the curved portion 33, the joint point 36 between the deformed metal plate 32 and the connection metal plate 34 is cut. As a result, the conductive path from the positive electrode terminal 70 to the electrode body 80 is cut, and the overcharge current is cut off.

  The current interrupt mechanism 30 is not limited to the positive terminal 70 side, and may be provided on the negative terminal 72 side. Further, the current interrupt mechanism 30 is not limited to the mechanical cutting accompanied by the deformation of the deformed metal plate 32 described above. For example, the internal pressure of the battery case 50 is detected by a sensor, and the internal pressure detected by the sensor sets the set pressure. An external circuit that cuts off the charging current when exceeded can be provided as a current cut-off mechanism.

  FIG. 3 is a diagram schematically showing a long sheet structure (electrode sheet) in a stage before assembling the wound electrode body 80. A positive electrode sheet 10 in which a positive electrode mixture layer 14 is formed along the longitudinal direction on one or both surfaces (typically both surfaces) of a long positive electrode current collector 12, and a long negative electrode current collector 22. The negative electrode sheet 20 on which the negative electrode mixture layer 24 is formed along the longitudinal direction on one side or both sides (typically both sides) is overlapped with the long separators 40A and 40B and wound in the longitudinal direction. A wound electrode body is prepared. A flat wound electrode body 80 is obtained by squashing and winding the wound electrode body from the side surface direction. Since a battery including a wound electrode body has a high capacity among lithium secondary batteries, it is particularly important to improve reliability (for example, safety measures in the case of malfunction). According to the technology disclosed herein, the safety of such a battery (for example, safety when an overcharge or an internal short-circuit occurs) can be improved as compared with the related art.

  The sealed lithium secondary battery disclosed herein can be used for various applications, but is characterized by improved safety in a wide range of temperature environments. This is particularly effective when the battery is a large-capacity, high-power large battery and the use and / or storage environment can become high temperature. For example, the sealed lithium secondary battery 100 disclosed herein can be suitably used as a power source (drive power source) for a motor mounted on a vehicle 1 such as an automobile as shown in FIG. Although the kind of vehicle 1 is not specifically limited, Typically, a plug-in hybrid vehicle (PHV), a hybrid vehicle (HV), and an electric vehicle (EV) are mentioned. Moreover, the sealed lithium secondary battery 100 may be used alone, or may be used in the form of an assembled battery that is connected in series and / or in parallel.

  Hereinafter, as a specific test example, whether or not there is a difference in safety of the battery was evaluated by the method disclosed herein. It should be noted that the present invention is not intended to be limited to those shown in the specific examples.

[Construction of sealed lithium secondary battery]
<Example 1>
LiNi _0.33 Co _0.33 Mn _0.33 Zr _0.005 O ₂ powder as a positive electrode active material powder, acetylene black (AB) as a conductive material, and polyvinylidene fluoride (PVdF) as a binder, The material is introduced into a kneader (planetary mixer) so that the mass ratio is about 100: 5: 5, and N-methylpyrrolidone (NMP) is used so that the solid content concentration (NV) is about 50% by mass. While preparing the viscosity, the mixture was kneaded to prepare a slurry-like composition (positive electrode mixture slurry) for forming the positive electrode mixture layer. This positive electrode mixture slurry was applied to both sides of a long aluminum foil (positive electrode current collector) having a thickness of about 15 μm and dried to form a positive electrode mixture layer. The obtained positive electrode was roll-pressed to produce a sheet-like positive electrode (positive electrode sheet (Example 1)) having a width of 94 mm and a length of 4500 mm.

  Similarly to the above, natural graphite (powder) as the negative electrode active material and polyvinylidene fluoride (PVdF) as the binder are mixed in a kneader (planetary mixer) so that the mass ratio of these materials is about 100: 7. The slurry composition for forming a negative electrode mixture layer (negative electrode mixture slurry) was added and kneaded while adjusting the viscosity with N-methylpyrrolidone (NMP) so that the solid content concentration (NV) was about 45% by mass. ) Was prepared. This negative electrode mixture slurry was applied to both sides of a long copper foil (negative electrode current collector) having a thickness of about 20 μm and dried to form a negative electrode mixture layer. The obtained negative electrode was roll-pressed to produce a sheet-like negative electrode (negative electrode sheet) having a width of 100 mm and a length of 4700 mm.

Next, the prepared positive electrode sheet and negative electrode sheet are composed of two separators (here, a three-layer structure in which a polypropylene (PP) layer is laminated on both sides of a polyethylene (PE) layer and has a thickness of 20 μm. And then rolled. The obtained wound electrode body was crushed from the side surface direction and crushed to produce a flat wound electrode body. And the positive electrode terminal was joined to the edge part of the positive electrode electrical power collector of this electrode body, and the negative electrode terminal was joined to the edge part of the negative electrode electrical power collector, respectively.
Such a wound electrode body and a nonaqueous electrolyte (here, a mixed solvent containing ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of 3: 4: 3) are used as an electrolyte. LiPF ₆ was dissolved at a concentration of about 1.0 mol / L, and biphenyl (BP) and methyl propionate (MP) as overcharge inhibitors were added at concentrations of 2% by mass and 5% by mass, respectively. The electrolyte (125 g) was housed in a rectangular battery case, and a current interruption mechanism (CID) was installed in the battery case. Here, the operating pressure of CID was set to 0.7 MPa. A lid was attached to the opening of the battery case and welded and joined to construct a sealed lithium secondary battery (Example 1) having a rated capacity of 24 Ah.

<Example 2>
In Example 2, a sealed lithium secondary battery (example) was used in the same manner as in Example 1 except that cyclohexylbenzene (CHB: 1% by mass) and ethyl propionate (EP: 3% by mass) were used as overcharge inhibitors. 2) was constructed.

<Example 3>
In Example 3, as in Example 1, except that BP (3% by mass), CHB (2% by mass), and MP (10% by mass) were used as overcharge inhibitors, a sealed lithium secondary battery ( Example 3) was constructed.

<Example 4>
In Example 4, a sealed lithium secondary battery (as in Example 1 except that BP (3% by mass), CHB (2% by mass), and MP (15% by mass) were used as overcharge inhibitors. Example 4) was constructed.

<Example 5>
In Example 5, a sealed lithium secondary battery (Example 5) was constructed in the same manner as in Example 1 except that LiNi _0.33 Co _0.33 Mn _0.33 O ₂ was used as the positive electrode active material.

<Example 6>
In Example 6, a sealed lithium secondary battery similar to that in Example 1 was newly constructed in order to determine the difference depending on the operating voltage range (charging upper limit voltage) later. For distinction later, the battery is referred to as a sealed lithium secondary battery (Example 6).

<Example 7>
In Example 7, a sealed lithium secondary battery (Example 7) was constructed in the same manner as in Example 1 except that no overcharge inhibitor was contained.

<Example 8>
In Example 8, a sealed lithium secondary battery (Example 8) was constructed in the same manner as Example 1 except that only BP (2% by mass) was used as the overcharge inhibitor.

<Example 9>
In Example 8, a sealed lithium secondary battery (Example 9) was constructed in the same manner as Example 1 except that only MP (5% by mass) was used as the overcharge inhibitor.

<Example 10>
In Example 10, when producing the positive electrode mixture layer, LiNi _0.33 Co _0.33 Mn _0.33 Zr _0.005 O ₂ powder as the positive electrode active material powder, acetylene black (AB) as the conductive material, Sealed in the same manner as in Example 8 except that polyvinylidene fluoride (PVdF) as a binder and lithium carbonate as an additive were used so that the mass ratio of these materials was approximately 100: 5: 5: 3. Type lithium secondary battery (Example 10) was constructed.
Table 1 summarizes the characteristics of the battery constructed as described above.

[Overcharge test]
Each battery (Examples 1 to 10) constructed as described above was subjected to a conditioning process under a temperature environment of 25 ° C. (here, an operation of charging with constant current up to 4.1 V at a charge rate of 0.3 C, Under the temperature environment of 25 ° C. (low temperature to medium temperature range) and 60 ° C. (high temperature range) after performing an initial charge / discharge treatment that repeats the operation of discharging at a constant current up to 3.0 V at a discharge rate of 0.3 C twice) The overcharge test was conducted. In this example, first, each battery after the conditioning process is adjusted to a state of SOC 30%, and the battery is charged with a constant current at a charging rate of 1 C (charging current of about 24 A) until the voltage reaches 20 V, It was confirmed whether the electric current interruption mechanism worked normally.
The obtained measurement results are shown in the corresponding places in Table 2. In the column of “CID operation” in Table 2, the external appearance of each battery during the overcharge test is observed, and the battery state changes (specifically, the heat generation of the battery accompanying a rapid temperature rise or the deformation of the case). Etc.) and the current interrupting mechanism operated normally, it was judged that the current interrupting mechanism operated sufficiently quickly, and was evaluated as “◯”.

  As shown in Table 2, in Examples 1 to 6 including both two types of overcharge inhibitors (ie, aromatic compound and aliphatic carboxylic acid ester) having different gas generation temperatures, the temperatures of 25 ° C. and 60 ° C. In the overcharge test under the temperature environment, the current interruption mechanism operated normally. On the other hand, in Example 7 where no overcharge inhibitor was added, the current interruption mechanism did not operate at 25 ° C. and 60 ° C. Further, in Examples 8 and 10 containing only an aromatic compound as an overcharge inhibitor, the current interruption mechanism did not work normally in an environment of 60 ° C. This is because the redox shuttle reaction of aromatic compounds has preferentially occurred, so that the decomposition reaction is suppressed (that is, the amount of gas generation is reduced) and the internal pressure rise sufficient to operate the current interrupt mechanism does not occur. This is probably because In Example 10, although sodium carbonate capable of generating carbon dioxide gas was added in advance in the positive electrode mixture layer, the effect was not recognized in this example. Furthermore, in Example 9 containing only an aliphatic carboxylic acid ester as an overcharge inhibitor, the current interruption mechanism did not work normally in an environment at 25 ° C. This is considered to be because the oxidative decomposition of the aliphatic carboxylic acid ester hardly occurs in a low temperature to medium temperature environment (for example, 0 ° C. to 40 ° C.). Such a result shows the effect of applying the present invention (that is, stable gas generation under any environment). Furthermore, in the sealed lithium secondary battery, since the reaction efficiency of the overcharge inhibitor is high, it is necessary to reliably operate the current interruption mechanism even at high temperatures with a smaller amount of overcharge inhibitor than in the past. It is considered that a large amount of gas can be obtained.

[High temperature storage test]
Three cycles of charging / discharging of each battery after the above conditioning treatment (in this case, the operation of charging at a constant current up to 4.1 V at a current of 4 A and the operation of discharging at a constant current up to 3.0 V at a current of 4 A) Repeated charge / discharge). Thereafter, a charge (or discharge) operation was performed so that the SOC was 80%, the voltage was adjusted, and the battery was stored in an environment of 60 ° C. for 480 hours (approximately 20 days). The battery after high temperature storage was again charged and discharged for 3 cycles under the same conditions.
The obtained results are shown in the corresponding places in Table 2. The capacity recovery rate (%) was calculated as a ratio (B / A × 100 (%)) of the discharge capacity (B) after high temperature storage to the discharge capacity (A) before high temperature storage.

  As shown in Table 2, 1 to 5% by mass of an aromatic compound and 3 to 10% by mass of an aliphatic carboxylic acid ester were added to the electrolyte as an overcharge inhibitor. No. 3 exhibited excellent high-temperature storage characteristics of 90% or more. Therefore, by setting the addition amount of the overcharge inhibitor in the above range, it is possible to achieve both excellent high-temperature storage characteristics and the effect of the present invention (improvement of safety under a wide temperature environment) at a high level. Indicated. Moreover, when the charge upper limit voltage was 4.1 V (Example 1), the high-temperature storage characteristics were higher by 10% or more than when the voltage was 4.2 V (Example 6). This is probably because the oxidative decomposition of the aliphatic carboxylic acid ester could be suppressed by lowering the operating voltage range slightly. Furthermore, in Example 5 using lithium nickel cobalt manganese composite oxide as the positive electrode active material and Example 1 using lithium nickel cobalt manganese composite oxide containing Zr, Example 1 showed better high-temperature storage characteristics. Therefore, it was shown that excellent battery performance and the effects of the present invention can be achieved at a higher level by containing Zr.

  In order to clarify the contribution of two types of gases involved in the operation of the current interrupting mechanism (ie, carbon dioxide gas generated by oxidative decomposition of an aliphatic carboxylic acid ester and hydrogen gas generated by oxidative decomposition of an aromatic compound). Separately from the prismatic battery provided with the current interruption mechanism, a laminated sheet type sealed lithium secondary battery for quantifying the amount of gas generated was constructed.

<Example 1>
In the laminated sheet type battery of Example 1, first, the positive electrode sheet and the negative electrode sheet prepared in Example 1 were cut into dimensions of about 45 (mm) × 45 (mm), respectively. (Mm) × 47 (mm)) and arranged on the opposite side to produce an electrode body. And the positive electrode terminal was joined to the edge part of the positive electrode electrical power collector of this electrode body, and the negative electrode terminal was joined to the edge part of the negative electrode electrical power collector, respectively. Such an electrode body is mixed with a non-aqueous electrolyte (here, a mixed solvent containing ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of 3: 4: 3) and LiPF ₆ as an electrolyte. Is dissolved at a concentration of about 1.0 mol / L, and biphenyl (BP) and methyl propionate (MP) as an overcharge inhibitor are further contained at concentrations of 2 mass% and 5 mass%, respectively ( 0.8 g) was housed in a laminate sheet and a laminated sheet-type lithium secondary battery (Example 1) was constructed, and the conditioning treatment was performed in the same manner as the prismatic battery.

<Example 10>
In the laminated sheet type battery of Example 10, the positive electrode sheet prepared in Example 10 (that is, LiNi _0.33 Co _0.33 Mn _0.33 Zr ₀ as the positive electrode active material powder at the time of preparing the positive electrode mixture layer). _0.005 O ₂ powder, acetylene black (AB) as a conductive material, polyvinylidene fluoride (PVdF) as a binder, and lithium carbonate as an additive are approximately 100: 5: 5 in mass ratio of these materials. :) and a laminate sheet type lithium secondary battery (Example 10) in the same manner as in Example 1 except that only BP (2% by mass) was used as an overcharge inhibitor. ) And the conditioning treatment was performed in the same manner as the prismatic battery.

[Overcharge test (measurement of gas generation)]
For the laminated sheet type sealed lithium secondary battery (Examples 1 and 10) after the conditioning treatment, the volume of the cell was measured by Archimedes method. After that, the battery was charged at a constant current at a rate of 1 C under an environment of 25 ° C. and 60 ° C. until it was overcharged (in this example, SOC was 160%), and the volume of the cell was again measured by the Archimedes method. It was measured. Then, by subtracting the volume (B (cm ³ )) of the cell after the conditioning process from the volume (A (cm ³ )) of the cell after overcharging, the amount of gas generated during overcharging (A−B (cm ³ )) )) Was calculated. And the gas in a laminate sheet was analyzed by the general gas chromatograph (GC) -mass spectrometry (MS) method, and the composition of this gas was calculated | required. The obtained results are shown in FIG. In addition, the broken line shown in a figure represents the standard of the gas amount required in order to operate this electric current interruption mechanism, when the operating pressure of an electric current interruption mechanism is set to the typical value.
The Archimedes method is a method in which a measurement object (in this example, a laminated sheet type sealed lithium secondary battery) is immersed in a liquid medium (for example, distilled water or alcohol), and the buoyancy that the measurement object receives is measured. This is a technique for obtaining the volume of the measurement object by measuring.

  From the results shown in FIG. 5, in the battery of Example 1, the aromatic compound was mainly oxidized under the temperature environment of 25 ° C., and the aliphatic carboxylic acid ester was mainly oxidized and decomposed under the temperature environment of 60 ° C. It was thought that it was generated. On the other hand, in the battery of Example 10, the generation amount of carbon dioxide gas did not depend on the temperature environment, but the generation amount of hydrogen gas decreased under the temperature environment of 60 ° C., so that it was necessary for operating the current interruption mechanism. A sufficient amount of gas could not be obtained. This result shows the difference in the effect between the present invention and the prior art. That is, it was shown that the present invention can improve the reliability (safety) of the battery in a wider temperature environment than in the past.

  As mentioned above, although the specific example of this invention was demonstrated in detail, these are only illustrations and do not limit a claim. The technology described in the claims includes various modifications and changes of the specific examples illustrated above.

1 Automobile (vehicle)
10 Positive electrode sheet (positive electrode)
12 Positive electrode current collector 14 Positive electrode mixture layer 20 Negative electrode sheet (negative electrode)
22 Negative electrode current collector 24 Negative electrode composite material layer 30 Current interruption mechanism (CID)
32 Deformed metal plate (first member)
34 Connection metal plate (second member)
38 Insulating cases 40A and 40B Separator sheet 50 Battery case 52 Case body 54 Lid 70 Positive electrode terminal 72 Negative electrode terminal 74 Positive electrode current collector plate 76 Negative electrode current collector plate 80 Winding electrode body 100 Sealed lithium secondary battery

Claims (8)

A sealed lithium secondary battery in which an electrode body including a positive electrode and a negative electrode, and an electrolyte are housed in a predetermined battery case,
Here, the electrolyte includes
At least one aliphatic carboxylic acid ester selected from methyl propionate and ethyl propionate ;
At least one aromatic compound selected from cyclohexylbenzene and biphenyl ;
Is included,
The addition amount of the aliphatic carboxylic acid ester is 3% by mass or more and 15% by mass or less with respect to 100% by mass of the electrolyte.
The amount of the aromatic compound added is 1% by mass to 5% by mass with respect to 100% by mass of the electrolyte,
The battery case is characterized in that it comprises a current interruption mechanism operated when the pressure inside the battery case rises, sealed type lithium secondary battery. The sealed lithium secondary battery according to claim 1, wherein an addition amount of the aliphatic carboxylic acid ester is 3 % by mass or more and 10 % by mass or less with respect to 100% by mass of the electrolyte. The positive electrode includes a positive electrode current collector, and a positive electrode mixture layer including a positive electrode active material formed on the current collector,
3. The sealed lithium secondary battery according to claim 1, wherein the positive electrode active material includes a lithium nickel cobalt manganese composite oxide. 4. The sealed lithium secondary battery according to claim 3 , wherein at least a part of nickel element, cobalt element, and manganese element is substituted with zirconium element in the lithium nickel cobalt manganese composite oxide. The amount of the zirconium element with respect to the total 100 mass% of the zirconium element and the nickel and the cobalt element and the manganese element is 1.0 mass% or less than 0.1 mass%, according to claim 4 Sealed lithium secondary battery. The electrode body is
On a long positive electrode current collector, a positive electrode mixture layer having a predetermined width is formed along the longitudinal direction of the current collector, and
On the long negative electrode current collector, a negative electrode mixture layer having a predetermined width is formed along the longitudinal direction of the current collector.
The sealed lithium secondary battery according to any one of claims 1 to 5 , which is a wound electrode body in which the positive electrode and the negative electrode are laminated and wound in a longitudinal direction. The sealed lithium secondary battery according to any one of claims 1 to 6 , which is used at a charge upper limit voltage of 3.0 V or more and 4.1 V or less. A vehicle comprising the sealed lithium secondary battery according to any one of claims 1 to 7 as a driving power source.

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