JP2007087801A - Lithium ion secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a lithium ion secondary battery having improved safety and battery performance in large lithium ion secondary batteries using a laminate film for an external material. <P>SOLUTION: In the lithium ion secondary battery, where the battery capacity is 5 Ah or higher, and the battery capacity per cm<SP>2</SP>of positive and negative electrodes is 10 mAh or higher, a resin layer 8 is interposed between the external material (laminate film) 6 and lead terminals 7a, 7b. Cyclic ester is used for a nonaqueous electrolyte, where the cyclic ester has properties of both of high permittivity and low viscosity, improved oxidation-resistant properties, a high boiling point, low vapor pressure, a high flash point, and the like. Further, a positive electrode material with iron capable of realizing a low environment load and low costs as a main constituent, especially olivin type LiFePO<SB>4</SB>aiming at gaining both elements of low potential and high-energy density, and high safety and stability is used. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a chargeable / dischargeable secondary battery, and more particularly to a large lithium ion secondary battery.

  A lithium ion secondary battery using a metal oxide for the positive electrode, an organic electrolyte for the electrolyte, and a carbon material such as graphite for the negative electrode was first commercialized in 1991. Since then, its high energy density makes it compact and lightweight. It has rapidly become popular for portable electronic devices such as video cameras, mobile phones, notebook computers, and minidiscs.

  Many of the lithium ion secondary batteries that are power sources of these portable electronic devices have a thickness of 200 to 300 μm obtained by applying or pressing a composite material containing an electrode active material to a current collector made of a perforated metal plate or metal foil. A film-like electrode is wound or laminated together with a separator, and the wound or laminated film-like electrode is sealed in a cylindrical or square exterior material. Since the electrodes of these batteries are thin, the electrode area can be increased, and high rate charge / discharge is possible.

  In addition, as an exterior material for a lithium ion secondary battery, a laminate film is preferably used rather than a metal can in order to achieve further reduction in thickness and weight of the battery. As the laminate film, one surface of a metal foil such as an aluminum foil is usually coated with a hot-melt resin film, and the other surface is insulated and protected by resin film coating.

  As electrolytes for lithium ion secondary batteries, chain carbonates such as diethyl carbonate, ethyl methyl carbonate and dimethyl carbonate, cyclic carbonates such as propylene carbonate and ethylene carbonate, or cyclic esters such as γ-butyrolactone and γ-valerolactone Alternatively, a nonaqueous electrolytic solution in which a Li salt is dissolved in an organic solvent in which two or more of these are mixed is used. In particular, in order to smoothly move lithium ions in a low temperature environment, it is common to use a chain carbonate mixed with a cyclic carbonate.

  Among such lithium ion secondary batteries, a square battery can be used more efficiently when mounted on a device than a cylindrical battery, and can also be easily used as a battery as a battery pack. In the case of this rectangular battery, the stacked structure of strip electrodes is more advantageous than the wound structure in order to increase the filling rate of the electrode group with respect to the volume inside the battery.

  In the case of a stacked type square battery, when the electric capacity is increased, a technique for stacking a plurality of positive electrodes and negative electrodes accurately aligned is required. On the other hand, when the thickness of the electrode is increased, there is no need to align the positions of such a large number of positive electrodes and negative electrodes, but the electrode active material falls off during charging / discharging, and the capacity of the battery decreases. Problem arises. Therefore, for such problems, for example, as a conventional technique, a technique of using a porous sheet made of aluminum fibers as a core material of a positive electrode (see Patent Document 1), or holding a negative electrode active material in a metal porous body An electrode technology (see Patent Document 2) has been proposed.

In addition, lithium cobalt oxide (LiCoO ₂ ) containing cobalt (Co) that has been used for the positive electrode so far has less reserve than iron (Fe) and manganese (Mn), and is used continuously. Has a problem. In recent years, a positive electrode material containing iron as a main component has attracted attention as a low environmental load and ultra-low cost positive electrode material. For example, as a conventional technique, a technique for improving the electron conductivity of the positive electrode by mixing an active material of olivine type LiFePO ₄ and a noble conductive material having a higher redox potential (see Patent Document 3), or an olivine type A technique has been proposed in which a part of LiFePO ₄ is replaced with fluorine to reduce the electrical resistance of the positive electrode active material and increase the conductivity (see Patent Document 4).

  Further, as described above, an organic electrolyte is used as an electrolyte in the lithium ion secondary battery. For this reason, with regard to safety, several measures are taken, and it is designed not to cause an accident such as rupture or ignition even under severe use conditions. For example, it is designed to prevent accidents due to overcharge and overdischarge by attaching a protection circuit, and as a safety measure when the temperature of the battery rises, the conductive path from the terminal to the inside of the battery is In part, a PTC (Positive Temperature Coefficient) element is used in which the electrical resistance increases to almost infinite when the temperature exceeds a certain value.

  Even if such safety measures are implemented, current flows in a short-circuited location due to external factors (for example, when a nail is pierced) or an internal short circuit, and heat is generated by resistance heat generation. There is a possibility of so-called “thermal runaway” that causes a chemical reaction of the active material and electrolyte in the tubule, which may eventually lead to rupture and ignition. As one of the countermeasures, in a small lithium ion secondary battery, when the battery temperature rises, the separator melts, the hole of the separator is blocked and an insulating film is prevented from flowing current. Functions such as “shutdown function” are provided. In addition, a safety valve or the like is provided so that the battery does not burst when the internal pressure of the lithium ion secondary battery rises abnormally.

  Further, as described above, a laminate film is used as a packaging material for the lithium ion secondary battery. In a lithium ion secondary battery using a laminate film as an exterior material, an internal short circuit may occur through the metal foil layer. Such a lithium ion secondary battery generally has a battery element composed of a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte, and a heat-meltable resin layer formed by using two aluminum laminate films larger than the battery element. After being sandwiched inside, the outer peripheral portions of the laminate film are overlapped and heated, and sealed and assembled by welding of heat-meltable resins. The laminate film may be processed into a bag shape with a part opened, and after the battery element is inserted, the opening may be welded and sealed.

  In order to exchange electrons with the outside, it is necessary to extend the other end of the lead terminal whose one end is electrically joined to the positive electrode and the negative electrode through the sealing portion of the outer peripheral portion of the laminate film. As the lead terminal, a ribbon-like metal foil made of the same material as the current collector or a different material is used. In the lead terminal extending portion, the heat-meltable resin layers of the laminate film are heat-welded and sealed so as to sandwich the lead terminal. The metal foil layer and the lead terminal of the laminate film are insulated by the resin layer.

  However, if the resin layer is excessively melted, the metal foil layer is exposed and comes into contact with the lead terminal, and the positive electrode and the negative electrode may be short-circuited through this portion. With respect to this problem, in Patent Document 5, a resin having a structure in which both surfaces of an insulating film having a melting point higher than the heating temperature at the time of sealing is covered with a film that melts at the heating temperature is used between the laminate film and the lead terminal. It has been proposed to intervene. As a result, the low melting point layer melts and seals the extension, and the high melting point layer does not melt and the metal foil layer and the lead terminal are reliably insulated.

  Alternatively, the metal foil layer is exposed on the end face of the laminate film, and this portion is likely to come into contact with the lead terminal, so that an internal short circuit may occur. In order to solve this problem, it has been proposed that a part of the resin layer interposed between the laminate film and the lead terminal protrudes outside the exterior material (see Patent Document 6).

In addition, lithium ion secondary batteries have higher energy efficiency (power efficiency) in charge and discharge compared to lead-acid batteries and nickel metal hydride, so they are promising for electric vehicles and power storage applications. Has been promoted. In addition, medium-sized lithium ion secondary batteries are partly put into practical use in applications such as bicycles with electric assistance. The development of these medium-sized to large-sized batteries has been promoted by following the battery structure obtained in the development of small batteries.
JP-A-6-196170 JP-A-7-22021 JP 2001-110414 A JP 2003-187799 A JP 2000-268789 A JP 2000-268786 A

  In particular, in a large-sized lithium-ion battery using a laminate film as an exterior material, if a short circuit occurs, a very large current flows, which may cause overheating of the battery body and peripheral circuits. It is necessary to implement more reliable measures for the lead take-out part.

Then, while producing the lithium ion secondary battery by this prior art, the charging / discharging test of the produced lithium ion secondary battery was done. First, the produced lithium ion secondary battery will be described below. Lithium cobalt oxide (LiCoO ₂ ) was used as the positive electrode active material, 20 parts by weight of ketjen black as a conductive material, 10 parts by weight of polyvinylidene fluoride (hereinafter referred to as PVdF) as a binder (binder), and N-methyl as a solvent. A positive electrode paste was prepared using -2-pyrrolidone (hereinafter referred to as NMP). The paste thus obtained was filled in foamed aluminum (size: 10 cm × 20.5 cm, thickness 4 mm, porosity 92%) over 10 cm × 20 cm, leaving an unfilled portion of 5 mm × 10 cm. After drying, it was pressed using a hydraulic press to obtain an electrode having a thickness of 3.0 mm. The weight of the active material per area of the obtained electrode was 210 mg / cm ² , and the porosity of the positive electrode was 55%. An aluminum ribbon having a thickness of 100 μm was welded to the remaining unfilled portion to form a lead terminal. In the present specification, “parts by weight” is a value expressed as a percentage of the weight ratio of the positive electrode active material to the weight.

As the negative electrode active material, natural graphite powder made in China (average particle size 15 μm, d002 = 0.3357 nm, BET specific surface area 3 m ² / g) and vapor grown carbon fiber powder (trade name: VGCF) manufactured by Showa Denko KK (Registered Trademark)) (average particle diameter 15 μm, d002 = 0.359 nm, BET specific surface area 2 m ² / g) was mixed at a weight ratio of 50:50, and 12 parts by weight of PVdF was added as a binder to the solvent. A negative electrode paste was prepared using NMP. The obtained paste is used as a current collector 3b, and a non-woven part (size: 10.2 cm × 20.7 cm, thickness 2.5 mm, porosity 88%) of copper fiber is used, leaving an unfilled portion of 5 mm × 10 cm. .2 cm × 20.2 cm was filled, dried sufficiently, and then pressed using a hydraulic press to obtain an electrode having a thickness of 1.5 mm. The active material amount per area of the electrode thus obtained was 95 mg / cm ² and the porosity of the negative electrode was 50%. A nickel ribbon having a thickness of 100 μm was welded to the remaining unfilled portion to form a lead terminal. In this specification, “d002” is graphite having a layered crystal structure and is defined as an interval between adjacent layers (interval between (002) planes). Further, the “BET specific surface area” is an area per weight of pores capable of adsorbing nitrogen at 77 K, which is obtained by using the BET formula obtained by extending the monomolecular layer adsorption theory to a multimolecular layer.

  In addition, as a separator that is a member that separates both electrodes for preventing a short circuit due to direct contact between the positive electrode and the negative electrode, two positive microporous membranes made of polyethylene having a thickness of 25 μm were used. The sheets were placed and laminated at the positive electrode-negative electrode interface when sandwiched between two negative electrodes. This laminate was completely inserted into a square bag-shaped aluminum laminate film that was heat-sealed with the heat-meltable resin layer on the inside, leaving only one side, and only the positive and negative lead terminals were extended outward from the opening. I made it come out.

Then, as a non-aqueous electrolyte, borofluoride was added to a solvent in which ethylene carbonate (hereinafter EC) and dimethyl carbonate (hereinafter DMC) were mixed at a volume ratio of 1: 1 to a concentration of 1.5 mol / l. It was used by dissolving lithium (LiBF _4). After injecting this non-aqueous electrolyte into the bag-shaped exterior material, the opening is thermally welded so as to sandwich the lead terminal, thereby producing a lithium ion secondary battery having a design capacity of 5 Ah.

  Twenty batteries were produced as described above, and a charge / discharge test was performed under the following conditions. When charging, charging is performed until the charging current is 1.5 A and the voltage is 4.2 V, and then 15 hours elapses at the voltage 4.2 V, or the charging is terminated when the charging current is 0.1 A. When discharging, the discharge is performed until the discharge current is 1.5A and the voltage is 2.75V.

  If the battery voltage does not increase even when charging is started in this way, it is considered that a short circuit has occurred inside the battery. Therefore, the battery whose battery voltage did not reach 1.0 V even after 15 hours from the start of charging was counted as an internal short-circuiting battery. As a result, as shown in FIG. 5, as many as 13 batteries had defective charging with respect to 20 manufactured batteries, and it was confirmed that 65% internal short-circuiting was actually generated.

  In addition, after charging and discharging, a fully charged lithium ion secondary battery charged in the same manner once again was subjected to a nail penetration test that penetrates a 2.5 mmφ nail with the battery lying down sideways. did. As a result, as shown in FIG. 5, a dangerous state such as generation of white smoke is caused, which causes a safety problem.

  Thus, in the large-sized lithium ion secondary battery manufactured by simply enlarging the conventional small battery structure using the laminate film as an exterior material, the above-described small-sized lithium ion secondary battery is used. The safety measures that have been applied to are not sufficient.

  In addition, when a chain carbonate such as dimethyl carbonate is used for the electrolyte, the vapor pressure of the solvent increases, and a large amount of gas is generated in a high temperature environment. For this reason, in a lithium ion secondary battery containing a lot of chain carbonates, problems such as expansion and deformation of the outer package arise. Furthermore, when chain carbonate is included in the electrolytic solution, the amount of heat generated when a battery overcharge test or thermal test is performed during production is large. For this reason, in order to ensure sufficient safety, in addition to providing a protection circuit for preventing overcharge, it is necessary to use a plurality of protection mechanisms such as a safety valve, a current cutoff valve, a PTC element, etc. There is a problem that the battery becomes complicated and the energy density of the battery decreases. This problem is particularly noticeable in large-sized lithium ion secondary batteries.

In view of such a problem, the present invention uses a laminate film as an exterior material, particularly in a large-sized lithium ion secondary battery having a battery capacity of 5 Ah or more and an electric capacity per 1 cm ² of a positive electrode and a negative electrode of 10 mAh or more. The purpose is to propose a lithium ion secondary battery with excellent safety.

In order to achieve the above object, a lithium ion secondary battery of the present invention includes a positive electrode including a current collector having a positive electrode active material, a negative electrode including a current collector having a negative electrode active material, the positive electrode, and the positive electrode. A lithium ion secondary provided with a separator that prevents a short circuit due to physical contact with the negative electrode, a non-aqueous electrolyte containing a lithium salt, and a laminate film serving as an exterior material for housing these battery elements The battery has a battery capacity of 5 Ah or more, an electric capacity per 1 cm ^{2 of the} positive electrode and the negative electrode is 10 mAh or more, and the non-aqueous electrolyte contains a cyclic ester. When the lead terminal is taken out from the exterior material, a resin layer is provided at a contact portion between the lead terminal and the exterior material.

  In the lithium ion secondary battery of the present invention, the positive electrode active material is a compound containing iron as a main component.

Such a lithium ion secondary battery is characterized by a low environmental load and ultra-low cost positive electrode material. In order to continue to use LiCoO ₂ containing cobalt (Co) that has been used for the positive electrode so far, since Co has less reserve than iron (Fe) and manganese (Mn). There is no such thing as a problem.

The lithium ion secondary battery of the present invention is characterized in that the positive electrode active material is olivine type LiFePO ₄ .

Such a lithium ion secondary battery, like a commonly used positive electrode material such as LiCoO ₂ that has been used for the positive electrode so far, releases oxygen as the temperature rises, and the electrolyte solution burns and generates intense heat. Absent.

  In the lithium ion secondary battery of the present invention, the non-aqueous electrolyte contains at least one of γ-butyrolactone or γ-valerolactone.

  Such a lithium ion secondary battery has a high vapor pressure of the solvent as in the case where the chain carbonate is included in the electrolytic solution, and a large amount of gas is generated in a high temperature environment, and the outer body expands and deforms. Problems are prevented. Further, since the amount of heat generated is large, the problem that the battery manufacturing process is complicated in order to ensure sufficient safety and the energy density of the battery is reduced can be prevented.

  The lithium ion secondary battery according to the present invention has a structure in which the resin layer is formed by sandwiching and laminating a resin having a melting point equal to or higher than the welding temperature of the laminate film with a resin having a melting point equal to or lower than the welding temperature. Is protruded outside the exterior material.

  In such a lithium ion secondary battery, a resin layer having a melting point equal to or lower than the welding temperature is excessively melted, or the exposed metal foil layer and the lead terminal are in contact with each other on the end face of the laminate film. It is prevented that the positive electrode and the negative electrode are short-circuited through the portion.

  The lithium ion secondary battery of the present invention is characterized in that the content of the cyclic ester contained in the non-aqueous electrolyte is 50% or more in terms of volume fraction.

  In such a lithium ion secondary battery, since the content of the cyclic ester in the non-aqueous electrolyte is smaller than 50% in volume fraction, it is possible to prevent the safety of the large lithium ion secondary battery from being lowered. . A gel electrolyte or the like may be used as the non-aqueous electrolyte.

  In the lithium ion secondary battery of the present invention, the thickness of at least one of the positive electrode and the negative electrode is 1 mm or more.

  In such a lithium ion secondary battery, since the thickness of at least one of the positive electrode and the negative electrode is less than 1 mm, the porosity inside the electrode is low, and the weight of the positive electrode active material and the negative electrode active material is low. It does not decrease and the number of stacked layers does not increase.

  The lithium ion secondary battery of the present invention is characterized in that the separator has a porosity of 90% or less and the separator has a thickness of 5 μm or more.

  In such a lithium ion secondary battery, since the porosity of the separator is higher than 90%, the positive electrode and the negative electrode are brought into physical contact, and the lithium ion secondary battery is prevented from being internally short-circuited. Furthermore, since the thickness of the separator is less than 5 μm, the mechanical strength of the separator is insufficient, and it is possible to prevent an internal short circuit of the lithium ion secondary battery. The separator can be selected from non-woven fabric and microporous membrane made of polyethylene, polypropylene, polyester, etc., but when the separator is non-woven fabric made of polyester, the non-woven fabric is more than the microporous membrane. The non-aqueous electrolyte containing the cyclic ester has a high permeability and is more suitable.

  In the lithium ion secondary battery of the present invention, the current collector is a three-dimensional metal porous body having a plurality of holes.

  In such a lithium ion secondary battery, when the metal porous body having a three-dimensional structure including a plurality of pores is used for the current collector that constitutes the electrode, the thermal conductivity of the lithium ion secondary electrode is increased. Since excellent metal is present uniformly over the entire electrode, it is possible to improve heat dissipation within the electrode. This makes it possible to further increase safety.

  The lithium ion secondary battery of the present invention is characterized in that the porosity of the metal porous body constituting the current collector is 50% or more and 98% or less.

  In such a lithium ion secondary battery, if the porosity of the current collector is in the above range, the active material cannot be sufficiently filled because the porosity is low, and the energy density of the lithium ion secondary battery Is prevented from incurring a decrease. On the contrary, since the porosity is high, the strength of the electrode is weakened, and it is possible to prevent a sufficient heat dissipation effect from being obtained.

  By the structure of this invention, it becomes possible to improve the safety | security of the large sized lithium ion secondary battery which used the laminate film for the exterior material.

That is, since the lithium ion secondary battery of the present invention is large in size, if a short circuit occurs, a large current flows to reliably prevent the battery main body and peripheral circuits from overheating. In order to ensure insulation between the layer and the lead terminal, a resin layer having a structure is interposed between them. In addition, non-aqueous electrolytes are cyclic esters that have the properties of high dielectric constant and low viscosity, excellent oxidation resistance, high boiling point, low vapor pressure, high flash point, etc. Used for. In addition, a positive electrode material based on iron that can achieve low environmental load and low cost, especially olivine-type LiFePO ₄ with the characteristics that both high potential and high energy density and high safety and stability can be achieved at the same time. Use. By these things, compared with the lithium ion secondary battery as a conventional power supply for portable devices, the large sized lithium ion secondary battery excellent in safety can be provided.

Embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a schematic cross-sectional view of the lithium ion secondary battery of the present embodiment. The lithium ion secondary battery includes a positive electrode active material 1 and a negative electrode active material 2 that are mixed with a binder, a conductive material, and the like (not shown) to form a paste-like material, and a porous metal such as a sponge. Current collectors 3a and 3b, and separator 4 provided between current collectors 3a and 3b so that the positive electrode side and the negative electrode side are not in direct contact with each other, lithium borofluoride (LiBF ₄ ), etc. A non-aqueous electrolyte solution 5 in which an electrolyte salt (not shown) is dissolved, and a current collector 3a, 3b, a separator 4, and an exterior material 6 that covers the non-aqueous electrolyte solution 5 are provided.

  In such a lithium ion secondary battery, the positive electrode active material 1 is applied to the current collector 3a to form the positive electrode, and the negative electrode active material 2 is applied to the current collector 3b to form the negative electrode. . In the lithium ion secondary battery of the present embodiment, as shown in FIG. 1, two separators 4 are used, and one positive electrode is disposed at the positive electrode-negative electrode interface when sandwiched between two negative electrodes, and laminated. Yes.

  FIG. 2 is a projection view of the positive electrode and the negative electrode. When filling the positive electrode and the negative electrode active material, a part of the porous current collector is left unfilled, and a ribbon-shaped metal foil is welded to the current collectors 3a and 3b to form lead terminals 7a and 7b.

  FIG. 3 shows the lead terminal take-out portion. As the exterior material 6, one surface of the aluminum foil 6a is coated with a heat-meltable resin film 6b, and the other surface is usually insulated and protected by a resin film coating (not shown). At the outer periphery of the rectangular bag-shaped exterior material, the heat-meltable resin film 6b is placed inside, and the lead terminals 7a and 7b are sandwiched by heat welding so that the melting point is equal to or higher than the welding temperature. A resin layer 8 having a structure in which a resin film 8b having a melting point not higher than the welding temperature is sandwiched and laminated is interposed between the heat-meltable resin film 6b of the exterior material 6 and the lead terminals 7a and 7b. And a part thereof is protruded outside the exterior material. As a result, the resin layer having a melting point equal to or lower than the welding temperature is excessively melted, or the exposed aluminum foil 6a layer and the lead terminals 7a and 7b are reliably insulated at the end face of the laminate film.

  By providing the separator 4 between the current collectors 3a and 3b, the positive electrode side and the negative electrode side are in direct contact with each other, thereby preventing a short circuit. During charging, lithium ions escape from the positive electrode side and move to the negative electrode, and during discharging, lithium ions escape from the negative electrode side and return to the positive electrode side. That is, the charge / discharge operation is performed by the movement of lithium ions between the positive electrode and the negative electrode.

  Details of the lithium ion secondary battery configured as shown in FIGS. 1 to 3 will be described below. First, when such a lithium ion secondary battery is used in a system that requires a large capacity, such as a household distributed power source and a power storage system of a photovoltaic power generation system, it is necessary to use an assembled battery to obtain a large capacity. There is. However, when a small lithium ion secondary battery having a small charge / discharge capacity is used as a single battery, hundreds to thousands of single cells are required, and maintenance of the power storage system becomes very complicated. For this reason, the lithium ion secondary battery is large in size and large in charge / discharge capacity, and preferably the charge / discharge capacity as a single battery is 5 Ah or more.

At this time, when the electric capacity per cm ² is less than 10 mAh in the positive electrode composed of the positive electrode active material 1 and the current collector 3a and the negative electrode composed of the negative electrode active material 2 and the current collector 3b. The number of stacked layers per unit cell becomes ten to several tens, and the unit cell manufacturing operation becomes complicated. For this reason, the electric capacity per 1 cm < ² > of a positive electrode and a negative electrode shall be 10 mAh or more. The configuration of the lithium ion secondary battery having such a capacity value will be described below.

(Positive electrode and negative electrode)
First, when the thickness of the positive electrode and the negative electrode is smaller than 1 mm, the porosity inside the electrode is decreased, and the weight of the electrode active material is decreased, resulting in an increase in the number of laminated layers. Therefore, in the present embodiment, the thickness of the positive electrode and the negative electrode is 1 mm or more, although it depends on the density of the active material, the binder to be mixed, the type of conductive material, the press pressure of the electrode, and the like.

  Further, regarding the thickness of the positive electrode and the negative electrode used in the present embodiment, it is preferable to increase the thickness of the positive electrode when either one of the electrodes is a thick electrode. This is because in a lithium ion secondary battery, the negative electrode is charged and discharged at a potential close to that of lithium metal, and therefore lithium may be deposited when the polarization of the negative electrode increases. Further, by increasing the thickness of the positive electrode, a structure in which the negative electrode is disposed on both sides of the positive electrode and the polarization of the negative electrode can be reduced, so that lithium deposition can be avoided. In other words, the thick positive electrode is sandwiched between the negative electrodes having about half the capacity of the positive electrode.

Moreover, as for the positive electrode material such as LiCoO ₂ which has been used for the positive electrode so far, the oxygen is released as the temperature rises, and the electrolyte solution burns to generate intense heat. Further, LiCoO ₂ containing cobalt (Co) has a problem that it is difficult to keep using it because Co has less reserve than iron (Fe) and manganese (Mn).

On the other hand, a positive electrode material mainly composed of iron is preferable because of low environmental load and low cost. Of these, olivine-type LiFePO ₄ has attracted attention. This LiFePO ₄ is characterized by being able to satisfy both the high potential / high energy density and high safety / stability. In addition, since LiFePO ₄ has all oxygen bonded to phosphorus by a strong covalent bond, it does not generate heat like other positive electrode materials such as LiCoO ₂ described above, and oxygen release due to temperature rise is very unlikely to occur. It is preferable from the viewpoint of safety. Moreover, since it contains phosphorus, the positive electrode generates heat, and even when the electrolyte leaks, it can be expected to have an anti-flame effect, which is preferable. For this reason, the positive electrode of the present embodiment uses an iron-based positive electrode material, particularly olivine type LiFePO ₄ as the positive electrode active material 1.

As described above, the lithium ion secondary battery using olivine-type LiFePO ₄ as the positive electrode active material 1 has a charging voltage of about 3.5 V, and is almost completely charged at 3.8 V, which causes decomposition of the electrolytic solution. There is a margin to the voltage of about 4.5V. In addition, when the positive electrode material whose charging voltage reaches 4.0 V or higher is used for the positive electrode active material 1, it is not preferable because the electrolyte solution is easily decomposed when the charging voltage is further increased.

  In addition, the safety of large batteries greatly depends on the heat generation behavior and the heat dissipation rate. In a large-sized lithium ion secondary battery, since the battery size is large, heat tends to accumulate inside. However, when a metal three-dimensional structure is used for the current collectors 3a and 3b constituting the electrode, the metal having excellent thermal conductivity is uniformly present throughout the entire electrode within the lithium ion secondary electrode, so that It becomes possible to improve the heat dissipation of. For this reason, in this embodiment, the current collectors 3a and 3b are made of a porous metal body that is three-dimensionally connected. This makes it possible to further improve safety.

  Furthermore, when the porosity of the metal porous body described above is low, the active material cannot be sufficiently filled, resulting in a decrease in the energy density of the lithium ion secondary battery. Further, when the porosity is high, the strength of the electrode becomes weak and the effect of heat dissipation cannot be obtained sufficiently. For this reason, in this embodiment, the porosity of a metal porous body shall be 50% or more and 98% or less, More preferably, you may be 75% or more and 98% or less.

  Since the electrodes using the above-described three-dimensional structure as the current collectors 3a and 3b are arranged with a metal having a high three-dimensional thermal conductivity, it is possible to keep the temperature inside the electrode uniform and large. It is possible to suppress cycle deterioration caused by a local temperature rise that becomes a problem in the case of a lithium ion secondary battery. In the electrode used in this embodiment, metal fibers may be dispersed in the positive electrode active material 1 or the negative electrode active material 2 in order to improve conductivity between the active materials and heat dissipation. The length of the metal fiber is preferably about the same as the size of the gap of the three-dimensional structure used as the current collectors 3a and 3b.

  The “three-dimensional metal porous body” refers to a sponge-like metal structure, a nonwoven fabric made of metal fibers, a sintered metal powder, a metal foil molded into a honeycomb structure, and the like. Moreover, the material of the metal porous body used for each of these current collectors 3a and 3b is not particularly limited, but aluminum, titanium, stainless steel, etc. have oxidation resistance as materials used for the current collector 3a for the positive electrode. It is preferable because it is high, and copper, nickel, iron, stainless steel, and the like are preferable for the material used for the current collector 3b for the negative electrode because it is difficult to alloy with lithium and has high electrical conductivity.

(Lead terminal)
The lead terminals 7a and 7b for taking out current to the outside are often made of the same material as the current collector, but may be made of different materials. The thickness is suitably 30 μm or more and 100 μm or less in view of the necessary mechanical strength and airtightness.

(Non-aqueous electrolyte)
Cyclic esters have the properties of having both a high dielectric constant and low viscosity, and are excellent in oxidation resistance, high boiling point, low vapor pressure, and high flash point. For this reason, when used in non-aqueous electrolytes, the amount of heat generated when stored at high temperatures or overcharged is small, and the amount of gas generated is also small, which is very high compared to conventional small lithium ion secondary batteries. It is suitable as a solvent for an electrolytic solution of a large lithium ion secondary battery that requires safety. For this reason, it is assumed that the nonaqueous electrolytic solution 5 used in the present embodiment contains a cyclic ester. Examples of the cyclic ester include γ-butyrolactone (hereinafter referred to as GBL) and γ-valerolactone, and one or more of these may be used in combination.

  In the non-aqueous electrolyte 5, solvents that can be used by mixing with GBL include cyclic carbonates such as propylene carbonate (PC), ethylene carbonate (EC), and butylene carbonate, dimethyl carbonate (DMC), and diethyl carbonate. (DEC), chain carbonates such as ethyl methyl carbonate and dipropyl carbonate, lactones such as γ-valerolactone, furans such as tetrahydrofuran and 2-methyltetrahydrofuran, diethyl ether, 1,2-dimethoxyethane, 1, Examples include ethers such as 2-diethoxyethane, ethoxymethoxyethane, and dioxane, dimethyl sulfoxide, sulfolane, methyl sulfolane, acetonitrile, methyl formate, methyl acetate, and the like. It does not matter. In particular, cyclic carbonates such as PC, EC and butylene carbonate are preferred because they are high-boiling solvents.

  In addition, regarding the content of the cyclic ester in the non-aqueous electrolyte solution 5, if the content of the cyclic ester is less than 50% in terms of volume fraction, the safety of the large-sized lithium ion secondary battery is lowered. In the non-aqueous electrolyte solution 5 of the lithium ion secondary battery, the GBL content in the solvent for the non-aqueous electrolyte solution has a volume fraction of 50% or more. As the nonaqueous electrolytic solution 5, a gel electrolyte in which an electrolytic solution composed of the above solvent is held in a polymer matrix may be used.

As electrolyte salts dissolved in the non-aqueous electrolyte 5, lithium borofluoride (LiBF ₄ ), lithium hexafluorophosphate (LiPF ₆ ), lithium trifluoromethanesulfonate (LiCF ₃ SO ₃ ), trifluoro Examples thereof include lithium salts such as lithium acetate (LiCF ₃ COO) and lithium bis (trifluoromethanesulfone) imide (LiN (CF ₃ SO ₂ ) ₂ ), and one or more of these may be mixed and used.

(Separator)
The separator 4 used in the present embodiment can be selected from a nonwoven fabric made of polyethylene, polypropylene, polyester, or the like, or a microporous membrane. When the separator 4 is a nonwoven fabric made of polyester, the nonwoven fabric is fine. The nonaqueous electrolytic solution 5 containing GBL is higher in permeability than the porous membrane, and is more preferable.

  Further, when the porosity of the separator 4 is higher than 90%, the positive electrode and the negative electrode are brought into physical contact, causing an internal short circuit of the lithium ion secondary battery. On the other hand, when the thickness of the separator 4 is less than 5 μm, the mechanical strength of the separator 4 is insufficient, which causes an internal short circuit of the lithium ion secondary battery. For this reason, in this embodiment, the porosity of the separator 4 is 90% or less, and the thickness of the separator 4 is 5 μm or more.

(Exterior material)
In addition, as the outer packaging material 6 of the lithium ion secondary battery used in the present embodiment, a can made of iron, stainless steel, aluminum or the like has been used in the past, but in order to achieve further weight reduction and thickness reduction. A laminate film is more preferably used. The laminate film is usually coated on one surface of an aluminum foil 6a with a heat-meltable resin film 6b such as a modified polyolefin film, and the other surface is insulated and protected by a coating such as nylon or polyester film. Used. The thickness of each layer is suitably about several tens of μm in consideration of the workability and the purpose of reducing the thickness and weight while maintaining the necessary strength. Since the large-sized lithium ion secondary battery has many opportunities to be used as an assembled battery, the exterior material 6 is preferably rectangular or thin.

(Resin layer)
The resin layer 8 having a structure interposed between the exterior material 6 and the lead terminals 7a and 7b used in the present embodiment has a modified polyolefin film on both sides of the inner resin film 8b such as polyethylene terephthalate or nylon. Etc., which are coated with films 8a and 8c having a relatively low melting point. The coating on both surfaces may be films of different materials. Moreover, the inner layer and the coating layers on both sides thereof may each be composed of a plurality of layers. The low melting point layer contributes to the bonding at the exterior material / lead terminal interface. On the other hand, even if the low melting point layer is melted excessively, the inner layer resin film is formed between the aluminum foil 6a layer of the exterior material 6 and the lead terminals 7a, 7b. 8b ensures insulation.

  Below, the evaluation result in Example 1 and Example 1 of the lithium ion secondary battery produced by this embodiment is demonstrated.

An olivine type LiFePO ₄ is used for the positive electrode active material 1, 20 parts by weight of acetylene black as a conductive material, 10 parts by weight of PVdF as a binder, and NMP is used as a solvent to prepare a positive electrode paste. Filled foamed aluminum (size: 10 cm × 20.5 cm, thickness 4 mm, porosity 92%) using the obtained paste as current collector 3a over 10 cm × 20 cm leaving an unfilled portion of 5 mm × 10 cm, After sufficiently drying, it is pressed using a hydraulic press to obtain an electrode having a thickness of 3.0 mm. The active material weight per area of the electrode thus obtained was 210 mg / cm ² and the porosity of the positive electrode was 55%. An aluminum ribbon having a thickness of 100 μm is welded to the remaining unfilled portion to form lead terminals 7a.

The negative electrode active material 2 includes natural graphite powder made in China (average particle size 15 μm, d002 = 0.3357 nm, BET specific surface area 3 m ² / g) and vapor grown carbon fiber powder (trade name: manufactured by Showa Denko KK). VGCF (registered trademark)) (average particle size 15 μm, d002 = 0.3359 nm, BET specific surface area 2 m ² / g) mixed at a weight ratio of 50:50, 12 parts by weight of PVdF was added as a binder, A negative electrode paste is prepared using NMP. The obtained paste is used as a current collector 3b, and a non-woven part (size: 10.2 cm × 20.7 cm, thickness 2.5 mm, porosity 88%) of copper fiber is used, leaving an unfilled portion of 5 mm × 10 cm. .2 cm × 20.2 cm is filled, dried sufficiently, and then pressed using a hydraulic press to obtain an electrode having a thickness of 1.5 mm. The active material amount per area of the electrode thus obtained was 95 mg / cm ² and the porosity of the negative electrode was 50%. An aluminum ribbon having a thickness of 100 μm was welded to the remaining unfilled portion to form lead terminals 7b.

  Further, two polyester nonwoven fabrics having a thickness of 25 μm were used as the separator 4, and one obtained positive electrode was placed at the positive electrode-negative electrode interface when sandwiched between two negative electrodes, and laminated. The laminate was completely inserted into a square bag-shaped exterior material (aluminum laminate film) 6 with the layer of the heat-meltable resin 6b on the inner side and the outer peripheral portion being heat-sealed, leaving one side, and the positive electrode and the negative electrode Only the lead terminals 7a and 7b were extended outward from the opening. 40 μm thick modified polypropylene film (melting point: about 140 ° C.) −50 μm thick polyethylene terephthalate film (melting point: about 260 ° C.) on both surfaces of the lead terminals 7 a and 7 b facing the welded portion of the exterior material After arranging the resin film so as to protrude 2 mm outside the exterior material 6 in the order of the above polypropylene film, the opening was heated and welded to form the resin layer 8.

The non-aqueous electrolyte 5 was prepared by encapsulating LiBF ₄ dissolved in a solvent in which GBL and EC were mixed at a volume ratio of 7: 3 so that the concentration was 1.5 mol / l. In this way, a lithium ion secondary battery having a design capacity of 5 Ah according to this example was manufactured and tested. This test and test results will be described later.

  Twenty batteries were produced as in Example 1, and a charge / discharge test was performed under the following conditions. When charging, charging is performed until the charging current is 1.5 A and the voltage is 3.8 V, and then 15 hours elapses at the voltage 3.8 V, or the charging is terminated when the charging current is 0.1 A. These batteries are discharged until the discharge current is 1.5 A and the voltage is 2.25V.

  If the battery voltage does not increase even when charging is started in this way, it is considered that a short circuit has occurred inside the battery. Therefore, the battery whose battery voltage did not reach 1.0 V even after 15 hours from the start of charging was counted as an internal short-circuiting battery.

  In addition, after charging and discharging in this manner, a fully charged lithium ion secondary battery charged in the same manner once again was laid sideways, and a nail penetration test was conducted to penetrate a 2.5 mmφ nail. The result is shown in FIG.

Comparing the present invention and the prior art with reference to FIGS. 4 and 5, the lithium ion secondary battery of Example 1 in which a resin layer having a structure is interposed between the exterior material (laminate film) 6 and the lead terminals 7a and 7b is as follows. The occurrence of an internal short circuit immediately after creation can be suppressed very effectively (internal short circuit occurrence rate 0%). The lithium ion secondary battery of the present invention using GBL as the non-aqueous electrolyte 5 and olivine type LiFePO ₄ as the positive electrode active material 1 generates white smoke even when the nail penetration test is performed. Without any abnormal heat generation inside or volatilization of the organic solvent used in the non-aqueous electrolyte 5, which is a safety improvement over the conventional lithium ion secondary battery. . Of course, a reduction in the incidence of short-circuiting also leads to an improvement in safety.

These are schematic sectional drawing which shows the structure of the lithium ion secondary battery of this invention. These are the schematic projection figures which show the structure of the electrode of the lithium ion secondary battery of this invention. These are schematic sectional drawing which shows the structure of the lead terminal extraction part of the lithium ion secondary battery of this invention. These are figures which show the evaluation result of the lithium ion secondary battery in Example 1 of this invention. These are figures which show the evaluation result of the lithium ion secondary battery by a prior art.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Positive electrode active material 2 Negative electrode active material 3a Current collector for positive electrodes 3b Current collector for negative electrodes 4 Separator 5 Nonaqueous electrolyte 6 Exterior material (laminate film)
6a Aluminum foil 6b Heat-meltable resin film 7a Positive electrode lead terminal 7b Negative electrode lead terminal 8 Resin layer 8a, 8c Resin film having melting point not higher than welding temperature 8b Resin film having melting point not lower than welding temperature

Claims (10)

A positive electrode having a current collector having a positive electrode active material, a negative electrode having a current collector having a negative electrode active material, a separator for preventing the positive electrode and the negative electrode from being short-circuited due to physical contact; A lithium ion secondary battery comprising a non-aqueous electrolyte containing a lithium salt and a laminate film serving as an exterior material for housing these battery elements,
The battery capacity is 5 Ah or more, the electric capacity per 1 cm ^{2 of the} positive electrode and the negative electrode is 10 mAh or more, and the non-aqueous electrolyte contains a cyclic ester. A lithium ion secondary battery comprising a resin layer at a contact portion between a lead terminal and an exterior material when taken out from the exterior material.   The lithium ion secondary battery according to claim 1, wherein the positive electrode active material is a compound containing iron as a main component. The lithium ion secondary battery according to claim 1, wherein the positive electrode active material is olivine type LiFePO ₄ .   The lithium ion secondary battery according to any one of claims 1 to 3, wherein the nonaqueous electrolytic solution contains at least one of γ-butyrolactone and γ-valerolactone.   The resin layer has a structure in which a resin having a melting point equal to or higher than the welding temperature of the laminate film is sandwiched and laminated between resins having a melting point equal to or lower than the welding temperature, and a part of the resin layer protrudes outside the exterior material. The lithium ion secondary battery according to claim 4.   The lithium ion secondary battery according to claim 5, wherein a content rate of the cyclic ester contained in the non-aqueous electrolyte is 50% or more in terms of volume fraction.   The lithium ion secondary battery according to claim 6, wherein a thickness of at least one of the positive electrode and the negative electrode is 1 mm or more.   The lithium ion secondary battery according to claim 7, wherein a porosity of the separator is 90% or less and a thickness of the separator is 5 μm or more.   The lithium ion secondary battery according to claim 8, wherein the current collector is a three-dimensional porous metal body having a plurality of pores.   The lithium ion secondary battery according to claim 9, wherein the current collector has a porosity of 50% or more and 98% or less.

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