JP5344970B2 - Lithium ion secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a lithium ion secondary battery which includes a separator superior in oxidation resistance and makes both superior reliability and safety compatible even when carrying out charging at a higher voltage than before. <P>SOLUTION: The lithium ion secondary battery is equipped with a positive electrode 11 and a negative electrode 12 capable of reversively storing/releasing lithium, a separator 13 arranged between the positive electrode 11 and the negative electrode 12, a battery case to house the positive electrode 11, the negative electrode 12, and the separator 13, and a nonaqueous electrolytic solution filled into the battery case. The separator 13 has ultrahigh molecular weight polyethylene of 500,000 or more of the weight average molecular weight as the main component, and contains polyolefin resin composition obtained by chemically cross-linking ultra-high molecular weight polyethylene, polynorbornene, and grafted polyethylene, and gel fraction of the polyolefin resin composition is 42% or more and 57% or less. <P>COPYRIGHT: (C)2011,JPO&amp;INPIT

Description

  The present invention relates to a lithium ion secondary battery, and more particularly to improving the reliability of a lithium ion secondary battery.

  A lithium ion secondary battery, which is a type of non-aqueous electrolyte secondary battery, can be charged with a high energy density and has a high voltage during discharge. For this reason, it is widely used as a main power source for mobile communication devices and portable electronic devices. In recent years, these devices are required to be further reduced in size and performance, and accordingly, it is required to develop a higher performance lithium ion secondary battery.

  In the lithium ion secondary battery currently in practical use, a graphite material is mainly used as a negative electrode active material. However, since these lithium ion secondary batteries already have a capacity close to the theoretical capacity (about 370 mAh / g) of the graphite material, it is difficult to further increase the energy density. For this reason, the use of various novel materials has been studied as a negative electrode active material that enables further increase in capacity of lithium ion secondary batteries. For example, as a negative electrode active material, metals capable of occluding and releasing lithium, such as silicon and tin, and alloys containing these metals have been proposed, and a large increase in capacity is expected.

  As the positive electrode active material, lithium transition metal composite oxides having a layered structure or a spinel structure such as lithium cobaltate, lithium nickelate, and lithium manganate are currently widely used. Research on positive electrode active material that can achieve higher energy density than these materials has also been conducted. However, no material superior to these currently used materials has been found in comprehensive characteristics including reversibility and stability of reaction accompanying charge / discharge and electron conductivity.

  On the other hand, the layered lithium transition metal oxides such as lithium cobaltate and lithium nickelate that are currently used improve the utilization rate of lithium constituting the crystal, thereby further increasing the capacity of the lithium ion secondary battery. May be possible. For example, a method has been considered in which a lithium ion secondary battery is charged to a higher voltage and more lithium is released from the positive electrode active material at the time of charging, thereby improving the utilization rate of lithium and increasing the capacity. In this case, charging is performed by raising the positive electrode potential to a higher potential. As a result, the average voltage at the time of discharging the battery is increased, and the effect of improving the energy density of the battery can be expected from both the capacity and voltage aspects.

  However, when the charging voltage is increased, various materials constituting the lithium ion secondary battery are exposed to a high potential, and thus decompose or deteriorate. For this reason, it is difficult to set the charging voltage to a predetermined value or more. For example, in a general lithium ion secondary battery in which lithium cobaltate and a carbon material are used for the positive electrode and the negative electrode, the battery voltage at the end of charging is set to 4.2 V or less. One reason for this is that when the battery voltage at the end of charging exceeds 4.2 V, the polyethylene separator is oxidized, and the separator is deteriorated and gas due to oxidation is generated (Patent Document 1). ). In particular, when the lithium ion secondary battery is rectangular, there is a problem that the lithium ion secondary battery swells due to the generated gas.

  That is, when the battery voltage at the end of charging is set to 4.2 V, only about 60% of the positive electrode active material such as lithium cobaltate used for the positive electrode is utilized for its theoretical capacity. For this reason, in the conventional non-aqueous electrolyte secondary battery, the energy density can still be increased as the positive electrode active material, but the energy density of the lithium ion secondary battery is increased due to the material used for the separator. I could not.

  In order to solve this problem, Patent Documents 1 to 3 propose that a microporous membrane layer made of polypropylene having higher oxidation resistance than polyethylene is disposed on the surface of the separator facing the positive electrode. However, the melting point of polypropylene is higher than that of polyethylene. For this reason, when polypropylene is used alone as a separator, when the lithium ion secondary battery heats up abnormally, the separator melts and the separator hole closes, increasing the temperature at which the shutdown function is activated. Resulting in. In other words, a new problem arises that the battery function does not stop unless the temperature becomes higher.

  In order to solve this problem, it is conceivable to form a separator by laminating polyethylene and polypropylene, and to dispose the separator in a lithium ion secondary battery so that the polypropylene is in contact with the positive electrode. In this case, the shutdown function is exhibited by polyethylene at the same temperature as in the past. However, the separator becomes thicker due to the stacking, and the volume that does not contribute to energy storage in the lithium ion secondary battery increases, thereby reducing the energy density of the lithium ion secondary battery.

  Accordingly, there is a need to improve polyethylene materials and increase oxidation resistance. Patent Documents 4 to 7 disclose techniques related to modification of polyethylene materials.

JP 2001-273880 A JP 2006-164873 A JP 2006-286531 A International Publication No. 96/27633 Pamphlet JP 2004-263012 A International Publication No. 01/016219 Pamphlet JP 2001-59036 A

  Patent Documents 4 to 7 all disclose that a crosslinked structure is introduced into polyethylene. However, Patent Documents 4 to 7 are mainly aimed at improving the mechanical strength and permeability of the polyethylene material, and do not particularly focus on improving the oxidation resistance. Further, Patent Document 4 forms a cross-linked structure using an electron beam. According to this technique, residual radicals generated by an electron beam and not involved in the cross-linking reaction are generated at the time of manufacturing a secondary battery. May cause the electrolyte to deteriorate quickly.

  The present invention solves such a problem of the prior art, includes a separator having excellent oxidation resistance, and can achieve both excellent reliability and safety even when charged at a higher voltage than before. An object is to provide a secondary battery.

  A lithium ion secondary battery according to the present invention includes a positive electrode and a negative electrode capable of reversibly occluding and releasing lithium, a separator disposed between the positive electrode and the negative electrode, and a battery case containing the positive electrode, the negative electrode, and the separator. And a non-aqueous electrolyte filled in the battery case, wherein the separator is mainly composed of ultra high molecular weight polyethylene having a weight average molecular weight of 500,000 or more, and the ultra high molecular weight polyethylene, polynorbornene, and graft polyethylene are included. It contains a chemically crosslinked polyolefin resin composition, and the gel fraction of the polyolefin resin composition is 42% or more and 57% or less.

  In a preferred embodiment, the grafted polyethylene is maleic anhydride grafted polyethylene.

  In a preferred embodiment, the battery is charged with a charger having a maximum charging voltage of 4.3V to 4.8V.

  In a preferred embodiment, the polyolefin resin composition does not substantially decompose even at a potential of 4.3 V to 4.8 V with respect to lithium.

  According to the present invention, since the ultra-high molecular weight polyethylene as the main component of the separator has a three-dimensional structure that is chemically crosslinked, the rotation between carbon and carbon and the change in the bond angle that occur when polyethylene is oxidized are regulated. Is inhibited from oxidation. For this reason, even when the battery is charged at a voltage exceeding 4.2 V, oxidation of the separator is prevented and gas generation is suppressed. Moreover, since the main component is polyethylene, the melting point of the separator is equivalent to that of a conventional polyethylene separator, and high safety can be expressed. Therefore, according to the present invention, a lithium ion secondary battery capable of high voltage charging that achieves both excellent reliability and safety is realized.

1 is a perspective view showing an embodiment of a nonaqueous electrolyte secondary battery according to the present invention. FIG. 2 shows a cross-sectional view of an electrode group of the nonaqueous electrolyte secondary battery in FIG. 1. FIG. 3 shows a cross-sectional view of an electrode group of a model cell produced in Example 2. The dimension of the positive electrode of the model cell produced in Example 2 is shown. The dimension of the negative electrode of the model cell produced in Example 2 is shown. (A) And (b) has each shown the perspective view and sectional drawing of the model cell produced in the Example. It is a graph which shows the relationship between the gel fraction of the separator by an Example and a comparative example, and a side reaction electric energy. It is a graph which shows the relationship between the charging voltage by an Example and a comparative example, and the amount of swelling of a battery.

The inventor of the present application studied in detail the oxidation of polyethylene constituting a conventional separator. When polyethylene ((CH ₂ ) _n ) is oxidized, hydrogen bonded to two adjacent carbons is extracted one by one to generate protons, and the carbon chain from which protons are extracted forms a double bond. . This reaction occurs every other carbon chain in the polyethylene to form a conjugated double bond. As a result, the electrons can move relatively freely on the carbon chain of the conjugated double bond formed in the polyethylene. Thus, it is considered that the movement of electrons, that is, the oxidation reaction proceeds to the inside of the separator by converting the carbon chain so as to include a conjugated double bond in which the movement of electrons is relatively free.

  As described above, due to the oxidation of polyethylene, the carbon-carbon bond form needs to change from a single bond capable of free rotation to a double bond taking a planar form impossible to rotate. The inventor of the present application constrains the free rotational motion in the carbon-carbon bond by physically designing the polyethylene structure so that the molecular chain has a three-dimensional cross-linked structure, so that a double bond cannot be physically formed. For example, it was found that the above-described oxidation of polyethylene does not proceed.

  Furthermore, as a result of intensive studies by the inventors of the present application, in a polyolefin resin composition having an ultrahigh molecular weight polyethylene as a main component and having a crosslinked structure by chemical crosslinking, if the gel fraction of the polyolefin resin composition is 42% or more, It has become possible to effectively constrain the free rotational movement of polyethylene molecular chains, and has obtained the knowledge that no double bond is formed, leading to the present invention.

  Hereinafter, embodiments of a lithium ion secondary battery according to the present invention will be described in detail with reference to the drawings.

  FIG. 1 is a perspective view showing an embodiment of a non-aqueous electrolyte secondary battery according to the present invention, and a part thereof is cut so that an internal structure can be seen. As shown in FIG. 1, the lithium ion secondary battery of this embodiment includes an electrode group 1 and a battery case 4 that houses the electrode group 1. The electrode group 1 includes a negative electrode and a positive electrode, and a separator sandwiched between them, as will be described in detail below. In the present embodiment, each of the negative electrode, the separator, and the positive electrode has a sheet shape, and is wound in a spiral shape in a stacked state. A positive electrode lead and a negative electrode lead 3 made of nickel are welded to the positive electrode and the negative electrode, respectively, and are attached to an insulating frame (not shown) made of polypropylene resin provided on the upper part of the electrode group 1. The other end of 2 is spot welded to the inner surface of the aluminum sealing plate 8. The other end of the negative electrode lead 3 is spot welded to the lower part of a nickel negative electrode terminal 6 attached to the center of the sealing plate 8 via an insulating material 5.

  The open end of the battery case 4 and the peripheral edge of the sealing plate 8 are laser welded. A predetermined amount of non-aqueous electrolyte is injected into the battery case 4 from the inlet, and the inlet is sealed with an aluminum plug 7.

  FIG. 2 shows an enlarged cross-sectional structure of a part of the electrode group 1. As shown in FIG. 2, the electrode group 1 includes a positive electrode 11, a negative electrode 12, and a separator 13 sandwiched between the positive electrode 11 and the negative electrode 12. As described above, in the present embodiment, the electrode group 1 is wound in a spiral shape. In the wound state, the positive electrode 11 and the negative electrode 12 are laminated so as to be adjacent to each other. For this reason, in FIG. 2, although another separator is each provided in the upper side of the positive electrode 11, and the lower side of the negative electrode 12, this another separator is not shown.

  The positive electrode 11 includes a positive electrode current collector 11a and a positive electrode mixture 11b supported on both surfaces of the positive electrode current collector 11a. For the positive electrode current collector 11a, a material known as a positive electrode current collector for a lithium secondary battery, such as aluminum, titanium, and alloys thereof (main additive elements such as silicon, carbon, manganese, magnesium, copper, etc.) are used. be able to. The positive electrode mixture 11b contains a positive electrode active material and, if necessary, is made into a paste with a conductive material such as artificial graphite, natural graphite, fibrous carbon or carbon black, and a binder such as polyvinylidene fluoride or polytetrafluoroethylene, By being applied to the positive electrode current collector 11a and dried, it is carried on both surfaces of the positive electrode current collector 11a.

As the positive electrode active material, a known positive electrode active material that reversibly occludes and releases lithium is used. Preferably, the positive electrode active material is a lithium-containing transition metal oxide. Examples thereof include LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , LiNi _1/3 Mn _1/3 Co _1/3 O ₂ , LiMn ₂ O ₄ , Li ₂ MnO ₃ , LiNi _0.5 Mn _1.5 O _{4 and the} like.

  The negative electrode 12 includes a negative electrode current collector 12a and a negative electrode mixture 12b supported on both surfaces of the negative electrode current collector 12a. As the negative electrode current collector 12a, a material known as a current collector of a negative electrode for a lithium secondary battery, such as copper, nickel, and stainless steel, can be used. The negative electrode mixture 12b contains a negative electrode active material and, if necessary, a paste together with a conductive material such as artificial graphite, natural graphite, fibrous carbon or carbon black, and a binder such as polyvinylidene fluoride or styrene-butadiene rubber. Is applied to the negative electrode current collector 12a and dried to be carried on both surfaces of the negative electrode current collector 12a. A negative electrode mixture 12b containing a negative electrode active material may be formed on the foil-shaped negative electrode current collector 12a by vapor deposition or sputtering.

  As the negative electrode active material, a known negative electrode active material that reversibly occludes and releases lithium is used. Preferably, graphite, amorphous carbon capable of occluding and releasing Li, and compounds such as Sn, Si, and SiO capable of being alloyed with Li are used as the negative electrode active material.

  As shown in FIG. 2, the separator 13 is sandwiched between the positive electrode 11 and the negative electrode 12 so as to face the positive electrode mixture 11b and the negative electrode mixture 12b. The separator 13 includes a polyolefin resin composition having ultrahigh molecular weight polyethylene as a main component and having a crosslinked structure by chemical crosslinking. More specifically, the polyolefin resin composition has a crosslinked structure in which the carbon of the main chain of the polyethylene molecule is crosslinked by directly bonding to the carbon of another molecule, or is crosslinked through a functional group. I have. As a result, the lithium ion secondary battery of the present embodiment can be charged with a charger having a maximum charging voltage of 4.3 V or more and 4.8 V or less, and oxidation of the separator is substantially suppressed at a potential in this range. The Therefore, a high energy density lithium secondary battery is realized by improving the utilization efficiency of lithium and increasing the average voltage during discharge.

  As described above, in the polyolefin resin composition having such characteristics, the cross-linked structure constrains the free rotational motion and bond angle in the carbon-carbon bond generated when polyethylene is oxidized, so that it is physically a double bond. The molecular structure of polyethylene is regulated three-dimensionally so that cannot be formed. For this reason, the oxidation of polyethylene is suppressed.

  The crosslinked structure of the polyolefin resin composition is preferably formed not by radiation crosslinking but by chemical crosslinking. This is because residual radicals due to radiation cross-linking may react with the electrolytic solution during the production of the secondary battery to accelerate the deterioration of the electrolytic solution.

  The gel fraction of the polyolefin resin composition is preferably 42% or more. As shown below, according to the experimental results, if the gel fraction is less than 42%, the structure regulation of the polyethylene molecule is not sufficient, so that the oxidation inhibition effect of polyethylene cannot be sufficiently obtained. From the viewpoint of the effect of inhibiting oxidation of polyethylene, there is no particular limitation on the upper limit of the gel fraction. As the gel fraction increases and the cross-linking increases, the structural regulation of the polyethylene molecule is regulated and the oxidation of the polyethylene is suppressed. However, if the gel fraction is too high, the polyolefin resin composition becomes hard and the stretchability and the like are also lowered, so that it becomes difficult to mold the polyolefin resin composition as a thin film separator. In addition, the porosity is reduced, making it unsuitable as a separator for a lithium ion secondary battery. For this reason, the gel fraction of the polyolefin resin composition is more preferably 42% or more and 57% or less.

The gel fraction is determined as follows. First, the polyolefin resin composition is immersed in m-xylene (boiling point: 139 ° C.), and m-xylene is heated and boiled under reflux conditions for 5 hours. Then, the solid substance which remained without being dissolved in m-xylene is taken out, and the dried weight is measured. When the initial weight of the polyolefin resin composition is P ₀ and the weight after drying of the residual solid is P ₁ , the gel fraction R (%) is obtained by the following formula.
R (%) = 100 × P ₁ / P ₀

  Since the solubility in boiling m-xylene decreases due to an increase in molecular weight due to crosslinking, the value thus determined can be used as an indicator of the degree of crosslinking.

  The polyolefin resin composition can be obtained, for example, by chemically crosslinking ultrahigh molecular weight polyethylene, polynorbornene, and graft polyethylene. More specifically, the ultrahigh molecular weight polyethylene has a weight average molecular weight of 500,000 or more, and preferably has a weight average molecular weight of 1,000,000 or more from the viewpoint of increasing the mechanical strength of the composition.

  Polynorbornene is obtained by ring-opening polymerization of bicyclo [2,2,1] hept-5-ene (norbornene) alone, and has a carbon-carbon double bond (C = C) in the molecular chain. Yes. The molecular weight after polymerization is preferably 2 million or more.

  The graft polyethylene is a modified polyethylene having a polar group as a graph, and a maleic anhydride-grafted polyethylene having a maleic anhydride residue as a polar group as a graph is particularly preferable.

  These polymers (ultra high molecular weight polyethylene, polynorbornene and graft polyethylene) are mixed with alicyclic hydrocarbons such as liquid paraffin and kneaded at a temperature of 100 ° C to 200 ° C to uniformly disperse the polymers. Let After cooling and kneading the kneaded material, the liquid paraffin is removed by a solvent removal treatment. Thereafter, a chemically crosslinked polyolefin composition is obtained by heat treatment.

  The blending amount of the polymer in the kneaded product of the polymer and liquid paraffin is preferably 5 to 30% by weight, and more preferably 10 to 25% by weight. By setting the blending amount to 5% by weight or more, a separator having sufficient strength can be obtained. Moreover, by making a compounding quantity into 30 weight% or less, polyethylene can fully be dissolved in a liquid paraffin and sufficient entanglement of a polymer chain can be obtained. If necessary, additives such as antioxidants, ultraviolet absorbers, dyes, nucleating agents, pigments and antistatic agents may be added to the kneaded product as long as the object of the present invention is not impaired.

  Kneading and cooling can be performed by a known method. For example, the above-mentioned polymer and liquid paraffin are kneaded batch-wise using a Banbury mixer, a kneader, etc., and then the kneaded product is sandwiched between cooled metal plates to form a sheet. Or you may form a sheet | seat by extruding a kneaded material using the extruder etc. which attached T dies etc., and then making it contact with the cooled roll.

  Prior to the solvent removal treatment, a stretching treatment may be performed. There is no restriction | limiting in particular in the method of an extending | stretching process, You may combine a normal tender method, a roll method, an inflation method, or these methods. Furthermore, any method of uniaxial stretching and biaxial stretching may be used. When biaxial stretching is performed, any of longitudinal and lateral simultaneous stretching and sequential stretching may be used. However, from the viewpoint of uniformity and strength, it is preferable to perform stretching by simultaneous biaxial stretching. It is preferable that the temperature of an extending | stretching process is 100 to 140 degreeC.

  The solvent removal treatment is a step of removing liquid paraffin from the kneaded sheet to form a porous structure. For example, the liquid paraffin can be removed by washing the sheet with a solvent. Solvents include hydrocarbons such as pentane, hexane, heptane and decane, chlorinated hydrocarbons such as methylene chloride and carbon tetrachloride, fluorinated hydrocarbons such as ethane trifluorofluoride, ethers such as diethyl ether and dioxane, methanol, Examples of the volatile solvent include alcohols such as ethanol and ketones such as acetone and methyl ethyl ketone. These solvents may be used alone or in combination of two or more. There is no restriction | limiting in particular in the washing | cleaning method using a solvent, For example, the method of immersing the sheet | seat which performed the extending process in a solvent, and extracting a liquid paraffin from a sheet | seat etc. is mentioned.

  There is no restriction | limiting in particular in the heat processing of a sheet | seat, A well-known method can be used. A one-stage heat treatment method in which heat treatment is performed once may be used, or a temperature raising heat treatment method in which heat treatment is performed while raising the temperature may be used. The heat treatment temperature depends on the composition of the sheet, but is preferably 110 ° C to 140 ° C. The heat treatment time is about 0.5 to 2 hours.

  The heat treatment of the sheet is preferably performed in the presence of oxygen. By performing the heat treatment in the presence of oxygen, the polyolefin resin composition can be chemically crosslinked. The reaction mechanism of chemical cross-linking is complicated, and the reason for cross-linking is not necessarily clear, but oxygen acts on the carbon-carbon double bond site (C = C) of polynorbornene to generate a radical, and the radical site Are bonded directly to carbon atoms of other molecules or bonded through functional groups, and ultrahigh molecular weight polyethylene, polynorbornene and grafted polyethylene are considered to form a crosslinked structure.

  By adjusting the amount of carbon-carbon double bonds, that is, the blending amount of polynorbornene and the heat treatment temperature, the density and speed of the crosslinking reaction can be changed, and the gel fraction of the polyolefin resin composition can be adjusted. The gel fraction of the produced polyolefin composition can be measured by the method described above. The separator 13 is an insulating microporous film that includes the above-described polyolefin resin composition and has a large ion permeability and a predetermined mechanical strength as a whole. The pore diameter of the separator 13 is preferably in a range in which the active material, the binder, and the conductive agent detached from the electrode sheet do not permeate, and is preferably 0.01 to 1 μm, for example. The thickness of the separator 13 is 5 to 300 μm. Further, the porosity is determined according to the permeability of electrons and ions and the piercing strength of the material and the film, but is generally preferably 20 to 90%. As long as it has the hole of the range mentioned above, the separator 13 may be a microporous film sheet made of a bulk material or a non-woven fabric.

  In addition, from the viewpoint of safety of the lithium ion secondary battery, the separator 13 preferably has a function of closing the hole at 100 ° C. or higher and 140 ° C. or lower and increasing the resistance (shutdown function). If the hole is blocked at a temperature lower than 100 ° C, the battery function may be lost when used in a high temperature environment such as in a midsummer car. On the other hand, if the hole is blocked at a temperature higher than 140 ° C, the battery may be short-circuited. There is a possibility that self-heating caused by the above cannot be sufficiently suppressed. Since the above-mentioned polyolefin resin composition contains polyethylene as a main component, the pores are blocked in the same temperature range as that of a conventional polyethylene separator, and a shutdown function is exhibited.

  The non-aqueous electrolyte filled in the battery case 4 is composed of a solvent and a lithium salt dissolved in the solvent. Examples of the non-aqueous solvent include cyclic carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, and vinylene carbonate, and non-cyclic carbonates such as dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, and dipropyl carbonate. be able to. One of these or a mixture of two or more are used. In particular, it is preferable to use a mixed system of cyclic carbonate and acyclic carbonate as a main component.

Examples of the lithium salt dissolved in these solvents include LiClO ₄ , LiBF ₄ , LiPF ₆ , LiCF ₃ SO ₃ , LiCF ₃ CO ₂ , Li (CF ₃ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) _{2, and the} like. Can be used. These electrolytes using lithium salts can be used alone or in combination of two or more. In particular, it is more preferable to include LiPF ₆ .

  Further, the nonaqueous electrolytic solution can be supported on an organic polymer and used as a gel electrolyte. Examples of the organic polymer that supports the non-aqueous electrolyte include polyethylene oxide, polypropylene oxide, polyvinylidene fluoride, polyhexafluoropropylene, polyacrylate, polymethacrylate, and derivatives thereof.

  As the battery case 4, a metal battery can such as Al or Fe or a laminate film obtained by laminating a resin film on both surfaces of a metal foil in a bag shape can be used. In order to meet demands for further thinning and weight reduction of batteries in the market, it is preferable to use a battery case made of a laminate film. The shape of the battery defined in the battery case 4 is not particularly limited, and may be any of a cylindrical shape, a flat shape, and a square shape.

  According to the lithium ion secondary battery configured as described above, the polyethylene as the main component of the separator is chemically cross-linked and has a three-dimensional structure, rotation of carbon-carbon single bonds in the carbon chain, and carbon-carbon bond angle. Changes are regulated. For this reason, it is suppressed that the carbon-carbon single bond of polyethylene converts into the carbon-carbon double bond in an oxidation state. Thereby, the oxidation of polyethylene is prevented.

  Therefore, according to the lithium ion secondary battery of the present embodiment, even when the battery is charged at a voltage exceeding 4.2 V at which general polyethylene is oxidized, the separator is prevented from being oxidized and gas generation is suppressed. Is done. On the other hand, since the main component of the separator is polyethylene, a shutdown function is exhibited and high safety can be ensured in the same temperature range as in the past. Therefore, according to this embodiment, a high voltage rechargeable lithium ion secondary battery that achieves both excellent reliability and safety is realized.

  Examples of the present invention will be specifically described below. However, the present invention is not limited to these examples.

<Example 1>
A separator used in the lithium ion secondary battery of the present invention and a conventional separator were prepared and evaluated for oxidation resistance.

1. Production of Separator The separators A to C of Examples and the separators D to I of Comparative Examples used for the lithium ion secondary battery according to the present invention were produced as follows.

"Example"
(Separator A) Ultra-high molecular weight polyethylene with a weight average molecular weight of 1 million (melting point: 137 ° C.) 76% by weight, maleic anhydride graft polyethylene with a weight average molecular weight of 250,000 (manufactured by Nippon Polyethylene, ADDEX ER403A) 16% by weight, poly 15 parts by weight of a resin composition comprising 8% by weight of norbornene (manufactured by Nippon Zeon Co., Ltd., Nosorex NB weight average molecular weight of 2 million or more) and 85 parts by weight of liquid paraffin are uniformly mixed in a slurry state, It melt-kneaded with the twin-screw extruder at temperature, and extrusion-molded into a 4 mm-thick sheet. The sheet compact was taken out under a certain tension, cooled once with a roll cooled with 10 ° C. cooling water, and then pressed with a 120 ° C. belt press to produce a 1 mm thick resin sheet. . A stretched film was prepared by simultaneously biaxially stretching the resin sheet at a temperature of 123 ° C. at a speed of 10 mm / sec at a length and width of 4.0 × 4.0 times, and liquid paraffin was removed from the stretched film using heptane. . Thereafter, the stretched film was placed in a constant temperature oven at 122 ° C., held for 2 hours while applying an air current, and subjected to a crosslinking reaction, thereby producing a separator A. The thickness of the produced separator A was 17.5 μm, the porosity was 41%, and the gel fraction was 42%.

  (Separator B) 75% by weight of ultra high molecular weight polyethylene having a weight average molecular weight of 1 million (melting point: 137 ° C.), 15% by weight of maleic anhydride grafted polyethylene having a weight average molecular weight of 250,000 (manufactured by Nippon Polyethylene, ADDEX ER403A), poly Separator B was produced under the same conditions as separator A, except that a resin composition comprising 10% by weight of norbornene (manufactured by Nippon Zeon Co., Ltd., Northolex NB weight average molecular weight of 2 million or more) was used. The produced separator B had a film thickness of 16.5 μm, a porosity of 37%, and a gel fraction of 53%.

  (Separator C) Ultrahigh molecular weight polyethylene with a weight average molecular weight of 1,000,000 (melting point: 137 ° C.) 71% by weight, maleic anhydride grafted polyethylene with a weight average molecular weight of 250,000 (manufactured by Nippon Polyethylene, ADDEX ER403A) 14% by weight, poly Using a resin composition comprising 15% by weight of norbornene (manufactured by Nippon Zeon Co., Ltd., Nosolex NB weight average molecular weight of 2 million or more), the stretched film from which liquid paraffin has been removed was held in a constant temperature oven at 124 ° C. for 2 hours. A separator C was produced under the same conditions as for the separator A except for the above. The produced separator C had a film thickness of 14.5 μm, a porosity of 30%, and a gel fraction of 57%.

《Comparative example》
(Separator D) Ultra high molecular weight polyethylene having a weight average molecular weight of 1,000,000 (melting point: 137 ° C.) 83% by weight, maleic anhydride graft polyethylene having a weight average molecular weight of 250,000 (manufactured by Nippon Polyethylene, ADDEX ER403A) 17% by weight Separator D was produced under the same conditions as separator A except that the stretched film from which liquid paraffin was removed was held in a constant temperature oven at 120 ° C. for 2 hours using the resin composition. The thickness of the produced separator D was 21 μm, the porosity was 52%, and the gel fraction was 0%.

  (Separator E) Ultra high molecular weight polyethylene with a weight average molecular weight of 1,000,000 (melting point: 137 ° C.) 81% by weight, maleic anhydride grafted polyethylene with a weight average molecular weight of 250,000 (manufactured by Nippon Polyethylene, ADDEX ER403A) 16% by weight, poly Using a resin composition comprising 3% by weight of norbornene (manufactured by Nippon Zeon Co., Ltd., Northolex NB weight average molecular weight of 2 million or more), the stretched film from which liquid paraffin has been removed was held in a constant temperature oven at 120 ° C. for 2 hours. A separator E was produced under the same conditions as the separator A except for the above. The manufactured separator E had a film thickness of 20 μm, a porosity of 51%, and a gel fraction of 15%.

  (Separator F) Ultrahigh molecular weight polyethylene with a weight average molecular weight of 1 million (melting point: 137 ° C.) 78% by weight, maleic anhydride grafted polyethylene with a weight average molecular weight of 250,000 (manufactured by Nippon Polyethylene, ADDEX ER403A) 15% by weight, poly Using a resin composition comprising 7% by weight of norbornene (manufactured by Nippon Zeon Co., Ltd., Nosorex NB weight average molecular weight of 2 million or more), the resin sheet is simultaneously at a temperature of 125 ° C. and a speed of 10 mm / sec. The separator F was prepared under the same conditions as the separator A except that a stretched film was prepared by biaxially stretching 4.5 times and the stretched film from which liquid paraffin had been removed was kept in a constant temperature oven at 120 ° C. for 2 hours. Produced. The manufactured separator F had a film thickness of 15 μm, a porosity of 54%, and a gel fraction of 27%.

  (Separator G) A separator G was produced under the same conditions as the separator E, except that the stretched film from which the liquid paraffin had been removed was held in a constant temperature oven at 126 ° C. for 2 hours. The produced separator G had a film thickness of 17.5 μm, a porosity of 41%, and a gel fraction of 30%.

  (Separator H) A separator H was produced under the same conditions as the separator F except that the stretched film from which liquid paraffin had been removed was held in a constant temperature oven at 128 ° C. for 2 hours. The thickness of the produced separator G was 10.5 μm, the porosity was 35%, and the gel fraction was 34%.

  (Separator I) Ultra high molecular weight polyethylene having a weight average molecular weight of 1 million (melting point: 137 ° C.) 67% by weight, maleic anhydride grafted polyethylene having a weight average molecular weight of 250,000 (manufactured by Nippon Polyethylene, ADDEX ER403A) 13% by weight, poly Using a resin composition comprising 20% by weight of norbornene (manufactured by Nippon Zeon Co., Ltd., Northolex NB weight average molecular weight of 2 million or more), the stretched film from which liquid paraffin has been removed was held in a constant temperature oven at 126 ° C. for 8 hours. A separator I was produced under the same conditions as for the separator A except for the above. The produced separator I had a film thickness of 13 μm, a porosity of 22%, and a gel fraction of 63%. The separator I is hard and the porosity is too small. For this reason, the separator I was unsuitable as a separator for lithium ion secondary batteries.

2. Production of Model Cell Using the separators A to C of the examples and the separators D to H of the comparative examples, the following model cells were produced, and the oxidation resistance of the separators was evaluated using the model cells.

(A) Production of Positive Electrode A platinum (3N) sheet having a thickness of 100 μm was punched into the dimensions shown in FIG. 4 and used as the positive electrode 21 as shown in FIG.

(B) Production of Negative Electrode As shown in FIG. 5, a stainless steel (SUS304) mesh 2 was punched out to the dimensions shown in FIG. 5 and used as the current collector 22a shown in FIG. A negative electrode 22 was obtained by press-bonding 150 μm thick metal lithium 22b cut into a square shape with one side having a length of 22 mm.

(C) Assembly For positive electrode 21 and negative electrode 22, separators A to C of Examples and separators D to H of Comparative Examples as separators 23, for evaluation of oxidation resistance as shown in FIGS. The model cell was assembled. First, the positive electrode 21 and the negative electrode 22 were laminated | stacked through the separator 23, and the electrode group 33 was produced. An aluminum positive electrode lead 31 and a nickel negative electrode lead 32 were welded to the positive electrode 21 and the negative electrode 22, respectively. The electrode group 33 was housed inside an aluminum laminated film cell case 34 having a wall thickness of 0.12 mm and opened in three directions, and was fixed to the inner surface of the aluminum laminated film with PP tape. Two openings including the opening from which the positive electrode lead 31 and the negative electrode lead 32 protrude are thermally welded, and the cell case 34 made of an aluminum laminate film is formed into a bag shape. A predetermined amount of non-aqueous electrolyte was injected from the opening, and after depressurization and deaeration, the opening was thermally welded under reduced pressure to seal the inside of the battery. As the non-aqueous electrolyte, a solution of LiPF ₆ dissolved in a mixed solvent of ethylene carbonate and ethyl methyl carbonate in a volume ratio of 30:70 so as to have a concentration of 1.0 mol / L was used.

  The model cells produced using the separators A to H were designated as evaluation cells A1 to H1, respectively.

3. Evaluation of oxidation resistance of separator The prepared evaluation cells A1 to H1 were subjected to 4.3 V constant voltage charging for 100 hours in an environment of 60 ° C. with a pressure of 0.4 MPa applied uniformly to the electrodes in the evaluation cell. It was. The current value during constant voltage charging was measured, and the amount of electricity obtained by integrating the current values from the start of charging to 100 hours was defined as a side reaction amount of electricity, which was used as an index indicating the degree of oxidation of the separator.

  The original electric capacity of this evaluation cell is only a small electric double layer capacity, and a predetermined electric capacity is accumulated immediately after the start of charging. For this reason, the amount of electricity consumed during charging for 100 hours is an electrochemical side reaction current other than the charging reaction that occurs in the model cell, such as the oxidation of the separator or electrolyte. Table 1 shows the evaluation results.

The model cell after the test was disassembled, the separator was taken out, washed with dimethyl carbonate and dried, and then subjected to Raman spectroscopic analysis of the separator. A separator in which a Raman shift with respect to an incident laser of 530 nm is observed near 1600 cm ⁻¹ is marked with a circle in Table 1, and a separator that was not observed is marked with a cross. The Raman peak near 1600 cm ⁻¹ is attributed to a carbon-carbon double bond.

  Moreover, what plotted the side reaction electric quantity of the evaluation result of Table 1 with respect to the gel fraction is shown in FIG.

5. Evaluation Results As shown in Table 1 and FIG. 7, the evaluation cells E1 to H1 of the comparative example having a gel fraction of 15 to 34% with respect to the evaluation cell D1 of the comparative example having a gel fraction of 0% A decrease in the amount of electricity on the side reaction is observed. From this, it can be seen that the oxidation of polyethylene is somewhat suppressed by three-dimensionally crosslinking polyethylene. However, since the Raman peaks attributed to the carbon-carbon double bonds are observed in the separators D to H after the charge test of these evaluation cells, the carbon-carbon double bonds due to oxidation are generated. Thus, it can be seen that the oxidation inhibiting effect of polyethylene is not sufficient.

  On the other hand, in the evaluation cells A1 to C1 of the examples, the side reaction electricity amount is clearly smaller than that of the evaluation cells D1 to H1. This is considered to be because the oxidation reaction of the separator was suppressed. In the separators A to C after the test, since no Raman peak attributed to the carbon-carbon double bond was observed, almost no carbon-carbon double bond was formed, and almost no oxidation of the separator occurred. I understand that there is no.

  In addition, as shown in Table 1 and FIG. 7, in the evaluation cells A1 to C1 of the examples, the amount of side reaction is not zero even though the carbon-carbon double bond is not generated. . This is considered to be due to oxidation of substances other than the separator contained in the evaluation cell. Examples of the substance other than the separator include a solvent constituting the non-aqueous electrolyte, impurities contained in the non-aqueous electrolyte, and water contained in a minute amount in the evaluation cell.

  Moreover, as guessed from FIG. 7, if the gel fraction is 42% or more, the oxidation of the separator is sufficiently suppressed, and it is considered that the same tendency is exhibited even if the gel fraction exceeds 57%. Therefore, when adding a substance having improved stretchability and a high oxidation resistance property to the polyolefin resin composition constituting the separator, a polyolefin having a high molecular weight polyethylene as a main component and having a crosslinked structure by chemical crosslinking The gel fraction of the resin composition may be 57% or more. However, the separator I produced from the polyolefin resin composition having a gel fraction of 63% was not suitable as a separator for a lithium secondary battery because of its low porosity and poor plastic deformability. Therefore, in the case where the separator is mainly composed of only such a polyolefin resin composition, the lithium ion secondary battery has sufficient oxidation resistance by adjusting the gel fraction to 42 to 57%. It can be seen that the material has physical properties suitable as a separator for use.

<Example 2>
The shutdown characteristics of the separator used in the lithium ion secondary battery of the present invention and the conventional separator were evaluated.

1. Production of Separator The separators A to C of the example produced in Example 1 and the separator D of the comparative example were used.

2. Production of Model Cell Using the separators A to C of the examples and the separator J of the comparative example, the following model cells were produced.

  Using the separators A to C of the examples and the separator J of the comparative example, a model cell was manufactured by the following procedure. The model cells prepared using the separators A to C and J were set as evaluation cells A2 to C2 and J2, respectively.

  A SUS cell having a cylindrical test chamber of φ25 mm and capable of sealing the test chamber was used, and platinum plates (thickness: 1.0 mm) of φ20 mm and φ10 mm were used for the lower electrode and the upper electrode, respectively. Separators A to C and J punched to φ24 mm were immersed in an electrolytic solution, sandwiched between an upper electrode and a lower electrode, and set in a cell. The upper electrode and the lower electrode were applied with a constant surface pressure by a spring provided in the cell. As the electrolytic solution, a solution obtained by dissolving lithium borofluoride in a solvent in which polypropylene carbonate and dimethoxyethane were mixed at a volume ratio of 1: 1 to a concentration of 1.0 mol / L was used.

3. Evaluation of shutdown characteristics of separator The shutdown characteristics of the separator were evaluated by the following methods using the produced evaluation cells A2 to C2 and J2.

A thermocouple thermometer and an AC resistance meter were connected to the evaluation cells A2 to C2 and J2, so that the temperature and AC resistance of the separator could be measured. Evaluation cells A2 to C2 and J2 were stored in a thermostat heated to 180 ° C., and changes with time in temperature and resistance were measured. The average temperature increase rate at 100 ° C. to 150 ° C. was 10 ° C./min. Moreover, the average temperature increase rate in 160 to 170 degreeC was 4 degree-C / min. The temperature at which the resistance value reached 100 Ω · cm ² was taken as the shutdown temperature.

4). Evaluation results As shown in Table 2, the shutdown temperature in the evaluation cells A2 to C2 of the examples was 135 ° C or 136 ° C, and the shutdown function was exhibited in the same temperature range as that of the conventional polyethylene separator. On the other hand, the shutdown temperature of the evaluation cell J2 of the comparative example was 165 ° C. This is considered because the separator is made of polypropylene and the shrinkage temperature of the resin is high.

  From the above results, even with a polyolefin resin composition having a high molecular weight polyethylene as a main component and a crosslinked structure having a gel fraction of 42 to 57% by chemical crosslinking, a polyethylene separator having no crosslinked structure In the same temperature range, the shutdown function is exhibited. Therefore, it turns out that it has the shutdown characteristic which obstruct | occludes a hole in the temperature of 100 to 140 degreeC preferable as a separator for lithium ion secondary batteries.

<Example 3>
The lithium ion secondary battery shown in FIG. 1 was produced using the separator B of the example produced in Example 1 and the separator D of the comparative example, and the reliability was evaluated.

1. Production of Lithium Ion Secondary Battery (a) Production of Positive Electrode A positive electrode 11 shown in FIG. 2 was produced as follows.

LiCoO ₂ (average particle size: 8 μm, specific surface area by BET method: 4.2 m ² / g) is used as the positive electrode active material, and 3 parts by weight of acetylene black as a conductive agent is added to 100 parts by weight of this active material as a binder. 4 parts by weight of a certain polyvinylidene fluoride and an appropriate amount of N-methyl-2-pyrrolidone were added and stirred and mixed to obtain a slurry-like positive electrode mixture 11b. Polyvinylidene fluoride was used in a state of being previously dissolved in N-methyl-2-pyrrolidone.

  Next, the slurry-like positive electrode mixture 11b was applied to both surfaces of a current collector 11a made of an aluminum foil having a thickness of 20 μm, and the coating film was dried and rolled with a roller. The obtained electrode plate was cut into a predetermined size to obtain the positive electrode 11.

LiCoO ₂ used as the positive electrode active material was prepared by the following method. First, an aqueous metal salt solution in which cobalt sulfate was dissolved was prepared. The aqueous metal salt solution was maintained at 50 ° C. with stirring, and an aqueous solution containing 30% by weight of sodium hydroxide was dropped into the aqueous solution and neutralized to produce a precipitate of cobalt hydroxide.

  The cobalt hydroxide precipitate was filtered, washed with water, dried in air, and then calcined at 400 ° C. for 5 hours to obtain cobalt oxide. The obtained cobalt oxide was confirmed to be a single phase by powder X-ray diffraction.

Next, the obtained cobalt oxide and lithium carbonate were mixed so that the ratio of the number of Co atoms to the number of Li atoms was 1: 1. This mixture was calcined at 950 ° C. for 10 hours. Thereafter, the fired product was pulverized to obtain a powder having a cumulative 50% particle size of 8 μm obtained by a laser diffraction method, thereby obtaining the target LiCoO ₂ . The obtained LiCoO ₂ was confirmed to have a single-phase hexagonal crystal structure by powder X-ray diffraction.

(B) Production of Negative Electrode A negative electrode 12 shown in FIG. 2 was produced as follows.

  Using spherical natural graphite, 6 parts by weight of polyvinylidene fluoride as a binder and an appropriate amount of N-methyl-2-pyrrolidone are added to 100 parts by weight of this active material, and the mixture is stirred and mixed. A negative electrode mixture 12b was obtained. Polyvinylidene fluoride was used in a state of being previously dissolved in N-methyl-2-pyrrolidone.

Next, the slurry-like negative electrode mixture 12b was applied to both surfaces of a current collector 12a made of a copper foil having a thickness of 15 μm, the coating film was dried, and rolled with a roller. The filling density of the active material in the negative electrode mixture layer was 1.5 g / cm ³ . The obtained electrode plate was cut into a predetermined size to obtain a negative electrode 12.

Spherical natural graphite used as the negative electrode active material was prepared by the following procedure. First, a scale-like natural graphite (D50: 22.8 μm, specific gravity: 2.23 g / cm ³ ) having an average interplanar spacing d (002) in the C-axis direction determined from powder X-ray diffraction of 3.354 nm is hybridized. A spheroidizing treatment was performed by a mechanochemical method using a system (NHS-0 type, manufactured by Nara Machinery Co., Ltd.). This device consists of a rotor that rotates at high speed, a stator, and a circulation path. The powder charged into the device from the center of the rotor is mainly subjected to impact, compression, and shearing force caused by the collision between the rotor and the powder. Circulated. By this circulation, the scaly graphite particles were granulated to obtain the desired spheroidized natural graphite. The obtained spheroidized natural graphite was confirmed to be substantially spherical by a scanning electron microscope.

(C) Battery assembly A lithium ion secondary battery (thickness 5.2 mm, width 34 mm, height 50 mm, design) having the structure shown in FIG. 1 using the positive electrode 11 and the negative electrode 12 obtained by the procedure described above. A capacity of 800 mAh) was assembled.

  First, the strip-shaped positive electrode 11 and the negative electrode 12 were spirally wound through the separator 13 to produce the electrode plate group 1. An aluminum positive electrode lead 2 and a nickel negative electrode lead 3 were welded to the positive electrode 11 and the negative electrode 12, respectively. An insulating frame made of polypropylene resin (not shown) was mounted on the upper part of the electrode plate group, and housed inside an aluminum battery case 4 having a thickness of 0.25 mm. The other end of the positive electrode lead 2 was spot welded to the inner surface of the aluminum sealing plate 8. Further, the other end of the negative electrode lead 3 was spot welded to the lower part of the nickel negative electrode terminal 6 attached to the central portion of the sealing plate 8 via the insulating material 5.

  Next, the open end of the battery case 4 and the peripheral edge of the sealing plate 8 were laser welded. Then, a predetermined amount of non-aqueous electrolyte was injected from the injection port, and finally the injection port was sealed with an aluminum plug 7 to complete the battery.

In the non-aqueous electrolyte, 1 part by weight of vinylene carbonate is added to 100 parts by weight of a mixed solvent of ethylene carbonate and ethyl methyl carbonate in a volume ratio of 30:70, and LiPF ₆ is dissolved to a concentration of 1.0 mol / L. What was done was used.

  A battery manufactured using the separator B is referred to as a battery B3, and a battery manufactured using the separator D is referred to as a battery D3.

(D) Initial charge / discharge Battery B3 and D3 were conditioned and discharged by repeating the following charge / discharge cycle three times in a 25 ° C. environment. Charging was carried out by constant current charging with a current value of 80 mA and an end-of-charge voltage of 4.2V. The discharge was a constant current discharge with a current value of 80 mA and a discharge end voltage of 3.0 V. In this break-in charging / discharging, it was confirmed that the discharge capacity in the second cycle and the discharge capacity in the third cycle were almost the same, and a completed battery was obtained.

2. Evaluation of Lithium Secondary Battery Batteries B3 and D3 subjected to initial charge / discharge were charged, and the thickness of the battery was measured after the charge was completed. The thickness was defined as the initial thickness by measuring the portion where the thickness of the battery was maximum (the center of the battery). Charging was performed at a constant voltage of 4.2 V (maximum current value: 400 mA) in an environment of 25 ° C., and the charging was terminated when the current value was attenuated to 40 mA.

  Next, under a 45 ° C. environment, 4.2 to 5.0 V constant voltage charging (maximum current value: 400 mA) was performed for one week. After the constant voltage charging was completed, the battery was discharged after discharging to 3.0 V at a current value of 80 mA in an environment of 25 ° C., and after the charging was completed, the thickness of the battery was measured. The thickness was defined as the thickness after storage at a constant voltage and high temperature, by measuring the portion where the thickness of the battery was maximum (the central portion of the battery). Charging was performed at a constant voltage of 4.2 V (maximum current value: 400 mA) in an environment of 25 ° C., and the charging was terminated when the current value was attenuated to 40 mA.

  The amount of swelling was determined as an index for evaluating the reliability during high-temperature storage at a constant voltage. The results are shown in Table 3. Here, the swollen amount was defined as a value obtained by subtracting the initial thickness from the thickness after constant voltage high temperature storage (that is, swollen amount = thickness after constant voltage high temperature storage−initial thickness). Table 3 shows the results. FIG. 8 is a graph showing the relationship between the charging voltage and the amount of swelling.

3. Evaluation Results As is apparent from Table 3 and FIG. 8, when charged at a charging voltage higher than a general charging voltage of 4.2 V, the battery B of the example of the present invention is compared with the battery D3 of the comparative example. It can be seen that swelling during constant voltage high temperature storage can be suppressed. Moreover, even when charged with a general charging voltage of 4.2 V, although the effect is small, it can be seen that the swollen amount is suppressed as compared with the battery D3 of the comparative example.

  As can be seen from Table 3 and FIG. 8, in the battery B of the example, the amount of swelling is not zero. This is considered to be due to the influence of the gas generated by the oxidation of substances other than the separator contained in the battery B3. Examples of the substance other than the separator include a solvent constituting the non-aqueous electrolyte, impurities contained in the non-aqueous electrolyte, and water contained in a minute amount in the battery.

  As described in Example 1, when the charging voltage is 4.3 V, it has been confirmed by Raman spectrum that no carbon-carbon double bond is generated. Therefore, at a charging voltage of at least about 4.3 V, Little oxidation of the separator occurs. As shown by a broken line in FIG. 8, in the battery B of the example, the charging voltage and the swelling amount are well proportional in the range of the charging voltage from 4.2V to 4.6V. From this, it is considered that the generation of gas in the battery B is proportional to the voltage when the charging voltage is in the range of 4.2 V to 4.6 V, and there is no increase in the generation of gas due to a new reaction or the like. It is done. This is considered that the separator hardly oxidizes in this charging voltage range, and only the gas generated by the oxidation of substances other than the separator contributes to the amount of swelling. That is, from Table 3 and FIG. 8, it is inferred that according to the battery B of the example, the oxidation of the separator is almost completely suppressed at least in the range of the charging voltage from 4.2 V to 4.6 V. .

  When the charging voltage of the battery B3 of the example was set to 5.0V and when the charging voltage of the battery D3 of the comparative example was set to 4.8V or more, the cause was not clear, but an internal short circuit occurred. Interrupted on the way. Therefore, the amount of battery swelling after storage has not been measured. However, the swelling amount at the charging voltage of 4.6 V of the battery D3 of the comparative example is much larger than the swelling amount at 4.8 V of the battery B3 of the example. Therefore, even if the separator is slightly oxidized at the 4.8V of the battery B3 of the example and the battery is swollen due to the generation of gas, the gas generated by oxidation is far greater than that of the conventional polyethylene separator. Generation | occurrence | production is suppressed and it can be said that there exists a remarkable effect compared with a prior art.

  From the above results, it can be seen that a preferable charging voltage for effectively expressing the effects of the present invention is 4.2 V or more and 4.8 V or less, and a more preferable range is 4.3 V or more and 4.8 V or less.

  The present invention makes it possible to suppress the generation of gas even when charged at high temperature and high voltage. Therefore, the present invention is suitably used for a lithium ion secondary battery that achieves both high reliability, safety, and energy density. . For example, it can be used as a power source for portable electronic devices that require reliability. Further, the present invention can be widely applied to electric vehicles, large batteries used for power storage, and the like.

DESCRIPTION OF SYMBOLS 1 Electrode group 2 Positive electrode lead 3 Negative electrode lead 4 Battery case 5 Insulation material 6 Negative electrode terminal 7 Sealing 8 Sealing plate 11 Positive electrode 11a Positive electrode collector 11b Positive electrode mixture 12 Negative electrode 12a Negative electrode collector 12b Negative electrode mixture 13 Separator 21 Positive electrode 22 Negative electrode 22a Stainless steel mesh 22b Metallic lithium 23 Separator 31 Positive electrode lead 32 Negative electrode lead 33 Electrode group 34 Cell case

Claims (5)

A positive electrode and a negative electrode capable of reversibly inserting and extracting lithium; and
A separator disposed between the positive electrode and the negative electrode;
A battery case containing the positive electrode, negative electrode and separator;
A non-aqueous electrolyte filled in the battery case;
With
The separator is a polyolefin resin composition comprising, as a main component, ultrahigh molecular weight polyethylene having a weight average molecular weight of 500,000 or more, and chemically crosslinking (excluding radiation crosslinking) the ultrahigh molecular weight polyethylene, 10% by weight or more of polynorbornene and graft polyethylene. And a gel fraction of the polyolefin resin composition is 53 % or more and 57% or less.   The lithium ion secondary battery according to claim 1, wherein the graft polyethylene is maleic anhydride graft polyethylene.   3. The lithium ion secondary battery according to claim 1, wherein the lithium ion secondary battery is charged by a charger having a maximum charging voltage of 4.3 V or more and 4.8 V or less. The lithium ion secondary battery according to any one of claims 1 to 3, wherein the chemical crosslinking forms a crosslinked structure by heat treatment without irradiating radiation. The lithium ion secondary battery according to claim 1, wherein the polynorbornene is 15% by weight or less.

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