JP4142921B2 - Lithium ion secondary battery - Google Patents

Description

【００９５】
前記表１から明らかなように絶縁性被膜として５００℃以上の耐熱性を有する粉体がバインダ樹脂で結着された構造のものを用いた実施例１〜１６の二次電池は、過充電試験での発熱によってセパレータが収縮しても正負極の捲き方向に沿う両端部での内部短絡を防止して、熱暴走に至らないことがわかる。
【００９６】
これに対し、絶縁性被膜を形成しない比較例１の二次電池は勿論、正負極の捲き方向に沿う両端部に絶縁性被膜を固定した場合でも、その絶縁性被膜がポリフッ化ビニリデン（ＰＶｄＦ）のみの電解液に不溶の絶縁性樹脂から作られる比較例２の二次電池では、過充電によってセパレータが収縮すると熱暴走へ至ることがわかる。
【００９７】
【発明の効果】
以上詳述したように、本発明によれば電極群を構成するセパレータが熱で収縮し、セパレータの幅が正極と負極の幅よりも縮小した場合でも、高温の発熱に至ることを未然に防止することが可能な信頼性、安全性の高いリチウムイオン二次電池を提供することができる。
【図面の簡単な説明】
【図１】本発明に係る薄形のリチウムイオン二次電池を示す切欠斜視図。
【図２】図１のII−II線に沿う断面図。
【図３】図１のリチウムイオン二次電池に組み込まれる電極群を示す斜視図。
【図４】図３のIV−IV線に沿う断面図。
【図５】図１のリチウムイオン二次電池に組み込まれる電極群として捲回する前の正極、セパレータおよび負極の捲き始め端部側を示す斜視図。
【符号の説明】
１…電極群、
４…正極、
５…セパレータ、
８…負極、
９…正極側外部リード、
１０…負極側外部リード、
１１，１２…絶縁性被膜、
１３…外装フィルム、
１４…カップ、
１９ａ，１９ｂ，１９ｃ…シール部。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a lithium ion secondary battery.
[0002]
[Prior art]
Lithium ion secondary batteries use a material that can be doped / undoped with lithium ions, such as carbon materials, as a negative electrode, and use a lithium composite oxide, such as a lithium cobalt composite oxide, as the positive electrode. High, high energy density, and excellent cycle characteristics.
[0003]
The lithium ion secondary battery, for example, a prismatic lithium ion secondary battery, includes a positive electrode in which an active material layer is formed on both surfaces of a strip-shaped positive electrode current collector, and a negative electrode in which an active material layer is formed on both surfaces of the strip-shaped negative electrode current collector Are wound through a separator such as a polypropylene film to form an electrode group, and an insulator is placed on the upper and lower sides of the electrode group and housed in a container. In order to prevent lithium from depositing during charging and causing a short circuit therein, the width and length of the opposing negative electrode are increased by interposing a separator on the positive electrode.
[0004]
In such a lithium ion secondary battery, heat is generated by an electrochemical reaction when an excessive current flows for some reason, and if the separator contracts (especially in the width direction) with the heat, The vicinity of both ends along the length direction (the electrode group firing direction) faces each other directly without a separator, and insulation between the positive electrode and the negative electrode cannot be maintained, thereby causing an internal short circuit and promoting heat generation. There was sex.
[0005]
In order to simulate the above phenomenon, the inventors put a wound product composed of a positive electrode, a negative electrode, and a separator in a dryer and left it at 150 ° C. for 15 minutes. As a result, it became clear that the separator turned transparent and contracted in the vertical direction of the wound product, and the positive electrode and the negative electrode were in direct contact with the upper and lower ends of the wound product. Therefore, when such a separator contraction occurs during battery operation, an internal short circuit between the positive electrode and the negative electrode is caused, and there is a possibility that heat is generated to a high temperature due to Joule heat generation. In a lithium ion secondary battery, even if an accident that leads to heat generation is extremely rare, it is serious for the user, and it is desired to avoid it in advance.
[0006]
In JP-A-7-130389, a positive electrode and a negative electrode coated with an electrode mixture so that the length of the negative electrode facing the positive electrode is larger than the length of the positive electrode on both the front and back surfaces of a strip-shaped metal foil are separated from each other. Insulation insoluble in the electrolyte solution at least in part of the non-opposing portion of the negative electrode or the positive electrode located at the beginning and / or end of winding of the wound electrode group By covering the conductive resin, it is possible to maintain the coated portion in a state in which contact with the outside is cut off, so that the battery is held in a state of little involvement in the reaction with lithium ions in the electrolyte during charging of the battery. A non-aqueous electrolyte secondary battery such as a lithium ion secondary battery that prevents diffusion of lithium ions into the coating portion is disclosed.
[0007]
In such a non-aqueous electrolyte secondary battery, although the space between the negative electrode and the positive electrode located at the beginning and / or end of winding of the wound electrode group is covered with an insulating resin, the temperature due to overcharge reaction, etc. When the separator contracts due to the rise, it is expected that the positive electrode and the negative electrode are short-circuited, and there is a possibility that a large current flows and the above-described heat generation occurs.
[0008]
On the other hand, in JP-A-10-241655, in a battery including a positive electrode, a negative electrode, and a separator interposed therebetween, at least one of the active material layer of the positive electrode and the active material layer of the negative electrode is specified. A battery is described in which a separator is formed by fixing an insulating substance particle aggregate layer composed of insulating substance particles having a specific surface area and a binder that bonds the particles. Since the insulating substance particle aggregate layer is fixed to one of the positive and negative electrodes, it has an effect of preventing an internal short circuit between the positive and negative electrodes. However, unlike the conventional resin separator (for example, a microporous polyethylene separator), this insulating substance particle aggregate layer lacks flexibility, so that when there is a lack in a normal battery reaction, the positive and negative electrodes There is a risk that an internal short circuit will occur, resulting in high temperature heat generation. Therefore, the insulating substance particle aggregate layer exhibits an excellent effect in the case where a part of the insulating substance particle aggregate layer is reliably insulated, but is not suitable for a form covering the positive and negative electrode surfaces in a wide range due to weak stress. In order to cover the positive and negative electrode surfaces in a wide range, it is considered that a resin separator that is excellent in flexibility and has a shutdown function is suitable.
[0009]
[Problems to be solved by the invention]
According to the present invention, even when the separator constituting the electrode group is contracted by heat and the width of the separator is smaller than the width of the positive electrode and the negative electrode, it is possible to prevent the occurrence of high temperature heat generation. The next battery is to be provided.
[0010]
[Means for Solving the Problems]
The lithium ion secondary battery according to the present invention includes an electrode group in which a positive electrode having an active material layer formed on a current collector and a negative electrode having an active material layer formed on a current collector are wound with a separator interposed therebetween. Prepared,
An insulating film in which a powder having a heat resistance of 500 ° C. or higher is bound with a binder resin, and the binder resin is blended in an amount of 5 to 35 by weight with respect to the powder 100, The at least It fixes to the both ends along the rolling direction of the said positive / negative electrode containing.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described in detail.
[0012]
The non-aqueous electrolyte secondary battery according to the present invention is an electrode in which a positive electrode having an active material layer formed on a current collector and a negative electrode having an active material layer formed on the current collector are wound with a separator interposed therebetween. With groups. This electrode group is housed together with a non-aqueous electrolyte in an exterior member such as a plastic laminate film with an aluminum foil interposed therebetween.
[0013]
An insulating film in which a powder having a heat resistance of 500 ° C. or higher is bound with a binder resin is fixed to both ends along the rolling direction of the positive and negative electrodes including at least the starting end of the electrode group. .
[0014]
Next, the positive electrode, negative electrode, separator, electrode group, and non-aqueous electrolyte will be described.
[0015]
1) Positive electrode
The positive electrode includes a current collector and a positive electrode layer that is supported on one or both sides of the current collector and includes an active material.
[0016]
The positive electrode layer includes a positive electrode active material, a binder, and a conductive agent.
[0017]
Examples of the positive electrode active material include various oxides such as manganese dioxide, lithium manganese composite oxide, lithium-containing nickel oxide, lithium-containing cobalt oxide, lithium-containing nickel cobalt oxide, lithium-containing iron oxide, and lithium. Examples thereof include vanadium oxide and chalcogen compounds such as titanium disulfide and molybdenum disulfide. Among them, lithium-containing cobalt oxide (for example, LiCoO ₂ ), Lithium-containing nickel cobalt oxide (eg, LiNi _0.8 Co _0.2 O ₂ ), Lithium manganese composite oxide (for example, LiMn ₂ O _Four LiMnO ₂ ) Is preferable because a high voltage can be obtained. As the positive electrode active material, one kind of oxide may be used alone, or two or more kinds of oxides may be mixed and used.
[0018]
Examples of the conductive agent include acetylene black, carbon black, and graphite.
[0019]
The binder has a function of holding the active material on the current collector and connecting the active materials to each other. Examples of the binder include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), polyethersulfone, ethylene-propylene-diene copolymer (EPDM), and styrene-butadiene rubber (SBR). be able to.
[0020]
The mixing ratio of the positive electrode active material, the conductive agent and the binder is preferably in the range of 80 to 95% by weight of the positive electrode active material, 3 to 20% by weight of the conductive agent, and 2 to 7% by weight of the binder.
[0021]
As the current collector, a conductive substrate having a porous structure or a non-porous conductive substrate can be used. These conductive substrates can be formed from, for example, aluminum, stainless steel, or nickel.
[0022]
The positive electrode is produced, for example, by suspending a conductive agent and a binder in an appropriate solvent in a positive electrode active material, applying the suspension to a current collector, and drying to form a thin plate.
[0023]
2) Negative electrode
The negative electrode includes a current collector and a negative electrode layer carried on one side or both sides of the current collector.
[0024]
The negative electrode layer includes a negative electrode active material that absorbs and releases lithium ions and a binder.
[0025]
Examples of the negative electrode active material include graphite materials, carbonaceous materials such as graphite, coke, carbon fiber, spherical carbon, pyrolytic vapor phase carbonaceous material, and resin fired body; thermosetting resin, isotropic pitch, and mesophase. Graphite material obtained by subjecting pitch-based carbon, mesophase pitch-based carbon fiber, mesophase microspheres, etc. (especially mesophase pitch-based carbon fiber is preferable because of its high capacity and charge / discharge cycle characteristics) to 500-3000 ° C. Or a carbonaceous material; a chalcogen compound such as titanium disulfide, molybdenum disulfide, or niobium selenide; a light metal such as aluminum, an aluminum alloy, a magnesium alloy, lithium, or a lithium alloy; Among these, the (002) plane spacing d ₀₀₂ It is preferable to use a graphite material having a graphite crystal having a thickness of 0.34 nm or less. A non-aqueous electrolyte secondary battery including a negative electrode containing such a graphite material as a negative electrode active material can greatly improve battery capacity and large current discharge characteristics. The surface spacing d ₀₀₂ Is more preferably 0.337 nm or less.
[0026]
Examples of the binder include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), ethylene-propylene-diene copolymer (EPDM), styrene-butadiene rubber (SBR), carboxymethyl cellulose (CMC), and the like. Can be used.
[0027]
The blending ratio of the negative electrode active material and the binder is preferably in the range of 80 to 98% by weight of the carbonaceous material and 2 to 20% by weight of the binder.
[0028]
As the current collector, a conductive substrate having a porous structure or a non-porous conductive substrate can be used. These conductive substrates can be formed from, for example, copper, stainless steel, or nickel.
[0029]
The negative electrode is prepared by, for example, kneading a negative electrode active material and a binder in the presence of a solvent, applying the obtained suspension to a current collector, drying it, and then pressing it once at a desired pressure or 2 It is produced by multi-stage pressing ˜5 times.
[0030]
3) Separator
As this separator, a microporous film, a woven fabric, a non-woven fabric, a laminate of the same material or different materials among these can be used. In particular, the microporous membrane causes a so-called shutdown phenomenon in which the resin constituting the separator plastically deforms and closes the fine pores when the temperature of the electrode group rises abnormally due to heat generated by overcharging, etc., and the flow of lithium ions Is prevented, and further overheating is prevented, and the overcharge state can be safely terminated, which is preferable. Examples of the material for forming the separator include polyethylene, polypropylene, ethylene-propylene copolymer, and ethylene-butene copolymer. As a material for forming the separator, one type or two or more types selected from the types described above can be used.
[0031]
The separator has an air permeability of 200 to 600 seconds / 100 cm. ^Three It is preferable that Air permeability is 100cm ^Three Means the time (seconds) required for the air to pass through the separator, and can be measured by the method specified in JIS (Japanese Industrial Standards) P8117. The air permeability is 200 seconds / 100 cm. ^Three If it is less than this, since the pores of the separator are large and rough, the shutdown phenomenon does not occur promptly and it becomes difficult to prevent high-temperature heat generation. On the other hand, the air permeability is 600 seconds / 100 cm. ^Three Exceeding the limit, the separator hole is so small that the non-aqueous electrolyte does not completely impregnate the separator hole, and the shutdown phenomenon occurs only partially, thus completely blocking the flow of lithium ions. I can't. Air permeability is 600 seconds / 100cm ^Three If it exceeds 1, the ion movement resistance during normal charge / discharge increases, and therefore the internal resistance of the battery increases, which is not preferable. The air permeability value is 250 to 500 seconds / 100 cm. ^Three More preferably, the value is more preferably 300 to 450 seconds / 100 cm. ^Three It is.
[0032]
The temperature at which the separator causes a shutdown phenomenon, so-called shutdown temperature, is preferably 100 to 160 ° C. This shutdown temperature is measured by measuring the air permeability after heating the separator to a constant temperature, and the value of the air permeability is 100,000 seconds / 100 cm. ^Three It can be measured as the above temperature. When the shutdown temperature exceeds 160 ° C., the separator does not shut down quickly in an overcharged state, and it becomes difficult to prevent high temperature heat generation. On the other hand, when the shutdown temperature is lower than 100 ° C., the air permeability of the separator increases when the nonaqueous electrolyte secondary battery is used in a high temperature state such as in a car under the hot sun, for example. There is a possibility that problems such as deterioration of the large current discharge characteristics of the battery may occur. A more preferable shutdown temperature of the separator is 110 to 150 ° C.
[0033]
It is desirable that the separator has a temperature at which it melts and causes film breakage (melting temperature) of 160 ° C. or higher, or a temperature that is 15 ° C. higher than the shutdown temperature. When the melting temperature is less than 160 ° C. or the difference from the shutdown temperature is less than 15 ° C., even if the shutdown phenomenon once occurs due to the rise in battery temperature, the separator melts and breaks the film after that, so that the positive electrode and the negative electrode There is a greater risk of a so-called short-circuit condition. When the short circuit is reached, the electrical resistance of the shorted part is extremely small, so a large current continues to flow in the shorted part, and the temperature of the shorted part rises locally by generating Joule heat, and the surrounding separator There is a possibility of melting or heat generation to a high temperature.
[0034]
The separator preferably has a porosity in the range of 30 to 60%. A more preferable range of the porosity is 35 to 50%.
[0035]
The thickness of the separator is desirably 30 μm or less, more preferably 25 μm or less. Further, the lower limit value of the thickness of the separator is desirably 5 μm, more preferably 8 μm.
[0036]
The width of the separator is preferably wider than the width of the positive electrode and the negative electrode. By adopting such a configuration, it is possible to prevent the positive electrode and the negative electrode from coming into direct contact without a separator.
[0037]
4) Electrode group
This electrode group has a structure in which the positive electrode and the negative electrode are spirally wound with a separator interposed therebetween. For example, (i) the positive electrode and the negative electrode are wound in a flat shape or a spiral shape with a separator interposed therebetween, or (ii) the positive electrode and the negative electrode are wound in a spiral shape with a separator interposed therebetween. After turning, it is compressed in the radial direction or produced by any method.
[0038]
The electrode group need not be pressed, but is preferably pressed in order to increase the strength by integrating the positive electrode, the negative electrode and the separator. It is also possible to heat at the time of pressing.
[0039]
The electrode group is allowed to contain an adhesive polymer in order to integrate the positive electrode, the negative electrode, and the separator to increase the strength. The adhesive polymer is desirably one that can maintain high adhesiveness while holding the non-aqueous electrolyte. More preferably, the adhesive polymer has high lithium ion conductivity. Specifically, examples of the adhesive polymer include polyacrylonitrile (PAN), polyacrylate (PMMA), polyvinylidene fluoride (PVdF), polyvinyl chloride (PVC), and polyethylene oxide (PEO).
[0040]
5) Non-aqueous electrolyte
This non-aqueous electrolyte has a composition in which an electrolyte such as a lithium salt is dissolved in a non-aqueous solvent. These nonaqueous solvents and electrolytes will be described.
[0041]
(A) γ-butyrolactone (GBL)
Since GBL has low reactivity with the positive electrode in the charged state, that is, in the state where the potential of the positive electrode is high, the decomposition reaction and exothermic reaction of the nonaqueous solvent in the overcharged state can be suppressed. The ratio of GBL in the non-aqueous solvent is preferably 40% by weight or more. When the ratio of GBL is less than 40% by weight, the ratio of a non-aqueous solvent such as ethylene carbonate (EC) having a higher reactivity with the positive electrode than GBL is relatively high, and the non-aqueous solvent in an overcharged state. The decomposition reaction and the exothermic reaction easily occur, the temperature of the battery rises, and the risk of falling into a so-called thermal runaway state in which the decomposition reaction of the nonaqueous solvent is promoted is increased.
[0042]
The ratio of GBL is more preferably 50% by weight or more, and further preferably 55% by weight or more.
[0043]
(B) Cyclic carbonate
The cyclic carbonates such as ethylene carbonate (EC) and propylene carbonate (PC) can be used in the negative electrode active material without impairing the advantages of GBL, that is, the low freezing point, the high lithium ion conductivity, and the safety. This is preferable because the reaction between lithium ions occluded in GBL and GBL can be suppressed. In particular, EC is more preferable because it has a large effect of suppressing the reaction between lithium ions and GBL.
[0044]
When GBL is used as the solvent, the ratio of EC in the non-aqueous solvent is preferably in the range of 20 to 50% by weight. If the EC ratio is less than 20% by weight, the reaction between lithium ions and GBL, particularly in a high temperature environment, may not be suppressed. On the other hand, since EC has a higher reactivity with the positive electrode in the charged state than GBL, when the EC ratio exceeds 50% by weight, the decomposition reaction and exothermic reaction of the nonaqueous solvent in the overcharged state are likely to occur. Further, since EC has a high freezing point, if the EC ratio exceeds 50% by weight, the low-temperature discharge characteristics may be remarkably deteriorated. Furthermore, if the EC ratio exceeds 50% by weight, the viscosity of the non-aqueous electrolyte solution becomes high and the ionic conductivity is lowered, which may reduce the high current discharge characteristics. A more preferable EC ratio is 25 to 50% by weight, still more preferably 25 to 45% by weight.
[0045]
The non-aqueous solvent is allowed to contain a surfactant such as trioctyl phosphate (TOP) in order to improve the wettability with the separator. The addition amount of the surfactant is desirably 3% or less, more preferably 0.1 to 1%.
[0046]
Examples of the non-aqueous solvent include vinylene carbonate, vinyl ethylene carbonate, phenyl ethylene carbonate, diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, γ-valerolactone, methyl propionate, ethyl propionate, 2-methylfuran, furan, and thiophene. , Catechol carbonate, ethylene sulfite, 12-crown-4, tetraethylene glycol dimethyl ether and the like are allowed to be contained as accessory components. Among these, the secondary component containing vinylene carbonate (VC) produces a dense protective film on the negative electrode surface. Therefore, when GBL is used as a solvent, the reaction between lithium ions occluded in the negative electrode active material and GBL occurs. It becomes possible to suppress. The ratio of VC in the non-aqueous solvent is desirably 10% by weight or less. If the VC ratio exceeds 10% by weight, the lithium ion permeability of the protective film formed on the negative electrode surface may be reduced, and the low-temperature discharge characteristics may be significantly impaired. A more preferable VC ratio is 0.01 to 5% by weight, still more preferably 0.1 to 3% by weight.
[0047]
(C) Chain carbonate
Chain carbonates such as methyl ethyl carbonate (MEC), diethyl carbonate (DEC), and dimethyl carbonate (DMC) can be used as the non-aqueous solvent. In particular, by using it in combination with EC, which is one of the cyclic carbonates described above, it is possible to improve discharge characteristics at low temperatures and charge / discharge cycle characteristics.
[0048]
The chain carbonate preferably contains the MEC, and can be used in the form of, for example, MEC alone or a mixture of MEC and another chain carbonate.
[0049]
The other chain carbonate mixed with the MEC preferably has a low freezing point and a low viscosity. The other chain carbonates preferably have a relatively low molecular weight, and can improve discharge characteristics at low temperatures. As such other chain carbonate, at least one of DEC and DMC is preferable. In particular, a combination of MEC and other chain carbonates including DEC from the viewpoint of developing excellent charge / discharge cycle characteristics, and a combination of MEC and other chain carbonates including DMC from the viewpoint of exhibiting excellent low-temperature discharge characteristics. Is preferred.
[0050]
(D) Electrolyte
Examples of the electrolyte include lithium perchlorate (LiClO). _Four ), Lithium hexafluorophosphate (LiPF) ₆ ), Lithium tetrafluoroborate (LiBF) _Four ), Lithium hexafluoroarsenide (LiAsF) ₆ ), Lithium trifluorometasulfonate (LiCF) _Three SO _Three ), Bistrifluoromethylsulfonylimide lithium (LiN (CF _Three SO ₂ ) ₂ ), Bispentafluoroethylsulfonylimide lithium (LiN (C ₂ F _Five SO ₂ ) ₂ ) And the like. These electrolytes can be used in one kind or a mixture of two or more kinds.
[0051]
Among the electrolytes, LiBF _Four Is preferable because the reactivity with the positive electrode when the temperature of the secondary battery rises is low, and the amount of heat generated in the overcharged state becomes small. Also, (LiN (CF _Three SO ₂ ) ₂ And LiN (C ₂ F _Five SO ₂ ) ₂ A lithium salt comprising at least one of LiBF and LiBF _Four A mixed salt containing a lithium salt consisting of or LiBF _Four And LiPF ₆ When the mixed salt containing is used, the cycle life at a high temperature can be further improved.
[0052]
LiBF relative to the total weight of the electrolyte _Four The weight ratio is preferably 50% by weight or more. This is LiBF _Four When the weight ratio of Li is less than 50% by weight, LiBF _Four More reactive electrolytes (eg LiPF ₆ ) Ratio increases, an exothermic reaction is likely to occur in an overcharged state, and there is a possibility of a thermal runaway state from a high temperature state. Preferred LiBF _Four The ratio is 70% by weight or more, more preferably 80% by weight or more.
[0053]
The amount of the electrolyte dissolved in the non-aqueous solvent is preferably 0.5 to 2.5 mol / L. A more preferable dissolution amount is 1 to 2.5 mol / L.
[0054]
The amount of the non-aqueous electrolyte is preferably 0.2 to 0.6 g per 100 mAh of battery unit capacity. A more preferable amount of the non-aqueous electrolyte is 0.25 to 0.55 g / 100 mAh.
[0055]
6) Insulating coating
This insulating coating has a structure in which a powder having a heat resistance of 500 ° C. or higher is bound with a binder resin, and at least both ends along the rolling direction of the positive and negative electrodes including the rolling start end of the electrode group. Fixed.
[0056]
Here, “both ends along the winding direction of the positive and negative electrodes including the starting start portion of the electrode group” are formed on both sides of the exposed current collector located at the starting start portion of the positive and negative electrodes and the current collector. It means a region extending over the active material layer portion.
[0057]
In particular, by fixing the insulating coating to both ends along the winding direction of the positive and negative electrodes wound once from the starting end portion of the electrode group, the separator constituting the electrode group is contracted by heat, When the width of the separator is smaller than the width of the positive electrode and the negative electrode, it is possible to prevent the occurrence of high-temperature heat generation more effectively while suppressing the reduction of the battery reaction field between the positive and negative electrodes. . Of course, you may fix the said insulating film to the full length of the both ends along the winding direction of the said positive / negative electrode located in the winding end edge part from the winding start edge part of the said electrode group.
[0058]
When the width of the separator is larger than the width of the positive and negative electrodes, the insulating coating has a width corresponding to at least a contraction in the width direction of the separator due to heat generation of the secondary battery in the rolling direction of the positive and negative electrodes. It is preferable to fix to the both ends along. In particular, burrs associated with slitting to a predetermined dimension occur at both end portions along the positive and negative electrode rolling directions, and this is one of the causes of internal short circuit. From the viewpoint of covering the burrs at both ends along the line, it is desirable to fix the burrs at a width of 0.5% or more, more preferably 1.0% or more with respect to the entire width of the positive and negative electrodes. On the other hand, if the width of the insulating coating is too wide, the ratio of the positive and negative active material layers covered with the insulating coating increases, which may reduce the battery reaction efficiency. For this reason, it is desirable that the upper limit of the width of the insulating coating is 30% or less, preferably 15% or less, and most preferably 10% or less with respect to the entire width of the positive and negative electrodes.
[0059]
Examples of the powder include at least one inorganic powder selected from alumina, silica, zeolite, and titanium oxide.
[0060]
The powder preferably has an average particle size of 30 μm or less, more preferably 0.005 to 5 μm. When the average particle size of the powder exceeds 30 μm, it is difficult to form an insulating coating in the coating process, and it is difficult to form a relatively thin insulating coating suitable for manufacturing an electrode group. There is a fear.
[0061]
When a spherical powder is used as the powder, it becomes possible to improve the fluidity of the coating slurry and prevent the coating apparatus from being worn. As this spherical powder, alumina, silica and the like are commercially available.
[0062]
Examples of the binder resin that can be used include polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), ethylene-propylene-diene copolymer (EPDM), and styrene-butadiene rubber (SBR).
[0063]
The mixing ratio of the powder and the binder resin constituting the insulating coating is preferably 5 to 35 with respect to the powder 100 by weight. When the mixing ratio of the binder resin is less than 5, applicability when the insulating coating is formed by a coating means is decreased, the strength of the insulating coating is decreased, or the exposed surface of the current collector is exposed. There is a possibility that the fixing property of the insulating coating will deteriorate. On the other hand, when the mixing ratio of the binder resin exceeds 35, the amount of the binder resin in the insulating film becomes too large, and it is possible to prevent melting and damage of the insulating film due to Joule heat generation due to short-circuiting in the positive and negative electrodes. It can be difficult.
[0064]
The insulating film preferably has a thickness of 10 to 60 μm. If the thickness of the insulating coating is less than 10 μm, it may be difficult to prevent melting and damage of the insulating coating due to Joule heat generation due to a short circuit in the positive / negative polarity. On the other hand, when the thickness of the insulating coating exceeds 60 μm, it becomes thicker than the active material layers of the positive electrode and the negative electrode, which hinders the winding operation for producing the electrode group, or the electrode group of these active material layers. There is a risk that the proportion of
[0065]
The insulating film is formed by the following method, for example.
[0066]
The powder and the binder resin are added to an appropriate solvent, for example, N-methylpyrrolidone, and stirred to dissolve the binder resin, and the powder is dispersed in a binder resin solution to prepare a coating slurry. Subsequently, the coating slurry is sprayed on both ends along the winding direction of the positive and negative electrodes including at least the starting end portion of the electrode group, or is applied with a brush and then dried to form and fix an insulating film.
[0067]
Next, a lithium ion secondary battery according to the present invention, for example, a thin lithium ion secondary battery using a laminate film as an exterior member will be described with reference to FIGS.
[0068]
As shown in FIGS. 2 to 4, the electrode group 1 includes, for example, a positive electrode 4, a separator 5, and an active material and a binder in which a positive electrode active material layer 2 containing an active material and a binder is supported on both surfaces of the current collector 3. The negative electrode active material layer 6 containing the agent is wound around the negative electrode 8 supported on both surfaces of the current collector 7 and the separator 5 in a spiral shape, and further formed into a flat and rectangular shape. External leads 9, 10 connected to the positive and negative electrodes 4, 8 extend from the same side surface of the electrode group 1 to the outside. As shown in FIG. 5, the positive electrode side insulating coating 11 in which a powder having heat resistance of 500 ° C. or higher is bound with a binder resin is formed in the rolling direction of the positive electrode 4 including the rolling start end portion of the electrode group 1. For example, both ends of the electrode group 1 are fixed to both sides of the positive electrode 4 along the winding direction from the winding start end portion to the winding end end portion. Further, the negative electrode-side insulating coating 12 in which a powder having heat resistance of 500 ° C. or higher is bound with a binder resin is formed in the negative electrode 8 including the starting start portion of the electrode group 1 as shown in FIG. Both ends along the direction, for example, both sides of the entire length of both ends along the winding direction of the negative electrode 8 extending from the winding start end portion to the winding end end portion of the electrode group 1 are fixed.
[0069]
As shown in FIG. 1, the electrode group 1 has, for example, a folded portion in a cup 14 of a cup-shaped exterior film 13 that is folded in the opposite direction to the side surface on which the external leads 9 and 10 of the electrode group 1 are extended. It is wrapped so as to be located on the side surface side. As shown in FIG. 2, the exterior film 13 has an aluminum or aluminum alloy sheet 15 in the middle, and a sealant film 16 is attached to the inner surface side of the sheet 15 where the electrode group 1 is located via an adhesive. And a plastic film 17 having at least rigidity is attached to the outside of the sheet 15 through an adhesive. The adhesive insulating film 18 is between the upper and lower surfaces of the positive external lead 9 and the sealant film 16 of the exterior film 13 and between the upper and lower surfaces of the negative external lead 10 and the sealant film 16 of the exterior film 13. Respectively. Three side portions corresponding to the two long side surfaces and one short side surface of the electrode group 1 excluding the bent portion in the exterior film 13 are heat-sealed with the sealant films 16 and extended in the horizontal direction. Portions 19a, 19b, and 19c are formed, and the electrode group 1 is sealed by these seal portions 19a, 19b, and 19c. In the seal portion 19b, the sealant film 16 and the adhesive insulating film 18 are heat-sealed, and the external terminals connected to the molten adhesive insulating film 18 and the positive and negative electrodes 4 and 8 of the electrode group 1 9 and 10 are in close contact with each other, and the external terminals 9 and 10 are extended to the outside through the seal portion 19b. A nonaqueous electrolytic solution is impregnated and accommodated in the electrode group 1 and the exterior film 13 sealed by the seal portions 19a, 19b, and 19c.
[0070]
The sealant film is preferably an unstretched film that does not dissolve or swell in the non-aqueous electrolyte. For example, polyolefin polymer such as unstretched polypropylene (PP), ethylene / vinyl acetate (EVA) copolymer, ionomer (IO), polyamide (PA), nylon (Ny), polyethylene terephthalate (PET), polybutylene Terephthalate (PBT), polyvinyl chloride (PVC), polyvinylidene chloride (PVDC), polyvinyl alcohol (PVA), ethylene vinyl alcohol (EVOH), polycarbonate (PC), polystyrene (PS), polyacrylonitrile (PAN), ethylene Acrylic acid copolymer (EAA), ethylene / methacrylic acid copolymer (EMAA), ethylene / methyl acrylate copolymer (EMA), ethylene / methyl methacrylate copolymer (EMMA), ethylene / ethyl acrylate Preparative copolymer (EEA), it may be used polymethylpentene (PMP) and the like. In particular, a resin film obtained by graft polymerization of an acid anhydride such as maleic anhydride with these resins as a base polymer is suitable not only when an adhesive insulating film is not used, but also for increasing the adhesion to a metal. It is beneficial.
[0071]
The rigid plastic plastic film has a function of protecting the aluminum or aluminum alloy sheet and maintaining the mechanical structural characteristics of the battery. As the resin having this rigidity, for example, a polyolefin polymer such as biaxially stretched polyethylene (PE) or polypropylene (PP), polyethylene terephthalate (PET), nylon or the like can be used.
[0072]
As the adhesive insulating film, it is preferable to use a film that has characteristics and moldability equivalent to those of a sealant film and has excellent adhesion to a metal that is a positive and negative external terminal. Specifically, a polymer obtained by graft polymerization of an acid anhydride such as maleic anhydride to the base polymer made of the resin used for the sealant film described above can be used. The adhesive insulating film may be the same system as or different from the sealant film. In particular, if the adhesive insulating film and the sealant film are made of the same composition, a highly reliable sealing structure in which the positive and negative electrode side external terminal portions are homogeneous is obtained. However, the adhesive insulating film is not essential for the structure of the battery and may not be used as necessary.
[0073]
The lithium ion secondary battery according to the present invention is not limited to the thin lithium ion secondary battery described above, but can be similarly applied to a square lithium ion secondary battery and a cylindrical lithium ion secondary battery.
[0074]
As described above, according to the present invention, a powder having a heat resistance of 500 ° C. or higher is bound with a binder resin at both ends along the rolling direction of the positive and negative electrodes including at least the rolling start end of the electrode group. By fixing the insulating film, even if the secondary battery generates heat due to overcharging, etc., and the separator contracts due to the heat (especially in the width direction), the positive electrode and the negative electrode are directly opposed to each other. A short circuit can be prevented and high temperature heat generation and thermal runaway can be prevented.
[0075]
That is, the lithium secondary battery generates heat by an electrochemical reaction when an excessive current flows due to, for example, overcharging. As the heat is generated, the temperature of the electrode group rises, but the temperature at the end of the electrode group starts excessively as compared with the end of the electrode group. For this reason, especially the separator located in the vicinity of the starting end of the electrode group contracts in the length direction and the width direction due to a great heat effect. Separation of the separator (particularly in the width direction) is a state in which both end portions along the length direction of the positive and negative electrodes including the starting end portion of the electrode group (the winding direction of the electrode group) are directly opposed without a separator interposed therebetween. And the insulation between the both ends of the positive electrode and the negative electrode cannot be maintained, so that an internal short circuit may occur, leading to thermal runaway.
[0076]
Under such circumstances, the secondary battery of the present invention has a binder having a heat resistant powder of 500 ° C. or more at both ends along the rolling direction of the electrode group of the positive and negative electrodes including at least the starting start portion of the electrode group. Since it has a structure in which an insulating film bound with resin is fixed, the width direction of the separator In Even if it shrinks, the insulating film can ensure the insulation at both ends of the positive and negative electrodes. Ru Therefore, an internal short circuit between the positive and negative electrodes can be prevented. Therefore, it is possible to prevent thermal runaway due to an internal short circuit and to realize a highly reliable lithium ion secondary battery.
[0077]
In particular, if a powder having an average particle size of 30 μm or less is used as the powder constituting the insulating coating, the insulating coating in which the powder is more uniformly dispersed includes at least the starting start portion of the electrode group. Since it can fix to the both ends along the rolling direction of a positive / negative electrode, application | coating processability is favorable and can melt | dissolve the said insulating film by the heat_generation | fever mentioned above with the said powder effectively.
[0078]
In addition, if the mixing ratio of the powder constituting the insulating coating and the binder resin is set so that the binder resin is 5 to 35 with respect to the powder 100 in a weight ratio, the powder becomes high. Since the insulating coating dispersed in density can be firmly fixed to both ends along the rolling direction of the positive and negative electrodes including at least the starting start portion of the electrode group, the melting of the insulating coating due to the heat generation described above The powder can prevent it more effectively.
[0079]
【Example】
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
[0080]
(Example 1)
<Preparation of positive electrode>
First, lithium cobalt oxide (Li _x CoO ₂ Wherein X is 0 <X ≦ 1) and 90% by weight of powder, 5% by weight of acetylene black and 5% by weight of polyvinylidene fluoride (PVdF) in dimethylformamide (DMF) are added and mixed; A slurry was prepared. The slurry was applied to both sides of a current collector made of an aluminum foil having a thickness of 15 μm, then dried and pressed to produce a positive electrode having a structure in which the positive electrode layer was supported on both sides of the current collector. The positive electrode layer had a thickness of 60 μm per side and a width of 55 mm.
[0081]
<Production of negative electrode>
Mesophase pitch carbon fiber heat treated at 3000 ° C. as a carbonaceous material ((002) plane spacing (d determined by powder X-ray diffraction) (d ₀₀₂ ) 0.336 nm) was mixed with 95% by weight of a powder and 5% by weight of a polyvinylidene fluoride (PVdF) dimethylformamide (DMF) solution to prepare a slurry. The slurry was applied to both sides of a current collector made of a copper foil having a thickness of 12 μm, dried, and pressed to prepare a negative electrode having a structure in which the negative electrode layer was supported on the current collector. The negative electrode layer had a thickness of 55 μm per side and a width of 58 mm.
[0082]
Note that the spacing d of the (002) plane of the carbonaceous material ₀₀₂ Was determined from the powder X-ray diffraction spectrum by the half-width half-point method. At this time, scattering correction such as Lorentz scattering was not performed.
[0083]
<Formation of insulating coating>
About the negative electrode and the positive electrode, when it is an electrode group as shown in FIG. 5 described above, 20 parts by weight of PVdF on both sides of the positive electrode and the negative electrode in the entire length from the starting end to the end of the negative electrode. A coating solution containing 80 parts by weight of alumina was applied to a thickness corresponding to 5% of the total width of each of the positive electrode and the negative electrode so as to have a thickness of about 20 μm by a spray method. By sufficiently drying after application, it was confirmed that the insulating coating was firmly adhered to the entire length of both ends of the positive and negative electrodes.
[0084]
<Production of electrode group>
After the positive electrode lead made of a strip-shaped aluminum foil (thickness 100 μm) is ultrasonically welded to the positive electrode current collector, and the negative electrode lead made of a strip-shaped nickel foil (thickness 100 μm) is ultrasonically welded to the negative electrode current collector The positive electrode and the negative electrode were spirally wound through the separator between them to prepare an electrode group. The electrode group was formed into a flat shape by applying pressure with a press while heating. By changing the heating temperature and pressure during molding, the air permeability of the separator after molding can be adjusted. In Example 1, the temperature of the separator was molded at a temperature of 80 ° C. and a pressure of 1.4 MPa, and the air permeability of the molded separator was measured by a method defined in JIS (Japanese Industrial Standard) P8117. ^Three Met.
[0085]
<Preparation of non-aqueous electrolyte>
A nonaqueous solvent was prepared by mixing ethylene carbonate (EC) and γ-butyrolactone (GBL) so that the weight ratio (EC: GBL) was 40:60. Lithium tetrafluoroborate (LiBF) was added to the obtained non-aqueous solvent. _Four ) Was dissolved so that the concentration became 1.5 mol / L to prepare a non-aqueous electrolyte.
[0086]
Next, a laminate film having a thickness of 100 μm in which both surfaces of an aluminum sheet are covered with a polyethylene film is formed into a rectangular cup shape by a press machine to produce an exterior film material, and the electrode group is placed in the cup of the exterior film material. The positive and negative external leads were housed so as to protrude outside the exterior film material. Then, the moisture contained in the electrode group and the exterior film was removed by vacuum drying the exterior film material containing the electrode group in the cup at 80 ° C. for 12 hours.
[0087]
Next, an acid-modified LLDPE film having a thickness of 70 μm as an insulating adhesive film (trade name, manufactured by Nippon Polyolefin Co., Ltd .; Adtex ER615F) is interposed between the upper and lower surfaces of the outer film material and the positive and negative external leads. The outer film material cup was bent 180 ° at the molding end on the short side. The press head heated in this state was heated and pressurized to adhere the positive and negative external leads, the acid-modified LLDPE film, and the polyethylene film to form a seal portion on the side where the external leads are extended. One long side portion of the exterior film material having no positive and negative external leads was also heated and pressurized by a heated press head, and the polyethylene films were bonded to form a seal portion. These heat sealing orders may be performed simultaneously or first.
[0088]
After injecting the non-aqueous electrolyte of the above composition through the open long side portion of the exterior film material, the unsealed portion is heated and pressurized with a heated press head and sealed as described above with reference to FIG. 2, a thin lithium ion secondary battery having a thickness of 3.6 mm, a width of 35 mm, a height of 62 mm, and a nominal capacity of 0.65 Ah was assembled.
[0089]
(Examples 2 to 16)
The insulating film having the composition and configuration shown in Table 1 below is 5% of the total width of the positive and negative electrodes on both sides of the total length of both ends of the positive electrode and the negative electrode extending from the starting end portion to the ending end portion of the electrode group. 15 thin lithium ion secondary batteries having the structure shown in FIGS. 1 and 2 were assembled in the same manner as in Example 1 except that they were formed and fixed in a corresponding width.
[0090]
(Comparative Example 1)
1 and FIG. 2 described above in the same manner as in Example 1 except that the insulating coating is not formed on both sides of the entire length of both ends of the positive electrode and the negative electrode extending from the start end to the end end of the electrode group. A thin lithium ion secondary battery having the structure shown in FIG.
[0091]
(Comparative Example 2)
The insulating film having the composition and configuration shown in Table 1 below is 5% of the total width of the positive and negative electrodes on both sides of the total length of both ends of the positive electrode and the negative electrode extending from the starting end portion to the ending end portion of the electrode group. A thin lithium ion secondary battery having the structure shown in FIGS. 1 and 2 was assembled in the same manner as in Example 1 except that it was formed and fixed in a corresponding width.
[0092]
The following initial charge / discharge steps were performed on the obtained lithium ion secondary batteries of Examples 1 to 16 and Comparative Examples 1 and 2. First, constant current / constant voltage charging was performed for 15 hours at room temperature to 4.2 V at 0.2C. Then, it discharged to 3.0V at 0.2C at room temperature, and the nonaqueous electrolyte secondary battery was manufactured. Here, 1C is a current value necessary for discharging the nominal capacity (Ah) in one hour. Therefore, 0.2 C is a current value necessary for discharging the nominal capacity (Ah) in 5 hours.
[0093]
Lithium ion secondary that caused thermal runaway during an overcharge test that continued charging at a current value of 3 C for each of the secondary batteries of Examples 1 to 16 and Comparative Examples 1 and 2 after the initial charge The number of batteries was examined. The results are also shown in Table 1 below.
[0094]
[Table 1]

[0095]
As apparent from Table 1, the secondary batteries of Examples 1 to 16 using a structure in which a powder having a heat resistance of 500 ° C. or higher was bound with a binder resin as the insulating coating were overcharge tests. It can be seen that even if the separator contracts due to heat generation at, internal short-circuits at both ends along the rolling direction of the positive and negative electrodes are prevented and thermal runaway does not occur.
[0096]
On the other hand, not only the secondary battery of Comparative Example 1 in which an insulating film is not formed, but also when the insulating film is fixed to both ends along the winding direction of the positive and negative electrodes, the insulating film is polyvinylidene fluoride (PVdF). It can be seen that in the secondary battery of Comparative Example 2 made of an insulating resin that is insoluble only in the electrolyte, thermal runaway occurs when the separator contracts due to overcharging.
[0097]
【The invention's effect】
As described above in detail, according to the present invention, even when the separator constituting the electrode group is contracted by heat and the width of the separator is smaller than the width of the positive electrode and the negative electrode, it is possible to prevent high temperature heat generation. A lithium ion secondary battery with high reliability and safety that can be provided can be provided.
[Brief description of the drawings]
FIG. 1 is a cutaway perspective view showing a thin lithium ion secondary battery according to the present invention.
2 is a cross-sectional view taken along line II-II in FIG.
3 is a perspective view showing an electrode group incorporated in the lithium ion secondary battery of FIG. 1. FIG.
4 is a cross-sectional view taken along line IV-IV in FIG.
FIG. 5 is a perspective view showing a starting start side of a positive electrode, a separator, and a negative electrode before winding as an electrode group incorporated in the lithium ion secondary battery of FIG. 1;
[Explanation of symbols]
1 ... Electrode group,
4 ... positive electrode,
5 ... separator,
8 ... negative electrode,
9: Positive external lead,
10: negative electrode side external lead,
11, 12 ... Insulating coating,
13 ... exterior film,
14 ... Cup,
19a, 19b, 19c ... seal part.

Claims (5)

A positive electrode in which an active material layer is formed on a current collector and an electrode group in which a negative electrode in which an active material layer is formed on a current collector are wound with a separator interposed therebetween,
An insulating film in which a powder having a heat resistance of 500 ° C. or higher is bound with a binder resin, and the binder resin is blended in an amount of 5 to 35 by weight with respect to the powder 100, It fixes to the both ends along the rolling direction of the said positive / negative electrode containing at least lithium ion secondary battery characterized by the above-mentioned.   2. The lithium ion secondary battery according to claim 1, wherein the powder is at least one inorganic powder selected from alumina, silica, zeolite, and titanium oxide.   The lithium ion secondary battery according to claim 1, wherein the powder has an average particle size of 30 μm or less.   4. The lithium ion secondary battery according to claim 1, wherein the insulating coating has a thickness of 20 to 60 [mu] m.   When the width of the separator is larger than the width of the positive and negative electrodes, the insulating coating has a width corresponding to at least the amount of contraction in the width direction of the separator accompanying the heat generation of the secondary battery, along the winding direction of the positive and negative electrodes. The lithium ion secondary battery according to claim 1, wherein the lithium ion secondary battery is fixed to both ends.

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