JP6578148B2 - Lithium ion secondary battery - Google Patents

Description

  The present invention relates to a non-aqueous electrolyte battery, particularly a lithium ion secondary battery.

  Nonaqueous electrolyte batteries have been put to practical use as automobile batteries including hybrid cars and electric cars. Lithium ion secondary batteries are used as such on-vehicle power supply batteries. As the development of lithium ion secondary batteries progresses, higher output is achieved, and as a result, ensuring safety is essential.

  When a lithium ion secondary battery is destroyed by nail penetration or crushing, the battery may generate heat due to an internal short circuit. In order to prevent such heat generation, various methods such as providing a safety circuit have been proposed in addition to improving battery members including electrode materials, electrolytes, separators, and the like.

  In Patent Document 1, in a battery using a lightweight metal such as aluminum as a case, the resistance value of the battery is set to a predetermined value for the purpose of preventing heat generation of the battery when the battery is internally short-circuited or when the battery is destroyed. It is proposed that this be done. As in Patent Document 1, when the resistance value of the battery is increased, the output of the battery is also reduced, and thus it is difficult to apply to a battery that requires high output. In addition, when a battery is short-circuited, heat is usually generated by concentrating current locally in a minimum time, and it is difficult to prevent battery heat generation simply by increasing the resistance value of the battery. .

JP 2008-262832 A

  The present invention provides a lithium ion secondary battery that can prevent heat generation of the battery and maintain safety.

A lithium ion secondary battery according to an embodiment of the present invention includes a positive electrode in which a positive electrode active material layer is disposed on a positive electrode current collector, a negative electrode in which a negative electrode active material layer is disposed on a negative electrode current collector, a separator, and an electrolyte solution And a lithium ion secondary battery including a power generation element. Here, the value (W / Wh) of the battery output to capacity is 25 or more. The exothermic index obtained by dividing the reciprocal of the product of the DC resistance value and the total electrode area when a 50% charged battery is discharged for 1 second by the current collector heat capacity is 80 or more. And the thermal shrinkage rate of the separator to be used is 13% or less.

  The lithium ion secondary battery of the present invention is unlikely to generate heat when a short circuit occurs, and can suppress an increase in battery temperature. Thus, safety can be improved without reducing the output of the battery.

FIG. 1 is a schematic cross-sectional view showing a lithium ion secondary battery according to an embodiment of the present invention.

  Embodiments of the present invention will be described below. In the present embodiment, the positive electrode is a thin plate in which a positive electrode active material layer, a binder, and, if necessary, a mixture of a conductive additive are applied to a positive electrode current collector such as a metal foil or rolled and dried to form a positive electrode active material layer. Or a sheet-like battery member. The negative electrode is a thin plate-like or sheet-like battery member in which a negative electrode active material layer is formed by applying a mixture of a negative electrode active material, a binder, and, if necessary, a conductive additive to a negative electrode current collector. The separator is a film-like battery member for separating the positive electrode and the negative electrode and ensuring the conductivity of lithium ions between the negative electrode and the positive electrode. The electrolytic solution is an electrically conductive solution in which an ionic substance is dissolved in a solvent. In this embodiment, a nonaqueous electrolytic solution can be used in particular. The power generation element including the positive electrode, the negative electrode, the separator, and the electrolytic solution is a unit of the main constituent member of the battery. Usually, the positive electrode and the negative electrode are stacked (stacked) with the separator interposed therebetween. Is immersed in the electrolyte.

  The lithium ion secondary battery of the embodiment is configured such that the power generation element is included in the exterior body, and preferably the power generation element is sealed inside the exterior body. The term “sealed” means that the power generation element is encased in an exterior body material so as not to touch the outside air. That is, the exterior body has a bag shape capable of sealing the power generation element therein.

  The negative electrode that can be used in all embodiments includes a negative electrode in which a negative electrode active material layer containing a negative electrode active material is disposed on a negative electrode current collector. Preferably, the negative electrode has a negative electrode active material layer obtained by applying or rolling a mixture of a negative electrode active material, a binder, and optionally a conductive additive to a negative electrode current collector made of a metal foil such as copper foil, and drying. ing. In each embodiment, it is preferable that a negative electrode active material contains graphite. Graphite has a stable structure and is suitable as a negative electrode material for high capacity batteries. The negative electrode active material can also contain amorphous carbon in addition to graphite particles. When a mixed carbon material containing both graphite and amorphous carbon is used, the battery regeneration performance is improved.

  Graphite is a carbon material of hexagonal hexagonal plate crystal, and is sometimes referred to as graphite or graphite. The graphite is preferably in the form of particles. Amorphous carbon means a carbon material that may have a structure that partially resembles graphite, or a structure in which microcrystals are randomly networked, and is amorphous as a whole. To do. Examples of the amorphous carbon include carbon black, coke, activated carbon, carbon fiber, hard carbon, soft carbon, and mesoporous carbon. The amorphous carbon is preferably in the form of particles.

  Examples of conductive auxiliary agents used in the negative electrode active material layer include carbon fibers such as carbon nanofibers, carbon blacks such as acetylene black and ketjen black, carbon materials such as activated carbon, mesoporous carbon, fullerenes, and carbon nanotubes. It is done. In addition, additives generally used for electrode formation, such as a thickener, a dispersant, and a stabilizer, can be appropriately used for the negative electrode active material layer.

  As a binder used for the negative electrode active material layer, fluororesins such as polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), and polyvinyl fluoride (PVF), and conductive materials such as polyanilines, polythiophenes, polyacetylenes, and polypyrroles. Polymers, synthetic rubbers such as styrene butadiene rubber (SBR), butadiene rubber (BR), chloroprene rubber (CR), isoprene rubber (IR), acrylonitrile bradiene rubber (NBR), or carboxymethyl cellulose (CMC), xanthan gum, guar gum Polysaccharides such as pectin can be used.

The positive electrode that can be used in all embodiments includes a positive electrode in which a positive electrode active material layer including a positive electrode active material is disposed on a positive electrode current collector. Preferably, the positive electrode has a positive electrode active material layer obtained by applying or rolling a mixture of a positive electrode active material, a binder, and optionally a conductive additive to a positive electrode current collector made of a metal foil such as an aluminum foil, and drying. ing. As the positive electrode active material, a lithium transition metal oxide can be used. For example, a lithium / nickel-based oxide (for example, LiNiO ₂ ), a lithium cobalt-based oxide (for example, LiCoO ₂ ), a lithium manganese-based oxide (for example, LiMn _2). Preference is given to using O ₄ ) and mixtures thereof. As the positive electrode active material, a lithium nickel cobalt manganese composite oxide represented by a general formula Li _x Ni _y Co _z Mn _(1-yz) O ₂ can be used. Here, x in the general formula is 1 ≦ x ≦ 1.2, y and z are positive numbers that satisfy y + z <1, and the value of y is 0.5 or less. Note that when the proportion of manganese increases, it becomes difficult to synthesize a single-phase composite oxide, so 1-yz ≦ 0.4 is desirable. Further, since the cost increases and the capacity decreases when the proportion of cobalt increases, it is desirable to satisfy z <y and z <1-yz. In order to obtain a high capacity battery, it is particularly preferable to satisfy y> 1-yz and y> z. The lithium nickel cobalt manganese composite oxide preferably has a layered crystal structure.

  Carbon fibers such as carbon nanofibers, carbon blacks such as acetylene black and ketjen black, activated carbon, graphite, mesoporous carbon, fullerenes, carbon nanotubes and other carbon materials as conductive aids optionally used in the positive electrode active material layer Is mentioned. In addition, additives generally used for electrode formation, such as thickeners, dispersants, and stabilizers, can be appropriately used for the positive electrode active material layer.

  As a binder used for the positive electrode active material layer, fluororesins such as polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), and polyvinyl fluoride (PVF), and conductive materials such as polyanilines, polythiophenes, polyacetylenes, and polypyrroles. Polymers, synthetic rubbers such as styrene butadiene rubber (SBR), butadiene rubber (BR), chloroprene rubber (CR), isoprene rubber (IR), acrylonitrile bradiene rubber (NBR), or carboxymethyl cellulose (CMC), xanthan gum, guar gum Polysaccharides such as pectin can be used.

The electrolyte that can be used in all the embodiments is a non-aqueous electrolyte, which is dimethyl carbonate (hereinafter referred to as “DMC”), diethyl carbonate (hereinafter referred to as “DEC”), di-n-propyl. Chain carbonates such as carbonate, di-t-propyl carbonate, di-n-butyl carbonate, di-isobutyl carbonate, or di-t-butyl carbonate, propylene carbonate (hereinafter referred to as “PC”), ethylene carbonate ( Hereinafter, it is preferably a mixture containing a cyclic carbonate such as “EC”. The electrolytic solution is obtained by dissolving a lithium salt such as lithium hexafluorophosphate (LiPF ₆ ), lithium borofluoride (LiBF ₄ ), or lithium perchlorate (LiClO ₄ ) in such a carbonate mixture.

  The electrolytic solution can contain additives in addition to the above components. The additive that can be added to the electrolyte is preferably a substance that can be electrochemically decomposed to form a film on the electrode and the like in the process of charging and discharging the battery. In particular, it is particularly desirable to use an additive capable of stabilizing the negative electrode structure on the negative electrode surface. Such additives include cyclic disulfonates (eg, methylenemethane disulfonate, ethylenemethane disulfonate, propylenemethane disulfonate), cyclic sulfonates (eg, sultone), chain sulfonates (eg, , An additive containing a compound containing sulfur in the molecule, such as methylene bisbenzene sulfonate, methylene bisphenyl methane sulfonate, methylene bis ethane sulfonate, etc. (hereinafter referred to as “sulfur-containing additive”). ). In addition, vinylene carbonate, vinyl ethylene carbonate, methacrylic acid propylene carbonate, acrylic acid propylene carbonate, and the like can also be added as an additive capable of forming a protective film for the positive electrode and the negative electrode in the charge / discharge process of the battery. Further, other additives for forming a protective film for the positive electrode and the negative electrode in the charge / discharge process of the battery include fluoroethylene carbonate, difluoroethylene carbonate, trifluoroethylene carbonate, chloroethylene carbonate, dichloroethylene carbonate, trichloroethylene carbonate, and the like. . These additives are additives that can prevent the sulfur-containing additive from attacking the positive electrode active material containing the lithium / nickel composite oxide. The additive is contained in a proportion of 20% by weight or less, preferably 15% by weight or less, and more preferably 10% by weight or less, based on the weight of the whole electrolyte solution.

  In the embodiment, the separator is composed of an olefin resin layer. Here, the olefin resin layer is a layer composed of a polyolefin obtained by polymerizing or copolymerizing an α-olefin such as ethylene, propylene, butene, pentene, hexene or the like. In the embodiment, a structure having pores that are blocked when the battery temperature rises, that is, a layer composed of a porous or microporous polyolefin is preferable. Since the olefin resin layer has such a structure, even if the battery temperature rises, the separator is closed (shuts down), and the ion flow can be cut off. In order to exert a shutdown effect, it is very preferable to use a porous polyethylene film.

  It is preferable to use a separator obtained by crosslinking an olefin resin. Polymers that have been chemically cross-linked with polyfunctional substances (trimethylolpropane triacrylate, trimethylolpropane trimethacrylate, pentaerythritol triacrylate, pentaerythritol trimethacrylate, etc.) having multiple polymer groups in one molecule, ionizing radiation An electron-crosslinked polymer polymerized by irradiation to generate radicals can be used.

In each embodiment, it is particularly preferable that the thermal contraction rate of the separator is 13% or less. Here, the thermal contraction rate of the film material including the separator or the like is a numerical value indicating how much the dimension changes (decreases) when the film material is heated. For example, JIS-C-2151, JIS- It can be measured according to C-2318, ASTM DDD-1204, and the like. In this specification, the value of thermal shrinkage is measured after a film material test piece is suspended in a hot-air circulating thermostat, heated from 25 ° C. to 180 ° C. over 30 minutes, and then cooled to room temperature. It represents the shrinkage rate of the area of the test piece. That is, the heat shrinkage rate is expressed by the following formula:
100 × [(area of film material before test) − (area of film material after test)] / (area of film material before test)
Can be calculated with In a separator having a thermal shrinkage rate exceeding 13%, heat generation caused by a short-circuit may increase the film breaking area of the separator, and a large current may flow.

  The separator may have an olefin resin layer and a heat-resistant fine particle layer. The separator having the heat-resistant fine particle layer can prevent heat generation of the battery and can also suppress the thermal contraction of the separator. As the heat-resistant fine particles, inorganic fine particles having a heat resistance of 150 ° C. or more and stable in electrochemical reaction can be used. Examples of such inorganic fine particles include inorganic oxides such as silica, alumina (α-alumina, β-alumina, θ-alumina), iron oxide, titanium oxide, barium titanate, zirconium oxide; boehmite, zeolite, apatite, kaolin, Mention may be made of minerals such as spinel, mica and mullite. Thus, a separator having an olefin-based resin layer and a heat-resistant resin layer may be referred to as a “ceramic separator” in this specification.

  A ceramic separator having an olefin resin layer and a heat resistant fine particle layer has a form in which a heat resistant fine particle layer is laminated on the surface of an olefin resin film. The heat-resistant fine particle layer can be provided only on one side of the olefin-based resin film, or can be provided on both sides. The ratio of the thickness of the entire heat-resistant fine particle layer is preferably about 1/10 to 1/2, preferably about 1/8 to 1/3 of the thickness of the olefin resin layer. If the thickness of the heat-resistant fine particle layer is too thick, the decomposition product of the sulfur-containing additive contained in the electrolytic solution may increase, and if it is too thin, the effect of improving the heat resistance of the separator cannot be expected.

  Here, the structural example of the lithium ion secondary battery concerning this embodiment is demonstrated using drawing. FIG. 1 shows an example of a cross-sectional view of a lithium ion secondary battery. The lithium ion secondary battery 10 includes a negative electrode current collector 11, a negative electrode active material layer 13, a separator 17, a positive electrode current collector 12, and a positive electrode active material layer 15 as main components. In FIG. 1, the negative electrode active material layer 13 is provided on both surfaces of the negative electrode current collector 11 and the positive electrode active material layer 15 is provided on both surfaces of the positive electrode current collector 12, but only on one side of each current collector. An active material layer can also be formed. The negative electrode current collector 11, the positive electrode current collector 12, the negative electrode active material layer 13, the positive electrode active material layer 15, and the separator 17 are constituent units of one battery, that is, a power generation element (unit cell 19 in the figure). The separator 17 may be composed of a heat-resistant fine particle layer and an olefin resin film (both not shown). A plurality of such unit cells 19 are stacked via the separator 17. The extending portion extending from each negative electrode current collector 11 is collectively bonded onto the negative electrode lead 25, and the extending portion extending from each positive electrode current collector 12 is collectively bonded to the positive electrode lead 27. An aluminum plate is preferably used as the positive electrode lead, and a copper plate is preferably used as the negative electrode lead, and in some cases, it may have a partial coating with another metal (for example, nickel, tin, solder) or a polymer material. The positive electrode lead and the negative electrode lead are welded to the positive electrode and the negative electrode, respectively. A battery formed by laminating a plurality of single cells in this manner is packaged by an outer package 29 so that the welded negative electrode lead 25 and positive electrode lead 27 are drawn out to the outside. An electrolytic solution 31 is injected into the exterior body 29. The exterior body 29 has a shape in which two laminated bodies are overlapped and the peripheral portion is heat-sealed. In FIG. 1, the negative electrode lead 25 and the positive electrode lead 27 are respectively provided on opposite sides of the exterior body 29 (referred to as “both tabs”), but the negative electrode lead 25 and the positive electrode lead 27 are connected to the exterior body 29. (Ie, the negative electrode lead 25 and the positive electrode lead 27 are pulled out from one side of the outer package 29. This is referred to as “one-tab type”).

  It is preferable that the value of the ratio between the output and the capacity (hereinafter referred to as “output capacity ratio”, unit: W / Wh) of the lithium ion secondary battery manufactured as described above is 25 or more and less than 300. Here, the output of the battery represents an average output for 10 seconds. When the output capacity ratio is 25 or more, the electrode generally has a low electronic resistance, and thus a high output can be obtained. When the output capacity ratio is 25 or less, the electrode generally has a high electronic resistance, so that it may not be suitable for use as a high-power battery.

The exothermic index of the lithium ion secondary battery of all the embodiments may be 80 or more. In the present specification, the exothermic index is the product of the DC resistance value and the total electrode area when a battery having a state of charge of 50%, that is, a remaining capacity of the battery (hereinafter referred to as “SOC”) of 50% is discharged for 1 second. The reciprocal of is divided by the current collector heat capacity . The exothermic index is an index representing the tolerance for heat of the electrode (particularly the positive electrode) when the battery is discharged in a short time, and the higher the index, the higher the heat resistance of the battery. In order to increase the exothermic index of the battery to 80 or more, it is only necessary to increase the amount of the conductive auxiliary agent to reduce the resistance of the battery and to reduce the heat capacity of the current collector foil. The exothermic index is practically about 150 or less from the balance between the specific resistance of the battery and the positive electrode heat capacity.

  In another embodiment, the separator preferably has a tensile elongation of 2% or more. In this specification, the tensile elongation particularly refers to the piercing elongation. The piercing elongation is a puncture strength measurement measured according to JIS Z 1707 (1997). The piercing elongation is measured by measuring the moving distance of the pin tip from when the pin tip contacts the film until the piercing break occurs. X (length of the sample at the time of puncture breakage−length of the sample before the test) / value obtained by the length of the sample before the test. Preferably, the tensile elongation of the separator is 2% or more and 200% or less. If the tensile elongation is 2% or less, the separator may not be able to withstand the heat generation of the battery, and if it is 200% or more, the separator may be excessively extended during production, which may be difficult to handle.

<Production of negative electrode>
As a negative electrode active material, artificial graphite (specific capacity: 320 mA / g) and surface-coated natural graphite powder (specific capacity: 390 mAh / g) are mixed so as to be 8: 2 (weight ratio), and the carbon-based active material and conductive assistant are mixed. Carbon black powder (manufactured by SC65, TIMCAL) as an agent and polyvinylidene fluoride resin (PVDF, Kureha W # 7200, Kureha Battery Materials Japan Co., Ltd.) as a binder resin in a solid content mass ratio of 92: 2. Was added to ion-exchanged water at a ratio of 6 and stirred, and these materials were uniformly dispersed in water to prepare a slurry. The obtained slurry was applied onto a copper foil having a thickness of 8 μm serving as a negative electrode current collector. Next, the electrode was heated at 125 ° C. for 10 minutes to evaporate water, thereby forming a negative electrode active material layer. Furthermore, the negative electrode active material layer was pressed so as to have a porosity of 35%, and a negative electrode in which the negative electrode active material layer was applied on one surface of the negative electrode current collector was produced.

<Preparation of positive electrode>
Nickel / cobalt / lithium manganate (D50 particle size: 6 μm NCM433, that is, nickel: cobalt: manganese = 4: 3: 3) as a positive electrode active material, carbon black powder (SC65, manufactured by TIMCAL) as a conductive assistant, and binder PVDF (Kureha W # 7200, Kureha Battery Materials Japan Co., Ltd.) is used as the resin at a solid mass ratio of 89: 7: 4, and the solvent is N-methyl-pyrrolidone (hereinafter “NMP”). "). Furthermore, by adding 0.03 parts by mass of oxalic anhydride (molecular weight 90) as an organic moisture scavenger to the mixture with respect to 100 parts by mass of the solid content obtained by removing NMP from the above mixture, these were stirred. A material was uniformly dispersed to prepare a slurry. The obtained slurry was apply | coated on the aluminum foil (refer Table 1) used as a positive electrode electrical power collector. Next, the electrode was heated at 125 ° C. for 10 minutes to evaporate NMP, thereby forming a positive electrode active material layer. Furthermore, the positive electrode active material layer was pressed so as to have a porosity of 30%, and a positive electrode in which the positive electrode active material layer was applied on one surface of the positive electrode current collector was produced.
Next, a mixed positive electrode was produced. A composite oxide in which a part of lithium nickelate having a BET specific surface area of 0.7 m ² / g is substituted with cobalt and aluminum (D50 particle diameter: 5 μm, nickel: cobalt: aluminum = 80: 15: 5, hereinafter “NCA ) And LiMnO ₂ (D50 particle size: 10 μm, BET specific surface area 0.4 m ² / g, hereinafter referred to as “LMO1”) are mixed at a ratio described in the table, and used as a conductive assistant. Carbon black powder (SC65, manufactured by TIMCAL) and PVDF (Kureha W # 7200, Kureha Battery Materials Japan Co., Ltd.) as a binder resin are mixed in a solid mass ratio. Positive electrode active material: Conductive aid: Binder = 89: 7: 4 The ratio was added to NMP as a solvent. Furthermore, by adding 0.03 parts by mass of oxalic anhydride (molecular weight 90) as an organic moisture scavenger to the mixture with respect to 100 parts by mass of the solid content obtained by removing NMP from the above mixture, these were stirred. A material was uniformly dispersed to prepare a slurry. The obtained slurry was applied onto an aluminum foil having a thickness of 15 μm serving as a positive electrode current collector. Next, the electrode was heated at 125 ° C. for 10 minutes to evaporate NMP, thereby forming a positive electrode active material layer. Furthermore, the positive electrode active material layer was pressed so as to have a porosity of 30%, and a positive electrode in which a positive electrode active material layer having a weight per unit area (see Table 1) was applied on one surface of the positive electrode current collector was produced.
A mixed positive electrode different from the above was prepared. A composite oxide in which a part of lithium nickelate having a BET specific surface area of 0.7 m ² / g is substituted with cobalt and aluminum (nickel: cobalt: aluminum = 80: 15: 5, referred to as “NCA”), and LiMnO ₂ (D50 particle size: 6 μm, BET specific surface area 0.7 m ² / g, hereinafter referred to as “LMO2”) in a ratio described in the table, and carbon black powder (SC65, manufactured by TIMCAL, Ltd.) as a conductive aid. ) And PVDF (Kureha W # 7200, Kureha Battery Materials Japan Co., Ltd.) as a binder resin in a solid mass ratio of positive electrode active material: conductive auxiliary agent: binder = 89: 7: 4 Then, it was added to NMP as a solvent. Furthermore, by adding 0.03 parts by mass of oxalic anhydride (molecular weight 90) as an organic moisture scavenger to the mixture with respect to 100 parts by mass of the solid content obtained by removing NMP from the above mixture, these were stirred. A material was uniformly dispersed to prepare a slurry. The obtained slurry was applied onto an aluminum foil having a thickness of 15 μm serving as a positive electrode current collector. Next, the electrode was heated at 125 ° C. for 10 minutes to evaporate NMP, thereby forming a positive electrode active material layer. Furthermore, the positive electrode active material layer was pressed so as to have a porosity of 30%, and a positive electrode in which a positive electrode active material layer having a weight per unit area (see Table 1) was applied on one surface of the positive electrode current collector was produced.

<Separator>
The following five types of separators were prepared:
PP01: Polypropylene, thickness 25 μm, heat shrinkage 40%, piercing elongation 50%
CL01: Polypropylene (with electronic crosslinking), thickness 25 μm, heat shrinkage 12%, piercing elongation 2%
CL02: Polypropylene (with electronic crosslinking), thickness 25 μm, heat shrinkage 13%, piercing elongation 1%
CL03: Polypropylene (with electronic crosslinking), thickness 25 μm, heat shrinkage 17%, puncture elongation 5%
CC01: polypropylene / polyethylene / polypropylene laminate (with aluminum oxide coating), thickness 20 μm, heat shrinkage 13%, puncture elongation 30%
In addition, the measuring method of the thermal contraction rate and piercing elongation of a separator is mentioned later.

<Electrolyte>
Ethylene carbonate (hereinafter referred to as “EC”), propylene carbonate (hereinafter referred to as “PC”), and diethyl carbonate (hereinafter referred to as “DEC”) are EC: PC: DEC = 25: 5. : In a solution obtained by dissolving lithium hexafluorophosphate (LiPF ₆ ) as an electrolyte salt in a nonaqueous solvent mixed at a ratio of 70 (volume ratio) so that the concentration becomes 0.9 mol / L, As an additive, vinylene carbonate (VC) dissolved in a concentration of 1% by weight was used.

<Production of lithium ion secondary battery>
Each of the negative electrode plate and the positive electrode plate produced as described above was cut into a rectangle having a predetermined size (area: 253 mm ² ). Among these, an aluminum positive electrode lead terminal was ultrasonically welded to an uncoated portion for connecting the terminal. Similarly, a nickel negative electrode lead terminal having the same size as the positive electrode lead terminal was ultrasonically welded to an uncoated portion of the negative electrode plate. The negative electrode plate and the positive electrode plate were placed on both sides of the separator such that both active material layers overlapped with the separator therebetween. At this time, 13 positive electrode plates and 14 negative electrode plates are laminated so that the outermost layer is the negative electrode plate, and these are arranged so that the positive electrode lead and the negative electrode lead are both aligned on one side. Obtained. A bag-like laminate outer package was prepared by bonding one side of the two aluminum laminate films except for one of the long sides by heat fusion. The electrode laminate was inserted into the laminate outer package. After injecting the electrolyte solution and impregnating it in a vacuum, the opening was sealed by heat sealing under reduced pressure to obtain a so-called single-tab type laminated lithium ion battery. The multilayer lithium ion battery was initially charged and discharged, and then subjected to high temperature aging to obtain a multilayer lithium ion battery having a battery capacity of 5 Ah.

<Initial charge / discharge>
Initial charge / discharge was performed from 0% to 100% of the remaining capacity of the battery (hereinafter referred to as “SOC”) at an atmospheric temperature of 55 ° C. The charge / discharge conditions are as follows: constant current charge (CC charge) to 4.1V at 0.1C current, then constant voltage charge (CV charge) at 4.1V, then at 0.1C current Constant current discharge (CC discharge) is performed up to 2.5V.

<Measurement of SOC-OCV (remaining capacity-open voltage)>
CC charging was performed at a current of 0.2 CC until the SOC reached 50% from the state where the battery voltage was 3V. The battery voltage after standing in this state for 1 hour was defined as 50% SOC-OCV (unit: V), that is, the value of the open circuit voltage of the battery having a remaining capacity of 50%.

<Battery capacity>
The product of the above 50% SOC-OCV value (unit: V) and the battery capacity value (unit: Ah) by charging at 0.2 c current was defined as the battery capacity (unit: Wh).

<Measurement of output>
The maximum current value required to reach the lower limit voltage (3 V) in 10 seconds at 25 ° C. from the above 50% SOC-OCV state was measured. At this time, the product of the value of 50% SOC-OCV and the maximum current value (unit: A) was defined as the battery output (unit: W).

<Measurement of DC resistance>
The battery was CC-CV charged with a 0.2 C current so that the SOC was 50%. This battery was discharged at 1 C current, 2 C current, and 3 C current for 1 second, and left as it was for 10 minutes. The voltage after standing was measured, and the DC resistance value (unit: mΩ) at each discharge rate was calculated from the voltage drop amount and the applied current according to Ohm's law. These average values were used as 1-second DC resistance values (hereinafter referred to as “DCR”) of the respective batteries.

<Resistivity>
The product of the DCR value and the total electrode area was defined as the specific resistance (unit: Ωcm ² ).

<The heat capacity of the current collector foil>
The heat capacity of the current collector foil (electrode current collector) used per layer of the electrode laminate (unit: m coulomb).

<Fever index>
A value obtained by dividing the reciprocal of the specific resistance by the current collector foil heat capacity (electrode current collector heat capacity) was defined as an exothermic index.

<Heat shrinkage>
The separator was suspended in a hot-air circulating thermostat, heated from 25 ° C. to 180 ° C. over 30 minutes, and then cooled to room temperature, and then the area of the test piece measured was measured. The thermal shrinkage was calculated by 100 × [(area of separator before test) − (area of separator after test)] / (area of separator before test).

<Puncture strength>
A sample separator is fixed, a semicircular needle with a diameter of 1.0 mm and a tip shape radius of 0.5 mm is inserted into the sample surface at a speed of 50 ± 0.5 mm per minute, and the maximum load until the needle penetrates is measured. did.

<Puncture elongation>
In the measurement of the piercing strength measured according to JIS Z 1707 (1997), the movement distance of the pin tip from when the pin tip comes into contact with the separator sample until the piercing breakage occurs is measured by 100 × (at the time of piercing breakage The length obtained by the length of the sample-the length of the sample before the test) / the length of the sample before the test was defined as the piercing elongation.

<Tensile elongation>
Conforms to JIS-C-2151 and ASTM-D-882. Using a tensile tester, the tensile strength was obtained at a speed of 200 mm / min, the strength when the sample was cut (ruptured) (the value obtained by dividing the tensile load value by the cross-sectional area of the test piece), and the elongation. The tensile elongation was calculated by the following formula.

[Equation 1]
Tensile elongation (%) = 100 × (L−Lo) / Lo
(Lo: sample length before test L: sample length at break)

<Nail penetration test>
The battery was CC-CV charged at a 0.2 C current until it reached 100% SOC (4.2 V). A nail with a tip angle of 20 degrees of SUS303 (φ3 mm) was pierced into this charged battery at a speed of 10 mm / second. It was evaluated as “ignition” when ignition occurred, “smoke” when smoke occurred, and “no change” when neither smoke nor ignition occurred.

  A battery having a large heat generation index means that the heat generation exceeds the heat dissipation capability of the current collector, and indicates that the battery is likely to generate heat. In the development of a high-power battery, since the specific resistance of the battery is reduced and the discharge current is increased, the amount of heat generated by the current increases and the heat generation index tends to increase. In the battery having an exothermic index of 80 or more, smoke generation (for example, Comparative Examples 1 to 5) was observed in the nail penetration test when the thermal contraction rate of the separator exceeded 13%. However, even with a battery having a heat generation index of 80 or more, it was found that smoke was not observed in the nail penetration test when a separator having a heat shrinkage rate of 13% or less was used (Examples 1 to 6).

  As mentioned above, although the Example of this invention was described, the said Example was only an example of Embodiment of this invention, and in the meaning which limits the technical scope of this invention to specific embodiment or a specific structure. Absent.

DESCRIPTION OF SYMBOLS 10 Lithium ion secondary battery 11 Negative electrode collector 12 Positive electrode collector 13 Negative electrode active material layer 15 Positive electrode active material layer 17 Separator 25 Negative electrode lead 27 Positive electrode lead 29 Exterior body 31 Electrolyte

Claims (3)

A positive electrode in which a positive electrode active material layer including a lithium nickel cobalt aluminum composite oxide as a positive electrode active material is disposed on a positive electrode current collector;
A negative electrode having a negative electrode active material layer disposed on a negative electrode current collector;
A separator selected from a separator comprising an electron-crosslinked polymer or a separator having an olefin-based resin layer and a heat-resistant resin layer ;
An electrolyte,
A lithium ion secondary battery including a power generation element including:
The value of the battery output to capacity ratio (W / Wh) is 25 or more,
The exothermic index obtained by dividing the reciprocal of the product of the direct current resistance value and the total electrode area when the battery in a charged state of 50% is discharged for 1 second by the current collector heat capacity is 80 or more,
The thermal shrinkage rate of the separator was 13% or less ( the thermal shrinkage rate was determined by performing a thermal shrinkage test in which the temperature of the separator was raised from 25 ° C. to 180 ° C. over 30 minutes and then cooled to room temperature, Measure and formula:

[Equation 1]
Heat shrinkage rate (%) = 100 × [(area of separator before heat shrink test) − (area of separator after heat shrink test)] / (area of separator before heat shrink test)

Calculated by ) At which the lithium ion secondary battery. The piercing elongation of the separator is 2% or more (the piercing elongation is measured in the piercing test according to JIS Z 1707 (1997) until the piercing breakage occurs after the pin tip contacts the separator. Measure the distance traveled by the pin tip and use the following formula:

[Equation 2]
100 × [(Separator length at the time of piercing break) − (Separator length before test)] / (Separator length before test)

Calculated by ) Is a lithium ion secondary battery according to claim 1.   The lithium ion secondary battery according to claim 1 or 2, wherein the positive electrode current collector is an aluminum foil.

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