JP2008234879A - Lithium ion secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a lithium ion secondary battery excelling in short circuit resistance while having a thin separator. <P>SOLUTION: The above problem is solved by this lithium ion secondary battery including: a positive electrode composed by forming a positive electrode mix layer containing an active material on one surface or each of both surfaces of a collector; a negative electrode composed by forming a negative electrode mix layer containing an active material on one surface or each of both surfaces of a collector; a separator; and a nonaqueous electrolyte; and characterized in that, in at least either of the positive electrode mix layer surface of the positive electrode and the negative electrode mix layer surface of the negative electrode, insulating particles having an allowable temperature limit above 150°C and an organic polymer resin binder are included, and a porous layer having surface roughness Ra below 1.0 μm is also included; and the thickness of the separator is not larger than 5% with respect to the total of the total thickness of the positive electrode and that of the negative electrode including the porous layer. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a lithium ion secondary battery excellent in short circuit resistance.

  A lithium ion secondary battery, which is a type of nonaqueous electrolyte battery, is widely used as a power source for portable devices such as mobile phones and notebook personal computers because of its high energy density. As the performance of portable devices increases, the capacity of lithium ion secondary batteries tends to increase further, and ensuring safety is important.

  In current lithium ion secondary batteries, as a separator interposed between a positive electrode and a negative electrode, for example, a polyolefin-based microporous film having a thickness of about 20 to 30 μm is used. In addition, as separator material, the constituent resin of the separator is melted below the thermal runaway temperature of the battery to close the pores, thereby increasing the internal resistance of the battery and improving the safety of the battery in the event of a short circuit. In order to ensure a so-called shutdown function, polyethylene having a low melting point may be applied.

  By the way, as such a separator, for example, a uniaxially stretched film or a biaxially stretched film is used for increasing the porosity and improving the strength. Since such a separator is supplied as a single film, a certain strength is required in terms of workability and the like, and this is ensured by the above stretching. Therefore, the separator is distorted, and when it is exposed to a high temperature, there is a problem that shrinkage occurs due to residual stress. The shrinkage temperature is very close to the melting point, ie the shutdown temperature. For this reason, when a polyolefin-based porous film separator is used, when the battery temperature reaches the shutdown temperature in the case of abnormal charging, the current must be immediately decreased to prevent the battery temperature from rising. This is because if the pores are not sufficiently closed and the current cannot be reduced immediately, the battery temperature easily rises to the contraction temperature of the separator, and there is a risk of ignition due to an internal short circuit.

  From the viewpoint of increasing the capacity of a lithium ion secondary battery, for example, it is conceivable that the separator that does not participate in power generation is made thin so that the area in which the electrode can be accommodated in the battery is made larger. As the strength decreases, the workability during battery production is sufficiently secured, and further, in order to achieve sufficient separation between the positive electrode and the negative electrode in the battery, higher stretching is performed. It is necessary to increase the strength. A thin film separator that has been highly stretched in this manner has a high degree of shrinkage when exposed to high temperatures. Therefore, in a battery using such a separator, a short circuit due to the shrinkage of the separator occurs when the internal temperature becomes abnormally high. The probability of occurrence is greater.

  For example, Patent Document 1 discloses a battery in which a porous protective film made of a resin binder and solid particles is formed on the surface of either a positive electrode active material coating layer of a positive electrode or a negative electrode active material coating layer of a negative electrode. Has been. The porous protective layer according to the battery disclosed in Patent Document 1 is intended to prevent the active material from falling off the electrode during battery production and prevent the occurrence of an internal short circuit. Depending on the selection of the configuration, there is a possibility that the short circuit resistance of the battery can be improved.

Japanese Patent Laid-Open No. 7-220759

  This invention is made | formed in view of the said situation, The objective is to provide the lithium ion secondary battery excellent in short circuit resistance, providing a thin separator.

  The lithium ion secondary battery of the present invention that has achieved the above object is a positive electrode in which a positive electrode mixture layer containing an active material is formed on one or both sides of a current collector, and an active material on one or both sides of a current collector A lithium ion secondary battery having a negative electrode formed by forming a negative electrode mixture layer, a separator, and a nonaqueous electrolyte solution, wherein at least one of the positive electrode mixture layer surface of the positive electrode and the negative electrode mixture layer surface of the negative electrode And having a porous layer having an insulating particle having a heat-resistant temperature of 150 ° C. or higher and an organic polymer resin binder, and having a surface roughness Ra of 1.0 μm or less, including the porous layer. The thickness of the separator is 5% or less with respect to the total of the total thickness of the positive electrode and the total thickness of the negative electrode.

  In the lithium ion secondary battery of the present invention, the porous layer having a surface roughness Ra of 1.0 μm or less is formed on at least one surface of the positive electrode mixture layer of the positive electrode and the negative electrode mixture layer of the negative electrode. Even if the inside becomes high temperature and the separator shrinks, the porous layer can prevent contact between the positive electrode mixture layer of the positive electrode and the negative electrode mixture layer of the negative electrode, so that occurrence of a short circuit can be suppressed. In the present invention, by such an action, even if a thin separator that is more easily contracted is used, good short-circuit resistance can be secured, and the capacity of the battery can be increased.

  ADVANTAGE OF THE INVENTION According to this invention, the lithium ion secondary battery excellent in short circuit resistance can be provided, providing a thin separator.

  The lithium ion secondary battery of the present invention has insulating particles (hereinafter simply referred to as “insulating particles”) having a heat-resistant temperature of 150 ° C. or more on at least one surface of the positive electrode mixture layer of the positive electrode and the negative electrode mixture layer of the negative electrode. In some cases) and an organic polymer resin binder, and has a porous layer having a surface roughness Ra of 1.0 μm or less.

  In FIG. 1, the cross-sectional schematic diagram of an example of the electrode 1 (positive electrode or negative electrode) which has a porous layer is shown. In the electrode 1 shown in FIG. 1, the positive electrode mixture layer or the negative electrode mixture layer 3 is formed on both surfaces of the positive electrode current collector or the negative electrode current collector 4, and the mixture layer 3 on both sides of the current collector 4 is further formed. The porous layer 2 is formed on the surface.

  Due to the presence of the porous layer described above, even if the inside of the battery becomes hot and the separator shrinks or the battery is shocked and the separator is displaced, the positive electrode mixture layer of the positive electrode and the negative electrode of the negative electrode are mixed. Since the contact with the agent layer can be prevented, the short circuit resistance of the battery can be improved.

  In order to increase the short circuit resistance of the battery, it is preferable to increase the thickness of the porous layer. However, if the porous layer is too thick, the volume of the portion that does not participate in power generation in the battery increases. It becomes difficult. On the other hand, when the porous layer is simply thinned, it is difficult to sufficiently prevent the contact between the positive electrode mixture layer and the negative electrode mixture layer when the separator shrinks or the separator is displaced.

  As a result of intensive studies, the inventors of the present invention can successfully prevent contact between the positive electrode mixture layer and the negative electrode mixture layer even if the thickness of the porous layer is reduced by reducing the pores of the porous layer. I found. However, if the pores of the porous layer are made small, the non-aqueous electrolyte becomes difficult to contact the positive electrode mixture layer or the negative electrode mixture layer, the charge / discharge reaction becomes unstable, and the battery characteristics are impaired.

  Therefore, as a result of further studies, if the porous layer is formed so that the surface roughness Ra is 0.1 μm or less, the pores can be made small and uniform, thereby stabilizing the charge / discharge reaction. Thus, the inventors have found that excellent short circuit resistance can be secured while maintaining good battery characteristics, and have completed the present invention.

  That is, if the surface roughness of the porous layer is too large, a short circuit is likely to occur when an impact is applied to the battery or the battery is abnormally overheated. The reason for this is not clear, but when the surface roughness of the porous layer is large, the uniformity of the porous layer becomes insufficient, and there are spots where the insulation performance is insufficient. As a result, under conditions where the insulation performance of the separator deteriorates, such as when the battery is impacted or exposed to a high temperature, the electrode passes through a non-uniform portion of the porous layer (a defective portion with insufficient insulation performance). It is presumed that a short circuit is likely to occur.

  The surface roughness Ra of the porous layer referred to in this specification is an arithmetic average roughness specified in JIS B 0601. Specifically, a confocal laser microscope (“Real-time scanning laser microscope 1LM manufactured by Lasertec Corporation”) is used. −21D ”), a 1 mm × 1 mm field of view is measured with 512 × 512 pixels, and the absolute value from the average line at each point is arithmetically averaged. The surface roughness Ra of the porous layer is more preferably 0.6 μm or less.

  Further, by providing a certain degree of irregularities on the surface of the porous layer, it is possible to improve the circulation of the non-aqueous electrolyte on the surface of the porous layer and to promote the charge / discharge reaction more favorably. Therefore, the surface roughness Ra of the porous layer is preferably 0.1 μm or more.

  As the insulating particles constituting the porous layer, the heat-resistant temperature is 150 ° C. or higher, and it has electrical insulating properties, is electrochemically stable, and is used for forming a non-aqueous electrolyte or a porous layer. Any particles that are stable to the solvent used in the composition containing the insulating particles used in the above and that do not cause side reactions such as oxidation-reduction in the operating voltage range of the battery may be used. In this specification, “heat-resistant temperature is 150 ° C.” means that deformation such as softening is not observed at least at 150 ° C. Further, “electrochemically stable” as used in the present specification means that no chemical change occurs during charging / discharging of the battery.

Specific examples of such insulating particles include the following inorganic particles or organic particles, and these may be used alone or in combination of two or more. Examples of the inorganic particles (inorganic powder) include oxide particles such as iron oxide, silica (SiO ₂ ), alumina (Al ₂ O ₃ ), TiO ₂ , BaTiO ₂ , and ZrO; nitrides such as aluminum nitride and silicon nitride Particles: Slightly soluble ionic crystal particles such as calcium fluoride, barium fluoride and barium sulfate; Covalent crystal particles such as silicon and diamond; Clay particles such as montmorillonite, boehmite, zeolite, apatite, kaolin, mullite, spinel, And materials derived from mineral resources such as olivine or artificial products thereof. Further, the surface of conductive particles such as metal particles; oxide particles such as SnO ₂ and tin-indium oxide (ITO); carbonaceous particles such as carbon black and graphite; Fine particles imparted with electrical insulation properties by surface treatment with a material constituting the above non-electrically conductive inorganic particles or a material constituting the crosslinked polymer fine particles described later) may be used. Organic particles (organic powder) include acrylate resins such as crosslinked polymethyl methacrylate, crosslinked polystyrene, crosslinked polydivinylbenzene, crosslinked styrene-divinylbenzene copolymer, polyimide, melamine resin, phenol resin, benzoguanamine-formaldehyde condensation Examples thereof include various crosslinked polymer particles. The organic resin (polymer) constituting these organic particles is a mixture, modified product, derivative, or copolymer (random copolymer, alternating copolymer, block copolymer, graft copolymer) of the materials exemplified above. A polymer, a crosslinked body (in the case of thermoplastic polyimide) may be used, and among these insulating particles, alumina particles are particularly preferable.

  The average particle diameter of the insulating particles is 0.8 μm or less from the viewpoint of adjusting the surface roughness of the porous layer to the above predetermined value or less and facilitating the formation of a porous layer having a small and uniform structure. Preferably, it is 0.6 μm or less. In addition, if the particle size of the insulating particles is too small, the amount of the organic polymer resin binder used in the porous layer may increase and the heat resistance of the porous layer may decrease. The average particle size is preferably 0.2 μm or more, and more preferably 0.3 μm or more. In addition, the average particle diameter of the insulating particles referred to in this specification is the number measured by dispersing the fine particles in a medium that does not swell (for example, water) using a laser scattering particle size distribution diameter (“LA-920” manufactured by HORIBA). Average particle size.

  In addition, from the viewpoint of easily forming a porous layer having a small and uniform structure by adjusting the surface roughness of the porous layer to the predetermined value or less, the shape of the insulating particles is spherical. Is preferred. For example, when alumina particles are used as the insulating particles, alumina particles having an average particle size larger than that suitable for forming the porous layer are converted into planetary ball mills, ball mills, jet mills, biaxial dispersion mixers. It is preferable to use alumina particles that have been pulverized by, for example, the above-mentioned preferable average particle diameter, but this pulverization step makes the shape of the alumina particles spherical by smoothing the corners. be able to.

  The organic polymer resin binder constituting the porous layer may be any material that is electrochemically stable and stable with respect to the non-aqueous electrolyte, and can adhere the insulating particles well. For example, EVA (With a structural unit derived from vinyl acetate of 20 to 35 mol%), acrylic resin (ethylene-acrylate copolymer such as ethylene-ethyl acrylate copolymer), polyvinylidene fluoride (PVDF), fluorine-based rubber, styrene -Butadiene rubber (SBR), carboxymethyl cellulose (CMC), hydroxyethyl cellulose (HEC), polyvinyl alcohol (PVA), polyvinyl butyral (PVB), polyvinyl pyrrolidone (PVP), polyurethane, epoxy resin, and the like. You may use, and may use 2 or more types together. When these organic polymer resin binders are used, they can be used in the form of an emulsion dissolved or dispersed in a solvent of a composition for forming a porous layer described later.

  Since the organic polymer resin binder is inferior in heat resistance compared to the insulating particles, the amount of the organic polymer resin binder in the porous layer is reduced as much as possible, and the heat resistance of the entire porous layer is reduced. Is preferably increased. Therefore, the content of the organic polymer resin binder in the porous layer is preferably 20% by mass or less, and more preferably 10% by mass or less. Further, from the viewpoint of improving the binding between the insulating particles in the porous layer, the content of the organic polymer resin binder in the porous layer is preferably 2% by mass or more, It is more preferable that it is 4 mass% or more.

  In the porous layer according to the present invention, it is preferable to use insulating particles having a relatively small particle size from the viewpoint of adjusting the surface roughness, but such insulating particles have a large specific surface area. It is difficult to reduce the amount of the organic polymer resin binder that binds these insulating particles. However, for example, when an organic polymer resin binder having a functional group (for example, a polar functional group) effective for improving adhesiveness and having a weight average molecular weight of 500,000 or more is used, the tackiness is reduced. High, improving the ability to adhere insulating particles that are fine particles, ensuring good binding even if the amount of organic polymer resin binder in the porous layer is reduced and the ratio of insulating particles is increased it can. Therefore, the amount of the organic polymer resin binder in the porous layer can be further reduced, and a porous layer having better heat resistance can be formed.

  In addition, the weight average molecular weight of said organic polymer resin binder is a value (polystyrene conversion value) measured using gel permeation chromatography (GPC).

  Further, the content of the insulating particles in the porous layer is preferably 85% by mass or more and more preferably 90% by mass or more from the viewpoint of more effectively exerting the action of the insulating particles. . In addition, if the amount of insulating particles in the porous layer is too large, for example, the amount of the organic polymer resin binder is decreased, and the binding between the insulating particles may be insufficient. The content of the insulating particles therein is preferably 99.5% by mass or less, and more preferably 98% by mass or less.

  The thickness of the porous layer is preferably 5 μm or less, and more preferably 4 μm or less. By setting the thickness of the porous layer in this way, it is possible to prevent a decrease in battery capacity due to the formation of the porous layer. If the porous layer is too thin, the contact preventing action between the positive electrode mixture layer and the negative electrode mixture layer by the porous layer may be impaired. Therefore, the thickness of the porous layer is preferably 1 μm or more. More preferably, it is 2 μm or more.

  In the battery of the present invention, the electrode having the porous layer may be a positive electrode or a negative electrode. Moreover, both the positive electrode and the negative electrode may have a porous layer. Furthermore, a positive electrode having a positive electrode mixture layer on both sides of the current collector and a porous layer on the surface of the positive electrode mixture layer on one side, and a negative electrode layer on both sides of the current collector and having a negative electrode layer on one side A negative electrode having a porous layer on the surface of the mixture layer may be used, and the positive electrode and the negative electrode may be overlapped and used such that the porous layer is always interposed between the positive electrode and the negative electrode.

  The positive electrode according to the battery of the present invention is a positive electrode used in a conventionally known lithium ion secondary battery, that is, a positive electrode mixture containing a positive electrode active material capable of occluding and releasing lithium ions on one side or both sides of a current collector. A positive electrode having a layer can be used, and in the case of a positive electrode having a porous layer, a material in which a porous layer is further formed on the surface of the positive electrode mixture layer can be used.

As the positive electrode active material, lithium-containing transition metal oxide represented by Li _{1 + x} MO ₂ (−0.1 <x <0.1, M: Co, Ni, Mn, etc.); lithium manganese such as LiMn ₂ O ₄ Oxide; LiMn _x M _(1-x) O _{2 in} which part of Mn of LiMn ₂ O ₄ is substituted with another element; olivine type LiMPO ₄ (M: Co, Ni, Mn, Fe); LiMn _0.5 Ni _0.5 O ₂ ; Li _{(1 + a)} Mn _x Ni _y Co _(1-xy) O ₂ (−0.1 <a <0.1, 0 <x <0.5, 0 <y <0. 5); can be applied, and a known conductive additive (carbon material such as carbon black) or a binder such as polyvinylidene fluoride (PVDF) is appropriately added to these positive electrode active materials. Positive electrode mixture, molded body with current collector as core (positive electrode mixture Or the like can be used) to finish the thing. The thickness of the positive electrode mixture layer (thickness per side) is preferably 50 to 70 μm.

  As the current collector of the positive electrode, a metal foil such as aluminum, a punching metal, a net, an expanded metal, or the like can be used. Usually, an aluminum foil having a thickness of 10 to 30 μm is preferably used.

  The lead portion on the positive electrode side is normally provided by leaving the exposed portion of the current collector without forming the positive electrode mixture layer on a part of the current collector and forming the lead portion at the time of producing the positive electrode. However, the lead portion is not necessarily required to be integrated with the current collector from the beginning, and may be provided by connecting an aluminum foil or the like to the current collector later.

  The negative electrode according to the battery of the present invention is a negative electrode used in a conventionally known lithium ion secondary battery, that is, a negative electrode composite containing a negative electrode active material capable of occluding and releasing lithium ions on one side or both sides of a current collector. A negative electrode having an agent layer can be used, and in the case of a positive electrode having a porous layer, a material in which a porous layer is further formed on the surface of the negative electrode mixture layer can be used.

  As the negative electrode active material, for example, lithium, such as graphite, pyrolytic carbons, cokes, glassy carbons, fired organic polymer compounds, mesocarbon microbeads (MCMB), and carbon fibers, can be occluded and released. One kind or a mixture of two or more kinds of carbon-based materials is used. In addition, elements such as Si, Sn, Ge, Bi, Sb, In and their alloys, lithium-containing nitrides, oxides and other compounds that can be charged and discharged at a low voltage close to lithium metal, or lithium metals and lithium / aluminum alloys Can also be used as a negative electrode active material. For the negative electrode, a negative electrode mixture in which a conductive additive (carbon material such as carbon black) or a binder such as PVDF is appropriately added to these negative electrode active materials, and a molded body (negative electrode) using the current collector as a core material What was finished in the mixture layer) is used. The thickness of the negative electrode mixture layer (thickness per side) is preferably 45 to 80 μm.

  When a current collector is used for the negative electrode, a copper or nickel foil, a punching metal, a net, an expanded metal, or the like can be used as the current collector, but a copper foil is usually used. In the negative electrode current collector, when the thickness of the entire negative electrode is reduced in order to obtain a battery having a high energy density, the upper limit of the thickness is preferably 30 μm, and the lower limit is preferably 5 μm.

  Similarly to the lead portion on the positive electrode side, the negative electrode lead portion is usually exposed to the current collector without forming a negative electrode agent layer (a layer having a negative electrode active material) on a part of the current collector during negative electrode fabrication. It is provided by leaving a part and using it as a lead part. However, the lead portion on the negative electrode side is not necessarily integrated with the current collector from the beginning, and may be provided by connecting a copper foil or the like to the current collector later.

  The porous layer is formed, for example, by preparing a composition for forming a porous layer (slurry, paste, etc.) in which insulating particles and an organic polymer resin binder are dispersed in a solvent (organic polymer resin binder). The adhesive may be dissolved in a solvent), and this may be applied to the surface of the positive electrode mixture layer of the positive electrode or the negative electrode mixture layer of the negative electrode and dried.

  The solvent used in the composition for forming a porous layer may be any solvent that can uniformly disperse the insulating particles and can uniformly dissolve or disperse the organic polymer resin binder. For example, N-methyl -2-pyrrolidone (NMP); Amides such as N, N-dimethylformamide, N, N-dimethylacetamide; Dimethyl sulfoxide; Aromatic hydrocarbons such as toluene; Furans such as tetrahydrofuran; Methyl ethyl ketone, Methyl isobutyl ketone, etc. Organic solvents such as ketones are preferred, and among these, polar solvents [especially NMP, amides, dimethyl sulfoxide, etc.] have high affinity with the coating film formed by the composition for forming a porous layer. This is preferable. In addition, for the purpose of controlling the interfacial tension, alcohols (ethylene glycol, propylene glycol, etc.) or various propylene oxide glycol ethers such as monomethyl acetate may be appropriately added to these solvents. In addition, when the organic polymer resin binder is water-soluble or used as an emulsion, water may be used as a solvent. In this case, alcohols (methyl alcohol, ethyl alcohol, isopropyl alcohol, ethylene glycol) may be used. Etc.) can be added as appropriate to control the interfacial tension. The solid content concentration including the insulating particles and the organic polymer resin binder in the composition for forming a porous layer is preferably, for example, 10 to 40% by mass.

  The positive electrode mixture layer of the positive electrode is formed by applying a positive electrode mixture-containing composition (slurry, paste, etc.), in which a positive electrode mixture containing a positive electrode active material is dispersed in a solvent such as NMP, to a current collector and drying it. In the negative electrode mixture layer of the negative electrode, a negative electrode mixture-containing composition (slurry, paste, etc.) in which a negative electrode mixture containing a negative electrode active material is dispersed in a solvent such as NMP is used as a current collector. It may be formed by applying and drying. In the case of forming a porous layer on the surface of the positive electrode mixture layer or the negative electrode mixture layer formed through such a manufacturing method, the positive electrode mixture-containing composition before being completely dried after being applied on the current collector The positive electrode mixture layer, the negative electrode mixture layer, and the porous layer may be simultaneously formed by applying a porous layer forming composition on the product or the negative electrode mixture-containing composition and drying the composition. .

  The separator according to the battery of the present invention is a uniaxially stretched microporous film employed in a conventionally known lithium ion secondary battery, that is, a film formed by adding an inorganic filler to the constituent resin of the film. Alternatively, a separator made of a microporous film in which holes are formed by biaxial stretching can be applied.

  As resin which comprises the microporous film used as a separator, polyolefin, such as polyethylene (PE) and polypropylene (PP), etc. are mentioned, for example.

  The battery of the present invention uses a thin separator. Specifically, the thickness of the separator is 5% with respect to the total thickness of the positive electrode and the total thickness of the negative electrode (including the thickness of the porous layer). Hereinafter, it is preferably 4.5% or less. In the lithium ion secondary battery, a separator having a thickness of about 7% with respect to the total thickness of the positive electrode and the total thickness of the negative electrode is usually used, and the capacity of the battery of the present invention is higher than that of such a normal battery. Can do. However, if the separator is too thin, it is difficult to sufficiently secure the strength required at the time of manufacturing the battery and the strength required in the battery. Therefore, the thickness is preferably 4 μm or more.

  Moreover, it is preferable that the porosity of a separator is 25 to 55%.

  The lithium ion secondary battery of the present invention is, for example, a laminated electrode body in which the above positive electrode and the above negative electrode are overlapped with a separator interposed therebetween so that the porous layer is interposed between the positive electrode and the negative electrode. Further, this is wound into a spiral shape to form a wound electrode body, and after such an electrode body is inserted into the exterior body, a nonaqueous electrolyte is injected, and then the exterior body is sealed. .

  There is no restriction | limiting in particular as an exterior body of a battery, The steel can, aluminum can, etc. of the cylinder shape (square tube shape, cylindrical shape, etc.) employ | adopted as a conventionally well-known lithium ion secondary battery are mentioned. In addition, a laminate film obtained by vapor-depositing a metal on a resin film can be used for the exterior body.

Examples of the non-aqueous electrolyte include dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, methyl propionate, ethylene carbonate, propylene carbonate, butylene carbonate, γ-butyrolactone, ethylene glycol sulfite, 1,2-dimethoxyethane, 1, 3-dioxolane, tetrahydrofuran, 2-methyl - tetrahydrofuran, one composed of only an organic solvent such as diethyl ether or a mixture of two or more solvents, for _{example, LiClO 4, LiPF 6, LiBF} 4, LiAsF 6, LiSbF 6, _{LiCF 3 SO 3, LiCF 3 CO} 2, Li 2 C 2 F 4 (SO 3) 2, LiN (CF 3 SO 2) 2, LiC (CF 3 SO 2) 3, LiC n F 2n + 1 SO 3 (n ≧ ), LiN (RfOSO ₂₎ those ₂ [here Rf of fluoroalkyl group] was prepared by dissolving at least one selected from lithium salts such as are used. The concentration of the lithium salt in the electrolytic solution is preferably 0.5 to 1.5 mol / l, and more preferably 0.9 to 1.25 mol / l.

  In addition, non-aqueous electrolytes include esters with double bonds such as vinylene carbonate; sulfur-containing organic compounds such as propane sultone; fluorine-containing fluorine such as fluorobenzene for the purpose of improving charge / discharge cycle characteristics and load characteristics of the battery. It is preferable to add additives such as aromatic compounds.

  The lithium ion secondary battery of this invention can be used for the same use as the various uses to which a conventionally well-known lithium ion secondary battery is applied.

  Hereinafter, the present invention will be described in detail based on examples. However, the following examples are not intended to limit the present invention, and all modifications made without departing from the spirit of the preceding and following descriptions are included in the technical scope of the present invention.

<Preparation of positive electrode>
90 parts by mass of LiCoO _{2 as a} positive electrode active material, 6 parts by mass of carbon black as a conductive auxiliary agent, and 4 parts by mass of PVDF as a binder were mixed uniformly using NMP as a solvent, and a positive electrode mixture-containing paste was prepared. Prepared. This paste was applied on both sides of a 15 μm thick aluminum foil serving as a current collector so that the active material coating length on the front surface was 370 mm and the active material coating length on the back surface was 300 mm, dried, and then subjected to calendar treatment. The thickness of the positive electrode mixture layer was adjusted so that the total thickness was 135 μm and then cut to prepare a positive electrode having a length of 395 mm and a width of 43 mm. Furthermore, an aluminum tab (width 3 mm, thickness 80 μm) serving as a lead was connected to the exposed portion of the aluminum foil of the positive electrode by ultrasonic welding.

<Production of negative electrode>
Graphite was used as the negative electrode active material. As the binder, an SBR suspension and a 1.5 mass% CMC aqueous solution were used. The SBR suspension and the CMC aqueous solution were prepared so that the solid content was 1 part by mass (that is, 2 parts by mass as a whole of the binder solid content), and mixed with 98 parts by mass of the active material to contain the negative electrode mixture-containing composition A product was prepared. This negative electrode mixture-containing paste is uniformly applied to both sides of a copper foil having a thickness of 8 μm, dried, and then rolled by a roll press to adjust the thickness of the negative electrode mixture layer so that the total thickness becomes 134 μm. Then, it cut | disconnected and produced the negative electrode. Further, a tab was attached to the exposed portion of the copper foil of the negative electrode.

<Formation of porous layer>
100 g of alumina powder with an average particle size of 0.4 μm [“AA-04” manufactured by Sumitomo Chemical Co., Ltd.] and a PVDF NMP solution having functional groups effective for improving adhesion and having a weight average molecular weight of 1,000,000 [Kureha Co., Ltd. “KF Polymer L # 9305”, PVDF concentration 5 mass%] 80 g was put into a ball mill containing alumina balls and dispersed for 1 hour to obtain a slurry for forming a porous layer. Obtained. When this slurry was applied on the negative electrode mixture layer on the front and back surfaces of the negative electrode with a die coater (gap: 90 μm) and then dried to remove NMP, the thickness was 4 μm and the negative electrode surface roughness Ra was 0.6 μm. The condition of the coater was adjusted so that the porous layer was formed.

<Production of electrode body and battery>
The positive electrode obtained as described above and the negative electrode formed with a porous layer are wound as a separator through a microporous film made of polyolefin (thickness 12 μm, porosity 50%) to produce an electrode body. did. The electrode body was inserted into an aluminum alloy battery can and injected with an electrolytic solution, followed by sealing to produce a lithium ion secondary battery.

Example 2
A lithium ion secondary battery was produced in the same manner as in Example 1 except that the thickness of the porous film formed on the negative electrode was 2 μm. The surface roughness Ra of the porous layer according to the negative electrode formed at this time was 0.8 μm.

Example 3
A slurry for forming a porous layer was prepared in the same manner as in Example 1 except that an alumina powder having an average particle size of 0.8 μm [“AA-07” manufactured by Sumitomo Chemical Co., Ltd.] was used. A porous layer was formed on the negative electrode in the same manner as in Example 1 except that the thickness was 5 μm. The surface roughness Ra of the porous layer according to the negative electrode formed at this time was 0.8 μm. And the lithium ion secondary battery was produced like Example 1 except having used this negative electrode.

Example 4
A slurry for forming a porous layer was prepared in the same manner as in Example 1 except that alumina powder having an average particle size of 0.2 μm [“AKP-50” manufactured by Sumitomo Chemical Co., Ltd.] was used. A porous layer was formed on the negative electrode in the same manner as in Example 1 except that the thickness was 5 μm. The surface roughness Ra of the porous layer according to the negative electrode formed at this time was 0.4 μm. And the lithium ion secondary battery was produced like Example 1 except having changed the thickness of the separator made from polyolefin into 9 micrometers using this negative electrode.

Example 5
Using the same slurry for forming a porous layer as in Example 4, a porous layer having a thickness of 3 μm was formed on the negative electrode in the same manner as in Example 1. The surface roughness Ra of the porous layer according to the negative electrode formed at this time was 0.5 μm. And the lithium ion secondary battery was produced like Example 1 except having used this negative electrode.

Example 6
A slurry for forming a porous layer was prepared in the same manner as in Example 1 except that titanium oxide powder having an average particle size of 0.25 μm [“CR-50” manufactured by Ishihara Sangyo Co., Ltd.] was used, and this slurry was used. Except for the above, a porous layer having a thickness of 4 μm was formed on the negative electrode in the same manner as in Example 1. The surface roughness Ra of the porous layer according to the negative electrode formed at this time was 0.5 μm. And the lithium ion secondary battery was produced like Example 1 except having used this negative electrode.

Comparative Example 1
A polyolefin microporous film with a thickness of 16 μm was used as a separator, and the thickness of the mixture layer of the positive electrode and the negative electrode was adjusted so that the thickness of the wound electrode body was approximately equal so that it could be accommodated in the battery can. The positive electrode was made with a thickness of 131 μm, and the negative electrode was also made with a thickness of 131 μm. The porous layer on the negative electrode was formed in a thickness of 4 μm in the same manner as in Example 1. The surface roughness Ra of the porous layer was 0.6 μm. A lithium ion secondary battery was produced in the same manner as in Example 1 except that these positive electrode, negative electrode and separator were used.

Comparative Example 2
Using the same slurry for forming a porous layer as in Example 3, a porous layer having a thickness of 4 μm was formed on the negative electrode in the same manner as in Example 1. The surface roughness Ra of the porous layer according to the negative electrode formed at this time was 1.2 μm. And the lithium ion secondary battery was produced like Example 1 except having used this negative electrode.

Comparative Example 3
A slurry for forming a porous layer was prepared in the same manner as in Example 1 except that alumina powder having an average particle diameter of 1.6 μm was used. Using this slurry, a porous layer having a thickness of 5 μm was formed on the same negative electrode as that prepared in Comparative Example 1 by the same method as in Example 1. The surface roughness Ra of the porous layer according to the negative electrode formed at this time was 1.8 μm. A lithium ion secondary battery was produced in the same manner as in Comparative Example 1 except that this negative electrode was used.

Comparative Example 4
A lithium ion secondary battery was produced in the same manner as in Comparative Example 1 except that the porous layer was not formed on the negative electrode.

  The following characteristics evaluation was performed about the lithium ion secondary battery of Examples 1-6 and Comparative Examples 1-4. The results are shown in Table 1.

<Battery capacity>
The batteries of Examples 1 to 6 and Comparative Examples 1 to 4 were charged to 4.2 V with a constant current of 900 mA, and then charged with a constant voltage of 4.2 V (end current 20 mA). Then, the capacity | capacitance of each battery when discharging a 900 mA constant current discharge by the final voltage 3.0V was measured.

<Battery heating test>
The batteries of Examples 1 to 6 and Comparative Examples 1 to 4 were charged at a constant current of 900 mA up to 4.2 V, and then charged at a constant voltage of 4.2 V (end current 20 mA). Each battery after charging was placed in a constant temperature bath, heated to 130 ° C. at a temperature rising rate of 5 ° C./min, and kept at a temperature of 130 ° C. as it was. And the time until the short circuit phenomenon in which a voltage fell rapidly after the battery in a thermostat reached | attained 130 degreeC was measured.

<Battery impact test>
For each of the 10 batteries of Examples 1 to 6 and Comparative Examples 1 to 4, an iron round bar having a diameter of 15 mm and a weight of 6.9 kg was dropped at a height of 40 cm from the center of the wide surface on the battery. The impact was applied, the battery voltage change before and after the test was examined, the battery having a change of 0.5 V or more was judged as a short circuit, and the number of short-circuited batteries was examined.

  The “separator / electrode thickness ratio” in Table 1 means the ratio of the thickness of the separator to the sum of the total thickness of the positive electrode and the total thickness of the negative electrode (including the thickness of the porous layer). Further, the denominator in the column “number of short-circuited batteries during impact test” in Table 1 is the total number of test batteries (10), and the numerator is the number of batteries in which a short circuit has occurred. Furthermore, “Holding time during heating test” in Table 1 indicates the time during which the voltage can be held during the heating test until a short circuit phenomenon occurs in which the voltage suddenly drops after the battery reaches 130 ° C. Yes. Moreover, the thickness of the porous layer shown in Table 1 is the thickness per one side of the negative electrode.

  As shown in Table 1, in the lithium ion secondary batteries of Examples 1 to 6, the occurrence of a short circuit was not observed during the impact test, and the time until the short circuit occurred during the heating test was long and had a porous layer. It can be seen that the battery has better short-circuit resistance and higher capacity than the battery of Comparative Example 4 which is not present.

  In addition, in the batteries of Comparative Examples 1 to 3 where the surface roughness Ra of the porous layer is too large, a short circuit is likely to occur during an impact test or a heating test. This is, as described above, the porous layer is uneven and the insulation performance is insufficiently scattered, and under the situation where the separator insulation performance is reduced as in the impact test and the heat test, It is presumed that a short circuit between the electrodes is likely to occur through the nonuniform portion (defect portion) of the porous layer. In addition, when the porous layer is formed using insulating particles having a large particle size as in the battery of Comparative Example 3, the surface roughness can be reduced because defects formed between the particles are increased. It is difficult to cause a short circuit between the electrodes.

  As described above, a porous layer having a small surface roughness containing insulating particles having a heat-resistant temperature of 150 ° C. or higher and an organic polymer resin binder is provided on at least one of the positive electrode mixture layer surface and the negative electrode mixture layer surface. By forming, even if the separator was thinned, it was possible to suppress a reduction in heat resistance and impact protection characteristics, and to provide a high-capacity and safe battery.

It is a cross-sectional schematic diagram which shows an example of the electrode which has the porous layer which concerns on the lithium ion secondary battery of this invention.

Explanation of symbols

1 electrode (positive or negative)
2 Porous layer 3 Positive electrode mixture layer or negative electrode mixture layer 4 Positive electrode current collector or negative electrode current collector

Claims (4)

A positive electrode in which a positive electrode mixture layer containing an active material is formed on one or both sides of a current collector, a negative electrode in which a negative electrode mixture layer containing an active material is formed on one or both sides of a current collector, a separator, and non-water A lithium ion secondary battery having an electrolyte solution,
At least one of the positive electrode mixture layer surface of the positive electrode and the negative electrode mixture layer surface of the negative electrode contains insulating particles having an heat resistant temperature of 150 ° C. or higher and an organic polymer resin binder, and the surface roughness Ra is Having a porous layer of 1.0 μm or less,
A separator having a thickness of 5% or less with respect to a total of a total thickness of a positive electrode including the porous layer and a total thickness of a negative electrode.   The lithium ion secondary battery according to claim 1, wherein an average particle diameter of the insulating particles having a heat resistant temperature of 150 ° C or higher is 0.2 to 0.8 µm.   The lithium ion secondary battery according to claim 1 or 2, wherein the insulating particles having a heat resistant temperature of 150 ° C or higher are alumina particles.   The lithium ion secondary battery according to claim 1, wherein the porous layer has a thickness of 1 to 5 μm.

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