JP5398559B2 - Lithium ion secondary battery - Google Patents

Description

  The present invention relates to a lithium ion secondary battery having a small irreversible capacity at the time of first charge / discharge, good charge / discharge cycle characteristics, and excellent safety at high temperatures.

  Lithium ion secondary batteries are widely used as power sources for portable devices such as mobile phones and notebook personal computers, electric tools, and electric bicycles because of their high energy density. Recently, the performance of these devices has been remarkably improved, and accordingly, in a lithium ion secondary battery used as a power source, for example, a high level is required for load characteristics, capacity, and charge / discharge cycle characteristics (life characteristics). ing. In order to satisfy these various characteristics, various battery components such as a negative electrode material, a positive electrode material, a separator, and a non-aqueous electrolyte are being improved. In addition, as the various characteristics of the battery are improved, it is necessary to ensure high reliability and safety.

  Various improvement of the negative electrode material utilized as a negative electrode active material is also proposed. For example, Patent Document 1 discloses a technique for reducing irreversible capacity and improving discharge efficiency by coating the surface of phosphorus-like or flake-like graphite particles used as a negative electrode material with an organic substance such as a starch derivative. Proposed.

JP 2003-168433 A

  However, in the negative electrode material described in Patent Document 1, the graphite particles serving as the base material are in the form of phosphorus or flakes having a large aspect ratio, and therefore accompanying the insertion / extraction of lithium ions during charge / discharge of the battery. There is concern about damage due to expansion and contraction between graphite layers. In other words, by repeating charge and discharge of the battery, the surface of the graphite particles may be exposed due to peeling or cracking in the organic matter covering the surface of the graphite particles, which makes it irreversible for each charge and discharge cycle. There is a possibility that the capacity is generated and the charge / discharge cycle deterioration of the battery is caused.

  For this reason, in addition to reducing the irreversible capacity at the time of initial charge / discharge of the battery, development of a technique capable of suppressing the generation of the irreversible capacity even after repeated charge / discharge is required.

  In addition, in lithium ion secondary batteries applied to applications such as power tools where both charging and discharging are performed with a large current, the reaction at the electrodes tends to be non-uniform, and during repeated use, The large heat generation may cause local deterioration in the electrode, and there is a problem that the deterioration of the characteristics becomes large compared to the use in applications where a large current is not required like a mobile phone. Sometimes.

  And the heat_generation | fever at the time of the said charge / discharge may also affect battery members other than an electrode, and there exists a possibility of producing a problem. Lithium ion secondary batteries are often used in packs of several unit cells. Therefore, when the temperature inside the unit cell rises due to charging / discharging, the temperature of the unit cell further rises due to the heat inside the pack. . As a result, the internal temperature of the battery rises to near the melting point of the separator, and the separator is gradually clogged, so that charging and discharging cannot be performed with a large current. Therefore, there is a need for a lithium ion secondary battery that avoids the above problems and can maintain good characteristics even after repeated charging and discharging.

  The present invention has been made in view of the above circumstances, and its purpose is a lithium ion secondary battery having a small irreversible capacity at the time of initial charge / discharge, good charge / discharge cycle characteristics, and excellent safety at high temperatures. Is to provide.

  The lithium ion secondary battery of the present invention that has achieved the above object has a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte. The negative electrode has a solid-liquid surface contact angle (θ) with respect to water. A negative electrode material having graphite having a melting point of 0 to 50 ° and a natural polysaccharide or a derivative thereof covering the surface of the graphite, wherein the separator contains a resin having a melting point of 120 to 140 ° C. It is characterized by comprising a laminate having a layer (I) and a porous layer (II) mainly comprising a filler having a heat resistant temperature of 150 ° C. or higher.

  ADVANTAGE OF THE INVENTION According to this invention, the irreversible capacity | capacitance at the time of first charge / discharge is small, charging / discharging cycling characteristics are favorable, and the lithium ion secondary battery excellent in safety | security at high temperature can be provided.

  The negative electrode material used for the negative electrode according to the lithium ion secondary battery of the present invention has graphite and a natural polysaccharide or a derivative thereof covering the surface thereof.

  That is, in the negative electrode material, a coating layer is formed on the surface of graphite as a base material with natural polysaccharides or derivatives thereof, thereby reducing the irreversible capacity at the time of initial charge / discharge in the battery of the present invention. be able to.

  In addition, since the negative electrode material uses graphite having a wettability to water of 0 to 50 ° in terms of solid-liquid surface contact angle (θ), the surface of the graphite serving as the base material, Adhesion with natural polysaccharides or derivatives thereof covering the surface is good. For example, during charging / discharging of a battery, peeling or cracking of the coating layer due to natural polysaccharides or derivatives thereof formed on the surface of graphite is difficult to occur. . Therefore, in the lithium ion secondary battery of the present invention, even if charging / discharging is repeated, generation of a new irreversible capacity associated therewith is suppressed, and the effect of the coating layer by the natural polysaccharide or its derivative is sustained. The cycle characteristics are also good.

  The said wettability in graphite is calculated | required by measuring the contact angle of water by the sessile drop method based on prescription | regulation of JISR3257-6. Specifically, for example, there is a method of measuring in an air atmosphere at 25 ° C. using “WET-1200” manufactured by ULVAC-RIKO.

  In the negative electrode material, graphite as a base material is not particularly limited as long as it is a carbon material subjected to heat treatment (graphitization treatment) at 2500 ° C. or higher. For example, scaly or scaly natural graphite; scaly, scaly or Spherical artificial graphite; fibrous or cylindrical graphitized carbon. Moreover, the carbon material which coat | covered the surface of the said graphite with the low crystalline carbon material may be sufficient. These may be used alone or in combination of two or more.

  In addition, the graphite used for the negative electrode material may be subjected to various surface treatments in order to adjust the wettability to the preferable value. In particular, when the treatment is performed using oxygen or fluorine, the effect of improving the wettability by increasing the polarity of the graphite surface can be expected. Specific graphite surface treatment methods include, for example, gas treatment heated to 500 to 1200 ° C. in a carbon dioxide or water vapor atmosphere; oxidation treatment in an acidic aqueous solution such as nitric acid; fluorine treatment treated with various fluorides And so on.

  In the negative electrode material, natural polysaccharides or derivatives thereof are used for coating graphite. In order to produce the negative electrode material, for example, a method of dispersing the graphite in a solution (slurry or the like) in which the material for coating the graphite is dissolved in water and immersing it, and then taking out the graphite and drying it, A method of spraying the solution onto graphite and drying it may be employed. At this time, if the viscosity of the solution in which the material for coating graphite is dissolved is low, the solution adhered to the graphite surface may drip before drying, and thus the graphite surface may not be coated well. Therefore, it is desirable that the solution in which the material for coating the graphite is dissolved has a certain degree of viscosity. However, if the amount of the material dissolved in the solution is increased, a thin coating layer cannot be formed on the graphite surface. For example, the load characteristics of the battery may be reduced. However, since natural polysaccharides or their derivatives can be used to prepare highly viscous solutions with a relatively small amount of solution, it is easy to form a thin coating layer on the graphite surface. The deterioration of characteristics can be suppressed.

  Natural polysaccharides or derivatives thereof include, for example, natural polysaccharides such as xanthan gum, welan gum, gellan gum, guar gum, carrageenan, dextrin, pregelatinized starch, etc .; cellulose derivatives (carboxymethylcellulose, hydroxyethylcellulose, hydroxypropylcellulose, etc.) Derivatives of natural polysaccharides such as In producing the negative electrode material, it is preferable to use water as a solvent for the solution of the natural polysaccharide or derivative thereof used for coating graphite, from the viewpoint of environmental protection, etc. The derivative is water-soluble, and the viscosity of the solution can be made relatively high only by dissolving a small amount in water. Especially, the said natural polysaccharide is especially preferable from the fact that the thickening effect to water is higher, and a highly viscous solution can be prepared with a smaller amount. In addition, as a natural polysaccharide or its derivative (s), the thing of the said illustration may be used individually by 1 type, and 2 or more types may be used together.

  In the negative electrode material, the coating thickness of the natural polysaccharide or derivative on the graphite surface is preferably 1 nm or more, and more preferably 3 nm or more, from the viewpoint of better ensuring the effect of the coating. However, if the coating thickness of the natural polysaccharide or its derivative on the graphite surface is too large, the effect of suppressing the reduction in load characteristics due to the use of the natural polysaccharide or its derivative may be reduced in a battery using this. Therefore, the coating thickness of the natural polysaccharide or derivative on the graphite surface is preferably 20 nm or less, and more preferably 10 nm or less.

  The negative electrode material is prepared, for example, by preparing a solution (slurry or the like) in which a natural polysaccharide or a derivative thereof is dissolved in a solvent, immersing graphite as a base material in this solution, and then removing the graphite from the solution and drying it. It can be produced by a method or a method of spraying the solution onto graphite and drying. In addition, as above-mentioned, as a solvent which dissolves natural polysaccharide or its derivative (s), water is preferable.

  The solution obtained by dissolving the natural polysaccharide or its derivative in a solvent preferably has a surface tension at 25 ° C. of 4 to 40 mN / m. By using such a solution, the adhesion between graphite as a base material and the natural polysaccharide or derivative thereof covering the surface becomes better, and the above-mentioned effect by the natural polysaccharide or derivative thereof is sustained. More improved. Moreover, it becomes easy to adjust the thickness of the coating layer on the graphite surface to the above-mentioned preferable value. The “surface tension of a natural polysaccharide or its derivative at 25 ° C.” here is a value measured by a Wilhelmy method using a commercially available device (for example, “CBVP-Z” manufactured by Kyowa Interface Science Co., Ltd.). It is.

  The solution of the natural polysaccharide or derivative thereof dissolved in a solvent preferably has a viscosity at 25 ° C. (viscosity measured using a vibration viscometer) of 5 to 50 mPa · s. By using a solution having such a viscosity, it becomes easy to adjust the thickness of the coating layer on the graphite surface to the preferred value.

  The viscosity of the solution can be adjusted by adjusting the concentration of the natural polysaccharide or its derivative to be dissolved, but commercially available viscosity modifiers such as polyether and urethane-modified polyether (for example, “SN thickener series” manufactured by San Nopco) ) May be added to adjust.

  Additives such as surfactants are added for the purpose of improving the wettability with graphite in a solution in which natural polysaccharides or derivatives thereof are dissolved in a solvent, particularly when water is used as the solvent. be able to. Examples of such additives include commercially available surface conditioners such as organic solvents such as various alcohols and acetone, modified silicones and modified polyethers (for example, “SN wet series” and “SN clean act series manufactured by San Nopco). And “SN deformer series”). In the solution, the exemplified additive is preferably added so as to have a concentration of, for example, about 0.01 to 5% by mass, and thereby, a solution of a natural polysaccharide or a derivative thereof dissolved in water. The surface tension can be adjusted to the preferred value.

  As the negative electrode according to the battery of the present invention, for example, a negative electrode mixture layer containing the negative electrode material, a binder or the like formed on one side or both sides of a current collector can be used. In the negative electrode, graphite as the negative electrode material acts as a negative electrode active material.

  In addition, another negative electrode active material can also be used together with the said negative electrode material for a negative electrode active material. Examples of the other negative electrode active materials include graphite whose surface is not coated with a polysaccharide or a derivative thereof, pyrolytic carbons, cokes, glassy carbons, a fired body of an organic polymer compound, mesocarbon micro Examples thereof include carbon-based materials capable of occluding and releasing lithium, such as beads (MCMB) and carbon fibers.

  However, when another negative electrode active material is used in combination with the negative electrode material, the negative electrode material in the total amount of the negative electrode material and the other negative electrode active material from the viewpoint of favorably ensuring the effect of the use of the negative electrode material Is preferably 70% by mass or more, and more preferably 80% by mass or more.

  The negative electrode contains, for example, a negative electrode mixture in the form of a slurry or paste in which a negative electrode mixture containing the negative electrode material, other negative electrode active materials, a binder, and a conductive auxiliary agent used as necessary is dispersed in a solvent. After the composition is applied to one or both sides of the current collector and dried, the composition can be produced through a step of adjusting the thickness of the negative electrode mixture layer by applying a press treatment as necessary. In addition, you may produce the negative electrode which concerns on this invention by methods other than the above.

  Examples of the negative electrode binder include, but are not limited to, a fluororesin such as polyvinylidene fluoride (PVDF), a cellulose ether compound, and a rubber binder. Specific examples of the cellulose ether compound include carboxymethyl cellulose (CMC), carboxyethyl cellulose, hydroxyethyl cellulose, alkali metal salts such as lithium salts, sodium salts, and potassium salts, ammonium salts, and the like. Specific examples of rubber binders include, for example, styrene / conjugated diene copolymers such as styrene / butadiene copolymer rubber (SBR); nitrile / conjugated diene copolymers such as nitrile / butadiene copolymer rubber (NBR). Rubber; Silicone rubber such as polyorganosiloxane; Polymer of alkyl acrylate ester; Acrylic obtained by copolymerization of alkyl acrylate ester with ethylenically unsaturated carboxylic acid and / or other ethylenically unsaturated monomers Rubber; Fluororubber such as vinylidene fluoride copolymer rubber.

  Examples of the conductive assistant for the negative electrode include various carbon blacks such as acetylene black and carbon nanotubes.

  The thickness of the negative electrode mixture layer is preferably 10 to 100 μm per one surface of the current collector, for example. In addition, the composition of the negative electrode mixture layer is the amount of the negative electrode active material (in the case where only the negative electrode material is used, the amount thereof. When other negative electrode active materials are used in combination, the negative electrode material and the other negative electrode The total amount with the active material) is preferably 60 to 95% by mass, and the amount of the binder is preferably 1 to 15% by mass. Moreover, when using a conductive support agent, it is preferable that the quantity of the conductive support agent in a negative mix layer is 0.5-5 mass%.

  As the current collector for the negative electrode, a foil made of copper or nickel, a punching metal, a net, an expanded metal, or the like can be used, 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.

  The lead portion on the negative electrode side is usually provided by leaving the exposed portion of the current collector without forming the negative electrode mixture layer on a part of the current collector and forming the lead portion at the time of preparing the negative electrode. However, the lead portion 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.

  Next, the separator used for the lithium ion secondary battery of this invention is demonstrated. In general, a single porous film made of polyolefin used in lithium ion secondary batteries is a resin having a melting point near the shutdown temperature so that shutdown occurs at around 135 ° C. while maintaining a certain degree of heat resistance. Is used. However, due to the large strain of the film, depending on the use conditions, the film does not reach shutdown, but the battery heat generation tends to cause shrinkage or clogging of the film, resulting in a short circuit or deterioration of characteristics. Further, if the melting point of the resin is increased in consideration of heat resistance, it is difficult to cause a shutdown, which may cause a problem in terms of safety.

  On the other hand, the laminate used as a separator in the present invention mainly contains a filler having a heat resistant temperature of 150 ° C. or higher in addition to the porous layer (I) containing a resin having a melting point of 120 ° C. or higher and 140 ° C. or lower. Since it has a porous layer (II), even when it is used in applications where the internal temperature of the battery is likely to rise, such as a power tool, the thermal contraction of the separator is suppressed and clogging occurs. It is difficult to maintain the characteristics of the separator stably. For this reason, the characteristic which the negative electrode material mentioned above and the positive electrode material (positive electrode active material) mentioned later have can be exhibited effectively, there is little characteristic deterioration by charging / discharging by a large current, and it is in a comparatively high temperature environment. Can be a highly reliable battery. The separator is not particularly limited as long as it is a laminate having a porous layer (I) and a porous layer (II). A laminate composed of two layers with the porous layer (II), a laminate of three or more layers in which the porous layer (II) is disposed on both sides of the porous layer (I), and the like are suitable for the above purpose. More preferably used.

  The melting point of the resin contained in each layer of the separator referred to in this specification means the melting temperature measured using a differential scanning calorimeter (DSC) in accordance with the provisions of Japanese Industrial Standard (JIS) K7121. Yes.

The resin constituting the porous layer (I) is a resin having a melting point of 120 to 140 ° C., and specific examples include polyolefins such as polyethylene (PE), polybutene, and ethylene-propylene copolymer. High density polyethylene having a density of 0.94 g / cm ³ or more and 0.97 g / cm ³ or less is particularly preferable. For the porous layer (I), a porous film (a so-called microporous film widely used for a separator of a lithium ion secondary battery) formed of the above exemplified resin is preferably used.

  The porous layer (II) for enhancing the heat resistance of the separator contains a filler as a main component, but such a filler has a heat resistance of 150 ° C. or higher, that is, heat resistance that does not cause deformation such as softening at least at 150 ° C. Inorganic or resin particles that have electrical insulation properties and are electrochemically stable particles that are resistant to oxidation / reduction in the battery operating voltage range may be inorganic particles or organic particles. From the viewpoints of stability (particularly oxidation resistance), inorganic fine particles are more preferably used.

Examples of the inorganic fine particles include inorganic oxides such as iron oxide, SiO ₂ , Al ₂ O ₃ , TiO ₂ , BaTiO ₃ , and ZrO ₂ ; inorganic nitrides such as aluminum nitride and silicon nitride; calcium fluoride, barium fluoride, and sulfuric acid Examples thereof include poorly soluble ion binding compounds such as barium; covalent bonding compounds such as silicone and diamond; clays such as montmorillonite. Here, the inorganic oxide may be boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, mica, or other mineral resource-derived substances or artificial products thereof. Among the inorganic particles, Al ₂ O ₃ , SiO ₂ and boehmite are particularly preferably used.

  Organic particles (organic powder) include crosslinked polymethyl methacrylate, crosslinked polystyrene, crosslinked polydivinylbenzene, crosslinked styrene-divinylbenzene copolymer, polyimide, melamine resin, phenol resin, benzoguanamine-formaldehyde condensate, etc. Examples thereof include various crosslinked polymer particles and heat-resistant polymer particles such as polysulfone, polyacrylonitrile, polyaramid, polyacetal, and thermoplastic polyimide. The organic resin (polymer) constituting these organic particles is a mixture, modified body, derivative, or copolymer (random copolymer, alternating copolymer, block copolymer, graft copolymer) of the materials exemplified above. Polymer) or a crosslinked product (in the case of the above-mentioned heat-resistant polymer).

The shape of the filler having a heat resistant temperature of 150 ° C. or higher may be, for example, a nearly spherical shape or a plate shape, but is preferably a plate-like particle from the viewpoint of preventing a short circuit. Typical examples of the plate-like particles include plate-like Al ₂ O ₃ and plate-like boehmite. Moreover, the thing of the secondary particle structure which the primary particle aggregated can also be used. By using particles having a secondary particle structure, adhesion between particles can be prevented to some extent, and voids between particles can be appropriately maintained. Thereby, the path | route which ion permeate | transmits can be ensured, high ion permeability can be maintained, and it can be set as the structure more suitable for charging / discharging by a large current.

The particle size of the filler having a heat resistant temperature of 150 ° C. or higher is an average particle size, and is preferably 0.01 μm or more, more preferably 0.1 μm or more, preferably 15 μm or less, more preferably 5 μm or less. In addition, the average particle diameter of the filler as used in this specification is, for example, using a laser scattering particle size distribution meter (for example, “LA-920” manufactured by Horiba, Ltd.), and filling the filler into a medium that does not dissolve the filler (for example, water). It is a particle diameter (D _50% ) at a volume-based integrated fraction of 50% measured by dispersing.

  The porous layer (II) can be formed by binding fillers having a heat resistant temperature of 150 ° C. or higher with a binder or the like. The proportion of filler with a heat resistant temperature of 150 ° C or higher in the porous layer (II) is in the total volume of the constituent components of the porous layer (II) so that the filler is mainly contained (in the total volume excluding the pores) ), 50 volume% or more. Further, in order to improve the binding property by a binder or the like, the ratio of the filler having a heat resistant temperature of 150 ° C. or higher in the porous layer (II) is 99 volume in the total volume of the constituent components of the porous layer (II). % Or less is preferable.

  Binders that can be used for the porous layer (II) include ethylene-vinyl acetate copolymer, ethylene-acrylic acid copolymer, fluorine-based rubber, styrene-butadiene rubber and other highly flexible resins, as well as carboxymethyl cellulose. Hydroxyethyl cellulose, polyvinyl alcohol, polyvinyl butyral, polyvinyl pyrrolidone, cross-linked acrylic resin, polyurethane, epoxy resin and the like are used. In particular, a heat-resistant binder capable of maintaining excellent binding properties even at a temperature of 150 ° C. or higher and maintaining the shape of the porous layer (II) is preferably used.

  The porous layer (II) is, for example, a method in which a slurry formed by dispersing a filler, a binder or the like having a heat resistant temperature of 150 ° C. or higher in a solvent is applied to the surface of the porous layer (I) and dried. Can be formed.

  In the separator according to the battery of the present invention, the thickness of the porous layer (II) [when the separator has a plurality of porous layers (II), the total thickness thereof] is 1 μm from the viewpoint of suppressing thermal shrinkage of the separator. Preferably, the thickness is 3 μm or more, and from the viewpoint of suppressing an increase in the thickness of the entire separator, it is preferably 15 μm or less, and more preferably 10 μm or less.

  Furthermore, in the separator according to the battery of the present invention, the thickness of the porous layer (I) [when the separator has a plurality of porous layers (I), the total thickness thereof] ensures better shutdown characteristics. From the viewpoint, it is preferably 3 μm or more, more preferably 5 μm or more, and from the viewpoint of suppressing an increase in the thickness of the entire separator, it is preferably 20 μm or less, and more preferably 15 μm or less.

  The lithium ion secondary battery of the present invention only needs to include the above-described negative electrode and the above-described separator, and there is no particular limitation on the other configuration and structure, and it is adopted in conventionally known lithium-ion secondary batteries. Various configurations and structures can be applied.

The positive electrode according to the battery of the present invention is not particularly limited as long as it is a positive electrode used in a conventionally known lithium ion secondary battery, that is, a positive electrode containing an active material capable of occluding and releasing Li ions. For example, as the active material, Li _{1 + x} MO ₂ (−0.1 <x <0.1, M: Co, Ni, Mn, Al, Mg, etc. Note that the element M is a metal element other than Li and is 10 atoms. A lithium-containing transition metal oxide having a layered structure represented by the following formula: LiMn ₂ O ₄ and a lithium manganese oxide having a spinel structure in which a part of the element is substituted with another element, LiMPO ₄ An olivine type compound represented by (M: Co, Ni, Mn, Fe, etc.) can be used. Specific examples of the lithium-containing transition metal oxide having a layered structure include LiCoO ₂ and LiNi _1-x Co _xy Al _y O ₂ (0.1 ≦ x ≦ 0.3, 0.01 ≦ y ≦ 0. 2) and other oxides containing at least Co, Ni and Mn (LiMn _1/3 Ni _1/3 Co _1/3 O ₂ , LiMn _5/12 Ni _5/12 Co _1/6 O ₂ , LiNi _{3 / 5} Mn _1/5 Co _1/5 O ₂ etc.). In particular, an active material containing 40% or more of Ni is preferable because the battery has a high capacity, and O (oxygen atom) may be substituted with 1 atom% of fluorine or sulfur atom.

In particular, it is preferable to use lithium nickel oxide represented by the following general formula (1) as the positive electrode active material.
LiNi _(1-x) M _x O ₂ (1)
In the general formula (1), M is at least one metal element selected from the group consisting of Al, Mn, Co, Cr, Mg, Fe, Zr and Ti. X is the stoichiometric number of the metal element M, and is selected as 0 ≦ x <1. That is, the lithium nickel oxide is lithium nickelate (LiNiO ₂ ) when x is 0, and is a composite oxide in which a part of Ni is substituted with the metal element M when x exceeds 0. .

In addition, among the lithium nickel oxides represented by the general formula (1), LiNiO ₂ has a high capacity, but has a drawback that initial charging characteristics are difficult and the utilization rate is low. In addition, although the thermal stability is also difficult, the disadvantage of LiNiO _{2 is} maintained while maintaining a high capacity, such as by replacing a part of Ni with the metal element M to increase the thermal stability. Can be eliminated.

Specific examples of such lithium nickel oxide include nickel-lithium cobaltate (LiNi _0.8 Co _0.2 O ₂ ) in which the metal element M is Co and x is 0.2. It is done. In addition, the metal element M may be two or more, and examples thereof include LiNi _0.8 Co _0.1 Ti _0.1 O ₂ .

  When the lithium nickel oxide represented by the general formula (1) is used as the positive electrode active material, the content of the lithium nickel oxide represented by the general formula (1) in the total positive electrode active material is 70 masses. % Or more, preferably 80% by mass or more, and more preferably 100% by mass. The positive electrode active material here is a member that directly contributes to the reversible insertion and desorption reaction of Li ions as the positive electrode material, and does not include a binder and a conductive auxiliary agent described later. By making the content of the lithium nickel oxide represented by the general formula (1) in all the positive electrode active materials used for the positive electrode as described above, it is possible to achieve better battery capacity.

In addition, the size of the positive electrode active material used in the battery of the present invention is an average particle diameter (D _50% ) measured by the same method as a filler having a heat resistant temperature of 150 ° C. or higher, and is 0.1 μm or more. Preferably, it is 1 μm or more, more preferably 30 μm or less, and more preferably 15 μm or less.

  In the same way as the above-described method for preparing a negative electrode, a positive electrode is prepared by collecting a slurry-like or paste-like positive electrode mixture-containing composition in which a positive electrode mixture containing the positive electrode active material, a binder, and a conductive additive is dispersed in a solvent. After applying to one side or both sides of the electric body and drying, it can be produced through a step of adjusting the thickness of the positive electrode mixture layer by performing a press treatment as necessary. In addition, you may produce the positive electrode which concerns on this invention by methods other than the above.

  As the positive electrode binder, various materials exemplified above as a negative electrode binder or a separator binder can be used. Among them, PVDF is inexpensive and electrochemically stable, and is soluble in an organic solvent suitable as a solvent for a positive electrode mixture-containing composition such as N-methyl-2-pyrrolidone (NMP). It is preferably used because it facilitates the preparation of a uniform positive electrode mixture-containing composition.

  Note that PVDF is easily defluorinated under alkali. For example, when dissolved in NMP suitable for the solvent of the positive electrode mixture-containing composition, PVDF tends to react with a small amount of alkali component in NMP and olefinate. . Furthermore, some of the lithium-containing composite oxides such as lithium nickel oxide represented by the general formula (1) used as the positive electrode active material generally exhibit strong alkalinity, and this is an NMP solution of PVDF. When mixed, the PVDF defluorination further proceeds and the positive electrode mixture-containing composition may be gelled.

  Therefore, when a lithium-containing composite oxide such as lithium nickel oxide represented by the general formula (1) is used as a positive electrode active material and PVDF is used as a binder for a positive electrode, for example, an alkali component is previously obtained by distillation purification or the like. It is preferable to prepare a positive electrode mixture-containing composition using NMP from which NMP is removed as a solvent, and when the purified NMP is a 10% by mass aqueous solution, the pH is particularly preferably 5 to 7.

  As the conductive aid for the positive electrode, as in the case of the negative electrode, natural or artificial graphite, carbon materials such as carbon black, carbon fiber, and carbon nanotube are preferably used.

  The thickness of the positive electrode mixture layer is preferably, for example, 10 to 100 μm per one side of the current collector. Moreover, as a composition of a positive mix layer, it is preferable that the quantity of a positive electrode active material is 60-95 mass%, it is preferable that the quantity of a binder is 1-15 mass%, and the quantity of a conductive support agent is 3. It is preferable that it is -20 mass%.

  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 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 and the positive electrode can be used in the form of a laminated electrode body laminated via the separator or a wound electrode body obtained by winding the laminated electrode body.

As the nonaqueous electrolytic solution according to the battery of the present invention, a solution in which a lithium salt is dissolved in an organic solvent is used. The lithium salt is not particularly limited as long as it dissociates in a solvent to form Li ⁺ ions and does not cause a side reaction such as decomposition in a voltage range used as a battery. For example, inorganic lithium salts such as 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 ≧ 2), LiN (RfOSO ₂ ) ₂ [where Rf is a fluoroalkyl group]; it can.

  The concentration of the lithium salt in the non-aqueous electrolyte is preferably 0.5 to 1.5 mol / l, and more preferably 0.9 to 1.25 mol / l.

  The organic solvent used for the non-aqueous electrolyte is not particularly limited as long as it dissolves the lithium salt and does not cause a side reaction such as decomposition in a voltage range used as a battery. For example, cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate and vinylene carbonate; chain carbonates such as dimethyl carbonate, diethyl carbonate and methyl ethyl carbonate; chain esters such as methyl propionate; Cyclic esters such as butyrolactone; chain ethers such as dimethoxyethane, diethyl ether, 1,3-dioxolane, diglyme, triglyme and tetraglyme; cyclic ethers such as dioxane, tetrahydrofuran and 2-methyltetrahydrofuran; acetonitrile, propionitrile, methoxypro Nitriles such as pionitrile; sulfites such as ethylene glycol sulfite; and the like. May be used in, it may be used in combination of two or more thereof.

  In addition, in order to obtain a battery with better characteristics, it is preferable to use a mixed solvent of the above-mentioned cyclic carbonate and chain carbonate, and this provides good characteristics of each solvent such as high ionic conductivity. Can be pulled out.

  When graphite is used as the negative electrode active material, there is a problem of reactivity with PC, and EC is often used as the cyclic carbonate. However, in the battery of the present invention, the graphite used as the negative electrode active material has a good PC resistance because its surface is coated with a natural polysaccharide or a derivative thereof. Even if an electrolytic solution is used, the occurrence of problems due to this can be suppressed. For example, in a non-aqueous electrolyte solvent, by replacing at least a part of EC that is solid at room temperature with PC, the non-aqueous electrolyte can be reduced in viscosity and non-aqueous electrolyte exhibiting higher ionic conductivity. By using this, a lithium ion secondary battery having better load characteristics can be obtained. In addition, there is a merit in the battery manufacturing process, such as suppressing clogging of the nozzle when the nonaqueous electrolyte is injected into the battery outer body.

  In addition, vinylene carbonates, 1,3-propane sultone, diphenyl disulfide, cyclohexylbenzene, biphenyl, and fluorobenzene are used for the purpose of improving safety, charge / discharge cycleability, and high-temperature storage properties of these non-aqueous electrolytes. , T-butylbenzene, acid anhydride, sulfurated ester, vinyl ethylene carbonate, and derivatives thereof can also be appropriately added as additives. The addition amount varies depending on the additive used and the purpose of use, and is selected in the range of, for example, 0.1 to 5% by mass. These additives may be used individually by 1 type, and may use 2 or more types together.

  Also, instead of the organic solvent, melting at room temperature such as ethyl-methylimidazolium trifluoromethylsulfonium imide, heptyl-trimethylammonium trifluoromethylsulfonium imide, pyridinium trifluoromethylsulfonium imide, guanidinium trifluoromethylsulfonium imide A salt can also be used.

  Furthermore, a polymer material that contains the non-aqueous electrolyte and gels may be added to make the organic electrolyte into a gel and used for the battery. Polymeric materials for gelling organic electrolytes include PVDF, vinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP), polyacrylonitrile, polyethylene oxide, polypropylene oxide, ethylene oxide-propylene oxide copolymer And a known host polymer capable of forming a gel electrolyte, such as a crosslinked polymer having an ethylene oxide chain in the main chain or side chain, and a crosslinked poly (meth) acrylate.

  The lithium ion secondary battery of the present invention is a cylindrical electrode body using a laminated electrode body in which the above-described negative electrode, separator and positive electrode are laminated, or a wound electrode body in which these are wound, using a steel can or an aluminum can as an outer can. It can be manufactured by filling a can (such as a rectangular tube or cylinder), a soft package having a metal-deposited laminate film as an outer package, and injecting the non-aqueous electrolyte therein.

  In the battery, it is preferable that the porous layer (II) of the separator face the positive electrode. When, for example, a compound in which an alkyl group is bonded to a benzene ring (cyclohexylbenzene or the like) is added to the non-aqueous electrolyte, the lithium ion secondary battery is overcharged. Is polymerized to form a conductive path in the pores of the separator, thereby causing a soft short, and thus an effect of suppressing an increase in battery temperature is exhibited. However, at the time of overcharging, the separator is easily oxidized by the positive electrode. If the separator is deteriorated by this, the soft short cannot be caused stably, and the safety at the time of overcharging may not be satisfactorily secured. However, by placing the separator so that the porous layer (II), which has a heat resistance temperature of 150 ° C or higher as the main component and has better oxidation resistance, faces the positive electrode, the oxidative degradation of the separator during overcharge Therefore, the soft short can be generated more stably.

  Furthermore, the porous layer (I) of the separator is more preferably disposed so as to face the negative electrode. Although the detailed reason is unknown, when the separator is disposed so that the porous layer (I) faces at least the negative electrode, the porous layer is more likely to be shut down than when disposed on the positive electrode side. Since the melted resin from (I) is less effectively absorbed by the electrode mixture layer, and the melted resin is used more effectively to close the pores of the separator, the effect of shutdown is better. Become.

  The lithium ion secondary battery of the present invention can be applied to the same applications as conventionally known lithium ion secondary batteries.

  Hereinafter, the present invention will be described in detail based on examples. However, the following examples do not limit the present invention.

<Preparation of negative electrode material A>
Graphite having an average particle diameter D _50% of 16 μm, d ₀₀₂ of 0.3360 nm, and a specific surface area of 3.4 m ² / g was placed in an activation furnace, and surface treatment was performed at 1000 ° C. for 4 hours in a carbon dioxide gas stream. (Hereinafter, this surface-treated graphite is referred to as “graphite A”). For the graphite A after the surface treatment, the wettability [solid-liquid surface contact angle (θ)] with respect to water was determined by the above-described method.

  Xanthan gum, which is a natural polysaccharide: 100 g is dissolved in 50 L of water, and an appropriate amount of a hydrophobic silica aqueous tension adjuster is added to prepare a solution having a surface tension at 25 ° C. as shown in Table 1. After adding 5 kg of graphite A, it was taken out from the solution and dried to prepare a negative electrode material A in which the surface of the graphite was coated with xanthan gum. The coating thickness of xanthan gum in this negative electrode material A was measured with a transmission electron microscope (TEM).

<Preparation of negative electrode material B>
The same graphite (graphite before surface treatment) as that used for the production of the negative electrode material A was subjected to surface treatment at 1000 ° C. for 3 hours under a carbon dioxide gas stream (hereinafter, the graphite after the surface treatment is referred to as “ Called graphite B). About the graphite B after surface treatment, the wettability [solid-liquid surface contact angle ((theta))] with respect to water was calculated | required by the said method.

  Xanthan gum, which is a natural polysaccharide: 70 g is dissolved in 50 L of water, and an appropriate amount of a hydrophobic silica aqueous tension adjuster is added to prepare a solution having a surface tension at 25 ° C. as shown in Table 1. After introducing 5 kg of graphite B, it was taken out from the solution and dried to prepare a negative electrode material B in which the surface of the graphite was coated with xanthan gum. The coating thickness of xanthan gum in this negative electrode material B was measured with a transmission electron microscope (TEM).

<Preparation of negative electrode material C>
The same graphite (graphite before surface treatment) as that used for the preparation of the negative electrode material A was subjected to surface treatment at 1000 ° C. for 2 hours under a carbon dioxide gas stream (hereinafter, the surface treated graphite is referred to as “ Called graphite C). For the graphite C after the surface treatment, the wettability [solid-liquid surface contact angle (θ)] with respect to water was determined by the above-described method.

  Xanthan gum which is a natural polysaccharide: 150 g is dissolved in 50 L of water, and an appropriate amount of a hydrophobic silica aqueous tension adjuster is added to prepare a solution having a surface tension at 25 ° C. as shown in Table 1. After adding 5 kg of graphite C, it was taken out from the solution and dried to prepare a negative electrode material C in which the surface of the graphite was coated with xanthan gum. The coating thickness of xanthan gum in this negative electrode material C was measured with a transmission electron microscope (TEM).

<Preparation of negative electrode material D>
The same graphite (graphite before surface treatment) as that used for the preparation of the negative electrode material A was subjected to surface treatment at 1000 ° C. for 1 hour in a carbon dioxide gas stream (hereinafter, the graphite after the surface treatment is referred to as “ Called graphite D). For the graphite D after the surface treatment, the wettability [solid-liquid surface contact angle (θ)] with respect to water was determined by the above-described method.

  Xanthan gum which is a natural polysaccharide: 180 g of water is dissolved in 50 L of water, and an appropriate amount of a hydrophobic silica aqueous tension adjuster is added to prepare a solution having a surface tension at 25 ° C. as shown in Table 1. After adding 5 kg of graphite D, the graphite D was taken out from the solution and dried to prepare a negative electrode material D in which the surface of the graphite was coated with xanthan gum. The coating thickness of xanthan gum in this negative electrode material D was measured with a transmission electron microscope (TEM).

<Negative electrode material E>
The same graphite (graphite before surface treatment) as that used for preparation of the negative electrode material A was used as the negative electrode material E without performing the surface treatment with carbon dioxide gas and the coating treatment with xanthan gum.

  The negative electrode materials A to E were subjected to a charge / discharge test using a single electrode evaluation cell.

<Preparation of negative electrode for single electrode evaluation cell>
The negative electrode material: 90 parts by mass and PVDF as a binder: 10 parts by mass of a negative electrode mixture mixed with NMP in which 0.1 part by mass of oxalic acid was dissolved in advance to prepare a negative electrode mixture-containing slurry Prepared. The slurry was applied to one side of a current collector made of copper foil with a thickness of 10 μm, dried, and then subjected to a calendar treatment to adjust the thickness of the negative electrode mixture layer so that the total thickness became 70 μm. A negative electrode for a single electrode evaluation cell containing any of materials A to E was produced.

<Production of single electrode evaluation cell>
A single electrode evaluation cell was fabricated using the negative electrode for each single electrode evaluation cell. The single electrode evaluation cell has the negative electrode (the negative electrode mixture layer coated surface cut into a 2 × 2 cm square) as a working electrode, a Li metal foil as a counter electrode and a reference electrode, EC, PC, and non-aqueous electrolyte. A solution obtained by dissolving LiPF ₆ at a concentration of 1.2 mol / L in a mixed solution in which methyl ethyl carbonate was mixed at a ratio (volume ratio) of 3: 2: 5 was used, and a polyethylene microporous film (thickness) was used as a separator. 12 μm).

<Charge / discharge test using single electrode evaluation cell>
Each single electrode evaluation cell was charged with a constant current of 4 mA until the potential of the reference electrode was 0.03 V, and further charged with a constant voltage of 0.03 V until the total charging time was 3 hours. After charging, the single-electrode evaluation cell was discharged to a voltage of 2.5 V at a constant current of 4 mA, and the initial charge capacity and discharge capacity were measured. Charge / discharge efficiency (100 × discharge capacity ÷ charge capacity, Unit%) was calculated.

  Graphite wettability (solid-liquid contact angle) used for the negative electrode materials A to E, characteristics of xanthan gum dissolved in water (xanthan gum aqueous solution) (surface tension and viscosity at 25 ° C.), coating of xanthan gum on the graphite surface Table 1 shows the initial charge / discharge efficiency obtained by the thickness and the monopolar evaluation cell.

  As is apparent from Table 1, the negative electrode materials A to D constituted by coating the surface of specific graphite having wettability with water with natural polysaccharide are the negative electrode materials E not coated with natural polysaccharide. Compared to the initial charge / discharge efficiency, the irreversible capacity during the first charge / discharge is reduced.

Example 1
<Preparation of battery negative electrode A>
Negative electrode material A: 98 parts by mass, CMC: 1 part by mass, and SBR: 1 part by mass were mixed in the presence of water to prepare a negative electrode mixture-containing slurry. This slurry is intermittently applied to both sides of a current collector made of copper foil with a thickness of 10 μm, dried, and then subjected to a calender treatment so that the negative electrode mixture layer has a density of 1.54 g / cm ³ . The thickness of the agent layer was adjusted. Then, it cut | disconnected so that it might become width 57mm and length 1025mm, and the negative electrode A for batteries was obtained. Further, a tab was welded to the exposed portion of the copper foil of the negative electrode A for a battery to form a lead portion.

<Preparation of battery positive electrode A>
PVDF as a binder is dissolved in NMP (distilled product, pH 7 when an aqueous solution of 10% by mass is 7), and a positive electrode active material having an average particle diameter D _50% of LiNi _0.8 Co _{0. 2} O ₂ and acetylene black, which is a conductive additive, were added and mixed uniformly to prepare a positive electrode mixture-containing paste. The mixing ratio was positive electrode active material: binder: conducting aid = 85: 10: 5 (parts by mass). This paste is intermittently applied to both sides of an aluminum foil having a thickness of 15 μm serving as a current collector, dried, calendered, and the positive electrode mixture layer is pressure-molded to a thickness of 84 μm. The battery positive electrode A was produced by cutting to 55 mm and a length of 886 mm. Further, a tab was welded to the exposed portion of the aluminum foil of the battery positive electrode A to form a lead portion.

<Preparation of separator A>
To 5 kg of boehmite having an average particle diameter D50% of 1 μm, 5 kg of ion exchange water and 0.5 kg of a dispersing agent (aqueous polycarboxylic acid ammonium salt, solid content concentration 40% by mass) are added, and the internal volume is 20 L, the number of rotations is 40 times / A dispersion was prepared by dispersing for 1 hour with a ball mill for 1 minute. The prepared dispersion was vacuum dried at 120 ° C. and observed with a scanning electron microscope (SEM). As a result, the boehmite was almost plate-shaped.

  To 500 g of the above dispersion, 0.5 g of xanthan gum as a thickener and 17 g of a resin binder dispersion (modified polybutyl acrylate, solid content 45% by mass) as a binder are added and stirred with a three-one motor for 3 hours to form a uniform slurry. [Slurry for forming porous layer (II), solid content ratio 50 mass%] was prepared.

PE microporous separator for battery [Porous layer (I): thickness 16 μm, porosity 40%, average pore diameter 0.08 μm, PE melting point 135 ° C.] on one side corona discharge treatment (discharge amount 40 W · min / m ² ), and the slurry was applied to the corona discharge treated surface by a micro gravure coater and dried to form a porous layer (II), whereby a separator A was obtained. The mass per unit area of the porous layer (II) in the separator A is 3.4 g / m ² , and the amount of boehmite in the total volume of the constituent components of the porous layer (II) is 90% by volume. there were.

<Battery assembly>
The battery negative electrode A and the battery positive electrode A were overlapped via the separator A and wound into a spiral shape to obtain a wound electrode body. In the wound electrode body, the porous layer (II) of the separator A was made to face the battery positive electrode A. This wound electrode body was loaded into cylindrical cans each having a diameter of 18 mm and a height of 65 mm, and a non-aqueous electrolyte [EC, PC, and methyl ethyl carbonate were mixed in a solvent in a ratio (volume ratio) of 3: 2: 5]. , LiPF ₆ was dissolved at a concentration of 1.2 mol / l, and cyclohexylbenzene was added at a ratio of 1.0 part by mass with respect to 100 parts by mass of the solvent. A battery was produced. This battery is provided with a cleavage vent for lowering the pressure when the internal pressure rises at the top of the can.

Example 2
A battery positive electrode B was produced in the same manner as the battery positive electrode A in Example 1, except that the positive electrode active material was changed to LiCoO ₂ having an average particle diameter D _50% of 10 μm. Then, a lithium ion secondary battery was produced in the same manner as in Example 1 except that this battery positive electrode B was used instead of the battery positive electrode A.

Example 3
A mixture of LiNi _0.8 Co _0.2 O ₂ : 70% by mass, which is the same as that used for the positive electrode A for a battery, and LiCoO ₂ : 30% by mass, which is the same as that used for the positive electrode B for a battery. A battery positive electrode C was produced in the same manner as the battery positive electrode A in Example 1, except that the change was changed to. Then, a lithium ion secondary battery was produced in the same manner as in Example 1 except that this battery positive electrode C was used instead of the battery positive electrode A.

Example 4
A battery negative electrode B was produced in the same manner as the battery negative electrode A in Example 1, except that the negative electrode material B was used instead of the negative electrode material A. And it replaced with the negative electrode A for batteries, and produced the lithium ion secondary battery like Example 1 except having used this negative electrode B for batteries.

Example 5
A battery negative electrode C was produced in the same manner as the battery negative electrode A in Example 1, except that the negative electrode material C was used instead of the negative electrode material A. And it replaced with the negative electrode A for batteries, and produced the lithium ion secondary battery like Example 1 except having used this negative electrode C for batteries.

Example 6
A battery negative electrode D was produced in the same manner as the battery negative electrode A in Example 1, except that the negative electrode material D was used instead of the negative electrode material A. And it replaced with the negative electrode A for batteries, and produced the lithium ion secondary battery like Example 1 except having used this negative electrode D for batteries.

Example 7
A lithium ion secondary battery was produced in the same manner as in Example 1 except that the porous layer (I) of the separator faced the positive electrode in the wound electrode body.

Comparative Example 1
A battery negative electrode E was produced in the same manner as the battery negative electrode A in Example 1, except that the negative electrode material E was used instead of the negative electrode material A.

  The battery negative electrode E is used instead of the battery negative electrode A, and the battery PE microporous separator (separator B) used for the production of the separator A is used instead of the separator A. A lithium ion secondary battery was produced in the same manner as in Example 1 except that II) was used without forming.

Comparative Example 2
A lithium ion secondary battery was produced in the same manner as in Example 1 except that the separator B was used instead of the separator A.

Comparative Example 3
In the same manner as in Example 1, except that the negative electrode E for the battery was used instead of the negative electrode A for the battery, and the porous layer (I) of the separator faced the positive electrode in the wound electrode body, A secondary battery was produced.

  Table 2 summarizes the configurations of the lithium ion secondary batteries of Examples 1 to 7 and Comparative Examples 1 to 3. Note that Comparative Example 1 and Comparative Example 2 use the separator B having no porous layer (II) [that is, the separator made only of the porous layer (I)] as the separator. In the column of “layer facing the positive electrode” in Comparative Example 1, Comparative Example 1 and Comparative Example 2 are described as “porous layer (I)”.

  The following evaluation was performed about the lithium ion secondary battery of Examples 1-7 and Comparative Examples 1-3.

<Initial discharge capacity measurement>
For the lithium ion secondary batteries of Examples 1 to 7 and Comparative Examples 1 to 3, constant current-constant voltage charging (total charging) with a constant current of 0.75 A and a constant voltage of 4.2 V at room temperature (25 ° C.) (Time: 2.5 hours), followed by constant current discharge (discharge end voltage: 2.5 V) at 1.5 A (corresponding to 1 C), and the initial discharge capacity was measured.

<1C cycle test>
For the lithium ion secondary batteries of Examples 1 to 7 and Comparative Examples 1 to 3, constant current-constant voltage charging (total charging) with a constant current of 1.5 A and a constant voltage of 4.2 V at room temperature (25 ° C.) Time: 2.5 hours), and then a constant current discharge (discharge end voltage: 2.5 V) was performed at 1.5 A (corresponding to 1 C). With this as one cycle, 500 cycles of charge and discharge were repeated under the above conditions, and the capacity retention rate (100 × 500) was calculated from the capacity (discharge capacity) of the first cycle and the capacity (discharge capacity) of the 500th cycle obtained by these. Cycle capacity / cycle 1 capacity, unit%) was calculated.

<2C cycle test>
For the lithium ion secondary batteries of Examples 1 to 7 and Comparative Examples 1 to 3, constant current-constant voltage charging with a constant current of 4 A and a constant voltage of 4.2 V at room temperature (25 ° C.) (total charging time: 2.5 hours), a constant current discharge (discharge end voltage: 2.0 V) was performed at 3A (equivalent to 2C). With this as one cycle, 500 cycles of charge and discharge were repeated under the above conditions, and the capacity retention rate (100 × 500) was calculated from the capacity (discharge capacity) of the first cycle and the capacity (discharge capacity) of the 500th cycle obtained by these. Cycle capacity / cycle 1 capacity, unit%) was calculated.

<Heating test>
For the lithium ion secondary batteries of Examples 1 to 7 and Comparative Examples 1 to 3, constant current-constant voltage charging (total charging) with a constant current of 0.75 A and a constant voltage of 4.2 V at room temperature (25 ° C.) Time: 2.5 hours), each battery was put in a thermostatic bath, heated from 30 ° C. to 150 ° C. at a rate of 5 ° C. per minute, and then allowed to stand at 150 ° C. for 3 hours. The surface temperature was measured. And the presence or absence of the battery whose said battery surface temperature rose to 160 degreeC or more was investigated.

<Overcharge test>
About the lithium ion secondary battery of Examples 1-7 and Comparative Examples 1-3, after discharging a battery to 3.0V at 0.75A, normal temperature (25 degreeC) sets an upper limit voltage to 15V, and is 0.75A. Charging was performed, and the surface temperature of each battery at that time was measured. And the presence or absence of the battery whose said battery surface temperature rose to 130 degreeC or more was investigated.

  Each evaluation result is shown in Table 3. Of the data shown in Table 3, the initial discharge capacity, 1C cycle capacity retention rate, and 2C cycle capacity retention rate are all average values of the measurement results of 10 batteries. In addition, for the heating test and overcharge test, each of 10 batteries was evaluated. In the heating test, when there was even one battery whose surface temperature rose to 160 ° C. or higher, “Yes” was given. In the test, “Yes” is given when even one battery whose surface temperature has risen to 130 ° C. or higher occurs.

As is apparent from Table 3, a negative electrode containing a negative electrode material in which the surface of graphite having a specific value of water wettability is coated with a natural polysaccharide, a porous layer (I), and a porous layer (II) The lithium ion secondary batteries of Examples 1 to 7 using a separator made of a laminate having a large initial discharge capacity, a small irreversible capacity at the time of initial charge / discharge, and a 1C cycle test and a 2C cycle test Has a high capacity retention rate, and generation of irreversible capacity is suppressed even when charge and discharge are repeated, and charge and discharge cycle characteristics are good. In addition, the lithium ion secondary batteries of Examples 1 to 7 are also excellent in safety and reliability because an abnormal temperature rise in the heating test and the overcharge test is suppressed. Moreover, since the batteries of Examples 1 to 7 have a high capacity retention rate in the 2C cycle test, it can be said that the charge / discharge characteristics at a large current, that is, the load characteristics are also excellent. Further, the batteries of Examples 1 and 3 to 7 using lithium nickel oxide having a specific composition as the positive electrode active material had a larger initial discharge capacity than the battery of Example 2 using only LiCoO _2. Yes.

  On the other hand, the battery of Comparative Example 1 using the negative electrode using graphite whose surface is not coated with natural polysaccharide and the separator having no porous layer (II) has an initial discharge capacity and 1 C cycle. The capacity retention in the test and the 2C cycle test is inferior, and the temperature rises during the heating test and overcharge test. Moreover, the battery of the comparative example 2 using the separator which does not have a porous layer (II) has produced the temperature rise at the time of a heating test and an overcharge test. Furthermore, the battery of Comparative Example 3 using the negative electrode using graphite whose surface is not coated with natural polysaccharide or the like is inferior in initial discharge capacity and capacity retention in the 1C cycle test and 2C cycle test.

Claims (5)

A lithium ion secondary battery having a positive electrode, a negative electrode, a separator and a non-aqueous electrolyte,
The negative electrode contains a negative electrode material having graphite whose wettability to water is 0 to 50 ° in terms of solid-liquid surface contact angle (θ), and a natural polysaccharide or a derivative thereof covering the surface of the graphite. ,
The natural polysaccharide or derivative thereof is at least one selected from the group consisting of xanthan gum, welan gum, gellan gum, guar gum, carrageenan, dextrin, pregelatinized starch, carboxymethylcellulose, hydroxyethylcellulose and hydroxypropylcellulose;
The separator is composed of a laminate having a porous layer (I) containing a resin having a melting point of 120 to 140 ° C. and a porous layer (II) mainly containing a filler having a heat resistant temperature of 150 ° C. or higher. Lithium ion secondary battery. The lithium ion secondary battery according to claim 1, wherein the natural polysaccharide or a derivative thereof is xanthan gum. The lithium ion secondary battery according to claim 1 or 2 , wherein the negative electrode material contained in the negative electrode has a thickness of 1 to 20 nm of a natural polysaccharide or a derivative thereof covering the graphite surface. The lithium ion secondary battery according to any one of claims 1 to 3, wherein the porous layer (II) of the separator faces at least the positive electrode. The positive electrode has a general formula LiNi _(1-x) M _x O ₂ (where 0 ≦ x <1 as the positive electrode active material, where M is from Al, Mn, Co, Cr, Mg, Fe, Zr and Ti) the composed of at least one metal element selected from the group) lithium nickel oxide represented by the total positive electrode active material in any one of claims 1-4 which is contained in an amount of more than 70 wt% The lithium ion secondary battery described in 1.