JP6048312B2 - Negative electrode for lithium ion secondary battery and lithium ion secondary battery - Google Patents

Description

  The present invention relates to a negative electrode for a lithium ion secondary battery and a lithium ion secondary battery having the negative electrode.

  A lithium ion secondary battery is a secondary battery having a high charge / discharge capacity and capable of high output. At present, lithium ion secondary batteries are mainly used as a power source for portable electronic devices, and further expected as a power source for electric vehicles that are expected to be widely used in the future.

  In recent years, silicon-based materials such as silicon and silicon compounds, and tin-based materials such as tin and tin compounds, which have a charge / discharge capacity that greatly exceeds the theoretical capacity of carbon materials, have been studied as negative electrode active materials for lithium ion secondary batteries. . However, it is known that when a silicon-based material or a tin-based material is used as the negative electrode active material, the negative electrode active material expands and contracts with the insertion and extraction of lithium (Li) in the charge / discharge cycle. When the negative electrode active material expands or contracts, a load is applied to the binder that holds the negative electrode active material on the current collector, and the adhesion between the negative electrode active material and the current collector decreases. The conductive path in the electrode is destroyed, the resistance of the electrode is increased, the capacity is remarkably reduced, or the negative electrode active material is distorted by repeated expansion and contraction, and the negative electrode active material is refined and detached from the electrode. .

  Various studies have been made to enable electron transfer between the negative electrode active material layer containing the negative electrode active material and the current collector even if the negative electrode active material expands or contracts.

  For example, Patent Document 1 describes a negative electrode in which a conductive layer having a conductive agent and a binder is provided between a current collector and a negative electrode active material layer. The addition amount of the conductive agent in the conductive layer is 90% by mass to 98% by mass, and the conductive layer is intended to improve the conductivity between the negative electrode active material layer and the current collector. Graphite powder is used as the negative electrode active material, and in the technique described in Patent Document 1, a heat-resistant porous layer is disposed between the positive electrode and the negative electrode, and the heat-resistant porous layer serves as the negative electrode active material layer. It is assumed that the configuration is enclosed with the current collector.

  Patent Document 2 describes an electrode in which a conductive primer layer containing a chlorine-containing polymer is disposed between a current collector and an active material layer. In order for the electrode to have vibration resistance required for an electric device mounted on an automobile, a conductive primer layer is provided to increase the adhesion between the current collector and the active material layer. This conductive primer layer serves as a damping material that absorbs vibration. However, graphite powder is used as the negative electrode active material, and a negative electrode active material that expands and contracts with the insertion and extraction of Li is not used.

  Patent Document 3 includes an adhesion layer containing a binder and a conductive material between a negative electrode active material layer having a negative electrode active material that changes in volume with charge and discharge, and a current collector, and the binder. Discloses a negative electrode made of a modified fluorine-containing polymer compound. In the examples, it is described that the discharge capacity maintenance ratio of the third cycle is improved in the electrode having the adhesion layer as compared with the electrode without the adhesion layer, but it is difficult to say that the discharge capacity is sufficiently maintained. .

  Although various studies have been made as described above, sufficient studies have been made to enable electron transfer between the negative electrode active material layer and the current collector even if the negative electrode active material expands or contracts. Was not.

JP 2012-99385 A JP 2012-253000 A JP 2004-95264 A

  The present invention has been made in view of such circumstances, and even when a negative electrode active material that expands and contracts with the insertion and extraction of Li is used, the negative electrode active material layer, the current collector, An object of the present invention is to provide a negative electrode for a lithium ion secondary battery that is difficult to peel off and can improve storage characteristics, and a lithium ion secondary battery having the negative electrode.

  As a result of intensive studies by the present inventors, the present inventors have found a new negative electrode for lithium ion secondary batteries that can suppress an increase in resistance of the negative electrode and can improve storage characteristics. Here, the negative electrode resistance rises because the negative electrode active material layer is peeled off from the current collector body, and the negative electrode active material layer is less peeled off from the current collector body. We thought that it was possible to suppress the increase in resistance. Here, the resistance of the negative electrode was measured as an index of the difficulty of peeling the negative electrode active material layer from the current collector body.

  That is, the negative electrode for a lithium ion secondary battery of the present invention includes a current collector body, a coat layer disposed on the surface of the current collector body, and a negative electrode active material disposed on the surface of the coat layer and including a negative electrode active material The coating layer includes a coating layer binder and a conductive material. When the entire coating layer is 100% by mass, the content of the coating layer binder is more than 40% by mass and 99% by mass. %, And the negative electrode active material contains at least one selected from the group consisting of Si, Si compound, Sn and Sn compound.

The conductive material is at least one oxide selected from the group consisting of TiO ₂ , SnO ₂ , ZnO, NiO, VO ₂ , In ₂ O ₃ , CuO, Ag ₂ O, SnO, and Nb ₂ O ₅ , or these An oxide semiconductor made of a complex oxide containing an oxide is preferable.

Conductive material is preferably a SnO _2.

  The thickness of the coat layer is preferably 0.05 μm or more and 0.5 μm or less.

  The binder for the coat layer is preferably a fluorine-containing resin.

  The lithium ion secondary battery of this invention has the said negative electrode for lithium ion secondary batteries, It is characterized by the above-mentioned.

  Since the negative electrode for a lithium ion secondary battery of the present invention has a coat layer disposed on the surface of the current collector body, even when using a negative electrode active material that expands and contracts with the insertion and extraction of Li, It is possible to suppress the negative electrode active material layer from being peeled off from the current collector main body, and to prevent the negative electrode resistance from increasing, thereby improving storage characteristics.

It is a schematic cross section explaining the negative electrode for lithium ion secondary batteries of this embodiment.

<Anode for lithium ion secondary battery>
The negative electrode for a lithium ion secondary battery of the present invention is disposed on the surface opposite to the current collector body, the coat layer disposed on the surface of the current collector body, and the surface of the coat layer on the current collector body side. And a negative electrode active material layer containing a negative electrode active material.

  The current collector body refers to a chemically inert electronic high conductor that keeps a current flowing through an electrode during discharge or charging of a nonaqueous electrolyte secondary battery. Examples of materials that can be used for the current collector body include metal materials such as stainless steel, titanium, nickel, aluminum, and copper, or conductive resins. The current collector body can take the form of a foil, a sheet, a film and the like. Therefore, for example, a metal foil such as a copper foil, a nickel foil, an aluminum foil, and a stainless steel foil can be suitably used as the current collector body.

  The current collector body preferably has a thickness of 10 μm to 100 μm.

  A negative electrode active material layer contains a negative electrode active material and a binder, and contains a conductive support agent as needed.

  The negative electrode active material includes at least one selected from the group of Si, Si compound, Sn, and Sn compound.

Examples of the Si compound include SiB ₄ , SiB ₆ , Mg ₂ Si, Mg ₂ Sn, Ni ₂ Si, TiSi ₂ , MoSi ₂ , CoSi ₂ , NiSi ₂ , CaSi ₂ , CrSi ₂ , Cu ₅ Si, FeSi ₂ , MnSi ₂ , NbSi ₂ , TaSi ₂ , VSi ₂ , WSi ₂ , ZnSi ₂ , SiC, Si ₃ N ₄ , Si ₂ N ₂ O, SnSiO ₃ , LiSiO, SiO _v (0 <v ≦ 2) can be used. In particular, the Si compound is preferably SiO _x (0.5 ≦ x ≦ 1.5).

As the Sn compound, for example, SnO _w (0 <w ≦ 2), SnSiO ₃ , LiSnO, tin alloy (Cu—Sn alloy, Co—Sn alloy, etc.) can be used.

  As the negative electrode active material, in addition to at least one selected from the group consisting of Si, Si compound, Sn, and Sn compound, another negative electrode active material may be used.

  As the other negative electrode active material, a carbon-based material capable of inserting and extracting lithium, an element capable of being alloyed with lithium, an elemental compound having an element capable of being alloyed with lithium, a polymer material, or the like can be used.

  Examples of the carbon-based material include non-graphitizable carbon, artificial graphite, coke, graphite, glassy carbon, organic polymer compound fired body, carbon fiber, activated carbon, or carbon black. Here, the organic polymer compound fired body refers to a material obtained by firing and carbonizing a polymer material such as phenols and furans at an appropriate temperature.

  Elements that can be alloyed with lithium are Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Ba, Ra, Ti, Ag, Zn, Cd, Al, Ga, In, Ge, Pb, Sb. And at least one of Bi.

  As an element compound having an element that can be alloyed with lithium, for example, ZnLiAl, AlSb, or the like can be used.

  As the polymer material, polyacetylene, polypyrrole, or the like can be used.

The negative electrode active material is preferably in powder form. If the anode active material is in powder form, it is preferable that the average particle size _{D 50} of the negative electrode active material is 0.1μm or more 30μm or less. The average particle diameter D ₅₀ of the negative electrode active material 0.1μm smaller, the specific surface area of the powder of the negative electrode active material is increased, an increasing number of contact surfaces of the powder of the anode active material and the electrolyte solution, decomposition of the electrolyte solution Advances and the cycle characteristics deteriorate. Moreover, Si is an anode active material, Si compounds, Sn, since none of the Sn compound conductive poor, and the average particle diameter D ₅₀ of the negative electrode active material is larger than 30 [mu] m, the conductivity of the whole electrode becomes uneven The charge / discharge characteristics are deteriorated. The average particle diameter D ₅₀ can be measured by particle size distribution measurement method. The average particle diameter D ₅₀ refers to the particle size cumulative value of the volume distribution in the particle size distribution measurement by laser diffraction method is equivalent to 50%. That is, the average particle diameter D ₅₀ refers to the median diameter measured by volume.

  The binder plays a role of connecting the negative electrode active material and the conductive additive to the current collector body. As the binder, for example, fluorine-containing resins such as polyvinylidene fluoride, polytetrafluoroethylene and fluororubber, thermoplastic resins such as polypropylene, polyethylene and polyvinyl acetate resins, imide resins such as polyimide and polyamideimide, alkoxy Silyl group-containing resins and rubbers such as styrene butadiene rubber (SBR) can be used.

  A conductive additive is added to the negative electrode active material layer as necessary in order to increase the conductivity of the electrode. Carbon black, graphite, acetylene black (AB), ketjen black (registered trademark) (KB), vapor grown carbon fiber (VGCF), etc., which are carbonaceous fine particles, are used alone or in combination of two or more as conductive aids. Can be used. The amount of the conductive auxiliary agent used is not particularly limited, but can be, for example, about 1 part by mass to 30 parts by mass with respect to 100 parts by mass of the active material contained in the negative electrode.

  The coating layer includes a coating layer binder and a conductive material.

  The binder for the coat layer is preferably a resin resistant to an organic solvent, and is preferably durable even in a reducing atmosphere. The binder for the coat layer is preferably one having good adhesion to the current collector body. As the binder for the coat layer, for example, fluorine-containing resin, rubber, thermosetting resin, and thermoplastic resin can be used.

  Examples of the fluorine-containing resin include polytetrafluoroethylene (PTFE), tetrafluoroethylene / perfluoroalkyl vinyl ether copolymer (PFA), tetrafluoroethylene / hexafluoropropylene copolymer (FEP), and tetrafluoroethylene / ethylene. Examples thereof include a copolymer (ETFE), polyvinylidene fluoride (PVDF), and polychlorotrifluoroethylene (PCTFE).

  Examples of the rubber include natural rubber, isoprene rubber, butadiene rubber, styrene rubber, butyl rubber, nitrile rubber, acrylic rubber, urethane rubber, silicone rubber, and fluorine rubber.

  Examples of the thermosetting resin include phenol resin, melamine resin, xylene resin, and epoxy resin.

  Examples of the thermoplastic resin include polypropylene, polyethylene, and polyvinyl acetate resin.

  As the binder for the coat layer, a fluorine-containing resin is preferable. The fluorine-containing resin is highly resistant to organic solvents, hardly swells in the electrolyte, and easily maintains its shape. Therefore, even if the negative electrode active material expands or contracts, the shape of the binder for the coating layer is difficult to change, and the current collector body and the negative electrode active material layer are difficult to peel off.

  The shape of the conductive material is not particularly limited, and known shapes such as powder, fiber, plate, flat, lump, cloth, and mesh can be appropriately selected. It is particularly preferable to use a powdery conductive material. A conductive powder material that does not have a certain direction in shape is more uniform in the coating layer than a conductive material that has a certain directionality, such as a fiber, without making the coating layer unevenly conductive. Conductivity can be imparted.

Regarding the size of the conductive material, various sizes of conductive material can be used depending on the thickness of the coat layer and the shape of the conductive material. For example, if the conductive material is powdery, the average particle diameter _{D 50} can be used those 10 nm to 500 nm. If the average particle diameter D ₅₀ of the conductive material is 10nm or more, it can have an appropriate conductivity to the coating layer. The average particle diameter D ₅₀ and 500nm greater than the conductive material is not preferable because the film thickness of the coating layer becomes too thick.

  As long as the conductive material has conductivity, the degree of conductivity is not particularly limited. For example, when the conductive material is in a powder form, the conductive material preferably has a powder specific resistance of 1E + 3 Ωcm or less. The powder specific resistance is obtained by putting the powder as a sample on a sample stage and applying a certain pressure to calculate the specific resistance.

  If the conductive material is a material having high conductivity, the conductivity of the coat layer can be increased with a small amount. In addition, when the thickness of the coat layer is thin, a conductive material that is not so high in conductivity may be used, and the amount of the conductive material put into the coat layer can be reduced.

  The material used as the conductive material is not particularly limited. As the conductive material, for example, a carbon material, a metal material, or an oxide semiconductor material can be used.

  Examples of the carbon material include carbon black, graphite, acetylene black (AB), ketjen black (registered trademark) (KB), and vapor grown carbon fiber (VGCF).

  Examples of the metal material include at least one metal or alloy powder selected from the group consisting of Au, Ag, Cu, Al, Pt, Zn, Ni, Cr, Ti, Sn, and Fe.

Examples of the oxide semiconductor material include indium oxide (In ₂ O ₃ ), zinc oxide (ZnO), zinc peroxide (ZnO ₂ ), tin oxide (II) (SnO), tin oxide (IV) (SnO ₂ ), and oxidation. Tin (VI) (SnO ₃ ), titanium dioxide (TiO ₂ ), dititanium trioxide (Ti ₂ O ₃ ), ruthenium oxide (RuO ₂ ), nickel oxide (NiO), tantalum oxide (Ta ₂ O ₃ ), oxidation Tungsten (III) (W ₂ O ₃ ), tungsten oxide (IV) (WO ₂ ), tungsten oxide (VI) (WO ₃ ), chromium oxide (Cr ₂ O ₃ ), vanadium dioxide (VO ₂ ), copper oxide ( II) (CuO), silver oxide (I) (Ag ₂ O), niobium oxide (V) (Nb ₂ O ₅ ), indium oxide (In ₂ O ₃ ), Zn, Mo, W, Ti, Zr, Sn and At least one selected from H That the addition of the element, that by adding F, W, Ta, Sb, at least one element selected from P and B tin oxide _{(IV) (SnO 2),} Ga zinc oxide (ZnO), Al, and It is preferably at least one selected from those obtained by adding at least one element selected from B and those obtained by adding Nb element to titanium dioxide (TiO ₂ ).

Among indium oxide (In ₂ O ₃ ) added with at least one element selected from Zn, Mo, W, Ti, Zr, Sn, and H, indium oxide (In ₂ O ₃ ) contains Zn element or Sn element. Those added are preferred. As a material obtained by adding Zn element indium oxide _(In 2 _{O 3),} indium zinc oxide and the like, as the addition of Sn element indium oxide _(In 2 _{O 3),} it includes indium tin oxide. In ₂ O ₃ —ZnO (IZO) is preferable as the indium zinc oxide, and In ₂ O ₃ —SnO ₂ (ITO) is preferable as the indium tin oxide.

F tin oxide _{(IV) (SnO 2),} W, Ta, Sb, among those prepared by adding at least one element selected from P and B, tin oxide _{(IV) (SnO 2),} F element, Sb element , Ta elements or P elements added are preferable. As a material obtained by adding F element tin oxide _(IV) (SnO 2), include fluorine tin oxide, as the addition of Sb element tin oxide _(IV) (SnO 2), antimony tin oxide Examples of tin oxide (IV) (SnO ₂ ) added with Ta element include tantalum tin oxide, and tin oxide (IV) (SnO ₂ ) added with P element includes phosphorus tin oxide. Is mentioned. Fluorine tin oxide is preferably fluorine-added tin oxide (FTO), antimony tin oxide is preferably antimony-added tin oxide (ATO), tantalum tin oxide is preferably tantalum-added tin oxide (TaTO), and phosphorus tin oxide is preferred. Phosphorus-doped tin oxide (PTO) is preferred.

  Among those in which at least one element selected from Ga, Al and B is added to zinc oxide (ZnO), gallium zinc oxide can be cited as a material in which Ga element is added to zinc oxide (ZnO), and zinc oxide (ZnO ) Includes aluminum zinc oxide, and zinc oxide (ZnO) includes boron zinc oxide. Gallium-doped zinc oxide (GZO) is preferred as the gallium zinc oxide, aluminum-doped zinc oxide (AZO) is preferred as the aluminum zinc oxide, and boron-doped zinc oxide (BZO) is preferred as the boron zinc oxide.

An example of the titanium dioxide (TiO ₂ ) added with an Nb element is titanium niobium oxide. The titanium niobium oxide is preferably TiO ₂ ; Nb.

As the conductive material, at least one oxide selected from the group consisting of TiO ₂ , SnO ₂ , ZnO, NiO, VO ₂ , In ₂ O ₃ , CuO, Ag ₂ O, SnO, and Nb ₂ O ₅ , or these An oxide semiconductor made of a complex oxide containing an oxide is preferable.

Further, SnO ₂ is particularly preferable as the conductive material. SnO ₂ is a material that has reactivity with Li and at the same time has sufficient conductivity. SnO ₂ expands by reacting with Li during initial charging. Therefore, SnO ₂ can follow the expansion of the negative electrode active material during initial charging. Since the initial expansion of the negative electrode active material is the largest, the shape of the SnO ₂ changes so as to follow the expansion of the negative electrode active material during initial charging, so that the current collector and the negative electrode active material layer are less likely to be peeled off. Can keep the path.

As SnO _2, an average particle diameter _{D 50} is preferably used a powder of 10 nm to 100 nm. SnO ₂ can adjust the degree of volume change at the time of initial charging the average particle diameter _{D 50.} The average particle diameter D ₅₀ is suitable for use when it is desired to decrease it in this range, the thickness of the coating layer.

  Content of the binder for coat layers in a coat layer is more than 40 mass% and 99 mass% or less when the whole coat layer is 100 mass%. As for content of the binder for coat layers, it is more preferable that they are 50 to 90 mass%. The mass% at this time is a ratio (mass) when the respective materials are mixed.

  When the content of the binder for the coating layer is more than 40% by mass, the adhesion between the coating layer and the current collector body and between the coating layer and the negative electrode active material layer is increased. The coating layer, the negative electrode active material layer, the coating layer, and the current collector main body are prevented from peeling off. If content of the binder for coat layers is 99 mass% or less, content of the electroconductive material of a coat layer will become more than 1 mass%, and can provide electroconductivity to a coat layer.

  Moreover, when this content is seen by volume%, when the whole coating layer is 100 volume%, content of the binder for coating layers is more than 20 volume% and 99 volume% or less. As for content of the binder for coat layers, it is more preferable that they are 30 volume% or more and 70 volume% or less.

  The content represented by volume% at this time was calculated from the area ratio of the conductive material and the binder for the coating layer when the surface of the coating layer was observed with a scanning electron microscope (Scanning Electron Microscope) image.

  In the coating layer, when the binder for the coating layer is included within the above range, the current collecting body and the negative electrode active material layer can be satisfactorily adhered to each other while the coating layer has conductivity. When the current collector main body and the negative electrode active material layer are in good contact with each other by the coat layer, even if the negative electrode active material expands and contracts with the insertion and extraction of Li, the current collector main body extends from the negative electrode active material layer. Is difficult to peel. Although demonstrated in an Example, the storage characteristic of a lithium ion secondary battery can be improved by including the binder for coating layers in the said range in a coating layer. Since the coating layer has high adhesion, the coating layer can suppress the negative electrode active material from expanding and contracting along with the insertion and extraction of Li and peeling from the current collector body. When the negative electrode active material layer is peeled from the current collector body, the resistance of the negative electrode increases, the charge / discharge capacity decreases, and the storage characteristics deteriorate.

  The thickness of the coat layer is preferably 0.05 μm or more and 0.5 μm or less, and more preferably 100 nm to 300 nm. When the thickness of the coat layer is 0.05 μm or more, the adhesion between the current collector body and the negative electrode active material layer can be enhanced by the coat layer. If the thickness of the coat layer is 0.5 μm or less, the resistance of the negative electrode does not have to be increased even if the coat layer is disposed. Moreover, if the thickness of the coat layer is 0.5 μm or less, the thickness occupied in the battery is small, and the capacity reduction can be minimized.

  The arrangement of the conductive material and the coating layer binder in the coating layer is not particularly limited. For example, the arrangement state of the conductive material in the coat layer may be a plurality of conductive materials dispersed in the coat layer without contacting each other, or from the surface of the current collector body side of the coat layer to the negative electrode A plurality of conductive materials may be disposed in contact with each other as appropriate up to the surface on the active material layer side. Further, if there is one conductive material having a size equivalent to the thickness of the coat layer in the thickness direction of the coat layer, the single conductive material from the surface of the current collector body side of the coat layer to the negative electrode active material layer side. Electrons are transferred to the surface.

  In the coating layer, the shape of the binder for the coating layer is preferably a particulate shape having a certain size. When the shape of the binder for the coat layer is in the form of particles, the binder for the coat layer follows in the form of following the expansion and contraction of the negative electrode active material layer, and the adhesion can be further maintained.

  FIG. 1 is a schematic cross-sectional view illustrating a negative electrode for a lithium ion secondary battery according to the present invention. In FIG. 1, the coat layer 2 is disposed on the surface of the current collector body 1, and the negative electrode active material layer 3 is disposed on the surface of the coat layer 2. That is, the coat layer 2 is sandwiched between the current collector body 1 and the negative electrode active material layer 3. In the coat layer 2, the conductive material 22 is dispersed in the coat layer binder 21. By including the conductive material 22 in the coat layer 2, the resistance of the coat layer 2 can be lowered. In FIG. 1, the coat layer binder 21 is described as a film, but may be an aggregate of a plurality of particles.

  Although the method of arrange | positioning a coat layer to a collector main body is not specifically limited, It can form with the following method.

  The conductive material and the coat layer binder are dispersed in an organic solvent or water to make a paste, thereby obtaining a viscosity-adjusted coat layer forming paste. A paste for forming a coat layer is applied to the current collector body.

  As a method for applying the coating layer forming paste, a conventionally known method such as a roll coating method, a dip coating method, a doctor blade method, a spray coating method, or a curtain coating method may be used.

  As the organic solvent, ethanol, N-methyl-2-pyrrolidone (NMP), methanol, methyl isobutyl ketone (MIBK), ethyl acetate, ethylene glycol, isopropanol (IPA) and the like can be used. The water is preferably water from which impurities have been removed, such as distilled water or ion exchange water. The solvent used for the paste for forming the coat layer is preferably a different type of solvent from the solvent used for forming the negative electrode active material layer. When the same type of solvent is used, the composition of the coat layer may be eluted into the negative electrode active material layer when forming the negative electrode active material layer.

  The negative electrode can be produced as follows. A negative electrode active material layer, a binder, and, if necessary, a negative electrode active material layer-forming composition containing a conductive auxiliary agent are prepared, and an appropriate solvent is added to the composition to make a paste, and the negative electrode active material layer A forming paste is prepared. The negative electrode active material layer forming paste is applied to the surface of the current collector main body to which the coat layer forming paste is applied. The current collector body after application is dried. The negative electrode can be formed by being compressed as necessary to increase the electrode density. The drying after the coating is preferably performed after the coating layer forming paste is applied and then once dried, and after the drying, the negative electrode active material layer forming paste is applied and dried again. Alternatively, after applying the coating layer forming paste, the negative electrode active material forming paste may be continuously applied and then dried. In that case, it is preferable that the solvent used for the paste for forming the coating layer is a different type of solvent from the solvent used when forming the negative electrode active material layer.

  As a method for applying the negative electrode active material layer forming paste, a conventionally known method such as a roll coating method, a dip coating method, a doctor blade method, a spray coating method, or a curtain coating method may be used.

  As a solvent for adjusting the viscosity, N-methyl-2-pyrrolidone (NMP), methanol, methyl isobutyl ketone (MIBK) and the like can be used.

<Lithium ion secondary battery>
The lithium ion secondary battery of this invention has the said negative electrode for lithium ion secondary batteries, It is characterized by the above-mentioned. The lithium ion secondary battery which has the said negative electrode for lithium ion secondary batteries has the outstanding storage characteristic.

  The lithium ion secondary battery of the present invention includes a positive electrode, a separator, and an electrolytic solution as a battery component in addition to the above-described negative electrode for a lithium ion secondary battery.

  The positive electrode has a current collector body and a positive electrode active material layer bound to the surface of the current collector body. The positive electrode active material layer includes a positive electrode active material and a binder, and includes a conductive additive as necessary. The current collector body, the binder, and the conductive additive are the same as those described in the negative electrode for lithium ion secondary batteries.

As the positive electrode active material, a lithium-containing compound or another metal compound can be used. Examples of the lithium-containing compound, for example, lithium-cobalt composite oxide having a layered structure, the lithium nickel composite oxide having a layered structure, the lithium manganese composite oxide having a spinel structure represented by the general _{formula: Li a Co p Ni q Mn} r D _s O _x (D is a doping component, which is a component composed of Al, Mg, Ti, Sn, Zn, W, Zr, Mo, Fe, Na, and is added as necessary, p + q + r + s = 1, 0 < p <1, 0 ≦ q <1, 0 ≦ r <1, 0 ≦ s <1, 0.8 ≦ a <2.0, −0.2 ≦ x− (a + p + q + r + s) ≦ 0.2) Lithium cobalt-containing composite metal oxide having a layered structure, general formula: olivine-type lithium phosphate composite oxide represented by LiMPO ₄ (M is at least one of Mn, Fe, Co, and Ni), general formula: Fluorine olivine type lithium phosphate complex oxide represented by Li ₂ MPO ₄ F (M is at least one of Mn, Fe, Co and Ni), silicate type represented by general formula: Li ₂ MSiO ₄ A lithium composite oxide (M is at least one of Mn, Fe, Co, and Ni) can be used. Examples of other metal compounds include oxides such as titanium oxide, vanadium oxide, and manganese dioxide, and disulfides such as titanium sulfide and molybdenum sulfide.

The positive electrode active material is preferably made of a lithium-containing oxide represented by the chemical formula: LiMO ₂ (M is at least one selected from Ni, Co, and Mn), and further has a general formula: Li _a Co _{p Ni q Mn r D s O x} (D is doped component, Al, Mg, Ti, Sn , Zn, W, Zr, Mo, Fe, and a component consisting of Na, are added if necessary, p + q + r + s = 1, 0 <p <1, 0 ≦ q <1, 0 ≦ r <1, 0 ≦ s <1, 0.8 ≦ a <2.0, −0.2 ≦ x− (a + p + q + r + s) ≦ 0. It is preferably made of a lithium cobalt-containing composite metal oxide having a layered structure represented by 2).

Examples of the lithium-containing oxide include LiCo _1/3 Ni _1/3 Mn _1/3 O ₂ , LiNi _0.6 Co _0.2 Mn _0.2 O ₂ , LiNi _0.5 Co _0.2 Mn _{0. 3} O ₂ , LiCoO ₂ , LiNi _0.8 Co _0.2 O ₂ , LiCoMnO ₂ can be used. Among these, LiCo _1/3 Ni _1/3 Mn _1/3 O ₂ and LiNi _0.5 Co _0.2 Mn _0.3 O ₂ are preferable in terms of thermal stability.

The positive electrode active material preferably has an average particle diameter _{D 50} is a powder shape is 1 m to 20 m. The average particle diameter D ₅₀ of the positive electrode active material increases the reaction area of 1μm smaller than the specific surface area of the cathode active material increases the positive electrode active material and the electrolyte. When the average particle diameter D ₅₀ of the positive electrode active material is larger than 20 μm, the resistance of the lithium ion secondary battery is increased, and the output characteristics of the lithium ion secondary battery are lowered.

  The separator separates the positive electrode and the negative electrode and allows lithium ions to pass through while preventing a short circuit of current due to contact between the two electrodes. As the separator, for example, a porous film made of synthetic resin such as polytetrafluoroethylene, polypropylene, or polyethylene, or a porous film made of ceramics can be used.

  As the electrolytic solution, an electrolytic solution that can be used for a lithium ion secondary battery can be used. The electrolytic solution includes a solvent and an electrolyte dissolved in the solvent.

  For example, cyclic esters, chain esters, and ethers can be used as the solvent. Examples of cyclic esters include ethylene carbonate, propylene carbonate, butylene carbonate, gamma butyrolactone, vinylene carbonate, 2-methyl-gamma butyrolactone, acetyl-gamma butyrolactone, and gamma valerolactone. Examples of chain esters include dimethyl carbonate, diethyl carbonate, dibutyl carbonate, dipropyl carbonate, methyl ethyl carbonate, propionic acid alkyl ester, malonic acid dialkyl ester, and acetic acid alkyl ester. Examples of ethers that can be used include tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1,2-dibutoxyethane, and the like.

As the electrolyte dissolved in the electrolytic solution, for example, a lithium salt such as LiClO ₄ , LiAsF ₆ , LiPF ₆ , LiBF ₄ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ can be used.

For example, as an electrolytic solution, 0.5 mol / l to 1.7 mol / l of a lithium salt such as LiClO ₄ , LiPF ₆ , LiBF ₄ , LiCF ₃ SO _{3 in} a solvent such as ethylene carbonate, dimethyl carbonate, propylene carbonate, and dimethyl carbonate. A solution dissolved at a certain concentration can be used.

  The lithium ion secondary battery can be mounted on a vehicle. Since the lithium ion secondary battery has excellent cycle performance, a vehicle equipped with the lithium ion secondary battery has high performance in terms of life and output.

  The vehicle may be a vehicle that uses electric energy from a battery as a whole or a part of a power source. For example, an electric vehicle, a hybrid vehicle, a plug-in hybrid vehicle, a hybrid railway vehicle, an electric forklift, an electric wheelchair, and an electric assist. Bicycles and electric motorcycles are examples.

  As mentioned above, although embodiment of the negative electrode for lithium ion secondary batteries and lithium ion secondary battery of this invention was described, this invention is not limited to the said embodiment. The present invention can be implemented in various forms without departing from the gist of the present invention, with modifications and improvements that can be made by those skilled in the art.

  Hereinafter, the present invention will be described in more detail with reference to examples.

<Formation of coat layer on current collector body>
A copper foil having a thickness of 20 μm was prepared as a current collector body.

As the conductive material, SnO ₂ powder having an average particle diameter D ₅₀ of 20 nm and PTFE as a binder for the coating layer were prepared. Moreover, ion-exchange water and isopropanol (IPA) were prepared as viscosity adjusting solvents.

(Current collector A)
The mass ratio of SnO ₂ : PTFE was 25:75, and ion-exchanged water and IPA were added to prepare a slurry-like paste for forming a coat layer. The coating layer forming paste was placed on a copper foil and applied using a doctor blade so that the film thickness of the slurry was 200 nm. Thereafter, the copper foil coated with the coating layer forming paste was heated in air at 120 ° C. for 10 minutes to volatilize moisture and IPA. This was designated as current collector A.

(Current collector B)
A current collector B was produced in the same manner as the current collector A except that the mass ratio of SnO ₂ : PTFE was 10:90.

(Current collector C)
A current collector C was produced in the same manner as the current collector A except that the mass ratio of SnO ₂ : PTFE was 50:50.

(Current collector D)
A current collector D was produced in the same manner as the current collector A except that the mass ratio of SnO ₂ : PTFE was 80:20.

(Current collector E)
A copper foil itself was used as a current collector E without forming a coat layer.

<Production of laminated lithium-ion secondary battery>
Example 1
The laminated lithium ion secondary battery of Example 1 was produced as follows.

(Preparation of negative electrode)
As the negative electrode active material, SiO (manufactured by Aldrich) having an average particle diameter D ₅₀ of 4 μm and graphite (natural graphite (manufactured by Hitachi Chemical Co., Ltd.) having an average particle diameter D ₅₀ of 20 μm) were prepared. Alkoxy group-containing silane-modified polyamideimide resin as binder resin (Arakawa Chemical Industries, Ltd., trade name Composeran, product number H900-2, solvent composition: N-methylpyrrolidone (NMP) / xylene (Xyl), curing residue 30%, A viscosity of 8000 mPa · s, silica in the cured residue, 4 wt%, and the cured residue means a solid content obtained by curing the resin and excluding volatile components). KB (Ketjen Black) manufactured by Ketjen Black International Co., Ltd. was prepared as a conductive aid.

  The negative electrode active material, the conductive auxiliary agent, and the binder resin were mixed at a mass ratio of SiO: graphite: conductive auxiliary agent: binder resin = 32: 50: 3: 15. An appropriate amount of N-methyl-2-pyrrolidone (NMP) as a solvent was added to the above mixture to prepare a slurry, and a slurry for a negative electrode active material layer was prepared.

The negative electrode active material layer slurry was placed on the current collector A, and applied to a film using a doctor blade. The current collector A coated with the negative electrode active material layer slurry was dried at 80 ° C. for 20 minutes to volatilize and remove NMP, and then tightly bonded with a roll press. At this time, the density of the negative electrode active material layer was set to 1.6 g / cm ² . The joined product was heated with a vacuum dryer at 200 ° C. for 2 hours, cut into a predetermined shape (26 mm × 31 mm rectangular shape), and a negative electrode having a negative electrode active material layer thickness of 20 μm.

(Creation of positive electrode)
88 parts by mass, 6 parts by mass, and 6 parts by mass of LiNi _0.5 Co _0.2 Mn _0.3 O ₂ as a positive electrode active material, acetylene black as a conductive additive, and polyvinylidene fluoride (PVDF) as a binder, respectively. Mass parts were mixed, and this mixture was dispersed in an appropriate amount of N-methyl-2-pyrrolidone (NMP) to prepare a positive electrode active material layer slurry.

A 20 μm aluminum foil was prepared as a positive electrode current collector, and a slurry for a positive electrode active material layer was placed on the aluminum foil and applied to form a film using a doctor blade. The aluminum foil after the application of the positive electrode active material layer slurry was dried at 80 ° C. for 20 minutes to volatilize and remove NMP, and then tightly bonded with a roll press. At this time, the density of the positive electrode active material layer was set to 3.2 g / cm ² . The joined product was heated at 120 ° C. for 6 hours with a vacuum dryer, cut into a predetermined shape (rectangular shape of 25 mm × 30 mm), and formed into a positive electrode having a positive electrode active material layer having a thickness of about 45 μm.

A laminate type lithium ion secondary battery was manufactured using the positive electrode and the negative electrode. Specifically, a rectangular sheet (27 × 32 mm, thickness 25 μm) made of polypropylene resin as a separator was sandwiched between the positive electrode and the negative electrode to form an electrode plate group. The electrode plate group was covered with a set of two laminated films, and the three sides were sealed, and then an electrolyte solution was injected into the bag-like laminated film. As an electrolyte, 1 mol of LiPF ₆ was added to a solvent obtained by mixing ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in EC: EMC: DMC = 3: 3: 4 (volume ratio). A dissolved solution was used. Thereafter, the remaining one side was sealed to obtain a laminate type lithium ion secondary battery in which the four sides were hermetically sealed and the electrode plate group and the electrolyte were sealed. Note that the positive electrode and the negative electrode have a tab that can be electrically connected to the outside, and a part of the tab extends to the outside of the laminated lithium ion secondary battery. The laminated lithium ion secondary battery of Example 1 was produced through the above steps.

(Example 2)
A laminated lithium ion secondary battery of Example 2 was produced in the same manner as in Example 1 except that the current collector B was used instead of the current collector A in Example 1.

Example 3
A laminated lithium ion secondary battery of Example 3 was produced in the same manner as in Example 1 except that the current collector C was used instead of the current collector A in Example 1.

(Comparative Example 1)
A laminated lithium ion secondary battery of Comparative Example 1 was produced in the same manner as in Example 1 except that the current collector D was used instead of the current collector A in Example 1.

(Comparative Example 2)
A laminated lithium ion secondary battery of Comparative Example 2 was produced in the same manner as in Example 1 except that the current collector E was used instead of the current collector A in Example 1.

<Initial capacity measurement>
The initial capacities of the laminated lithium ion secondary batteries of Example 1, Example 2, Example 3, Comparative Example 1 and Comparative Example 2 were measured. At the time of charging, CC charging (constant current constant voltage charging) was performed up to a voltage of 0.33 C and a voltage of 4.5 V at 25 ° C. CV charging was held at a voltage of 4.5 V for 2.5 hours. When discharging, CC discharge (constant current discharge) was performed at a rate of 0.33 C up to 2.5V. The total charge / discharge time was 8.5 hours. The discharge capacity at this time was measured and used as the initial capacity.

<Cell resistance evaluation>
The cell resistances of the laminated lithium ion secondary batteries of Examples 1 to 3 and Comparative Examples 1 and 2 were measured. The cell resistance (Ω) was measured at 3C rate for 10 seconds at a voltage (3.6 V) at 20% SOC (State of charge). A smaller measured value of cell resistance indicates less internal loss of the lithium ion secondary battery. Further, since the cell resistance is measured at a 3C rate, the measured value of the cell resistance is also an index indicating a high rate characteristic.

<60 ° C storage test>
Using the laminated lithium ion secondary batteries of Examples 1 to 3 and Comparative Examples 1 to 2, a 60 ° C. storage test was performed. In this example, the laminate type lithium ion secondary batteries of Examples 1 to 3 and Comparative Examples 1 to 2 were subjected to a charge / discharge test once under the following conditions, and the recharged battery was left at 60 ° C. for 1 week. Charging was performed at 60 ° C. at 90% SOC (State of charge) (4.3 V), and CCCV charging (constant current constant voltage charging) was performed at a 1 C rate. The voltage was held for 1 hour. When discharging, CC discharge (constant current discharge) was performed at a rate of 2.5 V and 0.33 C.

  Here, since the storage test is stored in a charged state, Si contained in the negative electrode active material is stored in an expanded state. In such a state, the laminate type lithium ion secondary battery causes a side reaction between the electrolytic solution and the active material, and is easily self-discharged.

  The discharge capacity and the cell resistance after the storage test were measured by the above measuring methods, and were defined as the capacity after storage and the resistance after storage.

  The results are shown in Table 1. The capacity retention rate (%) was calculated by the formula of discharge capacity after storage test / initial capacity × 100. The resistance change rate (%) was calculated by the equation of resistance after storage / initial resistance × 100.

  As can be seen from Table 1, Example 1, Example 2 and Example using a negative electrode having a PTFE content in the coat layer of 50% by mass to 90% by mass and 30% by volume to 70% by volume by mass%. The laminate type lithium ion secondary battery of Example 3 has a significantly lower resistance after storage than the laminate type lithium ion secondary batteries of Comparative Example 1 and Comparative Example 2. Example 1, Example 2 and Example The capacity retention rate of the laminated lithium ion secondary battery of Example 3 was also significantly higher than the capacity retention rates of the laminated lithium ion secondary batteries of Comparative Example 1 and Comparative Example 2. The laminate type lithium ion secondary battery of Comparative Example 1 using a negative electrode in which the content of the binder for the coat layer in the coat layer is 20% by mass and 10% by volume is 10% by mass, and the resistance after storage becomes high. It was found that the capacity decreased after storage.

  Regarding the resistance, a low resistance indicates that the negative electrode active material layer and the current collector main body are in close contact with each other by the coating layer and are not peeled off. Therefore, the post-storage resistance of the laminate type lithium ion secondary battery of Comparative Example 1 is rapidly increased compared with the post-storage resistance of the laminate type lithium ion secondary battery of Example 3, and the high voltage is used at 60 ° C. for one week. It is considered that the negative electrode active material layer of the negative electrode of the laminate type lithium ion secondary battery of Comparative Example 1 was peeled off from the coat layer by a storage test under conditions under the environment.

  That is, if a negative electrode having a coating layer binder content of 20% by mass higher than 20% by mass in the coating layer, preferably a negative electrode having a coating layer binder content of 50% by mass to 90% by mass, high voltage is used. It was found that the negative electrode active material layer is less likely to be peeled off from the coat layer even under the use environment, and the negative electrode resistance can be prevented from increasing, and thus the decrease in capacity after storage can be suppressed.

  1: current collector body, 2: coat layer, 21: binder for coat layer, 22: conductive material, 3: negative electrode active material layer.

Claims (3)

A current collector body;
A coat layer disposed on the surface of the current collector body;
A negative electrode active material layer disposed on the surface of the coat layer and including a negative electrode active material and a binder ,
The coating layer includes a coating layer binder and a conductive material,
When the entire coating layer is 100% by mass, the content of the coating layer binder is more than 40% by mass and 99% by mass or less,
The negative active material Si, Si compounds, see containing at least one selected from the group of Sn and Sn compounds,
The binder is a polyamideimide resin,
The binder for the coat layer is polytetrafluoroethylene,
The negative electrode for a lithium ion secondary battery, wherein the conductive material is SnO _2. The negative electrode for a lithium ion secondary battery according to claim 1, wherein the coat layer has a thickness of 0.05 μm or more and 0.5 μm or less. The lithium ion secondary battery which has a negative electrode for lithium ion secondary batteries of Claim 1 or 2 .

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