JP2015011922A - Negative electrode for lithium ion secondary battery and lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a negative electrode for a lithium ion secondary battery and a lithium ion secondary battery, exhibiting excellent cycle characteristics and excellent charge/discharge capacity when used in high temperatures and high voltages.SOLUTION: A negative electrode for a lithium ion secondary battery includes: a collector; a negative electrode active material layer disposed on a surface of the collector and containing a negative electrode active material and a binder; and a coating layer disposed on a surface of the negative electrode active material layer. The coating layer contains insulating inorganic powder and a coating binder. When a copper foil is pressed to a surface of the coating layer at a pressure of 1.96×10N/φ16 mm to measure an electric resistance between the copper foil and the collector, the value of resistance is higher than 3×10Ω/φ16 mm and 2×10Ω/φ16 mm or less.

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.

In recent years, along with the development of portable electronic devices such as mobile phones and notebook computers, and the practical application of electric vehicles, secondary batteries with small and light weight and high capacity are required. Currently, lithium ion secondary batteries using lithium cobalt oxide (LiCoO ₂ ) as a positive electrode material and a carbon-based material as a negative electrode material are commercialized as high capacity secondary batteries.

  In order to improve the safety of the lithium ion secondary battery, it has been studied to prevent an internal short circuit by providing an insulating layer on the surface of the negative electrode active material layer. However, the provision of the insulating layer increases the electrode resistance and decreases the battery capacity. Therefore, it has been studied to suppress an increase in internal resistance due to the insulating layer. Patent Document 1 discloses a technique for suppressing an increase in internal resistance by providing an insulating layer on a negative electrode active material layer so that the negative electrode active material layer has an exposed portion. Here, in the technique described in Patent Document 1, an increase in internal resistance due to the insulating layer is suppressed in order to improve rapid discharge performance in a low temperature environment.

  By the way, when the lithium ion secondary battery is driven at a high temperature and a high voltage, there is a problem that the cycle characteristics are extremely deteriorated. Therefore, there is a demand for a lithium ion secondary battery that exhibits high performance even under high temperature and high voltage use environments. In this specification, use at a voltage of 4.3 V or higher is defined as high voltage use.

  However, for lithium ion secondary batteries in which an insulating layer is provided on the negative electrode active material layer to prevent internal short-circuiting, it has not been studied so far to suppress deterioration of cycle characteristics under high temperature and high voltage use environment. It was.

JP 2010-192365 A

  The present invention has been made in view of such circumstances, and a lithium ion secondary battery provided with an insulating layer on the negative electrode active material layer to prevent internal short circuit is used in a high temperature, high voltage use environment. The purpose is to exhibit excellent cycle characteristics even underneath.

  As a result of intensive studies by the present inventors, by arranging a coating layer having an insulating inorganic powder and a coating layer binder on the surface of the negative electrode active material layer, the resistance value of the negative electrode is set within a certain range. It has been found that a negative electrode for a lithium ion secondary battery having excellent cycle characteristics in a high temperature and high voltage use environment can be obtained.

That is, the negative electrode for a lithium ion secondary battery of the present invention is disposed on the surface of the current collector, the current collector, the negative electrode active material layer including the negative electrode active material and the binder, and the surface of the negative electrode active material layer. A coating layer disposed on the surface of the coating layer at a pressure of 1.96 × 10 ² N / φ16 mm. The coating layer includes an insulating inorganic powder and a binder for the coating layer. When the electric resistance between the copper foil and the current collector is measured by pressing the foil, the resistance value is greater than 3 × 10 ³ Ω / φ16 mm and 2 × 10 ⁶ Ω / φ16 mm or less. .

The resistance value is preferably 6 × 10 ³ Ω / φ16 mm or more and 1.5 × 10 ⁶ Ω / φ16 mm or less.

  The thickness of the coating layer is preferably 2 μm or more and 20% or less of the thickness of the negative electrode active material layer.

It is preferable that the average particle size _{D 50} of the inorganic powder is 100nm or more 1μm or less.

  The inorganic powder is preferably made of at least one selected from boron nitride, aluminum nitride, and silicon nitride.

  The coating layer binder is preferably an inorganic binder containing at least one selected from silicon element, aluminum element and zirconium element.

  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.

  In the negative electrode for a lithium ion secondary battery according to the present invention, a coating layer having an insulating inorganic powder and a coating layer binder is disposed on the surface of the negative electrode active material layer. The surface of is protected. The lithium ion secondary battery of the present invention having such a negative electrode can exhibit excellent cycle characteristics even in a high temperature and high voltage use environment.

It is a schematic diagram explaining the negative electrode for lithium ion secondary batteries of this embodiment. 3 is a graph showing cycle test results of laminated lithium ion secondary batteries of Example 1, Example 2, Comparative Example 1 and Comparative Example 2. FIG.

<Anode for lithium ion secondary battery>
The negative electrode for a lithium ion secondary battery of the present invention is disposed on the surface of a current collector, a negative electrode active material layer including a negative electrode active material and a binder, and disposed on the surface of the current collector. The coating layer has an insulating inorganic powder and a binder for the coating layer.

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

  As the negative electrode active material, a carbon-based material that can occlude and release lithium, an element that can be alloyed with lithium, a compound that includes an element that can be 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, Si, Ge, Sn. , Pb, Sb, Bi. Among them, the element that can be alloyed with lithium is preferably silicon (Si) or tin (Sn).

Examples of the compound having an element that can be alloyed with lithium include ZnLiAl, AlSb, SiB ₄ , SiB ₆ , Mg ₂ Si, Mg ₂ Sn, Ni ₂ Si, TiSi ₂ , MoSi ₂ , CoSi ₂ , NiSi ₂ , and 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, SiO v (0 <v ≦ 2), SnO _w (0 <w ≦ 2), SnSiO ₃ , LiSiO or LiSnO can be used. As the compound having an element that can be alloyed with lithium, a silicon compound or a tin compound is preferable. As the silicon compound, SiO _x (0.5 ≦ x ≦ 1.5) is preferable. As the tin compound, for example, a tin alloy (Cu—Sn alloy, Co—Sn alloy, etc.) is preferable.

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

The negative electrode active material is preferably in powder form. When the negative electrode active material is in a powder form, the average particle diameter D ₅₀ of the negative electrode active material is preferably 0.1 μm or more and 30 μm or less, and more preferably 0.2 μm or more and 20 μm 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, the contact area of the powder of the anode active material and the electrolyte solution is increased, the decomposition of the electrolyte solution Advances and the cycle characteristics deteriorate. Further, 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, charging and discharging characteristics are degraded. The average particle diameter D ₅₀ can be measured by particle size distribution measurement method. The average particle diameter D ₅₀ is that 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 means the median size measured by volume.

  The binder plays a role of connecting the negative electrode active material and the conductive additive to the current collector. 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.

  In order to dispose the negative electrode active material layer on the surface of the current collector, a composition for forming a negative electrode active material layer containing a negative electrode active material, a binder, and, if necessary, a conductive additive is prepared. An appropriate solvent is added to form a paste, which is then applied to the surface of the current collector and then dried. In addition, you may compress the electrical power collector in which the negative electrode active material layer is arrange | positioned so that an electrode density may be raised as needed.

  As a method for applying the composition for forming a negative electrode active material layer, 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.

  The coating layer is disposed on the surface of the negative electrode active material layer. Since the surface of the negative electrode active material layer is covered with the coating layer, the negative electrode active material is difficult to directly contact with the electrolytic solution. Therefore, the decomposition reaction of the electrolytic solution by the negative electrode active material is suppressed. Therefore, it can suppress that cycling characteristics deteriorate. Moreover, since the eluate of the metal component contained in the electrolytic solution and the decomposed product of the electrolytic solution can be physically trapped by the coating layer, it is possible to suppress the deposition of the decomposed material on the surface of the negative electrode active material. As a result, deterioration of the cycle characteristics of the lithium ion secondary battery can be suppressed.

The negative electrode for a lithium ion secondary battery of the present invention measures the electrical resistance between the copper foil and the current collector by pressing the copper foil against the surface of the coating layer with a pressure of 1.96 × 10 ² N / φ16 mm. In this case, the resistance value is larger than 3 × 10 ³ Ω / φ16 mm and equal to or less than 2 × 10 ⁶ Ω / φ16 mm. The resistance value is preferably 6 × 10 ³ Ω / φ16 mm or more and 1.5 × 10 ⁶ Ω / φ16 mm or less.

Moreover, if the said resistance value exists in the said range, deterioration of cycling characteristics when it is set as a lithium ion secondary battery can be suppressed. When the resistance value is larger than 3 × 10 ³ Ω / φ16 mm, the negative electrode active material layer is well protected by the coating layer, and decomposition of the electrolytic solution by the negative electrode active material is suppressed. Therefore, deterioration of cycle characteristics can be suppressed.

When the resistance value is 2 × 10 ⁶ Ω / φ16 mm or less, the cell resistance (Ω) does not increase so much when a lithium ion secondary battery is used. The cell resistance is, for example, a value measured at a 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. If the cell resistance does not become too large, the charge / discharge capacity of the lithium ion secondary battery is unlikely to decrease.

  The resistance value may be a value obtained by measuring the negative electrode immediately after the production of the negative electrode for a lithium ion secondary battery, or may be a value obtained by measuring the negative electrode of a charged / discharged lithium ion secondary battery. When measuring the resistance value of the negative electrode of a charged and discharged lithium ion secondary battery, disassemble the battery and take out the negative electrode, and after washing the negative electrode with a solvent that does not affect the negative electrode active material layer or the coating layer, such as dimethyl carbonate, After drying, measurement may be performed in the same manner as described above.

  The thickness of the coating layer is preferably 2 μm or more and 20% or less of the thickness of the negative electrode active material layer. When the thickness of the coating layer is less than 2 μm, there is a possibility that deterioration of the cycle characteristics of the lithium ion secondary battery cannot be suppressed under a high temperature and high voltage use environment. When the thickness of the coating layer exceeds 20% of the thickness of the negative electrode active material layer, the charge / discharge capacity of the lithium ion secondary battery may be reduced due to a decrease in the mass ratio of the negative electrode active material in the negative electrode.

  The coating layer includes an insulating inorganic powder and a binder for the coating layer. By arranging the coating layer containing the insulating inorganic powder on the surface of the negative electrode active material layer, it is possible to suppress a short circuit between the positive electrode and the negative electrode.

  Moreover, the binder for coating layers binds inorganic powders and inorganic powder and a negative electrode active material layer. Therefore, the coating layer is difficult to peel from the negative electrode active material layer. Therefore, the effect of the coating layer is likely to persist even after repeated cycles.

  The content of the binder for the coating layer in the coating layer is preferably 3 parts by mass or more and 50 parts by mass or less with respect to 100 parts by mass of the inorganic powder. When the content of the binder for the coating layer is excessive, the protective effect of the negative electrode active material layer by the inorganic powder is reduced. When the content of the binder for the coating layer is too small, it becomes difficult to obtain the binding effect of the binder for the coating layer.

  As the insulating inorganic powder, for example, a nitride, a carbide, or a metal oxide can be used. Examples of the nitride include boron nitride, aluminum nitride, silicon nitride, and carbon nitride. Examples of the carbide include silicon carbide and boron carbide. Examples of the metal oxide include silica, alumina, magnesium oxide, zirconium oxide, and hafnium oxide.

  In particular, the inorganic powder is preferably thermally stable. For this reason, the inorganic powder is preferably a nitride. In general, nitride is thermally stable up to about 1500 ° C. Since 1500 ° C. is a temperature sufficiently higher than the thermal runaway range of the lithium ion secondary battery, the nitride exists stably even in the temperature range where the lithium ion secondary battery runs out of heat.

  In general, nitrides have high chemical stability and do not cause excessive side reactions accompanying battery reactions. Among them, boron nitride, aluminum nitride, and silicon nitride are highly insulating, electrically inactive, and highly resistant to organic solvents. Therefore, the inorganic powder is at least selected from boron nitride, aluminum nitride, and silicon nitride. It preferably consists of one.

The average particle diameter D ₅₀ of the inorganic powder is preferably smaller than the average particle diameter D ₅₀ of the negative electrode active material. When the average particle diameter D ₅₀ of the inorganic powder is smaller than the average particle diameter D ₅₀ of the negative electrode active material, the inorganic powder is easily disposed along the unevenness of the surface of the negative electrode active material layer is present on the surface of the anode active material layer The surface of the negative electrode active material is easily covered with a coating layer containing inorganic powder. If the surface of the negative electrode active material is coated with a coating layer containing an inorganic powder, the negative electrode active material can be prevented from coming into direct contact with the electrolytic solution, and the electrolytic solution can be prevented from being decomposed by the negative electrode active material.

Inorganic powder preferably has an average particle diameter _{D 50} is 100nm or more 1μm or less, and more preferably 200nm or more 800nm or less. When the average particle diameter D ₅₀ of the inorganic powder is 100nm or 1μm or less, the coating layer containing an inorganic powder is advantageous to reliably cover the surface of the negative electrode active material layer.

  As the binder for the coating layer, an inorganic binder containing an element such as silicon element, aluminum element, zirconium element, phenol resin, polyimide resin, epoxy resin, or melamine resin can be used.

  In order to easily form the coating layer, it is preferable to use a binder that can be dissolved in an organic solvent and solidifies at room temperature when the organic solvent volatilizes. When such a binder is used, it is not necessary to heat the electrode in order to form the coating layer. For example, an inorganic binder that can be solidified at room temperature is preferably used as the binder for the coating layer.

  In addition, since the inorganic binder is chemically stable, the decomposition reaction of the electrolytic solution accompanying the battery reaction can be stably suppressed. The binder for the coating layer is preferably an inorganic binder containing at least one of silicon element, aluminum element and zirconium element. Moreover, as an inorganic binder, the inorganic binder containing a zirconium element, especially a zirconium oxide can be used preferably.

  As the inorganic binder, for example, metal alkoxide and phosphate can be used. Examples of the metal alkoxide include tetraethoxysilane, tetrabutoxysilane, tetrapropoxysilane, tetramethoxysilane, polymethoxysiloxane, aluminum triethoxide, aluminum isopropoxide, aluminum butoxide, tetra-n-butoxyzirconium, tetra-n. -Propoxy zirconium, tributoxy monoacetonato zirconium, octoxy tridecoxy zirconium, monobutoxy zirconium (IV) tri-i-stearate, tributoxy monoacetonato zirconium. Examples of the phosphate include aluminum phosphate and zirconium phosphate.

  Metal alkoxide is easily hydrolyzed and solidified by moisture in the atmosphere and moisture contained in the solvent. Therefore, if a metal alkoxide is used as the binder for the coating layer, the coating layer can be easily formed.

  The coating layer is disposed on the surface of the negative electrode active material layer. In order to prevent the coating layer and the negative electrode active material layer from being mixed, the coating layer is desirably disposed after the negative electrode active material layer is completed.

  The method for disposing the coating layer on the negative electrode active material layer is not particularly limited. For example, the coating layer can be disposed on the negative electrode active material layer by the following method. Dissolving the material of the coating layer in an organic solvent to create a solution, spraying the coating surface of the negative electrode active material layer using a sprayer, volatilizing and removing the organic solvent, and placing the coating layer on the negative electrode active material layer Can do. In this case, ethanol, N-methyl-2-pyrrolidone (NMP), methanol, butanol, methyl isobutyl ketone (MIBK), or the like can be used as the organic solvent.

  Also, the material of the coating layer is dissolved in an organic solvent for viscosity adjustment to create a paste-like mixture, the paste-like mixture is applied on the negative electrode active material layer, and dried after application to the negative electrode active material layer. A coating layer can be disposed. As a coating method, 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 for adjusting the viscosity, ethanol, NMP, methanol, butanol, MIBK and the like can be used.

  FIG. 1 is a schematic diagram illustrating a negative electrode for a lithium ion secondary battery according to this embodiment. In FIG. 1, a negative electrode active material 3 is bound on a current collector 1 by a binder 2. The negative electrode active material layer 4 includes a negative electrode active material 3 and a binder 2. The coating layer 5 is disposed on the negative electrode active material layer 4.

In the coating layer 5 of FIG. 1, the plurality of inorganic powders 51 are arranged along the irregularities on the surface of the negative electrode active material layer 4, and the coating layer binder 52 is between the inorganic powders 51 and between the inorganic powder 51 and the negative electrode active material layer. 4 is arranged. The plurality of inorganic powders 51 or the inorganic powder 51 and the negative electrode active material layer 4 are bound together by the coating layer binder 52. The negative electrode for a lithium ion secondary battery of the present embodiment presses the copper foil against the surface of the coating layer with a pressure of 1.96 × 10 ² N / φ16 mm, thereby reducing the electrical resistance between the copper foil and the current collector. When measured, the resistance value is greater than 3 × 10 ³ Ω / φ16 mm and less than or equal to 2 × 10 ⁶ Ω / φ16 mm. As shown in FIG. 1, the coating layer 5 does not have such a large gap that makes the resistance value lower than the above range.

<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 having the negative electrode for a lithium ion secondary battery has excellent cycle performance even in a high temperature and high voltage use environment.

  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 and a positive electrode active material layer bound to the surface of the current collector. 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, binder, and 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 at least one selected from Al, Mg, Ti, Sn, Zn, W, Zr, Mo, Fe and Na, 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) things, the general formula: olivine-type lithium phosphate compound oxide represented by LiMPO ₄ (at least one of M is Mn, Fe, Co and Ni), the general _formula: represented by Li 2 MPO ₄ F Fluoride olivine-type lithium phosphate compound oxide (M is Mn, Fe, at least one of Co and Ni), the general _formula: Li 2 MSiO silicate lithium composite oxide represented by ₄ (M is _{one or more of} Mn , Fe, Co, and Ni). Examples of other metal compounds include oxides such as titanium oxide, vanadium oxide, and manganese dioxide, and sulfides 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, Al, Mg, Ti, Sn , Zn, W, Zr, Mo, at least one, p + q + r + s = 1,0 <p <1,0 ≦ selected from Fe and Na Lithium having a layered structure represented by q <1, 0 ≦ r <1, 0 ≦ s <1, 0.8 ≦ a <2.0, −0.2 ≦ x− (a + p + q + r + s) ≦ 0.2) It is preferably made of a cobalt-containing composite metal oxide.

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. The average particle diameter D ₅₀ of the positive electrode active material becomes large 20μm greater than the resistance of the lithium ion secondary battery, there is a possibility that the output characteristics of the lithium ion secondary battery decreases.

  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.

  The electrolytic solution includes a solvent and an electrolyte dissolved in the solvent.

  As the solvent, for example, cyclic esters, chain esters, and ethers can be used. 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 the 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, and 1,2-dibutoxyethane.

Moreover, as an 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.

As an electrolytic solution, for example, a lithium salt such as LiClO ₄ , LiPF ₆ , LiBF ₄ , LiCF ₃ SO _{3 is} added to a solvent such as ethylene carbonate, dimethyl carbonate, propylene carbonate, dimethyl carbonate, and the like from 0.5 mol / l to 1.7 mol / l. 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.

<Creation of negative electrode for lithium ion secondary battery>
(Negative electrode A)
97 parts by mass of graphite powder having an average particle diameter D ₅₀ of 10 μm, 1 part by mass of acetylene black as a conductive additive, 1 part by mass of styrene-butadiene rubber (SBR) and 1 part by mass of carboxymethyl cellulose (CMC) as binders To make a mixture. This mixture was dispersed in an appropriate amount of ion-exchanged water to prepare a slurry. This slurry was applied to a copper foil having a thickness of 20 μm, which is a negative electrode current collector, in a film shape using a doctor blade. The current collector coated with the slurry was dried and pressed, and the bonded product was heated with a vacuum dryer at 120 ° C. for 6 hours. A negative electrode A was obtained by cutting it into a predetermined shape (rectangular shape of 25 mm × 30 mm). The thickness of the negative electrode active material layer of the negative electrode A was about 50 μm.

(Negative electrode B)
As insulating inorganic powder, the average particle diameter D ₅₀ was prepared powder of boron nitride 500 nm. Tetra-n-butoxyzirconium was prepared as a binder for the coating layer. Boron nitride powder and tetra-n-butoxyzirconium dissolved in butanol were weighed to a mass ratio of 3: 2 and mixed to obtain a mixture. The mixture dispersed in butanol as described above was sprayed on the surface of the negative electrode A using the sprayer as the number of sprays was one. The spraying time for one spraying was 5 seconds per unit area in the spraying range. Butanol was volatilized and removed, and a coating layer having a thickness of 1 μm composed of boron nitride powder and a coating layer binder was formed on the surface of the negative electrode A by hydrolysis of metal alkoxide. This is designated as negative electrode B. When the surface of the negative electrode B was observed with a scanning electron microscope (SEM), it was confirmed that the coating layer was disposed along the surface of the negative electrode active material layer. This is an average particle diameter D ₅₀ of 500nm boron nitride powder, the average particle diameter D ₅₀ of 10μm of graphite powder as a negative electrode active material, since the boron nitride powder was placed along the unevenness of the surface of the graphite powder I think that the.

(Negative electrode C)
A coating layer having a thickness of 2 μm composed of boron nitride powder and a coating layer binder was formed on the surface of the negative electrode A in the same manner as the negative electrode B, except that the number of sprays was changed to two. This is designated as negative electrode C. Specifically, when the number of sprays is set to 2 times, the mixture is sprayed once by the method described in the negative electrode B over the entire spray range, and the second spray is applied to the first sprayed portion in the same manner as the first spray. Went again. When the surface of the negative electrode C was observed with a scanning electron microscope (SEM), it was confirmed that the coating layer was disposed along the surface of the negative electrode active material layer.

(Negative electrode D)
A coating layer having a thickness of 5 μm composed of boron nitride powder and a coating layer binder was formed on the surface of the negative electrode A in the same manner as the negative electrode B except that the number of sprays was changed to five. This is designated as negative electrode D. When the surface of the negative electrode D was observed with a scanning electron microscope (SEM), it was confirmed that the coating layer was disposed along the surface of the negative electrode active material layer. Here, the thickness of the coating layer of 5 μm corresponds to 10% of the thickness of the negative electrode active material layer.

<Measurement of negative electrode resistance>
The electrode resistance of the negative electrode A, the negative electrode B, the negative electrode C, and the negative electrode D was measured. A sheet before cutting into a predetermined shape (25 mm × 30 mm rectangular shape) of each negative electrode was cut into a shape having a circular portion with a diameter of 16 mm and a terminal portion extending from one side of the circle into a strip shape with a width of 5 mm. The negative electrode active material layer and the coating layer were removed from the terminal portion to expose the negative electrode current collector. This was used as a test piece. A copper foil having a thickness of about 15 μm was cut into a shape having a circular part slightly larger than a diameter of 16 mm and a terminal part extending from one side of the square to a band shape having a width of 5 mm. The test piece and the copper foil of the above shape were overlapped so that the circular portions overlapped and the terminal portions were arranged on one end side and the other end side of the circle. The circular portion is sandwiched between jigs and compressed with a force of 20 kgf (about 1.96 × 10 ² N) / φ16 mm to bring the coating layer and the copper foil into close contact with each other. , Referred to as electrode resistance). The obtained results are shown in Table 1.

  From the results in Table 1, it was found that the electrode resistance increases as the coating layer thickness increases. As will be described later, when a lithium ion secondary battery was produced using these negative electrodes, the cell resistance did not increase so much.

<Production of laminated lithium-ion secondary battery>
Example 1
A laminate type lithium ion secondary battery of Example 1 using the negative electrode C as the negative electrode was produced as follows.

The positive electrode was produced as follows. LiNi _0.5 Co _0.2 Mn _0.3 O ₂ having an average particle diameter D ₅₀ of 5 μm as a positive electrode active material, acetylene black as a conductive additive, and polyvinylidene fluoride (PVDF) as a binder are each 94 The mixture was mixed as parts by mass, 3 parts by mass, and 3 parts by mass to obtain a mixture. This mixture was dispersed in an appropriate amount of N-methyl-2-pyrrolidone (NMP) to prepare a slurry.

An aluminum foil having a thickness of 20 μm was prepared as a current collector. The slurry was placed on the current collector, and applied using a doctor blade so that the slurry became a film. The obtained sheet was dried at 80 ° C. for 20 minutes to volatilize and remove NMP. Thereafter, the current collector and the coated material on the current collector were firmly and closely joined by a roll press. At this time, the electrode density was set to 3.2 g / cm ² . The bonded product was heated in a vacuum dryer at 120 ° C. for 6 hours. The bonded product after heating was cut into a predetermined shape (rectangular shape of 25 mm × 30 mm) to obtain a positive electrode. The thickness of the positive electrode active material layer was about 45 μm.

Using the positive electrode and the negative electrode C, a laminate type lithium ion secondary battery was manufactured. 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 C 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 electrolytic solution, a solution obtained by dissolving LiPF ₆ at 1 mol / l in a solvent obtained by mixing ethylene carbonate (EC) and diethyl carbonate (DEC) at EC: DEC = 3: 7 (volume ratio) 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 each 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 negative electrode C in Example 1 was changed to the negative electrode D.

(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 negative electrode C in Example 1 was changed to the negative electrode A.

(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 negative electrode C in Example 1 was changed to the negative electrode B.

<Cell resistance evaluation>
The cell resistance of the laminated lithium ion secondary batteries of Examples 1-2 and Comparative Examples 1-2 was 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. The results are shown in Table 2.

  From the results in Table 2, it was found that the cell resistance did not increase so much even when the coating layer was disposed on the surface of the negative electrode active material layer.

<Evaluation of cycle characteristics of laminated lithium ion secondary batteries of Example 1, Example 2, Comparative Example 1 and Comparative Example 2>
The cycle characteristics of the laminated lithium ion secondary batteries of Example 1, Example 2, Comparative Example 1 and Comparative Example 2 were evaluated. As an evaluation of the cycle characteristics, a cycle test in which charging and discharging were repeated under the following conditions was performed, and the discharge capacity of each cycle was measured. When charging, CC charging (constant current charging) was performed at 60 ° C. at a 1C rate and a voltage of 4.5V. When discharging, CC discharge (constant current discharge) was performed at 3.0 V and 1 C rate. This charging / discharging was made into 1 cycle, and the cycle test was done to 200 cycles. The discharge capacity at this time was used by calculating the discharge capacity per mass of the positive electrode active material. Therefore, the unit of discharge capacity is mAh / g.

  The capacity retention rate of each cycle was determined by the capacity retention rate (%) of each cycle = (discharge capacity of each cycle / initial discharge capacity) × 100.

  FIG. 2 shows the relationship between the number of cycles and the capacity retention rate (%) of the laminated lithium ion secondary batteries of Example 1, Example 2, Comparative Example 1 and Comparative Example 2.

  The initial discharge capacities of the laminated lithium ion secondary batteries of Example 1 and Example 2 were equal to or slightly lower than the initial discharge capacities of the laminated lithium ion secondary batteries of Comparative Example 1 and Comparative Example 2. Specifically, the initial discharge capacity of the laminated lithium ion secondary battery of Comparative Example 1 is 191.5 mAh / g, and the initial discharge capacity of the laminated lithium ion secondary battery of Comparative Example 2 is 193.2 mAh / g. On the other hand, the initial discharge capacity of the laminated lithium ion secondary battery of Example 1 was 186.4 mAh / g, and the initial discharge capacity of the laminated lithium ion secondary battery of Example 2 was 183.7 mAh / g. there were.

From this, the first discharge capacity of the laminated lithium ion secondary battery using the negative electrode in which the coating layer composed of the boron nitride powder and the coating layer binder is disposed on the surface of the negative electrode active material layer is the negative electrode without the coating layer. It was found that there was almost no change compared to the initial discharge capacity of the laminate type lithium ion secondary battery using. In addition, since the initial discharge capacity of the laminate type lithium ion secondary battery of Example 1 and the initial discharge capacity of the laminate type lithium ion secondary battery of Example 2 hardly change, the thickness of the coating layer is 2 μm to 5 μm. Thus, it was found that the initial discharge capacity was almost the same if the resistance value was 6 × 10 ³ Ω / φ16 mm or more and 1.5 × 10 ⁶ Ω / φ16 mm or less.

This is because the coating layer is composed of boron nitride powder having an average particle size _{D50 of} 500 nm and a binder for the coating layer, and therefore there is a fine space formed between the boron nitride powders in the coating layer, and the coating layer is thick. Even so, it is presumed that lithium ions can move in the coating layer through a fine space.

  As can be seen from FIG. 2, the capacity retention rate of the laminated lithium ion secondary batteries of Example 1 and Example 2 is that of the laminated lithium ion secondary batteries of Comparative Example 1 and Comparative Example 2 up to 40 cycles. It was almost the same as the capacity maintenance rate. After the 40th cycle, the capacity retention rates of the laminated lithium ion secondary batteries of Example 1 and Example 2 were higher than the capacity retention ratios of the laminated lithium ion secondary batteries of Comparative Example 1 and Comparative Example 2. .

  Comparing the discharge capacity at the 200th cycle, the discharge capacity at the 200th cycle of the laminate type lithium ion secondary battery of Comparative Example 1 was 95.9 mAh / g, whereas the laminate type lithium ion secondary battery of Comparative Example 2 was The discharge capacity at the 200th cycle was 117.8 mAh / g, the discharge capacity at the 200th cycle of the laminate type lithium ion secondary battery of Example 1 was 118.8 mAh / g, and the laminate type lithium ion secondary battery of Example 2 was The discharge capacity at the 200th cycle was 124.3 mAh / g. That is, the discharge capacity at the 200th cycle increased in the order of Comparative Example 1 <Comparative Example 2 <Example 1 <Example 2.

From this, it was found that when the resistance value of the coating layer was larger than 3 × 10 ³ Ω / φ16 mm and not more than 2.0 × 10 ⁶ Ω / φ16 mm, the cycle characteristics of the lithium ion secondary battery were high. Since the coating layer does not have a large gap that makes the resistance value lower than the above range, the negative electrode active material layer is entirely covered with the coating layer, and the decomposition reaction of the electrolytic solution by the negative electrode active material can be suppressed. When the coating is insufficient, the resistance becomes lower than a predetermined value, and it is presumed that the effect of improving the cycle characteristics is reduced without obtaining a sufficient protective effect. The coating layer includes boron nitride powder and an inorganic binder. Boron nitride is thermally and chemically stable, and the inorganic binder is also thermally and chemically stable. Therefore, since the coating layer is formed of the boron nitride powder and the inorganic binder, it is estimated that the coating layer more stably suppresses the decomposition reaction of the electrolytic solution.

  From the above results, when the coating layer having the insulating inorganic powder and the coating layer binder is disposed on the surface of the negative electrode active material layer, and the resistance value of the coating layer is within the above range, the lithium ion secondary battery Was found to have excellent cycle characteristics and charge / discharge capacity even under high temperature and high voltage use environments.

  1: current collector, 2: binder, 3: negative electrode active material, 4: negative electrode active material layer, 5: coating layer, 51: inorganic powder, 52: binder for coating layer.

Claims (7)

A current collector,
A negative electrode active material layer disposed on a surface of the current collector and including a negative electrode active material and a binder;
A coating layer disposed on the surface of the negative electrode active material layer;
Have
The coating layer has an insulating inorganic powder and a coating layer binder,
When a copper foil is pressed against the surface of the coating layer at a pressure of 1.96 × 10 ² N / φ16 mm and the electrical resistance between the copper foil and the current collector is measured, the resistance value is 3 × 10. ^A negative electrode for a lithium ion secondary battery, wherein the negative electrode is larger than ³ Ω / φ16 mm and 2 × 10 ⁶ Ω / φ16 mm or less. 2. The negative electrode for a lithium ion secondary battery according to claim 1, wherein the resistance value is 6 × 10 ³ Ω / φ16 mm or more and 1.5 × 10 ⁶ Ω / φ16 mm or less.   3. The negative electrode for a lithium ion secondary battery according to claim 1, wherein the coating layer has a thickness of 2 μm or more and 20% or less of the thickness of the negative electrode active material layer. 4. The negative electrode for a lithium ion secondary battery according to claim 1, wherein an average particle diameter D ₅₀ of the inorganic powder is 100 nm or more and 1 μm or less.   The negative electrode for a lithium ion secondary battery according to any one of claims 1 to 4, wherein the inorganic powder is made of at least one selected from boron nitride, aluminum nitride, and silicon nitride.   The negative electrode for a lithium ion secondary battery according to any one of claims 1 to 5, wherein the coating layer binder is an inorganic binder containing at least one selected from a silicon element, an aluminum element, and a zirconium element.   It has a negative electrode for lithium ion secondary batteries as described in any one of Claims 1-6, The lithium ion secondary battery characterized by the above-mentioned.

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