JP5927449B2 - Positive electrode for secondary battery and secondary battery using the same - Google Patents

Description

  The present invention relates to a positive electrode including an olivine type lithium silicate compound and a secondary battery using the positive electrode.

  A secondary battery such as a lithium ion battery is a kind of non-aqueous electrolyte battery, and is used in a wide range of fields such as a mobile phone, a digital camera, a notebook PC, a hybrid vehicle, and an electric vehicle. Lithium ion batteries mainly use lithium metal oxide as a positive electrode material and a carbon material such as graphite as a negative electrode material.

As this positive electrode material, lithium cobaltate (LiCoO ₂ ), lithium manganate (LiMnO ₂ ), lithium iron phosphate (LiFePO ₄ ), lithium iron silicate (Li ₂ FeSiO ₄ ) and the like are known. Among these, LiFePO ₄ and Li ₂ FeSiO ₄ have an olivine structure and are useful as a positive electrode material for a high capacity lithium ion battery.

  Since the discharge capacity of the secondary battery varies greatly depending on the type of positive electrode material, various olivine-type compounds have been reported (Patent Documents 1 and 2), and further by doping Mn and the like into the olivine-type lithium iron silicate. Improvement of discharge capacity has been studied (Non-Patent Documents 1 to 3).

  In order to form a positive electrode using these positive electrode materials, a method is widely used in which a conductive additive, a binder and a solvent are kneaded with the positive electrode material, and the resultant mixture is applied to a conductive substrate and then dried. The kneading means is also devised (Patent Documents 3 to 6).

JP 2001-266882 A JP 2008-186807 A JP 2000-123879 A JP 2004-14206 A JP 2007-220581 A JP 2012-160463 A

Electrochemical and Solid-State Letters 19, 12, A542-A544 (2006) GS Yuasa Technical Report June 2009, Vol. 6, No. 1, p21-26 R. Dominiko et al. Journal of Power Sources 184 (2008), P462-468

However, when the positive electrode formation by these conventional methods is applied to an olivine type lithium silicate compound, a positive electrode exhibiting a sufficiently high discharge capacity can be obtained although a theoretically high discharge capacity is expected. It turned out not to be.
Accordingly, an object of the present invention is to provide a positive electrode exhibiting a high discharge capacity from the first cycle and a secondary battery using the same, using an olivine type lithium silicate compound as a positive electrode active material.

  Thus, as a result of various studies on the formation method of the positive electrode using the olivine type lithium silicate compound as the positive electrode active material, the present inventor has found that in addition to the positive electrode active material, the conductive additive, the binder and the organic solvent, phosphoric acid, boron A positive electrode whose discharge capacity is dramatically improved from the first cycle by kneading a specific acid such as an acid, a fatty acid having 1 to 6 carbon atoms, or a salt thereof, and applying and drying the resultant mixture on a conductive substrate. Was found and the present invention was completed.

  That is, the present invention includes a positive electrode active material composed of particles containing an olivine-type lithium silicate compound, a conductive additive, a binder, an organic solvent, and phosphoric acid, boric acid, a C 1-6 fatty acid, and salts thereof. The present invention provides a positive electrode for a secondary battery having a positive electrode mixture layer formed by applying a kneaded material containing selected acids to a conductive substrate.

  Moreover, this invention provides the secondary battery which has said positive electrode for secondary batteries.

  Furthermore, the present invention kneads a positive electrode active material composed of particles containing an olivine type lithium silicate compound, a conductive additive, an organic solvent, and acids selected from phosphoric acid, boric acid, fatty acids having 1 to 6 carbon atoms and salts thereof. Then, a kneaded product obtained by adding a binder is applied to a conductive substrate and dried, and a method for producing a positive electrode for a secondary battery is provided.

  The secondary battery using the positive electrode of the present invention is an excellent secondary battery having a high discharge capacity from the first cycle and a high discharge capacity peculiar to the olivine type lithium silicate compound.

  The positive electrode for a secondary battery of the present invention includes (A) a positive electrode active material comprising particles containing an olivine type lithium silicate compound, (B) a conductive auxiliary agent, (C) a binder, (D) an organic solvent, and (E ) It has a positive electrode mixture layer formed by applying a kneaded material containing acids selected from phosphoric acid, boric acid, fatty acids having 1 to 6 carbon atoms and salts thereof onto a conductive substrate.

  The positive electrode active material used for the positive electrode of the present invention is composed of particles containing an olivine type lithium silicate compound.

(A) Examples of olivine type lithium silicate compounds include olivine type lithium silicate compounds containing lithium ions, metal (M) ions and silicate ions (SiO ₄ ⁴⁻ ), and in particular lithium ions and metal M (M is Fe An olivine type lithium silicate compound containing one or more selected from Ni, Co, Al, Zn, Mn, V and Zr) and containing a silicate ion (SiO ₄ ⁴⁻ ) is preferable.

Specific examples of the olivine type lithium silicate compound include the following.
Li ₂ Fe _x Mn _y Co _z SiO ₄ (1)
(In the formula, x, y, and z represent numbers satisfying 0 ≦ x <1, 0 ≦ y <1, 0 <z <1, x + y + z = 1, and x + y ≠ 0.)
_{Li a1 Fe x Mn y Al z} SiO 4 ··· (2)
(Wherein a ¹ , x, y and z are 1 <a ¹ ≦ 2, 0 ≦ x <1, 0 ≦ y <1, 0 <z <1, a ¹ + 2x + 2y + 3z = 4 and x + y ≠ 0. Indicates the number to satisfy.)
Li ₂ M ¹ SiO _4-x1 F _2x1 (3)
(In the formula, M ¹ represents one or more selected from Fe, Mn, Co and Ni, and x ¹ represents a number satisfying 0 <x ¹ ≦ 4.)
_{Li a2 Fe x Mn y V z} SiO 4 ··· (4)
(Wherein a ² , x, y and z are 1 <a ² ≦ 2, 0 ≦ x <1, 0 ≦ y <1, 0 <z <1, a ² + 2x + 2y + (2-5) z = 4 , And a number satisfying x + y ≠ 0.)
Li ₂ Fe _x Mn _y Zn _z SiO ₄ (5)
(In the formula, x, y, and z represent numbers satisfying 0 ≦ x <1, 0 ≦ y <1, 0 <z <1, x + y + z = 1, and x + y ≠ 0.)
Li ₂ Fe _a3 Ni _b Co _c Mn _d Zr _x2 SiO ₄ (6)
(Wherein, a, b, c and d satisfy a + b + c + d = 1−2x, and x ² represents a number satisfying 0 <x ² <0.5)

  The olivine type lithium silicate compound may contain F or the like by doping.

  (A) The olivine type lithium silicate compound can be produced by, for example, hydrothermal reaction of a mixture containing a lithium-containing compound, a metal (M) compound and a silicate compound.

Examples of the lithium-containing compound include lithium hydroxide (for example, LiOH.H ₂ O), lithium carbonate (Li ₂ CO ₃ ), lithium sulfate, and lithium acetate, and lithium hydroxide and lithium carbonate are particularly preferable. The concentration of the lithium-containing compound in the aqueous dispersion is preferably 0.30 to 3.00 mol / l, more preferably 1.00 to 1.50 mol / l.

  As the metal (M) compound, an iron compound, a manganese compound, a cobalt compound, a nickel compound, an aluminum compound, a zinc compound, a vanadium compound, or a zirconium compound may be used.

  The iron compound, manganese compound, cobalt compound, nickel compound, and zinc compound may be any divalent iron compound, divalent manganese compound, divalent cobalt compound, divalent nickel compound, or divalent zinc compound. , For example, halides such as iron halide, manganese halide, cobalt halide, nickel halide and zinc halide, sulfates such as iron sulfate, manganese sulfate, cobalt sulfate, nickel sulfate and zinc sulfate, iron oxalate, Examples thereof include organic acid salts such as iron acetate, manganese acetate, cobalt acetate, nickel acetate, and zinc acetate.

  The aluminum compound may be a trivalent compound, and examples thereof include halides such as aluminum halide, metal sulfates such as aluminum sulfate, and metal organic acid salts such as aluminum acetate and aluminum lactate. Examples of vanadium compounds include vanadium halides.

  The zirconium compound may be a tetravalent compound, and examples thereof include organic acid salts such as zirconium halide, zirconium sulfate, zirconium diacetate oxide, zirconium octoate, and zirconium laurate oxide.

  The concentration of the metal (M) compound in the reaction mixture is preferably 0.15 to 1.50 mol / l, more preferably 0.50 to 0.75 mol / l.

The silicic acid compound is not particularly limited as long as it is a reactive silica compound, and amorphous silica, Na ₄ SiO ₄ and Na ₄ SiO ₄ .nH ₂ O (for example, Na ₄ SiO ₄ .H ₂ O) can be used. preferable. Of these, the use of Na ₄ SiO ₄ is more preferable because the reaction mixture becomes basic.
The concentration of the silicate compound is preferably 0.15 to 1.50 mol / l, more preferably 0.50 to 0.75 mol / l.

The mixture containing the lithium-containing compound, the metal (M) compound and the silicate compound prevents side reactions and uses water to form a basic aqueous dispersion from the viewpoint of the solubility of the silicate compound. Good. Specifically, the pH of the aqueous dispersion is preferably 12.0 to 13.5. The pH of the aqueous dispersion may be adjusted by adding a base such as sodium hydroxide, but it is preferable to use Na ₄ SiO ₄ as the silicate compound.

  In producing an aqueous dispersion, as a metal (M) compound, for example, when a metal sulfate is used, a lithium compound, a silicate compound, and a surfactant are separately used from the metal sulfate in terms of suppressing side reactions. It is preferable to prepare in advance a basic aqueous dispersion containing an antioxidant. In this case, the aqueous dispersion and the metal sulfate are mixed and subjected to a hydrothermal reaction. The order of addition of the lithium compound, the silicate compound, the surfactant and the antioxidant is not particularly limited, and these four components may be added to water.

  In addition, when an organic acid salt is used as the metal (M) compound, for example, a basic aqueous dispersion containing a lithium compound, a silicic acid compound, a surfactant, and an antioxidant, and further containing an organic acid salt Is preferably prepared. Usually, an organic acid salt is a raw material used in a solid phase method, but side reactions can be suppressed by using it in a hydrothermal reaction.

Examples of the antioxidant include sodium hydrosulfite (Na ₂ S ₂ O ₄ ), aqueous ammonia, sodium sulfite and the like. The content of the antioxidant in the aqueous dispersion is preferably equal to or less than the metal (M) because the formation of the olivine type lithium silicate compound is suppressed when added in a large amount. The molar ratio is more preferably 0.5 or less.

  130-250 degreeC is preferable and the temperature at the time of attaching | subjecting to a hydrothermal reaction has more preferable 150-200 degreeC. The hydrothermal reaction is preferably performed in a pressure vessel. When the reaction is performed at 130 to 250 ° C, the pressure at this time is 0.3 to 4.0 MPa, and the pressure when the reaction is performed at 150 to 200 ° C is 0. 5 to 1.6 MPa. The hydrothermal reaction time is preferably 0.5 to 24 hours, and more preferably 3 to 12 hours.

  The preparation of the mixed solution and the hydrothermal reaction are preferably performed under non-oxygen conditions, for example, in a nitrogen atmosphere.

By the hydrothermal reaction, an olivine type lithium silicate compound is obtained in high yield, and its crystallinity is also high.
After the hydrothermal reaction, the produced olivine-type silicate compound is collected by filtration and washed to obtain primary particles. Washing is preferably carried out with water using a filtration device having a cake washing function.
Here, the particle size of the obtained primary particles is preferably as small as possible, preferably 5 to 300 nm, and more preferably 5 to 30 nm.

Then, drying is performed. As the drying means, a box-type dryer, freeze-drying, vacuum drying, fluidized bed dryer, or spray-type dryer (spray dryer) can be used.
About the obtained dried material, you may crush using a mortar, a pin mill, a roll mill, a crusher, etc.

  Thereafter, carbon is supported on primary particles or secondary particles of the obtained olivine type lithium silicate compound. By doing in this way, the electron conduction area (electron conduction path) of an olivine type lithium silicate compound will increase, and more sufficient electronic conductivity can be secured.

  Examples of the carbon-supporting treatment include a treatment in which a slurry containing the obtained olivine-type lithium silicate compound and a conductive carbon material is prepared and fired after granulation. The slurry may appropriately contain an organic binder and an inorganic binder. By performing such treatment, a carbon thin film can be formed on the surface of secondary particles formed from primary particles, and electron conductivity can be further enhanced.

  Examples of the conductive carbon material include glucose, polyvinyl alcohol, carboxymethyl cellulose, and carbon black.

Examples of the binder include glucose, polyvinyl alcohol and carboxymethyl cellulose which can also be used as a conductive carbon material, as well as fructose, polyethylene glycol, saccharose, starch, dextrin, citric acid and the like.
Among these, glucose, polyvinyl alcohol, and carboxymethyl cellulose are preferable, and glucose is more preferable because it functions as a carbon source by adjusting the amount used and can also be used as a conductive carbon material.

  Examples of the inorganic binder include scaly silica (silicon dioxide), silica-titania, silicon glass, colloidal silica, silica sol, and alumina sol.

  The amount of the conductive carbon material used in the process of preparing the slurry and firing after granulation is based on 100 parts by mass of the olivine type lithium silicate compound in the slurry from the viewpoint of good charge / discharge capacity and economy. The amount is preferably 0.5 to 20 parts by mass, more preferably 2 to 10 parts by mass in terms of carbon atoms.

  Moreover, you may use water or an organic solvent as a solvent, and water is preferable from an economical viewpoint. The content (slurry concentration) of the olivine type lithium silicate compound and the conductive carbon material in the slurry is preferably 30 to 60% by mass, and more preferably 45 to 55% by mass. The slurry viscosity at 25 ° C. is preferably 3 to 3000 m · Pa, more preferably 10 to 100 m · Pa. Furthermore, the pH of the slurry is preferably adjusted to 10.5 to 11.2.

The granulation treatment is not particularly limited as long as secondary particles having a desired particle diameter can be obtained, but it is preferably by spray drying, and most preferably by spray drying by a spray drying method. . The average particle size of the secondary particles obtained after the granulation treatment is preferably 1 μm to 500 μm, and more preferably 20 μm to 50 μm.
The obtained secondary particles can then be used as a positive electrode active material for a secondary battery by firing.

  In addition to the above granulation treatment, for example, a method of pulverizing / compositing / mixing a mixture containing an olivine type lithium silicate compound and a conductive carbon material may be used as a carbon supporting treatment. By performing such treatment, secondary particles in which the primary particles of the precursor and the conductive carbon material are combined can be formed, and the conductivity can be further increased.

  Examples of the conductive carbon material used in the pulverization treatment include the same conductive carbon materials that can be used in the granulation treatment. Of these, carbon black is preferable, and acetylene black and ketjen black are more preferable. The addition amount of the conductive carbon material in the pulverization treatment is 0.5 to 20 parts by mass in terms of carbon atoms with respect to 100 parts by mass of the primary particles of the olivine type lithium silicate compound from the viewpoint of good discharge capacity and economy. Preferably, 2-10 mass parts is further preferable.

  The mixture containing the olivine type lithium silicate compound and the conductive carbon material is pulverized / composited / mixed in a dry manner. At this time, a small amount of diethylene glycol, ethanol or the like may be added as an auxiliary agent.

  As an apparatus for pulverizing / combining / mixing, an ordinary ball mill may be used, but a planetary ball mill (manufactured by Fritsch) capable of rotating and revolving is preferred, and nobilta (manufactured by Hosokawa Micron), multipurpose mixer (manufactured by Nippon Coke Industries, Ltd.). ), Or a hybridizer (manufactured by Nara Machinery Co., Ltd.), etc., as long as it can perform a mechanochemical action / combination treatment on an object to be processed.

  Examples of the container of the device used in the planetary ball mill include steel, stainless steel, and nylon, and the inner wall includes alumina brick, magnetic material, natural silica, rubber, urethane, and the like. As the ball, alumina sphere, natural silica, iron ball, zirconia ball or the like is used. The size of the ball is preferably 0.1 mm to 20 mm, more preferably 0.5 mm to 5 mm. It is preferable that the filling amount of the ball is a ratio in which the filling volume of the ball occupies 5 to 50% with respect to the internal volume of the mill to be used.

  Mixing using a planetary ball mill is performed, for example, under conditions of revolution of 50 to 800 rpm and rotation of 100 to 1,600 rpm, preferably 5 minutes to 24 hours, more preferably 0.5 to 6 hours, and further preferably 1 to 3 hours.

  Firing of the secondary particles subjected to the treatment for supporting carbon as described above is preferably performed at 500 to 800 ° C. for 10 minutes to 24 hours, more preferably at 600 to 700 ° C. in an inert gas atmosphere or under reducing conditions. It is preferable to carry out for 0.5 to 3 hours. By this treatment, a positive electrode active material that is secondary particles in which carbon is further firmly supported on the surface of the olivine-type silicate compound can be obtained. The apparatus used for firing is not particularly limited as long as the firing atmosphere and temperature can be adjusted, and any of a batch system, a continuous system, and a heating system (indirect or direct) can be used. . Examples of such an apparatus include tubular electric furnaces such as an external heat kiln and a roller hearth.

  The average particle diameter of the obtained secondary particles is preferably 1 to 100 μm, more preferably 5 to 50 μm. The tap density is preferably 0.5 g / ml or more, more preferably 0.7 g / ml or more.

Examples of the conductive assistant (B) used in the present invention include carbon black conductive assistant, scaly graphite, fibrous carbon, and activated carbon. Of these, carbon black-based conductive assistants are preferable, and specific examples include furnace black, ketjen black, acetylene black, and thermal black. The specific surface area of the conductive auxiliary agent is preferably 50 to 2000 m ² / g from the viewpoint of preventing decrease in conductivity and coating properties.

  Examples of the (C) binder used in the present invention include resin binders, rubber binders, and cellulose binders. Examples of the resin binder include polytetrafluoroethylene, polyvinylidene fluoride (PVDF), polyethylene, polypropylene, polyethylene oxide, polyvinyl pyrrolidone, polyester resin, acrylic resin, phenol resin, and epoxy resin. Examples of the rubber binder include ethylene-propylene-diene copolymer resin, styrene butadiene rubber (SBR), polybutadiene, and fluorine rubber. Examples of the cellulose binder include hydroxypropyl cellulose and carboxymethyl cellulose (CMC). These binders may be used individually by 1 type, and may use 2 or more types together. Among these, PVDF is particularly preferable.

  (D) As the organic solvent, for example, an aprotic polar organic solvent such as N-methyl-2-pyrrolidone (NMP), dimethylacetamide, dimethylformamide or the like may be used alone, or two or more of them may be used in combination. May be.

  (E) acids used in the present invention are phosphoric acid, boric acid, fatty acids having 1 to 6 carbon atoms, or salts thereof. By using these (E) acids, a secondary battery having a high discharge capacity can be obtained from the first cycle. Examples of the fatty acid having 1 to 6 carbon atoms include formic acid, acetic acid, propionic acid, and butyric acid. Among these, formic acid, acetic acid, and propionic acid are more preferable, and acetic acid is more preferable. In addition, examples of salts of these acids include alkali metal salts such as sodium salt and potassium salt, ammonium and the like, and ammonium salts which are non-metal salts are preferable.

  Among the components (A) to (E) in the positive electrode of the present invention, the solid content concentration comprising (A) the positive electrode active material, (B) the conductive auxiliary agent, and (C) the binder is increased in density and applicability. 70 mass% or more is preferable, 75 mass% or more is more preferable, 85 mass% or less is preferable, and 82 mass% or less is more preferable.

  The amount of the positive electrode active material in the positive electrode mixture layer is preferably 90% by mass or more, more preferably 95% by mass or more, preferably 99% by mass or less, and 98% by mass or less from the viewpoint of increasing the battery capacity and conductivity. Is more preferable. From the viewpoint of conductivity, the amount of the conductive auxiliary agent is preferably 0.1% by mass or more, more preferably 0.5% by mass or more, preferably 8% by mass or less, and more preferably 5% by mass or less. Further, the amount of the binder is preferably 0.5% by mass or more, more preferably 1.0% by mass or more, and preferably 3.0% by mass or less from the viewpoints of binding properties and adhesiveness to the conductive substrate. 2.0 mass% or less is more preferable.

  In addition, the amount of (E) acids added is preferably 0.01% by mass or more, more preferably 0.1% by mass or more with respect to the positive electrode active material, from the viewpoint of the effect of improving the discharge capacity of the obtained secondary battery. 5 mass% or less is preferable, 3 mass% or less is more preferable, and 2 mass% or less is further more preferable.

  As the conductive substrate used in the positive electrode of the present invention, for example, foils such as aluminum, copper, nickel, and stainless steel, expanded metal, and nets can be used, and aluminum foil is particularly preferable. The thickness of the conductive substrate is, for example, 5 μm or more, more preferably 8 μm or more, and preferably 60 μm or less, more preferably 40 μm or less.

  The positive electrode mixture layer can be formed by kneading the components (A) to (E), applying the obtained kneaded material to a conductive substrate, and then drying. (A) Olivine type Cathode active material comprising particles containing lithium silicate compound, (B) conductive auxiliary agent, (D) organic solvent, and (E) phosphoric acid, boric acid, fatty acids having 1 to 6 carbon atoms, and salts thereof It is preferable from the viewpoint of coatability and homogeneity to produce a kneaded product obtained by kneading and then adding the binder (C) to a conductive substrate and drying.

  Here, the kneading means can be performed by a planetary mixer, a twin screw extruder, a single screw extruder, a centrifugal disk mixer, a double arm kneader, a ball mill, a planetary mill, a Henschel mixer, and a planetary mixer is more preferable. .

In producing the positive electrode, for example, the kneaded product is applied to one or both sides of a conductive substrate and dried. Then, it compresses by a press process and adjusts the thickness and density of a positive mix layer. As a method for applying the kneaded product, for example, various known application methods such as an extrusion coater, a reverse coater, and a doctor blade can be employed.
The thickness of the positive electrode mixture layer after compression by the press treatment is, for example, 30 μm or more, more preferably 50 μm or more, and 300 μm or less, more preferably 150 μm or less per side. Further, the density of the positive electrode mixture layer after compression by the press process, for example, 3.55 g / cm ³ or more, more preferably be at 3.75 g / cm ³ or more, 4.0 g / cm ³ or less, more preferably Is preferably 3.95 g / cm ³ or less. Therefore, it is preferable to adjust the compression conditions at the time of the press treatment so that the thickness and density of the positive electrode mixture layer after compression have such values.

By using the positive electrode thus obtained, a secondary battery with improved discharge capacity can be obtained.
The secondary battery of the present invention is not particularly limited as long as it is a lithium ion secondary battery and has a positive electrode, a negative electrode, an electrolytic solution, and a separator as essential components.

  Here, as long as lithium ions can be occluded at the time of charging and released at the time of discharging, the material structure is not particularly limited, and a known material structure can be used. For example, a carbon material such as lithium metal, graphite, or amorphous carbon. It is preferable to use an electrode formed of an intercalating material capable of electrochemically inserting and extracting lithium, particularly a carbon material.

  The electrolytic solution is obtained by dissolving a supporting salt in an organic solvent. The organic solvent is not particularly limited as long as it is an organic solvent that is usually used for an electrolyte solution of a lithium ion secondary battery. For example, carbonates, halogenated hydrocarbons, ethers, ketones, nitriles, lactones An oxolane compound or the like can be used.

The type of the supporting salt is not particularly limited, but an inorganic salt selected from LiPF ₆ , LiBF ₄ , LiClO ₄ and LiAsF ₆ , a derivative of the inorganic salt, LiSO ₃ CF ₃ , LiC (SO ₃ CF ₃ ) ₂ and LiN (SO ₃ CF ₃ ) ₂ , LiN (SO ₂ C ₂ F ₅ ) ₂ and LiN (SO ₂ CF ₃ ) (SO ₂ C ₄ F ₉ ), and organic salt derivatives It is preferable that it is at least 1 type of these.

  The separator plays a role of electrically insulating the positive electrode and the negative electrode and holding the electrolytic solution. For example, a porous synthetic resin film, particularly a polyolefin polymer (polyethylene, polypropylene) porous film may be used.

  EXAMPLES Hereinafter, although this invention is demonstrated concretely based on an Example, this invention is not limited to these Examples.

[Reference Example 1] (Olivine type lithium silicate compound)
30 cm ³ of ultrapure water was added to and mixed with 4.20 g (100 mmol) of LiOH.H ₂ O and 13.98 g (50 mmol) of Na ₄ SiO ₄ .nH ₂ O (pH at this time was about 13). To this aqueous dispersion, 8.94 g (32.2 mol) of FeSO ₄ .7H ₂ O, 3.82 g (15.8 mol) of MnSO ₄ .5H ₂ O and 0.28 g (1 mmol) of ZrSO ₄ .4H ₂ O were added. , Mixed. The obtained mixed solution was put into an autoclave and subjected to a hydrothermal reaction at 150 ° C. for 12 hours. The reaction solution was filtered and then lyophilized. Glucose (carbon concentration: 10%) and ultrapure water (10 cm ³⁾ were added to 8.4 g of the powder obtained by freeze-drying (about 12 hours), and calcined at 600 ° C. for 1 hr in a reducing atmosphere.

[Example 1] (when acetic acid is added)
Silicate positive electrode material (Reference Example 1) and ketjen black (conductive agent) are mixed at a blending ratio of 75:15, and 1.6 ml of N-methyl-2-pyrrolidone and 10 μl of acetic acid are added thereto and kneaded sufficiently. Slurry A was prepared. Add 1/9 weight polyvinylidene fluoride (binding agent) and 1.8 ml N-methyl-2-pyrrolidone to Slurry A and knead well with the total weight of the silicate cathode material and ketjen black. Thus, a positive electrode slurry was prepared. The positive electrode slurry was applied to a current collector made of an aluminum foil having a thickness of 20 μm using a coating machine, and vacuum dried at 120 ° C. for 12 hours. Thereafter, it was punched into a disk shape of φ14 mm and pressed at 16 MPa for 2 minutes using a hand press to obtain a positive electrode.

[Example 2] (when ammonium dihydrogen phosphate is added)
Silicate-based positive electrode material (Reference Example 1) and ketjen black (conductive agent) were mixed at a mixing ratio of 75:15, and 1.6 ml of N-methyl-2-pyrrolidone and 20 wt% ammonium dihydrogen phosphate were mixed therewith. 150 mg of an aqueous solution was added and sufficiently kneaded to prepare slurry A. 7/90 weight polyvinylidene fluoride (binding agent) and 1.8 ml of N-methyl-2-pyrrolidone are added to slurry A and kneaded sufficiently with respect to the total weight of the silicate positive electrode material and ketjen black. Thus, a positive electrode slurry was prepared. The positive electrode slurry was applied to a current collector made of an aluminum foil having a thickness of 20 μm using a coating machine, and vacuum dried at 80 ° C. for 12 hours. Thereafter, it was punched into a disk shape of φ14 mm and pressed at 16 MPa for 2 minutes using a hand press to obtain a positive electrode.

[Example 3] (when boric acid is added)
Silicate-based positive electrode material (Reference Example 1) and ketjen black (conductive agent) are mixed at a blending ratio of 75:15, and 1.6 ml of N-methyl-2-pyrrolidone and 100 mg of 10 wt% boric acid ethanol solution are mixed therewith. And kneaded well to prepare slurry A. 1/10 weight of polyvinylidene fluoride (binding agent) and 1.8 ml of N-methyl-2-pyrrolidone with respect to the total weight of the silicate cathode material and ketjen black are added to slurry A and kneaded sufficiently. Thus, a positive electrode slurry was prepared. The positive electrode slurry was applied to a current collector made of an aluminum foil having a thickness of 20 μm using a coating machine, and vacuum dried at 80 ° C. for 12 hours. Thereafter, it was punched into a disk shape of φ14 mm and pressed at 16 MPa for 2 minutes using a hand press to obtain a positive electrode.

[Example 4] (When formic acid is added)
Silicate-based positive electrode material (Reference Example 1) and ketjen black (conductive agent) are mixed at a mixing ratio of 75:15, and 1.6 ml of N-methyl-2-pyrrolidone and 5 mg of formic acid are added thereto and kneaded sufficiently. Slurry A was prepared. Add 100 ml of polyvinylidene fluoride (binder) and 1.8 ml of N-methyl-2-pyrrolidone to Slurry A to 100% of the total weight of the silicate cathode material and ketjen black. The mixture was kneaded to prepare a positive electrode slurry. The positive electrode slurry was applied to a current collector made of an aluminum foil having a thickness of 20 μm using a coating machine, and vacuum dried at 120 ° C. for 12 hours. Thereafter, it was punched into a disk shape of φ14 mm and pressed at 16 MPa for 2 minutes using a hand press to obtain a positive electrode.

[Example 5] (When propionic acid is added)
Silicate-based positive electrode material (Reference Example 1) and Ketjen black (conductive agent) are mixed at a mixing ratio of 75:15, and 1.6 ml of N-methyl-2-pyrrolidone and 5 mg of propionic acid are added to the mixture. The slurry A was prepared by kneading. To the slurry A was added 1.8 ml of polyvinylidene fluoride (binder) and 1.8 ml of N-methyl-2-pyrrolidone with a weight of 1/100 of the total weight of the silicate-based positive electrode material and ketjen black. The mixture was sufficiently kneaded to prepare a positive electrode slurry. The positive electrode slurry was applied to a current collector made of an aluminum foil having a thickness of 20 μm using a coating machine, and vacuum dried at 80 ° C. for 12 hours. Thereafter, it was punched into a disk shape of φ14 mm and pressed at 16 MPa for 2 minutes using a hand press to obtain a positive electrode.

[Comparative Example 1] (When no acid is added)
Silicate-based positive electrode material (Reference Example 1), ketjen black (conducting agent), and polyvinylidene fluoride (binding agent) are mixed at a weight ratio of 75:15:10, and this is mixed with N-methyl-2. -3.4 ml of pyrrolidone was added and sufficiently kneaded to prepare a positive electrode slurry. The positive electrode slurry was applied to a current collector made of an aluminum foil having a thickness of 20 μm using a coating machine, and vacuum dried at 80 ° C. for 12 hours. Thereafter, it was punched into a disk shape of φ14 mm and pressed at 16 MPa for 2 minutes using a hand press to obtain a positive electrode.

[Comparative Example 2] (when lactic acid is added)
Silicate positive electrode material (Reference Example 1) and ketjen black (conductive agent) are mixed at a blending ratio of 75:15, and 1.6 ml of N-methyl-2-pyrrolidone and 10 μl of lactic acid are added thereto and kneaded sufficiently. Slurry A was prepared. 1/10 weight of polyvinylidene fluoride (binding agent) and 1.8 ml of N-methyl-2-pyrrolidone with respect to the total weight of the silicate cathode material and ketjen black are added to slurry A and kneaded sufficiently. Thus, a positive electrode slurry was prepared. The positive electrode slurry was applied to a current collector made of an aluminum foil having a thickness of 20 μm using a coating machine, and vacuum dried at 120 ° C. for 12 hours. Thereafter, it was punched into a disk shape of φ14 mm and pressed at 16 MPa for 2 minutes using a hand press to obtain a positive electrode.

[Comparative Example 3] (When citric acid hydrate was added)
Silicate-based positive electrode material (Reference Example 1) and Ketjen black (conductive agent) were mixed at a mixing ratio of 75:15, and 1.6 ml of N-methyl-2-pyrrolidone and 30 wt% citric acid monohydrate were mixed therewith. 10 mg of an ethanol solution was added and kneaded sufficiently to prepare slurry A. Add 1/9 weight polyvinylidene fluoride (binding agent) and 1.8 ml N-methyl-2-pyrrolidone to Slurry A and knead well with the total weight of the silicate cathode material and ketjen black. Thus, a positive electrode slurry was prepared. The positive electrode slurry was applied to a current collector made of an aluminum foil having a thickness of 20 μm using a coating machine, and vacuum dried at 80 ° C. for 12 hours. Thereafter, it was punched into a disk shape of φ14 mm and pressed at 16 MPa for 2 minutes using a hand press to obtain a positive electrode.

[Test Example 1]
A coin-type lithium ion secondary battery was constructed using the positive electrode of the example or the comparative example. A lithium foil punched to φ15 mm was used for the negative electrode. As the electrolytic solution, a solution obtained by dissolving LIPF ₆ at a concentration of 1 mol / l in a mixed solvent in which ethylene carbonate and ethyl methyl carbonate were mixed at a volume ratio of 1: 1 was used. As the separator, a known one such as a polymer porous film such as polypropylene was used. These battery components were assembled and housed in a conventional manner in an atmosphere with a dew point of −50 ° C. or lower to produce a coin-type lithium secondary battery (CR-2032).

  Charging / discharging at a constant current density was performed using the manufactured lithium ion secondary battery. The charging conditions at this time were constant current and constant voltage charging with a current of 0.1 CA (33 mA / g) and a voltage of 4.5 V, and the discharging conditions were constant current discharging with a current of 0.1 CA and a final voltage of 1.5 V. All temperatures were 30 ° C.

  Table 1 shows the discharge capacity at the first cycle.

  As is apparent from Table 1, the positive electrode of the present invention obtained by using a kneaded product obtained by adding phosphoric acid, boric acid, a fatty acid having 1 to 6 carbon atoms or a salt thereof does not add an acid. In this case, the discharge capacity was higher than when other acids were added.

Claims (6)

Contains positive electrode active material consisting of particles containing olivine type lithium silicate compound, conductive additive, binder, organic solvent, and acids selected from phosphoric acid, boric acid, formic acid, acetic acid, propionic acid, butyric acid and their salts The positive electrode for secondary batteries which has the positive mix layer formed by apply | coating the kneaded material to apply | coat to an electroconductive base | substrate. The olivine-type lithium silicate compound contains lithium ions and metal M (M is one or more selected from Fe, Ni, Co, Al, Zn, Mn, V, and Zr) ions, and silicic acid The positive electrode for a secondary battery according to claim 1, which is an olivine type lithium silicate compound containing ions (SiO ₄ ⁴⁻ ).   The positive electrode for a secondary battery according to claim 1 or 2, wherein the conductive auxiliary agent is selected from carbon black conductive auxiliary agent, acetylene black, flaky graphite, fibrous carbon, and activated carbon.   The positive electrode for a secondary battery according to any one of claims 1 to 3, wherein the binder is selected from a resin-based binder, a rubber-based binder, and a cellulose-based binder.   The positive electrode for a secondary battery according to any one of claims 1 to 4, wherein the organic solvent is an aprotic polar organic solvent. A positive electrode active material composed of particles containing an olivine-type lithium silicate compound, a conductive aid, an organic solvent, and acids selected from phosphoric acid, boric acid, formic acid, acetic acid, propionic acid, butyric acid and their salts are kneaded and then combined. A method for producing a positive electrode for a secondary battery, wherein a kneaded product obtained by adding an adhesive is applied to a conductive substrate and dried.