WO2020059803A1 - Secondary battery - Google Patents

Abstract

This secondary battery is provided with a positive electrode, a negative electrode, and an electrolyte. The positive electrode is provided with a positive electrode active material layer which includes: positive electrode active material particles including Al on the surfaces; and a fluorine-based binder having a melting point in the range of 152-166˚C inclusive. The ratio y/x of the ratio y of Al in surface regions of the positive electrode active material particles, to the ratio x of Al in internal regions of the positive electrode active material particles satisfies the expression 149%≤y/x≤305%.

Description

Rechargeable battery

The present invention relates to a secondary battery.

(4) Since the characteristics of the secondary battery largely depend on the positive electrode active material to be used, various technologies for the positive electrode active material are being studied. For example, a technique for modifying the surface of the positive electrode active material particles has been studied.

In Patent Literature 1, an oxide containing lithium, aluminum, and boron is formed on the surface of a positive electrode active material, and a primary phase of a primary phase is formed at a grain boundary between primary particles existing near the surface of a secondary particle. A positive electrode active material for a non-aqueous electrolyte secondary battery containing aluminum at a higher concentration is described.

In Patent Document _{2, Li a Co b A c} B d O e F f (A is Al or Mg, B is group IV transition elements, 0.90 ≦ a ≦ 1.10,0.97 ≦ b ≦ 1.00 , 0.0001≤c≤0.03, 0.0001≤d≤0.03, 1.98≤e≤2.02, 0≤f≤0.02, 0.0001≤c + d≤0.03). The positive electrode active material for a lithium secondary battery, which is a particulate positive electrode active material for a lithium ion secondary battery, and in which the element A, the element B, and the fluorine are uniformly present near the surface of the particle, is described. I have.

JP-A-2015-76336 WO 2004-30126 pamphlet

In recent years, secondary batteries have been used in a variety of electronic devices. Therefore, there is a need for a technology capable of suppressing gas generation of batteries and improving safety.

目的 An object of the present invention is to provide a secondary battery capable of suppressing gas generation and improving safety.

In order to solve the above-mentioned problems, the present invention provides a positive electrode, a negative electrode, and an electrolytic solution, wherein the positive electrode has positive electrode active material particles containing Al on its surface, and fluorine having a melting point of 152 ° C. or more and 166 ° C. or less. And a ratio y / x of an Al ratio x in an inner region of the positive electrode active material particles and an Al ratio y in a surface region of the positive electrode active material particles is 149% ≦ y / x. It is a secondary battery that satisfies ≦ 305%.

According to the present invention, gas generation can be suppressed, and safety can be improved.

FIG. 1 is an exploded perspective view illustrating an example of a configuration of a nonaqueous electrolyte secondary battery according to a first embodiment of the present invention. FIG. 2 is a sectional view taken along the line II-II of FIG. 1. It is a graph which shows an example of a DSC curve of a fluorinated binder. FIG. 6 is a cross-sectional view illustrating an example of a configuration of a nonaqueous electrolyte secondary battery according to a second embodiment of the present invention. FIG. 5 is an enlarged cross-sectional view illustrating a part of the electrode body illustrated in FIG. 4. It is a block diagram showing an example of composition of electronic equipment concerning a 3rd embodiment of the present invention.

Embodiments of the present invention will be described in the following order.
1 First Embodiment (Example of Laminated Battery)
2. Second Embodiment (Example of Cylindrical Battery)
3. Third embodiment (example of electronic device)

<1 First embodiment>
[Configuration of Battery]
FIG. 1 shows an example of a configuration of a nonaqueous electrolyte secondary battery (hereinafter, simply referred to as “battery”) according to the first embodiment of the present invention. The battery according to the first embodiment is a so-called laminated battery, in which an electrode body 20 to which a positive electrode lead 11 and a negative electrode lead 12 are attached is housed inside a film-shaped exterior material 10, It is possible to reduce the weight and thickness.

(4) The positive electrode lead 11 and the negative electrode lead 12 are respectively directed from the inside of the exterior material 10 to the outside, for example, in the same direction. Each of the positive electrode lead 11 and the negative electrode lead 12 is made of, for example, a metal material such as Al, Cu, Ni, or stainless steel, and has a thin plate shape or a mesh shape, respectively.

The outer package 10 is made of, for example, a rectangular aluminum laminate film in which a nylon film, an aluminum foil, and a polyethylene film are laminated in this order. The exterior material 10 is disposed, for example, such that the polyethylene film side and the electrode body 20 face each other, and the respective outer edges are adhered to each other by fusion bonding or an adhesive. An adhesive film 13 is inserted between the exterior material 10 and the positive electrode lead 11 and the negative electrode lead 12 to suppress invasion of outside air. The adhesive film 13 is made of a material having adhesiveness to the positive electrode lead 11 and the negative electrode lead 12, for example, a polyolefin resin such as polyethylene, polypropylene, modified polyethylene or modified polypropylene.

The packaging material 10 may be made of a laminated film having another structure, a polymer film such as polypropylene, or a metal film, instead of the above-described aluminum laminated film. Alternatively, it may be constituted by a laminate film in which a polymer film is laminated on one or both sides of an aluminum film as a core material.

FIG. 2 is a cross-sectional view of the electrode body 20 shown in FIG. 1 along the line II-II. The electrode body 20 is of a wound type, and has a configuration in which a long positive electrode 21 and a long negative electrode 22 are laminated via a long separator 23 and wound flat and spirally. The outermost peripheral portion is protected by a protective tape 24. An electrolytic solution as an electrolyte is injected into the exterior material 10 and impregnated in the positive electrode 21, the negative electrode 22, and the separator 23.

Hereinafter, the positive electrode 21, the negative electrode 22, the separator 23, and the electrolyte constituting the battery will be sequentially described.

(Positive electrode)
The positive electrode 21 includes, for example, a positive electrode current collector 21A and positive electrode active material layers 21B provided on both surfaces of the positive electrode current collector 21A. The positive electrode current collector 21A is made of, for example, a metal foil such as an aluminum foil, a nickel foil, or a stainless steel foil. The positive electrode current collector 21A may have a plate shape or a mesh shape. The cathode lead 11 may be configured by extending a part of the periphery of the cathode current collector 21A. The positive electrode active material layer 21B contains a positive electrode active material and a binder. The positive electrode active material layer 21B may further include a conductive auxiliary as needed.

(Positive electrode active material)
The positive electrode active material includes positive electrode active material particles containing Al on the surface, and Al is dissolved in the positive electrode active material crystal. When the positive electrode active material particles contain Al on the surface, the electrochemical stability of the surface of the positive electrode active material particles is improved, and gas generation can be suppressed. In particular, it is possible to suppress gas generation in a high temperature and high SOC (States of Charge) state. The concentration of Al on the surface of the positive electrode active material particles is higher than the concentration of Al inside the positive electrode active material particles. More specifically, the concentration of the element in the positive electrode active material particles decreases from the surface to the inside of the positive electrode active material particles. In this case, the concentration of the element may gradually change from the surface of the positive electrode active material particles toward the inside, or may change abruptly in a substantially step-like manner.

In the present invention, the “surface of the positive electrode active material particles” refers to a region having a depth of 30 nm or less from the outermost surface of the positive electrode active material particles. On the other hand, “inside of the positive electrode active material particles” refers to the center of the positive electrode active material particles. In the present invention, for convenience, a position at a depth of 100 nm from the outermost surface of the positive electrode active material particles is defined as the inside (center). Consider In the present invention, the “depth from the outermost surface of the positive electrode active material particles” is a sputtering depth in terms of SiO ₂ . The sputtering depth in terms of SiO _2, when irradiated with X-rays in the SiO ₂ base in the same conditions, something can be sputtered to a depth of nm, refers to the indication that.

The ratio y / x of the Al ratio x in the inner region of the positive electrode active material particles and the Al ratio y in the surface region of the positive electrode active material particles is 149% ≦ y / x ≦ 305%, preferably 189% ≦ y / x. ≦ 305%, more preferably 201% ≦ y / x ≦ 263%. The ratio y / x is a parameter representing the uneven distribution ratio of Al on the surface of the positive electrode active material particles. If the ratio y / x is y / x <149%, the uneven distribution rate of Al on the surface is too low, and the effect of suppressing gas generation by Al decreases. On the other hand, if the ratio y / x is 305% <y / x, the uneven distribution rate of Al on the surface is too high. Therefore, even if the surface of the positive electrode active material particles is coated with a wide and thin binder film as described later, Cannot suppress the decrease in thermal stability inside the positive electrode active material particles due to the inclusion of Al. Therefore, safety is reduced.

比率 The above ratio y / x is measured as follows. First, the cathode 21 is taken out of the battery, washed and dried with dimethyl carbonate (DMC), and then heated and stirred in a solvent in which the binder is soluble (for example, N-methyl-pyrrolidone in the case of PVdF) to remove the binder. While dissolving in the conductive agent, the conductive additive is dispersed in the solvent, and the positive electrode active material is taken out by centrifugation.

Next, three samples were prepared by sputtering from the outermost surface of the positive electrode active material particles to a depth of 100 ± 5 nm by X-ray photoelectron spectroscopy (XPS). The ratio I _{1 of the} Al peak area (integrated intensity) I ₁ obtained when measuring at three different depths within the range and the peak area (integrated intensity) I ₂ of the transition metal species most contained in the active material. The average of / I ₂ is defined as Al ratio x. In addition, three samples were prepared by sputtering from the outermost surface of the positive electrode active material particles to a depth of 30 ± 5 nm by XPS, and each sample was measured at three different depths within the above 30 ± 5 nm range. The average of the ratio I ₃ / I ₄ of the Al peak area (integrated intensity) I ₃ obtained in this case and the peak area (integrated intensity) I ₄ of the transition metal species most contained in the positive electrode active material is defined as the Al ratio y. . Finally, the ratio y / x is calculated using the Al ratios x and y obtained as described above. Here, each of the depths of 30 nm and 100 nm is a depth in terms of SiO ₂ . The metal species most contained in the positive electrode active material is Ni for NCA, and Co for LCO.

The measurement apparatus and measurement conditions used for XPS are shown below.
Measuring device: Quantera SXM, manufactured by ULVAC-PHI, Inc.
X-ray source: monochromatic Al-Kα (1486.6 eV)
X-ray spot diameter: 100 μm
Sputtering rate: 1.4 nm / min in terms of SiO ₂

The positive electrode active material particles are primary particles. The positive electrode active material particles can occlude and release lithium as an electrode reactant, and include a lithium transition metal composite oxide having a layered rock salt type crystal structure. The lithium transition metal composite oxide contains, for example, Ni or Co as a main component as a metal element. It is particularly preferable that the lithium transition metal composite oxide contains Ni as a main component as a metal element. When the lithium transition metal composite oxide containing Ni as the main component is used, the numerical range of the above ratio y / x is compared with the case where the lithium transition metal composite oxide containing Co as the main component is used. This is because the expression of the effect of improving safety by combination with a fluorine-based binder becomes remarkable. Here, “containing Ni or Co as a main component as a metal element” means that the atomic ratio of Ni or Co is 50% or more with respect to the total amount of the metal elements contained in the lithium transition metal composite oxide. I do.

When the positive electrode active material particles include a lithium transition metal composite oxide containing Ni as a main component as a metal element, the average composition of the positive electrode active material particles is represented by, for example, the following formula (A).
_{LiNi 1-xyz Co x Al y} M1 z O 2 ··· (A)
(However, M1 is Mn, Mg, B, W, Zr, Ti, P, S, Fe, Si, Cr, Cu, Zn, Ge, Y, Mo, Ag, Ba, In, Sr, Sn, Pb and Sb is at least one selected from the group consisting of Sb, wherein x, y and z are 0.01 ≦ x ≦ 0.3, 0.01 ≦ y ≦ 0.3, 0 ≦ z ≦ 0.05, 1− xyz ≧ 0.5. Note that the composition of lithium varies depending on the state of charge and discharge, and represents a value in a completely discharged state.)

When the positive electrode active material particles include a lithium transition metal composite oxide containing Co as a main component as a metal element, the average composition of the positive electrode active material particles is represented, for example, by the following formula (B).
Li _r Co _(1-s) M2 _s O _{(2-t) Fu} ... (B)
(However, M2 is at least one selected from the group consisting of Ni, Mn, Mg, Al, B, Ti, V, Cr, Fe, Cu, Zn, Mo, Sn, Ca, Sr and W. r , S, t and u are values in the range of 0.8 ≦ r ≦ 1.2, 0 ≦ s <0.5, −0.1 ≦ t ≦ 0.2, 0 ≦ u ≦ 0.1. Note that the composition of lithium differs depending on the state of charge and discharge, and the value of r represents a value in a completely discharged state.)

When the positive electrode active material particles include a lithium transition metal composite oxide containing Ni as a main component as a metal element, the average particle size of the positive electrode active material particles is, for example, a primary particle diameter of 0.3 μm or more and 0.8 μm or less. It is. When the positive electrode active material particles include a lithium transition metal composite oxide containing Co as a main component as a metal element, the average particle size of the positive electrode active material particles is, for example, 3 μm or more and 30 μm or less.

平均 The average particle size of the positive electrode active material particles is measured as follows. First, the battery is disassembled, the positive electrode 21 is taken out, the taken out positive electrode 21 is washed with dimethyl carbonate (DMC), and dried, and then, the positive electrode 21 is cut to obtain a sample piece. SEM observation is performed. Then, 30 particles are randomly selected from the photographed SEM image, the area of the particle cross section is measured by image processing, and the particle diameter (diameter) of each particle is obtained assuming that the particle cross section is circular. Finally, the average particle diameter of the 30 measured particles is simply averaged (arithmetic average) to obtain an average particle diameter, which is defined as the average particle diameter of the positive electrode active material particles.

(binder)
The binder includes a fluorine-based binder. The melting point of the fluorine-based binder is from 152 ° C. to 166 ° C., preferably from 152 ° C. to 160 ° C., and more preferably from 152 ° C. to 156 ° C. If the melting point of the fluorine-based binder is less than 152 ° C., the positive electrode active material layer 21B becomes too hard when the positive electrode active material layer 21B is dried (heat treated) in the manufacturing process of the positive electrode 21; Becomes difficult. On the other hand, when the melting point of the fluorine-based binder exceeds 166 ° C., when the positive electrode active material layer 21 </ b> B is dried (heat treated) in the manufacturing process of the positive electrode 21, the binder becomes difficult to flow, and the surface of the positive electrode active material particles is wide and thin. It cannot be coated with a binder film. For this reason, the reaction surface of the positive electrode active material particles with the electrolyte cannot be reduced. Further, it becomes impossible to suppress a decrease in thermal stability inside the positive electrode active material particles due to the inclusion of Al on the surface. Therefore, gas generation cannot be suppressed, and safety cannot be improved.

融 点 The melting point of the above-mentioned fluorine-based binder is measured as follows. First, the cathode 21 is taken out of the battery, washed and dried with DMC, the cathode current collector 21A is removed, and the binder is dispersed by heating and stirring in an appropriate dispersion medium (eg, N-methylpyrrolidone). Dissolve in the medium. Thereafter, the positive electrode active material is removed by centrifugation, and the supernatant is filtered, and then the binder is removed by evaporation to dryness or reprecipitation in water.

Next, a sample of several to several tens mg was heated at a heating rate of 1 to 10 ° C./min by a differential scanning calorimeter (DSC, for example, Rigaku Thermoplus plus DSC 8230 manufactured by Rigaku Corporation), and then heated at 100 to 250 ° C. Of the endothermic peaks appearing in the temperature range up to (see FIG. 3), the temperature showing the maximum endothermic amount is defined as the melting point of the fluorine-based binder.

The fluorine-based binder is, for example, polyvinylidene fluoride (PVdF). As the polyvinylidene fluoride, it is preferable to use a homopolymer of vinylidene fluoride (VdF). As the polyvinylidene fluoride, a copolymer of vinylidene fluoride (VdF) and another monomer can be used. However, the polyvinylidene fluoride, which is a copolymer, swells and dissolves in the electrolytic solution. The characteristics of the positive electrode 21 may be deteriorated because the bonding is easy and the binding force is weak. As the polyvinylidene fluoride, one obtained by modifying a part of the terminal or the like with a carboxylic acid such as maleic acid may be used.

The content of the fluorine-based binder in the positive electrode active material layer 21B is 0.7% by mass to 4.0% by mass, preferably 2.0% by mass to 4.0% by mass, and more preferably 3.0% by mass. % Or more and 4.0% by mass or less. When the content of the fluorine-based binder is 0.7% by mass or more, the surface of the positive electrode active material particles can be effectively coated with the binder. Can be reduced. Further, since the surface of the positive electrode active material particles can be effectively coated with the binder as described above, it is possible to effectively suppress a decrease in thermal stability inside the positive electrode active material particles due to the inclusion of Al on the surface. Can also. Therefore, generation of gas can be further suppressed, and safety can be further improved. On the other hand, when the content of the binder is 4.0% by mass or less, a decrease in the exposure ratio of the surface of the positive electrode active material particles causes a decrease in the diffusion path of Li ions, thereby suppressing deterioration in load characteristics. . In addition, it is possible to suppress an increase in internal resistance of the battery during a charge / discharge cycle, and to suppress a decrease in charge / discharge cycle characteristics.

含有 The content of the above-mentioned fluorine-based binder is measured as follows. First, the cathode 21 is taken out of the battery, washed with DMC, and dried. Next, a sample of several to several tens of mg was subjected to a differential thermal balance (TG-DTA, for example, Rigaku Thermoplus TG8120 manufactured by Rigaku Corporation) at a temperature rising rate of 1 to 5 ° C./min in an air atmosphere at 600 ° C. C., and the content of the fluorine-based binder in the positive electrode active material layer 21B is determined from the weight loss at that time. The amount of weight loss due to the binder can be determined by isolating the binder as described in the method for measuring the melting point of the binder, performing TG-DTA measurement of the binder alone in an air atmosphere, Can be confirmed by examining how many times they burn.

(Conduction aid)
As the conductive additive, for example, at least one carbon material of graphite, carbon fiber, carbon black, acetylene black, Ketjen black, carbon nanotube, graphene, and the like can be used. Note that the conductive assistant may be any material having conductivity, and is not limited to a carbon material. For example, a metal material or a conductive polymer material may be used as the conductive assistant. Examples of the shape of the conductive additive include, but are not limited to, granules, scales, hollows, needles, and cylinders.

(Negative electrode)
The negative electrode 22 includes, for example, a negative electrode current collector 22A and negative electrode active material layers 22B provided on both surfaces of the negative electrode current collector 22A. The anode current collector 22A is made of, for example, a metal foil such as a copper foil, a nickel foil, or a stainless steel foil. The negative electrode current collector 22A may have a plate shape or a mesh shape. The negative electrode lead 12 may be configured by extending a part of the peripheral edge of the negative electrode current collector 22A. The anode active material layer 22B includes one or more anode active materials capable of inserting and extracting lithium. The negative electrode active material layer 22B may further include at least one of a binder and a conductive additive as necessary.

In this battery, the electrochemical equivalent of the negative electrode 22 or the negative electrode active material is larger than the electrochemical equivalent of the positive electrode 21. In theory, lithium metal does not precipitate on the negative electrode 22 during charging. Is preferred.

(Negative electrode active material)
Examples of the negative electrode active material include carbon materials such as non-graphitizable carbon, easily graphitizable carbon, graphite, pyrolytic carbons, cokes, glassy carbons, organic polymer compound fired bodies, carbon fibers, and activated carbon. Is mentioned. Among them, the coke includes pitch coke, needle coke, petroleum coke and the like. An organic polymer compound fired body is obtained by firing a polymer material such as a phenol resin or a furan resin at an appropriate temperature and carbonizing the material, and a part thereof is hardly graphitizable carbon or easily graphitizable carbon. Some are classified as. These carbon materials are preferable because a change in crystal structure that occurs during charge and discharge is very small, a high charge and discharge capacity can be obtained, and good cycle characteristics can be obtained. Particularly, graphite is preferable because it has a large electrochemical equivalent and can obtain a high energy density. Further, non-graphitizable carbon is preferable because excellent cycle characteristics can be obtained. Furthermore, a material having a low charge / discharge potential, specifically, a material having a charge / discharge potential close to lithium metal is preferable because a high energy density of the battery can be easily realized.

Other examples of the negative electrode active material capable of increasing the capacity include a material containing at least one of a metal element and a metalloid element as a constituent element (for example, an alloy, a compound, or a mixture). If such a material is used, a high energy density can be obtained. In particular, when used together with a carbon material, high energy density can be obtained and excellent cycle characteristics can be obtained, which is more preferable. In the present invention, alloys include alloys containing one or more metal elements and one or more metalloid elements in addition to alloys composed of two or more metal elements. Further, a nonmetallic element may be included. The structure includes a solid solution, a eutectic (eutectic mixture), an intermetallic compound, and a structure in which two or more of them coexist.

As such a negative electrode active material, for example, a metal element or a metalloid element capable of forming an alloy with lithium is given. Specific examples include Mg, B, Al, Ti, Ga, In, Si, Ge, Sn, Pb, Bi, Cd, Ag, Zn, Hf, Zr, Y, Pd and Pt. These may be crystalline or amorphous.

As the negative electrode active material, a material containing a metal element or a metalloid element belonging to the group 4B in the short-periodic periodic table as a constituent element is preferable, and a material containing at least one of Si and Sn as a constituent element is more preferable. This is because Si and Sn have a large ability to insert and extract lithium and can obtain a high energy density. Examples of such a negative electrode active material include a simple substance, an alloy, or a compound of Si, a simple substance, an alloy, or a compound of Sn, and a material having at least one or more of them.

As an alloy of Si, for example, Sn, Ni, Cu, Fe, Co, Mn, Zn, In, Ag, Ti, Ge, Bi, Sb, Nb, Mo, Al, as second constituent elements other than Si, Examples include those containing at least one selected from the group consisting of P, Ga, and Cr. As an alloy of Sn, for example, as a second constituent element other than Sn, Si, Ni, Cu, Fe, Co, Mn, Zn, In, Ag, Ti, Ge, Bi, Sb, Nb, Mo, Al, Examples include those containing at least one selected from the group consisting of P, Ga, and Cr.

Examples of the compound of Sn or the compound of Si include those containing O or C as a constituent element. These compounds may contain the second constituent element described above.

Above all, it is preferable that the Sn-based negative electrode active material contains Co, Sn, and C as constituent elements and has a low crystallinity or an amorphous structure.

Other negative electrode active materials include, for example, metal oxides or polymer compounds capable of inserting and extracting lithium. Examples of the metal oxide include lithium titanium oxide containing Li and Ti, such as lithium titanate (Li ₄ Ti ₅ O ₁₂ ), iron oxide, ruthenium oxide, and molybdenum oxide. Examples of the polymer compound include polyacetylene, polyaniline, and polypyrrole.

(binder)
Examples of the binder include resin materials such as polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), polyacrylonitrile (PAN), styrene butadiene rubber (SBR), and carboxymethyl cellulose (CMC), and these resin materials as main components. At least one selected from the group consisting of copolymers and the like is used.

(Conduction aid)
As the conductive auxiliary, the same as the positive electrode active material layer 21B can be used.

(Separator)
The separator 23 separates the positive electrode 21 and the negative electrode 22 and allows lithium ions to pass therethrough while preventing current short circuit due to contact between the two electrodes. The separator 23 is made of, for example, polytetrafluoroethylene, a polyolefin resin (such as polypropylene (PP) or polyethylene (PE)), an acrylic resin, a styrene resin, a polyester resin or a nylon resin, or a porous material formed by blending these resins. It may be formed of a porous film, and may have a structure in which two or more of these porous films are laminated.

Above all, a porous film made of polyolefin is preferable because it has an excellent short circuit prevention effect and can improve the safety of the battery by a shutdown effect. In particular, polyethylene is preferable as a material constituting the separator 23 because it can obtain a shutdown effect in the range of 100 ° C. or more and 160 ° C. or less and has excellent electrochemical stability. Among them, low-density polyethylene, high-density polyethylene, and linear polyethylene are suitably used because they have an appropriate melting temperature and are easily available. Alternatively, a material obtained by copolymerizing or blending a resin having chemical stability with polyethylene or polypropylene can be used. Alternatively, the porous membrane may have a structure of three or more layers in which a polypropylene layer, a polyethylene layer, and a polypropylene layer are sequentially laminated. For example, a three-layer structure of PP / PE / PP is preferable, and the mass ratio [wt%] of PP and PE is preferably set to PP: PE = 60: 40 to 75:25. Alternatively, from the viewpoint of cost, a single-layer substrate having 100 wt% of PP or 100 wt% of PE can be used. The method for producing the separator 23 may be either a wet method or a dry method.

不 織布 A non-woven fabric may be used as the separator 23. Aramid fiber, glass fiber, polyolefin fiber, polyethylene terephthalate (PET) fiber, nylon fiber, or the like can be used as the fiber constituting the nonwoven fabric. Further, these two or more kinds of fibers may be mixed to form a nonwoven fabric.

The separator 23 may have a configuration including a base material and a surface layer provided on one or both surfaces of the base material. The surface layer includes electrically insulating inorganic particles, and a resin material that binds the inorganic particles to the surface of the base material and binds the inorganic particles to each other. This resin material may be, for example, fibrillated and have a three-dimensional network structure in which a plurality of fibrils are connected. The inorganic particles are supported on the resin material having the three-dimensional network structure. In addition, the resin material may bind the surface of the base material or the inorganic particles without fibrillation. In this case, higher binding properties can be obtained. By providing a surface layer on one or both surfaces of the base material as described above, the oxidation resistance, heat resistance, and mechanical strength of the separator 23 can be increased.

The base material is a porous film composed of an insulating film that transmits lithium ions and has a predetermined mechanical strength. Since the electrolyte solution is held in the pores of the base material, the base material has resistance to the electrolyte solution. , High reactivity, low reactivity, and difficulty in expanding.

樹脂 As a material forming the base material, the above-described resin material or nonwoven fabric forming the separator 23 can be used.

The inorganic particles include at least one selected from the group consisting of metal oxides, metal nitrides, metal carbides, metal sulfides, and the like. Examples of the metal oxide include aluminum oxide (alumina, Al ₂ O ₃ ), boehmite (hydrated aluminum oxide), magnesium oxide (magnesia, MgO), titanium oxide (titania, TiO ₂ ), and zirconium oxide (zirconia, ZrO _2). ), Silicon oxide (silica, SiO ₂ ) or yttrium oxide (yttria, Y ₂ O ₃ ) or the like can be suitably used. As the metal nitride, silicon nitride (Si ₃ N ₄ ), aluminum nitride (AlN), boron nitride (BN), titanium nitride (TiN), or the like can be preferably used. As the metal carbide, silicon carbide (SiC) or boron carbide (B ₄ C) can be preferably used. As the metal sulfide, barium sulfate (BaSO ₄ ) or the like can be suitably used. Among the above metal oxides, it is preferable to use alumina, titania (especially those having a rutile structure), silica or magnesia, and more preferably to use alumina.

Further, the inorganic particles are made of a porous aluminosilicate such as zeolite (M _{2 / n} O.Al ₂ O ₃ .xSiO ₂ .yH ₂ O, M is a metal element, x ≧ 2, y ≧ 0); Salts, minerals such as barium titanate (BaTiO ₃ ) or strontium titanate (SrTiO ₃ ) may be included. The inorganic particles have oxidation resistance and heat resistance, and the surface layer on the side facing the positive electrode containing the inorganic particles has strong resistance to an oxidizing environment near the positive electrode during charging. The shape of the inorganic particles is not particularly limited, and any of a spherical shape, a plate shape, a fibrous shape, a cubic shape, and a random shape can be used.

粒径 The particle size of the inorganic particles is preferably in the range of 1 nm to 10 μm. If the thickness is less than 1 nm, it is difficult to obtain, and if the thickness is more than 10 μm, the distance between the electrodes becomes large, so that a sufficient amount of active material cannot be obtained in a limited space, and the battery capacity is reduced.

Examples of the resin material constituting the surface layer include fluorine-containing resins such as polyvinylidene fluoride and polytetrafluoroethylene, fluorine-containing rubbers such as vinylidene fluoride-tetrafluoroethylene copolymer and ethylene-tetrafluoroethylene copolymer, and styrene. -Butadiene copolymer or hydride thereof, acrylonitrile-butadiene copolymer or hydride thereof, acrylonitrile-butadiene-styrene copolymer or hydride thereof, methacrylate-acrylate copolymer, styrene-acrylate Copolymers, acrylonitrile-acrylate copolymers, rubbers such as ethylene propylene rubber, polyvinyl alcohol, polyvinyl acetate, etc., ethyl cellulose, methyl cellulose, hydroxyethyl cellulose, carboxymethyl Cellulose derivatives such as cellulose, polyphenylene ether, polysulfone, polyethersulfone, polyphenylene sulfide, polyetherimide, polyimide, polyamide such as wholly aromatic polyamide (aramid), polyamideimide, polyacrylonitrile, polyvinyl alcohol, polyether, acrylic resin Alternatively, a resin having high heat resistance such as polyester having at least one of a melting point and a glass transition temperature of 180 ° C. or more may be used. These resin materials may be used alone or as a mixture of two or more. Above all, from the viewpoint of oxidation resistance and flexibility, a fluororesin such as polyvinylidene fluoride is preferable, and from the viewpoint of heat resistance, it is preferable to contain aramid or polyamideimide.

As a method for forming the surface layer, for example, a slurry composed of a matrix resin, a solvent, and inorganic particles is applied on a substrate (porous film), and the slurry is passed through a poor solvent for the matrix resin and a solvent-friendly bath of the solvent. A method of phase separation and then drying can be used.

The inorganic particles described above may be contained in a porous film as a substrate. Further, the surface layer may not include the inorganic particles, and may be formed only of the resin material.

(Electrolyte)
The electrolyte is a so-called non-aqueous electrolyte, and includes an organic solvent (non-aqueous solvent) and an electrolyte salt dissolved in the organic solvent. The electrolytic solution may contain a known additive in order to improve battery characteristics. Note that, instead of the electrolytic solution, an electrolytic layer containing the electrolytic solution and a polymer compound serving as a holder for holding the electrolytic solution may be used. In this case, the electrolyte layer may be in a gel state.

環状 As the organic solvent, a cyclic carbonate such as ethylene carbonate or propylene carbonate can be used, and it is preferable to use one of ethylene carbonate and propylene carbonate, particularly a mixture of both. This is because the cycle characteristics can be further improved.

As the organic solvent, it is preferable to use a mixture of chain carbonates such as diethyl carbonate, dimethyl carbonate, ethylmethyl carbonate and methylpropyl carbonate in addition to these cyclic carbonates. This is because high ionic conductivity can be obtained.

It is preferable that the organic solvent further contains 2,4-difluoroanisole or vinylene carbonate. This is because 2,4-difluoroanisole can further improve the discharge capacity, and vinylene carbonate can further improve the cycle characteristics. Therefore, it is preferable to use a mixture of these, because the discharge capacity and cycle characteristics can be further improved.

Besides these, organic solvents include butylene carbonate, γ-butyrolactone, γ-valerolactone, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolan, and 4-methyl-1,3 -Dioxolan, methyl acetate, methyl propionate, acetonitrile, glutaronitrile, adiponitrile, methoxyacetonitrile, 3-methoxypropironitrile, N, N-dimethylformamide, N-methylpyrrolidinone, N-methyloxazolidinone, N, N- Examples thereof include dimethylimidazolidinone, nitromethane, nitroethane, sulfolane, dimethylsulfoxide, and trimethyl phosphate.

Note that compounds in which at least part of hydrogen in these organic solvents has been replaced with fluorine may be preferable because the reversibility of the electrode reaction can be improved depending on the type of electrode to be combined.

It is preferable that the electrolyte further contains a halogenated carbonate represented by the following formula (1) as an additive from the viewpoint of improving cycle characteristics. The consumption of the halogenated carbonate in the positive electrode 21 is suppressed by Al contained in the surface of the positive electrode active material particles and the binder film covering the surface of the positive electrode active material particles widely and thinly. As a result, more of the halogenated carbonate is consumed by the negative electrode 22, and the effect of improving the cycle characteristics by the halogenated carbonate becomes remarkable.

(Wherein R1 to R4 are each independently a hydrogen group, a halogen group, an alkyl group or a halogenated alkyl group, provided that at least one of R1 to R4 is a halogen group or a halogenated alkyl group. The halogen group is preferably a fluorine group, and the halogenated alkyl group is preferably a fluorinated alkyl group.)

Examples of the halogenated carbonate represented by the formula (1) include 4-fluoro-1,3-dioxolan-2-one, 4-chloro-1,3-dioxolan-2-one, and 4,5-difluoro -1,3-dioxolan-2-one, tetrafluoro-1,3-dioxolan-2-one, 4-chloro-5-fluoro-1,3-dioxolan-2-one, 4,5-dichloro-1, 3-oxolan-2-one, tetrachloro-1,3-dioxolan-2-one, 4,5-bistrifluoromethyl-1,3-dioxolan-2-one, 4-trifluoromethyl-1,3-dioxolan -2-one, 4,5-difluoro-4,5-dimethyl-1,3-dioxolan-2-one, 4,4-difluoro-5-methyl-1,3-dioxolan-2-one, 4-ethyl 5,5-difluoro-1,3-dioxolan-2-one, 4-fluoro-5-trifluoromethyl-1,3-dioxolan-2-one, 4-methyl-5-trifluoromethyl-1,3- Dioxolan-2-one, 4-fluoro-4,5-dimethyl-1,3-dioxolan-2-one, 5- (1,1-difluoroethyl) -4,4-difluoro-1,3-dioxolan-2 -One, 4,5-dichloro-4,5-dimethyl-1,3-dioxolan-2-one, 4-ethyl-5-fluoro-1,3-dioxolan-2-one, 4-ethyl-4,5 -Difluoro-1,3-dioxolan-2-one, 4-ethyl-4,5,5-trifluoro-1,3-dioxolan-2-one, 4-fluoro-4-methyl-1,3-dioxolan- 2-on, etc.These may be used alone or in combination of two or more.

Among them, 4-fluoro-1,3-dioxolan-2-one or 4,5-difluoro-1,3-dioxolan-2-one is preferred. This is because it is easily available and a high effect can be obtained.

The lower limit of the content of the halogenated carbonate represented by the formula (1) in the electrolytic solution is preferably 0.01% by mass or more, more preferably 1% by mass or more, and still more preferably 3% by mass or more. It is. When the lower limit of the content of the halogenated carbonate is 0.01% by mass or more, the charge / discharge cycle characteristics can be particularly improved. The upper limit of the content of the halogenated carbonate represented by the formula (1) in the electrolytic solution is preferably 6% by mass or less, more preferably 5% by mass or less, and even more preferably 4% by mass or less. . When the upper limit of the content of the halogenated carbonate is 6% by mass or less, an increase in gas generation due to the addition of the halogenated carbonate can be suppressed.

含有 The content of the halogenated carbonate is determined as follows. First, the battery is disassembled under an inert atmosphere such as a glove box, and an electrolyte component is extracted using DMC or a heavy solvent. Next, GC-MS (Gas-Chromatograph-Mass-Spectrometry) measurement and ICP (Inductively-Coupled-Plasma) measurement are performed on the obtained extract to determine the content of the halogenated carbonate in the electrolytic solution.

Examples of the electrolyte salt include a lithium salt, and one kind may be used alone, or two or more kinds may be used in combination. The lithium _{salt, LiPF 6, LiBF 4, LiAsF} 6, LiClO 4, LiB (C 6 H 5) 4, LiCH 3 SO 3, LiCF 3 SO 3, LiN (SO 2 CF 3) 2, LiC (SO 2 CF ₃ ) ₃ , LiAlCl ₄ , LiSiF ₆ , LiCl, lithium difluoro [oxolate-O, O ′] borate, lithium bisoxalate borate, or LiBr. Above all, LiPF ₆ is preferable because high ion conductivity can be obtained and cycle characteristics can be further improved.

[Battery operation]
In the battery having the above structure, when charged, for example, lithium ions are released from the positive electrode active material layer 21B and occluded in the negative electrode active material layer 22B via the electrolytic solution. Further, when discharging is performed, for example, lithium ions are released from the negative electrode active material layer 22B and occluded in the positive electrode active material layer 21B via the electrolytic solution.

[Battery manufacturing method]
Next, an example of the method for manufacturing the battery according to the first embodiment of the present invention will be described.

(Preparation process of positive electrode)
The positive electrode 21 is manufactured as follows. First, for example, a positive electrode mixture is prepared by mixing a positive electrode active material, a binder, and a conductive auxiliary, and the positive electrode mixture is dispersed in a solvent such as N-methyl-2-pyrrolidone (NMP) to form a paste. A positive electrode mixture slurry is prepared. Next, the positive electrode mixture slurry is applied to the positive electrode current collector 21A, the solvent is dried, and compression molding is performed by a roll press or the like to form the positive electrode active material layer 21B, and the positive electrode 21 is obtained.

When the positive electrode active material particles include a lithium transition metal composite oxide containing Ni as a metal element as a main component, the positive electrode active material particles are produced, for example, as follows. First, nickel sulfate, cobalt sulfate, and ammonium hydroxide are dissolved in water and co-precipitated to produce nickel and cobalt hydroxides. Next, a lithium compound such as lithium carbonate and lithium hydroxide and an aluminum compound such as aluminum hydroxide are added to the obtained hydroxide to prepare a precursor. When other additional elements are added, they are added at this stage. Thereafter, the obtained precursor is baked in a temperature range of, for example, 500 ° C. to 700 ° C. while changing the time conditions, so that the ratio y / x is in the range of 149% ≦ y / x ≦ 305%. Active material particles are obtained. The longer the firing time and the higher the temperature, the more Al diffuses into the crystal.

In the case where the positive electrode active material particles include a lithium transition metal composite oxide containing Co as a main component as a metal element, the positive electrode active material particles are produced, for example, as follows. First, cobalt sulfate and ammonium hydroxide are dissolved in water and coprecipitated to produce a cobalt hydroxide. Next, a lithium compound such as lithium carbonate and lithium hydroxide and an aluminum compound such as aluminum hydroxide are added to the obtained hydroxide to prepare a precursor. When other additional elements are added, they are added at this stage. Thereafter, the obtained precursor is baked in a temperature range of, for example, 600 ° C. to 800 ° C. while changing the time conditions, so that the ratio y / x is within a range of 149% ≦ y / x ≦ 305%. Active material particles are obtained. The longer the firing time and the higher the temperature, the more Al diffuses into the crystal.

(Negative electrode fabrication process)
The negative electrode 22 is manufactured as follows. First, for example, a negative electrode mixture is prepared by mixing a negative electrode active material and a binder, and this negative electrode mixture is dispersed in a solvent such as N-methyl-2-pyrrolidone to prepare a paste-like negative electrode mixture slurry. I do. Next, the negative electrode mixture slurry is applied to the negative electrode current collector 22A, the solvent is dried, and compression molding is performed by a roll press or the like to form the negative electrode active material layer 22B, and the negative electrode 22 is obtained.

(Production process of electrode body)
The wound electrode body 20 is manufactured as follows. First, the cathode lead 11 is attached to one end of the cathode current collector 21A by welding, and the anode lead 12 is attached to one end of the anode current collector 22A by welding. Next, the positive electrode 21 and the negative electrode 22 are wound around a flat core with a separator 23 interposed therebetween, and are wound many times in the longitudinal direction. Get.

(Sealing process)
The exterior body 10 seals the electrode body 20 as follows. First, the electrode body 20 is sandwiched between the package members 10, and the outer peripheral edge portion excluding one side is heat-fused into a bag shape, and is housed inside the package member 10. At that time, an adhesive film 13 is inserted between the positive electrode lead 11 and the negative electrode lead 12 and the exterior material 10. Note that the adhesive film 13 may be attached to each of the positive electrode lead 11 and the negative electrode lead 12 in advance. Next, after injecting the electrolytic solution into the exterior material 10 from one side of the unfused part, one side of the unfused part is heat-sealed in a vacuum atmosphere to be sealed. Thus, the batteries shown in FIGS. 1 and 2 are obtained.

[effect]
In the battery according to the first embodiment, the positive electrode active material layer 21B includes a positive electrode active material layer containing Al-containing positive electrode active material particles and a fluorine-based binder having a melting point of 152 ° C. or more and 166 ° C. or less. . The ratio y / x of the Al ratio x in the inner region of the positive electrode active material particles and the Al ratio y in the surface region of the positive electrode active material particles satisfies 149% ≦ y / x ≦ 305%. Thereby, Al can be unevenly distributed on the surface of the positive electrode active material particles in an appropriate amount, and the surface of the positive electrode active material particles can be covered with a wide and thin binder film. Therefore, gas generation, that is, battery swelling can be suppressed, and safety can be improved. In particular, gas generation at high temperature and high SOC, that is, battery swelling can be suppressed.

<2 Second Embodiment>
[Configuration of Battery]
FIG. 4 shows an example of a configuration of a secondary battery according to the second embodiment of the present invention. The battery according to the second embodiment is a so-called cylindrical type, and a pair of a band-shaped positive electrode 51 and a band-shaped negative electrode 52 are provided inside a substantially hollow cylindrical battery can (exterior material) 41 with a separator 53. And has a wound electrode body 50 after lamination. The battery can 41 is made of nickel-plated iron or aluminum or the like, and has one end closed and the other end open. An electrolytic solution as a liquid electrolyte is injected into the battery can 41, and is impregnated in the positive electrode 51, the negative electrode 52, and the separator 53. In addition, a pair of insulating plates 42 and 43 are respectively arranged perpendicular to the winding peripheral surface so as to sandwich the electrode body 50. The electrolyte is the same as the electrolyte in the first embodiment.

At the open end of the battery can 41, a battery cover 44, a safety valve mechanism 45 provided inside the battery cover 44, and a thermal resistance element (Positive Temperature Coefficient; PTC element) 46 are disposed via a sealing gasket 47. It is attached by being caulked. Thereby, the inside of the battery can 41 is sealed. The battery lid 44 is made of, for example, the same material as the battery can 41. The safety valve mechanism 45 is electrically connected to the battery cover 44. When the internal pressure of the battery becomes higher than a predetermined value due to an internal short circuit or external heating, the disk plate 45A is inverted and the battery cover 44 and the electrode are connected. The electrical connection with the body 50 is cut off. The sealing gasket 47 is made of, for example, an insulating material, and its surface is coated with asphalt.

例 え ば For example, a center pin 54 is inserted into the center of the electrode body 50. The positive electrode 51 of the electrode body 50 is connected to a positive electrode lead 55 made of aluminum or the like, and the negative electrode 52 is connected to a negative electrode lead 56 made of nickel or the like. The positive electrode lead 55 is electrically connected to the battery cover 44 by welding to the safety valve mechanism 45, and the negative electrode lead 56 is welded to and electrically connected to the battery can 41.

FIG. 5 is a cross-sectional view showing an enlarged part of the electrode body shown in FIG. The positive electrode 51 includes a positive electrode current collector 51A and positive electrode active material layers 51B provided on both surfaces of the positive electrode current collector 51A. The negative electrode 52 includes a negative electrode current collector 52A and negative electrode active material layers 52B provided on both surfaces of the negative electrode current collector 52A. The configurations of the positive electrode current collector 51A, the positive electrode active material layer 51B, the negative electrode current collector 52A, the negative electrode active material layer 52B, and the separator 53 are respectively the positive electrode current collector 21A, the positive electrode active material layer 21B, and the negative electrode in the first embodiment. The same applies to the current collector 22A, the negative electrode active material layer 22B, and the separator 53.

[effect]
In the battery according to the second embodiment, gas generation can be suppressed, and safety can be improved, as in the first embodiment. Further, in the state of high temperature and high SOC, it is possible to suppress the internal pressure of the battery from becoming high enough to operate the safety valve mechanism 45 due to gas generation.

<3 Third Embodiment>
In the third embodiment, an electronic device including the battery according to the first or second embodiment will be described.

FIG. 6 shows an example of the configuration of an electronic device 400 according to the third embodiment of the present invention. The electronic device 400 includes an electronic circuit 401 of the electronic device main body and the battery pack 300. Battery pack 300 is electrically connected to electronic circuit 401 via positive electrode terminal 331a and negative electrode terminal 331b. The electronic device 400 may have a configuration in which the battery pack 300 is detachable.

Examples of the electronic device 400 include a notebook personal computer, a tablet computer, a mobile phone (for example, a smartphone), a portable information terminal (Personal Digital Assistants: PDA), a display device (LCD (Liquid Crystal Display), and an EL (Electro Luminescence). A) Display, electronic paper, etc.), imaging device (eg, digital still camera, digital video camera, etc.), audio equipment (eg, portable audio player), game equipment, cordless phone handset, electronic book, electronic dictionary, radio, headphone, navigation System, memory card, pacemaker, hearing aid, power tool, electric shaver, refrigerator, air conditioner, TV, stereo, water heater, microwave oven, dishwasher, washing machine, dryer, lighting equipment, toy, medical equipment, robot Load conditioners, although traffic signals and the like, without such limited thereto.

(Electronic circuit)
The electronic circuit 401 includes, for example, a CPU (Central Processing Unit), a peripheral logic unit, an interface unit, a storage unit, and the like, and controls the entire electronic device 400.

(Battery pack)
The battery pack 300 includes an assembled battery 301 and a charge / discharge circuit 302. Battery pack 300 may further include an exterior material (not shown) that accommodates assembled battery 301 and charge / discharge circuit 302 as necessary.

The assembled battery 301 is configured by connecting a plurality of secondary batteries 301a in series and / or in parallel. The plurality of secondary batteries 301a are connected in, for example, n parallel and m series (n and m are positive integers). FIG. 6 shows an example in which six rechargeable batteries 301a are connected in two parallel and three series (2P3S). The battery according to the above-described first or second embodiment is used as the secondary battery 301a.

Here, a case will be described where the battery pack 300 includes an assembled battery 301 including a plurality of secondary batteries 301a. However, the battery pack 300 includes a single secondary battery 301a instead of the assembled battery 301. May be adopted.

The charging / discharging circuit 302 is a control unit that controls charging / discharging of the battery pack 301. Specifically, at the time of charging, the charge / discharge circuit 302 controls charging of the battery pack 301. On the other hand, at the time of discharging (that is, at the time of using the electronic device 400), the charging / discharging circuit 302 controls discharging to the electronic device 400.

ケ ー ス As the exterior material, for example, a case made of a metal, a polymer resin, or a composite material thereof can be used. Examples of the composite material include a laminate in which a metal layer and a polymer resin layer are laminated.

Hereinafter, the present invention will be described specifically with reference to Examples, but the present invention is not limited to these Examples.

In the following Examples and Comparative Examples, the ratio y / x, the melting point of the fluorine-based binder, and the content of FEC (halogenated carbonate) were determined by the measurement method described in the first embodiment. is there.

(Method of producing positive electrode active materials (1) to (5))
First, nickel sulfate, cobalt sulfate and ammonium hydroxide were dissolved in water and co-precipitated to produce nickel and cobalt hydroxides. Next, lithium carbonate and aluminum hydroxide were added to the obtained hydroxide to prepare a precursor. Thereafter, the obtained precursor is baked in a temperature range of 500 ° C. to 700 ° C. while changing the temperature and time conditions, whereby the positive electrode active material (1) to which the ratio y / x (Al uneven distribution ratio) is changed is (5) (LiNi _0.7 Co _0.2 Al _0.1 O ₂ ) was obtained.

(Method for producing positive electrode active materials (6) and (7))
First, cobalt sulfate and ammonium hydroxide were dissolved in water and co-precipitated to produce cobalt hydroxide. Next, lithium carbonate and aluminum hydroxide were added to the obtained hydroxide to prepare a precursor. Thereafter, the obtained precursor is fired in a temperature range of 600 ° C. to 800 ° C. while changing the temperature and time conditions, whereby the ratio y / x (Al uneven distribution ratio) of the positive electrode active material (6) to (7) (LiCo _0.98 Al _0.02 O ₂ ) was obtained.

[Examples 1 to 3, Comparative Examples 3 and 4]
(Preparation process of positive electrode)
A positive electrode was produced as follows. 96 parts by mass of LiNi _0.7 Co _0.2 Al _0.1 O ₂ as a positive electrode active material, ₂ parts by mass of polyvinylidene fluoride having a melting point of 166 ° C. as a binder (homopolymer of vinylidene fluoride), and 2 parts by mass of Ketjen black as a conductive aid Are mixed to form a positive electrode mixture, and this positive electrode mixture is dispersed in an organic solvent (N-methyl-2-pyrrolidone: NMP) to obtain a paste-like positive electrode mixture slurry. As shown in Table 1, the positive electrode active materials (1) to (5) having different ratios y / x were used as the positive electrode active materials. Subsequently, the positive electrode mixture slurry was applied to the positive electrode current collector (aluminum foil) using a coating device, and then dried to form a positive electrode active material layer. Finally, the positive electrode active material layer was compression-molded using a press.

(Negative electrode fabrication process)
A negative electrode was manufactured as follows. First, a negative electrode mixture was prepared by mixing 96% by mass of artificial graphite powder as a negative electrode active material and 4% by mass of polyvinylidene fluoride (PVdF) as a binder. Then, the negative electrode mixture was mixed with an organic solvent (N-methyl- 2-pyrrolidone: NMP) to give a paste-like negative electrode mixture slurry. Subsequently, the negative electrode mixture slurry was applied to the negative electrode current collector (copper foil) using a coating device and then dried. Finally, the negative electrode active material layer was compression molded using a press.

(Step of preparing electrolyte solution)
An electrolyte was prepared as follows. First, a mixed solvent was prepared by mixing ethylene carbonate (EC), propylene carbonate (PC), and diethyl carbonate (DEC) in a mass ratio of EC: PC: DEC = 15: 15: 70. Subsequently, in this mixed solvent, lithium hexafluorophosphate (LiPF ₆ ) as an electrolyte salt was dissolved at a concentration of 1 mol / l to prepare an electrolyte solution. Next, the amount of FEC was adjusted and added to the electrolyte solution so that the content of 4-fluoro-1,3-dioxolan-2-one (FEC) in the electrolyte solution in the completed battery was 1% by mass. .

(Battery assembly process)
The battery was assembled as follows. First, a positive electrode and a negative electrode obtained as described above are laminated through a separator made of a microporous polyethylene stretched film, a negative electrode, a separator, a positive electrode, and a separator in this order, and are wound many times to form a jelly roll type. Was obtained. Next, the wound electrode body was sandwiched between a pair of insulating plates, the inner wall was housed in a nickel-plated iron battery can, the negative electrode lead was welded to the bottom of the can, and the positive electrode lead was welded to the safety valve mechanism. Then, after injecting the electrolyte into the battery can by a reduced pressure method, the safety valve mechanism, the PTC element and the battery lid were fixed, and a cylindrical battery having an outer diameter (diameter) of 18.20 mm and a height of 65 mm was obtained.

[Examples 4 and 5, Comparative Example 1]
In the manufacturing process of the positive electrode, as shown in Table 1, as in Example 1, except that polyvinylidene fluoride (homopolymer of vinylidene fluoride) having a melting point of 152 ° C, 160 ° C, or 175 ° C was used as a binder. I got a battery.

[Comparative Example 2]
As shown in Table 1, a positive electrode was produced in the same manner as in Example 1 except that polyvinylidene fluoride having a melting point of 145 ° C. (homopolymer of vinylidene fluoride) was used as a binder. )There has occurred. Therefore, in Comparative Example 2, it was difficult to manufacture the battery.

[Examples 6 to 8, Comparative Example 6]
In the preparation process of the electrolytic solution, as shown in Table 2, the FEC content was adjusted so that the FEC content in the electrolytic solution in the completed battery was 0.01% by mass, 3% by mass, 6% by mass, and 10% by mass. A battery was obtained in the same manner as in Example 1, except that the amount was adjusted and added to the electrolytic solution.

[Comparative Example 5]
As shown in Table 2, in the preparation process of the electrolyte solution, a battery was obtained in the same manner as in Example 1 except that FEC was not added to the electrolyte solution.

[Comparative Example 7]
As shown in Table 2, a battery was obtained in the same manner as in Example 2 except that polyvinylidene fluoride having a melting point of 175 ° C. (homopolymer of vinylidene fluoride) was used as a binder in the process of manufacturing the positive electrode.

[Example 9]
As shown in Table 3, a battery was obtained in the same manner as in Example 1 except that the positive electrode active material (6) was used as the positive electrode active material in the process of manufacturing the positive electrode.

[Comparative Example 8]
As shown in Table 3, a battery was obtained in the same manner as in Comparative Example 4, except that polyvinylidene fluoride having a melting point of 175 ° C. (homopolymer of vinylidene fluoride) was used as a binder in the process of manufacturing the positive electrode.

[Comparative Example 9]
As shown in Table 3, in the manufacturing process of the positive electrode, the positive electrode active material (7) was used as the positive electrode active material, and polyvinylidene fluoride (homopolymer of vinylidene fluoride) having a melting point of 175 ° C. was used as the binder. A battery was obtained in the same manner as in Example 1 except for the above.

[Evaluation]
The following evaluation was performed on the battery obtained as described above.

(Heating limit temperature)
A battery charged with CCCV (Constant Current / Constant Voltage) for 4 hours at 0.5 C and 4.5 V was placed in an oven and heated at a specified temperature. After the sample reached the specified temperature, the temperature of the thermostat was maintained for 1 hour, and it was confirmed whether or not the battery would run out of heat. This procedure was repeated while increasing the specified temperature until thermal runaway was confirmed. The upper limit temperature at which thermal runaway was not confirmed was defined as the heating limit temperature.

(Interruption time)
The battery charged under the condition of a predetermined charging voltage +100 mV was stored in a thermostat at 90 ° C., and the time required until the safety valve was opened by the generated gas and the voltage was cut off was measured.

(Cycle characteristics)
After reaching 4.2 V at a charging current of 1 C, the battery was charged by reducing the current to 0.05 C, and then discharged at a discharging current of 1 C with 3 V as the initial voltage. This charge / discharge cycle was repeated, and the number of cycles until the discharge capacity was reduced to 80% of the discharge capacity in the first cycle was measured.

Table 1 shows the configurations and evaluation results of the batteries of Examples 1 to 5 and Comparative Examples 1 to 4.

Table 2 shows the configurations and evaluation results of the batteries of Examples 1, 6 to 8, and Comparative Examples 5 to 7.

Table 3 shows the configurations and evaluation results of the batteries of Examples 1 and 9 and Comparative Examples 8 and 9.

Table 1 shows the following. That is, the positive electrode includes a positive electrode active material layer including positive electrode active material particles containing Al on the surface and a fluorine-based binder having a melting point of 152 ° C. or more and 166 ° C. or less, and an Al ratio x in an internal region of the positive electrode active material particles. When the ratio y / x of the Al ratio y in the surface region of the active material particles satisfies 149% ≦ y / x ≦ 305%, the voltage cutoff time can be lengthened and the heating limit temperature can be reduced. Can be improved. Therefore, gas generation can be suppressed, and safety can be improved.

Table 2 shows the following. That is, from the viewpoint of improving the cycle characteristics, the content of FEC contained in the electrolytic solution is preferably 0.01% by mass or more, more preferably 1% by mass or more, and still more preferably 3% by mass or more. Further, from the viewpoint of suppressing a significant shortening of the voltage interruption time due to the addition of FEC, that is, from the viewpoint of suppressing a significant increase in gas generation due to the addition of FEC, the content of FEC contained in the electrolyte is preferably 6%. % By mass or less. In addition, it is preferable in terms of battery performance that the cut-off time exceeds 50 hours.

サ イ ク ル Further, by adding FEC to the battery in which the melting point of the binder and the ratio y / x are specified in the above ranges, particularly the cycle characteristics can be improved (see Example 1 and Comparative Example 7). This is considered to be because the FEC consumption in the positive electrode 21 is suppressed in the battery in which the melting point of the binder and the ratio y / x are defined in the above ranges.

Table 3 shows the following. That is, when the melting point and the ratio y / x of the binder are defined in the above range in the battery using NCA as the positive electrode active material, the melting point and the ratio y / x of the binder in the battery using LCO as the positive electrode active material are set as described above. The effect of improving the heating limit temperature, that is, the effect of improving safety, is more remarkable than in the case where the range is specified.

Although the embodiments of the present invention have been specifically described above, the present invention is not limited to the above-described embodiments, and various modifications based on the technical idea of the present invention are possible.

For example, the configurations, methods, steps, shapes, materials, numerical values, and the like mentioned in the above-described embodiments are merely examples, and if necessary, use different configurations, methods, steps, shapes, materials, numerical values, and the like. Is also good.

The configurations, methods, steps, shapes, materials, numerical values, and the like of the above-described embodiments can be combined with each other without departing from the gist of the present invention.

Reference Signs List 10 exterior material 11, 55 positive electrode lead 12, 56 negative electrode lead 13 adhesive film 20, 50 electrode body 21, 51 positive electrode 21A, 51A positive electrode current collector 21B, 51B positive electrode active material layer 22, 52 negative electrode 22A, 52A negative electrode current collector 22B, 52B Negative electrode active material layer 23, 53 Separator 24 Protective tape 41 Battery can 42, 43 Insulating plate 44 Battery cover 45 Safety valve mechanism 45A Disk plate 46 Thermal resistance element 47 Gasket 54 Center pin 300 Battery pack 400 Electronic device

Claims (3)

1. Comprising a positive electrode, a negative electrode, and an electrolytic solution,
   The positive electrode includes a positive electrode active material layer including positive electrode active material particles containing Al on the surface and a fluorine-based binder having a melting point of 152 ° C. or more and 166 ° C. or less,
   A secondary battery in which a ratio y / x of an Al ratio x in an inner region of the positive electrode active material particles and an Al ratio y in a surface region of the positive electrode active material particles satisfies 149% ≦ y / x ≦ 305%.
2. The electrolyte further includes a halogenated carbonate represented by the following formula (1),
   The secondary battery according to claim 1, wherein the content of the halogenated carbonate in the electrolytic solution is 0.01% by mass or more and 6% by mass or less.
   

   

   (Wherein R1 to R4 are each independently a hydrogen group, a halogen group, an alkyl group or a halogenated alkyl group, provided that at least one of R1 to R4 is a halogen group or a halogenated alkyl group. Is.)
3. The secondary battery according to claim 1, wherein the average composition of the positive electrode active material particles is represented by the following formula (A).
   _{LiNi 1-xyz Co x Al y} M z O 2 ··· (A)
   (Provided that x, y, and z are 0.01 ≦ x ≦ 0.3, 0.01 ≦ y ≦ 0.3, 0 ≦ z ≦ 0.05, 1−x−y−z ≧ 0.5) M is Mn, Mg, B, W, Zr, Ti, P, S, Fe, Si, Cr, Cu, Zn, Ge, Y, Mo, Ag, Ba, In, Sr, Sn, Pb and Sb. At least one selected from the group consisting of

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