JP2012089402A - Lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium ion secondary battery that is excellent in safety and high temperature storage characteristics, and is suitable for charging and discharging in a large current.SOLUTION: A lithium ion secondary battery of the present invention includes: a positive electrode 1 containing a manganese-containing lithium composite oxide as a positive electrode active material; a negative electrode 2 containing a substance capable of occluding and discharging lithium as a negative electrode active material; a separator 3; and a nonaqueous electrolyte 4. A BET specific area of the manganese-containing lithium composite oxide is 0.1 to 1.0 m/g, and at least one salt selected from among carboxylate, carbonate and phosphate is contained in at least one battery element selected from among the positive electrode 1, the negative electrode 2, the separator 3 and the nonaqueous electrolyte 4, as an additive.

Description

  The present invention relates to a lithium ion secondary battery used for various electric devices.

  A lithium ion secondary battery, which is a type of nonaqueous electrolyte battery, is widely used as a power source for portable devices such as mobile phones and notebook personal computers because of its high energy density. In addition, due to consideration for environmental issues, the importance of rechargeable secondary batteries is increasing. In addition to portable devices, automobiles, electric tools, electric chairs, home and commercial power storage systems, etc. The application of is being considered.

  So far, nickel-cadmium batteries, nickel-hydrogen batteries, etc. have been used for applications such as automobiles, electric tools, electric chairs, and household and commercial power storage systems. Has been.

  On the other hand, while regulations on hazardous chemical substances including the EU are spreading worldwide, there is a movement to replace nickel-cadmium batteries containing cadmium, which is a harmful chemical substance, with lithium ion secondary batteries.

  When a nickel-cadmium battery or the like is replaced with a lithium ion secondary battery as described above, the characteristics required for the lithium ion secondary battery are diverse, and various measures are required depending on the application. For applications that require use at large currents, such as high energy density and short charge time when used at large currents, that is, further improvement of input / output characteristics is required for adaptation to high-load devices. Has been.

Further, since power tools and the like are premised on outdoor use and a relatively rough working environment, higher safety and reliability are required for lithium ion secondary batteries. Under such circumstances, it has been studied to improve the safety of the lithium ion secondary battery with respect to the positive electrode active material. For example, a lithium-manganese composite oxide (LiMn ₂ O ₄ ) having a spinel structure with high thermal stability. ) And the like, and the use of a lithium composite oxide containing Mn as a positive electrode active material has been studied (Patent Document 1).

However, a lithium ion secondary battery using LiMn ₂ O ₄ as a positive electrode active material is improved in safety such as thermal stability, but has insufficient high-temperature storage characteristics and high-temperature cycle characteristics, and is used in a high-temperature environment. However, there is a problem that the characteristic deterioration is large. This is because manganese ions are eluted from LiMn ₂ O ₄ at high temperatures, the eluted manganese ions are reduced on the negative electrode surface and deposited as metallic manganese, and the precipitated metallic manganese inhibits lithium ion conduction on the negative electrode surface. It is thought to do. To synthesize LiMn ₂ O ₄ for this problem, the stability of the crystal structure is improved by adding trace elements such as magnesium (Mg) and aluminum (Al), and the elution of manganese ions is suppressed. Proposals have also been made (for example, Patent Document 2), but it is not sufficient to suppress performance deterioration due to the use of a lithium ion secondary battery in a high temperature environment.

  On the other hand, in order to trap the gas generated in the battery that becomes high during overcharge, it has been studied to contain a weakly acidic organic lithium salt in the electrolyte (Patent Document 3). However, all of the weakly acidic organic lithium salts do not effectively act on elution of manganese ions from the lithium-manganese composite oxide.

JP 2001-118569 A JP 2009-123715 A JP 2003-187863 A

Especially in applications where both charging and discharging are performed with a large current, such as power tools, the reaction at the electrodes tends to become non-uniform, and the battery tends to become hot due to large heat generated during charging and discharging over time. Compared to the use in applications where a large current is not required such as a mobile phone, manganese ions are liable to elute from LiMn ₂ O ₄ and the deterioration of characteristics in a high temperature environment increases. ing.

  The present invention has been made to solve the above-described problems, and improves the safety of lithium ion secondary batteries, improves high-temperature storage characteristics, and is suitable for applications that repeatedly charge and discharge with a large current such as an electric tool. A lithium ion secondary battery is provided.

A lithium ion secondary battery of the present invention includes a positive electrode including a manganese-containing lithium composite oxide as a positive electrode active material, a negative electrode including a material capable of occluding and releasing lithium as a negative electrode active material, a separator, a non-aqueous electrolyte, The manganese-containing lithium composite oxide has a BET specific area of 0.1 to 1.0 m ² / g, and the positive electrode, the negative electrode, the separator, and the non-aqueous electrolysis It is characterized in that at least one salt selected from carboxylate, carbonate and phosphate is contained as an additive in at least one battery element selected from a liquid.

In particular, the content of the additive is 0.3 × 10 ⁻³ or more in terms of molar ratio with respect to 1 mol of manganese element contained in the positive electrode.

  According to the present invention, it is possible to provide a lithium ion secondary battery that is excellent in safety and high-temperature storage characteristics and suitable for charging and discharging with a large current.

It is sectional drawing which shows an example of the lithium ion secondary battery of this invention.

  Hereinafter, embodiments of the lithium ion secondary battery of the present invention will be described.

A lithium ion secondary battery of the present invention includes a positive electrode including a manganese-containing lithium composite oxide as a positive electrode active material, a negative electrode including a material capable of occluding and releasing lithium as a negative electrode active material, a separator, a non-aqueous electrolyte, It has. In the lithium ion secondary battery of the present invention, the manganese-containing lithium composite oxide has a BET specific area of 0.1 to 1.0 m ² / g. Furthermore, the lithium ion secondary battery of the present invention includes a carboxylate, a carbonate, and a phosphate in at least one battery element selected from the positive electrode, the negative electrode, the separator, and the non-aqueous electrolyte. It contains at least one selected salt as an additive.

In the present invention, the content of the additive is preferably 0.3 × 10 ⁻³ or more in terms of molar ratio with respect to 1 mol of manganese element contained in the positive electrode.

  By containing at least one salt selected from carboxylate, carbonate and phosphate as an additive in the battery element, manganese ions eluted from the positive electrode active material react with the anion of the additive. Forming a manganese complex that is sparingly soluble in the battery's electrolyte solvent, blocking the migration of manganese ions to the negative electrode surface, and preventing the eluted manganese ions from being reduced and deposited as metallic manganese on the negative electrode surface it can. Thereby, the high temperature storage characteristic of a lithium ion secondary battery can be improved.

As the additive, at least one salt selected from carboxylate, carbonate and phosphate is used. As the carboxylate, carbonate, and phosphate, alkali metal salts, alkaline earth metal salts, and the like can be used, and sodium salts, lithium salts, magnesium salts, calcium salts, and the like are preferably used. Examples of the carboxylic acid constituting the carboxylate include acetic acid, propionic acid, butyric acid, and other dicarboxylic acids such as oxalic acid, malonic acid, succinic acid, fumaric acid, maleic acid; benzoic acid, phthalic acid, terephthalic acid Aromatic carboxylic acids such as acids; hydroxy acids such as lactic acid, malic acid and citric acid; Examples of the carbonates, for _example, can be used as _{NaHCO 3, Na 2 CO 3,} Li 2 CO 3. As the phosphates, for _example, it can be used as the _{Na 3 PO 4, Li 3 PO} 4.

For example, when the additive is sodium acetate, the reaction between sodium acetate and manganese ions is considered to proceed as shown in the following formula.
3CH ₃ COONa + Mn ³⁺ → (CH ₃ COO) ₃ Mn ↓ + 3Na ⁺

  The position in which the additive is contained is not particularly limited as long as it is inside the battery. However, if the additive can be dissolved in the non-aqueous electrolyte, the additive may be dissolved in the non-aqueous electrolyte. On the other hand, when the additive has a low solubility in a non-aqueous electrolyte solution such as sodium acetate, it is contained in the positive electrode or / and negative electrode mixture or / and held in the pores of the separator. Just do it.

The content of the additive is preferably 0.3 × 10 ⁻³ or more, and 2.0 × 10 ⁻³ or more in terms of molar ratio with respect to 1 mol of manganese element contained in the positive electrode. More preferably, it is particularly preferably 3.0 × 10 ⁻³ or more. On the other hand, it is preferably 15 × 10 ⁻³ or less, and more preferably 7.0 × 10 ⁻³ or less. This is because when the molar ratio is 0.3 × 10 ⁻³ or more, the effect of the additive becomes clearer, while when it exceeds 15 × 10 ⁻³ , the impedance of the battery tends to increase.

As the manganese-containing lithium composite oxide serving as the positive electrode active material, a spinel-type lithium-containing composite oxide represented by the general formula LiMn ₂ O ₄ (some of the constituent elements are Ni, Co, Al, Mg, Zr). In addition, a composite oxide substituted with an element such as Ti, B is also included.), A lithium-containing composite oxide having a layered structure represented by the general formula LiMnO ₂ (a part of the constituent elements is Ni, Co, Al, Mg) , Zr, Ti, B, etc.), lithium-containing composite oxides having an olivine structure represented by the general formula LiMnPO ₄ (some of the constituent elements are Ni, Co, Fe And a complex oxide substituted with an element such as).

More specifically, as the lithium-containing composite oxide having the above spinel structure, Li _{1 + x} Mn _2-xy Al _y O ₄ (−0.05 ≦ x ≦ 0.1, 0.01 ≦ y ≦ 0.2) ), Li _{1 + x} Mn _1.5 Ni _0.5 O ₄ (−0.05 ≦ x ≦ 0.1), and the like. As the lithium-containing composite oxide having the above layered structure, Li _{1 + x} Mn _1/3 Ni _1/3 Co _1/3 O ₂ , Li _{1 + x} Ni _0.6 Co _0.2 Mn _0.2 O ₂ , LiNi _0.7 Co _0.1 Mn _0.1 Mg _0.05 Ti _0.05 O ₂ Examples are compositions such as (each -0.05 ≦ x ≦ 0.1).

In order to enhance the effect of the additive, the manganese-containing lithium composite oxide may have a BET specific area of 1.0 m ² / g or less. On the other hand, the BET specific area is preferably 0.1 m ² / g or more so as not to reduce the reactivity in charge / discharge.

The manganese-containing lithium composite oxide can be used together with other active materials. Examples of positive electrode active materials other than the manganese-containing lithium composite oxide include lithium cobalt composite oxides represented by the general formula LiCoO ₂ (elements such as Ni, Al, Mg, Zr, Ti, and B are part of the constituent elements) In addition, a lithium-nickel composite oxide represented by a general formula LiNiO ₂ , Li _{1 + x} Ni _0.7 Co _0.25 Al _0.05 O _2, etc. , Including complex oxides substituted with elements such as Co, Al, Mg, Zr, Ti, and B.) complex oxides having a layered structure; lithium-titanium composites represented by the general formula Li ₄ Ti ₅ O ₁₂ representative general formula LiMPO _4; oxides (. some constituent elements, Ni, Co, Al, Mg , Zr, composite oxide substituted by elements such as B including) composite oxide having a spinel structure, such as Lithium composite oxide having an olivine structure (where, M is Ni, at least one selected from Co and Fe) and the like are exemplified.

  The positive electrode is formed on a positive electrode current collector such as an aluminum foil by applying a coating material containing the positive electrode active material, a conductive powder serving as a conductive auxiliary agent, and a binder, and drying to form a positive electrode mixture layer. It can be obtained by pressure molding.

  As the positive electrode current collector, a punching metal, a net, an expanded metal, or the like can be used in addition to a metal foil such as an aluminum foil. Usually, an aluminum foil having a thickness of 10 to 30 μm is preferably used.

  The conductive auxiliary agent may be added as necessary for the purpose of improving the conductivity of the positive electrode mixture layer. Carbon black, ketjen black, acetylene black, fibrous carbon may be used as the conductive powder as the conductive auxiliary agent. Carbon powder such as graphite and metal powder such as nickel powder can be used.

  Examples of the binder include, but are not limited to, polyvinylidene fluoride (PVDF) and polytetrafluoroethylene.

  The negative electrode is applied to a negative electrode current collector such as a copper foil by applying a coating material containing a negative electrode active material and, if necessary, a conductive powder and a binder as a conductive auxiliary agent, followed by drying. It is obtained by forming a layer and pressing.

  The negative electrode active material is not particularly limited as long as it is a negative electrode active material used in conventional lithium ion secondary batteries. For example, graphite, pyrolytic carbons, cokes, glassy carbons, organic polymer compounds 1 type, or a mixture of two or more types of carbon-based materials capable of occluding and releasing lithium, such as fired bodies, mesocarbon microbeads (MCMB), and carbon fibers.

  As the conductive auxiliary agent and binder used for the negative electrode, the same conductive auxiliary agent and binder used for the positive electrode can be used.

  As the negative electrode current collector, a punching metal, a net, an expanded metal, or the like can be used in addition to a metal foil such as a copper foil. Usually, a copper foil having a thickness of 10 to 30 μm is preferably used.

  A porous film formed by laminating a plurality of thermoplastic resin films each containing thermoplastic resins having different melting points is preferably disposed as a separator between the negative electrode and the positive electrode.

  In general, a single porous film made of polyolefin used in a lithium ion secondary battery has a certain degree of heat resistance and melts at around 135 ° C. to shut down the pores of the porous film. As occurs, a resin having a melting point near the shutdown temperature is used. However, due to the large strain of the film, when used in power tools, etc., it does not result in shutdown, but the film is likely to shrink or clog due to the heat generated by the battery, resulting in a short circuit or deterioration in characteristics. is there. Further, if the melting point of the resin is increased in consideration of heat resistance, it becomes difficult to cause a shutdown, which causes a problem in terms of safety.

  On the other hand, the laminate used as the separator has a porous layer containing a resin having a melting point of 150 ° C. or higher in addition to a porous layer (low melting point resin layer) containing a resin having a melting point of 120 to 140 ° C. If it contains a (high melting point resin layer), even if it is used in applications where the internal temperature of the battery is likely to rise, such as an electric tool, the thermal shrinkage of the separator is suppressed and clogging is unlikely to occur. Is maintained stably. For this reason, there is little characteristic deterioration by charging / discharging by a large current, and it can be set as a reliable battery also in a comparatively high temperature environment. The separator may be a laminate composed of two layers of a high melting point resin layer and a low melting point resin layer, but in particular, a laminate of three or more layers in which both surfaces have a high melting point resin layer and the inside has a low melting point resin layer. The body is more preferably used.

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

For the low melting point resin layer, a porous film made of a resin such as polyethylene, polybutene, or ethylene propylene copolymer (low melting point resin having a melting point of 120 to 140 ° C.) is used. As the low melting point resin, high density polyethylene having a density of 0.94 to 0.97 g / cm ³ is particularly preferable.

  In addition, a porous film formed of a resin such as polypropylene, poly-4-methylpentene-1 or poly-3-methylbutene-1 (a high melting point resin having a melting point of 150 ° C. or higher) is used for the high melting point resin layer. . Polypropylene is particularly preferable as the high melting point resin.

  As a porous film formed by laminating a plurality of thermoplastic resin films each containing a thermoplastic resin having a different melting point, for example, a resin having a melting point of 120 to 140 ° C. formed by a stretching method or an extraction method And a porous layer containing a resin having a melting point of 150 ° C. or higher, which is also formed by a stretching method or an extraction method, and is laminated by stretching, pressure bonding, or an adhesive. A commercially available laminate manufactured by a method, or a method in which a layer containing a resin having a melting point of 120 to 140 ° C. and a layer containing a resin having a melting point of 150 ° C. or more are thermocompression bonded and made porous by a stretching method or the like A film can be used.

  The non-aqueous electrolyte is not particularly limited, and a general-purpose non-aqueous electrolyte obtained by dissolving an electrolyte salt such as a lithium salt in a non-aqueous solvent such as an organic solvent is generally used.

  Examples of the non-aqueous solvent include dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, methyl propionate, ethylene carbonate, propylene carbonate, butylene carbonate, γ-butyrolactone, ethylene glycol sulfite, 1,2-dimethoxyethane, 1, A solvent mixture such as 3-dioxolane, tetrahydrofuran, 2-methyl-tetrahydrofuran, diethyl ether or the like can be used alone or as a mixed solvent.

As the electrolyte salt, for _{example, LiClO 4, LiPF 6, LiBF} 4, LiAsF 6, LiSbF 6, LiCF 3 SO 3, LiCF 3 CO 2, Li 2 C 2 F 4 (SO 3) 2, LiN (CF 3 SO ₂ ) ₂ , LiC (CF ₃ SO ₂ ) ₃ , LiC _n F _{2n + 1} SO ₃ (2 ≦ n ≦ 5), LiN (RfOSO ₂ ) ₂ [where Rf is a fluoroalkyl group] and the like. The concentration of the electrolyte salt in the electrolytic solution is preferably 0.3 to 1.7 mol / liter, particularly 0.5 to 1.5 mol / liter.

  In order to further improve the charge / discharge cycle characteristics and storage characteristics of the non-aqueous electrolyte, vinylene carbonate or a derivative thereof; alkylbenzenes such as cyclohexylbenzene and tertiary butylbenzene; cyclic sultone such as biphenyl and propane sultone An additive such as sulfides such as diphenyl disulfide may be contained. The additive amount of the additive may be 0.1 to 10% by mass in the nonaqueous electrolytic solution, more preferably 0.5% by mass or more, and more preferably 5% by mass or less.

  Next, an example of the lithium ion secondary battery of the present invention will be described with reference to the drawings. FIG. 1 is a cross-sectional view showing an example of the lithium ion secondary battery of the present invention. In FIG. 1, a lithium ion secondary battery includes a positive electrode 1 having a positive electrode mixture layer containing the positive electrode active material according to the present invention described above, a negative electrode 2 having a negative electrode mixture layer containing a negative electrode active material, and a separator. 3 and a non-aqueous electrolyte 4 are provided. The positive electrode 1 and the negative electrode 2 are spirally wound through a separator 3 and are housed in a cylindrical battery can 5 together with a nonaqueous electrolyte solution 4 as an electrode body having a wound structure.

  However, in FIG. 1, in order to avoid complication, the metal foil that is a current collector used for manufacturing the positive electrode 1 and the negative electrode 2 is not illustrated. Moreover, although the separator 3 shows the cut surface, it does not attach | subject the hatching which shows a cross section.

  The battery can 5 is made of, for example, iron and nickel-plated on the surface, and an insulator 6 made of, for example, polypropylene is disposed at the bottom of the battery can 5 prior to the insertion of the electrode body having the winding structure. The sealing plate 7 is made of, for example, aluminum and has a disk shape. A thin portion 7a is provided at the center of the sealing plate 7, and a pressure introduction port 7b for allowing the battery internal pressure to act on the explosion-proof valve 9 around the thin portion 7a. As a hole. And the protrusion part 9a of the explosion-proof valve 9 is welded to the upper surface of the thin part 7a, and the welding part 11 is comprised. The thin-walled portion 7a provided on the sealing plate 7 and the protruding portion 9a of the explosion-proof valve 9 are shown only on the cut surface for easy understanding on the drawing, and the contour line behind the cut surface is not shown. is doing. In addition, the welded portion 11 between the thin-walled portion 7a of the sealing plate 7 and the protruding portion 9a of the explosion-proof valve 9 is also shown in an exaggerated state so as to facilitate understanding on the drawing.

  The terminal plate 8 is made of, for example, rolled steel, has a nickel-plated surface, and has a hat-like shape with a peripheral edge portion, and the terminal plate 8 is provided with a gas discharge port 8a. The explosion-proof valve 9 is made of, for example, aluminum and has a disk shape. A projecting portion 9a having a tip portion is provided on the power generation element side (lower side in FIG. 1) at the center thereof, and the thin-walled portion 9b is provided. As described above, the lower surface of the protruding portion 9a is welded to the upper surface of the thin-walled portion 7a of the sealing plate 7 to form the welded portion 11. The insulating packing 10 is made of, for example, polypropylene and has an annular shape. The insulating packing 10 is arranged at the upper part of the peripheral edge of the sealing plate 7, and the explosion-proof valve 9 is arranged at the upper part thereof. At the same time, the gap between the two is sealed so that the electrolyte does not leak from between them. The annular gasket 12 is made of, for example, polypropylene. The lead body 13 is made of aluminum, for example, and connects the sealing plate 7 and the positive electrode 1. An insulator 14 is disposed on the upper part of the wound electrode body, and the negative electrode 2 and the bottom of the battery can 5 are connected to each other by a lead body 15 made of nickel, for example.

  In the battery of FIG. 1, the thin-walled portion 7a of the sealing plate 7 and the protruding portion 9a of the explosion-proof valve 9 are in contact with each other at the welded portion 11, and the peripheral portion of the explosion-proof valve 9 and the peripheral portion of the terminal plate 8 are in contact. 1 and the sealing plate 7 are connected by a lead body 13 on the positive electrode side. Therefore, in a normal state, the positive electrode 1 and the terminal plate 8 are connected to the lead body 13, the sealing plate 7, the explosion-proof valve 9 and their welded parts. The electrical connection is obtained by 11 and functions normally as an electric circuit.

  When an abnormal situation occurs in the battery, such as the battery is exposed to high temperature or generates heat due to overcharge, and gas is generated inside the battery and the internal pressure of the battery increases, the explosion-proof valve 9 The center part of the is deformed in the internal pressure direction (the upper direction in FIG. 1). Along with this, a shearing force is applied to the thin portion 7a of the sealing plate 7 integrated at the welded portion 11, and the thin portion 7a is broken, or the protrusion 9a of the explosion-proof valve 9 and the thin portion 7a of the sealing plate 7 are broken. After the welded portion 11 is peeled off, the thin-walled portion 9b provided in the explosion-proof valve 9 is cleaved to discharge the gas from the gas discharge port 8a of the terminal plate 8 to the outside of the battery, thereby preventing the battery from bursting. Designed to be able to.

  The lithium ion secondary battery of the present invention has little characteristic deterioration due to charging / discharging with a large current, can maintain stable characteristics over a long period of time, and has high reliability even in a relatively high temperature environment. Therefore, the lithium ion secondary battery of the present invention is suitable for applications in which charging / discharging is repeated with a large current or the battery is used in a relatively high temperature environment, such as a power supply application for an electric tool. Moreover, it can be used for various applications to which a conventional lithium ion secondary battery is applied.

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

Example 1
As a positive electrode active material, 94 parts by mass of LiMn ₂ O ₄ having a BET specific surface area of 0.26 m ² / g, 2.5 parts by mass of ketjen black as a conductive auxiliary agent, and 3.5 parts by mass of polyvinylidene fluoride as a binder , N-methyl-2-pyrrolidone was mixed uniformly as a solvent to prepare a positive electrode mixture-containing paste. After applying the paste on both sides of a positive electrode current collector made of an aluminum foil having a thickness of 15 μm, drying to form a positive electrode mixture layer, and pressing the positive electrode mixture layer with a roller until the thickness reaches 84 μm. Then, the positive electrode was manufactured by cutting so as to have a width of 55 mm and a length of 886 mm.

Graphite powder having a BET specific surface area of 2.2 m ² / g and an average particle size of 18 μm as the negative electrode active material, carboxymethyl cellulose (CMC) and styrene-butadiene copolymer rubber (SBR) as the binder, and acetic acid as the additive Using sodium (Na), graphite powder, CMC, SBR and Na acetate together with water as a solvent in a mass ratio of 96: 1: 1: 2, respectively, A strike was prepared.

In this example, the content of the additive added to the negative electrode was 4.4 × 10 ^{−3 in} terms of molar ratio with respect to 1 mol of the manganese (Mn) element present in the positive electrode.

The obtained negative electrode mixture-containing paste was applied to both sides of a negative electrode current collector made of a copper foil having a thickness of 10 μm, dried to form a negative electrode mixture layer, and the density of the negative electrode mixture layer was 1. After being pressure-molded to 5 g / cm ³ , the negative electrode was fabricated by cutting to a width of 57 mm and a length of 1025 mm.

  As a separator, a polypropylene film having a thickness of about 7 μm (high melting point resin layer, polypropylene melting point: 165 ° C.), a polyethylene film having a thickness of about 7 μm (low melting point resin layer, melting point of polyethylene: 125 ° C.), and a polypropylene having a thickness of about 7 μm. A porous laminated film was prepared by laminating a film (high melting point resin layer, polypropylene melting point: 165 ° C.) in this order. The porous laminated film (separator) had a total thickness of about 20 μm and an aperture ratio of 46%.

  Next, the separator was placed between the positive electrode and the negative electrode, wound in a spiral shape, and inserted into a cylindrical outer can.

LiPF ₆ was dissolved at a ratio of 1.2 mol / liter in a solvent in which ethylene carbonate and dimethyl carbonate were mixed in a non-aqueous electrolyte at a volume ratio of 1: 2, and 2% by mass of vinylene carbonate was further added. The solution was used and poured into the outer can, and then sealed to obtain a lithium ion secondary battery having a diameter of 18 mm and a height of 65 mm.

(Example 2)
A lithium ion secondary battery was produced in the same manner as in Example 1 except that the mass ratio of the graphite powder and Na acetate was changed to 93 and 5, respectively. In this example, the content of the additive added to the negative electrode was 1.1 × 10 ^{−2 in} terms of a molar ratio with respect to 1 mol of Mn element present in the positive electrode.

(Example 3)
A lithium ion secondary battery was produced in the same manner as in Example 1 except that LiMn ₂ O ₄ having a BET specific surface area of 0.41 m ² / g was used.

Example 4
A lithium ion secondary battery was produced in the same manner as in Example 2 except that LiMn ₂ O ₄ having a BET specific surface area of 0.41 m ² / g was used.

(Example 5)
A lithium ion secondary battery was produced in the same manner as in Example 1 except that LiMn ₂ O ₄ having a BET specific surface area of 0.63 m ² / g was used.

(Example 6)
A lithium ion secondary battery was produced in the same manner as in Example 2 except that LiMn ₂ O ₄ having a BET specific surface area of 0.63 m ² / g was used.

(Example 7)
A lithium ion secondary battery was produced in the same manner as in Example 5 except that Li ₃ PO ₄ was used as an additive instead of Na acetate. In this example, the content of the additive added to the negative electrode was 3.4 × 10 ^{−3 in} terms of molar ratio with respect to 1 mol of Mn element present in the positive electrode.

(Example 8)
As the positive electrode active material, _LiMn 1/3 _Ni of the LiMn ₂ O ₄ 47 parts by weight of the BET specific surface area of 0.63 m ² / g, a BET specific surface area ^{_{0.24m 2 / g 1/3 Co 1/3 O}} 2 A lithium ion secondary battery was produced in the same manner as in Example 1 except that 47 parts by mass was used. In this example, the content of the additive added to the negative electrode was 1.4 × 10 ^{−2 in} terms of molar ratio with respect to 1 mol of Mn element present in the positive electrode.

(Comparative Example 1)
A lithium ion secondary battery was produced in the same manner as in Example 5 except that Na acetate was not added and the mass ratio of graphite powder, CMC and SBR was 98: 1: 1 respectively.

(Comparative Example 2)
A lithium ion secondary battery was produced in the same manner as in Example 1 except that LiMn ₂ O ₄ having a BET specific surface area of 1.78 m ² / g was used.

(Comparative Example 3)
A lithium ion secondary battery was produced in the same manner as in Example 1 except that LiMn ₂ O ₄ having a BET specific surface area of 0.09 m ² / g was used.

Evaluation of battery characteristics For the lithium ion secondary batteries of Examples 1 to 8 and Comparative Examples 1 to 3, constant current-constant voltage charging with a constant current of 0.5 A and a constant voltage of 4.2 V at room temperature (total After charging time: 2.5 hours, constant current discharge (discharge end voltage: 2.5 V) was performed at 1.0 A, and the initial capacity was measured. Next, after charging the battery under the above conditions, it was stored in a constant temperature bath at 60 ° C. for 30 days. After 30 days, the stored battery was taken out and subjected to constant current discharge at 1.0 A at room temperature (discharge end voltage: 2.5 V). After discharge, a constant current of 0.5 A and a voltage of 4.2 V were constant at room temperature. After performing constant current-constant voltage charging by voltage (total charge time: 2.5 hours), constant current discharge (discharge end voltage: 2.5 V) was performed at 1.0 A, and the discharge capacity after storage was measured. .

  For each battery, the capacity recovery rate was calculated from the ratio of the discharge capacity after storage to the measured initial capacity, and the high-temperature storage characteristics were evaluated. The results are shown in Table 1.

  Capacity recovery rate (%) = (discharge capacity after storage / initial capacity) × 100

  Moreover, about the battery of Example 1, Example 3, Example 5, and Comparative Example 3, the load characteristic was evaluated on condition of the following using the battery different from the battery used for the said test.

  After performing constant current-constant voltage charging (total charging time: 2.5 hours) with a constant current of 0.5 A and a constant voltage of 4.2 V at room temperature, the discharge conditions were 1.0 A, 5 A, 10 A and The discharge capacities when 20 A constant current discharge (discharge end voltage: 2.5 V) was measured respectively, and 5 A discharge, 10 A discharge, and 20 A for a discharge capacity at 1.0 A discharge (this is the initial capacity). The ratio (capacity ratio) of the discharge capacity in the discharge was determined. The results are shown in Table 2.

As is apparent from Table 1, in the batteries of Examples 1 to 8, the BET specific area of the manganese-containing lithium composite oxide was in the range of 0.1 to 1.0 m ² / g, and a specific additive was added in the battery element. As a result, it was possible to increase the capacity recovery rate after high-temperature storage as compared with the battery of Comparative Example 1 in which no additive was added.

On the other hand, in the battery of Comparative Example 2 in which the BET specific area of the manganese-containing lithium composite oxide is larger than 1.0 m ² / g, the capacity recovery rate is low despite the addition of the additive, and the BET specific area is 0. In the battery of Comparative Example 3 smaller than 0.1 m ² / g, as shown in Table 2, the capacity drop due to the large current discharge was remarkable.

  According to the present invention, it is possible to improve the safety of a lithium ion secondary battery, improve high-temperature storage characteristics, and provide a lithium ion secondary battery suitable for applications in which charging and discharging are repeated with a large current such as an electric tool.

DESCRIPTION OF SYMBOLS 1 Positive electrode 2 Negative electrode 3 Separator 4 Nonaqueous electrolyte 5 Battery can

Claims (3)

A lithium ion secondary battery including a positive electrode including a manganese-containing lithium composite oxide as a positive electrode active material, a negative electrode including a material capable of occluding and releasing lithium as a negative electrode active material, a separator, and a non-aqueous electrolyte. ,
The manganese-containing lithium composite oxide has a BET specific area of 0.1 to 1.0 m ² / g,
In at least one battery element selected from the positive electrode, the negative electrode, the separator, and the non-aqueous electrolyte, at least one salt selected from carboxylate, carbonate, and phosphate is included as an additive. The lithium ion secondary battery characterized by the above-mentioned. 2. The lithium ion secondary battery according to claim 1, wherein the content of the additive is 0.3 × 10 ⁻³ or more in terms of a molar ratio with respect to 1 mol of the manganese element contained in the positive electrode. 2. The lithium ion secondary battery according to claim 1, wherein the content of the additive is 15 × 10 ⁻³ or less in molar ratio with respect to 1 mol of manganese element contained in the positive electrode.

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