JP6664148B2 - Lithium ion secondary battery - Google Patents

Description

  The present invention relates to a lithium ion secondary battery.

2. Description of the Related Art In recent years, with the spread of portable information electronic devices such as mobile phones, video cameras, and notebook personal computers, the performance, size, and weight of the devices have been rapidly developing.
As for power supplies used in these devices, demands for secondary batteries, particularly lithium ion secondary batteries, are growing due to their good balance of economy, high performance, small size and light weight. It is also used in some hybrid and electric vehicles. Further, these electronic devices have been further improved in performance and miniaturization, and lithium ion secondary batteries are also required to have improved safety and longer life.

  Recently, in order to further enhance safety, various types of thermoplastic porous membranes have been proposed as separators suitable for lithium ion secondary battery applications, and inorganic coated separators using inorganic particles as separators have been used. . This is because by using an inorganic coated separator, even if an abnormal current flows due to a short circuit or the like, and even if the temperature inside the battery rises to a certain high temperature, the shutdown state can be maintained without the film being broken by the temperature. It becomes possible.

  In addition, by specifying the positive electrode active material and the electrolyte additive as described in Patent Document 1, when a high end-of-charge voltage is applied, both the room temperature cycle characteristics and the high temperature cycle characteristics are excellent. It is described that a lithium ion secondary battery can be used.

Patent No. 5239302

However, in the case of the inorganic coated separator provided with the heat-resistant porous layer as described above, depending on the combination of the battery electrode and the non-aqueous electrolyte, the member used for the heat-resistant porous layer reacts with the lithium salt in the non-aqueous electrolyte. I knew it was possible. This effect tends to be more pronounced when stored at high temperatures. When the member of the heat-resistant porous layer is decomposed by the reaction with the lithium salt, the stored energy is consumed by the decomposition reaction itself, and the decomposed product influences other members (eg, gas generation, deposition of the decomposed product on the electrode surface). It is necessary to prevent this as much as possible because such a case may occur. An object of the present invention is to suppress the decomposition of these heat-resistant porous layers and prevent the capacity of the battery from deteriorating after storage at a high temperature.

In order to achieve the above object, a lithium ion secondary battery according to the present invention includes a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte. A battery, wherein the separator includes a base layer made of a porous film, and a heat-resistant porous layer formed on one or both sides of the base layer, and the heat-resistant porous layer contains inorganic particles and a resin. Wherein the non-aqueous electrolyte contains 1,3,2-dioxathiolane-2,2-dioxide (DTD).

According to the lithium ion secondary battery of the present invention, a film similar to an SEI film is formed on the surface of the heat-resistant porous layer by the electrolyte containing 1,3,2-dioxathiolane-2,2-dioxide (DTD). be able to. This coating suppresses the reaction between the inorganic particles and the lithium salt in the heat-resistant porous layer, and prevents the inorganic particles and the lithium salt from decomposing. It is speculated that a secondary battery can be provided.

The total amount of 1,3,2-dioxathiolane-2,2-dioxide (DTD) contained in the non-aqueous electrolyte according to the present invention is 0.5 to 10% by mass based on the whole non-aqueous electrolyte. Is preferred.

The capacity retention ratio of the lithium ion secondary battery according to the present invention after high-temperature storage is further improved.

The non-aqueous electrolyte according to the present invention preferably further contains 1,3-propane sultone (PS).

In the lithium ion secondary battery according to the present invention, it is inferred that, by using 1,3-propane sultone in combination, the film on the electrode surface is formed more densely, and the protective effect on the electrode, particularly on the negative electrode is improved. Therefore, deterioration of the negative electrode at high temperatures is reduced, and the capacity retention after storage at higher temperatures is improved.

The non-aqueous electrolyte according to the present invention contains 1,3,2-dioxathiolane-2,2-dioxide (DTD) and 1,3-propane sultone (PS), and comprises 1,3,2-dioxathiolane. When the total amount of -2,2-dioxide (DTD) and 1,3-propane sultone (PS) is 0.5 to 10% by mass, the capacity retention after high-temperature storage is further improved.

By setting the inorganic particles according to the present invention to at least one selected from the group consisting of alumina and boehmite, it is possible to avoid thermal shrinkage of the separator and to prevent short circuit beforehand, and to react with sulfur to obtain favorable SEI. This is preferable because it is considered that a film is formed.

The heat-resistant porous layer according to the present invention is preferably provided between the base layer and the positive electrode.

According to the present invention, the self-discharge of the lithium ion secondary battery can be suppressed, the consumption of charged energy can be prevented, the clogging of the separator by reaction products can be prevented, and the deterioration of the electrode active material can be reduced. It is possible to suppress the capacity deterioration after high-temperature storage by the action presumed to prevent.

  The maximum voltage of the lithium ion secondary battery according to the present invention is preferably 4.30 to 4.50V. This makes it possible to produce a safer battery with a higher energy density.

Preferably, LiCoO ₂ is used as the active material of the positive electrode of the lithium ion secondary battery according to the present invention. Thus, stable charging and discharging can be performed in a voltage range of 4.30 to 4.50 V.

ADVANTAGE OF THE INVENTION According to this invention, the lithium ion secondary battery which can prevent the capacity deterioration at the time of high temperature storage of a battery can be provided.

FIG. 2 is a schematic sectional view of a lithium ion secondary battery.

A preferred embodiment of the lithium ion secondary battery according to the present invention will be described in detail with reference to the drawings. However, the lithium ion secondary battery of the present invention is not limited to the following embodiments. Note that the dimensional ratios in the drawings are not limited to the illustrated ratios.

(Lithium ion secondary battery)
The electrode and the lithium ion secondary battery according to the present embodiment will be briefly described with reference to FIG. The lithium ion secondary battery 100 mainly includes a case 50 that accommodates the stacked body 40 in a sealed state, and a pair of leads 60 and 62 connected to the stacked body 40. Although not shown, a non-aqueous electrolyte is housed in the case 50 together with the laminate 40.

  The laminated body 40 has a pair of the positive electrode 20 and the negative electrode 30 that are opposed to each other with the separator 10 interposed therebetween. The positive electrode 20 has a positive electrode active material layer 24 provided on a plate-shaped (film-shaped) positive electrode current collector 22. The negative electrode 30 is obtained by providing a negative electrode active material layer 34 on a plate-shaped (film-shaped) negative electrode current collector 32. The positive electrode active material layer 24 and the negative electrode active material layer 34 are in contact with both sides of the separator 10 respectively. Leads 62, 60 are connected to ends of the positive electrode current collector 22 and the negative electrode current collector 32, respectively, and the ends of the leads 60, 62 extend to the outside of the case 50.

  Hereinafter, the positive electrode 20 and the negative electrode 30 are collectively referred to as electrodes 20 and 30, the positive electrode current collector 22 and the negative electrode current collector 32 are collectively referred to as current collectors 22 and 32, and the positive electrode active material layer 24 and the negative electrode The active material layers 34 are collectively referred to as active material layers 24 and 34.

Hereinafter, a specific configuration of the lithium ion secondary battery will be described.

(Non-aqueous electrolyte)
First, the non-aqueous electrolyte used in the lithium ion secondary battery according to the present embodiment will be described.
The non-aqueous electrolyte according to the present embodiment includes a solvent, a solute, and an additive.

(solvent)
The solvent of the non-aqueous electrolyte is not particularly limited, and a known compound having lithium ion conductivity can be used. For example, cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), and fluoroethylene carbonate (FEC) can be used alone or in an appropriate combination. In order to increase the electric conductivity and obtain an electrolyte having an appropriate viscosity, a chain carbonate such as dimethyl carbonate (DMC), methyl ethyl carbonate (MEC), and diethyl carbonate (DEC), and 1,2-dimethoxyethane And ester compounds such as methyl ether, methyl acetate and methyl propionate.

(Solute)
Can be used a known material solute is not particularly limited in the non-aqueous electrolyte solution, in particular LiPF _6, LiBF _4, LiClO _4, etc. can be used, to increase the electrical conductivity of LiPF ₆ It is preferable to include

(Additive)
The non-aqueous electrolyte of this embodiment further contains 1,3,2-dioxathiolane-2,2-dioxide (DTD) as an additive.
By using this material for a lithium ion secondary battery including a separator having a base layer made of a porous film and a heat-resistant porous layer containing inorganic particles and a resin, which will be described in detail later, by high-temperature storage. Capacity deterioration can be prevented. The mechanism of the effect manifestation by 1,3,2-dioxathiolane-2,2-dioxide (DTD) is not clear, but the present inventors consider as follows.

1,3,2-dioxathiolane-2,2-dioxide (DTD) not only forms an SEI film on the electrode surface but also forms a film similar to the SEI film on the heat-resistant porous layer surface of the separator. Have a job. This coating suppresses the reaction between the inorganic particles and the lithium salt in the heat-resistant porous layer, and consumes stored energy and generates gas due to the decomposition reaction itself, and deposits of inorganic particle decomposition products and lithium salt decomposition products on the electrode surface, It is presumed that it becomes possible to prevent a decrease in electric conductivity due to the decomposition of the lithium salt and a decrease in the concentration.

The content of 1,3,2-dioxathiolane-2,2-dioxide (DTD) is preferably 0.5 to 10% by mass based on the whole nonaqueous electrolyte. Since the SEI film on the electrode surface is not too thick, the film on the surface of the heat-resistant porous layer functions sufficiently and suppresses the reaction between the inorganic particles and the lithium salt, the lithium ion with less capacity deterioration after higher temperature storage It becomes a secondary battery.

The non-aqueous electrolyte preferably contains 1,3-propane sultone (PS) in addition to 1,3,2-dioxathiolane-2,2-dioxide (DTD). When 1,3-propane sultone (PS) is used in combination, the film on the electrode surface is formed more densely, and the protective effect on the electrode, particularly the negative electrode is improved, so that the deterioration of the negative electrode at high temperatures is reduced. In addition, the capacity retention rate after storage at higher temperatures is improved.

When the 1,3-propane sultone is used in combination, the total amount of 1,3,2-dioxathiolane-2,2-dioxide (DTD) and 1,3-propane sultone (PS) is 0.5 to 10% by mass. % Is preferable. In this case, the protection effect on the negative electrode is further enhanced, and the capacity retention at a high temperature is further improved.

In addition to 1,3,2-dioxathiolane-2,2-dioxide (DTD) and 1,3-propanesultone (PS), known compounds such as vinylene carbonate, vinylethylene carbonate, ethylene sulfide, fluorobenzene, and fluoroethylene carbonate It may contain additives.

(Positive electrode current collector)
The positive electrode current collector 22 may be a conductive plate material, and for example, a thin metal plate of aluminum, copper, or nickel foil can be used.

  The positive electrode active material layer 24 contains a positive electrode active material, a binder, and a necessary amount of a conductive material.

(Positive electrode active material)
As the positive electrode active material contains a lithium ion, may be any capable of absorbing and releasing compound lithium ions, for _{example, LiCoO 2, LiNiO 2, LiMn} 2 O 4, Li (Co x Ni y Mn z) O 2 _{, Li (Ni x Co y Al} z) O 2, Li (Mn x Al y) 2 O 4, Li (Li w Mn x Ni y Co z) O 2, LiVOPO 4, LiFePO lithium-containing metal oxides such as ₄ Is mentioned. The binder binds the active materials together and binds the positive electrode active material and the positive electrode current collector 22.

(binder)
The binder binds the active materials together and binds the positive electrode active material and the positive electrode current collector 22.
The material of the binder may be any material as long as the above-mentioned bonding is possible. For example, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene -Perfluoroalkyl vinyl ether copolymer (PFA), ethylene-tetrafluoroethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), ethylene-chlorotrifluoroethylene copolymer (ECTFE), polyvinyl fluoride ( (PVF) and the like.

  In addition to the above, as a binder, for example, polyethylene, polypropylene, polyethylene terephthalate, polyamide (PA), polyimide (PI), polyamideimide (PAI), aromatic polyamide, cellulose, styrene-butadiene rubber, isoprene rubber, butadiene Rubber, ethylene / propylene rubber, polyacrylic acid and its salts, alginic acid and its salts, and the like may be used. Thermoplastic elastomeric polymers such as styrene / butadiene / styrene block copolymers, hydrogenated products thereof, styrene / ethylene / butadiene / styrene copolymers, styrene / isoprene / styrene block copolymers, and hydrogenated products thereof May be used. Further, a syndiotactic 1,2-polybutadiene, an ethylene / vinyl acetate copolymer, a propylene / α-olefin (2 to 12 carbon atoms) copolymer, or the like may be used.

  Further, an electron conductive conductive polymer or an ion conductive conductive polymer may be used as the binder. Examples of the electron conductive conductive polymer include polyacetylene. In this case, since the binder also functions as a conductive material, the conductive material need not be added. Examples of the ion-conductive conductive polymer include a complex of a polymer compound such as polyethylene oxide and polypropylene oxide with a lithium salt or an alkali metal salt mainly composed of lithium.

(Conductive material)
Examples of the conductive material include carbon powders such as carbon blacks, carbon nanotubes, carbon materials, metal fine powders such as copper, nickel, stainless steel, and iron, mixtures of carbon materials and metal fine powders, and conductive oxides such as ITO. Can be

(Negative electrode current collector)
The negative electrode current collector 32 may be a conductive plate material, and for example, a thin metal plate of aluminum, copper, or nickel foil can be used.

(Negative electrode active material)
The negative electrode active material may be any compound capable of inserting and extracting lithium ions, and a known negative electrode active material for a battery can be used. Examples of the negative electrode active material include carbon materials such as graphite (natural graphite and artificial graphite) capable of occluding and releasing lithium ions, carbon nanotubes, non-graphitizable carbon, easily graphitizable carbon, low-temperature calcined carbon, aluminum, and silicon. , A metal that can be combined with lithium such as tin, an amorphous compound mainly composed of an oxide such as silicon dioxide and tin dioxide, and particles containing lithium titanate (Li ₄ Ti ₅ O ₁₂ ). . It is preferable to use graphite which has a high capacity per unit weight and is relatively stable.

  The binder and the conductive material used for the negative electrode can be the same as those for the positive electrode.

(Method of Manufacturing Electrodes 20, 30)
Next, a method for manufacturing the electrodes 20, 30 according to the present embodiment will be described.

The active material (the positive electrode active material or the negative electrode active material is collectively referred to as an active material), a binder, and a solvent are mixed. If necessary, a conductive material may be further added. As the solvent, for example, water, N-methyl-2-pyrrolidone and the like can be used. The method of mixing the components constituting the paint is not particularly limited, and the mixing order is not particularly limited.
The paint is applied to the current collectors 22 and 32. There is no particular limitation on the coating method, and a method usually used in the case of manufacturing an electrode can be used.

  Subsequently, the solvent in the paint applied on the current collectors 22 and 32 is removed. The removal method is not particularly limited, and the current collectors 22 and 32 to which the paint has been applied may be dried in an atmosphere of, for example, 80C to 150C.

  Then, if necessary, the electrode on which the positive electrode active material layer 24 and the negative electrode active material layer 34 are formed is subjected to press processing by a roll press device or the like. The linear pressure of the roll press can be, for example, 10 to 50 kgf / cm.

  Through the above steps, an electrode in which the electrode active material layers 24 and 34 are formed on the current collectors 22 and 32 is obtained.

(Separator)
The separator 10 will be specifically described. The separator 10 includes a base layer 14 and a heat-resistant porous layer 12 containing inorganic particles and a resin formed on at least one surface of the base layer 14.

(Base material layer)
The substrate layer 14 made of the porous film of the present embodiment is not particularly limited, and may be made of any material and by any manufacturing method as long as it is known.
Preferably, one composed of one or more resins selected from the group consisting of polyolefins having a weight average molecular weight of 500,000 to 1.5 million, thermoplastic elastomers and graft copolymers is used.
For example, polyolefin resins such as polyethylene and polypropylene, and modified polyolefin resins such as an ethylene-acrylic monomer copolymer and an ethylene-vinyl acetate copolymer are exemplified.
These resins may be used alone or in combination of two or more.

(Heat-resistant porous layer)
The heat-resistant porous layer 12 of the present embodiment is a film having air permeability due to gaps between the inorganic particles, and contains a resin for binding the inorganic particles. The content of the inorganic particles is not particularly limited, but is preferably 10% by mass or more and 99% by mass or less, more preferably 80% by mass or more and 98% by mass or less with respect to 100% by mass of the heat-resistant porous layer. When the content is 10% by mass or more, the lithium ion conductivity tends to be high. When the content is 98% by mass or less, the mechanical strength of the heat-resistant porous layer tends to be high.

(resin)
Examples of the resin for binding the inorganic particles include hydroxymethyl cellulose, ethyl cellulose, methyl cellulose, carboxymethyl cellulose, carboxyethyl cellulose, agar, carrageenan, furcellane, pectin, starch, mannan, curdlan, ernest gum, starch, pullulan, guar gum, Polysaccharides such as xanthan gum (xanthan gum); proteins such as gelatin; polyethers such as polyethylene oxide and polypropylene oxide; vinyl alcohols such as polyvinyl alcohol and polyvinyl butyral; polyacids such as polyacrylic acid and polymethacrylic acid; Examples of water-soluble polymers such as metal salts, and synthetic polymer emulsions include styrene-butadiene copolymer latex and polystyrene polymer latex. Polybutadiene-based polymer latex, acrylonitrile-butadiene-based copolymer latex, polyurethane-based polymer latex, polymethyl methacrylate-based polymer latex, methyl methacrylate-butadiene-based copolymer latex, polyacrylate-based polymer latex, vinyl chloride-based polymer latex Examples include coalesced latex, vinyl acetate-based polymer emulsion, vinyl acetate-ethylene-based copolymer emulsion, polyethylene emulsion, carboxy-modified styrene-butadiene copolymer resin emulsion, and acrylic resin emulsion. In addition, the binders mentioned in the description of the electrodes may be used. Among them, water-soluble polymer carboxymethylcellulose, polyacrylic acid, and styrene-butadiene copolymer latex are more preferable in view of cost and improvement in binding property.

(Inorganic particles)
Specific examples of the inorganic particles of the present embodiment include metal oxides such as alumina, boehmite, titania, silica, and zirconia; metal carbonates such as calcium carbonate; metal phosphates such as calcium phosphate; aluminum hydroxide; A metal hydroxide such as magnesium is preferably used, and alumina and boehmite are particularly preferable.
The average particle diameter of the inorganic particles is preferably in the range of 0.01 to 2 μm. When the average particle diameter of the inorganic particles exceeds 2 μm, there is a possibility that there is a problem in forming the heat-resistant porous layer with an appropriate thickness.
Further, when the average particle diameter of the inorganic particles is less than 0.01 μm, the strength of the coating film may be reduced and a problem of powder dropping may occur.

(Method of Manufacturing Separator 10)
Next, a method for manufacturing the separator 10 according to the present embodiment will be described.

  The inorganic particles, resin and solvent are mixed. As the solvent, for example, water, N-methyl-2-pyrrolidone and the like can be used. The method of mixing the components constituting the paint is not particularly limited, and the mixing order is not particularly limited.

  The above coating material is applied to a base material layer made of a porous film. The application method is not particularly limited, and for example, an application method using various coaters, sprays, or the like can be used.

  Subsequently, the solvent in the paint applied on the base layer made of the porous film is removed. The removal method is not particularly limited, and a method such as appropriate drying can be used.

  Through the above steps, the separator 10 is manufactured.

(Case)
The case 50 seals the laminate 40 and the electrolytic solution therein. The case 50 is not particularly limited as long as it can prevent leakage of the electrolytic solution to the outside, entry of moisture or the like from the outside into the electrochemical device 100, and the like. For example, as the case 50, as shown in FIG. 1, a metal laminate film in which a metal foil 52 is coated with a polymer film 54 from both sides can be used. For example, an aluminum foil can be used as the metal foil 52, and a film such as polypropylene can be used as the polymer film 54. For example, the material of the outer polymer film 54 is preferably a polymer having a high melting point, such as polyethylene terephthalate (PET) or polyamide, and the material of the inner polymer film 54 is polyethylene (PE), polypropylene (PP), or the like. preferable.

(Lead)
The leads 60 and 62 are formed from a conductive material such as aluminum.

  Then, the leads 62 and 60 are welded to the positive electrode current collector 22 and the negative electrode current collector 32 by a known method, and a separator is provided between the positive electrode active material layer 24 of the positive electrode 20 and the negative electrode active material layer 34 of the negative electrode 30. It is sufficient to insert the case 10 together with the electrolytic solution into the case 50 while sandwiching the case 10 and seal the entrance of the case 50.

Thereafter, initial charging is performed with a predetermined amount of current, and a lithium ion secondary battery can be obtained. In addition, a safe battery pack can be obtained by using this lithium ion secondary battery for part or all of the battery pack assembled in series and parallel.
The preferred embodiments of the non-aqueous electrolyte, the electrode, and the lithium ion secondary battery including the electrolyte and the electrode according to the present invention and the manufacturing method thereof have been described above in detail. However, the present invention is not limited to this.

  Hereinafter, the present invention will be described more specifically based on Examples and Comparative Examples, but the present invention is not limited to the following Examples.

(Example 1)
(Preparation of electrolyte)
LiPF ₆ was dissolved at a concentration of 1 M in a solution in which ethylene carbonate was mixed at a ratio of 30% by volume and diethyl carbonate at a ratio of 70% by volume. To the solution in which the electrolyte was dissolved, 1,3,2-dioxathiolane-2,2-dioxide (DTD) was added and dissolved at 0.5% by mass based on the amount of the solution. Thus, a non-aqueous electrolyte was prepared.

(Preparation of separator)
Alumina was used as the inorganic particles of the heat-resistant porous layer, and styrene-butadiene rubber (SBR) and carboxymethylcellulose were used as resins. This coating material was applied to one surface of a base layer of a porous film mainly composed of polyolefin using a commercially available bar coater, and dried at 60 ° C. for 5 minutes to obtain a separator.

(Preparation of negative electrode)
A coating was prepared by mixing artificial graphite as a negative electrode active material, styrene-butadiene rubber (SBR) and carboxymethyl cellulose as binders with water as a solvent. This paint was applied to a copper foil (thickness: 15 μm) as a current collector by a doctor blade method, dried at 80 ° C., and then rolled to form a negative electrode active material layer on the surface of the copper foil. The copper foil was provided with a portion to which no paint was applied in order to connect an external lead-out terminal. As the external lead-out terminal, for the purpose of improving the sealing property with the exterior body, a nickel foil was wound around polypropylene grafted with maleic anhydride. The nickel foil and the copper foil after applying and drying the above paint were ultrasonically welded.

(Preparation of positive electrode)
LiCoO ₂ as a positive electrode active material, polyvinylidene fluoride as a binder, carbon black and graphite as conductive aids, and N-methyl-2-pyrrolidone as a solvent were mixed to prepare a paint. The paint was applied to an aluminum foil (thickness: 20 μm) as a current collector by a doctor blade method, dried at 100 ° C., and then rolled to form a positive electrode active material layer on the aluminum foil surface. The aluminum foil was provided with a portion to which no paint was applied in order to connect the external lead-out terminal. As the external lead-out terminal, one obtained by winding a polypropylene grafted with maleic anhydride on an aluminum foil was prepared for the purpose of improving the sealing property with the exterior body. The aluminum foil and the aluminum foil after applying and drying the above paint were ultrasonically welded.

(Production of lithium ion secondary battery cell)
The positive electrode, the negative electrode, and the separator produced as described above were cut into predetermined dimensions. The cut positive electrode, negative electrode, and separator were laminated in the order of negative electrode, separator, (heat-resistant porous layer), positive electrode, (heat-resistant porous layer), separator, and negative electrode to produce a full-cell laminate. The heat-resistant porous layer of the separator was laminated so as to face the positive electrode side. The laminate was placed in an exterior body, an appropriate amount of the electrolyte solution prepared as described above was added, and the exterior body was vacuum-sealed to produce a lithium ion secondary battery cell (hereinafter, referred to as a cell).

(Initial charge / discharge)
The cell fabricated as described above was charged for 3 hours with a charge / discharge tester at a current value at which the current density with respect to the positive electrode active material was 8 mA / g. Thereafter, the gas generated inside the cell was removed, and charged again using a charge / discharge tester until the cell potential became 4.35 V at a current value at which the current density with respect to the positive electrode active material was 16 mA / g. Thereafter, discharge was performed at a current value at which the current density with respect to the positive electrode active material was 16 mA / g until the cell potential became 2.8 V.

(Discharge capacity measurement after high temperature storage)
The cell fabricated as described above was charged with a charge / discharge tester at room temperature at a charge / discharge rate of 0.5 C (current value at which discharge is completed in 2 hours when constant current discharge is performed at 25 ° C.) and the cell voltage is increased. The discharge capacity (unit: mAh) was measured when the battery was charged to 4.35 V and then discharged until the cell voltage reached 2.8 V. Then, the battery was charged again until the cell voltage became 4.35 V, and then stored for 30 days in a thermostat at an internal temperature of 60 ° C. Thereafter, the cell was allowed to stand until the cell temperature reached the same temperature as room temperature, and then discharged until the cell voltage reached 2.8V. Compare the discharge capacity measured before storage in a 60 ° C. constant temperature bath with the discharge capacity measured after storage in a 60 ° C. constant temperature bath, and compare the discharge capacity measured before storage in the 60 ° C. constant temperature bath with the discharge capacity after 30 days storage. Is shown in Table 1 as the capacity retention rate after storage at high temperature.

(Example 2)
LiPF ₆ was dissolved at a concentration of 1 M in a solution in which ethylene carbonate was mixed at a ratio of 30% by volume and diethyl carbonate at a ratio of 70% by volume. A solution prepared by adding 3.0% by mass of 1,3,2-dioxathiolane-2,2-dioxide (DTD) to the solution and dissolving the solution was used as a non-aqueous electrolyte. Other than that is the same as the first embodiment.

(Example 3)
LiPF ₆ was dissolved at a concentration of 1 M in a solution in which ethylene carbonate was mixed at a ratio of 30% by volume and diethyl carbonate at a ratio of 70% by volume. A solution prepared by adding 10.0% by mass of 1,3,2-dioxathiolane-2,2-dioxide (DTD) to the solution and dissolving the solution was used as a non-aqueous electrolyte. Other than that is the same as the first embodiment.

(Example 4)
LiPF ₆ was dissolved at a concentration of 1 M in a solution in which ethylene carbonate was mixed at a ratio of 30% by volume and diethyl carbonate at a ratio of 70% by volume. A solution prepared by adding 12.0% by mass of 1,3,2-dioxathiolane-2,2-dioxide (DTD) to the solution and dissolving the solution was used as a non-aqueous electrolyte. Other than that is the same as the first embodiment.

(Example 5)
LiPF ₆ was dissolved at a concentration of 1 M in a solution in which ethylene carbonate was mixed at a ratio of 30% by volume and diethyl carbonate at a ratio of 70% by volume. To this solution, 1,3,2-dioxathiolane-2,2-dioxide (DTD) was added at 1.5% by mass based on the solution amount, and 1,3-propanesultone (PS) was added at 1% based on the solution amount. The solution dissolved by adding 0.5% by mass was used as a non-aqueous electrolyte. Other than that is the same as the first embodiment.

(Example 6)
The cell was prepared by laminating the heat-resistant porous layer of the separator so as to face the negative electrode. Other than that is the same as the fifth embodiment.

(Example 7)
Instead of alumina, calcium carbonate was used as the inorganic particles of the separator. Other than that is the same as the second embodiment.

(Example 8)
Instead of alumina, boehmite was used as the inorganic particles of the separator. Other than that is the same as the fifth embodiment.

(Example 9)
Instead of alumina, silica was used as inorganic particles of the separator. Other than that is the same as the fifth embodiment.

(Example 10)
Instead of alumina, titania was used as the inorganic particles of the separator. Other than that is the same as the fifth embodiment.

(Comparative Example 1)
A solution obtained by dissolving LiPF6 at a concentration of 1 M in a solution in which ethylene carbonate was mixed at a ratio of 30% by volume and diethyl carbonate at a ratio of 70% by volume was used as a nonaqueous electrolyte. Other than that is the same as the first embodiment.

(Comparative Example 2)
Instead of alumina, calcium carbonate was used as the inorganic particles of the separator. Other than that is the same as Comparative Example 1.

Table 1 shows the types of inorganic particles (inorganic particles in the table), the types of additives (additives in the table), and the content ratios of additives to the entire nonaqueous electrolyte (in the table). , The additive content (% by mass)), the electrode against the heat-resistant porous layer (the surface opposite to the heat-resistant porous layer in the table), and the capacity retention rate (%) after high-temperature storage.
As shown in Table 1, it was found that the example had an improved capacity retention ratio after high-temperature storage as compared with the comparative example.
In addition, it was found that the positive electrode was more effective than the negative electrode on the electrode surface facing the heat-resistant porous layer (referred to as the heat-resistant porous layer opposite surface).

  As described above, according to the present invention, a favorable lithium ion secondary battery in which capacity deterioration after high-temperature storage is prevented can be manufactured.

DESCRIPTION OF SYMBOLS 10 ... separator, 12 ... heat resistant porous layer, 14 ... base material layer, 20 ... positive electrode, 30 ... negative electrode, 22 ... positive electrode current collector, 24 ... positive electrode active material layer, 32 , Negative electrode current collector, 34, negative electrode active material layer, 40, laminated body, 50, case, 52, metal foil, 54, polymer film, 60, 62, lead, 100, lithium ion secondary battery .

Claims (6)

A positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, a lithium ion secondary battery including a non-aqueous electrolyte, wherein the separator is a base layer made of a porous film, A heat-resistant porous layer formed between the base material layer and the positive electrode, the heat-resistant porous layer contains inorganic particles and a resin, and the non-aqueous electrolyte contains 1,4-dioxane. A lithium ion secondary battery containing 0.5 to 10% by mass of 1,3,2-dioxathiolane-2,2-dioxide (DTD) based on the whole non-aqueous electrolyte. battery. 2. The lithium ion secondary battery according to claim 1, wherein the non-aqueous electrolyte further contains 1,3-propane sultone (PS). 3. The non-aqueous electrolyte contains 1,3,2-dioxathiolane-2,2-dioxide (DTD) and 1,3-propanesultone (PS), and comprises 1,3,2-dioxathiolane-2,2. The lithium ion secondary battery according to claim 2, wherein the total amount of dioxide (DTD) and the 1,3-propane sultone (PS) is 0.5 to 10% by mass. The lithium ion secondary battery according to any one of claims 1 to 3, wherein the inorganic particles are at least one selected from the group consisting of alumina and boehmite. The lithium ion secondary battery according to any one of claims 1 to 4, wherein the maximum voltage of the battery is 4.30 to 4.50V. Lithium-ion secondary battery according to any one of claims 1 to 5, wherein the use of LiCoO ₂ as the active material of the positive electrode.

Images