JP7363443B2 - Lithium ion secondary battery - Google Patents

Description

The present invention relates to a lithium ion secondary battery.

Lithium-ion secondary batteries are already used in many electronic devices, but as the performance of these devices increases, lithium-ion secondary batteries are also required to have even higher energy density. There is an urgent need to develop positive electrodes with high capacity and high potential.

Currently, a lithium cobalt oxide called LiCoO ₂ is known as the most widely used positive electrode. The theoretical capacity of LiCoO ₂ is very high at 274 mAh/g, but normally only about half of the capacity is used, with a voltage of about 4.2 V and a capacity of about 150 mAh/g. In order to utilize more lithium ions and obtain higher capacity, it is necessary to charge and discharge at a voltage higher than 4.2V. In this case, phase transition of LiCoO ₂ and elution of cobalt ions pose problems, and there is also concern about oxidative decomposition of the electrolytic solution due to increase in oxidation voltage. Therefore, various surface modifications are being considered to solve these problems.

For example, in Non-Patent Document 1, various reports have been made, such as improvement of cycle characteristics at high voltage by surface modification with Al ₂ O ₃ , and Non-Patent Document 2, reduction of impedance by coating with Li ₂ CO ₃ . There is.

J. Electrochem. Soc. , 149, A466 (2002) J. Power Sources, 132 187 (2004)

Surface modification using Li ₂ CO ₃ described in Non-Patent Document 2 tends to promote decomposition of the electrolytic solution, leading to gas generation and increased resistance. This tendency is particularly noticeable in tests conducted under high temperatures, such as high-temperature storage tests.

The present invention has been made in view of the problems of the prior art described above, and an object of the present invention is to provide a lithium ion secondary battery in which an increase in internal resistance after a high-temperature storage test is suppressed.

(1) The lithium ion secondary battery according to the first aspect is a lithium ion secondary battery comprising a positive electrode and an electrolyte, wherein the positive electrode is Li _1-x CoO ₂ (0≦x≦1). A particle containing lithium cobalt oxide represented by the formula (1), a first coating layer provided on the surface of the particle and containing Li ₂ CO ₃ , and a first coating layer provided on the surface of the first coating layer, which is formed by the following general formula (1). A lithium ion secondary battery comprising a positive electrode active material and a second coating layer containing a compound represented by:

However, in general formula (1), Rf is a fluorine-containing alkyl group.

(2) In the lithium ion secondary battery according to the embodiment, the first coating layer may have a thickness in a range of 0.1 μm or more and 1 μm or less.

According to the present invention, it is possible to provide a lithium ion secondary battery in which an increase in internal resistance after a high temperature storage test is suppressed.

1 is a schematic cross-sectional view of a lithium ion secondary battery according to an embodiment of the present invention.

Hereinafter, embodiments will be described in detail with reference to figures as appropriate. In the drawings used in the following explanation, characteristic parts may be shown enlarged for convenience in order to make the characteristics easier to understand, and the dimensional ratios of each component may differ from the actual ones. The materials, dimensions, etc. exemplified in the following description are merely examples, and the present invention is not limited thereto, and can be implemented with appropriate changes within the scope of the invention.

"Lithium ion secondary battery"
FIG. 1 is a schematic cross-sectional view of a lithium ion secondary battery according to an embodiment of the present invention. The lithium ion secondary battery 100 shown in FIG. 1 includes a power generation element 40, an exterior body 50, and an electrolyte (not shown). Exterior body 50 covers the periphery of power generation element 40 . The power generating element 40 is connected to the outside through a pair of connected terminals 60 and 62. The electrolytic solution is housed in the exterior body 50 and impregnated into the power generation element 40.

(power generation element)
The power generating element 40 includes a positive electrode 20, a negative electrode 30, and a separator 10.

<Separator>
Separator 10 is sandwiched between positive electrode 20 and negative electrode 30. The separator 10 isolates the positive electrode 20 and the negative electrode 30 and prevents a short circuit between the positive electrode 20 and the negative electrode 30. Lithium ions can pass through the separator 10.

The separator 10 has, for example, an electrically insulating porous structure. The separator 10 is, for example, a monolayer or laminate of a film made of polyolefin such as polyethylene or polypropylene, a stretched film of a mixture of the above resins, or selected from the group consisting of cellulose, polyester, polyacrylonitrile, polyamide, polyethylene, and polypropylene. Examples include fibrous nonwoven fabrics made of at least one constituent material.

Separator 10 may be, for example, a solid electrolyte. The solid electrolyte is, for example, a polymer solid electrolyte, an oxide solid electrolyte, or a sulfide solid electrolyte. The solid polymer electrolyte is, for example, a polyethylene oxide polymer in which an alkali metal salt is dissolved. Examples of oxide-based solid electrolytes include Li _1.3 Al _0.3 Ti _1.7 (PO ₄ ) ₃ (Nashicon type), Li _1.07 Al _0.69 Ti _1.46 (PO ₄ ) ₃ (glass Ceramics), Li _0.34 La _0.51 TiO _2.94 (perovskite type), Li ₇ La ₃ Zr ₂ O ₁₂ (garnet type), Li _2.9 PO _3.3 N _0.46 (amorphous, LIPON) , 50Li ₄ SiO ₄ .50Li ₂ BO ₃ (glass), and 90Li ₃ BO ₃ .10Li ₂ SO ₄ (glass ceramics). Sulfide-based solid electrolytes include, for example, Li _3.25 Ge _0.25 P _0.75 S ₄ (crystal), Li ₁₀ GeP ₂ S ₁₂ (crystal, LGPS), Li ₆ PS ₅ Cl (crystal, argyrodite type) , Li _9.54 Si _1.74 P _1.44 S _11.7 Cl _0.3 (crystal), Li _3.25 P _0.95 S ₄ (glass ceramics), Li ₇ P ₃ S ₁₁ (glass ceramics) , 70Li ₂ S・30P ₂ S ₅ (glass), 30Li ₂ S・26B ₂ S ₃・44LiI (glass), 50Li ₂ S・17P ₂ S ₅・33LiBH ₄ (glass), 63Li ₂ S・36SiS ₂・Li _{3PO 4} (glass), 57Li ₂ S.38SiS 2.5Li ₄ SiO ₄ (glass) _.

<Positive electrode>
The positive electrode 20 includes a positive electrode current collector 22 and a positive electrode active material layer 24 . The positive electrode active material layer 24 is formed on at least one surface of the positive electrode current collector 22. The positive electrode active material layer 24 may be formed on both sides of the positive electrode current collector 22.

[Positive electrode current collector]
The positive electrode current collector 22 is, for example, a conductive plate material. The positive electrode current collector 22 is, for example, a thin metal plate made of aluminum, copper, nickel, titanium, stainless steel, or the like.

[Cathode active material layer]
The positive electrode active material layer 24 includes, for example, a positive electrode active material, a conductive additive, and a binder.

The positive electrode active material includes positive electrode active material particles, a first coating layer that covers at least a part of the surface of the positive electrode active material particles, and a second coating that covers at least a part of the surface of the first coating layer. It has a layer.

The positive electrode active material particles include lithium cobalt oxide represented by Li _1-x CoO ₂ (0≦x≦1). The positive electrode active material particles may be lithium cobalt oxide particles containing lithium cobalt oxide alone, or may be mixture particles containing lithium cobalt oxide particles and other positive electrode active material particles. Examples of positive electrode active materials other than lithium cobalt oxide include lithium nickel oxide (LiNiO ₂ ), lithium manganate (LiMnO ₂ ), lithium manganese spinel (LiMn ₂ O ₄ ), and general formula: _LiNix Co _y Mn _z M _a2 (x+y+z+a=1, 0≦x<1, 0≦y<1, 0≦z<1, 0≦a<1, M is 1 selected from Al, Mg, Nb, Ti, Cu, Zn, Cr complex metal oxides, lithium vanadium compounds (LiV ₂ O ₅ ), olivine-type LiMPO ₄ (where M is Co, Ni, Mn, Fe, Mg, Nb, Ti, Al, one or more elements selected from Zr or VO), lithium titanate (Li ₄ Ti ₅ O ₁₂ ), composite metals such as _LiNix Co _y Al _z O ₂ (0.9<x+y+z<1.1) These include oxides, polyacetylene, polyaniline, polypyrrole, polythiophene, and polyacene. These positive electrode active materials may be used alone or in combination of two or more. Preferably, the mixture particles contain 80% by mass or more of lithium cobalt oxide.

The positive electrode active material particles preferably have an average particle diameter in the range of 1 μm or more and 50 μm or less.

The first coating layer contains Li ₂ CO ₃ . The first coating layer may be a Li ₂ CO ₃ layer containing only Li ₂ CO ₃ or a mixture layer containing Li ₂ CO ₃ and a carbonate other than Li ₂ CO ₃ . Carbonates other than Li ₂ CO ₃ are, for example, CoCO ₃ , MnCO ₃ , and NiCO ₃ . These carbonates may be used alone or in combination of two or more. The mixture layer preferably contains 80% by mass or more of Li ₂ CO ₃ .

The thickness of the first coating layer is preferably in the range of 0.1 μm or more and 1 μm or less.

The second coating layer contains a compound represented by the following general formula (1). The second coating layer may be a single layer containing the compound of the following general formula (1) alone, or a mixture layer containing the compound of the following general formula (1) and another dicarbonate compound. It's okay. The mixture layer preferably contains 80% by mass or more of a compound represented by the following general formula (1).

However, in general formula (1), Rf is a fluorine-containing alkyl group. The fluorine-containing alkyl group preferably has 1 to 3 carbon atoms. The fluorine-containing alkyl group preferably has 1 to 3 fluorine elements. The fluorine-containing alkyl group may have a substituent. Examples of the substituent include a hydroxy group and an alkoxy group having 1 to 3 carbon atoms. Examples of fluorine-containing alkyl groups include -CHF-CH ₃ and -CHF-CH ₂ OH.

The positive electrode active material can be manufactured, for example, as follows.
First, positive electrode active material particles and Li ₂ CO ₃ are mixed and LiCO ₃ is attached to the surface of the positive electrode active material particles to form a first coating layer (LiCO ₃ layer). For mixing the positive electrode active material particles and Li ₂ CO ₃ , a device such as a ball mill that mixes the raw materials while applying a shearing force can be used.

Next, the positive electrode active material particles coated with the first coating layer and a cyclic carbonate compound having a fluorine-containing hydrocarbon group are mixed and heated to combine Li ₂ CO ₃ and the cyclic carbonate compound of the first coating layer. By reacting, a compound of general formula (1) is generated to form a second coating layer. As the cyclic carbonate compound, for example, fluoroethylene carbonate can be used. By forming the second coating layer by reacting the Li ₂ CO ₃ of the first coating layer with the cyclic carbonate compound, the adhesion between the first coating layer and the second coating layer is improved, and the lithium ion secondary battery The increase in internal resistance after a high temperature storage test can be suppressed more reliably.

The conductive support material is scattered within the positive electrode active material layer 24. The conductive additive increases the conductivity between the positive electrode active materials in the positive electrode active material layer 24. Examples of the conductive additive include carbon powder such as carbon black, carbon nanotubes, carbon materials, fine metal powder such as copper, nickel, stainless steel, and iron, a mixture of carbon materials and fine metal powder, and conductive oxides such as ITO. . The conductive auxiliary material is preferably a carbon material such as carbon black. If sufficient conductivity can be ensured with the active material alone, the positive electrode active material layer 24 does not need to contain a conductive additive.

The binder binds the positive electrode active materials 25 in the positive electrode active material layer 24 to each other. A known binder can be used. The binder is, for example, a fluororesin. Examples of the fluororesin include polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), These include ethylene-tetrafluoroethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), ethylene-chlorotrifluoroethylene copolymer (ECTFE), and polyvinyl fluoride (PVF).

In addition to the above, binders include, for example, vinylidene fluoride-hexafluoropropylene-based fluororubber (VDF-HFP-based fluororubber), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene-based fluororubber (VDF-HFP-TFE-based fluororubber), (fluororubber), vinylidene fluoride-pentafluoropropylene-based fluororubber (VDF-PFP-based fluororubber), vinylidene fluoride-pentafluoropropylene-tetrafluoroethylene-based fluororubber (VDF-PFP-TFE-based fluororubber), vinylidene fluoride Vinylidene fluoride fluorine rubber such as Ride-perfluoromethyl vinyl ether-tetrafluoroethylene fluorine rubber (VDF-PFMVE-TFE fluorine rubber), vinylidene fluoride-chlorotrifluoroethylene fluorine rubber (VDF-CTFE fluorine rubber), etc. It can also be made of rubber.

<Negative electrode>
The negative electrode 30 includes, for example, a negative electrode current collector 32 and a negative electrode active material layer 34. The negative electrode active material layer 34 is formed on at least one surface of the negative electrode current collector 32.

[Negative electrode current collector]
The negative electrode current collector 32 is, for example, a conductive plate material. As the negative electrode current collector 32, the same one as the positive electrode current collector 22 can be used.

[Negative electrode active material layer]
The negative electrode active material layer 34 contains a negative electrode active material. Further, a conductive aid and a binder may be included as necessary.

The negative electrode active material may be any compound that can absorb and release ions, and negative electrode active materials used in known lithium ion secondary batteries can be used. Examples of negative electrode active materials include carbon materials such as metallic lithium, lithium alloys, graphite (natural graphite, artificial graphite) capable of intercalating and releasing ions, carbon nanotubes, non-graphitizable carbon, easily graphitizable carbon, and low-temperature calcined carbon. , metals that can be combined with metals such as lithium such as aluminum, silicon, tin, and germanium, SiO _x (0<x<2), amorphous compounds mainly composed of oxides such as tin dioxide, titanic acid These are particles containing lithium (Li ₄ Ti ₅ O ₁₂ ) and the like.

The negative electrode active material layer 34 may contain, for example, silicon, tin, or germanium, as described above. Silicon, tin, and germanium may exist as single elements or as compounds. The compound is, for example, an alloy, an oxide, or the like. As an example, when the negative electrode active material is silicon, the negative electrode 30 may be referred to as a Si negative electrode. The negative electrode active material may be, for example, a mixture of silicon, tin, germanium alone or a compound and a carbon material. The carbon material is, for example, natural graphite. Further, the negative electrode active material may be, for example, silicon, tin, germanium alone or a compound whose surface is coated with carbon. The carbon material and the coated carbon increase the electrical conductivity between the negative electrode active material and the conductive additive. When the negative electrode active material layer contains silicon, tin, and germanium, the capacity of the lithium ion secondary battery 100 increases.

The negative electrode active material layer 34 may contain lithium. Lithium may be metallic lithium or a lithium alloy. The negative electrode active material layer 34 may be made of metallic lithium or a lithium alloy. Examples of lithium alloys include Si, Sn, C, Pt, Ir, Ni, Cu, Ti, Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Sb, Pb, In, Zn, Ba, It is an alloy of lithium and one or more elements selected from the group consisting of Ra, Ge, and Al. As an example, when the negative electrode active material is metallic lithium, the negative electrode 30 may be referred to as a Li negative electrode. The negative electrode active material layer 34 may be a lithium sheet.

The negative electrode 30 may include only the negative electrode current collector 32 without having the negative electrode active material layer 34 at the time of manufacture. When the lithium ion secondary battery 100 is charged, metallic lithium is deposited on the surface of the negative electrode current collector 32. Metallic lithium is single lithium in which lithium ions have been deposited, and metallic lithium functions as the negative electrode active material layer 34.

The same conductive additive and binder as those used in the positive electrode 20 can be used. In addition to those listed for the positive electrode 20, the binder in the negative electrode 30 may be, for example, cellulose, styrene/butadiene rubber, ethylene/propylene rubber, polyimide resin, polyamideimide resin, acrylic resin, or the like. The cellulose may be, for example, carboxymethylcellulose (CMC).

(terminal)
Terminals 60 and 62 are connected to positive electrode 20 and negative electrode 30, respectively. Terminal 60 connected to positive electrode 20 is a positive electrode terminal, and terminal 62 connected to negative electrode 30 is a negative electrode terminal. Terminals 60 and 62 are responsible for electrical connection with the outside. The terminals 60, 62 are made of a conductive material such as aluminum, nickel, copper, or the like. The connection method may be welding or screwing. The terminals 60, 62 are preferably protected with insulating tape to prevent short circuits.

(exterior body)
The exterior body 50 seals the power generation element 40 and the electrolyte therein. The exterior body 50 prevents electrolyte from leaking to the outside and moisture from entering into the lithium ion secondary battery 100 from the outside.

For example, as shown in FIG. 1, the exterior body 50 includes a metal foil 52 and a resin layer 54 laminated on each surface of the metal foil 52. The exterior body 50 is a metal laminate film in which a metal foil 52 is coated on both sides with a polymer film (resin layer 54).

For example, aluminum foil can be used as the metal foil 52. For the resin layer 54, a polymer film such as polypropylene can be used. The material constituting the resin layer 54 may be different between the inside and outside. For example, the outer material is a polymer with a high melting point, such as polyethylene terephthalate (PET), polyamide (PA), etc., and the inner polymer membrane material is polyethylene (PE), polypropylene (PP), etc. be able to.

(electrolyte)
The electrolytic solution is sealed in the exterior body 50 and impregnated into the power generation element 40. The electrolytic solution is a non-aqueous electrolytic solution, and includes, for example, a non-aqueous solvent and an electrolyte. The electrolyte is dissolved in a non-aqueous solvent.

The non-aqueous solvent contains, for example, a cyclic carbonate and a chain carbonate. Cyclic carbonates solvate electrolytes. Cyclic carbonates are, for example, ethylene carbonate, propylene carbonate and butylene carbonate. The chain carbonate reduces the viscosity of the cyclic carbonate. Examples of chain carbonates include diethyl carbonate, dimethyl carbonate, and ethylmethyl carbonate. Other non-aqueous solvents include methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, 1,2-dimethoxyethane, 1,2-diethoxyethane, fluoroethylene carbonate, vinylene carbonate. etc. may be included.

The ratio of cyclic carbonate to chain carbonate in the nonaqueous solvent is, for example, 1:9 to 1:1 in terms of volume ratio.

Examples of the electrolyte include _LiPF6 , LiClO4, _LiBF4 , _{LiCF3SO3 , LiCF3CF2SO3} , _LiC ( _{CF3SO2 ) 3 ,} LiN( _{CF3SO2 ) 2} , LiN( _{CF3CF2 ) . SO2} ) ₂ , LiN( _{CF3SO2 )} ( _{C4F9SO2 ) ,} LiN( _CF3CF2CO ) _{2 ,} LiBOB, and the like.

“Method for manufacturing lithium ion secondary batteries”
First, the positive electrode 20 is produced. The positive electrode 20 is prepared by mixing a positive electrode active material, a conductive additive, a binder, and a solvent to prepare a paste-like positive electrode slurry. The method of mixing these components constituting the positive electrode slurry is not particularly limited, and the mixing order is also not particularly limited. Next, the positive electrode slurry is applied to the positive electrode current collector 22 . There are no particular restrictions on the coating method. Examples include the slit die coating method and the doctor blade method.

Subsequently, the solvent in the positive electrode slurry applied onto the positive electrode current collector 22 is removed. The removal method is not particularly limited. For example, the positive electrode current collector 22 coated with the positive electrode slurry is dried in an atmosphere of 80° C. to 150° C. Next, the obtained coating film is pressed to increase the density of the positive electrode active material layer 24, thereby obtaining the positive electrode 20. As the pressing means, for example, a roll press machine, a hydrostatic press machine, etc. can be used.

Next, the negative electrode 30 is produced. The negative electrode 30 can be manufactured in the same manner as the positive electrode 20. The negative electrode 30 is prepared by mixing a negative electrode active material, a binder, and a solvent to prepare a paste negative electrode slurry. The negative electrode 30 is obtained by applying the negative electrode slurry to the negative electrode current collector 32 and drying it.

Next, the produced positive electrode 20 and negative electrode 30 are stacked so that the separator 10 is located between them, thereby producing the power generation element 40. When the power generation element 40 is a wound body, the positive electrode 20, the negative electrode 30, and the separator 10 are wound around one end side of the separator 10 as an axis.

Finally, the power generating element 40 is enclosed in the exterior body 50. The electrolytic solution is injected into the exterior body 50. By performing depressurization, heating, etc. after injecting the electrolytic solution, the electrolytic solution is impregnated into the power generation element 40. By applying heat or the like to seal the exterior body 50, the lithium ion secondary battery 100 is obtained.

Then, the manufactured lithium ion secondary battery 100 is aged. Aging is a process of preliminarily charging the manufactured lithium ion secondary battery, and can remove defective products. Aging is performed, for example, at a voltage of 4.5 V (vsLi/Li ⁺ ) or higher.

The lithium ion secondary battery 100 according to the present embodiment has a positive electrode including particles containing lithium cobalt oxide represented by Li _1-x CoO ₂ (0≦x≦1), and a positive electrode provided on the surface of the particles. Since it includes a first coating layer containing ₂ CO ₃ and a second coating layer provided on the surface of this first coating layer and containing a compound represented by the above general formula (1), Increase in resistance is suppressed. The reason why such an effect is obtained is not clear, but it is thought as follows. That is, the compound of general formula (1) contained in the second coating layer not only suppresses contact between Li ₂ CO ₃ contained in the first coating layer and the electrolytic solution, but also suppresses the contact of Li ₂ CO 3 in the structure. ₃ residue, the film has high adhesion to the first coating layer. Furthermore, by including fluorine ions in the structure, it becomes chemically stable and can suppress reactions with electrolytes during high-temperature storage. This is thought to suppress the increase in resistance after the high temperature storage test.

The embodiments of the present invention have been described above in detail with reference to the drawings, but each configuration and combination thereof in each embodiment is merely an example, and additions or omissions of configurations may be made within the scope of the spirit of the present invention. , substitutions, and other changes are possible.

(Preparation of positive electrode)
97 parts by mass of LiCoO ₂ (LCO) and 3 parts by mass of LiCO ₃ were mixed using a ball mill to coat the LCO particle surfaces with LiCO ₃ . The thickness of the LiCO ₃ layer of the obtained LiCO ₃ -coated LCO was measured using a scanning electron microscope (JEOL Co., Ltd.) after creating a particle cross-section sample using an ion milling device (manufactured by Hitachi High-Technologies Corporation). As a result of measurement, the thickness of the _three LiCO layers was 0.5 μm. Next, the LiCO ₃ -coated LCO, ethanol, and fluoroethylene carbonate were charged into a reaction vessel equipped with a reflux vessel, and heated at a temperature of 80° C. for 2 hours while stirring. After cooling to room temperature, the reaction product was taken out from the reaction vessel, washed with ethanol, and then dried. As a result of analyzing the surface of the obtained dried product using _NMR , it was found that the compound represented by the above general formula (1) (wherein Rf is - It was confirmed that a single physical phase (second coating layer) of CHF-CH ₃ ) was formed. As described above, a positive electrode active material was obtained.

80 parts by mass of a positive electrode active material, 10 parts by mass of carbon black, and 10 parts by mass of PVDF were dispersed in N-methyl-2-pyrrolidone (NMP) to prepare a slurry for forming a positive electrode active material layer. This slurry was applied onto one surface of a 20 μm thick aluminum metal foil so that the coating amount of the positive electrode active material was 9.0 mg/cm ² , and dried at 100° C. to form a positive electrode active material layer. Thereafter, pressure molding was performed using a roller press to produce a positive electrode.

(Preparation of negative electrode)
A negative electrode mixture was prepared by mixing a negative electrode active material, a conductive additive, and a binder. The negative electrode active material was graphite, the conductive additive was carbon black, and the binder was carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR). The mass ratio of the negative electrode active material, conductive additive, and binder was 90:5:5. This negative electrode mixture was dispersed in distilled water to prepare a negative electrode slurry. Then, the negative electrode slurry was applied to one side of the copper foil having a thickness of 10 μm. After coating, it was dried at 100° C. to remove the solvent and form a negative electrode active material layer.

(Preparation of lithium ion secondary battery)
The produced negative electrodes and positive electrodes were punched into a predetermined shape and alternately stacked with polypropylene separators having a thickness of 25 μm interposed therebetween, and a laminate was produced by stacking nine negative electrodes and eight positive electrodes.

The laminate was inserted into an aluminum laminate film exterior and heat-sealed except for one location around the periphery to form an opening. A non-aqueous electrolyte was injected into the exterior body. The nonaqueous electrolyte was prepared by dissolving 1.0 mol/L of lithium hexafluorophosphate (LiPF ₆ ) in a solvent in which equal amounts of ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed. Then, the remaining one location was sealed with a heat seal while reducing the pressure using a vacuum sealing machine, thereby producing a lithium ion secondary battery according to Example 1.

[Examples 2 to 5]
In producing the positive electrode, by changing the mixing ratio of LCO and LiCO ₃ , positive electrode active materials were obtained in which the thickness of the LiCO ₃ (first coating layer) was as shown in Table 1 below. Lithium ion secondary batteries according to Examples 2 to 5 were produced in the same manner as in Example 1 except that this positive electrode active material was used. The mixing ratio of LCO and LiCO ₃ (LCO:LiCO ₃ ) in Examples 2 to 5 was 99.5:0.5 in Example 2, 93:7 in Example 3, and 93:7 in mass ratio. In Example 4, the ratio was 87:13, and in Example 5, it was 80:20.

[Example 6]
In producing the positive electrode, the general formula (1 ) was obtained as a positive electrode active material in which Rf was a fluorine-containing alkyl group shown in Table 1. A lithium ion secondary battery according to Example 6 was produced in the same manner as Example 1 except that this positive electrode active material was used.

[Examples 7 to 9]
Lithium ion secondary batteries according to Examples 7 to 9 were produced in the same manner as in Example 1 except that LCO and other positive electrode active material particles shown in Table 1 were used instead of LCO in producing the positive electrode. did. In Table 1, NMC of Example 7 represents lithium nickel manganese cobalt oxide (LiNiMnCoO ₂ ), NCA of Example 8 represents lithium nickel cobalt aluminum oxide (LiNiCoAlO ₂ ), and LFP of Example 9 represents , represents lithium iron phosphate (LiFePO ₄ ).

[Comparative example 1]
In producing the positive electrode, a lithium ion secondary battery according to Comparative Example 1 was produced in the same manner as in Example 1 except that the second coating layer was not formed.

[Comparative example 2]
A lithium ion secondary battery according to Comparative Example 2 was produced in the same manner as in Example 1 except that the first coating layer and the second coating layer were not formed in the production of the positive electrode.

[evaluation]
(1) Cell resistance after initial charge/discharge The lithium ion secondary batteries produced in Examples 1 to 9 and Comparative Examples 1 to 2 were tested using a charge/discharge test device (manufactured by Hokuto Denko Co., Ltd.) in a constant temperature bath at 25°C. After charging with constant current charging at a current density of 80μA/cm2 until the battery voltage reaches 4.2V, the battery voltage becomes 2.8V with constant current discharging at a current density of 80μA/cm2. Discharge was performed until Thereafter, DC resistance was measured using a digital multimeter (manufactured by Hioki Electric Co., Ltd.). The results are shown in Table 1.

(2) Measurement of cell resistance after high temperature storage test The lithium ion secondary batteries produced in Examples 1 to 9 and Comparative Examples 1 to 2 were measured using a secondary battery charge/discharge test device at a charging rate current density of 80 μA/cm ² The battery was charged at a constant current until the battery voltage reached 4.2V, and the battery was allowed to stand for 6 hours in a constant temperature bath set at 80°C. After 6 hours, the battery was taken out and allowed to radiate heat at room temperature for 15 minutes, and then DC resistance was measured using a digital multimeter. The results are shown in Table 1.

From the results in Table 1, the lithium ion secondary batteries of Examples 1 to 9 having the first coating layer and the second coating layer have low cell resistance after initial charge and discharge, and low cell resistance after high temperature storage test. I understand. On the other hand, the lithium ion secondary battery of Comparative Example 1 having only the first coating layer had a low cell resistance after the first charge/discharge, but a high cell resistance after the high temperature storage test. In addition, the lithium ion secondary battery of Comparative Example 2, which did not have either the first coating layer or the second coating layer, had high cell resistance after the first charge/discharge and high cell resistance after the high temperature storage test.

10 separator 20 positive electrode 22 positive electrode current collector 24 positive electrode active material layer 30 negative electrode 32 negative electrode current collector 34 negative electrode active material layer 40 power generation element 50 exterior body 52 metal foil 54 resin layer 60, 62 terminal 100 lithium ion secondary battery

Claims (2)

A lithium ion secondary battery comprising a positive electrode and an electrolyte,
The positive electrode includes particles containing lithium cobalt oxide represented by Li _1-x CoO ₂ (0≦x≦1);
a first coating layer provided on the surface of the particles and containing Li ₂ CO ₃ ;
A lithium ion secondary battery comprising a positive electrode active material, and a second coating layer provided on the surface of the first coating layer and containing a compound represented by the following general formula (1).

However, in general formula (1), Rf is a fluorine-containing alkyl group having 1 to 3 carbon atoms . The lithium ion secondary battery according to claim 1, wherein the thickness of the first coating layer is within a range of 0.1 μm or more and 1 μm or less.

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