JP2022141192A - Positive electrode and non-aqueous electrolyte secondary battery including the same - Google Patents

Abstract

To provide a positive electrode using spinel-type manganese-containing composite oxide that is capable of attaching excellent capacity degradation resistance to a nonaqueous electrolyte secondary battery on repeated charge-discharge cycles.SOLUTION: A positive electrode includes a positive electrode collector and a positive electrode active material layer supported by the positive electrode collector. The positive electrode active material layer includes a positive electrode active material. The positive electrode active material is a lithium complex oxide having a spinel-type crystal structure and containing Mn. The lithium composite oxide has a phosphonic acid compound coating having fluorinated hydrocarbon groups on the surface thereof. The ratio of the phosphonic acid compound to the lithium composite oxide is between 0.1 mass% and 1.0 mass% inclusive.SELECTED DRAWING: Figure 1

Description

The present invention relates to positive electrodes. The present invention also relates to a non-aqueous electrolyte secondary battery including the positive electrode.

In recent years, non-aqueous electrolyte secondary batteries such as lithium ion secondary batteries have been used as portable power sources for personal computers and mobile terminals, and for driving vehicles such as electric vehicles (EV), hybrid vehicles (HV), and plug-in hybrid vehicles (PHV). It is suitable for use as a power source for electric appliances.

Non-aqueous electrolyte secondary batteries generally use an active material that can occlude and release ions that serve as charge carriers. Lithium composite oxides are generally used as the active material for the positive electrode, and examples of lithium composite oxides include composites having a spinel-type crystal structure and containing manganese, such as spinel-type lithium-manganese-based composite oxides. It is known that oxides (hereinafter sometimes referred to as "spinel-type manganese-containing composite oxides") can be used (see Patent Documents 1 and 2, for example).

The spinel-type manganese-containing composite oxide has the advantages of high thermal stability and low cost. However, as described in Patent Documents 1 and 2, when a non-aqueous electrolyte secondary battery using a spinel-type manganese-containing composite oxide is repeatedly charged and discharged, the capacity deteriorates significantly. exist across. On the other hand, Patent Literature 3 discloses a technique for suppressing deterioration of capacity during repeated charging and discharging by adding a phosphonic acid compound having a fluoroalkyl group to a non-aqueous electrolyte.

Japanese Unexamined Patent Application Publication No. 2001-76722 WO2014/175355 JP 2015-232972 A

However, as a result of intensive studies by the present inventors, even in the conventional technology described in Patent Document 3, when a non-aqueous electrolyte secondary battery using a spinel-type manganese-containing composite oxide is repeatedly charged and discharged, It has been found that there is still room for improvement in suppressing the capacity deterioration of the battery.

Accordingly, an object of the present invention is to provide a positive electrode using a spinel-type manganese-containing composite oxide, which is capable of providing a non-aqueous electrolyte secondary battery with excellent capacity deterioration resistance when charging and discharging are repeated. be.

The positive electrode disclosed herein includes a positive electrode current collector and a positive electrode active material layer supported by the positive electrode current collector. The positive electrode active material layer contains a positive electrode active material. The positive electrode active material is a lithium composite oxide having a spinel crystal structure and containing Mn. The lithium composite oxide has a coating of a phosphonic acid compound having a fluorinated hydrocarbon group on its surface. A ratio of the phosphonic acid compound to the lithium composite oxide is 0.1% by mass or more and 1.0% by mass or less. According to such a configuration, it is possible to provide a positive electrode using a spinel-type manganese-containing composite oxide, which can impart excellent resistance to capacity deterioration when charging and discharging are repeated to a non-aqueous electrolyte secondary battery. can be done.

In a preferred embodiment of the positive electrode disclosed herein, the positive electrode active material layer further contains trilithium phosphate. According to such a configuration, it is possible to impart higher resistance to capacity deterioration to the non-aqueous electrolyte secondary battery.

In a preferred embodiment of the positive electrode disclosed herein, the fluorinated hydrocarbon group of the phosphonic acid compound is a fluorinated alkyl group having 8 to 12 carbon atoms. According to such a configuration, it is possible to impart higher resistance to capacity deterioration to the non-aqueous electrolyte secondary battery.

In a preferred embodiment of the positive electrode disclosed herein, the lithium composite oxide is LiMn ₂ O ₄ . Since LiMn ₂ O ₄ has a particularly large capacity deterioration when the non-aqueous electrolyte secondary battery is repeatedly charged and discharged, according to such a configuration, the effect of suppressing capacity deterioration of the positive electrode disclosed herein is more remarkable. Become. Moreover, according to such a configuration, it is possible to improve the thermal stability of the non-aqueous electrolyte secondary battery and reduce the cost.

From another aspect, a non-aqueous electrolyte secondary battery disclosed herein includes the above positive electrode, negative electrode, and non-aqueous electrolyte. According to such a configuration, it is possible to provide a non-aqueous electrolyte secondary battery that is excellent in capacity deterioration resistance when charging and discharging are repeated.

1 is a cross-sectional view schematically showing a positive electrode according to an embodiment of the invention; FIG. 1 is a cross-sectional view schematically showing the internal structure of a lithium ion secondary battery using a positive electrode according to one embodiment of the present invention; FIG. 3 is a schematic exploded view showing the configuration of a wound electrode body of the lithium ion secondary battery of FIG. 2. FIG.

Hereinafter, embodiments according to the present invention will be described with reference to the drawings. Matters not mentioned in this specification but necessary for the implementation of the present invention can be grasped as design matters by those skilled in the art based on the prior art in the relevant field. The present invention can be implemented based on the contents disclosed in this specification and common general technical knowledge in the field. Further, in the following drawings, members and portions having the same function are denoted by the same reference numerals. Also, the dimensional relationships (length, width, thickness, etc.) in each drawing do not reflect the actual dimensional relationships.

In this specification, the term "secondary battery" refers to an electricity storage device that can be repeatedly charged and discharged, and is a term that includes so-called storage batteries and electricity storage elements such as electric double layer capacitors. In this specification, the term “lithium ion secondary battery” refers to a secondary battery that utilizes lithium ions as a charge carrier and is charged/discharged by the transfer of charge associated with the lithium ions between the positive and negative electrodes.

Hereinafter, the present invention will be described in detail using a positive electrode used in a lithium ion secondary battery as an example, but the present invention is not intended to be limited to those described in the embodiments. FIG. 1 is a schematic cross-sectional view perpendicular to the thickness direction of the positive electrode according to this embodiment.

As illustrated, the cathode 50 includes a cathode current collector 52 and a cathode active material layer 54 supported by the cathode current collector 52 . The positive electrode active material layer 54 may be provided on one side of the positive electrode current collector 52, or may be provided on both sides of the positive electrode current collector 52 as illustrated. It is preferably provided on both sides of 52 .

As the positive electrode current collector 52, a known positive electrode current collector used for lithium ion secondary batteries may be used, and examples thereof include metals with good conductivity (e.g., aluminum, nickel, titanium, stainless steel, etc.). ) sheets or foils. Aluminum foil is preferable as the positive electrode current collector 52 .

The dimensions of the positive electrode current collector 52 are not particularly limited, and may be appropriately determined according to the battery design. When an aluminum foil is used as the positive electrode current collector 52, its thickness is not particularly limited, but is, for example, 5 μm or more and 35 μm or less, preferably 7 μm or more and 20 μm or less.

The positive electrode active material layer 54 contains a positive electrode active material. In the present embodiment, a lithium composite oxide having a spinel-type crystal structure and containing Mn (spinel-type manganese-containing composite oxide) is used as the positive electrode active material. Examples of such a composite oxide include lithium manganate (LiMn ₂ O ₄ ) having a spinel crystal structure, and spinel crystal structure in which manganese in lithium manganate is partially replaced with lithium or other elements. Composite oxides (for example, LiNi _0.5 Mn _1.5 O ₄ etc.) and the like are included.

As the spinel-type manganese-containing composite oxide, for example, a composite oxide having a composition represented by the following formula (I) can be used.
Li _x (M1 _y M2 _z Mn _2-x-y-z )O _4-δ (I)

In formula (I), M1 is at least one element selected from the group consisting of Ni, Co and Fe, preferably Ni. M2 is the group consisting of Na, Mg, Al, P, K, Ca, Ba, Sr, Ti, V, Cr, Cu, Ga, Y, Zr, Nb, Mo, In, Ta, W, Re, and Ce Ti is at least one element selected from Ti, preferably Ti.

In formula (I), x satisfies 1.00≦x≦1.20, preferably 1.00≦x≦1.05, and more preferably 1.00. y satisfies 0≦y≦1.20, preferably 0≦y≦0.60, more preferably 0. z satisfies 0≦z≦0.5, preferably 0≦z≦0.10, more preferably 0. δ satisfies 0≦δ≦0.20, preferably 0≦δ≦0.05, more preferably 0.

In the present embodiment, a spinel-type manganese-containing composite oxide having a specific composition may be used alone, or two or more spinel-type manganese-containing composite oxides having different compositions may be used in combination. When a non-aqueous electrolyte secondary battery using LiMn ₂ O ₄ is repeatedly charged and discharged, its capacity deteriorates particularly significantly. Therefore, in the present embodiment, it is advantageous to use LiMn ₂ O ₄ as the spinel-type manganese-containing composite oxide because the effect of suppressing capacity deterioration of the positive electrode according to the present embodiment becomes more pronounced. In addition, the use of LiMn ₂ O ₄ has the advantage of being able to impart high thermal stability to the non-aqueous electrolyte secondary battery using the positive electrode 50 and to reduce the cost.

In this embodiment, the spinel-type manganese-containing composite oxide has a coating of a phosphonic acid compound having a fluorinated hydrocarbon group (hereinafter also referred to as a phosphonic acid compound (A)) on its surface.

The "hydrocarbon group" of the fluorinated hydrocarbon group of the phosphonic acid compound (A) may be either an aliphatic hydrocarbon group or an aromatic hydrocarbon group. Moreover, the hydrocarbon group may be a saturated hydrocarbon group or an unsaturated hydrocarbon group. Further, the hydrocarbon group may be linear, branched, or cyclic.

Specific examples of the hydrocarbon group include methyl group, ethyl group, propyl group, n-butyl group, isobutyl group, sec-butyl group, tert-butyl group, pentyl group, hexyl group, heptyl group, octyl group and nonyl. decyl group, undecyl group, dodecyl group; alkenyl groups such as vinyl group, allyl group, propenyl group, isopropenyl group, butenyl group, pentenyl group, hexenyl group; ethynyl group, propynyl group, butynyl group; Alkynyl groups such as pentynyl group and hexynyl group; aryl groups such as phenyl group, naphthyl group and anthryl group; arylalkyl groups such as benzyl group and phenylethyl group; alkylaryl groups such as tolyl group; The hydrocarbon group is preferably an alkyl group, an aryl group, an arylalkyl group or an alkylaryl group, more preferably an arylalkyl group or an alkyl group, still more preferably an alkyl group.

Hydrocarbon groups are fluorinated. Thus, the hydrocarbon group has at least some hydrogen atoms replaced by fluorine atoms. The number of fluorine atoms in the fluorinated hydrocarbon group is not particularly limited. In the hydrocarbon group, 30% or more of the hydrogen atoms are preferably substituted with fluorine atoms, more preferably 50% or more of the hydrogen atoms are substituted with fluorine atoms, and 70% or more of the hydrogen atoms are More preferably, it is substituted with a fluorine atom.

As the fluorinated hydrocarbon group, a fluorinated alkyl group having 8 to 12 carbon atoms is preferable because it can impart higher resistance to capacity deterioration to the non-aqueous electrolyte secondary battery.

Since higher capacity deterioration resistance can be imparted to the non-aqueous electrolyte secondary battery, the phosphonic acid compound (A) has a phosphonic acid group bonded to one bond of the alkylene group having 1 or 2 carbon atoms, and the other A compound in which a perfluorohydrocarbon group having 4 to 10 carbon atoms (especially a perfluorohydrocarbon group having 6 to 10 carbon atoms) is bonded to the bond is preferred.

Suitable examples of the phosphonic acid compound (A) having a fluorinated hydrocarbon group include phosphonic acid compounds represented by the following general formula (1).

In formula (1), the substituent Rf is a fluoroalkyl group having 1 to 12 carbon atoms, preferably a fluoroalkyl group having 4 to 10 carbon atoms, more preferably a fluoroalkyl group having 6 to 10 carbon atoms. be. A fluoroalkyl group may be linear, branched, or cyclic. The fluoroalkyl group represented by Rf may have one or more hydrogen atoms substituted with fluorine atoms, preferably an alkyl group having all hydrogen atoms substituted with fluorine atoms (i.e., perfluoroalkyl group) is. The fluoroalkyl group represented by Rf is a fluorine atom and other substituents (eg, a hydroxyl group, a methoxy group, a halogen atom other than a fluorine atom, etc.) in addition to a fluorine atom to the extent that the effects of the present invention are not impaired. Atoms may be substituted, but preferably hydrogen atoms are substituted only with fluorine atoms. Rf is particularly preferably a perfluoroalkyl group having 4 to 10 carbon atoms, and most preferably a perfluoroalkyl group having 6 to 10 carbon atoms.

Preferred specific examples of phosphonic acid compounds having a fluorinated hydrocarbon group include 1H,1H,2H,2H-perfluoro-n-hexylphosphonic acid (that is, Rf is (CF ₂ ) ₃ CF ₃ ). 1H,1H,2H,2H-perfluoro-n-octylphosphonic acid (ie, where Rf is (CF ₂ ) ₅ CF ₃ ); 3,3,4,4,5,5, 6,6,7,7,8,8,9,9,10,10,10-heptadecafluorodecylphosphonic acid (ie, where Rf is (CF ₂ ) ₇ CF ₃ ); 3,3,4 ,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,12,12,12-heneicosafluorododecylphosphonic acid (i.e., Rf is (CF ₂ ) ₉ CF ₃ )).

Another preferred example of the phosphonic acid compound (A) is 2,3,4,5,6-pentafluorobenzylphosphonic acid.

As the phosphonic acid oxide (A), a phosphonic acid compound having one fluorinated hydrocarbon group may be used alone, or two or more phosphonic acid compounds having two or more fluorinated hydrocarbon groups may be used in combination. may be used.

As long as the phosphonic acid compound (A) exists on the surface of the spinel-type manganese-containing composite oxide, there is no particular limitation on the coating form. The spinel-type manganese-containing composite oxide may be coated with the phosphonic acid compound (A) by adsorbing the phosphonic acid compound (A) by an intermolecular force. The phosphonic acid compound (A) and the spinel-type manganese-containing composite oxide may be bonded by hydrogen bonding to coat the spinel-type manganese-containing composite oxide. The phosphonic acid compound (A) may be bonded to the surface of the spinel-type manganese-containing composite oxide by reacting with it to coat the spinel-type manganese-containing composite oxide.

In the positive electrode according to the present embodiment, by using a spinel-type manganese-containing composite oxide having a surface coated with a phosphonic acid compound (A) as the positive electrode active material, excellent resistance to capacity deterioration when charging and discharging are repeated. It can be applied to non-aqueous electrolyte secondary batteries. It is considered that this is due to the following reasons.

Mn is eluted from the spinel-type manganese-containing composite oxide, and when the eluted Mn reaches the negative electrode, Li is deactivated on the negative electrode, resulting in a decrease in capacity. In contrast, the coating of the phosphonic acid compound (A) can serve as a barrier when Mn elutes from the spinel-type manganese-containing composite oxide, thereby suppressing the elution of Mn. In addition, the coating of the phosphonic acid compound (A) limits the access of hydrofluoric acid (HF) to the spinel-type manganese-containing composite oxide, which causes the elution of Mn, and also suppresses the elution of Mn from this point. be able to. As a result, capacity deterioration due to Mn elution can be suppressed.

however. In this embodiment, the ratio of the phosphonic acid compound (A) to the lithium composite oxide is 0.1% by mass or more and 1.0% by mass or less. If the proportion of the phosphonic acid compound (A) is less than 0.1% by mass, the effect of suppressing capacity deterioration by coating with the phosphonic acid compound (A) will be insufficient. When the proportion of the phosphonic acid compound (A) exceeded 1.0% by mass, the adhesion between the positive electrode active material layer 54 and the positive electrode current collector 52 deteriorated, and the non-aqueous electrolyte secondary battery was repeatedly charged and discharged. At this time, separation is likely to occur between the positive electrode active material layer 54 and the positive electrode current collector 52 . As a result, resistance to capacity deterioration is lowered.

The average particle diameter (median diameter D50) of the positive electrode active material is not particularly limited, but is, for example, 0.05 μm or more and 25 μm or less, preferably 0.5 μm or more and 23 μm or less, more preferably 3 μm or more and 22 μm or less. . The average particle diameter (median diameter D50) of the positive electrode active material can be determined by, for example, a laser diffraction scattering method.

The content of the positive electrode active material is not particularly limited, but is preferably 70% by mass or more, more preferably 80% by mass or more, in the positive electrode active material layer 54 (that is, with respect to the total mass of the positive electrode active material). , more preferably 85% by mass or more.

The positive electrode active material layer 54 may contain components other than the positive electrode active material. Examples thereof include trilithium phosphate (Li ₃ PO ₄ ), conductive materials, binders, and the like.

When the positive electrode active material layer 54 contains trilithium phosphate, it is possible to impart higher resistance to capacity deterioration to the non-aqueous electrolyte secondary battery. The content of trilithium phosphate is not particularly limited, but is, for example, 0.01% by mass or more and 10% by mass or less, preferably 0.1% by mass or more and 5% by mass or less, relative to the positive electrode active material, More preferably, it is 0.2% by mass or more and 3% by mass or less.

Carbon black such as acetylene black (AB) and other carbon materials (eg, graphite) can be suitably used as the conductive material. The content of the conductive material in the positive electrode active material layer 54 is not particularly limited, but is, for example, 0.1% by mass or more and 20% by mass or less, preferably 1% by mass or more and 15% by mass or less, and more preferably 2% by mass. It is more than mass % and below 10 mass %.

As the binder, for example, polyvinylidene fluoride (PVdF) or the like can be used. Although the content of the binder in the positive electrode active material layer 54 is not particularly limited, it is, for example, 0.5% by mass or more and 15% by mass or less, preferably 1% by mass or more and 10% by mass or less, and more preferably 1. It is 5 mass % or more and 8 mass % or less.

Although the thickness of the positive electrode active material layer 54 is not particularly limited, it is, for example, 10 μm or more and 300 μm or less, preferably 20 μm or more and 200 μm or less.

The positive electrode 50 according to this embodiment can be manufactured, for example, as follows. First, a positive electrode active material is prepared by coating a spinel-type manganese-containing composite oxide with a phosphonic acid compound (A). The coating method is not particularly limited, but as a suitable example thereof, a phosphonic acid compound (A) is dissolved in an appropriate solvent (eg, N-methylpyrrolidone) to prepare a diluted solution, and a spinel type There is a method of removing the solvent after dispersing the manganese-containing composite oxide.

Next, the positive electrode active material and arbitrary components (trilithium phosphate, conductive material, binder, etc.) are mixed in a solvent (dispersion medium) to prepare a positive electrode active material layer forming paste. The positive electrode active material layer forming paste is applied onto the positive electrode current collector 52 and dried to form the positive electrode active material layer 54 . If necessary, the positive electrode active material layer 54 is subjected to press treatment. Thus, the positive electrode 50 can be obtained.

A non-aqueous electrolyte secondary battery (particularly a lithium ion secondary battery) including the positive electrode 50 according to the present embodiment is inhibited from deteriorating in capacity when charging and discharging are repeated. Therefore, from another aspect, the non-aqueous electrolyte secondary battery according to the present embodiment is a non-aqueous electrolyte secondary battery including the above-described positive electrode, negative electrode, and non-aqueous electrolyte.

Hereinafter, a configuration example of a lithium ion secondary battery as an example of the non-aqueous electrolyte secondary battery according to the present embodiment will be described with reference to FIGS. 2 and 3. FIG.

The lithium ion secondary battery 100 shown in FIG. 2 is a closed type constructed by housing a flat wound electrode body 20 and a non-aqueous electrolyte 80 in a flat rectangular battery case (that is, an outer container) 30. Battery. The battery case 30 is provided with a positive terminal 42 and a negative terminal 44 for external connection, and a thin safety valve 36 set to release the internal pressure when the internal pressure of the battery case 30 rises above a predetermined level. ing. Further, the battery case 30 is provided with an injection port (not shown) for injecting the non-aqueous electrolyte 80 . The positive terminal 42 is electrically connected to the positive collector plate 42a. The negative terminal 44 is electrically connected to the negative collector plate 44a. As the material of the battery case 30, for example, a metal material such as aluminum that is lightweight and has good thermal conductivity is used. Note that FIG. 2 does not accurately represent the amount of non-aqueous electrolyte 80 .

As shown in FIGS. 2 and 3, the wound electrode body 20 is formed by stacking a positive electrode sheet 50 and a negative electrode sheet 60 with two long separator sheets 70 interposed therebetween and winding them in the longitudinal direction. morphology. The positive electrode sheet 50 has a configuration in which a positive electrode active material layer 54 is formed along the longitudinal direction on one side or both sides (here, both sides) of a long positive electrode current collector 52 . The negative electrode sheet 60 has a configuration in which a negative electrode active material layer 64 is formed along the longitudinal direction on one side or both sides (here, both sides) of a long negative electrode current collector 62 . The positive electrode active material layer non-formed portion 52a (that is, the portion where the positive electrode current collector 52 is exposed without the positive electrode active material layer 54 being formed) and the negative electrode active material layer non-formed portion 62a (that is, the negative electrode active material layer 64 is formed). The portion where the negative electrode current collector 62 is exposed without being wound) is formed so as to protrude outward from both ends of the wound electrode body 20 in the winding axial direction (that is, the sheet width direction orthogonal to the longitudinal direction). there is A positive collector plate 42a and a negative collector plate 44a are joined to the positive electrode active material layer non-formed portion 52a and the negative electrode active material layer non-formed portion 62a, respectively.

As the positive electrode sheet 50, the positive electrode 50 according to the present embodiment described above is used. In this configuration example, the positive electrode sheet 50 has the positive electrode active material layers 54 formed on both sides of the positive electrode current collector 52 .

As the negative electrode current collector 62 constituting the negative electrode sheet 60, a known negative electrode current collector used for lithium ion secondary batteries may be used. , titanium, stainless steel, etc.) sheets or foils. A copper foil is preferable as the negative electrode current collector 62 .

The dimensions of the negative electrode current collector 62 are not particularly limited, and may be appropriately determined according to the battery design. When a copper foil is used as the negative electrode current collector 62, its thickness is not particularly limited, but is, for example, 5 μm or more and 35 μm or less, preferably 7 μm or more and 20 μm or less.

The negative electrode active material layer 64 contains a negative electrode active material. As the negative electrode active material, for example, carbon materials such as graphite, hard carbon, and soft carbon can be used. Graphite may be natural graphite, artificial graphite, or amorphous carbon-coated graphite in which graphite is coated with an amorphous carbon material.

The average particle diameter (median diameter D50) of the negative electrode active material is not particularly limited, but is, for example, 0.1 μm or more and 50 μm or less, preferably 1 μm or more and 25 μm or less, more preferably 5 μm or more and 20 μm or less.

The content of the negative electrode active material in the negative electrode active material layer is not particularly limited, but is preferably 90% by mass or more, more preferably 95% by mass or more.

The negative electrode active material layer 64 may contain components other than the negative electrode active material, such as binders and thickeners.

As the binder, for example, styrene-butadiene rubber (SBR) and its modified products, acrylonitrile-butadiene rubber and its modified products, acrylic rubber and its modified products, fluororubber, and the like can be used. Among them, SBR is preferable. The content of the binder in the negative electrode active material layer 64 is not particularly limited, but is preferably 0.1% by mass or more and 8% by mass or less, and more preferably 0.2% by mass or more and 3% by mass or less.

Examples of thickeners that can be used include cellulose-based polymers such as carboxymethylcellulose (CMC), methylcellulose (MC), cellulose acetate phthalate (CAP), and hydroxypropylmethylcellulose (HPMC); polyvinyl alcohol (PVA) and the like. Among them, CMC is preferred. The content of the thickening agent in the negative electrode active material layer 64 is not particularly limited, but is preferably 0.3% by mass or more and 3% by mass or less, and more preferably 0.4% by mass or more and 2% by mass or less. .

Although the thickness of the negative electrode active material layer 64 is not particularly limited, it is, for example, 10 μm or more and 300 μm or less, preferably 20 μm or more and 200 μm or less.

Examples of the separator 70 include porous sheets (films) made of resin such as polyethylene (PE), polypropylene (PP), polyester, cellulose, and polyamide. Such a porous sheet may have a single-layer structure or a laminated structure of two or more layers (for example, a three-layer structure in which PP layers are laminated on both sides of a PE layer). A heat-resistant layer (HRL) may be provided on the surface of the separator 70 .

Although the thickness of the separator 70 is not particularly limited, it is, for example, 5 μm or more and 50 μm or less, preferably 10 μm or more and 30 μm or less.

Non-aqueous electrolyte 80 typically contains a non-aqueous solvent and an electrolyte salt (in other words, a supporting salt). As the non-aqueous solvent, organic solvents such as various carbonates, ethers, esters, nitriles, sulfones, lactones, etc., which are used in electrolytes of general lithium ion secondary batteries, can be used without particular limitation. can be done. Among them, carbonates are preferable, and specific examples thereof include ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethylmethyl carbonate (EMC), monofluoroethylene carbonate ( MFEC), difluoroethylene carbonate (DFEC), monofluoromethyldifluoromethyl carbonate (F-DMC), trifluorodimethyl carbonate (TFDMC) and the like. Such non-aqueous solvents can be used singly or in combination of two or more.

As the electrolyte salt, for example, lithium salts such as LiPF ₆ , LiBF ₄ and lithium bis(fluorosulfonyl)imide (LiFSI) can be used, among which LiPF ₆ is preferred. The concentration of the electrolyte salt is not particularly limited, but is preferably 0.7 mol/L or more and 1.3 mol/L or less.

Moreover, the non-aqueous electrolyte 80 may contain a phosphonic acid compound having a fluorinated hydrocarbon group. At this time, capacity deterioration of the lithium-ion secondary battery 100 when charging and discharging are repeated is further suppressed. The content of the phosphonic acid compound having a fluorinated hydrocarbon group in the nonaqueous electrolyte 80 is not particularly limited, but is, for example, 0.1% by mass or more and 10% by mass or less, preferably 0.1% by mass or more. It is 4.0 mass % or less, more preferably 0.1 mass % or more and 2.0 mass % or less.

In addition, the non-aqueous electrolyte 80 includes components other than the components described above, such as film-forming agents such as oxalato complexes, gases such as biphenyl (BP) and cyclohexylbenzene (CHB), as long as the effects of the present invention are not significantly impaired. Various additives such as generators; thickeners; and the like may be included.

When the positive electrode active material layer 54 of the positive electrode 50 contains trilithium phosphate and the spinel-type lithium composite oxide is LiMn ₂ O ₄ , a voltage of 4.7 V or more is applied to the lithium ion secondary battery 100. By performing the initial charging with , the effect of improving resistance to capacity deterioration due to trilithium phosphate can be further enhanced.

In the lithium-ion secondary battery 100 configured as described above, deterioration of capacity is less likely to occur when charging and discharging are repeated. That is, the lithium ion secondary battery 100 has excellent cycle characteristics.

The lithium ion secondary battery 100 can be used for various purposes. Suitable applications include drive power supplies mounted in vehicles such as electric vehicles (EV), hybrid vehicles (HV), and plug-in hybrid vehicles (PHV). Moreover, the lithium ion secondary battery 100 can be used as a storage battery such as a small power storage device. The lithium ion secondary battery 100 can also be typically used in the form of an assembled battery in which a plurality of batteries are connected in series and/or in parallel.

As an example, the prismatic lithium ion secondary battery 100 including the flattened wound electrode body 20 has been described. However, the nonaqueous electrolyte secondary battery disclosed herein is configured as a lithium ion secondary battery including a laminated electrode body (that is, an electrode body in which a plurality of positive electrodes and a plurality of negative electrodes are alternately laminated). can also The nonaqueous electrolyte secondary battery disclosed herein can also be configured as a cylindrical lithium ion secondary battery, a laminate case type lithium ion secondary battery, a coin type lithium ion secondary battery, or the like. The non-aqueous electrolyte secondary battery disclosed herein can also be configured as a non-aqueous electrolyte secondary battery other than the lithium-ion secondary battery according to known methods.

EXAMPLES Examples relating to the present invention will be described below, but the present invention is not intended to be limited to those shown in the examples.

<Production of lithium-ion secondary battery for evaluation of each example>
A phosphonic acid compound solution shown in Table 1 was dissolved in N-methyl-2-pyrrolidone (NMP) to obtain a phosphonic acid compound solution. This solution and LiMn ₂ O ₄ were mixed so that the amount of the phosphonic acid compound added to LiMn ₂ O ₄ was the amount shown in Table 1. As the phosphonic acid compounds shown in Table 1, commercially available reagents manufactured by Aldrich were used.

Carbon black (CB) as a conductive material and polyvinylidene fluoride (PVdF) as a binder were added to a mixture containing the obtained positive electrode active material (LMO) and a phosphonic acid compound, and LMO:CB:PVdF=90:8: A paste for forming a positive electrode active material layer was prepared by mixing in NMP at a mass ratio of 2. However, in Example 8, trilithium phosphate (LPO) was further added to the positive electrode active material so as to be 0.5% by mass to prepare the positive electrode active material layer forming paste.

This paste for forming a positive electrode active material layer was applied onto an aluminum foil, dried, and then subjected to densification treatment by roll pressing to prepare a positive electrode sheet. This positive electrode sheet was cut into a size of 120 mm×100 mm.

In addition, spheroidized graphite (C) as a negative electrode active material, styrene-butadiene rubber (SBR) as a binder, and carboxymethyl cellulose (CMC) as a thickening agent were combined into C:SBR:CMC=98:1:1. were mixed in water at a mass ratio of , to prepare a paste for forming a negative electrode active material layer. This paste for forming a negative electrode active material layer was applied onto a copper foil, dried, and then densified by roll pressing to prepare a negative electrode sheet. This negative electrode sheet was cut into a size of 122 mm×102 mm.

A porous polyolefin sheet was prepared as a separator sheet. An electrode body was produced using the positive electrode sheet, the negative electrode sheet, and the separator, and after attaching an electrode terminal to the electrode body, the electrode body was housed in a battery case together with a non-aqueous electrolyte. The non-aqueous electrolyte is a mixed solvent containing ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) at a volume ratio of 3:4:3, and LiPF ₆ at 1.1 mol/L. Lithium bisoxalatoborate (LiBOB) was dissolved at a concentration of 0.5% by mass. In this manner, a lithium ion secondary battery for evaluation of each example was produced.

<Production of lithium ion secondary battery for evaluation of Comparative Example 1>
A lithium ion secondary battery for evaluation was produced in the same manner as in Example 1, except that LiMn ₂ O ₄ was used as a positive electrode active material as it was without being coated with a phosphonic acid compound.

<Production of lithium ion secondary battery for evaluation of Comparative Example 2>
A lithium ion secondary battery for evaluation was produced in the same manner as in Example 1, except that the amount of fluorooctylsulfonic acid added was changed to 5% by mass.

<Production of lithium ion secondary battery for evaluation of Comparative Example 3>
In the same manner as in Example 1, except that LiMn ₂ O ₄ was used as a positive electrode active material without being coated with a phosphonic acid compound, and fluorooctyl sulfonic acid was added to the non-aqueous electrolyte at a concentration of 0.5% by mass. A lithium ion secondary battery for evaluation was produced.

<Activation and initial capacity measurement>
Each evaluation lithium ion secondary battery produced above was placed in an environment of 25°C. Activation (initial charge) is performed by a constant current-constant voltage method, and the current value of each lithium ion secondary battery for evaluation is 0.1 C, and 4.9 V in Example 8 in which trilithium phosphate (LPO) is added. In the other examples and each comparative example, constant-current charging was performed to 4.2 V, and then constant-voltage charging was performed until the current value became 1/50 C to reach a fully charged state. After that, each lithium ion secondary battery for evaluation was subjected to constant current discharge to 3.0 V at a current value of 0.1C. Then, the discharge capacity at this time was measured to obtain the initial capacity.

<Cycle characteristics evaluation>
Place each activated lithium ion secondary battery for evaluation in an environment of 60 ° C., charge and discharge with constant current charging at 0.5 C to 4.2 V and constant current discharging at 0.5 C to 3.0 V as one cycle was repeated for 50 cycles. The discharge capacity after 50 cycles was determined in the same manner as the initial capacity. As an index of the cycle characteristics (capacity deterioration resistance), the capacity retention rate (%) was obtained from (discharge capacity after 50 charge/discharge cycles/initial capacity)×100. Table 1 shows the results.

*: The official names of the phosphonic acid compounds listed in the table are as follows.
Fluorooctylphosphonic acid: 1H,1H,2H,2H-perfluoro-n-octylphosphonic acid Fluorohexylphosphonic acid: 1H,1H,2H,2H-perfluoro-n-hexylphosphonic acid Fluorodecylphosphonic acid: 3,3 , 4,4,5,5,6,6,7,7,8,8,9,9,10,10,10-heptadecafluorodecylphosphonic acid fluorododecylphosphonic acid: 3,3,4,4, 5,5,6,6,7,7,8,8,9,9,10,10,11,11,12,12,12-heneicosafluorododecylphosphonic acid fluorobenzylphosphonic acid: 2,3, 4,5,6-pentafluorobenzylphosphonic acid

From the comparison of Examples 1 to 3 and Comparative Examples 1 and 2, when the spinel-type lithium manganate was coated with 0.1% by mass or more and 1.0% by mass or less of fluorooctylphosphonic acid, the capacity retention rate increased. I know you are. That is, it can be seen that an effect of suppressing capacity deterioration is obtained. Furthermore, a comparison with Comparative Example 3 shows that this effect of suppressing capacity deterioration is greater than in the case of adding fluorooctylphosphonic acid to the electrolytic solution as in the prior art. In Comparative Example 2, partial peeling was observed between the positive electrode active material layer and the positive electrode current collector.

Further, from the results of Examples 4 to 7, even if the type of phosphonic acid compound was changed, the resistance to capacity deterioration was improved. It can be seen that the effect of suppressing deterioration is obtained. Therefore, it can be seen that the positive electrode disclosed herein can provide a non-aqueous electrolyte secondary battery with resistance to capacity deterioration when charging and discharging are repeated while using a spinel-type manganese-containing composite oxide. Further, from the results of Example 8, it can be seen that the effect of suppressing capacity deterioration is further enhanced by adding trilithium phosphate to the positive electrode active material layer.

Although specific examples of the present invention have been described in detail above, these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and changes of the specific examples illustrated above.

20 Wound electrode assembly 30 Battery case 36 Safety valve 42 Positive electrode terminal 42a Positive electrode current collector 44 Negative electrode terminal 44a Negative electrode current collector 50 Positive electrode sheet (positive electrode)
52 positive electrode current collector 52a positive electrode active material layer non-formed portion 54 positive electrode active material layer 60 negative electrode sheet (negative electrode)
62 Negative electrode current collector 62a Negative electrode active material layer non-formation portion 64 Negative electrode active material layer 70 Separator sheet (separator)
80 nonaqueous electrolyte 100 lithium ion secondary battery

Claims (5)

A positive electrode comprising a positive electrode current collector and a positive electrode active material layer supported by the positive electrode current collector,
The positive electrode active material layer contains a positive electrode active material,
The positive electrode active material is a lithium composite oxide having a spinel crystal structure and containing Mn,
The lithium composite oxide has a coating of a phosphonic acid compound having a fluorinated hydrocarbon group on its surface,
The positive electrode, wherein the ratio of the phosphonic acid compound to the lithium composite oxide is 0.1% by mass or more and 1.0% by mass or less. The positive electrode according to claim 1, wherein the positive electrode active material layer further contains trilithium phosphate. 3. The positive electrode according to claim 1, wherein the fluorinated hydrocarbon group of the phosphonic acid compound is a fluorinated alkyl group having 8 to 12 carbon atoms. The positive electrode according to any one of claims 1 to 3, wherein the lithium composite oxide is LiMn ₂ O ₄ . A non-aqueous electrolyte secondary battery comprising the positive electrode according to any one of claims 1 to 4, a negative electrode, and a non-aqueous electrolyte.

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