WO2022209058A1

WO2022209058A1 - Secondary battery

Info

Publication number: WO2022209058A1
Application number: PCT/JP2021/047212
Authority: WO
Inventors: 友章島村; 宗明安藤
Original assignee: 株式会社村田製作所
Priority date: 2021-03-31
Filing date: 2021-12-21
Publication date: 2022-10-06

Abstract

This secondary battery comprises a battery element that includes a positive electrode, a negative electrode, and an electrolyte, a storage member that stores said battery element in an interior thereof, and an electric current blocking mechanism that operates in accordance with an increase in pressure in the interior of said storage member and that blocks the energization of the battery element. The positive electrode includes a lithium iron phosphate compound represented by formula (1). The electrolyte includes a solvent and an electrolyte salt. The solvent has a boiling point of 100°C or greater, and includes a chain carboxylic acid ester having a viscosity of 0.9 mPa・s or less at 25°C. The electrolyte salt includes a lithium sulfonylimide salt represented by formula (2). The operation pressure of the electric current blocking mechanism is 20 kgf/cm² or greater.　Formula (1): LiFe_xM_1-xPO₄ (M is one or two or more transition metal elements (not including Fe); x satisfies the expression 0<x≤1.)　Formula (2): LiN(R1SO₂)(R2SO₂) (Each of R1 and R2 is one among a fluorine group and a perfluoroalkyl group.)

Description

secondary battery

This technology relates to secondary batteries.

Due to the widespread use of various electronic devices such as mobile phones, secondary batteries are being developed as power sources that are compact, lightweight, and have high energy density. The secondary battery includes a positive electrode, a negative electrode, and an electrolytic solution, and various studies have been made on the configuration of the secondary battery.

Specifically, in order to obtain a good cycle life, the electrolyte contains LiN(FSO ₂ ) ₂ together with the first ester compound and the second ester compound (see, for example, Patent Document 1). In order to improve battery swelling, the positive electrode contains LiFePO ₄ and the electrolyte contains LiN(CF ₃ SO ₂ ) ₂ together with a propionate compound (see, for example, Patent Document 2). In order to obtain good cycle characteristics even at high temperatures, the positive electrode contains LiMPO ₄ (where M is Fe, etc.), and the electrolyte contains LiN(FSO ₂ ) ₂ together with a carboxylic acid ester compound (see, for example, Patent Documents 3). In order to prevent interruption of battery use due to operation of the current interrupter, the positive electrode contains _LiFePO4 and the electrolyte contains LiN( _CF3SO2 ) ₂ _together with the propionate compound. (See Patent Document 4, for example).

Japanese translation of PCT publication No. 2014-523096 Japanese Patent Publication No. 2011-508956 International Publication No. 2019/150902 pamphlet JP 2014-225457 A

Various studies have been made on the configuration of secondary batteries, but the high-temperature operation characteristics and high-temperature storage characteristics of the secondary batteries are still insufficient, so there is room for improvement.

Therefore, a secondary battery capable of obtaining excellent high-temperature operating characteristics and excellent high-temperature storage characteristics is desired.

A secondary battery according to an embodiment of the present technology includes a battery element including a positive electrode, a negative electrode, and an electrolytic solution, a storage member that stores the battery element therein, and operates according to an increase in pressure inside the storage member. and a current interrupting mechanism for interrupting current supply to the battery element. The positive electrode contains a lithium iron phosphate compound represented by the following formula (1). The electrolyte solution contains a solvent and an electrolyte salt, the solvent having a boiling point of 100° C. or higher and a chain carboxylic acid ester having a viscosity of 0.9 mPa s or less at 25° C., and the electrolyte salt having the following contains a lithium sulfonylimide salt represented by the formula (2) of The working pressure of the current interrupting mechanism is 20 kgf/cm ² or more.

LiFe _x M _1-x PO ₄ (1)
(M is one or more transition metal elements (excluding Fe). x satisfies 0<x≦1.)

LiN(R1SO2)( _R2SO2 ) ( ₂ )
(Each of R1 and R2 is either a fluorine group or a perfluoroalkyl group.)

According to the secondary battery of one embodiment of the present technology, the secondary battery includes battery elements (a positive electrode, a negative electrode, and an electrolytic solution), a storage member, and a current interrupting mechanism, and the positive electrode contains a lithium iron phosphate compound. the solvent of the electrolyte contains a chain carboxylic acid ester having a boiling point of 100° C. or higher and a viscosity of 0.9 mPas or less at 25° C., and the electrolyte salt of the electrolyte is a lithium sulfonylimide salt and the operating pressure of the current interrupting mechanism is 20 kgf/cm ² or more, so excellent high temperature operation characteristics and excellent high temperature storage characteristics can be obtained.

It should be noted that the effects of the present technology are not necessarily limited to the effects described here, and may be any of a series of effects related to the present technology described below.

It is a sectional view showing the composition of the rechargeable battery in one embodiment of this art. FIG. 2 is a cross-sectional view showing the configuration of the battery element shown in FIG. 1; FIG. 3 is a block diagram showing the configuration of an application example of a secondary battery;

Hereinafter, one embodiment of the present technology will be described in detail with reference to the drawings. The order of explanation is as follows.

1. Secondary Battery 1-1. Configuration 1-2. Operation 1-3. Manufacturing method 1-4. Action and effect 2 . Modification 3. Applications of secondary batteries

<1. Secondary battery>
First, a secondary battery according to an embodiment of the present technology will be described.

The secondary battery described here is a secondary battery in which battery capacity is obtained by utilizing the absorption and release of electrode reactants, and is equipped with a positive electrode, a negative electrode, and an electrolytic solution, which is a liquid electrolyte.

The type of electrode reactant is not particularly limited, but specifically light metals such as alkali metals and alkaline earth metals. Alkali metals include lithium, sodium and potassium, and alkaline earth metals include beryllium, magnesium and calcium.

In the following, the case where the electrode reactant is lithium will be taken as an example. A secondary battery whose battery capacity is obtained by utilizing the absorption and release of lithium is a so-called lithium ion secondary battery. In this lithium ion secondary battery, lithium is intercalated and deintercalated in an ionic state.

Here, the charge capacity of the negative electrode is larger than the discharge capacity of the positive electrode. That is, the electrochemical capacity per unit area of the negative electrode is set to be larger than the electrochemical capacity per unit area of the positive electrode. This is to prevent electrode reactants from depositing on the surface of the negative electrode during charging.

<1-1. Configuration>
1 shows the cross-sectional structure of a secondary battery, and FIG. 2 shows the cross-sectional structure of the battery element 20 shown in FIG. However, in FIG. 2, only part of the battery element 20 is shown.

1 and 2, the secondary battery mainly includes a battery can 10, a battery element 20, a pair of

insulating plates

31 and 32, a positive electrode lead 41 and a negative electrode lead 42, and a current interrupting mechanism. 50. The secondary battery described here is a cylindrical secondary battery in which a battery element 20 is housed inside a cylindrical battery can 10 .

[Battery can]
The battery can 10 , as shown in FIG. 1 , is a housing member that houses the battery element 20 and the like, and includes a battery can main body 11 and a battery lid 12 .

The battery can body 11 is a container-like member that accommodates the battery element 20 and the like inside. Since the battery can body 11 has a hollow structure with one end open and the other end closed, it has an opening 11K at one end. In addition, the battery can body 11 contains one or more of metal materials such as iron, aluminum, iron alloys, and aluminum alloys. The metal material may be plated.

The battery lid 12 is a plate-like member that shields the opening 11K of the battery can main body 11, as shown in FIG. Since the battery lid 12 is crimped to the vicinity of the opening 11K of the battery can body 11 via the gasket 60, the battery can body 11 is sealed by the battery lid 12. As shown in FIG. The material for forming the battery lid 12 is the same as the material for forming the battery can main body 11 . Gasket 60 contains an insulating material, and the insulating material contains one or more of polymer compounds such as polybutylene terephthalate. However, the surface of the gasket 60 may be coated with asphalt.

[Battery element]
The battery element 20, as shown in FIGS. 1 and 2, is a power generation element including a positive electrode 21, a negative electrode 22, a separator 23, and an electrolytic solution (not shown).

This battery element 20 is a so-called wound electrode assembly. That is, in the battery element 20, the positive electrode 21 and the negative electrode 22 are laminated with the separator 23 interposed therebetween, and the positive electrode 21, the negative electrode 22 and the separator 23 are wound. Thus, the positive electrode 21 and the negative electrode 22 are wound while facing each other with the separator 23 interposed therebetween. A center pin 80 is inserted in a winding space 20S provided at the center of winding of the battery element 20 . However, the center pin 80 may be omitted.

(positive electrode)
The positive electrode 21 includes a positive electrode current collector 21A and a positive electrode active material layer 21B, as shown in FIG.

The positive electrode current collector 21A has a pair of surfaces on which the positive electrode active material layer 21B is provided. This positive electrode current collector 21A contains a conductive material such as a metal material, and the metal material is aluminum or the like.

The positive electrode active material layer 21B contains one or more of positive electrode active materials capable of intercalating and deintercalating lithium. However, the positive electrode active material layer 21B may further contain one or more of other materials such as a positive electrode binder and a positive electrode conductor.

Here, the positive electrode active material layer 21B is provided on both sides of the positive electrode current collector 21A. However, the positive electrode active material layer 21B may be provided only on one side of the positive electrode current collector 21A on the side where the positive electrode 21 faces the negative electrode 22 . A method for forming the positive electrode active material layer 21B is not particularly limited, but specifically, one or more of coating methods and the like are used.

The positive electrode active material contains one or more of the lithium iron phosphate compounds represented by the following formula (1). This lithium iron phosphate compound is a phosphate compound containing lithium (Li) and iron (Fe) as constituent elements, as shown in formula (1).

The positive electrode active material contains a lithium iron phosphate compound because the positive electrode active material (lithium iron phosphate compound), a solvent (chain carboxylic acid ester) described later, and an electrolyte salt (lithium sulfonylimide salt described later) ) and the condition regarding the operating pressure of the current interrupting mechanism 50 (the operating pressure is 20 kgf/cm ² or more), which will be described later, make it easier for the current to converge when the secondary battery is overcharged. be. This improves the high-temperature operating characteristics of the current interrupting mechanism 50, thereby improving the safety of the secondary battery during overcharging.

The type of the transition metal element (M) is not particularly limited as long as it is one or more of the transition metal elements excluding iron. Specifically, the transition metal element is manganese (Mn ) and cobalt (Co). This is because the average voltage of the secondary battery becomes high while there is a steep voltage rise in the overcharge region of the secondary battery.

As is clear from the possible range of the value of x (0<x≦1), the lithium iron phosphate compound may contain the transition metal element (M) as a constituent element, or the transition metal element (M) may not be included as a constituent element.

A lithium iron phosphate compound that does not contain a transition metal element (M) as a constituent element is _LiFePO4 . Specific examples of the lithium iron phosphate compound containing a transition metal element ₍ _M ) as a _constituent _element _include _{LiFe0.5Mn0.5PO4} , _{LiFe0.7Mn0.3PO4} _, _{LiFe0.3Mn0.7PO4} _, and _{LiFe0.5Co0.5PO4} _. _, _{LiFe0.7Co0.3PO4} _and _{LiFe0.3Co0.7PO4} _. _{_}

Note that the positive electrode active material may contain one or more of other materials together with the lithium iron phosphate compound. The type of other material is not particularly limited, but is specifically a lithium-containing compound. However, lithium iron phosphate compounds are excluded from the lithium-containing compounds described herein.

This lithium-containing compound is a compound containing lithium and one or more transition metal elements as constituent elements, and may further contain one or more other elements as constituent elements. The type of the other element is not particularly limited as long as it is an element other than lithium and transition metal elements. Specifically, the other element is an element belonging to Groups 2 to 15 in the long period periodic table. be. The type of lithium-containing compound is not particularly limited, but specific examples include oxides, phosphoric acid compounds, silicic acid compounds and boric acid compounds.

_Specific examples of _oxides _include _LiNiO2 , _LiCoO2 _, _{LiCo0.98Al0.01Mg0.01O2} _, _{LiNi0.5Co0.2Mn0.3O2} _and _LiMn2O4 _. A specific example of the phosphoric acid compound is LiMnPO ₄ and the like.

The positive electrode binder contains one or more of synthetic rubber and polymer compounds. Synthetic rubbers include styrene-butadiene-based rubber, fluorine-based rubber, and ethylene propylene diene. Polymer compounds include polyvinylidene fluoride, polyimide and carboxymethyl cellulose.

The positive electrode conductive agent contains one or more of conductive materials such as carbon materials, and the carbon materials include graphite, carbon black, acetylene black, and ketjen black. However, the conductive material may be a metal material, a polymer compound, or the like.

(negative electrode)
The negative electrode 22 includes a negative electrode current collector 22A and a negative electrode active material layer 22B, as shown in FIG.

The negative electrode current collector 22A has a pair of surfaces on which the negative electrode active material layer 22B is provided. This negative electrode current collector 22A contains a conductive material such as a metal material, and the metal material is copper or the like.

The negative electrode active material layer 22B contains one or more of negative electrode active materials capable of intercalating and deintercalating lithium. However, the negative electrode active material layer 22B may further contain one or more of other materials such as a negative electrode binder and a negative electrode conductor.

Here, the negative electrode active material layer 22B is provided on both sides of the negative electrode current collector 22A. However, the negative electrode active material layer 22B may be provided only on one side of the negative electrode current collector 22A on the side where the negative electrode 22 faces the positive electrode 21 . The method of forming the negative electrode active material layer 22B is not particularly limited, but specifically, any one of a coating method, a vapor phase method, a liquid phase method, a thermal spraying method, a firing method (sintering method), or the like, or Two or more types.

The type of negative electrode active material is not particularly limited, but specifically, the negative electrode active material is one or both of a carbon material and a metal-based material. This is because a high energy density can be obtained. Carbon materials include graphitizable carbon, non-graphitizable carbon and graphite (natural graphite and artificial graphite). A metallic material is a material containing as constituent elements one or more of metallic elements and semi-metallic elements capable of forming an alloy with lithium. , one or both of silicon and tin, and the like. This metallic material may be a single substance, an alloy, a compound, a mixture of two or more of them, or a material containing two or more of these phases. Specific examples of metallic materials include TiSi ₂ and SiO _x (0<x≦2, or 0.2<x<1.4).

The details of each of the negative electrode binder and the negative electrode conductive agent are the same as those of the positive electrode binder and the positive electrode conductive agent.

(separator)
The separator 23 is an insulating porous film interposed between the positive electrode 21 and the negative electrode 22, as shown in FIG. Allows lithium ions to pass through. This separator 23 contains a polymer compound such as polyethylene.

(Electrolyte)
The electrolyte is impregnated in each of the positive electrode 21, the negative electrode 22 and the separator 23 and contains a solvent and an electrolyte salt.

(solvent)
The solvent contains one or more of non-aqueous solvents (organic solvents), and the electrolytic solution containing the non-aqueous solvent is a so-called non-aqueous electrolytic solution.

Specifically, the solvent is a chain carboxylic acid ester having a boiling point of 100° C. or higher and a viscosity of 0.9 mPa·s or less at 25° C. (hereinafter simply referred to as “chain carboxylic acid ester”). contains any one or two or more of Below, the viscosity at 25°C is simply referred to as "viscosity".

The type of the chain carboxylic acid ester is not particularly limited as long as it satisfies the conditions regarding the boiling point and viscosity described above, so the chain carboxylic acid ester may be a chain acetate or a chain propionate. However, it may be a chain butyric acid ester, or may be other than these.

Below, chain carboxylic acid esters that do not satisfy the above boiling point and viscosity conditions are referred to as "other chain carboxylic acid esters." Other linear carboxylic acid esters include chain carboxylic acid esters having a boiling point of 100° C. or higher but not having a viscosity of 0.9 mPa·s or less at 25° C., and chain carboxylic acid esters having a boiling point of 100° C. or higher linear carboxylic acid esters that do not have a viscosity of 0.9 mPa·s or less at 25°C, and chain carboxylic acid esters that do not have a boiling point of 100°C or more and have a viscosity of 0.9 mPa·s or less at 25°C and a chain carboxylic acid ester that does not have

Specific examples of chain carboxylic acid esters include propyl acetate (boiling point = 101°C, viscosity = 0.64 mPa s, carbon number = 5), butyl acetate (boiling point = 126°C, viscosity = 0.73 mPa s, carbon number = 6), propyl propionate (boiling point = 122°C, viscosity = 0.72 mPa s, carbon number = 6), butyl propionate (boiling point = 144°C, viscosity = 0.76 mPa s, carbon number = 7), Methyl butyrate (boiling point = 102°C, viscosity = 0.52 mPa s, carbon number = 5), ethyl butyrate (boiling point = 119°C, viscosity = 0.61 mPa s, carbon number = 6) and propyl butyrate (boiling point = 143 °C, viscosity = 0.83 mPa·s, carbon number = 7), and so on.

For reference, specific examples of other chain carboxylic acid esters are methyl acetate (boiling point = 58°C, viscosity = 0.35 mPa·s, carbon number = 3), ethyl acetate (boiling point = 77°C, viscosity = 0.41 mPa·s, number of carbon atoms = 4), methyl propionate (boiling point = 79°C, viscosity = 0.51 mPa·s, number of carbon atoms = 4) and ethyl propionate (boiling point = 99°C, viscosity = 0.5°C). 50 mPa·s, carbon number=6), and the like.

The boiling point and viscosity values given here are based on the boiling point and viscosity values published in Gill, A. H., and F. P. Dexter et al., Ind. Eng. Chem., 26, 881 (1934). shall be compliant.

The reason why the solvent of the electrolytic solution contains the chain carboxylic acid ester is that the pressure inside the battery can 10 does not rise rapidly even when the secondary battery is used and stored, so that the current interrupting mechanism 50 does not work excessively. This is because it is inhibited from operating. In this secondary battery, as will be described later, the operating pressure of the current interrupting mechanism 50 is set to a relatively high pressure (operating pressure is 20 kgf/cm ² or more), so the pressure inside the battery can 10 rises, the pressure remains relatively high. In addition, the lithium sulfonylimide salt and the chain carboxylic acid ester react with each other under the condition that the internal pressure of the battery can 10 is maintained at a relatively high level, so that the charge/discharge reaction in the battery element 20 is stable and smooth. Therefore, even if the secondary battery is stored in a high-temperature environment, the discharge capacity is less likely to decrease.

Specifically, when the solvent contains other chain carboxylic acid esters, the boiling point of the other chain carboxylic acid esters is too low. Acid ester becomes easy to volatilize. In this case, the vapor pressure inside the battery can 10 tends to rise sharply, and the pressure inside the battery can 10 tends to rise sharply. easier to operate. As a result, when the secondary battery is in use, the current interrupting mechanism 50 excessively interrupts the energization of the battery element 20, making it difficult to stably and continuously use the secondary battery. The tendency for the current interrupting mechanism 50 to operate excessively due to the tendency for the pressure inside the battery can 10 to rise rapidly in this way is particularly significant when the secondary battery is used and stored in a high-temperature environment. becomes conspicuous when

Also, when the solvent contains other chain carboxylic acid esters, the viscosity of the other chain carboxylic acid esters is too high, so the viscosity of the electrolytic solution becomes excessively high. In this case, the electrolytic solution is less likely to be injected into the battery can 10 in the manufacturing process of the secondary battery, that is, the positive electrode 21, the negative electrode 22, and the separator 23 are each less likely to be impregnated with the electrolytic solution. The amount of electrolyte retained by 20 is reduced. This makes it difficult for the charge/discharge reaction to proceed stably and smoothly in the battery element 20, so that the discharge capacity tends to decrease when the secondary battery is stored. The tendency for the discharge capacity to decrease due to the decrease in the amount of electrolyte retained by the battery element 20 becomes particularly pronounced when the secondary battery is stored in a high-temperature environment.

On the other hand, when the solvent contains a chain carboxylic acid ester, the boiling point of the chain carboxylic acid ester is appropriately high. less likely to volatilize. In this case, it is difficult for the vapor pressure inside the battery can 10 to rise rapidly, and the pressure inside the battery can 10 does not rise sharply. Become. As a result, the current interrupting mechanism 50 does not excessively interrupt the energization of the battery element 20 when the secondary battery is used, so that the secondary battery can be stably and continuously used. The tendency for the current interrupting mechanism 50 to be less likely to operate excessively even when the internal pressure of the battery can 10 rises in this manner can be obtained even when the secondary battery is used and stored in a high-temperature environment. be done.

Also, when the solvent contains a chain carboxylic acid ester, the viscosity of the chain carboxylic acid ester is appropriately low, so the viscosity of the electrolytic solution is appropriately low. In this case, the electrolytic solution is easily injected into the battery can 10 in the manufacturing process of the secondary battery. The amount of electrolyte retained by 20 increases. As a result, the charging and discharging reactions in the battery element 20 tend to proceed stably and smoothly, so that the discharge capacity is less likely to decrease even when the secondary battery is stored. Such a tendency that the discharge capacity is less likely to decrease is similarly obtained particularly when the secondary battery is stored in a high-temperature environment.

The number of carbon atoms in the chain carboxylic acid ester is preferably 6 or less. This is because the viscosity of the chain carboxylic acid ester becomes sufficiently low, so that the positive electrode 21, the negative electrode 22, and the separator 23 are more easily impregnated with the electrolytic solution. As a result, the amount of electrolyte retained by the battery element 20 is further increased, so that the discharge capacity is less likely to decrease.

Among them, the chain carboxylic acid ester is preferably one or both of propyl acetate and propyl propionate. Since the boiling point is sufficiently high and the viscosity is sufficiently low, excessive operation of the current interrupting mechanism 50 is sufficiently suppressed, and the discharge capacity is less likely to decrease sufficiently even when the secondary battery is stored. Because it becomes

This solvent may contain only a chain carboxylic acid ester, but may also contain one or both of a cyclic carbonate and a chain carbonic acid ester together with the chain carboxylic acid ester. This is because, while the excessive operation of the current interrupting mechanism 50 is suppressed, it becomes difficult for the discharge capacity to decrease sufficiently even when the secondary battery is stored. The number of cyclic carbonates may be one, or two or more. Similarly, the number of chain carbonate esters may be one, or two or more. Specific examples of the cyclic carbonate include ethylene carbonate and propylene carbonate, and specific examples of the chain carbonate include dimethyl carbonate, diethyl carbonate and ethylmethyl carbonate.

Although the content of the chain carboxylic acid ester in the solvent is not particularly limited, it is preferably 10% by weight to 75% by weight. This is because the discharge capacity is less likely to decrease sufficiently even when the secondary battery is stored.

The solvent may further contain one or more of lactones and the like. Only one type of lactone may be used, or two or more types may be used. Specific examples of lactones include γ-butyrolactone and γ-valerolactone.

In addition, the solvent may contain one or more of the additives. This is because the chemical stability of the electrolytic solution is improved, so that the discharge capacity is less likely to decrease even when the secondary battery is stored.

The additives include unsaturated cyclic carbonates, fluorinated cyclic carbonates, sulfonic acid esters, acid anhydrides, nitrile compounds, isocyanate compounds and phosphate esters. The content of the additive in the electrolytic solution is not particularly limited and can be set arbitrarily.

Specific examples of unsaturated cyclic carbonates include vinylene carbonate, vinylethylene carbonate and methyleneethylene carbonate. Specific examples of fluorinated cyclic carbonates include ethylene fluorocarbonate and ethylene difluorocarbonate. Specific examples of sulfonic acid esters include 1,3-propanesultone and 1-propene-1,3-sultone. Specific examples of acid anhydrides include succinic anhydride, glutaric anhydride, 1,2-ethanedisulfonic anhydride, 1,3-propanedisulfonic anhydride and 2-sulfobenzoic anhydride. Specific examples of nitrile compounds include succinonitrile, glutaronitrile and adiponitrile. Specific examples of isocyanate compounds include hexamethylene diisocyanate. Specific examples of phosphate esters include trimethyl phosphate and triethyl phosphate.

Among them, the additive is preferably a fluorinated cyclic carbonate. This is because a coating film derived from the fluorinated cyclic carbonate is formed on the surface of the negative electrode 22 during charging and discharging, so that the electrolytic solution is less likely to be decomposed on the surface of the negative electrode 22 . This makes it more difficult for the discharge capacity to decrease even when the secondary battery is stored.

The content of the fluorinated cyclic carbonate in the electrolytic solution is not particularly limited, but is preferably 0.05% by weight to 3.5% by weight. This is because the discharge capacity is less likely to decrease sufficiently even when the secondary battery is stored. In particular, the content of the fluorinated cyclic carbonate in the electrolytic solution is more preferably 0.1% by weight to 3.0% by weight. This is because the discharge capacity is less likely to decrease while the excessive operation of the current interrupting mechanism 50 is further suppressed.

(electrolyte salt)
The electrolyte salt contains a lithium salt, more specifically, one or more of lithium sulfonylimide salts represented by the following formula (2). This lithium sulfonylimide salt is a lithium imide salt having two fluorinated sulfonyl groups ((R1SO ₂ )(R2SO ₂ )) as shown in formula (2).

Each of R1 and R2 is not particularly limited as long as it is either a fluorine group or a perfluoroalkyl group, as described above. The type of R1 and the type of R2 may be the same or different. Since the number of carbon atoms in the perfluoroalkyl group is not particularly limited, the perfluoroalkyl group includes a trifluoromethyl group (--CF ₃ ) and a pentafluoroethyl group (--C ₂ F ₅ ).

Specific examples of lithium sulfonylimide salts are lithium bis(fluorosulfonyl)imide (LiN(FSO2) ₂ ), lithium bis(trifluoromethanesulfonyl)imide (LiN( _CF3SO2 ) ₂ ) and _bis ₍ pentafluoroethanesulfonyl ) _imidelithium (Li _(C2F5SO2)2 ₎ _.

The reason why the electrolyte salt contains a lithium sulfonylimide salt is that the safety of the secondary battery during overcharging is improved for the reason described above for the case where the positive electrode active material contains a lithium iron phosphate compound. is.

Among them, the lithium sulfonylimide salt is preferably one or both of bis(fluorosulfonyl)imidelithium and bis(trifluoromethanesulfonyl)imidelithium. This is because the secondary battery is less likely to generate heat sufficiently, and the safety of the secondary battery at the time of overcharging is sufficiently improved.

Although the content of the lithium sulfonylimide salt in the solvent is not particularly limited, it is preferably 0.8 mol/kg or more, more preferably 0.8 mol/kg to 2.0 mol/kg. This is because the safety of the secondary battery during overcharge is sufficiently improved while high ion conductivity is obtained.

The electrolyte salt may contain only the lithium sulfonylimide salt, or may contain one or more of other lithium salts together with the lithium sulfonylimide salt. This is because the battery capacity and the like are improved.

Specific examples of other lithium salts include lithium hexafluorophosphate ( _LiPF6 ), lithium tetrafluoroborate ( _LiBF4 ), lithium tris(trifluoromethanesulfonyl)methide (LiC( _CF3SO2 ) ₃ ) _, Lithium bis(oxalato)borate (LiB ₍ _C2O4 ) ₂ ), lithium monofluorophosphate ₍ _Li2PFO3 ) and lithium difluorophosphate ₍ _LiPF2O2 ).

[Pair of insulating plates]
Since the insulating

plates

31 and 32 are arranged to face each other with the battery element 20 interposed therebetween, the battery element 20 is sandwiched between the insulating

plates

31 and 32 . As a result, the battery element 20 is accommodated inside the battery can 10 while being sandwiched between the insulating

plates

31 , 31 . Note that each of the insulating

plates

31 and 32 may be provided with an opening for partially exposing the center pin 80 .

[Positive lead and negative lead]
The cathode lead 41 is connected to the cathode current collector 21A of the cathode 21, as shown in FIGS. 1 and 2, and contains one or more of conductive materials such as aluminum. there is This positive electrode lead 41 is electrically connected to the battery lid 12 via a current interrupting mechanism 50 .

The negative electrode lead 42 is connected to the negative electrode current collector 22A of the negative electrode 22, as shown in FIGS. 1 and 2, and contains one or more of conductive materials such as nickel. there is This negative electrode lead 42 is electrically connected to the battery can body 11 .

[Current interrupting mechanism]
The current interrupting mechanism 50 is crimped through a gasket 60 together with the battery lid 12 and the thermal resistance element (PTC element) 70 to the vicinity of the opening 11K of the battery can body 11 . Thereby, the current interrupting mechanism 50 is fixed to the battery can 10 together with the PTC element 70 .

Each of the current interrupting mechanism 50 and the PTC element 70 is arranged inside the battery lid 12 , and the current interrupting mechanism 50 includes a disc plate 51 . Thus, the current interrupting mechanism 50 is electrically connected to the positive electrode lead 41 via the disk plate 51 and electrically connected to the battery lid 12 via the PTC element 70 . The electrical resistance of the PTC element 70 increases as the temperature rises.

The disk plate 51 has a thickness T and is bent so as to partially recess toward the battery element 20 . In this case, a portion of the disc plate 51 is folded back toward the side away from the center of the disc plate 51 (outside) and then folded back toward the side closer to the center of the disc plate 51 (inside). ing. As a result, the disc plate 51 has a portion (folded portion 51P) where the disc plate 51 is folded on itself in the middle.

The current cut-off mechanism 50 cuts off the energization of the battery element 20 by operating in response to an increase in pressure inside the battery can 10 . More specifically, when the pressure inside the battery can 10 rises and the pressure reaches a predetermined pressure, the disk plate 51 is reversed, so that the disk plate 51 is electrically separated from the positive electrode lead 41 . be done. In this case, the electrical connection between the battery cover 12 and the battery element 20 is cut off, so that the current flow through the battery element 20 is cut off. This prevents the secondary battery (battery element 20) from generating abnormal heat due to a large current.

In this case, the current interrupting mechanism 50 is partially cleaved in response to an increase in pressure inside the battery can 10, so that a pressure release gap (pressure release path) is formed in the current interrupting mechanism 50. be. As a result, the internal pressure of the battery can 10 is released, thereby preventing the secondary battery from bursting. In other words, the current cut-off mechanism 50 functions not only to cut off the energization of the battery element 20 when the pressure inside the battery can 10 rises, but also as a safety valve to release the pressure inside the battery can 10 .

Here, the current interrupting mechanism 50 operates when the pressure inside the battery can 10 reaches a predetermined pressure, more specifically, when the pressure reaches 20 kgf/cm ² or more. That is, the pressure (operating pressure) at which the current interrupting mechanism 50 operates is 20 kgf/cm ² or more. This is because the pressure inside the battery can 10 is maintained in a relatively high state, so that the current interrupting mechanism 50 is prevented from operating excessively. As a result, the secondary battery can be used stably and continuously.

In order to investigate the operating pressure of the current interrupting mechanism 50, the operating state of the current interrupting mechanism 50 is examined while increasing the internal pressure (MPa) of the battery can 10, thereby obtaining the pressure when the current interrupting mechanism 50 is operated. is specified, the pressure is converted to working pressure (kgf/cm ² ). In this case, hydraulic pressure is used to increase the pressure inside the battery can 10 . Further, by measuring the voltage of the secondary battery while increasing the pressure inside the battery can 10, it is determined whether or not the current interrupting mechanism 50 is activated based on the change in the voltage. When converting the pressure into working pressure, 1 kgf/cm ² =about 0.098 MPa.

Since the method of adjusting the operating pressure of the current interrupting mechanism 50 is not particularly limited, the operating pressure can be adjusted to a desired value using one or more of known methods. be. For example, the working pressure may be adjusted by changing the material of the disk plate 51 and the connection strength of the disk plate 51 to the positive electrode lead 41 . Further, the working pressure may be adjusted by changing the thickness T of the disc plate 51, more specifically, the thickness T of the disc plate 51 at the folded portion 51P. Of course, other methods not illustrated here may be used to adjust the actuation pressure.

<1-2. Operation>
During charging of the secondary battery, in the battery element 20, lithium is released from the positive electrode 21 and absorbed into the negative electrode 22 via the electrolyte. On the other hand, when the secondary battery is discharged, in the battery element 20, lithium is released from the negative electrode 22 and absorbed into the positive electrode 21 through the electrolyte. Lithium is intercalated and deintercalated in an ionic state during charging and discharging.

<1-3. Manufacturing method>
In the case of manufacturing a secondary battery, the positive electrode 21 and the negative electrode 22 are prepared according to the procedure described below, and an electrolytic solution is prepared. Make a battery.

[Preparation of positive electrode]
First, a mixture (positive electrode mixture) in which a positive electrode active material containing a lithium iron phosphate compound, a positive electrode binder, and a positive electrode conductive agent are mixed together is put into a solvent to obtain a pasty positive electrode mixture. Prepare slurry. This solvent may be an aqueous solvent or an organic solvent. Subsequently, the cathode active material layer 21B is formed by applying the cathode mixture slurry to both surfaces of the cathode current collector 21A. Finally, the cathode active material layer 21B is compression-molded using a roll press or the like. In this case, the positive electrode active material layer 21B may be heated, or compression molding may be repeated multiple times. As a result, the cathode active material layers 21B are formed on both surfaces of the cathode current collector 21A, so that the cathode 21 is produced.

[Preparation of negative electrode]
A negative electrode 22 is formed by the same procedure as that of the positive electrode 21 described above. Specifically, first, a paste-like negative electrode mixture slurry is prepared by putting a mixture (negative electrode mixture) in which a negative electrode active material, a negative electrode binder, and a negative electrode conductor are mixed together into a solvent. Subsequently, the anode active material layer 22B is formed by applying the anode mixture slurry to both surfaces of the anode current collector 22A. Finally, the negative electrode active material layer 22B is compression molded. As a result, the negative electrode 22 is manufactured because the negative electrode active material layers 22B are formed on both surfaces of the negative electrode current collector 22A.

[Preparation of electrolytic solution]
An electrolyte salt containing a lithium sulfonylimide salt is added to a solvent containing a chain carboxylic acid ester. This disperses or dissolves the electrolyte salt in the solvent, thus preparing an electrolytic solution.

[Assembly of secondary battery]
First, the positive electrode lead 41 is connected to the positive electrode current collector 21A of the positive electrode 21 by welding or the like, and the negative electrode lead 42 is connected to the negative electrode current collector 22A of the negative electrode 22 by welding or the like.

Subsequently, after the positive electrode 21 and the negative electrode 22 are laminated with the separator 23 interposed therebetween, the positive electrode 21, the negative electrode 22 and the separator 23 are wound to form a wound body (not shown) having a winding space 20S. to form This wound body has the same structure as the battery element 20 except that the positive electrode 21, the negative electrode 22 and the separator 23 are not impregnated with the electrolytic solution. Subsequently, the center pin 80 is inserted into the winding space 20S of the wound body.

Then, with the wound body sandwiched between the insulating

plates

31 and 32, the wound body is housed inside the battery can main body 11 through the opening 11K together with the insulating

plates

31 and 32. In this case, the positive electrode lead 41 is connected to the current interrupting mechanism 50 (disk plate 51) using a welding method or the like, and the negative electrode lead 42 is connected to the battery can body 11 using a welding method or the like. Subsequently, by injecting an electrolytic solution into the inside of the battery can main body 11, the wound body is impregnated with the electrolytic solution. As a result, each of the positive electrode 21, the negative electrode 22 and the separator 23 is impregnated with the electrolytic solution, so that the battery element 20 is produced.

Finally, after housing the battery lid 12, the current interrupting mechanism 50, and the PTC element 70 inside the battery can body 11 through the opening 11K, the portion of the battery can body 11 near the opening 11K is inserted through the gasket 60. and crimp. Thereby, the battery lid 12, the current interrupting mechanism 50 and the PTC element 70 are fixed to the battery can body 11, and the battery can 10 including the battery can body 11 and the battery lid 12 is formed. Accordingly, since the battery element 20 is sealed inside the battery can 10, the secondary battery is assembled.

When assembling this secondary battery, as described above, by adjusting the material of the disk plate 51 and the connection strength of the disk plate 51 to the positive electrode lead 41, the current interrupting mechanism is adjusted so that the current is 20 kgf/cm ² or more. 50 working pressure is set.

[Stabilization of secondary battery]
The secondary battery after assembly is charged and discharged. Various conditions such as environmental temperature, number of charge/discharge times (number of cycles), and charge/discharge conditions can be arbitrarily set. As a result, films are formed on the respective surfaces of the positive electrode 21 and the negative electrode 22, so that the state of the secondary battery is electrochemically stabilized. Thus, a secondary battery is completed.

<1-4. Action and effect>
According to this secondary battery, the secondary battery includes a battery can 10, a battery element 20 (a positive electrode 21, a negative electrode 22, and an electrolytic solution) and a current interrupting mechanism 50, and the positive electrode 21 contains a lithium iron phosphate compound. a solvent of the electrolytic solution contains a chain carboxylic ^acid ester; an electrolytic salt of the electrolytic solution contains a lithium sulfonylimide salt; be.

In this case, first, as described above, the positive electrode active material (lithium iron phosphate compound), the solvent (chain carboxylic acid ester), the electrolyte salt (lithium sulfonylimide salt), and the current interrupting mechanism 50 (the operating pressure is 20 kgf/cm 2 or more) and the condition related to the operating pressure (operating pressure is 20 kgf/cm ² or more), the current tends to converge when the secondary battery is overcharged.

Secondly, since the solvent of the electrolytic solution contains the chain carboxylic acid ester, the pressure inside the battery can 10 is less likely to rise sharply even when the secondary battery is used and stored, as described above. , the excessive operation of the current interrupting mechanism 50 is suppressed. In addition, the chain carboxylic acid ester reacts with the lithium sulfonylimide salt under the condition that the internal pressure of the battery can 10 is kept relatively high, so that the charging/discharging reaction in the battery element 20 proceeds stably and smoothly. Therefore, even if the secondary battery is stored in a high-temperature environment, the discharge capacity is less likely to decrease.

Third, the operating pressure of the current interrupting mechanism 50 is 20 kgf/cm ² or more, that is, the operating pressure is set to be relatively high. increases, the pressure remains relatively high. As a result, the current interrupting mechanism 50 is prevented from operating excessively, that is, the current interrupting mechanism 50 does not excessively interrupt the energization of the battery element 20, so that the secondary battery can be stably and continuously used.

For these reasons, while ensuring the safety of the secondary battery during overcharging, excessive operation of the current interrupting mechanism 50 is suppressed, and the discharge capacity decreases even when the secondary battery is stored. Therefore, excellent high temperature operation characteristics and excellent high temperature storage characteristics can be obtained.

In particular, the linear carboxylic acid ester comprises one or both of propyl acetate and propyl propionate, and the lithium sulfonylimide salt comprises lithium bis(fluorosulfonyl)imide and lithium bis(trifluoromethanesulfonyl)imide. If one or both of them are included, the excessive operation of the current interrupting mechanism 50 is sufficiently suppressed, the discharge capacity is less likely to decrease sufficiently even when the secondary battery is stored, and furthermore, the secondary battery during overcharging Since the safety of the secondary battery is sufficiently improved, a higher effect can be obtained.

Further, if the transition metal element (M) is one or both of Mn and Co in the formula (1) regarding the lithium iron phosphate compound, an average Since the voltage is higher, a higher effect can be obtained.

Further, when the content of the lithium sulfonylimide salt is 0.8 mol/kg to 2.0 mol/kg with respect to the solvent, high ionic conductivity is obtained, and the safety of the secondary battery during overcharging is sufficient. , a higher effect can be obtained.

In addition, if the content of the chain carboxylic acid ester in the solvent is 10% by weight to 75% by weight, the discharge capacity will not sufficiently decrease even if the secondary battery is stored, so that a higher effect can be obtained. can be done.

Further, when the electrolyte contains a fluorinated cyclic carbonate and the content of the fluorinated cyclic carbonate in the electrolyte is 0.1% by weight to 3.0% by weight, the secondary battery can be stored. However, the discharge capacity is not sufficiently reduced, and the excessive operation of the current interrupting mechanism 50 is further suppressed, so that a higher effect can be obtained.
It is from.

Also, if the secondary battery is a lithium-ion secondary battery, a sufficient battery capacity can be stably obtained by utilizing the absorption and release of lithium, so a higher effect can be obtained.

<2. Variation>
The configuration of the secondary battery described above can be changed as appropriate, as described below. However, the series of variants described below may be combined with each other.

[Modification 1]
A separator 23, which is a porous membrane, was used. However, although not specifically illustrated here, a laminated separator including a polymer compound layer may be used.

Specifically, a laminated separator includes a porous membrane having a pair of surfaces and a polymer compound layer provided on one or both sides of the porous membrane. This is because the adhesiveness of the separator to each of the positive electrode 21 and the negative electrode 22 is improved, so that positional deviation (winding deviation) of the battery element 20 is suppressed. As a result, the secondary battery is less likely to swell even if a decomposition reaction or the like occurs in the electrolytic solution. The polymer compound layer contains a polymer compound such as polyvinylidene fluoride. This is because polyvinylidene fluoride or the like has excellent physical strength and is electrochemically stable.

One or both of the porous film and the polymer compound layer may contain one or more of a plurality of insulating particles. This is because the plurality of insulating particles dissipate heat when the secondary battery generates heat, thereby improving the safety (heat resistance) of the secondary battery. The insulating particles contain one or both of an inorganic material and a resin material. Specific examples of inorganic materials are aluminum oxide, aluminum nitride, boehmite, silicon oxide, titanium oxide, magnesium oxide and zirconium oxide. Specific examples of resin materials include acrylic resins and styrene resins.

When manufacturing a laminated separator, after preparing a precursor solution containing a polymer compound, a solvent, etc., the precursor solution is applied to one or both sides of the porous membrane. In this case, if necessary, a plurality of insulating particles may be added to the precursor solution.

Even when this laminated separator is used, the same effect can be obtained because lithium ions can move between the positive electrode 21 and the negative electrode 22 . In this case, particularly, as described above, the safety of the secondary battery is improved, so that a higher effect can be obtained.

[Modification 2]
An electrolytic solution, which is a liquid electrolyte, was used. However, although not specifically illustrated here, an electrolyte layer that is a gel electrolyte may be used.

In the battery element 20 using the electrolyte layer, the positive electrode 21 and the negative electrode 22 are laminated with the separator 23 and the electrolyte layer interposed therebetween, and the positive electrode 21, the negative electrode 22, the separator 23 and the electrolyte layer are wound. This electrolyte layer is interposed between the positive electrode 21 and the separator 23 and interposed between the negative electrode 22 and the separator 23 .

Specifically, the electrolyte layer contains a polymer compound together with an electrolytic solution, and the electrolytic solution is held by the polymer compound. This is because leakage of the electrolytic solution is prevented. The composition of the electrolytic solution is as described above. Polymer compounds include polyvinylidene fluoride and the like. When forming the electrolyte layer, after preparing a precursor solution containing an electrolytic solution, a polymer compound, a solvent, and the like, the precursor solution is applied to one side or both sides of each of the positive electrode 21 and the negative electrode 22 .

Even when this electrolyte layer is used, lithium ions can move between the positive electrode 21 and the negative electrode 22 through the electrolyte layer, so a similar effect can be obtained. In this case, especially, as described above, leakage of the electrolytic solution is prevented, so that a higher effect can be obtained.

<3. Use of secondary battery>
The use (application example) of the secondary battery is not particularly limited. A secondary battery used as a power source may be a main power source for electronic devices and electric vehicles, or may be an auxiliary power source. A main power source is a power source that is preferentially used regardless of the presence or absence of other power sources. An auxiliary power supply is a power supply that is used in place of the main power supply or that is switched from the main power supply.

Specific examples of secondary battery applications are as follows. Electronic devices such as video cameras, digital still cameras, mobile phones, laptop computers, headphone stereos, portable radios and portable information terminals. Backup power and storage devices such as memory cards. Power tools such as power drills and power saws. It is a battery pack mounted on an electronic device. Medical electronic devices such as pacemakers and hearing aids. It is an electric vehicle such as an electric vehicle (including a hybrid vehicle). It is a power storage system such as a home or industrial battery system that stores power in preparation for emergencies. In these uses, one secondary battery may be used, or a plurality of secondary batteries may be used.

The battery pack may use a single cell or an assembled battery. An electric vehicle is a vehicle that operates (runs) using a secondary battery as a drive power source, and may be a hybrid vehicle that also includes a drive source other than the secondary battery. In a household electric power storage system, electric power stored in a secondary battery, which is an electric power storage source, can be used to use electric appliances for home use.

Here, an example of application of the secondary battery will be specifically described. The configuration of the application example described below is merely an example, and can be changed as appropriate.

Fig. 3 shows the block configuration of the battery pack. The battery pack described here is a battery pack (a so-called soft pack) using one secondary battery, and is mounted in an electronic device such as a smart phone.

This battery pack includes a power source 91 and a circuit board 92, as shown in FIG. The circuit board 92 is connected to the power supply 91 and includes a positive terminal 93 , a negative terminal 94 and a temperature detection terminal 95 .

The power supply 91 includes one secondary battery. In this secondary battery, the positive lead is connected to the positive terminal 93 and the negative lead is connected to the negative terminal 94 . The power supply 91 can be connected to the outside through a positive terminal 93 and a negative terminal 94, and can be charged and discharged. The circuit board 92 includes a control section 96 , a switch 97 , a PTC element 98 and a temperature detection section 99 . However, the PTC element 98 may be omitted.

The control unit 96 includes a central processing unit (CPU), memory, etc., and controls the operation of the entire battery pack. This control unit 96 detects and controls the use state of the power supply 91 as necessary.

When the voltage of the power supply 91 (secondary battery) reaches the overcharge detection voltage or the overdischarge detection voltage, the control unit 96 cuts off the switch 97 so that the charging current does not flow through the current path of the power supply 91. to The overcharge detection voltage is not particularly limited, but is specifically 4.2V±0.05V, and the overdischarge detection voltage is not particularly limited, but is specifically 2.4V±0.1V. is.

The switch 97 includes a charge control switch, a discharge control switch, a charge diode, a discharge diode, and the like, and switches connection/disconnection between the power supply 91 and an external device according to instructions from the control unit 96 . The switch 97 includes a field effect transistor (MOSFET) using a metal oxide semiconductor, etc., and the charge/discharge current is detected based on the ON resistance of the switch 97 .

The temperature detection unit 99 includes a temperature detection element such as a thermistor, measures the temperature of the power supply 91 using the temperature detection terminal 95 , and outputs the temperature measurement result to the control unit 96 . The measurement result of the temperature measured by the temperature detection unit 99 is used when the control unit 96 performs charging/discharging control at the time of abnormal heat generation and when the control unit 96 performs correction processing when calculating the remaining capacity.

An example of this technology will be explained.

<Experimental Examples 1 to 29 and Comparative Examples 1 to 6>
As described below, after the secondary battery was produced, the battery characteristics of the secondary battery were evaluated.

[Production of secondary battery]
The cylindrical lithium-ion secondary battery shown in FIGS. 1 and 2 was produced by the following procedure.

(Preparation of positive electrode)
First, 91 parts by mass of a positive electrode active material (LiFePO ₄ which is a lithium iron phosphate compound), 3 parts by mass of a positive electrode binder (polyvinylidene fluoride), and 6 parts by mass of a positive electrode conductive agent (carbon black) are mixed together. A positive electrode mixture was obtained by Subsequently, the positive electrode mixture was added to a solvent (N-methyl-2-pyrrolidone, which is an organic solvent), and the solvent was stirred to prepare a pasty positive electrode mixture slurry. Subsequently, the positive electrode mixture slurry is applied to both surfaces of the positive electrode current collector 21A (a strip-shaped aluminum foil having a thickness of 12 μm) using a coating device, and then the positive electrode mixture slurry is dried to obtain a positive electrode active material. A material layer 21B is formed. Finally, the positive electrode active material layer 21B was compression-molded using a roll press. Thus, the positive electrode 21 was produced.

(Preparation of negative electrode)
First, 93 parts by mass of a negative electrode active material (artificial graphite that is a carbon material) and 7 parts by mass of a negative electrode binder (polyvinylidene fluoride) were mixed together to obtain a negative electrode mixture. Subsequently, the negative electrode mixture was added to a solvent (N-methyl-2-pyrrolidone, which is an organic solvent), and the solvent was stirred to prepare a pasty negative electrode mixture slurry. Subsequently, the negative electrode mixture slurry is applied to both surfaces of the negative electrode current collector 22A (band-shaped copper foil having a thickness of 15 μm) using a coating device, and then the negative electrode mixture slurry is dried to obtain a negative electrode active material. A material layer 22B is formed. Finally, the negative electrode active material layer 22B was compression molded using a roll press. Thus, the negative electrode 22 was produced.

(Preparation of electrolytic solution)
The solvent was stirred by adding the electrolyte salt to the solvent and then by adding additives to the solvent as required.

As solvents, cyclic carbonates such as ethylene carbonate (EC) and propylene carbonate (PC), and chain carboxylic acid esters such as propyl propionate (PrPr) and propyl acetate (PrAc) are used. Dimethyl carbonate (DMC), ethyl methyl carbonate (EMC) and diethyl carbonate (DEC), which are chain carbonates, were used.

Lithium sulfonylimide salts bis(fluorosulfonyl)imide lithium (LiFSI) and bis(trifluoromethanesulfonyl)imide lithium (LiTFSI) were used as electrolyte salts.

As an additive, monofluoroethylene carbonate (FEC), which is a fluorinated cyclic carbonate, was used.

This prepared the electrolyte. The content (% by weight) representing the mixing ratio of the solvent, the content (mol/kg) of the electrolyte salt, and the content (% by weight) of the additive in the electrolytic solution are shown in Tables 1 to 3. Street.

For comparison, the following electrolytic solution was also prepared. Table 3 shows the content (% by weight) representing the mixing ratio of the solvent and the content (mol/kg) of the electrolyte salt.

An electrolytic solution was prepared in the same manner, except that chain carbonate was used as the solvent instead of chain carboxylate. Further, an electrolytic solution was prepared by the same procedure except that lithium hexafluorophosphate (LiPF ₆ ), which is another lithium salt, was used instead of the lithium sulfonylimide salt as the electrolyte salt. In addition, an electrolytic solution was prepared in the same manner, except that a chain carbonate was used as the solvent instead of the chain carboxylate, and lithium hexafluorophosphate was used as the electrolyte salt instead of the lithium sulfonylimide salt. was prepared. Furthermore, an electrolytic solution was prepared by the same procedure except that other chain carboxylic acid esters, methyl propionate (MePr) and ethyl propionate (EtPr), were used instead of the chain carboxylic acid ester as the solvent. did.

(Assembly of secondary battery)
First, the positive electrode lead 41 made of aluminum was welded to the positive electrode current collector 21A of the positive electrode 21 and the negative electrode lead 42 made of copper was welded to the negative electrode current collector 22A of the negative electrode 22 .

Subsequently, the positive electrode 21 and the negative electrode 22 are laminated with each other with a separator 23 (a microporous polyethylene film having a thickness of 15 μm) interposed therebetween, and then the positive electrode 21, the negative electrode 22 and the separator 23 are wound to obtain a winding. A wound body having a winding space 20S was produced. Subsequently, the center pin 80 was inserted into the winding space 20S of the wound body.

Subsequently, the insulating

plates

31 and 32 were accommodated together with the wound body inside the battery can main body 11 through the opening 11K. In this case, the positive electrode lead 41 was welded to the disk plate 51 of the current interrupting mechanism 50 and the negative electrode lead 42 was welded to the battery can main body 11 . Subsequently, an electrolytic solution was injected into the battery can main body 11 through the opening 11K. As a result, the wound body was impregnated with the electrolytic solution, and the battery element 20 was produced.

Finally, after housing the battery lid 12, the current interrupting mechanism 50 and the PTC element 70 inside the battery can main body 11 through the opening 11K, the portion of the battery can main body 11 near the opening 11K is covered with a gasket 60 (polyethylene). butylene terephthalate). As a result, the battery can 10 including the battery can main body 11 and the battery lid 12 was formed, and the battery element 20 was sealed inside the battery can 10, so that the secondary battery was assembled.

In this case, by changing the thickness T of the folded portion 51P of the disk plate 51, as shown in Tables 1 to 3, the operating pressure (kgf/cm ² ) of the current interrupting mechanism 50 can be changed. changed.

(Stabilization of secondary battery)
In a normal temperature environment (temperature=23° C.), the assembled secondary battery was charged and discharged for one cycle. During charging, constant-current charging was performed at a current of 0.1C until the voltage reached 4.2V, and then constant-voltage charging was performed at the voltage of 4.2V until the current reached 0.05C. During discharge, constant current discharge was performed at a current of 0.1C until the voltage reached 3.0V. 0.1C is a current value that can fully discharge the battery capacity (theoretical capacity) in 10 hours, and 0.05C is a current value that fully discharges the battery capacity in 20 hours. This completes the secondary battery.

[Evaluation of battery characteristics]
Evaluation of battery characteristics (high-temperature operating characteristics and high-temperature storage characteristics) of the secondary batteries yielded the results shown in Tables 1 to 3.

(High temperature operating characteristics)
First, the secondary battery was charged and discharged in a normal temperature environment (temperature = 23°C). During charging, constant current charging was performed at a current of 1 C until the voltage reached 3.6 V, and then constant voltage charging was performed at the voltage of 3.6 V until the current reached 5 mA. 1C is a current value that can discharge the battery capacity in 1 hour.

Subsequently, after charging the secondary battery in the same environment, the secondary battery in the charged state was placed in a constant temperature bath (temperature = 105°C). The charging conditions are as described above.

Finally, while measuring the voltage of the secondary battery, by storing the secondary battery in a charged state in a thermostat, it is an index for evaluating the high-temperature operating characteristics based on the voltage of the secondary battery. The operating state of the current interrupting mechanism 50 was investigated.

Specifically, when the voltage did not drop by 2.5 V or more during storage of the secondary battery (voltage drop <2.5 V), it was determined that the current interrupting mechanism 50 did not operate. On the other hand, when the voltage dropped by 2.5 V or more during storage of the secondary battery (amount of voltage drop≧2.5 V), it was determined that the current interrupting mechanism 50 was activated.

Here, the operating state of the current interrupting mechanism 50 was examined until the storage period reached one month (operating state 1).

In addition, when the current interrupting mechanism 50 did not operate even after the storage period reached 1 month, the operating state of the current interrupting mechanism 50 was continuously investigated until the total storage period reached 3 months (operating state 2). In this case, when the current interrupting mechanism 50 was activated in the middle, the total storage period until the current interrupting mechanism 50 was activated was investigated. 1.1M, 1.5M and 2M shown in Tables 2 and 3 mean 1.1 months, 1.5 months and 2 months, respectively.

(High temperature storage characteristics)
First, the discharge capacity (discharge capacity before storage) was measured by charging and discharging the secondary battery in a normal temperature environment (temperature = 23°C). The charging/discharging conditions were the same as the charging/discharging conditions when the high-temperature operating characteristics were examined, and the same was applied hereinafter.

Subsequently, after storing the secondary battery in a high-temperature environment (temperature = 90 ° C. constant temperature bath) (storage period = 30 days), by charging and discharging the secondary battery in the same environment, the discharge capacity ( discharge capacity after storage) was measured.

Finally, the capacity retention rate, which is an index for evaluating high-temperature storage characteristics, was calculated based on the formula: capacity retention rate (%) = (discharge capacity after storage/discharge capacity before storage) x 100.

[Discussion]
As shown in Tables 1 to 3, in the secondary battery in which the positive electrode 21 contains a lithium iron phosphate compound, the operating state and capacity retention rate of the current interrupting mechanism 50 depend on the composition of the electrolyte and the current interrupting mechanism, respectively. 50 operating conditions.

Specifically, even when the solvent in the electrolytic solution contains a chain carboxylic acid ester and the electrolyte salt contains a lithium sulfonylimide salt, the operating pressure is less than 20 kgf/cm ² (Comparative Example 1). A high capacity retention rate was obtained, but the current interrupting mechanism 50 was excessively operated.

In addition, when the electrolyte salt in the electrolyte contains a lithium sulfonylimide salt but the solvent does not contain a chain carboxylic acid ester (Comparative Example 2), the solvent in the electrolyte contains a chain carboxylic acid ester. However, in the case where the electrolyte salt did not contain a lithium sulfonylimide salt (Comparative Example 3), the current interrupting mechanism 50 excessively operated independently of the operating pressure, and the capacity retention rate decreased.

Furthermore, when the solvent in the electrolytic solution does not contain a chain carboxylic acid ester and the electrolyte salt does not contain a lithium sulfonylimide salt (Comparative Example 4), the current interrupting mechanism 50 is excessive regardless of the operating pressure. worked. In this case, since the current interrupting mechanism 50 was activated immediately after storage of the secondary battery, the capacity retention rate could not be calculated.

In addition, when the electrolytic solution contains a lithium sulfonylimide salt as the electrolyte salt but the solvent contains other chain carboxylic acid esters (Comparative Examples 5 and 6), a high capacity retention rate may be obtained in some cases. However, the current interrupting mechanism 50 was operated excessively.

On the other hand, in the electrolyte solution, the solvent contains a chain carboxylic acid ester, the electrolyte salt contains a lithium sulfonylimide salt, and the operating pressure is 20 kgf/cm ² or more (Examples 1 to 29). 2, the current interrupting mechanism 50 did not operate excessively, and a high capacity retention rate was obtained.

In this case, in particular, we obtained a series of trends that will be explained below. First, when the content of the chain carboxylic acid ester in the solvent was 10% by weight to 75% by weight, the current interrupting mechanism 50 did not operate excessively, or the capacity retention rate increased.

Second, a sufficient capacity retention rate was obtained when the content of the lithium sulfonylimide salt was 0.8 mol/kg to 2.0 mol/kg with respect to the solvent.

Third, when the solvent contains a fluorinated cyclic carbonate and the content of the fluorinated cyclic carbonate in the electrolytic solution is 0.1% by weight to 3.0% by weight, the current interrupting mechanism 50 is more effective. It became difficult to operate excessively, and the capacity retention rate increased more.

[summary]
From the results shown in Tables 1 to 3, the secondary battery includes the battery can 10, the battery element 20 (the positive electrode 21, the negative electrode 22, and the electrolytic solution), and the current interrupting mechanism 50, and the positive electrode 21 is lithium iron phosphate. The solvent of the electrolytic solution contains a chain carboxylic acid ester, the electrolyte salt of the electrolytic solution contains a lithium sulfonylimide salt, and the operating pressure of the current interrupting mechanism 50 is 20 kgf/cm ² . With the above, a high capacity retention rate was obtained while the excessive operation of the current interrupting mechanism 50 was suppressed. Therefore, it was possible to obtain excellent high-temperature operating characteristics and excellent high-temperature storage characteristics in the secondary battery.

Although the present technology has been described above while citing one embodiment and example, the configuration of this technology is not limited to the configuration described in the one embodiment and example, and can be variously modified.

Specifically, the case where the battery structure of the secondary battery is cylindrical has been described, but the type of battery structure is not particularly limited. Specifically, the battery structure may be rectangular, coin-shaped, button-shaped, or the like.

Also, the case where the element structure of the battery element is the wound type has been described, but the type of the element structure is not particularly limited. Specifically, the device structure may be a stacked type in which electrodes (positive and negative electrodes) are stacked, a zigzag-fold type in which electrodes are folded in a zigzag pattern, or other configurations.

Furthermore, the case where the electrode reactant is lithium has been described, but the type of the electrode reactant is not particularly limited. Specifically, the electrode reactants may be other alkali metals such as sodium and potassium, or alkaline earth metals such as beryllium, magnesium and calcium, as described above. Alternatively, the electrode reactant may be other light metals such as aluminum.

Since the effects described in this specification are merely examples, the effects of the present technology are not limited to the effects described in this specification. Accordingly, other advantages may be obtained with respect to the present technology.

Claims

a battery element comprising a positive electrode, a negative electrode and an electrolyte;
a housing member for housing the battery element therein;
a current interrupting mechanism that operates in response to an increase in pressure inside the storage member and interrupts current flow to the battery element,
The positive electrode contains a lithium iron phosphate compound represented by the following formula (1),
The electrolyte contains a solvent and an electrolyte salt,
The solvent contains a linear carboxylic acid ester having a boiling point of 100° C. or higher and a viscosity of 0.9 mPa s or lower at 25° C.,
The electrolyte salt includes a lithium sulfonylimide salt represented by the following formula (2),
The operating pressure of the current interrupting mechanism is 20 kgf/cm 2 or more.
secondary battery.
LiFe x M 1-x PO 4 (1)
(M is one or more transition metal elements (excluding Fe). x satisfies 0<x≦1.)
LiN(R1SO2)( R2SO2 ) ( 2 )
(Each of R1 and R2 is either a fluorine group or a perfluoroalkyl group.)
The chain carboxylic acid ester contains at least one of propyl acetate and propyl propionate,
The lithium sulfonylimide salt comprises at least one of bis(fluorosulfonyl)imidelithium and bis(trifluoromethanesulfonyl)imidelithium,
The secondary battery according to claim 1.
the transition metal element includes one or both of Mn and Co;
The secondary battery according to claim 1 or 2.
The content of the lithium sulfonylimide salt is 0.8 mol/kg or more and 2.0 mol/kg or less with respect to the solvent.
The secondary battery according to any one of claims 1 to 3.
The content of the chain carboxylic acid ester in the solvent is 10% by weight or more and 75% by weight or less.
The secondary battery according to any one of claims 1 to 4.
The electrolytic solution further contains a fluorinated cyclic carbonate,
The content of the fluorinated cyclic carbonate in the electrolytic solution is 0.1% by weight or more and 3.0% by weight or less.
The secondary battery according to any one of claims 1 to 5.
A lithium ion secondary battery,
The secondary battery according to any one of claims 1 to 6.