WO2015079893A1

WO2015079893A1 - Lithium secondary battery

Info

Publication number: WO2015079893A1
Application number: PCT/JP2014/079673
Authority: WO
Inventors: 貴子西田; 祐介中村; 龍太韓; 春樹上剃
Original assignee: 日立マクセル株式会社
Priority date: 2013-11-26
Filing date: 2014-11-10
Publication date: 2015-06-04
Also published as: JP6208560B2; JP2015103408A

Abstract

Provided is a lithium secondary battery which has excellent charge/discharge cycle characteristics and excellent storage characteristics in a discharged state. The above-mentioned problem is solved by a lithium secondary battery which uses a positive electrode, a negative electrode, a separator and a nonaqueous electrolyte, and which is characterized in that: the nonaqueous electrolyte contains a compound having a nitrile group in each molecule, lithium bisoxalate borate, a compound having a reduction potential at the negative electrode of 1.3 V or less with respect to the Li metal potential, and an electrolyte salt; the content of the compound having a nitrile group in each molecule in the nonaqueous electrolyte is 0.05-1.5% by mass; the content of the lithium bisoxalate borate in the nonaqueous electrolyte is 0.05-5% by mass; and the content of the compound having a reduction potential at the negative electrode of 1.3 V or less with respect to the Li metal potential is 10% by mass or less.

Description

Lithium secondary battery

The present invention relates to a lithium secondary battery excellent in charge / discharge cycle characteristics and storage characteristics in a discharged state.

In recent years, along with the development of portable electronic devices such as mobile phones and notebook computers, and the practical application of electric vehicles, small and light lithium secondary batteries with high capacity have been required.

In addition, lithium secondary batteries are required to improve various battery characteristics in accordance with the spread of applicable devices.

As one of means for improving the battery characteristics of the lithium secondary battery, there is improvement of a non-aqueous electrolyte. For example, Patent Document 1 discloses that a non-aqueous electrolyte using a dinitrile compound as an organic solvent and a specific type of lithium salt can suppress a decrease in charge / discharge amount due to repeated charge / discharge of the battery. Are listed.

JP 2011-222473 A

By the way, a nitrile compound such as a dinitrile compound is not used in a large amount as an organic solvent for a non-aqueous electrolyte in a lithium secondary battery, but is used in a small amount as an additive thereof, for example, during storage of a lithium secondary battery. It is known that it has an effect of suppressing the swelling (bulging when stored under high temperature in a charged state) and the effect of improving the safety of the lithium secondary battery. On the other hand, lithium secondary batteries are also required to have good charge / discharge cycle characteristics so that a large capacity can be maintained even if charge / discharge is repeated many times, but contain a nitrile compound as an additive. In a lithium secondary battery using a non-aqueous electrolyte, the charge / discharge cycle characteristics at room temperature are likely to deteriorate.

For these reasons, in lithium secondary batteries using a non-aqueous electrolyte containing a nitrile compound as an additive, development of a technology that enhances charge / discharge cycle characteristics is also required.

Also, for example, in portable devices such as portable game machines that use a lithium secondary battery as a power source, the lithium secondary battery may be left in a discharged state for a long period of time. As a result, the voltage of the battery drops to an overdischarged state, and the power cannot be turned on even if the device is used again.

Therefore, there is a demand for development of a technology for improving the storage characteristics in a discharged state so that the occurrence of the above-described phenomenon can be suppressed in the lithium secondary battery.

The present invention has been made in view of the above circumstances, and an object thereof is to provide a lithium secondary battery excellent in charge / discharge cycle characteristics and storage characteristics in a discharged state.

The lithium secondary battery of the present invention that has achieved the above object is a lithium secondary battery using a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte, and the non-aqueous electrolyte has a nitrile group in the molecule. (Hereinafter referred to as “nitrile compound”), lithium bisoxalate borate (hereinafter referred to as “LiBOB”), a compound whose reduction potential at the negative electrode is 1.3 V or less with respect to the Li potential, and an electrolyte salt. The content of the compound having a nitrile group in the molecule in the non-aqueous electrolyte is 0.05 to 1.5% by mass, and the content of LiBOB in the non-aqueous electrolyte is 0.05 to 5% by mass. The content of the compound in which the reduction potential at the negative electrode is 1.3 V or less with respect to the metal Li potential in the nonaqueous electrolyte is 10% by mass or less.

According to the present invention, a lithium secondary battery excellent in charge / discharge cycle characteristics and storage characteristics in a discharged state can be provided.

It is a fragmentary longitudinal cross-sectional view which represents typically an example of the lithium secondary battery of this invention. FIG. 2 is a perspective view of FIG. 1.

As described above, the nitrile compound is used as an additive for the non-aqueous electrolyte, so that the storage characteristics of the non-aqueous secondary battery (storage characteristics in a high temperature environment of the lithium secondary battery in a charged state) and safety can be improved. While contributing to the improvement, the charge / discharge cycle characteristics at room temperature are deteriorated.

Therefore, in the present invention, a nonaqueous electrolyte containing, as an additive, a specific amount of LiBOB and a compound having a reduction potential at the negative electrode of 1.3 V or less with respect to the metal Li potential is used together with the specific amount of nitrile compound. It was decided.

Both LiBOB and the compound whose reduction potential at the negative electrode is 1.3 V or less with respect to the metal Li potential have the effect of forming a film on the negative electrode surface when the battery is charged and discharged. Since the coating formed on the negative electrode surface by these compounds suppresses the decomposition reaction of non-aqueous electrolyte components including the nitrile compound on the negative electrode surface when the charge and discharge of the lithium secondary battery is repeated, It is possible to suppress a decrease in capacity that can be caused by such a decomposition reaction.

Moreover, since LiBOB has a reduction potential at the negative electrode of about 1.5 V with respect to the metal Li potential, the non-aqueous solution contains a compound with a reduction potential at LiBOB and the negative electrode of 1.3 V or less with respect to the metal Li potential. In a battery using an electrolyte, LiBOB preferentially forms a film on the negative electrode surface by charging and discharging. Since the LiBOB-derived film formed on the negative electrode surface is strong and has lithium ion conductivity, the decomposition reaction of the nonaqueous electrolyte component on the negative electrode surface is satisfactorily suppressed, while the battery reaction is Does not hinder.

In addition, the LiBOB-derived film that is preferentially formed on the negative electrode surface suppresses the film-forming reaction of a compound whose reduction potential at the negative electrode is 1.3 V or less with respect to the metal Li potential. Therefore, a compound having a reduction potential at the negative electrode of 1.3 V or less with respect to the metal Li potential remains in the nonaqueous electrolyte as it is for a relatively long period after the use of the nonaqueous secondary battery is started. And, when a defect occurs in the LiBOB-derived film formed on the negative electrode surface by repeating many charging and discharging of the non-aqueous secondary battery, the reduction potential at the negative electrode is lower than the metal Li potential at this defective portion. A compound having a voltage of 1.3 V or less reacts with the negative electrode to form a film, whereby the decomposition reaction of the nonaqueous electrolyte component on the negative electrode surface is continuously suppressed.

Thus, in the lithium secondary battery of the present invention, LiBOB and a compound whose reduction potential at the negative electrode is 1.3 V or less with respect to the metal Li potential form a film on the negative electrode surface step by step. Even if it repeats, since the decomposition reaction of the nonaqueous electrolyte component in the negative electrode can be suppressed over a long period of time, excellent charge / discharge cycle characteristics can be ensured.

In addition, for example, in a non-aqueous secondary battery using a non-aqueous electrolyte containing a compound whose reduction potential at the negative electrode is 1.3 V or less with respect to the metal Li potential, for example, use of a portable device using the non-aqueous electrolyte as a power source If the battery is left in a car in a discharged state, such as when left in a car with advanced, the potential of the negative electrode is high, so the reduction potential at the negative electrode is 1.3 V or less with respect to the metal Li potential. The reduction reaction by the compound proceeds, and the battery voltage is likely to decrease (that is, the overdischarge state is likely to occur). In a portable device having a battery in such a state, the power cannot be turned on. Further, as a result of studies by the present inventors, a battery using a nonaqueous electrolyte containing a nitrile compound as an additive together with a compound having a reduction potential at the negative electrode of 1.3 V or less with respect to the metal Li potential has been described above. It has been found that the phenomenon of a discharge state is more likely to occur.

However, when a nonaqueous electrolyte containing a specific amount of LiBOB is used together with a compound having a reduction potential of 1.3 V or less at the negative electrode and a specific amount of nitrile compound, the phenomenon of the overdischarge state as described above occurs. Occurrence can be suppressed. A compound having a reduction potential at LiBOB and a negative electrode of 1.3 V or less with respect to the metallic Li potential generates gas and causes swelling in a discharged battery, but a specific amount of nitrile compound and a specific amount When a nonaqueous electrolyte containing LiBOB and a compound having a reduction potential at the negative electrode of 1.3 V or less with respect to the metal Li potential is used, the occurrence of such blistering can also be suppressed.

Thus, in the non-aqueous secondary battery of the present invention, the specific additive added to the non-aqueous electrolyte acts in a complex manner, so that the charge / discharge cycle characteristics are enhanced and the storage characteristics in the discharged state are also excellent. Can be.

As the non-aqueous electrolyte according to the lithium secondary battery of the present invention, a solution (non-aqueous electrolyte) in which an electrolyte salt is dissolved in an organic solvent is usually used. The non-aqueous electrolyte contains, as additives, a compound having a nitrile group in the molecule, lithium bisoxalate borate (LiBOB), and a compound whose reduction potential at the negative electrode is 1.3 V or less with respect to the metal Li potential. is doing.

Specific examples of the nitrile compound include acetonitrile, propionitrile, butyronitrile, valeronitrile, benzonitrile, acrylonitrile and other mononitriles; malononitrile, the following general formula (1)
NC- (CH ₂ ) _n -CN (1)
[In the general formula (1), n is an integer of 2 to 4]
1,4-dicyanoheptane, 1,5-dicyanopentane, 1,6-dicyanohexane, 1,7-dicyanoheptane, 2,6-dicyanoheptane, 1,8-dicyanooctane, 2, Dinitriles such as 7-dicyanooctane, 1,9-dicyanononane, 2,8-dicyanononane, 1,10-dicyanodecane, 1,6-dicyanodecane and 2,4-dimethylglutaronitrile; cyclic nitriles such as benzonitrile; Alkoxy substituted nitriles such as methoxyacetonitrile; and the like, and only one of these may be used, or two or more may be used in combination.

Among the exemplified nitrile compounds, dinitriles are preferable, compounds represented by the general formula (1) (succinonitrile, glutaronitrile, adiponitrile) are more preferable, and charging / discharging of the battery by using in combination with other additives A compound (glutaronitrile, adiponitrile) represented by the general formula (1) and n in the general formula (1) is 3 or 4 is more preferable because the effect of improving the cycle characteristics becomes better.

Examples of the compound whose reduction potential at the negative electrode is 1.3 V or less with respect to the metal Li potential include, for example, 2-propynyl diethylphosphonoacetate (PDEA), 4-fluoro-1,3-dioxolan-2-one (FEC) , Vinylene carbonate (VC), vinyl ethylene carbonate (VEC), and the like, and only one of them may be used, or two or more may be used in combination.

The content of the nitrile compound in the non-aqueous electrolyte used for the lithium secondary battery is 0.05% by mass or more, and preferably 0.2% by mass or more, from the viewpoint of ensuring a good effect due to the use. . However, if the amount of the nitrile compound in the non-aqueous electrolyte is too large, for example, the amount of LiBOB required to suppress the deterioration of the charge / discharge cycle characteristics of the battery due to the nitrile compound becomes too large, and the discharge state of the battery The storage characteristics at the end will deteriorate. Therefore, the content of the nitrile compound in the non-aqueous electrolyte used for the lithium secondary battery is 1.5% by mass or less, and preferably 1% by mass or less.

The content of LiBOB in the non-aqueous electrolyte used for the lithium secondary battery is 0.05% by mass or more, and preferably 0.25% by mass or more, from the viewpoint of ensuring a good effect due to its use. However, if the amount of LiBOB in the non-aqueous electrolyte is too large, the storage characteristics of the battery in the discharged state will deteriorate. Therefore, the content of LiBOB in the non-aqueous electrolyte used for the lithium secondary battery is 5% by mass or less.

A compound whose reduction potential at the negative electrode is 1.3 V or less with respect to the metal Li potential is an additive that contributes to the improvement of the charge / discharge cycle characteristics of the battery as described above, but in the non-aqueous electrolyte used in the battery If the amount is too large, the charge / discharge cycle characteristics of the battery may be impaired, and the battery may swell during storage in a discharged state. Therefore, the content of a compound having a reduction potential at the negative electrode of 1.3 V or less with respect to the metal Li potential in the non-aqueous electrolyte used for the lithium secondary battery (the non-aqueous electrolyte has a reduction potential at the negative electrode that is a metal Li potential). In the case where a plurality of compounds of 1.3 V or less are contained, their total amount (the same applies hereinafter) is 10% by mass or less, and preferably 9% by mass or less. In addition, from the viewpoint of favorably ensuring the effect of using a compound having a reduction potential at the negative electrode of 1.3 V or less with respect to the metal Li potential, the content in the non-aqueous electrolyte used in the lithium secondary battery is 1 mass. % Or more, more preferably 1.5% by mass or more.

The electrolyte salt related to the non-aqueous electrolyte is not particularly limited as long as it dissociates in a solvent to form Li ⁺ ions and hardly causes a side reaction such as decomposition in a voltage range used as a battery. For example, inorganic lithium salts such as LiClO ₄ , LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiSbF ₆ ; LiCF ₃ SO ₃ , LiCF ₃ CO ₂ , Li ₂ C ₂ F ₄ (SO ₃ ) ₂ , LiN (CF ₃ SO ₂ ) ₂ , LiC (CF ₃ SO ₂ ) ₃ , LiC _n F _{2n + 1} SO ₃ (n ≧ 2), LiN (RfOSO ₂ ) ₂ [where Rf is a fluoroalkyl group]; it can.

The concentration of the lithium salt in the non-aqueous electrolyte is preferably 0.5 to 1.5 mol / l, more preferably 0.9 to 1.25 mol / l.

The organic solvent used for the non-aqueous electrolyte is not particularly limited as long as it dissolves the lithium salt and does not cause a side reaction such as decomposition in a voltage range used as a battery. For example, cyclic carbonates such as ethylene carbonate (EC), propylene carbonate, butylene carbonate; chain carbonates such as dimethyl carbonate, diethyl carbonate (DEC), and methyl ethyl carbonate; chain esters such as methyl propionate; γ-butyrolactone, etc. Cyclic esters; chain ethers such as dimethoxyethane, diethyl ether, 1,3-dioxolane, diglyme, triglyme and tetraglyme; cyclic ethers such as 1,4-dioxane, tetrahydrofuran and 2-methyltetrahydrofuran; acetonitrile, propionitrile, Nitriles such as methoxypropionitrile; sulfites such as ethylene glycol sulfite; and the like. And it can also be. In order to obtain a battery with better characteristics, it is desirable to use a combination that can obtain high conductivity, such as a mixed solvent of ethylene carbonate and chain carbonate.

The nonaqueous electrolyte used for the lithium secondary battery preferably contains 1,3-dioxane. Thereby, the charge / discharge cycle characteristics of the lithium secondary battery can be further enhanced.

The content of 1,3-dioxane in the non-aqueous electrolyte used for the lithium secondary battery is preferably 0.1% by mass or more from the viewpoint of better ensuring the effect of the use, and 0.5% by mass. % Or more is more preferable. However, if the amount of 1,3-dioxane in the non-aqueous electrolyte is too large, the load characteristics of the battery may be reduced, and the effect of improving the charge / discharge cycle characteristics may be reduced. Therefore, the content of 1,3-dioxane in the non-aqueous electrolyte used for the lithium secondary battery is preferably 5% by mass or less, and more preferably 2% by mass or less.

Non-aqueous electrolytes used in lithium secondary batteries include acid anhydrides, sulfonic acid esters, 1 for the purpose of further improving charge / discharge cycle characteristics and improving safety such as high-temperature storage and prevention of overcharge. , 3-propane sultone, diphenyl disulfide, cyclohexylbenzene, biphenyl, fluorobenzene, t-butylbenzene, and other additives (including derivatives thereof) can be added as appropriate.

Further, as the non-aqueous electrolyte of the lithium secondary battery, a gel (gel electrolyte) obtained by adding a known gelling agent such as a polymer to the non-aqueous electrolyte (non-aqueous electrolyte) is used. You can also.

The lithium secondary battery of the present invention has a positive electrode, a negative electrode, a non-aqueous electrolyte, and a separator, and the non-aqueous electrolyte may be used as the non-aqueous electrolyte, and other configurations and structures are particularly limited. Rather, various configurations and structures employed in conventionally known lithium secondary batteries can be applied.

As the positive electrode for the lithium secondary battery, for example, one having a structure having a positive electrode mixture layer containing a positive electrode active material, a binder, a conductive auxiliary agent and the like on one side or both sides of a current collector can be used.

For the positive electrode active material, lithium cobalt composite oxide such as LiCoO ₂ ; lithium manganese composite oxide such as LiMnO ₂ and Li ₂ MnO ₃ ; lithium nickel composite oxide such as LiNiO ₂ ; layered such as LiCo _1-x NiO ₂ Lithium-containing composite oxide having a structure; lithium-containing composite oxide having a spinel structure such as LiMn ₂ O ₄ , Li _4/3 Ti _5/3 O ₄ ; lithium-containing composite oxide having an olivine structure such as LiFePO ₄ ; One or two or more of lithium-containing composite oxides such as oxides having a basic composition and substituted with various elements can be used.

For example, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), styrene butadiene rubber (SBR), carboxymethyl cellulose (CMC), or the like is preferably used for the binder related to the positive electrode mixture layer. In addition, as the conductive auxiliary agent related to the positive electrode mixture layer, for example, graphite (graphite carbon material) such as natural graphite (flaky graphite), artificial graphite; acetylene black, ketjen black, channel black, furnace black, Examples thereof include carbon blacks such as carbon blacks such as lamp black and thermal black; carbon fibers.

For the positive electrode, for example, a paste-like or slurry-like positive electrode mixture-containing composition in which a positive electrode active material, a binder, a conductive auxiliary agent, and the like are dispersed in a solvent such as N-methyl-2-pyrrolidone (NMP) is prepared ( However, the binder may be dissolved in a solvent), and this is applied to one or both sides of the current collector, dried, and then subjected to a calendering process as necessary. However, the positive electrode is not limited to those manufactured by the above manufacturing method, and may be manufactured by other methods.

Further, a lead body for electrical connection with other members in the lithium secondary battery may be formed on the positive electrode according to a conventional method, if necessary.

The thickness of the positive electrode mixture layer is preferably, for example, 10 to 100 μm per one side of the current collector. As the composition of the positive electrode mixture layer, for example, the amount of the positive electrode active material is preferably 60 to 95% by mass, the amount of the binder is preferably 1 to 15% by mass, and the amount of the conductive auxiliary agent Is preferably 3 to 20% by mass.

The positive electrode current collector may be the same as that used for the positive electrode of a conventionally known lithium secondary battery, and for example, an aluminum foil having a thickness of 10 to 30 μm is preferable.

The negative electrode related to the lithium secondary battery has, for example, a negative electrode active material, a binder, and, if necessary, a negative electrode mixture layer including a negative electrode mixture containing a conductive auxiliary agent on one side or both sides of the current collector. A structure can be used.

Examples of the negative electrode active material include graphite, pyrolytic carbons, cokes, glassy carbons, fired bodies of organic polymer compounds, mesocarbon microbeads, carbon fibers, activated carbon, and metals that can be alloyed with lithium (Si , Sn, etc.) or alloys thereof, oxides, etc., and one or more of these can be used.

Further, the same negative electrode binder and conductive additive as those exemplified above as those that can be used for the positive electrode can be used.

For the negative electrode, for example, a negative electrode active material, a binder, and a conductive auxiliary agent used as necessary are prepared in a paste-like or slurry-like negative electrode mixture-containing composition in which a solvent such as NMP or water is dispersed. (However, the binder may be dissolved in a solvent), which is applied to one or both sides of the current collector, dried, and then subjected to a calendering process as necessary. However, the negative electrode is not limited to those manufactured by the above manufacturing method, and may be manufactured by other methods.

Further, a lead body for electrical connection with other members in the lithium secondary battery may be formed on the negative electrode according to a conventional method, if necessary.

The thickness of the negative electrode mixture layer is preferably, for example, 10 to 100 μm per one side of the current collector. The composition of the negative electrode mixture layer is preferably 80.0 to 99.8 mass% for the negative electrode active material and 0.1 to 10 mass% for the binder, for example. Further, when the negative electrode mixture layer contains a conductive additive, the amount of the conductive auxiliary in the negative electrode mixture layer is preferably 0.1 to 10% by mass.

As the negative electrode current collector, a copper or nickel foil, a punching metal, a net, an expanded metal, or the like can be used, but a copper foil is usually used. In the negative electrode current collector, when the thickness of the entire negative electrode is reduced in order to obtain a battery having a high energy density, the upper limit of the thickness is preferably 30 μm, and the lower limit is 5 μm in order to ensure mechanical strength. Is desirable.

The separator according to the lithium secondary battery has a property that the pores are closed at 80 ° C. or higher (more preferably 100 ° C. or higher) and 170 ° C. or lower (more preferably 150 ° C. or lower) (that is, a shutdown function). It is preferable that a separator used in a normal lithium secondary battery, for example, a microporous membrane made of polyolefin such as polyethylene (PE) or polypropylene (PP) can be used. The microporous film constituting the separator may be, for example, one using only PE or one using PP, or a laminate of a PE microporous film and a PP microporous film. There may be.

The thickness of the separator is preferably 10 to 30 μm, for example.

The positive electrode, the negative electrode, and the separator are formed in the form of a laminated electrode body in which a separator is interposed between the positive electrode and the negative electrode, or a wound electrode body in which the separator is wound in a spiral shape. It can be used for the lithium secondary battery of the invention.

Examples of the form of the lithium secondary battery of the present invention include a cylindrical shape (such as a square cylindrical shape or a cylindrical shape) using a steel can or an aluminum can as an outer can. Moreover, it can also be set as the soft package battery which used the laminated film which vapor-deposited the metal as an exterior body.

Lithium secondary batteries may use thin outer casings such as rectangular tube outer casings and laminate film outer casings as described above depending on their applications. If the volume occupied by the electrode bodies (positive electrode and negative electrode) in the battery is increased in order to increase the capacity, the content of the non-aqueous electrolyte decreases. The nonaqueous electrolyte is decomposed and reduced in accordance with the charge / discharge of the lithium secondary battery, so in a battery with a low content of the nonaqueous electrolyte, the nonaqueous electrolyte has a relatively small number of charge / discharge cycles. There is a risk that the amount becomes insufficient and sufficient capacity cannot be extracted (that is, charge / discharge cycle deterioration occurs).

However, in the lithium secondary battery of the present invention, the action of each additive in the non-aqueous electrolyte can suppress decomposition and weight loss of the non-aqueous electrolyte due to charge and discharge. Even when the capacity is increased while being used and the content of the non-aqueous electrolyte cannot be increased, good charge / discharge cycle characteristics can be secured.

Specifically, in the lithium secondary battery of the present invention, the amount of nonaqueous electrolyte per discharge capacity (hereinafter simply referred to as “the amount of nonaqueous electrolyte per discharge capacity”) is as small as 2.1 g / Ah or less. Even so, good charge / discharge cycle characteristics can be secured. In addition, it is preferable that the quantity of the nonaqueous electrolyte per discharge capacity in the lithium secondary battery of this invention is 1.8 g / Ah or more.

The discharge capacity used for the calculation of the amount of the nonaqueous electrolyte per discharge capacity is a constant current charge to 4.4 V at a current value of 0.5 C in an environment of 23 ° C., and subsequently a current at 4.4 V. The discharge capacity is obtained by performing constant voltage charging until the value reaches 0.05 C, and then discharging at 2.75 V with a constant current of 0.2 C.

Further, the amount of the nonaqueous electrolyte used for calculating the amount of the nonaqueous electrolyte per discharge capacity is a value obtained by the following procedures (1) to (6).
(1) The mass of the lithium secondary battery is measured.
(2) A hole is made in the lithium secondary battery after mass measurement (in the case of using an outer can, a hole is made in the bottom of the outer can), and the nonaqueous electrolyte in the battery is removed using a centrifuge. Extract.
(3) Take out the positive electrode and the negative electrode from the lithium secondary battery, and immerse them in dimethyl carbonate for 24 hours in order to extract the nonaqueous electrolyte soaked in the inside.
(4) The positive electrode and negative electrode after immersion and other members (exterior body, separator, sealing member, etc.) are dried.
(5) The masses of the positive electrode and negative electrode of the lithium secondary battery and other members (exterior body, separator, sealing member, etc.) are measured.
(6) Subtract the mass determined in (5) from the mass determined in (1) to calculate the amount of non-aqueous electrolyte.

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

Example 1
<Preparation of positive electrode>
100 parts by mass of a positive electrode active material obtained by mixing LiCoO ₂ and Li _1.0 Ni _0.5 Co _0.2 Mn _0.3 O _{2 at} a ratio (mass ratio) of 8: 2, and 10 parts by mass of PVDF as a binder. 20 parts by weight of NMP solution contained at a concentration of 1%, 1 part by weight of artificial graphite and 1 part by weight of ketjen black, which are conductive assistants, are kneaded using a biaxial kneader, and NMP is added to adjust the viscosity. Thus, a positive electrode mixture-containing paste was prepared.

After coating the positive electrode mixture-containing paste on both surfaces of an aluminum foil (positive electrode current collector) having a thickness of 15 μm, vacuum drying is performed at 120 ° C. for 12 hours to form a positive electrode mixture layer on both surfaces of the aluminum foil. did. Thereafter, press treatment was performed to adjust the thickness and density of the positive electrode mixture layer, and a nickel lead body was welded to the exposed portion of the aluminum foil to produce a strip-like positive electrode having a length of 375 mm and a width of 43 mm. . The positive electrode mixture layer in the obtained positive electrode had a thickness of 55 μm per one side.

<Production of negative electrode>
A composite in which the surface of SiO having an average particle diameter D50% of 8 μm, which is a negative electrode active material, is coated with a carbon material (the amount of the carbon material in the composite is 10% by mass), and graphite having an average particle diameter D50% of 16 μm A mixture in which the amount of the composite having the SiO surface coated with a carbon material is 3.75% by mass: 97.5 parts by mass, SBR as a binder: 1.5 parts by mass, and a thickener CMC: 1 part by mass of water was added and mixed to prepare a negative electrode mixture-containing paste.

After applying the negative electrode mixture-containing paste to both sides of a copper foil (negative electrode current collector) having a thickness of 8 μm, vacuum drying is performed at 120 ° C. for 12 hours to form a negative electrode mixture layer on both sides of the copper foil. did. Thereafter, press treatment was performed to adjust the thickness and density of the negative electrode mixture layer, and a nickel lead body was welded to the exposed portion of the copper foil to produce a strip-shaped negative electrode having a length of 380 mm and a width of 44 mm. . The negative electrode mixture layer in the obtained negative electrode had a thickness of 65 μm per one surface.

<Preparation of non-aqueous electrolyte>
LiPF ₆ is dissolved at a concentration of 1.1 mol / L in a mixed solvent of EC and DEC at a volume ratio of 3: 7, so that adiponitrile is 0.5 mass% and LiBOB is 0.25 mass%. The amount of PDEA is 2.0% by mass, the amount of FEC is 1.5% by mass, the amount of VC is 3.0% by mass, and the amount of 1,3-dioxane is 1.5% by mass. A non-aqueous electrolyte (non-aqueous electrolyte solution) was prepared by adding each in an amount of%.

<Battery assembly>
The belt-like positive electrode is stacked on the belt-like negative electrode through a microporous polyethylene separator (porosity: 41%) having a thickness of 16 μm, wound in a spiral shape, and then pressed so as to be flat. A wound electrode body having a flat wound structure was formed, and this electrode wound body was fixed with an insulating tape made of polypropylene. Next, the wound electrode body is inserted into a prismatic battery case made of aluminum alloy having an outer dimension of 4.0 mm in thickness, 34 mm in width, and 50 mm in height, and the lead body is welded, and the lid made of aluminum alloy The plate was welded to the open end of the battery case. Thereafter, the non-aqueous electrolyte is injected from the inlet provided in the cover plate, and left for 1 hour, and then the inlet is sealed. The lithium secondary battery having the structure shown in FIG. Obtained.

Here, the battery shown in FIGS. 1 and 2 will be described. FIG. 1 is a partial sectional view of the battery, and the positive electrode 1 and the negative electrode 2 are spirally wound via a separator 3 and then flattened. The flat wound electrode body 6 is pressurized and accommodated in a rectangular (rectangular tube) battery case 4 together with a nonaqueous electrolyte. However, in FIG. 1, in order to avoid complication, a metal foil, a non-aqueous electrolyte, and the like as a current collector used for manufacturing the positive electrode 1 and the negative electrode 2 are not illustrated.

The battery case 4 is made of an aluminum alloy and constitutes a battery outer body. The battery case 4 also serves as a positive electrode terminal. And the insulator 5 which consists of PE sheets is arrange | positioned at the bottom part of the battery case 4, and it connects to each one end of the positive electrode 1 and the negative electrode 2 from the flat wound electrode body 6 which consists of the positive electrode 1, the negative electrode 2, and the separator 3. The positive electrode lead body 7 and the negative electrode lead body 8 thus drawn are drawn out. A stainless steel terminal 11 is attached to a sealing lid plate 9 made of aluminum alloy for sealing the opening of the battery case 4 via a polypropylene insulating packing 10, and an insulator 12 is attached to the terminal 11. A stainless steel lead plate 13 is attached.

The cover plate 9 is inserted into the opening of the battery case 4, and the joint of the two is welded, whereby the opening of the battery case 4 is sealed and the inside of the battery is sealed. Further, in the battery of FIG. 1, a non-aqueous electrolyte inlet 14 is provided in the lid plate 9, and the non-aqueous electrolyte inlet 14 is welded by, for example, laser welding with a sealing member inserted. The battery is sealed to ensure the battery hermeticity. Further, the lid plate 9 is provided with a cleavage vent 15 as a mechanism for discharging the internal gas to the outside when the temperature of the battery rises.

In the battery of this Example 1, the outer can 5 and the cover plate 9 function as a positive electrode terminal by directly welding the positive electrode lead body 7 to the lid plate 9, and the negative electrode lead body 8 is welded to the lead plate 13, The terminal 11 functions as a negative electrode terminal by conducting the negative electrode lead body 8 and the terminal 11 through the lead plate 13, but depending on the material of the battery case 4, the sign may be reversed. There is also.

FIG. 2 is a perspective view schematically showing the external appearance of the battery shown in FIG. 1. FIG. 2 is shown for the purpose of showing that the battery is a square battery. FIG. 1 schematically shows a battery, and only specific members of the battery are shown. Also in FIG. 1, the inner peripheral portion of the electrode body is not cross-sectional.

Examples 2-7
A non-aqueous electrolyte was prepared in the same manner as in Example 1 except that the addition amount of adiponitrile, LiBOB, PDEA, FEC and VC was changed to the amounts shown in Tables 1 and 2, except that these non-aqueous electrolytes were used. A lithium secondary battery was produced in the same manner as in Example 1.

Example 8
A non-aqueous electrolyte was prepared in the same manner as in Example 2 except that succinonitrile was used instead of adiponitrile, and a lithium secondary battery was produced in the same manner as in Example 1 except that this non-aqueous electrolyte was used.

Example 9
A nonaqueous electrolyte was prepared in the same manner as in Example 2 except that VEC was added in an amount of 4.5% by mass without adding FEC and VC, and Example 1 except that this nonaqueous electrolyte was used. Similarly, a lithium secondary battery was produced.

Example 10
A nonaqueous electrolyte was prepared in the same manner as in Example 2 except that 1,3-dioxane was not added, and a lithium secondary battery was produced in the same manner as in Example 1 except that this nonaqueous electrolyte was used.

Example 11
A nonaqueous electrolyte was prepared in the same manner as in Example 2 except that PDEA was not added, and a lithium secondary battery was produced in the same manner as in Example 1 except that these nonaqueous electrolytes were used.

Example 12
A nonaqueous electrolyte was prepared in the same manner as in Example 2 except that FEC and VC were not added, and a lithium secondary battery was produced in the same manner as in Example 1 except that these nonaqueous electrolytes were used.

Comparative Example 1
A nonaqueous electrolyte was prepared in the same manner as in Example 1 except that adiponitrile and LiBOB were not added, and a lithium secondary battery was produced in the same manner as in Example 1 except that this nonaqueous electrolyte was used.

Comparative Example 2
A nonaqueous electrolyte was prepared in the same manner as in Example 1 except that LiBOB was not added, and a lithium secondary battery was produced in the same manner as in Example 1 except that this nonaqueous electrolyte was used.

Comparative Example 3
A nonaqueous electrolyte was prepared in the same manner as in Example 1 except that adiponitrile was not added, and a lithium secondary battery was produced in the same manner as in Example 1 except that this nonaqueous electrolyte was used.

Comparative Example 4
A non-aqueous electrolyte was prepared in the same manner as in Example 2 except that PDEA, FEC and VC were not added, and a lithium secondary battery was prepared in the same manner as in Example 1 except that this non-aqueous electrolyte was used.

Comparative Examples 5-9
A nonaqueous electrolyte was prepared in the same manner as in Example 1 except that the addition amount of adiponitrile, LiBOB, PDEA, FEC and VC was changed to the amounts shown in Table 3 and Table 4, and except that these nonaqueous electrolytes were used. A lithium secondary battery was produced in the same manner as in Example 1.

The following evaluations were performed on the lithium secondary batteries of Examples and Comparative Examples.

<Charge / discharge cycle characteristics evaluation>
Regarding the lithium secondary batteries of Examples and Comparative Examples, first, in a 23 ° C. environment, constant current charging was performed up to 4.4 V at a current value of 0.5 C, and subsequently, the current value was 0 at a constant voltage of 4.4 V. The battery was charged to 0.05 C, and then discharged to 2.75 V at a constant current of 0.2 C to determine the initial discharge capacity. Further, each battery was charged at a constant current of up to 4.4 V at a current value of 1 C at 23 ° C., and subsequently charged to a current value of 0.05 C at a constant voltage of 4.4 V, and then a current value of 1 C. A series of operations for discharging to 2.75 V in 1 cycle was made one cycle, and this was repeated many times. Then, the capacity retention rate was calculated by expressing the value obtained by dividing the discharge capacity at the 450th cycle by the initial discharge capacity as a percentage.

Further, for each of the lithium secondary batteries of Examples and Comparative Examples, the initial discharge capacity and the capacity maintenance ratio at the 500th cycle were obtained in the same manner as described above except that the environmental temperature was changed to 45 ° C.

<High temperature storage characteristics evaluation in the discharge state>
About each lithium secondary battery of an Example and a comparative example, it discharged until it became 3.0V with the constant current of 0.1C, and the open circuit voltage (OCV) measurement and the thickness measurement were performed after that. Then, each battery was put in a thermostat kept at 60 ° C. and stored for 30 days. Then, each battery was taken out from the thermostat, and OCV measurement and thickness measurement were performed after 2 hours.

And about OCV, the change amount ((DELTA) V) of OCV was computed by subtracting the value after storage from the value before storage.

Moreover, about the thickness of the battery, the change rate (%) of thickness was computed using the following formula | equation.
Change in thickness (%)
= 100 × thickness after storage ÷ thickness before storage

Tables 1 to 4 show the contents of the nonaqueous electrolyte additives related to the lithium secondary batteries of Examples and Comparative Examples, and Tables 5 and 6 show the evaluation results.

As shown in Tables 1 to 6, a nonaqueous electrolyte containing an appropriate amount of a nitrile compound, an appropriate amount of LiBOB, and an appropriate amount of a compound having a reduction potential at the negative electrode of 1.3 V or less with respect to the metal Li potential The lithium secondary batteries of Examples 1 to 11 using the above had a high capacity retention rate at the time of evaluation of charge / discharge cycle characteristics at both 23 ° C. and 45 ° C., and had good charge / discharge cycle characteristics. In addition, the lithium secondary batteries of Examples 1 and 2 have low OCV reduction and change in thickness (occurrence of blistering) after high temperature storage in a discharged state, and high temperature storage characteristics in a discharged state are good. Met.

On the other hand, the battery of Comparative Example 1 using the nonaqueous electrolyte containing no nitrile compound and LiBOB has inferior charge / discharge cycle characteristics at 23 ° C. and 45 ° C., and after high temperature storage in a discharged state. The amount of decrease in OCV was larger than that of the battery of the example. In addition, the battery of Comparative Example 2 using a non-aqueous electrolyte that does not contain LiBOB and the battery of Comparative Example 7 using a non-aqueous electrolyte with a low LiBOB content are those of OCV after high-temperature storage in a discharged state. The amount of decrease was larger than that of the battery of the example. Furthermore, the battery of Comparative Example 3 using a non-aqueous electrolyte containing no nitrile compound and the battery of Comparative Example 5 using a non-aqueous electrolyte with a low content of nitrile compound are obtained after high-temperature storage in a discharged state. The rate of change of thickness was larger than that of the battery of the example.

Further, the battery of Comparative Example 4 using a nonaqueous electrolyte that does not contain a compound whose reduction potential at the negative electrode is 1.3 V or less with respect to the metal Li potential, and a nonaqueous electrolyte with a high content of the compound are used. The battery of Comparative Example 9 was inferior in charge / discharge cycle characteristics at 23 ° C. and 45 ° C., and the rate of change in thickness after high-temperature storage in a discharged state was larger than that of the battery of the example. Furthermore, the battery of Comparative Example 6 using a non-aqueous electrolyte with a high adiponitrile content is inferior in charge / discharge cycle characteristics at 23 ° C. and 45 ° C., and the decrease in OCV after high-temperature storage in a discharged state is low. It was larger than the battery of the example. In addition, the battery of Comparative Example 8 using a nonaqueous electrolyte with a high LIBOB content had a greater rate of change in thickness after high-temperature storage in the discharged state than the battery of the example.

The present invention can be implemented in other forms as long as it does not depart from the spirit of the present invention. The embodiments disclosed in the present application are examples, and the present invention is not limited to these embodiments. The scope of the present invention is construed in preference to the description of the appended claims rather than the description of the above specification, and all modifications within the scope equivalent to the claims are construed in the scope of the claims. included.

The lithium secondary battery of the present invention is excellent in charge / discharge cycle characteristics and storage characteristics in a discharged state, and can maintain good charge / discharge cycle characteristics even in a thin form in which the nonaqueous electrolytic mass is limited. Therefore, the lithium secondary battery of the present invention is required to have a thin shape and is often left in a discharged state, such as a digital camera, a portable game machine, a mobile phone, a smart phone, etc. that are frequently charged and discharged. It can be preferably used for the same application as various applications to which a conventionally known lithium secondary battery is applied, including a power supply application for portable devices.

1 Positive electrode 2 Negative electrode 3 Separator

Claims

A lithium secondary battery using a positive electrode, a negative electrode, a separator and a non-aqueous electrolyte,
The non-aqueous electrolyte contains a compound having a nitrile group in the molecule, lithium bisoxalate borate, a compound having a reduction potential at the negative electrode of 1.3 V or less with respect to the metal Li potential, and an electrolyte salt.
The content of the compound having a nitrile group in the molecule in the non-aqueous electrolyte is 0.05 to 1.5% by mass,
The content of the lithium bisoxalate borate in the non-aqueous electrolyte is 0.05 to 5% by mass,
The lithium secondary battery, wherein a content of the compound having a reduction potential at the negative electrode of 1.3 V or less with respect to the metal Li potential in the nonaqueous electrolyte is 10% by mass or less.
The non-aqueous electrolyte is represented by the following general formula (1) as a compound having a nitrile group in the molecule.
NC- (CH 2 ) n -CN (1)
[In the general formula (1), n is an integer of 2 to 4]
The lithium secondary battery of Claim 1 containing the compound represented by these.
The nonaqueous electrolyte contains a compound represented by the general formula (1) as a compound having a nitrile group in the molecule and n in the general formula (1) being 3 or 4. 2. The lithium secondary battery according to 2.
The non-aqueous electrolyte is a compound having a reduction potential at the negative electrode of 1.3 V or less with respect to the metal Li potential, such as 2-propynyldiethylphosphonoacetate, 4-fluoro-1,3-dioxolan-2-one, vinylene The lithium secondary battery according to any one of claims 1 to 3, comprising at least one compound selected from the group consisting of carbonate and vinyl ethylene carbonate.
The lithium secondary battery according to claim 4, wherein the non-aqueous electrolyte contains 2-propynyl diethylphosphonoacetate as a compound having a reduction potential at the negative electrode of 1.3 V or less with respect to a metal Li potential.
6. The nonaqueous electrolyte contains 4-fluoro-1,3-dioxolan-2-one as a compound having a reduction potential at the negative electrode of 1.3 V or less with respect to a metal Li potential. The lithium secondary battery as described in.
The lithium secondary battery according to any one of claims 4 to 6, wherein the non-aqueous electrolyte contains vinylene carbonate as a compound having a reduction potential at the negative electrode of 1.3 V or less with respect to a metal Li potential.
The lithium secondary battery according to any one of claims 1 to 7, wherein the non-aqueous electrolyte further contains 1,3-dioxane.
The lithium secondary battery according to any one of claims 1 to 8, wherein the nonaqueous electrolyte contains LiPF 6 as the electrolyte salt.