JP2015111552A - Lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high-capacity nonaqueous secondary battery superior in high-temperature cycle characteristics, storage suitability, and reliability.SOLUTION: A lithium secondary battery comprises: a positive electrode; a negative electrode; a separator; and a nonaqueous electrolyte. The nonaqueous electrolyte includes: a compound having a nitrile group in its molecule; lithium tetrafluoroborate; and an electrolytic salt. In the nonaqueous electrolyte, the content of the compound having a nitrile group in its molecule is 0.05-5.0 mass%, and the content of the lithium tetrafluoroborate is 0.05-0.70 mass%.

Description

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

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.

And lithium secondary battery is calculated | required to improve various battery characteristics with the breadth of the application apparatus.

  One means for improving the battery characteristics of a lithium secondary battery is to improve the nonaqueous 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. Have been described.

  Further, Patent Document 2 discloses a secondary battery in which the non-aqueous electrolyte contains a nitrile compound and an S═O group-containing compound and thus has excellent battery cycle characteristics, electric capacity, storage characteristics, and the like. .

JP 2011-222473 A JP 2004-179146 A

Lithium secondary batteries are required to have good cycle characteristics that maintain capacity even after repeated charge and discharge, but storage characteristics and heating can be improved by adding nitrile compounds such as dinitrile compounds to non-aqueous electrolytes. While the characteristics and the high temperature cycle are improved, the cycle characteristics particularly at room temperature are deteriorated.

In addition, if the lithium secondary battery is discharged after use with a mobile phone or the like and left in a place where the temperature is high, such as in a car, for a long time, the battery may swell.

  Furthermore, there is a known method of increasing the discharge capacity of the battery by increasing the upper limit voltage for charging the battery. However, under a high voltage, the battery is placed in a harsher situation compared to the conventional case, and there are problems caused thereby. is there.

As described above, improvement in cycle characteristics and storage characteristics in a discharged state is required.

This invention is made | formed in view of the said situation, The objective is to provide the high capacity | capacitance and the non-aqueous secondary battery excellent in the cycling characteristics and the storage characteristics of the discharge 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, wherein the non-aqueous electrolyte is lithium borofluoride, the following general formula (1 ) Each containing a predetermined amount of a compound having a nitrile group in the molecule.
NC-R-CN (1)
[In General Formula (1), R is a linear or branched hydrocarbon difference chain having 1 to 10 carbon atoms. ]

INDUSTRIAL APPLICABILITY The present invention can provide a non-aqueous secondary battery having high capacity and excellent high-temperature cycle characteristics, storage characteristics, and reliability.

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.

In the lithium secondary battery of the present invention, for example, a solution (non-aqueous electrolyte) in which a lithium salt is dissolved in an organic solvent, lithium borofluoride, and a nitrile group in the molecule represented by the general formula (1) The nonaqueous electrolyte containing the compound which has is used.

As described above, using a compound having a nitrile group in the molecule as a non-aqueous electrolyte additive contributes to improving the reliability of the lithium secondary battery, while reducing the cycle characteristics at room temperature.

In the present invention, a non-aqueous electrolyte to which a compound having a nitrile group in the molecule and lithium borofluoride are added is used.

A lithium borofluoride-derived film is formed on the negative electrode surface by adding lithium borofluoride to the non-aqueous electrolyte. Since the coating derived from lithium borofluoride formed on the surface of the negative electrode is strong and has lithium ion conductivity, the battery effectively suppresses the decomposition reaction of the non-aqueous electrolyte component on the negative electrode surface. The reaction is not inhibited.

As described above, since the lithium borofluoride-derived coating is strong, the decomposition reaction of the nonaqueous electrolyte component in the negative electrode can be suppressed over a long period of time even after repeated charging and discharging, so that excellent charge / discharge cycle characteristics are ensured. Can do.

Further, when the non-aqueous secondary battery is stored in a discharged state at a high temperature for a long time, it is likely to be in an overdischarged state. The battery in such a state generates gas and causes swelling, but when a non-aqueous electrolyte to which a compound having a nitrile group in the molecule and lithium borofluoride are used, the occurrence of such swelling can be suppressed. .

Furthermore, in this invention, content of the said lithium borofluoride in nonaqueous electrolyte is limited to 0.05-0.70 mass%. When lithium borofluoride is added in a large amount, it may contribute to swell in storage (charged state) at a high temperature for a long period of time, or elution of metal in the positive electrode active material when charged for a long time to maintain a constant voltage. Therefore, by limiting the amount of lithium borofluoride, cycle characteristics can be improved, and deterioration of storage characteristics and metal elution can be prevented in a charged state.
Further, when the amount of lithium borofluoride is limited and used in combination with a compound having a nitrile group in the molecule, the effect of preventing metal elution is synergistically improved.

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.

In the lithium secondary battery of the present invention, for example, a solution (non-aqueous electrolyte) in which a lithium salt is dissolved in an organic solvent, lithium borofluoride as an additive, and nitrile in the molecule represented by the general formula (1) A nonaqueous electrolyte containing a compound having a group is used.

A compound having a nitrile group in the molecule is adsorbed on the positive electrode in the lithium secondary battery and can form a film to suppress elution of transition metal into the non-aqueous electrolyte under high voltage. Therefore, the lithium secondary battery of the present invention can be stably used under high voltage by adding a compound having a nitrile group in the molecule to the non-aqueous electrolyte.

A compound having a nitrile group in the molecule represented by the general formula (1) has a carbon number of 1 to 10 in consideration of an increase in viscosity. For example, malononitrile, succinonitrile, glutaronitrile, adiponitrile, 1 , 4-dicyanoheptane, 1,5-dicyanopentane, 1,6-dicyanohexane, 1,7-dicyanoheptane, 2,6-dicyanoheptane, 1,8-dicyanooctane, 2,7-dicyanooctane, 1, 9-dicyanononane, 2,8-dicyanononane, 1,10-dicyanodecane, 1,6-dicyanodecane, 2,4-dimethylglutaronitrile and the like may be used, and only one of these may be used, Two or more kinds may be used in combination.

Of the above-exemplified nitrile compounds, dinitrile compounds are more preferable, and among them, adiponitrile having a good effect of improving cycle characteristics is more preferable.

The content of the compound having a nitrile group in the molecule in the non-aqueous electrolyte used in the battery is required to be 0.05% by mass or more from the viewpoint of more effectively exerting the action due to the use of these compounds. The content is preferably 0.1% by mass or more, and more preferably 0.2% by mass or more. However, if the amount of the compound having a nitrile group in the molecule is too large, for example, the storage characteristics in the charged state of the battery are further improved, but the charge / discharge cycle characteristics at room temperature are reduced, or in the discharged state. There is a risk of contributing to swelling during storage. Therefore, the content of the compound having a nitrile group in the molecule in the non-aqueous electrolyte used for the battery is 5% by mass or less, and more preferably 2% by mass or less.

The content of lithium borofluoride is 0.05% by mass or more, more preferably 0.1% by mass or more from the viewpoint of obtaining the above-described effect. Moreover, it is 0.70 mass% or less, More preferably, it is 0.60 mass% or less.

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 , LiAsF ₆ , LiSbF ₆ ; LiN (FSO ₂ ) ₂ , LiC ₄ BO ₈ , 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] Organic lithium salt; etc. can be used.

The concentration of the lithium salt in the nonaqueous 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, propylene carbonate, butylene carbonate; chain carbonates such as dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate; chain esters such as methyl propionate; cyclic esters such as γ-butyrolactone; dimethoxyethane; Chain ethers such as diethyl ether, 1,3-dioxolane, diglyme, triglyme and tetraglyme; cyclic ethers such as 1,4-dioxane, tetrahydrofuran and 2-methyltetrahydrofuran; sulfites such as ethylene glycol sulfite; These may be used, and two or more of these may be used in combination. 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.

In addition, by including 1,3-dioxane in the non-aqueous electrolyte used in the lithium secondary battery, a film is formed on the negative electrode surface together with lithium borofluoride, and the charge / discharge cycle characteristics of the lithium secondary battery are deteriorated. Can be suppressed.

Furthermore, the nonaqueous electrolyte containing 1,3-dioxane and the compound having a nitrile group in the molecule also acts on the positive electrode surface. As described above, a compound having a nitrile group forms a film on the positive electrode, but 1,3-dioxane forms a film on the positive electrode before the compound having the nitrile group forms a film on the positive electrode. . Thereby, the elution of a transition metal can be suppressed. Further, the reaction between the positive electrode and the non-aqueous electrolyte can be suppressed, so that the decomposition of the electrolyte can be prevented and the generation of the resistance component can be suppressed.
However, under severe conditions such as high temperatures and high voltages, the positive electrode in a charged state has a strong oxidizing power, and the coating on the positive electrode derived from 1,3-dioxane may be destroyed. In that case, since the coating film is formed again on the positive electrode by the compound having a nitrile group remaining in the electrolyte, the elution of the transition metal and the decomposition of the electrolyte at the positive electrode can be suppressed.

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 ensuring the above-mentioned effects by using the better. More preferably, it is 0.5 mass% or more. On the other hand, if the amount of 1,3-dioxane in the nonaqueous electrolyte is too large, the load characteristics and charge / discharge cycle characteristics of the battery may be degraded. 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.

Further, by containing a phosphonoacetate compound represented by the following general formula (2) in the nonaqueous electrolyte, a film is formed on the negative electrode surface of the lithium secondary battery together with lithium borofluoride, and the negative electrode active material is Degradation of the water electrolyte can be suppressed.
Furthermore, when the nonaqueous electrolyte contains lithium borofluoride, a phosphonoacetate compound and 1,3-dioxane, the cycle characteristics are further improved.

［一般式（２）中、Ｒ^１、Ｒ^２およびＲ^３は、それぞれ独立して、ハロゲン原子で置換されていてもよい炭素数１〜１２のアルキル基、アルケニル基又はアルキニル基を示し、ｎは０〜６の整数を示す。］

[In General Formula (2), R ¹ , R ² and R ³ each independently represent an alkyl group, alkenyl group or alkynyl group having 1 to 12 carbon atoms which may be substituted with a halogen atom, and n Represents an integer of 0-6. ]

Specific examples of the phosphonoacetate compound represented by the general formula (2) include the following.

<Compound wherein n = 0 in the general formula (2)>
Trimethyl phosphonoformate, methyl diethyl phosphonoformate, methyl dipropyl phosphonoformate, methyl dibutyl phosphonoformate, triethyl phosphonoformate, ethyl dimethyl phosphonoformate, ethyl dipropyl phosphonoformate, ethyl Dibutylphosphonoformate, tripropylphosphonoformate, propyl dimethylphosphonoformate, propyldiethylphosphonoformate, propyl dibutylphosphonoformate, tributylphosphonoformate, butyldimethylphosphonoformate, butyldiethylphospho Noformate, butyl dipropylphosphonoformate, methyl bis (2,2,2-trifluoroethyl) phosphonoformate, ethyl bis (2,2 2-trifluoroethyl) phosphonoacetate formate, propyl bis (2,2,2-trifluoroethyl) phosphonoacetate formate, etc. butyl bis (2,2,2-trifluoroethyl) phosphonoacetate formate.

<Compound with n = 1 in the general formula (2)>
Trimethyl phosphonoacetate, methyl diethylphosphonoacetate, methyldipropylphosphonoacetate, methyldibutylphosphonoacetate, triethylphosphonoacetate, ethyldimethylphosphonoacetate, ethyldipropylphosphonoacetate, ethyldibutylphosphonoacetate, tripropyl Phosphonoacetate, propyl dimethylphosphonoacetate, propyldiethylphosphonoacetate, propyldibutylphosphonoacetate, tributylphosphonoacetate, butyldimethylphosphonoacetate, butyldiethylphosphonoacetate, butyldipropylphosphonoacetate, methylbis (2 , 2,2-trifluoroethyl) phosphonoacetate, ethyl bis (2,2,2-trifluoroethyl) phosphonoa Cetate, propyl bis (2,2,2-trifluoroethyl) phosphonoacetate, butyl bis (2,2,2-trifluoroethyl) phosphonoacetate, allyl dimethylphosphonoacetate, allyldiethylphosphonoacetate, 2- Propinyl dimethylphosphonoacetate, 2-propynyl diethylphosphonoacetate and the like.

<Compound wherein n = 2 in the general formula (2)>
Trimethyl 3-phosphonopropionate, methyl 3- (diethylphosphono) propionate, methyl 3- (dipropylphosphono) propionate, methyl 3- (dibutylphosphono) propionate, triethyl 3-phosphonopropionate, ethyl 3- (dimethylphosphono) propionate, ethyl 3- (dipropylphosphono) propionate, ethyl 3- (dibutylphosphono) propionate, tripropyl 3-phosphonopropionate, propyl 3- (dimethylphosphono) propionate, Propyl 3- (diethylphosphono) propionate, propyl 3- (dibutylphosphono) propionate, tributyl 3-phosphonopropionate, butyl 3- (dimethylphosphono) propionate, butyl 3- (diethylphosphono) propionate Onate, butyl 3- (dipropylphosphono) propionate, methyl 3- (bis (2,2,2-trifluoroethyl) phosphono) propionate, ethyl 3- (bis (2,2,2-trifluoroethyl) phosphono ) Propionate, propyl 3- (bis (2,2,2-trifluoroethyl) phosphono) propionate, butyl 3- (bis (2,2,2-trifluoroethyl) phosphono) propionate, and the like.

<Compound wherein n = 3 in the general formula (2)>
Trimethyl 4-phosphonobutyrate, methyl 4- (diethylphosphono) butyrate, methyl 4- (dipropylphosphono) butyrate, methyl 4- (dibutylphosphono) butyrate, triethyl 4-phosphonobutyrate, ethyl 4- (Dimethylphosphono) butyrate, ethyl 4- (dipropylphosphono) butyrate, ethyl 4- (dibutylphosphono) butyrate, tripropyl 4-phosphonobutyrate, propyl 4- (dimethylphosphono) butyrate, propyl 4- (Diethylphosphono) butyrate, propyl dibutylphosphono) butyrate, tributyl 4-phosphonobutyrate, butyl 4- (dimethylphosphono) butyrate, butyl 4- (diethylphosphono) butyrate, butyl 4- (dipropylphosphono) ) Butyrate etc.

Among the phosphonoacetate compounds, 2-propynyl diethylphosphonoacetate (PDEA) and ethyl diethylphosphonoacetate (EDPA) are preferably used.

The content of the phosphonoacetate compound in the non-aqueous electrolyte used in the lithium secondary battery is preferably 0.1% by mass or more from the viewpoint of ensuring better the effect of the use, The content is preferably at least mass%, more preferably at least 1 mass%. However, if the content of the phosphonoacetate compound in the non-aqueous electrolyte is too large, the charge / discharge cycle characteristics of the battery may be deteriorated. Therefore, the content of the phosphonoacetate compound in the non-aqueous electrolyte used for the lithium secondary battery is preferably 5.0% by mass or less.

In addition, deterioration of cycle characteristics can be suppressed by adding vinylene carbonate, 4-fluoro-1,3-dioxolan-2-one to the electrolyte used in the lithium secondary battery.

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, cyclohexyl benzene, biphenyl, fluorobenzene, t-butylbenzene and other additives (including these derivatives) can also 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 according to 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 additive and the like on one side or both sides of a current collector can be used.

The positive electrode active material includes a lithium-containing transition metal oxide having a layered structure represented by Li _{1 + x} MO ₂ (−0.1 <x <0.1, M: Co, Ni, Mn, Al, Mg, etc.); LiMn ₂ O ₄ , Li _4/3 Ti _5/3 O ₄ and other spinel-structure lithium-containing composite oxides; LiMPO ₄ (M: Co, Ni, Mn, Fe, etc.) olivine type compounds; 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. Specific examples of the lithium-containing transition metal oxide having a layered structure include LiCoO ₂ and the like, and oxides containing at least Co, Ni, and Mn (LiMn _1/3 Ni _1/3 Co _1/3 O ₂ , LiNi _{0 .5} Co _0.3 Mn _0.2 O ₂ , LiNi _0.85 Co _0.10 Al _0.05 O ₂ , etc.).

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, Carbon materials such as carbon black such as lamp black and thermal black; carbon fiber;

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.

Moreover, you may form the lead body for electrically connecting with the other member in a lithium secondary battery according to a conventional method as needed to a positive electrode.

The thickness of the positive electrode mixture layer is preferably, for example, 10 to 100 μm per one side of the current collector. Moreover, as a composition of a positive mix layer, it is preferable that the quantity of a positive electrode active material is 60-98 mass%, for example, it is preferable that the quantity of a binder is 1-15 mass%, and the quantity of a conductive support agent. Is preferably 1 to 20% by mass.

The positive electrode current collector can be the same as that used for the positive electrode of a conventionally known lithium secondary battery. 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.

Examples of the Si alloy include SiSn, SiCu, SiCr, and SiTi. The oxide of Si can be represented by SiOx.

SiOx may contain Si microcrystal or amorphous phase. In this case, the atomic ratio of Si and O is a ratio including Si microcrystal or amorphous phase Si. That is, the SiOx includes a structure in which Si (for example, microcrystalline Si) is dispersed in an amorphous SiO2 matrix, and the amorphous SiO2 and Si dispersed in the amorphous SiO2 matrix. In addition, it is sufficient that the atomic ratio x satisfies 0.5 ≦ x ≦ 1.5. For example, in the case of a material in which Si is dispersed in an amorphous SiO 2 matrix and the material has a molar ratio of SiO 2 to Si of 1: 1, x = 1, so that the structural formula is represented by SiO. In the case of a material having such a structure, for example, in X-ray diffraction analysis, a peak due to the presence of Si (microcrystalline Si) may not be observed, but when observed with a transmission electron microscope, the presence of fine Si Can be confirmed.

SiOx can be used as a composite compounded with a carbon material. For example, the surface of SiOx is preferably covered with a carbon material. Since SiOx has poor conductivity, when using it as a negative electrode active material, from the viewpoint of ensuring good battery characteristics, a conductive material (conductive aid) is used, and the SiOx and conductive material in the negative electrode are used. There is a need for good mixing and dispersion to form an excellent conductive network. If the composite is a composite of SiOx and a carbon material, for example, a conductive network in the negative electrode is formed better than when a material obtained by simply mixing SiOx and a conductive material such as a carbon material is used. Is done.

When a composite of SiOx and carbon material is used for the negative electrode, the ratio of SiOx and carbon material is carbon from SiOx: 100 parts by mass from the viewpoint of satisfactorily exerting the effect of the composite with the carbon material. The material is preferably 5 parts by mass or more, and more preferably 10 parts by mass or more. In the composite, if the ratio of the carbon material to be combined with SiOx is too large, the amount of SiOx in the negative electrode mixture layer may be reduced, and the effect of increasing the capacity may be reduced. The carbon material is preferably 50 parts by mass or less and more preferably 40 parts by mass or less with respect to 100 parts by mass.

As the negative electrode active material, it is preferable to use graphite together with Si, Si alloy or SiOx. Graphite is widely used as a negative electrode active material for non-aqueous electrolyte secondary batteries, and has a relatively large capacity, while its volume change accompanying charging / discharging of the battery is smaller than that of the high capacity negative electrode material. Therefore, by using the high-capacity negative electrode material and graphite in combination with the negative electrode active material, the effect of improving the capacity of the battery is reduced as the amount of the high-capacity negative electrode material used is reduced. The volume change amount of the mixture layer can be suppressed as much as possible.

As the graphite used as the negative electrode active material, for example, natural graphite such as flaky graphite; graphitized carbon such as pyrolytic carbons, mesophase carbon microbeads (MCMB), and carbon fiber was graphitized at 2800 ° C. or higher. Artificial graphite; and the like.

Content of Si, Si alloy or SiOx in all negative electrode active materials (when only one of them is used, this is the amount, and when using two or more of these, the total amount thereof) Is preferably 0.01% by mass or more, more preferably 1% by mass or more, and more preferably 3% by mass or more from the viewpoint of favorably securing the effect of increasing the capacity. More preferred. Further, from the viewpoint of better avoiding the problem due to the volume change of Si, Si alloy or SiOx accompanying charge / discharge, the content of Si, Si alloy or SiOx in the entire negative electrode active material is 20% by mass or less. Is preferable, and it is more preferable that it is 15 mass% or less.

As the negative electrode active material, in addition to SiOx, Si, Si alloy and graphite, other active materials may be used together with these active materials. Examples of such other active materials include pyrolytic carbons, cokes, glassy carbons, fired bodies of organic polymer compounds, MCMB, carbon fibers, activated carbon, metals that can be alloyed with lithium (Sn, etc. ) Or an alloy or oxide thereof. However, the amount of these other active materials used is preferably 10% by mass or less in the total negative electrode active material.

Moreover, the same thing as what was illustrated previously as what can be used for a positive electrode can be used for the binder and conductive support agent of a negative electrode.

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.

Moreover, you may form in the negative electrode the lead body for electrically connecting with the other member in a lithium secondary battery according to a conventional method as needed.

The thickness of the negative electrode mixture layer is preferably 10 to 100 μm per one surface of the current collector, for example. Moreover, as a composition of a negative mix layer, it is preferable that a negative electrode active material shall be 80.0-99.8 mass% and a binder shall be 0.1-10 mass%, for example. Furthermore, when making a negative mix layer contain a conductive support agent, it is preferable that the quantity of the conductive support agent in a negative mix layer shall be 0.1-10 mass%.

  As the current collector for the negative electrode, a foil made of copper or nickel, 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. Furthermore, you may use the separator which has resin porous layer (I) for ensuring a shutdown characteristic, and heat resistant porous layer (II) for improving the heat resistance of a separator. For example, the resin porous layer (I) uses a polyolefin microporous film such as polyethylene or polypropylene, and the heat-resistant porous layer (II) includes inorganic fine particles having a heat-resistant temperature of 150 ° C. or more bound with a binder. The resin porous layer (I) can be laminated.

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 battery of the invention.

Examples of the form of the lithium secondary battery of the present invention include a cylindrical shape (such as a rectangular tube 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.

The lithium secondary battery of the present invention is used by setting the upper limit voltage of charging to 4.3 V or higher, thereby achieving high capacity and high reliability even when used at such a high voltage. And exhibit storage properties. In addition, it is preferable that the upper limit voltage of charge in the lithium secondary battery of this invention is 4.7V or less.

The lithium secondary battery of the present invention can be used for the same applications as various applications to which conventionally known lithium secondary batteries are applied.

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>
In a mixed solvent of ethylene carbonate and diethyl carbonate in a volume ratio of 3: 7, LiPF ₆ is dissolved at a concentration of 1.1 mol / l, adiponitrile is 0.5 mass%, and lithium borofluoride is 0.3 mass. A nonaqueous electrolyte was prepared by adding VC in an amount of 2.75% by mass and FEC in an amount of 1.75% by mass.

<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 an outer package of the battery. 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.

And this cover plate 9 is inserted in the opening part of the battery case 4, and the opening part of the battery case 4 is sealed by welding the joint part of both, 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.

Example 2
An amount of 0.5% by mass of adiponitrile, an amount of 0.3% by mass of lithium borofluoride, an amount of 2.75% by mass of VC, and an amount of 1.75% by mass of FEC The nonaqueous electrolyte was prepared in the same manner as in Example 1 except that 1,3-dioxane was added in an amount of 0.75% by mass, and the same procedure as in Example 1 was performed except that this nonaqueous electrolyte was used. Thus, a lithium secondary battery was produced.

Example 3
An amount of 0.5% by mass of adiponitrile, an amount of 0.3% by mass of lithium borofluoride, an amount of 2.75% by mass of VC, and an amount of 1.75% by mass of FEC A non-aqueous electrolyte was prepared in the same manner as in Example 1 except that 0.75% by mass of 1,3-dioxane and 1.25% by mass of PDEA were added. Example 1 except that this non-aqueous electrolyte was used. In the same manner, a lithium secondary battery was produced.

Examples 4 to 13 and Comparative Examples 1 to 8
A non-aqueous electrolyte was prepared as shown in Table 1, and a lithium secondary battery was prepared in the same manner as in Example 1 except that this non-aqueous electrolyte was used.
The following evaluation was performed about the lithium secondary battery of the Example and the comparative example.

<Charge / discharge cycle characteristics evaluation>
For the lithium secondary batteries of Examples and Comparative Examples, first, constant current charging was performed up to 4.35 V at a current value of 0.5 C in an environment of 23 ° C., followed by constant voltage charging at 4.35 V (constant current). The total charge time of charging and constant voltage charging was 5 hours), and then discharging was performed at 2.75 V with a constant current of 0.2 C to obtain the initial discharge capacity. Next, each battery was charged at a constant current up to 4.35V at a current value of 1C at 23 ° C. and subsequently charged at a constant voltage of 4.35V until the current value reached 0.05C. A series of operations for discharging to a value of 3.0 V was taken as one cycle, and this was repeated many times. Each battery was subjected to 500 cycles, and a constant current-constant voltage charge and a constant current discharge were performed under the same conditions as in the initial discharge capacity measurement, to determine a discharge capacity. Then, the capacity retention rate was calculated by expressing the value obtained by dividing these discharge capacities by the initial discharge capacity as a percentage. Thereafter, charge and discharge were performed until the capacity retention rate reached 50%.

For each of the lithium secondary batteries of Examples and Comparative Examples, the initial discharge capacity measurement, the charge / discharge cycle, and the discharge capacity measurement were performed in the same manner as described above except that the environmental temperature was changed to 45 ° C. The value divided by the initial discharge capacity was expressed as a percentage, and the capacity maintenance rate was calculated. Thereafter, charge and discharge were performed until the capacity retention rate reached 50%.

<High temperature storage characteristics evaluation in the discharge state>
About each lithium secondary battery of the Example and the comparative example, it discharged until it became 2.75V with the constant current of 0.1C, and the impedance measurement and the thickness measurement were performed after that. Thereafter, each battery was placed in a thermostat kept at 60 ° C. and stored for 20 days. Then, each battery was taken out from the thermostat, and impedance measurement and thickness measurement were performed after 2 hours.
And about the impedance, the value before storage was subtracted from the value after storage, and the amount of change in impedance (Ω) was calculated.
Moreover, about the thickness of the battery, the change rate (%) of thickness was computed using the following formula | equation.
Change in thickness (%)
= 100 x thickness after storage ÷ thickness before storage -100

<1C discharge load characteristic evaluation>
The lithium secondary batteries of Examples and Comparative Examples were charged at a constant current up to 4.35 V at a current value of 0.5 C, subsequently charged at a constant voltage of 4.35 V, and then discharged at 0.2 C. Thereafter, the battery was charged at a constant current up to 4.35 V at a current value of 0.5 C, subsequently charged at a constant voltage of 4.35 V, and then discharged at 1 C. The rate characteristic was calculated by expressing the value obtained by dividing the discharge capacity at 1 C by the discharge capacity at 0.2 C as a percentage.

<Float charging evaluation>
About each lithium secondary battery of an Example and a comparative example, after carrying out the constant current charge to 4.35V with the electric current value of 1.0C in the environment of 60 degreeC, the constant voltage charge was performed with the voltage of 4.4V. The time until the current value increased was measured while continuing constant voltage charging. Specifically, after the current value in the constant voltage charging region was minimized, it was determined that the current value had increased by 1.5 mA or more.

Table 1 shows the contents of the nonaqueous electrolyte additives related to the lithium secondary batteries of Examples and Comparative Examples, and Tables 2 and 3 show the evaluation results.

As shown in Table 1 and Table 2, the lithium secondary battery of Example 1 using a nonaqueous electrolyte containing adiponitrile and lithium borofluoride has 500 cycles at a capacity of 45 ° C. at the time of charge / discharge cycle characteristics evaluation. The subsequent capacity retention rate had good charge / discharge cycle characteristics. Moreover, when 1,3-dioxane was added, it became more favorable, and when PDEA was added, even better cycle characteristics were shown. In addition, the lithium secondary batteries of Examples 1 and 2 have suppressed impedance increase and change in thickness (occurrence of swelling) after high temperature storage in a discharged state, and have high temperature storage characteristics in a discharged state. It was good. And when it was up to 0.7 mass% in the non-aqueous electrolyte of lithium borofluoride, the float test showed good characteristics.

In the battery of Comparative Example 1 not containing lithium borofluoride and the battery of Comparative Example 4 using a non-aqueous electrolyte containing 0.3% by mass of lithium borofluoride and 7.0% of adiponitrile, Although the cycle characteristics are good, the cycle characteristics at room temperature are lower than the result of Comparative Example 5 in which adiponitrile and lithium borofluoride are not added, and after high-temperature storage in a discharged state, the amount of increase in impedance is It was larger than the battery of the example. This is presumably because the protective film of the negative electrode is not strong or the amount of adiponitrile added is large, so that adiponitrile decomposes at the negative electrode and becomes a resistance component.

Although the battery of Comparative Example 2 to which no adiponitrile was added was an effect of lithium borofluoride, the cycle characteristics at room temperature and 45 ° C. were improved as compared with the battery of Comparative Example 5 not containing adiponitrile and lithium borofluoride. Compared with the examples, the cycle characteristics at 45 ° C. are inferior.

In addition, the battery of Comparative Example 3 using a nonaqueous electrolyte containing 4.0% lithium borofluoride has poor cycle characteristics but poor storage characteristics and 1C rate characteristics.

The battery of Comparative Example 5 using a non-aqueous electrolyte containing no adiponitrile and lithium borofluoride has inferior charge / discharge cycle characteristics at 45 ° C. compared to the Examples. The increase amount and the rate of change in thickness were larger than those of the batteries of the examples.

Further, Comparative Example 6 using a nonaqueous electrolyte containing 0.8% by mass of lithium borofluoride was inferior in the float test result.

1 Positive electrode 2 Negative electrode 3 Separator

Claims (10)

A lithium secondary battery using a positive electrode, a negative electrode, a separator, and a nonaqueous electrolyte, wherein the nonaqueous electrolyte contains lithium borofluoride and a compound having a nitrile group in a molecule represented by the following general formula (1) And
The content of the lithium borofluoride in the non-aqueous electrolyte is 0.05 to 0.70 mass%,
Content of the compound which has a nitrile group in the said molecule | numerator in the said nonaqueous electrolyte is 0.05-5.0 mass%, The lithium secondary battery characterized by the above-mentioned.
NC-R-CN (1)
[In General Formula (1), R is a linear or branched hydrocarbon chain having 1 to 10 carbon atoms. ] The lithium secondary battery according to claim 1, wherein the non-aqueous electrolyte contains LiPF ₆ as an electrolyte salt. The lithium secondary battery according to any one of claims 1 to 2, wherein a nonaqueous electrolyte having a content of the compound having a nitrile group of 0.05 to 2.0 mass% is used. The lithium secondary battery according to any one of claims 1 to 3, wherein a nonaqueous electrolyte having a content of the lithium borofluoride of 0.05 to 0.60 mass% is used. The lithium secondary battery according to claim 1, wherein a nonaqueous electrolyte containing 1,3-dioxane is used. The lithium secondary battery according to claim 5, wherein non-aqueous electrolytic non-water having a content of 1,3-dioxane of 0.1 to 5.0% by mass is used. The lithium secondary battery according to any one of claims 1 to 6, wherein the non-aqueous electrolyte contains a phosphonoacetate compound represented by the following general formula (2).

[In General Formula (2), R ¹ , R ² and R ³ each independently represent an alkyl group, alkenyl group or alkynyl group having 1 to 12 carbon atoms which may be substituted with a halogen atom, and n Represents an integer of 0-6. ]
The lithium secondary battery according to claim 7, wherein a nonaqueous electrolytic solution having a content of the phosphonoacetate compound represented by the general formula (2) of 0.1 to 5.0% by mass is used. The lithium secondary battery according to claim 1, wherein the nonaqueous electrolyte contains vinylene carbonate. The lithium secondary battery according to any one of claims 1 to 9, wherein the nonaqueous electrolyte contains 4-fluoro-1,3-dioxolan-2-one.

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