CN108199035B - Lithium secondary battery - Google Patents

Abstract

The invention provides a lithium secondary battery which can be stably used even if continuously charged at a high voltage of 4.3V or more. The lithium secondary battery comprises a lithium secondary battery having a composition represented by the general formula Li_{1+ y}Ni_{1‑a‑b‑c}Co_aMn_bM¹ _cO₂The lithium nickel composite oxide represented by the general formula Li_1+oCo_1‑p‑qMg_pM² _qO₂The lithium cobalt composite oxide is used as a positive electrode of a positive electrode active material, a negative electrode containing a material containing Si and O in constituent elements, a non-aqueous electrolyte containing a compound having a nitrile group in a molecule, and a separator.

Description

Lithium secondary battery

The application is a divisional application of an invention application with an application date of 2012, 12 and 11, an application number of 201280059747.3 and an invention name of lithium secondary battery.

Technical Field

The present invention relates to a lithium secondary battery which can be stably used even when continuously charged at a high voltage of 4.3V or more.

Background

In recent years, with the development of portable electronic devices such as cellular phones and notebook personal computers, and the practical use of electric vehicles, small-sized, lightweight, and high-capacity lithium secondary batteries have become necessary.

In a lithium secondary battery, LiCoO is generally used as a positive electrode active material₂、LiMnO₂Such lithium-containing composite oxides, and further, for the purpose of increasing the capacity of lithium secondary batteries, studies have been made to use a material having a structure represented by SiO in the negative electrode active material in addition to a graphitic carbon material or the like₂SiO having a structure in which Si is dispersed in ultrafine particles_x(for example, patent documents 1 to 3).

In order to increase the capacity of a lithium secondary battery, a method of increasing the charging voltage may be considered in addition to using an active material having a large capacity. For example, LiCoO is now widely used₂In a lithium secondary battery as a positive electrode active material, an upper limit value of a charging voltage is generally set to about 4.2V, and the above-described method is a method of increasing the capacity by increasing the charging voltage to be higher than the above-described value.

In addition, improvement of characteristics of the lithium secondary battery has been also being carried out by incorporating various additives in the nonaqueous electrolyte. For example, patent document 4 discloses a lithium secondary battery having improved charge-discharge cycle characteristics and the like by including a compound having 2 or more nitrile groups in the molecule in a nonaqueous electrolyte.

Documents of the prior art

Patent document

Patent document 1 Japanese patent laid-open publication No. 2004-047404

Patent document 2 Japanese laid-open patent publication No. 2005-259697

Patent document 3 Japanese laid-open patent publication No. 2008-210618

Patent document 4, Japanese patent laid-open No. 2008-108586

Disclosure of Invention

Problems to be solved by the invention

In the portable electronic device, continuous charging is sometimes performed continuously in actual use, for example, the device is left for a long time in a charged state, and the device is used while being charged.

In such a situation of use in which continuous charging is continuously performed, ions of transition metal elements such as Co and Mn are eluted from the lithium-containing composite oxide as the positive electrode active material into the nonaqueous electrolyte, thereby causing deterioration of the positive electrode active material and impairing battery characteristics, and if the amount of eluted ions is very large, internal short-circuiting of the battery occurs.

According to the studies of the present inventors, it was revealed that these problems caused by the deterioration of the positive electrode active material due to the continuous charging of the battery use SiO in the negative electrode active material_xSuch a high-capacity material is remarkably generated when the charging voltage is higher than the conventional charging voltage.

Therefore, in particular, in a lithium secondary battery used for a power source of a portable electronic device, when a high capacity is achieved by increasing a charging voltage using a negative electrode active material having a large capacity, it is necessary to improve stability when continuous charging is performed.

The present invention has been made in view of the above circumstances, and an object thereof is to provide a lithium secondary battery that can be stably used even when continuous charging is continuously performed at a high voltage of 4.3V or more.

Means for solving the problems

The lithium secondary battery of the present invention that can achieve the above object is characterized by comprising a positive electrode having a positive electrode mixture layer containing a positive electrode active material on one or both surfaces of a current collector, a negative electrode having a negative electrode mixture layer containing a negative electrode active material on one or both surfaces of a current collector, a nonaqueous electrolyte, and a separator, wherein the positive electrode mixture layer of the positive electrode contains a composition represented by the general formula Li_{1+ y}Ni_1-a-b-cCo_aMn_bM¹ _cO₂(wherein, M¹Is at least 1 element selected from the group consisting of Mg, Al, Ti, Fe, Cu, Zn, Ga, Ge, Zr, Nb, Mo, Sn, Sr, W, B, P and Bi, -0.15 ≦ y ≦ 0.15, 0.05 ≦ a ≦ 0.3, 0.05 ≦ B ≦ 0.3, 0 ≦ c ≦ 0.03 anda + b + c ≦ 0.5), the negative electrode mix layer of the negative electrode contains, as a negative electrode active material, a material containing Si and O in constituent elements (wherein the atomic ratio x of O to Si is 0.5 ≦ x ≦ 1.5), and the nonaqueous electrolyte contains a compound having a nitrile group in a molecule.

ADVANTAGEOUS EFFECTS OF INVENTION

According to the present invention, it is possible to provide a lithium secondary battery that can be stably used even when continuous charging is continuously performed at a high voltage of 4.3V or more.

Drawings

Fig. 1 is a schematic view showing an example of a lithium secondary battery according to the present invention, wherein (a) is a plan view thereof, and (b) is a partial vertical sectional view thereof.

Fig. 2 is a perspective view of the lithium secondary battery shown in fig. 1.

Detailed Description

As described above, if SiO is used by the negative electrode active material in the lithium secondary battery_xWhen continuous charging is continued as described above, ions of the transition metal element derived from the positive electrode active material are eluted into the nonaqueous electrolyte, and the battery characteristics are significantly impaired. This is believed to be due to: SiO is present in the negative electrode surface as ions of the transition metal element eluted from the positive electrode active material_xCauses SiO to be deposited selectively, which is responsible for most of the capacity of the negative electrode_xIs not needed.

In the lithium secondary battery of the present invention, SiO is used as the negative electrode active material_xIn particular, when the charging voltage is set to 4.3V or more, the elution of ions of the transition metal element from the positive electrode active material into the nonaqueous electrolyte, which is generated when continuous charging is continuously performed as described above, is suppressed by the following method: a lithium-containing composite oxide having a composition in which elution of these ions is less likely to occur is used for the positive electrode active material, and a component having an action of suppressing elution of the ions is contained in the nonaqueous electrolyte. Therefore, the lithium secondary battery of the present invention has a high capacityAnd can be stably used even by a method of continuously performing continuous charging.

Examples of the nonaqueous electrolyte in the lithium secondary battery of the present invention include a solution (nonaqueous electrolyte solution) in which a lithium salt is dissolved in an organic solvent, and a compound having a nitrile group in a molecule is used for the nonaqueous electrolyte.

The compound having a nitrile group in the molecule is adsorbed on the surface of the positive electrode in the lithium secondary battery to form a coating film, and has a function of suppressing elution of ions of a transition metal element of the positive electrode active material into the nonaqueous electrolyte in a state of being charged to a high voltage. Therefore, the lithium secondary battery of the present invention can be stably used even when continuous high-voltage charging is continuously performed, because the above-described effect obtained by the compound having a nitrile group in the molecule and the effect obtained by using the positive electrode active material in which the ion of the transition metal element described later is less likely to elute are synergistic with each other.

Further, since the compound having a nitrile group in the molecule forms a coating film on the surface of the positive electrode, direct contact between the positive electrode and the nonaqueous electrolyte can be suppressed, and therefore, the decomposition of the nonaqueous electrolyte component on the surface of the positive electrode and the generation of gas due to the decomposition can be suppressed with charge and discharge of the battery. Therefore, the lithium secondary battery of the present invention has good charge-discharge cycle characteristics, and also has good storage characteristics because it can suppress battery swelling during storage.

Examples of the compound having a nitrile group in a molecule include a mononitrile compound having 1 nitrile group in a molecule, a dinitrile compound having 2 nitrile groups in a molecule, and a trinitrile compound having 3 nitrile groups in a molecule. Among them, a dinitrile compound (i.e., a compound having 2 nitrile groups in the molecule) is preferable, and a dinitrile compound represented by the general formula NC-R-CN (wherein R is a linear or branched hydrocarbon chain having 1 to 10 carbon atoms) is more preferable, in terms of the above-described effects (the effect of suppressing elution of ions derived from the positive electrode active material obtained by forming a coating film on the surface of the positive electrode and the effect of suppressing the reaction of the positive electrode with the nonaqueous electrolyte component). R in the general formula is more preferably a linear or branched alkylene chain having 1 to 10 carbon atoms.

Specific examples of the mononitrile compound include lauronitrile. Specific examples of the dinitrile compound represented by the above general formula include 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 and 2, 4-dimethylglutaronitrile, and only 1 kind of them may be used, or 2 or more kinds of them may be used in combination. Among the dinitrile compounds exemplified above, adiponitrile is more preferable from the viewpoint of further enhancing the effect of suppressing elution of ions of the transition metal element from the positive electrode active material.

The content of the compound having a nitrile group in a molecule in the nonaqueous electrolyte used for the battery is preferably 0.1 mass% or more, more preferably 0.2 mass% or more, from the viewpoint of more effectively exerting the effect obtained by using these compounds. However, if the amount of the compound having a nitrile group in the molecule is too large, the storage characteristics of the battery may be further improved, but the effect of improving the charge-discharge cycle characteristics may be small. Therefore, the content of the compound having a nitrile group in a molecule in the nonaqueous electrolyte used for the battery is preferably 5.0% by mass or less, and more preferably 4.0% by mass or less.

Further, as the nonaqueous electrolyte, a nonaqueous electrolyte containing a phosphoryl acetate compound represented by the following general formula (1) is preferably used.

[ solution 1]

In the above general formula (1), R¹、R²And R³Each independently an alkyl group having 1 to 12 carbon atoms which may be substituted with a halogen atom, and n is an integer of 0 to 6.

Use of the lithium Secondary Battery of the inventionSiO_xAs the negative electrode active material, in such a battery, SiO is caused by a volume change accompanying charge and discharge_xThe particles are pulverized and highly active Si is exposed (with respect to SiO)_xDetailed information of the structure of (a) will be described later), which decomposes the nonaqueous electrolyte, there is a problem that charge-discharge cycle characteristics are liable to be degraded.

The phosphoryl acetate compound represented by the general formula (1) has a function of forming a coating film on the surface of the negative electrode even when SiO changes in volume with charge and discharge_xThe particles of (2) are pulverized to produce a fresh noodle, which is also coated well. Therefore, the reaction of the negative electrode active material with the nonaqueous electrolyte can be highly suppressed by the coating.

The phosphoryl acetate compound represented by the general formula (1) also has an effect of suppressing swelling of a lithium secondary battery. Therefore, when a nonaqueous electrolyte further containing a phosphoryl acetate compound represented by the above general formula (1) is used, the action synergistically acts with the action obtained from a compound having a nitrile group in the molecule, and the storage characteristics of the lithium secondary battery can be further improved.

In the above general formula (1), R¹、R²And R³Each independently a hydrocarbon group having 1 to 12 carbon atoms (for example, an alkyl group, an alkenyl group, an alkynyl group, etc.) which may be substituted with a halogen atom, and n is an integer of 0 to 6. Namely the aforementioned R¹、R²And R³May be different from each other or 2 or more.

Specific examples of the phosphoryl acetate compound represented by the general formula (1) include the following compounds.

A compound of the aforementioned general formula (1) wherein n is 0: trimethylphosphoryl methyl ester, methyldiethylphosphoryl methyl ester, methyldipropylphosphoryl methyl ester, methyldibutylphosphoryl methyl ester, triethylphosphoryl methyl ester, ethyldimethylphosphoryl methyl ester, ethyldipropylphosphoryl methyl ester, ethyldibutylphosphoryl methyl ester, tripropylphosphoryl methyl ester, propyldimethylphosphoryl methyl ester, propyl diethylphosphoryl methyl ester, propyl dibutylphosphoryl methyl ester, tributylphosphoryl methyl ester, butyl dimethylphosphoryl methyl ester, butyl diethylphosphoryl methyl ester, butyl dipropylphosphoryl methyl ester, methyl bis (2,2, 2-trifluoroethyl) phosphoryl methyl ester, ethyl bis (2,2, 2-trifluoroethyl) phosphoryl methyl ester, propyl bis (2,2, 2-trifluoroethyl) phosphoryl methyl ester, butyl bis (2,2, 2-trifluoroethyl) phosphoryl methyl ester, and the like.

A compound of the aforementioned general formula (1) wherein n is 1: trimethyl phosphoryl acetate, methyl diethyl phosphoryl acetate, methyl dipropyl phosphoryl acetate, methyl dibutyl phosphoryl acetate, triethyl phosphoryl acetate, ethyl dimethyl phosphoryl acetate, ethyl dipropyl phosphoryl acetate, ethyl dibutyl phosphoryl acetate, tripropyl phosphoryl acetate, propyl dimethyl phosphoryl acetate, propyl diethyl phosphoryl acetate, propyl dibutyl phosphoryl acetate, tributyl phosphoryl acetate, butyl dimethyl phosphoryl acetate, butyl diethyl phosphoryl acetate, butyl dipropyl phosphoryl acetate, methyl bis (2,2, 2-trifluoroethyl) phosphoryl acetate, ethyl bis (2,2, 2-trifluoroethyl) phosphoryl acetate, propyl bis (2,2, 2-trifluoroethyl) phosphoryl acetate, butyl bis (2,2, 2-trifluoroethyl) phosphoryl acetate, allyl dimethyl phosphoryl acetate, allyl diethyl phosphoryl acetate, 2-propynyl dimethyl phosphoryl acetate, 2-propynyl diethyl phosphoryl acetate, and the like.

A compound of the aforementioned general formula (1) wherein n is 2: trimethyl 3-phosphoryl propionate, methyl 3- (diethylphosphoryl) propionate, methyl 3- (dipropylphosphoryl) propionate, methyl 3- (dibutylphosphoryl) propionate, triethyl 3-phosphoryl propionate, ethyl 3- (dimethylphosphoryl) propionate, ethyl 3- (dipropylphosphoryl) propionate, ethyl 3- (dibutylphosphoryl) propionate, tripropyl 3-phosphoryl propionate, propyl 3- (dimethylphosphoryl) propionate, propyl 3- (diethylphosphoryl) propionate, propyl 3- (dibutylphosphoryl) propionate, tributyl 3-phosphoryl propionate, butyl 3- (dimethylphosphoryl) propionate, butyl 3- (diethylphosphoryl) propionate, butyl 3- (dipropylphosphoryl) propionate, methyl 3- (dibutylphosphoryl) propionate, ethyl 3- (dibutylphosphoryl) propionate, tripropyl 3-phosphoryl propionate, propyl 3- (dimethylphosphoryl) propionate, propyl 3- (dimethylp, Methyl 3- (bis (2,2, 2-trifluoroethyl) phosphoryl) propionate, ethyl 3- (bis (2,2, 2-trifluoroethyl) phosphoryl) propionate, propyl 3- (bis (2,2, 2-trifluoroethyl) phosphoryl) propionate, butyl 3- (bis (2,2, 2-trifluoroethyl) phosphoryl) propionate, and the like.

A compound of the aforementioned general formula (1) wherein n is 3: trimethyl 4-phosphoryl butyrate, methyl 4- (diethylphosphoryl) butyrate, methyl 4- (dipropylphosphoryl) butyrate, methyl 4- (dibutylphosphoryl) butyrate, triethyl 4-phosphoryl butyrate, ethyl 4- (dimethylphosphoryl) butyrate, ethyl 4- (dipropylphosphoryl) butyrate, ethyl 4- (dibutylphosphoryl) butyrate, tripropyl 4-phosphoryl butyrate, propyl 4- (dimethylphosphoryl) butyrate, propyl 4- (diethylphosphoryl) butyrate, propyl dibutylphosphoryl) butyrate, tributyl 4-phosphoryl butyrate, butyl 4- (dimethylphosphoryl) butyrate, butyl 4- (diethylphosphoryl) butyrate, butyl 4- (dipropylphosphoryl) butyrate, and the like.

Among the above-exemplified phosphoryl acetate compounds, 2-propynyl diethylphosphoryl acetate (PDPA) and ethyl diethylphosphoryl acetate (EDPA) are particularly preferable.

The content of the phosphoryl acetate compound represented by the general formula (1) in the nonaqueous electrolyte used for a lithium secondary battery is preferably 0.5% by mass or more, and more preferably 1.0% by mass or more, from the viewpoint of ensuring the effects of using the phosphoryl acetate compound more effectively. However, if the content of the phosphoryl acetate compound represented by the general formula (1) in the nonaqueous electrolyte is too large, the effect of improving the charge-discharge cycle characteristics of the battery may be small. Therefore, the content of the phosphoryl acetate compound represented by the general formula (1) in the nonaqueous electrolyte used for a lithium secondary battery is preferably 30% by mass or less, and more preferably 5.0% by mass or less.

It is presumed that R in the general formula (1) represents the phosphoryl acetate compound¹、R²And R³In the case where any of the above contains an unsaturated bond, a carbon-carbon double bond or a carbon-carbon triple bond is opened on the surface of the negative electrode, and polymerization occurs to form a coating film. Since the constituent molecules (constituent polymers) of the film formed in this case have soft carbon-carbon bonds as the main chain, the film has a high thermal conductivity,the flexibility is high. When a lithium secondary battery is charged and discharged, the negative electrode active material expands and contracts with the charge and discharge, and thus the volume of the negative electrode (negative electrode mixture layer) as a whole also changes. However, when a film derived from a phosphoryl acetate compound is formed on the surface of the negative electrode (negative electrode mixture layer), the film is rich in flexibility as described above, and therefore, the film is less likely to break, crack, or the like following the volume change of the negative electrode accompanying charge and discharge of the battery, and therefore, the aforementioned effects obtained by the film derived from a phosphoryl acetate compound can be satisfactorily maintained even when charge and discharge of the battery are repeated.

Further, the nonaqueous electrolyte preferably contains a halogen-substituted cyclic carbonate. The halogen-substituted cyclic carbonate acts on the negative electrode, and has an effect of suppressing the reaction of the negative electrode with the nonaqueous electrolyte component. Therefore, by using a nonaqueous electrolyte further containing a halogen-substituted cyclic carbonate, a lithium secondary battery having better charge-discharge cycle characteristics can be obtained.

As the halogen-substituted cyclic carbonate, a compound represented by the following general formula (2) can be used.

[ solution 2]

In the above general formula (2), R⁴、R⁵、R⁶And R⁷Represents hydrogen, halogen or alkyl with 1-10 carbon atoms, wherein part or all of hydrogen of the alkyl can be replaced by halogen, R⁴、R⁵、R⁶And R⁷At least 1 of which is a halogen element, R⁴、R⁵、R⁶And R⁷May be different from each other or 2 or more. R⁴、R⁵、R⁶And R⁷In the case of an alkyl group, the smaller the number of carbon atoms, the better. The halogen element is particularly preferably fluorine.

Among such cyclic carbonates substituted with halogen elements, 4-fluoro-1, 3-dioxolan-2-one (FEC) is particularly preferable.

The content of the halogen-substituted cyclic carbonate in the nonaqueous electrolyte used for a lithium secondary battery is preferably 0.1 mass% or more, and more preferably 0.5 mass% or more, from the viewpoint of ensuring the effects of using the same more effectively. However, if the content of the halogen-substituted cyclic carbonate in the nonaqueous electrolyte is too large, the effect of improving the storage characteristics may be small. Therefore, the content of the halogen-substituted cyclic carbonate in the nonaqueous electrolyte used for the lithium secondary battery is preferably 10% by mass or less, and more preferably 5% by mass or less.

Further, a nonaqueous electrolyte containing Vinylene Carbonate (VC) is preferably used. VC acts on a negative electrode (particularly, a negative electrode using a carbon material as a negative electrode active material) and has an effect of suppressing a reaction between the negative electrode and a nonaqueous electrolyte component. Therefore, by using a nonaqueous electrolyte further containing VC, a lithium secondary battery having better charge-discharge cycle characteristics can be obtained.

The content of VC in the nonaqueous electrolyte used for a lithium secondary battery is preferably 0.1 mass% or more, and more preferably 1.0 mass% or more, from the viewpoint of ensuring better the effects of using the nonaqueous electrolyte. However, if the content of VC in the nonaqueous electrolyte is too large, the effect of improving the storage characteristics may be small. Therefore, the content of VC in the nonaqueous electrolyte used for a lithium secondary battery is preferably 10 mass% or less, and more preferably 4.0 mass% or less.

The lithium salt used for the nonaqueous electrolyte is any salt that is dissociated in a solvent to form Li⁺The lithium salt is not particularly limited, and is one which is ionic and hardly undergoes a side reaction such as decomposition in a voltage range used as a battery. For example, LiClO can be used₄、LiPF₆、LiBF₄、LiAsF₆、LiSbF₆Equal inorganic lithium salt and LiCF₃SO₃、LiCF₃CO₂、Li₂C₂F₄(SO₃)₂、LiN(CF₃SO₂)₂、LiC(CF₃SO₂)₃、LiC_nF_2n+1SO₃(n≧2)、LiN(RfOSO₂)₂(here, Rf is fluoroalkyl) or the like.

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

The organic solvent used for the nonaqueous electrolyte is not particularly limited as long as it dissolves the lithium salt and does not cause side reactions such as decomposition in the voltage range used as a battery. Examples thereof include: cyclic carbonates such as ethylene carbonate, propylene carbonate, and butylene carbonate, chain carbonates such as dimethyl carbonate, diethyl carbonate, and methylethyl carbonate, chain esters such as methyl propionate, cyclic esters such as γ -butyrolactone, chain ethers such as dimethoxyethane, diethyl ether, 1, 3-dioxolane, diglyme, triglyme, and tetraglyme, cyclic ethers such as dioxane, tetrahydrofuran, and 2-methyltetrahydrofuran, and sulfurous acid esters such as ethyleneglycol sulfite, and these may be used in combination of 2 or more. In order to produce a battery having better characteristics, it is preferable to use a solvent capable of obtaining high conductivity, such as a mixed solvent of ethylene carbonate and a chain carbonate, in combination.

Further, for the purpose of further improving charge-discharge cycle characteristics, improving safety such as high-temperature storage properties and overcharge prevention, additives (including derivatives thereof) such as acid anhydride (salicylic acid), sulfonic acid ester, 1, 3-propane sultone, diphenyl sulfide, cyclohexylbenzene, biphenyl, fluorobenzene and tert-butylbenzene may be appropriately added to the nonaqueous electrolyte used for the lithium secondary battery.

Further, as the nonaqueous electrolyte of the lithium secondary battery, a nonaqueous electrolyte (gel electrolyte) that is gelled by adding a known gelling agent such as a polymer may be used.

The positive electrode in the lithium secondary battery of the present invention uses an electrode having the following structure: the current collector has a positive electrode mixture layer containing a positive electrode active material, a binder, a conductive assistant, and the like on one surface or both surfaces thereof.

The positive electrode active material uses a composition formula Li_1+yNi_1-a-b-cCo_aMn_bM¹ _cO₂(wherein, M¹A lithium nickel composite oxide (a) represented by at least 1 element selected from the group consisting of Mg, Al, Ti, Fe, Cu, Zn, Ga, Ge, Zr, Nb, Mo, Sn, W, B, P and Bi, y ≦ 0.15, a ≦ 0.05 ≦ 0.3, B ≦ 0.3, c ≦ 0.03, and a + B + c ≦ 0.5).

The lithium nickel composite oxide (a) has a high capacity, and is less likely to cause elution of metal ions in the nonaqueous electrolyte in the battery, and has high thermal stability. Therefore, the action obtained by the lithium nickel composite oxide (a) acts synergistically with the action of the compound having a nitrile group in the molecule contained in the nonaqueous electrolyte, and therefore the lithium secondary battery of the present invention has a high capacity and can be used stably even by a method such as continuous charging. Further, by using the lithium nickel composite oxide (a), the charge-discharge cycle characteristics, high-temperature storage characteristics, load characteristics, and safety of the battery can be improved.

In the lithium nickel composite oxide (a), Ni is a component contributing to increase in capacity of the lithium nickel composite oxide (b).

In the lithium nickel composite oxide (a), Co contributes to an increase in the capacity of the lithium nickel composite oxide (a) and also contributes to an increase in the packing density of the lithium nickel composite oxide (a) in the positive electrode mixture layer. In addition, Co suppresses the change in the valence number of Mn with doping and dedoping of Li during charge and discharge of the battery, stabilizes the average valence number of Mn at a value around 4, and has an effect of further improving the reversibility of charge and discharge. Therefore, in the general formula of the composition representing the lithium nickel composite oxide (a), a representing the amount of Co is 0.05 or more, preferably 0.1 or more, from the viewpoint of better exerting the above-described action obtained by Co.

However, if the amount of Co in the lithium nickel composite oxide (a) is too large, the amount of other elements is reduced, and there is a possibility that the effects obtained by these other elements cannot be sufficiently exhibited. Therefore, in the above-described general composition formula of the lithium nickel composite oxide (a), a representing the amount of Co is 0.3 or less, preferably 0.2 or less.

In the lithium nickel composite oxide (a), Mn has an effect of improving the thermal stability of the lithium nickel composite oxide (a). Therefore, from the viewpoint of better exerting the above-described action obtained from Mn, in the above-described general composition formula representing the lithium nickel composite oxide (a), b representing the amount of Mn is 0.05 or more, preferably 0.1 or more.

However, if the amount of Mn in the lithium nickel composite oxide (a) is too large, the amount of other elements is reduced, and there is a possibility that the effects obtained by these other elements cannot be sufficiently exhibited. Therefore, in the above composition formula of the lithium nickel composite oxide (a), b representing the amount of Mn is 0.3 or less, preferably 0.2 or less.

As for the lithium nickel composite oxide (a), in addition to Li, O, Ni, Co and Mn, at least 1 element M selected from the group consisting of Mg, Al, Ti, Fe, Cu, Zn, Ga, Ge, Zr, Nb, Mo, Sn, W, B, P and Bi may be contained¹. However, if the element M in the lithium nickel composite oxide (a)¹If the amount of (B) is too large, the amounts of Ni, Co and Mn are reduced, and there is a possibility that the effects obtained therefrom cannot be sufficiently exhibited. Therefore, in the above-mentioned general formula of the composition of the lithium nickel composite oxide (a), the element M is represented¹C is 0.03 or less, preferably 0.01 or less. Further, the lithium nickel composite oxide (a) may not contain the element M¹In the above composition formula, M represents an element¹C is 0 or more.

In the above-mentioned general formula of the lithium nickel composite oxide (a), if a represents the amount of Co, b represents the amount of Mn, and M represents the element¹If the total amount "a + b + c" of c in the amount of (a) is too large, the amount of Ni decreases, and the capacity of the lithium nickel composite oxide (a) may decrease. Therefore, the "a + b + c" is 0.5 or less, preferably 0.3 or less.

That is, "1-a-b-c" indicating the amount of Ni in the above-described general composition formula of the lithium nickel composite oxide (a) is 0.5 or more, preferably 0.6 or more, and 0.9 or less, preferably 0.8 or less from the viewpoint of ensuring the effects obtained by the addition of Co, Mn, and the like.

The lithium nickel composite oxide (a) has a higher true density and a higher energy bulk density particularly when it has a composition close to the stoichiometric ratio, and specifically, in the above composition formula, it is preferable to set-0.15 ≦ y ≦ 0.15, and by adjusting the value of y in this way, the true density and reversibility at the time of charge and discharge can be improved.

The lithium nickel composite oxide (a) can be produced by the following method: a Li-containing compound (lithium hydroxide, etc.), a Ni-containing compound (nickel sulfate, etc.), a Co-containing compound (cobalt sulfate, etc.), a Mn-containing compound (manganese sulfate, etc.), and, if necessary, an element M¹And (3) mixing the above-mentioned compounds (oxide, hydroxide, sulfate, etc.) and firing the raw material mixture. In order to synthesize the lithium nickel composite oxide (a) with higher purity, it is preferable that Ni, Co, Mn, and, if necessary, M are contained¹A composite compound (hydroxide, oxide, etc.) of a plurality of elements in (b) is mixed with other raw material compounds (Li-containing compound, etc.) and the raw material mixture is fired.

The firing conditions of the raw material mixture for synthesizing the lithium nickel composite oxide (a) may be set to 800 to 1050 ℃ for 1 to 24 hours, for example, and it is preferable to preheat the raw material mixture by temporarily heating the raw material mixture to a temperature lower than the firing temperature (for example, 250 to 850 ℃) and holding the temperature, and then raise the temperature to the firing temperature to allow the reaction to proceed. The time for the preheating is not particularly limited, and usually may be about 0.5 to 30 hours. The atmosphere at the time of firing may be an oxygen-containing atmosphere (i.e., in the atmosphere), a mixed atmosphere of an inert gas (argon, helium, nitrogen, or the like) and oxygen, an oxygen atmosphere, or the like, and the oxygen concentration (on a volume basis) at this time is preferably 15% or more, and preferably 18% or more.

In the positive electrode active material, in addition to the lithium nickel composite oxide (a), the following materials may be used together with the lithium nickel composite oxide (a): LiCoO₂Lithium cobalt oxide, LiMnO, etc₂、Li₂MnO₃Lithium manganese oxide, LiNiO₂Lithium nickel oxide, LiCo_1-xNiO₂Lithium-containing composite oxide having equilamellar structure, LiMn₂O₄、Li_4/3Ti_5/3O₄Lithium-containing composite oxide of iso-spinel structure, LiFePO₄Lithium-containing composite oxides of isoolivine structure, based on said oxidesOxides having a composition substituted with various elements (except for the lithium nickel composite oxide (a)), and the like.

In addition, the following substances may also be used together with the lithium nickel composite oxide (a): general formula of composition Li_1+oCo_{1-p- q}Mg_pM² _qO₂(wherein, M²A lithium cobalt composite oxide (B) represented by-0.3 ≦ o ≦ 0.3, 0.001 ≦ P ≦ 0.1, and 0 ≦ q ≦ 0.1, which is at least 1 element selected from the group consisting of Al, Ti, Fe, Cu, Zn, Ga, Ge, Zr, Nb, Mo, Sn, Sr, W, B, P, and Bi. The lithium cobalt composite oxide (b) contains Mg, and by utilizing its effect, for example, it is widely used as a positive electrode active material for lithium secondary batteries₂In contrast, the stability in the high voltage region is high, and elution of metal (mainly Co) ions into the nonaqueous electrolyte is less likely to occur in the battery, and the thermal stability is also high. Therefore, when the lithium cobalt composite oxide (b) is used in combination with the lithium nickel composite oxide (a), the stability is high even when charging is performed at a high voltage, and a lithium secondary battery having better charge-discharge cycle characteristics, high-temperature storage characteristics, load characteristics, and safety can be obtained.

In the lithium cobalt composite oxide (b), Co is a component contributing to the improvement of the capacity of the lithium cobalt composite oxide (b). As shown by the above-mentioned composition formula, the lithium cobalt composite oxide (b) contains Mg and an element M in addition to Li, O and Co²(the element M may not be contained)²) The amount of Co being determined by the amount p of Mg and the element M²The amount of (a) q is expressed as "1-p-q". The amount of Co "1-p-q" in the lithium cobalt composite oxide (b) is, specifically, preferably 0.9 or more, more preferably 0.95 or more from the viewpoint of improving the capacity thereof, and is preferably 0.999 or less, more preferably 0.05 or less from the viewpoint of satisfactorily ensuring the effect obtained by the addition of Mg or the like.

In the lithium cobalt composite oxide (b), Mg has an effect of improving the stability of the lithium cobalt composite oxide (b) in a high voltage region, suppressing elution of metal ions, and also has an effect of improving the thermal stability of the lithium cobalt composite oxide (b). Therefore, from the viewpoint of better exerting the above-described action obtained from Mg, in the above-described general composition formula representing the lithium cobalt composite oxide (b), p representing the amount of Mg is preferably 0.001 or more, and more preferably 0.002 or more.

However, Mg does not contribute to the increase in capacity of the lithium cobalt composite oxide (b), and if the amount of Mg in the lithium cobalt composite oxide (b) is too large, for example, the amount of Co decreases, and the capacity may decrease. Therefore, in the above-described general composition formula of the lithium cobalt composite oxide (b), p representing the amount of Mg is preferably 0.1 or less, and more preferably 0.05 or less.

The lithium cobalt composite oxide (B) may contain at least 1 element M selected from the group consisting of Al, Ti, Fe, Cu, Zn, Ga, Ge, Zr, Nb, Mo, Sn, Sr, W, B, P and Bi in addition to Li, O, Co and Mg². However, if the element M in the lithium cobalt composite oxide (b)²If the amount of (b) is too large, the amounts of Co and Mg are reduced, and there is a possibility that the effects obtained by them cannot be sufficiently exhibited. Therefore, in the above-mentioned general composition formula representing the lithium cobalt composite oxide (b), the element M is represented²The amount q of (b) is preferably 0.1 or less, more preferably 0.05 or less. Further, as described above, the lithium cobalt composite oxide (b) may not contain the element M²In the above composition formula, M represents an element²Q is 0 or more.

The lithium cobalt composite oxide (b) has a higher true density and a higher energy bulk density particularly when it has a composition close to the stoichiometric ratio, and specifically, in the above composition formula, it is preferable to set the value of-0.3 ≦ o ≦ 0.3, and by adjusting the value of o in this way, the true density and the reversibility at the time of charge and discharge can be improved.

The lithium cobalt composite oxide (b) may be synthesized by the following method: a Li-containing compound (lithium hydroxide, etc.), a Co-containing compound (cobalt sulfate, etc.), a Mg-containing compound (magnesium sulfate, etc.), and, if necessary, an element M²And (3) mixing the above-mentioned compounds (oxide, hydroxide, sulfate, etc.) and firing the raw material mixture. In order to synthesize the lithium cobalt composite oxide (b) with higher purity, it is preferable that the lithium cobalt composite oxide (b) contains Co and Mg, and further contains an element M as needed²The composite compound (hydroxide, oxide, etc.) of (A) is mixed with a Li-containing compound, etc. and the raw material is mixedAnd (5) sintering the material mixture.

As for the firing conditions of the raw material mixture for synthesizing the lithium cobalt composite oxide (b), as in the case of the lithium nickel composite oxide (a), for example, 800 to 1050 ℃ for 1 to 24 hours may be used, and it is preferable to preheat the raw material mixture by temporarily heating the raw material mixture to a temperature lower than the firing temperature (for example, 250 to 850 ℃) and holding the temperature at the temperature, and then raise the temperature to the firing temperature to allow the reaction to proceed. The time for the preheating is not particularly limited, and usually may be about 0.5 to 30 hours. The atmosphere at the time of firing may be an oxygen-containing atmosphere (i.e., in the atmosphere), a mixed atmosphere of an inert gas (argon, helium, nitrogen, or the like) and oxygen, an oxygen atmosphere, or the like, and the oxygen concentration (on a volume basis) at this time is preferably 15% or more, and preferably 18% or more.

When the lithium nickel composite oxide (a) and the other positive electrode active material are used in combination as the positive electrode active material, the lithium cobalt composite oxide (b) is more preferably used as the other positive electrode active material.

In the case where the lithium nickel composite oxide (a) and another positive electrode active material (for example, lithium cobalt composite oxide (b)) are used in combination as the positive electrode active material, the content of the lithium nickel composite oxide (a) in the total amount of the positive electrode active material is preferably 10 mass% or more from the viewpoint of more preferably ensuring the effects obtained by using the lithium nickel composite oxide (a).

In the case where the lithium nickel composite oxide (a) and the lithium cobalt composite oxide (b) are used in combination as the positive electrode active material, the content of the lithium nickel composite oxide (a) in the total amount of the positive electrode active material is preferably 80 mass% or less (that is, the content of the lithium cobalt composite oxide (b) in the total amount of the positive electrode active material is preferably 20 mass% or more).

As the binder in the positive electrode mixture layer, for example, polyvinylidene fluoride (PVDF), Polytetrafluoroethylene (PTFE), Styrene Butadiene Rubber (SBR), carboxymethyl cellulose (CMC), or the like can be used as appropriate. Further, as the conductive aid in the positive electrode mixture layer, the following carbon materials and the like can be mentioned: examples of the carbon black include graphite (graphite carbon material) such as natural graphite (flake graphite, etc.) and artificial graphite, carbon black such as acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black, and carbon fiber.

The positive electrode can be produced by the following steps: for example, a positive electrode mixture-containing composition in the form of a paste or slurry in which a positive electrode active material, a binder, a conductive assistant, and the like are dispersed in a solvent such as N-methyl-2-pyrrolidone (NMP) or the like (in which the binder may be dissolved in the solvent) is prepared, applied to one or both surfaces of a current collector, dried, and then, optionally, subjected to a calender treatment. However, the positive electrode should not be construed as being limited to the electrode manufactured by the foregoing manufacturing method, and may be an electrode manufactured by another method.

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

The thickness of the positive electrode mixture layer is preferably 10 to 100 μm per surface of the current collector, for example.

In addition, 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 additive is preferably 3 to 20% by mass.

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

The negative electrode in the lithium secondary battery of the present invention is, for example, an electrode having a structure in which a negative electrode mixture layer containing a negative electrode active material, a binder, and the like is provided on one or both surfaces of a current collector. SiO is used as the negative electrode active material in the negative electrode_x。

SiO_xA microcrystalline or amorphous phase of Si may be contained, and in this case, the atomic ratio of Si to O becomes the ratio of Si including the microcrystalline or amorphous phase of Si. I.e. SiO_xIncluding in amorphous SiO₂A substance having a structure in which Si (e.g., microcrystalline Si) is dispersed in a matrix, and the amorphous SiO₂And Si dispersed therein may be added so that the atomic ratio x is 0.5 ≦ x ≦ 1.5. For example, in amorphous SiO₂Structure in which Si and SiO are dispersed in matrix₂In the case of a material having a molar ratio of 1:1 to Si, x is 1, and the structural formula used isSiO represents. In the case of a material having such a structure, for example, a peak due to the presence of Si (microcrystalline Si) may not be observed in X-ray diffraction analysis, but if observation is performed with a transmission electron microscope, the presence of fine Si can be confirmed.

And, SiO_xIt is preferably a composite of a carbon material, for example, SiO_xThe surface of (2) is coated with a carbon material. As mentioned above, SiO_xSince it lacks conductivity, when it is used as a negative electrode active material, it is necessary to use a conductive material (conductive aid) to make SiO in the negative electrode so as to ensure good battery characteristics_xThe conductive material is well mixed and dispersed, so that an excellent conductive network is formed. If it is SiO_xComposites with carbon materials, e.g. using SiO alone_xThe conductive network is formed more favorably in the negative electrode than in the case of a material obtained by mixing a conductive material such as a carbon material.

As SiO_xComposite with carbon material, other than SiO as described above_xIn addition to the composite in which the surface is coated with a carbon material, SiO may be mentioned_xAnd granules of carbon materials.

Further, by using the above-mentioned SiO_xThe composite body having a surface coated with a carbon material of (a) is further used in combination with a conductive material (a carbon material or the like), and a better conductive network can be formed in the negative electrode, so that a lithium secondary battery having a higher capacity and more excellent battery characteristics (for example, charge-discharge cycle characteristics) can be realized. As SiO coated with carbon material_xThe composite with a carbon material includes, for example, SiO to be coated with a carbon material_xAnd granules obtained by further granulating the mixture with a carbon material.

Further, SiO coated with a carbon material on the surface_xSiO can be preferably used_xA composite (e.g., granules) of a carbon material having a resistivity lower than that of the above-described composite, the surface of which is further coated with a carbon material. If the inside of the granules is SiO_xAnd a carbon material dispersed therein, and thus has a better conductive networkContaining SiO_xIn a lithium secondary battery as a negative electrode of a negative electrode active material, battery characteristics such as heavy-load discharge characteristics can be further improved.

As can be used with SiO_xExamples of the carbon material forming the composite include carbon materials such as low crystalline carbon, carbon nanotubes, and vapor grown carbon fibers.

As details of the carbon material, at least 1 material selected from the group consisting of fibrous or spiral carbon materials, carbon black (including acetylene black and ketjen black), artificial graphite, graphitizable carbon, and graphitizable carbon is preferable. Fibrous or helical carbon materials are preferable in terms of easy formation of a conductive network and large surface area. Carbon black (including acetylene black, ketjen black), easily graphitized carbon and hardly graphitized carbon have high conductivity, high liquid retention property, and further even SiO_xThe property that the particles are easily kept in contact with the particles by expansion and contraction is preferable in this respect.

In the present invention, as described in detail later, it is preferable that a graphitic carbon material and SiO as a negative electrode active material are mixed_xThe graphite carbon material may be used together as SiO_xThe carbon material used in the composite with the carbon material. The graphite carbon material has high conductivity, high liquid retention, and even SiO, similarly to carbon black and the like_xThe particles are also easy to maintain the property of contacting with the particles by expansion and contraction, and therefore, can be used in SiO_xThe complex of (3) is preferably formed.

Among the above-exemplified carbon materials, the carbon material is mixed with SiO_xThe carbon material used when the composite of (a) is a granulated body is particularly preferably a fibrous carbon material. This is because the fibrous carbon material is thin and flexible, and thus can follow SiO_xThe battery can be expanded and contracted with charge and discharge of the battery, and has a large volume density, so that the battery can be mixed with SiO_xThe particles have more binding sites. Examples of the fibrous carbon include Polyacrylonitrile (PAN) carbon fiber, pitch carbon fiber, vapor grown carbon fiber, and carbon nanotube, and the fibrous carbon may beAny of them is used.

Further, the fibrous carbon material may be, for example, SiO by a vapor phase method_xThe surface of the particles is formed.

SiO_xIs generally 10³～10⁷k Ω cm, whereas the resistivity of the carbon materials exemplified above is generally 10^-5～10kΩcm。

Furthermore, SiO_xThe composite with a carbon material may further have a material layer (a material layer containing non-graphitizable carbon) for covering the carbon material coating layer on the particle surface.

Using SiO in the negative electrode_xIn the case of a composite with a carbon material, SiO_xThe ratio of the carbon material to the SiO solid is set so that the function of the SiO solid obtained by the combination with the carbon material can be satisfactorily exhibited_xThe carbon material is preferably 5 parts by mass or more, and more preferably 10 parts by mass or more, per 100 parts by mass. Furthermore, in the above-mentioned composite, if with SiO_xWhen the ratio of the carbon material to be combined is too high, SiO in the negative electrode mixture layer is generated_xThe effect of increasing the capacity may be reduced by the decrease in the amount, and therefore, the amount may be smaller than that of SiO_xThe carbon material is preferably 50 parts by mass or less, and more preferably 40 parts by mass or less, based on 100 parts by mass.

The aforementioned SiO_xThe complex with a carbon material can be obtained by, for example, the following method.

First, to make SiO_xThe method for producing the composite material will be described. Preparation of SiO_xThe dispersion liquid dispersed in the dispersion medium is spray-dried to prepare composite particles containing a plurality of particles. As the dispersion medium, for example, ethanol or the like can be used. The dispersion is preferably sprayed in an atmosphere of 50 to 300 ℃. In addition to the above-described methods, similar composite particles can be produced by a granulation method using a mechanical method using a vibration-type or planetary ball mill, a rod mill, or the like.

In addition, in the production of SiO_xResistivity ratio of SiO_xIn the case of granules of small carbon materials, in SiO_xIn a dispersion mediumAdding the carbon material to a dispersion liquid dispersed in a medium, and using the dispersion liquid, reacting with SiO_xIn the case of composite particles, composite particles (granules) may be prepared in the same manner as in the case of composite particles. Further, SiO can also be produced by a granulation method using the same mechanical method as described above_xAnd a granulated body of a carbon material.

Then, SiO_xParticles (SiO)_xComposite particles or SiO_xGranules of carbon material) is coated with a carbon material to form a composite, for example, SiO_xThe particles and the hydrocarbon-based gas are heated in a gas phase, and carbon generated by thermal decomposition of the hydrocarbon-based gas is deposited on the surfaces of the particles. In this way, a thin and uniform coating film (carbon material coating layer) containing a conductive carbon material can be formed on the surface of the composite particle and in the pores on the surface by vapor phase growth (CVD) method using a hydrocarbon gas over the corners of the particle, and therefore, SiO can be uniformly coated with a small amount of the carbon material with good uniformity_xThe particles impart conductivity.

In SiO coated with carbon material_xIn the production of (2), the treatment temperature (atmosphere temperature) in the vapor phase growth (CVD) method varies depending on the kind of the hydrocarbon-based gas, and is usually preferably 600 to 1200 ℃, more preferably 700 ℃ or higher, and still more preferably 800 ℃ or higher. This is because a coating layer containing carbon with little impurity residue and high conductivity can be formed at a high treatment temperature.

As the liquid source of the hydrocarbon-based gas, toluene, benzene, xylene, mesitylene, or the like can be used, and toluene which is easy to handle is particularly preferable. The hydrocarbon-based gas can be obtained by vaporizing (for example, foaming with nitrogen gas). Further, methane gas, acetylene gas, or the like may also be used.

Alternatively, SiO may be formed by vapor phase deposition (CVD)_xParticles (SiO)_xComposite particles or SiO_xAnd granules of a carbon material) is coated with a carbon material, and then at least 1 organic compound selected from the group consisting of petroleum pitch, coal pitch, thermosetting resin, and condensates of naphthalenesulfonates and aldehydes is attached to a coating layer containing a carbon materialThen, the particles to which the organic compound is attached are fired.

Specifically, SiO coated with a carbon material is prepared_xParticles (SiO)_xComposite particles, or SiO_xGranules formed of a carbonaceous material) and the organic compound dispersed in a dispersion medium, and spray-drying the dispersion to form particles coated with the organic compound, and firing the particles coated with the organic compound.

As the pitch, isotropic pitch can be used, and as the thermosetting resin, phenol resin, furan resin, furfural resin, or the like can be used. As the condensate of the naphthalenesulfonate and the aldehyde, a naphthalenesulfonate-formaldehyde condensate can be used.

As SiO for coating with carbon material_xAs the dispersion medium in which the particles and the organic compound are dispersed, for example, water or alcohols (e.g., ethanol) can be used. The dispersion is preferably sprayed in an atmosphere of 50 to 300 ℃. The firing temperature is usually preferably 600 to 1200 ℃, and among them, 700 ℃ or higher is preferable, and 800 ℃ or higher is more preferable. This is because a coating layer containing a carbon material having a small amount of impurity remaining and high conductivity and good quality can be formed at a high treatment temperature. However, the treatment temperature needs to be in SiO_xBelow the melting point of (a).

The negative electrode active material for a lithium secondary battery according to the present invention is preferably prepared by mixing a graphitic carbon material and SiO_xAre used together. Reduction of SiO in negative electrode active material by using graphitic carbon material_xOf SiO is suppressed as much as possible_xThe reduction in the amount of the negative electrode mixture layer reduces the effect of increasing the capacity, and the volume change of the negative electrode (negative electrode mixture layer) according to the charge and discharge of the battery is suppressed, whereby the reduction in the battery characteristics that can occur due to the volume change can be suppressed.

As with SiO_xExamples of the graphitic carbon material used together as the negative electrode active material include: for example, natural graphite such as flake graphite, artificial graphite obtained by graphitizing easily graphitizable carbon such as pyrolytic carbons, mesocarbon microbeads (MCMB), carbon fibers at 2800 ℃ or higher, and the like.

In addition, in the negative electrode of the present invention, SiO is used satisfactorily_xFrom the viewpoint of the obtained effect of increasing the capacity, SiO in the negative electrode active material_xThe content of (b) is preferably 0.01% by mass or more, more preferably 3% by mass or more. In addition, SiO in the negative electrode active material is preferable from the viewpoint of better avoiding the problem of volume change of the negative electrode with charge and discharge_xThe content of (b) is preferably 30% by mass or less, more preferably 20% by mass or less.

For the binder in the negative electrode mixture layer, for example, polyvinylidene fluoride (PVDF), Polytetrafluoroethylene (PTFE), styrene-butadiene rubber (SBR), carboxymethyl cellulose (CMC), or the like can be used as appropriate. Further, various carbon blacks such as acetylene black, carbon nanotubes, carbon fibers, and the like may be added as a conductive aid to the negative electrode mixture layer.

The negative electrode can be produced through the following steps: for example, a composition containing a negative electrode mixture in which a negative electrode active material, a binder, and further, if necessary, a conductive assistant are dispersed in a solvent such as NMP or water (in which the binder may be dissolved in the solvent) is prepared, applied to one or both surfaces of a current collector, dried, and then, if necessary, subjected to a calender treatment. However, the negative electrode should not be construed as being limited to the electrode manufactured by the foregoing manufacturing method, and may be an electrode manufactured by another method.

The thickness of the negative electrode mixture layer is preferably 10 to 100 μm per surface of the current collector, and the density of the negative electrode mixture layer (calculated from the mass and thickness of the negative electrode mixture layer per unit area laminated on the current collector) is preferably 1.0 to 1.9g/cm³. In addition, as the composition of the negative electrode mixture layer, for example, the amount of the negative electrode active material is preferably 80 to 95% by mass, the amount of the binder is preferably 1 to 20% by mass, and in the case of using the conductive auxiliary agent, the amount thereof is preferably 1 to 10% by mass.

As the current collector of the negative electrode, copper, nickel foil, punched metal, mesh, rolled metal, and the like can be used, and copper foil is generally used. In the case where the thickness of the entire negative electrode is made thin in order to obtain a battery with a high energy density, the upper limit of the thickness is preferably 30 μm, and the lower limit is preferably 5 μm in order to secure mechanical strength.

The separator according to the lithium secondary battery of the present invention preferably has a property of pore-closing (i.e., a shut-down function) at 80 ℃ or higher (more preferably 100 ℃ or higher) and 170 ℃ or lower (more preferably 150 ℃ or lower), and a separator used for a general lithium secondary battery or the like, for example, a microporous membrane made of polyolefin such as Polyethylene (PE) or polypropylene (PP) can be used. The microporous membrane constituting the separator may be, for example, a microporous membrane using only PE, a microporous membrane using only PP, or a laminate of a microporous membrane made of PE and a microporous membrane made of PP.

In addition, the separator in the lithium secondary battery of the present invention is preferably a laminated separator having a porous layer (I) mainly composed of a resin having a melting point of 140 ℃ or lower and a porous layer (II) mainly composed of a resin that does not melt at a temperature of 150 ℃ or lower or an inorganic filler having a heat-resistant temperature of 150 ℃ or higher. Here, the "melting point" means a melting temperature measured by a Differential Scanning Calorimeter (DSC) in accordance with JIS K7121. Further, "does not melt at a temperature of 150 ℃ or lower" means that the melting temperature measured by DSC exceeds 150 ℃ according to JIS K7121, and the melting behavior is not exhibited at a temperature of 150 ℃ or lower when the melting temperature is measured. Further, the phrase "heat resistant temperature is 150 ℃ or higher" means that at least 150 ℃ does not cause deformation such as softening.

The porous layer (I) in the laminated separator is a layer mainly for securing the shut-down function, and when the lithium secondary battery reaches a temperature equal to or higher than the melting point of the resin that is a main component of the porous layer (I), the resin in the porous layer (I) melts and blocks pores of the separator, thereby generating shut-down that suppresses the progress of the electrochemical reaction.

Examples of the resin having a melting point of 140 ℃ or lower, which is the main component of the porous layer (I), include PE, and examples of the form thereof include a resin obtained by coating a dispersion containing PE particles on a substrate such as a microporous membrane or a nonwoven fabric used in the lithium secondary battery and drying the coating. Here, the volume of the resin having a melting point of 140 ℃ or lower as a main component in the entire composition of the porous layer (I) is 50 vol% or more, and more preferably 70 vol% or more. For example, when the porous layer (I) is formed of the microporous film of PE, the volume of the resin having a melting point of 140 ℃ or less is 100 vol%.

The porous layer (II) in the laminated separator has a function of preventing a short circuit due to direct contact between the positive electrode and the negative electrode even when the internal temperature of the lithium secondary battery increases, and this function is ensured by a resin that does not melt at a temperature of 150 ℃ or lower or an inorganic filler having a heat resistance temperature of 150 ℃ or higher. That is, even if the porous layer (I) shrinks when the battery becomes hot, the porous layer (II) that is less likely to shrink can prevent short-circuiting due to direct contact between the positive and negative electrodes that can occur when the separator thermally shrinks. Further, since the heat-resistant porous layer (II) functions as a skeleton of the separator, the heat shrinkage of the porous layer (I), that is, the heat shrinkage itself of the entire separator can be suppressed.

When the porous layer (II) is formed mainly of a resin having a melting point of 150 ℃ or higher, examples thereof include: a form in which a microporous film made of a resin that does not melt at a temperature of 150 ℃ or lower (for example, the PP-made microporous film for a battery) is laminated on the porous layer (I); the coating layer laminated type form of the porous layer (II) is formed on the surface of the porous layer (I) by applying a dispersion liquid containing particles of a resin or the like that does not melt at a temperature of 150 ℃ or lower onto the porous layer (I) and drying the dispersion liquid.

Examples of the resin that does not melt at a temperature of 150 ℃ or lower include: PP, crosslinked polymethyl methacrylate, crosslinked polystyrene, crosslinked polydivinylbenzene, crosslinked styrene-divinylbenzene copolymer, polyimide, melamine resin, phenol resin, benzoguanamine-formaldehyde condensate, and other crosslinked polymer fine particles, polysulfone, polyether sulfone, polyphenylene sulfide, polytetrafluoroethylene, polyacrylonitrile, aramid, polyacetal, and the like.

When resin particles that do not melt at a temperature of 150 ℃ or lower are used, the particle diameter is preferably 0.01 μm or more, more preferably 0.1 μm or more, and preferably 10 μm or less, more preferably 2 μm or less, in terms of average particle diameter, for example. In addition, the average particle diameter of each particle referred to in the present specification is: for example, the average particle diameter D50% is determined by dispersing these fine particles in a medium which does not dissolve the resin, using a laser scattering particle size analyzer (for example, "LA-920" manufactured by horiba, Ltd.).

When the porous layer (II) is formed mainly of an inorganic filler having a heat resistant temperature of 150 ℃ or higher, for example, a coating layer stack type form in which a dispersion liquid containing an inorganic filler having a heat resistant temperature of 150 ℃ or higher or the like is applied to the porous layer (I) and dried to form the porous layer (II) can be cited.

The inorganic filler in the porous layer (II) may be any electrochemically stable inorganic filler that is stable to the nonaqueous electrolyte of the battery at a heat-resistant temperature of 150 ℃ or higher and is less likely to be oxidized and reduced in the operating voltage range of the battery, and is preferably fine particles, alumina, silica, or boehmite, from the viewpoint of dispersion or the like. Alumina, silica, and boehmite have high oxidation resistance and can adjust the particle diameter and shape to desired values, and therefore, the porosity of the porous layer (II) can be easily controlled with high accuracy. Further, for example, 1 kind of the inorganic filler exemplified above may be used alone, or 2 or more kinds may be used in combination, as the inorganic filler having a heat resistant temperature of 150 ℃. Further, the inorganic filler having a heat-resistant temperature of 150 ℃ may be used in combination with the resin which does not melt at a temperature of 150 ℃ or lower.

The shape of the inorganic filler having a heat resistance temperature of 150 ℃ or higher in the porous layer (II) is not particularly limited, and inorganic fillers having various shapes such as a substantially spherical shape (including a true spherical shape), a substantially elliptical shape (including an elliptical shape), and a plate shape can be used.

Further, if the average particle diameter of the inorganic filler having a heat resistant temperature of 150 ℃ or higher in the porous layer (II) is too small, the permeability of ions decreases, and therefore, it is preferably 0.3 μm or more, and more preferably 0.5 μm or more. Further, if the inorganic filler having a heat resistant temperature of 150 ℃ or higher is too large, the electrical characteristics are easily deteriorated, and therefore, the average particle diameter thereof is preferably 5 μm or less, more preferably 2 μm or less.

The porous layer (II) mainly contains a resin that does not melt at a temperature of 150 ℃ or lower and an inorganic filler having a heat resistant temperature of 150 ℃ or higher, therefore, the amount of them in the porous layer (II) (the amount thereof in the case where the porous layer (II) contains only either one of a resin that does not melt at a temperature of 150 ℃ or lower and an inorganic filler having a heat-resistant temperature of 150 ℃ or higher, the total amount thereof in the case where both are contained; the amount of the resin that does not melt at a temperature of 150 ℃ or lower and the inorganic filler having a heat-resistant temperature of 150 ℃ or higher in the porous layer (II) will be the same hereinafter) is in the entire volume of the constituent components of the porous layer (II), is 50% by volume or more, preferably 70% by volume or more, more preferably 80% by volume or more, and further preferably 90% by volume or more. By setting the content of the inorganic filler in the porous layer (II) to be high as described above, thermal shrinkage of the entire separator can be favorably suppressed even when the lithium secondary battery is at a high temperature, and occurrence of short circuit due to direct contact between the positive electrode and the negative electrode can be more favorably suppressed.

Further, as described later, since the porous layer (II) preferably further contains an organic binder, the amount of the resin that does not melt at a temperature of 150 ℃ or lower and the inorganic filler having a heat resistance temperature of 150 ℃ or higher in the porous layer (II) is preferably 99.5 vol% or less of the entire volume of the constituent components of the porous layer (II).

The porous layer (II) preferably contains an organic binder for the purpose of bonding a resin that does not melt at a temperature of 150 ℃ or lower or an inorganic filler having a heat resistance temperature of 150 ℃ or higher, and integrating the porous layer (II) and the porous layer (I). Examples of the organic binder include an ethylene-vinyl acetate copolymer (EVA, having 20 to 35 mol% of a structural unit derived from vinyl acetate), an ethylene-acrylic acid copolymer such as an ethylene-ethyl acrylate copolymer, a fluorine rubber, SBR, CMC, carboxyethyl cellulose (HEC), polyvinyl alcohol (PVA), polyvinyl butyral (PVB), polyvinyl pyrrolidone (PVP), a crosslinked acrylic resin, polyurethane, and an epoxy resin, and a heat-resistant binder having a heat resistance temperature of 150 ℃. The organic binder may be used alone in 1 kind or in combination of 2 or more kinds.

Among the above-exemplified organic binders, highly flexible binders such as EVA, ethylene-acrylic acid copolymers, fluorine-based rubbers, and SBR are preferable. Specific examples of such a highly flexible organic binder include "EVAFLEX series (EVA)" of dupont polymer chemical company, EVA of eunica japan, "EVAFLEX-EEA series (ethylene-acrylic acid copolymer)" of dupont polymer chemical company, EEA of eunica japan, "DAI-EL late series (fluororubber)" of major industries, TRD-2001(SBR) "of JSR company, and" BM-400b (SBR) "of rui japan.

When the organic binder is used for the porous layer (II), the composition for forming the porous layer (II) may be used in the form of a solution in a solvent or in the form of a dispersed emulsion.

The coated laminated separator can be produced by the following method: for example, a porous layer (II) is formed by applying a porous layer (II) forming composition (a liquid composition such as a slurry) containing particles of a resin that does not melt at a temperature of 150 ℃ or lower, an inorganic filler having a heat resistant temperature of 150 ℃ or higher, and the like, to the surface of a microporous membrane constituting the porous layer (I), and drying the composition at a predetermined temperature.

The composition for forming the porous layer (II) contains, in addition to particles of a resin that does not melt at a temperature of 150 ℃ or lower and/or an inorganic filler having a heat resistance temperature of 150 ℃ or higher, an organic binder and the like as necessary, and these are dispersed in a solvent (including a dispersion medium, the same applies hereinafter). In addition, the organic binder may also be dissolved in a solvent. The solvent used in the composition for forming the porous layer (II) may be any solvent as long as it can uniformly disperse particles of a resin, an inorganic filler, and the like that do not melt at a temperature of 150 ℃ or lower, and can uniformly dissolve or disperse the organic binder, and for example, general organic solvents such as aromatic hydrocarbons such as toluene, furans such as tetrahydrofuran, and ketones such as methyl ethyl ketone and methyl isobutyl ketone may be suitably used. For the purpose of controlling the surface tension, various propylene oxide-based alcohol ethers such as alcohols (ethylene glycol, propylene glycol, etc.) and monomethyl ether acetate may be added to these solvents as appropriate. In addition, when the organic binder is water-soluble, when used as an emulsion, or the like, water may be used as a solvent, and in this case, alcohols (methanol, ethanol, isopropanol, ethylene glycol, or the like) may be added as appropriate to control the surface tension.

The composition for forming the porous layer (II) preferably contains particles of a resin that does not melt at a temperature of 150 ℃ or lower, and/or an inorganic filler having a heat resistance temperature of 150 ℃ or higher, and further contains an organic binder or the like in a solid content of, for example, 10 to 80 mass%.

In the above-described laminated separator, the porous layer (I) and the porous layer (II) do not need to be 1 layer each, and a plurality of layers may be provided in the separator. For example, the porous layer (I) may be disposed on both surfaces of the porous layer (II) or the porous layer (II) may be disposed on both surfaces of the porous layer (I). However, since the thickness of the separator increases with the number of layers, which may increase the internal resistance of the battery and decrease the energy density, it is not preferable to increase the number of layers, and the total number of layers of the porous layer (I) and the porous layer (II) in the laminated separator is preferably 5 or less.

The thickness of the separator (the separator formed of a microporous film made of polyolefin, the aforementioned laminated separator) in the lithium secondary battery of the present invention is preferably 10 to 30 μm, for example.

In the multilayer separator, the thickness of the porous layer (II) (the total thickness of the porous layer (II) in the case of a separator having a plurality of porous layers) is preferably 3 μm or more, from the viewpoint of more effectively exhibiting each of the aforementioned functions obtained by the porous layer (II). However, if the porous layer (II) is too thick, there is a possibility of a decrease in energy density of the battery, and the like, and therefore the thickness of the porous layer (II) is preferably 8 μm or less.

Further, in the multilayer separator, from the viewpoint of more effectively exhibiting the above-described action (especially, the shut-down action) obtained by using the porous layer (I), the thickness of the porous layer (I) (the total thickness thereof in the case of a separator having a plurality of porous layers (I)) is preferably 6 μm or more, and more preferably 10 μm or more. However, if the porous layer (I) is too thick, there is a possibility that the energy density of the battery is lowered, and the force of thermal shrinkage of the porous layer (I) is increased, and the effect of suppressing thermal shrinkage of the entire separator is reduced. Therefore, the thickness of the porous layer (I) is preferably 25 μm or less, more preferably 20 μm or less, and still more preferably 14 μm or less.

The porosity of the entire separator is preferably 30% or more in a dry state in order to ensure the liquid retention amount of the electrolyte and to improve the ion permeability. On the other hand, the porosity of the separator in a dry state is preferably 70% or less from the viewpoint of ensuring the strength of the separator and preventing internal short circuits. The porosity P (%) of the separator can be calculated from the thickness of the separator, the mass per unit area, and the density of the constituent components by summing up the components i by using the following equation (3).

P＝{1-(m/t)/(Σa_i·ρ_i)}×100 (3)

Here, in the above formula (3), a_iThe ratio of the component i when the total mass is 1, ρ_iIs the density (g/cm) of the component i³) And m is the mass per unit area (g/cm) of the separator²) And t is the thickness (cm) of the separator.

In the case of the laminated separator, in the formula (3), m is defined as a mass per unit area (g/cm) of the porous layer (I)²) Let t be the thickness (cm) of the porous layer (I), and the porosity P (%) of the porous layer (I) can be obtained by using the above expression (3). The porosity of the porous layer (I) obtained by this method is preferably 30 to 70%.

In the case of the laminated separator, in the formula (3), m is defined as a mass per unit area (g/cm) of the porous layer (II)²) Let t be the thickness (cm) of the porous layer (II), and the porosity P (%) of the porous layer (II) can also be obtained using the above equation (3). Obtained by the methodThe porosity of the porous layer (II) is preferably 20 to 60%.

The separator is preferably a separator having high mechanical strength, and for example, the puncture strength is preferably 3N or more. For example, alloys and oxides of Si and Sn contribute to a high capacity of a battery as a negative electrode active material having a large capacity, but change in volume with charge and discharge is large. Therefore, when such a negative electrode active material is used, the negative electrode is expanded and contracted as a whole by repeated charge and discharge, and the separator facing the negative electrode is mechanically damaged. If the puncture strength of the separator is 3N or more, good mechanical strength is ensured, and mechanical damage to the separator can be alleviated.

The separator having a puncture strength of 3N or more includes the above-described laminated separator, and particularly, a separator in which a porous layer (II) mainly containing an inorganic filler having a heat-resistant temperature of 150 ℃ or more is laminated on a porous layer (I) mainly containing a resin having a melting point of 140 ℃ or less is preferable. This is believed to be due to: since the inorganic filler has high mechanical strength, the mechanical strength of the porous layer (I) is compensated for, and the mechanical strength of the entire separator can be improved.

The puncture strength can be measured by the following method. On a plate having a hole of 2 inches in diameter, a diaphragm was fixed without wrinkles and deflection, a hemispherical metal needle having a tip of 1.0mm in diameter was dropped on a measurement sample at a speed of 120mm/min, and the force at the time of generating a hole in the diaphragm was measured 5 times. Then, the average value of 3 measurements excluding the maximum value and the minimum value among the 5 measurements was determined, and this was used as the puncture strength of the separator.

The positive electrode, the negative electrode, and the separator may be used in the lithium secondary battery of the present invention in the following forms: the laminated electrode body in which the positive electrode and the negative electrode are stacked with the separator interposed therebetween is further wound into a spiral wound electrode body.

In the case where the laminated separator is used, particularly, in the case where a porous layer (I) mainly composed of a resin having a melting point of 140 ℃ or lower is laminated with a porous layer (II) mainly composed of an inorganic filler having a heat resistance temperature of 150 ℃ or higher, the laminated electrode body or the wound electrode body is preferably arranged such that the porous layer (II) faces at least the positive electrode. In this case, since the porous layer (II) having a higher oxidation resistance, which mainly contains an inorganic filler having a heat resistance temperature of 150 ℃. In addition, when an additive such as VC or cyclohexylbenzene is added to the nonaqueous electrolyte, a coating film is formed on the positive electrode side to block pores of the separator, and there is a possibility that the battery characteristics are degraded. Here, by orienting the relatively porous layer (II) toward the positive electrode, an effect of suppressing clogging of the micropores can also be expected.

On the other hand, when one surface of the laminated separator is the porous layer (I), the porous layer (I) is preferably oriented toward the negative electrode, and this suppresses, for example, absorption of the thermoplastic resin melted from the porous layer (I) into the electrode mixture layer during the shut-down, and can effectively utilize the blocking of the pores of the separator.

Examples of the form of the lithium secondary battery of the present invention include a cylindrical form (e.g., a square cylindrical form, a cylindrical form) using a steel can, an aluminum can, or the like as an outer packaging can. In addition, a flexible packaging battery having a laminate mold with a metal deposited thereon as an outer package can be produced.

The lithium secondary battery of the present invention can have an upper limit voltage during charging before use of about 4.2V which is generally used for a lithium secondary battery, but can be stably used even by a method of charging at a high voltage of 4.3V or more.

Examples

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

Example 1

And (3) manufacturing a positive electrode: mixing Li_1.0Ni_0.5Co_0.2Mn_0.3O₂(lithium-nickel composite oxide (a)) and Li_1.036Co_0.991Al_0.004Mg_0.002Sr_0.001Ti_0.002Zr_0.001O₂(lithium cobalt composite oxide (b)) 100 parts by mass of a positive electrode active material, 20 parts by mass of an NMP solution containing PVDF as a binder at a concentration of 10% by mass, 1 part by mass of artificial graphite as a conductive additive, and 1 part by mass of ketjen black were kneaded with a twin-screw kneader, and further NMP was added to adjust the viscosity, thereby preparing a positive electrode mixture-containing paste.

The paste containing the positive electrode mixture was applied to both surfaces of an aluminum foil (positive electrode current collector) having a thickness of 15 μm, and then vacuum-dried at 120 ℃ for 12 hours, thereby forming positive electrode mixture layers on both surfaces of the aluminum foil. Then, the thickness and density of the positive electrode mixture layer were adjusted by applying pressure treatment, and a lead body made of nickel was welded to the exposed portion of the aluminum foil, thereby producing a strip-shaped positive electrode having a length of 375mm and a width of 43 mm. The thickness of each positive electrode mixture layer in the obtained positive electrode was 55 μm.

And (3) manufacturing a negative electrode: a negative electrode mixture-containing paste was prepared by mixing 97.5 parts by mass of a mixture obtained by mixing a composite (10% by mass of the carbon material in the composite) in which the SiO surface having an average particle diameter D50% of 8 μm was coated with the carbon material and graphite having an average particle diameter D50% of 16 μm in an amount of 3.75% by mass of the composite in which the SiO surface was coated with the carbon material, 1.5 parts by mass of SBR as a binder, and 1 part by mass of CMC as a thickener, with water being added thereto.

The paste containing the negative electrode mixture was applied to both sides of a copper foil (negative electrode current collector) having a thickness of 8 μm, and then vacuum-dried at 120 ℃ for 12 hours, thereby forming negative electrode mixture layers on both sides of the copper foil. Then, the thickness and density of the negative electrode mixture layer were adjusted by applying pressure treatment, and a lead body made of nickel was welded to the exposed portion of the copper foil, thereby producing a strip-shaped negative electrode having a length of 380mm and a width of 44 mm. The thickness of each negative electrode mixture layer in the obtained negative electrode was 65 μm.

Preparation of nonaqueous electrolyte: in a mixed solvent of Ethylene Carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of 3:7, LiPF is added₆At a rate of 1.1mol/LThe above solutions were dissolved in an amount of 0.1 mass% for adiponitrile, 1.25 mass% for EDPA, 2.0 mass% for FEC, and 2.0 mass% for VC, respectively, to prepare nonaqueous electrolytes.

Assembling the battery: the above-mentioned strip-shaped positive electrode was stacked on the above-mentioned strip-shaped negative electrode with a separator having a thickness of 16 μm or less, wound spirally, and then pressed to be flat to prepare an electrode wound body having a flat wound structure, and the electrode wound body was fixed with an insulating tape made of polypropylene. Then, the electrode wound body was inserted into a rectangular battery case made of an aluminum alloy having an outer dimension of 4.0mm, a width of 34mm, and a height of 50mm, and a lead body was welded thereto, and a lid plate made of an aluminum alloy was welded to an open end of the battery case. Thereafter, the nonaqueous electrolyte was injected through an injection port provided in the lid plate, and after standing for 1 hour, the injection port was sealed, thereby obtaining a lithium secondary battery having the structure shown in fig. 1 and the appearance shown in fig. 2.

Manufacturing a diaphragm: 5kg of boehmite containing 5kg of 1 μm of an average particle size D50% and 5kg of ion exchange water and 0.5kg of a dispersant (an aqueous polycarboxylic acid ammonium salt, a solid content concentration of 40% by mass) were added, and the resultant mixture was pulverized for 10 hours by a ball mill having an internal volume of 20L and 40 revolutions per minute to prepare a dispersion. The treated dispersion was dried under vacuum at 120 ℃ and observed by a Scanning Electron Microscope (SEM), and as a result, the boehmite had a substantially plate-like shape.

To 500g of the dispersion, 0.5g of xanthan gum as a thickener and 17g of a resin binder dispersion (modified polybutyl acrylate, solid content: 45% by mass) as a binder were added, and the mixture was stirred for 3 hours by a Three-One Motor to prepare a uniform slurry (slurry for forming the porous layer (II), solid content: 50% by mass).

A microporous PE separator for a lithium secondary battery (porous layer (I): 12 μm thick, 40% porosity, 0.08 μm average pore diameter, 135 ℃ melting point of PE) was subjected to corona discharge treatment (discharge amount 40W. min/m)²) Coating the porous layer (II) forming slurry on the treated surface by a mini gravure coater, and drying to formA porous layer (II) having a thickness of 4 μm, thereby obtaining a laminated separator. The mass per unit area of the porous layer (II) in the separator was 5.5g/m²The boehmite volume content was 95 vol%, and the porosity was 45%.

Here, the battery shown in fig. 1 and 2 will be described, where fig. 1 (a) is a plan view and fig. 1 (b) is a partial cross-sectional view thereof, and as shown in fig. 1 (b), a positive electrode 1 and a negative electrode 2 are wound in a spiral shape with a separator 3 interposed therebetween, and then pressurized so as to be flat, and accommodated in a prismatic (rectangular cylindrical) battery case 4 as a flat electrode wound body 6 together with a nonaqueous electrolyte. However, in fig. 1, a metal foil, a nonaqueous electrolyte, and the like as a current collector used in producing the positive electrode 1 and the negative electrode 2 are not shown in order to avoid complication.

The battery case 4 is made of an aluminum alloy and constitutes an outer casing of the battery, and the battery case 4 also serves as a positive electrode terminal. An insulator 5 formed of a PE sheet is disposed at the bottom of the battery case 4, and a positive lead body 7 and a negative lead body 8 connected to one end of each of the positive electrode 1 and the negative electrode 2 are drawn from a flat electrode wound body 6 formed of the positive electrode 1, the negative electrode 2, and the separator 3. Further, a stainless steel terminal 11 is attached to the aluminum alloy sealing lid plate 9 for sealing the opening of the battery case 4 via an insulating gasket 10 made of polypropylene, and a lead plate 13 made of stainless steel is attached to the terminal 11 via an insulator 12.

The lid plate 9 is inserted into the opening of the battery case 4, and the joint between the two is welded, whereby the opening of the battery case 4 is sealed, and the battery interior is sealed. In the battery of fig. 1, the cover plate 9 is provided with a nonaqueous electrolyte injection port 14, and the nonaqueous electrolyte injection port 14 is sealed by welding, for example, laser welding, in a state where a sealing member is inserted, thereby ensuring the sealing property of the battery (therefore, in the battery of fig. 1 and 2, the nonaqueous electrolyte injection port 14 is actually a nonaqueous electrolyte injection port and a sealing member, but for ease of explanation, it is shown as the nonaqueous electrolyte injection port 14). Further, the lid plate 9 is provided with an explosion vent 15 as a mechanism for discharging internal gas to the outside when the temperature of the battery rises.

In the battery of example 1, the positive lead 7 is directly welded to the lid plate 9, and the outer can 5 and the lid plate 9 function as a positive terminal, and the negative lead 8 is welded to the lead plate 13, and the negative lead 8 is electrically connected to the terminal 11 via the lead plate 13, whereby the terminal 11 functions as a negative terminal, but the positive and negative polarities may be reversed depending on the material of the battery case 4.

Fig. 2 is a perspective view schematically showing the appearance of the battery shown in fig. 1, and fig. 2 is a view for illustrating the battery as a rectangular battery, and fig. 1 shows the battery as a whole, and shows only specific members among the constituent members of the battery. In fig. 1, the inner peripheral portion of the electrode body is not shown in cross section.

Example 2

A nonaqueous electrolyte was prepared in the same manner as in example 1 except that the amount of adiponitrile added was changed to 0.25 mass%, and a lithium secondary battery was produced in the same manner as in example 1 except that this nonaqueous electrolyte was used.

Example 3

A nonaqueous electrolyte was prepared in the same manner as in example 1 except that the amount of adiponitrile added was changed to 0.5 mass%, and a lithium secondary battery was produced in the same manner as in example 1 except that this nonaqueous electrolyte was used.

Example 4

A nonaqueous electrolyte was prepared in the same manner as in example 1 except that the amount of adiponitrile added was changed to 1.0 mass%, and a lithium secondary battery was produced in the same manner as in example 1 except that this nonaqueous electrolyte was used.

Example 5

A nonaqueous electrolyte was prepared in the same manner as in example 1 except that the amount of adiponitrile added was changed to 2.5 mass%, and a lithium secondary battery was produced in the same manner as in example 1 except that this nonaqueous electrolyte was used.

Example 6

A nonaqueous electrolyte was prepared in the same manner as in example 1 except that the amount of adiponitrile added was changed to 5.0 mass%, and a lithium secondary battery was produced in the same manner as in example 1 except that this nonaqueous electrolyte was used.

Example 7

A nonaqueous electrolyte was prepared in the same manner as in example 1 except that the amount of adiponitrile added was changed to 0.5 mass% and EPDA 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 8

A nonaqueous electrolyte was prepared in the same manner as in example 1 except that the amount of adiponitrile added was changed to 7.5 mass%, and a lithium secondary battery was produced in the same manner as in example 1 except that this nonaqueous electrolyte was used.

Example 9

A nonaqueous electrolyte was prepared in the same manner as in example 4 except that adiponitrile was changed to succinonitrile, and a lithium secondary battery was produced in the same manner as in example 4 except that this nonaqueous electrolyte was used.

Example 10

A nonaqueous electrolyte was prepared in the same manner as in example 4 except that EDPA was changed to PDPA, and a lithium secondary battery was produced in the same manner as in example 4 except that this nonaqueous electrolyte was used.

Comparative example 1

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.

50 ℃ DOD 5% test: the batteries of examples and comparative examples were charged at a current value of 1.0C until 4.35V was reached, and then, charged at a voltage of 4.35V. The total charging time of the constant current charging and the constant voltage charging was set to 2.5 hours. Thereafter, discharge was performed at a current value of 0.2C until the voltage reached 3.0V, and the initial capacity was measured.

Next, each battery after the initial capacity measurement was charged to a fully charged state under the same conditions as the initial capacity measurement. Then, each battery in a fully charged state was discharged at a current value of 1.0C to about 5% of the rated capacity (DOD 5%) at 50 ℃, and charged again at a current value of 1.0C until the battery reached the fully charged state, and the above-described series of operations was regarded as 1 cycle, and charge and discharge were repeated for 800 cycles. The subsequent batteries were charged and discharged under the same conditions as the initial capacity measurement, the capacity of each battery after 800 cycles was measured, and the value obtained by dividing the capacity by the initial capacity was expressed as a percentage to determine the capacity retention rate.

The 50 ℃ DOD 5% test is the following test: a method of continuously charging a battery in a fully charged state is assumed by repeating discharge of only a small part of the capacity and charge up to the fully charged state. Therefore, in this test, the higher the capacity retention rate, the higher the stability even when the battery is used by continuously charging the battery.

Short circuit test after continuous charging: for each of the batteries of examples and comparative examples (not the battery subjected to the 50 ℃. DOD 5% test), constant current-constant voltage charging was continued, and the presence or absence of short circuit of each battery after 500 hours was examined, the constant current-constant voltage charging being: charging was carried out at 45 ℃ at a current value of 1.0C until constant current charging of 4.4V, and then constant voltage charging was carried out at a voltage of 4.4V.

This test also assumes a method of use in which continuous charging is continuously performed, and a battery in which a short circuit is not observed after this test means that the stability is high even if the battery is used in a method in which continuous charging is continuously performed.

Charge-discharge cycle characteristics: the batteries of examples and comparative examples (not the batteries subjected to the above evaluations) were charged and discharged under the same conditions as those for the initial capacity measurement in the 50 ℃ DOD 5% test, and the initial capacity was measured.

Then, each battery after the initial capacity measurement was charged to 4.35V at a current value of 1C, then charged to 4.35V at a voltage of 4.35V (the total charging time of the constant current charging and the constant voltage charging was 2.5 hours), and thereafter discharged to 3.0V at a current value of 1C, and the above-described series of operations was repeated as 1 cycle of 500 cycles of charging and discharging. The subsequent batteries were charged and discharged under the same conditions as those in the initial capacity measurement, the discharge capacity of each battery after 500 cycles was measured, and the value obtained by dividing the capacity by the initial capacity was expressed as a percentage, to determine the capacity retention rate. It can be said that the higher the capacity retention rate is, the better the charge-discharge cycle characteristics of the battery are.

Storage test: the batteries of examples and comparative examples (not the batteries subjected to the above evaluations) were charged under the same conditions as those for the initial capacity measurement of the 50 ℃ DOD 5% test. The charged batteries were placed in a thermostatic bath, the temperature in the bath was set at 85 ℃ and left to stand for 24 hours. Thereafter, each battery was taken out from the thermostatic bath, left to cool until it reached room temperature, and then the thickness was measured, and the difference from the initial thickness (4.0mm) was calculated to determine the swelling amount of the battery after the storage test.

Load characteristics: the batteries of examples and comparative examples (not the batteries subjected to the above evaluations) were charged and discharged under the same conditions as those for the initial capacity measurement in the 50 ℃ DOD 5% test, and the discharge capacity (0.2C discharge capacity) was measured. Further, each of the batteries after 0.2C discharge capacity measurement was charged under the same conditions as in the 0.2C discharge capacity measurement, and was discharged at a current value of 1.5C up to 3.0V, thereby measuring the discharge capacity (1.5C discharge capacity). Then, the value obtained by dividing the 1.5C discharge capacity by the 0.2C discharge capacity was expressed by percentage for each battery, and the capacity retention rate was determined. It can be said that the higher the capacity retention rate is, the better the load characteristics of the battery are.

The contents of the additives in the nonaqueous electrolytes used in the lithium secondary batteries of examples and comparative examples are shown in table 1, and the evaluation results are shown in table 2.

TABLE 1

TABLE 2

As shown in tables 1 and 2, the lithium secondary batteries of examples 1 to 10 using the lithium nickel composite oxide (a) for the positive electrode active material and SiO for the negative electrode active material and using the nonaqueous electrolyte containing the compound having a nitrile group in the molecule had a higher capacity retention rate in the 50 ℃ DOD 5% test and did not cause short-circuiting even after continuous charging, compared with the battery of comparative example 1 using the nonaqueous electrolyte containing no compound having a nitrile group in the molecule. Therefore, the lithium secondary batteries of examples 1 to 10 can be stably used even by a method of continuously charging the batteries at a high voltage of 4.3V or more. Further, the lithium secondary batteries of examples 1 to 10 were also excellent in load characteristics.

The lithium secondary batteries of examples 1 to 6, 8, 9 and 10 using the nonaqueous electrolyte further containing a phosphoryl acetate compound represented by the general formula (1) had a smaller swelling amount after a storage test and also had excellent storage characteristics, compared with the battery of example 7 using a nonaqueous electrolyte not containing the compound.

Further, the lithium secondary batteries of examples 1 to 7, 9 and 10 using a nonaqueous electrolyte having an appropriate content of a compound having a nitrile group in the molecule had a higher capacity retention rate and superior charge-discharge cycle characteristics in the evaluation of charge-discharge cycle characteristics, as compared with the lithium secondary battery of example 8 using a nonaqueous electrolyte having a very large content of the compound.

The present invention may be practiced in other forms than those described above without departing from the spirit and scope of the present invention. The embodiments disclosed in the present application are merely examples, and the present invention is not limited to these embodiments. The scope of the present invention is defined by the appended claims rather than the description of the foregoing specification, and all changes which come within the meaning and range equivalent to the claims are intended to be embraced therein.

Industrial applicability

The lithium secondary battery of the present invention can be stably used even when continuously charged, and therefore, can be suitably used for various applications in which conventionally known lithium secondary batteries are applied, in addition to the power supply application of portable electronic devices which are highly likely to be used by such a method.

Description of the symbols

1 positive electrode

2 negative electrode

3 diaphragm

Claims (5)

1. A lithium secondary battery comprising a positive electrode, a negative electrode, a nonaqueous electrolyte and a separator,

the positive electrode has a positive electrode mixture layer containing a positive electrode active material on one or both surfaces of a current collector,

the negative electrode has a negative electrode mixture layer containing a negative electrode active material on one or both surfaces of a current collector,

the positive electrode mixture layer of the positive electrode contains a positive electrode material having a composition represented by the general formula Li_1+yNi_1-a-b-cCo_aMn_bM¹ _cO₂The lithium nickel composite oxide represented by the general formula Li_1+oCo_1-p-qMg_pM² _qO₂The lithium cobalt composite oxide shown is used as a positive electrode active material,

wherein M is¹Is at least 1 element selected from the group consisting of Mg, Al, Ti, Fe, Cu, Zn, Ga, Ge, Zr, Nb, Mo, Sn, W, B, P and Bi, -0.15 ≦ y ≦ 0.15, 0.05 ≦ a ≦ 0.2, 0.05 ≦ B ≦ 0.2, 0 ≦ c ≦ 0.03 and a + B + c ≦ 0.5,

M²is at least 1 element selected from the group consisting of Al, Ti, Fe, Cu, Zn, Ga, Ge, Zr, Nb, Mo, Sn, Sr, W, B, P and Bi, -0.3 ≦ o ≦ 0.3, 0.001 ≦ P ≦ 0.1 and 0 ≦ q ≦ 0.1,

the negative electrode mixture layer of the negative electrode contains, as a negative electrode active material, a material containing Si and O as constituent elements, wherein x is 0.5 ≦ x ≦ 1.5 for the atomic ratio of O to Si,

the non-aqueous electrolyte contains a compound having a nitrile group in a molecule, and the compound has 2 or 3 nitrile groups in a molecule,

the non-aqueous electrolyte contains a phosphoryl acetate compound represented by the following general formula (1),

in the general formula (1), R¹、R²And R³Each independently a hydrocarbon group of 1 to 12 carbon atoms which may be substituted with a halogen atom, and R¹、R²And R³Any one of which contains an unsaturated bond, n is an integer of 0 to 6,

the upper limit voltage of charging is set to 4.3V or more.

2. A lithium secondary battery comprising a positive electrode, a negative electrode, a nonaqueous electrolyte and a separator,

the positive electrode has a positive electrode mixture layer containing a positive electrode active material on one or both surfaces of a current collector,

the negative electrode has a negative electrode mixture layer containing a negative electrode active material on one or both surfaces of a current collector,

the positive electrode mixture layer of the positive electrode contains a positive electrode material having a composition represented by the general formula Li_1+yNi_1-a-b-cCo_aMn_bM¹ _cO₂The lithium nickel composite oxide shown is used as a positive electrode active material,

wherein M is¹Is at least 1 element selected from the group consisting of Mg, Al, Ti, Fe, Cu, Zn, Ga, Ge, Zr, Nb, Mo, Sn, W, B, P and Bi, -0.15 ≦ y ≦ 0.15, 0.05 ≦ a ≦ 0.2, 0.05 ≦ B ≦ 0.2, 0 ≦ c ≦ 0.03 and a + B + c ≦ 0.5,

the negative electrode mixture layer of the negative electrode contains, as a negative electrode active material, a material containing Si and O as constituent elements, wherein x is 0.5 ≦ x ≦ 1.5 for the atomic ratio of O to Si,

the non-aqueous electrolyte contains a compound having a nitrile group in a molecule, and the compound has 2 or 3 nitrile groups in a molecule,

the non-aqueous electrolyte contains a phosphoryl acetate compound represented by the following general formula (1),

in the general formula (1), R¹、R²And R³Each independently a hydrocarbon group of 1 to 12 carbon atoms which may be substituted with a halogen atom, and R¹、R²And R³Any one of which contains an unsaturated bond, n is an integer of 0 to 6,

the upper limit voltage of charging is set to 4.3V or more.

3. The lithium secondary battery according to claim 1 or 2, which uses a nonaqueous electrolyte having a content of a compound having a nitrile group in a molecule of 0.1 to 5.0 mass%.

4. The lithium secondary battery according to claim 1 or 2, wherein a nonaqueous electrolyte containing 0.5 to 30 mass% of the phosphoryl acetate compound represented by the general formula (1) is used.

5. The lithium secondary battery according to claim 1 or 2, wherein the negative electrode mixture layer of the negative electrode contains a composite of a material containing Si and O as constituent elements and a carbon material.

Images