JP6253411B2 - Lithium secondary battery - Google Patents

Description

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

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

In lithium secondary batteries, lithium-containing composite oxides such as LiCoO ₂ and LiMnO ₂ are usually used as the positive electrode active material, and in addition to the graphitic carbon material, lithium secondary secondary materials such as LiCoO ₂ and LiMnO ₂ are used. For the purpose of increasing the capacity of batteries, studies are underway to use SiO _x having a structure in which Si ultrafine particles are dispersed in SiO ₂ (for example, Patent Documents 1 to 3).

In order to increase the capacity of the lithium secondary battery, in addition to using an active material having a large capacity, a method for increasing the charging voltage is conceivable. For example, in a lithium secondary battery using LiCoO ₂ as a positive electrode active material that is currently widely used, the upper limit of the charging voltage is generally set to about 4.2 V. By increasing the value above, the capacity can be further increased.

  In addition, various additives are added to the non-aqueous electrolyte to improve the characteristics of the lithium secondary battery. For example, Patent Document 4 discloses a lithium secondary battery in which a compound having two or more nitrile groups in a molecule is contained in a non-aqueous electrolyte to improve charge / discharge cycle characteristics and the like.

Japanese Patent Laid-Open No. 2004-047404 Japanese Patent Laid-Open No. 2005-259697 JP 2008-210618A JP 2008-108586 A

  By the way, in a portable electronic device, for example, the device may be continuously charged during actual use, such as being left for a long time while being charged, or using the device while being charged.

  Under such usage conditions where continuous charging is continued, ions of transition metal elements such as Co and Mn are eluted from the lithium-containing composite oxide that is the positive electrode active material into the non-aqueous electrolyte, and the positive electrode active material When battery deterioration deteriorates, battery characteristics are impaired, or when the amount of ions to be eluted becomes extremely large, an internal short circuit of the battery may occur.

The problems caused by the deterioration of the positive electrode active material due to continuous charging of the battery are noticeable when a high-capacity material such as SiO _x is used for the negative electrode active material or the charging voltage is increased as compared with the conventional case. However, it became clear by examination of the present inventors.

  Therefore, in lithium secondary batteries that are applied to power sources for portable electronic devices in particular, charging was continued in order to increase the capacity by using a large capacity negative electrode active material and increasing the charging voltage. It is required to improve stability at the time.

  This invention is made | formed in view of the said situation, The objective is to provide the lithium secondary battery which can be used stably even if it continues charging continuously by the high voltage of 4.3V or more. is there.

The lithium secondary battery of the present invention that has achieved the above-mentioned object includes a positive electrode having a positive electrode mixture layer containing a positive electrode active material on one side or both sides of a current collector, and a negative electrode active material on one side or both sides of the current collector. A lithium secondary battery including a negative electrode having a negative electrode mixture layer, a non-aqueous electrolyte, and a separator, wherein the positive electrode mixture layer of the positive electrode has a general composition formula Li _{1 + y} Ni _1-a-b-c Co _a Mn _b M ¹ _c O ₂ [wherein M ¹ is selected from the group consisting of Mg, Al, Ti, Fe, Cu, Zn, Ga, Ge, Zr, Nb, Mo, Sn, Sr, W, B, P, and Bi. At least one element selected, -0.15 ≦ y ≦ 0.15, 0.05 ≦ a ≦ 0.3, 0.05 ≦ b ≦ 0.3, 0 ≦ c ≦ 0.03, And a + b + c ≦ 0.5]. The negative electrode mixture layer of the negative electrode is a material containing Si and O as constituent elements (provided that the atomic ratio x of O to Si is 0.5 ≦ x ≦ 1.5) ) As a negative electrode active material, and the non-aqueous electrolyte contains a compound having a nitrile group in the molecule.

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

BRIEF DESCRIPTION OF THE DRAWINGS It is a figure which shows typically an example of the lithium secondary battery of this invention, (a) is the top view, (b) is the fragmentary longitudinal cross-sectional view. It is a perspective view of the lithium secondary battery shown in FIG.

As described above, in a lithium secondary battery, when the capacity is increased by using SiO _x as the negative electrode active material and setting the charging voltage to 4.3 V or higher, the continuous charging as described above is continued. In such a case, battery characteristics are greatly impaired by elution of ions of transition metal elements from the positive electrode active material into the non-aqueous electrolyte. This is because transition metal element ions eluted from the positive electrode active material are selectively deposited at the location where SiO _x exists on the negative electrode surface, thereby causing deterioration of SiO _x that bears a large portion of the negative electrode capacity. It is thought that.

In the lithium secondary battery of the present invention, the positive electrode that can be generated when the charge is continuously continued as described above, particularly when SiO _x is used as the negative electrode active material and the charge voltage is set to 4.3 V or higher. Suppressing the elution of ions of transition metal elements from the active material into the non-aqueous electrolyte is performed by using a lithium-containing composite oxide having a composition in which the elution of ions is difficult to occur as the positive electrode active material, and the non-aqueous electrolyte. This is achieved by including a component having an action of suppressing ion elution. Therefore, the lithium secondary battery of the present invention has a high capacity and can be used stably even by a method in which charging is continued continuously.

  Examples of the non-aqueous electrolyte according to the lithium secondary battery of the present invention include a solution (non-aqueous electrolyte) in which a lithium salt is dissolved in an organic solvent. The non-aqueous electrolyte includes a nitrile group in the molecule. The thing containing the compound which has is used.

  The compound having a nitrile group in the molecule is adsorbed on the surface of the positive electrode to form a film in the lithium secondary battery, and the non-ion of transition metal element ions from the positive electrode active material in a charged state at a high voltage. It has a function to suppress elution into the water electrolyte. Therefore, the lithium secondary battery of the present invention functions synergistically by the above-described action of the compound having a nitrile group in the molecule and the action of the use of a positive electrode active material in which elution of transition metal element ions described later hardly occurs. Therefore, it can be used stably even if the high voltage is continuously charged.

  In addition, since the compound having a nitrile group in the molecule can suppress direct contact between the positive electrode and the non-aqueous electrolyte by forming a film on the surface of the positive electrode, the non-aqueous electrolyte component accompanying charge / discharge of the battery Decomposition on the positive electrode surface and gas generation due to this can be suppressed. Therefore, in the lithium secondary battery of the present invention, the charge / discharge cycle characteristics are good, and since the battery swelling during storage can be suppressed, the storage characteristics are also good.

  Examples of the compound having a nitrile group in the molecule include a mononitrile compound having one nitrile group in the molecule, a dinitrile compound having two nitrile groups in the molecule, and a trinitrile compound having three nitrile groups in the molecule. Is mentioned. Among these, the above-mentioned actions (the action of suppressing the elution of ions of transition metal elements from the positive electrode active material by the formation of a film on the surface of the positive electrode, and the action of suppressing the reaction between the positive electrode and the nonaqueous electrolyte component) are better. , Dinitrile compounds (that is, compounds having two nitrile groups in the molecule) are preferred, and are represented by the general formula NC-R-CN (wherein R is a linear or branched hydrocarbon chain having 1 to 10 carbon atoms). More preferred are dinitrile compounds. 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 lauryl nitrile. Specific examples of the dinitrile compound represented by the general formula include, for example, malononitrile, succinonitrile, glutaronitrile, adiponitrile, 1,4-dicyanoheptane, 1,5-dicyanopentane, 1,6-dicyano. Hexane, 1,7-dicyanoheptane, 2,6-dicyanoheptane, 1,8-dicyanooctane, 2,7-dicyanooctane, 1,9-dicyanononane, 2,8-dicyanononane, 1,10-dicyanodecane, 1 , 6-dicyanodecane, 2,4-dimethylglutaronitrile, etc., and only one of them may be used, or two or more may be used in combination. Among the above-exemplified dinitrile compounds, adiponitrile is more preferable because it has a stronger effect of suppressing elution of transition metal element ions from the positive electrode active material.

  The content of the compound having a nitrile group in the molecule in the non-aqueous electrolyte used in the battery is preferably 0.1% by mass or more from the viewpoint of more effectively exerting the effect of the use of these compounds. More preferably, it is 2% by mass or more. However, if the amount of the compound having a nitrile group in the molecule is too large, for example, although the storage characteristics of the battery are further improved, the effect of improving the charge / discharge cycle characteristics may be reduced. Therefore, the content of the compound having a nitrile group in the molecule in the non-aqueous electrolyte used for the battery is preferably 5.0% by mass or less, and more preferably 4.0% by mass or less.

  Moreover, it is preferable to use what contains the phosphono acetate compound represented by following General formula (1) as a non-aqueous electrolyte.

In the general formula (1), R ¹ , R ² and R ³ are each 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 It is.

In the lithium secondary battery of the present invention, SiO _x is used as a negative electrode active material, but in such a battery, highly active Si is exposed by grinding of SiO _x particles caused by the volume change accompanying charge / discharge. (Details of the structure of SiO _x will be described later), and this decomposes the nonaqueous electrolyte, so that there is a problem that the charge / discharge cycle characteristics are liable to deteriorate.

The phosphonoacetate compound represented by the general formula (1) has a function of forming a film on the surface of the negative electrode. Even _if a SiO _x particle is pulverized by a volume change associated with charge and discharge, a new surface is generated. Coating well. Therefore, such a coating can highly suppress the reaction between the negative electrode active material and the nonaqueous electrolyte.

  The phosphonoacetate compound represented by the general formula (1) also has an action of suppressing swelling of the lithium secondary battery. Therefore, when the non-aqueous electrolyte containing the phosphonoacetate compound represented by the general formula (1) is used, this action and the action of the compound containing a nitrile group in the molecule are synergistic. It functions and can further improve the storage characteristics of the lithium secondary battery.

In the general formula (1), R ¹ , R ² and R ³ each independently represent a hydrocarbon group having 1 to 12 carbon atoms which may be substituted with a halogen atom (for example, an alkyl group, an alkenyl group, N is an integer of 0-6. That is, R ¹ , R ² and R ³ may be different from each other, or two or more may be the same.

  Specific examples of the phosphonoacetate compound represented by the general formula (1) include the following.

<Compound wherein n = 0 in the general formula (1)>
Trimethyl phosphonoformate, methyl diethylphosphonoformate, methyl dipropylphosphonoformate, methyl dibutylphosphonoformate, triethyl phosphonoformate, ethyl dimethylphosphonoformate, ethyl dipropylphosphonoformate, ethyl Dibutyl phosphonoformate, tripropyl phosphonoformate, propyl dimethylphosphonoformate, propyl diethylphosphonoformate, propyl dibutylphosphonoformate, tributyl phosphonoformate, butyl dimethylphosphonoformate, butyl diethylphospho Noformate, butyl dipropylphosphonoformate, methyl bis (2,2,2-trifluoroethyl) phosphonoformate, ethyl bis (2 2,2-trifluoroethyl) phosphonoacetate formate, propyl bis (2,2,2-trifluoroethyl) phosphonoacetate formate, and butyl bis (2,2,2-trifluoroethyl) phosphonoacetate formate.

<Compound with n = 1 in the general formula (1)>
Trimethyl phosphonoacetate, methyl diethyl phosphonoacetate, methyl dipropyl phosphonoacetate, methyl dibutyl phosphonoacetate, triethyl phosphonoacetate, ethyl dimethylphosphonoacetate, ethyl dipropylphosphonoacetate, ethyl dibutylphosphonoacetate, tripropyl Phosphonoacetate, propyl dimethylphosphonoacetate, propyl diethylphosphonoacetate, propyl dibutylphosphonoacetate, tributyl phosphonoacetate, butyldimethylphosphonoacetate, butyldiethylphosphonoacetate, butyldipropylphosphonoacetate, methylbis (2 , 2,2-trifluoroethyl) phosphonoacetate, ethyl bis (2,2,2-trifluoroethyl) phos No acetate, propyl bis (2,2,2-trifluoroethyl) phosphonoacetate, butyl bis (2,2,2-trifluoroethyl) phosphonoacetate, allyl dimethylphosphonoacetate, allyl diethylphosphonoacetate, 2 -Propynyl dimethylphosphonoacetate, 2-propynyl diethylphosphonoacetate, etc.

<Compound wherein n = 2 in the general formula (1)>
Trimethyl 3-phosphonopropionate, methyl 3- (diethylphosphono) propionate, methyl 3- (dipropylphosphono) propionate, methyl 3- (dibutylphosphono) propionate, triethyl 3-phosphonopropionate, ethyl 3- (dimethylphosphono) propionate, ethyl 3- (dipropylphosphono) propionate, ethyl 3- (dibutylphosphono) propionate, tripropyl 3-phosphonopropionate, propyl 3- (dimethylphosphono) propionate, Propyl 3- (diethylphosphono) propionate, propyl 3- (dibutylphosphono) propionate, tributyl 3-phosphonopropionate, butyl 3- (dimethylphosphono) propionate, butyl 3- (diethylphosphono) prop Pionate, butyl 3- (dipropylphosphono) propionate, methyl 3- (bis (2,2,2-trifluoroethyl) phosphono) propionate, ethyl 3- (bis (2,2,2-trifluoroethyl) phosphono ) Propionate, propyl 3- (bis (2,2,2-trifluoroethyl) phosphono) propionate, butyl 3- (bis (2,2,2-trifluoroethyl) phosphono) propionate, and the like.

<Compound wherein n = 3 in the general formula (1)>
Trimethyl 4-phosphonobutyrate, methyl 4- (diethylphosphono) butyrate, methyl 4- (dipropylphosphono) butyrate, methyl 4- (dibutylphosphono) butyrate, triethyl 4-phosphonobutyrate, ethyl 4- (Dimethylphosphono) butyrate, ethyl 4- (dipropylphosphono) butyrate, ethyl 4- (dibutylphosphono) butyrate, tripropyl 4-phosphonobutyrate, propyl 4- (dimethylphosphono) butyrate, propyl 4- (Diethylphosphono) butyrate, propyl dibutylphosphono) butyrate, tributyl 4-phosphonobutyrate, butyl 4- (dimethylphosphono) butyrate, butyl 4- (diethylphosphono) butyrate, butyl 4- (dipropylphosphono) ) Butyrate etc.

  Among the exemplified phosphonoacetate compounds, 2-propynyl diethylphosphonoacetate (PDPA) and ethyl diethylphosphonoacetate (EDPA) are particularly preferable.

  The content of the phosphonoacetate compound represented by the general formula (1) in the non-aqueous electrolyte used for the lithium secondary battery is 0.5% by mass or more from the viewpoint of better ensuring the effect of the use. It is preferable that it is 1.0 mass% or more. However, if the content of the phosphonoacetate 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 reduced. Therefore, the content of the phosphonoacetate compound represented by the general formula (1) in the nonaqueous electrolyte used for the lithium secondary battery is preferably 30% by mass or less, and 5.0% by mass or less. More preferably.

When any of R ¹ , R ² , and R ³ of the general formula (1) representing the phosphonoacetate compound contains an unsaturated bond, a carbon-carbon double bond or a carbon-carbon triple is formed on the negative electrode surface. It is presumed that a film is formed by polymerization by opening the bond. The film formed in this case is highly flexible because the constituent molecule (constituent polymer) has a flexible carbon-carbon bond as the main chain. When the lithium secondary battery is charged / discharged, the negative electrode active material expands and contracts accordingly, so that the entire negative electrode (negative electrode mixture layer) also changes in volume. However, when a film derived from a phosphonoacetate compound is formed on the surface of the negative electrode (negative electrode mixture layer), since this film is rich in flexibility as described above, the volume of the negative electrode accompanying charging / discharging of the battery Following the change, cracks and cracks are less likely to occur, and therefore the above-described effect of the film derived from the phosphonoacetate compound can be maintained satisfactorily even when the battery is repeatedly charged and discharged.

  In addition, it is preferable to use a non-aqueous electrolyte that also contains a halogen-substituted cyclic carbonate. The halogen-substituted cyclic carbonate acts on the negative electrode and has a function of suppressing the reaction between the negative electrode and the nonaqueous electrolyte component. Therefore, by using a non-aqueous electrolyte that also contains a halogen-substituted cyclic carbonate, a lithium secondary battery with 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.

In the general formula (2), R ⁴ , R ⁵ , R ⁶ and R ⁷ represent hydrogen, a halogen element or an alkyl group having 1 to 10 carbon atoms, and a part or all of the hydrogen in the alkyl group is halogen. may be substituted with an ^element, at least one of R ^4, R 5, ^{R 6} and ^{R 7} are ^halogen, R ^4, R 5, ^{R 6} and ^{R 7} have different respective Two or more may be the same. When R ⁴ , R ⁵ , R ⁶ and R ⁷ are alkyl groups, the smaller the number of carbon atoms, the better. As the halogen element, fluorine is particularly preferable.

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

  The content of the halogen-substituted cyclic carbonate in the non-aqueous electrolyte used for the lithium secondary battery is preferably 0.1% by mass or more from the viewpoint of ensuring better the effect of the use, More preferably, it is at least mass%. However, if the content of the halogen-substituted cyclic carbonate in the non-aqueous electrolyte is too large, the effect of improving storage characteristics may be reduced. 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, it is preferable to use a non-aqueous electrolyte that also contains vinylene carbonate (VC). VC acts on a negative electrode (particularly a negative electrode using a carbon material as a negative electrode active material) and has a function of suppressing a reaction between the negative electrode and a nonaqueous electrolyte component. Therefore, by using a nonaqueous electrolyte containing VC, a lithium secondary battery with better charge / discharge cycle characteristics can be obtained.

  The content of VC in the non-aqueous electrolyte used for the lithium secondary battery is preferably 0.1% by mass or more, and 1.0% by mass or more from the viewpoint of better ensuring the effect of the use. It is more preferable. However, when there is too much content of VC in a nonaqueous electrolyte, there exists a possibility that the improvement effect of a storage characteristic may become small. Therefore, the content of VC in the non-aqueous electrolyte used for the lithium secondary battery is preferably 10% by mass or less, and more preferably 4.0% by mass or less.

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

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

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

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

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

  The positive electrode according to the lithium secondary battery of the present invention has a structure having a positive electrode mixture layer containing a positive electrode active material, a binder, a conductive additive and the like on one side or both sides of a current collector.

The positive electrode active material includes a general composition formula Li _{1 + y} Ni _1-a-b-c Co _a Mn _b M ¹ _c O ₂ (where M ¹ is Mg, Al, Ti, Fe, Cu, Zn, Ga, Ge, It is at least one element selected from the group consisting of Zr, Nb, Mo, Sn, W, B, P and Bi, -0.15 ≦ y ≦ 0.15, 0.05 ≦ a ≦ 0.3 , 0.05 ≦ b ≦ 0.3, 0 ≦ c ≦ 0.03, and a + b + c ≦ 0.5) are used.

  In addition to a high capacity, the lithium nickel composite oxide (a) hardly causes elution of metal ions into the non-aqueous electrolyte in the battery, and has high thermal stability. Therefore, the lithium secondary battery of the present invention functions synergistically with the action of the lithium nickel composite oxide (a) and the action of the compound having a nitrile group in the molecule contained in the non-aqueous electrolyte. In addition, it can be stably used even by a method in which charging is continued continuously. 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 that contributes to an increase in the capacity of the lithium nickel composite oxide (b).

  In the lithium nickel composite oxide (a), Co contributes to an increase in capacity of the lithium nickel composite oxide (a) and also contributes to an improvement in the packing density of the lithium nickel composite oxide (a) in the positive electrode mixture layer. In addition, Co suppresses fluctuations in the valence of Mn associated with Li doping and dedoping during battery charging / discharging, stabilizes the average valence of Mn at a value close to tetravalent, and further improves the reversibility of charging / discharging. It also has an enhancing effect. Therefore, from the viewpoint of satisfactorily exerting the above-described action by Co, in the general composition formula representing the lithium nickel composite oxide (a), a representing the amount of Co is 0.05 or more, 0.1 or more It is preferable that

  However, if the amount of Co in the lithium nickel composite oxide (a) is too large, the amount of other elements decreases, and the effects of these other elements may not be sufficiently exhibited. Therefore, in the general composition formula representing the lithium nickel composite oxide (a), a representing the amount of Co is 0.3 or less, and preferably 0.2 or less.

  In the lithium nickel composite oxide (a), Mn has an effect of increasing the thermal stability of the lithium nickel composite oxide (a). Therefore, in the general composition formula representing the lithium nickel composite oxide (a), b representing the amount of Mn is 0.05 or more, and 0.1 or more from the viewpoint of satisfactorily exerting the above-described action by Mn. It is preferable that

  However, if the amount of Mn in the lithium nickel composite oxide (a) is too large, the amount of other elements decreases, and the action of these other elements may not be sufficiently exhibited. Therefore, in the composition formula representing the lithium nickel composite oxide (a), b representing the amount of Mn is 0.3 or less, and preferably 0.2 or less.

In addition to Li, O, Ni, Co and Mn, the lithium nickel composite oxide (a) includes Mg, Al, Ti, Fe, Cu, Zn, Ga, Ge, Zr, Nb, Mo, Sn, W, B, may contain at least one element M ¹ is selected from the group consisting of P and Bi. However, when the amount of the element M ¹ in the lithium nickel composite oxide (a) is too large, Ni and Co, with the amount of Mn is low, the action of these there is a possibility that not sufficiently exhibited. Therefore, in the general composition formula representing the lithium nickel composite oxide (a), c representing the amount of the element M ¹ is 0.03 or less, and preferably 0.01 or less. Further, the lithium nickel composite oxide (a) may not contain the element M ^1, in the general formula, the c representing the amount of the element M ^1, is greater than zero.

In the general composition formula representing the lithium nickel composite oxide (a), the total amount “a + b + c” of a representing the amount of Co, b representing the amount of Mn, and c representing the amount of the element M ¹ is If the amount is too large, the amount of Ni is decreased, and the capacity of the lithium nickel composite oxide (a) may be decreased. Therefore, the “a + b + c” is 0.5 or less, and preferably 0.3 or less.

  That is, in the general composition formula representing the lithium-nickel composite oxide (a), “1-abc” representing the amount of Ni is 0.5 or more, and preferably 0.6 or more. In addition, from the viewpoint of satisfactorily ensuring the effect of addition of Co, Mn, etc., it is 0.9 or less, and preferably 0.8 or less.

  The lithium nickel composite oxide (a) has a higher true density and a higher energy volume density, particularly when the composition is close to the stoichiometric ratio. , −0.15 ≦ y ≦ 0.15, and by adjusting the value of y in this way, the true density and reversibility during charge / discharge can be enhanced.

Lithium nickel composite oxide (a) includes Li-containing compounds (such as lithium hydroxide), Ni-containing compounds (such as nickel sulfate), Co-containing compounds (such as cobalt sulfate), Mn-containing compounds (such as manganese sulfate), and as required depending compound containing an element M ¹ and (oxides, hydroxides, sulfates, etc.) may be mixed and prepared by, for example, firing the raw material mixture. In addition, in order to synthesize lithium nickel composite oxide (a) with higher purity, a composite compound containing a plurality of elements of Ni, Co, Mn, and element M ¹ to be contained as required (hydroxide, It is preferable to mix an oxide or the like) with another raw material compound (Li-containing compound or the like), and to fire this raw material mixture.

  The firing conditions of the raw material mixture for synthesizing the lithium nickel composite oxide (a) can be, for example, 800 to 1050 ° C. for 1 to 24 hours, but once lower than the firing temperature (for example, 250 to It is preferable to carry out preliminary heating by heating to 850 ° C. and holding at that temperature, and then proceed to the reaction by raising the temperature to the firing temperature. Although there is no restriction | limiting in particular about the time of preheating, Usually, what is necessary is just to be about 0.5 to 30 hours. The atmosphere during firing can be an atmosphere containing oxygen (that is, in the air), a mixed atmosphere of an inert gas (such as argon, helium, or nitrogen) and oxygen gas, or an oxygen gas atmosphere. The oxygen concentration (volume basis) is preferably 15% or more, and more preferably 18% or more.

In addition to the lithium nickel composite oxide (a), the positive electrode active material includes lithium cobalt oxide such as LiCoO ₂ ; lithium manganese oxide such as LiMnO ₂ and Li ₂ MnO ₃ ; lithium nickel oxide such as LiNiO ₂ ; Lithium-containing composite oxide having a layered structure such as LiCo _1-x NiO ₂ ; Lithium-containing composite oxide having a spinel structure such as LiMn ₂ O ₄ and Li _4/3 Ti _5/3 O ₄ ; Olivine structure such as LiFePO ₄ Lithium-containing composite oxides; oxides having the above-mentioned oxide as a basic composition and substituted with various elements (however, other than lithium-nickel composite oxides (a)); and the like are used together with lithium-nickel composite oxides (a) can do.

Further, the general composition _{formula Li 1 + o Co 1-p} -q Mg p M 2 q O 2 ( however, ^{M 2} is Al, Ti, Fe, Cu, Zn, Ga, Ge, Zr, Nb, Mo, Sn, Sr, At least one element selected from the group consisting of W, B, P and Bi, -0.3 ≦ o ≦ 0.3, 0.001 ≦ p ≦ 0.1, and 0 ≦ q ≦ 0. The lithium cobalt composite oxide (b) represented by 1) can also be used together with the lithium nickel composite oxide (a). Lithium cobalt composite oxide (b) contains Mg, and, due to its action, for example, stability in a high voltage region is higher than LiCoO ₂ which is widely used as a positive electrode active material of a lithium secondary battery. The elution of metal (mainly Co) ions into the non-aqueous electrolyte hardly occurs in the battery, and the thermal stability is also high. Therefore, even when the lithium cobalt composite oxide (b) is used in combination with the lithium nickel composite oxide (a), it is highly stable even when charged at a high voltage, charge / discharge cycle characteristics, high temperature storage characteristics, load characteristics. Thus, a lithium secondary battery with better safety can be obtained.

In the lithium cobalt composite oxide (b), Co is a component that contributes to an increase in capacity of the lithium cobalt composite oxide (b). The lithium cobalt composite oxide (b) contains Mg and element M ² in addition to Li, O and Co as shown in the general composition formula (the element M ² may not be contained). The amount of Co is represented by “1-pq” using the amount p of Mg and the amount q of element M ² . Specifically, the amount of Co “1-pq” in the lithium cobalt composite oxide (b) is preferably 0.9 or more and 0.95 or more from the viewpoint of increasing the capacity thereof. Further, from the viewpoint of ensuring a good effect by adding Mg or the like, it is preferably 0.999 or less, and more preferably 0.05 or less.

  In the lithium cobalt composite oxide (b), Mg has an effect of improving the stability of the lithium cobalt composite oxide (b) in the high voltage region and suppressing the elution of metal ions. It also has the effect of increasing the thermal stability of the composite oxide (b). Therefore, from the viewpoint of satisfactorily exerting the above-described action by Mg, in the general composition formula representing the lithium cobalt composite oxide (b), p representing the amount of Mg is preferably 0.001 or more, 0 More preferably, it is 0.002 or more.

  However, since Mg cannot contribute to the capacity improvement of the lithium cobalt composite oxide (b), if the amount of Mg in the lithium cobalt composite oxide (b) is too large, for example, the amount of Co is reduced. There is a risk that the capacity will decrease. Therefore, in the general composition formula representing 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.

In addition to Li, O, Co and Mg, the lithium cobalt composite oxide (b) includes Al, Ti, Fe, Cu, Zn, Ga, Ge, Zr, Nb, Mo, Sn, Sr, W, B, P and it may contain at least one element M ² selected from the group consisting of Bi. However, if the amount of the element M ^{2 in} the lithium cobalt composite oxide (b) is too large, the amounts of Co and Mg are decreased, and the effects of these may not be sufficiently exhibited. Therefore, in the general composition formula representing the lithium cobalt composite oxide (b), q representing the amount of the element M ² is preferably 0.1 or less, and more preferably 0.05 or less. Further, lithium-cobalt composite oxide (b) is, as described above, may not contain the element M ^2, in the general formula, the q indicating the amount of elemental M ^2, is greater than zero.

  The lithium cobalt composite oxide (b) has a higher true density and a higher energy volume density, particularly when the composition is close to the stoichiometric ratio. , −0.3 ≦ o ≦ 0.3, and by adjusting the value of o in this way, the true density and reversibility during charge / discharge can be enhanced.

The lithium-cobalt composite oxide (b) is composed of a Li-containing compound (such as lithium hydroxide), a Co-containing compound (such as cobalt sulfate), a Mg-containing compound (such as magnesium sulfate), and an element M ² as necessary. It can be synthesized by mixing (oxide, hydroxide, sulfate, etc.) and firing this raw material mixture. In order to synthesize lithium cobalt composite oxide (b) with higher purity, a composite compound (hydroxide, oxide, etc.) containing Co and Mg, and further element M ² as required, and a Li-containing compound It is preferable to sinter the raw material mixture.

  The firing conditions of the raw material mixture for synthesizing the lithium cobalt composite oxide (b) can be set to, for example, 800 to 1050 ° C. for 1 to 24 hours, as in the case of the lithium nickel composite oxide (a). However, it is preferable that the temperature is once lowered to a temperature lower than the firing temperature (for example, 250 to 850 ° C.), preheated by holding at that temperature, and then heated to the firing temperature to advance the reaction. Although there is no restriction | limiting in particular about the time of preheating, Usually, what is necessary is just to be about 0.5 to 30 hours. The atmosphere during firing can be an atmosphere containing oxygen (that is, in the air), a mixed atmosphere of an inert gas (such as argon, helium, or nitrogen) and oxygen gas, or an oxygen gas atmosphere. The oxygen concentration (volume basis) is preferably 15% or more, and more preferably 18% or more.

  It is more preferable to use the lithium cobalt composite oxide (b) for the other positive electrode active material when the lithium nickel composite oxide (a) and another positive electrode active material are used in combination for the positive electrode active material.

  When the lithium nickel composite oxide (a) and another positive electrode active material [for example, lithium cobalt composite oxide (b)] are used in combination with the positive electrode active material, the effect of using the lithium nickel composite oxide (a) From the viewpoint of ensuring better, the content of the lithium nickel composite oxide (a) in the total amount of the positive electrode active material is preferably 10% by mass or more.

  Further, when the lithium nickel composite oxide (a) and the lithium cobalt composite oxide (b) are used in combination with 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 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 20 mass% or more].

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

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

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

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

  Moreover, as a composition of a positive mix layer, it is preferable that the quantity of a positive electrode active material is 60-95 mass%, for example, it is preferable that the quantity of a binder is 1-15 mass%, and the quantity of a conductive support agent. Is preferably 3 to 20% by mass.

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

For the negative electrode according to the lithium secondary battery of the present invention, for example, a negative electrode mixture layer containing a negative electrode active material or a binder is used on one side or both sides of the current collector. Then, the negative active material according to the negative electrode, using the SiO _x.

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

Then, SiO _x is preferably a complex complexed with carbon materials, for example, it is desirable that the surface of the SiO _x is coated with a carbon material. As described above, since SiO _x has poor conductivity, when using it as a negative electrode active material, from the viewpoint of securing good battery characteristics, a conductive material (conductive aid) is used, and SiO _x in the negative electrode is used. It is necessary to form an excellent conductive network by mixing and dispersing the material and the conductive material well. If complexes complexed with carbon material SiO _x, for example, simply than with a material obtained by mixing a conductive material such as SiO _x and the carbon material, good conductive network in the negative electrode Formed.

The complex of the SiO _x and the carbon material, as described above, other although the surface of the SiO _x coated with carbon material, such as granules of SiO _x and the carbon material can be cited.

In addition, since the composite in which the surface of SiO _x is coated with a carbon material is further combined with a conductive material (carbon material or the like), a better conductive network can be formed in the negative electrode. Therefore, it is possible to realize a lithium secondary battery with higher capacity and more excellent battery characteristics (for example, charge / discharge cycle characteristics). The complex of the SiO _x and the carbon material coated with a carbon material, for example, like granules the mixture was further granulated with SiO _x and the carbon material coated with a carbon material.

Further, as SiO _x whose surface is coated with a carbon material, the surface of a composite (for example, a granulated body) of SiO _x and a carbon material having a smaller specific resistance value is further coated with a carbon material. Those can also be preferably used. In a state where SiO _x and the carbon material are dispersed inside the granulated body, a better conductive network can be formed. Therefore, in a lithium secondary battery having a negative electrode containing SiO _x as a negative electrode active material, a heavy load Battery characteristics such as discharge characteristics can be further improved.

Preferred examples of the carbon material that can be used to form a composite with SiO _x include carbon materials such as low crystalline carbon, carbon nanotubes, and vapor grown carbon fibers.

The details of the carbon material include at least one selected from the group consisting of fibrous or coiled carbon materials, carbon black (including acetylene black and ketjen black), artificial graphite, graphitizable carbon, and non-graphitizable carbon. A seed material is preferred. Fibrous or coil-like carbon materials are preferable in that they easily form a conductive network and have a large surface area. Carbon black (including acetylene black and ketjen black), graphitizable carbon, and non-graphitizable carbon have high electrical conductivity and high liquid retention, and even if SiO _x particles expand and contract. This is preferable in that it has a property of easily maintaining contact with the particles.

Further, as will be described in detail later, in the present invention, it is preferable to use a graphitic carbon material as a negative electrode active material together with SiO _x , but this graphitic carbon material is related to a composite of SiO _x and a carbon material. It can also be used as a carbon material. Graphite carbon material, like carbon black, has high electrical conductivity and high liquid retention, and even when SiO _x particles expand and contract, they easily maintain contact with the particles. Therefore, it can be preferably used for forming a complex with SiO _x .

Among the carbon materials exemplified above, a fibrous carbon material is particularly preferable for use when the composite with SiO _x is a granulated body. Fibrous carbon material can follow the expansion and contraction of SiO _x with the charging and discharging of the battery due to the high shape is thin threadlike flexibility, also because bulk density is large, many and SiO _x particles It is because it can have a junction. Examples of the fibrous carbon include polyacrylonitrile (PAN) -based carbon fiber, pitch-based carbon fiber, vapor-grown carbon fiber, and carbon nanotube, and any of these may be used.

The fibrous carbon material can also be formed on the surface of the SiO _x particles by, for example, a vapor phase method.

The specific resistance value of SiO _x is usually 10 ³ to 10 ⁷ kΩcm, while the specific resistance value of the carbon material exemplified above is usually 10 ^{−5 to} 10 kΩcm.

The composite of SiO _x and the carbon material may further have a material layer (a material layer containing non-graphitizable carbon) that covers the carbon material coating layer on the particle surface.

When a composite of SiO _x and a carbon material is used for the negative electrode, the ratio of SiO _x and the carbon material is based on SiO _x : 100 parts by mass from the viewpoint of satisfactorily exerting the effect of the composite with the carbon material. The carbon material is preferably 5 parts by mass or more, and more preferably 10 parts by mass or more. Further, in the composite, if the ratio of the carbon material to be combined with SiO _x is too large, it may lead to a decrease in the amount of SiO _x in the negative electrode mixture layer, and the effect of increasing the capacity may be reduced. SiO _x: relative to 100 parts by weight, the carbon material, and more preferably preferably not more than 50 parts by weight, more than 40 parts by weight.

The composite of the SiO _x and the carbon material can be obtained, for example, by the following method.

First, a manufacturing method in the case of combining SiO _x will be described. A dispersion liquid in which SiO _x is dispersed in a dispersion medium is prepared, and sprayed and dried to produce composite particles including a plurality of particles. For example, ethanol or the like can be used as the dispersion medium. It is appropriate to spray the dispersion in an atmosphere of 50 to 300 ° C. In addition to the above method, similar composite particles can be produced also by a granulation method by a mechanical method using a vibration type or planetary type ball mill or rod mill.

Incidentally, the SiO _x, in the case of manufacturing a granulated body with small carbon material resistivity value than SiO _x is adding the carbon material in the dispersion liquid of SiO _x are dispersed in a dispersion medium, the dispersion by using a liquid, by a similar method to the case of composite of SiO _x may be a composite particle (granule). Further, by granulation process according to the similar mechanical method, it is possible to produce a granular material of the SiO _x and the carbon material.

Next, when the surface of SiO _x particles (SiO _x composite particles or a granulated body of SiO _x and a carbon material) is coated with a carbon material to form a composite, for example, the SiO _x particles and the hydrocarbon-based material The gas is heated in the gas phase, and carbon generated by pyrolysis of the hydrocarbon-based gas is deposited on the surface of the particles. As described above, according to the vapor deposition (CVD) method, the hydrocarbon-based gas spreads to every corner of the composite particle, and the surface of the particle and the pores in the surface are thin and contain a conductive carbon material. Since a uniform film (carbon material coating layer) can be formed, the SiO _x particles can be imparted with good conductivity with a small amount of carbon material.

In the production of SiO _x coated with a carbon material, the processing temperature (atmosphere temperature) of the vapor deposition (CVD) method varies depending on the type of hydrocarbon gas, but usually 600 to 1200 ° C. is appropriate. Among these, the temperature is preferably 700 ° C. or higher, and more preferably 800 ° C. or higher. This is because the higher the treatment temperature, the less the remaining impurities, and the formation of a coating layer containing carbon having high conductivity.

  As the liquid source of the hydrocarbon-based gas, toluene, benzene, xylene, mesitylene and the like can be used, but toluene that is easy to handle is particularly preferable. A hydrocarbon-based gas can be obtained by vaporizing them (for example, bubbling with nitrogen gas). Moreover, methane gas, acetylene gas, etc. can also be used.

In addition, after the surface of SiO _x particles (SiO _x composite particles or a granulated body of SiO _x and a carbon material) is covered with a carbon material by a vapor deposition (CVD) method, a petroleum-based pitch or a coal-based pitch is used. At least one organic compound selected from the group consisting of a thermosetting resin and a condensate of naphthalene sulfonate and aldehydes is attached to a coating layer containing a carbon material, and then the organic compound is attached. The obtained particles may be fired.

Specifically, a dispersion liquid in which a SiO _x particle (SiO _x composite particle or a granulated body of SiO _x and a carbon material) coated with a carbon material and the organic compound are dispersed in a dispersion medium is prepared, The dispersion is sprayed and dried to form particles coated with the organic compound, and the particles coated with the organic compound are fired.

  An isotropic pitch can be used as the pitch, and a phenol resin, a furan resin, a furfural resin, or the like can be used as the thermosetting resin. As the condensate of naphthalene sulfonate and aldehydes, naphthalene sulfonic acid formaldehyde condensate can be used.

As a dispersion medium for dispersing the SiO _x particles coated with the carbon material and the organic compound, for example, water or alcohols (ethanol or the like) can be used. It is appropriate to spray the dispersion in an atmosphere of 50 to 300 ° C. The firing temperature is usually 600 to 1200 ° C., preferably 700 ° C. or higher, and more preferably 800 ° C. or higher. This is because the higher the processing temperature, the less the remaining impurities, and the formation of a coating layer containing a high-quality carbon material with high conductivity. However, the processing temperature needs to be lower than the melting point of SiO _x .

It is preferable to use a graphitic carbon material together with SiO _x for the negative electrode active material according to the lithium secondary battery of the present invention. By reducing the ratio of SiO _x in the negative electrode active material using a graphitic carbon material, while suppressing the decrease in the effect of increasing the capacity due to the reduction in SiO _x as much as possible, the negative electrode accompanying charging / discharging of the battery ( It is possible to suppress a change in volume of the negative electrode mixture layer) and suppress a decrease in battery characteristics that may be caused by the volume change.

Examples of the graphitic carbon material used as the negative electrode active material together with SiO _x include natural graphite such as scaly graphite; pyrolytic carbons, mesophase carbon microbeads (MCMB), carbon fiber and other graphitizable carbon at 2800 ° C. Examples thereof include artificial graphite graphitized as described above.

In the negative electrode according to the present invention, from the viewpoint of satisfactorily ensuring the effect of the high capacity by using a SiO _x, the content of SiO _x in the anode active material is 0.01 wt% or more It is preferably 3% by mass or more. Further, from the viewpoint of better avoiding the problem due to the volume change of the negative electrode due to charge / discharge, the content of SiO _x in the negative electrode active material is preferably 30% by mass or less, and 20% by mass or less. Is more preferable.

  For example, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), styrene butadiene rubber (SBR), carboxymethyl cellulose (CMC), or the like is preferably used for the binder related to the negative electrode mixture layer. Furthermore, you may add various carbon blacks, such as acetylene black, a carbon nanotube, carbon fiber, etc. as a conductive support agent to a negative mix layer.

  The negative electrode is prepared, for example, by preparing a negative electrode mixture-containing composition in which a negative electrode active material and a binder and, if necessary, a conductive additive are dispersed in a solvent such as NMP or water (however, the binder is dissolved in the solvent). It may be applied to one or both sides of the current collector, dried, and then subjected to a calendaring process as necessary. However, the negative electrode is not limited to those manufactured by the above manufacturing method, and may be manufactured by other manufacturing methods.

The thickness of the negative electrode mixture layer is preferably 10 to 100 μm per one side of the current collector, and the density of the negative electrode mixture layer (the negative electrode mixture per unit area laminated on the current collector) (Calculated from the mass and thickness of the layer) is preferably 1.0 to 1.9 g / cm ³ . Moreover, as a composition of a negative mix layer, it is preferable that the quantity of a negative electrode active material is 80-95 mass%, for example, it is preferable that the quantity of a binder is 1-20 mass%, and uses a conductive support agent. When it does, it is preferable that the quantity is 1-10 mass%.

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

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

  The separator according to the lithium secondary battery of the present invention includes a porous layer (I) mainly composed of a resin having a melting point of 140 ° C. or lower, a resin that does not melt at a temperature of 150 ° C. or lower, or a heat resistant temperature of 150 ° C. or higher. It is preferable to use a laminated separator having a porous layer (II) mainly containing the inorganic filler. Here, the “melting point” means a melting temperature measured using a differential scanning calorimeter (DSC) in accordance with JIS K 7121. In addition, “does not melt at a temperature of 150 ° C. or lower” means that the melting temperature measured using DSC exceeds 150 ° C. according to the provisions of JIS K 7121. This means that the melting behavior is not exhibited at the temperature. Furthermore, “the heat resistant temperature is 150 ° C. or higher” means that deformation such as softening is not observed at least at 150 ° C.

  The porous layer (I) according to the multilayer separator is mainly for ensuring a shutdown function, and the melting point of the resin, which is a component in which the lithium secondary battery is the main component of the porous layer (I) When the temperature reaches the value, the resin related to the porous layer (I) melts to close the pores of the separator, thereby causing a shutdown that suppresses the progress of the electrochemical reaction.

  Examples of the resin having a melting point of 140 ° C. or lower, which is the main component of the porous layer (I), include PE, and the form thereof is a substrate such as a microporous film used in the above-described lithium secondary battery or a nonwoven fabric. And a dispersion obtained by applying a dispersion containing PE particles and drying. Here, in all the constituent components of the porous layer (I), the volume of the resin having a main melting point of 140 ° C. or less is 50% by volume or more, and more preferably 70% by volume 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 ° C. or lower is 100% by volume.

  The porous layer (II) according to the multilayer 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 is increased. The function is secured by a resin that does not melt at a temperature of ℃ or less or an inorganic filler with a heat resistant temperature of 150 ℃ or more. That is, when the battery becomes hot, even if the porous layer (I) shrinks, the porous layer (II) that does not easily shrink can cause the positive and negative electrodes directly when the separator is thermally contracted. It is possible to prevent a short circuit due to the contact of. Moreover, since this heat-resistant porous layer (II) acts as a skeleton of the separator, the thermal contraction of the porous layer (I), that is, the thermal contraction of the entire separator itself can be suppressed.

  When the porous layer (II) is mainly composed of a resin having a melting point of 150 ° C. or higher, for example, a microporous film formed of a resin that does not melt at a temperature of 150 ° C. or lower (for example, the above-mentioned PP microporous for battery The film is laminated on the porous layer (I), and a dispersion containing resin particles that do not melt at a temperature of 150 ° C. or less is applied to the porous layer (I) and dried to form the porous layer (I). An application lamination type form in which the porous layer (II) is formed on the surface of the substrate is mentioned.

  Examples of resins that do not melt at a temperature of 150 ° C. or less include PP; crosslinked polymethyl methacrylate, crosslinked polystyrene, crosslinked polydivinylbenzene, styrene-divinylbenzene copolymer crosslinked product, polyimide, melamine resin, phenol resin, benzoguanamine-formaldehyde condensation And various crosslinked polymer fine particles; polysulfone; polyether sulfone; polyphenylene sulfide; polytetrafluoroethylene; polyacrylonitrile; aramid; polyacetal and the like.

  When using resin particles that do not melt at a temperature of 150 ° C. or lower, the average particle size is, for example, preferably 0.01 μm or more, more preferably 0.1 μm or more, It is preferably 10 μm or less, and more preferably 2 μm or less. In addition, the average particle diameter of the various particles referred to in the present specification is determined by, for example, using a laser scattering particle size distribution meter (for example, “LA-920” manufactured by HORIBA, Ltd.) and dispersing these fine particles in a medium that does not dissolve the resin. The measured average particle diameter D is 50%.

  When the porous layer (II) is formed mainly of an inorganic filler having a heat resistant temperature of 150 ° C. or higher, for example, a dispersion containing an inorganic filler having a heat resistant temperature of 150 ° C. or higher is applied to the porous layer (I). Examples of the coating laminated type in which the porous layer (II) is formed by drying.

  The inorganic filler related to the porous layer (II) has a heat-resistant temperature of 150 ° C. or higher, is stable with respect to the nonaqueous electrolyte of the battery, and is electrochemically stable that is not easily oxidized or reduced in the battery operating voltage range. However, fine particles are preferable from the viewpoint of dispersion, and alumina, silica, and boehmite are preferable. Alumina, silica, and boehmite have high oxidation resistance, and the particle size and shape can be adjusted to the desired numerical values, making it easy to accurately control the porosity of the porous layer (II). It becomes. In addition, as for the inorganic filler whose heat-resistant temperature is 150 degreeC or more, the thing of the said illustration may be used individually by 1 type, and may use 2 or more types together, for example. In addition, an inorganic filler having a heat resistant temperature of 150 ° C. may be used in combination with a resin that does not melt at a temperature of 150 ° C. or lower.

  The shape of the inorganic filler having a heat resistant temperature of 150 ° C. or higher related to the porous layer (II) is not particularly limited, and is substantially spherical (including true spherical), substantially elliptical (including elliptical), plate-like, etc. Various shapes can be used.

  Further, the average particle diameter of the inorganic filler having a heat resistant temperature of 150 ° C. or higher related to the porous layer (II) is preferably 0.3 μm or more because the ion permeability is lowered if it is too small. More preferably, it is 5 μm or more. In addition, if the inorganic filler having a heat resistant temperature of 150 ° C. or higher is too large, the electrical characteristics are likely to be deteriorated. Therefore, the average particle diameter is preferably 5 μm or less, and more preferably 2 μm or less.

  In the porous layer (II), the resin that does not melt at a temperature of 150 ° C. or lower and the inorganic filler having a heat resistant temperature of 150 ° C. or higher are mainly contained in the porous layer (II). The amount in (II) [when the porous layer (II) contains only one of a resin that does not melt at a temperature of 150 ° C. or less and an inorganic filler that has a heat resistant temperature of 150 ° C. or more, is the amount, If both are included, the total amount. The same applies to the amount of the resin that does not melt at a temperature of 150 ° C. or less and the amount of the inorganic filler having a heat resistant temperature of 150 ° C. or more in the porous layer (II). ] Is 50% by volume or more in the total volume of the constituent components of the porous layer (II), preferably 70% by volume or more, more preferably 80% by volume or more, and 90% by volume or more. More preferably it is. By making the inorganic filler in the porous layer (II) high as described above, even when the lithium secondary battery becomes high temperature, the thermal contraction of the entire separator can be satisfactorily suppressed, Generation | occurrence | production of the short circuit by the direct contact of a positive electrode and a negative electrode can be suppressed more favorably.

  As will be described later, since it is preferable that the porous layer (II) also contains an organic binder, a porous layer (II) of a resin that does not melt at a temperature of 150 ° C. or lower and an inorganic filler having a heat resistant temperature of 150 ° C. or higher. ) In the total volume of the constituent components of the porous layer (II) is preferably 99.5% by volume or less.

  In the porous layer (II), a resin that does not melt at a temperature of 150 ° C. or less, or an inorganic filler having a heat resistant temperature of 150 ° C. or more is bound, or the porous layer (II) and the porous layer (I) For integration or the like, it is preferable to contain an organic binder. Examples of organic binders include ethylene-vinyl acetate copolymers (EVA, those having a structural unit derived from vinyl acetate of 20 to 35 mol%), ethylene-acrylic acid copolymers such as ethylene-ethyl acrylate copolymers, and fluorine-based binders. Examples include rubber, SBR, CMC, hydroxyethyl cellulose (HEC), polyvinyl alcohol (PVA), polyvinyl butyral (PVB), polyvinyl pyrrolidone (PVP), cross-linked acrylic resin, polyurethane, and epoxy resin. A heat-resistant binder having a heat-resistant temperature is preferably used. As the organic binder, those exemplified above may be used singly or in combination of two or more.

  Among the organic binders exemplified above, highly flexible binders such as EVA, ethylene-acrylic acid copolymer, fluorine-based rubber, and SBR are preferable. Specific examples of such highly flexible organic binders include Mitsui DuPont Polychemical's “Evaflex Series (EVA)”, Nihon Unicar's EVA, Mitsui DuPont Polychemical's “Evaflex-EAA Series (Ethylene). -Acrylic acid copolymer) ", Nippon Unicar EEA, Daikin Industries" DAI-EL Latex Series (Fluororubber) ", JSR" TRD-2001 (SBR) ", Nippon Zeon" BM-400B " (SBR) ".

  When the organic binder is used for the porous layer (II), it can be used in the form of an emulsion dissolved or dispersed in the solvent for the composition for forming the porous layer (II) described later. Good.

  The coating laminate type separator is, for example, a composition for forming a porous layer (II) containing a resin particle that does not melt at a temperature of 150 ° C. or lower, an inorganic filler having a heat resistant temperature of 150 ° C. or higher (liquid such as slurry). The composition etc.) can be applied to the surface of the microporous membrane for constituting the porous layer (I) and dried at a predetermined temperature to form the porous layer (II).

  The composition for forming the porous layer (II) contains resin particles that do not melt at a temperature of 150 ° C. or lower and / or an inorganic filler having a heat resistant temperature of 150 ° C. or higher, and an organic binder as necessary. Is dispersed in a solvent (including a dispersion medium; the same shall apply hereinafter). The organic binder can be dissolved in a solvent. The solvent used in the composition for forming the porous layer (II) can uniformly disperse resin particles and inorganic filler that do not melt at a temperature of 150 ° C. or lower, and can dissolve or disperse the organic binder uniformly. For example, general organic solvents such as aromatic hydrocarbons such as toluene, furans such as tetrahydrofuran, ketones such as methyl ethyl ketone and methyl isobutyl ketone are preferably used. In addition, for the purpose of controlling the interfacial tension, alcohols (ethylene glycol, propylene glycol, etc.) or various propylene oxide glycol ethers such as monomethyl acetate may be appropriately added to these solvents. In addition, when the organic binder is water-soluble or used as an emulsion, water may be used as a solvent. In this case, alcohols (methyl alcohol, ethyl alcohol, isopropyl alcohol, ethylene glycol, etc.) are appropriately added. It is also possible to control the interfacial tension.

  The composition for forming the porous layer (II) has a solid content containing, for example, a resin particle that does not melt at a temperature of 150 ° C. or lower and / or an inorganic filler having a heat resistant temperature of 150 ° C. or higher, and an organic binder. It is preferable to set it as -80 mass%.

In the laminated separator, the porous layer (I) and the porous layer (II) do not have to be one each, and a plurality of layers may be present in the separator. For example, a configuration in which the porous layer (I) is disposed on both sides of the porous layer (II) or a configuration in which the porous layer (II) is disposed on both sides of the porous layer (I) may be employed. However, increasing the number of layers may increase the thickness of the separator and increase the internal resistance of the battery or decrease the energy density. Therefore, it is not preferable to increase the number of layers. The total number of the porous layers (I) and (II) is preferably 5 or less.

  The thickness of the separator (a separator made of a microporous membrane made of polyolefin or the laminated separator) according to the lithium secondary battery of the present invention is preferably 10 to 30 μm, for example.

  In the laminated separator, the thickness of the porous layer (II) [when the separator has a plurality of porous layers (II), the total thickness] is determined by each of the functions of the porous layer (II). From the viewpoint of exhibiting more effectively, it is preferably 3 μm or more. However, if the porous layer (II) is too thick, the energy density of the battery may be lowered. Therefore, the thickness of the porous layer (II) is preferably 8 μm or less.

  Further, in the laminated separator, the thickness of the porous layer (I) [when the separator has a plurality of porous layers (I), the total thickness thereof. same as below. ] Is preferably 6 μm or more, and more preferably 10 μm or more, from the viewpoint of more effectively exerting the above-described action (particularly shutdown action) by using the porous layer (I). However, if the porous layer (I) is too thick, there is a possibility that the energy density of the battery may be lowered. In addition, the force that the porous layer (I) tends to shrink is increased, and the heat of the entire separator is increased. There is a possibility that the action of suppressing the shrinkage becomes small. Therefore, the thickness of the porous layer (I) is preferably 25 μm or less, more preferably 20 μm or less, and further preferably 14 μm or less.

The porosity of the separator as a whole is preferably 30% or more in a dried state in order to secure the amount of electrolyte solution retained and to improve ion permeability. On the other hand, from the viewpoint of securing separator strength and preventing internal short circuit, the separator porosity is preferably 70% or less in a dry state. The porosity of the separator: P (%) can be calculated by obtaining the sum for each component i using the following equation (3) from the thickness of the separator, the mass per area, and the density of the constituent components.
P = {1- (m / t) / (Σa _i · ρ _i )} × 100 (3)
Here, in the formula (3), a _i : ratio of component i when the total mass is 1, ρ _i : density of component i (g / cm ³ ), m: mass per unit area of the separator (G / cm ² ), t: thickness (cm) of the separator.

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

Further, in the case of the laminated separator, in the formula (3), m is the mass per unit area (g / cm ² ) of the porous layer (II), and t is the thickness of the porous layer (II) ( cm), the porosity: P (%) of the porous layer (II) can also be obtained using the formula (3). The porosity of the porous layer (II) obtained by this method is preferably 20 to 60%.

  The separator preferably has high mechanical strength. For example, the puncture strength is preferably 3N or more. For example, Si and Sn alloys and oxides contribute to an increase in capacity of a battery as a negative electrode active material having a large capacity, but have a large volume change due to charge and discharge. Therefore, when such a negative electrode active material is used, mechanical damage is also applied to the facing separator due to expansion and contraction of the entire negative electrode by repeating charge and discharge. If the piercing strength of the separator is 3N or more, good mechanical strength is ensured, and mechanical damage to the separator can be reduced.

  Examples of the separator having a puncture strength of 3N or more include the above-described laminated separator, and in particular, an inorganic filler having a heat resistant temperature of 150 ° C. or higher in the porous layer (I) mainly composed of a resin having a melting point of 140 ° C. or lower. A separator in which a porous layer (II) containing as a main component is laminated is preferable. This is probably because the mechanical strength of the inorganic filler is high, so that the mechanical strength of the entire separator can be increased by supplementing the mechanical strength of the porous layer (I).

  The puncture strength can be measured by the following method. A separator is fixed on a plate having a hole with a diameter of 2 inches so as not to be wrinkled or bent, and a semispherical metal pin having a tip diameter of 1.0 mm is lowered onto a measurement sample at a speed of 120 mm / min. Measure the force when making a hole in the separator 5 times. And an average value is calculated | required about the measurement of 3 times except the maximum value and the minimum value among the said measurement values of 5 times, and this is made into the piercing strength of a separator.

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

  In the laminated electrode body and the wound electrode body, the laminated separator, particularly the porous layer (I) mainly composed of a resin having a melting point of 140 ° C. or less, mainly comprises an inorganic filler having a heat resistant temperature of 150 ° C. or more. In the case of using a separator in which the porous layer (II) contained as a laminate is used, it is preferable that the porous layer (II) is disposed so as to face at least the positive electrode. In this case, the porous layer (II), which mainly contains an inorganic filler having a heat-resistant temperature of 150 ° C. or more, and more excellent in oxidation resistance, faces the positive electrode, so that the oxidation of the separator by the positive electrode can be suppressed more favorably. Therefore, the storage characteristics and charge / discharge cycle characteristics of the battery at a high temperature can be improved. Further, when an additive such as VC or cyclohexylbenzene is added to the non-aqueous electrolyte, there is a possibility that a film is formed on the positive electrode side to clog the pores of the separator, resulting in deterioration of battery characteristics. Therefore, an effect of suppressing pore clogging can be expected by making the relatively porous porous layer (II) face the positive electrode.

  On the other hand, when one surface of the multilayer separator is the porous layer (I), it is preferable that the porous layer (I) faces the negative electrode. The thermoplastic resin melted from the layer (I) is suppressed from being absorbed by the electrode mixture layer, and can be efficiently used to close the pores of the separator.

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

  In the lithium secondary battery of the present invention, the upper limit voltage at the time of charging before use may be about 4.2 V, which is adopted in a normal lithium secondary battery, but is charged at a high voltage of 4.3 V or more. Even the method can be used stably.

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

Example 1
<Preparation of positive electrode>
Li _1.0 Ni _0.5 Co _0.2 Mn _0.3 O ₂ [lithium nickel composite oxide (a)] and Li _1.036 Co _0.991 Al _0.004 Mg _0.002 Sr _0.001 Ti 100 parts by mass of a positive electrode active material obtained by mixing _0.002 Zr _0.001 O ₂ [lithium cobalt composite oxide (b)] at a ratio (mass ratio) of 3: 7, and 10 parts by mass of PVDF as a binder. 20 parts by weight of NMP solution contained at a concentration of 1%, 1 part by weight of artificial graphite and 1 part by weight of ketjen black, which are conductive assistants, are kneaded using a biaxial kneader, and NMP is added to adjust the viscosity. Thus, a positive electrode mixture-containing paste was prepared.

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

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

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

<Preparation of non-aqueous electrolyte>
In a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of 3: 7, LiPF ₆ is dissolved at a concentration of 1.1 mol / L, and an amount of adiponitrile is 0.1% by mass, A nonaqueous electrolyte was prepared by adding EDPA in an amount of 1.25% by mass, FEC in an amount of 2.0% by mass, and VC in an amount of 2.0% by mass.

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

<Preparation of separator>
Add 5 kg of ion-exchanged water and 0.5 kg of a dispersant (aqueous polycarboxylic acid ammonium salt, solid content concentration 40 mass%) to 5 kg of boehmite with an average particle diameter D50% of 1 μm, and have an internal volume of 20 L and 40 turns. A dispersion was prepared by pulverizing with a ball mill for 10 hours per minute. The treated dispersion was vacuum-dried at 120 ° C. and observed with a scanning electron microscope (SEM). As a result, the boehmite was almost plate-shaped.

  To 500 g of the above dispersion, 0.5 g of xanthan gum as a thickener and 17 g of a resin binder dispersion (modified polybutyl acrylate, solid content 45% by mass) as a binder are added and stirred with a three-one motor for 3 hours to form a uniform slurry. [Slurry for forming porous layer (II), solid content ratio: 50% by mass] was prepared.

PE microporous separator for a lithium secondary battery (porous layer (I): thickness 12 μm, porosity 40%, average pore diameter 0.08 μm, PE melting point 135 ° C.) on one side corona discharge treatment (discharge amount 40 W)・ Min / m ² ), a porous layer (II) forming slurry is applied to the treated surface by a micro gravure coater, and dried to form a porous layer (II) having a thickness of 4 μm. A separator was obtained. The mass per unit area of the porous layer (II) in this separator was 5.5 g / m ² , the boehmite volume content was 95% by volume, and the porosity was 45%.

  Here, the battery shown in FIGS. 1 and 2 will be described. FIG. 1A is a plan view, and FIG. 1B is a partial cross-sectional view thereof. As shown in FIG. 2 is spirally wound through the separator 3 and then pressed so as to be flat, and accommodated in a rectangular (rectangular tube) battery case 4 together with a nonaqueous electrolyte as a flat electrode winding body 6. Has been. However, in FIG. 1, in order to avoid complication, a metal foil, a non-aqueous electrolyte, and the like as a current collector used for manufacturing the positive electrode 1 and the negative electrode 2 are not illustrated.

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

  And this cover plate 9 is inserted in the opening part of the battery case 4, and the opening part of the battery case 4 is sealed by welding the joint part of both, and the inside of the battery is sealed. Further, in the battery of FIG. 1, a non-aqueous electrolyte inlet 14 is provided in the lid plate 9, and the non-aqueous electrolyte inlet 14 is welded by, for example, laser welding with a sealing member inserted. The battery is sealed to ensure the hermeticity of the battery (therefore, in the battery of FIGS. 1 and 2, the nonaqueous electrolyte inlet 14 is actually a nonaqueous electrolyte inlet and a sealing member. , For ease of explanation, shown as non-aqueous electrolyte inlet 14). Further, the lid plate 9 is provided with a cleavage vent 15 as a mechanism for discharging the internal gas to the outside when the temperature of the battery rises.

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

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

Example 2
A non-aqueous electrolyte was prepared in the same manner as in Example 1 except that the addition amount of adiponitrile was changed to 0.25% by mass. A lithium secondary secondary was prepared in the same manner as in Example 1 except that this non-aqueous electrolyte was used. A battery was produced.

Example 3
A non-aqueous electrolyte was prepared in the same manner as in Example 1 except that the amount of adiponitrile added was changed to 0.5% by mass. A lithium secondary secondary was prepared in the same manner as in Example 1 except that this non-aqueous electrolyte was used. A battery was produced.

Example 4
A nonaqueous electrolyte was prepared in the same manner as in Example 1 except that the addition amount of adiponitrile was changed to 1.0% by mass, and the lithium secondary secondary was prepared in the same manner as in Example 1 except that this nonaqueous electrolyte was used. A battery was produced.

Example 5
A non-aqueous electrolyte was prepared in the same manner as in Example 1 except that the amount of adiponitrile added was changed to 2.5% by mass. A lithium secondary secondary was prepared in the same manner as in Example 1 except that this non-aqueous electrolyte was used. A battery was produced.

Example 6
A non-aqueous electrolyte was prepared in the same manner as in Example 1 except that the addition amount of adiponitrile was changed to 5.0% by mass. A lithium secondary secondary was prepared in the same manner as in Example 1 except that this non-aqueous electrolyte was used. A battery was produced.

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% by mass and EPDA was not added, and Example 1 except that this nonaqueous electrolyte was used. In the same manner, a lithium secondary battery was produced.

Example 8
A non-aqueous electrolyte was prepared in the same manner as in Example 1 except that the addition amount of adiponitrile was changed to 7.5% by mass, and a lithium secondary secondary was prepared in the same manner as in Example 1 except that this non-aqueous electrolyte was used. A battery was produced.

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 prepared in the same manner as in Example 4 except that this nonaqueous electrolyte was used.

Example 10
A non-aqueous electrolyte was prepared in the same manner as in Example 4 except that EDPA was changed to PDPA, and a lithium secondary battery was prepared in the same manner as in Example 4 except that this non-aqueous electrolyte was used.

Comparative Example 1
A non-aqueous 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 non-aqueous electrolyte was used.

<50 ° C / DOD 5% test>
About the battery of an Example and a comparative example, the constant current charge was performed to 4.35V with the electric current value of 1.0C, and the constant voltage charge was subsequently performed with the voltage of 4.35V. The total charging time for constant current charging and constant voltage charging was 2.5 hours. Thereafter, the battery was discharged to 3.0 V at a current value of 0.2 C, and the initial capacity was measured.

  Subsequently, each battery after the initial capacity measurement was charged under the same conditions as the initial capacity measurement to obtain a fully charged state. Then, each battery in a fully charged state is discharged by 5% of the rated capacity (DOD 5%) at a current value of 1.0 C at 50 ° C., and charged to a fully charged state again at a current value of 1.0 C. A series of operations was taken as one cycle, and 800 cycles of charge / discharge were repeated. For each subsequent battery, charge / discharge was performed under the same conditions as the initial capacity measurement, and the capacity of each battery after 800 cycles was measured. The value obtained by dividing this by the initial capacity was expressed as a percentage, and the capacity maintenance rate was Asked.

  This 50 ° C / DOD 5% test is a test in which only a small part of the capacity is discharged and the battery is fully charged until the battery is fully charged. It is a thing. Therefore, it means that the higher the capacity maintenance rate in this test, the higher the stability even when the battery is used in a method of continuously charging.

<Short-circuit test after continuous charging>
For each of the batteries of Examples and Comparative Examples (batteries different from those subjected to the 50 ° C./DOD 5% test), constant current charging at 45 ° C. and a current value of 1.0 C up to 4.4 V, followed by Then, constant current-constant voltage charging in which constant voltage charging was performed at a voltage of 4.4 V was continued, and the presence or absence of a short circuit of each battery after 500 hours was examined.

  This test also assumes a method of continuous charging, and batteries that have not been short-circuited after this test are highly stable even when used continuously. Means.

<Charge / discharge cycle characteristics>
About each battery of an Example and a comparative example (battery different from what performed each said evaluation), it charged / discharged on the same conditions as the initial capacity measurement of a 50 degreeC and DOD5% test, and measured the initial capacity. .

  Next, for each battery after the initial capacity measurement, a constant current charge is performed up to 4.35V at a current value of 1C, and then a constant voltage charge is performed at a voltage of 4.35V (constant current charge and constant voltage charge The total charge time was 2.5 hours), and then a series of operations for discharging to 3.0 V at a current value of 1 C was taken as one cycle, and 500 cycles of charge and discharge were repeated. For each subsequent battery, charge and discharge were performed under the same conditions as the initial capacity measurement, and the discharge capacity of each battery after 500 cycles was measured. The value obtained by dividing this by the initial capacity was expressed as a percentage. The maintenance rate was determined. It can be said that the higher the capacity retention rate, the better the charge / discharge cycle characteristics of the battery.

<Storage test>
About the battery of an Example and a comparative example (battery different from what performed said each evaluation), it charged on the same conditions as the initial stage capacity | capacitance measurement of a 50 degreeC * DOD5% test. Each battery after charging was placed in a thermostatic bath, and the bath temperature was set to 85 ° C. and left for 24 hours. Then, each battery was taken out from the thermostat, allowed to cool to room temperature, measured for thickness, and calculated the difference from the initial thickness (4.0 mm) to determine the amount of swelling of the battery after the storage test. .

<Load characteristics>
The batteries of the examples and comparative examples (batteries different from those evaluated above) were charged and discharged under the same conditions as in the initial capacity measurement of the 50 ° C./DOD 5% test, and the discharge capacity (0.2 C discharge) Capacity). In addition, each battery after measuring the 0.2C discharge capacity is charged under the same conditions as those at the time of measuring the 0.2C discharge capacity, and discharged to 3.0V at a current value of 1.5C to obtain a discharge capacity (1. 5C discharge capacity) was measured. And about each battery, the value which remove | divided 1.5C discharge capacity by 0.2C discharge capacity was represented by the percentage, and the capacity | capacitance maintenance factor was calculated | required. It can be said that the higher the capacity retention rate, the better the load characteristics of the battery.

  Table 1 shows the content of each additive in the nonaqueous electrolyte used in the lithium secondary batteries of Examples and Comparative Examples, and Table 2 shows the evaluation results.

  As shown in Table 1 and Table 2, a non-aqueous electrolyte containing a compound using lithium nickel composite oxide (a) as a positive electrode active material, SiO as a negative electrode active material, and having a nitrile group in the molecule The used lithium secondary batteries of Examples 1 to 10 have a capacity in a 50 ° C./DOD 5% test as compared with the battery of Comparative Example 1 using a nonaqueous electrolyte containing no compound having a nitrile group in the molecule. The maintenance rate is high, and no short circuit occurs after continuous charging. Therefore, the lithium secondary batteries of Examples 1 to 10 can be used stably even if the method is such that the charging is continuously continued at a high voltage of 4.3 V or higher. Furthermore, the lithium secondary batteries of Examples 1 to 10 have good load characteristics.

  In addition, the lithium secondary batteries of Examples 1 to 6, 8, 9, and 10 using the non-aqueous electrolyte that also contains the phosphonoacetate compound represented by the general formula (1) are non-aqueous. Compared to the battery of Example 7 using an electrolyte, the amount of swelling after the storage test is small, and the storage characteristics are also excellent.

  In addition, the lithium secondary batteries of Examples 1 to 7, 9, and 10 using the nonaqueous electrolyte in which the content of the compound having a nitrile group in the molecule is suitable use the nonaqueous electrolyte having a very high content. Compared to the lithium secondary battery of Example 8, the capacity retention rate at the time of charge / discharge cycle characteristics evaluation is high, and the charge / discharge cycle characteristics are also excellent.

  The present invention can be implemented in other forms without departing from the spirit of the present invention. The embodiments disclosed in the present application are examples, and the present invention is not limited to these embodiments. The scope of the present invention is construed in preference to the description of the appended claims rather than the description of the above specification, and all modifications within the scope equivalent to the claims are construed in the scope of the claims. included.

  Since the lithium secondary battery of the present invention can be used stably even if it is continuously charged, it can be suitably used for a power source application of a portable electronic device that is likely to be used in such a method. It can be used for the same applications as various applications to which conventionally known lithium secondary batteries are applied.

1 Positive electrode 2 Negative electrode 3 Separator

Claims (10)

A positive electrode having a positive electrode mixture layer containing a positive electrode active material on one side or both sides of a current collector, a negative electrode having a negative electrode mixture layer containing a negative electrode active material on one side or both sides of a current collector, a non-aqueous electrolyte, and a separator. A lithium secondary battery provided,
The positive electrode mixture layer of the positive electrode has a general composition formula Li _{1 + y} Ni _1-a-b-c Co _a Mn _b M ¹ _c O ₂ [where M ¹ is Mg, Al, Ti, Fe, Cu, Zn, Ga , Ge, Zr, Nb, Mo, Sn, W, B, P and Bi, at least one element selected from the group consisting of -0.15 ≦ y ≦ 0.15, 0.05 ≦ a ≦ 0.3, 0.05 ≦ b ≦ 0.3, 0 ≦ c ≦ 0.03, and a + b + c ≦ 0.5], as a positive electrode active material. ,
The negative electrode mixture layer of the negative electrode contains a material containing Si and O as constituent elements (however, the atomic ratio x of O to Si is 0.5 ≦ x ≦ 1.5) as a negative electrode active material. And
The non-aqueous electrolyte contains a compound having one or two nitrile groups in the molecule. The lithium secondary battery according to claim 1, wherein a nonaqueous electrolyte containing 0.5 to 30% by mass of a phosphonoacetate compound represented by the following general formula (1) is used.

In [Formula ^{(1), R 1, R} 2 and ^{R 3} are each independently a hydrocarbon group having 1 to 12 carbon atoms which may be substituted with a halogen atom, n represents 0-6 Is an integer. ] The lithium secondary battery according to claim 1 or 2, wherein a non-aqueous electrolyte having a content of a compound having one or two nitrile groups in the molecule is 0.1 to 5.0% by mass. One or more compounds having two nitrile groups in the molecule, a lithium secondary battery according to any one of claims 1 to 3 having two nitrile groups in the molecule. The lithium secondary battery according to any one of claims 1 to 4, wherein the compound having one or two nitrile groups in the molecule is adiponitrile.   The lithium secondary battery according to claim 1, 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.   The lithium secondary battery according to claim 1, wherein the negative electrode mixture layer of the negative electrode further contains a graphitic carbon material as a negative electrode active material.   The lithium secondary battery according to any one of claims 1 to 7, wherein a non-aqueous electrolyte further containing a halogen-substituted cyclic carbonate is used.   The lithium secondary battery according to any one of claims 1 to 8, wherein a non-aqueous electrolyte further containing vinylene carbonate is used.   The lithium secondary battery according to any one of claims 1 to 9, wherein an upper limit voltage of charging is set to 4.3 V or more.

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