JP6179511B2 - Lithium secondary battery - Google Patents

Description

  The present invention relates to a lithium secondary battery.

  Lithium secondary batteries are widely used in applications such as portable electronic devices and personal computers. Lithium secondary batteries are required to have improved safety such as flame retardancy, and secondary batteries using an electrolytic solution containing a phosphate ester compound have been proposed as described in the following documents.

  Patent Document 1 discloses a secondary battery using an electrolytic solution composed of a phosphate ester compound, a cyclic carbonate containing a halogen, a chain carbonate, and a lithium salt. Patent Document 1 shows that the use of this electrolytic solution can improve safety, and the irreversible capacity can be reduced by a combination of a carbon negative electrode and an electrolytic solution.

  Patent Document 2 shows that by mixing a phosphate ester, high safety can be ensured even when lithium metal is deposited on the negative electrode.

  Patent Document 3 discloses a secondary battery using an electrolytic solution containing a phosphate ester, a cyclic carbonate, and either a vinylene carbonate compound or a vinylethylene carbonate compound.

  In patent document 4, the secondary battery which has the electrolyte solution containing the phosphate ester containing a fluorine is disclosed.

In Patent Document 5 and Patent Document 6, R ¹ O— (R ² O) _n —R ³ (R ¹ , R ³ : an alkyl group having 1 to 8 carbon atoms which may be substituted with a halogen atom, R ² : an alkylene group having 1 to 8 carbon atoms which may be substituted with a halogen atom, provided that at least one of R ¹ , R ² and R ³ must be substituted with a halogen atom, 1 ≦ n ≦ A secondary battery using an electrolytic solution containing glycol diether represented by 4) is disclosed. Furthermore, Patent Document 5 and Patent Document 6 disclose that a phosphate ester is contained in an electrolytic solution, but there is no description regarding a fluorinated phosphate ester.

  Further, in the lithium secondary battery, safety is required, but improvement of the energy density of the battery is an important technical problem.

Several methods are conceivable as methods for increasing the energy density of the lithium secondary battery. Among them, it is effective to increase the operating potential of the battery. In a lithium secondary battery using conventional lithium cobaltate or lithium manganate as a positive electrode active material, the operating potential is 4V class (average operating potential = 3.6 to 3.8V: lithium potential). This is because the expression potential is defined by the redox reaction of Co ions or Mn ions (Co ³⁺ ← → Co ⁴⁺ or Mn ³⁺ ← → Mn ⁴⁺ ).

On the other hand, for example, it is known that an operating potential of 5 V class can be realized by using, as an active material, a spinel compound in which Mn of lithium manganate is substituted with Ni or the like. Specifically, as disclosed in Patent Document 7, it is known that a spinel compound such as LiNi _0.5 Mn _1.5 O ₄ exhibits a potential plateau in a region of 4.5 V or higher. In such a spinel compound, Mn exists in a tetravalent state, and the operating potential is defined by oxidation and reduction of Ni ²⁺ ← → Ni ⁴⁺ instead of oxidation reduction of Mn ³⁺ ← → Mn ⁴⁺ .

LiNi _0.5 Mn _1.5 O ₄ has a capacity of 130 mAh / g or more and an average operating voltage of 4.6 V or more with respect to metallic lithium. Although the capacity is smaller than LiCoO ₂ , the energy density of the battery is higher than LiCoO ₂ . For these reasons, LiNi _0.5 Mn _1.5 O ₄ is promising as a future positive electrode material.

Japanese Patent No. 39615197 Japanese Patent No. 3812495 Japanese Patent No. 4187965 JP 2008-021560 A JP 2001-023691 A JP 2001-085056 A JP 2009-123707 A

J. et al. Electrochem. Soc. , Vol. 144, 204 (1997)

  The electrolytic solution deteriorates as it is repeatedly charged and discharged, and in some cases, gas is generated and the battery may be swollen. As factors of deterioration of the electrolytic solution, oxidative decomposition at the positive electrode and reductive decomposition at the negative electrode are known.

When a spinel compound such as LiNi _0.5 Mn _1.5 O ₄ is used as the positive electrode active material, a high operating voltage can be obtained, while the decomposition reaction of the electrolytic solution tends to proceed at the contact portion between the positive electrode and the electrolytic solution. Since gas is generated by this decomposition reaction, there are problems in practical use such as an increase in the internal pressure of the cell or a swelling of the laminate cell in the cycle operation. In addition, there is a problem that capacity and cycle characteristics are reduced due to decomposition of the electrolytic solution.

  On the other hand, carbonate-based materials are generally used as the solvent of the electrolytic solution. However, secondary batteries using this electrolytic solution have the above-described electrolysis during high-voltage operation or long-term operation at high temperatures. The generation of gas and the decrease in capacity accompanying the decomposition of the liquid were remarkable.

The batteries disclosed in the above-mentioned Patent Documents 1 to 6 use a 4V-class positive electrode active material such as LiMn ₂ O ₄ or LiCoO _2, and use a positive electrode active material having a higher discharge potential. The problem was not solved.

  An object of one embodiment of the present invention is to provide a lithium secondary battery having excellent cycle characteristics.

The present embodiment is a lithium secondary battery having a positive electrode, a negative electrode, and an electrolytic solution containing a nonaqueous electrolytic solvent,
The non-aqueous electrolytic solvent relates to a lithium secondary battery comprising a fluorine-containing phosphate ester represented by the following formula (1) and a fluorinated diether compound represented by the following formula (20).

（式（１）において、Ｒ_１、Ｒ_２およびＲ_３は、それぞれ独立に、置換または無置換のアルキル基であって、Ｒ_１，Ｒ_２およびＲ_３の少なくとも１つはフッ素含有アルキル基である。）
Ｒ_４Ｏ−（Ｒ_５Ｏ）_ｎ−Ｒ_６ （２０）
（式（２０）中、Ｒ_４およびＲ_６は、独立してフッ素原子で置換されていてもよい炭素数１〜４のアルキル基であり、Ｒ_５はフッ素原子で置換されていてもよい炭素数１〜４のアルキレン基であり、ただしＲ_４、Ｒ_５およびＲ_６の少なくとも一つはフッ素原子で置換されている基であり、ｎは、１〜４の整数を表す。）

(In Formula (1), R ₁ , R ₂ and R ₃ are each independently a substituted or unsubstituted alkyl group, and at least one of R ₁ , R ₂ and R ₃ is a fluorine-containing alkyl group. is there.)
_{R 4 O- (R 5 O)} n -R 6 (20)
(In formula (20), R ₄ and R ₆ are each independently a C 1-4 alkyl group optionally substituted with a fluorine atom, and R ₅ is a carbon optionally substituted with a fluorine atom. An alkylene group of 1 to 4, provided that at least one of R ₄ , R ₅ and R ₆ is a group substituted with a fluorine atom, and n represents an integer of 1 to 4)

  According to this embodiment, a lithium secondary battery having excellent cycle characteristics can be provided.

It is a figure which shows the cross-section of the secondary battery which concerns on this embodiment.

  The lithium secondary battery of this embodiment has a positive electrode, a negative electrode, and an electrolytic solution containing a nonaqueous electrolytic solvent. The non-aqueous electrolytic solvent includes a phosphate ester represented by the above formula (1) (hereinafter also referred to as a fluorine-containing phosphate ester) and a fluorinated diether compound represented by the above formula (20) (hereinafter simply referred to as fluorine). A diether compound). In the lithium secondary battery of the present embodiment, a positive electrode active material that operates at a 4 V class (for example, an average operating potential of 3.6 to 3.8 V: lithium potential) may be used. However, even when a positive electrode active material that operates at a potential of 4.5 V or higher with respect to lithium is used, gas generation is prevented by using a nonaqueous electrolytic solvent containing a fluorine-containing phosphate ester and a fluorinated diether compound. Thus, a lithium secondary battery having excellent cycle characteristics is obtained.

(Electrolyte)
The electrolytic solution contains a supporting salt and a nonaqueous electrolytic solvent, and the nonaqueous electrolytic solvent contains a fluorine-containing phosphate ester represented by the above formula (1) and a fluorinated diether compound represented by the above formula (20).

  The content of the fluorine-containing phosphate ester contained in the nonaqueous electrolytic solvent is not particularly limited, but is preferably 10% by volume or more and 95% by volume or less in the nonaqueous electrolytic solvent. When the content of the fluorine-containing phosphate ester in the nonaqueous electrolytic solvent is 10% by volume or more, the effect of increasing the voltage resistance is further improved. Moreover, the ion conductivity of electrolyte solution improves that the content rate in the nonaqueous electrolytic solvent of fluorine-containing phosphate ester is 95 volume% or less, and the charging / discharging rate of a battery becomes more favorable. Further, the content of the fluorine-containing phosphate ester in the nonaqueous electrolytic solvent is more preferably 20% by volume or more, further preferably 31% by volume or more, and particularly preferably 35% by volume or more. Further, the content of the fluorine-containing phosphate ester in the nonaqueous electrolytic solvent is more preferably 80% by volume or less, and further preferably 70% by volume or less.

In the fluorine-containing phosphate represented by the formula (1), R ₁ , R ₂ and R ₃ are each independently a substituted or unsubstituted alkyl group, and at least _one of R ₁ , R ₂ and R ₃ One is a fluorine-containing alkyl group. The fluorine-containing alkyl group is an alkyl group having at least one fluorine atom. The carbon numbers of the alkyl groups R ₁ , R ₂ , and R ₃ are each independently preferably 1 or more and 4 or less, and more preferably 1 or more and 3 or less. When the carbon number of the alkyl group is 4 or less, the increase in the viscosity of the electrolytic solution is suppressed, and the electrolytic solution can easily penetrate into the pores in the electrode and the separator, and the ion conductivity is improved. This is because the current value becomes favorable in the discharge characteristics.

In Formula (1), it is preferable that all of R ₁ , R ₂ and R ₃ are fluorine-containing alkyl groups.

In addition, at least one of R ₁ , R ₂ and R ₃ is preferably a fluorine-containing alkyl group in which 50% or more of the hydrogen atoms of the corresponding unsubstituted alkyl group are substituted with fluorine atoms. Also, all of R ₁ , R ₂ and R ₃ are fluorine-containing alkyl groups, and 50% or more of the hydrogen atoms of the unsubstituted alkyl group to which R ₁ , R ₂ and R ₃ correspond are substituted with fluorine atoms. More preferably, it is a fluorine-containing alkyl group. When the content of fluorine atoms is large, the voltage resistance is further improved, and even when a positive electrode active material that operates at a potential of 4.5 V or higher with respect to lithium is used, the deterioration of battery capacity after cycling is further reduced. Because it can. The ratio of fluorine atoms in the substituent containing a hydrogen atom in the fluorine-containing alkyl group is more preferably 55% or more.

R _{1 to} R ₃ may have a substituent in addition to the fluorine atom. Examples of the substituent include an amino group, a carboxy group, a hydroxy group, a cyano group, and a halogen atom (for example, a chlorine atom, And at least one selected from the group consisting of bromine atoms). In addition, said carbon number is the concept also including a substituent.

  Examples of the fluorine-containing phosphate ester include tris phosphate (trifluoromethyl), tris phosphate (trifluoroethyl), tris phosphate (tetrafluoropropyl), tris phosphate (pentafluoropropyl), tris phosphate ( Heptafluorobutyl), tris phosphate (octafluoropentyl) and the like. Examples of the fluorine-containing phosphate ester include trifluoroethyldimethyl phosphate, bis (trifluoroethyl) methyl phosphate, bistrifluoroethylethyl phosphate, pentafluoropropyldimethyl phosphate, heptafluorobutyldimethyl phosphate, Trifluoroethylmethyl ethyl phosphate, pentafluoropropylmethyl ethyl phosphate, heptafluorobutylmethyl ethyl phosphate, trifluoroethyl methyl propyl phosphate, pentafluoropropyl methyl propyl phosphate, heptafluorobutyl methyl propyl phosphate, phosphoric acid Trifluoroethylmethylbutyl, pentafluoropropylmethylbutyl phosphate, heptafluorobutylmethylbutyl phosphate, trifluoroethyldiethyl phosphate, pentafluoropropyldiethyl phosphate , Heptafluorobutyldiethyl phosphate, trifluoroethylethylpropyl phosphate, pentafluoropropylethylpropyl phosphate, heptafluorobutylethylpropyl phosphate, trifluoroethylethylbutyl phosphate, pentafluoropropylethylbutyl phosphate, phosphoric acid Heptafluorobutylethylbutyl, trifluoroethyldipropyl phosphate, pentafluoropropyldipropyl phosphate, heptafluorobutyldipropyl phosphate, trifluoroethylpropylbutyl phosphate, pentafluoropropylpropylbutyl phosphate, heptafluorophosphate Examples thereof include butylpropylbutyl, trifluoroethyl dibutyl phosphate, pentafluoropropyl dibutyl phosphate, heptafluorobutyl dibutyl phosphate and the like. Examples of tris (tetrafluoropropyl) phosphate include tris (2,2,3,3-tetrafluoropropyl) phosphate. Examples of tris phosphate (pentafluoropropyl) include tris phosphate (2,2,3,3,3-pentafluoropropyl). Examples of tris (trifluoroethyl) phosphate include tris (2,2,2-trifluoroethyl) phosphate (hereinafter also abbreviated as PTTFE). Examples of tris phosphate (heptafluorobutyl) include tris phosphate (1H, 1H-heptafluorobutyl). Examples of lintris (octafluoropentyl) include trisphosphate (1H, 1H, 5H-octafluoropentyl) and the like. Among these, tris phosphate (2,2,2-trifluoroethyl) represented by the following formula (2) is preferable because the effect of suppressing decomposition of the electrolytic solution at a high potential is high. A fluorine-containing phosphate ester can be used individually by 1 type or in combination of 2 or more types.

Formula (20):
_{R 4 O- (R 5 O)} n -R 6 (20)
In the fluorinated diether compound represented by: R ₄ and R ₆ are each independently an alkyl group having 1 to 4 carbon atoms which may be substituted with a fluorine atom, and R ₅ is substituted with a fluorine atom. Or an alkylene group having 1 to 4 carbon atoms, provided that at least one of R ₄ , R ₅ and R ₆ is a group substituted with a fluorine atom.

The number of carbon atoms of R ₄ and R ₆ is more preferably 1 or more and 3 or less. In a preferred embodiment, R ₄ and R ₆ are fluorine-substituted alkyl groups such as trifluoromethyl, trifluoroethyl, tetrafluoropropyl, pentafluoropropyl and heptafluorobutyl. The fluorine substitution position is arbitrary, and examples thereof include 2,2,2-trifluoroethyl, 2,2,3,3-tetrafluoropropyl, 2,2,3,3,3-pentafluoropropyl and the like. Although it can, it is not limited to these.

The number of carbon atoms in R ₅ is more preferably 1 or more and 3 or less. Examples include methylene, ethylene, 1,2-propylene, 1,3-propylene, butylene, and fluorine-substituted compounds thereof. In particular, ethylene, 1,2-propylene and 1,3-propylene are preferable. In a preferred embodiment, R ₅ is an unsubstituted alkylene group. n is preferably 1 or 2, and more preferably 1.

  A preferred compound of the fluorinated diether compound is represented by the following formula (21).

CF ₃ CH ₂ OCH ₂ CH ₂ OCH ₂ CF ₃ (21)

  The content of the fluorinated diether compound contained in the non-aqueous electrolytic solvent is not particularly limited, and for example, 0.1% by volume or more, more preferably 0.5% by volume or more, Preferably, it is 0.9 volume% or more. On the other hand, the upper limit of the content rate can be appropriately changed depending on the content of the fluorine-containing phosphate ester and the content of other organic solvents, and is typically 90% by volume or less, preferably 50% by volume or less. It is. As shown in Examples described later, the content of the fluorinated diether compound may be relatively small. Accordingly, in one embodiment, the content of the fluorinated diether compound is preferably 20% by volume, more preferably 10% by volume or less.

  The nonaqueous electrolytic solvent preferably further contains a cyclic carbonate or a chain carbonate in addition to the fluorine-containing phosphate ester and the fluorinated diether compound.

  Since cyclic carbonate has a large relative dielectric constant, the dissociation of the supporting salt is improved by addition, and sufficient conductivity is easily imparted. In addition, since the chain carbonate has a low viscosity, the viscosity of the electrolytic solution is lowered by addition, so that ion mobility in the electrolytic solution is improved. Cyclic carbonates and chain carbonates are suitable for mixing with fluorine-containing phosphate esters because of their high voltage resistance and electrical conductivity.

  Although it does not restrict | limit especially as a cyclic carbonate, For example, ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), vinylene carbonate (VC) etc. can be mentioned. The cyclic carbonate includes a fluorinated cyclic carbonate. Examples of the fluorinated cyclic carbonate include compounds in which some or all of the hydrogen atoms such as ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), or vinylene carbonate (VC) are substituted with fluorine atoms. Can be mentioned. More specific examples of the fluorinated cyclic carbonate include 4-fluoro-1,3-dioxolan-2-one, (cis or trans) 4,5-difluoro-1,3-dioxolan-2-one, 4,4-difluoro-1,3-dioxolan-2-one, 4-fluoro-5-methyl-1,3-dioxolan-2-one, and the like can be used. Among the above listed cyclic carbonates, ethylene carbonate, propylene carbonate, or a compound obtained by fluorinating a part thereof is preferable, and ethylene carbonate is more preferable, from the viewpoint of voltage endurance and conductivity. A cyclic carbonate can be used individually by 1 type or in combination of 2 or more types.

  The content of the cyclic carbonate in the non-aqueous electrolytic solvent is preferably 0.1% by volume or more, more preferably 5% by volume or more from the viewpoint of the effect of increasing the dissociation degree of the supporting salt and the effect of increasing the conductivity of the electrolytic solution. 10 volume% or more is further more preferable, and 15 volume% or more is especially preferable. Further, from the same viewpoint, the content of the cyclic carbonate in the nonaqueous electrolytic solvent is preferably 70% by volume or less, more preferably 50% by volume or less, and further preferably 40% by volume or less.

  The chain carbonate is not particularly limited, and examples thereof include dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), and dipropyl carbonate (DPC). The chain carbonate includes a fluorinated chain carbonate. As the fluorinated chain carbonate, for example, a part or all of hydrogen atoms such as ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC) and the like are substituted with fluorine atoms. Examples include compounds having a structure. More specifically, examples of the fluorinated chain carbonate include bis (fluoroethyl) carbonate, 3-fluoropropyl methyl carbonate, 3,3,3-trifluoropropyl methyl carbonate, and 2,2,2-trifluoro. Ethyl methyl carbonate, 2,2,2-trifluoroethyl ethyl carbonate, monofluoromethyl methyl carbonate, methyl 2,2,3,3, tetrafluoropropyl carbonate, ethyl 2,2,3,3, tetrafluoropropyl carbonate, Bis (2,2,3,3, tetrafluoropropyl) carbonate, bis (2,2,2 trifluoroethyl) carbonate, 1-monofluoroethyl ethyl carbonate, 1-monofluoroethyl methyl carbonate, 2-monofluoroethyl Methyl car , Bis (1-mono-fluoroethyl) carbonate, bis (2-monofluoroethyl) carbonate, bis (monofluoromethyl) carbonate, and the like. Among these, dimethyl carbonate, 2,2,2-trifluoroethyl methyl carbonate, monofluoromethyl methyl carbonate, methyl 2,2,3,3, tetrafluoropropyl carbonate and the like are preferable from the viewpoint of voltage resistance and conductivity. . A chain carbonate can be used individually by 1 type or in combination of 2 or more types.

  The chain carbonate has an advantage of low viscosity when the number of carbon atoms of the substituent added to the “—OCOO—” structure is small. On the other hand, if the number of carbon atoms is too large, the viscosity of the electrolytic solution may increase and the conductivity of Li ions may decrease. For these reasons, the total number of carbon atoms of the two substituents added to the “—OCOO—” structure of the chain carbonate is preferably 2 or more and 6 or less. Further, when the substituent added to the “—OCOO—” structure contains a fluorine atom, the oxidation resistance of the electrolytic solution is improved. For these reasons, the chain carbonate is preferably a fluorinated chain carbonate represented by the following formula (3).

_{C n H 2n + 1-l} F l -OCOO-C m H 2m + 1-k F k (3)

  (In the formula (3), n is 1, 2 or 3, m is 1, 2 or 3, l is any integer from 0 to 2n + 1, and k is any from 0 to 2m + 1. And at least one of l and k is an integer of 1 or more.)

  In the fluorinated chain carbonate represented by the formula (3), if the amount of fluorine substitution is small, the capacity retention rate of the battery decreases or gas is generated due to the reaction of the fluorinated chain carbonate with the positive electrode having a high potential. Sometimes. On the other hand, if the amount of fluorine substitution is too large, the compatibility of the chain carbonate with other solvents may decrease, or the boiling point of the chain carbonate may decrease. For these reasons, the fluorine substitution amount is preferably 1% or more and 90% or less, more preferably 5% or more and 85% or less, and further preferably 10% or more and 80% or less. That is, it is preferable that l, m, and n in Expression (3) satisfy the following relational expression.

  0.01 ≦ (l + k) / (2n + 2m + 2) ≦ 0.9

  The chain carbonate has an effect of lowering the viscosity of the electrolytic solution, and can increase the conductivity of the electrolytic solution. From these viewpoints, the content of the chain carbonate in the nonaqueous electrolytic solvent is preferably 5% by volume or more, more preferably 10% by volume or more, and further preferably 15% by volume or more. Further, the content of the chain carbonate in the nonaqueous electrolytic solvent is preferably 90% by volume or less, more preferably 80% by volume or less, and further preferably 70% by volume or less.

  Further, the content of the fluorinated chain carbonate is not particularly limited, but is preferably 0.1% by volume or more and 70% by volume or less in the nonaqueous electrolytic solvent. When the content of the fluorinated chain carbonate in the nonaqueous electrolytic solvent is 0.1% by volume or more, the viscosity of the electrolytic solution can be lowered and the conductivity can be increased. Moreover, the effect which improves oxidation resistance is acquired. Further, when the content of the fluorinated chain carbonate in the nonaqueous electrolytic solvent is 70% by volume or less, the conductivity of the electrolytic solution can be kept high. Further, the content of the fluorinated chain carbonate in the nonaqueous electrolytic solvent is more preferably 1% by volume or more, further preferably 5% by volume or more, and particularly preferably 10% by volume or more. The content of the fluorinated chain carbonate in the nonaqueous electrolytic solvent is more preferably 65% by volume or less, further preferably 60% by volume or less, and particularly preferably 55% by volume or less.

  The nonaqueous electrolytic solvent may contain a carboxylic acid ester in addition to the fluorine-containing phosphate ester and the fluorinated diether compound.

  The carboxylic acid ester is not particularly limited, and examples thereof include ethyl acetate, methyl propionate, ethyl formate, ethyl propionate, methyl butyrate, ethyl butyrate, methyl acetate, and methyl formate. The carboxylic acid ester also includes a fluorinated carboxylic acid ester. Examples of the fluorinated carboxylic acid ester include ethyl acetate, methyl propionate, ethyl formate, ethyl propionate, methyl butyrate, ethyl butyrate, methyl acetate, or formic acid. Examples thereof include compounds having a structure in which part or all of the hydrogen atoms of methyl are substituted with fluorine atoms. Specific examples of the fluorinated carboxylic acid ester include, for example, ethyl pentafluoropropionate, ethyl 3,3,3-trifluoropropionate, methyl 2,2,3,3-tetrafluoropropionate, and acetic acid. 2,2-difluoroethyl, methyl heptafluoroisobutyrate, methyl 2,3,3,3-tetrafluoropropionate, methyl pentafluoropropionate, 2- (trifluoromethyl) -3,3,3-trifluoropropion Methyl acetate, ethyl heptafluorobutyrate, methyl 3,3,3-trifluoropropionate, 2,2,2-trifluoroethyl acetate, isopropyl trifluoroacetate, tert-butyl trifluoroacetate, 4,4,4-tri Ethyl fluorobutyrate, methyl 4,4,4-trifluorobutyrate, 2,2-difluoro Acid butyl, ethyl difluoroacetate, n-butyl trifluoroacetate, 2,2,3,3-tetrafluoropropyl acetate, ethyl 3- (trifluoromethyl) butyrate, methyl tetrafluoro-2- (methoxy) propionate, 3 , 3,3-trifluoropropionic acid 3,3,3 trifluoropropyl, methyl difluoroacetate, 2,2,3,3-tetrafluoropropyl trifluoroacetate, 1H, 1H-heptafluorobutyl acetate, methyl heptafluorobutyrate And ethyl trifluoroacetate. Among these, from the viewpoints of withstand voltage and boiling point, as the carboxylic acid ester, ethyl propionate, methyl acetate, methyl 2,2,3,3-tetrafluoropropionate, 2,2,3,3 trifluoroacetic acid -Tetrafluoropropyl is preferred. Carboxylic acid esters have the effect of reducing the viscosity of the electrolytic solution in the same manner as chain carbonates. Therefore, for example, the carboxylic acid ester can be used in place of the chain carbonate, and can also be used in combination with the chain carbonate.

  The chain carboxylic acid ester has a feature that the viscosity is low when the number of carbon atoms of the substituent added to the “—COO—” structure is small, but the boiling point tends to be low. The chain carboxylic acid ester having a low boiling point may be vaporized when the battery is operated at a high temperature. On the other hand, if the number of carbon atoms is too large, the viscosity of the electrolytic solution may increase and conductivity may decrease. For these reasons, the total number of carbon atoms of the two substituents added to the “—COO—” structure of the chain carboxylic acid ester is preferably 3 or more and 8 or less. Further, when the substituent added to the “—COO—” structure contains a fluorine atom, the oxidation resistance of the electrolytic solution can be improved. For these reasons, the chain carboxylic acid ester is preferably a fluorinated chain carboxylic acid ester represented by the following formula (4).

_{C n H 2n + 1-l} F l -COO-C m H 2m + 1-k F k (4)

  (In the formula (4), n is 1, 2, 3 or 4, m is 1, 2, 3 or 4, l is any integer from 0 to 2n + 1, and k is 0 to 2m + 1. And at least one of l and k is an integer of 1 or more.)

  In the fluorinated chain carboxylic acid ester represented by the formula (4), when the amount of fluorine substitution is small, the capacity retention rate of the battery decreases due to the reaction of the fluorinated chain carboxylic acid ester with the positive electrode of high potential, Gas may be generated. On the other hand, if the amount of fluorine substitution is too large, the compatibility of the chain carboxylic acid ester with other solvents may decrease, or the boiling point of the fluorinated chain carboxylic acid ester may decrease. For these reasons, the fluorine substitution amount is preferably 1% or more and 90% or less, more preferably 10% or more and 85% or less, and further preferably 20% or more and 80% or less. That is, it is preferable that l, m, and n in Expression (4) satisfy the following relational expression.

  0.01 ≦ (l + k) / (2n + 2m + 2) ≦ 0.9

  The content of the carboxylic acid ester in the non-aqueous electrolytic solvent is preferably 0.1 volume or more, more preferably 0.2 volume% or more, further preferably 0.5 volume% or more, and particularly preferably 1 volume% or more. The content of the carboxylic acid ester in the nonaqueous electrolytic solvent is preferably 50% by volume or less, more preferably 20% by volume or less, still more preferably 15% by volume or less, and particularly preferably 10% by volume or less. By setting the content of the carboxylic acid ester to 0.1% by volume or more, the low temperature characteristics can be further improved, and the electrical conductivity can be further improved. Further, by setting the content of the carboxylic acid ester to 50% by volume or less, it is possible to reduce the vapor pressure from becoming too high when the battery is left at a high temperature.

  The content of the fluorinated chain carboxylic acid ester is not particularly limited, but is preferably 0.1% by volume or more and 50% by volume or less in the nonaqueous electrolytic solvent. When the content of the fluorinated chain carboxylic acid ester in the nonaqueous electrolytic solvent is 0.1% by volume or more, the viscosity of the electrolytic solution can be lowered and the conductivity can be increased. Moreover, the effect which improves oxidation resistance is acquired. In addition, when the content of the fluorinated chain carboxylic acid ester in the nonaqueous electrolytic solvent is 50% by volume or less, the conductivity of the electrolytic solution can be kept high, and the compatibility of the electrolytic solution is ensured. Can do. The content of the fluorinated chain carboxylic acid ester in the nonaqueous electrolytic solvent is more preferably 1% by volume or more, further preferably 5% by volume or more, and particularly preferably 10% by volume or more. The content of the fluorinated chain carboxylic acid ester in the nonaqueous electrolytic solvent is more preferably 45% by volume or less, further preferably 40% by volume or less, and particularly preferably 35% by volume or less.

  The nonaqueous electrolytic solvent may contain an alkylene biscarbonate represented by the following formula (5) in addition to the fluorine-containing phosphate ester and the fluorinated diether compound. Since the oxidation resistance of the alkylene biscarbonate is equal to or slightly higher than that of the chain carbonate, the voltage resistance of the electrolytic solution can be improved.

(R ₄ and R ₆ each independently represents a substituted or unsubstituted alkyl group. R ₅ represents a substituted or unsubstituted alkylene group.)

  In Formula (5), the alkyl group includes linear or branched ones, preferably having 1 to 6 carbon atoms, and more preferably 1 to 4 carbon atoms. The alkylene group is a divalent saturated hydrocarbon group, including a linear or branched group, preferably having 1 to 4 carbon atoms, and more preferably 1 to 3 carbon atoms. .

  Examples of the alkylene biscarbonate represented by the formula (5) include 1,2-bis (methoxycarbonyloxy) ethane, 1,2-bis (ethoxycarbonyloxy) ethane, and 1,2-bis (methoxycarbonyloxy). Examples include propane and 1-ethoxycarbonyloxy-2-methoxycarbonyloxyethane. Of these, 1,2-bis (methoxycarbonyloxy) ethane is preferred.

  The content of the alkylene biscarbonate in the nonaqueous electrolytic solvent is preferably 0.1% by volume or more, more preferably 0.5% by volume or more, further preferably 1% by volume or more, and particularly preferably 1.5% by volume or more. . The content of the alkylene biscarbonate in the nonaqueous electrolytic solvent is preferably 70% by volume or less, more preferably 60% by volume or less, further preferably 50% by volume or less, and particularly preferably 40% by volume or less.

  Alkylene biscarbonate is a material having a low dielectric constant. Therefore, for example, it can be used in place of the chain carbonate, or can be used in combination with the chain carbonate.

  The non-aqueous electrolytic solvent can contain a chain ether in addition to the fluorine-containing phosphate ester and the fluorinated diether compound.

  The chain ether is not particularly limited, and examples thereof include 1,2-ethoxyethane (DEE) and ethoxymethoxyethane (EME). The chain ether also includes fluorinated chain ether. The fluorinated chain ether is preferably used in the case of a positive electrode having high oxidation resistance and operating at a high potential. Examples of the fluorinated chain ether include compounds having a structure in which some or all of the hydrogen atoms of 1,2-ethoxyethane (DEE) or ethoxymethoxyethane (EME) are substituted with fluorine atoms. Specific examples of the fluorinated chain ether include 2,2,3,3,3-pentafluoropropyl 1,1,2,2-tetrafluoroethyl ether, 1,1,2,2 and the like. -Tetrafluoroethyl 2,2,2-trifluoroethyl ether, 1H, 1H, 2'H, 3H-decafluorodipropyl ether, 1,1,1,2,3,3-hexafluoropropyl-2,2 -Difluoroethyl ether, isopropyl 1,1,2,2-tetrafluoroethyl ether, propyl 1,1,2,2-tetrafluoroethyl ether, 1,1,2,2-tetrafluoroethyl 2,2,3 3-tetrafluoropropyl ether, 1H, 1H, 5H-perfluoropentyl-1,1,2,2-tetrafluoroethyl ether, 1H, 1H, 2′H-per Fluorodipropyl ether, 1H-perfluorobutyl-1H-perfluoroethyl ether, methyl perfluoropentyl ether, methyl perfluorohexyl ether, methyl 1,1,3,3,3-pentafluoro-2- (trifluoro Methyl) propyl ether, 1,1,2,3,3,3-hexafluoropropyl 2,2,2-trifluoroethyl ether, ethyl nonafluorobutyl ether, ethyl 1,1,2,3,3,3-hexa Fluoropropyl ether, 1H, 1H, 5H-octafluoropentyl 1,1,2,2-tetrafluoroethyl ether, 1H, 1H, 2′H-perfluorodipropyl ether, heptafluoropropyl 1,2,2,2 -Tetrafluoroethyl ether, 1,1,2,2-tetrafluoroe 2,2,3,3-tetrafluoropropyl ether, 2,2,3,3,3-pentafluoropropyl-1,1,2,2-tetrafluoroethyl ether, ethyl nonafluorobutyl ether, methyl nonafluoro Examples include butyl ether. Among these, from the viewpoint of withstand voltage and boiling point, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, 1H, 1H, 2′H, 3H-decafluoro Dipropyl ether, 1H, 1H, 2′H-perfluorodipropyl ether, ethyl nonafluorobutyl ether and the like are preferable. The chain ether has the effect of reducing the viscosity of the electrolytic solution, like the chain carbonate. Therefore, for example, a chain ether can be used in place of a chain carbonate or carboxylic acid ester, and can also be used in combination with a chain carbonate or carboxylic acid ester.

  Since chain ether tends to have a low boiling point when the number of carbon atoms is small, the chain ether may vaporize during high-temperature operation of the battery. On the other hand, if the number of carbon atoms is too large, the viscosity of the chain ether increases, and the conductivity of the electrolytic solution may decrease. Accordingly, the number of carbon atoms is preferably 4 or more and 10 or less. For these reasons, the chain ether is preferably a fluorinated chain ether represented by the following formula (6).

_{C n H 2n + 1-l} F l -O-C m H 2m + 1-k F k (6)

  (In the formula (6), n is 1, 2, 3, 4, 5 or 6, m is 1, 2, 3 or 4, l is any integer from 0 to 2n + 1, and k Is any integer from 0 to 2m + 1, and at least one of l and k is an integer of 1 or more.)

  In the fluorinated chain ether represented by the formula (6), if the amount of fluorine substitution is small, the capacity retention rate of the battery decreases or gas is generated due to the reaction of the fluorinated chain ether with the positive electrode having a high potential. Sometimes. On the other hand, if the amount of fluorine substitution is too large, the compatibility of the fluorinated chain ether with the multi-solvent may decrease, or the boiling point of the fluorinated chain ether may decrease. For these reasons, the fluorine substitution amount is preferably 10% or more and 90% or less, more preferably 20% or more and 85% or less, and further preferably 30% or more and 80% or more. That is, it is preferable that l, m, and n in Expression (6) satisfy the following relational expression.

  0.1 ≦ (l + k) / (2n + 2m + 2) ≦ 0.9

  The content of the fluorinated chain ether is not particularly limited, but is preferably 0.1% by volume or more and 70% by volume or less in the nonaqueous electrolytic solvent. When the content of the fluorinated chain ether in the nonaqueous electrolytic solvent is 0.1% by volume or more, the viscosity of the electrolytic solution can be lowered and the conductivity can be increased. Moreover, the effect which improves oxidation resistance is acquired. Further, when the content of the fluorinated chain ether in the nonaqueous electrolytic solvent is 70% by volume or less, it is possible to keep the conductivity of the electrolytic solution high and to ensure the compatibility of the electrolytic solution. Can do. The content of the fluorinated chain ether in the nonaqueous electrolytic solvent is more preferably 1% by volume or more, further preferably 5% by volume or more, and particularly preferably 10% by volume or more. The content of the fluorinated chain ether in the nonaqueous electrolytic solvent is more preferably 65% by volume or less, further preferably 60% by volume or less, and particularly preferably 55% by volume or less.

  In addition to the above, the nonaqueous electrolytic solvent may contain the following. Nonaqueous electrolytic solvents include, for example, γ-lactones such as γ-butyrolactone, chain ethers such as 1,2-ethoxyethane (DEE) or ethoxymethoxyethane (EME), and cyclic rings such as tetrahydrofuran or 2-methyltetrahydrofuran. Ethers and the like can be included. Moreover, what substituted some hydrogen atoms of these materials by the fluorine atom may be included. In addition, dimethyl sulfoxide, 1,3-dioxolane, formamide, acetamide, dimethylformamide, dioxolane, acetonitrile, propylnitrile, nitromethane, ethyl monoglyme, trimethoxymethane, dioxolane derivatives, sulfolane, methylsulfolane, 1,3 -Including an aprotic organic solvent such as dimethyl-2-imidazolidinone, 3-methyl-2-oxazolidinone, propylene carbonate derivative, tetrahydrofuran derivative, ethyl ether, 1,3-propane sultone, anisole, N-methylpyrrolidone good.

Examples of the supporting salt include LiPF ₆ , LiAsF ₆ , LiAlCl ₄ , LiClO ₄ , LiBF ₄ , LiSbF ₆ , LiCF ₃ SO ₃ , LiC ₄ F ₉ CO ₃ , LiC (CF ₃ SO ₂ ) ₂ , LiN (CF ₃ Examples thereof include lithium salts such as SO ₂ ) ₂ , LiN (C ₂ F ₅ SO ₂ ) ₂ , and LiB ₁₀ Cl ₁₀ . Other examples of the supporting salt include lower aliphatic lithium carboxylate carboxylate, lithium chloroborane, lithium tetraphenylborate, LiBr, LiI, LiSCN, LiCl, and the like. The supporting salt can be used alone or in combination of two or more.

  Further, an ion conductive polymer can be added to the nonaqueous electrolytic solvent. Examples of the ion conductive polymer include polyethers such as polyethylene oxide and polypropylene oxide, and polyolefins such as polyethylene and polypropylene. Examples of the ion conductive polymer include polyvinylidene fluoride, polytetrafluoroethylene, polyvinyl fluoride, polyvinyl chloride, polyvinylidene chloride, polymethyl methacrylate, polymethyl acrylate, polyvinyl alcohol, polymethacrylonitrile, and polyvinyl chloride. Examples thereof include acetate, polyvinyl pyrrolidone, polycarbonate, polyethylene terephthalate, polyhexamethylene acipamide, polycaprolactam, polyurethane, polyethyleneimine, polybutadiene, polystyrene, or polyisoprene, or derivatives thereof. An ion conductive polymer can be used individually by 1 type or in combination of 2 or more types. Moreover, you may use the polymer containing the various monomers which comprise the said polymer.

(Positive electrode)
The positive electrode of the lithium secondary battery according to the present embodiment preferably includes a positive electrode active material capable of occluding or releasing lithium ions at a potential of 4.5 V or higher with respect to lithium metal from the viewpoint of obtaining a high energy density.

  As this positive electrode active material, a material having at least a part of the charge / discharge curve having a charge curve of 4.5 V or more with respect to lithium metal can be used. That is, an active material having at least a region of 4.5 V or more with respect to lithium metal only in the charge curve, or at least a region of 4.5 V or more with respect to lithium metal in both the charge curve and the discharge curve Can be used.

  As measurement conditions for this charge / discharge curve, the charge / discharge current can be set to 5 mA / g per mass of the positive electrode active material, the charge end voltage can be set to 5.2V, and the discharge end voltage can be set to 3V.

  Examples of such positive electrode active materials include spinel materials, layered materials, and olivine materials.

As a spinel-based material, LiNi _0.5 Mn _1.5 O ₄ , LiCoMnO ₄ , LiCrMnO ₄ , LiFeMnO ₄ , LiCu _0.5 Mn _1.5 O _{4 and the} like with a high potential of 4.5 V or higher. materials operates; a part of Mn of LiMn ₂ O ₄ with increased substitution to life with another _{element, LiM1 x Mn 2-x-} y M2 y O 4 (M1 is Ni, Fe, Co, Cr, and Cu At least one selected, 0.4 <x <1.1, M2 is at least one selected from Li, Al, B, Mg, Si, and transition metals, and 0 <y <0.5 ); And those obtained by substituting a part of oxygen of these materials with fluorine or chlorine.

  As the spinel-based material, a material represented by the following formula is particularly preferable.

_{Li a (M x Mn 2-} x-y Y y) (O 4-w Z w) (4)
(Wherein 0 ≦ x ≦ 1.2, 0 ≦ y, x + y <2, 0 ≦ a ≦ 1.2, 0 ≦ w ≦ 1, M is a transition metal, Co, Ni, Fe, Cr , Including at least one selected from Cu, Y being a metal element, including at least one selected from Li, B, Na, Al, Mg, Ti, Si, K, and Ca, Z being a halogen element, F And at least one of Cl.)
In particular, 0.4 ≦ x ≦ 1.1 is preferable.
In the formula (4), M preferably contains 80% or more, more preferably 90% or more, and may be 100%. Y and Z each preferably contain 80% or more of the above elements, more preferably 90% or more, and may be 100%.

The layered material is represented by a general formula LiMO ₂ , specifically, LiCoO ₂ , LiNi _1-x M _x O ₂ (M is an element containing at least Co or Al, 0.05 <x <0.3). in represented by _{material, Li (Ni x Co y Mn} 2-x-y) O 2 (0.1 <x <0.7,0 <y <0.5), Li (M 1-z Mn z) O ₂ (0.7 ≧ z ≧ 0.33, M is a metal, and includes at least one of Li, Co and Ni. Here, M preferably contains 80% or more of these elements, more preferably 90% or more, and may be 100%.).

Also the formula:
_{Li (Li x M 1-x} -z Mn z) O 2 (9)
(0 ≦ x <0.3, 0.3 ≦ z ≦ 0.7, M is a transition metal and includes at least one of Co and Ni)
Is particularly preferred. X in the formula of this material is preferably 0 ≦ x <0.2. In the formula (9), M preferably contains 80% or more of these elements, more preferably 90% or more, and may be 100%.

The olivine-based material has the general formula:
LiMPO ₄ (8)
Specifically, LiFePO ₄ , LiMnPO ₄ , LiCoPO ₄ , and LiNiPO ₄ may be mentioned. Those in which a part of these transition metals is replaced with another element or the oxygen part is replaced with fluorine can also be used. From the viewpoint of high energy density, a material represented by LiMPO ₄ (M is a transition metal and includes at least one of Co and Ni) operating at a high potential is preferable. M preferably contains 80% or more, more preferably 90% or more of these elements, and it is also preferable that M contains, for example, Fe as other elements contained therein.

  In addition, NASICON type, lithium transition metal silicon composite oxide, and the like can be used.

  The positive electrode active material that operates at the above high potential may be used in combination with other normal positive electrode active materials, but the content of the above positive electrode active material that operates at a high potential in the entire positive electrode active material is 60% by mass or more. Is preferable, 80 mass% or more is more preferable, and 90 mass% or more is further preferable.

Specific surface areas of these positive electrode active materials are, for example, 0.01 to ⁵ m ² / g, preferably 0.05 to ⁴ m ² / g, more preferably 0.1 to ³ m ² / g, and 0.2 to ² m. ² / g is more preferable. By setting the specific surface area in such a range, the contact area with the electrolytic solution can be adjusted to an appropriate range. That is, when the specific surface area is 0.01 m ² / g or more, lithium ions can be easily inserted and desorbed smoothly, and the resistance can be further reduced. Moreover, by making a specific surface area 5 m < ² > / g or less, decomposition | disassembly of electrolyte solution can be accelerated | stimulated and it can suppress more that the constituent element of an active material elutes. The specific surface area can be measured by a usual BET specific surface area measurement method.

The center particle diameter of the positive electrode active material is preferably 0.01 to 50 μm, and more preferably 0.02 to 40 μm. By setting the particle size to 0.02 μm or more, elution of constituent elements of the positive electrode active material can be further suppressed, and deterioration due to contact with the electrolytic solution can be further suppressed. In addition, when the particle size is 50 μm or less, lithium ions can be easily inserted and desorbed smoothly, and the resistance can be further reduced. The central particle diameter is 50% cumulative diameter D ₅₀ (median diameter), and can be measured by a laser diffraction / scattering particle size distribution analyzer.

  As the positive electrode binder, the same negative electrode binder can be used. Among these, polyvinylidene fluoride is preferable from the viewpoint of versatility and low cost. The amount of the binder for positive electrode to be used is preferably 2 to 10 parts by mass with respect to 100 parts by mass of the positive electrode active material from the viewpoints of binding force and energy density in a trade-off relationship.

  As binders other than polyvinylidene fluoride (PVdF), vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene copolymer, styrene-butadiene copolymer rubber, polytetrafluoroethylene, polypropylene, Examples include polyethylene, polyimide, and polyamideimide.

  A conductive auxiliary material may be added to the positive electrode active material layer containing the positive electrode active material for the purpose of reducing impedance. Examples of the conductive auxiliary material include carbonaceous fine particles such as graphite, carbon black, and acetylene black.

(Negative electrode)
A negative electrode will not be specifically limited if the negative electrode active material contains the material which can occlude and discharge | release lithium.

  The negative electrode active material is not particularly limited. For example, a carbon material (a) that can occlude and release lithium ions, a metal (b) that can be alloyed with lithium, or a metal that can occlude and release lithium ions. An oxide (c) etc. are mentioned.

  As the carbon material (a), graphite, amorphous carbon, diamond-like carbon, carbon nanotube, or a composite thereof can be used. Here, graphite with high crystallinity has high electrical conductivity, and is excellent in adhesiveness and voltage flatness with a positive electrode current collector made of a metal such as copper. On the other hand, since amorphous carbon having low crystallinity has a relatively small volume expansion, it has a high effect of relaxing the volume expansion of the entire negative electrode, and deterioration due to non-uniformity such as crystal grain boundaries and defects hardly occurs. The carbon material (a) can be used alone or in combination with other substances, but is preferably in the range of 2% by mass to 80% by mass in the negative electrode active material, and is preferably 2% by mass to 30% by mass. More preferably, it is in the range of% or less.

  As the metal (b), a metal mainly composed of Al, Si, Pb, Sn, Zn, Cd, Sb, In, Bi, Ag, Ba, Ca, Hg, Pd, Pt, Te, La, or the like, or these Two or more kinds of alloys, or an alloy of these metals or alloys and lithium can be used. In particular, silicon (Si) is preferably included as the metal (b). The metal (b) can be used alone or in combination with other substances, but is preferably in the range of 5% by mass to 90% by mass in the negative electrode active material, and is 20% by mass to 50% by mass. The following range is more preferable.

  As the metal oxide (c), silicon oxide, aluminum oxide, tin oxide, indium oxide, zinc oxide, lithium oxide, or a composite thereof can be used. In particular, silicon oxide is preferably included as the metal oxide (c). This is because silicon oxide is relatively stable and hardly causes a reaction with other compounds. Moreover, 0.1-5 mass% of 1 type, or 2 or more types of elements chosen from nitrogen, boron, and sulfur can also be added to a metal oxide (c). By carrying out like this, the electrical conductivity of a metal oxide (c) can be improved. The metal oxide (c) can be used alone or in combination with other substances, but is preferably in the range of 5% by mass to 90% by mass in the negative electrode active material, and 40% by mass to 70% by mass. More preferably, it is in the range of mass% or less.

Specific examples of the metal oxide (c) include, for example, LiFe ₂ O ₃ , WO ₂ , MoO ₂ , SiO, SiO ₂ , CuO, SnO, SnO ₂ , Nb ₃ O ₅ , Li _x Ti _2−x O _4. (1 ≦ x ≦ 4/3), PbO ₂ , Pb ₂ O _{5 and the} like.

Other examples of the negative electrode active material include metal sulfide (d) that can occlude and release lithium ions. Metal sulfide as (d) are, for example, SnS and FeS ₂ or the like. In addition, as the negative electrode active material, for example, metallic lithium or lithium alloy, polyacene or polythiophene, or Li ₅ (Li ₃ N), Li ₇ MnN ₄ , Li ₃ FeN ₂ , Li _2.5 Co _0. Examples thereof include lithium nitride such as ₅ N or Li ₃ CoN.

  The above negative electrode active materials can be used alone or in admixture of two or more.

  Moreover, a negative electrode active material can be set as the structure containing a carbon material (a), a metal (b), and a metal oxide (c). Hereinafter, this negative electrode active material will be described.

  All or part of the metal oxide (c) preferably has an amorphous structure. The amorphous metal oxide (c) can suppress the volume expansion of the carbon material (a) and the metal (b), and can suppress the decomposition of the electrolytic solution. This mechanism is presumed to have some influence on the film formation on the interface between the carbon material (a) and the electrolytic solution due to the amorphous structure of the metal oxide (c). The amorphous structure is considered to have relatively few elements due to non-uniformity such as crystal grain boundaries and defects. In addition, it can be confirmed by X-ray diffraction measurement (general XRD measurement) that all or part of the metal oxide (c) has an amorphous structure. Specifically, when the metal oxide (c) does not have an amorphous structure, a peak specific to the metal oxide (c) is observed, but all or part of the metal oxide (c) is amorphous. In the case of having a structure, the intrinsic peak of the metal oxide (c) is broad and observed.

  The metal oxide (c) is preferably a metal oxide constituting the metal (b). The metal (b) and the metal oxide (c) are preferably silicon (Si) and silicon oxide (SiO), respectively.

  It is preferable that all or part of the metal (b) is dispersed in the metal oxide (c). By dispersing at least a part of the metal (b) in the metal oxide (c), the volume expansion of the whole negative electrode can be further suppressed, and the decomposition of the electrolytic solution can also be suppressed. Note that all or part of the metal (b) is dispersed in the metal oxide (c) because it is observed with a transmission electron microscope (general TEM observation) and energy dispersive X-ray spectroscopy (general). This can be confirmed by using a combination of a standard EDX measurement. Specifically, the cross section of the sample containing the metal (b) particles is observed, the oxygen concentration of the metal (b) particles dispersed in the metal oxide (c) is measured, and the metal (b) particles are configured. It can be confirmed that the metal being used is not an oxide.

  As described above, the content of each carbon material (a), metal (b), and metal oxide (c) with respect to the total of the carbon material (a) metal (b) and metal oxide (c) is It is preferable that they are 2 mass% or more and 80 mass% or less, 5 mass% or more and 90 mass% or less, and 5 mass% or more and 90 mass% or less, respectively. Moreover, the content rate of each carbon material (a), a metal (b), and a metal oxide (c) with respect to the sum total of a carbon material (a), a metal (b), and a metal oxide (c), respectively, More preferably, they are 2 mass% or more and 30 mass% or less, 20 mass% or more and 50 mass% or less, and 40 mass% or more and 70 mass% or less.

  A negative electrode active material in which all or part of the metal oxide (c) has an amorphous structure and all or part of the metal (b) is dispersed in the metal oxide (c) is disclosed in, for example, It can be produced by a method as disclosed in 2004-47404. That is, by performing a CVD process on the metal oxide (c) in an atmosphere containing an organic gas such as methane gas, the metal (b) in the metal oxide (c) is nanoclustered and the surface is a carbon material (a ) Can be obtained. Moreover, the said negative electrode active material is producible also by mixing a carbon material (a), a metal (b), and a metal oxide (c) by mechanical milling.

  The carbon material (a), the metal (b), and the metal oxide (c) are not particularly limited, but particulate materials can be used. For example, the average particle diameter of the metal (b) may be smaller than the average particle diameter of the carbon material (a) and the average particle diameter of the metal oxide (c). In this way, the metal (b) having a large volume change during charging and discharging has a relatively small particle size, and the carbon material (a) and the metal oxide (c) having a small volume change have a relatively large particle size. Therefore, dendrite formation and alloy pulverization are more effectively suppressed. In addition, lithium is occluded and released in the order of large-diameter particles, small-diameter particles, and large-diameter particles during the charge / discharge process. This also suppresses the occurrence of residual stress and residual strain. Is done. The average particle diameter of the metal (b) can be, for example, 20 μm or less, and is preferably 15 μm or less.

  Moreover, it is preferable that the average particle diameter of a metal oxide (c) is 1/2 or less of the average particle diameter of a carbon material (a), and the average particle diameter of a metal (b) is an average of a metal oxide (c). It is preferable that it is 1/2 or less of a particle diameter. Furthermore, the average particle diameter of the metal oxide (c) is ½ or less of the average particle diameter of the carbon material (a), and the average particle diameter of the metal (b) is the average particle diameter of the metal oxide (c). It is more preferable that it is 1/2 or less. By controlling the average particle size in such a range, the effect of relaxing the volume expansion of the metal and alloy phases can be obtained more effectively, and a secondary battery having an excellent balance of energy density, cycle life and efficiency can be obtained. Can do. More specifically, the average particle diameter of the silicon oxide (c) is set to 1/2 or less of the average particle diameter of the graphite (a), and the average particle diameter of the silicon (b) is the average particle of the silicon oxide (c). It is preferable to make it 1/2 or less of the diameter. More specifically, the average particle diameter of silicon (b) can be, for example, 20 μm or less, and is preferably 15 μm or less.

  The binder for the negative electrode is not particularly limited, but polyvinylidene fluoride (PVdF), vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene copolymer, styrene-butadiene copolymer. Polymerized rubber, polytetrafluoroethylene, polypropylene, polyethylene, polyimide, polyamideimide and the like can be mentioned.

  The content of the negative electrode binder is preferably in the range of 1 to 30% by mass and more preferably 2 to 25% by mass with respect to the total amount of the negative electrode active material and the negative electrode binder. By setting the content to 1% by mass or more, the adhesion between the active materials or between the active material and the current collector is improved, and the cycle characteristics are improved. Moreover, by setting it as 30 mass% or less, an active material ratio can improve and a negative electrode capacity | capacitance can be improved.

  Although it does not restrict | limit especially as a negative electrode electrical power collector, Aluminum, nickel, copper, silver, and those alloys are preferable from electrochemical stability. Examples of the shape include foil, flat plate, and mesh.

  The negative electrode can be produced by forming a negative electrode active material layer containing a negative electrode active material and a negative electrode binder on a negative electrode current collector. Examples of the method for forming the negative electrode active material layer include a doctor blade method, a die coater method, a CVD method, and a sputtering method. After forming a negative electrode active material layer in advance, a thin film of aluminum, nickel, or an alloy thereof may be formed by a method such as vapor deposition or sputtering to form a negative electrode current collector.

(Separator)
The secondary battery can be composed of a combination of a positive electrode, a negative electrode, a separator, and a nonaqueous electrolyte as its configuration. Examples of the separator include a woven fabric, a nonwoven fabric, a polyolefin polymer such as polyethylene and polypropylene, a polyimide, a porous polymer film such as a porous polyvinylidene fluoride film, or an ion conductive polymer electrolyte film. These can be used alone or in combination.

(Battery shape and exterior)
Examples of the shape of the battery include a cylindrical shape, a square shape, a coin shape, a button shape, and a laminate shape.

  In the case of the laminate type, the electrodes and the separator are laminated in a planar shape, and there is no portion with a small R (a region close to the winding core of the wound structure or a region corresponding to a folded portion of the flat wound structure). Therefore, when an active material having a large volume change associated with charging / discharging is used, it is less likely to be adversely affected by the volume change of the electrode associated with charging / discharging than a battery having a wound structure.

  Examples of the battery outer package include stainless steel, iron, aluminum, titanium, alloys thereof, and plated products thereof. As the plating, for example, nickel plating can be used. When the battery is a laminate type, a laminate film is preferable as the outer package.

  Examples of the metal foil layer on the resin base layer of the laminate film include aluminum, an aluminum alloy, and a titanium foil. Examples of the material for the heat-welded layer of the laminate film include thermoplastic polymer materials such as polyethylene, polypropylene, and polyethylene terephthalate. Moreover, the resin base material layer and the metal foil layer of the laminate film are not limited to one layer, but may be two or more layers. From the viewpoint of versatility and cost, an aluminum laminate film is preferable.

  When a laminate film is used as the outer package, battery volume changes and electrode distortions due to the generation of gas are more likely to occur than when a metal can is used as the outer package. This is because the laminate film is more easily deformed by the internal pressure of the battery than the metal can. In addition, when sealing a secondary battery using a laminate film as an exterior body, the internal pressure of the battery is usually lower than atmospheric pressure, and there is no extra space inside, so when gas is generated in the battery Immediately leads to battery volume change and electrode deformation. In addition, in the case of a laminate type battery, compared to a battery having a wound structure, when gas is generated between the electrodes, the gap between the electrodes tends to be widened because the gas tends to stay between the electrodes. When used, this tendency becomes more prominent. According to this embodiment, the occurrence of such a problem can be suppressed, and a non-aqueous electrolyte secondary battery excellent in long-term reliability is provided even for a laminated battery using a laminate film outer package. be able to.

(Basic battery structure)
A cross-sectional view of a laminated lithium secondary battery according to this embodiment is shown in FIG. As shown in FIG. 1, the lithium secondary battery according to the present embodiment includes a positive electrode current collector 3 made of a metal such as an aluminum foil and a positive electrode active material layer 1 containing a positive electrode active material provided thereon. And a negative electrode current collector 4 made of a metal such as copper foil and a negative electrode active material layer 2 containing a negative electrode active material provided thereon. The positive electrode and the negative electrode are laminated via a separator 5 made of a nonwoven fabric or a polypropylene microporous film so that the positive electrode active material layer 1 and the negative electrode active material layer 2 face each other. This electrode pair is accommodated in a container formed of exterior bodies 6 and 7 such as an aluminum laminate film. A positive electrode tab 9 is connected to the positive electrode current collector 3, and a negative electrode tab 8 is connected to the negative electrode current collector 4, and these tabs are drawn out of the container. An electrolytic solution is injected into the container and sealed. It can also be set as the structure where the electrode group by which the several electrode pair was laminated | stacked was accommodated in the container.

  EXAMPLES Hereinafter, specific examples to which the present invention is applied will be described. However, the present invention is not limited to the examples, and can be appropriately modified and implemented without departing from the gist thereof. .

Example 1
LiNi _0.5 Mn _1.5 O ₄ (90 parts by mass) as a positive electrode active material, polyvinylidene fluoride (5% by mass) as a binder, and carbon black (5% by mass) as a conductive agent. A positive electrode mixture was prepared by mixing. A positive electrode slurry was prepared by dispersing the positive electrode mixture in N-methyl-2-pyrrolidone. This positive electrode slurry was uniformly applied to one side of an aluminum current collector having a thickness of 20 μm. The thickness of the coating film was adjusted so that the initial charge capacity per unit area was 2.5 mAh / cm ² . After drying, a positive electrode was produced by compression molding with a roll press.

Artificial graphite was used as the negative electrode active material. A negative electrode slurry was prepared by dispersing in artificial graphite and N-methylpyrrolidone in which PVDF was dissolved. The mass ratio of the negative electrode active material and the binder was 90/10. This negative electrode slurry was uniformly coated on a 10 μm thick Cu current collector. The thickness of the coating film was adjusted so that the initial charge capacity was 3.0 mAh / cm ² . After drying, a negative electrode was produced by compression molding with a roll press.

  A positive electrode and a negative electrode cut out to 3 cm × 3 cm were arranged so as to face each other with a separator interposed therebetween. As the separator, a microporous polypropylene film having a thickness of 25 μm was used.

Ethylene carbonate (EC), 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (TFETFPE), and tris (2,2,2-trifluoroethyl) phosphate (TTFEP) ) Was mixed in a volume ratio of 3/5/2 to prepare a nonaqueous electrolytic solvent containing 1.4% of a fluorinated diether compound represented by CF ₃ CH ₂ OCH ₂ CH ₂ OCH ₂ CF ₃ . LiPF ₆ was dissolved in this nonaqueous electrolytic solvent at a concentration of 1 mol / l to prepare an electrolytic solution.

  The positive electrode, the negative electrode, the separator, and the electrolytic solution were placed in a laminate outer package, the laminate was sealed, and a lithium secondary battery was produced. The positive electrode and the negative electrode were connected to a tab and electrically connected from the outside of the laminate.

  After charging the lithium secondary battery at 20 mA and the upper limit voltage reached 4.8 V, the lithium secondary battery was charged at a constant voltage until the total charging time reached 2.5 hours. Thereafter, the battery was discharged at a constant current at 20 mA until the lower limit voltage was 3V. This charging / discharging was repeated a predetermined number of times. This charging / discharging was implemented in a 45 degreeC thermostat.

(Gas generation evaluation)
The amount of gas generation was evaluated by measuring the change in cell volume after the charge / discharge cycle. The cell volume was measured using the Archimedes method, and the gas generation amount was calculated by examining the difference before and after the charge / discharge cycle. The evaluation results of gas generation are shown in Table 1.

(Example 2)
Example 1 was repeated except that the nonaqueous electrolytic solvent was changed to the composition shown in Table 1. The evaluation results of gas generation are shown in Table 1.

(Comparative Examples 1 and 2)
As shown in Table 1, Example 1 was repeated except for changing to a non-aqueous electrolytic solvent not containing a fluorinated diether compound, and gas generation was evaluated. The results are shown in Table 1.

Abbreviation TTFEP in Table: Tris phosphate (2,2,2-trifluoroethyl)
EC: ethylene carbonate FE1: 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (TFETFPE)
FEC: Fluoroethylene carbonate (4-fluoro-1,3-dioxolan-2-one)
DMC: Dimethyl carbonate

  As described above, according to this embodiment, it is possible to obtain a high voltage lithium secondary battery with improved life characteristics and gas generation.

  The lithium secondary battery of the present embodiment is a secondary battery with good cycle characteristics, and can be used in all industrial fields that require a power source and industrial fields related to the transport, storage, and supply of electrical energy. . Specifically, it can be used for a power source of a mobile device, a power source of a moving / transport medium, a backup power source, a solar power generation, a wind power generation, and a power storage facility for storing power generated by the power generation.

DESCRIPTION OF SYMBOLS 1 Positive electrode active material layer 2 Negative electrode active material layer 3 Positive electrode collector 4 Negative electrode collector 5 Separator 6 Laminate exterior 7 Laminate exterior 8 Negative electrode tab 9 Positive electrode tab

Claims (12)

A lithium secondary battery having a positive electrode, a negative electrode, and an electrolyte solution containing a nonaqueous electrolytic solvent,
The non-aqueous electrolytic solvent includes a fluorine-containing phosphate ester represented by the following formula (1) and a fluorinated diether compound represented by the following formula (20):

(In Formula (1), R ₁ , R ₂ and R ₃ are each independently a substituted or unsubstituted alkyl group, and at least one of R ₁ , R ₂ and R ₃ is a fluorine-containing alkyl group. is there.)
_{R 4 O- (R 5 O)} n -R 6 (20)
(In formula (20), R ₄ and R ₆ are each independently a C 1-4 alkyl group optionally substituted with a fluorine atom, and R ₅ is a carbon optionally substituted with a fluorine atom. An alkylene group of 1 to 4, provided that at least one of R ₄ , R ₅ and R ₆ is a group substituted with a fluorine atom, and n represents an integer of 1 to 4)
The positive electrode active material contained in the positive electrode is
(I) Lithium manganese composite oxide represented by the following formula ( 4-1 ) and containing at least Ni as M:
_{Li a (M x Mn 2-} x-y Y y) (O 4-w Z w) (4-1)
(Where 0.4 ≦ x ≦ 1.1 , 0 ≦ y, x + y <2, 0 ≦ a ≦ 1.2 , 0 ≦ w ≦ 1, M is a transition metal, Co, Ni, Fe , Including at least one selected from the group consisting of Cr and Cu, Y being a metal element, including at least one selected from the group consisting of Li, B, Na, Al, Mg, Ti, Si, K and Ca, Z is a halogen element and includes at least one selected from the group consisting of F and Cl).
Or (ii) lithium metal composite oxide represented by the following formula ( 8-1 ) or ( 9-1 ):
LiMPO ₄ ( 8-1 )
(In the formula, M is a transition metal and contains at least one element of Co and Ni in a proportion of 80% or more .)
_{Li (Li x M 1-x} -z Mn z) O 2 (9-1)
(0 ≦ x < 0.2 , 0.3 ≦ z ≦ 0.7, M is a transition metal and includes at least one of Co and Ni.)
Only including,
The lithium secondary battery, wherein a content rate of the fluorine-containing phosphate is 20% by volume or more and 80% by volume or less in the nonaqueous electrolytic solvent . A lithium secondary battery having a positive electrode, a negative electrode, and an electrolyte solution containing a nonaqueous electrolytic solvent,
The nonaqueous electrolytic solvent includes a fluorine-containing phosphate ester represented by the following formula (1) and a fluorinated diether compound represented by the following formula (21) :

(In Formula (1), R ₁ , R ₂ and R ₃ are each independently a substituted or unsubstituted alkyl group, and at least one of R ₁ , R ₂ and R ₃ is a fluorine-containing alkyl group. is there.)
CF ₃ CH ₂ OCH ₂ CH ₂ OCH ₂ CF ₃ (21)
The positive electrode active material contained in the positive electrode is
(I) Lithium manganese composite oxide represented by the following formula ( 4-1 ) and containing at least Ni as M:
_{Li a (M x Mn 2-} x-y Y y) (O 4-w Z w) (4-1)
(Where 0.4 ≦ x ≦ 1.1 , 0 ≦ y, x + y <2, 0 ≦ a ≦ 1.2 , 0 ≦ w ≦ 1, M is a transition metal, Co, Ni, Fe , Including at least one selected from the group consisting of Cr and Cu, Y being a metal element, including at least one selected from the group consisting of Li, B, Na, Al, Mg, Ti, Si, K and Ca, Z is a halogen element and includes at least one selected from the group consisting of F and Cl).
Or (ii) lithium metal composite oxide represented by the following formula ( 8-1 ) or ( 9-1 ):
LiMPO ₄ ( 8-1 )
(In the formula, M is a transition metal and contains at least one element of Co and Ni in a proportion of 80% or more .)
_{Li (Li x M 1-x} -z Mn z) O 2 (9-1)
(0 ≦ x < 0.2 , 0.3 ≦ z ≦ 0.7, M is a transition metal and includes at least one of Co and Ni.)
A lithium secondary battery comprising: The lithium secondary battery according to claim 2, wherein a content rate of the fluorine-containing phosphate ester is 10% by volume or more and 95% by volume or less in the nonaqueous electrolytic solvent. At least one of R _1, R ₂ and R _3, any one of claims 1 to 3 50% or more of the hydrogen atom of the corresponding unsubstituted alkyl radicals are substituted fluorine-containing alkyl group with a fluorine atom 2. The lithium secondary battery according to item 1. The lithium secondary battery according to claim 1, wherein the fluorine-containing phosphate ester is a compound represented by the following formula (2).

A fluorinated diether compound represented by the formula (20) wherein n is 1 is included, wherein the fluorinated diether compound is limited to the portion cited in claim 1. the lithium secondary battery according to any one of). The lithium secondary battery according to claim 6, wherein the fluorinated diether compound is a compound represented by the following formula (21).
CF ₃ CH ₂ OCH ₂ CH ₂ OCH ₂ CF ₃ (21)   The lithium secondary battery according to claim 1, wherein the non-aqueous electrolytic solvent includes a cyclic carbonate.   The cyclic carbonate is at least one selected from the group consisting of ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, and a compound having a structure in which a part or all of the hydrogen atoms thereof are substituted with fluorine atoms. The lithium secondary battery according to claim 8. The lithium secondary battery according to any one of claims 1 to 9, wherein the positive electrode active material includes a lithium manganese composite oxide represented by the formula (4-1) defined in claim 1 . 10. The lithium secondary material according to claim 1, wherein the positive electrode active material contains a lithium metal composite oxide represented by the formula (8-1) or (9-1) defined in claim 1. Next battery .   The lithium secondary battery according to any one of claims 1 to 11, further comprising an exterior body that encloses the positive electrode, the negative electrode, and the electrolytic solution, and the exterior body is made of an aluminum laminate.

Images