WO2014103893A1 - Lithium secondary battery and method for selecting same - Google Patents

Abstract

The present invention relates to a lithium secondary battery provided with a positive electrode including a positive electrode active material, a negative electrode including a negative electrode active material, and an electrolyte solution including a non-aqueous electrolyte solvent. On a plane defined by two orthogonal axes of which one is the change in charge transfer resistance at the positive electrode is applied and the other is the change in charge transfer resistance at the negative electrode, the points (ΔRct2, ΔRct) representing the change (ΔRct) in charge transfer resistance at the positive electrode and the change (ΔRct2) in charge transfer resistance at the negative electrode before and after a predetermined stress is applied are projected, in the radial line direction, on a circle having the origin as the center and a radius of 1Ω, and normalized as points (ΔRa, ΔRc). On a plane defined by two orthogonal axes of which one is the change in charge transfer resistance at the positive electrode after normalization and the other is the change in charge transfer resistance at the negative electrode after normalization, the distances to the origin from points obtained by projecting the points (ΔRa, ΔRc) perpendicularly to a line passing through the origin are within a predetermined range.

Description

Lithium secondary battery and its sorting method

The present invention relates to a lithium secondary battery and a selection method thereof.

Lithium secondary batteries are widely used for portable electronic devices and personal computers because of their small size and large capacity. However, since the characteristics deteriorate due to repeated charge and discharge, there is a lifetime. In recent years, the use of batteries for large capacity devices such as household storage batteries and electric vehicles has been realized, so it is not desirable to replace batteries in the short term. It has become.

As a cause of deterioration, for example, a decomposition reaction occurs at the time of charge / discharge at the contact portion between the positive electrode or the negative electrode and the electrolytic solution, and a reaction product is formed on the electrode surface, or the crystallinity changes in a part of the electrode. Is considered.

Several methods are conceivable as methods for increasing the capacity of the lithium secondary battery. Among them, increasing the operating potential of the battery is effective. In lithium secondary batteries using lithium cobalt oxide 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). is there. 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 a 5 V class operating potential can be realized by using, as an active material, a spinel compound in which Mn of lithium manganate is substituted with Ni, Co, Fe, Cu, Cr or the like. Specifically, it is known that spinel compounds such as LiNi _0.5 Mn _1.5 O ₄ exhibit a potential plateau in the 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 . Furthermore, spinel type lithium manganese oxide has an advantage that it has a three-dimensional lithium diffusion path, has excellent thermodynamic stability, and is easy to synthesize. For these reasons, LiNi _0.5 Mn _1.5 O ₄ is promising as a future positive electrode material.

Although it is expected that the capacity is increased by increasing the operating voltage in this way, since the reactivity in the battery is increased at the same time, it is more difficult to realize the charge / discharge cycle life.

Patent Document 1 discloses the diameter of a semicircular arc drawn when impedance is measured in a frequency region of 10 kHz to 10 mHz and a result is described on a complex plane in an electrochemical cell using a positive electrode as a working electrode and lithium metal as a game. In an electrochemical cell using R1 and a negative electrode plate as the working electrode and lithium metal as the counter electrode, when impedance is measured in the frequency range from 10 kHz to 10 mHz and the result is described on the complex plane, the diameter of the semicircular arc drawn as R2 It is stated that the deterioration in the charge / discharge cycle can be suppressed by setting the value of R2 / R1 to 0.01 to 15.

Patent Document 2 describes an increase rate of a negative electrode resistance component (impedance), an increase rate of a positive electrode resistance component (impedance), and the like of a lithium ion battery subjected to charge / discharge cycles. Patent Document 3 describes the ratio of the charge transfer resistance between the early discharge stage and the late discharge stage of the electrode plate for a nonaqueous electrolyte secondary battery. Patent Document 4 shows a comparison between the charge transfer resistance of the positive electrode and the charge transfer resistance of the negative electrode for a plurality of unused lithium ion batteries of the same type and a plurality of lithium ion batteries of the same type different in use state. Has been.

JP 2000-173609 A JP 2003-308885 A JP 2007-103065 A JP 2009-097878 A

However, the method of Patent Document 1 is insufficient for improving the charge / discharge cycle deterioration. FIG. 7 shows a result of evaluating the relationship between R2 / R1 in a state where the battery is once charged after the trial manufacture and the amount of change ΔRall of the internal resistance of the battery due to 50 cycles of charge / discharge. The data is experimental data of examples and comparative examples in the present specification, and conditions will be described later. The evaluation of R1 and R2 was obtained by fitting an AC impedance measurement between the electrodes and an equivalent circuit as described later.

As shown in FIG. 7, when R2 / R1, which is the indicated range of Patent Document 1, is around 1, the amount of increase in resistance spreads from 0.5Ω to 2Ω, and there is a four-fold difference. Such an increase in electrode resistance causes a voltage drop. As the charge / discharge cycle progresses, the voltage actually applied to the battery is lowered, causing a decrease in capacity. As described above, the method of Patent Document 1 cannot sufficiently suppress the increase in electrode resistance, and the life is not improved sufficiently.

Therefore, the lithium secondary battery of the present invention aims to realize excellent charge / discharge cycle characteristics.

One of the embodiments is
A lithium secondary battery having a positive electrode including a positive electrode active material, a negative electrode including a negative electrode active material, and an electrolytic solution including a nonaqueous electrolytic solvent,
A change amount (ΔRct) of the positive electrode charge transfer resistance before and after a predetermined stress application on a plane having two axes orthogonal to each other, the change amount of the charge transfer resistance of the positive electrode and the change amount of the charge transfer resistance of the negative electrode ) And a point (ΔRct2, ΔRct) representing the amount of change (ΔRct2) in the charge transfer resistance of the negative electrode is projected to a point (ΔRa , ΔRc)
A point (ΔRa, ΔRc) is defined with respect to a straight line passing through the origin on a plane having two axes orthogonal to each other. The lithium secondary battery is characterized in that the distance from the vertically projected point to the origin is within a predetermined range.

According to the present invention, a secondary battery having excellent cycle characteristics can be provided. In particular, even when a positive electrode active material that operates at a potential of 4.5 V or higher with respect to lithium is used, a secondary battery having excellent cycle characteristics can be provided.

It is an example of sectional drawing of the secondary battery of this invention. It is an example of the equivalent circuit of the lithium secondary battery used by this invention. It is the figure which showed the method of extracting electrode resistance from the impedance measurement result used by this invention. A method of normalizing by projecting in a radiation direction on a circle with a radius of 1Ω centered on the origin on a plane having two axes orthogonal to each other, the amount of change in the positive electrode side charge transfer resistance and the amount of change in the negative electrode side charge transfer resistance. FIG. The points (ΔRa, ΔRc) obtained by normalization in FIG. 4 on a plane having two orthogonal axes of the change amount of the charge transfer resistance of the positive electrode after normalization and the change amount of the charge transfer resistance of the negative electrode after normalization. ) On a straight line represented by ΔRa = −0.6ΔRc. It is a figure which shows the relationship between resistance increase amount (DELTA) Rall by charging / discharging cycle, and the projection value ((DELTA) Rx) of battery resistance. It is the figure which showed the relationship between the resistance ratio of a positive electrode and a negative electrode, and resistance increase amount (DELTA) Rall by a charging / discharging cycle.

One aspect of this embodiment will be described below.

The lithium secondary battery of the present embodiment has a positive electrode including a positive electrode active material, a negative electrode including a negative electrode active material, and an electrolytic solution including a nonaqueous electrolytic solvent. Charge transfer resistances Rct and Rct2 exist in the positive electrode and the negative electrode, respectively. The amount of change in the charge transfer resistance of the positive electrode before and after the predetermined stress application [(charge transfer resistance value of the positive electrode after stress application) − (charge transfer resistance value of the positive electrode before stress application)] is ΔRct, The amount of change in the charge transfer resistance of the negative electrode [(charge transfer resistance value of negative electrode after applying stress) − (charge transfer resistance value of negative electrode before applying stress)] is represented by ΔRct2. First, on a plane having two axes perpendicular to the amount of change in the charge transfer resistance of the positive electrode and the amount of change in the charge transfer resistance of the negative electrode, the circle centered on the origin from the point (ΔRct2, ΔRct) is 1 Ω. Are projected in the radiation direction and normalized, and the point on the circle after normalization is defined as a point (ΔRa, ΔRc). Subsequently, the point (ΔRa, ΔRc) is set to the origin on a plane having two axes perpendicular to the amount of change in charge transfer resistance of the positive electrode after normalization and the amount of change in charge transfer resistance of the negative electrode after normalization. Project perpendicular to the straight line that passes through. When the distance ΔRx from the projected point (on the straight line passing through the origin) to the origin is within a predetermined range, the cycle characteristics of the lithium ion secondary battery are compared to when the distance ΔRx is not within the predetermined range. Has been found by the inventors of the present invention.

A method for calculating ΔRx will be described with reference to FIGS. 4 to 6 as an example. 4, the vertical axis represents the amount of change in charge transfer resistance of the positive electrode, the horizontal axis represents the amount of change in charge transfer resistance of the negative electrode, and after plotting (ΔRct2, ΔRct) of a plurality of lithium ion secondary batteries, Is a method of normalizing by projecting in the radiation direction onto a circle having a radius of 1Ω centered on. 5, the vertical axis represents the change amount of the charge transfer resistance of the positive electrode after normalization according to FIG. 4, and the horizontal axis represents the change amount of the charge transfer resistance of the negative electrode after normalization according to FIG. 4. The points (ΔRa, ΔRc) obtained in this way are plotted, and a method of projecting these points perpendicularly to a straight line represented by ΔRa = −0.6ΔRc is shown. The distance from the point on the straight line when projected perpendicularly to the straight line to the origin is ΔRx. FIG. 6 shows the relationship between the distance ΔRx from the point on the straight line to the origin and the actual resistance value ΔRall obtained by vertically projecting onto the straight line represented by ΔRa = −0.6ΔRc in FIG. In the figure, it can be seen that the value of ΔRall decreases when ΔRx is in the vicinity of zero. In the present specification, ΔRx may be referred to as “projection value”.

Examples of the stress include charge / discharge cycles, high temperature storage (for example, storage at 45 ° C. or higher for 5 minutes or longer), high voltage application (for example, a value higher by 0.05 V or more than the voltage on the higher potential side of the battery), and the like. Can be mentioned. According to the present invention, an increase in the internal resistance of the battery can be suppressed, and a lithium secondary battery with good charge / discharge cycle resistance can be realized. In particular, the present invention is more effective when the resistance increase due to stress is large, such as a lithium secondary battery using a positive electrode active material that operates at a potential of 4.5 V or higher with respect to lithium. .

When the type or magnitude of stress applied to the lithium secondary battery is changed, ΔRct2 and ΔRct also change. However, since ΔRct2 and ΔRct change in a substantially proportional relationship, the values change in a radiation direction centered on the origin. Increase or decrease. Therefore, in the present embodiment, as described above, the point (ΔRct2, ΔRct) is set to the origin on the plane with the change amount of the charge transfer resistance of the positive electrode on the vertical axis and the change amount of the charge transfer resistance of the negative electrode on the horizontal axis. Can be normalized by projecting in a radiation direction onto a circle having a radius of 1Ω around the center of. A point obtained by this normalization on a circle having a radius of 1Ω centered on the origin is defined as a point (ΔRa, ΔRc).

Subsequently, a straight line passing through the origin is set as a new axis on a plane with the vertical axis representing the amount of change in charge transfer resistance of the positive electrode after normalization and the horizontal axis representing the amount of change in charge transfer resistance of the negative electrode after normalization. Then, the projection is perpendicular to the straight line from the point (ΔRa, ΔRc). The slope of the straight line passing through the origin can be set from the measurement results of the charge transfer resistances of a plurality of lithium ion secondary batteries. The straight line passing through the origin is not particularly limited, but when represented by ΔRa = m · ΔRc, m is preferably a negative value, more preferably −3 ≦ m ≦ −1 / 3, More preferably, m is −0.6. The distance from the point (ΔRa, ΔRc) to the origin when projected perpendicularly to the straight line passing through the origin is assumed to be ΔRx.

Since ΔRa and ΔRc are already standardized values, the value ΔRx obtained by projecting perpendicularly to the straight line passing through the origin is in the range of 0 ± 1Ω. Among these, a predetermined range of ΔRx is determined, and a lithium secondary battery having excellent cycle characteristics can be obtained. In the present embodiment, the predetermined range ΔRx is not particularly limited, but is preferably within 0 ± 0.5Ω, and more preferably within 0 ± 0.3Ω.

As a measuring method of Rct and Rct2, there is a method of measuring and evaluating an impedance between a positive electrode and a negative electrode by an AC impedance method. When the measurement result is plotted on the complex plane, two arcs or elliptical arcs are observed. Of these arcs (or elliptical arcs), the actual resistance side diameter (or elliptical diameter) of the high frequency side arc (or elliptical arc) is Rct2, and the actual resistance side diameter or elliptical diameter of the low frequency side arc (or elliptical arc) is It may be regarded as Rct (see FIG. 3). If it is unknown which of the high frequency side and the low frequency side corresponds to Rct2, when Rct2 and Rct are interchanged, one of them may satisfy the condition that ΔRx is within a predetermined range. In addition, when there are three or more arcs, the larger one is considered. As a diameter extraction method, fitting to an equivalent circuit may be performed to extract Rct2 and Rct. For example, fitting with an equivalent circuit as shown in FIG. 2 may be performed to extract Rct2 and Rct. In the AC impedance method, an AC voltage is applied between terminals whose impedance is to be evaluated, a response current and its phase are evaluated, and real resistance and imaginary resistance at that frequency are extracted. This is a method of knowing the frequency response of the impedance by performing this at several frequencies from a desired frequency range, for example, 100 to 300 kHz to 10 to 100 mHz. In this method, it is possible to separate and evaluate resistors having different frequency responses, so that it is possible to separate the charge transfer resistances of the positive electrode and the negative electrode.

Another method for measuring Rct and Rct2 includes the following method. First, a conductive electrode made of lithium or the like is provided on the battery, and the impedance between the conductive electrode and the positive electrode is measured and evaluated by an AC impedance method. When the measurement result is plotted on a complex surface, an arc or an elliptical arc is observed. The actual resistance side diameter (or elliptical diameter) is Rct. Similarly, Rct2 is obtained from the impedance measurement result between the conductive electrode and the negative electrode.

This embodiment can be used as a means for selecting a cell having high resistance to stress and a cell having low resistance from a plurality of lithium ion secondary batteries (cells). For example, when the projection value ΔRx deviates from the design range or the amount of difference (standard deviation) from the distribution of other cells, the cell may be selected as a cell that is likely to become defective due to an abnormality. It becomes possible. Thus, the technology of the present invention can also be used as a means for selecting a lithium ion secondary battery.

Next, components of the lithium secondary battery of the present embodiment will be described.

(Electrolyte)
In the present embodiment, the electrolytic solution includes a supporting salt and a nonaqueous electrolytic solvent. The composition of the nonaqueous electrolytic solvent of the present invention can be adjusted as appropriate. For example, it preferably contains a fluorine-containing phosphate ester, and more preferably contains a cyclic sulfonate ester as an additive.

By containing the fluorine-containing phosphate ester compound in the electrolytic solution, the oxidation resistance of the electrolytic solution can be enhanced, and the compatibility with other solvent components and the ionic conductivity of the electrolytic solution can be enhanced. . In particular, when a fluorine-containing phosphate compound is used as an electrolyte solution solvent for a battery including a positive electrode that operates at a high potential, decomposition of the electrolyte solution at a high potential is suppressed, which is preferable. 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. Furthermore, examples of the fluorine-containing phosphate ester include trifluoroethyl dimethyl phosphate, bis (trifluoroethyl) methyl phosphate, bistrifluoroethyl ethyl phosphate, pentafluoropropyl dimethyl phosphate, heptafluorobutyl dimethyl 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, pentafluoropropyldie phosphate , Heptafluorobutyl diethyl phosphate, trifluoroethyl ethyl propyl phosphate, pentafluoropropyl ethyl propyl phosphate, heptafluorobutyl ethyl propyl phosphate, trifluoroethyl ethyl butyl phosphate, pentafluoropropyl ethyl butyl phosphate, phosphorus Heptafluorobutylethyl butyl phosphate, trifluoroethyl dipropyl phosphate, pentafluoropropyl dipropyl phosphate, heptafluorobutyl dipropyl phosphate, trifluoroethylpropyl butyl phosphate, pentafluoropropylpropyl butyl phosphate, hepta phosphate Examples thereof include fluorobutylpropylbutyl, trifluoroethyldibutyl phosphate, pentafluoropropyldibutyl phosphate, heptafluorobutyldibutyl phosphate, and the like. Examples of tris (tetrafluoropropyl) phosphate include tris (2,2,3,3-tetrafluoropropyl) phosphate. Examples of tris (pentafluoropropyl) phosphate include tris (2,2,3,3,3-pentafluoropropyl) phosphate. 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. These fluorine-containing phosphate esters can be used alone or in combination of two or more.

The content of the fluorine-containing phosphate ester is preferably in the range of 0.1 to 95% by volume, more preferably 0.2% by volume or more, and more preferably 0.5% by volume or more in the total nonaqueous electrolytic solvent. Preferably, 5 volume% or more is more preferable, it is more preferable that it is larger than 10 volume%, 20 volume% or more is further more preferable, 90 volume% or less is more preferable, and 80 volume% or less is further more preferable. When the content of the fluorine-containing phosphate is within this range, a lithium secondary battery excellent in cycle characteristics can be obtained.

The nonaqueous electrolytic solvent preferably contains a cyclic carbonate and / or a chain carbonate.

Since cyclic carbonate or chain carbonate has a large relative dielectric constant, the addition of these improves the dissociation property of the supporting salt and makes it easy to impart sufficient conductivity. In addition, cyclic carbonates and chain carbonates are suitable for mixing with fluorine-containing phosphate esters because of their high voltage resistance and electrical conductivity. Furthermore, it is possible to improve the ion mobility in the electrolytic solution by selecting a material having an effect of lowering the viscosity of the electrolytic solution.

The cyclic carbonate is not particularly limited, and examples thereof include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), and vinylene carbonate (VC). 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, for example, 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 resistance and conductivity. A cyclic carbonate can be used individually by 1 type or in combination of 2 or more types.

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-monofluoro Ethylmethylca Boneto, 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 linear carbonate can be used individually by 1 type or in combination of 2 or more types.

The nonaqueous electrolytic solvent may contain a carboxylic acid ester.

The carboxylate 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 viewpoint of withstand voltage and boiling point, the carboxylic acid esters include 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 nonaqueous electrolytic solvent can contain an alkylene biscarbonate represented by the following formula (1) in addition to the fluorine-containing phosphate ester. 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 the formula (1), 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 chain 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 (1) 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.

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

The nonaqueous electrolytic solvent may contain a chain ether.

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.

The nonaqueous electrolytic solvent may contain the following in addition to the above. 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.

The concentration of the supporting salt is preferably 0.5 to 1.5 mol / l in the electrolyte solution of the lithium salt. By setting the concentration of the lithium salt within this range, it is easy to adjust the density, viscosity, electrical conductivity, and the like to an appropriate range.

(Cyclic sulfonate ester)
The lithium secondary battery of the present invention may contain a cyclic sulfonate ester in the electrolytic solution. As cyclic sulfonic acid ester which can be used for this invention, the cyclic sulfonic acid ester represented by following formula (2) is mentioned, for example.

In the cyclic sulfonate ester represented by the formula (2), A and B each independently represent an alkylene group or a fluorinated alkylene group, and X represents a C—C single bond or —OSO ₂ — group. In the formula (2), the number of carbon atoms of the alkylene group is, for example, 1 to 8, preferably 1 to 6, and more preferably 1 to 4.

(In Formula (2), A and B are each independently an alkylene group or a fluoroalkylene group, and X is a C—C single bond or —OSO ₂ — group).

The fluorinated alkylene group represents a substituted alkylene group having a structure in which at least one hydrogen atom of the unsubstituted alkylene group is substituted with a fluorine atom. In the formula (2), the carbon number of the fluorinated alkylene group is, for example, 1 to 8, preferably 1 to 6, and more preferably 1 to 4.

The -OSO ₂ -group may be in any direction.

In the formula (2), when X is a single bond, the cyclic sulfonic acid ester is a cyclic monosulfonic acid ester, and the cyclic monosulfonic acid ester is preferably a compound represented by the following formula (4). .

(In Formula (4), R ₁₀₁ and R ₁₀₂ each independently represent a hydrogen atom, a fluorine atom, or an alkyl group having 1 to 4 carbon atoms. N is 0, 1, 2, 3, or 4. .)

In the formula (2), when X is a —OSO ₂ — group, the cyclic sulfonic acid ester is a cyclic disulfonic acid ester, and the cyclic disulfonic acid ester is preferably a compound represented by the following formula (5) .

(In the formula (5), R _{201 to} R ₂₀₄ each independently represents a hydrogen atom, a fluorine atom, or an alkyl group having 1 to 4 carbon atoms, and n is 1, 2, 3, or 4. , N is 2, 3, or 4, n R _203s may be the same or different from each other, and n R _204s may be the same or different from each other. May be.)

Examples of the cyclic sulfonate ester include 1,3-propane sultone, 1,2-propane sultone, 1,4-butane sultone, 1,2-butane sultone, 1,3-butane sultone, 2,4-butane sultone, 1,3 -Monosulfonic acid esters such as pentansultone (when X in formula (2) is a single bond), methylenemethane disulfonic acid ester (1,5,2,4-dioxadithian-2,2,4,4-tetraoxide ), Disulfonic acid esters such as ethylenemethane disulfonic acid ester (when X in the formula (2) is —OSO ₂ — group), and the like. Among these, 1,3-propane sultone, 1,4-butane sultone, and methylene methane disulfonic acid ester are preferable from the viewpoint of film forming effect, availability, and cost.

The content of the cyclic sulfonic acid ester in the electrolytic solution is preferably 0.01 to 10% by mass, and more preferably 0.1 to 5% by mass. When the content of the cyclic sulfonic acid ester is 0.01% by mass or more, a coating can be more effectively formed on the positive electrode surface to suppress decomposition of the electrolytic solution.

Further, an ion conductive polymer may 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.

Moreover, fluoroethylene carbonate (FEC) may be included as an additive in the nonaqueous electrolytic solvent.

(Positive electrode)
The positive electrode includes a positive electrode active material. The positive electrode is formed, for example, by binding a positive electrode active material so as to cover the positive electrode current collector with a positive electrode binder.

The positive electrode of the lithium secondary battery according to the present embodiment preferably contains a positive electrode active material capable of inserting or extracting 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.

The positive electrode active material that operates at a potential of 4.5 V or higher with respect to lithium can be selected by the following method, for example. First, a positive electrode containing a positive electrode active material and Li metal are placed in a state of facing each other with a separator interposed therebetween, and an electrolytic solution is injected to produce a battery. When charging / discharging is performed at a constant current of, for example, 5 mAh / g per positive electrode active material mass in the positive electrode, a charge / discharge capacity of 10 mAh / g or more per mass of active material is a potential of 4.5 V or more with respect to lithium. Can be a positive electrode active material that operates at a potential of 4.5 V or higher with respect to lithium. Moreover, when charging / discharging is performed at a constant current of 5 mAh / g per positive electrode active material mass in the positive electrode, the charge / discharge capacity per active material mass at a potential of 4.5 V or higher with respect to lithium is 20 mAh / g or higher. It is preferable that it is 50 mAh / g or more, and it is further more preferable that it is 100 mAh / g or more. The shape of the battery can be, for example, a coin type.

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

Examples of spinel-based materials include LiNi _0.5 Mn _1.5 O ₄ , LiCr _x Mn _2-x O ₄ (0.4 ≦ x ≦ 1.1), LiFe _x Mn _2-x O ₄ (0.4 ≦ x ≦ 1.1), LiCu _x Mn _2-x O ₄ (0.3 ≦ x ≦ 0.6), LiCo _x Mn _2-x O ₄ (0.4 ≦ x ≦ 1.1), and the like A material that operates at a high potential of 4.5 V or higher with respect to lithium, such as LiCoMnO ₄ , LiCrMnO ₄ , LiFeMnO ₄ , LiCu _0.5 Mn _1.5 O ₄ ; LiMn ₂ O ₄ a part of Mn increased the substituted to life with another _{element, LiM1 x Mn 2-x-} y M2 y O 4 (M1 represents at least one selected Ni, Fe, Co, Cr, and Cu, 0 .4 <x <1.1 and M2 is 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. These may be used alone or in combination of two or more. The positive electrode active material may have a composition in which LiNi _0.5 Mn _1.5 O ₄ and a positive electrode active material other than this are mixed.

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

Li _a (M _x Mn _2-xy Y _y ) (O _4-w Z _w ) (6)
(In the formula (6), 0.4 ≦ x ≦ 1.2, 0 ≦ y, x + y <2, 0 ≦ a ≦ 1.2, 0 ≦ w ≦ 1, M is Co, Ni, Fe, Y is at least one selected from the group consisting of Cr and Cu, Y is at least one selected from the group consisting of Li, B, Na, Al, Mg, Ti, Si, K, and Ca. And at least one selected from the group consisting of Cl.)

In the formula (6), M preferably contains Ni, and preferably only Ni. This is because when M contains Ni, a high-capacity active material can be obtained relatively easily. In the case where M consists only of Ni, x is preferably 0.4 or more and 0.6 or less from the viewpoint of obtaining a high-capacity active material. Moreover, it is more preferable that the positive electrode active material is LiNi _0.5 Mn _1.5 O ₄ because a high capacity of 130 mAh / g or more can be obtained.

In addition, the lifetime may be improved by replacing a part of the Mn portion of these active materials with Li, B, Na, Al, Mg, Ti, SiK, Ca, or the like. For example, in Equation (6), when 0 <y, the life may be improved. Among these, when Y is Al, Mg, Ti, or Si, the life improvement effect is high. Moreover, when Y is Ti, it is more preferable because it provides a life improvement effect while maintaining a high capacity. The range of y is preferably greater than 0 and less than or equal to 0.3. By setting y to 0.3 or less, it becomes easy to suppress a decrease in capacity.

It is also possible to replace the oxygen part with F or Cl. In Expression (6), by setting w to be greater than 0 and 1 or less, a decrease in capacity can be suppressed.

Layered materials have the general formula:
LiMO ₂
(In the formula, M is at least one of Co and Ni.)
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), 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 (8)
(In formula (8), 0.7 ≧ z ≧ 0.33, and M is at least one of Li, Co, and Ni). Further, Li (M1 _x M2 _y Mn _1-xy ) O ₂ (M1: at least one selected from the group consisting of Ni, Co and Fe, M2: at least one selected from the group consisting of Li, Mg and Al, 0.1 <x <0.5, 0.05 <y <0.3) and the like.

Also the formula:
_{Li (Li x M 1-x} -z Mn z) O 2 (9)
(In Formula (9), 0 ≦ x <0.3, 0.33 ≦ z ≦ 0.7, M is at least one of Co and Ni)
Is particularly preferred. X in the formula of this material is preferably 0.1 ≦ x <0.3.

The olivine-based material has the general formula:
LiMPO ₄ (7)
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.

In addition, examples of the active material that operates at a potential of 4.5 V or more with respect to lithium include Si composite oxide. Examples of the Si composite oxide include Li ₂ MSiO ₄ (M: Mn, Fe, Co). At least one of them).

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

The specific surface area of lithium-manganese composite oxide represented by the formula (6) is, for example, 0.01 ~ ^5m 2 / g, preferably ^{0.05 ~ 4m 2 / g, 0.1} ~ 3m 2 / g Is more preferable, 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 center particle size of the lithium manganese composite oxide is preferably 0.1 to 50 μm, more preferably 0.2 to 40 μm. By setting the particle size to 0.1 μm or more, elution of constituent elements such as Mn 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 particle diameter can be measured by a laser diffraction / scattering particle size distribution measuring apparatus.

As described above, the positive electrode active material preferably includes an active material that operates at a potential of 4.5 V or more with respect to lithium, but may include a 4 V class active material.

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 positive electrode binder 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 “sufficient binding force” and “higher energy” which are in a trade-off relationship. .

In addition, the positive electrode binder other than polyvinylidene fluoride is not particularly limited, but polyvinylidene fluoride (PVdF), vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene copolymer. Polymers, styrene-butadiene copolymer rubber, polytetrafluoroethylene, polypropylene, polyethylene, polyimide, polyamideimide and the like can also be used.

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.

As the positive electrode current collector, for example, aluminum, nickel, silver, stainless steel (SUS), valve metal, or an alloy thereof can be used from the viewpoint of electrochemical stability. Examples of the shape include foil, flat plate, and mesh. In particular, an aluminum foil can be suitably used.

(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, and for example, a carbon material (a) that can occlude and release lithium ions, a metal that can be alloyed with lithium (b), 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. In addition, one or more elements selected from nitrogen, boron, and sulfur may be added to the metal oxide (c), for example, 0.1 to 5% by mass. 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 or more and 90% by mass or less in the negative electrode active material, and is 40% by mass or more and 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. Other examples of the negative electrode active material include metal 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.

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

Further, the negative electrode active material can include a carbon material (a), a metal (b), and a metal oxide (c). Hereinafter, this negative electrode active material will be described.

It is preferable that all or part of the metal oxide (c) 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.

The metal (b) is preferably dispersed entirely or partially 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 These are preferably 2% by mass or more and 80% by mass or less, 5% by mass or more and 90% by mass or less, and 5% by mass or more and 90% by mass or less. Moreover, the content rate of each carbon material (a), metal (b), and metal oxide (c) with respect to the sum total of a carbon material (a), a metal (b), and a metal oxide (c) is 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 the method 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.

Further, 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 small particle size, large particle size, and small particle size 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.

As the negative electrode active material, graphite whose surface is covered with a low crystalline carbon material can be used. The surface of the graphite is covered with a low crystalline carbon material, which suppresses the reaction between the negative electrode active material and the electrolyte even when high energy density and high conductivity graphite is used as the negative electrode active material. can do. Therefore, by using the graphite covered with the low crystalline carbon material as the negative electrode active material, the capacity retention rate of the battery can be improved, and the battery capacity can be improved.

The graphite covered with the low crystalline carbon material can be obtained, for example, by coating particulate graphite with the low crystalline carbon material. The average particle diameter (volume average: D ₅₀ ) of the graphite particles is preferably 5 μm or more and 30 μm or less. The graphite preferably has crystallinity, and the I _D / _IG value of the graphite is more preferably 0.01 or more and 0.08 or less.

The thickness of the low crystalline carbon material is preferably 0.01 μm or more and 5 μm or less, and more preferably 0.02 μm or more and 1 μm or less.

The average particle diameter (D ₅₀ ) can be measured using, for example, a laser diffraction / scattering particle diameter / particle size distribution measuring apparatus Microtrac MT3300EX (Nikkiso).

The low crystalline carbon material can be formed on the surface of graphite by using a vapor phase method in which hydrocarbons such as propane and acetylene are thermally decomposed to deposit carbon. The low crystalline carbon material can be formed, for example, by using a method in which pitch or tar is attached to the surface of graphite and firing at 800 to 1500 ° C.

Graphite has a crystal structure in which the 002 plane layer spacing d ₀₀₂ is preferably 0.33 nm or more and 0.34 nm or less, more preferably 0.333 nm or more and 0.337 nm or less, and still more preferably 0.336 nm. It is as follows. Such highly crystalline graphite has a high lithium storage capacity and can improve charge and discharge efficiency.

The interlayer distance of graphite can be measured by, for example, X-ray diffraction.

The specific surface area of the graphite covered with the low crystalline carbon material is, for example, 0.01 to 20 m ² / g, preferably 0.05 to 10 m ² / g, and 0.1 to 5 m ² / g. More preferably, it is 0.2 to 3 m ² / g. By setting the specific surface area of the graphite covered with the low crystalline carbon to 0.01 m ² / g or more, the lithium ions can be inserted and desorbed smoothly, so that the resistance can be further reduced. By setting the specific surface area of graphite covered with low crystalline carbon to 20 m ² / g or less, decomposition of the electrolytic solution can be further suppressed, and elution of constituent elements of the active material into the electrolytic solution can be further suppressed. Can do.

The graphite used as the base material is preferably highly crystalline. For example, artificial graphite or natural graphite can be used, but is not particularly limited thereto. As the material of low crystalline carbon, for example, coal tar, pitch coke, phenol resin and mixed with high crystalline carbon can be used. A mixture is prepared by mixing 5 to 50% by mass of low crystalline carbon material with high crystalline carbon. After the mixture is heated to 150 ° C. to 300 ° C., heat treatment is further performed in the range of 600 ° C. to 1500 ° C. As a result, surface-treated graphite having a surface coated with low crystalline carbon can be obtained. The heat treatment is preferably performed in an inert gas atmosphere such as argon, helium, or nitrogen.

The negative electrode active material may contain other active materials besides graphite covered with the low crystalline carbon material.

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, 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.

The negative electrode current collector is not particularly limited, but aluminum, nickel, copper, silver, and alloys thereof are preferable from the viewpoint of 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 such as polyethylene and polypropylene, a porous polymer film such as polyimide, a porous polyvinylidene fluoride film, and an ion conductive polymer electrolyte film. These can be used alone or in combination.

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

Also, examples of the laminate resin film used for the laminate mold include aluminum, aluminum alloy, and titanium foil. Examples of the material of the heat-welded portion of the metal laminate resin film include thermoplastic polymer materials such as polyethylene, polypropylene, and polyethylene terephthalate. Further, the metal laminate resin layer and the metal foil layer are not limited to one layer, and may be two or more layers.

The exterior body can be appropriately selected as long as it is stable to the electrolyte and has a sufficient water vapor barrier property. For example, in the case of a laminated laminate type secondary battery, a laminate film made of aluminum, silica-coated polypropylene, polyethylene, or the like can be used as the outer package. In particular, it is preferable to use an aluminum laminate film from the viewpoint of suppressing volume expansion.

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. . FIG. 1 is a schematic diagram showing the configuration of a lithium secondary battery produced in this example.

As shown in FIG. 1, a lithium secondary battery includes a positive electrode active material layer 1 containing a positive electrode active material on a positive electrode current collector 3 made of metal such as aluminum foil, and a negative electrode current collector made of metal such as copper foil. And a negative electrode active material layer 2 containing a negative electrode active material on the body 4. The positive electrode active material layer 1 and the negative electrode active material layer 2 are disposed to face each other with a separator 5 made of an electrolytic solution, a nonwoven fabric containing the electrolyte, a polypropylene microporous film, and the like. In FIG. 1, 6 and 7 are exterior bodies, 8 is a negative electrode tab, 9 is a positive electrode tab.

(Example 1)
The positive electrode active material of this example was produced as follows. As a raw material, a material is selected from MnO ₂ , NiO, Fe ₂ O ₃ , TiO ₂ , B ₂ O ₃ , CoO, Li ₂ CO ₃ , MgO, Al ₂ O ₃ , and LiF so as to have a desired metal composition ratio. And weighed and mixed. LiNi _0.5 Mn _1.5 O ₄ was produced by firing the powder after mixing the raw materials at a firing temperature of 500 to 1000 ° C. for 8 hours. LiNi _0.5 Mn _1.5 O ₄ as a positive electrode active material, polyvinylidene fluoride (PVDF) (5 mass%) as a binder, and carbon black (5 mass%) as a conductive agent are mixed. Thus, a positive electrode mixture was obtained. 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 as a binder. The mass ratio between 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. After drying, a negative electrode was produced by compression molding with a roll press.

A positive electrode and a negative electrode cut out to 1.5 cm × 3 cm were arranged so as to face each other with a separator interposed therebetween. Five positive electrodes and six negative electrodes were alternately stacked. As the separator, a microporous polypropylene film having a thickness of 25 μm was used.

LiPF ₆ as 1 mol / l, non-aqueous electrolytic solvent, ethylene carbonate (EC) and fluorinated chain ether (1,1,2,2- tetrafluoroethyl 2,2,3,3 tetrafluoroethyl ether) ( TFETFPE), tris (2,2,2-trifluoroethyl) phosphate (PTTFE), and dimethyl carbonate (DMC) were mixed at a volume ratio EC / TFETFPE / PTTFE / DMC = 3/2/4/1. A solvent and a solvent containing 2% by volume of fluoroethylene carbonate (FEC) as an additive were used.

The above positive electrode, negative electrode, separator, and electrolyte 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.

First, after charging and discharging under the following charging conditions, the impedance was evaluated.

Charging conditions: constant current constant voltage method, charging end voltage 4.75 V, charging current 10 mA, total charging time 10 hours Discharging conditions: constant current discharging, discharge end voltage 3.0 V, discharge current 50 mA

Next, 50 cycles of charge / discharge cycle tests were performed at a temperature of 45 ° C. under the following conditions, and impedance was evaluated again.

Charging conditions: constant current constant voltage method, charging end voltage 4.75 V, charging current 50 mA, total charging time 2.5 hours Discharging conditions: constant current discharging, discharge end voltage 3.0 V, discharge current 50 mA

The actual resistance measured by the AC impedance method at 100 mHz increased by 0.59Ω. The high frequency side negative charge transfer resistance Rct2 obtained by fitting to the circuit model is 0.22Ω from 0.28Ω to 0.50Ω, and the low frequency side positive charge transfer resistance Rct is 0.90Ω to 0.97Ω from 0.90Ω. Changed by 07Ω. The projection value ΔRx is 0.36Ω.

(Example 2)
LiPF ₆ is 0.8 mol / l, EC / TFETFPE / PTTFE / propylene carbonate (PC) = 2/2/5/1, and the additive is cyclic disulfonate (1,5,2,4-dioxodithiane-2,2 , 4,4-tetraoxide) A lithium secondary battery was prepared and measured in the same manner as in Example 1 except that 0.8 wt% electrolytic solution was used. The actual resistance measured by the AC impedance method at 100 mHz increased by 0.58Ω. The high-frequency side negative charge transfer resistance Rct2 obtained by fitting to the circuit model is 0.24Ω from 0.57Ω to 0.81Ω, and the low-frequency side positive charge transfer resistance Rct is 0.40Ω to 0.76Ω. It changed by 34Ω. The projection value ΔRx is −0.26Ω.

(Reference Example 1)
Lithium secondary as in Example 1, except that an electrolytic solution of LiPF ₆ of 0.8 mol / l, TFETFPE / PTTFE / PC = 5/4, and an additive of cyclic disulfonic acid ester of 0.8 wt% was used. A battery was prepared and measured. The actual resistance measured at 100 mHz increased by 1.95Ω. The high-frequency side negative charge transfer resistance Rct2 changed from 0.14Ω to 1.76Ω to 1.62Ω, and the low-frequency side positive charge transfer resistance Rc changed from 1.45Ω to 1.51Ω by 0.06Ω. The projection value ΔRx is 0.62Ω.

For the evaluated lithium secondary battery levels including the above-mentioned examples and reference examples, the resistance is evaluated by the AC impedance method from 100 mHz to 300 kHz, and the relationship between ΔRct and ΔRct2 extracted by fitting the equivalent circuit shown in FIG. As shown in FIG. Despite changing the lithium salt concentration, electrolyte composition, additives, etc., ΔRct and ΔRct2 showed a strong negative correlation. In FIG. 4, normalized values ΔRc and ΔRa can be obtained by projecting in a radiation direction from a point (ΔRct2, ΔRct) onto a circle having a radius of 1Ω centered on the origin and normalizing. Subsequently, as shown in FIG. 5, ΔRc and ΔRa normalized in FIG. 4 are plotted and projected perpendicularly to a straight line represented by ΔRa = −0.6ΔRc. A distance (projection value) ΔRx was obtained. FIG. 6 shows a distribution of the change amount ΔRall of the actual resistance value measured by the AC impedance method at 100 mHz. The horizontal axis of the graph of FIG. 6 is a value (ΔRx) obtained by projecting the data of FIG. 4 perpendicularly to the line of ΔRa = −0.6ΔRc. The aforementioned straight line (ΔRa = −0.6ΔRc) was visually selected from straight lines passing through the origin so that the data in FIG. -0.6 is the meaning of the average value of all evaluation data of various electrolytes, and it can be easily estimated that this value changes if it is classified for each electrolyte. For this reason, for example, it is considered that a value in the range from -1/3, which is approximately half the size, to -3, which is the reciprocal, can be taken as a value. FIG. 5 shows that ΔRall tends to be minimum when the horizontal axis is near zero. For example, in order to set ΔRall to 1.7Ω or less, the projection value ΔRx needs to be between 0Ω ± 0.5Ω. By setting ΔRx between 0Ω ± 0.3Ω, ΔRall can be further reduced. Here, 50 charge / discharge cycles were performed and the resistance change was evaluated. As stress, in addition to charge / discharge cycles, high temperature storage, high voltage application, and the like are possible. In this case, it is necessary to separately evaluate the correlation with the charge / discharge cycle life. As a method of changing ΔRc and ΔRa, in addition to the above-described lithium salt concentration, electrolytic solution material, electrolytic solution composition, additive type, addition amount, electrode active material, active material thickness, active material area, electrode binder material Etc.

Further, if the projection value ΔRx deviates from the design range or the difference (standard deviation) from the distribution of other cells, the cell is selected as a cell that is likely to become defective due to an abnormality, and ΔRx is Those within a predetermined range can be selected as good products. Thus, the technology of the present invention can also be used as a means for selecting a lithium ion secondary battery.

The selection method of the lithium ion secondary battery of the present invention is LiCoPO ₄ , Li (Co _0.5 Mn _0.5 ) O ₂ , Li (Li _0.2 M _0. it is suggested also effective in ₃ Mn _{0.5) O 2} cathode material or the like.

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 (16)

1. A lithium secondary battery having a positive electrode including a positive electrode active material, a negative electrode including a negative electrode active material, and an electrolytic solution including a nonaqueous electrolytic solvent,
   A change amount (ΔRct) of the positive electrode charge transfer resistance before and after a predetermined stress application on a plane having two axes orthogonal to each other, the change amount of the charge transfer resistance of the positive electrode and the change amount of the charge transfer resistance of the negative electrode ) And a point (ΔRct2, ΔRct) representing the amount of change (ΔRct2) in the charge transfer resistance of the negative electrode is projected to a point (ΔRa , ΔRc)
   A point (ΔRa, ΔRc) is defined with respect to a straight line passing through the origin on a plane having two axes orthogonal to each other. A lithium secondary battery characterized in that the distance from the vertically projected point to the origin is within a predetermined range.
2. 2. The lithium secondary battery according to claim 1, wherein a straight line passing through the origin is represented by ΔRa = m · ΔRc (where −3 ≦ m ≦ −1 / 3).
3. 3. The lithium secondary battery according to claim 1, wherein a straight line passing through the origin is expressed by ΔRa = −0.6ΔRc.
4. The lithium secondary battery according to any one of claims 1 to 3, wherein the predetermined range is 0 ± 0.5Ω.
5. 5. The lithium secondary battery according to claim 1, wherein the predetermined range is 0 ± 0.3Ω.
6. The charge transfer resistor measures the impedance between the positive electrode and the negative electrode in a predetermined frequency region and draws a curve drawn when describing the result on the complex plane, the element 1 in which the resistor R1 and the capacitor C1 are connected in parallel, and the resistor 6. R1 and R2 obtained by fitting into a circuit in which an element 2 in which R2 and a capacitor C2 are connected in parallel and a resistor R0 are connected in series are provided. The lithium secondary battery as described.
7. A curve drawn when the charge transfer resistance measures the impedance between the positive electrode and the negative electrode in a predetermined frequency range and describes the result on the complex plane, the resistor R1 and the capacitor C1 expressed by a distributed constant are connected in parallel. R1 and R2 obtained by fitting into a circuit in which the element 1, the resistor R2 and the capacitor C2 represented by the distributed constant are connected in parallel, and the resistor R0 are connected in series The lithium secondary battery according to any one of claims 1 to 5, wherein:
8. The charge transfer resistance further installs a conductive electrode, measures the impedance in a predetermined frequency region between the positive electrode and the conductive electrode, and the negative electrode and the conductive electrode, and describes the result on a complex plane 6. The lithium secondary battery according to claim 1, wherein the lithium secondary battery is obtained as a diameter or an ellipse diameter of a semicircular arc or a semielliptical arc drawn at the time.
9. The lithium secondary battery according to any one of claims 1 to 8, wherein the positive electrode active material operates at a potential of 4.5 V or more with respect to lithium.
10. The lithium secondary battery according to any one of claims 1 to 9, wherein the positive electrode active material contains a lithium manganese composite oxide represented by the following formula (6).
    Li _a (M _x Mn _2-xy Y _y ) (O _4-w Z _w ) (6)
    (In the formula, 0.4 ≦ x ≦ 1.2, 0 ≦ y, x + y <2, 0 ≦ a ≦ 1.2, 0 ≦ w ≦ 1. M is Co, Ni, Fe, Cr and Cu. Y is at least one selected from the group consisting of Li, B, Na, Al, Mg, Ti, Si, K and Ca.Z is composed of F and Cl. At least one selected from the group.)
11. The lithium secondary battery according to claim 10, wherein the lithium manganese composite oxide represented by the formula (6) contains at least Ni as M.
12. The lithium secondary battery according to any one of claims 1 to 11, wherein the positive electrode active material includes a lithium metal composite oxide represented by the following formula (7), (8), or (9).
    LiMPO ₄ (7)
    (In Formula (7), M is at least one of Co and Ni.)
    Li (M _1-z Mn _z ) O ₂ (8)
    (In formula (8), 0.7 ≧ z ≧ 0.33, and M is at least one of Li, Co, and Ni.)
    _{Li (Li x M 1-x} -z Mn z) O 2 (9)
    (In formula (9), 0.3> x ≧ 0.1, 0.7 ≧ z ≧ 0.33, and M is at least one of Co and Ni.)
13. The lithium secondary battery according to any one of claims 1 to 12, further comprising an outer package including the positive electrode, the negative electrode, and the electrolyte, wherein the outer package is an aluminum laminate film.
14. A method for selecting a lithium secondary battery, comprising: a positive electrode including a positive electrode active material; a negative electrode including a negative electrode active material; and an electrolytic solution including a nonaqueous electrolytic solvent,
    A change amount (ΔRct) of the positive electrode charge transfer resistance before and after a predetermined stress application on a plane having two axes orthogonal to each other, the change amount of the charge transfer resistance of the positive electrode and the change amount of the charge transfer resistance of the negative electrode ) And a point (ΔRct2, ΔRct) representing the amount of change (ΔRct2) in the charge transfer resistance of the negative electrode is projected to a point (ΔRa , ΔRc)
    A point (ΔRa, ΔRc) is defined with respect to a straight line passing through the origin on a plane having two axes perpendicular to the amount of change in charge transfer resistance of the positive electrode after normalization and the amount of change in charge transfer resistance of the negative electrode after normalization. A method for selecting a lithium secondary battery, wherein the distance between the point projected vertically and the origin is within a predetermined range.
15. 15. The method of selecting a lithium secondary battery according to claim 14, wherein the predetermined range is set using a plurality of statistical values of battery characteristics.
16. A method for producing a lithium secondary battery, comprising: a positive electrode including a positive electrode active material; a negative electrode including a negative electrode active material; and an electrolytic solution including a nonaqueous electrolytic solvent,
    A change amount (ΔRct) of the positive electrode charge transfer resistance before and after a predetermined stress application on a plane having two axes orthogonal to each other, the change amount of the charge transfer resistance of the positive electrode and the change amount of the charge transfer resistance of the negative electrode ) And a point (ΔRct2, ΔRct) representing the amount of change (ΔRct2) in the charge transfer resistance of the negative electrode is projected to a point (ΔRa , ΔRc)
    A point (ΔRa, ΔRc) is defined with respect to a straight line passing through the origin on a plane having two axes orthogonal to each other. A method for manufacturing a lithium secondary battery, including a step of adjusting a distance from a vertically projected point to an origin so as to be within a predetermined range.

Images