JP2006114285A - Nonaqueous electrolyte for lithium secondary battery, the lithium secondary battery, and secondary battery system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte having superior voltage resistance and lithium ionic conductivity equal to that of the conventional nonaqueous electrolyte, and to provide a secondary battery and a secondary battery system, both equipped with the nonaqueous electrolyte. <P>SOLUTION: The nonaqueous electrolyte for the lithium secondary battery contains at least one kind of aromatic compounds polymerizable at 4.2-4.5 V of the potential of a working electrode, when metallic lithium is used as the counter electrode with respect to the platinum working electrode. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a non-aqueous electrolyte for a lithium secondary battery, a lithium secondary battery, and a secondary battery system.

  In a conventional lithium secondary battery, the end-of-charge voltage is generally set to about 4.2 V with respect to lithium from the viewpoint of the oxidation resistance of the nonaqueous electrolyte. This is because when the battery is charged to 4.2 V or higher with respect to lithium, the potential of the positive electrode rises and the nonaqueous electrolyte solution is decomposed, resulting in problems such as deterioration of cycle characteristics and deterioration of battery characteristics at high temperatures. is there. However, if the charging voltage is set to 4.2 V or higher and the average operating voltage can be further increased, further improvement in energy density can be desired.

Recently, as shown in Patent Document 1 below, a technique has been proposed that can realize a non-aqueous electrolyte with a high withstand voltage by adding a sulfite having a halogenated alkyl group as an electrolyte solvent.
JP 9-167635 A

  However, in Patent Document 1, although the withstand voltage of the electrolytic solution is improved by adding a sulfite, there is a problem that the conductivity of lithium ions is significantly reduced.

  The present invention has been made in view of the above circumstances, and has a non-aqueous electrolyte excellent in voltage resistance and having a lithium ion conductivity equivalent to that of a conventional one, and a lithium secondary battery provided with this non-aqueous electrolyte. It aims at providing a secondary battery and a secondary battery system.

In order to achieve the above object, the present invention employs the following configuration.
The nonaqueous electrolytic solution for a lithium secondary battery according to the present invention comprises at least a polymerizable aromatic compound having a working electrode potential of 4.2 V or more and 4.5 V or less when the counter electrode for the platinum working electrode is metallic lithium. It is characterized by containing 1 or more types.

  By using the above non-aqueous electrolyte as the electrolyte of the lithium secondary battery, the aromatic compound is polymerized on the surface of the positive electrode of the lithium secondary battery during the initial charge to form a film. By this film formation, the positive electrode and the non-aqueous electrolyte are not in direct contact. Thereby, the oxidative decomposition of the nonaqueous electrolytic solution by the positive electrode can be prevented, and the withstand voltage characteristic of the nonaqueous electrolytic solution can be enhanced. In addition, the high temperature characteristics of the lithium secondary battery can be improved.

  In the non-aqueous electrolyte for a lithium secondary battery of the present invention, the aromatic compound is preferably at least one of phenanthrene, terphenyl, and dimethylbiphenyl, particularly phenanthrene and terphenyl. Either one or both are preferred.

Next, the non-aqueous electrolyte for a lithium secondary battery of the present invention has a structural formula consisting of R ¹ —O—R ² or R ¹ —O—CO—O—R ² (where R ¹ and R ² are alkyls). One or both of organic fluorinated compounds represented by a group or a fluorinated alkyl group).

  The organic fluorinated compound does not decompose against oxidation by the positive electrode of the lithium secondary battery. By containing such an organic fluorinated compound, a nonaqueous electrolytic solution having excellent withstand voltage characteristics can be configured.

Moreover, in the non-aqueous electrolyte for lithium secondary batteries of this invention, it is preferable that the said organic fluorinated compound is any 1 or more types represented by the structural formula shown below.
_{HCF 2 CF 2 CH 2 OCF 2} CF 2 H, CF 3 CH 2 OCOOCH 2 CF 3, CH 3 OCOOCH 2 CHFCH 3, CH 3 OCOOCH (CH 3) CH 2 F, CH 3 OCOOCH 2 CH 2 CH 2 F.
In particular, the organic fluorinated compound is preferably one or both of HCF ₂ CF ₂ CH ₂ OCF ₂ CF ₂ H and CH ₃ OCOOCH ₂ CHFCH ₃ .

The non-aqueous electrolyte for a lithium secondary battery of the present invention is the non-aqueous electrolyte described above, and is selected from the group consisting of fluoroethylene carbonate, vinylene carbonate, propane sultone, trifluoropropylmethyl sulfone, fluorobenzene, and LiBF ₄ . Any one or more negative electrode film improvement additives are included.

  In addition to the above aromatic compound or organic fluorinated compound, a film formed on the negative electrode surface of the lithium secondary battery can be protected by adding a negative electrode film improving additive. Thereby, a rapid reaction between the negative electrode and the non-aqueous electrolyte is suppressed particularly during charging and discharging at a high temperature, and thermal runaway of the lithium secondary battery can be prevented in advance.

  Next, a lithium secondary battery of the present invention is characterized by comprising any of the non-aqueous electrolytes described above.

  According to said structure, since the non-aqueous electrolyte excellent in the withstand voltage characteristic is provided, even if the battery voltage at the time of a full charge is 4.2V or more and 4.5V or less, a non-aqueous electrolyte is decomposed | disassembled. There is no fear, and there is no risk of thermal runaway at high temperatures, and a lithium secondary battery with high capacity and high safety can be realized.

  Next, the secondary battery system of the present invention includes the above-described lithium secondary battery and a charging device that sets the battery voltage when the lithium battery is fully charged to 4.2 V or more and 4.5 V or less. It is characterized by becoming.

  According to the above configuration, the lithium secondary battery having the above configuration is provided, and the battery voltage when the lithium secondary battery is fully charged can be controlled to 4.2 V or more and 4.5 V or less. There is no possibility that the non-aqueous electrolyte is decomposed, and there is no risk of thermal runaway at a high temperature, and a high-capacity and highly safe secondary battery system can be realized.

  ADVANTAGE OF THE INVENTION According to this invention, the lithium secondary battery and secondary battery system provided with the non-aqueous electrolyte excellent in withstand voltage property and this non-aqueous electrolyte can be provided.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
The lithium secondary battery of this embodiment includes a positive electrode capable of inserting and extracting lithium, a negative electrode capable of inserting and extracting lithium, a separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte. It is provided and is roughly configured.

The positive electrode is formed by mixing a positive electrode active material powder with a binder such as polyvinylidene fluoride and a conductive additive such as carbon black, and forming the sheet into a sheet or the like. The positive electrode is bonded to the positive electrode current collector. Examples of the positive electrode current collector include metal foils or metal nets such as aluminum and stainless steel.
Further, as the positive electrode active material, at least one selected from the group consisting of a complex oxide of lithium and at least one selected from cobalt, manganese, and nickel is preferable. Specifically, LiMn ₂ O ₄ , LiCoO ₂ , LiNiO _{2, LiCoNiMnO 2, LiFeO 2,} V 2 O 5 and the like are _{preferable, LiCoO 2, LiNiO 2, LiCoNiMnO} 2 is particularly preferred. Moreover, you may use what can occlude / release lithium, such as TiS, MoS, an organic disulfide compound, or an organic polysulfide compound.

The negative electrode is formed by mixing a negative electrode active material powder with a binder such as polyvinylidene fluoride and, if necessary, a conductive additive such as carbon black, and forming the sheet. This negative electrode is joined to the negative electrode current collector. Examples of the negative electrode current collector include a metal foil or a metal net made of Cu or a Cu alloy.
Examples of the negative electrode active material include carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber, graphitized mesocarbon microbeads, and amorphous carbon. Moreover, the metal substance simple substance which can be alloyed with lithium, and the composite containing this metal substance and carbonaceous material can be illustrated as a negative electrode active material. Examples of metals that can be alloyed with lithium include Al, Si, Sn, Pb, Zn, Bi, In, Mg, Ga, and Cd. A metal lithium foil can also be used as the negative electrode active material.

  As the separator, a known separator such as a porous polypropylene film or a porous polyethylene film can be appropriately used.

  Next, the nonaqueous electrolytic solution conducts lithium ions. As an example, the nonaqueous electrolytic solution contains a nonaqueous solvent, a lithium salt, and a polymerizable aromatic compound.

The polymerizable aromatic compound is an aromatic compound that can be polymerized on the surface of the working electrode when the working electrode potential is 4.2 V or more and 4.5 V or less when the counter electrode for the platinum working electrode is metallic lithium. Is at least one of phenanthrene, terphenyl, and dimethylbiphenyl, and among these, one or both of phenanthrene and terphenyl are particularly preferred.
By using the above non-aqueous electrolyte as an electrolyte of a lithium secondary battery, an aromatic compound is polymerized on the surface of the positive electrode (corresponding to the working electrode) of the lithium secondary battery during initial charging to form a film. . By this film formation, the positive electrode and the non-aqueous electrolyte are not in direct contact. Thereby, the oxidative decomposition of the nonaqueous electrolytic solution by the positive electrode can be prevented, and the withstand voltage characteristic of the nonaqueous electrolytic solution can be enhanced. In addition, the high temperature characteristics of the lithium secondary battery can be improved.

  The amount of the aromatic compound added to the non-aqueous electrolyte is preferably in the range of 0.01% by mass to 5% by mass, and more preferably in the range of 0.1% by mass to 0.5% by mass. When the addition amount of the aromatic compound is 0.1% by mass or more, a sufficient amount of a film can be formed on the surface of the positive electrode, and the oxidative decomposition of the nonaqueous electrolytic solution by the positive electrode can be prevented. Moreover, if the addition amount of an aromatic compound is 0.5 mass% or less, the lithium ion conductivity of a non-aqueous electrolyte will not fall, and the charge / discharge efficiency of a lithium secondary battery can be improved.

Next, as another example of the non-aqueous electrolyte, a structural formula (provided that R ¹ and R ¹ -O—R ² or R ¹ —O—CO—O—R ^{2) are used.} R ² may be exemplified by one or both of organic fluorinated compounds represented by an alkyl group or a fluorinated alkyl group.

Examples of the organic fluorine _{compound, HCF 2 CF 2 CH 2 OCF} 2 CF 2 H, CF 3 CH 2 OCOOCH 2 CF 3, CH 3 OCOOCH 2 CHFCH 3, CH 3 OCOOCH (CH 3) CH 2 F, CH 3 It is preferable to use any one or more of OCOOCH ₂ CH ₂ CH ₂ F, and in particular, one or both of HCF ₂ CF ₂ CH ₂ OCF ₂ CF ₂ H and CH ₃ OCOOCH ₂ CHFCH ₃ are preferable.
The organic fluorinated compound does not decompose against oxidation by the positive electrode of the lithium secondary battery. By containing such an organic fluorinated compound, a nonaqueous electrolytic solution having excellent withstand voltage characteristics can be configured.

  The amount of the organic fluorinated compound added to the nonaqueous electrolytic solution is preferably in the range of 0.1% by mass to 50% by mass, and more preferably in the range of 1% by mass to 20% by mass. When the addition amount of the organic fluorinated compound is 1% by mass or more, the withstand voltage characteristic of the non-aqueous electrolyte is improved, and the oxidative decomposition of the non-aqueous electrolyte by the positive electrode can be prevented. Moreover, if the addition amount of the organic fluorinated compound is 20% by mass or less, the lithium ion conductivity of the non-aqueous electrolyte does not decrease, and the charge / discharge efficiency of the lithium secondary battery can be increased.

Further, any one of fluoroethylene carbonate, vinylene carbonate, propane sultone, trifluoropropylmethylsulfone, fluorobenzene, and LiBF ₄ is added to each of the two types of non-aqueous electrolytes containing an aromatic compound or an organic fluorinated compound. More than one type of negative electrode coating improving additive may be added.
By adding the negative electrode film improving additive, the film formed on the negative electrode surface of the lithium secondary battery with charge / discharge can be protected. Thereby, a rapid reaction between the negative electrode and the non-aqueous electrolyte is suppressed particularly during charging and discharging at a high temperature, and thermal runaway of the lithium secondary battery can be prevented in advance.

  The addition amount of the negative electrode film improving additive in the non-aqueous electrolyte is preferably in the range of 0.05% by mass to 20% by mass, and more preferably in the range of 0.5% by mass to 10% by mass. If the addition amount of the negative electrode film improving additive is 0.5% by mass or more, the film formed on the negative electrode surface can be sufficiently protected. Moreover, if the addition amount of a negative electrode film improvement additive is 10 mass% or less, the lithium ion conductivity of a non-aqueous electrolyte will not fall, and the charge / discharge efficiency of a lithium secondary battery can be improved.

Next, as a solvent contained in said non-aqueous electrolyte, the mixture of a cyclic carbonate and a chain carbonate can be illustrated.
As cyclic carbonate, what contains 1 or more types in ethylene carbonate, butylene carbonate, propylene carbonate, (gamma) -butyrolactone, etc. is preferable, for example. Since these cyclic carbonates easily solvate with lithium ions, the ionic conductivity of the electrolyte itself can be increased.
Moreover, as chain carbonate, what contains 1 or more types in dimethyl carbonate, methyl ethyl carbonate, and diethyl carbonate, for example is preferable. Since these chain carbonates have a low viscosity, the viscosity of the electrolyte itself can be lowered to increase the ionic conductivity. However, since these chain carbonates have a low flash point, if added excessively, the flash point of the electrolyte is lowered, so care must be taken not to add excessively.

Furthermore, LiPF ₆ , LiBF ₄ , Li [N (SO ₂ C ₂ F ₅ ) ₂ ], Li [B (OCOCF ₃ ) ₄ ], Li [B (OCOC ₂ F ₅ ) ₄ ] should be used as the lithium salt. Can do. The concentration of these lithium salts in the electrolyte is preferably 0.5 mol / L or more and 2.0 mol / L or less. By including these lithium salts in the electrolyte, the ionic conductivity of the electrolyte itself can be increased.

  According to the lithium secondary battery of this embodiment, since the non-aqueous electrolyte having excellent withstand voltage characteristics is provided, non-aqueous electrolysis is possible even when the battery voltage at full charge is 4.2 V or more and 4.5 V or less. There is no risk of the liquid being decomposed, and there is no risk of thermal runaway at a high temperature, and a lithium secondary battery with high capacity and high safety can be realized.

  Moreover, a secondary battery system can be configured by combining the lithium secondary battery of the present embodiment and a charging device that sets the battery voltage when the lithium battery is fully charged to 4.2 V or more and 4.5 V or less. it can. According to this secondary battery system, since the battery voltage when the lithium secondary battery is fully charged can be controlled to 4.2 V or more and 4.5 V or less, there is no possibility that the nonaqueous electrolyte of the lithium secondary battery is decomposed. In addition, there is no risk of thermal runaway at high temperatures, and a high-capacity and highly safe secondary battery system can be realized.

FIG. 1 shows an example of a secondary battery system. In FIG. 1, reference numeral 1 denotes a DC power supply, reference numeral 2 denotes a lithium secondary battery according to the present invention to be charged, reference numeral 4 denotes switch means inserted in series in a constant current circuit, and reference numeral 5 denotes control. Device. The DC power supply device 1 incorporates a charging voltage adjustment mechanism, and the charge end voltage is set in the range of 4.2 to 4.5V. The switch means 4 is controlled by the control device 5. The control device 5 monitors the voltage of the battery 2 and controls the charging current by turning on and off the switch means 4 according to the battery voltage. In this secondary battery system, charging current is supplied from the DC power supply device 1 to the lithium secondary battery 2 via the switch means 4 to perform constant current charging. Then, after the battery voltage reaches the end-of-charge voltage of 4.2 to 4.5V, the battery voltage is switched to constant voltage charging and charged until a predetermined time.
With the above configuration, the battery voltage when the lithium secondary battery is fully charged can be controlled to 4.2 V or more and 4.5 V or less.

Hereinafter, the present invention will be described in more detail with reference to experimental examples.
(Experimental example 1)
In Experimental Example 1, a tripolar cell was produced by adding a polymerizable aromatic compound or organic fluorinated compound to a non-aqueous electrolyte, and a cyclic voltammogram test was performed to measure a current-voltage curve. The reaction potential of the compound and the organic fluorinated compound was measured.

The triode cell was manufactured as follows. First, a working electrode made of a platinum wire having a diameter of 0.5 mm, a counter electrode made of metallic lithium, and a reference electrode were prepared.
Further, LiPF ₆ having a concentration of 1.3 mol / L is mixed with a mixed solvent in which ethylene carbonate (EC) and diethyl carbonate (DEC) are mixed at a ratio of EC: DEC = 30: 70 to obtain an electrolytic solution. Prepared. Examples 1 to Examples in which 0.2% by mass of phenanthrene, 0.2% by mass of m-terphenyl, and 10% by mass of CF ₃ CH ₂ OCOOCH ₂ CF ₃ were respectively added to the electrolyte. 3 non-aqueous electrolyte was prepared. As Comparative Example 1, a non-aqueous electrolyte solution to which nothing other than EC, DEC, and LiPF ₆ was added was prepared.

Next, the non-aqueous electrolyte was poured into a glass container, and the working electrode, counter electrode, and reference electrode were inserted into the non-aqueous electrolyte to assemble a three-electrode cell. After standing for 15 hours at 25 ° C. after assembling the cell, and the potential scanning in open circuit potential (about 3V (vs Li)) from 8V (vs Li) to a speed of 5 mV / sec for the working electrode area of 0.31 cm ² Thereafter, the current-voltage curve was measured by scanning the potential from 8 V (vsLi) to 0 V (vsLi) at a rate of 5 mV / sec. However, the non-aqueous electrolyte to which the aromatic compound was added was held at a potential at which a peak due to the aromatic compound was observed during the potential search for 8 hours.
Table 1 below shows the current value and the slope of the current-voltage curve at a voltage of 7 V, and the voltage (peak voltage) at which the differential coefficient of the current-voltage curve is maximized.
As shown in Table 1,
As shown in Table 1, in Example 1-3, the peak voltage was higher than that in Comparative Example 1, and it was found that the decomposition potential of the electrolytic solution was increased by the additive. Moreover, it turned out that the electric current value in 7V of Example 1-3 is smaller than the case of the comparative example 1. Therefore, in Examples 1 to 3, it was found that the decomposition of the electrolyte near 7 V was suppressed by the addition of phenanthrene, m-terphenyl, and CF ₃ CH ₂ OCOOCH ₂ CF ₃ .

(Experimental example 2)
In Experimental Example 2, a coin-type lithium secondary battery was manufactured by adding a polymerizable aromatic compound or organic fluorinated compound to a nonaqueous electrolytic solution, and various battery characteristics were evaluated.
The lithium secondary battery was manufactured as follows. First, the LiNiO ₂ having an average particle size of 10 [mu] m, to prepare a LiCoO ₂ having an average particle size of 10 [mu] m, these LiNiO a mass ratio _2: LiCoO ₂ = 30: were mixed in a ratio of 70 were positive electrode active material. A positive electrode active material made only of LiCoO ₂ was also prepared. These positive electrode active materials, a binder made of polyvinylidene fluoride, and a conductive additive made of carbon powder having an average particle size of 3 μm were mixed, and further N-methyl-2-pyrrolidone was mixed to make a positive electrode slurry. This positive electrode slurry was applied onto a positive electrode current collector made of an aluminum foil having a thickness of 20 μm by a doctor blade method and dried in a vacuum atmosphere at 120 ° C. for 24 hours to volatilize N-methyl-2-pyrrolidone. Rolled. Thus, the positive electrode which consists of a positive electrode active material, a binder, and a conductive support material was formed on the positive electrode current collector.

  Next, 95 parts by weight of artificial graphite having an average particle size of 15 μm was mixed with a binder composed of 5 parts by weight of polyvinylidene fluoride, and further mixed with N-methyl-2-pyrrolidone to form a negative electrode slurry. . This negative electrode slurry was applied onto a negative electrode current collector made of Cu foil having a thickness of 14 μm by a doctor blade method, and dried in a vacuum atmosphere at 120 ° C. for 24 hours to volatilize N-methyl-2-pyrrolidone. Rolled. In this way, a negative electrode composed of a negative electrode active material and a binder was formed on the negative electrode current collector.

Next, an electrolytic solution in which LiPF ₆ having a concentration of 1.3 mol / L is added to a mixed solvent in which ethylene carbonate (EC) and diethyl carbonate (DEC) are mixed at a ratio of EC: DEC = 30: 70. Prepared. 0.2% by mass of phenanthrene, 0.2% by mass of 3,3′-dimethylbiphenyl, 0.15% by mass of phenanthrene and 0.05% by mass of 3,3′-dimethylbiphenyl with respect to the electrolyte solution. mixture, 10 wt% of _{HCF 2 CF 2 CH 2 OCF 2} CF 2 H, 0.2 wt% m-terphenyl, 10 wt% of _{CF 3 CH 2} OCOOCH 2 _CF 3 added to example 4, respectively A non-aqueous electrolyte solution of Example 9 was prepared. As a comparative example, a non-aqueous electrolyte solution to which nothing other than EC, DEC, and LiPF ₆ was added was prepared.

  Next, a polypropylene porous separator is disposed between the positive electrode and the negative electrode, and these are laminated. Next, these are accommodated in a coin-type battery case, and the non-aqueous electrolyte is injected. The lithium secondary batteries of Examples 4 to 9 and Comparative Examples 2 and 3 were manufactured by sealing the battery case. In addition, the design value of the discharge capacity of each battery is 5 mAh. Table 2 shows the configuration of each battery.

About the obtained lithium secondary battery, after charging / discharging was repeated 2 cycles and it was made into the charge state, it stored for 4 hours in a 90 degreeC thermostat, and the discharge capacity (storage capacity) after storage was measured. Next, after measuring the discharge capacity, one cycle of charge and discharge was performed at room temperature to measure the discharge capacity (recovery capacity). In addition, charge / discharge is performed at a constant current until the battery voltage reaches 4.2V to 4.5V at a charge current of 0.5 C, and then a constant voltage charge is performed for 2 hours. It was performed under the condition of constant current discharge to 0.0V.
Table 3 shows the open circuit voltage and the percentage of the storage capacity and the recovery capacity when the initial capacity is 100%.

  As shown in Table 3, it can be seen that the lithium secondary batteries of Examples 4 to 9 have higher storage capacity and recovery capacity than those of Comparative Examples 2 and 3, and good high temperature characteristics. In particular, it can be seen that Examples 8 and 9 show good high temperature characteristics even when the charging voltage is 4.5V.

(Experimental example 3)
In Experimental Example 3, a cylindrical lithium secondary battery was manufactured by adding a negative electrode film improving additive and a polymerizable aromatic compound or organic fluorinated compound to a non-aqueous electrolyte, and various battery characteristics were obtained. Evaluated.
The lithium secondary battery was manufactured as follows. First, a positive electrode and a negative electrode were prepared in the same manner as in Experimental Example 2.
Next, an electrolytic solution in which LiPF ₆ having a concentration of 1.3 mol / L is added to a mixed solvent in which ethylene carbonate (EC) and diethyl carbonate (DEC) are mixed at a ratio of EC: DEC = 30: 70. Prepared. Nonaqueous electrolytes of Examples 10 to 18 were prepared by adding a negative electrode coating improving additive and a polymerizable aromatic compound or organic fluorinated compound to the electrolyte. As a comparative example, a non-aqueous electrolyte solution to which nothing other than EC, DEC, and LiPF ₆ was added was prepared.

  Next, a polypropylene porous separator is disposed between the positive electrode and the negative electrode, and these are laminated. Then, these are wound into a cylindrical shape and housed in a cylindrical battery case, and the nonaqueous electrolysis is further performed. The lithium secondary batteries of Examples 10 to 18 and Comparative Examples 4 to 6 were manufactured by sealing the battery case after pouring the liquid. In addition, the design value of the discharge capacity of each battery is 2400-2600 mAh. Table 4 shows the configuration of each battery. In Table 4, FEC is fluorinated ethylene carbonate, and VC is vinylene carbonate.

  About the obtained lithium secondary battery, charging / discharging was repeated 2 cycles and it was made into the charge state, Then, it stored for 4 hours in a 90 degreeC thermostat, and the operation state of the current cutoff valve was investigated. Moreover, about the obtained lithium secondary battery, charging / discharging was repeated 2 cycles and it was made into the charge state, Then, it stored in a 150 degreeC thermostat, and the presence or absence of the burst of the battery was investigated. The results are shown in Table 5.

  As shown in Table 5, in Comparative Examples 4 to 6, the current cutoff valve was operated, and further, battery rupture occurred due to storage at 150 ° C. In Examples 10 to 18, there was no abnormality.

(Experimental example 4)
In Experimental Example 4, a cylindrical lithium secondary battery was manufactured by adding a negative electrode film improving additive alone or a mixture of a negative electrode film improving additive and an aromatic compound to a non-aqueous electrolyte. The battery characteristics of were evaluated.
The production of the lithium secondary battery was performed in the same manner as in Experimental Example 3 except that all the positive electrodes were LiCoO ₂ and the configuration of the non-aqueous electrolyte was as shown in Table 6. In addition, all the design values of the discharge capacity of each battery are 2600 mAh. Table 6 shows the configuration of each battery. In Table 6, FEC is fluorinated ethylene carbonate, VC is vinylene carbonate, PS is propane sultone, TFPMS is trifluoropropylmethylsulfone, FB is fluorobenzene, and TP is m-terphenyl.

About the obtained lithium secondary battery, charging / discharging was repeated 100 cycles in a 45 degreeC thermostat, and the discharge capacity of the 100th cycle was measured.
Table 6 shows the discharge capacity at the 100th cycle as a percentage when the initial capacity is 100%.

  As shown in Table 6, compared with Comparative Example 7, Examples 19 to 25 exhibited excellent high-temperature cycle characteristics.

FIG. 1 is a schematic diagram showing a configuration of a secondary battery system according to an embodiment of the present invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... DC power supply device, 2 ... Lithium secondary battery, 4 ... Switch means, 5 ... Control apparatus

Claims (7)

  A lithium secondary battery comprising at least one aromatic compound that can be polymerized when the counter electrode for the platinum working electrode is metallic lithium and the working electrode potential is in the range of 4.2 V to 4.5 V. Non-aqueous electrolyte for use.   The non-aqueous electrolyte for a lithium secondary battery according to claim 1, wherein the aromatic compound is at least one of phenanthrene, terphenyl, and dimethylbiphenyl. Any of the organic fluorinated compounds represented by the structural formula consisting of R ¹ —O—R ² or R ¹ —O—CO—O—R ² (where R ¹ and R ² are alkyl groups or fluorinated alkyl groups) A non-aqueous electrolyte for a lithium secondary battery, characterized in that either or both of them are contained. The non-aqueous electrolyte for a lithium secondary battery according to claim 3, wherein the organic fluorinated compound is at least one of the following structural formulas.
HCF ₂ CF ₂ CH ₂ OCF ₂ CF ₂ H,
CF ₃ CH ₂ OCOOCH ₂ CF ₃ ,
CH ₃ OCOOCH ₂ CHFCH ₃ ,
CH ₃ OCOOCH (CH ₃ ) CH ₂ F,
CH ₃ OCOOCH ₂ CH ₂ CH ₂ F Fluoroethylene carbonate, vinylene carbonate, propane sultone, trifluoropropyl methyl sulfone, fluorobenzene, of claims 1 to 4, characterized in that it comprises a negative electrode coating improving additive of any one or more of LiBF ₄ The nonaqueous electrolyte for lithium secondary batteries in any one.   A lithium secondary battery comprising the nonaqueous electrolytic solution according to any one of claims 1 to 5. A secondary battery comprising: the lithium secondary battery according to claim 6; and a charging device that sets a battery voltage when the lithium battery is fully charged to 4.2 V or more and 4.5 V or less. system.

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