JP2013105643A - Lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium secondary battery capable of being used without suffering reduction in discharge voltage even under conditions of rapid charging and discharging, and offering excellent discharge capacity retention.SOLUTION: The lithium secondary battery includes a positive electrode containing a positive electrode active material that contains a complex metal oxide of lithium and cobalt, a negative electrode containing a negative electrode active material that can absorb and desorb lithium ions, and a nonaqueous electrolytic solution. The nonaqueous electrolytic solution contains an electrolyte (1); 0.01-2.0 mol/L of a compound represented by the formula (2): (XSO)(X'SO)NLi, where X and X' each represent a fluorine atom, a C1-C6 alkyl group, or a C1-C6 fluoroalkyl group, and at least one of X and X' is a fluorine atom; and a solvent.

Description

  The present invention relates to a lithium secondary battery using a non-aqueous electrolyte.

  In recent years, since lithium secondary batteries have high energy density, they have been used as power sources for mobile communication devices, portable information terminals, etc., and the market has grown rapidly with the spread of terminals, ensuring safety. Various studies have been conducted for the purpose of improving cycle characteristics, energy density, high-temperature storage characteristics, and the like.

  Among the lithium secondary batteries described above, in the field of lithium ion batteries using a non-aqueous electrolyte, by adding a cyclic carbonate having an unsaturated bond such as vinylene carbonate or vinyl ethylene carbonate to the non-aqueous electrolyte, the negative electrode It is known to improve the cycle characteristics by forming a coating film on the top and suppressing decomposition of the electrolyte contained in the non-aqueous electrolyte (Patent Documents 1 and 2). However, when a cyclic carbonate having such an unsaturated bond forms a film on the negative electrode, the internal resistance of the battery (of all the resistances, intentionally added to the circuit as a resistor or visible wiring in the circuit) There is a problem of causing a voltage drop in the initial stage of discharge (IR drop), and the effect becomes remarkable particularly during high-speed charge / discharge, so there is room for improvement.

JP 2004-165151 A JP 2009-293474 A

  The present invention has been made by paying attention to the above-described circumstances, and its purpose is to prevent the lithium secondary battery having a high discharge capacity maintenance rate and a discharge voltage from being lowered even when used under fast charge / discharge conditions. The next battery is to provide.

The lithium secondary battery of the present invention that has achieved the above object is:
A lithium secondary battery comprising a positive electrode containing a positive electrode active material containing a composite metal oxide of lithium and cobalt, a negative electrode containing a negative electrode active material capable of occluding and releasing lithium ions, and a non-aqueous electrolyte. The non-aqueous electrolyte is electrolyte (1), general formula (2); (XSO2) (X′SO2) N ^— Li ⁺ (X and X ′ are fluorine atoms, alkyl groups having 1 to 6 carbon atoms, or carbon A compound represented by formula 1-6, wherein at least one of X and X ′ is a fluorine atom), and a solvent,
It is characterized in that the 2C average discharge voltage is 3.64V or more.

Or a lithium secondary battery including a positive electrode containing a positive electrode active material containing a composite metal oxide of lithium and cobalt, a negative electrode containing a negative electrode active material capable of occluding and releasing lithium ions, and a non-aqueous electrolyte. The non-aqueous electrolyte is electrolyte (1), general formula (2); (XSO2) (X′SO2) N ^— Li ⁺ (X and X ′ are fluorine atoms, alkyl groups having 1 to 6 carbon atoms) Or a C1-C6 fluoroalkyl group, wherein at least one of X and X ′ is a fluorine atom.) A compound represented by 0.01 mol / L to 2.0 mol / L, and a solvent. It is characterized by.

  The lithium secondary battery of the present invention maintains a high discharge voltage and achieves a long life by suppressing IR drop under high-speed charge / discharge and keeping the internal resistance low. The lithium secondary battery of the present invention exhibits high performance even in fields where high-speed charging / discharging is required, such as lithium batteries for power tools, such as for automobiles (EV, PHEV, HEV) and power tools. it is conceivable that.

FIG. 1 is a graph showing the results of cycle tests in Experimental Examples 1 to 6.

The lithium secondary battery of the present invention that has solved the above problems is
A lithium secondary battery comprising a positive electrode containing a positive electrode active material containing a composite metal oxide of lithium and cobalt, a negative electrode containing a negative electrode active material capable of occluding and releasing lithium ions, and a non-aqueous electrolyte. The non-aqueous electrolyte is an electrolyte (1), general formula (2); (XSO ₂ ) (X′SO ₂ ) N ^— Li ⁺ (X and X ′ are fluorine atoms, alkyl groups having 1 to 6 carbon atoms) Or a C 1-6 fluoroalkyl group, wherein at least one of X and X ′ is a fluorine atom.) And a solvent,
The lithium secondary battery is characterized in that the 2C average discharge voltage is 3.64 V or more, or
A lithium secondary battery comprising a positive electrode containing a positive electrode active material containing a composite metal oxide of lithium and cobalt, a negative electrode containing a negative electrode active material capable of occluding and releasing lithium ions, and a non-aqueous electrolyte. The non-aqueous electrolyte is an electrolyte (1), the general formula (2); (XSO ₂ ) (X′SO ₂ ) N ^— Li ⁺ (X and X ′ are fluorine atoms, alkyl groups having 1 to 6 carbon atoms) Or a C 1-6 fluoroalkyl group, and at least one of X and X ′ is a fluorine atom.) 0.01 mol / L to 2.0 mol / L of a compound represented by:
A lithium secondary battery comprising:
It has the characteristic in that.

  The present inventors have repeatedly studied to develop a higher performance lithium secondary battery. By using the compound (2) in addition to the electrolyte (1) in the non-aqueous electrolyte, It has been found that charging / discharging at high speed normally suppresses the decrease in discharge capacity compared to charging at low speed and can maintain sufficient capacity even during high speed charging / discharging. The present invention has been completed. The present invention will be described below.

<Average discharge voltage>
The lithium secondary battery of the present invention has a 2C average discharge voltage of 3.64V or more. The lithium secondary battery of the present invention has a high average discharge voltage as described above, and can provide a high output battery according to the present invention. The said discharge capacity maintenance factor is measured on the conditions of charging / discharging speed | rate 2C (constant current mode) and 3.0-4.2V, for example using a charging / discharging test apparatus (ACD-01, product made from Asuka Electronics Co., Ltd.). It is a value obtained by doing. During each charge / discharge, a charge / discharge pause time of 10 minutes was provided.

<Lithium secondary battery>
The lithium secondary battery according to the present invention has a structure in which a positive electrode and a negative electrode are housed in a case via a separator impregnated with the electrolytic solution according to the present invention. The shape of the lithium secondary battery according to the present invention is arbitrary, and may be any of a cylindrical shape, a square shape, a laminate shape, a coin shape, a large size, and the like. Moreover, it is good also as a battery module comprised by connecting each battery in series as a high voltage power supply which requires several 10V-several 100V for mounting in an electric vehicle, a hybrid electric vehicle, etc.

  The lithium secondary battery of the present invention has a discharge capacity maintenance rate at 2C at 30 ° C. of 94% or more. The lithium secondary battery of the present invention has a high discharge capacity retention rate as described above, and it has become possible to provide a high output battery according to the present invention. The said discharge capacity maintenance factor is measured on the conditions of charging / discharging speed | rate 2C (constant current mode) and 3.0-4.2V, for example using a charging / discharging test apparatus (ACD-01, product made from Asuka Electronics Co., Ltd.). It is a value obtained by doing. During each charge / discharge, a charge / discharge pause time of 10 minutes was provided. Here, 1C represents the amount of current when the reference capacity of the battery is discharged in one hour, and 2C represents that charging / discharging is performed with the amount of current twice that amount.

<Positive electrode>
The positive electrode is a sheet in which a positive electrode active material composition containing a positive electrode active material, a conductive additive, a binder, a dispersion solvent and the like is supported on a positive electrode current collector, and is usually in a sheet form.

  The method for producing the positive electrode is, for example, a method in which the positive electrode active material composition is applied to the positive electrode current collector by the doctor blade method or the like and then dried; the positive electrode active material composition is kneaded, molded and dried. A method of joining the sheet to the positive electrode current collector via a conductive adhesive, pressing and drying; forming a positive electrode active material composition to which a liquid lubricant has been added on the positive electrode current collector; The method of removing and then extending | stretching to a uniaxial or multiaxial direction etc. is mentioned.

  The material of the positive electrode current collector is not particularly limited, and for example, a conductive metal such as aluminum, an aluminum alloy, SUS, or titanium can be used. Among these, aluminum is preferable from the viewpoint of being easily processed into a thin film and being inexpensive.

  Examples of the conductive assistant include acetylene black, carbon black, graphite, metal powder material, single-phase carbon nanotube, multi-walled carbon nanotube, and vapor grown carbon fiber.

  As binders, fluorine resins such as polyvinylidene fluoride and polytetrafluoroethylene; synthetic rubbers such as styrene-butadiene rubber and nitrile butadiene rubber; polyamide resins such as polyamideimide; polyolefin resins such as polyethylene and polypropylene A poly (meth) acrylic resin; a polyacrylic acid; a cellulose resin such as carboxymethylcellulose; These binders may be used alone or in combination of two or more. These binders may be dissolved in a solvent or dispersed in a solvent.

  The blending amount of the conductive auxiliary agent or the binder can be appropriately adjusted in consideration of the intended use of the battery (emphasis on output, importance on energy, etc.), ion conductivity, and the like.

  Examples of the solvent used for dispersing the positive electrode material when producing the positive electrode include N-methylpyrrolidone, dimethylformamide, dimethylacetamide, methyl ethyl ketone, tetrahydrofuran, acetone, ethanol, ethyl acetate, and water.

<Positive electrode active material>
In the lithium secondary battery of the present invention, the positive electrode active material contains a positive electrode active material containing a composite metal oxide of lithium and cobalt. Specifically, it is a composite metal oxide of lithium and cobalt, and may contain a transition metal other than cobalt,
It is a complex oxide represented by LiM _1-x Co _x O 2 (M is a transition metal, 0 ≦ x <1). Examples of M include Ni, Al, Mn, and the like, and one or more of them may be included. Also, a solid solution material incorporating a plurality of transition metals containing cobalt (electrochemically inactive layered Li ₂ MnO ₃ and electrochemically active layered LiMO (transition metals such as M = Co and Ni)) The solid solution may be used alone or in combination.

<Negative electrode>
The negative electrode is a sheet in which a negative electrode active material composition containing a negative electrode active material, a dispersion solvent, a binder, and a conductive auxiliary agent as necessary is supported on a negative electrode current collector, and is usually in the form of a sheet.

  As a material of the negative electrode current collector, a conductive metal such as copper, iron, nickel, silver, and stainless steel SUS (SUS stainless steel) can be used. However, copper is preferable from the viewpoint of easy processing into a thin film.

  As a negative electrode active material, the negative electrode active material used with a conventionally well-known lithium secondary battery can be used, What is necessary is just what can occlude / release lithium ion. Specifically, graphite materials such as artificial graphite and natural graphite, mesophase fired bodies made from coal / petroleum pitch, carbon materials such as non-graphitizable carbon, silicon Si-based negative electrode materials such as Si, Si alloy, and SiO Sn-based negative electrode materials such as Sn alloy, and lithium alloys such as lithium metal and lithium-aluminum alloy can be used.

In addition, the manufacturing method of a negative electrode can employ | adopt the method similar to the manufacturing method of a positive electrode. As the conductive auxiliary agent, binder, and material dispersion solvent that can be used for the negative electrode, the same ones as those used for the positive electrode can be used.
<Separator>
The separator is disposed so as to separate the positive and negative electrodes. The separator is not particularly limited, and a conventionally known separator can be used. For example, a porous sheet (for example, a polyolefin-based microporous separator or a cellulose-based separator) made of a polymer that absorbs and retains a nonaqueous electrolytic solution, a nonwoven fabric separator, a porous metal body, and the like can be given. Among these, a polyolefin microporous separator having a property of being chemically stable with respect to an organic solvent is preferable.

  Examples of the material for the porous sheet include polyethylene, polypropylene, and a laminate having a three-layer structure of polypropylene / polyethylene / polypropylene.

  Examples of the material of the nonwoven fabric separator include cotton, rayon, acetate, nylon, polyester, polypropylene, polyethylene, polyimide, aramid, glass, and the like, depending on the mechanical strength required for the nonaqueous electrolyte layer or the like. Used by mixing.

<Non-aqueous electrolyte>
[Electrolyte (1)]
The electrolyte (1) is a substance that functions as a main electrolyte in the nonaqueous electrolytic solution of the lithium secondary battery of the present invention. Since the present invention has a gist in that the compound (2) is used in addition to the electrolyte (1), the electrolyte (1) is not particularly limited, and is used as an electrolyte in electrolytes of various power storage devices. Any conventionally known electrolyte can be used. The electrolyte (1) is preferably one that has a large dissociation constant in the electrolyte and generates an anion that is difficult to solvate with the solvent described below. Specifically, one or more compounds selected from the group consisting of LiPF ₆ , LiClO ₄ , LiBF ₄ , LiAsF ₆ , LiSbF ₆ , LiAlO ₄ , LiAlCl ₄ , LiCl, and LiI, etc. may be used as the electrolyte (1 ). Among these, LiPF ₆ and LiBF ₄ are preferable, and LiPF ₆ is more preferable. The electrolyte (1) may be used alone or in combination of two or more.

  Moreover, it is preferable that the density | concentration of the electrolyte (1) in a non-aqueous electrolyte is 0.1 mol / L or more and below a saturation concentration. More preferably, it is 0.1 mol / L to 2.5 mol / L, more preferably 0.3 mol / L to 2 mol / L, still more preferably 0.4 mol / L to 1.5 mol / L, and most preferably Is 0.5 mol / L to 1.2 mol / L. If the concentration of the electrolyte (1) is too high, the viscosity of the non-aqueous electrolyte solution becomes high, the conductivity is lowered, and the battery performance may not be sufficiently exhibited.

  The proportion of the electrolyte (1) in the nonaqueous electrolytic solution is preferably 30 mol% or more in the total amount of the electrolyte contained in the nonaqueous electrolytic solution of the present invention. More preferably, it is 50 mol% or more. The upper limit may be 99 mol% or less.

[Compound (2)]
The nonaqueous electrolytic solution of the present invention contains a compound (2) represented by the general formula (2); (XSO ₂ ) (X′SO ₂ ) N ^— Li ⁺ . Similar to the electrolyte (1), the compound (2) dissociates anions and cations in the electrolytic solution, and therefore can be a charge carrier supply source. Therefore, it is conceivable to use the compound (2) as the electrolyte (1). However, the present inventors have used a specific amount of the compound (2) as the charge carrier by using the compound (2) with respect to the electrolyte (1). In addition to the function, it has been found that it acts on a film on the electrode described later, suppresses an increase in internal resistance, and allows the discharge voltage to be maintained at a high value.

  In the general formula (2), X and X ′ represent a fluorine atom, an alkyl group having 1 to 6 carbon atoms or a fluoroalkyl group having 1 to 6 carbon atoms, and at least one of X and X ′ is a fluorine atom. . The alkyl group having 1 to 6 carbon atoms is preferably a linear alkyl group, and examples thereof include a methyl group, an ethyl group, a propyl group, an isopropyl group, a butyl group, a pentyl group, and a hexyl group. Examples of the fluoroalkyl group having 1 to 6 carbon atoms include those in which some or all of the hydrogen atoms of the alkyl group are substituted with fluorine atoms, such as a fluoromethyl group, a difluoromethyl group, and a trifluoromethyl group. , Fluoroethyl group, difluoroethyl group, trifluoroethyl group, pentafluoroethyl group and the like. Among the alkyl groups or fluoroalkyl groups, a fluorine atom, a trifluoromethyl group, and a pentafluoroethyl group are preferable. Preferred compounds (2) include lithium bis (fluorosulfonyl) imide, lithium (fluorosulfonyl) (trifluoromethylsulfonyl) imide, lithium (fluorosulfonyl) (methylsulfonyl) imide, lithium (fluorosulfonyl) (pentafluoroethylsulfonyl) ) Imide, lithium (fluorosulfonyl) (ethylsulfonyl) imide, and more preferably lithium bis (fluorosulfonyl) imide, lithium (fluorosulfonyl) (trifluoromethylsulfonyl) imide, lithium (fluorosulfonyl) (pentafluoroethyl) Sulfonyl) imide, more preferably lithium bis (fluorosulfonyl) imide.

  The concentration of the compound (2) in the nonaqueous electrolytic solution of the present invention is preferably in the range of 0.01 mol / L to 2.0 mol / L, more preferably 0.05 mol / L to 1.0 mol / L. More preferably, it is 0.1 mol / L to 0.5 mol / L. When the concentration of the compound (2) is 0.01 mol / L or less, the effect on the film on the electrode is not sufficient, and the desired battery performance may not be obtained. When the concentration of the compound (2) is 2.0 mol / L or more, the viscosity of the nonaqueous electrolytic solution is increased, the conductivity is lowered, and the battery performance may not be sufficiently exhibited.

  The compounding amount of the compound (2) in the nonaqueous electrolytic solution of the present invention is such that the compound (2) is 50 mol% or less and 0.1 mol% when the total of the electrolyte (1) and the compound (2) is 100 mol%. The above is preferable. More preferably, they are 0.5 mol% or more and 30 mol% or less, More preferably, they are 1 mol% or more and 20 mol% or less. When the compounding amount of the compound (2) is too small, the effect on the film on the electrode is not sufficient, and the desired battery performance may not be obtained.

  As compound (2), a commercially available product may be used, or a compound synthesized by a conventionally known method may be used.

[Cyclic carbonate having an unsaturated bond]
The nonaqueous electrolytic solution of the lithium secondary battery of the present invention preferably contains a cyclic carbonate having an unsaturated bond in addition to the electrolyte (1) and the compound (2). By including a cyclic carbonate having an unsaturated bond, a film can be formed on the electrode surface of the lithium secondary battery, and the deterioration of the electrode and further the decomposition of the electrolyte can be suppressed, and charging and discharging can be performed stably. . Examples of the cyclic carbonate having an unsaturated bond include vinylene carbonate (VC), vinyl ethylene carbonate (VEC), methyl vinylene carbonate (MVC), and ethyl vinylene carbonate (EVC). Preferred are vinylene carbonate and vinyl ethylene carbonate, and more preferred is vinylene carbonate.

  It is preferable that the density | concentration of the cyclic carbonate which has the said unsaturated bond in the non-aqueous electrolyte of this invention is 0.01 mass%-5 mass%. More preferably, it is 0.05 mass%-4 mass%. More preferably, it is 0.1 mass%-3 mass%. When the amount of the cyclic carbonate having an unsaturated bond is more than 5% by mass, charging / discharging the battery may cause gas generation, which may lead to battery swelling. When the amount of the cyclic carbonate having an unsaturated bond is less than 0.01% by mass, the film formation on the electrode is insufficient and the cycle characteristics may be deteriorated.

[solvent]
In the non-aqueous electrolyte of the present invention, various non-aqueous solvents conventionally used in non-aqueous electrolytes can be used as the solvent for dissolving the electrolytes. For example, cyclic carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, and chloroethylene carbonate; chain carbonates such as dimethyl carbonate, ethylmethyl carbonate, and diethyl carbonate; tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, 1 Ethers such as 1,2-dimethoxyethane, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1,2-dibutoxyethane; γ-butyrolactone, γ-valerolactone, α-methyl-γ-butyrolactone, etc. Lactones of the above; chain carboxylic acid esters such as methyl propionate and methyl butyrate can be used. Among these non-aqueous solvents, carbonate solvents such as cyclic carbonates and chain carbonates are preferably used because they are difficult to decompose upon application of voltage and are stable. In addition, the said non-aqueous solvent may be used independently and may mix and use 2 or more types.

[Additive]
In addition to the electrolyte (1), the compound (2), the solvent, and the cyclic carbonate having an unsaturated bond, an additive is added to the non-aqueous electrolyte of the lithium secondary battery of the present invention in order to improve cycle characteristics and safety. It may be included. As additives, carbonate compounds such as fluoroethylene carbonate, trifluoropropylene carbonate, phenylethylene carbonate and erythritan carbonate; succinic anhydride, glutaric anhydride, maleic anhydride, citraconic anhydride, glutaconic anhydride, itaconic anhydride, Carboxylic anhydrides such as diglycolic anhydride, cyclohexanedicarboxylic anhydride, cyclopentanetetracarboxylic dianhydride, phenylsuccinic anhydride; ethylene sulfite, 1,3-propane sultone, 1,4-butane sultone, methane Sulfur-containing compounds such as methyl sulfonate, busulfan, sulfolane, sulfolene, dimethyl sulfone, tetramethylthiuram monosulfide; 1-methyl-2-pyrrolidinone, 1-methyl-2-piperidone, 3-methyl Nitrogenous compounds such as lu-2-oxazolidinone, 1,3-dimethyl-2-imidazolidinone, N-methylsuccinimide; phosphates such as monofluorophosphate and difluorophosphate; heptane, octane, And hydrocarbon compounds such as cycloheptane. When these additives are used in the non-aqueous electrolyte, the concentration is preferably 0.1% by weight to 5% by weight.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

Experimental example 1
[Preparation of electrolyte]
1.82 g (1.2 mol) of lithium hexafluorophosphate (LiPF ₆ , manufactured by Kishida Chemical Co., Ltd., LBG grade) is weighed into a 10 mL measuring flask, ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (both An electrolytic solution was prepared by volume-up with a mixed solvent having a volume ratio of 3/7 ((EC / EMC), manufactured by Kishida Chemical Co., Ltd., LBG grade).

[Production of laminate cell and charge / discharge before test]
One commercially available positive electrode sheet (active material: lithium cobaltate, 3 mAh / cm ² ) having an area of the positive electrode active material layer of 12 cm ² (3 × 4 cm) and an area of the negative electrode active material layer of 13.44 cm ² (3. 2 × 4.2 cm) of a commercially available negative electrode sheet (active material: graphite, 3.2 mAh / cm ² ) was laminated so as to face each other, and one polyolefin separator was sandwiched therebetween. The positive and negative electrode sheets are sandwiched between two aluminum laminate films, the outer sides of the positive and negative electrodes are sandwiched between two polypropylene sheets, the outermost is sandwiched between two aluminum laminate films, and three of the four sides are heat sealers. Was adhered using. The inside of the aluminum laminate film was filled with 1 mL of electrolytic solution, and the remaining one side was sealed in a vacuum state.

  2. Using a charge / discharge test apparatus (ACD-01, manufactured by Asuka Electronics Co., Ltd.), charge to 4.2 V at a charge / discharge rate of 0.2 C (constant current mode) at 30 ° C. and pause for 10 minutes. The battery was discharged to 0 V and rested for 10 minutes. After the first charge / discharge, the laminate cell was opened and then sealed again in a vacuum state. Under the same conditions, charging / discharging was repeated 5 times to complete a laminated lithium battery. The results of the load test and the cycle test are average values of n = 3.

[Load test]
The obtained laminated lithium battery is charged to 4.2 V per cycle at a charge / discharge rate of 0.2 C (constant current mode) at 30 ° C. using a charge / discharge test apparatus (ACD-01, manufactured by Asuka Electronics Co., Ltd.). Thereafter, the battery was discharged to 3.0 V, and the discharge capacity at this time was 100%. A charge / discharge pause time of 10 minutes was provided after each charge / discharge. Furthermore, charging / discharging was performed under the conditions of a discharge rate of 2.0 C and 5.0 C, and the discharge capacity retention rate, IR drop, internal resistance, and average discharge voltage were measured. The results are shown in Tables 1 and 2. After discharging, the battery was completely discharged at 0.2 C before proceeding to the next charge.

[Cycle test]
3. Regarding a laminated lithium battery produced separately from the load test, using a charge / discharge test apparatus (ACD-01, manufactured by Asuka Electronics Co., Ltd.) at a charge / discharge rate of 0.2 C (constant current mode) at 30 ° C. A cycle test was conducted by charging to 2V and discharging to 3.0V. A charge / discharge pause time of 10 minutes was provided after each charge / discharge. The results are shown in FIG.

Experimental example 2
Laminated lithium as in Experimental Example 1 except that 2% by weight of vinylene carbonate (VC, manufactured by Kishida Chemical Co., Ltd., LBG grade) was added to the electrolytic solution prepared in the same manner as in Experimental Example 1 in the preparation of the electrolytic solution. A battery was fabricated and evaluated.

Experimental example 3
In the preparation of the electrolytic solution, 1.52 g (1.0 mol) of lithium hexafluorophosphate (LiPF ₆ , manufactured by Kishida Chemical Co., Ltd., LBG grade) and lithium bis (fluorosulfonyl) imide (LiFSI, manufactured by Nippon Shokubai Co., Ltd.) as the electrolyte ) An electrolytic solution and a laminated lithium battery were prepared and evaluated in the same manner as in Experimental Example 1 except that 0.37 g (0.2 mol) was used.

Experimental Example 4
Laminated lithium as in Experimental Example 1 except that 2% by weight of vinylene carbonate (VC, manufactured by Kishida Chemical Co., Ltd., LBG grade) was added to the electrolytic solution prepared in the same manner as in Experimental Example 3 in the preparation of the electrolytic solution. A battery was fabricated and evaluated.

Experimental Example 5
In the preparation of the electrolytic solution, 1.52 g (1.0 mol) of lithium hexafluorophosphate (LiPF ₆ , manufactured by Kishida Chemical Co., Ltd., LBG grade) and lithium bis (trifluoromethanesulfonyl) imide (LiTFSI, Kishida Chemical Co., Ltd.) were used as electrolytes. Manufactured, LBG grade) Other than using 0.57 g (0.2 mol), an electrolytic solution and a laminated lithium battery were prepared and evaluated in the same manner as in Experimental Example 1.

Experimental Example 6
Laminated lithium as in Experimental Example 1 except that 2% by weight of vinylene carbonate (VC, manufactured by Kishida Chemical Co., Ltd., LBG grade) was added to the electrolytic solution prepared in the same manner as in Experimental Example 5 in the preparation of the electrolytic solution. A battery was fabricated and evaluated.

From the results of Table 1, compared to Experimental Example 1 using only LiPF ₆ as the electrolyte, in Experimental Examples 2, 3, and 4 using LiFSI and VC together, the discharge capacity retention rate and the average discharge voltage increased, and the battery Although the output is large, it can be seen that in Experimental Example 2 using LiPF ₆ and VC, the IR drop and the internal resistance are increased. However, in Experimental Examples 3 and 4 using LiFSI, both the IR drop and the internal resistance are lower than those of Experimental Example 1, and it was found that the internal resistance can be suppressed to a low level by using LiFSI. Further, in Experimental Example 4 in which LiFSI and VC were used in combination, it was found that a battery having both a high discharge capacity retention rate and a low internal resistance can be obtained.

  The same tendency was observed when the discharge rate was 5C.

From FIG. 1, compared with Experimental Example 1 using only LiPF ₆ as the electrolyte, Experimental Examples 2, 3, 4, and 6 using LiFSI and VC showed improved cycle characteristics. Of these, Experimental Example 4 using LiFSI and VC in combination was found to have the best cycle characteristics. In Experimental Example 5 using LiTFSI, the cycle characteristics were not improved at the same level as in Experimental Example 1.

Claims (8)

A lithium secondary battery comprising a positive electrode containing a positive electrode active material containing a composite metal oxide of lithium and cobalt, a negative electrode containing a negative electrode active material capable of occluding and releasing lithium ions, and a non-aqueous electrolyte. The non-aqueous electrolyte is an electrolyte (1), general formula (2); (XSO ₂ ) (X′SO ₂ ) N ^— Li ⁺ (X and X ′ are fluorine atoms, alkyl groups having 1 to 6 carbon atoms) Or a C 1-6 fluoroalkyl group, wherein at least one of X and X ′ is a fluorine atom.) And a solvent,
A lithium secondary battery having a 2C average discharge voltage of 3.64 V or more. A lithium secondary battery comprising a positive electrode containing a positive electrode active material containing a composite metal oxide of lithium and cobalt, a negative electrode containing a negative electrode active material capable of occluding and releasing lithium ions, and a non-aqueous electrolyte. The non-aqueous electrolyte is an electrolyte (1),
(XSO ₂ ) (X′SO ₂ ) N ^— Li ⁺ (X and X ′ represent a fluorine atom, an alkyl group having 1 to 6 carbon atoms, or a fluoroalkyl group having 1 to 6 carbon atoms. , X, and X ′ are fluorine atoms.) 0.01 mol / L to 2.0 mol / L of a compound represented by:
A lithium secondary battery comprising:   3. The lithium secondary battery according to claim 1, wherein the non-aqueous electrolyte further contains 0.01% by mass to 5% by mass of a cyclic carbonate having an unsaturated bond. The lithium secondary battery according to claim 1, wherein the electrolyte (1) is LiPF ₆ .   The lithium secondary battery according to any one of claims 3 to 4, wherein the cyclic carbonate having an unsaturated bond is vinylene carbonate.   The lithium secondary battery according to claim 1, wherein the solvent contains ethylene carbonate.   The lithium secondary battery according to any one of claims 1 to 6, wherein the compound represented by the general formula (2) is lithium bis (fluorosulfonyl) imide. The lithium secondary battery according to claim 1, wherein the positive electrode is lithium cobalt oxide.

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