JP6483943B2 - Lithium secondary battery - Google Patents

Description

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

  Since lithium secondary batteries have high energy density, they are used as power sources for mobile communication devices, power sources for portable information terminals, etc., and the market is rapidly expanding with the spread of terminals, securing safety, cycle Various studies have been made for the purpose of improving characteristics, high rate characteristics, energy density, high temperature storage characteristics and the like.

  In recent years, with the development of smaller and lighter electronic devices and the development of electric vehicles, the development of large-capacity, high-power lithium secondary batteries is desired, and therefore, for nonaqueous electrolytes of lithium secondary batteries It has been considered to use an ionic liquid as a flame retardant liquid.

Patent Document 1 discloses a lithium secondary battery including a non-aqueous electrolyte solution containing a lithium salt and a negative electrode of non-graphitizable carbon, but the cycle characteristics are not satisfactory.
Patent Document 2 discloses a lithium battery having an electrolyte in which a non-aqueous electrolytic solution containing a lithium salt is solidified with a crosslinkable polymer, but the discharge capacity is reduced due to the solidification of the solvent and the cycle characteristics are not sufficient.
Patent Document 3 discloses a lithium secondary battery in which the non-aqueous electrolyte solution is an ionic liquid containing a lithium salt, but the cycle characteristics are not satisfactory.

JP, 2009-26542, A JP, 2009-277413, A JP 2007-207675 A

  Among various investigations as described above, for example, a lithium secondary battery using a combination of a carbon negative electrode and an ionic liquid has high internal resistance as compared to an organic solvent-based battery, and output characteristics And the cycle characteristics etc. are insufficient.

  In addition, when the lithium-containing manganic acid-based material is used as the positive electrode active material, there is a problem that the cycle characteristics at high temperatures are deteriorated. This is considered to be due to the elution of the Mn component from the positive electrode under high temperature. Therefore, various efforts have been made such as a method of doping and replacing a part of Mn with another element, a method of synthesizing a lithium-containing manganic acid-based material, and modification of the particle surface of a lithium-containing manganic acid-based material In the secondary battery, it is a fact that further improvement of battery performance such as high temperature storage characteristics and cycle characteristics and suppression of metal component elution is required.

  The present invention has been made focusing on the above circumstances, and the object of the present invention is to form a high quality film on the positive and negative electrodes by suppressing the elution of the Mn component from the positive electrode and having excellent high temperature storage characteristics. To provide a lithium ion secondary battery having excellent cycle characteristics.

  The present inventors formed a film on an electrode by containing a specific electrolyte in a non-aqueous electrolyte when a lithium-containing manganic acid-based material is used as a positive electrode active material in a lithium secondary battery. As a result, it has been found that the deterioration of the electrode can be suppressed and the elution of the Mn component from the positive electrode can be suppressed, and the present invention has been completed.

That is, the lithium secondary battery of the present invention is
The element M is at least one element selected from the group consisting of Ni, Co, Al, Si, Zr, Ti, Fe, Mg and V, which is represented by the general formula Li _x Mn _a M _b O _y ; Lithium transition metal complex oxide satisfying 9 ≦ (a + b) ≦ 2.1, 0 <x ≦ 2, 1 ≦ a ≦ 3, 0 ≦ b ≦ 1.0, 3 ≦ y ≦ 4.5 as a positive electrode active material Containing positive electrode,
1. A lithium secondary battery comprising a negative electrode containing a negative electrode active material capable of absorbing and desorbing lithium ions, and a non-aqueous electrolyte,
The non-aqueous electrolyte has a general formula (1): (XSO ₂ ) (X′SO ₂ ) N ⁻ Li ⁺ as the electrolyte (1)
(X, X ′ each represents 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) And a carbonate solvent.

The non-aqueous electrolyte further includes an electrolyte (2) as an electrolyte, and the electrolyte (2) is
M′PF _c (C _m F _{2m + 1} ) _6-c (M ′ is an alkali metal ion, 0 ≦ c ≦ 6, 1 ≦ m ≦ 2),
M′BF _d (C _n F _{2n + 1} ) _4-d (where M ′ is an alkali metal ion, 0 ≦ d ≦ 4, 1 ≦ n ≦ 2), and M′AsF ₆ (M ′ is an alkali metal ion) It is preferable that it is at least one salt compound selected from the group consisting of

  At this time, the concentration of the electrolyte (2) contained in the non-aqueous electrolyte solution is preferably 10 to 99.9 mol% with respect to the total amount of the electrolyte (1) and the electrolyte (2).

The aspect in which the electrolyte (2) is LiPF ₆ and the aspect in which the carbonate solvent contains cyclic carbonates and linear carbonates are all preferred embodiments of the present invention.

  The compound represented by the general formula (1) is preferably lithium bis (fluorosulfonyl) imide.

The lithium transition metal composite oxide is preferably a LiMn ₂ O ₄ (lithium manganate).

  According to the present invention, it is possible to provide a lithium ion secondary battery excellent in high-temperature storage characteristics by suppressing the elution of the Mn component from the positive electrode, and excellent in cycle characteristics by forming a good film on the positive and negative electrodes. .

FIG. 1 is a graph showing the high temperature storage characteristics of the fully charged lithium secondary battery of the present invention (the horizontal axis represents the period of storage at 60 ° C. (month), and the vertical axis represents the capacity retention rate). FIG. 2 is a graph showing the high-temperature storage characteristics of the fully charged lithium secondary battery of the present invention (the horizontal axis represents the period of storage at 60 ° C. (month), and the vertical axis represents the capacity retention rate). FIG. 3 is a graph showing the high temperature storage characteristics of the fully charged lithium secondary battery of the present invention (the horizontal axis represents the period of storage at 60 ° C. (month), and the vertical axis represents the cell OCV). FIG. 4 is a graph showing the high-temperature storage characteristics of the fully charged lithium secondary battery of the present invention (the horizontal axis represents the period of storage at 60 ° C. (month), and the vertical axis represents the increase in resistance). FIG. 5 is a diagram showing improvement of cycle characteristics by the lithium secondary battery of the present invention at 30 ° C. (the horizontal axis shows the number of cycles and the vertical axis shows the capacity retention rate).

<Lithium rechargeable battery>
The lithium secondary battery of the present invention is
The element M is at least one element selected from the group consisting of Ni, Co, Al, Si, Zr, Ti, Fe, Mg and V, which is represented by the general formula Li _x Mn _a M _b O _y ; Lithium transition metal complex oxide satisfying 9 ≦ (a + b) ≦ 2.1, 0 <x ≦ 2, 1 ≦ a ≦ 3, 0 ≦ b ≦ 1.0, 3 ≦ y ≦ 4.5 as a positive electrode active material Containing positive electrode,
1. A lithium secondary battery comprising a negative electrode containing a negative electrode active material capable of absorbing and desorbing lithium ions, and a non-aqueous electrolyte,
The above non-aqueous electrolyte has a general formula (1); (XSO ₂ ) (X′SO ₂ ) N ⁻ Li ⁺ as the electrolyte (1)
(X, X ′ each represents 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) And a carbonate-based solvent.
According to the lithium secondary battery, when the non-aqueous electrolyte contains the electrolyte (1), a film can be formed on the electrode, and the decomposition of the non-aqueous electrolyte on the electrode can be suppressed. Further, it is possible to suppress the elution of the Mn component of the positive electrode, to suppress the rise of the internal resistance, and to maintain the discharge voltage at a high value. Furthermore, when the non-aqueous electrolytic solution further contains the electrolyte (2), the electric conductivity of the lithium salt can be increased, and the movement of lithium ions can be improved between the positive electrode and the negative electrode in charge and discharge. In addition, passivation of the positive electrode current collector can be promoted and deterioration can be suppressed.

<Positive electrode>
Below, the positive electrode used for the lithium secondary battery of this invention is demonstrated.

<Positive electrode active material>
In the lithium secondary battery of the present invention, the positive electrode active material is represented by the general formula Li _x Mn _a M _b O _y , and the element M consists of Ni, Co, Al, Si, Zr, Ti, Fe, Mg and V At least one element selected from the group consisting of 1.9 ≦ (a + b) ≦ 2.1, 0 <x ≦ 2, 1 ≦ a ≦ 3, 0 ≦ b ≦ 1.0, 3 ≦ y ≦ 4. 5 contains a lithium transition metal complex oxide. The lithium transition metal complex oxides can be used alone or in combination of two or more.

  In the general formula, 1.9 ≦ (a + b) ≦ 2.1, and in view of the lithium secondary battery exhibiting suitable characteristics, preferably 2.0 ≦ (a + b) ≦ 2.1, and more preferably (A + b) = 2.

  In the above general formula, 0 <x ≦ 2, preferably 0.5 ≦ x ≦ 2, more preferably 0.5 ≦ x ≦ 1.5, still more preferably 0.5 ≦ x ≦ 1, still more preferably It is x = 1. When x is 0, the Li site in the oxide is substituted by an element such as Ni, which may cause a substantial capacity reduction, and when x is larger than 2, the first time irreversible capacity increases. Efficiency may be reduced.

  In the above general formula, 1 ≦ a ≦ 3, preferably 1 ≦ a ≦ 2.5, more preferably 1.2 ≦ a ≦ 2.5, still more preferably 1.5 ≦ a ≦ 2.0 Preferably, a = 2. If a is less than 1, the thermal stability at the time of charge of the oxide may be reduced, and if a is more than 3, the capacity of the positive electrode active material may be reduced.

  In the above general formula, 0 ≦ b ≦ 1.0, preferably 0 ≦ b ≦ 0.8, more preferably 0 ≦ b ≦ 0.7, still more preferably 0 ≦ b ≦ 0.6, still more preferably 0 ≦ b ≦ 0.5, still more preferably b = 0.

  In the above general formula, 3 ≦ y ≦ 4.5, preferably 3.5 ≦ y ≦ 4.5, and more preferably y = 4.

In the lithium transition metal complex oxide, the most preferable combination of x, a, b and y is (a + b) = 2, a = 2, b = 0, x = 1, y = 4 or (a + b) = 2, a = 1.5, b = 0.5, x = 1, y = 4 Therefore, the most preferable lithium transition metal composite oxide is LiMn ₂ O ₄ or LiNi _0.5 Mn _1.5 O ₄ .

  The surface of the positive electrode active material of the lithium transition metal complex oxide may be doped with a specific material, and the materials used for the doping include Ni, Co, Al, Si, Zr, Ti, Fe, Mg and V Etc.

  The positive electrode active material contains the lithium transition metal complex oxide as a main component. The positive electrode active material preferably contains 50 to 100% by mass, more preferably 70 to 100% by mass, and still more preferably 90 to 100% by mass of the lithium transition metal composite oxide.

In the present invention, as long as the positive electrode active material exhibits the effects of the present invention, a plurality of compounds having an olivine structure such as lithium cobaltate, lithium nickelate, LiAPO ₄ (A = Fe, Mn, Ni, Co), and a plurality of transition metals A solid solution material (a solid solution of an electrochemically inactive layered Li ₂ MnO ₃ and an electrochemically active layered LiM′′O (M ′ ′ = transition metal such as Co, Ni)) Or the like may be used. These may be used alone or in combination of two or more.

The positive electrode active material may be manufactured by a manufacturing method known to those skilled in the art, and as the manufacturing method, for example, the electrosynthesized MnO ₂ , Li ₂ CO ₃ , and LiOH are mixed to obtain 750 to 800 ° C. A method of baking by, a method of mixing electrolytically synthesized MnO ₂ and Li ₂ CO ₃ , baking at about 800 ° C., slow cooling and pulverizing several times, mixing hydrothermally synthesized Mn ₃ O ₄ and LiOH, The method of baking at 750-820 degreeC etc. are mentioned.

The positive electrode is one in which a positive electrode active material composition containing a positive electrode active material, a conductive additive, a binder, a dispersing agent and the like is supported on a positive electrode current collector, and is usually in the form of a sheet.
The method of manufacturing the positive electrode is, for example, a method of coating or immersing the positive electrode active material composition on the positive electrode current collector by the doctor blade method or the like and then drying it; it is obtained by kneading, forming and drying the positive electrode active material composition. Method of bonding the obtained sheet to the positive electrode current collector via the conductive adhesive, pressing and drying; after forming the positive electrode active material composition to which the liquid lubricant is added on the positive electrode current collector, the liquid lubricant It may be removed and then uniaxially or multiaxially stretched.

  It does not specifically limit as a material of a positive electrode collector, For example, electroconductive metals, such as aluminum, aluminum alloy, and titanium, can be used. Among them, aluminum is preferable from the viewpoint of being easily processed into a thin film and being inexpensive.

  As a conductive support agent, acetylene black, carbon black, graphite, metal powder material, single-walled carbon nanotube, multi-walled carbon nanotube, vapor grown carbon fiber and the like can be mentioned.

  As a binder (binder), fluorinated resins such as polyvinylidene difluoride (PVDF) and polytetrafluoroethylene (PTFE); Synthetic rubbers such as styrene-butadiene rubber (SBR) and nitrile butadiene rubber; such as polyamide imide Polyamide resins; Polyolefin resins such as polyethylene and polypropylene; Poly (meth) acrylic resins; Polyacrylic acids; Cellulose resins such as carboxymethyl cellulose (CMC). These binders may be used alone or in combination of two or more. Further, these binders may be in a state of being dissolved in a solvent or in a state of being dispersed in a solvent.

  The compounding amount of the conductive support agent and the binder can be appropriately adjusted in consideration of the purpose of use of the battery (such as emphasis on output and emphasis on energy), ion conductivity, and the like.

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

<Non-aqueous electrolyte>
<Compound represented by General Formula (1) (hereinafter, also referred to as electrolyte (1))>
The non-aqueous electrolyte used in the present invention is a compound represented by the general formula (1); (XSO ₂ ) (X′SO ₂ ) N ⁻ Li ⁺ (wherein X and X ′ each represents a fluorine atom, It represents a C1-C6 alkyl group or a C1-C6 fluoroalkyl group, and at least one of X and X 'is a fluorine atom. } (Electrolyte (1)). By including the compound represented by the above general formula (1) in the non-aqueous electrolyte, a film can be formed on the electrode, and the decomposition of the non-aqueous electrolyte on the electrode can be suppressed. Further, it is possible to suppress the elution of the Mn component of the positive electrode, to suppress the rise of the internal resistance, and to maintain the discharge voltage at a high value. As a result, high temperature storage characteristics and cycle characteristics can be improved. In addition, the non-aqueous electrolytic solution essentially includes an electrolyte (1) and a carbonate-based solvent, and may further contain an electrolyte (2) described later, but triethylammonium, ethylmethylimidazolium, butyl commonly used for ionic liquids It is preferable not to include methyl imidazolium, 1-methyl-1-propyl pyrrolidinium, methyl propyl piperidinium and the like.

  In the above general formula (1), X and X 'each represents 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 . As a C1-C6 alkyl group, it is preferable that it is a linear alkyl group, for example, a methyl group, an ethyl group, a propyl group, an isopropyl group, a butyl group, a pentyl group, a hexyl group is mentioned. As a C1-C6 fluoroalkyl group, what the one part or all part of the hydrogen atom which the said alkyl group has was substituted by the fluorine atom is mentioned, For example, a fluoromethyl group, a difluoromethyl group, a trifluoromethyl group And fluoroethyl group, difluoroethyl group, trifluoroethyl group, pentafluoroethyl group and the like. Among the above alkyl group or fluoroalkyl group, trifluoromethyl group and pentafluoroethyl group are preferable. Preferred compounds represented by the general formula (1) 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 (fluoro sulfonyl) imide And sulfonyl) (pentafluoroethylsulfonyl) imide, more preferably lithium bis (fluorosulfonyl) imide (LiFSI).

In the non-aqueous electrolyte solution of the lithium secondary battery of the present invention, the electrolyte (1) may be a main electrolyte or another electrolyte may be a main electrolyte. When using another electrolyte as a main electrolyte, it is preferable that electrolyte (2) mentioned later becomes a main electrolyte.
The concentration of the compound (electrolyte (1)) represented by the general formula (1) in the non-aqueous electrolytic solution in the lithium secondary battery of the present invention is 0.1 mol when the electrolyte (1) is the main electrolyte It is preferable that the concentration is not less than / L and not more than the saturation concentration. More preferably, it is 0.1 mol / L to 2.5 mol / L, still more preferably 0.3 mol / L to 2 mol / L, particularly preferably 0.4 mol / L to 1.5 mol / L, and most preferably It 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 will increase and the electrical conductivity may decrease, and battery performance may not be sufficiently exhibited. If the concentration of the electrolyte (1) is low, the ion amount Tends to decrease, and the electric conductivity of the electrolyte may be insufficient. When an electrolyte other than the electrolyte (1) is used as the main electrolyte, the concentration of the electrolyte (1) is preferably 0.01 mol / L to 2 mol / L, more preferably 0.05 mol / L to 1 mol / L, further preferably Is 0.1 mol / L to 0.5 mol / L. If the concentration of the electrolyte (1) is less than 0.01 mol / L, the film formation on the electrode may not be sufficient, and the desired battery performance may not be obtained. When the concentration of the electrolyte (1) exceeds 2 mol / L, the viscosity of the non-aqueous electrolytic solution may be increased, the electrical conductivity may be decreased, and the battery performance may not be sufficiently exhibited.

  As the electrolyte (1), a commercially available product may be used, or one synthesized by a conventionally known method may be used.

<Electrolyte (2)>
In the lithium secondary battery of the present invention, the non-aqueous electrolyte further contains an electrolyte (2) as an electrolyte, and the electrolyte (2) is
M′PF _c (C _m F _{2m + 1} ) _6-c (M ′ is an alkali metal ion, 0 ≦ c ≦ 6, 1 ≦ m ≦ 2),
M′BF _d (C _n F _{2n + 1} ) _4-d (where M ′ is an alkali metal ion, 0 ≦ d ≦ 4, 1 ≦ n ≦ 2), and M′AsF ₆ (M ′ is an alkali metal ion) It is preferable that it is at least one salt compound selected from the group consisting of

M 'represents an alkali metal ion, and examples of the alkali metal ion include Li, Na, K, Rb, Cs and Fr.
In M′PF _c (C _m F _{2m + 1} ) _6-c , M ′ is preferably selected from the group consisting of Li, Na, K, Rb, Cs and Fr, and more preferably Li. Moreover, it is preferable that c = 6. In M′BF _d (C _n F _{2n + 1} ) _4-d , M ′ is preferably selected from the group consisting of Li, Na, K, Rb, Cs and Fr, and more preferably Li. Moreover, it is preferable that d = 4. In M'AsF _6, M 'is Li, Na, K, Rb, is preferably selected from the group consisting of Cs and Fr, and more preferably Li. These electrolytes (2) have a large dissociation constant in the electrolytic solution and produce anions which are difficult to solvate with the solvent described later. Specifically, the electrolyte (2) is preferably at least one lithium salt selected from the group consisting of LiPF ₆ , LiBF ₄ , and LiAsF ₆ . Among these, preferably LiPF ₆ and LiBF ₄ , more preferably LiPF ₆ . The above electrolyte (2) may be used alone or in combination of two or more.

  The electrolyte (2) has a large dissociation constant in the electrolyte solution, exhibits appropriate electrical conductivity, and can enhance battery performance.

In the non-aqueous electrolyte solution of the lithium secondary battery of the present invention, the electrolyte (2) may be a main electrolyte or the electrolyte (1) may be a main electrolyte.
When the electrolyte (2) is a main electrolyte, the concentration of the electrolyte (2) in the non-aqueous electrolytic solution is preferably 0.1 mol / L or more and the saturation concentration or less. More preferably, it is 0.1 mol / L to 2.5 mol / L, still more preferably 0.3 mol / L to 2 mol / L, particularly preferably 0.4 mol / L to 1.5 mol / L, and most preferably It is 0.5 mol / L to 1.2 mol / L. If the concentration of the electrolyte (2) is too high, the viscosity of the non-aqueous electrolyte solution may be increased to lower the electrical conductivity, and the battery performance may not be sufficiently exhibited. If the concentration of the electrolyte (2) is low, the ion amount Tends to decrease, and the electric conductivity of the electrolyte may be insufficient.
When the electrolyte (1) is a main electrolyte, the concentration of the electrolyte (2) is preferably 0.01 mol / L to 2 mol / L, more preferably 0.05 mol / L to 1 mol / L, still more preferably 0. It is 1 mol / L to 0.5 mol / L. If the concentration of the electrolyte (2) is less than 0.01 mol / L, the film formation on the electrode may not be sufficient and desired battery performance may not be obtained. When the concentration of the electrolyte (2) exceeds 2 mol / L, the viscosity of the non-aqueous electrolytic solution may be high, the electrical conductivity may be reduced, and the battery performance may not be sufficiently exhibited.

  Further, the concentration of the electrolyte (1) and the electrolyte (2) in the non-aqueous electrolytic solution is preferably 0.8 mol / L to 2.5 mol / L, as the total concentration of the electrolyte (1) and the electrolyte (2). More preferably, it is 0.8 mol / L to 2.0 mol / L, further preferably 0.8 mol / L to 1.5 mol / L.

  The concentration of the electrolyte (2) contained in the non-aqueous electrolytic solution is preferably 10 to 99 with respect to the total amount of the electrolyte (1) and the electrolyte (2) from the viewpoint of improvement of high temperature storage characteristics and cycle characteristics. It is .9 mol%. More preferably, it is 85 mol% or less, more preferably 80 mol% or less, still more preferably 75 mol% or less, still more preferably 70 mol% or less, particularly preferably 65 mol% or less, and most preferably 60 mol% or less. Since it becomes a moderate electrical conductivity that it is this range, it is thought that cycle characteristics also improve.

  The concentration of the electrolyte (1) in the non-aqueous electrolyte used in the present invention is 100 mol% of the total of the electrolyte (1) and the electrolyte (2) from the viewpoint of improving high-temperature storage characteristics and cycle characteristics. Preferably, it is 0.01 to 90 mol%, more preferably 15 to 90 mol%, still more preferably 20 to 90 mol%, still more preferably 25 to 90 mol%, still more preferably 30 to 90 mol%, particularly preferably 35 to 50 mol%. It is 90 mol%, most preferably 40 to 90 mol%. If the amount of the electrolyte (1) is too small, the film formation on the electrode may not be sufficient, and the desired battery performance may not be obtained.

<Carbonate solvent>
In the non-aqueous electrolyte used in the present invention, as solvents for dissolving the above-mentioned electrolytes, carbonate solvents of various non-aqueous solvents conventionally used in non-aqueous electrolytes can be used. For example, cyclic carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate and chloroethylene carbonate; linear carbonates such as dimethyl carbonate, methyl ethyl carbonate and diethyl carbonate can be used. These carbonate solvents can be preferably used because they are difficult to be decomposed when a voltage is applied and they are stable. Moreover, you may use together various non-aqueous solvents conventionally used for the non-aqueous electrolyte other than the said carbonate type solvent to such an extent that the effect of this invention is not impaired. As a solvent used in combination, for example, tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, 1,1-dimethoxyethane, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1,2-dibutoxyethane Ethers such as: γ-butyrolactone, γ-valerolactone, lactones such as α-methyl-γ-butyrolactone; and chain carboxylic acid esters such as methyl propionate and methyl butyrate can be used.
Moreover, you may use a fluorine-containing carbonate as a carbonate type solvent. As said fluorine-containing carbonate, a fluorine-containing cyclic carbonate or a fluorine-containing linear carbonate is mentioned.
As the fluorine-containing cyclic carbonate, fluoroethylene carbonate, 4,5-difluoroethylene carbonate, 4,4-difluoroethylene carbonate, trifluoroethylene carbonate, tetrafluoroethylene carbonate, 4-fluoro-5-methylethylene carbonate, difluoromethylethylene Carbonate, bis (fluoromethyl) ethylene carbonate, bis (difluoromethyl) ethylene carbonate, bis (trifluoromethyl) ethylene carbonate, fluoroethyl ethylene carbonate, difluoroethyl ethylene carbonate, trifluoroethyl ethylene carbonate, tetrafluoroethyl ethylene carbonate, etc. It can be mentioned.
Examples of fluorine-containing chain carbonates include monofluorodimethyl carbonate, monofluorodiethyl carbonate, monofluoroethyl methyl carbonate, bis (monofluoromethyl) carbonate, bis (monofluoroethyl) carbonate, trifluorodimethyl carbonate, trifluorodiethyl carbonate, Trifluoroethyl methyl carbonate etc. are mentioned. By using a fluorine-containing carbonate (for example, a fluorine-containing cyclic carbonate or a fluorine-containing chain carbonate) as described above in combination with the electrolyte (1) of the present invention, a good film is formed on the electrode and the electrode is degraded The mode of using these fluorine-containing carbonates (for example, fluorine-containing cyclic carbonate or fluorine-containing linear carbonate) as a solvent is one of the preferable embodiments of the present invention, since the effect to be suppressed can be further expected. When these fluorine-containing carbonates are used as a solvent, the concentration of the fluorine-containing carbonate in the non-aqueous electrolyte is 0.1% by mass to 30% by mass from the viewpoint of the viscosity, ion conductivity, etc. of the non-aqueous electrolyte It is preferably 0.1% by mass to 10% by mass. Moreover, you may use 2 or more types of these fluorine-containing carbonates.
Among the above-mentioned carbonate solvents, it is preferable to use a mixture of linear carbonates and cyclic carbonates. Among cyclic carbonates, ethylene carbonate and propylene carbonate are preferred, and ethylene carbonate is more preferred. Among the linear carbonates, dimethyl carbonate and ethyl methyl carbonate are preferred, and more preferably ethyl methyl carbonate.

  The concentration of the carbonate solvent in the non-aqueous electrolytic solution of the present invention is preferably 70% by mass to 95% by mass, more preferably 75% by mass to 95% by mass, and still more preferably 80% by mass to 90% by mass.

<Additives>
The non-aqueous electrolyte solution of the lithium secondary battery according to the present invention contains an additive other than the electrolyte (1), the carbonate-based solvent, and the electrolyte (2) optionally to improve high-temperature storage characteristics and cycle characteristics and improve safety. You may include it. Additives include cyclic carbonates having unsaturated bonds such as vinylene carbonate (VC), vinyl ethylene carbonate (VEC), methyl vinylene carbonate (MVC), ethyl vinylene carbonate (EVC), etc .; such as phenyl ethylene carbonate and erythritan carbonate Carbonate compounds; succinic anhydride, glutaric anhydride, maleic anhydride, citraconic anhydride, glutaconic anhydride, itaconic anhydride, diglycolic anhydride, cyclohexanedicarboxylic acid anhydride, cyclopentanetetracarboxylic acid dianhydride, phenylsuccinic acid Carboxylic anhydrides such as anhydrides; ethylene sulfite, 1,4-butane sultone, methyl methanesulfonate, busulfan, sulfolane, sulfolene, dimethyl sulfone, tetramethylthiuram monos Sulfur-containing compounds such as fide; 1-methyl-2-pyrrolidinone, 1-methyl-2-piperidone, 3-methyl-2-oxazolidinone, 1,3-dimethyl-2-imidazolidinone, N-methylsuccinimide and the like Nitrogen compounds; phosphates such as monofluorophosphate and difluorophosphate; and hydrocarbon compounds such as heptane, octane and cycloheptane. When these additives are used in the non-aqueous electrolytic solution, the concentration is preferably 0.1% by mass to 5% by mass, and two or more kinds of additives may be used.

<Negative electrode>
The negative electrode is a negative electrode current collector on which a negative electrode active material composition containing a negative electrode active material, a dispersing solvent, a binder, a thickener, and, if necessary, a conductive auxiliary agent is supported by a negative electrode current collector. It is a state.

  As a material of the negative electrode current collector, conductive metals such as copper, iron, nickel, silver, stainless steel (SUS) and the like can be used, but 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 by a conventionally well-known lithium secondary battery can be used, and it should just be a thing which can occlude / discharge lithium ion. Specifically, graphite materials such as graphite, artificial graphite and natural graphite, mesophase fired bodies made from coal and petroleum pitch, carbon materials such as non-graphitizable carbon, Si, Si alloys, silicon Si such as SiO, etc. A negative electrode material, a Sn-based negative electrode material such as a Sn alloy, or a lithium alloy such as lithium metal or a lithium-aluminum alloy can be used.

  In addition, as a manufacturing method of a negative electrode, the method similar to the manufacturing method of a positive electrode is employable. As the conductive auxiliary agent, the binder, the solvent for dispersing the material, and the like that can be used for the negative electrode, the same ones as those used for the positive electrode can be used.

  In addition, a thickener may be used in preparation of the negative electrode active material composition, and the thickener is a non-ionic polymer used to control the negative electrode active material composition to a viscosity that allows application on the current collector. Is preferred. Examples of the thickener include polyethylene glycols, celluloses, polyacrylamides, etc. Among them, polyethylene glycols, celluloses such as carboxymethyl cellulose (CMC), etc. are preferable, and suitable viscosity can be given. Particularly preferred is carboxymethylcellulose (CMC), which also has good dispersibility.

<Separator>
The separator is disposed to separate the positive and negative electrodes. As a separator, it should not be restrict | limited in particular, A conventionally well-known thing can be used. For example, a porous sheet (for example, a polyolefin-based fine porous separator or a cellulose-based separator) made of a polymer that absorbs and holds a non-aqueous electrolytic solution, a nonwoven fabric separator, a porous metal body and the like can be mentioned. Among them, a microporous polyolefin separator having a property of being chemically stable to an organic solvent is preferable.

  Examples of the material of the porous sheet include polyethylene, polypropylene, and a laminate having a three-layer structure of polypropylene / polyethylene / polypropylene. From the viewpoint of high oxidation voltage and high voltage, a polypropylene (PP) separator is preferable.

  Examples of the material of the non-woven separator include cotton, rayon, acetate, nylon, polyester, polypropylene, polyethylene, polyimide, aramid, glass and the like, and may be used alone or in accordance with the mechanical strength required for the non-aqueous electrolyte layer. It mixes and is used.

  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 a non-aqueous electrolytic solution. The shape of the lithium secondary battery according to the present invention is arbitrary, and may be, for example, any of cylindrical, square, laminate, coin and large.

  In the lithium secondary battery of the present invention, the elution amount of the Mn component from the positive electrode is preferably less than 300 ppm, more preferably 250 ppm or less, still more preferably 200 ppm or less, still more preferably 190 ppm or less, still more preferably 180 ppm or less Preferably it is 150 ppm or less, most preferably 140 ppm or less. In such a range, the elution of the Mn component from the positive electrode is suppressed, the deterioration of the electrode can be prevented, the increase in internal resistance can be suppressed, and the discharge voltage can be maintained at a high value. As a result, high temperature storage characteristics and cycle characteristics can be improved. The amount of elution of the Mn component from the positive electrode can be measured by the method described later.

  In the lithium secondary battery of the present invention, the 0.2 C recovery capacity when stored at 60 ° C. for 3 months is preferably 40% or more, more preferably 45% or more, and still more preferably 50% or more. The 0.2 C recovery capacity can be measured by the method described later.

  In the lithium secondary battery of the present invention, the 1 C recovery capacity when stored at 60 ° C. for 3 months is preferably 22% or more, more preferably 25% or more, and still more preferably 30% or more. The 1 C recovery capacity can be measured by the method described later.

  In the lithium secondary battery of the present invention, the cell OCV (open circuit voltage) when stored at 60 ° C. for 3 months is preferably 3.0 V or more, more preferably 3.1 V or more, still more preferably 3.2 V or more . The cell OCV can be measured by the method described later.

  In the lithium secondary battery of the present invention, the internal resistance increase rate (IR increase rate) when stored at 60 ° C. for 3 months is preferably less than 400%, more preferably 300% or less, still more preferably 250% or less, particularly Preferably it is 200% or less. In addition, the said internal resistance increase rate can be measured by the below-mentioned method.

  EXAMPLES Hereinafter, the present invention will be more specifically described by way of examples. However, the present invention is of course not limited by the following examples, and appropriate modifications may be made as long as the present invention can be applied to the purpose. Of course, implementation is also possible, and all of them are included in the technical scope of the present invention.

1. Preparation of Nonaqueous Electrolyte Preparation Example 1 of Nonaqueous Electrolyte
Lithium bis (fluorosulfonyl) imide (LiFSI, 0.94 g (5 mmol) synthesized by a conventionally known method) as a compound (electrolyte (1)) represented by the general formula (1), hexafluorophosphorus as the electrolyte (2) 0.76 g (5 mmol) of lithium lithium (LiPF ₆ , manufactured by Kishida Chemical Co., Ltd., LBG grade) is measured in a 10 mL measuring flask, and ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (both manufactured by Kishida Chemical Co., Ltd.) , And a volume ratio of LBG grade) was 3/7 (EC / EMC) to prepare a non-aqueous electrolyte 1.

Nonaqueous Electrolyte Preparation Example 2
In the same manner as in Nonaqueous Electrolyte Preparation Example 1 except that 0.75 g (4 mmol) of LiFSI as described above was used as the electrolyte (1) and 0.91 g (6 mmol) of LiPF ₆ as described above as the electrolyte (2). Nonaqueous electrolyte 2 was prepared.

Nonaqueous Electrolyte Preparation Example 3
In the same manner as in Nonaqueous Electrolyte Preparation Example 1, except that 0.56 g (3 mmol) of LiFSI as described above was used as the electrolyte (1) and 1.06 g (7 mmol) of LiPF ₆ as described above as the electrolyte (2). Nonaqueous electrolyte 3 was prepared.

Nonaqueous Electrolyte Preparation Example 4
In the same manner as in Nonaqueous Electrolyte Preparation Example 1 except that 0.37 g (2 mmol) of LiFSI as described above was used as the electrolyte (1) and 1.22 g (8 mmol) of LiPF ₆ as described above as the electrolyte (2). Nonaqueous electrolyte 4 was prepared.

Nonaqueous Electrolyte Preparation Example 5
In the same manner as in Nonaqueous Electrolyte Preparation Example 1 except that 0.19 g (1 mmol) of LiFSI as described above was used as the electrolyte (1) and 1.37 g (9 mmol) of LiPF ₆ as described above as the electrolyte (2). The non-aqueous electrolyte solution 5 was prepared.

Nonaqueous Electrolyte Preparation Example 6
The non-aqueous electrolyte 6 was prepared in the same manner as in Non-aqueous Electrolyte Preparation Example 1 except that 1.52 g (10 mmol) of LiPF _{6 described above} was used as the electrolyte (2) without using the electrolyte (1).

2. Preparation of Laminated Cell and Charging / Discharging Before Testing A positive electrode sheet (positive electrode active material: LiMn ₂ O ₄ , binder: PVDF, conductive additive: acetylene black 96) having an area of the positive electrode active material layer of 12 cm ² (3 cm × 4 cm): The mixture is kneaded at a ratio of 5: 4, dissolved in NMP solvent, slurried at 60% solids, coated on aluminum foil, dried on a hot plate at 100 ° C., and then fixed density with a roll press machine 1 sheet of ³ mAh / cm ² ) and a negative electrode active material layer area of 13.44 cm ² (3.2 cm × 4.2 cm) (negative electrode active material: artificial graphite, binder) : SBR, thickener: CMC, conductive auxiliary: carbon black is mixed at a ratio of 95.7: 2: 1.8: 0.5, dissolved in pure water and slurried at a solid content of 48% , Coated on copper foil, 00 ° C. After drying on a hot plate, a roll press machine .3.2mAh / cm ² which is manufactured by the press until a constant density) laminating the one to face, one of the polyolefin in the meantime A system separator (polyethylene 20 μm) was sandwiched. The positive electrode and the negative electrode sheet were sandwiched between two aluminum laminate films, and the inside of the aluminum laminate film was filled with each of the non-aqueous electrolytes 1 to 6 and sealed in a vacuum state to produce a laminate cell.

  Using a charge / discharge test apparatus (ACD-01, manufactured by Aska Electronics Co., Ltd.), charge / discharge speed was 0.2 C (constant current mode) at 30 ° C. and charge / discharge was performed once at 3.5 V to 4.2 V After that, the laminate cell was opened and sealed again under vacuum. Under the same conditions, charge and discharge were repeated 5 times to complete a laminate type lithium battery corresponding to the non-aqueous electrolyte.

3. Mn component dissolution test For each laminated lithium battery prepared using non-aqueous electrolytes 1 to 6, 4.2 V constant current measurement at 0.5 C under conditions of 30 ° C. using the above-mentioned charge and discharge test apparatus After 5 hours of voltage charging, only the positive electrode was taken out. Separately, an electrolytic solution having the same composition as that of the non-aqueous electrolytic solution used in the preparation of each battery was separately prepared, and each positive electrode taken out was put in each electrolytic solution, and left at 60 ° C. for 30 days. The electrolyte solution after standing was diluted with ultrapure water to 1% concentration to conduct ICP analysis (measuring device: ICP emission analyzer ICPE-9000 manufactured by Shimadzu Corporation). The results are shown in Table 1.

From the results in Table 1, it is understood that the Mn component elution is suppressed in the example in which LiFSI and LiPF ₆ are used in combination as compared with the example in which LiFSI and LiPF ₆ are used in combination and the example in which LiFSI is not used. In addition, it can be seen that when the concentration of LiFSI is increased, the elution of the Mn component is further suppressed. It is considered that this is because a film is formed on the positive electrode by LiFSI, the deterioration of the positive electrode is suppressed, and the elution of the Mn component from the positive electrode is suppressed.

4. High temperature storage characteristic test 4-1. Capacity retention rate Each laminated lithium battery obtained using non-aqueous electrolytes 1 to 3 was subjected to 4.2 V constant current constant voltage charging at 1 C at 30 ° C. using the above-mentioned charge / discharge test device. The discharge is performed with time and a 2.75 V cutoff at 0.2 C, and the discharge capacity at this time is defined as an initial discharge capacity (A). Thereafter, for each laminate type lithium battery, 4.2 V constant current constant voltage charging at 1 C at 30 ° C. is performed for 3 hours, and each of 0.5, 1.0, 2.0 and 3.0 months at 60 ° C. Stored at. Thereafter, after 2.75 V cutoff discharge at 30 ° C. and 0.2 C, 4.2 V constant current constant voltage charging at 1 C for 3 hours and 2.75 V cutoff discharge at 0.2 C Did. The discharge capacity at this time is taken as the discharge capacity (B) after high temperature storage. The capacity retention rate before and after high temperature storage was calculated by the following equation, and the results are shown in FIG. 1 and Table 2.
Capacity retention rate = Discharge capacity after high temperature storage (B) / Initial discharge capacity (A) × 100
The capacity retention rate before and after high temperature storage was calculated under the same conditions as above except that the discharge rate at the time of measurement of the discharge capacity was 1 C. The results are shown in FIG. 2 and Table 3.

From the results of FIGS. 1 and 2 and Tables 2 and 3, compared with the example using LiFSI and LiPF ₆ in combination and the example not using LiFSI, the example using LiFSI and LiPF ₆ in combination improves the capacity retention rate. I understand that. This is because when LiFSI is contained in the non-aqueous electrolyte, a film is formed on the positive electrode, and the elution of the Mn component from the positive electrode is suppressed, the deterioration of the electrode (positive electrode) is prevented, and the decrease in capacity is suppressed. It is believed that there is.

4-2. Cell OCV (open voltage)
For each laminated lithium battery obtained using non-aqueous electrolytes 1 to 6, 4.2 V constant current constant voltage charging at 1 C at 30 ° C. is performed for 3 hours, and the battery is performed for 6 hours or more and 24 hours after the end of charging The circuit voltage was measured (measuring device: low resistance meter manufactured by Tsuruga Denki Co., Ltd. Model No. 3566). After measurement, store for 3.0 months at 60 ° C, return to room temperature at 0.25, 0.5, 1.0, and 2.0 months in storage and leave for 3 hours, then measure battery circuit voltage did. The results are shown in Table 4 and FIG.

The results in FIG. 3 and Table 4 show that the cell OCV is improved in the example in which LiFSI and LiPF ₆ are used in combination as compared with the example in which LiFSI and LiPF ₆ are used in combination and the example in which LiFSI is not used. This is because when LiFSI is contained in the non-aqueous electrolyte, a film is formed on the positive electrode, and the elution of the Mn component from the positive electrode can be suppressed, the deterioration of the electrode (positive electrode) can be suppressed, and the discharge voltage becomes high. It is considered to be because it can be maintained.

4-3. Impedance Each laminated lithium battery obtained using non-aqueous electrolytes 1 to 3 was subjected to 4.2 V constant current constant voltage charging at 1 C for 3 hours at 30 ° C. for 6 hours to 24 hours after the end of charging The alternating current impedance was measured at 1 KHz (measuring device: manufactured by Tsuruga Denki Co., Ltd., low resistance meter, model number 3566). After measurement, it was stored at 60 ° C. for 3.0 months, and 1 KHz AC impedance was measured at 0.5, 1.0, and 2.0 months after storage. The measurement was carried out after returning each battery to the room temperature atmosphere and leaving it for 3 hours. The results are shown in Table 5 and FIG.

From the results in FIG. 4 and Table 5, it is understood that the increase in internal resistance is suppressed in the example in which LiFSI and LiPF ₆ are used in combination, as compared with the example in which LiFSI and LiPF ₆ are used in combination and the example in which LiFSI is not used. . This is because when LiFSI is contained in the non-aqueous electrolytic solution, a film is formed on the positive electrode, the elution of the Mn component from the positive electrode is suppressed, the deterioration of the positive electrode is suppressed, and the increase in internal resistance can be suppressed. Conceivable.

5. Cycle Characteristic Test For each laminate type lithium battery prepared using non-aqueous electrolytes 1 to 6, charging of 4.2 V constant current constant voltage at 1 C at 30 ° C. is started using the charge / discharge test apparatus. The battery was charged to a 0.02 C cutoff and discharged at 2.75 V cutoff at 1 C to conduct a cycle test. After each charge and discharge, a charge and discharge rest time of 10 minutes was provided. The results are shown in FIG.

From the results of FIG. _{5, LiFSI 0.5mol + LiPF 6 0.5mol} , LiFSI 0.4mol + LiPF 6 0.6mol, LiFSI 0.3mol + LiPF 6 0.7mol, LiFSI 0.2mol + LiPF 6 0.8mol, LiFSI 0.1mol + LiPF 6 0.9mol In the case of using the electrolyte composition (non-aqueous electrolytes 1 to 5) of the above, it is understood that the cycle characteristics are improved as compared with 1 mol of LiPF ₆ (non-aqueous electrolyte 6). This is because when LiFSI is contained in the non-aqueous electrolyte, a film is formed on the electrode, and elution of the Mn component from the positive electrode is suppressed, and deterioration of the positive electrode is suppressed. It is thought that the cycle performance is improved because the decomposition above is suppressed.

Claims (3)

The element M is at least one element selected from the group consisting of Ni, Co, Al, Si, Zr, Ti, Fe, Mg and V, which is represented by the general formula Li _x Mn _a M _b O _y ; Lithium transition metal complex oxide satisfying 9 ≦ (a + b) ≦ 2.1, 0 <x ≦ 2, 1 ≦ a ≦ 3, 0 ≦ b ≦ 1.0, 3 ≦ y ≦ 4.5 as a positive electrode active material Containing positive electrode,
1. A lithium secondary battery comprising a negative electrode containing a negative electrode active material capable of absorbing and desorbing lithium ions, and a non-aqueous electrolyte,
The non-aqueous electrolytic solution contains lithium bis (fluorosulfonyl) imide as an electrolyte (1) and a carbonate-based solvent, and the non-aqueous electrolytic solution further contains an electrolyte (2) as an electrolyte, and the electrolyte (2) Is LiPF ₆ , and
The concentration of the electrolyte (1) contained in the non-aqueous electrolytic solution is 0.1 mol / L or more,
The lithium secondary battery, wherein the concentration of the electrolyte (2) contained in the non-aqueous electrolytic solution is 10 to 75 mol% with respect to the total amount of the electrolyte (1) and the electrolyte (2). The lithium secondary battery according to claim 1 , wherein the carbonate-based solvent contains cyclic carbonates and linear carbonates. The lithium transition metal complex oxide, a lithium secondary battery according to claim 1 or 2 which is LiMn ₂ O ₄ (lithium manganate).