JP6113496B2 - Lithium secondary battery - Google Patents

Description

  The present invention relates to a positive electrode containing a lithium transition metal composite oxide as a positive electrode active material, a negative electrode containing a negative electrode active material capable of occluding and releasing lithium ions, a non-aqueous electrolyte and a lithium secondary battery using a solvent.

  Lithium secondary batteries have high energy density, so they are used as power sources for mobile communication devices, power sources for portable information terminals, etc., and the market is growing rapidly with the spread of terminals. Various studies have been conducted for the purpose of improving characteristics, high rate characteristics, energy density, low temperature characteristics, high temperature storage characteristics, and the like.

  In recent years, along with the downsizing and weight reduction of electronic devices or the development of electric vehicles, development of large capacity, high output lithium secondary batteries has been desired. For example, for non-aqueous electrolytes of lithium secondary batteries The use of ionic liquids as flame retardant liquids has been studied.

Patent Document 1 discloses a lithium secondary battery including a non-aqueous electrolyte containing a lithium salt and a negative electrode of non-graphitizable carbon, but is not a satisfactory cycle characteristic.
Patent Document 2 discloses a lithium battery having an electrolyte obtained by solidifying a nonaqueous electrolytic solution containing a lithium salt with a crosslinkable polymer, but the cycle characteristics are not sufficient due to solidification of the solvent.
Patent Document 3 discloses a lithium secondary battery that is an ionic liquid containing a lithium salt, but the cycle characteristics are not satisfactory.
Patent Document 4 discloses that a lithium secondary battery using an ionic liquid is a non-woven fabric made of fibers such as organic fibers and glass fibers, and has a porosity of 70% or more. The purpose is to improve battery output characteristics and cycle characteristics.
Patent Document 5 discloses a lithium secondary battery using a non-aqueous electrolyte solution containing a lithium salt and a negative electrode containing a Si—C composite as a negative electrode active material, and aims to improve high capacity retention.
Patent Document 6 discloses a non-aqueous electrolyte battery using a positive electrode using a positive electrode active material of a lithium-containing phosphate compound and a non-aqueous electrolyte containing a chain ether and a cyclic carbonate. Such a combination does not have sufficient cycle characteristics.
Patent Document 7 discloses that a secondary battery module using a plurality of electrolytes in a non-aqueous electrolyte exhibits safe properties even when overcharged under a voltage of several tens of volts.

JP 2009-26542 A JP 2009-277413 A JP 2007-207675 A JP 2010-287380 A JP 2010-97922 A JP 2011-96643 A JP 2011-82033 A

  In various studies as described above, for example, a lithium secondary battery using a combination of a carbon negative electrode and an ionic liquid has higher internal resistance and output characteristics than an organic solvent battery. There is a problem that the cycle characteristics and the high rate characteristics are insufficient. Because of these problems, a lithium ion battery using an ionic liquid has not been put into practical use at this stage.

  The present invention has been made paying attention to the above situation, and an object of the present invention is to provide a lithium secondary battery having improved cycle characteristics, high rate characteristics, and low temperature characteristics. Another object of the present invention is to provide a lithium secondary battery in which elution of metal components in an uncharged state is suppressed.

  In the lithium secondary battery, the present inventors use a specific electrolyte in the non-aqueous electrolyte so that a film is formed on the electrode, suppressing an increase in internal resistance, and maintaining the discharge voltage at a high value. At the same time, by using a specific solvent, it was found that the solvent is difficult to be decomposed and stable when a voltage is applied, and the present invention has been completed.

That is, the lithium secondary battery of the present invention is
Is represented by the general formula _{Li x Ni a Co b Mn c} M d O y, element M is at least one element Al, Si, Zr, Ti, Fe, selected from the group consisting of Mg and V, 0. 9 ≦ (a + b + c + d) ≦ 1.1, 1.0 ≦ x ≦ 1.3, 0 <a ≦ 0.8, 0 ≦ b ≦ 0.5, 0 ≦ c ≦ 0.5, 0 ≦ d ≦ 0. 5, a positive electrode containing a lithium transition metal composite oxide satisfying 1.9 ≦ y ≦ 2.1 as a positive electrode active material,
A lithium secondary battery comprising a negative electrode containing a negative electrode active material capable of inserting and extracting lithium ions, and a non-aqueous electrolyte,
The non-aqueous electrolyte is the general formula (1); (XSO ₂ ) (X′SO ₂ ) N ⁻ Li ⁺ as the electrolyte (1).
(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). And a carbonate-based solvent.

The non-aqueous electrolyte further includes an electrolyte (2) as an electrolyte, and the electrolyte (2)
LiPF _e (C _m F _{2m + 1} ) _6-e (0 ≦ e ≦ 6, 1 ≦ m ≦ 2),
LiBF _f (C _n F _{2n + 1} ) _4-f (0 ≦ f ≦ 4, 1 ≦ n ≦ 2) and at least one lithium salt selected from the group consisting of LiAsF ₆ are preferred.

  At this time, it is preferable that the density | concentration of the electrolyte (1) contained in the said non-aqueous electrolyte is 30-99.9 mol% with respect to the total amount of electrolyte (1) and electrolyte (2).

The embodiment in which the electrolyte (2) is LiPF ₆ and the embodiment in which the carbonate solvent contains a cyclic carbonate are preferred embodiments of the present invention.

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

  According to the present invention, it is possible to provide a lithium secondary battery having improved cycle characteristics, high rate characteristics, and low temperature characteristics. In addition, it is possible to provide a lithium secondary battery in which elution of metal components in an uncharged state is suppressed.

FIG. 1 is a diagram showing the improvement of cycle characteristics by the lithium secondary battery of the present invention (the horizontal axis indicates the number of cycles, and the vertical axis indicates the discharge retention rate). FIG. 2 is a diagram showing improvement in high rate characteristics by the lithium secondary battery of the present invention at 25 ° C. (the horizontal axis indicates capacity, and the vertical axis indicates voltage). FIG. 3 is a graph showing improvement in high rate characteristics and low temperature characteristics by the lithium secondary battery of the present invention at −30 ° C. (the horizontal axis indicates capacity and the vertical axis indicates voltage).

<Lithium secondary battery>
The lithium secondary battery of the present invention is
Is represented by the general formula _{Li x Ni a Co b Mn c} M d O y, element M is at least one element Al, Si, Zr, Ti, Fe, selected from the group consisting of Mg and V, 0. 9 ≦ (a + b + c + d) ≦ 1.1, 1.0 ≦ x ≦ 1.3, 0 <a ≦ 0.8, 0 ≦ b ≦ 0.5, 0 ≦ c ≦ 0.5, 0 ≦ d ≦ 0. 5, a positive electrode containing a lithium transition metal composite oxide satisfying 1.9 ≦ y ≦ 2.1 as a positive electrode active material,
A lithium secondary battery comprising a negative electrode containing a negative electrode active material capable of inserting and extracting lithium ions, and a non-aqueous electrolyte,
The non-aqueous electrolyte is the general formula (1); (XSO ₂ ) (X′SO ₂ ) N ⁻ Li ⁺ as the electrolyte (1).
(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). And a carbonate-based solvent.
According to such a lithium secondary battery, when the non-aqueous electrolyte contains the electrolyte (1), a film is formed on the electrode even at a low temperature or normal temperature, and the deterioration of the electrolyte is suppressed. The discharge voltage can be maintained at a high value. In addition, when the non-aqueous electrolyte contains a carbonate-based solvent, the solvent is hardly decomposed when a voltage is applied, and the stability can be improved. Furthermore, when the non-aqueous electrolyte further contains the electrolyte (2), corrosion of the current collector can be suppressed, and thus the cycle characteristics of the lithium ion battery can be improved.

<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 Ni a Co b Mn c} M d O y, the element M consists of Al, Si, Zr, Ti, Fe, Mg and V At least one element selected from the group, 0.9 ≦ (a + b + c + d) ≦ 1.1, 1.0 ≦ x ≦ 1.3, 0 <a ≦ 0.8, 0 ≦ b ≦ 0.5, A lithium transition metal composite oxide satisfying 0 ≦ c ≦ 0.5, 0 ≦ d ≦ 0.5, and 1.9 ≦ y ≦ 2.1 is contained. Such a lithium transition metal composite oxide is suitably used as a positive electrode active material because of its high electronic conductivity, high cycle characteristics and high rate characteristics. The lithium transition metal composite oxide can be used alone or in combination of two or more.

  In the general formula, 0.9 ≦ (a + b + c + d) ≦ 1.1, and from the viewpoint that the lithium secondary battery exhibits suitable characteristics, preferably 0.95 ≦ (a + b + c + d) ≦ 1.05, and more preferably (A + b + c + d) = 1.0.

  In the above general formula, 1.0 ≦ x ≦ 1.3, preferably 1.0 ≦ x ≦ 1.2, more preferably 1.0 ≦ x ≦ 1.1, and further preferably x = 1. If x is less than 1.0, the capacity may be substantially reduced, and if x is greater than 1.3, the initial irreversible capacity may increase and efficiency may be reduced.

  In the general formula, 0 <a ≦ 0.8, preferably 0 <a ≦ 0.7, more preferably 0 <a ≦ 0.6, and still more preferably 0 <a ≦ 0.5. If a is 0, the thermal stability during charging of the oxide may be reduced, and if a is greater than 0.8, the capacity of the positive electrode active material may be reduced.

  In the above general formula, 0 ≦ b ≦ 0.5, preferably 0 ≦ b ≦ 0.45, and more preferably 0 ≦ b ≦ 0.4. If b is larger than 0.5, the capacity of the positive electrode active material may be reduced.

  In the general formula, 0 ≦ c ≦ 0.5, preferably 0 ≦ c ≦ 0.45, and more preferably 0 ≦ c ≦ 0.4. When c is larger than 0.5, the capacity of the positive electrode active material may be reduced.

  In the above general formula, 0 ≦ d ≦ 0.5, preferably 0 ≦ d ≦ 0.4, more preferably 0 ≦ d ≦ 0.3, and further preferably 0 ≦ d ≦ 0.2. Yes, more preferably 0 ≦ d ≦ 0.1, and even more preferably d = 0. In the above general formula, 1.9 ≦ y ≦ 2.1, preferably 1.9 ≦ y ≦ 2.0, and more preferably y = 2.

Examples of the lithium transition metal composite oxide represented by the general formula Li _x Ni _a Co _b Mn _c M _d O _y include LiNi _1/3 Co _1/3 Mn _1/3 O _{2 and} LiNi _0.5 Co _0.3 Mn _0.2 O. ₂ and LiNi _0.5 Co _0.2 Mn _0.3 O ₂ .

Wherein the general formula _{Li x Ni a Co b Mn c} M d O surface of the positive electrode active material of lithium transition metal composite oxide represented by _y, may also be coated with a specific material doped and / or surface-doped Examples of substances used for coating on the surface include Al, Si, Zr, Ti, Fe, Mg, and V.

The positive electrode active material is mainly composed of a lithium transition metal composite oxide represented by the Li _x Ni _a Co _b Mn _c M _d O _y . The positive electrode active material, the Li _x Ni _a Co _b Mn _c lithium transition metal composite oxide represented by M _d O _y, preferably from 50 to 100 wt%, more preferably 70 to 100 wt%, more preferably 90 Contains ~ 100% by mass.

In the present invention, as a positive electrode active material, a compound having an olivine structure such as lithium cobaltate, lithium nickelate, lithium manganate, LiAPO ₄ (A = Fe, Mn, Ni, Co), as long as the effects of the present invention are exhibited. Solid solution materials incorporating a plurality of transition metals (electrochemically inactive layered Li ₂ MnO ₃ and electrochemically active layered LiMO ₂ (transition metals such as M = Co, Ni, Fe)) (Solid solution) or the like may be used. These may be used alone or in combination.

  The positive electrode active material only needs to be manufactured by a manufacturing method known to those skilled in the art. Examples of the manufacturing method include mixing a lithium raw material, a manganese raw material, a nickel raw material, a cobalt raw material, and an oxide raw material thereof. Examples thereof include a method of performing wet pulverization, granulating and drying, baking, and classifying as necessary.

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 dispersing solvent, and the like is supported on a positive electrode current collector.
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 dried after being immersed; 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, 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-walled carbon nanotube, multi-walled carbon nanotube, and vapor grown carbon fiber.

  As binder (binder), fluorocarbon resins such as polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyhexafluoropropylene and copolymers thereof; styrene-butadiene rubber (SBR), nitrile Synthetic rubber such as butadiene rubber; Polyamide resin such as polyamideimide; Polyolefin resin such as polyethylene and polypropylene; Poly (meth) acrylic resin; Polyacrylic acid; Cellulose resin such as carboxymethylcellulose (CMC) . 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 amounts of the conductive auxiliary agent and 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 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 in the lithium secondary battery of the present invention is a compound represented by the general formula (1); (XSO ₂ ) (X′SO ₂ ) N ⁻ Li ⁺ , wherein X and X ′ are fluorine An atom, an alkyl group having 1 to 6 carbon atoms or a fluoroalkyl group having 1 to 6 carbon atoms is represented, and at least one of X and X ′ is a fluorine atom. } (Electrolyte (1)). By including the compound represented by the general formula (1) in the electrolyte, a film is formed on the electrode even at a low temperature or normal temperature, and the deterioration of the electrode and the decomposition of the nonaqueous electrolytic solution can be suppressed. The rise in resistance can be suppressed and the discharge voltage can be maintained at a high value. Moreover, the elution of metal components from the current collector and the positive electrode active material can be suppressed even after storage at 20 ° C. for one month in an uncharged state. In addition, the non-aqueous electrolytic solution essentially includes an electrolyte (1) and a carbonate-based solvent, and may further include an electrolyte (2) described later, but triethylammonium, ethylmethylimidazolium, butyl, which are usually used for ionic liquids. Methyl imidazolium, 1-methyl-1-propylpyrrolidinium, methylpropylpiperidinium and the like are preferably not included. The electrolyte (1) may have a conductivity and a thermal decomposition temperature higher than those of the electrolyte (2) described later. When used in combination with a carbonate-based solvent, the electrolyte (1) exhibits conductivity similar to room temperature even at low temperatures. Anything is acceptable.

  In the general formula (1), 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. Of the alkyl group or fluoroalkyl group, a trifluoromethyl group and a 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. More preferable compounds represented by the general formula (1) are lithium bis (fluorosulfonyl) imide, lithium (fluorosulfonyl) (trifluoromethylsulfonyl) imide, lithium (fluorosulfonyl) (pentafluoroethylsulfonyl) imide, Lithium bis (fluorosulfonyl) imide (LiFSI) is preferred.

  In the nonaqueous electrolytic solution in the lithium secondary battery of the present invention, the electrolyte (1) may be a main electrolyte, or another electrolyte may be a main electrolyte. However, when other electrolyte is a main electrolyte, it will be described later. The electrolyte (2) to be used may be the main electrolyte. From the viewpoint of improving high rate characteristics, cycle characteristics, and low temperature characteristics, it is preferable to use the electrolyte (1) as a main electrolyte. In addition, the “main electrolyte” means at least 50 mol% or more of the total electrolyte content contained in the nonaqueous electrolytic solution.

  The concentration of the compound (electrolyte (1)) represented by the general formula (1) in the non-aqueous electrolyte in the lithium secondary battery of the present invention is 0.1 mol when the electrolyte (1) is the main electrolyte. / L or more and the saturation concentration or less is preferable. More preferably, it is 0.1 mol / L to 2.5 mol / L, further preferably 0.3 mol / L to 2 mol / L, particularly preferably 0.4 mol / L to 1.5 mol / L, most preferably 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 is increased and the electric conductivity may be lowered, and the battery performance may not be sufficiently exhibited. If the concentration of the electrolyte (1) is low, the amount of ions There is a tendency that the electric conductivity of the electrolytic solution becomes 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, and even more preferably. Is 0.1 mol / L to 0.7 mol / L. When the concentration of the electrolyte (1) is less than 0.01 mol / L, film formation on the electrode is not sufficient, and desired battery performance may not be obtained. When the concentration of the electrolyte (1) exceeds 2 mol / L, the viscosity of the non-aqueous electrolyte is increased and the electric conductivity is lowered, 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 includes an electrolyte (2) as an electrolyte, and the electrolyte (2)
LiPF _e (C _m F _{2m + 1} ) _6-e (0 ≦ e ≦ 6, 1 ≦ m ≦ 2),
LiBF _f (C _n F _{2n + 1} ) _4-f (0 ≦ f ≦ 4, 1 ≦ n ≦ 2) and at least one lithium salt selected from the group consisting of LiAsF ₆ are preferred.

In LiPF _e (C _m F _{2m + 1} ) _6-e , e = 6 is preferable, and in LiBF _f (C _n F _{2n + 1} ) _4-f , f = 4 is preferable. These electrolytes (2) have large dissociation constants in the electrolytic solution and generate anions that 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, LiPF ₆ and LiBF ₄ are preferable, and LiPF ₆ is more preferable. The electrolyte (2) may be used alone or in combination of two or more.

  The electrolyte (2) has a large dissociation constant in the electrolytic solution, exhibits an appropriate electrical conductivity, and can suppress deterioration of the current collector. When the electrolyte (2) is used in combination with the electrolyte (1), the electrical conductivity can be further increased and the performance of the lithium secondary battery can be improved.

In the non-aqueous electrolyte of the lithium secondary battery of the present invention, the electrolyte (2) may be used as a main electrolyte. From the viewpoint of improving high rate characteristics, cycle characteristics and low temperature characteristics, the electrolyte (1) is mainly used. It is preferable to use it as an electrolyte.
When the electrolyte (2) is the main electrolyte, the concentration of the electrolyte (2) in the nonaqueous electrolytic solution is preferably 0.1 mol / L or more and not more than the saturation concentration. The concentration of the electrolyte (2) in the nonaqueous electrolytic solution is more preferably 0.1 mol / L to 2.5 mol / L, still more preferably 0.3 mol / L to 2 mol / L, and particularly preferably 0.4 mol / L. ˜1.5 mol / L, most preferably 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 is increased and the electrical conductivity may be lowered, and the battery performance may not be sufficiently exhibited. If the concentration of the electrolyte (2) is low, The amount tends to decrease, and the electric conductivity of the electrolytic solution may be insufficient.

  When the main electrolyte other than the electrolyte (2) is used, the concentration of the electrolyte (2) in the non-aqueous electrolyte is preferably 0.01 mol / L to 2 mol / L, more preferably 0.05 mol / L to 1 mol / L. L, more preferably 0.1 mol / L to 0.5 mol / L. When the concentration of the electrolyte (2) is less than 0.01 mol / L, film formation on the electrode is not 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 electrolyte is increased and the electric conductivity is lowered, and the battery performance may not be sufficiently exhibited.

  The concentration of the electrolyte (1) and the electrolyte (2) in the nonaqueous 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, they are 1.0 mol / L-2.0 mol / L, More preferably, they are 1.0 mol / L-1.5 mol / L.

  The concentration of the electrolyte (1) contained in the non-aqueous electrolyte is preferably 30 with respect to the total amount of the electrolyte (1) and the electrolyte (2) from the viewpoint of improving cycle characteristics, high rate characteristics, and low temperature characteristics. ˜99.9 mol%. The lower limit is more preferably 40 mol% or more, and still more preferably 50 mol% or more. Within such a range, a film is formed on the electrode even at a low temperature or at a normal temperature, so that the deterioration of the electrode and the decomposition of the non-aqueous electrolyte can be suppressed, the increase in internal resistance is suppressed, and the discharge voltage is increased to a high value. As a result, cycle characteristics, high rate characteristics, and low temperature characteristics can be improved.

  The concentration of the electrolyte (2) contained in the non-aqueous electrolyte is preferably 0 with respect to the total amount of the electrolyte (1) and the electrolyte (2) from the viewpoint of improving cycle characteristics, high rate characteristics and low temperature characteristics. The upper limit is more preferably 70 mol% or less, still more preferably 60 mol% or less, and still more preferably 50 mol% or less.

<Carbonate solvent>
In the nonaqueous electrolytic solution of the present invention, as the solvent for dissolving the electrolytes, carbonate solvents of various nonaqueous solvents conventionally used in nonaqueous electrolytic solutions can be used. Such a carbonate-based solvent is not used as a solvent for solidifying the polymer as in Patent Document 2, but may be any solvent that simply dissolves the electrolyte and enhances the solubility of the electrolyte. Examples of carbonate solvents include cyclic carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, and chloroethylene carbonate; fluorine-containing cyclics such as fluoroethylene carbonate, 4,4-difluoroethylene carbonate, and 4,5-difluoroethylene carbonate. Carbonates; chain carbonates such as dimethyl carbonate, methyl ethyl carbonate, and diethyl carbonate; and fluorine-containing chain carbonates such as 2-fluoropropyl methyl carbonate and ethyl 2-fluoroethyl carbonate. These carbonate solvents can be suitably used because they are stable and resistant to decomposition when a voltage is applied. Among these, from the viewpoint of thermal stability, cyclic carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, and chloroethylene carbonate, and chain carbonates such as dimethyl carbonate, methyl ethyl carbonate, and diethyl carbonate are preferable. From the viewpoint of electrochemical stability, thermal stability, and relative dielectric constant of the solvent, it is more preferable to contain ethylene carbonate.
In addition, the said carbonate type solvent may be used independently, and it may be used in mixture of 2 or more types from a viewpoint of the chemical and electrochemical stability with respect to an electrode, thermal stability, and the dielectric constant of a solvent. preferable.

  The concentration of the carbonate solvent in the nonaqueous 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 further preferably 80% by mass to 90% by mass.

<Additives>
The non-aqueous electrolyte of the lithium secondary battery of the present invention contains an additive in addition to the electrolyte (1), the carbonate-based solvent, and optionally the electrolyte (2) in order to improve cycle characteristics, high rate characteristics, and safety. You can leave. As additives, cyclic carbonates having an unsaturated bond such as vinylene carbonate (VC), vinyl ethylene carbonate (VEC), methyl vinylene carbonate (MVC), ethyl vinylene carbonate (EVC); trifluoropropylene carbonate, phenylethylene carbonate And carbonate compounds such as erythritan carbonate; succinic anhydride, glutaric anhydride, maleic anhydride, citraconic anhydride, glutaconic anhydride, itaconic anhydride, diglycolic anhydride, cyclohexanedicarboxylic anhydride, cyclopentanetetracarboxylic acid Carboxylic anhydrides such as anhydrides and phenylsuccinic anhydrides; ethylene sulfite, 1,3-propane sultone, 1,4-butane sultone, methyl methanesulfonate, busulfan, sulfo Sulfur-containing compounds such as lan, sulfolene, dimethylsulfone, tetramethylthiuram monosulfide; 1-methyl-2-pyrrolidinone, 1-methyl-2-piperidone, 3-methyl-2-oxazolidinone, 1,3-dimethyl-2- Nitrogen-containing compounds such as imidazolidinone and N-methylsuccinimide; phosphates such as monofluorophosphate and difluorophosphate; hydrocarbon compounds such as heptane, octane and cycloheptane. When these additives are used in the non-aqueous electrolyte, the concentration is preferably 0.1% by mass to 10% by mass.

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

  As a material for the negative electrode current collector, a conductive metal such as copper, iron, nickel, silver, and stainless steel 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 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 graphite, artificial graphite, natural graphite, mesophase fired bodies made from coal / petroleum pitch, carbon materials such as non-graphitizable carbon, silicon-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, material dispersing solvent and the like that can be used for the negative electrode, the same materials as those used for the positive electrode can be used.

  Moreover, you may use a thickener for preparation of a negative electrode active material composition. Examples of the thickener include polyethylene glycols, celluloses, polyacrylamides, poly (meth) acrylates, etc. Among these, celluloses such as polyethylene glycols, carboxymethyl cellulose (CMC), and poly ( (Meth) acrylates and the like are preferable, and carboxymethyl cellulose (CMC) that can impart a suitable viscosity and has good dispersibility is particularly preferable.

<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 holds a non-aqueous electrolyte, 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 of 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.

  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 electrolyte. 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.

  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.

1. Nonaqueous Electrolyte Preparation Nonaqueous Electrolyte Preparation Example 1
0.37 g (2 mmol) of lithium bis (fluorosulfonyl) imide (LiFSI, synthesized by a conventionally known method) as the compound represented by the general formula (1) (electrolyte (1)), hexafluoro as the electrolyte (2) 1.52 g (10 mmol) of lithium phosphate (LiPF ₆ , manufactured by Kishida Chemical Co., Ltd., LBG grade) is weighed into a 10 mL volumetric flask and measured with a solvent of ethylene carbonate (EC) (manufactured by Kishida Chemical Co., Ltd., LBG grade). The nonaqueous electrolyte solution 1 was prepared.

Non-aqueous electrolyte preparation example 2
The same procedure as in Nonaqueous Electrolyte Preparation Example 1 except that 1.14 g (6 mmol) of LiFSI described above was used as the electrolyte (1) and 0.91 g (6 mmol) of LiPF ₆ was used as the electrolyte (2). A nonaqueous electrolytic solution 2 was prepared.

Non-aqueous electrolyte preparation example 3
Except for using LiFSI 1.9 g (10 mmol) as the electrolyte (1) and LiPF ₆ 0.3 g (2 mmol) as the electrolyte (2), the same as in Nonaqueous Electrolyte Preparation Example 1 A nonaqueous electrolytic solution 3 was prepared.

Nonaqueous electrolyte preparation example 4
Except that 1.50 g (8 mmol) of the above LiFSI was used as the electrolyte (1) and 0.3 g (2 mmol) of the above LiPF ₆ as the electrolyte (2), the same procedure as in Nonaqueous Electrolyte Preparation Example 1 was used. A non-aqueous electrolyte solution 4 was prepared.

Nonaqueous electrolyte preparation example 5
The same procedure as in Nonaqueous Electrolyte Preparation Example 1, except that 1.71 g (9 mmol) of LiFSI described above was used as the electrolyte (1) and 0.15 g (1 mmol) of LiPF ₆ was used as the electrolyte (2). A nonaqueous electrolytic solution 5 was prepared.

Nonaqueous electrolyte preparation example 6
A non-aqueous electrolyte solution 6 was prepared in the same manner as in the non-aqueous electrolyte preparation example 1, except that the electrolyte (1) was not used and the above-described LiPF ₆ 1.8 g (12 mmol) was used as the electrolyte (2).

2. Preparation of laminate cell and charge / discharge before test LiNi _1/3 Co _1/3 Mn _1/3 O ₂ , PVDF, acetylene black were kneaded at a ratio of 96: 5: 4, diluted with NMP solvent, and solid content Slurried at 60%, coated on aluminum foil, dried, then pressed to a constant density with a roll press machine, and 97: 1 artificial graphite, SBR, and CMC Kneaded at a ratio of 5: 1.5, diluted with pure water to make a slurry with a solid content of 48%, coated on copper foil, dried, and then until a constant density with a roll press One negative electrode sheet produced by pressing was laminated so as to face each other, and one polyolefin separator (20 μm made of polyethylene) was sandwiched therebetween. A laminate cell was produced by sandwiching the positive and negative electrode sheets with two aluminum laminate films, filling the aluminum laminate film with each of the non-aqueous electrolytes 1 to 6 and sealing in a vacuum state.

3. Cycle characteristics test Each laminated lithium battery using non-aqueous electrolytes 4 to 6 is charged to 4.2 V at a charge / discharge rate of 0.2 C (constant current mode) at 30 ° C. using the charge / discharge test apparatus. Then, the battery was discharged to 3.0 V and a cycle test was conducted. A charge / discharge pause time of 10 minutes was provided after each charge / discharge. The results are shown in FIG.

From the results of FIG. 1, it can be seen that the cycle characteristics are improved in the example using LiFSI and LiPF ₆ in comparison with the example using LiFSI and LiPF ₆ in combination with the example using no LiFSI. When LiFSI is included in the non-aqueous electrolyte, a film is formed on the electrode, which is considered to prevent electrode deterioration and solvent decomposition, and when LiPF ₆ is used, current collector deterioration. It is thought that this is because of the suppression.

4). High rate characteristic and low temperature characteristic test For each of the laminated lithium batteries using the non-aqueous electrolytes 1 to 3 and 1 cycle, the charge / discharge test apparatus is used, and the charge rate is 0.2 C (constant current mode) at 1 cycle. After charging to 4.2 V, 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, discharge was performed under the same conditions as described above except that the temperature was −30 ° C. or 25 ° C. and the discharge rate was 0.5 C, 1.0 C, and 2.0 C, and the voltage with respect to the capacity was measured. The results are shown in FIGS.

From the results of FIGS. 2 and 3, as compared with the example using no examples and LiFSI in combination with LiFSI and LiPF _6, an example in which a combination of LiFSI and LiPF ₆ indicates that the high rate characteristics are improved. This is because when LiFSI is included in the nonaqueous electrolyte, a film is formed on the electrode even at room temperature or low temperature, suppressing deterioration of the electrode and decomposition of the solvent, suppressing increase in internal resistance, and increasing discharge voltage. This is considered to maintain the value. It can also be seen that when LiPF ₆ and LiFSI are added, if the amount of LiPF ₆ is the same as the amount of LiFSI, or the amount of LiPF ₆ is less than the amount of LiFSI, the high rate characteristics are improved. This is considered to be because the conductivity is improved if the amount of LiFSI is appropriate. Furthermore, it can be seen from the results of FIG. 3 that not only the high rate characteristics but also the low temperature characteristics can be improved.

5. ICP measurement of non-aqueous electrolyte after storage for 1 month at 20 ° C in an uncharged state (metal component elution test)
Each laminated lithium battery produced using the non-aqueous electrolyte shown in Table 1 below was stored in an uncharged state at 20 ° C. for 1 month, and then taken out. Each non-aqueous electrolyte used for battery preparation was diluted to 1% with ultrapure water and subjected to ICP measurement (measuring device: ICP emission analyzer ICPE-9000 manufactured by Shimadzu Corporation). The results are shown in Table 1.

  According to Table 1, when LiFSI is included, it can be seen that elution of Al of the current collector, Co and Mn components of the positive electrode active material is suppressed. Moreover, when the density | concentration of LiFSI is higher, it turns out that metal component elution is suppressed more. This is considered to be because a film is formed on the electrode by LiFSI and the elution of metal components such as Al, Co, and Mn is suppressed. Therefore, if the lithium secondary battery of the present invention is used, the performance of the lithium secondary battery can be maintained even when stored at 20 ° C. for one month in an uncharged state.

Claims (6)

Is represented by the general formula _{Li x Ni a Co b Mn c} M d O y, element M is at least one element Al, Si, Zr, Ti, Fe, selected from the group consisting of Mg and V, 0. 9 ≦ (a + b + c + d) ≦ 1.1, 1.0 ≦ x ≦ 1.3, 0 <a ≦ 0.8, 0 ≦ b ≦ 0.5, 0 ≦ c ≦ 0.5, 0 ≦ d ≦ 0. 5, a positive electrode containing a lithium transition metal composite oxide satisfying 1.9 ≦ y ≦ 2.1 as a positive electrode active material,
A lithium secondary battery comprising a negative electrode containing a negative electrode active material capable of inserting and extracting lithium ions, and a non-aqueous electrolyte,
The non-aqueous electrolyte is the general formula (1); (XSO ₂ ) (X′SO ₂ ) N ⁻ Li ⁺ as the electrolyte (1).
(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). And a carbonate-based solvent,
The non-aqueous electrolyte further includes an electrolyte (2) as an electrolyte, and the electrolyte (2)
LiPF _e (C _m F _{2m + 1} ) _6-e (0 ≦ e ≦ 6, 1 ≦ m ≦ 2),
LiBF _f (C _n F _{2n + 1} ) _4−f (0 ≦ f ≦ 4, 1 ≦ n ≦ 2), and at least one lithium salt selected from the group consisting of LiAsF ₆ ,
The concentration of the electrolyte (1) contained in the nonaqueous electrolytic solution, Ri 30～99.9Mol% der on the total amount of the electrolyte (1) and the electrolyte (2),
The concentration of the compound represented by the general formula (1) (electrolyte (1)) in the non-aqueous electrolyte is 0.1 mol / L to 2.5 mol / L,
The lithium secondary battery, wherein the concentration of the electrolyte (2) in the non-aqueous electrolyte is 0.01 mol / L to 2 mol / L. The lithium secondary battery according to claim 1 wherein the electrolyte (2) is LiPF _6. The lithium secondary battery according to claim 1 or 2 wherein the carbonate-based solvent comprises a cyclic carbonate. The lithium secondary battery according to any one of claims 1 to 3, further comprising a chain carbonate. The compound represented by the general formula (1) is a lithium secondary battery according to any one of claims 1 to 4, which is a lithium bis (fluorosulfonyl) imide. The negative electrode, the graphite material, carboxymethyl cellulose and styrene - lithium secondary battery according to any one of claims 1 to 5 including a butadiene rubber.