JP2016189239A - Lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium ion secondary battery that is excellent in large current characteristic even when an ionic liquid is used as electrolytic solution.SOLUTION: A lithium ion secondary battery includes a positive electrode, a negative electrode, a separator, and an electrolytic solution containing an ionic liquid and a lithium salt. The porosity of the separator ranges from 80% to 98%. The positive electrode has a first current collector, and a positive electrode mixture which is provided on at least one surface of the first current collector and comprises a positive electrode active material, conductive agent, and binder. The coating amount of the positive electrode mixture on one surface of the first current collector ranges from 1 mg/cmto 20 mg/cm, and the conductive agent contains furnace black.SELECTED DRAWING: None

Description

  The present invention relates to a lithium ion secondary battery.

  Nonaqueous electrolyte secondary batteries such as lithium ion batteries have the advantages of high energy density, low self-discharge, and good cycle performance. Therefore, in recent years, it is expected that the non-aqueous electrolyte secondary battery is used as a power source for various industrial machines and industrial instruments by increasing the size or capacity.

  As a non-aqueous solvent used for such a non-aqueous electrolyte solution of a lithium ion secondary battery, a carbonate solvent such as ethylene carbonate or diethyl carbonate, which easily dissolves a lithium salt and hardly electrolyzes, is used.

  Recently, various studies have been made on the use of an ionic liquid as a non-aqueous electrolyte for a lithium ion secondary battery from the viewpoint of safety (for example, see Patent Document 1).

JP 2010-287380 A

  An ionic liquid is an ionic substance that is in a liquid state even at room temperature (about 30 ° C.), and not only has high ionic conductivity, but also has a low vapor pressure, non-volatility and flame retardancy, and is a lithium ion secondary. It also has excellent characteristics for battery safety. Further, the non-aqueous electrolyte solution of the lithium ion secondary battery is required to be electrochemically stable, and the ionic liquid has a stable potential window equal to or higher than that of the carbonate solvent.

  On the other hand, an ionic liquid has a problem that it is inferior in charge / discharge characteristics of a large current because it has higher viscosity and lower conductivity than a carbonate solvent.

  In view of the above circumstances, the present invention solves these problems of the prior art, and provides a lithium ion secondary battery having excellent large current characteristics even when an ionic liquid is used as an electrolytic solution. Objective.

Specific means for achieving the above object are as follows.
<1> A positive electrode, a negative electrode, a separator, and an electrolytic solution containing an ionic liquid and a lithium salt, wherein the separator has a porosity of 80% to 98%, and the positive electrode is a first current collector A positive electrode mixture comprising a positive electrode active material, a conductive agent, and a binder applied to at least one surface of the body and the first current collector, and the positive electrode on one surface of the first current collector the coating amount of the mixture is ^{1mg / cm 2 ~20mg / cm 2} , and a lithium ion secondary battery including a furnace black to said conductive material.
<2> The lithium ion secondary battery according to <1>, wherein the conductive agent further contains acetylene black.
<3> The lithium ion secondary battery according to <1> or <2>, wherein the separator is a nonwoven fabric including at least one selected from the group consisting of polyolefin fibers, glass fibers, cellulose fibers, and polyimide fibers.
<4> the anion component of the ionic ^{_{liquid, N (C 4 F 9 SO}} 2) 2 -, CF 3 SO 3 -, N (SO 2 F) 2 -, N (SO 2 CF 3) 2 -, and The lithium ion secondary battery according to any one of <1> to <3>, including at least one selected from the group consisting of N (SO ₂ CF ₂ CF ₃ ) ₂ ^— .
<5> The cation component of the ionic liquid includes at least one selected from the group consisting of a chain quaternary ammonium cation, a piperidinium cation, a pyrrolidinium cation, and an imidazolium cation. <1> to <4 > The lithium ion secondary battery of any one of>.
<6> The lithium ion according to any one of <1> to <5>, wherein the positive electrode mixture or the negative electrode mixture includes an active material having a median diameter determined by a laser diffraction method of 0.3 μm to 30 μm. Secondary battery.

  According to the present invention, it is possible to provide a lithium ion secondary battery having excellent large current characteristics even when an ionic liquid is used as an electrolytic solution.

  Hereinafter, the lithium ion secondary battery of the present invention will be described in detail.

  In addition, in this specification, the numerical range shown using "to" shows the range which includes the numerical value described before and behind "to" as a minimum value and a maximum value, respectively. Further, the amount of each component in the composition means the total amount of the plurality of substances present in the composition unless there is a specific notice when there are a plurality of substances corresponding to each component in the composition. The positive electrode is the side that releases (desorbs) lithium ions during charging, and the side that stores (inserts) lithium ions during discharging. The negative electrode stores (inserts) lithium ions during charging and releases lithium ions during discharging. (Desorption) side.

  The lithium ion secondary battery of the present invention has a positive electrode, a negative electrode, a separator, and an electrolytic solution containing an ionic liquid and a lithium salt.

As a result of intensive studies, the inventors have a separator porosity of 80% to 98%, and the positive electrode is applied to at least one surface of the first current collector and the first current collector. A positive electrode mixture comprising a positive electrode active material, a conductive agent, and a binder, and the coating amount of the positive electrode mixture on one surface of the first current collector is 1 mg / cm ^{2 to} 20 mg / cm ² and the inclusion of furnace black in the conductive agent, a lithium ion secondary battery having excellent large current characteristics can be provided even when an ionic liquid is used as the electrolyte, and the present invention is completed. I came to let you.

Hereinafter, each element which comprises the lithium ion secondary battery of this invention is demonstrated.
-Positive electrode-
The positive electrode will be described.

  The positive electrode has a first current collector and a positive electrode mixture applied to at least one surface of the first current collector. Specifically, as the positive electrode, for example, a positive electrode plate formed by applying a positive electrode mixture to at least one surface of the first current collector, drying, and pressing is used.

  As a material of the first current collector (also referred to as a positive electrode current collector), metals such as aluminum, titanium, and tantalum, and alloys thereof are used. Among these, the material of the first current collector is preferably lightweight aluminum and its alloy from the viewpoint of weight energy density.

  The positive electrode mixture includes a positive electrode active material. The positive electrode mixture may further contain a conductive agent, a binder and the like.

  As the positive electrode active material, a lithium transition metal compound or the like is used.

  Examples of lithium transition metal compounds include lithium transition metal oxides and lithium transition metal phosphates.

As the lithium transition metal oxide, a lithium transition metal oxide represented by the chemical formula LiMO ₂ (M is at least one transition metal) is used.

  As the lithium transition metal oxide, a part of transition metals such as Mn, Ni, Co, etc. contained in lithium manganate, lithium nickelate, lithium cobaltate, etc., which are one kind of lithium transition metal oxide, are used. Lithium transition metal oxides substituted with two or more other transition metals are also used.

  As the lithium transition metal oxide, one obtained by replacing a part of the transition metal of the lithium transition metal oxide with a metal element (typical element) such as Mg or Al is also used. In the present invention, the lithium transition metal oxide includes a part of the transition metal of the lithium transition metal oxide substituted with a metal element (typical element).

Specific examples of the lithium transition metal oxide include Li (Co _1/3 Ni _1/3 Mn _1/3 ) O ₂ , LiNi _1/2 Mn _1/2 O ₂ , LiNi _1/2 Mn _3/2 O _4. Etc.

Lithium transition metal phosphates include LiFePO ₄ , LiMnPO ₄ , LiMn _X M _1-X PO ₄ (0.3 ≦ x ≦ 1, M is Fe, Ni, Co, Ti, Cu, Zn, Mg and Zr) And at least one element selected from the group consisting of:

Among these, from the viewpoint of battery characteristics and safety, it is preferable to use a lithium nickel manganese composite oxide such as Li (Co _1/3 Ni _1/3 Mn _1/3 ) O ₂ as the positive electrode active material.

  The positive electrode active material preferably has a median diameter determined by a laser diffraction method in the range of 0.3 μm to 30 μm, more preferably in the range of 0.5 μm to 25 μm, and in the range of 0.5 μm to 10 μm. Further preferred. By using a positive electrode active material having a median diameter in the range of 0.3 μm to 30 μm, the reaction specific surface area can be increased, the internal resistance can be reduced, and the deterioration of the large current characteristics can be further suppressed.

  Here, the median diameter of the positive electrode active material refers to a value obtained by the following method.

  The positive electrode active material is put into pure water so as to be 1% by mass, dispersed with ultrasonic waves for 15 minutes, and then the particle diameter at which the volume-based cumulative distribution becomes 50% is measured by laser diffraction. And this particle diameter is made into the median diameter of a positive electrode active material.

  Furnace black is included as a conductive agent of the positive electrode mixture. Among the furnace blacks, it is particularly preferable to include ketjen black (registered trademark). Moreover, it is more preferable that acetylene black is further included. By including acetylene black, adjustment of the positive electrode mixture is facilitated and the binding force of the electrode is improved.

  When furnace black and acetylene black are included as the conductive agent, the composition ratio is preferably furnace black / acetylene black = 90/10 to 30/70 (mass ratio), and 85/15 to 40/60 (mass ratio). It is more preferable that it is 80 / 20-50 / 50 (mass ratio).

  The conductive agent may further contain carbon materials such as carbon black and graphite other than furnace black and acetylene black.

  The content of the conductive agent is preferably 1 to 10% by mass, more preferably 2 to 8% by mass, and still more preferably 3 to 5% by mass based on the total amount of the positive electrode mixture.

  A known binder is used as the binder for the positive electrode mixture. Specifically, polyvinylidene fluoride, styrene-butadiene rubber, isoprene rubber, acrylic rubber or the like is used as the binder for the positive electrode mixture. However, it is not limited to these materials. In the present invention, the binder for the positive electrode is preferably polyvinylidene fluoride.

  When the positive electrode mixture is applied to one surface of the first current collector, it is preferably dispersed in a dispersion medium to form a slurry. As the dispersion medium, a known dispersion medium can be appropriately selected and used. In the present invention, the dispersion medium is preferably an organic solvent such as N-methyl-2-pyrrolidone.

  The mixing ratio of the positive electrode active material, the conductive agent and the binder in the positive electrode mixture is, for example, 1: 0.05 in mass ratio (positive electrode active material: conductive agent: binder) when the positive electrode active material is 1. -0.20: 0.02-0.10. However, it is not limited to this range.

The coating amount of the positive electrode mixture (the coating amount of the positive electrode mixture on one surface of the first current collector: also referred to as the coating amount) is 1 mg / cm ^{2 to} 20 mg / cm ² , and 2 mg / cm ^2. preferably ^{~15mg / cm 2, 3mg / cm} 2 ~10mg / cm 2 is more preferable. When the coating amount of the positive electrode mixture is 1 mg / cm ² or more, it is easy to make the thickness of the positive electrode mixture uniform at the time of pressing, and it is advantageous because high energy density can be achieved. When the coating amount of the positive electrode mixture is 20 mg / cm ² or less, it is advantageous because the distance between the positive electrode and the negative electrode (ion conduction diffusion distance) is shortened.

  The coating amount of the positive electrode mixture can be obtained by subtracting the mass of the first current collector from the mass of the positive electrode cut out in a predetermined area.

  The volume porosity of the positive electrode mixture is preferably 20% by volume to 45% by volume, more preferably 30% by volume to 45% by volume, and still more preferably 35% by volume to 45% by volume. When the volume porosity of the positive electrode mixture is 20% by volume or more, the impregnation property of the ionic liquid is improved, which is advantageous. When the volume porosity of the positive electrode mixture is 45% by volume or less, the adhesion between the positive electrode current collector and the mixture is improved, which is advantageous. Moreover, when the volume porosity of a positive mix is 45 volume% or less, since the electronic network of a electrically conductive agent is formed and an electronic resistance can be reduced, it is advantageous.

  The volume porosity of the positive electrode mixture is calculated from the blending ratio of materials used for the positive electrode mixture, the true specific gravity of each material, the thickness, area, density, and the like of the positive electrode mixture. Specifically, for example, the volume porosity of the positive electrode mixture can be calculated from the following formula when the positive electrode mixture includes a positive electrode active material, a conductive agent, and a binder.

Formula: Volume porosity of positive electrode mixture (volume%) = [1-{(i) + (ii) + (iii) / (width of positive electrode mixture × length × thickness)}] × 100
Here, (i) represents the volume of the positive electrode active material in the positive electrode mixture, (ii) represents the volume of the conductive agent in the positive electrode mixture, and (iii) represents the binding in the positive electrode mixture. Represents the volume of the agent. (I), (ii), and (iii) can each be calculated from the following equations.

Formula: (i) = (total mass of positive electrode mixture × ratio of mass of positive electrode active material in positive electrode mixture) / true specific gravity of positive electrode active material Formula: (ii) = (total mass of positive electrode mixture × conductivity Ratio of the mass of the agent in the positive electrode mixture) / true specific gravity of the conductive agent: (iii) = (total mass of the positive electrode mixture × ratio of the mass of the binder in the positive electrode mixture) / binder The true specific gravity can be measured by the method for measuring the density and specific gravity of chemical products described in JIS K 0061 (2001).

The thickness of the positive electrode mixture (also referred to as coating thickness) is preferably 20 μm to 80 μm, and more preferably 20 μm to 50 μm. When the thickness of the positive electrode mixture is 20 μm or more, it is easy to make the thickness of the positive electrode mixture uniform at the time of pressing, and it is advantageous because the Li ⁺ concentration distribution in the positive electrode accompanying charge / discharge is less likely to occur. When the thickness of the positive electrode mixture is 80 μm or less, it is advantageous because the decrease in conductivity of the ionic liquid in the voids in the positive electrode mixture can be suppressed.
-Negative electrode-
The negative electrode has a second current collector and a negative electrode mixture applied to at least one surface of the second current collector. Specifically, as the negative electrode, for example, a negative electrode plate formed by applying a negative electrode mixture to at least one surface of the second current collector, drying, and pressing is used.

  As the material of the second current collector (also referred to as a negative electrode current collector), metals such as aluminum, copper, nickel, stainless steel, and alloys thereof are used. Among these, the material of the second current collector is preferably lightweight aluminum and its alloy from the viewpoint of weight energy density. The material of the second current collector is preferably copper from the viewpoint of easy processing into a thin film and cost.

  The negative electrode mixture includes a negative electrode active material. The negative electrode mixture may further contain a conductive agent, a binder and the like.

  Examples of the negative electrode active material include (1) lithium titanate, (2) carbon materials such as graphite and amorphous carbon, (3) metal materials including tin and silicon, and (4) metal lithium. From the viewpoint of energy density, it is preferable to use graphite.

  A known conductive agent can be used for the conductive agent of the negative electrode mixture. Specifically, carbon materials such as graphite, acetylene black, carbon black, and carbon fiber are used. However, it is not limited to these materials.

  A known binder can be used as the binder for the negative electrode mixture, and specific examples thereof are the same as those used for the positive electrode mixture.

  When the negative electrode mixture is applied to one surface of the second current collector, it is preferably dispersed in a dispersion medium to form a slurry. Specific examples of the dispersion medium are the same as the dispersion medium used for the positive electrode mixture. In the present invention, the dispersion medium is preferably water.

  In particular, when an aqueous solvent is used, it is preferable to use a thickener. A dispersing agent or the like is added to the thickener, and a slurry such as SBR is made into a slurry. The thickener is used to adjust the viscosity of the slurry. The thickener is not particularly limited, and examples thereof include carboxymethyl cellulose, methyl cellulose, hydroxymethyl cellulose, ethyl cellulose, polyvinyl alcohol, oxidized starch, phosphorylated starch, casein, and salts thereof. These may be used alone or in combination of two or more.

  The mixing ratio of the negative electrode active material, the conductive agent and the binder in the negative electrode mixture is, for example, 1: 0.00 in mass ratio (negative electrode active material: conductive agent: binder) when the negative electrode active material is 1. -0.20: 0.02-0.10. However, it is not limited to this range.

The coating amount of the negative electrode mixture (second current collector one coating amount of the positive electrode mixture to the surface of: also referred to as coating amount) ^is preferably from ^{1mg / cm 2 ~20mg / cm 2} , 1mg / ^{cm 2} ~10mg / ^cm 2 and more ^{preferably, 2mg / cm 2 ~7mg / cm} 2 is more preferable. When the coating amount of the negative electrode mixture is 1 mg / cm ² or more, it is easy to make the thickness of the positive electrode mixture uniform at the time of pressing, and it is advantageous because high energy density can be achieved. When the coating amount of the negative electrode mixture is 20 mg / cm ² or less, it is advantageous because the distance between the positive electrode and the negative electrode (ion conduction diffusion distance) is shortened.

  In addition, the coating amount of the negative electrode mixture can be obtained by subtracting the mass of the second current collector from the mass of the negative electrode cut out in a predetermined area.

  The volume porosity of the negative electrode mixture is 20% by volume to 45% by volume, preferably 30% by volume to 45% by volume, and more preferably 35% by volume to 45% by volume. This is for the same reason as the positive electrode mixture.

  The volume porosity of the negative electrode mixture is calculated from the blending ratio of the materials used for the negative electrode mixture, the true specific gravity of each material, the thickness, area, density, and the like of the negative electrode mixture. Specifically, for example, the volume porosity of the negative electrode mixture can be calculated from the following formula when the negative electrode mixture includes a negative electrode active material, a conductive agent, and a binder.

Formula: Volume porosity of negative electrode mixture (volume%) = [1-{(i) + (ii) + (iii) + (iV) / (width × length × thickness of negative electrode mixture)}] × 100
Here, (i) represents the volume of the active material in the negative electrode mixture, (ii) represents the volume of the conductive agent in the negative electrode mixture, and (iii) represents the binder in the negative electrode mixture. (IV) represents the volume of the thickener in the negative electrode mixture. (I) to (iV) can be calculated from the following equations, respectively.

Formula: (i) = (total mass of negative electrode mixture × ratio of mass occupied in negative electrode mixture of negative electrode active material) / true specific gravity of negative electrode active material Formula: (ii) = (total mass of negative electrode mixture × conductivity Ratio of the mass of the agent in the negative electrode mixture) / true specific gravity of the conductive agent: (iii) = (total mass of the negative electrode mixture × ratio of the mass of the binder in the negative electrode mixture) / binder True specific gravity formula: (iV) = (total mass of negative electrode mixture × ratio of mass occupied in negative electrode mixture of thickener) / true specific gravity of thickener Note that the true specific gravity is JIS K 0061 (2001) ) Can be measured by the method for measuring the density and specific gravity of chemical products.

  The thickness (also referred to as coating thickness) of the negative electrode mixture is preferably 10 μm to 80 μm, and more preferably 10 μm to 50 μm. This is for the same reason as the positive electrode.

In the negative electrode, for example, when metallic lithium is used as the negative electrode active material, the volume porosity of the negative electrode mixture may not satisfy the above.
-Separator-
The material and shape of the separator are not particularly limited. However, as the material of the separator, it is preferable to use a material that is stable with respect to the electrolytic solution and excellent in liquid retention. Specifically, as the separator, it is preferable to use a polyolefin porous film containing polyethylene, polypropylene, etc .; a nonwoven fabric containing polyolefin fibers (polyethylene fibers, polypropylene fibers, etc.), glass fibers, cellulose fibers, polyimide fibers, etc. . Among these, the nonwoven fabric is preferable as the separator because it is stable with respect to the electrolytic solution and has excellent liquid retention, and at least one selected from the group consisting of polyolefin fibers, glass fibers, cellulose fibers, and polyimide fibers. The nonwoven fabric containing is more preferable.

  The porosity of the separator is 80% to 98%. When a separator having a porosity of 80% to 98% is used, in a lithium ion secondary battery using an ionic liquid as an electrolyte, the ionic conductivity is excellent and the large current characteristics are improved. From such a viewpoint, the porosity of the separator is preferably 85% to 98%, more preferably 90 to 98%.

  The total pore volume of the separator is preferably 2 ml / g or more from the viewpoint of rate characteristics. The upper limit of the total pore volume of the separator is not particularly limited, and is preferably 10 ml / g from a practical viewpoint. From the viewpoint of rate characteristics, the total pore volume of the separator is more preferably 3 ml / g to 10 ml / g, further preferably 5 ml / g to 10 ml / g.

The porosity and total pore volume of the separator are values obtained from mercury porosimeter measurement. Conditions for mercury porosimeter measurement are as follows.・ Equipment: Autopore IV 9500 manufactured by Shimadzu Corporation ・ Mercury pressure: 0.51 psia ・ Pressure holding time at each measurement pressure: 10 s ・ Contact angle between sample and mercury: 140 ° ・ Surface tension of mercury: 485 dynes / cm ・ Mercury Density: 13.5335 g / mL
-Electrolyte-
The electrolyte is a non-aqueous electrolyte and includes an ionic liquid and a lithium salt. Specifically, for example, it is preferable to use an electrolytic solution obtained by dissolving a lithium salt in an ionic liquid that exhibits liquid properties at −20 ° C. or higher.

  The electrolytic solution may include a compound having a carbonate structure. When a compound having a carbonate structure is included, a film derived from the carbonate structure can be formed on the negative electrode mixture by lowering the charging voltage to the reductive decomposition potential of the compound having the carbonate structure at the first charge. Examples of the carbonate compound include ethylene carbonate, propylene carbonate, vinylene carbonate, and the like. It is more preferable to use vinylene carbonate as the carbonate compound from the viewpoint of forming a film derived from the carbonate structure on the negative electrode without increasing the charging voltage.

  When the compound having a carbonate structure is included, the content is preferably 0.1% by mass to 10% by mass, more preferably 0.2% by mass to 5% by mass, and still more preferably 0.5% by mass to 3% by mass. .

  The cation component of the ionic liquid is not particularly limited, and is at least one selected from the group consisting of a chain quaternary ammonium cation, a piperidinium cation, a pyrrolidinium cation, and an imidazolium cation. Is preferred.

  Examples of the chain quaternary ammonium cation include a chain quaternary ammonium cation (X is a nitrogen atom or a phosphorus atom) represented by the following general formula [1]. Examples of the piperidinium cation include a piperidinium cation that is a six-membered cyclic compound containing nitrogen represented by the following general formula [2]. Examples of the pyrrolidinium cation include a pyrrolidinium cation that is a five-membered cyclic compound represented by the general formula [3]. Examples of the imidazolium cation include an imidazolium cation represented by the general formula [4].

Here, R ¹ , R ² , R ³ and R ⁴ in the general formulas [1] to [3] are each independently an alkyl group having 1 to 20 carbon atoms, or R ⁶ —O— (CH ₂ ). _n - an alkoxyalkyl group represented by (R ⁶ represents a methyl group or an ethyl group, n is an integer of 1 to 4) is. However, in the general formula [1], the alkyl group is a chain alkyl group, and the alkoxyalkyl group is a chain alkoxyalkyl group. R ¹ , R ² , R ³ , R ⁴ and R ⁵ in the general formula [4] are each independently represented by an alkyl group having 1 to 20 carbon atoms, R ⁶ —O— (CH ₂ ) _n —. An alkoxyalkyl group (R ⁶ represents a methyl group or an ethyl group, and n represents an integer of 1 to 4), or a hydrogen atom.

The anion component of the ionic liquid is not particularly limited, and is an anion of a halogen such as Cl ⁻ , Br ⁻ or I ⁻ , an inorganic anion such as BF ₄ ⁻ or N (SO ₂ F) ₂ ^— , and B (C _{^{6 H 5) 4 -, CH}} 3 SO 3 -, CF 3 SO 3 -, N (C 4 F 9 SO 2) 2 -, N (SO 2 CF 3) 2 -, N (SO 2 CF 2 CF 3) ₂ ^- organic anions and the like.

Among them, as the anionic component of the ionic ^{_{liquid, B (C 6 H 5)}} 4 -, CH 3 SO 3 -, N (C 4 F 9 SO 2) 2 -, CF 3 SO 3 -, N (SO ₂ F) ₂ ⁻ , N (SO ₂ CF ₃ ) ₂ ^— and N (SO ₂ CF ₂ CF ₃ ) ₂ ^— are preferably included, and N (C ₄ F ₉ SO _{2) 2} ^- at least selected from the group consisting of ^{_{-, CF 3 SO 3 -,}} N (SO 2 F) 2 -, N (SO 2 CF 3) 2 -, and _{N (SO 2 CF 2 CF 3} ) 2 It is more preferable to include one type, and it is more preferable to include N (SO ₂ F) ₂ ^— .

As an anion ^{_{component, N (C 4 F 9 SO}} 2) 2 -, CF 3 SO 3 -, N (SO 2 F) 2 -, N (SO 2 CF 3) 2 -, and _N (SO 2 _CF 2 _{CF 3} ) ₂ ^- ionic liquid containing at least one selected from the group consisting of, in particular N (SO 2 _{F) 2} ^- for ionic liquids containing is relatively low viscosity, the use of this, charge and discharge characteristics Will be improved.

In the ionic liquid, a preferable combination of the anion component and the cation component includes a combination of N-methyl-N-propylpyrrolidinium and bis (fluorosulfonyl) imide (N (SO ₂ F) ₂ ⁻ ), N— Examples thereof include a combination of methyl-N-propylpyrrolidinium and bis (trifluoromethylsulfonyl) imide (N (SO ₂ CF ₃ ) ₂ ⁻ ).

  An ionic liquid may be used individually by 1 type, and may use 2 or more types together.

Lithium salts include LiBF ₄ , LiClO ₄ , LiB (C ₆ H ₅ ) ₄ , LiCH ₃ SO ₃ , LiCF ₃ SO ₃ , LiN (SO ₂ F) ₂ , LiN (SO ₂ CF ₃ ) ₂ , and LiN ( And at least one selected from the group consisting of SO ₂ CF ₂ CF ₃ ) ₂ . However, it is not limited to these materials.

  The concentration of the lithium salt is preferably 0.5 mol / L to 1.5 mol / L, more preferably 0.7 mol / L to 1.3 mol / L with respect to the ionic liquid, 0.8 mol More preferably, it is /L-1.2 mol / L. By setting the concentration of the lithium salt to 0.5 mol / L to 1.5 mol / L, the charge / discharge characteristics can be further improved.

  The manufacturing method of the lithium ion secondary battery of this invention which has a positive electrode, a negative electrode, a separator, and electrolyte solution is not specifically limited, A well-known method can be used. The shape of the lithium ion secondary battery is not particularly limited, and a stacked type, a wound type, or the like can be used.

EXAMPLES Hereinafter, the present invention will be specifically described with reference to examples and comparative examples. However, the present invention is not limited to these examples unless it exceeds the gist.
[Example 1]
As a positive electrode active material, a layered lithium / nickel / manganese / cobalt composite oxide (Li (Co _1/3 Ni _1/3 Mn _1/3 ) O ₂ having a median diameter of 6.5 μm measured by a laser diffraction method, (Hereinafter referred to as NMC) 90% by mass, 4.5% by mass of Ketjen Black (registered trademark) as a conductive agent, and 5.5% by mass of polyvinylidene fluoride as a binder are mixed and mixed. Prepared. The cathode mixture is dispersed in N-methyl-2-pyrrolidone as a dispersion medium, and the slurry is applied onto an aluminum foil having a thickness of 20 μm so that the coating amount after drying of the dispersion medium is 10 mg / cm ^2. And dried at 120 ° C. for 1 hour. After drying, pressing was performed to produce a positive electrode having a positive electrode mixture density of 2.4 g / cc, a positive electrode mixture coating thickness of 41 μm, and a positive electrode mixture volumetric porosity of 41 vol%. The density of the positive electrode mixture was calculated from the formula: density of positive electrode mixture = (mass of positive electrode−mass of current collector [aluminum foil]) / (thickness of positive electrode mixture × area of positive electrode mixture). .

  The positive electrode was cut into a 2.5 cm × 3.0 cm rectangle, and the positive electrode mixture was scraped from the aluminum foil, leaving a 2.0 cm × 2.0 cm positive electrode mixture. The aluminum tab was connected to the aluminum foil after scraping off the positive electrode mixture by spot welding.

As a negative electrode active material, natural graphite having a median diameter of 13 μm measured by laser diffraction method: 98% by mass, carboxymethylcellulose: 1% by mass as a thickener, and styrene-bradiene rubber: 1% by mass as a binder And mixed to prepare a negative electrode mixture. The negative electrode mixture was dispersed in water as a dispersion medium, and a slurry was applied on a copper foil having a thickness of 10 μm so that the coating amount after drying of the dispersion medium was 4.0 mg / cm ² . It dried at 80 degreeC for 1 hour. After drying, pressing was performed to produce a negative electrode having a negative electrode mixture density of 1.75 g / ml, a negative electrode mixture coating thickness of 23 μm, and a negative electrode mixture volume porosity of 20% by volume. The density of the negative electrode mixture was calculated from the formula: density of the negative electrode mixture = (mass of negative electrode−mass of current collector [copper foil]) / (thickness of negative electrode mixture / area of negative electrode mixture). .

  The negative electrode was cut into a 3.0 cm × 3.5 cm rectangle, and the negative electrode mixture was scraped from the copper foil, leaving a 2.5 cm × 2.5 cm negative electrode mixture. A nickel tab was connected to the copper foil after scraping off the negative electrode mixture by spot welding.

  As an electrolyte, lithium bis (fluorosulfonyl) imide (hereinafter referred to as LiFSI) dried under a dry argon atmosphere was used as a solute (lithium salt), and N-methyl-N-propylpyrrolidinium bis (fluoro) as an ionic liquid. What was melt | dissolved in the ratio of 1 mol / L in the sulfonyl) imide (Py13FSI) was used.

These positive electrode and negative electrode were inserted into an aluminum laminated bag through a glass fiber nonwoven fabric (manufactured by Nippon Sheet Glass Co., Ltd.) having a porosity of 90% as a separator, and sealed by thermal welding leaving a part of the opening. An electrolytic solution was poured from the remaining opening, and the inside of the aluminum laminate bag was evacuated, and then the remaining opening was sealed by heat welding to obtain a laminate cell.
[Example 2]
As a positive electrode active material, 90% by mass of layered lithium / nickel / manganese / cobalt composite oxide (NMC) having a median diameter of 6.5 μm measured by a laser diffraction method and Ketjen Black (registered trademark) 3 as a conductive agent .6% by mass, acetylene black (trade name: HS-100, Denki Kagaku Kogyo Co., Ltd.) 0.9% by mass, and polyvinylidene fluoride 5.5% by mass as a binder were added and mixed to obtain a positive electrode mixture. Prepared.

A laminate cell was produced in the same manner as in Example 1 except for the blending ratio of the positive electrode mixture.
[Example 3]
As a positive electrode active material, 90% by mass of layered lithium / nickel / manganese / cobalt composite oxide (NMC) having a median diameter of 6.5 μm measured by a laser diffraction method, and Ketjen Black (registered trademark) 2 as a conductive agent. .25% by mass, acetylene black (trade name: HS-100, Denki Kagaku Kogyo Co., Ltd.) 2.25% by mass, and polyvinylidene fluoride 5.5% by mass as a binder were added and mixed to obtain a positive electrode mixture. Prepared.

A laminate cell was produced in the same manner as in Example 1 except for the blending ratio of the positive electrode mixture.
[Comparative Example 1]
A laminate cell was produced in the same manner as in Example 3 except that a cellulose fiber nonwoven fabric having a porosity of 76% was used as the separator.
[Comparative Example 2]
As a positive electrode active material, 90% by mass of layered type lithium / nickel / manganese / cobalt composite oxide (NMC) having a median diameter of 6.5 μm measured by a laser diffraction method and acetylene black (trade name: HS-) as a conductive agent 100, Denki Kagaku Kogyo Co., Ltd.) 4.5% by mass and 5.5% by mass of polyvinylidene fluoride as a binder were added and mixed to prepare a positive electrode mixture.

A laminate cell was produced in the same manner as in Example 1 except for the blending ratio of the positive electrode mixture.
(Characteristic Evaluation) -Characteristic Evaluation of Examples 1-3 and Comparative Examples 1-2-
The batteries (laminate cells) produced in Examples 1 to 3 and Comparative Example 1 were subjected to a 3-cycle charge / discharge test at a constant current of 1.0 mA at 25 ° C. with a charge end voltage of 4.2 V and a discharge end voltage of 3.0 V. .

  Then, after charging to a final charge voltage of 4.2 V with a constant current of 1.0 mA, the battery was discharged to a final discharge voltage of 3.0 V with a constant current of 0.5 mA, and the discharge capacity was measured.

  Furthermore, after charging to a final charge voltage of 4.2 V with a constant current of 1.0 mA, the battery was discharged to a final discharge voltage of 3.0 V with a constant current of 10.0 mA, and the discharge capacity was measured.

  The initial discharge capacity (initial capacity), initial Coulomb efficiency (Coulomb efficiency), and 10.0 mA / 0.5 mA constant current discharge capacity ratio (rate characteristics) of the batteries prepared in Examples 1 to 3 and Comparative Examples 1 and 2 Table 1 shows.

  As shown in Table 1, the batteries of Examples 1 to 3 have an excellent rate characteristic of 80% or more compared to the battery of Comparative Example 2. In addition, the batteries of Examples 1 to 3 are excellent in rate characteristics by including a separator having a porosity of 80% or more as compared with the battery of Comparative Example 1.

Claims (6)

A positive electrode, a negative electrode, a separator, and an electrolyte containing an ionic liquid and a lithium salt, and the porosity of the separator is 80% to 98%;
The positive electrode has a positive electrode mixture composed of a positive electrode active material, a conductive agent, and a binder applied to at least one surface of the first current collector and the first current collector, and the first current collector while the coating amount of the positive electrode mixture to the surface is ^{1mg / cm 2 ~20mg / cm 2} , and a lithium ion secondary battery including a furnace black to said conductive agent.   The lithium ion secondary battery according to claim 1, wherein the conductive agent further contains acetylene black.   The lithium ion secondary battery according to claim 1 or 2, wherein the separator is a nonwoven fabric containing at least one selected from the group consisting of polyolefin fibers, glass fibers, cellulose fibers, and polyimide fibers. Anion component of the ionic ^{_{liquid, N (C 4 F 9 SO}} 2) 2 -, CF 3 SO 3 -, N (SO 2 F) 2 -, N (SO 2 CF 3) 2 -, and N (SO ₂ CF ₂ CF _{3) 2} ^- lithium ion secondary battery according to any one of claim 1 to claim 3 comprising at least one selected from the group consisting of.   The cation component of the ionic liquid includes at least one selected from the group consisting of a chain quaternary ammonium cation, a piperidinium cation, a pyrrolidinium cation, and an imidazolium cation. 2. The lithium ion secondary battery according to item 1.   The lithium ion secondary battery according to claim 1, wherein the positive electrode mixture or the negative electrode mixture includes an active material having a median diameter of 0.3 μm to 30 μm determined by a laser diffraction method. .