JP2017139087A - Lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium ion secondary battery which is superior in cycle and input characteristics and safety.SOLUTION: A lithium ion secondary battery comprises: a positive electrode; a negative electrode; and an electrolytic solution. The negative electrode includes easily graphitizable carbon as a negative electrode active material. The electrolytic solution includes: a cyclic carbonate and a chain carbonate as a nonaqueous solvent; and lithium bis(oxalato)borate and vinylene carbonate as an additive agent. The content of each of the lithium bis(oxalato)borate and vinylene carbonate is 0.1-1.5 mass% relative to a total amount of the electrolyte.SELECTED DRAWING: None

Description

  The present invention relates to a lithium ion secondary battery.

A lithium ion secondary battery is a high energy density secondary battery, and is used as a power source for portable devices such as notebook computers and mobile phones by taking advantage of its characteristics. In recent years, lithium-ion secondary batteries are expected to be used not only in consumer applications such as portable devices, but also in automotive applications and large-scale power storage systems for natural energy such as solar and wind power generation.
Here, Patent Document 1 discloses a lithium ion secondary battery having excellent output / regeneration characteristics in a high capacity lithium ion secondary battery. This lithium ion battery has a battery capacity of 21 Ah or more and contains lithium bis (oxalato) borate in the electrolyte.

JP 2014-35922 A

However, in the lithium ion secondary battery described in Patent Document 1, it has become clear from the examination results of the present inventors that there is room for further improvement in cycle characteristics and safety.
This invention is made | formed in view of the said situation, and is providing the lithium ion secondary battery which is excellent in the outstanding cycling characteristics and safety | security.

Specific means for solving the above problems are as follows.
<1> A lithium ion secondary battery comprising a positive electrode, a negative electrode, and an electrolytic solution, wherein the negative electrode includes graphitizable carbon as a negative electrode active material, and the electrolytic solution is a cyclic carbonate, a chain-like material as a non-aqueous solvent. Lithium bis (oxalato) borate and vinylene carbonate are included as carbonates and additives, and the content of each of the lithium bis (oxalato) borate and vinylene carbonate is 0.1% by mass to 1% with respect to the total amount of the electrolytic solution. Lithium ion secondary battery which is 0.5 mass%.
<2> The lithium ion secondary battery according to <1>, wherein the electrolytic solution contains lithium hexafluorophosphate as an electrolyte.
<3> The lithium ion secondary battery according to <1> or <2>, wherein the positive electrode includes a layered lithium / nickel / manganese / cobalt composite oxide as a positive electrode active material.

  According to the present invention, it is possible to provide a lithium ion secondary battery that is excellent in cycle characteristics, input characteristics, and safety.

It is sectional drawing of the lithium ion secondary battery of embodiment which can apply this invention.

  Hereinafter, embodiments of the present invention will be described in detail. Note that matters other than matters specifically mentioned in the present specification and necessary for the implementation of the present invention can be grasped as design matters of those skilled in the art based on the prior art in this field. The present invention can be carried out based on the contents disclosed in this specification and common technical knowledge in the field. In addition, dimensional relationships (length, width, thickness, etc.) in the following drawings do not reflect actual dimensional relationships.

In the present specification, a numerical range indicated using “to” indicates a range including the numerical values described before and after “to” as the minimum value and the maximum value, respectively.
In the numerical ranges described stepwise in this specification, the upper limit value or the lower limit value described in one numerical range may be replaced with the upper limit value or the lower limit value of another numerical range. Good. Further, in the numerical ranges described in this specification, the upper limit value or the lower limit value of the numerical range may be replaced with the values shown in the examples.

(Embodiment)
First, the outline of the lithium ion secondary battery will be briefly described. The lithium ion secondary battery has a positive electrode, a negative electrode, a separator, and an electrolytic solution in a battery container. A separator is disposed between the positive electrode and the negative electrode.
When charging the lithium ion secondary battery, a charger is connected between the positive electrode and the negative electrode. At the time of charging, lithium ions inserted into the positive electrode active material are desorbed and released into the electrolytic solution. The lithium ions released into the electrolytic solution move in the electrolytic solution, pass through a separator made of a microporous film, and reach the negative electrode. The lithium ions that have reached the negative electrode are inserted into the negative electrode active material constituting the negative electrode.
When discharging, an external load is connected between the positive electrode and the negative electrode. At the time of discharging, the lithium ions inserted into the negative electrode active material are desorbed and released into the electrolytic solution. At this time, electrons are emitted from the negative electrode. Then, the lithium ions released into the electrolytic solution move in the electrolytic solution, pass through a separator made of a microporous film, and reach the positive electrode. The lithium ions reaching the positive electrode are inserted into the positive electrode active material constituting the positive electrode. At this time, when lithium ions are inserted into the positive electrode active material, electrons flow into the positive electrode. In this way, discharge is performed by the movement of electrons from the negative electrode to the positive electrode.
In this manner, charging / discharging can be performed by inserting and desorbing lithium ions between the positive electrode active material and the negative electrode active material. A configuration example of an actual lithium ion battery will be described later (see, for example, FIG. 1).
Next, a positive electrode, a negative electrode, an electrolytic solution, a separator, and other constituent members that are constituent elements of the lithium ion secondary battery of the present embodiment will be sequentially described.

1. Positive electrode In this embodiment mode, the positive electrode shown below is applicable to a high-capacity, high-input / output lithium ion battery. The positive electrode (positive electrode plate) of the present embodiment is made of a current collector and a positive electrode mixture formed on the current collector. The positive electrode mixture is a layer including at least a positive electrode active material provided on the current collector.
The positive electrode active material includes a layered lithium / nickel / manganese / cobalt composite oxide (hereinafter also referred to as NMC). NMC has a high capacity and excellent safety.
From the viewpoint of further improving safety, a mixed active material of NMC and spinel type lithium manganese oxide (hereinafter sometimes referred to as sp-Mn) may be used.
The content of NMC is preferably 65% by mass or more, more preferably 70% by mass or more, and more preferably 80% by mass or more based on the total amount of the positive electrode mixture from the viewpoint of increasing the capacity of the battery. Is more preferable.

As said NMC, it is preferable to use what is represented by the following compositional formula (Formula 1).
Li _{(1 + δ)} MnxNiyCo _(1-xyz) MzO ₂ (Formula 1)
In the above composition formula (Formula 1), (1 + δ) is a composition ratio of Li (lithium), x is a composition ratio of Mn (manganese), y is a composition ratio of Ni (nickel), and (1-xyz) ) Indicates the composition ratio of Co (cobalt). z represents the composition ratio of the element M. The composition ratio of O (oxygen) is 2.
The elements M are Ti (titanium), Zr (zirconium), Nb (niobium), Mo (molybdenum), W (tungsten), Al (aluminum), Si (silicon), Ga (gallium), Ge (germanium), and Sn. It is at least one element selected from the group consisting of (tin).
-0.15 <δ <0.15, 0.1 <x ≦ 0.5, 0.6 <x + y + z ≦ 1.0, 0 ≦ z ≦ 0.1.

As the sp-Mn, it is preferable to use one represented by the following composition formula (Formula 2).
Li _{(1 + η)} Mn _(2-λ) M ′ _λ O _4.
In the above composition formula (Formula 2), (1 + η) represents the composition ratio of Li, (2-λ) represents the composition ratio of Mn, and λ represents the composition ratio of the element M ′. The composition ratio of O (oxygen) is 4.
Element M ′ includes Mg (magnesium), Ca (calcium), Sr (strontium),
It is preferably at least one element selected from the group consisting of Al, Ga, Zn (zinc), and Cu (copper).
0 ≦ η ≦ 0.2 and 0 ≦ λ ≦ 0.1.
Mg or Al is preferably used as the element M ′ in the composition formula (Chemical Formula 2). By using Mg or Al, the battery life can be extended. In addition, the safety of the battery can be improved. Furthermore, since the elution of Mn can be reduced by adding the element M ′, storage characteristics and charge / discharge cycle characteristics can be improved.
Further, as the positive electrode active material, materials other than the above NMC and sp-Mn may be used.
A positive electrode active material can be used individually by 1 type, or can use 2 or more types together.

  Next, the positive electrode mixture and the current collector will be described in detail. The positive electrode mixture contains a positive electrode active material, a binder, and the like, and is formed on the current collector. Although there is no restriction | limiting in the formation method, For example, it forms as follows. A positive electrode active material, a binder, and other materials such as a conductive material and a thickener used as needed are mixed in a dry form to form a sheet, which is pressure-bonded to a current collector (dry method). In addition, a positive electrode active material, a binder, and other materials such as a conductive material and a thickener used as necessary are dissolved or dispersed in a dispersion solvent to form a slurry, which is applied to a current collector and dried. (Wet method).

Examples of the conductive material for the positive electrode include metal materials such as copper and nickel; graphite such as natural graphite and artificial graphite (graphite); carbon black such as acetylene black; and carbonaceous materials such as amorphous carbon such as needle coke. It is done. Of these, one type may be used alone, or two or more types may be used in combination.
About the content (addition amount, ratio, amount) of the conductive material, the range of the content of the conductive material with respect to the mass of the positive electrode mixture is as follows. The lower limit of the range is 0.01% by mass or more, preferably 0.1% by mass or more, more preferably 1% by mass or more, and the upper limit is 50% by mass or less, preferably 30% by mass or less, more preferably 15%. It is below mass%. If it is less than the said minimum, electroconductivity may become inadequate. Moreover, when the said upper limit is exceeded, battery capacity may fall.

  The binder for the positive electrode active material is not particularly limited, and when the positive electrode mixture is formed by a coating method, a material having good solubility and dispersibility in the dispersion solvent is selected. Specific examples include polyethylene, polypropylene, polyethylene terephthalate, and polyvinylidene fluoride (PVdF).

In order to improve the packing density of the positive electrode active material, the layer formed on the current collector using the wet method or the dry method is preferably consolidated by a hand press, a roller press, or the like.
The density of the positive electrode mixture consolidated as described above is 2.5 to 2.8 g / cm ³ , but from the viewpoint of further improving input / output characteristics and safety, it is 2.55 g / cm ³ or more. , preferably 2.75 g / cm ³ or less, 2.6 g / cm ³ or more, 2.7 g / cm ³ or less is more preferable.
Moreover, the coating amount on one side of the positive electrode mixture to the positive electrode current collector is 100 to 160 g / m ² , but from the viewpoint of further improving the energy density and input / output characteristics, 110 g / m ² or more and 150 g. / M ² or less, more preferably 120 g / m ² or more and 140 g / m ² or less.
Considering the amount of coating on one side of the positive electrode mixture as described above and the density of the positive electrode mixture, the thickness of the single-sided coating film on the positive electrode current collector of the positive electrode mixture ([positive electrode thickness−positive electrode collector The thickness of the electric conductor] / 2) is preferably 39 to 68 μm, more preferably 43 to 64 μm, and still more preferably 46 to 60 μm.

2. Negative electrode In this embodiment, the negative electrode shown below is applicable to a high-output and high-capacity lithium ion secondary battery. The negative electrode (negative electrode plate) of the present embodiment is composed of a current collector and a negative electrode mixture (negative electrode mixture) formed on both surfaces thereof. The negative electrode mixture contains a negative electrode active material that can electrochemically occlude and release lithium ions.

The negative electrode active material used in the present invention includes graphitizable carbon (hereinafter sometimes referred to as soft carbon). In addition, carbon materials other than graphitizable carbon may be included. Carbon materials are broadly divided into graphite materials with a uniform crystal structure and non-graphite materials with a disordered crystal structure. The former includes natural graphite and artificial graphite, and the latter has a disordered crystal structure. Yes, but 2000
There are easily graphitized carbon that is likely to become graphite by heating at ˜3000 ° C. and non-graphitizable carbon that is difficult to become graphite (hereinafter sometimes referred to as hard carbon). Specifically, graphite, cokes (petroleum-based coke, pitch coke, needle coke, etc.), resin film-fired carbon, fiber-fired carbon, vapor-grown carbon, and the like are used. The non-graphite-based carbon material can be produced, for example, by heat treating petroleum pitch, polyacene, polyparaphenylene, polyfurfuryl alcohol, or polysiloxane. Or soft carbon. For example, from 500 ° C
A firing temperature of about 800 ° C. is suitable for producing hard carbon, and a firing temperature of about 800 ° C. to 1000 ° C. is suitable for producing soft carbon. The non-graphitizable carbon is defined as having a surface spacing d002 value in the C-axis direction obtained by an X-ray wide angle diffraction method of 0.36 nm or more and 0.40 nm or less.
The graphitizable carbon has a surface spacing d002 value in the C-axis direction obtained by an X-ray wide angle diffraction method.
The thickness is preferably 0.34 nm or more and less than 0.36 nm, 0.341 nm or more, 0.
It is more preferably 355 nm or less, and further preferably 0.342 nm or more and 0.35 nm or less.

The content of graphitizable carbon (soft carbon) is preferably 20% by mass or more, more preferably 50% by mass or more, and still more preferably 70% by mass or more based on the total amount of the negative electrode active material. Further, graphitizable carbon (soft carbon) and non-graphitizable carbon (hard carbon) may be used in combination. The mixing ratio (mass ratio) of graphitizable carbon (soft carbon) and non-graphitizable carbon (hard carbon) is graphitizable carbon (soft carbon) / non-graphitizable carbon (hard carbon) =
100/0 to 10/90 is preferable, 100/0 to 20/80 is more preferable, and 100 /
0-50 / 50 is more preferable, and 100 / 0-70 / 30 is particularly preferable.
The average particle diameter (d50) of the graphitizable carbon (soft carbon) is 2 to 50 μm.
It is preferable that When the average particle diameter is 2 μm or more, the specific surface area can be in an appropriate range, the initial charge / discharge efficiency of the lithium ion secondary battery is excellent, and the contact between particles tends to be excellent and the input / output characteristics tend to be excellent. On the other hand, when the average particle diameter is 50 μm or less, unevenness on the electrode surface is unlikely to occur and the short circuit of the battery can be suppressed and the diffusion distance of Li from the particle surface to the inside becomes relatively short, so The output characteristics tend to improve. From this viewpoint, the average particle diameter is more preferably 5 to 30 μm, and further preferably 10 to 20 μm. For example, for the particle size distribution, a sample is dispersed in purified water containing a surfactant, and a laser diffraction particle size distribution measuring apparatus (for example, SALD-manufactured by Shimadzu Corporation) is used.
3000J), and the average particle diameter is calculated as the median diameter (d50).

In addition, as the negative electrode active material, a metal oxide such as tin oxide or silicon oxide, a metal composite oxide, a lithium alloy such as lithium alone or a lithium aluminum alloy, or a material capable of forming an alloy with lithium such as Sn or Si is used in combination. May be. These may be used alone or in combination of two or more.
The metal composite oxide is not particularly limited as long as it can occlude and release lithium. However, a material containing both Ti (titanium), Li (lithium), and Ti and Li has a high current density. This is preferable from the viewpoint of discharge characteristics.

The SOC (Sta at a potential of 0.1 V with respect to the lithium potential of the negative electrode of the present embodiment.
te Of Charge) is preferably 65% or more and 90% or less, and more preferably 68% or more and 85% or less, from the viewpoint of input characteristics and practical use. The SOC can be measured at a potential of 0.1 V with respect to the lithium potential, for example, as follows. First, a negative electrode having a negative electrode mixture coated on one side of a current collector is punched into a diameter of 15 mm, metallic lithium punched into a diameter of 16 mm, and a separator punched into a diameter of 19 mm (for example, made of polyethylene) Porous sheet), electrolyte (for example,
A CR2032-type coin cell is produced under an argon atmosphere together with a carbonate system. The coin cell is set to 0 V (V vs Li /) at a constant current of 0.1 C at 25 ° C.
Li +) until the current density reaches 0.01C at a constant voltage of 0V. next,
The battery is discharged to 1.5 V (V vs Li / Li +) at a constant current of a current density of 0.1 C, and the charging and discharging are performed for 3 cycles. Then, assuming that the charge capacity in the third cycle is SOC 100%, the charge capacity when reaching 0.1 V in the third cycle is measured and calculated from the following formula 1.
SOC = CC charge capacity up to 0.1V in the third cycle / CCCV charge capacity up to 0V in the third cycle (Equation 1)
Here, in the coin cell, the direction in which lithium ions are inserted into the negative electrode active material is defined as charging, and the direction in which lithium ions inserted into the negative electrode active material are desorbed is defined as discharging.
C means “current value (A) / battery discharge capacity (Ah)”.

There is no restriction | limiting in particular as a material of the collector for negative electrodes, Metal materials, such as copper, nickel, stainless steel, nickel plating steel, are mentioned. Among these, copper is preferable from the viewpoint of ease of processing and cost.
There is no restriction | limiting in particular as a shape of an electrical power collector, The material processed into various shapes can be used. A metal foil, a metal plate, a metal thin film, an expanded metal, etc. are mentioned. Among these, a metal thin film is preferable, and a copper foil is more preferable. The copper foil includes a rolled copper foil formed by a rolling method and an electrolytic copper foil formed by an electrolytic method, both of which are suitable for use as a current collector.

The amount of single-sided application of the negative electrode mixture to the current collector is 50 from the viewpoint of energy density and input / output characteristics.
g / m ² or more, preferably 120 g / m ² or less, 60 g / m ² or more, 100 g
/ M ² or less is more preferable.
Although there is no restriction | limiting in particular in the structure of the negative electrode compound material formed using the negative electrode active material, The range of the negative electrode compound material density is as follows. The lower limit of the negative electrode composite density is preferably 0.7 g / cm ³ or more, more preferably 0.8 g / cm ³ , and still more preferably 0.9 g / cm ³ , and the upper limit is 2
g / cm ³ or less, preferably 1.9 g / cm ³ or less, more preferably 1.8 g / cm ³ or less, and even more preferably 1.7 g / cm ³ or less.
When the above upper limit is exceeded, particles of the negative electrode active material are likely to be destroyed, and a high current is generated due to an increase in the initial irreversible capacity and a decrease in the permeability of the non-aqueous electrolyte solution near the interface between the current collector and the negative electrode active material. There is a possibility of deteriorating the density charge / discharge characteristics. In addition, if it is less than the above lower limit, the conductivity between the negative electrode active materials decreases, so the battery resistance increases, and the capacity per unit volume may decrease.

The binder for the negative electrode active material is not particularly limited as long as it is a material that is stable to the non-aqueous electrolyte and the dispersion solvent used when forming the electrode. Resin polymers such as polyethylene, polypropylene, polyethylene terephthalate, polymethyl methacrylate, aromatic polyamide, cellulose, nitrocellulose; SBR (styrene-butadiene rubber), isoprene rubber, butadiene rubber, fluororubber, NBR (acrylonitrile-butadiene rubber) , Ethylene
Rubber-like polymers such as propylene rubber; styrene / butadiene / styrene block copolymer or hydrogenated product thereof; EPDM (ethylene / propylene / diene terpolymer), styrene / ethylene / butadiene / styrene copolymer, styrene・ Thermoplastic elastomeric polymers such as isoprene / styrene block copolymer or hydrogenated products thereof; syndiotactic-1,2-polybutadiene, polyvinyl acetate, ethylene / vinyl acetate copolymer, propylene / α-olefin copolymer Soft resinous polymers such as polymers; Fluoropolymers such as polyvinylidene fluoride, polytetrafluoroethylene, fluorinated polyvinylidene fluoride, polytetrafluoroethylene / ethylene copolymers; alkali metal ions (especially lithium ions) Polymer composition with ionic conductivity Etc. The. These may be used alone or in combination of two or more.

As the dispersion solvent for forming the slurry, any type of solvent can be used as long as it can dissolve or disperse the negative electrode active material, the binder, and the conductive material and the thickener used as necessary. There is no restriction, and either an aqueous solvent or an organic solvent may be used. Examples of the aqueous solvent include water, a mixed solvent of alcohol and water, and examples of the organic solvent include N-methylpyrrolidone (NMP), dimethylformamide, dimethylacetamide, methyl ethyl ketone, cyclohexanone, methyl acetate, and acrylic. Methyl acid, diethyltriamine, N, N
-Dimethylaminopropylamine, tetrahydrofuran (THF), toluene, acetone, diethyl ether, dimethylacetamide, hexamethylphosphalamide, dimethyl sulfoxide, benzene, xylene, quinoline, pyridine, methylnaphthalene, hexane. 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. In addition, the said dispersion solvent may be used individually by 1 type, or may be used in combination of 2 or more type.

Regarding the content (addition amount, ratio, amount) of the binder, the range of the content of the binder with respect to the mass of the negative electrode mixture is as follows. The lower limit of the range is preferably 0.1% by mass or more, more preferably 0.5% by mass or more, and further preferably 0.6% by mass or more. The upper limit is 20% by mass
Or less, preferably 15% by mass or less, more preferably 10% by mass or less, and further preferably 8% by mass or less.
It is below mass%.
When the upper limit is exceeded, the proportion of the binder that does not contribute to the battery capacity increases, which may lead to a decrease in battery capacity. Moreover, if it is less than the said minimum, the fall of the intensity | strength of a negative electrode compound material may be caused.
In particular, the range of the content of the binder with respect to the mass of the negative electrode mixture when a rubbery polymer typified by SBR is used as the main component as the binder is as follows. The lower limit of the range is 0.
1% by mass or more, preferably 0.5% by mass or more, more preferably 0.6% by mass or more,
The upper limit is 5% by mass or less, preferably 3% by mass or less, more preferably 2% by mass or less.
The range of the binder content with respect to the mass of the negative electrode mixture when a fluorine-based polymer typified by polyvinylidene fluoride is used as the main component as the binder is as follows. The lower limit of the range is 1% by mass or more, preferably 2% by mass or more, more preferably 3% by mass or more, and the upper limit is 15% by mass or less, preferably 10% by mass or less, more preferably 8% by mass or less. is there.

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 range of the content of the thickener relative to the mass of the negative electrode mixture when the thickener is used is as follows. The lower limit of the range is 0.1% by mass or more, preferably 0.5% by mass or more, and more preferably 0%.
. The upper limit is 5% by mass or less, preferably 3% by mass or less, and more preferably 2% by mass or less.
If it is less than the said minimum, the applicability | paintability of a slurry may fall. Moreover, when the said upper limit is exceeded, the ratio of the negative electrode active material to a negative electrode compound material will fall, and there exists a possibility of the fall of battery capacity and the raise between resistances of a negative electrode active material.

3. Electrolytic Solution The electrolytic solution of the present embodiment includes an electrolyte and a non-aqueous solvent that dissolves the electrolyte, and lithium bis (oxalato) borate and vinylene carbonate are added to the electrolytic solution.

The electrolyte is not particularly limited as long as it is a lithium salt that can be used as an electrolyte of a non-aqueous electrolyte solution for a lithium ion battery, and examples thereof include inorganic lithium salts and fluorine-containing organic lithium salts shown below.
Examples of the inorganic lithium _{salt, LiPF 6, LiBF 4, LiAsF} 6, LiSbF inorganic fluoride salts and the like _6, LiClO _4, Libro _4, LiIO and perhalogenate such as _4, an inorganic chloride salts such as LiAlCl _4, etc. Is mentioned.

Examples of the fluorine-containing organic lithium salt include perfluoroalkane sulfonates such as LiCF ₃ SO ₃ ; LiN (FSO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) ₂ , LiN (CF ₃ CF ₂ SO ₂ ) ₂ , LiN ( Perfluoroalkanesulfonylimide salts such as CF ₃ SO ₂ ) (C ₄ F ₉ SO ₉ ); perfluoroalkanesulfonylmethide salts such as LiC (CF ₃ SO ₂ ) ₃ ; Li [PF ₅ (CF ₂ CF ₂ CF _{3)], Li [PF 4} (CF 2 CF 2 CF 3) 2], Li [PF 3 (CF 2 CF 2 CF 3) 3], Li [PF 5 (CF 2 CF 2 CF 2 CF 3)], Examples thereof include fluoroalkyl fluorophosphates such as Li [PF ₄ (CF ₂ CF ₂ CF ₂ CF ₃ ) ₂ ] and Li [PF ₃ (CF ₂ CF ₂ CF ₂ CF ₃ ) ₃ ].

  Although there is no restriction | limiting in particular in the density | concentration of the electrolyte in nonaqueous electrolyte solution, The density | concentration range of an electrolyte is as follows. The lower limit of the concentration is 0.5 mol / L or more, preferably 0.6 mol / L or more, more preferably 0.7 mol / L or more. Further, the upper limit of the concentration is 2 mol / L or less, preferably 1.8 mol / L or less, more preferably 1.7 mol / L or less. If the concentration is too low, the electrical conductivity of the electrolyte may be insufficient. On the other hand, if the concentration is too high, the viscosity increases and the electrical conductivity may decrease. Such a decrease in electrical conductivity may reduce the performance of the lithium ion battery.

Nonaqueous solvents include cyclic carbonates and chain carbonates.
As the cyclic carbonate, those having 2 to 6 carbon atoms of the alkylene group constituting the cyclic carbonate are preferable, and those having 2 to 4 are more preferable. Examples include ethylene carbonate, propylene carbonate, butylene carbonate. Of these, ethylene carbonate and propylene carbonate are preferable.
As the chain carbonate, dialkyl carbonate is preferable, and the number of carbon atoms of the two alkyl groups is preferably 1 to 5, and more preferably 1 to 4. Examples include symmetric chain carbonates such as dimethyl carbonate, diethyl carbonate, and di-n-propyl carbonate; and asymmetric chain carbonates such as ethyl methyl carbonate, methyl-n-propyl carbonate, and ethyl-n-propyl carbonate. Of these, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate are preferable.
From the viewpoint of battery characteristics, the content of the cyclic carbonate and the chain carbonate is preferably 85% by mass or more, more preferably 90% by mass or more, and more preferably 95% by mass or more based on the total amount of the non-aqueous solvent. More preferably.
In addition, the mixing ratio of the cyclic carbonate and the chain carbonate is preferably 1/9 to 6/4 of cyclic carbonate / chain carbonate (volume ratio) from the viewpoint of battery characteristics, and 2/8 to 5 More preferably, it is / 5.

Additives include lithium bis (oxalato) borate and vinylene carbonate. By including the two kinds of additives, cycle characteristics and safety can be improved.
When lithium bis (oxalato) borate and vinylene carbonate are used in combination, 0.1 to 1.5% by mass is preferable at each concentration based on the total amount of the non-aqueous solvent, and 0.7 to 1.5% by mass. Is more preferable.
When lithium bis (oxalato) borate and vinylene carbonate are used in combination, cycle characteristics and safety are improved when the concentration of lithium bis (oxalato) borate is 0.1% by mass or more, and lithium bis (oxalato) borate And the vinylene carbonate concentration is 1.5% by mass or less, and excellent input characteristics and cycle characteristics are exhibited.
In addition to the above additives, other additives such as an overcharge preventing material, a negative electrode film forming material, a positive electrode protective material, and a high input / output material may be used depending on the required function.
By using the other additives, it is possible to suppress a rapid electrode reaction at the time of abnormality due to overcharge, improve capacity maintenance characteristics and cycle characteristics after high temperature storage, and improve input / output characteristics.

4). Separator The separator has mechanical strength that electronically insulates between both electrodes of the positive electrode and the negative electrode, has high ion permeability, and has resistance to oxidation on the side in contact with the positive electrode and reduction on the negative electrode side. Material is used. As a material for the separator, a resin material is usually used, and examples thereof include olefin polymers (for example, polypropylene and polyethylene). The separator is preferably selected from materials that are stable with respect to the electrolyte and excellent in liquid retention. For example, examples of the separator include a thin film-shaped porous film, specifically, a porous sheet made of at least one of polypropylene and polyethylene. As the thin film-like porous film, a film having a pore diameter of 0.01 μm to 1 μm and a thickness of 15 μm to 50 μm is preferable. Further, the porosity is preferably 30% to 50%, and more preferably 35% to 45%. The separator may be a single separator or a laminate of two or more separators.

(Capacity ratio of negative electrode to positive electrode of lithium ion secondary battery)
In the present invention, the capacity ratio of the negative electrode to the positive electrode (negative electrode capacity / positive electrode capacity) is preferably 1 or more and less than 1.3 from the viewpoint of safety and energy density, and more preferably 1.05 to 1.25. 1.1 to 1.2 are more preferable. If it is 1.3 or more, the positive electrode potential may be higher than 4.2 V during charging, which may reduce safety. (At this time, the positive electrode potential refers to the Li potential)

Here, the negative electrode capacity indicates [negative electrode discharge capacity], and the positive electrode capacity indicates [positive electrode initial charge capacity-negative electrode or positive electrode, whichever is greater, irreversible capacity]. Here, the “negative electrode discharge capacity” is defined to be calculated by the charge / discharge device when the lithium ions inserted into the negative electrode active material are desorbed. Further, the “initial charge capacity of the positive electrode” is defined as that calculated by the charge / discharge device when lithium ions are desorbed from the positive electrode active material.
The capacity ratio between the negative electrode and the positive electrode can be calculated from, for example, “discharge capacity of the lithium ion secondary battery / discharge capacity of the negative electrode”. The discharge capacity of the lithium ion secondary battery is, for example, 4.2 V, 0.1 C to 0.5 C, 0.1 C to 0.5 C after performing constant current and constant voltage (CCCV) charging with an end time of 2 to 10 hours. It can be measured under conditions when a constant current (CC) is discharged to 2.7 V at 0.5 C. For the discharge capacity of the negative electrode, the negative electrode whose discharge capacity was measured for the lithium ion secondary battery was cut into a predetermined area, lithium metal was used as the counter electrode, and a single electrode cell was prepared via a separator impregnated with an electrolyte. , 0V, 0.1C, constant current constant voltage (CCCV) charge at a final current of 0.01C, then discharge per predetermined area under the condition of constant current (CC) discharge to 1.5V at 0.1C It can be calculated by measuring the capacity and converting this to the total area used as the negative electrode of the lithium ion battery. In this single electrode cell, the direction in which lithium ions are inserted into the negative electrode active material is defined as charging, and the direction in which lithium ions inserted into the negative electrode active material are desorbed is defined as discharging.
C means “current value (A) / battery discharge capacity (Ah)”.

Hereinafter, the present embodiment will be described in more detail based on examples. The present invention is not limited to the following examples.
[Production of positive electrode plate]
The positive electrode plate was produced as follows. A layered lithium / nickel / manganese / cobalt composite oxide (BET specific surface area of 0.4 m ² / g, average particle diameter (d50) of 6.5 μm) was used as the positive electrode active material. In this mixture of positive electrode active materials, acetylene black (trade name: HS-100, average particle size 48 nm (electrochemical industry catalog value), electrochemical industry) is used as a conductive material, and polyvinylidene fluoride as a binder. Were sequentially added and mixed to obtain a mixture of positive electrode materials. The mass ratio was active material: conductive material: binder = 90: 5: 5. Further, N-methyl-2-pyrrolidone (NMP) as a dispersion solvent was added to the above mixture and kneaded to form a slurry. This slurry was applied substantially evenly and uniformly to both surfaces of a 20 μm thick aluminum foil as a positive electrode current collector. Then, the drying process was performed and it consolidated by the press to the predetermined density. The density of the positive electrode mixture was 2.5 g / cm ³ .

[Production of negative electrode plate]
The negative electrode plate was produced as follows. Graphitizable carbon (d002 = 0.35 nm, average particle size (d50) = 17 μm) was used as the negative electrode active material. Polyvinylidene fluoride was added as a binder to this negative electrode active material. These mass ratios were negative electrode active material: binder = 92: 8. A dispersion solvent N-methyl-2-pyrrolidone (NMP) was added thereto and kneaded to form a slurry. This slurry was applied substantially uniformly and uniformly on both surfaces of a rolled copper foil having a thickness of 10 μm, which is a current collector for a negative electrode. Then, the drying process was performed and it consolidated by the press to the predetermined density. The density of the negative electrode mixture was 1.15 g / cm ³ .
In Comparative Examples 5 and 6, non-graphitizable carbon (d002 = 0.375 nm, average particle diameter (D50) = 10 μm) was used as the negative electrode active material. Polyvinylidene fluoride was used as the binder, and the mass ratio of the negative electrode active material to the binder was active material: binder = 92: 8. A dispersion solvent N-methyl-2-pyrrolidone (NMP) was added thereto and kneaded to form a slurry. This slurry was applied substantially uniformly and uniformly on both surfaces of a rolled copper foil having a thickness of 10 μm, which is a current collector for a negative electrode. Then, the drying process was performed and it consolidated by the press to the predetermined density. The negative electrode mixture density was 1.0 g / cm ³ .
In Comparative Example 7, artificial graphite (d002 = 0.337 nm, average particle diameter (D50) = 20 μm) was used as the negative electrode active material. Polyvinylidene fluoride was used as the binder, and the mass ratio of the negative electrode active material and the binder was active material: binder = 91: 9. A dispersion solvent N-methyl-2-pyrrolidone (NMP) was added thereto and kneaded to form a slurry. This slurry was applied substantially uniformly and uniformly on both surfaces of a rolled copper foil having a thickness of 10 μm, which is a current collector for a negative electrode. Then, the drying process was performed and it consolidated by the press to the predetermined density. The negative electrode mixture density was 1.4 g / cm ³ .

[Electrolyte adjustment]
As the electrolytic solution, lithium hexafluorophosphate (LiPF ₆ ) was added at a concentration of 1.2 mol / L in a mixed solution in which ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate were mixed at a volume ratio of 2: 3: 2. A predetermined amount of lithium bis (oxalato) borate and vinylene carbonate shown in Table 1 was added.

[Production of lithium-ion batteries]
The positive electrode and the negative electrode were each cut into a predetermined size, and the cut positive electrode and negative electrode were wound with a single-layer separator made of polyethylene having a thickness of 30 μm interposed therebetween to form a roll-shaped electrode body. At this time, the lengths of the positive electrode, the negative electrode, and the separator were adjusted so that the diameter of the electrode body was 17.15 mm. A current collecting lead was attached to this electrode body, inserted into a 18650 type battery case, and then a non-aqueous electrolyte was injected into the battery case. Finally, the battery case was sealed to complete the lithium ion battery.

[Evaluation of battery characteristics (cycle characteristics, DC resistance, overcharge characteristics)]
The produced battery was charged at a constant current of up to 4.2 V at 0.5 CA in an environment of 25 ° C., and charged at a constant voltage until reaching a current value of 0.01 CA at that voltage after reaching 4.2 V. Then, it discharged to 2.7V by 0.5 CA constant current discharge. Further, a pause of 30 minutes was put between charge and discharge. This was carried out for 3 cycles. This state was defined as the initial state.

(Cycle characteristics)
The above lithium ion battery is charged at a constant current of up to 4.2 V at a current value of 1 CA in a 25 ° C. environment and charged at a constant voltage until reaching a current value of 0.01 CA at a constant voltage after reaching 4.2 V. A cycle test was repeated in which the battery was discharged to 2.7 V after being fully charged. Further, a pause of 30 minutes was put between charge and discharge. The discharge capacity was measured during the first cycle of charge / discharge, and the discharge capacity was also measured during the charge / discharge of the 1000th cycle. The ratio of the discharge capacity at the 1000th cycle to the discharge capacity at the 1st cycle was calculated as the capacity retention rate. It can be said that the higher the capacity retention rate, the higher the life characteristics. The results are shown in Table 1 as cycle characteristics.

(DC resistance)
The direct current resistance (DCR) is measured by first charging a constant current of 0.5 C to a voltage that reaches 50% SOC after discharging to 2.7 V, and the current value at that voltage is reached when the voltage reaches 50% SOC. The battery was charged at a constant voltage until it reached 0.01C. After that, it is charged at 0.2 CA for 11 seconds, discharged at 0.5 CA to a 50% SOC voltage, charged at 1.0 CA for 11 seconds, discharged at 0.5 CA to a 50% SOC voltage, and then at 1.3 CA for 11 seconds. The battery was charged and discharged at 0.5 CA to a voltage of SOC 50%. The charging current value at this time is plotted on the horizontal axis and the amount of voltage change in 10 seconds is plotted on the vertical axis, and the slope of the straight line is defined as DCR at 50% SOC. The ratio of the 1000th cycle DCR to the initial DCR was calculated as the DCR increase rate. DCR indicates the resistance value of the battery. It can be said that the lower the DCR increase rate of the battery that has passed the cycle test, the higher the input characteristics of the battery. The results are shown in Table 1 as DCR.

(Overcharge characteristics)
An overcharge test was performed on the lithium ion battery in the initial state (battery voltage 2.7 V). In the overcharge test, 3CA constant current charging was performed in an environment of 25 ° C., and the behavior was observed. At this time, “cleavage” represents a phenomenon in which the gas discharge valve operates and the non-aqueous electrolyte is released from the battery case. It can be said that the longer the time until the gas discharge valve is opened, the longer it can withstand overcharge and the higher the safety. The results are shown in Table 1.

  As shown in Examples 1 to 5 of the present invention, graphitizable carbon is used as the negative electrode active material, lithium bis (oxalato) borate and vinylene carbonate are included in the electrolyte solution, and lithium bis (oxalato) borate and When the content of each vinylene carbonate is within 0.1% by mass to 1.5% by mass with respect to the total amount of the electrolytic solution, a lithium ion secondary battery excellent in cycle characteristics, input characteristics, and safety can be obtained. .

DESCRIPTION OF SYMBOLS 1 ... Lithium ion secondary battery, 2 ... Positive electrode plate, 3 ... Negative electrode plate, 4 ... Separator, 5 ... Electrode winding group, 6 ... Battery container

Claims (3)

A lithium ion secondary battery comprising a positive electrode, a negative electrode, and an electrolyte solution,
The negative electrode includes graphitizable carbon as a negative electrode active material, and the electrolytic solution includes a cyclic carbonate as a non-aqueous solvent, a chain carbonate, and lithium bis (oxalato) borate and vinylene carbonate as additives, and the lithium bis (oxalato). ) A lithium ion secondary battery in which each content of borate and vinylene carbonate is 0.1% by mass to 1.5% by mass with respect to the total amount of the electrolytic solution. The lithium ion secondary battery according to claim 1, wherein the electrolytic solution contains lithium hexafluorophosphate as an electrolyte.   The lithium ion secondary battery according to claim 1, wherein the positive electrode includes a layered lithium / nickel / manganese / cobalt composite oxide as a positive electrode active material.

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