JP6168051B2 - Lithium ion secondary battery - Google Patents

Description

  The present invention relates to a lithium ion secondary battery.

  Generally, a lithium ion secondary battery includes a positive electrode, a negative electrode, an electrolytic solution, and a separator. The electrolytic solution may be decomposed by charging / discharging of the lithium ion secondary battery, which affects the performance of the lithium ion secondary battery. In view of this, for the purpose of improving various performances of lithium ion secondary batteries, studies have been made on electrolytic solutions (see Patent Documents 1 to 3).

JP 2008-066062 A JP 2009-076433 A JP 2011-124039 A

  If the specific surface area of the negative electrode active material of the lithium ion secondary battery is large, the field where charge can be transferred increases, so that the resistance tends to decrease and the output characteristics tend to improve. However, in a lithium ion secondary battery having high output characteristics, generally, a large amount of gas is generated due to decomposition of the solvent of the electrolyte. When a gas is generated, the negative electrode active material is destroyed by the gas, or ions cannot be exchanged between the negative electrode active material and the solvent in a place where gas bubbles exist, and the capacity of the lithium ion secondary battery is reduced. May decrease. Therefore, the capacity of a lithium ion secondary battery that can generate a large amount of gas tends to greatly decrease with charge and discharge.

  Further, in a negative electrode active material using, for example, a carbonaceous active material such as graphite, a bond between graphite layers may be broken during an intercalation reaction to the negative electrode active material, and a conductive path may be cut. . In a lithium ion secondary battery with high output characteristics, since the cutting of the conductive path proceeds rapidly, this also tends to greatly reduce the capacity with charge / discharge.

  As described above, when the capacity decrease due to charging / discharging is large, the cycle characteristics become low. Therefore, the lithium ion secondary battery generally has low cycle characteristics when the output characteristics are high, and conversely, when the cycle characteristics are high, the output characteristics are low. That is, it is difficult to improve both the output characteristics and the cycle characteristics in the lithium ion secondary battery.

  Therefore, it has been studied to devise an electrolytic solution as a means for improving both output characteristics and cycle characteristics. For example, it has been found that when vinylene carbonate (VC) is included in the solvent of the electrolytic solution, the cycle characteristics of the lithium ion secondary battery are improved. This is presumably because decomposition of the solvent of the electrolytic solution is suppressed by using vinylene carbonate. For example, the use of a mixed solvent in which vinylene carbonate and a solvent other than vinylene carbonate such as propylene carbonate (PC) are combined in a lithium ion secondary battery has been studied.

  By the way, propylene carbonate has a low viscosity at a low temperature, and precipitation at a low temperature is difficult to occur compared to a solvent such as ethylene carbonate. Therefore, a lithium ion secondary battery using propylene carbonate as a solvent for an electrolytic solution has been expected to improve output characteristics (low temperature characteristics) at low temperatures. However, when a mixed solvent in which propylene carbonate and vinylene carbonate are combined as described above, the low temperature characteristics of the lithium ion secondary battery are low. This is because if the amount of vinylene carbonate is increased to such an extent that improvement of cycle characteristics can be realized, the effect of increasing the viscosity of the electrolyte by vinylene carbonate is increased at low temperatures, and the viscosity of the electrolyte is increased. It is presumed that it has become lower.

Therefore, in the conventional lithium ion secondary battery, when the low temperature characteristic is high, the cycle characteristic is low, and conversely, when the cycle characteristic is high, the low temperature characteristic is low. Therefore, development of a lithium ion secondary battery that is excellent in both low temperature characteristics and cycle characteristics has been desired.
The present invention has been made in view of the above problems, and an object thereof is to provide a lithium ion secondary battery that is excellent in both low temperature characteristics and cycle characteristics.

As a result of intensive studies in view of the above-mentioned problems, the present inventors have found that when the negative electrode includes a negative electrode active material layer formed of a composition containing a negative electrode active material, a particulate binder, and a water-soluble polymer, A polymer having a specific surface area within a predetermined range, a copolymer containing an ethylenically unsaturated carboxylic acid monomer unit and a fluorine-containing (meth) acrylate monomer unit as a water-soluble polymer, and electrolysis By using a combination of propylene carbonate and vinylene carbonate at a predetermined volume ratio as a liquid solvent, it was found that a lithium ion secondary battery excellent in both low temperature characteristics and cycle characteristics can be realized, and the present invention was completed. It was.
That is, the present invention is as follows.

[1] A positive electrode, a negative electrode, an electrolytic solution, and a separator are provided.
The negative electrode includes a negative electrode active material layer formed of a composition containing a negative electrode active material, a particulate binder, and a water-soluble polymer,
The specific surface area of the negative electrode active material is ^{2m 2} / ^g~15m 2 / g,
The water-soluble polymer is a copolymer comprising an ethylenically unsaturated carboxylic acid monomer unit and a fluorine-containing (meth) acrylic acid ester monomer unit,
The lithium ion secondary battery in which the solvent of the electrolytic solution contains 50% by volume to 80% by volume of propylene carbonate and 0.05% by volume to 1% by volume of vinylene carbonate.
[2] The lithium ion secondary battery according to [1], wherein the negative electrode active material is one or both of a carbonaceous active material and a Si compound.
[3] The lithium ion secondary battery according to [1] or [2], wherein the particulate binder comprises a copolymer containing an aliphatic conjugated diene monomer unit and an ethylenically unsaturated carboxylic acid monomer unit. .
[4] The lithium according to any one of [1] to [3], wherein a content ratio of the ethylenically unsaturated carboxylic acid monomer unit in the water-soluble polymer is 20% by weight to 50% by weight. Ion secondary battery.
[5] The content ratio of the fluorine-containing (meth) acrylic acid ester monomer unit in the water-soluble polymer is 1% to 30% by weight, according to any one of [1] to [4]. Lithium ion secondary battery.
[6] Any of [1] to [5], wherein the weight ratio of the particulate binder and the water-soluble polymer is water-soluble polymer / particulate binder = 0.5 / 99.5 to 40/60. A lithium ion secondary battery according to claim 1.
[7] Any one of [1] to [6], wherein the water-soluble polymer contains a crosslinkable monomer unit, and the content ratio thereof is 0.1 wt% or more and 2 wt% or less. The lithium ion secondary battery described in 1.
[8] The water-soluble polymer contains a (meth) acrylic acid ester monomer unit other than the fluorine-containing (meth) acrylic acid ester monomer, and the content ratio is 30 wt% or more and 70 wt%. The lithium ion secondary battery according to any one of [1] to [7], which is:
[9] The lithium according to any one of [1] to [8], wherein the amount of the particulate binder is 0.1 parts by weight or more and 10 parts by weight or less with respect to 100 parts by weight of the negative electrode active material. Ion secondary battery.
[10] The content ratio of the aliphatic conjugated diene monomer unit in the particulate binder is 20 wt% or more and 60 wt% or less, and the content ratio of the ethylenically unsaturated carboxylic acid monomer unit is 0.00. The lithium ion secondary battery according to [3], which is 1% by weight or more and 15% by weight or less.

  According to the present invention, a lithium ion secondary battery excellent in both low temperature characteristics and cycle characteristics can be realized.

  Hereinafter, the present invention will be described in detail with reference to embodiments and examples. However, the present invention is not limited to the embodiments and examples described below, and may be arbitrarily modified and implemented without departing from the scope of the claims of the present invention and the equivalents thereof.

In the following description, (meth) acrylic acid includes acrylic acid and methacrylic acid. The (meth) acrylate includes acrylate and methacrylate.
Furthermore, that a substance is water-soluble means that an insoluble content is less than 0.5% by weight when 0.5 g of the substance is dissolved in 100 g of water at 25 ° C. On the other hand, that a certain substance is water-insoluble means that an insoluble content is 90% by weight or more when 0.5 g of the substance is dissolved in 100 g of water at 25 ° C.

[1. Overview]
The lithium ion secondary battery of the present invention includes a positive electrode, a negative electrode, an electrolytic solution, and a separator. The negative electrode includes a negative electrode active material layer formed of a composition including a negative electrode active material, a particulate binder, and a water-soluble polymer. The specific surface area of the negative electrode active material is ^{2m 2} / ^g~15m 2 / g. Further, the water-soluble polymer is a copolymer containing an ethylenically unsaturated carboxylic acid monomer unit and a fluorine-containing (meth) acrylic acid ester monomer unit. Moreover, the solvent of electrolyte solution contains propylene carbonate 50 volume%-80 volume% and vinylene carbonate 0.05 volume%-1 volume%.

  With such a configuration, the lithium ion secondary battery of the present invention is excellent in both low temperature characteristics and cycle characteristics. Furthermore, the lithium ion secondary battery of the present invention is usually excellent in high-temperature storage characteristics, and can normally suppress the swelling of the battery cell due to charge / discharge. The reason why such an excellent advantage can be obtained is not clear, but according to the study of the present inventor, it is presumed that the reason is as follows.

i. Low temperature characteristics Propylene carbonate is a solvent having a low viscosity at low temperatures. Since the lithium ion secondary battery of the present invention contains a large amount of propylene carbonate as a solvent for the electrolytic solution, the viscosity of the electrolytic solution at a low temperature can be lowered. Therefore, it is difficult for the solvent component of the electrolytic solution to precipitate in the solvent, so that the internal resistance can be lowered. Therefore, it is possible to improve the output characteristics of the lithium ion secondary battery at a low temperature, and the low temperature characteristics can be improved.

  In the lithium ion secondary battery of the present invention, the amount of vinylene carbonate in the electrolytic solution is reduced. For this reason, since the fall of the output in the low temperature by vinylene carbonate can be suppressed, the improvement of a low temperature characteristic is also possible by this.

  Furthermore, in the lithium ion secondary battery of the present invention, cycle characteristics can be improved even when a negative electrode active material having a large specific surface area is used. For this reason, since a negative electrode active material with a large specific surface area can be used, output characteristics can be improved. Therefore, the low temperature characteristics can be improved.

  Further, as described later, it is considered that the water-soluble polymer forms a film in the negative electrode active material layer and covers the negative electrode active material. Therefore, in the lithium ion secondary battery of the present invention, at first glance, it seems that the resistance is increased by the amount of the water-soluble polymer film, and the low-temperature characteristics are decreased. However, since the water-soluble polymer contains a fluorine-containing (meth) acrylate monomer unit, the water-soluble polymer film is excellent in ion conductivity. Therefore, actually, in the lithium ion secondary battery of the present invention, there is no significant increase in resistance due to the water-soluble polymer coating.

ii. Cycle characteristics In the lithium ion secondary battery of the present invention, the negative electrode active material layer is a water-soluble copolymer containing an ethylenically unsaturated carboxylic acid monomer unit and a fluorine-containing (meth) acrylate monomer unit. Including as a polymer. This water-soluble polymer can form a film in the negative electrode active material layer of the lithium ion secondary battery. Since the negative electrode active material is covered with this film, it is difficult for the negative electrode active material and propylene carbonate to be in direct contact with each other in the negative electrode. For this reason, in the lithium ion secondary battery of this invention, decomposition | disassembly of propylene carbonate is suppressed and it is hard to produce gas. Further, even when a carbonaceous active material such as graphite is used as the negative electrode active material, the bond between the graphite layers is hardly broken by propylene carbonate during the intercalation reaction. Therefore, even if charging / discharging is repeated, the capacity of the lithium ion secondary battery is unlikely to decrease, so that the cycle characteristics can be improved.

  Moreover, in the lithium ion secondary battery of this invention, since vinylene carbonate is contained in electrolyte solution, it is thought that cycling characteristics are improving also by the effect of this vinylene carbonate. Since the amount of vinylene carbonate is smaller than before, there is little or no deterioration in low-temperature characteristics due to vinylene carbonate. Moreover, even if there is little vinylene carbonate, since the effect by a water-soluble polymer is acquired as mentioned above, cycling characteristics can be made high enough.

  Furthermore, propylene carbonate is less likely to be decomposed than other solvents such as ethylene carbonate, and hardly produces gas. Therefore, it is considered that the use of a large amount of propylene carbonate as a solvent also contributes to the improvement of cycle characteristics.

iii. As described above, in the conventional lithium ion secondary battery, for example, the specific surface area of the negative electrode active material is increased or the amount of vinylene carbonate is decreased to reduce the low temperature characteristics. When the value was good, the cycle characteristics were inferior. Conversely, when the cycle characteristics are improved by reducing the specific surface area of the negative electrode active material or increasing the amount of vinylene carbonate, for example, the low temperature characteristics are inferior. Therefore, conventionally, in order to achieve both low temperature characteristics and cycle characteristics, optimization has been made by adjusting the specific surface area of the negative electrode active material and the amount of vinylene carbonate within a range where acceptable performance can be obtained. It was.
On the other hand, in the lithium ion secondary battery of the present invention, a copolymer containing an ethylenically unsaturated carboxylic acid monomer unit and a fluorine-containing (meth) acrylic acid ester monomer unit is used as a water-soluble polymer. As a result, both the low-temperature characteristics and the cycle characteristics can be improved more than expected, and an effect exceeding mere optimization can be obtained.

  The reason why both the low-temperature characteristics and the cycle characteristics can be improved more than expected is as follows. That is, since the ethylenically unsaturated carboxylic acid monomer unit contained in the water-soluble polymer is excellent in affinity with the negative electrode active material, a film having excellent binding properties to the surface of the negative electrode active material can be formed. For this reason, even if the negative electrode active material expands and contracts with charge and discharge, the coating is difficult to peel off, and the coating can be maintained stably. Furthermore, since the film is difficult to peel off, it is possible to suppress an increase in the resistance of the lithium ion secondary battery due to the film being dispersed at a place other than the surface of the negative electrode active material. Moreover, since the fluorine-containing (meth) acrylic acid ester monomer unit contained in the water-soluble polymer increases the ion conductivity of the film, an increase in resistance due to the film can be suppressed. It is speculated that these actions can effectively improve both the low temperature characteristics and the cycle characteristics.

  Further, according to the study of the present inventor, even when a water-soluble polymer is used, it is known that when vinylene carbonate is not used, output characteristics and cycle characteristics are inferior (see Comparative Example 6). In view of this, in the lithium ion secondary battery of the present invention, it is possible to improve both the low temperature characteristics and the cycle characteristics more than expected, the effect obtained by simply using a water-soluble polymer. Not only by combining a mixed solvent containing propylene carbonate and vinylene carbonate with a water-soluble polymer containing an ethylenically unsaturated carboxylic acid monomer unit and a fluorine-containing (meth) acrylic acid ester monomer unit. It is considered to be an effect obtained by involving any action that appears.

iv. High-temperature storage characteristics In the lithium ion secondary battery of the present invention, as described above, the negative electrode active material is covered with a water-soluble polymer film. For this reason, decomposition | disassembly of electrolyte solution is prevented not only in a low temperature environment but in a high temperature environment. Therefore, even when stored in a high temperature environment, the capacity of the lithium ion secondary battery of the present invention is difficult to decrease.

v. Inhibition of swelling of battery cell In the lithium ion secondary battery of the present invention, since the decomposition of the electrolytic solution due to charge / discharge can be prevented as described above, the generation of gas can be suppressed. Therefore, it is possible to suppress the swelling of the battery cell due to the generation of gas.
Further, since the water-soluble polymer film covers the negative electrode active material, the rigidity of the negative electrode active material layer can be increased. Therefore, expansion of the negative electrode due to repeated charge / discharge can be suppressed. Therefore, this also makes it possible to suppress the swelling of the battery cells.

[2. Negative electrode]
The negative electrode includes a negative electrode active material layer. Usually, the negative electrode includes a current collector, and a negative electrode active material layer is provided on the surface of the current collector. Under the present circumstances, the negative electrode active material layer may be provided in the single side | surface of the electrical power collector, and may be provided in both surfaces.

[2.1. Current collector]
As the current collector, one formed of a material having electrical conductivity and electrochemical durability is usually used. As a material for the current collector, a metal material is preferable since it has heat resistance. For example, iron, copper, aluminum, nickel, stainless steel, titanium, tantalum, gold, platinum, and the like can be given. Among these, copper is preferable as the current collector for the negative electrode.

The shape of the current collector is not particularly limited, and a sheet shape having a thickness of about 0.001 mm to 0.5 mm is preferable.
In order to increase the binding strength with the negative electrode active material layer, the current collector may be subjected to a roughening treatment prior to forming the negative electrode active material layer thereon. In addition, when there is a layer interposed between the current collector and the negative electrode active material layer, the current collector should be roughened prior to forming the layer on the current collector. May be. Examples of the roughening method include a mechanical polishing method, an electrolytic polishing method, and a chemical polishing method. In the mechanical polishing method, for example, abrasive cloth paper, abrasive wheels, emery buffs, wire brushes equipped with steel wires, etc., to which abrasive particles are fixed are used.
Further, an intermediate layer may be formed on the surface of the current collector in order to increase the binding strength between the current collector and the negative electrode active material layer or increase the conductivity.

[2.2. Negative electrode active material layer]
The negative electrode active material layer is a layer formed of a composition containing a negative electrode active material, a particulate binder, and a water-soluble polymer.

[2.2.1. Negative electrode active material]
The negative electrode active material is an electrode active material for a negative electrode, and is a material that can transfer electrons in the negative electrode of the secondary battery.

The specific surface area of the negative electrode active material is usually 2 m ² / g or more, usually 15 m ² / g or less, preferably 13 m ² / g or less, more preferably 10 m ² / g or less. By using a negative electrode active material having a specific surface area equal to or greater than the lower limit of the above range, the output characteristics of the lithium ion secondary battery can be improved. Further, by using a negative electrode active material having a specific surface area equal to or less than the upper limit of the above range, cycle characteristics can be improved and the life of the lithium ion secondary battery can be extended.

  The specific surface area of the negative electrode active material can be measured by a BET method (apparatus; Tristar II 3020 series, manufactured by Shimadzu Corporation) by nitrogen gas adsorption.

  As the negative electrode active material, a material that can occlude and release lithium is usually used. Examples of the negative electrode active material include a carbonaceous active material and a metal-based active material.

  The carbonaceous active material refers to a negative electrode active material having carbon as a main skeleton capable of inserting (also referred to as doping) and detaching (also referred to as dedoping) lithium ions. Specific examples of the carbonaceous active material include carbonaceous materials and graphite materials.

  The carbonaceous material generally indicates a material with low graphitization (that is, low crystallinity) obtained by carbonizing a carbon precursor by heat treatment at 2000 ° C. or lower. Although the minimum of the said heat processing temperature is not specifically limited, For example, it is good also as 500 degreeC or more. Examples of the carbonaceous material include graphitizable carbon that easily changes the carbon structure depending on the heat treatment temperature, non-graphitic carbon having a structure close to an amorphous structure typified by glassy carbon, and the like.

  Examples of graphitizable carbon include carbon materials made from tar pitch obtained from petroleum or coal. Specific examples include coke, mesocarbon microbeads (MCMB), mesophase pitch carbon fibers, pyrolytic vapor grown carbon fibers, and the like. Here, MCMB is carbon fine particles obtained by separating and extracting mesophase microspheres generated in the process of heating pitches at around 400 ° C. The mesophase pitch-based carbon fiber is a carbon fiber made from mesophase pitch obtained by growing and coalescing the mesophase microspheres. Furthermore, pyrolytic vapor-grown carbon fibers are (1) a method of thermally decomposing acrylic polymer fibers, (2) a method of thermally decomposing by spinning a pitch, or (3) catalyzing nanoparticles such as iron. It is a carbon fiber obtained by a catalytic vapor phase growth (catalytic CVD) method in which hydrocarbon is vapor-phase pyrolyzed.

  Examples of the non-graphitizable carbon include a phenol resin fired body, polyacrylonitrile-based carbon fiber, pseudo-isotropic carbon, furfuryl alcohol resin fired body (PFA), and hard carbon.

  The graphite material refers to a graphite material having high crystallinity close to that of graphite obtained by heat-treating graphitizable carbon at 2000 ° C. or higher. Although the upper limit of the said heat processing temperature is not specifically limited, For example, it is good also as 5000 degrees C or less. Examples of the graphite material include natural graphite and artificial graphite. Examples of artificial graphite include artificial graphite mainly heat-treated at 2800 ° C. or higher, graphitized MCMB heat-treated at 2000 ° C. or higher, graphitized mesophase pitch-based carbon fiber heat-treated mesophase pitch-based carbon fiber at 2000 ° C. or higher, etc. Is mentioned.

  The graphite interlayer distance of the carbonaceous active material is preferably 0.340 nm or more, more preferably 0.345 nm or more, particularly preferably 0.350 nm or more, more preferably 0.370 nm or less, and more preferably 0.365 nm or less. Preferably, it is 0.360 nm or less. Here, the graphite interlayer distance indicates a surface spacing (d value) of the (002) plane by an X-ray diffraction method. By using such a carbonaceous active material called so-called soft carbon, a lithium ion secondary battery having excellent output characteristics can be obtained without excessively reducing the capacity per volume.

  Among these carbonaceous active materials, a graphite material is preferable. Among them, a graphite material whose surface is coated with a carbonaceous material is particularly preferable. For example, graphite whose surface is coated with an amorphous carbonaceous material is preferable because it can improve the output characteristics of a lithium ion secondary battery. Conventionally, the use of a graphite material whose surface is coated with a carbonaceous material has resulted in significant gas generation. However, in the lithium ion secondary battery of the present invention, since the generation of gas can be suppressed, it is possible to suppress the performance degradation due to the generation of gas as in the conventional case.

  A metal-based active material refers to an active material containing a metal. In general, a metal-based active material refers to an active material that includes an element capable of inserting lithium in the structure and has a theoretical electric capacity per weight of 500 mAh / g or more when lithium is inserted. The upper limit of the theoretical electric capacity is not particularly limited, but may be, for example, 5000 mAh / g or less. As the metal-based active material, for example, lithium metal, a single metal forming a lithium alloy and an alloy thereof, and a compound thereof (for example, oxide, sulfide, nitride, silicide, carbide, phosphide, etc.) are used. It is done.

  Examples of the single metal forming the lithium alloy include single metals such as Ag, Al, Ba, Bi, Cu, Ga, Ge, In, Ni, P, Pb, Sb, Si, Sn, Sr, Zn, and Ti. Can be mentioned. Moreover, as a single metal alloy which forms a lithium alloy, the compound containing the said single metal is mentioned, for example. Among these, silicon (Si), tin (Sn), lead (Pb), and titanium (Ti) are preferable, and silicon, tin, and titanium are more preferable. Therefore, the metal-based active material is preferably a single metal of silicon (Si), tin (Sn), or titanium (Ti), an alloy containing these single metals, or a compound thereof.

The metal-based active material may contain one or more nonmetallic elements. For example, SiC, SiO _x C _y (0 <x ≦ 3, 0 <y ≦ 5), Si ₃ N ₄ , Si ₂ N ₂ O, SiO _x (0 <x ≦ 2), SnO _x (0 <x ≦ 2), LiSiO, LiSnO and the like. Among them, the insertion of lithium at low potential and desorption capable _SiO x, SiC and _SiO x _{C y} is preferred. For example, SiO _x C _y can be obtained by firing a polymer material containing silicon. Among SiO _x C _y, the balance of the capacity and cycle characteristics, preferably in the range of 0.8 ≦ x ≦ 3,2 ≦ y ≦ 4.

  Lithium metal, elemental metal forming a lithium alloy, and oxides, sulfides, nitrides, silicides, carbides, and phosphides of the alloys include, for example, oxides, sulfides, and nitrides of elements into which lithium can be inserted. , Silicides, carbides, phosphides and the like. Among these, an oxide is particularly preferable. For example, a lithium-containing metal composite oxide containing an oxide such as tin oxide, manganese oxide, titanium oxide, niobium oxide, and vanadium oxide and a metal element selected from the group consisting of Si, Sn, Pb, and Ti atoms is used. .

As the lithium-containing metal composite oxide, a lithium titanium composite oxide represented by Li _x Ti _y M _z O ₄ (0.7 ≦ x ≦ 1.5, 1.5 ≦ y ≦ 2.3, 0 ≦ z ≦ 1.6, and M represents an element selected from the group consisting of Na, K, Co, Al, Fe, Ti, Mg, Cr, Ga, Cu, Zn, and Nb.), Li _x Mn _y M A lithium manganese composite oxide represented by _z O ₄ (x, y, z and M are the same as defined in the lithium titanium composite oxide). Among these, Li _4/3 Ti _5/3 O ₄ , Li ₁ Ti ₂ O ₄ , Li _4/5 Ti _11/5 O ₄ , and Li _4/3 Mn _5/3 O ₄ are preferable.

Among these, a Si compound is preferable as the metal-based active material. Here, the Si compound refers to a compound containing silicon. By using the Si compound, the electric capacity of the secondary battery can be increased. Of the Si compounds, SiC, SiO _x and SiO _x C _y are preferable. In an active material containing a combination of these Si and C, when Li is inserted into and desorbed from Si (silicon) at a high potential, and Li is inserted into and desorbed from C (carbon) at a low potential Guessed. For this reason, since expansion and contraction are suppressed as compared with other metal-based active materials, the charge / discharge cycle characteristics of the secondary battery can be improved.

Moreover, a negative electrode active material may be used individually by 1 type, and may combine 2 or more types by arbitrary ratios. For example, a carbonaceous active material and a metal-based active material may be used in combination.
Among the negative electrode active materials described above, it is preferable to use one or both of a carbonaceous active material and a Si compound from the viewpoint of improving the balance between cycle characteristics and output characteristics. Among these, it is particularly preferable to use (i) only a carbonaceous active material and (ii) a combination of a carbonaceous active material and a Si compound.

  The negative electrode active material is preferably sized in the form of particles. When the shape of the particles is spherical, a higher density electrode can be formed during electrode molding. When the negative electrode active material is particles, the volume average particle diameter is appropriately selected in consideration of other constituent requirements of the lithium ion secondary battery. The specific volume average particle diameter of the negative electrode active material is preferably 0.1 μm or more, more preferably 1 μm or more, further preferably 2 μm or more, particularly preferably 5 μm or more, preferably 50 μm or less, more preferably 30 μm or less. Particularly preferably, it is 25 μm or less. When the volume average particle diameter of the carbonaceous active material as the negative electrode active material is in the above range, the amount of the particulate binder can be reduced when preparing the slurry composition for the negative electrode. For this reason, the capacity | capacitance fall of a lithium ion secondary battery can be suppressed, and the viscosity of a slurry composition can be easily adjusted to an appropriate range. Here, the volume average particle diameter is a particle diameter at which the cumulative volume calculated from the small diameter side in the measured particle diameter distribution is 50% when the particle diameter distribution is measured by a laser diffraction method.

[2.2.2. Particulate binder]
The particulate binder is a component capable of binding the electrode active materials to each other or binding the electrode active material and the current collector. In the negative electrode, the particulate binder binds the negative electrode active material, whereby the detachment of the negative electrode active material from the negative electrode active material layer is suppressed. In addition, the particulate binder usually binds particles other than the negative electrode active material contained in the negative electrode active material layer, and also plays a role of maintaining the strength of the negative electrode active material layer.

  As the particulate binder, it is preferable to use a binder excellent in performance for holding the negative electrode active material and having high binding property to the current collector. Usually, a polymer is used as the material of the particulate binder. The polymer as the material for the particulate binder may be a homopolymer or a copolymer. Among these, a copolymer containing an aliphatic conjugated diene monomer unit and an ethylenically unsaturated carboxylic acid monomer unit is preferable.

  The aliphatic conjugated diene monomer unit is a structural unit obtained by polymerizing an aliphatic conjugated diene monomer. Since the aliphatic conjugated diene monomer unit has a low rigidity and is a flexible structural unit, the flexibility of the particulate binder is increased by forming the particulate binder with a polymer containing the aliphatic conjugated diene monomer unit. be able to. Therefore, sufficient binding between the negative electrode active material layer and the current collector can be obtained.

  Examples of aliphatic conjugated diene monomers include 1,3-butadiene, 2-methyl-1,3-butadiene, 2,3-dimethyl-1,3 butadiene, 2-chloro-1,3-butadiene, substituted Examples include straight chain conjugated pentadienes, and substituted and side chain conjugated hexadienes. Among these, 1,3-butadiene and 2-methyl-1,3-butadiene are preferable, and 1,3-butadiene is particularly preferable. Moreover, these may be used individually by 1 type and may be used combining two or more types by arbitrary ratios.

  In the polymer forming the particulate binder, the content of the aliphatic conjugated diene monomer unit is preferably 20% by weight or more, more preferably 25% by weight or more, preferably 60% by weight or less, more preferably 50% by weight or less. By setting the content ratio of the aliphatic conjugated diene monomer unit to the lower limit or more of the above range, the binding property can be improved, and by setting the content ratio to the upper limit or less, the flexibility of the negative electrode can be ensured. Usually, the content ratio of the aliphatic conjugated diene monomer unit in the polymer coincides with the ratio (preparation ratio) of the aliphatic conjugated diene monomer in all monomers used when the polymer is produced.

  An ethylenically unsaturated carboxylic acid monomer unit is a structural unit obtained by polymerizing an ethylenically unsaturated carboxylic acid monomer. Since the ethylenically unsaturated carboxylic acid monomer unit includes a carboxy group (—COOH group) which is an acidic functional group, the adsorptivity of the particulate binder to the negative electrode active material and the current collector can be improved. The ethylenically unsaturated carboxylic acid monomer unit is a structural unit having high strength. By these, if the particulate binder is formed of a copolymer containing an ethylenically unsaturated carboxylic acid monomer unit, the detachment of the negative electrode active material from the negative electrode active material layer can be stably prevented, and the negative electrode Strength can be improved.

  Examples of the ethylenically unsaturated carboxylic acid monomer include monocarboxylic and dicarboxylic acids such as acrylic acid, methacrylic acid, crotonic acid, maleic acid, fumaric acid and itaconic acid, and anhydrides thereof. Of these, acrylic acid, methacrylic acid and itaconic acid are preferred, and itaconic acid is particularly preferred from the viewpoint of the stability of the slurry composition for the negative electrode. Moreover, these may be used individually by 1 type and may be used combining two or more types by arbitrary ratios.

  In the polymer forming the particulate binder, the content of the ethylenically unsaturated carboxylic acid monomer unit is preferably 0.1% by weight or more, more preferably 1% by weight or more, and preferably 15% by weight or less. More preferably, it is 10% by weight or less. By making the content ratio of the ethylenically unsaturated carboxylic acid monomer unit at least the lower limit of the above range, the binding property can be improved, and by making it the upper limit or less, the electrochemical stability of the negative electrode is improved. Can do. Usually, the content ratio of the ethylenically unsaturated carboxylic acid monomer unit in the polymer is the same as the ratio of the ethylenically unsaturated carboxylic acid monomer in all the monomers used for producing the polymer (feed ratio). To do.

  The polymer forming the particulate binder contains an arbitrary structural unit in addition to the aliphatic conjugated diene monomer unit and the ethylenically unsaturated carboxylic acid monomer unit described above, as long as the effects of the present invention are not significantly impaired. You may go out. Examples of optional monomers corresponding to the above arbitrary structural units include aromatic vinyl monomers, vinyl cyanide monomers, unsaturated carboxylic acid alkyl ester monomers, and hydroxyalkyl groups. And unsaturated carboxylic acid amide monomers. One of these may be used alone, or two or more of these may be used in combination at any ratio.

Examples of the aromatic vinyl monomer include styrene, α-methylstyrene, vinyl toluene, and divinylbenzene. Of these, styrene is preferred.
Examples of the vinyl cyanide monomer include acrylonitrile, methacrylonitrile, α-chloroacrylonitrile, and α-ethylacrylonitrile. Of these, acrylonitrile and methacrylonitrile are preferable.
Examples of unsaturated carboxylic acid alkyl ester monomers include methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, butyl acrylate, glycidyl methacrylate, dimethyl fumarate, diethyl fumarate, dimethyl maleate, diethyl maleate, dimethyl itaco Nates, monomethyl fumarate, monoethyl fumarate, and 2-ethylhexyl acrylate. Of these, methyl methacrylate is preferable.
Examples of unsaturated monomers containing a hydroxyalkyl group include β-hydroxyethyl acrylate, β-hydroxyethyl methacrylate, hydroxypropyl acrylate, hydroxypropyl methacrylate, hydroxybutyl acrylate, hydroxybutyl methacrylate, 3-chloro-2- Examples include hydroxypropyl methacrylate, di- (ethylene glycol) maleate, di- (ethylene glycol) itaconate, 2-hydroxyethyl maleate, bis (2-hydroxyethyl) maleate, and 2-hydroxyethyl methyl fumarate. Among these, β-hydroxyethyl acrylate is preferable.
Examples of unsaturated carboxylic acid amide monomers include acrylamide, methacrylamide, N-methylol acrylamide, N-methylol methacrylamide, and N, N-dimethylacrylamide. Of these, acrylamide and methacrylamide are preferable.
These arbitrary monomers may be used individually by 1 type, and may be used combining two or more types by arbitrary ratios.

  Further, the polymer forming the particulate binder is formed by polymerizing monomers used in ordinary emulsion polymerization such as ethylene, propylene, vinyl acetate, vinyl propionate, vinyl chloride, vinylidene chloride, and the like. A structural unit having a structure may be included. One of these may be used alone, or two or more of these may be used in combination at any ratio.

  In the polymer forming the particulate binder, the content ratio of an arbitrary structural unit is preferably 0.1% by weight or more, more preferably 0.2% by weight or more, particularly preferably 0.5% by weight or more, Preferably it is 75 weight% or less, More preferably, it is 70 weight% or less, Most preferably, it is 65 weight% or less. Usually, the content ratio of an arbitrary structural unit in a polymer matches the ratio (preparation ratio) of an arbitrary monomer in all monomers used when the polymer is produced.

The weight average molecular weight of the polymer forming the particulate binder is preferably 10,000 or more, more preferably 20,000 or more, preferably 5,000,000 or less, more preferably 1,000,000 or less. When the weight average molecular weight is in the above range, the strength of the negative electrode and the dispersibility of the negative electrode active material of the present invention are easily improved.
The weight average molecular weight of the particulate binder can be determined as a value in terms of polystyrene using tetrahydrofuran as a developing solvent by gel permeation chromatography (GPC).

  The glass transition temperature of the particulate binder is preferably −40 ° C. or higher, preferably 50 ° C. or lower, more preferably 0 ° C. or lower. When the glass transition temperature of the particulate binder is within the above range, the binding property and the electrode strength can be increased.

  The number average particle size of the particulate binder is preferably 50 nm or more, more preferably 70 nm or more, preferably 500 nm or less, more preferably 400 nm or less. When the number average particle diameter of the particulate binder is in the above range, the strength and flexibility of the negative electrode can be improved. The presence of particles can be easily measured by transmission electron microscopy, Coulter counter, laser diffraction scattering, or the like.

  The particulate binder can be produced, for example, by polymerizing a monomer composition containing the above-described monomer in an aqueous solvent to form polymer particles. By using a particulate binder polymerized in an aqueous solvent, it is possible to prevent the organic solvent from remaining in the lithium ion secondary battery. As a result, the decomposition gas of the residual organic solvent in the use of the lithium ion secondary battery can be prevented. Cell deformation due to occurrence can be avoided.

  The ratio of each monomer in the monomer composition is usually the content ratio of structural units (for example, aliphatic conjugated diene monomer units and ethylenically unsaturated carboxylic acid monomer units) in the polymer to be produced. And so on.

  The aqueous solvent is not particularly limited as long as the particulate binder can be dispersed. The aqueous solvent usually has a boiling point at normal pressure of preferably 80 ° C. or higher, more preferably 100 ° C. or higher, preferably 350 ° C. or lower, more preferably 300 ° C. or lower. Examples of the aqueous solvent will be given below. In the following examples, the number in parentheses after the solvent name is the boiling point (unit: ° C) at normal pressure, and the value after the decimal point is a value rounded off or rounded down.

  Examples of aqueous solvents include water (100); ketones such as diacetone alcohol (169) and γ-butyrolactone (204); ethyl alcohol (78), isopropyl alcohol (82), normal propyl alcohol (97) and the like. Alcohols: propylene glycol monomethyl ether (120), methyl cellosolve (124), ethyl cellosolve (136), ethylene glycol tertiary butyl ether (152), butyl cellosolve (171), 3-methoxy-3-methyl-1-butanol (174), Ethylene glycol monopropyl ether (150), diethylene glycol monobutyl pyrether (230), triethylene glycol monobutyl ether (271), dipropylene glycol monomethyl ether (188) Glycol ethers and the like; and 1,3-dioxolane (75), 1,4-dioxolane (101), ethers such as tetrahydrofuran (66) and the like. Among them, water is particularly preferable from the viewpoint that it is not flammable and a dispersion of a particulate binder can be easily obtained. Further, water may be used as the main solvent, and an aqueous solvent other than water may be mixed and used within a range in which the dispersed state of the particulate binder can be ensured.

  The polymerization method is not particularly limited, and any method such as a solution polymerization method, a suspension polymerization method, a bulk polymerization method, and an emulsion polymerization method may be used. As the polymerization method, any method such as ionic polymerization, radical polymerization, and living radical polymerization may be used. It is easy to obtain a high molecular weight product, and since the polymer is obtained in a state of being dispersed in water as it is, no redispersion treatment is required, and it can be used for production of a slurry composition for a negative electrode. From the viewpoint of efficiency, the emulsion polymerization method is particularly preferable.

  The emulsion polymerization method is usually performed by a conventional method. For example, the method is described in “Experimental Chemistry Course” Vol. 28, (Publisher: Maruzen Co., Ltd., edited by The Chemical Society of Japan). That is, water, an additive such as a dispersant and a crosslinking agent, a polymerization initiator, and a monomer are mixed in a closed container equipped with a stirrer and a heating device so as to have a predetermined composition, and the composition in the container Is stirred to emulsify the monomer or the like in water, and the temperature is increased while stirring to initiate polymerization. Or after emulsifying the said composition, it is the method of putting into a sealed container and starting reaction similarly.

  Examples of polymerization initiators include organic compounds such as lauroyl peroxide, diisopropyl peroxydicarbonate, di-2-ethylhexyl peroxydicarbonate, t-butyl peroxypivalate, 3,3,5-trimethylhexanoyl peroxide, and the like. Peroxides; azo compounds such as α, α′-azobisisobutyronitrile; ammonium persulfate; and potassium persulfate. A polymerization initiator may be used individually by 1 type, and may be used combining two or more types by arbitrary ratios. The amount of the polymerization initiator may be 0.01 to 5 parts by weight with respect to 100 parts by weight of the monomer.

  Examples of dispersants include benzene sulfonates such as sodium dodecyl benzene sulfonate and sodium dodecyl phenyl ether sulfonate; alkyl sulfates such as sodium lauryl sulfate and sodium tetradodecyl sulfate; sodium dioctyl sulfosuccinate, sodium dihexyl sulfosuccinate, etc. Sulfosuccinates; fatty acid salts such as sodium laurate; ethoxy sulfate salts such as polyoxyethylene lauryl ether sulfate sodium salt and polyoxyethylene nonylphenyl ether sulfate sodium salt; alkane sulfonates; sodium alkyl ether phosphates Salt; polyoxyethylene nonylphenyl ether, polyoxyethylene sorbitan lauryl ester, polyoxyethylene-polyoxypro Nonionic emulsifier such as renblock copolymer; gelatin, maleic anhydride-styrene copolymer, polyvinyl pyrrolidone, sodium polyacrylate, polyvinyl alcohol having a polymerization degree of 700 or more and a saponification degree of 75% or more. Examples include molecules. Among these, benzenesulfonates such as sodium dodecylbenzenesulfonate and sodium dodecylphenylethersulfonate; alkyl sulfates such as sodium lauryl sulfate and sodium tetradodecylsulfate are preferable. More preferred are benzene sulfonates such as sodium dodecyl benzene sulfonate and sodium dodecyl phenyl ether sulfonate from the viewpoint of excellent oxidation resistance. A dispersing agent may be used individually by 1 type, and may be used combining two or more types by arbitrary ratios. The amount of the dispersant may be 0.01 to 10 parts by weight with respect to 100 parts by weight of the monomer.

  The polymerization temperature and polymerization time can be arbitrarily selected depending on the polymerization method and the type of polymerization initiator. The polymerization temperature is preferably 0 ° C. or higher, more preferably 25 ° C. or higher, particularly preferably 30 ° C. or higher, preferably 100 ° C. or lower, more preferably 80 ° C. or lower. The polymerization time is preferably 0.5 hours or more, more preferably 1 hour or more, particularly preferably 5 hours or more, preferably 50 hours or less, more preferably 30 hours or less, particularly preferably 20 hours or less. is there.

In the polymerization, seed polymerization may be performed using seed particles.
Further, additives such as amines may be used as a polymerization aid.

Further, the pH of the aqueous dispersion containing the particulate binder obtained by these methods may be adjusted to be preferably in the range of 5 to 10, more preferably 5 to 9. In this case, as a method for adjusting the pH, for example, hydroxides of alkali metals (for example, Li, Na, K, Rb, Cs), ammonia, inorganic ammonium compounds (for example, NH ₄ Cl), organic amine compounds (for example, And a basic aqueous solution containing ethanolamine, diethylamine, etc.) and an aqueous dispersion. Among these, pH adjustment with an alkali metal hydroxide is preferable because it improves the binding property (peel strength) between the current collector and the negative electrode active material.

  The particulate binder may be a composite polymer particle composed of two or more kinds of polymers. The composite polymer particles are prepared by polymerizing at least one monomer component by a conventional method, then polymerizing at least one other monomer component, and polymerizing by a conventional method (two-stage polymerization method), etc. Can also be obtained. In this way, by polymerizing the monomer stepwise, it is possible to obtain core-shell structured particles having a core layer present inside the particle and a shell layer covering the core layer.

  The amount of the particulate binder is preferably 0.1 parts by weight or more, more preferably 0.5 parts by weight or more, particularly preferably 0.8 parts by weight or more with respect to 100 parts by weight of the negative electrode active material, It is 10 parts by weight or less, more preferably 8 parts by weight or less, and particularly preferably 5 parts by weight or less. By making the amount of the particulate binder not less than the lower limit of the above range, it is possible to prevent detachment of the negative electrode active material from the negative electrode active material layer and to reduce the occurrence of short-circuiting in the lithium ion secondary battery. Further, by setting the value to the upper limit or less, the internal resistance can be kept low and the output characteristics can be improved.

[2.2.3. Water-soluble polymer]
It is considered that the water-soluble polymer according to the present invention forms a film covering the negative electrode active material in the lithium ion secondary battery, and decomposition of the electrolyte is suppressed by the action of the film.

  The water-soluble polymer contains an ethylenically unsaturated carboxylic acid monomer unit. For this reason, the water-soluble polymer usually has a carboxy group (—COOH group) as an acidic functional group. When this carboxy group functions as an acidic functional group, the water-soluble polymer can exhibit excellent binding properties. That is, the polar group present on the surface of the negative electrode active material interacts with the acidic functional group of the water-soluble polymer, so that the water-soluble polymer is held on the surface of the negative electrode active material and forms a stable film. It has become possible.

Examples of the ethylenically unsaturated carboxylic acid monomer include ethylenically unsaturated monocarboxylic acid and derivatives thereof, ethylenically unsaturated dicarboxylic acid and acid anhydrides thereof, and derivatives thereof. Examples of ethylenically unsaturated monocarboxylic acids include acrylic acid, methacrylic acid, and crotonic acid. Examples of ethylenically unsaturated monocarboxylic acid derivatives include 2-ethylacrylic acid, isocrotonic acid, α-acetoxyacrylic acid, β-trans-aryloxyacrylic acid, α-chloro-β-E-methoxyacrylic acid, And β-diaminoacrylic acid. Examples of ethylenically unsaturated dicarboxylic acids include maleic acid, fumaric acid, and itaconic acid. Examples of acid anhydrides of ethylenically unsaturated dicarboxylic acids include maleic anhydride, acrylic anhydride, methyl maleic anhydride, and dimethyl maleic anhydride. Examples of derivatives of ethylenically unsaturated dicarboxylic acids include methyl maleate such as methylmaleic acid, dimethylmaleic acid, phenylmaleic acid, chloromaleic acid, dichloromaleic acid, fluoromaleic acid; and diphenyl maleate, nonyl maleate And maleate esters such as decyl maleate, dodecyl maleate, octadecyl maleate and fluoroalkyl maleate. One of these may be used alone, or two or more of these may be used in combination at any ratio. Among these, ethylenically unsaturated monocarboxylic acids such as acrylic acid and methacrylic acid are preferable. Solubility in water of the resulting water-soluble polymer it is possible to increase more.

  In the water-soluble polymer, the content of the ethylenically unsaturated carboxylic acid monomer unit is preferably 20% by weight or more, more preferably 25% by weight or more, particularly preferably 30% by weight or more, preferably 50% by weight. % Or less, more preferably 45% by weight or less, and particularly preferably 40% by weight or less. Improving the binding property of the negative electrode (that is, the binding property between the negative electrode active material layer and the current collector) by setting the content ratio of the ethylenically unsaturated carboxylic acid monomer unit to the lower limit of the above range, or The life characteristics of the lithium ion secondary battery can be improved, and the flexibility of the negative electrode can be ensured by setting it to the upper limit or less. Usually, the content ratio of the ethylenically unsaturated carboxylic acid monomer unit in the water-soluble polymer is the ratio of the ethylenically unsaturated carboxylic acid monomer in all the monomers used for producing the water-soluble polymer (preparation). Ratio).

  The water-soluble polymer is a copolymer containing a fluorine-containing (meth) acrylic acid ester monomer unit in addition to the ethylenically unsaturated carboxylic acid monomer. Here, the fluorine-containing (meth) acrylic acid ester monomer unit is a structural unit obtained by polymerizing a fluorine-containing (meth) acrylic acid ester monomer. Fluorine-containing (meth) acrylic acid ester monomer unit has high ionic conductivity, so it suppresses the increase in resistance due to the coating of water-soluble polymer and improves both the output characteristics and cycle characteristics of lithium ion secondary battery. Can work.

  Examples of the fluorine-containing (meth) acrylic acid ester monomer include monomers represented by the following formula (I).

In the above formula (I), R ¹ represents a hydrogen atom or a methyl group.
In the above formula (I), R ² represents a hydrocarbon group containing a fluorine atom. The carbon number of the hydrocarbon group is preferably 1 or more, and preferably 18 or less. Moreover, the number of fluorine atoms contained in R ² may be one or two or more.

  Examples of fluorine-containing (meth) acrylic acid ester monomers represented by formula (I) include (meth) acrylic acid alkyl fluoride, (meth) acrylic acid fluoride aryl, and (meth) acrylic acid fluoride. Aralkyl is mentioned. Of these, alkyl fluoride (meth) acrylate is preferable. Specific examples of such monomers include 2,2,2-trifluoroethyl (meth) acrylate, β- (perfluorooctyl) ethyl (meth) acrylate, 2,2, (meth) acrylic acid. 3,3-tetrafluoropropyl, (meth) acrylic acid 2,2,3,4,4,4-hexafluorobutyl, (meth) acrylic acid 1H, 1H, 9H-perfluoro-1-nonyl, (meth) 1H, 1H, 11H-perfluoroundecyl acrylate, perfluorooctyl (meth) acrylate, trifluoromethyl (meth) acrylate, 3 [4] (1-trifluoromethyl-2, 2- (Meth) acrylic acid perfluoroalkyl esters such as bis [bis (trifluoromethyl) fluoromethyl] ethynyloxy] benzooxy] 2-hydroxypropyl Le, and the like. Among these, 2,2,2-trifluoroethyl methacrylate is preferable from the viewpoint of balance between cycle characteristics and output characteristics. Moreover, a fluorine-containing (meth) acrylic acid ester monomer may be used individually by 1 type, and may be used combining two or more types by arbitrary ratios.

  In the water-soluble polymer, the content ratio of the fluorine-containing (meth) acrylic acid ester monomer unit is preferably 1% by weight or more, more preferably 1.5% by weight or more, and particularly preferably 2% by weight or more. Preferably it is 30 weight% or less, More preferably, it is 25 weight% or less, Most preferably, it is 20 weight% or less. By setting the content ratio of the fluorine-containing (meth) acrylic acid ester monomer unit to be equal to or higher than the lower limit of the above range, the output characteristics of the lithium ion secondary battery can be improved, and thus the low temperature characteristics can be improved. it can. Moreover, electrochemical stability is securable by setting it as an upper limit or less. Usually, the content ratio of the fluorine-containing (meth) acrylate monomer unit in the water-soluble polymer is the fluorine-containing (meth) acrylate monomer in all monomers used for producing the water-soluble polymer. This is consistent with the ratio (preparation ratio).

  The water-soluble polymer may contain a crosslinkable monomer unit. The crosslinkable monomer unit is a structural unit obtained by polymerizing a crosslinkable monomer. By including a crosslinkable monomer unit, the water-soluble polymer can be cross-linked, so that the strength and stability of the film formed of the water-soluble polymer can be increased.

  As the crosslinkable monomer, a monomer capable of forming a crosslinked structure upon polymerization can be used. Examples of the crosslinkable monomer include monomers having two or more reactive groups per molecule. More specifically, a monofunctional monomer having a heat-crosslinkable crosslinkable group and one olefinic double bond per molecule, and a polyfunctional having two or more olefinic double bonds per molecule. Ionic monomers.

  Examples of thermally crosslinkable groups contained in the monofunctional monomer include epoxy groups, N-methylolamide groups, oxetanyl groups, oxazoline groups, and combinations thereof. Among these, an epoxy group is more preferable in terms of easy adjustment of crosslinking and crosslinking density.

  Examples of the crosslinkable monomer having an epoxy group as a thermally crosslinkable group and having an olefinic double bond include vinyl glycidyl ether, allyl glycidyl ether, butenyl glycidyl ether, o-allylphenyl glycidyl. Unsaturated glycidyl ethers such as ether; butadiene monoepoxide, chloroprene monoepoxide, 4,5-epoxy-2-pentene, 3,4-epoxy-1-vinylcyclohexene, 1,2-epoxy-5,9-cyclododecadiene Monoepoxides of dienes or polyenes such as; alkenyl epoxides such as 3,4-epoxy-1-butene, 1,2-epoxy-5-hexene, 1,2-epoxy-9-decene; and glycidyl acrylate, glycidyl methacrylate, Glycidyl crotonate, glycidyl Unsaturated carboxylic acids such as -4-heptenoate, glycidyl sorbate, glycidyl linoleate, glycidyl-4-methyl-3-pentenoate, glycidyl ester of 3-cyclohexene carboxylic acid, glycidyl ester of 4-methyl-3-cyclohexene carboxylic acid Glycidyl esters; and the like.

  Examples of the crosslinkable monomer having an N-methylolamide group as a thermally crosslinkable group and having an olefinic double bond have a methylol group such as N-methylol (meth) acrylamide (meta ) Acrylamides.

  Examples of the crosslinkable monomer having an oxetanyl group as a heat crosslinkable group and having an olefinic double bond include 3-((meth) acryloyloxymethyl) oxetane, 3-((meth) Acryloyloxymethyl) -2-trifluoromethyloxetane, 3-((meth) acryloyloxymethyl) -2-phenyloxetane, 2-((meth) acryloyloxymethyl) oxetane, and 2-((meth) acryloyloxymethyl ) -4-trifluoromethyloxetane.

  Examples of the crosslinkable monomer having an oxazoline group as a thermally crosslinkable group and having an olefinic double bond include 2-vinyl-2-oxazoline, 2-vinyl-4-methyl-2- Oxazoline, 2-vinyl-5-methyl-2-oxazoline, 2-isopropenyl-2-oxazoline, 2-isopropenyl-4-methyl-2-oxazoline, 2-isopropenyl-5-methyl-2-oxazoline, and And 2-isopropenyl-5-ethyl-2-oxazoline.

  Examples of multifunctional monomers having two or more olefinic double bonds include allyl (meth) acrylate, ethylene di (meth) acrylate, diethylene glycol di (meth) acrylate, triethylene glycol di (meth) acrylate, Tetraethylene glycol di (meth) acrylate, trimethylolpropane-tri (meth) acrylate, dipropylene glycol diallyl ether, polyglycol diallyl ether, triethylene glycol divinyl ether, hydroquinone diallyl ether, tetraallyloxyethane, trimethylolpropane-diallyl Examples include ethers, allyl or vinyl ethers of polyfunctional alcohols other than those described above, triallylamine, methylenebisacrylamide, and divinylbenzene.

  Of these, ethylene dimethacrylate, allyl glycidyl ether, and glycidyl methacrylate are particularly preferable as the crosslinkable monomer.

  In the water-soluble polymer, the content of the crosslinkable monomer unit is preferably 0.1% by weight or more, more preferably 0.2% by weight or more, particularly preferably 0.5% by weight or more, preferably It is 2% by weight or less, more preferably 1.5% by weight or less, and particularly preferably 1% by weight or less. By making the content rate of a crosslinkable monomer unit into the said range, swelling degree can be suppressed and durability of an electrode can be improved. Usually, the content ratio of the crosslinkable monomer unit in the water-soluble polymer coincides with the ratio (charge ratio) of the crosslinkable monomer in all monomers used in producing the water-soluble polymer.

  The water-soluble polymer may contain a reactive surfactant unit. The reactive surfactant unit is a structural unit obtained by polymerizing a reactive surfactant monomer. The reactive surfactant unit forms part of the water-soluble polymer and can function as a surfactant.

  The reactive surfactant monomer is a monomer having a polymerizable group that can be copolymerized with another monomer and having a surface active group (hydrophilic group and hydrophobic group). Usually, the reactive surfactant monomer has a polymerizable unsaturated group, and this group also acts as a hydrophobic group after polymerization. Examples of the polymerizable unsaturated group that the reactive surfactant monomer has include a vinyl group, an allyl group, a vinylidene group, a propenyl group, an isopropenyl group, and an isobutylidene group. One kind of the polymerizable unsaturated group may be used alone, or two or more kinds may be used in combination at any ratio.

  The reactive surfactant monomer usually has a hydrophilic group as a portion that exhibits hydrophilicity. Reactive surfactant monomers are classified into anionic, cationic and nonionic surfactants depending on the type of hydrophilic group.

Examples of the anionic hydrophilic group include —SO ₃ M, —COOM, and —PO (OH) ₂ . Here, M represents a hydrogen atom or a cation. Examples of cations include alkali metal ions such as lithium, sodium and potassium; alkaline earth metal ions such as calcium and magnesium; ammonium ions; ammonium ions of alkylamines such as monomethylamine, dimethylamine, monoethylamine and triethylamine; and And ammonium ions of alkanolamines such as monoethanolamine, diethanolamine, and triethanolamine.
Examples of the cationic hydrophilic group include —Cl, —Br, —I, and —SO ₃ OR ^X. Here, R ^X represents an alkyl group. Examples of R ^X is methyl group, an ethyl group, a propyl group, and isopropyl group.
-OH is mentioned as an example of a nonionic hydrophilic group.

  Examples of suitable reactive surfactant monomers include compounds represented by the following formula (II).

In the formula (II), R represents a divalent linking group. Examples of R include -Si-O- group, methylene group and phenylene group.
In the formula (II), R ³ represents a hydrophilic group. An example of R ³ includes —SO ₃ NH ₄ .
In the formula (II), n represents an integer of 1 to 100.

Another example of a suitable reactive surfactant monomer has polymerized units based on the polymerization units and butylene oxide based on ethylene oxide, the addition terminal, alkenyl and -SO ₃ NH having a terminal double bond ₄ (for example, trade names “Latemul PD-104” and “Latemul PD-105” manufactured by Kao Corporation).
A reactive surfactant monomer may be used individually by 1 type, and may be used combining two or more types by arbitrary ratios.

  In the water-soluble polymer, the content of the reactive surfactant unit is preferably 0.1% by weight or more, more preferably 0.2% by weight or more, particularly preferably 0.5% by weight or more, preferably It is 5% by weight or less, more preferably 4% by weight or less, and particularly preferably 2% by weight or less. The dispersibility of the slurry composition for negative electrodes can be improved by making the content rate of a reactive surfactant unit more than the lower limit of the said range. Moreover, durability of a negative electrode can be improved by setting it as below an upper limit.

  The water-soluble polymer may contain (meth) acrylic acid ester monomer units other than fluorine-containing (meth) acrylic acid ester monomer units. A (meth) acrylic acid ester monomer unit is a structural unit obtained by polymerizing a (meth) acrylic acid ester monomer. However, among the (meth) acrylate monomers, those containing fluorine are distinguished from (meth) acrylate monomers as fluorine-containing (meth) acrylate monomers.

  Examples of (meth) acrylic acid ester monomers include methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, n-butyl acrylate, t-butyl acrylate, pentyl acrylate, hexyl acrylate, heptyl acrylate, octyl acrylate, Acrylic acid alkyl esters such as 2-ethylhexyl acrylate, nonyl acrylate, decyl acrylate, lauryl acrylate, n-tetradecyl acrylate, stearyl acrylate; and methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, t -Butyl methacrylate, pentyl methacrylate, hexyl methacrylate, heptyl Methacrylate, octyl methacrylate, 2-ethylhexyl methacrylate, nonyl methacrylate, decyl methacrylate, lauryl methacrylate, n- tetradecyl methacrylate, and methacrylic acid alkyl esters such as stearyl methacrylate. A (meth) acrylic acid ester monomer may be used individually by 1 type, and may be used combining two or more types by arbitrary ratios.

  In the water-soluble polymer, the content ratio of the (meth) acrylic acid ester monomer unit is preferably 30% by weight or more, more preferably 35% by weight or more, particularly preferably 40% by weight or more, and preferably 70% by weight or less. By making the amount of the (meth) acrylic acid ester monomer unit equal to or higher than the lower limit of the above range, the binding property of the negative electrode active material to the current collector can be increased, and the upper limit of the above range is set. Thus, the flexibility of the negative electrode can be increased.

The water-soluble polymer may contain an arbitrary structural unit in addition to the above-described structural unit. Examples of arbitrary structural units include structural units obtained by polymerizing the following arbitrary monomers. Moreover, arbitrary monomers may be used individually by 1 type, and may be used combining two or more types by arbitrary ratios.
Examples of the optional monomer include styrene, chlorostyrene, vinyl toluene, t-butyl styrene, vinyl benzoic acid, methyl vinyl benzoate, vinyl naphthalene, chloromethyl styrene, hydroxymethyl styrene, α-methyl styrene, divinyl benzene. Styrene monomers such as acrylamide; amide monomers such as acrylamide; α, β-unsaturated nitrile compound monomers such as acrylonitrile and methacrylonitrile; olefin monomers such as ethylene and propylene; vinyl chloride; Monomers containing halogen atoms such as vinylidene chloride; vinyl ester monomers such as vinyl acetate, vinyl propionate, vinyl butyrate and vinyl benzoate; vinyl ether monomers such as methyl vinyl ether, ethyl vinyl ether and butyl vinyl ether; methyl Vinyl ketone, ethyl vinyl Ketone, butyl vinyl ketone, hexyl vinyl ketone, monomers such as isopropenyl vinyl ketone; and N- vinylpyrrolidone, vinylpyridine, and the like heterocycle-containing vinyl compound monomers such as vinyl imidazole. Moreover, as an arbitrary monomer, the monomer containing a phosphoric acid group, such as a compound containing a phosphoric acid group and an allyloxy group, and a phosphoric acid group containing (meth) acrylic acid ester, can be mentioned, for example. Examples of the compound containing a phosphoric acid group and an allyloxy group include 3-allyloxy-2-hydroxypropane phosphoric acid. Examples of the phosphoric acid group-containing (meth) acrylic acid ester include dioctyl-2-methacryloyloxyethyl phosphate, diphenyl-2-methacryloyloxyethyl phosphate, monomethyl-2-methacryloyloxyethyl phosphate, and dimethyl-2. -Methacryloyloxyethyl phosphate, monoethyl-2-methacryloyloxyethyl phosphate, diethyl-2-methacryloyloxyethyl phosphate, monoisopropyl-2-methacryloyloxyethyl phosphate, diisopropyl-2-methacryloyloxyethyl phosphate, mono-n -Butyl-2-methacryloyloxyethyl phosphate, di-n-butyl-2-methacryloyloxyethyl phosphate, monobutoxyethyl-2-methacryloyloxyethyl phosphate Eto, dibutoxyethyl-2-methacryloyloxyethyl phosphate, mono (2-ethylhexyl) -2-methacryloyloxyethyl phosphate, and di (2-ethylhexyl) -2-methacryloyloxyethyl phosphate. Furthermore, as an arbitrary monomer, the monomer containing a sulfonic acid group can be mentioned, for example. Examples of the monomer containing a sulfonic acid group include monomers obtained by sulfonated one of conjugated double bonds of diene compounds such as isoprene and butadiene, vinyl sulfonic acid, styrene sulfonic acid, allyl sulfonic acid, sulfone. A sulfonic acid group-containing monomer such as ethyl methacrylate or sulfopropyl methacrylate or a salt thereof; a monomer containing an amide group and a sulfonic acid group such as 2-acrylamido-2-methylpropanesulfonic acid (AMPS); A salt thereof; a monomer containing a hydroxyl group and a sulfonic acid group, such as 3-allyloxy-2-hydroxypropanesulfonic acid (HAPS), or a salt thereof;

  In the water-soluble polymer, the content of any structural unit is preferably 20% by weight or more, more preferably 25% by weight or more, particularly preferably 30% by weight or more, preferably 70% by weight or less, more preferably It is 65% by weight or less, particularly preferably 60% by weight or less. Usually, the content ratio of an arbitrary structural unit in a polymer matches the ratio (preparation ratio) of an arbitrary monomer in all monomers used when the polymer is produced.

The weight average molecular weight of the water-soluble polymer is usually smaller than the polymer that forms the particulate binder. The weight average molecular weight of the water-soluble polymer is preferably 500 or more, more preferably 1000 or more, particularly preferably 5000 or more, preferably 500,000 or less, more preferably 250,000 or less, and particularly preferably 100,000 or less. By setting the weight average molecular weight of the water-soluble polymer to be equal to or higher than the lower limit of the above range, the strength of the water-soluble polymer can be increased and the coating covering the negative electrode active material can be stabilized. For this reason, the cycle characteristics and output characteristics of the lithium ion secondary battery can be improved. Moreover, a water-soluble polymer can be softened by setting it as an upper limit or less. For this reason, for example, it is possible to suppress swelling of the negative electrode and improve the binding property of the negative electrode active material layer to the current collector.
The weight average molecular weight of the water-soluble polymer can be determined by GPC as a value in terms of polystyrene using a solution obtained by dissolving 0.85 g / ml sodium nitrate in a 10% by volume aqueous solution of dimethylformamide.

  The glass transition temperature of the water-soluble polymer is preferably 0 ° C. or higher, more preferably 5 ° C. or higher, preferably 100 ° C. or lower, more preferably 70 ° C. or lower. When the glass transition temperature of the water-soluble polymer is in the above range, both the binding property and flexibility of the negative electrode can be achieved. The glass transition temperature of the water-soluble polymer can be adjusted by combining various monomers.

  The water-soluble polymer has a viscosity of 0.1 mPa · s or more, more preferably 0.5 mPa · s or more, particularly preferably 1 mPa · s or more, preferably 20000 mPa · s when a 1% by weight aqueous solution is used. s or less, more preferably 15000 mPa · s or less, particularly preferably 10,000 mPa · s or less. By setting the viscosity to be equal to or higher than the lower limit of the above range, the strength of the water-soluble polymer can be increased and the durability of the negative electrode can be improved. Moreover, the coating property of the slurry composition for negative electrodes can be made favorable by setting it as an upper limit or less, and the binding strength of a collector and a negative electrode active material layer can be improved. The viscosity can be adjusted by, for example, the molecular weight of the water-soluble polymer. In addition, the said viscosity is a value when it measures at 25 degreeC and rotation speed 60rpm using an E-type viscosity meter. The pH of the aqueous solution at the time of measuring the viscosity is 8.

There is no special restriction | limiting in the manufacturing method of a water-soluble polymer. For example, a monomer composition containing an ethylenically unsaturated carboxylic acid monomer and a fluorine-containing (meth) acrylic acid ester monomer and, if necessary, other monomers in an aqueous solvent. A water-soluble polymer may be produced by polymerization.
The ratio of each monomer in the monomer composition is usually a structural unit in a water-soluble polymer (for example, an ethylenically unsaturated carboxylic acid monomer unit, a fluorine-containing (meth) acrylic acid ester monomer unit). , Crosslinkable monomer units, reactive surfactant units, etc.).

  The aqueous solvent used for the polymerization reaction can be the same as in the production of the particulate binder, for example. The procedure for the polymerization reaction can be the same as the procedure for producing the particulate binder. Thereby, normally, the reaction liquid containing a water-soluble polymer is obtained. The obtained reaction solution is usually acidic, and the water-soluble polymer is often dispersed in an aqueous solvent. The water-soluble polymer dispersed in the water-soluble solvent as described above can be usually made soluble in an aqueous solvent by adjusting the pH of the reaction solution to, for example, 7 to 13. You may take out a water-soluble polymer from the reaction liquid obtained in this way. However, usually, water can be used as an aqueous medium, a slurry composition for a negative electrode can be produced using a water-soluble polymer dissolved in this water, and a negative electrode can be produced using the slurry composition.

  The method for alkalinizing the pH to 7 to 13 is, for example, an alkali metal aqueous solution such as a lithium hydroxide aqueous solution, a sodium hydroxide aqueous solution, or a potassium hydroxide aqueous solution; an alkaline earth metal such as a calcium hydroxide aqueous solution or a magnesium hydroxide aqueous solution. Aqueous solution: A method of mixing an alkaline aqueous solution such as an aqueous ammonia solution with the reaction solution. One kind of the alkaline aqueous solution may be used alone, or two or more kinds may be used in combination at any ratio.

  The amount of the water-soluble polymer is preferably 0.01 parts by weight or more, more preferably 0.03 parts by weight or more, particularly preferably 0.05 parts by weight or more, preferably 100 parts by weight of the negative electrode active material. Is 20 parts by weight or less, more preferably 15 parts by weight or less, and particularly preferably 10 parts by weight or less. By setting the amount of the water-soluble polymer to be equal to or higher than the lower limit of the above range, the cycle characteristics of the lithium ion secondary battery can be improved. Moreover, the low temperature characteristic of a lithium ion secondary battery can be improved by setting it as below an upper limit.

  The weight ratio of the particulate binder and the water-soluble polymer is “water-soluble polymer / particulate binder”, preferably 0.5 / 99.5 or more, more preferably 1.0 / 99.0 or more, particularly preferably. Is 1.5 / 98.5 or more, preferably 40/60 or less, more preferably 30/70 or less, and particularly preferably 20/80 or less. By making the content rate of a particulate binder and a water-soluble polymer more than the lower limit of the said range, the binding property of a negative electrode and the lifetime characteristic of a lithium ion secondary battery can be improved. Moreover, the flexibility of a negative electrode and the low temperature characteristic of a lithium ion secondary battery can be improved by setting it as an upper limit or less.

[2.2.4. Arbitrary ingredients]
The composition forming the negative electrode active material layer may contain any component other than the negative electrode active material, the particulate binder, and the water-soluble polymer as long as the effects of the present invention are not significantly impaired. For example, a conductivity imparting material (also referred to as a conductive material), a reinforcing material, a dispersant, a leveling agent, an antioxidant, and the like can be given.

The conductivity imparting material is a component that can improve electrical contact between the negative electrode active materials. By including the conductivity-imparting material, the discharge load characteristics of the lithium ion secondary battery can be improved.
Examples of the conductivity imparting material include conductive carbon such as acetylene black, ketjen black, carbon black, vapor grown carbon fiber, and carbon nanotube. Further, carbon powder such as graphite, fibers or foils of various metals may be used. One type of conductivity imparting material may be used alone, or two or more types may be used in combination at any ratio.
The amount of the conductive material is preferably 0.01 parts by weight or more, more preferably 1 part by weight or more, preferably 20 parts by weight or less, more preferably 10 parts by weight with respect to 100 parts by weight of the negative electrode active material. Or less.

As the reinforcing material, for example, various inorganic and organic spherical, plate-like, rod-like, or fibrous fillers may be used. Moreover, a reinforcing material may be used individually by 1 type, and may be used combining two or more types by arbitrary ratios. By using the reinforcing material, a tough and flexible electrode can be obtained, and excellent long-term cycle characteristics can be obtained.
The amount of the reinforcing material is preferably 0.01 parts by weight or more, more preferably 1 part by weight or more, preferably 20 parts by weight or less, more preferably 10 parts by weight or less with respect to 100 parts by weight of the negative electrode active material. is there. By setting the amount of the reinforcing material within the above range, high capacity and high load characteristics can be obtained.

Examples of the dispersant include an anionic compound, a cationic compound, a nonionic compound, and a polymer compound. Specific types of the dispersant can be selected according to the negative electrode active material and the conductivity-imparting material to be used. Moreover, a dispersing agent may be used individually by 1 type, and may be used combining two or more types by arbitrary ratios.
The amount of the dispersant is preferably 0.01% by weight to 10% by weight in the negative electrode active material layer. When the amount of the dispersant is within the above range, the stability of the negative electrode slurry composition can be improved, a smooth electrode can be obtained, and a high battery capacity can be realized.

Examples of the leveling agent include surfactants such as alkyl surfactants, silicon surfactants, fluorine surfactants, and metal surfactants. A leveling agent may be used individually by 1 type, and may be used combining two or more types by arbitrary ratios. By using a leveling agent, it is possible to prevent the repelling that occurs during coating or to improve the smoothness of the negative electrode.
The amount of the leveling agent is preferably 0.01% by weight to 10% by weight in the negative electrode active material layer. When the amount of the leveling agent is within the above range, the productivity, smoothness, and battery characteristics during electrode production are excellent.

Examples of the antioxidant include a phenol compound, a hydroquinone compound, an organic phosphorus compound, a sulfur compound, a phenylenediamine compound, and a polymer type phenol compound. Among these, the polymer type phenol compound is a polymer having a phenol structure in the molecule. The polymer-type phenol compound preferably has a weight average molecular weight of 200 or more, more preferably 600 or more, and preferably 1000 or less, more preferably 700 or less.
The amount of the antioxidant in the negative electrode active material layer is preferably 0.01% by weight or more, more preferably 0.05% by weight or more, preferably 10% by weight or less, more preferably 5% by weight or less. . Thereby, it is excellent in stability of a slurry composition, battery capacity, and cycling characteristics.

  Furthermore, the composition for forming the negative electrode active material layer may include a component included in the slurry composition for the negative electrode used in the production of the negative electrode active material layer.

[2.2.5. Negative electrode active material layer thickness]
The thickness of the negative electrode active material layer is preferably 1 μm or more, more preferably 5 μm or more, particularly preferably 30 μm or more, preferably 300 μm or less, more preferably 250 μm or less, still more preferably 200 μm or less, particularly preferably 100 μm or less. is there. When the thickness of the negative electrode active material layer is in the above range, the power density and the energy density can be balanced.

[2.3. Method for producing negative electrode]
The method for producing the negative electrode is not particularly limited. For example, a negative electrode active material layer may be formed on the surface of the current collector by preparing a slurry composition for the negative electrode, applying the slurry composition to the surface of the current collector, and drying to obtain a negative electrode. Good.

  The slurry composition for a negative electrode is a slurry-like composition containing a negative electrode active material, a particulate binder, a water-soluble polymer, and an aqueous solvent. Moreover, the slurry composition may contain components other than a negative electrode active material, a binder, a water-soluble polymer, and an aqueous solvent as needed. The amount of the negative electrode active material, the binder, the water-soluble polymer, and the components included as necessary is usually the same as the amount of each component included in the negative electrode active material layer. In such a slurry composition, a part of the water-soluble polymer is usually dissolved in the aqueous solvent, but another part of the water-soluble polymer is adsorbed on the surface of the negative electrode active material, thereby The substance is covered with a stable layer (film) of a water-soluble polymer, and the dispersibility of the negative electrode active material in the solvent is improved. For this reason, the slurry composition for negative electrodes has good coating properties when applied to the current collector.

As the aqueous solvent for the slurry composition for the negative electrode, the same aqueous solvent used in the polymerization of the particulate binder and the water-soluble polymer can be used. Of these, water is preferably used as the aqueous solvent.
The amount of the aqueous solvent is preferably adjusted as appropriate so that the slurry composition has properties suitable for the subsequent steps. Specifically, the solid content concentration of the slurry composition is preferably 30% by weight or more, more preferably 40% by weight or more, preferably 70% by weight or less, more preferably 60% by weight or less. Used by adjusting. Here, the solid content of the slurry composition indicates a substance remaining as a constituent component of the negative electrode active material layer after drying and heating of the slurry composition.

Moreover, the slurry composition for negative electrodes may contain compounding agents, such as antiseptic | preservative and a thickener, for example.
As the preservative, it is preferable to use a benzoisothiazoline compound represented by the following formula (III), 2-methyl-4-isothiazolin-3-one, or a mixture thereof, and more preferably a mixture thereof. preferable.

In formula (III), R ⁴ represents a hydrogen atom or an alkyl group having 1 to 8 carbon atoms. When the benzisothiazoline compound represented by the formula (III) and 2-methyl-4-isothiazolin-3-one are used in combination, the ratio of these should be 1:10 to 10: 1 by weight. Is preferred. Further, the amount of the preservative in the slurry composition is preferably 0.001 to 0.1 parts by weight, preferably 0.001 parts by weight to 100 parts by weight in total of the particulate binder and the water-soluble polymer. 0.05 part by weight is more preferable, and 0.001 to 0.01 part by weight is particularly preferable.

As described above, the particulate binder and the water-soluble polymer are preferably obtained by polymerization in an aqueous solvent. Therefore, particulate binder over is generally stored as a dispersion in water. The water-soluble polymer is usually stored as an aqueous solution. Therefore, in general, the quality is likely to deteriorate due to the propagation of microorganisms. On the other hand, such quality deterioration can be prevented by using a preservative.

  Examples of the thickener include cellulose polymers such as carboxymethylcellulose, methylcellulose, and hydroxypropylcellulose, and ammonium salts and alkali metal salts thereof; (modified) poly (meth) acrylic acid and ammonium salts and alkali metal salts thereof; (Modified) Polyvinyl alcohols such as polyvinyl alcohol, copolymers of acrylic acid or acrylates and vinyl alcohol, maleic anhydride or copolymers of maleic acid or fumaric acid and vinyl alcohol; polyethylene glycol, polyethylene oxide, polyvinyl pyrrolidone , Modified polyacrylic acid, oxidized starch, phosphate starch, casein, various modified starches, acrylonitrile-butadiene copolymer hydride, and the like. Among these, cellulose polymers and ammonium salts and alkali metal salts thereof; (modified) poly (meth) acrylic acid and ammonium salts and alkali metal salts thereof are preferable. One of these may be used alone, or two or more of these may be used in combination at any ratio. Here, “(modified) poly” means “unmodified poly” or “modified poly”. The amount of thickener in the slurry composition is preferably 0.1 wt% to 10 wt%. Since the dispersibility of the negative electrode active material in a slurry composition can be improved because a thickener is the said range, a smooth electrode can be obtained. For this reason, it is possible to realize excellent load characteristics and cycle characteristics.

The negative electrode slurry composition may be produced by mixing the negative electrode active material, the particulate binder, the water-soluble polymer, the aqueous solvent, and any components used as necessary.
The apparatus used for mixing may be any apparatus that can uniformly mix the above components. Examples include bead mill, ball mill, roll mill, sand mill, pigment disperser, crusher, ultrasonic disperser, homogenizer, planetary mixer, fill mix and the like. Among these, it is particularly preferable to use a ball mill, a roll mill, a pigment disperser, a crusher, or a planetary mixer because dispersion at a high concentration is possible.

The viscosity of the slurry composition is preferably 10 mPa · s or more, more preferably 100 mPa · s or more, preferably 100,000 mPa · s or less, from the viewpoint of uniform coating properties and stability over time of the slurry composition. More preferably, it is 50,000 mPa · s or less. Here, the viscosity is a value measured using a B-type viscometer at 25 ° C. and a rotation speed of 60 rpm.
Further, the solid content concentration of the slurry composition is not particularly limited as long as it can be applied and immersed and has a fluid viscosity, but is generally 10% by weight to 80% by weight.

  A negative electrode active material layer can be formed by applying a slurry composition for a negative electrode onto a member such as a current collector, and further drying and heating as necessary. Further, in the lithium ion secondary battery, when there is a layer interposed between the current collector and the negative electrode active material layer, the slurry composition may be applied on the layer. The method of application is not particularly limited. Examples thereof include a doctor blade method, a zip method, a reverse roll method, a direct roll method, a gravure method, an extrusion method, and a brush coating method.

  The conditions for drying the slurry composition layer formed on the current collector are not particularly limited. For example, it is good also as 1 hour or more above 120 degreeC. Examples of the drying method include drying with warm air, hot air, and low-humidity air; vacuum drying: drying method by irradiation with energy rays such as infrared rays, far infrared rays, and electron beams.

After drying the layer of the slurry composition, it is preferable to apply a pressure treatment using, for example, a die press or a roll press as necessary. By the pressure treatment, a negative electrode active material layer having a low porosity can be obtained. The porosity is preferably 5% or more, more preferably 7% or more, preferably 15% or less, more preferably 13% or less. By setting the porosity to be equal to or higher than the lower limit of the above range, a high volume capacity can be easily obtained, and the negative electrode active material layer can be made difficult to peel from the current collector. Moreover, high charging efficiency and discharge efficiency are acquired by setting it as an upper limit or less.
Furthermore, when a negative electrode active material layer contains a curable polymer, it is preferable to harden the said polymer after formation of a negative electrode active material layer.

[3. Positive electrode]
The positive electrode usually includes a current collector and a positive electrode active material layer formed on the surface of the current collector. The positive electrode active material layer includes a positive electrode active material and a binder for the positive electrode.

  As the current collector for the positive electrode, one made of a material having electrical conductivity and electrochemical durability is usually used. As the current collector for the positive electrode, for example, the same current collector used for the negative electrode of the present invention may be used. Among these, aluminum is particularly preferable.

  As the positive electrode active material, a material capable of inserting and removing lithium ions is used. Such positive electrode active materials are roughly classified into inorganic compounds and organic compounds.

Examples of the positive electrode active material made of an inorganic compound include transition metal oxides, transition metal sulfides, lithium-containing composite metal oxides of lithium and transition metals, and the like.
Examples of the transition metal include Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Mo.

Examples of the transition metal oxide include MnO, MnO ₂ , V ₂ O ₅ , V ₆ O ₁₃ , TiO ₂ , Cu ₂ V ₂ O ₃ , amorphous V ₂ O—P ₂ O ₅ , MoO ₃ , V ₂ O ₅ , V ₆ O _{13 and the} like can be mentioned. Among them, MnO, V ₂ O ₅ , V ₆ O ₁₃ and TiO ₂ are preferable from the viewpoint of cycle stability and capacity.
Examples of the transition metal sulfide include TiS ₂ , TiS ₃ , amorphous MoS ₂ , FeS, and the like.

Examples of the lithium-containing composite metal oxide include a lithium-containing composite metal oxide having a layered structure, a lithium-containing composite metal oxide having a spinel structure, and a lithium-containing composite metal oxide having an olivine structure.
Examples of the lithium-containing composite metal oxide having a layered structure include lithium-containing cobalt oxide (LiCoO ₂ ), lithium-containing nickel oxide (LiNiO ₂ ), lithium composite oxide of Co—Ni—Mn, Ni—Mn— Examples thereof include a lithium composite oxide of Al and a lithium composite oxide of Ni—Co—Al.
Examples of the lithium-containing composite metal oxide having a spinel structure include Li [Mn _3/2 M _1/2 ] O _{4 in which} lithium manganate (LiMn ₂ O ₄ ) or a part of Mn is substituted with another transition metal. (Where M is Cr, Fe, Co, Ni, Cu, etc.).
Examples of the lithium-containing composite metal oxide having an olivine type structure include Li _X MPO ₄ (wherein M is Mn, Fe, Co, Ni, Cu, Mg, Zn, V, Ca, Sr, Ba, Ti). Olivine type lithium phosphate compound represented by the formula: at least one selected from the group consisting of Al, Si, B and Mo, wherein X represents a number satisfying 0 ≦ X ≦ 2.

  Examples of the positive electrode active material made of an organic compound include conductive polymer compounds such as polyacetylene and poly-p-phenylene.

Moreover, you may use the positive electrode active material which consists of a composite material which combined the inorganic compound and the organic compound.
Alternatively, for example, a composite material covered with a carbon material may be produced by reducing and firing an iron-based oxide in the presence of a carbon source material, and the composite material may be used as a positive electrode active material. Iron-based oxides tend to have poor electrical conductivity, but can be used as a high-performance positive electrode active material by using a composite material as described above.
Furthermore, you may use as a positive electrode active material what carried out the element substitution of the said compound partially.
Moreover, you may use the mixture of said inorganic compound and organic compound as a positive electrode active material.
As the positive electrode active material, one type may be used alone, or two or more types may be used in combination at any ratio.

  The volume average particle diameter of the positive electrode active material particles is preferably 1 μm or more, more preferably 2 μm or more, preferably 50 μm or less, more preferably 30 μm or less. By setting the volume average particle diameter of the positive electrode active material particles in the above range, the amount of the binder when preparing the positive electrode active material layer can be reduced, and the decrease in the capacity of the secondary battery can be suppressed. In order to form the positive electrode active material layer, a positive electrode slurry composition containing a positive electrode active material and a binder is usually prepared. The viscosity of the positive electrode slurry composition is set to an appropriate viscosity that is easy to apply. Adjustment becomes easy and a uniform positive electrode can be obtained.

  The amount of the positive electrode active material in the positive electrode active material layer is preferably 90% by weight or more, more preferably 95% by weight or more, preferably 99.9% by weight or less, more preferably 99% by weight or less. By setting the content of the positive electrode active material in the above range, the capacity of the lithium ion secondary battery can be increased, and the flexibility of the positive electrode and the binding property between the current collector and the positive electrode active material layer can be improved. it can.

  Examples of the positive electrode binder include polyethylene, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), polyacrylic acid derivatives, and polyacrylonitrile derivatives. Resins; Soft polymers such as acrylic soft polymers, diene soft polymers, olefin soft polymers, and vinyl soft polymers can be used. A binder may be used individually by 1 type and may be used combining two or more types by arbitrary ratios.

  In addition, the positive electrode active material layer may contain components other than the positive electrode active material and the binder as necessary. When the example is given, a conductive material, a reinforcing material, a leveling agent, antioxidant, a thickener etc. will be mentioned, for example. Moreover, these components may be used individually by 1 type, and may be used combining two or more types by arbitrary ratios.

  The thickness of the positive electrode active material layer is preferably 5 μm or more, more preferably 10 μm or more, preferably 300 μm or less, more preferably 250 μm or less. When the thickness of the positive electrode active material layer is in the above range, high characteristics can be realized in both load characteristics and energy density.

  The positive electrode can be produced, for example, in the same manner as the above-described negative electrode.

[4. Electrolyte]
The electrolytic solution includes a solvent and a supporting electrolyte dissolved in the solvent.

As the electrolyte contained in the electrolytic solution, a lithium salt is usually used. Examples of the lithium salt include LiPF ₆ , LiAsF ₆ , LiBF ₄ , LiSbF ₆ , LiAlCl ₄ , LiClO ₄ , CF ₃ SO ₃ Li, C ₄ F ₉ SO ₃ Li, CF ₃ COOLi, (CF ₃ CO) ₂ NLi , (CF ₃ SO ₂ ) ₂ NLi, (C ₂ F ₅ SO ₂ ) NLi, and the like. Among these, LiPF ₆ , LiClO ₄ , and CF ₃ SO ₃ Li are preferably used because they are particularly soluble in a solvent and exhibit a high degree of dissociation. One of these may be used alone, or two or more of these may be used in combination at any ratio. Generally, the higher the dissociation of the supporting electrolyte, the higher the lithium ion conductivity. Therefore, the lithium ion conductivity can be adjusted depending on the type of the supporting electrolyte.

  The concentration of the supporting electrolyte in the electrolytic solution is preferably 1% by weight or more, more preferably 5% by weight or more, and preferably 30% by weight or less, more preferably 20% by weight or less. If the amount of the supporting electrolyte is too small or too large, the ionic conductivity is lowered, and the charging characteristics and discharging characteristics of the lithium ion secondary battery may be lowered.

  As the solvent contained in the electrolytic solution, a solvent containing propylene carbonate and vinylene carbonate is used.

  The amount of propylene carbonate in the solvent is usually 50% by volume or more, usually 80% by volume or less, preferably 75% by volume or less, more preferably 70% by volume or less. The output characteristic of a lithium ion secondary battery can be improved by making the quantity of propylene carbonate more than the lower limit of the said range. Moreover, by setting it as the upper limit value or less, cycle characteristics can be improved and the life of the battery can be extended.

  The amount of vinylene carbonate in the solvent is usually 0.05% by volume or more, preferably 0.1% by volume or more, more preferably 0.15% by volume or more, and usually 1.0% by volume or less, preferably 0. .8% by volume or less, more preferably 0.6% by volume or less. By using vinylene carbonate more than the lower limit of the above range, the cycle characteristics of the lithium ion secondary battery can be improved. Moreover, since the water-soluble polymer is used in the lithium ion secondary battery of this invention, it is possible to improve cycling characteristics even if the amount of vinylene carbonate is reduced. For this reason, since the quantity of vinylene carbonate can be decreased and the viscosity increase of electrolyte solution can be suppressed, it is also possible to improve the low temperature characteristic of a lithium ion secondary battery.

  Moreover, as a solvent for the electrolytic solution, any solvent other than propylene carbonate and vinylene carbonate may be used in combination with propylene carbonate and vinylene carbonate. Examples of arbitrary solvents include alkyl carbonates such as dimethyl carbonate (DMC), ethylene carbonate (EC), diethyl carbonate (DEC), butylene carbonate (BC), methyl ethyl carbonate (MEC); γ-butyrolactone, formic acid Esters such as methyl; ethers such as 1,2-dimethoxyethane and tetrahydrofuran; sulfur-containing compounds such as sulfolane and dimethyl sulfoxide; In particular, carbonates such as dimethyl carbonate, ethylene carbonate, diethyl carbonate, and methyl ethyl carbonate are preferred because high ion conductivity is easily obtained and the use temperature range is wide. These solvents may be used alone or in combination of two or more at any ratio. The lower the viscosity of the solvent used, the higher the lithium ion conductivity. Therefore, the lithium ion conductivity can be adjusted depending on the type of solvent.

  Furthermore, you may include arbitrary compounding agents in electrolyte solution as needed.

[5. Separator]
As the separator, a porous substrate having a pore portion is usually used. Examples of separators include (a) a porous separator having pores, (b) a porous separator having a polymer coating layer formed on one or both sides, and (c) a porous resin coat containing inorganic ceramic powder. Examples thereof include a porous separator having a layer formed thereon. Examples of these include solid polymer electrolytes such as polypropylene, polyethylene, polyolefin, or aramid porous separators, polyvinylidene fluoride, polyethylene oxide, polyacrylonitrile, or polyvinylidene fluoride hexafluoropropylene copolymers. Or a polymer film for a gel polymer electrolyte; a separator coated with a gelled polymer coat layer; a separator coated with a porous film layer composed of an inorganic filler and a dispersant for inorganic filler;

[6. Manufacturing method of lithium ion secondary battery]
The manufacturing method of a lithium ion secondary battery is not specifically limited. For example, the above-described negative electrode and positive electrode may be overlapped via a separator, and this may be wound or folded in accordance with the shape of the battery and placed in the battery container, and the electrolyte may be injected into the battery container and sealed. Furthermore, if necessary, for example, expanded metal; an overcurrent prevention element such as a fuse or a PTC element; a lead plate or the like may be inserted to prevent an increase in pressure inside the battery or overcharge / discharge. The shape of the battery may be any of, for example, a laminate cell type, a coin type, a button type, a sheet type, a cylindrical type, a square shape, and a flat type.

Hereinafter, the present invention will be specifically described with reference to examples. However, the present invention is not limited to the embodiments shown below, and may be arbitrarily modified and implemented without departing from the scope of the claims of the present invention and its equivalent scope. In the following description, “%” and “parts” representing amounts are based on weight unless otherwise specified. Further, the operations described below were performed under normal temperature and normal pressure conditions unless otherwise specified.
In addition, the artificial graphite used in the following examples and comparative examples has a structure in which the surface of a portion formed of a graphite material having high crystallinity is covered with an amorphous carbonaceous material having low crystallinity. It was.

[Measuring method]
(1) Evaluation Method of High-Temperature Storage Characteristics A lithium ion secondary battery of a laminate type cell was prepared and allowed to stand for 24 hours in an environment at 25 ° C. Thereafter, under an environment of 25 ° C., charge / discharge operation of charging to 4.2 volts and discharging to 3.0 volts was performed by a constant current method of 0.1 C, and an initial capacity C0 was measured. Further, the battery was charged to 4.2 volts in a 25 ° C. environment and stored at 60 ° C. for 30 days. Then, the charge / discharge operation of charging to 4.2 volts and discharging to 3.0 volts was performed by a constant current method of 0.1 C, and the capacity C1 after high temperature storage was measured. The high temperature storage characteristics were evaluated by a capacity retention ratio ΔC1 represented by ΔC1 = C1 / C0 × 100 (%). It shows that it is excellent in a high temperature storage characteristic, so that this value is high.

(2) Evaluation method of high-temperature cycle characteristics A lithium ion secondary battery of a laminate type cell was prepared and allowed to stand for 24 hours in an environment at 25 ° C. After that, under an environment of 25 ° C., charge / discharge operation of charging to 4.2 volts and discharging to 3.0 volts was performed by a constant current method of 0.1 C, and an initial capacity C0 was measured. Furthermore, in a 60 ° C. environment, the charge / discharge operation of charging to 4.2 volts and discharging to 3.0 volts is repeated 1000 times (1000 cycles) by a constant current method of 1 C, and the capacity C2 after 1000 cycles is calculated. It was measured. The high-temperature cycle characteristics were evaluated by a capacity retention ratio ΔC2 represented by ΔC2 = C2 / C0 × 100 (%). It shows that it is excellent in high temperature cycling characteristics, so that this value is high.

(3) Evaluation method of low-temperature characteristics A lithium ion secondary battery of a laminate type cell was produced and allowed to stand in an environment of 25 ° C. for 24 hours. Thereafter, the charging operation was performed up to 4.2 volts by a constant current method of 0.1 C. Then, discharge operation was performed by a 1 C constant current method in -10 degreeC environment, and the voltage V 15 seconds after the discharge start was measured. The low temperature characteristics were evaluated by a voltage change ΔV indicated by ΔV = 4.2 volts-V. It shows that it is excellent in a low temperature characteristic, so that this value is small.

(4) Evaluation Method of Cell Swelling Rate A lithium ion secondary battery of a laminate type cell was prepared and allowed to stand in an environment at 25 ° C. for 24 hours. Thereafter, in an environment of 25 ° C., charge / discharge operation of charging to 4.2 volts and discharging to 3.0 volts was performed by a constant current method of 0.1 C, and an initial cell volume M0 was measured. Further, the battery was charged to 4.2 volts in a 25 ° C. environment and stored at 60 ° C. for 30 days. After that, under an environment of 25 ° C., a charge / discharge operation of charging to 4.2 volts and discharging to 3.0 volts was performed by a constant current method of 0.1 C, and the cell volume M1 after high-temperature storage was measured. . The cell swelling ratio ΔM was calculated by ΔM = (M1−M0) / M0 × 100 (%). The lower this value, the smaller the bulge and the better.

(5) Method for Measuring Specific Surface Area of Negative Electrode Active Material The specific surface area of the negative electrode active material was measured by a BET method by nitrogen gas adsorption using a measuring device “Tristar II 3020 series” manufactured by Shimadzu Corporation.

[Example 1]
(1-1. Method for producing water-soluble polymer)
6. In a 5 MPa pressure vessel with a stirrer, 32.5 parts of methacrylic acid (ethylenically unsaturated carboxylic acid monomer) and 2,2,2-trifluoroethyl methacrylate (fluorine-containing (meth) acrylic acid ester monomer) 5 parts, ethylene dimethacrylate (crosslinkable monomer) 0.8 part, butyl acrylate (optional monomer) 58.0 parts, polyoxyalkylene alkenyl ether ammonium sulfate (reactive surfactant monomer, Kao Corporation) Product, product name “Latemul PD-104”) 1.2 parts, 0.6 parts of t-dodecyl mercaptan, 150 parts of ion-exchanged water, and 0.5 parts of potassium persulfate (polymerization initiator), and sufficiently stirred did. Then, it heated to 60 degreeC and superposition | polymerization was started. When the polymerization conversion reached 96%, the reaction was stopped by cooling to obtain a mixture containing a water-soluble polymer. 10% aqueous ammonia was added to the mixture containing the water-soluble polymer to adjust the pH to 8. As a result, an aqueous solution containing a desired water-soluble polymer was obtained.

(1-2. Production method of particulate binder)
In a 5 MPa pressure vessel with a stirrer, 33.5 parts of 1,3-butadiene (aliphatic conjugated diene monomer), 3.5 parts of itaconic acid (ethylenically unsaturated carboxylic acid monomer), styrene (arbitrary unit amount) Body) 63 parts, sodium dodecylbenzenesulfonate (emulsifier) 4 parts, ion exchanged water 150 parts, and potassium persulfate (polymerization initiator) 0.5 part were added and sufficiently stirred. Then, it heated to 50 degreeC and superposition | polymerization was started. When the polymerization conversion reached 96%, the reaction was stopped by cooling to obtain a mixture containing a particulate binder. A 5% aqueous sodium hydroxide solution was added to the mixture containing the particulate binder to adjust the pH to 8. Then, the unreacted monomer was removed by heating under reduced pressure. Then, it cooled to 30 degrees C or less. Thereby, an aqueous dispersion containing a desired particulate binder was obtained.

(1-3. Method for producing binder composition containing water-soluble polymer and particulate binder)
The aqueous solution containing the water-soluble polymer obtained above was diluted with ion exchange water to adjust the concentration to 5%. This was mixed with the aqueous dispersion containing the particulate binder obtained above so that the weight ratio corresponding to the solid content was particulate binder: water-soluble polymer = 95.0: 5.0. A composition was obtained.

(1-4. Production of slurry composition for negative electrode)
As a negative electrode active material, artificial graphite (volume average particle diameter: 24.5 μm) having a specific surface area of 5.5 m ² / g was prepared.
To a planetary mixer with a disper, add 100 parts of the above artificial graphite and 1 part of a 1% aqueous solution of carboxymethyl cellulose (“BSH-12” manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) as a thickener. It was. Further, ion exchange water was added to adjust the solid content concentration to 55%. Then, it mixed for 60 minutes at 25 degreeC. Next, the solid content concentration was adjusted to 52% with ion-exchanged water. Then, it further mixed for 15 minutes at 25 degreeC, and obtained the liquid mixture.

  1.0 part of the binder composition obtained in the above (1-3) and ion-exchanged water were added to the mixed solution, and the final solid content concentration was adjusted to 50%. Further mixed for 10 minutes. This was defoamed under reduced pressure to obtain a negative electrode slurry composition having good fluidity.

(1-5. Production of negative electrode)
The slurry composition for negative electrode obtained in the above (1-4) is applied on a copper foil having a thickness of 20 μm, which is a current collector, with a comma coater so that the film thickness after drying becomes about 150 μm. , Dried. This drying was performed by conveying the copper foil in an oven at 60 ° C. at a speed of 0.5 m / min for 2 minutes. Thereafter, heat treatment was performed at 120 ° C. for 2 minutes to obtain a negative electrode raw material. This negative electrode raw material was rolled with a roll press to obtain a negative electrode having a negative electrode active material layer having a thickness of 80 μm.

(1-6. Production of positive electrode)
As a binder for the positive electrode, a 40% aqueous dispersion containing an acrylate polymer having a glass transition temperature Tg of −40 ° C. and a number average particle diameter of 0.20 μm was prepared. The acrylate polymer is a copolymer obtained by emulsion polymerization of a monomer mixture containing 2-ethylhexyl acrylate 78%, acrylonitrile 20%, and methacrylic acid 2%.

100 parts of LiCoO ₂ having a volume average particle diameter of 12 μm as a positive electrode active material, and 1 part of a 1% aqueous solution of carboxymethyl cellulose (“BSH-12” manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) as a thickener, As a positive electrode binder, 5 parts of a 40% aqueous dispersion of the above acrylate polymer corresponding to the solid content was mixed with ion-exchanged water. The amount of ion-exchanged water was such that the total solid concentration was 40%. These were mixed by a planetary mixer to prepare a positive electrode slurry composition.

  The above slurry composition for positive electrode was applied with a comma coater onto an aluminum foil having a thickness of 20 μm as a current collector so that the film thickness after drying was about 200 μm and dried. This drying was performed by conveying the aluminum foil in an oven at 60 ° C. at a speed of 0.5 m / min for 2 minutes. Thereafter, heat treatment was performed at 120 ° C. for 2 minutes to obtain a positive electrode raw material. This positive electrode raw material was rolled with a roll press to obtain a positive electrode.

(1-7. Preparation of separator)
A single-layer polypropylene separator (“Celguard 2500” manufactured by Celgard) was cut into a 5 × 5 cm ² square.

(1-8. Production of lithium ion secondary battery)
An aluminum packaging exterior was prepared as the battery exterior. The positive electrode obtained in the above (1-6) was cut into a 4 × 4 cm ² square and arranged so that the surface on the current collector side was in contact with the aluminum packaging exterior. The square separator obtained in (1-7) above was placed on the surface of the positive electrode active material layer of the positive electrode. Furthermore, the negative electrode obtained in the above (1-5) was cut into a square of 4.2 × 4.2 cm ² and arranged on the separator so that the surface on the negative electrode active material layer side faces the separator.

The electrolyte was injected so that no air remained. In the electrolytic solution, LiPF ₆ having a concentration of 1M was used as the electrolyte. Moreover, the solvent of electrolyte solution is ethylene carbonate (EC), diethyl carbonate (DEC), propylene carbonate (PC), and vinylene carbonate (VC), volume ratio EC: DEC: PC: VC = 19.5: 10: 70. : A mixed solvent containing 0.5.

Furthermore, in order to seal the opening of the aluminum packaging material, heat sealing at 150 ° C. was performed to close the aluminum exterior, and a lithium ion secondary battery was manufactured.
About the obtained lithium ion secondary battery, the high temperature storage characteristic, the high temperature cycling characteristic, the low temperature characteristic, and the swelling rate of the cell were evaluated.

[Example 2]
In the above (1-1), the procedure of Example 1 was repeated except that the amount of 2,2,2-trifluoroethyl methacrylate was changed to 3 parts and the amount of butyl acrylate was changed to 62.5 parts. A lithium ion secondary battery was manufactured and evaluated.

[Example 3]
In the above (1-1), the procedure of Example 1 was repeated except that the amount of 2,2,2-trifluoroethyl methacrylate was changed to 18 parts and the amount of butyl acrylate was changed to 47.5 parts. A lithium ion secondary battery was manufactured and evaluated.

[Example 4]
In the above (1-3), the mixing ratio of the aqueous solution containing the water-soluble polymer and the aqueous dispersion of the particulate binder is equivalent to the solid content on a weight basis, and the particulate binder: water-soluble polymer = 98.0: 2 A lithium ion secondary battery was manufactured and evaluated in the same manner as in Example 1 except that it was changed to 0.0.

[Example 5]
In the above (1-3), the mixing ratio of the aqueous solution containing the water-soluble polymer and the aqueous dispersion of the particulate binder is equivalent to the solid content on a weight basis, and the particulate binder: water-soluble polymer = 90.0: 10. A lithium ion secondary battery was manufactured and evaluated in the same manner as in Example 1 except that it was changed to 0.0.

[Example 6]
In the above (1-8), ethylene carbonate (EC), diethyl carbonate (DEC), propylene carbonate (PC) and vinylene carbonate (VC) are used as the solvent of the electrolytic solution, and the volume ratio EC: DEC: PC: VC = 19. A lithium ion secondary battery was manufactured and evaluated in the same manner as in Example 1 except that the mixed solvent contained at 8: 10: 70: 0.2 was used.

[Example 7]
In the above (1-8), ethylene carbonate (EC), diethyl carbonate (DEC), propylene carbonate (PC) and vinylene carbonate (VC) are used as the solvent of the electrolytic solution, and the volume ratio EC: DEC: PC: VC = 19. .2: 10: 70: A lithium ion secondary battery was produced and evaluated in the same manner as in Example 1 except that the mixed solvent contained at 10: 70: 0.8 was used.

[Example 8]
In the above (1-8), ethylene carbonate (EC), diethyl carbonate (DEC), propylene carbonate (PC) and vinylene carbonate (VC) are used as the solvent of the electrolytic solution, and the volume ratio EC: DEC: PC: VC = 24. A lithium ion secondary battery was manufactured and evaluated in the same manner as in Example 1 except that the mixed solvent contained at 5: 20: 55: 0.5 was used.

[Example 9]
In the above (1-8), ethylene carbonate (EC), diethyl carbonate (DEC), propylene carbonate (PC) and vinylene carbonate (VC) are used as the solvent of the electrolytic solution, and the volume ratio EC: DEC: PC: VC = 14. A lithium ion secondary battery was manufactured and evaluated in the same manner as in Example 1 except that the mixed solvent contained at 5: 10: 75: 0.5 was used.

[Example 10]
In the above (1-1), a lithium ion secondary battery was produced in the same manner as in Example 1, except that the amount of methacrylic acid was changed to 22 parts and the amount of butyl acrylate was changed to 68.5 parts. And evaluated.

[Example 11]
In the above (1-1), a lithium ion secondary battery was produced in the same manner as in Example 1 except that the amount of methacrylic acid was changed to 48 parts and the amount of butyl acrylate was changed to 42.5 parts. And evaluated.

[Example 12]
In the above (1-4), a lithium ion secondary battery was produced in the same manner as in Example 1 except that artificial graphite (volume average particle diameter: 12 μm) having a specific surface area of 2.5 m ² / g was used as the negative electrode active material. Were manufactured and evaluated.

[Example 13]
In the above (1-4), a lithium ion secondary battery was obtained in the same manner as in Example 1 except that artificial graphite (volume average particle diameter: 13 μm) having a specific surface area of 8.9 m ² / g was used as the negative electrode active material. Were manufactured and evaluated.

[Example 14]
A mixture containing artificial graphite and SiOC at a ratio of graphite / SiOC = 85/15 (weight ratio) was prepared. The specific surface area of this mixture was 6.4 m ² / g.
In the above (1-4), a lithium ion secondary battery was produced and evaluated in the same manner as in Example 1 except that the mixture containing artificial graphite and SiOC was used as the negative electrode active material.

[Example 15]
A mixture containing artificial graphite and SiC at a ratio of graphite / SiC = 85/15 (weight ratio) was prepared. The specific surface area of this mixture was 7.1 m ² / g.
In the above (1-4), a lithium ion secondary battery was produced and evaluated in the same manner as in Example 1 except that the mixture containing artificial graphite and SiC was used as the negative electrode active material.

[Comparative Example 1]
Water containing a particulate binder in the same manner as in (1-2) of Example 1 except that the amount of 1,3-butadiene was changed to 65.5 parts and the amount of styrene was changed to 31 parts. A dispersion was obtained.
Example 1 except that 0.95 parts by weight of the aqueous dispersion containing the particulate binder produced in Comparative Example 1 was used instead of the binder composition obtained in (1-3) above in terms of solid content. In the same manner as in (1-4) above, a negative electrode slurry composition was produced.
Using the negative electrode slurry composition thus obtained, a negative electrode was produced in the same manner as in the above (1-5), and further using this negative electrode in the same manner as in the above (1-8), a lithium ion secondary battery was produced. Manufactured and evaluated.

[Comparative Example 2]
Instead of the binder composition obtained in the above (1-3), an aqueous dispersion containing the particulate binder produced in the above (1-2) was used in an amount of 0.95 parts by weight corresponding to the solid content. Further, artificial graphite (volume average particle diameter: 21 μm) having a specific surface area of 1.2 m ² / g was used as the negative electrode active material. A slurry composition for a negative electrode was produced in the same manner as in (1-4) of Example 1 except for these matters.
Using the negative electrode slurry composition thus obtained, a negative electrode was produced in the same manner as in the above (1-5), and further using this negative electrode in the same manner as in the above (1-8), a lithium ion secondary battery was produced. Manufactured and evaluated.

[Comparative Example 3]
Instead of the binder composition obtained in the above (1-3), an aqueous dispersion containing the particulate binder produced in the above (1-2) was used in an amount of 0.95 parts by weight corresponding to the solid content. Further, artificial graphite (volume average particle diameter: 3.2 μm) having a specific surface area of 18 m ² / g was used as the negative electrode active material. A slurry composition for a negative electrode was produced in the same manner as in (1-4) of Example 1 except for these matters.
Using the negative electrode slurry composition thus obtained, a negative electrode was produced in the same manner as in the above (1-5), and further using this negative electrode in the same manner as in the above (1-8), a lithium ion secondary battery was produced. Manufactured and evaluated.

[Comparative Example 4]
As a negative electrode active material, artificial graphite (volume average particle diameter: 24.5 μm) having a specific surface area of 5.5 m ² / g was prepared. To a planetary mixer with a disper, 100 parts of the above artificial graphite and 2 parts of a 12% N-methylpyrrolidone solution of polyvinylidene fluoride (“7208” manufactured by Kureha Co., Ltd.) corresponding to the solid content were added as a soluble binder. . Further, N-methylpyrrolidone was added to adjust the solid content concentration to 55%. This was defoamed under reduced pressure to obtain a negative electrode slurry composition having good fluidity.
Using the negative electrode slurry composition thus obtained, a negative electrode was produced in the same manner as in the above (1-5) of Example 1, and further using this negative electrode in the same manner as in the above (1-8), lithium ion Secondary batteries were manufactured and evaluated.

[Comparative Example 5]
A water-soluble polymer was prepared in the same manner as in (1-1) of Example 1 except that 2,2,2-trifluoroethyl methacrylate was not used and the amount of butyl acrylate was changed to 65.5 parts. An aqueous solution containing was obtained.
Further, the particulate binder was changed in the same manner as in (1-2) of Example 1 except that the amount of 1,3-butadiene was changed to 65.5 parts and the amount of styrene was changed to 31 parts. An aqueous dispersion containing was obtained.

  A water-soluble polymer and particles in the same manner as in (1-3) of Example 1 except that the aqueous solution containing the water-soluble polymer produced in Comparative Example 5 and the aqueous dispersion containing the particulate binder were used. A binder composition containing a binder was obtained. Using this binder composition, a lithium ion secondary battery was produced and evaluated in the same manner as in the above (1-4) to (1-8).

[Comparative Example 6]
In the above (1-8), a mixed solvent containing ethylene carbonate (EC), diethyl carbonate (DEC) and propylene carbonate (PC) in a volume ratio EC: DEC: PC = 20: 10: 70 as a solvent for the electrolytic solution A lithium ion secondary battery was produced and evaluated in the same manner as in Example 1 except that was used.

[Comparative Example 7]
In the above (1-8), ethylene carbonate (EC), diethyl carbonate (DEC), propylene carbonate (PC) and vinylene carbonate (VC) are used as the solvent of the electrolytic solution, and the volume ratio EC: DEC: PC: VC = 18. A lithium ion secondary battery was manufactured and evaluated in the same manner as in Example 1 except that the mixed solvent contained at 8: 10: 70: 1.2 was used.

[Comparative Example 8]
In the above (1-8), ethylene carbonate (EC), diethyl carbonate (DEC), propylene carbonate (PC) and vinylene carbonate (VC) are used as the solvent of the electrolytic solution, and the volume ratio EC: DEC: PC: VC = 39. A lithium ion secondary battery was manufactured and evaluated in the same manner as in Example 1 except that the mixed solvent contained at 5: 30: 30: 0.5 was used.

[result]
The results of Examples and Comparative Examples are shown in Tables 1 to 6. Here, the meanings of the abbreviations described in the table are as follows. In Tables 1 to 6, the unit “%” in the “VC amount” and “PC amount” columns is based on volume.
Monomer A: Aliphatic conjugated diene monomer BD: 1,3-butadiene Monomer B: Ethylenically unsaturated carboxylic acid monomer IA: Itaconic acid Monomer I: Fluorine-containing (meth) acrylic acid ester Monomer TFEMA: 2,2,2-trifluoroethyl methacrylate Monomer II: Ethylenically unsaturated carboxylic acid monomer MAA: Methacrylic acid VC: Vinylene carbonate PC: Propylene carbonate EC: Ethylene carbonate DEC: Diethyl carbonate PVDF : Polyvinylidene fluoride ΔC1: Capacity maintenance ratio indicating evaluation results of high-temperature storage characteristics ΔC2: Capacity maintenance ratio indicating evaluation results of high-temperature cycle characteristics ΔV: Voltage change ΔM: Cell swelling ratio

[Consideration]
In the examples, superior results were obtained in both high temperature cycle characteristics and low temperature characteristics as compared with the comparative examples. It can also be seen that in any of the examples, a lithium ion secondary battery having excellent high-temperature storage characteristics can be realized. Furthermore, since the swelling rate of the cell was smaller in the example than in the comparative example, it was confirmed that the generation of gas could be suppressed in the example.

Claims (10)

A positive electrode, a negative electrode, an electrolytic solution and a separator are provided.
The negative electrode includes a negative electrode active material layer formed of a composition containing a negative electrode active material, a particulate binder, and a water-soluble polymer,
The specific surface area of the negative electrode active material is ^{2m 2} / ^g~15m 2 / g,
The water-soluble polymer is a copolymer comprising an ethylenically unsaturated carboxylic acid monomer unit and a fluorine-containing (meth) acrylic acid ester monomer unit,
The lithium ion secondary battery in which the solvent of the electrolytic solution contains 50% by volume to 80% by volume of propylene carbonate and 0.05% by volume to 1% by volume of vinylene carbonate.   The lithium ion secondary battery according to claim 1, wherein the negative electrode active material is one or both of a carbonaceous active material and a Si compound.   The lithium ion secondary battery according to claim 1 or 2, wherein the particulate binder is made of a copolymer containing an aliphatic conjugated diene monomer unit and an ethylenically unsaturated carboxylic acid monomer unit.   The lithium ion secondary battery as described in any one of Claims 1-3 whose content rate of the ethylenically unsaturated carboxylic acid monomer unit in the said water-soluble polymer is 20 weight%-50 weight%.   The lithium ion secondary as described in any one of Claims 1-4 whose content rate of the fluorine-containing (meth) acrylic acid ester monomer unit in the said water-soluble polymer is 1 to 30 weight%. battery.   The weight ratio of the particulate binder and the water-soluble polymer is a water-soluble polymer / particulate binder = 0.5 / 99.5 to 40/60, according to any one of claims 1 to 5. Lithium ion secondary battery.   The lithium ion according to any one of claims 1 to 6, wherein the water-soluble polymer contains a crosslinkable monomer unit, and the content ratio thereof is 0.1 wt% or more and 2 wt% or less. Secondary battery.   The water-soluble polymer contains (meth) acrylic acid ester monomer units other than the fluorine-containing (meth) acrylic acid ester monomer, and the content ratio is 30 wt% or more and 70 wt% or less. The lithium ion secondary battery as described in any one of Claims 1-7.   The lithium ion secondary battery according to any one of claims 1 to 8, wherein an amount of the particulate binder is 0.1 parts by weight or more and 10 parts by weight or less with respect to 100 parts by weight of the negative electrode active material.   In the particulate binder, the content of the aliphatic conjugated diene monomer unit is 20% by weight or more and 60% by weight or less, and the content of the ethylenically unsaturated carboxylic acid monomer unit is 0.1% by weight. The lithium ion secondary battery according to claim 3, wherein the content is 15% by weight or less.