JP2018067407A - Lithium ion battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium ion battery containing a positive electrode material high in ratio of nickel and having a high energy density, which is high in safety and excellent in battery cycle life.SOLUTION: A lithium ion battery includes a positive electrode material having a composition formula: LiNiMnCoO, and a separator in which inorganic particles are supported on a nonwoven fabric substrate. In the positive electrode material, x+y+z=1 and x≥0.3 are satisfied. Aramid fibers are contained as constituent fibers of the nonwoven fabric substrate.SELECTED DRAWING: None

Description

  The present invention relates to a lithium ion battery.

  In recent years, as the use of lithium-ion batteries (hereinafter sometimes abbreviated as “battery”) has expanded from portable electronic devices to in-vehicle applications and land-use power supply applications, demands for higher energy density of lithium-ion batteries Is growing. Nickel-based positive electrode materials have a high capacity density and are suitable for such high energy density demands, but on the other hand, there are problems such as low thermal stability in the charged state and poor safety, and charge / discharge There is a problem that the volume change of the electrode accompanying the expansion, the expansion and compression of the separator is repeated, the separator deteriorates, and the battery life is shortened. Currently, large-sized lithium-ion batteries suitable for use are not being put into practical use.

  Conventionally, as a separator for a lithium ion battery (hereinafter sometimes abbreviated as “separator”), a polyolefin porous film having fine pores that have penetrated has been used. The separator of the polyolefin porous film suppresses the battery temperature rise by increasing the internal resistance of the battery by melting and penetrating the fine pores through which the polyolefin melted when the lithium ion battery generates heat . However, when the temperature rises due to external heat, or when a chemical reaction occurs inside the battery due to the temperature rise, the porous film contracts, causing an internal short circuit, which can lead to serious events such as ignition and rupture. is there. In addition, in the heat generation environment accompanying charging / discharging of the battery, the separator is repeatedly expanded / compressed due to the contraction / expansion of the electrode, which causes degradation of the partially thermoplasticized polyolefin, resulting in a short battery life. was there.

  In order to solve such problems, a separator is proposed in which inorganic particles are supported on a nonwoven fabric containing fibers having high heat resistance such as polyethylene terephthalate (PET) (see, for example, Patent Documents 1 to 4). A fiber having high heat resistance such as PET has an advantage that the fiber is hardly partially plasticized due to heat generated by charging / discharging of the battery, and is hardly deteriorated even if it is repeatedly expanded and compressed. Further, since the heat resistance of the inorganic particles is added to the heat resistance of the fibers, the battery is excellent in heat resistance, and a battery using a separator formed by supporting inorganic particles on such a nonwoven fabric has an advantage of excellent safety. On the other hand, when the nickel ratio of the positive electrode material is further increased, the contraction / expansion of the electrode becomes violent, and the excessive volume change of the separator is repeated, so that even a fiber such as PET that hardly undergoes thermoplasticization can be obtained. During repeated use, the fiber shape is difficult to restore, and the adhesiveness at the electrode-separator interface decreases, causing local electrolyte depletion and the like, making it difficult to obtain a sufficient battery cycle life. There was a case.

JP 2007-294437 A Special table 2011-505663 gazette JP 2005-536658 Gazette International Publication No. 2013/176276 Pamphlet

  The subject of this invention is related with the lithium ion battery which has a high energy density containing the positive electrode material with a high nickel ratio, and is providing a lithium ion battery with high safety | security and excellent in the cycle life of a battery.

As a result of intensive studies, the present inventors have invented a lithium ion battery that can solve the problem. That is, a lithium ion battery including a positive electrode material having the composition formula LiNi _x Mn _y Co _z O ₂ and a separator in which inorganic particles are supported on a nonwoven fabric base material, where x + y + z = 1 and x ≧ 0. 3 and an aramid fiber as a constituent fiber of the nonwoven fabric base material.

  According to the present invention, in a lithium ion battery having a high energy density that includes a positive electrode material having a high nickel ratio, the lithium ion battery is highly safe and has an excellent cycle life.

The lithium ion battery of the present invention is a lithium ion battery including a positive electrode material having the composition formula LiNi _x Mn _y Co _z O ₂ and a separator in which inorganic particles are supported on a non-woven fabric substrate. In the positive electrode material, x + y + z = 1 and x ≧ 0.3, and an aramid fiber is included as a constituent fiber of the nonwoven fabric substrate.

In the present invention, the positive electrode material includes an active material having the composition formula LiNi _x Mn _y Co _z O ₂ , where x + y + z = 1 and x ≧ 0.3. In order to increase the energy density of the battery, x ≧ 0.5 is preferable. However, if the nickel ratio is increased, the cycle life of the battery is likely to be reduced. Therefore, x ≦ 0.95 is preferable. Note that y ≧ 0 and z ≧ 0.

  In the present invention, the separator for a lithium ion battery is formed by supporting inorganic particles on a nonwoven fabric base material, and includes an aramid fiber as a constituent fiber of the nonwoven fabric base material.

  In this invention, it is preferable that it is 1-10 mass% of a nonwoven fabric base material as content of the aramid fiber contained in a nonwoven fabric base material, More preferably, it is 3-8 mass%. By setting the content of the aramid fiber to 1% by mass or more, the compression recovery resistance is improved and the cycle life of the battery is easily improved. Moreover, by adjusting the content of the aramid fiber to 10% by mass or less, it is easy to easily adjust the performance balance such as strength and pore diameter of the nonwoven fabric substrate.

  In the present invention, the nonwoven fabric substrate can contain fibers other than aramid, and the constituent materials thereof include polyethylene terephthalate, polybutylene terephthalate and derivatives thereof, semi-aromatic polyester, wholly aromatic polyester, and the like. , Polyolefin, acrylic, polyacetal, polycarbonate, aliphatic polyketone, aromatic polyketone, aliphatic polyamide, semi-aromatic polyamide, polyimide, polyamideimide, polyphenylene sulfide, polybenzimidazole, polyetheretherketone, polyethersulfone, poly (para -Phenylenebenzobisthiazole), poly (para-phenylene-2,6-benzobisoxazole), polyvinylidene fluoride, polytetrafluoroethylene, polyvinyl alcohol Le, resins such as polyurethane and polyvinyl chloride; and cellulose. Two or more kinds of fibers other than these aramids may be used in combination in the nonwoven fabric substrate. Polyethylene terephthalate having excellent heat resistance is particularly preferable.

  In the present invention, the nonwoven fabric substrate is preferably a wet nonwoven fabric produced by a wet papermaking method. In the wet papermaking method, fibers are dispersed in water to form a uniform papermaking slurry, and this papermaking slurry is rolled up with a paper machine to produce a wet nonwoven fabric. Examples of the paper machine include a circular net paper machine, a long net paper machine, an inclined paper machine, an inclined short net paper machine, and a composite machine of these. A dispersing agent, a thickener, an antifoaming agent and the like can be appropriately added to the papermaking slurry, if necessary, in addition to the fibers, and the papermaking slurry has a solid content concentration of about 0.001 to 5% by mass. To prepare. The papermaking slurry is further diluted to a predetermined concentration to make paper, and then dried. In the process of manufacturing a wet nonwoven fabric, hydroentanglement treatment may be performed as necessary. The wet nonwoven fabric obtained by papermaking is subjected to calendering, thermal calendering, heat treatment and the like as necessary.

In the present invention, the basis weight of the nonwoven substrate is preferably 4~30g / ^{m 2,} more preferably from 5 to 20 g / ^{m 2.} When the basis weight is 4 g / m ² or more, it is easy to obtain uniformity as a nonwoven fabric substrate, and when it is 30 g / m ² or less, the thickness is suitable for a lithium ion battery separator. The basis weight means the basis weight based on the method defined in JIS P 8124.

  Examples of inorganic particles that can be used in the present invention include kaolin, calcined kaolin, heavy calcium carbonate, light calcium carbonate, magnesium carbonate, zinc oxide, alumina, boehmite, aluminum hydroxide, magnesium hydroxide, titanium dioxide, barium sulfate, Zinc sulfate, amorphous silica, calcium silicate and the like can be mentioned. These may be used alone or in combination of two or more. Of these, alumina, boehmite or magnesium hydroxide is preferably used from the viewpoint of thermal stability. Only one type of inorganic particles may be used, or two or more types may be used in combination. Moreover, it is preferable that the inorganic particle contained in the separator of this invention is 30-70 mass% in the total solid of a separator from the point of thermal stability.

  The particle size of the inorganic particles used in the present invention is preferably 0.02 to 10.0 μm, more preferably 0.1 to 7.5 μm. By setting the particle size to 0.02 μm or more, the stability of the coating liquid used when supporting the inorganic particles on the nonwoven fabric substrate is likely to be high, and by setting the particle size to 10.0 μm or less, a flat coating surface Becomes easier to obtain. In addition, the average particle diameter here refers to the average particle diameter (D50) measured by a laser diffraction scattering method.

  In the present invention, a binder may be used when the inorganic particles are supported on the nonwoven fabric substrate. As the binder, latex polymer is preferably used. Specific examples include, for example, styrene / butadiene copolymers, acrylonitrile / butadiene copolymers, methyl acrylate / butadiene copolymers, acrylonitrile / butadiene / styrene terpolymers, polyvinyl acetate, vinyl acetate / acrylate esters. Examples include, but are not limited to, latex polymers such as copolymers, ethylene / vinyl acetate copolymers, polyacrylates, styrene / acrylate copolymers, and polyurethanes. In the present invention, from the viewpoint of the high rate characteristics of the separator and the strength of the inorganic particle layer, the amount of the binder with respect to the total amount of the inorganic particles and the binder is preferably 2 to 15% by mass on a solid basis.

  In the present invention, various additives such as a dispersant, a wetting agent, and a thickener can be used when the inorganic particles are supported on the nonwoven fabric substrate within a range that does not impair the effects of the invention.

  In this invention, there is no restriction | limiting in particular in the method by which an inorganic particle is carry | supported by the nonwoven fabric base material. For example, coating liquid containing inorganic particles by air doctor coater, blade coater, knife coater, rod coater, squeeze coater, impregnation coater, gravure coater, kiss roll coater, die coater, reverse roll coater, transfer roll coater, spray coater, etc. By coating and drying, the inorganic particles can be supported on the nonwoven fabric substrate to form an inorganic particle layer.

In the present invention, the coating amount (absolute dry coating amount) of the inorganic particle layer containing inorganic particles is preferably 5 to 30 g / m ² , more preferably 10 to 20 g / m ² . When the coating amount is 5 g / m ² or more, it becomes easy to sufficiently cover the surface of the nonwoven fabric base material, and it is easy to prevent a minute short circuit. Moreover, when the coating amount is 30 g / m ² or less, an increase in the thickness of the separator can be easily suppressed.

In the lithium ion battery separator of the present invention, the basis weight of the separator is preferably 10 to 50 g / m ² , more preferably 17 to 40 g / m ² . Moreover, 10-50 micrometers is preferable and, as for the thickness of a separator, More preferably, it is 15-40 micrometers. The density of the separator is preferably 0.4 to 1.2 g / cm ³ , more preferably 0.5 to 1.0 g / cm ³ . The basis weight means a basis weight based on the method defined in JIS P 8124. The density is a value obtained by dividing the basis weight by the thickness. The thickness means a value measured by an outer micrometer defined in JIS B 7502.

The negative electrode active material of the lithium ion battery in the present invention includes carbon materials such as graphite and coke, metallic lithium, aluminum, silica, tin, nickel, an alloy of lithium and lithium, SiO, SnO , Fe ₂ O ₂ , WO ₂ , Nb ₂ O ₅ , metal oxides such as Li _4/3 Ti _5/3 O ₄ , and nitrides such as Li _0.4 CoN.

  Examples of the electrolytic solution of the lithium ion battery of the present invention include a solution obtained by dissolving a lithium salt in an organic solvent such as propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, dimethoxyethane, dimethoxymethane, and a mixed solvent thereof. Examples of the lithium salt include lithium hexafluorophosphate and lithium tetrafluoroborate. Examples of the solid electrolyte include those obtained by dissolving a lithium salt in a gel polymer such as polyethylene glycol and derivatives thereof, polymethacrylic acid derivatives, polysiloxane and derivatives thereof, and polyvinylidene fluoride.

  The positive electrode of the lithium ion battery of the present invention is produced by providing a positive electrode active material layer on a positive electrode current collector. Similarly, the negative electrode is produced by providing a negative electrode active material layer on a negative electrode current collector. The current collector is not particularly limited as long as it has conductivity, but for the positive electrode, for example, a metal such as aluminum (Al), an alloy containing a metal such as Al, for the negative electrode, for example, Al or copper ( An alloy containing a metal such as Cu) or a metal such as Al or Cu can be given.

  EXAMPLES The present invention will be described below with reference to examples, but the present invention is not limited to these examples. In Examples,% and parts are based on mass unless otherwise specified.

Production of Non-woven Fabric Base A Fineness 0.06 dtex (average fiber diameter 2.4 μm), 56 parts of oriented crystallized polyethylene terephthalate (PET) short fibers having a fiber length of 3 mm and fineness 0.2 dtex (average fiber diameter 4.3 μm), 1% slurry 400 obtained by fibrillating 40.0 parts of PET short fibers for a single-component binder with a fiber length of 3 mm and an aramid fiber pulp (average fiber length of 1.7 mm, average fiber diameter of 10 μm) with a high-pressure homogenizer The slurry was dispersed in water with a pulper to prepare a uniform papermaking slurry having a concentration of 1%. This slurry for paper making is made by a wet paper making method, dried by a 130 ° C. Yankee dryer, and then heated using a 1 nip heat calender comprising a dielectric heating jacket roll (metal hot roll) and an elastic roll. Thermal calendering was performed under the conditions of a temperature of 100 ° C., a linear pressure of 100 kN / m, and a processing speed of 40 m / min to prepare a nonwoven fabric substrate A having a basis weight of 10 g / m ² and a thickness of 15 μm.

Production of Non-woven Fabric Base B 53 parts of oriented crystallized polyethylene terephthalate (PET) short fibers having a fineness of 0.06 dtex (average fiber diameter of 2.4 μm) and a fiber length of 3 mm, and a pulp of aramid fibers (average fiber length of 1. Nonwoven fabric substrate B was prepared in the same manner as nonwoven fabric substrate A, except that 700 parts of 1% slurry obtained by fibrillating 7 mm and an average fiber diameter of 10 μm with a high-pressure homogenizer was used.

Fabrication of nonwoven fabric substrate C Instead of 400 parts of 1% slurry in which aramid fibers (average fiber length 1.7 mm, average fiber diameter 10 μm) were fibrillated by a high-pressure homogenizer, fineness 0.5 dtex (average fiber diameter 20 μm), fiber A nonwoven fabric substrate C was prepared in the same manner as the nonwoven fabric substrate A, except that 4 parts of polypropylene fibers having a length of 3 mm were used.

Preparation of coating liquid As inorganic particles, 100 parts of magnesium hydroxide having an average particle diameter of 2.0 μm is dispersed in 120 parts of a 0.3% aqueous solution of carboxymethyl cellulose sodium salt having a viscosity of 200 mPa · s at 25 ° C. And stirred well to prepare a magnesium hydroxide dispersion. Subsequently, 300 parts of a 0.5% aqueous solution of carboxymethylcellulose sodium salt having a viscosity of 7000 mPa · s at 25 ° C. in a 1% by mass aqueous solution is mixed and stirred, and a latex polymer of 45% styrene / butadiene copolymer is used as a binder. 15 parts were mixed and stirred to prepare a coating solution.

Production of Separator A On one side of the nonwoven fabric substrate A, a coating liquid was applied and dried so that the dry coating amount was 10 g / m ² , thereby producing a separator A.

Production of Separator B A separator B was produced on one side of the nonwoven fabric substrate B by coating and drying the coating solution so that the dry coating amount was 10 g / m ² .

Production of Separator C A separator C was produced on one side of the nonwoven fabric substrate C by coating and drying the coating solution so that the dry coating amount was 10 g / m ² .

Preparation of Positive Electrode A As a positive electrode active material, LiNi _1/3 Mn _1/3 Co _1/3 O ₂ powder 100 parts, conductive material 3 parts acetylene black 3 parts graphite 3 parts polyvinylidene fluoride 4 parts A slurry dispersed in N-methyl-2-pyrrolidone was prepared. The slurry was applied to both sides of a current collector made of an aluminum foil having a thickness of 15 μm and rolled, and then vacuum-dried at 150 ° C. for 2 hours to prepare a positive electrode A for a lithium ion battery having a thickness of 100 μm.

Production of Positive Electrode B As a positive electrode active material, 100 parts of LiNi _0.5 Mn _0.3 Co _0.2 O ₂ powder, 3 parts of acetylene black, 3 parts of graphite and 4 parts of polyvinylidene fluoride as a conductive material were mixed. A slurry dispersed in N-methyl-2-pyrrolidone was prepared. The slurry was applied to both sides of a current collector made of an aluminum foil having a thickness of 15 μm and rolled, and then vacuum-dried at 150 ° C. for 2 hours to prepare a positive electrode B for a lithium ion battery having a thickness of 100 μm.

Preparation of Positive Electrode C As a positive electrode active material, 100 parts of LiNi _0.6 Mn _0.2 Co _0.2 O ₂ powder, 3 parts of acetylene black, 3 parts of graphite and 4 parts of polyvinylidene fluoride as a conductive material were mixed. A slurry dispersed in N-methyl-2-pyrrolidone was prepared. The slurry was applied to both sides of a current collector made of an aluminum foil having a thickness of 15 μm and rolled, and then vacuum-dried at 150 ° C. for 2 hours to prepare a positive electrode C for a lithium ion battery having a thickness of 100 μm.

Production of Positive Electrode D As a positive electrode active material, 100 parts of LiNi _0.8 Mn _0.1 Co _0.1 O ₂ powder, 3 parts of acetylene black, 3 parts of graphite and 4 parts of polyvinylidene fluoride as a conductive material were mixed. A slurry dispersed in N-methyl-2-pyrrolidone was prepared. The slurry was applied to both sides of a current collector made of an aluminum foil having a thickness of 15 μm and rolled, and then vacuum-dried at 150 ° C. for 2 hours to produce a positive electrode D for a lithium ion battery having a thickness of 100 μm.

Production of Negative Electrode As a negative electrode active material, 100 parts of graphite powder was dispersed in 100 parts of a 1% carboxymethylcellulose sodium salt aqueous solution, and then mixed with 6 parts of a 45% styrene / butadiene copolymer latex polymer, and this was 8 μm thick. After applying and rolling on both sides of a current collector made of copper foil, the electrode was dried in a vacuum at 150 ° C. for 2 hours to prepare a negative electrode for a lithium ion battery having a thickness of 100 μm.

Example 1
The produced separator A, positive electrode A, negative electrode, and an electrolyte solution was a mixed solvent solution (1 mol / L) of 1/1/1 (volume ratio) of lithium hexafluorophosphate ethylene carbonate, diethyl carbonate, and dimethyl carbonate. The laminated lithium ion battery A having a design capacity of 100 mAh was prepared by laminating the separator A so that the coating side surface was the negative electrode side.

Example 2
A lithium ion battery B was produced in the same manner as in Example 1 except that the positive electrode B was used as the positive electrode.

Example 3
A lithium ion battery C was produced in the same manner as in Example 1 except that the positive electrode C was used as the positive electrode.

Example 4
A lithium ion battery D was produced in the same manner as in Example 1 except that the positive electrode D was used as the positive electrode.

Example 5
A lithium ion battery E was produced in the same manner as in Example 1 except that the separator B was used as the separator and the positive electrode D was used as the positive electrode.

Comparative Example 1
A lithium ion battery F was produced in the same manner as in Example 1 except that the separator C was used as the separator.

<Evaluation>

[Separator heat resistance]
A sheet sample of 50 mm × 50 mm is cut out from each separator produced, and the CD (cross direction, lateral direction) side of the sheet sample is fixed with a clip and sandwiched between heat resistant glass plates, and 1 in a thermostatic chamber at 150 ° C. and 180 ° C. After holding for each time, the sample was taken out, the width of the sample was measured, and the shrinkage before and after heating was calculated. The evaluation was as follows.

A: The shrinkage rate is less than 2% and almost no shrinkage is observed.
○: The shrinkage rate is 2 to 5%, which is a practically acceptable level.
(Triangle | delta): A shrinkage rate is 5 to 8%, and there is some concern about shrinkage by local overheating.
X: The shrinkage rate exceeds 8%, and there is a concern about shrinkage during local overheating.

[Cycle life]
For each lithium ion battery produced, under 55 ° C., 100 mA constant current charge → 4.2 V constant voltage charge → charge current 10 mA, 100 mA constant current discharge → 2.8 V Then, charge / discharge of 500 cycles was performed, and the capacity reduction rate was determined as [1- (discharge capacity at 500th cycle / discharge capacity at 4th cycle)] × 100 (%). The evaluation was as follows.

A: Capacity reduction rate is less than 10%.
◯: The capacity reduction rate is 10 to 15%, which is a level that is not problematic in practice.
(Triangle | delta): A capacity | capacitance fall rate is 15 to 25%, and we are anxious about the capacity | capacitance fall at the time of long-term use.
X: The capacity reduction rate exceeds 25%, and there is a concern about capacity reduction in a relatively short time.

  As is clear from Table 1, the lithium ion batteries of Examples 1 to 5 include a positive electrode material having a high nickel ratio, but the nonwoven fabric substrate in the separator in which inorganic particles are supported on the nonwoven fabric substrate is a constituent fiber. Since the aramid fiber is included, the heat resistance of the separator is excellent and the cycle life of the battery is also excellent.

Claims (1)

A lithium ion battery including a positive electrode material having a composition formula LiNi _x Mn _y Co _z O ₂ and a separator in which inorganic particles are supported on a nonwoven fabric base material, wherein x + y + z = 1 and x ≧ 0.3. And a lithium ion battery comprising an aramid fiber as a constituent fiber of the nonwoven fabric substrate.