JP2018147624A - Negative electrode for lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve cycle durability while increasing short circuit resistance.SOLUTION: A negative electrode 10 for a lithium ion secondary battery includes at least a negative electrode mixture layer 12 and a surface layer 13. The surface layer 13 is disposed on the surface of the negative electrode mixture layer 12. The surface layer 13 contains a composite particle 3. The composite particle 3 contains a resin particle 1 and a lithium titanate particle 2. The lithium titanate particle 2 coats the resin particle 1.SELECTED DRAWING: Figure 1

Description

  The present disclosure relates to a negative electrode for a lithium ion secondary battery.

  Japanese Patent Laying-Open No. 2015-115168 (Patent Document 1) discloses a negative electrode for a lithium ion secondary battery (hereinafter sometimes simply referred to as “negative electrode”) in which a negative electrode composite material layer includes resin particles.

JP2015-115168A

  In the negative electrode of Patent Document 1, resin particles are dispersed in the negative electrode mixture layer. In this negative electrode, when the battery temperature exceeds the melting point of the resin particles, the resin particles melt. Thereby, the electrical resistance of the whole negative electrode increases, and it is thought that the temperature rise by a current flowing into the negative electrode is suppressed.

  However, it is difficult to increase the short-circuit resistance between the conductive foreign material and the negative electrode in an abnormal mode in which the positive electrode and the negative electrode are short-circuited through the conductive foreign material, for example, as in a nail penetration test. When the short-circuit resistance is small, current flows through the conductive foreign matter, and Joule heat is generated. Since the separator contracts due to Joule heat, it is considered that the short-circuit area is expanded and the temperature rise is promoted. Further, since the resin particles do not have electronic conductivity and lithium (Li) ion conductivity, the cycle durability may be reduced by including the resin particles in the negative electrode mixture layer.

  An object of the present disclosure is to improve cycle durability while increasing short-circuit resistance.

  Hereinafter, the technical configuration and effects of the present disclosure will be described. However, the mechanism of action of the present disclosure includes estimation. The scope of the claims should not be limited by the correctness of the mechanism of action.

  The negative electrode for a lithium ion secondary battery of the present disclosure includes at least a negative electrode mixture layer and a surface layer. The surface layer is disposed on the surface of the negative electrode mixture layer. The surface layer includes composite particles. The composite particles include resin particles and lithium titanate particles. Lithium titanate particles coat resin particles.

  In the negative electrode of the present disclosure, a surface layer containing resin particles is disposed on the surface of the negative electrode mixture layer. In order to cause a short circuit through the conductive foreign matter, the conductive foreign matter needs to penetrate the surface layer. When the conductive foreign matter penetrates the surface layer, the resin particles enter between the conductive foreign matter and the negative electrode mixture layer, so that the short circuit resistance is expected to increase.

  It is considered that when Joule heat is generated due to a short-circuit current flowing through the conductive foreign matter, the resin particles melt and spread around the conductive foreign matter, and an insulating film is formed around the conductive foreign matter. This is expected to suppress the expansion of the short circuit area.

  Furthermore, in the negative electrode of the present disclosure, the resin particles are coated with lithium titanate (LTO) particles. LTO particles can occlude and release Li ions. It is considered that the LTO particles come into contact with each other and are easily continuous when the resin particles are covered with the LTO particles. It is considered that a network-like Li ion diffusion path is formed when the LTO particles come into contact with each other and are continuous. As a result, it is expected that in the surface layer of the negative electrode, the diffusion of Li ions is promoted and the cycle durability is improved.

FIG. 1 is a schematic cross-sectional view illustrating an example of a configuration of a negative electrode for a lithium ion secondary battery according to an embodiment of the present disclosure.

  Hereinafter, an embodiment of the present disclosure (hereinafter also referred to as “the present embodiment”) will be described. However, the following description does not limit the scope of the claims.

<Anode for lithium ion secondary battery>
FIG. 1 is a schematic cross-sectional view illustrating an example of a configuration of a negative electrode for a lithium ion secondary battery according to an embodiment of the present disclosure. The negative electrode 10 includes a current collector 11, a negative electrode mixture layer 12, and a surface layer 13. That is, the negative electrode 10 includes at least a negative electrode mixture layer 12 and a surface layer 13. The current collector 11 may be a copper (Cu) foil or the like, for example. The Cu foil may be a pure Cu foil or a Cu alloy foil. For example, the current collector 11 may have a thickness of 5 to 20 μm.

(Negative electrode mixture layer)
The negative electrode mixture layer 12 is disposed on the surface of the current collector 11. The negative electrode mixture layer 12 is formed, for example, by applying a negative electrode mixture paste on the surface of the current collector 11 and drying it. The application method may be, for example, a slot die method. The negative electrode mixture paste is prepared by mixing a negative electrode active material, a binder, and a solvent. The solvent may be water, for example. After drying, the thickness of the negative electrode mixture layer 12 may be adjusted by compressing the negative electrode mixture layer 12. The negative electrode mixture layer 12 may have a thickness of 10 to 100 μm, for example.

  The negative electrode mixture layer 12 includes, for example, 95 to 99.5% by mass of a negative electrode active material and 0.5 to 5% by mass of a binder. The negative electrode active material may be, for example, graphite, graphitizable carbon, non-graphitizable carbon, silicon, silicon oxide, tin, tin oxide, or the like. One type of negative electrode active material may be used alone, or two or more types of negative electrode active materials may be used in combination. The binder may be, for example, carboxymethyl cellulose (CMC), styrene butadiene rubber (SBR), polyacrylic acid (PAA), alginic acid, and the like. One kind of binder may be used alone, or two or more kinds of binders may be used in combination.

(Surface layer)
The surface layer 13 is disposed on the surface of the negative electrode mixture layer 12. The surface layer 13 desirably covers the entire surface of the negative electrode mixture layer 12. However, as long as the effect of increasing the short-circuit resistance and the effect of improving the cycle durability are obtained, there may be a portion that is not covered with the surface layer 13 on the surface of the negative electrode mixture layer 12. The surface layer 13 includes the composite particles 3. Composite particles 3 include resin particles 1 and LTO particles 2. The surface layer 13 is formed, for example, by applying a particle dispersion to the surface of the compressed negative electrode mixture layer 12 and drying it. The application method may be a gravure method, for example.

  The surface layer 13 may have a thickness of 5 to 20 μm, for example. If the surface layer 13 is thick, the distance between the positive and negative electrodes is increased, and thus the battery resistance may be increased. From the viewpoint of suppressing an increase in battery resistance, the surface layer 13 preferably has a thickness not less than D50 of the composite particles 3 and not more than 1/10 (1/10) of the thickness of the negative electrode mixture layer 12. “D50” in the present specification indicates a particle size of 50% cumulative from the fine particle side in the volume-based particle size distribution. The volume-based particle size distribution is measured by, for example, a laser diffraction scattering method or a Coulter counter method. The surface layer 13 may have a thickness of 5 to 10 μm, for example.

  The composite particle 3 is a so-called core-shell type particle. The core is resin particles 1 and the shell is LTO particles 2. The composite particle 3 may have a D50 of 3 to 20 μm, for example. As long as the surface layer 13 includes the composite particles 3, the surface layer 13 may include other components. The surface layer 13 may include, for example, 90 to 99.5% by mass of the composite particles 3 and 0.5 to 10% by mass of a binder (not shown). As the binder, a binder similar to the binder exemplified as the binder of the negative electrode mixture layer 12 can be used. As long as the surface layer 13 includes the composite particles 3, for example, the surface layer 13 may include the resin particles 1 and the LTO particles 2 that are not combined.

  The resin particle 1 constitutes the core of the composite particle 3. The resin particle 1 may have D50 of 1 to 10 μm, for example. The resin particle 1 is made of a thermoplastic resin. The resin particles 1 preferably have a melting point of 150 ° C. or higher and 300 ° C. or lower. The melting point indicates the melting temperature measured by a method in accordance with “JIS K7121: Plastic Transition Temperature Measurement Method”. When a short-circuit current flows through the conductive foreign matter, the surrounding area is considered to be 150 ° C. or higher. At that time, it is considered that the resin film 1 has a melting point of 150 ° C. or higher and 300 ° C. or lower, so that after the insulating film is formed by the resin particle 1, the insulating film easily maintains its form. That is, it is considered that the temperature rise suppressing effect lasts for a long time.

  Examples of the thermoplastic resin having a melting point of 150 ° C. or higher include polypropylene (PP), polyvinyl alcohol (PVA), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polyphenylene sulfide (PPS), polyester, and nylon. Etc. One kind of thermoplastic resin may be used alone, or two or more kinds of thermoplastic resins may be used in combination.

The LTO particle 2 constitutes a shell of the composite particle 3. The LTO particles 2 cover the resin particles 1 that are cores. The LTO particles 2 are lithium titanate (Li ₄ Ti ₅ O ₁₂ ) particles. The LTO particles 2 can occlude and release Li ions. It is considered that when the resin particles 1 are covered with the LTO particles 2, the LTO particles 2 come into contact with each other and are continuous to form a network-like Li ion diffusion path. As a result, it is expected that the diffusion of Li ions is promoted in the surface layer of the negative electrode 10 and the cycle durability is improved.

In addition, lithium titanate is 1.55 V vs. Li ions are occluded and released near Li / Li ⁺ . That is, lithium titanate reacts with Li ions at a potential nobler than the deposition potential of Li metal (0 V vs. Li / Li ⁺ ). Therefore, it is expected that the deposition of Li metal is suppressed in the surface layer of the negative electrode 10.

  The LTO particle 2 may cover the entire surface of the resin particle 1 or may cover a part of the surface of the resin particle 1. It is considered that the LTO particle 2 covers at least a part of the surface of the resin particle 1 so that an effect of improving the cycle durability can be obtained. From the viewpoint of increasing the contact ratio between the LTO particles 2, the LTO particles 2 preferably have a D50 that is ¼ (one quarter) or less of the D50 of the resin particles 1. It is expected that the cycle durability is further improved by increasing the contact rate between the LTO particles 2. The LTO particles 2 may have a D50 of 0.2 to 2 μm, for example.

  The method for combining the resin particles 1 and the LTO particles 2 should not be particularly limited. For example, it is conceivable to use a jet mill. In the jet mill, a large energy can be given to the surface of the resin particle 1. Therefore, it is expected that the resin particles 1 and the LTO particles 2 are combined in a short time.

  Examples will be described below. However, the following examples do not limit the scope of the claims.

<Comparative Example 1>
1. Production of negative electrode The following materials were prepared.
Negative electrode active material: Natural graphite Binder: CMC (Nippon Paper Industries) and SBR (JSR)
Solvent: Ion exchange water Current collector: Cu foil

A negative electrode mixture paste was prepared by mixing the negative electrode active material, the binder, and the solvent. The composition of the solid content was natural graphite: CMC: SBR = 98: 1: 1 (mass ratio). The negative electrode mixture paste was applied to the surface of the current collector and dried. As a result, a negative electrode mixture layer was formed. The coating amount (after drying) was 20 mg / cm ² . The negative electrode mixture layer was compressed. The thickness of the negative electrode mixture layer (one side) after compression was 66.5 μm. The density of the negative electrode mixture layer after compression was 1.5 g / cm ³ . From the above, a negative electrode was produced.

2. Production of positive electrode The following materials were prepared.
Positive electrode active material: nickel cobalt lithium manganate (hereinafter abbreviated as “NCM”)
Conductive material: Acetylene black (manufactured by Denka, hereinafter abbreviated as “AB”)
Binder: Polyvinylidene fluoride (manufactured by Kureha, hereinafter abbreviated as “PVdF”)
Solvent: N-methyl-2-pyrrolidone Current collector: Aluminum foil

A positive electrode mixture paste was prepared by mixing a positive electrode active material, a conductive material, a binder, and a solvent. The composition of the solid content was NCM: AB: PVdF = 90: 8: 2 (mass ratio). The positive electrode mixture paste was applied to the surface of the current collector and dried. As a result, a positive electrode mixture layer was formed. The coating amount (after drying) was 40 mg / cm ² . The positive electrode mixture layer was compressed. The density of the positive electrode mixture layer after compression was 3.0 g / cm ³ . From the above, a positive electrode was manufactured.

3. Production of Battery A polyethylene porous membrane having a thickness of 20 μm was prepared as a separator. The positive electrode and the negative electrode were disposed opposite to each other with the separator interposed therebetween. Thus, an electrode group was configured. An aluminum laminate film bag was prepared as an exterior body. An electrode group was inserted into the outer package. The electrolyte was injected into the outer package. The outer package was sealed. From the above, a battery (laminated lithium ion secondary battery) was manufactured. This battery is designed to operate in the range of 3.0 to 4.1V.

<Comparative Example 2>
A resin particle dispersion (product name “Chemical 400W”, manufactured by Mitsui Chemicals, Inc.) was prepared. The resin particle dispersion was heated in an oven set at 80 ° C. Thereby, the solvent was evaporated and polyethylene (PE) particles were obtained. The PE particles had a melting point of 110 ° C. and a D50 of 4 μm. A negative electrode mixture paste was prepared by mixing the negative electrode active material, the PE particles, the binder, and the solvent. The composition of the solid content was natural graphite: PE particles: CMC: SBR = 92: 2: 1: 1 (mass ratio). Except for these, the negative electrode was produced and the battery was produced by the same production method as in Comparative Example 1.

<Comparative Example 3>
A resin particle dispersion (product name “Chemical 4005W”, manufactured by Mitsui Chemicals, Inc.) was prepared. The resin particle dispersion was heated in an oven set at 80 ° C. As a result, the solvent was evaporated and PE particles were obtained. The PE particles had a melting point of 110 ° C. and a D50 of 0.6 μm. A negative electrode mixture paste was prepared by mixing the negative electrode active material, the PE particles, the binder, and the solvent. The composition of the solid content was natural graphite: PE particles: CMC: SBR = 96: 2: 1: 1 (mass ratio). Except for these, the negative electrode was produced and the battery was produced by the same production method as in Comparative Example 1.

<Comparative example 4>
Polyester elastomer (TPC) particles (product name “Trepearl TPC”, manufactured by Toray Industries, Inc.) were prepared as resin particles. The TPC particles had a melting point of 220 ° C. and a D50 of 5 μm.

  LTO particles (Ishihara Sangyo Co., Ltd.) were prepared. LTO particles (secondary particles) were crushed by a desktop spiral jet mill (model “50AS”, manufactured by Hosokawa Micron Corporation). The pulverized LTO particles (primary particles) had a D50 of 1.1 μm.

  A particle dispersion was prepared by mixing TPC particles, LTO particles, CMC and ion-exchanged water. The composition of the solid content was TPC particles: LTO particles: CMC = 60: 35: 5 (mass ratio). The solid content ratio of the particle dispersion was 40% by mass. The particle dispersion was applied to the surface of the negative electrode mixture layer after compression by a gravure method and dried. As a result, a surface layer having a thickness of 6.5 μm was formed. Except for these, the negative electrode was produced and the battery was produced by the same production method as in Comparative Example 1.

<Example 1>
The TPC particles and the pulverized LTO particles were combined by a desktop spiral jet mill. Thereby, TPC particles were covered with LTO particles, and composite particles were prepared. A particle dispersion was prepared by mixing the composite particles, CMC and ion-exchanged water. The composition of the solid content was composite particles: CMC = 95: 5 (mass ratio). The solid content ratio of the particle dispersion was 40% by mass. The particle dispersion was applied to the surface of the negative electrode mixture layer after compression by a gravure method and dried. As a result, a surface layer having a thickness of 6.5 μm was formed. Except for these, the negative electrode was produced and the battery was produced by the same production method as in Comparative Example 1.

<Example 2>
A negative electrode was manufactured and a battery was manufactured by the same manufacturing method as in Example 1 except that a surface layer having a thickness of 20 μm was formed.

<Example 3>
The PE particles used in Comparative Example 2 were prepared. PE particles and pulverized LTO particles were combined by a desktop spiral jet mill. Thereby, composite particles were prepared. A particle dispersion was prepared by mixing the composite particles, CMC and ion-exchanged water. The composition of the solid content was composite particles: CMC = 95: 5 (mass ratio). The solid content ratio of the particle dispersion was 40% by mass. The particle dispersion was applied to the surface of the negative electrode mixture layer after compression by a gravure method and dried. As a result, a surface layer having a thickness of 5.5 μm was formed. Except for these, the negative electrode was produced and the battery was produced by the same production method as in Example 1.

<Evaluation>
(Short circuit resistance)
A nail (made of metal) having a body diameter of 6 mm was prepared. A resistance measuring instrument was connected to the current collector and the nail. The nail was pierced into the negative electrode at a speed of 0.2 cm / sec. The value when the electrical resistance between the current collector and the nail became substantially constant was taken as the short-circuit resistance. The results are shown in Table 1 below.

(Battery resistance)
The SOC (State Of Charge) of the battery was adjusted to 60%. In a thermostat set at 25 ° C., the battery was discharged with a current of 5 C for 10 seconds. The amount of voltage drop after 10 seconds of discharge was measured. The battery resistance was calculated by dividing the voltage drop by the discharge current. The results are shown in Table 1 below. Here, “5C” indicates a current capable of discharging the full charge capacity of the battery in 0.2 hours.

(Cycle durability)
The battery was placed in a thermostat set at 0 ° C. One cycle of the following charging and discharging was taken as one cycle, and 500 cycles were carried out. Here, “10C” indicates a current capable of discharging the full charge capacity of the battery in 0.1 hour.
Charging: Current = 10C, end voltage = 4.1V
Discharge: Current = 10C, end voltage = 3.0V

  The capacity retention rate was calculated by dividing the discharge capacity after 500 cycles by the initial discharge capacity. The results are shown in Table 1 below.

<Result>
Since Comparative Example 1 does not include resin particles, the short circuit resistance is small. In Comparative Examples 2 and 3, the short circuit resistance is increased as compared with Comparative Example 1. However, the increase width is small. This is probably because the resin particles are dispersed in the negative electrode mixture layer. Moreover, the cycle durability of Comparative Examples 2 and 3 is lower than that of Comparative Example 1. Comparative Example 3 has a particularly large reduction in cycle durability. Since the resin particles have a small particle size, it is considered that the resin particles easily block the reaction surface of the negative electrode active material.

  In Comparative Example 4, the short circuit resistance was significantly increased. This is probably because a surface layer containing resin particles is formed on the surface of the negative electrode mixture layer. However, the cycle durability of Comparative Example 4 was lower than that of Comparative Example 1. In Comparative Example 4, resin particles and LTO particles are not combined. Since LTO particles aggregate in the particle dispersion, it is considered that a network-like Li ion diffusion path is not formed in the surface layer of the negative electrode.

  In Examples 1 to 3, the short circuit resistance was significantly increased as compared with Comparative Example 1, and the cycle durability was improved. Since the resin particles are covered with the LTO particles, it is considered that the LTO particles are in contact with each other and are continuous to form a network-like Li ion diffusion path in the surface layer of the negative electrode.

  From the results of Examples 1 and 2, it is considered that the surface layer preferably has a thickness of 1/10 or less of the thickness of the negative electrode mixture layer in consideration of battery resistance.

  The above embodiments and examples are illustrative in all respects and not restrictive. The technical scope defined by the claims includes meanings equivalent to the claims and all modifications within the scope.

  DESCRIPTION OF SYMBOLS 1 Resin particle, 2 Lithium titanate particle, 3 Composite particle, 10 Negative electrode (negative electrode for lithium ion secondary batteries), 11 Current collector, 12 Negative electrode compound material layer, 13 Surface layer.

Claims (1)

Including at least a negative electrode mixture layer and a surface layer,
The surface layer is disposed on the surface of the negative electrode mixture layer,
The surface layer includes composite particles;
The composite particles include resin particles and lithium titanate particles,
The lithium titanate particles coat the resin particles,
Negative electrode for lithium ion secondary battery.

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