JP2015185229A - Electrode of lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electrode of a lithium ion secondary battery, capable of improving conductivity and capable of maintaining discharge capacity even when charge and discharge are repeated.SOLUTION: An electrode of a lithium ion secondary battery includes conductive paths 1 formed of carbon nanotubes and active material particles 2. One ends of two conductive paths 1 are connected to at least one of the active material particles 2, and other ends of the two conductive paths 1 are connected to different other active material particles 2, respectively.

Description

  The present invention relates to an electrode of a lithium ion secondary battery.

  In an electrode of a conventional lithium ion secondary battery, carbon black typified by particulate acetylene black or ketjen black is in contact with active material particles in the electrode as an aggregate. By this contact, the active material particles in the electrode are connected to each other through the carbon black to form a conductive path (see, for example, Patent Document 1).

  Moreover, in the electrode of the conventional lithium ion secondary battery, as a binder constituting the electrode, organic solvent-based polyvinylidene fluoride (PVDF) and polyimide, water-based styrene butadiene rubber (SBR), carboxymethyl cellulose (CMC), and the like Is used.

JP 2013-77479 A (see page 4 [0016])

  However, in the case of the above-mentioned Patent Document 1, there is a concern that the carbon black and the active material particles are in point contact so that there are few contact points and a good conductive path may not be formed. In particular, when carbon black and a particulate SBR binder are combined, the binder enters the contact portion between the active material particles and the carbon black, resulting in an increase in resistance between the active material particles.

  In addition, when there is no binder between the carbon black and the active material, the carbon black and the active material particles are in point contact, and are not firmly bonded. The number of contact points between the carbon black and the active material is reduced. If it does so, in the location which carbon black and an active material have not contacted, since it does not have electroconductivity, it will not react and discharge capacity will fall.

  Therefore, an object of the present invention is to provide an electrode of a lithium ion secondary battery that can improve electrical conductivity and can maintain a discharge capacity even after repeated charge and discharge.

  A first aspect of the present invention is an electrode of a lithium ion secondary battery including a conductive path formed of carbon nanotubes and active material particles, wherein at least one of the active material particles includes two conductive paths. One end is coupled, and the other ends of the two conductive paths are coupled to different active material particles.

  A second aspect of the present invention is an invention based on the first aspect. The conductive path has a thickness of 20 nm or more and a length of 2 to 5 μm.

  A third aspect of the present invention is the invention based on the first or second aspect, wherein 10% or more of the surface of the active material particle is covered with carbon nanotubes in the cross section of the electrode. And

  According to the present invention, since the active material particles are connected to each other by the conductive path, the conductivity can be improved. Moreover, a capacity | capacitance can be maintained even if charging / discharging is repeated.

3 is a SEM photograph in a cross section of the composite electrode according to Example 1. 4 is a SEM photograph in a cross section of a composite electrode according to Example 2. 4 is a SEM photograph of a cross section of a composite electrode according to Comparative Example 1. 5 is a SEM photograph of a cross section of a composite electrode according to Comparative Example 2.

  Hereinafter, embodiments of the present invention will be described in detail.

1. Embodiment (configuration)
The electrode of the lithium ion secondary battery according to the present embodiment includes a current collector and a composite electrode. Among the current collectors, an aluminum foil or the like is used for the positive electrode current collector, and a copper foil or the like is used for the negative electrode current collector.

  The composite electrode includes a conductive path and active material particles connected to each other through the conductive path. The conductive path is made of a linear member, and is formed by adding an additive such as a binder with carbon nanotubes as a conductive auxiliary agent as a main component. The conductive path has one end bonded to one active material particle and the other end bonded to another active material particle. The conductive path preferably has a thickness of 20 nm or more and a length of 2 to 5 μm. When the conductive path has a thickness and a length within the above ranges, the active material particles can be more reliably connected to each other.

  Carbon nanotubes can be synthesized by methods such as chemical vapor deposition, arc discharge, and laser evaporation. In the present embodiment, carbon nanotubes having a diameter of 1 to 900 nm can be used.

  In the present embodiment, the diameter of the carbon nanotube is preferably 5 to 50 nm. The fiber length of the carbon nanotube is preferably 0.1 to 10 μm.

  Moreover, the vapor grown carbon fiber (VGCF) of the carbon nanotubes preferably has a diameter of 100 to 200 nm and a fiber length of 0.1 to 10 μm.

  As the fiber length of the carbon nanotube is longer and the diameter is larger, a better conductive path is formed. However, when the fiber length of the carbon nanotube becomes too long, it becomes entangled in a slurry described later and tends to aggregate. When the diameter of the carbon nanotube becomes too large, it becomes difficult to bend, and there is a possibility that the number of contacts with the active material particles is reduced.

  The diameter of the carbon nanotube can be adjusted by changing the catalyst powder diameter during synthesis. The fiber length of the carbon nanotube becomes longer as the synthesis time is increased. The specific surface area of the carbon nanotube can be adjusted by the treatment time with nitric acid / sulfuric acid after synthesis. Carbon nanotubes can be synthesized, for example, by the method described in JP-A-2006-152490.

  At least a part of the active material particles contained in the composite electrode has one end of two conductive paths bonded to one active material particle. The other ends of the two conductive paths are bonded to two different active material particles. In this way, a network of active material particles connected by a conductive path is formed as a whole composite electrode.

  The active material particles are formed of a positive electrode active material or a negative electrode active material. It is preferable that 10% or more of the surface of the active material particles is covered with carbon nanotubes. Thereby, the conductive path formed of the carbon nanotubes can be easily joined to the active material particles.

  More preferably, the active material particles are entirely covered with carbon nanotubes. As a result, the resistance of the entire electrode can be lowered, and the coated portion can react uniformly, so that the structure of the electrode can be maintained.

Examples of the positive electrode active material include lithium-containing transition metal oxides such as LiCoO ₂ , LiMn ₂ O ₄ , LiNiO ₄ , LiNi _1/3 Mn _1/3 Co _1/3 O _2, and LiFePO ₄ . You may use these in combination of 2 or more type. The average particle diameter of the active material particles used for the positive electrode is preferably 0.5 to 10 μm.

Examples of the negative electrode active material include silicon (Si), silicon oxide (SiO), tin (Sn), tin-cobalt compound (Sn—Co), stannic oxide (SnO ₂ ), natural graphite, artificial graphite, and lithium titanate. (Li ₄ Ti ₅ O ₁₂ ) and the like. You may use these in combination of 2 or more type. The average particle diameter of the active material particles used for the negative electrode is preferably 1 to 20 μm, and more preferably 5 to 10 μm.

  The electrode according to this embodiment may contain a binder. Examples of the binder include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), ethylene-propylene-diene copolymer (EPDM), styrene butadiene rubber (SBR), and carboxymethyl cellulose (CMC). Carboxymethylcellulose (CMC) is a thickener but also functions as a binder. Among these, polyvinylidene fluoride (PVDF), styrene butadiene rubber (SBR), and carboxymethyl cellulose (CMC) are preferable.

(Production method)
Hereinafter, the manufacturing method of the electrode which concerns on this embodiment is demonstrated. The electrode according to the present embodiment can be produced by applying a predetermined amount of heat to a slurry containing active material particles, a binder, and carbon nanotubes coated on a current collector and drying the slurry.

  When producing a positive electrode, a positive electrode active material and a positive electrode current collector are used, and when producing a negative electrode, a negative electrode active material and a negative electrode current collector are used.

  The carbon nanotube is used as a carbon nanotube dispersion in which the carbon nanotube is dispersed in a solvent. The carbon nanotube dispersion is prepared by adding a solvent about 10 to 30 times the solid mass to carbon nanotubes, and stirring for about 3 hours with a stirrer, for example. By such preparation, a carbon nanotube dispersion liquid in which carbon nanotubes are uniformly dispersed in a solvent can be obtained. Examples of the solvent include water, N-methyl-2-pyrrolidone (NMP), and the like.

  In this embodiment, for example, a slurry is prepared by the following method. First, the active material particles, the binder, and the carbon nanotube dispersion liquid are weighed so as to have a predetermined mass ratio. When producing a positive electrode, the content rate of the positive electrode active material in a positive electrode slurry is about 80-98 mass% normally, Preferably it is 90-95 mass%. When producing a negative electrode, the content rate of the negative electrode active material in a negative electrode slurry is about 85-99 mass% normally, Preferably it is 93-98 mass%. The content ratio of carbon nanotubes in the negative electrode slurry is usually about 0.1 to 5.0% by mass, preferably 0.5 to 3.0% by mass.

  As a general compounding ratio in the case of removing the carbon nanotube which is a conductive auxiliary agent, active material: CMC: SBR = 98: 1: 1 (mass ratio) is often used, and CMC which is a thickener is 0.3. -5% by mass and SBR can be varied depending on the application within the range of 0.3-5% by mass.

  Next, a binder and a solvent are added to the carbon nanotube dispersion, and the mixture is stirred and mixed with a hybrid mixer that rotates and revolves until the solid binder is completely dissolved. Here, the solvent added to the carbon nanotube dispersion liquid is preferably the same as the solvent used for the preparation of the carbon nanotube dispersion liquid. Then, a homogeneous slurry is prepared by adding active material particles and further stirring and mixing.

  If the prepared slurry is a positive electrode, the prepared slurry is applied to an aluminum foil, and if it is a negative electrode, it is applied to a copper foil. Examples of the coating method include a doctor blade method or a die coating method.

  In the step of drying the slurry containing the active material, the binder, and the carbon nanotubes, the carbon nanotubes are dried so as to collect between the active material particles. For example, the carbon nanotubes are easily gathered between the active material particles by drying slowly by setting the heating temperature to a lower temperature and increasing the heating time. In the present embodiment, it is preferable to use a far-infrared heater from the viewpoint of reducing convection in the slurry at the time of evaporation of the solvent and enabling relatively gentle drying.

  By changing the drying conditions of the slurry applied to the current collector, the amount of carbon nanotubes that move between the active materials changes.

  In this embodiment, the slurry containing the solvent, the active material, the binder, and the carbon nanotubes applied to the current collector is heated at a temperature 10 to 50 ° C. lower than the boiling point of the solvent for 1 to 10 minutes, and then the solvent The mixture is dried by heating at a temperature 10 to 100 ° C. higher than the boiling point of 1 to 20 minutes.

  Incidentally, in the positive electrode slurry, when the binder is polyvinylidene fluoride (PVDF), N-methyl-2-pyrrolidone (NMP) is used as a solvent for dissolving PVDF. A positive electrode slurry containing LCO as an active material, carbon nanotubes as a conductive additive, and PVDF as a binder is usually dried at a drying furnace set temperature of about 250 ° C., which is higher than 202 ° C., which is the boiling point of NMP.

  On the other hand, in this embodiment, in order to dry the slurry gently, a drying furnace having a set temperature of 150 ° C. is installed before the final drying furnace having a set temperature of 250 ° C. By evaporating a large amount of NMP in this drying oven at a set temperature of 150 ° C. and drying it slowly, the time during which the slurry is in a high viscosity state can be increased.

(Method for producing negative electrode)
The case where a negative electrode slurry is prepared using graphite as the negative electrode active material and CMC and SBR as the binder will be described below. In this case, the blending ratio is graphite: SiO: CMC: SBR: carbon nanotube = 87: 10: 1: 1: 1 (mass ratio).

  First, 1 equivalent of CMC is sufficiently dissolved in water to obtain a 2% by mass aqueous solution. Thereto, 87 equivalents of graphite and 10 equivalents of SiO are charged, and mixed for 5 minutes at a rotation speed of 2000 rpm with a rotation / revolution hybrid mixer (model: ARE-310, manufactured by Shinkey Co., Ltd.). Thereafter, 1 equivalent of carbon nanotubes is added and mixed again for 5 minutes with the same mixer. Thereafter, the SBR emulsion is added so that the SBR solid content becomes 1 equivalent of the total electrode solid ratio, and again mixed with the same mixer for 5 minutes. The solid content ratio of the aqueous solution of the negative electrode slurry after adding all the constituent materials is 44%. That is, a negative electrode slurry containing 56% water is prepared.

  This negative electrode slurry was coated on a copper foil as a current collector using a comma roll coater, dried at 100 ° C. with a far infrared heater, then dried at 150 ° C. with a far infrared heater, and a negative electrode sheet Is made. This negative electrode sheet can be cut into a predetermined size to produce a negative electrode.

  Even when graphite or graphite + Sn is used as the active material, the negative electrode can be produced by the above method.

(Production method of positive electrode)
A case where a positive electrode slurry is prepared using LiCoO ₂ as a positive electrode active material and PVDF as a binder will be described below. In this case, the blending ratio is LiCoO ₂ : PVDF: carbon nanotube = 95: 3: 2 (mass ratio).

LiCoO ₂ powder, PVDF, carbon nanotubes are weighed, put into a planetary mixer, put into N-methylpyrrolidone (NMP) as a solvent, mixed in a kneaded state, and then further added with NMP, Stir until a uniform slurry is obtained. The solid content ratio of the positive electrode slurry solution after all the constituent materials are added is 75%. That is, a positive electrode slurry containing 25% NMP is prepared.

Thereafter, the positive electrode slurry is applied onto an aluminum foil, and an electrode film having a constant thickness is formed using an applicator having a gap of 50 μm. Thereafter, the sheet is transferred to a dryer, and NMP as a solvent is dried at 130 ° C. to produce a positive electrode sheet. Then, a positive electrode can be produced by cutting the positive electrode sheet with an area of 10 cm ² and compressing and crushing the electrode with a press so that the porosity is about 25%.

Even when an active material other than LiCoO ₂ is used, the positive electrode can be produced by the above method.

(Function and effect)
In the electrode according to the present embodiment, the carbon nanotubes are easily collected between the active material particles when the slurry is dried by heating the slurry applied to the current collector by applying a moderate amount of heat. By holding for a predetermined heating time, the carbon nanotubes are solidified in a state of being arranged between the active material particles, and a composite electrode is formed.

  In the case of the composite electrode formed in this way, at least one of the active material particles has one end of the two conductive paths coupled to each other, and the other end of the two conductive paths has different other active material particles. Is bound to. That is, two conductive paths extend from one active material particle, and the conductive paths are respectively coupled to different active material particles. In this way, a network of active material particles connected by a conductive path is formed as a whole composite electrode. Therefore, the electrode according to the present embodiment can improve the conductivity because the active material particles are connected to each other by the conductive path.

  In the present embodiment, when a conductive path is formed by carbon nanotubes, the thickness of the conductive path is 20 nm or more. The conductive path is preferably formed between the active material particles, but it is preferable that the conductive path having no charge / discharge capacity is as small as possible and the active material particles are densely stacked.

  When the particle diameter of the active material particles is 5 to 10 μm, the interval between the active material particles is preferably 5 μm or less. In this case, the length of the conductive path is 5 μm or less, similarly to the interval between the active material particles. The space in the electrode where the interval between the active material particles exceeds 5 μm is preferably filled with the active material particles. When the interval between the active material particles is more than 5 μm, it is possible to form a conductive path between the active material particles even in this structure. However, since many carbon nanotubes are consumed in proportion to the interval between the active material particles, The number of conductive paths and the coverage of the active material are reduced, and the deterioration of the electrode is accelerated.

  When the active material is SiO, the volume expands by 40% during charging and contracts during discharging. When the particle diameter of SiO is 10 μm, it expands and contracts by 1.2 μm in the radial direction. Therefore, if the length of the conductive path is 2 μm or more, the state where the particles are connected can be maintained even if the active material particles contract.

  In the case of the present embodiment, the conductive path has a certain length, so that the active material particles can be kept connected even if the active material expands and contracts. By covering 10% or more of the surface of the active material particles with carbon nanotubes, the conductive path formed of the carbon nanotubes can be joined to the active material particles.

  Incidentally, when the conductivity of the active material particles is low, the resistance of the electrode is increased even if a conductive path is formed between the active material particles. In addition, the reaction of the active material particles becomes non-uniform, and the stress at the time of expansion and contraction is biased, resulting in fine pulverization of the electrode.

  On the other hand, in the case of this embodiment, the active material particles are coated with carbon nanotubes, so that the resistance of the entire electrode can be lowered, and the coated portion reacts uniformly, so that the structure of the electrode is maintained. You can also.

2. Example (Sample)
A negative electrode produced by the procedure shown in the above “Method for producing negative electrode” was defined as Example 1. A negative electrode produced in the same manner as in Example 1 except that graphite: CMC: SBR: carbon nanotube = 97: 1: 1: 1 (mass ratio) was used as Example 2.

  For comparison, Example 1 except that the carbon nanotubes were changed to acetylene black and the drying conditions were dried with a far infrared heater at 70 ° C., then dried with a far infrared heater at 120 ° C., and then vacuum dried at room temperature. A negative electrode produced in the same manner as in Example 1 was designated as Comparative Example 1. Further, a negative electrode produced in the same manner as in Example 2 except that carbon nanotubes were not included, and the drying condition was dried at 100 ° C. with a far infrared heater and then dried at 200 ° C. with a far infrared heater. It was set as Comparative Example 2.

  Scanning electron microscope (SEM) photographs of the cross sections of the negative electrodes of Examples 1 and 2 and Comparative Examples 1 and 2 are shown in FIGS. In addition, a normal electrode is formed by forming a composite electrode on a current collector and then press-working to apply a force in the thickness direction. This figure is a photograph taken in a state before the press-working. is there. As shown in FIGS. 1 and 2, two conductive paths 1 and 1 extend from one active material particle 2 in different directions, and the conductive paths 1 and 1 are bonded to other different active material particles 2. I can confirm. Further, it was confirmed that the periphery of the active material particles 2 was covered with carbon nanotubes.

  On the other hand, as shown in FIG. 3, in the comparative example 1, since the conductive path 10 is less than 2 μm, it cannot be said that the active material particles 2 are bonded to each other. Moreover, as shown in FIG. 4, since the comparative example 2 does not contain a carbon nanotube, the conductive path is not formed.

Next, a laminate cell was prepared using the negative electrode, and a charge / discharge cycle test was performed. As the positive electrode, an electrode produced according to the procedure shown in the above-mentioned “Method for producing positive electrode” was used. The negative electrode and the positive electrode according to Example 1 each have a vertical and horizontal length of 25 mm × 35 mm. The negative electrode and the positive electrode were arranged with a microporous polyethylene separator in between, and inserted into an aluminum laminate pack. Thereafter, an electrolyte containing 1M LiPF _{6 in} a solvent of ethylene carbonate (EC): diethyl carbonate (DEC) = 1: 1 was injected as an electrolyte, vacuum packed, and a laminate cell was produced. A laminate cell was similarly prepared for the negative electrodes according to Example 2 and Comparative Examples 1 and 2.

Charging was performed by the CC-CV (constant current-constant voltage) method under the condition of a constant 0.2 C rate and a voltage of 0.005 V (Li ⁺ / Li). Discharge was performed by a CC (constant current) method at a constant 0.2 C rate and a cut-off voltage of 1.0 V (Li ⁺ / Li). Charging / discharging was measured every cycle, and the discharge capacity was repeated 5 cycles. The measurement temperature at this time was constant at 25 ° C. The results are shown in Table 1. A value obtained by dividing the discharge capacity after the end of each cycle by the maximum value of the discharge capacity obtained by the measurement of each sample was calculated as the discharge capacity retention rate.

  From this table, it was confirmed that in the case of the negative electrodes according to Example 1 and Example 2, the discharge capacity retention rate did not decrease even after 5 cycles. From this, even if charging / discharging is repeated, it can be said that active material particles are joined by the conductive path. On the other hand, in Comparative Example 1 and Comparative Example 2, it was confirmed that the discharge capacity retention rate decreased with an increase in the number of cycles.

3. Modifications The present invention is not limited to the above-described embodiment, and can be appropriately modified within the scope of the gist of the present invention. For example, in the above embodiment, the case where a metal foil is used as a current collector has been described, but the present invention is not limited to this. The current collector may be, for example, a foamed material of aluminum, an aluminum alloy, stainless steel, or titanium.

1 conductive path 2 active material particles

Claims (3)

An electrode of a lithium ion secondary battery comprising a conductive path formed of carbon nanotubes and active material particles,
At least one of the active material particles has one end of the two conductive paths coupled to each other,
An electrode of a lithium ion secondary battery, wherein the other ends of the two conductive paths are bonded to other different active material particles. 2. The electrode of a lithium ion secondary battery according to claim 1, wherein the conductive path has a thickness of 20 nm or more and a length of 2 to 5 μm. 3. The electrode of a lithium ion secondary battery according to claim 1, wherein 10% or more of the surface of the active material particles is covered with carbon nanotubes in a cross section of the electrode.

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