JP6437348B2 - Electrode active material, secondary battery electrode, secondary battery - Google Patents

Description

  The present invention relates to an electrode active material, an electrode for a secondary battery using the same, and a secondary battery.

In recent years, secondary batteries have been used in a very wide range of fields and industries such as electric vehicles and mobile phones, and with the expansion of applications, demands for performance such as high energy density, high output density, and high-speed charge / discharge Is getting higher.
In order to satisfy these requirements, various researches and developments on various materials have been conducted, and one of them is to improve the conductivity of the electrode active material.
In general, the electrode active material alone has a low electrical conductivity, and therefore a conductive agent is used at the time of electrode preparation. This conductive agent forms a conductive path between individual electrode active material particles or the surface of the electrode active material. By increasing the current collecting effect by covering the electrode, it has the effect of increasing the conductivity in the electrode.

On the other hand, methods for improving the conductivity by atomizing the electrode active material itself or supporting a carbonaceous material on the electrode active material have been studied. For example, a gas, liquid or solid carbonaceous material is used as the electrode active material. In addition to the above, a method of coating (carbon coating) is known (Non-patent Document 1, Patent Documents 1 and 2).
However, hydrocarbons have been studied as carbonaceous materials for carbon coats, but other preferred carbonaceous materials (such as carbon black) have not yet been examined, and the structure of carbon black that is optimal for carbon coats is also not available. It remains unexamined.
Further, in the future, an electrode active material [LFP (LiFePO ₄ ), which is excellent in discharge capacity, cost, safety, etc., but has a tendency to be inferior to LCO (LiCoO ₂ ) and LNO (LiNiO ₄ ), which have been conventionally used. Since use of LVP [Li ₃ V ₂ (PO ₄ ) ₃ ] and the like] is also expected, development of a better method for improving conductivity is desired.

JP 2014-143041 A JP 2011-014368 A

Journal of Smart Process Society Vol. 1 No. 5 (2012, 9) 196-203p

  In view of the present situation as described above, the present invention provides an electrode active material whose surface is efficiently coated with a suitable carbonaceous material and has an improved current collecting effect, an electrode for a secondary battery using the same, and improved battery characteristics. The purpose is to provide a secondary battery.

As a result of intensive studies on a carbonaceous material suitable for coating the electrode active material of a secondary battery, the present inventors have found that the combined use of carbon black having a specific aggregate shape and a hydrocarbon is effective. The inventors have found that the combined use is suitable and have arrived at the present invention.
That is, the said subject is solved by the following invention 1)-5).
1) An electrode active material coated with a carbonaceous material, wherein the carbonaceous material is carbon black and hydrocarbon, and the carbon black includes a number PPA (number) of primary particles forming the aggregate; An electrode active material , wherein the ratio “PPA / d” to the primary particle diameter d (nm) is 8 or more, and the primary particle diameter d is 15 nm or less .
2) The electrode active material according to 1), wherein the hydrocarbon is polystyrene.
3) The carbon black is characterized in that the ratio “PPA / d” of the number PPA (number) of primary particles forming the aggregate and the primary particle diameter d (nm) is 12 or less 1) Or the electrode active material as described in 2).
4) An electrode for a secondary battery comprising at least an electrode active material according to any one of 1) to 3) and a material containing acetylene black as a conductive material.
5) A secondary battery, wherein the positive electrode and / or the negative electrode comprises the secondary battery electrode according to 4).

  According to the present invention, it is possible to provide an electrode active material whose surface is efficiently coated with a suitable carbonaceous material and has an improved current collecting effect, a secondary battery electrode using the same, and a secondary battery with improved battery characteristics. .

It is a schematic diagram for demonstrating the shape of carbon black used by this invention.

Hereinafter, the present invention will be described in detail.
Conventionally, the physical properties of carbon black for electrode use are specified by specific surface area and DBP absorption. Therefore, as in the present invention, the shape of the carbon black aggregate is measured from the viewpoint of the development degree of the aggregate evaluated by the particle size of the primary particles constituting the aggregate and the number of primary particles, and these characteristics are measured. Thus, as far as the present applicant knows, there is no example that defines the optimum shape of the carbon black aggregate itself for coating the electrode active material for a secondary battery.
That is, the carbon black used in the present invention is a carbon black that defines the optimum conditions for the shape of the aggregate, and cannot be achieved by using a conventional carbonaceous material alone or a carbon coat that uses a carbon black that is not specified in this application. There is an effect.

FIG. 1 shows a schematic diagram of the shape of carbon black used in the present invention.
The carbon black aggregate 2 is obtained by aggregating many primary particles 1 as shown in the figure.
The primary particle diameter d refers to the diameter (nm) of the primary particles 1 constituting the carbon black aggregate 2, and is an average particle diameter derived from CB morphological analysis by an electron microscope as defined in ASTM D3849-13 M).
Further, PPA (Particle number Per Aggregate) indicates the number of primary particles 1 contained in one carbon black aggregate 2 and is a total particle derived by CB morphological analysis by an electron microscope defined by ASTM D3849-13. It is a value obtained by dividing the number (nt in the regulation) by the number of observed carbon black aggregates 2.

In the present invention, it shall be the 15nm or less primary particle diameter d. When the primary particle diameter d is 15 nm or less, the contact area with the electrode active material and the hydrocarbon used together increases, and the surface conductive effect is improved. As for the lower limit of the primary particle diameter d, it is very difficult to produce carbon black of less than 8 nm with the current production technology, and 8 nm is the lower limit in production, but 8 nm is the lower limit as a characteristic of carbon black. Do not mean.

In the present invention, the ratio “PPA / d” of the number PPA (number) of primary particles to the primary particle diameter d (nm) is 8 or more, preferably 8-12. If it is this range, it will become a shape suitable for the carbon black aggregate to coat | cover the electrode active material of a secondary battery efficiently, and the preparation of the electrode which has a favorable conductive path is attained.
When “PPA / d” is less than 8, that is, when the primary particle diameter d is larger than the size of the carbon black aggregate, or when the number of primary particles is small and the length of the carbon black aggregate is short. In the carbon coating process, it is difficult to apply a shearing stress, and it becomes difficult to disperse the carbon black. In addition, since a strong carbon skeleton is not formed between carbon blacks, the carbon coat cannot be retained due to the volume fluctuation of the electrode active material due to rapid charge and discharge, and the conductivity improvement effect cannot be sufficiently obtained.
Further, when “PPA / d” is larger than 12, the primary particle diameter d is extremely smaller than the size of the carbon black aggregate, and the surface energy is increased. It is easy to condense and it becomes difficult to disperse.

  The carbon black can be produced using a general carbon black production furnace. Specifically, from the upstream part, a fuel introduction part, a raw material introduction part, a narrow cylindrical part, and a quench water pressure spraying device for stopping the reaction are provided. It can be produced using a carbon black production furnace in which the reaction continuation and cooling chamber is connected. Production conditions include air amount, air preheating temperature, fuel amount, raw material introduction amount, reaction stop water amount, and the like, and these may be controlled as appropriate.

The carbon black having the specific shape has excellent covering properties and is suitable as a carbonaceous material for coating an electrode active material. However, in the present invention, the entire surface is further strengthened by using a hydrocarbon as a carbonaceous material. Thus, an electrode active material that is coated with carbon and has improved conductivity and surface current collection effect can be obtained. Examples of the hydrocarbon include polystyrene, polyethylene glycol, ascorbic acid, glucose, pitch, acetylene, and methane.
And, by using the electrode for a secondary battery produced using this electrode active material for the positive electrode and / or the negative electrode, the electrode stability of the secondary battery is improved and the current collection effect of the secondary battery is increased. A secondary battery having excellent battery characteristics (such as discharge capacity and rate characteristics) can be manufactured.

The electrode active material of the present invention can be produced, for example, by the following two-stage firing method.
That is, the electrode active material LFP (LiFePO ₄ ) is temporarily fired at 320 ° C. for 5 hours in an atmosphere of argon: hydrogen = 95: 5, and then pulverized and mixed with the specific shape carbon black and hydrocarbon by a ball mill. . Next, if the main baking is performed at 700 ° C. for 10 hours in an argon atmosphere, a carbon-coated LFP can be obtained. In this LFP, particle growth is suppressed, and the electron and Li ion diffusion path in the LFP is shortened. Therefore, the discharge capacity and output characteristics (rate characteristics) of a secondary battery using the LFP can be improved.
In addition, other electrode active materials such as LVP [Li ₃ V ₂ (PO ₄ ) ₃ ], LTO (lithium titanate), LiMnPO ₄ are also subjected to temporary firing, pulverization mixing, and main firing in the same manner as described above. Can be carbon coated.

  EXAMPLES Hereinafter, although an Example and a comparative example are shown and this invention is demonstrated further more concretely, this invention is not limited by these Examples.

Table 1 shows the production conditions (operation conditions) of carbon black used in the examples and comparative examples.
In Table 1, CB1 to CB6 are for examples, and CB11 to CB12 are for comparative examples.

The DBP absorption amount, nitrogen adsorption specific surface area (N2SA), PPA, and primary particle diameter d of carbon black obtained under the production conditions shown in Table 1 were measured. The results are shown in Table 2.
For comparison, Table 2 also shows the characteristics of AB (acetylene black: Denka Black, manufactured by Denki Kagaku Kogyo Co., Ltd.), which is carbon black conventionally used for conductive materials.

The DBP absorption amount is the amount of dibutyl phthalate absorbed per 100 g of carbon black (mL / 100 g carbon black), and is a general index for evaluating the structural characteristics of carbon black aggregates. Here, JIS K6217 is used. -4: measured by the method described in 2008.
The N2SA is a specific surface area per unit weight (m ² / g), and was measured by the method described in JIS K6217-2: 2001 here.
The PPA (the number of primary particles 1 constituting the carbon black aggregate 2) and the primary particle diameter d are CB morphological analysis with a field emission scanning microscope (JSM-6700F: manufactured by JEOL Ltd.) according to ASTM D3849-13. The average particle size (m in the specification) and the total number of particles (nt in the specification) are derived, the m is the primary particle size d, and the nt is divided by the number of carbon black aggregates observed. The value obtained in this way was defined as PPA.

As shown in Table 2, CB1 to CB6 satisfy the definition of the present invention, “PPA / d” is in the range of 8 or more, and the primary particle diameter d is 15 nm or less.
On the other hand, in CB11 to CB12 and AB, “PPA / d” is less than 8 and the primary particle diameter d is larger than 15 nm.

Examples 1-6, Comparative Examples 1-2
Using each carbon black and / or polystyrene (MW = 2000 manufactured by Wako Pure Chemical Industries, Ltd.) shown in Table 1 and Table 2, each electrode active material coated with a carbonaceous material is prepared, and these electrode active materials are Each secondary battery electrode was fabricated using these electrodes, and each secondary battery was fabricated using these electrodes as positive electrodes. The production procedure is as follows.
Table 3 shows the types of carbonaceous materials used, the ratio of conductive materials, electrode active materials, and binders (% by mass). In the table, “←” means “same on the left”.

[Production of carbon-coated LFP (electrode active material)]
LFP (LiFePO ₄ ) was calcined at 320 ° C. for 5 hours in an atmosphere of argon: hydrogen = 95: 5, 5-5% by mass of each carbon black was dry-mixed with a ball mill, and then 20% by mass of polystyrene. After adding xylene as an organic solvent and wet mixing with a ball mill, xylene was evaporated. Next, this was fired at 700 ° C. for 10 hours under an argon atmosphere to obtain each carbon-coated LFP.

[Production of secondary battery electrode]
A conductive material, a binder (polyvinylidene fluoride), and a solvent (N-methylpyrrolidone) were mixed with the carbon-coated LFP at a predetermined ratio to form an electrode slurry. The slurry was applied to an aluminum foil, dried, and then pressed to prepare each secondary battery electrode.

[Production of secondary battery]
Each of the secondary battery electrodes was used as a positive electrode, lithium metal was used as a negative electrode, and each was punched into the shape of an evaluation cell.
Subsequently, each secondary battery was produced by combining each positive electrode, negative electrode, separator (polyolefin-based porous film), and electrolytic solution (manufactured by Kishida Chemical Co., Ltd., LBG-00022).
For each of these secondary batteries, charge / discharge characteristics (discharge capacity, rate characteristics) were evaluated. The results are shown in Table 4.
The charge / discharge characteristics were measured using a charge / discharge battery test system PFX2011 (manufactured by Kikusui Electronics Co., Ltd.). The measurement conditions are a temperature of 25 ° C., a cut-off voltage of 2.0 to 4.0 V, and the C rate is gradually increased to 0.5, 1, 2, 5, 10, 20, 30, 40, 50, 60. I let you. The initial capacity is a measured value at 0.5C, and the rate characteristic is a value of each rate when 0.5C is 100.

Reference examples 1-4
Reference Examples 1 and 2 are examples using only polystyrene as a carbonaceous material for carbon coating, except that the dry mixing step of carbon black in [Production of carbon coated LFP] in the example was omitted. In the same manner as in the example, a carbon-coated LFP was produced, and then a secondary battery electrode and a secondary battery were produced.
Reference Examples 3 to 4 are examples in which only CB1 or AB was used as the carbonaceous material for carbon coating, and the wet mixing step of polystyrene in [Production of carbon-coated LFP] in the example was omitted. Except for the above, a carbon-coated LFP was produced in the same manner as in the example, and then a secondary battery electrode and a secondary battery were produced.
About each said secondary battery, it carried out similarly to the Example, and evaluated charge / discharge characteristics (discharge capacity, rate characteristics). The results are shown in Table 4.

  From the results of Table 4 above, it can be seen that the secondary batteries of the examples maintain higher rate characteristics than the secondary batteries of the comparative examples, and exhibit a high charge / discharge capacity. That is, it can be seen that the secondary battery having excellent battery characteristics can be obtained by the carbon coating using the specific shape of carbon black and polystyrene according to the present invention.

1 Primary particle 2 Carbon black aggregate

Claims (5)

An electrode active material coated with a carbonaceous material, wherein the carbonaceous material is carbon black and hydrocarbon, and the carbon black is composed of a number PPA (units) of primary particles forming the aggregate, and primary particles. An electrode active material , wherein the ratio “PPA / d” to the diameter d (nm) is 8 or more, and the primary particle diameter d is 15 nm or less .   The electrode active material according to claim 1, wherein the hydrocarbon is polystyrene. The carbon black has a ratio "PPA / d" of the number PPA (number) of primary particles forming the aggregate and the primary particle diameter d (nm) of 12 or less. 2. The electrode active material according to 2.   An electrode for a secondary battery comprising at least a material containing the electrode active material according to any one of claims 1 to 3 and acetylene black as a conductive material.   A secondary battery, wherein the positive electrode and / or the negative electrode is constituted by the electrode for a secondary battery according to claim 4.

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