JP2018081787A - Method for manufacturing positive electrode for lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for manufacturing a positive electrode for a lithium ion secondary battery, by which the thermal stability can be increased efficiently even with a low mixing rate of an olivine type phosphate.SOLUTION: A method for manufacturing a positive electrode for a lithium ion secondary battery comprises the steps of: (A) preparing first wet granules by mixing a first positive electrode active material, a conductive material, a binder and a solvent; (B) preparing second wet granules by mixing the first wet granules and a second positive electrode active material; and (C) disposing the second wet granules on a current collector. The first positive electrode active material is a lamellar rock salt type oxide. The second positive electrode active material is an olivine type phosphate. The second wet granules are prepared so that the second positive electrode active material has a mass percentage of 5 mass% or more and 30 mass% or less to a total quantity of the first positive electrode active material and the second positive electrode active material.SELECTED DRAWING: Figure 1

Description

  The present disclosure relates to a method for manufacturing a positive electrode for a lithium ion secondary battery.

  Japanese Patent Laying-Open No. 2006-076317 (Patent Document 1) discloses a positive electrode containing two kinds of positive electrode active materials (layered rock salt type oxide and olivine type phosphate). The positive electrode is manufactured by applying a paste to the surface of the current collector.

JP, 2006-076317, A

  A layered rock salt type oxide is known as a positive electrode active material of a lithium ion secondary battery. Since the layered rock salt type oxide has a relatively high theoretical capacity, it is advantageous for increasing the capacity of the battery. However, the layered rock salt type oxide has an unstable crystal structure in a charged state. Therefore, the positive electrode containing the layered rock salt type oxide has room for improvement in the thermal stability of the charged state.

  An olivine-type phosphate is also known as a positive electrode active material of a lithium ion secondary battery. Since olivine-type phosphate has a strong crystal structure, it has excellent thermal stability in a charged state. Furthermore, olivine-type phosphate has a remarkable increase in resistance at the time of an internal short circuit, for example. Thereby, suppression of the heat_generation | fever at the time of an internal short circuit is anticipated, for example.

  Therefore, as shown in Patent Document 1, it is conceivable to use a mixture of a layered rock salt type oxide and an olivine type phosphate. However, olivine phosphate has a lower theoretical capacity than layered rock salt oxide. Therefore, it is difficult to achieve both high capacity and thermal stability.

  The objective of this indication is providing the manufacturing method of the positive electrode for lithium ion secondary batteries which can improve thermal stability efficiently, even if the mixing ratio of an olivine type phosphate is low.

  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 invention of the present disclosure should not be limited by the correctness of the mechanism of action.

The manufacturing method of the positive electrode for lithium ion secondary batteries of this indication contains the following (A)-(C).
(A) A first wet granule is prepared by mixing a first positive electrode active material, a conductive material, a binder, and a solvent.
(B) A second wet granule is prepared by mixing the first wet granule and the second positive electrode active material.
(C) A positive electrode for a lithium ion secondary battery is manufactured by disposing the second wet granule on the surface of the current collector.
The first positive electrode active material is a layered rock salt type oxide. The second positive electrode active material is olivine-type phosphate. The second wet granule is prepared such that the second positive electrode active material has a mass ratio of 5% by mass to 30% by mass with respect to the total of the first positive electrode active material and the second positive electrode active material.

Hereinafter, the “positive electrode for lithium ion secondary battery” may be abbreviated as “positive electrode”.
Conventionally, pastes are widely used as positive electrode precursors. A “paste” is a dispersion prepared by dispersing a positive electrode active material, a conductive material, and a binder in a solvent. The paste is applied to the surface of the current collector and dried to produce the positive electrode. The olivine-type phosphate has a property of being easily aggregated. In the paste containing both the layered rock salt type oxide and the olivine type phosphate, since the olivine type phosphate aggregates in the solvent, a positive electrode in which the olivine type phosphate is unevenly distributed is manufactured. When olivine-type phosphate is unevenly distributed, it is considered that the excellent thermal stability of olivine-type phosphate is difficult to be reflected in the whole positive electrode. Furthermore, the increase in resistance of olivine-type phosphate during an internal short circuit is considered to be difficult to be reflected in the entire positive electrode. For this reason, it is considered that a large amount of olivine-type phosphate needs to be added in order to obtain the desired thermal stability.

  In the manufacturing method of the present disclosure, wet granules are used as the precursor of the positive electrode. As described above, in the paste, powder particles (positive electrode active material and the like) are dispersed in the solvent. On the other hand, in wet granules, it may be considered that the solvent is dispersed in the powder.

  The first wet granule is prepared by wet granulating a first positive electrode active material (layered rock salt type oxide), a conductive material and a binder. The first wet granule is an aggregate of composite particles. Each composite particle included in the first wet granule includes a first positive electrode active material, a conductive material, a binder, and a solvent. Next, the first wet granules and the second positive electrode active material (olivine phosphate) are stirred to prepare second wet granules. At this time, the second positive electrode active material is considered to adhere to the surface of each composite particle. That is, it is considered that the second positive electrode active material is disposed on the surface of the composite particle. Finally, the second wet granule is disposed on the surface of the current collector to manufacture the positive electrode.

  In the positive electrode manufactured in this way, it is considered that the second positive electrode active material is distributed evenly without being unevenly distributed. Therefore, it is considered that the excellent thermal stability of the second positive electrode active material (olivine phosphate) is easily reflected in the entire positive electrode. Furthermore, it is considered that this positive electrode includes a structure in which the first positive electrode active material gathered (parts derived from the composite particles) is partitioned by the second positive electrode active material. Thereby, at the time of an internal short circuit, it is thought that the resistance increase of olivine type phosphate is reflected in the whole positive electrode efficiently, and heat generation is suppressed.

  From the above, the production method of the present disclosure can efficiently improve the thermal stability even when the mixing ratio of the olivine-type phosphate is low (that is, 5% by mass or more and 30% by mass or less). Conceivable.

FIG. 1 is a flowchart illustrating an outline of a method for manufacturing a positive electrode for a lithium ion secondary battery according to an embodiment of the present disclosure. FIG. 2 is a conceptual diagram showing the first wet granule. FIG. 3 is a conceptual diagram showing the second wet granule. FIG. 4 is a schematic diagram illustrating an example of a coating apparatus. FIG. 5 is a conceptual cross-sectional view illustrating a configuration of a positive electrode for a lithium ion secondary battery according to an embodiment of the present disclosure.

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

<Method for producing positive electrode for lithium ion secondary battery>
FIG. 1 is a flowchart showing an outline of a method for producing a positive electrode for a lithium ion secondary battery according to the present embodiment. The manufacturing method of this embodiment includes (A) preparation of the first wet granule, (B) preparation of the second wet granule, and (C) manufacture of the positive electrode.

<< (A) Preparation of first wet granule >>
The manufacturing method of the present embodiment includes (A) preparing a first wet granule by mixing a first positive electrode active material, a conductive material, a binder, and a solvent.

  The first wet granule is prepared by wet granulation. The granulation operation can be performed by a general stirring and mixing apparatus. That is, the first wet granule can be prepared by stirring and mixing the first positive electrode active material, the conductive material, the binder and the solvent in the stirring tank of the stirring and mixing device. The first wet granule may be prepared to have a solid content of 72 to 85% by mass, for example. A solid content rate shows the mass ratio of components other than a solvent. FIG. 2 is a conceptual diagram showing the first wet granule. The first wet granule 10 is an aggregate of composite particles 5. Each composite particle 5 includes a first positive electrode active material 1, a conductive material 3, and a solvent (not shown).

(First positive electrode active material)
The first positive electrode active material is a layered rock salt type oxide. The layered rock salt type oxide of the present embodiment is a lithium transition metal composite oxide having a layered rock salt type crystal structure.
The layered rock salt type oxide has the following formula (I):
LiMaO ₂ (I)
(In the formula (I), Ma is at least one selected from the group consisting of Ni, Co, Mn and Al)
Represented by

Specific examples of the layered rock salt type oxide include, for example, LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , LiNi _0.5 Mn _0.5 O ₂ , LiNi _0.5 Co _0.3 Mn _0.2 O ₂ , LiNi _0.6 Co _0.2 Mn _0.2 O ₂ , LiNi _0.4 Co _0.2 Mn _0.4 O ₂ , LiNi _0.4 Co _0.4 Mn _0.2 O ₂ , LiNi _1/3 Co _1/3 Mn _1/3 O ₂ , LiNi _0.8 Co _0.15 Al _0.05 O ₂ , LiNi _0.7 Co _0.2 Al _0.1 O ₂ , LiNi _0.6 Co _0.2 Al _0.2 O _{2 and the} like can be mentioned.

  From the viewpoint of increasing the capacity, Ma in the above formula (I) preferably contains Ni. For example, Ni may occupy 0.3 mol or more of 1 mol of Ma, may occupy 0.5 mol or more, or may occupy 0.7 mol or more. In the above formula (I), 1 mol of Ma may contain, for example, less than 0.05 mol of elements other than Ni, Co, Mn and Al. Examples of elements other than Ni, Co, Mn, and Al include F, S, Si, Ti, Cr, V, P, Zn, Ga, Ge, Zr, Nb, Mo, Hf, and W. The first positive electrode active material may contain one kind of layered rock salt type oxide alone or may contain two or more kinds of layered rock salt type oxides.

  The first positive electrode active material may have an average particle diameter of 1 to 20 μm, for example. The average particle diameter is assumed to indicate a 50% cumulative particle diameter from the fine particle side in the volume-based particle size distribution measured by the laser diffraction scattering method.

  The conductive material should not be particularly limited. The conductive material includes, for example, a conductive carbon material. Examples of the carbon material include acetylene black (AB), thermal black, furnace black, graphite, vapor grown carbon fiber (VGCF), and carbon nanotube (CNT). The conductive material may include one type of carbon material alone, or may include two or more types of carbon materials. For example, the conductive material may be 1 to 15 parts by mass with respect to 100 parts by mass of the positive electrode active material (the total of the first positive electrode active material and the second positive electrode active material).

  The binder should not be particularly limited. The binder includes, for example, a binding polymer compound. Examples of the polymer compound include polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), carboxymethyl cellulose (CMC), polyacrylic acid (PAA), and the like. The binder may contain one kind of polymer compound alone, or may contain two or more kinds of polymer compounds. For example, the binder may be 1 to 5 parts by mass with respect to 100 parts by mass of the positive electrode active material.

  The solvent should not be particularly limited. However, a solvent that is easily compatible with the binder is desirable. For example, when PVdF is used as the binder, N-methyl-2-pyrrolidone (NMP) can be used as the solvent.

<< (B) Preparation of second wet granule >>
The manufacturing method of the present embodiment includes (B) preparing the second wet granule by mixing the first wet granule and the second positive electrode active material.

  The second wet granule can also be prepared by a stirring and mixing device. That is, the second wet granule can be prepared by stirring and mixing the first wet granule and the second positive electrode active material in the stirring tank of the stirring and mixing device. At this time, the solid content may be adjusted by adding a solvent. The second wet granule may be prepared to have a solid content of 72 to 82% by mass, for example. FIG. 3 is a conceptual diagram showing the second wet granule. In the second wet granule 20, the second positive electrode active material 2 is disposed on the surface of the composite particle 5.

(Second positive electrode active material)
The second positive electrode active material is olivine type phosphate. The olivine-type phosphate of this embodiment is a lithium transition metal composite phosphate having an olivine-type crystal structure.
The olivine type phosphate has the following formula (II):
LiMbPO ₄ (II)
(In the formula (II), Mb is at least one selected from the group consisting of Fe, Ni, Co and Mn)
Represented by

Specific examples of the olivine type phosphate include, for example, LiFePO ₄ , LiMnPO ₄ , LiMn _0.5 Fe _0.5 PO ₄ , LiMn _0.8 Fe _0.2 PO ₄ , LiMn _0.2 Fe _0.8 PO ₄ , LiCoPO ₄ , LiNiPO _{4 and the} like.

  From the viewpoint of thermal stability, in the above formula (II), Mb preferably contains Fe and Mn. For example, Fe may occupy 0.3 mol or more of 1 mol of Mb, may occupy 0.5 mol or more, or may occupy 0.8 mol or more. In the above formula (II), 1 mol of Mb may contain less than 0.05 mol of elements other than Fe, Ni, Co and Mn. Examples of elements other than Fe, Ni, Co, and Mn include F, S, Al, Si, Ti, Cr, V, P, Zn, Ga, Ge, Zr, Nb, Mo, Hf, and W. . The olivine-type phosphate may include a carbon film on the surface thereof. The second positive electrode active material may contain one kind of olivine-type phosphate alone, or may contain two or more kinds of olivine-type phosphate. For example, the second positive electrode active material may have an average particle diameter of 1 to 20 μm.

  In the present embodiment, the second wet granule is prepared so that the second positive electrode active material has a mass ratio of 5% by mass to 30% by mass with respect to the total of the first positive electrode active material and the second positive electrode active material. Is done. If the mass ratio of the second positive electrode active material is less than 5 mass%, the desired thermal stability may not be obtained. When the mass ratio of the second positive electrode active material exceeds 30 mass%, the capacity reduction is large and undesirable. The mass ratio of the second positive electrode active material is preferably 15% by mass or less, and more preferably 10% by mass or less. When the mass ratio of the second positive electrode active material is within these ranges, a higher capacity positive electrode for a lithium ion secondary battery can be provided.

<< (C) Production of positive electrode >>
The manufacturing method of this embodiment includes manufacturing the positive electrode for lithium ion secondary batteries by arrange | positioning (C) 2nd wet granule on the surface of an electrical power collector.

  FIG. 4 is a schematic diagram illustrating an example of a coating apparatus. The coating device 200 is composed of three rotating rolls. That is, the coating apparatus 200 includes a first rotating roll 201, a second rotating roll 202, and a third rotating roll 203. Each rotating roll is connected to a driving device (not shown). Each rotating roll is driven to rotate in the direction of a curved arrow drawn on each rotating roll. For example, the second rotating roll 202 may have a faster peripheral speed than the first rotating roll 201. For example, the third rotating roll 203 may have a faster peripheral speed than the second rotating roll 202.

  The second wet granule 20 is supplied to the roll gap between the first rotary roll 201 and the second rotary roll 202. In the roll gap, the second wet granule 20 is formed into a sheet shape. As a result, the sheet 102 is obtained. The second rotating roll 202 supplies the sheet 102 to the roll gap between the second rotating roll 202 and the third rotating roll 203. The third rotating roll 203 supplies the current collector 101 to the roll gap between the second rotating roll 202 and the third rotating roll 203. In the roll gap, the sheet 102 is pressed on the surface of the current collector 101, whereby the sheet 102 is disposed on the surface of the current collector 101. That is, the second wet granule 20 is disposed on the surface of the current collector 101. As described above, the positive electrode 100 is manufactured.

  Thereafter, the positive electrode 100 may be dried. The sheet 102 can be disposed on both sides of the current collector 101. The positive electrode 100 is processed into a predetermined dimension in accordance with the specifications of the lithium ion secondary battery. Processing here includes rolling and cutting.

《Positive electrode for lithium ion secondary battery》
FIG. 5 is a conceptual cross-sectional view showing the configuration of the positive electrode for a lithium ion secondary battery according to this embodiment. The positive electrode 100 includes a current collector 101 and a sheet 102. The current collector 101 may have a thickness of 1 to 30 μm, for example. The current collector 101 may be, for example, an aluminum (Al) foil. The Al foil may be a pure Al foil or an Al alloy foil.

  The sheet 102 is disposed on the surface of the current collector 101. The sheet 102 may have a thickness of 10 to 150 μm, for example. The sheet 102 corresponds to a so-called positive electrode mixture layer. The sheet 102 includes a first positive electrode active material 1, a second positive electrode active material 2, a conductive material 3, and a binder (not shown). The second positive electrode active material 2 has a mass ratio of 5% by mass to 30% by mass with respect to the total of the first positive electrode active material 1 and the second positive electrode active material 2.

  The second positive electrode active material 2 is distributed substantially uniformly throughout the sheet 102. Therefore, even if the mixing ratio of the second positive electrode active material 2 (olivine phosphate) is low, it is considered that the excellent thermal stability of the second positive electrode active material 2 is easily reflected in the entire sheet 102. Further, the sheet 102 includes compartments derived from the composite particles 5 (see FIGS. 1 and 2). The first positive electrode active material 1 is gathered in the compartment. The second positive electrode active material 2 is distributed so as to partition the compartments. Thereby, at the time of an internal short circuit, it is thought that the resistance rise of the 2nd positive electrode active material 2 is reflected in the whole positive electrode efficiently, and heat_generation | fever is suppressed.

  The positive electrode 100 can achieve both high capacity and thermal stability. The lithium ion secondary battery including this positive electrode is suitable for a power source of a plug-in hybrid vehicle (PHV), a hybrid vehicle (HV), an electric vehicle (EV), and the like. However, the use of the lithium ion secondary battery including the positive electrode according to the present embodiment should not be limited to such in-vehicle use. The lithium ion secondary battery including the positive electrode according to the present embodiment can be applied to any application.

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

<Example 1>
The following materials were prepared:
The first positive electrode active _{material: LiNi x Co y Al z O} 2 (x> 0, y> 0, z> 0, x + y + z = 1)
Second positive electrode active material: LiFePO ₄
Conductive material: AB
Binder: PVdF
Solvent: NMP
Current collector: Al foil

<< (A) Preparation of first wet granule >>
The first wet granule was prepared by mixing 95 parts by mass of the first positive electrode active material, 4.3 parts by mass of the conductive material, 3.2 parts by mass of the binder, and a predetermined amount of solvent.

<< (B) Preparation of second wet granule >>
The second wet granule was prepared by mixing the total amount of the first wet granule obtained above and 5 parts by mass of the second positive electrode active material. That is, the second wet granule was prepared so that the second positive electrode active material had a mass ratio of 5 mass% with respect to the total of the first positive electrode active material and the second positive electrode active material. The solid content of the second wet granule was 78% by mass. The composition of the solid content of the second wet granule is “positive electrode active material: conductive material: binder = 93: 4: 3” by mass ratio. The positive electrode active material here indicates the total of the first positive electrode active material and the second positive electrode active material.

<< (C) Production of positive electrode >>
The second wet granule was placed on the surface of the current collector and dried by the coating apparatus shown in FIG. The positive electrode was manufactured from the above.

<Examples 2 to 4, Comparative Example 1>
As shown in Table 1 below, the positive electrode was manufactured by the same manufacturing method as in Example 1 except that the mass ratio of the second positive electrode active material to the total of the first positive electrode active material and the second positive electrode active material was changed. Was manufactured. Examples 1 to 4 and Comparative Example 1 correspond to examples in which wet granules were prepared by “two-stage granulation”.

<Comparative example 2>
A positive electrode paste was prepared by mixing 100 parts by mass of the first positive electrode active material, 4.3 parts by mass of a conductive material, 3.2 parts by mass of a binder, and a predetermined amount of solvent. The solid content of the positive electrode paste was 70% by mass. The composition of the solid content of the positive electrode paste is “first positive electrode active material: conductive material: binder = 93: 4: 3” by mass ratio. The positive electrode paste was applied to the surface of the current collector by a die coater and dried. This produced a positive electrode.

<Comparative Example 3>
95 parts by mass of a first positive electrode active material, 5 parts by mass of a second positive electrode active material, 4.3 parts by mass of a conductive material, 3.2 parts by mass of a binder, and a predetermined amount of a solvent are mixed to obtain a positive electrode paste Was prepared. Except for this, a positive electrode was produced by the same production method as in Comparative Example 2.

<Comparative Examples 4-6>
As shown in Table 1 below, the positive electrode is manufactured by the same manufacturing method as in Comparative Example 3 except that the mass ratio of the second positive electrode active material to the total of the first positive electrode active material and the second positive electrode active material is changed. Was manufactured. Comparative Examples 3 to 6 correspond to examples in which the first positive electrode active material and the second positive electrode active material were mixed together in the paste preparation.

<Evaluation>
A lithium ion secondary battery including the positive electrode manufactured in each example was manufactured. Battery capacity was measured. The results are shown in the “Capacity” column of Table 1 below. The numerical value shown in the column “capacity” is a relative value when the battery capacity of Comparative Example 2 is 100. The internal resistance of the battery was measured. The results are shown in the “resistance” column of Table 1 below. The maximum temperature reached on the battery surface in the nail penetration test was measured. The results are shown in the “Achieved temperature” column of Table 1 below. The lower the temperature reached, the higher the thermal stability.

<Result>
Comparative Example 2 has a high ultimate temperature in the nail penetration test. This is probably because the positive electrode does not contain the second positive electrode active material (olivine phosphate) and contains only the first positive electrode active material (layered rock salt type oxide).

  In Examples 1 to 4 (wet granules), the effect of suppressing the temperature rise (the effect of improving the thermal stability) is remarkable as compared with Comparative Examples 3 to 6 (paste). In the positive electrode of the example, the second positive electrode active material is disposed so that the thermal stability of the second positive electrode active material is easily reflected in the entire positive electrode and the increase in resistance of the second positive electrode active material is easily reflected in the entire positive electrode. It is thought that. Therefore, according to the structure of an Example, the thermal stability of a battery can be improved, keeping the mixing ratio of a 2nd positive electrode active material low. That is, it is possible to achieve both high capacity and thermal stability.

  From the results of Comparative Example 1 and Example 1, it is considered that the mass ratio of the second positive electrode active material needs to be 5% by mass or more in order to obtain the temperature rise suppressing effect. In the present example, the effect of improving the thermal stability was recognized when the mass ratio of the second positive electrode active material was up to 30 mass%. When the mass ratio of the second positive electrode active material exceeds 30% by mass, the capacity reduction is increased, so that it is expected that it is difficult to increase the battery capacity. Therefore, the mass ratio of the second positive electrode active material needs to be 5 mass% or more and 30 mass% or less.

  The embodiments and examples disclosed herein are illustrative in all aspects and should not be construed as being restrictive. The scope of the present disclosure is defined by the terms of the claims, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

  DESCRIPTION OF SYMBOLS 1 1st positive electrode active material, 2nd positive electrode active material, 3 electrically conductive material, 5 composite particle, 10 1st wet granule, 20 2nd wet granule, 100 positive electrode, 101 Current collector, 102 sheet | seat, 200 coating apparatus, 201 First rotating roll, 202 Second rotating roll, 203 Third rotating roll.

Claims (1)

Preparing a first wet granule by mixing a first positive electrode active material, a conductive material, a binder and a solvent;
The second wet granule is prepared by mixing the first wet granule and the second positive electrode active material, and the second wet granule is disposed on the surface of the current collector, whereby the lithium ion secondary battery is used. Producing a positive electrode,
Including
The first positive electrode active material is a layered rock salt type oxide,
The second positive electrode active material is olivine-type phosphate,
The second wet granule is prepared such that the second positive electrode active material has a mass ratio of 5% by mass to 30% by mass with respect to the total of the first positive electrode active material and the second positive electrode active material. The
A method for producing a positive electrode for a lithium ion secondary battery.

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