WO2012114502A1

WO2012114502A1 - Positive electrode for lithium-ion rechargeable batteries, lithium-ion rechargeable battery, and battery module

Info

Publication number: WO2012114502A1
Application number: PCT/JP2011/054262
Authority: WO
Inventors: 豊隆湯浅; 小林　満; 宰小川
Original assignee: 日立ビークルエナジー株式会社
Priority date: 2011-02-25
Filing date: 2011-02-25
Publication date: 2012-08-30
Also published as: JPWO2012114502A1

Abstract

The purpose of the present invention is to provide: a positive electrode for lithium-ion rechargeable batteries which is capable of improving capacity per electrode volume; and a lithium-ion rechargeable battery that uses the positive electrode. This positive electrode for lithium-ion rechargeable batteries is characterized in that a positive electrode active material (3), a mixed layer containing a conductive material and a binder, and a current collector (1), upon the surface of which the mixed layer is formed, are provided; the positive electrode active material (3) is a complex oxide having an olivine structure represented by the chemical formula Li_aM_xPO₄ (where M is a transition metal containing Fe and/or Mn, 0 < a ≤ 1.1, and 0.9 ≤ x ≤ 1.1); and the conductive material contains fibrous carbon (4), pits (2) are formed on the surface of the current collector (1); and part of the positive electrode active material (3) and part of the fibrous carbon (4) are placed in the pits (2).

Description

Positive electrode for lithium ion secondary battery, lithium ion secondary battery, and battery module

The present invention relates to a positive electrode for a lithium ion secondary battery using a non-aqueous electrolyte, and more particularly, to improvement of the positive electrode structure.

Development of a plug-in hybrid vehicle (hereinafter referred to as "PHEV") is required to further improve the energy efficiency of the vehicle. PHEV uses energy charged by a household power source for traveling, and travels a certain distance only with a motor. Therefore, as the performance of the battery used for PHEV, high output and high battery in a short time necessary for hybrid vehicles Capacity and is required.

As described above, in the battery characteristics required for PHEVs, it is important to increase the output as well as to increase the capacity. For this reason, since the lithium ion secondary battery for PHEV becomes a large sized large capacity battery, ensuring safety is important. In the large sized large capacity lithium ion secondary battery for vehicles, improvement of volume energy density and weight energy density is called for for size reduction and weight reduction of a battery. In addition, in a large-sized large-capacity lithium ion secondary battery, since the energy to be stored is large, a positive electrode active material having high thermal stability and high safety is required.

An olivine-type positive electrode active material (LiMPO ₄ , M is a transition metal containing at least one of Fe and Mn as a positive electrode material of a lithium ion secondary battery for PHEV and is composed of Fe or Mn as a transition metal. Is called attention). In the olivine positive electrode material, the bond between oxygen and phosphorus in the crystal structure is strong, and oxygen is hardly released from the crystal structure at the time of overcharge, so the safety is high. However, it is reported that the olivine positive electrode material has low electron conductivity and a low lithium ion diffusion coefficient into the positive electrode material.

With regard to olivine positive electrode materials, as a means for solving the above problems for practical use, the diffusibility of lithium ions is improved by making the material a high specific surface area, and conductivity is achieved by coating with carbon (carbon coating) It gives the sex. The carbon coating can impart conductivity, can suppress crystal growth, and can contribute to high specific surface area by reducing the primary particle size to a submicron size.

The above olivine positive electrode material has the following problems in the point of volume energy density improvement. For example, since the true density of olivine Fe is 3.6 g / cc (g / cm ³ ), it has the same volume energy density as a positive electrode material using a layered LiNiMnCoO ₂ system having a true density of 5.1 g / cc. In order to obtain, olivine positive electrode material becomes bulky. For this reason, olivine positive electrode material is a material in which high volume density formation is difficult. In addition, in the carbon-coated olivine positive electrode material, the density is further reduced because the total amount of active material is reduced. Further, as described above, since the olivine positive electrode material has a high specific surface area, the amount of binder per surface area required at the time of electrode formation is increased. However, in order to secure the battery capacity, it is desirable to reduce the amount of binder in the electrode composition and to increase the mass of the active material.

Generally, in high-capacity batteries, it is necessary to improve the content of the positive electrode active material in the positive electrode and to improve the thickness of the mixture layer composed of the positive electrode active material, the conductive material, and the binder to improve volume energy density. is there. In the preparation of an olivine positive electrode, a slurry composed of an olivine positive electrode material dispersed in a solvent, a conductive material and a binder is applied on an aluminum current collector, and then dried to obtain a positive electrode. When the mixture layer is a thick electrode, in this drying step, the phenomenon that the binder resin moves to the surface layer as the solvent evaporates becomes remarkable. As a result, the amount of binder decreases at the interface between the aluminum current collector and the mixture layer. When the amount of binder is reduced, the mixture layer peels off from this interface by consolidation by electrode press or roll rolling. As described above, in the olivine positive electrode, consolidation of the electrode is essential to improve volumetric energy density, but it is necessary to suppress peeling of the mixture layer from the interface between the aluminum current collector and the mixture layer. is there.

In Patent Document 1, a method of improving the volume energy density and load characteristics by increasing the application capacity of the active material using a current collector having a surface roughness Ra of 0.1 μm or more with respect to high volumetric energy density of the olivine positive electrode Is disclosed. In particular, with regard to the improvement of load characteristics, it is disclosed that the discharge capacity retention rate at high discharge current is improved in the surface-roughened aluminum current collector. The composition of the electrode described in Patent Document 1 is an olivine positive electrode active material, a conductive carbon powder used as a conductive material, and a binder.

JP 2004-335344 A

It is considered that peeling of the mixture layer can be suppressed to a certain extent by using an aluminum current collector having a surface roughened as in the technique described in Patent Document 1. However, peeling can not be sufficiently suppressed by simply using a surface-roughened aluminum current collector. Therefore, it is necessary to increase the amount of binder in order to suppress peeling, and there is a problem that the volume energy density of the electrode (which is an amount representing the capacity per volume of the electrode) decreases.

An object of the present invention is to provide a positive electrode for a lithium ion secondary battery capable of improving the capacity per volume of the electrode, and a lithium ion secondary battery using the same.

The positive electrode for a lithium ion secondary battery according to the present invention has the following features. A positive electrode active material, a mixture layer containing a conductive material and a binder, and a current collector having the mixture layer formed on the surface, the positive electrode active material has a chemical formula Li _a M _x PO ₄ (where M is Fe and A transition metal containing at least one of Mn.A positive electrode for a lithium ion secondary battery, which is a composite oxide having an olivine structure represented by 0 <a ≦ 1.1, 0.9 ≦ x ≦ 1.1), The conductive material contains fibrous carbon, and a pit is formed on the surface of the current collector, and a part of the positive electrode active material and a part of the fibrous carbon are in the pit.

ADVANTAGE OF THE INVENTION According to this invention, the lithium ion secondary battery positive electrode which can improve the capacity | capacitance per volume of an electrode, and a lithium ion secondary battery using the same can be provided.

It is a figure which shows surface roughness Ra of an aluminum base material collector, and the relationship of electrode density. FIG. 2 is a cutaway cross-sectional view schematically showing a cylindrical lithium ion secondary battery. It is sectional drawing of the positive electrode for lithium ion secondary batteries.

The inventors of the present invention conducted intensive studies to solve the above-mentioned problems, and as a result, the positive electrode active material constituting the positive electrode mixture, the member and composition of the conductive material and the binder, and also the base material of the mixture. By examining the surface roughness of the aluminum base current collector used, the content of the positive electrode active material in the positive electrode is improved, and the density of the positive electrode is improved, and the energy density per unit volume (volume energy density) ) Found that improved. The capacity per volume of the electrode is represented by the electrode volume energy density.

The positive electrode for a lithium ion secondary battery according to the present invention comprises a mixture layer containing a positive electrode active material, a conductive material, and a binder, and a current collector, and the positive electrode active material has the chemical formula Li _a M _x PO ₄ (M is A transition metal containing at least one of Fe and Mn, which is a composite oxide having an olivine structure represented by 0 <a ≦ 1.1, 0.9 ≦ x ≦ 1.1).

The positive electrode active material (olivine positive electrode material) has a specific surface area of 10 m ² / g or more and 30 m ² / g or less (10 to 30 m ² / g), and an average primary particle diameter of 0.05 μm or more and 0.3 μm The average secondary particle diameter is in the range of 0.2 to 1 μm (0.2 to 1 μm). In the present specification, the mean primary particle size and the mean secondary particle size are also referred to simply as the primary particle size and the secondary particle size, respectively. The conductive material is a mixture of carbon black and fibrous carbon. The current collector is made of an aluminum base having a defined surface roughness.

Furthermore, in the positive electrode for a lithium ion secondary battery according to the present invention, the content of the positive electrode active material in the mixture layer is preferably 90% or more and 93% or less by weight, but is limited to this range Absent. The weight percentage of fibrous carbon in the conductive material is preferably 20% or more and less than 60%, but is not limited to this range. The electrode density is preferably 2.0 g / cc (g / cm ³ ) or more and 2.3 g / cc (g / cm ³ ) or less, but is not limited to this range.

In order to increase the capacity of the battery, it is necessary to increase the thickness of the positive electrode, increase the content of the positive electrode active material in the positive electrode, and increase the density of the positive electrode. In order to achieve this positive electrode specification using an olivine positive electrode material composed of fine primary particles, a positive electrode configuration having high binding properties is required. Although it is conceivable to improve the binding property by examining the binder, in the present invention, attention was paid to the binding property of the interface between the current collector and the mixture layer as described above. In general, attempts have been made to roughen the surface of the aluminum current collector to improve the binding property of the interface between the current collector and the mixture layer. In the present invention, the improvement of the binding property of the olivine positive electrode material composed of fine primary particles was studied from the following viewpoints. That is, pits are formed on the surface of the aluminum base current collector, and the relationship between the pit diameter and the average secondary particle diameter of the olivine positive electrode material, and further, the effect of fibrous carbon used as a conductive material dispersed in the positive electrode is there.

The "pit" is a hole formed on the surface of the aluminum base current collector, and the shape of the opening and the shape in the depth direction are arbitrary. The "pit diameter" is the maximum length (maximum width) of the opening at the pit opening. In the present specification, the average pit diameter, which is the average of the pit diameters of the respective pits, is also simply referred to as the pit diameter.

Since a part of the positive electrode active material (olivine positive electrode material) and a part of fibrous carbon enter the pits, it is possible to increase the binding property of the interface between the current collector and the mixture layer by the anchor effect. The olivine positive electrode material that enters the pits may be either primary particles or secondary particles. However, the relationship between the pit diameter and the particle diameter of the olivine positive electrode material is determined by secondary particles having a large particle diameter.

The relationship between the pit diameter and the average secondary particle diameter of the olivine positive electrode material is shown below. Generally, in a current collector using an aluminum base, pits having a diameter of several μm and a depth of several μm can be formed on the surface of the base by surface treatment using an acid or an alkali. Secondary particles of the olivine positive electrode material intrude into the pits to generate an anchor effect, and the binding property of the interface between the current collector and the mixture layer is increased. Here, the binding ability differs depending on the relative relationship between the pit diameter and the secondary particle diameter. For example, if the pit diameter and the secondary particle diameter are substantially the same, it is difficult for the secondary particles of the olivine positive electrode material to enter the pit. On the other hand, if the secondary particle diameter of the olivine positive electrode material is too small relative to the pit diameter, the anchor effect is reduced.

For this reason, in the present invention, the secondary particle diameter of the olivine positive electrode material suitable for the pit diameter on the surface of the current collector is specified as follows. That is, the average secondary particle diameter of the olivine positive electrode material is 0.2 μm or more and 1 μm or less, and the ratio of the average secondary particle diameter to the average pit diameter is 0.1 or more and 0.5 or less. It is assumed. According to this definition, the binding property of the interface between the current collector and the mixture layer can be increased and the density of the positive electrode can be increased by using an appropriate amount of olivine positive electrode material in the pit.

Next, in order to explain the effect of fibrous carbon dispersed in the positive electrode, the conductive material used for the positive electrode will be described. In the positive electrode, a conductive material is dispersed in the positive electrode in order to secure electron conductivity. As the conductive material, fine granular acetylene black and fibrous carbon can be mentioned.

The effect of fibrous carbon dispersed in the positive electrode is shown below using FIG. FIG. 3 is a cross-sectional view of a positive electrode for a lithium ion secondary battery according to the present invention, showing pits 2 formed on the surface of an aluminum base current collector 1, secondary particles 3 of an olivine positive electrode material and fibrous carbon. 4 is shown. In FIG. 3, only secondary particles are shown as a representative among particles (primary particles and secondary particles) of the olivine positive electrode material. The same explanation applies to primary particles as to secondary particles.

The secondary particles 3 of the olivine positive electrode material can enter into the pits 2. Here, if fibrous carbon 4 used as the conductive material is dispersed in the mixture slurry, fibrous carbon 4 also enters pit 2 and fibrous carbon 4 is distributed in the thickness direction of the mixture layer. Thus, it is possible to improve the binding property of the combination. However, when the amount of fibrous carbon 4 contained in the conductive material is too large, the fibrous carbon 4 and the olivine positive electrode material form an aggregate and can not enter the pit 2. When the amount of fibrous carbon 4 is small, the anchoring effect of the fibrous carbon 4 is reduced. For this reason, it is necessary to define the content of fibrous carbon 4 contained in all the conductive materials. The proportion of fibrous carbon 4 in the entire conductive material was defined as 20% or more and less than 60% by weight.

Specific fibrous carbons used herein include vapor grown carbon fibers, carbon nanotubes (CNTs) and carbon nanofibers (CNF). Although fibrous carbon has excellent properties, it is difficult to disperse in a mixture slurry and may form aggregates in the slurry. Since the aggregates in the slurry inhibit the mixture layer from becoming constant in the electrode coating process, a mixture composition in which no aggregates are formed is desirable.

Next, the effects of acetylene black are shown below. Acetylene black is fine particulate particles having a particle diameter of several tens of nm, and is excellent in dispersibility in a slurry. For this reason, it is effective in order to ensure the electronic conductivity in a positive electrode, suppressing formation of an aggregate.

In consideration of such characteristics of fibrous carbon and acetylene black, it was specified that the ratio of fibrous carbon in the entire conductive material was 20% or more and less than 60% by weight. Here, if the addition amount of fibrous carbon is less than 20%, the above effect is small, and if it is 60% or more, a large amount of agglomerates in the slurry makes it difficult to form a positive electrode.

With the above electrode configuration, the content of the positive electrode active material (olivine positive electrode material) occupied in the mixture layer is 90 to 93% by weight percentage, and even in a high density positive electrode having an electrode density of 2.0 to 2.3 g / cc, A positive electrode excellent in high volume energy density and high rate discharge can be obtained.

As described above, the present invention aims to obtain a high-safety large-capacity large-capacity lithium ion secondary battery, and defines the positive electrode configuration of the olivine positive electrode material.

The positive electrode for a lithium ion secondary battery, the lithium ion secondary battery, and the battery module according to the present invention have the following features.
(1) A mixture layer containing a positive electrode active material, a conductive material and a binder, and a current collector having the mixture layer formed on the surface, the positive electrode active material has a chemical formula Li _a M _x PO ₄ (M is Fe A transition metal containing at least one of the following: and Mn, in a positive electrode for a lithium ion secondary battery, which is a composite oxide having an olivine structure represented by 0 <a ≦ 1.1, 0.9 ≦ x ≦ 1.1) The conductive material contains fibrous carbon, and a pit is formed on the surface of the current collector, and a part of the positive electrode active material and a part of the fibrous carbon are in the pit.
(2) In the positive electrode for a lithium ion secondary battery according to (1), the positive electrode active material has an average secondary particle diameter of 0.2 μm or more and 1 μm or less, and an average secondary particle diameter and an average pit diameter of pits It is preferable that the average secondary particle diameter / average pit diameter, which is the ratio thereof, be 0.1 or more and 0.5 or less.
(3) In the positive electrode for a lithium ion secondary battery according to (1) or (2), the surface roughness Ra of the current collector is preferably 0.3 μm or more and 1 μm or less.
(4) In the positive electrode for a lithium ion secondary battery according to any one of (1) to (3), the weight percentage of fibrous carbon in the conductive material is 20% or more and less than 60%. preferable.
(5) In the positive electrode for a lithium ion secondary battery according to any one of (1) to (4), the content of the positive electrode active material in the mixture layer is 90% or more and 93% or less by weight Is preferred.
(6) In the positive electrode for a lithium ion secondary battery according to any one of (1) to (5), the electrode density is preferably 2.0 g / cc or more and 2.3 g / cc or less.
(7) A lithium ion secondary battery using the positive electrode for a lithium ion secondary battery according to any one of (1) to (6).
(8) A battery module in which a plurality of lithium ion secondary batteries according to (7) are electrically connected.

With regard to the above features (1) to (6), the effects of the present invention can be obtained even if the conditions described in (2) to (6) are not necessarily satisfied as long as the conditions described in (1) are satisfied. be able to. For example, the surface roughness Ra of the current collector is an average value for the entire current collector, and does not necessarily correspond one-to-one with the average pit diameter described in (2). That is, in addition to pits satisfying the condition of (2), in the case where a large number of fine pits not satisfying the condition of (2) exist, the surface roughness Ra may exceed 1 μm. Even in any case, the present invention is effective. Of course, if the conditions of (2) to (6) are satisfied in addition to the condition of (1), the effects of the present invention will be remarkable.

According to the present invention, it is possible to provide a lithium ion secondary battery suitable for application to devices requiring high capacity and high safety, such as plug-in hybrid vehicles or electric vehicles.

Hereinafter, examples of the positive electrode for a lithium ion secondary battery according to the present invention will be described in detail.

[Material of positive electrode for lithium ion secondary battery]
The positive electrode for lithium ion secondary batteries has an olivine positive electrode material (positive electrode active material) having the following features.

The specific surface area of the olivine positive electrode material is 10 to 30 m ² / g. Here, when the specific surface area is less than 10 m ² / g, the electrode resistance increases because the reaction area between the positive electrode material and the lithium ion is small. When the specific surface area exceeds 30 m ² / g, it is not possible to simultaneously achieve the improvement of the electrode density and the formation of a conductive network in the positive electrode. In particular, in the case of the olivine positive electrode material, since the electron conductivity is low, if the conductive network can not be formed, the resistance becomes high and a desired discharge capacity can not be obtained.

The average primary particle diameter of the olivine positive electrode material is 0.05 to 0.3 μm. If the average primary particle size is less than 0.05 μm, aggregates are formed at the time of electrode coating, resulting in coating failure. On the other hand, when the average primary particle size exceeds 0.3 μm, the reactivity of the positive electrode active material itself is reduced, and a desired discharge capacity can not be obtained.

The average secondary particle diameter of the olivine positive electrode material is 0.2 to 1 μm. If the average secondary particle diameter is less than 0.2 μm, aggregates are formed at the time of electrode coating, resulting in coating failure. On the other hand, when the average secondary particle diameter is 1.1 μm or more, it is difficult to obtain a high density electrode for improving the battery capacity.

The composition of the olivine cathode material has the formula _Li a _M x _PO 4 _(M is a transition metal containing at least one of Fe and Mn .0 <a ≦ 1.1,0.9 ≦ x ≦ 1.1) It is a complex oxide having an olivine structure represented by Here, the range of a indicating the composition of Li is 0 <a ≦ 1.1, and the reason is shown below. Li content in the olivine positive electrode material which comprises an electrode becomes 0 <a <= 1.0 according to the charge condition of a positive electrode. Furthermore, since Li may be excessive in the olivine positive electrode material and Li may enter the M site, the range of a showing the composition of Li was set to 0 <a ≦ 1.1. In addition, the range of x representing the composition of the transition metal M is 0.9 ≦ x ≦ 1.1 in consideration of the case where Li is excessive, and is 0.9 ≦ x, and the transition metal M is excessive It is because it is referred to as x <= 1.1 in consideration of the case where it became.

Next, the aluminum-based current collector of the positive electrode for a lithium ion secondary battery has the following features. That is, it is an aluminum base current collector having pits having a pit diameter of 2 to 7 μm on the surface and having a surface roughness Ra of 0.3 to 1 μm according to JIS 2001. Since the relationship between the pit diameter on the aluminum base material surface and the secondary particle diameter of the positive electrode material has been described above, the surface roughness Ra defined in the present invention will be described below.

When Ra is less than 0.3 μm, the density of pits formed on the surface of the aluminum-based current collector is low, and the anchor effect acting on the interface between the positive electrode mixture and the aluminum-based current collector is small. For this reason, the positive electrode of desired electrode density can not be obtained, but peeling will generate | occur | produce in consolidation process. On the other hand, when Ra exceeds 1 μm, the strength of the aluminum base current collector is lowered due to the pits formed in the depth direction of the aluminum base current collector and the density of the pits, so The breakage of the electrode occurs, and the yield of electrode fabrication decreases. Therefore, the surface roughness Ra of the aluminum base current collector is desirably 0.3 μm or more and 1 μm or less.

Next, the roughening process on the aluminum base current collector will be described. Roughening can be performed by sand blasting on an aluminum base current collector or etching with acid or alkali. In order to form pits effective for the anchor effect on the aluminum base current collector, it is desirable to perform roughening by etching or a combination of etching and another processing method (for example, sand blast).

An outline of the method for producing the olivine positive electrode, the battery and the module is shown below.

[Method for producing olivine positive electrode material]
Finely pulverized iron oxalate dihydrate, ammonium dihydrogen phosphate and lithium carbonate are mixed so as to have a molar ratio of 2: 2: 1.0, and this is calcined under a nitrogen atmosphere at 300 ° C. The precursor was obtained. Thereafter, the precursor and polyvinyl alcohol were mixed, and heat treatment was performed for 8 hours in a nitrogen atmosphere at 700 ° C. to obtain an olivine positive electrode material.

[Method of manufacturing lithium ion secondary battery]
The lithium ion secondary battery may be any type of cylindrical type, laminated type, coin type, card type and the like, and is not particularly limited. In this specification, a method of manufacturing a cylindrical lithium ion secondary battery is described as an example.

1) Production Method of Positive Electrode A conductive material such as acetylene black and fibrous carbon is added to and mixed with the olivine positive electrode material produced as described above. The olivine positive electrode material described in the present specification has a high specific surface area, and the liquid absorbability of the organic solvent used at the time of producing the electrode is high. For this reason, N-methyl-2-pyrrolidinone (hereinafter abbreviated as "NMP"), which is an organic solvent, is mixed with the positive electrode active material in advance and the positive electrode active material absorbs NMP, and then the conductive material is made the positive electrode active material. Distribute Thereafter, a binder dissolved in a solvent such as NMP is added to the mixture, and the mixture is kneaded to obtain a positive electrode slurry. Here, polyvinylidene fluoride (hereinafter, abbreviated as "PVDF") is used as a binder. Next, this slurry is applied onto an aluminum base current collector and then dried to prepare a positive electrode plate.

2) Production Method of Negative Electrode A conductive material such as acetylene black and carbon fiber is added to and mixed with an amorphous carbon material which is a negative electrode active material. After adding PVDF or a rubber-based binder (SBR etc.) dissolved in NMP as a binder to this, it knead | mixes, and obtains a negative electrode slurry. Next, this slurry is applied onto a copper foil and then dried to prepare a negative electrode plate.

3) Method of Forming Battery The positive electrode plate and the negative electrode plate are dried after the slurry is applied to both sides of the electrode. Furthermore, it is densified by rolling and cut into a desired shape to produce an electrode. Next, lead pieces for flowing current to these electrodes are formed. A porous insulating material separator is sandwiched between the positive electrode and the negative electrode, and after being wound, it is inserted into a battery can made of stainless steel or aluminum. Next, after the lead pieces and the battery can are connected, a non-aqueous electrolytic solution is injected, and finally, the battery can is sealed to obtain a lithium ion secondary battery.

4) Modularization of Battery As one of the embodiments using the above lithium ion secondary battery, a battery module in which a plurality of batteries are connected in series can be mentioned. The battery module using the lithium ion secondary battery of the present invention can have a high capacity.

〔Example〕
EXAMPLES Hereinafter, the present invention will be specifically described by way of examples, but the following examples do not limit the scope of the present invention. In the following examples, the case of using only Fe and the case of using Fe and Mn as the transition metal M constituting the olivine positive electrode material will be described. Even when only Mn is used as the transition metal M, the same effect as that of the following embodiment can be obtained. This is because the olivine cathode material using only Mn as the transition metal M has the same crystal structure as the olivine cathode material using only Fe as the transition metal M and the olivine cathode material using Fe and Mn.

Example 1
<Fabrication of olivine cathode material>
Iron oxalate dihydrate, ammonium dihydrogen phosphate and lithium carbonate which have been finely ground in a ball mill for 3 hours are mixed in a molar ratio of 2: 2: 1.0, and this is mixed with nitrogen at 300 ° C. It was calcined under an atmosphere to obtain a precursor. Thereafter, the precursor and polyvinyl alcohol were mixed, and heat treatment was performed for 8 hours in a nitrogen atmosphere at 700 ° C., to obtain an olivine positive electrode material (1) made of carbon-coated LiFePO ₄ . The amount of carbon coated was 2 wt%.

<Method of measuring specific surface area>
The olivine positive electrode material (1) was previously dried at 120 ° C., filled in a sample cell, and dried in nitrogen gas at 300 ° C. for 30 minutes. Next, the sample cell was attached to the measurement unit, and after counting signals at the time of desorption with a He / N ₂ mixed gas, the specific surface area was calculated by the BET method. As a result, the specific surface area of the secondary particles was 30 m ² / g.

<Method of measuring secondary particle diameter>
The olivine positive electrode material (1) which is a positive electrode active material was disperse | distributed in hexametaphosphoric acid aqueous solution, and the average secondary particle diameter (D50) of the olivine positive electrode material was computed from scattering of a laser beam. As a result, D50 was 0.8 μm. Since the average pit diameter was determined to be 5.3 μm, the ratio of the average secondary particle diameter to the average pit diameter (average secondary particle diameter / average pit diameter) is 0.15.

<Fabrication of positive electrode>
Using the olivine positive electrode material (1), a positive electrode plate was produced by the following procedure. A solution of a binder dissolved in NMP as a solvent, an olivine positive electrode material (1), acetylene black which is a carbon-based conductive material having an average particle diameter of 35 nm, and VGCF (registered trademark. Diameter: 150 nm) which is vapor grown carbon fiber. , Fiber length: 10 to 20 μm) to prepare a positive electrode mixture slurry. At this time, the two conductive materials were equal in weight ratio. Therefore, the weight percentage of fibrous carbon in the conductive material is 50%.

The olivine positive electrode material (1), the carbon-based conductive material, and the binder were mixed so as to have a ratio of 91: 4: 5, respectively, as represented by a weight percentage ratio. Therefore, the content of the positive electrode active material (olivine positive electrode material) of the positive electrode occupied in the mixture layer is 91% by weight.

The slurry is uniformly coated on an aluminum sheet (aluminum base current collector) with a thickness of 30 μm and a surface roughness of Ra = 0.7 μm, and then dried at 100 ° C. Pressure was applied at / cm ² to form a coating having a thickness of about 60 μm, and a positive electrode plate having an electrode density of 2.2 g / cc (g / cm ³ ) was obtained. Next, in order to remove the moisture of the positive electrode plate, vacuum heat treatment was performed at 140 ° C. for 2 hours.

The surface roughness Ra of the aluminum sheet (aluminum base current collector) used here was evaluated according to JIS 2001 using a surface roughness measuring machine (Mitsutoyo, SURFTEST SV-2100).

<Evaluation of positive electrode>
The positive electrode plate was punched into φ15, and the counter electrode and the reference electrode were made of metallic lithium, to prepare a cylindrical lithium ion secondary battery as a test battery. At this time, a mixed solvent of ethyl carbonate and dimethyl carbonate in which 1.0 mol of LiPF ₆ was used as an electrolyte was used as an electrolytic solution.

This test battery was initialized three times by repeating charging and discharging up to an upper limit voltage of 3.6 V and a lower limit voltage of 2.0 V at 0.3C. Furthermore, after performing constant current constant voltage charging for 5 hours with an upper limit voltage of 3.6V equivalent to 0.3C, constant current discharge up to a lower limit voltage of 2.0V equivalent to 0.3C is performed to obtain a discharge capacity I asked.

Next, the volumetric energy density (unit: mAh / cc (mAh / cm ³ )) of the electrode was calculated. After dividing the discharge capacity by the mixture weight of this electrode (the total weight of the olivine positive electrode material, the conductive material and the binder), the electrode density (2.2 g / cc) and the content of the positive electrode active material (91% by weight) The product was taken as volume energy density. This value represents the energy per unit volume and is an indicator of the high packing of the battery.

The row of Example 1 in Table 1 shows the ratio of the average secondary particle diameter to the average pit diameter (average secondary particle diameter / average pit diameter) of the olivine positive electrode material (positive electrode active material) as a result of evaluation of this positive electrode. Surface roughness Ra of aluminum base current collector, weight percentage of fibrous carbon in conductive material, average secondary particle diameter, content of positive electrode active material in mixture layer, electrode density, electrode volume energy density, current discharge capacity at 0.125mA / cm ² (a), discharge capacity at current 0.5mA / cm ² (B), shows the discharge capacity retention ratio of (B / a). The discharge capacity retention rate was determined by dividing the discharge capacity (B) by the discharge capacity (A). The volumetric energy density was 285 mAh / cc (285 mAh / cm ³ ), and the discharge capacity retention rate was 0.98, both of which were good.

<Evaluation of cylindrical lithium ion secondary battery>
In order to produce a cylindrical lithium ion secondary battery which is a test battery, the positive electrode plate using the olivine positive electrode material (1) was cut to have a coating width of 5.4 cm and a coating length of 60 cm. An aluminum foil lead piece was welded to the positive electrode plate in order to extract the current.

Next, in order to produce a cylindrical lithium ion secondary battery in combination with a positive electrode plate, a negative electrode plate was produced. The negative electrode mixture slurry was prepared by dissolving and mixing the negative electrode active material graphite carbon material in NMP as a binder. At this time, the dry weight ratio of the graphite carbon material to the binder was made to be 92: 8. The slurry was uniformly applied to a rolled copper foil having a thickness of 10 μm. Thereafter, compression molding was performed by a roll press machine, cutting was performed so as to have an application width of 5.6 cm and an application length of 64 cm, and a copper foil lead piece was welded to produce a negative electrode plate.

FIG. 2 is a cutaway cross-sectional view schematically showing the produced cylindrical lithium ion secondary battery. The cylindrical lithium ion secondary battery was manufactured in the following procedure using the positive electrode plate and negative electrode plate which were produced as mentioned above.

First, the separator 9 was disposed between the positive electrode plate 7 and the negative electrode plate 8 so that the positive electrode plate 7 and the negative electrode plate 8 were not in direct contact with each other, and wound to fabricate an electrode group. At this time, the lead piece (positive electrode lead piece) 13 of the positive electrode plate 7 and the lead piece (negative electrode lead piece) 11 of the negative electrode plate 8 were positioned on the opposite end faces of the electrode assembly. Furthermore, the arrangement of the positive electrode plate 7 and the negative electrode plate 8 prevents the mixture application portion of the positive electrode from protruding from the mixture application portion of the negative electrode. The separator 9 used here was a microporous polypropylene film 25 μm thick and 5.8 cm wide.

Next, the electrode group was inserted into the battery can 10 made of SUS, the negative electrode lead piece 11 was welded to the can bottom, and the positive electrode lead piece 13 was welded to the sealing lid 12. The sealing lid 12 doubles as a positive electrode current terminal. The non-aqueous electrolytic solution was injected into the battery can 10 in which the electrode group was disposed. The non-aqueous electrolytic solution was prepared by dissolving 1.0 mol / liter of LiPF ₆ in a mixed solvent of ethylene carbonate (EC) and dimethyl carbonate (DMC) in a volume ratio of 1: 2. Thereafter, the sealing lid 12 to which the packing 15 is attached is crimped to the battery can 10 and sealed to form a cylindrical battery having a diameter of 18 mm and a length of 65 mm.

The sealing lid 12 has a cleavage valve which is split when the pressure in the battery rises and releases the pressure in the battery. The insulating plate 14 was disposed between the sealing lid 12 and the electrode group, and between the can bottom of the battery can 10 and the electrode group.

This cylindrical battery was initialized three times by charging and discharging up to an upper limit voltage of 3.6 V and a lower limit voltage of 2.0 V at 0.3C. Furthermore, charging / discharging to the upper limit voltage 3.6V and the lower limit voltage 2.0V was performed at 0.3 C, and the battery discharge capacity was measured. The battery discharge capacity was 1.3 Ah.

As described above, in the cylindrical lithium ion secondary battery using the positive electrode according to this example, the capacity could be increased.

Next, 10 cylindrical lithium ion secondary batteries were connected in series to obtain a battery module having a high capacity.

FIG. 1 shows the results of examining the relationship between the surface roughness Ra and the electrode density by changing the surface roughness Ra of the aluminum base current collector in the electrode configuration of Example 1. FIG. When Ra is 1 μm, the electrode is densified as the surface roughness Ra increases. However, when the surface roughness Ra was 1.2 μm, the interface between the mixture layer and the current collector became uneven, and the electrode density was lowered due to the occurrence of peeling.

Example 2
In Example 1, the surface roughness Ra of the aluminum-based current collector is changed to 0.3 μm, the average secondary particle diameter of the olivine positive electrode material is changed to 0.2 μm, and the other conditions are the same as in Example 1. And evaluation of the battery. In Example 2, the ratio of the average secondary particle diameter to the average pit diameter of the olivine positive electrode material (secondary particle diameter / average pit diameter) was set to 0.1.

Since the surface roughness Ra was 0.3 μm, the electrode density was slightly reduced to 2.0 g / cc (g / cm ³ ). In addition, as a result of evaluating the electrode volume energy density, it was 240 mAh / cc (mAh / cm ³ ), and the discharge capacity retention rate was 0.95. The results are shown in the row of Example 2 in Table 1.

[Example 3]
In Example 1, the surface roughness Ra of the aluminum base current collector is changed to 1 μm, the average secondary particle diameter of the olivine positive electrode material is changed to 1 μm, and the other processes are the same as in Example 1. The evaluation of In Example 3, the ratio (secondary particle diameter / average pit diameter) of the average secondary particle diameter to the average pit diameter of the olivine positive electrode material was set to 0.2.

Since the surface roughness Ra was 1 μm, the electrode density slightly increased to 2.3 g / cc. In addition, as a result of evaluating the electrode volume energy density, it was 291 mAh / cc, and the discharge capacity retention rate was 0.97. The results are shown in the row of Example 3 in Table 1.

Comparative Example 1
In Example 1, preparation of the positive electrode and evaluation of the battery were performed in the same manner as in Example 1 except that the surface roughness Ra of the aluminum base current collector was changed to 0.2 μm. In Comparative Example 1, the ratio (secondary particle diameter / average pit diameter) of the average secondary particle diameter to the average pit diameter of the olivine positive electrode material was set to 0.6.

Since the surface roughness Ra was 0.2 μm, the electrode density decreased to 1.9 g / cc. Moreover, as a result of evaluating a volume energy density of electrodes, it was set to 206 mAh / cc, a discharge capacity maintenance rate was set to 0.66, and battery characteristics fell. The results are shown in the row of Comparative Example 1 in Table 1.

Comparative Example 2
In Example 1, the surface roughness Ra of the aluminum-based current collector was changed to 1.1 μm, and in the same manner as in Example 1 except for the above, preparation of a positive electrode and evaluation of a battery were performed. In Comparative Example 2, the ratio (secondary particle diameter / average pit diameter) of the average secondary particle diameter to the average pit diameter of the olivine positive electrode material is set to 0.15.

Since the surface roughness Ra was 1.1 μm, the electrode density increased to 2.4 g / cc. However, at the time of electrode processing, there were local breakages. Therefore, as a result of evaluating the electrode volume energy density, it was 205 mAh / cc, the discharge capacity retention rate was 0.67, and the battery characteristics were deteriorated. The results are shown in the row of Comparative Example 2 in Table 1.

Example 4
In Example 1, the surface roughness Ra of the aluminum base current collector is changed to 0.3 μm, the average secondary particle diameter of the olivine positive electrode material is changed to 1 μm, and the other processes are performed in the same manner as in Example 1 to prepare a positive electrode. And the battery was evaluated. In Example 4, the ratio of the average secondary particle diameter to the average pit diameter of the olivine positive electrode material (secondary particle diameter / average pit diameter) was set to 0.5.

Since the surface roughness Ra was 0.3 μm, the electrode density slightly decreased to 2.1 g / cc. In addition, as a result of evaluating the electrode volume energy density, it was 258 mAh / cc, and the discharge capacity retention rate was 0.95. The results are shown in the row of Example 4 in Table 1.

Comparative Example 3
In Example 1, preparation of the positive electrode and evaluation of the battery were performed in the same manner as in Example 1 except that the average secondary particle diameter of the olivine positive electrode material was changed to 0.48 μm. In Comparative Example 3, the ratio of the average secondary particle diameter to the average pit diameter (secondary particle diameter / average pit diameter) of the olivine positive electrode material was set to 0.09.

Since the ratio of the average secondary particle diameter to the average pit diameter decreased, the anchoring effect decreased, and the electrode density decreased to 1.9 g / cc. Moreover, as a result of evaluating an electrode volume energy density, it was set to 207 mAh / cc, and the discharge capacity maintenance factor was 0.65. The results are shown in the row of Comparative Example 3 in Table 1.

[Example 5]
In Example 1, preparation of a positive electrode and evaluation of a battery were performed in the same manner as in Example 1 except that the weight percentage of fibrous carbon in the conductive material was changed to 20%.

Since the acetylene black has a bulk density higher than that of fibrous carbon, VGCF, the electrode density is slightly increased to 2.3 g / cc. In addition, as a result of evaluating the electrode volume energy density, it was 290 mAh / cc, and the discharge capacity retention rate was 0.95. Since the amount of fibrous carbon added decreased, the discharge retention rate slightly decreased. The results are shown in the row of Example 5 in Table 1.

Comparative Example 4
In Example 1, preparation of a positive electrode and evaluation of a battery were performed in the same manner as in Example 1 except that the weight percentage of fibrous carbon in the conductive material was changed to 10%.

Since the amount of fibrous carbon was small, the anchoring effect decreased, and the electrode density decreased to 1.9 g / cc. In addition, as a result of evaluating the electrode volume energy density, it was 208 mAh / cc, and the discharge capacity retention rate was 0.67. Since the amount of fibrous carbon added decreased, the discharge retention rate slightly decreased. The results are shown in the row of Comparative Example 4 in Table 1.

Comparative Example 5
In Example 1, the weight percentage of fibrous carbon in the conductive material was changed to 60%, and the other processes were performed in the same manner as in Example 1 to fabricate a positive electrode.

Since a large amount of fibrous carbon was generated, a large amount of aggregates were generated in the slurry, and a positive electrode could not be formed. The results are shown in the row of Comparative Example 5 of Table 1.

[Example 6]
In Example 1, preparation of the positive electrode and evaluation of the battery were performed in the same manner as in Example 1 except that the content of the olivine positive electrode material was changed to 90%.

Since the content of the olivine positive electrode material decreased, the electrode density slightly increased to 2.3 g / cc. In addition, as a result of evaluating the electrode volume energy density, it was 290 mAh / cc, and the discharge capacity retention rate was 0.97. The results are shown in the row of Example 6 in Table 1.

[Example 7]
In Example 1, preparation of the positive electrode and evaluation of the battery were performed in the same manner as in Example 1 except that the content of the olivine positive electrode material was changed to 93%.

Since the content of the olivine positive electrode material increased, the electrode density slightly decreased to 2 g / cc. In addition, as a result of evaluating the electrode volume energy density, it was 271 mAh / cc, and the discharge capacity retention rate was 0.94. The results are shown in the row of Example 7 in Table 1.

Comparative Example 6
In Example 1, preparation of the positive electrode and evaluation of the battery were performed in the same manner as in Example 1 except that the content of the olivine positive electrode material was changed to 89%.

As the content of the olivine positive electrode material decreased, the electrode density slightly increased to 2.3 g / cc. In addition, as a result of evaluating the electrode volume energy density, it was 230 mAh / cc, and the discharge capacity retention rate was 0.70. Due to the low content of the olivine cathode material, the desired high volumetric energy density could not be achieved. The results are shown in the row of Comparative Example 6 in Table 1.

Comparative Example 7
In Example 1, preparation of the positive electrode and evaluation of the battery were performed in the same manner as in Example 1 except that the content of the olivine positive electrode material was changed to 94%.

Since the content of the olivine positive electrode material increased, the electrode density decreased due to peeling and became 1.9 g / cc. In addition, as a result of evaluating the electrode volume energy density, it was 208 mAh / cc, and the discharge capacity retention rate was 0.65. The results are shown in the row of Comparative Example 7 in Table 1.

Comparative Example 8
In Example 1, the average secondary particle diameter of the olivine positive electrode material was changed to 0.1 μm, and the other processes were performed in the same manner as in Example 1 to fabricate a positive electrode.

A lot of aggregates were generated in the slurry, and the positive electrode could not be formed. The results are shown in the row of Comparative Example 8 in Table 1.

Comparative Example 9
In Example 1, the average secondary particle diameter of the olivine positive electrode material is changed to 1.1 μm, the ratio of the average secondary particle diameter to the average pit diameter is changed to 0.2, and the other conditions are the same as in Example 1. And evaluation of the battery. As the average secondary particle size increased, the ratio of the average secondary particle size to the average pit size also slightly increased.

The electrode density decreased to 1.9 g / cc because the average secondary particle size of the olivine positive electrode material increased. Moreover, as a result of evaluating an electrode volume energy density, it was set to 209 mAh / cc, and the discharge capacity maintenance factor was 0.66. The results are shown in the row of Comparative Example 9 in Table 1.

Example 8
In Example 8, instead of the olivine positive electrode material LiFePO ₄ produced in Example 1, an olivine positive electrode material represented by a composition formula LiMn _0.8 Fe _0.2 PO ₄ was produced. The preparation method is described below.

And _NH ₄ _H 2 PO 4 in 7.2 g, and LiOH · _{H 2} O of 2.27 g, and _MnC ₂ _O 4 · 2H 2 _O of 9 g, and _FeC ₂ _O 4 · 2H 2 _O of 2.25g mixing did. Sucrose was added to this so that it might be 12 mass%, the zirconia grinding ball was thrown into the zirconia pot, and it mixed using the planetary ball mill. The mixed powder was put into an alumina crucible and subjected to temporary firing at 400 ° C. for 10 hours under an argon flow of 0.3 L / min.

The obtained calcined body was once crushed in an agate mortar, again put into an alumina crucible, and subjected to main firing at 700 ° C. for 10 hours under an argon flow of 0.3 L / min. After the main firing, the obtained powder was crushed in an agate mortar, and the particle size was adjusted with a 40 μm mesh sieve to obtain an olivine positive electrode material represented by the composition formula LiMn _0.8 Fe _0.2 PO ₄ .

Next, in the same manner as in Example 1, a positive electrode was produced and evaluated. The electrode density was 2 g / cc because LiMn _0.8 Fe _0.2 PO ₄ has a lower true density compared to LiFePO ₄ .

Next, in the same manner as in Example 1, a cylindrical lithium ion secondary battery as a test battery was produced, and the battery was evaluated. However, in the evaluation of the battery, the charging voltage was 4.1V. As a result of evaluating the electrode volume energy density, it was 257 mAh / cc, and the discharge capacity retention rate was 0.97. The results are shown in the row of Example 8 in Table 1.

Comparative Example 10
In Example 8, preparation of the positive electrode and evaluation of the battery were performed in the same manner as in Example 1 except that the surface roughness Ra of the aluminum base current collector was changed to 0.2 μm. In Comparative Example 10, the ratio (secondary particle diameter / average pit diameter) of the average secondary particle diameter to the average pit diameter of the olivine positive electrode material was set to 0.6.

Since the surface roughness Ra was 0.2 μm, the electrode density decreased to 1.7 g / cc. Moreover, as a result of evaluating a electrode volume energy density, it was set to 206 mAh / cc, and the discharge capacity maintenance factor was 0.68. These results are shown in the row of Comparative Example 10 in Table 1.

The present invention can be applied to devices that require high capacity, such as electric vehicles and plug-in hybrid vehicles.

DESCRIPTION OF SYMBOLS 1 ... Aluminum base material collector, 2 ... pit, 3 ... secondary particle of olivine positive electrode material, 4: ... fibrous carbon, 7: ... positive electrode plate, 8 ... negative electrode plate, 9 ... separator, 10 ... battery can, 11 ... Negative electrode lead piece, 12: Sealing lid, 13: Positive electrode lead piece, 14: Insulating plate, 15: Packing.

Claims

A positive electrode active material, a mixture layer containing a conductive material and a binder, and a current collector having the mixture layer formed on the surface, the positive electrode active material has a chemical formula Li a M x PO 4 (where M is Fe and A transition metal containing at least one of Mn.A positive electrode for a lithium ion secondary battery, which is a composite oxide having an olivine structure represented by 0 <a ≦ 1.1, 0.9 ≦ x ≦ 1.1),
The conductive material contains fibrous carbon,
Pits are formed on the surface of the current collector,
A positive electrode for a lithium ion secondary battery, wherein a part of the positive electrode active material and a part of the fibrous carbon enter the pits.
In the positive electrode for a lithium ion secondary battery according to claim 1,
The positive electrode active material has an average secondary particle diameter of 0.2 μm or more and 1 μm or less,
A positive electrode for a lithium ion secondary battery, wherein an average secondary particle diameter / average pit diameter, which is a ratio of the average secondary particle diameter to the average pit diameter of the pits, is 0.1 or more and 0.5 or less.
In the positive electrode for a lithium ion secondary battery according to claim 1 or 2,
The current collector is a positive electrode for a lithium ion secondary battery having a surface roughness Ra of 0.3 μm or more and 1 μm or less.
The positive electrode for a lithium ion secondary battery according to any one of claims 1 to 3,
The positive electrode for a lithium ion secondary battery, wherein a weight percentage of the fibrous carbon in the conductive material is 20% or more and less than 60%.
The positive electrode for a lithium ion secondary battery according to any one of claims 1 to 4,
The positive electrode for a lithium ion secondary battery in which the content of the positive electrode active material in the mixture layer is 90% to 93% by weight.
The positive electrode for a lithium ion secondary battery according to any one of claims 1 to 5,
The positive electrode for lithium ion secondary batteries whose electrode density is 2.0 g / cc or more and 2.3 g / cc or less.
A lithium ion secondary battery using the positive electrode for a lithium ion secondary battery according to any one of claims 1 to 6.
A battery module comprising a plurality of the lithium ion secondary batteries according to claim 7 electrically connected.