JPWO2012114502A1 - Positive electrode for lithium ion secondary battery, lithium ion secondary battery, and battery module - Google Patents

Abstract

The objective of this invention is providing the positive electrode for lithium ion secondary batteries which can improve the capacity | capacitance per volume of an electrode, and a lithium ion secondary battery using the same. A positive electrode for a lithium ion secondary battery according to the present invention includes a positive electrode active material (3), a mixture layer containing a conductive material and a binder, and a current collector (1) having a mixture layer formed on the surface thereof. The active material (3) has an olivine structure represented by the chemical formula LiaMxPO4 (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 composite oxide, the conductive material contains fibrous carbon (4), pits (2) are formed on the surface of the current collector (1), a part of the positive electrode active material (3) and fibrous carbon ( Part of 4) is characterized by entering the pit (2).

Description

  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.

  In order to further improve the energy efficiency of automobiles, development of plug-in hybrid automobiles (hereinafter abbreviated as “PHEV”) is required. PHEV uses energy charged by a household power source for running and runs only a motor for a certain distance, so the battery used in PHEV has high output and high battery in a short time required for hybrid vehicles Capacity is required.

  As described above, in the battery characteristics required for PHEV, it is important to increase the output as well as increase the capacity. For this reason, since the lithium ion secondary battery for PHEV becomes a large sized large capacity battery, ensuring safety | security is important. In a large-capacity large-capacity lithium ion secondary battery for in-vehicle use, an improvement in volume energy density and weight energy density is required in order to reduce the size and weight of the battery. In addition, since a large-capacity lithium-ion secondary battery stores a large amount of energy, a positive electrode active material having high thermal stability and high safety is required.

As a positive electrode material for a lithium ion secondary battery for PHEV, a positive electrode active material having an olivine structure composed of Fe or Mn as a transition metal (LiMPO ₄ , M is a transition metal containing at least one of Fe and Mn. ")" Is attracting attention. The olivine cathode material is highly safe because the bond between oxygen and phosphorus in the crystal structure is strong and oxygen is not easily released from the crystal structure during overcharge. However, it has been reported that the olivine positive electrode material has low electronic conductivity and a low lithium ion diffusion coefficient into the positive electrode material.

  For the olivine cathode material, as a means for solving the above problems for practical use, the material has a high specific surface area to improve the diffusibility of lithium ions and to be conductive by coating with carbon (carbon coating). Has been given sex. When carbon coating is applied, conductivity can be imparted, crystal growth can be suppressed, and the primary surface can be contributed to a high specific surface area by reducing the particle size to a submicron size.

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

  Generally, in a high capacity battery, in order to improve the volume energy density, it is necessary to improve the positive electrode active material content in the positive electrode and to increase the thickness of the mixture layer composed of the positive electrode active material, the conductive material, and the binder. is there. In the preparation of the olivine positive electrode, a slurry composed of an olivine positive electrode material, a conductive material and a binder dispersed in a solvent is applied on an aluminum current collector, and then dried to obtain a positive electrode. In the case of an electrode having a thick mixture layer, the phenomenon that the binder resin moves to the surface layer along with the evaporation of the solvent becomes significant in this drying step. For this reason, the amount of binder decreases at the interface between the aluminum current collector and the mixture layer. When the amount of the binder decreases, the mixture layer peels from this interface by consolidation by electrode pressing or roll rolling. As described above, in the olivine positive electrode, it is essential to consolidate the electrode in order to improve the volume energy density, but it is necessary to suppress separation of the mixture layer from the interface between the aluminum current collector and the mixture layer. is there.

  In Patent Document 1, for increasing the volume energy density of the olivine positive electrode, a method of improving the volume energy density and load characteristics by using a current collector having a surface roughness Ra of 0.1 μm or more and increasing the coating capacity of the active material. Is disclosed. In particular, regarding the improvement of the load characteristics, it is disclosed that the surface-roughened aluminum current collector has improved the discharge capacity maintenance rate at a high discharge current. 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

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

  The objective of this invention is providing the positive electrode for lithium ion secondary batteries which can improve the capacity | capacitance per volume of an 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 characteristics. A mixture layer including a positive electrode active material, a conductive material, and a binder; and a current collector formed on the surface of the mixture layer, wherein the positive electrode active material has a chemical formula Li _a M _x PO ₄ (M is Fe and Transition metal containing at least one of Mn. 0 <a ≦ 1.1, 0.9 ≦ x ≦ 1.1) In the positive electrode for a lithium ion secondary battery, which is a composite oxide having an olivine structure, The conductive material includes fibrous carbon, pits are formed on the surface of the current collector, and a part of the positive electrode active material and a part of the fibrous carbon enter the pits.

  ADVANTAGE OF THE INVENTION According to this invention, the positive electrode for lithium ion secondary batteries 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 the relationship between surface roughness Ra of an aluminum base material electrical power collector, and an electrode density. It is a notch sectional view which shows typically a cylindrical lithium ion secondary battery. It is sectional drawing of the positive electrode for lithium ion secondary batteries.

  As a result of intensive studies to solve the above-mentioned problems, the present inventors have found that the positive electrode active material, the conductive material, and the binder composition and the composition of the positive electrode mixture, and further, as the base material of the mixture By examining the surface roughness of the aluminum base current collector to be used, the content of the positive electrode active material in the positive electrode is improved, and the density of the positive electrode is improved. The energy density per unit volume (volume energy density) ) Was found to improve. The capacity per volume of the electrode is represented by the electrode volume energy density.

A positive electrode for a lithium ion secondary battery according to the present invention includes a mixture layer including a positive electrode active material, a conductive material and a binder, and a current collector. The positive electrode active material has a chemical formula Li _a M _x PO ₄ (M is Transition metal containing at least one of Fe and Mn, which is a complex 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 0.2 μm or more and 1 μm or less (0.2 to 1 μm). In the present specification, the average primary particle diameter and the average secondary particle diameter are also simply referred to as primary particle diameter and secondary particle diameter, respectively. The conductive material is a mixture of carbon black and fibrous carbon. The current collector is made of an aluminum base material having a specified surface roughness.

Further, 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 percentage, but is not 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, to increase the content of the positive electrode active material in the positive electrode, and to 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 the binding property can be improved by examining the binder, the present invention focuses on the binding property at the interface between the current collector and the mixture layer as described above. In general, attempts have been made to roughen the surface of an aluminum current collector to improve the binding property at 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 examined from the following viewpoints. That is, by forming pits on the surface of the aluminum base current collector, the relationship between the pit diameter and the average secondary particle diameter of the olivine positive electrode material, and the effect of fibrous carbon used as a conductive material dispersed in the positive electrode is there.

  The “pit” is a hole formed in 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 opening of the pit. In this specification, the average pit diameter that is the average of the pit diameters of the pits is also simply referred to as a pit diameter.

  By allowing a part of the positive electrode active material (olivine positive electrode material) and a part of the fibrous carbon to enter the pit, the binding property at the interface between the current collector and the mixture layer can be increased by the anchor effect. The olivine cathode material that enters the pit may be either primary particles or secondary particles. However, the relationship between the pit diameter and the particle diameter of the olivine cathode 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 cathode material is shown below. Generally, in a current collector using an aluminum substrate, pits having a pit diameter of several μm and a depth of several μm can be formed on the surface of the substrate by surface treatment using acid or alkali. The secondary particles of the olivine positive electrode material enter into the pits, an anchor effect is generated, and the binding property of the interface between the current collector and the mixture layer is increased. Here, the binding property varies 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 almost the same, it becomes difficult for the secondary particles of the olivine cathode material to enter the pit. On the other hand, if the secondary particle diameter of the olivine positive electrode material is too small with respect to the pit diameter, the anchor effect is reduced.

  For this reason, in this invention, the secondary particle diameter of the olivine positive electrode material suitable for the pit diameter of the collector surface was prescribed | regulated 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 average secondary particle diameter / average pit diameter, which is a ratio to the average pit diameter, is 0.1 or more and 0.5 or less. It was said that. According to this regulation, it is possible to increase the binding property of the interface between the current collector and the mixture layer and to increase the density of the positive electrode with an appropriate amount of the olivine positive electrode material entering the pits.

  Next, in order to explain the effect of fibrous carbon dispersed in the positive electrode, a 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 ensure electronic conductivity. Examples of the conductive material include fine granular acetylene black and fibrous carbon.

  The effect of fibrous carbon dispersed in the positive electrode will be described below with reference to FIG. FIG. 3 is a cross-sectional view of a positive electrode for a lithium ion secondary battery according to the present invention, in which 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 the secondary particles are representatively shown among the particles (primary particles and secondary particles) of the olivine positive electrode material. For the primary particles, the same explanation as for the secondary particles applies.

  The secondary particles 3 of the olivine cathode material can enter the pits 2. Here, if the fibrous carbon 4 used as the conductive material is dispersed in the mixture slurry, the fibrous carbon 4 also enters the pit 2, and the fibrous carbon 4 is distributed in the thickness direction of the mixture layer. Thus, the binding property of the mixture can be improved. However, when too much fibrous carbon 4 is contained in the conductive material, the fibrous carbon 4 and the olivine cathode material form an aggregate and cannot enter the pit 2. Moreover, when there is little fibrous carbon 4, the anchor effect by fibrous carbon 4 will reduce. For this reason, it is necessary to prescribe | regulate content of the fibrous carbon 4 contained in all the electrically conductive materials. The proportion of the fibrous carbon 4 in the total conductive material was defined as 20% or more and less than 60% by weight percentage.

  Specific fibrous carbon used here includes vapor grown carbon fiber, carbon nanotube (CNT) and carbon nanofiber (CNF). Fibrous carbon has excellent properties, but is difficult to disperse in the mixture slurry and may form aggregates in the slurry. Aggregates in the slurry hinder the formation of a mixture layer in the electrode application step, and therefore a mixture composition in which no aggregates are formed is desirable.

  Next, the effect of acetylene black is shown below. Acetylene black is a fine granular particle having a particle diameter of several tens of nanometers and is excellent in dispersibility in a slurry. For this reason, it is effective for ensuring the electronic conductivity in the positive electrode while suppressing the formation of aggregates.

  Considering the characteristics of such fibrous carbon and acetylene black, it was specified that the proportion of fibrous carbon in the total conductive material was 20% or more and less than 60% in weight percentage. Here, if the amount of fibrous carbon added is less than 20%, the above effect is small, and if it is 60% or more, there are many aggregates in the slurry, making it difficult to produce a positive electrode.

  With the above electrode configuration, the content of the positive electrode active material (olivine positive electrode material) in the mixture layer is 90 to 93% by weight, and the high density positive electrode has 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, an object of the present invention is to define a positive electrode configuration of an olivine positive electrode material for the purpose of obtaining a high-safety large-capacity lithium ion secondary battery.

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 characteristics.
(1) A mixture layer including a positive electrode active material, a conductive material and a binder, and a current collector having a mixture layer formed on a surface thereof, wherein the positive electrode active material has a chemical formula Li _a M _x PO ₄ (M is Fe And a transition metal containing at least one of 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, pits are formed on the surface of the current collector, and part of the positive electrode active material and part of the fibrous carbon enter the pits.
(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 the average secondary particle diameter and the average pit diameter of pits The average secondary particle diameter / average pit diameter, which is the ratio of the above, is preferably 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 current collector has a surface roughness Ra of 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 percentage. 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 the lithium ion secondary batteries according to (7) are electrically connected.

  About the above characteristics (1)-(6), if the conditions as described in (1) are satisfy | filled, even if the conditions as described in (2)-(6) are not necessarily satisfied, the effect of this invention is acquired. 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 the pits satisfying the condition (2), when there are many fine pits that do not satisfy the condition (2), the surface roughness Ra may exceed 1 μm. Even in this case, the present invention is effective. Of course, if the conditions (2) to (6) are satisfied in addition to the condition (1), the effect of the present invention is remarkably exhibited.

  ADVANTAGE OF THE INVENTION According to this invention, the lithium ion secondary battery suitable for the application to apparatus with high capacity | capacitance and high safety | security required, such as a plug-in hybrid vehicle or an electric vehicle, can be provided.

  Hereinafter, the example of the positive electrode for lithium ion secondary batteries by this invention is demonstrated 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 characteristics.

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 ions is small. When the specific surface area exceeds 30 m ² / g, it is impossible to simultaneously achieve improvement in electrode density and 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 a conductive network cannot be formed, the resistance becomes high and a desired discharge capacity cannot 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 during electrode application, resulting in poor coating. On the other hand, if the average primary particle diameter exceeds 0.3 μm, the reactivity of the positive electrode active material itself is lowered and a desired discharge capacity cannot 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 during electrode application, resulting in poor coating. 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 battery capacity.

The composition of the olivine positive electrode material is chemical 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) Is a complex oxide having an olivine structure represented by Here, the range of a indicating the composition of Li is set to 0 <a ≦ 1.1, and the reason is described below. The Li content in the olivine positive electrode material constituting the electrode is 0 <a ≦ 1.0 depending on the state of charge of the positive electrode. Furthermore, since Li may be excessive in the olivine cathode material and Li may enter the M site, the range of a indicating the composition of Li is set to 0 <a ≦ 1.1. In addition, the range of x indicating the composition of the transition metal M is set to 0.9 ≦ x ≦ 1.1 in consideration of the case where Li becomes excessive, 0.9 ≦ x, and the transition metal M is excessive. This is because x ≦ 1.1 in consideration of the case where

  Next, the aluminum base current collector of the positive electrode for a lithium ion secondary battery has the following characteristics. That is, the aluminum base current collector has pits with a pit diameter of 2 to 7 μm on the surface and a surface roughness Ra according to JIS2001 of 0.3 to 1 μm. Since the relationship between the pit diameter on the surface of the aluminum substrate 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 base current collector is low, and the anchor effect acting on the interface between the positive electrode mixture and the aluminum base current collector is small. For this reason, a positive electrode having a desired electrode density cannot be obtained, and peeling occurs in the consolidation process. On the other hand, when Ra exceeds 1 μm, the strength of the aluminum base current collector decreases due to the pits formed in the depth direction of the aluminum base current collector and the high density of the pits. Electrode breakage occurs and the yield of electrode fabrication decreases. For this reason, the surface roughness Ra of the aluminum base current collector is desirably 0.3 μm or more and 1 μm or less.

  Next, a roughening process on the aluminum base current collector will be described. Surface roughening can be performed by sandblasting the 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 treatment or a combination of etching treatment and other treatment methods (for example, sandblasting).

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

[Manufacturing method of olivine cathode material]
Finely pulverized iron oxalate dihydrate, ammonium dihydrogen phosphate and lithium carbonate are mixed at a molar ratio of 2: 2: 1.0, and calcined in a nitrogen atmosphere at 300 ° C. Thus, a precursor was obtained. Then, the precursor and polyvinyl alcohol were mixed, and the olivine positive electrode material was obtained by performing heat processing for 8 hours in 700 degreeC nitrogen atmosphere.

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

1) Method for producing 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 has a high liquid absorption property of an organic solvent used for electrode preparation. Therefore, N-methyl-2-pyrrolidinone (hereinafter abbreviated as “NMP”), which is an organic solvent, is mixed with the positive electrode active material in advance to absorb NMP in the positive electrode active material, and then the conductive material is used as the positive electrode active material. To disperse. Thereafter, a binder dissolved in a solvent such as NMP is added to the mixture and kneaded to obtain a positive electrode slurry. Here, polyvinylidene fluoride (hereinafter abbreviated as “PVDF”) is used as the binder. Next, after apply | coating this slurry on the aluminum base material electrical power collector, it dries and produces a positive electrode plate.

2) Method for producing 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. To this, PVDF or a rubber-based binder (SBR or the like) dissolved in NMP is added as a binder and then kneaded to obtain a negative electrode slurry. Next, after apply | coating this slurry on copper foil, it dries and produces a negative electrode plate.

3) Battery Formation Method The positive electrode plate and the negative electrode plate are dried after applying the slurry to both surfaces of the electrode. Further, it is densified by rolling and cut into a desired shape to produce an electrode. Next, lead pieces for passing a current through these electrodes are formed. A porous insulating material separator is sandwiched between the positive electrode and the negative electrode, wound, and then inserted into a battery can molded of stainless steel or aluminum. Next, after connecting the lead piece and the battery can, a non-aqueous electrolyte 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 lithium ion secondary battery, there is a battery module in which a plurality of batteries are connected in series. The battery module using the lithium ion secondary battery of the present invention can be increased in capacity.

〔Example〕
EXAMPLES Hereinafter, although an Example demonstrates this invention concretely, a following example does not limit the scope of the present invention. In the following examples, the case where only Fe and the case where Fe and Mn are used will be described as the transition metal M constituting the olivine positive electrode material. Even if only Mn is used as the transition metal M, the same effects as in the following examples can be obtained. This is because the olivine positive electrode material using only Mn as the transition metal M has the same crystal structure as the olivine positive electrode material using only Fe as the transition metal M and the olivine positive electrode material using Fe and Mn.

[Example 1]
<Production of olivine cathode material>
Iron oxalate dihydrate, ammonium dihydrogen phosphate and lithium carbonate, which were finely pulverized for 3 hours in a ball mill, were mixed at a molar ratio of 2: 2: 1.0. The precursor was obtained by calcination in an atmosphere. Thereafter, the precursor and polyvinyl alcohol were mixed and heat-treated 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 for 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 the specific surface area was calculated by the BET method after counting the signals at the time of desorption with the He / N ₂ mixed gas. As a result, the specific surface area of the secondary particles was 30 m ² / g.

<Method for measuring secondary particle size>
The olivine positive electrode material (1), which is a positive electrode active material, was dispersed in a hexametaphosphoric acid aqueous solution, and the average secondary particle diameter (D50) of the olivine positive electrode material was calculated from the scattering of laser light. As a result, D50 was 0.8 μm. Since the average pit diameter was 5.3 μm, the ratio of the average secondary particle diameter to the average pit diameter (average secondary particle diameter / average pit diameter) was 0.15.

<Preparation of positive electrode>
Using the olivine positive electrode material (1), a positive electrode plate was prepared by the following procedure. A solution obtained by dissolving a binder 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 a vapor-grown carbon fiber. , Fiber length: 10 to 20 μm) was mixed to prepare a positive electrode mixture slurry. At this time, the two kinds of conductive materials were made equal in weight ratio. Accordingly, 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 at a weight percentage ratio of 91: 4: 5. Therefore, the content of the positive electrode active material (olivine positive electrode material) of the positive electrode in the mixture layer is 91% by weight.

This slurry was uniformly applied onto an aluminum sheet (aluminum base current collector) having a thickness of 30 μm and a surface roughness Ra = 0.7 μm, then dried at 100 ° C., and about 1.5 ton by press. / Cm ² to form a coating film having a film 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 moisture from 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 JIS2001 using a surface roughness measuring machine (Mitutoyo Corporation, SURFTEST SV-2100).

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

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

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

In the row of Example 1 in Table 1, as a result of the evaluation of this positive electrode, the ratio of the average secondary particle diameter and the average pit diameter of the olivine positive electrode material (positive electrode active material) (average secondary particle diameter / average pit diameter), 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 obtained by dividing the discharge capacity (B) by the discharge capacity (A). The volume energy density was 285 mAh / cc (285 mAh / cm ³ ) and the discharge capacity retention rate was 0.98, both of which were good.

＜円筒型リチウムイオン二次電池の評価＞
試験用電池である円筒型リチウムイオン二次電池を作製するため、オリビン正極材（１）を用いた正極板を、塗布幅が５．４ｃｍで、塗布長さが６０ｃｍとなるよう切断した。電流を取り出すために、アルミニウム箔製のリード片を正極板に溶接した。

<Evaluation of cylindrical lithium ion secondary battery>
In order to produce a cylindrical lithium ion secondary battery as a test battery, a positive electrode plate using the olivine positive electrode material (1) was cut so that the coating width was 5.4 cm and the coating length was 60 cm. In order to take out an electric current, the lead piece made from aluminum foil was welded to the positive electrode plate.

  Next, in order to produce a cylindrical lithium ion secondary battery in combination with the positive electrode plate, a negative electrode plate was produced. The negative electrode mixture slurry was prepared by dissolving and mixing a graphite carbon material as a negative electrode active material in NMP as a binder. At this time, the dry weight ratio of the graphite carbon material and the binder was set to 92: 8. This slurry was uniformly applied to a rolled copper foil having a thickness of 10 μm. Then, it was compressed and shaped by a roll press machine, cut so that the coating width was 5.6 cm and the coating length was 64 cm, and a lead piece made of copper foil was welded to produce a negative electrode plate.

  FIG. 2 is a cut-away sectional view schematically showing the produced cylindrical lithium ion secondary battery. Using the positive electrode plate and the negative electrode plate produced as described above, a cylindrical lithium ion secondary battery was produced in the following procedure.

  First, a 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, and the electrode group was produced. 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 surfaces of the electrode group. Further, the arrangement of the positive electrode plate 7 and the negative electrode plate 8 prevents the positive electrode mixture application part from protruding from the negative electrode mixture application part. The separator 9 used here was a microporous polypropylene film having a thickness of 25 μm and a width of 5.8 cm.

Next, the electrode group was inserted into a battery can 10 made of SUS, the negative electrode lead piece 11 was welded to the bottom of the can, and the positive electrode lead piece 13 was welded to the sealing lid 12. The sealing lid 12 also serves as a positive electrode current terminal. A non-aqueous electrolyte was injected into the battery can 10 in which this electrode group was arranged. As the nonaqueous electrolytic solution, a solution in which 1.0 mol / liter of LiPF ₆ was dissolved in a mixed solvent of ethylene carbonate (EC) and dimethyl carbonate (DMC) in a volume ratio of 1: 2 was used. Thereafter, the sealing lid portion 12 to which the packing 15 was attached was caulked and sealed to the battery can 10 to obtain a cylindrical battery having a diameter of 18 mm and a length of 65 mm.

  The sealing lid portion 12 has a cleavage valve that cleaves when the pressure in the battery rises and releases the pressure inside the battery. An 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 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 0.3 C three times. Furthermore, charging / discharging to 0.3 C and upper limit voltage 3.6V and the lower limit voltage 2.0V was performed, and the battery discharge capacity was measured. The battery discharge capacity was 1.3 Ah.

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

  Next, 10 cylindrical lithium ion secondary batteries were connected in series to obtain a battery module with an increased 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. When Ra is up to 1 μm, the electrode density increases 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 non-uniform, and the electrode density decreased due to the occurrence of peeling.

[Example 2]
In Example 1, the surface roughness Ra of the aluminum base current collector was changed to 0.3 μm, and the average secondary particle diameter of the olivine positive electrode material was changed to 0.2 μm. And the battery were evaluated. In Example 2, 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 0.1.

Since the surface roughness Ra was 0.3 μm, the electrode density slightly decreased to 2.0 g / cc (g / cm ³ ). Moreover, as a result of evaluating electrode volume energy density, it was set to 240 mAh / cc (mAh / cm < ³ >), and the discharge capacity maintenance factor was 0.95. These 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 was changed to 1 μm, and the average secondary particle diameter of the olivine positive electrode material was changed to 1 μm. Was evaluated. 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 increased slightly to 2.3 g / cc. Moreover, as a result of evaluating an electrode volume energy density, it was 291 mAh / cc and the discharge capacity maintenance factor was 0.97. These results are shown in the row of Example 3 in Table 1.

[Comparative Example 1]
In Example 1, the surface roughness Ra of the aluminum base current collector was changed to 0.2 μm, and the other processes were performed in the same manner as in Example 1 to produce a positive electrode and evaluate the battery. In Comparative Example 1, 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 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 electrode volume energy density, it became 206 mAh / cc, the discharge capacity maintenance factor became 0.66, and the battery characteristic fell. These 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 base current collector was changed to 1.1 μm, and the other processes were performed in the same manner as in Example 1 to produce a positive electrode and evaluate the battery. In Comparative Example 2, 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 0.15.

  Since the surface roughness Ra was 1.1 μm, the electrode density was increased to 2.4 g / cc. However, there was a local break at the time of electrode processing. For this reason, as a result of evaluating an electrode volume energy density, it became 205 mAh / cc, the discharge capacity maintenance factor became 0.67, and the battery characteristic fell. These 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 was changed to 0.3 μm, and the average secondary particle diameter of the olivine positive electrode material was changed to 1 μm. And the battery was evaluated. In Example 4, 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 0.5.

  Since the surface roughness Ra was 0.3 μm, the electrode density slightly decreased to 2.1 g / cc. Moreover, as a result of evaluating an electrode volume energy density, it was set to 258 mAh / cc, and the discharge capacity maintenance factor was 0.95. These results are shown in the row of Example 4 in Table 1.

[Comparative Example 3]
In Example 1, the average secondary particle diameter of the olivine positive electrode material was changed to 0.48 μm, and the other processes were carried out in the same manner as in Example 1 to produce a positive electrode and evaluate the battery. 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 0.09.

  Since the ratio of the average secondary particle diameter to the average pit diameter was reduced, the anchor effect was reduced, and the electrode density was reduced 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. These results are shown in the row of Comparative Example 3 in Table 1.

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

  Since acetylene black has a higher bulk density than VGCF, which is fibrous carbon, the electrode density was slightly higher to 2.3 g / cc. Moreover, as a result of evaluating an electrode volume energy density, it was 290 mAh / cc and the discharge capacity maintenance factor was 0.95. Since the amount of fibrous carbon added decreased, the discharge maintenance ratio slightly decreased. These results are shown in the row of Example 5 in Table 1.

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

  Since there was little fibrous carbon, the anchor 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 208 mAh / cc, and the discharge capacity maintenance factor was 0.67. Since the amount of fibrous carbon added decreased, the discharge maintenance ratio slightly decreased. These results are shown in the row of Comparative Example 4 in Table 1.

[Comparative Example 5]
In Example 1, the positive electrode was produced in the same manner as in Example 1 except that the weight percentage of fibrous carbon in the conductive material was changed to 60%.

  Since there were many fibrous carbons, many aggregates generate | occur | produced in the slurry and the positive electrode could not be formed. The results are shown in the row of Comparative Example 5 in Table 1.

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

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

Example 7
In Example 1, the content of the olivine positive electrode material was changed to 93%, and the production of the positive electrode and the battery were evaluated in the same manner as in Example 1 except for that.

  Since the content of the olivine positive electrode material increased, the electrode density slightly decreased to 2 g / cc. Moreover, as a result of evaluating an electrode volume energy density, it was set to 271 mAh / cc and the discharge capacity maintenance factor was 0.94. These results are shown in the row of Example 7 in Table 1.

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

  Since the content of the olivine positive electrode material decreased, the electrode density slightly increased to 2.3 g / cc. Moreover, as a result of evaluating an electrode volume energy density, it was set to 230 mAh / cc, and the discharge capacity maintenance factor was 0.70. Since the content of the olivine positive electrode material was low, the desired high volume energy density could not be achieved. These results are shown in the row of Comparative Example 6 in Table 1.

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

  Since the content of the olivine positive electrode material was increased, the electrode density was reduced by peeling to 1.9 g / cc. Moreover, as a result of evaluating an electrode volume energy density, it was set to 208 mAh / cc, and the discharge capacity maintenance factor was 0.65. These 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 others were made in the same manner as in Example 1 to produce a positive electrode.

  Many aggregates were generated in the slurry, and a 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 was changed to 1.1 μm, and the ratio of the average secondary particle diameter to the average pit diameter was changed to 0.2. And the battery were evaluated. Since the average secondary particle diameter increased, the ratio of the average secondary particle diameter to the average pit diameter also increased slightly.

  Since the average secondary particle diameter of the olivine positive electrode material was increased, the electrode density was decreased to 1.9 g / cc. 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. These results are shown in the row of Comparative Example 9 in Table 1.

Example 8
In Example 8, instead of the olivine cathode material LiFePO ₄ produced in Example 1, an olivine cathode material represented by the composition formula LiMn _0.8 Fe _0.2 PO ₄ was produced. A manufacturing method will be described below.

Mix 7.2 g NH ₄ H ₂ PO ₄ , 2.27 g LiOH.H ₂ O, 9 g MnC ₂ O ₄ .2H ₂ O and 2.25 g FeC ₂ O ₄ .2H ₂ O did. Sucrose was added to this so that it might become 12 mass%, the ball | bowl for zirconia grinding | pulverization was thrown into the pot made from zirconia, and it mixed using the planetary ball mill. This mixed powder was put into an alumina crucible and pre-baked at 400 ° C. for 10 hours under an argon flow of 0.3 L / min.

The obtained calcined product was once crushed in an agate mortar, charged again 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 a composition formula LiMn _0.8 Fe _0.2 PO ₄ . .

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

  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 battery evaluation, 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. These results are shown in the row of Example 8 in Table 1.

[Comparative Example 10]
In Example 8, the surface roughness Ra of the aluminum base current collector was changed to 0.2 μm, and the other processes were performed in the same manner as in Example 1 to produce a positive electrode and evaluate the battery. In Comparative Example 10, 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 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 an 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 used for 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 (8)

A mixture layer including a positive electrode active material, a conductive material, and a binder; and a current collector formed on the surface of the mixture layer, wherein the positive electrode active material has a chemical formula Li _a M _x PO ₄ (M is Fe and Transition metal containing at least one of Mn. 0 <a ≦ 1.1, 0.9 ≦ x ≦ 1.1) In the positive electrode for a lithium ion secondary battery, which is a composite oxide having an olivine structure,
The conductive material includes 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 pit. The positive electrode for a lithium ion secondary battery according to claim 1,
The positive electrode active material has an average secondary particle size of 0.2 μm or more and 1 μm or less,
The positive electrode for a lithium ion secondary battery, wherein an average secondary particle diameter / average pit diameter, which is a ratio between the average secondary particle diameter and the average pit diameter of the pits, is 0.1 or more and 0.5 or less. 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, wherein the content of the positive electrode active material in the mixture layer is 90% or more and 93% or less by weight percentage. The positive electrode for a lithium ion secondary battery according to any one of claims 1 to 5,
A positive electrode for a lithium ion secondary battery having an electrode density of 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, wherein a plurality of lithium ion secondary batteries according to claim 7 are electrically connected.

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