JP2012074246A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary battery having high capacity and high output characteristics simultaneously.SOLUTION: In the nonaqueous electrolyte secondary battery, a mixture of a positive electrode active material particle (A) and a positive electrode active material particle (B) is used as a positive electrode active material. The positive electrode active material particle (A) is represented as LiNiMnCoO(where, 1.05≤α≤1.15, 0.5≤x≤0.6 and 0.2≤y≤0.3), and has an average particle size of 10 to 20 μm and a specific surface area of 0.4 to 0.6 m/g, while the positive electrode active material particle (B) is represented by general formula: LiNiMnCoO(where, 1.05≤β≤1.10, 0.2≤s≤0.4 and s+t=1.0), and has an average particle size of 5 to 10 μm and a specific surface area of 0.8 to 1.2 m/g. Further, a specific surface area of the mixed positive electrode active material is maintained in a range of 0.6 to 1.0 m/g. Accordingly, a nonaqueous electrolyte secondary battery having high capacity and high output characteristics can be provided.

Description

  The present invention relates to a positive electrode for a non-aqueous electrolyte secondary battery and a non-aqueous electrolyte secondary battery.

  In recent years, development of lithium ion secondary batteries having high capacity density and high output characteristics has been promoted. Lithium-manganese composite oxides with high thermal stability are widely used as positive electrode active materials for in-vehicle lithium ion batteries. Further, it has been studied to increase the capacity of a battery by using a lithium / nickel composite oxide having a large capacity per unit weight as a positive electrode active material. However, the lithium / nickel composite oxide generates more heat in the charged state than the lithium / manganese composite oxide, and there is a problem in thermal stability.

  JP 2005-197004 (Patent Document 1) describes the development of an in-vehicle lithium ion battery using a lithium / manganese / nickel / cobalt composite oxide for the purpose of achieving both high capacity and thermal stability. Has been made. Lithium / nickel / manganese / cobalt composite oxide is attracting attention as a positive electrode active material for in-vehicle batteries because it can control capacity, output characteristics and thermal stability characteristics by the composition ratio of nickel, manganese and cobalt. A lithium / nickel / manganese / cobalt composite oxide (hereinafter referred to as Ni-rich positive electrode material) containing a large amount of Ni is considered promising because of its large capacity per weight.

JP 2005-197004 A

  Since the Ni-rich positive electrode material has a large amount of residual alkali component in the active material, the produced coating material is gelled and has poor stability and has a problem in mass productivity. However, as an attempt to reduce the residual alkali component, when the particle size of the active material is increased to reduce the specific surface area, the output characteristics of the battery are deteriorated.

  Therefore, an object of the present invention is to provide a vehicle-mounted nonaqueous electrolyte secondary battery having a large capacity and high output characteristics.

In the present invention, Li _α Ni _x Mn _(1-xy) Co _y O ₂ (where 1.05 ≦ α ≦ 1.15, 0.5 ≦ x ≦ 0 _{) is} used as a positive electrode active material of a nonaqueous electrolyte secondary battery. Positive active material particles A having an average particle size of 10 to 20 μm and a specific surface area of 0.4 to 0.6 m ² / g, represented by the following formula: 0.6, 0.2 ≦ y ≦ 0.3) Li _β Ni _(1-st) Mn _s Co _t O ₂ (where 1.05 ≦ β ≦ 1.10, 0.2 ≦ s ≦ 0.4), the average particle size is 5 to 10 μm, A mixture of positive electrode active material particles B having a specific surface area of 0.8 to 1.2 m ² / g is used. By adjusting the specific surface area of such a mixed positive electrode active material from 0.6 to 1.0 m ² / g, a non-aqueous electrolyte secondary battery having high capacity and high output characteristics can be provided.

  ADVANTAGE OF THE INVENTION According to this invention, a high capacity | capacitance and a high output nonaqueous electrolyte secondary battery can be provided.

The schematic sectional drawing of a cylindrical battery. The figure which shows the mixing process of a positive electrode active material. The conceptual diagram of a mixed positive electrode active material. The figure which shows the relationship between a capacity density and Ni composition ratio. The figure which shows the relationship between the specific surface area of a mixed positive electrode active material, and DCR.

  A hybrid that combines an electric vehicle (EV) and two types of driving sources (gasoline engine and motor) in a situation where global warming due to greenhouse gases such as carbon dioxide and the problem of exhaustion of fossil fuels are becoming more serious. Cars (HEV) are attracting attention. Recently, it is expected that a plug-in hybrid vehicle (PHEV) that can be charged, such as from a household outlet, will be put to practical use.

Such a lithium ion secondary battery used for in-vehicle use is required to have a high output both when the remaining amount of the battery is low and when it is high (in a wide state of charge (SOC)). Moreover, in order to lengthen the distance which can be drive | worked by one charge, it is important that the energy density of a battery is high.
In particular, EV travel distance is long in PHEV. Therefore, development of a lithium ion secondary battery having a high capacity density and high output characteristics is in progress.

  Various attempts have been made to achieve high capacity of nonaqueous electrolyte secondary batteries. For example, in the positive electrode, the capacity can be increased by increasing the density of the positive electrode mixture layer to increase the filling amount of the positive electrode active material, increasing the charging voltage, or the like. In the method of increasing the density of the positive electrode mixture layer, the permeability of the electrolytic solution to the positive electrode mixture layer is lowered, and there is a possibility that the battery characteristics are deteriorated due to an increase in direct current resistance (DCR). Moreover, the method for increasing the charging voltage has a problem in maintaining safety during high-voltage charging.

  As described above, although the Ni-rich cathode material having a high capacity per unit weight is considered promising, there is a problem that a large amount of residual alkali component is contained. When there are many alkali components, for example, when the pH is 11.0 or more, when polyvinylidene fluoride (PVdF) is used as the binder component of the slurry, a polymerization reaction occurs over time, gelation occurs, and the fluidity of the slurry decreases. Therefore, mass production becomes difficult. On the other hand, if the specific surface area is reduced by increasing the particle size of the active material in order to reduce the residual alkali component, the reaction area is decreased, which makes it difficult for current to flow and increases the apparent resistance value, thereby increasing the output of the battery. The characteristics will deteriorate.

  Accordingly, the present inventors formed a mixture containing a high-capacity Ni-rich positive electrode active material A and a high-power nickel-manganese-cobalt ternary positive electrode active material B as active materials on an Al current collector. A nonaqueous electrolyte secondary battery comprising a positive electrode, a negative electrode formed from a mixture containing a negative electrode active material on a Cu current collector, and an electrolytic solution was invented. By using lithium / nickel / manganese / cobalt composite oxide and controlling its powder properties as well as its composition, we have achieved a large capacity lithium-ion battery with excellent output characteristics. A lithium-nickel-manganese-cobalt composite oxide with a high nickel content and a high capacity, a positive electrode active material (positive electrode active material A) having a large particle size to reduce the effect of its residual alkali value, A positive electrode mixture of a lithium / nickel / manganese / cobalt composite oxide having a low alkali value and a high specific surface area by reducing the particle size is prepared. The specific surface area and the residual alkali value By defining the above, the capacity is large, and high output of the nonaqueous electrolyte secondary battery can be achieved.

  Furthermore, when the particle diameter of the positive electrode active material B is approximately ½ of the particle diameter of the positive electrode active material A, the positive electrode active material B / (positive electrode active material) can be used to increase the active material filling rate and increase the capacity. The weight ratio represented by the substance A + the positive electrode active material B) is preferably 20 to 50%, and more preferably 30 to 40%. In particular, in the mixture of the positive electrode active materials A and B, it is desirable to adjust the pH value (residual alkali value) of the mixture from pH 10.5 to 11.0. By using the remaining alkali component of the positive electrode active material at this level, (lithium hydroxide component) can be reduced and mass productivity can be improved.

The direct current resistance (DCR) of a battery at a low temperature tends to depend on a material having a high specific surface area. On the other hand, when the reaction area becomes too large, it is necessary to increase the content of the carbon conductive auxiliary agent for ensuring the electron conductivity, and the electrode density is lowered, which has an adverse effect on the capacity density. Therefore, the specific surface area of the positive electrode active material is preferably in the range of 0.6 to 1.0 m ² / g.

  A nonaqueous electrolyte secondary battery to which the present invention is applied includes a positive electrode including a mixture containing an active material on a current collector, a negative electrode including a mixture containing a negative electrode active material on a current collector, Has electrolyte. FIG. 1 shows a schematic sectional view of a cylindrical battery. Positive electrode and negative electrode current collector plates 5 and 6 are attached to an electrode group 8 wound around a resin-made shaft core 7 and housed in the battery container 1. In the electrode group 8, the negative electrode is connected to the negative collector plate 6 by welding or the like, and is electrically connected to the battery container 1 via the negative lead 10.

  After the electrode group 8 and the positive and negative current collecting plates 5 and 6 are housed in the battery container 1, the bottom of the battery container 1 and the negative electrode lead 10 are welded to the center of the shaft core 7 through a welding jig. Thereafter, an electrolytic solution is injected into the battery container 1. On the positive current collecting plate 5, there is an upper lid portion having conductivity provided so as to seal the opening of the battery container 1, and the upper lid portion is composed of an upper lid 3 and an upper lid case 4. One of the positive electrode leads 9 is welded to the upper lid case 4 and the other is welded to the positive electrode current collector component 5, whereby the upper lid portion and the positive electrode of the electrode group 8 are electrically connected. A gasket 2 is provided between the battery container 1 and the upper cover case 4, and the gasket 2 seals the opening of the battery container 1 and electrically insulates the battery container 1 and the upper cover case 4. Thereby, a secondary battery is comprised.

  The conductive auxiliary agent used for the positive electrode is not limited as long as it is a substance that can assist the transmission of electrons generated by the occlusion / release reaction of lithium ions in the positive electrode mixture to the positive electrode. Examples of the positive electrode conductive assistant include graphite and acetylene black. The binder used for the positive electrode can bind the active material and the conductive additive, and the mixture and the current collector, and is preferably not deteriorated by contact with the non-aqueous electrolyte. Examples of the positive electrode binder include polyvinylidene fluoride (PVdF) and fluororubber. Examples of the dispersion solution include N-methyl-2-pyrrolidone (NMP) and water.

  The negative electrode is composed of a negative electrode active material, a negative electrode binder, and a thickener. In the present invention, it is preferable to use hard carbon, soft carbon, and graphite carbon as the negative electrode active material, and among them, secondary batteries for plug-in hybrids and electric vehicles that require large capacity by using graphite carbon. Can be made.

  As the non-aqueous electrolyte, for example, it is preferable to use a non-aqueous electrolytic solution prepared by dissolving a lithium salt in the following non-aqueous solvent.

  As the solvent, for example, an aprotic organic solvent such as ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), methyl ethyl carbonate (MEC) or the like is used alone, or two or more kinds are mixed. It can be used as a mixed solvent.

Examples of the lithium salt related to the nonaqueous electrolyte include LiPF ₆ and LiBF ₄ . These electrolyte salts can be used alone or in combination of two or more.

The positive electrode active material A contained in the positive electrode mixture has a general formula Li _α Ni _x M n _y Co _(1-xy) O ₂ (where 1.05 ≦ α ≦ 1.15, 0.5 ≦ x ≦ 0.0. 6, 0.2 ≦ y ≦ 0.3), the specific surface area is 0.4 to 0.6 m ² / g, and the particle size is 10 to 20 μm. The positive electrode active material B is represented by the general formula _{Li β Ni s Mn t Co (} 1-st) O 2 ( where, 1.05 ≦ β ≦ 1.10,0.3 ≦ s ≦ 0.5,0.2 ≦ t ≦ 0.4), the specific surface area is 0.8 to 1.2 m ² / g, and the particle size is 5 to 10 μm. When these composite oxides are used as a mixture, the specific surface area of the mixed composite oxide as a whole is in the range of 0.6 to 1.0 m ² / g.

  FIG. 2 shows a process for mixing the positive electrode active material, and FIG. 3 shows a conceptual diagram for mixing the positive electrode active material. The positive electrode active material A and the positive electrode active material B were weighed, and a mixed positive electrode mixture was prepared through dry mixing in which only the active material was mixed. Since the positive electrode active material A has a Ni / (Ni + Mn + Co) ratio of 0.5 or more and 0.6 or less, it easily reacts with moisture in the atmosphere to generate lithium hydroxide (LiOH), and the pH is from 10.8. 11.0. On the other hand, since the Ni / (Ni + Mn + Co) ratio of the positive electrode active material B is 0.3 or more and 0.5 or less, the reaction with moisture is suppressed, and the pH is in the range of 10.3 to 10.6. The pH value is obtained by measuring the pH of a filtrate obtained by adding 1 g of an active material to 50 g of pure water and stirring the mixture for 10 minutes.

Since the positive electrode active material A (symbol: 31) has a high residual alkali value, it is necessary to reduce the specific surface area in order to reduce the reactivity, and is preferably 0.4 to 0.6 m ² / g. . In order to increase the filling amount of the positive electrode active material and increase the capacity, the average particle size of the positive electrode active material A (31) is desirably in the range of 10 to 20 μm. On the other hand, the positive electrode active material B (symbol: 32) has a low residual alkali value and can increase the specific surface area, so that the specific surface area was set to 0.8 to 1.2 m ² / g in order to improve output characteristics. . In order to increase the filling rate of the active material and increase the electrode density, the average particle size is desirably in the range of 5 to 10 μm so as to be 1/2 or less of the positive electrode active material A. When these are mixed (symbol: 33), both high capacity and high output characteristics can be achieved.

  Hereinafter, although details are described based on examples, the present invention is not limited to the following examples.

Example 1
In this example, an active material (average particle size: 15 μm) represented by the general formula Li _1.05 Ni _0.60 Mn _0.20 Co _0.20 O ₂ as the positive electrode active material A, and a general formula Li _1.05 Ni _0.34 as the positive electrode active material B. An active material average particle diameter represented by Mn _0.33 Co _0.33 O ₂ : 5 μm) was used. Positive electrode active material B / (positive electrode active material A + positive electrode active material B) = 1000 g of positive electrode active material A and 428.6 g of positive electrode active material B were weighed and mixed so as to satisfy 30%.

  In addition to the mixed positive electrode active material A and positive electrode active material B, 1428.6 g of the positive electrode active material and the conductive assistant so that the weight composition of the binder PVdF serving as the conductive auxiliary agent and binder satisfies 85: 9: 6. 151.3 g of the agent and 840.4 g of PVdF were weighed and mixed. Further, N-methyl-2-pyrrolidone (NMP) was added as an organic solvent and kneaded using a planetary mixer to prepare a paste-like mixture.

This mixture was applied to an aluminum foil having a thickness of 15 μm serving as a current collector at a coating amount of 100 g / m ² , and after removing the solvent by drying, both ends were cut off to a width of 54 mm with a slitter. Then, the positive electrode was produced by compressing with a hot roll press at a roll temperature of 60 ° C.

Using the obtained positive electrode, a 18650 small cylindrical prototype battery (18φ, height 65 mm) was prepared. The graphite negative electrode (56 mm wide) is composed of 97% graphite, 1.5% carboxymethylcellulose (CMC) and styrene butadiene rubber (SBR) on both sides of a 10 μm thick Cu foil, and has a coating amount of 50 g / m ² . The density was 1.5 g / cm ³ . As the electrolytic solution, a mixed solvent having a volume ratio of EC, EMC, and DMC of 2: 4: 4, respectively, having a concentration of 1 mol / l with LiPF ₆ as a solute was used. A separator having a thickness of 30 μm and a width of 58 mm manufactured by Ube Industries was used. The design capacity of the prototype battery was 600 mAh with 18650 battery specifications.

<< Examples 2 and 3, Comparative Examples 1 to 4 >>
The following examples and comparative examples in which the compositions of the positive electrode active materials A and B were changed were similarly weighed and mixed. Table 1 summarizes the configurations of the studied examples and comparative examples.

≪Performance evaluation method≫
The output characteristics are obtained from the relational expression of output = voltage difference / resistance. Therefore, the direct current resistance (DCR) was measured in order to remove the influence of the potential difference due to the change of the positive electrode active material. In order to eliminate the influence of the charging depth of the negative electrode, DCR was charged to 4.1 V at room temperature, discharged at −30 ° C. at 600 mA, 1200 mA, and 2400 mA, and the resistance value was obtained from the voltage at 10 seconds. Table 2 shows the results of measuring DCR for the batteries of Examples and Comparative Examples.

  In addition, the mixing ratio, specific surface area, and capacity density of the positive electrode active material B are shown.

  From the above results, it can be seen that in the battery of the example, the capacity density is high and the DCR is not large.

≪Performance evaluation results≫
FIG. 4 shows a characteristic diagram in which the capacity density (mAh / cm ³ ) is plotted against the Ni composition ratio based on the results obtained in Examples 1 to 3 and Comparative Examples 1 to 4. As is clear from FIG. 4, the capacity density tends to increase as the Ni content increases and the particle size increases.

Further, from the results obtained in Examples 1 to 3 and Comparative Examples 1 to 4, DCR (mΩ) was plotted against the specific surface area (m ² / g) of the positive electrode mixture of the positive electrode active material A and the positive electrode active material B. The characteristic diagram is shown in FIG. As can be seen from FIG. 5, the positive electrode active material A and the positive electrode active material B having a reduced particle size by controlling the physical properties of the powder are mixed to produce a positive electrode mixture. From 0.6 m ² / g to 1.6 m ² / g, it was possible to increase the specific surface area to 1.0 m ² / g, and it was confirmed that the DCR was lowered. In the region where the specific surface area is smaller than 0.6 m ² / g, the DCR tends to be high, and therefore the specific surface area is preferably in the range of 0.6 to 1.0 m ² / g.

  As described above, by using two types of lithium-nickel-manganese-cobalt composite oxides with different compositions and pH, the powder ratio and pH are controlled by setting the mixing ratio to a predetermined value. While maintaining or improving the capacity per volume as the positive electrode for a battery, the DCR can be lowered and the output can be improved.

DESCRIPTION OF SYMBOLS 1 Battery can 2 Gasket 3 Upper cover 4 Upper cover case 5 Positive electrode current collection component 6 Negative electrode current collection component 7 Axial core 8 Electrode group 11 Secondary battery 12 Positive electrode tab 13 Negative electrode tab 14 Positive electrode 15 Negative electrode 16 Positive electrode mixture 17 Negative electrode mixture 18a, 18b Separator 19 Tape 31 Positive electrode active material A (average particle size: 15 μm, specific surface area: 0.4 to 0.6 m ² / g)
32 Positive electrode active material B (average particle diameter: 5 μm, specific surface area: 0.8 to 1.2 m ² / g)
33 Positive electrode mixture (average particle size: 10 to 20 μm, specific surface area: 0.6 to 1.0 m ² / g)

Claims (5)

In a non-aqueous electrolyte secondary battery having a positive electrode including a positive electrode mixture layer including a positive electrode active material, a binder, and a conductive agent, a negative electrode, and a non-aqueous electrolyte,
The positive electrode active material has a specific surface area of 0.6 to 1.0 m ² / g;
The positive electrode active material has a general formula of Li _α Ni _x Mn _y Co _(1-xy) O ₂ (where 1.0 ≦ α ≦ 1.15, 0.5 ≦ x ≦ 0.6, 0.2 ≦ y). a positive electrode active material particles A represented by ≦ 0.3), the general formula _{Li β Ni s Mn t Co (} 1-st) O 2 ( where, 1.05 ≦ β ≦ 1.10,0.3 ≦ s ≦ 0.5, 0.2 ≦ t ≦ 0.4) and positive electrode active material particles B
The non-aqueous electrolyte secondary battery, wherein the positive electrode active material particles B have a particle size smaller than that of the positive electrode active material particles A. The nonaqueous electrolyte secondary battery according to claim 1,
The positive electrode active material particles A have an average particle size of 10 to 20 μm,
The nonaqueous electrolyte secondary battery, wherein the positive electrode active material particles B have an average particle size of 5 to 10 μm. The nonaqueous electrolyte secondary battery according to claim 1 or 2,
The positive electrode active material particles A have a specific surface area of 0.4 to 0.6 m ² / g,
The non-aqueous electrolyte secondary battery, wherein the positive electrode active material particle B has a specific surface area of 0.8 to 1.2 m ² / g. The nonaqueous electrolyte secondary battery according to any one of claims 1 to 3,
A nonaqueous electrolyte secondary battery, wherein the content (B / (A + B) wt%) of the positive electrode active material particles B contained in the positive electrode active material is 20 or more and 50% or less. A nonaqueous electrolyte secondary battery according to any one of claims 1 to 4,
The non-aqueous electrolyte secondary battery, wherein a pH value of the mixture of the positive electrode active materials A and B is pH 10.5 to 11.0.

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