JP5794753B2 - Positive electrode for secondary battery - Google Patents

Description

  The present invention relates to a positive electrode for a secondary battery.

A lithium secondary battery in which a negative electrode is formed using a material capable of occluding and releasing lithium ions can suppress the deposition of dendride compared to a lithium battery in which a negative electrode is formed using metallic lithium. Therefore, there is an advantage that a battery having a high capacity and a high energy density can be provided while safety is improved by preventing a short circuit of the battery.
In recent years, while further increasing the capacity of this lithium secondary battery is demanded, as a battery for power applications, there is a demand for improvement in large current charge / discharge performance accompanying reduction in battery resistance. In this regard, the lithium metal oxide positive electrode material and the carbon-based negative electrode material itself that are battery reactants in the past have been increased in capacity, or the particle size of these reactant particles has been reduced, the electrode surface area has increased due to particle specific surface area and battery design, Furthermore, some contrivances such as reduction of liquid diffusion resistance by thinning the separator have been made. However, on the other hand, the binder is increased by reducing the particle size and increasing the specific surface area. As a result, the capacity increases and the positive and negative electrode materials are peeled off from the metal foil as the current collector. A battery internal short circuit may occur, and the safety of the lithium secondary battery, such as battery voltage drop or heat runaway, may be impaired. Therefore, in order to increase the binding property with the foil, the binder type has been changed (Patent Document 1). In addition, there is a device that uses a carbon conductive material to reduce the electrode resistance with respect to the large current charge / discharge of a lithium ion battery (Patent Documents 2, 3, and 4).

However, the means of changing the binder that has been proposed so far can be increased in terms of capacity, but is insufficient in terms of improving large current charge / discharge characteristics by reducing resistance, such as a Nicad battery or a nickel metal hydride battery. Compared to a secondary battery, it is difficult to develop a power tool or a hybrid car that requires large current charge / discharge, which is a large performance barrier of a lithium secondary battery. In addition, when a charge / discharge cycle with a large current is repeated, the conductive path of the positive / negative electrode particles is damaged by the expansion / contraction of the positive / negative electrode material, and as a result, a large current cannot flow at an early stage. On the other hand, in recent years, olivine-type lithium iron phosphate has been attracting attention as a positive electrode material for lithium ion batteries from the viewpoint of safety and cost. However, this material has a large resistance and its low resistance is a big problem. Yes (literature numbers 5, 6).
JP-A-5-226004 JP-A-2005-19399 JP 2001-126733 A JP 2003-168429 A Special table 2000-509193 JP-A-9-134724 WO / 2007/013678

  The present invention was made in order to cope with the low resistance and large current charge / discharge of the olivine type lithium iron phosphate, and for the lithium secondary battery capable of maintaining the large current charge / discharge for a long period of time. An object of the present invention is to provide a positive electrode.

The positive electrode for a secondary battery of the present invention is a lithium comprising an electrode group formed by laminating or winding a negative electrode and a positive electrode via a separator, and an electrolyte solution in which the electrode group is immersed. in the secondary battery, the fibrous carbon is a carbon nanotube having an average diameter of 5 to 100 nm, a carbon black composite fibrous carbon and carbon black ing coupled, ash defined by JIS K 1469 is 1 A carbon black composite having a mass ratio of 0.0 mass% or less, a surface having a primary particle size of 10 to 150 nm and an electric resistance of 2 to 10 Ωcm when measured by a method defined in JIS K 1469 carbon film comprising the olivine-type lithium iron phosphate particles that are coated, a positive electrode for a lithium secondary battery, the carbon black double Body which is a positive electrode for a lithium ion secondary battery, characterized by containing 1 to 30 wt%.

The positive electrode for a secondary battery of the present invention is a lithium comprising an electrode group formed by laminating or winding a negative electrode and a positive electrode via a separator, and an electrolyte solution in which the electrode group is immersed. In the secondary battery, the fibrous carbon is a carbon nanotube having an average diameter of 20 nm, or a carbon nanotube having an average diameter of 20 nm and a vapor-grown carbon fiber having an average diameter of 100 nm, and the fibrous carbon and carbon black are connected, and An olivine-type iron phosphate having 2 % by mass of a carbon black composite having an ash content defined by JIS K 1469 of 1.0% by mass or less, a particle size of 80 nm, and an electrical resistance defined by JIS K 1469 of 5 Ωcm A positive electrode for a lithium ion secondary battery comprising lithium.

The positive electrode of the present invention improves the electron conduction network in the electrode, reduces the positive electrode resistance, and enables large current charge / discharge.
In addition, even if the positive electrode particles expand and contract during charge and discharge, the positive electrode plate maintains improved contact between the positive electrode particles and the conductive material, and can prevent a sudden decrease in capacity and output.

Hereinafter, the present invention will be described in detail.
The positive electrode for the lithium secondary battery of the present invention uses a carbon black composite as a conductive material, and this carbon black composite is composed of a combination of fibrous carbon and carbon black connected, The carbon black composite is characterized by having an ash content defined by JIS K 1469 of 1.0% by mass or less. Furthermore, the composition is contained in at least one of the case where the composite is composite-integrated with olivine-type lithium iron phosphate or the case where a carbon composite is mixed. It is not specified. The olivine-type lithium iron phosphate has a particle diameter of 10 to 150 nm, preferably 50 to 120 nm, and the electric resistance of the olivine-type lithium iron phosphate is defined by JIS K 1469. 2 to 10 Ωcm, preferably 3 to 6 Ωcm, and 1 to 30% by weight, preferably 1 to 10% by weight of a carbon black composite. This is a positive electrode for use. By the way, as a battery constituent material that greatly contributes to the resistance at the time of charging / discharging of the battery, a positive electrode is mainly used, and the positive electrode of the present invention improves the electron conduction network in the electrode and reduces the positive electrode resistance. , Large current charging / discharging becomes possible.
In addition, even if the positive electrode particles expand and contract during charge and discharge, the positive electrode plate maintains improved contact between the positive electrode particles and the conductive material, and can prevent a sudden decrease in capacity and output.
The electric resistance of the olivine-type lithium iron phosphate particles can be reduced by carbon coating or the like.

  The carbon black composite is obtained by connecting fibrous carbon and carbon black. The connection between fibrous carbon and carbon black is not just a contact, it means that the carbon is physically fused, and it is not easily separated by normal mechanical operation, but the connected fibrous Electrons can move freely between carbon and carbon black without contact resistance. Therefore, even when mixed with an active material, the carbon black composite remains as it is, and good dispersibility can be obtained and at the same time high conductivity can be maintained. When fibrous carbon alone is mixed with an active material or other materials, it is difficult to obtain good dispersibility due to orientation and entanglement between fibers, resulting in variations in conductivity. When carbon black and fibrous carbon are simply mixed, the shape is different and the variation further increases. However, one feature of the carbon black composite of the present invention is that it has excellent conductivity stability. Here, the fibrous carbon is preferably 1 to 50% by mass. When the fibrous carbon is less than 1% by mass, sufficient conductivity cannot be obtained, and when it exceeds 50% by mass, the connection with the carbon black is not sufficient, and at the same time, the dispersibility is due to aggregation of the fibrous carbon. It drops significantly.

The fibrous carbon used in the present invention is carbon fiber (carbon fiber), vapor-grown carbon fiber, carbon nanotube, carbon nanofiber, or the like. In the present invention, fibrous carbon can be selected as appropriate. However, since fibrous carbon effectively exchanges electrons with the active material, it is preferable that the fibrous carbon is smaller than the particle size of the active material. For example, when the average particle diameter of the active material is less than 20 μm, the average diameter is preferably 100 nm or less. The average diameter of the fibrous carbon is preferably 5 nm or more in order to give a certain level of strength.
Furthermore, it is also possible to use a combination of two or more types of fibrous carbon. For example, a combination of carbon nanotubes with an average diameter of 20 nm and vapor grown carbon fibers with a thickness of 100 nm provides a relationship between capillaries and arteries, and enables more effective electron exchange and movement. Also in this case, the fibrous carbon having the smaller average diameter is preferably smaller than the particle diameter of the active material, and the amount of fibrous carbon having the larger average diameter is 100 masses of fibrous carbon having the smaller average diameter. It is preferable that it is 100 mass parts or less with respect to a part.

  The carbon black used in the present invention maintains the conductivity of the entire electrode and plays a role as a buffer for expansion / contraction of the active material. For example, thermal black, furnace black, lamp black, channel black, acetylene Black etc. are mentioned. Among these, acetylene black is preferable because it has a reaction in a reducing atmosphere called thermal decomposition of acetylene gas, and therefore, when fibrous carbon is introduced and combined, there is little combustion loss.

In the method for producing a carbon black composite, fibrous carbon and carbon black are connected. Although the production method is not particularly limited, for example, a method of introducing and combining fibrous carbon during hydrocarbon pyrolysis, as described in WO / 2007/013678, during pyrolysis of acetylene gas And / or a method in which a hydrocarbon containing a fibrous carbonization catalyst is supplied and combined in a state where the acetylene gas is thermally decomposed. In addition, fibrous carbon and carbon black are dispersed in a carbonization raw material liquid such as hydrocarbon or alcohol, and carbonized by an operation such as heating in a liquid or gasified state of the carbonized raw material liquid. May be connected. This carbon black composite has an ash content defined by JIS K 1469 of 1.0% by mass or less. The ash is mainly composed of a catalyst at the time of fibrous carbon production, impurities metals (for example, Fe, Ni, etc.) and oxides thereof, and when the ash content exceeds 1.0% by mass, for example, a Li ion secondary battery, There is a risk that metal deposits on the negative electrode during charging and the charge / discharge capacity decreases, and there is a risk of ignition by breaking through the separator and short-circuiting.

  The separator which can be used for the lithium secondary battery using the positive electrode of the present invention is to electrically insulate the positive electrode and the negative electrode to hold the electrolytic solution. Examples of the separator include a synthetic resin, and specific examples thereof include polyethylene and polypropylene. It is preferable to use a porous film because the electrolyte retainability is good.

  In the lithium secondary battery using the positive electrode of the present invention, it is preferable to use a non-aqueous electrolyte containing a lithium salt or an ion conductive polymer as the electrolyte into which the electrode group is immersed.

Examples of the nonaqueous solvent for the nonaqueous electrolyte in the nonaqueous electrolyte containing a lithium salt include ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), and methyl ethyl carbonate (MEC). Is mentioned. Examples of lithium salts that can be dissolved in the nonaqueous solvent include lithium hexafluorophosphate (LiPF ₆ ), lithium borotetrafluoride (LiBF ₄ ), lithium trifluoromethanesulfonate (LiSO ₃ CF ₄ ), and the like. .

  Hereinafter, the positive electrode for a lithium secondary battery according to the present invention will be described in detail with reference to Examples and Comparative Examples. However, the present invention is not limited to the following examples unless it exceeds the gist. An example of the positive electrode and coin battery manufacturing method in the present invention is shown below.

<Preparation of positive electrode>
Olivine-type lithium iron phosphate was used as the positive electrode active material, and polyvinylidene fluoride was used as the binder. A positive electrode mixture (slurry) in which N-methylpyrrolidone was added and kneaded as a dispersion solvent was prepared. The positive electrode mixture slurry was applied to both sides of an aluminum foil having a thickness of 20 μm, dried, then pressed and cut to obtain a positive electrode for a lithium secondary battery.
However, primary olivine-type lithium iron phosphate particles having a primary particle size of 5, 10, 80, 150 and 200 nm were prepared, and those having a primary particle size of 80 nm were used unless otherwise specified. In addition, since the olivine type lithium iron phosphate alone has a large resistance, the particle surface is generally coated with conductive carbon to reduce the resistance. For this coating, a vapor deposition method using carbonized gas was used this time, but this method is not specified, and the electrical resistance value of the active material particles after carbon coating (JIS K 1469) is defined. Those having 1, 2, 5, 10 and 13 Ωcm were prepared and used for electrode preparation. Unless otherwise specified in the specification, 5 Ωcm as a reference was used in the implementation and comparative examples.
Next, a method for producing a carbon black composite that contributes to reducing the electrode resistance of the invention will be described. However, the type of carbon black or fibrous carbon used for the composite or the method for forming the composite is not limited to the following, as long as the gist of the present invention is not exceeded. In this example, acetylene black (average primary particle diameter 35 nm) was used as carbon black, and carbon nanotubes (average diameter 20 nm) were used as fibrous carbon. The following firing method was used as a method for producing the composite. A slurry obtained by mixing 1% by mass of acetylene black and carbon nanotubes in ethanol (reagent: purity 99.5%, manufactured by Kanto Chemical Co.) at a mass ratio of 1: 1 is 1500 ° C. under a flow of nitrogen (10 L / min) in an electric furnace. Was baked to obtain a carbon black composite. In addition, when the carbon black composite is simply mixed with a predetermined amount by weight of the carbon-coated olivine-type lithium iron phosphate, the carbon-coated olivine-type lithium iron phosphate is combined with the following calcination method. What was integrated was also used as Example 11 of the present invention. A slurry in which 1% by mass of a carbon black composite and olivine type lithium iron phosphate is mixed with ethanol (as described above) is baked at 600 ° C. in a flow of nitrogen (10 L / min) in an electric furnace so as to be combined and integrated. Got. Unless otherwise specified in the specification, 2% by mass of carbon black composite, 90% by mass of carbon-coated olivine-type lithium iron phosphate, and 8% by mass of polyvinylidene fluoride were used as standards. Example 12 was the same as Example 11 except that carbon nanotubes (average diameter 20 nm) and vapor-grown carbon fibers (average diameter 100 nm) were mixed as the fibrous carbon in a mass ratio of 1: 1. Was evaluated.

As shown in the table below, according to each electrode specification, the above-described positive plates of Examples to Examples and Comparative Examples to Comparative Examples and positive plates with various carbon black composite addition amounts were made as prototypes. And 2032 type coin battery was produced using each produced positive electrode plate. Metallic lithium was used as the counter electrode with respect to the positive electrode plate, and an olefin fiber non-woven fabric was used as a separator to electrically isolate them. As the electrolytic solution, a solution obtained by dissolving 1 mol / L of lithium hexafluorophosphate (LiPF6) in a solution in which EC and MEC were mixed at a volume ratio of 30:70 was used.
As a battery discharge performance test, after confirming that the charge / discharge efficiency was close to 100% after the initial charge of the battery, constant current discharge was performed up to 2.1 V at a current density of 0.7 mA / cm ^2. Was measured, and the capacity density (mAh / g) divided by the amount of the positive electrode active material was calculated. First, the effect of the carbon-coated olivine-type lithium iron phosphate positive electrode active material itself used in the present invention on the battery characteristics due to the specifications (primary particle size and active material electrical resistance) and the ash content specification of the carbon black composite itself Table 1 shows the test results.
Next, the discharge capacity density when various predetermined amounts of the carbon black composite are contained in the positive electrode plate is summarized in FIG. The discharge capacity test was performed as described above.
As a further cycle performance test, charging 4.1V (3.5mA / cm ² current limit) in the constant current constant voltage charging (0.175mA / cm ² at termination), the discharge is 3.5mA / cm ² constant current The capacity ratio with respect to the discharge capacity in the first cycle of the cycle test when 50 cycles were repeated with a pause of 10 minutes between them and 10 cycles between them was expressed as a discharge capacity retention rate and is shown in Table 2.

The physical properties of the carbon black composite were measured by the following method.
(1) The average diameter of the fibrous carbon of the carbon black composite was measured with a transmission electron microscope (TEM) at a magnification of 30,000, and the average value was taken as the average diameter.
(2) The ash content of the carbon black composite was measured according to JIS K 1469.

From the examples and comparative examples in Table 1, it can be seen that the present invention is excellent in both capacity density and capacity retention. Regarding the primary particle diameter of the olivine-type lithium iron phosphate of the positive electrode for lithium ion secondary batteries of the present invention, in the case of fine particles of 5 nm, the specific surface area theoretically increases, and the charge transfer of Li ions on the active material surface It is thought that the electrochemical reaction sites related to the increase, resulting in an increase in capacity, but as the particles become finer, the olivine crystal structure collapsed and an amorphous phase precipitated on a part of the surface. Moreover, since the particles were fine, the dispersibility of the coating slurry at the time of electrode coating deteriorated, and the unevenness of the coated surface was noticeable. As a result, the capacity tends to decrease. On the other hand, when the particle diameter is 200 nm, the electrochemical reaction sites described above are reduced, and thus the capacity tends to be reduced. The primary particle size is preferably 10 to 150 nm.
When the electric resistance of the olivine-type lithium iron phosphate particles is 1.5 Ωcm, the amount of the active material tends to decrease because the amount of the carbon coating in the active material mixture is large. On the other hand, in the case of 13 Ωcm with a small amount of carbon coating, although the amount of active material itself is large, the electric resistance is large, so that the voltage drop during battery discharge is large and the capacity tends to decrease. The battery resistance is preferably 2 to 10 Ωcm.
With respect to the carbon black composite, it is considered that the amount of ash, which is an impurity, is attributed to resistance and capacity because the carbon atom components are connected and combined. A thing with much ash content is considered that resistance increases, and there exists a tendency for a capacity | capacitance to fall. The ash content defined by JIS K 1469 of the carbon black composite is required to be 1.0% by mass or less.
Next, the blending ratio of the carbon black composite and olivine-type lithium iron phosphate was examined. FIG. 1 shows the capacity transition with respect to the blending mass ratio. As a result, from 1 to 5% by mass, the capacity of the positive electrode increases and becomes saturated due to a decrease in the resistance of the positive electrode, and gradually decreases from 5% by mass to 30% by mass. It turned out that it falls rapidly. Therefore, the blending ratio region is preferably 1 to 30% by mass, and more preferably 1 to 10% by mass.
Furthermore, the difference depending on the content of the carbon black composite of the present invention was examined, and as a comparative example, a comparison was made with a positive electrode in which carbon black was mixed with an olivine type lithium iron phosphate active material. From Table 1, the capacity can be maintained even with the carbon black alone of the comparative example, but the capacity reduction is larger than that obtained by mixing carbon black and a composite of fibrous carbon such as carbon nanotubes in the cycle life. I understood it. This is because, at the initial stage of charge / discharge, carbon black alone forms an electron conduction network between olivine-type lithium iron phosphate active materials, and the capacity is not reduced. However, if the charge / discharge cycle is repeated, the electron conduction network between the active materials is not maintained as it is due to the expansion and contraction of the active material, and the electron conduction contact between the carbon black and the active material changes, resulting in a collapse of the network and a decrease in capacity. I think that came. Compared to the mixture of the carbon black composite and the active material, the positive electrode plate formed by integrally combining the active material and the firing method is more excellent in capacity and cycle characteristics. From this, it is considered that the maintenance of the electron conduction network was achieved by the composite with the carbon black composite and further the active material.

The positive electrode for a lithium secondary battery of the present invention has a high capacity and can be repeatedly charged and discharged with a large current, and can be suitably used for applications requiring large current charging and discharging such as electric tools and hybrid cars. .

Relationship between carbon black composite content and capacity density in the positive electrode

Claims (3)

A carbon black composite fibrous carbon and carbon black ing coupled, carbon black composite ash is defined in JIS K 1469 is equal to or less than 1.0 wt%, primary particle size of 10~150nm , and the when the electrical resistance was measured by the method specified in JIS K 1469, contains an olivine-type lithium iron phosphate particles carbon film on the surface so that 2~10Ωcm is coated A positive electrode for a lithium secondary battery, comprising 1 to 30% by mass of the carbon black composite . The fibrous carbon is a carbon nanotube having an average diameter of 20 nm, the carbon black composite is 2% by mass, and the olivine-type lithium iron phosphate is contained by 90% by mass. Positive electrode for lithium ion secondary battery. The fibrous carbon is a carbon nanotube having an average diameter of 20 nm and a vapor-grown carbon fiber having an average diameter of 100 nm, the carbon black composite is 2% by mass, the average particle diameter is 80 nm, and the electric resistance is 5 Ωcm. 2. The positive electrode for a lithium ion secondary battery according to claim 1, comprising 90% by mass of olivine type lithium iron phosphate.

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