JP2010097813A - Positive electrode for lithium ion secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a positive electrode to be used in a lithium ion secondary battery, which satisfies a capacitance characteristic and an output characteristic sufficient for power assist in start-up, departure, and acceleration. <P>SOLUTION: The positive electrode for lithium ion secondary battery includes a current collector, and a positive electrode active material layer formed on the current collector, the positive electrode active material layer including a positive electrode active material, a conductive aid, and a binder. The positive electrode active material is composed of a single aggregate of primary particles, the porosity in the positive active material layer is 40 to 60 vol.%, and the average particle size (D50) of the single aggregate is 10 μm or more. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a positive electrode for a lithium ion secondary battery. More specifically, the present invention relates to a lithium ion secondary battery positive electrode including a positive electrode active material having a single-bond structure of primary particles.

  In recent years, air pollution by automobile exhaust gas has become a global problem. Under such circumstances, electric vehicles using electricity as a power source, hybrid vehicles that run by combining an engine and a motor, and fuel cell vehicles that use a fuel cell as a power source are attracting attention. The development of high energy density and high power density batteries mounted on them occupies an important industrial position. In addition, automobiles powered only by an engine have been put into practical use with vehicles equipped with a high-voltage battery that enables various electric devices to be installed. Secondary batteries such as lithium ion secondary batteries are considered to be suitable for such vehicles because of their high energy density and high discharge voltage, and various developments are underway.

  When a lithium secondary battery is put to practical use as a battery for a hybrid electric vehicle, good high-rate discharge characteristics and a large output are required. Therefore, in order to reduce the internal resistance of the battery and take out a large output, the electrode area to be stored in the battery is made as small as possible by reducing the electrode thickness, or the particle size of the positive electrode active material or the negative electrode material is reduced. Therefore, the specific surface area of the electrode was increased to increase the output of the battery.

  Thus, in a battery, in order to charge and discharge with a large current, it is necessary to reduce the particle diameter of the active material and increase its surface area. However, in the case of a positive electrode active material that is inferior in electronic conductivity to the negative electrode material, the number of active materials per unit mass in the positive electrode mixture increases as the particle size of the active material is reduced. Therefore, in order to increase the conductivity in the positive electrode mixture, a large amount of conductive auxiliary agent is required, and the amount of active material per unit volume is reduced by that amount, resulting in a problem that the battery capacity is reduced. .

In order to solve such problems, a technique is disclosed in which active material primary particles are sintered to produce active material secondary particles, and electron conductivity between the active material primary particles is secured (patent). Reference 1).
JP 2002-270173 A

  However, in the positive electrode disclosed in Patent Document 1, since the secondary particle diameter of the positive electrode active material in the positive electrode active material layer is small, sufficient electron conductivity cannot be ensured between the positive electrode active material secondary particles (electronic resistance). There is a problem that the overall resistance of the positive electrode is increased.

  The present inventor has intensively studied to solve the above problems. As a result, by using a positive electrode comprising a positive electrode active material primary particle having a particle size in a predetermined range and having a positive electrode active material layer in which the porosity is controlled in a predetermined range, It has been found that the above problems can be solved.

  According to the present invention, there is provided a positive electrode for a lithium secondary battery in which the overall resistance of the positive electrode is reduced by reducing the electronic resistance in the positive electrode active material layer in a range where the porosity in the positive electrode active material layer is high. I can do it.

  Hereinafter, several suitable embodiments of the lithium ion secondary battery electrode of the present invention will be described in detail with reference to the drawings as appropriate. Each drawing is exaggerated for convenience of explanation, and the dimensional ratio of each component in each drawing may be different from the actual one.

  The type of the battery of the present invention is not particularly limited. However, when the lithium ion secondary battery is distinguished by its form and structure, a conventionally known type such as a stacked (flat) battery or a wound (cylindrical) battery is known. It can be applied to any form / structure. Also, when viewed in terms of electrical connection form (electrode structure) in a lithium ion secondary battery, it applies to both the non-bipolar (internal parallel connection type) battery and the bipolar (internal series connection type) battery described above. It is possible. In the bipolar battery, a single cell voltage is higher than that of a normal battery, and a battery excellent in capacity and output characteristics can be configured. In addition, by adopting a laminated (flat) battery structure, long-term reliability can be secured by a sealing technique such as simple thermocompression bonding, which is advantageous in terms of cost and workability.

  FIG. 1 shows a schematic cross-sectional view of a flat (stacked) non-bipolar lithium ion secondary battery. In the lithium ion secondary battery 31 shown in FIG. 1, a laminate film in which a polymer-metal is combined with a battery exterior material 32 is used. Then, the entire peripheral portion is joined by heat fusion. The lithium ion secondary battery 31 includes a positive electrode plate, an electrolyte layer 35, and a negative electrode plate. The positive electrode plate has a positive electrode active material layer 34 (hereinafter simply referred to as “positive electrode active material layer”) on both sides of the positive electrode current collector 33 (one side for the lowermost layer and the uppermost layer of the power generation element). 34 ") is formed. In the negative electrode plate, negative electrode active material layers 37 are formed on both surfaces of the negative electrode current collector 36 (one side for the lowermost layer and the uppermost layer of the power generation element). The lithium ion secondary battery 31 has a configuration in which a power generation element 38 in which these are stacked is housed and sealed.

  In addition, the positive electrode (terminal) lead 39 and the negative electrode (terminal) lead 40 that are electrically connected to the electrode plates (positive electrode plate and negative electrode plate) are connected to the positive electrode current collector 33 and the negative electrode current collector 36 of each electrode plate. It is attached by sonic welding or resistance welding. Thus, it has the structure which is pinched | interposed into the said heat-fusion part and exposed to the exterior of said battery exterior material 32. FIG.

  FIG. 2 is a schematic cross-sectional view schematically showing the entire structure of a bipolar lithium ion secondary battery (hereinafter also simply referred to as a bipolar battery). The bipolar battery 2 shown in FIG. 2 has a structure in which a substantially rectangular battery element 21 in which a charge / discharge reaction actually proceeds is sealed inside a laminate sheet 29 that is an exterior. The positive electrode active material layer 22 and the negative electrode active material layer 23 were formed on the respective surfaces of the current collector 24 (the first current collector and the second current collector are the same one). A plurality of bipolar electrodes (not shown) are provided. Each bipolar electrode is stacked via an electrolyte layer 25 to form a battery element 21. At this time, each bipolar type is formed such that the positive electrode active material layer 22 of one bipolar electrode and the negative electrode active material layer 23 of another bipolar electrode adjacent to the one bipolar electrode face each other through the electrolyte layer 25. An electrode and an electrolyte layer 25 are laminated.

  The adjacent positive electrode active material layer 22, electrolyte layer 25, and negative electrode active material layer 23 constitute one unit cell layer 26. Therefore, it can be said that the bipolar battery 2 has a configuration in which the single battery layers 26 are stacked. In addition, an insulating layer 27 for insulating the adjacent current collectors 24 is provided on the outer periphery of the unit cell layer 26. The current collector (outermost layer current collector) (24a, 24b) located in the outermost layer of the battery element 21 has a positive electrode active material layer 22 (positive electrode side outermost layer current collector 24a) or a negative electrode only on one side. One of the active material layers 23 (the negative electrode side outermost layer current collector 24b) is formed.

  Current collector plates 24a 'and 24b' are provided on the outer sides of the outermost layer current collectors 24a and 24b. The current collector plates 24a 'and 24b' are extended to form a negative electrode tab 28b and a positive electrode tab 28a, respectively. Further, the current collector plates 24a 'and 24b' are formed thicker than the current collector so that the current from the plurality of stacked unit cell layers 26 can be easily taken out.

  And the electric power generation element 21 is sealed in the exterior material which consists of the laminate sheet 29 so that these positive electrode tabs 28a and negative electrode tabs 28b may lead out outside.

  Instead of the current collector plate, the outermost layer current collectors 24a and 24b may be electrically connected to the positive electrode tab 28a and the negative electrode tab 28b as they are. At this time, the outermost layer current collector (24a, 24b) and the tab (28a, 28b) may be electrically connected via a positive terminal lead and a negative terminal lead. Further, instead of the current collector plate, the outermost layer current collectors 24a and 24b may be thickened and directly extended outside the laminate sheet 29 to form the negative electrode tab 28b and the positive electrode tab 28a. Further, an electrode active material layer may be provided between the outermost layer current collectors 24a and 24b and the current collector plates 24a 'and 24b'. That is, the current collectors 24a ′ and 24b ′ dedicated to the outermost layer provided with the electrode active material only on one side are used as the current collector of the outermost layer as it is, instead of the current collector having the electrode active material on both sides. It is good. The basic configuration of the bipolar battery 2 can be said to be a configuration in which a plurality of stacked single battery layers (single cells) are connected in series.

  The following is a summary of each component.

[Current collector]
The current collector that can be used in the present invention is not particularly limited, and conventionally known current collectors can be used. For example, aluminum foil, stainless steel (SUS) foil, nickel-aluminum clad material, copper-aluminum clad material, SUS-aluminum clad material, or a plated material of a combination of these metals can be preferably used. Further, a current collector in which a metal surface is coated with aluminum may be used. Moreover, you may use the electrical power collector which bonded 2 or more metal foil depending on the case. When the composite current collector is used, as a material for the positive electrode current collector, for example, a conductive metal such as aluminum, an aluminum alloy, SUS, or titanium can be used, and aluminum is particularly preferable. On the other hand, as the material of the negative electrode current collector, for example, conductive metals such as copper, nickel, silver, and SUS can be used, and SUS and nickel are particularly preferable. Further, in the composite current collector, the positive electrode current collector and the negative electrode current collector may be electrically connected to each other directly or via a conductive intermediate layer made of a third material. In addition to the flat plate (foil), the positive electrode current collector and the negative electrode current collector are constituted by a lath plate, that is, a plate in which a mesh space is formed by expanding a plate with a cut. Can also be used. Although the thickness of a collector is not specifically limited, Usually, it is about 1-100 micrometers.

[Positive electrode]
A positive electrode for a lithium ion secondary battery according to the present invention includes a current collector and a positive electrode active material layer formed on the current collector and having a positive electrode active material, a conductive additive, and a binder. It is a secondary battery positive electrode, wherein the positive electrode active material is composed of a single bond of primary particles, the porosity in the positive electrode active material layer is 40% by volume to 60% by volume, and the single bond The average particle size (D50) of the body is 10 μm or more.

  As described above, in order to increase the reaction field, conventionally, attempts have been made to increase the output of the battery by using an electrode active material having a small particle size of the active material. As a result, the number of active materials per unit mass in the positive electrode mixture (including the positive electrode active material, the conductive additive, and the binder) increases. As a result, a large amount of conductive aid is required to increase conductivity, and a large amount of binder is required to bind a large number of active materials, thereby reducing the amount of active material per unit volume. Thus, the above problem has occurred.

  Therefore, if the amount of the binder, the conductive auxiliary agent, etc. is reduced, the properties such as the binding property and the conductive property are reduced accordingly. Therefore, after applying the positive electrode mixture to the current collector, it is necessary to press at a high pressure. It will be. In that case, the adhesion of the electrode constituent materials such as the electrode active material and the conductive auxiliary agent is improved, so that the electron conductivity is improved, but the filling ratio of the electrode active material in the electrode active material layer is increased, and the electrode active materials are not separated. The voids (holes) are crushed. Then, there arises a problem that it becomes difficult to sufficiently impregnate the electrolyte solution in the electrode active material layer. As a result, since the contact area between the electrolyte and the electrode active material is remarkably reduced and the diffusion resistance of Li ions is increased, there is a problem that it is difficult to obtain a high output secondary battery. On the other hand, if the pressing is performed under an excessively high pressure condition, the positive electrode active material layer containing the positive electrode mixture may be peeled off. In particular, this problem is caused in a lithium ion secondary battery using a non-aqueous electrolyte having a low Li ion conductivity compared to an electrolyte containing an aqueous solution and using a positive electrode active material having a low electronic conductivity compared to the negative electrode active material. This is particularly noticeable in the positive electrode. Therefore, in order to increase the output of the lithium ion secondary battery, it is an effective means to solve the above-mentioned problem occurring in the positive electrode of the lithium ion secondary battery.

The present invention has been completed in view of such problems, and is characterized mainly by the following points (1) and (2). That is,
(1) A positive electrode active material composed of “significantly large single bonds” of “primary particles having a small particle size” is used.

  By doing so, the surface area increases, the reaction field increases, and the single particles as the positive electrode active material become significantly larger due to the aggregation of the primary particles. Therefore, the amount of conductive auxiliary agent and binder can be reduced accordingly.

  (2) The porosity in the positive electrode active material layer is controlled within a predetermined range.

  By doing so, the contact area between the electrolyte and the electrode active material is significantly increased. This minimizes the diffusion resistance of Li ions (that is, improves diffusibility). Then, by controlling the upper limit value to a predetermined value, it is possible to improve the adhesion of electrode constituent materials such as an electrode active material and a conductive additive, and to significantly increase the electron conductivity.

  The positive electrode according to the present invention employing such a configuration mainly has the above characteristics. Therefore, as a secondary battery used for a power source for automobiles such as a hybrid, it is used for a lithium ion secondary battery with improved capacity characteristics and output characteristics sufficient for power assist at the time of starting, starting and accelerating.

(Positive electrode active material)
Although it does not restrict | limit especially as a composition (material) which can be used for the positive electrode active material which comprises the lithium ion secondary battery positive electrode of this invention, Preferably the lithium nickel cobalt manganese composite shown by following Chemical formula (1) It is an oxide.

  In the above formula, 0 <a ≦ 1.2, 0.3 ≦ b ≦ 0.9, 0 ≦ c ≦ 0.6, 0.25 ≦ d ≦ 0.6, 0 ≦ e ≦ 0.3, 1. 5 ≦ f ≦ 2.2 and 0 ≦ g ≦ 0.5. M is at least one selected from the group consisting of Al or Mg, Ca, Ti, V, Cr, Fe, and Ga, and X is at least one of F, Cl, and S. The composition of these positive electrode active materials can be measured by ICP, atomic absorption method, fluorescent X-ray method, chelate titration, and particle analyzer.

  FIG. 3 schematically shows a state in which a plurality of spherical primary particles 13 are collected to form a single bond 11. Of course, the shape of the primary particles is not particularly limited. Even if it is a cube shape, it may be a rectangular parallelepiped, an elliptical sphere, a needle shape, a plate shape, a square shape or a column shape, or may be a non-spherical shape or a non-cubic shape (see FIGS. 4 and 5). .

  In addition, as long as the effect of this invention is not impaired, the positive electrode active material layer may contain the positive electrode active material which exists as a primary particle.

(Total porosity)
The porosity (total porosity) in the positive electrode active material layer in the present invention is the total volume of the voids (voids) in the positive electrode active material layer with respect to the volume of the positive electrode active material layer.

  The porosity in the positive electrode active material layer in the electrode according to the present invention is 40% by volume to 60% by volume. When the porosity in the positive electrode active material layer is 40% by volume to 60% by volume, both electron conductivity and diffusivity are improved, and the battery is provided with a significantly improved capacity characteristic / output characteristic. An electrode can be provided. On the other hand, if it is less than 40% by volume, the effect of the present invention may not be achieved in terms of diffusibility. On the other hand, if it exceeds 60% by volume, the electronic resistance increases rapidly, the electronic conductivity cannot be secured, and as a result, a positive electrode used for a battery having excellent output characteristics and capacity characteristics cannot be provided. There is a risk of being connected.

  In the present invention, the porosity in the positive electrode active material layer is 40 to 60% by volume, preferably 40 to 50% by volume, more preferably 40 to 45% by volume. In such a range, it is preferable from the viewpoint of energy density per volume.

  The overall porosity control method and measurement method will be described in detail below.

(Average particle size)
<Average particle size of single bond>
The average particle diameter (D50) of the single bond in the positive electrode according to the present invention is 10 μm or more. If the average particle size (D50) of the single bond is less than 10 μm, the positive electrode active material layer may be peeled off. If it exceeds 50 μm, since the positive electrode active material is large, the positive electrode active material layer may be uneven, and a flat electrode may not be obtained. In addition, the average particle diameter (D50) of the single bond is preferably 10 to 25 μm, more preferably 10 to 20 μm, from the viewpoint of the degree of freedom of the electrode film thickness.

  The average particle diameter of such a single bond can be measured by, for example, SEM observation or TEM observation. The average particle diameter mentioned above is expressed by an absolute maximum length because the shape of the particles may not be uniform. Here, the absolute maximum length is an average of the maximum length L among the distances between any two points on the outline of the single bond. The value is an average value obtained from 10 single bonds.

  The method for controlling the average particle size of the single bond will be described in “Method for producing single bond” described later.

  In the present invention, the shape of the single bond of the positive electrode active material is formed in a substantially spherical shape, although the shape that can be taken varies depending on the type and manufacturing method. However, in addition to this, it may be formed into an indefinite shape close to a spherical shape, etc., and is not particularly limited, and any shape can be used without any problem, and it is possible to improve battery characteristics such as charge / discharge characteristics. It is desirable to select the shape appropriately.

<Average particle size of primary particles>
The average particle diameter (D50) of the primary particles constituting the single bond is preferably in the range of 0.3 to 10 μm, more preferably 0.3 to 10 m from the viewpoint of increasing the surface area contributing to the reaction. The range is 3.0 μm.

  As the measuring method of the average particle size of the primary particles, the measuring method of the above-mentioned “average particle size of the single bond” is similarly valid, and the description here is omitted. In addition, the control method of the average particle diameter of primary particle | grains is demonstrated in "the manufacturing method of a single bond" mentioned later.

(Intergranular porosity)
Intergranular porosity in the present invention, when it is assumed that the positive electrode active material is not a single bond (porous body), that is, when it is assumed to be formed only by primary particles, in the volume of the positive electrode active material layer, This is the volume ratio of the portion other than the positive electrode active material, the conductive additive and the binder (that is, intergranular voids).

  The porosity (intergranular porosity) between single bonds in the electrode according to the present invention is preferably 9 to 40% by volume. When the intergranular porosity is 9% by volume or more, the diffusibility is improved, and when it is 40% by volume or less, the contact resistance between single bonds is reduced (the electronic resistance is reduced). By controlling in such a range, it is possible to provide an electrode for use in a battery having significantly improved capacity characteristics and output characteristics. On the other hand, if it is attempted to control the intergranular porosity to less than 9% by volume, the positive electrode active material layer may peel off from the current collector (that is, the positive electrode active material layer may be collected depending on the roll press). May not be able to form on the body.) That is, if it is attempted to control the intergranular porosity to less than 9% by volume, the electrode may be peeled off from the foil and the battery may not be assembled. Moreover, there exists a possibility that collapse may arise in the shape of a single bond. On the other hand, if it exceeds 40% by volume, the contact resistance between the single bonds increases, and the electron conductivity may decrease.

  In the present invention, as described above, the intergranular porosity is preferably 9 to 40% by volume, more preferably 15 to 30% by volume, still more preferably 15 to 28% by volume, and particularly preferably 20 to 20%. It is in the range of 25% by volume. What is preferable in such a range is from the viewpoint of energy density per volume and electronic conductivity between grains.

  The control method and measurement method of the intergranular porosity will be described in detail below.

(Intragranular porosity)
The intragranular porosity in the present invention is the ratio of the volume of pores (that is, intragranular vacancies) in the volume of one single bond composed of a plurality of primary particles in the volume of the positive electrode active material layer. It is.

  The porosity (intragranular porosity) in the single bond in the electrode according to the present invention is preferably 20% by volume to 30% by volume. When the intragranular porosity is 20% by volume or more, a reaction field of a single bond is significantly ensured, which leads to an effect of improving electron conductivity. And if it controls to 40 volume% or less, a diffusivity will improve, Furthermore, the intensity | strength of a single bond will improve and it will lead to the effect of the capacity maintenance per volume. Therefore, the capacity | capacitance per unit volume is ensured, a capacity | capacitance characteristic improves, and by extension, the electrode used for the battery which improved the output characteristic significantly can be provided.

  On the other hand, if it is attempted to control the intragranular porosity to less than 20% by volume, the positive electrode active material layer may have a decrease in the electrolyte impregnation rate. If it exceeds 40% by volume, the energy density per volume in the electrode may be lowered.

  In the present invention, the intragranular porosity is 20% by volume to 30% by volume, but from the viewpoint of maintaining the capacity per volume, it is preferably in the range of 22-30% by volume, more preferably 24-26% by volume. It is.

<Measurement method of intragranular porosity>
Examples of the method for measuring the intragranular porosity include a mercury intrusion method. The mercury intrusion method is a method of theoretically calculating how much pressure is applied to how much mercury enters the pores. This is a method of measuring the size and volume of the pores on the sample surface by detecting changes in the surface of the liquid surface (that is, the amount of mercury entering the pores).

Specifically, a single bond according to the present invention is formed. This single bond is put into a measurement cell, the measurement cell containing this sample is filled with mercury, the inside of the cell is pressurized, and the mercury intrusion volume (W (cm ³ )) in the process is detected by capacitance. Detect with instrument. This is the pore volume of the single bond, and the value obtained by dividing the mass (g) of the single bond by the mercury intrusion volume (W (cm ³ )) is the intragranular porosity. In addition, the value of the intragranular porosity is an average value obtained from 10 single bonds.

  In addition, the control method of the said intragranular porosity is explained in full detail below.

<Method for producing single bond>
In the electrode of the present invention, the positive electrode active material is composed of a single bond of primary particles. The method for synthesizing a positive electrode active material having a single bond composed of primary particles of the present invention is a method in which single bonds (secondary particles) are formed so that a plurality of primary particles aggregate (aggregate). .

  For example, the following method is mentioned. That is, the lithium manganate powder is pulverized to an average particle size of 0.3 to 1.0 μm with a general pulverizer or the like. Preferably, D50 = 0.5 μm.

  Here, although there is no restriction | limiting in particular with a general grinder, A jet mill, a disk type mill, a vibration ball mill etc. are mentioned as an example.

  Subsequently, the powder is dissolved in water to produce a slurry. The viscosity is preferably adjusted to 100 cp to 1000 cp, more preferably 200 to 500 cp. The solid concentration is preferably adjusted to 10 to 50%, more preferably 20 to 40%. Put the slurry in an atomizer spray dryer. Fine particles (less than 10 μm) can be removed by a cyclone method or a sieve.

<Control method of overall porosity, intergranular porosity and intragranular porosity>
1. Control Method of Intragranular Porosity There are various methods for controlling the intragranular porosity. As an example, there may be mentioned a method of firing a single bond produced by the above method in an air atmosphere furnace. Specifically, it is preferably 800 to 900 ° C., preferably 5 to 15 hours, more preferably 830 to 860 ° C. for 7 to 10 hours by firing in an air atmosphere furnace to obtain a desired intragranular space. A single bond having a porosity can be obtained. Of course, a person skilled in the art can confirm whether or not the desired intragranular porosity has been achieved by using, for example, a mercury intrusion method. Since it can control to a porosity, it is not restrict | limited to these baking conditions.

2. Methods for controlling overall porosity and intergranular porosity There are various methods for controlling the overall porosity and intergranular porosity. An example will be described in detail below.

  First, an electrode is produced using a single bond whose intragranular porosity is controlled by the above method. Here, it is assumed that, for example, one having an intragranular porosity of 25 volume% is used.

  Subsequently, the positive electrode mixture density is calculated from the blending ratio. For example, when using 85% by mass of a single bond of Mn-based material, 10% by mass of hard carbon as a conductive additive, and 5% of PVdF as a binder (binder), the following formula (1):

It can ask for.

The composition of the active material is estimated by chemical analysis. The true density of the active material is estimated by XRF, XRD, XPS or the like. If Mn based active material, if 4.2 g / ^cm 3, Ni-based active material, if 4.8 g / ^cm 3, 3-way-based active material, 4.75 g / cm ^3, there in olivine For example, 3.5 g / cm ³ .

Here, for example, a: true density of lithium manganate powder = 4.2 (g / cm ³ ), b: true density of hard carbon (HC) = 1.5 (g / cm ³ ), c: of PVdF When the true density is 1.7 (g / cm ³ ), the positive electrode mixture density is 3.81 (g / cm ³ ).

On the other hand, when the positive electrode mixture amount per unit area is A (mg / cm ² ) and the thickness of the desired positive electrode active material layer in the thickness direction is B (μm),

And

And

It is.

More specifically, the content of the positive electrode single bond is 85% by mass, and the “positive electrode mixture amount per unit area” is 20.5 (mg / cm ² ). When the “porosity” is 51% by volume,

Then, solving the above equation, B = 110 μm.

  That is, B = 110 (μm) is the desired thickness in the thickness direction of the positive electrode active material layer.

  When the film thickness is determined by the above method, the porosity between single bonds is

It becomes.

  By the above operation, it is possible to control the whole porosity: 51% by volume, intergranular porosity: 30% by volume, and intragranular porosity: 25% by volume.

  As described above, desired “total porosity”, “intragranular porosity”, and “intergranular porosity” can be obtained.

<Measurement method of overall porosity, intergranular porosity and intragranular porosity>
On the other hand, the method of measuring the whole porosity, intergranular porosity, and intragranular porosity can also be performed by applying the above.

  Even after the positive electrode and the negative electrode are combined and formed as an electrode, the positive electrode can be taken out by disassembling the separator and the negative electrode. Further, even if the positive electrode active material layer constituting the positive electrode is impregnated with an electrolytic solution or the like, it is washed with a solvent such as dimethyl carbonate (DMC), diethyl carbonate (DEC), etc. It can also be taken out as a positive electrode comprising a positive electrode active material layer composed of an active material, a conductive additive and a binder, and a current collector.

  Thereafter, in order to measure the intragranular porosity, a plurality (for example, 10) of single-bonded particles are taken out, and the intragranular porosity is measured by the mercury pressure method described above.

  Next, the compounding ratio of the active material / conductive aid / binder is estimated. The ratio can be estimated by TG-MASS or the like. The binder estimates the components by applying the generated gas to a gas chromatograph. The true density can be estimated from the component. Moreover, about a conductive support agent (carbon), a crystal state and a shape are observed from XRD or SEM. Estimate the true density from the carbon used. From the above, the ratio and the true density of the active material, the conductive additive, and the binder to be formed can be understood. After that, as described above, the total porosity, intergranular porosity, and intragranular porosity can be found from the film thickness and mercury porosimetry results.

[Positive electrode active material content]
The positive electrode according to the present invention uses a positive electrode active material composed of a single bond of primary particles, and controls the average particle size of the single bond to a predetermined size. By doing so, it is possible to significantly reduce the amount of the conductive auxiliary for increasing the conductivity in the positive electrode mixture and the amount of the binder for increasing the binding property while increasing the reaction field. it can.

  When the content of the positive electrode active material is 84% by mass or more with respect to the total mass of all the components constituting the positive electrode active material layer, the capacity characteristics are improved and the battery is provided with significantly improved output characteristics. This is preferable in that an electrode can be provided. The positive electrode active material content is more preferably 90% by mass with respect to the total mass of all components constituting the positive electrode active material layer in that a positive electrode having further improved capacity characteristics and further improved output characteristics can be supplied. % Or more, more preferably 93% by mass or more, and particularly preferably 95% by mass.

[Binder]
The binder that can be used in the present invention is not particularly limited, and conventionally known binders can be used. Examples thereof include polyvinylidene fluoride (PVdF), styrene butadiene rubber (SBR) and CMC, acrylonitrile, and a synthetic rubber binder.

[Conductive aid]
The conductive auxiliary agent that can be used in the present invention is not particularly limited, and conventionally known ones can be used.

  A conductive assistant means the additive mix | blended in order to improve electroconductivity. Examples of the conductive auxiliary agent include carbon materials such as carbon black such as acetylene black, graphite, vapor-grown carbon fiber, and graphite. Moreover, metal powder can also be used suitably as a conductive support agent.

[Negative electrode active material layer]
The negative electrode active material layer includes a negative electrode active material. Other examples of the negative electrode material include electrolytes such as an electrolytic solution (electrolyte supporting salt and plasticizer), a polymer gel or a solid electrolyte (host polymer, electrolytic solution, etc.). Since the contents other than the type of the negative electrode active material are the same as described above, the description thereof is omitted here.

  As the negative electrode material active material, a negative electrode active material that is also used in a conventionally known solution-type lithium ion battery can be used. Specifically, a negative electrode mainly composed of at least one selected from carbon materials such as natural graphite, artificial graphite, amorphous carbon, coke and mesophase pitch carbon fiber, graphite, and hard carbon which is amorphous carbon. Although it is desirable to use an active material, it is not particularly limited. In addition, a metal oxide (particularly a transition metal oxide, specifically titanium oxide), a composite oxide of a metal (particularly a transition metal, specifically titanium) and lithium, or the like can also be used.

[Electrolyte layer]
The present invention can be applied to any of (a) a separator impregnated with an electrolytic solution, (b) a polymer gel electrolyte, and (c) a polymer solid electrolyte depending on the purpose of use. Among these, (a) is particularly preferable.

(A) Separator soaked with electrolyte (electrolyte)
Other positive electrode mixtures that can be used for the positive electrode active material layer constituting the positive electrode of the lithium ion secondary battery according to the present invention include an electrolyte solution (electrolyte supporting salt and plasticizer), a polymer gel or a solid electrolyte (host polymer). Electrolytes) and the like. The electrolyte of the present application is particularly preferably composed of an electrolytic solution (electrolyte supporting salt and plasticizer) from the viewpoint of permeability to a single bond.

  The electrolyte solution (electrolyte supporting salt and plasticizer) is not particularly limited, and various conventionally known electrolyte solutions can be used as appropriate.

For example, inorganic acid anion salts such as LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ , LiTaF ₆ , LiAlCl ₄ , Li ₂ B ₁₀ Cl ₁₀ , LiCF ₃ SO ₃ , Li (CF ₃ SO ₂ ) ₂ N, Li (C 2 _F 5 _{SO 2)} selected from organic acid anion salts such as 2 _N, wherein at least one lithium salt (electrolyte support salt), propylene carbonate, cyclic carbonates such as ethylene carbonate; dimethyl carbonate Chain carbonates such as methyl ethyl carbonate and diethyl carbonate; ethers such as tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, 1,2-dimethoxyethane, 1,2-dibutoxyethane; γ-butyrolactone, etc. Lactones; Nitto such as acetonitrile Lyls; Esters such as methyl propionate; Amides such as dimethylformamide; Plasticizers such as aprotic solvents (organic) mixed with at least one selected from methyl acetate and methyl formate Those using a solvent) can be used. However, it is not necessarily limited to these.

(Separator)
The separator is not particularly limited, and a conventionally known separator can be used. For example, a porous sheet made of a polymer that absorbs and holds the electrolytic solution (for example, a polyolefin-based microporous separator), a nonwoven fabric separator, or the like can be used. The polyolefin microporous separator having the property of being chemically stable to an organic solvent has an excellent effect that the reactivity with the electrolyte (electrolytic solution) can be kept low.

  Examples of the material for the porous sheet such as the polyolefin-based microporous separator include a laminate having a three-layer structure of polyethylene (PE), polypropylene (PP), PP / PE / PP, and polyimide.

  As a material of the nonwoven fabric separator, for example, conventionally known materials such as cotton, rayon, acetate, nylon, polyester, polypropylene, polyethylene such as polyethylene, polyimide, and aramid can be used. Depending on the purpose of use (such as mechanical strength required for the electrolyte layer), these are used alone or in combination.

  The bulk density of the non-woven fabric is not particularly limited as long as sufficient battery characteristics can be obtained by the impregnated polymer gel electrolyte. That is, if the bulk density of the nonwoven fabric is too large, the proportion of the non-electrolyte material in the electrolyte layer becomes too large, and the ionic conductivity and the like in the electrolyte layer may be impaired.

  The amount of the electrolytic solution impregnated in the separator may be impregnated to the range of the liquid retention capacity of the separator, but may be impregnated beyond the liquid retention capacity range. This can prevent impregnation of the electrolyte from the electrolyte layer by injecting a resin into the electrolyte seal portion, and therefore can be impregnated as long as the electrolyte layer can be retained. The electrolyte can be impregnated in the separator by a conventionally known method, for example, the electrolyte can be completely sealed after being injected by a vacuum injection method or the like.

[Insulation layer]
The insulating layer is mainly used in the case of a bipolar battery. This insulating layer is formed around each electrode for the purpose of preventing adjacent collectors in the battery from contacting each other and short-circuiting due to slight unevenness at the end of the laminated electrode. It is. In this invention, you may provide an insulating layer around an electrode as needed. When this is used as a vehicle driving or auxiliary power source, it is necessary to completely prevent a short circuit (liquid drop) due to the electrolytic solution. Furthermore, vibrations and impacts on the battery are loaded for a long time. Therefore, from the viewpoint of prolonging battery life, it is desirable to install an insulating layer in order to ensure longer-term reliability and safety, and to provide a high-quality, large-capacity power supply. .

  The insulating layer only needs to have insulating properties, sealing properties against falling off of the solid electrolyte, sealing properties against moisture permeation from the outside (sealing properties), heat resistance at the battery operating temperature, and the like. For example, epoxy resin, rubber, polyethylene, polypropylene, polyimide and the like can be used, but epoxy resin is preferable from the viewpoint of corrosion resistance, chemical resistance, ease of production (film forming property), economy, and the like.

[Positive electrode and negative electrode lead]
The positive electrode and the negative electrode lead are not limited to the bipolar type, and the same leads as those used in conventionally known non-bipolar lithium ion batteries can be used. In addition, the part taken out from the battery exterior material (battery case) does not affect the product (for example, automobile parts, especially electronic devices, etc.) by contacting with peripheral devices or wiring and leaking electricity. It is preferable to coat with a heat-resistant insulating heat-shrinkable tube or the like.

[Battery exterior material (battery case)]
Lithium-ion batteries are not limited to bipolar types, and the entire battery stack or battery winding body, which is the main body of the battery, is housed in a battery case or battery case in order to prevent external impact and environmental degradation during use. It is desirable to do. From the viewpoint of weight reduction, a polymer-metal composite laminate film in which both surfaces of metals (including alloys) such as aluminum, stainless steel, nickel, and copper are covered with an insulator (preferably a heat-resistant insulator) such as a polypropylene film. A conventionally known battery exterior material may be used. And it is preferable to make it the structure which accommodated and sealed the battery laminated body by joining a part or all of the peripheral part by heat sealing | fusion. In this case, the positive electrode and the negative electrode lead may be structured to be sandwiched between the heat-sealed portions and exposed to the outside of the battery exterior material. In addition, it is preferable to use a polymer-metal composite laminate film having excellent thermal conductivity because heat can be efficiently transmitted from a heat source of an automobile and the inside of the battery can be quickly heated to the battery operating temperature. The polymer-metal composite laminate film is not particularly limited, and a conventionally known film in which a metal film is disposed between polymer films and the whole is laminated and integrated can be used. As specific examples, for example, an outer protective layer made of a polymer film (laminate outermost layer), a metal film layer, a heat-sealing layer made of a polymer film (laminate innermost layer), and the whole is laminated and integrated. The thing which becomes. Specifically, in the polymer-metal composite laminate film used for the exterior material, a heat-resistant insulating resin film is first formed as a polymer film on both surfaces of the metal film, and heat fusion is performed on at least one heat-resistant insulating resin film. An insulating insulating film is laminated. Such a laminate film is heat-sealed by an appropriate method, whereby the heat-welding insulating film portion is fused and joined to form a heat-sealing portion. An example of the metal film is an aluminum film. Insulating resin films include polyethylene tetraphthalate film (heat insulation film), nylon film (heat insulation film), polyethylene film (heat fusion insulation film), polypropylene film (heat fusion insulation film). Can be illustrated. However, the exterior material of the present invention should not be limited to these. In such a laminate film, it is possible to easily and surely join one to one (bag-like) laminate films by heat fusion using a heat fusion insulating film by ultrasonic welding or the like. In order to maximize the long-term reliability of the battery, metal films that are constituent elements of the laminate sheet may be directly joined. Ultrasonic welding can be used to join the metal films by removing or destroying the heat-fusible resin between the metal films.

[External structure of lithium-ion battery]
FIG. 6 is a perspective view showing the appearance of a laminated flat (non-bipolar) lithium ion secondary battery which is a typical embodiment of the lithium ion battery according to the present invention.

  As shown in FIG. 6, the laminated flat lithium ion secondary battery 50 has a rectangular flat shape, and a positive electrode tab 58 and a negative electrode tab 59 for taking out electric power from both sides thereof. Has been pulled out. The power generation element (battery element) 57 is encased in the battery exterior material 52 of the lithium ion secondary battery 50. The surroundings are heat-sealed, and the power generation element (battery element) 57 is sealed with the positive electrode tab 58 and the negative electrode tab 59 pulled out to the outside. Here, the power generation element (battery element) 57 corresponds to the power generation element (battery element) 17 of the non-bipolar non-bipolar lithium ion secondary battery 10a shown in FIG. A plurality of single battery layers (single cells) 16 composed of a positive electrode (positive electrode active material layer) 12, an electrolyte layer 13, and a negative electrode (negative electrode active material layer) 15 are laminated.

  Note that the lithium ion battery of the present invention is not limited to the stacked flat shape as shown in FIG. 6, and the wound lithium ion battery may have a cylindrical shape. Good. Such a cylindrical shape may be deformed into a rectangular flat shape. In the said cylindrical shape thing, a laminate film may be used for the exterior material, and the conventional cylindrical can (metal can) may be used, for example, It does not restrict | limit.

  Further, the removal of the tabs 68 and 69 shown in FIG. 6 is not particularly limited, and the positive electrode tab 68 and the negative electrode tab 69 may be pulled out from the same side. The positive electrode tab 68 and the negative electrode tab 69 may be divided into a plurality of parts and taken out from each side, and the present invention is not limited to the one shown in FIG. Further, in a wound type lithium ion battery, instead of a tab, for example, a terminal may be formed using a cylindrical can (metal can).

  The following are mentioned as a use of the lithium ion secondary battery which concerns on this invention. For example, as a large-capacity power source for an electric vehicle (EV), a hybrid electric vehicle (HEV), a fuel cell vehicle, a hybrid fuel cell vehicle, etc., suitable for a vehicle driving power source or an auxiliary power source that requires high energy density and high output density Can be used. In this case, it is desirable that the assembled battery is constructed by connecting a plurality of lithium ion secondary batteries of the present invention. That is, in the present embodiment, a plurality of lithium ion secondary batteries according to the present invention are assembled using at least one of a parallel connection, a series connection, a parallel-series connection, or a series-parallel connection. ). This makes it possible to meet the demands for capacity and voltage for various types of vehicles by combining basic batteries. As a result, it becomes possible to facilitate the design selectivity of required energy and output. For this reason, it is not necessary to design and produce different batteries for various vehicles, and it becomes possible to mass-produce basic batteries and to reduce costs by mass production. Hereinafter, a representative embodiment of the assembled battery (vehicle submodule) will be briefly described with reference to the drawings.

  The assembled battery of the present invention is constituted by connecting a plurality of lithium ion batteries of the present invention. Specifically, at least two or more are used, and are configured by serialization, parallelization, or both. Capacitance and voltage can be freely adjusted by paralleling in series. In the assembled battery of the present invention, the non-bipolar lithium ion battery and the bipolar lithium ion battery of the present invention are used, and a plurality of these are combined in series, in parallel, or in series and in parallel. Can also be configured.

  7 is an external view of a typical embodiment of the assembled battery according to the present invention, FIG. 7A is a plan view of the assembled battery, FIG. 7B is a front view of the assembled battery, and FIG. 7C is an assembled battery. FIG.

  As shown in FIG. 7, an assembled battery 300 according to the present invention forms a small assembled battery 250 in which a plurality of lithium ion batteries of the present invention are connected in series or in parallel and can be attached and detached. A plurality of the detachable small assembled batteries 250 are further connected in series or in parallel. Thereby, the assembled battery 300 having a large capacity and a large output suitable for a power source for driving a vehicle and an auxiliary power source that require a high volume energy density and a high volume output density can be formed. 7A is a plan view of the assembled battery, FIG. 7A is a front view, and FIG. 7C is a side view. The small assembled battery 250 that can be attached and detached is connected to each other using an electrical connection means such as a bus bar, and the assembled battery 250 is stacked in a plurality of stages using a connection jig 310. How many non-bipolar or bipolar lithium-ion batteries are connected to create the assembled battery 250, and how many assembled batteries 250 are stacked to produce the assembled battery 300 depends on the vehicle mounted. What is necessary is just to determine according to the battery capacity and output of (electric vehicle).

  When the lithium ion secondary battery using the lithium ion secondary battery electrode of the present invention or the assembled battery formed by combining a plurality of them is used in a vehicle, a vehicle having the above-described effects can be obtained.

  The vehicle is not particularly limited. For example, in the case of a car, a hybrid car, a fuel cell car, an electric car (all are four-wheeled vehicles (passenger cars, commercial vehicles such as trucks and buses, light vehicles, etc.), and motorcycles (motorcycles). ) And tricycles). Further, it can be applied to various power sources for other vehicles such as a moving body such as a train, and can also be used as a mounting power source for an uninterruptible power supply.

  FIG. 8 is a conceptual diagram of a vehicle equipped with the assembled battery of the present invention.

  As shown in FIG. 8, in order to mount the assembled battery 70 on a vehicle such as the electric vehicle 80, the battery pack 70 is mounted under the seat at the center of the vehicle body of the electric vehicle 80. This is because if it is installed under the seat, the interior space and the trunk room can be widened. The place where the assembled battery 70 is mounted is not limited to under the seat, but may be a lower part of the rear trunk room or an engine room in front of the vehicle. The electric vehicle 80 using the assembled battery 70 as described above has high durability and can provide sufficient output even when used for a long period of time. Furthermore, it is possible to provide electric vehicles and hybrid vehicles that are excellent in fuel efficiency and running performance. A vehicle equipped with the assembled battery of the present invention can be widely applied to a hybrid vehicle, a fuel cell vehicle and the like in addition to an electric vehicle as shown in FIG.

  The effects of the embodiments of the present invention are summarized as follows.

  ・ The measured total resistance value is classified into electronic resistance, diffusion resistance, reaction resistance, etc. The electronic resistance described in the present invention indicates a difficulty in the flow of electrons. When the porosity in the positive electrode active material layer is increased, the conduction path is separated and the resistance is increased with a predetermined amount of the conductive auxiliary agent. Therefore, it is necessary to set the porosity as low as possible in the positive electrode active material layer. On the other hand, diffusion resistance indicates difficulty in ion flow. If the porosity in the positive electrode active material layer is reduced, the impregnation state of the electrolyte is poor and the diffusion resistance increases. Therefore, it is necessary to set the porosity as large as possible. In other words, electronic resistance and diffusion resistance are in a trade-off relationship. However, as shown in the examples, by controlling the grain size of the single crystal and the empty efficiency of the positive electrode within a predetermined range, the electronic resistance can be reduced even with the same empty efficiency as in the comparative example, so the overall resistance is reduced. Can be made.

  -Since the said positive electrode active material is comprised by the single bond of the primary particle, the surface area increases, the reaction field increases, and the battery which the capacity | capacitance characteristic and the output characteristic improved significantly can be provided.

  -Since the average particle size of the single bond is controlled within the predetermined range, the amount of the conductive auxiliary agent and the binder can be significantly reduced, and the battery having significantly improved capacity characteristics and output characteristics can be obtained. Can be provided.

  -Since the porosity in the positive electrode active material layer is controlled within the predetermined range, the diffusion resistance of Li ions is minimized (that is, the diffusibility is improved). Therefore, it is possible to provide a battery in which the adhesion of electrode constituent materials such as an electrode active material and a conductive additive is improved, the electron conductivity is significantly increased, and the capacity characteristics and output characteristics are significantly improved.

  That is, as a secondary battery used for a power source for automobiles such as a hybrid, it is possible to provide a battery with improved capacity characteristics and output characteristics sufficient to perform power assist during starting, starting, and acceleration.

  Hereinafter, the content of the present invention will be described with reference to examples and comparative examples, but the present invention is not limited to these examples.

1. Production of Electrode <Example 1>
Lithium manganate powder (LiMn ₂ O ₄ ) was pulverized with a jet mill to an average particle size of 0.3 to 1.0 μm, and primary particles having D50 = 1 μm were prepared.

  Subsequently, the powder was put into water to prepare a slurry. The viscosity was adjusted to 3000 cp and the solid content concentration was adjusted to 43%.

  The slurry was put into an atomizer spray dryer. Fine particles (less than 10 μm) were removed by a cyclone method or a sieve to prepare a single bond (D50 = 10 μm).

  The produced single bond was fired at 850 ° C. for 10 hours in an air atmosphere furnace, and the intragranular porosity was controlled to 25% by volume.

  As a positive electrode active material, 84% by mass of the single bond prepared above, 6% by mass of polyvinylidene fluoride (PVdF) as a binder, 10% by mass of hard carbon as a conductive additive, N-methyl-2-pyrrolidone (NMP as a solvent) ) To prepare a positive electrode mixture (solid content concentration: 43.4%).

  Specifically, NMP was charged into the dispersing mixer, and then PVdF was charged and sufficiently dissolved in the NMP solvent. Thereafter, the single bond and hard carbon were added little by little to make it familiar with the solvent in which PVdF was dissolved. When all of the single bond and hard carbon were added, a solvent was appropriately added to adjust the viscosity (10000 cp).

  The slurry obtained above was applied to one side of an Al foil (film thickness: 20 μm) as a positive electrode current collector using a coating apparatus.

At the time of application, the squeegee height of the coating apparatus was adjusted so that the basis weight per unit area was 20.5 mg / cm ² . The height of the squeegee depends on the paste viscosity and the solid content ratio. Punch the electrode after volatilizing the solvent, etc., and measure the weight of the electrode with the foil pulled. Were determined.

  Subsequently, the thickness of the positive electrode active material layer is adjusted by using a doctor blade having a constant thickness, and is dried on a hot stirrer, so that the total porosity is set to 54% by volume. By setting the film thickness (the desired thickness of the positive electrode active material layer of 117 μm) and pressing with a roll press, the intergranular porosity is set to 33%, and the positive electrode active material layer having a predetermined overall porosity is used. It was set as the comprised positive electrode.

<Example 2>
In order to set the overall porosity to 45% by volume, a target film thickness (desired thickness of the positive electrode active material layer is 98 μm) is set for the positive electrode active material layer, and the intergranular porosity is set to (23.8 volume). %) Except that the positive electrode was produced in the same manner as in Example 1.

<Example 3>
In order to set the total porosity to 60% by volume, a target film thickness (desired thickness of the positive electrode active material layer is 135 μm) is set for the positive electrode active material layer, and the intergranular porosity is set to (38.8 volume). %) Except that the positive electrode was produced in the same manner as in Example 1.

<Comparative Example 1>
In order to set the total porosity to 30% by volume, a target film thickness (desired thickness of the positive electrode active material layer 77 μm) is set for the positive electrode active material layer, and the intergranular porosity is set to 8.8 volumes. %) Except that the positive electrode was made in the same manner as in Example 1, but the battery could not be assembled with the negative electrode described later.

<Comparative example 2>
In order to set the total porosity to 34.3% by volume, a target film thickness (a desired positive electrode active material layer thickness of 82 μm) is set for the positive electrode active material layer, and the intergranular porosity is set to (13. A positive electrode was produced in the same manner as in Example 1 except that the volume was changed to 0% by volume.

<Comparative Example 3>
In order to set the total porosity to 38.0% by volume, a target film thickness (desired thickness of the positive electrode active material layer: 87 μm) is set for the positive electrode active material layer, and the intergranular porosity is set to (16. A positive electrode was produced in the same manner as in Example 1 except that the volume was 8% by volume.

<Comparative example 4>
In order to set the entire porosity to 69% by volume, a target film thickness (desired thickness of the positive electrode active material layer 174 μm) is set for the positive electrode active material layer, and the intergranular porosity is set to (47.8 volume). %) Except that the positive electrode was produced in the same manner as in Example 1.

<Example 4>
D50 = 0.5 μm was prepared, the viscosity was adjusted to 3500 cp, the solid content concentration was adjusted to 44%, baked at 850 ° C. for 10 hours, and the intra-granular porosity was controlled to 25% by volume. , Single bond 84, PVdF6, and HC4. In order to set the overall porosity to 45% by volume, the target film thickness (desired thickness of the positive electrode active material layer is 98 μm) is set, and the intergranular porosity is set to 24% by volume. A positive electrode was produced in the same manner as in Example 1 except that.

<Example 5>
D50 = 0.5 μm is prepared, the viscosity is adjusted to 3000 cp, the solid content concentration is adjusted to 45%, and calcined at 800 ° C. for 10 hours, and the intra-granular porosity is controlled to 35% by volume. In order to set the overall porosity to 56.5% by volume, a target film thickness (a desired positive electrode active material layer thickness 125 μm) is set for the positive electrode active material layer, and the intergranular porosity is set to (27 A positive electrode was produced in the same manner as in Example 1 except that the volume%).

<Comparative Example 5>
Lithium manganate powder (LiMn ₂ O ₄ ) D50 = 5 μm 84% by mass as a positive electrode active material, 6% by mass of polyvinylidene fluoride (PVdF) as a binder, 10% by mass of hard carbon as a conductive additive, N-methyl- A positive electrode mixture was prepared using 2-pyrrolidone (NMP) (solid content concentration 50.0%).

  Specifically, NMP was charged into the dispersing mixer, and then PVdF was charged and sufficiently dissolved in the NMP solvent. Thereafter, the single bond and hard carbon were added little by little to make it familiar with the solvent in which PVdF was dissolved. When all of the single bond and hard carbon were added, a solvent was appropriately added to adjust the viscosity (6000 cp).

  The slurry obtained above was applied to one side of an Al foil (film thickness: 20 μm) as a positive electrode current collector using a coating apparatus.

  Subsequently, the thickness of the positive electrode active material layer is adjusted using a doctor blade having a constant thickness, dried on a hot stirrer, and the total porosity should be set to 23% by volume (since it is not a single bond) And there is no intra-granular porosity), a positive active material having a predetermined overall porosity with a target positive active material layer thickness (desired positive active material layer thickness: 70.5 μm) It was set as the positive electrode comprised from a layer.

<Comparative Example 6>
In order to set the overall porosity (intergranular porosity) to 31% by volume, except for setting the target thickness of the positive electrode active material layer (the desired thickness of the positive electrode active material layer 78.5 μm), A positive electrode was produced in the same manner as in Comparative Example 5.

<Comparative Example 7>
In order to set the overall porosity (intergranular porosity) to 35% by volume, except for setting the target thickness of the positive electrode active material layer (the desired thickness of the positive electrode active material layer 83.5 μm), A positive electrode was produced in the same manner as in Comparative Example 5.

<Comparative Example 8>
Comparative Example, except that the target positive electrode active material layer (thickness of the desired positive electrode active material layer 95 μm) was set in order to set the overall porosity (intergranular porosity) to 43% by volume. A positive electrode was produced in the same manner as in Example 5.

<Comparative Example 9>
In order to set the overall porosity (intergranular porosity) to 55% by volume, except for setting a target film thickness (a desired positive electrode active material layer thickness of 120.5 μm), A positive electrode was produced in the same manner as in Comparative Example 5.

<Comparative Example 10>
Comparative Example, except that the thickness of the positive electrode active material layer (desired thickness of the positive electrode active material layer is 180 μm) was set to set the overall porosity (intergranular porosity) to 70% by volume. A positive electrode was produced in the same manner as in Example 5.

2. Production and Evaluation of Battery The positive electrode obtained above was die-cut with a φ16 mm punching jig, and using a separator (5 μm) with φ17 mm and a metal Li with a thickness of 1 mm and φ16 mm as the negative electrode, LiPF ₆ EC / PC / DEC A coin cell was prepared by adding an electrolytic solution and evaluated.

(1) Electronic resistance measurement About Examples 1-3 and Comparative Examples 1-10, the electronic resistance value was computed from the intersection with the real axis on the high-weeks side of the Nyquist plot display by the AC impedance method from SOC 50%.

(2) Total resistance measurement For Examples 1 to 3 and Comparative Examples 1 to 10, discharge was performed from an arbitrary SOC (50%) at a constant rate (1C) for a fixed time (10 seconds). R = V / I was calculated.

(3) Energy density measurement For Examples 4 and 5, the discharge capacity was measured as follows, and the energy density was measured. Charging up to 4.2V at 1.0C in CC-CV mode was performed for 1.5 hours and discharging up to 2.5V at 0.2C. The amount of electricity at that time was calculated.

The results are shown in the table below. The unit in the table is volume% for intergranular, intragranular, and overall porosity, and is Ωcm ² for electronic resistance and overall resistance, and is assigned to the active material capacity per unit. Is mAh / cm ³ .

  As described above, when an active material using a single bond is used, there is no void between the single bonds when the porosity is less than 40% by volume, there is an increase in diffusion resistance due to deterioration of diffusivity, and the overall resistance value is It is thought that it rose.

1 is a schematic cross-sectional view of a flat (stacked) non-bipolar lithium ion secondary battery. It is the schematic sectional drawing which represented typically the whole structure of the bipolar lithium ion secondary battery. A state in which a plurality of spherical primary particles are collected to form a single bond is schematically shown. This is a schematic representation of a state in which a plurality of elliptical primary particles gather to form a single bond. This is a schematic representation of a state in which a plurality of irregularly shaped primary particles are collected to form a single bond. 1 is a perspective view showing an appearance of a stacked flat (non-bipolar) lithium ion secondary battery that is a typical embodiment of a lithium ion battery according to the present invention. FIG. 7A is a plan view of the assembled battery according to the present invention, FIG. 7B is a front view of the assembled battery according to the present invention, and FIG. 7C is a side view of the assembled battery according to the present invention. It is a schematic diagram which shows the vehicle carrying the assembled battery of this invention.

Explanation of symbols

31 Non-bipolar lithium ion secondary battery,
32 Battery exterior material,
33 positive electrode current collector,
34 positive electrode active material layer,
35 electrolyte layer,
36 negative electrode current collector,
37 negative electrode active material layer,
38 Power generation elements,
39 Positive electrode (terminal) lead,
2 Bipolar battery,
21 battery elements,
22 positive electrode active material layer,
23 negative electrode active material layer,
24 current collector,
24a, 24b outermost layer current collector,
24a ', 24b' current collector plate,
25 electrolyte layer,
26 cell layer,
27 insulating layer,
28a positive electrode tab,
28b negative electrode tab,
29 Laminate sheet,
50 Battery exterior material (exterior package),
67 Power generation element (battery element; laminate),
68 positive electrode tab,
69 negative electrode tab,
60 lithium ion battery,
11 single bond,
13 primary particles,
250 small battery pack,
300 battery packs,
310 connection jig,
70 battery pack 80 electric car.

Claims (7)

A positive electrode for a lithium ion secondary battery, comprising: a current collector; and a positive electrode active material layer formed on the current collector and having a positive electrode active material, a conductive additive, and a binder,
The positive electrode active material is composed of a single bond of primary particles,
The porosity in the positive electrode active material layer is 40 vol% to 60 vol%, and
The positive electrode for lithium ion secondary batteries whose average particle diameter (D50) of the said single bond is 10 micrometers or more.   The positive electrode according to claim 1, wherein a porosity between single bonds is 9% by volume to 40% by volume.   The positive electrode according to claim 1 or 2, wherein a porosity in the single bond is 20% by volume to 30% by volume.   The positive electrode according to any one of claims 1 to 3, wherein a content of the positive electrode active material is 84% by mass or more based on a total mass of components constituting the positive electrode active material layer.   The lithium ion secondary battery which uses the positive electrode of any one of Claims 1-4.   An assembled battery comprising the lithium ion secondary battery according to claim 5.   A vehicle equipped with the lithium ion secondary battery according to claim 5 or the assembled battery according to claim 6 as a motor driving power source.

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