JP6206259B2 - Method for producing positive electrode for lithium secondary battery, positive electrode for lithium secondary battery and granulated product - Google Patents

Description

  The present invention relates to a method for producing a positive electrode for a lithium secondary battery, a positive electrode for a lithium secondary battery, and a granulated product.

  In Japanese Patent Application Laid-Open No. 2008-112594 (Patent Document 1), a positive electrode mixture includes a positive electrode active material containing activated carbon, powdered graphite as a conductive material, flaky graphite, and amorphous carbon (carbon black). There is disclosed a lithium secondary battery characterized by having an adhesion material.

JP 2008-112594 A

  An in-vehicle lithium secondary battery is required to have excellent low temperature characteristics (large current characteristics in a low temperature environment). Conventionally, graphite particles such as flake graphite and amorphous carbon (that is, carbonaceous particles) such as acetylene black (AB) have been used as a conductive additive for the positive electrode in order to improve the low temperature characteristics of the lithium secondary battery. ing.

  Graphite particles are characterized by high conductivity derived from the crystal structure of graphite and a large average particle diameter. For this reason, the graphite particles have the advantage that the amount required to build a conductive network in the electrode is small. However, on the other hand, the graphite particles tend to have a low contact rate with the positive electrode active material depending on the particle shape. When the charge / discharge cycle is repeated, the expansion and contraction of the positive electrode active material cannot be followed, and the contact rate further decreases, leading to deterioration of charge / discharge characteristics. That is, when graphite particles are used as a conductive additive, there is a problem in cycle characteristics.

  On the other hand, although carbonaceous particles are low in conductivity compared to graphite particles, they are small-diameter particles having an average particle diameter of about 10 to 100 nm, and therefore can enter fine voids between the positive electrode active materials. Suitable for the construction of dense conductive networks. However, in order to construct such a dense conductive network, it is necessary to fill the carbonaceous particles not only between the voids between the particles but also between the positive electrode active material and the current collector. That is, when carbonaceous particles are used as a conductive additive, there is a problem that the necessary amount of conductive additive becomes excessive.

  For this reason, currently, a positive electrode is used in which graphite particles and carbonaceous particles are used in combination to compensate for each other's drawbacks and exhibit moderate characteristics. However, further improvements in large current characteristics, particularly low temperature characteristics are desired for in-vehicle lithium secondary batteries.

  Patent Document 1 proposes a method of improving the large current characteristics of a lithium secondary battery by using a third conductive additive called activated carbon in addition to graphite particles and carbonaceous particles. However, the use of the third conductive additive may cause a decrease in the mass and volume ratio of the positive electrode active material in the positive electrode, that is, a decrease in energy density. Even if the third conductive additive is added while maintaining the ratio of the positive electrode active material, the functions of the graphite particles and the carbonaceous particles are inevitably lowered.

If the filling rate (positive electrode density) of the positive electrode is increased, it is considered that the third conductive additive can be added while maintaining the volume energy density. This means a decrease in the amount of electrolyte solution retained in the positive electrode. Therefore, in this case, the diffusion of lithium ions (Li ⁺ ) in the positive electrode is hindered, resulting in deterioration of the low temperature characteristics.

  The present invention has been made in view of the above problems, and an object of the present invention is to provide a positive electrode for a lithium secondary battery excellent in low-temperature characteristics and cycle characteristics.

  The present inventor controls the arrangement of the positive electrode active material, the graphite particles and the carbonaceous particles in the positive electrode, so that the conductive network in the positive electrode can be used without the use of the third conductive aid and the decrease in energy density. The present invention has been completed by obtaining an idea that it is possible to construct a model and further research based on the idea. That is, the manufacturing method of the positive electrode for lithium secondary batteries of this invention is equipped with the following structures.

  (1) A method for producing a positive electrode for a lithium secondary battery includes a first step of mixing a positive electrode active material and carbonaceous particles to obtain a primary granulated product, and attaching graphite particles to the primary granulated product. A second step of obtaining a secondary granulated product, a third step of molding the secondary granulated product into a sheet-like molded product, and a fourth step of arranging the molded product on a current collector. .

  Conventionally, a manufacturing method (hereinafter also referred to as a “coating process”) in which a coating material (also referred to as slurry or paste) is applied to a current collector and dried is used to manufacture a positive electrode for a lithium secondary battery.

  Unlike the paint process, the method for producing a positive electrode for a lithium secondary battery of the present invention obtains a sheet-like molded body which is a positive electrode active material layer from a granulated powder containing a positive electrode active material (third step). Then, a process of arranging the molded body on the current collector is employed (fourth step). Further, the manufacturing method is different from the conventional one-step granulation process in which necessary materials are mixed and granulated at once, focusing on the shape and function of the conductive auxiliary agent, and granulating step by step. A two-stage granulation process is employed (first step and second step).

  According to the inventor's research, by adopting such a manufacturing method, it is easier to control the positional relationship between the positive electrode active material, the graphite particles, and the carbonaceous particles in the positive electrode than in the conventional process.

  First, the primary granulated product obtained in the first step is a granulated product obtained by mixing a positive electrode active material and carbonaceous particles. Here, since the carbonaceous particles are small-diameter particles, the contact ratio between the positive electrode active material and the carbonaceous particles (conducting aid) is easily ensured inside the primary granulated product. Further, since the carbonaceous particles are small-diameter particles, they can follow the expansion and contraction of the positive electrode active material accompanying the charge / discharge cycle.

  Next, the secondary granulated product obtained in the second step is a granulated product obtained by attaching graphite particles to the primary granulated product. That is, the secondary granulated product has graphite particles on its surface. Thereby, in a molded object (positive electrode active material layer), the conductive path between secondary granulated materials is ensured by highly conductive graphite particles (conductive auxiliary material). And unlike the case where the graphite particles are in direct contact with the positive electrode active material, the expansion and contraction of the positive electrode active material is buffered by voids in the primary granulated material, so that the primary granulated product accompanying the charge / discharge cycle and A decrease in the contact ratio with the graphite particles is suppressed.

  In addition, the conductive path between the secondary granulated product and the current collector in the positive electrode can be secured by the highly conductive graphite particles. Since the graphite particles have high conductivity and are large-diameter particles, the amount required for securing a conductive path with the current collector is small. Thereby, unnecessary increase of the conductive additive can be avoided.

  As described above, according to the method for manufacturing a lithium secondary battery of the present invention, the conductive path from the positive electrode active material to the current collector, that is, the conductive network is stably and efficiently built, and the cycle is performed without impairing the low temperature characteristics. Characteristics can be improved.

(2) The solid content concentration of the secondary granulated product is preferably 73% by mass or more.
Thereby, it can prevent that arrangement | positioning of each material in a secondary granulated material changes with time. And more preferably, the third step and the fourth step are performed while the state where the solid content concentration is 73% by mass or more is maintained. Thereby, a positive electrode can be manufactured, maintaining the arrangement | positioning of each material in a secondary granulated material stably. The solid content concentration of the secondary granulated product is more preferably 75% by mass or more.

  (3) The first step is a step of mixing the positive electrode active material, the carbonaceous particles, and the thickener to obtain a primary granulated product, and the second step is a process of graphitized particles and binding to the primary granulated product. It is preferable that the step is a step of attaching a material to obtain a secondary granulated product.

  As a result, a primary granulated product containing the positive electrode active material and the carbonaceous particles can be obtained more reliably, and a secondary granulated product having graphite particles attached to the surface of the primary granulated product can be obtained.

  (4) According to another aspect, the present invention is a granulated product used for a positive electrode for a lithium secondary battery, and the granulated product is a secondary granulated product in which a conductive layer is attached to the surface of the primary granulated product. The primary granulated product includes a positive electrode active material and carbonaceous particles, and the conductive layer includes graphite particles and a binder.

  The carbonaceous particles that are small-diameter particles can follow the expansion and contraction of the positive electrode active material. Therefore, the fall of the contact rate of the conductive support material and positive electrode active material accompanying a charging / discharging cycle can be suppressed. The binder and the graphite particles can constitute a conductive layer. Graphite particles have high conductivity. Therefore, when a positive electrode active material layer is formed from the granulated material, a stable conductive network is constructed between the granulated materials or current collectors.

  The conductive layer only needs to be attached to at least a part of the surface of the primary granulated product, and does not necessarily cover the entire surface.

  (5) According to still another aspect of the present invention, there is provided a positive electrode for a lithium secondary battery comprising the granulated product described in (4) above.

  In the positive electrode including the granulated material, a stable conductive network is constructed. Therefore, the positive electrode can be excellent in low temperature characteristics and cycle characteristics.

  ADVANTAGE OF THE INVENTION According to this invention, the positive electrode for lithium secondary batteries which is excellent in a low temperature characteristic and cycling characteristics is provided.

It is a flowchart which shows the outline of the manufacturing method of the positive electrode for lithium secondary batteries which concerns on one Embodiment of this invention. It is a schematic diagram which shows the structure of the primary granulated material concerning one Embodiment of this invention. It is a schematic diagram which shows the structure of the secondary granulated material concerning one Embodiment of this invention. It is a typical fragmentary sectional view which shows the structure of the positive electrode for lithium secondary batteries concerning one Embodiment of this invention. It is a schematic diagram which shows an example of a granulation apparatus. It is a mimetic diagram illustrating an example of the 3rd process. It is a mimetic diagram illustrating an example of the 4th process. It is a typical top view which shows an example of the positive electrode for evaluation concerning one Embodiment of this invention. It is a typical top view which shows an example of the negative electrode for evaluation concerning one Embodiment of this invention. It is a schematic diagram which shows an example of a structure of the battery for evaluation concerning one Embodiment of this invention. It is a typical top view which shows an example of a structure of the battery for evaluation concerning one Embodiment of this invention. It is a scatter diagram which shows an example of the relationship between the low temperature IV resistance in the lithium secondary battery concerning one Embodiment of this invention, and the capacity maintenance rate after a cycle. It is a scatter diagram which shows an example of the relationship between the solid content concentration at the time of positive electrode preparation in the lithium secondary battery concerning one Embodiment of this invention, and low temperature IV resistance. It is a scatter diagram which shows an example of the relationship between the compounding quantity of the graphite particle | grains in the positive electrode in the lithium secondary battery concerning one Embodiment of this invention, and low temperature IV resistance. It is a typical section perspective view showing an example of the composition of the lithium secondary battery concerning one embodiment of the present invention. It is a flowchart which shows the outline of the manufacturing method of the lithium secondary battery concerning one Embodiment of this invention.

  Hereinafter, one embodiment of the present invention (also referred to as “this embodiment”) will be described, but the present embodiment is not limited thereto. In the following description, the average particle diameter represents a median diameter (so-called d50) measured by a laser diffraction scattering method.

[First Embodiment: Method for Producing Positive Electrode for Lithium Secondary Battery]
FIG. 1 is a flowchart showing an outline of the manufacturing method of this embodiment. As shown in FIG. 1, the manufacturing method includes a first step S110, a second step S120, a third step S130, and a fourth step S140.

  Here, the first step S110 can be rephrased as a primary granulation step, the second step S120 as a secondary granulation step, the third step S130 as a forming step, and the fourth step S140 as an arrangement (crimping) step.

First, materials and a granulating apparatus used in this embodiment will be described.
(Positive electrode active material)
The positive electrode active material 2 is not particularly limited as long as it functions as a positive electrode active material of a lithium secondary battery. Examples of the positive electrode active material 2 include LiCoO ₂ , LiNiO ₂ , LiNi _a Co _b O ₂ (a + b = 1, 0 <a <1, 0 <b <1), LiMnO ₂ , LiMn ₂ O ₄ , LiNi _a Co _b. Mn _c O ₂ (a + b + c = 1, 0 <a <1, 0 <b <1, 0 <c <1), LiFePO _{4 and the} like can be exemplified.

  From the viewpoint of fluidity during granulation, the average particle diameter of the positive electrode active material 2 is preferably 3 μm or more and 20 μm or less, more preferably 5 μm or more and 18 μm or less, and particularly preferably 8 μm or more and 15 μm or less. Two or more kinds of materials may be used as the positive electrode active material 2.

(Carbonaceous particles)
The carbonaceous particle 4 has a function as a conductive additive. The carbonaceous particles 4 are so-called amorphous carbon and are particles made of a carbon material having no graphite crystal structure. Examples of the carbonaceous particles 4 include carbon blacks such as thermal black, acetylene black (AB), channel black, furnace black, lamp black, and ketjen black (registered trademark).

  From the viewpoint of securing the contact ratio with the positive electrode active material 2, the average particle diameter of the carbonaceous particles 4 is preferably 10 nm or more and 100 nm or less, more preferably 20 nm or more and 80 nm or less, and particularly preferably 30 nm or more and 50 nm. It is as follows. Two or more kinds of materials may be used as the carbonaceous particles 4.

(Graphite particles)
The graphite particles 6 have a function as a conductive additive. The graphite particles 6 are particles made of a carbon material having a graphite crystal structure. The graphite particles 6 may be natural graphite or artificial graphite. For example, flaky graphite, earthy graphite, spheroidized graphite, expanded graphite, and the like can be used. Of these, scaly graphite is particularly preferable from the viewpoint of conductivity.

  From the viewpoint of suppressing the amount of conductive aid used, the average particle size of the graphite particles 6 is preferably 1 μm to 20 μm, more preferably 5 μm to 18 μm, and particularly preferably 8 μm to 15 μm. is there.

(Thickener)
In this embodiment, a thickener (not shown) can be used at the time of granulation. A conventionally well-known thickener can be used for a thickener. For example, sodium carboxymethylcellulose (CMC), methylcellulose (MC), sodium alginate, polyvinyl alcohol (PVA), polyethylene oxide (PEO), polyvinylpyrrolidone (PVD) and the like can be used. Among these, CMC is particularly preferable because of easy handling.

(Binder)
The binder 8 has a function of adhering the secondary granulated materials 10 to each other or the secondary granulated material 10 and the current collector 101 in the molded body 102. In the secondary granulated product 10, the conductive layer 10 b is configured together with the graphite particles 6.

  As the binder 8, for example, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), polyvinyl butyral (PVB), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), or the like can be used. Among these, a fibrous binder such as PTFE is preferable from the viewpoint that the conductive layer 10b is easily formed.

(solvent)
The solvent may be appropriately selected in consideration of, for example, the thickener or the dispersibility of the binder 8. Usable solvents include, for example, water (including ion-exchanged water and ultrapure water), N-methylpyrrolidone (NMP), dimethylformamide (DMF), dimethyl sulfoxide (DMSO), tetrahydrofuran (THF), acetone, diethyl ether , Benzene, hexane, toluene and the like.

(Granulating device)
A conventionally known powder mixing device can be used as the granulating device. For example, like the granulation apparatus 50 shown in FIG. 5, the thing provided with the container 52, the main stirring blade | wing 54 (agitator), and the crushing blade | wing 56 (chopper) is suitable. In the granulating apparatus 50, the particle diameter or structure of the granulated product can be controlled by adjusting the number of rotations of the main stirring blade 54 and the crushing blade 56, the stirring time, the material charging timing, and the like. An example of such an apparatus is “High Speed Mixer” manufactured by Earth Technica Co., Ltd.

Hereinafter, each step will be described.
<First step: primary granulation step>
In 1st process S110, the positive electrode active material 2 and the carbonaceous particle 4 are mixed, and the primary granulated material 10a is obtained. Specifically, the primary granulated material 10a can be obtained by measuring these materials, putting them into the containers 52 of the granulating apparatus 50, and stirring and mixing them under predetermined conditions.

  In the first step S110, it is preferable to mix a thickener (not shown) in addition to the positive electrode active material 2 and the carbonaceous particles 4. And after mixing these powder materials by a dry process, it is more preferable to add a solvent and perform granulation. Thereby, as shown in FIG. 2, it is easy to obtain a primary granulated product 10 a in which the contact ratio between the positive electrode active material 2 and the carbonaceous particles 4 is ensured.

  Here, the amount of the solvent is preferably adjusted so that the solid content concentration of the secondary granulated product 10 described later is 73% by mass or more. When the solid content concentration is less than 73% by mass, the arrangement of the material in the granulated product cannot be stably maintained, and the material arrangement in the granulated product is changed in a later process, and the desired low-temperature characteristics are exhibited. This is because there may be no case. The solid content concentration is more preferably 75% by mass or more, and particularly preferably 80% by mass or more. The upper limit of the solid content concentration is not particularly limited, but is, for example, 90% by mass or less from the viewpoint of reducing the load (torque) during granulation.

<Second step: secondary granulation step>
In 2nd process S120, the graphite particle 6 is made to adhere to the primary granulated material 10a obtained by 1st process S110, and the secondary granulated material 10 is obtained. The method for adhering the graphite particles 6 to the primary granulated material 10a is not particularly limited. For example, by bringing the primary granulated material 10a into contact with the graphite particles 6 and the binder 8, primary granulation is performed. The conductive layer 10b including the graphite particles 6 and the binder 8 can be attached to the surface of the object 10a. Specifically, after the primary granulated product 10a is obtained in the granulating apparatus 50, a predetermined amount of the graphite particles 6 and the binder 8 are charged and mixed under predetermined conditions to perform secondary granulation. Granules 10 can be obtained.

  Here, the binder 8 may be in the form of powder, but from the viewpoint of increasing the coverage with the conductive layer 10b, it is preferable that the binder 8 is previously dispersed in a solvent and dropped into the apparatus together with the solvent.

<Third step: molding step>
In the third step S <b> 130, the powder of the secondary granulated product 10 is formed into a sheet-like molded body 102. Here, the molding method is not particularly limited, and a conventionally known molding method can be employed. For example, as shown in FIG. 6, a sheet-like molded body 102 can be obtained by compressing and stretching the powder of the secondary granulated product 10 between two rolls.

  The basis weight and thickness of the molded body 102 are the load applied to the molded body 102 (secondary granulated product 10) from the first roll 62a and the second roll 62b, and the first roll 62a and the second roll 62b. By adjusting the gap (gap) with the roll 62b, it can be controlled within a desired range.

<4th process: Arrangement process>
In the fourth step S <b> 140, the sheet-like molded body 102 is disposed on the current collector 101. The method for disposing the molded body 102 on the current collector 101 is not particularly limited, and a conventionally known method can be employed. For example, as shown in FIG. 7, the molded body 102 and the current collector 101 are supplied between two rolls, and they are pressed with a roll so that the molded body 102 and the current collector 101 are pressed against each other. The molded body 102 can be pressure-bonded to the current collector 101.

In addition, after 4th process S140 is performed, drying operation using a hot air etc. may be performed.
As the current collector 101, a conventionally known material can be used. For example, Al foil or Al alloy foil is preferably used.

  By sequentially executing the first step S110 to the fourth step S140 as described above, a positive electrode for a lithium secondary battery excellent in low temperature characteristics and cycle characteristics is manufactured.

[Second Embodiment: Granulated Product]
2nd Embodiment is the granulated material used for the positive electrode for lithium secondary batteries. As described above, the granulated product is manufactured by executing the first step S110 and the second step S120.

  FIG. 3 is a schematic diagram showing the granulated product (that is, secondary granulated product 10) of the present embodiment. With reference to FIG. 3, the granulated product of this embodiment is a secondary granulated product 10 in which a conductive layer 10b is attached to the surface of the primary granulated product 10a, and the primary granulated product 10a is a positive electrode active material. 2 and carbonaceous particles 4, and the conductive layer 10 b includes graphite particles 6 and a binder 8.

  In the primary granulated product 10a, since the carbonaceous particles 4 can follow the expansion and contraction of the positive electrode active material 2, contact between the carbonaceous particles 4 and the positive electrode active material 2 can be ensured even during cycle durability. Further, by forming the conductive layer 10b including the highly conductive graphite particles 6, in the molded body 102 including the secondary granulated product 10, between the secondary granulated products 10 and the secondary granulated product 10 and A good conductive network can be constructed with the current collector 101.

  The primary granulated product 10a preferably further contains a thickener such as CMC. This is because the shape stability of the primary granulated product 10a and the secondary granulated product 10 is increased.

  Here, as long as the conductive layer 10b adheres to at least a part of the surface of the primary granulated product 10a, it can contribute to the construction of a conductive network. Therefore, the conductive layer 10b only needs to adhere to at least a part of the surface of the primary granulated product 10a, and does not necessarily have to cover the entire surface.

  From the viewpoint of fillability in the molded body 102, the average particle size of the secondary granulated product 10 is preferably 10 μm or more and 300 μm or less, more preferably 20 μm or more and 200 μm or less, and particularly preferably 30 μm or more and 150 μm or less. . The average particle diameter of the secondary granulated product 10 can be adjusted according to the conditions during granulation.

  As described above, the solid content concentration of the secondary granulated product 10 is preferably 73% by mass or more. Here, the charging ratio of each component constituting the solid content can be set as follows. This is because the low temperature characteristics and cycle characteristics can be improved without lowering the energy density in the following manner.

  The proportion of the positive electrode active material 2 in the solid content is, for example, 80% by mass or more and 99% by mass or less, and preferably 85% by mass or more and 95% by mass or less.

  The proportion of the carbonaceous particles 4 in the solid content is, for example, 1% by mass or more and 5% by mass or less, and preferably 2% by mass or more and 4% by mass or less.

  The proportion of the graphite particles 6 in the solid content is, for example, 1% by mass to 5% by mass, preferably 2% by mass to 4% by mass, and more preferably 2% by mass to 3% by mass. Especially preferably, it is 2.5 mass% or more and 3 mass% or less. In the present embodiment, since the graphite particles 6 are disposed in the outer layer (conductive layer 10b) of the granulated product, a sufficient conductive effect can be exhibited even with such a small amount of addition.

  The proportion of the thickener in the solid content is, for example, 1% by mass to 5% by mass, preferably 1% by mass to 4% by mass, and particularly preferably 1% by mass to 3% by mass.

  The proportion of the binder in the solid content is, for example, 0.5% by mass or more and 3% by mass or less, preferably 0.5% by mass or more and 2.5% by mass or less, and particularly preferably 0.5% by mass. More than 2.0 mass%.

[Third Embodiment: Positive Electrode for Lithium Secondary Battery]
3rd Embodiment is a positive electrode for lithium secondary batteries provided with the granulated material which concerns on 2nd Embodiment. For the production of the positive electrode, the production method according to the first embodiment (that is, the first step S110 to the fourth step S140) is preferably used.

  FIG. 4 is a schematic partial cross-sectional view illustrating the cross-sectional structure of the positive electrode 100 for a lithium secondary battery according to this embodiment. Referring to FIG. 4, positive electrode 100 includes current collector 101 and formed body 102 that is a positive electrode active material layer on current collector 101. And the molded object 102 contains the some secondary granulated material 10. FIG.

  As shown in FIG. 4, in the molded body 102, the secondary granules 10 are in close contact with each other via the conductive layer 10b. The conductive layer 10b includes a binder 8 and highly conductive graphite particles 6. Furthermore, the contact between the positive electrode active material 2 and the carbonaceous particles 4 is ensured inside the secondary granulated product 10 (primary granulated product 10a). Therefore, a conductive network is constructed that spreads from the positive electrode active material 2 and the carbonaceous particles 4 to another secondary granulated product 10 or the current collector 101 through the conductive layer 10b. This realizes conductivity that can withstand large current charging and discharging at low temperatures.

  The positive electrode 100 can exhibit the above-described low-temperature characteristics and cycle characteristics as long as the secondary granulated product 10 is included, and other granulated products (for example, the primary granulated material having no conductive layer 10b). It may contain granules 10a) and the like.

  It can be confirmed by a conventionally known method that the positive electrode 100 includes the secondary granulated product 10 of the present embodiment. For example, a cross-section sample of the positive electrode 100 is prepared using a cross-section processing apparatus such as a cross-section polisher apparatus (CP) or a focused ion beam apparatus (FIB), and the cross-section sample is observed with an electron microscope (SEM) and an electron beam microanalyzer. By performing the elemental analysis by (EPMA), the structure of the granulated material contained in the positive electrode 100 can be confirmed.

  For example, when a fluorine (F) -containing resin is used as the binder 8 as in PTFE, the conductive layer 10b containing PTFE and the graphite particles 6 is mapped by EPF mapping of fluorine (F). The existence can be confirmed.

  Also, the binder can be identified by a conventionally known method. For example, a part of the positive electrode active material layer (molded body 102) is peeled off from the current collector 101, a sample obtained by solvent extraction is subjected to Fourier transform infrared spectroscopy (FT-IR) analysis, and the FT-IR spectrum is obtained. The binder can also be identified from the above.

[Fourth Embodiment: Lithium Secondary Battery]
4th Embodiment is a lithium secondary battery provided with the positive electrode for lithium secondary batteries which concerns on 3rd Embodiment. Hereinafter, although a cylindrical battery will be described as an example, the present embodiment may be a prismatic battery or a pouch battery (also referred to as a laminate battery).

  FIG. 15 is a schematic cross-sectional perspective view showing an example of the configuration of the lithium secondary battery of the present embodiment. As shown in FIG. 15, the lithium secondary battery 1000 includes an electrode body 400 and an electrolytic solution (not shown) inside a cylindrical exterior body 500. The electrode body 400 includes a positive electrode 100, a negative electrode 200, and a separator 300. The positive electrode 100 and the negative electrode 200 are long belt-like sheet members. The electrode body 400 is configured by winding the separator 300 so that the positive electrode 100 and the negative electrode 200 face each other. The separator 300 is a microporous film made of, for example, polyethylene (PE) or polypropylene (PP).

  Here, in the positive electrode 100, the positive electrode active material layer is formed of a molded body 102 including the secondary granulated product 10. Therefore, the lithium secondary battery 1000 can be excellent in low temperature characteristics and cycle characteristics.

  The configuration of the negative electrode 200 is not particularly limited. As the configuration of the negative electrode 200, for example, a negative electrode active material layer containing a carbon-based negative electrode active material such as natural graphite, artificial graphite, or coke or an alloy-based negative electrode active material such as silicon or tin on a negative electrode current collector such as a Cu foil. The thing provided with can be illustrated.

The component of the electrolytic solution is not particularly limited. Examples of the electrolyte include cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), γ-butyrolactone (GBL) and vinylene carbonate (VC), dimethyl carbonate (DMC), ethyl For example, LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ , Li (CF ₃ SO ₂ ) ₂ N, LiCF ₃ SO ₃ are mixed in a mixed solvent composed of chain carbonates such as methyl carbonate (EMC) and diethyl carbonate (DEC). What dissolved Li salts, such as, can be used. A solid or gel electrolyte can also be used instead of the electrolyte.

[Fifth Embodiment: Manufacturing Method of Lithium Secondary Battery]
5th Embodiment is a manufacturing method of a lithium secondary battery including the manufacturing method of the positive electrode for lithium secondary batteries which concerns on 1st Embodiment.

  FIG. 16 is a flowchart showing an outline of a method for manufacturing the lithium secondary battery of the present embodiment. As shown in FIG. 16, the manufacturing method includes a step S100, a step S200, a step S300, a step S400, a step S500, and a step S600. Here, step S100 is a method for manufacturing a positive electrode for a lithium secondary battery according to the first embodiment.

  In step S200, the negative electrode 200 is produced. For example, a negative electrode paint containing a negative electrode active material can be produced by a conventional coating process, and the negative electrode paint can be applied onto a negative electrode current collector to produce a negative electrode.

  In step S300, the electrode body 400 is produced. The electrode body 400 is obtained by winding or laminating so that the positive electrode 100 and the negative electrode 200 face each other with the separator 300 interposed therebetween. Next, the electrode body 400 is inserted into the exterior body 500 (step S400). Then, the lithium secondary battery 1000 can be manufactured by injecting an electrolytic solution into the outer package 500 (step S500) and further sealing the outer package 500 (step S600).

  Hereinafter, although an example is given and this embodiment is described in detail, this embodiment is not limited to these.

  In the following manner, a positive electrode was produced via a paint or a granulated material by changing to various conditions and forming a paint, one-stage granulation, or two-stage granulation. Then, an evaluation battery (lithium secondary battery) using the positive electrode was produced, and the low-temperature characteristics (IV resistance in a −10 ° C. environment) and the cycle characteristics were evaluated.

  Hereinafter, the production method that passes through the coating process is called “Process A”, the production method that goes through one-step granulation is called “Process B”, and the production method that goes through two-step granulation is called “Process C”.

<Common items>
First, common items in the following examples and comparative examples will be described. Unless otherwise specified in the following description, these materials, conditions, manufacturing methods, and the like are used in the examples and comparative examples. In the following description, the average particle diameter is a median diameter measured by a laser diffraction scattering method.

(Material used for positive electrode)
[1] Cathode Active Material LiNi _0.33 Co _0.33 Mn _0.33 O ₂ powder (average particle size: 10 μm) was used as the cathode active material.

[2] Carbonaceous particles Acetylene black (AB) powder [average particle diameter: 35 nm, (product name “DENKA BLACK”, manufactured by Denki Kagaku Kogyo Co., Ltd.), which is amorphous carbon, was used as the carbonaceous particles. .

[3] Graphite particles As graphite particles, scaly graphite powder [average particle size: 3.6 μm (variety “KS-4”, manufactured by Timical)] was used.

[4] Thickening material CMC (variety “BSH-6”, manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) was used as the thickening material.

[5] Binder The PTFE (product number “D-210C”, manufactured by Daikin Industries, Ltd.) dispersed in an aqueous solvent (hereinafter referred to as “PTFE aqueous dispersion”) was used as the binder.

[6] Solvent Ion exchange water was used as the solvent.

(Preparation of negative electrode)
The negative electrode used for the evaluation battery is natural graphite, CMC as a thickener (variety “BSH-6”, manufactured by Daiichi Kogyo Seiyaku Co., Ltd.), and styrene butadiene rubber (SBR) as a binder (JSR shares) The negative electrode paint obtained by mixing natural graphite: CMC: SBR = 100: 1: 1 (mass ratio) and kneading these in water is applied onto the Cu foil. Made.

(Configuration of evaluation battery)
With reference to FIGS. 8 to 11, an evaluation battery 80 (laminated battery having a rated capacity of 16 mAh) was produced.

[1] Positive electrode for evaluation The positive electrode for evaluation 81 was produced by cutting out the positive electrode obtained in each of Examples and Comparative Examples described later to a predetermined size. Referring to FIG. 8, a positive electrode for evaluation 81 is obtained by forming a positive electrode active material layer 81b on one surface (main surface) of an Al foil 81a (current collector). The specific configuration is as follows.

Weight per unit area of the positive electrode active material layer 81b: 14 mg / cm ²
The thickness of the positive electrode active material layer 81b: 50 μm
Area of positive electrode active material layer 81b: 400 mm ² (vertical 20 mm × horizontal 20 mm)
The thickness of the Al foil 81a: 15 μm
Current collector: A positive electrode current collector lead 81c (with a resin welded portion) was welded to a portion (5 mm long × 5 mm wide) from which the positive electrode active material layer 81b was peeled to form a current collecting portion.

[2] Negative electrode for evaluation The negative electrode for evaluation 82 was prepared by cutting the negative electrode described above into a predetermined size. Referring to FIG. 9, evaluation negative electrode 82 is one in which negative electrode active material layer 82 b is formed on both surfaces (main surfaces) of Cu foil 82 a (current collector). The specific configuration is as follows.

Weight per unit area of negative electrode active material layer 82b (value on one side): 7.5 mg / cm ²
Negative electrode active material layer 82b thickness (single-sided value): 60 μm
Area of negative electrode active material layer 82b: 441 mm ² (length 21 mm × width 21 mm)
Cu foil 82a thickness: 10 μm
Current collector: Cu negative electrode current collector lead 82c (with resin welded portion) was welded to the portion (5 mm long × 5 mm wide) from which negative electrode active material layer 82b was peeled to form a current collecting portion.

[3] Evaluation Separator As the evaluation separator 83, a separator (thickness 20 μm) having a three-layer structure of PP / PE / PP was used.

[4] Evaluation Electrolytic Solution The evaluation electrolytic solution is a solution in which a Li salt is dissolved in a mixed solvent [LiPF ₆ = 1.0 mol / L, EC: DMC: EMC = 3: 4: 3 (volume ratio)]. Using.

[5] Assembly Referring to FIG. 10, laminate film 85, evaluation separator (not shown), evaluation positive electrode 81, evaluation negative electrode 82, evaluation separator 83, evaluation positive electrode 81, evaluation separator (FIG. Not shown), and a laminate film 85 were laminated in this order. Subsequently, the peripheral part of the laminate film 85 was heat-welded using the commercially available heat sealer, leaving the space which can be poured. Further, an electrolytic solution for evaluation (2 ml) was injected, and the space left for the injection was thermally welded while degassing the inside under reduced pressure.

(Evaluation method for low temperature characteristics and cycle characteristics)
The evaluation battery 80 was evaluated as follows. The unit of current value “It” is assumed to indicate a current value for discharging the rated capacity of the battery in one hour.

[1] Finishing charging / discharging In a 25 ° C. environment, five charging / discharging cycles with a constant current of 0.2 It (3.2 mA) and an upper limit voltage of 4.2 V and a lower limit voltage of 3.0 V were executed. The discharge capacity at the fifth cycle was set as the initial capacity.

[2] Evaluation of low-temperature characteristics The state of charge (SOC) of the evaluation battery 80 was adjusted to 60% in a room temperature environment. Next, the evaluation battery 80 was stored in a thermostat set at −10 ° C. for 5 hours. And in this thermostat, the discharge for 10 second was performed with the constant current of 5.0 It (80 mA), and the amount of voltage drops was measured. And IV resistance in a -10 degreeC environment was computed from the relationship between a discharge current value and a voltage drop amount.

[3] Cycle characteristics A charge / discharge cycle having an upper limit voltage of 4.2 V and a lower limit voltage of 3.0 V at a constant current of 5.0 It was executed 500 times. After 500 cycles, a charge / discharge cycle with an upper limit voltage of 4.2 V and a lower limit voltage of 3.0 V was performed again at a constant current of 0.2 It, and the post-cycle discharge capacity was measured. Then, the capacity retention rate was calculated by dividing the discharge capacity after the cycle by the initial capacity.

<Process A (coating): Comparative Examples 1-3>
(Comparative Example 1)
The following operations [1] to [6] were sequentially performed to produce an evaluation battery, and the low temperature characteristics and cycle characteristics were evaluated. The results are shown in Table 1.

[1] LiNi _0.33 Co _0.33 Mn _0.33 O ₂ , scaly graphite, AB, and CMC were put into a commercially available planetary mixer, and dry mixed for a predetermined time.

  [2] Ion exchange water (a part of the required amount) was charged into the planetary mixer and kneaded for 30 minutes.

  [3] The remaining ion-exchanged water was put into a planetary mixer and kneaded for 10 minutes.

  [4] A PTFE aqueous dispersion was charged into the planetary mixer, and kneaded while performing degassing under reduced pressure for 10 minutes to obtain a paint.

  [5] Using a comma coater (registered trademark), the paint was coated on an Al foil (current collector), dried, and further rolled using a roll rolling mill to obtain a positive electrode.

  [6] The positive electrode was cut into a predetermined size, an evaluation battery was prepared according to the above-described method, and the low-temperature characteristics and cycle characteristics were evaluated.

  Here, in Comparative Example 1, the solid content concentration (concentration of Non-Volatile contents: NV (hereinafter sometimes abbreviated as “NV”)) of the paint was 60% by mass. Moreover, the charging ratio of the solid content of the paint is as follows: positive electrode active material (91 parts by mass), carbonaceous particles (3 parts by mass), graphite particles (3 parts by mass), thickener (2 parts by mass), binder ( 1 part by mass).

(Comparative Examples 2 and 3)
As shown in Table 1, except that the charging ratio of the solid content was changed, a positive electrode was obtained in the same manner as in Comparative Example 1, an evaluation battery was prepared, and the low temperature characteristics and cycle characteristics were evaluated. The results are shown in Table 1.

<Process B (one-step granulation): Comparative Examples 4 to 9>
(Comparative Example 4)
The following operations [1] to [6] were sequentially performed to produce an evaluation battery, and the low temperature characteristics and cycle characteristics were evaluated. The results are shown in Table 1.

[1] LiNi _0.33 Co _0.33 Mn _0.33 O ₂ , scaly graphite, AB, and CMC are charged all at once into a high-speed mixer (manufactured by Earth Technica Co., Ltd.) and dry-mixed for 1 minute. I did it.

  [2] Charge ion exchange water (total amount) and PTFE aqueous dispersion into a high-speed mixer, set the rotational speed of the agitator to 800 rpm and the rotational speed of the chopper to 2000 rpm, and mix and granulate for 5 minutes. I did it.

[3] The obtained granulated powder was sieved (aperture 300 μm) to exclude coarse particles.
[4] Referring to FIG. 6, the granulated powder was supplied between the rolls of manual press device 60, and the granulated product was compressed and stretched to obtain a sheet-like molded body.

  [5] Referring to FIG. 7, a sheet-like molded body and an Al foil (current collector) were supplied between the rolls of manual press device 60, and the molded body was pressure-bonded to the Al foil. And the integrated object which consists of the obtained molded object and Al foil was dried, and the positive electrode which concerns on the comparative example 4 was obtained.

  [6] The positive electrode was cut into a predetermined size, an evaluation battery was prepared according to the above-described method, and the low-temperature characteristics and cycle characteristics were evaluated.

  Here, in Comparative Example 4, the solid content concentration (NV) of the granulated product was 80% by mass. Moreover, the charged ratio of the solid content of the granulated product is as follows: positive electrode active material (91 parts by mass), carbonaceous particles (3 parts by mass), graphite particles (3 parts by mass), thickener (2 parts by mass), binding A material (1 part by mass) was used.

(Comparative Examples 5-9)
As shown in Table 1, a positive electrode was obtained in the same manner as in Comparative Example 4 except that the solid content ratio and the solid content concentration (NV) were changed. Characteristics were evaluated. The results are shown in Table 1.

  In Comparative Example 7 in which the solid content concentration (NV) was 70% by mass, the granulated material was turned into a paint, and it was difficult to form a sheet by the above-described method, so the battery evaluation was not performed.

<Process C (two-stage granulation): Examples 1 to 5>
Example 1
The following operations [1] to [7] were sequentially executed to manufacture an evaluation battery, and the low temperature characteristics and cycle characteristics were evaluated. The results are shown in Table 1.

[1] LiNi _0.33 Co _0.33 Mn _0.33 O ₂ , AB and CMC were put into a high-speed mixer (manufactured by Earth Technica Co., Ltd.), and dry mixing was performed for 1 minute.

  [2] Charge ion-exchanged water (total amount) into a high-speed mixer, set the rotational speed of the agitator to 800 rpm and the rotational speed of the chopper to 2000 rpm, and mix and granulate for 3 minutes to produce the primary granulated product. Was obtained (first step).

  [3] The graphite flake and the PTFE aqueous dispersion are charged into the high speed mixer, and the mixture and granulation are performed for 2 minutes while maintaining the rotational speed of the agitator and chopper to obtain the secondary granulated product 10. (Second step).

  [4] The powder of the obtained secondary granulated product 10 was sieved (aperture 300 μm) to exclude coarse particles.

  [5] Referring to FIG. 6, the powder of the secondary granulated product 10 is supplied between the rolls of the manual press device 60, and the secondary granulated product 10 is compressed and stretched to form the sheet-like compact 102. Obtained (third step).

  [6] Referring to FIG. 7, the sheet-like molded body 102 and the Al foil (current collector 101) are supplied between the rolls of the manual press device 60, and the molded body 102 is crimped to the Al foil. The molded body 102 was disposed on the current collector 101 (fourth step). Then, the obtained integral body made of the molded body 102 and the Al foil was dried to obtain the positive electrode 100 according to Example 1.

  [7] The positive electrode 100 was cut into a predetermined size, an evaluation battery was produced according to the above-described method, and the low-temperature characteristics and cycle characteristics were evaluated.

  Here, in Example 1, the solid content concentration (NV) of the secondary granulated product 10 was set to 80% by mass. Moreover, the solid content (processing amount) at the time of granulation shall be 0.8 kg, and the preparation ratio of solid content is a positive electrode active material (91 mass parts), carbonaceous particles (3 mass parts), and graphite particles (3 mass parts). , Thickener (2 parts by mass) and binder (1 part by mass).

(Examples 2 to 5)
As shown in Table 1, a positive electrode was obtained in the same manner as in Example 1 except that the charge ratio of solid content and the solid content concentration (NV) were changed, and an evaluation battery was prepared to produce low temperature characteristics and cycle. Characteristics were evaluated. The results are shown in Table 1.

<Results and discussion>
(I) Positive Electrode Manufacturing Process FIG. 12 is a scatter diagram in which the measurement results in each example and comparative example are plotted with the IV resistance in a −10 ° C. environment as the horizontal axis and the post-cycle capacity retention rate as the vertical axis. In this scatter diagram, the closer the coordinates are to the upper left region, the better the low-temperature characteristics and the better the cycle characteristics.

  From FIG. 12, Examples 1-5 which employ | adopted process C (two-stage granulation) are compared with Comparative Examples 1-9 which employ | adopted process A (coating) and process B (one-stage granulation), There is a tendency that IV resistance at −10 ° C. is low and capacity retention after cycling is high.

  In the positive electrode obtained by the process A and the process B, since the graphite particles cannot follow the expansion and contraction of the positive electrode active material, the contact ratio between the positive electrode active material and the graphite particles decreases with the charge / discharge cycle. It is thought that the capacity maintenance rate has decreased. Moreover, since it is difficult to ensure the contact ratio between the graphite particles and the positive electrode active material, it is considered that the IV resistance is high.

  On the other hand, in the positive electrode obtained by the process C (two-stage granulation) as an example, contact between the positive electrode active material and the carbonaceous particles is ensured in the primary granulated product. Therefore, it is considered that the capacity after the cycle can be maintained high. Further, the conductive network is efficiently constructed by arranging the graphite particles on the surface of the primary granulated material. Therefore, it is considered that IV resistance at a low temperature can be reduced.

(Ii) Solid Content Concentration (NV) FIG. 13 shows Comparative Example 2 (Process A), Comparative Examples 5, 8 and 9 in which the solid content is the same (graphite particles: 2.5 parts by mass). It is the scatter diagram which showed the relationship between the low temperature IV resistance and NV in Process B) and Examples 2, 4 and 5 (Process C).

  From FIG. 13, it is recognized that the process C (Example) tends to have a lower IV resistance than the processes A and B (Comparative Example). In Process C (Example), when the solid content concentration (NV) is increased from 73 mass% (Example 4) to 75 mass% (Example 5), the IV resistance is significantly reduced. The reason for this is presumed that the arrangement of the materials in the secondary granulated product cannot be stably maintained because the solid content concentration in Example 4 is slightly low. Therefore, the solid content concentration (NV) of the secondary granulated product is preferably 73% by mass or more, more preferably 75% by mass or more, and particularly preferably 80% by mass or more.

(Iii) Compounding amount of graphitic particles FIG. 14 is a scatter diagram plotting the relationship between the compounding amount of graphite particles and low-temperature IV resistance in each positive electrode manufacturing process.

  From FIG. 14, it is recognized that the IV resistance is lower in Process C (Example) than in Process A and C (Comparative Example) in the blending amount of each graphite particle. That is, in the positive electrode according to the example, it can be said that the amount of graphite particles necessary for obtaining desired IV resistance (output characteristics) is small.

From the above experimental results, the following conclusions (i) to (iii) are derived.
(I) a first step in which a positive active material and carbonaceous particles are mixed to obtain a primary granulated product, and a second step in which graphite particles are adhered to the primary granulated product to obtain a secondary granulated product, A method for producing a positive electrode for a lithium secondary battery, comprising: a third step of forming the secondary granulated product into a sheet-like molded body; and a fourth step of arranging the molded body on a current collector. According to this, a positive electrode for a lithium secondary battery excellent in low temperature characteristics and cycle characteristics is provided.

  (Ii) A secondary granulated product in which a conductive layer is attached to the surface of the primary granulated product, the primary granulated product including a positive electrode active material and carbonaceous particles, and the conductive layer is composed of graphite particles. By forming a molded body (positive electrode active material layer) from a granulated product containing a binder and a binder, a positive electrode for a lithium secondary battery excellent in low temperature characteristics and cycle characteristics is provided.

  (Iii) The positive electrode for a lithium secondary battery provided with the above-mentioned secondary granulated product is excellent in low temperature characteristics and cycle characteristics.

  Although the present embodiment and examples have been described above, it is also planned from the beginning to combine the configurations of the above-described embodiments and examples as appropriate.

  It should be understood that the embodiments and examples disclosed herein are illustrative and non-restrictive in every respect. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

  2 positive electrode active material, 4 carbonaceous particles, 6 graphite particles, 8 binder, 10 secondary granulated product, 10a primary granulated product, 10b conductive layer, 50 granulator, 52 container, 54 main stirring blade, 56 Crushing blade, 60 Manual pressing device, 61 Handle, 62a First roll, 62b Second roll, 80 Evaluation battery, 81 Evaluation positive electrode, 81a Al foil, 81b Positive electrode active material layer, 81c Positive electrode current collecting lead , 82 Evaluation negative electrode, 82a Cu foil, 82b Negative electrode active material layer, 82c Negative electrode current collector lead, 83 Evaluation separator, 85 Laminate film, 100 Positive electrode, 101 Current collector, 102 Molded body, 200 Negative electrode, 300 Separator, 400 Electrode body, 500 exterior body, 1000 lithium secondary battery.

Claims (5)

A first step of mixing a positive electrode active material and carbonaceous particles to obtain a primary granulated product,
A second step of obtaining a secondary granulated product by attaching graphite particles to the surface of the primary granulated product,
A third step of forming the secondary granulated product into a sheet-like molded body;
A fourth step of disposing the molded body on a current collector ,
The said primary granulated material is a manufacturing method of the positive electrode for lithium secondary batteries containing several said positive electrode active material .   The manufacturing method of the positive electrode for lithium secondary batteries of Claim 1 whose solid content concentration of the said secondary granulated material is 73 mass% or more. The first step is a step of obtaining the primary granulated material by mixing the positive electrode active material, the carbonaceous particles, and a thickener.
3. The lithium according to claim 1, wherein the second step is a step of obtaining the secondary granulated product by attaching the graphite particles and a binder to the surface of the primary granulated product. A method for producing a positive electrode for a secondary battery. A secondary granulated product with a conductive layer attached to the surface of the primary granulated product,
It said primary granules may include a plurality of positive electrode active material, a carbonaceous particle,
The conductive layer is a granulated product containing graphite particles and a binder.   A positive electrode for a lithium secondary battery, comprising the granulated product according to claim 4.

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