JP5708526B2 - Method for producing positive electrode plate for secondary battery - Google Patents

Description

  The present invention relates to a method for producing a positive electrode plate for a secondary battery, comprising a positive electrode foil and a positive electrode active material layer formed on the positive electrode foil.

  2. Description of the Related Art Conventionally, a positive electrode plate constituting a secondary battery is known in which a positive electrode active material layer is formed by coating on a positive electrode foil. Such a positive electrode plate, for example, produces a positive electrode paste in which a positive electrode active material, a conductive material, a binder, and a dispersant for dispersing the conductive material are dispersed in a solvent, and the positive electrode paste is used as a positive electrode foil. It is manufactured by a method in which a positive electrode active material layer is formed by coating. Patent Documents 1 and 2 disclose the use of polyvinyl acetal (hereinafter also referred to as PVA) as a dispersant or the like (refer to the claims of Patent Documents 1 and 2, respectively).

JP 2006-253091 A JP 2011-184664 A

  By the way, according to the research of the present inventors, it has been found that the charge / discharge characteristics of a battery can be improved by forming a positive electrode active material layer using two kinds of carbon black and graphite as a conductive material. However, when these two kinds of conductive materials are used, the adhesion between the positive electrode active material layer and the positive electrode foil is improved, the viscosity of the positive electrode paste is appropriately adjusted so as not to become too high, and the carbon It was difficult to simultaneously establish a positive electrode paste in which the aggregation of black and graphite was suppressed.

  The present invention has been made in view of the present situation, and is intended to improve the adhesion between the positive electrode active material layer and the positive electrode foil, appropriately adjust the viscosity of the positive electrode paste, carbon black and graphite. An object of the present invention is to provide a method for producing a positive electrode plate for a secondary battery, which can be simultaneously established with the production of a positive electrode paste that suppresses the aggregation of.

One aspect of the present invention for solving the above problems is a first conductive material made of carbon black, a second conductive material made of graphite, and a polyvinyl acetal having a molecular weight of 5000 to 45000 and an acetalization degree of 40 to 70 mol%. A positive electrode paste preparation step of preparing a positive electrode paste by dispersing a dispersing agent, a binder, and a positive electrode active material in a solvent, and applying the positive electrode paste to a positive electrode foil to form a positive electrode active material layer A positive electrode active material layer forming step , wherein the positive electrode paste manufacturing step includes a first step of dissolving the dispersant in the solvent, and the first conductive material and the first step after the first step. A positive electrode plate for a secondary battery , comprising: a second step of adding and dispersing two conductive materials and the binder; and a third step of adding and dispersing the positive electrode active material after the second step . It is a manufacturing method.

In this method for producing a positive electrode plate for a secondary battery, while using two types of conductive materials, carbon black (first conductive material) and graphite (second conductive material), the dispersant has a molecular weight of 5000 to 45000 and an acetal. PVA having a degree of conversion of 40 to 70 mol% is used. For this reason, the adhesiveness of a positive electrode active material layer and positive electrode electrode foil can be improved. Moreover, the viscosity of the positive electrode paste can be adjusted appropriately. Furthermore, a positive electrode paste in which aggregation of carbon black (first conductive material) and graphite (second conductive material) is suppressed can be produced.
Furthermore, in this positive electrode plate manufacturing method, since the dispersant is dissolved in the solvent in advance in the first step of the positive electrode paste manufacturing step, the first conductive material, the second conductive material, etc. are added to the solvent in the second step. Sometimes the viscosity of the mixture can be prevented from becoming too high. Therefore, it is easy to disperse the first conductive material and the second conductive material in the solvent, and the dispersion time can be shortened. Further, by adding the positive electrode active material after the first step and the second step (third step), it is possible to shorten the time required for dispersion of the positive electrode active material, or to weaken the shearing force applied to the positive electrode active material. Therefore, it is possible to prevent problems such as deterioration of the positive electrode active material, cracking of the active material particles, and reduction in crystallinity due to long dispersion or kneading (kneading).

Examples of the “first conductive material (carbon black)” include acetylene black, furnace black, channel black, thermal black, and ketjen black.
Examples of the “second conductive material (graphite (graphite))” include natural graphite such as spheroidized graphite, flaky graphite, massive graphite, and earthy graphite, and artificial graphite.

  In addition, “polyvinyl acetal (PVA)” is a random copolymer in which three types of structural units including a structural unit containing an acetal group, a structural unit containing a hydroxyl group, and a structural unit containing an acetyl group are randomly bonded. It refers to a polymer compound. Specific examples include polyvinyl formal and polyvinyl butyral. Among such PVAs, in the above method for producing a positive electrode plate, the molecular weight is 5000 to 45000, and the degree of acetalization (ratio of structural units containing acetal groups among the three types of structural units) is 40 to 70 mol%. Use things.

Examples of the “binder” include polyvinylidene fluoride (PVDF), carboxymethyl cellulose (CMC), polyethylene oxide (PEO), polyacrylonitrile (PAN), polytetrafluoroethylene (PTFE), and the like.
Examples of the “solvent” include N-methyl-2-pyrrolidone (NMP), n-ethyl-pyrrolidone, water and the like.

The “positive electrode active material” includes a Li-containing positive electrode active material, specifically, lithium-cobalt composite oxide (Li _x CoO ₂ (0 <x ≦ 1.0)), lithium-nickel composite oxide ( _{Li x NiO 2 (0 <x} ≦ 1.0)), lithium-cobalt-nickel-manganese composite oxide _{(Li x Co y Ni z Mn} (1-yz) O 2 (0 <x ≦ 1.0,0 <Y ≦ 1.0, 0 <z ≦ 1.0)). In addition, layered compound materials such as compounds obtained by adding one or more alkaline earth metal elements to these, and spinel of lithium / manganese composite oxide (Li _x Mn ₂ O ₄ (0 <x ≦ 1.0)) System materials. Further, lithium transition metal composite oxides made of olivine-based materials such as lithium / iron / phosphorus composite oxide (LiFePO ₄ ) can be used.

  In addition, the “positive electrode paste manufacturing step” can be performed using, for example, a medialess disperser such as a homogenizer, a planetary mixer, a jet mill, an ultrasonic disperser, a disperser (stirring blade), or an extrusion kneader. Alternatively, the positive electrode paste may be produced using media dispersion in which ceramic beads such as glass and zirconia are introduced into a dispersing machine such as a bead mill or a ball mill. In producing the positive electrode paste, a plurality of dispersers can be used.

  In the “second step”, as a method of “adding and dispersing the first conductive material, the second conductive material, and the binder” (in the solvent in which the dispersant is dissolved), (1) the first conductive material is used. And a method of adding and dispersing the second conductive material and the binder at the same time. In this case, the first conductive material, the second conductive material, and the dispersant may be simply added simultaneously, or these materials may be mixed in advance and the mixture added. (2) Further, there is a method in which the first conductive material, the second conductive material, and the binder are added one by one (the order of addition is not limited) and dispersed. (3) Also, among the first conductive material, the second conductive material, and the binder, two materials are added and stirred at the same time, and the remaining one material is added separately (the order of addition is the order of the two materials. It may be later) or a method of dispersing. In this case, the two materials to be added simultaneously may be mixed in advance.

  Furthermore, in the method of manufacturing the positive electrode plate, the second step includes any one of a mixture obtained by previously mixing the first conductive material and the binder in a dry manner, and the second conductive material. It is good to set it as the manufacturing method of the positive electrode plate which is the process of adding and stirring the other among the said mixture and said 2nd electrically-conductive material after adding and stirring.

  If the first conductive material (carbon black) is added to the solvent alone, the wettability of the carbon black is low, so it is difficult to disperse the first conductive material (carbon black) in the solvent, and the dispersion time of the positive electrode paste is long. There is. Moreover, when a binder is added to a solvent by itself, it is easy to be in a state where a part of the binder remains undissolved (so-called spatter, lumps). On the other hand, in the manufacturing method of the positive electrode plate, the first conductive material and the binder are mixed in a dry manner, and this mixture is added. Thereby, dispersion | distribution of a 1st electrically conductive material becomes easy, and while being able to shorten the dispersion | distribution time of a positive electrode paste, it can prevent that a binder turns into a powder.

  Furthermore, it is a manufacturing method of the positive electrode plate according to any one of the above, wherein the positive electrode paste has a solid content concentration of 50 wt% or more.

  If the solid content concentration of the positive electrode paste is low, a large amount of heat is required to dry the positive electrode paste applied to the positive electrode foil, which requires a long drying time and a long drying furnace, resulting in high equipment costs. Become. Moreover, since the amount of the positive electrode paste is larger than the amount of the positive electrode active material necessary to form the positive electrode plate, the facilities necessary for the production and handling of the positive electrode paste such as a disperser, a pump, and a tank are increased in size, Cost increases. In addition, since a large amount of solvent is required, the cost is also increased in this respect. Therefore, it is preferable that the solid content concentration of the positive electrode paste is high, specifically, 50 wt% or more.

However, when a positive electrode paste having a high solid content concentration is produced by a conventional method, the viscosity of the positive electrode paste tends to be high, and the filter permeability of the positive electrode paste may be deteriorated or the coating property may be deteriorated. On the other hand, if the positive electrode paste preparation process is performed as described above, the filter permeability and the coating property of the positive electrode paste can be improved even if the solid content concentration of the positive electrode paste is increased. Therefore, when producing a positive electrode paste having a solid content concentration of 50 wt% or more, it is particularly effective to perform the above-described positive electrode paste production step.
The solid content concentration of the positive electrode paste is preferably 80 wt% or less. The reason is that when the solid content concentration is too high, the handling and coating properties of the positive electrode paste are deteriorated.

  Furthermore, it is a manufacturing method of the positive electrode plate in any one of said, Comprising: The said polyvinyl acetal is good to set it as the manufacturing method of the positive electrode plate whose molecular weight is 8000-21000.

  When the molecular weight of PVA is less than 8000, the effect of dispersing the conductive material is reduced. That is, the first conductive material and the second conductive material are difficult to disperse in the solvent, and the first conductive material and the second conductive material are easily aggregated. On the other hand, when the molecular weight of PVA is larger than 21000, the viscosity of the positive electrode paste tends to be high. On the other hand, in the positive electrode plate manufacturing method, the molecular weight of PVA is set to 8000 or more, so that the first conductive material and the second conductive material are particularly easily dispersed, and the first conductive material and the second conductive material are aggregated. Can be effectively suppressed. On the other hand, since the molecular weight of PVA is 21000 or less, it is possible to effectively prevent the viscosity of the positive electrode paste from becoming too high.

  Furthermore, it is a manufacturing method of the positive electrode plate in any one of said, Comprising: The said polyvinyl acetal is good to set it as the manufacturing method of the positive electrode plate whose acetalization degree is 50-65 mol%.

  When the degree of acetalization of PVA is less than 50 mol%, the effect of dispersing the conductive material is reduced. That is, the first conductive material and the second conductive material are difficult to disperse in the solvent, and the first conductive material and the second conductive material are easily aggregated. On the other hand, when the degree of acetalization of PVA is larger than 65 mol%, the adhesion between the positive electrode active material layer and the positive electrode foil tends to be low. On the other hand, in the manufacturing method of the positive electrode plate described above, since the degree of acetalization is 50 mol% or more, the first conductive material and the second conductive material are particularly easily dispersed, and the first conductive material and the second conductive material are not easily dispersed. Aggregation can be effectively suppressed. On the other hand, since the degree of acetalization is 65 mol% or less, the adhesion between the positive electrode active material layer and the positive electrode foil can be particularly enhanced.

It is a perspective view which shows the lithium ion secondary battery which concerns on embodiment. It is a longitudinal cross-sectional view which shows the lithium ion secondary battery which concerns on embodiment. It is an expanded view of an electrode body which concerns on embodiment and shows the state which mutually accumulated the positive electrode plate and the negative electrode plate through the separator. It is explanatory drawing which shows a positive electrode paste preparation process regarding the manufacturing method of the positive electrode plate which concerns on embodiment.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. 1 and 2 show a lithium ion secondary battery 100 (hereinafter also simply referred to as a battery 100) according to the present embodiment. FIG. 3 shows a state where the wound electrode body 120 constituting the battery 100 is developed. In the following description, the thickness direction BH, the width direction CH, and the height direction DH of the battery 100 are defined as the directions shown in FIGS.

  The battery 100 is a square battery that is mounted on a vehicle such as a hybrid vehicle or an electric vehicle, or a battery-powered device such as a hammer drill. The battery 100 includes a rectangular parallelepiped battery case 110, a flat wound electrode body 120 accommodated in the battery case 110, a positive terminal 150 and a negative terminal 160 supported by the battery case 110, and the like. (See FIGS. 1 and 2). In addition, a non-aqueous electrolyte solution 117 is held in the battery case 110.

Among these, the battery case 110 is made of metal (specifically, aluminum). The battery case 110 includes a box-shaped case main body member 111 opened only on the upper side, and a rectangular plate-shaped case cover member 113 welded in a form to close the opening 111h of the case main body member 111. (See FIGS. 1 and 2). The case lid member 113 is provided with a non-returnable safety valve 113v. Further, the case lid member 113 is provided with a liquid injection hole 113 h for the electrolytic solution 117, and is hermetically sealed with the sealing member 115.

  Further, in the case lid member 113, in the vicinity of both ends in the longitudinal direction (the width direction CH of the battery 100), a positive terminal (positive terminal member) 150 and a negative terminal that extend from the inside of the battery case 110 to the outside. (Negative electrode terminal member) 160 is fixed. Specifically, these terminals 150 and 160 are connected to the case lid via insulating members 155 and 165 made of resin together with bolts 153 and 163 for fastening connection terminals outside the battery, such as bus bars and crimp terminals. It is fixed to the member 113.

  Next, the electrode body 120 will be described. The electrode body 120 is housed in the battery case 110 in a state where the electrode body 120 is laid down so that its axis (winding axis) is parallel to the width direction CH of the battery 100 (see FIG. 2). In this electrode body 120, a belt-like positive electrode plate 121 and a belt-like negative electrode plate 131 are overlapped with each other via two belt-like separators 141 and 141 made of a resin porous film (see FIG. 3). It is wound around and compressed into a flat shape. A part of the positive electrode plate 121 in the width direction protrudes from the separators 141 and 141 in a spiral shape to one side AC (in FIG. 2, left, upper in FIG. 3) in the axial direction. (Positive electrode terminal member) 150 is connected (welded). Further, a part of the negative electrode plate 131 in the width direction protrudes in a spiral shape from the separators 141 and 141 to the other side AD in the axial direction (in FIG. 2, right side, in FIG. 3, downward). The negative electrode terminal (negative electrode terminal member) 160 is connected (welded).

  Among these, the positive electrode plate 121 has the strip | belt-shaped positive electrode foil 122 which consists of aluminum as a core material. A positive electrode active material layer (positive electrode mixture layer) 123 extending in a strip shape in the longitudinal direction EH (left and right direction in FIG. 3) is formed on a part of the other side AD in the width direction of both main surfaces of the positive electrode foil 122. , 123 are formed.

The positive electrode active material layers 123 and 123 are formed of the positive electrode active material 60, the first conductive material 30, the second conductive material 40, the binder 50, and the dispersant 20. In this embodiment, lithium-cobalt-nickel-manganese composite oxide (specifically, LiCo _1/3 Ni _1/3 Mn _1/3 O ₂ ) is used as the positive electrode active material 60, and carbon black is used as the first conductive material 30. (Specifically, acetylene black), graphite (graphite) as the second conductive material, and polyvinylidene fluoride (PVDF) as the binder 50 are used.

  Further, as the dispersant 20 for dispersing the first conductive material 30 and the second conductive material 40 in the solvent 10 described later, polyvinyl acetal (PVA) having a molecular weight of 5000 to 45000 and an acetalization degree of 40 to 70 mol% is used. . Specifically, this PVA has a molecular weight of 8500 and an acetalization degree of 61 mol%. The ratio of the positive electrode active material 60, the conductive material (the total of the first conductive material 30 and the second conductive material 40), the binder 50, and the dispersant 20 is 90.8: 6.0: 3 in weight ratio. 0.0: 0.2. The mixing ratio of the first conductive material (carbon black) 30 and the second conductive material (graphite) 40 in the conductive material is 4.0: 2.0 by weight.

  The negative electrode plate 131 has a strip-shaped negative electrode foil 132 made of copper as a core material. Negative electrode active material layers (negative electrode mixture layers) 133 and 133 extending in a strip shape in the longitudinal direction EH are respectively formed on a part of one side AC in the width direction of both main surfaces of the negative electrode foil 132. These negative electrode active material layers 133 and 133 are formed of a negative electrode active material, a thickener, and a binder.

Next, a method for manufacturing the positive electrode plate 121 and the battery 100 will be described.
First, a method for manufacturing the positive electrode plate 121 will be described. In the positive electrode paste manufacturing step, the first conductive material 30, the second conductive material 40, the dispersant 20, the binder 50, and the positive electrode active material 60 are dispersed in the solvent (specifically, NMP) 10, A positive electrode paste 80 is produced (see FIG. 4).

Specifically, first, in the first step S <b> 1 in the positive electrode paste manufacturing step, the solvent 10 is put in a bead mill, and the dispersant 20 is further added to the solvent 10 and stirred to dissolve the dispersant 20 in the solvent 10. .
Thereafter, in the second a step S2 of the second step, the second conductive material (graphite) 40 is added to the solvent 10 in which the dispersant 20 is dissolved, and dispersed by stirring.
Further, in the second step 2b S3 of the second step, the first conductive material (carbon black) 30 and the binder 50 are previously mixed in the solvent 10 containing the dispersant 20 and the second conductive material 40 in a dry manner. Add the mixture 35 and stir to disperse.
Thereby, the conductive paste 70 in which the dispersant 20, the first conductive material 30, the second conductive material 40, and the binder 50 are dispersed in the solvent 10 is produced.

Next, in the third step S4 of the positive electrode paste manufacturing step, the positive electrode active material 60 is added to the conductive paste 70 and dispersed by stirring using a high shear homogenizer. Thus, the positive electrode paste 80 is produced.
The mixing ratio of the positive electrode active material 60, the first conductive material (carbon black) 30, the second conductive material (graphite) 40, the binder 50, and the dispersant 20 is 90. 8: 4.0: 2.0: 3.0: 0.2. Moreover, the mixing ratio of these solid content 20, 30, 40, 50, 60 and the solvent 10 is 31:69 by weight ratio. Therefore, this positive electrode paste 80 has a solid content concentration of 69 wt%.

  Next, a strip-like positive electrode foil 122 made of aluminum is prepared, and in the positive electrode active material layer forming step, the positive electrode paste 80 is applied to the positive electrode foil 122 to form the positive electrode active material layers 123 and 123. Specifically, the positive electrode paste 80 is applied to a part of one main surface in the width direction of the positive electrode foil 122 using a die coater. Then, this is dried with hot air to form the positive electrode active material layer 123. Similarly, the positive electrode paste 80 is applied to a part of the main surface on the opposite side of the positive electrode foil 122 in the width direction, and dried with hot air to form the positive electrode active material layer 123. Thereafter, the positive electrode active material layers 123 and 123 are compressed by a pressure roll to increase the density. Thus, the positive electrode plate 121 is formed.

  Separately, the negative electrode plate 131 is manufactured. That is, a strip-shaped negative electrode foil 132 made of copper is prepared. Then, a negative electrode paste containing a negative electrode active material, a thickener, and a binder is applied to a part of the width direction of one main surface of the negative electrode electrode foil 132, and dried with hot air to form a negative electrode active material layer 133 is formed. Similarly, the negative electrode paste is applied to part of the main surface on the opposite side of the negative electrode foil 132 in the width direction and dried with hot air to form the negative electrode active material layer 133. Thereafter, the negative electrode active material layers 133 and 133 are compressed by a pressure roll to increase the density. Thus, the negative electrode plate 131 is formed.

Next, two strip-shaped separators 141 and 141 are prepared, and the above-described positive electrode plate 121 and negative electrode plate 131 are overlapped with each other through the separators 141 and 141 (see FIG. 3). Turn. Thereafter, the electrode body 120 is formed by compressing it into a flat shape.
Separately, a case lid member 113, a positive electrode terminal member 150, a negative electrode terminal member 160, and bolts 153 and 163 are prepared, and these are set in a mold for injection molding. Then, the insulating members 155 and 165 are integrally formed by injection molding, and the positive terminal member (positive terminal) 150 and the negative terminal member (negative terminal) 160 are fixed to the case lid member 113.

Next, the positive electrode terminal 150 and the negative electrode terminal 160 are welded to the electrode body 120, respectively. Thereafter, the case body member 111 is prepared, the electrode body 120 is accommodated in the case body member 111, and the opening 111 h of the case body member 111 is closed with the case lid member 113. Then, the case main body member 111 and the case lid member 113 are welded. Next, the electrolytic solution 117 is injected into the battery case 110 through the injection hole 113h, and the injection hole 113h is hermetically sealed with the sealing member 115. Thereafter, the battery 100 is subjected to initial charging, aging, and various inspections. Thus, the battery 100 is completed.

(Examples and Comparative Examples)
Next, the results of tests performed for verifying the effects of the method for manufacturing the positive electrode plate 121 according to the embodiment will be described. As Example 2, the positive electrode plate 121 was manufactured by the manufacturing method of the positive electrode plate 121 according to the embodiment. In the method for manufacturing the positive electrode plate 121, as described above, PVA having a molecular weight of 8500 and an acetalization degree of 61 mol% was used for the dispersant 20.

  Moreover, in the following Examples 1, 3-8, and Comparative Examples 1-8, as shown in Table 1, only the molecular weight and the degree of acetalization of PVA which is a dispersing agent were changed, and other than that embodiment (Example) A positive electrode plate was produced in the same manner as 2). Specifically, the PVA used in Example 1 has a molecular weight of 5800 and an acetalization degree of 62 mol%. The PVA used in Example 3 has a molecular weight of 19000 and an acetalization degree of 54 mol%. The PVA used in Example 4 has a molecular weight of 19000 and an acetalization degree of 63 mol%. The PVA used in Example 5 has a molecular weight of 19500 and an acetalization degree of 45 mol%. The PVA used in Example 6 has a molecular weight of 20000 and an acetalization degree of 61 mol%. The PVA used in Example 7 has a molecular weight of 20000 and an acetalization degree of 69 mol%. The PVA used in Example 8 has a molecular weight of 40,000 and a degree of acetalization of 65 mol%.

  On the other hand, the PVA used in Comparative Example 1 has a molecular weight of 4500 and an acetalization degree of 62 mol%. The PVA used in Comparative Example 2 has a molecular weight of 17000 and an acetalization degree of 74 mol%. The PVA used in Comparative Example 3 has a molecular weight of 19000 and an acetalization degree of 38 mol%. The PVA used in Comparative Example 4 has a molecular weight of 23000 and an acetalization degree of 74 mol%. The PVA used in Comparative Example 5 has a molecular weight of 53000 and an acetalization degree of 65 mol%. The PVA used in Comparative Example 6 has a molecular weight of 66000 and an acetalization degree of 73 mol%. The PVA used in Comparative Example 7 has a molecular weight of 92000 and an acetalization degree of 69 mol%. The PVA used in Comparative Example 8 has a molecular weight of 200000 and an acetalization degree of 63 mol%.

  About each positive electrode paste which concerns on these Examples 1-8 and Comparative Examples 1-8, the viscosity (mPa * s) was measured, respectively. The viscosity was measured using a rotary viscometer, specifically, a rheometer (Anton Paar, shear rate 40 / s). The viscosity was 3900 mPa · s or less, and the viscosity was larger than 3900 mPa · s and 4500 mPa · s or less was acceptable (Δ mark), and the case where it was larger than 4500 mPa · s was evaluated as NG (x mark).

  Moreover, about each positive electrode paste which concerns on Examples 1-8 and Comparative Examples 1-8, the magnitude | size (micrometer) of the aggregated particle which the 1st electrically conductive material, the 2nd electrically conductive material, etc. agglomerated and formed was measured, respectively. Specifically, the size of aggregated grains present in the positive electrode paste was measured with a grindometer (grain gauge). The size of the agglomerated particles was evaluated as good when less than 70 μm, 70 μm or more and less than 90 μm was acceptable (Δ mark), and 90 μm or more was evaluated as NG (× mark).

  Moreover, about each positive electrode plate which concerns on Examples 1-8 and Comparative Examples 1-8, the adhesiveness (peeling strength, N / m) of a positive electrode active material layer and positive electrode electrode foil was measured, respectively. Specifically, a strip-shaped piece having a predetermined size was cut out from each positive electrode plate. Among the pieces of the positive electrode plate, one side was fixed to a horizontal base through a double-sided tape, and the other side was lifted vertically by a peel strength measuring machine (manufactured by A & D). And the force (N) applied when the positive electrode active material layer stuck on the double-sided tape was peeled off from the positive electrode foil was measured by this peel strength measuring device, and the peel strength (N / m) was determined from this. The adhesion was evaluated as 2.0 N / m or better, less than 2.0 N / m and 1.5 N / m or more was acceptable (Δ mark), and less than 1.5 N / m was evaluated as NG (x mark).

  Moreover, about each battery manufactured using each positive electrode plate which concerns on Examples 1-8 and Comparative Examples 1-8, IV resistance (mohm) was measured, respectively. Specifically, IV resistance was measured with a charger / discharger. The IV resistance was 2.3 mΩ or less, and the case where the IV resistance was larger than 2.3 mΩ was evaluated as NG (x mark).

  And as comprehensive evaluation, the viscosity of the positive electrode paste, the size of the aggregated particles in the positive electrode paste, the IV resistance of the battery, and the adhesion of the positive electrode active material layer are all good (Examples 2 to 4 and 6). Was evaluated as particularly good (marked with ◎). Moreover, although there existed possible ((triangle | delta) mark) in any, the others (Example 1, 5, 7, 8) were evaluated as favorable (circle mark). Moreover, the thing (Comparative Examples 1-8) which has NG (x mark) in any one was evaluated as NG (x mark). These results are shown in Table 1.

  As can be seen from Table 1, the viscosity of the positive electrode paste was 4890 to 9260 mPa · s in Comparative Examples 5 to 8, which was higher than 4500 mPa · s (the evaluation “x” is shown in the table). The reason is considered that the molecular weight of PVA is too large (specifically, more than 45,000), and the viscosity of the positive electrode paste is increased due to the presence of the PVA itself.

  On the other hand, it was 4210 mPa · s in Example 8 and 3950 mPa · s in Comparative Example 4, and the viscosity was lower than Comparative Examples 5 to 8 (evaluation “Δ” is shown in the table). In these Example 8 and Comparative Example 4, the molecular weight of PVA is in the range of more than 21000 and 45000 or less, and is smaller than Comparative Examples 5 to 8, so the viscosity of the positive electrode paste is considered to be relatively low. It is done.

  In Examples 1 to 7 and Comparative Examples 1 to 3, the viscosity was 3650 to 3860 mPa · s, which was a good result of 3900 mPa · s or less. The reason is that the molecular weight of PVA is sufficiently small (specifically, 21,000 or less), and thus the viscosity of the positive electrode paste is considered to be sufficiently low. From this, it is understood that the molecular weight of PVA is preferably 45000 or less, more preferably 21000 or less.

  Further, as can be seen from Table 1, the size of the aggregated particles such as the conductive material was so large as to exceed 90 μm in Comparative Example 1 and 100 μm in Comparative Example 3 (evaluation “x” is shown in the table). When large agglomerated grains are present in the positive electrode paste, filter clogging may occur when the positive electrode paste is passed through the filter, or coating failure (so-called streaking) may occur when the positive electrode paste is applied to the positive electrode foil. Therefore, it is necessary to make the aggregated grains small (specifically, less than 90 μm, preferably less than 70 μm).

  The reason why the agglomerated grains are increased in Comparative Example 1 is that the molecular weight of PVA is too small (specifically, less than 5000), so that the dispersion effect of dispersing the first conductive material and the second conductive material is insufficient. It is thought that the conductive material and the second conductive material aggregated to form large aggregated particles. In addition, the reason why the agglomerated grains are increased in Comparative Example 3 is that the degree of acetalization of PVA is too low (specifically, less than 40 mol%), so that the solubility (affinity) of PVA in the solvent 10 decreases. It is considered that the dispersion effect of dispersing the first conductive material and the second conductive material is insufficient, and the first conductive material and the second conductive material are aggregated to form large aggregated particles.

  In contrast, in Examples 1 and 5, the aggregated particles were 70 μm, respectively, which was smaller than Comparative Examples 1 and 3 (evaluation “Δ” is shown in the table). In Example 1, since the molecular weight of PVA is relatively large (specifically 5000 or more and less than 8000), the first conductive material and the second conductive material are less likely to aggregate compared to Comparative Example 1, and the aggregated particles Is considered to be relatively small. Moreover, in Example 5, since the degree of acetalization of PVA is relatively high (specifically, 40 mol% or more and less than 50 mol%), the first conductive material and the second conductive material are compared with Comparative Example 3. It is difficult to agglomerate and it is considered that the agglomerated grains are relatively small.

  Further, in Examples 2 to 4, 6 to 8 and Comparative Examples 2 and 4 to 8, the agglomerated grains were 60 to 65 μm, which was a smaller and better result. In these, since the molecular weight of PVA is sufficiently large (specifically, 8000 or more) and the degree of acetalization of PVA is sufficiently high (specifically, 50 mol% or more), the molecular weight of PVA is further higher than those of Examples 1 and 5. It is considered that the first conductive material and the second conductive material are less likely to aggregate and the aggregated grains are reduced. From this, it can be seen that it is preferable that the molecular weight of PVA is 5000 or more and the degree of acetalization is 40 mol% or more. Furthermore, it is understood that it is particularly preferable that the molecular weight of PVA is 8000 or more and the degree of acetalization is 50 mol% or more.

Further, as can be seen from Table 1, the IV resistance was 2.6 mΩ in Comparative Example 1 and 2.5 mΩ in Comparative Example 3, which was larger than 2.3 mΩ (indicated by “x” in the table). The reason is that the first conductive material and the second conductive material are aggregated and the aggregated particles are too large (specifically, 90 μm or more). It is thought that the IV resistance increased due to insufficient conductive paths.
In contrast, in Examples 1 to 8 and Comparative Examples 2 and 4 to 8, the IV resistance was 2.0 to 2.2 mΩ, which was a sufficiently low and good result. In these, since the aggregation of the first conductive material and the second conductive material is relatively small (specifically, the aggregated grains are less than 90 μm), and the conductive path can be sufficiently secured, the IV resistance is considered to be sufficiently low. It is done.

Further, as can be seen from Table 1, the adhesion between the positive electrode active material layer and the positive electrode foil was a low value of 0.6 to 0.8 N / m in Comparative Examples 2, 4 and 6 (evaluated in the table). "X"). If the adhesion of the positive electrode active material layer is low, the positive electrode active material layer may be peeled off when the positive electrode plate is handled. Further, the positive electrode active material may be peeled off during repeated charging / discharging of the battery, resulting in a decrease in battery capacity and a decrease in battery output. Therefore, it is necessary to increase the adhesion of the positive electrode active material layer (specifically, 1.5 N / m or more, preferably 2.0 N / m or more).
The reason why the adhesion of the positive electrode active material layer was low in these Comparative Examples 2, 4, and 6 is that the degree of acetalization of PVA is high (specifically, higher than 70 mol%), and the proportion of hydroxyl groups in PVA is too low. It is. Since the hydroxyl group in PVA interacts with the positive electrode electrode foil, the adhesiveness is increased when the hydroxyl group is large. Therefore, in Comparative Examples 2, 4, and 6, it is considered that the adhesion of the positive electrode active material layer was lowered.

  On the other hand, it was 1.5 N / m in Example 7 and 1.6 N / m in Comparative Example 7, and the adhesion of the positive electrode active material layer was higher than that in Comparative Examples 2, 4 and 6 (in the table). Shows the evaluation “△”). The reason is that the degree of acetalization of PVA is slightly low (specifically, in the range of more than 65 mol% and not more than 70 mol%) and the ratio of hydroxyl groups in PVA is high. Thus, it is considered that the adhesion of the positive electrode active material layer is increased.

  Further, in Examples 1 to 6, 8 and Comparative Examples 1, 3, 5, and 8, it was 2.0 to 3.2 N / m, and the adhesiveness of the positive electrode active material layer was high and good results. In these, the degree of acetalization of PVA is sufficiently low (specifically, 65 mol% or less), and the proportion of the hydroxyl group in PVA is sufficiently high, so that the adhesion of the positive electrode active material layer is considered to be sufficiently high. . From this, it can be seen that the degree of acetalization of PVA is preferably 70 mol% or less, more preferably 65 mol% or less.

  As described above, in the method of manufacturing the positive electrode plate 121, the positive electrode paste manufacturing steps S1 to S4 are performed using PVA having a molecular weight of 5000 to 45000 and an acetalization degree of 40 to 70 mol% as the dispersant 20. Yes. This improves the adhesion between the positive electrode active material layer 123 and the positive electrode foil 122 while using two types of conductive materials, carbon black (first conductive material) 30 and graphite (second conductive material) 40. Can do. Moreover, the viscosity of the positive electrode paste 80 can be adjusted appropriately. Furthermore, the positive electrode paste 80 in which aggregation of the first conductive material 30 and the second conductive material 40 is suppressed can be produced. In particular, in the present embodiment, since the solid content concentration of the positive electrode paste 80 is 50 wt% or more (specifically 69 wt%), in order to keep the viscosity of the positive electrode paste 80 low, in particular, the positive electrode paste preparation step S1 described above. It is effective to perform ~ S4.

Furthermore, when the molecular weight of PVA is 8000 or more, the first conductive material 30 and the second conductive material 40 are particularly easily dispersed, and aggregation of the first conductive material 30 and the second conductive material 40 can be effectively suppressed. . On the other hand, by setting the molecular weight of PVA to 21000 or less, it is possible to particularly effectively prevent the viscosity of the positive electrode paste 80 from becoming too high.
Further, by setting the degree of acetalization to 50 mol% or more, the first conductive material 30 and the second conductive material 40 are particularly easily dispersed, and the aggregation of the first conductive material 30 and the second conductive material 40 is effectively suppressed. it can. On the other hand, when the degree of acetalization is 65 mol% or less, the adhesion between the positive electrode active material layer 123 and the positive electrode foil 122 can be particularly increased.

  Furthermore, in this embodiment, since the dispersing agent 20 is previously dissolved in the solvent in the first step S1 of the positive electrode paste manufacturing steps S1 to S4, the first conductive material 30, the second conductive material 40, etc. are added to the solvent. The viscosity of the mixture can be prevented from becoming too high. Accordingly, the dispersion of the first conductive material 30 and the second conductive material 40 in the solvent is easy and the dispersion time can be shortened. Further, by adding the positive electrode active material 60 after the first step S1 and the second steps S2 and S3, the time required for dispersion of the positive electrode active material 60 can be shortened, or the shearing force applied to the positive electrode active material 60 can be weakened. . Accordingly, it is possible to prevent problems such as deterioration of the positive electrode active material 60, cracking of the positive electrode active material particles, and reduction in crystallinity due to long dispersion or kneading (kneading).

  In the second step S3 of the second steps S2 and S3, the first conductive material (carbon black) 30 and the binder 50 are previously mixed in a dry manner, and the mixture 35 is added. Thereby, the dispersion | distribution of the 1st electrically-conductive material 30 becomes easy, while being able to shorten the dispersion | distribution time of the positive electrode paste 80 (conductive paste 70), it can prevent that the binder 50 becomes a spatter.

In the above, the present invention has been described with reference to the embodiment. However, the present invention is not limited to the above-described embodiment, and it is needless to say that the present invention can be appropriately modified and applied without departing from the gist thereof.
For example, in the embodiment, in the second steps S2 and S3 of the positive electrode paste preparation steps S1 to S4, the second a step S2 in which the second conductive material (graphite) 40 is added first is performed, and then the first conductive material (carbon black) ) Step 2b S3 in which a mixture 35 of 30 and the binder 50 was added was performed. However, the 2b process S3 which adds the mixture 35 previously can be performed, and the 2a process S2 which adds the 2nd electrically-conductive material (graphite) 40 after that can also be performed.

10 Solvent (NMP)
20 Dispersant (PVA)
30 First conductive material (carbon black)
40 Second conductive material (graphite)
50 Binder (PVDF)
60 Positive electrode active material (lithium, cobalt, nickel, manganese composite oxide)
70 conductive paste 80 positive electrode paste 100 lithium ion secondary battery (battery)
DESCRIPTION OF SYMBOLS 120 Electrode body 121 Positive electrode plate 122 Positive electrode electrode foil 123 Positive electrode active material layer 131 Negative electrode plate 141 Separator

Claims (5)

A first conductive material made of carbon black, a second conductive material made of graphite, a dispersant made of polyvinyl acetal having a molecular weight of 5000 to 45000 and an acetalization degree of 40 to 70 mol%, a binder, and a positive electrode active material Is dispersed in a solvent to produce a positive electrode paste,
A positive electrode active material layer forming step of coating the positive electrode paste on a positive electrode foil to form a positive electrode active material layer , and
The positive electrode paste preparation step includes:
A first step of dissolving the dispersant in the solvent;
A second step of adding and dispersing the first conductive material, the second conductive material and the binder after the first step;
A third step of adding and dispersing the positive electrode active material after the second step, and a method of manufacturing a positive electrode plate for a secondary battery. It is a manufacturing method of the positive electrode plate according to claim 1 ,
The second step includes
A mixture obtained by previously mixing the first conductive material and the binder in a dry manner and the second conductive material are added and stirred, and then the other of the mixture and the second conductive material is mixed. A method for producing a positive electrode plate, which is a step of additionally stirring. It is a manufacturing method of the positive electrode plate according to claim 1 or 2 ,
The positive electrode paste has a solid content concentration of 50 wt% or more. It is a manufacturing method of the positive electrode plate as described in any one of Claims 1-3 ,
The polyvinyl acetal is a method for producing a positive electrode plate having a molecular weight of 8000 to 21,000. It is a manufacturing method of the positive electrode plate as described in any one of Claims 1-4 ,
The said polyvinyl acetal is a manufacturing method of the positive electrode plate whose acetalization degree is 50-65 mol%.

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