JP2013114747A - Lithium ion secondary battery manufacturing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for manufacturing a lithium ion secondary battery which has an anode plate with high adhesion strength and excels in cycle characteristic.SOLUTION: The present invention applies to a lithium ion secondary battery manufacturing method which uses an anode plate consisting of an anode active material layer formed by applying a coating of anode mixture material paste to the surface on at least one side of a metal foil and then drying the coating. The lithium ion secondary battery manufacturing method of the present invention further uses, as an anode mixture material paste, what is manufactured by first mixing in which an anode active material and a first thickener, both in powder form, are mixed together with a solvent, second mixing in which the mixed material deriving from the first mixing and a second thickener and a solvent are mixed together, and third mixing in which the mixed material deriving from the second mixing and a binder are mixed together. Also, the manufacturing method involves using carboxymethyl cellulose in molecular weight of 330,000 or less as the first thickener and carboxymethyl cellulose in molecular weight of 330,000 or more as the second thickener.

Description

  The present invention relates to a method for manufacturing a lithium ion secondary battery including a negative electrode plate formed by forming a negative electrode active material layer by applying a negative electrode mixture paste on the surface of a metal foil and then drying the paste.

  In recent years, lithium ion secondary batteries have attracted attention as a power source for vehicles such as mobile phones and notebook computers, as well as hybrid vehicles and electric vehicles. A lithium ion secondary battery includes an electrode body in which a sheet-like positive electrode plate and a negative electrode plate are wound or laminated while sandwiching a sheet-like separator therebetween.

  An electrode plate of a lithium ion secondary battery is manufactured by forming an active material layer on the surface of a metal foil that is a current collector. The electrode plate is required to have high adhesion strength between the current collector and the active material layer. Where the active material layer is peeled off, the resistance between the current collector and the active material layer is increased. For this reason, the lithium ion secondary battery cannot fully exhibit charge / discharge performance.

  The active material layer is generally formed by applying a mixture paste containing an electrode material such as an active material on the surface of the current collector and then drying it. For this reason, the adhesion strength of the electrode plate is affected by the quality of the composite paste. In order to produce an electrode plate with high adhesion strength, it is particularly important to produce a homogeneous mixture paste with no bias in the distribution of components.

  For example, Patent Document 1 discloses a method of manufacturing a negative electrode mixture paste used for manufacturing a negative electrode plate by three kneading steps, an initial kneading step, a dilution kneading step, and a final kneading step. Here, in the initial kneading step, graphite as an active material is kneaded together with an aqueous solution of a thickener. In the dilution kneading process, an aqueous solution of a thickening agent is further added to the kneaded product after the initial kneading process, and the mixture is diluted while being kneaded. In the final kneading step, kneading is performed while adding a binder to the kneaded product after the dilution kneading step. And it is supposed that the negative electrode plate with the high adhesive strength of an electrical power collector and an active material layer can be manufactured by using the negative mix paste manufactured by these kneading | mixing processes. Furthermore, it is said that a lithium ion secondary battery with high cycle characteristics can be manufactured by using a negative electrode plate with high adhesion strength.

JP 2006-092760 A

  By the way, powder components such as graphite and thickener absorb this when mixed with water as a solvent. The amount of water absorption varies depending on the manufacturer that manufactured the powder component and the manufacturing lot. That is, the amount of water relative to the powder component must be adjusted appropriately during kneading according to the amount of water absorbed by the powder component.

  However, in the above-described conventional technique, the amount of water with respect to the graphite and the thickener as powder components cannot be adjusted appropriately during kneading. This is because all the solvent water is added in the form of a thickener aqueous solution. For this reason, the viscosity of the completed negative electrode mixture paste may vary, and a negative electrode plate having high adhesion strength may not be stably produced. When the adhesion strength of the negative electrode plate is low, in the lithium ion secondary battery, peeling of the negative electrode plate is likely to occur with charge / discharge. That is, the cycle characteristics may be deteriorated.

  The present invention has been made for the purpose of solving the problems of the prior art described above. That is, the problem is to provide a method for producing a lithium ion secondary battery having a negative electrode plate with high adhesion strength and high cycle characteristics.

  The method of manufacturing a lithium ion secondary battery of the present invention, which has been made for the purpose of solving this problem, forms a negative electrode active material layer by applying a negative electrode mixture paste to at least one surface of a metal foil and then drying it. A lithium ion secondary battery manufacturing method using a negative electrode plate comprising: a first kneading method comprising kneading a powdery negative electrode active material and a first thickener together with a solvent as a negative electrode mixture paste; , Second kneading by adding a second thickener and a solvent to the kneaded product after the first kneading, and third kneading by adding a binder to the kneaded product after the second kneading. The first thickener is carboxymethylcellulose having a molecular weight of 330,000 or less, and the second thickener is carboxymethylcellulose having a molecular weight of 330,000 or more. Lithium ion secondary It is a manufacturing method of the pond.

  That is, in the first kneading, both graphite and CMC are used as powders. Thereby, the quantity of water with respect to graphite and CMC can be adjusted at the time of kneading, and a negative electrode mixture paste having an appropriate viscosity can be stably produced. In addition, carboxymethylcellulose (CMC), which is a thickening agent, is more likely to adhere to the negative electrode active material and to disperse in water as the molecular weight decreases. Furthermore, the negative electrode active material is easily dispersed by coating its surface uniformly with CMC. CMC having a molecular weight of 330,000 or less can uniformly coat the surface of the negative electrode active material and can be suitably dispersed in a solvent. That is, the first kneading can easily disperse the negative electrode active material. In the first kneading in which CMC is difficult to disperse, the unadhered CMC that is not coated with the negative electrode active material can be suitably dispersed in the solvent. On the other hand, CMC increases the viscosity of the kneaded product as the molecular weight increases. And CMC whose molecular weight is 330,000 or more can raise the viscosity of the kneaded material moderately. That is, by the second kneading, the kneaded product can be brought into a state in which each component is difficult to settle. Thereby, a negative electrode mixture paste in which each component is suitably dispersed and hardly settled can be produced. That is, it is possible to produce a homogeneous negative electrode mixture paste with no uneven distribution of components. Therefore, a lithium ion secondary battery having high adhesion strength of the negative electrode plate and high cycle characteristics can be manufactured.

  In the method for manufacturing a lithium ion secondary battery described above, the kneading torque in the first kneading is not less than 1 and not more than 2.3 times the kneading torque in the second and third kneading. Is preferred. The first kneading is a step of first kneading a powdered negative electrode active material and CMC together with a solvent. That is, in the first kneading, the negative electrode active material and CMC are not yet completely dispersed in the solvent, and a shearing force is applied to them. Therefore, when the first kneading torque is too large, the negative electrode active material may be damaged. On the other hand, when the torque of the first kneading is too small, the non-adhered CMC that is not coated with the negative electrode active material cannot be uniformly dispersed in the kneaded product. Therefore, the kneading torque in the first kneading is not preferably too large or too small, and is preferably within the above range.

  According to the present invention, there is provided a method for producing a lithium ion secondary battery having a negative electrode plate with high adhesion strength and high cycle characteristics.

It is sectional drawing of the battery which concerns on embodiment. It is sectional drawing (AA sectional drawing of FIG. 1) of the battery which concerns on embodiment. It is sectional drawing (BB sectional drawing of FIG. 1) of the battery which concerns on embodiment. It is an expanded sectional view (C section of Drawing 3) of the battery concerning an embodiment. It is a figure which shows the schematic process of the manufacturing method of the negative electrode compound paste which concerns on embodiment. It is a graph which illustrates the difference in the viscosity of the kneaded material by the mixed powder and solid content rate in a 1st kneading | mixing process. It is a graph which shows the result of the peeling test of a negative electrode plate. It is a graph which shows the result of the cycle test of a secondary battery.

  DESCRIPTION OF THE PREFERRED EMBODIMENTS The best mode for embodying the present invention will be described below in detail with reference to the drawings.

[Secondary battery]
FIG. 1 shows a cross-sectional view of a secondary battery 1 according to this embodiment. The secondary battery 1 is manufactured by the method for manufacturing a lithium ion secondary battery of the present invention. As shown in FIG. 1, the secondary battery 1 is a lithium ion secondary battery including an electrode body 10, an electrolytic solution 50, and a battery case 60 that accommodates the electrode body 10 and the electrolytic solution 50. The battery case 60 includes a battery case main body 61 and a sealing plate 62. Further, the sealing plate 62 includes an insulating member 63.

The electrolytic solution 50 is obtained by dissolving an electrolyte in an organic solvent. Although not specifically limited, the organic solvent in this embodiment is a mixture of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC). In the electrolytic solution 50, these organic solvents are mixed in the following volume ratio.
EC: 3
DMC: 3
EMC: 4
Furthermore, the electrolytic solution 50 is an organic electrolytic solution in which lithium hexafluorophosphate (LiPF6) is added as a lithium salt as an electrolyte to the above mixed organic solvent to make the concentration of Li ions 1 mol / l.

  FIG. 2 is a cross-sectional view of the secondary battery 1 taken along the line AA shown in FIG. As shown in FIG. 2, the electrode body 10 is a flat wound electrode body. 3 is a cross-sectional view of the secondary battery 1 taken along the line BB shown in FIG. As shown in FIGS. 2 and 3, the electrode body 10 includes a belt-like positive electrode plate 20 and a belt-like negative electrode plate 30, and a belt-like separator 40 that is narrower than the positive electrode plate 20 and the negative electrode plate 30. Wrapped while pinching.

  As the separator 40, a porous film such as polypropylene (PP) or polyethylene (PE), or a composite material obtained by laminating a plurality of these in the thickness direction can be used. In this embodiment, a composite material made of PP / PE / PP is used as the separator 40.

  4 is an enlarged cross-sectional view of a portion C of the secondary battery 1 shown in FIG. As shown in FIG. 4, the positive electrode plate 20 has a positive electrode active material layer 22 formed on both surfaces of an aluminum foil 21 that is a positive electrode current collector. On the other hand, the negative electrode plate 30 is obtained by forming a negative electrode active material layer 32 on both surfaces of a copper foil 31 that is a negative electrode current collector.

  Further, as shown in FIG. 3, the positive electrode plate 20 has a portion protruding upward in the figure (rightward in FIG. 1) from the separator 40 and the negative electrode plate 30. The positive electrode active material layer 22 is not formed on this portion of the positive electrode plate 20, and the aluminum foil 21 is exposed. The positive electrode plate 20 is connected to the positive electrode terminal 70 at this portion.

  On the other hand, the negative electrode plate 30 has a portion protruding downward in FIG. 3 (leftward in FIG. 1) from the separator 40 and the positive electrode plate 20. In this portion of the negative electrode plate 30, the negative electrode active material layer 32 is not formed, and the copper foil 31 is exposed. The negative electrode plate 30 is connected to the negative electrode terminal 80 at this portion.

  Further, as shown in FIG. 1, the positive terminal 70 and the negative terminal 80 have ends 71 and 81 on the side not connected to the electrode plate, respectively, protruding outside the battery case 60 via the insulating member 63. Yes.

[Method for producing positive electrode plate]
A method for manufacturing the positive electrode plate 20 according to this embodiment will be described. The positive electrode plate 20 is formed by forming a positive electrode active material layer 22 on the surface of an aluminum foil 21. In this embodiment, the positive electrode active material layer 22 is formed by applying a positive electrode mixture paste obtained by dispersing a positive electrode material in a solvent on the surface of the aluminum foil 21 and then drying it.

As positive electrode materials, ternary LiNi _1/3 Co _1/3 Mn _1/3 O ₂ as a positive electrode active material, acetylene black (AB) as a conductive auxiliary agent, and polyfluoride as a binder Vinylidene (PVdF) is used. LiNi _1/3 Co _1/3 Mn _1/3 O _{2, which} is a positive electrode active material, is a component that contributes to charge and discharge in the positive electrode active material layer 22. AB, which is a conductive aid, is a component for increasing the conductivity in the positive electrode active material layer 22. PVdF as a binder is a component for binding the positive electrode active material layer 22 to the surface of the aluminum foil 21 while binding the components in the positive electrode active material layer 22 to each other.

The blending ratio of these positive electrode materials in the positive electrode mixture paste in this embodiment is shown below.
LiNi _1/3 Co _1/3 Mn _1/3 O ₂ : 90 wt%
AB: 5 wt%
PVdF: 5 wt%

[Method for producing negative electrode plate]
A method for manufacturing the negative electrode plate 30 according to the present invention will be described. The negative electrode plate 30 is formed by forming a negative electrode active material layer 32 on the surface of a copper foil 31. In this embodiment, the negative electrode active material layer 32 is formed by coating a negative electrode mixture paste obtained by dispersing a negative electrode material in a solvent on the surface of the copper foil 31 and then drying.

  As the negative electrode material, graphite as a negative electrode active material, carboxymethyl cellulose (CMC) as a thickener, and styrene butadiene rubber (SBR) as a binder are used. Graphite, which is a negative electrode active material, is a component that contributes to charge and discharge in the negative electrode active material layer 32. Specifically, the graphite used in this embodiment is natural graphite having an average particle size of 10 μm. CMC as a thickener is a component for uniformly dispersing each component in the negative electrode mixture paste. The SBR as a binder is a component for binding the negative electrode active material layer 32 to the surface of the copper foil 31 while binding the components in the negative electrode active material layer 32 to each other. In this embodiment, water is used as the solvent.

  A method for producing the negative electrode composite paste of this embodiment will be described with reference to the schematic process chart shown in FIG. As shown in FIG. 5, the negative electrode mixture paste of the present embodiment uses the negative electrode material and water to mix three kneading steps, “first kneading step”, “second kneading step”, and “third kneading step”. It is manufactured by sequentially performing the steps. The negative electrode mixture paste in this embodiment is manufactured by completing all the first to third kneading steps. What is in the middle of kneading in each kneading step is described as a kneaded product.

The blending ratio of the negative electrode material in the negative electrode mixture paste in this embodiment is shown below.
Graphite: 98.2 wt% (first kneading step)
CMC: 1.0 wt% (first kneading step 0.5 wt% + second kneading step 0.5 wt%)
SBR: 0.8 wt% (third kneading step)
In addition, as shown in parentheses above, all the amount of graphite is charged in the first kneading step. CMC is divided into a first kneading step and a second kneading step, and all the amounts are charged. All the amounts of SBR are charged in the third kneading step.

  First, the “first kneading step” will be described. The first kneading step is a kneading step performed first by storing graphite, CMC, and water in a kneading apparatus. Further, it is a kneading step for uniformly covering graphite with CMC by attaching CMC to the surface thereof. Furthermore, it is a kneading step for uniformly dispersing the graphite coated with CMC and the non-adhered CMC adhering to the graphite in water. Graphite is easily dispersed in water because its surface is uniformly coated with CMC. Thereby, finally, a homogeneous negative electrode mixture paste in which graphite is uniformly distributed can be produced.

  As described above, the amount of graphite used in the first kneading step is 98.2 wt% with respect to the weight of the negative electrode material in the negative electrode mixture paste. Further, the amount of CMC is 0.5 wt% with respect to the weight of the negative electrode material in the negative electrode mixture paste.

  In the first kneading step, it is preferable that graphite and CMC are first stored in powder form in a kneader and mixed to form a mixed powder, and then water is added and kneaded. This is because it is easy to uniformly coat the surface of graphite with CMC. Moreover, it is because the graphite coated with CMC and the unadhered CMC are easily dispersed uniformly in water.

  In the first kneading step, both graphite and CMC are used as powders. This is because the amount of water relative to graphite and CMC is adjusted to stably produce a kneaded material having an appropriate viscosity. Furthermore, by using this, a negative electrode mixture paste having an appropriate viscosity can be stably produced. For example, when graphite and a CMC aqueous solution in which CMC is previously dispersed in water are used, the amount of water relative to graphite and CMC cannot be adjusted. This is because the mixing ratio of graphite and CMC for forming the negative electrode active material layer 32 is determined in advance according to the specifications required for the secondary battery 1.

  Here, the quality of graphite and CMC varies in the manufacturing process. Specifically, variations in water absorption occur in graphite and CMC. The amount of water absorption is the amount of water that graphite and CMC absorb during kneading. These absorption amounts differ depending on the manufacturer even if they have the same specifications. Furthermore, even if it is the same specification of the same manufacturer, it differs depending on the production lot. For this reason, for example, even when graphite and CMC produced in different lots are mixed at the same blending ratio, a mixed powder having a large water absorption amount and a mixed powder having a small water absorption amount are produced.

  FIG. 6 is a graph illustrating the difference in viscosity of the kneaded product depending on the mixed powder and the solid content in the first kneading step. The solid content rate here is a ratio of the mixed powder, which is a solid content, in the kneaded product in the first kneading step. In FIG. 6, the vertical axis represents the viscosity of the kneaded product. The horizontal axis is the solid content rate. FIG. 6 shows the first and second mixed powders obtained by mixing graphite and CMC manufactured in different lots by the same manufacturer.

  As shown in FIG. 6, the viscosity of the kneaded product does not fall within the proper range if the amount of water relative to the mixed powder is too large or too small. And in any mixed powder, it is preferable to adjust the solid content rate with the center value of the range of the solid content rate when the viscosity of the kneaded material is in an appropriate range as a target. This is because the viscosity of the kneaded product falls within an appropriate range even if the adjustment causes a slight deviation from the target solid content. That is, in the first mixed powder, it is optimal to adjust the amount of water added so that the solid content rate is S1. On the other hand, in the second mixed powder, it is optimal to adjust the amount of water to be added so that the solid content is S2.

  And when the quantity of water whose solid content rate becomes S1 is added to the 2nd mixed powder, since there is too much quantity of water, a viscosity becomes lower than an appropriate range. On the other hand, when an amount of water having a solid content of S2 is added to the first mixed powder, the amount of water is too small and the viscosity becomes lower than the appropriate range. This is because the first mixed powder and the second mixed powder have different amounts of water absorption due to the different lots of graphite and CMC.

  Therefore, in this embodiment, before the first kneading step, the optimum water ratio is investigated for the mixed powder obtained by mixing graphite and CMC in the same lot and ratio as those used in the first kneading step. doing. The first kneading step is performed while adding the optimum ratio of water to the mixed powder obtained by mixing graphite and CMC in the same lot and ratio as in this investigation.

  Moreover, the same investigation is conducted again every time one of the lots of graphite and CMC changes, and every time the ratio of graphite and CMC mixed in the first kneading step changes. Each time, the ratio of water to be added is examined again. Thereby, a kneaded material having an appropriate viscosity can be stably produced.

  In addition, the kneading torque at the time of performing the first kneading step is not preferable whether it is too large or too small. The first kneading step is a step of first kneading powdered graphite and CMC together with water. That is, in the middle of the first kneading step, graphite and CMC are not yet completely dispersed in water. For this reason, when graphite and CMC rub against each other, a shearing force is applied to them. If the kneading torque is too large, an excessive shearing force is applied to the graphite in the kneaded product, which may damage the graphite. Further, the graphite, which is the negative electrode active material, may be damaged, and the cycle characteristics of the secondary battery 1 may be deteriorated.

  On the other hand, when the kneading torque is too small, graphite and CMC cannot be uniformly dispersed in the kneaded product. In particular, the unadhered CMC that is not coated with graphite may coagulate without being dispersed in the kneaded product. That is, in the first kneading step, the kneaded product must be kneaded with an appropriate torque. Specifically, the kneading torque in the first kneading step is preferably in the range of 1 to 2.3 times the kneading torque in the second and third kneading steps.

  Moreover, it is preferable that the molecular weight of CMC used for a 1st kneading process is 330,000 or less. In general, the smaller the molecular weight of CMC, the easier it is to adhere to graphite and to disperse in water. CMC having a molecular weight of 330,000 or less can uniformly coat the surface of graphite. Furthermore, the unadhered CMC that is not coated with graphite must be suitably dispersed in water. This is because when this coagulates without being dispersed, a homogeneous negative electrode mixture paste cannot be produced. CMC having a molecular weight of 330,000 or less can be suitably dispersed in water.

  In addition, when CMC having a molecular weight larger than 330,000 is used in the first kneading step, CMC is difficult to adhere to graphite. Also, CMC having a molecular weight greater than 330,000 is difficult to disperse in water itself. Therefore, in this case, it is necessary to knead with a large torque in order to disperse graphite and unadhered CMC in water. In other words, there is a risk of damaging the graphite.

  Further, the amount of CMC added in the first kneading step may be an amount sufficient to uniformly coat graphite. That is, it is preferable that the amount of unadhered CMC that does not cover graphite is small at this stage. This is because even if the kneading torque is the same, the shear force applied to the kneaded material increases as the amount of CMC increases. That is, the risk of damaging graphite increases.

  Next, the “second kneading step” will be described. The second kneading step is a kneading step performed by adding CMC and water to the kneaded material after the first kneading step. In addition, this is a kneading step for increasing the viscosity of the kneaded product in order to suppress sedimentation of components in the kneaded product. In the negative electrode mixture paste, the lower the viscosity, the more easily components such as graphite settle. And after the negative electrode mixture paste is manufactured, it is not always applied immediately. It may be stored for a while after manufacturing until it is applied. Therefore, the negative electrode mixture paste having an excessively low viscosity may not be homogeneous when applied. Note that the amount of CMC added in the second kneading step is 0.5 wt% with respect to the weight of the negative electrode material in the negative electrode mixture paste.

  In the second kneading step, powdery CMC and water may be separately added to the kneading apparatus in which the kneading part in which the first kneading step has been completed is housed, or CMC in which CMC is dispersed in water in advance. It may be added as an aqueous solution. However, the adjustment of the solid content ratio due to the variation in the water absorption amount may be insufficient only with the first kneading step. In this case, the solid content rate may be finely adjusted again while adding powdery CMC and water separately in the second kneading step.

  The molecular weight of CMC added in the second kneading step is preferably 330,000 or more. In general, the higher the molecular weight of CMC, the higher the viscosity of the kneaded product. CMC having a molecular weight of 330,000 or more can appropriately increase the viscosity of the kneaded product. As a result, the components are less likely to settle, and a homogeneous negative electrode mixture paste can be produced even during coating.

  When CMC having a molecular weight of less than 330,000 is used in the second kneading step, the viscosity of the kneaded product is difficult to increase. Therefore, in order to increase the viscosity of the kneaded product, a large amount of CMC having a molecular weight of less than 330,000 may be added. This is because the viscosity increases as the ratio of CMC to the kneaded product increases. However, excessive addition of CMC is not preferable because the internal resistance of the negative electrode active material layer 32 is increased. This is because CMC is a resistance component that does not contribute to charge / discharge.

  As described above, CMC having a molecular weight of more than 330,000 cannot be used in the first kneading step. This is because the first kneading step is a kneading step whose main purpose is to uniformly coat the surface of graphite with CMC. However, the graphite in the second kneading step is uniformly coated with CMC having a molecular weight of 330,000 or less in the first kneading step. Therefore, it is not necessary to use CMC that easily adheres to graphite in the second kneading step.

  The second kneading step is a step in which CMC and water are added and kneaded to the kneaded material in which the first kneading step is completed and graphite and CMC are suitably dispersed. For this reason, in the kneaded material in the second kneading step, even when CMC is added in the form of powder, the component in the powder state is only that. Thereby, even CMC having a molecular weight of 330,000 or more is easily dispersed and there is no fear of coagulation.

  Next, the “third kneading step” will be described. The third kneading step is a step for performing final kneading while adding SBR as a binder to the kneaded material after the second kneading step. In the third kneading step, the powdered SBR was added to the kneading apparatus containing the kneading section where the second kneading step was completed, and kneading was performed. The amount of SBR added in the third kneading step is 0.8 wt% with respect to the weight of the negative electrode material in the negative electrode mixture paste.

As described above, the negative electrode mixture paste of this embodiment was manufactured by the three kneading steps of “first kneading step”, “second kneading step”, and “third kneading step”. Moreover, the viscosity of the negative electrode mixture paste completed through these kneading steps is adjusted to be in the range of 2000 to 3000 mPa · s when the shear rate is 2 sec ⁻¹ .

[Confirmation of effect]
The present inventor conducted the following two tests in order to confirm the effect of the present invention.
1. Peel test 2. Cycle test

  The “1. Peel test” will be described. In order to perform this test, first, negative electrode plates of Examples 1 to 3, Comparative Examples 1 and 2, and a conventional example were prepared. Table 1 shows the respective production conditions of the produced negative electrode plate. The “kneading torque ratio” in Table 1 is the ratio of the kneading torque in the first kneading step to the kneading torque in the second and third kneading steps. In addition, the conditions that are not particularly mentioned below are manufactured under the same conditions as those for the method of manufacturing the negative electrode plate of the present embodiment. For example, the type of negative electrode material used, the mixing ratio thereof, and the kneading apparatus used for kneading are the same.

  The negative electrode plates of Examples 1 to 3 and Comparative Examples 1 and 2 were produced by the same procedure as the method for producing the negative electrode plate of this embodiment. However, the negative electrode plates of Examples 1 to 3 were produced by a method satisfying all the above-described conditions. On the other hand, the negative electrode plates of Comparative Examples 1 and 2 were manufactured by a method that did not satisfy at least one of the above-described conditions. That is, in the negative electrode plate of Comparative Example 1, the molecular weight of CMC used in the second kneading step is 280,000 and does not satisfy the above condition “330,000 or more”. In the negative electrode plate of Comparative Example 2, the molecular weight of CMC used in the first kneading step is 350,000, and does not satisfy the above condition “330,000 or less”. As for the kneading torque ratio, the negative electrode plates of Comparative Examples 1 and 2 both satisfy the above-mentioned condition “within a range of 1 to 2.3 times”.

  In addition, the negative electrode plate of the conventional example was manufactured by a procedure partially different from the method for manufacturing the negative electrode plate of this embodiment. That is, all of the negative electrode material graphite and CMC were added in the first kneading step. Therefore, CMC was not added at all in the second kneading step, and only water was added to the kneaded product after the completion of the first kneading step. Furthermore, for the negative electrode plate of the conventional example, the kneading torque in the first kneading step is 10 times the kneading torque in the second and third kneading steps, and is within the range of “1 to 2.3 times the above-mentioned conditions”. Is not satisfied.

  In the peel test, 90 ° peel tests were performed between the copper foil and the negative electrode active material layer using the negative electrodes of Examples 1 to 3, Comparative Examples 1 and 2, and the conventional example prepared under the above conditions. . FIG. 7 shows the result of the peel test. As shown in FIG. 7, it was confirmed that the peel strengths of the negative electrode plates of Examples 1 to 3 according to the present invention were all higher than the peel strengths of the negative electrode plates of Comparative Examples 1 and 2 and the conventional example.

  The following can be considered from the results of the peel test. First, in the negative electrode plate of Comparative Example 1, the molecular weight of CMC used in the second kneading step does not satisfy “330,000 or more”. As a result, the viscosity of the completed negative electrode mixture paste is low, and it is considered that the components have settled. In the negative electrode plate of Comparative Example 2, the molecular weight of CMC used in the first kneading step does not satisfy “330,000 or less”. As a result, it is considered that the non-adhered CMC not coated with graphite coagulated. In the negative electrode plate of the conventional example, all CMC is added in the first kneading step. For this reason, it is considered that the CMC in the first kneading step is excessive and coagulates despite being kneaded with a large torque. That is, all of the negative electrode plates of Comparative Examples 1 and 2 and the conventional example are manufactured using a heterogeneous negative electrode mixture paste in which the negative electrode material is biased. Therefore, in these negative electrode plates, it is considered that the adhesion strength between the copper foil and the negative electrode active material layer was low.

  On the other hand, all of the negative electrode plates of Examples 1 to 3 are manufactured using a homogeneous negative electrode mixture paste in which the distribution of the negative electrode material is not biased in accordance with the manufacturing method of the negative electrode plate of this embodiment. As a result, the negative electrode plate has high adhesion strength between the copper foil and the negative electrode active material layer.

  Next, “2. Cycle test” will be described. In order to perform this test, first, secondary batteries of Examples 1 to 3, Comparative Examples 1 and 2, and a conventional example were fabricated. In each of these secondary batteries, the negative electrode plates of Examples 1 to 3, Comparative Examples 1 and 2, and a conventional example manufactured by the same method as those used in “1. Peel test” are used. The configurations of these secondary batteries are the same as those of the secondary battery 1 except for the negative electrode plate.

In the cycle test, the secondary batteries of Examples 1 to 3, Comparative Examples 1 and 2, and the conventional example were repeatedly charged and discharged. As a common condition for the cycle test, the C rate represented by the ratio of the current value (A) to the full charge capacity (Ah) was charged / discharged at a current of 2C. Further, the SOC (Stat), which represents the range to be charged / discharged, is expressed by the ratio of the remaining battery capacity to the fully charged battery capacity.
e Of Charge) was in the range of 30-90%. Furthermore, charging / discharging under these conditions was repeated 500 cycles.

  FIG. 8 shows the results of the cycle test. The capacity maintenance ratio shown on the vertical axis in FIG. 8 is fully charged after the cycle test with respect to the battery capacity when the batteries are fully charged at the first time for each of the secondary batteries of Examples 1 to 3, Comparative Examples 1 and 2, and the conventional example Is the ratio of the battery capacity at the time. As shown in FIG. 8, the capacity maintenance rates of the secondary batteries of Examples 1 to 3 according to the present invention are all higher than the capacity maintenance rates of the secondary batteries of Comparative Examples 1 and 2 and the conventional example. confirmed.

  The following can be considered from the results of the cycle test. First, from the results of the peel test, the adhesion strength between the copper foil and the negative electrode active material layer is low in each of Comparative Examples 1 and 2 and the conventional negative electrode plate. In other words, in the secondary battery manufactured using the negative electrode plate having low adhesion strength, it is considered that the negative electrode active material layer expanded and contracted due to charging / discharging partially peeled off from the copper foil. Therefore, it is considered that the charge / discharge reaction of the peeled portion does not occur and the capacity retention rate is lowered.

  Further, in the negative electrode plate of Comparative Example 2, it is considered that the molecular weight of CMC used in the first kneading step was larger than 330,000, and the CMC dispersion was insufficient. Also, in the conventional negative electrode plate, it is considered that CMC was insufficiently dispersed because of excessive CMC in the first kneading step. For this reason, in the negative electrode plates of the secondary batteries of Comparative Example 2 and the conventional example, it is considered that Li was precipitated while charging and discharging were repeated with CMC that was insufficiently dispersed as a base point. Therefore, in the secondary batteries of Comparative Example 2 and the conventional example, it is considered that Li ions contributing to charging / discharging gradually became insufficient, and the capacity maintenance rate was lowered.

  On the other hand, all of the secondary batteries of Examples 1 to 3 are manufactured using a negative electrode plate having high adhesion strength. Furthermore, the secondary batteries of Examples 1 to 3 are each configured using a negative electrode plate manufactured using a homogeneous negative electrode mixture paste with no bias in the distribution of the negative electrode material. Therefore, the secondary battery has a high capacity maintenance rate and excellent cycle characteristics.

  As described above in detail, the negative electrode plate 30 according to the present embodiment is applied to the surface of the copper foil 31 by applying a negative electrode mixture paste in which the negative electrode material is dispersed in water, followed by drying. The active material layer 32 is formed. The negative electrode mixture paste is kneaded by adding a first kneading step for kneading powdered graphite and CMC together with water, a second kneading step for kneading by adding CMC and water, and further adding SBR. It is manufactured by the third kneading step. Further, the molecular weight of CMC used in the first kneading step is 330,000 or less. The molecular weight of CMC added in the second kneading step is 330,000 or more. In addition, the kneading torque in the first kneading step is in the range of 1 to 2.3 times the kneading torque in the second and third kneading steps. Thereby, a negative electrode plate with high adhesion strength is manufactured. In addition, by using the negative electrode plate, a lithium ion secondary battery with high cycle characteristics is realized.

  Note that this embodiment is merely an example, and does not limit the present invention. Therefore, the present invention can naturally be improved and modified in various ways without departing from the gist thereof. For example, the present invention is not limited to a flat lithium ion secondary battery, and can be similarly applied to any battery using a wound electrode body. For example, the present invention can also be applied to a lithium ion secondary battery having a stacked electrode body in which a negative electrode plate and a positive electrode plate are stacked without winding.

  In addition, for example, various materials including a positive electrode material and a negative electrode material are merely examples, and can be selected from various materials conventionally used in lithium ion secondary batteries. In addition, the solvent constituting the negative electrode mixture paste is not limited to water, and any mixed solvent mainly composed of water can be preferably used. As the solvent other than water constituting the mixed solvent, one or more organic solvents (lower alcohol, lower ketone, etc.) that can be uniformly mixed with water can be appropriately selected and used. Of these solvents, different solvents can be used for the first kneading step and the second kneading step.

DESCRIPTION OF SYMBOLS 1 ... Secondary battery 30 ... Negative electrode plate 31 ... Copper foil 32 ... Negative electrode active material layer

Claims (2)

In a method for producing a lithium ion secondary battery using a negative electrode plate formed by forming a negative electrode active material layer by applying a negative electrode mixture paste on at least one surface of a metal foil and then drying the paste;
As the negative electrode mixture paste,
Both of the first kneading of the powdered negative electrode active material and the first thickener together with the solvent,
A second kneading by adding a second thickener and a solvent to the kneaded product after the first kneading;
Using a product produced by adding a binder to the kneaded product after the second kneading and kneading the third kneading,
The first thickener is carboxymethylcellulose having a molecular weight of 330,000 or less, and the second thickener is carboxymethylcellulose having a molecular weight of 330,000 or more. Production method. In the manufacturing method of the lithium ion secondary battery of Claim 1,
A method for producing a lithium ion secondary battery, wherein the kneading torque in the first kneading is 1 to 2.3 times the kneading torque in the second and third kneading.

Images