Negative electrode and method for manufacturing the same

Abstract

To provide a negative electrode capable of improving an initial discharge capacity and a capacity retention rate of a lithium ion secondary battery, and a method for manufacturing the same.SOLUTION: A method of manufacturing a negative electrode includes the steps of: manufacturing granulated particles each including a Si-based active material, a conductive material, and a first binder including an imide skeleton; manufacturing a negative electrode mixture including the granulated particles, graphite particles, and a second binder that does not include an imide skeleton; and coating the negative electrode mixture on a current collector and drying the negative electrode mixture. The weight ratio of the first binder included in each granulated particle is 5% or more and 15% or less, and the average particle diameter of the granulated particles is 50 μm or less.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to a negative electrode and a method for producing the same. [Background technology]

[0002] Various techniques have been proposed for negative electrodes such as those disclosed in Patent Document 1. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2016-225079 A [Patent Document 2] JP 2016-207318 A Summary of the Invention [Problem to be solved by the invention]

[0004] Active materials that expand and contract, such as Si, require the use of high-strength binders such as polyimide binders. However, polyimide binders have low electrical conductivity and are prone to increasing resistance, so they cannot be added in large amounts to electrodes that use Si-based active materials. As a result, conventional technologies have not been able to adequately suppress the deterioration of capacity due to the expansion and contraction of Si-based active materials.

[0005] The present disclosure has been made in consideration of the above-described circumstances, and has as its main object to provide a negative electrode capable of improving the initial discharge capacity and capacity retention rate of a lithium-ion secondary battery, and a method for manufacturing the same. [Means for solving the problem]

[0006] That is, the present disclosure includes the following aspects. <1> A method for producing a negative electrode, comprising the steps of: A step of preparing granulated particles including a Si-based active material, a conductive material, and a first binder including an imide skeleton; preparing a negative electrode mixture containing the granulated particles, graphite particles, and a second binder not containing an imide skeleton; applying the negative electrode mixture to a current collector and drying the same; The weight ratio of the first binder contained in the granule particles is 5% or more and 15% or less, The method for producing a negative electrode, wherein the granulated particles have an average diameter of 50 μm or less.

[0007] <2> The first binder is at least one of polyimide and polyamideimide. <1> 2. A method for producing the negative electrode according to claim 1 .

[0008] <3> The second binder is at least one selected from the group consisting of carboxymethyl cellulose, styrene butadiene rubber, and polyacrylic acid. <1> or <2> 2. A method for producing the negative electrode according to claim 1 .

[0009] <4> The Si-based active material is at least one selected from the group consisting of simple Si, Si oxide, and Si alloy. <1> ~ <3> 13. A method for producing a negative electrode according to claim 12.

[0010] <5> A negative electrode including a current collector and a negative electrode mixture disposed on the current collector, the negative electrode mixture includes granulated particles including a Si-based active material, a conductive material, and a first binder including an imide skeleton, graphite particles, and a second binder not including an imide skeleton; The weight ratio of the first binder contained in the granule particles is 5% or more and 15% or less, The negative electrode, wherein the granulated particles have an average diameter of 50 μm or less. Effect of the Invention

[0011] The negative electrode and the manufacturing method thereof according to the present disclosure can improve the initial discharge capacity and capacity retention rate of a lithium ion secondary battery. [Brief description of the drawings]

[0012] [Figure 1] FIG. 1 is a schematic cross-sectional view showing an example of a granulated particle according to the present disclosure. [Diagram 2] FIG. 2 is a schematic cross-sectional view showing an example of a negative electrode according to the present disclosure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] Hereinafter, the embodiments of the present disclosure will be described. In addition, matters other than those specifically mentioned in this specification and necessary for the implementation of the present disclosure (for example, the general configuration and manufacturing process of the negative electrode that do not characterize the present disclosure) can be understood as design matters of a person skilled in the art based on the conventional technology in the field. The present disclosure can be implemented based on the contents disclosed in this specification and the technical common sense in the field. In this disclosure, the average particle size of particles is the volume-based median diameter (D50) value measured by laser diffraction / scattering particle size distribution measurement, unless otherwise specified. In this disclosure, the median diameter (D50) is the diameter (volume average diameter) at which the cumulative volume of particles is half (50%) of the total volume when the particles are arranged in order from the smallest particle size.

[0014] The present disclosure provides a method for producing a negative electrode, comprising the steps of: A step of preparing granulated particles including a Si-based active material, a conductive material, and a first binder including an imide skeleton; preparing a negative electrode mixture containing the granulated particles, graphite particles, and a second binder not containing an imide skeleton; applying the negative electrode mixture to a current collector and drying the same; The weight ratio of the first binder contained in the granule particles is 5% or more and 15% or less, The method for producing a negative electrode, wherein the granulated particles have an average diameter of 50 μm or less, is provided.

[0015] To prevent capacity degradation due to expansion and contraction of silicon-based active materials, it is essential to use both a high-strength binder that prevents the distance between particles from increasing and a conductive material that helps maintain the electronic paths between particles. Although it depends on the type and usage of the Si-based active material, generally, for electrodes using only Si-based active material (not composited with graphite particles), a weight ratio of Si-based active material: high-strength binder: conductive material in the composite of approximately 80:15:5 is often used in research areas.

[0016] Representative high-strength binders that can be used in the negative electrodes of lithium-ion secondary batteries include polyimide and polyacrylic acid, each of which has the following characteristics.

[0017] Polyimide is said to be the most suitable Si-based active material because of its high elasticity and ductility. In addition, it has ion permeability due to the fact that some of its skeleton reacts with Li, and even if it is mixed into the composite at a relatively high ratio (5% or more), the electrode resistance does not increase significantly. The trade-offs are that the material is expensive, it is difficult to dissolve in water and the paste tends to become NMP-based, it is in the form of polyamic acid in the binder solution and requires heating at about 250°C or higher to turn it into polyimide, and it reacts with Li, which increases the irreversible capacity of the negative electrode.

[0018] Although polyacrylic acid is inferior to polyimide in terms of ductility, it has high elasticity comparable to polyimide. In addition, it is easy to dissolve the polymer in an aqueous solution, and does not require heat treatment for hardening. However, since it has poor ion permeability and is prone to increasing electrode resistance, it is often added to the mixture at 3% or less (characteristics change slightly depending on molecular weight, etc.).

[0019] Based on the above premise, in the present disclosure, when the above high-strength binder is used in a mixed electrode in which a Si-based active material and graphite particles are combined, the structure is such that the binder is present only around the Si-based active material where it is required. In this disclosure, a granule is first created by firmly bonding the Si-based active material and conductive material together using polyimide, which is a relatively widely available material, and then this granule is bonded together with graphite particles and a small amount of polyacrylic acid. The advantages of this method are that the granule manufacturing process using polyimide is separated from the electrode coating process, eliminating the need to sinter the electrode itself, making it advantageous for mass production; that the polyimide and conductive material are only present around the Si-based active material, so the amount used can be kept to a minimum for the entire electrode; and that the granules, which have firmly bonded the particles together, are further bonded to the graphite and current collector with another binder, making it easier to achieve high strength with less binder overall.

[0020] In the present disclosure, the charge / discharge efficiency can be improved by having a specific amount of polyimide and conductive material necessary for improving durability present around the active material. The irreversible capacity can be minimized by having the polyimide and conductive material necessary for improving the durability of Si present only around the Si-based active material. Although polyacrylic acid is a relatively strong binder, its ion permeability is lower than that of polyimide, and the amount added cannot be increased. The polyimide in the granules compensates for the binder shortage, thereby improving the charge / discharge efficiency. In addition, polyimide generally requires heat treatment at 250°C or higher to imidize polyamic acid, but in the present disclosure, only the granules can be heat treated, so there is no need to take measures against oxidation of the current collector due to heating or decomposition of polyacrylic acid, etc.

[0021] The method for producing a negative electrode according to the present disclosure includes a granulated particle preparation step, a negative electrode mixture preparation step, and a drying step.

[0022] [Granule particle production process] The granulated particle preparation step is a step of preparing granulated particles containing a Si-based active material, a conductive material, and a first binder containing an imide skeleton. The granulated particles may be prepared by preparing a granulated particle paste containing a Si-based active material, a conductive material, a first binder containing an imide skeleton, and a solvent such as N-methylpyrrolidone (NMP), and then performing a spray-type granulation process using the granulated particle paste. The weight ratio of the first binder contained in the granule particles is 5% or more and 15% or less. The average diameter of the granulated particles may be 50 μm or less, may be 47 μm or less, or may be 14 μm or more. The average diameter of the granulated particles is determined by observing them with a SEM, and the longest straight line connecting two points on the periphery is taken as the diameter of the granulated particle. The average diameter is calculated by observing 20 granulated particles.

[0023] The first binder may be at least one of a polyimide and a polyamideimide.

[0024] The Si-based active material may be at least one selected from the group consisting of simple Si, Si oxide, Si-C composite, and Si alloy.

[0025] The conductive material may be a known material, such as a carbon material and metal particles. The carbon material may be, for example, acetylene black (AB), furnace black, VGCF, carbon nanotubes (CNT), and carbon nanofibers. Among them, from the viewpoint of electronic conductivity, at least one selected from the group consisting of VGCF, carbon nanotubes, and carbon nanofibers may be used. The metal particles may be particles of Ni, Cu, Fe, SUS, etc. The content of the conductive material in the granulated particles is not particularly limited, and the weight ratio of the conductive material contained in the granulated particles may be 1% or more and 5% or less.

[0026] [Negative electrode mixture preparation process] The negative electrode mixture preparation step is a step of preparing a negative electrode mixture containing the granulated particles, graphite particles, and a second binder not containing an imide skeleton. The graphite particles may be at least one selected from the group consisting of natural graphite particles and artificial graphite particles. The second binder may be at least one selected from the group consisting of carboxymethyl cellulose (CMC), styrene butadiene rubber (SBR), and polyacrylic acid. The weight ratio of the granulated particles contained in the negative electrode mixture may be 6% or more and 21.6% or less. The weight ratio of the graphite particles contained in the negative electrode mixture may be 76.7% or more and 92.2% or less. The weight ratio of the second binder contained in the negative electrode mixture may be 0.2% or more and 1.6% or less. The negative electrode mixture contains the above-mentioned conductive material as necessary.

[0027] [Drying process] The drying step is a step of applying the negative electrode mixture onto a current collector and drying it.

[0028] The material of the current collector may be a material that does not alloy with Li, and examples of such materials include SUS, copper, and nickel. Examples of the shape of the current collector include foil and plate. The shape of the current collector in plan view is not particularly limited, and examples of such shapes include a circle, an ellipse, a rectangle, and any polygonal shape. The thickness of the current collector varies depending on the shape, and may be, for example, in the range of 1 μm to 50 μm, or in the range of 5 μm to 20 μm.

[0029] The method of applying the negative electrode mixture to the current collector and drying it includes, for example, putting the negative electrode mixture into a solvent and stirring the mixture to prepare a slurry for the negative electrode layer, applying the slurry for the negative electrode layer onto one side of a support such as a current collector, and drying the slurry to obtain the negative electrode layer. Examples of the solvent include butyl acetate, butyl butyrate, heptane, and N-methyl-2-pyrrolidone. The method for applying the negative electrode layer slurry onto one surface of a support such as a current collector is not particularly limited, and examples thereof include a doctor blade method, a metal mask printing method, an electrostatic application method, a dip coating method, a spray coating method, a roll coating method, a gravure coating method, and a screen printing method. The support can be appropriately selected from those having self-supporting properties and is not particularly limited. For example, metal foils such as Cu and Al can be used.

[0030] The negative electrode of the present disclosure is a negative electrode including a current collector and a negative electrode composite material disposed on the current collector. The negative electrode composite material has granulated particles, graphite particles, and a second binder not including an imide skeleton, the granulated particles including an Si-based active material, a conductive material, and a first binder including an imide skeleton. The weight ratio of the first binder contained in the granulated particles is 5% or more and 15% or less. The average diameter of the granulated particles is 50 μm or less.

[0031] The negative electrode includes a current collector and a negative electrode composite material disposed on the current collector. The current collector and the negative electrode composite material are as described above.

[0032] FIG. 1 is a schematic cross-sectional view showing an example of the granulated particles of the present disclosure. As shown in FIG. 1, the granulated particles include an Si-based active material 10, a conductive material 20, and a first binder 30 including an imide skeleton. FIG. 2 is a schematic cross-sectional view showing an example of the negative electrode of the present disclosure. As shown in FIG. 2, the negative electrode includes a negative electrode current collector 70 and a negative electrode composite material disposed on the negative electrode current collector 70, and the negative electrode composite material has granulated particles 40, graphite particles 50, and a second binder 60 not including an imide skeleton.

[0033] The negative electrode of the present disclosure is used in a lithium-ion secondary battery. The lithium-ion secondary battery includes a positive electrode, the negative electrode of the present disclosure, and an electrolyte layer between the positive electrode and the negative electrode.

[0034] The lithium-ion secondary battery may include an exterior body that houses the positive electrode, the negative electrode, the electrolyte layer, etc., as necessary. The material of the exterior body is not particularly limited as long as it is stable to the electrolyte, and examples thereof include resins such as polypropylene, polyethylene, and acrylic resin.

[0035] Examples of the shape of the lithium-ion secondary battery include a coin type, a laminate type, a cylindrical type, and a square type.

[0036] The lithium ion secondary battery may be a liquid lithium ion secondary battery using an electrolytic solution as an electrolyte, or may be a solid lithium ion secondary battery using a solid electrolyte as an electrolyte. Examples of applications of the lithium ion secondary battery include power sources for vehicles such as hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), electric vehicles (BEVs), gasoline automobiles, and diesel automobiles. In particular, the lithium ion secondary battery may be used as a driving power source for hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), or electric vehicles (BEVs). The lithium ion secondary battery may also be used as a power source for moving objects other than vehicles (for example, railways, ships, and aircraft), and may also be used as a power source for electrical products such as information processing devices. EXAMPLES

[0037] Example 1 [Positive electrode production] Cathode active material (average particle size: 10μm, LiNi 0.8 Co 0.1 Mn 0.1 O 2 ), conductive material (granular acetylene black), and binder (PVdF) were mixed with NMP in a ratio of 93:4:3 to prepare a positive electrode paste. The solid content of the paste was adjusted to 65% with NMP. The prepared positive electrode paste was applied to a 15 μm thick aluminum foil using a blade coater and dried in a drying furnace at 120°C for 10 minutes to obtain a coated body. After drying, the weight per surface area on one side was 22 mg / cm. 2 The coated body was then pressed with a roll press machine. The density of the positive electrode mixture after pressing was 2.9 g / cc. [Preparation of negative electrode granules] Silicon monoxide particles with an average particle size of 6 μm were used as the negative electrode active material, polyamic acid (UBE U Varnish A) was used as the binder (first binder), and Ketjen Black was used as the conductive material. These were mixed to 85:10:5, NMP was added and kneaded to prepare a paste with a solid content of 52%. Next, using this paste, a spray-type granulation process was performed using a Buchi Mini Spray Dryer B290. The drying temperature after spraying was 200°C. The obtained granulated particles were placed in an atmospheric furnace and baked at 400°C for 30 minutes under an argon atmosphere. The average diameter of the granulated particles after the process was observed by SEM. Since the granulated particles were irregular, the longest straight line connecting two points on the periphery was taken as the diameter of the granulated particles, and the average diameter was calculated by observing 20 granulated particles. In this result, the average diameter of the granulated particles was 28 μm. [Negative electrode production] 16 g of spherical natural graphite with an average particle size of 17 μm, 4.5 g of the above granulated particles, 3 g of polyacrylic acid solution (SW-100 manufactured by Sumitomo Seika Chemicals) as a binder (second binder), and 15 g of ion-exchanged water were mixed and kneaded for 20 minutes with a planetary mixer to prepare a coating paste. Next, this paste was applied to a copper foil with a thickness of 15 μm with a blade coater and dried at 120° C. for 10 minutes. This negative electrode was pressed with a roll press machine. The coating gap of the blade coater was adjusted to the desired basis weight. The basis weight of the obtained negative electrode was 6.3 mg / cm 2 It was. [Coin cell battery production] The positive and negative electrodes were punched out into disks with a diameter of 16 mm, and placed opposite each other through a disk-shaped separator (porosity 55%, thickness 20 μm) with a diameter of 19 mm, and placed in a coin-type battery can. The electrode body was filled with an electrolyte (EC:FEC:EMC:DMC=0.2:0.1:0.3:0.4 (vol ratio), LIPF 6 1 [mol / kg]) was added to the coin can, which was then crimped and sealed to create a coin battery. [Evaluation of battery characteristics] A charge / discharge test was carried out as follows. First charge / discharge Charging: 4.2V, CCCV, 0.1mA cutoff, current value: 1mA Discharge: 2.5V, CCCV, 0.1mA cutoff, current value: 1mA Capacity calculation The initial discharge capacity [mAh] was used. -DC resistance measurement It was calculated from the voltage drop at the start of the first discharge. Cycle test (100 cycles) Charging: 4.2V, CCCV, 0.5mA cutoff, current value: 5mA Discharge: CC2.5V cut, current value: 0.5mA The composition of the granulated particles is shown in Table 1, the negative electrode mixture configuration in Table 2, and the results of the charge / discharge test in Table 3.

[0038] (Example 2, Comparative Example 6) The same procedure as in Example 1 was repeated except that the ratio of polyimide in the granulated particles was changed as shown in Table 1.

[0039] (Example 3, Example 4, Comparative Example 7) The same procedure as in Example 1 was repeated except that the average diameter of the granulated particles was changed as shown in Table 1 by adjusting the solid content of the spray paste during granulation.

[0040] Example 5 The same procedure as in Example 1 was repeated except that the silicon-based active material used for the granulated particles was changed to silicon particles having an average particle size of 0.7 μm.

[0041] Example 6 The silicon particles used in Example 5 were mixed with a polyamic acid (UBE U Varnish A) solution and polyvinylidene fluoride (#7300) in a weight ratio of 1:15:3, and then fired in an atmosphere furnace at 1200°C for 1 hour under an Ar atmosphere. The fired solid was pulverized in a ball mill and sieved to obtain a Si-C composite with an average particle size of 3 μm. The same procedure as in Example 1 was repeated, except that this Si-C composite was used as the active material for the granulated particles.

[0042] Example 7 The same procedure as in Example 1 was repeated except that the first binder used for the granulated particles was changed to polyamideimide by using a polyamideimide solution (Viromax HR-11MM, manufactured by Toyobo Co., Ltd.).

[0043] Example 8 The same procedure as in Example 1 was repeated except that the conductive material used for the granulated particles was changed to CNT (TUBALL manufactured by OCSIAL), and the same CNT was added to the negative electrode mixture in the composition shown in Table 2.

[0044] Example 9 The same procedure as in Example 1 was repeated, except that the second binder used in the negative electrode mixture was changed to CMC and SBR to give the composition shown in Table 2.

[0045] Example 10 The same procedure as in Example 1 was repeated except that the graphite particles used in the negative electrode mixture were changed to artificial graphite having an average particle size of 10 μm.

[0046] (Comparative Examples 1 to 5) The same procedure as in Example 1 was repeated except that no granulated particles were used and the SiO used for the granulated particles in Example 1 was directly mixed with the negative electrode mixture to produce the composition shown in Table 2.

[0047] [Table 1]

[0048] [Table 2]

[0049] [Table 3]

[0050] [Discussion of the results] From a comparison between Example 1, Example 2, and Comparative Example 6, it was found that if the ratio of binder 1 in the granules was too low, the cycle characteristics deteriorated. From a comparison of Examples 1, 3, and 4 and Comparative Example 7, it was found that when the size of the granules was greater than 50 μm, short-circuit behavior occurred during cycling. In Comparative Example 7, when the electrode after short circuit was observed, uneven precipitation of lithium was observed. It is considered that if the granules are too large, the in-plane unevenness of the lithium acceptance capacity in the negative electrode mixture becomes too large, resulting in non-uniform charge / discharge reactions. In Examples 6 to 10, some of the materials were changed, and the effects of the present disclosure were confirmed even in these cases. From a comparison of Example 1 with Comparative Example 1 and Comparative Example 2, it was found that when granules were not used and simple mixing was performed as in Comparative Example 1 and Comparative Example 2, the cycle characteristics deteriorated in Comparative Example 1, in which the polyimide content was extremely low, and when the polyimide ratio was increased to almost the same level as the polyimide ratio in the granules in Example 1 as in Comparative Example 2, the amount of polyimide in the entire negative electrode mixture became too large, and the initial capacity decreased due to the occurrence of irreversible capacity. Note that when the capacity is reduced to that of Comparative Example 2, the energy density of the battery becomes lower than when a graphite single electrode without SiO is used. In Comparative Examples 3 and 4, similarly to Comparative Example 1, no granules were used and the binder in the negative electrode mixture was polyacrylic acid. It was found that when the amount of binder was small as in Comparative Example 3, the cycle characteristics deteriorated, and when the amount of binder was large, the battery resistance became extremely high, and both the initial capacity and the cycle characteristics were significantly deteriorated. In Comparative Example 5, like Comparative Example 1, no granules were used, and the binder was changed to CMC and SBR, which are commonly used in graphite negative electrodes. However, like Comparative Examples 1 and 3, the cycle characteristics were significantly deteriorated. Comparison of Example 1 with Comparative Examples 1 to 5 reveals that the use of the granules can improve both the initial discharge capacity and the capacity retention rate. [Explanation of symbols]

[0051] 10 Si-based active material 20 Conductive materials 30 First Binder 40 Granule particles 50 Graphite particles 60 Second Binder 70 Negative electrode current collector

Claims

1. A method for producing a negative electrode, comprising the steps of: A step of preparing granulated particles including a Si-based active material, a conductive material, and a first binder including an imide skeleton; preparing a negative electrode mixture containing the granulated particles, graphite particles, and a second binder not containing an imide skeleton; applying the negative electrode mixture to a current collector and drying the same; The weight ratio of the first binder contained in the granule particles is 5% or more and 15% or less, The method for producing a negative electrode, wherein the granulated particles have an average diameter of 50 μm or less.

2. 2. The method for producing a negative electrode according to claim 1, wherein the first binder is at least one of a polyimide and a polyamideimide.

3. 2. The method for producing a negative electrode according to claim 1, wherein the second binder is at least one selected from the group consisting of carboxymethyl cellulose, styrene-butadiene rubber, and polyacrylic acid.

4. The method for producing a negative electrode according to claim 1 , wherein the Si-based active material is at least one selected from the group consisting of simple Si, Si oxides, and Si alloys.

5. A negative electrode including a current collector and a negative electrode mixture disposed on the current collector, the negative electrode mixture includes granulated particles including a Si-based active material, a conductive material, and a first binder including an imide skeleton, graphite particles, and a second binder not including an imide skeleton; The weight ratio of the first binder contained in the granule particles is 5% or more and 15% or less, The average diameter of the granulated particles is 50 μm or less.

Images