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 manufacturing the same.

Background Art

[0002] Various techniques have been proposed regarding negative electrodes as disclosed in Patent Document 1.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0004] Active materials that expand and contract, such as Si, need to use high - strength binders such as polyimide binders. However, polyimide binders have low conductivity and are likely to increase resistance, so they cannot be added too much in an electrode using a Si - based active material. In the prior art, capacity degradation due to the expansion and contraction of the Si - based active material could not be sufficiently suppressed.

[0005] The present disclosure has been made in view of the above circumstances, and the main object is 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 Problems

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

[0007] <2> The first binder is at least one of polyimide and polyamideimide. <1> A method for manufacturing the negative electrode described above.

[0008] <3> The second binder is at least one selected from the group consisting of carboxymethylcellulose, styrene-butadiene rubber, and polyacrylic acid. <1> or <2> A method for manufacturing the negative electrode described above.

[0009] <4> The Si-based active material is at least one selected from the group consisting of elemental Si, Si oxide, and Si alloy. <1> ~ <3> A method for manufacturing a negative electrode as described in any one of the following.

[0010] <5> A negative electrode comprising a current collector and a negative electrode composite material disposed on the current collector, The negative electrode composite material comprises granular particles, graphite particles, and a second binder that does not contain an imide skeleton, comprising a Si-based active material, a conductive material, and a first binder containing an imide skeleton. The weight ratio of the first binder contained in the granular particles is 5% or more and 15% or less. A negative electrode having an average diameter of 50 μm or less for the granular particles. [Effects of the Invention]

[0011] The negative electrode and method for manufacturing the same described herein can improve the initial discharge capacity and capacity retention rate of lithium-ion secondary batteries. [Brief explanation of the drawing]

[0012] [Figure 1] FIG. 1 is a schematic cross-sectional view showing an example of the granulated particles of the present disclosure. [Figure 2] FIG. 2 is a schematic cross-sectional view showing an example of the negative electrode of the present disclosure.

MODE FOR CARRYING OUT THE INVENTION

[0013] Hereinafter, embodiments according to 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 grasped as design matters of those skilled in the art based on the prior art in the relevant field. The present disclosure can be implemented based on the content disclosed in this specification and the common technical knowledge in the relevant field. In the present disclosure, unless otherwise specified, the average particle size of the particles is the value of the volume-based median diameter (D50) measured by laser diffraction / scattering particle size distribution measurement. Further, in the present disclosure, the median diameter (D50) is the diameter (volume average diameter) at which the cumulative volume of the particles becomes half (50%) of the total volume when the particles are arranged in order from the particles with a small particle size.

[0014] In the present disclosure, a method for manufacturing a negative electrode, a step of creating granulated particles containing a Si-based active material, a conductive material, and a first binder containing an imide skeleton; a step of creating a negative electrode composite material containing the granulated particles, graphite particles, and a second binder not containing an imide skeleton; a step of applying and drying the negative electrode composite material to a current collector, wherein the weight ratio of the first binder contained in the granulated particles is 5% or more and 15% or less; and the average diameter of the granulated particles is 50 μm or less, and a method for manufacturing a negative electrode is provided.

[0015] In order to suppress capacity deterioration due to expansion and contraction of the Si-based active material, it is essential to use a high-strength binder that suppresses the formation of gaps between particles and a conductive material that assists in maintaining the electron path between particles in combination. Although it depends on the usage and type of Si-based active materials, generally in the case of an electrode made of a single Si-based active material (not compounded with graphite particles), the weight ratio within the composite material of Si-based active material: high-strength binder: conductive material is about 80:15:5, which is widely used in the research field.

[0016] Typical high-strength binders that can be used for the negative electrode of a lithium-ion secondary battery include polyimide and polyacrylic acid, which have the following characteristics respectively.

[0017] Polyimide is said to have high elasticity and high ductility and is most suitable for Si-based active materials. In addition, since some of its skeletons react with Li, it has ion permeability, and even when mixed in the composite material at a relatively high ratio (5% or more), the electrode resistance will not increase significantly. On the contrary, the material is expensive, it is difficult to dissolve in water and the paste tends to be in an NMP system, it is in the state of polyamic acid in the binder solution and requires heat treatment at about 250 °C or higher to be polyimidized, and it reacts with Li, which is a factor increasing 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 can be easily polymer-dissolved in an aqueous solution and does not require heat treatment for curing. However, due to its poor ion permeability and easy increase in electrode resistance, it is often used within 3% when introduced into the composite material (the characteristics vary slightly depending on the molecular weight, etc.).

[0019] Based on the above premise, in the present disclosure, in the case of using the above high-strength binder in a mixed electrode in which Si-based active material and graphite particles are composite, a structure is adopted in which it exists only around the necessary Si-based active material. In the present disclosure, first, a granule in which Si-based active material and conductive material are firmly fixed with polyimide, which can be used relatively often, is made, and this granule is fixed with graphite particles and a small amount of polyacrylic acid. s The advantages of this method are that, because the granulation manufacturing process using polyimide and the electrode coating process are separated, there is no need to fire the electrode itself, making it advantageous for mass production; because polyimide and conductive material are present only around the Si-based active material, the amount used in the entire electrode can be kept to a minimum; and because the granules, in which the particles are firmly bonded together, are further bound to graphite and current collectors with another binder, it is easy to achieve high strength with a small amount of binder overall.

[0020] In this disclosure, charge and discharge efficiency can be improved by placing a specific amount of polyimide and conductive material, necessary for improving durability, around the active material. By ensuring that the polyimide and conductive materials necessary for improving the durability of Si are present only around the Si-based active material, irreversible capacity can be minimized. Furthermore, although polyacrylic acid is a relatively strong binder, its ion permeability is lower than that of polyimide, so the amount added cannot be increased. The polyimide within the granules compensates for this binder deficiency, thereby improving charge and discharge efficiency. In addition, while polyimide generally requires heat treatment at 250°C or higher for the imidization of polyamic acid, in this disclosure only the granules can be heat-treated, eliminating the need for countermeasures against oxidation of the current collector or decomposition of polyacrylic acid due to heating.

[0021] The method for manufacturing a negative electrode according to this disclosure includes a granular particle preparation step, a negative electrode composite preparation step, and a drying step.

[0022] [Granulated particle creation process] The granular particle preparation process is a process for creating granular particles containing a Si-based active material, a conductive material, and a first binder containing an imide skeleton. Granulated particles may be prepared by creating 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 treatment using the granulated particle paste. The weight ratio of the first binder contained within the granular particles is 5% or more and 15% or less. The average diameter of the granulated particles may be 50 μm or less, 47 μm or less, or 14 μm or more. The average diameter of granular particles is determined by observing them using a scanning electron microscope (SEM). The longest line connecting two points on the outer circumference is taken as the diameter of that granular particle, and the average diameter is calculated by observing 20 granular particles.

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

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

[0025] As conductive materials, known materials can be used, such as carbon materials and metal particles. Examples of carbon materials include acetylene black (AB), furnace black, VGCF, carbon nanotubes (CNT), and carbon nanofibers. In particular, from the viewpoint of electronic conductivity, at least one selected from the group consisting of VGCF, carbon nanotubes, and carbon nanofibers may be used. Examples of metal particles include particles of Ni, Cu, Fe, and SUS. The content of the conductive material in the granular particles is not particularly limited, and the weight ratio of the conductive material contained in the granular particles may be 1% or more and 5% or less.

[0026] [Negative electrode composite material preparation process] The negative electrode mixture preparation process is a process for preparing a negative electrode mixture comprising the granulated particles, graphite particles, and a second binder that does not contain 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 carboxymethylcellulose (CMC), styrene-butadiene rubber (SBR), and polyacrylic acid. The weight ratio of the granular particles contained in the negative electrode mixture may be 6% or more and 21.6% or less. The weight ratio of 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 composite material may be 0.2% or more and 1.6% or less. The negative electrode composite material may include the conductive material described above, if necessary.

[0027] [Drying process] The drying process involves applying the negative electrode mixture to the current collector and drying it.

[0028] The material of the current collector may be a material that does not alloy with Li, and examples include SUS, copper, and nickel. Examples of the form of the current collector include foil-like and plate-like shapes. The planar shape of the current collector is not particularly limited, but examples include circular, elliptical, rectangular, and any polygonal shape. The thickness of the current collector varies depending on the shape, but may be in the range of 1 μm to 50 μm, or in the range of 5 μm to 20 μm.

[0029] A method for applying and drying the negative electrode mixture to a current collector involves, for example, preparing a slurry for the negative electrode layer by adding the negative electrode mixture to a solvent and stirring it, and then applying the slurry for the negative electrode layer to one surface of a support such as a current collector and drying it to obtain the negative electrode layer. Examples of solvents include butyl acetate, butyl butyrate, heptane, and N-methyl-2-pyrrolidone. The method for applying the slurry for the negative electrode layer onto one surface of a support such as a current collector is not particularly limited and includes methods such as the doctor blade method, metal mask printing method, electrostatic coating method, dip coating method, spray coating method, roll coating method, gravure coating method, and screen printing method. As a support, any self-supporting material can be appropriately selected and used, and is not particularly limited; for example, metal foils such as Cu and Al can be used.

[0030] The negative electrode of this disclosure is a negative electrode comprising a current collector and a negative electrode composite material disposed on the current collector, The negative electrode composite material comprises granular particles, graphite particles, and a second binder that does not contain an imide skeleton, comprising a Si-based active material, a conductive material, and a first binder containing an imide skeleton. The weight ratio of the first binder contained in the granular 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] Figure 1 is a schematic cross-sectional view showing an example of granulated particles of the present disclosure. As shown in Figure 1, the granular particles comprise a Si-based active material 10, a conductive material 20, and a first binder 30 containing an imide skeleton. Figure 2 is a schematic cross-sectional view showing an example of the negative electrode of this disclosure. As shown in Figure 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, the negative electrode composite material having granular particles 40, graphite particles 50, and a second binder 60 that does not contain an imide skeleton.

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

[0034] Lithium-ion secondary batteries include an outer casing that, if necessary, houses a positive electrode, a negative electrode, an electrolyte layer, and the like. The material of the outer casing is not particularly limited as long as it is stable in electrolytes, but examples include polypropylene, polyethylene, and resins such as acrylic resin.

[0035] Examples of lithium-ion secondary battery shapes include coin-type, laminate-type, cylindrical, and prismatic types.

[0036] Lithium-ion secondary batteries may be liquid-type lithium-ion secondary batteries using an electrolyte solution, or solid-type lithium-ion secondary batteries using a solid electrolyte solution. Applications of lithium-ion secondary batteries include, for example, power sources for vehicles such as hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), battery electric vehicles (BEVs), gasoline cars, and diesel cars. In particular, they may be used as power sources for hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), or battery electric vehicles (BEVs). Furthermore, lithium-ion secondary batteries may be used as power sources for mobile devices other than vehicles (e.g., trains, ships, aircraft), and as power sources for electrical products such as information processing devices. [Examples]

[0037] (Example 1) [Positive electrode fabrication] Cathode active material (average particle size: 10 μm, LiNi 0.8 Co 0.1 Mn 0.1 A positive electrode paste was prepared by kneading O2, a conductive material (granular acetylene black), and a binder (PVdF) with NMP in a ratio of 93:4:3. The solid content of the paste was adjusted with NMP to 65%. The prepared positive electrode paste was coated onto 15 μm thick aluminum foil using a blade coater and dried in a drying oven at 120°C for 10 minutes to obtain a coated body. The basis weight on one side after drying was 22 mg / cm². 2 The adjustments were made accordingly. Next, the coated body was pressed using a roll press. The positive electrode composite density of the positive electrode after pressing was 2.9 g / cc. [Fabrication of negative electrode granular particles] As the negative electrode active material, silicon monoxide particles with an average particle size of 6 μm were used, polyamic acid (UBE U-Varnish A) was used as the binder (first binder), and Ketjenblack was used as the conductive material. These were mixed in a ratio of 85:10:5, and NMP was added and kneaded to prepare a paste with a solid content of 52%. Next, this paste was used for spray granulation using a Büch B290 mini spray dryer. The drying temperature after spraying was set to 200°C. The obtained granular particles were placed in an atmospheric furnace and fired at 400°C for 30 minutes under an argon atmosphere. The average diameter of the granular particles after treatment was observed by SEM. Since the granular particles were irregular in shape, the diameter of each granular particle was determined by the longest line connecting two points on the outer circumference, and the average diameter was calculated by observing 20 granular particles. In this result, the average diameter of the granular particles was 28 μm. [Negative electrode fabrication] A coating paste was prepared by mixing 16 g of spheroidized 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 Co., Ltd.) as a binder (second binder), and 15 g of deionized water. The mixture was kneaded for 20 minutes using a planetary mixer. Next, this paste was applied to a 15 μm thick copper foil using a blade coater and dried at 120°C for 10 minutes. This negative electrode was then pressed using a roll press. The coating gap of the blade coater was adjusted to achieve the desired basis weight. The basis weight of the resulting negative electrode was 6.3 mg / cm². 2 That was the case. [Making coin-type batteries] The positive and negative electrodes described above were punched out into 16mm diameter discs and placed opposite each other via a 19mm diameter disc-shaped separator (a porous polyethylene material with 55% porosity and 20μm thickness) and placed inside a coin-type battery casing. 100μl of electrolyte (EC:FEC:EMC:DMC = 0.2:0.1:0.3:0.4 (vol ratio), LIPF61 [mol / kg]) was added to these electrode bodies, and the coin casing was crimped and sealed to fabricate a coin-type battery. [Evaluation of battery characteristics] The charge and discharge tests were conducted according to the following procedure. • First charge and discharge Charging: 4.2V, CCCV, 0.1mA cutoff, Current value: 1mA Discharge: 2.5V, CCCV, 0.1mA cutoff, Current value: 1mA • Calculation of capacity The initial discharge capacity [mAh] was used. • Measurement of DC resistance It was calculated from the voltage drop at the start of the initial discharge. • Cycle test (100 cycles) Charging: 4.2V, CCCV, 0.5mA cutoff, Current: 5mA Discharge: CC2.5V cutoff, Current value: 0.5mA Table 1 shows the composition of the granular particles, Table 2 shows the composition of the negative electrode composite, and Table 3 shows the results of the charge-discharge test.

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

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

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

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

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

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

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

[0045] (Example 10) The procedure was the same as in Example 1, except that the graphite particles used in the negative electrode composite were replaced with artificial graphite with an average particle size of 10 μm.

[0046] (Comparative Examples 1-5) The procedure was the same as in Example 1, except that instead of using granular particles, the SiO used in the granular particles of Example 1 was directly mixed into the negative electrode mixture, resulting in the composition shown in Table 2.

[0047] [Table 1]

[0048] [Table 2]

[0049] [Table 3]

[0050] [Discussion of the results] From a comparison of Example 1, Example 2, and Comparative Example 6, it was found that if the ratio of binder 1 in the granules is too low, the cycle characteristics deteriorate. From a comparison of Examples 1, 3, 4, and Comparative Example 7, it was found that when the size of the granulated material exceeds 50 μm, short-circuit behavior occurs during the cycle. In Comparative Example 7, observation of the electrode after short-circuiting revealed unevenness in lithium deposition. This suggests that if the granules are too large, the in-plane unevenness of lithium acceptance capacity in the negative electrode mixture becomes too large, leading to an uneven charge-discharge reaction. In Examples 6 to 10, some of the materials were changed, and the effects of this disclosure were confirmed even in these cases. From a comparison of Example 1 with Comparative Examples 1 and 2, it was found that when a simple mixture was used without granules, as in Comparative Examples 1 and 2, the cycle characteristics deteriorated in Comparative Example 1, which had an extremely low amount of polyimide. When the polyimide ratio was increased to almost the same level as that in the granules of Example 1, as in Comparative Example 2, the amount of polyimide in the entire negative electrode mixture became too high, and the initial capacity decreased due to the generation of irreversible capacity. Furthermore, when the capacity decreased to that of Comparative Example 2, the energy density of the battery became lower than when a graphite-only electrode without SiO was used. Comparative Examples 3 and 4, like Comparative Example 1, did not use granules, and the binder in the negative electrode mixture was polyacrylic acid. However, 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 cycle characteristics deteriorated significantly. In Comparative Example 5, similar to Comparative Example 1, granules were not used, and the binder was changed to CMC and SBR, which are commonly used in graphite anodes. However, the cycle characteristics deteriorated significantly, similar to Comparative Examples 1 and 3. A comparison of Example 1 with Comparative Examples 1-5 revealed that using granulated material 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 manufacturing a negative electrode, A process for creating granular particles comprising a Si-based active material, a conductive material, and a first binder containing an imide skeleton, A step of preparing a negative electrode composite material comprising the granular particles, graphite particles, and a second binder that does not contain an imide skeleton, The process includes applying the negative electrode mixture to a current collector and drying it, The weight ratio of the first binder contained in the granular particles is 5% or more and 15% or less. A method for manufacturing a negative electrode, wherein the average diameter of the granulated particles is 50 μm or less.

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

3. 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 carboxymethylcellulose, styrene-butadiene rubber, and polyacrylic acid.

4. The method for manufacturing a negative electrode according to claim 1, wherein the Si-based active material is at least one selected from the group consisting of elemental Si, Si oxide, and Si alloy.

5. A negative electrode comprising a current collector and a negative electrode composite material disposed on the current collector, The negative electrode composite material comprises granular particles, graphite particles, and a second binder that does not contain an imide skeleton, comprising a Si-based active material, a conductive material, and a first binder containing an imide skeleton. The weight ratio of the first binder contained in the granular particles is 5% or more and 15% or less. A negative electrode having an average diameter of 50 μm or less for the granular particles.

Images