JP7733693B2 - Negative electrode mixture slurry and its manufacturing method - Google Patents

Description

The present invention relates to a negative electrode mixture slurry and a method for producing the same.

The negative electrode of a non-aqueous electrolyte secondary battery has a negative electrode active material layer. The negative electrode active material layer is formed using a negative electrode mixture slurry containing the negative electrode active material. It is known to use a carbon-based active material as well as a Si-based active material as the negative electrode active material (for example, Patent Documents 1 and 2). Patent Document 1 discloses the use of Si-based alloy particles, and Patent Document 2 discloses the use of silicon oxide SiO _x (x = 1.01) as the silicon-based active material.

Japanese Patent Application Laid-Open No. 2021-163637 Patent No. 5754855

As the Si-based active material, a Si-containing active material containing oxygen but with a low oxygen content may be used. When a negative electrode mixture slurry containing such a Si-containing active material is stored, gas may be generated. Note that the Si-based alloy particles described in Patent Document 1 are alloys, and therefore contain almost no oxygen (less than 1 mass%), while the SiO _x (x = 1.01) described in Patent Document 2 is thought to have a high oxygen content (more than 30 mass%).

The purpose of this disclosure is to provide a negative electrode mixture slurry that can suppress gas generation even when using a Si-containing active material with a low oxygen content, and a method for producing the same.

[1] A method for producing a negative electrode mixture slurry, comprising:
A mixing step of mixing an active material, polyacrylic acid, and water,
the active material includes a Si-containing active material having an oxygen content of 1% by mass or more and 8% by mass or less,
The polyacrylic acid has a pH of 2 or more and 5.5 or less at 25°C when made into a 2% by mass aqueous solution,
The negative electrode mixture slurry has a pH of 3.5 or more and 6.0 or less at 25°C.
[2] The manufacturing method according to [1], wherein the Si-containing active material is SiC particles.
[3] The manufacturing method according to [1] or [2], wherein the specific surface area of the Si-containing active material is 3 m ² /g or more and 15 m ² /g or less.
[4] The manufacturing method according to any one of [1] to [3], wherein the content of the polyacrylic acid in the negative electrode mixture slurry is 0.3 mass% or more and 1.2 mass% or less with respect to the total amount of the active material.
[5] The manufacturing method according to any one of [1] to [4], wherein the active material further contains a carbon-based active material.
[6] The mixing step
mixing the active material and carboxymethyl cellulose to obtain a mixture;
a step of mixing the mixture, a conductive material, the polyacrylic acid, and water and kneading the mixture to obtain a kneaded body;
[6] The method for producing a styrene-butadiene rubber composition according to any one of [1] to [5], further comprising: kneading the kneaded body, styrene-butadiene rubber, and water.
[7] The manufacturing method according to [6], wherein the conductive material is fibrous carbon.
[8] A battery comprising an active material, polyacrylic acid, and water,
the active material includes a Si-containing active material having an oxygen content of 1% by mass or more and 8% by mass or less,
the content of the polyacrylic acid is 0.3% by mass or more and 1.2% by mass or less with respect to the total amount of the active material,
The negative electrode mixture slurry has a pH at 25°C of 3.5 or more and 6.0 or less.
[9] The negative electrode mixture slurry according to [8], wherein the Si-containing active material is SiC particles.
[10] The negative electrode mixture slurry according to [8] or [9], wherein the specific surface area of the Si-containing active material is 3 m ² /g or more and 15 m ² /g or less.
[11] The negative electrode mixture slurry according to any one of [8] to [10], wherein the active material further contains a carbon-based active material.
[12] The negative electrode mixture slurry according to any one of [8] to [11], further comprising at least one of carboxymethyl cellulose, styrene butadiene rubber, and fibrous carbon.

The negative electrode mixture slurry disclosed herein can suppress gas generation.

[Embodiment 1 (Method for Producing Negative Electrode Mixture Slurry)]
The negative electrode mixture slurry (1) (hereinafter also referred to as "the present slurry (1)") produced by the method for producing a negative electrode mixture slurry of this embodiment can be used to form a negative electrode active material layer of a negative electrode included in a nonaqueous electrolyte secondary battery (hereinafter also referred to as "secondary battery") such as a lithium ion secondary battery. The present slurry (1) contains an active material, polyacrylic acid (hereinafter also referred to as "PAA"), and water.

In addition to the active material, PAA, and water, the slurry (1) may contain binders other than PAA and conductive materials. Examples of binders include cellulose-based binders such as carboxymethyl cellulose (hereinafter also referred to as "CMC"), methyl cellulose (MC), and hydroxypropyl cellulose; styrene butadiene rubber (hereinafter also referred to as "SBR"), acrylonitrile butadiene rubber (NBR), polyvinylidene fluoride (PVdF), and polytetrafluoroethylene (PTFE). Examples of conductive materials include carbon materials such as fibrous carbon, carbon black (acetylene black, ketjen black, etc.), coke, and activated carbon. Examples of fibrous carbon include carbon nanotubes (hereinafter also referred to as "CNTs"). CNTs may be single-walled carbon nanotubes (SWCNTs) or multi-walled carbon nanotubes such as double-walled carbon tubes (DWCNTs).

The method for producing the slurry (1) includes a mixing step of mixing an active material, PAA, and water. The active material includes a Si-containing active material having an oxygen content of 1% by mass or more and 8% by mass or less. The PAA, when made into a 2% by mass aqueous solution, has a pH of 2 or more and 5.5 or less at 25°C. The pH of the slurry at 25°C is 3.5 or more and 6.0 or less.

The pH of the slurry (1) at 25°C may be 3.7 or more and 6.0 or less, 3.7 or more and 5.7 or less, or 3.8 or more and 5.5 or less. The pH described in this specification can be measured using a pH meter.

Si-containing active materials with an oxygen content within the above range are prone to gas generation when the pH of the negative electrode mixture slurry becomes alkaline. The gas generated from the negative electrode mixture slurry is presumably hydrogen gas generated by a reaction between the silicon contained in the Si-containing active material and alkaline components, and/or a charge compensation reaction that occurs when silicate ions are eluted from the Si-containing active material. This slurry (1) uses a PAA solution with a pH at 25°C within the above range when prepared as a 2% by mass aqueous solution. Because the pH of this slurry (1) is within the above range, the amount of gas generated from this slurry (1) can be reduced. This is thought to prevent point defects from occurring in the negative electrode active material layer formed using this slurry (1).

The oxygen content of the Si-containing active material may be 2% by mass or more and 7% by mass or less, or 3% by mass or more and 6% by mass or less. The Si-containing active material is preferably SiC particles. The SiC particles are a composite material of silicon (Si) and carbon (C). The SiC particles are preferably particles in which silicon nanoparticles are dispersed within porous carbon particles. The SiC particles are preferably particles in which silicon or a silicon oxide film (passive film) is exposed on the surface. When the Si-containing active material is SiC particles and silicon or a silicon oxide film is exposed on the surface, the silicon and the alkaline component are likely to react. However, according to the manufacturing method of the present slurry (1), the generation of gas from the present slurry (1) can be suppressed. The oxygen content of the Si-containing active material can be determined by the amount of oxygen extracted by a heat-melting method in an inert gas using an oxygen analyzer.

The specific surface area (BET) of the Si-containing active material may be 1 m ² /g or more and 20 m ² /g or less, 3 m ² /g or more and 15 m ² /g or less, or 3 m ² /g or more and 10 m ² /g or less. The specific surface area of the Si-containing active material can be calculated by inserting a predetermined mass of the Si-containing active material into a cell and measuring using a fully automatic specific surface area meter.

The content of the Si-containing active material in the slurry (1) is preferably 2% by mass or more and 50% by mass or less, and may be 3% by mass or more and 20% by mass or less, or 5% by mass or more and 15% by mass or less, relative to the total amount of active material. The content of the Si-containing active material in the active material used in the mixing step may also be within the above range.

The active material may contain a carbon-based active material such as graphite particles in addition to the Si-containing active material. Examples of carbon-based active materials include carbon (C) such as graphite, hard carbon, soft carbon, and amorphous-coated graphite. The carbon-based active material is preferably graphite particles. The content of the carbon-based active material in the present slurry (1) is preferably 50% by mass or more and 98% by mass or less, and may be 80% by mass or more and 97% by mass or less, or 85% by mass or more and 95% by mass or less, based on the total amount of active material. The content of the carbon-based active material in the active material used in the mixing step may also be within the above range.

The pH of a 2% by mass aqueous solution of PAA at 25°C may be 2 or more and 5.3 or less, 2.0 or more and 5.0 or less, or 2.0 or more and 4.0 or less. When the pH of the 2% by mass aqueous solution of PAA is within the above range, it becomes easier to adjust the pH of the present slurry (1) to the above range.

The PAA used in the method for producing the present slurry (1) may be in the acid form, the salt form, or a mixture thereof, but preferably contains the acid form (unneutralized PAA) or is in the acid form. When the PAA is in the salt form, examples of such salts include alkali metal salts such as lithium and sodium.

The PAA content in the present slurry (1) may be 3% by mass or more and 1.5% by mass or less, relative to the total amount of active material, but is preferably 0.3% by mass or more and 1.2% by mass or less, more preferably 0.3% by mass or more and 1.1% by mass or less, and may be 0.5% by mass or more and 1.0% by mass or less. The PAA content is calculated assuming that all of the PAA in the present slurry (1) is present in the form of an acid. By keeping the PAA content within the above range, it is possible to suppress an increase in the resistance of a secondary battery including a negative electrode active material layer formed using the present slurry (1), while adjusting the pH of the present slurry (1) to the above range.

The mixing step may be a step of mixing an active material with an aqueous solution of PAA, or a step of mixing an active material with an aqueous solution of PAA and water. The pH of the aqueous solution of PAA depends on the concentration of PAA in the aqueous solution. When an aqueous solution of PAA is used in the mixing step, the aqueous solution of PAA preferably contains 2% by mass or more and 40% by mass or less of PAA.

The mixing process is
Step (a) of mixing an active material and a CMC to obtain a mixture;
A step (b) of mixing the mixture, the conductive material, the PAA, and water and kneading the mixture to obtain a kneaded body;
and (c) kneading the kneaded body, SBR, and water.

The CMC used in step (a) may be in the form of a salt or an acid, but is preferably in the form of a salt. Step (a) may be a dry mixing process, in which the active material and CMC are mixed using a mixer such as a planetary mixer. When the CMC is in the form of a salt, examples of the salt include alkali metal salts such as lithium and sodium. The CMC used in step (a) is preferably a powder. The CMC used in the method for producing this slurry (1) may be such that, when the CMC is prepared into a 1% by mass aqueous solution, the pH at 25°C is 6.2 to 8.0, 6.5 to 7.5, or 6.8 to 7.3.

Step (b) may be a step of kneading the mixture obtained in step (a), an aqueous dispersion of a conductive material, an aqueous solution of PAA, and, if necessary, water. Step (b) may be performed using a mixer such as a planetary mixer. The aqueous dispersion of a conductive material used in step (b) is preferably an aqueous dispersion of fibrous carbon, more preferably an aqueous dispersion of CNT, and may be an aqueous dispersion of SWCNT. When an aqueous dispersion of CNT is used in step (b), the pH of the aqueous dispersion of CNT at a temperature of 25°C may be 7.0 or more and 10.0 or less, or 8.0 or more and 9.0 or less. The aqueous solution of PAA used in step (b) may have the same pH as the above-mentioned aqueous solution of PAA. In step (b), the amount of water added is adjusted to enable kneading.

The kneaded body obtained by the hard kneading in step (b) is considered to be in a state before it becomes a slurry state, for example, in a funicular state. A slurry state refers to a state in which the powder phase forms a discontinuous phase (is dispersed) in the continuous liquid phase. A funicular state refers to a state in which the liquid phase is a continuous phase, a gas phase (air) is present, and the powder phase is a continuous phase in which the powder particles are in contact with each other. The hard kneading performed in step (b) refers to mixing and kneading the mixture, conductive material, PAA, and water so as to obtain a kneaded body in a state before it becomes a slurry state (for example, a funicular state).

Step (c) may be a step of kneading the kneaded product obtained in step (b), an aqueous dispersion of SBR, and water. Step (c) may include a step of diluting the kneaded product with water and a step of kneading the diluted kneaded product with the aqueous dispersion of SBR. Step (c) may be performed using a mixer such as a planetary mixer. The aqueous SBR solution used in step (c) may have a pH at 25°C of 6.0 to 9.0, or 7.0 to 8.0. The amount of water added in step (c) is preferably adjusted to achieve the viscosity required for slurry (1), and the viscosity of the kneaded product obtained through step (c) is preferably the same as that of slurry (1).

[Embodiment 2 (Negative Electrode Mixture Slurry)]
The negative electrode mixture slurry of this embodiment (hereinafter also referred to as "the present slurry (2)") can be used to form a negative electrode active material layer of a negative electrode included in a secondary battery. The present slurry (2) contains an active material, PAA, and water. The active material includes a Si-containing active material having an oxygen content of 1 mass% or more and 8 mass% or less. The content of PAA in the present slurry (2) is 0.3 mass% or more and 1.2 mass% or less with respect to the total amount of the active material. The pH of the present slurry (2) at 25°C is 3.5 or more and 6.0 or less.

Si-containing active materials with an oxygen content within the above range are prone to gas generation when the pH of the negative electrode mixture slurry becomes alkaline. Because the pH of this slurry (2) is within the above range, the amount of gas generation can be suppressed. This is thought to prevent the occurrence of point defects in the negative electrode active material layer formed using this slurry (2). The pH of this slurry (2) at 25°C may be 3.7 or more and 6.0 or less, 3.7 or more and 5.7 or less, or 3.8 or more and 5.5 or less.

Since the PAA content of the present slurry (2) is within the above range, it is possible to suppress an increase in the resistance of a secondary battery including a negative electrode active material layer formed using the present slurry (2) while adjusting the pH of the present slurry (2) to the above range. The PAA content in the present slurry (2) is preferably 0.3% by mass or more and 1.1% by mass or less, and may be 0.5% by mass or more and 1.0% by mass or less, relative to the total amount of active material.

The active material, Si-containing active material, and PAA contained in this slurry (2) may be those described in the section on the manufacturing method for this slurry (1). The oxygen content of the Si-containing active material, the specific surface area (BET) of the Si-containing active material, and the pH range of a 2% by mass aqueous solution of PAA may be within the ranges described in the section on the manufacturing method for this slurry (1).

In addition to the active material, PAA, and water, the present slurry (2) may contain binders and conductive materials other than PAA. Examples of binders and conductive materials include those described in the section on the manufacturing method of the present slurry (1). The present slurry (2) preferably contains CMC, SBR, and fibrous carbon, and more preferably contains at least one of CMC, SBR, and CNT, and may contain two or all of these. The present slurry (2) may contain at least one of CMC, SBR, and SWCNT, or may contain all of these.

The present slurry (2) may be the present slurry (1) produced by the above-described method for producing the present slurry (1).

(Method for manufacturing negative electrode)
As described above, the present slurry (1) and the present slurry (2) can be used to form a negative electrode active material layer in a negative electrode of a secondary battery. A method for producing a negative electrode includes, for example, applying the present slurry (1) or the present slurry (2) onto a negative electrode current collector, drying and compressing the applied slurry, thereby forming a negative electrode active material layer on the negative electrode current collector. As described above, the present slurry (1) and the present slurry (2) have a reduced amount of gas generation. Therefore, it is believed that the occurrence of point-like defects in a coating film formed by applying the present slurry onto a negative electrode current collector can be suppressed, and a negative electrode having a negative electrode active material layer in which the occurrence of point-like defects is suppressed can be obtained.

(Nonaqueous electrolyte secondary battery)
The secondary battery includes an electrode assembly and an electrolyte. The secondary battery may include an exterior body that houses the electrode assembly and the electrolyte, and a resin sheet serving as an electrode holder may be disposed between the electrode assembly and the exterior body.

The electrode assembly includes the above-mentioned negative electrode, positive electrode, and separator. In the electrode assembly, the negative electrode active material layer of the negative electrode and the positive electrode active material layer of the positive electrode face each other with the separator interposed between them. The electrode assembly may be a laminated type in which the negative electrode, positive electrode, and separator are stacked, or a wound type in which a laminate in which the negative electrode, positive electrode, and separator are stacked is wound.

The positive electrode typically includes a positive electrode current collector and a positive electrode active material layer. The positive electrode current collector is typically a metal foil made of an aluminum material such as aluminum or an aluminum alloy. The positive electrode active material layer includes a positive electrode active material. Examples of the positive electrode active material include layered or spinel-based _lithium transition metal oxides (e.g., _LiNiCoMnO2 , _LiNiO2 , _LiCoO2 , _LiFeO2 , _LiMn2O4 , _{LiNi0.5Mn1.5O4} , _LiCrMnO4 , _LiFePO4 , and _{LiNi1 /3Co1 / 3Mn1/ 3O2} ). _The lithium transition metal oxide may be a lithium nickel cobalt manganese composite oxide (NCM).

The separator has a substrate and may have a functional layer on at least one side of the substrate. The substrate may be a film made of a resin such as polyethylene, polypropylene, polyester, cellulose, or polyamide, or a porous sheet such as a nonwoven fabric. The substrate may have a single-layer structure or a multi-layer structure. Examples of functional layers include an adhesive layer and a heat-resistant layer, and the separator may have one or both of these. The adhesive layer may be formed, for example, from an adhesive. The heat-resistant layer may contain, for example, a filler and a binder.

The electrolyte is usually a non-aqueous electrolyte, preferably a non-aqueous solvent such as an organic solvent containing a supporting salt. Examples of supporting salts include LiPF ₆ , LiBF ₄ , LiClO ₄ , LiFSO ₃ , and LiBOB (lithium bis(oxalato)borate). The electrolyte may contain one or more of these supporting salts. Examples of non-aqueous solvents include ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), propylene carbonate (PC), butylene carbonate (BC), and diethyl carbonate (DEC). The electrolyte may contain one or more of these non-aqueous solvents.

Hereinafter, the present disclosure will be described more specifically with reference to examples and comparative examples.
[Preparation of PAA aqueous solution]
(Preparation of PAA aqueous solution (1))
A PAA aqueous solution (1) was prepared using PAA (unneutralized PAA (PAA in the form of acid)) having a pH of 2 at 25°C when made into a 2% by mass aqueous solution.

(Preparation of PAA aqueous solution (2))
A PAA aqueous solution (2) was prepared using PAA (neutralized PAA (PAA in the form of a salt)) having a pH of 6.8 at 25°C when made into a 2% by mass aqueous solution.

(Preparation of PAA aqueous solution (3))
The PAA aqueous solution (1) and the PAA aqueous solution (2) were mixed to prepare the PAA aqueous solution (3). The PAA aqueous solution (3) was prepared so that the pH of the PAA aqueous solution (3) at 25°C was 5.3 when the PAA contained in the PAA aqueous solution (3) was made into a 2 mass% aqueous solution.
Example 1
(Preparation of active material)
As active materials, graphite particles as a carbon-based active material and SiC particles as a Si-containing active material were prepared. The SiC particles were porous carbon particles in which silicon nanoparticles were dispersed, and Si or a Si oxide film was exposed on at least a portion of the surface. Using an oxygen analyzer, the oxygen content of the SiC was calculated from the amount of oxygen extracted by a heat melting method in an inert gas, and was within the range of 1% by mass to 8% by mass. Using an automatic specific surface area meter, a predetermined mass of SiC was inserted into a cell to measure the specific surface area (BET) of the SiC, which was 8 m ² /g.

(Preparation of negative electrode mixture slurry)
Graphite particles, SiC particles, and powdered CMC were weighed and dry mixed using a planetary mixer to obtain a mixed powder (mixture). To this mixed powder, PAA aqueous solution (1), SWCNT aqueous dispersion, and water were weighed and added, and the mixture was kneaded using a planetary mixer to obtain a kneaded body. The kneaded body was diluted with water, and then an aqueous SBR dispersion was weighed and added and kneaded to obtain a negative electrode mixture slurry. The composition of the negative electrode mixture slurry was graphite particles / SiC particles / PAA / CMC / SBR / SWCNT / water / = 90 / 10 / 1 / 1 / 1 / 0.05 / 106 (mass ratio), and the content of PAA relative to the total amount of active material (100 mass%) was 1 mass%.

(Fabrication of Laminated Cells)
A negative electrode was fabricated using the negative electrode mixture slurry obtained above, and a laminate cell was obtained using this negative electrode.

[Examples 2 to 5, Comparative Examples 1 and 2]
A negative electrode mixture slurry was prepared in the same manner as in Example 1, except that the PAA aqueous solution shown in Table 1 was used and the content of PAA relative to the total amount (100 mass%) of the active material was set to the content shown in Table 1, and a laminate cell was obtained.

[pH Measurement]
The pH of the 2 mass % aqueous solution of PAA, the pH of the PAA aqueous solutions (1) to (3), and the pH of the negative electrode mixture slurry were all measured using a pH meter in an environment of 25° C. The results are shown in Table 1.

[Measurement of gas generation amount]
1 g of the negative electrode mixture slurry was placed in a laminated bag. After creating a vacuum inside the laminated bag, the laminated bag was sealed and used as a measurement sample. The volume V1 of the measurement sample was measured using a buoyancy method in a room at a temperature of 22±1°C. The measurement sample was left in a thermostatic chamber at a temperature of 45°C for two days. The measurement sample was removed from the thermostatic chamber and left in a room at a temperature of 22±1°C for one hour, after which the volume V2 of the measurement sample was measured using a buoyancy method. The value obtained by subtracting the volume V1 from the volume V2 was taken as the amount of gas generated from the negative electrode mixture slurry [cc/g]. The results are shown in Table 1.

[Measurement of Laminate Cell Resistance]
The DCIR (direct current internal resistance) of the laminated cells was measured at an SOC (state of charge) of 50% in an environment of -10°C. The DCIR of the laminated cells obtained in Examples 2 to 5 and Comparative Examples 1 and 2 was calculated as a relative value when the DCIR of the laminated cell obtained in Example 1 was set to 1. The results are shown in Table 1.

Claims (8)

A method for producing a negative electrode mixture slurry, comprising:
A mixing step of mixing an active material, polyacrylic acid, and water,
The active material includes a Si-containing active material having an oxygen content of 1% by mass or more and 8% by mass or less , and a carbon-based active material ,
The Si-containing active material is SiC particles in which silicon nanoparticles are dispersed within porous carbon particles,
The polyacrylic acid has a pH of 2 or more and 5.5 or less at 25°C when made into a 2% by mass aqueous solution,
The negative electrode mixture slurry has a pH of 3.5 or more and 6.0 or less at 25°C. The method according to claim 1 , wherein the specific surface area of the Si-containing active material is 3 m ² /g or more and 15 m ² /g or less. The manufacturing method described in claim 1 or 2, wherein the content of the polyacrylic acid in the negative electrode mixture slurry is 0.3 mass% or more and 1.2 mass% or less relative to the total amount of the active material. The mixing step
mixing the active material and carboxymethyl cellulose to obtain a mixture;
a step of mixing the mixture, a conductive material, the polyacrylic acid, and water and kneading the mixture to obtain a kneaded body;
The method according to claim 1 or 2, further comprising the step of kneading the kneaded body, styrene-butadiene rubber, and water. The manufacturing method according to claim 4 , wherein the conductive material is fibrous carbon. an active material, polyacrylic acid, and water;
The active material includes a Si-containing active material having an oxygen content of 1% by mass or more and 8% by mass or less , and a carbon-based active material ,
The Si-containing active material is SiC particles in which silicon nanoparticles are dispersed within porous carbon particles,
the content of the polyacrylic acid is 0.3% by mass or more and 1.2% by mass or less with respect to the total amount of the active material,
The negative electrode mixture slurry has a pH at 25°C of 3.5 or more and 6.0 or less. The negative electrode mixture slurry according to claim 6 , wherein the specific surface area of the Si-containing active material is 3 m ² /g or more and 15 m ² /g or less. The negative electrode mixture slurry according to claim 6 or 7 , further comprising at least one of carboxymethyl cellulose, styrene butadiene rubber, and fibrous carbon.