Preparation method of graphite-silicon mixed negative electrode slurry, negative electrode plate and lithium ion battery

Abstract

The invention belongs to the technical field of lithium ion batteries, and particularly relates to a preparation method of graphite-silicon mixed negative electrode slurry, a negative electrode plate and a lithium ion battery. Comprising the following steps: firstly, adding a polyacrylic acid PAA glue solution, conductive agent slurry and conductive agent powder into a pulping machine, and quickly stirring to obtain conductive adhesive slurry; then adding silicon powder into the obtained conductive adhesive slurry, and rapidly stirring to obtain silicon slurry; adding the graphite powder into another pulping machine, slowly stirring for a short time, adding a sodium carboxymethyl cellulose (CMC) glue solution and water, slowly stirring for a long time, and adjusting the solid content of the slurry to 62-68% to obtain graphite slurry; and adding the silicon slurry into the obtained graphite slurry in three times, and adding water to adjust the solid content of the slurry to be 45-55%. And finally, adding a styrene butadiene rubber SBR emulsion into the obtained slurry, slowly stirring, vacuumizing and sieving to obtain the final slurry. According to the preparation method of the slurry, the dispersion uniformity of silicon in graphite can be effectively improved, the volume expansion in the silicon circulation process is inhibited, the connection of the conductive agent and the silicon particles is promoted and the silicon conductivity is improved through the hydrogen bond bonding between the polyacrylic acid PAA with strong bonding force and the silicon particles, so that the resistance of a pole piece is reduced, and the cycle performance of the whole battery is improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-ion batteries, and in particular relates to a method for preparing a graphite-silicon mixed negative electrode slurry, a negative electrode sheet and a lithium-ion battery. Background Art

[0002] Silicon negative electrode materials have become a key material for improving the energy density of lithium-ion batteries due to their advantages such as high theoretical capacity and low operating potential. However, silicon has problems such as poor conductivity and large volume expansion during charging and discharging. Therefore, mixing silicon with graphite can take advantage of the low expansion characteristics of graphite to absorb the high expansion stress of silicon. At the same time, the high conductivity of graphite can build a conductive network, which is the preferred solution for the commercial application of silicon at this stage. However, in the application of mixed silicon and graphite negative electrodes, the uniformity of silicon dispersion in graphite is a key factor that restricts the performance and consistency of the entire battery. Due to the agglomeration of silicon particles, local expansion will increase, causing silicon powdering and repeated rupture and growth of the solid electrolyte membrane (SEI), resulting in excessive consumption of electrolyte, thereby causing deterioration of the performance of the entire battery.

[0003] There are three main types of negative electrode slurrying processes in the currently disclosed patents: (1) dry process, first mix the dry powders of active material, conductive agent and binder, then add solvent and knead them, and finally add solvent and SBR to disperse them to obtain slurry, such as disclosed in Chinese Patent Publication No.: CN108807997A; (2) semi-dry process, first mix the dry powders of active material and conductive agent evenly, then add the glue prepared in advance to the dry powder and knead them, and finally add solvent and SBR to disperse them to obtain slurry, such as disclosed in Chinese Patent Publication No.: CN111834623A; (3) wet process, first prepare glue, add conductive agent to the glue to disperse it to obtain conductive glue, then add active material to disperse it, and finally add solvent and SBR to disperse it to obtain slurry, such as disclosed in Chinese Patent Publication No.: CN111900391 B.

[0004] None of the above existing pulping processes fully consider the differences between silicon and graphite in particle size, conductivity, and affinity with different types of binders, as well as the risk of local agglomeration of silicon particles in application scenarios with high silicon content formulas above 10%. Summary of the Invention

[0005] In view of this, the present application provides a method for preparing a graphite-silicon mixed negative electrode slurry, a negative electrode sheet and a lithium-ion battery to overcome the defects of the prior art.

[0006] To achieve the above object, the technical solution adopted by the present invention is:

[0007] In a first aspect, the present invention provides a method for preparing a graphite-silicon mixed negative electrode slurry, comprising the following steps:

[0008] (1) Adding polyacrylic acid (PAA) glue, conductive agent slurry, and conductive agent powder into a pulping machine and rapidly stirring to obtain a conductive adhesive slurry;

[0009] (2) adding silicon powder to the conductive adhesive slurry obtained in step (1) and stirring rapidly to obtain silicon slurry;

[0010] It should be noted that the wet process of preparing silicon slurry by mixing polyacrylic acid PAA with a conductive agent and then adding silicon can enable the polyacrylic acid PAA with strong adhesion to be coated on the surface of silicon particles, while promoting the connection between the conductive agent and the silicon particles to form an effective conductive network, thereby improving the conductivity of silicon.

[0011] (3) adding the graphite powder to another pulping machine, stirring slowly for a short time, adding sodium carboxymethyl cellulose (CMC) glue and water, and then stirring slowly for a long time to adjust the solid content of the slurry to 62-68% to obtain a graphite slurry;

[0012] It should be noted that sodium carboxymethyl cellulose (CMC) glue forms a protective layer by adsorbing on the graphite surface to prevent graphite from agglomerating.

[0013] (4) Adding silicon slurry to the graphite slurry obtained in step (3) in three times, stirring slowly for a short time each time, and then stirring quickly for a long time, and adding water to adjust the solid content of the slurry to 45-55%.

[0014] It should be noted that by adding the silicon slurry into the graphite slurry three times, the small silicon particles can be gradually dispersed in the large graphite particles, achieving a good dispersion effect.

[0015] (5) Add styrene-butadiene rubber (SBR) emulsion to the slurry obtained in step (4), stir slowly, and obtain a final slurry after vacuum sieving.

[0016] The solid content of the polyacrylic acid PAA glue is 2-10%, preferably 4%;

[0017] The conductive agent slurry is one or more of single-walled carbon nanotube slurry, multi-walled carbon nanotube slurry, and graphene slurry, and has a solid content of 0.1-10%, preferably 0.4%;

[0018] The conductive agent powder is one or more of Super P, ECP, SFG-6, and KS-6.

[0019] The silicon powder is at least one of silicon-oxygen and silicon-carbon materials.

[0020] The graphite powder is one or more of natural graphite, artificial graphite, soft carbon and hard carbon; the solid content of the sodium carboxymethyl cellulose (CMC) glue is 1.0-1.5%, preferably 1.2%.

[0021] The solid content of the styrene-butadiene rubber (SBR) emulsion is 30-50%, preferably 40%.

[0022] The dry weight ratio of graphite: silicon: conductive agent powder: conductive agent slurry: CMC: PAA: SBR is (67.0-92.1): (4.8-28.7): (0.5-1.0): (0.01-0.1): (0.6-1.0): (0.5-1.4): (0.8-1.5), preferably 82.2: 14.5: 0.67: 0.03: 0.7: 0.7: 1.2.

[0023] In the step (4), the amounts of silicon slurry added three times are respectively 25-35%: 25-35%: 35-45% of the total amount added, preferably 25%: 35%: 40%.

[0024] In the step (1) and the step (2), the rapid stirring speed is 1000-3000 rpm, and the stirring time is 20-60 min; in the step (3), the two slow stirring speeds are 15-35 rpm, the short slow stirring time is 10-30 min, and the long slow stirring time is 60-90 min; in the step (4), the short slow stirring speed is 15-35 rpm, the short slow stirring time is 10-30 min, the long rapid stirring speed is 1000-3000 rpm, and the long rapid stirring time is 20-60 min; in the step (5), the slow stirring speed is 10-20 rpm, and the stirring time is 10-30 min.

[0025] In a second aspect, the present invention provides a negative electrode sheet, which is prepared by uniformly coating the negative electrode slurry obtained by the preparation method of the first aspect on a copper foil, and then drying, rolling and shearing.

[0026] In a third aspect, the present invention provides a lithium-ion battery comprising the negative electrode sheet as described in the second aspect.

[0027] Compared with the prior art, the present invention has the following beneficial effects:

[0028] The present invention takes into account the differences between silicon and graphite in terms of particle size, conductivity, and affinity with different types of binders. A wet process is used to prepare silicon slurry by mixing polyacrylic acid (PAA) with a conductive agent and then adding silicon. This allows the polyacrylic acid (PAA) with strong adhesive force to coat the surface of the silicon particles, while promoting the connection between the conductive agent and the silicon particles to form an effective conductive network, thereby improving the conductivity of silicon. A semi-dry process is used to disperse graphite with sodium carboxymethyl cellulose (CMC) glue to prepare graphite slurry. At the same time, by adding the silicon slurry to the graphite slurry three times, small silicon particles are gradually dispersed in large graphite particles, achieving a good dispersion effect. This preparation method can improve the uniformity of silicon dispersion in graphite. The strong adhesive polyacrylic acid (PAA) hydrogen bonds with silicon particles, suppressing the volume expansion of silicon during cycling. The conductive agent connects to silicon particles to improve silicon conductivity, thereby reducing electrode sheet resistance and improving full battery cycle performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 This is a SEM scanning electron microscope image of the negative electrode sheet of Example 1;

[0030] Figure 2 This is a SEM scanning electron microscope image of the negative electrode sheet of Example 2;

[0031] Figure 3 This is a SEM image of the negative electrode sheet of Example 3;

[0032] Figure 4 This is the SEM scanning electron microscope image of the negative electrode sheet of Comparative Example 1;

[0033] Figure 5 This is the SEM scanning electron microscope image of the negative electrode sheet of Comparative Example 2;

[0034] Figure 6 This is the SEM scanning electron microscope image of the negative electrode sheet of Comparative Example 3. DETAILED DESCRIPTION

[0035] In order to enable those skilled in the art to better understand the technical solution of the present invention, the present invention is further described in detail below with reference to the accompanying drawings and the best embodiments.

[0036] Figure 1 、 Figure 2 and Figure 3 Shown are SEM images of silicon dispersed in graphite in Example 1, Example 2 and Example 3, respectively.

[0037] Figure 4 、 Figure 5 and Figure 6 Shown are SEM images of silicon particles agglomerated in Comparative Example 1, Comparative Example 2 and Comparative Example 3, respectively.

[0038] Example 1:

[0039] A method for preparing a graphite-silicon mixed negative electrode slurry comprises the following steps:

[0040] (1) Add PAA glue with a solid content of 4%, conductive agent slurry with a solid content of 0.4%, and conductive agent powder into a pulping machine and stir rapidly at 2000 rpm for 40 minutes to fully mix them;

[0041] (2) adding silicon powder to the slurry obtained in step (1) and rapidly stirring at 2000 rpm for 40 min to obtain silicon slurry;

[0042] (3) adding the graphite powder to another pulping machine, stirring slowly at 35 rpm for 20 min, adding CMC glue with a solid content of 1.2% and water, stirring slowly at 35 rpm for 90 min to wet the powder, and adjusting the solid content of the slurry to 65% to obtain graphite slurry;

[0043] (4) Adding silicon slurry to the graphite slurry in three portions, with the added amounts accounting for 25%, 35%, and 40% of the total amount. Each time, stir slowly at 35 rpm for 20 minutes, then stir rapidly at 2000 rpm for 40 minutes, and add water to adjust the solid content of the slurry to 50%;

[0044] (5) Add SBR emulsion with a solid content of 40% to the slurry obtained in step (4), stir slowly at 15 rpm for 15 min, and vacuum-sieve to obtain the final slurry.

[0045] Among them, the dry weight ratio of graphite: silicon: conductive agent powder: conductive agent slurry: CMC: PAA: SBR is 67.0:28.7:1.0:0.1:1.0:1.4:0.8.

[0046] A negative electrode sheet is prepared by coating the negative electrode slurry obtained by the above preparation method on a copper foil, and then performing drying, rolling and shearing processes.

[0047] A lithium-ion battery comprises a negative electrode sheet prepared by the above method.

[0048] Example 2:

[0049] A method for preparing a graphite-silicon mixed negative electrode slurry comprises the following steps, which differ from Example 1 in that the proportion of the formula is different:

[0050] (1) Add PAA glue with a solid content of 4%, conductive agent slurry with a solid content of 0.4%, and conductive agent powder into a pulping machine and stir rapidly at 2000 rpm for 40 minutes to fully mix them;

[0051] (2) adding silicon powder to the slurry obtained in step (1) and rapidly stirring at 2000 rpm for 40 min to obtain silicon slurry;

[0052] (3) adding the graphite powder to another pulping machine, stirring slowly at 35 rpm for 20 min, adding CMC glue with a solid content of 1.2% and water, stirring slowly at 35 rpm for 90 min to wet the powder, and adjusting the solid content of the slurry to 65% to obtain graphite slurry;

[0053] (4) Adding silicon slurry to the graphite slurry in three portions, with the added amounts accounting for 25%, 35%, and 40% of the total amount. Each time, stir slowly at 35 rpm for 20 minutes, then stir rapidly at 2000 rpm for 40 minutes, and add water to adjust the solid content of the slurry to 50%;

[0054] (5) Add SBR emulsion with a solid content of 40% to the slurry obtained in step (4), stir slowly at 15 rpm for 15 min, and vacuum-sieve to obtain the final slurry.

[0055] The dry weight ratio of graphite: silicon: conductive agent powder: conductive agent slurry: CMC: PAA: SBR is 92.1:4.8:0.49:0.01:0.6:0.5:1.5. The preparation method of the negative electrode sheet and the lithium ion battery is the same as that of Example 1.

[0056] Example 3:

[0057] A method for preparing a graphite-silicon mixed negative electrode slurry comprises the following steps, which differ from Example 1 in that the proportion of the formula is different:

[0058] (1) Add PAA glue with a solid content of 4%, conductive agent slurry with a solid content of 0.4%, and conductive agent powder into a pulping machine and stir rapidly at 2000 rpm for 40 minutes to fully mix them;

[0059] (2) adding silicon powder to the slurry obtained in step (1) and rapidly stirring at 2000 rpm for 40 min to obtain silicon slurry;

[0060] (3) adding the graphite powder to another pulping machine, stirring slowly at 35 rpm for 20 min, adding CMC glue with a solid content of 1.2% and water, stirring slowly at 35 rpm for 90 min to wet the powder, and adjusting the solid content of the slurry to 65% to obtain graphite slurry;

[0061] (4) Adding silicon slurry to the graphite slurry in three portions, with the added amounts accounting for 25%, 35%, and 40% of the total amount. Each time, stir slowly at 35 rpm for 20 minutes, then stir rapidly at 2000 rpm for 40 minutes, and add water to adjust the solid content of the slurry to 50%;

[0062] (5) Add 40% SBR emulsion to the slurry obtained in step (4), stir slowly at 15 rpm for 15 min, and vacuum-sieve to obtain the final slurry.

[0063] The dry weight ratio of graphite: silicon: conductive agent powder: conductive agent slurry: CMC: PAA: SBR is 82.2:14.5:0.67:0.03:0.7:0.7:1.2. The preparation method of the negative electrode sheet and the lithium ion battery is the same as that of Example 1.

[0064] Comparative Example 1:

[0065] A method for preparing a graphite-silicon mixed negative electrode slurry comprises the following steps, which differs from Example 1 in that a semi-dry process is used:

[0066] (1) Add graphite powder, silicon powder, and conductive agent powder into a pulping machine and stir slowly at 15 rpm for 15 minutes;

[0067] (2) Add CMC glue and PAA glue to the mixed powder in step (1), wherein the amount of CMC glue added in this step accounts for 40% of the total amount added, and the amount of PAA glue added in this step accounts for 40% of the total amount added, and stir slowly at 25 rpm for 30 minutes; after scraping, stir at the same speed for 60 minutes;

[0068] (3) Add the remaining CMC glue and PAA glue to the slurry obtained in step (2), stir slowly at 200 rpm for 60 minutes, then stir rapidly at 1000 rpm for 120 minutes, and add water to adjust the solid content of the slurry to 50%;

[0069] (4) Add SBR emulsion to the slurry obtained in step (3), stir slowly at 15 rpm for 15 min, and vacuum-sieve to obtain the final slurry.

[0070] The formulation, dosage, negative electrode sheet and lithium ion battery preparation method are the same as those in Example 1.

[0071] Comparative Example 2:

[0072] A method for preparing a graphite-silicon mixed negative electrode slurry comprises the following steps, which differs from Example 1 in that the silicon slurry is prepared using a semi-dry process, and the remaining steps are the same as Example 1:

[0073] (1) Add silicon powder and conductive agent powder into a pulping machine, stir slowly at 35 rpm for 20 min, add PAA glue and conductive agent slurry, stir slowly at 35 rpm for 90 min, and obtain silicon slurry;

[0074] (2) adding graphite powder to another pulping machine, stirring slowly at 35 rpm for 20 min, adding CMC glue and water, stirring slowly at 35 rpm for 90 min to wet the powder, and adjusting the solid content of the slurry to 65% to obtain graphite slurry;

[0075] (3) Adding silicon slurry to the graphite slurry in three times, with the added amount accounting for 25%: 35%: 40% of the total amount. Stirring slowly at 35 rpm for 20 minutes each time, then stirring rapidly at 2000 rpm for 40 minutes, and adding water to adjust the solid content of the slurry to 50%;

[0076] (4) Add SBR emulsion to the slurry obtained in step (3), stir slowly at 15 rpm for 15 min, and vacuum-sieve to obtain the final slurry.

[0077] The formulation, dosage, negative electrode sheet and lithium ion battery preparation method are the same as those in Example 1.

[0078] Comparative Example 3:

[0079] A method for preparing a graphite-silicon mixed negative electrode slurry comprises the following steps, which differs from Example 1 in that the silicon slurry is added to the graphite slurry at one time, and the remaining steps are the same as Example 1:

[0080] (1) Add PAA glue, conductive agent slurry, and conductive agent powder into a pulping machine and stir rapidly at 2000 rpm for 40 minutes to fully mix them;

[0081] (2) adding silicon powder to the slurry obtained in step (1) and rapidly stirring at 2000 rpm for 40 min to obtain silicon slurry;

[0082] (3) adding the graphite powder to another pulping machine, stirring slowly at 35 rpm for 20 min, adding CMC glue and water, stirring slowly at 35 rpm for 90 min to wet the powder, and adjusting the solid content of the slurry to 65% to obtain graphite slurry;

[0083] (4) Adding silicon slurry to the graphite slurry, stirring slowly at 35 rpm for 20 min, then stirring rapidly at 2000 rpm for 40 min, and adding water to adjust the solid content of the slurry to 50%;

[0084] (5) Add SBR emulsion to the slurry obtained in step (4), stir slowly at 15 rpm for 15 min, and vacuum-sieve to obtain the final slurry.

[0085] The formulation, dosage, negative electrode sheet and lithium ion battery preparation method are the same as those in Example 1.

[0086] The performance test of the lithium ion battery of the present invention is as follows:

[0087] The negative electrode sheet adhesion test was performed using the method described in GB / T 2792-2014;

[0088] Use the four-probe method to test the electrode resistance and calculate the resistance average value and standard deviation. It is generally believed that the dispersion is good when the standard deviation is less than 0.05%.

[0089] Scanning electron microscope (JSM-6360LV) was used to analyze the microstructure of the electrode. Figure 1-6 As shown;

[0090] The BT 2000 equipment from Arbin Company of the United States was used to test and analyze the cycle performance of the full battery. The test temperature was 25°C and the test conditions were 2C constant voltage charging to 4.5V, then constant voltage charging to 0.05C, resting for 5 minutes, and discharging at 0.7C to 3V. The capacity retention rate was calculated by dividing the discharge capacity of the current cycle by the discharge capacity of the first cycle.

[0091] The test results are shown in Table 1:

[0092] Table 1

[0093]

[0094] The test results of Examples 1, 2, and 3 show that when the silicon content of the formula is in the range of 4.8% to 28.7%, the electrode sheet adhesion, resistance, and full battery cycle performance obtained by the slurry preparation method provided by the present invention are good, and the standard deviation of the electrode sheet resistance is less than 0.05%. Silicon is evenly dispersed in the graphite without obvious agglomeration. Figure 1 、 Figure 2 and Figure 3 The SEM scanning electron microscope image shown in the figure. The electrode prepared by the conventional semi-dry process in Comparative Example 1 has a lower bonding strength and an increased average resistance compared to Example 1. The cycle capacity retention rate of the full battery after 300 cycles is reduced. Figure 4 The SEM scanning electron microscope image shows that silicon particles are clearly agglomerated, the standard deviation of the electrode resistance is greater than 0.05%, and the silicon distribution uniformity is poor. Compared with Example 1, the semi-dry method for preparing the silicon slurry in Example 2 and the addition of the silicon slurry to the graphite slurry in Example 3 will affect the silicon distribution uniformity. Figure 5 and Figure 6 The SEM scanning electron microscope image shown shows the agglomeration of silicon particles, the increase in the average value of the electrode resistance, the standard deviation > 0.05%, the decrease in the electrode bonding force, and the corresponding decrease in the cycle performance of the entire battery.

Claims

1. A method for preparing a graphite-silicon mixed negative electrode slurry, characterized in that: The following steps are involved: (1) Adding polyacrylic acid (PAA) glue, conductive agent slurry, and conductive agent powder into a pulping machine and rapidly stirring to obtain a conductive adhesive slurry; (2) adding silicon powder to the conductive adhesive slurry obtained in step (1) and stirring rapidly to obtain silicon slurry; (3) adding the graphite powder to another pulping machine, stirring slowly for a short time, adding sodium carboxymethyl cellulose (CMC) glue and water, and then stirring slowly for a long time to adjust the solid content of the slurry to 62-68% to obtain a graphite slurry; (4) Adding silicon slurry to the graphite slurry obtained in step (3) in three times, stirring slowly for a short time each time, and then stirring quickly for a long time, and adding water to adjust the solid content of the slurry to 45-55%. (5) Add styrene-butadiene rubber (SBR) emulsion to the slurry obtained in step (4), stir slowly, and obtain a final slurry after vacuum sieving.

2. The method for preparing the graphite silicon mixed negative electrode slurry according to claim 1, wherein: The solid content of the polyacrylic acid PAA glue is 2-10%, preferably 4%; The conductive agent slurry is one or more of single-walled carbon nanotube slurry, multi-walled carbon nanotube slurry, and graphene slurry, and has a solid content of 0.1-10%, preferably 0.4%; The conductive agent powder is one or more of Super P, ECP, SFG-6, and KS-6.

3. The method for preparing the graphite silicon mixed negative electrode slurry according to claim 1, wherein: The silicon powder is at least one of silicon-oxygen and silicon-carbon materials.

4. The method for preparing the graphite-silicon mixed negative electrode slurry according to claim 1, wherein: The graphite powder is one or more of natural graphite, artificial graphite, soft carbon and hard carbon; the solid content of the sodium carboxymethyl cellulose (CMC) glue is 1.0-1.5%, preferably 1.2%.

5. The method for preparing the graphite silicon mixed negative electrode slurry according to claim 1, wherein: The solid content of the styrene-butadiene rubber (SBR) emulsion is 30-50%, preferably 40%.

6. The method for preparing the graphite-silicon mixed negative electrode slurry according to claim 1, wherein: The dry weight ratio of graphite: silicon: conductive agent powder: conductive agent slurry: CMC: PAA: SBR is (67.0-92.1): (4.8-28.7): (0.5-1.0): (0.01-0.1): (0.6-1.0): (0.5-1.4): (0.8-1.5), preferably 82.2: 14.5: 0.67: 0.03: 0.7: 0.7: 1.

2.

7. The method for preparing the graphite-silicon mixed negative electrode slurry according to claim 1, characterized in that: In the step (4), the amounts of silicon slurry added three times are respectively 25-35%: 25-35%: 35-45% of the total amount added, preferably 25%: 35%: 40%.

8. The method for preparing the graphite-silicon mixed negative electrode slurry according to claim 1, wherein: In the step (1) and the step (2), the rapid stirring speed is 1000-3000 rpm, and the stirring time is 20-60 min; in the step (3), the two slow stirring speeds are 15-35 rpm, the short slow stirring time is 10-30 min, and the long slow stirring time is 60-90 min; in the step (4), the short slow stirring speed is 15-35 rpm, the short slow stirring time is 10-30 min, the long rapid stirring speed is 1000-3000 rpm, and the long rapid stirring time is 20-60 min; in the step (5), the slow stirring speed is 10-20 rpm, and the stirring time is 10-30 min.

9. A negative electrode sheet, characterized in that: The negative electrode sheet is prepared by uniformly coating the negative electrode slurry obtained by the preparation method according to claims 1 to 8 on a copper foil, and then drying, rolling and shearing.

10. A lithium ion battery, characterized in that: The lithium-ion battery comprises the negative electrode sheet according to claim 9.

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