Double-layer coated negative plate and preparation method and application thereof

Abstract

The invention discloses a double-layer coated negative plate as well as a preparation method and application thereof, and belongs to the technical field of lithium ion batteries. The double-layer coated negative plate comprises a negative current collector, a first coating coated on the surface of the current collector and a second coating coated on the surface of the first coating, the first coating comprises artificial graphite; and the second coating comprises carbon microspheres. According to the double-layer coated negative plate provided by the invention, the first coating containing the artificial graphite and the second coating containing the carbon microspheres are formed on the surface of the current collector in a double-layer coating manner, and the artificial graphite provides higher capacity and compaction, and bears larger mechanical stress during rolling; and the carbon microspheres with better power performance are in direct contact with the electrolyte, so that the solid-liquid interface impedance with the electrolyte is reduced, the outer-layer solid-phase transmission path is shortened, and the lithium ion transmission resistance is slowed down, thereby improving the power performance of the negative electrode and the whole battery cell.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium ion batteries, and in particular to a double-layer coated negative electrode sheet, a preparation method and an application thereof. Background Art

[0002] In recent years, in addition to their current large-scale applications in electric vehicles, home energy storage, and industrial energy storage, research on high-power lithium-ion batteries has also intensified in niche applications such as competitive drones, lightweight track racing, electric water skateboards, and supercars, driven by the demand for high-performance, fast-response, and long-lasting batteries. The anode material of lithium-ion batteries significantly impacts the battery's overall power output, making the development of high-power anodes imperative. Ordinary artificial graphite has a small interlayer spacing, making it susceptible to lithium deposition during high-current charge and discharge, leading to performance degradation and potential safety hazards. Therefore, the power of batteries made from ordinary artificial graphite has a certain upper limit.

[0003] Mesocarbon microbeads (MCMB) have a unique graphite-like lamellar structure. In addition to the advantages of general carbon negative electrode materials, their spherical layered structure also gives them high density, easy lithium insertion and extraction, and low boundary loss. They are a better choice as negative electrode materials for high-power batteries. However, their use is subject to greater restrictions due to their relatively low capacity and a price that is 3-5 times that of ordinary artificial graphite.

[0004] CN117199323A proposes a carbon negative electrode material for ultra-high power lithium-ion batteries and a preparation method thereof: mesophase carbon microspheres are crushed and then graded to remove fine powder to obtain powder A; powder A is mixed and coated with a coating agent to obtain a mixed material B; mixed powder B is heat-treated to obtain a heat-treated product, and the heat-treated product is broken up and sieved to obtain a small particle high-rate negative electrode material C coated with carbon on the surface. By utilizing the mesophase carbon microsphere crushing small particles combined with carbon coating technology, the material's ability to transmit lithium ions from the inside to the outside is significantly improved, while also having a stable solid-liquid interface and good processing performance, thereby obtaining a carbon negative electrode material for ultra-high power lithium-ion batteries. However, the material prepared by this method has a capacity of only about 330mAh / g, and its compaction is also less than 1.3g / cc. The cost is several times that of ordinary graphite, and its application scenarios are very limited.

[0005] Patent CN201711333907.X proposes a method for preparing a high-capacity and high-rate composite graphite negative electrode material. The method comprises the following steps: low-temperature carbonization of artificial graphite granular material is followed by crushing to 8-11 μm, and surface modification of the raw mesophase carbon microspheres is performed at room temperature; the two materials are then mixed with a binder and subjected to secondary coating modification and granulation, followed by high-temperature graphitization treatment to obtain a high-capacity and high-rate composite graphite negative electrode material. This technology processes two different specifications of graphite, artificial graphite single particles and graphitized mesophase carbon microspheres, separately, mixes them in a certain proportion, and then performs coating modification and secondary granulation. The result is a product that has both the high capacity density of artificial graphite and the high tap density and high-rate cycle performance of mesophase carbon microspheres. However, the particle size D50 of the composite graphite negative electrode material obtained by this preparation method is controlled at 14-19μm, which is significantly larger than the current high-rate graphite negative electrode material D50 of 6-7μm, and is not conducive to ion diffusion. From this point of view, the application of this composite graphite negative electrode in high-power lithium batteries is difficult to promote. Summary of the Invention

[0006] The purpose of the present invention is to provide a high-power double-layer coated negative electrode sheet; another purpose of the present invention is to provide a method for preparing a high-power double-layer coated negative electrode sheet; another purpose of the present invention is to provide an application of a high-power double-layer coated negative electrode sheet.

[0007] The invention discloses a double-layer coated negative electrode sheet, comprising a negative electrode current collector, a first coating coated on the surface of the current collector, and a second coating coated on the surface of the first coating; the first coating comprises artificial graphite; the second coating comprises carbon microspheres.

[0008] The material of the current collector can be copper, nickel, stainless steel, etc.

[0009] Introducing crushed carbon microspheres into the negative electrode sheet can increase the solid-phase transmission rate of lithium ions and thus improve the power performance of the battery. Since the artificial graphite layer accounts for the majority, the cost of the negative electrode sheet can be greatly reduced compared to pure carbon microsphere graphite, and the capacity and compaction can also be improved.

[0010] A double-layer coating method is adopted, and the second coating containing carbon microspheres is coated on the side away from the current collector. When lithium ions are transmitted, they are preferentially embedded in the carbon microsphere layer, reducing the solid-liquid interface impedance, greatly improving the transmission rate of lithium ions from the electrolyte to the graphite layer, and improving power performance.

[0011] Furthermore, the particle size D50 of the artificial graphite is 3-12 μm, and the capacity is 340-350 mAh / g; the raw material of the artificial graphite is a mixture of one or more of petroleum coke, needle coke, isotropic coke and medium sulfur coke; and the artificial graphite is a carbonized artificial graphite product.

[0012] Artificial graphite carbonization products, due to the amorphous carbon layer coated on the graphite surface, have a higher degree of surface disorder and faster lithium ion transmission rate, making them more suitable for use in high-power projects. This artificial graphite carbonization product can be purchased directly. It is mainly carbonized and coated after graphitization. The carbon source for the coating includes resin, pitch, glucose, etc.

[0013] Furthermore, the carbon microspheres are obtained by graphitizing spherical mesophase carbon microspheres after crushing, and the particle size D50 of the carbon microspheres is 1-5 μm.

[0014] The second coating layer utilizes carbon microspheres that are easily deintercalated and deintercalated with lithium. These microspheres, broken into small particles, further shorten the lithium-ion transmission path, reducing the resistance of lithium-ion transfer from the liquid phase to the solid phase. This significantly increases the lithium-ion transfer rate and enhances the negative electrode's rate performance. Due to the transition from the first coating layer to the second coating layer, the first coating layer has less stringent rate performance requirements. Therefore, artificial graphite with higher capacity and compaction density is selected to increase the battery's overall capacity and volumetric density. This also addresses the issue of overpressure caused by greater mechanical stress on the inner electrode during rolling, which can affect the electrode's liquid retention, impacting the cell's capacity and power performance.

[0015] Furthermore, the mass of the first coating layer accounts for 50-95% of the total mass of the first coating layer and the second coating layer; the first coating layer and the second coating layer also include a binder.

[0016] Binders include water-based binders and oil-based binders. Water-based binders include one or a mixture of CMC (carboxymethyl cellulose), SBR (styrene-butadiene rubber), PAA (polyacrylic acid), PVA (polyvinyl alcohol), etc.; oil-based binders are mainly PVDF (polyvinylidene fluoride).

[0017] Furthermore, the binder is an oil-based binder.

[0018] When using oil-based adhesives, the ultimate compaction of the electrode is the highest, and the power of the lithium-ion battery is higher, so the effect of using oil-based adhesives is better.

[0019] The present invention also discloses a method for preparing a double-layer coated negative electrode sheet, comprising the following steps:

[0020] S1: preparing a first coating slurry: uniformly dispersing artificial graphite, a binder, and a conductive agent in a solvent to form a slurry;

[0021] S2: preparing a second coating slurry: uniformly dispersing the carbon microspheres, the binder, and the conductive agent in a solvent to form a slurry;

[0022] S3 coating: coating the first coating slurry on the surface of the negative electrode current collector, and then coating the second coating slurry on the surface of the first coating slurry, and then drying, rolling, and cutting to obtain the double-layer coated negative electrode sheet as described above.

[0023] Furthermore, in step S3, the coating single-side density of the first coating slurry or the second coating slurry is 5-55 g / m 3 .

[0024] Furthermore, in step S3, the first coating slurry and the second coating slurry also include additives, and the mass ratio of the artificial graphite or the carbon microspheres in the first coating slurry or the second coating slurry: the binder: the conductive agent: the additive is (90-95):(4-6):(1-3):(0.1-0.4).

[0025] Furthermore, the additive includes oxalic acid; and the conductive agent includes carbon nanotubes or carbon fibers.

[0026] The purpose of adding additives is to adjust the pH of the slurry and improve the dispersibility of the slurry.

[0027] The present invention also discloses a lithium ion battery, comprising a positive electrode sheet, a separator, an electrolyte and a double-layer coated negative electrode sheet obtained by the preparation method described above, which is used for preparing the lithium ion battery.

[0028] The present invention provides a double-layer coated negative electrode sheet, which forms a first coating layer containing artificial graphite and a second coating layer containing carbon microspheres on the surface of the current collector by double-layer coating. The artificial graphite provides greater capacity and compaction, and bears higher mechanical stress during rolling; while the carbon microspheres with better power performance are in direct contact with the electrolyte, reducing the solid-liquid interface impedance with the electrolyte, shortening the outer solid phase transmission path, and reducing the resistance to lithium ion transmission, thereby improving the power performance of the negative electrode and the entire battery cell. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 1 is a graph showing the HPPC test results of normal temperature discharge of Example 1, Example 4, Example 5 and Comparative Example 1 of the present invention;

[0030] Figure 2 1 is a graph showing the HPPC test results of room temperature charging of Example 1, Example 4, Example 5 and Comparative Example 1 of the present invention. DETAILED DESCRIPTION

[0031] In order to make the technical solution of the present invention clearer, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0032] Example 1

[0033] The specific steps for preparing the negative electrode sheet are:

[0034] S1 Preparation of the first coating slurry: Using NMP (N-methyl-2-pyrrolidone) as a solvent, a 6% solid content PVDF (polyvinylidene fluoride) glue solution was prepared. Oxalic acid was added to adjust the solid content to 0.26%, and the mixture was stirred for 5 hours. VGCF (vapor-grown carbon fiber) was added to the glue solution and stirred for 3 hours. A petroleum coke artificial graphite carbonized product with a D50 of 10 μm was added and stirred for 4 hours. NMP was added to adjust the viscosity to 2000-5000 mPa·s. After uniform stirring, the first coating slurry was obtained, wherein the mass ratio of artificial graphite: VGCF: PVDF: oxalic acid was 92.5:1.5:5.75:0.25.

[0035] S2 Preparation of the second coating slurry: Using NMP as a solvent, a PVDF glue solution with a solid content of 6% was prepared. Oxalic acid was added to adjust the solid content to 0.26%, and the mixture was stirred for 5 hours. VGCF was added to the above glue solution and stirred for 3 hours. Carbon microsphere graphite with a D50 of 3 μm was added and stirred for 4 hours. NMP was added to adjust the viscosity to 2000-5000 mPa·s. After stirring evenly, the second coating slurry was obtained. The mass ratio of carbon microsphere graphite: VGCF: PVDF: oxalic acid was 92.5:1.5:5.75:0.25.

[0036] S3 coating: The first coating slurry and the second coating slurry are coated on the current collector copper foil using a double-layer coating and double-die feeding method. The first coating slurry is coated on the surface of the current collector with a single surface density of 30g / m 2 The second coating slurry is coated on the surface of the first coating slurry, and the single-sided surface density is 10g / m 2 After coating one side of the current collector, the other side is coated in the same way. The coated negative electrode sheet is dried, rolled, and cut to obtain a double-layer coated negative electrode sheet.

[0037] Example 2

[0038] The specific steps for preparing the negative electrode sheet are:

[0039] S1: Preparation of the first coating slurry: Using deionized water as the solvent, prepare a CMC (carboxymethyl cellulose) glue solution with a solid content of 1.5%, and stir for 5 hours. Take the glue solution, add carbon black, and stir for 3 hours. Then, add petroleum coke artificial graphite carbonization product with a D50 of 10 μm, and stir for 4 hours. After stirring, add deionized water to adjust the viscosity to 2000-6000 mPa･s. Then, add SBR (styrene-butadiene rubber) with a solid content of 45%, and stir evenly to obtain the first coating slurry, wherein the mass ratio of artificial graphite: carbon black: CMC: SBR is 94.5:1.5:1.5:2.5.

[0040] S2: Preparation of the second coating slurry: Using NMP as the solvent, prepare a 6% solids PVDF glue solution. Add oxalic acid to a 0.26% solids content and stir for 5 hours. Add VGCF to the glue solution and stir for 3 hours. Add carbon microspheres with a D50 of 3 μm and stir for 4 hours. Adjust the viscosity to 2000-5000 mPa·s with NMP. Stir thoroughly to obtain the second coating slurry. The mass ratio of carbon microspheres: VGCF: PVDF: oxalic acid is 92.5:1.5:5.75:0.25.

[0041] S3 coating: The first coating slurry and the second coating slurry are coated on the current collector copper foil using a double-layer coating and double-die feeding method. The first coating slurry is coated on the surface of the current collector with a single surface density of 30g / m 2 The second coating slurry is coated on the surface of the first coating slurry, with a single-side surface density of 10 g / m 2 After coating one side of the current collector, the other side is coated in the same way. The coated negative electrode sheet is dried, rolled, and cut to obtain a double-layer coated negative electrode sheet.

[0042] Examples 3-7 and Comparative Examples 1-2

[0043] The artificial graphite particle size, carbon microsphere particle size, binder of the first coating, binder of the second coating, single-sided surface density of the first coating, and single-sided surface density of the second coating of Examples 3-7 and Comparative Examples 1-2 are set according to Table 1, respectively. The rest are consistent with Example 1: If it is a water-based binder, the slurry method is consistent with the steps in Example 2.

[0044] Table 1. Examples and Comparative Examples

[0045] Artificial graphite particle size μm Carbon microsphere particle size μm Binder for the first coating Binder for the second coating <![CDATA[The single-sided areal density of the first coating, g / m 2 > <![CDATA[The areal density per side of the second coating, g / m 2 > Example 1 10 3 PVDF PVDF 30 10 Example 2 10 3 CMC / SBR PVDF 30 10 Example 3 10 3 PVDF CMC / SBR 30 10 Example 4 10 3 PVDF PVDF 35 5 Example 5 10 3 PVDF PVDF 22 18 Example 6 12 3 PVDF PVDF 30 10 Example 7 10 5 PVDF PVDF 30 10 Comparative Example 1 10 3 PVDF PVDF 40 0 Comparative Example 2 10 3 PVDF PVDF 0 40 Comparative Example 3 10 3 / / / /

[0046] Comparative Example 3

[0047] Preparation of the negative electrode coating slurry: Prepare a 6% PVDF gel solution using NMP as the solvent. Add oxalic acid to a 0.26% solids content and stir for 5 hours. Add VGCF to the gel solution and stir for 3 hours. Then, add carbon microspheres with a D50 of 3 μm and artificial graphite with a D50 of 10 μm and stir for 4 hours. The mass ratio of carbon microspheres to artificial graphite is 1:3. Adjust the viscosity to 2000-5000 mPa·s with NMP. Stir thoroughly to obtain the outer layer slurry. The mass ratio of active material: VGCF: PVDF: oxalic acid is 92.5:1.5:5.75:0.25.

[0048] Coating: Coat the negative electrode coating slurry on the current collector copper foil with a single-side density of 40g / m 2After coating one side of the current collector, the other side is coated in the same way. The coated negative electrode sheet is dried, rolled (compacted 1.55), and cut to obtain the negative electrode sheet.

[0049] The negative electrode sheets prepared in Examples 1-7 and Comparative Examples 1-3 were used to prepare lithium-ion batteries using lithium iron phosphate as the active material of the positive electrode.

[0050] Performance testing:

[0051] 1. Take the electrode obtained in step 3 of the above embodiment and comparative example, and test the ultimate compaction in turn. Adjust the roller gap and pressure until the thickness of the electrode no longer changes or the electrode wrinkles and bulges. The compaction of the electrode at this time is the ultimate compaction. The results are recorded in Table 2.

[0052] 2. Button test: The negative electrode sheets (compacted 1.5 g / cc) of the above-mentioned embodiment and comparative example were used as working electrodes, metallic lithium was used as the counter electrode, Celgard 2400 was used as the separator, and the electrolyte contained LiPF6, DMC, DEC and EC. The concentration of LiPF6 in the electrolyte was 1 mol / L, and the ratio of DMC:DEC:EC (wt%) was 1:1:1 to prepare button batteries. The button batteries were then subjected to charge and discharge tests. The measured capacity and initial efficiency of the materials in the electrode sheets are shown in Table 2.

[0053] Table 2 Test results of examples and comparative examples

[0054] Ultimate compaction (g / cc) 0.1C capacity (mAh / g) First effect (%) DCR(mΩ) Example 1 1.61 347.2 93.5 1.45 Example 2 1.52 346.6 93.2 1.46 Example 3 1.58 346.8 93.1 1.48 Example 4 1.64 348.4 94.1 1.49 Example 5 1.59 344.2 93.1 1.41 Example 6 1.67 347.7 94.2 1.51 Example 7 1.63 346.5 93.4 1.49 Comparative Example 1 1.68 350.9 94.6 1.61 Comparative Example 2 1.49 336.1 92.4 1.38 Comparative Example 3 1.58 347.5 93.7 1.50

[0055] 3. Full electric test: Using the negative electrode sheets obtained in the above examples and comparative examples as the negative electrode and iron lithium as the positive electrode, a 5Ah soft-pack battery cell was prepared by Z-shaped lamination for electrochemical performance testing. Conventional DCR test was first performed, and the results are shown in Table 2. Then, 25C 10s pulse discharge and charge HPPC test was performed on Example 1, Example 4, Example 5 and Comparative Example 1, and the results are shown in Table 2. Figure 1-Figure 2 shown.

[0056] As shown in Table 2, comparing Example 1 and Comparative Example 3, it can be seen that while simply mixing carbon microspheres into the artificial graphite slurry can reduce the DCR of the negative electrode sheet, the magnitude is not as significant as using a double-layer coating. This is because the solid-liquid interface in double-layer coating consists entirely of carbon microsphere graphite layers. These results demonstrate that the double-layer coating of the present invention significantly improves battery power and is superior to simply mixing the two materials.

[0057] From Comparative Example 1, Example 1, Example 4 and Example 5, it can be seen that after the carbon microsphere layer is coated on the outer layer of the artificial graphite layer in the form of double-layer coating, although the half-cell gram capacity of the pole piece is reduced and the compaction is also reduced to a certain extent, the DCR is significantly reduced after the full battery is prepared. When the carbon microsphere layer density is 5g / m 2 When the carbon microspheres are used, the DCR decreases from 1.61 to 1.49, a decrease of nearly 10%. The decrease in DCR slows down as the carbon microspheres are further increased. This is also confirmed by the results of the room temperature HPPC test. This shows that the ratio of the areal density of the two layers of the double-layer coating affects the compaction and power performance of the electrode. The higher the outer layer density, the higher the power. However, the increase in the amount of carbon microspheres will lead to higher costs. In addition, the compaction of the electrode will also decrease. During use, the ratio of the areal density of the inner and outer layers can be reasonably adjusted according to the cost and power requirements.

[0058] From the results of Example 1, Example 2 and Example 3, when both layers use oil-based binders, the electrode ultimate compaction is the highest, and the DCR is slightly lower than that using water-based binders, indicating that the power of the lithium-ion battery is slightly higher when the oil-based binder is used.

[0059] From Example 1, Example 6 and Example 7, whether adjusting the particle size of artificial graphite or carbon microspheres, it has an impact on the DCR of the pole piece. The smaller the particle size, the smaller the DCR. The influence of particle size is greater than the influence of surface density ratio, which is greater than the influence of binder system. It needs to be comprehensively considered according to compaction requirements and power requirements.

[0060] The above-described embodiments merely illustrate several implementations of the present invention, and while their descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that a person skilled in the art would be able to make numerous variations and improvements without departing from the spirit of the present invention, all of which fall within the scope of protection of the present invention. Therefore, the scope of protection of the present invention shall be determined by the appended claims.

Claims

1. A double-layer coated negative electrode sheet, characterized in that: The invention comprises a negative electrode current collector, a first coating layer coated on the surface of the current collector, and a second coating layer coated on the surface of the first coating layer; the first coating layer comprises artificial graphite; and the second coating layer comprises carbon microspheres.

2. The double-layer coated negative electrode sheet according to claim 1, characterized in that: The particle size D50 of the artificial graphite is 3-12 μm, and the capacity is 340-350 mAh / g; the raw material of the artificial graphite is a mixture of one or more of petroleum coke, needle coke, isotropic coke and medium-sulfur coke; and the artificial graphite is a carbonized artificial graphite product.

3. The double-layer coated negative electrode sheet according to claim 1, characterized in that: The carbon microspheres are obtained by graphitizing spherical mesophase carbon microspheres after crushing, and the particle size D50 of the carbon microspheres is 1-5 μm.

4. The double-layer coated negative electrode sheet according to claim 1, characterized in that: The mass of the second coating layer accounts for 5-50% of the mass of the first coating layer and the second coating layer; the first coating layer and the second coating layer also include a binder.

5. The double-layer coated negative electrode sheet according to claim 1, characterized in that: The binder is an oil-based binder.

6. A method for preparing a double-layer coated negative electrode sheet, characterized in that: The following steps are involved: S1: preparing a first coating slurry: uniformly dispersing artificial graphite, a binder, and a conductive agent in a solvent to form a slurry; S2: preparing a second coating slurry: uniformly dispersing the carbon microspheres, the binder, and the conductive agent in a solvent to form a slurry; S3 coating: coating the first coating slurry on the surface of the negative electrode current collector, and then coating the second coating slurry on the surface of the first coating slurry, and then drying, rolling, and cutting to obtain a double-layer coated negative electrode sheet as described in any one of claims 1-5.

7. The method for preparing a double-layer coated negative electrode sheet according to claim 6, characterized in that: In step S3, the coating density of the first coating slurry or the second coating slurry is 5-55 g / m 3 .

8. The method for preparing a double-layer coated negative electrode sheet according to claim 6, characterized in that: In step S3, the first coating slurry and the second coating slurry further include additives, and the mass ratio of the artificial graphite or the carbon microspheres in the first coating slurry or the second coating slurry: the binder: the conductive agent: the additive is 90-95:4-6:1-3:0.1-0.

4.

9. The method for preparing a double-layer coated negative electrode sheet according to claim 8, characterized in that: The additive includes oxalic acid; and the conductive agent includes carbon nanotubes or carbon fibers.

10. A lithium ion battery, characterized in that: The invention comprises a positive electrode sheet, a separator, an electrolyte and a double-layer coated negative electrode sheet obtained by the preparation method according to any one of claims 6 to 9.

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