Negative plate, preparation method thereof and lithium ion battery

Abstract

The invention relates to the technical field of batteries, in particular to a negative plate, a preparation method thereof and a lithium ion battery. The negative plate comprises a negative active material, a negative conductive agent and a negative binder; the negative electrode active material comprises a carbon material and a silicon material; the carbon material comprises first active carbon material high-dynamic graphite and second active carbon material high-compaction graphite. According to the technical scheme, by regulating and controlling the composition and proportion of the negative electrode system, on one hand, high-dynamic graphite is added, conduction of lithium ions is accelerated, and the fast charging performance of the battery is ensured, and on the other hand, graphite with high compaction capacity is added, so that the stress of the pole piece is smaller, silicon particles can be protected during rolling, the breaking phenomenon is avoided, and the service life of the battery is prolonged. And the advantages of high capacity and high dynamics of the battery cell design are exerted.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of battery, in particular to a negative electrode sheet, a preparation method thereof and a lithium ion battery. BACKGROUND

[0002] With the increasing dependence on unsustainable fossil fuels, environmental pollution needs to be addressed. Lithium ion batteries (LIBs) are widely used in electric vehicles and portable electronic devices due to their high energy density and no memory effect, but their performance is limited by the low theoretical capacity and poor structural stability of traditional graphite anodes. Therefore, developing new high-capacity anode materials is key to improving the performance of lithium ion batteries. Silicon anodes have a theoretical specific capacity of 4200 mAh / g and may become the next generation of anode materials, but the material has poor conductivity and cycle performance, and is prone to pulverization during charging and discharging, accompanied by a large volume expansion effect. The volume expansion of the silicon anode during lithium intercalation can reach 300%, and the volume shrinks during lithium extraction, resulting in a large stress within the particles. This volume expansion not only causes the silicon particles to break, but also causes the particles to lose contact with the current collector or the binder, resulting in the active material falling off; in addition, the volume expansion of silicon causes the SEI film to break, exposing the fresh silicon surface to the electrolyte, further consuming lithium ions. To solve this problem, by regulating the composition of the anode to increase the compaction density of the high-capacity silicon anode sheet, the breaking of the silicon anode particles is suppressed, which is a problem to be solved in the field. With increasing global attention to sustainable energy use and technological advancements. SUMMARY

[0003] The purpose of the present application is to overcome the deficiencies and shortcomings of the prior art, and to provide a negative electrode sheet, a preparation method thereof and a lithium ion battery.

[0004] To achieve the above-mentioned purpose, the following solutions are adopted in the present application:

[0005] A negative electrode sheet, comprising a negative electrode active material, a negative electrode conductive agent and a negative electrode binder; the negative electrode active material comprises a carbon material and a silicon material; the carbon material comprises a first active carbon material and a second active carbon material.

[0006] The first active carbon material is a secondary particle formed by aggregation of primary particles, and the high kinetic graphite; preferably, the first active carbon material accounts for ≥50%, preferably 50%-80%, and more preferably 50%-65% of the negative electrode active material;

[0007] The first active carbon material has a volume distribution particle size Dv50 of 9-18 μm, preferably 10-16 μm, and a Dv90 of 16-35 μm, preferably 18-28 μm; a specific surface area of 0.8-1.5 m 2 / g, preferably 0.9-1.2m 2 / g, graphitization degree ≥94%, preferably 94.5-96%; discharge capacity ≥350mAh / g, preferably 350-355mAh / g; compaction density 1.5-2.0g / cm³ 3 The preferred value is 1.6-1.8 g / cm³. 3 .

[0008] The second activated carbon material is high-pressure compacted primary particles, or high-pressure compacted graphite; the second activated carbon material accounts for ≥10% of the negative electrode active material, preferably 20%-50%, and more preferably 30%-45%.

[0009] The second activated carbon material has a Dv50 of 6-12 μm, preferably 8-10 μm, and a Dv90 of 12-24 μm, preferably 12-20 μm; its specific surface area is 0.8-3 m². 2 / g, preferably 1.1-2.0m 2 / g, graphitization degree ≥95%, preferably 95%-96.5%, discharge capacity ≥355mAh / g, preferably 358-365mAh / g, compaction density 1.8-2.2g / cm³ 3 The preferred concentration is 1.9-2.1 g / cm³. 3 .

[0010] The silicon material accounts for ≥1% of the negative electrode active material, preferably 2%-5%, and more preferably 5%.

[0011] The silicon material has a Dv50 of 3-10 μm, preferably 4-8 μm, a Dv90 of 10-20 μm, preferably 10-16 μm, and a specific surface area of ​​1.2-3.0 m². 2 / g, preferably 1.4-2.0m 2 / g; carbon content ≥40%, preferably 45-55%; silicon content ≥40%, preferably 45-55%; silicon crystal size ≤10nm, preferably ≤5nm; discharge capacity ≥1800mAh / g, preferably 1800-2000mAh / g.

[0012] The negative electrode conductive agent is conductive carbon black and / or single-walled carbon nanotubes; preferably, the negative electrode binder is one or a mixture of sodium carboxymethyl cellulose, polyacrylic acid, and styrene-butadiene rubber.

[0013] The present invention also includes a method for preparing the negative electrode sheet, comprising the following steps: mixing negative electrode active material, negative electrode conductive agent, and negative electrode binder, adding deionized water, and obtaining a negative electrode slurry under the action of a vacuum stirrer; uniformly coating the negative electrode slurry onto a negative electrode current collector copper foil, drying it, cold pressing it, trimming the edges, cutting it into sheets, slitting it, slitting it, and welding the tabs to obtain the negative electrode sheet.

[0014] The application also provides a lithium ion battery comprising the negative electrode sheet, the positive electrode sheet, the separator and the electrolyte.

[0015] Preferably, the positive electrode sheet is prepared in the following manner: the positive electrode active material lithium iron phosphate, the conductive agent and the binder are mixed uniformly, N-methyl pyrrolidone (NMP) is added, and the mixture is stirred under the action of a vacuum stirrer until the mixture system becomes a positive electrode slurry with uniform fluidity; the positive electrode slurry is uniformly coated on the positive electrode current collector aluminum foil, dried and then cold-pressed, and then subjected to edge cutting, sheet cutting and strip dividing; after the strip dividing, the tab is welded to obtain the positive electrode sheet.

[0016] Preferably, the electrolyte is prepared in the following manner: ethylene carbonate (EC), methyl ethyl carbonate (EMC) and diethyl carbonate (DEC) are mixed according to a volume ratio of 1:1:1 to obtain an organic solvent, and then LiPF6 is dissolved in the organic solvent to prepare an electrolyte with a concentration of 1 mol / L.

[0017] Preferably, the separator is a rubber-coated separator.

[0018] The lithium ion battery is prepared in the following manner: the positive electrode sheet and the negative electrode sheet are sequentially placed, the separator is placed between the positive electrode sheet and the negative electrode sheet, and the positive electrode sheet, the negative electrode sheet and the separator are wound to form a pole group; the pole group is placed in an outer package, dried, and then the electrolyte is injected; and the secondary battery is obtained after the processes of standing, formation, aging and load adjustment.

[0019] Compared with the prior art, the application has the following beneficial effects:

[0020] The technical scheme of the application regulates the composition and ratio of the negative electrode system; on one hand, high-kinetics graphite is added to accelerate the conduction of lithium ions and ensure the fast-charging performance of the battery; on the other hand, graphite with high compacting capacity is added to make the stress of the electrode sheet smaller and protect the silicon particles during rolling to avoid the phenomenon of breakage, thereby exerting the advantages of high capacity and high kinetics of the battery design. Therefore, the negative electrode sheet of the application can make the secondary battery have high capacity, high kinetics, fast charging and excellent cycle performance. BRIEF DESCRIPTION OF DRAWINGS

[0021] Figure 1 The scanning electron microscope images of the examples and the comparative examples of the application. DETAILED DESCRIPTION

[0022] The application will be further described in detail below with reference to the drawings and specific examples. It should be understood that the specific examples described herein are only used to explain the application and do not limit the application.

[0023] The first active carbon material used in the present application comprises high kinetic secondary particles formed by aggregation of primary particles, accounts for ≥50% in the negative active component, which can be selected from 50%-80%, the present application is 50-65%, which can effectively improve the electronic conductivity of the negative electrode sheet and provide good fast charging performance. The volume distribution particle size Dv50 is 9-18 μm, which can be selected from 10-16 μm, the present application is 13.8 μm, and the Dv90 is 16-35 μm, which can be selected from 18-28 μm, the present application is 20.3 μm. The volume distribution particle size of the first carbon material is in the above range, the material particle consistency is good, which is beneficial to the conduction and transmission of ions and electrons, and effectively improves the fast charging capacity of the battery; the specific surface area is 0.8-1.5 m 2 / g, which can be selected from 0.9-1.2 m 2 / g, the present application is 1.18 m 2 / g, the specific surface area of the first carbon material in this range can significantly reduce the reaction active sites of the electrode sheet, reduce the irreversible consumption of lithium ions during the first formation of the electrolyte, improve the capacity of the battery, and is beneficial to the rapid diffusion of lithium ions from the surface to the bulk, and improves the fast charging performance of the battery; the graphitization degree is ≥94%, which can be selected from 94.5-96%; the present application is 95.1%, the discharge capacity is ≥350 mAh / g, which can be selected from 350-355 mAh / g; the present application is 353 mAh / g, the compaction density is 1.5-2.0 g / cm3, which can be selected from 1.6-1.8 g / cm3, the present application is 1.65 g / cm3.

[0024] The second active carbon material comprises high compaction primary particles, accounts for ≥10% in the negative active component, which can be selected from 20%-50%, the present application is 30-45%, the volume distribution particle size Dv50 is 6-12 μm, which can be selected from 8-10 μm, the present application is 9.5 μm, and the Dv90 is 12-24 μm, which can be selected from 12-20 μm, the present application is 14.6 μm. The particle size of the second active carbon material in the above range is beneficial to improve the fast charging capacity of the battery; the specific surface area is 0.8-3 m 2 / g, which can be selected from 1.1-2.0 m 2 / g, the present application is 1.32 m2 / g, the specific surface area of the second active carbon material in this range can effectively expand the lithium ion transmission channel and improve the fast charging capacity of the battery; the graphitization degree is ≥95%, which can be selected from 95%-96.5%, the present application is 95.8%, the discharge capacity is ≥355 mAh / g, which can be selected from 358-365 mAh / g, the present application is 361 mAh / g, the capacity of the second active carbon material in this range is beneficial to improve the energy density of the secondary battery; the compaction density is 1.8-2.2 g / cm 3 , which can be selected from 1.9-2.1 g / cm 3The compacted density of the second active carbon material is in the range of 1.95 g / cm3, which can effectively improve the pressure resistance of the negative electrode material. The first active carbon material and the first active silicon material can alleviate the rolling pressure, and prevent the particles from being crushed by rolling.

[0025] The silicon material contains a silicon-carbon material, has a stable porous framework, good mechanical strength, and can reduce the volume change of silicon before and after the charge and discharge. The content of the first active silicon material in the negative electrode active component is ≥1%, and can be 2%-5%. In this application, the content is 5%, which can give the battery system high capacity and improve the reaction activity. The volume distribution particle size Dv50 is 3-10 μm, and can be 4-8 μm. In this application, the volume distribution particle size Dv50 is 8.2 μm. The Dv90 is 10-20 μm, and can be 10-16 μm. In this application, the Dv90 is 13 μm. The particle size of the first active silicon material in the above range is beneficial to reduce the lithium ion diffusion path and improve the battery fast charging capacity. The specific surface area is 1.2-3.0 m2 / g, and can be 1.4-2.0 m2 / g. In this application, the specific surface area is 1.8 m2 / g. The specific surface area of the first active silicon material in the above range prevents the specific surface area from being too large, which leads to the irreversible consumption of lithium ions during the first formation of the electrolyte, improves the battery capacity, and ensures sufficient lithium ion transmission channels. The carbon content is ≥40%, and can be 45-55%. In this application, the carbon content is 46.3%. The silicon content is ≥40%, and can be 45-55%. In this application, the silicon content is 47.5%. The silicon crystal size is ≤10 nm, and can be ≤5 nm. In this application, the silicon crystal size is 3.7 nm. The discharge capacity is ≥1800 mAh / g, and can be 1800-2000 mAh / g. In this application, the discharge capacity is 1922 mAh / g.

[0026] The secondary batteries of Examples 1-5 and Comparative Example 1 were prepared as follows.

[0027] Preparation of the positive electrode sheet: the positive electrode active material lithium iron phosphate, conductive carbon nanotubes, conductive carbon black, binder polyvinylidene fluoride, dispersant were mixed uniformly according to the weight ratio of 97.25%:0.5%:1%:1.2%:0.05%, and N-methyl pyrrolidone (NMP) was added. Stirring was carried out under the action of a vacuum stirrer until the mixed system became a homogeneous positive electrode slurry. The positive electrode slurry was uniformly coated on the positive electrode current collector aluminum foil, and the coating amount was 42.5 g / cm 2 After drying at 85°C, cold pressing was carried out, and edge cutting, sheet cutting, and striping were carried out. After striping, the sample was dried at 85°C for 4 hours under vacuum conditions, and the tab was welded to obtain the positive electrode sheet.

[0028] Negative electrode preparation: Negative electrode active materials (including first activated carbon material, second activated carbon material, and first activated silicon material), conductive agent carbon black, thickener sodium carboxymethyl cellulose, polyacrylic acid, binder styrene-butadiene rubber, and single-walled carbon nanotubes are mixed in a weight ratio of 96.15%:0.9%:0.8%:1.2%:0.9%:0.05%. Deionized water is added, and a negative electrode slurry is obtained under vacuum stirring. The negative electrode slurry is then uniformly coated onto the negative electrode current collector copper foil with a coating amount of 18 g / cm³. 2 After drying at 85℃, the material is cold-pressed, trimmed, cut into pieces, and slit. After slitting, it is dried at 85℃ for 4 hours under vacuum conditions, and then the tabs are welded to obtain the negative electrode sheet.

[0029] The weight parts of each component in the electrolyte are as follows: ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) are mixed in a volume ratio of 1:1:1 to obtain an organic solvent. LiPF6 is then dissolved in the above organic solvent to prepare an electrolyte with a concentration of 1 mol / L.

[0030] Preparation of the separator: The base membrane is selected as a coated separator with a thickness of not less than 4 micrometers.

[0031] The positive and negative electrode sheets prepared above are arranged in order, and the separator is placed between the positive and negative electrode sheets. The electrode assembly is prepared by winding. The electrode assembly is placed in the outer packaging, dried, and then injected with electrolyte. After standing, formation, aging, and load adjustment, a secondary battery is obtained.

[0032] Table 1. Composition and compaction density of negative electrode sheet

[0033]

[0034]

[0035] Scanning electron microscopy test, such as Figure 1 As shown, by Figure 1 It can be seen that the compaction density of the electrode sheets in Examples 1-5 is all between 1.55 and 1.6 g / cm³. 3 In the control group (Example 1), the morphology of the silicon particles remained relatively good, with no breakage or pulverization. In contrast, in the control group (Example 1) without the second activated carbon material, the silicon particles were severely broken and pulverized, with blurred particle edges. These results indicate that adding a high-compacted second activated carbon material to the negative electrode active material system can significantly improve the silicon particle breakage of the electrode and avoid subsequent performance degradation caused by silicon particle breakage.

[0036] Performance testing

[0037] (1) Fast charging performance test of secondary batteries

[0038] The test was carried out at room temperature, first capacity calibration, charged to 3.65V at 0.33C, then discharged to 2.0V at 0.33C, cycled three times, record the average of the three discharge capacities as the battery calibration capacity Cx.

[0039] Fast charging time: respectively charged to 3.65V at 0.2Cx, 0.5Cx, 1Cx, 1.5Cx and 2Cx, after each charging, it needs to be discharged to 2.0V at 1Cx, record the negative electrode potential corresponding to 10%, 15%, 20%, 30%, 40%, 50%, 60%, 70%, 80% SOC state under different rates, draw the charging rate-negative electrode potential curve under different SOC states, linear fitting to get the charging rate corresponding to the lithium extraction potential of the negative electrode under different SOC states, respectively recorded as C10, C20, C30, C40, C50, C60, C70, C80, according to the formula (60 / C20+60 / C30+60 / C40+60 / C50+60 / C60+60 / C70+60 / C80) x 10% to calculate the charging time Tmin of the secondary battery from 10% SOC to 80% SOC. The smaller Tmin is, the shorter the time is, and the better the fast charging performance of the battery is.

[0040] Fast charging cycle: respectively charged to 0.1Cx at 1Cx, 0.3Cx at 6Cx, 0.4Cx at 5Cx, 0.5Cx at 4.2Cx, 0.6Cx at 3.8Cx, 0.7Cx at 3.6Cx, 0.8Cx at 2.9Cx, 3.65V at 0.33Cx, then discharged to 2.0V at 0.33C rate, cycled to Cx=70%SOH to stop testing, record the cycle capacity retention rate of the battery.

[0041] (2) Cycle performance test of secondary battery

[0042] The test was carried out at 45℃, first capacity calibration, charged to 3.65V at 0.33C, then discharged to 2.0V at 0.33C, cycled three times, record the average of the three discharge capacities as the battery calibration capacity Cx. Then, constant current charging at 1Cx, charged to 3.65V, then discharged to 2.0V at 1Cx, record as a circle, cycle to Cx=70%SOH to stop testing, record the cycle capacity retention rate of the battery.

[0043] Table 2 Fast charging performance and cycle performance of examples 1-5 and comparative example 1

[0044]

[0045] Based on the test results in Table 2, the compaction density of the negative electrode sheets in Examples 1-5 is 1.55-1.6 g / cm³. 3 The batteries exhibited shorter fast-charging times, superior capacity retention after 500 fast-charging cycles, and excellent 45°C fast-charging cycle performance, significantly outperforming the comparative example without the addition of a second activated carbon material. This indicates that introducing high-compact graphite in this system allows the battery's capacity and performance to be fully realized. The comprehensive scanning results suggest that the superior performance of Examples 1-5 can be attributed to the high-compact second activated carbon material reducing stress on the negative electrode sheet during rolling, alleviating silicon particle breakage, and preventing performance degradation caused by silicon particle pulverization as seen in Comparative Example 1. Therefore, the negative electrode sheet proposed in this invention can ensure excellent fast-charging and cycle performance while maintaining battery energy density, demonstrating promising application prospects.

[0046] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. It will be apparent to those skilled in the art that the present invention is not limited to the details of the above exemplary embodiments, and that the present invention can be implemented in other specific forms without departing from the spirit or basic features of the present invention.

[0047] Therefore, the embodiments should be regarded as exemplary and non-limiting in all respects, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, it is intended that all variations falling within the meaning and scope of the equivalents of the claims be included within the invention.

[0048] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A negative electrode sheet characterized by comprising: The negative electrode active material, the negative electrode conductive agent and the negative electrode binder; the negative electrode active material comprises a carbon material and a silicon material; the carbon material comprises a first active carbon material and a second active carbon material.

2. The negative electrode sheet according to claim 1, characterized by The first active carbon material is a secondary particle formed by aggregation of primary particles; preferably, the first active carbon material accounts for ≥50% in the negative electrode active material, preferably 50%-80%, more preferably 50%-65%.

3. The negative electrode sheet according to claim 1, characterized by The first activated carbon material has a volume distribution particle size Dv50 of 9-18 μm, preferably 10-16 μm, a Dv90 of 16-35 μm, preferably 18-28 μm; a specific surface area of 0.8-1.5 m 2 / g, preferably 0.9-1.2 m 2 / g, a graphitization degree of ≥94%, preferably 94.5-96%; a discharge capacity of ≥350 mAh / g, preferably 350-355 mAh / g; and a compacted density of 1.5-2.0 g / cm 3 , preferably 1.6-1.8 g / cm 3 .

4. The negative electrode sheet according to claim 1, characterized by The second active carbon material is a high-compaction primary particle; the second active carbon material accounts for ≥10% in the negative electrode active material, preferably 20%-50%, more preferably 30%-45%.

5. The negative electrode sheet according to claim 1, characterized by The second activated carbon material has a Dv50 of 6-12 μm, preferably 8-10 μm, and a Dv90 of 12-24 μm, preferably 12-20 μm; a specific surface area of 0.8-3 m 2 / g, preferably 1.1-2.0 m 2 / g, a graphitization degree of ≥ 95%, preferably 95%-96.5%, a discharge capacity of ≥ 355 mAh / g, preferably 358-365 mAh / g, and a compacted density of 1.8-2.2 g / cm 3 , preferably 1.9-2.1 g / cm 3 .

6. The negative electrode sheet according to claim 1, wherein The silicon material accounts for ≥1% in the negative electrode active material, preferably 2%-5%, more preferably 5%.

7. The negative electrode sheet according to claim 1, wherein The silicon material has a Dv50 of 3-10 μm, preferably 4-8 μm, a Dv90 of 10-20 μm, preferably 10-16 μm, a specific surface area of 1.2-3.0 m 2 / g, preferably 1.4-2.0 m 2 / g; a carbon content of > 40%, preferably 45-55%, a silicon content of > 40%, preferably 45-55%, a silicon crystallite size of < 10 nm, preferably < 5 nm, a discharge capacity of > 1800 mAh / g, preferably 1800-2000 mAh / g.

8. The negative electrode sheet according to claim 1, characterized by The negative electrode conductive agent is conductive carbon black and / or single-walled carbon nanotubes; preferably, the negative electrode binder is one or a mixture of sodium carboxymethyl cellulose, polyacrylic acid and styrene-butadiene rubber.

9. A method for producing the negative electrode sheet according to any one of claims 1 to 8, characterized by, The method comprises the following steps: mixing the negative electrode active material, the negative electrode conductive agent and the negative electrode binder, adding deionized water, and obtaining a negative electrode slurry under the action of a vacuum stirrer; uniformly coating the negative electrode slurry on a negative electrode current collector copper foil, cold-pressing after drying, and then performing edge cutting, piece cutting and striping, welding of the tabs to obtain a negative electrode piece.

10. A lithium-ion battery, characterized by, The method comprises the negative electrode piece, the positive electrode piece, the separator and the electrolyte according to any one of claims 1-8. Preferably, the positive electrode piece is prepared in the following manner: uniformly mixing the positive electrode active material lithium iron phosphate, the conductive agent and the binder, adding N-methyl pyrrolidone NMP, and stirring under the action of a vacuum stirrer until the mixture system becomes a positive electrode slurry with uniform fluidity; uniformly coating the positive electrode slurry on a positive electrode current collector aluminum foil, cold-pressing after drying, and then performing edge cutting, piece cutting and striping, welding of the tabs after striping to obtain a positive electrode piece. Preferably, the electrolyte is prepared in the following manner: mixing ethylene carbonate EC, methyl ethyl carbonate EMC and diethyl carbonate DEC according to a volume ratio of 1:1:1 to obtain an organic solvent, and then dissolving LiPF6 in the organic solvent to prepare an electrolyte with a concentration of 1 mol / L; Preferably, the separator is a coated separator. The lithium ion battery is prepared in the following manner: placing the positive electrode piece and the negative electrode piece in sequence, placing the separator between the positive electrode piece and the negative electrode piece, preparing an electrode group in a wound form, placing the electrode group in an outer package, injecting the electrolyte after drying, and then performing the processes of standing, formation, aging and load adjustment to obtain a secondary battery.

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