Method for improving rate capability by modifying graphite negative electrode

Abstract

The invention discloses a method for improving rate capability by modifying a graphite negative electrode and a negative electrode pole piece, the method comprises the following steps: S1, mixing graphite with dopamine, and carrying out auto-polymerization on the dopamine on the surface of the graphite to obtain a polydopamine / graphite composite material; then carbonizing to obtain carbon-coated graphite; and S2, mixing the carbon-coated graphite obtained in S1 as an active material with a conductive agent, an adhesive and sodium carboxymethyl cellulose, adding a solvent to prepare negative electrode slurry, coating a current collector with the negative electrode slurry, and removing the solvent to obtain a negative electrode plate. Dopamine is adopted as a coating material to obtain the rate type graphite negative electrode material, and a layer of artificial SEI film is further constructed on the basis, so that the coulombic performance and the safety of the lithium ion battery are improved.

Description

Technical Field

[0001] The invention belongs to the field of lithium ion batteries, and in particular relates to a method for modifying a graphite negative electrode to improve rate performance. Background Art

[0002] Graphite negative electrode materials for lithium-ion batteries have become the mainstream negative electrode materials due to their advantages such as low voltage platform, good cycle stability, low price, wide source, and non-toxicity. However, the market demand for fast charging is becoming increasingly strong, and the rate performance of artificial graphite negative electrodes is becoming increasingly difficult to meet the current fast charging market demand. Summary of the invention

[0003] Based on the above technical problems, the present invention uses dopamine as a coating material to carbonize and coat graphite to obtain a rate-type graphite negative electrode material. However, while this method improves the rate performance, it also reduces the coulombic efficiency. In order to compensate for the safety issues caused by the reduction in the first efficiency and the uncontrollable growth of lithium dendrites after the carbon layer is coated, the present invention constructs a layer of artificial SEI film based on the research of the rate-type graphite negative electrode material, thereby improving the coulombic performance and safety of lithium-ion batteries.

[0004] The specific scheme of the present invention is as follows:

[0005] The present invention provides a method for modifying a graphite negative electrode to improve rate performance, comprising the following steps: S1, mixing graphite with dopamine, allowing dopamine to self-polymerize on the surface of graphite to obtain a polydopamine / graphite composite material; then carbonizing to obtain carbon-coated graphite; S2, mixing the carbon-coated graphite obtained in S1 as an active material with a conductive agent, a binder, and sodium carboxymethyl cellulose, adding a solvent to prepare a negative electrode slurry, coating the negative electrode slurry on a current collector, and removing the solvent to obtain a negative electrode sheet.

[0006] Preferably, in S1, the mass ratio of graphite to dopamine is 1:0.1-0.5; more preferably, in S1, the mass ratio of graphite to dopamine is 1:0.2-0.3.

[0007] Preferably, in S1, the carbonization temperature is 900-1000° C., and the carbonization time is 7-9 hours.

[0008] Preferably, in S1, the graphite is artificial graphite or natural graphite.

[0009] Preferably, in S2, the mass ratio of carbon-coated graphite, conductive agent, binder and sodium carboxymethyl cellulose is 7.0-7.8:1.0-1.2:1.0-1.2:0.2-1.

[0010] Preferably, in S2, the conductive agent is selected from at least one of acetylene black, carbon black, carbon nanotubes and graphene.

[0011] Preferably, in S2, the binder is selected from at least one of polyvinylidene fluoride, polyvinyl alcohol, polytetrafluoroethylene, and sodium carboxymethyl cellulose.

[0012] Preferably, in S2, the solvent is N-methylpyrrolidone or dimethylformamide.

[0013] Preferably, the current collector is copper foil or aluminum foil; more preferably, the copper foil has a thickness of 0.1 mm to 0.5 mm.

[0014] The present invention also provides a lithium battery negative electrode plate, which is prepared by any of the above methods.

[0015] The beneficial effects of the present invention are:

[0016] In order to improve the rate performance of the artificial graphite negative electrode, the present invention first uses dopamine for coating, but the coating will cause the reduction of Coulomb efficiency and the safety risk of lithium dendrite growth. Therefore, sodium carboxymethyl cellulose (CMC-Na) is further used to prepare a carboxymethyl cellulose-lithium (CMC-Li) film to stabilize the lithium negative electrode and the rich hydrogen bond structure makes the CMC-Li macromolecules cross-linked into a uniform dense layer. Replacing lithium ions in carboxymethyl cellulose can improve ion conductivity and redistribute lithium ion flux. DETAILED DESCRIPTION

[0017] The technical solutions of the present invention are described in detail below through specific embodiments. However, it should be clearly stated that these embodiments are for illustration only and are not to be construed as limiting the scope of the present invention.

[0018] Example 1

[0019] A rate-type artificial graphite negative electrode material, the preparation method of which comprises:

[0020] 355 artificial graphite of conventional capacity was mixed with dopamine in mass ratios of 1:0.1, 1:0.2, 1:0.3, 1:0.4 and 1:0.5, respectively. Dopamine was self-polymerized on the graphite surface to obtain a polydopamine / graphite composite material. It was then carbonized at 950°C for 8h to obtain carbon-coated graphite, named FG-PDA-X, where X is the coating amount of dopamine.

[0021] An LIR2016 button battery is prepared using the prepared rate-type graphite negative electrode material, and the method comprises: fully grinding FG-PDA-X, acetylene black (ACET) and polyvinylidene fluoride (PVDF) in a mass ratio of 8:1:1 on agate to fully mix them, then transferring them into a pulping vessel, adding an appropriate amount of N-methylpyrrolidone (NMP) to prepare a slurry, uniformly coating the slurry on a copper foil current collector with a thickness of 0.2 mm, then placing it in a blast drying oven, drying it at 100° C. for 1 hour to remove moisture; then transferring it to a vacuum drying oven at 100° C. for vacuum drying for 10 hours to remove NMP, then punching and cutting it into a circular electrode sheet with a diameter of 14 mm, weighing and recording it as a negative electrode; and assembling it into an LIR2016 button battery.

[0022] The 355 artificial graphite without dopamine modification was used as the control (FG), and the LIR2016 button cell was assembled in the same manner as above. The performance of the assembled LIR2016 button cell was tested as follows:

[0023] The conditions for rate performance testing are: current density range is 0.5C-4C, different current densities are cycled for 200 cycles, and the test results are shown in Table 1 below.

[0024] Table 1. Rate performance test results of different materials (unit: mAh / g)

[0025] Material Name 0.5C 1C 2C 3C 4C FG 354.3 318.6 284.4 128.3 65.5 FG-PDA-0.1 353.3 320.1 300.4 168.6 104.7 FG-PDA-0.2 353.1 322.5 305.7 179.0 113.2 FG-PDA-0.3 352.4 338.4 319 198.7 125.5 FG-PDA-0.4 351.7 323.3 288.7 158.3 109.9 FG-PDA-0.5 351.1 309.2 277.5 133.8 66.7

[0026] It can be seen that the capacity of FG without dopamine modification and coating at 0.5C is 354.3mAh / g, and the capacity decreases rapidly after high current charge and discharge. For FG-PDA-0.1 coated at a ratio of 1:0.1, the capacity loss after high current charge and discharge is relatively slow. Among them, the capacity after 200 cycles under 2C conditions is nearly 20mAh / g higher than that of FG without dopamine coating.

[0027] As the coating ratio of dopamine increases, the rate of capacity loss of the material after high current charge and discharge becomes slower and slower. In particular, for FG-PDA-0.3 coated at a ratio of 1:0.3, the capacity after 200 cycles under 2C conditions is 34.6mAh / g higher than that of FG without dopamine coating, and the capacity after 200 cycles under 4C conditions is 60mAh / g higher than that of FG without dopamine coating. As the coating ratio of dopamine continues to increase, the rate of capacity loss of the material after high current charge and discharge gradually becomes faster. When the coating ratio of dopamine reaches 0.5, the capacity loss of high current charge and discharge is basically the same as that of the material without dopamine coating.

[0028] However, the study found that with the coating of dopamine, the low Coulombic efficiency decreased. Therefore, an artificial SEI film was constructed based on the research on the rate negative electrode to further improve the performance and safety of lithium-ion batteries.

[0029] Example 2

[0030] In order to improve the coulombic efficiency and safety performance of the rate-type artificial graphite negative electrode material described in Example 1, FG-PDA-0.3 with excellent rate performance was used as the base material for further modification.

[0031] A method for modifying a graphite negative electrode to improve rate performance and coulombic efficiency comprises the following steps:

[0032] S1, preparing carbon-coated graphite FG-PDA-0.3, the preparation method is the same as that in Example 1;

[0033] S2, the FG-PDA-0.3 obtained in S1 is mixed as an active material with acetylene black, polyvinylidene fluoride, and sodium carboxymethyl cellulose in a mass ratio of 7.8:1:1:0.2, and fully ground on agate to make them fully mixed. The mixture is named FG-PDA-0.3-0.2;

[0034] The mixture was transferred into a slurry preparation vessel, and an appropriate amount of N-methylpyrrolidone (NMP) was added to prepare a slurry. The slurry was evenly coated on a copper foil current collector with a thickness of 0.2 mm, and then placed in a blast drying oven and dried at 100°C for 1 h to remove moisture; then transferred to a vacuum drying oven and vacuum dried at 100°C for 10 h to remove NMP, and then punched and cut into circular electrode sheets with a diameter of 14 mm, weighed and recorded as negative electrode sheets; assembled into LIR2016 button batteries.

[0035] Example 3

[0036] A method for modifying a graphite negative electrode to improve rate performance and coulombic efficiency, the difference from Example 2 is that in S2, FG-PDA-0.3 is used as an active material and adjusted to a mass ratio of 7.6:1:1:0.4 with acetylene black, polyvinylidene fluoride, and sodium carboxymethyl cellulose, and the other methods and parameters are the same as those in Example 2. The resulting mixture is recorded as FG-PDA-0.3-0.4.

[0037] Example 4

[0038] A method for modifying a graphite negative electrode to improve rate performance and coulombic efficiency, the difference from Example 2 is that in S2, FG-PDA-0.3 is used as an active material and the mass ratio of acetylene black, polyvinylidene fluoride, and sodium carboxymethyl cellulose is adjusted to 7.4:1:1:0.6, and other methods and parameters are the same as in Example 2. The resulting mixture is recorded as FG-PDA-0.3-0.6.

[0039] Example 5

[0040] A method for modifying a graphite negative electrode to improve rate performance and coulombic efficiency, the difference from Example 2 is that in S2, FG-PDA-0.3 is used as an active material and the mass ratio of acetylene black, polyvinylidene fluoride, and sodium carboxymethyl cellulose is adjusted to 7.2:1:1:0.8, and other methods and parameters are the same as in Example 2. The resulting mixture is recorded as FG-PDA-0.3-0.8.

[0041] Example 6

[0042] A method for modifying a graphite negative electrode to improve rate performance and coulombic efficiency, the difference from Example 2 is that FG-PDA-0.3 in S2 is used as an active material and adjusted to a mass ratio of 7.0:1:1:1 with acetylene black, polyvinylidene fluoride, and sodium carboxymethyl cellulose, and the other methods and parameters are the same as those in Example 2. The resulting mixture is recorded as FG-PDA-0.3-1.

[0043] The coulombic efficiency and rate performance of the LIR2016 button cell obtained in the above embodiment were tested. The test conditions were: the current density was 2C, and the first coulombic efficiency was tested after 20 cycles of the current density of 2C. The test results are shown in Table 2 below.

[0044] Table 2. First coulombic efficiency (%) and rate performance test data of different materials (mAh / g)

[0045] Material Name 2C 2C FG 284.4 95.45 FG-PDA-0.3 319 89.88 FG-PDA-0.3-0.2 313 91.32 FG-PDA-0.3-0.4 317 93.47 FG-PDA-0.3-0.6 308 94.98 FG-PDA-0.3-0.8 305 92.11 FG-PDA-0.3-1.0 311 91.17

[0046] It can be seen that the first coulombic efficiency of FG without dopamine coating is 95.45%, and the first coulombic efficiency of FG-PDA-0.6 with the best rate performance drops to 89.88% due to dopamine coating.

[0047] By adding sodium carboxymethyl cellulose (CMC-Na), it can be cross-linked into a uniform dense layer to form an artificial SEI film, thereby increasing the number of lithium ion transmission and the first coulomb efficiency. Among them, the first coulomb efficiency of FG-PDA-0.3-0.6 reached 94.98%, close to FG, basically eliminating the effect of dopamine coating on the decrease in coulomb efficiency, and obtaining a qualified rate negative electrode material.

[0048] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can make equivalent replacements or changes according to the technical scheme and inventive concept of the present invention within the technical scope disclosed by the present invention, which should be covered by the protection scope of the present invention.

Claims

1. A method for modifying a graphite negative electrode to improve rate performance, characterized in that: The method comprises the following steps: S1, mixing graphite and dopamine, allowing dopamine to self-polymerize on the graphite surface to obtain a polydopamine / graphite composite material; and then carbonizing to obtain carbon-coated graphite; S2, mixing the carbon-coated graphite obtained in S1 as an active material with a conductive agent, a binder, and sodium carboxymethyl cellulose, adding a solvent to prepare a negative electrode slurry, coating the negative electrode slurry on a current collector, and removing the solvent to obtain a negative electrode sheet.

2. The method for improving the rate performance of a graphite negative electrode according to claim 1, characterized in that: In S1, the mass ratio of graphite to dopamine is 1:0.1-0.5; preferably, in S1, the mass ratio of graphite to dopamine is 1:0.2-0.

3.

3. The method for modifying a graphite negative electrode to improve rate performance according to claim 1 or 2, characterized in that: In S1, the carbonization temperature is 900-1000°C, and the carbonization time is 7-9h.

4. The method for improving the rate performance of a graphite negative electrode according to claim 1 or 2, characterized in that: In S1, the graphite is artificial graphite or natural graphite.

5. The method for modifying a graphite negative electrode to improve rate performance according to claim 1 or 2, characterized in that: In S2, the mass ratio of carbon-coated graphite, conductive agent, binder and sodium carboxymethyl cellulose is 7.0-7.8:1.0-1.2:1.0-1.2:0.2-1.

6. The method for modifying a graphite negative electrode to improve rate performance according to claim 1 or 2, characterized in that: In S2, the conductive agent is selected from at least one of acetylene black, carbon black, carbon nanotubes, and graphene.

7. The method for modifying a graphite negative electrode to improve rate performance according to claim 1 or 2, characterized in that: In S2, the binder is selected from at least one of polyvinylidene fluoride, polyvinyl alcohol, polytetrafluoroethylene, and sodium carboxymethyl cellulose.

8. The method for modifying a graphite negative electrode to improve rate performance according to claim 1 or 2, characterized in that: In S2, the solvent is N-methylpyrrolidone or dimethylformamide.

9. The method for modifying a graphite negative electrode to improve rate performance according to claim 1 or 2, characterized in that: The current collector is copper foil or aluminum foil.

10. A negative electrode plate for a lithium battery, characterized in that: The method is prepared by any one of claims 1 to 9.