Lithium ion battery negative electrode sheet preparation method, negative electrode sheet and lithium ion battery

Abstract

The present invention discloses a method for preparing a negative electrode sheet of a lithium ion battery, comprising the following steps: S01: uniformly mixing lignin, polyethylene oxide and dimethylformamide; S02: adding nano silicon powder to the precursor solution obtained in S01, maintaining a constant temperature, continuously stirring during heating, and stopping when the viscosity of the mixture is 2000-3000cp; S03: coating the precursor solution obtained in S02 on the negative electrode collector, and then drying naturally in an atmospheric environment; S04: sintering the negative electrode collector obtained in S03 under the protection of an inert atmosphere. The present invention provides a novel method for preparing a negative electrode sheet of a lithium ion battery using a carbon-coated silicon material, and a novel negative electrode sheet of a lithium ion battery and a lithium ion battery containing the negative electrode sheet are obtained by the preparation method. The negative electrode sheet of a lithium ion battery prepared using the above preparation method increases the cycle performance due to the structure of nano silicon-coated carbon, thereby improving the overall performance of the lithium ion battery.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium ion battery manufacturing, and specifically relates to a method for preparing a negative electrode plate of a lithium ion battery and a negative electrode plate prepared by the preparation method, and also relates to a lithium ion battery comprising the negative electrode plate. Background Art

[0002] Since lithium-ion batteries have the advantages of high energy density, low self-discharge, and no memory effect, they have great application prospects and wide demand in mobile electronic devices, electric vehicles, and large-scale energy storage. Lithium-ion batteries contain positive and negative electrode plates, and the positive and negative electrode plates are coated with a conductive slurry prepared by mixing a binder, a conductive agent, and an electrode active material in a certain proportion. In traditional technology, lithium-ion batteries mostly use graphite as the active material of the negative electrode, but the theoretical specific capacity of graphite is low, only 372 mAh / g, which is difficult to meet the market's growing demand for battery capacity. The development of new negative electrode material systems has become an inevitable trend.

[0003] Silicon has become the most popular and promising material in the new generation of negative electrode materials due to its high theoretical specific capacity (4200 mAh / g), low lithium reaction potential, and wide sources. However, silicon has a large volume change rate (>300%) during the charge and discharge process, which will cause the silicon active material negative electrode to pulverize and rupture, thereby reducing the capacity of the battery. This pulverization and rupture will lead to the formation of new SEI (solid electrolyte interphase) and continuously consume recyclable lithium, thereby reducing the battery cycle efficiency. In order to solve the inherent defects of silicon negative electrodes, researchers have made numerous improvements on silicon-based negative electrode active materials in recent years to improve the stability of silicon-based negative electrode active materials. Among them, carbon coating and composite of silicon materials have been proven to be the most effective method. On the one hand, carbon coating can protect the silicon surface from excessive contact with the electrolyte and reduce the formation of SEI. On the other hand, carbon is relatively soft and abundant in source, and can well absorb the volume change of silicon materials during the cycle. Graphite and structurally related carbon are relatively soft, have very good electrical conductivity, are low in mass and are characterized by a small volume change during charge / discharge. For these reasons, as is known, carbon-based negative electrodes have very good electrochemical stability. By combining the advantages of both elements (Si has a large capacity and C has a high stability), Si / C-based electrode active materials have more stable cycling characteristics than pure silicon with increased capacity.

[0004] The Chinese patent application number 201710052050.8 entitled "A carbon-coated nano-silicon composite material and its preparation method" provides a core-shell structure powder with nano-silicon particles as content and carbon as shell as a negative electrode material for lithium-ion batteries. In this scheme, asphalt is used as a carbon coating precursor to prepare silicon for carbon coating. This method can improve the cycle performance of silicon, but the process is relatively complicated, and the asphalt carbon precursor used is mainly extracted from petroleum and coal, which is a non-renewable energy source. Because the traditional electrode preparation process is relatively complicated, and a binder without lithium storage activity is introduced at the same time, this will lead to a loss of electrode capacity. Compared with the traditional process, growing an active material composite structure that does not require a binder directly on a copper foil current collector can reduce the process flow of electrode preparation and increase the capacity of the electrode. For example, the invention patent application number 20151070849.8 entitled "A lithium-ion battery negative electrode material without a binder and a conductive agent and a preparation method" discloses a method for preparing a battery negative electrode material without a binder and a conductive agent directly on a nickel foam current collector. This binder-free negative electrode preparation method can simplify the process, but the prepared Na 3 VO 4 / Ni has no significant performance improvement compared to graphite or silicon materials. Summary of the invention

[0005] In order to solve the deficiencies of the prior art, the present invention provides a method for preparing a new type of carbon-coated silicon material as a negative electrode plate for a lithium-ion battery, and a new type of negative electrode plate for a lithium-ion battery and a lithium-ion battery containing the negative electrode plate are obtained by the preparation method. The present invention provides a method for preparing a negative electrode plate for a lithium-ion battery using nano-silicon particles as an active material and heat-treated lignin as a coating structure and a structural frame on the surface of the silicon material. The negative electrode plate for a lithium-ion battery prepared using the above preparation method has increased cycle performance due to the use of a nano-silicon-coated carbon structure, thereby improving the overall performance of the lithium-ion battery.

[0006] The technical effects to be achieved by the present invention are achieved through the following solutions:

[0007] The method for preparing a negative electrode sheet of a lithium ion battery provided in the present invention comprises the following steps:

[0008] S01: Preparation of precursor solution: uniformly mix lignin, polyethylene oxide and dimethylformamide, wherein the mass ratio of lignin, polyethylene oxide and dimethylformamide is (150-250):1:(1800-2200), heat the mixed solution to 50-70°C, and stir for 1-2 hours;

[0009] S02: Adding nano silicon powder: Adding nano silicon powder to the precursor solution obtained in S01, the mass ratio of the added nano silicon powder to the lignin is 1: (0.9-1.5); maintaining a constant temperature, stirring continuously during the heating process, and stopping when the viscosity of the mixture reaches 2000-3000cp;

[0010] S03: Drying: coating the precursor solution obtained in S02 on the negative electrode current collector, and then drying it naturally in an atmospheric environment for 4-8 hours;

[0011] S04: Sintering: Sinter the negative electrode current collector obtained in S03 at 500-600° C. for 1-2 hours under the protection of an inert atmosphere to obtain the desired lithium ion negative electrode sheet.

[0012] In the present invention, compared with the prior art, due to the addition of polyethylene oxide to increase the complex viscosity, that is, due to the hydrogen bonding between polyethylene oxide and lignin, during the temperature carbonization process, the lignin solution can better maintain its morphology coated on the silicon surface and finally obtain a uniform coating. At the same time, the increase in complex viscosity also enables the composite electrode to maintain its ability to disperse silicon in lignin during the volatilization of the dimethylacetamide solvent, while reducing the stratified precipitation of silicon particles in the mixed solution, and finally obtaining a more excellent silicon particle capacity utilization and cycle performance. Moreover, the method of the present invention adopts a one-step molding method, that is, the coating is directly completed on the surface of the copper foil and the aluminum foil and then sintered, which reduces the complexity of the preparation process.

[0013] Furthermore, in S01, the lignin is one or more of hydrolyzed lignin, kraft lignin, sulfonate lignin, Alcell lignin, and acetic acid lignin. Among them, kraft lignin is the most stable, cheap and easy to obtain, and is used as a preferred raw material.

[0014] Furthermore, in S01, the relative molecular mass of the polyethylene oxide is 5.0×10 5 -1.0×10 6 The polyethylene oxide within the above relative molecular weight range can not only ensure the viscosity during the preparation process and make the nano-silicon powder and lignin form a stable uniform phase, but also can volatilize within a limited time during the drying process without affecting the subsequent sintering process.

[0015] Furthermore, in S02, the precursor solution and the nano-silicon powder mixture are continuously magnetically stirred, and the planetary mixer is switched to stir for 5-8 minutes every 20-30 minutes.

[0016] Furthermore, in S02, the particle size of the nano silicon powder is in the range of 30-150 nm. When the particle size of the silicon nanoparticles is greater than 150 nm, they are easily pulverized and broken, and the particles are too large to be dispersed in the solution. If the particles are too small, the cost of raw materials is increased, and it is difficult to apply to large-scale industrial production.

[0017] Furthermore, in S03, the negative electrode current collector is copper foil or carbon-coated copper foil.

[0018] The present invention also provides a negative electrode plate for a lithium ion battery, and the method for preparing the negative electrode plate is as described above.

[0019] The present invention also provides a lithium ion battery, wherein the negative electrode plate used in the lithium ion battery is as described above.

[0020] Furthermore, the lithium-ion battery is a button cell; a metal lithium sheet is used as a counter electrode, a Celgard membrane is used as a diaphragm, and an electrolyte of 1 mol / L LiPF6 / EC+DEC and 10 wt% FEC additive, wherein the volume ratio of EC to DEC is 1:1.

[0021] Furthermore, under the test conditions of a test voltage of 0.005-1.2V and a test rate of 0.1A / g, the lithium-ion battery has a first-week reversible capacity greater than 3000mAh / g and an initial efficiency greater than 80%; after 100 cycles, the capacity is greater than 2000mAh / g and the capacity retention rate is greater than 80%.

[0022] The present invention has the following advantages:

[0023] 1. The present invention provides a method for preparing a negative electrode sheet of a lithium-ion battery using nano-silicon particles as an active material and heat-treated lignin as both a coating structure and a structural framework on the surface of the silicon material. The negative electrode sheet of a lithium-ion battery prepared using the above preparation method has increased cycle performance due to the use of a nano-silicon-coated carbon structure, thereby improving the overall performance of the lithium-ion battery.

[0024] 2. The carbon coating precursor used in the present invention is derived from lignin, which is a renewable organic matter widely derived from fabrics. It is not only cheap and easy to obtain, but also green and environmentally friendly.

[0025] 3. The basic structural framework of the negative electrode of the lithium-ion battery in the present invention is derived from the heat-treated lignin, which replaces the traditional binder and conductive agent under the premise of having a certain lithium storage capacity, and simplifies the process flow of electrode preparation. The method of the present invention adopts a one-step molding method, that is, directly coating the copper foil surface and then sintering, which reduces the complexity of the preparation process.

[0026] 4. The lithium-ion battery of the present invention has excellent electrochemical cycle performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is a SEM image of the surface of the negative electrode of the lithium ion battery in the present invention;

[0028] Figure 2 The charge and discharge curves (2-a) and the cycle performance diagram (2-b) of the first to fifth cycles (including the first cycle) obtained by testing another lithium-ion battery in the present invention. DETAILED DESCRIPTION

[0029] The present invention is described in detail below with reference to the accompanying drawings and embodiments.

[0030] 1. Preparation of negative electrode sheets for lithium-ion batteries

[0031] The negative electrode sheets of lithium-ion batteries are prepared according to the following three groups.

[0032] Group 1:

[0033]

[0034] The steps for preparing the negative electrode of a lithium-ion battery are as follows:

[0035] S01: Preparation of precursor solution: Take lignin, polyethylene oxide and dimethylformamide according to the formula in the above table and mix them evenly, heat the mixed solution to 60° C. and stir for 1 hour.

[0036] S02: Add nano silicon powder to the precursor solution obtained in S01, with the mass ratio of the added nano silicon powder to the mass ratio of lignin being 1:1; maintain a constant temperature, and continuously magnetically stir the precursor solution and nano silicon powder mixture, and switch to a planetary mixer for stirring for 5 minutes every half an hour. The particle size of the nano silicon powder used in this step is 80nm.

[0037] S03: The precursor solution obtained in S02 is coated on the negative electrode current collector copper foil, and then naturally dried in an atmospheric environment for 6 hours.

[0038] S04: Sinter the negative electrode current collector copper foil obtained in S03 under the protection of argon atmosphere for 1.5 hours to obtain the required lithium ion negative electrode plate.

[0039] Group 2:

[0040]

[0041] The steps for preparing the negative electrode of a lithium-ion battery are as follows:

[0042] S01: Preparation of precursor solution: Take lignin, polyethylene oxide and dimethylformamide according to the formula in the above table and mix them evenly, heat the mixed solution to 50° C. and stir for 1 hour.

[0043] S02: Add nano silicon powder to the precursor solution obtained in S01, the mass ratio of the added nano silicon powder to the mass ratio of lignin is 1:0.9; maintain a constant temperature, and continuously magnetically stir the precursor solution and nano silicon powder mixture, and change the planetary mixer to stir for 5 minutes every half an hour. The particle size of the nano silicon powder used in this step is 30nm.

[0044] S03: The precursor solution obtained in S02 is coated on the negative electrode current collector copper foil, and then naturally dried in an atmospheric environment for 4 hours.

[0045] S04: Sinter the negative electrode current collector copper foil obtained in S03 for 2 hours under the protection of argon atmosphere to obtain the required lithium ion negative electrode sheet.

[0046] Group 3:

[0047]

[0048] The steps for preparing the negative electrode of a lithium-ion battery are as follows:

[0049] S01: Preparation of precursor solution: Take lignin, polyethylene oxide and dimethylformamide according to the formula in the above table and mix them evenly, heat the mixed solution to 70° C. and stir for 1 hour.

[0050] S02: Add nano silicon powder to the precursor solution obtained in S01, with the mass ratio of the added nano silicon powder to the mass ratio of lignin being 1:1.5; maintain a constant temperature, and continuously magnetically stir the precursor solution and nano silicon powder mixture, and switch to a planetary mixer for stirring for 5 minutes every half an hour. The particle size of the nano silicon powder used in this step is 150nm.

[0051] S03: The precursor solution obtained in S02 is coated on the negative electrode current collector copper foil, and then naturally dried in an atmospheric environment for 8 hours.

[0052] S04: Sinter the negative electrode current collector copper foil obtained in S03 for 2 hours under the protection of argon atmosphere to obtain the required lithium ion negative electrode sheet.

[0053] 2. Preparation of lithium-ion batteries

[0054] The prepared silicon composite electrode sheet was directly used as the negative electrode sheet and cut into an electrode sheet with a diameter of 12 mm, and then vacuum dried at 120°C for 12 h. Then, a metal lithium sheet was used as the counter electrode, a Celgard membrane was used as the separator, and an electrolyte with 1 mol / L LiPF6 / EC+DEC and 10 wt% FEC additive, in which the volume ratio of EC to DEC was 1:1, was assembled into a CR2025 button battery in an argon-protected glove box.

[0055] 3. Testing

[0056] The prepared lithium-ion battery was subjected to constant current charge and discharge and cycle tests using a Biologic (VMP-3) battery test system, with a test voltage of 0.005-1.2V. Under the test condition of 0.1A / g rate buckling point, the test results of the embodiment are shown in the following table. For comparison, the comparative examples used are lithium-ion batteries made by the same preparation method, wherein comparative examples 1, 3, and 5 use graphite as the negative electrode material, and comparative examples 2, 4, and 6 use nano-silicon as the negative electrode material, and the negative electrode material of the same material is used in each group as the comparative example.

[0057] The first set of test results:

[0058]

[0059] As shown in the attached figure, Figure 1 This is a SEM image of the surface of the negative electrode sheet of the lithium battery prepared in Example 1. Figure 2 The charge and discharge curves (3-a) and the cycle performance diagram (3-b) of the first to fifth cycles (including the first cycle) obtained from the lithium-ion battery test in Example 3.

[0060] The second set of test results:

[0061]

[0062] The third set of test results:

[0063]

[0064] It can be seen from the test process that the mass ratio of lignin, polyethylene oxide and dimethylformamide has little effect on the final result, as long as it is within the mass ratio range provided in the present invention. Under the same other conditions, the use of lignin with a high relative molecular mass and the increase of the heat treatment temperature help to improve the performance of the negative electrode to a certain extent. In the present invention, compared with the prior art, due to the addition of polyethylene oxide to increase the complex viscosity, that is, due to the hydrogen bonding between polyethylene oxide and lignin, during the temperature increase carbonization process, the lignin solution can better maintain its morphology coated on the silicon surface and finally obtain a uniform coating. At the same time, the increase in complex viscosity also enables the composite electrode to maintain its ability to disperse silicon in lignin during the volatilization of the dimethylacetamide solvent, while reducing the stratified precipitation of silicon particles in the mixed solution, and finally obtaining a more excellent silicon particle capacity utilization and cycle performance. Moreover, the method of the present invention adopts a one-step molding method, that is, the coating is directly completed on the copper foil surface and then sintered, which reduces the complexity of the preparation process.

[0065] It can be seen from the test results that the lithium-ion battery in the embodiment has excellent electrochemical cycle performance. The battery in the comparative example, such as the graphite negative electrode battery, has a large value in the first efficiency and capacity retention rate after 100 cycles, but the capacity of the battery itself is small, almost only 1 / 7 to 1 / 8 of the battery in this embodiment, and cannot be compared with the battery in this embodiment in terms of practicality. Although the ordinary nano-silicon negative electrode battery has a high reversible capacity in the first week, the first efficiency and cycle efficiency are low, and the capacity retention rate after 100 cycles falls to less than 30%, which cannot meet the needs of long-term use and has a short battery life.

[0066] Taking Example 3 in the first group of experiments as an example, this example is the best parameter result in the first group of examples. Its first-week reversible capacity reaches 3086 mAh / g, far exceeding the lithium battery with a traditional graphite negative electrode sheet in Comparative Example 1. The first efficiency is greater than 80%. After 100 cycles, the capacity reaches 2378 mAh / g, far exceeding the comparative example, and the capacity retention rate exceeds 80%.

[0067] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the embodiments of the present invention rather than to limit them. Although the embodiments of the present invention have been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the embodiments of the present invention can still be modified or replaced by equivalents, and these modifications or equivalent replacements cannot cause the modified technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing a negative electrode sheet of a lithium-ion battery, Features The preparation steps are as follows: S01: Preparation of precursor solution: Take lignin, polyethylene oxide and dimethylformamide and mix them evenly, wherein the mass ratio of lignin, polyethylene oxide and dimethylformamide is (150-250):1:(1800-2200), heat the mixed solution to 50-70°C and stir for 1-2 hours; the relative molecular mass of the polyethylene oxide is 5.0×10 5 -1.0×10 6 ; S02: Adding nano silicon powder: Adding nano silicon powder to the precursor solution obtained in S01, the mass ratio of the added nano silicon powder to the lignin is 1: (0.9-1.5); maintaining a constant temperature, stirring continuously during the heating process, and stopping when the viscosity of the mixture reaches 2000-3000cp; S03: Drying: coating the precursor solution obtained in S02 on the negative electrode current collector, and then drying it naturally in an atmospheric environment for 4-8 hours; S04: Sintering: Sinter the negative electrode current collector obtained in S03 at 500-600° C. for 1-2 hours under the protection of an inert atmosphere to obtain the desired lithium ion negative electrode sheet.

2. The method for preparing the negative electrode sheet of a lithium ion battery according to claim 1, Features: In S01, the lignin is one or a combination of hydrolyzed lignin, sulfate lignin, sulfonate lignin, Alcell lignin, and acetic acid lignin.

3. The method for preparing the negative electrode sheet of a lithium ion battery according to claim 1, Features: In S02, the precursor solution and the nano-silicon powder mixture are continuously magnetically stirred, and the planetary mixer is switched to stir for 5-8 minutes every 20-30 minutes.

4. The method for preparing the negative electrode sheet of a lithium ion battery according to claim 1, Features: In S02, the particle size range of nano silicon powder is 30-150nm.

5. The method for preparing the negative electrode sheet of a lithium ion battery according to claim 1, Features: In S03, the negative electrode current collector is copper foil or carbon-coated copper foil.

6. A negative electrode plate for a lithium-ion battery, Features: The method for preparing the negative electrode sheet is as described in any one of claims 1-5.

7. A lithium ion battery, Features: The negative electrode plate in the lithium-ion battery is as described in claim 6.

8. The lithium ion battery according to claim 7, Features: The lithium-ion battery is a button cell; a metal lithium sheet is used as a counter electrode, a Celgard membrane is used as a diaphragm, and an electrolyte of 1 mol / L LiPF6 / EC+DEC and 10 wt% FEC additive, wherein the volume ratio of EC to DEC is 1:

1.

9. The lithium ion battery according to claim 8, Features: The lithium-ion battery has a first-week reversible capacity greater than 3000mAh / g and a first-week efficiency greater than 80% under the test voltage of 0.005-1.2V and a test rate of 0.1A / g; After 100 cycles, the capacity is greater than 2000mAh / g, and the capacity retention rate is greater than 80%.

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