CN108470912B

CN108470912B - Preparation method of lithium ion battery cathode applying adhesive

Info

Publication number: CN108470912B
Application number: CN201810324451.9A
Authority: CN
Inventors: 刘健; 甘礼惠; 王信心; 龙敏南
Original assignee: Xiamen University
Current assignee: Xiamen University
Priority date: 2018-04-12
Filing date: 2018-04-12
Publication date: 2020-07-03
Anticipated expiration: 2038-04-12
Also published as: CN108470912A

Abstract

A preparation method of a lithium ion battery cathode applying a binder comprises the following steps of 1) taking cellulose as a raw material, reducing the polymerization degree of the cellulose, and confirming that the cellulose in the polymerization degree range accounts for the total amount of the cellulose in a sample; 2) adding ethanol, adding NaOH under stirring, adding cellulose powder, alkalifying, adding sodium chloroacetate, washing with ethanol water solution, filtering, and drying to constant weight to obtain adhesive; 3) the resulting adhesive was tested: the degree of substitution is 0.45-0.70, and the substituents are mainly distributed in C6 and are more than or equal to 90% as verified by nuclear magnetic resonance; if yes, entering the step 4), if not, replacing the cellulose powder with the adhesive, and repeating the step 2); 4) ball-milling silicon powder, a conductive agent and an adhesive, adding water and stirring to prepare slurry; 5) coating the slurry on a metal foil, drying, pressing, and adding electrolyte under the protection of inert gas by using metal lithium as a counter electrode and polyethylene or polypropylene as a diaphragm.

Description

Preparation method of lithium ion battery cathode applying adhesive

Technical Field

The invention relates to a lithium ion battery, in particular to a preparation method of a lithium ion battery cathode applying a binder.

Background

The binder is a key material necessary for the manufacture of lithium ion battery electrodes, and has a main function of connecting a current collector, a conductive component and an active component, and has an important function in improving the performance of the lithium ion battery, particularly the cycle performance (HOCHGATTERER N S, SCHWEIGER M R, KOLLER S, et al. silicon/graphite composition for high-capacity antibodies: flexibility of binder chemistry on cycle stability. electrochemical and Solid-State letters.2008,11(5): A76-A80). Carboxymethyl cellulose (CMC) is a carboxymethylated derivative of cellulose, is a high molecular chemical substance, is easy to expand after absorbing water, and can form transparent viscous glue solution when swelling in water, so that the CMC can be used as a water-based adhesive, has relatively low price and is harmless to the environment; when the battery is prepared, the requirement on humidity is not strict, and the drying speed of the prepared electrode is higher, so that the electrode manufacturing process is simpler and more convenient (GUERF A, KANEKO M, PETTCLERC M, et al₄Water-soluble binding for Li-ion batteries, journal of Power Sources,2007,163: 1047-. The water-soluble adhesive CMC replaces an oily adhesive and is used for the silicon-based lithium ion battery, so that the use of an organic solvent can be avoided, the environment is protected, and the cost is reduced. Meanwhile, the environmental humidity has no great influence on the production process, so the preparation process is relatively simple, and the electricity can be improvedElectrochemical cell Performance, extended cycle life (Jun-Tao Li, Zhan-Yu Wu, Yan-Qiu Lu, et al, Water solvent Binder, an Electrochemical Performance Booster for electrode Materials with High Energy density. advanced Energy materials.2017,7,1701185).

CMC is a water-soluble cellulose ether, and when hydroxyl hydrogen on glucose molecule is substituted by carboxymethyl group, it becomes carboxymethyl cellulose (CMC refers to sodium carboxymethyl cellulose unless specified otherwise in this specification), and its structural formula is as follows:

the main physicochemical indices determining the quality of CMC are: degree of Polymerization (DP), Degree of Substitution (DS), distribution of substituents, and the like.

Polymerization degree: an indicator of molecular size of a polymer. The number of the repeating units is taken as a reference, namely the average value of the number of the repeating units contained in the macromolecular chain of the polymer is expressed by n; based on the number of structural units, i.e., the number of individual structural units contained in the macromolecular chain of the polymer.

Degree of substitution: in the molecular structure of sodium carboxymethylcellulose, each glucose unit has 3 hydroxyl groups, the number of the hydrogen on the hydroxyl group of the glucose unit substituted by the carboxymethyl group is defined by the substitution degree, and theoretically, the maximum value of the substitution degree is 3 (Wangxiang, Zhai Yu, Zhanwei, etc.. methods for measuring the substitution degree of sodium carboxymethylcellulose, journal of food safety quality detection 2015, 6 (8): 3145-. The degree of substitution of CMC directly affects the solubility, emulsibility, thickening property, stability, acid resistance, salt resistance and other properties of CMC. The degree of substitution of CMC, as a characteristic parameter, has a great influence on its application in the electrochemical field.

Distribution of substituents: a total of 3 hydroxyl groups are present per glucose unit, these being the secondary hydroxyl groups of C2, C3 and the primary hydroxyl group of C6, respectively, the activity sizes of which are theoretically C6 > C2 > C3 in this order. In addition, many researchers have studied this aspect because of the unique advantages in application properties of CMC with a specific range of degrees of substitution (Hiroyuki Kono, Kazuhiro Oshima, Hisaho Hashimoto, et al.NMR characterization of a colloidal carbon cellulose 2: Chemical shift analysis and correlation analysis of substrate groups. carbohydrate Polymers 241, 150,2016, 249). The performance of CMC is not only related to the degree of substitution, but also the uniformity of the distribution of the substituents at each substitution site has a significant influence on the performance of CMC (Zhixing Cai, Juan Wu, Bai qiao Du, et al, impact of distribution of carboxmethyl substrates in the stabilizer of carboxmethyl cell on the stability of acid fine coatings, 76,2018, 150-157).

The inventor of the invention compares the cyclic performance, rate performance, impedance performance and cyclic voltammetry characteristic of CMC preparation electrodes with different degrees of substitution and polymerization, optimizes the degree of substitution and polymerization of CMC, and finds that for CMC with specific degree of substitution (150-350) and specific degree of substitution (0.45-0.70), each electrochemical performance of the prepared electrode is superior to that of a commodity CMC preparation electrode. Therefore, the CMC with a specific polymerization degree (150-350) and a specific substitution degree (0.45-0.70) has good advantages in the application of the lithium ion battery, and provides good potential for further improving the comprehensive electrochemical performance of the lithium ion battery.

Disclosure of Invention

Aiming at the defects in the prior art, the invention provides the preparation method of the lithium ion battery cathode using the adhesive, which has simple preparation process, can improve the electrochemical performance of the battery and prolong the cycle life.

The invention comprises the following steps:

1) reducing the polymerization degree of cellulose to 150-350 by using the cellulose as a raw material, and confirming that the cellulose in the polymerization degree range accounts for more than 90% of the total amount of the cellulose in a sample;

2) adding ethanol into a container, adding NaOH while stirring, adding cellulose powder, alkalizing, adding sodium chloroacetate, performing etherification reaction, neutralizing to be neutral, washing with an ethanol water solution, filtering, and drying to constant weight to obtain an adhesive;

3) the resulting adhesive was checked for presence in the following range: the degree of substitution is 0.45-0.70, and the substituents are mainly distributed in C6 and are more than or equal to 90% as verified by nuclear magnetic resonance; if yes, entering step 4), if not, replacing the cellulose powder with the prepared adhesive, and repeating the step 2) until the obtained adhesive meets the requirement;

4) ball-milling silicon powder, a conductive agent and an adhesive, adding water and stirring to prepare slurry;

5) and coating the slurry on a metal foil, drying, pressing, drying again, and adding electrolyte under the protection of inert gas by taking metal lithium as a counter electrode and polyethylene or polypropylene as a diaphragm to obtain the cathode of the lithium ion battery using the adhesive.

In the step 1), the cellulose may be natural cellulose, the polymerization degree of the cellulose is reduced to 150-350, and the polymerization degree of the cellulose can be reduced to 150-350 by a pulping method, wherein the pulping method is a process for reducing the molecular weight of a cellulose raw material by adopting a physical or chemical means, and the pulping method may comprise a mechanical method, a chemical-mechanical method, an enzymatic method and the like.

In the step 2), the mass ratio of the ethanol, the NaOH, the cellulose powder and the sodium chloroacetate can be (10-80): 1-4): 1-5): 6; the cellulose powder can adopt the cellulose powder with the polymerization degree of 150-350; the alkalization temperature can be 10-40 ℃, and the alkalization time can be 1-60 min; the temperature of the etherification reaction can be 40-70 ℃, and the time of the etherification reaction can be 1-120 min; the neutralization to neutrality can be performed by using glacial acetic acid; the ethanol water solution can adopt 50-100% of ethanol water solution in percentage by mass; the drying temperature can be 40-110 ℃.

In the step 4), the mass ratio of the silicon powder, the conductive agent, the adhesive and the water can be (1-5): 1-2): 5-100; the conductive agent can be selected from at least one of acetylene black, graphene and the like; the ball milling time can be 0.5-4 h.

In the step 5), the drying can be carried out in a vacuum drying oven for 1-24 h; the pressing can be performed on a pressing machine under the pressure of 5-20 MPa; the re-drying can be carried out in a vacuum drying oven for 1-10 h; the electrolyte takes lithium salt and organic solvent as main components; the lithium salt may include at least one of lithium perchlorate, lithium hexafluorophosphate, and the like; the organic solvent may be at least one selected from ethylene carbonate, propylene carbonate, diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, and the like.

The invention adopts a slurry method to prepare CMC. The CMC silicon cathode with specific polymerization degree (150-350) and specific substitution degree (0.45-0.70) synthesized by the method is prepared on the basis of the CMC silicon cathode with specific polymerization degree and substitution degree, and is subjected to a series of electrochemical performance tests with various commercial products CMC, and the silicon electrode prepared by the CMC synthesized by the method is found to have comprehensive performance obviously superior to that of the commercial products CMC. In the slurry method, the amount of the organic solvent is about 10-30 times of the amount of the cellulose, and in a reaction system, the reaction solid is in a slurry or suspension state, so the suspension method is also called. When the reaction bath ratio is high, the raw materials can react with the alkalizer and the etherifying agent more fully. Because the medium has uniform and fast heat and mass transfer in the reaction process, the main reaction is fast, the side reaction is reduced, and the utilization rate of the etherifying agent is higher, the reaction uniformity and the stability are improved, thereby greatly improving the substitution degree and the substitution uniformity of the product, better playing the service performance of the CMC, and becoming the direction of the key excavation of the whole cellulose ether industry.

Drawings

FIG. 1 is an XRD spectrum of cellulose with polymerization degree and CMC.

FIG. 2 is an infrared spectrum of polymerized cellulose and CMC.

Fig. 3 shows the cycling performance of electrodes prepared from CMC with home-made degree of polymerization and substitution and commercial CMC.

Detailed Description

The following examples will further illustrate the present invention with reference to the accompanying drawings.

Example 1

Adding 80mL of ethanol into a flat-bottomed flask, adding 26.67g of 15% (mass fraction) NaOH solution while stirring, adding 5.0g of cellulose with polymerization degree, alkalizing for 0.5h at 30 ℃, adding 6.0g of sodium chloroacetate, and heating to 60 ℃ for etherification reaction for 1 h. After the etherification is finished, glacial acetic acid is added to neutralize to be neutral, ethanol with the volume fraction of 80 percent is used for washing for 3 times, and the CMC sample (recorded as L1) is obtained after filtration and drying. Taking L1 as a raw material, adding an equivalent amount of NaOH solution, alkalizing for 0.5h at 30 ℃, adding an equivalent amount of sodium chloroacetate, heating to 60 ℃, carrying out etherification reaction for 1h, then carrying out neutralization, alcohol washing, filtering and drying, repeating the operations for 1-4 times respectively to obtain CMC samples L2, L3, L4 and L5, and measuring the substitution degree by a sulfuric acid titration method to find that the substitution degrees of L2, L3 and L4 are within the range of 0.45-0.70. Representative examples of the results were L3, and XRD, infrared and nuclear magnetic characterization was performed.

The XRD spectrum of the polymerized cellulose (MCC) and the CMC (L3) is shown in figure 1. As can be seen from fig. 1, the diffraction peak appearing at 2 θ ═ 15.7 ° is a typical crystalline form of cellulose i, and the strong diffraction peak at 2 θ ═ 22.5 ° is a crystalline form of cellulose i with the 002 crystal plane. As shown in the spectrum of CMC, the diffraction peak at 15.7 ° 2 θ disappears, the strong diffraction peak at 22.2 ° 2 θ shifts to a low angle, and a new distinct characteristic derivative peak appears at 20 ° 2 θ. These all indicate the generation of carboxymethyl groups during the basification.

The infrared spectra of polymerized cellulose (MCC) and CMC (L3) are shown in FIG. 2, and the cellulose molecules and CMC molecules are at 3410cm^-1And 1100-1000 cm^-1The absorption peaks of the two parts respectively correspond to stretching vibration of-OH and C-O, C-O-C vibration on molecular skeletons of cellulose and CMC. However, the CMC was 1604cm^-1、1430cm^-1、1324cm^-1There is also an absorption peak corresponding to the coincidence of antisymmetric and symmetric stretching vibrations of-COO-, and stretching vibrations of C-H on carboxymethyl group, which is the main difference from the raw material cellulose. This indicates that the cellulose, upon alkalization and etherification, produces a large number of carboxymethyl functional groups, indicating that the synthesized product is CMC.

In conclusion, carboxymethyl groups are connected to the cellulose molecular chains, and CMC with the degree of substitution can be obtained from the cellulose with the polymerization degree through a synthesis method.

Weighing 1.0g of CMC dried to a constant amount at 105 ℃, respectively adding into 39mL of deionized water, stirring until the solution is clear and transparent, standing without precipitation, and preparing into a CMC binder solution. Weighing 0.28g of nano silicon, 0.06g of acetylene black and 2.4g of CMC binder solution, and performing ball milling for 3.5h according to the ratio of m (nano silicon) to m (acetylene black) to m (CMC) of 7 to 1.5 to prepare slurry.

And respectively uniformly coating the slurry on copper foils, and placing the coated copper foils in a vacuum drying oven at 80 ℃ for 24 hours. In a glove box, lithium metal is used as a counter electrode, Celgard 2400 membrane is used as a diaphragm, and 1mol/L LiPO4/EC (ethylene carbonate) + DMC (dimethyl carbonate) (volume ratio EC: DMC ═ 1: 1, battery grade)/10% FEC (fluoroethylene carbonate) is used as an electrolyte to prepare a button cell.

The L1-L5 series CMC is prepared, and electrodes are respectively prepared with 4 kinds of commercial CMC (H1-H4), and the cycle performance is measured. 4 commercial CMCs (H1-H4, degree of substitution 0.7, 0.9, and 1.2, respectively, and degree of polymerization of about 1250) were purchased from Shanghai Aladdin Chemicals, Inc. Referring to fig. 3, the electrode prepared using the self-made CMC (L1-L5) has better cycle performance than 4 commercial CMCs (H1-H4). The specific capacity of the electrode prepared from L1-L5 is seriously attenuated in the first 3-5 cycles, but the attenuation of the electrode prepared from L3 tends to be moderate after the 3 rd cycle; at the 100 th cycle, the specific capacity of the electrode prepared from L3 was 1407.2mAh/g, which is significantly higher than that of the other electrodes.

The degree of polymerization and degree of substitution CMCs of the present invention and the specific capacity of electrodes made from commercial CMCs at cycle 100 are shown in table 1. As can be seen from table 1, the cycling stability of the electrode prepared with L3 was the best.

TABLE 1

The invention discloses a lithium ion battery silicon cathode applying an adhesive, which realizes better electrochemical performance. The method comprises the steps of alkalization and etherification by using a slurry method with specific cellulose raw materials, sodium chloroacetate, sodium hydroxide and the like as raw materials, and alkalization and etherification are carried out for multiple times to prepare the carboxymethyl cellulose (CMC) with a specific polymerization degree (150-350) and a specific substitution degree (0.45-0.70). The prepared CMC is used as a silicon-based adhesive of a lithium ion battery cathode to assemble a battery, and characterization modes such as constant current charging and discharging, cyclic voltammetry, alternating current impedance, multiplying power performance and the like are adopted for testing the silicon cathode. The experimental result shows that the electrochemical performance is superior to that of a silicon negative electrode prepared by taking common commodity CMC as a binder.

Claims

1. A preparation method of a lithium ion battery cathode applying a binder is characterized by comprising the following steps:

1) reducing the polymerization degree of cellulose to 150-350 by using the cellulose as a raw material, and confirming that the cellulose in the polymerization degree range accounts for more than 90% of the total mass of the cellulose in a sample;

3) the resulting adhesive was checked for presence in the following range: the substitution degree is 0.45-0.70, and the mass percent of the carboxymethyl cellulose with the substituent distributed on the No. 6 carbon is more than or equal to 90% of the total sample mass through nuclear magnetic resonance verification; if yes, entering step 4), if not, replacing the cellulose powder with the prepared adhesive, and repeating the step 2) until the obtained adhesive meets the requirement;

2. The method for preparing the negative electrode of the lithium ion battery using the binder according to claim 1, wherein in the step 1), the cellulose is natural cellulose, and the step of reducing the polymerization degree of the cellulose to 150 to 350 is to reduce the polymerization degree of the cellulose to 150 to 350 by a pulping method.

3. The method of claim 2, wherein the pulping process is a process for reducing the molecular weight of the cellulose material by physical or chemical means, and the pulping process comprises mechanical, chemical, chemimechanical and enzymatic methods.

4. The method for preparing the negative electrode of the lithium ion battery using the binder according to claim 1, wherein in the step 2), the mass ratio of the ethanol, the NaOH, the cellulose powder and the sodium chloroacetate is (10-80): 1-4): 1-5): 6.

5. The method for preparing the negative electrode of the lithium ion battery using the binder according to claim 1, wherein in the step 2), the cellulose powder with the polymerization degree of 150-350 is adopted as the cellulose powder.

6. The method for preparing the negative electrode of the lithium ion battery using the binder according to claim 1, wherein in the step 2), the alkalization temperature is 10 to 40 ℃, and the alkalization time is 1 to 60 min; the temperature of the etherification reaction is 40-70 ℃, and the time of the etherification reaction is 1-120 min; the neutralization to the neutrality is performed by using glacial acetic acid; the ethanol water solution adopts 50 to 100 mass percent ethanol water solution; the drying temperature is 40-110 ℃.

7. The method for preparing the negative electrode of the lithium ion battery using the binder according to claim 1, wherein in the step 4), the mass ratio of the silicon powder, the conductive agent, the binder and the water is (1-5): 1-2): 5-100.

8. The method for preparing a negative electrode of a lithium ion battery using a binder according to claim 1, wherein in the step 4), the conductive agent is at least one selected from acetylene black and graphene; the ball milling time is 0.5-4 h.

9. The method for preparing the negative electrode of the lithium ion battery using the binder according to claim 1, wherein in the step 5), the drying is performed for 1 to 24 hours in a vacuum drying oven; the pressing is carried out on a pressing machine under the pressure of 5-20 MPa; and the re-drying is to place the mixture into a vacuum drying oven for drying for 1-10 h.

10. The method of preparing a negative electrode for a lithium ion battery using a binder according to claim 1, wherein in the step 5), the electrolyte solution comprises a lithium salt and an organic solvent as main components; the lithium salt comprises at least one of lithium perchlorate and lithium hexafluorophosphate; the organic solvent is at least one of ethylene carbonate, propylene carbonate, diethyl carbonate, dimethyl carbonate and ethyl methyl carbonate.