Modified sodium carboxymethyl cellulose-based binder and application thereof in lithium ion battery

Abstract

The invention discloses a modified sodium carboxymethyl cellulose-based binder, and belongs to the technical field of binders for lithium ion batteries. According to the modified sodium carboxymethyl cellulose-based binder, CMC-Na is used as a raw material, and a modifier is added, so that the modified sodium carboxymethyl cellulose-based binder is obtained; the modifier is any one of a mixture of PA (polyamide) and CaCl2 or PEI (polyetherimide). The specific capacity of the prepared C-PEI-10% pole piece is 2249.1 mAh g <-1 > after 100 cycles under the current density of 0.2 C, and a compact SEI layer can be formed on the electrode. The specific capacity of Si-coated C-PA-Ca < 2 + > is 1879.7 mAh g <-1 > after 300 cycles under the current density of 0.5 C, and the capacity retention ratio is 70.85%.

Description

Technical Field

[0001] The invention belongs to the technical field of binders for lithium ion batteries, and in particular relates to a modified sodium carboxymethyl cellulose-based binder and application thereof in lithium ion batteries. Background Art

[0002] Lithium-ion batteries have been widely used in various fields such as automobiles, electricity, and new energy due to their high energy density, clean and environmentally friendly, and long life. However, the widely used graphite negative electrode has the problem of low specific capacity, which also restricts the energy density of the battery prepared with it. The current silicon-based and its derivative materials have attracted much attention due to their advantages such as higher theoretical specific capacity, more stable voltage platform, and abundant raw material sources. However, the electrical conductivity of silicon materials is low, and the expansion and contraction effects are serious during the charging and discharging process, which will cause cracking and shedding of active materials, as well as continuous growth of solid electrolyte membrane (SEI), which seriously damages the performance of the battery. By adding a binder, the adhesion of the electrode to the copper foil can be enhanced, the integrity of the electrode can be ensured, and the stability of the battery during cycling can be increased.

[0003] Sodium carboxymethyl cellulose (CMC-Na) is a natural biomass polymer, one of the most common anionic polysaccharides derived from natural cellulose. It is a water-soluble, colorless and odorless white solid powder. CMC-Na was discovered as an excellent natural binder for silicon-based negative electrodes, and was first introduced into people's field of vision and has been commercialized in graphite lithium batteries. Its molecular structure contains a large number of carboxyl groups (-COOH) and hydroxyl groups (-OH). Because it contains many polar carboxyl functional groups, it can interact well with the surface oxide layer of Si particles to form strong and stable chemical bonds through active substances, so that Si particles are still not easy to fall off after multiple cycles, reducing the problems caused by the volume expansion of Si particles during cycling. However, when CMC-Na is used as a binder for silicon-based materials with large volume expansion, it still has the disadvantages of poor rate performance, poor long cycle effect, and difficulty in forming a stable SEI layer. Summary of the invention

[0004] One technical problem solved by the present invention is to provide a modified sodium carboxymethyl cellulose-based binder, which uses a mixture of PA and CaCl2 or PEI as a modifier, has a wide source of raw materials, is low in price, and has a simple preparation method, and is a binder with excellent application prospects. Another technical problem to be solved by the present invention is to provide the application of the modified sodium carboxymethyl cellulose-based binder in lithium-ion batteries, which has excellent performance as a battery negative electrode binder, is conducive to the formation of a dense SEI layer, and improves battery stability.

[0005] Technical solution: In order to solve the above technical problems, the technical solution adopted by the present invention is as follows:

[0006] A modified sodium carboxymethyl cellulose-based binder is prepared by taking sodium carboxymethyl cellulose (CMC-Na) as a raw material and adding a modifier to obtain the modified sodium carboxymethyl cellulose-based binder; the modifier is a mixture of phytic acid (PA) and calcium chloride (CaCl2) or polyethyleneimine (PEI).

[0007] The modified sodium carboxymethyl cellulose-based binder is prepared by uniformly mixing a polyethyleneimine solution and a sodium carboxymethyl cellulose solution to obtain a modified polyethyleneimine-sodium carboxymethyl cellulose-based binder.

[0008] The modified sodium carboxymethyl cellulose-based binder, the molecular weight of the polyethyleneimine is 70,000; the mass concentration of the polyethyleneimine solution is 2%-5%, the concentration of the sodium carboxymethyl cellulose solution is 2%; the mass ratio of polyethyleneimine to sodium carboxymethyl cellulose is 1:5-1:19.

[0009] The modified sodium carboxymethyl cellulose-based binder, the mass concentration of the polyethyleneimine solution is 2%, and the mass ratio of polyethyleneimine to sodium carboxymethyl cellulose is 1:9.

[0010] The modified sodium carboxymethyl cellulose-based binder, the mass concentration of the polyethyleneimine solution is 5%, and the mass ratio of polyethyleneimine to sodium carboxymethyl cellulose is 1:19.

[0011] The modified sodium carboxymethyl cellulose-based binder is prepared by adding a phytic acid aqueous solution to a sodium carboxymethyl cellulose aqueous solution, mixing them uniformly at room temperature, and obtaining a sodium carboxymethyl cellulose-phytic acid aqueous solution after the reaction is completed; and then adding a CaCl2 aqueous solution to obtain a modified sodium carboxymethyl cellulose-phytic acid-CaCl2 aqueous solution. 2+ Binder.

[0012] The modified sodium carboxymethyl cellulose-based binder comprises: a mass concentration of 2% sodium carboxymethyl cellulose, a mass concentration of 2% to 10% phytic acid aqueous solution, and a mass concentration of 2% to 10% CaCl2 aqueous solution; a mass ratio of sodium carboxymethyl cellulose, phytic acid, and calcium ions is 90:5:5; preferably, the mass concentration of phytic acid aqueous solution is 2.5%, and the mass concentration of CaCl2 aqueous solution is 10%.

[0013] Application of the modified sodium carboxymethyl cellulose-based binder in lithium-ion batteries.

[0014] Beneficial effects: Compared with the prior art, the present invention has the following advantages:

[0015] (1) Polyethyleneimine (PEI) is a water-soluble polymer with a large number of highly polar nitrogen-containing functional groups inside its molecular chain, such as primary, secondary, and tertiary amine groups. The amine groups in the long chain can generate electrostatic interactions with negatively charged sodium carboxymethyl cellulose (CMC-Na). It also has excellent adhesion properties, which is conducive to forming stable hydrogen bonds with the silicon oxide layer on the surface of silicon-based materials. The resulting force is stronger than the van der Waals force, which can effectively inhibit the volume expansion of silicon-based materials during charging and discharging. In addition, the branched structure of PEI can maintain the stability of the electrode structure. The modified polyethyleneimine-sodium carboxymethyl cellulose-based binder has excellent adhesion strength, can achieve a good bonding effect, and improve the battery rate performance. It is also a water-soluble substance, low-priced, environmentally friendly, and meets industrial needs. It is a binder with excellent application prospects.

[0016] (2) The peel strength of the C-PEI-10% electrode of the present invention reaches 2.42 N. When the surface loading of the electrode is 0.5~1.0 mgcm -2 At a current density of 0.2C, the specific capacity is 2249.1 mAh g after 100 cycles. -1 , and is conducive to the formation of a dense SEI layer on the electrode, indicating the excellent performance of C-PEI-10% as a silicon negative electrode binder.

[0017] (3) C-PA-Ca prepared by cross-linking 2+ The binder has better mechanical properties, can effectively alleviate the impact of volume expansion, and significantly improve the electrochemical performance of Si electrodes. 2+ The electrode peel strength reaches 2.39 N, Si@C-PA-Ca 2+ After 200 cycles at 0.2C current density, Si@C-PA-Ca 2+ The specific capacity of the electrode is 2035.2 mAh g -1 , it has good long-term cycle stability under 0.5C conditions. After 300 cycles, the specific capacity is 1879.7 mAh g -1 The capacity retention rate is 70.85%. After 600 cycles at 1C current density, the specific capacity of the electrode is 1139.7 mAh g -1 . BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 The cyclic voltammetry curve (1a) and rate curve (1b) of the lithium-ion battery provided in Example 1;

[0019] Figure 2 The SEM image (2a) and the SEM image (2b) of the electrode provided in Example 1 before cycling;

[0020] Figure 3 The EDS graph (3a) of the electrode before cycling and the EDS graph (3b) after cycling provided in Example 1;

[0021] Figure 4 The cyclic voltammetry curve (4a) and the first cycle charge-discharge curve (4b) of the electrode provided in Example 3;

[0022] Figure 5 SEM images (5a and 5c) of the electrode provided in Comparative Example 1 and SEM images (5b and 5d) of the electrode provided in Example 3;

[0023] Figure 6 The C 1s high-resolution XPS graph (6a) and the F 1s high-resolution XPS graph (6b) of the electrode provided in Comparative Example 1, and the C 1s high-resolution XPS graph (6c) and the F 1s high-resolution XPS graph (6d) of the electrode provided in Example 3;

[0024] Figure 7 The first cycle charge-discharge curves (7a and 7d) of the electrode provided in Example 4, with a constant discharge capacity of 1000 mAh g -1 Long cycle performance at different cycle times (7b and 7e), charge and discharge curves at different cycle numbers (7c and 7f) [(7a-7c) current density is 0.5 C; (7d-7f) current density is 1 C];

[0025] Figure 8 The SEM image of the electrode provided in Example 4, wherein: Figure 8 a is before the cycle, Figure 8 b is after the cycle. DETAILED DESCRIPTION

[0026] The present invention is further illustrated below in conjunction with specific examples. The examples are implemented based on the technical solutions of the present invention. It should be understood that these examples are only used to illustrate the present invention and are not used to limit the scope of the present invention.

[0027] Example 1

[0028] The application of modified polyethyleneimine-sodium carboxymethylcellulose based binder in Si / C negative electrode of lithium ion battery is as follows:

[0029] (1) Weigh 1.0 g of sodium carboxymethyl cellulose (CMC-Na) powder and 49.0 g of deionized water, stir overnight to obtain a clear CMC-Na colloidal aqueous solution; weigh 2.0 g of polyethyleneimine (PEI, MW70000, 50% aqueous solution), dissolve it in 48.0 g of deionized water to obtain a 2% PEI aqueous solution;

[0030] (2) Si / C composite material, conductive carbon black, CMC-Na and PEI were mixed into a slurry in a mass ratio of 80:10:9:1, and ball-milled in a rotary mixer for 30 min to obtain a slurry with uniform color. The slurry was then evenly coated on a copper foil (20 cm × 8 cm) and dried naturally for 30 min.

[0031] (3) The entire electrode was transferred to a vacuum drying oven and dried at 80 °C for 8 h. It was then cut into a circular electrode with a diameter of d = 14 mm. The electrode was named Si / C@C-PEI.

[0032] (4) The battery was assembled in an argon-filled glove box. The battery model was CR-2032, with a metal lithium sheet as the counter electrode. 1C = 950 mAh g -1 .

[0033] The electrode prepared with the polyethyleneimine-sodium carboxymethylcellulose binder was subjected to a 180° peel force test on a universal tensile testing machine. The results showed that the average peel force was 2.33 N.

[0034] The Si / C@C-PEI electrode was subjected to CV test with a test voltage range of 0.01-1.5 V and a scan rate of 0.2 mVs -1 , the results are as follows Figure 1 (a) The results show that the reduction peak of Si / C@C-PEI appears at 0.14 V, and the oxidation peak appears at 0.22-0.56 V. The response current value in the second to fifth CV curves gradually increases, and the positions of the redox peaks are similar, and the peak values ​​do not shift, which proves that the Si / C@C-PEI electrode has good reversibility.

[0035] The initial coulombic efficiency (ICE) performance test of the above battery showed that the average ICE of the three Si / C@C-PEI electrodes was 84.62%, indicating that less Li was consumed in the first cycle of discharge-charge process. + To form the SEI layer.

[0036] Battery rate performance test: The current density gradually increases from 0.1C to 3C, and then gradually decreases to 0.2C. Figure 1 (b) It can be seen that the discharge capacity of Si / C@C-PEI electrode at a high rate of 3C is 419.2 mAh g -1 After 54 cycles, the reversible discharge capacity of Si / C@C-PEI is 811.0 mAh g -1 , indicating that the binder C-PEI helps to improve the battery rate performance.

[0037] Constant current charge and discharge test: In the voltage range of 0.01V~1.5V, the Si / C button cell with modified polyethyleneimine-sodium carboxymethylcellulose binder was tested with the Blue Battery Test System. Before the long cycle process, the battery was pre-cycled at a low current density of 0.05C for three cycles. Under 0.5C conditions, the discharge capacity of the Si / C@C-PEI electrode after 240 cycles of charge and discharge was 418.9 mAh g -1 The capacity retention rate is 44.1%. The experiment proves that the composite binder C-PEI can improve the battery cycle performance.

[0038] The current density was increased to 1C and 2C to test the battery performance of the electrode at higher current density. The discharge capacity of Si / C@C-PEI was 397.2 mAh g after 300 cycles at a current density of 1C. -1 After 500 cycles at a current density of 2C, the discharge capacity of the Si / C@C-PEI electrode was 364.3 mAh g -1 The Si / C@C-PEI electrode has excellent cycling performance, which can be attributed to the electrostatic interaction between the amine groups on the PEI molecular chain and the negatively charged carboxyl groups on the CMC-Na molecular chain, which improves the stability of the binder structure.

[0039] SEM was used to observe the surface morphology changes of silicon carbon electrode before and after cycling. The electrode morphology features are as follows: Figure 2 As shown. Figure 2 It can be seen that the Si / C@C-PEI electrode has holes caused by the evaporation of the wet slurry solvent before cycling, but the whole electrode is flat and has no obvious cracks. After 240 cycles at a current density of 0.5C, the electrode is flat, uniform and dense, which shows that the C-PEI binder enhances the stability of the electrode structure and ensures the Li + Continuity of migration and stability of electrode electrochemistry.

[0040] like Figure 3 As shown in Figure 2, the elements in the Si / C@C-PEI electrode micro-region were qualitatively analyzed by EDS. Figure 3 (a) shows that the C, N, O, Si and other elements are evenly distributed before the electrode cycle. Figure 3 (b) It can be seen that after the electrode cycle, elements such as C, N, O, and Si are still evenly distributed. The results prove that the Si / C@C-PEI electrode has a stable structure.

[0041] Example 2

[0042] The application of modified polyethyleneimine-carboxymethylcellulose-based sodium binder in the Si negative electrode of lithium-ion batteries is as follows:

[0043] (1) Mix 18.0 g of deionized water with 2.0 g of PEI (MW70000, 50% aqueous solution) to prepare a 5% PEI aqueous solution; mix 1.0 g of CMC-Na with 49.0 g of deionized water to prepare a 2% CMC-Na colloidal aqueous solution;

[0044] (2) mixing the PEI solution and the CMC-Na solution in an effective mass ratio of 5:95, and labeling the mixture as C-PEI-5%, and placing the mixture in a cool place to obtain a modified polyethyleneimine-sodium carboxymethylcellulose binder;

[0045] (3) Mix silicon, modified polyethyleneimine-sodium carboxymethylcellulose binder and conductive carbon black in a ratio of 6:2:2 and stir until the mixed material is in a slurry state with uniform color and no obvious separation or granularity;

[0046] (4) Use a scraper to evenly apply the mixed slurry on the cut copper foil (8 cm × 20 cm), with a blade thickness of 60-70 μm; after drying under natural ventilation conditions, place the electrode in a vacuum drying oven at 80 °C overnight;

[0047] (5) Use a cutting machine to cut the electrode sheet into a circular electrode sheet with a radius of 7 mm. The electrode silicon surface loading is 0.5~1.0 mgcm -2 ;

[0048] (6) Place the weighed electrode in a vacuum drying oven for 2 hours, then move the electrode into an argon atmosphere glove box for 6 hours before assembling the battery;

[0049] (7) The water oxygen value in the glove box is less than 1 mg / L, the lithium sheet is used as the counter electrode, the battery model is CR2032, 1C=3800mAh g -1 , the silicon electrode prepared with C-PEI-5% as binder is marked as Si@C-PEI-5%.

[0050] The 180° peeling test was carried out using a tensile testing machine. The average peeling force of the C-PEI-5% composite silicon electrode was 2.22 N. Therefore, mixing PEI and CMC-Na into a mixed adhesive can improve its overall bonding performance, indicating that the modified adhesive can adhere nanocarbon and conductive carbon black to the copper foil well, thereby ensuring the stability of the electrode.

[0051] The constant current charge and discharge test was carried out at a current density of 0.2C and a voltage range of 0.01-1.2 V. The silicon electrode (Si@C-PEI-5%) composed of the above binder showed an excellent initial coulombic efficiency of up to 91.30%, indicating that only a small amount of lithium ions were consumed in the formation of the SEI film during one charge and discharge. The Si@C-PEI-5% electrode showed excellent long-cycle characteristics, with a charge capacity of 2185.4 mAh g after 100 cycles at a current density of 0.2C. -1 .

[0052] Example 3

[0053] The application of modified polyethyleneimine-carboxymethylcellulose-based sodium binder in the Si negative electrode of lithium-ion batteries is as follows:

[0054] (1) Mix polyethyleneimine solution and sodium carboxymethylcellulose solution with a mass concentration of 2% in a ratio of 10:90 and label it as C-PEI-10%;

[0055] (2) The remaining steps are the same as steps (3) to (7) of Example 2. The silicon electrode prepared using C-PEI-10% as a binder is labeled Si@C-PEI-10%.

[0056] Comparative Example 1

[0057] The preparation of the electrode Si@CMC-Na comprises the following steps:

[0058] (1) Using the CMC-Na solution prepared in step (1) of Example 2, silicon, sodium carboxymethyl cellulose and conductive carbon black were mixed together in an effective mass ratio of 6:2:2 and stirred until the mixed material was in a slurry with uniform color;

[0059] (2) The remaining steps are the same as steps (4) to (7) of Example 2. The silicon electrode prepared using CMC-Na as a binder is labeled Si@CMC-Na.

[0060] The 180° peeling test was carried out using a tensile testing machine, and the peeling strength of the C-PEI-10% electrode reached 2.42 N. When the surface loading of the electrode was 0.5~1.0 mg cm -2 At a current density of 0.2C, the specific capacity is 2249.1 mAh g after 100 cycles. -1 , indicating that C-PEI-10% has excellent long cycle performance as a silicon negative electrode binder.

[0061] Cyclic voltammetry test was performed on Si@C-PEI-10% electrode. Figure 4(a). The results show that with the increase of the number of cycles, the oxidation peak and reduction peak of Si@C-PEI-10% only slightly shift, indicating that the C-PEI-10% binder can help ensure the reversibility of the electrode. Figure 4 (b) Si@C-PEI-10% electrode shows a high ICE value (91.54%), indicating that C-PEI-10% as a binder only consumes a small amount of electrolyte and Li to form the SEI layer. + .

[0062] In order to study the changes in the morphological characteristics of the silicon electrode with C-PEI-10% as a binder before and after cycling, the morphological analysis was performed. The microscopic morphological structure is as follows: Figure 5 As shown. Figure 5 As can be seen from (a) and (b), before cycling, the electrode (Si@CMC-Na) provided in Comparative Example 1 and the Si@C-PEI-10% electrode prepared in Example 3 both exhibited good uniformity, and no obvious cracking was observed. After 100 cycles at a current density of 0.5C, the electrodes showed a huge difference: Figure 5 As shown in (c), the surface of the electrode (Si@CMC-Na) provided in Comparative Example 1 is fragmented, with a large number of cracks and unevenness; the Si@C-PEI-10% electrode ( Figure 5 (d)), only a few narrow cracks appeared, and a uniform and dense SEI layer was formed on the electrode surface, further proving that the C-PEI-10% binder is beneficial to improving the long-term cycling performance of the electrode.

[0063] observe Figure 6 (a), the C 1s spectrum of the Si@CMC-Na electrode provided in Comparative Example 1 before cycling has three peaks at 284.8, 286.5 and 287.8 eV, which belong to C-C / C-H, C-O / C-O-C and O-C-O, respectively. The C 1s spectrum of the electrode after cycling has four peaks at 284.8, 286.5, 288.6 and 289.9 eV, which correspond to C-C / C-H, C-O / C-O-C, C=O and Li2CO3, respectively. By comparison, it is found that the electrode after cycling generates Li2CO3, which accounts for 15.62%, indicating that the electrode consumes electrolyte to form SEI layer during the charge and discharge process. Analysis of the C 1s spectrum of the Si@C-PEI-10% electrode before and after cycling ( Figure 6 (c) The spectrum before cycling can be decomposed into three peaks, namely C-C / C-H, C-O / C-O-C / C-N and O-C-O. The spectrum after cycling can be decomposed into four peaks, namely C-C / C-H, C-O / C-O-C / C-N, C=O and Li2CO3, among which the content of Li2CO3 is 16.52%. Comparison of the F 1s spectra of the two electrodes before and after cycling ( Figure 6 (b) and (d)), because the binder does not contain F element, no F element was detected in the two electrodes before cycling. The two peaks at 684.8 and 687.5 eV in the spectrum of Si@CMC-Na electrode after cycling belong to LiF and Li x PF y , the contents are 69.07% and 30.93% respectively. The two peaks at 684.8 and 687.1 eV in the spectrum of Si@C-PEI-10% electrode after cycling correspond to LiF and Li x PO y F z , accounting for 92.05% and 7.95% respectively. The Si@C-PEI-10% electrode has a higher content of LiF after cycling, and the rich inorganic salt LiF is conducive to the formation of a stable SEI layer and rapid Li + transmission, indicating the superior advantage of using C-PEI-10% binder in forming a strong and reliable interface layer.

[0064] Example 4

[0065] Sodium Carboxymethyl Cellulose-Phytic Acid-Ca 2+ The application of the binder in the lithium-ion battery comprises the following steps:

[0066] (1) Weigh 1.0 g of CMC-Na solid and add 49.0 g of deionized water to prepare a 2% CMC-Na aqueous solution; weigh 0.5 g of PA (50 wt.%) and add 9.5 g of deionized water to prepare a 2.5% PA aqueous solution; weigh 1.0 g of anhydrous calcium chloride solid and add 9.0 g of deionized water to prepare a 10% CaCl2 aqueous solution;

[0067] (2) Add phytic acid solution and CaCl2 aqueous solution into sodium carboxymethyl cellulose solution in proportion and stir at room temperature for 2 h to obtain sodium carboxymethyl cellulose-phytic acid-CaCl2 solution. 2+ Aqueous solution (denoted as C-PA-Ca 2+ , CMC-Na, PA and Ca 2+ The mass ratio is 90:5:5);

[0068] (3) Si, conductive carbon black and binder (C-PA-Ca) were mixed in a ratio of 6:2:2. 2+ ) and then add an appropriate amount of deionized water according to the consistency of the slurry. After 45 minutes of grinding and stirring, a uniformly dispersed slurry is obtained. The thickness of the scraper is adjusted to evenly coat the mixed slurry on the copper foil. After it is naturally dried, the copper foil coated with the slurry is then placed in a vacuum oven and dried at 150 ° C for 6 h;

[0069] (4) After the temperature of the vacuum oven drops to room temperature, take out the electrode and use a cutting machine to cut the electrode into electrode pieces with a diameter of 14 mm; the negative electrode material is recorded as Si@C-PA-Ca 2+ ;

[0070] (5) Place the cut electrode pieces in an oven at 70 °C for 2 h to dry, and then assemble the button battery in a glove box. The battery materials used are as follows: positive electrode shell (CR2032), negative electrode shell (CR2032), separator (20 μm), antistatic tweezers, gasket, shrapnel, electrolyte (1.0 mol / L LiPF6, V(EC):V(EMC)=3:7 with 10 vol% FEC), lithium sheet (15.6 mm*0.6 mm);

[0071] (6) The battery assembly process is as follows: first put the positive electrode shell at the bottom, then put the electrode sheet in the center of the positive electrode shell, add the separator, use a pipette to drip about 20 μL of electrolyte, and then put the lithium sheet, gasket, and spring sheet in order just above the electrode sheet. After ensuring that they are all in the center, use a pipette to take 60 μL of electrolyte, and finally install the negative electrode shell. Place the battery on a tablet press for compression and sealing to obtain a complete button battery. Place the battery at 28 °C for 24 h and then test its electrochemical performance. 1C = 3800 mAh g -1 .

[0072] C-PA-Ca on a universal tensile testing machine 2+ The test results of the adhesion between the binder and Si particles show that the electrode Si@C-PA-Ca 2+ The average peeling force of the electrode is 2.39 N.

[0073] The Si button half-cell made of the modified binder was tested with a blue electric system in a voltage range of 0.01-1.2 V. During the long cycle of the battery, the battery was pre-cycled at a low current density of 0.05C for 3 cycles, and then the corresponding long cycle test was performed. After 200 cycles at a current density of 0.2C, the Si@C-PA-Ca 2+ The specific capacity of the electrode is 2035.2 mAh g -1 This is because after adding PA, CMC-Na and PA form a large number of strong and stable hydrogen bonds, which effectively alleviate the impact of volume expansion and stabilize the electrode structure. 2+ After, Ca 2+ The two positive charges it carries can coordinate with two PA molecules to form ionic bonds, making the conductive network more complete, improving the mechanical properties of the negative electrode material, and greatly improving the cycle performance.

[0074] The current density was increased to 0.5C and Si@C-PA-Ca was tested. 2+ Electrode cycling performance. Si@C-PA-Ca 2+ The electrode has good stability at 0.5C. After 300 cycles, the specific capacity is 1879.7 mAh g -1 The capacity retention rate is 70.85%. After 600 cycles at 1C current density, the specific capacity of the electrode is 1139.7 mAh g -1 .

[0075] Si@C-PA-Ca 2+ The electrode performed well under the three current density conditions and maintained an excellent capacity retention rate after cycling, which is due to the C-PA-Ca 2+ The 3D structure of the binder can effectively buffer the huge volume expansion of Si particles.

[0076] Si@C-PA-Ca 2+ The Li insertion capacity of the electrode is limited to 1000 mAh g at current densities of 0.5C and 1C. -1 Cyclic performance of Si@C-PA-Ca 2+ Before the electrode undergoes the capacity-limiting cycling process, the ICE of the 0.05C current density pre-cycling is as high as 89.36% ( Figure 7 (a)) and 89.66% ( Figure 7 (d) ), it can stably cycle 1000 times at a current density of 0.5C ( Figure 7 (b) ), it can stably cycle for 700 cycles at a current density of 1C ( Figure 7 (e)). Figure 7 (c) and (f) show the charge / discharge curves of some integer cycles at current densities of 0.5C and 1C, respectively. 2+ The charge and discharge platform of the electrode is very stable, with the charging platform stable at 0.20~0.60 V and the discharging platform stable at 0.05~0.3 V. The charge and discharge platform is also stable at 1C current density. Due to the higher current density, the charging platform is slightly higher at 0.35~0.65 V, and the discharging platform is about 0.02~0.3 V. In general, under the condition of limiting capacity, Si@C-PA-Ca 2+ The performance of the electrode at both 0.5C and 1C current densities is very satisfactory, which proves that the binder C-PA-Ca 2+ It has excellent long-cycle stability.

[0077] like Figure 8As shown in Figure 2, SEM was used to study the changes in the surface morphology of the silicon electrode before and after 150 cycles. From the SEM image, it can be observed that the electrode before the cycle ( Figure 8 (a)), the surface is relatively flat and has no obvious defects. After 100 cycles at a current density of 0.5C ( Figure 8 (b) ), C-PA-Ca 2+ The surface morphology is very dense and has no cracks. This is because the hydrogen bonds formed by the cross-linking of PA and CMC-Na form excellent mechanical stress on the surface of silicon particles, which improves the problem of surface cracking caused by volume expansion and 2+ After modification, the newly added ionic bonds strengthen the entire electrode system, and the binder C-PA-Ca 2+ It has an excellent effect in maintaining electrode stability and integrity, thereby ensuring the continuity of electron migration and conduction as well as the stability of electrode cycling.

[0078] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A modified sodium carboxymethyl cellulose based binder, characterized in that: Sodium carboxymethyl cellulose is used as a raw material, and a modifier is added to obtain a modified sodium carboxymethyl cellulose-based binder; the modifier is a mixture of phytic acid and calcium chloride or any one of polyethyleneimine.

2. The modified sodium carboxymethyl cellulose based binder according to claim 1, characterized in that: The polyethyleneimine solution and the sodium carboxymethylcellulose solution are mixed evenly to obtain a modified polyethyleneimine-sodium carboxymethylcellulose binder.

3. The modified sodium carboxymethyl cellulose based binder according to claim 1 or 2, characterized in that: The molecular weight of the polyethyleneimine is 70,000; the mass concentration of the polyethyleneimine solution is 2% to 5%, the concentration of the sodium carboxymethylcellulose solution is 2%; and the mass ratio of polyethyleneimine to sodium carboxymethylcellulose is 1:5 to 1:

19.

4. The modified sodium carboxymethyl cellulose based binder according to claim 3, characterized in that: The mass concentration of the polyethyleneimine solution is 2%, and the mass ratio of polyethyleneimine to sodium carboxymethylcellulose is 1:

9.

5. The modified sodium carboxymethyl cellulose based binder according to claim 3, characterized in that: The mass concentration of the polyethyleneimine solution is 5%, and the mass ratio of polyethyleneimine to sodium carboxymethylcellulose is 1:

19.

6. The modified sodium carboxymethyl cellulose based binder according to claim 1, characterized in that: The phytic acid aqueous solution is added to the sodium carboxymethyl cellulose aqueous solution, and mixed evenly at room temperature to obtain a carboxymethyl cellulose-phytic acid aqueous solution after the reaction is completed; and then a CaCl2 aqueous solution is added to obtain a modified carboxymethyl cellulose-phytic acid-CaCl2 aqueous solution. 2+ Binder.

7. The modified sodium carboxymethyl cellulose based binder according to claim 1 or 6, characterized in that: The mass concentration of the sodium carboxymethyl cellulose is 2%, the mass concentration of the phytic acid aqueous solution is 2%-10%, and the mass concentration of the CaCl2 aqueous solution is 2%-10%; the mass ratio of sodium carboxymethyl cellulose, phytic acid, and calcium ions is 90:5:

5.

8. Use of the modified sodium carboxymethyl cellulose-based binder according to claim 1 in lithium ion batteries.

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