Negative electrode slurry for lithium-ion battery and preparation method thereof

Abstract

The present invention discloses a negative electrode paste for lithium-ion batteries and a preparation method thereof, belonging to the technical field of lithium-ion batteries. The preparation method of the negative electrode paste of the present invention includes: preparing a polyamic acid prepolymer solution capped with amino groups or anhydride groups by using an asymmetric diamine and an asymmetric dianhydride as raw materials; mixing the obtained polyamic acid prepolymer solution with a negative electrode active material and a conductive agent uniformly, adding or not adding a polar solvent, and mixing evenly to obtain a slurry precursor; then adding a dianhydride capping agent or a diamine capping agent containing a bulky group to the slurry precursor, and stirring and reacting to obtain the product. The negative electrode paste of the present invention can effectively inhibit the swelling effect of the negative electrode material, thereby improving the initial efficiency and cycle stability of the battery at the same time.

Description

Technical Field

[0001] The present invention relates to a lithium ion battery, and more particularly to a negative electrode paste for a lithium ion battery and a preparation method thereof. Background Art

[0002] Lithium ion batteries have the advantages of high working voltage, long cycle life, low self-discharge rate, no memory effect, high specific capacity, etc., and are thus widely used and the market is still expanding. Among all battery structures, the positive and negative electrode materials have the greatest impact on the battery capacity. Among them, the currently widely commercialized negative electrode material is graphite, and its specific capacity has approached its theoretical value of 372 mAh / g. The silicon-based negative electrode material has a higher specific capacity, up to 4200 mAh / g, and is considered the most likely material to replace graphite. However, the silicon-based material will undergo severe volume expansion and contraction (up to about 400% expansion) during the charge and discharge process of the battery, making the material prone to cracking, pulverization and peeling, resulting in capacity loss and battery failure.

[0003] Using a negative electrode with carbon-coated silicon can reduce the volume effect of silicon. On this basis, if an adhesive that can bond with both the copper foil and the silicon-carbon material is developed, the expansion effect of the negative electrode material can be reduced to a large extent, thereby improving the cycle stability of the silicon-carbon negative electrode and the service life of the battery.

[0004] To alleviate drawbacks such as the volume expansion effect during the charge and discharge process of silicon-containing anodes, the prior art proposes using polyimide resin as an adhesive in the anode active material. For example, the invention patent with publication number CN114773599A discloses a block polyimide adhesive prepared from a block polyamic acid solution. In the block polyamic acid solution, the block polyamic acid is composed of a rigid polyamic acid chain segment and a flexible polyamic acid chain segment in a block form; the molar ratio of the rigid polyamic acid chain segment to the flexible polyamic acid chain segment is (0.05 - 20):1; the solid content of the polyamic acid solution is 0.5 - 40 wt%; the intrinsic viscosity of the polyamic acid solution is 0.5 - 6.0 dL / g. The polyimide adhesive prepared from the block polyamic acid provided by this invention has stronger adhesion. The lithium-ion battery assembled with the lithium-ion battery electrode sheet containing this block polyimide adhesive has a higher discharge specific capacity, capacity retention rate, and better high-temperature safety performance. Specifically, when it is applied to the positive electrode, the capacity retention rate of the lithium-ion battery is about 80% after 100 cycles at 25°C. The invention patent with publication number CN108701831B discloses an electrode mixture paste for secondary batteries, which includes an adhesive composition for secondary batteries and a negative electrode active material containing Si and / or silicon oxide. Among them, the adhesive composition includes: a polyamic acid containing a skeleton based on aromatic tetracarboxylic dianhydride and repeating units composed of bicyclo[2.2.1]heptane dimethylamine (NBDA). This invention points out that the adhesive with a cyclohexane structure can withstand the stress generated by the expansion and contraction of the active material during cycling, and through specific examples, it shows that by defining the structures of the tetracarboxylic acid component and diamine component of the polyamic acid, the initial charge / discharge efficiency (first efficiency) and cycling characteristics can be improved. Its initial charge / discharge efficiency is 66%, and the capacity retention rate is 91% after 150 cycles. When the above polyimide resin adhesive is applied to the negative electrode, either the improvement of the first efficiency is not ideal, or the improvement of the cycling stability is not ideal, and it is difficult to have both. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a negative electrode slurry for lithium-ion batteries and its preparation method that can effectively inhibit the expansion effect of the negative electrode material and thus improve both the first efficiency and cycling stability of the battery.

[0006] To solve the above technical problems, the present invention adopts the following technical solutions:

[0007] 1) Using an asymmetric diamine and an asymmetric dianhydride as raw materials, prepare a polyamic acid prepolymer solution terminated with an amino group or an anhydride group;

[0008] 2) Mix the polyamic acid prepolymer solution terminated with amino groups or anhydride groups evenly with the negative electrode active material and the conductive agent, with or without adding a polar solvent, and obtain a slurry precursor after mixing evenly;

[0009] 3) Add a dianhydride capping agent or a diamine capping agent containing a bulky group to the slurry precursor, stir and react to obtain the negative electrode slurry for the lithium ion battery; wherein,

[0010] When the polyamic acid prepolymer solution contained in the slurry precursor is terminated with amino groups, add a dianhydride capping agent containing a bulky group, and the addition amount is controlled so that the total amount of the dianhydride capping agent containing a bulky group and the amount of the asymmetric dianhydride is in excess relative to the molar amount of the asymmetric diamine;

[0011] When the polyamic acid prepolymer solution contained in the slurry precursor is terminated with anhydride groups, add a diamine capping agent containing a bulky group, and the addition amount is controlled so that the total amount of the diamine capping agent containing a bulky group and the amount of the asymmetric diamine is in excess relative to the molar amount of the asymmetric dianhydride.

[0012] In step 1) of the above preparation method, the asymmetric diamine specifically may be selected from any one or a combination of two or more of 3,4'-diaminodiphenyl ether (3,4'-ODA), m-phenylenediamine (m-PDA), 3,4'-diaminodiphenyl sulfone (3,4'-DDS), 3,4'-diaminobiphenyl (3,4'-DAB), 3,4'-diaminodiphenylmethane (3,4'-MDA), 3,4'-diaminodiphenyl disulfide and 3,4'-diaminodiphenyl sulfide, further preferably 3,4'-ODA or 3,4'-diaminodiphenyl sulfide, particularly preferably 3,4'-ODA. The asymmetric dianhydride specifically may be selected from any one or a combination of two or more of 2,3,3',4'-biphenyltetracarboxylic dianhydride (α-BPDA), 2,3,3',4'-benzophenone tetracarboxylic dianhydride (2,3,3',4'-BTDA), 2,3,3',4'-diphenylsulfone tetracarboxylic dianhydride (2,3,3',4'-DSDA), 2,3,3',4'-diphenylsulfide tetracarboxylic dianhydride (2,3,3',4'-TDPA) and 2,3,3’,4’-diphenylether tetracarboxylic dianhydride (α-ODPA), further preferably 2,3,3',4'-DSDA or 2,3,3',4'-TDPA, particularly preferably 2,3,3',4'-TDPA. In this step, the molar ratio of the asymmetric diamine to the asymmetric dianhydride is determined according to whether the prepared polyamic acid prepolymer solution is terminated with amino groups or anhydride groups. Generally, when the molar amount of one of the monomer raw materials is 1, the molar amount of the other monomer raw material is preferably 0.96 - 0.99.

[0013] In step 1) of the above preparation method, a polyamic acid prepolymer solution capped with amino groups or anhydride groups is prepared by using existing conventional methods (such as in-situ polymerization method), for example, by subjecting an asymmetric diamine and an asymmetric dianhydride to a polycondensation reaction in a polar aprotic solvent. Among them, the selection and dosage of the polar aprotic solvent, the temperature and time of the polycondensation reaction, etc. are the same as those in the prior art. For example, for the polar aprotic solvent, it can specifically be any one or a combination of two or more of N,N-dimethylformamide, N,N-dimethylacetamide, N,N-diethylacetamide, N-methyl-2-pyrrolidone, N-ethyl-2-pyrrolidone, 1,3-dimethyl-2-imidazolidinone, N-methylcaprolactam, hexamethylphosphoric triamide, 1,2-dimethoxyethane, bis(2-methoxyethyl) ether, 1,2-bis(2-methoxyethoxy)ethane, tetrahydrofuran, bis[2-(2-methoxyethoxy)ethyl] ether, 1,4-dioxane, dimethyl sulfoxide, dimethyl sulfone, diphenyl ether, sulfolane, diphenyl sulfone, tetramethylurea, m-cresol, phenol, and γ-butyrolactone. Further preferably, it is N,N-dimethylacetamide, N,N-diethylacetamide, N-methyl-2-pyrrolidone, N-ethyl-2-pyrrolidone, 1,3-dimethyl-2-imidazolidinone, or γ-butyrolactone, and particularly preferably N-methyl-2-pyrrolidone, N-ethyl-2-pyrrolidone, or γ-butyrolactone. The selection of other parameters will not be elaborated one by one here.

[0014] In step 2) of the above preparation method, both the negative electrode active material and the conductive agent are conventional selections in the prior art. For the negative electrode active material, it is preferably one or a combination of two or more of carbon, silicon, and silicon alloy, and further preferably a porous silicon-carbon composite powder containing porous silicon powder or silicon alloy powder; for the conductive agent, it is usually conductive carbon black. The ratio of the polyamic acid prepolymer solution capped with amino groups or anhydride groups, the negative electrode active material, and the conductive agent is also the same as that in the prior art. In this application, the weight ratio of the polyamic acid prepolymer solution capped with amino groups or anhydride groups, the negative electrode active material, and the conductive agent is preferably 15-2:65-96:20-2, where the polyamic acid prepolymer solution capped with amino groups or anhydride groups is calculated based on the amount of the solid component in the prepolymer solution.

[0015] The polar solvent involved in step 2) of the above preparation method is a conventional solvent used in the prior art for preparing the negative electrode slurry. Specifically, it can be the same as the selection of the polar aprotic solvent used in the preparation of the polyamic acid prepolymer solution mentioned above, and is preferably NMP and / or DMAc. The amount of the polar solvent in this step is such that the viscosity of the resulting slurry precursor meets the requirements for easy coating. Generally, the viscosity of the resulting slurry precursor is controlled to be 2000-10000 cp, preferably 4000-7000 cp. If the viscosity of the polyamic acid prepolymer solution capped with amino or anhydride prepared in the previous stage is relatively low, and the viscosity of the system just falls within the above-defined range after adding the negative electrode active material and the conductive agent and stirring evenly, then there is no need to add a polar solvent again.

[0016] In step 3) of the above preparation method, the dianhydride capping agent containing a bulky group is preferably 9,9-bis(3,4-dicarboxyphenyl)fluorene dianhydride (BPAF) and / or 3,4,9,10-perylene tetracarboxylic dianhydride. The diamine capping agent containing a bulky group is preferably selected from one or a combination of more than two of 9,9-bis(3-amino-4-hydroxyphenyl)fluorene, 9,9'-bis[4-(4-amino-2-hydroxyphenoxy)phenyl]fluorene, 9,9-bis(4-aminophenyl)fluorene (BAFL), 9,9-bis(4-amino-3-fluorophenyl)fluorene (BFAF), 9,9-bis[4-(4-aminophenoxy)phenyl]fluorene, 4,6-bis(1-adamantyl)-1,3-bis(4-amino-3-hydroxyphenoxy)benzene, and 9,9'-spirobi[9H-fluorene]-2,2'-diamine, and is further preferably 9,9-bis(3-amino-4-hydroxyphenyl)fluorene and 9,9'-bis[4-(4-amino-2-hydroxyphenoxy)phenyl]fluorene.

[0017] In step 3) of the above preparation method, when the polyamic acid prepolymer solution contained in the slurry precursor is capped with amino, a dianhydride capping agent containing a bulky group needs to be added, and its addition amount is controlled such that the total amount of the dianhydride capping agent containing a bulky group and the amount of the asymmetric dianhydride used in step 1) is 0.1-6% in excess of the molar amount of the asymmetric diamine, and further preferably 1-3% in excess. When the polyamic acid prepolymer solution contained in the slurry precursor is capped with anhydride, a diamine capping agent containing a bulky group needs to be added, and its addition amount is controlled such that the total amount of the diamine capping agent containing a bulky group and the amount of the asymmetric diamine used in step 1) is 0.1-6% in excess of the molar amount of the asymmetric dianhydride, and further preferably 1-3% in excess.

[0018] The present invention also includes a negative electrode slurry for a lithium ion battery prepared by the above method.

[0019] The present invention further includes a negative electrode of a lithium-ion battery, which comprises a negative electrode current collector and an electrode layer loaded on the negative electrode current collector. The electrode layer is formed by heat-treating the negative electrode paste for lithium-ion batteries described in the present application. When manufacturing the negative electrode by using the existing conventional method, specifically, the prepared negative electrode paste is cast or coated on a current collector (such as copper foil or aluminum foil), and is heat-treated under heating conditions (usually 150-400 °C in the prior art) for 2 min to 10 h (preferably 10 min to 6 h, particularly preferably 20 min to 3 h) to remove the solvent and carry out an imidization reaction to convert polyamic acid into polyimide, thereby obtaining the negative electrode.

[0020] Compared with the prior art, the characteristics of the present invention are as follows:

[0021] 1. In the polyimide structure formed by the imidization reaction of polyamic acid synthesized by the polycondensation of asymmetric structure monomers, there is a high degree of amorphous phase, which can improve the lithium-ion conductivity in the lithium battery system, that is, lithium ions can rapidly migrate in the amorphous phase of the locally relaxed chain segments in the polyimide structure, enabling the binder to have good mechanical properties, relatively high room-temperature conductivity, and a relatively wide chemical stability window, etc., further optimizing the cycle stability, initial Coulomb efficiency (first efficiency), and high-rate performance of the lithium-ion battery.

[0022] 2. After mixing the polyamic acid prepolymer solution with the negative electrode active material and the conductive agent, a dianhydride capping agent or a diamine capping agent containing a bulky group is further used to cap the prepolymer to obtain a polyamic acid with a "dumbbell" shape (small in the middle and large at both ends). On the one hand, the molecular chains of the polyamic acid with the "dumbbell" shape are "anchored" or "constrained" to the micro-nano porous silicon powder or the porous silicon-carbon composite active material body by the dianhydride or diamine residues containing bulky groups at both ends (especially interpenetrating the nano-sized pores of the micro-nano sized active material and forming a "self-locking" effect); on the other hand, the polyamic acid and / or the polyimide molecular chains themselves form a strong adhesive force by chemically bonding with the surface active groups of the silicon powder through chemical bonds such as covalent bonds and hydrogen bonds. The combined effects of the above two aspects effectively inhibit the influence of the volume expansion and contraction during charge and discharge of the silicon-based material, which may cause cracks or pulverization, and can simultaneously optimize the cycle stability characteristics and the first efficiency of the lithium-ion battery.

[0023] 3. The negative electrode paste for lithium-ion batteries capped with a diamine capping agent containing a hydroxyl functional group can further optimize the lithium battery characteristics (such as cycle stability characteristics and first efficiency);

[0024] 4. When using the negative electrode paste described in the present invention to prepare the negative electrode electrode layer and further applying it to a lithium-ion battery, the first Coulomb efficiency of the prepared battery is ≥92%, the capacity retention rate after 150 cycles is ≥96%, and the swelling rate of the electrode in the Z direction (thickness direction) before and after 150 cycles is ≤62%. Detailed implementation mode

[0025] In order to better explain the technical solution of the present invention, the present invention will be further described in detail below in conjunction with embodiments, but the implementation modes of the present invention are not limited thereto.

[0026] Embodiment 1

[0027] 1. Preparation of negative electrode paste

[0028] 1) Under a nitrogen atmosphere and at room temperature, take 28.523 g (0.142 mol) of asymmetric diamine 3,4'-diaminodiphenyl ether (3,4'-ODA) and stir it to dissolve in 425 g of DMAC, then add a total of 46.012 g (0.141 mol, added in 3 portions) of asymmetric dianhydride 2,3,3',4'-diphenyl sulfide tetracarboxylic dianhydride (2,3,3',4'-TDPA) (the molar ratio of diamine to dianhydride is 1:0.99), and stir and react for 16 h to obtain a polyamic acid prepolymer solution capped with amino groups (the solid content of the obtained polyamic acid prepolymer solution is about 15%, and the total amount of solid components is about 75 g);

[0029] 2) Take 13.5 g of the polyamic acid prepolymer solution capped with amino groups prepared in step 1) (the total amount of solid components is 2.0 g), 96.0 g of negative electrode active material (57.6 g of graphite, 38.4 g of nano-porous silicon powder (specific surface area is about 58 m 2 / g, the average mesopore diameter is about 27.0 nm, the same below), graphite:nano-porous silicon powder = 6:4) and 2.0 g of conductive agent (conductive carbon black), where the active material:binder:conductive agent = 96:2:2 (weight ratio), mix evenly, add 150 g of NMP, and grind and blend and stir the obtained mixture to obtain a slurry precursor (viscosity is about 5600 cp);

[0030] 3) Add 0.517 g (1.134 mmol) of 9,9-bis(3,4-dicarboxyphenyl)fluorene dianhydride (BPAF) bulky dianhydride capping agent (the molar ratio of the total amount of diamine raw material to the total amount of dianhydride raw material is 1:1.03) to the obtained slurry precursor, and stir and react for 8 h to obtain the negative electrode paste.

[0031] 2. Preparation of positive and negative electrodes of lithium battery

[0032] 2.1 Negative electrode

[0033] The negative electrode paste obtained in this embodiment was evenly coated on a copper foil, and the gap of the coating roll (doctor blade) was adjusted to control the thickness of the solidified negative electrode paste to be 35 μm ± 2.0 μm. The copper foil evenly coated with the negative electrode paste was placed in an oven, and heated at 80 °C for 1 h under the condition of flowing nitrogen with an oxygen concentration lower than 18 ppm, and then raised to 300 °C at a rate of 3.0 °C / min and held at 300 °C for 1.5 h to obtain a negative electrode sheet.

[0034] 2.2 Positive Electrode

[0035] The active material ternary positive electrode (NCM811): polyvinylidene fluoride: conductive carbon black were stirred and mixed evenly according to a weight ratio of 95:2:3, and the solvent NMP was added to adjust the system to an appropriate viscosity (6000 ± 500 cp), placed in a three-roll mill and ground for 3 h and dispersed at high speed for 2 h to obtain a positive electrode paste. The positive electrode paste was coated on an aluminum foil using a doctor blade coating method, and the gap of the coating roll (doctor blade) was adjusted to control the thickness of the solidified positive electrode paste to be 100 μm ± 3.0 μm. The coated aluminum foil was placed in an oven and held at 120 °C for 2 h under the condition of air circulation to obtain a positive electrode sheet.

[0036] 3. Preparation of Battery

[0037] To reduce the gap between the active materials, the above-mentioned lithium battery negative electrode sheet and positive electrode sheet were appropriately roll-pressed using a roll press. The roll-pressed negative electrode sheet and positive electrode sheet were cut into circular pieces with a diameter of 14 mm using a punching machine. A CR2032 button battery was assembled in an argon glove box (H2O < 0.01 ppm, O2 < 0.01 ppm), and the negative electrode shell, negative electrode sheet, separator, positive electrode sheet, nickel foam, spring sheet, and positive electrode shell were assembled in sequence. About 1 mL of electrolyte was dropped at both ends of the separator. The electrolyte was a mixed solution of 1.0 mol / L LiPF6 dissolved in EC and DMC (EC:DMC = 1:1, volume ratio). The assembled battery was placed in a sealing machine for encapsulation, and the encapsulation pressure was 75 MPa. After standing for 24 h, the corresponding electrochemical performance tests were carried out.

[0038] 4. Charge and Discharge Characteristics Test

[0039] The charge and discharge characteristics of each group of batteries prepared by the above method were tested. The batteries were charged, discharged, and cycled at 25 °C. The experimental charge and discharge test was carried out at a current of 0.2C, and the voltage window was 0.005 - 1.5V. The electric quantity flowing from the start to the end of charging or discharging was defined as the charge capacity or discharge capacity.

[0040] The charge and discharge efficiency after the first and 150th cycles was tested [where the charge and discharge efficiency = (discharge capacity / charge capacity) * 100%].

[0041] Taking the thickness in the cross-sectional SEM image of the negative electrode sheet of the battery before and after charge and discharge with the applied adhesive and after 150 cycles as a reference, the swelling rate of the negative electrode sheet was calculated based on the thickness of the active material of the electrode sheet before charging and the thickness of the active material of the electrode sheet after 150 cycles. Measure the initial thickness H0 of the negative electrode sheet and measure the thickness H of the negative electrode after 150 cycles 150 , and calculate the swelling rate of the electrode sheet according to the calculation formula (in the thickness direction Z): swelling rate = (H 150 -H0) / H0*100%.

[0042] The test results were as follows: the initial Coulomb efficiency was about 95%, the capacity retention rate after 150 cycles was about 98%, and the swelling rate of the electrode sheet in the Z direction (thickness direction) after 150 cycles was about 51%.

[0043] Comparative Example 1

[0044] Same as Example 1, the difference is only that: when preparing the negative electrode slurry, the slurry precursor obtained in step 2) was used as the negative electrode slurry to prepare the negative electrode, that is, step 3) was omitted in the preparation of the negative electrode slurry.

[0045] The test results were as follows: the initial Coulomb efficiency was about 84%, the capacity retention rate after 150 cycles was about 80%, and the swelling rate of the electrode sheet in the Z direction (thickness direction) after 150 cycles was about 96%.

[0046] Comparative Example 2

[0047] Same as Example 1, the difference is only that: when preparing the negative electrode slurry, the symmetric diamine 4,4'-diaminodiphenyl ether (4,4'-ODA) with the same molar amount was used to replace the diamine in Example 1.

[0048] The test results were as follows: the initial Coulomb efficiency was about 87%, the capacity retention rate after 150 cycles was about 92%, and the swelling rate of the electrode sheet in the Z direction (thickness direction) after 150 cycles was about 72%.

[0049] Comparative Example 3

[0050] Same as Example 1, the difference is only that: the symmetric diamine 4,4'-diaminodiphenyl ether (4,4'-ODA) with the same molar amount was used to replace the diamine in Example 1, and the symmetric dianhydride 3,3',4,4'-diphenylsulfide tetracarboxylic dianhydride (3,3',4,4'-TDPA) with the same molar amount was used to replace the dianhydride in Example 1.

[0051] The test results were as follows: the initial Coulomb efficiency was about 89%, the capacity retention rate after 150 cycles was about 93%, and the swelling rate of the electrode sheet in the Z direction (thickness direction) after 150 cycles was about 80%.

[0052] Comparative Example 4

[0053] Same as Example 1, with the only difference being that in the preparation of the negative electrode slurry, steps 1) and 2) are carried out as follows:

[0054] 1) Under a nitrogen atmosphere and at room temperature, take 28.523 g (0.142 mol) of the asymmetric diamine 3,4'-diaminodiphenyl ether (3,4'-ODA) and stir it to dissolve in 425 g of DMAC. Then add a total of 43.688 g (0.134 mol, added in 3 portions) of the asymmetric dianhydride 2,3,3',4'-diphenylsulfide tetracarboxylic dianhydride (2,3,3',4'-TDPA) (the molar ratio of diamine to dianhydride is 1:0.94), and stir and react for 16 h to obtain a polyamic acid prepolymer solution capped with amino groups (the solid content of the obtained polyamic acid prepolymer solution is about 15%, and the total designed amount of the solid component is about 75 g);

[0055] 2) Take 14.0 g of the polyamic acid prepolymer solution capped with amino groups prepared in step 1) (the total amount of the solid component is 2.0 g), 96.0 g of the negative electrode active material (57.6 g of graphite, 38.4 g of nano-porous silicon powder, graphite:nano-porous silicon powder = 6:4), and 2.0 g of the conductive agent (conductive carbon black), where the active material:binder:conductive agent = 96:2:2 (by weight), mix them evenly, add 150 g of NMP, and grind and blend and stir the obtained mixture to obtain a slurry precursor (viscosity is about 5500 cp).

[0056] The test results are as follows: the initial Coulombic efficiency is about 88%, the capacity retention rate after 150 cycles is about 92%, and the swelling rate of the electrode in the Z direction (thickness direction) after 150 cycles is about 68%.

[0057] Comparative Example 5

[0058] Same as Example 1, with the only difference being that in the preparation of the negative electrode slurry, step 3) is carried out as follows:

[0059] 3) Add 0.132 g (0.297 mmol) of 9,9-bis(3,4-dicarboxyphenyl)fluorene dianhydride (BPAF) bulky dianhydride capping agent to the obtained slurry precursor (the molar ratio of the total amount of diamine raw materials to the total amount of dianhydride raw materials is 1:1.07), and stir and react for 8 h to obtain the negative electrode slurry.

[0060] The test results are as follows: the initial Coulombic efficiency is about 85%, the capacity retention rate after 150 cycles is about 96%, and the swelling rate of the electrode in the Z direction (thickness direction) after 150 cycles is about 75%.

[0061] Example 2

[0062] 1) Under a nitrogen atmosphere and at room temperature, 53.386 g (0.267 mol) of the asymmetric diamine 3,4'-diaminodiphenyl ether (3,4'-ODA) was taken and stirred to dissolve in 656 g of DMAC. Then, a total of 90.614 g (0.278 mol, added in two portions) of the asymmetric dianhydride 2,3,3',4'-diphenylsulfide tetracarboxylic dianhydride (2,3,3',4'-TDPA) (the molar ratio of diamine to dianhydride was 0.96:1) was added, and the mixture was stirred and reacted for 16 h to obtain a polyamic acid prepolymer solution capped with anhydride groups (the solid content of the obtained polyamic acid prepolymer solution was about 18%, and the total amount of solid components was about 144 g);

[0063] 2) 84.0 g of the polyamic acid prepolymer solution capped with anhydride groups prepared in step 1) (the total amount of solid components was 15.0 g), 65.0 g of the negative electrode active material (39 g of graphite and 26 g of nano-porous silicon powder, graphite:nano-porous silicon powder = 6:4), and 20.0 g of the conductive agent (conductive carbon black) were taken, where the active material:binder:conductive agent = 65:15:20 (by weight). The mixture was uniformly mixed, 180 g of NMP was added, and the obtained mixture was ground and subjected to co-blending and stirring to obtain a slurry precursor (viscosity about 4000 cp);

[0064] 3) 0.781 g (1.383 mmol) of the bulky diamine capping agent 9,9'-bis[4-(4-amino-2-hydroxyphenoxy)phenyl]fluorene (the molar ratio of the total amount of diamine raw material to the total amount of dianhydride raw material was 1.01:1) was added to the obtained slurry precursor, and the mixture was stirred and reacted for 3 h to obtain the negative electrode slurry.

[0065] The preparation of the positive and negative electrodes of the lithium battery, the preparation of the battery, and the charge and discharge characteristic tests were the same as those in Example 1.

[0066] The test results were as follows: the initial Coulombic efficiency was about 94%, the capacity retention rate after 150 cycles was about 97%, and the swelling rate of the electrode in the Z direction (thickness direction) after 150 cycles was about 55%.

[0067] Comparative Example 6

[0068] Same as Example 2, the difference was only that in the preparation of the negative electrode slurry, step 3) was carried out as follows:

[0069] 3) 0.625 g (1.118 mmol) of the bulky dianhydride capping agent 9,9-bis(3,4-dicarboxyphenyl)fluorene dianhydride (BPAF) (the molar ratio of the total amount of diamine raw material to the total amount of dianhydride raw material was 1:1) was added to the obtained slurry precursor, and the mixture was stirred and reacted for 8 h to obtain the negative electrode slurry.

[0070] The test results are as follows: the initial Coulombic efficiency is approximately 80%, the capacity retention rate after 150 cycles is approximately 96%, and the swelling rate of the electrode in the Z direction (thickness direction) after 150 cycles is approximately 70%.

[0071] Example 3

[0072] 1. Preparation of the negative electrode slurry

[0073] 1) Under a nitrogen atmosphere and at room temperature, 63.399 g (0.293 mol) of the asymmetric diamine 3,4'-diaminodiphenyl ether (3,4'-ODA) was taken and stirred to dissolve in 840 g of DMAC. Then, a total of 96.601 g (0.296 mol, added in 3 portions) of the asymmetric dianhydride 2,3,3',4'-diphenylsulfide tetracarboxylic dianhydride (2,3,3',4'-TDPA) (the molar ratio of diamine to dianhydride is 0.99:1) was added, and the mixture was stirred and reacted for 24 h to obtain a polyamic acid prepolymer solution capped with anhydride groups (the solid content of the obtained polyamic acid prepolymer solution is approximately 16%, and the total amount of solid components is approximately 160 g);

[0074] 2) 37.5 g of the polyamic acid prepolymer solution capped with anhydride groups prepared in step 1) (the total amount of solid components is 6.0 g), 90.0 g of negative electrode active materials (45 g of graphite and 45 g of nano-porous silicon powder, graphite:nano-porous silicon powder = 5:5), and 4.0 g of a conductive agent (conductive carbon black) were taken, where the ratio of active material:binder:conductive agent = 90:6:4 (by weight). The mixture was uniformly mixed, and about 150 g of NMP was added. The obtained mixture was ground and subjected to co-blending and stirring to obtain a slurry precursor (viscosity is approximately 10,000 cp);

[0075] 3) 0.282 g (0.741 mmol) of 9,9-bis(3-amino-4-hydroxyphenyl)fluorene large-volume diamine capping agent (the molar ratio of the total amount of diamine raw materials to the total amount of dianhydride raw materials is 1.06:1) was added to the obtained slurry precursor, and the mixture was stirred and reacted for 2 h to obtain the negative electrode slurry.

[0076] The preparation of the positive and negative electrodes of the lithium battery, the preparation of the battery, and the charge and discharge characteristic tests are the same as those in Example 1.

[0077] The test results are as follows: the initial Coulombic efficiency is approximately 95%, the capacity retention rate after 150 cycles is approximately 96%, and the swelling rate of the electrode in the Z direction (thickness direction) after 150 cycles is approximately 59%.

[0078] Example 4

[0079] 1. Preparation of the negative electrode slurry

[0080] 1) Under a nitrogen atmosphere and at room temperature, 34.569 g (0.173 mol) of the asymmetric diamine 3,4'-diaminodiphenyl sulfide was taken and stirred to dissolve in 634 g of DMAC. Then, a total of 51.831 g (0.176 mol, added in one portion) of the asymmetric dianhydride 2,3,3',4'-biphenyltetracarboxylic dianhydride (α-BPDA) (the molar ratio of diamine to dianhydride was 0.98:1) was added, and the mixture was stirred and reacted for 12 h to obtain a polyamic acid prepolymer solution capped with anhydride groups (the solid content of the obtained polyamic acid prepolymer solution was about 12%, and the total amount of solid components was about 86.0 g);

[0081] 2) 76 g of the polyamic acid prepolymer solution capped with anhydride groups prepared in step 1) (the total amount of solid components was 9.0 g), 88.0 g of the negative electrode active material (44 g of graphite, 44 g of nano-porous silicon powder, graphite:nano-porous silicon powder = 5:5), and 3.0 g of the conductive agent (conductive carbon black) were taken, where the active material:binder:conductive agent = 88:9:3 (weight ratio). The mixture was uniformly mixed, 280 g of NMP was added, and the obtained mixture was ground and subjected to co-blending and stirring to obtain a slurry precursor (viscosity was about 2000 cp);

[0082] 3) 0.135 g (0.384 mmol) of 9,9-bis(4-aminophenyl)fluorene (BAFL) large-volume diamine capping agent (the molar ratio of the total amount of diamine raw material used to the total amount of dianhydride raw material used was 1.001:1) was added to the obtained slurry precursor, and the mixture was stirred and reacted for 2 h to obtain the negative electrode slurry.

[0083] The preparation of the positive and negative electrodes of the lithium battery, the preparation of the battery, and the charge and discharge characteristic tests were the same as those in Example 1.

[0084] The test results were as follows: the initial Coulombic efficiency was about 92%, the capacity retention rate after 150 cycles was about 97%, and the swelling rate of the electrode in the Z direction (thickness direction) after 150 cycles was about 60%.

[0085] Example 5

[0086] 1. Preparation of the negative electrode slurry

[0087] 1) Under a nitrogen atmosphere and at room temperature, 20.306 g (0.188 mol) of the asymmetric diamine m-phenylenediamine (m-PDA) was taken and stirred to dissolve in 634 g of DMAC. Then, a total of 54.694 g (0.186 mol, added in two portions) of the asymmetric dianhydride 2,3,3',4'-biphenyltetracarboxylic dianhydride (α-BPDA) (the molar ratio of diamine to dianhydride was 1:0.99) was added, and the mixture was stirred and reacted for 20 h to obtain a polyamic acid prepolymer solution capped with amino groups (the solid content of the obtained polyamic acid prepolymer solution was about 15%, and the total amount of solid components was about 75.0 g);

[0088] 2) Take 67 g of the amino-terminated polyamic acid prepolymer solution prepared in step 1) (total solid content is 10.0 g), 70.0 g of negative electrode active material (42 g of graphite, 28 g of nano-porous silicon powder, graphite:nano-porous silicon powder = 6:4), and 20.0 g of conductive agent (conductive carbon black), where the ratio of active material:binder:conductive agent = 70:10:20 (by weight), mix evenly, add about 250 g of NMP, and grind and blend the obtained mixture to obtain a slurry precursor (viscosity is about 3000 cp);

[0089] 3) Add 0.226 g (0.492 mmol) of 9,9-bis(3,4-dicarboxyphenyl)fluorene dianhydride (BPAF) bulky dianhydride capping agent (the molar ratio of the total amount of diamine raw material to the total amount of dianhydride raw material is 1:1.01) to the obtained slurry precursor, stir and react for 6 h to obtain the negative electrode slurry.

[0090] The preparation of the positive and negative electrodes of the lithium battery, the preparation of the battery, and the charge and discharge characteristic tests are the same as those in Example 1.

[0091] The test results are as follows: the initial Coulomb efficiency is about 93%, the capacity retention rate after 150 cycles is about 96%, and the swelling rate of the electrode in the Z direction (thickness direction) after 150 cycles is about 60%.

[0092] Example 6

[0093] Same as Example 5, the difference is only that: when preparing the negative electrode slurry, use 3,4,9,10-perylene tetracarboxylic dianhydride bulky dianhydride capping agent with the same molar amount to replace 9,9-bis(3,4-dicarboxyphenyl)fluorene dianhydride (BPAF) bulky dianhydride capping agent in Example 5.

[0094] The test results are as follows: the initial Coulomb efficiency is about 92%, the capacity retention rate after 150 cycles is about 96%, and the swelling rate of the electrode in the Z direction (thickness direction) after 150 cycles is about 62%.

Claims

1. Preparation method of anode slurry for lithium-ion battery, comprising the following steps: 1) Using an asymmetric diamine and an asymmetric dianhydride as raw materials, preparing a polyamic acid prepolymer solution capped with an amino group or an anhydride group; 2) Mixing the polyamic acid prepolymer solution capped with an amino group or an anhydride group with the anode active material and a conductive agent uniformly, adding or not adding a polar solvent, and obtaining a slurry precursor after mixing uniformly; 3) Adding a dianhydride capping agent or a diamine capping agent containing a bulky group to the slurry precursor, stirring and reacting to obtain the anode slurry for lithium-ion battery; wherein, When the polyamic acid prepolymer solution contained in the slurry precursor is capped with an amino group, adding a dianhydride capping agent containing a bulky group, and the addition amount is controlled such that the total amount of the dianhydride capping agent containing a bulky group and the amount of the asymmetric dianhydride is in excess of the molar amount of the asymmetric diamine; When the polyamic acid prepolymer solution contained in the slurry precursor is capped with an anhydride group, adding a diamine capping agent containing a bulky group, and the addition amount is controlled such that the total amount of the diamine capping agent containing a bulky group and the amount of the asymmetric diamine is in excess of the molar amount of the asymmetric dianhydride; The dianhydride capping agent containing a bulky group is 9,9-bis(3,4-dicarboxyphenyl)fluorene dianhydride and / or 3,4,9,10-perylene tetracarboxylic dianhydride; The diamine capping agent containing a bulky group is selected from one or a combination of two or more of 9,9-bis(3-amino-4-hydroxyphenyl)fluorene, 9,9'-bis[4-(4-amino-2-hydroxyphenoxy)phenyl]fluorene, 9,9-bis(4-aminophenyl)fluorene, 9,9-bis(4-amino-3-fluorophenyl)fluorene, 9,9-bis[4-(4-aminophenoxy)phenyl]fluorene, 4,6-bis(1-adamantyl)-1,3-bis(4-amino-3-hydroxyphenoxy)benzene, and 9,9'-spirobi[9H-fluorene]-2,2'-diamine.

2. The preparation method according to claim 1, wherein In step 1), the asymmetric diamine is selected from any one or a combination of two or more of 3,4'-diaminodiphenyl ether, m-phenylenediamine, 3,4'-diaminodiphenyl sulfone, 3,4'-diaminobiphenyl, 3,4'-diaminodiphenyl methane, 3,4'-diaminodiphenyl disulfide, and 3,4'-diaminodiphenyl sulfide.

3. The preparation method according to claim 1, characterized in that, In step 1), the asymmetric dianhydride is selected from any one or a combination of two or more of 2,3,3',4'-biphenyltetracarboxylic dianhydride, 2,3,3',4'-benzophenone tetracarboxylic dianhydride, 2,3,3',4'-diphenylsulfone tetracarboxylic dianhydride, 2,3,3',4'-diphenylsulfide tetracarboxylic dianhydride, and 2,3,3',4'-diphenylether tetracarboxylic dianhydride.

4. The preparation method according to claim 1, characterized in that, In step 2), the anode active material is a combination of one or two or more of carbon, silicon, and silicon alloy.

5. The preparation method according to any one of claims 1 to 4, characterized in that, In step 3), the addition amount of the dianhydride capping agent containing a bulky group is controlled such that the total amount of the dianhydride capping agent containing a bulky group and the amount of the asymmetric dianhydride is in excess of the molar amount of the asymmetric diamine by 0.1 to 6%.

6. The preparation method according to any one of claims 1 to 4, characterized in that, In step 3), the addition amount of the diamine terminator containing a bulky group is controlled such that the total amount of the diamine terminator containing a bulky group and the asymmetric diamine is in excess of the molar amount of the asymmetric dianhydride by 0.1 to 6%.

7. The negative electrode paste for a lithium ion battery prepared by the method according to any one of claims 1 to 6.

8. A negative electrode of a lithium-ion battery, comprising a negative electrode current collector and an electrode layer loaded on the negative electrode current collector, characterized in that, The electrode layer is formed by heat-treating the negative electrode paste for a lithium ion battery according to claim 7.