A negative electrode slurry for lithium ion batteries and a method for preparing the same

Abstract

The application discloses a kind of negative electrode slurry for lithium ion battery and its preparation method, the preparation method of the negative electrode slurry includes the following steps: 1) with 3,4'-ODA or 3,4'-ASD and alpha-ODPA as raw material preparation solid component content≤5wt% with amino-terminated polyamic acid prepolymer solution;2) the polyamic acid prepolymer obtained is mixed with negative electrode active material and conductive agent uniformly, add or not add polar solvent, mix evenly to obtain slurry precursor;3) in slurry precursor, add asymmetric structure isomerization dianhydride end-capping agent, stir polymerization reaction, namely obtained;The asymmetric structure isomerization dianhydride end-capping agent is preferably MPDA, 3,3'-ODPA or alpha-ODPA, and the molar ratio of the total amount of all dianhydride in the entire preparation process to diamine is 1:1.The negative electrode slurry can effectively inhibit the expansion effect of negative electrode material, thereby improving the initial efficiency and cycle stability of the battery.

Description

Technical Field

[0001] This invention relates to lithium-ion batteries, and more specifically to a negative electrode slurry for lithium-ion batteries and its preparation method. Background Technology

[0002] Lithium-ion batteries possess advantages such as high operating voltage, long cycle life, low self-discharge rate, no memory effect, and high specific capacity, leading to their widespread application and continued market expansion. Among all battery structures, the positive and negative electrode materials have the greatest impact on battery capacity. Currently, graphite is the most widely used commercially available negative electrode material, with a specific capacity approaching its theoretical value of 372 mAh / g. Silicon-based negative electrode materials, however, have a higher specific capacity, reaching 4200 mAh / g, and are considered the most likely material to replace graphite. However, silicon-based materials undergo severe volume expansion and contraction during battery charging and discharging (expansion can reach up to approximately 400%), making them highly susceptible to cracking, pulverization, and peeling, resulting in capacity loss and battery failure.

[0003] Using silicon-carbon composite anodes with carbon-coated silicon can reduce the volume expansion and contraction effect of silicon. Furthermore, by developing an adhesive that can bond to both copper foil and silicon-carbon materials, the volume expansion effect of the anode material can be mitigated to a greater extent, thereby improving the cycle stability of silicon-carbon anodes and battery life.

[0004] To mitigate the drawbacks of silicon-containing anodes, such as volume expansion and contraction during charge and discharge, existing technologies propose using polyimide resin as a binder in the anode active material. Patent CN108701831B discloses an electrode mixture paste for secondary batteries, comprising a binder composition for secondary batteries and an anode active material containing Si and / or silicon oxide. The binder composition comprises a polyamic acid with a backbone based on an aromatic tetracarboxylic acid dianhydride and repeating units based on bicyclic [2.2.1]heptanedimethylamine (NBDA). This invention indicates that a binder with a cyclohexane structure can withstand the internal stress generated by the expansion and contraction of the active material during cycling. Specific examples demonstrate that by defining the structure of the tetracarboxylic acid and diamine components of the polyamic acid, the initial charge / discharge efficiency (first-time efficiency) and cycle characteristics can be improved, achieving an initial charge / discharge efficiency of 66% and a capacity retention of 91% after 150 cycles. While the electrode mixture paste described in this invention exhibits good cycle stability, its improvement in first-time efficiency is not ideal. 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 a method for preparing the same, which can effectively improve the initial coulombic efficiency (first efficiency) and cycle stability of lithium-ion batteries.

[0006] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0007] A method for preparing a negative electrode slurry for lithium-ion batteries includes the following steps:

[0008] 1) An amino-terminated polyamic acid prepolymer solution was prepared by condensation polymerization of an asymmetric aromatic diamine and 2,3,3',4'-diphenyl ether tetracarboxylic dianhydride (α-ODPA) in a polar aprotic solvent; wherein,

[0009] The asymmetric aromatic diamine is 3,4'-diaminodiphenyl ether (3,4'-ODA) or 3,4'-diaminodiphenyl sulfide (3,4'-ASD), or a combination of both;

[0010] The amount of the polar aprotic solvent is such that the solid content in the obtained polyamic acid prepolymer solution is ≤5wt%.

[0011] 2) Mix the amino-terminated polyamic acid prepolymer solution with the negative electrode active material and conductive agent evenly, with or without adding polar solvent, and mix evenly to obtain the slurry precursor;

[0012] 3) An asymmetric isomeric dianhydride end-capping agent is added to the slurry precursor, and the polymerization reaction is stirred to obtain the aforementioned negative electrode slurry for lithium-ion batteries; wherein,

[0013] The amount of the asymmetric isomeric dianhydride end-capping agent added is controlled to be 1:1, with the total amount of the asymmetric isomeric dianhydride end-capping agent and 2,3,3',4'-diphenyl ether tetracarboxylic dianhydride being used in a molar ratio of 1:1 to the asymmetric aromatic diamine.

[0014] The asymmetric structural isomeric dianhydride end-capping agent is selected from any one or a combination of two or more of 1,2,3,4-benzenetetracarboxylic dianhydride (MPDA), 2,3,2',3'-diphenyl ether tetracarboxylic dianhydride (3,3'-ODPA), and 2,3,3',4'-diphenyl ether tetracarboxylic dianhydride (α-ODPA).

[0015] In step 1) of the above preparation method, the condensation polymerization reaction time is greater than or equal to 72 h, preferably 84–120 h. In this step, the asymmetric aromatic diamine is further preferably 3,4'-diaminodiphenyl ether, and the molar ratio of the asymmetric aromatic diamine and 2,3,3',4'-diphenyl ether tetracarboxylic dianhydride is generally limited to 1:0.98–0.99. The amount of the polar aprotic solvent is further preferably such that the solid content in the obtained polyamic acid prepolymer solution is 1–3 wt%.

[0016] In step 2) of the above preparation method, the negative electrode active material is preferably a combination of one or more of carbon, silicon, and silicon alloy.

[0017] In step 3) of the above preparation method, the asymmetric structural isomeric dianhydride end-capping agent is further preferably 1,2,3,4-benzenetetracarboxylic dianhydride or 2,3,2',3'-diphenyl ether tetracarboxylic dianhydride, or a combination of both. 1,2,3,4-benzenetetracarboxylic dianhydride is particularly preferred.

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

[0019] Compared with the prior art, the negative electrode slurry of the present invention can effectively improve the cycle stability and first-cycle efficiency of lithium-ion batteries. The applicant's test results show that the battery prepared by further using the negative electrode slurry of the present invention has a first-cycle efficiency of ≥93%, a cycle stability efficiency of ≥96% after 500 cycles, and an electrode Z-direction (thickness direction) expansion rate of ≤65% after 500 cycles. Detailed Implementation

[0020] The method for preparing the negative electrode slurry for lithium-ion batteries according to the present invention includes the following steps:

[0021] 1) An amino-terminated polyamic acid prepolymer solution was prepared by condensation polymerization of an asymmetric aromatic diamine and 2,3,3',4'-diphenyl ether tetracarboxylic dianhydride in a polar aprotic solvent; wherein,

[0022] The asymmetric aromatic diamine is 3,4'-diaminodiphenyl ether or 3,4'-diaminodiphenyl sulfide, or a combination of both;

[0023] The amount of the polar aprotic solvent is such that the solid content in the obtained polyamic acid prepolymer solution is ≤5wt%.

[0024] 2) Mix the amino-terminated polyamic acid prepolymer solution with the negative electrode active material and conductive agent evenly, with or without adding polar solvent, and mix evenly to obtain the slurry precursor;

[0025] 3) An asymmetric isomeric dianhydride end-capping agent is added to the slurry precursor, and the polymerization reaction is stirred to obtain the aforementioned negative electrode slurry for lithium-ion batteries; wherein,

[0026] The amount of the asymmetric isomeric dianhydride end-capping agent added is controlled to be 1:1, with the total amount of the asymmetric isomeric dianhydride end-capping agent and 2,3,3',4'-diphenyl ether tetracarboxylic dianhydride being used in a molar ratio of 1:1 to the asymmetric aromatic diamine.

[0027] The asymmetric structural isomeric dianhydride end-capping agent is selected from any one or a combination of two or more of 1,2,3,4-benzenetetracarboxylic dianhydride (MPDA), 2,3,2',3'-diphenyl ether tetracarboxylic dianhydride (3,3'-ODPA), and 2,3,3',4'-diphenyl ether tetracarboxylic dianhydride (α-ODPA).

[0028] In step 1) of the preparation method of this invention, the selection of the polar aprotic solvent used in the condensation polymerization reaction of the 2,3,3',4'-diphenyl ether tetracarboxylic dianhydride and the asymmetric aromatic diamine, as well as the temperature of the condensation reaction, are the same as in the prior art. For example, the preferred polar aprotic solvents are N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAC), N,N-diethylacetamide, or N-methyl-2-pyrrolidone (NMP). The amount of the polar aprotic solvent is further preferably such that the solid content in the resulting polyamic acid prepolymer solution is 1-3 wt%. The condensation reaction time is preferably greater than or equal to 72 h, and more preferably 84-120 h.

[0029] In step 1) of the preparation method of the present invention, the asymmetric aromatic diamine is further preferably 3,4'-diaminodiphenyl ether, and the molar ratio of the asymmetric aromatic diamine and 2,3,3',4'-diphenyl ether tetracarboxylic dianhydride is usually limited to 1:0.98-0.99.

[0030] In step 2) of the preparation method of this invention, the negative electrode active material and the conductive agent are conventional choices in the prior art. The negative electrode active material is preferably a combination of one or more of carbon, silicon, and silicon alloys, and more preferably a porous silicon-carbon composite powder containing porous silicon powder or silicon alloy powder. The conductive agent is usually conductive carbon black. The ratio of the amino-terminated polyamic acid prepolymer solution, the negative electrode active material, and the conductive agent is also the same as in the prior art. In this application, the preferred weight ratio of the amino-terminated polyamic acid prepolymer solution, the negative electrode active material, and the conductive agent is 15–2:65–96:20–2, where the amino-terminated polyamic acid prepolymer solution is calculated based on the amount of solid components in the prepolymer solution.

[0031] In step 2) of the preparation method described in this invention, the selection of the polar aprotic solvent is the same as described above and will not be detailed here. The amount of polar solvent used in this step is such that the viscosity of the resulting slurry precursor meets the requirements for easy coating; typically, the viscosity of the resulting slurry precursor is controlled to be 2000–10000 cp, preferably 4000–7000 cp. If the viscosity of the amino-terminated polyamic acid prepolymer solution prepared in the previous step is low, and the viscosity of the system after adding the negative electrode active material and conductive agent and stirring evenly is just within the above-mentioned range, then there is no need to add polar solvent.

[0032] In step 3) of the preparation method of the present invention, the asymmetric structural isomeric dianhydride end-capping agent is further preferably 1,2,3,4-benzenetetracarboxylic dianhydride or 2,3,2',3'-diphenyl ether tetracarboxylic dianhydride, or a combination of both. 1,2,3,4-benzenetetracarboxylic dianhydride is particularly preferred. The stirring polymerization time after adding the asymmetric structural isomeric dianhydride end-capping agent is typically controlled to be between 0.5 and 8 hours.

[0033] The battery prepared using the negative electrode slurry described in this invention exhibits excellent initial coulombic efficiency and cycle stability. The applicant believes, based on research and analysis, that this may be due to the following reasons:

[0034] First, under the premise of preparing polyamic acid prepolymers through polycondensation under low solid content conditions (weakening the interaction between the main chains of polyamic acid molecules, increasing the probability of changes in the geometric conformation of polyamic acid molecules, and reducing the possibility of polyamic acid molecules forming an ordered structure), by controlling the molar ratio of all dianhydride raw materials (α-ODPA, asymmetric isomeric dianhydrides) and all asymmetric aromatic diamine raw materials, and extending the polycondensation reaction time to greater than or equal to 72 hours, the polycondensation reaction shows a tendency to form rings, that is, it can form polyamic acid molecular chains with cyclic (or semi-cyclic) structures. This is achieved using α-ODPA (containing twisted structures and...) The non-coplanar configuration of benzene rings favors the formation of cyclic polymers. Simultaneously, the absence of other atoms surrounding the oxygen atom in the ether bond facilitates internal rotation. Using dianhydride monomers effectively promotes the formation of unique spatial structures (conformations) and chain flexibility in the polyamic acid molecular chains generated by the condensation polymerization reaction. This ease of molecular chain configuration modification reduces the complexity and diversity of molecular structure shapes. Furthermore, the use of asymmetric 3,4'-ODA and 3,4'-ASD as aromatic diamine monomers further influences conformational changes (chemical bond rotation) in the polyamic acid molecular chains, leading to cyclic or semi-cyclic polyamic acid molecular chains. These cyclic or semi-cyclic polyamic acid molecular chains exist in a "ring" and / or "hoop" manner on the micro / nano silica active material (i.e., surrounding the surface of the micro / nano-sized active material or weaving through its own nano-sized pores). Combined with the covalent bonds, hydrogen bonds, and other chemical bonds inherent in the polyamic acid, they work together to form a strong adhesive force on the surface active groups of the silica powder.

[0035] Secondly, the selection of asymmetric isomeric dianhydride end-capping agents with special conformations, especially MPDA and 3,3'-ODPA dianhydrides, can increase the probability of further polymerization reaction with polyamic acid prepolymer on the surface of silicon-carbon anode active materials to form cyclic polyamic acid, that is, more effectively "close the ring" to form cyclic polymer molecular chains. At the same time, it ensures that the polyimide formed by imidization of the prepared polyamic acid has high cohesive strength. The combined effect makes it more conducive to suppressing the volume expansion of micro-nano silicon powder active materials (i.e., effectively absorbing the volume expansion during the lithium intercalation process) after application, so as to obtain the desired electrochemical performance.

[0036] Furthermore, this application uses α-ODPA as the dianhydride raw material for polymerization and specific 3,4'-ODA and / or 3,4'-ASD as the diamine raw material for polymerization. Both the dianhydride and diamine raw materials have asymmetric structures, and the polyimide resin adhesive prepared from them exhibits a significant cavity structure effect, that is, it is easy to form loose molecular chain stacking, which is beneficial to lithium-ion transport. At the same time, the polyimide structure formed by the condensation polymerization of asymmetric structure monomers has a high degree of amorphous phase, which can further optimize the lithium-ion conduction in the lithium battery system. It can rapidly migrate through the amorphous phase of locally relaxed chain segments of polyimide, further promoting the polyimide resin mixture to have good mechanical properties, high room temperature conductivity, and a wide chemical stability window, effectively improving the cycle stability and first-efficiency of lithium-ion batteries.

[0037] To better explain the technical solution of the present invention, the present invention will be further described in detail below with reference to the embodiments, but the implementation of the present invention is not limited thereto.

[0038] Example 1

[0039] 1. Preparation of negative electrode slurry

[0040] 1.1) Under a nitrogen atmosphere and at room temperature, 2.354 g (11.75 mmol) of 3,4'-ODA was dissolved in 594 g of NMP by stirring. Then, a total of 3.574 g (11.52 mmol, added in 12 portions) of α-ODPA (with a molar ratio of dianhydride to diamine of 0.98:1) was added. The mixture was stirred and reacted for 72 h under a nitrogen atmosphere and at room temperature to obtain a polyamic acid prepolymer solution with amino-terminated ends and a solid content of 1% (total solid content of approximately 6 g).

[0041] 1.2) Take 200.0g of the amino-terminated polyamic acid prepolymer solution (total solid content 2.0g) prepared in step 1), 96.0g of the negative electrode active material (67.2g of graphite), and nanoporous silica powder (specific surface area approximately 58m²). 2 / g, with an average mesopore diameter of approximately 27.0nm (the same below) 28.8g, graphite: nanoporous silica powder = 7:3) and 2.0g conductive agent (conductive carbon black), wherein the active material: binder: conductive agent = 96:2:2 (weight ratio), are mixed evenly without the need for additional NMP. The resulting mixture is then ground and blended to obtain a slurry precursor (viscosity approximately 5180cp).

[0042] 1.3) Add 0.024 g (0.078 mmol) 3,3'-ODPA (the molar ratio of the total amount of diamine raw material to the total amount of dianhydride raw material is 1:1) to the obtained slurry precursor, stir and react for 8 h to obtain the negative electrode slurry.

[0043] 2. Preparation of positive and negative electrodes for lithium batteries

[0044] negative electrode:

[0045] The negative electrode slurry prepared in this embodiment was uniformly coated onto a copper foil. The gap of the coating roller (scraper) was adjusted to control the thickness of the negative electrode slurry after curing to be 35 μm ± 3.0 μm. The copper foil uniformly coated with the negative electrode slurry was placed in an oven and heated at 80°C for 1 hour under flowing nitrogen gas and an oxygen concentration of less than 20 ppm. Then, the temperature was increased to 320°C at a rate of 3.5°C / min and held at 320°C for 1.8 hours to obtain the negative electrode sheet.

[0046] positive electrode:

[0047] The active material ternary cathode (NCM721), polyvinylidene fluoride, and conductive carbon black were mixed evenly in a weight ratio of 94:3:3. NMP solvent was added to adjust the viscosity to an appropriate level (7000±500 cp). The mixture was then ground in a three-roll mill for 3 hours and dispersed at high speed for 2 hours to obtain the cathode slurry. The cathode slurry was coated onto aluminum foil using a doctor blade. The gap between the coating rollers (doctor blade) was adjusted to control the thickness of the cured cathode slurry to be 110 μm±3.0 μm. The coated aluminum foil was placed in an oven and kept at 120℃ for 2 hours under air circulation to obtain the cathode sheet.

[0048] 3. Battery manufacturing

[0049] To reduce the gaps between active materials, the aforementioned lithium battery negative and positive electrode sheets were appropriately rolled using a rolling mill. The rolled negative and positive electrode sheets were then cut into 14mm diameter round pieces using a punching machine. CR2032 coin cells were assembled in an argon glove box (H2O < 0.01ppm, O2 < 0.01ppm). The negative electrode shell, negative electrode sheet, separator, positive electrode sheet, nickel foam, spring sheet, and positive electrode shell were assembled sequentially. 1ml of electrolyte was added to each end of the separator. The electrolyte was a 1.0mol / L LiPF6 solution dissolved in a mixture of EC and DMC (EC:DMC = 1:1, volume ratio). The assembled battery was then sealed in a sealing machine at a pressure of 75MPa. After standing for 24 hours, the corresponding electrochemical performance was tested.

[0050] 4. Charge and discharge characteristics test

[0051] The batteries prepared by the above method were subjected to cyclic charge-discharge characteristic tests. The batteries were charged and discharged at 25°C and cycled. The experiment used a 0.2C current charge-discharge test with a voltage window of 0.005 to 1.5V. The amount of electricity flowing from the start of charging or discharging to the end was defined as the charging capacity or discharging capacity.

[0052] Test its charge-discharge efficiency after the first and 500 cycles [where charge-discharge efficiency = (discharge capacity / charge capacity) * 100%].

[0053] Using the thickness of the negative electrode sheet in the scanning electron microscope (SEM) cross-section images before and after 500 cycles with adhesive as a reference, the expansion rate of the negative electrode sheet was calculated based on the thickness of the active material before charging and after 500 cycles. The initial thickness H0 of the negative electrode sheet was measured, and the thickness H after 500 cycles was also measured. 500 And according to the calculation formula (thickness direction Z) expansion rate = (H 500 The electrode expansion rate is calculated as (-H0) / H0*100%.

[0054] The test results are as follows: the initial coulomb efficiency is 94%, the capacity retention rate after 500 cycles is 98%, and the Z-axis (thickness direction) expansion rate of the electrode after 500 cycles is 62%.

[0055] Comparative Example 1

[0056] Same as Example 1, except that in step 1.1), α-ODPA is replaced with an equimolar amount of another asymmetric dianhydride, 2,3,3',4'-benzophenone tetracarboxylic dianhydride (2,3,3',4'-BTDA).

[0057] The test results are as follows: the initial coulomb efficiency is 90%, the capacity retention rate after 500 cycles is 86%, and the Z-axis (thickness direction) expansion rate of the electrode after 500 cycles is 102%.

[0058] Comparative Example 2

[0059] Same as Example 1, except that in step 1.1), 3,4'-ODA is replaced with an equimolar amount of the symmetrical diamine 4,4'-diaminodiphenyl ether (4,4'-ODA).

[0060] The test results are as follows: the initial coulombic efficiency is 88%, the capacity retention rate after 500 cycles is 91%, and the Z-axis (thickness direction) expansion rate of the electrode after 500 cycles is 89%.

[0061] Comparative Example 3

[0062] Same as Example 1, except that in step 1.1), 3,4'-ODA is replaced with an equimolar amount of the symmetrical diamine 4,4'-diaminodiphenyl ether (4,4'-ODA), and α-ODPA is replaced with an equimolar amount of other asymmetrical dianhydrides 2,3,3',4'-benzophenone tetracarboxylic dianhydride (2,3,3',4'-BTDA).

[0063] The test results are as follows: the initial coulomb efficiency is 81%, the capacity retention rate after 500 cycles is 84%, and the electrode Z-axis (thickness direction) expansion rate after 500 cycles is 113%.

[0064] Comparative Example 4

[0065] Same as Example 1, except that in step 1.3), 3,3'-ODPA is replaced with 2,3,3',4'-biphenyltetracarboxylic dianhydride (α-BPDA) of other asymmetric structures in the same molar amount.

[0066] The test results are as follows: the initial coulombic efficiency is 90%, the capacity retention rate after 500 cycles is 90.5%, and the electrode Z-axis (thickness direction) expansion rate after 500 cycles is 81%.

[0067] Comparative Example 5

[0068] 1. Preparation of negative electrode slurry

[0069] 1.1) Under a nitrogen atmosphere and at room temperature, 2.354 g (11.75 mmol) of 3,4'-ODA was dissolved in 594 g of NMP by stirring. Then, a total of 3.646 g (11.75 mmol, added in 12 portions) of α-ODPA (with a molar ratio of dianhydride to diamine of 1:1) was added. The mixture was stirred and reacted for 72 h under a nitrogen atmosphere and at room temperature to obtain a polyamic acid prepolymer solution with amino-terminated ends and a solid content of 1% (total solid content of approximately 6 g).

[0070] 1.2) Take 200.0g of the amino-terminated polyamic acid prepolymer solution (total solid content 2.0g) prepared in step 1), 96.0g of the negative electrode active material (67.2g of graphite), and nanoporous silica powder (specific surface area approximately 58m²). 2 / g, with an average mesopore diameter of approximately 27.0nm (the same below) 28.8g, graphite: nanoporous silicon powder = 7:3) and 2.0g conductive agent (conductive carbon black), wherein the active material: binder: conductive agent = 96:2:2 (weight ratio), are mixed evenly without the need for additional NMP. The resulting mixture is ground and stirred, and the resulting material is used as the negative electrode slurry.

[0071] The preparation of the positive and negative electrodes of the lithium battery, the preparation of the battery itself, and the testing of its charge and discharge characteristics were all the same as in Example 1.

[0072] The test results are as follows: the initial coulomb efficiency is approximately 90%, the capacity retention rate after 500 cycles is approximately 94%, and the Z-axis (thickness direction) expansion rate of the electrode after 500 cycles is approximately 73%.

[0073] Comparative Example 6

[0074] 1. Preparation of negative electrode slurry

[0075] 1.1) Under a nitrogen atmosphere and at room temperature, 2.307 g (11.52 mmol) of 3,4'-ODA was dissolved in 594 g of NMP by stirring. Then, a total of 3.646 g (11.75 mmol, added in 12 portions) of α-ODPA (dianhydride to diamine molar ratio of 1:0.98) was added. The mixture was stirred and reacted for 72 h under a nitrogen atmosphere and at room temperature to obtain a polyamic acid prepolymer solution with anhydride end caps and a solid content of 1% (total solid content of approximately 6 g).

[0076] 1.2) Same as Example 1.

[0077] 1.3) Add 0.016 g (0.078 mmol) of 3,4'-ODA (the molar ratio of the total amount of diamine raw material to the total amount of dianhydride raw material is 1:1) to the obtained slurry precursor, stir and react for 8 h to obtain the negative electrode slurry.

[0078] The preparation of the positive and negative electrodes of the lithium battery, the preparation of the battery itself, and the testing of its charge and discharge characteristics were all the same as in Example 1.

[0079] The test results are as follows: the initial coulomb efficiency is about 85%, the capacity retention rate after 500 cycles is about 89.7%, and the Z-axis (thickness direction) expansion rate of the electrode after 500 cycles is about 167%.

[0080] Example 2

[0081] Same as Example 1, except that in step 1.3), 3,3'-ODPA is replaced with an equimolar amount of MPDA.

[0082] The test results are as follows: the initial coulomb efficiency is 96%, the capacity retention rate after 500 cycles is 98.8%, and the Z-axis (thickness direction) expansion rate of the electrode after 500 cycles is 55%.

[0083] Example 3

[0084] Same as Example 1, except that in step 1.3), 3,3'-ODPA is replaced with an equimolar amount of α-ODPA.

[0085] The test results are as follows: the initial coulomb efficiency is 93%, the capacity retention rate after 500 cycles is 96%, and the Z-axis (thickness direction) expansion rate of the electrode after 500 cycles is 65%.

[0086] Example 4

[0087] Same as Example 1, except that in step 1.1), 3,4'-ASD is used instead of 3,4'-ODA in the same molar amount.

[0088] The test results are as follows: the initial coulomb efficiency is 95%, the capacity retention rate is 98% after 500 cycles, and the Z-axis (thickness direction) expansion rate of the electrode is 61% after 500 cycles.

[0089] Example 5

[0090] 1. Preparation of negative electrode slurry

[0091] 1.1) Under a nitrogen atmosphere and at room temperature, 6.973 g (34.83 mmol) of 3,4'-ODA and 5.022 g (23.22 mmol) of 3,4'-ASD were mixed and dissolved in 970 g of NMP. Then, a total of 17.825 g (57.46 mmol, added in 6 portions) of α-ODPA (the molar ratio of dianhydride to diamine was 0.99:1) was added. The mixture was stirred and reacted for 120 h under a nitrogen atmosphere and at room temperature to obtain a polyamic acid prepolymer solution with amino-terminated ends and a solid content of 3% (the total solid content was approximately 30 g).

[0092] 1.2) Take 500.0g of the amino-terminated polyamic acid prepolymer solution (total solid content of 15.0g) prepared in step 1), 65.0g of the negative electrode active material (32.5g of graphite), and nanoporous silica powder (specific surface area of ​​approximately 58m²). 2 / g, with an average mesopore diameter of approximately 27.0nm (the same below) 32.5g, graphite: nanoporous silica powder = 5:5) and 20.0g conductive agent (conductive carbon black), wherein the active material: binder: conductive agent = 65:15:20 (weight ratio), are mixed evenly without the need for additional NMP. The resulting mixture is then ground and blended to obtain a slurry precursor (viscosity approximately 4018cp).

[0093] 1.3) Add 0.063 g (0.581 mmol) of MPDA (the molar ratio of the total amount of diamine raw material to the total amount of dianhydride raw material is 1:1) to the obtained slurry precursor, stir and react for 0.5 h to obtain the negative electrode slurry.

[0094] The preparation of the positive and negative electrodes of the lithium battery, the preparation of the battery itself, and the testing of its charge and discharge characteristics were all the same as in Example 1.

[0095] The test results are as follows: the initial coulomb efficiency is approximately 95%, the capacity retention rate after 500 cycles is approximately 98.5%, and the Z-axis (thickness direction) expansion rate of the electrode after 500 cycles is approximately 58%.

Claims

1. A method for preparing a negative electrode slurry for lithium-ion batteries, comprising the following steps: 1) An amino-terminated polyamic acid prepolymer solution was prepared by condensation polymerization of an asymmetric aromatic diamine and 2,3,3',4'-diphenyl ether tetracarboxylic dianhydride in a polar aprotic solvent; wherein, The asymmetric aromatic diamine is 3,4'-diaminodiphenyl ether or 3,4'-diaminodiphenyl sulfide, or a combination of both; The amount of the polar aprotic solvent used is such that the solid content in the obtained polyamic acid prepolymer solution is ≤5 wt%; 2) Mix the amino-terminated polyamic acid prepolymer solution with the negative electrode active material and conductive agent evenly, with or without adding polar solvent, and mix evenly to obtain the slurry precursor; 3) An asymmetric isomeric dianhydride end-capping agent is added to the slurry precursor, and the polymerization reaction is stirred to obtain the aforementioned negative electrode slurry for lithium-ion batteries; wherein, The amount of the asymmetric isomeric dianhydride end-capping agent added is controlled to be 1:1, with the total amount of the asymmetric isomeric dianhydride end-capping agent and 2,3,3',4'-diphenyl ether tetracarboxylic dianhydride being used in a molar ratio of 1:1 to the asymmetric aromatic diamine. The asymmetric structural isomeric dianhydride end-capping agent is selected from any one or a combination of two or more of 1,2,3,4-benzenetetracarboxylic dianhydride, 2,3,2',3'-diphenyl ether tetracarboxylic dianhydride, and 2,3,3',4'-diphenyl ether tetracarboxylic dianhydride.

2. The preparation method according to claim 1, characterized in that, In step 1), the condensation polymerization reaction takes 72 hours or more.

3. The preparation method according to claim 2, characterized in that, In step 1), the condensation polymerization reaction takes 84-120 hours.

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

5. The preparation method according to any one of claims 1 to 3, characterized in that, In step 1), the amount of the polar aprotic solvent used is such that the solid content in the obtained polyamic acid prepolymer solution is 1~3wt%.

6. The preparation method according to any one of claims 1 to 3, characterized in that, In step 1), the molar ratio of the asymmetric aromatic diamine and 2,3,3',4'-diphenyl ether tetracarboxylic dianhydride is 1:0.98~0.

99.

7. The negative electrode slurry for lithium-ion batteries prepared by the method according to any one of claims 1 to 6.