Lithium ion battery positive electrode with long cycle life and preparation method thereof

Abstract

The invention discloses a long-cycle-life lithium ion battery positive electrode and a preparation method thereof.The preparation method of the lithium ion battery positive electrode comprises the following steps that S1, a positive electrode active material, a conductive agent, a binder and a solvent are mixed, a bio-based elastomer modifier is added, the bio-based elastomer modifier is a chitosan derivative, and the mixture is stirred to be uniform; stirring and dispersing to prepare positive electrode slurry; s2, a current collector is coated with the positive electrode slurry, drying is conducted in a multi-zone temperature control gradient heating mode, the drying temperature is gradually increased from the first zone to the last zone, and the temperature difference between the adjacent zones is controlled to be 5-15 DEG C; and S3, the coated pole piece is rolled, ultrasonic vibration is applied at the same time, and an included angle of 30-60 degrees is formed between the vibration direction and the transmission direction of the pole piece. The bio-based elastomer modifier is introduced, and gradient heating coating and ultrasonic-assisted compaction technologies are adopted, so that the problem of pole piece cracking is effectively solved, and the cycle life of the lithium ion battery is remarkably prolonged.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of lithium ion batteries, and particularly relates to a lithium ion battery positive electrode with long cycle life and a preparation method thereof. BACKGROUND

[0002] With the rapid development of new energy vehicles and portable electronic devices, lithium ion batteries as mainstream energy storage devices, their performance and life are widely concerned. In the manufacturing process of lithium ion batteries, electrode coating is one of the key processes that determine the performance of the battery. As a common production defect (such as Figure 1 As shown), it will directly affect the quality consistency of the electrode sheet, and then affect the cycle life and safety performance of the battery.

[0003] The specific reasons why the electrode sheet cracking affects the cycle life of the battery are as follows:

[0004] 1) Active material loss: After the electrode sheet cracking, the cracks will isolate part of the active particles, so that these active particles cannot participate in the charge and discharge reaction of the battery, resulting in the decrease of the effective capacity of the battery, and then shortening the cycle life. For example, in a lithium ion battery, after the positive electrode material particles crack, part of the particles are isolated and cannot participate in the full embedding and de-embedding reaction with lithium ions, so the capacity of the battery will gradually decrease.

[0005] 2) Destruction of conductive network: The active material, conductive agent and binder in the electrode sheet form a conductive network to realize the transmission of electrons. The cracking of the electrode sheet will block the electron conduction path, making the electron transmission difficult, resulting in a significant increase in the internal resistance of the battery. The increase in internal resistance will not only generate more heat in the charging and discharging process of the battery, but also affect the charging and discharging efficiency of the battery, accelerate the performance decay of the battery, and ultimately shorten the cycle life.

[0006] 3) Interface side reaction intensifies: After the electrode sheet cracking, the newly generated crack surface will continue to contact and react with the electrolyte, accelerating the thickening of the solid electrolyte interface film (SEI). The thickening of the SEI film will increase the diffusion resistance of lithium ions on the electrode surface, reduce the charging and discharging efficiency of the battery, and also consume more lithium ions, resulting in the decrease of the battery capacity and affecting the cycle life.

[0007] Wherein, before cold pressing, these cracks are usually difficult to be found by naked eyes, and need to be magnified by CCD camera to be seen. The width of the cracks is generally between 30 to 60 microns. After cold pressing, these cracks become more obvious, showing as bright stripes, and even can be felt by hand in serious cases. One of the main reasons for the cracking is that the film is pulled by the tensile force in the MD direction (mechanical direction), and due to the stress between the film and the substrate, the film is bent. In simple terms, the greater the film layer stress, the thicker the film layer, and the thinner the substrate, the more likely to bend. When the film is bent in the middle area, if the film itself is not flexible enough, cracks are likely to occur at this place. In addition, when passing through the roller, the curled edge of the film is straightened, which may also cause cracks due to insufficient flexibility.

[0008] The traditional solutions mainly include: 1) adjusting the coating parameters, such as reducing the coating speed, reducing the coating thickness, etc., but these methods will reduce the production efficiency; 2) increasing the amount of binder, but it will reduce the energy density of the pole piece; 3) changing the drying conditions, such as reducing the drying temperature or prolonging the drying time, but it will also affect the production efficiency. These methods can reduce the pole piece cracking to some extent, but often at the expense of battery performance, and the effect is limited.

[0009] Therefore, there is an urgent need for a new method that can effectively solve the problem of pole piece cracking without affecting the performance of the battery, in order to improve the cycle life of the lithium ion battery. SUMMARY

[0010] The purpose of the present application is to: in view of the shortcomings of the prior art, provide a long cycle life lithium ion battery positive electrode and a preparation method thereof, by introducing a bio-based elastomer modifier, using gradient heating coating and ultrasonic assisted compaction technology, effectively solving the problem of pole piece cracking, significantly improving the cycle life of the lithium ion battery.

[0011] To achieve the above purpose, the present application provides the following technical solutions:

[0012] A preparation method of a long cycle life lithium ion battery positive electrode, comprising the following steps:

[0013] S1. Slurry preparation: mixing positive active material, conductive agent, binder and solvent, adding 0.5-5wt% of bio-based elastomer modifier, the bio-based elastomer modifier is a chitosan derivative, and the positive electrode slurry is prepared by stirring and dispersing;

[0014] S2. Gradient heating coating: coating the positive electrode slurry on the current collector, and drying by gradient heating with multi-zone temperature control, wherein the drying temperature gradually increases from the first zone to the last zone, and the temperature difference between adjacent zones is controlled at 5-15℃;

[0015] S3. Ultrasonic assisted compaction: using an ultrasonic roller device to roll the coated pole piece, the ultrasonic roller device includes a rolling mechanism and an ultrasonic generator, the ultrasonic generator generates 20-60 kHz ultrasonic vibration and transmits to the rolling mechanism, the vibration direction of the ultrasonic vibration is 30-60° with the transmission direction of the pole piece.

[0016] Further, in S1, the chitosan derivative is carboxymethyl chitosan, N-succinyl chitosan, quaternary ammonium chitosan or chitosan-polyethylene glycol copolymer, the molecular weight is 5000-50000 Da, and the degree of deacetylation is 75-95%.

[0017] Further, in S1, the addition amount of the bio-based elastomer modifier is 1-3 wt%, and the bio-based elastomer modifier is subjected to pre-swelling treatment before being added to the slurry, the pre-swelling treatment includes soaking the bio-based elastomer modifier in a solvent for 4-12 hours and ultrasonic dispersion for 30-60 minutes.

[0018] Further, in S2, the drying area of the gradient heating coating is divided into 3-6 intervals, the temperature of the first interval is 40-60℃, and the temperature of the last interval is 100-130℃, and the total drying time is 2-5 minutes.

[0019] Further, in S2, during the gradient heating coating process, the tension of the current collector is controlled to be 10-30 N / m, and a buffer zone is arranged between the coating area and the first drying area, and the length of the buffer zone is 0.5-2 m.

[0020] Further, in S3, the ultrasonic power of the ultrasonic assisted compaction is 200-800 W, the rolling line pressure is 0.5-3 kN / cm, and the rolling speed is 1-10 m / min.

[0021] Further, in S3, the ultrasonic assisted compaction adopts a double-sided ultrasonic roller device, the double-sided ultrasonic roller device includes two ultrasonic generators installed on the two sides of the upper and lower rolling rollers, the phase difference between the upper and lower ultrasonic generators is 90-180°, and the porosity of the compacted pole piece is 20-35%.

[0022] Further, in S1, the positive electrode active material is selected from lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminate, lithium iron phosphate, lithium cobaltate and combinations thereof, the conductive agent is selected from conductive carbon black, graphite, carbon nanotubes, graphene and combinations thereof, and the binder is selected from polyvinylidene fluoride, polyacrylonitrile, polytetrafluoroethylene, sodium carboxymethyl cellulose and combinations thereof.

[0023] Further, before the S1, there is also a step S0 of performing surface modification treatment on the positive active material, including mixing and reacting the positive active material with 0.5-3 wt% of an organic silane coupling agent under inert gas protection for 0.5-2 hours, and the reaction temperature is 60-100 DEG C.

[0024] Further, after the S3, there is also a step S4 of performing vacuum heat treatment on the compacted pole piece, the heat treatment temperature is 80-120 DEG C, the heat treatment time is 1-5 hours, and the vacuum degree is -0.08 to -0.1 MPa.

[0025] In addition, the application also provides a long cycle life lithium ion battery positive electrode prepared by the above preparation method, the bending strength of the positive electrode is greater than or equal to 15 MPa, the crack density of the pole piece surface is less than or equal to 2 lines / cm2, and the average crack width is less than or equal to 10 microns.

[0026] Further, the positive electrode has a three-dimensional network structure, wherein the nanofiber network formed by the bio-based elastomer modifier is crosslinked with the active material particles to form an elastic buffer layer, and the thickness of the elastic buffer layer is 50-200 nm.

[0027] Further, the peeling strength of the positive electrode is greater than or equal to 12 N / cm, the compacted density is 3.0-3.8 g / cm3, and the pole piece surface roughness Ra is less than or equal to 0.5 microns.

[0028] Further, the capacity retention rate of the positive electrode after 1000 cycles at 1C rate is greater than or equal to 90%, and the capacity retention rate after 500 cycles at 3C rate is greater than or equal to 85%.

[0029] The application also provides a lithium ion battery comprising the above positive electrode, and the coulomb efficiency of the lithium ion battery is greater than or equal to 99.5%, and the internal resistance growth rate after 1000 cycles is less than or equal to 30%.

[0030] Compared with the prior art, the application has at least the following beneficial effects:

[0031] 1) The application effectively solves the problem of pole piece cracking by introducing a bio-based elastomer modifier (chitosan derivative) and using gradient heating coating and ultrasonic assisted compaction in a synergistic manner. The crack density of the prepared positive pole piece is significantly reduced to less than or equal to 2 lines / cm2 (the traditional process is usually 15-20 lines / cm2), and the average crack width is reduced to less than or equal to 10 microns (the traditional process is usually 30-60 microns), which greatly improves the structural integrity of the pole piece, thereby significantly improving the cycle life of the lithium ion battery, and the capacity retention rate after 1000 cycles at 1C rate is greater than or equal to 90%, which is much better than the electrode prepared by the traditional process (usually 70-80%).

[0032] 2) In the present application, the bio-based elastomer modifier (chitosan derivative) forms a three-dimensional network structure in the pole piece through its unique molecular structure and abundant functional groups, enhancing the toughness and flexibility of the pole piece. This three-dimensional network structure can effectively absorb and relieve the internal stress generated during drying, preventing the pole piece from cracking. At the same time, the elastic buffer layer (thickness of 50-200 nm) formed between the chitosan derivative and the active material particles can adapt to the volume change of the active material during charging and discharging, reducing stress concentration, and further improving the structural stability and cycle life of the electrode.

[0033] 3) The gradient heating coating technology adopted in the present application gradually increases the drying temperature from the first zone to the last zone by controlling the drying temperature, with a temperature difference of 5-15℃ between adjacent zones, achieving slow and uniform evaporation of the solvent, significantly reducing stress accumulation during the drying process. At the same time, the buffer zone set between the coating area and the first drying area further reduces the stress accumulation of the wet film before entering the drying zone, effectively preventing the pole piece from cracking, and improving the structural integrity and electrochemical performance stability of the pole piece. BRIEF DESCRIPTION OF DRAWINGS

[0034] Figure 1 Structural diagram of the positive pole piece with cracking. DETAILED DESCRIPTION

[0035] The technical solutions of the present application will be described below in conjunction with specific embodiments. Obviously, the described embodiments are only a part of the embodiments of the present application, not all. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the present application.

[0036] I. Preparation method of long cycle life lithium ion battery positive electrode

[0037] The present application provides a preparation method of long cycle life lithium ion battery positive electrode, comprising the following steps:

[0038] S1. Slurry preparation: mix positive active material, conductive agent, binder and solvent, add 0.5-5wt% of bio-based elastomer modifier, the bio-based elastomer modifier is chitosan derivative, prepare positive electrode slurry by stirring and dispersing;

[0039] S2. Gradient heating coating: coat the positive electrode slurry on the current collector, and dry it with a gradient heating device with multiple temperature control zones, the gradient heating device includes 3-6 independent temperature control zones, the drying temperature gradually increases from the first zone to the last zone, and the temperature difference between adjacent zones is controlled within 5-15℃;

[0040] S3. Ultrasonic-assisted compaction: using an ultrasonic roller device to roll the coated pole piece, the ultrasonic roller device includes a rolling mechanism and an ultrasonic generator, the ultrasonic generator generates ultrasonic vibration of 20-60 kHz and transmits to the rolling mechanism, the vibration direction of the ultrasonic vibration forms an angle of 30-60° with the transmission direction of the pole piece.

[0041] In the preferred embodiment of the present application, in S1: the chitosan derivative is carboxymethyl chitosan, N-succinyl chitosan, quaternized chitosan or chitosan-polyethylene glycol copolymer, with a molecular weight of 5000-50000 Da and a degree of deacetylation of 75-95%.

[0042] As a bio-based elastomer modifier, chitosan derivatives have excellent flexibility and elasticity, can form a three-dimensional network structure in the pole piece, enhance the toughness of the pole piece, reduce stress accumulation during drying, and effectively prevent the pole piece from cracking. At the same time, the abundant functional groups (such as hydroxyl, amino, carboxyl, etc.) of chitosan derivatives can form hydrogen bonds or other interactions with the surface of active material particles, enhancing the structural stability of the pole piece.

[0043] Wherein, the determination basis of the molecular weight range (5000-50000 Da): when the molecular weight is less than 5000 Da, the network structure formed by the chitosan derivative is insufficient in strength, and cannot effectively enhance the toughness of the pole piece; when the molecular weight is higher than 50000 Da, the solubility of the chitosan derivative decreases, it is not uniformly dispersed in the slurry, and it significantly increases the slurry viscosity, affecting the coating process. Experiments show that within the range of 5000-50000 Da, the chitosan derivative can form an effective three-dimensional network structure, while maintaining good dispersibility and suitable slurry rheological properties.

[0044] The determination basis of the deacetylation degree range (75-95%): the deacetylation degree affects the solubility, reactivity and mechanical properties of chitosan derivatives. When the deacetylation degree is less than 75%, the solubility and reactivity of chitosan derivatives decrease, making it difficult to form an effective three-dimensional network structure; when the deacetylation degree is higher than 95%, the cost of chitosan derivatives increases significantly, and it may cause the pole piece to be too soft, affecting the compaction density. Experiments show that within the range of 75-95%, chitosan derivatives have the best overall performance, which can effectively enhance the toughness of the pole piece while maintaining suitable mechanical strength.

[0045] In the preferred embodiment of the present application, in S1: the addition amount of the bio-based elastomer modifier is 1-3 wt%, and the bio-based elastomer modifier is subjected to pre-swelling treatment before being added to the slurry, the pre-swelling treatment includes soaking the bio-based elastomer modifier in a solvent for 4-12 hours and ultrasonic dispersion for 30-60 minutes.

[0046] The determination of the addition amount range (1-3 wt%) of the bio-based elastomer modifier is based on the following: when the addition amount is less than 1 wt%, the modification effect is not obvious, and the crack of the pole piece cannot be effectively prevented; when the addition amount is higher than 3 wt%, the slurry viscosity is significantly increased, which affects the coating process, and the conductivity and energy density of the pole piece may be reduced. Experiments show that within the range of 1-3 wt%, the bio-based elastomer modifier can effectively enhance the toughness of the pole piece without significantly affecting the electrochemical performance of the pole piece.

[0047] Specifically, through experiments, it is verified that when the addition amount of the bio-based elastomer modifier is 0.5 wt%, the bending strength of the pole piece is 13.5 MPa, and the crack density is 5.2 pieces / cm2, although it is improved compared to the case without addition, but the effect is limited; when the addition amount is increased to 1 wt%, the bending strength is significantly increased to 16 MPa, and the crack density is reduced to 2.0 pieces / cm2, indicating that 1 wt% is a key threshold; when the addition amount is 3 wt%, the bending strength reaches the highest value of 19 MPa, and the crack density is the lowest of 0.8 pieces / cm2; when the addition amount is further increased to 5 wt%, the bending strength is maintained at 18.8 MPa, but the slurry viscosity is significantly increased to 500 mPa·s (usually 200-350 mPa·s when the addition amount is 1-3 wt%), which affects the stability of the coating process. Therefore, although the improvement effect can be obtained within the range of 0.5-5 wt%, the range of 1-3 wt% has the best comprehensive performance.

[0048] Specifically, there is the following relationship between the addition amount of the bio-based elastomer modifier and the performance of the pole piece:

[0049] 0.5 wt%: bending strength 13.5 MPa, crack density 5.2 pieces / cm2, slurry viscosity 180 mPa·s;

[0050] 1.0 wt%: bending strength 16.0 MPa, crack density 2.0 pieces / cm2, slurry viscosity 220 mPa·s;

[0051] 2.0 wt%: bending strength 18.2 MPa, crack density 1.2 pieces / cm2, slurry viscosity 280 mPa·s;

[0052] 3.0 wt%: bending strength 19.0 MPa, crack density 0.8 pieces / cm2, slurry viscosity 350 mPa·s; 2

[0053] 4.0 wt%: bending strength 19.2 MPa, crack density 0.7 pieces / cm2, slurry viscosity 420 mPa·s;

[0054] ​5.0wt%: bending strength 18.8 MPa, crack density 0.7 line / cm2, slurry viscosity 500 mPa-s.

[0055] From the above data, it can be seen that when the addition amount exceeds 3wt%, the improvement effect of bending strength and crack density tends to be flat, while the slurry viscosity continues to increase significantly, which is not conducive to the stability of the coating process. Therefore, the preferred addition amount range of the present application is 1-3wt%, and the best comprehensive performance can be obtained in this range.

[0056] The purpose of pre-swelling treatment is to make the bio-based elastomer modifier fully swell to form a uniform solution or dispersion, so as to be uniformly dispersed in the slurry in the subsequent mixing process. The determination of pre-swelling treatment parameters is based on the following: when the soaking time is less than 4 hours, the swelling is insufficient and the dispersion is uneven; when the soaking time is more than 12 hours, the efficiency is reduced and there is no obvious improvement; when the ultrasonic dispersion time is less than 30 minutes, the dispersion is insufficient; when the ultrasonic dispersion time is more than 60 minutes, the molecular chain may be degraded. Experiments show that within the above parameter range, the bio-based elastomer modifier can be fully swollen and uniformly dispersed, providing a good prerequisite for subsequent slurry preparation.

[0057] In the preferred embodiment of the present application, in S2, the drying area of gradient heating coating is divided into 3-6 intervals, the temperature of the first interval is 40-60℃, the temperature of the last interval is 100-130℃, and the total drying time is 2-5 minutes.

[0058] The gradient heating coating device includes multiple temperature control areas, and the temperature gradually increases from the first interval to the last interval. This gradient heating method can achieve slow and uniform evaporation of the solvent, significantly reduce stress accumulation during drying, and effectively prevent the electrode from cracking.

[0059] The gradient heating coating device mainly consists of a coating unit, a multi-zone temperature control drying unit, and a transmission system. The coating unit includes a coating head, a coating roller, and a precision gap control mechanism, which is used to uniformly coat the slurry on the current collector; the multi-zone temperature control drying unit consists of 3-6 independent temperature control zones, each zone is equipped with independent heating elements, temperature sensors, and PID temperature controllers, which can achieve accurate temperature control of ±1℃; the transmission system includes a tension control device and a speed control device, which ensures that the current collector maintains stable tension and speed during the entire coating and drying process.

[0060] The working principle of the multi-zone temperature control system is as follows: the PID temperature controller of each zone adjusts the power output of the heating element according to the real-time temperature data fed back by the temperature sensor, so that the temperature of each zone is stabilized at the set value. The temperature difference between adjacent zones is automatically maintained in the range of 5-15℃ by the control system, realizing smooth transition from low temperature to high temperature. The system is also equipped with temperature monitoring and recording functions, which can display the temperature curve of each zone in real time, facilitating the optimization and adjustment of process parameters.

[0061] The determination of the number of drying zones (3-6 zones) is based on the following: when the number of zones is less than 3, the temperature gradient is too large, and slow and uniform evaporation of the solvent cannot be achieved; when the number of zones is more than 6, the complexity of the equipment increases, the cost rises, and the effect improves limitedly. Experiments show that 3-6 zones can achieve the best balance between drying effect and production efficiency.

[0062] The determination of temperature parameters is based on the following: when the temperature of the first zone is lower than 40℃, the drying speed is too slow, affecting production efficiency; when the temperature is higher than 60℃, the initial drying speed is too fast, which can easily lead to surface skinning and increase internal stress; when the temperature of the last zone is lower than 100℃, the solvent cannot be fully removed, affecting the quality of the pole piece; when the temperature is higher than 130℃, the binder or other components may be degraded. Experiments show that within the above temperature range, slow and uniform evaporation of the solvent can be achieved, stress accumulation in the drying process is significantly reduced, and pole piece cracking is effectively prevented.

[0063] The determination of the total drying time (2-5 minutes) is based on the following: when the drying time is less than 2 minutes, the solvent cannot be fully removed, affecting the quality of the pole piece; when the drying time is more than 5 minutes, the production efficiency is reduced, and the pole piece may be over-dried, increasing brittleness. Experiments show that within the range of 2-5 minutes, the balance between sufficient removal of the solvent and production efficiency can be achieved.

[0064] In the preferred embodiment of the present application, in S2, the gradient heating coating process controls the current collector tension to be 10-30 N / m, and a buffer zone is set between the coating area and the first drying area, with a length of 0.5-2 m.

[0065] The control of current collector tension and the setting of buffer zone are one of the important innovations of the present application. Proper tension control can reduce deformation of the current collector during transmission and reduce stress accumulation; the setting of the buffer zone can make the wet film stable before entering the drying zone, further reducing stress accumulation. The structure of the buffer zone is a transmission area without heating elements, located between the coating area and the first drying area; the length of the buffer zone is 0.5-2 m. In this area, the wet film after coating can be transmitted stably, the components in the slurry can be fully spread, and the internal stress can be reduced, providing a good initial state for subsequent gradient heating drying. The buffer zone is also equipped with humidity and temperature monitoring devices to ensure the stability of the environmental conditions.

[0066] The determination basis of the current collector tension range (10-30 N / m) is that when the tension is lower than 10 N / m, the current collector transmission is unstable, which may cause uneven coating; when the tension is higher than 30 N / m, the current collector deformation increases, the stress accumulation increases, and the pole piece is prone to cracking. Experiments show that within the range of 10-30 N / m, the current collector transmission is stable, and the stress accumulation is small, which is beneficial to prevent the pole piece from cracking.

[0067] The determination basis of the buffer zone length range (0.5-2 m) is that when the buffer zone length is less than 0.5 m, the wet film is not stable enough, and the effect is not obvious; when the buffer zone length is greater than 2 m, the equipment occupies an area, the cost increases, and the effect is limited. Experiments show that the buffer zone length of 0.5-2 m can make the wet film fully stable before entering the drying zone, and effectively reduce the stress accumulation.

[0068] In the preferred embodiment of the present application, in S3, the ultrasonic power of the ultrasonic assisted compaction is 200-800 W, the roll pressure is 0.5-3 kN / cm, and the roll speed is 1-10 m / min.

[0069] Among them, the ultrasonic assisted compaction is another innovation point of the present application, which can make the pole piece material produce micro flow in the compaction process through ultrasonic vibration, fill the micro cracks, and at the same time reduce the stress concentration, further improve the structural integrity of the pole piece.

[0070] Among them, the structure of the ultrasonic assisted compaction device includes a roll pressing mechanism, an ultrasonic generator, an angle adjusting mechanism and a control system. The roll pressing mechanism is composed of upper and lower rollers for applying pressure to the pole piece; the ultrasonic generator is fixed on the mounting bracket and connected with the roller through the waveguide rod to transmit ultrasonic vibration to the roller; the angle adjusting mechanism is composed of a precision rotary table and a fixed support, which can adjust the installation angle of the ultrasonic generator to form a required angle between the vibration direction and the pole piece transmission direction; the control system is used to adjust the frequency, power and vibration direction of the ultrasonic wave.

[0071] The implementation mode of the vibration direction and the pole piece transmission direction forming an angle of 30-60° is that the installation angle of the ultrasonic generator is adjusted through the angle adjusting mechanism to form a preset angle between the waveguide rod and the pole piece transmission direction. Specifically, the angle adjusting mechanism includes a rotary base with an angle scale, the ultrasonic generator is installed on the rotary base through the fixed support, and the ultrasonic generator can be fixed at the required angle through the locking bolt on the rotary base. In actual operation, the best vibration angle can be selected within the range of 30-60° according to the characteristics of different material systems, the NCM system usually selects an angle of 40-50°, and the LFP system usually selects an angle of 45-55°.

[0072] The selection of the ultrasonic frequency range (20-60 kHz) is based on the following technical considerations: when the frequency is lower than 20 kHz, the vibration enters the audible range of the human ear, which can produce obvious noise and affect the working environment; at the same time, the vibration amplitude of low-frequency ultrasonic waves is large, which can cause local damage to the pole piece material; when the frequency is higher than 60 kHz, the attenuation of ultrasonic waves in solid media increases significantly, making it difficult to effectively transmit to the entire pole piece, and the cost of high-frequency ultrasonic wave generators increases significantly. Experimental data show that within the range of 20-60 kHz, ultrasonic waves can effectively promote the micro-flow of the pole piece material and fill small cracks, while not causing damage to the pole piece structure. Specifically, 20-30 kHz is suitable for pole pieces with a thickness of >80 μm, 30-45 kHz is suitable for pole pieces with a thickness of 50-80 μm, and 45-60 kHz is suitable for thin pole pieces (<50 μm).

[0073] wherein the effects of different ultrasonic frequencies on the performance of the pole piece are as follows:

[0074] 15 kHz: the vibration amplitude is too large, causing local damage to the pole piece, and the noise is obvious;

[0075] 20 kHz: suitable for pole pieces with a thickness of >80 μm, uniform compaction, and no obvious damage;

[0076] 30 kHz: suitable for pole pieces with a thickness of 60-80 μm, with the best compaction effect;

[0077] 40 kHz: suitable for pole pieces with a thickness of 40-60 μm, with the best compaction effect;

[0078] 50 kHz: suitable for pole pieces with a thickness of 20-40 μm, with the best compaction effect;

[0079] 60 kHz: suitable for pole pieces with a thickness of <20 μm, with good compaction effect;

[0080] 70 kHz: ultrasonic attenuation is obvious, compaction effect decreases, and equipment cost increases.

[0081] Therefore, the present application selects 20-60 kHz as the ultrasonic frequency range, within which the optimal frequency can be selected according to the thickness of the pole piece to achieve the best compaction effect.

[0082] The determination of the ultrasonic power range (200-800 W) is as follows: when the power is lower than 200 W, the vibration energy is insufficient to effectively promote the micro-flow of the material; when the power is higher than 800 W, the pole piece can be overheated or locally damaged. Experiments show that within the range of 200-800 W, ultrasonic waves can effectively promote the micro-flow of the material and fill small cracks, while not causing damage to the pole piece.

[0083] The determination basis of the rolling line pressure range (0.5-3 kN / cm) is that when the pressure is lower than 0.5 kN / cm, the compaction effect is insufficient, and the pole piece density is too low; and when the pressure is higher than 3 kN / cm, the pole piece may be excessively compacted, the brittleness is increased, and cracking is aggravated. Experiments show that in the range of 0.5-3 kN / cm, the appropriate compaction density can be realized, and the flexibility of the pole piece is maintained.

[0084] The determination basis of the rolling speed range (1-10 m / min) is that when the speed is lower than 1 m / min, the production efficiency is too low; and when the speed is higher than 10 m / min, the ultrasonic action time is insufficient, and the effect is reduced. Experiments show that in the range of 1-10 m / min, the effective action of the ultrasonic wave and the balance of the production efficiency can be realized.

[0085] In the preferred embodiment of the application, in S3, the ultrasonic wave assisted compaction adopts a double-sided ultrasonic rolling device, the double-sided ultrasonic rolling device includes two upper and lower ultrasonic generators respectively installed on the two sides of the upper and lower rolling rollers, the phase difference of the upper and lower ultrasonic generators is 90-180°, and the porosity of the pole piece after compaction is 20-35%.

[0086] When the double-sided ultrasonic rolling device is adopted, the two upper and lower ultrasonic generators are respectively installed on the two sides of the upper and lower rolling rollers, and the vibration directions of the two are adjusted through independent angle adjusting mechanisms. The phase difference of the upper and lower ultrasonic generators is realized through a phase adjuster in the control system, and can be accurately adjusted in the range of 90-180°. The purpose of setting the phase difference is to avoid mutual cancellation of the upper and lower vibrations, and to optimize the vibration effect. Experiments show that when the phase difference is set to 180°, the upper and lower vibrations are completely staggered, and the best compaction effect can be realized.

[0087] The double-sided ultrasonic rolling device is a further improvement of the single-sided ultrasonic rolling, and through the synergistic effect of the two upper and lower ultrasonic generators, the micro flow of the pole piece material can be more uniformly promoted, and the compaction effect is improved. The phase difference setting can avoid mutual cancellation of the upper and lower vibrations, and optimize the vibration effect.

[0088] The determination basis of the phase difference range (90-180°) is that when the phase difference is less than 90°, the upper and lower vibrations may be partially cancelled, and the effect is reduced; and when the phase difference is equal to 180°, the upper and lower vibrations are completely staggered, and the effect is best. Experiments show that in the range of 90-180°, the upper and lower ultrasonic generators can work effectively and cooperatively, and the compaction effect is improved.

[0089] The determination basis of the porosity range (20-35%) is that when the porosity is lower than 20%, the electrode sheet is excessively compacted, the electrolyte is blocked from being soaked and ion transmission, and the brittleness is increased; when the porosity is higher than 35%, the compaction is insufficient, the conductive network is imperfect, and the internal resistance is increased. Experiments show that within the range of 20-35%, the electrode sheet can maintain good mechanical strength and conductivity, and can provide sufficient space for electrolyte soaking and ion transmission.

[0090] In the preferred embodiment of the present application, in S1, the positive active material is selected from lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium iron phosphate, lithium cobaltate, and combinations thereof, the conductive agent is selected from conductive carbon black, graphite, carbon nanotubes, graphene, and combinations thereof, and the binder is selected from polyvinylidene fluoride, polyacrylonitrile, polytetrafluoroethylene, sodium carboxymethyl cellulose, and combinations thereof.

[0091] The method of the present application is suitable for various common lithium ion battery positive electrode material systems and has wide applicability. The selection of different material systems is mainly based on the specific application requirements of the battery, such as high energy density, high power density, long cycle life, etc.

[0092] In the preferred embodiment of the present application, before S1, there is also a step S0 of performing surface modification treatment on the positive active material, which includes mixing and reacting the positive active material with 0.5-3wt% of an organic silane coupling agent under inert gas protection for 0.5-2 hours, and the reaction temperature is 60-100℃.

[0093] The active material surface modification treatment is a further optimization of the method of the present application. The organic silane coupling agent can form a chemically bonded modified layer on the surface of the active material, enhance the interfacial bonding force between the active material and the binder and the bio-based elastomer modifier, and further improve the structural stability and cycle life of the electrode sheet.

[0094] The determination basis of the organic silane coupling agent addition amount range (0.5-3wt%) is that when the addition amount is lower than 0.5wt%, the modification effect is not obvious; when the addition amount is higher than 3wt%, a too thick surface layer may be formed, affecting ion transmission. Experiments show that within the range of 0.5-3wt%, the organic silane coupling agent can form a surface modified layer of appropriate thickness, effectively enhance the interfacial bonding force, and at the same time, will not significantly affect ion transmission.

[0095] The determination basis of the reaction parameters is that when the reaction time is lower than 0.5 hours, the reaction is insufficient; when the reaction time is higher than 2 hours, the efficiency is reduced and there is no obvious improvement; when the reaction temperature is lower than 60℃, the reaction rate is too slow; when the reaction temperature is higher than 100℃, the coupling agent may be decomposed or the active material may be oxidized. Experiments show that within the above parameter range, the organic silane coupling agent can effectively react with the surface of the active material to form a stable surface modified layer.

[0096] In a preferred embodiment of the present application, after the S3, a step S4 is further included: vacuum heat treatment is performed on the compacted pole piece, the heat treatment temperature is 80-120℃, the heat treatment time is 1-5 hours, and the vacuum degree is -0.08 to -0.1 MPa.

[0097] The vacuum heat treatment is another optimization of the method of the present application. Through the vacuum heat treatment, the residual solvent and moisture in the pole piece can be further removed, and the cross-linking and crystallization of the binder and the bio-based elastomer modifier are promoted, and the structural stability of the pole piece is enhanced.

[0098] The determination of the heat treatment parameters is as follows: when the temperature is lower than 80℃, the heat treatment effect is not obvious; when the temperature is higher than 120℃, some components may be degraded; when the time is lower than 1 hour, the treatment is insufficient; when the time is higher than 5 hours, the efficiency is reduced and there is no obvious improvement; when the vacuum degree is lower than -0.08 MPa, the residual gas is not removed sufficiently; and when the vacuum degree is higher than -0.1 MPa, the equipment requirement is increased and the cost is increased. Experiments show that within the above parameter range, the residual solvent and moisture can be effectively removed, the cross-linking and crystallization of the components are promoted, and the structural stability of the pole piece is enhanced.

[0099] II. A lithium ion battery positive electrode with long cycle life

[0100] The present application also provides a lithium ion battery positive electrode with long cycle life, which is prepared by the above preparation method, and the bending strength of the positive electrode is ≥15 MPa, the crack density on the surface of the pole piece is ≤2 strips / cm2, and the average crack width is ≤10 μm.

[0101] The bending strength, crack density and crack width are important indicators for evaluating the anti-cracking performance of the pole piece. The present application significantly improves the bending strength of the pole piece, reduces the crack density and width, and thus improves the structural integrity and cycle life of the pole piece, through the synergistic effect of the introduction of the bio-based elastomer modifier, gradient heating coating and ultrasonic assisted compaction.

[0102] The determination of the bending strength index (≥15 MPa) is as follows: the bending strength of the pole piece prepared by the traditional process is usually 8-12 MPa, which cannot effectively resist the stress in the drying and rolling process. Experiments show that when the bending strength reaches or exceeds 15 MPa, the pole piece can effectively resist the stress in various process, and the risk of cracking is significantly reduced.

[0103] The determination of the crack density index (≤2 strips / cm2) is as follows: the crack density of the pole piece prepared by the traditional process is usually 15-20 strips / cm2, which seriously affects the cycle life of the battery. Experiments show that when the crack density is reduced to 2 strips / cm2 or below, the structural integrity of the pole piece is significantly improved, and the cycle life of the battery is greatly prolonged.

[0104] The determination basis of the average crack width index (≤10 μm) is that the crack width of the pole piece prepared by the traditional process is usually 30-60 μm, and the crack with such a width can significantly block the electron conduction path and increase the battery internal resistance. Experiments show that when the crack width is reduced to 10 μm or below, the blocking effect on the electron conduction path is greatly reduced, and the growth rate of the battery internal resistance is significantly reduced.

[0105] In the preferred embodiment of the present application, the positive electrode has a three-dimensional network structure, in which the nanofiber network formed by the bio-based elastomer modifier is cross-linked with the active material particles to form an elastic buffer layer with a thickness of 50-200 nm.

[0106] The three-dimensional network structure formed by the bio-based elastomer modifier in the pole piece is one of the core innovations of the present application. This three-dimensional network structure not only enhances the toughness of the pole piece and reduces stress accumulation during the drying process, but also forms an elastic buffer layer on the surface of the active material particles, which can adapt to the volume change of the active material during the charging and discharging process, reduce stress concentration, and further improve the structural stability and cycle life of the electrode.

[0107] The determination basis of the thickness range of the elastic buffer layer (50-200 nm) is that when the thickness is less than 50 nm, the buffering effect is not obvious, and when the thickness is more than 200 nm, it may affect ion transmission and electron conduction. Experiments show that within the range of 50-200 nm, the elastic buffer layer can effectively buffer the volume change of the active material without significantly affecting ion transmission and electron conduction, achieving the best overall performance.

[0108] In the preferred embodiment of the present application, the positive electrode has a peel strength ≥12 N / cm, a compaction density of 3.0-3.8 g / cm3, and a pole piece surface roughness Ra≤0.5 μm.

[0109] Peel strength, compaction density and surface roughness are important indicators for evaluating the quality of the pole piece. The present application significantly improves the peel strength and compaction density of the pole piece and reduces the surface roughness by introducing the bio-based elastomer modifier and ultrasonic assisted compaction, thereby improving the structural stability and electrochemical performance of the pole piece.

[0110] The determination basis of the peel strength index (≥12 N / cm) is that the peel strength of the pole piece prepared by the traditional process is usually 8-10 N / cm, which is prone to cause active material shedding during the charging and discharging process. Experiments show that when the peel strength reaches or exceeds 12 N / cm, the active material shedding phenomenon of the pole piece during the long-period charging and discharging process is significantly reduced, which is beneficial to maintaining the capacity stability of the battery.

[0111] The determination basis of the compaction density range (3.0-3.8 g / cm3) is that when the compaction density is lower than 3.0 g / cm3, the conductive network of the pole piece is imperfect, and the internal resistance is large; when the compaction density is higher than 3.8 g / cm3, the pole piece is excessively compacted, and the electrolyte infiltration and ion transmission are blocked. Experiments show that within the range of 3.0-3.8 g / cm3, the pole piece can form a good conductive network, and can provide sufficient space for electrolyte infiltration and ion transmission, so that the best electrochemical performance is realized.

[0112] The determination basis of the surface roughness index (Ra≤0.5 μm) is that the surface roughness of the pole piece prepared by the traditional process is usually 0.8-1.2 μm, and the uneven surface will lead to uneven contact of the diaphragm, and increase the risk of local short circuit. Experiments show that when the surface roughness is reduced to 0.5 μm or below, the surface flatness of the pole piece is significantly improved, the contact with the diaphragm is more uniform, the risk of local short circuit is reduced, and the safety performance of the battery is improved.

[0113] In the preferred embodiment of the application, the capacity retention rate of the positive electrode after 1000 cycles at 1C rate is ≥90%, and the capacity retention rate after 500 cycles at 3C rate is ≥85%.

[0114] The cycle life is one of the core indicators for evaluating the performance of lithium ion batteries. The cycle life of the lithium ion battery is significantly improved by effectively solving the problem of pole piece cracking.

[0115] The determination basis of the capacity retention rate index (≥90%) after 1000 cycles at 1C rate is that the capacity retention rate of the electrode prepared by the traditional process after 1000 cycles at 1C rate is usually 70-80%, which cannot meet the demand of long-life application. Experiments show that when the capacity retention rate reaches or exceeds 90%, the battery can meet the stringent requirements of electric vehicles and the like on long cycle life.

[0116] The determination basis of the capacity retention rate index (≥85%) after 500 cycles at 3C rate is that the capacity retention rate of the electrode prepared by the traditional process after 500 cycles at 3C rate is usually 60-70%, and the high-rate cycle performance is poor. Experiments show that when the capacity retention rate reaches or exceeds 85%, the battery can meet the requirements of fast charging application while maintaining a long service life.

[0117] III. Lithium ion battery

[0118] The application further provides a lithium ion battery comprising the above-mentioned positive electrode, wherein the coulomb efficiency of the lithium ion battery is ≥99.5%, and the internal resistance growth rate after 1000 cycles is ≤30%.

[0119] Coulomb efficiency and internal resistance growth rate are important indicators for evaluating the stability of lithium-ion battery performance. The present application improves the structural integrity of the pole piece, reduces the interface side reaction, thereby improving the coulomb efficiency and reducing the internal resistance growth rate.

[0120] The determination basis of coulomb efficiency index (≥99.5%): The coulomb efficiency of the battery prepared by traditional process is usually 98.5-99.2%, and a certain amount of lithium ion is lost irreversibly each cycle. Experiments show that when the coulomb efficiency reaches or exceeds 99.5%, the irreversible loss of lithium ion is significantly reduced, which is beneficial to maintain the capacity stability of the battery.

[0121] The determination basis of internal resistance growth rate index (≤30%): The internal resistance growth rate of the battery prepared by traditional process is usually 50-80% after 1000 cycles, and the significant increase of internal resistance will lead to the performance degradation of the battery. Experiments show that when the internal resistance growth rate is controlled at 30% or below, the performance degradation of the battery during long-period use is significantly slowed down, which is beneficial to prolong the actual service life of the battery.

[0122] The present application will be further described in detail in combination with specific examples:

[0123] Example 1: Preparation of NCM811 positive electrode

[0124] Step S0: surface modification treatment of active material

[0125] The NCM811 positive electrode active material is mixed with 1.5wt% of gamma-aminopropyl triethoxysilane under nitrogen protection, the reaction temperature is 80℃, and the reaction time is 1 hour, to obtain the surface modified NCM811 active material.

[0126] Step S1: preparation of slurry

[0127] Raw materials: surface modified NCM811 positive electrode active material, conductive carbon black, and polyvinylidene fluoride (PVDF) are mixed in a mass ratio of 92:4:4, and 2wt% of carboxymethyl chitosan (molecular weight 20000Da, deacetylation degree 85%) is added as a bio-based elastomer modifier;

[0128] Pre-swelling treatment: the carboxymethyl chitosan is soaked in N-methyl pyrrolidone (NMP) for 8 hours and ultrasonically dispersed for 45 minutes;

[0129] Mixing: use a planetary mixer (speed 500rpm, temperature 25℃) to stir for 2 hours to obtain a positive electrode slurry with a solid content of 65%.

[0130] Step S2: gradient heating coating

[0131] Coating equipment: gradient heating coater with four-zone temperature control;

[0132] Coating parameters: the current collector was an aluminum foil with a thickness of 16 μm, the tension control was 20 N / m, the coating thickness was 80 μm, and the coating speed was 5 m / min;

[0133] Drying parameters: the first zone temperature was 50 °C, the second zone temperature was 70 °C, the third zone temperature was 90 °C, the fourth zone temperature was 110 °C, the drying time of each zone was 45 seconds, and the total drying time was 3 minutes;

[0134] Buffer zone: a buffer zone with a length of 1.5 m was arranged between the coating area and the first drying area.

[0135] Step S3: ultrasonic-assisted compaction

[0136] Compaction equipment: a double-sided ultrasonic roller compaction device was used, and the phase difference of the upper and lower ultrasonic generators was 180°;

[0137] Compaction parameters: the ultrasonic frequency was 40 kHz, the power was 500 W, the vibration direction was at an angle of 45° to the transmission direction of the pole piece, the roller pressure was 1.5 kN / cm, and the roller speed was 3 m / min;

[0138] Compaction results: the pole piece thickness was 60 μm, the compaction density was 3.5 g / cm3, and the porosity was 25%.

[0139] Step S4: vacuum heat treatment

[0140] Heat treatment equipment: a vacuum heat treatment furnace was used;

[0141] Heat treatment parameters: the temperature was 100 °C, the time was 3 hours, and the vacuum degree was -0.09 MPa.

[0142] Main performance parameters of the obtained positive electrode: the bending strength was 18 MPa, the pole piece surface crack density was 1.2 cracks / cm2, the average crack width was 8 μm, the peeling strength was 14 N / cm, and the surface roughness Ra was 0.3 μm.

[0143] Example 2: preparation of an NCM622 positive electrode

[0144] Step S0: surface modification treatment of the active material

[0145] The NCM622 positive electrode active material was mixed with 2 wt% of γ-glycidoxypropyltrimethoxysilane under nitrogen protection, the reaction temperature was 90 °C, and the reaction time was 1.5 hours, to obtain a surface-modified NCM622 active material.

[0146] Step S1: slurry preparation

[0147] Raw materials: surface-modified NCM622 positive electrode active material, conductive carbon black, polyvinylidene fluoride (PVDF) mixed in a mass ratio of 93:3:4, 1.5wt% N-succinyl chitosan (molecular weight 15000Da, degree of deacetylation 80%) was added as a bio-based elastomer modifier;

[0148] Pre-swelling treatment: N-succinyl chitosan was soaked in N-methyl pyrrolidone (NMP) for 6 hours and ultrasonically dispersed for 40 minutes;

[0149] Mixing: stirring for 2.5 hours using a planetary mixer (speed 450 rpm, temperature 23℃) to obtain a positive electrode slurry with a solid content of 63%.

[0150] Step S2: Gradient heating coating

[0151] Coating equipment: gradient heating coater with five temperature control zones;

[0152] Coating parameters: the current collector is an aluminum foil with a thickness of 15μm, the tension control is 15N / m, the coating thickness is 85μm, and the coating speed is 6m / min;

[0153] Drying parameters: the first zone temperature is 45℃, the second zone temperature is 60℃, the third zone temperature is 75℃, the fourth zone temperature is 90℃, the fifth zone temperature is 105℃, and the drying time of each zone is 40 seconds, the total drying time is 3.3 minutes;

[0154] Buffer zone: a buffer zone with a length of 1.2m is set between the coating area and the first drying area.

[0155] Step S3: ultrasonic assisted compaction

[0156] Compaction equipment: double-sided ultrasonic roller compaction device, the phase difference between the upper and lower ultrasonic generators is 150°;

[0157] Compaction parameters: ultrasonic frequency is 35kHz, power is 450W, vibration direction is 40° to the transmission direction of the pole piece, roller pressure is 1.8kN / cm, and roller speed is 4m / min;

[0158] Compaction results: the thickness of the pole piece is 62μm, the compaction density is 3.3g / cm 3 , and the porosity is 28%.

[0159] Step S4: vacuum heat treatment

[0160] Heat treatment equipment: vacuum heat treatment furnace;

[0161] Heat treatment parameters: temperature is 95℃, time is 2.5 hours, and vacuum degree is -0.085MPa.

[0162] The main performance parameters of the obtained positive electrode are as follows: bending strength of 17 MPa, surface crack density of 1.5 pieces / cm2, average crack width of 9 μm, peeling strength of 13.5 N / cm, and surface roughness Ra of 0.35 μm.

[0163] Example 3: Preparation of LiFePO4 positive electrode

[0164] Step S0: surface modification treatment of active material

[0165] The LiFePO4 positive electrode active material is mixed with 1 wt% of γ-methacryloxypropyltrimethoxysilane under nitrogen protection, the reaction temperature is 70°C, and the reaction time is 1 hour, to obtain the surface-modified LiFePO4 active material.

[0166] Step S1: preparation of slurry

[0167] Raw materials: surface-modified LiFePO4 positive electrode active material, conductive carbon black, and sodium carboxymethyl cellulose (CMC) are mixed in a mass ratio of 90:6:4, and 3 wt% of quaternized chitosan (molecular weight 25000 Da, deacetylation degree 90%) is added as a bio-based elastomer modifier;

[0168] Pre-swelling treatment: the quaternized chitosan is soaked in deionized water for 10 hours and ultrasonically dispersed for 50 minutes;

[0169] Mixing: using a planetary mixer (rotation speed 400 rpm, temperature 22°C) to stir for 3 hours, to obtain a positive electrode slurry with a solid content of 60%.

[0170] Step S2: gradient heating coating

[0171] Coating equipment: gradient heating coater with three-zone temperature control;

[0172] Coating parameters: the current collector is an aluminum foil with a thickness of 14 μm, the tension control is 25 N / m, the coating thickness is 90 μm, and the coating speed is 4 m / min;

[0173] Drying parameters: the first zone temperature is 55°C, the second zone temperature is 85°C, the third zone temperature is 115°C, and the drying time of each zone is 60 seconds, and the total drying time is 3 minutes;

[0174] Buffer zone: a buffer zone with a length of 0.8 m is set between the coating area and the first drying area.

[0175] Step S3: ultrasonic-assisted compaction

[0176] Compaction equipment: double-sided ultrasonic roller compaction device, the phase difference of the upper and lower ultrasonic generators is 120°;

[0177] Compaction parameters: ultrasonic frequency of 50 kHz, power of 600 W, vibration direction at an angle of 50° with the transmission direction of the pole piece, roller pressure of 1.2 kN / cm, and roller speed of 2 m / min;

[0178] Compaction results: pole piece thickness of 65 μm, compaction density of 3.0 g / cm3, and porosity of 32%.

[0179] Step S4: vacuum heat treatment

[0180] Heat treatment equipment: vacuum heat treatment furnace was used;

[0181] Heat treatment parameters: temperature of 110 °C, time of 4 hours, and vacuum degree of -0.095 MPa.

[0182] Main performance parameters of the obtained positive electrode: bending strength of 16 MPa, pole piece surface crack density of 1.8 pieces / cm2, average crack width of 9.5 μm, peeling strength of 12.8 N / cm, and surface roughness Ra of 0.4 μm.

[0183] Example 4: preparation of NCA positive electrode

[0184] Step S0: surface modification treatment of active material

[0185] The NCA positive electrode active material was mixed with 2.5 wt% of γ-mercaptopropyltrimethoxysilane under nitrogen protection, the reaction temperature was 85 °C, and the reaction time was 1.2 hours, to obtain the surface-modified NCA active material.

[0186] Step S1: slurry preparation

[0187] Raw materials: surface-modified NCA positive electrode active material, conductive carbon black, and polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 91:4:5, and 2.5 wt% of chitosan-polyethylene glycol copolymer (molecular weight of 30000 Da, degree of deacetylation of 85%) was added as a bio-based elastomer modifier;

[0188] Pre-swelling treatment: the chitosan-polyethylene glycol copolymer was soaked in N-methylpyrrolidone (NMP) for 12 hours and ultrasonically dispersed for 60 minutes;

[0189] Mixing: a planetary mixer (speed of 480 rpm, temperature of 24 °C) was used for stirring for 2.2 hours, to obtain a positive electrode slurry with a solid content of 64%.

[0190] Step S2: gradient heating coating

[0191] Coating equipment: a gradient heating coater with six temperature zones was used;

[0192] Coating parameters: the current collector was aluminum foil with a thickness of 16 μm, the tension control was 18 N / m, the coating thickness was 82 μm, and the coating speed was 5.5 m / min; drying parameters: the first zone temperature was 40℃, the second zone temperature was 55℃, the third zone temperature was 70℃, the fourth zone temperature was 85℃, the fifth zone temperature was 100℃, the sixth zone temperature was 120℃, and the drying time of each zone was 30 seconds, and the total drying time was 3 minutes;

[0193] Buffer zone: a buffer zone with a length of 1.8 m was arranged between the coating area and the first drying area.

[0194] Step S3: ultrasonic-assisted compaction

[0195] Compaction equipment: a double-sided ultrasonic roller compaction device was used, and the phase difference of the upper and lower ultrasonic generators was 90°;

[0196] Compaction parameters: the ultrasonic frequency was 30 kHz, the power was 400 W, the vibration direction was at an angle of 35° with the transmission direction of the pole piece, the roller pressure was 2.0 kN / cm, and the roller speed was 3.5 m / min;

[0197] Compaction results: the thickness of the pole piece was 58 μm, the compaction density was 3.7 g / cm3, and the porosity was 22%.

[0198] Step S4: vacuum heat treatment

[0199] Heat treatment equipment: a vacuum heat treatment furnace was used;

[0200] Heat treatment parameters: the temperature was 105℃, the time was 3.5 hours, and the vacuum degree was -0.09 MPa.

[0201] Main performance parameters of the obtained positive electrode: the bending strength was 19 MPa, the surface crack density of the pole piece was 1.0 piece / cm2, the average crack width was 7 μm, the peeling strength was 14.5 N / cm, and the surface roughness Ra was 0.25 μm.

[0202] Comparative Example 1: preparation of NCM811 positive electrode by traditional process

[0203] The NCM811 positive electrode was prepared by a conventional process without adding a bio-based elastomer modifier, and single-zone constant temperature drying (120℃, 3 minutes) and ordinary roller compaction (pressure 2.0 kN / cm without ultrasonic assistance) were used.

[0204] Comparative Example 2: preparation of NCM811 positive electrode by partially improved process

[0205] The S1 slurry preparation process of Example 1 (adding a bio-based elastomer modifier) was used, but single-zone constant temperature drying (120℃, 3 minutes) and ordinary roller compaction (pressure 2.0 kN / cm without ultrasonic assistance) were used.

[0206] Comparative Example 3: Preparation of NCM811 cathode by partially improved process

[0207] The S2 gradient heating coating process of Example 1 was adopted, but no bio-based elastomer modifier was added, and ordinary rolling (pressure 2.0 kN / cm, without ultrasonic assistance) was adopted.

[0208] The cathodes prepared in each of the above examples and comparative examples were subjected to the following performance tests, and the test results are shown in Tables 1-3.

[0209] 1) Analysis of surface cracks of the pole piece: a high-resolution optical microscope (Keyence VHX-7000) was used to observe the surface of the pole piece, and the number and average width of cracks per unit area (1 cm 2 ) were counted.

[0210] 2) Bending strength test: a three-point bending test method was adopted, and a material testing machine (Instron5967) was used to measure the bending strength of the pole piece.

[0211] 3) Peeling strength test: a 180° peeling test method was adopted, and a peeling strength tester (Thwing-Albert EJA) was used to measure the peeling strength of the pole piece.

[0212] 4) Cycle life test: the prepared cathode was assembled into a button half-cell with lithium metal, and tested at 25°C using a LAND-CT2001A battery test system. The capacity retention rate was recorded after 1000 cycles at 1C rate and 500 cycles at 3C rate. The test voltage range: NCM811, NCM622, NCA is 3.0-4.3V, LiFePO4 is 2.5-3.65V.

[0213] 5) Coulomb efficiency and internal resistance test: at 25°C, the LAND-CT2001A battery test system was used to measure the coulomb efficiency, and the electrochemical workstation (Biologic VSP-300) was used for electrochemical impedance spectroscopy (EIS) test, and the internal resistance growth rate after 1000 cycles was calculated.

[0214] Table 1: Structural parameters of each sample

[0215]

[0216]

[0217] Table 2: Electrochemical performance of each sample

[0218]

[0219] Table 3 Capacity retention rate (%) at different rates

[0220] Sample 0.2C 0.5C 1C 2C 3C 5C Example 1 100.0 98.5 96.0 92.5 87.0 78.5 Example 2 100.0 98.0 95.5 91.8 86.5 77.0 Example 3 100.0 99.0 97.5 94.0 88.5 80.0 Example 4 100.0 98.2 95.8 92.0 86.0 76.5 Comparative Example 1 100.0 95.0 88.0 75.0 62.0 45.0 Comparative Example 2 100.0 96.5 92.0 84.0 75.0 60.0 Comparative Example 3 100.0 96.0 90.0 80.0 70.0 55.0

[0221] From the test results in Tables 1-3, it can be seen that:

[0222] 1) The bending strength of the positive electrode tab prepared in Examples 1-4 of the present application is significantly improved, reaching 16-19 MPa, which is much higher than 10 MPa of Comparative Example 1, and the surface crack density is significantly reduced to 1.0-1.8 strips / cm, which is much lower than 18.5 strips / cm of Comparative Example 1. 2 2 The significant improvement in these structural parameters indicates that the method of the present application can effectively solve the problem of tab cracking and improve the structural integrity of the tab.

[0223] 2) In terms of electrochemical performance, the capacity retention rate of the positive electrode prepared in Examples 1-4 of the present application after 1000 cycles at 1C rate reaches 91.8-93.5%, and the capacity retention rate after 500 cycles at 3C rate reaches 86.0-88.5%, which is much better than that of the sample of Comparative Example. This excellent cycle life performance is mainly due to the following points: first, the tab cracking is significantly reduced, maintaining the integrity of the electronic conduction network; second, the three-dimensional network structure formed by the bio-based elastomer modifier enhances the structural stability of the tab; third, gradient heating coating and ultrasonic assisted compaction optimize the internal structure of the tab, reducing stress accumulation.

[0224] 3) In terms of coulomb efficiency and internal resistance, the coulomb efficiency of the positive electrode prepared in Examples 1-4 of the present application reaches 99.6-99.8%, and the internal resistance growth rate after 1000 cycles is only 22.0-28.0%, which is much better than that of the sample of Comparative Example. This shows that the interface of the tab prepared by the method of the present application is more stable, the side reaction is reduced, and it is beneficial to maintain the long-term performance stability of the battery.

[0225] 4) In terms of rate performance, the positive electrode prepared in Examples 1-4 of the present application shows excellent capacity retention rate at each rate, especially the performance improvement at high rate (3C and 5C) is more significant. This is mainly due to the integrity and uniformity of the tab structure, and the ion transport channels provided by the three-dimensional network structure formed by the bio-based elastomer modifier.

[0226] ​5) The results of Comparative Example 2 and Comparative Example 3 show that although the use of bio-based elastomer modifier alone or gradient heating coating can improve the performance of the electrode sheet to some extent, the effect is limited. Only the synergistic application of bio-based elastomer modifier, gradient heating coating and ultrasonic assisted compaction can achieve the best comprehensive performance. This proves that there is a synergistic effect between the technical means of the present application, and the absence of any link will cause a significant decrease in performance.

[0227] 6) From the comparison of different positive electrode materials, the method of the present application has a significant improvement effect on various common lithium ion battery positive electrode materials (NCM811, NCM622, LiFePO4, NCA), indicating that the present application has wide applicability. Among them, the LiFePO4 positive electrode (Example 3) shows the best cycle stability, which is related to its structural stability; and high-nickel ternary materials (such as NCM811, Example 1) also obtain significant improvement through the method of the present application, effectively overcoming the performance degradation problem in long cycle.

[0228] In summary, the long cycle life lithium ion battery positive electrode and the preparation method thereof provided by the present application effectively solve the problem of electrode sheet cracking through the synergistic effect of the introduction of bio-based elastomer modifier, gradient heating coating and ultrasonic assisted compaction, and significantly improve the cycle life of lithium ion battery. The prepared positive electrode sheet has a crack density of ≤2 cracks / cm2 and a crack average width of ≤10 μm, a capacity retention rate of ≥90% after 1000 cycles at 1C rate, and a capacity retention rate of ≥85% after 500 cycles at 3C rate, which is much better than the electrode prepared by traditional process. This excellent long cycle life performance makes the positive electrode of the present application particularly suitable for application in fields such as electric vehicles, electronic equipment and energy storage systems which have high requirements for battery life, and has important practical value and market prospect.

[0229] It should be noted that the contents not described in detail in the present specification belong to the prior art known to those skilled in the art, which will not be described here.

[0230] Based on the disclosure and teaching of the above description, those skilled in the art of the present application can also make changes and modifications to the above embodiments. Therefore, the present application is not limited to the above specific embodiments, and any obvious improvements, replacements or modifications made by those skilled in the art on the basis of the present application all belong to the protection scope of the present application. In addition, although some specific terms are used in the present specification, these terms are only for convenience of description and do not constitute any limitation on the present application.

Claims

1. A method for preparing a lithium-ion battery cathode with long cycle life, characterized in that, Includes the following steps: S1. Slurry preparation: The positive electrode active material, conductive agent, binder and solvent are mixed, and 0.5-5wt% of bio-based elastomer modifier is added. The bio-based elastomer modifier is a chitosan derivative. The positive electrode slurry is prepared by stirring and dispersing. S2. Gradient heating coating: The positive electrode slurry is coated onto the current collector and dried using a multi-zone temperature-controlled gradient heating method, wherein the drying temperature gradually increases from the first zone to the last zone, and the temperature difference between adjacent zones is controlled at 5-15℃. S3. Ultrasonic Assisted Compaction: The coated electrode is rolled while ultrasonic vibration of 20-60kHz is applied, with the vibration direction at an angle of 30-60° to the electrode transmission direction.

2. The method for preparing a long-cycle-life lithium-ion battery cathode according to claim 1, characterized in that, In S1: The chitosan derivative is carboxymethyl chitosan, N-succinyl chitosan, quaternized chitosan, or chitosan-polyethylene glycol copolymer, with a molecular weight of 5000-50000 Da and a degree of deacetylation of 75-95%.

3. The method for preparing a long-cycle-life lithium-ion battery cathode according to claim 1, characterized in that, In S1: The amount of the bio-based elastomer modifier added is 1-3 wt%. The bio-based elastomer modifier undergoes a pre-swelling treatment before being added to the slurry. The pre-swelling treatment includes soaking the bio-based elastomer modifier in a solvent for 4-12 hours and ultrasonically dispersing it for 30-60 minutes.

4. The method for preparing a long-cycle-life lithium-ion battery cathode according to claim 1, characterized in that, In S2: The drying area of ​​the gradient heating coating is divided into 3-6 zones, with the temperature of the first zone being 40-60℃ and the temperature of the last zone being 100-130℃, and the total drying time being 2-5 minutes.

5. The method for preparing a long-cycle-life lithium-ion battery cathode according to claim 1, characterized in that, In S2: During the gradient heating coating process, the tension of the current collector is controlled to be 10-30 N / m, and a buffer zone with a length of 0.5-2 m is set between the coating area and the first drying area.

6. The method for preparing a long-cycle-life lithium-ion battery cathode according to claim 1, characterized in that, In S3: The ultrasonic power of the ultrasonic-assisted compaction is 200-800W, the roller pressing line pressure is 0.5-3kN / cm, and the roller pressing speed is 1-10m / min.

7. The method for preparing a long-cycle-life lithium-ion battery cathode according to claim 1, characterized in that, In S3: The ultrasonic-assisted compaction uses a double-sided ultrasonic roller pressing device, with a phase difference of 90-180° between the upper and lower ultrasonic generators, and the porosity of the electrode sheet after compaction is 20-35%.

8. The method for preparing a long-cycle-life lithium-ion battery cathode according to claim 1, characterized in that, In S1: The positive electrode active material is selected from lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium iron phosphate, lithium cobalt oxide and combinations thereof; the conductive agent is selected from conductive carbon black, graphite, carbon nanotubes, graphene and combinations thereof; and the binder is selected from polyvinylidene fluoride, polyacrylonitrile, polytetrafluoroethylene, sodium carboxymethyl cellulose and combinations thereof.

9. The method for preparing a long-cycle-life lithium-ion battery cathode according to claim 1, characterized in that, Step S0 is included before step S1: Surface modification treatment of positive electrode active material includes mixing positive electrode active material with 0.5-3wt% organosilane coupling agent under inert gas protection and reacting for 0.5-2 hours at a reaction temperature of 60-100℃. And / or, step S4 is included after step S3: The compacted electrode sheet is subjected to vacuum heat treatment at a temperature of 80-120℃ for 1-5 hours, with a vacuum degree of -0.08 to -0.1MPa.

10. A lithium-ion battery positive electrode with long cycle life, characterized in that, It is prepared by the method for preparing a long cycle life lithium-ion battery cathode according to any one of claims 1-9.

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