Dynamic lithium supplement diaphragm and preparation method thereof, lithium battery and electric equipment

Abstract

The invention discloses a dynamic lithium supplement diaphragm and a preparation method thereof, a lithium battery and electric equipment, the dynamic lithium supplement diaphragm comprises a base film layer, the base film layer comprises a first side and a second side which are opposite to each other; the first side of the base film layer is coated with the thermal stability layer; the bonding layer covers the second side of the base film layer and the thermal stable layer; the lithium supplementing functional layer is located on the second side of the base film layer and comprises a lithium oxalate-carbon nano tube composite material and a conductive agent, and the surfaces of lithium oxalate particles are coated with carbon nano tubes. In the dynamic lithium supplement diaphragm, the lithium supplement functional layer adopts a composite structure that lithium oxalate particles are fully coated with carbon nanotubes, and controllable release of lithium ions is realized through a conductive network and a physical barrier effect of the carbon nanotubes.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium ion batteries, and in particular to a dynamic lithium replenishing diaphragm and a preparation method thereof, a lithium battery and an electrical device. Background Art

[0002] During the cycle of lithium-ion batteries, the irreversible loss of active lithium is the main cause of capacity decay. In the prior art, the initial lithium loss is compensated by adding a lithium supplement to the positive electrode slurry, but there are the following significant defects: 1. The efficiency of lithium supplement is limited. If metal lithium powder or lithium-rich compounds are used as lithium supplements, the active lithium is quickly released during the battery formation stage, which can only alleviate the capacity decay in the early stage of the cycle and cannot cover the continuous lithium consumption in the middle and late stages. The repeated rupture and regeneration of the negative electrode solid electrolyte interface film leads to continuous loss of active lithium, and traditional lithium supplements cannot dynamically respond to the lithium demand during the cycle. 2. Poor side reactions and interface stability. The lithium supplement is in direct contact with the electrolyte, and side reactions (such as gas generation and electrolyte decomposition) are triggered under high pressure or high temperature conditions, resulting in expansion of the battery cell and increased interface impedance. The catalytic effect of the lithium supplement on the positive electrode side and the positive electrode material (such as lithium iron phosphate) accelerates decomposition, further exacerbating the gas production problem. Summary of the invention

[0003] In order to overcome the defects in the prior art, the first purpose of the present invention is to provide a dynamic lithium replenishing diaphragm, the second purpose of the present invention is to provide a method for preparing a dynamic lithium replenishing diaphragm, the third purpose of the present invention is to provide a lithium battery, and the fourth purpose of the present invention is to provide an electrical device.

[0004] In order to achieve the above object, the technical solution adopted by the present invention is:

[0005] In a first aspect, a dynamic lithium replenishing diaphragm comprises:

[0006] a base film layer, the base film layer comprising a first side and a second side opposite to each other;

[0007] A thermally stable layer coated on a first side of the base film layer;

[0008] a bonding layer covering the second side of the base film layer and the thermally stable layer;

[0009] A lithium supplementation functional layer is located on the second side of the base film layer, the lithium supplementation functional layer comprises a lithium oxalate-carbon nanotube composite material and a conductive agent, wherein the carbon nanotubes cover the surface of lithium oxalate particles. The first side is opposite to the positive electrode, and the second side is opposite to the negative electrode.

[0010] The dynamic lithium replenishment diaphragm of the present invention realizes dynamic compensation of lithium ions through multi-layer structural design and coordinated optimization of materials, and its core includes functional integration of base film layer, thermal stability layer, bonding layer and lithium replenishment functional layer. The base film layer serves as a supporting body, and the thermal stability layer coated on the first side improves the high temperature stability of the diaphragm, and the second side realizes interface strengthening bonding with the lithium replenishment functional layer through the bonding layer; the lithium replenishment functional layer is arranged on the bonding layer surface on the second side of the base film layer, and is composed of a fully coated composite structure formed by lithium oxalate and carbon nanotubes, wherein the carbon nanotubes cover the surface of the lithium oxalate particles in a continuous and dense coating form to form a three-dimensional conductive network. The fully coated surface coating structure inhibits the direct contact between lithium oxalate and the electrolyte through the physical barrier effect of carbon nanotubes, avoids the gas production problem caused by the decomposition of lithium oxalate under high pressure, and uses the high conductivity of carbon nanotubes to establish an electron transmission channel, so that the lithium ion release rate of lithium oxalate is dynamically matched with the battery cycle demand. The lithium replenishment functional layer delays the release of excessive lithium through the barrier effect of the carbon nanotube layer at the beginning of the cycle to prevent the reduction of the first effect. As the number of cycles increases, the carbon nanotube layer forms controllable microcracks under mechanical stress, and the lithium oxalate is gradually exposed and releases active lithium in stages, continuously compensating for the lithium loss caused by the regeneration of the negative electrode SEI film. The spatial layout and material collaborative design of each functional layer not only ensure the ion conduction capacity of the diaphragm, but also block the direct contact between the lithium replenisher and the positive electrode material from the structural level, achieving the dual optimization of side reaction inhibition and dynamic lithium replenishment.

[0011] In a second aspect, a method for preparing the above-mentioned dynamic lithium replenishment diaphragm comprises the following steps:

[0012] Dissolving squaric acid and lithium carbonate in deionized water to react and generate a lithium oxalate solution;

[0013] Adding a carbon nanotube dispersion into a lithium oxalate solution, mixing, and spray drying to obtain a lithium oxalate-carbon nanotube composite material;

[0014] A thermal stability layer is coated on the first side of the base film layer, a bonding layer is coated on the second side of the base film layer and on the surface of the thermal stability layer, and finally a lithium replenishing functional layer slurry is coated on the second side of the base film layer and on the surface of the bonding layer, wherein the lithium replenishing functional layer slurry is composed of the composite material, a conductive agent and a bonding agent.

[0015] The full surface coating of lithium oxalate particles by carbon nanotubes is achieved through a spray drying process. During the spray drying process, the mixed solution (lithium oxalate solution and carbon nanotube dispersion) is dispersed into micron-sized droplets through an atomizer. Inside the droplets, due to the rapid evaporation of the solvent, a concentration gradient is formed on the surface of the droplets, driving the solute to migrate to the surface of the droplets. Carbon nanotubes are preferentially adsorbed on the surface of the droplets due to their high specific surface area and surface hydrophobicity, while lithium oxalate precipitates and crystallizes in the core area of ​​the droplets due to its low solubility. This process causes carbon nanotubes to spontaneously coat the periphery of lithium oxalate particles to form a core-shell structure. This process optimizes the conductivity and interface stability of the material, and also gives it a dynamic lithium replenishment function through structural design, providing a reliable solution for the industrial production of high-performance lithium battery separators.

[0016] Preferably, the feed rate of the spray drying is 15 to 20 ml / min. A pressure atomizer can be used for the spray drying. The characteristics of the pressure atomizer show significant differences under the change of the feed rate. When the feed rate is in the range of 15-20 ml / min, the liquid is fully atomized under high pressure to form fine droplets; the larger droplet surface area accelerates the solvent evaporation process, and the carbon nanotubes have sufficient time to migrate to the droplet surface during the drying process and tightly coat the lithium oxalate particles, eventually forming composite particles with uniform particle size distribution. When the feed rate is lower than 15 ml / min, the droplet size is further reduced, and the excessively fast drying rate causes the carbon nanotubes to solidify before completing the surface migration, and local holes appear in the coating layer. At the same time, the atomizing nozzle is blocked due to the residual droplets that are not completely atomized under low pressure, which not only reduces the production efficiency, but also increases the wear rate of the nozzle. When the feed rate exceeds 20 ml / min, the liquid flow rate exceeds the atomization pressure carrying capacity, forming large droplets. The solvent evaporation time is prolonged, resulting in insufficient concentration gradient inside the droplets, and the migration path of the carbon nanotubes is blocked and retained in the core area of ​​the particles. The particles that are not completely dried adhere to each other to form a hollow or porous structure, the uniformity of the carbon nanotube distribution decreases, the conductivity of the composite material decreases, and the mechanical properties significantly deteriorate.

[0017] Preferably, the solid content of the carbon nanotube dispersion is 4%. The uniform dispersion and effective coating of the carbon nanotubes in the lithium oxalate solution are achieved by optimizing the ratio of carbon nanotubes to solvent. At this solid content, the dispersion system of carbon nanotubes in deionized water has suitable viscosity and fluidity, which not only ensures the uniform atomization of droplets during spray drying, but also provides sufficient kinetic conditions for the migration of carbon nanotubes to the surface of droplets. When the solid content is lower than 4%, the concentration of carbon nanotubes in the dispersion is too low, and the number of carbon nanotubes migrating to the surface of droplets is insufficient, resulting in a decrease in the continuity of the coating layer and a decrease in the density of the conductive network between particles; and when the solid content is higher than 4%, the viscosity of the dispersion is too high, the droplet size distribution becomes wider during the atomization process, and the carbon nanotubes form local enrichment areas inside the droplets due to agglomeration. After drying, the coating layer on the surface of the particles is uneven in thickness, and some areas hinder the diffusion of lithium ions due to the dense stacking of carbon nanotubes. Experimental verification shows that in the composite material formed by spray drying of a 4% solid content dispersion, carbon nanotubes uniformly coat lithium oxalate particles in the form of single to multiple layers. The coating layer has a stable porosity, which not only maintains the transmission channel of lithium ions, but also effectively blocks the penetration of electrolyte, thereby reducing the gas production caused by the decomposition of lithium oxalate. At the same time, the electronic conductivity of the composite material is improved, laying a material foundation for the efficient realization of the dynamic lithium replenishment function.

[0018] Preferably, the diameter of the carbon nanotubes is 1.0 to 2.2 nanometers. The conductivity and structural stability of the lithium supplement functional layer are balanced by optimizing the diameter of the carbon nanotubes. The carbon nanotubes in this diameter range have both flexibility and mechanical strength, and can conform to the surface morphology of lithium oxalate particles during the spray drying process to form a continuous and dense coating. Smaller diameters (<1.0 nanometers) cause carbon nanotubes to break or agglomerate easily, weakening the integrity of the coating and the connectivity of the conductive network. Larger diameters (>2.2 nanometers) are difficult to fit tightly to the particle surface due to enhanced rigidity, resulting in a loose and porous coating that cannot effectively block electrolyte penetration. By selecting carbon nanotubes of 1.0 to 2.2 nanometers, the capillary force of the solvent is used to drive their directional arrangement during the drying process to form a uniform conductive network, while maintaining the physical barrier effect of the coating to inhibit direct contact between lithium oxalate and the electrolyte, thereby achieving the dual functions of dynamic lithium supplementation and side reaction inhibition during the cycle.

[0019] Preferably, the length of the carbon nanotubes is ≥5 microns. Carbon nanotubes in this length range can form a ductile coating across the surface of the lithium oxalate particles during the spray drying process, effectively bridging the electron transfer path between adjacent particles, and avoiding the problem of conductive network breakage caused by too short tube segments. Longer carbon nanotubes produce a directional arrangement trend through capillary forces when the droplets are drying, closely fit the surface of the particles to form a continuous coverage, reduce the pore defects in the coating layer caused by local folding or stacking, and thus strengthen the physical barrier to electrolyte penetration. Compared with the uneven dispersion and increased contact resistance that are prone to occur in short carbon nanotubes (<5 microns), carbon nanotubes with a length of ≥5 microns build a three-dimensional interconnected conductive framework in the lithium replenishment layer, significantly improving the uniformity of the lithium ion release rate, while enhancing the coating layer's resistance to fracture under cyclic stress, ensuring the long-term effectiveness of the dynamic lithium replenishment function.

[0020] Preferably, the lithium supplement functional layer slurry contains 80wt% lithium oxalate-carbon nanotube composite material, 10wt% conductive carbon black and 10wt% polyvinylidene fluoride. The electrochemical performance and structural stability of the lithium supplement layer are optimized through the synergistic effect of the proportion of each component. As the core active material, the lithium oxalate-carbon nanotube composite material provides a lithium source and constructs a three-dimensional conductive network; the conductive carbon black further strengthens the electron transmission path, makes up for the defect of poor local contact of the carbon nanotubes, and ensures the charge balance of the lithium ion release process; polyvinylidene fluoride is used as a binder to fix the active material and the conductive agent while maintaining the interface bonding strength between the lithium supplement layer and the diaphragm matrix to prevent the coating from peeling off during the cycle. Under the premise of ensuring a high active material content in the lithium supplement layer, this ratio design achieves a comprehensive balance of electronic conduction, ion transmission and mechanical stability through the reasonable addition of conductive agents and binders, avoiding excessive binders from hindering ion diffusion and preventing the local polarization caused by insufficient conductive agents from intensifying, thereby ensuring the efficiency and durability of the dynamic lithium supplement function while suppressing side reactions.

[0021] Preferably, the thermally stable layer contains aluminum oxide or boehmite, and the coating thickness is 2 to 5 microns. The high melting point characteristics of aluminum oxide and boehmite can effectively inhibit the thermal shrinkage behavior of the diaphragm in a high temperature environment and avoid the risk of internal short circuit caused by increased temperature. Its porous structure provides a stable channel for lithium ion transmission while maintaining the wettability of the electrolyte. The coating thickness is set to 2 to 5 microns, which can not only ensure that the ceramic particles evenly cover the surface of the base membrane to form a continuous protective layer, but also avoid increasing the ion transmission resistance or reducing the mechanical flexibility of the diaphragm due to excessive thickness of the coating. The thermally stable layer within this thickness range maintains the structural integrity of the diaphragm under high temperature conditions by closely fitting with the base membrane layer, blocking the chain reaction caused by thermal runaway, while taking into account the dynamic cycle performance and long-term safety of the battery.

[0022] Preferably, the bonding layer is polyvinylidene fluoride, and the coating thickness is 1 to 3 microns. With its excellent chemical stability and bonding properties, polyvinylidene fluoride forms a continuous and flexible bonding interface between the base film and the lithium replenishment functional layer, ensuring the mechanical integrity of the multilayer structure during the battery cycle. Its thickness range has been optimized to provide sufficient bonding force to prevent coating peeling, while avoiding excessive thickness that leads to an extension of the ion transmission path or a decrease in electrolyte wettability. The bonding layer of this thickness effectively alleviates the stress difference between the thermal stability layer and the lithium replenishment functional layer by evenly covering the surface of the base film, while maintaining the overall flexibility and pore structure of the diaphragm, providing stable support for the efficient transmission of lithium ions, thereby ensuring the interface stability and cycle performance of the battery during dynamic lithium replenishment.

[0023] In a third aspect, a lithium battery includes a positive electrode, a negative electrode, an electrolyte and the above-mentioned dynamic lithium replenishment diaphragm.

[0024] In a fourth aspect, an electrical device comprises the above-mentioned lithium battery.

[0025] Due to the application of the above technical solution, the present invention has the following advantages compared with the prior art:

[0026] 1. The dynamic lithium replenishment diaphragm of the present invention realizes the precise division of labor and synergistic efficiency of functional modules through the multi-layer structure of base film layer, thermal stability layer, bonding layer and lithium replenishment functional layer. The thermal stability layer is arranged on the side of the base film layer facing the positive electrode, and adopts high temperature resistant ceramic material to suppress dimensional deformation under high temperature; the bonding layer covers the base film layer and the thermal stability layer on both sides to ensure the interlayer interface bonding strength; the lithium replenishment functional layer is directionally coated on the side of the bonding layer facing the negative electrode, and the structure of lithium oxalate fully covered by carbon nanotubes is used to construct a dual mechanism of dynamic lithium replenishment and side reaction inhibition. The spatial layout and thickness matching of each functional layer not only ensure the efficiency of lithium ion transmission, but also solve the interface instability problem of traditional lithium replenishment technology from a structural point of view.

[0027] 2. The lithium replenishment functional layer adopts a composite structure of lithium oxalate particles fully coated with carbon nanotubes, and the controlled release of lithium ions is achieved through the conductive network and physical barrier effect of carbon nanotubes. The carbon nanotube coating layer acts as a barrier to delay the release of excessive lithium in the early stage of the cycle, avoiding the decrease in the initial efficiency due to the excessive thickness of the SEI film; as the cycle progresses, the coating layer produces controllable microcracks under mechanical stress, and the lithium oxalate gradually releases active lithium in stages, dynamically compensating for the lithium loss caused by the regeneration of the negative electrode SEI film. This mechanism breaks through the limitation of one-time release of traditional lithium replenishment technology and significantly extends the lithium replenishment cycle.

[0028] 3. The preparation of lithium oxalate-carbon nanotube composite materials is achieved through a spray drying process. The precise control of the feed rate, carbon nanotube size and solid content of the dispersion ensures the uniformity of the particle morphology and the integrity of the coating layer. The thermal stability layer uses aluminum oxide or boehmite, combined with an optimized coating thickness, to form a stable porous ceramic skeleton at high temperatures; the bonding layer uses polyvinylidene fluoride combined with its thickness range to balance the interface bonding strength and ion transmission resistance. The coordinated design of material properties and process parameters provides a highly consistent technical path for industrial mass production.

[0029] 4. The dynamic lithium replenishment diaphragm can be directly integrated into the existing lithium-ion battery system without modifying the electrode structure or adjusting the electrolyte formula, and is compatible with mainstream battery manufacturing processes such as winding and lamination. It has excellent adaptability in different positive electrode systems such as lithium iron phosphate and ternary materials and negative electrode systems such as graphite and silicon carbon. It is suitable for a variety of electrical equipment such as new energy vehicles, energy storage systems and consumer electronics, and has broad market application prospects.

[0030] In order to make the above and other purposes, features and advantages of the present invention more obvious and easy to understand, preferred embodiments are specifically cited below and described in detail with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0032] Figure 1 is a SEM image of the lithium oxalate-carbon nanotube composite material in Example 1 of the present invention;

[0033] Figure 2 is a TEM image of lithium oxalate-carbon nanotubes in Example 1 of the present invention;

[0034] Figure 3 It is the contact angle measurement result diagram of the present invention;

[0035] Figure 4 This is a data chart of the cyclic release of dynamic lithium replenishment of lithium iron phosphate and lithium oxalate-carbon nanotubes. DETAILED DESCRIPTION

[0036] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0037] Embodiment 1:

[0038] This embodiment discloses a dynamic lithium replenishment diaphragm, comprising:

[0039] a base film layer, the base film layer comprising a first side and a second side opposite to each other;

[0040] A thermally stable layer coated on a first side of the base film layer;

[0041] a bonding layer covering the second side of the base film layer and the thermally stable layer;

[0042] The lithium supplement functional layer is located on the second side of the base film layer, and the lithium supplement functional layer comprises a lithium oxalate-carbon nanotube composite material and a conductive agent, wherein the carbon nanotubes cover the surface of the lithium oxalate particles.

[0043] This embodiment discloses a method for preparing the above-mentioned dynamic lithium replenishment diaphragm, comprising the following steps:

[0044] The carbon nanotube powder was added to deionized water, the solid content was controlled to be 4%, a dispersant (polyvinyl pyrrolidone) was added, and the mixture was dispersed for 30 minutes under ultrasonic treatment to obtain a uniform carbon nanotube dispersion.

[0045] Weigh 1.14 g of squaric acid and 0.74 g of lithium carbonate, dissolve in 50 ml of deionized water, and stir at room temperature for 2 h to obtain a uniform lithium oxalate solution.

[0046] Add 50 ml of the prepared carbon nanotube dispersion into the lithium oxalate solution and continue stirring for 1 hour. Transfer the mixed solution to a spray drying device, set the feed rate to 15 ml / min and the drying temperature to 180°C, and obtain a lithium oxalate-carbon nanotube composite material after spray drying.

[0047] A 2-micron aluminum oxide or boehmite heat-stabilizing layer is coated on the side of a 7-micron thick PE (polyethylene) base film opposite to the positive electrode, and then a 1-micron PVDF (polyvinylidene fluoride) bonding layer is coated on both sides of the base film.

[0048] 80wt% lithium oxalate-carbon nanotube composite material, 10wt% SuperP (conductive carbon black), 10wt% PVDF and NMP (N-methylpyrrolidone) were mixed to form a slurry. The slurry was applied to the surface of the bonding layer opposite to the base film and the negative electrode with a thickness of 2 microns. The slurry was vacuum dried at 60°C for 12 hours to obtain a multilayer separator.

[0049] The microstructure of the lithium oxalate-carbon nanotube composite material prepared above was observed by scanning electron microscopy (SEM, QUANTA250FEG). Figure 1 As shown, the SEM image at 5 μm scale shows that the composite material is spherical particles of uniform size, indicating the homogenization effect of the spray drying process. Its regular morphology is conducive to reducing the lithium ion transmission resistance and improving the kinetic performance of the battery cell.

[0050] The microstructure of the lithium oxalate-carbon nanotube composite material prepared above was observed by transmission electron microscopy (TEM, Tecnai F30). Figure 2 As shown, at a resolution of 200nm, carbon nanotubes are tightly wrapped around the lithium oxalate particles to form a core-shell structure, which greatly enhances the conductivity of the lithium source and inhibits the catalytic side reactions of lithium oxalate and lithium iron phosphate positive electrode materials through physical barrier effects.

[0051] Comparative Example 1:

[0052] This comparative example discloses a diaphragm, which directly uses a PE base film.

[0053] Comparative Example 2:

[0054] This comparative example discloses a diaphragm, which is prepared by the following steps:

[0055] A 2-micron aluminum oxide or boehmite heat-stabilizing layer is coated on the side of a 7-micron thick PE (polyethylene) base film opposite to the positive electrode, and then a 1-micron PVDF (polyvinylidene fluoride) bonding layer is coated on both sides of the base film to obtain a separator.

[0056] Embodiment 3:

[0057] The separators in Example 1 and Comparative Examples 1 and 2 were used to prepare lithium batteries, and the battery composition was as follows:

[0058] The positive electrode is prepared using lithium iron phosphate (LiFePO 4 ) as the active material, and mixed with conductive carbon black, polyvinylidene fluoride and carbon nanotubes in a mass ratio of 96:1.8:1.7:0.5 to form a slurry, and the coating amount of the positive electrode active material on each side is 16.0 mg / cm 2 , evenly coated on the surface of the aluminum foil current collector.

[0059] The negative electrode was prepared by mixing graphite as the active material with aqueous binder, conductive carbon black, carbon nanotubes and sodium carboxymethyl cellulose in a mass ratio of 96:2.3:0.9:0.4:0.4 to prepare a slurry. The coating amount of the negative electrode active material on each side was 10.1 mg / cm 2 , coated on the surface of the copper foil current collector with a thickness of 6.0μm.

[0060] The electrolyte is composed of 1 mol / L lithium hexafluorophosphate (LiPF 6 ) as the lithium salt, and the solvent is composed of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 3:7.

[0061] The diaphragms used are those in Example 1 and Comparative Examples 1 and 2, respectively.

[0062] The capacity ratio of the negative electrode to the positive electrode (N / P ratio) is 1.13, and the charge and discharge voltage range is 2.0-4.5V.

[0063] The contact angles of the membranes of Example 1, Comparative Examples 1 and 2 were measured. The contact angle measuring instrument used was XG-CAMB3. Figure 3 As shown in the figure, the contact angle (16.23°) of the separator in Example 1 is the smallest, indicating that its wettability is the best. This is mainly due to the synergistic effect of the regular arrangement of the spherical particles (lithium oxalate-carbon nanotube composite particles) in the lithium supplementation functional layer and the polar groups (PVDF binder). The pore structure between the spherical particles provides space for electrolyte storage, further accelerating the wetting rate.

[0064] Thermogravimetric analysis was performed using a thermogravimetric analyzer (TGA, Q600SDT) on the separators of Example 1 and Comparative Examples 1 and 2. The thermal stability of lithium oxalate was studied at a heating rate of 5°C / min from room temperature to 800°C under air flow.

[0065] Cyclic voltammetry test was performed on the lithium oxalate-carbon nanotube composite material in Example 1. The lithium oxalate-carbon nanotube composite material was made into a working electrode, and a reference electrode (lithium sheet) and a counter electrode (platinum sheet) were prepared at the same time to assemble a three-electrode system. An electrolyte similar to the actual battery system was selected to ensure good ion conductivity. The three-electrode system was connected to an electrochemical workstation (GAMRY INTERFACE 1010E), and the scan rate (0.03mVs -1 ).

[0066] The lithium-ion batteries prepared in Example 1 and Comparative Examples 1 and 2 were subjected to room temperature cycle tests. According to the battery composition and assembly method of Example 3, the positive electrode, negative electrode, separator, electrolyte, etc. were assembled into LiFePO 4||Gr full battery. Connect the assembled battery to the Xinwei test system, ensuring that the connection is firm and the contact is good. Set the charge and discharge voltage range (2.5-4.3V), current density (0.1C charge to 3.7V, then 0.02C charge to 4.3V, 1C = 160mAhg -1 ) and other parameters. Start the test system, make the battery charge and discharge cyclically under the set conditions, record the capacity and other performance data of the battery at different cycle times, until it reaches 700 cycles, and analyze the cycle stability of the battery.

[0067] The test results of the above tests are shown in the following table:

[0068] Test items Example 1 Comparative Example 1 Comparative Example 2 Thermogravimetric testing 190℃ 135℃ 150℃ Cyclic voltammetry 4.25V none none Normal temperature cycle test 700 laps @ 98.5% 700 laps @ 96.2% 700 laps @ 96.9%

[0069] It can be seen from the thermogravimetric test results that the starting temperature of decomposition of the diaphragm in Example 1 is 190°C, which is significantly higher than that of Comparative Example 1 (135°C) and Comparative Example 2 (150°C), proving that the coating of the lithium oxalate-carbon nanotube composite material greatly enhances the thermal stability of the diaphragm, mainly because the high-temperature stability of both is very strong, which can slow down the thermal decomposition of the PE diaphragm.

[0070] The results of cyclic voltammetry tests showed that the lithium oxalate-carbon nanotube composite material had a characteristic oxidation peak at 4.25V (vs. Li+ / Li), indicating that its electrochemical reaction potential was higher than the operating voltage window (2.5-4.3V) of the lithium iron phosphate positive electrode, thereby triggering a lithium replenishment reaction in the middle and late stages of the battery cycle (when the positive electrode potential rises to above 4.25V due to polarization), thereby releasing active lithium on demand.

[0071] The results of the room temperature cycle test show that the LiFePO 4 || After 700 cycles in the voltage range of 2.5-4.3V, the capacity retention rate of the Gr full battery reached 98.5%, which is significantly better than Comparative Example 1 (96.2%) and Comparative Example 2 (96.9%). The lithium replenishment functional layer releases lithium ions in stages in the middle and late stages of the cycle, continuously compensating for the lithium loss caused by the regeneration of the negative electrode SEI film, thereby significantly improving the cycle stability.

[0072] like Figure 4As shown, the comparison chart of the cyclic release data of lithium iron phosphate and lithium oxalate-carbon nanotube composite materials in the dynamic lithium replenishment process shows that the normalized current of the two at different potentials shows significant differences. Specifically, the lithium oxalate-carbon nanotube composite material can realize the dynamic lithium replenishment function through the battery management system (BMS) because of its unique reaction potential characteristics. In the battery cell integrated system, the state of charge (SOC) of the battery cell is regulated by BMS, and it can be observed that the potential gradually rises with the increase of the number of cycles. For example, the potential is about 3.8V when the cycle is 100 times, and the potential shifts when the cycle reaches 300 times. When the number of cycles reaches 3000 times, the potential is close to 3.9V-4.0V. At this time, the reaction activity of the material is significantly enhanced, and the number of lithium ions released to the positive electrode is also increased accordingly. The above data and phenomena not only deepen the understanding of the dynamic lithium replenishment mechanism, but also fully verify the significant advantages of the dynamic lithium replenishment diaphragm of the present invention in practical applications-it can accurately regulate the release of lithium ions according to the cycle state of the battery cell, thereby significantly improving the cycle stability and overall performance of the battery.

[0073] The present invention uses specific embodiments to illustrate the principles and implementation methods of the present invention. The description of the above embodiments is only used to help understand the method of the present invention and its core idea. At the same time, for those skilled in the art, according to the idea of ​​the present invention, there will be changes in the specific implementation methods and application scope. In summary, the content of this specification should not be understood as a limitation on the present invention.

Claims

1. A dynamic lithium replenishing diaphragm, characterized in that: include: a base film layer, the base film layer comprising a first side and a second side opposite to each other; A thermally stable layer coated on a first side of the base film layer; a bonding layer covering the second side of the base film layer and the thermally stable layer; The lithium supplement functional layer is located on the second side of the base film layer, and the lithium supplement functional layer comprises a lithium oxalate-carbon nanotube composite material and a conductive agent, wherein the carbon nanotubes cover the surface of the lithium oxalate particles.

2. A method for preparing a dynamic lithium replenishment diaphragm according to claim 1, characterized in that: The following steps are involved: Dissolving squaric acid and lithium carbonate in deionized water to react and generate a lithium oxalate solution; Adding a carbon nanotube dispersion into a lithium oxalate solution, mixing, and spray drying to obtain a lithium oxalate-carbon nanotube composite material; A thermal stability layer is coated on the first side of the base film layer, a bonding layer is coated on the second side of the base film layer and on the surface of the thermal stability layer, and finally a lithium replenishing functional layer slurry is coated on the second side of the base film layer and on the surface of the bonding layer, wherein the lithium replenishing functional layer slurry is composed of the composite material, a conductive agent and a bonding agent.

3. The preparation method according to claim 2, characterized in that: The feed rate of the spray drying is 15-20 ml / min.

4. The preparation method according to claim 2, characterized in that: The solid content of the carbon nanotube dispersion is 4%.

5. The preparation method according to claim 2, characterized in that: The diameter of the carbon nanotube is 1.0 to 2.2 nanometers.

6. The preparation method according to claim 2, characterized in that: The length of the carbon nanotubes is ≥5 microns.

7. The preparation method according to claim 2, characterized in that: The lithium supplement functional layer slurry comprises 80 wt % of lithium oxalate-carbon nanotube composite material, 10 wt % of conductive carbon black and 10 wt % of polyvinylidene fluoride.

8. The preparation method according to claim 2, characterized in that: The thermally stable layer comprises aluminum oxide or boehmite, and has a coating thickness of 2 to 5 microns.

9. The preparation method according to claim 2, characterized in that: The bonding layer is polyvinylidene fluoride, and the coating thickness is 1 to 3 microns.

10. A lithium battery, characterized in that: It comprises a positive electrode, a negative electrode, an electrolyte and the dynamic lithium replenishment diaphragm as claimed in claim 1.

11. An electrical device, characterized in that: Comprising the lithium battery as claimed in claim 10.

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