Modified PE diaphragm with low decomposition voltage and high lithium supplement capacity and preparation method thereof

Abstract

The invention relates to the technical field of lithium battery diaphragms, in particular to a modified PE diaphragm with low decomposition voltage and high lithium supplement capacity and a preparation method of the modified PE diaphragm. The preparation method comprises the following steps: mixing C3N4, a molybdenum source, a cobalt source and citric acid to obtain a precursor; sintering the precursor in a protective atmosphere to obtain a composite catalyst; mixing the composite catalyst, a conductive agent, a binder, lithium oxalate and a solvent to obtain slurry; and coating the surface of a PE diaphragm with the slurry, and drying to obtain the modified PE diaphragm with low decomposition voltage and high lithium supplement capacity. The composite catalyst, the mixed conductive agent, the binder, the lithium oxalate and other homogenates are obtained by sintering the multi-phase precursor and are coated on the PE diaphragm, so that the decomposition potential of the lithium oxalate can be greatly reduced, the problem that the lithium oxalate in a ternary positive electrode system is difficult to efficiently supplement lithium is effectively solved, the specific capacity released by the lithium oxalate under low voltage is improved, and the service life of the ternary positive electrode system is prolonged. And the effect of high lithium supplement capacity is achieved.

Description

Technical Field

[0001] This invention relates to the field of lithium battery separator technology, and in particular to a modified PE separator with low decomposition voltage and high lithium replenishment capacity, and its preparation method. Background Technology

[0002] With the rise of electric vehicles and the rapid development of portable electronic devices, traditional battery technology can no longer meet the ever-increasing performance demands. Against this backdrop, lithium-ion batteries, with their cleanliness, safety, and relatively higher energy density compared to traditional batteries, are widely used in various electronic devices. However, during the first charge and discharge cycle of a lithium battery, a layer of SEI (solid electrolyte interphase) film inevitably forms on the surface of the negative electrode material. This film formation process irreversibly consumes lithium ions, directly leading to a decrease in the battery's initial coulombic efficiency and a reduction in the battery's actual overall reversible capacity (i.e., energy density).

[0003] To overcome this problem, pre-lithiation technology has emerged. This technology introduces an additional lithium source before the battery's first charge and discharge cycle, pre-compensating for the lithium ions consumed during subsequent SEI film formation, thereby effectively improving the battery's first-cycle coulombic efficiency and final energy density, while also contributing to improved cycle performance. Currently, common pre-lithiation methods mainly include lithium metal sheet pre-lithiation, chemical pre-lithiation, and electrochemical pre-lithiation. Among them, chemical pre-lithiation has received considerable attention due to its relative ease of integration into existing production processes. This method typically utilizes lithium-containing reagents with strong reducing properties (such as n-butyllithium) to directly deposit active lithium onto the surface of the negative electrode material through a redox reaction. However, these strongly reducing reagents (such as n-butyllithium) are extremely chemically reactive, highly sensitive to air and moisture, and flammable and explosive, posing serious safety hazards and limiting their practical application.

[0004] In contrast, lithium oxalate (Li₂C₂O₄) is relatively stable in air and does not readily undergo violent oxidation or hydrolysis reactions, making it a more promising and safer candidate for chemical pre-lithiation. However, lithium oxalate suffers from an excessively high decomposition voltage (typically above 4.7V vs. Li₂C₂O₄). + The problem of lithium oxalate (Li₂O₃) has hindered its further application. To solve this problem, researchers have tried to introduce catalysts to reduce its decomposition voltage, but the decomposition voltage of lithium oxalate has not yet dropped to a level that allows for efficient lithium replenishment in ternary cathode lithium batteries. Summary of the Invention

[0005] The purpose of this invention is to address the problem that existing methods for reducing the decomposition voltage of lithium oxalate are insufficient in reducing its decomposition potential. This invention provides a modified PE membrane with low decomposition voltage and high lithium replenishment capacity, along with its preparation method. A composite catalyst is obtained by sintering a multiphase precursor, and a slurry is formed by mixing a conductive agent, binder, and lithium oxalate, which is then coated onto the PE membrane. This significantly reduces the decomposition potential of lithium oxalate, effectively solving the problem of efficient lithium replenishment in ternary cathode systems.

[0006] To achieve the above objectives, the present invention provides a method for preparing a modified PE separator with low decomposition voltage and high lithium replenishment capacity, comprising the following steps: S1. Mix C3N4, molybdenum source, cobalt source and citric acid to obtain the precursor; S2. The precursor is sintered under a protective atmosphere to obtain a composite catalyst; S3. Mix the composite catalyst, conductive agent, binder, lithium oxalate and solvent to obtain a slurry; S4. The slurry is coated on the surface of the PE membrane and dried to obtain a modified PE membrane with low decomposition voltage and high lithium replenishment capacity.

[0007] Preferably, the molybdenum source in S1 includes one or more of ammonium molybdate and molybdenum oxide, and the cobalt source includes one or more of cobalt nitrate, cobalt chloride, and cobalt acetate.

[0008] Preferably, in S1, the ratio of the amount of carbon in C3N4 to the amount of molybdenum in the molybdenum source is (10-12):(1-1.2); the ratio of the amount of molybdenum in the molybdenum source to the amount of cobalt in the cobalt source is 10:(1-7); and the ratio of the total amount of molybdenum in the molybdenum source and cobalt in the cobalt source to the amount of citric acid is (0.5-1.5):(1-2).

[0009] Preferably, in S1, the mixing is carried out in a mixed solution of ethanol and water, wherein the volume ratio of ethanol to water is (0.5-1.5):(0.5-1.5).

[0010] Preferably, the protective atmosphere in S2 includes one or more of argon and hydrogen; the heating rate of the sintering is 8℃ / min-12℃ / min, the target sintering temperature is 780℃-820℃, and the holding time of the sintering is 1.5h-2.5h.

[0011] Preferably, the conductive agent in S3 includes one or more of carbon black conductive agents and graphene composite conductive agents; the binder includes polyvinylidene fluoride; and the solvent includes one or more of N-methylpyrrolidone and dimethylformamide.

[0012] Preferably, the mass ratio of the composite catalyst, conductive agent, binder, and lithium oxalate in S3 is (15-25):(5-15):(2-8):(60-70); the mass fraction of solids in the slurry is 15%-25%.

[0013] Preferably, in S4, the ratio of the coating thickness to the PE membrane thickness is (1-2):(10-20).

[0014] Preferably, in S4, the drying includes a first drying and a second drying performed sequentially; The first drying temperature is 20℃-30℃, and the first drying time is 2h-4h; the second drying temperature is 50℃-70℃, and the second drying time is 11h-13h.

[0015] The present invention also provides a modified PE membrane with low decomposition voltage and high lithium replenishment capacity, which is prepared by the above-mentioned method for preparing the modified PE membrane with low decomposition voltage and high lithium replenishment capacity.

[0016] The beneficial effects of this invention are as follows: 1. This invention provides a method for preparing a modified PE membrane with low decomposition voltage and high lithium replenishment capacity. A composite catalyst is obtained by sintering a multiphase precursor, and a slurry is prepared by mixing a conductive agent, a binder, lithium oxalate, etc., and then coating it onto a PE membrane. This method can significantly reduce the decomposition potential of lithium oxalate, effectively solve the problem of efficient lithium replenishment of lithium oxalate in ternary cathode systems, improve the specific capacity released by lithium oxalate at low voltage, and achieve the effect of high lithium replenishment capacity.

[0017] 2. In the composite catalyst of this invention, the empty orbitals of molybdenum carbide play an adsorption catalytic role on the lone pair electrons of lithium oxalate, the highly conductive carbon structure establishes a fast transport channel for electron and ion migration, and the introduced cobalt element can change the electron cloud distribution and surface chemical environment of molybdenum carbide, increase the number of active centers, reduce the energy barrier for lithium release from lithium oxalate, accelerate the reaction kinetics between lithium ions and active materials, promote the insertion and extraction of lithium ions, thereby improving the lithium replenishment efficiency, enabling the battery to complete the charge and discharge process more quickly, and improving the rate performance of the battery.

[0018] Meanwhile, cobalt can fill the lattice defects or interstitial sites of molybdenum carbide, supporting and reinforcing the lattice structure, enhancing the crystal structure stability of molybdenum carbide, and thus improving the overall stability of the coating. In addition, the presence of the lithium-filling coating (the coating formed after the slurry dries) significantly improves the mechanical strength of the PE separator. Combined with the promoting effect of the composite catalyst on reaction kinetics, the assembled battery exhibits excellent cycle capacity retention and high specific capacity, as well as superior cycle stability.

[0019] 3. Lithium oxalate, catalysts, etc. are loaded onto the PE separator by surface coating. This lithium replenishment method is highly flexible. The load and distribution can be adjusted according to the actual needs of the battery. Moreover, the lithium replenishment function can be achieved without significantly increasing the battery volume and weight, which is conducive to the realization of high energy density of lithium batteries. Attached Figure Description

[0020] Figure 1 This is a TEM characterization image of the composite catalyst in Example 1 of the present invention; Figure 2 This is the EDS mapping diagram of the composite catalyst in Example 1 of the present invention; Figure 3 This is a magnified TEM image of the composite catalyst in Example 1 of this invention. Figure 4 This is a SEM characterization image of the composite catalyst in Example 1 of this invention; Figure 5 This is the XRD characterization diagram of the composite catalyst in Example 1 of the present invention; Figure 6 These are XRD characterization comparison diagrams of the composite catalysts in Examples 1-2 and Comparative Example 1 of this invention; Figure 7 This is the XPS characterization image of the composite catalyst in Comparative Example 1 of this invention. Figure 7 In the figure, 'a' represents the C1s spectrum of the composite catalyst in Comparative Example 1. Figure 7 In the diagram, b represents the Mo 3d spectrum of the composite catalyst in Comparative Example 1. Figure 7 In the figure, c represents the Co 2p spectrum of the composite catalyst in Comparative Example 1; Figure 8 This is the XPS characterization image of the composite catalyst in Example 1 of this invention. Figure 8 In this diagram, 'a' represents the C1s spectrum of the composite catalyst in Example 1. Figure 8 In the image, b represents the Mo 3d spectrum of the composite catalyst in Example 1. Figure 8 In this context, 'c' represents the Co 2p spectrum of the composite catalyst in Example 1. Figure 9 This is a contact angle diagram of the modified PE separator with low decomposition voltage and high lithium replenishment capacity in Embodiment 1 of the present invention; Figure 10 This is a contact angle diagram of the modified PE separator with low decomposition voltage and high lithium replenishment capacity in Embodiment 2 of the present invention; Figure 11 This is a contact angle diagram of the modified PE membrane in Comparative Example 1 of the present invention; Figure 12 This is a comparison diagram of stress-strain curves of different modified PE membranes of the present invention; Figure 13This is a comparison chart of the critical current densities of different modified PE membranes of the present invention; Figure 14 This is a graph showing the change in lithium-ion transference number of the modified PE membrane with low decomposition voltage and high lithium replenishment capacity in Example 1 of the present invention. Figure 14 The large image in the image is the it curve corresponding to Example 1. Figure 14 The small figures in the diagram are impedance diagrams before and after polarization corresponding to Example 1; Figure 15 This is a graph showing the lithium-ion transference number variation of the modified PE membrane with low decomposition voltage and high lithium replenishment capacity in Example 2 of the present invention. Figure 15 The large image in the image is the it curve corresponding to Example 2. Figure 15 The small figures in the diagram are impedance diagrams before and after polarization corresponding to Example 2; Figure 16 This is a graph showing the change in lithium-ion transference number of the modified PE membrane in Comparative Example 1 of this invention; Figure 16 The larger image in the chart is the it curve corresponding to Comparative Example 1. Figure 16 The smaller figures in the diagram are impedance diagrams before and after polarization, corresponding to Comparative Example 1. Figure 17 This is a comparison chart of lithium oxalate decomposition voltage plateau curves during different button battery activation processes according to the present invention. Figure 18 This is a comparison chart of dQ / dV curves during the activation process of different button batteries according to the present invention; Figure 19 This is a comparison chart of charge and discharge curves during the activation process of different button batteries according to the present invention; Figure 20 This is a comparison chart of the first-cycle lithium replenishment capacity during the activation process of different button batteries according to the present invention; Figure 21 This is a comparison chart of the cycle stability of different lithium symmetric batteries of the present invention; Figure 22 This is a comparison chart of the cycle curves of different full cells of the present invention. Detailed Implementation

[0021] This invention provides a method for preparing a modified PE separator with low decomposition voltage and high lithium replenishment capacity, comprising the following steps: S1. Mix C3N4, molybdenum source, cobalt source and citric acid to obtain the precursor; S2. The precursor is sintered under a protective atmosphere to obtain a composite catalyst; S3. Mix the composite catalyst, conductive agent, binder, lithium oxalate and solvent to obtain a slurry; S4. The slurry is coated on the surface of the PE membrane and dried to obtain a modified PE membrane with low decomposition voltage and high lithium replenishment capacity.

[0022] In this invention, the preparation method of C3N4 described in S1 includes the following steps: melamine is thoroughly ground and sintered to obtain C3N4.

[0023] In this invention, the sintering heating rate is 2℃ / min-3℃ / min, the target sintering temperature is 500℃-600℃, and the sintering holding time is 5h-7h.

[0024] In this invention, the molybdenum source S1 includes one or more of ammonium molybdate and molybdenum oxide, and the cobalt source includes one or more of cobalt nitrate, cobalt chloride, and cobalt acetate.

[0025] In this invention, ammonium molybdate includes ammonium molybdate tetrahydrate, and cobalt nitrate includes cobalt nitrate hexahydrate.

[0026] In this invention, in S1, the ratio of the amount of carbon in C3N4 to the amount of molybdenum in the molybdenum source is (10-12):(1-1.2); the ratio of the amount of molybdenum in the molybdenum source to the amount of cobalt in the cobalt source is 10:(1-7); and the ratio of the total amount of molybdenum in the molybdenum source and cobalt in the cobalt source to the amount of citric acid is (0.5-1.5):(1-2).

[0027] In this invention, the molar ratio of molybdenum in the molybdenum source to cobalt in the cobalt source is 10:(1-7), preferably 10:3. The cobalt source assists the molybdenum source in electron transfer. When the cobalt content is too low, the ability to assist electron transfer is weakened, thereby reducing the catalytic effect. When the cobalt content is too high, the relative content of molybdenum carbide per unit volume decreases when the total amount of composite catalyst is constant, thereby reducing the catalytic effect.

[0028] In this invention, in S1, the mixing is carried out in a mixed solution of ethanol and water, with a volume ratio of ethanol to water of (0.5-1.5):(0.5-1.5), and the ratio of the total mass of the mixed solution of ethanol and water to the mass of C3N4 is (15-20):1.

[0029] In this invention, in S1, the mixing temperature is 50℃-70℃, the stirring speed is 500rpm-700rpm, and the time is 1h-3h.

[0030] In this invention, in step S1, after mixing, the precursor is obtained by drying and ball milling in sequence.

[0031] In this invention, the drying temperature is 110℃-130℃, the drying time is 11h-13h, the ball-to-material ratio of the ball mill is (3-5):1, the ball milling speed is 300rpm-500rpm, and the ball milling time is 2h-4h.

[0032] In this invention, the protective atmosphere in S2 includes one or more of argon and hydrogen; the heating rate of the sintering is 8℃ / min-12℃ / min, the target sintering temperature is 780℃-820℃, and the holding time of the sintering is 1.5h-2.5h.

[0033] In this invention, the conductive agent in S3 includes one or more of carbon black conductive agent and graphene composite conductive agent; the binder includes polyvinylidene fluoride; and the solvent includes one or more of N-methylpyrrolidone and dimethylformamide.

[0034] In this invention, the mass ratio of the composite catalyst, conductive agent, binder, and lithium oxalate in S3 is (15-25):(5-15):(2-8):(60-70); the mass fraction of solids in the slurry is 15%-25%.

[0035] In this invention, step S3, mixing includes the following steps: first mixing the composite catalyst, conductive agent, binder, and lithium oxalate, then adding a solvent, and ball milling to complete the mixing.

[0036] In this invention, the ball-to-material ratio of the ball mill is (2-4):1, the ball milling speed is 300rpm-500rpm, and the ball milling time is 5h-7h.

[0037] In this invention, in step S4, the coating method includes single-sided coating using a bar coating method, wherein the bar type used in the bar coating method includes OSP-1.5.

[0038] In this invention, in S4, the ratio of the coating thickness to the thickness of the PE separator is (1-2):(10-20).

[0039] In this invention, S4, drying includes a first drying and a second drying performed sequentially.

[0040] In this invention, the temperature of the first drying is 20℃-30℃ and the time of the first drying is 2h-4h; the second drying is carried out under vacuum conditions, the temperature of the second drying is 50℃-70℃ and the time of the second drying is 11h-13h.

[0041] The present invention also provides a modified PE membrane with low decomposition voltage and high lithium replenishment capacity, which is prepared by the above-described method for preparing a modified PE membrane with low decomposition voltage and high lithium replenishment capacity.

[0042] The present invention will be further described below with reference to the accompanying drawings and embodiments. Unless otherwise defined, the technical or scientific terms used in this invention should be understood in their ordinary sense by those skilled in the art. The features mentioned above or in the specific examples mentioned in this invention can be combined arbitrarily, and these specific embodiments are only used to illustrate the invention and are not intended to limit the scope of the invention.

[0043] All raw materials used in the embodiments of this invention are commercially available, including: conductive agents such as acetylene black, Ketjen black, Kaina D5315, and Lier LA205. Kaina D5315 was purchased from Xiamen Kaina Graphene Technology Co., Ltd.; and Lier LA205 was purchased from Sichuan Lier New Material Technology Co., Ltd. The PE diaphragm was purchased from Dongguan Kelude New Energy Technology Co., Ltd., and its thickness is 12μm.

[0044] Example 1 This embodiment provides a method for preparing a modified PE separator with low decomposition voltage and high lithium replenishment capacity, comprising the following steps: Melamine was thoroughly ground and then sintered in air. The sintering heating rate was set at 2.5℃ / min, the target temperature was 550℃, and the holding time to reach the target temperature was 6h, resulting in C3N4.

[0045] C3N4, ammonium molybdate tetrahydrate, cobalt nitrate hexahydrate, citric acid, ethanol, and water were mixed at 60°C and 600 rpm for 2 hours. The molar ratio of carbon in C3N4 to molar molybdenum in the molybdenum source was 11:1.1; the molar ratio of molybdenum in the molybdenum source to cobalt in the cobalt source was 10:3; the molar ratio of the total molar amount of molybdenum and cobalt in the molybdenum source to the molar amount of citric acid was 1:1.5; the volume ratio of ethanol to water was 1:1; and the mass ratio of the total mass of the ethanol and water mixture to the mass of C3N4 was 17.5:1. After mixing, the mixture was dried at 120°C for 12 hours. Finally, the dried powder was ball-milled at a ball-to-material ratio of 4:1, a speed of 400 rpm, and a time of 3 hours to obtain the precursor.

[0046] The precursor was heated to 800℃ in an argon atmosphere at a heating rate of 10℃ / min and sintered. The holding time at 800℃ was set to 2h to obtain the composite catalyst.

[0047] A mixture of composite catalyst, acetylene black, Ketjen black, Kayner D5315, Lyle LA205, polyvinylidene fluoride, and lithium oxalate (mass ratio of composite catalyst, acetylene black, Ketjen black, Kayner D5315, Lyle LA205, polyvinylidene fluoride, and lithium oxalate was 20.36:6:2:0.2:0.2:5.08:66.16) was added, and the mixture was ball-milled at a ball-to-material ratio of 3:1, a rotation speed of 400 rpm, and a time of 6 hours to obtain a slurry (the mass fraction of solids in the slurry was 20%).

[0048] The slurry was coated on the surface of the PE membrane (single-sided coating was performed using a wire bar coating method, with the wire bar model being OSP-1.5), and the coating thickness was 1.5 μm. After coating, the membrane was dried, first at 25°C for 3 h, and then under vacuum at 60°C for 12 h, to obtain a modified PE membrane with low decomposition voltage and high lithium replenishment capacity.

[0049] Example 2 This embodiment provides a method for preparing a modified PE membrane with low decomposition voltage and high lithium replenishment capacity, which is basically the same as that in Example 1, except that the ratio of the amount of molybdenum in the molybdenum source to the amount of cobalt in the cobalt source is modified to 10:6.

[0050] Comparative Example 1 This comparative example provides a method for preparing a modified PE separator, comprising the following steps: Melamine was thoroughly ground and then sintered in air. The sintering heating rate was set at 2.5℃ / min, the target temperature was 550℃, and the holding time to reach the target temperature was 6h, resulting in C3N4.

[0051] C3N4 and ammonium molybdate tetrahydrate were mixed, wherein the molar ratio of carbon in C3N4 to molar molybdenum in the molybdenum source was 11:1.1. The resulting powder was ball-milled with a ball-to-material ratio of 4:1, a rotation speed of 400 rpm, and a time of 3 hours to obtain the precursor.

[0052] The precursor was heated to 800℃ in an argon atmosphere at a heating rate of 10℃ / min and sintered. The holding time at 800℃ was set to 2h to obtain the composite catalyst.

[0053] A mixture of composite catalyst, acetylene black, Ketjen black, Kayner D5315, Lyle LA205, polyvinylidene fluoride, and lithium oxalate (mass ratio of composite catalyst, acetylene black, Ketjen black, Kayner D5315, Lyle LA205, polyvinylidene fluoride, and lithium oxalate was 20.36:6:2:0.2:0.2:5.08:66.16) was added, and the mixture was ball-milled at a ball-to-material ratio of 3:1, a rotation speed of 400 rpm, and a time of 6 hours to obtain a slurry (the mass fraction of solids in the slurry was 20%).

[0054] The slurry was coated on the surface of the PE membrane (single-sided coating was performed using a wire bar coating method, with the wire bar model being OSP-1.5), and the coating thickness was 1.5 μm. After coating, the membrane was dried, first at 25°C for 3 hours, and then under vacuum at 60°C for 12 hours to obtain the modified PE membrane.

[0055] Experimental Example 1 The composite catalyst prepared in Example 1 was characterized by transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), and scanning electron microscopy (SEM). The TEM characterization image of the composite catalyst in Example 1 is shown below. Figure 1 As shown; EDS mapping diagram of the composite catalyst in Example 1, as shown. Figure 2 As shown; a magnified TEM image of the composite catalyst in Example 1, as shown. Figure 3 As shown; SEM characterization image of the composite catalyst in Example 1, as shown. Figure 4 As shown. From Figures 1-3 As can be seen, Mo2C and Co are in close contact, and the Mo2C and Co particles are interspersed, which is conducive to electron transfer. From... Figure 4 As can be seen, the composite catalyst powder has a spherical morphology, which is more conducive to contact catalysis with lithium oxalate.

[0056] Experimental Example 2 The composite catalysts prepared in Examples 1-2 and Comparative Example 1 were characterized by X-ray diffraction (XRD). The XRD pattern of the composite catalyst in Example 1 is shown below. Figure 5 As shown; XRD characterization comparison diagrams of the composite catalysts in Examples 1-2 and Comparative Example 1, as shown. Figure 6 As shown. From Figures 5-6 As can be seen from the data, the obvious crystal structure of the composite catalyst in Example 1 is Mo2C, Co, and MoC. Comparing Example 1 and Example 2 with Comparative Example 1, it was found that after introducing Co, the crystallinity of MoC decreased, while the crystallinity of Mo2C and Co increased.

[0057] Experimental Example 3 The composite catalysts prepared in Example 1 and Comparative Example 1 were characterized by X-ray photoelectron spectroscopy (XPS). The XPS characterization patterns of the composite catalysts in Example 1 and Comparative Example 1 are shown below. Figure 7 As shown; Figure 7 In the figure, 'a' represents the C1s spectrum of the composite catalyst in Comparative Example 1. Figure 7 In the diagram, b represents the Mo 3d spectrum of the composite catalyst in Comparative Example 1. Figure 7 In the figure, c represents the Co 2p spectrum of the composite catalyst in Comparative Example 1.

[0058] XPS characterization of the composite catalyst in Example 1, as shown below. Figure 8 As shown; Figure 8 In this diagram, 'a' represents the C1s spectrum of the composite catalyst in Example 1. Figure 8 In the image, b represents the Mo 3d spectrum of the composite catalyst in Example 1. Figure 8 In the figure, c represents the Co 2p spectrum of the composite catalyst in Example 1.

[0059] from Figures 7-8 From the results, we can see that the C 1s spectrum shows that, compared to Comparative Example 1, the peak intensity of C-Mo decreased after the introduction of Co in Example 1, indicating that the introduction of Co affected the bonding environment between C and Mo. The Mo 3d spectrum shows that... 2+ and Mo 4+ The peak is a characteristic peak of Mo-C, Mo 6+ The peak is a result of partial surface oxidation. Mo 2+ Mo 4+ 3D 5 / 2 The orbital (main peak on the right) binding energies underwent negative shifts of 0.1 eV and 0.53 eV, respectively, indicating an increase in the electron density around Mo atoms in Mo2C.

[0060] The mechanism is as follows: when the surrounding electron density increases, the shielding effect between electrons is enhanced, weakening the attraction of the atomic nucleus to the outer electrons, leading to a decrease in binding energy and a rightward shift of the peak. Co 2p spectrum: After introducing Co in Example 1, the Co element exhibits Co 2p spectrum. 0 Co 2+ Co 3+ The state of Co 0 This represents elemental Co. 2+ Co 3+ The presence of Co indicates that it exists not only as an element, but also that a small number of Co atoms enter the Mo₂C lattice and interact with it. Co is reduced by cobalt nitrate hexahydrate, tending towards metallization, which reduces the surrounding electron density. The opposite shift in the binding energy between Co and Mo confirms electron transfer in the Co-Mo₂C system.

[0061] Experiment Example 4 The modified PE membranes with low decomposition voltage and high lithium replenishment capacity prepared in Examples 1-2 and the modified PE membrane prepared in Comparative Example 1 were subjected to electrolyte contact angle tests (the electrolyte was (KLD-LP06): 1.0 mol / L lithium hexafluorophosphate (LiPF6), the solvent was ethylene carbonate (EC) and dimethyl carbonate (DMC) mixed at a volume ratio of 1:1, and 5% fluoroethylene carbonate (FEC) was added).

[0062] The contact angle diagram of the modified PE separator with low decomposition voltage and high lithium replenishment capacity in Example 1 is obtained, as shown below. Figure 9 As shown; the contact angle diagram of the modified PE separator with low decomposition voltage and high lithium replenishment capacity in Example 2, as shown. Figure 10 As shown; the contact angle diagram of the modified PE membrane in Comparative Example 1, as shown. Figure 11 As shown. From Figures 9-11 It can be seen that the modified PE membrane in Example 1 has a smaller electrolyte contact angle and better wettability to the electrolyte.

[0063] Experimental Example 5 The modified PE membranes with low decomposition voltage and high lithium replenishment capacity prepared in Examples 1-2, and the modified PE membrane prepared in Comparative Example 1, were subjected to stress-strain tests. Taking Example 1 as an example, a membrane with a width of 2 cm and a length of 10 cm was used for testing, and the strain rate was set to 10 mm / min. Finally, a comparison graph of stress-strain curves of different modified PE membranes was obtained, as shown in the figure. Figure 12 As shown. From Figure 12 It can be seen that, compared with Comparative Example 1, the mechanical properties of the modified PE membrane in Examples 1 and 2, especially in Example 1, have been greatly improved.

[0064] Experimental Example 6 The modified PE membranes with low decomposition voltage and high lithium replenishment capacity prepared in Examples 1-2, and the modified PE membrane prepared in Comparative Example 1, were respectively assembled into lithium symmetric batteries for critical current density (CCD) testing. During the test, the current density was set from 0.01 mAh / cm². 2 Gradually increase to 20mAh / cm 2 The voltage variation with current density was recorded. A comparison graph of the critical current densities of different modified PE membranes was obtained, as shown below. Figure 13 As shown. From Figure 13 It can be seen that the modified PE membrane in Example 1 has a higher critical current density and a lower overpotential. In addition, considering the critical current density, overpotential and mechanical properties, the modified PE membrane in Example 1 has the best overall performance.

[0065] Experimental Example 7 The modified PE membranes with low decomposition voltage and high lithium replenishment capacity prepared in Examples 1-2, and the modified PE membrane prepared in Comparative Example 1, were subjected to lithium-ion transference number tests. Each membrane was assembled into a lithium symmetric battery, and its impedance value before polarization was measured. Then, a constant voltage polarization of 10mV DC voltage was applied for 10000s and the it curve was recorded. After polarization, the impedance value (impedance after polarization) was measured again.

[0066] The lithium-ion transference number variation diagram of the modified PE membrane with low decomposition voltage and high lithium replenishment capacity in Example 1 is obtained, as shown in the figure. Figure 14 As shown; Figure 14 The large image in the image is the it curve corresponding to Example 1. Figure 14 The small figures in the diagram are impedance diagrams before and after polarization corresponding to Example 1.

[0067] The lithium-ion transference number variation of the modified PE membrane with low decomposition voltage and high lithium replenishment capacity in Example 2 is shown in the figure below. Figure 15 As shown; Figure 15 The large image in the image is the it curve corresponding to Example 2. Figure 15 The small figures in the diagram are impedance diagrams before and after polarization for Example 2.

[0068] The graph showing the change in lithium-ion transference number of the modified PE membrane in Comparative Example 1 is shown below. Figure 16 As shown; Figure 16 The larger image in the chart is the it curve corresponding to Comparative Example 1. Figure 16 The small figure in the diagram is the impedance diagram before and after polarization, corresponding to Comparative Example 1.

[0069] contrast Figures 14-16 It can be seen that the lithium-ion transference number of the modified PE membrane in Example 1 is 0.52, and the lithium-ion transference number of the modified PE membrane in Example 2 is 0.35, which is higher than the lithium-ion transference number of the modified PE membrane 2 in Comparative Example 1 (0.22).

[0070] Experimental Example 8 The modified PE separators with low decomposition voltage and high lithium replenishment capacity prepared in Examples 1-2, and the modified PE separator prepared in Comparative Example 1, were respectively assembled into button cells with aluminum foil as the positive electrode and lithium sheet as the negative electrode (the separator coating side faces the positive electrode). These button cells were activated at a rate of 0.05C with a charge-discharge voltage range of 2.8V-4.3V.

[0071] A comparison of lithium oxalate decomposition voltage plateau curves during the activation process of different button batteries was obtained, as shown in the figure. Figure 17 As shown; will Figure 17 Differential processing was performed to obtain a comparison graph of dQ / dV curves during the activation process of different button cells, as shown below. Figure 18 As shown. By Figures 17-18 It can be seen that the battery assembled in Example 1 exhibits the lowest lithium oxalate decomposition voltage plateau, with a value of approximately 4.15V. This indicates that the composite catalyst used in Example 1 has the best catalytic effect on lithium oxalate decomposition, significantly reducing the decomposition overpotential of lithium oxalate, allowing it to decompose at 4.15V.

[0072] Experimental Example 9 PE separators, modified PE separators with low decomposition voltage and high lithium replenishment capacity prepared in Examples 1-2, and modified PE separators prepared in Comparative Example 1 were assembled into button cells with nickel-cobalt-manganese 811 as the positive electrode and lithium foil as the negative electrode (separator coating side facing the positive electrode). These button cells were activated at a 0.05C rate with a charge / discharge voltage range of 2.8V-4.3V. Comparison of charge / discharge curves for different button cells during activation was obtained, as shown in the figure. Figure 19 As shown in the figure. A comparison chart of the first-cycle lithium replenishment capacity during the activation process of different coin cells, as shown in the figure. Figure 20 As shown. Figure 19 The results show that the battery assembled in Example 1 exhibits the highest specific charge capacity during the activation process. Figure 20 It can be seen that the battery assembled in Example 1 has the highest lithium replenishment capacity during the first charge cycle in the activation process.

[0073] Experimental Example 10 The modified PE membranes with low decomposition voltage and high lithium replenishment capacity prepared in Examples 1-2, and the modified PE membrane prepared in Comparative Example 1, were respectively assembled into lithium symmetric batteries. During cycle performance testing, the constant current charging time was set to 1 hour, the constant current charging current to 0.5 mA, the constant current discharging time to 1 hour, the constant current discharging current to 0.5 mA, and the number of cycles to 1000. These batteries were subjected to cycle performance testing, and a comparison chart of the cycle stability of different lithium symmetric batteries was obtained, as shown below. Figure 21 As shown. By Figure 21 It can be seen that, under the same cycling conditions, the lithium symmetric battery assembled in Example 1 has the best cycle stability.

[0074] Experimental Example 11 The modified PE separators with low decomposition voltage and high lithium replenishment capacity prepared in Examples 1-2, and the modified PE separator prepared in Comparative Example 1, were respectively assembled into full cells with nickel-cobalt-manganese 811 as the positive electrode and silicon-carbon material as the negative electrode. These cells were subjected to cycle performance tests at a 0.5C rate, with the charge / discharge voltage range set at 2.8V-4.3V. Comparison graphs of the cycle curves of different full cells were obtained, as shown below. Figure 22 As shown. By Figure 22 It can be seen that the full cell assembled in Example 1 has better cycle stability and capacity retention.

[0075] Therefore, the present invention adopts the above preparation method to obtain a composite catalyst by sintering a multiphase precursor, mixing conductive agent, binder, lithium oxalate, etc. into a slurry, and coating it on a PE separator. This can significantly reduce the decomposition potential of lithium oxalate, effectively solve the problem of lithium oxalate being difficult to efficiently replenish lithium in the ternary cathode system, improve the specific capacity released by lithium oxalate at low voltage, and achieve the effect of high lithium replenishment capacity.

[0076] In the composite catalyst of this invention, the empty orbitals of molybdenum carbide play an adsorption catalytic role on the lone pair electrons of lithium oxalate, the highly conductive carbon structure establishes a fast transport channel for electron and ion migration, and the introduced cobalt element can change the electron cloud distribution and surface chemical environment of molybdenum carbide, increase active centers, reduce the energy barrier for lithium release from lithium oxalate, accelerate the reaction kinetics between lithium ions and active materials, promote the insertion and extraction of lithium ions, thereby improving the lithium replenishment efficiency, enabling the battery to complete the charge and discharge process more quickly, and improving the rate performance of the battery.

[0077] Meanwhile, cobalt can fill the lattice defects or interstitial sites of molybdenum carbide, supporting and reinforcing the lattice structure, enhancing the crystal structure stability of molybdenum carbide, and thus improving the overall stability of the coating. In addition, the presence of the lithium-filling coating (the coating formed after the slurry dries) significantly improves the mechanical strength of the PE separator. Combined with the promoting effect of the composite catalyst on reaction kinetics, the assembled battery exhibits excellent cycle capacity retention and high specific capacity, as well as superior cycle stability.

Claims

1. A method for preparing a modified PE separator having a low decomposition voltage and a high lithium supplement capacity, characterized by, The method comprises the following steps: S1. mixing C3N4, a molybdenum source, a cobalt source and citric acid to obtain a precursor; S2. sintering the precursor under a protective atmosphere to obtain a composite catalyst; S3. mixing the composite catalyst, a conductive agent, a binder, lithium oxalate and a solvent to obtain a slurry; S4. coating the slurry on the surface of a PE separator, and drying to obtain a modified PE separator with low decomposition voltage and high lithium supplement capacity.

2. The method of claim 1, wherein the modified PE separator having a low decomposition voltage and a high lithium supplement capacity is prepared by the steps of: (a) preparing a PE separator; (b) coating the PE separator with a lithium supplement material; and (c) drying the coated PE separator. The molybdenum source in S1 includes one or more of ammonium molybdate and molybdenum oxide, and the cobalt source includes one or more of cobalt nitrate, cobalt chloride and cobalt acetate.

3. The method of claim 1 or 2, wherein the modified PE separator having a low decomposition voltage and a high lithium supplement capacity is prepared by the steps of: (a) preparing a PE separator; (b) coating the PE separator with a lithium supplement material; and (c) drying the PE separator coated with the lithium supplement material. In S1, the ratio of the amount of substance of carbon in C3N4 to the amount of substance of molybdenum in the molybdenum source is (10-12):(1-1.2); the ratio of the amount of substance of molybdenum in the molybdenum source to the amount of substance of cobalt in the cobalt source is 10:(1-7); and the total amount of substance of molybdenum in the molybdenum source and cobalt in the cobalt source to the amount of substance of citric acid is (0.5-1.5):(1-2).

4. The method of claim 1, wherein the modified PE separator having a low decomposition voltage and a high lithium supplement capacity is prepared by the steps of: (a) preparing a PE separator; (b) coating the PE separator with a lithium supplement material; and (c) drying the coated PE separator. In S1, the mixing is performed in a mixed solution of ethanol and water, and the volume ratio of ethanol to water is (0.5-1.5):(0.5-1.5).

5. The method of claim 1, wherein the modified PE separator having a low decomposition voltage and a high lithium supplement capacity is prepared by the steps of: (a) preparing a PE separator; (b) coating the PE separator with a lithium supplement material; and (c) drying the coated PE separator. The protective atmosphere in S2 includes one or more of argon and hydrogen; the sintering has a heating rate of 8-12 ℃ / min, a target temperature of 780-820 ℃ and a holding time of 1.5-2.5 h.

6. The method of claim 1, wherein the modified PE separator having a low decomposition voltage and a high lithium supplement capacity is prepared by the steps of: (a) preparing a PE separator; (b) coating the PE separator with a lithium supplement material; and (c) drying the coated PE separator. The conductive agent in S3 includes one or more of carbon black conductive agent and graphene composite conductive agent; the binder includes polyvinylidene fluoride; and the solvent includes one or more of N-methylpyrrolidone and dimethylformamide.

7. The method of claim 1 or 6, wherein the modified PE separator having a low decomposition voltage and a high lithium supplement capacity is prepared by the steps of: (a) preparing a PE separator; (b) coating the PE separator with a lithium supplement material; and (c) drying the coated PE separator. In S3, the mass ratio of the composite catalyst, the conductive agent, the binder and lithium oxalate is (15-25):(5-15):(2-8):(60-70); and the mass fraction of solid substances in the slurry is 15%-25%.

8. The method of claim 1, wherein the modified PE separator having a low decomposition voltage and a high lithium supplement capacity is prepared by the steps of: (a) preparing a PE separator; (b) coating the PE separator with a lithium supplement material; and (c) drying the coated PE separator. In S4, the ratio of the coating thickness to the thickness of the PE separator is (1-2):(10-20).

9. The method of claim 1 or 8, wherein the modified PE separator having a low decomposition voltage and a high lithium supplement capacity is prepared by the steps of: (a) preparing a PE separator; (b) coating the PE separator with a lithium supplement material; and (c) drying the PE separator coated with the lithium supplement material. In S4, the drying includes first drying and second drying performed in sequence; The first drying has a temperature of 20-30 ℃ and a time of 2-4 h; and the second drying has a temperature of 50-70 ℃ and a time of 11-13 h.

10. A modified PE separator having a low decomposition voltage and a high lithium supplement capacity, characterized by, The modified PE separator with low decomposition voltage and high lithium supplement capacity is prepared by the method according to any one of claims 1-9.

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