Lithium replenishing membrane and preparation method thereof, composite lithium replenishing diaphragm and lithium ion battery

Abstract

The present invention belongs to the field of lithium-ion battery technology, specifically relating to a lithium-replenishing membrane and its preparation method, a composite lithium-replenishing separator, and a lithium-ion battery. The lithium-replenishing membrane comprises a binder, a conductive polymer, and lithium doped in the conductive polymer. The binder is fiberized and then pressed to form a networked binder structure, with the conductive polymer dispersed on the binder. The lithium-replenishing membrane provided by the present invention can improve the initial coulombic efficiency and cycle performance of lithium-ion batteries. Its preparation process does not require solvents and is simple to operate.

Description

Technical Field

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

[0002] The basic components of lithium-ion batteries include positive and negative electrodes, separators, electrolytes, and other components. The separator's primary function is to isolate the positive and negative electrodes. Its porous structure acts as a channel for lithium ion conduction between the positive and negative electrodes, and can also adsorb electrolyte to ensure the battery's long-term cycle life. During the initial battery cycle, the solid electrolyte membrane generated on the negative electrode surface consumes lithium ions in the system, resulting in a decrease in the battery's initial coulombic efficiency. To replenish the lost lithium ions in the system, researchers in related technical fields use lithium replenishment technology, which involves using additional lithium sources to replenish these lithium ions and provide more lithium ions for the battery's long-term cycle life.

[0003] Chinese patent CN113078417A discloses a composite lithium-supplementing diaphragm and its secondary battery. The diaphragm uses progressive lithium-supplementing layers. The first lithium-supplementing layer contains a small amount of lithium, the second layer is a high-content lithium layer, and the outermost layer is a ceramic layer. Its preparation process requires multiple slurry mixing and coating. The preparation process is very cumbersome and does not meet the needs of mass production.

[0004] Therefore, it is of great significance to develop a lithium-replenishing membrane that does not require solvents in the preparation process, is simple to operate, and can improve the initial coulombic efficiency and cycle performance of lithium-ion batteries. Summary of the Invention

[0005] In view of the above-mentioned deficiencies in the prior art, the purpose of the present invention is to provide a lithium replenishing membrane and its preparation method, a composite lithium replenishing separator and a lithium ion battery. The lithium replenishing membrane provided by the present invention can improve the initial coulombic efficiency and cycle performance of lithium ion batteries, and its preparation process does not require solvents and is simple to operate.

[0006] In order to achieve the above object, the present invention adopts the following technical solutions:

[0007] In a first aspect, the present invention provides a lithium replenishing membrane, which includes a binder, a conductive polymer and lithium doped in the conductive polymer. The binder is fiberized and then pressed to form a binder with a network structure, and the conductive polymer is dispersed on the binder.

[0008] The lithium replenishing membrane provided by the present invention can improve the initial coulombic efficiency and cycle performance of lithium-ion batteries, and its preparation process does not require a solvent and is simple to operate.

[0009] In the above lithium replenishing membrane, as a preferred embodiment, the binder includes at least one of polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), sodium carboxymethyl cellulose (CMC), polyethylene (PE), and polypropylene (PP).

[0010] In the above lithium replenishing film, as a preferred embodiment, the conductive polymer includes at least one of polyacetylene, polythiophene, polypyrrole, polyaniline, polyparaphenylene vinylene, and polyparaphenylene.

[0011] In the above-mentioned lithium replenishing membrane, as a preferred embodiment, the lithium source is metallic lithium powder.

[0012] In the above lithium replenishing film, as a preferred embodiment, the mass ratio of the conductive polymer, the lithium doped in the conductive polymer and the binder is (65-97): (1-30): (2-5), for example, it can be 68:30:2, 78:20:2, 88:10:2 or 90:5:5, etc.

[0013] In the above lithium replenishing film, as a preferred embodiment, the thickness of the lithium replenishing film is 1-10 μm (for example, 1 μm, 3 μm, 5 μm, 7 μm or 10 μm, etc.), preferably 1-5 μm.

[0014] In a second aspect, the present invention provides a method for preparing the lithium-replenishing membrane according to the first aspect, comprising: subjecting the conductive polymer, the lithium source, and the binder to a mixing process, a fiberizing process, and a sheeting process to obtain an initial thin sheet; then applying an electric current to the initial thin sheet to complete the doping of lithium in the conductive polymer to obtain the lithium-replenishing membrane.

[0015] The preparation method of the lithium replenishing membrane provided by the present invention does not use a solvent, thereby avoiding the influence of the solvent on lithium metal and the production of lithium by-products. It has high safety and simple operation and meets the needs of mass production.

[0016] In the above-mentioned method for preparing the lithium replenishing membrane, as a preferred embodiment, the preparation method comprises the following steps:

[0017] S1 (mixing step), uniformly mixing the conductive polymer, the lithium source and the binder in a mixer to obtain a first mixture;

[0018] S2 (fiberization step), fiberizing the first mixed material to obtain a second mixed material;

[0019] S3 (sheeting process), pressing the second mixed material into an initial film sheet through a pressing roller;

[0020] S4. Connecting the initial membrane to an external power source to apply current to the initial membrane to complete lithium doping in the conductive polymer and obtain the lithium-replenishing membrane.

[0021] In the above-mentioned method for preparing the lithium replenishing film, as a preferred embodiment, the mass ratio of the conductive polymer, the lithium source and the binder is (65-97): (1-30): (2-5), for example, it can be 68:30:2, 78:20:2, 88:10:2 or 90:5:5, etc.

[0022] In the above-mentioned method for preparing the lithium replenishing membrane, as a preferred embodiment, in step S1, the rotation speed of the mixer is 1000-8000 rpm (for example, it can be 1000 rpm, 3000 rpm, 5000 rpm or 8000 rpm, etc.), and the mixing time is 10-240 min (for example, it can be 10 min, 50 min, 100 min, 150 min or 240 min, etc.).

[0023] In the above-mentioned method for preparing the lithium replenishing membrane, as a preferred embodiment, in step S2, the fiberization method is air flow milling, the air flow pressure of the air flow milling is 0.1-1MPa (for example, it can be 0.1MPa, 0.3MPa, 0.5MPa, 0.7MPa or 1MPa, etc.), and the gas used in the air flow milling is nitrogen and / or argon.

[0024] In the above-mentioned method for preparing the lithium-replenishing membrane, as a preferred embodiment, in step S3, the second mixture is pressed into an initial membrane sheet through a vertical hot pressing roller and a horizontal hot pressing roller, wherein the temperature of the hot pressing roller is 30-80°C (for example, 30°C, 50°C or 80°C, etc.), preferably 40-60°C.

[0025] In the above-mentioned method for preparing the lithium-replenishing film, as a preferred embodiment, in step S4, the two ends of the initial membrane are connected to an external power supply (external circuit) to apply current to the initial membrane to complete the lithium doping in the conductive polymer and obtain the lithium-replenishing film (lithium-doped conductive polymer sheet).

[0026] In the above-mentioned method for preparing the lithium replenishing membrane, as a preferred embodiment, in step S4, the current is 0.1-100mA (for example, it can be 0.1mA, 1mA, 5mA, 10mA, 30mA, 50mA, 70mA or 100mA, etc.), preferably 10-50mA, and the time for applying the current is 1-24h (for example, it can be 1h, 5h, 10h, 15h, 20h or 24h, etc.).

[0027] In a third aspect, the present invention provides a composite lithium-replenishing diaphragm, comprising a porous base membrane and a lithium-replenishing membrane arranged on at least one side of the porous base membrane, wherein the lithium-replenishing membrane is the lithium-replenishing membrane described in the first aspect, or a lithium-replenishing membrane prepared by the preparation method of the lithium-replenishing membrane described in the second aspect.

[0028] Here, the porous base membrane is a conventional separator disposed between the positive electrode sheet and the negative electrode sheet.

[0029] The lithium replenishing membrane provided by the present invention is a thin film prepared after fiberization. The lithium replenishing membrane is constructed from many fibers and has high tensile strength. The network structure has a high porosity and will not block the pores of the porous base membrane, which can ensure the smooth passage of the electrolyte. The lithium replenishing film | porous base membrane structure has a good liquid retention capacity, which can ensure that the battery has sufficient electrolyte during long cycles.

[0030] Compared with conventional separators of the same thickness, the composite lithium-supplementing separator obtained by compounding the lithium-supplementing membrane with the porous base membrane has a lower shrinkage rate and can improve the safety of the battery.

[0031] In the above-mentioned composite lithium-supplementing separator, as a preferred embodiment, the material of the porous base membrane is PE (polyethylene) or PP (polypropylene).

[0032] In the above-mentioned composite lithium-supplementing separator, as a preferred embodiment, only one side of the porous base membrane is coated with ceramic or has no ceramic coating.

[0033] Here, when only one side of the porous base membrane is coated with ceramic, the lithium replenishing membrane provided by the first aspect or the lithium replenishing membrane prepared by the preparation method of the lithium replenishing membrane provided by the second aspect is arranged on the side of the porous base membrane without ceramic coating.

[0034] When the surface of the porous base membrane has no ceramic coating, both sides of the porous base membrane are provided with the lithium replenishing membrane provided by the first aspect, or the lithium replenishing membrane prepared by the preparation method of the lithium replenishing membrane provided by the second aspect.

[0035] In the above-mentioned composite lithium-replenishing separator, as a preferred embodiment, the lithium-replenishing membrane is provided on at least one side of the porous base membrane by thermal composite method.

[0036] The lithium-replenishing film is die-cut into the size that meets the pre-lithium battery cell, and then thermally composited with the diaphragm through a hot pressing roller to prepare a composite lithium-replenishing diaphragm.

[0037] In the above-mentioned composite lithium-supplementing separator, as a preferred embodiment, the temperature of the hot pressing roller used in the thermal composite process is 40-75°C (for example, it can be 40°C, 45°C, 50°C, 60°C, 70°C or 75°C, etc.), preferably 50-60°C, and the pressure of the hot pressing roller used in the thermal composite process is 0.1-0.5MPa (for example, it can be 0.1MPa, 0.2MPa, 0.3MPa or 0.5MPa, etc.), preferably 0.2-0.3MPa.

[0038] In a fourth aspect, the present invention provides a lithium-ion battery comprising a positive electrode sheet, a negative electrode sheet, a separator disposed between the positive electrode sheet and the negative electrode sheet, and an electrolyte, wherein the separator is the composite lithium-supplementing separator provided in the third aspect.

[0039] Compared with the prior art, the present invention has at least one of the following advantages:

[0040] (1) The lithium-replenishing membrane provided by the present invention can improve the initial coulombic efficiency and cycle performance of lithium-ion batteries, and its preparation process does not require solvents and is simple to operate.

[0041] (2) The lithium replenishing membrane provided by the present invention is a thin film prepared after fiberization. The lithium replenishing membrane is constructed of many fibers and has high tensile strength. The network structure has a high porosity and will not block the pores of the diaphragm, which can ensure the smooth passage of electrolyte. The basis for the long cycle of the battery is sufficient electrolyte in the battery. The fiberized structure of the lithium replenishing film has the function of a capillary, which can store more electrolyte and ensure that the battery has sufficient electrolyte during the long cycle.

[0042] (3) Because lithium has a strong reducing property, according to the characteristic of conductive polymers that can be doped with electron acceptors, the lithium in the film made by fiberizing the conductive polymer and lithium will provide electrons to the conductive polymer, thereby enhancing the conductivity of the conductive polymer. After lithium loses electrons, lithium ions will be doped into the molecular chain of the conductive polymer. When the film is energized, the doping of lithium can be accelerated.

[0043] (4) Conductive polymers have high conductivity after being doped with lithium, which can improve the conductivity of the battery, reduce internal resistance, and reduce polarization.

[0044] (5) The lithium-replenishing membrane improves the mechanical strength, thermal shrinkage and corrosion resistance of conventional isolation membranes.

[0045] (6) The lithium doped in the conductive polymer can adaptively replenish the lithium consumed by the negative electrode in the initial stage and the lithium consumed during the battery cycle, thereby achieving effective and safe lithium replenishment of the battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Figure 1 A circuit diagram of the lithium doping process of the present invention;

[0047] Figure 2 A schematic structural diagram of a composite lithium-supplementing diaphragm provided by the present invention;

[0048] Figure 3 A schematic structural diagram of another composite lithium-supplementing diaphragm provided by the present invention;

[0049] Figure 4 This is a schematic structural diagram of a lithium-ion battery cell provided in Example 1 of the present invention.

[0050] The attached figure indicates: 1. Ceramic layer; 2. Porous base membrane; 3. Lithium replenishing membrane; 4. Positive electrode; 5. Negative electrode. DETAILED DESCRIPTION

[0051] In order to make the purpose, technical solutions and advantages of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. It should be understood by those skilled in the art that the embodiments are merely to help understand the present invention and should not be regarded as specific limitations of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative work are within the scope of protection of the present invention.

[0052] The embodiments of the present invention are implemented on the premise of the technical solution of the present invention, and detailed implementation methods and processes are given. However, the protection scope of the present invention is not limited to the following embodiments. The process parameters in the following embodiments that do not specify specific conditions are generally based on conventional conditions.

[0053] The endpoints of the ranges and any values ​​disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoints of each range, the endpoints of each range and individual point values, and the individual point values ​​can be combined with each other to form one or more new numerical ranges, and these numerical ranges should be considered to be specifically disclosed in the present invention.

[0054] Throughout this disclosure, unless otherwise specified and / or explained, all references to component amounts are in parts by weight. Process parameters in the following examples, where specific conditions are not specified, generally follow conventional conditions. The experimental reagents used in the following examples, unless otherwise noted, are conventional biochemical reagents; and the amounts of experimental reagents used, unless otherwise noted, are those used in routine experimental procedures.

[0055] In a first aspect, an embodiment of the present invention provides a lithium replenishing membrane, comprising a binder, a conductive polymer, and lithium doped in the conductive polymer. The binder is fiberized and then pressed to form a binder with a network structure. The conductive polymer is dispersed on the binder. The binder comprises at least one of polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), sodium carboxymethyl cellulose (CMC), polyethylene (PE), and polypropylene (PP). The conductive polymer comprises at least one of polyacetylene, polythiophene, polypyrrole, polyaniline, polyparaphenylene vinylene, and polyparaphenylene. The lithium source is metallic lithium powder. The mass ratio of the conductive polymer, the lithium doped in the conductive polymer, and the binder is (65-97): (1-30): (2-5). The thickness of the lithium replenishing membrane is 1-10 μm.

[0056] In a second aspect, the present invention provides a method for preparing the lithium replenishing membrane according to the first aspect, the preparation method comprising the following steps:

[0057] S1 (mixing step), uniformly mixing the conductive polymer, the lithium source and the binder in a mixer to obtain a first mixture, wherein the mass ratio of the conductive polymer, the lithium source and the binder is (65-97): (1-30): (2-5), the rotation speed of the mixer is 1000-8000 rpm, and the mixing time is 10-240 min.

[0058] S2 (fiberization process), the first mixture is fiberized to obtain a second mixture, the fiberization method is air flow milling, the air flow pressure of the air flow milling is 0.1-1MPa, and the gas used in the air flow milling is nitrogen and / or argon.

[0059] S3 (sheeting process), pressing the second mixed material into an initial film sheet through a vertical hot pressing roller and a horizontal hot pressing roller, wherein the temperature of the hot pressing roller is 30-80°C.

[0060] S4, Figure 1 The circuit diagram of the lithium doping process in the present invention is as follows: Figure 1 As shown, the two ends of the initial membrane are connected to an external power supply (external circuit) to apply current to the initial membrane to complete the lithium doping in the conductive polymer and obtain the lithium-supplementing membrane (lithium-doped conductive polymer sheet), wherein the current is 0.1-100mA and the time of applying the current is 1-24h.

[0061] Compared with the current lithium replenishment technology, the lithium replenishment membrane provided by the present invention has a simple preparation process, a high lithium replenishment amount, a light weight, and a relatively high lithium replenishment amount and weight increase ratio, and is suitable for mass production.

[0062] In a third aspect, the present invention provides a composite lithium-replenishing membrane, comprising a porous base membrane and a lithium-replenishing membrane arranged on at least one side of the porous base membrane, wherein the lithium-replenishing membrane is the lithium-replenishing membrane described in the first aspect, or a lithium-replenishing membrane prepared by the preparation method of the lithium-replenishing membrane described in the second aspect, wherein the material of the porous base membrane is PE (polyethylene) or PP (polypropylene), and only one side of the porous base membrane is coated with ceramic or has no ceramic coating, wherein the lithium-replenishing membrane is arranged on at least one side of the porous base membrane by thermal composite, the lithium-replenishing membrane is die-cut into a size that meets the size of the pre-lithium battery cell, and then thermally composited with the diaphragm by a hot pressing roller to prepare a composite lithium-replenishing membrane, the temperature of the hot pressing roller used in the thermal composite process is 40-75°C, and the pressure of the hot pressing roller used in the thermal composite process is 0.1-0.5MPa.

[0063] Figure 2 A schematic structural diagram of a composite lithium supplement diaphragm provided by the present invention is shown in FIG. Figure 2 As shown, when the ceramic layer 1 is provided on only one side of the porous base membrane 2, the lithium replenishing membrane 3 provided by the first aspect or the lithium replenishing membrane 3 prepared by the preparation method of the lithium replenishing membrane provided by the second aspect is provided on the side of the porous base membrane 2 without the ceramic layer.

[0064] Figure 3 This is a schematic structural diagram of another composite lithium supplement membrane provided by the present invention, such as Figure 3 As shown, when there is no ceramic layer on the surface of the porous base membrane 2 , lithium replenishing membranes 3 are provided on both sides of the porous base membrane 2 .

[0065] In a fourth aspect, the present invention provides a lithium-ion battery comprising a positive electrode sheet, a negative electrode sheet, a separator disposed between the positive electrode sheet and the negative electrode sheet, and an electrolyte, wherein the separator is the composite lithium-supplementing separator provided in the third aspect.

[0066] In order to further understand the present invention, the lithium replenishing membrane and its preparation method, the composite lithium replenishing separator and the lithium ion battery provided by the present invention are described in detail below with reference to the examples. The protection scope of the present invention is not limited by the following examples.

[0067] Example 1

[0068] The lithium-replenishing membrane provided in this embodiment includes a binder, a conductive polymer, and lithium doped in the conductive polymer. The binder is fiberized and then overlapped with each other through a pressing process to form a binder with a network structure. The conductive polymer is dispersed on the binder, wherein the binder is PTFE, the conductive polymer is polyacetylene, and the thickness of the lithium-replenishing membrane is 5 μm.

[0069] The preparation method of the lithium replenishing membrane provided in this embodiment comprises the following steps:

[0070] S1. Add polyacetylene, lithium powder and PTFE in a mass ratio of 88:10:2 into a high-speed mixer and mix them evenly. The speed of the high-speed mixer is 7000 rpm and the mixing time is 20 minutes to obtain a first mixture.

[0071] S2. The first mixed material is fiberized by a jet mill, using high-purity argon gas at a pressure of 0.6 MPa to obtain a second mixed material.

[0072] S3. Press the second mixed material into an initial sheet (initial film) with a thickness of 5 μm through a vertical hot pressing roller and a horizontal hot pressing roller, wherein the temperature of the hot pressing roller is 50° C.

[0073] S4, such as Figure 1 As shown, the two ends of the initial thin film are connected to an external circuit (external power supply), and a current of 60 mA is applied to the initial thin film for 8 hours to complete the lithium doping in the conductive polymer and obtain a pre-lithium thin film (lithium replenishing film).

[0074] The preparation method of the composite lithium-replenishing separator provided in this embodiment includes: thermally compounding the pre-lithium thin sheet (lithium-replenishing membrane) prepared in this embodiment and the PP porous base membrane through a hot pressing roller, laminating the lithium-replenishing membrane on one side of the porous base membrane, and providing a ceramic layer on the other side of the porous base membrane. The ceramic layer has a thickness of 1 μm, and the porous base membrane has a thickness of 9 μm. The thermal compounding pressure is 0.15 MPa and the temperature is 50° C. to obtain the composite lithium-replenishing separator, wherein the length and width of the lithium-replenishing membrane are respectively the same as the length and width of the porous base membrane.

[0075] Figure 4 This is a schematic diagram of the structure of the lithium-ion battery cell provided in this embodiment, as shown in FIG. Figure 4 As shown, the lithium-ion battery provided in this embodiment includes: a positive electrode plate 4 with lithium iron phosphate as the active material, a negative electrode plate 5 with graphite as the active material, an electrolyte of LiPF6 / EC+DEC (volume ratio of 1:1), and a separator that is a composite lithium-replenishing separator prepared in this embodiment (including a ceramic layer 1, a porous base film 2 and a lithium-replenishing membrane 3), and the lithium-replenishing membrane 3 is opposite to the negative electrode plate 5.

[0076] The preparation method of the lithium ion battery provided in this embodiment includes: Figure 4 The structural diagram is used to prepare the battery cells, and the battery cells are assembled into a battery according to the battery preparation process.

[0077] Example 2

[0078] The lithium-replenishing membrane provided in this embodiment includes a binder, a conductive polymer, and lithium doped in the conductive polymer. The binder is fiberized and then overlapped with each other through a pressing process to form a binder with a network structure. The conductive polymer is dispersed on the binder, wherein the binder is PTFE, the conductive polymer is polyaniline, and the thickness of the lithium-replenishing membrane is 3 μm.

[0079] The preparation method of the lithium replenishing membrane provided in this embodiment comprises the following steps:

[0080] S1. Add polyaniline, lithium powder and PTFE in a mass ratio of 78:20:2 into a high-speed mixer at a speed of 6000 rpm for 30 min to obtain a first mixture.

[0081] S2. The first mixed material is fiberized by a jet mill, using high-purity argon gas at a pressure of 0.5 MPa to obtain a second mixed material.

[0082] S3. The second mixed material is pressed into an initial sheet (initial film) with a thickness of 3 μm by a vertical hot pressing roller and a horizontal hot pressing roller, wherein the temperature of the hot roller is 55° C.

[0083] S4, such as Figure 1 As shown, the two ends of the initial thin film are connected to an external circuit (external power supply), and a current of 40 mA is applied to the initial thin film for 6 hours to complete the lithium doping in the conductive polymer and obtain a pre-lithium thin film (lithium replenishing film).

[0084] The preparation method of the composite lithium-replenishing separator provided in this embodiment includes: thermally compounding the pre-lithium thin sheet (lithium-replenishing membrane) prepared in this embodiment and a PP porous base membrane without a ceramic coating by a hot pressing roller, wherein the thickness of the PP porous base membrane is 9 μm, and the pre-lithium thin sheet is laminated on both sides of the PP porous base membrane, the thermal compounding pressure is 0.3 MPa, and the temperature is 60° C., to obtain a composite lithium-replenishing separator, wherein the length and width of the lithium-replenishing membrane are respectively the same as the length and width of the porous base membrane.

[0085] The lithium-ion battery provided in this embodiment includes: a positive electrode plate with lithium iron phosphate as the active material, a negative electrode plate with graphite as the active material, an electrolyte of LiPF6 / EC+DEC (volume ratio 1:1), and an isolation membrane of the composite lithium-supplementing membrane prepared in this embodiment.

[0086] The method for preparing a lithium-ion battery provided in this embodiment includes: assembling a positive electrode sheet, a negative electrode sheet, and a separator into a battery cell, and assembling the battery cell into a battery according to a battery preparation process.

[0087] Example 3

[0088] The lithium-replenishing membrane provided in this embodiment includes a binder, a conductive polymer, and lithium doped in the conductive polymer. The binder is fiberized and then overlapped with each other through a pressing process to form a binder with a network structure. The conductive polymer is dispersed on the binder, wherein the binder is PTFE, the conductive polymer is polypyrrole, and the thickness of the lithium-replenishing membrane is 1 μm.

[0089] The preparation method of the lithium replenishing membrane provided in this embodiment comprises the following steps:

[0090] S1. Add polypyrrole, lithium powder and PTFE in a mass ratio of 68:30:2 into a high-speed mixer and mix them evenly. The speed of the high-speed mixer is 5000 rpm and the mixing time is 50 min to obtain a first mixture.

[0091] S2. The first mixed material is fiberized by a jet mill, using high-purity argon gas at a pressure of 0.3 MPa to obtain a second mixed material.

[0092] S3. Press the second mixed material into an initial sheet (initial film) with a thickness of 1 μm through a vertical hot pressing roller and a horizontal hot pressing roller, wherein the temperature of the hot pressing roller is 40°C.

[0093] S4, such as Figure 1 As shown, the two ends of the initial thin film are connected to an external circuit (external power supply), and a current of 10 mA is applied to the initial thin film for 12 hours to complete the lithium doping in the conductive polymer and obtain a pre-lithium thin film (lithium replenishing film).

[0094] The preparation method of the composite lithium-replenishing separator provided in this embodiment includes: thermally compounding the pre-lithium thin sheet (lithium-replenishing membrane) prepared in this embodiment and a PP porous base membrane without a ceramic coating through a hot pressing roller, wherein the pre-lithium thin sheet is laminated to both sides of the PP porous base membrane, the thickness of the PP porous base membrane is 9 μm, the thermal compounding pressure is 0.1 MPa, and the temperature is 40° C., to obtain a composite lithium-replenishing separator, wherein the length and width of the lithium-replenishing membrane are respectively the same as the length and width of the porous base membrane.

[0095] The lithium-ion battery provided in this embodiment includes: a positive electrode plate with lithium iron phosphate as the active material, a negative electrode plate with graphite as the active material, an electrolyte of LiPF6 / EC+DEC (volume ratio 1:1), and an isolation membrane of the composite lithium-supplementing membrane prepared in this embodiment.

[0096] The method for preparing a lithium-ion battery provided in this embodiment includes: assembling a positive electrode sheet, a negative electrode sheet, and a separator into a battery cell, and assembling the battery cell into a battery according to a battery preparation process.

[0097] Example 4

[0098] The preparation method of the lithium replenishing membrane provided in this embodiment is basically the same as that of Example 1, except that the mass ratio of polyacetylene, lithium powder and PTFE is 70:28:2.

[0099] A composite lithium-supplementing separator was prepared by referring to the preparation method of the composite lithium-supplementing separator provided in Example 1.

[0100] A lithium-ion battery was prepared by referring to the preparation method of the lithium-ion battery provided in Example 1.

[0101] Example 5

[0102] The preparation method of the lithium replenishing film provided in this embodiment is basically the same as that in embodiment 1, except that the thickness of the lithium replenishing film is 3 μm.

[0103] A composite lithium-supplementing separator was prepared by referring to the preparation method of the composite lithium-supplementing separator provided in Example 1.

[0104] A lithium-ion battery was prepared by referring to the preparation method of the lithium-ion battery provided in Example 1.

[0105] Comparative Example 1

[0106] The preparation method of the lithium-ion battery provided in this comparative example is basically the same as that in Example 1, except that the isolation membrane is a PP porous base membrane coated with a ceramic layer on one side (a ceramic layer is provided on one side of the porous base membrane), the thickness of the ceramic layer is 1 μm, and the thickness of the isolation membrane (PP porous base membrane + ceramic layer) is the same as the thickness of the composite lithium-supplementing separator in Example 1, and there is no pre-lithium sheet.

[0107] Comparative Example 2

[0108] The preparation method of the lithium replenishing membrane provided in this comparative example is basically the same as that of Example 1, except that the raw materials for preparing the lithium replenishing membrane are a conductive agent (conductive carbon black), lithium powder and PTFE in a mass ratio of 88:10:2, and specifically comprises the following steps:

[0109] S1. Add conductive carbon black, lithium powder and PTFE in a mass ratio of 88:10:2 into a high-speed mixer and mix them evenly. The speed of the high-speed mixer is 7000 rpm and the mixing time is 20 min to obtain a first mixture.

[0110] S2. The first mixed material is fiberized by a jet mill, using high-purity argon gas at a pressure of 0.6 MPa to obtain a second mixed material.

[0111] S3. Press the second mixed material into a pre-lithium sheet (lithium supplement film) with a thickness of 5 μm through a vertical hot pressing roller and a horizontal hot pressing roller, wherein the temperature of the hot pressing roller is 50° C.

[0112] A composite lithium-supplementing membrane was prepared by referring to the preparation method of the composite lithium-supplementing membrane provided in Example 1. A lithium-ion battery was prepared by referring to the preparation method of the lithium-ion battery provided in Example 1.

[0113] Performance Testing

[0114] Thermal shrinkage tests were performed on the composite lithium-supplementing separator provided in the examples and the separator and composite lithium-supplementing separator without pre-lithium sheets in the comparative examples. Internal resistance tests, initial coulombic efficiency tests, and cycle performance tests were performed on the batteries prepared in the examples and comparative examples. The test results are shown in Table 1.

[0115] Thermal shrinkage test: The composite lithium-supplementing separators of Example 1 and Comparative Example 2 and the isolation membrane of Comparative Example 1 were cut into 10 mm × 100 mm strips. The thermal shrinkage of the separators after heat treatment at 130°C for 30 min was tested using an FST-02 film thermal shrinkage tester.

[0116] DC internal resistance test: Charge the battery to 60% SOC at 0.33C constant current and constant voltage. After standing for 12 hours, use a four-probe internal resistance meter to test the internal resistance at both ends of the battery, which is the battery's DC internal resistance R.

[0117] Cycle life test: The batteries in the examples and comparative examples were charged at a constant temperature of 25°C at a constant current and constant voltage of 0.2C to 3.6V, with a cutoff current of 0.05C; then discharged at 0.2C to 2.5V. This charge and discharge cycle was repeated until the battery's capacity retention reached 80%. The number of cycles was recorded (if the number of cycles was less than this, the capacity retention was not less than 80%). Capacity retention = discharge capacity at the Nth cycle / discharge capacity at the first cycle.

[0118] The test method for the first coulombic efficiency is as follows: the batteries in the embodiments and comparative examples are subjected to charge and discharge tests, and constant current charging is performed at a current of 0.2C. The charging limit is 50% SOC of the battery design capacity, and the charging capacity FC1 of the battery is recorded. The battery is left to stand at 45°C for 12 hours, and the battery is continued to be charged and discharged. The process steps are: constant current charging at 0.2C to 3.6V, and then constant voltage charging to a cutoff current of 0.05C, and the charging capacity is recorded as FC2; discharging at a constant current of 0.2C to a cutoff voltage of 2.5V, and the discharge capacity DC3 is recorded. The first coulombic efficiency = DC3 / (FC2+FC1).

[0119] Table 1

[0120]

[0121] The following points can be seen from Table 1:

[0122] (1) Comparing Examples 1-5 with Comparative Example 1, since the isolation membranes in Examples 1-5 include lithium replenishing membranes, the initial coulombic efficiency and cycle performance of the lithium-ion battery are improved, the resistance of the battery is reduced, and the shrinkage rate of the isolation membrane is reduced, thereby improving the safety of the battery.

[0123] (2) Comparing Example 1 with Comparative Examples 1-2, since the conductive polymer was replaced with a conductive agent (conductive carbon black) in Comparative Example 2, the shrinkage rate of the isolation membrane could not be reduced. Moreover, due to the high content of the conductive agent, the conductive agent consumed more lithium in the battery, and the consumed lithium could not be released, resulting in a decrease in the first coulombic efficiency and cycle performance of the battery.

[0124] Obviously, the above embodiments are merely examples for clarity of explanation and are not intended to limit the implementation methods. Those skilled in the art will readily appreciate that other variations or modifications based on the above descriptions are possible. It is not necessary and impossible to enumerate all implementation methods here. Obvious variations or modifications arising therefrom remain within the scope of protection of the present invention.

Claims

1. A lithium replenishing membrane, characterized in that The lithium replenishing film comprises a binder, a conductive polymer and lithium doped in the conductive polymer, wherein the binder is fiberized and then pressed to form a binder with a network structure, and the conductive polymer is dispersed on the binder; The conductive polymer includes at least one of polyacetylene, polythiophene, polypyrrole, polyaniline, polyparaphenylene vinylene, and polyparaphenylene; The preparation method of the lithium replenishing membrane comprises: The conductive polymer, the lithium source and the binder are subjected to a mixing process, a fiberizing process and a sheeting process to obtain an initial thin sheet, and then an electric current is applied to the initial thin sheet to complete the doping of lithium in the conductive polymer to obtain the lithium replenishing film; the mass ratio of the conductive polymer, the lithium source and the binder is (65-97): (1-30): (2-5).

2. The lithium replenishing membrane according to claim 1, characterized in that The binder includes at least one of polytetrafluoroethylene, polyvinylidene fluoride, sodium carboxymethyl cellulose, polyethylene, and polypropylene; and / or, the lithium source is metallic lithium powder; And / or, the thickness of the lithium replenishing film is 1-10 μm.

3. The lithium replenishing membrane according to claim 1, characterized in that The mass ratio of the conductive polymer, the lithium doped in the conductive polymer and the binder is (65-97): (1-30): (2-5).

4. The method for preparing a lithium replenishing membrane according to claim 1, wherein: The preparation method comprises the following steps: S1, uniformly mixing the conductive polymer, the lithium source and the binder in a mixer to obtain a first mixture; S2, fiberizing the first mixture to obtain a second mixture; S3, pressing the second mixed material into an initial film through a pressing roller; S4. Connecting the initial membrane to an external power source to apply current to the initial membrane to complete lithium doping in the conductive polymer and obtain the lithium-replenishing membrane.

5. The method for preparing a lithium replenishing membrane according to claim 4, wherein: In step S1, the speed of the mixer is 1000-8000 rpm, and the mixing time is 10-240 min; And / or, in step S2, the fiberization method is air flow milling, the air flow pressure of the air flow milling is 0.1-1 MPa, and the gas used in the air flow milling is nitrogen and / or argon; And / or, in step S3, the second mixed material is pressed into an initial film by a vertical hot pressing roller and a horizontal hot pressing roller, wherein the temperature of the hot pressing roller is 30-80° C.; And / or, in step S4, the magnitude of the current is 0.1-100 mA, and the time for applying the current is 1-24 hours.

6. A composite lithium supplementing diaphragm, characterized in that: The composite lithium-replenishing membrane comprises a porous base membrane and a lithium-replenishing membrane disposed on at least one side of the porous base membrane, wherein the lithium-replenishing membrane is the lithium-replenishing membrane according to any one of claims 1 to 3, or a lithium-replenishing membrane prepared by the method for preparing the lithium-replenishing membrane according to any one of claims 4 to 5.

7. The composite lithium supplementing diaphragm according to claim 6, characterized in that: The lithium supplement film is arranged on at least one side of the porous base film by thermal compounding.

8. A lithium-ion battery comprising a positive electrode sheet, a negative electrode sheet, a separator disposed between the positive electrode sheet and the negative electrode sheet, and an electrolyte, wherein: The isolation membrane is the composite lithium-supplementing membrane according to claim 6 or 7.

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