Diaphragm, preparation method thereof and lithium ion battery

Abstract

The invention discloses a diaphragm and a preparation method thereof and a lithium ion battery, the diaphragm comprises a base membrane and a functional layer arranged on at least one side surface of the base membrane, the functional layer comprises pre-lithiated halloysite nanotubes (Li-HNTs) and a carboxyl-containing binder, the Li-HNTs comprise halloysite nanotubes (HNTs) and a lithium source loaded on the halloysite nanotubes, the surface of the Li-HNTs has-O-Li < + > sites, and the surface of the Li-HNTs has-O-Li < + > sites. An ionic bond is formed by carboxyl contained in the binding agent, so that the binding force of the coating can be improved; a releasable lithium source is loaded on the Li-HNTs, and irreversible lithium loss formed by a negative electrode SEI film in first circulation can be compensated; the surface of the Li-HNTs is passivated, so that the contact with electrolyte can be reduced, and the corrosion of acid generated by decomposition of lithium salt in the electrolyte to the material is reduced; and the binder is preferentially distributed at the Li-HNTs pipe orifice to form a bottle brush-shaped structure, so that the electrolyte wettability of the diaphragm can be optimized, the power density and the safety of the battery are improved, and the service life of the battery is prolonged.

Description

Technical Field

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

[0002] Currently, the mainstream lithium-ion battery separators on the market are still traditional polyolefin separators, supplemented by double-sided coatings of ceramic inorganic particles (such as Al2O3) and binders (such as PVDF) to enhance separator performance. However, these existing separators often suffer from the following drawbacks: they are unable to provide lithium ions that are lost during cycling, resulting in a decrease in coulombic efficiency; they also suffer from poor interfacial stability, and the large difference in thermal expansion coefficients between the ceramic coating (Al2O3) and the polyolefin substrate makes the coating susceptible to delamination at high temperatures. Consequently, there is an urgent need for lithium-ion battery separators with high mechanical strength, excellent thermal stability, and a long cycle life. Summary of the Invention

[0003] The present invention aims to solve at least one of the technical problems existing in the prior art. To this end, the present invention provides a separator, a preparation method thereof, and a lithium-ion battery.

[0004] In a first aspect of the present invention, a separator is provided, comprising a base film and a functional layer provided on at least one surface of the base film, wherein the components of the functional layer include pre-lithiated halloysite nanotubes (Li-HNTs) and a carboxyl-containing binder, the pre-lithiated halloysite nanotubes include halloysite nanotubes (HNTs) and a lithium source loaded on the halloysite nanotubes, and the surface of the pre-lithiated halloysite nanotubes has -O-Li + sites, the surface of the pre-lithiated halloysite nanotubes contains -O-Li + The sites form ionic bonds (-COO-Li + ).

[0005] According to the diaphragm of the embodiment of the present invention, there are at least the following beneficial effects: the diaphragm can be used in lithium-ion batteries, wherein the functional layer provided on at least one side of the base film contains pre-lithiated halloysite nanotubes (Li-HNTs) including halloysite nanotubes (HNTs) and a lithium source loaded on the HNTs, the Li-HNTs are loaded with a releasable lithium source, and the released lithium source can compensate for the irreversible lithium loss formed by the negative electrode SEI film in the first cycle; the surface of the pre-lithiated halloysite nanotubes has a large amount of active -O-Li + sites, which form ionic bonds (-COO-Li +), which can improve the adhesion of the coating, thereby improving the mechanical strength and thermal stability of the diaphragm; and compared with HNTs, the surface passivation of pre-lithiation halloysite nanotubes (Li-HNTs) can reduce subsequent direct contact with the electrolyte, reduce the corrosion of the material caused by the decomposition of lithium salts (such as LiPF6) in the electrolyte to produce acid, thereby extending the cycle life; in addition, the binder in the functional layer is preferentially distributed at the tube mouth of Li-HNTs to form a "bottle brush" structure, which can reduce the contact angle between the electrolyte and the diaphragm, optimize the wettability of the electrolyte, increase the liquid flux, enhance the electrochemical stability and heat resistance of the diaphragm, and thus improve the power density, safety performance and service life of the battery.

[0006] In some embodiments of the present invention, the lithium source is supported on the surface and inside the halloysite nanotubes.

[0007] In some embodiments of the present invention, the pre-lithiated halloysite nanotubes are prepared by an ion exchange method.

[0008] In some embodiments of the present invention, the pre-lithiated halloysite nanotubes are prepared by a preparation method comprising the following steps: mixing halloysite nanotubes with a lithium salt solution, performing solid-liquid separation to obtain a solid phase material, and then calcining the solid phase material.

[0009] In the above preparation method of pre-lithiated halloysite nanotubes, the halloysite nanotubes are first mixed with lithium salts for pre-lithiation, which can induce the formation of lithium ion diffusion paths between and on the surface of the halloysite nanotubes. Subsequently, calcination can further fix these path channels, which are then applied to the functional layer on the diaphragm, thereby improving the selective transmission ability of the diaphragm surface coating for lithium ions and reducing the interface impedance. The stable lithium ion channels and low-impedance interface can reduce the polarization of the battery during high-rate charge and discharge, and achieve a higher capacity retention rate.

[0010] In some embodiments of the present invention, in the method for preparing pre-lithiated halloysite nanotubes, solid-liquid separation may be performed by centrifugal separation.

[0011] In some embodiments of the present invention, in the method for preparing pre-lithiated halloysite nanotubes, the solid-phase material is washed before calcining. The washing can be performed with deionized water, and the number of washes can be one or more, for example, within the range of 1, 2, 3, 4, 5, or 7 times, or any two of these. In some embodiments, the solid-phase material can be washed in deionized water three times.

[0012] In some embodiments of the present invention, after washing the solid-phase material and before calcining, the washed solid-phase material may be dried. The drying temperature may be controlled at 70°C to 90°C, for example, any one of 70°C, 75°C, 78°C, 80°C, 82°C, 85°C, and 90°C, or any range of two thereof.

[0013] In the method for preparing pre-lithiated halloysite nanotubes, calcination can remove bound water in the solid phase material and fix the lithium ion diffusion path channels formed between and on the surface of the halloysite nanotubes by pre-lithiation. In some embodiments of the present invention, the temperature of the calcination process is controlled to be 250°C to 400°C. For example, the calcination temperature can be any value among 250°C, 260°C, 270°C, 280°C, 285°C, 290°C, 300°C, 310°C, 320°C, 350°C, 360°C, 380°C, and 400°C, or a range of any two of them.

[0014] In some embodiments of the present invention, the calcination time is controlled to be 5 h to 10 h. For example, the calcination time can be any one of 5 h, 5.5 h, 6 h, 7 h, 8 h, 8.5 h, 9 h, and 10 h, or a range of any two of the values.

[0015] In some embodiments of the present invention, the molecular weight of the carboxyl-containing binder is 300,000 to 500,000. For example, the molecular weight of the carboxyl-containing binder can be any value among 300,000, 320,000, 350,000, 360,000, 380,000, 400,000, 410,000, 420,000, 450,000, 460,000, 470,000, 480,000, 490,000, and 500,000, or any two range values.

[0016] The molecular weight of the carboxyl-containing binder will affect the battery performance after the separator is applied to the battery. The effect of molecular weight on the performance of the binder is mainly reflected in the chain length and functional group density. There are a large number of active -O-Li on the surface of the pre-lithiated halloysite nanotubes. + sites, which form ionic bonds with the carboxyl groups contained in the binder in contact with Li-HNTs through condensation reaction, which can improve the adhesion of the coating. When the molecular weight is too low, it will affect the bonding between Li-HNTs and the binder. When the molecular weight is too high, the chain segment mobility of the binder is weakened, which will also affect the bonding between Li-HNTs and the binder. By controlling the molecular weight of the carboxyl-containing binder within the above range, the bonding between Li-HNTs and the binder can be effectively guaranteed, and the adhesion of the coating can be improved.

[0017] Some existing binders (such as the currently commonly used PVDF) have high crystallinity and low ionic conductivity. The room temperature ionic conductivity is only 0.3-0.5mS / cm, which is not conducive to improving the performance of the diaphragm. In some embodiments of the present invention, the carboxyl-containing binder contains a flexible segment. The above carboxyl-containing binder containing a flexible segment is used in combination with pre-lithiated halloysite nanotubes (Li-HNTs). The Li-HNTs are loaded with a releasable lithium source, Li + The flexible chain segments that migrate through the binder produce a sliding friction effect with HNTs, which improves the ionic conductivity and can quickly compensate for the irreversible lithium loss formed by the negative electrode SEI film in the first cycle; moreover, the nanochannels of HNTs and the flexible chain segments in the binder that can moderately swell in the electrolyte form a synergistic hierarchical ion path, which makes Li + The migration number is further improved, achieving improved rate performance.

[0018] In some embodiments of the present invention, the binder is selected from at least one of polyacrylic acid (PAA) and its derivatives, carboxylated styrene-butadiene rubber, sodium oleate (SO), carboxylated polyurethane, carboxylated polybutadiene, and ethylene-vinyl acetate-acrylic acid terpolymer. The above binders contain carboxyl groups and soft segments, with the soft segments being the main component.

[0019] In some embodiments of the present invention, the binder contains carboxyl groups and soft segments, and has a molecular weight of 300,000 to 500,000.

[0020] In some embodiments of the present invention, the binder is selected from polyacrylic acid (PAA), and its molecular weight is 300,000 to 500,000.

[0021] In some embodiments of the present invention, the composition of the functional layer includes 50 wt% to 90 wt% of pre-lithiated halloysite nanotubes and 10 wt% to 50 wt% of a carboxyl-containing binder. For example, in the functional layer, the content of pre-lithiated halloysite nanotubes may be any value or any two of 50 wt%, 52 wt%, 55 wt%, 57 wt%, 60 wt%, 63 wt%, 65 wt%, 68 wt%, 70 wt%, 72.5 wt%, 74 wt%, 75 wt%, 77 wt%, 80 wt%, 82 wt%, 83 wt%, 85 wt%, 86.5 wt%, 88 wt%, 90 wt%. the content of the carboxyl-containing binder may be any one of 10wt%, 12wt%, 13.5wt%, 15wt%, 17wt%, 18wt%, 20wt%, 23wt%, 25wt%, 26wt%, 27.5wt%, 30wt%, 32wt%, 35wt%, 37wt%, 40wt%, 43wt%, 45wt%, 48wt%, 50wt% or any two of the range values.

[0022] In some embodiments of the present invention, the base film is a polyolefin base film.

[0023] In some embodiments of the present invention, the base film is made of a material selected from polypropylene (PP), polyethylene (PE), or a composite of the two.

[0024] In some embodiments of the present invention, the functional layer is provided on both side surfaces of the base film, specifically on both side surfaces of the base film that are arranged in opposite directions.

[0025] In some embodiments of the present invention, the thickness of the functional layer is 0.5 μm to 2 μm. For example, the thickness of the functional layer may be 0.5 μm, 0.6 μm, 0.7 μm, 0.75 μm, 0.8 μm, 0.85 μm, 0.9 μm, 0.95 μm, 1 μm, 1.1 μm, 1.15 μm, 1.2 μm, 1.3 μm, 1.35 μm, 1.4 μm, 1.5 μm, 1.6 μm, 1.7 μm, or 2 μm, or any two of the range values.

[0026] The second aspect of the present invention provides a method for preparing any of the aforementioned diaphragms of the present invention, comprising the following steps:

[0027] Preparing pre-lithiated halloysite nanotubes, comprising: mixing halloysite nanotubes (HNTs) with a lithium salt solution, then performing solid-liquid separation to obtain a solid phase material, and calcining the solid phase material to obtain pre-lithiated halloysite nanotubes (Li-HNTs);

[0028] Preparing a coating slurry, comprising: preparing a coating slurry by mixing the pre-lithiated halloysite nanotubes, a carboxyl-containing binder and a solvent;

[0029] The coating slurry is coated on at least one side of the surface of the base film and dried to obtain a diaphragm.

[0030] In the above preparation method, HNTs are first mixed with a lithium salt solution for pre-lithiation, and then calcined to obtain Li-HNTs, and then the Li-HNTs are mixed with a binder. Compared with directly mixing HNTs, lithium salts and binders, the former can induce the formation of lithium ion diffusion paths between and on the surface of halloysite nanotubes after pre-lithiation, and calcination can further fix these paths and channels, which can then be applied to the functional layer on the diaphragm, thereby improving the selective transmission capacity of the diaphragm surface coating for lithium ions and reducing the interfacial impedance. The stable lithium ion channels and low-impedance interfaces can reduce the polarization of the battery during high-rate charge and discharge, and achieve a higher capacity retention rate. The prepared Li-HNTs include HNTs and a lithium source loaded on the HNTs. The load on the Li-HNTs can release the lithium source, which can compensate for the irreversible lithium loss formed by the negative electrode SEI film in the first cycle; there are a large number of active -O-Li on the surface of the pre-lithiation halloysite nanotubes. + sites, which form ionic bonds with the carboxyl groups contained in the binder in contact with Li-HNTs through condensation reaction, thereby improving the adhesion of the coating; compared with HNTs, the surface passivation of pre-lithiated halloysite nanotubes (Li-HNTs) can reduce subsequent direct contact with the electrolyte, reduce the corrosion of the material caused by the decomposition of lithium salts (such as LiPF6) in the electrolyte to produce acid, thereby extending the cycle life; in addition, after the binder is mixed with Li-HNTs, the binder is preferentially distributed at the mouth of the Li-HNTs to form a "bottle brush" structure, which can reduce the contact angle between the electrolyte and the diaphragm, optimize the electrolyte wettability, thereby increasing the liquid flux, enhancing the electrochemical stability and heat resistance of the diaphragm, and thus improving the power density, safety performance and service life of the battery.

[0031] In some embodiments of the present invention, during the preparation of pre-lithiated halloysite nanotubes, the solid phase material is washed before calcining, wherein the washing can be performed with deionized water, and the washing frequency can be one or more.

[0032] In some embodiments of the present invention, after the solid phase material is cleaned and before calcination, the cleaned solid phase can be dried, wherein the drying temperature can be controlled at 70°C to 90°C.

[0033] In some embodiments of the present invention, during the preparation of pre-lithiated halloysite nanotubes, the temperature of the calcination process can be controlled to be 250° C. to 350° C.; in addition, the calcination time can be 5 h to 10 h.

[0034] In some embodiments of the present invention, preparing the coating slurry includes: dispersing the pre-lithiated halloysite nanotubes in a portion of the solvent to obtain a dispersion A; dissolving a carboxyl-containing binder in the remaining solvent to obtain a solution B; and mixing the dispersion A and the solution B to obtain a coating slurry.

[0035] In some embodiments of the present invention, the amount of solvent used to prepare dispersion A accounts for 40% to 70% of the total amount of solvent used to prepare the coating slurry. For example, the ratio of the amount of solvent used to prepare dispersion A to the total amount of solvent used to prepare the coating slurry is 40%, 45%, 50%, 55%, 58%, 60%, 62%, 65%, 68%, 70%, or any two of the above.

[0036] In a third aspect of the present invention, a lithium-ion battery is proposed, which includes a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte, wherein the separator is sandwiched between the positive electrode sheet and the negative electrode sheet, and the separator is any of the aforementioned separators of the present invention or a separator prepared by the preparation method of any of the aforementioned separators of the present invention.

[0037] In some embodiments of the present invention, the negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer disposed on at least one side of the negative electrode current collector, and the negative electrode active material layer is disposed toward the functional layer on the separator.

[0038] In some embodiments of the present invention, the surface of the negative electrode active material layer has hydroxyl groups, and the carboxyl groups contained in the binder on the surface of the functional layer form hydrogen bonds with the hydroxyl groups on the surface of the negative electrode active material layer in contact with the functional layer. These hydrogen bonds stabilize the SEI film structure, inhibit electrolyte decomposition, and synergistically reduce interfacial impedance by forming lithium ion diffusion channels between and on the surface of the Li-HNT nanotubes and by the surface passivation of the Li-HNTs.

[0039] In some embodiments of the present invention, the negative electrode active material layer includes a negative electrode active material selected from a silicon-based negative electrode active material. These negative electrode active materials have hydroxyl groups on their surfaces that can form hydrogen bonds with carboxyl groups in the binder, thereby stabilizing the SEI film structure, inhibiting electrolyte decomposition, and reducing interfacial impedance. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] The present invention will be further described below with reference to the accompanying drawings and embodiments, in which:

[0041] Figure 1 IR spectra of the pre-lithiated halloysite nanotubes (Li-HNTs) prepared in Example 1 and the functional layer PAA@Li-HNTs scraped off the separator. DETAILED DESCRIPTION

[0042] The following will clearly and completely describe the concept and technical effects of the present invention in conjunction with the embodiments to fully understand the purpose, features and effects of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, other embodiments obtained by those skilled in the art without creative work are all within the scope of protection of the present invention.

[0043] Example 1

[0044] This embodiment proposes a separator, including a base film and a functional layer provided on both sides of the base film. The thickness of the functional layer on a single side is 1 μm, and its composition includes 80 wt% of pre-lithiated halloysite nanotubes (Li-HNTs) and 20 wt% of a binder polyacrylic acid (PAA), that is, the mass ratio of PAA to Li-HNTs is 2:8 (i.e., 1:4); wherein, Li-HNTs include halloysite nanotubes (HNTs) and a lithium source loaded on the surface and inside the HNTs. The binder PAA contains carboxyl groups and flexible segments, and the molecular weight of PAA is 400,000. The surface of Li-HNTs has a large number of active -O-Li + sites, -O-Li contained on the surface of Li-HNTs + The sites form ionic bonds with the carboxyl groups contained in the binder PAA in contact with Li-HNTs.

[0045] The above diaphragm is prepared by a preparation method comprising the following steps:

[0046] S1. Preparation of pre-lithiated halloysite nanotubes (Li-HNTs).

[0047] Specifically, 10g of LiCl was completely dissolved in 500mL of deionized water to form a clear solution (pH = 5). The solution was then stirred at room temperature for 24 hours. Then, 5g of HNTs were added to the clear solution, mixed thoroughly, and centrifuged using a high-speed centrifuge to separate the solid phase. The solid phase was then washed three times in deionized water and dried in an 80°C oven for 8 hours. Finally, the dried solid phase was calcined in a muffle furnace at 300°C for 3 hours to remove bound water, producing pre-lithiated halloysite nanotubes (Li-HNTs).

[0048] S2. Prepare coating slurry.

[0049] Specifically, 2g of Li-HNTs were weighed and placed in a beaker. 100mL of deionized water was added, and a stirrer was added to stir at room temperature for 24 hours to prepare a 20mg / mL dispersion A. 2g of PAA (molecular weight 400,000) was weighed and placed in another beaker. 100mL of deionized water was added, and a stirrer was added to stir at room temperature for 24 hours until completely dissolved. This prepared a 20mg / mL solution B. 8mL of dispersion A and 2mL of solution B were then added to a new beaker, stirred at room temperature for 3 hours with a stirrer to prepare the coating slurry (PAA@Li-HNTs).

[0050] S3. Take the coating slurry prepared in step S2 and apply it on both sides of the polyolefin base film (PE / PP composite film). After drying, a functional layer with a single-side thickness of 1 μm is formed on both sides of the polyolefin base film to prepare a product diaphragm.

[0051] Example 2

[0052] This embodiment provides a separator. The difference between this embodiment and Example 1 is that the mass ratio of PAA to Li-HNTs in the functional layer of the separator of this embodiment is adjusted from 1:4 in Example 1 to 1:9; accordingly, in step S2 of the preparation method, the amounts of dispersion A and solution B are adjusted from 8 mL and 2 mL in Example 1 to 9 mL and 1 mL, respectively. Other steps are the same as in Example 1.

[0053] Example 3

[0054] This embodiment provides a separator. The difference between this embodiment and Example 1 is that the mass ratio of PAA to Li-HNTs in the functional layer of the separator of this embodiment is adjusted from 1:4 in Example 1 to 3:7; accordingly, in step S2 of the preparation method, the amounts of dispersion A and solution B are adjusted from 8 mL and 2 mL in Example 1 to 7 mL and 3 mL, respectively. Other steps are the same as in Example 1.

[0055] Example 4

[0056] This embodiment provides a separator, which differs from Example 1 in that the mass ratio of PAA to Li-HNTs in the functional layer of the separator of this embodiment is adjusted from 1:4 in Example 1 to 4:6 (i.e., 2:3); accordingly, in step S2 of the preparation method, the amounts of dispersion A and solution B are adjusted from 8 mL and 2 mL in Example 1 to 6 mL and 4 mL, respectively. Other steps are the same as in Example 1.

[0057] Example 5

[0058] This embodiment provides a separator, which differs from Example 1 in that the mass ratio of PAA to Li-HNTs in the functional layer of the separator of this embodiment is adjusted from 1:4 in Example 1 to 5:5 (i.e., 1:1); accordingly, in step S2 of the preparation method, the amounts of dispersion A and solution B are adjusted from 8 mL and 2 mL in Example 1 to 5 mL and 5 mL, respectively. Other steps are the same as in Example 1.

[0059] Example 6

[0060] This embodiment provides a diaphragm, which differs from Example 1 in that the binder in the functional layer of the diaphragm of this embodiment is adjusted from PAA with a molecular weight of 400,000 in Example 1 to PAA with a molecular weight of 300,000, and the rest is the same as Example 1.

[0061] Example 7

[0062] This embodiment provides a diaphragm, which differs from Example 1 in that the binder in the functional layer of the diaphragm of this embodiment is adjusted from PAA with a molecular weight of 400,000 in Example 1 to PAA with a molecular weight of 500,000, and the rest is the same as Example 1.

[0063] Comparative Example 1

[0064] This comparative example proposes a separator that differs from Example 1 in that the functional layer of this comparative example omits Li-HNTs and instead contains only PAA with a molecular weight of 400,000. Accordingly, the preparation method for Li-HNTs in Step S1 of Example 1 is omitted. In Step S2, 2g of PAA (molecular weight of 400,000) is directly weighed and placed in a separate beaker. 100mL of deionized water is added, and a stirrer is added, stirring at room temperature for 24 hours until completely dissolved. This results in a 20mg / mL solution B, which serves as the coating slurry. All other aspects are the same as in Example 1.

[0065] Comparative Example 2

[0066] This comparative example provides a separator, which differs from Example 1 in that the mass ratio of PAA to Li-HNTs in the functional layer of the separator of this comparative example is adjusted from 1:4 in Example 1 to 0.5:9.5; accordingly, in step S2 of the preparation method, the amounts of dispersion A and solution B are adjusted from 8 mL and 2 mL in Example 1 to 9.5 mL and 0.5 mL, respectively. Other steps are the same as in Example 1.

[0067] Comparative Example 3

[0068] This comparative example proposes a diaphragm, which differs from Example 1 in that the mass ratio of PAA to Li-HNTs in the functional layer of the diaphragm of this comparative example is adjusted from 1:4 in Example 1 to 6:4 (i.e., 3:2); accordingly, in step S2 of the preparation method, the amounts of dispersion A and solution B are adjusted from 8 mL and 2 mL in Example 1 to 4 mL and 6 mL, respectively. Other steps are the same as in Example 1.

[0069] Comparative Example 4

[0070] This comparative example proposes a separator, which differs from Example 1 in that: non-pre-lithiated halloysite nanotubes (HNTs) are used in the functional layer of the separator in this comparative example instead of the pre-lithiated halloysite nanotubes (Li-HNTs) in Example 1; accordingly, the step of preparing Li-HNTs in step S1 of Example 1 is omitted from the preparation method, and in step S2 of preparing the coating slurry, an equal amount of HNTs is used instead of Li-HNTs to prepare dispersion A in the same manner. All other aspects are the same as in Example 1.

[0071] Comparative Example 5

[0072] This comparative example proposes a diaphragm, which differs from Example 1 in that the binder in the functional layer of the comparative example diaphragm is adjusted from PAA with a molecular weight of 400,000 in Example 1 to PAA with a molecular weight of 100,000, and the rest is the same as Example 1.

[0073] Comparative Example 6

[0074] This comparative example proposes a diaphragm, which differs from Example 1 in that the binder in the functional layer of the comparative example diaphragm is adjusted from PAA with a molecular weight of 400,000 in Example 1 to PAA with a molecular weight of 800,000, and the rest is the same as Example 1.

[0075] Comparative Example 7

[0076] This comparative example proposes a diaphragm, which differs from Example 1 in that the binder used in the functional layer of the comparative example diaphragm is adjusted from PAA with a molecular weight of 400,000 in Example 1 to polyvinylidene fluoride (PVDF) with a molecular weight of 400,000, wherein PVDF does not contain carboxyl groups but is mainly composed of flexible chain segments; the rest is the same as Example 1.

[0077] Comparative Example 8

[0078] This comparative example proposes a diaphragm, which differs from Example 1 in that the binder used in the functional layer of the comparative example diaphragm is adjusted from PAA with a molecular weight of 400,000 in Example 1 to polyimide (PI) with a molecular weight of 400,000, and PI does not contain carboxyl groups and is mainly composed of rigid segments; the rest is the same as Example 1.

[0079] Comparative Example 9

[0080] This comparative example proposes a diaphragm, which differs from Example 1 in that the binder used in the functional layer of the comparative example diaphragm is adjusted from PAA with a molecular weight of 400,000 in Example 1 to polymaleic anhydride (PMA) with a molecular weight of 400,000, and PMA contains carboxyl groups but is mainly composed of rigid chain segments; the rest is the same as Example 1.

[0081] Example 8

[0082] This embodiment proposes a lithium-ion battery, specifically a button battery, comprising a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte; wherein the separator adopts the separator of Example 1; the positive electrode sheet comprises a positive electrode collector and a positive electrode active material layer provided on one side surface of the positive electrode collector (specifically one side surface of the separator), the positive electrode collector adopts aluminum foil, and the positive electrode active material layer comprises a positive electrode active material lithium iron phosphate, a conductive agent superconducting carbon (Super-P), and a binder polyvinylidene fluoride (PVDF) in a mass ratio of 9:0.5:0.5; the negative electrode sheet adopts a metallic lithium sheet; the electrolyte adopts a LiPF6 electrolyte with a concentration of 1 mol / L, and its solvent is ethylene carbonate (EC), dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC) in a mass ratio of 1:2:1.

[0083] The above lithium-ion battery is prepared by a preparation method comprising the following steps:

[0084] S1. Prepare the positive electrode sheet.

[0085] Lithium iron phosphate (LiFePO4), conductive agent superconducting carbon (Super-P), binder polyvinylidene fluoride (PVDF) are mixed evenly with solvent N-methylpyrrolidone (NMP) in a mass ratio of 9:0.5:0.5 to prepare a positive electrode slurry with a solid content of 70±1% and a viscosity of 3000-8000 mPa·s. The positive electrode slurry is coated on one surface of the positive electrode current collector aluminum foil, dried at 85°C and then cold pressed; then the edges are trimmed, cut and slit, and after slitting, the strips are dried at 110°C for 4 hours under vacuum conditions, and the tabs are welded to form positive electrode sheets.

[0086] S2. Prepare the electrolyte.

[0087] Lithium hexafluorophosphate (LiPF6) was dissolved in a mixed solvent consisting of ethylene carbonate (EC), dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC) in a mass ratio of 1:2:1 to obtain an electrolyte with a concentration of 1 mol / L.

[0088] S3. Assemble and prepare lithium-ion batteries.

[0089] A button cell was assembled using a metal lithium sheet as the negative electrode and the separator from Example 1. The positive electrode sheet prepared in step S1, the separator from Example 1, and the negative electrode sheet (i.e., the metal lithium sheet) were assembled, with the separator sandwiched between the positive and negative electrodes. Specifically, the button cell's positive electrode casing was placed upward, followed by the positive electrode sheet, with the positive active material layer facing upward. The separator was then placed, followed by the negative electrode sheet. The electrolyte prepared in step S2 was then added dropwise, with the electrolyte added accounting for 25% of the separator's mass. The button cell's negative electrode casing was then attached, completing the lithium-ion battery.

[0090] Examples 9 to 14

[0091] Examples 9 to 14 respectively propose a lithium-ion battery, which differs from Example 8 in that the lithium-ion batteries of Examples 9 to 14 respectively use the separators of Examples 2 to 7 instead of the separator of Example 1 used in the lithium-ion battery of Example 8, and are otherwise the same as Example 8.

[0092] Example 15

[0093] This embodiment proposes a lithium-ion battery, which differs from Example 8 in that: the negative electrode sheet in this embodiment includes a negative electrode current collector copper foil and a negative electrode active material layer provided on one side surface of the negative electrode current collector copper foil. The negative electrode current collector is copper foil, and the negative electrode active material layer includes a negative electrode active material hydroxylated graphite, a dispersant CMC, and a binder polystyrene acrylate in a mass ratio of 97.7:1.1:1.2. The other structures are the same as those in Example 8.

[0094] Among them, hydroxylated graphite can be treated with strong oxidizing acid (such as concentrated nitric acid, mixed acid composed of concentrated nitric acid and concentrated sulfuric acid, etc.) to destroy the sp 2 structure, generating oxygen-containing groups (including carboxyl groups) at the edges or defects. The hydroxylated graphite used in this embodiment can be prepared by the following method: graphite powder is dispersed in a mixed acid consisting of concentrated sulfuric acid and concentrated nitric acid in a volume ratio of 3:1, heated to 60-80°C, and vigorously stirred for 2-4 hours; then a strong oxidant KMnO4 is added for further oxidation (similar to the Hummers method for preparing graphene oxide, but the target product retains some graphite structure); H2O2 is then added to terminate the reaction, and the solution turns bright yellow. Finally, it is centrifuged and washed with dilute hydrochloric acid and deionized water until neutral, and then vacuum-dried at 60°C to obtain carboxylated graphite.

[0095] Correspondingly, the preparation method of the lithium-ion battery also includes the steps of preparing the negative electrode sheet, specifically including:

[0096] Hydroxylated graphite, a dispersant, and a binder are uniformly mixed in a mass ratio of 97.7:1.1:1.2 to prepare a negative electrode slurry with a solid content of 45% to 55% and a viscosity of 3000 to 7000 mPa*s; the negative electrode slurry is coated on one surface of a negative electrode current collector copper foil to prepare a negative electrode sheet.

[0097] Then, when assembling and preparing a lithium-ion battery, the negative electrode sheet prepared above was used to replace the metal lithium sheet in Example 8, and the other operations were the same as in Example 8.

[0098] Example 16

[0099] This embodiment proposes a lithium-ion battery, which differs from Example 15 in that: in this embodiment, the negative electrode active material of the negative electrode active material layer on the negative electrode sheet uses an equal amount of graphite (without hydroxyl groups) instead of the hydroxylated graphite used in Example 15, and the other structures and preparation operations are the same as those in Example 15.

[0100] Comparative Examples 10-18

[0101] Comparative Examples 10 to 18 respectively propose a lithium ion battery, which differs from Example 8 in that the lithium ion batteries of Comparative Examples 10 to 18 respectively use the separators of Comparative Examples 1 to 9 instead of the separator of Example 1 used in the lithium ion battery of Example 8, and are otherwise the same as Example 8.

[0102] Performance Testing

[0103] (1) The raw material halloysite nanotubes (HNTs) used in Example 1 and the functional layer (PAA@Li-HNT) on the diaphragm of Example 1 were scraped off and tested using an infrared spectrometer.

[0104] (2) Contact angle test

[0105] The morphology of the droplet (ie, the electrolyte in Example 8) on the surface of the diaphragm of Examples 1 to 7 and Comparative Examples 1 to 9 was observed through a magnifying glass, and the contact angle at the interface between the droplet and the diaphragm was measured.

[0106] (3) Diaphragm adhesion test

[0107] The membrane separators of Examples 1 to 7 and Comparative Examples 1 to 9 were respectively cut into small pieces of 25 mm * 150 mm, ensuring that there were no bubbles or wrinkles in the bonding area; and the membrane separators were allowed to stand for 24 hours in a constant temperature and humidity environment at 23±2°C and 50±5% RH to eliminate the effects of temperature and humidity on the bonding interface. A 180° peel test was performed using a universal material testing machine with a pneumatic fixture or a double-sided tape fixture to prevent slipping during the test, and the peel force-displacement curve was recorded. The test was repeated for 5 sets of valid data, and the peel force was averaged.

[0108] (4) Rate test

[0109] Rate testing was performed on the lithium-ion batteries of Examples 8-16 and Comparative Examples 10-18. Specifically, five lithium-ion batteries of Examples 8-16 and Comparative Examples 10-18 were each placed in a room temperature, natural environment for 24 hours. They were then subjected to 400 charge-discharge cycles at a 3C rate using a battery testing system from Newwell Electronics Co., Ltd. at room temperature and a voltage range of 2.5-4.2V to test their charge-discharge specific capacity.

[0110] (5) Cyclic performance test

[0111] Five lithium-ion batteries from Examples 8 to 16 and Comparative Examples 10 to 18 were taken respectively, and the lithium-ion batteries were repeatedly charged and discharged through the following steps, and the cycle capacity retention rates of the lithium-ion batteries were calculated.

[0112] 400 charge and discharge cycles were performed at a rate of 0.5 C, and the discharge capacity of the first cycle and the discharge capacity of the 400th cycle were recorded.

[0113] Capacity retention rate after 400 cycles = (discharge capacity at the 400th cycle / discharge capacity at the first cycle) × 100%.

[0114] The above methods were used to test the separators of the examples and comparative examples and their corresponding lithium-ion batteries. The results are as follows: Figure 1 As shown in Table 1, the content in the brackets after PAA in the second column of Table 1 is the molecular weight of the binder.

[0115] Table 1

[0116]

[0117]

[0118] pass Figure 1 The infrared spectrum analysis shows that the infrared spectrum of Li-HNTs is at 3625cm -1 and 3450cm -1 There are obvious characteristic peaks, which are the internal surface hydroxyl (AL-OH) and structural water hydroxyl. After Li-HNTs and PAA are composited, Li-HNTs has a peak at 3625cm -1 The characteristic peak of the inner surface hydroxyl group (AL-OH) disappears, and the peak at 3450 cm -1 The characteristic peak of the structural water hydroxyl group is weakened, while the peak at 1741cm -1 The characteristic peak of ester group appears at the position of the Li-HNTs, which is due to the chemical bonding between Li-HNTs and PAA.

[0119] Comparing the performance test results of the separators of Examples 1-5 and Comparative Examples 1-3 and their corresponding batteries in Table 1 shows that when the mass ratio of PAA (molecular weight 400,000) to Li-HNTs in the functional layer of the separator of Example 1 is 2:8, the corresponding lithium-ion battery has an initial discharge capacity of 146 mAh / g at 0.5C, and a capacity retention rate of 92% after 400 cycles. As the PAA content increases, the initial discharge capacity decreases. This is primarily because the functional layer of the separator can compensate for the lithium loss of the battery. Specifically, by pre-lithiation of the HNTs, the HNT lumen load can release lithium sources. Li+ migrates through the flexible segments of the PAA, generating a sliding friction effect with the HNTs, increasing ionic conductivity and quickly compensating for the irreversible lithium loss caused by the formation of the negative electrode SEI film during the initial cycle. However, when the PAA content is below 20%, performance also decreases. This is primarily because the reduced PAA content reduces the number of flexible segments, which reduces ionic conductivity.

[0120] In addition, the functional layer of the separator in Example 1 has a mass ratio of PAA (molecular weight of 400,000) to Li-HNTs of 2:8. The corresponding discharge capacity of the button cell after 400 cycles at a 3C rate is 123 mAh / g, which is much higher than that of the other examples and comparative examples. This is mainly because the nanochannels of the HNTs and the flexible chain segments in the PAA, which can moderately swell in the electrolyte, form a synergistic hierarchical ion pathway, further increasing the Li+ migration number and achieving improved rate performance.

[0121] From the performance test results of the separators of Example 1 and Comparative Example 4 and their corresponding batteries in Table 1, it can be seen that compared with Example 1 (the functional layer uses Li-HNTs), the functional layer of the separator of Comparative Example 4 uses HNTs that have not been pre-lithiated, and its performance decreases sharply. This is mainly because the pre-lithiated HNTs (i.e., Li-HNTs) can compensate the battery for lithium, thereby improving the battery performance of the separator.

[0122] From the performance test results of the separators of Example 1, Examples 6-7 and Comparative Examples 5-6 and their corresponding batteries in Table 1, it can be seen that the molecular weight of PAA also affects the battery performance of the composite separator. The influence of molecular weight on PAA performance is mainly reflected in the chain length and functional group density. When the molecular weight of PAA is between 300,000 and 500,000, its battery performance is the best. This is mainly because the active -O-Li + The sites form ionic bonds with the carboxyl groups contained in PAA through condensation reactions, which can improve the adhesion of the coating. However, when the molecular weight of PAA is too low, it will affect the bonding between PAA and Li-HNTs. When the molecular weight of PAA is too high, its chain segment mobility is reduced, also affecting its bonding with Li-HNTs.

[0123] From the performance test structures of the separators of Examples 1-5 and Comparative Examples 1-3 and their corresponding batteries in Table 1, it can be seen that the mass ratio of PAA to Li-HNTs in the functional layer of the separator of Example 1 is 2:8, and the separator has the smallest contact angle of only 65°. This is because PAA is preferentially distributed at the mouth of the Li-HNTs to form a "bottle brush" structure, which can reduce the contact angle between the electrolyte and the separator, optimize the wettability of the electrolyte, thereby reducing the interfacial impedance and improving the battery performance.

[0124] From the performance test results of the separators of Example 1 and Comparative Examples 7 to 9 and their corresponding batteries in Table 1, it can be seen that when the adhesive PVDF containing no carboxyl groups and mainly flexible segments, the adhesive PI containing no carboxyl groups and mainly rigid segments, and the adhesive PMA containing carboxyl groups but mainly rigid segments replace the PAA containing carboxyl groups and mainly flexible segments, the adhesion of the separator itself is reduced and the battery cycle performance is reduced. This is because the carboxyl groups (-COOH) in the adhesive have -O-Li on the surface of the pre-lithiated halloysite nanotubes. + The formation of ionic bonds at the sites improves the adhesion of the separator coating, which is beneficial to optimizing the distribution of the electrolyte in the battery and prolonging the battery life. This is because the separator with low adhesion is easy to separate from the electrode sheet during the cycle, resulting in local electrolyte drying and blocking the transmission channel of lithium ions. In addition, the lithium source can be released by loading on Li-HNTs, Li + The flexible chain segments that migrate through the binder produce a sliding friction effect with HNTs, which improves the ionic conductivity and can quickly compensate for the irreversible lithium loss formed by the negative electrode SEI film in the first cycle; moreover, the nanochannels of HNTs and the flexible chain segments in the binder that can moderately swell in the electrolyte form a synergistic hierarchical ion path, which makes Li + The migration number is further improved, achieving improved rate performance.

[0125] From the performance test results of the batteries of Examples 8 to 16 in Table 1, it can be seen that compared with Example 16, the lithium ion battery of Example 15 can increase the discharge specific capacity by 7 mAh / g under 0.5C cycle, and Example 15 has more advantages than the lithium ion batteries of Examples 8 to 14 using metal lithium sheets as negative electrodes. This is mainly due to the formation of hydrogen bonds between the carboxyl groups contained in the binder and the hydroxyl groups contained on the surface of the negative electrode active material layer.

[0126] The above-described embodiments merely illustrate several implementations of the present invention. While the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that a person skilled in the art would be able to make numerous variations and improvements without departing from the spirit of the present invention, and all such variations and improvements fall within the scope of protection of the present invention.

Claims

1. A diaphragm, characterized in that: The invention comprises a base film and a functional layer provided on at least one side of the base film, wherein the functional layer comprises pre-lithiated halloysite nanotubes and a binder containing a carboxyl group, wherein the pre-lithiated halloysite nanotubes comprise halloysite nanotubes and a lithium source loaded on the halloysite nanotubes, and wherein the surface of the pre-lithiated halloysite nanotubes has -O-Li + sites, the surface of the pre-lithiated halloysite nanotubes contains -O-Li + The sites form ionic bonds with carboxyl groups contained in the binder in contact with the pre-lithiated halloysite nanotubes.

2. The diaphragm according to claim 1, characterized in that The lithium source is loaded on the surface and inside of the halloysite nanotube; And / or, the carboxyl-containing binder satisfies at least one of the following conditions: The molecular weight of the carboxyl-containing binder is 300,000 to 500,000; The carboxyl group-containing binder contains a soft segment.

3. The diaphragm according to claim 2, characterized in that The carboxyl-containing binder is selected from at least one of polyacrylic acid and its derivatives, carboxylated styrene-butadiene rubber, sodium oleate, carboxylated polyurethane, carboxylated polybutadiene, and ethylene-vinyl acetate-acrylic acid terpolymer.

4. The diaphragm according to claim 1, characterized in that The functional layer comprises 50 wt% to 90 wt% of pre-lithiated halloysite nanotubes and 10 wt% to 50 wt% of a carboxyl-containing binder; And / or, the base film is a polyolefin base film.

5. The method for preparing the diaphragm according to any one of claims 1 to 4, characterized in that: The following steps are involved: Preparing pre-lithiated halloysite nanotubes, comprising: mixing the halloysite nanotubes with a lithium salt solution, then performing solid-liquid separation to obtain a solid phase material, and calcining the solid phase material to obtain the pre-lithiated halloysite nanotubes; Preparing a coating slurry, comprising: preparing a coating slurry by mixing the pre-lithiated halloysite nanotubes, a carboxyl-containing binder and a solvent; The coating slurry is coated on at least one side of the surface of the base film and dried to obtain a diaphragm.

6. The method for preparing the diaphragm according to claim 5, characterized in that: During the preparation of pre-lithiated halloysite nanotubes, the solid phase material is cleaned before being calcined; and / or the temperature of the calcination process is controlled at 250° C. to 350° C.

7. The method for preparing a diaphragm according to claim 5, characterized in that: The coating slurry is prepared by dispersing the pre-lithiated halloysite nanotubes in a portion of the solvent to obtain a dispersion A; dissolving a carboxyl-containing binder in the remaining solvent to obtain a solution B; and mixing the dispersion A and the solution B to obtain the coating slurry.

8. A lithium-ion battery, characterized in that: The invention comprises a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte; the separator is sandwiched between the positive electrode sheet and the negative electrode sheet, and the separator is a separator according to any one of claims 1 to 4 or a separator prepared by the preparation method of the separator according to any one of claims 5 to 7.

9. The lithium-ion battery according to claim 8, characterized in that The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer provided on at least one side surface of the negative electrode current collector, the negative electrode active material layer is arranged toward the functional layer on the separator, and the surface of the negative electrode active material layer has hydroxyl groups, and the carboxyl groups contained in the binder on the surface of the functional layer form hydrogen bonds with the hydroxyl groups contained in the surface of the negative electrode active material layer in contact with the functional layer.

10. The lithium-ion battery according to claim 9, characterized in that The negative electrode active material layer includes a negative electrode active material, and the negative electrode active material is selected from silicon-based negative electrode active materials.

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