Composite separator, and preparation method therefor and use thereof

Abstract

A composite separator, and a preparation method therefor and the use thereof. The composite separator comprises a porous base membrane and a coating, wherein the coating is arranged on the surface of the porous base membrane and comprises a base layer and a non-viscous polymer C arranged on the base layer; the base layer comprises inorganic particles A and a viscous polymer B; the maximum particle size of the non-viscous polymer C is greater than the thickness of the base layer, and the particle size of the non-viscous polymer C is 0.3-30 μm; the viscous polymer B comprises a first component and a second component; the first component comprises at least one of an ethylene-based polymer, a propylene-based polymer, an amide-based polymer and an epoxy-based polymer; and the second component comprises a cellulose-based polymer.

Description

Composite diaphragm and preparation method and application thereof

[0001] This application claims priority to Chinese Patent Application No. 202410549312.1, filed on April 30, 2024, and Chinese Patent Application No. 202410622507.4, filed on May 17, 2024, the contents of which are incorporated herein by reference in their entirety. TECHNICAL FIELD

[0002] The present application relates to the technical field of lithium batteries, in particular to a composite diaphragm and a preparation method and application thereof. BACKGROUND

[0003] As a key safety component of lithium ion batteries, the diaphragm has a rich pore structure, and its role is to block the direct contact of the positive and negative electrode sheets and allow ions to pass through. The traditional diaphragm uses an adhesive polymer and inorganic particles mixed to coat the surface of a polyolefin-based film to form a porous active layer, and the inorganic particles in the porous active layer are connected and fixed to each other by the adhesive polymer. PROBLEMS TO BE SOLVED BY THE INVENTION

[0004] The porous active layer of the traditional diaphragm has no adhesive effect with the electrode sheet, which reduces the stability of the diaphragm and the electrode sheet in the roll core, and as the electrode area and the roll core size increase, the poor stability of the diaphragm and the electrode sheet in the roll core will be further amplified, resulting in an increase in the looseness of the roll core and a decrease in the mechanical strength of the roll core. The increase in the looseness of the roll core will result in the roll core being unable to enter the shell, and the poor mechanical strength of the roll core will result in the electrode sheet being wrinkled. In addition, during the charging and discharging process of the lithium battery, the electrode sheet expands, which will intensify the degree of deformation of the roll core, which will seriously affect the cycle performance of the lithium battery. TECHNICAL SOLUTION

[0005] In a first aspect, the present application provides a composite diaphragm, which adopts the following technical solution:

[0006] A composite diaphragm, comprising a porous base film and a coating layer; the coating layer is arranged on the surface of the porous base film, and the coating layer comprises a base layer and a non-adhesive polymer C arranged on the base layer, and the base layer comprises inorganic particles A and an adhesive polymer B;

[0007] The maximum particle size of the non-adhesive polymer C is greater than the thickness of the base layer, and the particle size of the non-adhesive polymer C is 0.3-30 μm;

[0008] The viscous polymer B comprises a first component and a second component; the first component comprises at least one of an ethylene-based polymer, a propylene-based polymer, an amide-based polymer, an epoxy-based polymer; the second component comprises a cellulose-based polymer;

[0009] The ethylene-based polymer comprises at least one of polyvinylidene fluoride, polyvinylidene fluoride-co-trichloroethylene, polyvinyl acetate, polyethylene-co-vinyl acetate, polyvinyl alcohol, polyvinylpyrrolidone, and polymaleic anhydride.

[0010] The propylene-based polymer comprises at least one of polymethyl methacrylate and polyacrylonitrile.

[0011] The amide-based polymer comprises at least one of polyimide, acrylamide, methacrylamide, hydroxymethyl acrylamide, diacetone acrylamide, N,N-diethyl acrylamide, N,N-diethyl methacrylamide, N-ethyl acrylamide, N-ethyl methacrylamide, N,N-dimethyl acrylamide, N,N-dimethyl acrylamide, N-methyl acrylamide, N-isopropyl acrylamide, N-isopropyl methacrylamide, and 2-acrylamido-2-phenylethanesulfonic acid.

[0012] The epoxy-based polymer comprises at least one of polyethylene oxide and polypropylene oxide.

[0013] The cellulose-based polymer comprises at least one of sodium carboxymethyl cellulose and lithium carboxymethyl cellulose.

[0014] In a second aspect, the application provides a preparation method of a composite separator, which adopts the following technical scheme:

[0015] A preparation method of a composite separator comprises the following steps:

[0016] Step 1: mixing the inorganic particles A, the viscous polymer B, and the non-viscous polymer C with a solvent, and stirring to obtain a mixture slurry;

[0017] Step 2: coating the mixture slurry on the surface of the porous base film, and drying to obtain the composite separator.

[0018] In a third aspect, the application provides a lithium ion battery, which adopts the following technical scheme:

[0019] The lithium ion battery comprises the composite separator as described above. Advantages

[0020] First, by selecting a suitable adhesive polymer B, the adhesive polymer B can interact with the inorganic particles A, improve the adhesion of the base layer to the porous base film, better fix the non-adhesive polymer C, and not affect the uniformity of the distribution of the non-adhesive polymer C with a larger particle size in the base layer. Second, the maximum particle size of the non-adhesive polymer C distributed in the base layer is greater than the thickness of the base layer, so that the non-adhesive polymer C protrudes from the base layer, and a gap for filling the electrolyte is formed between the base layer and the pole piece, which is conducive to the more sufficient infiltration of the electrolyte into the pole piece and improves the lithium precipitation performance of the pole piece. Third, the non-adhesive polymer C used in the present application has no or weak adhesion, but the non-adhesive polymer C can produce physical adhesion between the pole piece under certain temperature and pressure conditions; since the non-adhesive polymer C can protrude from the surface of the base layer close to the pole piece, the non-adhesive polymer C can contact and produce physical adhesion with the pole piece, and at the same time, the adhesive polymer B in the base layer produces chemical adhesion with the porous base film, that is, one side of the base layer is physically adhered to the pole piece through the non-adhesive polymer C, and the other side of the base layer is chemically adhered to the porous base film, thereby balancing the stress distribution of the two sides of the base layer, realizing the stable adhesion of the pole piece-non-adhesive polymer C-base layer (inorganic particles A)-adhesive polymer B-porous base film, that is, realizing the stable adhesion of the pole piece and the porous base film, which helps to improve the mechanical strength of the roll core, avoid the pole piece from wrinkling, and thus improve the cycle performance of the lithium ion battery. Fourth, the adhesive polymer B and the non-adhesive polymer C can further improve the adhesion stability of the pole piece-non-adhesive polymer C-base layer (inorganic particles A)-adhesive polymer B-porous base film under the restraint of the inorganic particles A, thereby further improving the mechanical strength of the roll core and reducing the expansion degree of the pole piece during the charging and discharging process, which is conducive to further improving the cycle performance of the lithium ion battery.

[0021] It is worth mentioning that chemical adhesion mainly combines materials together through chemical bonding force, that is, through chemical reaction to achieve the adhesion effect; and physical adhesion mainly absorbs through the intermolecular force between the surfaces of materials to form force accumulation and thus achieve the adhesion effect. BRIEF DESCRIPTION OF DRAWINGS

[0022] FIG. 1 is a schematic view of the structure of a composite separator.

[0023] BRIEF DESCRIPTION OF DRAWINGS

[0024] 1, porous base film; 2, coating layer; 21, base layer; 22, non-adhesive polymer C. EMBODIMENTS OF THE INVENTION

[0025] In some embodiments, the thickness of the base layer is denoted as d, and the thickness of the base layer satisfies d≥1 μm.

[0026] In some embodiments, the thickness of the base layer satisfies 1 μm ≤ d ≤ 4 μm.

[0027] The base layer of the present application is thin, and uses less material, and can act with the non-adhesive polymer C to improve the adhesion stability of the pole piece and the porous base film.

[0028] In some embodiments, the non-adhesive polymer C includes at least one of an unsaturated nitrile monomer unit copolymer, a vinyl monomer unit copolymer, an alkenyl amine monomer unit copolymer, an acrylate monomer unit copolymer, a methacrylate monomer unit copolymer, a vinyl sulfonic acid monomer unit copolymer, a vinyl acetate monomer unit copolymer, a vinyl chloride monomer unit copolymer, a diene monomer unit copolymer, and a modified compound of the above copolymers.

[0029] In some embodiments, the unsaturated nitrile monomer unit includes at least one of acrylonitrile, methacrylonitrile;

[0030] The vinyl monomer unit includes at least one of styrene, α-methylstyrene, butoxystyrene, a vinyl naphthalene aromatic cluster vinyl monomer;

[0031] The alkenyl amine monomer unit includes at least one of acrylamide, methacrylamide, phenylmaleimide, and derivatives thereof;

[0032] The acrylate monomer unit includes at least one of methyl acrylate, ethyl acrylate, propyl acrylate, isopropyl acrylate, n-butyl acrylate, t-butyl acrylate, n-amyl acrylate, n-hexyl acrylate, heptyl acrylate, isooctyl acrylate, 2-ethylethyl acrylate, nonyl acrylate, decyl acrylate, lauryl acrylate, n-tetracosyl acrylate, stearyl acrylate, 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, and alkali metal salts thereof;

[0033] The methacrylate monomer unit includes at least one of methyl methacrylate, ethyl methacrylate, propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, t-butyl methacrylate, n-amyl methacrylate, n-hexyl methacrylate, heptyl methacrylate, isooctyl methacrylate, 2-ethylethyl methacrylate, nonyl methacrylate, decyl methacrylate, lauryl methacrylate, n-tetracosyl methacrylate, stearyl methacrylate, 2-hydroxyethyl methacrylate, 2-hydroxypropyl methacrylate, glycidyl methacrylate, cyclohexyl methacrylate, isobornyl methacrylate, allyl methacrylate, ethyleneglycol dimethacrylate, and alkali metal salts thereof;

[0034] The vinyl sulfonic monomer unit includes at least one of vinyl sulfonic acid, methyl vinyl sulfonic acid, styrene sulfonic acid, and alkali metal salts thereof;

[0035] The vinyl acetate monomer unit copolymer includes at least one of vinyl acetate and alkali metal salts thereof;

[0036] The chlorovinyl monomer unit includes at least one of chlorovinyl and vinylidene chloride;

[0037] The diene monomer unit includes at least one of phenyl maleimide, 1,4-butadiene, and isoprene;

[0038] In some embodiments, the non-adhesive polymer C includes a vinyl sulfonic monomer unit copolymer.

[0039] When the non-adhesive polymer C includes a vinyl sulfonic monomer unit copolymer, the sulfonic acid group (-SO3H) contained therein has excellent ion conduction performance, which can cooperate with the non-adhesive polymer B to enhance the lithium ion conduction ability of the composite separator, and is beneficial to improve the electrical performance of the lithium ion battery.

[0040] In some embodiments, the weight percentages of the inorganic particles A, the adhesive polymer B, and the non-adhesive polymer C are 65%-94%: 3%-10%: 3%-25%, based on 100% of the weight of the coating.

[0041] By adjusting the amounts of the inorganic particles A, the adhesive polymer B, and the non-adhesive polymer C, the inorganic particles A in the base layer can fully play a restraining role on the adhesive polymer B and the non-adhesive polymer C, significantly improving the strength of the core and improving the cycle performance of the core.

[0042] In some embodiments, the inorganic particles A include at least one of SrTiO3, SnO2, CeO2, MgO, NiO, CaO, ZnO, ZrO2, Y2O3, Al2O3, TiO2, SiC, and AlOOH.

[0043] Example 1

[0044] 1. Preparation of a composite separator

[0045] A composite separator includes a polyethylene separator and a coating layer, the polyethylene separator has a pore size of 10-400 nm and a thickness of 10 μm.

[0046] The coating layer comprises a base layer and a non-adhesive polymer C (vinyl chloride-vinylidene chloride copolymer) disposed on the base layer, the particle size of the non-adhesive polymer C is 0.3-15.0 μm, the particle size D50 is 1.0 μm, the base layer comprises inorganic particles A (Al2O3) and adhesive polymer B (polyvinylidene fluoride, polyvinyl alcohol and sodium carboxymethyl cellulose in a mass ratio of 1:1:1); the ratio of the maximum particle size of the non-adhesive polymer C to the thickness of the base layer in the coating layer is 9:1;

[0047] The weight percentage of the inorganic particles A, the adhesive polymer B and the non-adhesive polymer C is 80%:7%:13%;

[0048] The composite separator is prepared by the following steps:

[0049] Step one: the inorganic particles A, the adhesive polymer B and the non-adhesive polymer C are mixed with pure water according to the above weight percentage, and a mixture slurry is obtained by stirring;

[0050] Step two: the above mixture slurry is coated on both sides of the polyethylene separator, and a composite separator is obtained by drying.

[0051] The structure diagram of the composite separator in this embodiment is shown in FIG. 1.

[0052] 2. Preparation of lithium battery

[0053] 2.1 Preparation of positive electrode sheet

[0054] Lithium iron phosphate, polyvinylidene fluoride and acetylene black are mixed uniformly in a proper amount of N-methyl pyrrolidone at a mass ratio of 98:1:1 to obtain a positive electrode slurry, the positive electrode slurry is coated on a positive electrode current collector aluminum, and a positive electrode sheet is obtained through the processes of drying, cold pressing, slitting and cutting.

[0055] 2.2 Preparation of negative electrode sheet

[0056] Artificial graphite, sodium carboxymethyl cellulose and acetylene black are mixed uniformly in a proper amount of deionized water at a mass ratio of 98:1.5:0.5 to obtain a negative electrode slurry, the negative electrode slurry is coated on a negative electrode current collector copper foil, and a negative electrode sheet is obtained through the processes of drying, cold pressing, slitting and cutting.

[0057] 2.3 Preparation of separator

[0058] The separator is the composite separator prepared above.

[0059] 2.4 Preparation of electrolyte

[0060] Mix ethylene carbonate (EC), methyl ethyl carbonate (EMC) and diethyl carbonate (DEC) in a volume ratio of 1:1:1 to obtain a mixed solvent; dissolve the fully dried electrolyte salt LiPF6 in the mixed solvent, and obtain an electrolyte solution with a concentration of 1.0 mol / L after uniform mixing.

[0061] 2.5 Assembly of lithium battery

[0062] Stack the positive electrode sheet, the separator membrane and the negative electrode sheet in order, so that the composite separator membrane is between the positive and negative electrode sheets to play a role of isolation, and then roll to obtain an electrode assembly: heat press the electrode assembly at 90℃ under a pressure of 1.0 tons for 1s; place the electrode assembly in an outer package, inject the electrolyte prepared above into the dried secondary battery, and obtain a lithium battery after vacuum packaging, standing, formation and shaping processes.

[0063] Example 2

[0064] 1. Preparation of composite separator membrane

[0065] A composite separator membrane comprises a polypropylene separator membrane and a coating layer, the polypropylene separator membrane has a pore size of 10-400 nm and a thickness of 5 μm;

[0066] The coating layer comprises a base layer and a non-adhesive polymer C (acrylamide copolymer) arranged on the base layer, the non-adhesive polymer C has a particle size of 0.5-20.0 μm and a particle size D50 of 3.0 μm, the base layer comprises inorganic particles A (NiO) and an adhesive polymer B (polyacrylonitrile and lithium carboxymethyl cellulose in a mass ratio of 2:1); the ratio of the maximum particle size of the non-adhesive polymer C to the thickness of the base layer is 10:1 in the coating layer;

[0067] The weight percentage of the inorganic particles A, the adhesive polymer B and the non-adhesive polymer C is 65%:10%:25%;

[0068] The composite separator membrane is prepared by the following steps:

[0069] Step one: mix the inorganic particles A, the adhesive polymer B and the non-adhesive polymer C with pure water according to the above weight percentage, and stir to obtain a mixture slurry;

[0070] Step two: coat the mixture slurry on both sides of the polypropylene separator membrane, and dry to obtain the composite separator membrane.

[0071] 2. Preparation of lithium battery

[0072] 2.1 Preparation of positive electrode sheet

[0073] Lithium iron phosphate, polyvinylidene fluoride and acetylene black are mixed uniformly in a proper amount of N-methyl pyrrolidone at a mass ratio of 98:1:1 to obtain a positive electrode slurry, the positive electrode slurry is coated on a positive electrode current collector aluminum, and the positive electrode slurry is obtained through the processes of drying, cold pressing, slitting and cutting.

[0074] 2.2 Preparation of negative electrode sheet

[0075] Artificial graphite, sodium carboxymethyl cellulose and acetylene black are mixed uniformly in a proper amount of deionized water at a mass ratio of 98:1.5:0.5 to obtain a negative electrode slurry, the negative electrode slurry is coated on a negative electrode current collector copper foil, and the negative electrode sheet is obtained through the processes of drying, cold pressing, slitting and cutting.

[0076] 2.3 Preparation of separator film

[0077] The separator film is the composite separator film prepared above.

[0078] 2.4 Preparation of electrolyte

[0079] Vinyl carbonate (EC), methyl ethyl carbonate (EMC) and diethyl carbonate (DEC) are mixed at a volume ratio of 1:1:1 to obtain a mixed solvent; the fully dried electrolyte salt LiPF6 is dissolved in the mixed solvent, and the electrolyte with a concentration of 1.0 mol / L is obtained after uniform mixing.

[0080] 2.5 Assembly of lithium battery

[0081] The positive electrode sheet, the separator film and the negative electrode sheet are stacked in sequence, the composite separator film is located between the positive electrode sheet and the negative electrode sheet to play a role of isolation, and then the electrode assembly is obtained by winding; the electrode assembly is hot-pressed at 90°C and a pressure of 1.0 tons for 1s; the electrode assembly is placed in an outer package, the electrolyte prepared above is injected into the dried secondary battery, and the lithium battery is obtained after the processes of vacuum packaging, standing, formation and shaping.

[0082] Example 3

[0083] 1. Preparation of composite separator film

[0084] A composite separator film, comprising a polyethylene separator film and a coating layer, the polyethylene separator film has a pore size of 10-400 nm and a thickness of 20 μm.

[0085] The coating layer comprises a base layer and a non-adhesive polymer C (phenyl maleimide copolymer) disposed on the base layer, the particle size of the non-adhesive polymer C is 1.0-22.0 μm, the particle size D50 is 10.0 μm, the base layer comprises inorganic particles A (ZrO2) and adhesive polymer B (polyimide, acrylamide and sodium carboxymethyl cellulose in a mass ratio of 1:1:1); the ratio of the maximum particle size of the non-adhesive polymer C to the thickness of the base layer in the coating layer is 12:1;

[0086] The weight percentage of the inorganic particles A, the adhesive polymer B and the non-adhesive polymer C is 94%:3%:3%;

[0087] The composite diaphragm is prepared by the following steps:

[0088] Step one: the inorganic particles A, the adhesive polymer B and the non-adhesive polymer C are mixed with pure water according to the above weight percentage, and a mixture slurry is obtained by stirring;

[0089] Step two: the above mixture slurry is coated on both sides of the polyethylene diaphragm, and a composite diaphragm is obtained by drying.

[0090] 2. Preparation of lithium battery

[0091] 2.1 Preparation of positive electrode sheet

[0092] Lithium iron phosphate, polyvinylidene fluoride and acetylene black are uniformly mixed in a proper amount of N-methyl pyrrolidone at a mass ratio of 98:1:1 to obtain a positive electrode slurry, the positive electrode slurry is coated on a positive electrode current collector aluminum, and a positive electrode sheet is obtained through the processes of drying, cold pressing, slitting and cutting.

[0093] 2.2 Preparation of negative electrode sheet

[0094] Artificial graphite, sodium carboxymethyl cellulose and acetylene black are uniformly mixed in a proper amount of deionized water at a mass ratio of 98:1.5:0.5 to obtain a negative electrode slurry, the negative electrode slurry is coated on a negative electrode current collector copper foil, and a negative electrode sheet is obtained through the processes of drying, cold pressing, slitting and cutting.

[0095] 2.3 Preparation of isolation film

[0096] The isolation film uses the composite diaphragm prepared above.

[0097] 2.4 Preparation of electrolyte

[0098] Vinyl carbonate (EC), methyl ethyl carbonate (EMC) and diethyl carbonate (DEC) are mixed at a volume ratio of 1:1:1 to obtain a mixed solvent; the fully dried electrolyte salt LiPF6 is dissolved in the above mixed solvent, and the electrolyte with a concentration of 1.0 mol / L is obtained after uniform mixing.

[0099] 2.5 Lithium battery assembly

[0100] The positive electrode sheet, the separator, and the negative electrode sheet are stacked in order, the composite separator is arranged between the positive and negative electrode sheets to play a role of isolation, and then the electrode assembly is obtained by winding: the electrode assembly is hot-pressed at 90℃ under a pressure of 1.0 ton for 1s; the electrode assembly is placed in an outer package, the prepared electrolyte is injected into the dried secondary battery, and after vacuum packaging, standing, formation, and shaping processes, a lithium battery is obtained.

[0101] Example 4

[0102] 1. Preparation of composite separator

[0103] A composite separator comprises a polyethylene separator and a coating layer, the polyethylene separator has a pore size of 10-400nm and a thickness of 20μm;

[0104] The coating layer comprises a base layer and a non-adhesive polymer C (acrylonitrile-methacrylonitrile copolymer) arranged on the base layer, the particle size of the non-adhesive polymer C is 3.0-30.0μm and the particle size D50 is 15.0μm, the base layer comprises inorganic particles A (SnO2) and adhesive polymer B (polyethylene oxide and sodium carboxymethyl cellulose with a mass ratio of 2:1); the ratio of the maximum particle size of the non-adhesive polymer C to the thickness of the base layer in the coating layer is 15:1;

[0105] The weight percentage of inorganic particles A, adhesive polymer B, and non-adhesive polymer C is 90%:4%:6%;

[0106] The composite separator is prepared by the following steps:

[0107] Step one: inorganic particles A, adhesive polymer B, and non-adhesive polymer C are mixed with pure water according to the above weight percentage, and the mixture slurry is obtained by stirring;

[0108] Step two: the mixture slurry is coated on both sides of the polyethylene separator, and the composite separator is obtained by drying.

[0109] 2. Preparation of lithium battery

[0110] 2.1 Preparation of positive electrode sheet

[0111] Lithium iron phosphate, polyvinylidene fluoride, and acetylene black are uniformly mixed in an appropriate amount of N-methyl pyrrolidone at a mass ratio of 98:1:1 to obtain a positive electrode slurry, the positive electrode slurry is coated on the positive electrode current collector aluminum, and the positive electrode sheet is obtained by drying, cold pressing, slitting, and cutting processes.

[0112] 2.2 Preparation of negative electrode sheet

[0113] The artificial graphite, sodium carboxymethyl cellulose and acetylene black are mixed uniformly in a proper amount of deionized water at a mass ratio of 98:1.5:0.5 to obtain a negative electrode slurry, the negative electrode slurry is coated on a negative electrode current collector copper foil, and through the processes of drying, cold pressing, slitting and cutting, a negative electrode sheet is obtained.

[0114] 2.3 Preparation of the separation film

[0115] The separation film is the composite separation film prepared above.

[0116] 2.4 Preparation of the electrolyte

[0117] Vinyl carbonate (EC), methyl ethyl carbonate (EMC) and diethyl carbonate (DEC) are mixed at a volume ratio of 1:1:1 to obtain a mixed solvent; the fully dried electrolyte salt LiPF6 is dissolved in the mixed solvent, and an electrolyte with a concentration of 1.0 mol / L is obtained after uniform mixing.

[0118] 2.5 Assembly of the lithium battery

[0119] The positive electrode sheet, the separation film and the negative electrode sheet are stacked in sequence, the composite separation film is located between the positive electrode sheet and the negative electrode sheet to play a separation role, and then the electrode assembly is obtained by winding: the electrode assembly is hot-pressed at 90°C under a pressure of 1.0 tons for 1s; the electrode assembly is placed in an outer package, the electrolyte prepared above is injected into the dried secondary battery, and the lithium battery is obtained after the processes of vacuum packaging, standing, formation and shaping.

[0120] Example 5

[0121] The difference between this example and Example 1 is that the particle size of the non-adhesive polymer C is 2.0-6.0 μm, and the particle size D50 is 4.0 μm; the other steps and parameters are consistent with those of Example 1.

[0122] Example 6

[0123] The difference between this example and Example 2 is that the particle size of the non-adhesive polymer C is 5.0-10.0 μm, and the particle size D50 is 6.5 μm; the other steps and parameters are consistent with those of Example 2.

[0124] Example 7

[0125] The difference between this example and Example 3 is that the particle size of the non-adhesive polymer C is 13.0-18.0 μm, and the particle size D50 is 15.0 μm; the other steps and parameters are consistent with those of Example 3.

[0126] Example 8

[0127] The difference between this embodiment and embodiment 4 is that the particle size of the non-adhesive polymer C is 15.0-25.0 μm, and the particle size D50 is 20.0 μm; other steps and parameters are consistent with embodiment 4.

[0128] Example 9

[0129] The difference between this embodiment and embodiment 1 is that the ratio of the maximum particle size of the non-adhesive polymer C to the thickness of the base layer is adjusted to 18:1 while the thickness of the base layer remains unchanged; other steps and parameters are consistent with embodiment 1.

[0130] Example 10

[0131] The difference between this embodiment and embodiment 1 is that the ratio of the maximum particle size of the non-adhesive polymer C to the thickness of the base layer is adjusted to 1.05:1 while the thickness of the base layer remains unchanged; other steps and parameters are consistent with embodiment 1.

[0132] Example 11

[0133] The difference between this embodiment and embodiment 1 is that the non-adhesive polymer C is a vinyl amine monomer unit copolymer (acrylamide-methyl acrylamide copolymer); other steps and parameters are consistent with embodiment 1.

[0134] Example 12

[0135] The difference between this embodiment and embodiment 1 is that the non-adhesive polymer C is an alkenyl sulfonic acid monomer unit copolymer (vinyl sulfonic acid-methyl vinyl sulfonic acid copolymer); other steps and parameters are consistent with embodiment 1.

[0136] Example 13

[0137] The difference between this embodiment and embodiment 1 is that the weight percentage of inorganic particles A, adhesive polymer B and non-adhesive polymer C is 80%:15%:5%; other steps and parameters are consistent with embodiment 1.

[0138] Example 14

[0139] The difference between this embodiment and embodiment 1 is that the weight percentage of inorganic particles A, adhesive polymer B and non-adhesive polymer C is 55%:15%:30%; other steps and parameters are consistent with embodiment 1.

[0140] Comparative Example 1

[0141] The difference between this comparative example and embodiment 1 is that the maximum particle size of the non-adhesive polymer C is less than the thickness of the base layer, and other steps and parameters are consistent with embodiment 1.

[0142] Comparative Example 2

[0143] The difference between this comparative example and Example 1 is that the adhesive polymer B is selected from aromatic polyketone, and other steps and parameters are consistent with Example 1.

[0144] Test method

[0145] I. Cycle performance of lithium battery

[0146] The lithium batteries in the above examples and comparative examples were tested for cycle performance, and the specific test steps were as follows: at 25°C, the lithium battery was charged at 1C rate to the charge cut-off voltage of 3.65V, then charged at constant voltage to the current ≤0.05C, and rested for 5min, then discharged at 1C rate to the discharge cut-off voltage of 2.5V, and rested for 30min, and the battery capacity C0at this time was recorded; the lithium battery was subjected to 1500 cycles of charge and discharge according to this method, and the battery capacity C1after 1500 cycles was recorded.

[0147] The cycle capacity retention rate of the battery at 25°C = C1 / C0x 100%.

[0148] II. Test of lithium precipitation performance

[0149] The lithium batteries in the above examples and comparative examples were tested for cycle performance, and the specific test steps were as follows: using a new will (BTSDA) test system, the test temperature was 25°C, and the specific test steps were as follows: charged at 1C constant current to 3.65V, charged at constant voltage to 0.05C, rested for 10min, discharged at 1C to 2.5V, and rested for 10min, and the above charge and discharge steps were repeated for 200 cycles; the lithium battery after 200 cycles was disassembled, and the presence or absence of lithium precipitation on the negative electrode sheet was observed, and the proportion of lithium precipitation area to the entire negative electrode sheet area was estimated, the proportion of lithium precipitation area to the entire electrode sheet area within 5% was not lithium precipitation, the proportion of lithium precipitation area to the entire electrode sheet area within 20% was slight lithium precipitation, and the proportion of lithium precipitation area to the entire electrode sheet area of 20% or more was severe lithium precipitation.

[0150] Table 1

[0151] No. Cycle capacity retention rate / % Lithium precipitation performance / % Example 1 89.3 3.3 Example 2 88.6 3.6 Example 3 89.1 3.5 Example 4 88.8 3.4 Example 5 91.1 2.4 Example 6 90.5 2.7 Example 7 91.2 2.6 Example 8 90.8 2.5 Example 9 85.8 6.3 Example 10 86.9 5.8 Example 11 88.5 3.8 Example 12 90.9 3.2 Example 13 81.6 9.1 Comparative Example 1 82.9 8.2 Comparative Example 2 77.8 14.3

[0152] In combination with Examples 1-3, Comparative Examples 1-2 and Table 1, it can be seen that, by selecting suitable types of adhesive polymer B and suitable sizes of non-adhesive polymer C, and by the combined action of inorganic particles A, the present application can significantly improve the adhesion stability between the pole piece and the composite separator, avoid the swelling of the core during the application of the lithium battery, and improve the lithium precipitation performance of the lithium battery; the cycle capacity retention rate of the lithium ion battery in Example 1-4 is higher than that of the lithium ion battery in Comparative Example 1-2, and the lithium precipitation area of the pole piece in Example 1-3 is smaller than that of the pole piece in Comparative Example 1-2.

[0153] This is because the adhesive polymer B can form a basic layer with the inorganic particles A and the porous base film, and the large-particle non-adhesive polymer C can also be stably embedded in the basic layer. Under certain temperature and pressure conditions, the non-adhesive polymer C forms a physical adhesion between the pole piece, thereby realizing the stable adhesion of the coating layer to the pole piece and the porous base film on the opposite sides, avoiding the pole piece from wrinkling and improving the mechanical strength of the core; and the large-particle non-adhesive polymer C can also protrude from the surface of the basic layer, increasing the gap between the pole piece and the porous base film, which not only improves the degree of electrolyte infiltration of the pole piece to a certain extent, but also increases the lithium ion transmission path, which all helps to improve the lithium precipitation performance of the pole piece.

[0154] In combination with Example 1, Examples 5-8 and Table 1, it can be seen that the capacity retention rate of the lithium ion battery in Examples 5-8 is slightly higher than that of the lithium ion battery in Example 1, which is because by further optimizing the particle size of the non-adhesive polymer C and the adhesive polymer B, the compatibility between the polymer C and the basic layer can be adjusted in time, the adhesion stability of the pole piece-non-adhesive polymer C-basic layer-adhesive polymer B-porous base film system can be improved, the mechanical strength of the core can be further optimized and improved, and the cycle stability of the lithium ion battery can be improved.

[0155] In combination with Example 2, Examples 9-10 and Table 1, it can be seen that the cycle capacity retention rate of the lithium ion battery in Example 1 is higher than that of the lithium ion battery in Example 9, and the lithium precipitation area of the pole piece in Example 1 is smaller than that of the pole piece in Example 9; this is because the electrolyte channel jointly constructed between the coating and the pole piece can increase the liquid retention capacity of the composite separator, form an effective lithium ion passage, compared with the case that the ratio of the maximum particle size of the non-adhesive polymer C to the thickness of the base layer in Example 9 is too large, the ratio of the maximum particle size of the non-adhesive polymer C to the thickness of the base layer in Example 1 is controlled within a suitable range, which helps to maintain a strong liquid retention capacity in the gap between the composite separator and the pole piece, so that the electrolyte in the above-mentioned electrolyte channel always remains in a relatively sufficient state, thereby maintaining the lithium ion transmission capacity of the lithium ion battery in Example 1 at a relatively good level, the corresponding part of the pole piece is small and the pole piece has a small risk of lithium precipitation, which shows the improvement of the comprehensive performance of the lithium ion battery.

[0156] The cycle capacity retention rate of the lithium ion battery in Example 1 is higher than that of the lithium ion battery in Example 10, and the lithium precipitation area of the pole piece in Example 1 is smaller than that of the pole piece in Example 10; this is because when the ratio of the maximum particle size of the non-adhesive polymer C to the thickness of the base layer is controlled within a suitable range (such as Example 1), the area of the non-adhesive polymer C protruding from the base layer is larger, and under the subsequent action of temperature and pressure, the area of the effective region where the non-adhesive polymer C and the pole piece produce physical adhesion is large, which is conducive to improving the adhesion between the coating and the pole piece, and further improving the adhesion between the porous base film and the pole piece, and also conducive to improving the stability of the coating with the porous base film and the pole piece on the two opposite surfaces respectively, so that the mechanical strength of the roll core in Example 1 is improved, thereby improving its resistance to swelling and deformation of the pole piece during use, and improving the cycle stability of the lithium ion battery; at the same time, the electrolyte channel formed by the gap between the pole piece and the composite separator is conducive to the passage of electrolyte with sufficient volume, so that the degree of electrolyte infiltration of the pole piece in Example 1 is high, thereby the lithium precipitation performance of the pole piece in Example 1 is superior to that of the pole piece in Example 10.

[0157] In combination with Example 1, Examples 11-12 and Table 1, it can be seen that when the non-adhesive polymer C includes a vinyl sulfonic acid monomer unit copolymer, the cycle capacity retention rate of the lithium battery is increased and the lithium precipitation area is decreased, which is because the sulfonic acid group contained in the vinyl sulfonic acid monomer unit copolymer can cooperate with the non-adhesive polymer B to enhance the lithium ion conduction capacity of the composite separator, thereby facilitating the improvement of the cycle performance of the lithium ion battery and the reduction of the lithium precipitation area of the pole piece.

[0158] In combination with Example 1, Example 13 and Table 1, it can be seen that the cycle capacity retention rate of the lithium ion battery in Example 1 is higher than that of the lithium ion battery in Example 13, and the lithium precipitation area of the pole piece in Example 1 is smaller than that of the pole piece in Example 13; this is because when the amount of non-adhesive polymer C is higher than that of adhesive polymer B (Example 1), the physical adhesion between non-adhesive polymer C-pole pieces and the chemical adhesion between adhesive polymer B-porous base film can achieve a better balance, which helps to improve the mechanical strength of the roll core, avoids the expansion of the pole piece in the application process, and thus exhibits excellent cycle performance of the lithium ion battery and small lithium precipitation area of the pole piece.

[0159] In combination with Example 1, Example 14 and Table 1, it can be seen that the cycle capacity retention rate of the lithium ion battery in Example 1 is higher than that of the lithium ion battery in Example 14, and the lithium precipitation area of the pole piece in Example 1 is smaller than that of the pole piece in Example 14; this is because when the amount of inorganic particles A is kept within a suitable range (Example 1), the restraint of inorganic particles A on adhesive polymer B and non-adhesive polymer C is kept at a high level, which is conducive to achieving stable adhesion of the coating between the pole piece and the porous base film, increasing the adhesion stability of the pole piece and the porous base film, and avoiding the pole piece from appearing wrinkles, thereby exhibiting excellent cycle performance of the lithium ion battery and small lithium precipitation area of the pole piece.

Claims

1. A composite separator comprising a porous base film and a coating layer; the coating layer is disposed on a surface of the porous base film, the coating layer comprises a base layer and a non-adhesive polymer C disposed on the base layer, the base layer comprises inorganic particles A and an adhesive polymer B; a maximum particle size of the non-adhesive polymer C is greater than a thickness of the base layer, the particle size of the non-adhesive polymer C is 0.3-30 μm; the adhesive polymer B comprises a first component and a second component; the first component comprises at least one of an ethylene-based polymer, a propylene-based polymer, an amide-based polymer, an epoxy-based polymer; the second component comprises a cellulose-based polymer; wherein the ethylene-based polymer comprises at least one of polyvinylidene fluoride, polyvinylidene fluoride-co-trichloroethylene, polyvinyl acetate, polyethylene-co-vinyl acetate, polyvinyl alcohol, polyvinylpyrrolidone, polymaleic anhydride; the propylene-based polymer comprises at least one of polymethyl methacrylate, polyacrylonitrile; the amide-based polymer comprises at least one of polyimide, acrylamide, methacrylamide, hydroxymethyl acrylamide, diacetone acrylamide, N,N-diethyl acrylamide, N,N-diethyl methacrylamide, N-ethyl acrylamide, N-ethyl methacrylamide, N,N-dimethyl acrylamide, N,N-dimethyl acrylamide, N-methyl acrylamide, N-isopropyl acrylamide, N-isopropyl methacrylamide, 2-acrylamido-2-phenylethanesulfonic acid; the epoxy-based polymer comprises at least one of polyethylene oxide, polypropylene oxide; the cellulose-based polymer comprises at least one of sodium carboxymethyl cellulose, lithium carboxymethyl cellulose.

2. A composite separator according to claim 1, wherein: when the adhesive polymer B comprises an ethylene-based polymer and a cellulose-based polymer, the particle size of the non-adhesive polymer C is 0.3-15.0 μm.

3. The composite separator of claim 1, wherein: when the adhesive polymer B comprises a propylene-based polymer and a cellulose-based polymer, the particle size of the non-adhesive polymer C is 0.5-20.0 μm.

4. The composite separator of claim 1, wherein: when the adhesive polymer B comprises an amide-based polymer and a cellulose-based polymer, the particle size of the non-adhesive polymer C is 1.0-22.0 μm.

5. The composite separator of claim 1, wherein: when the adhesive polymer B comprises an epoxy-based polymer and a cellulose-based polymer, the particle size of the non-adhesive polymer C is 3.0-30.0 μm.

6. The composite separator of claim 1, wherein: a ratio of a maximum particle size of the non-adhesive polymer C to a thickness of the base layer satisfies 1.2-15:

1.

7. A composite separator according to claim 1, wherein: the thickness of the base layer is denoted as d, and the thickness of the base layer satisfies d≥1 μm.

8. The composite separator of claim 1, wherein: the non-adhesive polymer C comprises at least one of an unsaturated nitrile-based monomer unit copolymer, a vinyl monomer unit copolymer, an alkenyl amine-based monomer unit copolymer, an acrylate-based monomer unit copolymer, a methacrylate-based monomer unit copolymer, a vinyl sulfonic acid-based monomer unit copolymer, a vinyl acetate monomer unit copolymer, a chlorovinyl monomer unit copolymer, a diene-based monomer unit copolymer, and a modified compound of the above copolymers.

9. A composite separator according to claim 8, wherein: the non-adhesive polymer C comprises a vinyl sulfonic acid-based monomer unit copolymer.

10. A composite separator as set forth in claim 1, wherein: The weight percentage of the inorganic particles A, the adhesive polymer B and the non-adhesive polymer C is 65%-94%:3%-10%:3%-25%, based on 100% of the weight of the coating.

11. The composite separator of claim 1, wherein: The inorganic particles A comprise at least one of SrTiO3, SnO2, CeO2, MgO, NiO, CaO, ZnO, ZrO2, Y2O3, Al2O3, TiO2, SiC, AlOOH.

12. A method for preparing the composite separator according to any one of claims 1-11, comprising the following steps: Step one: mixing the inorganic particles A, the adhesive polymer B and the non-adhesive polymer C with a solvent, and stirring to obtain a mixture slurry; Step two: coating the mixture slurry on the surface of the porous base film, and drying to obtain the composite separator.

13. A lithium ion battery comprising the composite separator according to any one of claims 1-11.

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