Composite diaphragm as well as preparation method and application thereof

Abstract

The invention relates to the technical field of lithium ion batteries, in particular to a composite diaphragm as well as a preparation method and application thereof. The composite diaphragm comprises a base membrane and a coating layer formed on the base membrane, the coating layer is prepared from raw material components including a dispersion liquid of a composite solid electrolyte, a wetting agent and a binder; the composite solid electrolyte comprises: a solid electrolyte; the surface of the solid electrolyte is coated with the barium titanate dielectric material layer; the surface of the barium titanate dielectric material layer is coated with the silane coupling agent layer. The barium titanate dielectric material layer is adopted as the first functional layer, then the silane coupling agent is coupled to the surface of the barium titanate dielectric material layer, the solid electrolyte modified by the double functional layers can effectively reduce powder agglomeration, and dissociation of lithium salt and lithium ion transmission at a solid-liquid interface are promoted.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium-ion batteries, and particularly to a composite separator, a preparation method thereof, and an application thereof. Background Art

[0002] The separator plays a role of isolating the positive and negative electrodes and transporting lithium ions in a lithium-ion battery. As one of the four major main materials of a lithium-ion battery, the separator plays an important role in the performance of the entire battery cell, significantly affecting the cycle, rate, and safety performance of the battery. The commonly used polyethylene / polypropylene separator has a certain mechanical strength, which is convenient for winding and pressing during the assembly of the battery cell. It has micropores of different sizes inside, allowing the electrolyte to fill and lithium ions to pass through. However, due to its poor performance such as thermal shrinkage, closed pores, and thermal melting, it is extremely easy to be pierced by lithium dendrites, leading to major safety hazards. Therefore, it is difficult to meet the requirements of high electrical performance and high safety battery cells. Thus, it is necessary to modify the basic separator. The commonly used modification method is to coat one or more layers of ceramic inorganic substances / organic polymers on one or both sides of the separator to improve the wettability of the separator to the electrolyte and the thermal stability, thereby improving its application performance in the battery cell. Compared with using other modification methods (non-woven separator, chemical grafting, etc.), the simple coating process is simple and has good economic benefits.

[0003] Compared with the base film and inert ceramic coating, coating with fast ion conductors (LATP, LLTO, LLZO, etc.) not only improves the wettability of the separator to the electrolyte, but also can transport lithium ions synergistically with the electrolyte, while enhancing the overall thermal stability of the separator and also significantly improving the safety performance. In addition, compounding or other modification treatments of the coating material can maximize the role of the coating, and optimizing some combinations of additives used in the preparation of the coating slurry is also an important way to improve the performance of the separator.

[0004] The patent application document CN115548576A discloses that a mixed liquid of lithium aluminum titanium phosphate, PMMA polymer collagen, and an organic solvent is coated on the separator. After coating, the organic solvent in the separator is extracted with water, and the final finished film is obtained after drying. The rich hydroxyl groups on the polymer surface interact with the oxygen atoms in the carbonate in the electrolyte to lock a large amount of liquid electrolyte. At the same time, PMMA has a bonding effect, realizing the tight connection between the coating and the base film, and maintaining the cycling ability in the later cycle of the lithium-ion battery when the electrolyte is lacking.

[0005] The patent application document CN109119573A uses a mixture of LLZO and a binder to make aqueous slurry and oil-based slurry respectively, which are coated on the separator. LLZO is synthesized by the sol-gel method and has a high ionic conductivity. The LLZO coating layer reduces the original pores of the separator, and smaller pores can absorb more electrolyte under capillary action. In addition, the LLZO coating layer can inhibit the growth of lithium dendrites and the shuttle effect in lithium-sulfur batteries.

[0006] Therefore, coating a functional layer on the separator is an effective method to improve the wettability of the electrolyte and the thermal stability of the separator body. However, there is a difference in the chemical potential between the coated functional layer and the electrolyte, which will cause the formation of a double-layer capacitance at the solid-liquid interface, having a certain hindering effect on the migration of lithium ions at the interface. In addition, due to the presence of polar groups on the surface of the powder in the coating slurry, there is an agglomeration phenomenon between small-sized particles after the coating layer dries. Coating the separator with polymer-coated oxide solid electrolyte powder effectively hinders the agglomeration of oxide solid electrolyte in water, but there is also a space charge between the oxide solid electrolyte and the polymer. Therefore, the dissociation of lithium salts in the polymer is hindered, limiting the improvement of ionic conductivity. A similar situation occurs when using a polymer binder. At the same time, as a poor ionic conductor, the binder will change chemically when soaked in the electrolyte, affecting the cycling performance. Summary of the Invention

[0007] Aiming at the problems of space charge hindrance, coating powder agglomeration, and additive restricting ion dissociation and transport existing in the existing separator coating, the present invention provides a composite separator, its preparation method and application, which can effectively reduce powder agglomeration, promote the dissociation of lithium salts and the lithium ion transport at the solid-liquid interface.

[0008] The present invention provides a composite separator, characterized in that it includes:

[0009] a base film, and a coating layer formed on the base film; the coating layer is prepared from raw material components including a dispersion liquid of composite solid electrolyte, a wetting agent, and a binder;

[0010] The composite solid electrolyte includes:

[0011] a solid electrolyte;

[0012] a barium titanate dielectric material layer coated on the surface of the solid electrolyte;

[0013] a silane coupling agent layer coated on the surface of the barium titanate dielectric material layer.

[0014] Preferably, the silane coupling agent includes at least one of nonafluorohexyltrimethoxysilane, trimethoxyhexadecylsilane, trimethoxyperfluorodecylsilane, perfluorooctyltrimethoxysilane, triethoxyperfluorodecylsilane, trimethoxyoctadecylsilane, dodecyltrimethoxysilane, n-propyltrimethoxysilane, and trimethoxymethylsilane.

[0015] Preferably, the preparation method of the composite solid electrolyte includes the following steps:

[0016] S1) After mixing the mixture liquid A and the mixture liquid B evenly, disperse them at a high speed. After adjusting the pH value to 3-4, continue to disperse at a high speed, carry out ultrasonic treatment, and then react at 30-60 °C under stirring conditions until a gel is formed;

[0017] The mixture liquid A is obtained by dispersing solid electrolyte nanopowder in a first solvent;

[0018] The mixture liquid B is obtained by dispersing tetrabutyl titanate and barium acetate powder in a second solvent;

[0019] S2) After drying the gel, a precursor material is obtained;

[0020] S3) Calcinate the precursor material to obtain a barium titanate-coated solid electrolyte material;

[0021] S4) Mix the ethanol dispersion of the barium titanate-coated solid electrolyte material with the hydrolysis solution of the silane coupling agent, carry out modification under the conditions of heating and stirring, and then dry to obtain the composite solid electrolyte.

[0022] Preferably, in step S1), the solid electrolyte includes LATP, LLTO or LLZO; the median particle size D50 of the solid electrolyte nanopowder is 500-600 nm.

[0023] Preferably, in the mixture liquid B, the mass content of tetrabutyl titanate is 7%-10%, and the mass content of barium acetate is 4%-7%;

[0024] The molar ratio of titanium element in the tetrabutyl titanate to barium element in the barium acetate powder is (1-1.3):1;

[0025] The mass ratio of the tetrabutyl titanate to the solid electrolyte nanopowder is 1:(2-4).

[0026] Preferably, in step S3), the calcination temperature is 400-700 °C and the time is 5-10 h.

[0027] Preferably, in step S4), the mass ratio of the silane coupling agent to the barium titanate-coated solid electrolyte material is 1:(20-60).

[0028] Preferably, in step S4), the temperature of the heating and stirring is 40-80°C.

[0029] The present invention also provides a method for preparing the above-mentioned composite separator, comprising the following steps:

[0030] Mix the dispersion of the composite solid electrolyte, the wetting agent and the binder to obtain a mixed slurry, coat it on the base film, and after drying, obtain the composite separator.

[0031] Preferably, the wetting agent includes at least one of sodium dodecyl sulfate, the condensate of fatty alcohol and ethylene oxide, and alkynediols;

[0032] The binder includes at least one of polyacrylate, carboxymethyl acrylate salt and carboxymethyl cellulose salt.

[0033] Preferably, the mass ratio of the composite solid electrolyte, the wetting agent and the binder is (15-50):(1-3):(2.5-5.5).

[0034] The present invention also provides a lithium-ion battery, comprising the above-mentioned composite separator, or the composite separator prepared by the above-mentioned preparation method.

[0035] The present invention provides a composite separator, comprising: a base film, and a coating layer formed on the base film; the coating layer is prepared from raw material components including a dispersion of a composite solid electrolyte, a wetting agent and a binder; the composite solid electrolyte includes: a solid electrolyte; a barium titanate dielectric material layer coated on the surface of the solid electrolyte; a silane coupling agent layer coated on the surface of the barium titanate dielectric material layer. The present invention uses the barium titanate dielectric material layer as the first functional layer, in-situ coats the solid electrolyte by the sol-gel method, there is a dipole moment inside the dielectric material, forming a spontaneous built-in electric field, coating on the surface of the solid electrolyte, and under the action of the electric field, polarization occurs, promoting the dissociation of the lithium salt in the electrolyte and the polymer and the transport of lithium ions at the solid-liquid interface, creating a high-throughput lithium ion transport path. Then, a silane coupling agent is coupled on the surface of the barium titanate dielectric material layer, and the non-polar group of the alkyl at the other end of the silane coupling agent is used to achieve full contact with the electrolyte and the base film, enhancing the wettability of the solid electrolyte with the electrolyte and the adhesiveness with the base film. The solid electrolyte modified by the double-functional layer can effectively reduce powder agglomeration, promote the dissociation of the lithium salt and the transport of lithium ions at the solid-liquid interface. The solid electrolyte coated with the double-functional layer promotes the dissociation of the lithium salt and the transport of lithium ions at the solid-liquid interface due to the piezoelectric effect of the inner barium titanate, and the outer silane coupling agent makes the solid electrolyte fully wetted by the electrolyte. In addition, due to the presence of the coupling agent, the amount of the binder can be reduced while the bonding strength between the powder and the base film is improved. Detailed Embodiments

[0036] The technical solutions of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0037] The present invention provides a composite separator, comprising:

[0038] A base film;

[0039] A coating layer formed on the base film;

[0040] The coating layer is prepared from raw material components including a dispersion liquid of a composite solid electrolyte, a wetting agent, and a binder;

[0041] The composite solid electrolyte includes:

[0042] A solid electrolyte;

[0043] A barium titanate dielectric material layer coated on the surface of the solid electrolyte;

[0044] A silane coupling agent layer coated on the surface of the barium titanate dielectric material layer.

[0045] In some embodiments of the present invention, the solid electrolyte includes LATP, LLTO, or LLZO.

[0046] The material of the barium titanate dielectric material layer is barium titanate.

[0047] In some embodiments of the present invention, the silane coupling agent is selected from at least one of nonafluorohexyltrimethoxysilane, trimethoxyhexadecylsilane, trimethoxyperfluorodecylsilane, perfluorooctyltrimethoxysilane, triethoxyperfluorodecylsilane, trimethoxyoctadecylsilane, dodecyltrimethoxysilane, n-propyltrimethoxysilane, and trimethoxymethylsilane.

[0048] Specifically, the composite solid electrolyte includes:

[0049] A solid electrolyte;

[0050] A barium titanate dielectric material layer in-situ coated on the surface of the solid electrolyte by a sol-gel method;

[0051] A silane coupling agent layer coupled to the surface of the barium titanate dielectric material layer.

[0052] The present invention uses a barium titanate dielectric material layer as the first functional layer, and in-situ coats a solid electrolyte by the sol-gel method. There is a dipole moment inside the dielectric material, forming a spontaneous built-in electric field. Coated on the surface of the solid electrolyte, it is polarized under the action of the electric field, promoting the dissociation of the lithium salt in the electrolyte and the polymer and the transport of lithium ions at the solid-liquid interface, creating a high-throughput lithium ion transport path. Then, a silane coupling agent is coupled on the surface of the barium titanate dielectric material layer, and the non-polar group of the alkyl at the other end of the silane coupling agent is used to achieve full contact with the electrolyte and the base film, enhancing the wettability of the solid electrolyte with the electrolyte and the adhesion to the base film. The solid electrolyte modified with the dual-functional layer can effectively reduce powder agglomeration, promote the dissociation of the lithium salt and the transport of lithium ions at the solid-liquid interface.

[0053] In the present invention, the solid electrolyte coated with the dual-functional layer promotes the dissociation of the lithium salt and the transport of lithium ions at the solid-liquid interface due to the piezoelectric effect of the inner layer of barium titanate. The outer layer of silane coupling agent makes the solid electrolyte fully wetted by the electrolyte. In addition, due to the presence of the coupling agent, the amount of binder can be reduced while the bonding strength between the powder and the base film is improved.

[0054] In some embodiments of the present invention, the preparation method of the composite solid electrolyte includes the following steps:

[0055] S1) After mixing the mixture solution A and the mixture solution B evenly, disperse them at high speed, adjust the pH value to 3-4, then continue to disperse at high speed, perform ultrasonic treatment, and then react at 30-60 °C under stirring conditions until a gel is formed;

[0056] The mixture solution A is obtained by dispersing solid electrolyte nanopowder in a first solvent;

[0057] The mixture solution B is obtained by dispersing tetrabutyl titanate and barium acetate powder in a second solvent;

[0058] S2) After drying the gel, a precursor material is obtained;

[0059] S3) The precursor material is calcined to obtain a barium titanate-coated solid electrolyte material;

[0060] S4) The ethanol dispersion of the barium titanate-coated solid electrolyte material is mixed with the hydrolysis solution of the silane coupling agent, modified under heating and stirring conditions, and then dried to obtain the composite solid electrolyte.

[0061] Regarding step S1):

[0062] The mixture solution A is obtained by dispersing solid electrolyte nanopowder in a first solvent.

[0063] The median particle size D50 of the solid electrolyte nanopowder is 500 - 600 nm, such as 562 nm.

[0064] The first solvent is ethanol.

[0065] The dispersion is high-speed dispersion, with a rotation speed of 1000 - 1500 r / min, such as 1200 r / min; the time is 0.5 - 1 h, such as 0.5 h. The dispersion is carried out in a high-speed disperser.

[0066] The solid content of the mixed material liquid A is 25% - 35%, such as 30%.

[0067] The mixed material liquid B is obtained by dispersing tetrabutyl titanate and barium acetate powder in the second solvent.

[0068] In the mixed material liquid B, the mass content of tetrabutyl titanate is 7% - 10%, such as 7.4%, 8.1%, 8.8%; the mass content of barium acetate is 4% - 7%, such as 5.6%, 5.5%, 5.47%.

[0069] The molar ratio of titanium element in the tetrabutyl titanate to barium element in the barium acetate powder is (1 - 1.3):1, such as 1:1.

[0070] The mass ratio of tetrabutyl titanate to the solid electrolyte nanopowder is 1:(2 - 4), such as 1:3.53, 1:3.2, 1:2.9.

[0071] The second solvent is ethanol.

[0072] The dispersion is high-speed dispersion, with a rotation speed of 1000 - 1500 r / min, such as 1200 r / min; the time is 0.5 - 1 h, such as 1 h. The dispersion is carried out in a high-speed disperser.

[0073] After mixing the mixed material liquid A and the mixed material liquid B, perform high-speed dispersion, adjust the pH value to 3 - 4, then continue high-speed dispersion, followed by ultrasonic treatment, and then react at 30 - 60 °C under stirring conditions until a gel is formed.

[0074] The rotation speed of the high-speed dispersion is 500 - 1000 r / min, such as 600 r / min; the time is 25 - 35 min, such as 30 min.

[0075] The reagent used to adjust the pH value is glacial acetic acid. The pH value is adjusted to 3.4.

[0076] The rotation speed of the continued high-speed dispersion is 500 - 1000 r / min, and the time is 25 - 35 min, such as 30 min.

[0077] The time of the ultrasonic treatment is 8 to 12 minutes, for example, 10 minutes. The function of the ultrasonic treatment is to disperse the powder evenly.

[0078] The stirring is carried out in a magnetic stirrer.

[0079] The temperature of the reaction is 45 °C.

[0080] Regarding step S2):

[0081] After drying the gel, a precursor material is obtained.

[0082] The temperature of the drying is 110 to 130 °C, for example, 120 °C. The drying can be carried out in an oven.

[0083] After the drying, it further includes: pulverization. The method of pulverization can be jet milling. The particle size distribution D50 of the pulverized particles (precursor material) is < 10 μm.

[0084] The precursor material is a precursor material of barium titanate-coated solid electrolyte.

[0085] Regarding step S3):

[0086] The precursor material is calcined to obtain a barium titanate-coated solid electrolyte material (barium titanate@LATP).

[0087] The temperature of the calcination is 400 to 700 °C, for example, 500 °C; the time is 5 to 10 hours, for example, 6 hours. The calcination can be carried out in a muffle furnace.

[0088] After the calcination, it further includes: cooling and pulverization. The cooling is: natural cooling to room temperature. The method of pulverization is jet pulverization.

[0089] The particle size distribution D50 of the barium titanate-coated solid electrolyte material is < 700 nm.

[0090] Regarding step S4):

[0091] The hydrolysis solution of the silane coupling agent is prepared by the following method:

[0092] In stirred anhydrous ethanol, glacial acetic acid is first added dropwise until the pH value is 2 to 4, and then the silane coupling agent is added dropwise, and a hydrolysis reaction is carried out at 50 to 90 °C to obtain a hydrolysis solution of the silane coupling agent.

[0093] The rotation speed of the stirring is 1000 to 3000 r / min. The dropwise addition can be carried out drop by drop.

[0094] First, glacial acetic acid is added dropwise until the pH value is 3.

[0095] The mass ratio of the silane coupling agent to absolute ethanol is (0.8 - 1.2):(23 - 27), such as 1:25.

[0096] The temperature of the hydrolysis reaction is 60 °C; the time is 1 - 2 h.

[0097] After obtaining the hydrolysis solution of the silane coupling agent, the ethanol dispersion of the barium titanate-coated solid electrolyte material is mixed with the hydrolysis solution of the silane coupling agent, and modification is carried out under the conditions of heating and stirring, and then drying is performed to obtain a composite solid electrolyte.

[0098] The solid content of the ethanol dispersion of the barium titanate-coated solid electrolyte material is 25% - 35%, such as 30%.

[0099] The mass ratio of the silane coupling agent to the barium titanate-coated solid electrolyte material is 1:(20 - 60), such as 1:27.5, 1:55.

[0100] The temperature of the heating and stirring is 40 - 80 °C, such as 65 °C; the rotation speed is 400 - 800 r / min, such as 500 r / min; the time is 0.5 - 1.5 h, such as 1 h. The heating and stirring are carried out in a magnetic stirrer.

[0101] The temperature of the drying is 150 - 200 °C, and the time is 1 - 4 h. The drying is carried out in a forced-air oven.

[0102] After the drying, it further includes: pulverization. The method of pulverization is jet milling.

[0103] The obtained composite solid electrolyte is a solid electrolyte powder (SCA / barium titanate@LATP) modified with a silane coupling agent and a barium titanate bifunctional layer. The particle size distribution D50 of the composite solid electrolyte is < 700 nm.

[0104] In some embodiments of the present invention, the material of the base film is selected from polyethylene PE, polypropylene PP or aramid. The thickness of the base film is 7 - 12 μm, such as 9 μm. The thickness of the coating layer is 1 - 3 μm, such as 2 μm.

[0105] The present invention also provides a method for preparing the composite separator described above, including the following steps:

[0106] The dispersion of the composite solid electrolyte, the wetting agent and the binder are mixed evenly to obtain a mixed slurry, which is coated on the base film, and after drying, a composite separator is obtained.

[0107] In some embodiments of the present invention, the solvent of the dispersion of the composite solid electrolyte is deionized water; the solid content is 15% - 25%, preferably 20%.

[0108] In some embodiments of the present invention, the wetting agent is selected from at least one of sodium dodecyl sulfate, the condensate of fatty alcohol and ethylene oxide, and alkynediols.

[0109] In some embodiments of the present invention, the binder is selected from at least one of polyacrylate, carboxymethyl acrylate salt, and carboxymethyl cellulose salt.

[0110] In some embodiments of the present invention, the mass ratio of the composite solid electrolyte, the wetting agent, and the binder is (15 - 50):(1 - 3):(2.5 - 5.5), such as 20:1:2.5, 35:2:4.5, 50:3:5.5.

[0111] In some embodiments of the present invention, the drying temperature is 75 - 85 °C, such as 80 °C.

[0112] The present invention also provides a lithium-ion battery, which includes the composite separator described above, or the composite separator prepared by the preparation method described above.

[0113] Specifically, the lithium-ion battery includes a positive electrode, a negative electrode, an electrolyte, and a separator, and the separator is the composite separator described above, or the composite separator prepared by the preparation method described above.

[0114] Compared with the prior art, the present invention has the following beneficial effects:

[0115] For the surface modification of the solid electrolyte powder used for separator coating, the present invention first uses the sol-gel method to in-situ coat a uniform barium titanate dielectric material on the surface of the solid electrolyte powder. The positive and negative charge centers of barium titanate are asymmetric, and there is a spontaneous electric dipole moment, so polarization occurs to generate an internal built-in electric field. By controlling the direction of the external electric field, the polarization direction is enhanced to strengthen the internal built-in electric field, so as to eliminate the space charge existing at the solid-liquid interface. At the same time, in the hot pressing and pressurized cycling stages, under the combined action of pressure and external electric field, the internal built-in electric field strength of the barium titanate coating layer is enhanced, which can promote the transport of lithium ions at the interface.

[0116] Space charge will also be formed at the contact interface between the polymer binder and the solid electrolyte. After coating with barium titanate, the anionic groups and lithium ions in the electrolyte in the binder are adsorbed by the positive and negative charges of the internal built-in electric field, effectively promoting the dissociation of the anions and cations of the electrolyte in the polymer, reducing the adverse effects of the binder as a poor ionic conductor, and reducing the interfacial impedance between the solid electrolyte and the binder.

[0117] In the present invention, a layer of silane coupling agent is coated on the first layer of barium titanate. By using the polar and non-polar groups at both ends of the silane coupling agent, one end is tightly adsorbed on the surface of barium titanate, and the other end is in full contact with the electrolyte according to the principle of "like dissolves like", enabling the electrolyte to wet each solid powder particle. At the same time, the low surface energy makes the coupling agent form an interfacial film on the powder particles in the form of a charged layer or a hydrated layer, reducing powder agglomeration. In addition, it is also found that the non-polar group of the coupling agent has good adhesion to the base film, which can reduce the dosage of the binder.

[0118] The silane coupling agent is used to first fully wet the solid electrolyte powder with the electrolyte, and then barium titanate is used to improve the lithium ion transfer efficiency at the interface. The two fully play a synergistic role, jointly improving the wettability of the solid electrolyte-coated separator to the electrolyte and promoting the dissociation and transfer of lithium ions, improving the cycle and rate performance of the battery cell, and maintaining more electrolyte in the later stage of the cycle.

[0119] The present invention has no special restrictions on the raw material sources used above, and they can be commercially available generally.

[0120] To further illustrate the present invention, the following describes in detail a composite separator, its preparation method and application provided by the present invention in combination with embodiments, but it should not be construed as a limitation to the protection scope of the present invention.

[0121] Example 1

[0122] 1) Preparation of the composite solid electrolyte:

[0123] 1-1) Disperse 600 g of LATP solid electrolyte nanopowder (median particle size D50 = 562 nm) in 1400 g of ethanol solvent, with a solid content of 30%. Use a high-speed disperser to disperse at a speed of 1200 r / min for 0.5 h to obtain a mixed material liquid A;

[0124] Sequentially add 170.16 g of tetrabutyl titanate (the mass ratio of the tetrabutyl titanate to the solid electrolyte nanopowder is 1:3.53) and 127.71 g of barium acetate powder (the molar ratio of titanium element in the tetrabutyl titanate to barium element in the barium acetate powder is 1:1) into 2000 g of ethanol solvent, and use a high-speed disperser to disperse at a speed of 1200 r / min for 1 h to obtain a mixed material liquid B; in the mixed material liquid B, the mass content of tetrabutyl titanate is 7.4%, and the mass content of barium acetate is 5.6%;

[0125] Mix the mixed material liquid A and the mixed material liquid B evenly, and use a high-speed disperser to disperse at a speed of 600 r / min for 30 min. Add glacial acetic acid to adjust the pH value to 3.4 and then continue to disperse at high speed (600 r / min) for 30 min, then ultrasonicate for 10 min, and finally react in a magnetic stirrer at 45 °C until a gel is formed.

[0126] 1-2) Place the gel in an oven at 120 °C for drying. After drying, crush it with a jet mill to obtain the precursor of the barium titanate-coated LATP solid electrolyte, with a particle size distribution D50 < 10 μm.

[0127] 1-3) Calcinate the precursor of the barium titanate-coated LATP solid electrolyte using a muffle furnace. The calcination temperature is 500 °C and the time is 6 h. Then cool it naturally to room temperature and crush it with a jet mill again to obtain the barium titanate-coated LATP solid electrolyte material (barium titanate@LATP). The particle size distribution D50 of the barium titanate-coated solid electrolyte material is < 700 nm.

[0128] 1-4) Take 500 g of absolute ethanol and place it on a magnetic stirrer with a stirring speed of 1000 r / min. First, dropwise add glacial acetic acid until the pH value reaches 3, and then add 20 g of dodecyltrimethoxysilane coupling agent and hydrolyze it at 60 °C for 1 h to obtain the hydrolysis solution of the coupling agent.

[0129] Prepare an ethanol dispersion of barium titanate@LATP with a solid content of 30%, which includes 1283 g of absolute ethanol and 550 g of barium titanate@LATP powder (the mass ratio of the silane coupling agent to barium titanate@LATP is 1:27.5). Add the hydrolysis solution of the coupling agent to the dispersion, and use a magnetic stirrer to heat (65 °C) and stir at a speed of 500 r / min for 1 h for modification. Then dry the dispersion in a forced-air oven at a drying temperature of 200 °C for 4 h. After drying, crush it with a jet mill to obtain the LATP solid electrolyte powder modified with a bifunctional layer of silane coupling agent and barium titanate (SCA / barium titanate@LATP), with a particle size distribution D50 < 700 nm.

[0130] 2) Preparation of the composite separator:

[0131] Take 500 g of SCA / barium titanate@LATP powder and 2000 g of deionized water to prepare a dispersion with a solid content of 20%. Add 25 g of a wetting agent (sodium dodecyl sulfate) and disperse it with a high-speed disperser at 600 r / min for 1 h. Then add a binder (polyacrylate, the solid mass in the binder is 25 g, and it needs to be converted to the total mass of the binder according to a solid content of 40%, that is, 62.5 g), and disperse it with a high-speed disperser at 600 r / min for 1 h to obtain a mixed slurry; the mass ratio of the SCA / barium titanate@LATP powder, the wetting agent, and the binder is 20:1:2.5.

[0132] Coating the mixed slurry onto a base film (PE film, with a thickness of 9 μm) on a machine. After drying at 80 °C, a composite separator is obtained (in the composite separator, the thickness of the coating layer SCA / barium titanate@LATP is 2 μm).

[0133] Example 2

[0134] The difference from Example 1 is as follows:

[0135] In step 2):

[0136] Preparation of the composite separator:

[0137] Take 500 g of SCA / barium titanate@LATP powder and 2000 g of deionized water to prepare a dispersion with a solid content of 20%. Add 28.6 g of a wetting agent (sodium dodecyl sulfate), disperse for 1 h at 600 r / min using a high-speed disperser, then add a binder (polyacrylate, the solid mass in the binder is 25.74 g, and it needs to be converted to the total mass of the binder according to a 40% solid content, that is, 64.35 g), and disperse for 1 h at 600 r / min using a high-speed disperser to obtain a mixed slurry; the mass ratio of the SCA / barium titanate@LATP powder, the wetting agent, and the binder is 35:2:4.5;

[0138] Coat the above-mentioned mixed slurry onto a base film (PE film with a thickness of 9 μm) on a machine, and after drying at 80°C, obtain a composite separator (in the composite separator, the thickness of the coated layer of SCA / barium titanate@LATP is 2 μm).

[0139] Example 3

[0140] The difference from Example 1 is as follows:

[0141] Preparation of the composite separator:

[0142] Take 500 g of SCA / barium titanate@LATP powder and 2000 g of deionized water to prepare a dispersion with a solid content of 20%. Add 30 g of a wetting agent (sodium dodecyl sulfate), disperse for 1 h at 600 r / min using a high-speed disperser, then add a binder (polyacrylate, the solid mass in the binder is 22 g, and it needs to be converted to the total mass of the binder according to a 40% solid content, that is, 55 g), and disperse for 1 h at 600 r / min using a high-speed disperser to obtain a mixed slurry; the mass ratio of the SCA / barium titanate@LATP powder, the wetting agent, and the binder is 50:3: 5.5 ;

[0143] Coat the above-mentioned mixed slurry onto a base film (PE film with a thickness of 9 μm) on a machine, and after drying at 80°C, obtain a composite separator (in the composite separator, the thickness of the coated layer of SCA / barium titanate@LATP is 2 μm).

[0144] Example 4

[0145] The difference from Example 1 is as follows:

[0146] In steps 1-4), replace the dosage of dodecyltrimethoxysilane coupling agent with 10 g; the mass ratio of the silane coupling agent to barium titanate@LATP is 1:55.

[0147] For the remaining steps and parameters, they are the same as those in Example 1, and a composite separator is obtained (in the composite separator, the thickness of the coating layer SCA / barium titanate@LATP is 2 μm).

[0148] Example 5

[0149] The difference from Example 2 is as follows:

[0150] In step 1-1), replace the dosage of tetrabutyl titanate with 187.17 g; the mass ratio of tetrabutyl titanate to the solid electrolyte nanopowder is 1:3.2; the dosage of barium acetate powder is still 127.71 g; the molar ratio of barium element in the barium acetate powder to titanium element in the tetrabutyl titanate is 1:1.1; in the mixed material liquid B, the mass content of tetrabutyl titanate is 8.1%, and the mass content of barium acetate is 5.5%.

[0151] For the remaining steps and parameters, they are the same as those in Example 2, and a composite separator is obtained (in the composite separator, the thickness of the coating layer SCA / barium titanate@LATP is 2 μm).

[0152] Example 6

[0153] The difference from Example 2 is as follows:

[0154] In step 1-1), replace the dosage of tetrabutyl titanate with 204.2 g; the mass ratio of tetrabutyl titanate to the solid electrolyte nanopowder is 1:2.9; the dosage of barium acetate powder is still 127.71 g; the molar ratio of barium element in the barium acetate powder to titanium element in the tetrabutyl titanate is 1:1.2; in the mixed material liquid B, the mass content of tetrabutyl titanate is 8.8%, and the mass content of barium acetate is 5.47%.

[0155] For the remaining steps and parameters, they are the same as those in Example 2, and a composite separator is obtained (in the composite separator, the thickness of the coating layer SCA / barium titanate@LATP is 2 μm).

[0156] Example 7

[0157] The difference from Example 2 is as follows:

[0158] In step 1-4), replace the dodecyltrimethoxysilane coupling agent with a nonafluorohexyltrimethoxysilane coupling agent.

[0159] For the remaining steps and parameters, they are the same as those in Example 1, and a composite separator is obtained (in the composite separator, the thickness of the coating layer SCA / barium titanate@LATP is 2 μm).

[0160] Example 8

[0161] The difference from Example 2 is that:

[0162] In steps 1-4), the dodecyltrimethoxysilane coupling agent is replaced with hexadecyltrimethoxysilane.

[0163] The remaining steps and parameters are the same as those in Example 1, and a composite separator is obtained (in the composite separator, the thickness of the coating layer SCA / barium titanate@LATP is 2 μm).

[0164] Example 9

[0165] The difference from Example 2 is that:

[0166] In steps 1-4), the dodecyltrimethoxysilane coupling agent is replaced with perfluorooctyltrimethoxysilane.

[0167] The remaining steps and parameters are the same as those in Example 1, and a composite separator is obtained (in the composite separator, the thickness of the coating layer SCA / barium titanate@LATP is 2 μm).

[0168] Example 10

[0169] The difference from Example 2 is that:

[0170] In steps 1-4), the dodecyltrimethoxysilane coupling agent is replaced with triethoxyperfluorodecylsilane.

[0171] The remaining steps and parameters are the same as those in Example 1, and a composite separator is obtained (in the composite separator, the thickness of the coating layer SCA / barium titanate@LATP is 2 μm).

[0172] Comparative Example 1

[0173] The difference from Example 4 is that:

[0174] In steps 1-1), the amounts of tetrabutyl titanate and barium acetate powder are both replaced with 0.

[0175] The remaining steps and parameters are the same as those in Example 4, and a composite separator is obtained (in the composite separator, the thickness of the coating layer SCA / barium titanate@LATP is 2 μm).

[0176] Comparative Example 2

[0177] The difference from Example 4 is that:

[0178] In steps 1-1), the amounts of tetrabutyl titanate and barium acetate powder are both replaced with 0.

[0179] In steps 1-4), replace the dosage of dodecyltrimethoxysilane coupling agent with 0.

[0180] For the remaining steps and parameters, they are the same as those in Example 4, and a composite separator is obtained (in the composite separator, the thickness of the coating layer SCA / barium titanate@LATP is 2 μm).

[0181] Comparative Example 3

[0182] The difference from Example 4 is that:

[0183] In steps 1-4), replace the dosage of dodecyltrimethoxysilane coupling agent with 0.

[0184] For the remaining steps and parameters, they are the same as those in Example 4, and a composite separator is obtained (in the composite separator, the thickness of the coating layer SCA / barium titanate@LATP is 2 μm).

[0185] Comparative Example 4

[0186] The difference from Example 4 is that:

[0187] In steps 1-4), replace dodecyltrimethoxysilane coupling agent with methyltrimethoxysilane.

[0188] For the remaining steps and parameters, they are the same as those in Example 4, and a composite separator is obtained (in the composite separator, the thickness of the coating layer SCA / barium titanate@LATP is 2 μm).

[0189] Perform peel strength, EIS AC impedance, contact angle between the composite separator and the electrolyte tests on the composite separators obtained in Examples 1-10 and Comparative Examples 1-4, and assemble them into coin cells for electrochemical performance tests.

[0190] Peel strength test of the composite separator:

[0191] The instrument used is a universal tensile testing machine. Cut the separator into strips with a size of 20 mm×200 mm. Bond the separator to a graphite electrode sheet with a much larger area than the separator sample through double-sided tape, and press it with a pressure of 0.6 MPa to simulate the processes of pressurization and thermal pressing during the assembly of the battery cell; control the temperature at 25 °C. Bond the graphite electrode sheet to a stainless steel plate and fix it on the universal tensile testing machine. Pull open the extended side of the separator sample to complete the peel force test and calculate the peel strength.

[0192] Test of the EIS AC impedance of the composite separator:

[0193] The dried composite separator was cut into separator discs with a diameter of 16 mm using a punching machine, and a stainless-steel ion-blocking electrode symmetric cell was assembled in the glove box filled with argon in the order of the positive electrode housing, stainless-steel gasket, composite separator, stainless-steel gasket, shrapnel, and negative electrode housing group. The electrolyte used was 1 mol / L LiPF 6 solution (the solvent was EC, EMC, and DMC, with a volume ratio of 1:1:1). 40 μL of the electrolyte was dropped on each side of the composite separator and fully infiltrated for 12 h. The whole process was carried out in a dry room with a dew point of -40°C.

[0194] Contact angle test of the composite separator and the electrolyte:

[0195] Dataphysics OCA20 was selected to conduct the contact angle test of the composite separator and the electrolyte. A separator sample of appropriate size was placed on the contact angle test platform, and the electrolyte was dropped using the automatic titration system of the equipment. Test photos were taken, and the contact angle was measured by the protractor method.

[0196] Assembly and testing of the coin cell:

[0197] Using N-methylpyrrolidone (NMP) as the solvent, the ternary cathode material NCM811, conductive carbon black SP, and PVDF with a mass ratio of 97.8:1.2:1 were added and fully mixed under vacuum in a high-speed mixer. The designed theoretical solid content was 72.5%, and it was stirred at a speed of 2000 r / min for 2 h. After full mixing, it was evenly coated on a 17-μm-thick aluminum foil using an automatic coater and cut into cathode discs with a diameter of 12 mm using a punching machine, and then baked at 80°C for 24 h to obtain the cathode sheet. The electrolyte used was 1 mol / L LiPF 6 solution (the solvent was EC, EMC, and DMC, with a volume ratio of 1:1:1). The cathode sheet, the composite separator, lithium sheet (anode sheet), shrapnel, gasket, and positive and negative electrode housings were assembled into a coin cell in the glove box filled with argon in sequence and fully infiltrated for 12 h. The whole homogenization, coating, cutting, and assembly were carried out in a dry room with a dew point of -40°C.

[0198] The prepared lithium-ion battery was placed in a constant-temperature environment at 25°C for 3 h to keep the temperature inside and outside the battery stable. It was charged at a constant current and constant voltage of 1C (1C = 192 mAh / g) to 4.2 V and discharged at a constant current of 1C to 3 V for 500 cycles, and the capacity retention rate was calculated. The capacity retention rate (%) of the nth cycle = (the discharge specific capacity of the nth cycle / the discharge specific capacity of the first cycle) × 100%.

[0199] The data of the peel strength, EIS AC impedance, contact angle test of the composite separator and the electrolyte, and the electrochemical performance test of the assembled coin cell are shown in Table 1.

[0200] Table 1 Data of the peel strength, EIS AC impedance, contact angle test of the composite separator with the electrolyte, and electrochemical performance test of the assembled coin cell

[0201]

[0202] As can be seen from Table 1, by comparing Examples 1-6 with Comparative Examples 1-2, the impedance measured by the stainless steel symmetric coin cell assembled with the LATP-coated separator modified with the bifunctional layer is lower than that of the unmodified LATP-coated separator. The built-in electric field spontaneously existing in the inner layer of barium titanate promotes the dissociation of the lithium salt, generating more mobile lithium ions, while weakening the interfacial resistance between LATP and the binder and the solid-liquid space charge resistance.

[0203] In Examples 1-4, the molar ratio of barium to titanium in the raw materials was 1:1. Due to the volatilization of tetrabutyl titanate, the molar ratio was reduced to 1:1.1 and 1:1.2 in Examples 5 and 6. It can be found that when the molar mass ratio of barium to titanium in the raw materials is 1:1.1, the coin cell has a lower internal resistance and a higher capacity retention rate.

[0204] From Examples 1-3, when the dosage of the binder is further reduced, the peel strength of the separator decreases, and there is a slight powder dropping phenomenon in Example 3. The uneven coating structure makes the lithium ion flux non-uniform, restricting the capacity and the migration of lithium ions, resulting in a low capacity retention rate.

[0205] At the same time, by comparing Examples 1-6 with Comparative Examples 2 and 3, it is found that when there is a silane coupling agent on the surface of the LATP powder, the wetting angle between the separator coating and the electrolyte can be significantly reduced, and the dosage of the coupling agent also affects the size of the wetting angle. A smaller wetting angle means that the contact between the separator coating and the electrolyte in the battery is closer, which is beneficial to the cycle of the battery. When there is no silane coupling agent in the outer layer, the wetting angle increases significantly, and the cell performance of Comparative Examples 2 and 3 deteriorates compared with that of the examples.

[0206] The interaction between the silane coupling agent of Comparative Example 4 and the electrolyte and the separator is weak, and it is difficult to achieve effective contact. The impedance between the powder and the separator increases, restricting the performance of the solid electrolyte coating.

[0207] The capacity retention rate of the coin cells prepared with the separators in Examples 1-10 after 500 cycles at 1C / 1C is above 85%, which is better than that of Comparative Examples 1-4, showing excellent interfacial stability and cycle performance.

[0208] The description of the above embodiments is only used to help understand the method of the present invention and its core idea. Various modifications to these embodiments will be obvious to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to these embodiments shown herein, but rather should be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A composite diaphragm, characterized in that: include: A base film, and a coating layer formed on the base film; The coating layer is prepared from raw material components including a dispersion of a composite solid electrolyte, a wetting agent and a binder; The composite solid electrolyte comprises: Solid electrolytes; A barium titanate dielectric material layer coated on the surface of the solid electrolyte; A silane coupling agent layer is coated on the surface of the barium titanate dielectric material layer.

2. The composite diaphragm according to claim 1, characterized in that: The silane coupling agent includes at least one of nonafluorohexyltrimethoxysilane, trimethoxyhexadecylsilane, trimethoxyperfluorodecylsilane, perfluorooctyltrimethoxysilane, triethoxyperfluorodecylsilane, trimethoxyoctadecylsilane, dodecyltrimethoxysilane, n-propyltrimethoxysilane and trimethoxymethylsilane.

3. The composite diaphragm according to claim 1, characterized in that: The preparation method of the composite solid electrolyte comprises the following steps: S1) mixing the mixed liquid A and the mixed liquid B, dispersing at high speed, adjusting the pH value to 3-4, continuing to disperse at high speed, ultrasonicating, and then reacting at 30-60° C. under stirring until a gel is formed; The mixed liquid A is obtained by dispersing solid electrolyte nanopowder in a first solvent; The mixed liquid B is obtained by dispersing tetrabutyl titanate and barium acetate powder in a second solvent; S2) drying the gel to obtain a precursor material; S3) calcining the precursor material to obtain a barium titanate-coated solid electrolyte material; S4) mixing the ethanol dispersion of the barium titanate coated solid electrolyte material with the hydrolyzate of the silane coupling agent, modifying the mixture under heating and stirring, and then drying the mixture to obtain a composite solid electrolyte.

4. The composite diaphragm according to claim 3, characterized in that: In step S1), the solid electrolyte includes LATP, LLTO or LLZO; the median particle size D50 of the solid electrolyte nanopowder is 500-600 nm.

5. The composite diaphragm according to claim 3, characterized in that: In step S1), in the mixed liquid B, the mass content of tetrabutyl titanate is 7% to 10%, and the mass content of barium acetate is 4% to 7%; The molar ratio of the titanium element in the tetrabutyl titanate to the barium element in the barium acetate powder is (1-1.3):1; The mass ratio of tetrabutyl titanate to solid electrolyte nanopowder is 1:(2-4).

6. The composite diaphragm according to claim 3, characterized in that: In step S3), the calcination temperature is 400-700° C. and the calcination time is 5-10 hours.

7. The composite diaphragm according to claim 3, characterized in that: In step S4), the mass ratio of the silane coupling agent to the barium titanate coated solid electrolyte material is 1:(20-60).

8. The composite diaphragm according to claim 3, characterized in that: In step S4), the temperature of the heating and stirring is 40-80°C.

9. A method for preparing the composite diaphragm according to any one of claims 1 to 8, comprising the following steps: The dispersion liquid of the composite solid electrolyte, a wetting agent and a binder are mixed to obtain a mixed slurry, which is then coated on a base film and dried to obtain a composite diaphragm.

10. The preparation method according to claim 9, characterized in that: The wetting agent includes at least one of sodium lauryl sulfate, a condensate of a fatty alcohol and ethylene oxide, and acetylenic diols; The binder includes at least one of polyacrylate, carboxymethylacrylate and carboxymethylcellulose salt.

11. The preparation method according to claim 9, characterized in that: The mass ratio of the composite solid electrolyte, the wetting agent and the binder is (15-50): (1-3): (2.5-5.5).

12. A lithium ion battery, comprising the composite diaphragm according to any one of claims 1 to 8, or the composite diaphragm prepared by the preparation method according to any one of claims 9 to 11.

Images