A composite separator doped with composite elements, its preparation method and application

Abstract

The present invention discloses a composite separator doped with composite elements, a preparation method and an application thereof. The composite separator is a multi-layer separator obtained by rolling a cellulose film on one or both sides of a polyolefin separator. The multi-layer separator is further filled with metal hydroxide, and finally a nano-layer of metal fluoride is deposited on the surface and pores of the multi-layer separator. Different elements are incorporated into both the inner and outer layers of the composite separator. By means of the cellulose layer, the thermal shrinkage of the polyolefin layer is restricted, and the thermal stability of the separator is improved, so that it has stability in a high-temperature environment. The introduction of metal hydroxide and fluoride enhances the flame retardancy and ion affinity of the composite separator. Fluoride ions and hydroxide ions anchor anions in the electrolyte, improving the battery stability. Moreover, due to the strong electrolyte affinity of the composite separator, it can well desolvate ions, and the ions can uniformly pass through the composite separator, significantly enhancing the high-rate performance of the battery.

Description

Technical Field

[0001] The present invention belongs to the technical field of battery separator modification, and particularly relates to a preparation method and application of a gradient battery separator doped with composite elements. Background Art

[0002] With the continuous growth of the global demand for clean energy, the development of new energy technologies, especially high-performance battery technologies, has become a research hotspot. Among many battery technologies, batteries with metals such as lithium, sodium, and zinc as the negative electrode have attracted much attention due to their high theoretical specific capacity and low reduction potential, and are expected to significantly improve the energy density of the battery. However, the high activity of these metals also brings significant challenges, especially the formation of dendrites on the surface of the metal negative electrode. The continuous growth of these dendrites will not only penetrate the battery separator, causing short circuits and potential safety accidents, but also accelerate the consumption of the electrolyte, reducing the charge-discharge efficiency and lifespan of the battery. Therefore, solving the dendrite problem in metal negative electrode batteries and improving the safety and performance of the battery have become the focus of research.

[0003] Although many current studies focus on the modification of the negative electrode surface, the uniform ion transport at the solid-liquid interface is also crucial for preventing dendrite formation. Current solutions, such as electrolyte additives or artificial solid electrolytes, although effective to a certain extent, their high cost and complex preparation process limit their commercial applications. Therefore, there is a need to find a simple, low-cost, and environmentally friendly modification method.

[0004] In this regard, we have found that the polyolefin separators (such as polyethylene) used in traditional lithium-ion batteries have deficiencies in terms of the affinity between lithium ions and the electrolyte, the uniformity of pore distribution, and mechanical strength, and these problems may all lead to the formation of dendrites and battery short circuits. Therefore, it is not only necessary to modify the battery separator, but also the separator modification method we proposed can effectively solve the challenges in terms of cost and the environment, providing a new way to improve the performance and safety of the battery. Summary of the Invention

[0005] The purpose of the present invention is to provide a composite separator doped with composite elements, its preparation method and application, to solve the problems such as dendrite growth and low Coulomb efficiency existing in the existing lithium, sodium, and zinc metal negative electrodes during the cycling process.

[0006] The modification provided by the present invention consists of a gradient battery separator doped with composite elements. Due to the unique effect of introducing and modifying composite elements inside and outside the modified separator, it can effectively regulate the distribution of lithium ions, promote the uniform deposition of lithium metal, inhibit the growth of lithium dendrites, significantly improve its ionic conductivity and high-rate performance, and also has a significant improvement in flame retardancy and high temperature resistance. This composite separator is a new generation of innovative separator product independently developed by us, and both the equipment and the process have independent intellectual property rights. The product has excellent wettability, liquid retention, high temperature resistance, anti-aging and other properties of nanofiber membranes, as well as good tensile and puncture resistance of polyolefin membranes. This product combines the advantages of lithium battery and supercapacitor separators, and can perfectly adapt to scenarios such as sodium-ion batteries, energy storage lithium batteries, high-rate batteries, and power vehicle batteries, with extremely broad application prospects. In addition, this modified separator has a mature preparation process, can be mass-produced industrially, and has a significant improvement in modifying the electrochemical performance of batteries.

[0007] To achieve the above object, the present invention adopts the following technical solutions:

[0008] A composite separator doped with composite elements, which is formed by sandwiching a cellulose membrane on one or both sides of a polyolefin separator and compounding them through a binder, and obtaining a composite separator by depositing a nano-coating layer of metal fluoride on the surface and pores of the compounded multi-layer separator. Metal hydroxides are attached inside the compounded multi-layer separator. The pore size of the cellulose membrane is larger than that of the polyolefin separator, and the pore size of the polyolefin separator is larger than the pore size of the polyolefin separator after coating with a nano-coating layer of metal halide. The thickness of the composite separator is 10 - 35 μm.

[0009] Furthermore, the thickness ranges of the polyolefin separator and the cellulose membrane are 4 - 20 μm and 4 - 21 μm respectively. The pore size ranges of the polyolefin separator and the cellulose membrane are: 0.05 μm - 0.5 μm, 0.1 μm - 15 μm.

[0010] Furthermore, the metal elements in the metal hydroxides and metal halides are selected from at least one of Fe, Ni, Al, Zn, Sn, Cu, Cr, Ti, Mn, Li.

[0011] The thickness of the nano-coating layer of metal halide is not less than 5 nm and not more than 25 nm.

[0012] The pore size of the polyolefin separator with a nano-coating layer of metal halide in the composite separator is nanoscale or sub-nanoscale.

[0013] The present invention also provides a method for preparing the above-mentioned composite separator doped with composite elements and having a pore size gradient, including the following steps:

[0014] Step 1: Prepare the binder and metal hydroxide solution separately, mix the two and stir evenly to obtain a mixed solution;

[0015] Step 2: Add two types of diaphragms, the polyolefin diaphragm and the cellulose membrane, into the adhesive and metal hydroxide solution, and fully soak them;

[0016] Step 3: Composite the cellulose membrane fully soaked in Step 2 on both sides or one side of the polyolefin diaphragm, then perform rolling pressing, control the thickness of the multi-layer diaphragm to be 10 - 35 μm, and then let the prepared multi-layer diaphragm stand and dry in vacuum;

[0017] Step 4: Use an ALD device to deposit a nano-coated layer of metal fluoride on the surface and inner surface of the pores of the multi-layer diaphragm obtained in Step 3;

[0018] Step 5: Place the composite diaphragm prepared in Step 4 in a vacuum oven for storage and standby.

[0019] In the above preparation method, the atomic layer deposition method in Step 4 includes the following steps:

[0020] Step 1: Place the prepared multi-layer film in an atomic layer deposition device;

[0021] Step 2: Introduce an inert carrier gas into the atomic layer deposition system and evacuate it, adjust the outlet valve of the reaction chamber of the system to make the pressure in the chamber lower than 0.01 atmospheres; and heat to make the deposition temperature 1 - 150 °C;

[0022] Step 3: Inject a metal source into the reaction chamber for 5 seconds and hold for 60 s; introduce an inert carrier gas to wash away the excess metal source and by-products for 30 seconds; inject a halogen source into the reaction chamber to react with the metal source adsorbed on the surface of the substrate and deposit the reaction product on the surface of the substrate to be coated for 60 seconds; then introduce an inert carrier gas to wash away the unreacted halogen source and by-products for 30 seconds;

[0023] Step 4: Repeat Step 3, set the number of cycles to be 50 - 100 times until the thickness of the nano-coated layer of metal halide deposited is not less than 5 nm and not more than 25 nm;

[0024] The inert carrier gas is one of nitrogen, helium or argon;

[0025] The halogen source is one of tungsten hexafluoride, silicon tetrafluoride, boron trifluoride, sulfur hexafluoride, pyridine hydrogen halide;

[0026] The halogen source is one of tungsten hexafluoride, silicon tetrafluoride, boron trifluoride, sulfur hexafluoride, pyridine hydrogen fluoride;

[0027] The metal element in the metal source is selected from at least one of Fe, Ni, Al, Zn, Sn, Cu, Cr, Ti, Mn, and Li.

[0028] Furthermore, the concentration of the metal hydroxide solution in step 1 is 0.1 mg - 1 mg / ml, and the solvent in the metal hydroxide solution is one of isopropanol, absolute ethanol, and N-methylpyrrolidone; the concentration of the binder solution is 1 - 5 mg / ml, the solute of the adhesive is polyvinylidene fluoride, the solvent of the adhesive is one of N-methylpyrrolidone, isopropanol, and absolute ethanol, and the metal element of the metal hydroxide is selected from at least one of Fe, Ni, Al, Zn, Sn, Cu, Cr, Ti, Mn, and Li; the molecular weight of the polyvinylidene fluoride is 250,000 - 500,000.

[0029] Furthermore, the thickness of the polyolefin separator in step 2 is 4 - 20 μm, and the thickness of the cellulose-based membrane is 4 - 21 μm; the infiltration temperature in step 2 is room temperature, and the infiltration time is 4 - 8 h.

[0030] Furthermore, after the prepared composite separator in step 3 is left standing for 12 h, it is placed in a vacuum environment at 60 o °C for 12 h to be completely dried.

[0031] A composite separator with composite element doping and pore size gradient prepared by the present invention can be applied to batteries with negative electrodes including but not limited to lithium metal, sodium metal, and zinc metal.

[0032] A composite separator with composite element doping and pore size gradient prepared by the present invention can be applied to energy storage devices including metal batteries, fuel cells, or supercapacitors containing lithium / sodium / zinc, and the energy storage devices operate at high power and a rate not lower than 3C.

[0033] Advantages and beneficial effects of the present invention:

[0034] The present invention provides a composite separator doped with composite elements. The composite separator is mainly composed of a polyolefin separator with a cellulose film sandwiched on one or both sides, infiltrated with metal hydroxide inside, and coated with a nanolayer of metal fluoride outside. Fluoride ions can largely anchor anions present in the electrolyte, and the lithiophilic element fluorine can reduce the migration energy barrier of lithium ions. Compared with traditional separators, this composite separator has extremely strong electrolyte affinity, and the polyolefin separator coated with metal fluoride has a nanoscale pore structure, which can well desolvate ions. In addition, the combined presence of hydroxide ions in the metal hydroxide and fluoride ions in the metal fluoride can accelerate the ion migration rate and anchor anions present in the electrolyte to maintain the stability and high efficiency of the battery; the metal hydroxide and metal fluoride in the inner and outer layers can greatly improve the mechanical strength and flame retardancy. The composite separator contains abundant lithiophilic elements, which can reduce the migration energy barrier of lithium ions; and with the introduction of polyvinylidene fluoride, the composition of beneficial components of the SEI film is greatly enhanced, enabling the overall liquid surface modification to be presented. Under the synergistic action of various factors, the lithium metal anode can achieve a uniform deposition and stripping process, thus avoiding the growth of lithium dendrites and significantly improving its ionic conductivity and high-rate performance. Finally, the preparation process of this composite separator is simple, the preparation cost is low, and it has little impact on the energy density of the battery. Therefore, the metal anode battery based on this composite separator exhibits good rate performance, extremely high ionic conductivity and ion mobility. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 is a schematic structural diagram of the composite separator in Example 1; 1 is a polyolefin separator doped with metal fluoride, 2 is a cellulose film doped with metal hydroxide, and 3 is a nanoscale metal fluoride coating layer;

[0036] Figure 2 is the SEM test diagram of the polyolefin separator;

[0037] Figure 3 is the SEM test diagram of the cellulose separator;

[0038] Figure 4 is the EIS diagram in Example 21. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0039] In the embodiments of the present application, by providing a composite separator with gradient doping of composite elements, the disadvantages of commercial separators applied to metal anode batteries in the prior art are solved.

[0040] To better understand the above technical solutions, the above technical solutions will be described in detail below in conjunction with the accompanying drawings of the specification and specific implementation manners. It should be understood that the embodiments of the present invention and the specific features in the embodiments are detailed descriptions of the technical solutions of the present application, rather than limitations on the technical solutions of the present application. Without conflict, the technical features in the embodiments of the present application and the embodiments can be combined with each other.

[0041] The following will be described through specific implementation schemes and product test results.

[0042] In the following examples, the polyolefin separator used is from Asahi KASEI Corporation. The thickness of the polyolefin separator is 4 - 20 μm, it has a porous structure, the pore size is 0.05 μm - 0.5 μm, and the connectivity between pores is good. In addition, the porosity of the polyolefin separator is 40%.

[0043] In the following examples, the cellulose membrane (CN201911416124.7) used is self-developed by Ningbo Rouchuang Nano Technology Co., Ltd. Cellulose-based natural fibers and high-temperature resistant and electrochemically stable materials such as aramid, polyimide fiber, poly(p-phenylene benzobisoxazole) (PBO) fiber, and polyacrylonitrile are selected. The fibers are radially peeled through processing methods such as chemical enzyme catalysis, high-pressure homogenization, and mechanical grinding to achieve nanometerization of the fibers in fineness, while maintaining good aspect ratio, thickness, pore characteristics, porosity, and other parameters. Then, the above-mentioned various fibers are dispersed in water and formed into a wet membrane by papermaking. Then, a crosslinking agent is introduced onto the wet membrane composed of porous nanofibers to in-situ polymerize the crosslinking agent, further enhancing the strength at the fiber nodes of the base membrane, and thus enhancing the mechanical properties of the separator. At the same time, solvent evaporation induces self-assembly of the crosslinking agent to generate a three-dimensional network-like uniform pore structure inside the separator, further enhancing the porosity and pore uniformity of the separator. Finally, the lithium ion conductivity is improved by coating or impregnating the cellulose membrane with a polymer solid electrolyte and an inorganic ceramic solid electrolyte. The thickness of the cellulose membrane is 4 - 21 μm, the porosity is 40% - 75%, and the pore size is 0.1 - 15 μm.

[0044] One or more technical solutions provided in the embodiments of the present application have at least the following technical effects or advantages:

[0045] Example 1

[0046] Prepare a composite separator doped with composite elements.

[0047] In the order of cellulose membrane, polyolefin separator, and cellulose membrane, the three layers of membranes are simultaneously immersed in a mixed solution of aluminum hydroxide and PVDF. The concentration of the aluminum hydroxide solution is 1 mg / ml, and the concentration of the PVDF solution is 2 mg / ml. After soaking for 8 h, the cellulose membranes are sandwiched on both sides of the polyolefin separator (such as Figure 1As shown in the figure), then roll pressing is carried out until the thickness of the composite diaphragm is within 35 μm. Subsequently, the roll-pressed diaphragm is left to stand undisturbed for 12 h and then placed in an oven at 60 °C for standby. After 24 h, the composite diaphragm is taken out, and a 5-nm-thick ALF3 layer is uniformly deposited on its surface and the inner surface of the pores by using an ALD device.

[0048] The preparation steps are as follows: on the surface and the inner surface of the pores of the pretreated composite membrane, aluminum fluoride is coated by using atomic layer deposition technology, the reaction temperature is 1-150 °C, and the number of coating cycles per layer is 1 cycle (the total thickness of the aluminum fluoride coated for 50-100 cycles is about 5-10 nm).

[0049] The atomic layer deposition technology specifically includes the following steps:

[0050] Step 1, place the prepared composite membrane in an atomic layer deposition device;

[0051] Step 2, introduce an inert carrier gas into the atomic layer deposition system and evacuate it, adjust the outlet valve of the reaction chamber of the system so that the pressure in the chamber is lower than 0.01 atmospheres; and heat to make the deposition temperature 1-150 °C; this step belongs to low-temperature coating.

[0052] Step 3, inject trimethylaluminum into the reaction chamber for 5 s and keep it for 60 s; introduce an inert carrier gas to flush out the excessive trimethylaluminum and by-products for 30 s; inject pyridine hydrogen fluoride into the reaction chamber to react with the trimethylaluminum adsorbed on the surface of the substrate, and deposit the reaction product ALF3 on the surface of the substrate to be coated for 60 s; then introduce an inert carrier gas to flush out the unreacted pyridine hydrogen fluoride and by-products for 30 s;

[0053] Step 4, repeatedly execute Step 3, set the number of cycles to be repeated to 50-100 times until the thickness of the aluminum fluoride deposited on the inner surface of the pores on the surface of the composite membrane reaches 5-10 nm.

[0054] As Figure 2 and Figure 3 shown, the SEM diagram results of the polyolefin diaphragm and the cellulose membrane used in this embodiment are distributed. It can be seen from the figure that the pore diameters of the two are between 0.05-0.5 μm and 0.1-15 μm respectively.

[0055] After that, the modified composite diaphragms are respectively combined with different battery electrodes and assembled into half-cells and full-cells for electrochemical performance testing.

[0056] Example 2-Example 20

[0057] In Examples 2 to 5, the method steps for preparing the composite diaphragm are basically the same as those in Example 1. The difference is that in atomic layer deposition, the types of metal fluorides deposited are different.

[0058] In Example 6, the method steps for preparing the composite separator are basically the same as those in Example 1. The difference is that the cellulose membrane and the polyolefin separator are simultaneously immersed in a mixed solution of aluminum hydroxide and PVDF, and the cellulose membrane is laminated on one side of the polyolefin separator. Subsequently, rolling is carried out to form a composite separator until the thickness of the composite separator is within 35 μm. Subsequently, an atomic layer deposition nano-layer coating is carried out on the surface of the composite separator.

[0059] In Example 7, in the order of cellulose membrane, polyolefin separator, and cellulose membrane, the three layers of membranes are simultaneously immersed in a PVDF solution with a concentration of 2 mg / ml. After soaking for 8 h, the cellulose membranes are sandwiched on both sides of the polyolefin separator, and then rolling is carried out until the thickness of the composite separator is within 35 μm. In this example, no metal hydroxide is added in the soaking step, and no nano-coated layer of metal fluoride is deposited subsequently.

[0060] In Examples 8 to 12, the method steps for preparing the composite separator are basically the same as those in Example 1. The difference is that in the order of cellulose membrane, polyolefin separator, and cellulose membrane, the three layers of membranes are simultaneously immersed in a mixed solution of metal hydroxide and PVDF. The types of metal hydroxides are different in this soaking step, which are magnesium hydroxide, copper hydroxide, zinc hydroxide, nickel hydroxide, and chromium hydroxide in sequence. In atomic layer deposition, the deposited metal fluorides are all zinc fluoride.

[0061] In Examples 13 to 15, the basic operations of the method steps for preparing the composite separator are the same as those in Example 1. The difference is that only in the order of cellulose membrane, polyolefin separator, and cellulose membrane, the three layers of membranes are simultaneously immersed in different mixed solutions of metal hydroxide and PVDF. In this step, the metal hydroxides are nickel hydroxide, magnesium hydroxide, and zinc hydroxide respectively. After forming a membrane by laminating in the order of cellulose membrane, polyolefin separator, and cellulose membrane, no nano-coated layer of metal fluoride is deposited subsequently.

[0062] In Examples 16 to 18, the basic operations of the method steps for preparing the composite separator are the same as those in Example 1. The difference is that in the soaking step, only PVDF solution is used, and no metal hydroxide solution is used; in the atomic layer deposition step, the finally formed nano-layer coatings are zinc fluoride, magnesium fluoride, and nickel fluoride respectively.

[0063] In Example 19, the method steps for preparing the composite separator are basically the same as those in Example 1. The difference is that only in the order of cellulose membrane, polyolefin separator, and cellulose membrane, the three layers of membranes are simultaneously immersed in a mixed solution of metal hydroxide and PVDF. In this step, the metal hydroxide is aluminum hydroxide. After forming a membrane by laminating in the order of cellulose membrane, polyolefin separator, and cellulose membrane, the whole separator is then immersed in an aluminum fluoride solution subsequently.

[0064] Examples 20 and 21 are a single-layer polyolefin separator and a single-layer cellulose membrane respectively, which are not immersed in the mixed solution of metal hydroxide and PVDF, nor are they deposited with a nano-coating layer of metal fluoride.

[0065] For Examples 1 to 21, the coatings used in the infiltration step and the coatings deposited in the atomic layer deposition step are shown in Table 1 below respectively.

[0066] It should be noted that this patent only lists the doping of some metal fluorides and metal hydroxides, and one or more metal fluorides and metal hydroxides fall within the protection scope of this patent.

[0067] Table 1

[0068]

[0069] Example 22

[0070] Taking the composite separator prepared in Example 1, the cellulose membrane of Example 20, and the polyolefin separator of Example 21 as examples, a half-cell composed of a lithium metal sheet and a copper foil was used, 1M LiTFSI DOL / DME (volume ratio 1:1) was used as the electrolyte, and three separators were used to assemble batteries respectively in an argon atmosphere glove box, and interfacial impedance tests were carried out on an electrochemical workstation.

[0071] Figure 4 These are the relevant test results of the battery in Example 22. It can be seen that the composite separator prepared in Example 1 exhibits a smaller interfacial impedance.

[0072] Example 23

[0073] In this example, the lithium ion mobility reflected by lithium symmetric batteries based on different modified separators (Examples 1 to 19) and commercial separators (Examples 20 to 21), and the test results of the ionic conductivity reflected by steel-to-steel batteries were tested. The above test results are shown in Table 2.

[0074] Table 2

[0075]

[0076] The assembly method of the symmetric battery is to use lithium sheets (steel sheets) as both the positive and negative electrodes of the battery at the same time, assemble with different separators, and carry out corresponding tests using a Metrohm electrochemical workstation. The ion transport number is used to evaluate the ion migration ability. A low ion transport number will not only reduce the ionic conductivity of the electrolyte, but also cause serious concentration polarization phenomena, resulting in uneven deposition of lithium ions and affecting the performance of the battery. In addition, whether the separator can be fully in contact with the electrolyte also reflects the affinity of the separator for the electrolyte, and whether it will deform at high temperatures reflects the safety and stability of the battery.

[0077] Example 24

[0078] In this example, the cycling rate performance of NCM811 batteries based on different separators was tested. Using lithium metal as the negative electrode, NCM811 as the positive electrode, and 1M LiPF6 EC / DEC (volume ratio 1:1) as the electrolyte, different separators were used (the modified separators were Examples 1-19, and the commercial separators were Examples 20-21), and NCM811 batteries were assembled separately in an argon atmosphere glove box. The preparation method of the positive electrode sheet was as follows: NCM, SuperP, and polyvinylidene fluoride (PVDF) binder were mixed in a mass ratio of 8:1:1, ground evenly, dispersed in polyvinylpyrrolidone (NMP), and then homogenized in a homogenizer for 20 min to obtain a uniform slurry. The slurry was scraped onto aluminum foil using a coater and dried in a vacuum drying oven at 80 °C for 12 h.

[0079] Table 3 is the cycling performance graph of the Li-NCM811 battery in this example at different rates. It can be seen from Table 3 that the battery prepared using the composite separator has significantly higher rate performance than the battery using the commercial separator. The voltage of this positive electrode full battery is 3V-4.4V, and the test temperature is 25 °C.

[0080] The above data shows that in terms of effect: the cycling specific capacity at high rates of the three-layer composite separators treated by infiltration and atomic layer deposition of metal fluorides in Examples 1-5 and Examples 8-12 > the two-layer composite separator treated by infiltration and atomic layer deposition of metal fluorides in Example 6 > the three-layer composite separators treated by infiltration in Examples 13-15, the three-layer composite separators treated by infiltration and impregnation in Example 19, and the three-layer composite separators treated by atomic layer deposition of metal fluorides in Examples 16-18 > the three-layer composite separator in Example 7 > the single-layer separators in Examples 20-21.

[0081] It can be seen from Tables 1-3 that groups such as -OH on the surface of metal hydroxides have strong hydrophilicity, thus improving the wettability of the separator for the electrolyte. The introduction of metal fluorides plays a role in anchoring anions, which can make the positively charged Li + shuttle more easily therein, thereby improving the mobility of lithium ions.

[0082] The pores of the composite separator have good affinity with the electrolyte. The metal oxides in the pores of the composite separator have affinity for lithium ions in the lithium electrolyte, enabling the lithium ions to pass through the pores of the composite separator uniformly, so that the lithium electrolyte in each pore of the composite separator is evenly distributed, avoiding the problem that lithium ions only pass through the separator from a local area of the composite separator, resulting in a decrease in the rate of ion migration under high current. The coating of metal fluoride reduces the pore size of the polyolefin separator to the nanoscale. Anions such as fluoride ions in the pores of the composite separator can inhibit the anion shuttle of the electrolyte through the separator, enabling the pores of the composite separator to shuttle more lithium ions. The nanoscale pore size of the polyolefin separator is smaller than the molecular size of the solvent and larger than the radius of lithium ions. The polyolefin separator with a nanoscale pore size can remove the solvent molecules around the lithium ions, enabling the lithium ions to migrate freely in the pore structure of the polyolefin separator, thus greatly accelerating the rate of ion migration. This pore structure has an ideal effect in the application of high-power devices.

[0083] Table 3

[0084]

[0085] The metal fluoride in the above embodiments can also be replaced by metal halide, and the halogen ions can also anchor anions. The number of cycles of the metal fluoride coated by ALD in the above embodiments can be increased to 100 - 240 cycles, which can further reduce the pore size of the polyolefin separator after coating with metal fluoride. When the number of cycles of the metal fluoride coated by ALD in the above embodiments can be increased to 240 - 250 cycles, the polyolefin separator after coating with metal fluoride has a sub-nanoscale pore size, which can also achieve the effect of removing the solvent molecules around the lithium ions. This pore structure has an ideal effect in the application of high-power devices.

[0086] Finally, it should be noted that the above specific embodiments are only used to illustrate the technical solutions of the present invention and are not restrictive. Although the present invention has been described in detail with reference to the examples, those of ordinary skill in the art should understand that the technical solutions of the present invention can be modified or equivalently replaced without departing from the spirit and scope of the technical solutions of the present invention, and they should all be covered within the scope of the claims of the present invention.

Claims

1. A preparation method of a composite battery separator doped with composite elements, characterized in that, It includes the following preparation steps: Step 1: Prepare a binder solution and a metal hydroxide solution separately, mix the two and stir evenly to obtain a mixed solution. Step 2: Add two types of diaphragms, a polyolefin diaphragm and a cellulose membrane, into the binder and metal hydroxide solution, and fully immerse them. Step 3: Composite the cellulose membrane fully immersed in Step 2 on both sides or one side of the polyolefin diaphragm, then roll press it, control the thickness of the multi-layer diaphragm to be 10 - 35 μm, and then let the prepared multi-layer diaphragm stand and dry in vacuum. Step 4: Use an ALD device to deposit a nano-coated layer of metal halide on the surface and inner surface of the pores of the multi-layer diaphragm obtained in Step 3. Step 5: Place the composite diaphragm prepared in Step 4 in a vacuum oven for storage and standby.

2. The preparation method according to claim 1, wherein The specific steps of Step 4 are as follows: Step 1: Place the prepared multi-layer film in an atomic layer deposition device. Step 2: Introduce an inert carrier gas into the atomic layer deposition system and evacuate it, adjust the outlet valve of the reaction chamber of the system to make the pressure in the chamber lower than 0.01 atmospheres; and heat to make the deposition temperature 1 - 150 °C. Step 3: Inject a metal source into the reaction chamber for 5 seconds and hold for 60 s. Introduce an inert carrier gas to flush out the excessive metal source and by-products for 30 seconds; inject a halogen source into the reaction chamber to react with the metal source adsorbed on the surface of the substrate and deposit the reaction product on the surface of the substrate to be coated for 60 seconds; then introduce an inert carrier gas to flush out the unreacted halogen source and by-products for 30 seconds. Step 4: Repeat Step 3 for 50 - 100 times until the thickness of the nano-coated layer of metal halide deposited is not less than 5 nm and not more than 25 nm; the inert carrier gas is one of nitrogen, helium or argon.

3. The preparation method according to claim 2, characterized in that, The halogen source is one of tungsten hexahalide, silicon tetrahalide, boron trihalide, sulfur hexahalide, pyridine hydrogen halide. The metal element in the metal source is selected from at least one of Fe, Ni, Al, Zn, Sn, Cu, Cr, Ti, Mn, Li.

4. The preparation method according to claim 1, characterized in that, In Step 1, the concentration of the metal hydroxide solution is 0.1 mg - 1 mg / ml, and the solvent in the metal hydroxide solution is one of isopropyl alcohol, absolute ethanol, N-methylpyrrolidone; the concentration of the binder solution is 1 - 5 mg / ml, the solute of the binder is polyvinylidene fluoride, and the solvent of the binder is one of N-methylpyrrolidone, isopropyl alcohol, absolute ethanol. The metal element of the metal hydroxide is selected from at least one of Fe, Ni, Al, Zn, Sn, Cu, Cr, Ti, Mn, Li.

5. The preparation method according to claim 1, characterized in that, In Step 2, the thickness of the polyolefin diaphragm is 4 - 10 μm, and the thickness of the cellulose-based membrane is 4 - 21 μm; the immersion temperature in Step 2 is room temperature, and the immersion time is 4 - 8 h.

6. The preparation method according to claim 1, wherein In Step 3, after letting the prepared composite diaphragm stand for 12 h, place it in a vacuum environment at 60 °C for 12 h to be completely dried.

7. A composite battery separator doped with composite elements, characterized in that, Prepared by the method according to any one of claims 1 to 6, the composite separator is a composite separator obtained by sandwiching a cellulose membrane on one or both sides of a polyolefin separator and compounding through an adhesive, and depositing a nano-coated layer of metal halide on the surface and pores of the compounded multi-layer separator. Metal hydroxides are attached inside the cellulose membrane and the polyolefin separator. The pore size of the cellulose membrane is larger than that of the polyolefin separator, and the pore size of the polyolefin separator is larger than the pore size of the polyolefin separator after being coated with the nano-coated layer of metal halide. The thickness of the composite separator is 10 to 35 μm; The thickness ranges of the polyolefin separator and the cellulose membrane are 4 to 20 μm and 4 to 21 μm respectively, and the pore sizes of the polyolefin separator and the cellulose membrane are 0.05 μm - 0.5 μm and 0.1 μm - 15 μm respectively.

8. The composite separator according to claim 7, wherein The metal elements in the metal hydroxide and the metal halide are selected from at least one of Fe, Ni, Al, Zn, Sn, Cu, Cr, Ti, Mn, and Li.

9. An energy storage device for a lithium metal battery using the composite separator according to any one of claims 7 to 8.

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