Functional coating modified diaphragm as well as preparation method and application thereof

Abstract

The invention provides a functional coating modified diaphragm as well as a preparation method and application thereof, and belongs to the technical field of lithium batteries. The Zn-doped transition metal selenide is used for modifying the diaphragm, CoSe2 can effectively capture lithium polysulfide and accelerate the oxidation-reduction reaction of sulfur, the CoSe2 is used as a host material to inhibit shuttling of lithium polysulfide and improve the conversion kinetics of lithium polysulfide, and good catalytic ability is shown; after Zn is doped, the concentration of cations is changed in the hydrothermal process, so that a structure which is more beneficial to catalytic reaction can be generated; and the two are combined to play a catalysis-adsorption synergistic effect, so that higher ionic conductivity and better adsorption performance are obtained, and the electrochemical performance of the lithium-sulfur battery is improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium batteries, and particularly relates to a separator modified with a functional coating, a preparation method thereof, and an application thereof. Background Art

[0002] With the wide application of portable electronic devices and electric vehicles, the requirements for energy storage devices are also getting higher and higher, and the development of new sustainable energy storage / conversion devices has become particularly important. In addition to considering the battery energy density, safety has also become an important factor restricting the development of the battery industry.

[0003] As an important component of lithium batteries, the separator serves to separate the positive and negative electrodes of the battery to prevent direct contact between the two poles, which may cause a short circuit. At the same time, the separator also needs to allow lithium ions to pass through smoothly. Conventional basic separators are made of polyethylene (PE) or polypropylene (PP). To improve the performance of lithium batteries, an important research direction in the prior art is to develop functional separators, that is, to add a functional coating to the separator to inhibit the growth of lithium dendrites, thereby improving the safety performance of the battery.

[0004] Among the selection of modification materials, carbon materials such as graphene oxide, carbon nanotubes, and porous carbon have been first applied to functional separators due to their large surface area and excellent conductivity. [1,2] However, non-polar carbon materials with weak physical adsorption of polar polysulfide lithium cannot meet the requirements of long-term cycle stability. Therefore, there is an urgent need to propose a new functional coating separator that can better adsorb polysulfide lithium and inhibit the growth of lithium dendrites, thereby improving the safety performance of lithium batteries.

[0005] [1]Yuan C, Yang X, Zeng P, Mao J, Dai K, Zhang L, Sun X. Recent progress of functional separators with catalytic effects for high-performance lithium-sulfur batteries.

[0006] [2] Weihua Jin, Yunpeng Guo, Taorong Gan, Zhengyuan Shen, Xuebing Zhu, Peng Zhang, Yong Zhao, Cooperation of Multifunctional Redox Mediator and Separator Modification to Enhance Li-S Batteries Performance under Low Electrolyte / Sulfur Ratio, Angewandte Chemie International Edition. Summary of the Invention

[0007] Aiming at the problems existing in the background technology, the purpose of the present invention is to provide a separator modified with a functional coating, its preparation method and application. The present invention uses bimetallic selenide to modify the separator. The "catalytic-adsorption" synergistic effect of the bimetal can obtain higher ionic conductivity and better adsorption performance. By chemically adsorbing and catalytically converting polysulfides, dendritic growth is inhibited, and the cycle reversibility of lithium batteries is improved, thereby enhancing the electrochemical performance of lithium batteries.

[0008] To achieve the above purpose, the technical solution of the present invention is as follows:

[0009] A separator modified with a functional coating, comprising a base separator and a bimetallic selenide coating coated on at least one surface of the base separator, and the bimetallic selenide coating is a Zn-doped CoSe2 coating.

[0010] Further, the base separator is a polyethylene (PE) separator, a polypropylene (PP) separator, etc.

[0011] Further, the thickness of the bimetallic selenide coating is 1-300 μm, and the doping amount of Zn is 1%-30% of the Co element content.

[0012] A preparation method of a separator modified with a functional coating, comprising the following steps:

[0013] Step 1: Weigh cobalt source and zinc source according to the molar ratio of element Co:Zn = x:y, place the cobalt source, zinc source, urea and ammonium fluoride in pure water, and then stir to fully dissolve each component to obtain solution A; where 0 < x ≤ 10, 0 < y ≤ 10;

[0014] Step 2: Place solution A in a hydrothermal autoclave for hydrothermal reaction, the heating temperature is 80-180 °C, and the heating time is 8-18 h; after the reaction, naturally cool to room temperature;

[0015] Step 3: Separate the supernatant from the precipitate in the solution obtained in Step 2, and then wash the precipitate multiple times to obtain a double-metal hydrate;

[0016] Step 4: Grind the double-metal hydrate obtained in Step 3, then mix it with Se powder in a certain proportion and place it in a tube furnace, and sinter it in an inert atmosphere at a sintering temperature of 300-700 °C for 2-6 hours; after the sintering reaction ends, obtain the product;

[0017] Step 5: Grind, wash, and dry the product obtained in Step 4 to obtain double-metal selenide powder;

[0018] Step 6: Mix the double-metal selenide powder obtained in Step 5 with a conductive additive, a binder, and a solvent in a certain proportion to form a slurry, coat it on a basic separator, and dry it to obtain the required separator.

[0019] Further, in Step 1, the cobalt source is cobalt nitrate hexahydrate, cobalt tetroxide, cobalt chloride hexahydrate, etc., and the zinc source is zinc nitrate standard solution, nano-zinc oxide, zinc sulfate, etc.

[0020] Further, in Step 4, the molar ratio of the double-metal hydrate to Se powder is (1-5):1.

[0021] Further, in Step 4, the inert atmosphere is an argon or nitrogen atmosphere, and the gas flow rate is 60-180 sccm.

[0022] Further, in Step 6, the conductive additive is any one of acetylene black, Ketjen black, and carbon nanotubes; the binder is any one of polyvinylidene fluoride, polyvinylidene difluoride, polyethylene oxide, and polyacrylic acid, and the solvent is dimethylformamide (DMF) or N-methylpyrrolidone (NMP), etc.; the mass ratio of the double-metal selenide precursor powder to the conductive additive, the binder, and the solvent is x:y:z, where 0≤x≤1, 0≤y≤1, 0≤z≤1.

[0023] The present invention also provides a lithium battery, which includes the separator modified with the above functional coating.

[0024] In summary, due to the adoption of the above technical solutions, the beneficial effects of the present invention are:

[0025] 1. The present invention uses Zn-doped transition metal selenide to modify the separator. ZnCoSe is combined with a polymer substrate (PP) to form a flexible modification layer. On the one hand, it can enhance the mechanical properties of the separator. The dense structure of the modification layer can effectively block the shuttle of polysulfides, reduce its corrosion of the lithium negative electrode, effectively resist the piercing of lithium dendrites, and avoid the risk of short circuit. On the other hand, after Zn doping, the change in cation concentration during the hydrothermal process can result in the formation of a structure that is more conducive to catalytic reactions. In short, the application of ZnCoSe material in the modification of lithium battery separators effectively inhibits the growth of lithium dendrites and improves the cycle stability and safety of the lithium battery negative electrode through the synergistic effects of multiple mechanisms such as homogenizing the lithium ion flux, inducing uniform lithium deposition, inhibiting side reactions of the electrolyte, and enhancing mechanical strength.

[0026] 2. The present invention uses the hydrothermal method to synthesize a hydroxide precursor at low temperature, and then combines it with selenization to prepare a bimetallic selenide. The overall process is simple and has low energy consumption. The synthesized product has a larger specific surface area, exposing more active sites, which is beneficial to the adsorption and conversion of polysulfide lithium and improves the utilization rate of active substances, effectively enhancing the electrocatalytic performance of the material. Brief Description of the Drawings

[0027] Figure 1 SEM images of the bimetallic precursor hydrate ZnCo(OH) x and the bimetallic selenide in Example 1.

[0028] Figure 2 XRD pattern of the bimetallic selenide in Example 1.

[0029] Figure 3 SEM and corresponding elemental distribution mapping of the bimetallic selenide in Example 1.

[0030] Figure 4 Photos and folding test diagrams of the separator modified with the bimetallic selenide in Example 1.

[0031] Figure 5 Cycling comparison data diagrams of the separator modified with the bimetallic selenide and the unmodified separator in Example 1.

[0032] Figure 6 SEM of the bimetallic precursor hydrate ZnCo(OH) x in Comparative Example 1.

[0033] Figure 7 SEM of the monometallic precursor hydrate Co(OH)x and the final product CoSe2 in Comparative Example 2.

[0034] Figure 8 Electrochemical performance diagrams of the bimetallic selenide separator obtained in Example 1 and the ZnCo2O4 separator obtained in Comparative Example 3;

[0035] (a) is the rate performance diagram; (b) is the impedance data.

[0036] Figure 9 The bimetallic selenide and bimetallic precursor hydrate ZnCo(OH) obtained in Example 1 x , battery impedance diagram of ZnCo2O4 and basic diaphragm PP obtained in comparative example 3. DETAILED DESCRIPTION

[0037] In order to make the objectives, technical solutions and advantages of the present invention more clear, the present invention is further described in detail below in conjunction with the implementation modes and the accompanying drawings.

[0038] A functional coating modified diaphragm comprises a basic diaphragm and a bimetallic selenide coating coated on at least one side surface of the basic diaphragm, wherein the bimetallic selenide coating is a Zn-doped CoSe coating.

[0039] Example 1

[0040] A method for preparing a functional coating modified diaphragm comprises the following steps:

[0041] Step 1: Cobalt nitrate hexahydrate and zinc nitrate were weighed according to the molar ratio of element Co:Zn=10:1, 2.328g of cobalt nitrate hexahydrate, 8ml of zinc nitrate, 1.2g of urea and 0.148g of ammonium fluoride were added to 100ml of deionized water, and then stirred to fully dissolve the components to obtain solution A;

[0042] Step 2: Place solution A in a hydrothermal reactor for hydrothermal reaction at a heating temperature of 120°C for 12 hours; after the reaction is completed, cool naturally to room temperature;

[0043] Step 3: Separate the supernatant and the precipitate in the solution obtained in step 2, and then wash the precipitate multiple times with pure water and ethanol to obtain a bimetallic precursor hydrate ZnCo(OH)x;

[0044] Step 4: Grind the bimetallic hydrate ZnCo(OH)x obtained in step 3, mix it with Se powder in a molar ratio of 1:3, place it in a tube furnace, and sinter it in a N2 atmosphere with a gas flow rate of 120sccm, a sintering temperature of 500°C, and a sintering time of 3 hours; after the sintering reaction is completed, a product is obtained;

[0045] Step 5: Grind the product obtained in step 4, wash it with hot water and ethanol for multiple times, and dry it at low temperature to obtain bimetallic selenide powder;

[0046] Step 6: Mix the bimetallic selenide powder obtained in Step 5 with conductive carbon black, binder, solvent, etc. in a ratio of 5:4:1 to form a slurry, coat it on the PP separator, and after drying, the required separator can be obtained. The physical picture of the separator is as shown in Figure 4 shown.

[0047] Comparative Example 1

[0048] Prepare the separator modified with the functional coating according to the steps of Example 1, only adjust the molar ratio of Co and Zn elements in Step 1 to 1:1, and keep the rest of the steps unchanged.

[0049] Comparative Example 2

[0050] Prepare the separator modified with the functional coating according to the steps of Example 1, only remove the zinc source in Step 1, and keep the rest of the steps unchanged. The obtained product is CoSe2.

[0051] Comparative Example 3

[0052] Prepare the separator modified with the functional coating according to the steps of Example 1, only adjust the molar ratio of the bimetallic hydrate ZnCo(OH) x to Se powder to 1:0, and keep the rest of the steps unchanged. The obtained product is ZnCo2O4.

[0053] Figure 1 are the SEM images of the bimetallic precursor hydrate ZnCo(OH)x and the finally obtained bimetallic selenide in Example 1. Among them, (a) is the SEM image of the bimetallic precursor hydrate ZnCo(OH) x , and (b) is the SEM image of the bimetallic selenide. The morphology of the ZnCo(OH) x precursor is doped with nanocrystalline whiskers with uniform particle size and regular shape and layered flakes. The Zn-Co-Se powder obtained after selenization retains the whisker structure of the precursor, but the whiskers become thicker as shown in Figure 1 (b). It can be seen from the figure that the nanoscale flower-shaped whiskers become thicker after selenization, the gaps between the whiskers decrease, and the corresponding specific surface area also decreases.

[0054] Figure 2XRD pattern of the bimetallic selenide in Example 1. The seleniumized product was compared with the standard comparison card #88-1712, and it had obvious characteristic peaks of CoSe2. The diffraction peaks at diffraction angles of 30.62, 34.30, 37.68, 43.76, 51.81, 54.29, 56.69, 59.03, and 63.58° corresponded to the (200), (210), (211), (220), (311), (222), (230), (321), and (400), (421), (322) crystal planes of cubic CoSe2, respectively, confirming the successful synthesis of Zn-Co-Se; and the diffraction peaks shown in the XRD pattern were relatively sharp, indicating that the synthesized bimetallic selenide had good crystallinity. No additional diffraction peaks were detected by XRD. Combining with Figure 3 the elemental distribution map, it was shown that Zn element was successfully doped into the CoSe2 lattice and no impurity phase was generated.

[0055] Figure 3 SEM and corresponding elemental distribution mapping of the bimetallic selenide in Example 1. It can be seen from the figure that Co, Se, and Zn elements were evenly distributed in Zn-Co-Se. Combining with Figure 2 it was known that there was no peak of Zn in the XRD of the bimetallic selenide, but it could be clearly seen in the elemental distribution mapping that Zn element was evenly distributed in the sample, indicating that Zn existed in the CoSe2 crystal in a doped form.

[0056] Figure 4 Photos of the separator modified with the bimetallic selenide and the folding test diagram in Example 1. After the ZnCoSe material was modified on the surface of the PP separator by the coating method, it was dried and cut into small round pieces with a diameter of 19 mm, as shown in Figure 4 (a). The tested thickness was only 0.004 mm, which had little impact on the volume specific capacity of the battery. Through the folding test in Figure 4 (b), it could be seen that the material still maintained good integrity after repeated bending, and there was no case of peeling or powdering, indicating that the ZnCoSe material could adhere well to the PP separator after coating, so the material was not prone to peeling of the separator modification layer.

[0057] The electrochemical performance of the separator modified with the functional coating prepared in Example 1 was tested using a half-cell. The separator material modified with the bimetallic selenide was assembled with a sulfur cathode, an electrolyte, and a lithium metal wafer into a coin cell, and cyclic charge-discharge tests were carried out between 1.7 V and 2.8 V. It was activated for 3 cycles, and the activation current density was 0.1C; it was cycled for 240 cycles, and the cyclic current density was 1C. The charge-discharge test results were as shown in Figure 5 shown. From Figure 5It can be seen that the battery prepared based on the separator modified with bimetallic selenide has more stable electrochemical performance, higher initial capacity, and longer cycle performance compared to the battery prepared based on the conventional PP separator.

[0058] Figure 6 For the SEM of the bimetallic precursor hydrate ZnCo(OH) in Comparative Example 1 x If Zn is in excess, the synthesized ZnCo(OH) x The precursor hydrate is spherical obtained from nanosheet clusters, and the clusters result in a smaller specific surface area of the precursor hydrate compared to that in the example. Figure 1 Among them, the specific surface area of the precursor hydrate is smaller, and after selenization, Zn is in excess. The excess Zn cannot be completely doped into CoSe2, nor can it react with Se to form ZnSe, but ZnO is formed, indicating that the selection of the doping amount of Zn is specific.

[0059] Figure 7 For the SEM of the monometallic precursor hydrate Co(OH)x and the final product CoSe2 in Comparative Example 2; among them, (a) is the SEM image of the monometallic precursor hydrate Co(OH)x. It can be seen from the figure that if Zn doping is not carried out, the obtained precursor hydrate is all single whiskers without nanosheets; the SEM image after selenization is shown in (b).

[0060] Due to the difference in the morphology of the precursor hydrate, the morphology of the final selenized product is also different. Figure 1 and Figure 7 By comparison, it can be seen that Figure 1 With Zn doping, the final product is composed of nanolayer sheet units and coarsened nanowhisker units, so the obtained product ZnCoSe has a larger specific surface area, providing more active sites for increasing the loading of active sulfur; compared with CoSe2, ZnCoSe has more obvious surface effects, stronger adsorption, higher surface activity, and higher surface atom utilization rate, which can provide rich adsorption base points for the catalyst.

[0061] Figure 8 For the electrochemical performance diagrams of the bimetallic selenide separator obtained in Example 1 and the ZnCo2O4 separator obtained in Comparative Example 3; (a) is the rate performance diagram; (b) is the impedance data. From Figure 8 The rate performance comparison diagram in a shows that under the same environment and charge-discharge rate, the lithium-sulfur battery based on the ZnCoSe@PP separator shows a higher initial discharge specific capacity, and its capacity retention is higher and more stable. Especially at a current density of 3C, the ZnCoSe@PP battery still maintains a specific capacity of 772.273 mAh g -1 while the CoSe2@PP battery drops to 616.543 mAh·g -1. It shows that the doping of Zn can effectively improve the rate performance of the battery and has good reversibility at different current densities. In Figure 8 In the comparison diagram of the electrochemical impedance spectra of the two separator batteries in b), it can be seen that compared with the CoSe2@PP battery, the impedance spectrum of the ZnCoSe@PP battery has a smaller semicircle diameter, indicating that the doping of Zn makes the battery have lower interfacial resistance and charge transfer resistance, which can effectively improve the interfacial electrochemical reaction behavior and is more conducive to the performance of the lithium-sulfur battery. Figure 9 The battery impedance diagrams of the bimetallic selenide, the bimetallic precursor hydrate ZnCo(OH) obtained in Example 1 x , the ZnCo2O4 obtained in Comparative Example 3, and the basic separator PP. It can be seen from the figure that the impedance of the bimetallic selenide coating prepared by the method of the present invention is significantly smaller than that of the bimetallic precursor hydrate ZnCo(OH)x, ZnCo2O4, and PP, and thus has the highest conductivity.

[0062] As mentioned above, only the specific embodiments of the present invention are described. Any feature disclosed in this specification, unless specifically stated, can be replaced by other equivalent or similar-purpose alternative features; all the features disclosed, or all the steps in any method or process, except for mutually exclusive features and / or steps, can be combined in any way.

Claims

1. A separator modified with a functional coating, characterized in that, It includes a basic separator and a bimetallic selenide coating coated on at least one surface of the basic separator, and the bimetallic selenide coating is a Zn-doped CoSe2 coating.

2. The diaphragm modified with the functional coating according to claim 1, wherein The basic separator is a polyethylene separator or a polypropylene separator.

3. The separator modified with the functional coating according to claim 1, characterized in that, The thickness of the bimetallic selenide coating is 1-300 μm, and the doping amount of Zn is 1%-30% of the content of Co element.

4. A preparation method of a separator modified with a functional coating, characterized in that, It includes the following steps: Step 1: Weigh a cobalt source and a zinc source according to the molar ratio of element Co:Zn = x:y, place the cobalt source, zinc source, urea and ammonium fluoride in pure water, and then stir to fully dissolve each component to obtain solution A; where 0 < x ≤ 10, 0 < y ≤ 10; Step 2: Place solution A in a hydrothermal autoclave for hydrothermal reaction, the heating temperature is 80-180 °C, and the heating time is 8-18 h; after the reaction, it is naturally cooled to room temperature; Step 3: Separate the supernatant from the precipitate in the solution obtained in Step 2, and then wash the precipitate multiple times to obtain a bimetallic hydrate; Step 4: Grind the bimetallic hydrate obtained in Step 3, then mix it with Se powder in a certain proportion and place it in a tube furnace, and sinter it in an inert atmosphere, the sintering temperature is 300-700 °C, and the sintering time is 2-6 hours; after the sintering reaction is completed, a product is obtained; Step 5: Grind, wash and dry the product obtained in Step 4 to obtain bimetallic selenide powder; Step 6: Mix the bimetallic selenide powder obtained in Step 5 with a conductive additive, a binder and a solvent in a certain proportion to form a slurry, coat it on the basic separator, and after drying, the required separator can be obtained.

5. The preparation method according to claim 4, characterized in that, In Step 1, the cobalt source is cobalt nitrate hexahydrate, cobalt tetroxide, cobalt chloride hexahydrate, and the zinc source is zinc nitrate standard solution, nano-zinc oxide, zinc sulfate.

6. The preparation method according to claim 4, characterized in that, In Step 4, the molar ratio of the bimetallic hydrate to Se powder is (1-5):

1.

7. The preparation method according to claim 4, characterized in that, In Step 4, the inert atmosphere is an argon or nitrogen atmosphere, and the gas flow rate is 60-180 sccm.

8. The preparation method according to claim 4, characterized in that, In Step 6, the conductive additive is any one of acetylene black, Ketjen black, carbon nanotubes; the binder is any one of polyvinylidene fluoride, polyvinylidene difluoride, polyethylene oxide, polyacrylic acid, and the solvent is dimethylformamide or N-methylpyrrolidone; the mass ratio of the bimetallic selenide precursor powder to the conductive additive, binder, and solvent is x:y:z, where 0 ≤ x ≤ 1, 0 ≤ y ≤ 1, 0 ≤ z ≤ 1.

9. A lithium battery, characterized in that, The separator of the lithium battery is a separator modified with a functional coating obtained by the preparation method according to any one of claims 4-8.

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