A battery diaphragm, a preparation method thereof, and an aqueous sodium ion battery

Abstract

The present invention relates to the field of battery technology, and in particular, to a battery diaphragm, a preparation method thereof, and an aqueous sodium ion battery. The battery diaphragm comprises a diaphragm substrate and a coating provided on the diaphragm substrate, wherein the coating comprises carbon-coated sodium ferrocyanide, a conductive agent, and a binder. The preparation method of the battery diaphragm comprises the following steps: mixing carbon-coated sodium ferrocyanide, a conductive agent, a binder, and a solvent to form a slurry, coating the slurry on the diaphragm substrate, and drying the slurry. The present invention obtains a high-performance battery diaphragm by providing a modified coating containing carbon-coated sodium ferrocyanide, a conductive agent, and a binder on a conventional diaphragm substrate. Using the modified coating for aqueous sodium ion batteries can improve the long-cycle stability, capacity, coulombic efficiency, and energy density of the battery.

Description

Technical Field

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

[0002] Although lithium-ion batteries (LIBs) have been optimized to meet the requirements of portable electronic devices, some intrinsic properties make current LIBs less feasible for large-scale fixed-electrodes, where cost, safety, and long cycle life are more important than energy density. One of the main issues is safety. The high risk of safety accidents in LIBs is due to flammable organic electrolytes and thermal runaway caused by the reactivity of electrode materials with electrolytes. In addition, due to the special battery assembly technology, the strict dry environment required during the manufacturing process, and the high prices of transition metals, organic electrolytes, and lithium salts, the cost of lithium batteries is relatively high. The development of highly safe, low-cost, and long-cycle battery systems has become a current research hotspot.

[0003] Aqueous sodium-ion batteries perfectly meet the above requirements and are promising alternatives for large-scale applications. They can solve the safety issues of flammable organic electrolytes in traditional batteries, avoid strict manufacturing conditions, and have relatively low prices for electrolyte solvents and salts. The ionic conductivity of aqueous electrolytes is 2 orders of magnitude higher than that of organic electrolytes. In addition, they are based on environmentally friendly aqueous electrolytes.

[0004] The most commonly used aqueous energy storage system abroad is the Prussian-type symmetrical battery, but its poor cycling stability, low energy density, narrow voltage window, and slow electrochemical kinetics make it difficult to scale up aqueous sodium-ion batteries. Therefore, it is of great significance to develop a modified battery separator that can contribute to the battery's inherent capacity and improve cycling performance and coulombic efficiency to empower aqueous sodium-ion batteries.

[0005] In view of this, the present invention is proposed. Summary of the Invention

[0006] The first purpose of the present invention is to provide a battery separator to solve the technical problems of low interface conductivity, low reaction activity, poor long-cycle stability and low energy density in existing aqueous sodium ion batteries; a modifying coating containing carbon-coated sodium ferrocyanide, a conductive agent and a binder is provided on a conventional separator substrate to modify the separator substrate to obtain a high-performance battery separator, which can be used in aqueous sodium ion batteries to improve the long-cycle stability, capacity, coulombic efficiency and energy density of the battery.

[0007] The second object of the present invention is to provide a method for preparing the battery separator as described above, which has a simple preparation process, controllable thickness of the modified layer, and is easy to achieve mass production.

[0008] A third object of the present invention is to provide an aqueous sodium ion battery comprising the battery separator described above.

[0009] In order to achieve the above-mentioned purpose of the present invention, the following technical solutions are adopted:

[0010] A battery diaphragm comprises a diaphragm substrate and a coating arranged on the diaphragm substrate, wherein the coating comprises carbon-coated sodium ferrocyanide, a conductive agent and a binder.

[0011] Preferably, the carbon-coated sodium ferrocyanide is a double-layer carbon-coated sodium ferrocyanide, comprising sodium ferrocyanide and a first carbon layer and a second carbon layer sequentially coated on the outside of the sodium ferrocyanide.

[0012] Preferably, the carbon source for forming the first carbon layer includes at least one of polyvinyl alcohol, glacial acetic acid, tannic acid and oxalic acid.

[0013] Preferably, the carbon source for forming the second carbon layer includes sorbitan monooleate polyoxyethylene ether.

[0014] Preferably, the preparation method of the carbon-coated sodium ferrocyanide comprises the following steps:

[0015] Sodium ferrocyanide is mixed with the carbon source of the first carbon layer, ethanol is added dropwise for grinding, and the mixture is subjected to a first drying treatment after grinding. The material subjected to the first drying treatment is then mixed with the carbon source of the second carbon layer, ethanol is added dropwise for grinding, and the mixture is subjected to a second drying treatment after grinding. The material subjected to the second drying treatment is carbonized under an inert atmosphere to obtain double-layer carbon-coated anhydrous sodium ferrocyanide.

[0016] Preferably, the sodium ferrocyanide is anhydrous sodium ferrocyanide obtained by grinding and drying sodium ferrocyanide decahydrate;

[0017] The mass ratio of the carbon source of the first carbon layer to the anhydrous sodium ferrocyanide is 2%-8%; and / or the mass ratio of the carbon source of the second carbon layer to the anhydrous sodium ferrocyanide is 1%-5%.

[0018] Preferably, the temperature of the carbonization treatment is 260-300° C., and the time of the carbonization treatment is 6-10 h.

[0019] Preferably, the coating comprises, by mass percentage, 70%-90% of the carbon-coated sodium ferrocyanide, 5%-20% of the conductive agent, and 5%-10% of the binder.

[0020] Preferably, the binder comprises PVDF and / or CMC.

[0021] Preferably, the coating has a thickness of 5-30 μm.

[0022] A method for preparing the battery separator as described above comprises the following steps:

[0023] The carbon-coated sodium ferrocyanide, a conductive agent, a binder and a solvent are mixed to prepare a slurry, which is then coated on a diaphragm substrate and dried to obtain the slurry.

[0024] An aqueous sodium ion battery comprises the battery separator described above.

[0025] Compared with the prior art, the present invention has the following beneficial effects:

[0026] (1) The present invention modifies the diaphragm substrate by using a modification coating containing carbon-coated sodium ferrocyanide, a conductive agent and a binder. Sodium ferrocyanide can be used to in situ regulate the coordinated metal ions, thereby improving the specific capacity and cycle performance. However, since sodium ferrocyanide is water-soluble, conventional methods of directly attaching sodium ferrocyanide to the diaphragm substrate often result in a large amount of loss of the modified material. The present invention uses carbon-coated sodium ferrocyanide for modification, thereby protecting the internal substance and optimizing the electronic structure. The release rate and conductivity of sodium ferrocyanide can be regulated in situ, thereby avoiding the loss of sodium ferrocyanide due to dissolution and improving the electrochemical performance.

[0027] (2) The modified coating of the present invention contains a conductive agent and a binder. The conductive agent increases the ion transmission efficiency and electronic conductivity; the binder allows the sodium ferrocyanide to contact the diaphragm matrix more closely, slowing down its loss and increasing the long cycle performance.

[0028] (3) The aqueous sodium ion battery provided by the present invention has good long-cycle stability, large capacity, high coulombic efficiency and energy density, which solves the problem of rapid water dissolution of sodium ferrocyanide, as well as the problems of low interface conductivity, low reaction activity, poor long-cycle stability and low energy density of traditional aqueous sodium ion batteries. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the specific embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0030] Figure 1 The full battery assembled with the diaphragm prepared in Example 1 of the present invention is -1 The charge and discharge curves of the 98th-100th cycle at the current density;

[0031] Figure 2The full battery assembled with the diaphragm prepared in Example 17 of the present invention is -1 The charge and discharge curves of the 98th-100th cycle at the current density. DETAILED DESCRIPTION

[0032] The technical scheme of the present invention will be clearly and completely described below in conjunction with the accompanying drawings and specific embodiments, but it will be understood by those skilled in the art that the following described embodiments are part of embodiments of the present invention, rather than all embodiments, and are only used to illustrate the present invention, and should not be considered as limiting the scope of the present invention. Based on the embodiments in the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative work are within the scope of protection of the present invention. Those who do not specify specific conditions in the embodiments are carried out according to normal conditions or the conditions recommended by the manufacturer. Those whose reagents or instruments are not specified by the manufacturer are conventional products that can be purchased commercially.

[0033] A first aspect of the present invention provides a battery separator, comprising a separator substrate and a coating disposed on the separator substrate, wherein the coating comprises carbon-coated sodium ferrocyanide, a conductive agent, and a binder.

[0034] The present invention uses coated modified sodium ferrocyanide with a conductive agent and a binder to modify the diaphragm matrix, forming an interface modified conductive layer on the diaphragm matrix, thereby solving the problem of rapid water dissolution of sodium ferrocyanide, as well as the problems of low interface conductivity, low reaction activity, poor long-cycle stability and low energy density of traditional aqueous sodium ion batteries.

[0035] Sodium ferrocyanide can in situ regulate the coordinated metal ions and enhance the crystal structure stability of the Prussian positive electrode material during the discharge process. However, since sodium ferrocyanide is water-soluble, conventional adsorption modification methods often cause a large amount of sodium ferrocyanide loss. Sodium ferrocyanide is released quickly in the early stage, with a large capacity growth rate, but it is unstable in the middle and late cycles. Therefore, the present invention uses a multi-porous structure obtained by carbon coating and sodium ferrocyanide modified with multiple active groups to modify the diaphragm matrix. The carbon coating layer structure protects the internal substance while optimizing the electronic structure and enhancing the electron migration ability. The release rate of sodium ferrocyanide during charge and discharge can be regulated by adjusting the carbon content, and the release rate and conductivity of sodium ferrocyanide can be regulated in situ. The slowly released sodium ferrocyanide can effectively prevent the capacity reduction caused by the Jahn-Teller effect caused by the dissolution of transition metal ions in the Prussian blue analogue material, thereby increasing the long cycle capacity.

[0036] The interface modified by carbon-coated sodium ferrocyanide, conductive agent and binder can form an interface modified conductive layer, which increases ion mobility and electronic conductivity, enhances the adhesion of the modified material, and effectively reduces the loss of the modified material (sodium ferrocyanide). Under the action of the in-situ interface conductive layer, the water decomposition problem in the aqueous electrolyte can be effectively prevented, and the decomposed H + Will move to the Fe(CN)6-rich 4- The resulting membrane has good wettability, and the conductive agent and binder are rich in multi-electron structures, which are conducive to the rapid passage of ions and the rapid migration of electrons.

[0037] In some specific embodiments of the present invention, the carbon-coated sodium ferrocyanide used is double-layer carbon-coated sodium ferrocyanide, including sodium ferrocyanide and a first carbon layer and a second carbon layer sequentially coated on the outside of the sodium ferrocyanide.

[0038] In some specific embodiments of the present invention, the carbon source used to form the first carbon layer includes at least one of polyvinyl alcohol, glacial acetic acid, tannic acid, and oxalic acid.

[0039] The carbon source used to form the first carbon layer of the present invention has a low decomposition temperature, which can achieve low-temperature carbonization and avoid the decomposition of sodium ferrocyanide. In addition, the coating raw material will etch the cathode material and slightly etch the sodium ferrocyanide. During low-temperature carbonization, a contact interface with a deeper fit layer is formed, which is rich in functional groups and is conducive to electron adsorption and ion escape.

[0040] In some embodiments of the present invention, the carbon source used to form the second carbon layer includes sorbitan monooleate polyoxyethylene ether. The outer carbon layer (second carbon layer) can optimize the pore structure, further enhance the spatial caging effect, and be rich in functional groups, which is conducive to increased cyclic stability.

[0041] The carbon source of the second carbon layer is polyoxyethylene sorbitan monooleate, which is a non-ionic surfactant with good emulsifying effect and a wide spatial occupancy. Its introduction can increase the carbon layer spacing during carbonization, thereby further enriching the pore structure, and its surface is rich in active groups. The introduction of C=O bonds, COC and other bonds will not be lost due to high temperature when low-temperature cracking is adopted, and a large number of active groups are retained, which increases its surface charge distribution and optimizes the electron transmission channel. During carbonization, it covers the surface of the first carbon layer to form a second carbon layer, forming a double carbon layer structure, which optimizes the pore structure while further stabilizing the internal space.

[0042] In some specific embodiments of the present invention, the preparation method of the carbon-coated sodium ferrocyanide comprises the following steps:

[0043] Sodium ferrocyanide is mixed with the carbon source of the first carbon layer, ethanol is added dropwise for grinding, the grinding is followed by a first drying process, the material after the first drying process is mixed with the carbon source of the second carbon layer, ethanol is added dropwise for grinding, the grinding is followed by a second drying process, and the material after the second drying process is carbonized under an inert atmosphere to obtain double-layer carbon-coated anhydrous sodium ferrocyanide. The coating method of sodium ferrocyanide of the present invention is simple and the conditions are mild and controllable.

[0044] In some specific embodiments of the present invention, the sodium ferrocyanide used for carbon coating is anhydrous sodium ferrocyanide obtained by grinding and drying sodium ferrocyanide decahydrate;

[0045] The mass ratio of the carbon source of the first carbon layer used to the anhydrous sodium ferrocyanide is 2%-8%, for example, it can be any value among 2%, 3%, 4%, 5%, 6%, 7%, and 8, or a range consisting of any two values; and / or, the mass ratio of the carbon source of the second carbon layer used to the anhydrous sodium ferrocyanide is 1%-5%, for example, it can be any value among 1%, 2%, 3%, 4%, and 5, or a range consisting of any two values.

[0046] The amount of sorbitan monooleate polyoxyethylene ether, the carbon source of the second carbon layer, is too small to optimize the pore structure. Excessive sorbitan monooleate polyoxyethylene ether will react with sodium ferrocyanide to produce byproducts such as Na2O.

[0047] In some specific embodiments of the present invention, the temperature of the carbonization treatment is 260-300°C, for example, it can be any point value among 260°C, 270°C, 280°C, 290°C, and 300°C, or a range value consisting of any two point values; the time of the carbonization treatment is 6-10h, for example, it can be any point value among 6h, 7h, 8h, 9h, and 10h, or a range value consisting of any two point values.

[0048] Since the decomposition temperature of sodium ferrocyanide is 435°C, the present invention uses a carbon source that can be cracked at low temperature to coat the sodium ferrocyanide. After a low-temperature carbonization coating treatment at 260-300°C, sodium ferrocyanide modified with a multi-porous structure / multi-active groups can be obtained. The double carbon layer structure protects the internal substance while optimizing the electronic structure, enhancing the electron migration ability, and can in situ adjust the release rate and conductivity of the sodium ferrocyanide.

[0049] In some embodiments of the present invention, the coating comprises, by mass percentage, 70%-90% carbon-coated sodium ferrocyanide, 5%-20% conductive agent, and 5%-10% binder. Excessive carbon-coated sodium ferrocyanide content results in low conductive agent concentration and poor conductivity; insufficient carbon-coated sodium ferrocyanide content may affect the battery's cycling stability.

[0050] In some embodiments, typically but not limiting, for example, the mass percentage of carbon-coated sodium ferrocyanide in the coating can be any value among 70%, 75%, 80%, 85%, 90%, or a range consisting of any two values; the mass percentage of the conductive agent in the coating can be any value among 5%, 10%, 15%, 20%, or a range consisting of any two values; the mass percentage of the binder in the coating can be any value among 5%, 6%, 7%, 8%, 9%, 10%, or a range consisting of any two values.

[0051] In some specific embodiments of the present invention, the conductive agent used may be conductive carbon black.

[0052] In some embodiments of the present invention, the binder used includes PVDF and / or CMC.

[0053] In some specific embodiments of the present invention, the thickness of the coating is 5-30 μm, for example, it can be any value among 5 μm, 10 μm, 15 μm, 20 μm, 25 μm, 30 μm, or a range consisting of any two values.

[0054] In some specific embodiments of the present invention, the membrane matrix includes any one of a glass fiber membrane, a cellulose membrane, a PP membrane, and a PE membrane.

[0055] A second aspect of the present invention provides a method for preparing the battery separator according to any one of the aforementioned embodiments, comprising the following steps:

[0056] The carbon-coated sodium ferrocyanide, a conductive agent, a binder and a solvent are mixed to prepare a slurry, which is then coated on a diaphragm substrate and dried to obtain the slurry.

[0057] The method of the present invention has simple preparation process, low cost, controllable coating thickness, and can quantitatively adjust the thickness of the modified coating layer, thereby adjusting the content of the modified substance.

[0058] In the present invention, the drying conditions are not specifically limited, and are subject to complete evaporation of the solvent, and technicians can select them according to their needs.

[0059] A third aspect of the present invention provides an aqueous sodium ion battery, comprising the battery separator according to any one of the aforementioned embodiments, or a separator produced by the method for producing the battery separator according to the aforementioned embodiments.

[0060] In some specific embodiments of the present invention, the aqueous sodium ion battery further comprises a positive electrode sheet, and the positive electrode active material for the positive electrode sheet comprises a Prussian blue analogue. As an example, the positive electrode active material may be a manganese-based Prussian white material.

[0061] In some specific embodiments of the present invention, the aqueous sodium ion battery further comprises a negative electrode sheet, and the negative electrode active material for the negative electrode sheet may be a sodium titanium phosphate negative electrode material.

[0062] In some specific embodiments of the present invention, the electrolyte of the aqueous sodium ion battery uses a 17M NaClO4 aqueous solution as the aqueous electrolyte.

[0063] The following describes some embodiments of the present invention in detail with reference to specific examples. The raw materials used in the examples can be purchased from the market unless otherwise specified.

[0064] Example 1

[0065] S1. Preparation of Carbon-Coated Sodium Ferrocyanide

[0066] Take 5g of sodium ferrocyanide decahydrate, ball mill it to about 10 microns (600r / min, 2h), dry it at 100°C for 24h to remove the crystal water, take it out immediately, grind and mix it with polyvinyl alcohol, the amount of polyvinyl alcohol is 5% of the mass of anhydrous sodium ferrocyanide, add ethanol dropwise and grind it evenly, and dry it at 60°C; then grind and mix it with anhydrous sorbitan monooleate polyoxyethylene ether, the amount of anhydrous sorbitan monooleate polyoxyethylene ether is 3% of the mass of anhydrous sodium ferrocyanide, add ethanol dropwise and grind it evenly, dry it at 60°C, take out the powder and place it in a tubular furnace, and carbonize it at 280°C under an argon atmosphere for 8h to obtain anhydrous sodium ferrocyanide coated with a double carbon layer.

[0067] S2. Preparation of battery separators

[0068] A glass fiber diaphragm was used as the diaphragm matrix. The double-layer carbon-coated anhydrous sodium ferrocyanide, conductive carbon black and PVDF prepared in this embodiment were taken in a mass ratio of 8:1:1, and NMP was added to prepare a black slurry. The slurry was then applied to a glass fiber diaphragm with a thickness of 260 μm using a scraper to a coating thickness of 10 μm. The slurry was then vacuum-dried at 120°C for 12 h to obtain the battery diaphragm.

[0069] Example 2

[0070] Example 2 is similar to Example 1, with the only difference being that in step S1, polyvinyl alcohol is replaced with an equal amount of glacial acetic acid, and the remaining conditions are the same as those in Example 1.

[0071] Example 3

[0072] Example 3 is similar to Example 1, with the only difference being that in step S1, polyvinyl alcohol is replaced with an equal amount of tannic acid, and the remaining conditions are the same as those in Example 1.

[0073] Example 4

[0074] Example 4 is similar to Example 1, with the only difference being that in step S1, polyvinyl alcohol is replaced with an equal amount of oxalic acid, and the remaining conditions are the same as those in Example 1.

[0075] Example 5

[0076] Example 5 is similar to Example 1, with the only difference being that in step S1, the amount of sorbitan monooleate polyoxyethylene ether is 1% by mass of anhydrous sodium ferrocyanide, and the other conditions are the same as those in Example 1.

[0077] Example 6

[0078] Example 6 is similar to Example 1, with the only difference being that in step S1, polyvinyl alcohol is replaced with an equal amount of sorbitan monooleate polyoxyethylene ether, and the other conditions are the same as those in Example 1.

[0079] Example 7

[0080] Example 7 is similar to Example 1, with the only difference being that in step S1, the amount of polyvinyl alcohol is changed to 2% of the mass of anhydrous sodium ferrocyanide, and the other conditions are the same as those in Example 1.

[0081] Example 8

[0082] Example 8 is similar to Example 1, with the only difference being that in step S1, the amount of polyvinyl alcohol is changed to 8% of the mass of anhydrous sodium ferrocyanide, and the other conditions are the same as those in Example 1.

[0083] Example 9

[0084] Example 9 is similar to Example 1, with the only difference being that in step S2, the coating thickness is changed to 5 μm, and the other conditions are the same as those in Example 1.

[0085] Example 10

[0086] Example 10 is similar to Example 1, with the only difference being that in step S2, the coating thickness is changed to 15 μm, and the other conditions are the same as those in Example 1.

[0087] Example 11

[0088] Example 11 is similar to Example 1, with the only difference being that in step S1, the carbonization time is changed to 10 h, and the other conditions are the same as those in Example 1.

[0089] Example 12

[0090] Example 12 is similar to Example 1, with the only difference being that in step S1, the carbonization time is changed to 6 h, and the other conditions are the same as those in Example 1.

[0091] Example 13

[0092] Example 13 is similar to Example 1, with the only difference being that in step S1, the carbonization temperature is changed to 260° C., and the other conditions are the same as those in Example 1.

[0093] Example 14

[0094] Example 14 is similar to Example 1, with the only difference being that in step S1, the carbonization temperature is changed to 300° C., and the other conditions are the same as those in Example 1.

[0095] Example 15

[0096] Example 15 is similar to Example 1, with the only difference being that in step S2, the mass ratio of double-layer carbon-coated anhydrous sodium ferrocyanide, conductive carbon black, and PVDF is changed to 9:0.5:0.5, and the other conditions are the same as those in Example 1.

[0097] Example 16

[0098] Example 16 is similar to Example 1, with the only difference being that in step S2, the mass ratio of double-layer carbon-coated anhydrous sodium ferrocyanide, conductive carbon black, and PVDF is changed to 7:2:1, and the other conditions are the same as those in Example 1.

[0099] Example 17

[0100] Example 17 is similar to Example 1, except that in step S2, PVDF is replaced with an equal amount of CMC, and the other conditions are the same as those in Example 1.

[0101] Comparative Example 1

[0102] The diaphragm in Comparative Example 1 is the diaphragm substrate in Example 1 (260 μm glass fiber diaphragm), without any finishing coating.

[0103] Comparative Example 2

[0104] Comparative Example 2 is similar to Example 1, except that in step S2, an equal amount of sodium ferrocyanide decahydrate is used instead of the double-layer carbon-coated anhydrous sodium ferrocyanide. The preparation method of the battery separator is the same as that of Example 1, that is, the sodium ferrocyanide in this comparative example is not coated. The specific steps are as follows:

[0105] A glass fiber separator was used as the separator matrix. Sodium ferrocyanide decahydrate, conductive carbon black and PVDF were taken in a mass ratio of 8:1:1, and NMP was added to prepare a black slurry. The slurry was then coated on a glass fiber separator with a thickness of 260 μm using a scraper to a coating thickness of 10 μm. The slurry was then vacuum dried at 120°C for 12 hours to obtain the battery separator.

[0106] Comparative Example 3

[0107] A 260 μm glass fiber diaphragm was used as the diaphragm matrix, and the diaphragm matrix was modified directly using unmodified sodium ferrocyanide through an infiltration process. The steps are as follows:

[0108] A 0.5 M sodium ferrocyanide solution was evenly dripped onto the glass fiber membrane using a pipette to wet it. The amount of sodium ferrocyanide used was the same as that in Example 1. The membrane was dried at 60° C. for 2 h and then at 120° C. for 12 h.

[0109] Test example

[0110] The diaphragms prepared in each embodiment and each comparative example were respectively used to assemble aqueous sodium ion batteries and the electrochemical performance tests were performed. The battery assembly process and test conditions were as follows:

[0111] Preparation of the positive electrode active material: 5 mmol of Mn salt and 25 mmol of trisodium citrate were dissolved in 50 ml of deionized water to obtain solution A. 5 mmol of Na₄Fe(CN)₆ ...

[0112] Preparation of positive electrode sheet: Take the positive electrode active material prepared by the above method, PVDF (polyvinylidene fluoride), and conductive carbon black in a mass ratio of 8:1:1, weigh the above materials according to the proportion and place them in a small weighing bottle (2.5cm*2.5cm), add NMP (N-methylpyrrolidone) solution dropwise, stir for 4 hours, and apply the stirred slurry on stainless steel foil. After coating, place it in a vacuum drying oven and dry it for 8 hours. Then use a cutting machine to cut into small circular electrode sheets with a diameter of 16mm and compact them under a pressure of 10Mpa. Ensure that the mass of the Prussian blue analog active material on this electrode is approximately 2.5-3mg cm -2 , thereby obtaining the positive electrode of the button battery.

[0113] Preparation of the negative electrode active material: Sodium titanium phosphate was selected as the negative electrode material. The preparation steps were as follows: 1.0g NaH2PO2, 1.0g H2TiO3, and 1.0g H3PO4 were added to 1.0ml of deionized water and stirred at 800 rpm for 5 minutes to mix thoroughly. The mixture was then transferred to a 50ml autoclave and incubated at 140°C for 8 hours at a heating rate of 5°C / min. A white precipitate was obtained by centrifugation and dried overnight in a vacuum oven at 60°C to obtain a NaTi2(PO4)3 precursor. 1.0g of the prepared NaTi2(PO4)3 precursor was placed in a mortar with 0.05g of glucose and ground with 5ml of anhydrous ethanol for 10 minutes. The mixture was then transferred to a ball mill and milled for 60 minutes. The powder was removed and placed in a vacuum oven. After drying, the temperature was increased to 650°C at 5°C / min and carbonized for 2 hours to obtain the carbon-coated NaTi2(PO4)3 negative electrode active material.

[0114] Preparation of negative electrode sheet: Take the negative electrode active material prepared by the above method, conductive carbon black, and PVDF (polyvinylidene fluoride) in a mass ratio of 7:2:1, weigh the above materials according to the proportion and place them in a small weighing bottle (2.5cm*2.5cm), add 17 drops of NMP (N-methylpyrrolidone) solution, stir for 4 hours, and apply the stirred slurry on copper foil. After coating, place it in a vacuum drying oven and dry it for 8 hours. Then use a cutting machine to cut into small circular electrode sheets with a diameter of 16mm and compact them under a pressure of 5Mpa. Ensure that the mass of active material on this electrode is about 2.5-3.6mg cm -2 , ensure that the positive and negative electrode capacity ratio is between 1: (1.2-2.0).

[0115] Electrolyte: 17M NaClO4 aqueous solution was used as the aqueous electrolyte.

[0116] Full battery assembly: The full battery uses 2025 button cells. The assembly order is negative electrode shell, spring, gasket, negative electrode plate, electrolyte, diaphragm (diaphragm prepared in each embodiment and each comparative example), electrolyte, positive electrode plate, gasket, positive electrode shell; the mass ratio of positive and negative active materials is about 1:1.2. The mass of positive active material is about 2.5-3 mg / cm 2 .

[0117] Full battery test: A charge and discharge cycle test was performed on a full battery assembled with the battery separators prepared in each embodiment and comparative example. The test was performed on a Newwell battery testing system with a test voltage range of 0.01-2.0 V, a test current density of 0.2 A / g, and a constant temperature of 25°C. The capacity attenuation rate from the 100th to 2000th cycle was obtained by subtracting the specific capacity of the 2000th cycle from the specific capacity of the 100th cycle and then dividing it by the specific capacity of the 100th cycle. The results are shown in Table 1.

[0118] Table 1

[0119]

[0120]

[0121] The data in Table 1 show that the electrochemical performance of the modified diaphragm with a double-layer carbon coating is relatively stable, with minimal attenuation during the 100-2000 cycle. The effect of a single-layer coating is significantly inferior to that of a double-layer coating. In addition, the appropriate thickness affects the overall electrochemical performance. When the coating thickness is reduced, the electrochemical performance of the material decreases. When the coating thickness is too thick, the electrochemical performance of the material also decreases. This is because the amount of sodium ferrocyanide is too small, resulting in poor repair effect. The reason for being too thick is that the thickness affects the ion transport efficiency and reduces the conductivity. In addition, the carbonization temperature and time affect the coating effect. The appropriate carbonization temperature can obtain a uniform carbon layer thickness and a suitable pore structure. Too low a carbonization temperature or too short a time leads to incomplete carbonization. Too high a carbonization temperature or too long a time leads to excessive carbonization and the disappearance of multifunctional group activity. Obviously, the electrochemical performance of the uncoated diaphragm is extremely unstable, which is caused by the lack of sodium ferrocyanide repair. After replacing the binder with CMC, long-cycle performance decreased. This is due to CMC's water solubility, which hinders long-term adhesion. Uncoated sodium ferrocyanide showed poor results due to the large amount of sodium ferrocyanide dissolved. Furthermore, the membrane modified by adding 0.5M sodium ferrocyanide showed an initial increase in specific capacity, but poor long-term stability. This was also due to the dissolution of sodium ferrocyanide, which reduced the repair effect.

[0122] Depend on Figure 1 and Figure 2 It can be seen that the charge and discharge curves of the modified diaphragm are well matched and basically overlap, indicating that its electrochemical reaction stability is excellent, but there is an obvious Figure 2 There is no obvious voltage platform around 0.9 V, which is related to the dissolution of CMC in water. After dissolution, its repair effect gradually deteriorates.

[0123] Although the present invention has been illustrated and described using specific embodiments, it should be appreciated that the above embodiments are merely intended to illustrate the technical solutions of the present invention rather than to limit them. Those skilled in the art should understand that the technical solutions described in the above embodiments may be modified, or some or all of the technical features thereof may be replaced by equivalents, without departing from the spirit and scope of the present invention. However, these modifications or replacements do not deviate from the essence of the corresponding technical solutions within the scope of the technical solutions of the embodiments of the present invention. Therefore, this means that all such replacements and modifications within the scope of the present invention are included in the appended claims.

Claims

1. A separator for an aqueous sodium ion battery, characterized in that The invention comprises a diaphragm substrate and a coating provided on the diaphragm substrate, wherein the coating comprises carbon-coated sodium ferrocyanide, a conductive agent and a binder; The carbon-coated sodium ferrocyanide is a double-layer carbon-coated sodium ferrocyanide, comprising sodium ferrocyanide and a first carbon layer and a second carbon layer sequentially coated on the outside of the sodium ferrocyanide; The carbon source for forming the first carbon layer includes at least one of polyvinyl alcohol, glacial acetic acid, tannic acid and oxalic acid; The carbon source for forming the second carbon layer includes sorbitan monooleate polyoxyethylene ether.

2. The aqueous sodium ion battery separator according to claim 1, wherein The preparation method of the carbon-coated sodium ferrocyanide comprises the following steps: Sodium ferrocyanide is mixed with the carbon source of the first carbon layer, ethanol is added dropwise for grinding, and the mixture is subjected to a first drying treatment after grinding. The material subjected to the first drying treatment is then mixed with the carbon source of the second carbon layer, ethanol is added dropwise for grinding, and the mixture is subjected to a second drying treatment after grinding. The material subjected to the second drying treatment is carbonized under an inert atmosphere to obtain double-layer carbon-coated anhydrous sodium ferrocyanide.

3. The aqueous sodium ion battery separator according to claim 2, wherein The sodium ferrocyanide is anhydrous sodium ferrocyanide obtained by grinding and drying sodium ferrocyanide decahydrate; The mass ratio of the carbon source of the first carbon layer to the anhydrous sodium ferrocyanide is 2%-8%; and / or the mass ratio of the carbon source of the second carbon layer to the anhydrous sodium ferrocyanide is 1%-5%.

4. The aqueous sodium ion battery separator according to claim 2, wherein The temperature of the carbonization treatment is 260-300° C., and the time of the carbonization treatment is 6-10 hours.

5. The aqueous sodium ion battery separator according to claim 1, wherein Contains at least one of the following characteristics: (1) Calculated by mass percentage, the coating comprises 70%-90% of the carbon-coated sodium ferrocyanide, 5%-20% of the conductive agent, and 5%-10% of the binder; (2) The binder includes PVDF and / or CMC; (3) The thickness of the coating is 5-30 μm.

6. The method for preparing an aqueous sodium ion battery separator according to any one of claims 1 to 5, wherein: The following steps are involved: The carbon-coated sodium ferrocyanide, a conductive agent, a binder and a solvent are mixed to prepare a slurry, which is then coated on a diaphragm substrate and dried to obtain the slurry.

7. An aqueous sodium ion battery, characterized in that: The invention relates to a diaphragm for an aqueous sodium ion battery according to any one of claims 1 to 5, or a diaphragm prepared by the method for preparing a diaphragm for an aqueous sodium ion battery according to claim 6.

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