CN110556493B

CN110556493B - Functional composite diaphragm of lithium/sodium secondary battery and preparation method thereof

Info

Publication number: CN110556493B
Application number: CN201910746299.8A
Authority: CN
Inventors: 郭玉国; 王文鹏; 殷雅侠
Original assignee: Institute of Chemistry CAS
Current assignee: Institute of Chemistry CAS
Priority date: 2019-08-13
Filing date: 2019-08-13
Publication date: 2021-04-27
Anticipated expiration: 2039-08-13
Also published as: CN110556493A

Abstract

The invention discloses a functional composite diaphragm of a secondary battery and a preparation method thereof. The functionality of the lithium/sodium salt coating layer is embodied in that the lithium/sodium salt coating layer can catalyze the lithium/sodium secondary battery electrolyte to generate in-situ cationic polymerization and gelation transformation, the gelation of the electrolyte has an obvious improvement effect on the dendritic crystal problem of a metal cathode, and the lithium/sodium secondary battery electrolyte specially has a domain limiting and blocking effect on a polysulfide (selenium) compound intermediate generated in the charging and discharging processes of a sulfur (selenium) anode, so that the cycle stability of the lithium sulfur and lithium selenium battery is improved. The invention also discloses a secondary battery applying the composite diaphragm, and the battery shows excellent cycle and rate performance. The functional composite diaphragm provided by the invention has the advantages of simple preparation technology, easily obtained raw materials and extremely high practical and large-scale prospects.

Description

Functional composite diaphragm of lithium/sodium secondary battery and preparation method thereof

Technical Field

The invention belongs to the field of electrochemical power sources, and particularly relates to a functional composite diaphragm of a secondary battery, a preparation method of the functional composite diaphragm, a lithium and sodium metal secondary battery assembled by using the functional composite diaphragm, and application of the functional composite diaphragm in an energy storage device.

Background

The electrochemical energy storage system converts electric energy into chemical energy for storage, is an important component of the current large-scale energy storage system, and is also one of key technologies with strategic significance. Electrochemical energy storage has the advantages of high efficiency, cleanness and the like, and various battery systems including lead-acid batteries, nickel-cadmium batteries, nickel-hydrogen batteries, lithium ion batteries and the like are developed in the past decades, thereby playing an important role in industrial production and social life. The commercialized lithium ion battery makes a significant contribution in the fields of solving the energy environment problem, improving the quality of electronic equipment and the like. The conventional lithium ion battery is developed for years, the cycle life, the process and the like are nearly mature, however, the arrival of the era of 'electric vehicles' means that the field of lithium batteries must break the existing energy density upper limit to meet the requirement of longer endurance of the vehicles, and the research of the high-specific-energy metal secondary battery has a profound meaning facing the future and is also one of important systems on which a large-scale energy storage technology can be relied in the future.

However, although lithium and sodium metal secondary batteries have higher energy densities than those of current lithium and sodium ion batteries, it is undeniable that the metal as the negative electrode is easily pulverized and generates dendrites during long cycles, piercing the separator, causing safety problems such as short-circuiting and ignition. These factors limit the progress of commercialization of batteries, and new materials or new battery systems need to be developed to solve them. Polymer-based electrolytes have been extensively studied in the past few years as one of new electrolyte systems that can be widely used in future metal secondary batteries in place of liquid electrolytes. For a metal cathode, the polymer-based electrolyte can effectively relieve the dendritic crystal problem generated in the circulation process of the cathode, and in addition, the polymer-based electrolyte has the characteristic of difficult leakage, so the polymer-based electrolyte also has more important function in the aspect of improving the safety of the battery. In particular, for some metal secondary battery systems, such as lithium sulfur and lithium selenium batteries, because the polymer electrolyte has extremely low fluidity and low solubility to sulfur (selenium), the polymer electrolyte is expected to play a fundamental role in inhibiting dissolution and shuttling of polysulfide (selenium) compounds and improving the stability of the lithium sulfur (selenium) batteries, and thus has obvious advantages in the application of the polymer electrolyte in the lithium sulfur (selenium) batteries. Lithium sulfur batteries tend to exhibit greater polarization in pure solid polymer electrolytes given the extremely low conductivity of sulfur, and do not perform well at room temperature. Therefore, the construction of the lithium-sulfur battery based on the gel-state electrolyte has high practical value.

Wang et al [ Wang, q.s., et al, Chemical Communications (2016)52(8),1637 ] report a composite electrolyte of a sandwich structure of inorganic ceramic-gel state polymer-inorganic ceramic. The gel layer is a polyethylene oxide (PEO) based polymer film soaked with ether electrolyte. In such organic-inorganic composite gel-state electrolytes, the suppression of the polysulfide shuttle effect is largely borne by the additionally introduced ceramic layer, while the PEO polymer provides a storage space for the electrolyte. However, the application of the ex-situ gel electrolyte based on the electrolyte-polymer framework in the lithium sulfur battery cannot completely change the basic structure of "positive electrode-membrane/polymer framework (liquid electrolyte) -negative electrode" causing the dissolution and shuttling of polysulfide in the existing liquid system, and further, polysulfide can still be dissolved in the high-fluidity liquid electrolyte and passes through the gel electrolyte (membrane) through the shuttling effect to cause the side reaction at the negative electrode. This drawback can be solved by means of in-situ gelation of the electrolyte. However, in situ gelling precursors must be "ready-to-use" and cannot be stored for long periods of time.

CN101090164A discloses a porous polyethylene propylene membrane, which is prepared by dissolving liquid electrolyte in organic solvent, and forming a polymer lithium secondary battery together with a positive electrode, a negative electrode and the porous polyethylene propylene membrane. However, in order to prepare the composite separator, the polymer lithium secondary battery can be obtained only after the initial battery is subjected to cyclic charge and discharge for 7 weeks. The production cycle is long, the process is complex, and the defects of low utilization efficiency and inconvenient storage of the in-situ gelation precursor are still not solved.

CN106410098A discloses a composite lithium-sulfur battery diaphragm, and in particular relates to a polyvinylidene fluoride membrane doped with graphene, which is easy to form gelation with electrolyte, but the preparation process of the membrane is complex, the membrane needs to be modified by electrostatic spinning technology, the preparation process is complex, the price is high, and the membrane is not suitable for large-scale industrial production.

WO2017/167195a1 discloses a composite non-porous membrane comprising two or more polymeric materials, at least one of which is capable of being gelled by an organic solvent. However, batteries using such composite separators are inefficient.

Therefore, it is highly desirable to develop a functional composite membrane, which can reduce the waste of the precursor, simplify the process flow, and save the cost.

Disclosure of Invention

In order to solve the problems of complex preparation and low efficiency of the in-situ gelation membrane in the prior art and the problem of preparation in situ, the invention provides a functional composite membrane of a secondary battery and a preparation method thereof. The composite diaphragm provided by the invention is simple to prepare, the raw materials are easy to obtain, and complex procedures and instruments are not needed. Specifically, the invention provides the following technical scheme:

a functional composite diaphragm of a lithium/sodium secondary battery is composed of a common battery diaphragm and a lithium/sodium salt coating layer loaded on the surface, wherein the lithium salt or sodium salt coating layer can catalyze the lithium/sodium secondary battery electrolyte to generate in-situ cationic polymerization and gelation conversion.

The lithium/sodium salt loaded on the composite diaphragm is specially selected functional lithium/sodium salt, has a catalytic effect, and can catalyze part of solvent molecules in specific electrolyte to carry out cationic polymerization. Therefore, the composite diaphragm is functionally represented in that when the composite diaphragm is used as a battery diaphragm and contacts a specific liquid electrolyte, the supported lithium/sodium salt can be gradually dissolved in the electrolyte by a slow release method, and the electrolyte of the lithium/sodium secondary battery is catalyzed to undergo in-situ cationic polymerization and gelation to convert the functionality of the electrolyte. The generated gel polymer electrolyte can effectively improve the dendritic crystal problem of the metal cathode and improve the safety and the stability of the battery. Particularly, for lithium-sulfur and lithium-selenium batteries, the polymer electrolyte has a confinement and barrier effect on a polysulfide (selenide) intermediate generated by a sulfur (selenide) positive electrode in the charging and discharging process, so that an active substance is confined on the positive electrode side, the irreversible capacity attenuation caused by the dissolution and shuttle effects of the polysulfide (selenide) intermediate in a conventional liquid electrolyte is reduced, and the cycle stability of the lithium-sulfur and lithium-selenium batteries is improved.

The lithium/salt of the coating layer on the composite diaphragm is a lithium salt or a sodium salt of a strong Lewis acid radical, and preferably at least one of lithium difluoro oxalate borate, lithium hexafluorophosphate, lithium tetrafluoroborate, sodium tetrafluoroborate and sodium hexafluorophosphate.

Further preferably, the lithium/sodium salt coating layer has a thickness of 1 μm to 30 μm, preferably 5 μm to 10 μm.

The general battery separator is not particularly limited, and may be a separator used for a lithium/sodium secondary battery, which is generally used in the art. Such as polyethylene membranes, polypropylene membranes, polyethylene/polypropylene membranes, alumina polyethylene membranes, ceramic fiber paper membranes.

The invention also provides a preparation method of the functional composite diaphragm, which comprises the following steps:

(1) dissolving lithium/sodium salt and a binder for constituting a coating layer in an organic solvent under inert gas conditions;

(2) and coating the obtained solution on the surface of a common battery diaphragm by adopting a blade coating method, and drying to obtain the composite diaphragm.

Wherein the lithium/sodium salt is selected from one or more of lithium difluoro oxalate borate, lithium hexafluorophosphate, lithium tetrafluoroborate, sodium tetrafluoroborate and sodium hexafluorophosphate; the binder is selected from one or more of polyvinylidene fluoride, carboxymethyl cellulose, sodium carboxymethyl cellulose, styrene butadiene rubber, sodium alginate, polyethylene oxide, polyvinyl alcohol, polytetrafluoroethylene and polyamide; the organic solvent is selected from one or more of dimethyl sulfoxide, N-dimethylformamide, N-dimethylacetamide, diethyl ether, acetonitrile, cyclohexane, dichloromethane, acetone, ethanol and methanol.

Preferably, the molar concentration of the lithium/sodium salt is from 0.1 to 3M, preferably from 0.5 to 1.5M; the mass fraction of binder in the solution is 1-20 wt.%, preferably 5-10 wt.%; the drying temperature is 10-80 ℃, preferably 30-40 ℃.

The inert gas comprises various gases which do not react with the lithium/sodium salt, and comprises one or more of argon, nitrogen and helium.

The invention also provides a lithium/sodium secondary battery which comprises the functional composite diaphragm.

The components of the lithium/sodium secondary battery include a positive electrode, a lithium/sodium negative electrode, and an electrolyte, in addition to the functional composite separator.

The positive electrode is selected from a sulfur-carbon positive electrode, a selenium-carbon positive electrode, a lithium iron phosphate positive electrode, a lithium nickel cobalt manganese oxide positive electrode and a sodium nickelate positive electrode. The positive electrode may further include an active material, a conductive additive, a binder, and the like.

The active substance is one or more of a sulfur-coping black composite material, a sulfur-Super P composite material, a sulfur-carbon nano tube composite material, a sulfur-acidified carbon nano tube composite material, a selenium-coping black composite material, a selenium-Super P composite material, a selenium-carbon nano tube composite material, a selenium-acidified carbon nano tube composite material, lithium iron phosphate, lithium nickel cobalt manganese oxide and sodium nickelate; the conductive additive is one or more of Super P, ketjen black, graphene and a conductive carbon nano tube; the binder is one or more of polyvinylidene fluoride (PVDF), sodium carboxymethylcellulose (CMC), styrene butadiene rubber/sodium carboxymethylcellulose and Sodium Alginate (SA); the mass of the active substances in the sulfur-carbon and selenium-carbon positive electrode accounts for 70-99% of the total mass of the positive electrode, the mass of the conductive additive accounts for 0.5-20% of the total mass of the positive electrode, and the mass of the binder accounts for 0.5-20% of the total mass of the positive electrode.

Preferably, the negative electrode is one of metallic lithium and metallic sodium.

Preferably, the electrolyte solvent includes a first solvent selected from at least one of 1, 3 dioxolane, 1, 4 dioxane, tetrahydrofuran, trimethylolpropane triglycidyl ether, a second solvent selected from at least one of tetrahydropyran, ethylene glycol dimethyl ether, triethylene glycol dimethyl ether, methyl carbonate, ethylene carbonate, propylene carbonate, and an electrolyte; the electrolyte solute is at least one of lithium bis (oxalato) borate, lithium trifluoromethanesulfonate, lithium bis (trifluoromethanesulfonate) imide, lithium perchlorate and sodium perchlorate. In addition, the applicant of the present invention unexpectedly found that, in the system of the present invention, when the first solvent is trimethylolpropane triglycidyl ether and the second solvent is ethylene glycol dimethyl ether, the capacity retention rate is 90% or more regardless of whether the positive electrode material is selected from a sulfo-acidified carbon nanotube, a seleno-acidified carbon nanotube, lithium iron phosphate, or a nickel cobalt lithium manganate positive electrode.

In the electrolyte, the first solvent: the volume ratio of the second solvent is 1-2:1-2, and the concentration of the electrolyte solute is 0.5-1M.

The present invention also provides an energy storage device comprising the above lithium/sodium secondary battery.

Compared with the prior art, the functional composite diaphragm of the secondary battery provided by the invention has the following advantages: compared with the commonly used process of ' preparing precursor solution, injecting a battery core and ' in-situ gelation ' in the traditional electrolyte in-situ gelation, the method loads the catalyst on the diaphragm, and the diaphragm is used as a conventional component of the battery for storage and use. After the obtained functional composite diaphragm is assembled into a battery core as a secondary battery diaphragm, the lithium/sodium salt with catalytic effect can be slowly released in situ into the electrolyte to induce the electrolyte to be gelatinized. The invention changes the defects that the gelled precursor must be prepared just before use and is difficult to store, can realize the in-situ gelation of the electrolyte by the conventional assembly method, greatly simplifies the process, saves the cost and has high commercial prospect.

Drawings

Fig. 1 (a) is a surface SEM of the functional composite separator obtained in example 1, and fig. 1 (b) is a cross-sectional SEM of the functional composite separator obtained in example 1.

Fig. 2 (a) is a 0.1C charge-discharge curve of the lithium-sulfur battery obtained in example 1, and fig. 2 (b) is a 20-cycle capacity and coulombic efficiency of the lithium-sulfur battery obtained in example 1.

Fig. 3 (a) is a 0.1C charge-discharge curve of the lithium-sulfur battery obtained in example 12, and fig. 3 (b) is a 20-cycle capacity and coulombic efficiency of the lithium-sulfur battery obtained in example 12.

FIG. 4 is a 0.1C charge-discharge curve of the Li-Se battery obtained in example 18.

FIG. 5 is a 0.1C charge-discharge curve of the sodium battery obtained in example 21.

Fig. 6 (a) shows the 0.1C charge-discharge curve of the lithium-sulfur battery obtained in comparative example 1, and fig. 6 (b) shows the 20-cycle capacity and coulombic efficiency of the lithium-sulfur battery obtained in comparative example 1.

FIG. 7: schematic diagram of a functional composite separator for a secondary battery.

Detailed Description

The present invention will be further described with reference to the following specific examples.

The experimental methods described in the following examples are all conventional methods unless otherwise specified; the reagents and materials are commercially available.

Example 1

Preparation of (I) functional composite diaphragm and application of diaphragm in lithium-sulfur battery

Step 1) dissolving lithium difluorooxalato borate and polyvinylidene fluoride in acetonitrile under a high-purity argon atmosphere, wherein the molar concentration of the lithium difluorooxalato borate is 1M, the mass fraction of the polyvinylidene fluoride is 10 wt.%, and after the lithium difluorooxalato borate and the polyvinylidene fluoride are completely dissolved, scraping the solution on the surface of a commercial polyethylene diaphragm by using a scraper with a gap of 200 mu M. And (3) placing the diaphragm at 30 ℃ for drying treatment to obtain the composite diaphragm coated with the lithium difluoro-oxalato-borate on the surface. Fig. 1a is a surface SEM picture of the resulting composite separator, and fig. 1b is a cross-sectional SEM picture of the resulting composite separator.

Step 2) assembly of a lithium-sulfur battery using the composite separator: under high-purity argon, taking a sulfur-acidified carbon nano tube as a positive electrode active substance, Super P as a conductive additive, PVDF as a binder, and dropwise adding a1, 3-Dioxolane (DOL) solvent on the positive electrode side: ethylene glycol dimethyl ether (DME) (v/v ═ 1:1), an electrolyte solution with a solute of lithium bis (trifluoromethanesulfonate) imide (with a molar concentration of 1M), and then the composite separator obtained in step 1) and a lithium negative electrode were sequentially added and stacked in this order in a battery case.

And 3) completely sealing the battery case, and testing the battery performance after the in-situ polymerization is finished.

(II) Performance testing of lithium-sulfur batteries

The electrochemical performance of the cells was tested in a cell test system. The test temperature was 25o C, and the battery capacity and charge and discharge current were calculated as the mass of sulfur. Fig. 2 is a charge-discharge curve of the lithium-sulfur battery in example 1 at a rate of 0.1C, and the first-cycle discharge capacity, the first-cycle coulombic efficiency, the discharge capacity after 20 cycles, and the capacity retention rate thereof are recorded. The test results of the obtained battery are shown in table 1.

Example 2

The other conditions were the same as in example 1, except that the lithium/sodium salt used in step 1) was lithium hexafluorophosphate. All test results are listed in table 1.

Example 3

The other conditions were the same as in example 1, except that the lithium/sodium salt used in step 1) was lithium tetrafluoroborate. All test results are listed in table 1.

Example 4

The other conditions were the same as in example 1, except that the lithium/sodium salt used in step 1) was the same. All test results are listed in table 1.

Example 5

The other conditions were the same as in example 2, except that the binder used in step 1) was polyethylene oxide. All test results are listed in table 1.

Example 6

The other conditions were the same as in example 2 except that the binder used in step 1) was polyvinyl alcohol. All test results are listed in table 1.

Example 7

The other conditions were the same as in example 5 except that the molar concentration of lithium/sodium salt in the solution of step 1) was 0.5M, and the thickness of the resulting coating film was 2 μ M.

Example 8

The other conditions were the same as in example 5 except that the molar concentration of lithium/sodium salt in the solution of step 1) was 1.5M, and the thickness of the resulting coating film was 10 μ M.

Example 9

The other conditions were the same as in example 5 except that the general battery separator used in step 1) was a polyethylene/polypropylene separator. All test results are listed in table 1.

Example 10

The other conditions were the same as in example 5 except that the general battery separator used in step 1) was an alumina polyethylene separator. All test results are listed in table 1.

Example 11

The other conditions were the same as in example 5 except that the ordinary separator used in step 1) was a cellulose film. The test results for the obtained batteries are shown in table 1.

Example 12

The other conditions were the same as in example 5 except that the general separator used in step 1) was a ceramic cellulose membrane. The test results for the obtained batteries are shown in table 1.

Example 13

The same as example 10 except that the electrolyte solvent used in step 2) was 1, 4-dioxane: ethylene glycol dimethyl ether (DME) (v/v ═ 1: 1). All test results are listed in table 1.

Example 14

The other conditions were the same as in example 10, except that the electrolyte solvent used in step 2) was tetrahydrofuran: ethylene glycol dimethyl ether (DME) (v/v ═ 1: 1). All test results are listed in table 1.

Example 15

The other conditions were the same as in example 10, except that the electrolyte solvent used in step 2) was trimethylolpropane triglycidyl ether: ethylene glycol dimethyl ether (DME) (v/v ═ 1: 1). All test results are listed in table 1.

Example 16

The other conditions were the same as in example 15, except that the positive electrode selected in step 2) was a selenium-acidified carbon nanotube positive electrode. All test results are listed in table 1.

Example 17

The other conditions are the same as those in example 15, except that the positive electrode selected in step 2) is a lithium iron phosphate positive electrode. All test results are listed in table 1.

Example 18

The other conditions are the same as those of the example 15, except that the positive electrode selected in the step 2) is a lithium nickel cobalt manganese oxide positive electrode. All test results are listed in table 1.

Example 19

The other conditions were the same as in example 15 except that

(1) The lithium/sodium salt selected in the step 1) is sodium hexafluorophosphate;

(2) the anode selected in the step 2) is a sodium nickelate anode, and the electrolyte solvent is 1, 3 Dioxolane (DOL): propylene Carbonate (PC) (v/v 1:1), the solute was sodium perchlorate (1M), and the selected negative electrode was sodium metal. All test results are listed in table 1.

Example 20

The other conditions were the same as in example 5 except that the molar concentration of lithium/sodium salt in the solution of step 1) was 3M, and the thickness of the resulting coating film was 17 μ M.

Comparative example 1

Preparation of lithium-sulfur Battery Using untreated Normal Battery separator

Step 1) preparation of an electric core: and sequentially stacking a sulfur-acidized carbon nanotube composite material anode, a polyethylene/polypropylene diaphragm and a metal lithium cathode in a battery shell under high-purity argon to form a battery cell to be injected.

Step 2), liquid injection: the solvent is trimethylolpropane triglycidyl ether: 1, 3 Dioxolane (DOL): ethylene glycol dimethyl ether (DME) (v/v/v ═ 1:1:1), the cell obtained in step 1) was infiltrated with an electrolyte solution whose solute was lithium bis (trifluoromethanesulfonate), and the cell was subsequently encapsulated.

(II) chemical performance test of lithium-sulfur battery

The electrochemical performance of the battery is tested in a battery test system, and the test voltage interval is 1.8-3V. The test temperature was 25o C, and the battery capacity and charge and discharge current were calculated as the mass of sulfur.

Comparative example 2

Preparation of lithium selenium battery using untreated common battery separator

Step 1) preparation of an electric core: and sequentially stacking a selenium-acidified carbon nanotube composite material anode, a polyethylene/polypropylene diaphragm and a metal lithium cathode in a battery shell under high-purity argon to form a battery core to be injected.

Chemical performance test of lithium selenium battery

Comparative example 3

Preparation of sodium Battery Using untreated common Battery separator

Step 1) preparation of an electric core: and sequentially stacking a sodium nickelate material anode, a polyethylene/polypropylene diaphragm and a metal sodium cathode in a battery shell under high-purity argon to form a battery cell to be injected.

Step 2), liquid injection: the solvent is trimethylolpropane triglycidyl ether: 1, 3 Dioxolane (DOL): Propylene Carbonate (PC) (v/v/v 1:1:1), and the electrolyte with a solute of sodium perchlorate soaks the cell obtained in step 1), and the cell is then encapsulated.

Comparative example 4

The other conditions were the same as in example 1, except that the electrolyte solvent used in step 2) was triethylene glycol dimethyl ether: ethylene glycol dimethyl ether (DME) (v/v ═ 1: 1).

Comparative example 5

The other conditions were the same as in example 5, except that the electrolyte solvent used in step 2) was triethylene glycol dimethyl ether: ethylene glycol dimethyl ether (DME) (v/v ═ 1: 1).

Comparative example 6

The other conditions were the same as in example 5, except that lithium hexafluorophosphate was replaced with lithium dioxalate borate in the step 1), to obtain a composite separator having a lithium dioxalate borate coating layer.

Chemical performance test of sodium battery

And testing the electrochemical performance of the battery in a battery testing system, wherein the testing voltage interval is 2-4V. The test temperature was 25o C, and the battery capacity and charge and discharge current were calculated as the mass of sulfur.

Table 1 was analyzed: based on the data of examples 1-4, it is demonstrated that: the functionality of the composite separator depends on the kind of the coated lithium salt, which must have specificity because it needs to have an effect of catalyzing cationic polymerization, such as lithium difluorooxalato borate, lithium tetrafluoroborate, lithium hexafluorophosphate, etc. the composite separator has an effect of significantly catalyzing the in-situ gelation of the electrolyte, inhibiting the dissolution and shuttling of polysulfides, and further example 19, a sodium salt containing a lewis acid group, such as sodium hexafluorophosphate, sodium tetrafluoroborate, a functional composite separator for a sodium secondary battery, also has a catalytic effect; (2) according to examples 2, 5 and 6, if the binder itself has a function of conducting lithium ions, such as polyethylene oxide and polyvinylidene fluoride, the capacity of the lithium-sulfur battery can be favorably exerted, and the stronger the ionic conductivity of the binder, such as polyethylene oxide with higher ionic conductivity, the more favorably the battery performance can be exerted. (3) According to the embodiments 5, 7, 8 and 20, it can be seen that the lithium/sodium salt concentration affects the thickness of the coating and further affects the effect of the lithium/sodium salt coating layer, the coating layer is too thin, and after the slow release is finished, the content of the catalyst in the electrolyte is low, so that the gelation effect of the electrolyte is not obvious, and the barrier capability to polysulfide is poor; if the coating layer is too thick, the slow release is not complete enough, and the redundant coating layer blocks the ion transmission, so that the battery capacity exertion is influenced. (4) As can be seen from examples and comparative examples 4 and 5, the electrolyte solvent for a lithium sulfur battery, its own polymerization activity, affects the effect of gelation: solvent molecules which do not have polymerization activity, such as tetrahydropyrane, ethylene glycol dimethyl ether, triethylene glycol dimethyl ether, methyl carbonate, ethylene carbonate and propylene carbonate, cannot generate gelation transformation under the catalytic action of the functional composite membrane, and therefore do not have the function of blocking polysulfide dissolution; in addition, solvent molecules that can be polymerized and are liable to form a crosslinked state after polymerization, such as trimethylolpropane triglycidyl ether, are more advantageous for maintaining the stability of the battery. (5) Therefore, lithium hexafluorophosphate containing Lewis acid radicals and having a remarkable catalytic effect is preferably selected as a coating layer, polyethylene oxide with good ionic conductivity is selected as a binder, and the thickness of the coating layer is moderate (about 5 μm); the trimethylolpropane triglycidyl ether which can be polymerized and is easy to form a cross-linked state after polymerization is selected as one of solvents and a polymerization monomer, so that the composite diaphragm with the most excellent performance and the battery using the diaphragm can be obtained. (6) Comparing all the above examples with comparative example 1, it can be seen that the functional lithium/sodium salt layer is coated on the surface of the common diaphragm, and the obtained composite diaphragm can induce gelation of the electrolyte in situ, inhibit dissolution and shuttle effects of dendrites of the metal cathode and positive electrode polysulfide (selenide), and greatly improve the cycle stability of the secondary battery.

In conclusion, the invention creatively coats the lithium/sodium salt capable of catalyzing the gelation of the electrolyte on the surface of the commercial diaphragm to obtain the functional composite diaphragm. When the composite diaphragm is used for assembling a secondary battery core and injecting electrolyte, the composite diaphragm is contacted with electrode liquid to release lithium/sodium salt with a catalytic effect through dissolution, so that the effect of catalyzing the in-situ gelation of the electrolyte is achieved. Thereby greatly inhibiting the dissolution and shuttle effects of dendritic crystals of the metal cathode growing in polysulfide (selenide) compounds and improving the cycle stability of various secondary batteries. The method has the advantages of simple operation, easily obtained raw materials and obvious effect, and is suitable for commercial and large-scale production.

The above embodiments are merely illustrative of the present disclosure and do not represent a limitation of the present disclosure. Other variations of the specific structure of the invention will occur to those skilled in the art.

Claims

1. A lithium/sodium secondary battery comprises a lithium/sodium secondary battery functional composite diaphragm, a positive electrode, a negative electrode and electrolyte;

the functional composite diaphragm is composed of a common battery diaphragm and a lithium/sodium salt coating layer loaded on the surface, the lithium salt or sodium salt coating layer can catalyze the lithium/sodium secondary battery electrolyte to generate in-situ cationic polymerization and gelation transformation, and the lithium/sodium salt coating layer is selected from at least one of lithium difluorooxalato borate, lithium tetrafluoroborate and sodium tetrafluoroborate;

the electrolyte solvent comprises a first solvent, a second solvent and an electrolyte, wherein the first solvent is trimethylolpropane triglycidyl ether, and the second solvent is ethylene glycol dimethyl ether; the electrolyte solute is at least one of lithium bis (oxalato) borate, lithium trifluoromethanesulfonate, lithium bis (trifluoromethanesulfonate) imide, lithium perchlorate and sodium perchlorate;

the volume ratio of the first solvent to the second solvent is 1:1, and the concentration of the electrolyte solute is 0.5-1M.

2. The lithium/sodium secondary battery according to claim 1, wherein the general battery separator is selected from a polyethylene separator, a polypropylene separator, a polyethylene/polypropylene separator, an alumina polyethylene separator, a ceramic fiber paper separator.

3. The lithium/sodium secondary battery according to claim 1, characterized in that the lithium/sodium salt coating layer has a thickness of 1 μm to 30 μm.

4. The lithium/sodium secondary battery according to claim 3, wherein the lithium/sodium salt coating layer has a thickness of 5 μm to 10 μm.

5. The lithium/sodium secondary battery as claimed in any one of claims 1 to 4, wherein the composite separator is prepared by a preparation method comprising the steps of:

6. The lithium/sodium secondary battery according to claim 5, wherein the binder is one or more selected from polyvinylidene fluoride, carboxymethyl cellulose, sodium carboxymethyl cellulose, styrene-butadiene rubber, sodium alginate, polyethylene oxide, polyvinyl alcohol, polytetrafluoroethylene, and polyamide; the organic solvent is selected from one or more of dimethyl sulfoxide, N-dimethylformamide, N-dimethylacetamide, diethyl ether, acetonitrile, cyclohexane, dichloromethane, acetone, ethanol and methanol.