Pyrophosphate crosslinker and preparation method thereof, semi-solid electrolyte and battery

Abstract

The present invention relates to the field of semi-solid electrolyte technology, and more particularly to a pyrophosphate crosslinker, a preparation method thereof, a semi-solid electrolyte, and a battery. This pyrophosphate crosslinker, a symmetrical structure and a phosphate crosslinker containing two phosphorus atoms, not only exhibits excellent flame retardancy when used in semi-solid electrolytes, but also enables batteries made from it to achieve a first-cycle capacity of over 100 mAh / g at 0.2C, high capacity retention after 500 cycles, and excellent electrical performance.

Description

Technical Field

[0001] The present invention relates to the technical field of semi-solid electrolytes, and in particular to a pyrophosphate cross-linking agent and a preparation method thereof, a semi-solid electrolyte and a battery. Background Art

[0002] With the frequent occurrence of battery fire accidents, how to improve the safety performance of batteries has become the current research focus. Traditional liquid batteries contain a large amount of free liquids. These liquids have disadvantages such as high volatility, low flash point, and easy leakage, which can easily cause safety accidents such as battery combustion and explosion. In order to make up for the shortcomings of commercial liquid electrolytes, people have taken a lot of measures: 1) Introducing flame retardant additives into commercial liquid electrolytes to enhance the flame retardant properties of batteries; 2) Replacing liquid electrolytes with all-solid electrolytes or semi-solid electrolytes. Among them, the ability to suppress battery combustion is very limited when only flame retardant additives are introduced into liquid electrolytes; and all-solid electrolytes have electrical performance defects such as large interface resistance and poor lithium ion transmission performance, which are difficult to commercialize; therefore, at this stage, semi-solid electrolytes are most likely to replace commercial liquid electrolytes.

[0003] Introducing flame-retardant groups into semi-solid electrolytes can further enhance battery safety. Phosphate-based organic compounds are a common flame retardant. On the one hand, phosphate decomposes when heated, forming a sticky film of phosphoric acid and polyphosphoric acid on the surface of the material, thus achieving a flame retardant effect. On the other hand, phosphate generates a large number of phosphorus oxide free radicals when heated, which can combine with hydrogen radicals and hydroxyl radicals and quench them. Applying phosphate-based organic compounds to semi-solid electrolytes can further enhance battery safety and has become a hot topic in current research. However, the currently prepared phosphate-based semi-solid electrolytes mainly include TAEP, PFTnGA, (M)AnEPP, (M)AnEMP, etc. These electrolyte structure fragments contain only one phosphorus atom, which limits the flame retardant effect.

[0004] Patent document with publication number CN111499663A discloses a phosphate cross-linking agent and its preparation method, a phosphate-based cross-linked gel polymer electrolyte and its preparation method and application. In some embodiments, spiropentaerythritol diphosphoryl chloride and an acrylate compound are combined to prepare a phosphate cross-linking agent containing diphosphorus atoms. This cross-linking agent is added to a sodium ion liquid electrolyte at a certain concentration to have flame retardancy, but the electrical properties, such as specific capacity, and flexibility still need to be improved.

[0005] Patent document CN104380519A discloses a secondary battery whose electrolyte contains a phosphate-based compound as an additive. In some embodiments, the phosphate-based compound is a pyrophosphate-based acrylate. The electrolyte using this additive requires the addition of a flame retardant to improve flame retardancy, indicating that the inherent flame retardancy is poor. Furthermore, the pyrophosphate-based acrylate is acidic, which not only induces the dissolution of transition metals in the positive electrode material and the loss of positive electrode active material, but also promotes the continuous decomposition of the SEI film, which is not conducive to the formation of a stable interfacial SEI film. Both of these effects can reduce the battery's cycling performance. Summary of the Invention

[0006] The present invention aims to solve the above problems and provides a pyrophosphate crosslinking agent having good flame retardancy and electrical properties, a preparation method thereof, a semi-solid electrolyte and a battery.

[0007] The technical solution to solve the problem of the present invention is: first , providing a pyrophosphate crosslinking agent, characterized in that it is a compound with the following structural formula:

[0008]

[0009] The present application provides a symmetrically structured phosphate crosslinker containing two phosphorus atoms. When used in a semi-solid electrolyte, it not only has a good flame retardant effect, but also the battery prepared therefrom has a 0.2C first-week capacity of more than 100 mAh / g, a high capacity retention rate after 500 cycles, and good electrical performance.

[0010] The inventor speculates that this may be because the spiro ring in the structure is removed while retaining the symmetrical structure and the diphosphorus atom. In terms of electrical properties, the spiro ring has structural rigidity and stability, which may limit the movement and distribution of charge inside the spiro ring, affecting the electrochemical properties of the material; it may also be due to the connection between the diphosphorus atoms in the present application through the oxygen group, which has a strong polarity and is easy to interact with other polar molecules or ions, thereby facilitating the accumulation and release of charge and improving the electrochemical properties of the material. In terms of flame retardancy, except for the effect of the diphosphorus atom, the high molecular weight compound of the symmetrical structure may not be easy to migrate, has good thermal stability, has a strong endothermic effect, and helps to improve the flame retardant effect.

[0011] Here, n is a natural number and n≥0, for example, n can be 0, 1, 2, 3, 4, 5, etc.

[0012] As a preferred embodiment of the present invention, n=3; when n=3, the resulting pyrophosphate crosslinker, when used in batteries, exhibits optimal electrical and flame retardant properties. This may be because ethoxy segments of a certain length facilitate ion transport. However, as n increases, the proportion of phosphate flame-retardant segments decreases, suppressing the flame retardant effect and weakening the battery's safety performance.

[0013] R is hydrogen or methyl, and the difference in R groups has no effect on the electrical properties of the battery.

[0014] Secondly The present application also provides a method for preparing the above-mentioned pyrophosphate cross-linking agent, comprising the following steps:

[0015] S1. In the presence of an acid scavenger, reacting an ethylene glycol compound and an acryloyl chloride compound in a solvent to obtain an intermediate product;

[0016] S2. In the presence of an acid scavenger, reacting the intermediate product with pyrophosphoryl chloride in a solvent to obtain the pyrophosphate crosslinking agent.

[0017] In step S1 , ethylene glycol compounds are compounds with oxygen atoms and hydroxyl groups connected to ethyl groups, and their general structural formula is as follows:

[0018]

[0019] As described above, n is a natural number and n ≥ 0. As a preferred embodiment of the present invention, the glycol compound is selected from at least one of ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, pentaethylene glycol, and hexaethylene glycol. Furthermore, as a preferred embodiment of the present invention, as described above, n = 3, meaning tetraethylene glycol is preferably used as the raw material.

[0020] The general structural formula of acryloyl chloride compounds is as follows:

[0021]

[0022] As mentioned above, R is hydrogen or methyl. As a preferred embodiment of the present invention, the acryloyl chloride compound is selected from at least one of acryloyl chloride and methacryloyl chloride.

[0023] The reaction equations involved are:

[0024]

[0025] The acid scavenger is used to neutralize acidic byproducts such as HCl produced by the reaction. Preferably, the acid scavenger is selected from at least one of triethylamine, potassium carbonate, sodium carbonate, pyridine, and N,N-dimethylaniline. To avoid the introduction of impurities, preferably, the acid scavenger is triethylamine.

[0026] In this reaction, the acid scavenger needs to be in an appropriate amount and the ethylene glycol compound needs to be in an excess amount. As a preferred embodiment of the present invention, the molar ratio of the ethylene glycol compound, the acryloyl chloride compound, and the acid scavenger is (1.1-2):1:1. For example, it can be 1.1:1:1, 1.2:1:1, 1.3:1:1, 1.4:1:1, 1.5:1:1, 1.6:1:1, 1.7:1:1, 1.8:1:1, 1.9:1:1, 2.0:1:1; preferably, it is 1.5:1:1.

[0027] In this reaction, due to the high activity of acrylic acid chloride compounds, as a preferred method of the present invention, during operation, acrylic acid chloride compounds are slowly added dropwise to the mixed solution of ethylene glycol compounds and acid scavengers to avoid excessive local concentration caused by adding too much at one time and producing side reactions.

[0028] In this reaction, the reaction environment temperature is preferably low to prevent unnecessary conversion of sensitive functional groups in the reactants, avoid volatilization of the reactants, and slow down the reaction rate, making the reaction more controllable and reducing the occurrence of side reactions. As a preferred embodiment of the present invention, the reaction environment temperature is 0 to 10°C, such as 0°C, 1°C, 2°C, 3°C, 4°C, 5°C, 6°C, 7°C, 8°C, 9°C, 10°C; preferably 4°C.

[0029] In this reaction, the solvent should be stable and well-dissolved in the ethylene glycol compound and the acryloyl chloride compound to ensure that the reactants are fully mixed and contacted in the solvent. Preferably, the solvent is selected from at least one of dichloromethane, tetrahydrofuran, chloroform, and diethyl ether. Furthermore, preferably, the solvent is tetrahydrofuran.

[0030] After the reaction, the intermediate product obtained should be washed and purified before being used in step S2. As a preferred method of the present invention, after the reaction is completed, the mixed system is washed with water and dichloromethane solvent in sequence, dried with calcium chloride, and the solvent is removed. The final intermediate product is accurately purified by column chromatography to obtain the final intermediate product.

[0031] In step S2 , the reaction equation involved is:

[0032]

[0033] In this reaction, an acid scavenger is also used to neutralize acidic byproducts such as HCl produced by the reaction. Preferably, the acid scavenger is selected from at least one of triethylamine, sodium carbonate, potassium carbonate, pyridine, and N,N-dimethylaniline. To avoid the introduction of impurities, preferably, triethylamine is used as the acid scavenger.

[0034] In this reaction, the acid scavenger needs to be in an appropriate amount and the intermediate product needs to be in excess. As a preferred embodiment of the present invention, the molar ratio of the intermediate product, pyrophosphoryl chloride, and acid scavenger is (2.5-3.5):0.5:2. For example, it can be 2.5:0.5:2, 2.6:0.5:2, 2.7:0.5:2, 2.8:0.5:2, 2.9:0.5:2, 3.0:0.5:2, 3.1:0.5:2, 3.2:0.5:2, 3.3:0.5:2, 3.4:0.5:2, 3.5:0.5:2; preferably 3:0.5:2.

[0035] In this reaction, the reaction environment temperature is preferably low to prevent unnecessary conversion of sensitive functional groups in the reactants, avoid volatilization of the reactants, and slow down the reaction rate, making the reaction more controllable and reducing the occurrence of side reactions. As a preferred embodiment of the present invention, the reaction environment temperature is 0 to 10°C, such as 0°C, 1°C, 2°C, 3°C, 4°C, 5°C, 6°C, 7°C, 8°C, 9°C, 10°C; preferably 4°C.

[0036] In this reaction, the solvent should be stable and capable of dissolving the intermediate product and pyrophosphoryl chloride well to ensure that the reactants are fully mixed and contacted in the solvent. As a preferred embodiment of the present invention, the solvent is selected from at least one of dichloromethane, tetrahydrofuran, chloroform, and diethyl ether. Further preferred embodiment of the present invention, the solvent is tetrahydrofuran.

[0037] After the reaction, the obtained pyrophosphate crosslinker should be washed and purified. As a preferred method of the present invention, after the reaction is completed, the mixed system is washed with water and dichloromethane solvents in sequence, dried with calcium chloride, and then the solvent is removed and accurately purified by column chromatography to obtain the final pyrophosphate crosslinker.

[0038] Secondly Another object of the present invention is to provide a semi-solid electrolyte using the pyrophosphate crosslinker. The semi-solid electrolyte is prepared by mixing the pyrophosphate crosslinker with an acrylate monomer, an electrolyte, and an initiator to obtain a precursor, and initiating a polymerization reaction of the precursor to obtain an electrolyte.

[0039] In this application, the above-mentioned pyrophosphate crosslinker is used to obtain a semi-solid electrolyte. Compared with existing liquid electrolytes, it is less likely to react with electrodes, does not leak and cause safety accidents, has good flame retardancy, and has high safety. At the same time, compared with some semi-solid electrolytes in the prior art, it has improved flexibility and electrical performance.

[0040] Acrylate monomers are used to complement the electrochemical and mechanical properties of the pyrophosphate crosslinker, improving overall performance. The choice of acrylate monomer is not limited, as long as it can be used in the electrolyte and can be polymerized. Preferably, the acrylate monomer is selected from at least one of methyl acrylate, ethyl acrylate, butyl acrylate, and 2-ethylethyl acrylate. Furthermore, preferably, the acrylate monomer is methyl acrylate.

[0041] The amount of pyrophosphate crosslinker and acrylate monomer should be limited. If the pyrophosphate crosslinker content is too high, the electrolyte will have insufficient elasticity and toughness and poor stability, resulting in reduced electrical performance (capacity retention after 500 cycles). If the pyrophosphate crosslinker content is too low, free liquid will be present, and a semi-solid electrolyte cannot be formed, which can easily cause battery leakage and cause safety issues. As a preferred embodiment of the present invention, the molar ratio of the pyrophosphate crosslinker to the acrylate monomer is (0.5-2):1, for example, 0.5:1, 0.6:1, 0.7:1, 0.8:1, 0.9:1, 1.0:1, 1.1:1, 1.2:1, 1.3:1, 1.4:1, 1.5:1, 1.6:1, 1.7:1, 1.8:1, 1.9:1, 2.0:1; preferably 1:1.

[0042] As a preferred embodiment of the present invention, the electrolyte is an ester liquid electrolyte. Specifically, the ester liquid electrolyte includes a metal salt and an ester solvent. The metal salt can be selected from at least one of a sodium salt and a lithium salt. The ester solvent can be selected from at least one of dimethyl carbonate, ethyl acetate, diethyl carbonate, propylene carbonate, and ethylene carbonate.

[0043] The amount of electrolyte used should be limited. If the amount of electrolyte used is too high, the polymer cannot effectively wrap the liquid electrolyte, free liquid will exist, and a semi-solid electrolyte cannot be formed, which is prone to battery leakage and causes safety problems. In addition, if the amount of electrolyte used is too high, the proportion of phosphorus atoms in the precursor will be low, which will affect flame retardancy and further reduce safety performance. If the amount of electrolyte used is too low, the electrolyte density will increase, affecting conductivity. As a preferred embodiment of the present invention, the mass fraction of the electrolyte in the precursor is 80% to 95%, for example, it can be 80%, 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%; preferably 90%.

[0044] The choice of initiator is not limited, as long as it can form free radicals to react with the monomer. Preferably, the initiator is selected from at least one of azobisisobutyronitrile (AIBN), azobisisoheptanenitrile (ABVN), benzoyl peroxide, and diisopropyl peroxydicarbonate. Further preferably, the initiator is azobisisobutyronitrile.

[0045] The amount of initiator used should be limited. When the initiator content is too low, complete curing cannot be achieved, and leakage may occur. When the initiator content is too high, the excess initiator does not contribute to the overall electrolyte and does not improve performance, which is equivalent to introducing impurities. As a preferred embodiment of the present invention, the initiator is used in an amount of 0.1% to 0.5% of the total amount of the pyrophosphate crosslinker and acrylate monomer, for example, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, and preferably 0.3%.

[0046] The initiator decomposes at a certain temperature to form free radicals, which react with the monomers through free radical polymerization to obtain a semi-solid electrolyte. As a preferred embodiment of the present invention, the initiation temperature is 40 to 80°C, preferably 50 to 70°C, preferably 55 to 65°C, such as 55°C, 56°C, 57°C, 58°C, 59°C, 60°C, 61°C, 62°C, 63°C, 64°C, 65°C, and preferably 60°C.

[0047] at last Another object of the present invention is to provide a battery using the aforementioned semi-solid electrolyte. The battery is prepared by the following steps: assembling the electrolyte precursor into an ion battery, and polymerizing the electrolyte at an initiation temperature to obtain a semi-solid battery.

[0048] As a preferred embodiment of the present invention, the battery is prepared by the following steps: coating the positive electrode active material, conductive material, and binder on the current collector and then drying to obtain the positive electrode; installing the positive electrode and the negative electrode into the battery shell; injecting the precursor into the battery shell and sealing it; and polymerizing at the initiation temperature to obtain a semi-solid battery.

[0049] As a preferred embodiment of the present invention, the positive electrode active material includes at least one of sodium vanadium phosphate, sodium iron phosphate, lithium cobaltate, and lithium iron phosphate, preferably sodium vanadium phosphate; the conductive material includes at least one of carbon black and conductive graphite, preferably carbon black; the binder includes at least one of PVDF and C-PVDF, preferably PVDF.

[0050] Beneficial effects of the present invention:

[0051] 1. The present application provides a pyrophosphate crosslinking agent and a preparation method thereof. This pyrophosphate can be used in semi-solid electrolytes to provide good flame retardancy and electrical properties.

[0052] 2. The present application provides an electrolyte and a battery using the above-mentioned pyrophosphate crosslinker. The obtained electrolyte has been comprehensively improved in terms of flame retardancy, flexibility, and electrical properties, and the battery made from this electrolyte is non-flammable and has high safety performance; at the same time, the first-week capacity at 0.2C reaches more than 100mAh / g, the capacity retention rate is high after 500 cycles, and it has good electrical performance. DETAILED DESCRIPTION

[0053] The following are specific embodiments of the present invention and further describe the technical solutions of the present invention, but the present invention is not limited to these embodiments.

[0054] Example 1

[0055] A pyrophosphate crosslinker , is a compound with the following structural formula (n=0, R is hydrogen):

[0056]

[0057] This pyrophosphate crosslinker is prepared by the following steps:

[0058] S1. Triethylamine is used as the scavenger, and ethylene glycol and acryloyl chloride are used as the raw materials. At 4°C, 0.15 mol of ethylene glycol is poured into a mixed solution of 100 mL of tetrahydrofuran and 0.1 mol of triethylamine, and 0.1 mol of acryloyl chloride is slowly added dropwise. Stirring is continued until the acryloyl chloride reacts completely. The mixture is washed sequentially with deionized water and dichloromethane, dried over calcium chloride, and the solvent is removed. Purification by column chromatography is performed to obtain the final intermediate product, hydroxyethyl acryloyl ester.

[0059] S2. Triethylamine is used as the acid scavenger. At 4°C, 0.3 mol of hydroxyethyl acryloyl ester is poured into a mixed solution of 100 mL of tetrahydrofuran and 0.2 mol of triethylamine, and 0.05 mol of pyrophosphoryl chloride is slowly added dropwise. After the addition is complete, stirring is continued until the pyrophosphoryl chloride reacts completely. The mixture is washed sequentially with deionized water and dichloromethane, dried with calcium chloride, and the solvent is removed. The mixture is then accurately purified by column chromatography to obtain the final pyrophosphate crosslinker.

[0060] Example 2

[0061] A pyrophosphate crosslinker , is a compound with the following structural formula (n=3, R is hydrogen):

[0062]

[0063] This pyrophosphate crosslinker is prepared by the following steps:

[0064] S1. The acid scavenger is triethylamine, and the raw materials are tetraethylene glycol and acryloyl chloride. At 4°C, 0.15 mol of tetraethylene glycol is poured into a mixed solution of 100 mL of tetrahydrofuran and 0.1 mol of triethylamine, and 0.1 mol of acryloyl chloride is slowly added dropwise. Stirring is continued until the acryloyl chloride reacts completely. The mixture is washed sequentially with deionized water and dichloromethane, dried with calcium chloride, and the solvent is removed. The final intermediate product is then accurately purified by column chromatography.

[0065] S2. Triethylamine is used as the acid scavenger. At 4°C, 0.3 mol of the intermediate product is poured into a mixed solution of 100 mL of tetrahydrofuran and 0.2 mol of triethylamine, and 0.05 mol of pyrophosphoryl chloride is slowly added dropwise. After the addition is complete, stirring is continued until the pyrophosphoryl chloride reacts completely. The mixture is washed sequentially with deionized water and dichloromethane, dried with calcium chloride, and the solvent is removed. Purification by column chromatography is then performed to obtain the final pyrophosphate crosslinker.

[0066] Example 3

[0067] A pyrophosphate crosslinker , is a compound with the following structural formula (n=3, R is methyl):

[0068]

[0069] This pyrophosphate crosslinker is prepared by the following steps:

[0070] S1. The acid scavenger is triethylamine, and the raw materials are tetraethylene glycol and methacryloyl chloride. At 4°C, 0.15 mol of tetraethylene glycol is poured into a mixed solution of 100 mL of tetrahydrofuran and 0.1 mol of triethylamine, and 0.1 mol of methacryloyl chloride is slowly added dropwise. Stirring is continued until the methacryloyl chloride reacts completely. The mixture is washed sequentially with deionized water and dichloromethane, dried with calcium chloride, and the solvent is removed. The final intermediate product is accurately purified by column chromatography.

[0071] S2. Triethylamine is used as the acid scavenger. At 4°C, 0.3 mol of the intermediate product is poured into a mixed solution of 100 mL of tetrahydrofuran and 0.2 mol of triethylamine, and 0.05 mol of pyrophosphoryl chloride is slowly added dropwise. After the addition is complete, stirring is continued until the pyrophosphoryl chloride reacts completely. The mixture is washed sequentially with deionized water and dichloromethane, dried with calcium chloride, and the solvent is removed. Purification by column chromatography is then performed to obtain the final pyrophosphate crosslinker.

[0072] Example 4

[0073] Semi-solid electrolyte and battery , prepared by the following steps:

[0074] Electrolyte precursor preparation: 12.300 g of liquid ester electrolyte, 2.672 g (0.0047 mol) of the pyrophosphate crosslinker prepared in Example 1, 0.403 g (0.0047 mol) of methyl acrylate, and 0.009 g of AIBN were mixed to obtain a semi-solid electrolyte precursor solution. The mass fraction of the electrolyte in the precursor was 80%.

[0075] Battery Assembly: 0.8g of sodium vanadium phosphate (NVP) powder, 0.1g of carbon black, and 0.1g of PVDF were added to an agate mortar and pestle, followed by an appropriate amount of NMP solvent and ground until uniform. The slurry was then evenly coated on the surface of aluminum foil (the positive electrode current collector) and vacuum-dried at 60°C for 12 hours. The dried electrode sheet was cut into 12mm diameter discs and compacted to form the positive electrode. A button cell was assembled using the sodium foil as the counter electrode and the aforementioned semi-solid electrolyte precursor solution as the electrolyte.

[0076] Curing of the electrolyte: The assembled battery was heated and cured at 60°C for 1 hour to obtain the final semi-solid-state battery.

[0077] Example 5

[0078] This embodiment is substantially the same as embodiment 4, except that the molar ratio of the pyrophosphate crosslinking agent to methyl acrylate is 0.5:1.

[0079] Specifically, 12.300 g of liquid ester electrolyte, 2.362 g (0.0041 mol) of the pyrophosphate crosslinker prepared in Example 1, 0.713 g (0.0083 mol) of methyl acrylate, and 0.009 g of AIBN were mixed to obtain a semi-solid electrolyte precursor solution.

[0080] Example 6

[0081] This embodiment is substantially the same as embodiment 4, except that the molar ratio of the pyrophosphate crosslinking agent to methyl acrylate is 2:1.

[0082] Specifically, 12.300 g of liquid ester electrolyte, 2.859 g (0.0050 mol) of the pyrophosphate crosslinker prepared in Example 1, 0.216 g (0.0025 mol) of methyl acrylate, and 0.009 g of AIBN were mixed to obtain a semi-solid electrolyte precursor solution.

[0083] Example 7

[0084] This embodiment is basically the same as embodiment 4, with the only difference being that the mass fraction of the electrolyte in the precursor is 90%.

[0085] Specifically, 13.838 g of liquid ester electrolyte, 1.336 g (0.0023 mol) of the pyrophosphate crosslinker prepared in Example 1, 0.202 g (0.0023 mol) of methyl acrylate, and 0.005 g of AIBN were mixed to obtain a semi-solid electrolyte precursor solution.

[0086] Example 8

[0087] This embodiment is basically the same as embodiment 4, with the only difference being that the mass fraction of the electrolyte in the precursor is 95%.

[0088] Specifically, 14.606 g of liquid ester electrolyte, 0.668 g (0.0012 mol) of the pyrophosphate crosslinker prepared in Example 1, 0.101 g (0.0012 mol) of methyl acrylate, and 0.002 g of AIBN were mixed to obtain a semi-solid electrolyte precursor solution.

[0089] Example 9

[0090] Semi-solid electrolyte and battery , prepared by the following steps:

[0091] Electrolyte precursor preparation: 13.838 g of liquid ester electrolyte, 1.426 g (0.0013 mol) of the pyrophosphate crosslinker prepared in Example 2, 0.112 g (0.0013 mol) of methyl acrylate, and 0.005 g of AIBN were mixed to obtain a semi-solid electrolyte precursor solution. The mass fraction of the electrolyte in the precursor was 90%.

[0092] Battery Assembly: 0.8g of sodium vanadium phosphate (NVP) powder, 0.1g of carbon black, and 0.1g of PVDF were added to an agate mortar and pestle, followed by an appropriate amount of NMP solvent and ground until uniform. The slurry was then evenly coated on the surface of aluminum foil (the positive electrode current collector) and vacuum-dried at 60°C for 12 hours. The dried electrode sheet was cut into 12mm diameter discs and compacted to form the positive electrode. A button cell was assembled using the sodium foil as the counter electrode and the aforementioned semi-solid electrolyte precursor solution as the electrolyte.

[0093] Curing of the electrolyte: The assembled battery was heated and cured at 60°C for 1 hour to obtain the final semi-solid-state battery.

[0094] Example 10

[0095] Semi-solid electrolyte and battery , prepared by the following steps:

[0096] Electrolyte precursor preparation: 13.838 g of liquid ester electrolyte, 1.431 g (0.0012 mol) of the pyrophosphate crosslinker prepared in Example 3, 0.107 g (0.0012 mol) of methyl acrylate, and 0.005 g of AIBN were mixed to obtain a semi-solid electrolyte precursor solution. The mass fraction of the electrolyte in the precursor was 90%.

[0097] Battery Assembly: 0.8g of sodium vanadium phosphate (NVP) powder, 0.1g of carbon black, and 0.1g of PVDF were added to an agate mortar and pestle, followed by an appropriate amount of NMP solvent and ground until uniform. The slurry was then evenly coated on the surface of aluminum foil (the positive electrode current collector) and vacuum-dried at 60°C for 12 hours. The dried electrode sheet was cut into 12mm diameter discs and compacted to form the positive electrode. A button cell was assembled using the sodium foil as the counter electrode and the aforementioned semi-solid electrolyte precursor solution as the electrolyte.

[0098] Curing of the electrolyte: The assembled battery was heated and cured at 60°C for 1 hour to obtain the final semi-solid-state battery.

[0099] Comparative Example 1

[0100] This comparative example is basically the same as Example 4, except that the electrolyte is 15.375 g of liquid ester electrolyte.

[0101] Comparative Example 2

[0102] This comparative example is basically the same as Example 4, except that the pyrophosphate crosslinking agent prepared in Example 1 is replaced by a phosphate ester having the following structural formula in the prior art:

[0103]

[0104] Comparative Example 3

[0105] This comparative example is basically the same as Example 4, except that the pyrophosphate crosslinking agent prepared in Example 1 is replaced by a phosphate ester having the following structural formula in the prior art:

[0106]

[0107] Comparative Example 4

[0108] This comparative example is basically the same as Example 4, except that the pyrophosphate crosslinking agent prepared in Example 1 is replaced by a phosphate ester having the following structural formula in the prior art:

[0109]

[0110] Battery performance test: The batteries obtained in Examples 4 to 10 and Comparative Examples 1 to 4 were subjected to electrical performance tests; the obtained batteries were disassembled, and a combustion test was performed on the disassembled separator and electrolyte mixture; the test results are shown in Table 1 below.

[0111] Table 1.

[0112]

[0113] As shown in Table 1, the first-week capacity of the batteries obtained in Examples 4 to 10 all reached more than 100 mAh / g, the capacity retention rate all reached more than 92%, and the degree of combustion was not large, which proves that the electrolyte and battery prepared by the cross-linking agent of the present application have good electrical properties and flame retardant properties.

[0114] By comparing Example 4 and Comparative Example 1, it can be seen that the semi-solid electrolyte has significantly improved flame retardancy and cycle performance (capacity retention after 500 cycles) compared with the liquid electrolyte. This is because the liquid battery pack does not contain flame retardant components and the degree of combustion is greater; the liquid electrolyte is more likely to undergo side reactions with the electrode, thereby reducing the cycle performance of the battery.

[0115] By comparing Example 4 with Comparative Examples 2, 3, and 4, it can be seen that the electrolyte and battery prepared by the pyrophosphate crosslinker of the present application have advantages in flame retardancy and electrical properties compared to the phosphates of the prior art. This may be because the present application removes the spiro ring in the structure while retaining the symmetrical structure and the diphosphorus atom (or diphosphate structural fragment). Compared with Comparative Example 2, the present application eliminates the adverse effects of the spiro ring on charge movement and improves electrical performance. Compared with Comparative Example 3, the present application not only eliminates the adverse effects of the asymmetric structure on endothermic flame retardancy and improves flame retardancy; but also the structure of the present application is non-acidic, thereby improving the cycle performance of the battery. Compared with Comparative Example 4, the flame retardant structural fragment of the present application accounts for a high proportion and improves flame retardancy.

[0116] In addition, within the examples, by comparing Example 4 with Examples 5 and 6, it can be seen that the molar ratio of the cross-linker to the monomer is different, and the difference in the first-week capacity is not much. This is because: the ion transport of the semi-solid electrolyte can be transmitted in two forms. One is the three-dimensional network structure formed by the polymer, which can transport ions in the form of coordination (the same as the all-solid polymer electrolyte); the other is the electrolyte wrapped inside, which is still flowable at the microscopic level, so it can be transported in the form of a liquid electrolyte; regardless of the degree of cross-linking, ion transport can be achieved, so it has little effect on the first-week capacity electrical performance. However, there is a difference in the capacity retention rate after 500 cycles. This is because: the degree of cross-linking in Example 5 is insufficient, resulting in the presence of free liquid in the electrolyte, which is closer to the performance of a liquid electrolyte. As mentioned above, it is easy to have side reactions with the electrode, thereby reducing the cycle performance of the battery. In Example 6, the degree of cross-linking is higher, the electrolyte elasticity and toughness are insufficient, and the stability is poor, resulting in a decrease in the capacity retention rate after 500 cycles.

[0117] By comparing Example 4 with Examples 7 and 8, it can be seen that the change in the proportion of the liquid ester electrolyte has little effect on the electrical properties. This is because, as mentioned above, the ion transport of the semi-solid electrolyte can be transmitted in the form of polymers and liquid electrolytes. Since both polymers and electrolytes can achieve ion transport, the proportion of the liquid electrolyte has little effect on the electrical properties. However, the proportion of the liquid ester electrolyte affects the proportion of the phosphate flame retardant fragment, and therefore affects the flame retardant effect. When the mass proportion of the liquid ester electrolyte in the precursor does not exceed 90%, the flame retardant performance reaches the best. Furthermore, under the condition of ensuring flame retardant performance, when the mass of the liquid ester electrolyte in the precursor is 90%, the electrical performance of the battery also reaches the best. This may be because the ion transport effect of the liquid electrolyte is relatively high relative to the polymer.

[0118] By comparing Example 7 and Example 9, it can be seen that when n increases, the capacity retention rate increases, which may be because the ethoxy segment is conducive to the transmission of ions.

[0119] From the comparison between Example 9 and Example 10, it can be seen that the difference in R groups has no significant effect on the electrical properties.

[0120] The specific embodiments described herein are merely illustrative of the spirit of the present invention. Persons skilled in the art may make various modifications, additions, or substitutions to the described specific embodiments without departing from the spirit of the present invention or exceeding the scope of the appended claims.

Claims

1. A semi-solid electrolyte, characterized in that: Prepared by the following steps: Mixing a pyrophosphate crosslinking agent with an acrylate monomer, an electrolyte, and an initiator to obtain a precursor, and initiating a polymerization reaction of the precursor to obtain the semi-solid electrolyte; The pyrophosphate cross-linking agent is a compound with the following general structural formula: Wherein, n is a natural number and n≥0; R is hydrogen or methyl.

2. A semi-solid electrolyte according to claim 1, characterized in that: n=3。 3. A semi-solid electrolyte according to claim 1 or 2, characterized in that: The preparation method of the pyrophosphate cross-linking agent comprises the following steps: S1. In the presence of an acid scavenger, reacting an ethylene glycol compound and an acryloyl chloride compound in a solvent to obtain an intermediate product; S2. In the presence of an acid scavenger, reacting the intermediate product with pyrophosphoryl chloride in a solvent to obtain the pyrophosphate crosslinking agent.

4. A semi-solid electrolyte according to claim 3, characterized in that: The molar ratio of the ethylene glycol compound, the acryloyl chloride compound, and the acid scavenger is (1.1~2):1:

1.

5. A semi-solid electrolyte according to claim 3, characterized in that: The molar ratio of the intermediate product, pyrophosphoryl chloride and acid scavenger is (2.5~3.5):0.5:

2.

6. A semi-solid electrolyte according to claim 3, characterized in that: The reaction environment temperature of steps S1 and S2 is 0-10°C respectively.

7. A semi-solid electrolyte according to claim 1, characterized in that: The molar ratio of the pyrophosphate crosslinking agent to the acrylic ester monomer is (0.5-2):

1.

8. A semi-solid electrolyte according to claim 1, characterized in that: The mass fraction of the electrolyte in the precursor is 80% to 95%.

9. A battery, characterized in that: Comprising the semi-solid electrolyte as claimed in claim 1 or 7 or 8.

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