Cyclic nitroxide free radical compound as well as preparation method and application thereof

Abstract

The invention discloses a cyclic nitroxide free radical compound as well as a preparation method and application thereof, and belongs to the technical field of flow battery electrolyte materials. According to the cyclic nitroxide free radical compound disclosed by the invention, functional groups are introduced to positions 3 and 5 of a ring structure of the cyclic nitroxide free radical compound to form a synergistic coupling effect with a traditional functional group at a position 4, so that the electron delocalization range of the cyclic structure is expanded, the electron cloud distribution of the cyclic structure is regulated and controlled, and the effect that traditional cyclic nitroxide free radicals are in the position 3, 5 of the cyclic nitroxide free radical compound can be effectively inhibited. According to the invention, molecular structure disproportionation, decomposition and ring-opening reactions caused by Michael addition are carried out at the site 5 and the site 5, and a water-soluble functional group can be further introduced, so that the solubility, the energy density, the electrochemical cycle stability and other properties of the compound as an aqueous battery electrolyte material can be effectively improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of electrolyte materials for liquid flow batteries, and in particular relates to a cyclic nitroxide free radical compound and a preparation method and application thereof. Background Art

[0002] Developing advanced energy storage technologies is key to addressing the grid connection difficulties associated with the intermittent and unstable nature of renewable energy sources such as wind and solar power. Among various energy storage technologies, flow batteries have become a preferred choice for large-scale energy storage due to their high safety factor, long cycle life, and scalable power / energy. All-vanadium flow batteries are currently the most mature flow battery technology. However, limited vanadium metal resources and the continued rise in their price have increased the cost of the batteries, limiting their large-scale application. Therefore, there is an urgent need to develop new, low-cost flow battery energy storage systems.

[0003] Cyclic nitroxides are a type of electrolyte material containing nitroxides. These nitroxides undergo reversible redox reactions, resulting in excellent cycling stability during battery charge and discharge cycles. These compounds are primarily composed of elements abundant in the Earth's crust, such as carbon, hydrogen, oxygen, and nitrogen, offering the advantages of a wide range of raw material sources and low cost. Using cyclic nitroxide compounds as flow battery electrode materials also offers the advantage of using inexpensive anion exchange membranes and neutral sodium chloride and potassium chloride aqueous solutions as supporting electrolytes.

[0004] Currently, only a limited number of cyclic nitroxide compounds have been reported. Existing cathode materials based on these compounds suffer from low redox potentials, poor water solubility, and unstable electrochemical performance, resulting in low energy density and poor cycling stability in flow batteries. Therefore, the development of novel, low-cost, highly stable nitroxide compounds is crucial for the development of flow batteries. Summary of the Invention

[0005] The purpose of the present invention is to overcome the shortcomings of the above-mentioned prior art and provide a cyclic nitroxide free radical compound and its preparation method and application, so as to solve the problems of low energy density and poor cycle stability of the existing cyclic nitroxide free radical electrode materials in liquid flow battery charge and discharge tests.

[0006] To achieve the above object, the present invention provides the following technical solutions: The present invention provides a cyclic nitroxide free radical compound, the general structural formula of the cyclic nitroxide free radical compound is:

[0007] Wherein, R is one of -OH, C=O and amino; X1 and X2 each include one of a halogen, an alkyl chain, an alkoxy group, an alkyl chain group containing an amide group, an alkyl chain group containing a quaternary ammonium cation, an alkyl chain group containing a sulfonic acid anion, an alkyl chain group containing a phosphate anion, an alkyl chain group containing a terminal hydroxyl group, an alkyl chain group containing an ether group, or an alkyl chain group containing a carboxylic acid anion.

[0008] The molecular structure of the cyclic nitroxide free radical compound is a symmetrical structure or an asymmetrical structure.

[0009] The present invention also provides a method for preparing the cyclic nitroxide free radical compound, comprising: Reactants A, B, and C are mixed uniformly in a solvent to obtain a first solution, and the reaction system is placed in an oxygen-free state by vacuuming and replacing with an inert gas several times. Reactant D is then added to the first solution, stirred, and reacted to obtain a second solution. Reactant E is added to the second solution, stirred, and reacted to obtain a third solution. Acetonitrile is added to the third solution to precipitate a product, which is filtered to obtain a target product. The target product is subjected to anion exchange treatment to obtain a cyclic nitroxide free radical compound. The reactant A is one of 4-oxy-tetramethylpiperidinyloxy free radical, 4-hydroxy-tetramethylpiperidinyloxy free radical or 4-amino-tetramethylpiperidinyloxy free radical; The reactant B is formaldehyde; The reactant C is one or a combination of sodium hydroxide, potassium hydroxide, lithium hydroxide or calcium hydride; The reactant D is one of dimethylamine, diethylamine, dipropylamine, cyclic N compound or imidazole; The reactant E is one of a halogenated alkane, a halogenated alkyl chain containing a quaternary ammonium cation, an alkyl chain containing a sulfonic acid anion, a halogenated alkyl chain containing a phosphate anion, a halogenated alkyl chain containing a terminal hydroxyl group, an alkyl chain containing an ether group, or an alkyl chain containing a carboxylic acid anion.

[0010] The molar ratio of the reactant A to the reactant C is 1: (0.03-0.2).

[0011] The molar ratio of the reactant A to the reactant B is 1:(4-10).

[0012] The molar ratio of the reactant A to the reactant D is 1:(4-6).

[0013] The molar ratio of the reactant A to the reactant E is 1:(2.5-10).

[0014] The reactant E is one of methyl iodide, ethyl iodide, butyl iodide, butyl bromide, tribromopropyltrimethylammonium bromide, tribromobutyltrimethylammonium bromide, propyl sultone, butyl sultone, dibromoethanol, and 3-bromo-1-propanol.

[0015] The solvent is one or more of water, acetone, acetonitrile, toluene, tetrahydrofuran, N,N-dimethylformamide or dimethyl sulfoxide.

[0016] The present invention also provides an application of the cyclic nitroxide free radical compound, wherein the cyclic nitroxide free radical compound is used as a positive electrode electrolyte active substance in an aqueous liquid flow battery.

[0017] Compared with the prior art, the present invention has the following beneficial effects: The present invention provides a cyclic nitroxide compound. Compared to traditional cyclic nitroxide compounds, this invention introduces functional groups at the 3 and 5 positions, in addition to modifying the 4-position of the cyclic nitroxide compound with a functional group. The synergistic coupling of the functional groups at the 4-position and the 3 and 5-positions expands the electron delocalization range of the cyclic structure and modulates the electron cloud distribution. This effectively inhibits the molecular disproportionation, decomposition, and ring-opening reactions caused by Michael addition at the 3 and 5 positions of traditional cyclic nitroxide compounds, thereby improving the structural stability of these compounds. Furthermore, the introduction of water-soluble quaternary ammonium salts, sulfonates, carboxylates, phosphates, and other functional groups at the 3 and 5 positions effectively improves the solubility, intermolecular steric hindrance, and intermolecular Coulomb repulsion of these cyclic nitroxide compounds. This increases the molecular size and the Coulomb repulsion between the molecule and the flow battery separator, thereby improving the energy density and cycling stability of aqueous flow batteries constructed using these compounds. This is of great significance for the development of high-performance cathode materials for aqueous flow batteries.

[0018] The invention also discloses a method for synthesizing a cyclic nitrogen oxide free radical compound. The method is simple, fast and has high yield.

[0019] The present invention discloses the application of a cyclic nitroxide radical compound as an active material in the cathode electrolyte of an aqueous flow battery. When the cyclic nitroxide radical compound provided by the present invention is applied to an aqueous flow battery, the battery device exhibits excellent redox reversibility, along with superior charge-discharge performance and cycling stability. The battery device exhibits excellent overall electrical performance, high safety, and broad market application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for use in the embodiments: Figure 1is the cyclic voltammetry curve of the positive electrode electrolyte solution measured in Example 2; Figure 2 is the charge and discharge curve of the battery in Example 3; Figure 3 This is the 500-cycle charge-discharge curve of the battery in Example 3. DETAILED DESCRIPTION

[0021] The present invention will be further described below with reference to the accompanying drawings: In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Generally, the components of the embodiments of the present invention described and shown in the drawings herein can be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present invention provided in the drawings is not intended to limit the scope of the claimed invention, but merely represents selected embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0022] The present invention discloses a cyclic nitrogen oxide free radical compound, the general structural formula of which is:

[0023] Wherein R is one of -OH, C=O or amino; X1 and X2 each include one of an alkyl chain, an alkoxy group, an alkyl chain group containing an amide group, an alkyl chain group containing a quaternary ammonium cation, an alkyl chain group containing a sulfonic acid anion, an alkyl chain group containing a phosphate anion, an alkyl chain group containing a terminal hydroxyl group, an alkyl chain group containing an ether group, or an alkyl chain group containing a carboxylic acid anion.

[0024] The present invention provides a method for preparing a cyclic nitroxide free radical compound, comprising: Reactants A, B, and C are mixed uniformly in a solvent to obtain a first solution. The reaction system is placed in an oxygen-free state by repeated vacuuming and inert gas replacement. Reactant D is then added to the first solution, stirred, and reacted to obtain a second solution. Reactant E is added to the second solution, stirred, and reacted to obtain a third solution. Acetonitrile is added to the third solution to precipitate a product, which is then filtered to obtain a target product. The target product is subjected to anion exchange treatment to obtain a cyclic nitroxide free radical compound.

[0025] In this preparation method, reactant A is the basic structure of the cyclic nitroxide free radical compound, reactant B is an auxiliary reactant, reactant C provides an alkaline environment for the reaction, and reactants D and E are the reaction raw materials for the 3- and 5-position functional groups, respectively.

[0026] In some embodiments, the reactant A is one of 4-oxy-tetramethylpiperidinyloxy free radical (4-O-TEMPO), 4-hydroxy-tetramethylpiperidinyloxy free radical (4-HO-TEMPO) or 4-amino-tetramethylpiperidinyloxy free radical (4-NH2-TEMPO).

[0027] In some embodiments, the reactant B is formaldehyde.

[0028] In some embodiments, the reactant C is one or a combination of sodium hydroxide (NaOH), potassium hydroxide (KOH), lithium hydroxide (LiOH), or calcium hydride (CaH2).

[0029] In some embodiments, the reactant D is one of dimethylamine, diethylamine, dipropylamine, cyclic N-compound or imidazole.

[0030] In some embodiments, the reactant E is one of a halogenated alkane, a halogenated alkyl chain containing a quaternary ammonium cation, an alkyl chain containing a sulfonic acid anion, a halogenated alkyl chain containing a phosphate anion, a halogenated alkyl chain containing a terminal hydroxyl group, an alkyl chain containing an ether group, or an alkyl chain containing a carboxylic acid anion. Preferably, the reactant E is one of iodomethane, iodoethane, iodobutyl, bromobutyl, tribromopropyltrimethylammonium bromide, tribromobutyltrimethylammonium bromide, propyl sultone, butyl sultone, dibromoethanol, and 3-bromo-1-propanol.

[0031] In some embodiments, the reaction solvent is one or a combination of water, acetone, acetonitrile, toluene, tetrahydrofuran, N,N-dimethylformamide or dimethyl sulfoxide.

[0032] In some embodiments, the molar ratio of reactant A to reactant C is 1:(0.03-0.2). Limiting the usage ratio of reactants A and C can provide a suitable alkaline environment for the reaction system, which is beneficial to the reaction and reduces the occurrence of side reactions.

[0033] In some embodiments, the molar ratio of reactant A to reactant B is 1:(4-10). The main function of formaldehyde in this reaction is to improve the structural stability of the cyclic nitroxide radical compound by regulating electronic effects and preventing side reactions.

[0034] In some embodiments, the molar ratio of reactant A to reactant D is 1:(4-6), and the molar ratio of reactant A to reactant E is 1:(2.5-10); limiting the usage ratio of reactant A to reactant D and reactant E respectively helps to generate an appropriate electron cloud density at the 3 and 5 positions of the ring structure, thereby reducing structural instability caused by excessive electron concentration or sparseness; too high electron density may lead to unnecessary reactivity, while too low may fail to inhibit the disproportionation reaction.

[0035] The present invention also provides the use of cyclic nitroxide free radical compounds as positive electrode electrolyte active substances in aqueous flow batteries. The cyclic nitroxide free radical compound is dissolved in an aqueous solution containing a neutral salt supporting electrolyte to form a positive electrode electrolyte; the negative electrode active substance is dissolved in an aqueous solution containing a neutral salt supporting electrolyte to form a negative electrode electrolyte. The neutral salt supporting electrolyte is one or more of sodium chloride, potassium chloride, ammonium chloride, lithium chloride, sodium bromide, potassium bromide, ammonium bromide, lithium bromide, potassium sulfate, sodium sulfate, lithium sulfate, and ammonium sulfate. The negative electrode active substance is one or more of anthraquinone compounds, viologen compounds, phenazine compounds, alloxazine compounds, zinc chloride, zinc sulfate, and zinc trifluoromethanesulfonate.

[0036] In some embodiments, the aqueous liquid flow battery device is composed of a stainless steel metal end plate, a copper plate current collector, a graphite bipolar plate with a serpentine flow channel, an electrode, a positive electrode electrolyte, a diaphragm, a negative electrode electrolyte, an electrode, a copper plate current collector, a graphite bipolar plate with a serpentine flow channel, and a stainless steel metal end plate stacked in sequence and fastened with bolts.

[0037] In some embodiments, the membrane is an anion exchange membrane, a cation exchange membrane, or a porous membrane.

[0038] In the following examples, unless otherwise specified, all materials used can be obtained through common channels; and the testing methods adopted are conventional methods in the art.

[0039] Example 1 The reactants, 4-O-TEMPO (10 mmol) and NaOH (2 mmol), were dissolved in 100 ml of water and replaced with inert gas three times by vacuum pumping to maintain an oxygen-free state. The reaction temperature was lowered to below 10°C and stirred for 2 hours. Formaldehyde solution (34%, 100 mmol) was added to the mixed solution and stirred for 0.5 hours to obtain a first solution. Dimethylamine (40 mmol) was then added dropwise to the first solution at room temperature and stirred for 2 hours to obtain a second solution. Methyl iodide (100 mmol) was then slowly added dropwise to the second solution and stirred for 5 hours to obtain a third solution. The reaction was terminated by adding excess acetonitrile. The precipitate was filtered to obtain the iodide salt of the target product. The target product was then subjected to anion exchange resin to obtain the chloride salt of the target product, a cyclic nitroxide free radical compound named 3,5'-bis(trimethylammonium chloride methyl)-4-O-TEMPO.

[0040]

[0041] Example 2 The cyclic nitroxide radical compound obtained in Example 1 was weighed and dissolved in 10 mL of 1 mol / L potassium chloride aqueous solution to prepare a uniform solution with a concentration of 2 mmol / L. Cyclic voltammetry was performed on the solution using an electrochemical workstation three-electrode system, wherein a glassy carbon electrode was used as the working electrode, a platinum electrode was used as the counter electrode, and an Ag / AgCl electrode was used as the reference electrode. The test scan rate was 100 mV / s, and the test results were converted into standard hydrogen electrode potentials to obtain Figure 1 The cyclic voltammetry curve shown in the figure shows good redox reversibility with an average potential of 0.8 V (vs SHE (standard hydrogen electrode)).

[0042] Example 3 Weigh the compound obtained in Example 1 and dissolve it in a 1 mol / L potassium chloride solution to make a 0.1 mol / L homogeneous solution. Take 7 mL of this solution as the positive electrolyte solution. Weigh the methyl viologen molecule and dissolve it in a 1 mol / L potassium chloride solution to make a 0.1 mol / L homogeneous solution. Take 10 mL of this solution as the negative electrolyte solution. Assemble the flow battery according to the aforementioned flow battery assembly method, adding the positive and negative electrolyte solutions as required. Start the peristaltic pump to circulate the electrolyte solutions through the battery system.

[0043] The battery was tested for charge and discharge performance and cycle stability. 2 The battery was charged and discharged at a constant current density of 0.6~1.2 V. Figure 2This is the battery single cycle charge and discharge curve. The battery can work normally, and the coulomb efficiency is close to 100%. The battery is tested for 400 cycles of continuous charge and discharge. Figure 3 As shown in the figure, the capacity attenuation rate is only 0.002% / cycle, and the cycle performance is excellent.

[0044] Example 4 The reactant 4-O-TEMPO (10 mmol) was dissolved in KOH (1 mmol) in 80 ml of water, and the reaction system was evacuated and replaced with inert gas three times to make it oxygen-free. The mixture was stirred at room temperature for 20 minutes, then the reaction temperature was lowered to 10°C, and formaldehyde solution (34%, 60 mmol) was added to the mixed solution within 10 minutes, and stirring was continued for 0.5 hours to obtain a first solution; dimethylamine (40 mmol) was then added dropwise to the first solution at room temperature, and stirred at room temperature for 1 hour to obtain a second solution; 1-methylpiperidine (25 mmol) was then added dropwise to the second solution within 5 minutes, the solution was heated to 60°C, and stirred at 60°C for 4 hours to obtain a third solution, and excess acetonitrile solution was added to the third solution to stop the reaction. The precipitate was filtered after precipitation, and the filter cake was the iodide ion salt structure of the target product; the target product was exchanged with an anion exchange resin to obtain the chloride ion salt structure of the target product, which was retained by drying, namely a cyclic nitroxide free radical compound named: 3,5'-bis(1-methylpiperidinium chloride methyl)-4-O-TEMPO.

[0045]

[0046] Example 5 The reactants 4-O-TEMPO (10 mmol) and NaOH (2 mmol) were added to 50 ml of a water / toluene (v / v = 1:1) mixed solution, and the reaction system was placed in an oxygen-free state by vacuuming and replacing with inert gas three times. The mixture was stirred in an ice-water bath for 0.5 hour, and formaldehyde solution (34%, 50 mmol) was added to the mixed solution, and stirring was continued for 0.5 hour to obtain a first solution. Dimethylamine (40 mmol) was then added dropwise to the first solution at room temperature, and stirred at room temperature for 2 hours to obtain a second solution. Tribromopropyltrimethylammonium bromide (40 mmol) was then added to the second solution, and the mixed solution was heated to reflux for 12 hours to obtain a third solution. Excess acetonitrile solution was added to the third solution to stop the reaction. After the precipitate was precipitated, it was filtered at room temperature. The filter cake was the bromide ion salt structure of the target product. The filter cake was washed three times with acetonitrile solution. The target product was exchanged with an anion exchange resin to obtain the chloride ion salt structure of the target product, which is a cyclic nitroxide free radical compound named: 3,5'-bis(1-methyl-N,N-dimethylammonium chloride-propyltrimethylammonium chloride)-4-O-TEMPO.

[0047]

[0048] Example 6 The reactants 4-O-TEMPO (10 mmol) and lithium hydroxide (0.3 mmol) were added to 60 ml of a water / acetonitrile (v / v=1:1) mixed solution, and the reaction system was placed in an oxygen-free state by vacuuming and replacing with inert gas three times. The mixture was stirred at low temperature (below 0°C) for 0.5 hours, and formaldehyde solution (34%, 100 mmol) was added to the mixed solution, and stirring was continued for 0.5 hours to obtain a first solution; dimethylamine (40 mmol) was then added dropwise to the first solution at room temperature, and stirred at room temperature for 2 hours to obtain a second solution; propyl sultone (40 mmol) was then slowly added dropwise to the second solution, heated to 80°C, and stirred at 80°C for 5 hours to obtain a third solution, and an excess of acetonitrile solution was added to the third solution. After the precipitate was precipitated, it was filtered, and the filter cake was the iodide ion salt structure of the target product; the target product was exchanged with an anion exchange resin to obtain the chloride ion salt structure of the target product, which is a cyclic nitroxide free radical compound named: 3,5'-bis(1-methyl-N,N-dimethylammonium chloride-propylsulfonic acid)-4-O-TEMPO.

[0049]

[0050] Example 7 The reactants 4-O-TEMPO (10 mmol) and lithium hydroxide (0.2 mmol) were added to 50 ml of a water / acetonitrile (v / v=1:1) mixed solution, and the reaction system was placed in an oxygen-free state by vacuuming and replacing with inert gas three times. The mixture was stirred at low temperature (below 0°C) for 0.5 hours, and formaldehyde solution (34%, 80 mmol) was added to the mixed solution, and stirring was continued for 1 hour to obtain a first solution; diethylamine (60 mmol) was then added dropwise to the first solution at room temperature, and stirred at room temperature for 2 hours to obtain a second solution; 3-bromo-1-propanol (60 mmol) was then slowly added dropwise to the second solution, heated to 80°C, and stirred at 80°C for 6 hours to obtain a third solution, and excess acetonitrile solution was added to the third solution to stop the reaction. The precipitate was filtered after precipitation, and the filter cake was the iodide ion salt structure of the target product; the target product was exchanged with an anion exchange resin to obtain the chloride ion salt structure of the target product, which is a cyclic nitroxide free radical compound named: 3,5'-bis(1-methyl-N,N-dimethylammonium chloride-propanol)-4-O-TEMPO.

[0051]

[0052] Example 8 The reactants 4-HO-TEMPO (10 mmol) and NaOH (0.5 mmol) were dissolved in 60 ml of aqueous solution. The reaction system was evacuated and replaced with inert gas three times to maintain an oxygen-free state. Stirring was performed in an ice-water bath for 0.5 hour. Formaldehyde solution (34%, 100 mmol) was added to the mixed solution and stirring continued for 0.5 hour to obtain a first solution. Dimethylamine (60 mmol) was then added dropwise to the first solution at room temperature and stirred for 2 hours to obtain a second solution. 1-Methylaziridine (60 mmol) was then added dropwise to the second solution and stirred for 10 hours to obtain a third solution. Excess acetonitrile was added to the third solution to terminate the reaction. The precipitate was filtered after precipitation. The filter cake was the iodide salt structure of the target product. The target product was then exchanged with an anion exchange resin to obtain the chloride salt structure of the target product, which is a cyclic nitroxide free radical compound named: 3,5'-bis(1-methylcycloethylammonium chloridemethyl)-4-HO-TEMPO

[0053] Example 9 The reactants 4-HO-TEMPO (10 mmol) and KOH (1 mmol) were dissolved in 50 ml of aqueous solution, and the reaction system was placed in an oxygen-free state by vacuuming and replacing with inert gas three times. The mixture was stirred in an ice-water bath for 0.5 hour, and formaldehyde solution (34%, 80 mmol) was added to the mixed solution, and stirring was continued for 0.5 hour to obtain a first solution; dipropylamine (120 mmol) was then added dropwise to the first solution at room temperature, and stirred at room temperature for 2 hours to obtain a second solution; tribromobutyltrimethylammonium bromide (50 mmol) was then added to the second solution, and the mixed solution was heated to reflux for 12 hours to obtain a third solution, and an excess of acetonitrile solution was added to the third solution to stop the reaction. After the precipitate was precipitated, it was filtered at room temperature, and the filter cake was the bromide ion salt structure of the target product; the filter cake was washed three times with acetonitrile solution; the target product was exchanged with an anion exchange resin to obtain the chloride ion salt structure of the target product, which is a cyclic nitroxide free radical compound named: 3,5'-bis(1-methyl-N,N-dipropylammonium chloride-butyltrimethylammonium chloride)-4-HO-TEMPO.

[0054]

[0055] In summary, the cyclic nitroxide compounds prepared in the Examples are based on functional group modification at the 4-position of the cyclic nitroxide compound, with functional groups introduced at the 3- and 5-positions. The synergistic coupling of the functional groups at the 4-position and the 3- and 5-positions expands the electron delocalization range of the cyclic structure and regulates the electron cloud distribution of the cyclic structure. This effectively inhibits the molecular disproportionation, decomposition, and ring-opening reactions caused by Michael addition at the 3- and 5-positions of conventional cyclic nitroxides, thereby improving the structural stability of the compounds. Furthermore, the introduction of water-soluble quaternary ammonium salts, sulfonates, carboxylates, and phosphates at the 3- and 5-positions effectively improves the solubility, intermolecular steric hindrance, and intermolecular Coulomb repulsion of the cyclic nitroxide compounds. This increases the molecular size and the Coulomb repulsion between the molecule and the flow battery separator, thereby improving the energy density and cycling stability of aqueous flow batteries constructed based on these compounds. This is of great significance for the development of high-performance cathode materials for aqueous flow batteries.

[0056] The endpoints and any values ​​of the ranges disclosed in the present invention are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of each range, the endpoint values ​​of each range and a separate point value, and the separate point values ​​can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be considered as specifically disclosed in this article. Hereinafter, in principle, each technical solution can be combined with each other to obtain a new technical solution, which should also be considered as specifically disclosed in this article.

[0057] The above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the same. Although the present invention has been described in detail with reference to the above embodiments, a person skilled in the art may still modify or make equivalent substitutions to the specific implementations of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention are within the scope of protection of the claims of the present invention to be approved.

Claims

1. A cyclic nitrogen oxide free radical compound, characterized in that The general structural formula of the cyclic nitroxide free radical compound is: Wherein, R is one of -OH, C=O and amino; X1 and X2 each include one of a halogen, an alkyl chain, an alkoxy group, an alkyl chain group containing an amide group, an alkyl chain group containing a quaternary ammonium cation, an alkyl chain group containing a sulfonic acid anion, an alkyl chain group containing a phosphate anion, an alkyl chain group containing a terminal hydroxyl group, an alkyl chain group containing an ether group, or an alkyl chain group containing a carboxylic acid anion.

2. A cyclic nitroxide free radical compound according to claim 1, characterized in that The molecular structure of the cyclic nitroxide free radical compound is a symmetrical structure or an asymmetrical structure.

3. A method for preparing a cyclic nitroxide free radical compound according to any one of claims 1 or 2, characterized in that: include: Reactants A, B, and C are mixed uniformly in a solvent to obtain a first solution, and the reaction system is placed in an oxygen-free state by vacuuming and replacing with an inert gas several times. Reactant D is then added to the first solution, stirred, and reacted to obtain a second solution. Reactant E is added to the second solution, stirred, and reacted to obtain a third solution. Acetonitrile is added to the third solution to precipitate a product, which is filtered to obtain a target product. The target product is subjected to anion exchange treatment to obtain a cyclic nitroxide free radical compound. The reactant A is one of 4-oxy-tetramethylpiperidinyloxy free radical, 4-hydroxy-tetramethylpiperidinyloxy free radical or 4-amino-tetramethylpiperidinyloxy free radical; The reactant B is formaldehyde; The reactant C is one or a combination of sodium hydroxide, potassium hydroxide, lithium hydroxide or calcium hydride; The reactant D is one of dimethylamine, diethylamine, dipropylamine, cyclic N compound or imidazole; The reactant E is one of a halogenated alkane, a halogenated alkyl chain containing a quaternary ammonium cation, an alkyl chain containing a sulfonic acid anion, a halogenated alkyl chain containing a phosphate anion, a halogenated alkyl chain containing a terminal hydroxyl group, an alkyl chain containing an ether group, or an alkyl chain containing a carboxylic acid anion.

4. The method for preparing a cyclic nitroxide free radical compound according to claim 3, wherein: The molar ratio of the reactant A to the reactant C is 1: (0.03-0.2).

5. The method for preparing a cyclic nitroxide free radical compound according to claim 3, wherein: The molar ratio of the reactant A to the reactant B is 1:(4-10).

6. The method for preparing a cyclic nitroxide free radical compound according to claim 3, wherein: The molar ratio of the reactant A to the reactant D is 1:(4-6).

7. The method for preparing a cyclic nitroxide free radical compound according to claim 3, wherein: The molar ratio of the reactant A to the reactant E is 1:(2.5-10).

8. The method for preparing a cyclic nitroxide free radical compound according to claim 3, wherein: The reactant E is one of methyl iodide, ethyl iodide, butyl iodide, butyl bromide, tribromopropyltrimethylammonium bromide, tribromobutyltrimethylammonium bromide, propyl sultone, butyl sultone, dibromoethanol, and 3-bromo-1-propanol.

9. The method for preparing a cyclic nitroxide free radical compound according to claim 3, wherein: The solvent is one or more of water, acetone, acetonitrile, toluene, tetrahydrofuran, N,N-dimethylformamide or dimethyl sulfoxide.

10. Use of the cyclic nitroxide free radical compound according to any one of claims 1 or 2, characterized in that: The cyclic nitroxide free radical compound is used as a positive electrode electrolyte active material in an aqueous liquid flow battery.

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