Cyclotriphosphazene compound and application thereof

Abstract

The invention discloses a cyclotriphosphazene compound and application thereof, and belongs to the field of secondary batteries. The cyclotriphosphazene compound has a structure represented by a formula M, and in the formula M, R1 is fluorine, a C1-2 alkyl group, CH2SCH2COOCH3 or CH2SCH2COOCH2CH3, and R2 is hydrogen or a methyl group. The cyclotriphosphazene compound disclosed by the invention is applied as an electrolyte additive, and the multiple synergistic effects of the cyclotriphosphazene compound promote excellent discharge performance and capacity retention rate of a battery; a series of positive improvement results are generated by improving the main problem of sodium precipitation of the hard carbon negative electrode, so that a thought is provided for subsequently improving the performance of the hard carbon negative electrode.

Description

Technical Field

[0001] This invention belongs to the field of secondary batteries and mainly relates to a cyclic triphosphazene compound and its application as an electrolyte additive in secondary batteries, including: a new electrolyte additive and its preparation method, a sodium-ion battery additive, a sodium-ion battery electrolyte, and a sodium-ion battery. Background Technology

[0002] As a promising energy storage technology, sodium-ion batteries currently offer advantages primarily due to their abundant resources and low cost. Sodium is far more abundant in the Earth's crust than lithium, significantly reducing the raw material costs for sodium-ion batteries. Furthermore, sodium-ion batteries exhibit good stability under both high and low temperature conditions, demonstrating strong safety. With technological advancements, the energy density and cycle life of sodium-ion batteries are gradually improving, making them increasingly competitive in large-scale energy storage systems and electric vehicles. However, some limitations remain in areas such as fast charging performance, primarily due to the combined effects of electrode materials and electrolytes.

[0003] Currently, the main obstacles restricting the fast-charging performance of batteries include: ① Ion diffusion rate: Sodium ions have a larger radius than lithium ions, resulting in slower movement within the electrode material, especially during fast charging, where ion diffusion rate becomes a limiting factor. ② Structural stability of electrode materials: During fast charging and discharging, electrode materials need to rapidly and repeatedly absorb and release ions, posing a risk of structural collapse and degradation to sodium-ion battery electrode materials. ③ Electrolyte stability: Under fast charging conditions, electrolyte decomposition and electrode material reactions are more intense, potentially leading to performance degradation and shortened lifespan. ④ Electrode / electrolyte interface stability: Fast charging requires good contact and compatibility between the electrode and electrolyte, while interface instability can degrade overall battery performance, such as charging efficiency. ⑤ Safety concerns: Currently, high-rate charging and discharging results in significant overall battery polarization and increased overall impedance, easily generating large amounts of heat in a short time. Although current battery management systems (BMS) can manage this, the risk of combustion and fire remains.

[0004] Recently, the interaction between sodium electrode materials and electrolytes has been addressed primarily through two approaches. One is developing high-conductivity, low-resistance negative electrode materials, such as silicon-carbon composite negative electrodes. Currently, lithium batteries mainly use improved silicon-carbon negative electrode materials, which allow lithium ions to intercalate from multiple directions, reducing ion migration resistance during fast charging. However, sodium-ion battery negative electrodes are generally amorphous hard carbon materials, and interface problems during charge and discharge still exist, affecting performance and safety. The other approach is optimizing the electrolyte solvent system, mainly focusing on developing low-viscosity, high-ionic-conductivity electrolyte solvents. This reduces the viscosity of the formulation system without affecting ionic conductivity, increases ion mobility, and mitigates various problems caused by high polarization under high-rate charge and discharge conditions, such as high impedance exothermic reactions, sodium deposition, and abnormal capacity decay. This type of solution directly applies advanced solutions from lithium-ion battery systems. The above-mentioned improvement schemes have a certain improvement on the fast charging performance of sodium-ion batteries, but there are some problems such as the volume expansion of silicon-carbon composite anodes during charging, and the current low viscosity solvent system is difficult to meet the main salt dissolution performance, low dielectric constant, and high unit cost. These are all quite "fatal" for the sodium-ion battery system that focuses on safety and low cost, making it difficult to be widely used in various scenarios. Summary of the Invention

[0005] In view of the shortcomings of the prior art, the purpose of this invention is to provide a cyclotriphosphazene compound and its application as an electrolyte additive in secondary batteries. The provided cyclotriphosphazene compound, as an electrolyte additive, can improve the fast charging performance of sodium-ion batteries and the like while ensuring low cost.

[0006] The first aspect of this invention provides a cyclotriphosphazene compound having the structure of formula M:

[0007]

[0008] Wherein, R1 is fluorine, C1-2 alkyl, CH2SCH2COOCH3, or CH2SCH2COOCH2CH3, and R2 is hydrogen or methyl. The cyclotriphosphazene compound having the structure of formula M can be referred to as compound M; in formula M, the main component is a fluorine-containing cyclotriphosphazene structure; R1 can be fluorine (F) or a C1-2 alkyl group, or a structure containing a mercaptocarboxylic acid ester group, i.e., CH2SCH2COOCH3 or CH2SCH2COOCH2CH3. The mercaptocarboxylic acid ester group is simultaneously attached to the phosphorus atom connected to R1, wherein R2 is hydrogen (H) or methyl (CH3), preferably methyl.

[0009] Alkyl groups are the groups remaining after removing a hydrogen atom from alkane molecules such as methane. They are open-chain saturated organic groups containing only carbon and hydrogen atoms. C1-2 alkyl groups are alkane groups with one or two carbon atoms, namely methyl (CH3) or ethyl (CH2CH3), where methyl can be simplified to a single bond. The mercapto group, also known as a thiol group, is generally a negatively charged functional group consisting of a sulfur atom and a hydrogen atom bonded together, with the chemical formula -SH. The structural formula of an ester group is -COOR, where R is generally an alkyl group or other non-H group.

[0010] For example, R1 is F and R2 is methyl; R1 and R2 are both methyl; or R1 is CH2SCH2COOCH2CH3 and R2 is methyl. In some preferred embodiments, the cyclotriphosphazene compound has any of the following structures, which may be referred to as compound M1, compound M2, and compound M3 respectively;

[0011]

[0012] Compared to existing improvement schemes for hard carbon anodes, the innovation of this invention lies in proposing a novel class of electrolyte additives and their preparation routes, providing new options for future additive screening. This invention designs and synthesizes a novel class of electrolyte additives with a structure of cyclotriphosphazene compounds as shown in Formula M. This type of additive structure is unique to this invention. In particular, this invention designs this novel electrolyte additive primarily for performance improvement in sodium-ion batteries.

[0013] The cyclotriphosphazene compounds prepared in this invention can serve as novel electrolyte additives, specifically for adjusting system viscosity, improving the dissolution efficiency of the main salt, inhibiting sodium deposition at the hard carbon anode, and enhancing safety performance. Corresponding to the M-structure, the overall structure of the cyclotriphosphazene compounds is a combination of polar groups (cyclotriphosphazene structure) and non-polar groups (derived structures such as thioglycolic acid). This allows them to reduce the viscosity and improve the wetting properties of the electrolyte in the initial stages of addition, ensuring good interfacial contact and ion transport performance even at high magnification.

[0014] Furthermore, the fluorinated functional groups in the aforementioned structure act as co-solvents for the fluorinated main salt, effectively dissolving high-viscosity main salts with excellent performance, which is more beneficial to the battery's internal interface. During formation and cycling, the carboxylate ions formed after the decomposition of carboxylic acid ester groups contain oxygen atoms that can act as coordination sites, forming stable coordination bonds with sodium ions and reducing interfacial migration resistance. Simultaneously, the contained cyclic ether groups enhance the orientation of sodium ions, allowing them to uniformly fill the defect pores of the hard carbon anode, suppressing sodium supersaturation and precipitation in certain areas at high rates, and improving high-rate charge / discharge performance. In addition, the sulfide groups enhance the sodium ion complexation ability, improving the ionic conductivity and temperature range of the electrolyte, and optimizing the performance of the battery interfacial film during battery cycling, reducing the increase in battery DC internal resistance (DCR), resulting in a significant improvement in overall cell performance and stability.

[0015] The novel electrolyte additive prepared in this embodiment contains a flame-retardant cyclotriphosphazene structure, which can effectively retard flames during thermal runaway of the battery cell, further improving the safety performance of the battery.

[0016] The present invention provides a method for preparing cyclotriphosphazene compounds as described above, comprising the following steps: reacting the compound shown in formula Y with methyl mercaptoacetate or ethyl mercaptoacetate in an organic solvent in the presence of a catalyst to obtain a cyclotriphosphazene compound having the structure of formula M;

[0017]

[0018] Wherein, R1 is fluorine, C1-2 alkyl, CH2SCH2COOCH3 or CH2SCH2COOCH2CH3, X is a halogen element, and R2 is hydrogen or methyl.

[0019] Taking the preparation of compound M1 as an example, the present invention employs the following novel method to prepare the cyclotriphosphazene compound, the specific steps of which include: using hexafluorocyclotriphosphazene as the starting reactant, which can be dissolved in a first solvent at a temperature of -20°C, and after thorough stirring, preferably at room temperature, slowly adding the nucleophile lithium methyl, and continuing to stir thoroughly, and also at room temperature, to generate a monomethyl-substituted polyfluorocyclotriphosphazene, denoted as substance Z1.

[0020] The reaction formula involved in the first step of this embodiment of the invention is as follows; it can also generate dimethyl-substituted polyfluorocyclic triphosphazene, which can be denoted as substance Z2, used to prepare compound M2 / M3, and so on (according to the reaction equivalence ratio provided by the reaction activity, impurities that substitute onto the other two phosphorus atoms will appear during the reaction, but the proportion is small and will be removed during purification). In this step, the reactants hexafluorocyclic triphosphazene and methyllithium are both commercially available reagents; the equivalence ratio of hexafluorocyclic triphosphazene to methyllithium can be 1:(1-6), preferably 1:(2-4). The first solvent can be one or more of diethyl ether, tetrahydrofuran, dichloromethane and chloroform, preferably diethyl ether. Furthermore, the initial feeding temperature of the reaction system is preferably -30℃ to -10℃, more preferably -20℃; the subsequent continuous stirring reaction temperature is preferably 0℃ to 30℃, more preferably 15℃ to 25℃ (room temperature conditions). In addition, the continuous reaction time can be 0 to 1 h, preferably 0.5 h.

[0021]

[0022] Subsequently, in this embodiment of the invention, the obtained substance Z1 is used as one of the reactants in the second step. It can be added to the second solvent along with an iodine source (preferably iodine) at room temperature. After stirring, catalyst A is added, and the mixture is stirred thoroughly. Preferably, the temperature of the reaction system is raised to 40°C and the reaction is carried out for 10-15 hours to generate the target product Y1 (that is, the compound shown in formula Y, wherein the halogen element is iodine).

[0023] In the third step of this embodiment, ethyl mercaptoacetate and catalyst B are preferably added to a third solvent at room temperature. After thorough stirring, reactant Y1 is added. The entire reaction system can be reacted at room temperature, and the target product M1 (a cyclotriphosphazene compound with the structure of formula M, where R1 is F and R2 is methyl) is generated after 24-48 hours.

[0024] In embodiments of the present invention, the reaction formulas involved in the above two steps are as follows:

[0025]

[0026] The main change in reactant Z1 in this embodiment of the invention is the change in the number of fluorine atoms on the phosphorus atoms of the cyclotriphosphazene in step one that are replaced by methyl groups. However, the unsubstituted fluorine atoms do not participate in the main synthesis reaction, so they are described together here. The equivalence ratio of reactant Z1 to iodine (I2) can be 1:(5-10), preferably 1:(8-10). This step is preferably carried out under the action of catalyst A, and the equivalence ratio of reactant Z1 to catalyst A can be 1:(5-10), preferably 1:(8-10). Catalyst A is preferably one or more of sodium tert-butoxide, potassium tert-butoxide, and potassium carbonate, more preferably sodium tert-butoxide. The second solvent can be one or more of N,N-dimethylformamide, acetone, 2-butanone, toluene, and 1,4-dioxane; preferably acetone. Furthermore, the reaction temperature of the second step reaction system is preferably 20℃-50℃, more preferably 30℃-40℃. The reaction time in the second step is preferably 10h to 20h, and more preferably 12h to 15h.

[0027] The third step of the preparation method described in this embodiment of the invention satisfies at least one of the following: the catalyst (referred to as catalyst B to distinguish it from the second step) can be a hydroxide and / or a carbonate, and has a certain degree of alkalinity; the organic solvent used in this step is referred to as the third solvent, and can be an alcohol; the third solvent is used after being redistilled or dehydrated by molecular sieves; the reaction temperature can be 15–50°C; and the reaction time can be 24–50 h. In the embodiments of the present invention, the main change in reactant Y1 is the change in the number of iodine atoms on the substituted methyl groups of the phosphorus atoms in the cyclotriphosphazene in step one, but the unsubstituted methyl groups do not participate in the main synthesis reaction, so they are described together here.

[0028] Specifically, the equivalence ratio of reactant Y1 to ethyl mercaptoacetate can be 1:(1-1.5), preferably 1:(1.1-1.2). The equivalence ratio of reactant Y1 to catalyst B can be 1:(2-3), preferably 1:2.2. Catalyst B is further one or more of sodium hydroxide, potassium hydroxide, sodium carbonate, and potassium carbonate; preferably, catalyst B is sodium hydroxide. The third solvent in the third step is preferably one or more of methanol, ethanol, propanol, isopropanol, glycerol, ethylene glycol, and propylene glycol; more preferably, methanol. The temperature of the reaction system in the third step is preferably 15℃-50℃, more preferably 15-25℃; the reaction time can be 24-50h, more preferably 45h.

[0029] Furthermore, for ease of differentiation, the reaction solvents are sequentially designated as the first, second, and third solvents. Preferably, all solvents undergo a redistillation step or are dehydrated using molecular sieves before being used in the reaction. In embodiments of the present invention, the reaction product can be obtained through conventional purification or refining processes. For example, after the reaction is complete, acetic acid is added to adjust the pH value, the reaction system is subjected to vacuum distillation, the residue is extracted with methane using a triple-filtration process, the extract is concentrated again, and purified by column chromatography to obtain compound M1 molecules.

[0030] Experiments unexpectedly revealed that, compared to the main salt and solvent systems that affect the viscosity and stability of the electrolyte, the additive system is more efficient and convenient to adjust, and additives that affect the synergistic performance of the solvent system and the system viscosity are easier to obtain. Therefore, based on the new electrolyte system design concept, this invention innovatively designed a series of novel cyclotriphosphazene additive molecules, and used electrolyte regulation methods to synergistically modify the battery interface layer and electrolyte system. This not only ensures the advantage of low cost, but also helps to improve the battery's fast charging performance.

[0031] A second aspect of the present invention provides a non-aqueous electrolyte for a secondary battery, comprising: an electrolyte salt, a non-aqueous organic solvent, and a first additive, wherein the first additive is a cyclotriphosphazene compound as described above.

[0032] The cyclotriphosphazene compounds described above in this invention have a general structure of formula M and can be used as an additive product for sodium-ion battery electrolytes. This additive not only reduces the viscosity of sodium-ion battery electrolytes, ensuring ion transport kinetics under extreme conditions, but also effectively dissolves high-viscosity main salts with excellent performance, which is more beneficial to the battery interface. Furthermore, it can form stable complex coordination bonds with sodium ions, reducing interfacial migration resistance, improving the ionic conductivity and temperature range of the electrolyte, and optimizing the performance of the battery interface film during battery cycling, thus significantly improving the overall cell performance.

[0033] Preferably, the non-aqueous electrolyte satisfies at least one of the following: the electrolyte salt is a sodium salt; the content of the electrolyte salt accounts for 8-25% of the weight of the non-aqueous electrolyte; the non-aqueous organic solvent (non-aqueous solvent) is one or more of organic esters, C1-10 alkyl ethers, cyclic ethers, sulfones, and dinitrile; the non-aqueous organic solvent accounts for 60-90% of the weight of the non-aqueous electrolyte; and the first additive accounts for 1-5% of the weight of the non-aqueous electrolyte. Further, the electrolyte salt is selected from at least one of sodium hexafluorophosphate (NaPF6), sodium difluorosulfonamide (NaFSI), sodium perchlorate (NaClO4), sodium tetrafluoroborate (NaBF4), sodium difluorophosphate (NaPO2F2), and sodium difluorooxalate borate (NaODFB), for example, sodium difluorosulfonamide.

[0034] For the aforementioned non-aqueous organic solvents, further, the organic esters are carbonates and / or carboxylic acid esters, specifically at least one selected from ethylene carbonate, propylene carbonate, butenyl carbonate, dimethyl carbonate, diethyl carbonate, dipropyl carbonate, methyl ethyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, 1,4-butyrolactone, methyl formate, ethyl acetate, methyl propionate, ethyl propionate, propyl propionate, butyl propionate, and ethyl butyrate. The C1-10 alkyl ethers are at least one selected from dimethyl ether, diethyl ether, and methyl ethyl ether; the dinitrile is at least one selected from adiponitrile, succinic anion, glutaronitrile, or their fluorinated derivatives, preferably adiponitrile or their fluorinated derivatives; the sulfones are at least one selected from dimethyl sulfoxide, sulfolane, or their derivatives; the cyclic ethers are furan, dioxane, or their derivatives. In some embodiments, the non-aqueous organic solvent is a mixture of propylene carbonate (PC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC), wherein the mass ratio of PC, EMC, and DEC is 3:5:2.

[0035] Preferably, the content of the electrolyte salt accounts for 8-25% of the weight of the non-aqueous electrolyte. More preferably, the content of the electrolyte salt accounts for 13-15% of the weight of the non-aqueous electrolyte. Preferably, the non-aqueous organic solvent accounts for 60-90% of the weight of the non-aqueous electrolyte. More preferably, the non-aqueous organic solvent accounts for 74-90% of the weight of the non-aqueous electrolyte. Preferably, the additive of structural formula M accounts for 1-5% of the weight of the non-aqueous electrolyte.

[0036] A third aspect of the present invention provides a secondary battery, comprising a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the electrolyte is the non-aqueous electrolyte described above.

[0037] Furthermore, the secondary battery satisfies at least one of the following: the active material of the positive electrode is one or more of sodium manganate, sodium copper iron manganate, sodium nickel iron manganate, sodium copper nickel iron manganate, sodium iron phosphate, sodium vanadium phosphate, and composite sodium iron phosphate (sodium nickel iron manganate is preferred); the active material of the negative electrode is a hard carbon material, any product known in the art is acceptable; the separator is a polyolefin separator and / or a glass fiber separator, with a thickness of 15-20 μm.

[0038] Hard carbon refers to carbon that is difficult to graphitize and is a material obtained after the thermal decomposition of polymers. The hard carbon materials described in the embodiments of this invention include resin-based hard carbon, pitch-based hard carbon, and biomass-based hard carbon, and can also be expressed as hard carbon (biomass), hard carbon (pitch), or hard carbon (resin). This invention further selects biomass-based hard carbon. Furthermore, the separator can be selected from polypropylene (PP), polyethylene (PE), or glass fiber separators. In addition, the embodiments of this invention do not impose special limitations on the preparation of sodium-ion battery electrolytes, electrodes, and secondary batteries; all battery components are commercially available products. For example, commonly used conductive agents and binders (such as polyvinylidene fluoride and styrene-butadiene rubber) can be used to coat and form corresponding electrodes, thereby assembling the secondary battery.

[0039] Tests have shown that the multiple synergistic effects of the first additive in this embodiment of the invention contribute to the excellent discharge performance and capacity retention of sodium-ion batteries. Detailed Implementation

[0040] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. Unless otherwise defined, all technical terms used herein have the same meaning as commonly understood by those skilled in the art. The technical terms used herein are for the purpose of describing specific embodiments only and are not intended to limit the scope of protection of this invention.

[0041] To better understand the technical content of this application, specific embodiments are provided below to further illustrate the application. All raw materials involved are commercially available conventional products and are not subject to any special restrictions.

[0042] Example 1 - Preparation Example 1 (Preparation of M1 molecule)

[0043] The synthesis path is shown below:

[0044]

[0045] 10 g (1 eq, 0.04 mol) of 99% pure hexafluorocyclotriphosphazene was added to a reaction flask equipped with a thermometer, followed by 50 mL of diethyl ether after dehydration using 5A molecular sieves. After thorough stirring and dissolution, the system temperature was lowered to -20°C. 1.76 g (2 eq, 0.08 mol) of the nucleophile lithium methyl was weighed using a dry syringe, and slowly injected into the reaction system under controlled-dry conditions. After the addition was complete, the reaction system temperature was controlled to room temperature, and the reaction was stirred. The reaction was stopped after 0.5 h. Unreacted lithium methyl was quenched by slowly adding excess ethanol, followed by vacuum distillation. The residue was purified by recrystallization from a mixture of chloroform and carbon tetrachloride, yielding 7.1 g of substance Z1 with a yield of 72.5% and a purity of 99%. TOF-MS (ESI) m / z calcd for CH3F5N3P3: 244.95; [M+H] + Found: 245.61.

[0046] Add 10 g (1 eq, 0.041 mol) of substance Z1 prepared in the previous step to a reaction flask equipped with a thermometer, then add 200 mL of acetone (after dehydration using 5A molecular sieves). Weigh 83 g (8 eq, 0.328 mol) of elemental iodine and add it to the reaction system, stirring thoroughly to dissolve. Add 31.5 g (8 eq, 0.328 mol) of sodium tert-butoxide to create an alkaline environment, then raise the temperature of the reaction system to 30 °C and stir for 12 h. After the reaction is complete, wash with 0.2% sodium thiosulfate aqueous solution to separate the organic phase. Dry with anhydrous magnesium sulfate and filter. The obtained liquid is subjected to vacuum distillation, concentrated, and then separated by column chromatography with an eluent volume ratio of ethyl acetate:methanol = 1:3. After purification, 10.2 g of substance Y1 is obtained, with a yield of 67% and a purity of 99%. TOF-MS (ESI) m / z calcd for CH2F5IN3P3: 370.84; [M+H] + Found: 370.11.

[0047] Add 3.60 g (1.1 eq, 0.03 mol) of ethyl mercaptoside and 2.4 g (2.2 eq, 0.06 mol) of sodium hydroxide catalyst to a reaction flask equipped with a thermometer. Slowly add 60 mL of methanol solvent (after dehydration using 5A molecular sieve), stir thoroughly to dissolve, then add 10 g (1 eq, 0.027 mol) of substance Y1 prepared in the previous step, stir thoroughly again to dissolve, and react at room temperature for 45 h. After the reaction is complete, add acetic acid to adjust the pH value, and perform vacuum distillation on the reaction system. Extract the residue with methane using a triple filter, concentrate the extract again, and separate by column chromatography with an eluent ratio of ethyl acetate:petroleum ether = 1:9. After purification, 5.68 g of substance M1 is obtained, with a yield of 58% and a purity of 99%. TOF-MS (ESI) m / z calcd for C5H9F5N3O2P3S: 362.95; [M+H] + Found: 363.74.

[0048] Example 2 - Preparation Example 2 (Preparation of M2 molecule)

[0049] The synthesis path is shown below:

[0050]

[0051] 10 g (1 eq, 0.04 mol) of 99% pure hexafluorocyclotriphosphazene was added to a reaction flask equipped with a thermometer, followed by 50 mL of diethyl ether after dehydration using 5A molecular sieves. After thorough stirring and dissolution, the system temperature was lowered to -20°C. 2.63 g (3 eq, 0.12 mol) of the nucleophile lithium methyl was weighed using a dry syringe, and slowly injected into the reaction system under controlled-dry conditions. After the addition was complete, the reaction system temperature was controlled to room temperature, and the reaction was stirred. The reaction was stopped after 0.5 h. Unreacted lithium methyl was quenched by slowly adding excess ethanol, followed by vacuum distillation. The residue was purified by recrystallization from a mixture of chloroform and carbon tetrachloride, yielding 7.8 g of substance Z2, with a yield of 81% and a purity of 99%. TOF-MS (ESI) m / z calcd for C2H6F4N3P3: 240.97; [M+H] + Found: 241.72.

[0052] Add 10 g (1 eq, 0.041 mol) of substance Z2 prepared in the previous step to a reaction flask equipped with a thermometer, then add 250 mL of acetone (after dehydration using 5A molecular sieves). Weigh 93.6 g (9 eq, 0.369 mol) of elemental iodine and add it to the reaction system, stirring thoroughly to dissolve. Add 39.4 g (10 eq, 0.41 mol) of sodium tert-butoxide to create an alkaline environment, then raise the temperature of the reaction system to 40 °C and stir for 13 h. After the reaction is complete, wash with 0.2% sodium thiosulfate aqueous solution to separate the organic phase. Dry with anhydrous magnesium sulfate and filter. The obtained liquid is subjected to vacuum distillation, concentrated, and then separated by column chromatography with an eluent ratio of ethyl acetate:methanol = 1:3. After purification, 10.5 g of substance Y2 is obtained, with a yield of 70% and a purity of 99%. TOF-MS (ESI) m / z calcd for C2H5F4IN3P3: 366.87; [M+H] + Found: 367.42.

[0053] Add 3.85 g (1.2 eq, 0.032 mol) of ethyl mercaptoacetate and 2.4 g (2.2 eq, 0.06 mol) of sodium hydroxide catalyst to a reaction flask equipped with a thermometer. Slowly add 60 mL of methanol solvent (after dehydration using 5A molecular sieves), stir thoroughly to dissolve, then add 10 g (1 eq, 0.027 mol) of substance Y2 prepared in the previous step, stir thoroughly again to dissolve, and react at room temperature for 45 h. After the reaction is complete, add acetic acid to adjust the pH value, and perform vacuum distillation on the reaction system. Extract the residue with methane using a triple filter, concentrate the extract again, and separate by column chromatography with an eluent ratio of ethyl acetate:petroleum ether = 1:9. After purification, 6 g of substance M2 is obtained, with a yield of 62% and a purity of 99%. TOF-MS (ESI) m / z calcd for C6H 12 F4N3O2P3S: 358.98; [M+H] + Found: 359.94.

[0054] Example 3 - Preparation Example 3 (Preparation of M3 molecules)

[0055] The synthesis path is shown below:

[0056]

[0057] 10 g (1 eq, 0.04 mol) of 99% pure hexafluorocyclotriphosphazene was added to a reaction flask equipped with a thermometer, followed by 50 mL of diethyl ether after dehydration using 5A molecular sieves. After thorough stirring and dissolution, the system temperature was lowered to -20°C. 3.52 g (4 eq, 0.16 mol) of the nucleophile lithium methyl was weighed using a dry syringe, and slowly injected into the reaction system under controlled-dry conditions. After the addition was complete, the reaction system temperature was controlled to room temperature, and the reaction was stirred. The reaction was stopped after 0.5 h. Unreacted lithium methyl was quenched by slowly adding excess ethanol, followed by vacuum distillation. The residue was purified by recrystallization from a mixture of chloroform and carbon tetrachloride, yielding 7.42 g of substance Z2, with a yield of 77% and a purity of 99%. TOF-MS (ESI) m / z calcd for C2H6F4N3P3: 240.97; [M+H] + Found: 241.57.

[0058] Add 10 g (1 eq, 0.042 mol) of substance Z2 prepared in the previous step to a reaction flask equipped with a thermometer, then add 200 mL of acetone (after dehydration using 5A molecular sieves). Weigh 83 g (10 eq, 0.42 mol) of elemental iodine and add it to the reaction system, stirring thoroughly to dissolve. Add 40.4 g (10 eq, 0.42 mol) of sodium tert-butoxide to create an alkaline environment, then raise the temperature of the reaction system to 40 °C and stir for 15 h. After the reaction is complete, wash with 0.2% sodium thiosulfate aqueous solution to separate the organic phase. Dry with anhydrous magnesium sulfate and filter. The obtained liquid is subjected to vacuum distillation, concentrated, and then separated by column chromatography with an eluent ratio of ethyl acetate:methanol = 1:3. After purification, 13.86 g of substance Y3 is obtained, with a yield of 67% and a purity of 99%. TOF-MS (ESI) m / z calcd for C2H4F4I2N3P3: 492.76; [M+H] + Found: 493.54.

[0059] Add 5.29 g (2.2 eq, 0.044 mol) of ethyl mercaptoacetate and 1.76 g (2.2 eq, 0.044 mol) of sodium hydroxide catalyst to a reaction flask equipped with a thermometer. Slowly add 60 mL of methanol solvent (after dehydration using 5A molecular sieves), stir thoroughly to dissolve, then add 10 g (1 eq, 0.02 mol) of substance Y3 prepared in the previous step, stir thoroughly again to dissolve, and react at room temperature for 45 h. After the reaction is complete, add acetic acid to adjust the pH value, and perform vacuum distillation on the reaction system. Extract the residue with methane using a triple filter, concentrate the extract again, and separate by column chromatography with an eluent ratio of ethyl acetate:petroleum ether = 1:9. After purification, 5.86 g of substance M3 is obtained, with a yield of 59% and a purity of 99%. TOF-MS (ESI) m / z calcd for C10 H 18 F4N3O4P3S2: 476.99; [M+H] + Found: 477.97.

[0060] Example 4

[0061] Preparation of standard electrolyte for sodium-ion batteries:

[0062] In a glove box filled with N2, propylene carbonate (PC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) are mixed uniformly in a mass ratio of PC:EMC:DEC = 3:5:2. Sodium difluorosulfonamide (NaFSI) is slowly added to the mixed solution until the molar concentration is 1 mol / L to obtain a standard electrolyte.

[0063] Adding 1 wt% of M1 from Preparation Example 1 to the standard electrolyte yielded the electrolyte of Example 4.

[0064] Example 5

[0065] In the standard electrolyte of Example 4, 1 wt% of M2 from Preparation Example 2 was added to obtain the electrolyte of Example 5.

[0066] Example 6

[0067] In the standard electrolyte of Example 4, 1 wt% of M3 from Preparation Example 3 was added to obtain the electrolyte of Example 6.

[0068] Comparative Example 1

[0069] Use the standard electrolyte as the electrolyte for Comparative Example 1.

[0070] Comparative Example 2

[0071] To a standard electrolyte, 1 wt% of an existing unsaturated carbonate additive, vinylene carbonate (VC), was added to obtain the electrolyte of Comparative Example 2.

[0072] Comparative Example 3

[0073] To a standard electrolyte, 1 wt% of an existing fluorocarbonate additive, fluoroethylene carbonate (FEC), was added to obtain the electrolyte of Comparative Example 3.

[0074] Comparative Example 4

[0075] To a standard electrolyte, 1 wt% of an existing sulfone additive, sulfolane, was added to obtain the electrolyte of Comparative Example 4.

[0076] Comparative Example 5

[0077] To a standard electrolyte, 1 wt% of the existing ether additive 18-crown ether-6 was added to obtain the electrolyte of Comparative Example 5.

[0078] Comparative Example 6

[0079] To a standard electrolyte, 1 wt% of an existing additive with a similar structure to that of the present invention, ethoxy(pentafluoro)cyclotriphosphazene, was added to obtain the electrolyte of Comparative Example 6.

[0080] Comparative Example 7

[0081] To a standard electrolyte, 1 wt% of an existing additive with a similar structure to that of the present invention, phenoxy(pentafluoro)cyclotriphosphazene, was added to obtain the electrolyte of Comparative Example 7.

[0082] After preparing the above-mentioned comparative sodium electrolyte, an external viscosity test was conducted on it. The test environment was a constant temperature glove box with an oxygen content of less than 50 ppm, a water content of less than 20 ppm, and a temperature maintained at 25°C. The results are summarized in the table below. It can be seen that the sodium electrolyte containing the novel structural additive in this embodiment of the invention has a relatively low viscosity (3.94~3.74 cp).

[0083] Table 1 Comparison of viscosity of sodium electrolyte

[0084] Test case Viscosity / cP Test case Viscosity / cP Comparative Example 1 4.31 Comparative Example 3 4.63 Example 4 3.94 Comparative Example 4 4.26 Example 5 3.87 Comparative Example 5 3.93 Example 6 3.74 Comparative Example 6 3.99 Comparative Example 2 4.33 Comparative Example 7 4.12

[0085] Note: The purpose of the external viscosity test of the battery is to test whether the additive of this invention has a beneficial effect on the viscosity of the electrolyte system, since the viscosity of the electrolyte plays a decisive role in the overall ion transport performance at low temperature and high rate.

[0086] Application Example 1

[0087] Preparation of sodium-ion pouch cells: Weigh out the active material Na in a dry room. 0.9 Ni 1 / 3 Fe 1 / 3 Mn 1 / 3 O2, conductive carbon black, and PVDF binder (a 5wt% PVDF solution prepared with NMP as solvent) in a mass ratio of 95:2.5:2.5 are dissolved in N-methylpyrrolidone (NMP) to form a slurry. This slurry is then uniformly coated onto an aluminum current collector, dried, and rolled into an electrode core using a roller press. After cutting and drying, the positive electrode of the pouch battery is obtained. Simultaneously, the negative electrode active material (biomass-based), conductive carbon black, sodium carboxymethyl cellulose (CMC), and SBR binder in a mass ratio of 94:2:2:2 are weighed and prepared into a slurry using deionized water. This slurry is then uniformly coated onto an aluminum current collector, dried, and rolled into an electrode core using a roller press. After cutting and drying, the negative electrode of the pouch battery is obtained. Finally, the battery was semi-encapsulated according to the ratio of positive electrode to negative electrode of 12:13 and dried to facilitate the subsequent preparation and injection of electrolyte. The subsequent electrolyte injection coefficient was 6.5g / Ah.

[0088] The electrolyte used in the prepared soft-pack battery is a solution of sodium bis(fluorosulfonyl)imide (1 mol / L) in propylene carbonate (PC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DEC) (mass ratio 3:5:2), with 1 wt% of the electrolyte additive provided in Preparation Example 1 of this invention added; the separator is a PE membrane.

[0089] Application Example 2

[0090] The only difference between Application Example 2 and Application Example 1 is that 1 wt% of the electrolyte additive prepared in Preparation Example 2 of the present invention is added, while the other parameters and steps are the same as in Application Example 1.

[0091] Application Example 3

[0092] The only difference between Application Example 3 and Application Example 1 is that 1 wt% of the electrolyte additive prepared in Preparation Example 3 of the present invention is added, while the other parameters and steps are the same as in Application Example 1.

[0093] Comparative Application Example 1

[0094] The only difference between Application Example 1 and Application Example 2 is that 1 wt% of the existing unsaturated carbonate additive vinylene carbonate is added; the other parameters and steps are the same as in Application Example 1.

[0095] Comparative Application Example 2

[0096] The only difference between Application Example 2 and Application Example 1 is that 1 wt% of the existing fluorocarbonate additive fluoroethylene carbonate is added, while the other parameters and steps remain the same as in Application Example 1.

[0097] Comparative Application Example 3

[0098] The only difference between Application Example 3 and Application Example 1 is that 1 wt% of the existing sulfone additive sulfolane is added, while the other parameters and steps remain the same as in Application Example 1.

[0099] Comparative Application Example 4

[0100] The only difference between Application Example 4 and Application Example 1 is that 1 wt% of the existing ether additive 18-crown ether-6 is added, while the other parameters and steps are the same as in Application Example 1.

[0101] Comparative Application Example 5

[0102] The only difference between Application Example 5 and Application Example 1 is that 1 wt% of an existing additive with a similar structure to that of the present invention, ethoxy(pentafluoro)cyclotriphosphazene, is added; the remaining parameters and steps are consistent with those of Application Example 1.

[0103] Comparative Application Example 6

[0104] The only difference between Application Example 6 and Application Example 1 is that 1 wt% of an existing additive with a similar structure to that of the present invention, phenoxy(pentafluoro)cyclotriphosphazene, is added; the remaining parameters and steps are consistent with those of Application Example 1.

[0105] Performance testing

[0106] Corresponding to use cases 1-3, and compared with the soft-pack batteries obtained in application examples 1-6, performance tests were conducted on the following: room temperature (25℃) cycle (1000 cycles), low temperature (0℃) cycle, low temperature (-40℃) discharge performance, and negative electrode surface state performance after high rate (10C) charge and discharge. The test results are shown in Table 2 below:

[0107] Table 2 Comparison of Electrochemical Performance of Sodium Battery Pouch Cells

[0108]

[0109] The data above clearly show that: a) the addition of the novel additives of this invention significantly reduces the viscosity of the existing basic electrolyte formulation, ensuring high kinetic performance of the battery under low temperature and high rate (fast charging) conditions, thus improving the battery's low temperature and rate performance (Table 1). b) In tests on sodium-ion battery pouch cells, it was found that after 1000 cycles at room temperature, the capacity retention rate was above 93.2%; after 500 cycles at low temperature (0°C), the capacity retention rate was above 88.1%; after 200 cycles at high rate (15C), the capacity retention rate was above 85.2%. No sodium deposition was observed on the negative electrode after a 10C high rate charge-discharge, and a high capacity retention rate was observed after cycling. Therefore, it can be concluded that the multiple synergistic effects of the additives of this invention contribute to the excellent discharge performance and capacity retention of the battery; this is mainly due to the improvement of the main problem of sodium deposition on the hard carbon negative electrode, resulting in a series of positive improvements. This provides a framework for further improving the performance of hard carbon negative electrodes through electrolytes. Batteries prepared using existing positive and negative electrode film-forming additives exhibit significantly lower performance under harsh conditions such as low temperature and high rate of operation compared to the additives of this invention.

[0110] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit the scope of protection of the present invention. The description of the above embodiments is only used to help understand the method and core idea of ​​this application. At the same time, for those skilled in the art, there will be changes in the specific implementation and application scope based on the idea of ​​this application. Therefore, the content of this specification should not be construed as a limitation of this application.

Claims

1. A cyclotriphosphazene compound, characterized in that, It has an M-shaped structure: Wherein, R1 is fluorine, C1-2 alkyl, CH2SCH2COOCH3 or CH2SCH2COOCH2CH3, and R2 is hydrogen or methyl.

2. The cyclotriphosphazene compound according to claim 1, characterized in that, The cyclotriphosphazene compound has any of the following structures:

3. The method for preparing the cyclotriphosphazene compound as described in claim 1 or 2, comprising the following steps: The compound shown in formula Y is reacted with methyl mercaptoacetate or ethyl mercaptoacetate in an organic solvent in the presence of a catalyst to obtain a cyclotriphosphazene compound having the structure of formula M. Wherein, R1 is fluorine, C1-2 alkyl, CH2SCH2COOCH3 or CH2SCH2COOCH2CH3, X is a halogen element, and R2 is hydrogen or methyl.

4. The preparation method according to claim 3, characterized in that, The reaction shall meet at least one of the following conditions: the catalyst is a hydroxide and / or carbonate; the organic solvent is an alcohol; the organic solvent is used after being redistilled or dehydrated by molecular sieve; the reaction temperature is 15–50°C; and the reaction time is 24–50 h.

5. The preparation method according to claim 3, characterized in that, The catalyst is one or more of sodium hydroxide, potassium hydroxide, sodium carbonate, and potassium carbonate; the organic solvent is one or more of methanol, ethanol, propanol, isopropanol, glycerol, ethylene glycol, and propylene glycol.

6. A non-aqueous electrolyte, characterized in that, include: The electrolyte salt, the non-aqueous organic solvent, and the first additive, wherein the first additive is a cyclotriphosphazene compound as described in claim 1 or 2.

7. The non-aqueous electrolyte according to claim 6, characterized in that, The electrolyte salt is a sodium salt; the content of the electrolyte salt accounts for 8-25% of the weight of the non-aqueous electrolyte; the non-aqueous organic solvent is one or more of organic esters, C1-10 alkyl ethers, cyclic ethers, sulfones, and dinitrile; the non-aqueous organic solvent accounts for 60-90% of the weight of the non-aqueous electrolyte; and the first additive accounts for 1-5% of the weight of the non-aqueous electrolyte.

8. The non-aqueous electrolyte according to claim 6, characterized in that, The electrolyte salt is selected from at least one of sodium hexafluorophosphate, sodium difluorosulfonamide, sodium perchlorate, sodium tetrafluoroborate, sodium difluorophosphate, and sodium difluorooxalate borate.

9. A secondary battery, comprising a positive electrode, a negative electrode, a separator, and an electrolyte, characterized in that, The electrolyte is the non-aqueous electrolyte according to any one of claims 6-8.

10. The secondary battery according to claim 9, characterized in that, The positive electrode active material is one or more of sodium manganate, sodium copper iron manganate, sodium nickel iron manganate, sodium copper nickel iron manganate, sodium iron phosphate, sodium vanadium phosphate, and composite sodium iron phosphate; the negative electrode active material is a hard carbon material; and the separator is a polyolefin separator and / or a glass fiber separator.

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