Sodium-ion battery electrolyte containing phosphorus-containing benzene flame retardant as well as preparation method and application thereof

Abstract

The application discloses a sodium-ion battery electrolyte containing phosphorus benzene flame retardant as well as a preparation method and application thereof, and belongs to the technical field of sodium-ion batteries.The electrolyte components include alkali metal inorganic salt, organic solvent and additives, wherein the additives include phosphorus benzene flame retardant and second additives; the application provides two types of phosphorus benzene flame retardant structures, the two types of structures respectively have the structural characteristics of introducing benzene rings, ester groups on the phosphorus benzene branch and forming a closed ring structure with the benzene ring and the phosphorus, and have good structural stability and high flame retardant performance; the phosphorus benzene flame retardant can also be modified with groups containing nitrogen, sulfur, fluorine or carboxyl, and under the synergistic effect of the second additives, the alkali metal inorganic salt and the organic solvent, the electrolyte containing the phosphorus benzene flame retardant has good flame retardant effect and high ionic conductivity, and is expected to overcome the problems of poor capacity release and cycle stability commonly existing in sodium-ion batteries, and has a wide application prospect in the field of sodium-ion batteries.

Description

Technical Field

[0001] This invention specifically relates to a sodium-ion battery electrolyte containing phosphorus-benzene flame retardant, its preparation method, and its application, belonging to the field of sodium-ion battery technology. Background Technology

[0002] Currently, lithium-ion batteries are experiencing strong growth in consumer electronics, new energy vehicles, and grid energy storage. However, lithium-ion batteries cannot simultaneously support the development of both the electric vehicle and grid energy storage industries. In recent years, sodium-ion batteries, with their advantages of abundant resources, cleanliness, high energy density, and low cost, are expected to become an important supplement to lithium-ion batteries in the new energy field. However, in addition to low cost and high energy density, the safety of sodium-ion batteries is also a prominent issue. The non-aqueous electrolyte of sodium-ion batteries is generally composed of one or more sodium salts dissolved in a mixture of two or more organic solvents and additives. Commonly used solvents include ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and methyl ethyl carbonate (EMC). The flammability of organic solvents makes the thermal stability and non-flammability of sodium-ion battery electrolytes the main factors affecting the safety of sodium-ion batteries.

[0003] Currently, adding phosphorus-containing flame retardants to electrolytes is an important means to solve the problem of electrolyte flammability. Chinese patent CN105655631A discloses a high-safety, non-flammable sodium secondary battery system. This system uses non-flammable phosphate esters and sodium salts as electrolytes. The non-flammable phosphate ester solvent has the structural characteristic R1OP(O)OR2OR3, where R1, R2, or R3 are alkyl aryl groups, halogen-substituted alkyl or aryl groups, etc. R1, R2, and R3 can be symmetrical, asymmetrical, or cyclic. Halogen substitution can be partial or complete substitution, and the halogen is F, Cl, or Br, etc. This electrolyte has high safety and good cycle performance. Chinese patent CN114122516A discloses a non-flammable, high-safety sodium-ion battery. The battery includes a positive electrode, a negative electrode, and an electrolyte. The electrolyte includes a sodium salt, an ester solvent, and a flame retardant. The flame retardant is selected from one or more of trimethyl phosphate, triethyl phosphate, tributyl phosphate, dimethyl methyl phosphate, diethyl ethyl phosphate, diphenyl isopropyl phosphate, or hexamethylphosphoramide. Chinese patent CN117525596A discloses a sodium battery electrolyte, its preparation method, and a sodium battery. The sodium battery electrolyte includes a sodium salt, a non-aqueous solvent, and additives. The additives include fluorophosphate compounds and fluorocarbonate compounds. This electrolyte can reduce the capacity decay of sodium batteries and improve the cycle performance of sodium batteries to a certain extent.

[0004] It is evident that phosphorus-containing flame retardant electrolytes have been widely used in the field of sodium-ion batteries. However, most of the existing phosphorus-containing flame retardants are short-chain molecules. Although they have a strong ability to capture combustion free radicals and have excellent flame retardant effects, the electrolytes containing such flame retardants have a high viscosity and poor compatibility with the battery anode, which leads to low ionic conductivity of the electrolyte and accelerated battery capacity decay. This has been reported in many previous articles ([1] Hiroe Nakagawa, Manabu Ochida, Yasuhiro Domi, et al. Journal of Power Sources[J], 2012, 212: 148-153. [2] Daiying Zhou, Weishan Li, Chunlin Tan, et al. Journal of Power Sources[J], 2008, 184: 589-592.). These problems further increase the cost of sodium-ion batteries and limit their use. Therefore, developing phosphorus-containing flame-retardant electrolytes with high safety, excellent flame retardancy, good conductivity, and good stability in the field of sodium-ion batteries has broad development prospects. Summary of the Invention

[0005] To address the challenges of simultaneously achieving both flame retardant and conductivity properties in current sodium-ion battery electrolytes, as well as the strong anode decomposition effect of phosphorus-containing flame retardants, this invention provides a sodium-ion battery electrolyte containing a phosphorus-benzene flame retardant. This electrolyte effectively utilizes the flame retardant properties of existing phosphorus-containing flame retardants by improving the structural stability of the phosphorus-benzene through the introduction of benzene rings and ester groups onto the phosphorus-benzene branch chain, and by forming a closed-ring structure with phosphorus. This mitigates the strong anode decomposition effect of existing phosphorus-containing flame retardants. Furthermore, the phosphorus-benzene flame retardant in this electrolyte can be structurally modified with nitrogen, sulfur, fluorine, or carboxyl groups to further enhance its flame retardant properties and sodium content. + With the synergistic effect of other additives, alkali metal inorganic salts, and organic solvents in the electrolyte, this electrolyte has both good flame retardancy and ionic conductivity, and is expected to overcome the problems of poor capacity utilization and cycle stability that are common in sodium-ion batteries. It has broad application prospects in the field of sodium-ion batteries.

[0006] The technical solution of the present invention is as follows:

[0007] One objective of this invention is to provide a sodium-ion battery electrolyte, the components of which include additives, alkali metal inorganic salts, and organic solvents, as detailed below:

[0008] (1) The additive is composed of a phosphorobenzene flame retardant and a second additive; the phosphorobenzene flame retardant is one or more compounds with the following structural formulas 1 and 2:

[0009] (i) Formula 1: R1 is one of hydroxyl, amino, carboxylic acid, oxybenzene ring, pyrrole, pyrazole, imidazole, thiophene, or a hydroxyl, amino, or carboxylic acid derivative with 1 to 6 carbon atoms; R2 is one of hydrogen or an alkyl, haloalkyl, olefin, alkyne, haloolefin, haloalkynyl, amino, sulfone, or silyl group composed of 1 to 4 carbon atoms.

[0010] (ii) Equation 2: R1 is one of the following: hydroxyl, amino, carboxylic acid, five-membered heterocycle, or a hydroxyl, amino, carboxylic acid, five-membered heterocycle derivative with 1 to 6 carbon atoms, or an alkyl, haloalkyl, olefinic, alkyneic, haloolefinic, haloalkynic, phenyl, halobenzene, amino, sulfone, or silyl group.

[0011] The phosphorobenzene flame retardant provided by this invention is a phosphate ester composed of aryl substituents. Its high benzene ring content gives it a higher flame retardant efficiency than linear phosphate esters. In the electrolyte, this phosphorobenzene flame retardant, due to the strong electronegativity and easy reduction of the phosphorus atoms in its structure, preferentially reacts with the negative electrode surface during sodium-ion battery formation to form a phosphorus-containing organic SEI film. This improves the stability of the SEI film on the negative electrode surface, enhances the cycle performance of the sodium-ion battery, and suppresses gas generation during cycling. Under the high-temperature environment of battery operation, the phosphorus-containing flame retardant additive can release phosphorus-containing free radicals with flame-retardant properties, which then capture hydrogen in the organic free radical chain combustion reaction, terminating the chain reaction. This effectively improves the flame retardant effect of the electrolyte and enhances the thermal stability and thermal shock resistance of the sodium-ion battery under high-temperature conditions.

[0012] When adding phosphorus-containing flame retardant additives to the electrolyte, if the amount added is too large, although it can shorten the battery's self-extinguishing time (SET) and improve the battery's flame retardant performance, it also results in a thicker SEI film formed at the electrode, increasing the battery's internal resistance and reducing the cycle performance of the sodium-ion battery. If the amount added is too small, the phosphorus-containing flame retardant additives do not fully perform their function in the electrolyte, resulting in a long self-extinguishing time, poor flame retardant performance, and an inability to maximize the improvement of the sodium-ion battery's cycle performance.

[0013] Furthermore, the mass fraction of the phosphorobenzene flame retardant in the electrolyte is 0.1% to 1.5%, preferably 0.3% to 1.0%; the mass fraction of the second additive in the electrolyte is 0.5% to 7%.

[0014] Preferably, the phosphorobenzene flame retardant shown in Formula 1 includes any one or more of the following structures.

[0015]

[0016]

[0017] Preferably, the phosphorobenzene flame retardant shown in Formula 2 includes any one or more of the following structures.

[0018]

[0019]

[0020] The present invention further includes a second additive on the basis of phosphorus-benzene flame retardant; the second additive is one or more of fluoroethylene carbonate (FEC), vinylene carbonate (VC), ethylene sulfate (DTD), 1,3-propane sulpholactone (PS), and aliphatic crown ether, wherein the aliphatic crown ether is one of 18-crown ether-6 (18-C-6), 15-crown ether-5 (15-C-5), and 12-crown ether-4 (12-C-4).

[0021] Preferably, the second additive is composed of fluoroethylene carbonate (FEC) or vinylene carbonate (VC), 1,3-propane sulpholactone (PS) or ethylene sulfate (DTD), and an aliphatic crown ether, with a mass ratio of (2-5):(1-3):(0.5-1). Among the selected second additives, FEC and VC can promote the formation of a tight and stable SEI film at the cathode, reduce side reactions between the electrode and the electrolyte, and improve the cycle stability of the battery. DTD and PS can decompose at the cathode to form organic sulfonates with good ionic conductivity, reduce interfacial impedance, suppress the decrease in battery initial capacity, and improve the battery's capacity performance and high-temperature performance. In addition to the above additives, the addition of aliphatic crown ether can also reduce anodic polarization and improve the cycle stability of the electrolyte. The strong complexing ability of aliphatic crown ether with sodium ions can improve the solubility of sodium salts and improve the solvation structure of sodium ions, thereby improving the battery's capacity performance and cycle stability.

[0022] (2) Alkali metal inorganic salts consist of a main salt electrolyte and an auxiliary salt additive;

[0023] The main salt electrolyte is one of sodium hexafluorophosphate (NaPF6), sodium perchlorate (NaClO4), sodium trifluoromethanesulfonate (NaOTF), sodium bis(trifluoromethylsulfonyl)imide (NaFSI), and sodium bis(trifluoromethylsulfonyl)imide (NaTFSI); the mass percentage of the main salt electrolyte in the electrolyte is 8-30%.

[0024] Preferably, the main salt electrolyte is sodium hexafluorophosphate (NaPF6). NaPF6 has a high ion transport medium, which helps the charge and discharge reaction of sodium-ion batteries and can stabilize the battery electrolyte, thus giving sodium-ion batteries excellent electrochemical performance.

[0025] Preferably, the mass percentage of the main salt electrolyte in the electrolyte is 10-20%;

[0026] The auxiliary salt additive is composed of sodium metal inorganic salt additive and / or lithium metal inorganic salt additive; the anion type of the sodium metal inorganic salt additive and the lithium metal inorganic salt additive is one or more of the following: difluorophosphate, difluorosulfonyl imide, difluorooxalate borate, tetrafluoroborate, bis(trifluoromethanesulfonyl)imide, hexafluorophosphate, difluorooxalate phosphate, difluorooxalate borate, and trifluoromethanesulfonate; the amount of the auxiliary salt additive added meets one or more of the following conditions:

[0027] (i) The mass percentage of sodium metal inorganic salt additive in the electrolyte is 0.01 to 6%; preferably, the mass percentage of sodium metal inorganic salt additive in the electrolyte is 0.01 to 3%.

[0028] (ii) The mass percentage of lithium metal inorganic salt additive in the electrolyte is 0.0001 to 0.01%; preferably, the mass percentage of lithium metal inorganic salt additive in the electrolyte is 0.0001 to 0.003%.

[0029] Preferably, the auxiliary salt additive is composed of sodium metal inorganic salt additive and lithium metal inorganic salt additive, wherein the anion type of the sodium metal inorganic salt additive and the lithium metal inorganic salt additive is the same, and the anion type is one of difluorophosphate, difluorosulfonyl imide, bis(trifluoromethylsulfonyl)imide, and difluorooxalate borate.

[0030] Sodium metal inorganic salt additives can promote SEI film formation, effectively avoid side reactions between the electrolyte and the electrode, and improve the stability of the electrolyte. At the same time, its anions can further weaken the sodium ion solvation structure, improve the ionic conductivity of the electrolyte, reduce sodium ion transport resistance, and improve the charge-discharge specific capacity of sodium ions. On the basis of sodium metal inorganic salt additives, lithium metal inorganic salt additives are added. The introduced lithium ions can be inserted into positions where sodium ions cannot be inserted, improving the utilization rate of active materials. At the same time, lithium ions are inserted into the nanoporous structure, reducing the irreversible reaction of sodium ions, further reducing the self-discharge and cycle decay rate. In addition, lithium ions can realize the reversible reaction of insertion and deinsertion of negative electrode active materials, further improving the battery's capacity and cycle performance.

[0031] Preferably, the sodium metal inorganic salt additive has a mass percentage of 0.01 to 3% in the electrolyte;

[0032] Preferably, the lithium metal inorganic salt additive has a mass percentage of 0.0001 to 0.003% in the electrolyte.

[0033] (3) The organic solvent is a cyclic or chain-like carbonate solvent or a mixed solvent of cyclic or chain-like carbonates and ethers;

[0034] The carbonate solvent is one or more of the following: cyclic propylene carbonate (PC), ethylene carbonate (EC), and chain-like ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), and diethyl carbonate (DEC); the ether solvent is one or more of the following: crown ether, dimethyl glycol ether (DME), diethylene glycol dimethyl ether (G2), triethylene glycol dimethyl ether (G3), and tetraethylene glycol dimethyl ether (G4); the carbonate solvent has a mass fraction of 30-80% in the electrolyte; and the ether organic solvent has a mass fraction of 1-15% in the electrolyte.

[0035] Chain-like carbonate solvents have lower dielectric constants, lower viscosity, and narrower liquid temperature ranges; cyclic carbonate solvents have higher dielectric constants, higher viscosity, and higher melting and boiling points; ether solvents have higher ionic conductivity, sodium transfer number, and suitable stability window, providing high wettability and low viscosity, reducing impedance to electrodes, and minimizing sodium deposition.

[0036] Preferably, the organic solvent is a mixed solvent of carbonates and ethers; wherein the carbonate solvent used is two or three of cyclic propylene carbonate (PC), chain ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC), and the mass ratio of propylene carbonate (PC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) is (1-2):(1-1.5):(1-1.5), and the ether solvent used is diethylene glycol dimethyl ether (G2) and / or triethylene glycol dimethyl ether (G3).

[0037] The second objective of this invention is to provide a method for preparing the sodium-ion battery electrolyte containing the above-mentioned phosphorus-benzene flame retardant, the method comprising the following steps:

[0038] S1: Weigh out the required amounts of organic solvent, alkali metal inorganic salt, and additives according to the electrolyte solution formula;

[0039] S2: In an argon atmosphere glove box with oxygen and moisture content ≤1ppm, mix the organic solvent evenly according to a certain mass ratio, add the molecular sieve dehydrating agent, and let stand for 2 days;

[0040] S3: Then dissolve the dry alkali metal inorganic salt in an organic solvent and stir until it is completely dissolved and homogeneous, and the liquid is clear and transparent;

[0041] S4: Add the additive to the above liquid, mix well, and pour into a filter to remove impurities, thereby obtaining an impurity-free electrolyte.

[0042] A third objective of this invention is to provide a sodium-ion battery using the sodium-ion battery electrolyte containing the aforementioned phosphorus-benzene flame retardant as the electrolyte; the sodium-ion battery includes a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and the aforementioned sodium-ion battery electrolyte containing the aforementioned phosphorus-benzene flame retardant.

[0043] Unlike existing technologies, this invention has the following advantages:

[0044] 1. The two phosphorus-benzene flame retardants provided by this invention improve the structural stability of phosphorus-benzene by introducing benzene rings and ester groups on the phosphorus-benzene side chains and forming closed-ring structures with phosphorus. Compared with previous phosphorus-containing flame retardants, the phosphorus-benzene flame retardant of Formula 1 contains benzene rings and ester groups in its phosphate side chains, which can release PO2· and HPO2· free radicals. While further capturing H· and O· in the electrolyte, a polymerization reaction occurs, thereby generating a polyphosphate ester structure, which can effectively optimize the flame retardant performance of the battery and reduce the decomposition of the flame retardant on the battery anode. The phosphorus-benzene flame retardant of Formula 2 forms a closed ring with phosphorus, which improves the structural stability of phosphorus-benzene while retaining the flame retardant properties of phosphorus, further increases the boiling point of phosphorus-benzene, optimizes the flame retardant performance of the battery, and reduces the decomposition of the flame retardant on the anode.

[0045] 2. Unlike previous technologies, the phosphorus-benzene flame retardant of the present invention can be further structurally modified. In some preferred embodiments, nitrogen-, sulfur-, fluorine-, or carboxyl groups can be modified on the phosphorus-benzene structure. When the branched chains of the phosphorus-benzene structure modified with nitrogen and sulfur groups decompose under heat, they can generate non-flammable gases (nitrogen, ammonia, sulfur dioxide), diluting the concentration of combustion-supporting gases in the battery and inhibiting battery combustion. The F-containing groups act as "scavengers" during degradation, acting on free radicals to cut off internal thermal decomposition of the battery. The hydroxyl-containing groups undergo carboxyl dehydration condensation during degradation, diluting the concentration of combustion-supporting gases to inhibit battery combustion. The degradation reactions of the modified groups are all endothermic reactions, consuming heat to slow the rapid rise in internal battery temperature. All of the above characteristics can significantly improve the flame retardant performance of the battery. Furthermore, the phosphorus-benzene additive modified with nitrogen-, sulfur-, fluorine-, or carboxyl groups, during normal battery charging and discharging, is superior to EC and PC in forming an inorganic SEI film containing P, N, S, or F through reductive decomposition, while simultaneously optimizing and improving Na... + The solvation structure forms more contact ion pairs (CIPs), improving the Na+... + Ion transport rate improves the capacity utilization and cycle stability of sodium-ion batteries.

[0046] 3. The sodium-ion battery electrolyte of the present invention introduces crown ether as a second additive on the basis of phosphorus benzene flame retardant. Crown ether additive can reduce anodic polarization, inhibit gas production in sodium-ion batteries during cycling, improve the cycle stability of electrolyte, increase the solubility of sodium salt, improve the solvation structure of sodium ions, and effectively improve the thermal stability and thermal shock resistance of the battery, thereby improving the battery capacity and cycle stability.

[0047] 4. The sodium-ion battery electrolyte of this invention adds sodium metal inorganic salt and / or lithium metal inorganic salt as auxiliary additives to the existing main salt electrolyte. The sodium metal inorganic salt additive can further promote the formation of SEI film, effectively avoid side reactions between electrolyte and electrode, and improve the stability of electrolyte. At the same time, the anions in the sodium salt additive can further weaken the sodium ion solvation structure, improve the ionic conductivity of electrolyte, reduce sodium ion transport resistance, and improve the charge and discharge specific capacity of sodium ions. On this basis, the addition of lithium metal inorganic salt additive allows the introduced lithium ions to be inserted into positions where sodium ions cannot be inserted, improving the utilization rate of active materials. At the same time, the lithium ions are inserted into the nanoporous structure, reducing the irreversible reaction of sodium ions, further reducing the self-discharge and cycle decay rate. In addition, lithium ions can realize the reversible reaction of insertion and detachment of negative electrode active material, further improving the battery capacity and cycle performance. Detailed Implementation

[0048] The present invention will be further described below with reference to preferred embodiments. The embodiments given are only for illustrating the present invention and are not intended to limit the scope of the present invention.

[0049] Unless otherwise specified, all materials and reagents used in the following examples are commercially available; and all methods used in the following examples are conventional methods.

[0050] Example 1

[0051] This embodiment provides a sodium-ion battery electrolyte, the components of which include alkali metal inorganic salts, organic solvents and additives. The formulation is shown in Table 1. The content of each component is a mass percentage calculated based on the mass of the electrolyte.

[0052] The preparation method of the electrolyte includes the following steps: according to the types and mass ratios of each component shown in Table 1, the organic solvent is mixed evenly in an argon atmosphere with oxygen content and moisture content ≤1ppm and allowed to stand in a molecular sieve for at least 2 days to remove water. Then, the alkali metal inorganic salt is added and stirred until the liquid is clear and transparent. Finally, the additive is added, and the mixture is mixed and filtered to obtain the electrolyte.

[0053] This embodiment also provides a sodium-ion battery using the above-described electrolyte;

[0054] The sodium-ion battery includes a positive electrode, a negative electrode, a separator disposed between the positive and negative electrodes, and the sodium-ion battery electrolyte containing the aforementioned phosphorus-benzene flame retardant. Its preparation includes the following steps:

[0055] (1) Preparation of the positive electrode sheet: The positive electrode active material, acetylene black conductive agent SP, carbon nanotube conductive agent CNT, and polyvinylidene fluoride PVDF are dispersed in the solvent N-methylpyrrolidone NMP to prepare the positive electrode slurry; 12μm carbon-coated aluminum foil is used as the positive electrode current collector, and the positive electrode slurry is uniformly coated on the aluminum foil. After baking, rolling, die cutting, and slitting, the positive electrode sheet is obtained.

[0056] (2) Preparation of negative electrode sheet: Negative electrode active material, polyacrylic acid PAA, acetylene black conductive agent SP, sodium carboxymethyl cellulose CMC-Na, and styrene-butadiene rubber SBR are dispersed in deionized water to prepare negative electrode slurry; 12μm water-based carbon-coated aluminum foil is used as negative electrode current collector, negative electrode slurry is coated on aluminum foil, and after baking, rolling, die cutting and slitting, the negative electrode sheet is obtained;

[0057] (3) Preparation of sodium-ion battery: The positive electrode, separator and negative electrode are wound or stacked in the order of positive electrode-separator-negative electrode-separator to obtain bare cell. Then the bare cell is assembled, packaged, injected with electrolyte, formed and tested for capacity to obtain sodium-ion battery.

[0058] In this embodiment, the positive electrode active material can be any one of layered oxides, polyanionic oxides, or Prussian blue / white oxides; preferably, the positive electrode active material is a sodium-containing transition metal oxide, including sodium nickel iron manganate (NaNi). x Fe y Mn z O2), sodium nickel cobalt manganate (NaNi) x Co y Mn z O2); In this embodiment, sodium nickel iron manganate (NaNi) is used. 0.33 Fe 0.33 Mn 0.33 O2) is used as the positive electrode active material, and the weight ratio between sodium nickel iron manganese oxide, acetylene black conductive agent, carbon nanotube conductive agent and polyvinylidene fluoride is 95:0.5:1.5:3.

[0059] The negative electrode active material described in this embodiment can be one of graphite, hard carbon, or sodium metal; in this embodiment, hard carbon (HC) is used as the negative electrode active material, and the weight ratio between hard carbon, polyacrylic acid, acetylene black conductive agent, sodium carboxymethyl cellulose, and styrene-butadiene rubber is 95.5:1:0.5:2:1.

[0060] The separator described in this embodiment is a single-layer ceramic PE separator. In this embodiment, a separator with a thickness of 9+3μm and a single-sided ceramic + PE coating is used as the separator.

[0061] Examples 2-30 and Comparative Examples 1-5

[0062] The electrolyte formulations for Examples 2-30 and Comparative Examples 1-5 are shown in Table 1. The difference between each example and the comparative example lies in the type and / or content of salts, solvents, and additives used in the electrolyte. The specific types of additives and their mass percentage content in the electrolyte are shown in Table 1 below. The content of the additives is a mass percentage calculated based on the mass of the electrolyte. The preparation methods of the electrolyte and the sodium-ion battery are the same as in Example 1.

[0063] Table 1 lists the types and percentages of inorganic salts, solvents, and additives used in the electrolyte examples.

[0064]

[0065]

[0066]

[0067] In Table 1, the first additive is a phosphorobenzene flame retardant, and the structure of the first additive, formulas 1-1 to 1-15, is as follows:

[0068]

[0069] The structures of the first additive, formulas 2-1 to 2-9, are as follows:

[0070]

[0071]

[0072] The first additive used in Comparative Example 5 in Table 1 was diphenyl dimethyl phosphate (CDP).

[0073] Performance testing

[0074] To better illustrate the performance of the sodium-ion battery electrolyte containing phosphorus-benzene flame retardant of the present invention, the sodium-ion batteries prepared in the above embodiments and comparative examples were subjected to cycle performance, high-temperature storage performance, and combustion self-extinguishing time (SET) tests. The steps for the cycle performance, high-temperature storage performance, and combustion self-extinguishing time tests are as follows:

[0075] (1) Cyclic performance test: Under room temperature conditions, the sodium secondary battery was charged to 4.0V at a constant current-constant voltage rate of 1C, with a cutoff current of 0.05C; then discharged to 1.5V at a constant current of 1C to complete one charge-discharge cycle. The above charge-discharge process was repeated to perform 500 charge-discharge cycle tests on the battery.

[0076] Capacity retention rate after 500 discharges (%) = Capacity after 500 discharges / Capacity after initial discharges × 100%.

[0077] (2) High-temperature storage test:

[0078] First, the formed battery was charged and discharged three times at 0.2C at room temperature. Then, it was charged at 1C with constant current and constant voltage to 4.0V, with a cutoff current of 0.05C. The initial capacity of the battery was measured. After storing it at 60℃ for 30 days, it was discharged at 1C with constant current to 1.5V at room temperature, and the discharge capacity of the sodium-ion battery was measured. The capacity retention rate (%) was calculated as: discharge capacity after 30 days / initial battery capacity × 100%.

[0079] (3) Electrolyte self-extinguishing time (SET) test

[0080] The weighed electrolyte (400 mg each time in this experiment) was placed inside the CR2025 positive electrode battery casing, and the combustion process was recorded. This was repeated 5 times. The formula for SET is shown below:

[0081] SET=T b (Time required for the electrolyte to self-extinguish after ignition) / m (Mass of electrolyte used)

[0082] The test results are shown in Table 2.

[0083] Table 2 Results of Sodium-ion Battery Cyclic Performance Tests

[0084]

[0085]

[0086] Compared with Comparative Example 1, Examples 1-20 showed that the 500-cycle capacity retention rate and high-temperature storage capacity retention rate of Examples 1-20 were significantly better than those of Comparative Example 1, and the SET time was significantly shorter. This demonstrates that adding a phosphorus-containing flame retardant additive to the electrolyte can significantly improve the flame retardant effect of the electrolyte and effectively enhance the thermal stability and thermal shock resistance of sodium-ion batteries under high-temperature conditions. This is attributed to the chemical structural characteristics of the phosphorus-containing flame retardant of this invention. In Formula 1, the phosphate branch contains a benzene ring and an ester group. When the phosphate ester releases PO2· and HPO2· free radicals, it can further capture H· and O· in the electrolyte while undergoing a polymerization reaction to generate polyphosphate ester, which helps to further optimize the flame retardancy of the battery and reduce the decomposition of the anode. In Formula 2, the benzene ring forms a closed ring with phosphorus, improving the structural stability of the phosphorus-containing benzene while retaining the flame retardant properties of phosphorus, further increasing the boiling point of the phosphorus-containing benzene, and further optimizing the flame retardancy of the battery and reducing the decomposition of the anode.

[0087] The first additive used in Comparative Example 5 was diphenyl dimethyl phosphate (CDP), a common additive in the prior art. Compared with Comparative Example 5, Examples 1-20 also showed significantly improved 500-cycle capacity retention and high-temperature storage capacity retention. This is attributed to the structural characteristics of the phosphorobenzene flame retardant of the present invention and the modified nitrogen-, sulfur-, fluorine-, and carboxyl-containing groups. During normal battery charging and discharging, it can outperform EC and PC in forming an inorganic SEI film containing P, N, S, or F through reduction and decomposition, while simultaneously optimizing and improving Na. + Solvation structure, forming more CIPs, improving Na + The increased ion transport rate enhances the capacity utilization and cycle stability of sodium-ion batteries.

[0088] Compared with Examples 22-24, Example 1 showed some improvement in the 500-cycle capacity retention and high-temperature storage capacity. The results showed that adding phosphorus-containing flame retardant additives to the electrolyte, although adding too much can further shorten the battery's self-extinguishing time (SET) and improve its flame retardant performance, also results in a thicker SEI film formed at the electrode, increasing the battery's internal resistance and reducing the cycle performance of the sodium-ion battery. When the amount added is too low, the phosphorus-containing flame retardant additives do not fully perform their function in the electrolyte, resulting in a large self-extinguishing time (SET) and poor flame retardant performance, failing to maximize the improvement of the sodium-ion battery's cycle performance.

[0089] Compared with Comparative Examples 2 and 3, Example 1, based on the addition of a phosphorus-containing benzene flame retardant additive to the electrolyte, also incorporated lithium and sodium salts as auxiliary salts of alkali metal inorganic salts. This significantly improved the capacity utilization and cycle performance of the sodium-ion battery. This improvement is attributed to the synergistic effect of the sodium and lithium salts in Example 1. On one hand, the sodium salt additive enhances the stability of the SEI film while weakening the sodium ion solvation structure, increasing the ionic conductivity of the electrolyte, reducing sodium ion transport resistance, and improving the charge / discharge specific capacity of sodium ions. On the other hand, the lithium ions in the lithium salt additive can reduce the irreversible reaction of sodium ions, lower the self-discharge decay rate of the sodium-ion battery, and improve its cycle performance. The addition of both in a specific ratio can greatly optimize and enhance the capacity utilization and cycle performance of the sodium-ion battery.

[0090] Compared to Comparative Example 4, Example 1, which added a phosphorus-containing benzene flame retardant additive to the electrolyte but did not add ether solvents or a second ether additive, only using a carbonate system, showed relatively poor capacity utilization and cycle performance in its sodium-ion battery. Crown ethers possess certain coordination and inclusion capabilities, forming stable complexes with strong coordination ability. When combined with crown ether additives, they can reduce anodic polarization and improve the cycle stability of the electrolyte. The strong coordination ability of crown ethers with sodium ions can increase the solubility of sodium salts and improve the solvation structure of sodium ions, thereby enhancing battery capacity and cycle stability.

[0091] Compared with Examples 25-26, the amount of sodium salt added in Example 1 needs to be controlled within a reasonable range. If it is too low or too high, it will increase the internal resistance of the battery to a certain extent, thereby reducing the sodium ion transport rate. At the same time, too much or too little sodium salt will affect the number of CIPs in the sodium ion solvation structure, thus affecting the capacity of the sodium ion battery.

[0092] Compared with Examples 27-30, in Example 1, sodium hexafluorophosphate (NaPF6) is preferred as the main electrolyte over other types of sodium salts. NaPF6 has a high ion transport medium, which is conducive to the charge and discharge reaction of sodium-ion batteries and can stabilize the battery electrolyte, thus giving sodium-ion batteries excellent electrochemical performance.

Claims

1. A sodium-ion battery electrolyte, comprising an alkali metal inorganic salt, an organic solvent, and additives, characterized in that, The additive consists of a phosphorobenzene flame retardant and a second additive. The phosphorobenzene flame retardant is one or more compounds with the following structural formulas 1 and 2: (1) Equation 1: R1 is one of hydroxyl, amino, carboxylic acid or a hydroxyl, amino, or carboxylic acid derivative with 1 to 6 carbon atoms, and R2 is one of hydrogen or an alkyl, haloalkyl, olefin, alkyne, haloolefin, haloalkynyl, amino, sulfone, or silyl group composed of 1 to 4 carbon atoms. (2) Equation 2: R1 is one of benzene ring, pyrrole, pyrazole, imidazole, and thiophene; R2 is one of hydrogen or alkyl, haloalkyl, olefin, alkyne, haloolefin, haloalkynyl, amino, sulfone, and silyl groups composed of 1 to 4 carbon atoms. The phosphorobenzene flame retardant has a mass fraction of 0.1-1.5% in the electrolyte; the second additive has a mass fraction of 0.5-7% in the electrolyte. The second additive is composed of fluoroethylene carbonate or vinylene carbonate, 1,3-propane sulpholactone or ethylene sulfate, and an aliphatic crown ether, in a mass ratio of (2~5):(1~3):(0.5~1), wherein the aliphatic crown ether is one of 18-crown ether-6, 15-crown ether-5, and 12-crown ether-4.

2. The sodium-ion battery electrolyte according to claim 1, characterized in that, The alkali metal inorganic salt is composed of a main electrolyte and an auxiliary salt additive; the main electrolyte is one of sodium hexafluorophosphate, sodium perchlorate, sodium trifluoromethanesulfonate, sodium difluorosulfonylimide, and sodium bis(trifluoromethanesulfonyl)imide; the main electrolyte has a mass percentage of 8-30% in the electrolyte; the auxiliary salt additive is composed of sodium metal inorganic salt additive and / or lithium metal inorganic salt additive; the anion type of the sodium metal inorganic salt additive and the lithium metal inorganic salt additive is one or more of difluorophosphate, difluorosulfonylimide, difluorooxalateborate, tetrafluoroborate, bis(trifluoromethanesulfonyl)imide, hexafluorophosphate, difluorooxalate phosphate, difluorooxalateborate, and trifluoromethanesulfonate; the amount of the auxiliary salt additive added meets one or more of the following conditions: (1) The mass percentage of sodium metal inorganic salt additive in the electrolyte is 0.01~6%; (2) The mass percentage of lithium metal inorganic salt additive in the electrolyte is 0.0001~0.01%.

3. The sodium-ion battery electrolyte according to claim 2, characterized in that, The auxiliary salt additive is composed of sodium metal inorganic salt additive and lithium metal inorganic salt additive, wherein the anion type of the sodium metal inorganic salt additive and the lithium metal inorganic salt additive is the same, and the anion type is one of difluorophosphate, difluorosulfonyl imide, bis(trifluoromethylsulfonyl)imide, and difluorooxalate borate; the mass percentage of the sodium metal inorganic salt additive in the electrolyte is 0.01~3%; the mass percentage of the lithium metal inorganic salt additive in the electrolyte is 0.0001~0.003%.

4. The sodium-ion battery electrolyte according to claim 1, characterized in that, The organic solvent is a carbonate solvent or a mixture of carbonate and ether solvents; the carbonate solvent is one or more of propylene carbonate, ethylene carbonate, methyl ethyl carbonate, dimethyl carbonate, and diethyl carbonate; the ether solvent is one or more of crown ether, ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, and tetraethylene glycol dimethyl ether; the carbonate solvent has a mass fraction of 30-80% in the electrolyte; and the ether solvent has a mass fraction of 1-15% in the electrolyte.

5. The sodium-ion battery electrolyte according to claim 4, characterized in that, The organic solvent is a mixture of carbonate solvents and ether solvents; the carbonate solvent is two or three of propylene carbonate, ethyl methyl carbonate and dimethyl carbonate; the ether solvent is diethylene glycol dimethyl ether and / or triethylene glycol dimethyl ether, wherein the mass ratio of propylene carbonate, ethyl methyl carbonate and dimethyl carbonate is (1~2):(1~1.5):(1~1.5).

6. A method for preparing the sodium-ion battery electrolyte as described in any one of claims 1 to 5, comprising the following steps: In an argon atmosphere with oxygen and moisture content ≤1 ppm, a uniformly mixed organic solvent is dehydrated in a molecular sieve and allowed to stand for 2 days. The alkali metal inorganic salt is then added, and the mixture is stirred until the liquid is clear and transparent. The additives are then added, and the mixture is mixed and filtered to obtain the sodium-ion battery electrolyte.

7. A sodium-ion battery, characterized in that, It includes a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and a sodium-ion battery electrolyte as described in any one of claims 1 to 5 or a sodium-ion battery electrolyte prepared by the preparation method described in claim 6.

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