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

Abstract

The invention discloses a sodium-ion battery electrolyte containing a phosphorus-benzene flame retardant and a preparation method and application thereof, and belongs to the technical field of sodium-ion batteries. The electrolyte comprises alkali metal inorganic salt, an organic solvent and an additive, and the additive comprises a phosphorus-benzene flame retardant and a second additive; the invention provides two phosphorus-benzene flame retardant structures which respectively have the structural characteristics that a benzene ring and an ester group are introduced on a phosphorus-benzene branched chain and the benzene ring and phosphorus form a closed-loop structure, and the phosphorus-benzene flame retardant has good structural stability and high flame retardance; the phosphorus-benzene flame retardant can also modify groups or carboxyl containing nitrogen, sulfur and fluorine, and under the cooperation of the second additive, the alkali metal inorganic salt and the organic solvent, the electrolyte containing the phosphorus-benzene flame retardant has a good flame retardant effect and high ionic conductivity, and is expected to overcome the ubiquitous problems of poor capacity exertion and cycling stability and the like of a sodium-ion battery. Wide application prospects are realized in the field of sodium-ion batteries.

Description

Technical Field

[0001] The present invention specifically relates to a sodium ion battery electrolyte containing a phosphobenzene flame retardant and a preparation method and application thereof, belonging to the technical field of sodium ion batteries. Background Art

[0002] At present, lithium-ion batteries are developing strongly in the fields of consumer electronics, new energy vehicles, and grid energy storage. However, due to the limited reserves of lithium resources (only 17ppm) and uneven distribution (70% in South America), lithium-ion batteries are difficult to support the development of both electric vehicles and grid energy storage. The reserves of sodium in the earth's crust are about 23,000ppm, which is more than a thousand times that of lithium, and the distribution is relatively uniform. In recent years, sodium-ion batteries are expected to become an important supplement to lithium-ion batteries in the field of new energy due to their rich resources, cleanliness, high energy density, and low cost. However, for sodium-ion batteries, in addition to low cost and high energy density, their safety issues are also particularly prominent. 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), ethyl methyl carbonate (EMC), etc. 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] At present, adding phosphorus-containing flame retardants to the electrolyte is an important means to solve the problem of flammability of the electrolyte. Chinese patent CN105655631A discloses a highly safe non-flammable sodium secondary battery system, which uses non-flammable phosphate esters and sodium salts as electrolytes. The non-flammable phosphate ester solvent structure is characterized by R1OP(O)OR2OR3, wherein R1, R2 or R3 is an alkyl aryl, and a halogen-substituted alkyl or aryl, etc. R1, R2 and R3 can be symmetrical, asymmetrical and cyclic, and the halogen substitution can be partial or full substitution. The halogen is F, Cl or Br, etc. The electrolyte has high safety and good cycle performance; Chinese patent CN114122516A discloses a non-flammable, highly safe sodium ion battery, which The battery comprises a positive electrode, a negative electrode and an electrolyte, the electrolyte comprises a sodium salt, an ester solvent and a flame retardant, wherein the flame retardant is selected from one or more of trimethyl phosphate, triethyl phosphate, tributyl phosphate, dimethyl methyl phosphate, diethyl ethyl phosphate, isopropyl benzene diphenyl phosphate or hexamethylphosphoramide; Chinese patent CN117525596A discloses a sodium battery electrolyte, a preparation method thereof and a sodium battery, the sodium battery electrolyte comprises: a sodium salt, a non-aqueous solvent and an additive, the additive comprises a fluorinated phosphate compound and a fluorinated carbonate compound, the electrolyte can reduce the problem of sodium battery capacity decay and improve the cycle performance of the sodium battery to a certain extent.

[0004] It can be seen 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 viscosity of the electrolyte containing such flame retardants is relatively high, and the compatibility with the battery anode is poor, which leads to low ionic conductivity of the electrolyte and accelerated battery capacity decay. This has been reported in many previous reports ([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 have further increased the cost of sodium-ion batteries and limited the use of sodium-ion batteries. Therefore, in the field of sodium ion batteries, there is a broad development prospect for the development of phosphorus-containing flame retardant electrolytes with high safety, excellent flame retardancy, good conductivity and good stability. Summary of the invention

[0005] In view of the problems that it is difficult to take both flame retardancy and electrical conductivity into consideration in current sodium ion battery electrolytes and that phosphorus-containing flame retardants have strong decomposition properties on battery anodes, the present invention provides a sodium ion battery electrolyte containing a phosphorus-benzene flame retardant. The phosphorus-benzene flame retardant in the electrolyte effectively utilizes the flame retardancy of existing phosphorus-containing flame retardants, improves the stability of the phosphorus-benzene structure by introducing benzene rings and ester groups on the phosphorus-benzene side chains and forming a closed-loop structure with phosphorus, and improves the situation that existing phosphorus-containing flame retardants have strong decomposition properties on battery anodes; at the same time, the phosphorus-benzene flame retardant in the electrolyte can also be modified in structure with nitrogen-, sulfur-, fluorine-containing groups or carboxyl groups to further improve the flame retardancy and Na + Transfer rate. With the cooperation of other electrolyte additives, alkali metal inorganic salts and organic solvents, the electrolyte has good flame retardant effect and ionic conductivity, which is expected to overcome the common problems of sodium ion batteries such as poor capacity utilization and cycle stability, and has broad application prospects in the field of sodium ion batteries.

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

[0007] One of the purposes of the present invention is to provide a sodium ion battery electrolyte, the components of which include additives, alkali metal inorganic salts and organic solvents, and the specific components are as follows:

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

[0009] (i) Formula 1: R1 is hydroxyl, amino, carboxylic acid, oxybenzene ring, pyrrole, pyrazole, imidazole, thiophene or one of hydroxyl, amino and carboxylic acid derivatives of 1 to 6 carbon atoms, and R2 is hydrogen or one of alkyl, halogenated alkyl, olefin, alkynyl, halogenated olefin, halogenated alkynyl, amine, sulfone and silane groups composed of 1 to 4 carbon atoms.

[0010] (ii) Formula 2: R1 is one of hydroxyl, amino, carboxylic acid, five-membered heterocyclic ring or hydroxyl, amino, carboxylic acid, five-membered heterocyclic ring derivatives or alkyl, halogenated alkyl, olefin, alkynyl, halogenated olefin, halogenated alkynyl, phenyl, halogenated benzene, amine, sulfone and silane groups having 1 to 6 carbon atoms.

[0011] The phosphophenyl flame retardant provided by the present invention is a phosphate composed of aromatic substituents, and its high benzene ring content makes it show a more efficient flame retardant efficiency than a linear phosphate structure. The phosphophenyl flame retardant can rely on the strong electronegativity and easy reducibility of the P atom in its structure in the electrolyte, and preferentially react with the negative electrode surface to form a SEI film containing P organic matter during the formation of the sodium ion battery, thereby improving the stability of the SEI film on the negative electrode surface, improving the cyclability of the sodium ion battery, and inhibiting the gas production of the sodium ion battery during the cycle. In the high temperature environment of battery operation, the P-containing flame retardant additive can release P-containing free radicals with flame retardant properties and then capture hydrogen in the organic free radical chain combustion reaction, terminating the chain reaction, which can effectively improve the flame retardant effect of the electrolyte and improve the thermal stability and thermal shock resistance of the sodium ion battery under high temperature conditions.

[0012] When adding a flame retardant additive containing phosphorus benzene to the electrolyte, if the added amount is too large, although the battery's combustion self-extinguishing time (SET) can be shortened and the battery's flame retardant performance can be improved, the SEI film formed at the electrode end is also thicker, which increases the battery's internal resistance and reduces the cycle performance of the sodium ion battery; if the added amount is too low, the phosphorus benzene flame retardant additive does not fully exert its function in the electrolyte, the battery's combustion self-extinguishing time is long, the flame retardant performance is poor, and the cycle performance of the sodium ion battery cannot be improved to the greatest extent.

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

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

[0015]

[0016]

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

[0018]

[0019]

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

[0021] Preferably, the second additive is composed of fluoroethylene carbonate (FEC) or vinylene carbonate (VC), 1,3-propane sultone (PS) or ethylene sulfate (DTD), and aliphatic crown ether, and the mass ratio of the three is (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 the side reaction 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 ion conductivity, reduce the interface impedance, inhibit the decrease in the initialization capacity of the battery, and improve the capacity performance and high temperature performance of the battery; on the basis of the above-mentioned additives, adding aliphatic crown ethers can also reduce anode polarization and improve the cycle stability of the electrolyte. The strong coordination ability of aliphatic crown ethers with sodium ions can increase the solubility of sodium salts and improve the solvation structure of sodium ions, thereby improving the battery capacity performance and battery cycle stability.

[0022] (2) Alkali metal inorganic salts are composed of main salt electrolyte and auxiliary salt additives;

[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), which has a high ion transport medium, is conducive to the charge and discharge reaction of the sodium ion battery, and can stabilize the battery electrolyte, so that the sodium ion battery has 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 a sodium metal inorganic salt additive and / or a 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, bisfluorosulfonyl imide, difluorooxalatoborate, tetrafluoroborate, bis(trifluoromethylsulfonyl)imide, hexafluorophosphate, difluorooxalatophosphate, bisfluorooxalatoborate, and trifluoromethylsulfonate; the addition amount of the auxiliary salt additive satisfies one or more of the following conditions:

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

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

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

[0030] Sodium metal inorganic salt additives can promote the formation of SEI film, effectively avoid side reactions between electrolyte and electrode, and improve the stability of electrolyte. At the same time, its anions can further weaken the sodium ion solvation structure, improve the ionic conductivity of electrolyte, reduce the sodium ion transmission resistance, and improve the charge and 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 embedded in the position where sodium ions cannot be embedded, so as to improve the utilization rate of active materials. At the same time, lithium ions are embedded in the nanopore structure to reduce the irreversible reaction of sodium ions, further reduce the self-discharge and cycle attenuation rate, and at the same time, lithium ions can realize the reversible reaction of embedding and detaching of negative electrode active materials, further improving the capacity and cycle performance of the battery.

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

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

[0033] (3) The organic solvent is a cyclic or chain carbonate solvent or a mixed solvent of a cyclic or chain carbonate and an ether;

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

[0035] Chain carbonate solvents have a lower dielectric constant, lower viscosity and a narrower liquid temperature range; cyclic carbonate solvents have a higher dielectric constant, 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 with electrodes and reducing 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), 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 object of the present 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 a certain amount of organic solvent, alkali metal inorganic salt and additive according to the electrolyte solution plan;

[0039] S2: In an argon atmosphere glove box with an oxygen content and a moisture content of ≤1ppm, the organic solvents were mixed evenly in a certain mass ratio, and then a molecular sieve dehydrating agent was added and allowed to stand for 2 days;

[0040] S3: Then dissolving the dry alkali metal inorganic salt into the organic solvent, stirring until the solution is completely uniform 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 electrolyte without impurities.

[0042] The third object of the present invention is to provide a sodium ion battery using the above-mentioned sodium ion battery electrolyte containing phosphobenzene flame retardant as an electrolyte; the sodium ion battery comprises a positive electrode, a negative electrode, a separator arranged between the positive electrode and the negative electrode, and the above-mentioned sodium ion battery electrolyte containing phosphobenzene flame retardant.

[0043] Different from the prior art, the present invention has the following beneficial effects:

[0044] 1. The two phosphophenyl flame retardants provided by the present invention respectively improve the stability of the phosphophenyl structure by introducing a benzene ring and an ester group on the phosphophenyl side chain and forming a closed-loop structure with the benzene ring and phosphorus; compared with the previous phosphorus-containing flame retardants, the phosphate side chain of the phosphophenyl flame retardant of formula 1 contains a benzene ring and an ester group, which can release PO2· and HPO2· free radicals, and while further capturing H· and O· in the electrolyte, a polymerization reaction occurs, thereby generating a polyphosphate 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 benzene ring of the phosphophenyl flame retardant of formula 2 forms a closed loop with phosphorus, which improves the stability of the phosphophenyl structure while retaining the flame retardant properties of phosphorus, further improves the boiling point of phosphophenyl, optimizes the flame retardant performance of the battery, and reduces the decomposition of the flame retardant on the anode.

[0045] 2. Different from the prior art, the phosphophenyl flame retardant of the present invention can be further structurally modified; in some preferred embodiments, the phosphophenyl structure can be modified with nitrogen, sulfur, fluorine and other groups or carboxyl groups, wherein the phosphophenyl structure side chains modified with nitrogen and sulfur groups can produce non-flammable gases (nitrogen, ammonia, sulfur dioxide) when thermally decomposed, diluting the concentration of the battery's combustion-supporting gas and inhibiting battery combustion; the F-containing group acts as a "scavenger" during degradation, acting on free radicals to cut off thermal decomposition inside the battery; the hydroxyl-containing group undergoes carboxyl dehydration condensation during degradation, which can dilute the concentration of the combustion-supporting gas to inhibit battery combustion; the degradation reactions of the modified groups are all endothermic reactions, which can consume heat to slow down the rapid rise in the temperature inside the battery; the above characteristics can significantly improve the flame retardant properties of the battery. In addition, the phosphophenyl additives modified with nitrogen, sulfur, fluorine and other groups or carboxyl groups are superior to EC, PC and other reduction decompositions to form an inorganic component SEI film containing P, N, S or F during the normal charge and discharge process of the battery, and can simultaneously optimize and improve Na + Solvation structure, forming more contact ion pairs (CIPs), increasing Na + The ion transfer rate can improve the capacity and cycle stability of sodium-ion batteries.

[0046] 3. The sodium ion battery electrolyte of the present invention further introduces a crown ether as a second additive on the basis of the phosphophenyl flame retardant. The crown ether additive can reduce anode polarization, inhibit gas production of the sodium ion battery during the cycle, improve the cycle stability of the electrolyte, increase the solubility of the sodium salt, improve the solvation structure of the sodium ion, and effectively improve the thermal stability and thermal shock resistance of the battery, thereby improving the development of the battery capacity and the battery cycle stability.

[0047] 4. The sodium ion battery electrolyte of the present invention further adds sodium metal inorganic salt and / or lithium metal inorganic salt as auxiliary additives on the basis of the existing main salt electrolyte. The sodium metal inorganic salt additive can further promote the formation of SEI film, effectively avoid the side reaction between the electrolyte and the electrode, and improve the stability of the 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 the electrolyte, reduce the sodium ion transmission resistance, and improve the charge and discharge specific capacity of the sodium ion; on this basis, the lithium metal inorganic salt additive is added, and the introduced lithium ions can be embedded in the position where the sodium ions cannot be embedded, thereby improving the utilization rate of the active material. At the same time, the lithium ions are embedded in the nanopore structure, reducing the irreversible reaction of the sodium ions, further reducing the self-discharge and cycle attenuation rate, and at the same time, the lithium ions can realize the reversible reaction of the negative electrode active material embedding and detaching, further improving the capacity and cycle performance of the battery. DETAILED DESCRIPTION

[0048] The present invention is further described below in conjunction with preferred embodiments. The embodiments provided are only for illustrating the present invention, but not for limiting the scope of the present invention.

[0049] Unless otherwise specified, the materials, reagents, etc. used in the following examples can be obtained from commercial sources; the methods in the following examples are conventional methods unless otherwise specified.

[0050] Example 1

[0051] This embodiment provides a sodium ion battery electrolyte, the components of which include an alkali metal inorganic salt, an organic solvent and an additive, and the formula is shown in Table 1. The content of each component is the mass percentage calculated based on the mass of the electrolyte;

[0052] The preparation method of the electrolyte comprises the following steps: according to the types and mass proportions of the components shown in Table 1, in an argon atmosphere with an oxygen content and a moisture content of ≤1 ppm, the organic solvent is mixed evenly and dehydrated in a molecular sieve and allowed to stand for at least 2 days, and then the alkali metal inorganic salt is added, stirred until the liquid is clear and transparent, and finally the additive is added, and the electrolyte is obtained after mixing and filtering.

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

[0054] The sodium ion battery comprises a positive electrode, a negative electrode, a separator arranged between the positive electrode and the negative electrode, and the sodium ion battery electrolyte containing the phosphorus benzene flame retardant, and the preparation thereof comprises the following steps:

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

[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 as a solvent to prepare negative electrode slurry; 12 μm water-based carbon-coated aluminum foil is used as the negative electrode current collector, the negative electrode slurry is coated on the aluminum foil, and the negative electrode sheet is obtained after baking, rolling, die-cutting, and slitting;

[0057] (3) Preparation of sodium ion battery: The positive electrode sheet, separator, and negative electrode sheet are wound or stacked in the order of positive electrode-separator-negative electrode-separator to obtain a bare cell, and then the bare cell is assembled, packaged, injected, formed, and subjected to capacity division testing to obtain the sodium ion battery.

[0058] The positive electrode active material in this embodiment can be any one of layered oxides, polyanions, and Prussian blue / white. Preferably, the positive electrode active material is a sodium-containing transition metal oxide, including sodium nickel iron manganese oxide (NaNi x Fe y Mn z O2), sodium nickel cobalt manganate (NaNi x Co y Mn z O2); in this embodiment, sodium nickel iron manganese oxide (NaNi 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, and sodium metal; in this embodiment, hard carbon (HC) is used as the negative electrode active material, and the weight ratio of 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 isolation membrane described in this embodiment is a single-layer ceramic PE membrane. In this embodiment, a 9+3 μm thick, ceramic+PE single-sided coated membrane is used as the isolation membrane.

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

[0062] The electrolyte formulas of Examples 2 to 30 and Comparative Examples 1 to 5 are shown in Table 1; the difference between the examples and the comparative examples is that the types and / or contents of salts, solvents, additives used in the electrolyte are different, and the specific types of additives and their mass percentages in the electrolyte are shown in Table 1 below, and the content of the additives is the mass percentage calculated based on the mass of the electrolyte; the preparation method of the electrolyte and the sodium ion battery is the same as that of Example 1.

[0063] Table 1 List of inorganic salts, solvents and additives used in the electrolyte case and their added mass percentage

[0064]

[0065]

[0066]

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

[0068]

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

[0070]

[0071]

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

[0073] Performance Testing

[0074] In order 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 tested for cycle performance, high temperature storage performance and combustion self-extinguishing time (SET). The steps of the cycle performance, high temperature storage performance and combustion self-extinguishing time tests are as follows:

[0075] (1) Cycle performance test: At room temperature, the sodium secondary battery was charged to 4.0V in a 1C constant current-constant voltage mode with a cut-off current of 0.05C; then discharged to 1.5V at a 1C constant current to complete a charge-discharge cycle. The above charge-discharge process was repeated for 500 charge-discharge cycles.

[0076] 500th discharge capacity retention rate (%) = 500th discharge capacity / first discharge capacity × 100%.

[0077] (2) High temperature storage test:

[0078] First, the formed battery was charged and discharged at 0.2C for 3 times at room temperature, and then the battery was charged to 4.0V at 1C constant current and constant voltage, with a cut-off current of 0.05C, and the initial capacity of the battery was measured. After being stored at 60°C for 30 days, the battery was discharged at 1C constant current to 1.5V at room temperature, and the discharge capacity of the sodium ion battery was measured. Calculate the capacity retention rate (%) = discharge capacity after 30 days / initial capacity of the battery × 100%.

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

[0080] Place a weighed amount of electrolyte (400 mg in this experiment) into the CR2025 positive battery case, record its combustion process, and repeat 5 times. The formula of SET is as follows:

[0081] SET=T b (The time required for the electrolyte to ignite and self-extinguish) / m (mass of electrolyte used)

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

[0083] Table 2 Sodium ion battery cycle performance test results

[0084]

[0085]

[0086] Compared with Comparative Example 1, Examples 1 to 20 show that the 500-cycle capacity retention rate and high-temperature storage capacity retention rate of Examples 1 to 20 are significantly better than those of Comparative Example 1, and the SET time is significantly shorter than that of Comparative Example 1. It can be seen that adding a flame retardant additive containing phosphobenzene to the electrolyte can significantly improve the flame retardant effect of the electrolyte, and can effectively improve the thermal stability and thermal shock resistance of the sodium ion battery under high temperature conditions. This is due to the chemical structure characteristics of the phosphobenzene flame retardant of the present invention. In the structure of Formula 1, the phosphate side chain contains a benzene ring and an ester group. When the phosphate ester releases PO2· and HPO2· free radicals, which can further capture H· and O· in the electrolyte, a polymerization reaction can occur to generate polyphosphate ester, which helps to further optimize the flame retardancy of the battery and reduce the decomposition of the anode. In the structure of Formula 2, the benzene ring and phosphorus form a closed ring, which improves the stability of the phosphobenzene structure while retaining the flame retardant properties of phosphorus, further improves the boiling point of phosphobenzene, and optimizes the flame retardancy of the battery and reduces the decomposition of the anode.

[0087] The first additive used in Comparative Example 5 is a common diphenyl dimethyl phosphate (CDP) additive in the prior art. Compared with Comparative Example 5, the 500-cycle capacity retention rate and high-temperature storage capacity retention rate of Examples 1 to 20 are also significantly improved. This is due to the structural characteristics of the phosphophenyl flame retardant of the present invention and the modified nitrogen, sulfur, fluorine, carboxyl and other groups. During the normal charge and discharge process of the battery, it can be better than EC, PC, etc. in reducing and decomposing to form an inorganic component SEI film containing P, N, S or F, and can simultaneously optimize and improve Na + Solvation structure, forming more CIPs, increasing Na + The ion transfer rate is improved, thereby enhancing the capacity and cycle stability of sodium-ion batteries.

[0088] Compared with Examples 22 to 24, Example 1 has a certain improvement in the 500-cycle capacity retention rate and high-temperature storage capacity of Example 1. The results show that when a flame retardant additive containing phosphorus benzene is added to the electrolyte, when the addition amount is too large, the combustion self-extinguishing time (SET) of the battery can be further shortened and the flame retardant performance of the battery can be improved. However, at the same time, the SEI film formed at the electrode end is relatively thick, which increases the internal resistance of the battery and reduces the cycle performance of the sodium ion battery. When the addition amount is too low, the flame retardant additive containing phosphorus benzene does not fully exert its function in the electrolyte, the combustion self-extinguishing time (SET) of the battery is long, the flame retardant performance is poor, and the cycle performance of the sodium ion battery cannot be improved to the greatest extent.

[0089] Compared with Comparative Examples 2 to 3, Example 1 adds a flame retardant additive containing phosphorus benzene to the electrolyte, and lithium salt and sodium salt additives are added to the auxiliary salt additive of the alkali metal inorganic salt at the same time, and the capacity and cycle performance of the sodium ion battery are significantly improved, which is due to the synergistic effect of the sodium salt and the lithium salt additive in Example 1; on the one hand, the sodium salt additive improves the stability of the SEI film, while weakening the sodium ion solvation structure, improving the ionic conductivity of the electrolyte, reducing the sodium ion transmission resistance, and improving the charge and discharge specific capacity of the sodium ion; on the other hand, the lithium ions in the lithium salt additive can reduce the irreversible reaction of the sodium ions, reduce the self-discharge attenuation rate of the sodium ion battery, and improve the cycle performance of the sodium ion; the addition of the two in a certain ratio can greatly optimize and improve the capacity and cycle performance of the sodium ion battery.

[0090] Compared with Example 4, Example 1 adds a flame retardant additive containing phosphorus benzene to the electrolyte, and no ether solvent and ether second additive are added to the electrolyte. Only the carbonate system is used. The capacity and cycle performance of the sodium ion battery are relatively poor. Crown ether has certain coordination and inclusion capabilities, generates stable complexes, and has strong coordination ability. The crown ether additive can reduce the anode polarization and improve the cycle stability of the electrolyte. The strong coordination ability of crown ether and sodium ions can increase the solubility of sodium salts and improve the solvation structure of sodium ions, thereby improving the development of battery capacity and battery cycle stability.

[0091] Compared with Examples 25 to 26, the amount of sodium salt added in Example 1 needs to be controlled within a reasonable range. A lower limit or an upper limit will increase the internal resistance of the battery to a certain extent, thereby reducing the sodium ion transmission rate. At the same time, too much or too little sodium salt affects the number of CIPs in the sodium ion solvation structure, thereby affecting the capacity of the sodium ion battery.

[0092] Compared with Examples 27 to 30, in Example 1, the main salt electrolyte is preferably sodium hexafluorophosphate (NaPF6) relative to other types of sodium salts. NaPF6 has a higher ion transport medium, which is helpful for the charge and discharge reaction of the sodium ion battery and can stabilize the battery electrolyte, so that the sodium ion battery has excellent electrochemical properties.

Claims

1. A sodium ion battery electrolyte, comprising an alkali metal inorganic salt, an organic solvent and an additive, characterized in that: The additive consists of a phosphine flame retardant and a second additive; The phosphine flame retardant is one or more compounds of the following formula 1 and formula 2: (1) Formula 1: R1 is hydroxyl, amino, carboxylic acid, oxybenzene ring, pyrrole, pyrazole, imidazole, thiophene or one of hydroxyl, amino and carboxylic acid derivatives of 1 to 6 carbon atoms, and R2 is hydrogen or one of alkyl, halogenated alkyl, olefin, alkynyl, halogenated olefin, halogenated alkynyl, amine, sulfone and silane groups composed of 1 to 4 carbon atoms. (2) Formula 2: R1 is one of hydroxyl, amino, carboxylic acid, five-membered heterocyclic ring or hydroxyl, amino, carboxylic acid, five-membered heterocyclic ring derivatives or alkyl, halogenated alkyl, olefin, alkynyl, halogenated olefin, halogenated alkynyl, phenyl, halogenated benzene, amine, sulfone and silane groups having 1 to 6 carbon atoms. The second additive is one or more of fluoroethylene carbonate, vinylene carbonate, ethylene sulfate, 1,3-propane sultone, and aliphatic crown ether, wherein the aliphatic crown ether is one of 18-crown-6, 15-crown-5, and 12-crown-4.

2. The sodium ion battery electrolyte according to claim 1, characterized in that The mass fraction of the phosphophenyl flame retardant in the electrolyte is 0.1-1.5%; the mass fraction of the second additive in the electrolyte is 0.5-7%.

3. The sodium ion battery electrolyte according to claim 2, characterized in that The second additive is composed of fluoroethylene carbonate or vinylene carbonate, 1,3-propane sultone or ethylene sulfate, and aliphatic crown ether, and the mass ratio of the three is (2-5):(1-3):(0.5-1).

4. The sodium ion battery electrolyte according to claim 1, characterized in that The alkali metal inorganic salt is composed of a main salt electrolyte and an auxiliary salt additive; the main salt electrolyte is one of sodium hexafluorophosphate, sodium perchlorate, sodium trifluoromethanesulfonate, sodium bis(trifluoromethylsulfonyl)imide, and sodium bis(trifluoromethylsulfonyl)imide; the mass percentage of the main salt electrolyte in the electrolyte is 8-30%; the auxiliary salt additive is composed of a sodium metal inorganic salt additive and / or a 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, bis(fluorosulfonyl)imide, difluorooxalatoborate, tetrafluoroborate, bis(trifluoromethylsulfonyl)imide, hexafluorophosphate, difluorooxalatophosphate, difluorooxalatoborate, and trifluoromethylsulfonate; the addition amount of the auxiliary salt additive satisfies one or more of the following conditions: (1) The mass percentage of the sodium metal inorganic salt additive in the electrolyte is 0.01 to 6%; (2) The mass percentage of the lithium metal inorganic salt additive in the electrolyte is 0.0001 to 0.01%.

5. The sodium ion battery electrolyte according to claim 4, characterized in that: The auxiliary salt additive consists of a sodium metal inorganic salt additive and a lithium metal inorganic salt additive, wherein the sodium metal inorganic salt additive and the lithium metal inorganic salt additive have the same anion type, and the anion type is one of difluorophosphate, bisfluorosulfonyl 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%.

6. The sodium ion battery electrolyte according to claim 1, characterized in that The organic solvent is a carbonate solvent or a mixed solvent of carbonates and ethers; the carbonate solvent is one or more of propylene carbonate, ethylene carbonate, ethyl methyl 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 mass fraction of the carbonate organic solvent in the electrolyte is 30-80%; the mass fraction of the ether organic solvent in the electrolyte is 1-15%.

7. The sodium ion battery electrolyte according to claim 6, characterized in that: The organic solvent is a mixed solvent of carbonates and ethers; the carbonate organic 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).

8. A method for preparing a sodium ion battery electrolyte as claimed in any one of claims 1 to 7, comprising the following steps: In an argon atmosphere with an oxygen content and a moisture content of ≤1ppm, the uniformly mixed organic solvent is dehydrated in a molecular sieve and allowed to stand for 2 days, the alkali metal inorganic salt is added, stirred until the liquid is clear and transparent, and then the additive is added, and the sodium ion battery electrolyte is obtained after mixing and filtering.

9. A sodium ion battery, characterized in that: The invention comprises a positive electrode, a negative electrode, a separator arranged between the positive electrode and the negative electrode, and a sodium ion battery electrolyte as claimed in any one of claims 1 to 7 or a sodium ion battery electrolyte prepared by the preparation method as claimed in claim 8.

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