Non-combustible and ultralow-temperature sodium-ion battery electrolyte as well as preparation method and application thereof

Abstract

The invention provides a non-combustible and ultralow-temperature sodium-ion battery electrolyte as well as a preparation method and application thereof, and belongs to the technical field of sodium-ion batteries. The phosphate ester solvent with excellent flame retardance is matched with the weak-polarity diluent to serve as the main solvent, and the main solvent, the ionic liquid and the sodium salt obtain the non-combustible wide-temperature sodium-ion battery electrolyte capable of operating at the temperature ranging from the room temperature to-60 DEG C. Besides, the solvent-solvent strong interaction between the weakly solvated diluent and the strongly solvated phosphate in the system weakens the interaction between Na < + > and the strong solvent, so that the sodium ion kinetics is improved, the Na < + > solvated structure is further regulated and controlled by anions and cations of the ionic liquid, and the stability of the system is improved. And the safety performance of the battery can be improved due to the flame retardance and high electrochemical window of the ionic liquid. The electrolyte provided by the invention realizes high magnification and high cycle stability at room temperature, and also shows relatively high capacity at a low temperature of-60 DEG C.

Description

Technical Field

[0001] The present invention relates to the technical field of sodium ion batteries, and in particular to a non-flammable and ultra-low temperature sodium ion battery electrolyte and a preparation method and application thereof. Background Art

[0002] With the rapid development of renewable energy and the booming electric vehicle market, the demand for lithium-ion batteries continues to increase. However, the scarcity and cost of lithium resources have prompted researchers to explore alternative technologies, among which sodium-ion batteries have become one of the important research directions due to their abundant resources, low cost and high theoretical energy density. As the next generation of energy storage technology, sodium-ion batteries have unique advantages in safety, environmental friendliness and economy, but their commercialization still faces the performance bottleneck of electrolytes. The main advantage of sodium-ion batteries lies in the abundance and low cost of sodium resources. The abundance of sodium in the earth's crust is dozens of times that of lithium, and it is widely present in seawater and salt mines, which makes sodium-ion batteries have good economy and sustainability in long-term applications. However, the electrochemical performance and cycle life of sodium-ion batteries in low-temperature environments still need to be further improved, especially in the selection and design of electrolytes.

[0003] The electrolyte is a key component of battery performance, affecting the battery's ionic conductivity, cycle stability and safety. The electrolyte of a sodium-ion battery needs to have high ionic conductivity, a wide electrochemical window, good thermal stability, and excellent performance at low temperatures. At the same time, considering safety, non-combustible electrolytes are particularly important to reduce the risk of accidents under extreme conditions such as high temperature or short circuits. In practical applications, sodium-ion batteries often operate in low-temperature environments, such as electric vehicles, energy storage systems, and portable electronic devices in cold regions. Therefore, the development of non-flammable organic electrolytes with excellent low-temperature performance has become the key to improving the overall performance of sodium-ion batteries.

[0004] In order to solve the combustion risk, researchers have introduced non-combustible compounds into the design of organic electrolytes. For example, certain fluorinated organic compounds and high flash point substances show good electrochemical stability and high flash point, which can effectively reduce the combustion risk of electrolytes in extreme environments. However, most of the current non-combustible electrolytes have a high freezing point, increased viscosity, and poor performance at low temperatures due to the introduction of some flame retardant solvents or additives, making it difficult to take into account both non-combustible properties and ultra-low temperature operation. Summary of the invention

[0005] The purpose of the present invention is to provide a non-flammable and ultra-low temperature sodium ion battery electrolyte and a preparation method and application thereof, so as to solve the problem that the existing sodium ion battery organic electrolyte cannot simultaneously take into account the non-flammable characteristics and ultra-low temperature (below -40°C) operation and the poor rate performance at low temperature.

[0006] In order to achieve the above-mentioned object of the invention, the present invention provides the following technical solutions:

[0007] The present invention provides a non-flammable and ultra-low temperature sodium ion battery electrolyte, comprising the following raw materials: sodium salt, a main solvent and an ionic liquid additive; the main solvent is a phosphate ester solvent and a diluent;

[0008] Wherein, the volume ratio of the phosphate ester solvent to the diluent is 30-70:10-30;

[0009] When the ionic liquid additive is a liquid ionic liquid, the volume ratio of the ionic liquid additive to the main solvent is 1-5:70-30;

[0010] When the ionic liquid additive is a solid ionic liquid, the mass of the ionic liquid additive is 1 to 5% of the mass of the sodium salt;

[0011] The phosphate ester solvent includes trimethyl phosphate, dimethyl phosphate, triethyl phosphate, tributyl phosphate, dibutyl phosphate, monomethyl phosphate, triphenyl phosphate, trihexyl phosphate, di-n-butyl phosphate or triisopropyl phosphate;

[0012] The diluent includes diethyl carbonate, methyl formate, methyl acetate, methyl butyrate or ethyl propionate.

[0013] Preferably, the sodium salt includes one or more of sodium hexafluorophosphate, sodium bis(trifluoromethylsulfonyl)imide, sodium bis(fluorosulfonyl)imide, sodium perchlorate, sodium difluorooxalatoborate and sodium tetrafluoroborate.

[0014] Preferably, the concentration of the sodium salt in the main solvent is 0.5 to 2 mol / L.

[0015] Preferably, the ionic liquid additive includes 1-ethyl-3-methylimidazolium hexafluorophosphate, 1-butyl-3-methylimidazolium chloride, 1-ethyl-3-methylimidazolium bisfluorosulfonyl imide, pyridinium trifluoroacetate, 1-ethyl-3-methylimidazolium nitrate, and 1-butyl-3-methylimidazolium tetrafluoroborate.

[0016] The present invention provides a method for preparing the non-flammable and ultra-low temperature sodium ion battery electrolyte described in the above technical solution, comprising the following steps:

[0017] Sodium salt, phosphate ester solvent, diluent and ionic liquid additive are mixed to obtain non-flammable and ultra-low temperature sodium ion battery electrolyte.

[0018] The present invention provides the use of the non-flammable and ultra-low temperature sodium ion battery electrolyte described in the above technical scheme or the non-flammable and ultra-low temperature sodium ion battery electrolyte prepared by the preparation method described in the above technical scheme in a sodium ion battery, and the application temperature of the non-flammable and ultra-low temperature sodium ion battery electrolyte is 35 to -60°C.

[0019] Preferably, the sodium ion battery comprises a positive electrode, a negative electrode, a separator and a non-flammable and ultra-low temperature sodium ion battery electrolyte.

[0020] Preferably, the positive electrode includes a positive electrode current collector and a positive electrode membrane; the positive electrode membrane includes a positive electrode active material, a conductive agent and a binder; the positive electrode active material includes one or more of sodium vanadium phosphate, sodium iron phosphate, sodium nickel iron manganate and sodium vanadium fluorophosphate.

[0021] Compared with the prior art, the present invention has the following beneficial effects:

[0022] The present invention uses a strong solvating phosphate solvent as a main solvent, which has excellent flame retardant properties, can promote the electrolyte to achieve non-combustible properties, and can ensure that the electrolyte has high ionic conductivity and high solubility.

[0023] The present invention uses a low freezing point weak solvent diluent to regulate the phosphate ester solvent and Na + The strong solvent-solvent interactions between the weakly solvating diluent and the strongly solvating phosphates weaken the Na + The interaction between the electrolyte and the strong solvent increases the mixing entropy of the electrolyte, thus improving the sodium ion dynamics and achieving high rate characteristics. Taking the diluent methyl acetate (MA) as an example, its melting point is -98°C, which is miscible with phosphate solvents and ionic liquid additives but basically does not react with Na + Combine, consume solvent and increase Na + The effect with anions further weakens the Na + The interaction with the solvent promotes Na + desolvation, ultimately improving the low temperature and fast charging performance of the battery.

[0024] The present invention uses the anions and cations of the ionic liquid additive to further regulate the Na + Solvation structure can improve the interface stability of the electrode, and the flame retardancy and high electrochemical window of the ionic liquid itself can improve the safety performance of the battery. Taking pyridinium trifluoroacetate (PyF) as an example, it has electron-withdrawing properties, making Na +-Trifluoroacetic acid pyridine (PyF) has a weak interaction, forming an inorganic film rich in NaF inside, and can also increase the organic components in the solid electrolyte interface film (SEI film) and electrochemical interface film (CEI film); the formed organic and inorganic composite SEI film and CEI film have high toughness and high ionic conductivity.

[0025] The ultra-low temperature sodium ion battery electrolyte provided by the present invention can operate in a wide temperature range of room temperature to -60°C, has a low freezing point and high electrochemical performance, and not only achieves far superior performance than commercial electrolytes (such as 1M NaClO 4 -EC / DMC+5%FEC) has high rate capability and high cycle stability, and exhibits excellent electrochemical performance even at low temperature of -60°C. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 The appearance of the electrolytes obtained in Example 1 and Comparative Example 1 at room temperature;

[0027] Figure 2 The first three charge and discharge curves of Application Example 1 at room temperature and 0.1C current density;

[0028] Figure 3 The charge and discharge curves of the first three cycles of comparative application example 1 at room temperature and 0.1C current density are shown;

[0029] Figure 4 This is a comparison chart of the rate performance of Application Example 1 and Comparative Application Example 1 at room temperature;

[0030] Figure 5 This is a comparison chart of the cycle performance of Application Example 1 and Comparative Application Example 1 at room temperature;

[0031] Figure 6 The combustion conditions of the commercial ester electrolyte in Comparative Example 1 (a) and the electrolyte in Example 1 after burning for 3 seconds (b) and 5 seconds (c);

[0032] Figure 7 The stripping effect diagram (a) of sodium metal deposited on Al foil in the commercial electrolyte of Comparative Example 1 and the stripping effect diagram (b) of sodium metal deposited on Al foil in the N-TM electrolyte of Example 1;

[0033] Figure 8 The first three charge-discharge curves (a) and rate performance diagram (b) of Application Example 1 at low temperature -40°C;

[0034] Fig. 9 The appearance of the electrolyte in Comparative Example 1 at room temperature and low temperature -40°C;

[0035] Fig.10The charge and discharge curves of the first three cycles of comparative application example 1 at -40°C are shown;

[0036] Fig.11 The appearance of the electrolyte in Example 1 and Comparative Example 1 at a low temperature of -70°C;

[0037] Fig.12 This is the charge and discharge curve of Application Example 1 at -60°C. DETAILED DESCRIPTION

[0038] In the present invention, unless otherwise specified, the required raw materials or reagents are commercially available products well known to those skilled in the art.

[0039] The present invention provides a non-flammable and ultra-low temperature sodium ion battery electrolyte, comprising the following raw materials: sodium salt, a main solvent and an ionic liquid additive; the main solvent is a phosphate ester solvent and a diluent;

[0040] Wherein, the volume ratio of the phosphate ester solvent to the diluent is 30-70:10-30;

[0041] When the ionic liquid additive is a liquid ionic liquid, the volume ratio of the ionic liquid additive to the main solvent is 1-5:70-30;

[0042] When the ionic liquid additive is a solid ionic liquid, the mass of the ionic liquid additive is 1 to 5% of the mass of the sodium salt;

[0043] The phosphate ester solvent includes trimethyl phosphate, dimethyl phosphate, triethyl phosphate, tributyl phosphate, dibutyl phosphate, monomethyl phosphate, triphenyl phosphate, trihexyl phosphate, di-n-butyl phosphate or triisopropyl phosphate;

[0044] The diluent includes diethyl carbonate, methyl formate, methyl acetate (MA), methyl butyrate or ethyl propionate.

[0045] In the present invention, the sodium salt preferably includes one or more of sodium hexafluorophosphate, sodium bis(trifluoromethylsulfonyl)imide, sodium bis(fluorosulfonyl)imide, sodium perchlorate, sodium difluorooxalate borate and sodium tetrafluoroborate, and further preferably includes one of sodium hexafluorophosphate, sodium bis(trifluoromethylsulfonyl)imide, sodium bis(fluorosulfonyl)imide and sodium difluorooxalate borate, and more preferably sodium bis(trifluoromethylsulfonyl)imide (NaTFSI). When the sodium salt is two or more of the above, the present invention has no special limitation on the ratio of different types of sodium salts, and any ratio is acceptable.

[0046] In the present invention, the concentration of the sodium salt in the main solvent is preferably 0.5 to 2 mol / L, more preferably 0.8 to 1.5 mol / L, and further preferably 1 mol / L.

[0047] In the present invention, the phosphate solvent includes trimethyl phosphate (TMP), dimethyl phosphate, triethyl phosphate, tributyl phosphate, dibutyl phosphate, monomethyl phosphate, triphenyl phosphate, trihexyl phosphate, di-n-butyl phosphate, triisopropyl phosphate or, more preferably trimethyl phosphate or dimethyl phosphate, further preferably trimethyl phosphate.

[0048] In the present invention, the diluent includes diethyl carbonate, methyl formate, methyl acetate (MA), methyl butyrate or ethyl propionate, and more preferably methyl acetate (MA).

[0049] In the present invention, the ionic liquid additive preferably includes 1-ethyl-3-methylimidazolium hexafluorophosphate, 1-butyl-3-methylimidazolium chloride, 1-ethyl-3-methylimidazolium bisfluorosulfonyl imide salt, pyridinium trifluoroacetate (PyF), 1-ethyl-3-methylimidazolium nitrate, 1-butyl-3-methylimidazolium tetrafluoroborate, more preferably 1-ethyl-3-methylimidazolium hexafluorophosphate or pyridinium trifluoroacetate (PyF), and further preferably pyridinium trifluoroacetate (PyF).

[0050] In the present invention, the volume ratio of the phosphate ester solvent to the diluent is 30-70:10-30; preferably 35-60:15-25.

[0051] In the present invention, when the ionic liquid additive is a liquid ionic liquid, the volume ratio of the ionic liquid additive to the main solvent is 1-5:70-30;

[0052] When the ionic liquid additive is a solid ionic liquid, the mass of the ionic liquid additive is 1-5% of the mass of the sodium salt, more preferably 2%.

[0053] The present invention provides a method for preparing the non-flammable and ultra-low temperature sodium ion battery electrolyte described in the above technical solution, comprising the following steps:

[0054] Sodium salt, phosphate ester solvent, diluent and ionic liquid additive are mixed to obtain non-flammable and ultra-low temperature sodium ion battery electrolyte.

[0055] In the present invention, the sodium salt, phosphate ester solvent, diluent and ionic liquid additive are preferably mixed in the following manner: the phosphate ester solvent is mixed with the diluent, stirred for 10 minutes, then mixed with the ionic liquid additive, and then the sodium salt is added, and stirred for 10 hours. The present invention does not limit the mixing conditions, and the mixing can be carried out according to a process well known in the art.

[0056] The present invention provides the use of the non-flammable and ultra-low temperature sodium ion battery electrolyte described in the above technical scheme or the non-flammable and ultra-low temperature sodium ion battery electrolyte prepared by the preparation method described in the above technical scheme in a sodium ion battery, and the application temperature of the non-flammable and ultra-low temperature sodium ion battery electrolyte is 35 to -60°C.

[0057] In the present invention, the sodium ion battery comprises a positive electrode, a negative electrode, a separator and a non-flammable and ultra-low temperature sodium ion battery electrolyte.

[0058] The present invention does not limit the materials and preparation methods of the negative electrode and the separator, and any method familiar to those skilled in the art may be used.

[0059] In the present invention, the positive electrode includes a positive electrode current collector and a positive electrode membrane; the present invention does not limit the positive electrode current collector, and materials well known to those skilled in the art may be used.

[0060] In the present invention, the positive electrode membrane preferably includes a positive electrode active material, a conductive agent and a binder; the positive electrode active material includes one or more of sodium vanadium phosphate, sodium iron phosphate, sodium nickel iron manganate and sodium vanadium fluorophosphate, more preferably one of sodium vanadium phosphate, sodium iron phosphate and sodium vanadium fluorophosphate, and further preferably sodium vanadium phosphate.

[0061] The present invention has no special restrictions on the types and sources of the conductive agent and the binder, and the preparation method of the positive electrode active material, and corresponding materials well known to those skilled in the art can be used.

[0062] The technical solutions provided by the present invention are described in detail below in conjunction with the embodiments, but they should not be construed as limiting the protection scope of the present invention.

[0063] Unless otherwise specified, the reagents and materials used in the examples and comparative examples can be purchased commercially or prepared by conventional methods, and the instruments used can be purchased commercially.

[0064] Example 1

[0065] The ultra-low temperature sodium ion battery electrolyte provided in this embodiment comprises the following raw materials: sodium bis(trifluoromethylsulfonyl)imide (NaTFSI), a main solvent (trimethyl phosphate (TMP) and methyl acetate (MA)) and pyridinium trifluoroacetate (PyF); the concentration of NaFSI in the main solvent is 1 mol / L; the volume ratio of TMP:MA is 70:30; the mass of the ionic liquid PyF is 2% of the mass of NTFSI.

[0066] The preparation method of the above electrolyte comprises the following steps:

[0067] MA solvent was added to TMP, the mixture was stirred for 10 minutes, and then PyF was added to obtain a mixed solvent; NaTFSI was dissolved in the mixed solvent, and the mixture was stirred for 10 hours to obtain a non-flammable and ultra-low temperature sodium ion battery electrolyte, which was recorded as N-TM.

[0068] Example 2

[0069] The ultra-low temperature sodium ion battery electrolyte provided in this embodiment comprises sodium bis(trifluoromethylsulfonyl)imide (NaTFSI), a main solvent (trimethyl phosphate (TMP) and methyl acetate (MA)) and pyridinium trifluoroacetate (PyF); the concentration of NaFSI in the main solvent is 1 mol / L; the volume ratio of TMP:MA is 60:40; and the mass of the ionic liquid is 2% of the mass of NTFSI.

[0070] The preparation method of the above electrolyte comprises the following steps:

[0071] MA solvent was added to TMP, the mixture was stirred for 10 minutes, and PyF was added to obtain a mixed solvent; NaTFSI was dissolved in the mixed solvent, and the mixture was stirred for 10 hours to obtain a non-flammable and ultra-low temperature sodium ion battery electrolyte, which was recorded as N-TM-2.

[0072] Example 3

[0073] The ultra-low temperature sodium ion battery electrolyte provided in this embodiment comprises the following raw materials: sodium bis(trifluoromethylsulfonyl)imide (NaTFSI), a main solvent (trimethyl phosphate (TMP) and methyl acetate (MA)) and pyridinium trifluoroacetate (PyF); the concentration of NaFSI in the main solvent is 1 mol / L; the volume ratio of TMP:MA is 80:20; and the mass of the ionic liquid is 2% of the mass of NTFSI.

[0074] The preparation method of the above electrolyte comprises the following steps:

[0075] MA solvent was added to TMP, the mixture was stirred for 10 minutes, and PyF was added to obtain a mixed solvent; NaTFSI was dissolved in the mixed solvent, and the mixture was stirred for 10 hours to obtain a non-flammable and ultra-low temperature sodium ion battery electrolyte, which was recorded as N-TM-3.

[0076] Comparative Example 1

[0077] The carbonate sodium ion battery electrolyte provided in this comparative example has the following raw materials: sodium perchlorate (NaClO 4 ), ethylene carbonate (EC), dimethyl carbonate (DMC), FEC; NaClO 4The concentration of EC is 1 mol / L; the volume ratio of EC to DMC is 1:1; the volume fraction of FEC is 5% of the total volume of EC+DMC solvent; the electrolyte is a commercial ester electrolyte (i.e., 1M NaClO 4 -EC / DMC+5%FEC).

[0078] The appearance of the sodium ion battery electrolyte obtained in Example 1 and Comparative Example 1 at room temperature is as follows: Figure 1 As shown, both are transparent liquids.

[0079] Application Example 1

[0080] This application example provides a method for preparing a 2032 sodium ion battery, comprising the following steps:

[0081] (1) Preparation of positive electrode sheet: Sodium vanadium phosphate, binder polyvinylidene fluoride, and conductive agent acetylene black are mixed in a mass ratio of 8:1:1, and then N-methylpyrrolidone (NMP) solvent (the mass volume ratio of binder polyvinylidene fluoride and NMP is 40 mg:1 mL) is added and mixed evenly to obtain positive electrode slurry. The positive electrode slurry is coated on the aluminum foil current collector using a coating apparatus with a coating thickness of 100 μm. The heating plate of the coating apparatus is heated to 80°C to preliminarily dry the positive electrode. The preliminarily dried electrode is placed in a vacuum drying oven and dried at 120°C for 10 hours. Finally, the dried electrode is cut into a 10 mm diameter positive electrode sheet using a wafer slicer.

[0082] Preparation of negative electrode sheet: Mix hard carbon, binder polyvinylidene fluoride, and conductive agent acetylene black in a mass ratio of 8:1:1, then add NMP (the mass volume ratio of binder polyvinylidene fluoride and NMP is 40mg:1mL) and mix evenly to obtain negative electrode slurry. The negative electrode slurry is coated on the current collector copper foil with a coating apparatus, and the coating thickness is 100μm. The heating plate of the coating apparatus is heated to 80℃ to preliminarily dry the negative electrode. Place the preliminarily dried electrode in a vacuum drying oven and dry at 120℃ for 10h. Finally, the dried electrode is cut into a 12mm diameter negative electrode sheet with a wafer slicer.

[0083] Isolation membrane: Adopt CROOD commercial glass fiber membrane (GF / D), model: MA-EN-SE-02.

[0084] (2) In a glove box, a battery was assembled in the order of a negative electrode shell, a negative electrode sheet, a separator, the electrolyte of Example 1, a gasket, a positive electrode sheet, and a positive electrode shell. The amount of electrolyte added was 200 μL. The battery was left to stand for 10 h before an electrochemical test.

[0085] Comparative application example 1

[0086] This comparative example adopts the carbonate sodium ion battery electrolyte of comparative example 1.

[0087] The electrochemical performance of batteries assembled with different electrolytes was tested.

[0088] Figure 2 The first three charge and discharge curves of Application Example 1 at room temperature and 0.1C current density are shown. It can be found that Application Example 1 can be stabilized after one cycle at 0.1C, forming an efficient CEI, thereby showing high reversibility.

[0089] Figure 3 For comparison, the charge and discharge curves of the first three cycles of Application Example 1 at room temperature and 0.1C current density show obvious capacity decay in the first three cycles, with low coulombic efficiency, indicating that it has low reversibility and it is difficult to form a stable CEI film to effectively inhibit the decomposition of the electrolyte.

[0090] Figure 4 The figure is a comparison of the rate performance of Application Example 1 and Comparative Application Example 1 at room temperature. Sodium vanadium phosphate exhibits a high rate in the electrolyte of Application Example 1 (N-TM), and still has a high current density of 80.6 mAh g at a high current density of 50C. -1 The discharge specific capacity is far superior to the rate performance of sodium vanadium phosphate in commercial ester electrolytes.

[0091] Figure 5 The figure is a comparison of the cycle performance at room temperature of Application Example 1 and Comparative Application Example 1. Sodium vanadium phosphate exhibits high capacity and high cycle stability at 10C current density in the electrolyte of Application Example 1 (N-TM), which is better than the cycle performance of sodium vanadium phosphate in commercial ester electrolytes.

[0092] The battery separator (glass fiber membrane (GF / D), model: MA-EN-SE-02) was completely soaked in the electrolyte in Example 1 and the commercial electrolyte in Comparative Example 1, and then ignited with a lighter. After 3 seconds, a photo was taken. The results are shown in Figure 6 ; Figure 6 (a) shows the burning of commercial ester electrolyte, and it is found that the electrolyte is very flammable. (b) and (c) show the burning of 3 seconds and 5 seconds after the fire source is removed under the same operation as in Example 1, respectively. It can be seen that the N-TM electrolyte in Example 1 is non-flammable and has good flame retardancy.

[0093] The deposition and stripping effect of sodium metal on Al foil was studied using the commercial electrolyte of Comparative Example 1 and the N-TM electrolyte of Example 1. Specifically, Na / / Al batteries were assembled using the commercial electrolyte of Comparative Example 1 and the N-TM electrolyte of Example 1 at room temperature, and then the Na / / Al batteries were charged at 0.5 mA cm -2 The current density was 0.5 mAh cm-2 deposited on Al foil. -2 The capacity is then 0.5 mA cm -2 The current density was stripped to 1V to observe the efficiency between the deposition capacity and the stripping capacity. The results are shown in Figure 7 .

[0094] Figure 7 The stripping effect diagram (a) of sodium metal deposited on Al foil in the commercial electrolyte of Comparative Example 1 and the stripping effect diagram (b) of sodium metal deposited on Al foil in the N-TM electrolyte of Example 1;

[0095] like Figure 7 As shown in (a), the Coulomb efficiency of deposition stripping is low and difficult to stabilize; Figure 7 As shown in (b), the coulombic efficiency of the N-TM electrolyte deposition and stripping in Example 1 is relatively high and can be quickly stabilized.

[0096] Figure 8 The first three charge-discharge curves (a) and rate performance diagram (b) of Application Example 1 at low temperature -40°C show that even at low temperature -40°C, sodium vanadium phosphate still has high capacity in the electrolyte of Application Example 1, reaching 77% of the room temperature capacity. As can be seen from (b), Application Example 1 has a high rate performance.

[0097] Fig. 9 The following are the appearance diagrams of the electrolyte in Comparative Example 1 at room temperature and low temperature of -40°C. It can be seen that at -40°C, Comparative Example 1 is frozen.

[0098] Fig.10 This is the charge and discharge curve of the first three cycles at -40°C for comparative application example 1. It can be seen that the capacity decay is serious.

[0099] Fig.11 The following are the appearances of the electrolytes in Example 1 and Comparative Example 1 at a low temperature of -70°C. It can be seen that at -70°C, the electrolyte in Example 1 has not frozen, while the electrolyte in Comparative Example 1 is completely frozen.

[0100] Fig.12 The charge and discharge curve of Application Example 1 at -60°C is shown in FIG. 1. It can be seen that at the ultra-low temperature of -60°C, sodium vanadium phosphate has a relatively high capacity in the electrolyte of Application Example 1.

[0101] In summary, the present invention achieves non-flammability of the electrolyte by matching a strongly solvating non-flammable phosphate ester solvent as the main solvent, matching a weakly solvating diluent and an ionic liquid additive to form a non-flammable and ultra-low temperature sodium ion battery electrolyte, and realizes high-rate, long-cycle, and ultra-low temperature operation in the sodium vanadium phosphate electrode material, which provides a design idea for sodium ion batteries used in extreme climates, and provides new ideas for other non-flammable and ultra-low temperature battery fields.

[0102] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A non-flammable and ultra-low temperature sodium ion battery electrolyte, characterized in that: The method comprises the following raw materials: sodium salt, main solvent and ionic liquid additive; the main solvent is a phosphate ester solvent and a diluent; Wherein, the volume ratio of the phosphate ester solvent to the diluent is 30-70:10-30; When the ionic liquid additive is a liquid ionic liquid, the volume ratio of the ionic liquid additive to the main solvent is 1-5:70-30; When the ionic liquid additive is a solid ionic liquid, the mass of the ionic liquid additive is 1 to 5% of the mass of the sodium salt; The phosphate ester solvent includes trimethyl phosphate, dimethyl phosphate, triethyl phosphate, tributyl phosphate, dibutyl phosphate, monomethyl phosphate, triphenyl phosphate, trihexyl phosphate, di-n-butyl phosphate or triisopropyl phosphate; The diluent includes diethyl carbonate, methyl formate, methyl acetate, methyl butyrate or ethyl propionate.

2. The non-flammable and ultra-low temperature sodium ion battery electrolyte according to claim 1, characterized in that: The sodium salt includes one or more of sodium hexafluorophosphate, sodium bis(trifluoromethylsulfonyl)imide, sodium bis(fluorosulfonyl)imide, sodium perchlorate, sodium difluorooxalatoborate and sodium tetrafluoroborate.

3. The non-flammable and ultra-low temperature sodium ion battery electrolyte according to claim 1 or 2, characterized in that: The concentration of the sodium salt in the main solvent is 0.5-2 mol / L.

4. The non-flammable and ultra-low temperature sodium ion battery electrolyte according to claim 2, characterized in that: The ionic liquid additive includes 1-ethyl-3-methylimidazolium hexafluorophosphate, 1-butyl-3-methylimidazolium chloride, 1-ethyl-3-methylimidazolium bisfluorosulfonyl imide salt, pyridinium trifluoroacetate, 1-ethyl-3-methylimidazolium nitrate or 1-butyl-3-methylimidazolium tetrafluoroborate.

5. The method for preparing the non-flammable and ultra-low temperature sodium ion battery electrolyte according to any one of claims 1 to 4, characterized in that: The following steps are involved: Sodium salt, phosphate ester solvent, diluent and ionic liquid additive are mixed to obtain non-flammable and ultra-low temperature sodium ion battery electrolyte.

6. Use of the non-flammable and ultra-low temperature sodium ion battery electrolyte according to any one of claims 1 to 4 or the non-flammable and ultra-low temperature sodium ion battery electrolyte prepared by the preparation method according to claim 5 in a sodium ion battery, characterized in that: The application temperature of the non-flammable and ultra-low temperature sodium ion battery electrolyte is 35 to -60°C.

7. The use according to claim 6, characterized in that: The sodium ion battery comprises a positive electrode, a negative electrode, a separator and a non-flammable and ultra-low temperature sodium ion battery electrolyte.

8. The use according to claim 7, characterized in that: The positive electrode includes a positive electrode current collector and a positive electrode membrane; the positive electrode membrane includes a positive electrode active material, a conductive agent and a binder; the positive electrode active material includes one or more of sodium vanadium phosphate, sodium iron phosphate, sodium nickel iron manganese oxide and sodium vanadium fluorophosphate.

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