Flame-retardant sodium ion electrolyte and preparation method and application thereof

Abstract

The invention relates to a flame-retardant sodium ion electrolyte as well as a preparation method and application thereof, and belongs to the technical field of electrolytes. The flame-retardant sodium ion electrolyte comprises the following components in percentage by mass: 13-15% of sodium salt, 3-10% of a first additive, 1-3% of a second additive and the balance of an organic solvent, and the first additive is pentafluorobenzene dimethyl silicon diethylamine. According to the flame-retardant sodium-ion electrolyte provided by the invention, the specific first additive is selected, and the effect of the flame-retardant sodium-ion electrolyte is comprehensively superior to that of traditional phosphazene and phosphite flame-retardant additives when the flame-retardant sodium-ion electrolyte is used in a sodium-ion battery; according to the present invention, the lithium ion battery electrolyte has the flame retardation effect, the NaF component-rich SEI film can be formed on the negative electrode side so as to easily provide the electrochemical performance, and the Si-N bond in the structure can remove hydrogen fluoride (HF) and phosphorus pentafluoride (PF5), such that the impurities in the electrolyte can be removed, the cycle performance and the stability of the battery can be improved, and the high application value can be provided.

Description

Technical Field

[0001] The present invention relates to the technical field of electrolytes, and in particular to a flame retardant sodium ion electrolyte and a preparation method and application thereof. Background Art

[0002] At present, the organic electrolytes commonly used in sodium-ion batteries contain a large amount of flammable organic solvents. When the battery has abnormal conditions such as thermal runaway, these organic solvents are easy to burn or even cause explosions, posing a great safety hazard to the battery.

[0003] At present, the main additives used to improve the flame retardant properties of sodium ion battery electrolytes are phosphazenes and phosphites. However, the addition of these flame retardant additives may interfere with the transmission of ions in the electrolyte, causing the ionic conductivity of the electrolyte to decrease, and some flame retardant additives may react with the electrode material or form a poor interface layer on the electrode surface, thereby reducing the reversible capacity of the battery, thereby affecting the battery's rate performance and endurance; at the same time, some additives have poor solubility in the electrolyte, making it difficult to disperse evenly in the electrolyte, and are prone to precipitation or stratification. This will not only affect the uniformity of the flame retardant effect, but may also block the pore structure inside the battery, affecting the penetration of the electrolyte and ion transmission.

[0004] Therefore, it is urgent to develop a sodium ion electrolyte with a suitable flame retardant additive so that it can take into account both electrochemical performance and safety performance when used in sodium ion batteries and have a high application prospect. Summary of the invention

[0005] The purpose of the present invention is to overcome the shortcomings of the prior art and provide a flame retardant sodium ion electrolyte and its preparation method and application. The sodium ion electrolyte provided by the present invention has good flame retardant properties, can be used to prepare batteries with good safety performance and electrochemical performance, and has high application value.

[0006] To achieve the above object, the technical solution adopted by the present invention is:

[0007] In a first aspect, the present invention provides a flame-retardant sodium ion electrolyte, which comprises the following components, by mass percentage: 13-15% sodium salt, 3-10% first additive, 1-3% second additive, and the balance organic solvent; the first additive is pentafluorophenyldimethylsilyldiethylamine.

[0008] The flame retardant sodium ion electrolyte provided by the present invention mainly achieves good flame retardant properties of the electrolyte by proportioning a first additive (i.e., a flame retardant additive) with a sodium salt and a second additive (i.e., a conventional additive), and is simultaneously applied to batteries to improve electrochemical properties.

[0009] The first additive used in the present invention is pentafluorophenyldimethylsilyldiethylamine, whose chemical structure contains silicon, nitrogen and abundant fluorine elements. It can not only play a flame retardant effect, but also form a SEI film rich in NaF components on the negative electrode side. Compared with traditional phosphazene and phosphite flame retardants, it is more conducive to the electrochemical performance. At the same time, the Si-N bond in the structure can remove hydrogen fluoride (HF) and phosphorus pentafluoride (PF5), thereby removing impurities in the electrolyte and improving the cycle performance and stability of the battery.

[0010] The sodium ion electrolyte with the specific ratio of the first additive and the second additive of the present invention is used in the preparation of sodium ion batteries, which can shorten the self-extinguishing time of the battery after being ignited, thereby improving the safety performance of the battery; at the same time, it has good battery reaction efficiency and cycle performance. However, when the ratio of the first additive is too high, the electrochemical performance of the battery will deteriorate; when the ratio of the second additive is too high, it will affect the flame retardant performance of the first additive.

[0011] The flame-retardant sodium ion electrolyte provided by the present invention can give the sodium ion battery good electrochemical performance, with the first-week coulomb efficiency being above 84.5%, the 500-week cycle capacity retention rate being above 90%, and the 500-week cycle capacity retention rate being above 90% after being left to stand for 15 days; at the same time, good flame retardant performance can be taken into account, and the self-extinguishing time after ignition is 6-23s / g. Compared with the electrolyte using the traditional flame retardant, it has better performance and has high application value in the sodium ion battery.

[0012] Preferably, the flame retardant sodium ion electrolyte comprises the following components, measured in percentage by mass: 14-15% sodium salt, 5-10% first additive, 1.5-2% second additive, and the balance organic solvent.

[0013] The flame-retardant sodium ion electrolyte with this ratio is used in sodium ion batteries, and its overall effect is significantly better than traditional flame retardants or other Si-N compounds, and the self-extinguishing time after ignition can be controlled within 17s / g.

[0014] Further preferably, the flame retardant sodium ion electrolyte comprises the following components, in percentage by mass: 15% sodium salt, 5% first additive, 2% second additive, and the balance organic solvent.

[0015] The flame-retardant sodium ion electrolyte with this ratio is used in sodium ion batteries, which can take into account excellent safety performance and electrochemical performance, and the self-extinguishing time after ignition is 6s / g.

[0016] Preferably, the sodium salt is at least one of sodium hexafluorophosphate, sodium perchlorate, sodium bis(trifluoromethylsulfonyl)imide, sodium trifluoromethanesulfonate, sodium bis(fluorosulfonyl)imide, sodium bis(oxalatoborate), and sodium difluorooxalatoborate.

[0017] Preferably, the second additive is at least one of vinylene carbonate, fluoroethylene carbonate, 1,3-propane sultone, ethylene sulfate, and vinyl ethylene carbonate.

[0018] Preferably, the organic solvent is at least one of ethylene carbonate, diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, propionate carbonate, methyl formate, methyl acetate, methyl butyrate and ethyl propionate.

[0019] Further preferably, the organic solvent includes dimethyl carbonate, ethyl methyl carbonate and ethylene carbonate.

[0020] In a second aspect, the present invention provides a method for preparing the flame retardant sodium ion electrolyte, comprising the following steps:

[0021] Under an inert atmosphere, a second solvent, a sodium salt and a first solvent are added to an organic solvent and mixed to obtain the flame-retardant sodium ion electrolyte.

[0022] Preferably, in the inert atmosphere, the contents of water and oxygen are both less than 1 ppm.

[0023] In a third aspect, the present invention provides the use of the flame-retardant sodium ion electrolyte in a sodium ion battery.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] The flame-retardant sodium ion electrolyte provided by the present invention contains silicon, nitrogen and abundant fluorine elements by adopting a specific first additive, which can not only play a flame-retardant effect, but also can form a SEI film rich in NaF components on the negative electrode side, which is beneficial to the electrochemical performance; the Si-N bond in the structure can remove hydrogen fluoride (HF) and phosphorus pentafluoride (PF5), thereby removing impurities in the electrolyte, and further improving the cycle performance and stability of the battery; the present invention gives the sodium ion battery better comprehensive performance through the appropriate ratio of components in the sodium ion electrolyte, and has a high application value. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 The chemical structure diagram of the first additive in the flame-retardant sodium ion electrolyte of the present invention.

[0027] Figure 2 This is a graph showing the test results of the 25°C-1C / 1C cycle performance of a battery made with the flame-retardant sodium ion electrolyte of Example 1. DETAILED DESCRIPTION

[0028] In order to better illustrate the purpose, technical scheme and advantages of the present invention, the present invention will be further described below in conjunction with specific examples. The test methods used in the following examples are conventional methods unless otherwise specified; the materials and reagents used are reagents and materials that can be obtained from commercial channels unless otherwise specified.

[0029] Example 1

[0030] An embodiment of the flame-retardant sodium ion electrolyte of the present invention, the flame-retardant sodium ion electrolyte described in this embodiment, is the following components by mass percentage: 14% sodium hexafluorophosphate, 3% pentafluorophenyldimethylsilyldiethylamine, 2% fluoroethylene carbonate, and the balance organic solvent;

[0031] The organic solvent is a mixed solvent of dimethyl carbonate, ethyl methyl carbonate and ethylene carbonate, and the mass ratio of dimethyl carbonate, ethyl methyl carbonate and ethylene carbonate is dimethyl carbonate: ethyl methyl carbonate: ethylene carbonate = 1:1:1; the chemical structure of the first additive pentafluorophenyldimethylsilyldiethylamine is as follows Figure 1 .

[0032] The preparation method of the flame retardant sodium ion electrolyte described in this embodiment is:

[0033] Materials with corresponding mass percentage ratios are prepared by weighing, and in a glove box filled with argon and with water and oxygen contents less than 1 ppm, dimethyl carbonate, ethyl methyl carbonate, and ethylene carbonate are mixed to obtain an organic solvent, and a second additive, fluoroethylene carbonate, is added to the organic solvent, and then sodium salt sodium hexafluorophosphate is slowly added, and stirred evenly to completely dissolve the sodium hexafluorophosphate, and finally the first additive pentafluorophenyldimethylsilyldiethylamine is added to the mixed solution, and dissolved and stirred evenly to obtain the flame-retardant sodium ion electrolyte.

[0034] Example 2

[0035] An embodiment of the flame retardant sodium ion electrolyte of the present invention, the flame retardant sodium ion electrolyte described in this embodiment, is the following components, by mass percentage: 14% sodium hexafluorophosphate, 5% pentafluorophenyldimethylsilyldiethylamine, 2% fluoroethylene carbonate, and the balance organic solvent; the composition of the organic solvent is the same as that of Example 1.

[0036] The preparation method of the flame-retardant sodium ion electrolyte described in this embodiment is the same as that in Example 1.

[0037] Example 3

[0038] An embodiment of the flame retardant sodium ion electrolyte of the present invention, the flame retardant sodium ion electrolyte described in this embodiment, comprises the following components, by mass percentage: 14% sodium hexafluorophosphate, 7% pentafluorophenyldimethylsilyldiethylamine, 2% fluoroethylene carbonate, and the balance organic solvent; the composition of the organic solvent is the same as that of Example 1.

[0039] The preparation method of the flame-retardant sodium ion electrolyte described in this embodiment is the same as that in Example 1.

[0040] Example 4

[0041] An embodiment of the flame retardant sodium ion electrolyte of the present invention, the flame retardant sodium ion electrolyte described in this embodiment, comprises the following components, by mass percentage: 14% sodium hexafluorophosphate, 10% pentafluorophenyldimethylsilyldiethylamine, 1.5% fluoroethylene carbonate, and the balance organic solvent; the composition of the organic solvent is the same as that of Example 1.

[0042] The preparation method of the flame-retardant sodium ion electrolyte described in this embodiment is the same as that in Example 1.

[0043] Example 5

[0044] An embodiment of the flame retardant sodium ion electrolyte of the present invention, the flame retardant sodium ion electrolyte described in this embodiment, comprises the following components, by mass percentage: 15% sodium hexafluorophosphate, 5% pentafluorophenyldimethylsilyldiethylamine, 2% fluoroethylene carbonate, and the balance organic solvent; the composition of the organic solvent is the same as that of Example 1.

[0045] The preparation method of the flame-retardant sodium ion electrolyte described in this embodiment is the same as that in Example 1.

[0046] Comparative Example 1

[0047] A sodium ion electrolyte, comprising the following components, by mass percentage: 14% sodium hexafluorophosphate, 2% fluoroethylene carbonate, and the balance organic solvent; the composition of the organic solvent is the same as that of Example 1;

[0048] The preparation method of sodium ion electrolyte is shown in Example 1.

[0049] Comparative Example 2

[0050] A flame retardant sodium ion electrolyte comprises the following components, measured by mass percentage: 14% sodium hexafluorophosphate, 5% trimethyl phosphite, 2% fluoroethylene carbonate, and the balance organic solvent; the composition of the organic solvent is the same as that of Example 1.

[0051] The preparation method of the flame retardant sodium ion electrolyte is referred to Example 1.

[0052] Comparative Example 3

[0053] A sodium ion electrolyte comprises the following components, measured by mass percentage: 14% sodium hexafluorophosphate, 3% 1,1-dichloro-1-ethyl-N,N-dimethylsilylamine, 2% fluoroethylene carbonate, and the balance organic solvent; the composition of the organic solvent is the same as that of Example 1.

[0054] The preparation method of sodium ion electrolyte is shown in Example 1.

[0055] Comparative Example 4

[0056] A sodium ion electrolyte comprises the following components, measured by mass percentage: 14% sodium hexafluorophosphate, 15% pentafluorophenyldimethylsilyldiethylamine, 0.5% fluoroethylene carbonate, and the balance organic solvent; the composition of the organic solvent is the same as that of Example 1.

[0057] The preparation method of sodium ion electrolyte is shown in Example 1.

[0058] Comparative Example 5

[0059] A sodium ion electrolyte comprises the following components, measured by mass percentage: 14% sodium hexafluorophosphate, 2% pentafluorophenyldimethylsilyldiethylamine, 5% fluoroethylene carbonate, and the balance organic solvent; the composition of the organic solvent is the same as that of Example 1.

[0060] The preparation method of sodium ion electrolyte is shown in Example 1.

[0061] Effect Example 1

[0062] In order to explore the flame retardant properties and electrochemical properties of the flame retardant sodium ion electrolyte provided by the present invention, the electrolytes in the embodiments and comparative examples were used to prepare batteries, and the self-extinguishing time of the electrolyte, the first week coulomb efficiency of the prepared battery, the 500-week cycle capacity retention rate, and the 500-week cycle capacity retention rate after standing for 15 days were tested respectively. The specific steps are as follows:

[0063] The steps for preparing the battery are:

[0064] S1. Cathode Preparation

[0065] The sodium ion positive electrode material ternary material sodium nickel iron manganese oxide (NaNi 1 / 3 Fe 1 / 3 Mn 1 / 3 O2), adhesive PVDF and conductive agent SuperP are uniformly mixed in a mass ratio of 96:2:2 to form a positive electrode slurry for a sodium ion battery, the mixed slurry is coated on both sides of an aluminum foil, and the positive electrode sheet is obtained after drying and rolling.

[0066] S2. Negative Electrode Preparation

[0067] The sodium ion negative electrode material hard carbon, the conductive agent SuperP, and the adhesive PVDF are made into slurry in a mass ratio of 91:2:7, mixed evenly, and the mixed slurry is coated on both sides of the copper foil, and the negative electrode sheet is obtained after drying and rolling.

[0068] S3. Preparation of batteries

[0069] The positive electrode, the negative electrode and the separator are wound into a battery cell, which is packaged with an aluminum-plastic film. The electrolyte prepared in the embodiment and the comparative example is injected into a glove box filled with argon gas and having a water and oxygen content of less than 1 ppm. After the steps of formation and capacity separation, a sodium ion battery with a capacity of 1 Ah is prepared.

[0070] The electrolytes in the embodiments and comparative examples and the batteries prepared therefrom were subjected to the following performance tests:

[0071] (1) Electrolyte flame retardancy test

[0072] The flame retardant properties of the electrolytes in the embodiments and comparative examples were tested by the self-extinguishing method. Uniform ceramic fiber paper was selected, cut into strips of 4 mm thick, 10 mm wide and 20 mm long, and baked in a constant temperature drying oven at 90-100°C for 12 hours. The baked ceramic fiber paper was hooked with a copper wire, and its total mass was weighed as m1. The weighed ceramic fiber paper was soaked in the electrolyte for a certain period of time, taken out, and its total mass was weighed as m2. It was then hung on an iron frame, ignited, and the time t from the time the fire source left until the flame of the ceramic fiber paper went out was recorded. The flame retardant properties of the electrolyte are measured by the self-extinguishing time set, which is calculated by the following formula: set = t / (m2-m1);

[0073] (2) Battery first week coulombic efficiency test

[0074] At 25°C, the prepared sodium ion battery was charged to 4.0V at 0.5C constant current and constant voltage to obtain a charging capacity Q0, and then left for 5 minutes; then discharged to 1.8V at 0.5C current, and the capacity at this time was recorded as Q1; the first cycle coulomb efficiency was calculated by the following formula: first cycle coulomb efficiency = (Q1 / Q0) × 100%;

[0075] (3) Battery cycle performance test

[0076] At 25°C, the prepared battery was charged to 4.0V at 1C constant current and constant voltage, and left for 5 minutes; then discharged to 1.8V at 1C current, and left for 5 minutes. The above steps were repeated 500 times, and the capacity retention rate before and after the cycle was calculated by the following formula:

[0077] Capacity retention rate = (500th cycle discharge capacity / first cycle discharge capacity) × 100%

[0078] (4) Cycle test of batteries after long-term storage

[0079] After the newly manufactured battery was left to stand at room temperature for 15 days, the cycle performance was tested using the method in (3) above.

[0080] The performance test results of (1)-(4) are summarized in Table 1, where the 25°C-1C / 1C cycle performance results of the battery prepared with the flame-retardant sodium ion electrolyte in Example 1 are as follows: Figure 2 ; From Table 1, we can see that:

[0081] The flame-retardant sodium ion electrolyte in Examples 1-5, in which the ratio of sodium salt, the first additive, the second additive and the organic solvent is within the specified range, has a self-extinguishing time of 6-23 s / g, a first-week coulombic efficiency of more than 84.6%, and a 500-week cycle capacity retention rate and a 500-week cycle capacity retention rate after standing for 15 days are both greater than 90%, taking into account both good flame retardant properties and electrochemical stability.

[0082] In Comparative Example 1, the first additive is not added, and the flame retardant performance deteriorates significantly; in Comparative Examples 2 and 3, the first additive is replaced by other additives, and the flame retardant performance and the electrochemical performance cannot be taken into account at the same time; in Comparative Example 4, when the first additive ratio is too high and the second additive is insufficient, the stability of the electrochemical performance is reduced; in Comparative Example 5, when the first additive is insufficient and the second additive is too high, the flame retardant performance deteriorates significantly.

[0083] The specific first additive and its proportion defined in the present invention can make the flame-retardant sodium ion electrolyte have both good safety performance and electrochemical performance, overcome the defects of traditional additives, and have high application value in sodium ion batteries.

[0084] Table 1 Performance test results of the electrolytes of the embodiments and comparative examples and the batteries obtained therefrom

[0085]

[0086] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solution of the present invention can be modified or replaced by equivalents without departing from the essence and scope of the technical solution of the present invention.

Claims

1. A flame retardant sodium ion electrolyte, characterized in that: The invention comprises the following components by mass percentage: 13-15% sodium salt, 3-10% first additive, 1-3% second additive and the balance organic solvent; the first additive is pentafluorophenyldimethylsilyldiethylamine.

2. The flame retardant sodium ion electrolyte according to claim 1, characterized in that: The invention comprises the following components by mass percentage: 14-15% sodium salt, 5-10% first additive, 1.5-2% second additive and the balance organic solvent.

3. The flame retardant sodium ion electrolyte according to claim 2, characterized in that: The composition comprises the following components by mass percentage: 15% sodium salt, 5% first additive, 2% second additive and the balance organic solvent.

4. The flame retardant sodium ion electrolyte according to claim 1, characterized in that: The sodium salt is at least one of sodium hexafluorophosphate, sodium perchlorate, sodium bis(trifluoromethylsulfonyl)imide, sodium trifluoromethanesulfonate, sodium bis(fluorosulfonyl)imide, sodium bis(oxalatoborate), and sodium difluorooxalatoborate.

5. The flame retardant sodium ion electrolyte according to claim 1, characterized in that: The second additive is at least one of vinylene carbonate, fluoroethylene carbonate, 1,3-propane sultone, ethylene sulfate, and vinyl ethylene carbonate.

6. The flame retardant sodium ion electrolyte according to claim 1, characterized in that: The organic solvent is at least one of ethylene carbonate, diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, propionate carbonate, methyl formate, methyl acetate, methyl butyrate and ethyl propionate.

7. The flame retardant sodium ion electrolyte according to claim 6, characterized in that: The organic solvent includes dimethyl carbonate, ethyl methyl carbonate and ethylene carbonate.

8. The method for preparing a flame-retardant sodium ion electrolyte according to any one of claims 1 to 7, characterized in that: The following steps are involved: Under an inert atmosphere, a second solvent, a sodium salt and a first solvent are added to an organic solvent and mixed to obtain the flame-retardant sodium ion electrolyte.

9. The method for preparing a flame retardant sodium ion electrolyte according to claim 8, characterized in that: In the inert atmosphere, the contents of water and oxygen are both less than 1 ppm.

10. Use of the flame-retardant sodium ion electrolyte according to any one of claims 1 to 7 in a sodium ion battery.

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