Low-temperature local high-concentration electrolyte and secondary sodium ion battery

Abstract

The invention relates to a low-temperature local high-concentration electrolyte and a secondary sodium-ion battery, and belongs to the technical field of secondary sodium-ion batteries, and the low-temperature local high-concentration electrolyte comprises a functional diluent, an ether solvent and a sodium salt; the functional diluent is prepared by mixing two diluents as shown in a structural formula I and a structural formula II: # imgabs0 #. By adding the diluent as shown in the structural formula I, the intrinsic freezing point of the electrolyte can be effectively reduced, the liquid path of the electrolyte is widened, and ion transmission of the electrolyte at low temperature is promoted; strong intermolecular interaction exists between fluorinated groups of the diluent shown in the structural formula II and the diluent shown in the structural formula I, unstable phase precipitation is avoided, and the low-temperature performance of the sodium ion battery is improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of secondary sodium-ion batteries, and particularly relates to a low-temperature type locally high-concentration electrolyte and a secondary sodium-ion battery. Background Art

[0002] In recent years, sodium-ion batteries have been widely used in various fields such as electronic products, starting power supplies, low-speed electric vehicles, and power grid energy storage due to their advantages of rich resources, high safety, and low cost. With the continuous expansion of the application scenarios of sodium-ion batteries, the electrochemical performance requirements for sodium-ion batteries in special scenarios are getting higher and higher. China has a vast territory, and there are large temperature differences between the north and the south. In winter in the northern regions, the single-trip mileage of pure electric vehicles is often greatly reduced compared to that in summer. In some extremely cold regions (such as some high-altitude regions and some northeastern regions, etc.), the outdoor temperature may even reach -30°C. At this time, most intelligent mobile devices can hardly be used, and often there will be a situation of "freezing shutdown". In addition, with the development of polar scientific research, aerospace, and underwater exploration and other undertakings, the low-temperature performance requirements for sodium-ion batteries are further improved, and even need to reach below -50°C.

[0003] As the medium for ion transport and charge transfer inside the battery, the electrolyte has an important impact on the low-temperature performance of the battery. Ether-based electrolytes have a low freezing point, moderate sodium salt solubility, and low desolvation energy, and are good choices for low-temperature electrolytes of sodium-ion batteries. The oxidation resistance of ether-based electrolytes is poor. To broaden the applicable voltage range of ether-based electrolytes, a locally high-concentration method is often constructed through diluents. However, the diluents selected for locally high-concentration electrolytes are often highly fluorinated solvents, which completely do not have the ability to dissolve sodium salts and are extremely likely to separate from other components in the electrolyte. In a low-temperature environment, this instability will be further enhanced, resulting in problems such as sodium salt precipitation, which causes the electrolyte to fail. Therefore, improving the stability of the diluent in the electrolyte and strengthening the interaction between the diluent and other components of the electrolyte are particularly important for improving the low-temperature applicability of locally high-concentration electrolytes and broadening the low-temperature applications in sodium-ion batteries. Summary of the Invention

[0004] In order to improve the stability of the diluent in the electrolyte and strengthen the interaction between the diluent and other components in the electrolyte, the present invention provides a low-temperature type locally high-concentration electrolyte and a secondary sodium-ion battery.

[0005] In order to achieve the above object, the present invention adopts the following technical solutions:

[0006] A low-temperature type locally high-concentration electrolyte, comprising a functional diluent, an ether solvent, and a sodium salt; the functional diluent comprises two diluents shown in Structural Formula I and Structural Formula II;

[0007]

[0008] In Structural Formula I, R 1 and R 2 each independently selects F, CH 2 F, CHF 2 or CF 3 ; in Structural Formula II, R 3 and R 4 each independently selects H, CH 3 , CH 2 CH 3 or OCH 3 .

[0009] Furthermore, the volume ratio of the diluents shown in Structural Formula I and Structural Formula II is 1:3 to 3:1.

[0010] Furthermore, the ether solvent includes one or a combination of more of ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, tetrahydrofuran, 2-methyltetrahydrofuran, and ethyl methyl ether.

[0011] Furthermore, the sodium salt is one or a combination of more of sodium hexafluorophosphate (NaPF 6 ), sodium difluoro(oxalato)borate (NaODFB), sodium bis(fluorosulfonyl)imide (NaFSI), sodium bis(trifluoromethylsulfonyl)imide (NaTFSI), and sodium tetrafluoroborate (NaBF 4 ).

[0012] Furthermore, the total molar concentration range of the sodium salt in the ether solvent is 1 mol / L - 4 mol / L.

[0013] Furthermore, the volume ratio of the ether solvent to the functional diluent is 1:3 to 3:1.

[0014] A secondary sodium-ion battery with good low-temperature performance and long cycle life, comprising a positive electrode, an electrolyte, and a negative electrode, wherein the electrolyte is the low-temperature type local high-concentration electrolyte described above.

[0015] Furthermore, the positive electrode includes any one of a layered oxide positive electrode, a Prussian blue positive electrode, a polyanion positive electrode, and an organic positive electrode.

[0016] Furthermore, the negative electrode includes any one of a hard carbon negative electrode, a sodium metal negative electrode, an alloy negative electrode, a titanium-based negative electrode, and an organic negative electrode.

[0017] Compared with the prior art, the beneficial effects of the present invention are:

[0018] A diluent shown in Structural Formula I is obtained by substituting a fluorinated group at the meta position of the epoxy atoms in 1,3-dioxane. The fluorine group can not only lower the freezing point of the molecule, but also disperse the local electron cloud density of the two oxygen atoms, reduce the molecular polarity, and enhance its intrinsic oxidation resistance. The short-chain fluorinated group, namely F, CH 2 F, CHF 2 or CF 3 , can effectively avoid the influence of molecular steric hindrance and promote the rapid migration of sodium ions in the electrolyte. Further, by substituting a short-chain electron-donating group, namely H, CH 3 , CH 2 CH 3 or OCH 3 , at the ortho position of 1,3-dioxane, a diluent of Structural Formula II is obtained. The short-chain electron-donating group can increase the local electron cloud density of the oxygen atom. When the two are mixed, the diluent of Structural Formula I can effectively lower the freezing point of the electrolyte and broaden the liquid range of the electrolyte. The increase in the local electron cloud density of the oxygen atom on the diluent molecule shown in Structural Formula II strengthens the intermolecular interaction with the fluorinated group of the diluent shown in Structural Formula I, thereby promoting the molecular-level dispersion of the diluent shown in Structural Formula I in the liquid phase of the electrolyte, avoiding the precipitation of unstable phases, promoting the low-temperature applicability of the locally high-concentration electrolyte formed, and thus improving the low-temperature performance of the sodium-ion battery. Description of the Drawings

[0019] Figure 1 In [the figure], a, b, and c are the optical photos of Example 1, Comparative Example 1, and Comparative Example 2 after being left at -40°C for 4 h, respectively;

[0020] Figure 2 In [the figure], a and b are the charge-discharge curves of the Na||HC half-cells of Example 2 and Comparative Example 3 at -40°C and a current density of 100 mA / g, respectively;

[0021] Figure 3 is the long cycle curve of the HC||Fe-PB full cell prepared in Example 3 at -30°C and a current density of 0.1C;

[0022] Figure 4 is the long cycle curve of the HC||Fe-PB full cell prepared in Comparative Example 4 at -30°C and a current density of 0.1C;

[0023] Figure 5 In [the figure], a and b are the discharge curves of the HC||Fe-PB full cells of Example 4 and Comparative Example 5 at -50°C and a current density of 0.1C, respectively;

[0024] Figure 6Among them, a and b are the discharge curves of the HC||NVP@C soft-pack batteries in Example 5 and Comparative Example 6 at -30°C and a current density of 0.1C. Detailed implementation manners

[0025] Next, the technical solutions in the present invention will be clearly and completely described in conjunction with the accompanying drawings and embodiments. Obviously, the described embodiments are only a part of the embodiments of the invention, rather than all embodiments. Based on the embodiments in the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present invention.

[0026] Example 1

[0027] A low-temperature type locally high-concentration electrolyte is composed of NaPF 6 , diethylene glycol dimethyl ether, 3-trifluoromethyl-1,3-dioxane and 2-methyl-1,3-dioxane. Among them, NaPF 6 is a sodium salt, diethylene glycol dimethyl ether is used as a solvent, and the mixture of 3-trifluoromethyl-1,3-dioxane and 2-methyl-1,3-dioxane is used as a functional diluent. The volume ratio of 3-trifluoromethyl-1,3-dioxane to 2-methyl-1,3-dioxane is 1:1. The molar concentration of NaPF 6 in diethylene glycol dimethyl ether is 2 mol / L, and the volume ratio of the functional diluent to the solvent diethylene glycol dimethyl ether is 1:1. It can be understood that after adding the functional diluent, the molar concentration of NaPF 6 is reduced to 1 mol / L.

[0028] The specific preparation method and low-temperature tolerance test are as follows:

[0029] Add NaPF 6 to the diethylene glycol dimethyl ether solvent according to the molar ratio, and stir well until completely dissolved to obtain a 2 mol / L high-concentration sodium-ion battery electrolyte. Mix 3-trifluoromethyl-1,3-dioxane and 2-methyl-1,3-dioxane according to a volume ratio of 1:1 to obtain a functional diluent, and then add it to the above 2 mol / L high-concentration sodium-ion battery electrolyte. The volume ratio of the added functional diluent to the solvent diethylene glycol dimethyl ether is 1:1. Stir well again until the electrolyte is uniformly mixed to obtain a low-temperature type locally high-concentration electrolyte; place the obtained low-temperature type locally high-concentration electrolyte in a -40°C low-temperature box for 4 h; the experimental results of Example 1 are as Figure 1 (a) shows that the electrolyte remains in a liquid state and does not solidify.

[0030] Example 2

[0031] A low-temperature local high-concentration electrolyte is composed of NaFSI, ethylene glycol dimethyl ether, 3-difluoromethyl-1,3-dioxane and 2-ethyl-1,3-dioxane, wherein NaFSI is a sodium salt, ethylene glycol dimethyl ether is used as a solvent, a mixture of 3-difluoromethyl-1,3-dioxane and 2-ethyl-1,3-dioxane is used as a functional diluent, the volume ratio of 3-difluoromethyl-1,3-dioxane to 2-ethyl-1,3-dioxane is 1:2, the molar concentration of NaFSI in ethylene glycol dimethyl ether is 3 mol / L, and the volume ratio of the functional diluent to the solvent ethylene glycol dimethyl ether is 2:1. It can be understood that after adding the functional diluent, the molar concentration of NaFSI is reduced to 1 mol / L; hard carbon (HC) is used as a research electrode, a sodium metal sheet is used as a counter electrode, and a glass fiber separator and the electrolyte described in Example 2 are assembled into a Na||HC 2025 button half-cell.

[0032] The specific preparation method and low-temperature electrochemical performance test are as follows:

[0033] NaFSI is added to the ethylene glycol dimethyl ether solvent according to the molar ratio, and stirred thoroughly until it is completely dissolved to obtain a 3 mol / L high-concentration sodium ion battery electrolyte. 3-Difluoromethyl-1,3-dioxane and 2-ethyl-1,3-dioxane are mixed at a volume ratio of 1:2 to obtain a functional diluent, which is then added to the above 3 mol / L high-concentration sodium ion battery electrolyte. The volume ratio of the added functional diluent to the solvent ethylene glycol dimethyl ether is 2:1, and the electrolyte is stirred thoroughly again until the electrolyte is mixed uniformly to obtain a low-temperature local high-concentration electrolyte. The test results of the Na||HC 2025 button half-cell described in Example 2 are as follows: Figure 2 As shown in (a), after activation at room temperature, the charge and discharge reversible capacity is 255 mAh / g at -40°C and a current density of 100 mA / g.

[0034] Example 3

[0035] A low-temperature, locally high-concentration electrolyte composed of NaPF 6 , diethylene glycol dimethyl ether, 3-trifluoromethyl-1,3-dioxane and 1,3-dioxane, among which NaPF 6 It is sodium salt, diethylene glycol dimethyl ether is used as solvent, a mixture of 3-trifluoromethyl-1,3-dioxane and 1,3-dioxane is used as functional diluent, the volume ratio of 3-trifluoromethyl-1,3-dioxane and 1,3-dioxane is 1:3, NaPF 6 The molar concentration in diethylene glycol dimethyl ether is 2 mol / L, and the volume ratio of the functional diluent to the solvent diethylene glycol dimethyl ether is 1:1. It can be understood that after adding the functional diluent, NaPF 6The molar concentration is reduced to 1 mol / L; Prussian blue (Fe-PB) is used as the positive electrode, HC is used as the negative electrode, and a glass fiber separator and the electrolyte described in Example 3 are assembled into an HC||Fe-PB 2025-type button full cell.

[0036] The specific preparation method and low-temperature electrochemical performance test are as follows:

[0037] According to the molar ratio, NaPF 6 is added to the diethylene glycol dimethyl ether solvent, and stirred thoroughly until completely dissolved to obtain a 2 mol / L high-concentration sodium-ion battery electrolyte. 3-Trifluoromethyl-1,3-dioxolane and 1,3-dioxolane are mixed in a volume ratio of 1:3 to obtain a functional diluent, which is then added to the above 2 mol / L high-concentration sodium-ion battery electrolyte. The volume ratio of the added functional diluent to the solvent diethylene glycol dimethyl ether is 1:1, and stirred again thoroughly until the electrolyte is uniformly mixed to obtain a low-temperature type locally high-concentration electrolyte; the test results of the HC||Fe-PB 2025-type button full cell described in Example 3 are as Figure 3 shown. After activation at room temperature, charge and discharge cycling is carried out at -30 °C and a current density of 0.1 C. The first charge-discharge reversible capacity is 133.4 mAh / g, and after 100 cycles, the reversible capacity is still 125.9 mAh / g, and the capacity retention rate reaches 94.4%.

[0038] Example 4

[0039] A low-temperature type locally high-concentration electrolyte is composed of NaFSI, tetrahydrofuran, 3-fluoromethyl-1,3-dioxolane and 2-methoxy-1,3-dioxolane. Among them, NaFSI is a sodium salt, tetrahydrofuran is used as a solvent, and the mixture of 3-fluoromethyl-1,3-dioxolane and 2-methoxy-1,3-dioxolane is used as a functional diluent. The volume ratio of 3-fluoromethyl-1,3-dioxolane and 2-methoxy-1,3-dioxolane is 2:1. The molar concentration of NaFSI in tetrahydrofuran is 3 mol / L, and the volume ratio of the functional diluent to the solvent tetrahydrofuran is 1:1. It can be understood that after adding the functional diluent, the molar concentration of NaFSI is reduced to 1.5 mol / L; Prussian blue (Fe-PB) is used as the positive electrode, HC is used as the negative electrode, and a glass fiber separator and the electrolyte described in Example 4 are assembled into an HC||Fe-PB 2025-type button full cell.

[0040] The specific preparation method and low-temperature electrochemical performance test are as follows:

[0041] Add NaFSI to the tetrahydrofuran solvent according to the molar ratio, and stir well until completely dissolved to obtain a high-concentration sodium-ion battery electrolyte with a concentration of 3 mol / L. Mix 3-(fluoromethyl)-1,3-dioxane and 2-methoxy-1,3-dioxane in a volume ratio of 2:1 to obtain a functional diluent, and then add it to the above-mentioned 3 mol / L high-concentration sodium-ion battery electrolyte. The volume ratio of the added functional diluent to the volume of the solvent tetrahydrofuran is 1:1. Stir well again until the electrolyte is uniformly mixed to obtain a low-temperature type locally high-concentration electrolyte; the test results of the HC||Fe-PB 2025 type button full cell described in Example 4 are as Figure 5 (a) As shown, after activation and charging at room temperature, discharge at -50 °C and a current density of 0.1 C, and the discharge capacity is 94.8 mAh / g.

[0042] Example 5

[0043] A low-temperature type locally high-concentration electrolyte is composed of NaTFSI, 2-methyltetrahydrofuran, 3-(difluoromethyl)-1,3-dioxane and 2-ethyl-1,3-dioxane. Among them, NaTFSI is a sodium salt, 2-methyltetrahydrofuran is used as a solvent, and the mixture of 3-(difluoromethyl)-1,3-dioxane and 2-ethyl-1,3-dioxane is used as a functional diluent. The volume ratio of 3-(difluoromethyl)-1,3-dioxane to 2-ethyl-1,3-dioxane is 3:1. The molar concentration of NaTFSI in 2-methyltetrahydrofuran is 2 mol / L. The volume ratio of the functional diluent to the volume of the solvent 2-methyltetrahydrofuran is 1:1. It can be understood that after adding the functional diluent, the molar concentration of NaTFSI is reduced to 1 mol / L; using carbon-coated sodium vanadium phosphate (NVP@C) as the positive electrode, HC as the negative electrode, and a polypropylene separator to prepare a 1.2 Ah HC||NVP@C soft-pack battery with the electrolyte described in Example 5.

[0044] The specific preparation method and low-temperature electrochemical performance test are as follows:

[0045] Add NaTFSI to 2-methyltetrahydrofuran according to the molar ratio, and stir well until completely dissolved to obtain a high-concentration sodium-ion battery electrolyte with a concentration of 2 mol / L. Mix 3-(difluoromethyl)-1,3-dioxane and 2-ethyl-1,3-dioxane in a volume ratio of 3:1 to obtain a functional diluent, and then add it to the above-mentioned 2 mol / L high-concentration sodium-ion battery electrolyte. The volume ratio of the added functional diluent to the volume of the solvent 2-methyltetrahydrofuran is 1:1. Stir well again until the electrolyte is uniformly mixed to obtain a low-temperature type locally high-concentration electrolyte; the test results of the 1.2 Ah HC||NVP@C soft-pack battery described in Example 5 are as Figure 6As shown in (a), after being activated and charged at room temperature, it is discharged at -30 °C and a current density of 0.1 C, and the discharge capacity is 1.10 Ah.

[0046] Comparative Example 1

[0047] The difference between this comparative example and Example 1 is that only 3-trifluoromethyl-1,3-dioxane is used as the diluent, and the other parameters are the same as those in Example 1. The electrolyte prepared in Comparative Example 1 is placed in a low-temperature box at -40 °C for 4 h, and the experimental results are as Figure 1 shown in (b), and the electrolyte solidifies.

[0048] Comparative Example 2

[0049] The difference between this comparative example and Example 1 is that no functional diluent is used, and the other parameters are the same as those in Example 1. The electrolyte prepared in Comparative Example 2 is placed in a low-temperature box at -40 °C for 4 h, and the experimental results are as Figure 1 shown in (c), and the electrolyte solidifies.

[0050] Comparative Example 3

[0051] The difference between this comparative example and Example 2 is that no functional diluent is used, and the other parameters are the same as those in Example 2. The test results of the Na||HC 2025 type button half-cell assembled in Comparative Example 3 are as Figure 2 shown in (b). After being activated at room temperature, it is tested at -40 °C and a current density of 100 mA / g, and the charge-discharge reversible capacity is 145 mAh / g.

[0052] Comparative Example 4

[0053] The difference between this comparative example and Example 3 is that no functional diluent is used, and the other parameters are the same as those in Example 3. The test results of the HC||Fe-PB 2025 type button full cell assembled in Comparative Example 4 are as Figure 4 shown. After being activated at room temperature, it is charged and discharged at -30 °C and a current density of 0.1 C. The first charge-discharge reversible capacity is 64.2 mAh / g, and the electrolyte fails after less than 20 cycles.

[0054] Comparative Example 5

[0055] The difference between this comparative example and Example 4 is that no functional diluent is used, and the other parameters are the same as those in Example 4. The test results of the HC||Fe-PB 2025 type button full cell assembled in Comparative Example 5 are as Figure 5 shown in (b). After being activated and charged at room temperature, it is discharged at -50 °C and a current density of 0.1 C, and the discharge capacity is 61.1 mAh / g.

[0056] Comparative Example 6

[0057] The difference between this comparative example and Example 5 is that no functional diluent is used, and the other parameters are the same as those in Example 5. The test results of the 1.2 Ah HC||NVP@C soft-pack battery assembled in Comparative Example 6 are as follows Figure 6 (b) After activation and charging at room temperature, the battery is discharged at -30 °C and a current density of 0.1 C, and the discharge capacity is 0.41 Ah.

[0058] In addition, it should be understood that although this specification is described according to the implementation manners, not every implementation manner only contains an independent technical solution. This narrative manner of the specification is only for clarity. Those skilled in the art should regard the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other implementation manners that can be understood by those skilled in the art.

Claims

1. A low-temperature local high-concentration electrolyte, characterized in that: It includes a functional diluent, an ether solvent and a sodium salt; the functional diluent includes two diluents shown in structural formula I and structural formula II; In structural formula I, R1 and R2 are each independently selected from F, CH2F, CHF2 or CF3; in structural formula II, R3 and R4 are each independently selected from H, CH3, CH2CH3 or OCH3.

2. A low-temperature local high-concentration electrolyte according to claim 1, characterized in that: The volume ratio of the diluents shown in the structural formula I and the structural formula II is 1:3 to 3:

1.

3. The low-temperature local high-concentration electrolyte according to claim 1, characterized in that: The ether solvent includes one or more of ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, tetrahydrofuran, 2-methyltetrahydrofuran and ethyl methyl ether.

4. The low-temperature local high-concentration electrolyte according to claim 1, characterized in that: The sodium salt includes one or more of sodium hexafluorophosphate, sodium bis(fluorooxalato)borate, sodium bis(fluorosulfonyl)imide, sodium bis(trifluoromethylsulfonyl)imide and sodium tetrafluoroborate.

5. The low-temperature local high-concentration electrolyte according to claim 1, characterized in that: The total molar concentration of the sodium salt in the ether solvent is in the range of 1 mol / L to 4 mol / L.

6. The low-temperature local high-concentration electrolyte according to claim 1, characterized in that: The volume ratio of the ether solvent to the functional diluent is 1:3 to 3:

1.

7. A secondary sodium ion battery comprising a positive electrode, an electrolyte and a negative electrode, characterized in that: The electrolyte is a low-temperature, locally high-concentration electrolyte as described in any one of claims 1 to 6.

8. A secondary sodium ion battery according to claim 7, characterized in that: The positive electrode includes any one of a layered oxide positive electrode, a Prussian blue positive electrode, a polyanion positive electrode and an organic positive electrode.

9. A secondary sodium ion battery according to claim 7, characterized in that: The negative electrode includes any one of a hard carbon negative electrode, a sodium metal negative electrode, an alloy negative electrode, a titanium-based negative electrode and an organic negative electrode.

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