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

Abstract

The application discloses a low-temperature type local high-concentration electrolyte and a secondary sodium ion battery, and belongs to the technical field of the secondary sodium ion battery, and the specific scheme is as follows: a low-temperature type local high-concentration electrolyte, which comprises a functional diluent, an ether solvent and a sodium salt; the functional diluent is prepared by mixing two diluents shown in structural formula I and structural formula II; by adding the diluent shown in structural formula I, the intrinsic freezing point of the electrolyte can be effectively reduced, the electrolyte liquid range is widened, and ion transmission of the electrolyte at low temperature is promoted; the diluent shown in structural formula II and the fluorine group between the diluent shown in structural formula I have strong intermolecular interaction, unstable phases are avoided, and the low-temperature performance of the sodium ion battery is improved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of secondary sodium ion batteries, and particularly relates to a low-temperature type local high-concentration electrolyte and a secondary sodium ion battery. BACKGROUND

[0002] In recent years, sodium ion batteries have been widely applied in various electronic products, starting power sources, low-speed electric vehicles and power grid energy storage and the like due to advantages of abundant resources, high safety and low cost. With continuous expansion of application scenarios of sodium ion batteries, the requirement for electrochemical performance of sodium ion batteries in special scenarios is higher and higher. China has a vast territory, and there is a large temperature difference between the north and the south. In the northern region, the single cruising range of a pure electric vehicle in winter is often greatly discounted compared with that in summer, and in some extremely cold regions (such as some high-altitude regions and some northeast regions), the outdoor temperature may even reach-30℃. At this time, most intelligent mobile devices are almost unusable, and often appear to be "frozen out of service". In addition, with the development of polar exploration, aerospace and submarine exploration and the like, the requirement for low-temperature performance of sodium ion batteries is further improved, and even needs to reach below-50℃.

[0003] The electrolyte is a medium for ion transmission and charge transfer inside the battery. It has an important influence on the low-temperature performance of the battery. Ether-based electrolyte has a low freezing point, moderate sodium salt solubility and low desolvation energy, and is a good choice for low-temperature electrolyte of sodium ion batteries. The oxidation resistance of ether-based electrolyte is poor. In order to broaden the applicable voltage range of ether-based electrolyte, a local high-concentration method is often used by diluent. However, the diluent selected for the local high-concentration electrolyte is often a highly fluorinated solvent, which has no ability to dissolve sodium salt at all, and is extremely easy 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 precipitation and failure of the electrolyte. Therefore, it is particularly important to improve the stability of the diluent in the electrolyte and to strengthen the interaction between the diluent and other components in the electrolyte in order to improve the low-temperature applicability of the local high-concentration electrolyte and broaden the low-temperature application in sodium ion batteries. SUMMARY

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

[0005] In order to achieve the above-mentioned purpose, the application adopts the following technical solutions:

[0006] A low-temperature type local high-concentration electrolyte comprises a functional diluent, an ether solvent and a sodium salt; the functional diluent comprises two diluents represented by structural formula I and structural formula II.

[0007]

[0008] In the structural formula I, R1 and R2 are each independently selected from F, CH2F, CHF2 or CF3; in the structural formula II, R3 and R4 are each independently selected from H, CH3, CH2CH3 or OCH3.

[0009] Further, the diluent volume ratio of the structural formula I and the structural formula II is 1:3-3:1.

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

[0011] Further, the sodium salt is a combination of one or more of sodium hexafluorophosphate (NaPF6), sodium difluoro(oxalato)borate (NaODFB), sodium difluorooxalate (NaFSI), sodium bis(trifluoromethylsulfonyl)imide (NaTFSI) and sodium tetrafluoroborate (NaBF4).

[0012] Further, the total molar concentration of the sodium salt in the ether solvent ranges from 1 mol / L to 4 mol / L.

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

[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 partial high-concentration electrolyte.

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

[0016] Further, the negative electrode comprises 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 application are:

[0018] The diluent shown in structural formula I is obtained by substituting fluorinated groups on the 1,3-dioxane oxygen atom interposition, the fluorine group not only can reduce the freezing point of the molecule, but also can disperse the local electron cloud density of two oxygen atoms, reduce the molecular polarity, and improve the intrinsic oxidation resistance, while the short-chain fluorinated group, i.e. F, CH2F, CHF2 or CF3, can effectively avoid the influence of molecular steric hindrance, promote the rapid migration of sodium ions in the electrolyte; further, by substituting short-chain electron-donating groups, i.e. H, CH3, CH2CH3 or OCH3, on the 1,3-dioxane ortho position, the diluent of structural formula II is obtained, the short-chain electron-donating group can improve the local electron cloud density of oxygen atoms; when the two are mixed, the diluent of structural formula I can effectively reduce the freezing point of the electrolyte and broaden the liquid range of the electrolyte, while the improvement of the local electron cloud density of oxygen atoms on the diluent molecule shown in structural formula II strengthens the intermolecular interaction between the fluorinated groups of the diluent shown in structural formula I, thereby promoting the molecular-level dispersion of the diluent shown in structural formula I in the electrolyte liquid phase, avoiding the precipitation of unstable phases, promoting the low-temperature applicability of the formed local high-concentration electrolyte, and thereby improving the low-temperature performance of the sodium ion battery. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1 In the figure, a, b, c are optical photographs of Example 1, Comparative Example 1 and Comparative Example 2 after standing at -40℃ for 4h respectively;

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

[0021] Figure 3 The long cycle curve of HC||Fe-PB full cell prepared for Example 3 at -30℃ and a current density of 0.1C;

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

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

[0024] Figure 6 In the figure, a, b are the discharge curves of HC||NVP@C pouch cell of Example 5 and Comparative Example 6 at -30℃ and a current density of 0.1C respectively. DETAILED DESCRIPTION

[0025] The technical solutions in the present application will be described clearly and completely in connection with the drawings and embodiments. Obviously, the described embodiments are only some of the embodiments of the present application, but not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work fall within the scope of the present application.

[0026] Embodiment 1

[0027] A low-temperature type local high-concentration electrolyte is composed of NaPF6, diethylene glycol dimethyl ether, 3-trifluoromethyl-1,3-dioxane and 2-methyl-1,3-dioxane, wherein NaPF6 is a sodium salt, diethylene glycol dimethyl ether is used as a solvent, 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 and 2-methyl-1,3-dioxane is 1:1, the molar concentration of NaPF6 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 the addition of the functional diluent, the molar concentration of NaPF6 is reduced to 1 mol / L.

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

[0029] NaPF6 is added to the diethylene glycol dimethyl ether solvent according to the molar ratio, and fully stirred until completely dissolved, to obtain a 2 mol / L high-concentration sodium ion battery electrolyte. 3-trifluoromethyl-1,3-dioxane and 2-methyl-1,3-dioxane are mixed in a volume ratio of 1:1 to obtain a functional diluent, which is then added to the above-mentioned 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. The electrolyte is fully stirred again until the electrolyte is uniformly mixed, to obtain a low-temperature type local high-concentration electrolyte. The obtained low-temperature type local high-concentration electrolyte is placed in a -40℃ low-temperature box for 4h. The experimental results of Embodiment 1 are shown in Figure 1 (a), and the electrolyte remains in a liquid state without freezing.

[0030] Embodiment 2

[0031] A low-temperature type 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 and 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 the addition of the functional diluent, the molar concentration of NaFSI is reduced to 1 mol / L; a hard carbon (HC) is used as a research electrode, a sodium metal sheet is used as a counter electrode, and a glass fiber separator is assembled with the electrolyte described in Example 2 to form a Na||HC 2025 type 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 in a molar ratio, and fully stirred until 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 in a volume ratio of 1:2 to obtain a functional diluent, which is then added to the above-mentioned 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. The electrolyte is fully stirred again until it is uniformly mixed, to obtain a low-temperature type local high-concentration electrolyte. The test results of the Na||HC 2025 type button half-cell described in Example 2 are as follows: Figure 2 (a) After activation at room temperature, the reversible capacity of charge and discharge is 255 mAh / g at -40℃ and a current density of 100 mA / g.

[0034] Example 3

[0035] A low-temperature type local high-concentration electrolyte is composed of NaPF6, diethylene glycol dimethyl ether, 3-trifluoromethyl-1,3-dioxane and 1,3-dioxane, wherein NaPF6 is a sodium salt, diethylene glycol dimethyl ether is used as a solvent, the mixture of 3-trifluoromethyl-1,3-dioxane and 1,3-dioxane is used as a functional diluent, the volume ratio of 3-trifluoromethyl-1,3-dioxane and 1,3-dioxane is 1:3, the molar concentration of NaPF6 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 the addition of the functional diluent, the molar concentration of NaPF6 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 is assembled with the electrolyte described in Example 3 to form a HC||Fe-PB 2025 type button full cell.

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

[0037] NaPF6 is added to diethylene glycol dimethyl ether solvent according to the molar ratio, and fully stirred until completely dissolved to obtain a 2 mol / L high-concentration sodium ion battery electrolyte. 3-trifluoromethyl-1,3-dioxane and 1,3-dioxane are mixed in a volume ratio of 1:3 to obtain a functional diluent, which is then added to the above-mentioned 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. The electrolyte is fully stirred again until it is uniformly mixed, and a low-temperature type local high-concentration electrolyte is obtained. The test results of the HC||Fe-PB 2025 type button full cell described in Example 3 are shown in Figure 3 After activation at room temperature, the first charge-discharge reversible capacity is 133.4 mAh / g at -30°C and 0.1C current density, and the reversible capacity is still 125.9 mAh / g after 100 cycles, with a capacity retention rate of 94.4%.

[0038] Example 4

[0039] A low-temperature type local high-concentration electrolyte is composed of NaFSI, tetrahydrofuran, 3-monofluoromethyl-1,3-dioxane and 2-methoxy-1,3-dioxane, wherein NaFSI is a sodium salt, tetrahydrofuran is used as a solvent, the mixture of 3-monofluoromethyl-1,3-dioxane and 2-methoxy-1,3-dioxane is used as a functional diluent, the volume ratio of 3-monofluoromethyl-1,3-dioxane and 2-methoxy-1,3-dioxane 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 the addition of 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 is assembled with the electrolyte described in Example 4 to form a HC||Fe-PB 2025 type button full cell.

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

[0041] NaFSI is added to the tetrahydrofuran solvent in a molar ratio, and fully stirred until completely dissolved, to obtain a 3 mol / L high-concentration sodium ion battery electrolyte. 3-monofluoromethyl-1,3-dioxane and 2-methoxy-1,3-dioxane are mixed in a volume ratio of 2:1 to obtain a functional diluent, which is then added to the above-mentioned 3 mol / L high-concentration sodium ion battery electrolyte. The volume ratio of the added functional diluent to the solvent tetrahydrofuran is 1:1. The electrolyte is fully stirred again until it is uniformly mixed, to obtain a low-temperature type local high-concentration electrolyte. The test results of the HC||Fe-PB 2025 type button full cell described in Example 4 are as follows: Figure 5 (a) After activation and charging at room temperature, discharging at -50℃ and 0.1C current density, the discharge capacity is 94.8 mAh / g.

[0042] Example 5

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

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

[0045] NaTFSI is added to 2-methyltetrahydrofuran according to the molar ratio, and fully stirred until completely dissolved, to obtain a 2 mol / L high-concentration sodium ion battery electrolyte. A functional diluent is obtained by mixing 3-difluoromethyl-1,3-dioxane and 2-ethyl-1,3-dioxane in a volume ratio of 3:1, which is then added to the above-mentioned 2 mol / L high-concentration sodium ion battery electrolyte. The volume ratio of the added functional diluent to the solvent 2-methyltetrahydrofuran is 1:1. The electrolyte is fully stirred again until it is uniformly mixed, to obtain a low-temperature type local high-concentration electrolyte. The test results of the 1.2 Ah HC||NVP@C soft pack battery described in Example 5 are as shown in Figure 6 (a) After activation and charging at room temperature, discharging at -30℃ and 0.1C current density, 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 -40℃ low-temperature box for 4h, and the experimental results are as shown in Figure 1 (b) 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 -40℃ low-temperature box for 4h, and the experimental results are as shown in Figure 1 (c) 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 rest of the parameters are the same as Example 2. The test results of the Na||HC 2025 type button half-cell assembled in Comparative Example 3 are shown in Figure 2 (b). After activation at room temperature, the charge-discharge reversible capacity is 145 mAh / g at -40°C and a current density of 100 mA / g.

[0052] Comparative Example 4

[0053] The difference between this comparative example and Example 3 is that no functional diluent is used, and the rest of the parameters are the same as Example 3. The test results of the HC||Fe-PB 2025 type button full cell assembled in Comparative Example 4 are shown in Figure 4 (b). After activation and charging at room temperature, the discharge capacity is 61.1 mAh / g at -50°C and a current density of 0.1C.

[0054] Comparative Example 5

[0055] The difference between this comparative example and Example 4 is that no functional diluent is used, and the rest of the parameters are the same as Example 4. The test results of the 1.2 Ah HC||NVP@C soft pack battery assembled in Comparative Example 5 are shown in Figure 5 (b). After activation and charging at room temperature, the discharge capacity is 0.41 Ah at -30°C and a current density of 0.1C.

[0056] Comparative Example 6

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

[0058] In addition, it should be understood that although the present specification is described in terms of embodiments, not every embodiment contains only one independent technical solution, and the description of the specification is only for the sake of clarity, and those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can be appropriately combined to form other embodiments that those skilled in the art can understand.

Claims

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

1.

3. The low-temperature, locally 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, locally high-concentration electrolyte according to claim 1, characterized in that: The sodium salt includes one or more of sodium hexafluorophosphate, sodium bisfluorooxalatoborate, sodium bisfluorosulfonyl imide, sodium bis(trifluoromethylsulfonyl)imide and sodium tetrafluoroborate.

5. The low-temperature, locally 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 locally 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 the low-temperature, locally high-concentration electrolyte 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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