Ester-based low-temperature electrolyte for sodium metal battery

Abstract

The invention discloses an ester-based low-temperature electrolyte for a sodium metal battery. The ester-based low-temperature electrolyte comprises an ester-based organic solvent, a solute and an additive, the ester-based organic solvent is a mixture of a carboxylic ester solvent and fluoroethylene carbonate; the carboxylic ester solvent is ethyl difluoroacetate; the solute is a mixture of bis (fluorosulfonyl) imide sodium and sodium trifluoroacetate; the additive is tris (trimethylsilyl) phosphite ester. Through specific combination of the components of the ester-based electrolyte, the sodium metal battery can normally work in an extreme low-temperature environment (lower than-40 DEG C) of-60 DEG C.

Description

Technical Field

[0001] The present invention belongs to the technical field of sodium battery electrolytes, and particularly relates to an ester-based low-temperature electrolyte for sodium metal batteries. Background Art

[0002] As a next-generation emerging battery system, sodium metal batteries have received extensive attention due to their rich sodium resource reserves and high-specific-energy sodium metal anodes. And because the radius of sodium ions is larger than that of lithium ions, sodium metal batteries have more advantages at low temperatures compared to lithium batteries. However, when in an extremely low-temperature environment (below -40°C), sodium metal batteries still face problems such as severe internal polarization, sharp capacity decline, dendrite growth, and even short circuits, which limit the use of sodium metal batteries in environments such as high latitudes, high altitudes, space, and deep seas, and urgently need to be solved.

[0003] The transport of sodium ions at the electrode / electrolyte interface is a key factor affecting the low-temperature performance of the battery. The transport of sodium ions at the electrode / electrolyte interface is mainly divided into two parts. One is the desolvation process of sodium ions, and the other is the migration of sodium ions in the solid-electrolyte interface film after desolvation. For the existing commercial ester-based electrolytes for sodium metal batteries, there are problems such as difficult desolvation processes and slow migration of sodium ions in the solid-electrolyte interface film, which affect the electrochemical performance of sodium metal batteries in extremely low-temperature environments (below -40°C). Summary of the Invention

[0004] Aiming at the deficiencies of existing ester-based electrolytes in extremely low-temperature environments, the present invention provides an ester-based low-temperature electrolyte for sodium metal batteries, which enables sodium metal batteries to operate normally in extremely low-temperature environments of -40°C and even -60°C through a specific combination of electrolyte components.

[0005] To achieve the above technical objectives, the present invention adopts the following technical solutions:

[0006] An ester-based low-temperature electrolyte for sodium metal batteries, comprising an ester-based organic solvent, a solute, and an additive;

[0007] The ester-based organic solvent is a mixture of a carboxylic acid ester solvent and fluoroethylene carbonate; the carboxylic acid ester solvent is ethyl difluoroacetate;

[0008] The solute is a mixture of sodium bis(fluorosulfonyl)imide and sodium trifluoroacetate;

[0009] The additive is tris(trimethylsilyl) phosphite.

[0010] Further, the volume ratio of the carboxylic acid ester solvent to fluoroethylene carbonate is 8-10:1.

[0011] Furthermore, the molar ratio of sodium bis(fluorosulfonyl)imide to sodium trifluoroacetate is 8-10:1.

[0012] Furthermore, the concentration of the solute is 0.5-2 mol / L.

[0013] Furthermore, based on the electrolyte, the mass fraction of the additive is 0.5-1.5 wt%.

[0014] In the present invention, the carboxylic ester solvent has an extremely low freezing point. For example, the freezing point of ethyl acetate is as low as -84°C. Using ethyl acetate as the main solvent can enable the electrolyte to remain liquid in an extremely low-temperature environment. Due to the electron-withdrawing effect of fluorine atoms, appropriate fluorination of ethyl acetate can reduce the interaction energy between sodium ions and solvent molecules, thereby enabling the construction of an anion-dominated solvation structure. By adjusting the solute and introducing trifluoroacetate anions with strong interaction with sodium ions, the interaction of anions in the solvation structure can be further enhanced. Due to the strong electrostatic repulsion of the negative electrode to anions, a rich-anion solvation structure can reduce the desolvation energy of sodium ions, and the trifluoroacetate anions in the solvation structure can reduce the energy level value of the lowest unoccupied orbital of the overall solvation structure, making the entire solvation cluster easier to be reduced. This results in the fluorinated ethyl acetate molecules in the solvation cluster being more likely to de-fluorinate, thereby deriving a solid-electrolyte interface film rich in inorganic components, which is beneficial to the diffusion of sodium ions. Therefore, by adjusting the ester-based organic solvent and the solute, the kinetic process of the battery at low temperature can be effectively accelerated, the formation of sodium dendrites at the negative electrode can be reduced, the polarization inside the battery can be lowered, and thus the battery capacity can be improved. However, highly fluorinated solvents and solutes will increase the risk of generating hydrogen fluoride by-products during the operation of the electrolyte. Hydrogen fluoride will attack the electrode / electrolyte interface and the electrode body structure, resulting in the decline of battery performance. By using tris(trimethylsilyl) phosphite as an additive, the spontaneous elimination of hydrogen fluoride by-products in the electrolyte can be achieved, and tris(trimethylsilyl) phosphite can also participate in the formation of a more stable solid-electrolyte interface film, thereby extending the working life of the battery. In summary, through the specific combination of each component in the ester-based electrolyte, the present invention enables the sodium metal battery to operate normally in an extremely low-temperature environment of -40°C or even -60°C.

[0015] Compared with the prior art, the present invention has the following advantages

[0016] 1) The present invention uses a moderately fluorinated carboxylic ester solvent, which can construct an anion-dominated solvation structure while preventing the electrolyte from freezing in a low-temperature environment at the low end.

[0017] 2) The present invention introduces sodium trifluoroacetate to further enhance the interaction of anions in the solvation structure, promote the desolvation process of sodium ions, and facilitate the defluorination of ethyl fluoroacetate in the solvation structure, thereby deriving a solid-electrolyte interface film that is conducive to the migration of sodium ions.

[0018] 3) The present invention uses tris(trimethylsilyl) phosphite additive to overcome the defect that highly fluorinated electrolytes are prone to form hydrogen fluoride by-products, enhance the stability of the electrode / electrolyte interface and the electrode body structure, and achieve the extension of the battery working life.

[0019] 4) The present invention realizes the normal operation of sodium metal batteries in extreme low-temperature environments of -40°C and even -60°C through specific combinations of the components in the ester-based electrolyte. Description of the Drawings

[0020] Figure 1 It is an optical photograph of the electrolyte in Example 1 frozen at -60°C for 12 h.

[0021] Figure 2 It is an electrochemical performance graph of a sodium metal battery using the electrolyte in Example 1 at -40°C and -60°C.

[0022] Figure 3 It is an electrochemical performance graph of a symmetric battery using the electrolyte in Example 1 at -40°C.

[0023] Figure 4 It is a scanning electron microscope morphology graph of the positive and negative electrodes after cycling at -40°C using the electrolyte in Example 1.

[0024] Figure 5 It is an optical photograph of the electrolyte in Comparative Example 1 frozen at -60°C for 12 h.

[0025] Figure 6 It is an electrochemical performance graph of a sodium metal battery using the electrolyte in Comparative Example 1 at -40°C and -60°C.

[0026] Figure 7 It is an electrochemical performance graph of a symmetric battery using the electrolyte in Comparative Example 1 at -40°C.

[0027] Figure 8 It is a scanning electron microscope morphology graph of the positive and negative electrodes after cycling at -40°C using the electrolyte in Comparative Example 1.

[0028] Figure 9 It is an optical photograph of the electrolyte in Comparative Example 2 frozen at -60°C for 12 h.

[0029] Figure 10Electrochemical performance graphs of a sodium metal battery using the electrolyte in Comparative Example 2 at -40 °C and -60 °C.

[0030] Figure 11 Electrochemical performance graph of a symmetric battery using the electrolyte in Comparative Example 2 at -40 °C.

[0031] Figure 12 Scanning electron microscope morphology graphs of the positive electrode and negative electrode after cycling at -40 °C using the electrolyte in Comparative Example 2.

[0032] Figure 13 Optical photograph of the electrolyte in Comparative Example 3 frozen at -60 °C for 12 h.

[0033] Figure 14 Electrochemical performance graphs of a sodium metal battery using the electrolyte in Comparative Example 3 at -60 °C.

[0034] Figure 15 Optical photograph of the electrolyte in Comparative Example 4 frozen at -60 °C for 12 h.

[0035] Figure 16 Electrochemical performance graphs of a sodium metal battery using the electrolyte in Comparative Example 4 at -60 °C.

[0036] Figure 17 Optical photograph of the electrolyte in Comparative Example 5 stirred for 24 h. Detailed implementation mode

[0037] The present invention will be further described below in conjunction with the accompanying drawings and embodiments. The raw materials described in the present invention are all obtained through commercial channels. The preparation methods described in the present invention are all conventional preparation methods in the art unless otherwise specified. The following embodiments are intended to illustrate the present invention rather than further limit the present invention.

[0038] Example 1

[0039] 1) Prepare 0.180 g of sodium bis(fluorosulfonyl)imide and 0.0136 g of sodium trifluoroacetate, and the molar ratio of sodium bis(fluorosulfonyl)imide to sodium trifluoroacetate is 9:1;

[0040] 2) Prepare ethyl difluoroacetate and fluoroethylene carbonate, and the volume ratio is 9:1

[0041] 3) Add sodium bis(fluorosulfonyl)imide and sodium trifluoroacetate to ethyl difluoroacetate and fluoroethylene carbonate.

[0042] 4) Then add 1 wt% (based on the electrolyte) of tris(trimethylsilyl) phosphite and stir evenly for 24 h to prepare a 1 mol / L electrolyte.

[0043] As Figure 1As shown, the electrolyte in Example 1 remained liquid after being frozen at -60°C for 12 hours.

[0044] As Figure 2 shown, sodium chromate was selected as the positive electrode to assemble a sodium metal battery. Using the electrolyte in Example 1, the initial specific capacity of the battery was 120.6 mAh / g at -40°C (100 mA / g), and the battery could stably cycle 300 times with a capacity retention rate of 68.6%; the initial specific capacity of the battery was 92.4 mAh / g at -60°C (10 mA / g), and the battery could stably cycle 100 times with a capacity retention rate of 85.6%.

[0045] As Figure 3 shown, the symmetric battery assembled with the electrolyte in Example 1 could work for more than 260 hours at -40°C (0.2 mA / cm 2 , 0.2 mAh / cm 2 ).

[0046] As Figure 4 shown, using the electrolyte in Example 1, the morphology of the negative electrode remained uniform and dense at -40°C, and the morphology of the positive electrode was also dense without cracks and by-products, indicating that the electrolyte in Example 1 could enhance the negative electrode interface kinetics and had a protective effect on the positive electrode interface.

[0047] Comparative Example 1

[0048] 1) Prepare 0.202 g of sodium bis(fluorosulfonyl)imide;

[0049] 2) Prepare ethyl difluoroacetate and fluoroethylene carbonate with a volume ratio of 9:1

[0050] 3) Add sodium bis(fluorosulfonyl)imide to ethyl difluoroacetate and fluoroethylene carbonate and stir evenly for 24 hours to prepare a 1 mol / L electrolyte.

[0051] As Figure 5 shown, the electrolyte in Comparative Example 1 remained liquid after being frozen at -60°C for 12 hours.

[0052] As Figure 6 shown, sodium chromate was selected as the positive electrode to assemble a sodium metal battery. Using the electrolyte in Comparative Example 1, the initial specific capacity of the battery was 104.5 mAh / g at -40°C (100 mA / g), and the capacity of the battery rapidly decayed after cycling 70 times; the initial specific capacity of the battery was 39.9 mAh / g at -60°C (10 mA / g), and then the capacity continued to decay, and the capacity retention rate was 19.3% after cycling 80 times.

[0053] As Figure 7 shown, the symmetric battery assembled with the electrolyte in Comparative Example 1 could work at -40°C (0.2 mA / cm 2 , 0.2 mAh / cm2 ) The operation is no more than 120 h.

[0054] As Figure 8 shown, when using the electrolyte in Comparative Example 1, at -40 °C, the negative electrode morphology shows severe sodium dendrites, and cracks and flaky by-products appear on the positive electrode morphology.

[0055] Comparative Example 2

[0056] 1) Prepare 0.180 g of sodium bis(fluorosulfonyl)imide and 0.0136 g of sodium trifluoroacetate, and the molar ratio of sodium bis(fluorosulfonyl)imide to sodium trifluoroacetate is 9:1;

[0057] 2) Prepare ethyl difluoroacetate and fluoroethylene carbonate, and the volume ratio is 9:1

[0058] 3) Add sodium bis(fluorosulfonyl)imide and sodium trifluoroacetate to ethyl difluoroacetate and fluoroethylene carbonate, and stir evenly for 24 h to prepare a 1 mol / L electrolyte.

[0059] As Figure 9 shown, the electrolyte in Comparative Example 2 remains liquid after being frozen at -60 °C for 12 h.

[0060] As Figure 10 shown, sodium chromate is selected as the positive electrode to assemble a sodium metal battery. When using the electrolyte in Comparative Example 2, at -40 °C (100 mA / g), the initial specific capacity of the battery is 120.2 mAh / g, and the capacity of the battery decays rapidly after 115 cycles; at -60 °C (10 mA / g), the initial specific capacity of the battery is 78.4 mAh / g, and the capacity decays rapidly after 68 cycles.

[0061] As Figure 11 shown, the symmetric battery assembled with the electrolyte in Comparative Example 2 operates no more than 130 h at -40 °C (0.2 mA / cm 2 , 0.2 mAh / cm 2 ).

[0062] As Figure 12 shown, when using the electrolyte in Comparative Example 2, at -40 °C, the negative electrode morphology is uneven, showing abnormal sodium grain growth, and cracks and flaky by-products appear on the positive electrode morphology.

[0063] Comparative Example 3

[0064] 1) Prepare 0.202 g of sodium bis(fluorosulfonyl)imide;

[0065] 2) Prepare ethyl acetate and fluoroethylene carbonate, and the volume ratio is 9:1

[0066] 3) Add sodium bis(fluorosulfonyl)imide to ethyl acetate and vinyl fluorocarbonate and stir evenly for 24 h to prepare an electrolyte with a concentration of 1 mol / L.

[0067] As Figure 13 shown, the electrolyte in Comparative Example 3 remained liquid after being frozen at -60 °C for 12 h.

[0068] As Figure 14 shown, sodium chromate was selected as the positive electrode to assemble a sodium metal battery. Using the electrolyte in Comparative Example 3, the battery could not be charged and discharged normally at -60 °C.

[0069] Comparative Example 4

[0070] 1) Prepare 0.202 g of sodium bis(fluorosulfonyl)imide;

[0071] 2) Prepare ethyl trifluoroacetate and vinyl fluorocarbonate with a volume ratio of 9:1

[0072] 3) Add sodium bis(fluorosulfonyl)imide to ethyl trifluoroacetate and vinyl fluorocarbonate and stir evenly for 24 h to prepare an electrolyte with a concentration of 1 mol / L.

[0073] As Figure 15 shown, due to the excessive degree of fluorination, the solubility of ethyl trifluoroacetate was too weak, and solute precipitation occurred in the electrolyte of Comparative Example 4 at -60 °C.

[0074] As Figure 16 shown, sodium chromate was selected as the positive electrode to assemble a sodium metal battery. Using the electrolyte in Comparative Example 4, the battery could not be charged and discharged normally at -60 °C.

[0075] Comparative Example 5

[0076] 1) Prepare 0.136 g of sodium trifluoroacetate;

[0077] 2) Prepare ethyl difluoroacetate and vinyl fluorocarbonate with a volume ratio of 9:1

[0078] 3) Add sodium trifluoroacetate to ethyl difluoroacetate and vinyl fluorocarbonate and stir evenly for 24 h to prepare an electrolyte with a concentration of 1 mol / L.

[0079] As Figure 17 shown, due to the too strong sodium ion-anion interaction in sodium trifluoroacetate, the electrolyte in Comparative Example 5 was turbid and the solute could not be completely dissolved, so it could not be used for the normal use of the battery.

Claims

1. An ester-based low-temperature electrolyte for a sodium metal battery, characterized in that: It includes an ester-based organic solvent, a solute, and an additive; The ester-based organic solvent is a mixture of a carboxylic acid ester solvent and fluoroethylene carbonate; the carboxylic acid ester solvent is ethyl difluoroacetate; The solute is a mixture of sodium bis(fluorosulfonyl)imide and sodium trifluoroacetate; The additive is tris(trimethylsilyl) phosphite.

2. The ester-based low-temperature electrolyte for sodium metal battery according to claim 1, wherein: The volume ratio of the carboxylic acid ester solvent to fluoroethylene carbonate is 8 to 10:

1.

3. An ester-based low-temperature electrolyte for a sodium metal battery according to claim 1, wherein: The molar ratio of sodium bis(fluorosulfonyl)imide to sodium trifluoroacetate is 8 to 10:

1.

4. The ester-based low-temperature electrolyte for a sodium metal battery according to claim 1, wherein: The concentration of the solute is 0.5 to 2 mol / L.

5. The ester-based low-temperature electrolyte for a sodium metal battery according to claim 1, wherein: Based on the electrolyte, the mass fraction of the additive is 0.5 to 1.5 wt%.

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