A low-concentration, low-temperature electrolyte for sodium-ion batteries, its preparation method, and its application.

Abstract

A low-concentration and low-temperature electrolyte for sodium ion batteries and a preparation method and application thereof, which relates to an electrolyte for sodium ion batteries and a preparation method and application thereof. It aims to solve the technical problems of high cost and low specific capacity of existing low-concentration electrolytes for sodium ion batteries. The electrolyte of the present invention is a mixture of sodium salt, a strong solvating ether solvent and a weak solvating ether solvent, wherein the strong solvating ether solvent is ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether or tetraethylene glycol dimethyl ether; the weak solvating ether solvent is dimethoxymethane, dimethoxyethane, trimethoxymethane or trimethoxyethane; the preparation method is: dissolving the sodium salt in a solvent in an anhydrous and oxygen-free argon glove box; the sodium ion battery assembled with the electrolyte has an initial specific capacity of 300 mAh g at -20°C. ‑1 , 230mAh g after 120 cycles ‑1 The battery has an operating temperature of ‑20~25℃ and can be used in the field of sodium ion batteries.

Description

Technical Field

[0001] The invention relates to a sodium ion battery electrolyte and a preparation method and application thereof. Background Art

[0002] Sodium-ion batteries have attracted widespread attention in recent years due to their advantages such as abundant resources, low price and high safety, and are expected to become a substitute for lithium-ion batteries in some fields. As the core component of rechargeable batteries, sodium-ion battery electrolytes directly affect the performance, life and safety of batteries. Reliable electrolytes can effectively regulate electrochemical reaction behaviors and interface properties, which is extremely important for the development of sodium-ion batteries with high coulombic efficiency, stable cycle performance and high rate performance.

[0003] The solid electrolyte interface (SEI) formed in traditional ester electrolytes has poor stability and faces many difficulties when used in sodium ion batteries. In contrast, the application of ether electrolytes in the field of secondary batteries has gradually increased, especially in improving the performance of sodium ion battery electrodes. Its stable solvation structure can achieve highly reversible solvent co-intercalation reaction and form a thin and stable electrolyte interface. In addition, ether electrolytes are conducive to stable electrode circulation and rapid sodium storage kinetics, which is mainly due to its excellent electrolyte / electrode interaction, good chemical compatibility and wettability. However, the cost of electrolytes used in sodium ion batteries is generally expensive, among which the cost of sodium salt can reach more than ten times that of solvent, accounting for about one-third of the total battery production cost. Such high electrolyte costs have seriously restricted the commercial development of sodium ion batteries. At the same time, when the salt concentration in the electrolyte is higher than 1.5M or lower than 1M, its ionic conductivity will be significantly reduced, but the diffusion coefficient of ions in lower concentration electrolytes is much higher than that in high concentration electrolytes. At present, the performance of most rechargeable batteries is greatly affected by the ambient temperature. In extremely low temperature environments, the battery performance will decay rapidly, and even safety accidents will occur, which seriously limits the application of rechargeable batteries in extreme environments. The Chinese patent "A low-concentration electrolyte and its application in low-temperature sodium-ion batteries" with application number CN202310477286.1 discloses a low-concentration electrolyte and its application in low-temperature sodium-ion batteries, which is a sodium salt dissolved in an ether organic solvent at a concentration of 0.05-0.9 mol / L, wherein the ether organic solvent is methyl ether, ethyl ether, propyl ether, butyl ether, dimethyl ether, diethyl ether, dipropyl ether, dibutyl ether, ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, and dioxolane. The operating temperature is, but the capacity of the electrolyte at -25°C is only 230-250mAh g -1 , low capacitance. Summary of the invention

[0004] The present invention aims to solve the technical problems of high cost and low specific capacity of the existing low-concentration electrolyte for sodium ion batteries, and to provide a low-concentration and low-temperature electrolyte for sodium ion batteries and a preparation method and application thereof.

[0005] The sodium ion battery low-concentration low-temperature electrolyte of the present invention is mixed with sodium salt, strong solvating ether solvent and weak solvating ether solvent, wherein the sodium salt is a combination of one or more of phosphate sodium salt, sulfonic acid sodium salt and sulfonimide sodium salt; the strong solvating ether solvent is ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether or tetraethylene glycol dimethyl ether; the weak solvating ether solvent is dimethoxymethane, trimethoxymethane or trimethoxyethane; the concentration of the sodium salt in the solvent is 0.1-0.8 mol / L.

[0006] Furthermore, the volume ratio of the strong solvating ether solvent to the weak solvating ether solvent is 1:(1.5-9).

[0007] The preparation method of the sodium ion battery low-concentration low-temperature electrolyte is carried out according to the following steps:

[0008] In an argon atmosphere glove box with a water content of <0.01ppm and an oxygen content of <0.01ppm, a sodium salt is placed in a brown glass bottle, and then a strong solvating ether solvent and a weak solvating ether solvent are added into the brown glass bottle at a concentration of 0.1 to 0.8 mol / L of the sodium salt, wherein the volume ratio of the strong solvating ether solvent to the weak solvating ether solvent is 1:(1.5 to 9), and then stirred with a magnetic stirrer for 12 hours until the sodium salt is completely dissolved, to obtain a low-concentration electrolyte for a sodium ion battery.

[0009] The application of the low-concentration and low-temperature electrolyte for sodium ion batteries is to use the electrolyte in sodium ion batteries.

[0010] In the sodium ion battery low-concentration low-temperature electrolyte of the present invention, the high dielectric constant of the strong solvating solvent provides it with a strong sodium salt solubility, while the weak solvating solvent makes the desolvation process of sodium ions easier to occur. The low-concentration low-temperature electrolyte system with a strong / weak solvating solvent combination has the advantages of low freezing point and low viscosity, making it an ideal solvent for low-temperature electrolytes. The two solvents can not only provide sufficient sodium salt solubility and suitable ionic conductivity, but also effectively reduce the Na in the electrolyte. + The desolvation energy promotes Na +The desolvation process occurs faster. The low-concentration and low-temperature electrolyte for the sodium ion battery of the present invention has high ionic conductivity and low viscosity at both room temperature and low temperature. At the same time, due to the weak solvation between sodium ions and solvent molecules, the interfacial charge transfer kinetics are accelerated, which is conducive to interfacial ion transport and has a significant improvement effect on the low-temperature cycle performance of the sodium ion battery. The low-concentration and low-temperature electrolyte for the sodium ion battery of the present invention can reduce the amount of sodium salt in the electrolyte, thereby greatly reducing the cost of the electrolyte, which is conducive to the construction and large-scale application of low-cost sodium ion batteries.

[0011] The sodium ion battery low-concentration low-temperature electrolyte of the present invention has a first-cycle specific capacity of 320 mAh g at room temperature. -1 , maintained at 300mAh g after 200 cycles -1 The initial specific capacity at -20℃ is close to 300mAh g -1 , and can still maintain 230mAh g after 120 cycles -1 It is a low-cost, high-performance sodium-ion battery electrolyte. It is a solvent system designed by utilizing the dissolution of sodium salts and the synergistic effect between solvents, and adopting a combination of strong / weak solvating solvents. It is a weakly solvated, low-salt concentration ether-based electrolyte. When the electrolyte is applied to sodium-ion batteries, it exhibits a specific capacity comparable to that of a traditional electrolyte with a concentration of 1 mol / L at room temperature, and exhibits better electrochemical properties than commercial electrolytes under low temperature conditions. The operating temperature is -20 to 25°C, and it can be used in the field of sodium-ion batteries. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 It is a polarization voltage curve diagram of the Na||Na symmetric sodium ion battery of the low-concentration and low-temperature electrolyte of the sodium ion battery prepared in Example 1 and the commercial electrolyte prepared in Comparative Example 1;

[0013] Figure 2 1 is an electrochemical window curve diagram of a Na||Na symmetric sodium ion battery of a low-concentration and low-temperature electrolyte for a sodium ion battery prepared in Example 1 and a commercial electrolyte prepared in Comparative Example 1;

[0014] Figure 3 Cyclic voltammetry curves of the low-concentration and low-temperature electrolyte for sodium ion batteries prepared in Example 1 and the commercial electrolyte in Comparative Example 1;

[0015] Figure 4 The impedance spectra of the sodium ion battery low-concentration low-temperature electrolyte prepared in Example 1, the commercial electrolyte prepared in Comparative Example 1, and the commercial low-concentration electrolyte prepared in Comparative Example 2;

[0016] Figure 5It is a comparison chart of the cycle performance at room temperature of the low-concentration and low-temperature electrolyte for sodium ion batteries prepared in Example 1, the commercial electrolyte prepared in Comparative Example 1, and the commercial low-concentration electrolyte prepared in Comparative Example 2;

[0017] Figure 6 It is a comparison chart of the cycle performance of the low-concentration and low-temperature electrolyte of the sodium ion battery of Example 1 and the commercial electrolyte of Comparative Example 1 under -20°C conditions. DETAILED DESCRIPTION

[0018] The beneficial effects of the present invention are verified by the following examples.

[0019] Example 1: The preparation method of the sodium ion battery low-concentration low-temperature electrolyte of this embodiment is carried out according to the following steps:

[0020] In an argon atmosphere glove box with a water content of 0.005ppm and an oxygen content of 0.005ppm, 0.8398g of sodium hexafluorophosphate was weighed and placed in a brown glass bottle, and then 8mL of dimethoxymethane and 2mL of ethylene glycol dimethyl ether were pipetted into the glass bottle with a pipette, and a magnetic stirrer was used to stir for 12 hours to completely dissolve the sodium salt to obtain a low-concentration, low-temperature electrolyte for sodium ion batteries. The concentration of sodium hexafluorophosphate in the electrolyte was 0.5mol / L, and the volume ratio of dimethoxymethane to ethylene glycol dimethyl ether was 8:2.

[0021] Comparative Example 1: This comparative example is to prepare a commercial electrolyte. The specific preparation method is as follows: In an argon atmosphere glove box with a water content of 0.005ppm and an oxygen content of 0.005ppm, 1.6795g of sodium hexafluorophosphate is accurately weighed and placed in a brown glass bottle, and then 10mL of ethylene glycol dimethyl ether is pipetted into the glass bottle with a pipette, and stirred with a magnetic stirrer for 12 hours to completely dissolve the sodium salt, thereby obtaining a commercial electrolyte. The concentration of sodium hexafluorophosphate in the electrolyte is 1mol / L.

[0022] Comparative Example 2: This comparative example is to prepare a commercial low-concentration electrolyte, and the specific preparation method is as follows:

[0023] In an argon atmosphere glove box with a water content of 0.005 ppm and an oxygen content of 0.005 ppm, accurately weigh 0.8398 g of sodium hexafluorophosphate and place it in a brown glass bottle, then use a pipette to transfer 10 mL of ethylene glycol dimethyl ether into the glass bottle, and stir it thoroughly with a magnetic stirrer for 12 hours to completely dissolve the sodium salt, thereby obtaining a commercial low-concentration electrolyte in which the concentration of sodium hexafluorophosphate is 0.5 mol / L.

[0024] The sodium ion battery low-concentration low-temperature electrolyte prepared in Example 1 and the commercial electrolyte prepared in Comparative Example 1 were used as the positive electrode and the negative electrode, respectively, to assemble a Na||Na symmetric sodium ion battery, and then a polarization voltage test was performed. The obtained polarization voltage curve is shown in Figure 1 As shown, the electrochemical window curve is Figure 2 As shown, from Figure 1 and Figure 2 It can be seen that the commercial electrolyte system exhibits a higher polarization voltage fluctuation amplitude, which indicates that its electrochemical performance is relatively unstable during long-term polarization, and the polarization voltage changes greatly. The low-concentration and low-temperature electrolyte system of the sodium ion battery exhibits a lower polarization voltage fluctuation amplitude, and its electrochemical performance is more stable during long-term polarization, and the polarization voltage changes less. From the fluctuation range of the polarization voltage, the low-concentration and low-temperature electrolyte of the sodium ion battery exhibits better stability and reversibility. This is because the solvent system in Example 1 is more conducive to ion conduction and electrochemical reactions at the electrode interface. Although the commercial electrolyte system contains a higher concentration of sodium salt, its electrochemical stability and cycle performance are poor during long-term polarization. In addition, the low-concentration and low-temperature electrolyte system of the sodium ion battery has a wider electrochemical window than the commercial electrolyte system, indicating that it is suitable for a wider voltage range of use.

[0025] Figure 3 The cyclic voltammetry (CV) test results of the sodium ion battery low-concentration low-temperature electrolyte prepared in Example 1 and the commercial electrolyte in Comparative Example 1 are compared. Figure 3 It can be seen that the current peak of the low-concentration and low-temperature electrolyte system of the sodium ion battery at 0-3V is significantly higher than that of the commercial electrolyte system. This shows that within the same voltage range, the electrochemical activity of the low-concentration electrolyte system is stronger. It has an obvious redox peak near 0V, indicating that the electrolyte has good redox reaction activity at low potentials. The peak value of the commercial electrolyte system in the same potential range is lower, indicating that its electrochemical reaction activity is weaker. The current changes of both electrolytes are small in the entire voltage range, indicating that they are relatively stable in this voltage range and no obvious decomposition reaction occurs. Compared with the commercial electrolyte, although the sodium salt concentration of the low-concentration electrolyte system is greatly reduced, its peak current is higher than that of the commercial electrolyte. This further illustrates the huge influence of the solvent system on the electrochemical performance of the electrolyte, and the electrolyte system of strong / weak solvation solvent mixture can provide an excellent electrochemical environment even at lower concentrations.

[0026] In order to compare the differences in ionic conductivity of different electrolytes, impedance tests were performed on the sodium ion battery low-concentration low-temperature electrolyte prepared in Example 1, the commercial electrolyte prepared in Comparative Example 1, and the commercial low-concentration electrolyte prepared in Comparative Example 2 at room temperature (25°C) and extremely low temperature (-20°C), and the ionic conductivities of the three electrolytes at different temperatures were calculated. The results are as follows: Figure 4 As shown in Table 1. Figure 4 It can be seen that at room temperature, the impedance value of the commercial electrolyte is the smallest and has the highest ionic conductivity, because it contains a higher ion concentration. The ionic conductivity of the low-concentration and low-temperature electrolyte system for sodium-ion batteries is second, and the ionic conductivity of the commercial low-concentration electrolyte is the smallest. Thanks to the synergistic effect between strong solvating ether solvents and weak solvating ether solvents, even if the sodium salt concentration is greatly reduced at room temperature, the ionic conductivity of the low-concentration and low-temperature electrolyte for sodium-ion batteries is relatively moderate, while the commercial low-concentration electrolyte shows a more obvious decrease in conductivity. At an extremely low temperature of -20°C, the low-concentration and low-temperature electrolyte for sodium-ion batteries has lower viscosity, excellent wettability and smaller Na + Due to the advantage of desolvation energy, its ion conductivity did not change significantly. However, commercial electrolytes and commercial low-concentration electrolytes showed a significant conductivity attenuation at low temperatures. Since the reduction in sodium salt concentration resulted in lower viscosity, the conductivity of commercial low-concentration electrolytes was higher than that of commercial electrolytes with a concentration of 1 mol / L. This result further illustrates the excellence of low-concentration low-temperature electrolytes for sodium-ion batteries at low temperatures.

[0027] Table 2. Comparison of conductivity of different electrolytes at room temperature and low temperature

[0028]

[0029] In order to explore the electrochemical performance of different electrolytes, sodium ion batteries were assembled using the three electrolytes of Example 1, Comparative Examples 1 and 2, and the cycling performance of the three batteries at room temperature was compared. The cycling performance of the assembled batteries at a current density of 0.5C for 200 cycles was compared. Figure 5 Even if the sodium salt concentration is greatly reduced, the sodium ion battery low-concentration low-temperature electrolyte system still has sufficient sodium ion conductivity due to the synergistic effect between the strong solvating solvent and the weak solvating solvent to form a special solvent-separated ion pair solvation structure. At the same time, a larger proportion of weak solvating solvents makes the electrolyte have a smaller viscosity. These all promote Na + The reversible migration rate in this system is faster and the desolvation process is easier to occur. Even if the commercial electrolyte has a higher sodium salt concentration and has a higher ionic conductivity at room temperature, it is not conducive to Na +Directional migration in the solution. Therefore, at room temperature, the cycle performance of commercial electrolytes and low-concentration and low-temperature electrolytes for sodium-ion batteries is almost the same, and the first cycle specific capacity can reach 320 mAh g -1 , maintained at 300mAh g after 200 cycles -1 After reducing the sodium salt concentration, commercial low-concentration electrolyte cannot provide sufficient ion conductivity. Although its first-cycle specific capacity can reach about 320mAh g-1, it cannot continuously replenish Na during the cycle due to insufficient ion concentration. + Due to the consumption, the specific capacity performance has been greatly attenuated compared with commercial electrolytes and low-concentration and low-temperature electrolytes for sodium-ion batteries.

[0030] The cycle performance comparison of the sodium ion battery low-concentration low-temperature electrolyte of Example 1 and the commercial electrolyte of Comparative Example 1 at -20°C is shown in the figure below: Figure 6 As shown, from Figure 6 It can be seen that the low-concentration low-temperature electrolyte of the sodium ion battery in Example 1 has excellent specific capacity performance at low temperature, and exhibits a high discharge capacity during the entire cycle, with an initial specific capacity of nearly 300 mAh g -1 , and can still maintain 230mAh g after 120 cycles -1 The commercial electrolyte has poor cycling performance at low temperatures, with an initial capacity of about 250 mAh g -1 The capacity decreased significantly with the increase of cycle number, and dropped to 190 mAh g after 120 cycles. -1 About. Under low temperature conditions of -20°C, the viscosity and solvation structure of the electrolyte will significantly affect the ion mobility. The system of strong / weak solvation solvent mixture can form a more favorable solvation structure at low temperature, reducing the viscosity of the electrolyte, thereby improving the ion conductivity and electrochemical stability, and showing better electrochemical performance. The higher sodium salt concentration of commercial electrolytes leads to an increase in the viscosity of the solvent, which affects the ion migration rate, resulting in poor low-temperature cycle performance.

[0031] Example 2: The preparation method of the sodium ion battery low-concentration low-temperature electrolyte of this embodiment is carried out according to the following steps:

[0032] In an argon atmosphere glove box with a water content of 0.005ppm and an oxygen content of 0.005ppm, 0.6093g of sodium bis(fluorosulfonyl)imide was weighed and placed in a brown glass bottle, and 7mL of dimethoxymethane and 3mL of diethylene glycol dimethyl ether were pipetted into the glass bottle with a pipette, and a magnetic stirrer was used to stir for 12 hours to completely dissolve the sodium salt to obtain a low-concentration, low-temperature electrolyte for sodium ion batteries. The concentration of sodium bis(fluorosulfonyl)imide in the electrolyte was 0.3mol / L, and the volume ratio of dimethoxyethane and diethylene glycol dimethyl ether was 7:3.

[0033] The conductivity of the sodium ion battery low-concentration low-temperature electrolyte in this embodiment at -20°C is 0.0239mS / m. The initial specific capacity at -20°C is 289mAh g -1 , and can still maintain 224mAh g after 120 cycles -1 .

[0034] Embodiment 3: The preparation method of the sodium ion battery low-concentration low-temperature electrolyte of this embodiment is carried out according to the following steps:

[0035] In an argon atmosphere glove box with a water content of 0.005ppm and an oxygen content of 0.005ppm, 1.2044g of sodium trifluoromethanesulfonate was weighed and placed in a brown glass bottle, and 9mL of trimethoxyethane and 1mL of triethylene glycol dimethyl ether were pipetted into the glass bottle with a pipette, and a magnetic stirrer was used to stir for 12 hours to completely dissolve the sodium salt, thereby obtaining a low-concentration, low-temperature electrolyte for sodium ion batteries. The concentration of sodium trifluoromethanesulfonate in the electrolyte was 0.7mol / L, and the volume ratio of trimethoxyethane to triethylene glycol dimethyl ether was 9:1.

[0036] The conductivity of the sodium ion battery low-concentration low-temperature electrolyte in this embodiment at -20°C is 0.0247mS / m. The initial specific capacity at -20°C is 295mAh g -1 , and can still maintain 234mAh g after 120 cycles -1 .

Claims

1. A low-concentration, low-temperature electrolyte for sodium ion batteries, characterized in that The electrolyte is formed by mixing sodium salt, strong solvating ether solvent and weak solvating ether solvent, wherein the sodium salt is a combination of one or more of phosphate sodium salt, sulfonic acid sodium salt and sulfonimide sodium salt; the strong solvating ether solvent is ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether or tetraethylene glycol dimethyl ether; the weak solvating ether solvent is dimethoxymethane, trimethoxymethane or trimethoxyethane; the concentration of the sodium salt is 0.1-0.8 mol / L.

2. A sodium ion battery low-concentration low-temperature electrolyte according to claim 1, characterized in that The volume ratio of the strong solvating ether solvent to the weak solvating ether solvent is 1:(1.5-9).

3. A method for preparing a low-concentration and low-temperature electrolyte for sodium ion batteries according to claim 1, characterized in that The method is carried out according to the following steps: in an argon atmosphere glove box with a water content of less than 0.01 ppm and an oxygen content of less than 0.01 ppm, placing a sodium salt in a brown glass bottle, then adding a strong solvating ether solvent and a weak solvating ether solvent into the brown glass bottle according to a sodium salt concentration of 0.1 to 0.8 mol / L, wherein the volume ratio of the strong solvating ether solvent to the weak solvating ether solvent is 1:(1.5 to 9), and then stirring with a magnetic stirrer until the sodium salt is completely dissolved to obtain a low-concentration low-temperature electrolyte for a sodium ion battery.

4. The use of a low-concentration and low-temperature electrolyte for sodium ion batteries according to claim 1, characterized in that The application is to use low-concentration and low-temperature electrolyte for sodium-ion batteries in sodium-ion batteries.

Images