Low-temperature aqueous electrolyte based on ethylene glycol and MgCl2 as well as preparation method and application of low-temperature aqueous electrolyte

Abstract

The invention relates to the field of magnesium ion batteries, and discloses a low-temperature aqueous electrolyte based on ethylene glycol and MgCl2, the low-temperature aqueous electrolyte comprises a soluble metal salt, an additive and a solvent, the additive is ethylene glycol, and the volume ratio of ethylene glycol to the solvent is (2-8): 10; the preparation method comprises the following steps: S1, weighing a certain amount of soluble metal salt for later use; s2, measuring a certain amount of additive, and uniformly mixing the additive with the solvent to prepare a mixed solvent; s3, dissolving the soluble metal salt weighed in the step S1 into the mixed solvent in the step S2, and uniformly mixing to prepare a low-temperature aqueous electrolyte based on ethylene glycol and MgCl2; the low-temperature aqueous electrolyte based on ethylene glycol and MgCl2 is applied to an aqueous ion battery. According to the technical scheme, the bottleneck that the aqueous electrolyte is easy to freeze at low temperature can be broken through, collaborative optimization of high safety performance, low cost and wide temperature application range is realized, and an effective solution is provided for commercialized popularization of the aqueous magnesium ion battery.

Description

Technical Field

[0001] The present invention relates to the field of magnesium ion batteries, and in particular to a low-temperature aqueous electrolyte based on ethylene glycol and MgCl2, and a preparation method and application thereof. Background Art

[0002] Compared to traditional organic electrolytes, aqueous electrolytes have become a research hotspot in the energy storage field due to their advantages such as simple processing, low cost, and high ionic conductivity (>20mS / cm). Their environmentally friendly properties (non-toxic and non-flammable) make them particularly suitable for large-scale applications. However, due to the inherent freezing point of water (0°C), aqueous electrolytes (usually with a freezing point >-20°C) are prone to freezing at low temperatures, thus hindering the improvement of energy density. This contradiction has become a core bottleneck for their commercialization. In recent years, to address the bottleneck problem of aqueous magnesium-ion battery electrolytes under extreme conditions, researchers have explored high-concentration electrolytes, deep eutectic electrolytes, co-soluble additives, and gel electrolyte systems. However, each method has limitations: (1) High-concentration electrolytes have high salt concentrations (>20M), which leads to a surge in raw material costs and increased viscosity (>100mPa·s). Ion migration is hindered at low temperatures, making salt precipitation crystallization more likely to occur, and long-term cycle stability is poor. (2) The ionic conductivity of some deep eutectic electrolytes is low (<5mS / cm), and some components (such as ClO4 - ) It may corrode the electrode, the optimization of the eutectic ratio is complex, and large-scale preparation is difficult; (3) Although the gel electrolyte system has mechanical flexibility to adapt to flexible devices, solid-state properties to avoid leakage, and is inherently non-flammable, it can also achieve self-repair and resistance to dehydration through dynamic bonds such as hydrogen bonds and ionic crosslinking, and can work at lower temperatures, the gel network pores shrink at extreme temperatures, which will limit ion transport and require coordinated regulation with anti-low-temperature co-solvents; (4) Co-solvent additives are low in cost and small in dosage. They can synergistically improve low-temperature performance through hydrogen bond regulation and solvation optimization. They are also compatible with various salt systems such as Li / Na / K / Zn / Mg and have strong universality. However, excessive addition will dilute the effective ion concentration and reduce the energy density. Some additives (such as methanol) have the risk of toxicity or side reactions. Summary of the Invention

[0003] The present invention aims to provide a low-temperature aqueous electrolyte based on ethylene glycol and MgCl2, as well as its preparation method and application, to overcome the bottleneck of easy freezing of aqueous electrolytes at low temperatures, achieve the coordinated optimization of high safety performance, low cost and a wide temperature adaptability range, and provide an effective solution for the commercial promotion of aqueous magnesium-ion batteries.

[0004] To achieve the above-mentioned purpose, the present invention adopts the following technical solution: a low-temperature aqueous electrolyte based on ethylene glycol and MgCl2, comprising a soluble metal salt, an additive and a solvent, wherein the additive is ethylene glycol, and the volume ratio of ethylene glycol to solvent is (2-8):10.

[0005] Preferably, the soluble metal salt is a soluble magnesium salt.

[0006] Preferably, the concentration of the soluble metal salt is 1 to 5 mol / L.

[0007] Preferably, the soluble magnesium salt is MgCl2.

[0008] Preferably, the operating temperature of the electrolyte is -20 to 25°C.

[0009] The present invention also provides another technical solution, a method for preparing a low-temperature aqueous electrolyte based on ethylene glycol and MgCl2, comprising the following steps: S1: Weigh a certain amount of soluble metal salt for use; S2: taking a certain amount of additive and mixing it evenly with the solvent to prepare a mixed solvent; S3: dissolving the soluble metal salt weighed in S1 in the mixed solvent of S2, mixing them evenly, and preparing a low-temperature aqueous electrolyte based on ethylene glycol and MgCl2.

[0010] The present invention also provides another technical solution, an application of a low-temperature aqueous electrolyte based on ethylene glycol and MgCl2 in aqueous ion batteries.

[0011] Preferably, the aqueous ion battery is an aqueous magnesium ion battery.

[0012] Preferably, the positive electrode material of the aqueous ion battery is a vanadium-based positive electrode material; and the negative electrode material includes PTCDA.

[0013] Preferably, the vanadium-based positive electrode material is a multivalent magnesium vanadate compound.

[0014] Compared with the existing technology, the beneficial effects of this solution are: (1) Low-temperature stability and significant expansion of the operating temperature range: Traditional aqueous electrolytes are limited by the freezing point of water and are prone to freezing in environments above -20°C. However, this solution forms strong hydrogen bonds between ethylene glycol and water, effectively inhibiting the crystallization of water molecules and lowering the freezing point of the electrolyte to below -33°C, ensuring that it remains liquid in an environment of -20°C. It successfully expands the operating temperature to -20~25°C, breaking through the application bottleneck of aqueous electrolytes in cold environments.

[0015] (2) Synergistic optimization of low-temperature ionic conductivity and electrochemical performance: At -20°C, the ionic conductivity of the electrolyte of this scheme can reach 23.8mS / cm, which is 57.5% higher than that of the system without ethylene glycol, and the performance is better than most deep eutectic electrolytes. In the three-electrode system, at -20°C and 0.05Ag -1 At the current density, the discharge capacity reaches 187.3mAhg -1 , with 1Ag -1 After 500 cycles of current density, the capacity retention rate is 77.3%; when the full battery uses PTCDA as the negative electrode and MVOH as the positive electrode, the capacity retention rate is 77.3% after 500 cycles of current density. -1 At this current density, the discharge capacity is 76.6 mAh g -1 , 1Ag -1 After 5000 cycles of current density cycling, the capacity retention rate is 71.1%, which is close to the performance at room temperature (72.8% at 25°C), effectively solving the problem of ion transport obstruction in existing gel electrolytes under low temperature environments.

[0016] (3) Cost advantage and simplicity of preparation process: Using 1 mol / LMgCl2 as the soluble metal salt and the volume ratio of ethylene glycol to water controlled at (2-8):10, the raw material cost is significantly lower than that of traditional high-concentration electrolytes (salt concentration > 20M). The preparation process only requires simple steps such as dissolution and stirring, without the need for complex eutectic proportioning or gel synthesis processes, making it well suited for industrial batch production and overcoming the difficulty of large-scale preparation of deep eutectic systems.

[0017] (4) Dual improvement in safety and environmental friendliness: The electrolyte of this technical solution is non-flammable and does not contain toxic additives. The Tafel polarization curve shows that its corrosion current density is lower than that of the traditional system. The hydrogen bonding between ethylene glycol and water can inhibit the hydrogen evolution side reaction (reducing water activity) and reduce the occurrence of interfacial side reactions. At the same time, DFT calculations confirm that ethylene glycol participates in the regulation of the magnesium ion solvation structure, further improving the stability of the system and solving the problems of organic additive toxicity and high-concentration electrolyte corrosion.

[0018] (5) Theoretical mechanism support: Through DFT theoretical calculations, the optimization mechanism of ethylene glycol on the solvation structure of magnesium ions was clarified, providing quantitative guidance for the design of electrolyte formulations. This mechanism of action can be extended to other aqueous ion battery systems, providing a universal solution for wide-temperature aqueous energy storage. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 Graph showing the actual states of the electrolytes prepared in Examples 1-2 and Comparative Examples 1-3 of the present invention at 25°C and -20°C; Figure 2A comparison chart of the freezing points of the electrolytes prepared in Examples 1-2 and Comparative Examples 1-3 of the present invention; Figure 3 This is a comparison chart of the ionic conductivities of the electrolytes prepared in Examples 1-2 and Comparative Examples 1-3 of the present invention at different temperatures; Figure 4 GCD curves of the electrolytes prepared in Examples 1-2 and Comparative Examples 1-3 of the present invention; Figure 5 This is a comparison chart of the rate performance and cycle performance of the electrolytes prepared in Examples 1-2 of the present invention and Comparative Examples 1-3 at -20°C; Figure 6 Electrochemical performance diagram of a full battery assembled with the electrolyte prepared in Example 1 of the present invention; Figure 7 A safety performance comparison chart of the electrolytes prepared in Example 1 and Comparative Example 3 of the present invention; Figure 8 This is a DFT theoretical calculation diagram of the electrolyte prepared in Example 1 of the present invention. DETAILED DESCRIPTION

[0020] The following is further described in detail through specific implementation methods: Example 1 A low-temperature aqueous electrolyte based on ethylene glycol and MgCl₂ comprises a soluble metal salt, an additive, and a solvent. The soluble metal salt is a soluble magnesium salt, MgCl₂, with a concentration of 1 to 5 mol / L; the solvent is deionized water; the additive is ethylene glycol, and the volume ratio of ethylene glycol to deionized water is (2 to 8):10. The operating temperature of the electrolyte is -20 to 25°C. In this embodiment, the concentration of MgCl₂ is 1 mol / L, and the volume ratio of ethylene glycol to deionized water is 2:8.

[0021] A method for preparing a low-temperature aqueous electrolyte based on ethylene glycol and MgCl2 comprises the following steps: S1: Weigh 2.033 g of MgCl2·6H2O as a soluble metal salt; S2: Measure 2 mL of ethylene glycol (EG) as an additive, add deionized water to make up to 10 mL, and mix well to prepare a mixed solvent; S3: Dissolve the MgCl2·6H2O weighed in S1 in the mixed solvent of S2, add a magnetic stirrer and transfer to a magnetic stirrer, stir for 10 minutes until completely dissolved, and prepare a low-temperature aqueous electrolyte based on ethylene glycol and MgCl2, recorded as 2EG.

[0022] An application of a low-temperature aqueous electrolyte based on ethylene glycol and MgCl2, wherein the electrolyte is applied to an aqueous ion battery, wherein the aqueous ion battery is an aqueous magnesium ion battery. The positive electrode material of the aqueous ion battery is a vanadium-based positive electrode material, and the negative electrode material is PTCDA, wherein the vanadium-based positive electrode material is a multivalent magnesium vanadate compound, wherein the multivalent magnesium vanadate compound is specifically Mg x V 10 O 24 nH2O nanoflower cathode material, denoted as MVOH.

[0023] Example 2 Different from Example 1, a low-temperature aqueous electrolyte based on ethylene glycol and MgCl2 has a volume ratio of ethylene glycol to deionized water of 4:6. A method for preparing a low-temperature aqueous electrolyte based on ethylene glycol and MgCl2 comprises measuring 4 mL of ethylene glycol in S2, adding deionized water to make the volume to 10 mL, and preparing a low-temperature aqueous electrolyte based on ethylene glycol and MgCl2, which is recorded as 4EG.

[0024] Comparative Example 1 Different from Example 1, a low-temperature aqueous electrolyte based on ethylene glycol and MgCl2, the volume ratio of ethylene glycol to deionized water is 6:4; a preparation method of a low-temperature aqueous electrolyte based on ethylene glycol and MgCl2, measuring 6 mL of ethylene glycol in S2, adding deionized water to make the volume to 10 mL, and the obtained low-temperature aqueous electrolyte based on ethylene glycol and MgCl2 is recorded as 6EG.

[0025] Comparative Example 2 Different from Example 1, a low-temperature aqueous electrolyte based on ethylene glycol and MgCl2, the volume ratio of ethylene glycol to deionized water is 8:2; a preparation method of a low-temperature aqueous electrolyte based on ethylene glycol and MgCl2, measuring 8 mL of ethylene glycol in S2, adding deionized water to make the volume to 10 mL, and the obtained low-temperature aqueous electrolyte based on ethylene glycol and MgCl2 is recorded as 8EG.

[0026] Comparative Example 3 Unlike Example 1, the electrolyte comprises MgCl2 and deionized water, without the ethylene glycol additive. Preparation method: Excluding S2, the MgCl2·6H2O weighed in S1 was directly dissolved in 10 mL of deionized water. The solution was transferred to a magnetic stirrer equipped with a magnetic stirrer and stirred for 10 minutes to obtain an aqueous MgCl2 electrolyte, designated as 0EG.

[0027] The performance of the electrolytes prepared in Examples 1-2 and Comparative Examples 1-3 was tested.

[0028] The two-electrode system and the three-electrode system were used in the performance test, specifically: two-electrode electrolytic cell: Pt sheet was used as the working electrode and counter electrode to test the ionic conductivity. Three-electrode electrolytic cell: the working electrode was Mg x V 10 O 24 nH2O, a Pt counter electrode, and an Ag / AgCl reference electrode are used to test the electrochemical performance of the electrolyte. Full-cell testing: Using a Swagelok cell mold, MVOH as the positive electrode and PTCDA as the negative electrode, the full-cell electrochemical performance is evaluated.

[0029] (1) Freezing point test of electrolyte Depend on Figure 1 It can be seen that at 25°C, the electrolytes prepared in Examples 1-2 and Comparative Examples 1-3 are all liquid; at -20°C, the electrolyte prepared in Comparative Example 3 freezes, while the electrolytes prepared in Examples 1-2 and Comparative Example 1-2 remain liquid, indicating that the addition of EG can significantly lower the freezing point of the electrolyte.

[0030] The freezing points of the electrolytes prepared in Examples 1-2 and Comparative Examples 1-3 were calculated using differential scanning calorimetry (DSC). Figure 2 It can be seen that when no EG is added, the freezing point of the electrolyte is -15°C, indicating that although the addition of magnesium chloride can lower the freezing point, the effect is limited. When the EG addition amount is 2mL, the freezing point of the electrolyte drops to -33°C, indicating that after the addition of EG, EG forms hydrogen bonds with water, which are stronger than the interaction between water molecules, reducing water activity, inhibiting solidification, and significantly lowering the freezing point. When the EG addition amount is 4mL, the freezing point of the electrolyte further drops to -53°C, indicating that with the increase of EG content, its regulatory effect on the solvation structure of magnesium ions becomes more obvious, optimizing the transport environment of magnesium ions, and further enhancing the inhibition of water activity, causing the freezing point to continue to drop significantly. When the EG addition amount is 6mL and 8mL, no obvious freezing point appears in the test temperature range of -60°C to 10°C, indicating that with the increase of EG addition, the electrolyte forms a more stable low-temperature liquid structure.

[0031] (2) Low-temperature conductivity of electrolyte The ionic conductivities of the electrolytes prepared in Examples 1-2 and Comparative Examples 1-3 at room temperature (25° C.) and low temperature (−20° C.) were tested by two-electrode electrochemical impedance spectroscopy (EIS) (Pt sheets were used as working electrodes and counter electrodes).

[0032] Depend on Figure 3 It can be seen that at 25°C, as the amount of EG added increases, the ionic conductivity of the electrolyte continues to decrease. This shows that under normal temperature conditions, EG mainly plays a "negative" role.

[0033] At -20°C, the electrolyte without EG freezes, and the ionic conductivity is only 15.11 mS / cm. When the EG addition amount is 2 mL, the electrolyte remains liquid, and the ionic conductivity reaches 23.8 mS / cm. However, as the EG addition amount continues to increase, the electrolyte ionic conductivity shows a downward trend. It can be seen that under low temperature conditions, EG has a "two-way" effect on the electrolyte ionic conductivity: it can improve the ionic conductivity when added in a low proportion, but the negative effect will be more prominent when added in a high proportion.

[0034] (3) Capacity and cycle performance test The electrolytes prepared in Examples 1-2 and Comparative Examples 1-3 were used to test the GCD, rate performance, and cycle performance of the vanadium-based positive electrode material MOVH at a low temperature of -20°C using a three-electrode system.

[0035] Depend on Figure 4-5 It can be seen that in the three-electrode system, when the amount of EG added is 2 mL, at -20 ° C, 0.05Ag -1 Under current density conditions, the battery discharge capacity can reach 187.3mAhg -1 , and showed excellent rate performance and cycle performance. -1 After 500 cycles at a current density of -20°C, the capacity retention rate reached 77.3%. Compared with the battery without EG (0EG), the battery with 2mL EG (2EG) showed a significant improvement in discharge specific capacity, with better rate performance and cycling performance. However, as the EG addition continued to increase, the battery discharge specific capacity continued to decline, and the rate and cycling performance deteriorated simultaneously. When the EG addition reached 8mL, the battery discharge specific capacity dropped to the lowest level.

[0036] A full battery was assembled with PTCDA as the negative electrode, MVOH as the positive electrode, and 2EG as the electrolyte, and its electrochemical performance was tested at -20℃ and 25℃. Figure 6 It can be seen that at -20℃, 0.2Ag -1 At this current density, the battery discharge capacity can reach 76.6 mAh g -1 , showing excellent rate performance and cycle performance, after testing, with 1Ag -1 After 5000 cycles at a current density of 1.5447 W, the capacity retention rate was 71.1% in a -20°C environment and 72.8% in a 25°C environment, indicating that the system has excellent long-cycle stability. Although it is slightly inferior to room temperature at low temperatures, the attenuation is small, making it suitable for long-life and wide-temperature range application scenarios.

[0037] (4) Test the safety performance of the electrolyte The safety of the electrolytes prepared in Example 1 and Comparative Example 3 was evaluated by combustion experiments and Tafe polarization curves. Figure 7As can be seen, after subjecting a glass fiber separator soaked in 2EG electrolyte to an open flame for 8 seconds, no combustion was observed. However, when subjecting a glass fiber separator soaked in pure EG to the same combustion conditions, a pale blue flame was clearly visible, and the combustion gradually intensified. Therefore, this result demonstrates that, although EG is flammable, its addition as an electrolyte additive in trace amounts does not result in flammability, maintaining a certain level of safety. Tafel testing revealed that the addition of EG increased the corrosion potential and decreased the corrosion current density of the electrolyte. This suggests that the addition of EG inhibits metal corrosion in magnesium chloride-based electrolytes, thereby reducing side reactions and improving electrolyte stability.

[0038] (5) The electrolyte system prepared in Example 1 was calculated based on DFT theory. Depend on Figure 8 It can be seen that the calculation of Mg 2+ Respectively with H2O, EG, Cl - The binding energy of Mg is -1.155eV, -1.964eV and -1.615eV respectively. 2+ The binding energy of -EG is significantly greater than that of Mg 2+ -H2O binding energy, which indicates that EG can change the Mg 2+ The existence form of Mg is optimized, its solvation structure is optimized, and the new Mg 2+ Complex formation improves the diffusion kinetics of magnesium ions. At the same time, calculate the reaction of H2O with H2O, EG, and Cl - The binding energies of the two atoms are -0.142eV, -0.189eV, and -0.252eV, respectively. Figure 8 Also presented EG and EG, Cl - The binding energies between EG and H2O are -0.399eV and -0.275eV, respectively. A comparison shows that the H2O-EG binding energy is greater than the H2O-H2O binding energy, meaning that EG can form hydrogen bonds with H2O, and the strength of the interaction exceeds that between water molecules. This can reduce the activity of water in the electrolyte, inhibit side reactions such as hydrogen evolution, and effectively lower its freezing point while improving the stability of the electrolyte.

[0039] In summary, the addition of EG to the MgCl2 electrolyte can effectively lower the freezing point of the electrolyte and regulate ionic conductivity and electrochemical performance. Comprehensive test results show that when the EG addition amount is 2mL, the electrolyte performance is optimal, with a significantly lower low-temperature freezing point, suitable conductivity, and excellent battery rate and cycle performance. DFT calculations also confirm that it improves system performance by optimizing the solvation structure and hydrogen bonding, providing a reliable electrolyte formulation basis for low-temperature electrochemical energy storage applications.

[0040] The above is only an embodiment of the present invention, and the common knowledge such as the specific technical solutions and / or characteristics in the solution are not described in detail here. It should be pointed out that for those skilled in the art, without departing from the technical solution of the present invention, several variations and improvements can be made, which should also be regarded as the scope of protection of the present invention, and these will not affect the effect of the implementation of the present invention and the practicality of the patent. The scope of protection required by this application shall be based on the content of its claims, and the specific implementation methods and other records in the description can be used to interpret the content of the claims.

Claims

1. A low-temperature aqueous electrolyte based on ethylene glycol and MgCl2, characterized in that: The invention comprises a soluble metal salt, an additive and a solvent, wherein the additive is ethylene glycol, and the volume ratio of ethylene glycol to the solvent is (2-8):

10.

2. A low-temperature aqueous electrolyte based on ethylene glycol and MgCl2 according to claim 1, characterized in that: The soluble metal salt is a soluble magnesium salt.

3. The low-temperature aqueous electrolyte based on ethylene glycol and MgCl2 according to claim 2, characterized in that: The concentration of the soluble metal salt is 1 to 5 mol / L.

4. The low-temperature aqueous electrolyte based on ethylene glycol and MgCl2 according to claim 3, characterized in that: The soluble magnesium salt is MgCl2.

5. The low-temperature aqueous electrolyte based on ethylene glycol and MgCl2 according to claim 4, characterized in that: The working temperature of the electrolyte is -20 to 25°C.

6. A method for preparing a low-temperature aqueous electrolyte based on ethylene glycol and MgCl2, characterized in that: The following steps are involved: S1: Weigh a certain amount of soluble metal salt for use; S2: taking a certain amount of additive and mixing it evenly with the solvent to prepare a mixed solvent; S3: dissolving the soluble metal salt weighed in S1 in the mixed solvent of S2, mixing them evenly, and preparing a low-temperature aqueous electrolyte based on ethylene glycol and MgCl2.

7. An application of a low-temperature aqueous electrolyte based on ethylene glycol and MgCl2, characterized in that: Used in aqueous ion batteries.

8. The use of a low-temperature aqueous electrolyte based on ethylene glycol and MgCl2 according to claim 7, characterized in that: The aqueous ion battery is an aqueous magnesium ion battery.

9. The use of a low-temperature aqueous electrolyte based on ethylene glycol and MgCl2 according to claim 8, characterized in that: The positive electrode material of the aqueous ion battery includes a vanadium-based positive electrode material; the negative electrode material includes PTCDA.

10. The use of a low-temperature aqueous electrolyte based on ethylene glycol and MgCl2 according to claim 9, characterized in that: The vanadium-based positive electrode material is a polyvalent magnesium vanadate compound.

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