Low-temperature electrolyte for water-based rechargeable magnesium metal battery and preparation method of low-temperature electrolyte

Abstract

The invention provides a low-temperature electrolyte for a water-based rechargeable magnesium metal battery and a preparation method of the low-temperature electrolyte. The electrolyte is prepared by taking water, magnesium salt, a polymer monomer and a low-temperature additive as raw materials through thermal initiation polymerization. The electrolyte is simple in preparation process and easy for large-scale industrial production; according to the prepared electrolyte, by destroying a hydrogen bond network of solvent water, adjusting the hydrogen bond type, reducing the ordered movement tendency of water molecules and inhibiting liquid-solid phase change kinetics of water at low temperature, the electrolyte still has high conductivity at low temperature and even does not solidify in a working environment as low as 40 DEG C; and the electrolyte has a wide electrochemical stable window, can realize long-time stable circulation of a magnesium metal negative electrode, is suitable for being applied to a water system rechargeable magnesium metal battery in a low-temperature working environment, and has a wide application prospect.

Description

Technical Field

[0001] The present invention belongs to the technical field of magnesium secondary batteries, and particularly relates to a low-temperature electrolyte for aqueous rechargeable magnesium metal batteries and a preparation method thereof. Background Art

[0002] New energy storage is an important equipment foundation and key supporting technology for building a new power system and promoting the green and low-carbon transformation of energy. The rechargeable magnesium metal battery with a magnesium foil negative electrode is a new energy storage technology with great development prospects, having advantages such as high volumetric energy density, good safety, and low cost. Especially the aqueous rechargeable magnesium metal battery has significant advantages of intrinsic safety, environmental friendliness, and low cost compared with the organic rechargeable magnesium metal battery that uses an organic electrolyte and requires an anhydrous and anaerobic assembly environment. However, the electrolyte applied to the aqueous rechargeable magnesium metal battery faces severe problems due to the large amount of solvent water, such as a narrow electrochemical window, easy corrosion of the magnesium metal negative electrode to generate an ion-insulating Mg(OH)2 passivation layer, which increases polarization and then prevents subsequent electrochemical reactions from proceeding; especially affected by the freezing point of the solvent water, the aqueous electrolyte is extremely easy to solidify at low temperatures (below 0 °C under standard atmospheric pressure), resulting in a sharp decrease in the electrolyte conductivity and inability to normally transport ions, so it is difficult to be applied to a low-temperature working environment. Thus, the lack of an electrolyte suitable for a low-temperature working environment greatly restricts the development and application of aqueous rechargeable magnesium metal batteries.

[0003] Under this background, developing a new electrolyte with a wide electrochemical stability window, capable of realizing stable cycling of the magnesium metal negative electrode, and having low-temperature adaptability has become the key to breaking through the low-temperature application bottleneck of aqueous rechargeable magnesium metal batteries. Summary of the Invention

[0004] Aiming at the above deficiencies existing in the prior art, the present invention aims to provide a low-temperature electrolyte for aqueous rechargeable magnesium metal batteries and a preparation method thereof to solve the technical problem that the existing electrolyte of aqueous rechargeable magnesium metal batteries is not suitable for a low-temperature working environment, and at the same time meet the requirements of the aqueous rechargeable magnesium metal battery for a wide electrochemical window of the electrolyte and the ability to realize stable cycling of the magnesium metal negative electrode.

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

[0006] A low-temperature electrolyte for aqueous rechargeable magnesium metal batteries, characterized in that, by mass percentage, it includes 10 wt% - 25 wt% of magnesium salt, 15 wt% - 25 wt% of polymer monomer, 2 wt% - 10 wt% of low-temperature additive, and the balance is water; and it is formed by thermal-initiated polymerization;

[0007] The magnesium salt is composed of one or more of magnesium chloride, magnesium sulfate, magnesium trifluoromethanesulfonate, magnesium perchlorate, and magnesium bromide;

[0008] The polymer monomer is one or two of acrylamide, acrylic acid, dimethylaminoethyl methacrylate, and methyl methacrylate;

[0009] The low-temperature additive includes one or more of metal salt additives, eutectic additives, and organic additives;

[0010] Furthermore, the metal salt additive is one of calcium chloride and lithium nitrate.

[0011] Furthermore, the eutectic additive is composed of a hydrogen bond acceptor and a hydrogen bond donor, with choline chloride as the hydrogen bond acceptor and one of urea, acetamide, and citric acid as the hydrogen bond donor.

[0012] Furthermore, the organic additive is one of ethylene glycol, glycerol, and polyethylene glycol 400.

[0013] The present invention also provides a method for preparing a low-temperature electrolyte for a water-based rechargeable magnesium metal battery. Materials are prepared according to the above components and ratios, and the specific steps include:

[0014] ① Dissolve the magnesium salt and the additive in water, then add the polymer monomer, and magnetically stir under water bath heating conditions to fully mix them;

[0015] ② Pass nitrogen into the solution obtained in step ① for 15 minutes to remove dissolved oxygen to obtain a precursor solution;

[0016] ③ Add an initiator and a crosslinking agent to the precursor solution obtained in step ②, fully stir and mix, and then drop it into a mold;

[0017] ④ Heat to cause thermal polymerization of the raw materials in the mold to obtain a low-temperature electrolyte for a water-based rechargeable magnesium metal battery.

[0018] Furthermore, the initiator is ammonium persulfate or benzoyl peroxide, and the molar ratio of the initiator to the polymer monomer is 1:200 to 1:50.

[0019] Furthermore, the crosslinking agent is N,N'-methylenebisacrylamide, and the molar ratio of the crosslinking agent to the polymer monomer is 1:200 to 1:25.

[0020] Furthermore, the water bath heating temperature is 25 to 90 °C, and the magnetic stirring time is 0.5 to 3 hours.

[0021] Furthermore, the thermal polymerization temperature is 50 to 70 °C, and the polymerization time is 1 to 5 hours.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] (1) The electrolyte of the present invention is prepared by thermally initiated polymerization using water, magnesium salt, polymer monomer and various low-temperature additives as raw materials. The cross-linked network of the polymer and various low-temperature additives cooperate to disrupt the hydrogen bond network of the solvent water, regulate the type of hydrogen bonds, reduce the tendency of water molecules to move orderly, and inhibit the liquid-solid phase transition kinetics of water at low temperatures, so as to ensure that the electrolyte still has high conductivity at low temperatures and does not solidify even in a working environment as low as -40°C.

[0024] (2) The synergistic effect among the components of the electrolyte of the present invention endows the electrolyte with the characteristics of a wide electrochemical stability window, small overpotential, and long-term stable cycling of the magnesium metal negative electrode. Therefore, it is suitable for use in aqueous rechargeable magnesium metal batteries in low-temperature working environments and has good application prospects.

[0025] (3) The preparation method of the low-temperature electrolyte for the aqueous rechargeable magnesium metal battery provided by the present invention is simple, efficient, and has a simple process, which is easy for large-scale industrial production. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 is the electrochemical stability window of the electrolyte in Example 1 of the present invention;

[0027] Figure 2 is the polarization curve of magnesium foil in the electrolyte of Example 1 of the present invention;

[0028] Figure 3 is the galvanostatic charge-discharge curve of the magnesium-magnesium symmetric battery using the electrolyte of Example 1 of the present invention;

[0029] Figure 4 is the digital photo of the electrolyte in Example 1 of the present invention after being kept at -40°C for 6 hours;

[0030] Figure 5 is the electrochemical stability window of the electrolyte in Comparative Example 1 of the present invention;

[0031] Figure 6 is the polarization curve of magnesium foil in the electrolyte of Comparative Example 1 of the present invention;

[0032] Figure 7 is the galvanostatic charge-discharge curve of the magnesium-magnesium symmetric battery using the electrolyte of Comparative Example 1 of the present invention;

[0033] Figure 8 is the digital photo of the electrolyte in Comparative Example 1 of the present invention after being kept at -20°C for 6 hours. DETAILED DESCRIPTION OF THE INVENTION

[0034] The present invention will be further described in detail below with reference to specific examples. These examples are only used to illustrate the present invention and do not limit the present invention. Any improvement to this method under the premise of the concept of the present invention belongs to the scope of protection required by the present invention.

[0035] Example 1:

[0036] I. Preparation of low-temperature electrolyte

[0037] Weigh choline chloride and urea in a molar ratio of 1:2 to form a eutectic additive, then add magnesium chloride and water to it, and subsequently add the polymer monomer acrylic acid. Under the condition of a 25°C water bath, stir magnetically for 0.5 h to form a uniformly mixed solution of the above raw materials; by mass percentage of the above raw materials, the magnesium chloride accounts for 20%, the polymer monomer accounts for 20%, the eutectic additive accounts for 10%, and water accounts for 50%. Pass nitrogen into the above solution for about 15 min to remove dissolved oxygen to obtain a precursor solution. Add ammonium persulfate initiator to the precursor solution according to a molar ratio of 1:100 with the polymer monomer, and stir well until completely dissolved and uniformly dispersed. Subsequently, accurately weigh N,N'-methylenebisacrylamide as a crosslinking agent according to a molar ratio of 1:50 with the polymer monomer, add it to the above mixture, continuously stir to form a uniformly dispersed mixed system, and then quantitatively transfer it to a preset mold and react at 70°C for 3 h to ensure that the thermal polymerization reaction is fully completed, and finally obtain a low-temperature electrolyte.

[0038] II. Characterization and testing methods

[0039] 1. Conductivity measurement

[0040] The ionic conductivity σ (S cm -1 ) of the gel electrolyte is obtained by measuring the electrochemical impedance spectrum. According to calculate, where L (cm) is the thickness of the gel electrolyte, S (cm 2 ) is the contact area between the electrode and the electrolyte, and R (Ω) is the bulk resistance of the hydrogel (obtained by the electrochemical impedance spectrum). When testing, set the AC amplitude to 5 mV, and the measurement range is 10 5 Hz ~ 0.1 Hz.

[0041] 2. Electrochemical stability window measurement

[0042] The electrochemical stability window is measured by linear sweep voltammetry (LSV) and is carried out on a Shanghai Chenhua CHI 660e electrochemical workstation. The measurement is carried out through a three-electrode system. A three-electrode system is assembled with a glassy carbon electrode as the working electrode, an Ag / AgCl electrode as the reference electrode, and a platinum wire as the counter electrode. The scanning rate is 1 mV / s, and the potential range between the current densities of ±0.1 mA cm -2 is the electrochemical stability window of the electrolyte.

[0043] 3. Corrosion current density measurement

[0044] The natural corrosion current density of the magnesium sheet in the electrolyte was measured by a steady-state polarization curve. The test was carried out on a Shanghai Chenhua CHI 660e electrochemical workstation. A symmetrical cell was assembled for the test. A clean magnesium sheet was used as the working electrode and the counter electrode. The test range was ±250 mV of the open-circuit potential, and the scanning rate was 1 mV / s. The natural corrosion current density of the magnesium sheet in the electrolyte was obtained by extrapolating the polarization curve.

[0045] 4. Test of magnesium deposition / dissolution performance

[0046] The magnesium deposition / dissolution performance was detected by constant current charge-discharge test. The test was carried out on a Neware charge-discharge test system. A symmetrical cell was assembled with a magnesium foil as the working electrode and the counter electrode. Constant current charge-discharge test was carried out under the condition of 0.05 mA cm -2 to determine the polarization voltage and cycle life of magnesium deposition / dissolution.

[0047] 5. Test of full cell performance

[0048] For the full cell, a magnesium foil was used as the negative electrode, the low-temperature electrolyte of the present invention or an aqueous solution of magnesium chloride was used as the electrolyte, and CuHCF was used as the positive electrode material. The full cell was assembled and the performance test was carried out after standing for 24 h. The constant current charge-discharge test was carried out on a Neware charge-discharge test system, and the current was constant at 0.05 A g -1 .

[0049] The electrolyte prepared in Example 1 was tested by the above test method. The electrochemical stability window of the electrolyte was obtained by linear sweep voltammetry (LSV). As Figure 1 shown, its electrochemical stability window was as high as 4.04 V. The polarization curve showed that this electrolyte could reduce the natural corrosion current density of magnesium (i corr ) to 1.73×10 -5 A cm -2 ( Figure 2 ). A magnesium-magnesium symmetrical cell was assembled for the test. Figure 3 It was shown that the polarization voltage remained within 0.5 V during the 500-hour constant current charge-discharge process. In particular, this electrolyte had a conductivity as high as 4.78 mS cm -1 even in a low-temperature environment of -20°C. Compared with the conductivity of 5.31 mS cm -1 at room temperature, the decrease was very small; and in a low-temperature environment of -20°C, the symmetrical cell using this electrolyte could maintain a cycle life of more than 350 hours. Figure 4 The state of the electrolyte after being kept in a -40°C environmental test chamber for 6 hours was shown. The electrolyte did not solidify at all, confirming its outstanding low-temperature adaptability.

[0050] The electrolyte preparation process of the present invention is simple and easy for large-scale industrial production; the prepared electrolyte can reduce the tendency of water molecules to move orderly by destroying the hydrogen bond network of solvent water and adjusting the hydrogen bond type, inhibit the liquid-solid phase change kinetics of water at low temperatures, not only ensure that the electrolyte still has high conductivity at low temperatures and does not freeze even in a working environment as low as -40°C, but also endows the electrolyte with a wide electrochemical stability window and enables long-term stable cycling of the magnesium metal anode, making it suitable for aqueous rechargeable magnesium metal batteries in low-temperature working environments and having broad application prospects.

[0051] Examples 2-7:

[0052] Examples 2-7 are the same as Example 1 in terms of the electrolyte preparation steps and performance testing methods, but the compositions and dosages of various substances and the preparation conditions are different, so the performance testing results are different. Please refer to Tables 1-4 for details.

[0053] Table 1. Dosages of magnesium salts and additives in each example

[0054]

[0055]

[0056] Table 2. Compositions of polymer monomers, initiators, and crosslinking agents in each example

[0057]

[0058] Table 3. Heat treatment conditions in each example

[0059]

[0060]

[0061] Table 4. Performance testing results in each example

[0062]

[0063] Comparative Example 1:

[0064] The comparative example is an aqueous magnesium chloride solution with the same concentration as that in Example 1. The electrochemical stability window, natural corrosion current density, and magnesium deposition / dissolution performance are tested by the same method as in Example 1, and its low-temperature state is observed at -20°C.

[0065] In the comparative example, an aqueous MgCl2 solution is used as the electrolyte, and its electrochemical stability window is only 2.59 V ( Figure 5 ). Figure 6The polarization curves shown indicate that the corrosion current of the magnesium foil in the MgCl2 solution is 30 times that when using the electrolyte of Example 1. The polarization voltage of the symmetric battery assembled therefrom shows obvious fluctuations after only less than 20 h of cycling ( Figure 7 ). The electrolyte was placed in an environmental test chamber at -20 °C for 6 hours, taken out and observed for its state ( Figure 8 ). The electrolyte was completely frozen and solidified, unable to support its normal operation, indicating that it does not have low-temperature adaptability.

[0066] Comparing the results of the above examples and control examples, it can be concluded that the aqueous rechargeable magnesium battery low-temperature electrolyte prepared by the present invention can effectively broaden the electrochemical stability window, improve the reversibility of magnesium deposition / dissolution, achieve long-term stable cycling of the magnesium metal negative electrode, and extend the cycle life of the battery. In particular, this electrolyte still has a high conductivity at low temperatures and does not solidify even in a working environment as low as -40 °C, solving the technical problem that the current aqueous rechargeable magnesium metal battery lacks an electrolyte suitable for low-temperature working environments.

[0067] Finally, it should be noted that the above examples are only used to illustrate the technical solutions of the present invention and not to limit the technical solutions. Those of ordinary skill in the art should understand that any modifications or equivalent replacements made to the technical solutions of the present invention without departing from the purpose and scope of the present technical solution should be covered by the scope of the claims of the present invention.

Claims

1. A low-temperature electrolyte for an aqueous rechargeable magnesium metal battery, characterized in that: In terms of mass percentage, it comprises 10 wt% to 25 wt% of magnesium salt, 15 wt% to 25 wt% of polymer monomer, 2 wt% to 10 wt% of low-temperature additive, and the balance is water; it is formed by thermally initiated polymerization; The magnesium salt is one or more of magnesium chloride, magnesium sulfate, magnesium trifluoromethanesulfonate, magnesium perchlorate and magnesium bromide; The polymer monomer is one or two of acrylamide, acrylic acid, dimethylaminoethyl methacrylate, and methyl methacrylate; The low temperature additive includes one or more of a metal salt additive, a low eutectic additive and an organic additive.

2. The aqueous rechargeable magnesium metal battery low-temperature electrolyte according to claim 1, characterized in that: The metal salt additive is one of calcium chloride and lithium nitrate; The eutectic additive is composed of a hydrogen bond acceptor and a hydrogen bond donor, with choline chloride serving as the hydrogen bond acceptor and one of urea, acetamide and citric acid serving as the hydrogen bond donor; The organic additive is one of ethylene glycol, glycerol and polyethylene glycol 400.

3. A method for preparing a low-temperature electrolyte for an aqueous rechargeable magnesium metal battery, characterized in that: The ingredients and proportions are prepared according to claim 1 or 2, and the specific steps include: ① Dissolve the magnesium salt and additives in water, then add the polymer monomer, and stir magnetically in a water bath to fully mix; ② Passing nitrogen gas into the solution obtained in step ① for 15 min to remove dissolved oxygen to obtain a precursor solution; ③ Add initiator and crosslinker to the precursor solution obtained in step ②, stir and mix thoroughly, and then drip into the mold; ④ Heating causes the raw materials in the mold to undergo thermal polymerization to obtain a low-temperature electrolyte for an aqueous rechargeable magnesium metal battery.

4. The method for preparing a low-temperature electrolyte for an aqueous rechargeable magnesium metal battery according to claim 3, characterized in that: The initiator is ammonium persulfate or dibenzoyl peroxide, and the molar ratio of the initiator to the polymer monomer is 1:200-1:

50.

5. The method for preparing a low-temperature electrolyte for an aqueous rechargeable magnesium metal battery according to claim 3, characterized in that: The crosslinking agent is N,N'-methylenebisacrylamide, and the molar ratio of the crosslinking agent to the polymer monomer is 1:200-1:

25.

6. The method for preparing a low-temperature electrolyte for an aqueous rechargeable magnesium metal battery according to claim 3, characterized in that: The water bath heating temperature is 25-90° C., and the magnetic stirring time is 0.5-3 hours.

7. The method for preparing a low-temperature electrolyte for an aqueous rechargeable magnesium metal battery according to claim 3, characterized in that: The thermal polymerization temperature is 50-70° C., and the polymerization time is 1-5 hours.

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