Application of an Extremely Cold-Resistant Aqueous Electrolyte in an Iron Metal Battery

Abstract

The present invention discloses the application of an extremely cold-resistant aqueous electrolyte in an iron metal battery. By introducing an inorganic metal bromide salt into an aqueous solution of ferrous chloride and regulating the concentration ratio of chloride ions to bromide ions, an aqueous electrolyte with an ultra-low freezing temperature (-42.5 oC) can be obtained. The principle is that halogen ions can form a unique bonding interaction with water molecules, thereby breaking the original hydrogen bond network structure and reducing the freezing temperature of the mixed aqueous solution. An iron metal battery assembled from this electrolyte, a foam carbon positive electrode, and an iron foil negative electrode can exhibit excellent electrochemical energy storage characteristics at -40 oC. The technical solution involved in the present invention effectively broadens the low-temperature working range of iron metal batteries. The assembled battery has low cost, is green and environmentally friendly, and has potential application value in the fields of energy storage applications and low-temperature special batteries, etc.

Description

Technical Field

[0001] The present invention belongs to the technical field of aqueous batteries, and in particular relates to an extreme cold resistant aqueous electrolyte, and an extreme cold resistant iron metal battery constructed by using the electrolyte. Background Art

[0002] In order to meet the power demand of mobile electronic devices in extreme environments, it is urgent to develop high-performance low-temperature battery technology. At present, the capacity of lithium-ion batteries will drop sharply under low temperature conditions, and they may even fail to operate normally. In addition, it contains a large amount of highly toxic and flammable electrolyte, which poses environmental pollution and safety hazards. In comparison, aqueous batteries have attracted much attention because they use low-toxic, non-flammable, safe and reliable aqueous electrolytes. Among them, iron metal batteries, as an emerging type of aqueous battery, are composed of an iron metal negative electrode, a high specific surface area carbon positive electrode, and a ferrous salt electrolyte. The materials used are all environmentally non-toxic substances and are suitable for the development of green energy storage batteries. The working mechanism of this battery is similar to that of lithium metal batteries. The deposition and stripping reactions of ferrous ions occur on the surface of the iron negative electrode, while the conversion of ferrous salts to trivalent iron salts occurs at the carbon positive electrode. However, all ferrous salt solutions have not reached the extremely cold temperature zone (i.e. -40 o C and below) is completely frozen, resulting in obstruction of ion transmission and complete failure of the battery system. Therefore, it is of great application value to develop a new type of extremely cold-resistant ferrous salt aqueous electrolyte to improve the energy storage characteristics of iron metal battery devices in extreme environments.

[0003] The development approaches of aqueous low-temperature resistant electrolytes mainly include: 1. Adding organic solvents to destroy the hydrogen bond network structure of the original solution. Some organic solvents (such as dimethyl sulfoxide, ethyl acetate, acetonitrile, etc.) can form hydrogen bonds with water molecules, significantly reducing the freezing temperature of the solution. However, organic additives are toxic, volatile and flammable, and do not meet the environmental protection and safety requirements of energy storage equipment. 2. Using gel electrolytes. The polymer molecular chain ends in the gel are rich in a large number of hydrophilic functional groups (such as hydroxyl groups, carboxyl groups, etc.), which will form new intermolecular / internal hydrogen bond structures with water, increasing the low temperature resistance limit of the electrolyte. However, the gel electrolyte configuration process is cumbersome and costly, and it has been difficult to put into practical use so far. 3. Using ultra-high concentration salt solutions to significantly reduce the water content. Although the use of high concentration salt solutions can reduce the freezing temperature of the electrolyte, it will also cause problems such as increased electrolyte viscosity and reduced ionic conductivity, which increases the concentration difference polarization of the battery during the charge and discharge process and reduces the rapid charge and discharge capability. In summary, in order to achieve high-performance low-temperature-resistant iron metal batteries, it is crucial to develop new aqueous electrolytes with low freezing temperature, low viscosity and fast ion conduction.

[0004] According to the Hofmeister effect theory, introducing halogen anions into a solution can disrupt the hydrogen bond density and self-association degree among water molecules, thereby reducing the freezing temperature of the aqueous solution. Based on this, in the present invention, ferrous chloride is selected as the basic electrolyte, and an inorganic metal bromide is added to its aqueous solution. By regulating the concentration ratio of chloride ions to bromide ions, an aqueous electrolyte with an ultra-low freezing temperature (-42.5 o °C) can be obtained. When the total concentration of halogen ions in the mixed solution is controlled at 3 - 4 mol / L, the freezing temperature of the solution decreases significantly, and at the same time, excellent physical properties such as low viscosity and high ionic conductivity are maintained. The test results show that an iron metal battery assembled with a foam carbon positive electrode, an iron foil negative electrode, and this solution can operate normally and have excellent performance at -40 o °C. This solution is simple, easy to implement, and low in cost, and has certain practical potential and commercial prospects in the field of low-temperature energy storage batteries. Summary of the Invention

[0005] In view of this, the purpose of the present invention is to develop an aqueous electrolyte with a low freezing temperature, low viscosity, and fast ion conduction, and use this electrolyte to construct a green and environmentally friendly iron metal battery with excellent performance in extremely cold environments.

[0006] To achieve the above object, the technical solution provided by the present invention is as follows:

[0007] 1. Develop an aqueous electrolyte resistant to extremely cold environments for use in iron metal batteries, including the following steps:

[0008] (1) Preparation of the electrolyte: Add an appropriate amount of ferrous chloride to deionized water to prepare a solution with a concentration range of 2.5 - 3 mol / L. Subsequently, add an appropriate amount of metal bromide (including one or several of potassium bromide, sodium bromide, calcium bromide, and magnesium bromide) additive to the above solution, and its concentration range is 0.5 - 1 mol / L. The preferred concentration ratio of chloride ions to bromide ions is 3:1, and at this time, the freezing temperature of the electrolyte is -42.5 o °C.

[0009] (2) Preparation of the positive electrode material and the electrode sheet: The positive electrode active material is foam carbon, which is prepared by a liquid-phase reaction and high-temperature calcination method, and the positive electrode sheet is prepared by a traditional electrode coating process. The specific steps are as follows:

[0010] a) Weigh dopamine hydrochloride and iron(III) oxide powder (average particle size: 50 nm) in a mass ratio of 1:1, and add them to an appropriate amount of tris(hydroxymethyl)aminomethane hydrochloride ( Tris solution) ( Tris solution concentration: 10 mmol / L, pH 8.5; the mass of dopamine hydrochloride and iron(III) oxide and TrisThe mass ratio of the solution is: 1:200). After mixing and stirring evenly, let it stand for 1 to 5 hours to obtain the polydopamine-coated iron oxide composite precursor.

[0011] b) Immerse the carbon-coated iron oxide powder in concentrated hydrochloric acid (mass fraction: 37%) for pickling for 10 to 60 minutes. Subsequently, after vacuum filtration and washing with deionized water several times, the foam carbon cathode material is obtained.

[0012] c) Mix the foam carbon, acetylene black conductive agent, and polyvinylidene fluoride PVDF binder in a ratio of 8:1:1, and then add an appropriate amount of N-methylpyrrolidone organic solvent, and stir at room temperature for 8 to 12 hours to obtain a black viscous slurry. Use a scraper to coat the slurry on the carbon cloth current collector electrode, and after drying, the positive electrode sheet is obtained.

[0013] (3) Preparation and performance testing of the negative electrode sheet: The negative electrode material is a commercially available iron foil after cutting and cleaning, and its specific treatment process is as follows:

[0014] Cut the commercially available iron foil (purity: 99.9%; thickness: 0.15 mm) into appropriate sizes (1×1 cm 2 ). After soaking in acetone solution for 10 minutes, rinsing with deionized water and drying, the iron foil can be used as the battery negative electrode.

[0015] (4) Assembly and performance testing of the half-cell: Using the iron foil as the working electrode and reference electrode, a platinum sheet as the counter electrode, and a mixed solution of ferrous chloride and metal bromide as the electrolyte to assemble a half-cell, and perform electrochemical performance testing on this half-cell system at -40 o °C environment.

[0016] (5) Assembly and performance testing of the full cell: Using the iron foil as the negative electrode, the foam carbon as the positive electrode, the mixed solution of ferrous chloride / metal bromide as the electrolyte, and the glass fiber as the battery separator to assemble a soft-pack battery, and perform electrochemical performance testing on this full-cell system at -40 o °C environment. In the full-cell system, the mass ratio of the iron foil negative electrode to the foam carbon positive electrode is 3:2.

[0017] The beneficial effect of the present invention is that we disclose an aqueous electrolyte with an ultra-low freezing temperature. This electrolyte uses ferrous chloride and metal bromide as electrolytes, and by reasonably regulating the concentration ratio of chloride ions and bromide ions, the freezing temperature limit of the solution is reduced to -42.5 o °C. The preparation of this mixed aqueous solution is simple, and it still has the characteristics of low viscosity and high ionic conductivity in a low-temperature environment. The iron metal battery assembled from this solution, the foam carbon positive electrode, and the iron foil negative electrode can operate at -40 oIt operates stably in a C environment, showing characteristics such as high capacity, high Coulombic efficiency, and long cycle life. This electrolyte can effectively broaden the working temperature range of iron metal batteries. The assembled full battery has low cost, is green and environmentally friendly, and has certain practical value. Brief Description of the Drawings

[0018] To make the objectives, technical solutions, and beneficial effects of the present invention clearer, the following drawings are provided for the description of the present invention:

[0019] Figure 1 Optical photo comparison of an aqueous solution of ferrous chloride and metal bromide with a certain concentration ratio in Example 1 at 25 o °C and -40 o °C;

[0020] Figure 2 Differential scanning calorimetry test result diagram (a), ionic conductivity (b), activation energy test result diagram (c), and viscosity vs. temperature relationship curve diagram (d) of aqueous solutions of ferrous chloride and metal bromide with different concentrations in Example 1;

[0021] Figure 3 Linear cyclic voltammogram (a), galvanostatic charge-discharge voltage curve (b), and long-cycle durability test result diagram (c) of an iron foil negative electrode in an extreme cold-resistant electrolyte at -40 o °C in Example 2;

[0022] Figure 4 Galvanostatic charge-discharge voltage curve (a) and long-cycle durability test result diagram (b) of the assembled iron metal full battery at -40 o °C in Example 3. Detailed Embodiments

[0023] The preferred embodiments of the present invention will be described in detail below in conjunction with the drawings.

[0024] Example 1 Preparation of Extreme Cold-Resistant Aqueous Electrolyte and Study on Its Related Physicochemical Properties

[0025] This example provides an extreme cold-resistant aqueous electrolyte, the solute components of which are ferrous chloride and inorganic metal bromide. By regulating the concentration ratio of chloride ions and bromide ions, physicochemical properties such as the freezing temperature, ionic conductivity, activation energy, and viscosity of the electrolyte can be changed. Research shows that when the total concentration of chloride ions and bromide ions is 3 - 4 mol / L, the electrolyte can exhibit excellent extreme cold resistance. As Figure 1 shown, an aqueous solution of halogen salts with a certain concentration ratio at 25 o °C ( Figure 1 a) and -40 o °C( Figure 1b) Optical photo comparison under the condition. The halogen salt mixed solution with a specific concentration ratio remains liquid at -40 o °C and does not freeze. The differential scanning calorimetry test results show that ( Figure 2 a), by adjusting the halogen salt concentration, the lowest freezing temperature limit of the mixed salt solution can reach -42.5 o °C. In addition, the ion conductivity analysis and detection results of different salt solutions show that ( Figure 2 b), when the concentration of ferrous chloride in the solution is 3 mol / L and the metal bromide is 1 mol / L, the ion conductivity of the solution reaches the maximum value of 41.26 mS / cm at -40 o °C. In addition, as shown in Figure 2 c and d, the lowest reaction activation energy of the mixed salt solution is 12.35 kJ / mol, and the corresponding viscosity coefficient is 3.41 cSt, still maintaining at a relatively low level. The above data show that the prepared electrolyte has the advantages of high ion conductivity, low activation energy and low viscosity.

[0026] Example 2 - Electrochemical performance detection of iron foil negative electrode in mixed halogen salt electrolyte at -40 °C

[0027] We comparatively studied the electrochemical behavior of the iron foil negative electrode in halogen mixed salt solutions with different concentration ratios. Specifically, we assembled a half-cell with an iron foil as the working electrode and reference electrode and a platinum wire as the counter electrode, and carried out a series of electrochemical performance detections on it at -40 °C. The linear cyclic voltammetry scan test (scan rate: 5 mV / s; as shown in Figure 3 a) results show that the electrochemical oxidation-reduction peaks of the iron negative electrode are very obvious, which fully confirms that even at extremely low temperatures, the electroplating and stripping reactions of iron can still proceed smoothly. To further detect the charge-discharge Coulomb efficiency of the iron negative electrode under extremely cold conditions, we carried out a constant current charge-discharge test on the battery (current density: 1 mA / cm 2 ; areal capacity: 1 mAh / cm 2 ). As shown in Figure 3 b, the electrochemical reaction platform of the iron negative electrode during charge and discharge is obvious, the Coulomb efficiency of the first cycle is about 64%, and its Coulomb efficiency value is higher than ~98% during subsequent cycles. The battery cycle durability test (current density: 5 mA / cm 2 , areal capacity: 1 mAh / cm 2 ; as shown in Figure 3 c) shows that the cycle stability time of this half-cell at -40 o °C remains above 1000 hours, and no distortion occurs in the voltage platform. The above detection results all confirm that the iron negative electrode has excellent electrochemical reaction characteristics in the mixed halogen salt electrolyte.

[0028] Electrochemical Performance Detection of the Assembled Iron Metal Battery at -40 °C

[0029] The carbon foam positive electrode sheet, glass fiber separator, and iron foil negative electrode sheet were tightly stacked in sequence, and then thermally sealed in an aluminum-plastic film. Then, an appropriate amount of mixed halogen salt electrolyte was injected into the aluminum-plastic film. After secondary packaging and ear welding, the iron metal battery was obtained. The constant current charge-discharge test of this battery at -40 o °C (as shown in Figure 4 a) showed that as the current density gradually increased, the discharge platform of the battery always remained stable at about ~1 V without obvious change, which fully reflected that the battery had excellent low-temperature reaction kinetic characteristics and superior rate performance. The long-cycle charge-discharge test (current density: ~10 mA / cm 2 ; as shown in Figure 4 b) showed that after 1000 cycles, the battery could still maintain ~80% of its specific capacity, and its Coulomb efficiency could always be stable above 98%. The above test results all confirmed that the assembled iron metal battery had excellent electrochemical energy storage properties.

Claims

1. Application of an extremely cold-resistant aqueous electrolyte in an iron metal battery, characterized in that, The described iron metal battery system includes a positive electrode, a negative electrode, and an extremely cold-resistant aqueous electrolyte; the positive electrode is foam carbon, the negative electrode is high-purity iron foil, and the extremely cold-resistant aqueous electrolyte is a mixed aqueous solution of ferrous chloride and inorganic metal bromide; the preparation method of the extremely cold-resistant aqueous electrolyte is: dissolving a certain amount of ferrous chloride and inorganic metal bromide in deionized water, and obtaining a mixed salt solution after stirring evenly; the inorganic metal bromide includes one or several of potassium bromide, sodium bromide, calcium bromide, and magnesium bromide, and the sum of the chloride ion and bromide ion concentrations in the extremely cold-resistant aqueous electrolyte is 3-4 mol / L; the mass ratio of the iron foil negative electrode to the foam carbon positive electrode is 3:

2.

2. The application of an extremely cold-resistant aqueous electrolyte in an iron metal battery according to claim 1, wherein The foam carbon is obtained by liquid-phase reaction and high-temperature calcination, and then the positive electrode sheet is obtained by using the traditional coating process, including the following preparation steps: (1)Weigh dopamine hydrochloride and iron(III) oxide powder with an average particle size of 50 nm at a mass ratio of 1:1, and add them to an appropriate amount of tris(hydroxymethyl)aminomethane hydrochloride Tris solution. After mixing and stirring evenly, let it stand for 1 to 5 hours to obtain a polydopamine-coated iron(III) oxide composite precursor; the Tris solution concentration is 10 mmol / L and the pH is 8.5; the total mass of the dopamine hydrochloride and iron(III) oxide and Tris the ratio of the solution mass is 1:200; (2)Place the polydopamine-coated iron oxide composite precursor powder in a tube furnace and heat it at a heating rate of 1-10 o °C / min to reach 800 o °C, and then keep it at a constant temperature for 1 hour to obtain carbon-coated iron oxide powder; (3) Pickling and soaking the carbon-coated iron oxide powder with 37% concentrated hydrochloric acid for 10-60 minutes; subsequently, obtaining the foam carbon positive electrode material after vacuum filtration and washing with deionized water several times; (4) Mixing the foam carbon, acetylene black conductive agent, and polyvinylidene fluoride PVDF binder in a ratio of 8:1:1, and then adding an appropriate amount of N-methylpyrrolidone organic solvent, and stirring at room temperature for 8-12 hours to obtain a black viscous slurry; coating the slurry onto a carbon cloth current collector electrode with a scraper, and obtaining the positive electrode sheet after drying treatment.

3. The application of an extremely cold-resistant aqueous electrolyte in a ferrous metal battery according to claim 1, characterized in that, The negative electrode is high-purity iron foil, including the following preparation steps: Select a commercial iron foil with a purity of 99.9% and a thickness of 0.15 mm and cut it into 1×1 cm 2 dimensions. Then soak it in an acetone solution for 10 minutes. After rinsing with deionized water and drying, the iron foil can be used as the battery's negative electrode.

4. Use of an extremely cold-resistant aqueous electrolyte according to claim 1 in a ferrous metal battery, characterized in that, The described iron metal battery includes the following assembly and performance detection: (1)Half-cell assembly and performance testing: Using an iron foil as the working electrode and reference electrode, a platinum sheet as the counter electrode, and a mixed solution of ferrous chloride and metal bromide as the electrolyte, a half-cell is assembled, and the electrochemical performance of this half-cell system is tested under the condition of -40 o °C; (2)Full cell assembly and performance testing: Using iron foil as the negative electrode, foam carbon as the positive electrode, a mixed solution of ferrous chloride and metal bromide as the electrolyte, and glass fiber as the battery separator, a soft-pack battery was assembled and its electrochemical performance was tested at -40 o °C.

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