Liquid molten salt electrolyte and molten salt battery comprising the same

Abstract

The application provides a liquid molten salt electrolyte, which comprises a quaternary phosphonium salt and a metal chloride, the substituent group on the quaternary phosphonium salt is an alkyl group with 1-14 main chain carbon atoms, and the metal chloride is anhydrous aluminum chloride; the liquid molten salt electrolyte is a light yellow liquid at room temperature. The optimal molar ratio of the quaternary phosphonium salt and the anhydrous aluminum chloride in the electrolyte is 1:1.5, and the quaternary phosphonium salt is preferably tetrabutylphosphonium chloride. The molten salt battery made of the electrolyte provided by the application has a minimum working temperature of 30 DEG C, a high operating potential, an energy density, a cycle stability and a low overall battery cost.

Description

Technical Field

[0001] This invention relates to the field of molten salt batteries, and more specifically to a liquid molten salt electrolyte and a molten salt battery containing the same. Background Technology

[0002] Molten salt batteries, also known as thermal batteries, are high-temperature backup batteries. Typically, their electrolyte is solid and non-conductive at room temperature. During use, a heating element is ignited through electrical or mechanical activation. This rapid heating causes the electrolyte to melt, forming a highly conductive ionic conductor, thus bringing the molten salt battery into operation and providing electrical energy to the power system within a short time. Molten salt batteries are characterized by short activation time, high output power, excellent specific characteristics, and good storage resistance. They typically operate within a temperature range of 350–550°C and exhibit good discharge performance after activation, making them widely used as power supplies in special environments.

[0003] Once activated, a molten salt battery remains operational as long as the molten salt electrolyte remains in a molten state, until the active materials involved in the reaction are completely exhausted. However, due to heat loss, the battery will prematurely cease operation once the temperature drops below the electrolyte's melting point, causing the molten salt to re-solidify. When the amount of electroactive material is sufficient, the thermal lifetime determines the operational life of the molten salt battery.

[0004] Common methods to extend the operating time of molten salt batteries include: 1) Enhancing insulation by using high-performance insulation materials or thickening the insulation layer. However, high-performance insulation materials are expensive, and thickened insulation layers take up a lot of space. 2) Increasing the heat capacity of the molten salt battery stack, for example, by adding heat storage plates at appropriate locations in the battery structure. However, adding heat storage plates will reduce the specific energy and specific power of the battery. 3) Selecting a heat-discharge reaction to ensure that the heat generated during battery startup is sufficient to prevent the electrolyte from solidifying, achieving a self-sustaining effect. However, this will lead to energy loss as heat. 4) Using low-melting-point molten salts to widen the operating temperature range of the molten salt battery and reduce the internal and external temperature difference during operation.

[0005] Traditional molten salt batteries are mostly primary storage batteries, including: magnesium / vanadium pentoxide batteries, calcium / lead sulfate batteries, LAN sulfide batteries, lithium alloy / iron sulfide batteries, etc. Commonly used molten salts include K2CO3, LiCl-KCl, LiF-LiCl-LiBr, and LiF-KCl-LiBr. Among them, LiF-LiCl-LiBr is often used in thermal batteries that require high current density output due to its high conductivity, but its melting point is 449℃, resulting in a relatively high operating temperature.

[0006] Compared to organic solvent electrolytes (such as carbonate organic solvents) widely used in secondary batteries, molten salt electrolytes have the following advantages when used as electrolyte solvents in secondary batteries: low vapor pressure, non-flammability, good stability, high heat capacity, and good conductivity. They can be used in mobile power supplies, grid peak shaving and energy storage, solar thermal power generation and other fields.

[0007] However, the high melting point of molten salt electrolytes leads to higher battery operating temperatures. At the same time, high temperatures also accelerate the dissolution and failure of positive and negative electrode materials and the aging of battery coating materials, resulting in low battery cycle rates and making it difficult to promote and apply them on a large scale.

[0008] Therefore, it is indeed necessary to provide a molten salt electrolyte with a lower melting point in order to reduce the operating temperature of molten salt batteries and improve battery cycle performance. Summary of the Invention

[0009] To address the problems existing in the prior art, the present invention provides a liquid molten salt electrolyte comprising a quaternary phosphate salt and a metal chloride, wherein the substituent group on the quaternary phosphate salt is one or more of alkyl, alkenyl, and aromatic groups, and the metal chloride is one or more of aluminum chloride, ferric chloride, zinc chloride, copper chloride, manganese chloride, and chromium chloride.

[0010] Furthermore, the metal chloride is anhydrous aluminum chloride.

[0011] Furthermore, the substituent group of the quaternary phosphate salt is an alkyl group with 1 to 14 carbon atoms in the main carbon chain, that is, selected from one or more of methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, tridecyl, tetradecyl or its isomers.

[0012] Furthermore, the quaternary phosphate salt is one or more of methyltributylphosphonium chloride, ethyltributylphosphonium chloride, hexyltributylphosphonium chloride, tetrabutylphosphonium chloride, octyltributylphosphonium chloride, and tetradecyltributylphosphonium chloride.

[0013] Furthermore, the quaternary phosphate salt is tetrabutylphosphonium chloride.

[0014] Furthermore, the molar ratio of quaternary phosphorus salt to metal chloride in the liquid molten salt electrolyte is 1:1.5 to 1:2.5.

[0015] The present invention also provides a molten salt battery comprising a liquid molten salt electrolyte, including a negative electrode, a positive electrode, a separator and a liquid electrolyte.

[0016] Furthermore, the separator is wrapped around the outside of the positive electrode and can selectively allow only anions to pass through. The separator includes a separator layer and a support layer located on the side of the separator layer. The support layer is connected to the positive electrode of the DC power supply on the side closer to the negative electrode. The separator is positively charged, and the support layer is used to support and fix the separator layer.

[0017] Furthermore, the support layer is a steel plate, and the support layer has a plurality of through-holes corresponding to the isolation membrane layer, with a porosity of 40%.

[0018] (1) The electrolyte provided by the present invention is liquid at room temperature and has a working temperature of 30°C to 300°C as a molten salt electrolyte. It has high conductivity, stability and redox reactivity, and can improve the operating potential, energy density and cycle stability of molten salt batteries containing the electrolyte composition.

[0019] (2) The electrolyte provided by the present invention has extremely low vapor pressure. The battery using the electrolyte at high temperature has a low expansion rate, which can effectively reduce safety hazards such as explosion and leakage and improve battery safety.

[0020] (3) The negative electrode of the battery using this liquid low-temperature molten salt electrolyte is aluminum, which greatly improves the energy storage density, which can reach 750Wh / kg. (4) The materials of the positive electrode, negative electrode and low temperature molten salt electrolyte in this invention are abundant and inexpensive on Earth, which greatly reduces the manufacturing cost of the battery.

[0021] It should be understood that the above general description and the following detailed description are exemplary and explanatory only, and are not intended to limit the invention. Attached Figure Description

[0022] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the accompanying drawings used in the description of the specific embodiments or the prior art will be briefly introduced below.

[0023] Figure 1 This is a schematic diagram of a molten salt battery according to an embodiment of the present disclosure; Among them, 100 is the molten salt battery; 10 is the positive electrode of the battery; 11 is the negative electrode of the battery; 12 is the separator; and 20 is the electrolyte. Detailed Implementation

[0024] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0025] Figure 1 This is a schematic diagram of a molten salt battery 100 according to an embodiment of the present invention. The metal-ion battery 100 may include a positive electrode 10, a negative electrode 11, and a separator 12, wherein the separator 12 may be disposed between the positive electrode 10 and the negative electrode 11. The metal-ion battery 100 includes the aforementioned electrolyte 20 disposed within the metal-ion battery 100 and located between the positive electrode and the negative electrode, such that the electrolyte composition 20 is in contact with the positive electrode 10 and the negative electrode 11. The molten salt battery 100 may be a rechargeable secondary battery, but the present invention also covers primary batteries.

[0026] According to embodiments of the present invention, the positive electrode 10 may include a positive electrode current collector layer and a positive electrode active material. According to embodiments of the present invention, the positive electrode current collector layer may be a conductive carbon substrate, a porous metal material, or a composite layer of a conductive carbon substrate and a metal material. According to embodiments of the present invention, the positive electrode active material may be a layered carbon material, a layered double hydroxide, a layered oxide, a layered chalcogenide, a vanadium oxide, or a metal sulfide, or an agglomerate of the above materials. According to embodiments of the present invention, the layered carbon material may be graphite, carbon nanotubes, graphene, or a combination thereof. According to embodiments of the present invention, the positive electrode active material may be directly grown (e.g., by chemical vapor deposition) on the positive electrode current collector layer, or the positive electrode active material may be fixed to the positive electrode current collector layer using an adhesive (e.g., polyvinyl alcohol, polytetrafluoroethylene, sodium carboxymethyl cellulose, polyvinylidene fluoride, or a combination thereof). According to embodiments of the present invention, when the positive electrode current collector layer is a porous metal material, the positive electrode active material may further fill the pores of the metal material.

[0027] According to an embodiment of the present invention, the separator membrane covers the outside of the positive electrode and can selectively allow only anions to pass through. The separator membrane includes a separator membrane layer and a support layer located on the side of the separator membrane layer. The side of the support layer closer to the negative electrode is connected to the positive electrode of the DC power supply, so that the separator membrane is positively charged, which can attract anions in the electrolyte to move towards the separator membrane. Ions with the same charge as the separator membrane attach to the support layer, while ions with different charges pass through the separator membrane.

[0028] In this embodiment, the separator layer of the separator is a macromolecular membrane, and the raw materials include at least one selected from polytetrafluoroethylene, polyethylene, polypropylene, glass fiber, nonwoven fabric, wood fiber, polyethersulfone resin, and ceramic fiber. The support layer is a steel plate, and the support layer has a plurality of through-holes corresponding to the separator layer, with a porosity of 40%. The support layer of the separator provided by the present invention is used to support and fix the separator layer, prevent the separator layer from deforming, reduce the shrinkage of the separator micropores, reduce the shrinkage rate, and ensure the stability of ion penetration; at the same time, the support layer has good heat insulation effect, can protect the separator layer, and improve safety.

[0029] According to an embodiment of the present invention, the negative electrode 11 comprises a negative electrode active material, wherein the negative electrode active material may be a metal or an alloy of the metal, a carbon material with a layered structure, a metal sulfide, or an agglomerate of the above materials. According to an embodiment of the present invention, the metal may be aluminum, copper, iron, indium, nickel, tin, chromium, yttrium, titanium, manganese, or molybdenum. According to an embodiment of the present invention, the carbon material with a layered structure may be graphite, carbon nanotubes, graphene, or a combination thereof. The negative electrode 11 may comprise a negative electrode current collector layer, and the negative electrode active material may be directly grown on the negative electrode current collector layer, or the negative electrode active material may be fixed on the negative electrode current collector layer using an adhesive. The negative electrode current collector layer may be a conductive carbon substrate, a metal material with a porous structure, or a composite layer of a conductive carbon substrate and a metal material. Example 1

[0030] Step 1: Preparation of Quaternary Phosphorus Salt: 202.32 g (1 mol) of ethanol and 400 mL of ethanol were added to a four-necked flask equipped with a mechanical stirrer, reflux condenser, constant-pressure dropping funnel, and thermometer. Stirring was then started, and the liquid phase temperature was raised to 65 °C. 92.58 g (1 mol) of chloromethane was added dropwise using the constant-pressure dropping funnel. After the addition was complete, the reaction was carried out under reflux for 12 hours. Heating and stirring were then stopped, and the mixture was cooled and discharged. The resulting reaction mixture was subjected to vacuum distillation to remove the solvent. The remaining material was then extracted with n-octane to remove unreacted raw materials and other impurities. The separated product layer was dried in a vacuum drying oven. The obtained product was characterized by phosphorus NMR spectroscopy and proton NMR spectroscopy, confirming it as methyltributylphosphonium chloride.

[0031] Step 2, Electrolyte Preparation: 126.4 g (0.5 mol) of methyltributylphosphonium chloride and 133.34 g (1 mol) of anhydrous AlCl3 (95% purity) were slowly mixed in a glass beaker. The mixture was magnetically stirred continuously in an argon-filled glove box (O2 < 1 ppm and H2O < 1 ppm). During the preparation of the mixed ionic liquid, ice gel patches were used to maintain the temperature of the mixture at a low level to prevent electrolyte decomposition. After stirring overnight at room temperature, a transparent, pale yellow, viscous liquid was obtained. To further purify the electrolyte, aluminum granules (99.999% purity) were added, and the mixture was heated under vacuum at 60°C for 1 hour to remove residual gases.

[0032] Step 3, Battery Making: ① Preparation of negative electrode The negative electrode is made of aluminum, which is prepared into aluminum foil electrodes by rolling, cutting and cleaning.

[0033] ② Preparation of the positive electrode 100g of porous carbon material and 50g of sulfur were mixed and ball-milled for 1–10 hours at a ball-to-material mass ratio of 1:0.5–10 at a rotation speed of 50–600 r / min. The mixed cathode material was then dried in a tube furnace filled with an inert atmosphere at 60°C for 3–10 hours, followed by cooling to room temperature to remove moisture and organic residues, yielding a sulfur / carbon composite material. The sulfur / carbon composite material and the binder polyvinylidene fluoride were added to N-methylpyrrolidone and stirred to form a suspension. The suspension was coated onto a current collector, which was then dried in a drying chamber or oven to remove N-methylpyrrolidone, allowing the cathode to dry and solidify. The current collector was a foamed metal with a thickness of 0.2–3 mm and a porosity of 70–98%.

[0034] ③ Preparation of aluminum-sulfur batteries The separator consists of a single layer of glass fiber membrane (6 layers of 1 / 2-inch, Whatman 934-AH) and a steel plate support layer. The positive electrode, separator, and negative electrode are sequentially placed in the outer packaging, and molten salt electrolyte is injected between the positive and negative electrodes. Then, encapsulation, settling, formation, shaping, and capacity testing are performed to fabricate an aluminum-sulfur battery with dimensions of 40×100×100mm. Example 2

[0035] Step 1: Preparation of Quaternary Phosphorus Salt: 202.32 g (1 mol) of tributylphosphine and 400 mL of ethanol were added to a four-necked flask equipped with a mechanical stirrer, reflux condenser, constant-pressure dropping funnel, and thermometer. Stirring was then started, and the liquid phase temperature was raised to 65 °C. 64.52 g (1 mol) of chloroethane was added dropwise using the constant-pressure dropping funnel. After the addition was complete, the reaction was carried out under reflux for 12 hours. Heating and stirring were then stopped, and the mixture was cooled and discharged. The resulting reaction mixture was subjected to vacuum distillation to remove the solvent. The remaining material was then extracted with n-octane to remove unreacted raw materials and other impurities. The separated product layer was dried in a vacuum drying oven. The obtained product was characterized by phosphorus NMR spectroscopy and proton NMR spectroscopy, confirming it as ethyltributylphosphine chloride.

[0036] Step 2, Electrolyte Preparation: 113.42 g (0.5 mol) of ethyltributylphosphonium chloride and 133.34 g (1 mol) of anhydrous AlCl3 (95% purity) were slowly mixed in a glass beaker. The mixture was magnetically stirred continuously in an argon-filled glove box (O2 < 1 ppm and H2O < 1 ppm). During the preparation of the mixed ionic liquid, ice gel patches were used to maintain the temperature of the mixture at a low level to prevent electrolyte decomposition. After stirring overnight at room temperature, a transparent, pale yellow, viscous liquid was obtained. To further purify the electrolyte, aluminum granules (99.999% purity) were added, and the mixture was heated under vacuum at 60°C for 1 hour to remove residual gases.

[0037] Step 3 is the same as step 3 in Example 1. Example 3

[0038] Step 1: Preparation of Quaternary Phosphorus Salt: 202.32 g (1 mol) of tributylphosphine and 400 mL of ethanol were added to a four-necked flask equipped with a mechanical stirrer, reflux condenser, constant-pressure dropping funnel, and thermometer. Stirring was then started, and the liquid phase temperature was raised to 65 °C. 120.62 g (1 mol) of chlorohexane was added dropwise using the constant-pressure dropping funnel. After the addition was complete, the reaction was carried out under reflux for 12 hours. Heating and stirring were then stopped, and the mixture was cooled and discharged. The resulting reaction mixture was subjected to vacuum distillation to remove the solvent. The remaining material was then extracted with n-octane to remove unreacted raw materials and other impurities. The separated product layer was dried in a vacuum drying oven. The obtained product was characterized by phosphorus NMR and proton NMR spectroscopy, confirming it as hexyltributylphosphine chloride.

[0039] Step 2, Electrolyte Preparation: 161.47 g (0.5 mol) of hexyltributylphosphonium chloride and 133.34 g of anhydrous AlCl3 (95% purity) were slowly mixed in a glass beaker. The mixture was magnetically stirred continuously in an argon-filled glove box (O2 < 1 ppm and H2O < 1 ppm). During the preparation of the mixed ionic liquid, ice gel patches were used to maintain the temperature of the mixture at a low level to prevent electrolyte decomposition. After stirring overnight at room temperature, a transparent, pale yellow, viscous liquid was obtained. To further purify the electrolyte, aluminum granules (99.999% purity) were added, and the mixture was heated under vacuum at 60°C for 1 hour to remove residual gases.

[0040] Step 3 is the same as step 3 in Example 1. Example 4

[0041] Step 1: Preparation of Quaternary Phosphorus Salt: 202.32 g (1 mol) of tributylphosphine and 400 mL of ethanol were added to a four-necked flask equipped with a mechanical stirrer, reflux condenser, constant-pressure dropping funnel, and thermometer. Stirring was then started, and the liquid phase temperature was raised to 65 °C. 92.58 g (1 mol) of 1-chlorobutane was added dropwise using the constant-pressure dropping funnel. After the addition was complete, the reaction was carried out under reflux for 12 hours. Heating and stirring were then stopped, and the mixture was cooled and discharged. The resulting reaction mixture was subjected to vacuum distillation to remove the solvent. The remaining material was then extracted with n-octane to remove unreacted raw materials and other impurities. The separated product layer was dried in a vacuum drying oven. The obtained product was characterized by phosphorus NMR and proton NMR spectroscopy, confirming it as tetrabutylphosphine chloride.

[0042] Step 2, Electrolyte preparation: 147.44 g (0.5 mol) of tetrabutylphosphonium chloride and 133.34 g of anhydrous AlCl3 (95% purity) were slowly mixed in a glass beaker. The mixture was magnetically stirred continuously in an argon-filled glove box (O2 < 1 ppm and H2O < 1 ppm). During the preparation of the mixed ionic liquid, ice gel patches were used to maintain the temperature of the mixture at a low level to prevent electrolyte decomposition. After stirring overnight at room temperature, a transparent, pale yellow, viscous liquid was obtained. To further purify the electrolyte, aluminum granules (99.999% purity) were added, and the mixture was heated under vacuum at 60°C for 1 hour to remove residual gases.

[0043] Step 3 is the same as step 3 in Example 1. Example 5

[0044] Step 1: Preparation of Quaternary Phosphorus Salt: 202.32 g (1 mol) of tributylphosphine and 400 mL of ethanol were added to a four-necked flask equipped with a mechanical stirrer, reflux condenser, constant-pressure dropping funnel, and thermometer. Stirring was then started, and the liquid phase temperature was raised to 65 °C. 148.67 g (1 mol) of 1-chlorooctane was added dropwise using the constant-pressure dropping funnel. After the addition was complete, the reaction was carried out under reflux for 12 hours. Heating and stirring were then stopped, and the mixture was cooled and discharged. The resulting reaction mixture was subjected to vacuum distillation to remove the solvent. The remaining material was then extracted with n-octane to remove unreacted raw materials and other impurities. The separated product layer was dried in a vacuum drying oven. The obtained product was characterized by phosphorus NMR spectroscopy and proton NMR spectroscopy, confirming it as octyltributylphosphine chloride.

[0045] Step 2, Electrolyte Preparation: 175.495 g (0.5 mol) of octyltributylphosphonium chloride and 133.34 g of anhydrous AlCl3 (95% purity) were slowly mixed in a glass beaker. The mixture was magnetically stirred continuously in an argon-filled glove box (O2 < 1 ppm and H2O < 1 ppm). During the preparation of the mixed ionic liquid, ice gel patches were used to maintain the temperature of the mixture at a low level to prevent electrolyte decomposition. After stirring overnight at room temperature, a transparent, pale yellow, viscous liquid was obtained. To further purify the electrolyte, aluminum granules (99.999% purity) were added, and the mixture was heated under vacuum at 60°C for 1 hour to remove residual gases.

[0046] Step 3 is the same as step 3 in Example 1. Example 6

[0047] Step 1: Preparation of Quaternary Phosphorus Salt: 202.32 g (1 mol) of tributylphosphine and 400 mL of ethanol were added to a four-necked flask equipped with a mechanical stirrer, reflux condenser, constant-pressure dropping funnel, and thermometer. Stirring was then started, and the liquid phase temperature was raised to 65 °C. 232.83 g (1 mol) of tetradecyl chloride was added dropwise using the constant-pressure dropping funnel. After the addition was complete, the reaction was carried out under reflux for 12 hours. Heating and stirring were then stopped, and the mixture was cooled and discharged. The resulting reaction mixture was subjected to vacuum distillation to remove the solvent. The remaining material was then extracted with n-octane to remove unreacted raw materials and other impurities. The separated product layer was dried in a vacuum drying oven. The obtained product was characterized by phosphorus NMR spectroscopy and proton NMR spectroscopy, confirming it as tetradecyltributylphosphine chloride.

[0048] Step 2, Electrolyte Preparation: 217.575 g (0.5 mol) of tetradecyltributylphosphonium chloride and 133.34 g of anhydrous AlCl3 (95% purity) were slowly mixed in a glass beaker. The mixture was magnetically stirred continuously in an argon-filled glove box (O2 < 1 ppm and H2O < 1 ppm). During the preparation of the mixed ionic liquid, ice gel patches were used to maintain the temperature of the mixture at a low level to prevent electrolyte decomposition. After stirring overnight at room temperature, a transparent, pale yellow, viscous liquid was obtained. To further purify the electrolyte, aluminum granules (99.999% purity) were added, and the mixture was heated under vacuum at 60°C for 1 hour to remove residual gases.

[0049] Step 3 is the same as step 3 in Example 1.

[0050] The melting point and conductivity of the synthesized electrolytes in different embodiments were determined using a STA409PC differential scanning calorimeter and a DDSJ-308A conductivity meter, respectively. The test results are shown in Table 1.

[0051] Table 1 Serial Number <![CDATA[Viscosity (10 3 Pa·s)]]> <![CDATA[Conductivity (S·m -1 )]]> Example 1 211 1.1 Example 2 141 1.4 Example 3 90 1.9 Example 4 43 2.8 Example 5 125 1.6 Example 6 176 0.89

[0052] A comprehensive analysis of the six embodiments revealed that the ionic liquid formed by tetrabutylphosphonium chloride and AlCl3 exhibits higher conductivity and a significantly lower melting point than the other ionic liquids. This is presumably because the tetrabutylphosphonium chloride molecule is more spherical, resulting in less resistance to intermolecular motion, thus leading to lower viscosity and higher conductivity. Furthermore, the table shows that compared to other quaternary phosphonium chloride salts, tetrabutylphosphonium chloride has a lower melting point and higher conductivity, making it more suitable as an electrolyte in molten salt batteries. Example 7

[0053] The operation steps of Example 7 are the same as those of Example 4, except that 98.30g (0.667mol) of tetrabutylphosphonium chloride is added in step 2, and the molar ratio of aluminum chloride to quaternary phosphonium salt is 1.5:1. Example 8

[0054] The operation steps of Example 8 are the same as those of Example 4, except that 58.98g (0.4mol) of tetrabutylphosphonium chloride is added in step 2, and the molar ratio of aluminum chloride to quaternary phosphonium salt is 2.5:1.

[0055] The viscosity and conductivity of the ionic liquids prepared in Examples 4, 7, and 8 were measured within the temperature range of 313K to 353K, and the results are shown in Table 2.

[0056] Table 2

[0057] As shown in the table, within the temperature range of 313K to 353K, the viscosity gradually decreases with increasing temperature. This is because higher temperatures intensify molecular motion and collisions, increasing the velocity between particles and their internal energy. This makes it easier for particles to overcome the polymerization effect, increasing the diffusion coefficient and accelerating the diffusion rate between particles, thus reducing viscosity. Furthermore, the relative values ​​in the table show that the viscosity of the ionic liquid increases with the molar ratio of anhydrous aluminum chloride to tetrabutylphosphonium chloride. This is because anhydrous aluminum chloride polymerizes into a viscous liquid dimer at high temperatures, indicating that ionic liquids containing lower concentrations of aluminum chloride are more suitable for use as electrolytes.

[0058] As shown in the table, within the temperature range of 313K to 353K, the conductivity increases with increasing temperature. This is because the viscosity of the ionic liquid decreases and the ion movement speed increases with increasing temperature, leading to increased conductivity. Simultaneously, the relative magnitudes of the data in the table show that the conductivity of the ionic liquid decreases with increasing molar ratio of anhydrous aluminum chloride to tetrabutylphosphonium chloride. This is because anhydrous aluminum chloride polymerizes into a non-conductive dimer at high temperatures, indicating that ionic liquids containing lower concentrations of aluminum chloride are more suitable for use as electrolytes.

[0059] After activating the molten salt battery prepared in the embodiment of the present invention with a current of 80 mA / g, the molten salt battery was subjected to charge-discharge cycle test with a current of 500 mA / g (charged to 4.2V), and the median discharge voltage and energy density of the molten salt battery at the 10th cycle, as well as the cycle life (discharge energy density at the 20th cycle / maximum energy density ★100%), were measured. The results are shown in Table 3.

[0060] Table 3 Serial Number Discharge median voltage / V <![CDATA[Energy density / mWh·g -1 > Cycle life / % Example 1 1.52 31.2 65.1 Example 2 2.01 91.4 47.3 Example 3 2.27 74.3 86.1 Example 4 3.13 153.1 98.4 Example 5 2.40 22.8 79.5 Example 6 2.39 87.3 83.4 Example 7 3.47 178.1 99.3 Example 8 3.21 164.3 98.1

[0061] As shown in Table 3, since tetrabutylphosphonium chloride was used in Examples 4, 7, and 8, its discharge median voltage (greater than 3.1V) and energy density (greater than 150mWh·g) were significantly higher. -1 Both the conductivity and cycle life (greater than 98%) are higher than in other embodiments, therefore tetrabutylphosphonium chloride is more suitable for mixing with anhydrous aluminum chloride as an electrolyte for molten salt batteries. The electrolyte of this invention possesses high conductivity, stability, and redox reactivity, which can improve the operating potential (median discharge voltage can be no less than 3.0V), energy density, and cycle stability of molten salt batteries containing this electrolyte. Furthermore, a horizontal comparison of Examples 4, 7, and 8 reveals that the electrolyte obtained in Example 7 has lower viscosity and higher conductivity, resulting in higher discharge voltage, energy density, and cycle life for molten salt batteries containing this electrolyte. The electrolyte of this invention is liquid at room temperature, therefore the operating temperature of molten salt batteries containing this electrolyte is 30°C to 300°C, which helps reduce heat loss in maintaining the molten state of the molten salt battery, making it more suitable for large-scale applications. The materials for the positive electrode, negative electrode, and low-temperature molten salt electrolyte in this invention are abundant and inexpensive on Earth, resulting in lower battery manufacturing costs.

[0062] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions conceived without inventive effort should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope defined in the claims.

Claims

1. A liquid molten salt electrolyte, characterized in that, It includes quaternary phosphate salts and metal chlorides, wherein the substituent groups on the quaternary phosphate salts are one or more of alkyl, alkenyl, and aromatic groups, and the metal chlorides are one or more of aluminum chloride, ferric chloride, zinc chloride, copper chloride, manganese chloride, and chromium chloride.

2. The liquid molten salt electrolyte according to claim 1, characterized in that, The metal chloride is anhydrous aluminum chloride.

3. The liquid molten salt electrolyte according to claim 1, characterized in that, The substituent group of the quaternary phosphate salt is an alkyl group with 1 to 14 carbon atoms in the main carbon chain, that is, one or more selected from methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, tridecyl, tetradecyl or its isomers.

4. The liquid molten salt electrolyte according to claim 3, characterized in that, The quaternary phosphorus salt is one or more of the following: methyltributylphosphonium chloride, ethyltributylphosphonium chloride, hexyltributylphosphonium chloride, tetrabutylphosphonium chloride, octyltributylphosphonium chloride, and tetradecyltributylphosphonium chloride.

5. The liquid molten salt electrolyte according to claim 4, characterized in that, The quaternary phosphate salt is tetrabutylphosphonium chloride.

6. The liquid molten salt electrolyte according to claim 1, characterized in that, The molar ratio of quaternary phosphate salt to metal chloride in the liquid molten salt electrolyte is 1:1.5 to 1:2.

5.

7. A molten salt battery comprising a liquid molten salt electrolyte, characterized in that, It includes a negative electrode (20), a positive electrode (10), a separator, and a liquid electrolyte as described in any one of claims 1 to 6.

8. The molten salt battery comprising a liquid molten salt electrolyte as described in claim 7, characterized in that, The separator is wrapped around the outside of the positive electrode and can selectively allow only anions to pass through. The separator includes a separator layer and a support layer located on the side of the separator layer. The support layer is connected to the positive electrode of the DC power supply on the side closer to the negative electrode. The separator is positively charged, and the support layer is used to support and fix the separator layer.

9. The molten salt battery comprising a liquid molten salt electrolyte as described in claim 8, characterized in that, The support layer is a steel plate, and the support layer has a number of through micropores corresponding to the isolation membrane layer, with a porosity of 40%.

Images