Aqueous electrolyte and preparation method and application thereof

Abstract

The invention discloses an aqueous electrolyte and a preparation method and application thereof, and belongs to the technical field of secondary batteries. The aqueous electrolyte comprises a lithium salt, water and an additive, and the additive is selected from at least one of gamma-valerolactone, fluorocarbonate and sulfonate; the content of the lithium salt and the water meets the condition that the concentration of a lithium salt aqueous solution formed by the lithium salt and the water is 15 mol / kg-22 mol / kg; the mass ratio of the lithium salt aqueous solution to the additive is 1: (1-5). On the basis of maintaining the advantages of high safety, low cost, environmental friendliness and the like, the electrochemical energy storage device has the advantages of high safety, high working voltage, high energy density, high capacity retention rate, longer cycle life and the like by widening an electrochemical window and restraining side reactions.

Description

Technical Field

[0001] The present invention relates to the technical field of secondary batteries, and in particular to an aqueous electrolyte and a preparation method and application thereof. Background Art

[0002] Lithium-ion batteries use an electrolyte as a medium for the transfer of lithium ions between the positive and negative electrodes, ensuring efficient ion transport during the battery's charge and discharge processes. During charging, Li+ is released from the positive electrode, migrates through the electrolyte, and is embedded in the negative electrode. During discharge, Li+ is released from the negative electrode and returns to the positive electrode through the electrolyte. Therefore, the electrolyte significantly impacts the charge and discharge speed and efficiency of lithium-ion batteries.

[0003] Currently, the electrolytes used in lithium-ion batteries include organic and aqueous electrolytes. Although organic electrolytes have many advantages, they are subject to problems such as leakage, volatilization, oxidative decomposition, and thermal runaway during use. Aqueous electrolytes are safer than organic electrolytes because they are non-flammable, significantly reducing the risk of battery combustion and explosion.

[0004] However, the inherently narrow electrochemical window of water (1.23V) makes the electrochemical window of aqueous electrolytes also correspondingly narrow, thereby limiting the operating voltage and energy density of lithium-ion batteries. Summary of the Invention

[0005] In view of this, the present invention provides an aqueous electrolyte and a preparation method and application thereof, which can solve the technical problems existing in the related art.

[0006] Specifically, the following technical solutions are included:

[0007] In one aspect, an aqueous electrolyte is provided, comprising a lithium salt, water, and an additive, wherein the additive is selected from at least one of γ-valerolactone, a fluorocarbonate, and a sulfonate.

[0008] The contents of the lithium salt and the water satisfy that the concentration of the lithium salt aqueous solution formed by the lithium salt and the water is 15 mol / kg-22 mol / kg;

[0009] The mass ratio of the lithium salt aqueous solution to the additive is 1:1-5.

[0010] In some possible implementations, the concentration of the lithium salt aqueous solution is 15 mol / kg-20 mol / kg; and the mass ratio of the lithium salt aqueous solution to the additive is 1:1.3-4.7.

[0011] In some possible implementations, the lithium salt is selected from at least one of lithium perchlorate, lithium hexafluorophosphate, lithium tetrafluoroborate, lithium trifluoromethanesulfonate, lithium bistrifluoromethanesulfonyl imide, lithium bisoxalatoborate, lithium difluorophosphate, lithium fluorosulfonate, lithium difluorobisoxalatophosphate, and lithium bisfluorosulfonyl imide.

[0012] In some possible implementations, the electrochemical window of the aqueous electrolyte is 2.5V to 3.5V.

[0013] In another aspect, a method for preparing an aqueous electrolyte is provided. The aqueous electrolyte is as described above, and the preparation method comprises:

[0014] Dissolving lithium salt in water to prepare a lithium salt aqueous solution with a concentration of 15 mol / kg to 22 mol / kg;

[0015] The lithium salt aqueous solution is mixed with an additive, and subjected to heat treatment in a sealed state to obtain the aqueous electrolyte, wherein the mass ratio of the lithium salt aqueous solution to the additive is 1:1-5.

[0016] In some possible implementations, the temperature of the heating treatment is 50° C. to 70° C., and the time of the heating treatment is 1 hour to 3 hours.

[0017] In some possible implementations, when preparing the lithium salt aqueous solution, the solution is kept sealed and kept at a constant temperature, and the lithium salt is dissolved by standing for a set time and / or stirring.

[0018] In some possible implementations, the aqueous electrolyte is as described above, or the aqueous electrolyte is prepared by any of the methods described above.

[0019] On the other hand, an electrochemical energy storage device is provided, characterized in that the electrochemical energy storage device includes a positive electrode, a negative electrode, and an aqueous electrolyte filled between the positive electrode and the negative electrode, wherein the aqueous electrolyte is as described above, or the aqueous electrolyte is prepared by any of the methods described above.

[0020] In some possible implementations, the electrochemical energy storage device is an aqueous lithium-ion battery or an aqueous electrochemical supercapacitor.

[0021] The beneficial effects of the technical solution provided by the embodiment of the present invention include at least:

[0022] The aqueous electrolyte provided by the embodiment of the present invention has a lithium salt and water content that satisfies the requirement that the concentration of the lithium salt aqueous solution formed by the lithium salt and water is 15 mol / kg-22 mol / kg, so that the aqueous electrolyte reaches a high concentration of salt-in-water state, and by increasing the concentration of the lithium salt, the content of free water molecules in the aqueous electrolyte is reduced, thereby suppressing the activity of water and achieving the purpose of broadening the electrochemical window of the aqueous electrolyte. Due to the reduction of free water molecules, the electrolyte with a high lithium salt concentration, on the one hand, has a reduced water activity and reduces the decomposition of water, which is beneficial to improving the safety of electrochemical energy storage devices such as lithium ion batteries. On the other hand, the aqueous electrolyte with a high lithium salt concentration is also beneficial to reducing the dissolution of organic electrode materials, so that electrochemical energy storage devices such as lithium ion batteries exhibit excellent cycle life and high capacity retention. On the other hand, the aqueous electrolyte with a high lithium salt concentration can suppress the growth of lithium dendrites.

[0023] Furthermore, at least one of γ-valerolactone, fluorocarbonate, and sulfonate is used as an additive, and the mass ratio of the lithium salt aqueous solution to the additive is adjusted to 1:1-5, which not only further improves the safety of the electrochemical energy storage device, but also further improves its cycle life and high capacity retention rate.

[0024] In summary, the aqueous electrolyte provided by the embodiments of the present invention, while maintaining the advantages of high safety, low cost, and environmental friendliness, broadens its electrochemical window and curbs side reactions, enabling electrochemical energy storage devices to have the advantages of high safety, high operating voltage, high energy density, high capacity retention rate, and longer cycle life. DETAILED DESCRIPTION

[0025] The following is a clear and complete description of the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts are within the scope of protection of the present invention.

[0026] To address the narrow electrochemical window of aqueous electrolytes in related technologies, embodiments of the present invention provide an aqueous electrolyte comprising a lithium salt, water, and an additive, wherein the additive is selected from at least one of γ-valerolactone, a fluorocarbonate, and a sulfonate. The lithium salt and water are present in an aqueous solution with a concentration of 15 mol / kg to 22 mol / kg, and a mass ratio of the aqueous lithium salt solution to the additive of 1:1 to 5.

[0027] The aqueous electrolyte provided by the embodiment of the present invention has a lithium salt and water content that satisfies the requirement that the concentration of the lithium salt aqueous solution formed by the lithium salt and water is 15 mol / kg-22 mol / kg, so that the aqueous electrolyte reaches a high concentration of salt-in-water state, and by increasing the concentration of the lithium salt, the content of free water molecules in the aqueous electrolyte is reduced, thereby suppressing the activity of water and achieving the purpose of broadening the electrochemical window of the aqueous electrolyte. Due to the reduction of free water molecules, the electrolyte with a high lithium salt concentration, on the one hand, has a reduced water activity and reduces the decomposition of water, which is beneficial to improving the safety of electrochemical energy storage devices such as lithium ion batteries. On the other hand, the aqueous electrolyte with a high lithium salt concentration is also beneficial to reducing the dissolution of organic electrode materials, so that electrochemical energy storage devices such as lithium ion batteries exhibit excellent cycle life and high capacity retention. On the other hand, the aqueous electrolyte with a high lithium salt concentration can suppress the growth of lithium dendrites.

[0028] Furthermore, at least one of γ-valerolactone, fluorocarbonate, and sulfonate is used as an additive, and the mass ratio of the lithium salt aqueous solution to the additive is adjusted to 1:1-5, which not only further improves the safety of the electrochemical energy storage device, but also further improves its cycle life and high capacity retention rate.

[0029] In summary, the aqueous electrolyte provided by the embodiments of the present invention, while maintaining the advantages of high safety, low cost, and environmental friendliness, broadens its electrochemical window and curbs side reactions, enabling electrochemical energy storage devices to have the advantages of high safety, high operating voltage, high energy density, high capacity retention rate, and longer cycle life.

[0030] As described above, to achieve the salt-in-water state, the contents of the lithium salt and the water satisfy that the concentration of the lithium salt aqueous solution formed by the lithium salt and the water is 15 mol / kg-22 mol / kg, including but not limited to 15 mol / kg, 16 mol / kg, 17 mol / kg, 18 mol / kg, 19 mol / kg, 20 mol / kg, 21 mol / kg, 22 mol / kg, etc. In some examples, the concentration of the lithium salt aqueous solution can be 15 mol / kg-20 mol / kg.

[0031] The additive can be at least one of γ-valerolactone, fluorocarbonate, and sulfonate. Research has found that the addition of γ-valerolactone can reduce the activity of water molecules and induce the formation of a special SEI film within the electrolyte solvation structure to provide protection, effectively inhibiting the occurrence of side reactions at the electrode interface. This not only further improves the safety of electrochemical energy storage devices, but also further enhances their cycle life and high capacity retention.

[0032] Fluorine in fluorocarbonates is a strong electron-withdrawing atom. Replacing the hydrogen in the carbonate group with fluorine can lower its HOMO energy level, improving its antioxidant capacity. Fluorocarbonates can be used as electrolyte additives with high stability, high conductivity, and good compatibility with graphite electrodes. For example, fluorocarbonates can include fluoroethylene carbonate, bis(2,2,2-trifluoroethyl) carbonate, methyl trifluoroethyl carbonate, and trifluoromethyl ethylene carbonate.

[0033] Sulfonate esters can also be used as electrolyte additives in aqueous lithium-ion batteries. By controlling their dosage, they can improve the electrolyte's ionic conductivity, enhance electrode interface stability, and improve battery cycling performance and safety. Examples of sulfonate esters include p-toluenesulfonate and vinyl sulfite.

[0034] The mass ratio of the lithium salt aqueous solution to the additive is 1:1-5, which includes but is not limited to: 1:1.5, 1:1.6, 1:1.7, 1:1.8, 1:1.9, 1:2, 1:2.1, 1:2.2, 1:2.3, 1:2.4, 1:2.5, 1:2.6, 1:2.7, 1:2.8, 1:2.9, 1:3, 1:3.1, 1:3.2, 1:3.3, 1:3.4, 1:3.5, 1:3.6, 1:3.7, 1:3.8, 1:3.9, 1:4, 1:4.1, 1:4.2, 1:4.3, 1:4.4, 1:4.5, 1:4.6, 1:4.7, 1:4.8, 1:4.9, etc. In some examples, the mass ratio of the lithium salt aqueous solution to the additive can be set to 1:1.3-4.7 to achieve better results.

[0035] The aqueous electrolyte provided by the embodiments of the present invention can be applied to aqueous lithium-ion batteries. Suitable lithium salts are selected from at least one of lithium perchlorate, lithium hexafluorophosphate, lithium tetrafluoroborate, lithium trifluoromethanesulfonate, lithium bistrifluoromethanesulfonyl imide, lithium bisoxalatoborate, lithium difluorophosphate, lithium fluorosulfonate, lithium difluorobisoxalatophosphate, and lithium bisfluorosulfonyl imide. For example, the lithium salt can be lithium perchlorate.

[0036] The aqueous electrolyte provided in the embodiment of the present invention has an electrochemical window of 2.5V to 3.5V. It can be seen that compared with the 2.2V voltage window of a common lead-acid battery, the aqueous electrolyte can further widen the electrochemical window.

[0037] On the other hand, an embodiment of the present invention provides a method for preparing an aqueous electrolyte, wherein the aqueous electrolyte is as described above, and the method for preparing the aqueous electrolyte comprises the following steps:

[0038] Step S1: dissolving lithium salt in water to prepare a lithium salt aqueous solution with a concentration of 15 mol / kg to 22 mol / kg. In some examples, the purity of the lithium salt is greater than or equal to 99%.

[0039] Step S2: mixing the lithium salt aqueous solution with the additive, and performing a heat treatment in a sealed state to obtain an aqueous electrolyte, wherein the mass ratio of the lithium salt aqueous solution to the additive is 1:1-5.

[0040] In step S1, lithium perchlorate is dissolved in water to form a high-concentration lithium salt aqueous solution with a salt-in-water structure. Then, in step S2, the lithium salt aqueous solution is mixed evenly with an additive, such as γ-valerolactone, under heating conditions. After heating treatment, a clear aqueous electrolyte is obtained.

[0041] In short, the electrolyte preparation process begins with dissolving lithium perchlorate in water to form the desired salt-in-water electrolyte. Lithium perchlorate is dissolved in deionized water to create the salt-in-water electrolyte. γ-Valerolactone is then added and heated to mix thoroughly, creating a clear aqueous electrolyte.

[0042] For step S2, the temperature of the heating treatment is 50℃~70℃, including but not limited to 50℃, 55℃, 60℃, 65℃, 70℃, etc., and the time of the heating treatment is 1 hour~3 hours, including but not limited to 1 hour, 2 hours, 3 hours, etc.

[0043] The above-mentioned heating treatment conditions ensure that the lithium salt aqueous solution and the additives are fully and thoroughly mixed to form a clear aqueous electrolyte.

[0044] In some examples, when preparing the lithium salt aqueous solution, the solution is sealed and kept at a constant temperature, and the lithium salt is dissolved by allowing the solution to stand for a set time and / or stirring. That is, the heating process is performed under sealed conditions and at a constant temperature, for example, in a sealed oven. Furthermore, complete dissolution of the lithium salt can be achieved by increasing the standing time and / or stirring time, adjusting the stirring speed, and / or adjusting the stirring speed.

[0045] In summary, the preparation method of the aqueous electrolyte provided by the embodiment of the present disclosure, wherein the high-concentration salt-in-water lithium salt aqueous solution reduces the content of free water molecules in the electrolyte by increasing the salt concentration, thereby suppressing the activity of water and widening the electrochemical window. Compared with the 2.2V voltage window of the common lead-acid battery, the salt-in-water electrolyte can widen the electrochemical window to above 2.5V. The high-concentration aqueous electrolyte reduces the activity of water due to the reduction of free water molecules, thereby reducing water decomposition and improving the safety of the battery. Moreover, due to the low content of free water molecules, the concentrated electrolyte significantly reduces the dissolution of organic electrode materials, so that the full battery exhibits excellent cycle life and high capacity retention. At the same time, the high-concentration electrolyte can inhibit the growth of lithium dendrites, avoid side reactions, and improve the safety of the battery. The high-concentration salt-in-water not only reduces the cost and production conditions of the electrolyte, but also broadens the electrochemical window and operating temperature range of the electrolyte, reduces the viscosity of the electrolyte, and at the same time enables the assembled lithium-ion battery to have a higher operating voltage and long cycle life.

[0046] Specifically, in terms of safety, aqueous electrolytes significantly reduce the risk of battery combustion due to their non-flammability, which is crucial for improving battery safety. Furthermore, their non-toxic and harmless properties and low-cost preparation conditions give them significant advantages in terms of environmental protection and economic efficiency. Furthermore, the relatively easy preparation conditions for aqueous electrolytes facilitate large-scale production and application, further reducing the overall cost of batteries.

[0047] The preparation method of the aqueous electrolyte has the advantages of simplicity, low cost, and ease of implementation, and has strong application prospects in lithium-ion batteries.

[0048] In combination with the above scheme, a typical preparation step of an aqueous electrolyte can be as follows:

[0049] According to the mass ratio, a specific mass of lithium salt powder with a purity of 99%, such as lithium perchlorate powder, is weighed and then transferred to a glass beaker. Subsequently, a certain mass of deionized water is added to the glass beaker, and the beaker mouth is carefully sealed with plastic wrap to prevent contamination of the solution. The beaker is then left to stand for 24 hours to allow the lithium salt to completely dissolve, forming a clear lithium salt-in-water aqueous solution. The lithium salt aqueous solution is then transferred to a sample tube, and a specific mass of an additive, such as γ-valerolactone, is added to the sample tube. The sample tube is then sealed again to ensure a tight seal during the mixing process. Finally, the sample tube is placed in an airtight oven at a set temperature and heated to allow the additive to fully mix and dissolve with the lithium salt aqueous solution, ultimately obtaining a clear aqueous electrolyte. This aqueous electrolyte significantly broadens the electrochemical window and liquid range of the electrolyte while reducing costs and simplifying production conditions. At the same time, it also effectively reduces the viscosity of the electrolyte.

[0050] On the other hand, an embodiment of the present invention provides an application of an aqueous electrolyte in an electrochemical energy storage device, wherein the aqueous electrolyte is as described above, or the aqueous electrolyte is prepared by any of the methods described above.

[0051] When aqueous electrolytes are used in electrochemical energy storage devices, they can enable the electrochemical energy storage devices to have the advantages brought by aqueous electrolytes, which will not be elaborated here.

[0052] On the other hand, an embodiment of the present invention provides an electrochemical energy storage device, which includes a positive electrode, a negative electrode, and an aqueous electrolyte filled between the positive electrode and the negative electrode, wherein the aqueous electrolyte is as described above, or the aqueous electrolyte is prepared by any of the methods described above.

[0053] The electrochemical energy storage device provided by the embodiment of the present invention has the advantages brought by aqueous electrolytes. The electrochemical energy storage device at least exhibits a higher operating voltage and an excellent long cycle life. These characteristics give it significant application potential and market competitiveness in the field of energy storage.

[0054] Exemplarily, the electrochemical energy storage device is an aqueous lithium-ion battery or an aqueous electrochemical supercapacitor.

[0055] In combination with any of the above-mentioned technical solutions, the aqueous electrolyte, preparation method, and application thereof according to the embodiments of the present invention have at least the following advantages:

[0056] (1) High safety: Aqueous electrolyte is non-flammable, which greatly reduces the risk of battery combustion and explosion and improves battery safety.

[0057] (2) Low cost: The preparation conditions of aqueous electrolytes are relatively loose and the cost is low, which is conducive to large-scale production and application.

[0058] (3) Environmentally friendly: Aqueous electrolytes are non-toxic and harmless, have little impact on the environment, and are a green and environmentally friendly battery technology.

[0059] (4) High ionic conductivity: The ionic conductivity of aqueous electrolytes is two orders of magnitude higher than that of organic electrolytes, which improves the rate and fast charging performance of lithium-ion batteries.

[0060] (5) Inhibition of side reactions: By adding additives such as γ-valerolactone to reduce the activity of water molecules and the protective effect of the special SEI film induced by the electrolyte solvation structure, the occurrence of side reactions at the electrode interface is effectively inhibited.

[0061] Below will be described in more detail exemplary embodiments of the present invention. Although the following describes exemplary embodiments of the present invention, it should be understood that the present invention can be implemented in various forms and should not be limited by the embodiments set forth herein. In the examples, if specific techniques or conditions are not indicated, they are carried out according to the techniques or conditions described in the literature in this area or according to the product specifications. Reagents or instruments used that do not indicate the manufacturer are conventional products that can be obtained commercially.

[0062] Example 1

[0063] Example 1 provides an aqueous electrolyte, which is prepared by the following method:

[0064] Step 1. Accurately weigh 22.03 g of 99% pure lithium perchlorate powder using a balance. Ensure the balance is level and free from air currents. Use a dry, clean weighing boat to prevent moisture absorption and contamination. Carefully transfer the weighed lithium perchlorate powder to a clean, dry 50 ml glass beaker.

[0065] Step 2. Use a pipette to accurately measure the required amount of deionized water and slowly add it to the lithium perchlorate powder in the beaker. During the addition, carefully control the water flow rate to avoid splattering the powder. Carefully cover the beaker with plastic wrap and secure it with a rubber band or tape to ensure a good seal.

[0066] Step 3. Place the sealed beaker in a well-ventilated, temperature-stable environment and let it sit for 24 hours to allow the lithium perchlorate to completely dissolve. During this process, the lithium perchlorate will gradually dissolve, eventually forming a clear lithium salt solution with a concentration of 18 mol / kg. If any undissolved solids remain in the solution, allow the solution to sit for an extended period or stir gently.

[0067] Step 4. Use a balance to accurately measure 9.61 g of the lithium salt solution. Then, use a balance to accurately weigh 17.23 g of γ-valerolactone and carefully add them to the sample tube. Carefully seal the sample tube with plastic wrap to prevent solvent evaporation and the ingress of foreign matter. While sealing, gently press the plastic wrap to expel air from the tube and ensure a secure seal.

[0068] Step 5. Place the sealed sample tube in an oven preheated to 60°C and heat for 1 hour. During the heating process, γ-valerolactone will thoroughly mix with the lithium perchlorate aqueous solution and dissolve. Be careful to ensure that the sample tube is placed firmly to prevent it from rolling or tipping over during heating. After heating is complete, carefully remove the sample tube and observe whether the solution is clear and transparent. If the solution is still turbid or contains undissolved solids, further heating or stirring is required to prepare a clear aqueous electrolyte.

[0069] Example 2

[0070] Example 2 provides an aqueous electrolyte. The preparation method of the aqueous electrolyte can refer to Example 1. The difference between Example 2 and Example 1 is that the concentration of the lithium salt aqueous solution is 15 mol / kg.

[0071] Example 3

[0072] Example 3 provides an aqueous electrolyte. The preparation method of the aqueous electrolyte can refer to Example 1. The difference between Example 3 and Example 1 is that the mass ratio of the lithium salt aqueous solution to γ-valerolactone is 1:3.2.

[0073] Example 4

[0074] Example 4 provides an aqueous electrolyte. The preparation method of the aqueous electrolyte can refer to Example 1. The difference between Example 4 and Example 1 is that the mass ratio of the lithium salt aqueous solution to γ-valerolactone is 1:2.7.

[0075] Example 5

[0076] Example 5 provides an aqueous electrolyte. The preparation method of the aqueous electrolyte can refer to Example 1. The difference between Example 5 and Example 1 is that the mass ratio of the lithium salt aqueous solution to γ-valerolactone is 1:1.3.

[0077] Example 6

[0078] Example 6 provides an aqueous electrolyte. The preparation method of the aqueous electrolyte can refer to Example 1. The difference between Example 6 and Example 1 is that the mass ratio of the lithium salt aqueous solution to γ-valerolactone is 1:4.7.

[0079] Example 7

[0080] Example 7 provides an aqueous electrolyte. The preparation method of the aqueous electrolyte can be referred to Example 1. The difference between Example 7 and Example 1 is that during the heating treatment, the temperature is 40° C. and the heating time is 6 hours.

[0081] Test Case

[0082] The aqueous electrolytes provided in Examples 1 to 7 were tested, and the test items were as follows:

[0083] Test item (1): A three-electrode system was assembled using lithium iron phosphate as the positive electrode, a lithium sheet as the negative electrode, and a saturated Ag / AgCl electrode as the reference electrode. The test equipment was an electrochemical workstation, and the test technique was linear sweep voltammetry (LSV) at a scan rate of 10 mV / s. The test showed that Example 1 had an electrochemical window of 3 V. Specifically, the negative electrode window was 1.6 V, and the positive electrode window was 4.6 V. Based on the same method, the electrochemical windows of the electrolytes shown in Examples 2 to 7 were 2.6 V, 2.75 V, 2.9 V, 2.5 V, 2.63 V, and 3 V, respectively.

[0084] Test Item (2): Lithium-ion batteries were assembled using lithium iron phosphate as the positive electrode and graphene as the negative electrode. Cycling tests were conducted using a constant current charge-discharge method at a current density of 5 A / g. The results showed that the batteries prepared using the aqueous electrolytes provided in Examples 1-7 all maintained energy retention rates greater than 85% after 5000 cycles, demonstrating excellent cycle life.

[0085] The above description is only for the purpose of facilitating those skilled in the art to understand the technical solution of the present invention and is not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention shall be included in the scope of protection of the present invention.

Claims

1. An aqueous electrolyte, characterized in that The aqueous electrolyte comprises a lithium salt, water and an additive, wherein the additive is selected from at least one of γ-valerolactone, a fluorocarbonate and a sulfonate; The contents of the lithium salt and the water satisfy that the concentration of the lithium salt aqueous solution formed by the lithium salt and the water is 15 mol / kg-22 mol / kg; The mass ratio of the lithium salt aqueous solution to the additive is 1:1-5.

2. The aqueous electrolyte according to claim 1, characterized in that The concentration of the lithium salt aqueous solution is 15 mol / kg-20 mol / kg; and the mass ratio of the lithium salt aqueous solution to the additive is 1:1.3-4.

7.

3. The aqueous electrolyte according to claim 1, characterized in that The lithium salt is selected from at least one of lithium perchlorate, lithium hexafluorophosphate, lithium tetrafluoroborate, lithium trifluoromethanesulfonate, lithium bistrifluoromethanesulfonyl imide, lithium bisoxalatoborate, lithium difluorophosphate, lithium fluorosulfonate, lithium difluorobisoxalatophosphate and lithium bisfluorosulfonyl imide.

4. The aqueous electrolyte according to any one of claims 1 to 3, characterized in that The electrochemical window of the aqueous electrolyte is 2.5V to 3.5V.

5. A method for preparing an aqueous electrolyte, characterized in that: The aqueous electrolyte according to any one of claims 1 to 4, wherein the preparation method comprises: Dissolving lithium salt in water to prepare a lithium salt aqueous solution with a concentration of 15 mol / kg to 22 mol / kg; The lithium salt aqueous solution is mixed with an additive, and subjected to heat treatment in a sealed state to obtain the aqueous electrolyte, wherein the mass ratio of the lithium salt aqueous solution to the additive is 1:1-5.

6. The method for preparing an aqueous electrolyte according to claim 5, wherein: The temperature of the heating treatment is 50° C. to 70° C., and the time of the heating treatment is 1 hour to 3 hours.

7. The method for preparing an aqueous electrolyte according to claim 5, wherein: When preparing the lithium salt aqueous solution, the solution is kept sealed and at a constant temperature, and the lithium salt is dissolved by standing for a set time and / or stirring.

8. Application of aqueous electrolyte in electrochemical energy storage device, characterized in that: The aqueous electrolyte is as described in any one of claims 1-4, or the aqueous electrolyte is prepared according to the method according to any one of claims 5-7.

9. An electrochemical energy storage device, characterized in that: The electrochemical energy storage device includes a positive electrode, a negative electrode, and an aqueous electrolyte filled between the positive electrode and the negative electrode, wherein the aqueous electrolyte is as described in any one of claims 1-4, or the aqueous electrolyte is prepared according to the method according to any one of claims 5-7.

10. The electrochemical energy storage device according to claim 8, characterized in that: The electrochemical energy storage device is an aqueous lithium-ion battery or an aqueous electrochemical supercapacitor.