Reference electrode for three-electrode lithium-ion batteries and its preparation method, and lithium-ion batteries.

Abstract

This invention belongs to the field of lithium-ion battery technology, specifically relating to a lithium metal reference electrode for a three-electrode lithium-ion battery. The invention also relates to a method for preparing the lithium metal reference electrode and a three-electrode lithium-ion battery comprising the lithium metal reference electrode. The lithium metal reference electrode comprises a lithium metal component encapsulated by a polymer solid electrolyte membrane as a first protective layer and a Li-IL@MOFs / polymer composite film based on a metal-organic framework as a second protective layer. This reduces the risk of corrosion during use and improves the stability of the lithium metal reference electrode when the entire reference electrode is placed outside the battery cell.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery technology, specifically relating to a lithium metal reference electrode for a three-electrode lithium-ion battery. This invention also relates to a method for preparing the lithium metal reference electrode and a three-electrode lithium-ion battery containing the lithium metal reference electrode. Background Technology

[0002] New energy vehicles, as an effective means of energy conservation and carbon reduction, play a vital role in the green, low-carbon, and circular economy system, thus providing ample room for the development of pure electric vehicles powered by lithium-ion batteries. The slow lithium intercalation process in the graphite anode of lithium-ion batteries limits the further development of fast-charging technology. Therefore, evaluating the lithium intercalation capability of graphite anodes and reducing the risk of lithium plating during fast charging have become key research areas in the application of fast-charging technology.

[0003] In the early 21st century, to compensate for measurement errors caused by various factors such as electrolyte, electrode materials, and current paths in two-electrode systems, researchers began to explore the introduction of a reference electrode that is stable in the electrolyte and has a known potential to construct a three-electrode system. This allows for independent measurement and precise potential control of the working electrode (the analyte). In short, a three-electrode system involves constructing a current testing loop between the working electrode and the counter electrode, and a voltage testing loop between the working electrode and the reference electrode, forming the classic "three-electrode, two-loop" system. Based on the characteristic that the reference electrode can provide a stable potential reference, the potential of the working electrode can be independently measured. Introducing it into lithium-ion battery systems allows for the analysis of the electrochemical behavior of lithium-ion battery anode materials during charging and discharging, particularly the measurement of potential changes at the anode during fast charging. This provides more reliable data support for detecting the lithium plating boundary current (maximum charging current) and lithium plating behavior at the anode. Therefore, the three-electrode system has become a commonly used electrochemical method in fast-charging technology research.

[0004] Chinese invention patent CN 114284564 B discloses a method for manufacturing a soft-pack battery with a constant potential reference electrode by placing a lithium metal sheet on the outside of the battery cell. By using a lithium metal sheet as the reference electrode, an electrochemical treatment of the oxide layer on the lithium metal surface is performed through constant current discharge to obtain a reference electrode with a constant potential, and the changes in lithium potential and impedance of the positive and negative electrodes under different states are analyzed. Although this method considers the oxidation of the lithium metal sheet, the long manufacturing cycle of the battery cell leads to a further increase in the degree of lithium metal oxidation. Although electrochemical treatment is performed later to gradually peel off the oxide layer through a dissolution reaction, excessive oxygen is inevitably introduced into the entire sealed battery system and electrolyte, increasing the adverse effects on the battery system and consequently affecting the battery capacity.

[0005] Chinese invention patent application CN 117007658 A discloses a method for fabricating a reference electrode coated with a solid electrolyte. The method involves partially treating a metal wire containing an insulating layer to remove the insulating layer, then wrapping a lithium foil around the treated area to form a lithium layer. A solid electrolyte is then coated onto the outside of the lithium metal layer, or the solid electrolyte is coated onto the metal wire followed by lithium plating to form the lithium layer. The solid electrolyte reduces the probability of metal component detachment and isolates the reference electrode from the positive and negative electrodes, preventing direct contact and mitigating side reactions between the reference electrode and the electrolyte, thus ensuring the stability of the reference electrode. However, because the reference electrode containing the solid electrolyte layer is placed between the positive and negative electrodes inside the battery, the reference electrode and the outermost solid electrolyte layer can affect lithium-ion transport during charge-discharge testing, particularly causing uneven lithium intercalation in the negative electrode portion shielded by the reference electrode, affecting the accuracy of experimental test results. Therefore, further optimization of the entire three-electrode test structure is needed.

[0006] In summary, choosing lithium metal as the reference electrode, compared to electrochemical lithium plating, does not require a long lithium plating time and offers greater flexibility in placement. Electrochemical lithium plating, on the other hand, requires placement between the positive and negative electrodes containing the lithium source inside the cell; otherwise, lithium plating is difficult to achieve. However, this also hinders lithium-ion transport, a disadvantage that lithium metal can effectively avoid.

[0007] Because lithium metal is a highly reactive alkali metal, its surface is immediately corroded by oxygen and moisture in the air when exposed, causing various uncontrollable adverse reactions and generating various heterogeneous contaminants. This can affect the accuracy of test results when used as a three-electrode reference electrode. Therefore, the isolation and protection of lithium metal when used as a reference electrode is particularly important. It is necessary to construct an air / water stable protective layer on its surface using appropriate technology to reduce its reactivity with ambient air. Summary of the Invention

[0008] Purpose of the invention

[0009] In view of the problems existing in the prior art described in the background section above, the object of the present invention is to provide a lithium metal reference electrode for a three-electrode lithium-ion battery. The present invention also relates to a method for preparing the lithium metal reference electrode and a three-electrode lithium-ion battery comprising the lithium metal reference electrode.

[0010] Technical Solution

[0011] To achieve the above objectives, the present invention adopts the following technical solution:

[0012] Option 1: A lithium metal reference electrode for a three-electrode lithium-ion battery, comprising a lithium metal component, a wire, a first protective layer, and a second protective layer, wherein...

[0013] The lithium metal component contains approximately ≥98.0% by weight, preferably approximately ≥99.0% by weight, and more preferably approximately ≥99.9% by weight of elemental lithium.

[0014] The second protective layer comprises a Li-IL@MOFs / polymer composite film formed by metal-organic frameworks (MOFs) adsorbing a second lithium salt ionic liquid (IL) solution and a film-forming polymer, which encapsulates the lithium metal component.

[0015] The first protective layer comprises a polymer solid electrolyte membrane formed of polyvinylidene fluoride (PVDF) and a first lithium salt, which encapsulates the lithium metal component and the second protective layer therein; and

[0016] One end of the wire is electrically connected to the lithium metal component, and the other end extends out to encapsulate the lithium metal component through the second protective layer and the first protective layer.

[0017] Option 2: The lithium metal reference electrode according to Option 1 above, wherein the lithium metal component is selected from lithium foil, lithium sheet, lithium plate or lithium mesh.

[0018] Option 3: The lithium metal reference electrode according to Option 1 or 2 above, wherein the lithium metal component has a thickness in the range of about 0.1 to about 1 mm.

[0019] Option 4: A lithium metal reference electrode according to any one of Options 1 to 3 above, wherein the wire comprises a copper wire.

[0020] Option 5: A lithium metal reference electrode according to any one of Options 1 to 4 above, wherein the thickness of the first protective layer is in the range of about 0.1 to about 0.5 mm.

[0021] Option 6: A lithium metal reference electrode according to any one of Options 1 to 5 above, wherein the thickness of the second protective layer is in the range of about 0.5 mm to about 1 mm.

[0022] Option 7: A lithium metal reference electrode according to any one of Options 1 to 6 above, wherein the first lithium salt and the second lithium salt each independently comprise one or more of lithium perchlorate, lithium tetrafluoroborate, lithium hexafluorophosphate, lithium bis(oxalate)borate, lithium difluorooxalateborate, lithium bis(difluorosulfonyl)imide, and lithium bis(trifluoromethanesulfonyl)imide.

[0023] Option 8: A lithium metal reference electrode according to any one of Options 1 to 7 above, wherein the first lithium salt and the second lithium salt are the same and are selected from lithium hexafluorophosphate, lithium bis(difluorosulfonyl)imide, and lithium bis(trifluoromethylsulfonyl)imide.

[0024] Option 9: A lithium metal reference electrode according to any one of Options 1 to 8 above, wherein the molecular weight of the polyvinylidene fluoride is in the range of about 300,000 to about 1,100,000 Daltons.

[0025] Option 10: A lithium metal reference electrode according to any one of Options 1 to 9 above, wherein the mass ratio of polyvinylidene fluoride to the first lithium salt is in the range of about (2.9-3.1):1.

[0026] Scheme 11: A lithium metal reference electrode according to any one of Schemes 1 to 10 above, wherein the ionic liquid comprises one or more of 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt, 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt and 1-butyl-1-methylpyrrolidine bis(trifluoromethanesulfonyl)imide salt.

[0027] Option 12: A lithium metal reference electrode according to any one of Options 1 to 11 above, wherein the film-forming polymer comprises polyvinyl alcohol (PVA) with a molecular weight in the range of about 120,000 to about 150,000 Daltons.

[0028] Scheme 13: A lithium metal reference electrode according to any one of Schemes 1 to 12 above, wherein the metal-organic framework material is prepared by zinc nitrate and dimethylimidazole in methanol.

[0029] Scheme 14: A lithium metal reference electrode according to any one of Schemes 1 to 13 above, wherein the molar concentration of the second lithium salt ionic liquid solution used in the formation of the Li-IL@MOFs / polymer composite film of the second protective layer is in the range of about 1 to about 3 mol / L.

[0030] Option 15: A lithium metal reference electrode according to any one of Options 1 to 14 above, wherein during the formation of the Li-IL@MOFs / polymer composite film of the second protective layer, the mass ratio of the metal-organic framework material to the second lithium salt ionic liquid solution is in the range of about (1.4-1.6):1.

[0031] Option 16: A lithium metal reference electrode according to any one of Options 1 to 15 above, wherein during the formation of the Li-IL@MOFs / polymer composite film of the second protective layer, the mass ratio of the metal-organic framework material adsorbed with the second lithium salt ionic liquid solution to the film-forming polymer is in the range of about (2-2.5):1.

[0032] Scheme 17: A method for preparing a lithium metal reference electrode according to any one of Schemes 1 to 16 above, wherein the method comprises the following steps:

[0033] Step 1: Provide the lithium metal component electrically connected to one end of the wire;

[0034] Step 2: Providing a Li-IL@MOFs / polymer composite film as the second protective layer includes dissolving the second lithium salt in the ionic liquid to obtain a second lithium salt ionic liquid solution, mixing the second lithium salt ionic liquid solution with the metal-organic framework material to adsorb the second lithium salt ionic liquid solution into the metal-organic framework material, then mixing the metal-organic framework material adsorbed with the second lithium salt ionic liquid solution with the film-forming polymer and grinding it, and hot-pressing the obtained ground mixture into a film to form the Li-IL@MOFs / polymer composite film, wherein the size of the Li-IL@MOFs / polymer composite film is sufficient to encapsulate the lithium metal component;

[0035] Step 3: Provide a polymer solid electrolyte membrane as the first protective layer, comprising dissolving the polyvinylidene fluoride and the first lithium salt in a volatile organic solvent to form a solution, and evaporating the organic solvent in the solution to form the polymer solid electrolyte membrane, wherein the polymer solid electrolyte membrane is large enough to encapsulate the lithium metal component encapsulated by the Li-IL@MOFs / polymer composite membrane;

[0036] Step 4: The lithium metal component obtained from Step 1 is sequentially encapsulated with the Li-IL@MOFs / polymer composite film obtained from Step 2 and the polymer solid electrolyte film obtained from Step 3 to form the lithium metal reference electrode. The lithium metal reference electrode includes the polymer solid electrolyte film, the Li-IL@MOFs / polymer composite film and the lithium metal component in the order from the outside to the inside. The other end of the wire electrically connected to the lithium metal component extends out of the polymer solid electrolyte film and the Li-IL@MOFs / polymer composite film encapsulating the lithium metal component.

[0037] Scheme 18: According to the preparation method described in Scheme 17 above, wherein step 4 includes: stacking the Li-IL@MOFs / polymer composite film obtained in step 2 and the lithium metal component obtained in step 1 on the polymer solid electrolyte film obtained in step 3 in the order of "Li-IL@MOFs / polymer composite film", "lithium metal component" and "Li-IL@MOFs / polymer composite film", then further providing a polymer solid electrolyte film as the first protective layer on the uppermost Li-IL@MOFs / polymer composite film to form an assembly, and then fusing the polymer solid electrolyte film and the Li-IL@MOFs / polymer in the encapsulation region of the obtained assembly to complete the encapsulation.

[0038] Scheme 19: According to the preparation method described in Scheme 17 above, wherein step 4 includes: stacking the polymer solid electrolyte membrane obtained from step 3, the Li-IL@MOFs / polymer composite membrane obtained from step 2, and the lithium metal component obtained from step 1 in the order of "polymer solid electrolyte membrane", "Li-IL@MOFs / polymer composite membrane", "lithium metal component", "Li-IL@MOFs / polymer composite membrane" and "polymer solid electrolyte membrane" to form an assembly, and then fusing the polymer solid electrolyte membrane and the Li-IL@MOFs / polymer located in the encapsulation region of the obtained assembly to complete the encapsulation.

[0039] Option 20: The preparation method according to Option 18 or 19 above, wherein the fusion composite includes hot pressing for about 15 to about 30 seconds at a temperature of about 40 to about 70°C and a pressure of about 5 to about 10 MPa.

[0040] Option 21: The preparation method according to any one of Options 17 to 20 above, wherein the adsorption process in step 2 includes maintaining a mixture comprising the second lithium salt ionic liquid solution and the metal-organic framework material at a temperature of about 100 to about 120°C and a vacuum of about -0.06 MPa to about -0.08 MPa for about 12 to about 24 hours to allow the second lithium salt ionic liquid solution to adsorb into the metal-organic framework material.

[0041] Option 22: The preparation method according to any one of Options 17 to 21 above, wherein the hot pressing film formation in step 2 includes holding at a temperature of about 70 to about 80°C and a pressure of about 18 to about 30 MPa for about 2 to about 5 minutes.

[0042] Scheme 23: The preparation method according to any one of Schemes 17 to 22 above, wherein the volatile organic solvent used in step 3 comprises one or more of acetonitrile, dimethyl sulfoxide, N,N-dimethylformamide and N-methylpyrrolidone.

[0043] Option 24: The preparation method according to any one of Options 17 to 23 above, wherein the formation of the polymer solid electrolyte membrane in step 3 includes film formation at a temperature of about 40 to about 60°C and at a vacuum of about -0.04 to about -0.08 MPa.

[0044] Option 25: The preparation method according to any one of Options 17 to 24 above, wherein the solid content of the solution formed in step 3 is controlled in the range of about 5 to about 10% by weight.

[0045] Scheme 26: A three-electrode lithium-ion battery comprising a cell, wherein the cell comprises a separator, a negative electrode and a positive electrode, wherein the three-electrode lithium-ion battery further comprises a lithium metal reference electrode prepared according to any one of Schemes 1 to 16 above or a lithium metal reference electrode prepared according to any one of claims 17 to 25.

[0046] Option 27: The three-electrode lithium-ion battery according to Option 26 above, wherein the lithium metal reference electrode is preferably placed on the outside of the cell and closer to the negative electrode than the positive electrode.

[0047] Option 28: A three-electrode lithium-ion battery according to Option 26 or 27 above, wherein the size of the lithium metal reference electrode is smaller than the size of the negative electrode.

[0048] Technical effect

[0049] This invention protects the lithium metal component in a lithium metal reference electrode through a composite protective layer. Specifically, this invention utilizes a bilayer structure formed by a solid electrolyte membrane based on polyvinylidene fluoride (PVDF) (as the first protective layer) and a metal-organic framework material (Li-IL@MOFs material) / polymer composite membrane adsorbed with lithium salt ion liquid solution (as the second protective layer) to encapsulate and protect the lithium metal component. This composite protective layer reduces the possibility of the lithium metal component coming into contact with air, improves the stability of the lithium metal component in air, and thus improves the stability of the lithium metal reference electrode.

[0050] In addition, the lithium metal reference electrode of the present invention can be located entirely on the outside of the cell of a three-electrode lithium-ion battery, thereby reducing the impact of the introduction of the reference electrode on the entire cell system.

[0051] The composite protective layer constructed in this invention solves some problems associated with the preparation of three-electrode systems based on lithium metal as a reference electrode, and specifically has the following outstanding advantages:

[0052] (1) From the perspective of the three-electrode system structure, compared with the common existing technology of placing copper wires or other metal current collectors between the positive and negative electrodes inside the battery cell and then electrochemically depositing lithium, in the design of the present invention, lithium metal is placed between the battery cell and the aluminum-plastic film so that the battery can be tested immediately after it is off the production line, without the need for long-term forward and reverse lithium plating, making it more convenient and faster to use.

[0053] (2) From the perspective of material properties, polyvinyl alcohol (PVA), as a film-forming polymer, is almost unaffected by weak acids, weak bases or organic solvents and has a certain chemical stability. At the same time, PVA is easy to form films, has good mechanical strength and processing performance. PVA can play a cross-linking role, which can effectively improve the brittle fracture defect in the process of pressing single Li-IL@MOFs materials into films, and can effectively enhance the flexibility of Li-IL@MOFs / polymer composite films.

[0054] (3) Metal-organic framework materials (MOFs) have advantages such as high specific surface area, high porosity, adjustable pore size and ability to accommodate a large number of gas or liquid molecules. The abundant pore space can form ion migration pathways after adsorbing lithium salts, ensuring ion conduction between the reference electrode and the working electrode and forming an effective test circuit. At the same time, the microporous structure of MOFs has high surface energy and strong adsorption capacity, which can adsorb air, water and other substances to a certain extent, forming a protective effect on the lithium metal interface and reducing the risk of lithium metal oxidation.

[0055] (4) Since polyvinylidene fluoride (PVDF) is chemically stable to water and air, the double-layer membrane composed of PVDF-based solid electrolyte membrane and Li-IL@MOFs / polymer composite membrane can form an air / water stable protective layer on the lithium metal surface through synergistic effect, reduce its reactivity with ambient air, reduce its risk of corrosion by water and oxygen in the air, and achieve multiple protections for lithium metal.

[0056] (5) The encapsulation of the composite protective layer of the present invention can prevent lithium metal from contacting the electrolyte and reduce the occurrence of side reactions between lithium metal and electrolyte, thereby improving the stability of lithium metal reference electrode in corrosive electrolyte and ensuring the accuracy of lithium reference electrode potential monitoring.

[0057] (6) From the perspective of processing and preparation, the use of thermal / mechanical pressing to form a film reduces the use of organic solvents in the film formation process compared with wet casting. It does not require the dissolution and dispersion process of polymer and MOF mixed materials in organic solvents, shortening the film formation time. At the same time, the use of organic solvents can easily cause incomplete solvent evaporation and adsorption of organic solvents in the pores of MOFs, resulting in higher solvent removal temperature and difficulty in removal.

[0058] (7) Lithium metal components such as lithium sheets, lithium meshes and lithium foils have a higher lithium content than lithium layers formed by electrochemical lithium plating, and reference electrodes based on lithium metal components will also have a longer static life and dynamic life. Attached Figure Description

[0059] To more clearly illustrate the technical solutions of the specific embodiments of the present invention, the drawings used in the specific embodiments will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0060] Figure 1 An exploded view illustrating the components of the lithium metal reference electrode of the present invention;

[0061] Figure 2 This is a perspective view of the assembled lithium metal reference electrode of the present invention.

[0062] Figure 3 A top view of the assembled lithium metal reference electrode of the present invention;

[0063] Figure 4 An exploded view illustrating the components of the three-electrode lithium-ion battery of the present invention;

[0064] Figure 5 This is an assembly diagram of the three-electrode lithium-ion battery of the present invention after assembly;

[0065] Figure 6 The diagram shows the step charge / charge curves of the three-electrode lithium-ion battery of Embodiment 3 of the present invention; and

[0066] Figure 7 This is a test diagram of the reference electrode-negative electrode potential stability of the three-electrode lithium-ion battery in Embodiment 3 of the present invention.

[0067] Figure label:

[0068] 1: First protective layer; 2: Second protective layer; 3: Lithium metal component; 4: Wire; 5: Separator; 6: Negative electrode; 7: Positive electrode; 8: Positive electrode tab; 9: Negative electrode tab; 10: Tab adhesive; 11: Reference electrode tab; 12: Reference electrode Detailed Implementation

[0069] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below in conjunction with the embodiments of this invention. Those skilled in the art should understand that the embodiments described are merely for the purpose of aiding understanding of this invention and should not be considered as specific limitations on this invention. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention. Process parameters in the following embodiments that are not specifically specified are generally performed under conventional conditions.

[0070] In the description of this invention, the terms "upper," "lower," "front," "rear," "left," "right," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing the invention and do not require the invention to be constructed and operated in a specific orientation. Therefore, they should not be construed as limiting the invention. The terms "connected," "linked," and "set up" used in this invention should be interpreted broadly. For example, they can refer to a fixed connection or a detachable connection; a direct connection or an indirect connection through intermediate components; a wired connection, a radio connection, or a wireless communication signal connection. Those skilled in the art can understand the specific meaning of the above terms according to the specific circumstances.

[0071] The endpoints and any values ​​of the ranges disclosed in this invention are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. The term "about" as used in this invention indicates that the number it modifies may fluctuate within ±20%, ±15%, ±10%, ±5%, or ±2% of that number. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this invention.

[0072] According to a first aspect of the present invention, a lithium metal reference electrode for a three-electrode lithium-ion battery is provided, the lithium metal reference electrode comprising a lithium metal component, a wire, a first protective layer, and a second protective layer. The various components of the lithium metal reference electrode will be described in detail below.

[0073] Lithium metal components:

[0074] In the lithium metal reference electrode of the first aspect of the present invention described above, the lithium metal component contains at least about 98.0% by weight, preferably at least about 99.0% by weight, and more preferably at least about 99.9% by weight of elemental lithium.

[0075] The shape of the lithium metal component is not particularly limited, but in a preferred embodiment of the invention, the lithium metal component can be formed into any one of lithium foil, lithium sheet, lithium plate, and lithium mesh. Here, the thickness of the lithium foil, lithium sheet, lithium plate, and lithium mesh is not particularly limited, but in a preferred embodiment of the invention, the thickness can be set in the range commonly used in the art, from about 0.1 to about 1 mm, for example, about 0.2 mm, about 0.3 mm, about 0.4 mm, about 0.5 mm, about 0.6 mm, about 0.7 mm, about 0.8 mm, or about 0.9 mm.

[0076] First protective layer and second protective layer:

[0077] In the lithium metal reference electrode of the first aspect of the present invention, the second protective layer comprises a Li-IL@MOFs / polymer composite film formed by a metal-organic framework material adsorbed with a second lithium salt ionic liquid solution and a film-forming polymer, which encapsulates the lithium metal component.

[0078] In the lithium metal reference electrode of the first aspect of the present invention, the first protective layer comprises a polymer solid electrolyte membrane formed of polyvinylidene fluoride and a first lithium salt, which encapsulates the lithium metal component and the second protective layer therein.

[0079] The lithium salt used in the first and second protective layers:

[0080] In the lithium metal reference electrode of the first aspect of the present invention described above, the first lithium salt in the first protective layer and the second lithium salt in the second protective layer may be the same or different. However, since the reference electrode and the protective layer are both in the electrolyte system of the entire battery cell, different types of lithium salts dissolved in the electrolyte may affect the battery cell system, and may participate in reactions and generate various byproducts during charging and discharging, thereby affecting the battery cell testing. Therefore, in a preferred embodiment, the first lithium salt and the second lithium salt are the same.

[0081] Here, the first lithium salt and the second lithium salt may each independently comprise one or more of lithium perchlorate, lithium tetrafluoroborate, lithium hexafluorophosphate, lithium bis(oxalate-borate), lithium difluorooxalate-borate, lithium bis(difluorosulfonyl)imide, and lithium bis(trifluoromethanesulfonyl)imide. Particularly preferably, the first lithium salt and the second lithium salt are the same and are selected from any one of lithium hexafluorophosphate, lithium bis(difluorosulfonyl)imide, and lithium bis(trifluoromethanesulfonyl)imide.

[0082] The ionic liquid used in the second protective layer:

[0083] In the lithium metal reference electrode of the first aspect of the present invention described above, the ionic liquid used in the process of forming the second protective layer is an organic ionic liquid. For example, in a preferred embodiment, the ionic liquid used may contain one or more of 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt, 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt, and 1-butyl-1-methylpyrrolidine bis(trifluoromethanesulfonyl)imide salt.

[0084] The film-forming polymer used in the second protective layer:

[0085] In the lithium metal reference electrode of the first aspect of the present invention, the film-forming polymer used in the formation of the second protective layer preferably includes polyvinyl alcohol. From a material properties perspective, polyvinyl alcohol, as a film-forming polymer, is almost unaffected by weak acids, weak bases, or organic solvents, exhibiting a certain degree of chemical stability. Furthermore, polyvinyl alcohol is easy to form films, possesses good mechanical strength and processing performance, can act as a crosslinking agent, and can effectively improve the brittle fracture defects during the pressing of single Li-IL@MOFs materials into films, thus effectively enhancing the flexibility of the Li-IL@MOFs / polymer composite film.

[0086] Here, polyvinyl alcohol with a molecular weight in the range of about 120,000 to about 150,000 Daltons, for example, about 130,000 Daltons or about 140,000 Daltons, is particularly preferred. The molecular weight of the polyvinyl alcohol should not be too large, for example, not exceeding about 150,000 Daltons, otherwise the uniformity of the film layer will be difficult to control during the film formation process; nor should it be too small, for example, not below about 120,000 Daltons, otherwise the mechanical properties of the formed film may be poor, for example, the formed film may be too soft, thereby increasing the difficulty of demolding.

[0087] Process parameters for forming the second protective layer:

[0088] In a preferred embodiment, during the formation of the second protective layer, the concentration of the second lithium salt solution in 1L is approximately 1 to approximately 3 mol / L, for example, approximately 1.5 mol / L, approximately 2 mol / L, or approximately 2.5 mol / L. Here, the concentration of the solution should not be too high, for example, not exceeding approximately 3 mol / L, otherwise the lithium salt may not dissolve completely; nor should it be too low, for example, not below approximately 1 mol / L, otherwise it may affect the formation of the lithium-ion conductive network and affect ion transport kinetics.

[0089] In a further preferred embodiment, during the formation of the second protective layer, the mass ratio of the MOF material to the IL solution of the second lithium salt is controlled within the range of approximately (1.4-1.6):1, for example, approximately 1.5:1. Here, the mass ratio range should not be too large, for example, not exceeding approximately 1.6:1, otherwise, after the MOFs completely adsorb the Li-IL solution, the unadsorbed pores of the MOFs may adsorb too much electrolyte; nor should it be too small, for example, not lower than approximately 1.4:1, otherwise, the MOFs may not completely adsorb the Li-IL solution, resulting in an excess of Li-IL solution, causing the prepared sample to exhibit a clumped and wet state, which is not conducive to subsequent experimental preparation and sampling.

[0090] In another further preferred embodiment, during the formation of the second protective layer, the mass ratio of the metal-organic framework material (Li-IL@MOFs) adsorbed with the second lithium salt ionic liquid solution to the film-forming polymer is controlled within the range of approximately (2.0-2.5):1, for example, approximately 2.1:1, approximately 2.2:1, approximately 2.3:1, or approximately 2.4:1. Here, the mass ratio range should not be too large, for example, not exceeding approximately 2.5:1, otherwise the cross-linking effect of the film-forming polymer may be insignificant, resulting in poor flexibility after film formation, thus causing the formed film to be brittle and prone to breakage; nor should it be too small, for example, not lower than approximately 2.0:1, otherwise the mechanical properties and uniformity of the formed film may deteriorate.

[0091] The metal-organic framework material used in the second protective layer:

[0092] In the lithium metal reference electrode of the first aspect of the present invention described above, the metal-organic framework material used in the process of forming the second protective layer can be prepared by a preparation method commonly used in the art.

[0093] For example, in a particularly preferred embodiment, the metal-organic framework material used in this invention is prepared from zinc nitrate and dimethylimidazole in methanol. For instance, the metal-organic framework material can be obtained by dissolving zinc nitrate and dimethylimidazole in methanol, allowing it to stand at room temperature for approximately 12 hours, and then washing it by high-speed centrifugation.

[0094] Polyvinylidene fluoride in the first protective layer:

[0095] In a preferred embodiment, the polyvinylidene fluoride used in forming the first protective layer has a molecular weight in the range of about 300,000 to about 1,100,000 Daltons, for example, about 400,000, or about 500,000, or about 600,000, or about 700,000, or about 800,000, or about 900,000, or about 1,000,000 Daltons. Here, the molecular weight of the polyvinylidene fluoride should not be too high, for example, not exceeding about 1,100,000 Daltons, otherwise it will be difficult to control the uniformity of the formed film, and crystallization may occur in the formed film; nor should it be too low, for example, not below about 300,000 Daltons, otherwise it will easily lead to poor mechanical properties of the formed film.

[0096] Process parameters for forming the first protective layer:

[0097] In yet another preferred embodiment, during the formation of the first protective layer, the mass ratio of polyvinylidene fluoride (PVDF) to the first lithium salt is controlled within the range of approximately (2.9-3.1):1, for example, approximately 3.0:1. Here, the mass ratio range should not be too large, for example, not exceeding approximately 3.1:1, otherwise PVDF and lithium salt may not effectively form an ion-conducting network, affecting the formation of lithium-ion pathways; nor should it be too small, for example, not below approximately 2.9:1, otherwise it may lead to enhanced interactions between PVDF molecules, thereby hindering lithium-ion migration.

[0098] Thicknesses of the first protective layer and the second protective layer:

[0099] Furthermore, in the lithium metal reference electrode of the first aspect of the present invention, the thickness of the first protective layer can be in the range of about 0.1 to about 0.5 mm, for example, about 0.2 mm, about 0.3 mm, or about 0.4 mm. Here, the thickness of the first protective layer should not be too large or too small, for example, it should not exceed about 0.5 mm, otherwise it may affect the subsequent aluminum-plastic film casing of the battery; nor should it be less than about 0.1 mm, otherwise it may increase the risk of damage to the protective layer during assembly, thereby failing to provide sufficient protection for the underlying second protective layer, and the requirements for the manufacturing process will also be significantly increased.

[0100] On the other hand, in the lithium metal reference electrode of the first aspect of the present invention, the thickness of the second protective layer is in the range of about 0.5 mm to about 1 mm, for example, about 0.6 mm, or about 0.7 mm, or about 0.8 mm, or about 0.9 mm. Here, the thickness of the second protective layer should not be too large or too small, for example, it should not exceed about 1 mm, otherwise it will lead to an increase in the overall thickness of the reference electrode, which will affect the thickness of the entire cell and the subsequent battery packaging. It should also not be less than about 0.5 mm, otherwise it may cause insufficient mechanical properties of the second protective layer and insufficient protection effect on the reference electrode, and the processability will deteriorate.

[0101] wire:

[0102] In the lithium metal reference electrode of the first aspect of the present invention, one end of the wire is electrically connected to the lithium metal component, and the other end extends out to encapsulate the lithium metal component with a second protective layer and a first protective layer.

[0103] Here, the conductor can be a copper wire commonly used in the art. The copper wire can be enameled copper wire, and the end that contacts the lithium metal component has been stripped of its enamel coating.

[0104] According to a second aspect of the present invention, a method for preparing a lithium metal reference electrode according to the first aspect of the present invention described above is provided. Here, the general and preferred features described with respect to the lithium metal reference electrode in the first aspect of the present invention also apply to the second aspect of the present invention. The preparation method according to the second aspect of the present invention comprises steps 1 to 4.

[0105] Step 1:

[0106] In the preparation method of the second aspect of the present invention described above, step 1 includes providing the lithium metal component electrically connected to one end of the wire. The electrical connection can be achieved by means commonly used in the art, such as welding, riveting, bonding, and rolling.

[0107] Step 2:

[0108] In the preparation method of the second aspect of the present invention described above, step 2 includes providing a Li-IL@MOFs / polymer composite film as the second protective layer. Specifically, step 2 may include dissolving the second lithium salt in the ionic liquid to obtain an ionic liquid solution (Li-IL) of the second lithium salt, then mixing the second lithium salt ionic liquid solution with the metal-organic framework material, thereby adsorbing the second lithium salt ionic liquid solution into the metal-organic framework material, then mixing the metal-organic framework material adsorbed with the second lithium salt ionic liquid solution with the film-forming polymer and grinding it, and hot-pressing the obtained ground mixture into a film to form the Li-IL@MOFs / polymer composite film, wherein the size of the Li-IL@MOFs / polymer composite film is sufficient to encapsulate the lithium metal component.

[0109] In a preferred embodiment, the adsorption process in step 2 above includes maintaining a mixture comprising the second lithium salt ionic liquid solution and the metal-organic framework material at a temperature of about 100 to about 120°C (e.g., about 110°C) and a vacuum of about -0.06 MPa to about -0.08 MPa (e.g., about -0.07 MPa) for about 12 to about 24 hours (e.g., about 18 hours) to allow the second lithium salt ionic liquid solution to adsorb into the metal-organic framework material. Here, the temperature of the adsorption process should not be too high or too low; for example, it should not exceed about 120°C, otherwise the structure of the metal-organic framework material may be damaged, increasing the risk of structural collapse; nor should it be below about 100°C, otherwise it will be detrimental to the adsorption of the lithium salt ionic liquid solution. Furthermore, the vacuum degree of the adsorption process should not be too high or too low; for example, it should not exceed about -0.08 MPa, otherwise the requirements for the vacuum equipment will be higher, increasing costs; nor should it be below about -0.06 MPa, otherwise the adsorbed water, organic solvents, and other molecules cannot be completely removed, and the lithium salt ionic liquid solution will not be completely adsorbed. In addition, the adsorption process should not be too long or too short. For example, it should not exceed about 24 hours, otherwise it will increase the preparation cycle time and affect the preparation efficiency. It should also not be less than about 12 hours, otherwise the lithium salt ion liquid solution will not be completely adsorbed.

[0110] In another preferred embodiment, the hot pressing film formation in step 2 includes film formation for about 2 to 5 minutes (e.g., about 3 minutes or about 4 minutes) at a temperature of about 70 to about 80°C (e.g., about 75°C) and a pressure of about 18 to about 30 MPa (e.g., about 21 MPa, about 24 MPa, or about 27 MPa). Here, the temperature of the hot pressing film formation should not be too high or too low; for example, it should not exceed about 80°C, otherwise the uniformity of the film formation may be easily affected; nor should it be below about 70°C, otherwise the polymer molecules responsible for crosslinking may not reach the glass transition temperature, thus failing to form a film. Furthermore, the pressure of the hot pressing film formation should not be too high or too low; for example, it should not exceed about 30 MPa, otherwise it may be difficult to control the film thickness and may exceed the mold's pressure tolerance, affecting the mold; nor should it be below about 18 MPa, otherwise the film thickness may be too large, failing to achieve the target film thickness. In addition, the hot pressing film formation time should not be too long or too short. For example, it should not exceed about 5 minutes, otherwise it may cause film shrinkage and affect the uniformity of film formation. It should also not be less than about 2 minutes, otherwise it may cause more pores and defects inside the material.

[0111] Step 3:

[0112] In the preparation method of the second aspect of the present invention described above, step 3 includes providing a polymer solid electrolyte membrane for the first protective layer. Specifically, step 3 may include dissolving the polyvinylidene fluoride and the first lithium salt in a volatile organic solvent to form a solution, and evaporating the organic solvent in the solution to form the polymer solid electrolyte membrane, wherein the size of the polymer solid electrolyte membrane is sufficient to encapsulate the lithium metal component encapsulated by the Li-IL@MOFs / polymer composite membrane.

[0113] There are no particular limitations on the volatile organic solvent used in step 3 above, as long as it can effectively dissolve the polyvinylidene fluoride and the first lithium salt, and readily evaporates during the subsequent film formation process. For example, in a preferred embodiment, the volatile organic solvent that can be used includes one or more of acetonitrile, dimethyl sulfoxide, N,N-dimethylformamide, and N-methylpyrrolidone.

[0114] In another preferred embodiment, the solid content of the solution formed in step 3 above can be controlled in the range of about 5% to about 10% by weight, for example, about 6% by weight, about 7% by weight, about 8% by weight, or about 9% by weight. Here, the solid content should not be too high or too low, for example, it should not exceed about 10% by weight, otherwise the uniformity of film formation may be affected, and the film may be prone to unevenness; it should also not be lower than about 5% by weight, otherwise bubbles may be easily generated during solvent evaporation, and the uniformity of film formation may also be affected.

[0115] In yet another preferred embodiment, the process of forming the polymer solid electrolyte membrane in step 3 above includes film formation at a temperature of about 40 to about 60°C and a vacuum of about -0.04 to about -0.08 MPa. Here, the temperature of the film formation process should not be too high or too low; for example, it should not exceed about 60°C, otherwise the solvent may evaporate rapidly, easily leading to uneven film formation; nor should it be below about 40°C, otherwise incomplete solvent evaporation may occur. In addition, the vacuum level of the film formation process should not be too high or too low; for example, it should not exceed about -0.08 MPa, otherwise many bubbles may be generated inside the membrane; nor should it be below about -0.04 MPa, otherwise incomplete solvent removal may occur.

[0116] Step 4:

[0117] In the preparation method of the second aspect of the present invention described above, step 4 includes encapsulating the lithium metal component obtained from step 1 with the Li-IL@MOFs / polymer composite film obtained from step 2 and the polymer solid electrolyte film obtained from step 3 in sequence to form the lithium metal reference electrode. The lithium metal reference electrode obtained after encapsulation sequentially comprises the polymer solid electrolyte film, the Li-IL@MOFs / polymer composite film, and the lithium metal component in order from the outside to the inside, wherein the other end of the wire electrically connected to the lithium metal component extends out of the polymer solid electrolyte film and the Li-IL@MOFs / polymer composite film encapsulating the lithium metal component.

[0118] In a preferred embodiment, step 4 may include stacking the Li-IL@MOFs / polymer composite film obtained in step 2 and the lithium metal reference electrode current collector obtained in step 1 on the polymer solid electrolyte film obtained in step 3 in the order of "Li-IL@MOFs / polymer composite film", "lithium metal component" and "Li-IL@MOFs / polymer composite film", and then further providing a polymer solid electrolyte film with the first protective layer on the uppermost Li-IL@MOFs / polymer composite film to form an assembly; and then fusing the polymer solid electrolyte film and the Li-IL@MOFs / polymer composite film located in the encapsulation region of the obtained assembly to complete the encapsulation.

[0119] Alternatively, step 4 may include stacking the polymer solid electrolyte membrane obtained from step 3, the Li-IL@MOFs / polymer composite membrane obtained from step 2, and the lithium metal component obtained from step 1 in the order of "polymer solid electrolyte membrane", "Li-IL@MOFs / polymer composite membrane", "lithium metal component", "Li-IL@MOFs / polymer composite membrane", and "polymer solid electrolyte membrane" to form an assembly; and then fusing the polymer solid electrolyte membrane and the Li-IL@MOFs / polymer composite membrane located in the encapsulation region of the obtained assembly to complete the encapsulation.

[0120] Here, the meaning of the encapsulation area is as shown in the appendix to the specification. Figure 3 As shown, in the formed assembly, the regions of the polymer solid electrolyte membrane encapsulating the lithium metal component and the Li-IL@MOFs / polymer composite membrane that do not overlap with the lithium metal component.

[0121] Furthermore, the fusion bonding described above preferably involves hot pressing for approximately 15 to 30 seconds (e.g., approximately 20 seconds or approximately 25 seconds) at a temperature of approximately 40 to approximately 70°C (e.g., approximately 50°C or approximately 60°C) and a pressure of approximately 5 to approximately 10 MPa (e.g., approximately 6 MPa, approximately 7 MPa, approximately 8 MPa, or approximately 9 MPa). Here, the fusion bonding temperature should not be too high or too low; for example, it should not exceed approximately 70°C, otherwise it may easily lead to film shrinkage, resulting in incomplete encapsulation. It should also not be below approximately 40°C, otherwise the films of the first and second protective layers may not soften enough to achieve effective fusion, which may also affect the sealing performance of the encapsulation. Additionally, the fusion bonding pressure should not be too high or too low; for example, it should not exceed approximately 10 MPa, otherwise it may lead to a smaller encapsulation area thickness, increasing the risk of breakage. It should also not be below approximately 5 MPa, otherwise effective encapsulation fusion between the first and second protective layers may not be possible. In addition, the fusion and bonding time should not be too long or too short. For example, it should not exceed about 30 seconds, otherwise the thickness of the encapsulation area may become smaller, and it should not be less than about 15 seconds, otherwise the encapsulation area may not be properly sealed.

[0122] According to a third aspect of the present invention, a three-electrode lithium-ion battery is provided, wherein the three-electrode lithium-ion battery comprises a lithium metal reference electrode prepared according to the first aspect of the present invention described above or a lithium metal reference electrode prepared according to the preparation method described in the second aspect of the present invention described above. Here, the general and preferred features described with respect to the lithium metal reference electrode in the first and second aspects of the present invention also apply to the third aspect of the present invention.

[0123] In the three-electrode lithium-ion battery described in the third aspect of the present invention, the three-electrode lithium-ion battery includes a cell, wherein the cell includes a separator, a negative electrode and a positive electrode.

[0124] In a preferred embodiment, the battery includes a cell and a reference electrode, wherein the cell comprises a separator, a negative electrode, a separator, a positive electrode, a separator, a negative electrode, and a separator arranged alternately in the following order, and the reference electrode is located on the outside of the cell near the negative electrode, thereby forming a three-electrode battery system. The reference electrode, due to its double-layer interface protection layer, does not require an external separator.

[0125] In a particularly preferred embodiment, the size of the lithium metal reference electrode is smaller than the size of the negative electrode.

[0126] The present invention will now be described in further detail with reference to specific embodiments.

[0127] Examples 1 to 3: A general method for preparing a three-electrode lithium-ion battery:

[0128] The materials used in this method are shown in Table 1.

[0129] Combined with appendix Figures 1 to 5 :

[0130] (1) Provide a lithium metal component 3 with the specifications shown in Table 1, wherein the lithium metal component 3 is electrically connected to one end of a copper wire, which serves as a conductor 4, by a rolling process.

[0131] (2) Provide a Li-IL@MOFs / polymer composite film as the second protective layer 2:

[0132] Specifically, the second lithium salt is dissolved in an ionic liquid to obtain an ionic liquid solution of the second lithium salt. The metal-organic framework material and the ionic liquid solution of the second lithium salt are mixed at a weight ratio of about 1.5:1 at a temperature of about 110°C and a vacuum of -0.07 MPa for about 18 hours, so that the second lithium salt ionic liquid solution is fully adsorbed in the metal-organic framework material. The metal-organic framework material is obtained by washing by high-speed centrifugation after the zinc nitrate and dimethylimidazole are placed in methanol and allowed to stand at room temperature for about 12 hours.

[0133] The metal-organic framework material adsorbed with the second lithium salt ionic liquid solution is then mixed uniformly with polyvinyl alcohol having an average molecular weight of about 130,000 Daltons as a film-forming polymer and ground. The resulting ground mixture is then hot-pressed at a temperature of about 75°C and a certain pressure to form the Li-IL@MOFs / polymer composite film, wherein the size of the Li-IL@MOFs / polymer composite film is sufficient to encapsulate the lithium metal component.

[0134] (3) Provide a polymer solid electrolyte membrane as the first protective layer 1.

[0135] Specifically, polyvinylidene fluoride having an average molecular weight of about 400,000 Daltons and the first lithium salt are dissolved in a volatile organic solvent in a mass ratio of about 3:1 to form a solution, and the organic solvent in the solution is evaporated to form the polymer solid electrolyte membrane, wherein the polymer solid electrolyte membrane is large enough to encapsulate the lithium metal component encapsulated by the Li-IL@MOFs / polymer composite membrane;

[0136] (4) The lithium metal component obtained from step (1) is sequentially encapsulated with the Li-IL@MOFs / polymer composite film obtained from step (2) and the polymer solid electrolyte film obtained from step (3) to form the lithium metal reference electrode 12. The encapsulation is performed by hot-pressing the Li-IL@MOFs / polymer composite film and the polymer solid electrolyte film located in the encapsulation area at a temperature of approximately 55°C and a pressure of approximately 7.5 MPa for 25 seconds. The resulting lithium metal reference electrode sequentially comprises the polymer solid electrolyte film, the Li-IL@MOFs / polymer composite film, and the lithium metal component from the outside in, wherein the other end of the wire electrically connected to the lithium metal component extends out of the polymer solid electrolyte film and the Li-IL@MOFs / polymer composite film encapsulating the lithium metal component.

[0137] (5) A battery cell comprising a separator 5, a negative electrode 6, and a positive electrode 7 is provided, wherein the battery cell comprises separator 5, negative electrode 6, separator 5, positive electrode 7, separator 5, negative electrode 6, and separator 5 arranged alternately in the following order. The battery cell is combined with a lithium metal reference electrode prepared as described above to form a three-electrode lithium-ion battery, wherein the lithium metal reference electrode is placed outside the battery cell and close to the negative electrode, and the size of the lithium metal reference electrode is smaller than the size of the negative electrode (approximately 54 cm). 2 ).

[0138] The negative electrode 6 and negative electrode tab 9, and the positive electrode 7 and positive electrode tab 8 are all connected by ultrasonic welding. The negative electrode tab 9 and positive electrode tab 8 are both bonded to the aluminum-plastic film by top and side heat sealing with tab adhesive 10.

[0139] The reference electrode 12 and the reference electrode tab 11 are connected by a copper wire. The copper wire is soldered to the reference electrode tab 11 to form a conductive path from the lithium metal component 3 to the reference electrode tab 11.

[0140] The preparation process conditions used in the above embodiments are summarized in Table 1 below:

[0141] Table 1:

[0142]

[0143] The three-electrode lithium-ion batteries prepared in Examples 1 to 3 were subjected to stepped charging. To avoid lithium deposition on the negative electrode surface due to polarization during high-rate (high-current) direct charging, a multi-stage constant current (CC) charging method was used to charge the batteries. This ensured that the negative electrode potential remained within a safe range while reducing the risk of polarization and lithium deposition on the negative electrode surface during charging, thereby reducing battery damage and extending battery life. The negative electrode potential was monitored during charging using a circuit formed between the reference electrode and the negative electrode.

[0144] For example, the step-charge verification (multi-stage constant current charging) process of the three-electrode lithium-ion battery in Example 3 is as follows: Figure 6 As shown.

[0145] Figure 6 The negative electrode-reference potential value is actually the lithium potential (vs. Li / Li). + ), see Table 2 below for specific values. When the potential value is less than or equal to 0V (vs. Li / Li + There is a risk of lithium plating when the voltage is greater than 0V (vs. Li / Li). + There is no risk of lithium plating when it is used.

[0146] Table 2 shows the negative electrode-reference potential monitoring results of the three-electrode lithium-ion battery prepared in Example 3 at different cumulative SOC (state of charge).

[0147] Table 2:

[0148] Charging SOC (%) Cumulative SOC (%) Negative electrode-reference potential (mV) 10 10 179.0 30 40 16.0 10 50 13.0 15 65 12.5 15 80 12.5 10 90 21.0 8 98 29.0

[0149] Depend on Figure 6 As can be seen from the results in Table 2 above, the three-electrode battery of the present invention can accurately verify the step charging method (multi-stage constant current charging) during charging, without affecting the lithium battery, and can easily determine whether there is a risk of lithium plating on the negative electrode, and provide a reference for subsequent charging method optimization.

[0150] Stability tests were performed on the three-electrode lithium-ion batteries prepared in Examples 1 to 3. This included connecting the positive electrode of an electrochemical testing device to the negative electrode tab 9 and the negative electrode to the reference electrode tab 11, and testing the stability of the negative electrode-reference electrode within a voltage protection range of approximately -0.5V to 3V, wherein the battery state of charge (SOC) was controlled between approximately 10% and approximately 25%. Exemplarily, the stability test results of the three-electrode lithium-ion battery prepared in Example 3 are as follows: Figure 7As shown, due to the self-discharge characteristics of lithium-ion batteries, the voltage fluctuation range remained within approximately 1 to 3 mV during the approximately 400-hour test period. Self-discharge is an inherent characteristic of lithium-ion batteries, influenced by factors such as the formation and decomposition of the solid electrolyte interphase (SEI) film and temperature. Therefore, during long-term testing, the potential of the negative electrode-reference electrode will rise to some extent. In this embodiment, the potential change of the negative electrode-reference electrode during the 400-hour test was approximately 3 mV, which also reflects the low self-discharge rate of the battery.

[0151] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions claimed by the present invention.

Claims

1. A lithium metal reference electrode for a three-electrode lithium-ion battery, characterized in that, The lithium metal reference electrode comprises: a lithium metal component, a wire, a first protective layer, and a second protective layer, wherein... The lithium metal component contains ≥ 98.0% by weight of elemental lithium; The second protective layer comprises a Li-IL@MOFs / polymer composite film formed by a metal-organic framework material adsorbed with a second lithium salt ionic liquid solution and a film-forming polymer, wherein the Li-IL@MOFs / polymer composite film encapsulates the lithium metal component, wherein the molar concentration of the second lithium salt ionic liquid solution used is in the range of 1 to 3 mol / L, the mass ratio of the metal-organic framework material to the second lithium salt ionic liquid solution is in the range of (1.4-1.6):1, and the mass ratio of the metal-organic framework material adsorbed with the second lithium salt ionic liquid solution to the film-forming polymer is in the range of (2-2.5):1; The first protective layer comprises a polymer solid electrolyte membrane formed of polyvinylidene fluoride and a first lithium salt, which encapsulates the lithium metal component and the second protective layer therein, wherein the mass ratio of polyvinylidene fluoride to the first lithium salt is in the range of (2.9-3.1):1; and One end of the wire is electrically connected to the lithium metal component, and the other end extends out to encapsulate the lithium metal component through the second protective layer and the first protective layer.

2. The lithium metal reference electrode according to claim 1, characterized in that, The lithium metal component contains ≥ 99.0% by weight of elemental lithium.

3. The lithium metal reference electrode according to claim 1, characterized in that, The lithium metal component contains ≥ 99.9% by weight of elemental lithium.

4. The lithium metal reference electrode according to claim 1, characterized in that, The lithium metal component is selected from lithium foil, lithium sheet, lithium plate or lithium mesh.

5. The lithium metal reference electrode according to claim 4, characterized in that, The thickness of the lithium metal component is in the range of 0.1 to 1 mm.

6. The lithium metal reference electrode according to claim 1, characterized in that, The conductor contains copper wire.

7. The lithium metal reference electrode according to claim 1, characterized in that, The thickness of the first protective layer is in the range of 0.1 to 0.5 mm.

8. The lithium metal reference electrode according to claim 1, characterized in that, The thickness of the second protective layer is in the range of 0.5 mm to 1 mm.

9. The lithium metal reference electrode according to claim 1, characterized in that, The first lithium salt and the second lithium salt each independently comprise one or more of lithium perchlorate, lithium tetrafluoroborate, lithium hexafluorophosphate, lithium bis(oxalate)borate, lithium difluorooxalateborate, lithium bis(difluorosulfonyl)imide, and lithium bis(trifluoromethylsulfonyl)imide.

10. The lithium metal reference electrode according to claim 9, characterized in that, The first lithium salt and the second lithium salt are the same and are selected from lithium hexafluorophosphate, lithium bis(difluorosulfonyl)imide or lithium bis(trifluoromethylsulfonyl)imide.

11. The lithium metal reference electrode according to any one of claims 1 to 10, characterized in that, The molecular weight of the polyvinylidene fluoride is in the range of 300,000 to 1,100,000 Daltons.

12. The lithium metal reference electrode according to any one of claims 1 to 10, characterized in that, The ionic liquid comprises one or more of 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, and 1-butyl-1-methylpyrrolidine bis(trifluoromethanesulfonyl)imide.

13. The lithium metal reference electrode according to any one of claims 1 to 10, characterized in that, The film-forming polymer comprises polyvinyl alcohol with a molecular weight in the range of 120,000 to 150,000 Daltons.

14. The lithium metal reference electrode according to any one of claims 1 to 10, characterized in that, The metal-organic framework material is prepared by reacting zinc nitrate and dimethylimidazole in methanol.

15. The lithium metal reference electrode according to any one of claims 1 to 10, characterized in that, During the formation of the Li-IL@MOFs / polymer composite film of the second protective layer, the molar concentration of the second lithium salt ionic liquid solution used is in the range of 1.5 to 2.5 mol / L.

16. A method for preparing a lithium metal reference electrode according to any one of claims 1 to 15, characterized in that, The method includes the following steps: Step 1: Provide the lithium metal component electrically connected to one end of the wire; Step 2: Providing a Li-IL@MOFs / polymer composite film as the second protective layer includes dissolving the second lithium salt in the ionic liquid to obtain a second lithium salt ionic liquid solution, mixing the second lithium salt ionic liquid solution with the metal-organic framework material to adsorb the second lithium salt ionic liquid solution into the metal-organic framework material, then mixing the metal-organic framework material adsorbed with the second lithium salt ionic liquid solution with the film-forming polymer and grinding it, and hot-pressing the obtained ground mixture into a film to form the Li-IL@MOFs / polymer composite film, wherein the size of the Li-IL@MOFs / polymer composite film is sufficient to encapsulate the lithium metal component; Step 3: Provide a polymer solid electrolyte membrane as the first protective layer, comprising dissolving the polyvinylidene fluoride and the first lithium salt in a volatile organic solvent to form a solution, and evaporating the organic solvent in the solution to form the polymer solid electrolyte membrane, wherein the polymer solid electrolyte membrane is large enough to encapsulate the lithium metal component encapsulated by the Li-IL@MOFs / polymer composite membrane; Step 4: The lithium metal component obtained from Step 1 is sequentially encapsulated with the Li-IL@MOFs / polymer composite film obtained from Step 2 and the polymer solid electrolyte film obtained from Step 3 to form the lithium metal reference electrode. The lithium metal reference electrode includes the polymer solid electrolyte film, the Li-IL@MOFs / polymer composite film and the lithium metal component in the order from the outside to the inside. The other end of the wire electrically connected to the lithium metal component extends out of the polymer solid electrolyte film and the Li-IL@MOFs / polymer composite film encapsulating the lithium metal component.

17. The method for preparing a lithium metal reference electrode according to claim 16, characterized in that, Step 4 includes: On the polymer solid electrolyte membrane obtained in step 3, the Li-IL@MOFs / polymer composite membrane obtained in step 2 and the lithium metal component obtained in step 1 are stacked in the order of "Li-IL@MOFs / polymer composite membrane", "lithium metal component" and "Li-IL@MOFs / polymer composite membrane". Then, a polymer solid electrolyte membrane as the first protective layer is further provided on the uppermost Li-IL@MOFs / polymer composite membrane to form a composite.

18. The method for preparing a lithium metal reference electrode according to claim 16, characterized in that, Step 4 includes: The polymer solid electrolyte membrane obtained from step 3, the Li-IL@MOFs / polymer composite membrane obtained from step 2, and the lithium metal component obtained from step 1 are stacked in the order of "polymer solid electrolyte membrane", "Li-IL@MOFs / polymer composite membrane", "lithium metal component", "Li-IL@MOFs / polymer composite membrane", and "polymer solid electrolyte membrane" to form an assembly; and The polymer solid electrolyte membrane and the Li-IL@MOFs / polymer in the encapsulation region of the obtained assembly are then fused together to complete the encapsulation, wherein the fusion is preferably performed by hot pressing at a temperature of 40 to 70°C and a pressure of 5 to 10 MPa for 15 to 30 seconds.

19. The method for preparing a lithium metal reference electrode according to claim 16, characterized in that, The adsorption process in step 2 includes maintaining a mixture containing the second lithium salt ionic liquid solution and the metal-organic framework material at a temperature of 100 to 120°C and a vacuum of -0.06 MPa to -0.08 MPa for 12 to 24 hours to allow the second lithium salt ionic liquid solution to adsorb into the metal-organic framework material.

20. The method for preparing a lithium metal reference electrode according to claim 16, characterized in that, The hot pressing film formation in step 2 involves holding the film at a temperature of 70 to 80°C and a pressure of 18 to 30 MPa for 2 to 5 minutes.

21. The method for preparing a lithium metal reference electrode according to any one of claims 16 to 20, characterized in that, The volatile organic solvent used in step 3 includes one or more of acetonitrile, dimethyl sulfoxide, N,N-dimethylformamide, and N-methylpyrrolidone.

22. The method for preparing a lithium metal reference electrode according to any one of claims 16 to 20, characterized in that, The formation of the polymer solid electrolyte membrane in step 3 includes film formation at a temperature of 40 to 60°C and a vacuum of -0.04 to -0.08 MPa.

23. The method for preparing a lithium metal reference electrode according to any one of claims 16 to 20, characterized in that, The solid content of the solution formed in step 3 is controlled within the range of 5 to 10% by weight.

24. A three-electrode lithium-ion battery comprising a cell, wherein the cell comprises a separator, a negative electrode, and a positive electrode, characterized in that, The three-electrode lithium-ion battery further comprises a lithium metal reference electrode according to any one of claims 1 to 15 or a lithium metal reference electrode prepared by any one of claims 16 to 23.

25. The three-electrode lithium-ion battery according to claim 24, characterized in that, The lithium metal reference electrode is placed on the outside of the cell and is closer to the negative electrode than the positive electrode.

26. The three-electrode lithium-ion battery according to claim 24 or 25, characterized in that, The size of the lithium metal reference electrode is smaller than the size of the negative electrode.

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