Modified metal negative electrode and preparation method therefor, and lithium metal battery

Abstract

The present application relates to the technical field of lithium metal batteries, and in particular relates to a modified metal negative electrode and a preparation method therefor, and a lithium metal battery. The modified metal negative electrode comprises: a lithium metal negative electrode and an in-situ polymer film, which covers the lithium metal negative electrode, and the in-situ polymer film and the lithium metal negative electrode are fixedly connected via a chemical bond of fluorine and lithium, wherein the molecular structure of the in-situ polymer film comprises at least two fluorine atoms, and the content of hydrogen in the molecular structure of the in-situ polymer film is less than or equal to 10%.

Description

Modified metal negative electrode, preparation method thereof and lithium metal battery

[0001] Cross-reference to related applications

[0002] The embodiments of the present application are based on and claim priority from Chinese Patent Application No. 202410629937.9 filed on May 21, 2024, the entire contents of which are incorporated herein by reference. TECHNICAL FIELD

[0003] The present application relates to the technical field of lithium metal batteries, and particularly relates to a modified metal negative electrode, a preparation method thereof and a lithium metal battery. BACKGROUND

[0004] Currently, high energy density and thermal safety of lithium ion batteries (LIBs) have become the two most important requirements in the development process of batteries, and lithium metal batteries have the characteristics of super-high energy density and extremely low potential in lithium ion batteries, which promote lithium metal batteries to become the first choice of high energy density battery negative electrodes. However, high energy density lithium metal batteries are often at the expense of battery safety, so how to improve the safety of lithium metal batteries has become a research hotspot at present. The current research shows that the metal lithium in the lithium metal battery has high reactivity, and the high-reactivity metal lithium will react with the electrolyte in the lithium metal battery to form lithium dendrite phenomenon, which is the culprit affecting the safety of the lithium metal battery, which hinders the further practical development of the metal lithium battery. Although there are many optimization strategies for the safety of lithium metal batteries in the current research, such as using safer solid-state electrolyte, non-flammable electrolyte and using artificial interface (SEI) to block the side reaction between electrolyte and lithium metal. However, from the practical point of view, using SEI to improve the stability of the electrode interface has become the preferred solution to improve the safety of lithium metal batteries.

[0005] The mainstream way of improving the stability of the electrode interface based on SEI at the present stage is to use a large amount of anionic polymer on the surface of the lithium metal negative electrode, but the thermal stability of the anionic polymer is poor, which is easy to separate between the lithium metal negative electrode during the normal operation of the lithium metal battery, thereby affecting the cycle efficiency and cycle performance of the lithium metal battery and other working parameters. SUMMARY

[0006] The present application provides a modified metal negative electrode, a preparation method thereof and a lithium metal battery, to solve the technical problem of how to improve the thermal stability of the polymer during the normal operation of the lithium metal battery.

[0007] In a first aspect, the application provides a modified metal negative electrode, comprising: a lithium metal negative electrode, and an in-situ polymer film covering the lithium metal negative electrode, the in-situ polymer film being fixedly connected with the lithium metal negative electrode through a chemical bond between fluorine and lithium.

[0008] The molecular structure of the in-situ polymer film contains at least two fluorine atoms, and the hydrogen content of the molecular structure of the in-situ polymer film is ≤10% in terms of mass fraction.

[0009] In a second aspect, the application provides a method for preparing the modified metal negative electrode of the first aspect, the method comprising:

[0010] Preparation of a polymer-containing raw material solution;

[0011] Immersion of a lithium metal negative electrode in the raw material solution and standing, so that the polymer of the raw material solution undergoes an in-situ polymerization reaction on the surface of the lithium metal negative electrode to generate an in-situ polymer film, thereby obtaining a modified metal negative electrode.

[0012] In a third aspect, the application provides a lithium metal battery, the lithium metal battery comprising a positive electrode sheet, a composite separator, and the modified metal negative electrode of the first aspect, the composite separator being arranged between the positive electrode sheet and the modified metal negative electrode, and the positive electrode sheet, the composite separator, and the modified metal negative electrode being fixedly overlapped through stacking.

[0013] The above technical solutions provided by the embodiments of the application have the following advantages compared with the prior art:

[0014] The modified metal negative electrode provided by the embodiment of the present application comprises: a lithium metal negative electrode, and an in-situ polymer film covering the lithium metal negative electrode, the in-situ polymer film being fixedly connected with the lithium metal negative electrode through a chemical bond of fluorine and lithium; wherein the molecular structure of the in-situ polymer film comprises at least two fluorine atoms, and the hydrogen content in the molecular structure of the in-situ polymer film is less than or equal to 10%. The molecular structure of the in-situ polymer film comprises at least two fluorine atoms, and the hydrogen content in the molecular structure of the in-situ polymer film is less than or equal to 10%, which can cause sufficient fluorine and little hydrogen in the polymer film, the little hydrogen can avoid hydrogen and fluorine from competitively reacting with lithium of the lithium metal negative electrode, thereby causing sufficient fluorine to react with lithium of the lithium metal negative electrode to generate sufficient lithium fluoride, the sufficient lithium fluoride can provide sufficient chemical bond sites, the sufficient chemical bond sites can fixedly connect the lithium metal negative electrode and the in-situ polymer film, thereby improving the bonding strength between the lithium metal negative electrode and the in-situ polymer film, in addition, the sufficient fluorine has good heat resistance, thereby improving the heat resistance of the in-situ polymer film, and further using the in-situ polymer film can improve the thermal stability of the lithium metal negative electrode, avoiding separation between the in-situ polymer film and the lithium metal negative electrode during normal operation of the lithium metal battery. BRIEF DESCRIPTION OF DRAWINGS

[0015] The accompanying drawings, which are incorporated herein and constitute part of the specification, illustrate embodiments consistent with the present application and, together with the description, serve to explain the principles of the present application.

[0016] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the accompanying drawings needed to be used in the embodiments or prior art description will be briefly introduced. Obviously, those skilled in the art can obtain other drawings according to these drawings without any creative effort.

[0017] FIG. 1 is a schematic diagram of a modified metal negative electrode provided by an embodiment of the present application;

[0018] FIG. 2 is a flowchart of a preparation method of a modified metal negative electrode provided by an embodiment of the present application;

[0019] FIG. 3 is a product schematic diagram of a lithium metal battery provided by an embodiment of the present application. Embodiments of the present application

[0020] In order to make the objectives, technical solutions and advantages of the embodiments of the present application clearer, the following will be combined with the accompanying drawings for the embodiments of the present application to make a clear and complete description of the technical solutions in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by a person of ordinary skill in the art without creative work fall within the protection scope of the present application.

[0021] Various embodiments of the present application can exist in the form of a range; it should be understood that the description in the form of a range is merely for the convenience and brevity, and should not be understood as a hard limit to the scope of the present application; therefore, it should be considered that the described range has been specifically disclosed all possible sub-ranges and single values in the range; for example, it should be considered that the range description from 1 to 6 has specifically disclosed sub-ranges, such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., as well as single numbers in the range, such as 1, 2, 3, 4, 5 and 6, which applies to any range; in addition, whenever a numerical range is indicated in this document, it refers to any cited number (fraction or integer) in the indicated range.

[0022] In this document, the term includes “comprises” and the like means “including but not limited to”. “At least one”, “at least one of the following (one)” or the like means any combination of these items, including single item (one) or any combination of multiple items; for example, “at least one of a, b or c”, or “at least one of a, b and c”, can represent a, b, c, a-b (i.e. a and b), a-c, b-c, or a-b-c, where a, b, c can be single or multiple. Unless otherwise specified, various raw materials, reagents, instruments and equipment used in the present application can be purchased from the market or prepared by existing methods.

[0023] It should be noted that for the prior art of “using a large amount of anion polymer on the surface of the lithium metal negative electrode” in the background art, the inventors have found that although boron nitride can be added to the anion polymer to improve the thermal stability of the lithium metal negative electrode, the price of boron nitride is high, and adding it to the anion polymer will increase the preparation cost of the lithium metal negative electrode, in addition, since boron nitride is generally in the form of particles, it will reduce the contact area between the anion polymer and the lithium metal negative electrode, thereby reducing the bonding strength between the anion polymer and the lithium metal negative electrode.

[0024] FIG. 1 exemplarily shows a schematic diagram of a modified metal negative electrode provided by an embodiment of the present application;

[0025] As shown in FIG. 1, the modified metal negative electrode provided by the embodiments of the present application comprises: a lithium metal negative electrode, and an in-situ polymer film covering the lithium metal negative electrode, the in-situ polymer film being fixedly connected with the lithium metal negative electrode through chemical bonds between fluorine and lithium.

[0026] In some optional embodiments, the molecular structure of the in-situ polymer film contains at least two fluorine atoms, and the hydrogen content of the molecular structure of the in-situ polymer film is ≤10% in terms of mass fraction.

[0027] In some optional embodiments, the molecular structure of the in-situ polymer film contains at least four fluorine atoms, and the hydrogen content of the molecular structure of the in-situ polymer film is ≤5% in terms of mass fraction.

[0028] In these embodiments, the molecular structure of the in-situ polymer film can contain at least four fluorine atoms, and the hydrogen content of the molecular structure of the in-situ polymer film can be ≤5%, which can further increase the fluorine content and reduce the hydrogen content in the polymer film, so as to further promote the reaction between fluorine and lithium of the lithium metal negative electrode to generate sufficient lithium fluoride, and the sufficient lithium fluoride can be used to fixedly connect the lithium metal negative electrode and the in-situ polymer film through chemical bonds, so as to further improve the bonding strength between the lithium metal negative electrode and the in-situ polymer film. In addition, high content of fluorine has good heat resistance, so as to improve the heat resistance of the in-situ polymer film, and thus the use of the in-situ polymer film can improve the thermal stability of the lithium metal negative electrode, and avoid the separation between the in-situ polymer film and the lithium metal negative electrode during the normal operation of the lithium metal battery.

[0029] In some optional embodiments, the raw material of the in-situ polymer film comprises ethyl difluoroacetate and / or methyl difluoroacetate.

[0030] In these embodiments, the raw material of the in-situ polymer film can comprise ethyl difluoroacetate and / or methyl difluoroacetate, which can promote the in-situ polymer film to contain no hydrogen while having a relatively high fluorine content, so as to further improve the heat resistance of the in-situ polymer film and the bonding strength between the in-situ polymer film and the lithium metal negative electrode.

[0031] In some optional embodiments, the raw material comprises ethyl difluoroacetate and methyl difluoroacetate, and the mass ratio of the ethyl difluoroacetate to the methyl difluoroacetate is 1:0.5-1.5.

[0032] In these embodiments, the raw material of the in-situ polymer film can comprise ethyl difluoroacetate and methyl difluoroacetate, and the mass ratio of the ethyl difluoroacetate to the methyl difluoroacetate can be 1:0.5-1.5, which can be used to form an in-situ polymer film with sufficient thickness on the surface of the lithium metal negative electrode and improve the fluorine content of the in-situ polymer film.

[0033] The mass ratio of ethyl difluoroacetate and methyl difluoroacetate can be 1:0.5, 1:1.0 or 1:1.5.

[0034] In some alternative embodiments, the in-situ polymer film has a thickness of 5-15 μm.

[0035] In these embodiments, the in-situ polymer film has a thickness of 5-15 μm, which can ensure that the in-situ polymer film has sufficient thickness, the in-situ polymer film with sufficient thickness has sufficient fluorine, the sufficient fluorine can react with lithium of the lithium metal negative electrode to generate sufficient lithium fluoride, the sufficient lithium fluoride can provide sufficient chemical bond sites, the sufficient chemical bond sites can fixedly connect the lithium metal negative electrode and the in-situ polymer film, thereby the bonding strength between the lithium metal negative electrode and the in-situ polymer film can be improved, in addition, the sufficient fluorine has good heat resistance, thereby the heat resistance of the in-situ polymer film can be improved, and further the in-situ polymer film can improve the thermal stability of the lithium metal negative electrode.

[0036] The in-situ polymer film has a thickness of 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm or 15 μm.

[0037] In some alternative embodiments, the material of the lithium metal negative electrode comprises a copper-lithium composite tape, and the copper-lithium composite tape has a thickness of 10-20 μm.

[0038] In these embodiments, the material of the lithium metal negative electrode comprises a copper-lithium composite tape, and the copper-lithium composite tape has a thickness of 10-20 μm, which can ensure that the copper-lithium composite tape has sufficient lithium, thereby the sufficient lithium can react with fluorine in the in-situ polymer film to generate sufficient lithium fluoride, the sufficient lithium fluoride can provide sufficient chemical bond sites, the sufficient chemical bond sites can fixedly connect the lithium metal negative electrode and the in-situ polymer film, thereby the bonding strength between the lithium metal negative electrode and the in-situ polymer film can be improved, in addition, the sufficient fluorine has good heat resistance, thereby the heat resistance of the in-situ polymer film can be improved, and further the in-situ polymer film can improve the thermal stability of the lithium metal negative electrode.

[0039] The copper-lithium composite tape has a thickness of 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, 16 μm, 17 μm, 18 μm, 19 μm or 20 μm.

[0040] FIG. 2 exemplarily shows a preparation method flowchart of a modified metal negative electrode according to an embodiment of the present application;

[0041] As shown in FIG. 2, based on a general inventive concept, the present application provides a method for preparing the modified metal negative electrode, which comprises the following steps:

[0042] S1. preparing a raw material solution containing a polymer;

[0043] S2. immersing a lithium metal negative electrode into the raw material solution and standing, so that the polymer in the raw material solution undergoes an in-situ polymerization reaction on the surface of the lithium metal negative electrode to generate an in-situ polymer film, thereby obtaining a modified metal negative electrode.

[0044] The preparation method is directed to the preparation method of the modified metal negative electrode. The specific structure of the modified metal negative electrode can refer to the above embodiments. Since the preparation method adopts part or all of the technical solutions of the above embodiments, it at least has all the beneficial effects brought by the technical solutions of the above embodiments, which will not be repeated here.

[0045] In some optional embodiments, the standing temperature is 25-65°C, and the standing time is 0.1-1h.

[0046] In these embodiments, the standing temperature can be 25-65°C, and the standing time can be 0.1-1h. The standing method can be used to promote the radical in-situ polymerization reaction of the polymer in the raw material solution on the surface of the lithium metal negative electrode and generate an in-situ polymer film with sufficient thickness, so that the in-situ polymer film can be used to improve the thermal stability of the lithium metal negative electrode.

[0047] The standing temperature can be 25°C, 30°C, 35°C, 40°C, 45°C, 50°C, 55°C, 60°C or 65°C.

[0048] The standing time can be 0.1h, 0.2h, 0.3h, 0.4h, 0.5h, 0.6h, 0.7h, 0.8h, 0.9h or 1.0h.

[0049] FIG. 3 exemplarily shows a product schematic diagram of a lithium metal battery provided by the embodiments of the present application;

[0050] Based on a general inventive concept, as shown in FIG. 3, the present application provides a lithium metal battery, which comprises a positive electrode sheet, a composite separator and the modified metal negative electrode. The composite separator is arranged between the positive electrode sheet and the modified metal negative electrode, and the positive electrode sheet, the composite separator and the modified metal negative electrode are fixed by overlapping.

[0051] It should be noted that the overlapping method is to overlap the positive electrode sheet, the composite separator and the modified metal negative electrode by Z-shaped lamination.

[0052] It should be noted that the lithium metal battery can be a negative electrode-free soft-pack lithium metal battery, which can avoid the problem of excessive thickness of the lithium negative electrode.

[0053] The lithium metal battery is realized based on the modified metal negative electrode, and the specific structure and composition of the modified metal negative electrode can refer to the above embodiments. Since the lithium metal battery adopts part or all of the technical solutions of the above embodiments, it at least has all the beneficial effects brought by the technical solutions of the above embodiments, which will not be repeated here.

[0054] In some optional embodiments, the raw materials of the positive electrode sheet include a positive electrode material, a conductive agent, a binder, and an organic solvent; wherein the positive electrode material includes at least one of the following:

[0055] Nickel-cobalt-manganese, nickel-cobalt-aluminum, lithium cobaltate, lithium iron phosphate, lithium manganate, and lithium-rich solid solution;

[0056] The conductive agent includes at least one of the following:

[0057] Carbon black conductive agent, special high-conductive graphite, single-walled carbon nanotubes, and multi-walled carbon nanotubes;

[0058] The binder includes at least one of the following:

[0059] Sodium carboxymethyl cellulose, butadiene rubber, and polyacrylic acid;

[0060] The organic solvent includes at least one of the following:

[0061] Ethanol, N-methyl pyrrolidone, polyvinyl pyrrolidone, polyvinyl alcohol, polyethylene glycol, and polyvinylidene fluoride.

[0062] In the embodiments of the present application, the specific types of positive electrode material, conductive agent, binder, and organic solvent in the positive electrode sheet can obtain a positive electrode sheet with good conductivity.

[0063] The present application will be further described in conjunction with specific examples. The experimental methods in the following examples are not specified, and the industry standards are generally measured; if there is no corresponding industry standard, the international standard, the general condition, or the condition suggested by the manufacturer is used.

[0064] Example 1

[0065] Based on the one-step solution immersion method, an in-situ polymer film is formed on the surface of the composite copper lithium belt, and the in-situ polymer film is used to realize the surface modification and modification of the lithium metal negative electrode. Lithium iron phosphate (LFP) is used as a positive electrode material, and a modified metal negative electrode (Poly-Li / Cu-0.3) is used as a negative electrode material to prepare an electrode sheet. The specific steps are as follows:

[0066] 1. Positive electrode sheet preparation process:

[0067] Lithium iron phosphate is selected as the positive electrode material, single-walled carbon nanotubes are selected as the conductive agent, sodium carboxymethyl cellulose is selected as the binder, and N-methyl pyrrolidone is selected as the solvent. A positive electrode sheet is obtained through a traditional preparation process.

[0068] 2. Negative electrode sheet preparation process:

[0069] (1) A copper-lithium composite tape with a thickness of 15 μm is used, which is designed in an integrated manner with lithium foil and copper current collector;

[0070] (2) The copper-lithium composite tape is added to a mixed solution of methyl difluoroacetate (MFA) and ethyl difluoroacetate (DFEA) with a mass ratio of 1:1, and is simply soaked and placed in a sealed bottle. The temperature for standing is 45°C, and the standing time is 0.3 h, so as to obtain a Poly-MFA-DFE-Li / Cu-0.3 electrode negative sheet.

[0071] 3. Preparation process of soft-pack lithium metal battery without negative electrode:

[0072] The prepared positive electrode sheet, composite separator and negative electrode sheet are stacked in a Z-shaped stacking manner, and then go through the processes of welding, liquid injection, pre-charging, two-sealing and formation, so as to obtain a 2 Ah soft-pack lithium metal battery without negative electrode (Poly-MFA-DFE-Li / Cu-0.3||LFP).

[0073] Example 2

[0074] 1. Positive electrode sheet preparation process:

[0075] Lithium iron phosphate is selected as the positive electrode material, single-walled carbon nanotubes are selected as the conductive agent, sodium carboxymethyl cellulose is selected as the binder, and N-methyl pyrrolidone is selected as the solvent. A positive electrode sheet is obtained through a traditional preparation process.

[0076] 2. Negative electrode sheet preparation process:

[0077] (1) A copper-lithium composite tape with a thickness of 15 μm is used, which is designed in an integrated manner with lithium foil and copper current collector;

[0078] (2) The copper-lithium composite tape is added to a mixed solution of methyl difluoroacetate (MFA) and ethyl difluoroacetate (DFEA) with a mass ratio of 1:1, and is simply soaked and placed in a sealed bottle. The temperature for standing is 45°C, and the standing time is 0.6 h, so as to obtain a Poly-MFA-DFE-Li / Cu-0.6 electrode negative sheet.

[0079] 3. Preparation process of soft-pack lithium metal battery without negative electrode:

[0080] The prepared positive electrode sheet, composite separator and negative electrode sheet are stacked in a Z-shaped manner, and then subjected to welding, liquid injection, pre-charging, two-sealing and formation processes to obtain a 2 Ah soft-pack lithium metal battery without negative electrode (Poly-MFA-DFE-Li / Cu-0.6||LFP).

[0081] Example 3

[0082] Comparing Example 3 and Example 1, the difference between Example 3 and Example 1 is that:

[0083] 1. Positive electrode sheet preparation process:

[0084] Lithium iron phosphate is selected as the positive electrode material, single-walled carbon nanotubes are selected as the conductive agent, carboxymethyl cellulose sodium is selected as the binder, and N-methyl pyrrolidone is selected as the solvent. The positive electrode sheet is obtained by a traditional preparation process.

[0085] 2. Negative electrode sheet preparation process:

[0086] (1) A copper-lithium composite tape with a thickness of 15 μm is used, which is designed with lithium foil and copper current collector in one body;

[0087] (2) The copper-lithium composite tape is added to a mixed solution of methyl difluoroacetate (MFA) and ethyl difluoroacetate (DFEA) with a mass ratio of 1:1, and is simply soaked in a sealed bottle. The temperature for standing is 45°C, and the standing time is 1.0 h, to obtain a Poly-MFA-DFE-Li / Cu-1.0 electrode negative electrode sheet.

[0088] 3. Preparation process of soft-pack lithium metal battery without negative electrode:

[0089] The prepared positive electrode sheet, composite separator and negative electrode sheet are stacked in a Z-shaped manner, and then subjected to welding, liquid injection, pre-charging, two-sealing and formation processes to obtain a 2 Ah soft-pack lithium metal battery without negative electrode (Poly-MFA-DFE-Li / Cu-1.0||LFP).

[0090] Example 4

[0091] Comparing Example 4 and Example 1, the difference between Example 4 and Example 1 is that:

[0092] The mass ratio of ethyl difluoroacetate to methyl difluoroacetate is 1:0.5.

[0093] Example 5

[0094] Comparing Example 5 and Example 1, the difference between Example 5 and Example 1 is that:

[0095] The mass ratio of ethyl difluoroacetate to methyl difluoroacetate is 1:1.5.

[0096] Example 6

[0097] Comparing Example 6 and Example 1, the difference between Example 5 and Example 1 is that:

[0098] The temperature for standing is 25℃.

[0099] Example 7

[0100] Comparing Example 7 and Example 1, the difference between Example 5 and Example 1 is that:

[0101] The temperature for standing is 65℃.

[0102] Comparative Example 1

[0103] Comparing Comparative Example 1 and Example 1, the difference between Comparative Example 1 and Example 1 is that:

[0104] 1. Positive electrode tab preparation process:

[0105] Lithium iron phosphate is selected as the positive electrode material, single-walled carbon nanotubes are selected as the conductive agent, carboxymethyl cellulose sodium is selected as the binder, and N-methyl pyrrolidone is selected as the solvent. The positive electrode tab is prepared by a traditional preparation process.

[0106] 2. Negative electrode tab preparation process:

[0107] (1) A copper-lithium composite tape with a thickness of 15 μm is used, which is designed with lithium foil and copper current collector integration. The copper-lithium composite tape is cut into shape according to the requirements of lithium batteries, and the shaped copper-lithium composite tape is used as the positive electrode tab;

[0108] 3. Preparation process of soft-pack lithium metal battery without negative electrode:

[0109] The prepared positive electrode tab, composite separator and negative electrode tab are stacked in a Z-shaped manner, and then subjected to welding, liquid injection, pre-charging, two-sealing and formation processes to obtain a 2 Ah soft-pack lithium metal battery without negative electrode (Li / Cu || LFP).

[0110] Comparative Example 2

[0111] Comparing Comparative Example 2 and Example 1, the difference between Comparative Example 2 and Example 1 is that:

[0112] The standing time is 2h.

[0113] Comparative Example 3

[0114] Comparing Comparative Example 3 and Example 1, the difference between Comparative Example 3 and Example 1 is that:

[0115] The standing temperature is 10℃.

[0116] Comparative Example 4

[0117] Comparative Example 4 and Example 1 are compared, and the difference between Comparative Example 4 and Example 1 is that:

[0118] The temperature for standing is 80°C.

[0119] Related experiments and effect data:

[0120] 1. The no-negative-plate soft-pack lithium metal battery obtained in each example and each comparative example is subjected to ARC test, to verify the thermal performance of the working battery. Three key temperatures should be noted during thermal runaway: T1: when the self-heating rate of the battery is greater than 0.02°C·min -1 , it represents the critical temperature of the thermal safety of the battery, when the temperature of the battery exceeds T1, the chemical reaction in the battery will automatically increase, so that the battery cannot be safely operated or stored; T2: when the battery produces heat violently and starts thermal runaway (self-heating rate > 60°C·min -1 ); the battery ignites within a few seconds after reaching T2, and the thermal runaway temperature is represented by the “ignition” temperature; T3: when the battery reaches the highest temperature during thermal runaway, the heat release rate of all reactions in the battery reaches the maximum value, and T3 reflects the power of thermal runaway of a single battery.

[0121] Table 1

[0122]

[0123] As can be seen from Table 1, the thermal safety critical temperature of the 2.0 Ah no-negative-plate soft-pack lithium metal battery formed by the modified metal negative electrode provided in the application is increased by 20°C to 30°C compared with Comparative Example 1.

[0124] 2. The no-negative-plate soft-pack lithium metal battery obtained in each example and each comparative example is subjected to 1C charge-discharge cycle test, and the results are shown in Table 2.

[0125] Table 2: 1C charge-discharge cycle test results of no-negative-plate soft-pack lithium metal batteries obtained in each example and each comparative example

[0126]

[0127] As can be seen from Table 2, in the 1C charge-discharge cycle test, when the standing time of simple soaking reaches 0.6 h, the obtained Poly-MFA-DFE-Li / Cu-0.6||LFP battery (Example 2) has higher capacity retention rate and cycle life than the Li / Cu||LFP battery (Comparative Example 1); in addition, compared with the Poly-MFA-DFE-Li / Cu-0.3||LFP battery and the Poly-MFA-DFE-Li / Cu-1.0||LFP battery, the Poly-MFA-DFE-Li / Cu-0.6||LFP battery has higher capacity retention rate and cycle life.

[0128] In summary, the modified metal negative electrode provided by the embodiments of the present application promotes in-situ polymerization of methyl difluoroacetate and ethyl difluoroacetate to form an in-situ polymer film on the surface of the lithium metal power of the traditional lithium metal battery through one-step solution treatment. Since the in-situ polymer film has a large amount of fluorine and does not contain hydrogen, the in-situ polymer film has high thermal stability, which can effectively reduce the exothermic reaction between the electrolyte and the negative electrode and the positive electrode. Experimental results show that the thermal safety critical temperature of the 2.0 Ah negative electrode-free soft package lithium metal battery formed using the modified metal negative electrode is increased by 20℃ to 30℃, and the "ignition" point temperature (i.e., the starting temperature of the thermal runaway of the battery) of the lithium metal battery is sharply increased from 240.0℃ to 338.0℃.

[0129] In addition, the modified metal negative electrode provided by the embodiments of the present application forms an in-situ polymer film by using methyl difluoroacetate and ethyl difluoroacetate to in-situ polymerize on the surface of the lithium metal negative electrode, which can construct a dense and firm artificial interface film (SEI film). The SEI film not only helps the fast and stable deposition and stripping of Li + , but also effectively inhibits the growth of lithium dendrites, thereby effectively improving the cycle efficiency of the modified lithium metal negative electrode and the cycle life of the lithium metal battery.

[0130] The above description is merely a specific implementation of the present application, which enables those skilled in the art to understand or implement the present application. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined in the present application can be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application will not be limited to these embodiments shown in the present application, but will conform to the widest scope consistent with the principles and novel features of the present application.

Claims

1. A modified metal negative electrode, comprising: A lithium metal negative electrode, and an in-situ polymer film covering the lithium metal negative electrode, the in-situ polymer film being fixedly connected with the lithium metal negative electrode through a chemical bond between fluorine and lithium; The molecular structure of the in-situ polymer film contains at least two fluorine atoms, and the hydrogen content of the molecular structure of the in-situ polymer film is ≤10% in terms of mass fraction.

2. The modified metal anode of claim 1, wherein, The molecular structure of the in-situ polymer film contains at least four fluorine atoms, and the hydrogen content of the molecular structure of the in-situ polymer film is ≤5% in terms of mass fraction.

3. The modified metal anode of claim 2, wherein, The raw material of the in-situ polymer film includes ethyl difluoroacetate and / or methyl difluoroacetate.

4. The modified metal anode of claim 3, wherein, The raw material includes ethyl difluoroacetate and methyl difluoroacetate, and the mass ratio of the ethyl difluoroacetate to the methyl difluoroacetate is 1:0.5-1.

5.

5. The modified metal anode of claim 1, wherein, The thickness of the in-situ polymer film is 5-15 μm.

6. The modified metal anode of claim 1, wherein, The lithium metal negative electrode includes a copper-lithium composite tape, and the thickness of the copper-lithium composite tape is 10-20 μm.

7. A method for preparing the modified metal negative electrode according to any one of claims 1-6, the method comprising: preparing a polymer-containing raw material solution; immersing a lithium metal negative electrode into the raw material solution and standing to allow the polymer of the raw material solution to undergo an in-situ polymerization reaction on the surface of the lithium metal negative electrode to generate an in-situ polymer film, thereby obtaining a modified metal negative electrode.

8. The method of claim 7, wherein, The standing temperature is 25-65 °C, and the standing time is 0.1-1 h.

9. A lithium metal battery, the lithium metal battery including a positive electrode sheet, a composite separator, and a modified metal negative electrode according to any one of claims 1-6, the composite separator being arranged between the positive electrode sheet and the modified metal negative electrode, and the positive electrode sheet, the composite separator, and the modified metal negative electrode being fixedly overlapped by stacking.

10. The lithium metal battery of claim 9, wherein, The raw material of the positive electrode sheet includes a positive electrode material, a conductive agent, a binder, and an organic solvent; and the positive electrode material includes at least one of: nickel-cobalt-manganese, nickel-cobalt-aluminum, lithium cobaltate, lithium iron phosphate, lithium manganate, and lithium-rich solid solution; the conductive agent includes at least one of: carbon black conductive agent, super-tight conductive graphite, single-walled carbon nanotube, and multi-walled carbon nanotube; the binder includes at least one of: sodium carboxymethyl cellulose, butadiene rubber, and polyacrylic acid; the organic solvent includes at least one of: ethanol, N-methyl pyrrolidone, polyvinylpyrrolidone, polyvinyl alcohol, polyethylene glycol, and polyvinylidene fluoride.

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