Preparation method of lithium metal surface high-temperature-resistant protective layer, lithium metal and application of lithium metal

Abstract

The invention discloses a preparation method of a high-temperature-resistant protective layer on the surface of lithium metal, the lithium metal and application of the lithium metal, lithium salt, polymer monomers and phosphate ester are mixed, heated and stirred according to the mass ratio of 1: (0.5-2): (3-10), and a homogeneous solution is formed; and coating the surface of a lithium metal base material pretreated by fluoroether with the homogeneous solution, and standing to form a high-temperature-resistant protective layer on the surface of the lithium metal, the polymer protective layer is designed from the angle of intermolecular interaction, and the obtained protective layer can realize self-repairing in the battery, so that the service life of the battery is prolonged, and the service life of the battery is prolonged. The reaction consumption of the electrolyte on the surface of the electrode in the battery cycle process is relieved, the interface impedance of the electrode / electrolyte is greatly reduced, the migration of lithium ions is facilitated, and compared with unmodified lithium metal, the modified lithium metal has more excellent cycle stability and high-temperature performance.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-ion batteries, and in particular relates to a method for preparing a high-temperature resistant protective layer on the surface of a lithium metal, lithium metal and applications thereof. Background Art

[0002] Lithium-ion batteries have been widely used in new energy vehicles, energy storage base stations, digital electronic devices and other fields. With the development of society, people's demand for lithium-ion batteries has grown rapidly, and the development of higher-performance lithium-ion batteries has become more urgent. The energy density of graphite, the traditional negative electrode material for lithium-ion batteries, has approached the theoretical limit (372mAh / g), while lithium metal has an extremely high theoretical specific capacity (3860mAh / g) and is considered to be one of the ideal negative electrode materials for the next generation of high-energy-density batteries. However, lithium metal negative electrodes face many challenges in practical applications, mainly including the following aspects:

[0003] Lithium metal itself is highly reactive and requires a strict water-free and oxygen-free environment during production and storage. The high cost of this part hinders its large-scale application. It is also prone to side reactions with electrolytes in batteries, forming a thermodynamically unstable interface, which is not conducive to the uniform deposition of lithium ions. The side reactions are more intense in high-temperature environments, and the battery performance will be further deteriorated.

[0004] The volume of lithium metal changes greatly during the charge and discharge process of the battery, which will change the thickness of the electrode and the internal pressure, resulting in uneven current distribution at the interface, causing lithium dendrite growth, increased interface impedance, and reduced battery cycle performance. In severe cases, it will cause internal short circuits in the battery, causing safety problems and increasing the risk of thermal runaway of the battery in high-temperature environments.

[0005] To solve the above problems, there are two main solutions:

[0006] The first type is to construct an artificial interface layer on the surface of lithium metal, which includes three methods: electrochemical construction, chemical treatment, and physical coating.

[0007] The second category is electrolyte modification, which specifically includes changing the solvent composition and introducing film-forming additives.

[0008] The first method is simple and direct, but the inactive interface usually causes the interface impedance to increase, the battery capacity to decrease, and the cost is high; the second method is low in cost, but the resulting interface protection effect is limited and will gradually fail over time. Therefore, it is extremely important to rationally design and construct an effective electrolyte / lithium metal interface layer. Summary of the Invention

[0009] The purpose of this section is to summarize some aspects of the embodiments of the present invention and briefly introduce some preferred embodiments. Some simplifications or omissions may be made in this section and the abstract and title of this application to avoid obscuring the purpose of this section, the abstract and the title of the invention, and such simplifications or omissions should not be used to limit the scope of the present invention.

[0010] In view of the above problems and / or the problems existing in the prior art, the present invention is proposed.

[0011] Therefore, the purpose of the present invention is to overcome the deficiencies in the prior art and provide a method for preparing a high-temperature resistant protective layer on the surface of lithium metal.

[0012] In order to solve the above technical problems, the present invention provides the following technical solutions: comprising:

[0013] The lithium salt, polymer monomer, and phosphate are mixed, heated, and stirred in a mass ratio of 1:0.5 to 2:3 to 10 to form a homogeneous solution;

[0014] The homogeneous solution is applied to the surface of the lithium metal substrate pretreated with fluoroether, and a high-temperature resistant protective layer is formed on the surface of the lithium metal after standing.

[0015] Wherein, the polymer monomer includes one of polypropylene, polycarbonate and polyvinylidene fluoride, and the number average molecular weight is 2000 to 3000000.

[0016] As a preferred embodiment of the method for preparing the high-temperature resistant protective layer on the surface of lithium metal according to the present invention, the lithium salt comprises one of lithium hexafluorophosphate, lithium difluorooxalatoborate, and lithium bis(trifluoromethylsulfonyl)imide, and the concentration of the lithium salt in the homogeneous solution is 1 to 2.5 mol / L.

[0017] As a preferred embodiment of the method for preparing the high-temperature resistant protective layer on the surface of lithium metal according to the present invention, the phosphate ester includes one of trimethyl phosphate, triethyl phosphate, triphenyl phosphate, tris(2-chloroethyl) phosphate, and diphenyl octyl phosphate.

[0018] As a preferred embodiment of the method for preparing the high-temperature resistant protective layer on the surface of lithium metal according to the present invention, the homogeneous solution further includes an additive, and the additive includes one or more of ethylene carbonate, fluoroethylene carbonate, and vinyl sulfate.

[0019] As a preferred solution of the method for preparing the high-temperature resistant protective layer on the surface of lithium metal according to the present invention, the amount of the additive added is 0 to 8 wt % compared to the lithium salt, polymer monomer, and phosphate ester.

[0020] As a preferred embodiment of the method for preparing the high-temperature resistant protective layer on the surface of lithium metal according to the present invention, the rotation speed of the mixing, heating and stirring is 600 to 2000 r / min, and the temperature is 60 to 120°C.

[0021] As a preferred embodiment of the method for preparing the high-temperature resistant protective layer on the surface of lithium metal according to the present invention, the coating method includes dipping, brushing or spraying.

[0022] As a preferred embodiment of the method for preparing the high-temperature resistant protective layer on the surface of lithium metal according to the present invention, 0 to 0.5% of the mass of the substrate is added to the surface of the lithium metal, and the substrate is allowed to stand for 2 to 8 hours under vacuum conditions of ≤90 kPa and 10 to 30°C to obtain a lithium metal substrate pretreated with fluoroether.

[0023] As a preferred embodiment of the method for preparing the high-temperature resistant protective layer on the surface of lithium metal according to the present invention, the standing time is 1 to 4 hours.

[0024] Another object of the present invention is to provide a lithium metal having a surface high-temperature resistant protective layer, wherein the lithium metal has a porous polymer with a thickness of 0.5 to 50 μm as the protective layer.

[0025] Another object of the present invention is to provide an application of lithium metal as a negative electrode of a lithium battery or a lithium ion battery.

[0026] Another object of the present invention is to provide a lithium-ion battery using lithium metal having a surface high-temperature resistant protective layer as the negative electrode.

[0027] Beneficial effects of the present invention:

[0028] (1) The present invention provides a lithium metal protective layer that can be directly attached to a lithium metal negative electrode treated with fluoroether. It is easy to use. The porous polymer protective layer can isolate oxygen and moisture in the air, reduce side reactions, extend storage time, improve safety and reduce use costs.

[0029] (2) The present invention designs a polymer protective layer from the perspective of intermolecular interaction. The polymer layer and the electrolyte components can regulate the thermodynamic stability and kinetic properties of the interface through interaction. The resulting protective layer can achieve self-repair inside the battery, alleviate the reaction consumption of the electrolyte on the electrode surface during the battery cycle, greatly reduce the electrode / electrolyte interface impedance, and facilitate the migration of lithium ions. Compared with unmodified lithium metal, it has better cycle stability and high-temperature performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for describing the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. Those skilled in the art can also derive other drawings based on these drawings without inventive effort. Among them:

[0031] Figure 1 This is a microscopic image of lithium metal with a protective layer after storage at 60° C. for 30 days in Example 1 of the present invention.

[0032] Figure 2 This is a microscopic image of lithium metal without a protective layer in Comparative Example 1 of the present invention after being stored at 60° C. for 30 days.

[0033] Figure 3 The cycle performance at high temperature (60° C.) of the lithium ion batteries assembled with lithium metal in Examples 1 to 3 of the present invention and Comparative Examples 1 to 3 is shown. DETAILED DESCRIPTION

[0034] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the specific implementation methods of the present invention are described in detail below in conjunction with the embodiments of the specification.

[0035] In the following description, many specific details are set forth to facilitate a full understanding of the present invention. However, the present invention may also be implemented in other ways different from those described herein. Those skilled in the art may make similar generalizations without violating the connotation of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0036] Secondly, the term "one embodiment" or "embodiment" herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The phrase "in one embodiment" appearing in various places throughout this specification does not necessarily refer to the same embodiment, nor does it refer to a separate or selective embodiment that is mutually exclusive of other embodiments.

[0037] Unless otherwise specified, the raw materials used in the present invention are commonly available in the market.

[0038] The present invention performs a performance test on the electrolyte by assembling a lithium-ion battery according to the following method:

[0039] A lithium metal battery was made using the nickel-cobalt-containing lithium manganese oxide ternary cathode material NCM 622 as the positive electrode, a polymer solid electrolyte (LiDFOB, PVDF, and DEGDME were composited in a molar ratio of 1:2.5:4), and lithium metal as the negative electrode.

[0040] Specific test conditions:

[0041] Normal temperature test:

[0042] At room temperature (25°C), the battery was charged and discharged three times at a constant current of 0.2C (1C = 200mAh / g) in the voltage range of 2.8V to 4.3V. The subsequent long cycles were all charged and discharged at a constant current and voltage of 0.5C / discharge of 0.5C.

[0043] High temperature test:

[0044] The pre-cycle was carried out at a high temperature of 60°C with a constant current of 0.2C for three times, and the subsequent long cycles were all carried out with a constant current, constant voltage, charging and constant current discharge at 0.5C.

[0045] The capacity retention rate of the battery after 100 cycles was calculated, wherein the capacity retention rate (%) of the battery at the Nth cycle = the Nth discharge capacity / the 4th discharge capacity × 100%.

[0046] Example 1

[0047] This embodiment provides a method for preparing a high-temperature resistant protective layer on the surface of lithium metal, specifically:

[0048] 1) Add 0.5% of the substrate mass of fluoroether to the lithium surface, and let it stand for 5 hours under vacuum conditions of 90 kPa and 20° C. to obtain pretreated lithium metal.

[0049] 2) Lithium difluorooxalatoborate (LiDFOB), polyvinylidene fluoride (PVDF) with a molecular weight of 400,000, and trimethyl phosphate (TMP) were mixed in a container at a mass ratio of 1:1.2:6, and heated at 80°C at 900 r / min with stirring until a homogeneous solution was formed. At this point, the lithium salt concentration in the homogeneous solution was 2.35 mol / L;

[0050] 3) The homogeneous solution is applied to the pretreated lithium metal surface and allowed to stand at room temperature for 2 hours to obtain the lithium metal with a high temperature resistant protective layer formed in this embodiment.

[0051] Figure 1 This is a microscopic image of lithium metal with a protective layer in this example after being stored at 60°C for 30 days.

[0052] Comparative Example 1

[0053] This comparative example uses lithium metal without a high-temperature resistant protective layer as a comparison.

[0054] Figure 2 This is a microscopic image of lithium metal without a protective layer in this comparative example after being stored at 60°C for 30 days.

[0055] contrast Figure 1 、 Figure 2 It can be seen that the protective layer of the present invention has a good protective effect on lithium metal.

[0056] Comparative Example 2

[0057] The difference between this comparative example and Example 1 is that trimethyl phosphate (TMP) is omitted in step 2), and the remaining steps and processes are referred to Example 1 to obtain the protective layer of this comparative example.

[0058] Comparative Example 3

[0059] The difference between this comparative example and Example 1 is that in step 2), trimethyl phosphate (TMP) is replaced by ethylene carbonate (EC), and the remaining steps and processes are the same as those in Example 1 to obtain the protective layer of this comparative example.

[0060] The lithium metals obtained in Example 1 and Comparative Examples 1 to 3 were assembled into batteries and subjected to high-temperature cycle performance tests. The results are shown in Table 1.

[0061] Table 1 60℃ high temperature cycle test results

[0062] Discharge capacity after the 4th cycle 100th cycle capacity Capacity retention after 100 cycles Example 1 <![CDATA[168.28mAhg -1 ]]> <![CDATA[137.27mAhg -1 ]]> 81.57％ Comparative Example 1 <![CDATA[169.66mAhg -1 ]]> <![CDATA[36.74mAhg -1 ]]> 21.66％ Comparative Example 2 <![CDATA[113.74mAhg -1 ]]> <![CDATA[30.2mAhg -1 ]]> 26.55％ Comparative Example 3 <![CDATA[151.24mAhg -1 ]]> <![CDATA[72.81mAhg -1 ]]> 48.14％

[0063] As can be seen in Table 1, the overall performance of the protective film in Example 1 is significantly superior to that of Comparative Examples 1, 2, and 3. This demonstrates that the present invention achieves superior cycling stability and high-temperature performance compared to the unmodified protective film by introducing an organic solvent to regulate interactions. The protective layer formed in Comparative Example 2 is unable to regulate intermolecular interactions, providing some protection in the early stages of cycling, but failing in the later stages, resulting in severe capacity degradation. While the protective layer formed in Comparative Example 3 can regulate intermolecular interactions and improve cycling performance, its impedance is too high, resulting in a lower capacity at high temperatures than that of Example 1.

[0064] Comparative Example 4

[0065] The difference between this comparative example and Example 1 is that polyvinylidene fluoride (PVDF) is omitted in step 2), and the remaining steps and processes are the same as those in Example 1. As a result, no polymer protective layer can be formed.

[0066] Comparative Example 5

[0067] The difference between this comparative example and Example 1 is that lithium difluorooxalatoborate (LiDFOB) is omitted in step 2), and the remaining steps and processes are the same as those in Example 1. As a result, the formed polymer layer has low ionic conductivity and too large impedance, and cannot provide effective protection.

[0068] It can be seen from the results of Comparative Examples 4 and 5 that not any combination of components can form an effective polymer protective layer.

[0069] Example 2

[0070] This embodiment introduces propylene carbonate (PC) as an additive based on embodiment 1. Specifically:

[0071] 1) Adding 0.5% of the mass of the substrate to the lithium surface with fluoroether, and leaving it to stand for 5 hours under vacuum degree ≤ 90 kPa and 20° C. to obtain pretreated lithium metal.

[0072] 2) lithium difluorooxalatoborate (LiDFOB), polyvinylidene fluoride (PVDF), and trimethyl phosphate (TMP) are mixed in a container at a mass ratio of 1:1.2:6 to obtain a mixed solution I;

[0073] Propylene carbonate (PC) was added thereto at a mass ratio of 5% relative to the mass of the mixed solution I, and the mixture was heated and stirred at 80°C at a speed of 900 r / min until a homogeneous solution was formed. At this time, the lithium salt concentration in the homogeneous solution was 2.35 mol / L;

[0074] 3) The homogeneous solution is applied to the pretreated lithium metal surface and allowed to stand at room temperature for 2 hours to obtain the lithium metal with a high temperature resistant protective layer formed in this embodiment.

[0075] Example 3

[0076] This embodiment introduces the additive fluoroethylene carbonate (FEC) on the basis of embodiment 1, specifically:

[0077] 1) Adding 0.5% of the mass of the substrate to the lithium surface with fluoroether, and leaving it to stand for 5 hours under vacuum degree ≤ 90 kPa and 20° C. to obtain pretreated lithium metal.

[0078] 2) lithium difluorooxalatoborate (LiDFOB), polyvinylidene fluoride (PVDF), and trimethyl phosphate (TMP) are mixed in a container at a mass ratio of 1:1.2:6 to obtain a mixed solution I;

[0079] Add 5% by mass of fluoroethylene carbonate (FEC) to the mixed solution I, and heat and stir at 80°C at 900 r / min until a homogeneous solution is formed. At this time, the lithium salt concentration in the homogeneous solution is 2.35 mol / L;

[0080] 3) The homogeneous solution is applied to the pretreated lithium metal surface and allowed to stand at room temperature for 2 hours to obtain the lithium metal with a high temperature resistant protective layer formed in this embodiment.

[0081] The lithium metals obtained in Examples 2 and 3 were assembled into batteries and subjected to high-temperature cycle performance tests. The results were compared with those in Example 1. The results are shown in Table 2.

[0082] Table 2 60℃ high temperature cycle test results

[0083] Discharge capacity after the 4th cycle 100th cycle capacity Capacity retention after 100 cycles Example 1 <![CDATA[168.28mAhg -1 ]]> <![CDATA[137.27mAhg -1 ]]> 81.57％ Example 2 <![CDATA[166.91mAhg -1 ]]> <![CDATA[143.57mAhg -1 ]]> 86.02％ Example 3 <![CDATA[168.01mAhg -1 ]]> <![CDATA[147.98mAhg -1 ]]> 88.07％

[0084] It can be seen that the comprehensive performance of the electrolyte prepared in the embodiment of the present invention in the voltage range of 2.8V to 4.3V is significantly better than that of the comparative example, indicating that the protective layer of the present invention can significantly reduce the side reactions at the interface between lithium metal and the electrolyte at high temperature, thereby improving the battery cycle performance; in addition, different additives are introduced in Examples 2 and 3 of the present invention, respectively, which correspondingly improve the discharge capacity or cycle performance. This is because the introduction of different additives will produce different interactions with other components of the electrolyte, thereby affecting the self-repair function of the protective film and further affecting the battery performance.

[0085] Comparative Example 6

[0086] The difference between this comparative example and Example 1 is that the heating temperature in step 2) is 60° C., resulting in no polymer coating being formed and no protection for the lithium metal.

[0087] Comparative Example 7

[0088] The difference between this comparative example and Example 1 is that the heating temperature in step 2) is 60° C., resulting in no polymer coating being formed and no protection for the lithium metal.

[0089] Example 4

[0090] This embodiment differs from embodiment 1 in that the concentrations of lithium difluorooxalatoborate (LiDFOB) in step 2) are adjusted to 1, 2, 2.5, and 3 mol / L, respectively, and the remaining steps and processes are similar to those in embodiment 1, thereby obtaining lithium metal in a high-temperature resistant protective layer formed with different concentrations of lithium difluorooxalatoborate in this embodiment.

[0091] The lithium metal of this embodiment was assembled into a battery and subjected to a high temperature cycle performance test and compared with that of Example 1. The results are shown in Table 3.

[0092] Table 3

[0093] Lithium salt concentration Discharge capacity after the 4th cycle Discharge capacity at the 50th cycle Capacity retention after 50 cycles 1mol / L <![CDATA[162.87mAhg -1 ]]> 136.13mAhg-1 83.58％ 2mol / L <![CDATA[168.28mAhg -1 ]]> 141.47mAhg-1 84.07％ 2.35mol / L <![CDATA[168.28mAhg -1 ]]> 144.92mAhg-1 86.02％ 2.5mol / L <![CDATA[167.5mAhg -1 ]]> 142.32mAhg-1 84.97％ 3mol / L <![CDATA[160.4mAhg -1 ]]> 127.95mAhg-1 79.77％

[0094] As can be seen from Table 3, different lithium salt concentrations affect the effect of the protective layer. Too high or too low a lithium salt concentration will result in low capacity. As the lithium salt concentration increases, the capacity retention rate first increases and then decreases. Compared with Comparative Examples 6-1, 2, 3, and 4, the comprehensive performance of Example 1 is significantly better, indicating that Example 1 of the present invention successfully optimizes the cycle performance improvement effect by regulating the interaction between the lithium salt and the organic solvent.

[0095] In summary, the present invention provides a lithium metal high-temperature protective layer, which forms a self-repairing protective layer by optimizing organic solvents. Compared with the unmodified protective layer, it has better cycle stability and high-temperature performance, and can better meet the market demand for multifunctional batteries.

[0096] The present invention designs a lithium metal protective layer from the perspective of intermolecular interactions, and adjusts the kinetic characteristics and thermodynamic stability of the electrolyte through the interaction between additives and other components of the electrolyte. The resulting protective layer can achieve self-repair inside the battery, greatly reducing the interface impedance of the electrode / electrolyte, being more conducive to the migration of lithium ions, and significantly improving the high-temperature performance of the battery.

[0097] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the spirit and scope of the technical solutions of the present invention, which should all be included in the scope of the claims of the present invention.

Claims

1. A method for preparing a high-temperature resistant protective layer on the surface of lithium metal, characterized by: include, The lithium salt, polymer monomer, and phosphate are mixed, heated, and stirred in a mass ratio of 1:0.5 to 2:3 to 10 to form a homogeneous solution; The homogeneous solution is applied to the surface of the lithium metal substrate pretreated with fluoroether, and a high-temperature resistant protective layer is formed on the surface of the lithium metal after standing. Wherein, the polymer monomer includes one of polypropylene, polycarbonate and polyvinylidene fluoride, and the number average molecular weight is 2000 to 3000000.

2. The method for preparing a high-temperature resistant protective layer on a lithium metal surface according to claim 1, wherein: The lithium salt includes one of lithium hexafluorophosphate, lithium difluorooxalatoborate, and lithium bis(trifluoromethylsulfonyl)imide, and the concentration of the lithium salt in the homogeneous solution is 1-2.5 mol / L.

3. The method for preparing a high temperature resistant protective layer on a lithium metal surface according to claim 2, wherein: The phosphate ester includes one of trimethyl phosphate, triethyl phosphate, triphenyl phosphate, tris(2-chloroethyl) phosphate, and diphenyl octyl phosphate.

4. The method for preparing a high temperature resistant protective layer on a lithium metal surface according to claim 3, wherein: The homogeneous solution also includes additives, which include one or more of ethylene carbonate, fluoroethylene carbonate, and vinyl sulfate. The amount of the additives added is 0 to 8 wt % compared to the lithium salt, polymer monomer, and phosphate.

5. The method for preparing a high temperature resistant protective layer on a lithium metal surface according to claim 1, wherein: The rotation speed of the mixing, heating and stirring is 600-2000 r / min, and the temperature is 60-120°C.

6. The method for preparing a high temperature resistant protective layer on a lithium metal surface according to claim 1, wherein: Adding 0-0.5% of the mass of the substrate to the surface of the lithium metal is performed, and the substrate is allowed to stand for 2-8 hours under the conditions of vacuum degree ≤90kPa and 10-30°C to obtain a lithium metal substrate pretreated with the fluoroether.

7. The method for preparing a high temperature resistant protective layer on a lithium metal surface according to claim 1, wherein: The standing time is 1 to 4 hours.

8. The lithium metal prepared by the preparation method according to any one of claims 1 to 7, characterized in that: The lithium metal has a porous polymer with a thickness of 0.5 to 50 μm as a protective layer.

9. Use of the lithium metal as claimed in claim 8 as a negative electrode of a lithium battery or a lithium ion battery.

10. A lithium-ion battery, characterized in that: The lithium metal according to claim 8 is used as the negative electrode.

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