Lithium negative electrode inorganic protective layer as well as preparation method and application thereof

Abstract

The invention discloses a lithium negative electrode inorganic protective layer and a preparation method and application thereof, and belongs to the technical field of lithium metal batteries. According to the invention, lewis acid is dissolved in an organic solvent to form a uniform solution, and the uniform solution reacts with lithium metal by adopting a soaking, spin coating or spray coating method, so that the lithium metal negative electrode of which the oxide layer is removed and the surface is protected by an inorganic coating is obtained. The anion of the Lewis acid is tetrafluoroborate, has an empty p orbit, can be used as an acceptor of an electron pair, shows Lewis acidity, and reacts with lithium metal. And the reaction product can be uniformly and compactly attached to the surface of the negative electrode active material to form a layer of artificial SEI, so that the direct contact between the negative electrode active material and the electrolyte is prevented, the side reaction is inhibited, and the passivation of the lithium negative electrode is delayed.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium metal batteries, and particularly relates to an inorganic protective layer for a lithium negative electrode, a preparation method thereof, and an application thereof. Background Art

[0002] Traditional lithium-ion batteries have achieved excellent cycling performance through intercalation chemistry and played an important role in the development of new energy. However, the theoretical capacity of its graphite negative electrode is relatively low, which limits the further improvement of the battery energy density. Currently, the energy density of lithium-ion batteries has approached the theoretical limit. At the same time, the safety issues of lithium-ion batteries have gradually emerged. Therefore, searching for new power sources with high safety and high specific capacity has become a research hotspot at home and abroad.

[0003] The lithium metal negative electrode stores energy by reversibly plating and stripping metallic lithium, and its theoretical capacity (3860 mAh / g) is 10 times that of the graphite negative electrode (372 mAh / g). Therefore, lithium metal is considered an ideal negative electrode material for next-generation high-energy-density lithium batteries. However, during the charge and discharge process of lithium metal batteries, the lithium metal negative electrode still faces many challenges that need to be solved urgently. Lithium metal shows high reactivity towards most organic electrolytes, especially towards the currently most commonly used carbonate-based electrolytes. During the cycling of carbonate-based electrolytes, side reactions occur on the surface of the lithium negative electrode, leading to the formation of porous lithium dendrites and significant volume changes. These problems severely limit the cycle life and safety of lithium metal batteries (LMBs).

[0004] In order to maintain the interfacial stability of the lithium negative electrode and inhibit the occurrence of side reactions, establishing a stable solid electrolyte interphase (SEI) between the lithium metal and the electrolyte is an effective strategy. This can be achieved through various methods, such as applying artificial SEI coatings, ionic liquid electrolytes, and electrolyte additives, etc. However, when using carbonate-based electrolytes, side reactions still occur on the surface of the lithium negative electrode. In addition, the formation of an SEI rich in LiF components continuously consumes the fluorine-containing components in the electrolyte, resulting in an uneven SEI layer and reducing its ability to protect the lithium negative electrode. Finally, the natural passivation layer on the surface of the lithium negative electrode causes uneven lithium ion flux, thereby promoting the growth of lithium dendrites. Even with the most effective electrolyte design, these passivation layers cannot be completely removed from the lithium surface. Therefore, although significant progress has been made in the research on lithium metal negative electrode protection, constructing a multi-component artificial SEI with high lithium ion conductivity through simple surface chemistry methods and eliminating the natural passivation layer of the lithium negative electrode remains a challenge that needs to be solved urgently. Summary of the Invention

[0005] The purpose of this section is to outline 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, as well as in the abstract and title of the present application, to avoid obscuring the purpose of this section, the abstract, and the title, and such simplifications or omissions shall not be used to limit the scope of the present invention.

[0006] In view of the above and / or problems existing in the prior art, the present invention is proposed.

[0007] Therefore, an object of the present invention is to overcome the deficiencies in the prior art and provide a method for preparing an inorganic protective layer for a lithium anode.

[0008] To solve the above technical problems, the present invention provides the following technical solutions:

[0009] After dissolving a Lewis acid in an organic solvent to form a homogeneous solution, the solution is reacted with lithium metal by means of soaking, spin-coating or spraying and then washed with the organic solvent to obtain a lithium metal anode with the oxide layer removed and its surface protected by an inorganic coating;

[0010] Wherein, the volume ratio of the Lewis acid solution to the lithium metal is 2000 - 3000:1.

[0011] As a preferred embodiment of the method for preparing an inorganic protective layer for a lithium anode according to the present invention, wherein: the anion of the Lewis acid is tetrafluoroborate, which has an empty p orbital and can act as an acceptor of electron pairs, showing Lewis acidity and being able to react with lithium metal.

[0012] As a preferred embodiment of the method for preparing an inorganic protective layer for a lithium anode according to the present invention, wherein: the Lewis acid includes one of tributylphosphonium tetrafluoroborate, ammonium tetrafluorobutyrate, tetrabutylphosphonium tetrafluoroborate, and tetraethylphosphonium tetrafluoroborate.

[0013] As a preferred embodiment of the method for preparing an inorganic protective layer for a lithium anode according to the present invention, wherein: the positive charge center of the cation of the Lewis acid is connected to an alkyl group.

[0014] As a preferred embodiment of the method for preparing an inorganic protective layer for a lithium anode according to the present invention, wherein: the concentration of the Lewis acid solution is 0.5 - 1.5 mol / L.

[0015] As a preferred embodiment of the method for preparing an inorganic protective layer for a lithium anode according to the present invention, wherein: the organic solvent includes tetrahydrofuran.

[0016] As a preferred embodiment of the method for preparing an inorganic protective layer for a lithium anode according to the present invention, wherein: the reaction time is 5 - 10 min.

[0017] Another object of the present invention is to overcome the deficiencies in the prior art and provide an inorganic protective layer for a lithium negative electrode, and the thickness of the inorganic protective layer is 10-30 μm.

[0018] The third object of the present invention is to overcome the deficiencies in the prior art and provide an application of an inorganic protective layer for a lithium negative electrode in the preparation of a lithium metal battery.

[0019] The third object of the present invention is to overcome the deficiencies in the prior art and provide a lithium metal battery including an inorganic protective layer for a lithium negative electrode.

[0020] Advantages of the present invention:

[0021] (1) Since the anion of the Lewis acid is tetrafluoroborate, which has an empty p orbital and can act as an acceptor of electron pairs, showing Lewis acidity and reacting with lithium metal. The reaction product can uniformly and densely adhere to the surface of the negative electrode active material to form an artificial SEI, thereby preventing the direct contact between the negative electrode active material and the electrolyte, inhibiting the occurrence of side reactions during charge and discharge, and delaying the passivation of the lithium negative electrode.

[0022] (2) The multi-component inorganic protective layer prepared by the present invention is rich in inorganic substances such as LiF, has a high ionic conductivity and a low lithium deposition activation energy for lithium ions, can reduce the lithium deposition overpotential, enhance the lithium deposition kinetics, and improve the reversibility of lithium deposition, thereby significantly improving the cycle life of the battery.

[0023] (3) The present invention uses a Lewis acid to react with the lithium metal negative electrode, and the surface of the formed multi-component inorganic protective layer is flat and smooth, which can homogenize the lithium ion flux and promote the uniform deposition of lithium ions, thereby inhibiting the growth of lithium dendrites and improving the battery stability.

[0024] (4) Compared with the previous methods such as magnetron sputtering, the present invention forms an artificial SEI layer on the negative electrode by a simple and effective chemical method, with low energy consumption, low cost, and convenient and fast process, which is more conducive to large-scale production. Description of the drawings

[0025] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following will briefly introduce the drawings required for the description of the embodiments. Obviously, the following drawings are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained according to these drawings. Among them:

[0026] Figure 1 Photographs (a), scanning electron microscope images (b), scanning electron microscope images (c), and scanning electron microscope images (d) of the lithium negative electrode after being protected by the inorganic coating provided in Embodiment 1 of the present invention.

[0027] Figure 2 These are the long - cycle curves of the lithium / lithium symmetric battery after being protected by the inorganic coating provided in Example 1 of the present invention (a) and the long - cycle curves of the unprotected lithium / lithium symmetric battery provided in Comparative Example 1 (b).

[0028] Figure 3 These are the impedance curves of the lithium / lithium symmetric battery after being protected by the inorganic coating at different cycle numbers provided in Example 1 of the present invention (a) and the impedance curves of the unprotected lithium / lithium symmetric battery at different cycle numbers provided in Comparative Example 1 (b).

[0029] Figure 4 These are the surface scanning electron microscope images of the inorganic - coating - protected ones provided in Comparative Example 2 (a), Example 1 (b) and Comparative Example 3 (c) of the present invention.

[0030] Figure 5 These are the impedance curves of the lithium / lithium symmetric battery before and after protection provided in Example 2 of the present invention;

[0031] Figure 6 These are the scanning electron microscope images of the lithium negative electrode of the protected lithium / lithium symmetric battery before (a) and after (b) cycling provided in Example 3 of the present invention.

[0032] Figure 7 These are the scanning electron microscope images of the lithium negative electrode of the protected lithium / lithium symmetric battery before (a) and after (b) cycling provided in Example 4 of the present invention. Detailed implementation manners

[0033] To make the above - mentioned objects, features and advantages of the present invention more obvious and understandable, the following will describe the detailed implementation manners of the present invention in conjunction with the embodiments of the specification.

[0034] In the following description, many specific details are set forth to fully understand the present invention. However, the present invention can also be implemented in other ways different from those described here. Those skilled in the art can make similar extensions without departing from the connotation of the present invention. Therefore, the present invention is not limited by the specific embodiments disclosed below.

[0035] Secondly, the so - called "one embodiment" or "embodiment" herein refers to a specific feature, structure or characteristic that can be included in at least one implementation manner of the present invention. The phrase "in one embodiment" appearing in different places in this specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment that is mutually exclusive with other embodiments.

[0036] Unless otherwise specified, the raw materials used in the present invention are all commercially available.

[0037] The materials prepared in the embodiments and comparative examples of the present invention are tested for their electrochemical performance according to the following method:

[0038] Select an ester-based electrolyte and a PE separator. In a glove box filled with argon, assemble a symmetric battery using the lithium negative electrodes prepared in the examples and comparative examples as the negative electrode.

[0039] Example 1

[0040] This example provides a method for preparing an inorganic protective layer for a lithium negative electrode, specifically as follows:

[0041] In a glove box filled with argon, dissolve tributylphosphonium tetrafluoroborate (see formula 1) in tetrahydrofuran to prepare a tributylphosphonium tetrafluoroborate solution with a molar fraction of 1 mol / L.

[0042]

[0043] In the glove box, punch a lithium sheet into a circular piece with a diameter of 14 mm, and stick polytetrafluoroethylene tape on its back. Then, place the front side of the lithium sheet downward and immerse it in the prepared 1 mol / L tributylphosphonium tetrafluoroborate solution (the volume ratio of the Lewis acid solution to lithium metal is 2400:1). After reacting for 6 minutes, take out the lithium sheet, remove it from the tape, and wash the lithium sheet with tetrahydrofuran. After drying, obtain a lithium negative electrode protected by an inorganic coating A.

[0044] As Figure 1 shown in a, it can be seen that a uniform inorganic protective layer is coated on the surface of the lithium sheet. By observing the surface scanning electron microscope image of the lithium sheet after protection ( Figure 1 b), it is found that the protective layer is very smooth and uniformly covers the surface of the lithium sheet. Then, through Figure 1 the scanning electron microscope cross-sectional image in d, the thickness of the inorganic protective layer is measured to be approximately 30 μm.

[0045] Comparative Example 1

[0046] Use unprotected lithium metal as Comparative Example 1. Assemble a symmetric battery using the lithium negative electrode protected by the lithium-magnesium alloy coating prepared in Example 1 and unprotected lithium metal as the negative electrode. Specifically:

[0047] Dissolve 1 mol / L LiPF6 in a solution of DEC:EC = 1:1, and simultaneously add 10% by mass of FEC to prepare an electrolyte, and add molecular sieves to remove moisture;

[0048] Cut a PE film into a circular piece with a diameter of 16.5 mm on a cutting machine to make a separator, and dry it in a vacuum oven at 80 °C for 5 hours;

[0049] In a glove box filled with argon, assemble a symmetric battery with the electrolyte, separator, and lithium negative electrode protected by the lithium-magnesium alloy coating prepared in the example / comparative example. Drop 80 μl of electrolyte into each battery. At the same time, use an unprotected lithium sheet as the negative electrode to assemble a symmetric battery for comparison. The two assembled batteries are respectively at 1 mA / cm2 The charge and discharge cycles are carried out at a current density of , and the long cycle curve is as follows Figure 2 The results show that the unprotected lithium / lithium symmetric battery fails after 150 cycles, while the protected lithium / lithium symmetric battery can stably cycle for more than 1600 cycles, greatly extending the battery life.

[0050] Symmetrical cells were assembled in the same way, and the impedance of the cells was tested using an electrochemical workstation before cycling, after 1 cycle, 10 cycles, and 20 cycles. The test results are shown in Figure 2. Figure 3 As shown in the figure, the impedance of the protected battery is lower than that of the unprotected battery after charge and discharge cycles. The impedance test results confirm that the inorganic coating formed by the reaction of tri-n-butylphosphonium tetrafluoroborate with the lithium negative electrode can accelerate the transfer of lithium ions and charges, thereby promoting uniform deposition of lithium and faster interfacial reaction kinetics.

[0051] Comparative Example 2

[0052] The difference between this comparative example and Example 1 is that the concentration of the phenylmagnesium chloride solution spin-coated on the surface of the lithium sheet is adjusted to 0.1 mol / L, and the remaining steps and processes are all referred to Example 1 to obtain the lithium negative electrode protected by the inorganic coating of this comparative example.

[0053] Comparative Example 3

[0054] The difference between this comparative example and Example 1 is that the concentration of the phenylmagnesium chloride solution spin-coated on the surface of the lithium sheet is adjusted to 2 mol / L, and the remaining steps and processes are all referred to Example 1 to obtain the lithium negative electrode protected by the lithium inorganic layer of this comparative example.

[0055] The inorganic coating protection obtained in Example 1 and Comparative Examples 2 and 3 was observed by scanning electron microscopy. The results are as follows: Figure 4 As shown. After treatment with 0.1mol / L solution, there are still large uneven areas such as gullies and protrusions on the surface, which indicates that the reaction is not complete and the concentration of 0.1mol / L solution is too low. After treatment with 1mol / L solution, even when observed at a larger magnification, it can be clearly seen that the surface is smooth and flat with fewer defects. After treatment with 2mol / L solution, there are some crystalline gullies on the surface, which indicates that the reaction rate is too fast and the deposit forms crystals. The uneven surface will cause uneven lithium ion flux in the battery and aggravate dendrite growth. Therefore, 1mol / L is the optimal concentration.

[0056] Example 2

[0057] The difference between this embodiment and embodiment 1 is that the type of Lewis acid is adjusted to n-butylammonium tetrafluoroborate (see formula 2), and the rest of the preparation process is the same as that of embodiment 1, to obtain a lithium negative electrode protected by an inorganic coating B.

[0058]

[0059] Using an electrochemical workstation, the impedance of the battery after 10 cycles was measured (see Figure 5 ). The test results are as Figure 4 shown. The impedance of the protected battery is less than that of the unprotected battery after charge-discharge cycling.

[0060] Example 3

[0061] The difference between this example and Example 1 is that the type of Lewis acid was adjusted to tetrabutylphosphonium tetrafluoroborate (see Formula 3), and the rest of the preparation process was the same as that of Example 1, and a lithium negative electrode protected by an inorganic coating C was prepared.

[0062]

[0063] After 50 charge-discharge cycles at a current density of 1 mA / cm 2 , the battery was disassembled. The negative electrode before and after cycling was photographed by scanning electron microscopy ( Figure 6 ). By comparing the surface morphology of the negative electrode before and after cycling, it was found that the inorganic protective layer changed little before and after cycling, indicating its stability on the surface of the negative electrode.

[0064] Example 4

[0065] The difference between this example and Example 1 is that the type of Lewis acid was adjusted to tetraethylphosphonium tetrafluoroborate (see Formula 4), and the rest of the preparation process was the same as that of Example 1, and a lithium negative electrode protected by an inorganic coating D was prepared.

[0066]

[0067] After 50 charge-discharge cycles at a current density of 1 mA / cm 2 , the battery was disassembled. The negative electrode before and after cycling was photographed by scanning electron microscopy ( Figure 7 ). By comparing the surface morphology of the negative electrode before and after cycling, it was found that the inorganic protective layer changed little before and after cycling, indicating its stability on the surface of the negative electrode.

[0068] In summary, in the present invention, a Lewis acid is dissolved in an organic solvent to form a homogeneous solution, and then it is reacted with lithium metal by means of immersion, spin coating or spraying, and washed with the organic solvent, so as to obtain a lithium metal negative electrode with the oxide layer removed and the surface protected by an inorganic coating. Since the anion of the Lewis acid is tetrafluoroborate, which has an empty p orbital and can act as an acceptor of electron pairs, showing Lewis acidity and reacting with lithium metal. The reaction product can uniformly and densely adhere to the surface of the negative electrode active material to form an artificial SEI, thereby preventing the direct contact between the negative electrode active material and the electrolyte, inhibiting the occurrence of side reactions, and delaying the passivation of the lithium negative electrode. In addition, the generated multi-component inorganic protection layer has a high ionic conductivity and a low lithium deposition activation energy for lithium ions, can reduce the lithium deposition overpotential, enhance the lithium deposition kinetics, and improve the reversibility of lithium deposition.

[0069] 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 preferred embodiments, those of ordinary skill in the art should understand that the technical solutions of the present invention can be modified or equivalently replaced without departing from the spirit and scope of the technical solutions of the present invention, and they should all be covered within the scope of the claims of the present invention.

Claims

1. A method for preparing an inorganic protective layer for a lithium negative electrode, characterized in that: include, The Lewis acid is dissolved in an organic solvent to form a uniform solution, and then reacted with lithium metal by immersion, spin coating or spraying, and then washed with an organic solvent to obtain a lithium metal negative electrode with the oxide layer removed and the surface protected by an inorganic coating; Wherein, the volume ratio of the Lewis acid solution to the lithium metal is 2000-3000:

1.

2. The method for preparing the inorganic protective layer for lithium negative electrode according to claim 1, characterized in that: The anion of the Lewis acid is tetrafluoroborate, which has an empty p orbital, can serve as an acceptor of electron pairs, exhibits Lewis acidity, and can react with lithium metal.

3. The method for preparing the inorganic protective layer for lithium negative electrode according to claim 2, characterized in that: The Lewis acid includes one of tri-n-butylphosphonium tetrafluoroborate, n-butylammonium tetrafluoroborate, tetrabutylphosphonium tetrafluoroborate, and tetraethylphosphonium tetrafluoroborate.

4. The method for preparing the inorganic protective layer for lithium negative electrode according to claim 3, characterized in that: The positive center of the cation of the Lewis acid is connected to the alkyl group.

5. The method for preparing the inorganic protective layer for lithium negative electrode according to claim 1, characterized in that: The concentration of the Lewis acid solution is 0.5-1.5 mol / L.

6. The method for preparing the inorganic protective layer for lithium negative electrode according to claim 1, characterized in that: The organic solvent includes tetrahydrofuran.

7. The method for preparing the inorganic protective layer for lithium negative electrode according to claim 1, characterized in that: The reaction time is 5 to 10 minutes.

8. An inorganic protective layer for lithium negative electrode prepared by the method of any one of claims 1 to 7, characterized in that: The thickness of the inorganic protective layer is 10 to 30 μm.

9. Use of the inorganic protective layer for lithium negative electrode as claimed in claim 8 in the preparation of lithium metal batteries.

10. A lithium metal battery, characterized in that: It includes the lithium negative electrode inorganic protective layer as claimed in claim 9.

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