Lithium metal negative electrode and preparation method and application thereof

Abstract

The invention discloses a lithium metal negative electrode and a preparation method and application thereof.A Grignard reagent with the molecular formula being RMgX is dissolved in an organic solvent to form a uniform solution, X in the molecular formula RMgX represents chlorine or bromine, R represents an organic substituent and comprises one of methyl, ethyl, propyl, n-butyl or phenyl, the solution reacts with lithium metal in a spin coating or spraying mode, and the lithium metal negative electrode is obtained. And the lithium metal negative electrode with the oxide layer removed and the surface protected by the lithium-magnesium alloy coating is obtained through cleaning with an organic solvent, and the preparation method is simple and efficient. The obtained lithium-magnesium alloy protective layer can effectively prevent direct contact between a lithium negative electrode and electrolyte, and side reaction in the charging and discharging process is inhibited. Besides, compared with a pure lithium negative electrode, the lithium-magnesium alloy protective layer with the double-layer structure has higher strength and can inhibit SEI fracture caused by volume expansion of the lithium negative electrode in the charging and discharging process, so that generation of dendritic crystals and loss of electrolyte are inhibited, the reversibility of lithium deposition is improved, and the cycle life of the battery is further remarkably prolonged.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium metal negative electrodes, and in particular relates to a lithium metal negative electrode and a preparation method and application thereof. Background Art

[0002] Advanced energy storage technology is an important support for modern life. At present, there are many types of energy storage technologies, with significant differences in energy and power density, service life, efficiency, cost, etc. Among them, battery technology has achieved great success due to its low cost and convenience. Traditional lithium-ion batteries have achieved excellent cycle performance through intercalation chemistry and have played an important role in the field of new energy. However, the actual energy density of commercial lithium-ion batteries is currently increasing slowly and is close to the theoretical limit of its positive / negative electrode materials, especially graphite negative electrodes. Therefore, the development of advanced electrode materials to achieve higher energy density is crucial to meet the needs of emerging high-end electronic devices.

[0003] Lithium metal is considered to be an ideal negative electrode material and has received widespread attention due to its ultra-high theoretical capacity (3860mAh / g) and lowest electrochemical negative potential (-3.040Vvs. standard hydrogen electrode). However, lithium metal still faces many challenges during repeated charge and discharge. Dendrite growth during lithium plating / stripping leads to instability at the lithium metal / liquid electrolyte interface, and dendrite short circuiting may cause thermal runaway and even lead to safety hazards such as battery fire. These problems severely limit the cycle life and safety of lithium metal batteries, hindering their application in the commercial market.

[0004] In order to maintain the interfacial stability of the lithium negative electrode and inhibit side reactions, it is an effective strategy to construct a stable solid electrolyte interphase (SEI) between lithium metal and the electrolyte. At present, this strategy can be achieved through a variety of methods, such as the application of artificial SEI coatings, ionic liquid electrolytes, and LiX alloy strategies. However, the volume change of the lithium metal negative electrode during the charge and discharge process is significant, which will cause the SEI layer to rupture, and the electrolyte will re-contact with the lithium metal, triggering serious side reactions, consuming the components in the negative electrode and the electrolyte, and finally forming dendrites and dead lithium, leading to battery failure. Therefore, it is very important to construct a negative electrode protective layer that is stable in long cycles and can promote uniform deposition of lithium. However, most of the previous negative electrode protective layers have complex preparation processes and high energy consumption, making it difficult to achieve large-scale industrial production. Therefore, it is still a challenge to be solved to construct a negative electrode protective layer that is stable in long cycles through simple and low-cost surface chemical methods. 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. 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, but 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 a lithium metal negative electrode.

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

[0009] A Grignard reagent with the molecular formula RMgX is dissolved in an organic solvent to form a homogeneous solution;

[0010] The solution reacts with lithium metal by spin coating or spraying, and after being washed with the organic solvent, a lithium metal negative electrode with the oxide layer removed and its surface protected by a lithium-magnesium alloy coating is obtained;

[0011] Wherein, in the molecular formula RMgX, X represents chlorine or bromine, and R represents an organic substituent, including one of methyl, ethyl, propyl, n-butyl or phenyl.

[0012] As a preferred embodiment of the method for preparing a lithium metal negative electrode according to the present invention, wherein: the concentration of the Grignard reagent in the homogeneous solution is 0.5 - 1 mol / L.

[0013] As a preferred embodiment of the method for preparing a lithium metal negative electrode according to the present invention, wherein: the Grignard reagent includes one of n-butylmagnesium chloride, butylmagnesium bromide, or phenylmagnesium chloride.

[0014] As a preferred embodiment of the method for preparing a lithium metal negative electrode according to the present invention, wherein: the reaction time of the solution with lithium metal by spin coating or spraying is 20 - 30 min.

[0015] As a preferred embodiment of the method for preparing a lithium metal negative electrode according to the present invention, wherein: the preparation of the lithium metal negative electrode is carried out in a glove box filled with argon.

[0016] Another object of the present invention is to provide a lithium metal negative electrode.

[0017] To solve the above technical problems, the present invention provides the following technical solutions: including a lithium-magnesium alloy protective layer with a thickness of 30-40 μm attached to the surface of the lithium metal negative electrode. The lithium-magnesium alloy protective layer is obtained by reacting a Grignard reagent with the formula RMgX with lithium metal, where X in the formula RMgX represents chlorine or bromine, and R represents an organic substituent, including one of methyl, ethyl, propyl, n-butyl or phenyl.

[0018] As a preferred embodiment of the preparation method of the lithium metal negative electrode of the present invention, wherein: the components of the lithium-magnesium alloy protective layer include lithium-magnesium alloy, lithium chloride, and lithium fluoride, and the content ratio is about 2:3:5.

[0019] Another object of the present invention is to provide an application of a lithium metal negative electrode in the preparation of a lithium metal battery.

[0020] Another object of the present invention is to provide a lithium metal battery including a lithium metal negative electrode.

[0021] Advantages of the present invention:

[0022] The lithium negative electrode with a lithium-magnesium alloy protective layer described in the present invention has a simple and efficient preparation method. The lithium-magnesium alloy protective layer can effectively prevent the direct contact between the lithium negative electrode and the electrolyte and inhibit side reactions during charge and discharge. In addition, the lithium-magnesium alloy protective layer with a double-layer structure has higher strength than a pure lithium negative electrode, can inhibit the rupture of the SEI caused by the volume expansion of the lithium negative electrode during charge and discharge, thereby inhibiting the generation of dendrites and the loss of electrolyte, improving the reversibility of lithium deposition, and significantly improving the cycle life of the battery. Description of the drawings

[0023] 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 also be obtained based on these drawings. Among them:

[0024] Figure 1 It is a photo of the lithium negative electrode after being protected by the lithium-magnesium alloy coating provided in Example 1 of the present invention;

[0025] Figure 2 It is a scanning electron microscope photo of the cross-section of the lithium negative electrode after being protected by the lithium-magnesium alloy coating provided in Example 1 of the present invention;

[0026] Figure 3 It is an XRD curve of the lithium-magnesium alloy coating provided in Example 1 of the present invention;

[0027] Figure 4 It is an XPS curve of the lithium-magnesium alloy coating provided in Example 1 of the present invention;

[0028] Figure 5 Scanning electron microscope photographs of the cross-section of the lithium negative electrode after being protected by the lithium-magnesium alloy coating provided in Comparative Example 1 and Comparative Example 2 of the present invention;

[0029] Figure 6 Long cycle curves of the symmetrical cells assembled with the lithium-magnesium alloy coatings provided in Example 1 and Comparative Examples 1-3 of the present invention;

[0030] Figure 7 Scanning electron microscope photographs of the surface and cross-section of the lithium negative electrode after being protected by the lithium-magnesium alloy coating provided in Example 2 of the present invention;

[0031] Figure 8 Long cycle curve of the Li / Li symmetrical cell before being protected by the lithium-magnesium alloy coating provided in Example 2 of the present invention;

[0032] Figure 9 Long cycle curve of the Li / Li symmetrical cell after being protected by the lithium-magnesium alloy coating provided in Example 2 of the present invention;

[0033] Figure 10 Long cycle curves of the Li / Li symmetrical cell before and after being protected by the lithium-magnesium alloy coating provided in Example 3 of the present invention;

[0034] Figure 11 Impedance curve diagrams of the Li / Li symmetrical cells after being protected by inorganic coatings with different concentration treatments provided in Example 3 and Comparative Examples 4 and 5 of the present invention. Detailed implementation manners

[0035] To make the above 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.

[0036] 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 herein. 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.

[0037] 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 appearances of "in one embodiment" in different places in this specification do not all refer to the same embodiment, nor are they separate or selectively exclusive embodiments from other embodiments.

[0038] The raw materials used in the present invention are all commonly commercially available in the art without special instructions.

[0039] Example 1

[0040] This embodiment provides a method for preparing a lithium-magnesium alloy protective layer on a lithium metal anode. Specifically:

[0041] 1) In a glove box filled with argon, dissolve n-butylmagnesium chloride with the structural formula shown in Formula 1 in tetrahydrofuran to prepare an n-butylmagnesium chloride solution with a molar fraction of 0.5 mol / L.

[0042]

[0043] 2) In the glove box, punch the lithium sheet into a circular piece with a diameter of 14 mm, place it on a spin coater, spin coat the n-butylmagnesium chloride solution on the surface of the lithium sheet, and react for 20 min. At this time, the surface of the lithium sheet is completely covered by the reaction layer, and the metallic luster of lithium cannot be observed. After the reaction, wash the lithium sheet with tetrahydrofuran, and dry it to obtain a lithium anode protected by a lithium-magnesium alloy coating A (see Figure 1 ).

[0044] As Figure 1 shown, it can be seen that a lithium-magnesium alloy protective layer is evenly coated on the surface of the lithium sheet. By observing the surface scanning electron micrograph of the lithium sheet after protection ( Figure 2 ), it is found that the protective layer evenly and densely covers the surface of the lithium sheet, with a uniform thickness of about 40 μm.

[0045] Perform XRD testing on the protected lithium sheet, and the results are as Figure 3 shown. It can be seen that the upper layer of the protective layer is mainly LiCl. Through XPS testing, the results are as Figure 4 shown. The lower layer is the alloy phase of the lithium-magnesium alloy. Therefore, this protective layer is a homogeneous mixed phase of LiCl and LiMg alloy, and the protective layer presents a layered structure. After elemental analysis, the content ratios of lithium-magnesium alloy, lithium chloride, and lithium fluoride are 2:3:5. This double-layer lithium-magnesium alloy protective layer can prevent the direct contact between the negative electrode active material and the electrolyte, and inhibit the side reactions on the surface of the negative electrode. At the same time, compared with the pure lithium anode, this lithium-magnesium alloy protective layer has higher strength, can inhibit the rupture of SEI caused by the infinite volume expansion of the lithium anode during charge and discharge, thereby inhibiting the generation of dendrites and the loss of electrolyte, improving the reversibility of lithium deposition, and thus improving the cycle life of the battery.

[0046] Comparative Example 1

[0047] The difference between this comparative example and Example 1 is that the reaction time for spin coating the n-butylmagnesium chloride solution on the surface of the lithium sheet in step 2) is adjusted to 10 min, and the process of the remaining steps refers to Example 1 to obtain the lithium anode protected by the lithium-magnesium alloy coating of this comparative example.

[0048] Comparative Example 2

[0049] The difference between this comparative example and Example 1 is that the reaction time for spin-coating the n-butylmagnesium chloride solution on the surface of the lithium sheet in step 2) was adjusted to 30 min, and the processes of the remaining steps were all referred to Example 1 to obtain the lithium negative electrode protected by the lithium-magnesium alloy coating of this comparative example.

[0050] Figure 5 a is the cross-sectional scanning electron microscope image of the lithium negative electrode protected by the lithium-magnesium alloy coating prepared in Comparative Example 1. It can be seen that its protective layer is too thin and the protective effect on the lithium metal negative electrode is poor. Figure 5 b is the cross-sectional scanning electron microscope image of the lithium negative electrode protected by the lithium-magnesium alloy coating prepared in Comparative Example 2. The thickness of the protective layer is uneven and there are defects on the surface, while Figure 2 the protective layer prepared in Example 1 is dense, uniform in thickness, and has few defects. Therefore, the reaction time of 20 minutes is the best.

[0051] Comparative Example 3

[0052] The difference between this comparative example and Example 1 is that the reaction time for spin-coating the n-butylmagnesium chloride solution on the surface of the lithium sheet in step 2) was adjusted to 1 min, and the processes of the remaining steps were all referred to Example 1 to obtain the lithium negative electrode protected by the lithium-magnesium alloy coating of this comparative example.

[0053] The lithium negative electrodes protected by the lithium-magnesium alloy coatings prepared in Example 1 and Comparative Examples 1 to 3 were used as the negative electrodes to assemble symmetric cells. Specifically:

[0054] 1 mol / L LiPF6 was dissolved in a solution of DEC:EC = 1:1, and at the same time, 10% by mass of FEC was added to prepare an electrolyte solution, and molecular sieves were added to remove moisture;

[0055] The PE film was cut into a circular piece with a diameter of 16.5 mm on a cutting machine to make a separator, and it was dried in a vacuum oven at 80 °C for 5 hours;

[0056] In a glove box filled with argon, the electrolyte, the separator, and the lithium negative electrode protected by the lithium-magnesium alloy coating prepared in the example / comparative example were assembled into a symmetric cell. 80 μl of the electrolyte was added dropwise to each cell. At the same time, an unprotected lithium sheet was used as the negative electrode to assemble a symmetric cell for comparison. The two assembled cells were charged and discharged cycled at a current density of 1 mA / cm 2 and a charge-discharge capacity of 0.5 mAh / cm 2 . The results are as Figure 6 shown. The cells assembled with treatment times of 1 minute, 10 minutes, and 30 minutes were cycled for 170 hours, 190 hours, and 350 hours respectively, while the cells treated for 20 minutes were cycled for more than 1500 hours, greatly improving the cycle stability, indicating that the effect is the best after treatment for 20 minutes.

[0057] Example 2

[0058] The difference between this embodiment and Embodiment 1 lies in that n-butylmagnesium chloride with the structural formula shown in Formula 2 is replaced by butylmagnesium bromide with the structural formula shown in Formula 2;

[0059]

[0060] For the remaining steps, the process is referred to Embodiment 1, and the lithium negative electrode protected by the lithium-magnesium alloy coating B of this embodiment is obtained.

[0061] As Figure 7 shown, Figure 7 a is a scanning electron microscope photograph of the surface of the lithium negative electrode after being protected by the lithium-magnesium alloy coating, Figure 7 b is a scanning electron microscope photograph of the cross-section. It can be seen that a layer of lithium-magnesium alloy protective layer is evenly coated on the surface of the lithium sheet. By observing the scanning electron microscope image of the surface of the lithium sheet after protection, it is found that the protective layer evenly and densely covers the surface of the lithium sheet with a uniform thickness of about 40 μm.

[0062] Using the lithium negative electrode protected by the lithium-magnesium alloy coating B prepared in this embodiment as the negative electrode, a symmetric battery is assembled according to the above method. At the same time, an unprotected lithium sheet is used as the negative electrode to assemble a symmetric battery for comparison.

[0063] The two assembled batteries are respectively charged and discharged cycled at a current density of 1 mA / cm 2 and a charge-discharge capacity of 0.5 mAh / cm 2 . The long cycle curves of the lithium metal negative electrode before and after protection are respectively as Figure 8 and Figure 9 shown. Compared with the battery assembled with the unprotected lithium negative electrode, the symmetric battery assembled with the lithium sheet protected by the lithium-magnesium alloy protective layer has a lower polarization voltage, only 0.01 V, a smoother curve, and the cycle time increases by 1500 hours. This indicates that the lithium-magnesium alloy protective layer has a strong ability to adapt to high-throughput lithium-ion transport and has a significant protective effect on the lithium negative electrode.

[0064] Embodiment 3

[0065] The difference between this embodiment and Embodiment 1 lies in that n-butylmagnesium chloride with the structural formula shown in Formula 3 is replaced by phenylmagnesium chloride with the structural formula shown in Formula 3;

[0066]

[0067] For the remaining steps, the process is referred to Embodiment 1, and the lithium negative electrode protected by the lithium-magnesium alloy coating C of this embodiment is obtained.

[0068] Using the lithium negative electrode protected by the lithium-magnesium alloy coating C prepared in this embodiment as the negative electrode, a symmetric battery is assembled according to the above method. At the same time, an unprotected lithium sheet is used as the negative electrode to assemble a symmetric battery for comparison.

[0069] The two assembled batteries were respectively charged and discharged in the voltage range of 2.8 V to 4.3 V at a charging rate of 0.5 C. The long cycle curves before and after protection are respectively as Figure 10 shown. The capacity retention rate of the unprotected control group was only 50% after 200 cycles, while the long cycle curve of the experimental group battery was smoother and the capacity retention rate was higher, reaching 78%, improving the cycle stability of the full battery.

[0070] Comparative Example 4

[0071] The difference between this comparative example and Example 3 is that the concentration of the phenylmagnesium chloride solution spin-coated on the surface of the lithium sheet was adjusted to 0.1 mol / L, and the rest of the process steps were all referred to Example 3 to obtain the lithium negative electrode protected by the lithium-magnesium alloy coating in this comparative example.

[0072] Comparative Example 5

[0073] The difference between this comparative example and Example 3 is that the concentration of the phenylmagnesium chloride solution spin-coated on the surface of the lithium sheet was adjusted to 1 mol / L, and the rest of the process steps were all referred to Example 3 to obtain the lithium negative electrode protected by the lithium-magnesium alloy coating in this comparative example.

[0074] The lithium negative electrodes protected by the lithium-magnesium alloy coatings prepared in Comparative Examples 4 and 5 were used as the negative electrodes to assemble symmetric batteries according to the above method, and at the same time, the unprotected lithium sheet was used as the negative electrode to assemble a symmetric battery for comparison.

[0075] The impedance of the battery was tested with an electrochemical workstation. The test results are as Figure 11 shown. The impedances of the unprotected lithium negative electrode and the lithium negative electrode treated with the 0.1 mol / L solution were both large, indicating that the reaction between the 0.1 mol / L solution and the lithium sheet was too slow to form an effective lithium-magnesium alloy protective layer. The impedance of the lithium sheet treated with the 1 mol / L solution was larger than that of the lithium sheet treated with the 0.5 mol / L solution, indicating that the concentration of the 1 mol / L solution was too high, the reaction was too violent, and the lithium-magnesium alloy protective layer was too thick. The impedance test results confirmed that the lithium sheet treated with the 0.5 mol / L solution had the smallest impedance and the best effect of the lithium-magnesium alloy protective layer.

[0076] In summary, the lithium negative electrode with a lithium-magnesium alloy protective layer described in the present invention has a simple and efficient preparation method. The lithium-magnesium alloy protective layer can effectively prevent the direct contact between the lithium negative electrode and the electrolyte and inhibit side reactions during charge and discharge. In addition, the double-layer lithium-magnesium alloy protective layer has higher strength than the pure lithium negative electrode, can inhibit the rupture of the SEI caused by the volume expansion of the lithium negative electrode during charge and discharge, thereby inhibiting the generation of dendrites and the loss of electrolyte, improving the reversibility of lithium deposition, and further significantly improving the cycle life of the battery.

[0077] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than 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 by the scope of the claims of the present invention.

Claims

1. A method for preparing a lithium metal negative electrode, characterized in that: include, The Grignard reagent with the molecular formula RMgX is dissolved in an organic solvent to form a uniform solution; The solution reacts with lithium metal by spin coating or spray coating, and is washed with an organic solvent to obtain a lithium metal negative electrode with the oxide layer removed and the surface protected by a lithium-magnesium alloy coating; In the molecular formula RMgX, X represents chlorine or bromine, and R represents an organic substituent, including one of methyl, ethyl, propyl, n-butyl or phenyl.

2. The method for preparing a lithium metal negative electrode according to claim 1, characterized in that: The organic solvent is an ether solvent, including tetrahydrofuran.

3. The method for preparing a lithium metal negative electrode according to claim 1, characterized in that: The concentration of the Grignard reagent in the uniform solution is 0.5-1 mol / L.

4. The method for preparing a lithium metal negative electrode according to claim 1, characterized in that: The Grignard reagent includes one of n-butylmagnesium chloride, butylmagnesium bromide and phenylmagnesium chloride.

5. The method for preparing a lithium metal negative electrode according to claim 1, characterized in that: The solution reacts with lithium metal by spin coating or spray coating for 20 to 30 minutes.

6. The method for preparing a lithium metal negative electrode according to any one of claims 1 to 5, characterized in that: The preparation of lithium metal anode was carried out in a glove box filled with argon.

7. A lithium metal negative electrode, characterized in that: A lithium-magnesium alloy protective layer with a thickness of 30 to 40 μm is attached to the surface of the lithium metal negative electrode. The lithium-magnesium alloy protective layer is obtained by reacting a Grignard reagent with a molecular formula of RMgX with lithium metal, wherein X in the molecular formula RMgX represents chlorine or bromine, and R represents an organic substituent, including one of methyl, ethyl, propyl, n-butyl or phenyl.

8. The lithium metal negative electrode according to claim 7, characterized in that: The components of the lithium-magnesium alloy protective layer include lithium-magnesium alloy, lithium chloride and lithium fluoride.

9. Use of the lithium metal negative electrode as claimed in claim 7 in the preparation of a lithium metal battery.

10. A lithium metal battery, characterized in that: The negative electrode is the lithium metal negative electrode as claimed in claim 6.

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