Lithium metal negative electrode, preparation method thereof and secondary battery

Abstract

The invention belongs to the technical field of batteries, and particularly relates to a lithium metal negative electrode, a preparation method thereof and a secondary battery. The preparation method of the lithium metal negative electrode comprises the following steps: heating a mixed solution of soluble transition metal salt, vanillin and a carbon material under a weakly alkaline condition, and separating to obtain the carbon material loaded with the soluble transition metal salt; uniformly mixing the carbon material loaded with the soluble transition metal salt with a binder and an organic solvent, coating the surface of a negative electrode current collector with the mixture, and drying the mixture to obtain a substrate material; and depositing metal lithium on the surface of the substrate material to obtain the lithium metal negative electrode. According to the lithium metal negative electrode, a uniform and compact lithium-loving layer can be effectively formed on the current collector, and metal lithium is induced to be uniformly deposited on the copper foil, so that the dendritic crystal growth problem of lithium metal is solved, and the stability of the lithium metal as the negative electrode is improved.

Description

Technical Field

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

[0002] In recent years, in order to improve the development and utilization efficiency of traditional energy such as fossil fuels, it is necessary to change the energy structure and make it develop towards diversification and cleanness. Therefore, the development and utilization of clean energy has become an inevitable choice, such as wind energy, solar energy, geothermal energy, tidal energy, biomass energy, etc. However, due to the characteristics of these energy sources such as intermittent time and uneven geographical distribution, energy storage devices are required to store excess energy and release it on demand. Electrochemical energy storage devices represented by rechargeable batteries are considered to be one of the most important strategies to solve energy problems due to their advantages such as high energy conversion efficiency, environmental friendliness and ease of use. Among them, lithium-ion batteries have attracted great attention from academia and industry worldwide due to their advantages such as high rate, long life and low cost. However, with the leapfrog development of modern science and technology, higher requirements have been put forward for various technical indicators of batteries, and the energy density of lithium-ion batteries with carbon-based materials as negative electrodes can only reach ~250Wh Kg -1 The current level can no longer meet the requirements of various fields for high energy density energy storage systems. Therefore, it is urgent to develop a new generation of rechargeable batteries to meet and support the long-term development of related industries.

[0003] Lithium metal anode has a very high theoretical specific capacity (3860 mAh g -1 ) and the lowest redox potential (-3.04Vvs.SHE), making it an ideal negative electrode material for the next generation of secondary batteries. However, the metal lithium negative electrode has serious problems of dendrite growth and volume expansion during the battery cycle, which limits the further widespread application of lithium metal batteries.

[0004] Therefore, solving the problem of dendrite growth of lithium metal and improving its stability as a negative electrode are important issues in the current research of lithium battery systems. Summary of the invention

[0005] The present invention aims to solve the above-mentioned problems and provides a lithium metal negative electrode, a preparation method thereof and a secondary battery. The lithium metal negative electrode can effectively form a uniform and dense lithium-philic layer on the current collector, induce the uniform deposition of metallic lithium on the copper foil, thereby solving the problem of dendrite growth of lithium metal and improving its stability as a negative electrode.

[0006] According to the technical solution of the present invention, the method for preparing the lithium metal negative electrode comprises the following steps:

[0007] S1: Under weak alkaline conditions, a mixed solution of a soluble transition metal salt, vanillin and a carbon material is heated to separate and obtain a carbon material loaded with a soluble transition metal salt;

[0008] The carbon material has a three-dimensional porous hierarchical structure;

[0009] S2: mixing the carbon material loaded with the soluble transition metal salt with a binder and an organic solvent, coating the mixture on the surface of the negative electrode current collector, and drying the mixture to obtain a base material;

[0010] S3: depositing metallic lithium on the surface of the base material to obtain the lithium metal negative electrode.

[0011] Furthermore, in step S1, the pH of the weak alkaline condition is 7.2-8.7. Specifically, the pH can be adjusted to 7.2-8.7 by a weak base, such as sodium citrate, ammonia water, sodium bicarbonate, etc., and the mass ratio of the addition amount to the soluble transition metal salt is 1-2:1-2.

[0012] Furthermore, in step S1, the mass ratio of the soluble transition metal salt, vanillin and carbon material is 1:(0.05-0.5):(1-3). Specifically, the mass ratio of the soluble transition metal salt to the carbon material can be (1-2):(2-3), and the amount of vanillin added is 0.05-0.5 of the mass of the soluble transition metal salt.

[0013] Furthermore, in step S1, the heating treatment temperature is 60-120° C. and the time is 10-18 hours.

[0014] Furthermore, the transition metal in the soluble transition metal salt is a lithium-philic element, and the soluble transition metal salt is selected from one or more of tin salts, titanium salts, zinc salts and aluminum salts, for example, it can be sulfates, chlorides, nitrates, etc. of transition metals.

[0015] Furthermore, the carbon material is obtained by calcining biomass material after activation.

[0016] Furthermore, the activation comprises the following steps: mixing the biomass material with a strong alkali and performing a heating treatment, grinding after washing and drying, and then soaking in a strong alkali.

[0017] Furthermore, the biomass material is selected from one or more of cellulose, bamboo fiber, coconut shell, rice straw, cotton stalk skin, and sugarcane bagasse; the strong alkali can be KOH solution, NaOH solution, etc.; the heating treatment temperature is 120-200°C, the heating treatment time is 8-24h; the strong alkali soaking time is 8-20h.

[0018] Furthermore, the calcination temperature after activation is 600-1000° C., and the calcination time is 2-5 hours.

[0019] Furthermore, in step S2, the mass ratio of the carbon material loaded with the soluble transition metal salt to the binder is (7-9):(1-3); the organic solvent is N-methyl-2-pyrrolidone, or other commonly used solvents in the battery preparation process.

[0020] Furthermore, in the step S2, the coating thickness is 40-60 μm.

[0021] Furthermore, in step S3, metallic lithium is deposited by magnetron sputtering or melting.

[0022] Furthermore, in step S3, the amount of deposited lithium can be (0.1-0.4 g) / 1540.25 cm 2 .

[0023] The second aspect of the present invention provides a lithium metal negative electrode prepared by the above preparation method.

[0024] Furthermore, the lithium metal negative electrode includes a negative electrode current collector, a carbon layer and metallic lithium, the carbon layer includes a carbon material and a transition metal element (a lithium-philic element); the carbon material has a three-dimensional porous layered structure, the transition metal element is loaded in the pores of the carbon material, and the metallic lithium is loaded in the pores of the carbon material through the transition metal element.

[0025] Furthermore, the thickness of the carbon layer is 30-50 μm; the loading amount of the transition metal element is (0.5-1 g) / 15.4025 cm 2 , part of the metallic lithium forms an alloy with the transition metal element.

[0026] A third aspect of the present invention provides a secondary battery comprising the above-mentioned lithium metal negative electrode.

[0027] The technical solution of the present invention has the following advantages over the prior art:

[0028] The lithium metal negative electrode of the present invention can effectively form a uniform and dense lithium-philic layer on the current collector, inducing uniform deposition of metallic lithium on the copper foil;

[0029] Among them, porous layered carbon can provide more sites for lithium deposition, slow down the generation of lithium dendrites, and make the prepared lithium metal anode have good cycle stability;

[0030] The alloying lithium storage mechanism of lithium-philic elements (transition metals) can achieve rapid ion transfer kinetics. At the same time, it can enhance the adsorption of lithium, provide abundant nucleation sites, and promote the uniform nucleation and deposition of lithium. Specifically, taking tin as an example, the reaction equation is as follows: Sn+xLi=Li x Sn(22≥x≥2) alloy, Li x Sn alloys have low Li+ Diffusion barrier can achieve fast ion transfer kinetics. Its adsorption of lithium is mainly manifested in good deposition kinetics. Due to the huge difference in electronegativity between Sn and Li, Li can diffuse rapidly between them. In addition, the voltage difference between the alloy phase and the lithium layer is extremely small, which can make the adsorbed lithium deposit rapidly on the alloy layer to form a dense SEI film, thus improving its cycle performance.

[0031] Vanillin is added during the preparation of the lithium metal negative electrode. Taking tin as an example, when Sn undergoes an alloy reaction, vanillin will preferentially adsorb on the protrusions of the deposited Sn to avoid further deposition of metallic lithium. It can effectively inhibit the growth of lithium dendrites, and ultimately make the lithium form a uniform and dense morphology, thereby improving the stability of the lithium metal negative electrode. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 This is a discharge rate performance diagram of a button battery prepared from the material obtained in Example. DETAILED DESCRIPTION

[0033] The present invention provides a lithium metal negative electrode, comprising a negative electrode current collector, a carbon layer and metallic lithium, wherein the carbon layer comprises a carbon material and a transition metal element (a lithium-loving element). Specifically, the carbon material has a three-dimensional porous layered structure, the transition metal element is loaded in the pores of the carbon material, and the metallic lithium is loaded in the pores of the carbon material through the transition metal element.

[0034] Specifically, the soluble transition metal salt is selected from one or more of tin salts, titanium salts, zinc salts and aluminum salts, for example, it can be a sulfate, chloride, nitrate, etc. of a transition metal. Taking tin salt as an example, the soluble transition metal salt can be selected from one or more of tin sulfate, tin chloride and tin nitrate.

[0035] In some preferred embodiments, the carbon material is obtained by calcining a biomass material (selected from one or more of cellulose, bamboo fiber, coconut shell, rice straw, cotton stalk skin, and sugarcane bagasse) after activation.

[0036] Specifically, the preparation of the carbon material of the present invention uses low-cost biomass materials as carbon sources, and prepares three-dimensional layered porous carbon by hydrothermal reaction with strong alkali and high-temperature carbonization reaction. The strong alkali as an activator can react violently with the fiber to release (CO, CO 2 , H 2 O and H 2 ) gas, during the reaction process, the fibers will be destroyed and reorganized, part of the pore structure will collapse, and part of the pore structure will be retained and expanded.

[0037] The specific preparation method can be as follows:

[0038] Using biomass materials as carbon sources, stirring with strong base (such as KOH solution, NaOH solution, etc.) at 25-60° C. for 0.5-2h to obtain a mixed solution;

[0039] The mixed solution is transferred to a reactor and reacted at 120-200° C. for 8-24 hours. After being taken out, it is washed alternately with water and ethanol (or other organic solvents). The washed sample is placed in a blast drying oven and dried at 60-100° C. for 8-18 hours. After being taken out, it is ground and sieved to obtain a biomass material sample;

[0040] Then soak it (biomass material sample) in a strong alkali (such as KOH solution, NaOH solution, etc.) for 8-20 hours, dry it, take it out, put it into a high-temperature tube furnace, raise the temperature to 600-1000°C at 5-10°C / min, keep it for 2-5 hours, and then cool it to room temperature (25±5°C) with the furnace. After taking it out, grind and sieve it to obtain a carbon material with a three-dimensional porous layered structure.

[0041] In some preferred embodiments, the thickness of the carbon layer is 30-50 μm, and the loading amount of the transition metal element is 0.5-1 g / 15.4025 cm 2 , part of the metallic lithium forms an alloy with the transition metal elements in the carbon layer. Taking tin as an example, the Sn alloy formed by Sn and metallic lithium acts as a lithium-philic layer, which can effectively solve the problem of the large contact angle between lithium metal and copper foil, allowing lithium metal and copper foil to be better combined.

[0042] The lithium metal negative electrode can be prepared by the following method:

[0043] S1: Under weak alkaline conditions, a mixed solution of a soluble transition metal salt, vanillin and a carbon material is heated to separate and obtain a carbon material loaded with a soluble transition metal salt;

[0044] S2: mixing the obtained carbon material loaded with soluble transition metal salt with a binder and an organic solvent (such as N-methyl-2-pyrrolidone), coating the mixture on the surface of the negative electrode current collector, and drying the mixture to obtain a base material;

[0045] S3: depositing metallic lithium on the surface of the substrate material to obtain a lithium metal negative electrode.

[0046] Specifically, in step S1, the pH of the mixed solution can be adjusted to 7.2-8.7 by a weak base (selected from one or more of sodium citrate, ammonia water, sodium bicarbonate, etc.); the mixed solution is heated at a temperature of 60-120° C. for 10-18 hours.

[0047] In some preferred embodiments, the mass ratio of the soluble transition metal salt to the carbon material is (1-2): (2-3), and the added amount of vanillin is 0.05-0.5 of the mass of the soluble transition metal salt, for example, it can be 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, etc.

[0048] In some preferred embodiments, in step S2, the mass ratio of the carbon material loaded with a soluble transition metal salt to the binder is (7-9): (1-3); the carbon material and the binder are mixed and ground for 30-60 minutes to form a slurry, which is then coated on the surface of the negative electrode collector (such as copper foil) with a coating thickness of 40-60 μm, and the base material is obtained after vacuum drying.

[0049] In some preferred embodiments, metallic lithium is deposited by magnetron sputtering or melting, and the amount of deposited lithium can be (0.1-0.4 g) / 1540.25 cm 2 , for example, it can be 0.218g / 1540.25cm 2 .

[0050] The obtained lithium metal negative electrode can be used to prepare a secondary battery.

[0051] The present invention is further described below in conjunction with the accompanying drawings and specific embodiments so that those skilled in the art can better understand the present invention and implement it, but the embodiments are not intended to limit the present invention.

[0052] Example 1

[0053] (1) 20 chopsticks (wooden chopsticks) are bundled into a bundle and cut to a length of 3 cm, soaked in a KOH solution with a concentration of 0.1 mol / L, placed in a heated magnetic stirrer and stirred at 45° C. for 0.5 h to obtain a mixed solution; the mixed solution is transferred to a reaction kettle and placed in a forced air drying oven for reaction at 180° C. for 12 h, taken out and washed alternately with water and ethanol, placed in a drying oven for drying at 80° C., and then ground and sieved to obtain a fiber material;

[0054] (2) soaking the fiber material in KOH for 15 hours, drying it, placing it in a high-temperature tube furnace and heating it to 800°C at 5°C / min for 3 hours, then cooling it to room temperature with the furnace, taking out the sample, grinding and sieving it, and obtaining a carbon material with a three-dimensional porous hierarchical structure;

[0055] (3) SnSO 4 , sodium citrate, vanillin and carbon material were dissolved in distilled water in a ratio of 1:1:0.25:2, and stirred for 2 hours to obtain a mixed solution;

[0056] (4) The mixed solution was transferred into a reaction kettle and placed in a forced air drying oven for a hydrothermal reaction at 120° C. for 12 h. After the reaction was completed, the mixed solution was washed and dried to obtain a Sn@porous layered carbon sample;

[0057] (5) Sn@porous layered carbon and a binder were mixed in a ratio of 7:2, and N-methyl-2-pyrrolidone was used as a solvent. After mixing evenly, the mixture was ground for 40 minutes to prepare a slurry with a solid content of 50%, and then coated on a copper foil with a coating thickness of 50 μm. After vacuum drying, a Sn@porous layered carbon substrate material was obtained;

[0058] (6) The lithium metal is evenly sprayed on the surface of the Sn@porous layered carbon substrate material by magnetron sputtering to obtain the Sn@porous layered carbon modified lithium metal composite negative electrode.

[0059] Example 2

[0060] This embodiment provides a Sn@porous layered carbon modified lithium metal composite negative electrode, and its preparation method is basically the same as that of Example 1, except that in step (3), SnSO 4 , the mass ratio of sodium citrate, vanillin and carbon material was adjusted to 1:1:0.05:2.

[0061] Example 3

[0062] This embodiment provides a Sn@porous layered carbon modified lithium metal composite negative electrode, and its preparation method is basically the same as that of Example 1, except that in step (3), SnSO 4 , the mass ratio of sodium citrate, vanillin and carbon material was adjusted to 1:1:0.15:2.

[0063] Example 4

[0064] This embodiment provides a Sn@porous layered carbon modified lithium metal composite negative electrode, and its preparation method is basically the same as that of Example 1, except that in step (3), SnSO 4 , the mass ratio of sodium citrate, vanillin and carbon material was adjusted to 1:1:0.5:2.

[0065] Example 5

[0066] This embodiment provides an Al@porous layered carbon modified lithium metal composite negative electrode, and its preparation method is basically the same as that of Example 1, except that AlCl is used in step (3). 3 Specifically, AlCl 3 The mass ratio of sodium citrate, vanillin and carbon material is 1:1:0.25:2. The Al@porous layered carbon is obtained in step (4), and the Al@porous layered carbon substrate is obtained in step (5).

[0067] Example 6

[0068] This embodiment provides a Sn@porous layered carbon modified lithium metal composite negative electrode, and its preparation method is basically the same as that of Example 1, except that in step (6), lithium metal is infused on the surface of the Sn@porous layered carbon substrate material to obtain the Sn@porous layered carbon modified lithium metal composite negative electrode.

[0069] Example 7

[0070] This embodiment provides a Sn@porous layered carbon modified lithium metal composite negative electrode, and its preparation method is basically the same as that of Example 1, except that bamboo is used in step (1).

[0071] Comparative Example 1

[0072] This embodiment provides a Sn@porous layered carbon modified lithium metal composite negative electrode, and its preparation method is basically the same as that of Example 1, except that vanillin is not added in step (3).

[0073] Comparative Example 2

[0074] This embodiment provides a Sn@porous layered carbon substrate material, and its preparation method is the same as steps (1) to (5) in Example 1, that is, step (6) is not included.

[0075] Comparative Example 3

[0076] This embodiment provides a Sn@porous layered carbon modified lithium metal composite negative electrode, and its preparation method is basically the same as that of Example 1, except that steps (1) and (2) are not included, and carbon black is directly used as the carbon material (in step (3)).

[0077] Results Analysis

[0078] The negative electrodes obtained in the embodiment (including Sn@porous layered carbon modified lithium metal composite negative electrode, Al@porous layered carbon modified lithium metal composite negative electrode and Sn@porous layered carbon substrate material) were cut into 12 mm diameter discs as working electrodes, NCM613 was used as the counter electrode, and a double-sided ceramic diaphragm and lithium metal electrolyte were used to assemble into 2025 button batteries.

[0079] The button cell was connected to the charging cabinet for rate performance and cycle performance tests. The rate performance test method is as follows: first discharge to 2.8V with 0.1C constant current, let stand for 5 minutes, then charge to 4.2V with 0.1C constant current and let stand for 5 minutes, then discharge to 2.8V with 1C constant current, then completely discharge with 0.1C constant current and constant voltage, let stand for 5 minutes, then charge to 4.2V with 0.2C constant current and let stand for 5 minutes, then discharge to 2.8V with 1C constant current, then completely discharge with 0.1C constant current and constant voltage, and so on, and perform discharge rate tests at 0.5C, 1C, 2C, 3C, and 5C, respectively, and the tested specific capacity is 30mAh / cm2 , the result is as follows Figure 1 As shown; the cycle performance test was carried out with 0.2C constant current charging and 0.5C constant current discharging until the capacity was 80% of the initial capacity, and the number of cycles was recorded. The results are shown in Table 1.

[0080] Table 1

[0081]

[0082]

[0083] like Figure 1 As shown in Table 1, it can be seen that the rate performance and cycle performance of the Sn@porous layered carbon modified lithium metal negative electrode material treated with vanillin are better than those of the untreated lithium metal battery (Comparative Example 1). The rate performance and cycle performance of adding different amounts of vanillin are also different, among which the performance of the vanillin addition content of 25% (Example 1) is better than that of other groups. This is because the addition of a certain amount of vanillin can effectively inhibit the growth of lithium dendrites and make the modified layer interface uniformly deposited. Too little vanillin addition content does not significantly inhibit the growth of lithium dendrites. Too much vanillin addition content occupies part of the internal space of the porous layered carbon, making it impossible for lithium to be deposited in the internal space, and also has an opposite effect on the rate performance and cycle performance of the battery.

[0084] When bamboo fiber is used as the carbon material (Example 7), the carbon tubes in the bamboo fiber are more easily damaged than chopsticks, resulting in incomplete three-dimensional porous layered carbon, which affects the electrical performance of the battery. Although more lithium can be introduced into the three-dimensional material by lithium perfusion (Example 6), the uniformity of the surface cannot be controlled during perfusion. When the surface is uneven, lithium dendrites are more likely to form in thicker locations, resulting in decreased cycle performance of the battery. When the tin salt is replaced with an aluminum salt (Example 5), since both are lithium-philic elements, the cycling performance of the battery cell is not much different. The slight difference mainly comes from the different electronegativity of tin and aluminum, which leads to different lithium deposition rates, thus affecting the cycling performance of the lithium metal battery. In addition, in the later stage of the rate test, since the stability of aluminum atoms is less than that of tin atoms, the products produced by the side reactions between aluminum atoms and the electrolyte continue to react, resulting in a decrease in the lithium-philic effect of aluminum atoms, which makes it impossible for lithium atoms to be deposited on the negative electrode surface through aluminum atoms at high rates, resulting in uneven deposition of lithium atoms on the negative electrode surface, producing lithium dendrites that affect the rate stability of the battery. When carbon black is used as the carbon material (Comparative Example 3), due to its larger specific surface area (10-150m 2 / g), resulting in an increase in the electrode resistance of the prepared negative electrode sheet. In addition, a larger specific surface area will also cause the battery cell to generate a larger solid electrolyte membrane (SEI), which will consume more electrolyte and reduce the initial efficiency and cycle life of the battery cell.

[0085] Obviously, the above embodiments are merely examples for clear explanation and are not intended to limit the implementation methods. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the implementation methods here. The obvious changes or modifications derived from these are still within the protection scope of the invention.

Claims

1. A method for preparing a lithium metal negative electrode, characterized in that: The following steps are included: S1: Under weak alkaline conditions, a mixed solution of a soluble transition metal salt, vanillin and a carbon material is heated to separate and obtain a carbon material loaded with a soluble transition metal salt; The carbon material has a three-dimensional porous hierarchical structure; S2: mixing the carbon material loaded with the soluble transition metal salt with a binder and an organic solvent, coating the mixture on the surface of the negative electrode current collector, and drying the mixture to obtain a base material; S3: depositing metallic lithium on the surface of the base material to obtain the lithium metal negative electrode.

2. The preparation method according to claim 1, characterized in that The step S1 satisfies at least one of the following conditions: The pH of weakly alkaline conditions is 7.2-8.7; The mass ratio of the soluble transition metal salt, vanillin and carbon material is 1:(0.05-0.5):(1-3); The heating temperature is 60-120°C and the time is 10-18h.

3. The preparation method according to claim 2, characterized in that: The pH is adjusted to 7.2-8.7 by using a weak base, wherein the weak base is selected from one or more of sodium citrate, ammonia water and sodium bicarbonate.

4. The preparation method according to claim 1, characterized in that: The soluble transition metal salt is selected from one or more of a soluble tin salt, a soluble titanium salt, a soluble zinc salt and a soluble aluminum salt.

5. The preparation method according to claim 1, characterized in that: The carbon material is obtained by calcining the biomass material after activation; The activation comprises the following steps: heating the mixed solution of the biomass material and the strong alkali, grinding after washing and drying, and then soaking with the strong alkali.

6. The preparation method according to claim 5, characterized in that: At least one of the following conditions is met: The biomass material is selected from one or more of cellulose, bamboo fiber, coconut shell, rice straw, cotton stalk skin, and sugarcane bagasse; The strong base is selected from KOH solution and / or NaOH solution; The heating treatment temperature is 120-200°C and the time is 8-24h; The strong alkali soaking time is 8-20h; The calcination temperature after activation is 600-1000°C and the calcination time is 2-5h.

7. The preparation method according to claim 1, characterized in that: In the step S2, the mass ratio of the carbon material loaded with the soluble transition metal salt to the binder is (7-9): (1-3).

8. The preparation method according to claim 1, characterized in that: In the step S3, metallic lithium is deposited by magnetron sputtering or melting.

9. A lithium metal negative electrode prepared by the preparation method according to any one of claims 1 to 8.

10. A secondary battery, characterized in that: Comprising the lithium metal negative electrode as described in claim 9.

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