Lithium metal negative electrode based on fluoride protective layer and preparation method

Abstract

The invention provides a lithium metal negative electrode based on a fluoride protective layer and a preparation method, the lithium metal negative electrode is prepared by reacting a graphite fluoride dispersion liquid with molten lithium, and the lithium metal negative electrode comprises the fluoride protective layer and lithium metal; the fluoride protection layer is a mixed protection layer and comprises a graphite fluoride layer and a graphite fluoride / lithium fluoride mixed transition layer. After the graphite fluoride and the molten lithium uniformly react, a distinct fluoride protection layer is constructed on the surface of the lithium metal, and the growth of dendrites in the lithium metal negative electrode circulation process is inhibited. The invention further provides a preparation method of the lithium metal negative electrode based on the fluoride protection layer. A lithium battery containing the lithium metal negative electrode based on the fluoride protection layer can show excellent cycle performance. The lithium metal negative electrode based on the fluoride protective layer can be matched with various positive electrode materials, and can stably circulate under the conditions of high current density and high surface capacity.

Description

Technical Field

[0001] The present invention relates to the technical field of negative electrode materials for lithium metal batteries, and in particular to a lithium metal negative electrode based on a fluoride protective layer and a preparation method thereof. Background Art

[0002] The continuous development of electronic products has continuously increased the requirements for battery energy density. Lithium-ion batteries based on graphite negative electrodes have approached their theoretical specific energy limit (250Wh / kg), which has profoundly affected social development over the past 30 years. However, they can no longer meet the growing demand for battery energy density in electronic products. Lithium metal has the lowest electrochemical potential (-3.04V relative to standard hydrogen electrode), ultra-high theoretical specific capacity (3860mAh / g) and low density (0.53g / cm 3 ), high energy density rechargeable battery systems with lithium metal as negative electrode are considered to be the most promising rechargeable batteries of the next generation, such as lithium sulfur, lithium air and solid-state lithium metal batteries. Although lithium metal negative electrode has many advantages, it still needs to overcome many challenges, such as lithium dendrite growth caused by uneven lithium deposition, electrolyte corrosion, huge volume changes and unstable solid electrolyte interface film (SEI), among which the most influential ones are lithium dendrite growth and unstable solid electrolyte interface film that cause internal short circuit and thermal runaway and explosion.

[0003] During the charge and discharge process, lithium metal grows in the form of dendrites. The growth of lithium dendrites will not only cause the generation of dead lithium, leading to irreversible capacity loss, but also lithium dendrites may pierce the diaphragm and cause safety problems such as thermal runaway. In addition, due to the high reducibility of lithium, lithium metal will react with the electrolyte to generate fragile SEI, which is easily broken due to the volume change of lithium metal. The exposed lithium reacts with the electrolyte, resulting in irreversible capacity loss. Artificially preparing a layer of SEI on the surface of lithium metal to replace the SEI generated by the reaction of ordinary lithium metal and electrolyte solvent is an effective method to inhibit the growth of lithium dendrites and solve the problem of unstable SEI to protect the lithium metal negative electrode. The artificially constructed SEI can separate the electrolyte from the lithium metal, inhibit continuous side reactions, and improve the stability of the lithium metal. Lithium fluoride is a known ideal SEI material, but how to uniformly and large-scale prepare lithium fluoride artificial SEI is still a more difficult problem. Summary of the invention

[0004] In order to solve the above technical problems, the present invention discloses a lithium metal negative electrode based on a fluoride protective layer and a preparation method thereof.

[0005] The specific technical solutions are as follows:

[0006] A lithium metal negative electrode based on a fluoride protective layer, wherein the lithium metal negative electrode is made by reacting a graphite fluoride dispersion with molten lithium, and the lithium metal negative electrode comprises a fluoride protective layer and lithium metal; the fluoride protective layer is a mixed protective layer, comprising a graphite fluoride layer and a graphite fluoride / lithium fluoride mixed transition layer.

[0007] Furthermore, the solvent of the fluorinated graphite dispersion is one or a combination of N-methylpyrrolidone, tetrahydrofuran and dimethyl sulfoxide.

[0008] Furthermore, the solubility of the fluorinated graphite in the solvent of the fluorinated graphite dispersion is 0.5 mg / mL-50 mg / mL.

[0009] Furthermore, the mass fraction of the fluoride protective layer in the lithium metal negative electrode is 0.1%-20%.

[0010] Furthermore, the positive electrode material suitable for the lithium metal negative electrode is any one of lithium iron phosphate, lithium nickel manganese oxide, manganese nickel cobalt composite oxide, lithium nickel cobalt manganese oxide and lithium nickel cobalt aluminum oxide, sulfur carbon, and air.

[0011] A method for preparing a lithium metal negative electrode based on a fluoride protective layer comprises the following steps:

[0012] S1. Add the graphite fluoride dispersion dropwise onto the surface of the lithium metal and apply it evenly; the mass of the graphite fluoride in the graphite fluoride dispersion accounts for 0.1%-20% of the total mass of the graphite fluoride and the lithium metal;

[0013] S2. Raise the temperature of the lithium metal coated with the graphite fluoride dispersion to 181-200° C. to melt the lithium, maintain the temperature for 0.5 h to 6 h, and then cool naturally to obtain a lithium metal negative electrode with a uniform fluoride protective layer.

[0014] Furthermore, the lithium metal coated with the graphite fluoride dispersion was heated to 181°C.

[0015] The preparation method of the graphite fluoride dispersion comprises the following steps:

[0016] S1. In an argon-protected glove box, a certain amount of graphite fluoride is added to a solvent;

[0017] S2. The solution obtained in S1 is fully mixed to form a uniform graphite fluoride dispersion.

[0018] A method for preparing a lithium metal negative electrode based on a fluoride protective layer, wherein the method of fully mixing is stirring or ultrasonic oscillation for 30 minutes.

[0019] Compared with the prior art, the lithium metal negative electrode solution described in the present invention mainly has the following technical advantages:

[0020] (1) The lithium metal negative electrode based on the fluoride protective layer of the present invention can be stably cycled under high current density and high surface capacity, and can ensure that the lithium metal symmetric battery can be used at 5 mA cm -2 The current density and 5 mA h cm -2 It can stably cycle 300 times under high pressure.

[0021] (2) The lithium metal negative electrode based on the fluoride protective layer of the present invention can be matched with a variety of positive electrode materials, and the positive electrode material is any one of lithium iron phosphate, lithium nickel manganese oxide, manganese nickel cobalt composite oxide, lithium nickel cobalt manganese oxide and lithium nickel cobalt aluminum oxide, sulfur carbon, and air to improve the energy density of the battery.

[0022] (3) The fluoride protective layer of the present invention can prevent direct contact between lithium metal and the gas in the environment, so that the lithium metal negative electrode based on the fluoride protective layer can be stored in the air, reducing the cost of the battery preparation process.

[0023] (4) The preparation method of the lithium metal negative electrode based on the fluoride protective layer of the present invention is simple. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The symmetrical battery cycle diagram and the lithium iron phosphate battery cycle diagram in the embodiments and comparative examples of the present invention are shown in the accompanying drawings.

[0025] Figure 1 Symmetrical battery cycle diagrams of Example 1, Example 2, Example 3 and Comparative Example 1.

[0026] Figure 2 This is a cycle diagram of the lithium iron phosphate batteries of Example 1, Example 2, Example 3 and Comparative Example 1.

[0027] In the figure: 1 is a symmetrical battery cycle diagram of Example 1, 2 is a symmetrical battery cycle diagram of Example 2, 3 is a symmetrical battery cycle diagram of Example 3, 4 is a symmetrical battery cycle diagram of Comparative Example 1, 5 is a lithium iron phosphate battery cycle diagram of Example 1, 6 is a lithium iron phosphate battery cycle diagram of Example 2, 7 is a lithium iron phosphate battery cycle diagram of Example 3, and 8 is a lithium iron phosphate battery cycle diagram of Comparative Example 1. DETAILED DESCRIPTION

[0028] The present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0029] Example 1

[0030] This embodiment provides a lithium metal negative electrode based on a fluoride protective layer and a preparation method, specifically:

[0031] S1. In an argon-protected glove box, a certain amount of graphite fluoride was added to N-methylpyrrolidone, and ultrasonic vibration was performed for 30 minutes to form a uniform graphite fluoride dispersion.

[0032] S2. Under argon protection, a certain mass of graphite fluoride dispersion is evenly coated on the surface of lithium metal in a glove box, wherein the mass fraction of the graphite fluoride to the total mass of the lithium metal negative electrode is 1%;

[0033] S3. The lithium metal coated with the graphite fluoride dispersion is heated to 181° C. to melt the lithium metal, and the temperature is maintained for 2 hours, followed by natural cooling to obtain a lithium metal negative electrode with a uniform fluoride protective layer.

[0034] Example 2

[0035] This embodiment provides a lithium metal negative electrode based on a fluoride protective layer and a preparation method, specifically:

[0036] S1. In an argon-protected glove box, a certain amount of graphite fluoride was added to N-methylpyrrolidone, and ultrasonic vibration was performed for 30 minutes to form a uniform graphite fluoride dispersion.

[0037] S2. Under argon protection, a certain mass of graphite fluoride dispersion is evenly coated on the surface of lithium metal in a glove box, wherein the mass fraction of the graphite fluoride to the total mass of the lithium metal negative electrode is 5%;

[0038] S3. The lithium metal coated with the graphite fluoride dispersion is heated to 181° C. to melt the lithium metal, and the temperature is maintained for 2 hours, followed by natural cooling to obtain a lithium metal negative electrode with a uniform fluoride protective layer.

[0039] Example 3

[0040] This embodiment provides a lithium metal negative electrode based on a fluoride protective layer and a preparation method, specifically:

[0041] S1. In an argon-protected glove box, a certain amount of graphite fluoride was added to N-methylpyrrolidone, and ultrasonic vibration was performed for 30 minutes to form a uniform graphite fluoride dispersion.

[0042] S2. Under argon protection, a certain mass of graphite fluoride dispersion is evenly coated on the surface of lithium metal in a glove box, wherein the mass fraction of the graphite fluoride to the total mass of the lithium metal negative electrode is 10%;

[0043] S3. The lithium metal coated with the graphite fluoride dispersion is heated to 181° C. to melt the lithium metal, and the temperature is maintained for 2 hours, followed by natural cooling to obtain a lithium metal negative electrode with a uniform fluoride protective layer.

[0044] Comparative Example 1

[0045] S1. Under argon protection, a certain amount of pure N-methylpyrrolidone is evenly coated on the surface of lithium metal in a glove box;

[0046] S2. The lithium metal coated with pure N-methylpyrrolidone is heated to 181°C to melt the lithium metal, and the temperature is maintained for 2 hours, followed by natural cooling to obtain a lithium metal negative electrode with a pure N-methylpyrrolidone protective layer.

[0047] The lithium metal negative electrodes prepared in Example 1, Example 2, Example 3 and Comparative Example 1 are used to assemble corresponding lithium batteries, respectively, and the specific implementation steps are as follows:

[0048] S1. Add lithium iron phosphate positive electrode, acetylene black and polyvinylidene fluoride binder (PVDF) into N-methylpyrrolidone, wherein the mass ratio of lithium iron phosphate positive electrode, acetylene black and PVDF is 8:1:1, and stir at room temperature at a speed of 2000r / min overnight to obtain a prepared slurry;

[0049] S2, evenly coating the slurry described in S1 on aluminum foil, drying at 80° C. for 12 hours, cutting into round or square electrodes, and placing in a glove box for standby use;

[0050] S3. Assemble a 2032 button battery in a glove box using a lithium iron phosphate positive electrode, the metal lithium negative electrode prepared in Examples 1-3 and Comparative Example 1, a separator and an electrolyte.

[0051] The lithium metal symmetrical battery prepared above was subjected to constant current charge and discharge tests at room temperature on a blue battery tester with a current density of 5 mA cm -2 , with a surface capacity of 5 mAh cm -2 .Depend on Figure 1 It can be seen that the symmetrical battery with lithium metal negative electrode in Examples 1-3 prepared by the method of the present invention can be -2 The current density is 5 mA h cm -2 The lithium metal symmetric battery without the fluoride protective layer in Comparative Example 1 could not cycle normally after 10 cycles. Among Examples 1-3, Example 2 showed the best performance and could cycle stably for 300 times, while Examples 1 and 3 could only cycle stably for 200 and 120 times, respectively.

[0052] The lithium iron phosphate battery prepared above was subjected to constant current charge and discharge test at room temperature on a blue battery tester. Figure 2 This is the cycle curve of the lithium iron phosphate full battery, with a cut-off voltage of 2.5V-4.0V. Figure 2It can be seen that Example 2 can maintain a high capacity retention rate within 400 cycles, and the capacity retention rates of Examples 1 and 3 after 400 cycles are both reduced to below 80%, while the battery assembled in Comparative Example 1 has a large irreversible capacity after cycling, and the capacity retention rate drops to below 80% after 80 cycles.

[0053] The above embodiments are only for illustrating the technical concept and features of the present invention, and their purpose is to enable people familiar with this technology to understand the contents of the present invention and implement them accordingly. They cannot be used to limit the protection scope of the present invention. Any equivalent changes or modifications made according to the spirit of the present invention should be included in the protection scope of the present invention.

Claims

1. A lithium metal negative electrode based on a fluoride protective layer, characterized in that: The lithium metal negative electrode is made by reacting a graphite fluoride dispersion with molten lithium, and the lithium metal negative electrode comprises a fluoride protective layer and lithium metal; the fluoride protective layer is a mixed protective layer, comprising a graphite fluoride layer and a graphite fluoride / lithium fluoride mixed transition layer.

2. A lithium metal negative electrode based on a fluoride protective layer according to claim 1, characterized in that: The solvent of the fluorinated graphite dispersion is one or a combination of N-methylpyrrolidone, tetrahydrofuran and dimethyl sulfoxide.

3. A lithium metal negative electrode based on a fluoride protective layer according to claim 1, characterized in that: The solubility of graphite fluoride in the graphite fluoride dispersion is 0.5 mg / mL-50 mg / mL.

4. A lithium metal negative electrode based on a fluoride protective layer according to claim 1, characterized in that: The mass fraction of the fluoride protective layer in the lithium metal negative electrode is 0.1%-20%.

5. A lithium metal negative electrode based on a fluoride protective layer according to claim 1, characterized in that: The positive electrode material suitable for the lithium metal negative electrode is any one of lithium iron phosphate, lithium nickel manganese oxide, manganese nickel cobalt composite oxide, lithium nickel cobalt manganese oxide and lithium nickel cobalt aluminum oxide, sulfur carbon and air.

6. The method for preparing a lithium metal negative electrode based on a fluoride protective layer according to claim 1, characterized in that: The following steps are involved: S1. Add the graphite fluoride dispersion dropwise onto the surface of the lithium metal and apply it evenly; the mass of the graphite fluoride in the graphite fluoride dispersion accounts for 0.1%-20% of the total mass of the graphite fluoride and the lithium metal; S2. Heating the lithium metal coated with the graphite fluoride dispersion to 181-200° C. to melt the lithium, maintaining the temperature for 0.5-6 hours, and naturally cooling to room temperature to obtain the lithium metal negative electrode.

7. The method for preparing a lithium metal negative electrode based on a fluoride protective layer according to claim 6, characterized in that: The lithium metal coated with the graphite fluoride dispersion was heated to 181°C.

8. The method for preparing a lithium metal negative electrode based on a fluoride protective layer according to claim 1, characterized in that: The preparation method of the graphite fluoride dispersion comprises the following steps: S1. In an argon-protected glove box, a certain amount of graphite fluoride is added to a solvent; S2. The solution obtained in S1 is fully mixed to form a uniform graphite fluoride dispersion.

9. The method for preparing a lithium metal negative electrode based on a fluoride protective layer according to claim 8, characterized in that: The method of sufficient mixing is stirring or ultrasonic shaking for 30 minutes.

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