A coated lithium cobaltate cathode material for lithium-ion batteries and its preparation method

Abstract

The present invention discloses a coated lithium cobaltate cathode material for lithium-ion batteries and a preparation method thereof. By mixing lithium cobaltate with a fluorinated organic acid or an organic acid anhydride and calcining, lithium hydroxide and lithium carbonate on the surface of lithium cobaltate are converted into a lithium fluoride coating layer, thereby obtaining a lithium cobaltate material with a lithium fluoride coating on the surface. The method of the present invention is simple and efficient, and can greatly improve the cycle stability of lithium cobaltate.

Description

Technical Field

[0001] The present invention belongs to the field of cathode materials for lithium-ion batteries, and particularly relates to a coated lithium cobaltate cathode material for lithium-ion batteries and a preparation method thereof. Background Art

[0002] Lithium-ion batteries have the advantages of high energy density, low self-discharge, and no memory effect, and are widely used in the field of consumer electronics. Among them, lithium-ion batteries with lithium cobaltate as the cathode are widely used in mobile phone batteries, and the service life is an important evaluation index of mobile phone batteries. Compared with carbon-based anodes, the lithium cobaltate cathode has relatively low cycle stability, which is a key factor restricting the service life of mobile phone batteries.

[0003] During the charge and discharge cycles of the battery, the lithium cobaltate cathode is easily eroded by the electrolyte, resulting in phenomena such as structural decline and surface side reactions. For example, during deep charging, a large amount of lithium in the lithium cobaltate will be removed, inducing irreversible changes in the layered structure, and then leading to structural decline; when a large amount of lithium is removed, cobalt is in a high oxidation state and is prone to side reactions with the electrolyte, resulting in an increase in the impedance at the surface interface and a decrease in the electrochemical performance.

[0004] In addition, during the synthesis of lithium cobaltate, in order to ensure that there is enough lithium in the material, an excessive amount of lithium salt is often added. These lithium salts will be converted into substances such as lithium carbonate and lithium hydroxide during high-temperature calcination and exist on the surface of the lithium cobaltate, affecting the material performance.

[0005] CN 104409726 A discloses a preparation method of a coated lithium-ion battery cathode material. Using trifluoroacetic acid as a fluorine raw material, together with magnesium acetate and a solvent, a sol-gel system is formed, and this system is used to coat LiCoO2 particles; after heat treatment, a LiCoO2 cathode material coated with magnesium fluoride is obtained. Although the stability of the electrode material is improved to a certain extent, it is necessary to form a sol-gel system, the preparation method is complex, and it is coated with magnesium fluoride, which is not conducive to the transport of lithium ions at the interface of the electrode material. Summary of the Invention

[0006] Aiming at the problems in the prior art, the present application provides a method for constructing a lithium fluoride surface coating layer by using a fluorine-containing organic acid or organic anhydride to improve the cycle stability of lithium cobaltate.

[0007] The technical solution of this application is as follows: A fluorinated organic acid or an organic acid anhydride is mixed with lithium cobaltate. Through high-temperature calcination, the fluorinated organic acid or the organic acid anhydride reacts with lithium carbonate and lithium hydroxide existing on the surface of lithium cobaltate to generate lithium fluoride, thereby constructing an artificial SEI film, stabilizing the surface of the electrode material, and improving the stability of the electrode material. Due to the relatively high surface energy on the surface of lithium cobaltate, structural degradation and side reactions mainly start from the surface of the electrode material. This application performs surface and interface modification on the electrode material to inhibit surface side reactions from the source, which can improve the cycle stability of lithium cobaltate.

[0008] The preparation method of the present invention includes the following steps:

[0009] Dissolve the fluorinated organic acid or the organic acid anhydride in an organic solvent, add lithium cobaltate, stir, and evaporate to dryness to obtain a mixture of lithium cobaltate and the fluorinated organic acid or the organic acid anhydride;

[0010] Place the mixture in a muffle furnace for calcination to obtain lithium cobaltate with a lithium fluoride coating on its surface.

[0011] The fluorinated organic acid or the organic acid anhydride includes one or several of trifluoroacetic acid, trifluoromethanesulfonic acid, and trifluoroacetic anhydride.

[0012] The molar ratio of fluorine in the lithium cobaltate to the fluorinated organic acid or the organic acid anhydride is 10 - 100.

[0013] The organic solvent is a non-aqueous solvent, including one or several of methanol, ethanol, propanol, and isopropanol.

[0014] The evaporation temperature is 40 - 60 °C.

[0015] The calcination temperature is 600 °C - 800 °C.

[0016] The beneficial effects of the present invention are as follows:

[0017] Through the high-temperature solid-phase reaction of lithium carbonate and lithium hydroxide on the surface of lithium cobaltate with the fluorinated organic acid or the organic acid anhydride, the present invention constructs a lithium fluoride coating layer to obtain lithium cobaltate with a lithium fluoride coating on its surface. It can not only reduce the content of inert substances such as lithium carbonate and lithium hydroxide on the surface of lithium cobaltate, but also convert these substances into lithium fluoride with high lithium ion conductivity. The lithium fluoride coating layer can protect lithium cobaltate, effectively resist the erosion of the electrolyte, reduce surface side reactions, and improve the cycle stability of lithium cobaltate. Description of the Drawings

[0018] Figure 1 is the scanning electron microscope image (SEM image) of the material before treatment in Example 1

[0019] Figure 2 is the scanning electron microscope image of the material after treatment in Example 1.

[0020] Figure 3 It is a transmission electron microscope image of the material after being processed in Example 1.

[0021] Figure 4 It is a comparative X-ray photoelectron spectroscopy chart of the material before and after coating in Example 1.

[0022] Figure 5 It is a chart of the cycle performance of the battery assembled with the material before and after coating in Example 1.

[0023] Figure 6 It is a chart of the cycle performance of the battery assembled with the material after coating in Example 2.

[0024] Figure 7 It is a chart of the cycle performance of the battery assembled with the material after coating in Example 3.

[0025] Figure 8 It is a chart of the cycle performance of the battery assembled with the material after coating in Example 4. Detailed implementation manners

[0026] Example 1

[0027] I. Preparation of lithium fluoride-coated lithium cobaltate

[0028] Disperse 0.05 mL of trifluoroacetic acid into 20 mL of methanol, add 2 g of lithium cobaltate, stir, evaporate to dryness in a 40°C water bath, place the product in a muffle furnace, and calcine at 650°C for 2 h. Naturally cool to obtain a lithium cobaltate material with a lithium fluoride coating on its surface.

[0029] II. Characterize the structure and morphology of the material before and after treatment

[0030] The scanning electron microscope images (SEM images) of the material before and after treatment are as shown in Figure 1 , Figure 2 . The SEM images show that there is no obvious change in the morphology of the material before and after treatment. It shows that the treatment process has no adverse effects such as corrosion on the material. The transmission electron microscope image (TEM) of the material after treatment is as shown in Figure 3 . The existence of the coating layer can be seen, indicating that the LiF coating layer is successfully constructed. The X-ray photoelectron spectroscopy analysis of the lithium cobaltate coated with lithium fluoride on the surface of the material before and after coating is as shown in Figure 4 . Fluorine elements exist on the surface of the material, indicating that the lithium fluoride coating layer is successfully constructed. The characteristic peaks of the material before and after treatment have not changed significantly, indicating that the treatment process has no adverse effects on the material structure.

[0031] III. Assemble a button cell

[0032] Slurry is prepared in a ratio of electrode material: carbon black: PVDF = 8:1:1, coated on aluminum foil as the positive electrode, lithium metal is used as the negative electrode, the electrolyte is 1M LiPF6 electrolyte, and the electrolyte solvent is EC:DEC:DMC = 1:1:1. A coin cell is assembled and subjected to an electrochemical cycling test at 0.5C within a voltage range of 3 - 4.5V. The test temperature is 25°C. Figure 5 It shows that the cycling stability of the battery assembled with the treated material is enhanced. This method significantly improves the electrochemical cycling stability of lithium cobaltate.

[0033] Example 2

[0034] Disperse 0.05 mL of trifluoroacetic acid into 20 mL of ethanol, add 10 g of lithium cobaltate, stir, evaporate to dryness in a 40°C water bath, place the product in a muffle furnace, and calcine at 650°C for 2 h. Cool naturally to obtain lithium cobaltate material coated with lithium fluoride on the surface. Assemble the battery and conduct electrochemical tests in the same manner as in Example 1. The test results show ( Figure 6 ), the electrochemical cycling stability of lithium cobaltate is improved.

[0035] Example 3

[0036] Disperse 0.05 mL of trifluoroacetic anhydride into 20 mL of methanol, add 5 g of lithium cobaltate, stir, evaporate to dryness in a 40°C water bath, place the product in a muffle furnace, and calcine at 700°C for 2 h. Cool naturally to obtain lithium cobaltate material coated with lithium fluoride on the surface. Assemble the battery and conduct electrochemical tests in the same manner as in Example 1. The test results show ( Figure 7 ), the cycling stability of the coated lithium cobaltate becomes better.

[0037] Example 4

[0038] Disperse 0.08 mL of trifluoroacetic anhydride into 20 mL of methanol, add 5 g of lithium cobaltate, stir, evaporate to dryness in a 60°C water bath, place the product in a muffle furnace, and calcine at 700°C for 2 h. Cool naturally to obtain lithium cobaltate material coated with lithium fluoride on the surface. Assemble the battery and conduct electrochemical tests in the same manner as in Example 1. The results show ( Figure 8 ), the cycling stability of the material after lithium fluoride coating becomes even better.

[0039] The test results of different examples are summarized in the following table. It can be seen from the following table that under the conditions provided by the present invention, the cycling stability of lithium cobaltate is improved, which proves the effectiveness of the coating method.

[0040] Table 1 Comparison of Electrochemical Performance of Different Examples

[0041]

Claims

1. A preparation method of a coated lithium cobalt oxide cathode material for a lithium-ion battery, characterized in that: Mix a fluorinated organic acid or an organic acid anhydride with lithium cobaltate. Through high-temperature calcination, the fluorinated organic acid or the fluorinated organic acid anhydride reacts with lithium carbonate and lithium hydroxide existing on the surface of lithium cobaltate to generate lithium fluoride, thereby constructing an artificial SEI film to stabilize the surface of the electrode material; The method includes the following steps: Dissolve a fluorinated organic acid or an organic acid anhydride in an organic solvent, add lithium cobaltate, stir, and evaporate to dryness to obtain a mixture of lithium cobaltate and the fluorinated organic acid or the fluorinated organic acid anhydride; Calcinate the mixture to obtain lithium cobaltate with lithium fluoride coated on the surface; The fluorinated organic acid includes at least one of trifluoroacetic acid and trifluoromethanesulfonic acid, and the fluorinated organic acid anhydride includes trifluoroacetic anhydride; The evaporation temperature is 40 - 60 °C; the calcination temperature is 600 °C - 800 °C.

2. The preparation method of a coated lithium cobaltate cathode material for a lithium-ion battery according to claim 1, characterized in that: The molar ratio of lithium cobaltate to fluorine in the fluorinated organic acid or the fluorinated organic acid anhydride is 10 - 100.

3. The preparation method of the coated lithium cobalt oxide cathode material for lithium ion batteries according to claim 1, characterized in that: The organic solvent is a non-aqueous solvent, including one or several of methanol, ethanol, propanol, and isopropanol.

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