Method for improving environmental stability of sodium ion positive electrode material NaNi < 0.33 > Fe < 0.33 > Mn < 0.33 > O < 2 >

Abstract

The invention discloses a method for improving the environmental stability of a sodium ion positive electrode material NaNi < 0.33 > Fe < 0.33 > Mn < 0.33 > O < 2 >. The method comprises the following steps: dissolving a hydrophobic compound in an organic solvent to form a hydrophobic compound solution, adding the O3-NaNi < 0.33 > Fe < 0.33 > Mn < 0.33 > O2 sodium ion positive electrode material into the hydrophobic compound solution, and carrying out an oil bath reaction while stirring to obtain the hydrophobic compound coated O3-NaNi < 0.33 > Fe < 0.33 > Mn < 0.33 > O2 sodium ion positive electrode material. The sodium ion positive electrode material is subjected to hydrophobic molecular coating by adopting a molecular self-assembly method, the preparation method is simple and low in cost, the formed coating layer has a strong hydrophobic effect, is extremely thin and is uniform in coating, and after a battery assembled by the coated O3-NNFM electrode material is stored in air, the battery has a good hydrophobic effect. And compared with a battery assembled by an uncoated O3-NNFM material, the capacity retention ratio is obviously improved, and the material has a wide application prospect in the field of sodium ion batteries.

Description

Technical Field

[0001] The present invention belongs to the technical field of sodium-ion battery electrode materials, and relates to a method for improving the environmental stability of a sodium-ion cathode material NaNi 0.33 Fe 0.33 Mn 0.33 O 2 environment stability. Background Art

[0002] Secondary batteries, including lithium-ion batteries, sodium-ion batteries, etc., are high-energy-density secondary batteries with great development potential and application prospects. Due to the scarcity of Li resources, sodium-ion batteries with relatively rich raw materials and low costs, as a favorable supplement to lithium-ion batteries, have received extensive attention from researchers.

[0003] Although the cost of sodium-ion battery cathode materials is relatively low, the sodium-ion battery cathode materials basically have serious air sensitivity, that is, their chemical structures are unstable in the air. After storage under environmental conditions, the materials are prone to react with water and carbon dioxide, resulting in capacity loss and increased polarization and failure. Therefore, their preparation, storage, and assembly must be carried out in a dry or even inert atmosphere, which inevitably brings higher economic costs and limits the practical application prospects of sodium-ion batteries. Therefore, it is urgent to solve the environmental stability problem of sodium-ion battery cathode materials to reduce the production cost of sodium-ion batteries.

[0004] Generally, the methods for solving the air sensitivity problem of layered sodium oxides mainly include interface coating, removal of surface residual alkali, and structure regulation, etc. Currently, the most common modification strategy for the air stability of sodium-ion battery cathode materials is the regulation of the interface. Literature 1 used the high-temperature solid-phase method to construct a spinel-structured titanium-rich interface on the surface of layered manganese-based oxides to inhibit the contact between the material and environmental conditions. No new phase appeared after the sample was placed in humid air for 3 days, proving the protective effect of the titanium oxide interface (Yao, Hu-Rong et al. “Designing Air-Stable O3-Type Cathode Materials by Combined Structure Modulation for Na-Ion Batteries.” Journal of the American Chemical Society vol. 139, 25 (2017): 8440-8443.). Literature 2 used atomic deposition method to coat the surface of P2-Na 2 / 3 Mn 1 / 2 Fe 1 / 4 Co 1 / 4 O 2 material with nanoscale Al 2 O3 layer to improve the air stability of the electrode, ultrathin deposited Al 2 O 3 layer can prevent Na 2 / 3 Mn 1 / 2 Fe 1 / 4 Co 1 / 4 O 2 from being directly exposed to H 2 O and CO 2 environments, making it an air-insensitive electrode (Chu, S.; Jia, X.; Wang, J.; et al. “Reduced air sensitivity and improved electrochemical stability of P2-Na 2 / 3 Mn 1 / 2 Fe 1 / 4 Co 1 / 4 O 2 through atomic layer deposition-assisted Al 2 O 3 coating.” Compos. B. Eng. 2019, 173, 106913.). However, these coating methods have high requirements for the particle size of the active material, uneven coating, and high energy consumption. SUMMARY OF THE INVENTION

[0005] The object of the present invention is to provide a method for improving the environmental stability of sodium-ion cathode material NaNi 0.33 Fe 0.33 Mn 0.33 O 2 by using hydrophobic molecule coating on the air-sensitive O3-NaNi 0.33 Fe 0.33 Mn 0.33 O 2 sodium-ion cathode material (O3-NNFM), and the hydrophobic molecules form an ultrathin hydrophobic film on the surface of the electrode material through self-assembly.

[0006] The technical solution for achieving the object of the present invention is as follows:

[0007] A method for improving the environmental stability of sodium-ion cathode material NaNi 0.33 Fe 0.33 Mn 0.33 O 2 is as follows:

[0008] (1) Stir and dissolve the hydrophobic compound in an organic solvent to form a hydrophobic compound solution with a concentration of 2 wt% to 4 wt%. The hydrophobic compound is pentafluorophenyltriethoxysilane (PFPTES) or pentafluorophenyltrimethoxysilane (PFPTMS);

[0009] (2) Add the O3-NaNi 0.33 Fe 0.33 Mn 0.33 O 2 sodium ion cathode material into the hydrophobic compound solution, stir and react in an oil bath at 70 ± 5 °C. After the reaction, filter by suction, wash with the organic solvent, and dry in vacuum to obtain the O3-NaNi 0.33 Fe 0.33 Mn 0.33 O 2 sodium ion cathode material coated with the hydrophobic compound.

[0010] Further, in step (1) or (2), the organic solvent is selected from tetrahydrofuran (THF), N-methylpyrrolidone, ethanol, N,N-dimethylformamide or acetone. In the specific embodiment of the present invention, THF is taken as an example.

[0011] Further, in step (1), the stirring time is 1 to 2 h.

[0012] Further, in step (2), the stirring reaction time is 5 to 8 h.

[0013] Further, in step (2), the vacuum drying temperature is 105 to 110 °C, and the vacuum drying time is more than 12 h.

[0014] Compared with the prior art, the present invention has the following advantages:

[0015] (1) In the present invention, PFPTES or PFPTMS is used as the coating molecule, and the coating is carried out by the method of molecular self-assembly. The preparation method is simple and the cost is low. The formed coating layer has a strong hydrophobic effect, its hydrophobic groups face the outside air, the thickness is extremely thin, the size is 1 - 3 nm, the ionic conductivity is high, and it will not change in thickness due to the change of the material morphology, and the coating layer is uniform;

[0016] (2) After the battery assembled with the O3-NNFM electrode material coated by the method of the present invention is stored in the air, compared with the battery assembled with the uncoated O3-NNFM material, the capacity retention rate is significantly improved, and the capacity retention rate is as high as 97%, which significantly improves the environmental stability of the O3-NNFM material. Description of the Drawings

[0017] Figure 1 For the improvement of the sodium ion cathode material NaNi 0.33Fe 0.33 Mn 0.33 O 2 Schematic flow diagram of the method for environmental stability.

[0018] Figure 2 Schematic diagram of the simple structure of a sodium-ion battery.

[0019] Figure 3 Transmission electron microscopy (TEM) image of the PFPTES@O3-NNFM electrode material prepared in Example 1.

[0020] Figure 4 Hydrophobicity comparison chart of uncoated O3-NNFM and PFPTES@O3-NNFM.

[0021] Figure 5 Electrochemical performance chart of the battery assembled with uncoated O3-NNFM and the battery assembled with PFPTES@O3-NNFM.

[0022] Figure 6 TEM image of the PFPTMS@O3-NNFM electrode material.

[0023] Figure 7 Electrochemical performance chart of the battery assembled with PFPTMS@O3-NNFM.

[0024] Figure 8 TEM image of the PFPTES@O3-NNFM electrode material prepared in Example 3.

[0025] Figure 9 TEM image of the PFPTES@O3-NNFM electrode material prepared in Comparative Example 1.

[0026] Figure 10 TEM image of the PFPTES@O3-NNFM electrode material prepared in Comparative Example 2. Detailed implementation manners

[0027] The present invention will be further described in detail below in conjunction with specific embodiments and the accompanying drawings.

[0028] Example 1

[0029] Dissolve 0.4 g of commercially available PFPTES in 20 g of tetrahydrofuran, and stir for 1 hour at room temperature to obtain a PFPTES solution with a concentration of 2 wt.%. Dissolve 2 g of commercially available sodium-ion battery cathode material O3-NaNi 0.33 Fe 0.33 Mn 0.33 O 2The powder was added to a 2 wt.% PFPTES solution. While heating in an oil bath at 70 °C, a magnetic stirrer was used to stir at a speed of 600 r / min. After reacting for 5 hours, suction filtration was carried out using a Buchner flask to filter off the solution. During the suction filtration process, the powder was continuously rinsed with tetrahydrofuran to remove the excess PFPTES. The obtained powder was collected and placed in a vacuum oven, where it was dried under vacuum at 105 °C for 12 hours to obtain the PFPTES@O3-NNFM powder electrode material.

[0030] Figure 3 This is the transmission electron microscopy image of the PFPTES@O3-NNFM electrode material. It can be seen that there is a uniform coating layer about 1.5 nm thick at the outermost edge. Compared with the traditional coating method, the coating layer prepared by this method is thinner and more uniform.

[0031] Figure 4 This is the comparison chart of the hydrophobicity of uncoated O3-NNFM and PFPTES@O3-NNFM. It can be seen that the water contact angle of the original sample is almost 0, while the water contact angle of PFPTES@O3-NNFM reaches 91°, indicating strong hydrophobicity.

[0032] The PFPTES@O3-NNFM cathode materials (just coated, stored in air for 7 days, stored in air for 14 days) and the uncoated O3-NNFM cathode materials (just coated, stored in air for 7 days, stored in air for 14 days) were mixed and coated with Super P and polyvinylidene fluoride (PVDF) in a ratio of 8:1:1, using N-methylpyrrolidone (NMP) as the solvent, and baked in a vacuum oven at 105 °C for 10 h to make cathode sheets. The prepared cathode sheets were punched into cathode sheets with a diameter of 12 mm. The prepared cathode sheets were used together with sodium metal electrodes and Grade GF / A glass fiber diaphragms, and NaClO 4 electrolyte (1.0 M NaClO 4 in PC = 100 Vol% with 3.0% FEC) were assembled into a 2032 coin cell with a sandwich structure, as Figure 2 shown. Figure 5The figure shows the comparison of the capacity retention rates of the batteries assembled with uncoated O3-NNFM and the batteries assembled with PFPTES@O3-NNFM. It can be seen that for the batteries assembled with O3-NNFM, after being exposed to air for 7 days and 14 days, the specific capacity decays from the original 123 mAh / g to 107 mAh / g and 78 mAh / g, and the capacity retention rates are only 86% and 63% respectively. While for the batteries assembled with PFPTES@O3-NNFM, after being exposed for 7 days and 14 days, the specific capacity decays from the original 121 mAh / g to 118 mAh / g and 103 mAh / g, and the capacity retention rates are 97% and 85% respectively. The above results indicate that after being stored in air, the PFPTES@O3-NNFM material after coating has a higher capacity retention rate than the batteries assembled with the uncoated O3-NNFM material.

[0033] Example 2

[0034] Dissolve 0.4 g of commercially available PFPTMS in 20 g of tetrahydrofuran and stir at room temperature for 1 hour to obtain a PFPTMS solution with a concentration of 2 wt.%. Add 2 g of commercially available sodium-ion battery cathode material O3-NaNi 0.33 Fe 0.33 Mn 0.33 O 2 powder into the 2 wt.% PFPTMS solution. While heating in an oil bath at 70 °C, stir with a magnetic stirrer at a speed of 600 r / min. After reacting for 5 hours, carry out suction filtration with a suction flask, filter off the solution, and continuously rinse with tetrahydrofuran during the suction filtration process to wash away the excess PFPTMS. After collecting the obtained powder material, put it into a vacuum oven and vacuum dry at 105 °C for 12 hours to obtain the PFPTMS@O3-NNFM powder electrode material.

[0035] Figure 6 Figure shows the TEM image of the PFPTMS@O3-NNFM sample, and a coating layer of about 1 nm can be seen. Figure 7 Figure shows the electrochemical performance graph of the battery assembled with PFPTMS@O3-NNFM. After being exposed for 7 days, the specific capacity decays from the original 121 mAh / g to 118 mAh / g, and the capacity retention rate is 97%, showing a good capacity retention rate.

[0036] Example 3

[0037] This example is basically the same as Example 1, except that the concentration of the PFPTES solution is 4 wt.%.

[0038] The TEM image of the PFPTES@O3-NNFM electrode material prepared in Example 3 is as Figure 8 shown, and a uniform molecular layer of about 1.8 nm can be seen, indicating a good coating effect.

[0039] Comparative Example 1

[0040] This comparative example is basically the same as Example 1, except that the concentration of the PFPTES solution is 1 wt.%.

[0041] The TEM image of the PFPTES@O3-NNFM electrode material prepared in Comparative Example 1 is as Figure 9 shown. A large area on the surface of the electrode material did not form a molecular coating layer, indicating that when the concentration of the PFPTES solution is too low, it is difficult for PFPTES to coat on the material surface.

[0042] Comparative Example 2

[0043] This comparative example is basically the same as Example 1, except that the concentration of the PFPTES solution is 5 wt.%.

[0044] The TEM image of the PFPTES@O3-NNFM electrode material prepared in Comparative Example 2 is as Figure 10 shown. The thickness of its molecular coating layer exceeds 5 nm, indicating that when the concentration of the PFPTES solution is too high, it is difficult for PFPTES to form a thin and uniform molecular layer on the material surface.

Claims

1. Method for improving environmental stability of sodium ion cathode material NaNi 0.33 Fe 0.33 Mn 0.33 O 2 ​ It is characterized in that The specific steps are as follows: (1) Stir and dissolve the hydrophobic compound in an organic solvent to form a hydrophobic compound solution with a concentration of 2 wt% to 4 wt%. The hydrophobic compound is pentafluorophenyltriethoxysilane or pentafluorophenyltrimethoxysilane; (2) Add the O3-NaNi 0.33 Fe 0.33 Mn 0.33 O 2 sodium ion cathode material into a hydrophobic compound solution, stir and react in an oil bath at 70±5°C. After the reaction, perform suction filtration, wash with an organic solvent, and dry in vacuum to obtain the O3-NaNi 0.33 Fe 0.33 Mn 0.33 O 2 sodium ion cathode material coated with a hydrophobic compound.

2. The method according to claim 1, It is characterized in that In step (1) or (2), the organic solvent is selected from tetrahydrofuran, N-methylpyrrolidone, ethanol, N,N-dimethylformamide or acetone.

3. The method according to claim 1, It is characterized in that In step (1), the stirring time is 1 to 2 h.

4. The method according to claim 1, It is characterized in that In step (2), the stirring reaction time is 5 to 8 h.

5. The method according to claim 1, It is characterized in that In step (2), the vacuum drying temperature is 105 to 110 °C, and the vacuum drying time is more than 12 h.

Images