Bi2O3-coated Ag2WO4 composite positive electrode material and preparation and application thereof

Abstract

The invention relates to the technical field of lithium batteries, in particular to a Bi2O3 / Ag2WO4 composite positive electrode material as well as preparation and application thereof. The composite positive electrode material is compounded by coating flaky bismuth oxide with amorphous silver tungstate, and the composite positive electrode material has a PN heterojunction. The Bi2O3 and Ag2WO4 heterojunction composite positive electrode material is created, a new direction is developed for the applicable positive electrode material of the lithium oxygen battery, meanwhile, the lithium oxygen battery prepared from the positive electrode material is relatively low in overpotential, and the energy efficiency of the battery is greatly improved. The invention also provides a synthesis method of the Bi2O3 (at) Ag2WO4 heterojunction composite positive electrode material, the Bi2O3 (at) Ag2WO4 heterojunction composite positive electrode material is synthesized by adopting a one-step hydrothermal method, the method is simple and easy to operate, and popularization and application of a new material are facilitated.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium batteries, and particularly relates to a Bi2O3@Ag2WO4 composite cathode material and its preparation and application. Background Art

[0002] In recent years, with the increasing demand for new energy, lithium-air batteries, as a new type of clean energy storage battery, have continuously attracted the attention of people from all walks of life. During the research process of lithium-air batteries, researchers found that they have the advantages of ultra-high theoretical specific energy, low cost, environmental friendliness, light weight, etc., which provide an effective solution and approach to the problem of the increasing demand for chemical power sources. Its theoretical specific energy can reach 11400 Wh·kg -1 , which is about 5-10 times higher than the theoretical specific capacity of lithium-ion batteries.

[0003] Although lithium-air batteries have many advantages, there are still many scientific and technical problems that need to be solved urgently to apply them thoroughly in people's daily lives, such as: the slow conversion rate of lithium peroxide, low conductivity, and high discharge overpotential in lithium-oxygen batteries, resulting in poor actual performance of the batteries.

[0004] To solve the above problems, many researchers will choose materials with suitable all-round performance as electrode materials to solve the problem from the source.

[0005] Bismuth oxide has been widely concerned due to its small particle size, high theoretical specific capacity and energy density, and many oxygen vacancies. A large number of oxygen vacancies can help promote the separation of electrons and holes and enhance the conductivity of the material. However, its band gap is relatively wide, resulting in a still high recombination rate of photo-generated electron-hole pairs. This limits the electron conduction performance of bismuth oxide, reduces the discharge performance and cycle stability of lithium-oxygen batteries. In addition, the wide band gap also limits the diffusion rate of lithium ions in bismuth oxide, affecting the charge and discharge rate and power performance of the battery.

[0006] At present, there are ways to regulate the band gap of bismuth oxide by doping metals and introducing other vacancies, but the performance improvement of the finally prepared lithium-oxygen battery is not significant, and there is still much room for improvement. Summary of the Invention

[0007] The present invention aims to overcome the defect that the lithium-oxygen battery prepared with bismuth oxide with a wide band gap as the cathode material has poor electrochemical performance, and provides a Bi2O3@Ag2WO4 composite cathode material and its preparation and application to overcome the above defects.

[0008] To achieve the above object, the present invention provides the following technical solutions: A Bi2O3@Ag2WO4 composite cathode material, wherein the composite cathode material is obtained by coating amorphous silver tungstate on flaky bismuth oxide, and the composite cathode material has a PN heterojunction.

[0009] The inventor of the present application aims at the problem of the wide bandgap of current bismuth oxide, and regulates the bandgap of bismuth oxide by forming a heterojunction by compounding narrow-bandgap silver tungstate, so that the bandgap of the composite cathode material is shortened, and the recombination efficiency of photogenerated electron-hole pairs is further reduced to help improve the battery performance.

[0010] Furthermore, bismuth oxide is an N-type semiconductor, while silver tungstate is a P-type semiconductor, and the two are compounded to form a PN heterojunction. Since the electric field direction of the applied voltage is opposite to the internal electric field direction in the PN heterojunction. Under the action of the external electric field, the internal electric field will be weakened, the blocking layer will become narrower, and the diffusion motion will be enhanced accordingly. In this way, the majority carriers will continuously pass through the PN heterojunction under the drive of the external electric field force, forming a large diffusion current, and these diffusion currents are beneficial to the ORR and OER processes of the lithium-oxygen battery.

[0011] Moreover, silver tungstate itself has a moderate oxygen adsorption and desorption free energy, which can effectively promote the ORR and OER processes, and thus improve the electrochemical performance of the lithium-oxygen battery. In addition, silver tungstate contains a large amount of silver ions, and the excellent electronic structure of silver ions can help form a new charge distribution at the contact interface between silver tungstate and lithium peroxide and generate local charge transfer. This phenomenon can also be attributed to the mutual polarization of electron orbits caused by van der Waals forces. After the charge transfer occurs, it can help improve the reaction rate of the electrode, accelerate the ORR and OER processes, and enhance the electrochemical performance of the lithium-oxygen battery.

[0012] Secondly, amorphous silver tungstate has a larger specific surface area, which can broaden the interaction area between flaky bismuth oxide and the electrolyte, help improve the reaction rate, and is beneficial to the performance of the lithium-oxygen battery.

[0013] In summary, the positive electrode composite material prepared by the present invention can solve the problem of poor performance of single bismuth oxide as a positive electrode material in the application of lithium-oxygen batteries by reducing the bandgap of bismuth oxide and forming a PN heterojunction. In order to verify the excellent performance of the material designed by the present invention, it is experimentally verified that the lithium-oxygen battery assembled with the composite cathode material coated with silver tungstate has a lower overpotential and a higher discharge capacity.

[0014] A preparation method of a Bi2O3@Ag2WO4 composite cathode material, wherein after the flaky bismuth oxide powder is evenly dispersed in water, a silver source and a tungsten source are added and stirred evenly, and then transferred to a reaction kettle for heating reaction to obtain a composite cathode material of bismuth oxide and silver tungstate.

[0015] The inventors of the present application synthesized the Bi2O3@Ag2WO4 composite cathode material by a simple one-step hydrothermal method. Its preparation method is simple, and the Bi2O3@Ag2WO4 new material can be obtained, which is conducive to the popularization and application of the new material. During the hydrothermal synthesis process, the conditions of high temperature and high pressure promote the amorphous silver tungstate to wrap on the surface of the flaky bismuth oxide powder. The amorphous silver tungstate has a large specific surface area and excellent wrapping property for bismuth oxide. Thus, a new material with a heterojunction structure is obtained by compounding.

[0016] Preferably, the preparation method of the bismuth oxide includes the following steps: S1. Add the bismuth source and the oxygen source into water and stir evenly. Drop in the sodium hydroxide solution to ensure that the solution is alkaline. Transfer it to a reaction kettle for heating reaction to obtain a precursor. S2. Calcinate the precursor obtained in S1 to obtain flaky bismuth oxide.

[0017] The mixed solution of the bismuth source and the oxygen source can be maintained under alkaline conditions with the assistance of sodium hydroxide. In this way, the rate of the hydrothermal reaction and the formation of subsequent products can be effectively controlled, promoting the reaction to proceed in the direction of the product. Then, bismuth oxide is obtained through calcination. The advantage of calcination is to promote the nucleation and growth of bismuth oxide particles and improve the crystallinity of the crystal. Finally, flaky bismuth oxide can be obtained.

[0018] Preferably, in the step S1, the bismuth source is selected from one or more of bismuth nitrate, bismuth sulfate, bismuth carbonate, and bismuth phosphate, and the oxygen source is sodium sulfate.

[0019] There are no special requirements for the selection of the bismuth source. Any raw material that is easy to obtain and can synthesize bismuth oxide without synthetic by-products is acceptable, not limited to the several mentioned above. The selection of each raw material has no special influence on the performance and morphology of the synthesized bismuth oxide.

[0020] Preferably, in the step S1, the molar ratio of bismuth in the bismuth source to sodium hydroxide is 1:5 - 7.

[0021] Ensure that the amount of substance of sodium hydroxide is five to seven times that of bismuth ions. The setting of this condition is to ensure that there is enough sodium hydroxide in the solution to neutralize the acidic substances in the bismuth source and keep the solution alkaline. The setting of this condition helps to control the reaction process and is conducive to obtaining the desired product.

[0022] Preferably, in the step S2, the calcination temperature is 400 - 500 °C, and the calcination time is 1 - 3 h.

[0023] Preferably, the silver source is selected from one or more of silver nitrate, silver chloride, and silver sulfate, and the tungsten source is selected from one or more of ammonium tungstate, sodium tungstate, zinc tungstate, cobalt tungstate, and calcium tungstate.

[0024] There are no special requirements for the selection of silver source and tungsten source. Any raw materials that are easy to obtain and can synthesize silver tungstate without synthetic by-products are acceptable, not limited to the several mentioned above. The selection of each raw material has no special impact on the performance and morphology of the synthesized silver tungstate.

[0025] Preferably, the molar ratio of silver in the silver source to tungsten in the tungsten source is 2:1.

[0026] When the molar ratio of silver ions to tungsten ions is close to 2:1, the synthesized silver tungstate has higher purity.

[0027] Preferably, the molar ratio of the bismuth oxide to silver tungstate is 1:(0.5 - 2).

[0028] The molar ratio of bismuth oxide to silver tungstate is another design focus of this scheme. The addition amount of silver tungstate determines the performance of the Bi2O3@Ag2WO4 composite cathode material. The relative amount of the two is also determined by the different morphologies and structures of the two materials. Flaky bismuth oxide and amorphous silver tungstate will form a more stable structure when compounded. The selection of this relative amount may affect the morphology and structure of the composite cathode material and thus affect the performance of the composite cathode material. In this reaction, mixing bismuth oxide and silver tungstate according to a certain molar ratio helps to control the morphology and structure of the composite cathode material, thereby regulating the performance of the material.

[0029] After further verification, it is found that when the molar ratio of bismuth oxide to silver tungstate is controlled at 1:(0.5 - 2), the obtained material has excellent electrochemical performance. Due to the photocatalytic effect of the composite cathode material itself, this composite cathode material can be applied in a photo-assisted lithium-air battery.

[0030] Preferably, the heating temperature is 160 - 180 °C and the heating time is 12 - 24 h.

[0031] Under this hydrothermal reaction condition, it can ensure the synthesis of silver tungstate with a completely developed crystal form of the material, and can help silver tungstate grow tightly on the surface of bismuth oxide to obtain a completely compounded heterojunction composite catalyst.

[0032] The Bi2O3@Ag2WO4 composite cathode material provided by the present invention or the Bi2O3@Ag2WO4 composite cathode material prepared by the provided method can be applied in a photo-assisted lithium-air battery.

[0033] Verified: The cathode material prepared by the present invention itself has good photocatalytic effect. This discovery means that the cathode material can promote the performance and efficiency of lithium-oxygen batteries under light-assisted conditions. In a light-assisted lithium-oxygen battery, light energy can be utilized to stimulate the reactions on the cathode material, thereby enhancing the energy conversion efficiency and discharge performance of the battery. Moreover, by using the cathode material provided by the present invention, the cycle stability of the battery can be improved, the discharge capacity can be increased, and the polarization process of the battery can be reduced.

[0034] Therefore, the present invention has the following beneficial effects: (1) The present invention creates a Bi2O3@Ag2WO4 heterojunction composite cathode material, opening up a new direction for the applicable cathode materials of lithium-oxygen batteries. At the same time, the lithium-oxygen battery prepared with this cathode material has a lower overpotential, significantly improving the energy efficiency of the battery.

[0035] (2) The present invention provides a synthesis method for the Bi2O3@Ag2WO4 heterojunction composite cathode material, which is synthesized by a one-step hydrothermal method. The method is simple and easy to operate, facilitating the popularization and use of the new material.

[0036] (3) The present invention helps to improve the performance of wide-bandgap bismuth oxide by forming a heterojunction with the composite of an N-type semiconductor and a P-type semiconductor, providing a new idea for the modification of other wide-bandgap semiconductors.

[0037] (4) The present invention provides the application of the Bi2O3@Ag2WO4 heterojunction composite cathode material in lithium-oxygen batteries. With the photocatalytic effect and structural characteristics of the material itself, and by combining the advantages of individual materials, the composite cathode material exhibits unexpected effects in lithium-oxygen batteries. Description of the Drawings

[0038] Figure 1 It is a XRD comparison diagram; Figure 2 It is the SEM diagram of Example 2; Figure 3 It is the first charge-discharge curve comparison diagram; Figure 4 It is the first-cycle discharge capacity comparison diagram; Figure 5 It is the schematic diagram of the Bi2O3@Ag2WO4 heterojunction mechanism. Detailed Embodiments

[0039] The present invention will be further described below in conjunction with specific embodiments. Those of ordinary skill in the art will be able to implement the present invention based on these descriptions. In addition, the embodiments of the present invention described below generally only represent a part of the embodiments of the present invention, rather than all of the embodiments. Therefore, all other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the scope of protection of the present invention.

[0040]

Example

[0041] S2: Slowly add 40 mL of 0.5 mol / L NaOH solution to the solution in S1, then transfer it to a 100 mL hydrothermal autoclave and react at 150 °C for 12 h.

[0042] S3: Wait for the liquid in S2 to cool, collect the powder, wash it 3 times with deionized water, and then transfer it to a vacuum drying oven and dry overnight at 60 °C.

[0043] S4: Take the dried powder in S3 and place it in a tubular furnace, and obtain Bi2O3 under an air environment at 500 °C for 2 h.

[0044] S5: Take 1.86 g of Bi2O3 (4 mmol), add 40 mL of deionized water, ultrasonically disperse it for 1 h, then add 4 mmol of AgNO3, stir at 300 rpm until completely dissolved, and then add 0.168 mmol of (NH4) 10 H2(W2O7)6, stir at 300 rpm for 30 min.

[0045] S6: Transfer the above solution to a 50 mL hydrothermal autoclave and react at 160 °C for 16 h, centrifuge to collect the powder to obtain Bi2O3@Ag2WO4-1.

[0046] Example 2 S1: Place 1.94 g of Bi(NO)3·5H2O and 0.284 g of Na2SO4 in a beaker, then add 40 mL of deionized water and stir vigorously for 30 min.

[0047] S2: Slowly add 50 mL of 0.5 mol / L NaOH solution to the solution in S1, then transfer it to a 100 mL hydrothermal autoclave and react at 150 °C for 12 h.

[0048] S3: Wait for the liquid in S2 to cool, collect the powder, wash it 3 times with deionized water, and then transfer it to a vacuum drying oven and dry overnight at 60 °C.

[0049] S4: The dried powder obtained in S3 is placed in a tube furnace at 400°C in air for 3 hours to obtain Bi2O3.

[0050] S5: Take 1.86g Bi2O3 (4mmol), add 40mL deionized water, ultrasonicate for 1h to fully disperse, then add 8mmol AgNO3, stir at 300rpm until completely dissolved, and add 0.336mmol (NH4) 10 H2(W2O7)6, stirring at 300rpm for 30min.

[0051] S6: The above solution was transferred to a 50 mL hydrothermal reactor and reacted at 170 °C for 24 h. The powder was collected by centrifugation to obtain Bi2O3@Ag2WO4-2.

[0052] Example 3 S1: Place 1.94 g Bi(NO)3·5H2O and 0.284 g Na2SO4 in a beaker, then add 40 mL deionized water and stir vigorously for 30 min.

[0053] S2: Slowly add 55 mL of 0.5 mol / L NaOH solution to the S1 solution, then transfer it to a 100 mL hydrothermal reactor and react at 150 °C for 12 h.

[0054] S3: After the liquid in S2 cools down, collect the powder and wash it three times with deionized water, then transfer it to a vacuum drying oven at 60°C overnight.

[0055] S4: The dried powder obtained in S3 is placed in a tube furnace at 500°C in air for 1 hour to obtain Bi2O3.

[0056] S5: Take 1.86g Bi2O3 (4mmol), add 40mL deionized water, ultrasonicate for 1h to fully disperse, then add 12mmol AgNO3, stir at 300rpm until completely dissolved, add 0.504mmol (NH4) 10 H2(W2O7)6, stirring at 300rpm for 30min.

[0057] S6: The above solution was transferred to a 50 mL hydrothermal reactor and reacted at 180 °C for 12 h. The powder was collected by centrifugation to obtain Bi2O3@Ag2WO4-3.

[0058] Comparative Example 1 S1: Place 1.94 g Bi(NO)3·5H2O and 0.284 g Na2SO4 in a beaker, then add 40 mL deionized water and stir vigorously for 30 min.

[0059] S2: Slowly add 40 mL of 0.5 mol / L NaOH solution dropwise to the S1 solution, then transfer it to a 100 mL hydrothermal reactor and react at 150 °C for 12 h.

[0060] S3: Wait for the liquid in S2 to cool, collect the powder, wash it 3 times with deionized water, and then transfer it to a vacuum drying oven and dry it overnight at 60 °C.

[0061] S4: Place the dried powder obtained in S3 in a tubular furnace and heat it at 500 °C in an air atmosphere for 2 h to obtain Bi2O3.

[0062]

Performance Test

[0063] 2. SEM Test Perform SEM test on the material obtained in Example 2, and the results are as Figure 2 shown. The Bi2O3@Ag2WO4 heterojunction synthesized in Experimental Example 2 is a flaky Bi2O3 with a thin film of Ag2WO4 growing on its surface, and the structure is regular; it is consistent with the XRD results, and the Bi2O3@Ag2WO4 material is successfully synthesized.

[0064] 3. Electrochemical Performance Test In order to evaluate the practicality of the prepared Bi2O3@Ag2WO4 cathode material in the battery, 1M LiTFSI TEGDME is used as the electrolyte, Whatsman W / D glass fiber membrane (thickness 2.5 mm) is used as the separator, the cathode plates prepared in Examples 1-3 and Comparative Example 1, and lithium sheets are used as the anode plates. Assemble them in the order of the negative electrode shell, lithium sheet, separator, electrolyte, positive electrode sheet, gasket, and positive electrode shell, and use a button battery sealer to compact them to obtain a Li-O2 battery. The light-assisted condition during the test should be provided by a xenon lamp with a color temperature of 3500-5000 k, and the charge-discharge test should be carried out in an oxygen glove box.

[0065] Among them, the preparation method of the positive electrode sheet is as follows: Take Bi2O3@Ag2WO4 and Bi2O3 prepared in Examples 1 to 3 or Comparative Example 1 as positive electrode additives. After mixing them according to the mass ratio of additive: conductive agent (super p): binder (PVDF) of 7:2:1, grind them thoroughly in a mortar, add an appropriate amount of NMP and grind until it becomes a slurry. Then, use a scraper to evenly coat the slurry on carbon paper and dry it in vacuum for 12 h to obtain the positive electrode sheet.

[0066] Figure 3 The first charge-discharge curves of the lithium-oxygen batteries assembled with the materials prepared in Examples 1 to 3 and Comparative Example 1 as catalysts at a current density of 0.1 A / g are shown. The test conditions are a current density of 0.1 A / g and a voltage range of 2.2 - 4.5 V, and the set specific capacity is 500 mAh / g. Observe Figure 3 It can be seen that Example 2 has the lowest first overpotential of 0.73 V (1.07 V for Experimental Example 1, 1.10 V for Experimental Example 3, and 1.21 V for Comparative Example 1). This shows that the Bi2O3@Ag2WO4 heterojunction can effectively reduce the overpotential and improve the performance of the lithium-oxygen battery. And with the increase of the addition amount of Ag2WO4, the trend of the overpotential first decreases and then slightly increases.

[0067] Figure 4 The first discharge capacity curves of the full cells of the lithium-oxygen batteries using the materials prepared in Examples 1 to 3 and Comparative Example 1 as catalysts are shown. The test conditions are a current density of 0.1 A / g and a voltage range of 2.2 - 4.5 V. Observe Figure 4 It can be seen that Experimental Example 2 has the highest first discharge specific capacity of the lithium-oxygen battery, which is 12379 mAh / g (8856 mAh / g for Experimental Example 1, 10336 mAh / g for Experimental Example 3, and 6814 mAh / g for Comparative Example 1).

[0068] In summary, for the preparation method of a Bi2O3@Ag2WO4 heterojunction as a positive electrode catalyst of a lithium-oxygen battery in the present invention, the prepared Bi2O3@Ag2WO4 heterojunction has a flaky Bi2O3 on the surface and a thin film-like Ag2WO4 growing on it in terms of morphology. When used as a cathode catalyst of a lithium-oxygen battery, Bi2O3@Ag2WO4-2 has the lowest first overpotential of 0.73 V and the highest first discharge specific capacity of the lithium-oxygen battery, which is 12379 mAh / g. This shows that this electrode can effectively improve the conversion rate of lithium peroxide and improve the performance of the battery.

[0069] The above-described embodiments are only descriptions of the preferred embodiments of the present invention, and do not limit the scope of the present invention. Without departing from the design spirit of the present invention, various deformations and improvements made by those of ordinary skill in the art to the technical solutions of the present invention should fall within the protection scope of the present invention.

Claims

1. A Bi2O3@Ag2WO4 composite cathode material, characterized in that the composite cathode material is obtained by coating amorphous silver tungstate on flaky bismuth oxide, and the composite cathode material has a PN heterojunction.

2. A preparation method of the Bi2O3@Ag2WO4 composite cathode material according to claim 1, characterized in that after the flaky bismuth oxide powder is uniformly dispersed in water, a silver source and a tungsten source are added and stirred evenly, and then transferred to a reaction kettle for heating reaction to obtain a composite cathode material of bismuth oxide and silver tungstate.

3. According to the preparation method described in claim 2, characterized in that the preparation method of the bismuth oxide includes the following steps: S1. A bismuth source and an oxygen source are added to water and stirred evenly, and a sodium hydroxide solution is dropped to ensure that the solution is alkaline, and then transferred to a reaction kettle for heating reaction to obtain a precursor; S2. The precursor obtained in S1 is calcined to obtain flaky bismuth oxide.

4. According to the preparation method described in claim 3, characterized in that in the step S1, the bismuth source is selected from one or more of bismuth nitrate, bismuth sulfate, bismuth carbonate, and bismuth phosphate, and the oxygen source is sodium sulfate.

5. According to the preparation method described in claim 3, characterized in that in the step S1, the molar ratio of bismuth in the bismuth source to sodium hydroxide is 1:5 to 7.

6. According to the preparation method described in claim 3, characterized in that in the step S2, the calcination temperature is 400 to 500 °C, and the calcination time is 1 to 3 h.

7. According to the preparation method described in any one of claims 2 to 6, characterized in that the silver source is selected from one or more of silver nitrate, silver chloride, and silver sulfate, and the tungsten source is selected from one or more of ammonium tungstate, sodium tungstate, zinc tungstate, cobalt tungstate, and calcium tungstate.

8. According to the preparation method described in claim 2, characterized in that the molar ratio of bismuth oxide to silver tungstate is 1:(0.5 to 2).

9. According to the preparation method described in claim 2 or 8, characterized in that the heating temperature is 160 to 180 °C, and the heating time is 12 to 24 h.

10. Application of the Bi2O3@Ag2WO4 composite cathode material according to claim 1 or the Bi2O3@Ag2WO4 composite cathode material prepared by the method described in any one of claims 2 to 9 in a photo-assisted lithium-oxygen battery.

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