Manganese oxide nanomaterials obtained through phase transition-phase reduction and preparation method and application thereof

Abstract

This invention discloses a manganese oxide nanomaterial obtained through phase change-phase reduction, its preparation method, and its application. The preparation method includes: 1) oxidizing carbon fibers; 2) hydrothermally growing manganese dioxide nanosheets in a potassium permanganate solution to obtain an electrode sheet; 3) hydrothermally growing manganese dioxide / manganese tetroxide nanospheres in a manganese acetate solution; 4) performing charge-discharge cycling on the manganese dioxide / manganese tetroxide nanospheres at 0–1.2 V; 5) further performing charge-discharge cycling on the electrode obtained in step 4) at -1.0–0 V to obtain the manganese dioxide / manganese tetroxide nanosphere electrode material. The phase reduction method of this invention is simple, and the material obtained after reversible cycling improves the cycle performance of the electrode material by 120% during battery use, thereby achieving a performance recovery after the decline in cycle performance of manganese oxide electrode materials.

Description

Technical Field

[0001] This invention belongs to the field of supercapacitor technology, specifically relating to a manganese oxide nanomaterial obtained by phase change-phase reduction, its preparation method, and its application. Background Technology

[0002] Energy and environmental crises have become two major challenges facing the world today, and electrochemical conversion and storage technologies have emerged as one of the most promising energy supplements. Supercapacitors, as novel energy storage devices, are electronic devices that have the potential to compensate for battery deficiencies and even replace batteries in the future. They possess advantages such as fast charging and discharging speeds, good cycle stability, and high power density. Based on their energy storage mechanisms, supercapacitors can be classified into electric double-layer capacitors (EDLCs) and pseudocapacitors (PDCs). EDLCs primarily achieve ion and charge transport through ion adsorption and desorption at the interface between the electrode material and the electrolyte phase, while PDCs achieve energy storage through rapid transfer of Faraday charges at the electrode surface and near the surface, resulting in redox reactions, or through extremely rapid ion adsorption and desorption within the electrode material. Generally speaking, the key factor affecting the performance of supercapacitors is the electrode material. Common electrode materials include carbon materials, metal oxides / sulfides, conductive polymers, MOFs, COFs, and MXenes. Many transition metal oxides have attracted widespread attention from researchers in recent years due to their large theoretical capacitance, low cost, and non-toxicity, with manganese oxides being the most prominent. Mn3O4 nanospheres have attracted significant attention from researchers in recent years due to their ability to undergo phase transitions and deformations under electrochemical stimulation. For example, some researchers have hydrothermally grown nickel-based manganese tetroxide by immersing nickel foam in a solution of ethanol and ethylene glycol containing manganese acetate. This was then subjected to electrochemical cycling to induce different deformations, resulting in high specific capacitance. Another example is the voltammetric cycling of prepared Mn3O4 nanospheres in an electrolyte containing lithium sulfate, which resulted in the insertion of Li during the cyclic phase transition to form Li4Mn5O. 12 The compound increases the original lattice spacing, which is conducive to the entry and exit of ions, thus greatly improving the electrochemical performance. Although Mn3O4 undergoes deformation during the phase transition, increasing the specific surface area and thus increasing the capacitance, after multiple cycles, the excessive deformation of Mn3O4 causes the plates to fall off or petrify, resulting in a significant decrease in cycling performance. Summary of the Invention

[0003] To address the shortcomings and deficiencies of existing technologies, the primary objective of this invention is to provide a method for preparing manganese oxide nanomaterials obtained through phase transformation and phase reduction, specifically a method for highly reduced manganese tetroxide nanospheres (or nanospheres accompanied by rods or nanospheres accompanied by blocks). First, oxidized carbon fibers are reacted with potassium permanganate to grow manganese dioxide nanosheets. Then, a hydrothermal reaction is performed in a mixed solution of manganese acetate in ethanol and ethylene glycol to grow dense manganese dioxide / manganese tetroxide nanospheres. After charge-discharge cycling at 0-0.9 V, followed by charge-discharge cycling at -0.9-0 V, complete manganese tetroxide nanospheres (or nanospheres accompanied by rods / blocks) are formed, thus achieving a complete nanosphere-nanosheet-nanosphere cycle.

[0004] Another object of the present invention is to provide a manganese oxide nanomaterial obtained by the above preparation method through phase transformation-phase reduction.

[0005] Another object of the present invention is to provide the application of the above-mentioned manganese oxide nanomaterial obtained by phase change-phase reduction in supercapacitors.

[0006] To achieve this objective, the present invention employs the following technical solution:

[0007] In a first aspect, the present invention provides a method for preparing manganese oxide nanomaterials obtained by phase transformation-phase reduction, comprising the following steps:

[0008] (1) High-temperature oxidation of carbon fiber

[0009] After cleaning and drying, carbon fibers are oxidized in air to obtain oxidized carbon fibers.

[0010] (2) Hydrothermal growth of manganese oxides from potassium permanganate

[0011] The carbon dioxide from step (1) was placed in a potassium permanganate solution and manganese dioxide was grown by hydrothermal reaction. After washing and drying, the electrode sheet was obtained.

[0012] (3) Growth of manganese tetroxide

[0013] The electrode sheet from step (2) was placed in a manganese acetate solution to grow manganese tetroxide via a hydrothermal reaction. After cleaning and drying, manganese oxide electrode material was obtained.

[0014] (4) Phase transition of manganese oxide electrode materials

[0015] The manganese oxide electrode material from step (3) was subjected to charge-discharge cycles under a positive voltage window until the nanospheres underwent a complete phase transition and became nanosheets. The nanosheet electrode material was then cleaned and dried to obtain the nanosheet electrode material.

[0016] (5) Phase reduction of manganese oxide nanospheres

[0017] By subjecting the nanosheet electrode material from step (4) to charge-discharge cycles under a negative voltage window, the nanosheets can be regenerated to obtain nanosphere morphology, i.e., phase reduction yields manganese oxide nanomaterials.

[0018] Preferably, the carbon fiber described in step (1) is ultrasonically washed in anhydrous ethanol, then rinsed with deionized water and dried.

[0019] Preferably, the oxidation treatment temperature in step (1) is 200-600℃, more preferably 300-600℃; the oxidation treatment time is 1-6 h, more preferably 2-6 h.

[0020] Preferably, the concentration of the potassium permanganate solution in step (2) is 1 to 10 mg / mL, more preferably 1 to 8 mg / mL.

[0021] Preferably, the temperature of the hydrothermal reaction in step (2) is 140-190°C and the time of the hydrothermal reaction is 2-8 h.

[0022] Preferably, the concentration of manganese acetate in the manganese acetate solution in step (3) is 1 to 10 mg / mL, more preferably 2 to 9 mg / mL.

[0023] Preferably, the manganese acetate solution in step (3) is prepared from manganese acetate, ethanol, ethylene glycol and water.

[0024] More preferably, the volume ratio of ethanol, ethylene glycol and water is (1-4):(2-8):1.

[0025] Preferably, the temperature of the hydrothermal reaction in step (3) is 120-200°C, more preferably 120-190°C; and the time of the hydrothermal reaction is 4-8 h.

[0026] Preferably, the positive voltage window in step (4) is 0 to 1.2 V.

[0027] Preferably, the number of charge-discharge cycles in step (4) is 4 to 10,000, more preferably 50 to 10,000.

[0028] Preferably, the electrolyte in the charge-discharge cycle of step (4) is a 0.1-10 mol / L sodium sulfate electrolyte.

[0029] Preferably, the negative voltage window in step (5) is -1.0 to 0 V.

[0030] Preferably, the number of charge-discharge cycles in step (5) is 4 to 10,000, more preferably 50 to 10,000.

[0031] Preferably, the electrolyte in the charge-discharge cycle of step (5) is a 0.1-10 mol / L sodium sulfate electrolyte.

[0032] The process for switching the structure of manganese oxide from nanospheres to nanosheets to nanospheres in this invention is an electrochemical charge-discharge stimulation.

[0033] Secondly, the present invention provides a manganese oxide nanomaterial obtained by the above preparation method through phase transformation-phase reduction.

[0034] Thirdly, the present invention provides the application of the manganese oxide nanomaterial obtained by phase change-phase reduction in supercapacitors.

[0035] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0036] The phase reduction method of this invention is simple. Using this invention, the phase reduction of deformable manganese tetroxide nanospheres can be achieved, thereby realizing the reversible cycle of nanosphere-nanosheet-nanosphere. After the reversible cycle, the cycle performance of the electrode material is improved by 120% during the battery use process, thereby realizing the performance recovery of manganese oxide electrode material after the cycle performance decline. Attached Figure Description

[0037] Figure 1 The image is a scanning electron microscope image of the electrode sheet obtained in step (2) of Example 1.

[0038] Figure 2 and 3 The image is a scanning electron microscope image of the electrode sheet obtained in step (3) of Example 1.

[0039] Figure 4 The image shows a scanning electron microscope (SEM) image of the electrode sheet obtained in step (4) of Example 1. As can be seen from the image, the electrode undergoing electrochemical cycling under a positive voltage window changes from the original nanospheres to nanosheets.

[0040] Figure 5 , 6 Figures 7 and 8 are scanning electron microscope (SEM) images of the electrode material obtained in step (5) of Example 1. As can be seen from the figures, the electrode transforms from nanosheets to nanospheres and nanorods under the action of electrochemical cycling in the negative voltage window, indicating the reduction effect under the electrochemical cycling in the negative voltage window.

[0041] Figure 8 The graph shows the process of the first three charge-discharge cycles obtained in step (5) of Example 1. As can be seen from the graph, the discharge time decreases with the increase of the number of cycles under the negative voltage window, which further indicates that the electrode gradually changes from nanosheets to nanospheres.

[0042] Figure 9The figure shows the electrode charge-discharge cycle performance test results of the electrode material obtained in step (5) of Example 1 at a current density of 8 A / g. As shown in the figure, the electrode still has a complete and symmetrical charge-discharge curve under high current density, indicating the rapid charge migration rate of the electrode.

[0043] Figure 10 The graph shows a comparison of the charge-discharge cycle performance of the electrode obtained in step (4) of Example 1 at a current density of 20 A / g and the electrode obtained in step (5) at a current density of 20 A / g. As can be seen from the graph, the electrode treated with negative voltage window cycling can maintain its discharge time without decay during 50 charge-discharge cycles and has a longer discharge time, indicating that the electrode treated with negative voltage window cycling has better capacitance and rate performance.

[0044] Figure 11 and 12 The image shows a scanning electron microscope (SEM) image of the electrode material obtained in step (4) of Example 2. As can be seen from the image, the electrode undergoing electrochemical cycling under a positive voltage window transforms from nanospheres to nanosheets.

[0045] Figure 13 and 14 The image shows a scanning electron microscope (SEM) image of the electrode material obtained in step (5) of Example 2. As can be seen from the image, under the electrochemical cycling effect in the negative voltage window, the electrode changes from nanosheets to nanospheres and nanoblocks, indicating the reduction effect of electrochemical cycling in the negative voltage window.

[0046] Figure 15 The graph shows the process of the first three charge-discharge cycles obtained in step (5) of Example 2. As can be seen from the graph, the discharge time decreases with the increase of the number of cycles under the negative voltage window, which further indicates that the electrode gradually changes from nanosheets to nanospheres.

[0047] Figure 16 and 17 The image shows a scanning electron microscope (SEM) image of the electrode material obtained in step (4) of Example 3. As can be seen from the image, the electrode undergoing electrochemical cycling under a positive voltage window transforms from nanospheres to nanosheets.

[0048] Figure 18 and 19 The image shows a scanning electron microscope (SEM) image of the electrode material obtained in step (5) of Example 3. As can be seen from the image, under the electrochemical cycling effect in the negative voltage window, the electrode changes from nanosheets to nanospheres and nanoblocks, indicating the reduction effect of electrochemical cycling in the negative voltage window.

[0049] Figure 20 and 21 The image shows a scanning electron microscope (SEM) image of the electrode material obtained in step (4) of Example 4. As can be seen from the image, the electrode undergoing electrochemical cycling under a positive voltage window transforms from nanospheres to nanosheets.

[0050] Figure 22 , 23 Figures 24 and 25 are scanning electron microscope images of the electrode material obtained in step (5) of Example 4. As can be seen from the figures, the electrode changes from nanosheets to nanospheres under the action of electrochemical cycling in the negative voltage window, indicating the reduction effect of electrochemical cycling under the negative voltage window.

[0051] Figure 25 and 26 The image shows a scanning electron microscope (SEM) image of the electrode material obtained in step (4) of Example 5. As can be seen from the image, the electrode undergoing electrochemical cycling under a positive voltage window transforms from nanospheres to nanosheets.

[0052] Figure 27 The image shows a scanning electron microscope (SEM) image of the electrode material obtained in step (5) of Example 5. As can be seen from the image, under the action of electrochemical cycling in the negative voltage window, the electrode changes from nanosheets to thicker nanosheets and nanospheres, indicating the reduction effect of electrochemical cycling under the negative voltage window.

[0053] Figure 28 The graph shows the process of the first three charge-discharge cycles obtained in step (5) of Example 5. As can be seen from the graph, the discharge time decreases with the increase of the number of cycles under the negative voltage window, which further indicates that the electrode gradually changes from nanosheets to nanospheres.

[0054] Figure 29 and 30 The image shows a scanning electron microscope (SEM) image of the electrode material obtained in step (4) of Example 6. As can be seen from the image, the electrode undergoing electrochemical cycling under a positive voltage window transforms from nanospheres to nanosheets.

[0055] Figure 31 and 32 The image shows a scanning electron microscope (SEM) image of the electrode material obtained in step (5) of Example 6. As can be seen from the image, the electrode changes from nanosheets to nanospheres under the action of electrochemical cycling in the negative voltage window, indicating the reduction effect of electrochemical cycling under the negative voltage window.

[0056] Figure 33 The graph shows the process of the first 100 charge-discharge cycles obtained in step (5) of Example 6. As can be seen from the graph, the charge-discharge time decreases with the increase of the number of cycles under the negative voltage window, which further indicates that the electrode gradually changes from nanosheets to nanospheres.

[0057] Figure 34 The image shows a scanning electron microscope (SEM) image of the electrode sheet obtained in step (4) of Comparative Example 1. As can be seen from the image, the electrode exhibits a nanosphere morphology, indicating that the electrode remains a nanosphere under the action of electrochemical cycling through a negative voltage window in the nanosphere state.

[0058] Figure 35 and 36The image shows a scanning electron microscope (SEM) image of the electrode sheet obtained in step (5) of Comparative Example 1. As can be seen from the image, the electrode exhibits a nanosheet morphology, indicating that after undergoing electrochemical cycling in the nanosphere state through a negative voltage window and then through a positive voltage window electrochemical cycle, the electrode is transformed from a nanosphere into a nanosheet.

[0059] Figure 37 The graphs show the first three charge-discharge cycles obtained in step (5) of Example 1 and Comparative Example 1. As can be seen from the graphs, the discharge time increases with the number of cycles under the positive voltage window, indicating an improvement in the electrochemical performance of the electrode. This may be attributed to the increase in specific surface area, active sites, and ion migration rate of the nanomaterial as the nanospheres transform into nanosheets. Meanwhile, the discharge time decreases with the number of cycles under the negative voltage window, possibly due to the reduction in active sites and increased ion migration rate under negative voltage as the nanosheets transform into nanospheres. Detailed Implementation

[0060] The present invention will be further described in detail below with reference to the embodiments and accompanying drawings, but the implementation of the present invention is not limited thereto.

[0061] Unless otherwise specified in the embodiments of this invention, the conditions shall be performed according to conventional conditions or conditions recommended by the manufacturer. All raw materials and reagents used, unless otherwise specified, are commercially available conventional products.

[0062] Example 1

[0063] 1) The carbon fiber was ultrasonically washed in anhydrous ethanol, then washed with deionized water and dried; the dried carbon fiber was heated in air at 500°C for 4 h to obtain oxidized carbon fiber.

[0064] 2) Carbon oxidized fibers were placed in a 4 mg / mL potassium permanganate solution and hydrothermally reacted at 160℃ for 6 h to grow manganese dioxide nanosheets. After repeated washing with deionized water, the nanosheets were dried to obtain electrode sheets.

[0065] 3) Prepare a manganese acetate solution with a mass concentration of 5 mg / mL by adding manganese acetate to ethanol, ethylene glycol and water in a volume ratio of 2:7:1. Place the electrode sheet from step 2) in the manganese acetate solution and then perform a hydrothermal reaction at 180℃ for 5 h to grow manganese tetroxide. After the system cools to room temperature, remove the electrode, wash it repeatedly with deionized water and dry it to obtain nano-spherical manganese oxide electrode material.

[0066] 4) The obtained nanosphere manganese oxide electrode material was subjected to charge-discharge cycles 50 times in 1 mol / L sodium sulfate electrolyte within a voltage window of 0 to 0.9 V (i.e., the lower limit of the voltage was set to 0 and the upper limit to 0.9 V). At this time, the nanospheres completely underwent a phase change and became nanosheets. After the cycle was completed, the nanosheet manganese oxide electrode material was repeatedly washed with deionized water and then dried to obtain the nanosheet manganese oxide electrode material.

[0067] 5) The nanosheet manganese oxide electrode material obtained in step 4) can be phase-reduced and the nanosphere morphology can be obtained again by cycling it 50 times in 1 mol / L sodium sulfate electrolyte under a voltage window of -0.9 to 0 V (i.e., the upper limit of the voltage is set to 0 and the lower limit is set to -0.9 V).

[0068] Example 2

[0069] 1) The carbon fiber was ultrasonically washed in anhydrous ethanol, then washed with deionized water and dried; the dried carbon fiber was heated in air at 600℃ for 2 h to obtain oxidized carbon fiber.

[0070] 2) Carbon oxidized fibers were placed in a 2 mg / mL potassium permanganate solution and hydrothermally reacted at 140℃ for 8 h to grow manganese dioxide nanosheets. After repeated washing with deionized water, the nanosheets were dried to obtain electrode sheets.

[0071] 3) Prepare a manganese acetate solution with a mass concentration of 2 mg / mL by adding manganese acetate to ethanol, ethylene glycol and water in a volume ratio of 1:7:1. Place the electrode sheet from step 2) in the manganese acetate solution and then perform a hydrothermal reaction at 190℃ for 4 h to grow manganese tetroxide. After the system cools to room temperature, remove the electrode, wash it repeatedly with deionized water and dry it to obtain nano-spherical manganese oxide electrode material.

[0072] 4) The obtained nanosphere manganese oxide electrode material was subjected to 2000 charge-discharge cycles in 6 mol / L sodium sulfate electrolyte at a voltage window of 0 to 1.0 V (i.e., the lower limit of the voltage was set to 0 and the upper limit to 1.0 V). At this time, the nanospheres completely underwent a phase change and became nanosheets. After the cycle was completed, the nanosheet manganese oxide electrode material was repeatedly washed with deionized water and then dried to obtain the nanosheet manganese oxide electrode material.

[0073] 5) The nanosheet manganese oxide electrode material obtained in step 4) can be phase-reduced and the nanosphere morphology can be obtained again by cycling it 2000 times in a 6 mol / L sodium sulfate electrolyte with a voltage window of -1.0 to 0 V (i.e., setting the upper limit of voltage to 0 and the lower limit to -1.0 V).

[0074] Example 3

[0075] 1) The carbon fiber was ultrasonically washed in anhydrous ethanol, then washed with deionized water and dried; the dried carbon fiber was heated in air at 300°C for 6 h to obtain oxidized carbon fiber.

[0076] 2) Carbon oxide carbon fibers were placed in a 2 mg / mL potassium permanganate solution and hydrothermally reacted at 150 °C for 6 h to grow manganese dioxide nanosheets. After repeated washing with deionized water, the nanosheets were dried to obtain electrode sheets.

[0077] 3) Prepare a manganese acetate solution with a mass concentration of 2 mg / mL by adding manganese acetate to ethanol, ethylene glycol and water in a volume ratio of 1:6:1. Place the electrode sheet from step 2) in the manganese acetate solution and then perform a hydrothermal reaction at 130℃ for 8 h to grow manganese tetroxide. After the system cools to room temperature, remove the electrode, wash it repeatedly with deionized water and dry it to obtain nano-spherical manganese oxide electrode material.

[0078] 4) The obtained nanosphere manganese oxide electrode material was subjected to 5000 charge-discharge cycles in a 0.3 mol / L sodium sulfate electrolyte with a voltage window of 0 to 1.0 V (i.e., the lower limit of the voltage was set to 0 and the upper limit to 1.0 V). At this time, the nanospheres completely underwent a phase change and became nanosheets. After the cycle was completed, the nanospheres were repeatedly washed with deionized water and then dried to obtain the nanosheet manganese oxide electrode material.

[0079] 5) The nanosheet manganese oxide electrode material obtained in step 4) can be phase-reduced and the nanosphere morphology can be obtained again by cycling it 5000 times in 0.3 mol / L sodium sulfate electrolyte with a voltage window of -1.0 to 0 V (i.e., setting the upper limit of voltage to 0 and the lower limit to -1.0 V).

[0080] Example 4

[0081] 1) The carbon fiber was ultrasonically washed in anhydrous ethanol, then washed with deionized water and dried; the dried carbon fiber was heated in air at 600℃ for 2 h to obtain oxidized carbon fiber.

[0082] 2) Carbon oxidized fibers were placed in an 8 mg / mL potassium permanganate solution and hydrothermally reacted at 180℃ for 2 h to grow manganese dioxide nanosheets. After repeated washing with deionized water, the nanosheets were dried to obtain electrode sheets.

[0083] 3) Prepare a manganese acetate solution with a mass concentration of 5 mg / mL by adding manganese acetate to ethanol, ethylene glycol and water in a volume ratio of 2:7:1. Place the electrode sheet from step 2) in the manganese acetate solution and then perform a hydrothermal reaction at 180℃ for 4 h to grow manganese tetroxide. After the system cools to room temperature, remove the electrode, wash it repeatedly with deionized water and dry it to obtain nano-spherical manganese oxide electrode material.

[0084] 4) The obtained nanosphere manganese oxide electrode material was subjected to 10,000 charge-discharge cycles in an 8 mol / L sodium sulfate electrolyte with a voltage window of 0 to 0.8 V (i.e., the lower limit of the voltage was set to 0 and the upper limit to 0.8 V). At this time, the nanospheres completely underwent a phase change and became nanosheets. After the cycle was completed, the nanospheres were repeatedly washed with deionized water and then dried to obtain the nanosheet manganese oxide electrode material.

[0085] 5) The nanosheet manganese oxide electrode material obtained in step 4) can be phase-reduced and the nanosphere morphology can be obtained again by cycling it 10,000 times in an 8 mol / L sodium sulfate electrolyte with a voltage window of -0.8 to 0 V (i.e., the upper limit of the voltage is set to 0 and the lower limit is -0.8 V).

[0086] Example 5

[0087] 1) The carbon fiber was ultrasonically washed in anhydrous ethanol, then washed with deionized water and dried; the dried carbon fiber was heated in air at 600℃ for 2 h to obtain oxidized carbon fiber.

[0088] 2) Carbon oxidized fibers were placed in a 6 mg / mL potassium permanganate solution and hydrothermally reacted at 180℃ for 3 h to grow manganese dioxide nanosheets. After repeated washing with deionized water, the nanosheets were dried to obtain electrode sheets.

[0089] 3) Prepare a manganese acetate solution with a mass concentration of 2 mg / mL by adding manganese acetate to ethanol, ethylene glycol and water in a volume ratio of 4:2:1. Place the electrode sheet from step 2) in the manganese acetate solution and then perform a hydrothermal reaction at 120℃ for 8 h to grow manganese tetroxide. After the system cools to room temperature, remove the electrode, wash it repeatedly with deionized water and dry it to obtain nano-spherical manganese oxide electrode material.

[0090] 4) The obtained nanosphere manganese oxide electrode material was subjected to 6000 charge-discharge cycles in a 0.7 mol / L sodium sulfate electrolyte with a voltage window of 0 to 0.9 V (i.e., the lower limit of the voltage was set to 0 and the upper limit to 0.9 V). At this time, the nanospheres completely underwent a phase change and became nanosheets. After the cycle was completed, the nanospheres were repeatedly washed with deionized water and then dried to obtain the nanosheet manganese oxide electrode material.

[0091] 5) The nanosheet manganese oxide electrode material obtained in step 4) can be phase-reduced and the nanosphere morphology can be obtained again by cycling it 6000 times in 0.7 mol / L sodium sulfate electrolyte with a voltage window of -0.9 to 0 V (i.e., setting the upper limit of voltage to 0 and the lower limit to -0.9 V).

[0092] Example 6

[0093] 1) The carbon fiber was ultrasonically washed in anhydrous ethanol, then washed with deionized water and dried; the dried carbon fiber was heated in air at 500°C for 6 h to obtain oxidized carbon fiber.

[0094] 2) Carbon oxidized fibers were placed in a 1 mg / mL potassium permanganate solution and hydrothermally reacted at 190℃ for 2 h to grow manganese dioxide nanosheets. After repeated washing with deionized water, the nanosheets were dried to obtain electrode sheets.

[0095] 3) Prepare a manganese acetate solution with a mass concentration of 9 mg / mL by adding manganese acetate to ethanol, ethylene glycol and water in a volume ratio of 3:8:1. Place the electrode sheet from step 2) in the manganese acetate solution and then perform a hydrothermal reaction at 150℃ for 5 h to grow manganese tetroxide. After the system cools to room temperature, remove the electrode, wash it repeatedly with deionized water and dry it to obtain nano-spherical manganese oxide electrode material.

[0096] 4) The obtained nanosphere manganese oxide electrode material was subjected to charge-discharge cycles for 2000 times in a 10 mol / L sodium sulfate electrolyte with a voltage window of 0 to 1.0 V (i.e., the lower limit of the voltage was set to 0 and the upper limit to 1.0 V). At this time, the nanospheres completely underwent a phase change and became nanosheets. After the cycle was completed, the nanospheres were repeatedly washed with deionized water and then dried to obtain the nanosheet manganese oxide electrode material.

[0097] 5) The nanosheet manganese oxide electrode material obtained in step 4) can be phase-reduced and the nanosphere morphology can be obtained again by cycling it 2000 times in 10 mol / L sodium sulfate electrolyte with a voltage window of -1.0 to 0 V (i.e., the upper limit of the voltage is set to 0 and the lower limit is -1.0 V).

[0098] Comparative Example 1

[0099] 1) The carbon fiber was ultrasonically washed in anhydrous ethanol, then washed with deionized water and dried; the dried carbon fiber was heated in air at 500°C for 4 h to obtain oxidized carbon fiber.

[0100] 2) Carbon oxidized fibers were placed in a 4 mg / mL potassium permanganate solution and hydrothermally reacted at 160℃ for 6 h to grow manganese dioxide nanosheets. After repeated washing with deionized water, the nanosheets were dried to obtain electrode sheets.

[0101] 3) Prepare a manganese acetate solution with a mass concentration of 5 mg / mL by adding manganese acetate to ethanol, ethylene glycol and water in a volume ratio of 2:7:1. Place the electrode sheet from step 2) in the manganese acetate solution and then perform a hydrothermal reaction at 180℃ for 5 h to grow manganese tetroxide. After the system cools to room temperature, remove the electrode, wash it repeatedly with deionized water and dry it to obtain nano-spherical manganese oxide electrode material.

[0102] 4) The obtained nanosphere manganese oxide electrode material was subjected to charge-discharge cycles 50 times in a 1 mol / L sodium sulfate electrolyte with a voltage window of -0.9 to 0 V (i.e., the upper limit of the voltage was set to 0 and the lower limit to -0.9 V). After the cycle was completed, it was repeatedly washed with deionized water and then dried. The manganese oxide electrode material after the cycle was still nanospheres.

[0103] 5) The manganese oxide electrode material after step 4) was cycled 50 times in 1 mol / L sodium sulfate electrolyte with a voltage window of 0 to 0.9 V (i.e., the lower limit of the voltage was set to 0 and the upper limit to 0.9 V). The nanosphere morphology could not be obtained.

[0104] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. A method for preparing manganese oxide nanomaterials obtained through phase transformation-phase reduction, characterized in that, Includes the following steps: (1) After cleaning and drying the carbon fiber, it is oxidized in air to obtain oxidized carbon fiber; (2) Place the carbon dioxide from step (1) in a potassium permanganate solution and grow manganese dioxide by hydrothermal reaction. Then wash and dry to obtain an electrode sheet. (3) Place the electrode sheet from step (2) in a manganese acetate solution to perform a hydrothermal reaction to grow manganese tetroxide, clean and dry it to obtain manganese oxide electrode material; (4) The manganese oxide electrode material from step (3) is subjected to charge-discharge cycles under a positive voltage window until the nanospheres undergo a complete phase transition and become nanosheets. The nanosheet electrode material is then cleaned and dried to obtain the nanosheet electrode material. (5) The nanosheet electrode material from step (4) is subjected to charge-discharge cycles under a negative voltage window, i.e., phase reduction to obtain manganese oxide nanomaterials.

2. The preparation method according to claim 1, characterized in that, The negative voltage window in step (5) is -1.0 to 0 V; And / or, the number of charge-discharge cycles in step (5) is 4 to 10,000, more preferably 50 to 10,000.

3. The preparation method according to claim 1 or 2, characterized in that, The positive voltage window in step (4) is 0 to 1.2V; And / or, the number of charge-discharge cycles in step (4) is 4 to 10,000, more preferably 50 to 10,000.

4. The preparation method according to claim 1 or 2, characterized in that, The electrolyte used in the charge-discharge cycle in step (4) is a 0.1–10 mol / L sodium sulfate electrolyte; And / or, the electrolyte in the charge-discharge cycle of step (5) is a 0.1-10 mol / L sodium sulfate electrolyte.

5. The preparation method according to claim 1 or 2, characterized in that, The oxidation treatment temperature in step (1) is 200-600℃, more preferably 300-600℃; the oxidation treatment time is 1-6 h, more preferably 2-6 h.

6. The preparation method according to claim 1 or 2, characterized in that, The concentration of the potassium permanganate solution in step (2) is 1–10 mg / mL, more preferably 1–8 mg / mL; And / or, the temperature of the hydrothermal reaction in step (2) is 140 to 190°C, and the time of the hydrothermal reaction is 2 to 8 h.

7. The preparation method according to claim 1 or 2, characterized in that, In step (3), the concentration of manganese acetate in the manganese acetate solution is 1-10 mg / mL, more preferably 2-9 mg / mL; And / or, the temperature of the hydrothermal reaction in step (3) is 120-200°C, more preferably 120-190°C; the time of the hydrothermal reaction is 4-8 h.

8. The preparation method according to claim 1 or 2, characterized in that, The manganese acetate solution in step (3) is prepared from manganese acetate, ethanol, ethylene glycol and water; The volume ratio of ethanol, ethylene glycol and water is (1-4):(2-8):

1.

9. A manganese oxide nanomaterial obtained by phase transformation-phase reduction according to any one of claims 1 to 8.

10. The application of the manganese oxide nanomaterial obtained by phase change-phase reduction as described in claim 9 in a supercapacitor.

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