Mnnife oxide nanosheet and preparation method and application thereof

Abstract

The present application belongs to the technical field of lithium ion batteries, and particularly relates to a MnNiFe oxide nanosheet, a preparation method and application thereof. The present application first synthesizes NiFe LDHs nanosheets, then realizes manganese doping by using the nanosheets as a template and adopting an impregnation method, and further obtains MnNiFe oxide nanosheets after a further sintering process. The method has simple steps, low raw material cost, and potential market value. The doping of manganese ions can significantly adjust the crystallinity and structural stability of the three-metal nanomaterials. In the heat treatment process, the LDHs hydrotalcite structure is destroyed, and the anions and water disappear from the space between the layers, which promotes the formation of small pores in the material structure and also promotes the formation of nanosheets. The coordination among the three elements and the porous nanosheet structure can greatly improve the electrochemical performance, including the cycle performance and rate performance.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery technology, specifically relating to a MnNiFe oxide nanosheet, its preparation method, and its application. Background Technology

[0002] Sales of new energy vehicles have increased significantly, and the range of pure electric vehicles directly affects consumers' driving experience. Improving range is directly related to the cathode material of lithium batteries, so it is extremely important to prepare low-cost, high-capacity cathode materials.

[0003] Manganese (Mn) ore resources are more abundant and cheaper than cobalt metal. Therefore, if manganese can be used extensively in the development and application of lithium-ion cathode materials, the cost of lithium batteries can be reduced from the raw material stage. Thus, the development and application of manganese in cathode materials has always been a focus of research institutes and enterprises.

[0004] Layered double hydroxides (LDHs), also known as hydrotalcite-like structures, are a typical sandwich structure. The main body consists of a common-edge octahedral MO6 structure composed of divalent and trivalent transition metal cations, while the guest consists of anions with balanced charges and water molecules forming H bonds. This special structure facilitates the full exposure of active sites and enhances ion transport kinetics, resulting in excellent electrochemical performance.

[0005] For example, existing technology discloses a method for synthesizing nickel-iron layered bimetallic hydroxides. The preparation steps include preparing a bimetallic solution and a complexing agent solution of a certain concentration; adding the complexing agent solution dropwise to the bimetallic solution to obtain a stable solution to be treated; subjecting the solution to ultrasonic treatment; washing the precipitate with ultrapure water and ethanol by centrifugation after treatment; and finally vacuum drying to obtain nickel-iron layered bimetallic hydroxides with large interlayer spacing. However, bimetallic hydroxides themselves are unstable, which directly affects the lifespan of electrode materials. Summary of the Invention

[0006] Therefore, the technical problem to be solved by the present invention is to overcome the defects of nickel-iron bimetallic hydroxides in the prior art, such as instability and impact on service life, thereby providing a MnNiFe oxide nanosheet, its preparation method and application.

[0007] Therefore, the present invention provides the following technical solution:

[0008] This invention provides a method for preparing MnNiFe oxide nanosheets, comprising the following steps:

[0009] S1, preparing nickel-iron LDH nanosheets;

[0010] S2, the obtained nickel-iron LDHs nanosheets are immersed in potassium permanganate solution to obtain manganese-doped nickel-iron LDHs nanosheets;

[0011] S3, the obtained manganese-doped nickel-iron LDHs nanosheets are calcined to obtain the MnNiFe oxide nanosheets.

[0012] Optionally, in step S2, the mass ratio of nickel-iron LDH nanosheets to potassium permanganate is 1:(1-2).

[0013] Optionally, in step S2, the concentration of the potassium permanganate solution is 1-3 g / L.

[0014] Optionally, in step S2, the pH of the system is kept weakly alkaline during the impregnation process;

[0015] Optionally, the pH of the system can be maintained at 7.5-9.5.

[0016] Optionally, in step S3, the calcination temperature is 450-800℃ and the calcination time is 2-7h.

[0017] The method for preparing nickel-iron hydroxide nanosheets in this invention is a conventional method in the field. Typically, without limitation, in step S1, nickel-iron LDH nanosheets can be prepared by hydrothermal method or high-temperature reflux method.

[0018] Specifically, the hydrothermal method for preparing nickel-iron LDH nanosheets includes the following steps:

[0019] Nickel and iron salts are dissolved in water, urea and hexamethylenetetramine are added, and a hydrothermal reaction is carried out. The products are separated to obtain the nickel-iron carbonate layered nanosheets. Urea can be replaced by ammonia, or methanol can be used to replace urea and hexamethylenetetramine. These substances mainly play a role in morphology control.

[0020] The method for preparing nickel-iron LDH nanosheets by high-temperature reflux is the same as that for hydrothermal method. The preparation method includes the following steps: mixing the raw materials, refluxing at 140-180℃ under nitrogen protection for a period of time (e.g., 24 hours), cooling to room temperature under nitrogen protection, and then centrifuging, washing, and collecting.

[0021] Optionally, the method for preparing the MnNiFe oxide nanosheets satisfies at least one of the following (1)-(4):

[0022] (1) The molar ratio of nickel salt to iron salt is 3:1-1:3;

[0023] (2) The molar ratio of urea to hexamethylenetetramine is 5:1 to 1:1;

[0024] (3) In terms of metallic elements, the ratio of the total number of moles of nickel and iron to the number of moles of urea is 1:3-2:1;

[0025] (4) The temperature of the hydrothermal reaction is 100-180℃ and the time of the hydrothermal reaction is 24-52h.

[0026] Specifically, the preparation process of the present invention is as follows:

[0027] (1) Preparation of NiFe LDHs

[0028] Dissolve 5 mmol of FeCl2·6H2O and 10 mmol of NiCl2·6H2O in 35 mL of water (preferably, chloride salts are used, but other nickel-iron metal salts are also acceptable). Under stirring, add 30 mmol of urea and 10 mmol of hexamethylenetetramine (the molar ratio of each substance can be other ratios, ① the ratio of the two metals can be between 3:1 and 1:3, and the total molar amount of metal can be between 10 mmol and 30 mmol; ② the ratio of urea to hexamethylenetetramine can be between 5:1 and 1:1; ③ the ratio of the total molar amount of metal to urea can be between 1:3 and 2:1). Stir for 30 minutes and sonicate for 15 minutes to form a homogeneous and clear solution. Pour the solution into a high-pressure reactor lined with polytetrafluoroethylene. The high-pressure reactor was placed in a forced-air drying oven and (preferably) kept at 150℃ for 48 hours (the reaction temperature can be 100℃, 110℃, 120℃, 130℃, 140℃, 150℃, 160℃, 170℃, or 180℃; the reaction time can be 24 hours, 28 hours, 32 hours, 36 hours, 40 hours, 42 hours, 48 ​​hours, or 52 hours). After cooling, the reactor was removed, centrifuged several times with water and ethanol, and the sample was collected. Finally, the product was dried overnight in a drying oven to obtain the NiFe LDHs material. In this hydrothermal reaction, the slow and gradual hydrolysis of urea and hexamethylenetetramine makes the solution generally alkaline, and under high temperature conditions, metal ions are induced to form uniform crystals.

[0029] (2) Preparation of manganese-doped NiFe LDHs

[0030] Take 40 mg of the NiFe LDHs material obtained above and soak it in 30 mL of an aqueous solution containing 60 mg of potassium permanganate (the content of potassium permanganate can be 40 mg, 50 mg, 60 mg, or 70 mg). Under stirring, slowly add a 0.1 mol / L Na2CO3 / NaOH buffer solution to the solution until the pH of the solution is 8.5 (the pH can be between 7.5 and 9.5). Place the well-mixed solution in a vacuum drying oven at 55–85 °C. MnO4, which has strong oxidizing properties, will dry the solution in a weakly alkaline environment. 4-The sample undergoes a chemical reaction with LDHs. After the solution has completely evaporated, the sample is removed and crushed to obtain a composite material of manganese-doped NiFe LDHs.

[0031] (3) MnNiFe porous nanosheet oxide

[0032] The manganese ion-doped NiFe LDHs composite material obtained above was placed in a tube furnace and held in a nitrogen atmosphere at 650℃ (the tube furnace temperature can be 450℃, 500℃, 550℃, 600℃, 650℃, 700℃, 750℃, or 800℃) for 5 hours (holding time can be 2h, 3h, 4h, 5h, 6h, or 7h). During the heat treatment, the LDHs-like hydrotalcite structure was destroyed, and carbonate anions and water disappeared from the interlayer space, promoting the formation of fine pores and channels in the material structure, and also facilitating the formation of nanosheets. The addition of manganese ions can significantly regulate the crystallinity and structural stability of the trimetallic nanomaterials. The synergistic effect among the three elements and the porous nanosheet structure can also greatly improve its electrochemical performance.

[0033] The present invention also provides MnNiFe oxide nanosheets prepared by the above preparation method.

[0034] The present invention also provides a positive electrode sheet comprising the above-mentioned MnNiFe oxide nanosheets.

[0035] The present invention also provides a lithium-ion battery, including the above-described positive electrode sheet.

[0036] The positive electrode sheet and other components and preparation methods of the lithium-ion battery provided by this invention are all conventional in the field.

[0037] Typically, but not exclusively, the process of preparing the positive electrode and assembling the battery can be as follows:

[0038] The above-mentioned MnNiFe oxide nanosheets were combined with 5-8 wt% PVDF binder ((CH2CF2)). n The acetylene black was thoroughly ground in a mortar for at least 20 minutes at a mass ratio of 80-92:4-10:4-10. An appropriate amount of N-methyl-2-pyrrolidone (NMP) was added to the electrode material mixed with the conductive agent and binder, and the mixture was stirred at 1000-2000 rpm for 8-16 minutes to form a uniform slurry; alternatively, a segmented homogenization method using a combination of high and low speeds can be used. The slurry was then uniformly coated onto aluminum foil, which was then dried overnight in a vacuum drying oven at 80°C. The aluminum foil coated with the active material was cut into pieces with an area of ​​1.131 cm². 2 The circular electrode pads contain approximately 3-9 mg / cm³ of active material. -2 The positive electrode sheet is obtained and set aside for later use.

[0039] This positive electrode was used as the positive electrode for lithium-ion batteries in the assembly of 2032 button batteries in a glove box. Additionally, a solution containing 1.0 mol·L⁻¹ was used. -1 A commercially available electrolyte solution consisting of bis(fluoromethanesulfonyl)imide (LiTFSI), 0.5 wt% LiNO3, 1,3-dioxane (DOL), and 1,2-dimethoxyethane (DME) (volume ratio 1:1) was used as the electrolyte for the lithium-ion battery. Both the counter and reference electrodes were lithium foil, and the separator was a Celgard 2400 membrane. Prior to testing, the assembled batteries were placed in a 25°C oven for 4–12 hours to allow the electrolyte to fully wet the electrode materials.

[0040] The technical solution of this invention has the following advantages:

[0041] The present invention provides a method for preparing MnNiFe oxide nanosheets, comprising the following steps: S1, preparing nickel-iron LDHs nanosheets; S2, immersing the obtained nickel-iron LDHs nanosheets in a potassium permanganate solution to obtain manganese-doped nickel-iron LDHs nanosheets; S3, calcining the obtained manganese-doped nickel-iron LDHs nanosheets to obtain the MnNiFe oxide nanosheets. This invention first synthesizes NiFe LDHs nanosheets, then uses them as templates to achieve manganese doping via an impregnation method, and finally obtains MnNiFe oxide nanosheets after a further sintering process. This method is simple, has low raw material costs, and has potential market value. Manganese ion doping can significantly regulate the crystallinity and structural stability of trimetallic nanomaterials. During heat treatment, the LDHs-like hydrotalcite structure is destroyed, and anions and water disappear from the interlayer space, promoting the formation of fine channels in the material structure and also contributing to the formation of nanosheets. The provided MnNiFe oxide nanosheets can greatly improve their electrochemical performance, including cycling performance and rate performance, through the synergistic effect among the three elements and the porous nanosheet structure. Detailed Implementation

[0042] The following embodiments are provided to better understand the present invention and are not limited to the preferred embodiments described. They do not constitute a limitation on the content and scope of protection of the present invention. Any product that is the same as or similar to the present invention, derived by any person under the guidance of the present invention or by combining the features of the present invention with other prior art, falls within the protection scope of the present invention.

[0043] For experiments not specifically described in the examples, the procedures or conditions should be followed according to the conventional experimental procedures described in the literature in this field. Reagents or instruments whose manufacturers are not specified are all commercially available conventional reagent products.

[0044] Example 1

[0045] This embodiment provides a method for preparing MnNiFe oxide nanosheets, and the specific steps and operating parameters are as follows:

[0046] (1) Preparation of NiFe LDHs

[0047] 5 mmol of FeCl₂·6H₂O and 10 mmol of NiCl₂·6H₂O were dissolved in 35 mL of water. Under stirring, 30 mmol of urea and 10 mmol of hexamethylenetetramine were added. The mixture was stirred for 30 minutes and sonicated for 15 minutes to form a homogeneous, clear solution. The solution was then transferred to a high-pressure reactor lined with polytetrafluoroethylene (PTFE). The reactor was placed in a forced-air drying oven and kept at 150 °C for 48 hours. After cooling, the reactor was removed, centrifuged several times with water and ethanol, and the sample was collected. Finally, the product was dried overnight in a drying oven to obtain the NiFe LDHs material.

[0048] (2) Preparation of manganese ion-doped NiFe LDHs

[0049] Take 40 mg of the NiFe LDHs obtained above and soak it in 30 mL of aqueous solution containing 60 mg of potassium permanganate. Under stirring, slowly add 0.1 mol / L Na2CO3 / NaOH buffer solution to the solution until the pH of the solution is 8.5. Place the well mixed solution in a vacuum drying oven at 80 °C. After the solution has evaporated completely, take out the sample and crush it to obtain the composite material of manganese ion doped NiFeLDHs.

[0050] (3) MnNiFe porous nanosheet oxide

[0051] The manganese ion-doped NiFe LDHs composite material obtained above was placed in a tube furnace and kept in a nitrogen atmosphere at 650°C for 5 hours to obtain MnNiFe porous nanosheet oxide.

[0052] Tests showed that its crystallinity was 90%.

[0053] Example 2

[0054] This embodiment provides a method for preparing MnNiFe oxide nanosheets. The only difference from Example 1 is that the reaction temperature in step (1) is 120°C and the reaction time is 52h.

[0055] The crystallinity was tested and found to be 82%. The reason for the reduced crystallinity is that the lower hydrothermal temperature is not conducive to crystal growth.

[0056] Example 3

[0057] This embodiment provides a method for preparing MnNiFe oxide nanosheets. The only difference from Example 1 is that the reaction temperature in step (1) is 170°C and the reaction time is 28h.

[0058] The crystallinity was tested and found to be 78%. The reason for the decreased crystallinity is that the hydrothermal temperature was higher, the reaction was more vigorous, and the grown crystals were more prone to collapse.

[0059] Example 4

[0060] This embodiment provides a method for preparing MnNiFe oxide nanosheets. The only difference from Example 1 is that in step (2), the amount of potassium permanganate used is 45 mg, and the pH of the solution is adjusted to 7.5.

[0061] The crystallinity was tested and found to be 72%. The reason for the reduced crystallinity is the low pH value, which is unfavorable for the doping of metallic manganese.

[0062] Example 5

[0063] This embodiment provides a method for preparing MnNiFe oxide nanosheets. The only difference from Example 1 is that the amount of potassium permanganate used in step (2) is 70 mg, and the pH of the solution is adjusted to 9.5.

[0064] The crystallinity was tested and found to be 60%. The decrease in crystallinity was due to the high pH value, which increased the doping amount of metallic manganese, hindering the recombination of the crystal structure.

[0065] Example 6

[0066] This embodiment provides a method for preparing MnNiFe oxide nanosheets. The only difference from Example 1 is that the calcination temperature in the tube furnace in step (3) is 750°C and the calcination time is 3 hours.

[0067] The crystallinity was tested and found to be 75%. The decrease in crystallinity is due to the high calcination temperature, which caused significant structural collapse and reduced crystallinity.

[0068] Example 7

[0069] This embodiment provides a method for preparing MnNiFe oxide nanosheets. The only difference from Example 1 is that the calcination temperature in the tube furnace in step (3) is 500°C and the calcination time is 7h.

[0070] The crystallinity was tested and found to be 72%. The decrease in crystallinity was due to the low calcination temperature and insufficient crystallization.

[0071] Example 8

[0072] This embodiment provides a method for preparing MnNiFe oxide nanosheets. The only difference from Example 1 is that in step (1), the amount of FeCl2·6H2O is 10 mmol and the amount of NiCl2·6H2O is 5 mmol.

[0073] The crystallinity was tested and found to be 84%. The decrease in crystallinity is due to the influence of the metal ion ratio on the element coordination during crystal crystallization.

[0074] Comparative Example 1

[0075] This comparative example provides a method for preparing NiFe oxide nanosheets, which differs from Example 1 only in that step (2) is not included.

[0076] The crystallinity was tested and found to be 72%. The reduced crystallinity is due to the absence of manganese doping, which affects crystallinity, results in insufficient number of active metal sites, weak intermetallic synergy, low reactivity, and poor electrochemical performance.

[0077] Comparative Example 2

[0078] This comparative example provides a method for preparing MnNiFe oxide nanosheets. The only difference from Example 1 is that manganese dichloride is used instead of potassium permanganate in step (2).

[0079] The crystallinity was tested and found to be 75%. The decrease in crystallinity is due to the presence of Mn in the weakly alkaline solution. 2+ It has low chemical reactivity with NiFe LDHs and low elemental doping concentration.

[0080] Comparative Example 3

[0081] This comparative example provides a method for preparing MnNiFe nanosheets, which differs from Example 1 only in that step (3) is not included.

[0082] The crystallinity was tested and found to be 76%. The reason for the reduced crystallinity is that without calcination, the crystal structure is unstable and the active sites are not easily exposed.

[0083] Test case

[0084] The materials provided in the above embodiments and comparative examples, along with 7 wt% PVDF binder ((CH2CF2)) n The acetylene black was thoroughly ground in a mortar at a mass ratio of 90:6:4 for at least 20 minutes. An appropriate amount of N-methyl-2-pyrrolidone (NMP) was added to the electrode material containing the conductive agent and binder, and the mixture was stirred at 1000 rpm for 12 minutes to form a uniform slurry. The slurry was then uniformly coated onto aluminum foil, which was then dried overnight in a vacuum drying oven at 80°C. The aluminum foil coated with the active material was cut into pieces with an area of ​​1.131 cm².2 The circular electrode sheet contains approximately 3.5 mg / cm³ of active material. -2 The positive electrode sheet is obtained and set aside for later use.

[0085] This positive electrode was used as the positive electrode for lithium-ion batteries in the assembly of 2032 button batteries in a glove box. Additionally, a solution containing 1.0 mol·L⁻¹ was used. -1 A commercially available electrolyte solution consisting of bis(fluoromethanesulfonyl)imide (LiTFSI), 0.5 wt% LiNO3, 1,3-dioxane (DOL), and 1,2-dimethoxyethane (DME) (volume ratio 1:1) was used as the electrolyte for the lithium-ion battery. Both the counter and reference electrodes were lithium foil, and the separator was a Celgard 2400 membrane. Prior to testing, the assembled battery was placed in a 25°C oven for 10 hours to allow the electrolyte to fully wet the electrode materials.

[0086] Half-cell electrochemical performance

[0087] The battery testing system was Wuhan LAND, with a testing temperature of 25℃ and a voltage window set to 1.5–3.2V. Table 1 shows the charge / discharge specific capacity and efficiency details at a current density of 0.1C. The material provided in this embodiment of the invention retains more than 60% of its capacity after 2000 cycles at a current density of 0.1C, reaching a maximum of 81.20%. This material exhibits excellent electrochemical performance as a positive electrode for lithium-ion batteries, while the materials provided in the comparative examples retain less than 50% of their capacity after 2000 cycles at a current density of 0.1C. Table 2 shows the discharge specific capacity at different rates, indicating that this material has good rate performance.

[0088] Table 1

[0089]

[0090]

[0091]

[0092] Note: Capacity retention rate = discharge specific capacity of the nth cycle / discharge specific capacity of the 1st cycle × 100%; Efficiency = discharge specific capacity of the nth cycle / charge specific capacity of the nth cycle × 100%.

[0093] Table 2

[0094]

[0095]

[0096] Note: Capacity retention rate = discharge specific capacity at other rates / discharge specific capacity of the second cycle at 0.1C × 100%.

[0097] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A method for preparing MnNiFe oxide nanosheets for positive electrode plates, characterized in that, The specific steps are as follows: S1, preparing nickel-iron LDH nanosheets; S2, the obtained nickel-iron LDHs nanosheets are impregnated in potassium permanganate solution to obtain manganese-doped nickel-iron LDHs nanosheets; the concentration of the potassium permanganate solution is 1-2 g / L, and the pH of the system is maintained at 7.5-8.5 during the impregnation process; the mass ratio of nickel-iron LDHs nanosheets to potassium permanganate is 1:(1-2). S3, the obtained manganese-doped nickel-iron LDHs nanosheets are calcined to obtain the MnNiFe oxide nanosheets.

2. The method for preparing MnNiFe oxide nanosheets for positive electrode plates according to claim 1, characterized in that, In step S3, the roasting temperature is 450-800℃ and the roasting time is 2-7h.

3. The method for preparing MnNiFe oxide nanosheets for positive electrode plates according to any one of claims 1-2, characterized in that, In step S1, nickel-iron LDH nanosheets are prepared using a hydrothermal method or a high-temperature reflux method.

4. The method for preparing MnNiFe oxide nanosheets for positive electrode plates according to claim 3, characterized in that, The process for preparing nickel-iron LDH nanosheets satisfies at least one of the following (1)-(4): (1) The molar ratio of nickel salt to iron salt is 3:1-1:3; (2) The molar ratio of urea to hexamethylenetetramine is 5:1 to 1:1; (3) In terms of metallic elements, the ratio of the total number of moles of nickel and iron to the number of moles of urea is 1:3-2:1; (4) The temperature of the hydrothermal reaction is 100-180℃ and the time of the hydrothermal reaction is 24-52h.

5. A MnNiFe oxide nanosheet for a positive electrode prepared by the preparation method according to any one of claims 1-4.

6. A positive electrode sheet, characterized in that, Including the MnNiFe oxide nanosheets for positive electrode as described in claim 5.

7. A lithium-ion battery, characterized in that, Includes the positive electrode sheet as described in claim 6.

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