Magnesium-based cathode material and preparation method thereof

Abstract

The application relates to the technical field of secondary batteries, and discloses a magnesium-based battery positive electrode material and a preparation method thereof. X My[Fe(CN)5NO)]; wherein A is any one of Na, K, Mg and Ca, M is one or more transition metal elements, 0

Description

Technical Field

[0001] The present invention relates to the technical field of secondary batteries, and particularly to a cathode material for a magnesium-based battery and a preparation method thereof. Background Art

[0002] With the continuous increase of energy demand, people have begun to seek energy storage technologies with higher specific capacity, more safety and reliability. Among them, magnesium-ion batteries are one of them. However, there are still many problems in magnesium-ion batteries that hinder their practical applications. For example, the ion migration in magnesium-ion batteries is relatively slow, there is no suitable commercial magnesium-ion electrolyte, and the cathode materials are limited in use.

[0003] Currently, the common cathode materials for magnesium-ion batteries include Chevrel phase Mo6S8 and other sulfides and oxides. Chevrel phase Mo6S8 can form a battery with APC electrolyte and magnesium metal. The discharge specific capacity of this battery can reach up to 120 mAh / g -1 or so, and it has good cycle performance, but it has problems such as low working voltage and low energy density, and the synthesis difficulty is high and the yield is low. Therefore, it cannot be commercially applied at present. Prussian blue, as a common cathode material in sodium-ion batteries, has a rigid open structure, and has advantages such as a high voltage platform, simple and convenient synthesis, and low cost. Therefore, it has received extensive attention in practical applications.

[0004] Generally, the chemical formula of Prussian blue analogues is A x M y [Fe(CN)6] z , where A is any one of Na, K, Mg, Ca, M is selected from any one of transition metals, 0≤x≤2, 0<y<2, 0<z<1. However, compared with lithium and sodium ions, the diffusion energy barrier of magnesium ions is higher. Directly using the existing Prussian blue analogues as cathode materials has problems such as low battery capacity and poor cycle performance, and the solvation of magnesium ions is serious, making it difficult to embed them into the cathode materials and difficult to find a suitable electrolyte to match them. Summary of the Invention

[0005] The present invention aims to provide a cathode material for a magnesium-based battery and a preparation method thereof to solve the technical problem of low battery capacity existing in the existing cathode materials for magnesium-based batteries.

[0006] To achieve the above object, the present invention adopts the following technical scheme: A cathode material for a magnesium-based battery, whose structural formula is A X My[Fe(CN)5NO)]; where A is any one of Na, K, Mg, Ca, M is one or more transition metal elements, 0<x<2, 0<y<2.

[0007] The principles and advantages of this scheme are:

[0008] In this invention, the magnesium-based battery cathode material is doped with various transition metals and partially replaces the C≡N in the structure with N=O synthesized from sodium nitroprusside, thereby reducing the diffusion energy of magnesium ions in the structure and increasing the battery capacity. Specifically, replacing part of the C≡N in the structure with N=O reduces the resistance to magnesium ion transfer, promotes magnesium ion diffusion, and results in fewer structural water and vacancies, thus improving cycle stability. The resulting cathode material exhibits high specific capacity, high operating voltage, and high stability.

[0009] Preferably, as an improvement, the transition metal salt is one or more of manganese salt, zinc salt, cobalt salt, and nickel salt.

[0010] In this application, transition metal salts can be doped with various metals, utilizing the properties of different metal elements to give the prepared cathode material better performance. Among them, zinc doping can control the material morphology, giving it a stable structure; manganese doping can improve the discharge plateau; and cobalt doping can improve the cycle performance of the cathode material and increase the discharge capacity.

[0011] Preferably, as an improvement, the cathode material has a cubic octahedral morphology and the particle size of the cathode material is 2-3 μm.

[0012] In this application, the cubic octahedral morphology can bring a more stable structure to the cathode material and improve the material's cycle performance.

[0013] This application also provides a method for preparing magnesium-based battery cathode materials, including the following steps:

[0014] S1: The transition metal salt and chelating agent were added to deionized water one after another and stirred until homogeneous to obtain solution A;

[0015] S2: Add sodium nitroprusside, PVP and ascorbic acid to deionized water and stir until homogeneous to obtain solution B;

[0016] S3: Slowly add solution A dropwise to solution B and stir until homogeneous to obtain solution C;

[0017] S4: Heat solution C to a constant temperature, react at the constant temperature, and then cool to obtain a precipitate;

[0018] S5: Wash and dry the precipitate obtained in S4 to obtain the cathode material.

[0019] Preferably, as an improvement, the molar ratio of the transition metal salt, chelating agent, sodium nitroprusside, PVP and ascorbic acid is 0.8-1.2:2.4-3.6:0.8-1.2:2.4-3.6:2.4-3.6.

[0020] In this application, it is necessary to control the weight ratio of each raw material to ensure the preparation of a qualified cathode material. Sodium nitroprusside primarily serves to provide the structural framework for the metallic material; if its content is too low, cathode material preparation is impossible, while if its content is too high, the doping amount of transition metal salts in the synthesized cathode material will be too low, rendering the cathode material unusable. Ascorbic acid is used to prevent oxidation reactions; if the amount of ascorbic acid added is too high, it will cause an imbalance in the iron ion ratio in the cathode material, rendering the cathode material unusable.

[0021] Preferably, as an improvement, the chelating agent is EDTA or sodium citrate.

[0022] In this application, the addition of a chelating agent can reduce the nucleation rate, thereby controlling the content of structural water and vacancies and improving the stability of the cathode material.

[0023] Preferably, as an improvement, the dropping rate of solution A in step S3 is 5-10 ml / min.

[0024] In this application, controlling the dropping rate of the solution can ensure the synthesis of the cathode material. If the dropping rate is too fast or too slow, the cathode material cannot be successfully prepared.

[0025] Preferably, as an improvement, the constant temperature in step S4 is 100-200°C, and the constant temperature reaction time is 3-5 hours.

[0026] In this application, if the reaction temperature is too low, the raw materials will not react with each other, and if the temperature is too high, the cathode material structure will collapse. Attached Figure Description

[0027] Figure 1 This is a TEM (transmission electron microscope) image of the cathode material in Example 1 of the present invention;

[0028] Figure 2 This is a SEM (scanning electron microscope) image of the cathode material in Example 1 of the present invention;

[0029] Figure 3 The XRD diffraction pattern of the cathode material in Example 1 of this invention;

[0030] Figure 4 The TG (thermogravimetric) diagram of the cathode material in Embodiment 1 of the present invention;

[0031] Figure 5 This is a cycle curve diagram of the positive electrode material in Embodiment 1 of the present invention;

[0032] Figure 6This is a SEM image of the cathode material in Comparative Example 2 of the present invention;

[0033] Figure 7 This is a cycle curve diagram of the cathode material in Comparative Example 2 of the present invention. Detailed Implementation

[0034] The following detailed description illustrates the specific implementation method:

[0035] Example 1

[0036] The basics are as follows: Figure 1-5 As shown, the preparation method of magnesium-based battery cathode material includes the following steps:

[0037] S1: Add the transition metal salt and chelating agent to deionized water one after another, and stir until homogeneous to obtain solution A;

[0038] Specifically, weigh 2 mmol (0.272 g) of zinc chloride and 2 mmol (0.252 g) of manganese chloride, pour them into 80 ml and 20 ml of deionized water respectively and stir. Then add 3.2 g and 0.8 g of sodium citrate respectively and stir for 3 hours to obtain solution A1 and solution A2.

[0039] S2: Add sodium nitroprusside, PVP and ascorbic acid to deionized water and stir until homogeneous to obtain solution B;

[0040] Specifically, 3 mmol (0.789 g) of sodium nitroprusside, 2 g of PVP (polyvinylpyrrolidone), and 2.48 g of ascorbic acid were weighed and added to 100 ml of deionized water and stirred for 3 hours to obtain solution B.

[0041] S3: Slowly add solution A dropwise to solution B and stir until homogeneous to obtain solution C;

[0042] Specifically, solutions A1 and A2 are added dropwise to solution B at a rate of 5 ml / min while stirring. After the addition is complete, stirring is continued for 5 hours to obtain solution C.

[0043] S4: Heat solution C to a constant temperature, react at the constant temperature, and then cool to obtain a precipitate;

[0044] Specifically, solution C was transferred to the medium, heated to 110°C and kept at that temperature for 3 hours, and then cooled to obtain a precipitate.

[0045] S5: Wash and dry the precipitate obtained in S4 to obtain the cathode material.

[0046] Specifically, the obtained precipitate was repeatedly washed with deionized water and ethanol, and then placed in a vacuum oven and kept at 60°C for 12 hours to obtain the positive electrode material, whose structural formula is Na. 1.7 Zn0.36 Mn 0.15 Fe 0.724 [Fe 0.276 [(CN)5NO], its microstructure is as follows Figure 1 As shown, it has an octahedral morphology; the experimental results are as follows. Figures 2-3 As shown, the operating voltage can reach 3.25V, at 50mAg -1 The actual discharge specific capacity can reach 126mAhg. -1 It also exhibits good cycle stability at 100 mAg. -1 After 60 cycles, the specific capacity can still be maintained above 85%.

[0047] This embodiment also provides a method for preparing a magnesium-based battery, including the following steps:

[0048] Step 1: Preparation of the positive electrode sheet: The above-mentioned positive electrode material, binder, and conductive agent are mixed in a mass ratio of 7:2:1 and added to NMP (N-methylpyrrolidone). The mixture is stirred for 12 hours to obtain a positive electrode material slurry. The obtained positive electrode material slurry is coated onto molybdenum foil and placed in a vacuum oven to dry at 80°C for 12 hours. Finally, it is cut into 10mm diameter circular pieces as positive electrode sheets and stored in a glove box for later use.

[0049] Step 2: Preparation of electrolyte: Add 0.8 mmol magnesium chloride, 0.4 mmol aluminum chloride, 0.4 mmol magnesium bis(trifluoromethanesulfonyl)imide and 0.4 mmol sodium bis(trifluoromethanesulfonyl)imide to 2 ml of DME (ethylene glycol dimethyl ether) and stir for 2-3 hours to prepare the electrolyte.

[0050] Step 3: Preparation of the negative electrode sheet. A 0.1mm thick, 11mm diameter magnesium foil disc is polished using 1200 grit molybdenum sandpaper until it is smooth and free of scratches. It is then ultrasonically cleaned in anhydrous ethanol. Next, it is dried in a vacuum oven at 60℃ for 12 hours. Finally, it is transferred to a glove box for later use.

[0051] Step 4: Preparation of magnesium-based battery: Using a molded battery casing, the separator, the positive electrode, the negative electrode prepared by the above method, and the electrolyte are assembled in a conventional manner to obtain a magnesium-based battery and perform performance testing.

[0052] Example 2

[0053] The difference between this embodiment and Embodiment 1 lies in the type of transition metal used for doping. Specifically, in this embodiment, the transition metals used for doping are zinc and nickel, with the specific structural formula Na. 1.7 Zn 0.36 Ni 0.15 Fe 0.724 [Fe0.276 (CN)5NO].

[0054] Example 3

[0055] The difference between this embodiment and Embodiment 1 lies in the type of transition metal used for doping. Specifically, in this embodiment, the transition metals used for doping are zinc, cobalt, and manganese, with the specific structural formula Na. 0.7 Zn 0.36 Mn 0.15 Co 0.5 Fe 0.724 [Fe 0.276 (CN)5NO].

[0056] Comparative Example 1

[0057] The difference between this comparative example and Example 1 is that it does not contain any transition metal doping. Its specific structural formula is Fe[Fe(CN)5NO].

[0058] Comparative Example 2

[0059] The difference between this comparative example and Example 1 is that 4 mmol of sodium ferrocyanide was used instead of 3 mmol of sodium nitroprusside in step S2, and ascorbic acid was not added. Its specific structural formula is Na. 1.7 Zn 0.36 Mn 0.64 [Fe(CN)6], its microstructure is shown in the attached figure. Figure 6 As shown, this is a cube-shaped structure, and the experimental results are as follows. Figure 7 As shown, the current density used in the test was 50 mAg. -1 At low current densities, the specific capacity reaches a maximum of 48 mAh g. -1 Furthermore, the specific capacity gradually decreases with increasing cycle number, reaching only 22 mAh g after 15 cycles. -1 It is less than 50% of the initial specific capacity.

[0060] Comparative Example 3

[0061] The difference between this comparative example and Example 1 is that the transition metal used for doping is different. Specifically, this comparative example is doped with manganese, and its structural formula is Mn[Fe(CN)5NO].

[0062] The experimental data for the examples and comparative examples are recorded in Table 1 below. The specific capacity test used a current density of 200 mAg. -1 .

[0063]

[0064] Analysis of experimental results:

[0065] In this embodiment of the invention, the current density is 200 mAg -1 Under these conditions, its maximum specific capacitance can reach around 70mAh / g, and at 50mAg -1 Their maximum specific capacity can reach around 120mAh / g, which is a significant improvement compared to the comparative ratio. Meanwhile, from the attached... Figure 3 It can be seen that the adsorbed water content in the cathode material of the present invention is 10.4% and the bound water content is 4.2%, both of which are much lower than those in the comparative example and the prior art.

[0066] The difference between Comparative Example 1 and the Example is that the cathode material is not doped with transition metal elements, and the specific capacity of the prepared magnesium-based battery is reduced by about 50% compared with the Example.

[0067] The difference between Comparative Example 2 and the Example is that the cathode material did not use the N=O substitution structure synthesized with sodium nitroprusside to replace part of the C≡N. Therefore, the specific capacity of the magnesium-based battery prepared in Comparative Example 2 decreased by about 30% compared to the Example, resulting in a significant reduction in performance. Simultaneously, its adsorbed water content was measured to be 15%-16%, and its bound water content was approximately 5-6%, both higher than those in the Example.

[0068] The difference between Comparative Example 3 and the Example is that the cathode material is doped with only one transition metal, and the specific capacity of the magnesium-based battery prepared by it is reduced by about 30% compared with the Example, resulting in a significant reduction in performance.

[0069] The above descriptions are merely embodiments of the present invention, and common knowledge such as specific technical solutions and / or characteristics are not described in detail here. It should be noted that those skilled in the art can make various modifications and improvements without departing from the technical solutions of the present invention, and these should also be considered within the scope of protection of the present invention. These modifications and improvements will not affect the effectiveness of the implementation of the present invention or the practicality of the patent. The scope of protection claimed in this application should be determined by the content of its claims, and the specific embodiments described in the specification can be used to interpret the content of the claims.

Claims

1. A method for preparing magnesium-based battery cathode material, characterized in that: The structural formula of the positive electrode material is A X My[Fe(CN)5NO)]; where A is any one of Na, K, Mg, and Ca, M is one or more transition metal elements, 0 < x < 2, 0 < y < 2; the positive electrode material is a cubic octahedron, and the particle size of the positive electrode material is 2 - 3 μm; The preparation method includes the following steps: S1: The transition metal salt and chelating agent were added to deionized water one after another and stirred until homogeneous to obtain solution A; S2: Add sodium nitroprusside, PVP and ascorbic acid to deionized water and stir until homogeneous to obtain solution B; S3: Slowly add solution A dropwise to solution B and stir until homogeneous to obtain solution C; S4: Heat solution C to a constant temperature, react at the constant temperature, and then cool to obtain a precipitate; S5: Wash and dry the precipitate obtained in S4 to obtain the cathode material; The molar ratio of the transition metal salt, chelating agent, sodium nitroprusside, PVP and ascorbic acid is 0.8–1.2:2.4–3.6:0.8–1.2:2.4–3.6:2.4–3.

6.

2. The method for preparing the magnesium-based battery cathode material according to claim 1, characterized in that: The transition metal salt is one or more of manganese salt, zinc salt, cobalt salt, and nickel salt.

3. The method for preparing the magnesium-based battery cathode material according to claim 2, characterized in that: The chelating agent is EDTA or sodium citrate.

4. The method for preparing the magnesium-based battery cathode material according to claim 3, characterized in that: In step S3, the dropping rate of solution A is 5-10 ml / min.

5. The method for preparing the magnesium-based battery cathode material according to claim 4, characterized in that: In step S4, the constant temperature is 100-200℃, and the constant temperature reaction time is 3-5 hours.

Images