Method for synthesizing potassium-doped manganese dioxide by solid phase method, potassium-doped manganese dioxide, water-based zinc ion battery positive electrode, water-based zinc ion battery

Abstract

The application belongs to the technical field of energy storage batteries, and discloses a method for synthesizing potassium-doped manganese dioxide by a solid-phase method, potassium-doped manganese dioxide prepared by the method, a water-based zinc ion battery positive electrode, and a water-based zinc ion battery. The application adopts a solid-phase two-step method of high-energy ball milling and moist air oxygen-rich calcination, precisely defines a critical doping interval of a molar ratio of K elements in a potassium source to Mn elements in a manganese source of 0.05-0.3:1, and realizes lattice doping of K ions in alpha-MnO2 instead of surface adsorption. The application breaks through the technical path of a conventional liquid-phase method for preparing potassium-doped MnO2, does not need a solvent, has a simple process, and is easy to scale up and industrialize. The prepared potassium-doped manganese dioxide is porous spherical pure-phase alpha-MnO2, K ions are uniformly embedded in the lattice, and the specific capacity is greater than or equal to 260 mAh / g at a current density of 0.1 A / g. The material is applied to a water-based zinc ion battery positive electrode, and can significantly improve the specific capacity and cycle stability of the battery.

Description

Technical Field

[0001] This invention belongs to the field of energy storage battery technology, specifically relating to a method for solid-phase synthesis of potassium-doped manganese dioxide, and also to potassium-doped manganese dioxide, aqueous zinc-ion battery cathode, and aqueous zinc-ion battery prepared by this method. Background Technology

[0002] Aqueous zinc-ion batteries (AZIBs) have shown great application potential in large-scale energy storage due to their advantages such as high theoretical capacity (zinc anode capacity up to 820 mAh / g), abundant resources, low cost, and the inherent safety of aqueous electrolytes. The cathode material is one of the key factors determining the electrochemical performance of AZIBs. Among them, manganese dioxide (MnO2), with its diverse crystal structures (α, β, γ, δ, etc.), high operating voltage, environmental friendliness, and abundant reserves, is considered one of the most promising cathode materials for AZIBs.

[0003] However, MnO2 cathode materials face two major bottlenecks in practical applications: first, their poor conductivity limits electron transport and rate performance; second, during charging and discharging, Mn... 3+ The Jahn-Teller effect easily leads to crystal structure collapse, and some Mn elements dissolve in the electrolyte, causing rapid capacity decay. To address these issues, researchers have developed various modification strategies, including nanostructure manipulation, conductive material compositing, and metal ion pre-intercalation (doping). Among these, alkali metal ions (such as K+) are particularly effective. + Na + When pre-embedded in layered or tunnel-structured MnO2, it can act as a pillar to stabilize the crystal framework and broaden the ion diffusion channels, thereby significantly improving the cycling stability and rate performance of the material.

[0004] Currently, methods for preparing alkali metal-doped MnO2 mainly focus on liquid-phase methods, such as hydrothermal methods, co-precipitation methods, and sol-gel methods. For example, Chinese patent CN111592045B discloses a method for preparing K2O2 via co-precipitation-hydrothermal-calcination. X Chinese patent CN116924474B discloses a process for synthesizing alkali metal-doped MnO2 through co-precipitation combined with hydrothermal reaction. However, liquid-phase methods generally suffer from drawbacks such as long process flow, harsh reaction conditions (high temperature and high pressure), high waste liquid treatment costs due to the use of large amounts of solvent, and difficulty in continuous production. In addition, some methods use potassium permanganate (KMnO4) as a manganese and potassium source, but the procurement and use of KMnO4 are limited, and the reaction process is violent, posing safety hazards.

[0005] Solid-state methods have attracted much attention due to their simplicity, solvent-free nature, and suitability for large-scale production. Chinese patents CN107634215A and CN118851270A have reported on the preparation of K using a solid-state method combining ball milling and high-temperature calcination. X MnO2 materials. However, existing solid-state methods for preparing K... X MnO2 materials still suffer from several performance defects: their specific capacity is generally low, their rate performance is insufficient, and it is difficult to achieve both high capacity and high rate performance; their cycle stability is poor, with significant capacity decay during long-term charge and discharge, and their structure is prone to collapse; their crystal phase and morphology are difficult to control, and the products often contain impurities and are mostly irregular blocks, lacking porous structures that facilitate ion transport, resulting in poor product consistency; in addition, potassium ions mostly exist in the form of surface adsorption, with low lattice insertion efficiency, making it difficult to effectively play a supporting role in stabilizing the crystal structure, and thus having limited effect on improving electrochemical performance.

[0006] In summary, developing a simple, solvent-free solid-phase synthesis method suitable for large-scale production, capable of precisely achieving potassium ion lattice doping, obtaining pure α-MnO2 with excellent electrochemical performance, is of great significance for promoting the technological advancement and industrial application of aqueous zinc-ion battery cathode materials. Summary of the Invention

[0007] The purpose of this invention is to overcome the shortcomings of existing technologies and provide a solid-state method for synthesizing potassium-doped manganese dioxide. This method employs a two-step solid-state process, which is simple and solvent-free. Through precise molar ratio definition and unique calcination atmosphere control, potassium ion lattice doping is achieved, resulting in high-performance pure-phase potassium-doped manganese dioxide. Another objective of this invention is to provide potassium-doped manganese dioxide prepared by the above method, which is a porous spherical pure-phase α-MnO2 with excellent electrochemical performance. A further objective of this invention is to provide an aqueous zinc-ion battery cathode containing the above-mentioned potassium-doped manganese dioxide and an aqueous zinc-ion battery.

[0008] To achieve the above objectives, the present invention adopts the following technical solution:

[0009] A solid-state method for synthesizing potassium-doped manganese dioxide employs a two-step solid-state process, comprising the following steps:

[0010] (1) High-energy ball milling: The potassium source and manganese source are mixed and then subjected to high-energy ball milling to obtain precursor powder; the molar ratio of K element in the potassium source to Mn element in the manganese source is 0.05~0.3:1.

[0011] (2) Oxygen-enriched calcination in humid air: The precursor powder obtained in step (1) is placed in a sagger or crucible and calcined in an oxygen-enriched atmosphere with a relative humidity of 40% to 60%. The calcination temperature is 300 to 500°C and the calcination time is 2 to 4 hours. After calcination, the powder is cooled to room temperature in the furnace to obtain potassium-doped manganese dioxide. The volume fraction of O2 in the oxygen-enriched atmosphere is ≥80%.

[0012] This invention combines high-energy ball milling with oxygen-enriched calcination in humid air to construct a novel two-step solid-phase synthesis process, breaking through the existing liquid-phase method. It eliminates the need for solvents, significantly simplifies the process, reduces production costs, and is more suitable for large-scale industrial production. Through extensive experiments, this invention precisely defines the critical range of the molar ratio of K to Mn elements (0.05~0.3:1). This range is crucial for K ions to embed into the 2×2 channels of α-MnO2 to form lattice doping in the solid-phase method: when the molar ratio is below 0.05, no effective K ions embed into the lattice, and the doping modification effect cannot be achieved; when the molar ratio is above 0.3, excess K ions cannot enter the lattice, easily generating impurity phases such as K2CO3 and K2Mn4O8, leading to a significant decrease in the electrochemical performance of the material.

[0013] Meanwhile, this invention introduces a humid, oxygen-rich atmosphere with a relative humidity of 40% to 60% for the first time during solid-state calcination. This atmosphere works synergistically with the calcination parameters to achieve the dual effects of pure phase formation and porous morphology construction: the oxygen-rich atmosphere ensures that Mn is fully oxidized to +4 valence, avoiding the formation of low-valence manganese oxides such as Mn3O4, and ensuring the formation of the α-MnO2 pure phase; the trace amounts of H2O in the humid air can slightly etch the material surface during calcination, inducing the material to form a porous spherical morphology, which greatly improves the specific surface area and ion diffusion efficiency of the material; and the relative humidity of the humid air is controlled at 40% to 60%. If the humidity is too low, the etching effect will be insufficient and a porous morphology cannot be formed, while if the humidity is too high, the crystal form of the material will collapse.

[0014] Preferably, in step (1), the molar ratio of K element in the potassium source to Mn element in the manganese source is 0.1:1. This ratio is the optimal ratio for solid-phase lattice doping, which can achieve uniform embedding of K ions in the α-MnO2 lattice, taking into account both the crystal stability and electrochemical performance of the material.

[0015] Preferably, in step (1), the potassium source is one or more of potassium acetate, potassium sulfate, potassium carbonate, potassium bicarbonate, and potassium hydroxide; and the manganese source is one or more of manganese carbonate and manganese trioxide.

[0016] Preferably, in step (1), the high-energy ball milling speed is 800~1200 r / min, and the milling time is 10~20 min; the milling medium used in the high-energy ball milling is zirconia balls, and the ball-to-material ratio is 10:1~20:1. These milling parameters enable the potassium source and manganese source to achieve atomic-level uniform mixing, avoiding the problem of localized uneven doping that is prone to occur in the solid-state method, and laying the foundation for uniform lattice doping of K ions in the subsequent calcination process; as a milling medium, zirconia balls have high hardness and do not introduce impurities, ensuring the purity of the precursor powder.

[0017] Preferably, in step (2), the heating rate of calcination is 1~5℃ / min, and the oxygen flow rate of the oxygen-enriched atmosphere is 100~150L / min. The slow heating rate allows the precursor powder to react fully, avoiding crystal distortion caused by excessively high local temperatures; the oxygen flow rate of 100~150L / min ensures that a high concentration of oxygen-enriched atmosphere is maintained in the calcination system, ensuring the full oxidation of Mn.

[0018] Preferably, in step (2), the calcination temperature is 450℃ and the calcination time is 3h. These calcination parameters are the optimal parameters, which can fully decompose the precursor powder and complete the lattice doping of K ions, while avoiding material sintering caused by excessively high temperature or excessively long time, and ensuring the integrity of the porous spherical morphology.

[0019] Preferably, the potassium source is potassium acetate, and the manganese source is manganese carbonate. Potassium acetate has a moderate decomposition temperature, generates no harmful gases during calcination, and its K ion release rate is highly matched with the oxidation rate of Mn in manganese carbonate, enabling efficient lattice doping of K ions. After calcination and decomposition, manganese carbonate only generates MnO2 and CO2, leaving no impurities. It has the best compatibility with the potassium source, ensuring the purity of the final product.

[0020] The present invention also provides a potassium-doped manganese dioxide, which is prepared by any of the methods described above; the potassium-doped manganese dioxide is a porous spherical pure phase α-MnO2, in which K ions are embedded in the 2×2 channels of α-MnO2 to form lattice doping, and has a specific capacity ≥260mAh / g at a current density of 0.1A / g, which is far superior to commercial manganese dioxide, and has high ion diffusion efficiency and good cycle stability.

[0021] This invention also provides an aqueous zinc-ion battery cathode comprising the potassium-doped manganese dioxide described above. Using the potassium-doped manganese dioxide of this invention as the cathode active material can significantly improve the specific capacity and cycle performance of the aqueous zinc-ion battery cathode.

[0022] This invention also provides an aqueous zinc-ion battery comprising the aforementioned aqueous zinc-ion battery cathode. This aqueous zinc-ion battery exhibits high specific capacity, good cycle stability, and low manufacturing cost, making it suitable for large-scale energy storage applications.

[0023] Advantages and benefits of the present invention

[0024] 1. This invention provides a novel two-step solid-phase method for synthesizing potassium-doped manganese dioxide, which breaks through the existing liquid-phase method. It requires no solvent, is simple to operate, has low production cost, and is easy to scale up for industrial production.

[0025] 2. This invention precisely defines the critical doping range of K / Mn molar ratio of 0.05 to 0.3:1 through a large number of experiments. This range is the key to realizing K ion lattice doping under solid-state method, which solves the problem of ambiguous definition of doping ratio in existing solid-state methods. Moreover, this molar ratio is universally applicable to different potassium and manganese sources.

[0026] 3. This invention introduces a humid, oxygen-rich atmosphere into solid-state calcination for the first time. This atmosphere works synergistically with the calcination parameters to ensure the formation of the pure α-MnO2 phase and induce the material to form a porous spherical morphology, thereby significantly improving the specific surface area and ion diffusion efficiency of the material.

[0027] 4. The potassium-doped manganese dioxide prepared by this invention is a porous spherical pure phase α-MnO2 with K ions uniformly embedded in the crystal lattice. It has a specific capacity ≥260mAh / g at 0.1A / g, and its electrochemical performance is far superior to that of commercial manganese dioxide. When applied to the cathode of an aqueous zinc-ion battery, it can significantly improve the battery's specific capacity and cycle stability, providing material support for the large-scale application of aqueous zinc-ion batteries. Attached Figure Description

[0028] Figure 1 The image shows the XRD pattern of the manganese dioxide material synthesized in an embodiment of the present invention.

[0029] Figure 2 This is a SEM image of the manganese dioxide material synthesized in an embodiment of the present invention;

[0030] Figure 3 This is a charge-discharge curve of the manganese dioxide material synthesized in an embodiment of the present invention.

[0031] Figure 4 This is a performance comparison diagram of the manganese dioxide materials synthesized in the embodiments of the present invention;

[0032] Figure 5 The image shows the XRD pattern of the manganese dioxide material synthesized in Comparative Example 1 of this invention.

[0033] Figure 6 This is a cycle performance diagram of the manganese dioxide material synthesized in Comparative Example 1 of the present invention;

[0034] Figure 7 The image shows the XRD pattern of the manganese dioxide material synthesized in Comparative Example 2 of this invention.

[0035] Figure 8This is a cycle performance diagram of the manganese dioxide material synthesized in Comparative Example 2 of the present invention;

[0036] Figure 9 The image shows the XRD pattern of the manganese dioxide material synthesized in Comparative Example 3 of this invention.

[0037] Figure 10 This is a cycle performance diagram of the manganese dioxide material synthesized in Comparative Example 3 of the present invention. Detailed Implementation

[0038] To further illustrate the invention's content, features, and practical effects, the invention will be described in detail below with reference to embodiments. It should be noted that the modification methods of the invention are not limited to these specific implementation methods. Equivalent substitutions and modifications made by those skilled in the art based on their reading of the invention's content, without departing from the spirit and essence of the invention, are also within the scope of protection claimed by this invention.

[0039] Example 1

[0040] A solid-phase method for synthesizing potassium-doped manganese dioxide includes the following steps: (1) High-energy ball milling: Weigh 12g of manganese carbonate (0.097mol of Mn) and 1.5g of potassium acetate (0.0097mol of K), with a K / Mn molar ratio of 0.1:1. Mix them and place them in a high-energy ball mill. Use zirconia balls as the ball milling medium, with a ball-to-material ratio of 20:1 and a rotation speed of 1000r / min for 15min to obtain a uniformly mixed precursor powder; (2) Oxygen-enriched calcination in humid air: Place the precursor powder in a muffle furnace, control the relative humidity of the humid air to be 50%, the volume fraction of O2 in the oxygen-enriched atmosphere to be 90%, the oxygen flow rate to be 120L / min, and heat it to 450℃ at a heating rate of 5℃ / min. Calcinate for 3h, and after calcination, allow it to cool naturally to obtain black potassium-doped manganese dioxide powder.

[0041] Assembly and electrochemical performance testing of aqueous zinc-ion batteries: The obtained potassium-doped manganese dioxide powder was used as the positive electrode active material and mixed with conductive carbon black (Super P) and polyvinylidene fluoride (PVDF) at a mass ratio of 8:1:1. The mixture was thoroughly ground using N-methylpyrrolidone (NMP) as a solvent to prepare a uniform slurry. The slurry was uniformly coated onto the surface of a titanium foil current collector and dried in a vacuum drying oven at 80 ℃ for 12 h. It was then punched into circular electrode sheets with a diameter of 12 mm and pressed for later use. The active material loading was controlled at 1.0–2.0 mg·cm³. -2Using zinc foil as the negative electrode and a glass fiber membrane (Whatman GF / D) as the separator, a mixed aqueous solution of 1 mol / L ZnSO4 and 0.1 mol / L MnSO4 was used as the electrolyte. 60–80 μL of electrolyte was added to each coin cell, and assembly was completed in CR2032 coin cells. After assembly, the cells were placed at room temperature for 12 hours to allow the electrolyte to fully wet the electrodes and separator. The assembled aqueous zinc-ion batteries were subjected to constant current charge-discharge tests using a NEWARE battery testing system, with a test voltage range of 1.0–1.8 V, to evaluate the rate performance and cycle stability of the batteries.

[0042] The potassium-doped manganese dioxide prepared in this embodiment was characterized as follows: Figure 1 As shown, XRD test results indicate that the material is a pure phase α-MnO2; Figure 2 As shown, SEM test results indicate that the material has a porous spherical morphology; Figure 3 As shown, electrochemical performance testing revealed that the material has a specific capacity of 260 mAh / g at a current density of 0.1 A / g; Figure 4 As shown, after 40 cycles, the capacity is still far superior to that of commercial manganese dioxide.

[0043] Comparative Example 1

[0044] A method for synthesizing manganese dioxide, except that the K / Mn molar ratio is 0.4:1 (weighing 12g of manganese carbonate and 10.5g of potassium acetate), is completely consistent with the example in all other steps and parameters.

[0045] The product prepared in this comparative example was characterized as follows: XRD test results are shown in the figure. Figure 5 As shown, the product is a mixed phase of MnO2, K2CO3, and K2Mn4O8; electrochemical performance tests are as follows. Figure 6 As shown, the product exhibits a maximum specific capacity of less than 110 mAh / g at a current density of 0.1 A / g, and poor cycling stability.

[0046] This comparative example demonstrates that when the K / Mn molar ratio is higher than the upper limit of 0.3 defined in this invention, even with the optimal process of this invention, excess K ions cannot be embedded in the α-MnO2 lattice, easily generating impurity phases, which leads to a significant decrease in the electrochemical performance of the material. This verifies that 0.3 is the upper limit threshold for solid-phase lattice doping.

[0047] Comparative Example 2

[0048] A method for synthesizing manganese dioxide, except that the K / Mn molar ratio is 0.04:1 (weighing 12g of manganese carbonate and 0.375g of potassium acetate), is completely consistent with the example in all other steps and parameters.

[0049] The product prepared in this comparative example was characterized as follows: XRD test results are shown in the figure. Figure 7 As shown, the product is pure-phase α-MnO2; electrochemical performance tests are as follows. Figure 8 As shown, the specific capacity of this product is only 20 mAh / g at a current density of 0.1 A / g.

[0050] This comparative example demonstrates that when the K / Mn molar ratio is below the lower limit of 0.05 defined in this invention, no effective K ions are embedded in the α-MnO2 lattice, and the doping modification effect cannot be achieved. The material has low ion diffusion efficiency and extremely low specific capacity, thus verifying that 0.05 is the lower limit threshold for effective lattice doping.

[0051] Comparative Example 3

[0052] A method for synthesizing potassium-doped manganese dioxide is identical to the example except that the calcination atmosphere is a conventional air atmosphere (21% O2 volume fraction).

[0053] The product prepared in this comparative example was characterized as follows: XRD test results are shown in the figure. Figure 9 As shown, the product is a mixed phase of α-MnO2 and Mn3O4; electrochemical performance tests are as follows. Figure 10 As shown, the specific capacity of this product is only 80 mAh / g at a current density of 0.1 A / g.

[0054] This comparative example demonstrates that an oxygen-rich atmosphere is a necessary process feature for the present invention to ensure that Mn is fully oxidized to the +4 valence and to obtain the pure phase α-MnO2. Under conventional air atmosphere, Mn is not fully oxidized and is prone to generating low-valence manganese oxide impurities.

[0055] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for solid-state synthesis of potassium-doped manganese dioxide, characterized in that, Includes the following steps: (1) High-energy ball milling: The potassium source and manganese source are mixed and then subjected to high-energy ball milling to obtain precursor powder; the molar ratio of K element in the potassium source to Mn element in the manganese source is 0.05~0.3:1; (2) Oxygen-enriched calcination in humid air: The precursor powder obtained in step (1) is placed in a sagger or crucible and calcined in an oxygen-enriched atmosphere with a relative humidity of 40% to 60%. The calcination temperature is 300 to 500°C and the calcination time is 2 to 4 hours. After calcination, the powder is cooled to room temperature in the furnace to obtain potassium-doped manganese dioxide. The volume fraction of O2 in the oxygen-enriched atmosphere is ≥80%.

2. The method according to claim 1, characterized in that, In step (1), the molar ratio of K element in the potassium source to Mn element in the manganese source is 0.1:

1.

3. The method according to claim 1, characterized in that, The potassium source is one or more of potassium acetate, potassium sulfate, potassium carbonate, potassium bicarbonate, and potassium hydroxide; the manganese source is one or more of manganese carbonate and manganese trioxide.

4. The method according to claim 1, characterized in that, In step (1), the high-energy ball mill rotates at a speed of 800~1200 r / min and the milling time is 10~20 min; the high-energy ball mill uses zirconia balls as the milling medium and the ball-to-material ratio is 10:1~20:

1.

5. The method according to claim 1, characterized in that, In step (2), the heating rate of calcination is 1~5℃ / min, and the oxygen flow rate of the oxygen-rich atmosphere is 100~150L / min.

6. A potassium-doped manganese dioxide, characterized in that, It is prepared by the method according to any one of claims 1 to 5; the potassium-doped manganese dioxide is a porous spherical pure phase α-MnO2, with K ions embedded in the 2×2 channels of α-MnO2 to form lattice doping, and a specific capacity ≥260mAh / g at a current density of 0.1A / g.

7. A positive electrode for an aqueous zinc-ion battery, characterized in that, It includes the potassium-doped manganese dioxide as described in claim 6.

8. An aqueous zinc-ion battery, characterized in that, It includes the aqueous zinc-ion battery positive electrode as described in claim 7.

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