Prussian blue positive electrode material and preparation method and application thereof

Abstract

The invention discloses a Prussian blue positive electrode material as well as a preparation method and application thereof, and relates to the technical field of battery materials. The molecular formula of the Prussian blue positive electrode material is KxNayM [Fe (CN) 6] znH2O, M is a transition metal element, 1.4 < = x + y < = 1.95, 0.90 < = z < = 1, and 0 < = n < = 2. The Prussian blue positive electrode material is doped with the K element, the atomic radius of potassium is larger than that of Na, gaps of crystal lattices of the Prussian blue positive electrode material can be filled with the K element, vacancies in the crystal lattices are fully occupied, crystal water in the crystal lattices is prevented from being formed, and the service life of the Prussian blue positive electrode material is prolonged. Therefore, the influence of the crystal water on the performance of the Prussian blue positive electrode material and the sodium ion battery is greatly reduced, and the unification of the electrochemical test result and the theoretical expectation is ensured.

Description

Technical Field

[0001] The present invention relates to the technical field of battery materials, and in particular to a Prussian blue-based positive electrode material and a preparation method and application thereof. Background Art

[0002] Sodium ion battery is a secondary battery, that is, a rechargeable battery, which mainly relies on Na + It moves between the positive and negative electrodes to work. When charging, Na + The process deintercalates from the positive electrode and then intercalates into the negative electrode through the electrolyte; the reverse occurs during discharge. Due to the abundant sodium resources and low cost, sodium-ion batteries have great application prospects in large-scale energy storage, low-speed transportation, and 5G base station construction.

[0003] However, due to the large radius and molar mass of sodium ions, it is difficult to develop suitable host materials for sodium ions. Prussian blue materials have become very valuable sodium ion battery positive electrode materials due to their advantages such as three-dimensional open framework structure and large interstitial sites. However, sodium ion batteries are sensitive to factors such as Fe(CN)6 vacancies, crystal water, and lattice changes during charging and discharging in the crystal lattice of Prussian blue materials. The actual electrochemical test results deviate greatly from theoretical expectations, resulting in limited application of sodium ion batteries. Therefore, there is an urgent need to provide a Prussian blue material with low crystal water content and little influence of Fe(CN)6 vacancies to ensure that sodium ion batteries using it as raw material can exhibit better electrochemical performance.

[0004] In view of this, the present invention is proposed. Summary of the Invention

[0005] The purpose of the present invention is to provide a Prussian blue cathode material and a preparation method and application thereof.

[0006] The present invention is achieved in that:

[0007] In the first aspect, the present invention provides a Prussian blue cathode material, the molecular formula of which is K x Na y M[Fe(CN)6] z nH2O, wherein M is a transition metal element, 1.4≤x+y≤1.95, 0.05≤x≤0.3, 0.90≤z≤1, 0≤n≤2.

[0008] In an alternative embodiment, M is a positive divalent transition metal element.

[0009] Preferably, M includes at least one of Fe, Mn, Co, Ni and Cu.

[0010] In an optional embodiment, x+y satisfies: 1.7≤x+y≤1.9, z satisfies: 0.92≤z≤0.96, and n satisfies: 0≤n≤1.

[0011] Preferably, the molecular formula of the Prussian blue cathode material is K 0.17 Na 1.65 Mn[Fe(CN)6] 0.94 0.35H2O.

[0012] In a second aspect, the present invention provides a method for preparing a Prussian blue-based positive electrode material as described in any of the aforementioned embodiments, comprising placing ferrocyanide and a transition metal salt into a ball mill for low-temperature ball milling, and then increasing the temperature for high-temperature ball milling, wherein the ferrocyanide is sodium ferrocyanide and potassium ferrocyanide.

[0013] In an optional embodiment, the ball milling temperature of the high-temperature ball milling is 100-220° C., the ball milling speed is 200-500 rpm, and the ball milling time is 2-6 hours.

[0014] Preferably, the ball milling temperature of the high-temperature ball milling is 170° C., the ball milling speed is 300 rpm, and the ball milling time is 4 h.

[0015] Preferably, the ball milling temperature of the low-temperature ball milling is 15 to 80° C., the ball milling speed is 800 to 1400 rpm, and the ball milling time is 1 to 4 hours.

[0016] Preferably, the ball milling speed of the low-temperature ball milling is 1000 rpm, and the ball milling time is 3 hours.

[0017] Preferably, ball milling beads are added to the ball milling jar for ball milling, and the ratio of the mass of the ball milling beads to the total mass of the ferrocyanide and the transition metal salt is 8 to 12:1, more preferably 10:1.

[0018] In an optional embodiment, the molar ratio of the transition metal salt to the ferrocyanide is 0.5-1.5:0.5-1.5, preferably 1:1.

[0019] Preferably, the molar ratio of sodium ferrocyanide to potassium ferrocyanide is 8-9.5:0.5-2; preferably 9.5:0.5-8:2; more preferably 9:1.

[0020] Preferably, the transition metal salt includes any one of sulfate, nitrate, carbonate, acetate or phosphate of a divalent transition metal, more preferably sulfate of a divalent transition metal, and more preferably manganese sulfate.

[0021] In an optional embodiment, the method further includes cleaning and drying the material after high-temperature ball milling.

[0022] Preferably, ultrasonic cleaning is used for cleaning, and the ultrasonic cleaning time is 5 to 15 minutes. After ultrasonic cleaning, the precipitate is collected by centrifugation.

[0023] Preferably, ultrasonic cleaning and centrifugation are repeated 2 to 5 times.

[0024] Preferably, drying comprises placing the cleaned material in a vacuum drying oven for drying at a drying temperature of 100-150° C. for a drying time of 10-15 hours; more preferably, the drying temperature is 120° C. for a drying time of 12 hours.

[0025] In a third aspect, the present invention provides a sodium ion battery positive electrode plate, comprising a Prussian blue-based positive electrode material as described in any one of the aforementioned embodiments or a Prussian blue-based positive electrode material prepared by the preparation method as described in any one of the aforementioned embodiments.

[0026] In a fourth aspect, the present invention provides a sodium ion battery, comprising the sodium ion battery positive electrode sheet according to the aforementioned embodiment.

[0027] In a fifth aspect, the present invention provides a Prussian blue-based cathode material according to any one of the aforementioned embodiments or a Prussian blue-based cathode material prepared by the preparation method according to any one of the aforementioned embodiments, and its use in the field of batteries.

[0028] The present invention has the following beneficial effects:

[0029] The present invention provides a Prussian blue-based cathode material, a preparation method thereof, and an application thereof. By doping the Prussian blue-based cathode material with K element, the atomic radius of potassium is larger than that of Na, and the potassium can fill the gaps in the crystal lattice of the Prussian blue-based material, occupying the vacancies in the crystal lattice and preventing the formation of crystal water in the crystal lattice. This greatly reduces the influence of crystal water on the performance of the Prussian blue-based cathode material and the sodium ion battery, and ensures that the electrochemical test results thereof are consistent with theoretical expectations. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for use in the embodiments. It should be understood that the following drawings only illustrate certain embodiments of the present invention and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other relevant drawings can be obtained based on these drawings without paying any creative work.

[0031] Figure 1 Thermogravimetric curve of the Prussian blue cathode material provided by the present invention;

[0032] Figure 2 A diagram showing the charge and discharge capacity of the sodium ion battery provided by the present invention;

[0033] Figure 3This is a cyclic discharge capacity diagram of the Prussian blue-based positive electrode material provided by the present invention. DETAILED DESCRIPTION

[0034] To make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention are described clearly and completely below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer are used. Where the manufacturer of the reagents or instruments is not specified, all are conventional products that can be purchased commercially.

[0035] The features and performance of the present invention are further described in detail below with reference to the embodiments.

[0036] In the first aspect, the present invention provides a Prussian blue cathode material, the molecular formula of which is K x Na y M[Fe(CN)6] z nH2O, wherein M is a transition metal element, 1.4≤x+y≤1.95, 0.05≤x≤0.3, 0.90≤z≤1, 0≤n≤2.

[0037] Due to factors such as Fe(CN)6 vacancies and crystal water in the lattice of Prussian blue materials and lattice changes during charging and discharging, the electrochemical test results of actual sodium-ion batteries in actual applications deviate significantly from theoretical expectations. In order to solve the above technical problems, the inventors have found that the crystal water in Prussian blue can be subdivided into bound water, interstitial water and adsorbed water.

[0038] Adsorbed water is water adsorbed on the surface of Prussian blue-based materials and can be easily removed by heating. Interstitial water, located in the interstitial spaces of the crystal lattice, can also be removed by increasing the temperature. However, this process can damage the structure of the Prussian blue-based material, thus affecting its functionality. Bound water occupies Fe(CN)6 vacancies in Prussian blue-based materials. Due to its deep binding position, it is almost impossible to remove it by heating. If forced to remove it by heating, the material structure is severely damaged, making it unusable as an electrode material.

[0039] In addition, the content of crystalline water also affects the initial phase structure of Prussian blue-based materials. Generally, when a large amount of crystalline water is present, the most common sodium-poor cubic phase is formed, at which the charge-discharge performance of the Prussian blue-based material will be significantly reduced. When the crystalline water content is gradually reduced to only a small amount of interstitial water, the sodium-rich monoclinic phase is formed. Further reducing the crystalline water content and removing this interstitial water will lead to the sodium-rich rhombohedral phase, at which the capacity of the Prussian blue-based material will be significantly improved. The sodium-rich phase is pursued by cathode material research because future full-battery research requires cathode materials with high sodium content.

[0040] Based on the above principle, the present invention proposes a potassium-doped Prussian blue-based cathode material, wherein the atomic radius of potassium is larger than that of Na, and can fill the gaps in the crystal lattice of the Prussian blue-based material, occupying the vacancies in the crystal lattice and preventing the formation of crystal water in the crystal lattice. This greatly reduces the impact of crystal water on the performance of the Prussian blue-based cathode material and the sodium ion battery, ensuring that its electrochemical test results are consistent with theoretical expectations.

[0041] Preferably, since K is not electrochemically active, in order to ensure the point mutual performance of the Prussian blue-based positive electrode material, x<y. Due to the small atomic radius of Na, when it is doped in the Prussian blue-based positive electrode material, it cannot fill the lattice gap of the Prussian blue-based positive electrode material. If the preparation method is an aqueous phase preparation or the ambient humidity is high, interstitial water will enter the lattice gap around Na, resulting in the actual electrochemical performance of the sodium ion battery prepared from the Prussian blue-based positive electrode material being less than the theoretical electrochemical performance. In order to improve the actual electrochemical performance of the sodium ion battery prepared from the Prussian blue-based positive electrode material, the present invention uses potassium doping to enable the potassium element to completely fill the lattice gap of the Prussian blue-based positive electrode material, prevent the formation of interstitial water, ensure the consistency of its electrochemical test results with theoretical expectations, and obtain a sodium ion battery with more stable actual electrochemical performance.

[0042] In an alternative embodiment, M is a positive divalent transition metal element.

[0043] Preferably, M includes at least one of Fe, Mn, Co, Ni and Cu.

[0044] In an optional embodiment, x+y satisfies: 1.7≤x+y≤1.9, 0.05≤x≤0.3, z satisfies: 0.92≤z≤0.96, and n satisfies: 0≤n≤1.

[0045] Preferably, the molecular formula of the Prussian blue cathode material is K 0.17 Na 1.65 Mn[Fe(CN)6] 0.94 0.35H2O.

[0046] In a second aspect, the present invention provides a method for preparing a Prussian blue-based positive electrode material as described in any of the aforementioned embodiments, comprising placing ferrocyanide and a transition metal salt into a ball mill for low-temperature ball milling, and then increasing the temperature for high-temperature ball milling, wherein the ferrocyanide is sodium ferrocyanide and potassium ferrocyanide.

[0047] Currently, most existing methods for preparing Prussian blue cathode materials use aqueous synthesis methods. Since water is used as a medium for the auxiliary reaction during the reaction process, a large amount of water is introduced into the crystal lattice of the Prussian blue cathode material, which blocks the sodium ion channels of the battery and results in poor capacity. The present invention, however, adopts a ball milling method to prevent the introduction of water during the preparation process. At the same time, a high-temperature treatment is performed in the latter stage of the ball milling, which can further remove the crystal water in the Prussian blue cathode material. In addition, the doping effect of K in the reaction raw materials effectively avoids the formation of interstitial water in the Prussian blue cathode material from a triple angle, thereby providing a Prussian blue cathode material with low water content, high capacity, and simple preparation process. In addition, the ball milling method provided by the present invention is simple, low-cost, and the obtained product has excellent actual electrochemical performance and good application prospects.

[0048] In an optional embodiment, the ball milling temperature of the high-temperature ball milling is 100-220°C, for example, it can be 100°C, 120°C, 140°C, 160°C, 180°C, 200°C, or 220°C; the ball milling speed is 200-500rpm, for example, it can be 200rpm, 250rpm, 300rpm, 350rpm, 400rpm, 450rpm, or 500rpm; and the ball milling time is 2-6h, for example, it can be 2h, 3h, 4h, 5h, or 6h.

[0049] Preferably, the high-temperature ball milling temperature is 170° C., the ball milling speed is 300 rpm, and the ball milling time is 4 hours. High-temperature ball milling can remove adsorbed water on the surface of the Prussian blue-based cathode material and a small amount of interstitial water inside, thereby obtaining a Prussian blue-based cathode material with more stable electrochemical performance.

[0050] Preferably, the milling temperature of the low-temperature ball milling is 15 to 80°C, for example, it can be 15°C, 30°C, 45°C, 60°C or 80°C; the milling speed is 800 to 1400 rpm, for example, it can be 800 rpm, 900 rpm, 1000 rpm, 1100 rpm, 1200 rpm, 1300 rpm or 1400 rpm; the milling time is 1 to 4 h, for example, it can be 1 h, 2 h, 3 h or 4 h.

[0051] Preferably, the ball milling speed of the low-temperature ball milling is 1000 rpm, and the ball milling time is 3 hours.

[0052] Preferably, ball milling beads are added to the ball milling jar for ball milling, and the ratio of the mass of the ball milling beads to the total mass of the ferrocyanide and the transition metal salt is 8 to 12:1, more preferably 10:1.

[0053] In an optional embodiment, the molar ratio of the transition metal salt to the ferrocyanide is 0.5-1.5:0.5-1.5, preferably 1:1.

[0054] Preferably, the molar ratio of sodium ferrocyanide to potassium ferrocyanide is 8 to 9.5:0.5 to 2; preferably 9.5:0.5 to 8:2; and more preferably 9:1. It is understood that when the molar ratio of sodium ferrocyanide to potassium ferrocyanide is 9.5:0.5 to 8:2, it means that the content of sodium ferrocyanide decreases while the content of potassium ferrocyanide increases. For example, the molar ratio of sodium ferrocyanide to potassium ferrocyanide can be 9.5:0.5, 9:1, 8.5:1.5, or 8:2.

[0055] Preferably, the transition metal salt includes any one of sulfate, nitrate, carbonate, acetate or phosphate of a divalent transition metal, more preferably sulfate of a divalent transition metal, and more preferably manganese sulfate.

[0056] In an optional embodiment, the method further includes cleaning and drying the material after high-temperature ball milling to remove residues from the reaction process and obtain a clean Prussian blue-based positive electrode material.

[0057] Preferably, the cleaning is performed by ultrasonic cleaning with deionized water for 5 to 15 minutes, and the precipitate is collected by centrifugation after ultrasonic cleaning.

[0058] Preferably, ultrasonic cleaning and centrifugation are repeated 2 to 5 times.

[0059] Preferably, drying comprises placing the cleaned material in a vacuum drying oven for drying at a drying temperature of 100-150° C. for a drying time of 10-15 hours; more preferably, the drying temperature is 120° C. for a drying time of 12 hours.

[0060] Since the Prussian blue cathode material has been formed after the ball milling reaction, no interstitial water will be introduced into the Prussian blue cathode material during the cleaning process. Only a small amount of adsorbed water will be removed by centrifugation and drying, ensuring its electrochemical performance for subsequent use.

[0061] In a third aspect, the present invention provides a sodium ion battery positive electrode plate, comprising a Prussian blue-based positive electrode material as described in any one of the aforementioned embodiments or a Prussian blue-based positive electrode material prepared by the preparation method as described in any one of the aforementioned embodiments.

[0062] In an optional embodiment, the preparation of the positive electrode sheet of the sodium ion battery can be prepared by an existing method. For example, the above-mentioned Prussian blue positive electrode material is mixed with conductive carbon and a binder and ground, an organic solvent is added and placed in a homogenizer for vibration mixing to obtain a positive electrode slurry, the positive electrode slurry is evenly coated on a carbon-coated aluminum foil, and after drying, a slicer is used to prepare a sheet of a corresponding shape (round or square, etc.), and then a tablet press is used to press the sheet and placed in a vacuum oven, and vacuum dried at 120°C for 12 hours to obtain a sodium ion battery positive electrode sheet.

[0063] In a fourth aspect, the present invention provides a sodium ion battery, comprising the sodium ion battery positive electrode sheet as described in the aforementioned embodiment.

[0064] In an optional embodiment, the preparation of the sodium ion battery can also adopt the existing preparation method, for example, the sodium ion battery positive electrode plate prepared above is used as the working electrode, metallic sodium is used as the counter electrode, and an organic electrolyte is used to assemble it into a button battery in a glove box filled with argon atmosphere.

[0065] In a fifth aspect, the present invention provides a Prussian blue-based cathode material according to any one of the aforementioned embodiments or a Prussian blue-based cathode material prepared by the preparation method according to any one of the aforementioned embodiments, and its use in the field of batteries.

[0066] Example 1

[0067] This embodiment provides a Prussian blue-based positive electrode material, and the preparation method thereof is as follows:

[0068] S01. Low-temperature ball milling: Weigh 18 mmol of sodium ferrocyanide, 2 mmol of potassium ferrocyanide, and 20 mmol of manganese sulfate into a ball milling jar, add 129 g of ball milling beads, and ball mill at 25°C and 1000 rpm for 3 h.

[0069] S02. High-temperature ball milling: After the ball milling in step S01 is completed, the ball milling jar is heated to 170° C. and the ball milling is continued at a speed of 300 rpm for 4 h.

[0070] S03. Post-treatment: After ball milling, cool the mixture and ultrasonically disperse and stir the mixture in deionized water for 10 minutes. Then, centrifuge the mixture to collect the precipitate. Repeat this process three times. The precipitate collected from the final centrifugation step is dried in a vacuum drying oven at 120°C for 12 hours to produce a Prussian blue cathode material.

[0071] The content of each element in the Prussian blue cathode material prepared in this embodiment was determined by inductively coupled plasma atomic emission spectrometry (ICP-AES). The molecular formula was calculated based on the test results: K 0.17 Na 1.65 Mn[Fe(CN)6] 0.94 0.35H2O.

[0072] Example 2

[0073] This embodiment provides a Prussian blue-based cathode material, the preparation method of which is as follows:

[0074] S01. Low-temperature ball milling: Weigh 18 mmol of sodium ferrocyanide, 2 mmol of potassium ferrocyanide, and 20 mmol of ferrous sulfate into a ball milling jar, add 129 g of ball milling beads, and ball mill at 25°C and 1000 rpm for 3 h.

[0075] S02. High-temperature ball milling: After the ball milling in step S01 is completed, the ball milling jar is heated to 170° C. and the ball milling is continued at a speed of 300 rpm for 4 h.

[0076] S03. Post-treatment: After ball milling, cool the mixture and ultrasonically disperse and stir the mixture in deionized water for 10 minutes. Then, centrifuge the mixture to collect the precipitate. Repeat this process three times. The precipitate collected from the final centrifugation step is dried in a vacuum drying oven at 120°C for 12 hours to produce a Prussian blue cathode material.

[0077] The content of each element in the Prussian blue cathode material prepared in this embodiment was determined by inductively coupled plasma atomic emission spectrometry (ICP-AES). The molecular formula was calculated based on the test results: K 0.17 Na 1.64 Fe[Fe(CN)6] 0.94 0.36H2O.

[0078] Example 3

[0079] This embodiment provides a Prussian blue-based cathode material, the preparation method of which is as follows:

[0080] S01. Low-temperature ball milling: Weigh 19 mmol of sodium ferrocyanide, 1 mmol of potassium ferrocyanide, and 20 mmol of cobalt sulfate into a ball milling jar, add 140 g of ball milling beads, and ball mill at 25°C and 900 rpm for 4 h.

[0081] S02. High-temperature ball milling: After the ball milling in step S01 is completed, the ball milling jar is heated to 150° C. and the ball milling is continued at a speed of 400 rpm for 3 h.

[0082] S03. Post-treatment: After ball milling, cool the mixture and ultrasonically disperse and stir the mixture in deionized water for 10 minutes. Then, centrifuge the mixture to collect the precipitate. Repeat this process three times. The precipitate collected from the final centrifugation step is dried in a vacuum drying oven at 120°C for 12 hours to produce a Prussian blue cathode material.

[0083] The content of each element in the Prussian blue cathode material prepared in this embodiment was determined by inductively coupled plasma atomic emission spectrometry (ICP-AES). The molecular formula was calculated based on the test results: K 0.09 Na 1.71 Co[Fe(CN)6] 0.94 0.62H2O.

[0084] Comparative Example 1

[0085] This comparative example provides a Prussian blue-based positive electrode material, the preparation method of which is as follows:

[0086] S01. Low-temperature ball milling: Weigh 18 mmol of sodium ferrocyanide, 2 mmol of potassium ferrocyanide, and 20 mmol of manganese sulfate into a ball milling jar, add 129 g of ball milling beads, and ball mill at 25°C and 1000 rpm for 3 h.

[0087] S02. Low-temperature ball milling: After the ball milling in step S01 is completed, the ball milling is continued at a speed of 300 rpm for 4 hours.

[0088] S03. Post-treatment: After ball milling, cool the mixture and ultrasonically disperse and stir the mixture in deionized water for 10 minutes. Then, centrifuge the mixture to collect the precipitate. Repeat this process three times. The precipitate collected from the final centrifugation step is dried in a vacuum drying oven at 120°C for 12 hours to produce a Prussian blue cathode material.

[0089] The content of each element in the Prussian blue cathode material prepared in this embodiment was determined by inductively coupled plasma atomic emission spectrometry (ICP-AES). The molecular formula was calculated based on the test results: 0.17 Na 1.65 Mn[Fe(CN)6] 0.94 0.8H2O.

[0090] Comparative Example 2

[0091] This comparative example provides a Prussian blue-based positive electrode material, the preparation method of which is as follows:

[0092] S01. Low-temperature ball milling: Weigh 20 mmol of sodium ferrocyanide and 20 mmol of manganese sulfate and put them into a ball milling jar, add 129 g of ball milling beads, and ball mill at 25°C and 1000 rpm for 3 h.

[0093] S02. High-temperature ball milling: After the ball milling in step S01 is completed, the ball milling jar is heated to 170° C. and the ball milling is continued at a speed of 300 rpm for 4 h.

[0094] S03. Post-treatment: After ball milling, cool the mixture and ultrasonically disperse and stir the mixture in deionized water for 10 minutes. Then, centrifuge the mixture to collect the precipitate. Repeat this process three times. The precipitate collected from the final centrifugation step is dried in a vacuum drying oven at 120°C for 12 hours to produce a Prussian blue cathode material.

[0095] The content of each element in the Prussian blue cathode material prepared in this embodiment was determined by inductively coupled plasma atomic emission spectrometry (ICP-AES). The molecular formula was calculated based on the test results: Na 1.83 Mn[Fe(CN)6] 0.95 0.65H2O.

[0096] Comparative Example 3

[0097] This comparative example provides a Prussian blue-based positive electrode material, the preparation method of which is as follows:

[0098] S01. Aqueous phase synthesis: 18 mmol of sodium ferrocyanide and 2 mmol of potassium ferrocyanide were dissolved in 200 ml of deionized water to form solution A, and 20 mmol of manganese sulfate was dissolved in 200 mol of deionized water to form solution B.

[0099] S02. Heat solution A to 60°C, add solution B dropwise into solution A at a dropping rate of 400ul / min, and age for 10h after the addition is completed.

[0100] S03. Ultrasonic dispersion and stirring were performed with deionized water for 10 minutes, followed by centrifugation to collect the precipitate. This was repeated three times. The precipitate collected from the final centrifugation was dried in a vacuum drying oven at 120°C for 12 hours to obtain a Prussian blue cathode material.

[0101] The content of each element in the Prussian blue cathode material prepared in this embodiment was determined by inductively coupled plasma atomic emission spectrometry (ICP-AES). The molecular formula was calculated based on the test results: 0.16 Na 1.66 Mn[Fe(CN)6] 0.94 2.4H2O.

[0102] Comparative Example 4

[0103] This comparative example provides a Prussian blue-based positive electrode material, and its preparation method is similar to that of Example 1, except that during high-temperature ball milling, the ball milling temperature is 250°C.

[0104] The content of each element in the Prussian blue cathode material prepared in this embodiment was determined by inductively coupled plasma atomic emission spectrometry (ICP-AES). The molecular formula was calculated based on the test results: 0.11 Na 1.02 Mn[Fe(CN)6] 0.94 0.8H2O.

[0105] Comparative Example 5

[0106] This comparative example provides a Prussian blue-based positive electrode material, and its preparation method is similar to that of Example 1, except that during high-temperature ball milling, the ball milling speed is 1000 rpm.

[0107] The content of each element in the Prussian blue cathode material prepared in this embodiment was determined by inductively coupled plasma atomic emission spectrometry (ICP-AES). The molecular formula was calculated based on the test results: 0.17 Na1.65 Mn[Fe(CN)6] 0.94 0.3H2O.

[0108] Comparative Example 6

[0109] This comparative example provides a Prussian blue cathode material, the preparation method of which is similar to that of Example 1, except that the molar ratio of sodium ferrocyanide to potassium ferrocyanide added during low-temperature ball milling is 7:3.

[0110] The content of each element in the Prussian blue cathode material prepared in this embodiment was determined by inductively coupled plasma atomic emission spectrometry (ICP-AES). The molecular formula was calculated based on the test results: 0.55 Na 1.27 Mn[Fe(CN)6] 0.94 0.3H2O.

[0111] Test Example 1

[0112] The Prussian blue cathode materials prepared in Examples 1 to 3 and Comparative Examples 1 to 6 were subjected to thermogravimetric analysis at a heating rate of 5°C / min. Figure 1 Results shown.

[0113] Depend on Figure 1 It can be seen that the Prussian blue-based cathode material provided by the embodiment of the present invention only experiences a cliff-like weight drop after 400°C. Comparative Examples 1 to 6 all experience a cliff-like weight drop after 200°C and another cliff-like weight drop after 400°C. These two weight drops refer to the removal of interstitial water and bound water in the Prussian blue-based cathode material, respectively. Therefore, the thermogravimetric analysis diagram shows that the Prussian blue-based cathode material provided by the embodiment of the present invention has almost no interstitial water, which has a significant advantage in improving the actual performance of the sodium ion battery prepared therefrom.

[0114] Test Example 2

[0115] The Prussian blue cathode materials prepared in Examples 1 to 3 and Comparative Examples 1 to 6 were prepared into sodium ion batteries, and the preparation method was as follows:

[0116] S01. Preparation of positive electrode sheets for sodium ion batteries

[0117] The Prussian blue positive electrode material provided in the embodiment and the comparative example was mixed with conductive carbon and binder polyvinylidene fluoride (PVDF) in a mass ratio of 8:1:1 and ground. Then, an organic solvent 1-methyl-2-pyrrolidone (NMP) was added and the mixture was placed in a homogenizer and vibrated for 10 minutes to prepare a positive electrode slurry.

[0118] The positive electrode slurry was evenly coated on the carbon-coated aluminum foil, and after drying, it was prepared into a circular electrode with a diameter of 14 mm using a slicer. Then, it was pressed using a tablet press and placed in a vacuum oven and vacuum dried at 120°C for 12 hours to obtain the positive electrode of the sodium ion battery.

[0119] S02. Preparation of sodium ion batteries

[0120] The sodium ion battery positive electrode prepared in the above steps was used as the working electrode, metallic sodium was used as the counter electrode, and an organic electrolyte of 1 mol / L NaClO4 / PC:EMC:FEC (49:49:2) was used to assemble a button-type sodium ion battery in a glove box filled with argon atmosphere.

[0121] The sodium ion battery prepared by the above method was subjected to electrochemical performance testing. The test voltage range was 2.0-4.0 V and the charge and discharge current was 10 mA / g. The test results are shown in Table 1. Figure 2 and Figure 3 shown.

[0122] Table 1 Electrochemical performance of sodium ion batteries

[0123]

[0124] Depend on Figure 2 As shown in Table 1, the Prussian blue-based cathode materials provided by the embodiments of the present invention have high initial charge-discharge specific capacity at 10 mA and low water content. This indicates that the Prussian blue-based cathode materials prepared by the embodiments of the present invention, using a staged high-temperature ball milling method with a non-aqueous solution as the reaction and potassium doping of Prussian blue, have advantages such as low water content, high capacity, and a simple preparation process.

[0125] Depend on Figure 3 As shown in Table 1, the Prussian blue-based cathode material provided by the embodiment of the present invention has a good cycle retention rate at 200 mA, and still has a cycle retention rate of 81.62% after 300 cycles, and has high capacity and low water content.

[0126] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Those skilled in the art will readily appreciate that various modifications and variations of the present invention are possible. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention shall be included within the scope of protection of the present invention.

Claims

1. A Prussian blue cathode material, characterized in that: Its molecular formula is K x Na y M[Fe(CN)6] z nH2O, wherein M is a transition metal element, 1.4≤x+y≤1.95, 0.05≤x≤0.3, 0.90≤z≤1, 0≤n≤2.

2. The Prussian blue-based cathode material according to claim 1, characterized in that The M is a divalent transition metal element; Preferably, the M includes at least one of Fe, Mn, Co, Ni and Cu.

3. The Prussian blue-based cathode material according to claim 1 or 2, characterized in that x+y satisfies: 1.7≤x+y≤1.9, z satisfies: 0.92≤z≤0.96, n satisfies: 0≤n≤1; Preferably, the molecular formula of the Prussian blue cathode material is K 0.17 Na 1.65 Mn[Fe(CN)6] 0.94 0.35H2O.

4. A method for preparing a Prussian blue cathode material according to any one of claims 1 to 3, characterized in that: The method comprises placing ferrocyanide and a transition metal salt into a ball milling jar for low-temperature ball milling, and then raising the temperature for high-temperature ball milling; The ferrocyanide is sodium ferrocyanide and potassium ferrocyanide.

5. The preparation method according to claim 4, characterized in that The ball milling temperature of the high-temperature ball mill is 100-220° C., the ball milling speed is 200-500 rpm, and the ball milling time is 2-6 hours; Preferably, the ball milling temperature of the high-temperature ball milling is 170° C., the ball milling speed is 300 rpm, and the ball milling time is 4 h; Preferably, the ball milling temperature of the low-temperature ball milling is 15 to 80° C., the ball milling speed is 800 to 1400 rpm, and the ball milling time is 1 to 4 hours; Preferably, the ball milling speed of the low-temperature ball milling is 1000 rpm, and the ball milling time is 3 h; Preferably, ball milling beads are further added to the ball milling jar for ball milling, and the ratio of the mass of the ball milling beads to the total mass of ferrocyanide and the transition metal salt is 8 to 12:1, more preferably 10:

1.

6. The preparation method according to claim 4, characterized in that The molar ratio of the transition metal salt to the ferrocyanide is 0.5-1.5:0.5-1.5, preferably 1:1; Preferably, the molar ratio of sodium ferrocyanide to potassium ferrocyanide is 8-9.5:0.5-2; preferably 9.5:0.5-8:2; more preferably 9:1; Preferably, the transition metal salt includes any one of sulfate, nitrate, carbonate, acetate or phosphate of a divalent transition metal, more preferably sulfate of a divalent transition metal, more preferably manganese sulfate.

7. The preparation method according to claim 5 or 6, characterized in that: It also includes cleaning and drying the material after high-temperature ball milling; Preferably, the cleaning is performed by ultrasonic cleaning, the ultrasonic cleaning time is 5 to 15 minutes, and the precipitate is collected by centrifugation after ultrasonic cleaning; Preferably, ultrasonic cleaning and centrifugation are repeated 2 to 5 times; Preferably, drying comprises placing the cleaned material in a vacuum drying oven for drying at a drying temperature of 100-150° C. for a drying time of 10-15 hours; more preferably, the drying temperature is 120° C. for a drying time of 12 hours.

8. A sodium ion battery positive electrode plate, characterized in that: The invention comprises the Prussian blue positive electrode material according to any one of claims 1 to 3 or the Prussian blue positive electrode material prepared by the preparation method according to any one of claims 4 to 7.

9. A sodium ion battery, characterized in that: Comprising the sodium ion battery positive electrode sheet as claimed in claim 8.

10. Use of the Prussian blue-based cathode material according to any one of claims 1 to 3 or the Prussian blue-based cathode material prepared by the preparation method according to any one of claims 4 to 7 in the field of batteries.

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