A sodium-rich, low-water-content prussian blue positive electrode material and a preparation method thereof, and a sodium-ion battery

Abstract

The application provides a preparation method of a sodium-rich and low-water-content Prussian blue positive electrode material, which comprises the following steps: dissolving sodium ferrocyanide or a hydrate thereof and a complexing agent in deoxygenated water under an inert gas atmosphere and at a certain temperature to obtain a mixed solution A; dissolving a transition metal salt or a hydrate thereof, a sodium supplementing agent, a complexing agent and an auxiliary complexing agent in deoxygenated water under an inert gas atmosphere and at a certain temperature to obtain a mixed solution B; continuously stirring solution A and solution B by dropwise adding them into a container with a positive pressure inert gas atmosphere and at a certain temperature, continuing to stir for a period of time after the dropwise adding is completed, and then standing, centrifuging and vacuum drying to obtain the Prussian blue positive electrode material. The Prussian blue positive electrode material comprises a mixed phase structure of rhombic phases and cubic phases, has the characteristics of high sodium content and low water content. The battery assembled by using the positive electrode material obtained by the method has a theoretical specific capacity close to the theoretical specific capacity under a small current density and still has a high specific capacity under a large current density.

Description

Technical Field

[0001] This invention relates to the field of energy storage materials technology, and in particular to a sodium-rich, low-water-content mixed-phase Prussian blue cathode material, its preparation method, and its application in sodium-ion batteries. Background Technology

[0002] Since the commercialization of lithium-ion batteries in 1991, they have been widely used in portable electronic products and electric vehicles. However, the limited availability of lithium resources restricts their further application in electric vehicles and large-scale energy storage. Sodium, as a group element with lithium, has similar properties and is abundant, inexpensive, and readily available. Sodium-ion batteries have a similar working mechanism to lithium-ion batteries, making them the most promising battery energy storage device after lithium-ion batteries.

[0003] Currently, the commercially available cathode materials for sodium-ion batteries are mainly Prussian blue-based materials, polyanionic compounds, and layered metal oxides. Prussian blue-based materials have advantages such as a three-dimensional open framework, tunable structure and chemical composition, high theoretical specific capacity, non-toxicity, and ease of synthesis, which are conducive to large-scale applications.

[0004] Prussian blue materials can be prepared using hydrothermal and coprecipitation methods. However, the hydrothermal method generates toxic gases during production, which is not conducive to large-scale production. Coprecipitation can be considered a safe and environmentally friendly production method, but currently, Prussian blue materials prepared by coprecipitation are mostly cubic in structure, with low sodium content and high water content, resulting in low specific capacity, poor cycle stability, and an inability to realize the advantages of Prussian blue materials.

[0005] Therefore, the industry urgently needs a new technology for a Prussian blue cathode material with high sodium content and low water content, as well as its preparation method. Summary of the Invention

[0006] To address the problems existing in the prior art, this invention provides a sodium-rich, low-water-content mixed-phase Prussian blue cathode material and its preparation method. The method involves introducing a complexing agent into a sodium ferrocyanide solution, and introducing a sodium-supplementing agent, a complexing agent, and an auxiliary complexing agent into a transition metal salt solution, followed by co-precipitation to obtain the sodium-rich, low-water-content mixed-phase Prussian blue cathode material. This cathode material has a high initial sodium content and low water content, thus significantly improving its capacity. This material can approach its theoretical specific capacity at low current densities and maintains a high specific capacity at high current densities. The method provided by this invention is simple and easy to implement, does not produce toxic substances, and is easily achievable for large-scale preparation.

[0007] To achieve the above objectives, the present invention provides a method for preparing a sodium-rich, low-water-content Prussian blue cathode material, comprising the following steps:

[0008] Step 1: Dissolve sodium ferrocyanide or its hydrate and complexing agent in deoxygenated water under an inert gas atmosphere and at a certain temperature to obtain mixed solution A;

[0009] Step 2: Dissolve the transition metal salt or its hydrate, sodium supplement, complexing agent, and auxiliary complexing agent in deoxygenated water under an inert gas atmosphere and at a certain temperature to obtain mixed solution B;

[0010] Step 3: By dynamically controlling the rate, solution A obtained in step 1 and solution B obtained in step 2 are added dropwise to a container with a positive pressure inert atmosphere and a certain temperature while continuously stirring. After the addition is complete, stirring is continued for a period of time, followed by standing, centrifugation, and vacuum drying to obtain the Prussian blue cathode material.

[0011] Furthermore, the molecular formula of the Prussian blue cathode material is represented as Na. x M[Fe(CN)6] y ·□1-y·nH2O, where M is the transition metal element in the transition metal salt, which is any one of Fe, Mn, Ni, Co, and Cu, □ represents the Fe(CN)6 vacancy, 1.8<x≤2, 0.95<y≤1, 0<n<1;

[0012] The Prussian blue cathode material is a mixed phase rich in sodium and low in water content, and the mixed phase includes a rhombic phase and a cubic phase.

[0013] Furthermore, in step one, the concentration of sodium ferrocyanide or its hydrate is 0.1–1 mol / L.

[0014] Furthermore, in step two, the transition metal salt is selected from at least one of the sulfate, chloride, acetate, and nitrate salts of Fe, Mn, Ni, Co, and Cu; the concentration of the transition metal salt is 0.1–1 mol / L.

[0015] Furthermore, in steps one and two, the complexing agent is selected from one of sodium thiocyanate, diethanolamine, citric acid, sodium citrate, ammonia, ethylenediaminetetraacetic acid, disodium ethylenediaminetetraacetic acid, tetrasodium ethylenediaminetetraacetic acid, trisodium aminotriacetate, and sodium pyrophosphate; the concentration of the complexing agent in step one is 0.1 to 10 times the concentration of sodium ferrocyanide and its hydrate; the concentration of the complexing agent in step two is 0.1 to 10 times the concentration of the transition metal salt.

[0016] Furthermore, in step two, the sodium supplement is selected from any one of sodium tripolyphosphate, sodium acetate, sodium sulfate, sodium alginate, sodium chloride, and sodium carbonate; the concentration of the sodium supplement is 0.1 to 10 times the concentration of the transition metal salt.

[0017] Furthermore, in step two, the auxiliary complexing agent is selected from at least one of isoascorbic acid, sodium isoascorbate, ascorbic acid, and citric acid; the concentration of the auxiliary complexing agent is 0.1 to 10 times the concentration of the transition metal salt.

[0018] Furthermore, in step three, the dripping rate is 0.1–2 ml / min; the stirring speed is 100–1000 rpm / min; the stirring time is 1–12 h; the settling time is 0–24 h; the centrifugation speed is 1000–10000 rpm / min; and the vacuum drying temperature is 150–300 °C.

[0019] Furthermore, in steps one to three, the inert gas atmosphere is either N2 or Ar, and the specific temperature is 0–80°C; in steps one and two, the dissolved oxygen concentration of the deoxygenated water is 0–3 mg / L.

[0020] This invention provides a sodium-rich, low-water-content Prussian blue cathode material, which is prepared using the above-described method for preparing Prussian blue cathode materials.

[0021] The present invention also provides a sodium-ion battery, comprising a Prussian blue cathode material prepared by the above method.

[0022] The present invention has the following beneficial effects:

[0023] 1. This invention provides a method for preparing a sodium-rich, low-water-content mixed-phase Prussian blue cathode material. The method involves introducing a complexing agent into a sodium ferrocyanide solution, and introducing a sodium-supplementing agent, a complexing agent, and an auxiliary complexing agent into a transition metal salt solution, followed by co-precipitation to obtain the sodium-rich, low-water-content mixed-phase Prussian blue cathode material. Compared with Prussian blue cathode materials prepared by existing co-precipitation methods, the Prussian blue cathode material prepared by this invention has the advantages of high sodium content and low water content. This is attributed to the differential control of the crystallization rate by adding a complexing agent to the sodium ferrocyanide solution and adding a sodium-supplementing agent, a complexing agent, and an auxiliary complexing agent to the transition metal salt solution. The complexing agent can react with Fe... 2+ The formation of complexes slows down the crystallization rate, increases the crystallinity of the material, and reduces the number of vacancies. The reduction in vacancies leads to a corresponding decrease in the structural water content of the material. Simultaneously, the addition of sodium supplementation agents allows for the maximization of Na... + Occupying corresponding spatial sites increases the initial sodium content of the material; simultaneously, the addition of auxiliary complexing agents can hydrolyze the solution to maintain a weakly acidic environment, which is beneficial for Fe 2+ The stable existence of Fe can also oxidize Fe. 2+ Reduction allows Fe to remain in a lower oxidation state, accommodating more Na. +This also increases the Na content of the material; and under the final vacuum drying conditions of 150–300°C, the interstitial water and structural water in the product are further removed, resulting in the sodium-rich, low-water-content mixed-phase Prussian blue cathode material.

[0024] 2. One of the biggest challenges in Prussian blue materials is controlling the composition. The presence of vacancies leads to poor electrochemical performance and structural degradation during cycling, while the presence of water causes side reactions with non-aqueous electrolytes. To fully utilize the advantages of Prussian blue materials, the structure must have the highest sodium content (x≈2), be free of vacancies (y≈0), and correspondingly, contain no water molecules coordinated with Fe (n≈0). This invention can yield uniform, micron-sized Prussian blue cathode materials with the molecular formula Na. x M[Fe(CN)6] y ·□1-y·nH2O, where M is a transition metal element in the transition metal salt, any one of Fe, Mn, Ni, Co, and Cu, □ represents a Fe(CN)6 vacancy, 1.8<x≤2, 0.95<y≤1, 0<n<1; XRD tests show that the material has diffraction peaks of both rhombic and cubic phases, elemental analysis shows that the molar ratio of Na to Fe in this material is 1.86:1, and thermogravimetric analysis shows that the water content of this material is 3.44%. Compared with the cubic phase structure, the Prussian blue cathode material prepared by this invention has higher crystallinity and higher sodium content. The Prussian blue cathode material obtained by this invention contains a mixed phase of rhombic and cubic phases, and has the characteristics of high sodium content, low vacancy amount, and low water content. Therefore, the battery assembled using the cathode material obtained by the method of this invention has the advantage of high specific capacity.

[0025] Furthermore, the preparation method proposed in this invention is simple, low-cost, and does not produce toxic substances, making it suitable for large-scale production.

[0026] In addition to the objectives, features, and advantages described above, the present invention has other objectives, features, and advantages. The invention will now be described in further detail with reference to the figures. Attached Figure Description

[0027] The accompanying drawings, which form part of this application, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings:

[0028] Figure 1 This is the XRD pattern of the Prussian blue cathode material prepared in Example 1;

[0029] Figure 2 This is a SEM image of the Prussian blue cathode material prepared in Example 1;

[0030] Figure 3 This is a thermogravimetric diagram of the Prussian blue cathode material prepared in Example 1;

[0031] Figure 4 These are the XRD patterns of the Prussian blue cathode materials prepared in Examples 2-4;

[0032] Figure 5 The image shows the XRD pattern of the Prussian blue cathode material prepared in Comparative Example 1.

[0033] Figure 6 This is a SEM image of the Prussian blue cathode material prepared in Comparative Example 1;

[0034] Figure 7 Thermogravimetric diagram of the Prussian blue cathode material prepared in Comparative Example 1;

[0035] Figure 8 SEM image of the Prussian blue cathode material prepared in Comparative Example 3

[0036] Figure 9 The image shows the XRD pattern of the Prussian blue cathode material prepared in Comparative Example 6.

[0037] Figure 10 Thermogravimetric analysis (TGA) of the Prussian blue cathode material prepared in Comparative Example 6;

[0038] Figure 11 This is the first charge-discharge curve of the battery assembled with the Prussian blue cathode material prepared in Example 1 under 0.1C (17mA / g) conditions;

[0039] Figure 12 This is a cycling diagram of a battery assembled from the Prussian blue cathode material prepared in Example 1 under 1C (170mA / g) conditions for 30 cycles;

[0040] Figure 13 This is a graph showing the first charge-discharge curve of a battery assembled from the Prussian blue cathode material prepared in Comparative Example 1 under 0.1C (17mA / g) conditions.

[0041] Figure 14 This is a graph showing the battery assembled from the Prussian blue cathode material prepared in Comparative Example 1 under 30 cycles at 1C (170 mA / g).

[0042] Figure 15 This is a cycle diagram of a battery assembled from the Prussian blue cathode material prepared in Comparative Example 6 under 30 cycles at 1C (170mA / g). Detailed Implementation

[0043] The embodiments of the present invention will be described in detail below with reference to the accompanying drawings. However, the present invention can be implemented in many different ways as defined and covered by the claims.

[0044] Example 1

[0045] A method for preparing a sodium-rich, low-water-content Prussian blue cathode material includes the following steps:

[0046] Take 0.04 mol sodium citrate, 0.027 mol sodium ferrocyanide decahydrate and 80 ml deoxygenated water in a round-bottom flask, stir with argon gas until dissolved, and obtain solution A.

[0047] Take 0.07 mol sodium citrate, 0.03 mol sodium chloride, 0.028 mol ascorbic acid, 0.016 mol ferrous sulfate and 80 ml deoxygenated water into a round-bottom flask, and purge with argon gas until dissolved to obtain solution B.

[0048] 40 ml of deoxygenated water was added to a four-necked flask beforehand. Then, under argon gas at 25 °C, solutions A and B were transferred to constant pressure dropping funnels respectively. The stirring speed of the four-necked flask was 700 rpm / min. After solutions A and B were added dropwise, stirring was continued for 1 hour. After stirring, the mixture was allowed to stand for 1 hour.

[0049] After standing, the supernatant was removed, and the remaining slurry was washed three times with deoxygenated water. The sample obtained by centrifugation was vacuum dried at 180°C for 12 hours to obtain the Prussian blue cathode material.

[0050] Example 2

[0051] A method for preparing a sodium-rich, low-water-content Prussian blue cathode material includes the following steps:

[0052] Take 0.04 mol sodium citrate, 0.027 mol sodium ferrocyanide decahydrate and 80 ml deoxygenated water in a round-bottom flask, stir with argon gas until dissolved, and obtain solution A.

[0053] Take 0.07 mol sodium citrate, 0.03 mol sodium chloride, 0.028 mol ascorbic acid, 0.016 mol ferrous sulfate and 80 ml deoxygenated water into a round-bottom flask, and purge with argon gas until dissolved to obtain solution B.

[0054] 40 ml of deoxygenated water was added to a four-necked flask beforehand. Then, under argon gas at 25 °C, solutions A and B were transferred to constant pressure dropping funnels respectively. The stirring speed of the four-necked flask was 700 rpm / min. After solutions A and B were added dropwise, stirring was continued for 1 hour. After stirring, the mixture was allowed to stand for 1 hour.

[0055] After standing, the supernatant was removed, and the remaining slurry was washed three times with deoxygenated water. The sample obtained by centrifugation was vacuum dried at 150°C for 12 hours to obtain the Prussian blue cathode material.

[0056] Example 3

[0057] A method for preparing a sodium-rich, low-water-content Prussian blue cathode material includes the following steps:

[0058] Take 0.04 mol sodium citrate, 0.027 mol sodium ferrocyanide decahydrate and 80 ml deoxygenated water in a round-bottom flask, stir with argon gas until dissolved, and obtain solution A.

[0059] Take 0.07 mol sodium citrate, 0.03 mol sodium chloride, 0.028 mol ascorbic acid, 0.016 mol ferrous sulfate and 80 ml deoxygenated water into a round-bottom flask, and purge with argon gas until dissolved to obtain solution B.

[0060] 40 ml of deoxygenated water was added to a four-necked flask beforehand. Then, under argon gas at 25 °C, solutions A and B were transferred to constant pressure dropping funnels respectively. The stirring speed of the four-necked flask was 700 rpm / min. After solutions A and B were added dropwise, stirring was continued for 1 hour. After stirring, the mixture was allowed to stand for 1 hour.

[0061] After standing, the supernatant was removed, and the remaining slurry was washed three times with deoxygenated water. The sample obtained by centrifugation was vacuum dried at 160°C for 12 hours to obtain the Prussian blue cathode material.

[0062] Example 4

[0063] A method for preparing a sodium-rich, low-water-content Prussian blue cathode material includes the following steps:

[0064] Take 0.04 mol sodium citrate, 0.027 mol sodium ferrocyanide decahydrate and 80 ml deoxygenated water in a round-bottom flask, stir with argon gas until dissolved, and obtain solution A.

[0065] Take 0.07 mol sodium citrate, 0.03 mol sodium chloride, 0.028 mol ascorbic acid, 0.016 mol ferrous sulfate and 80 ml deoxygenated water into a round-bottom flask, and purge with argon gas until dissolved to obtain solution B.

[0066] 40 ml of deoxygenated water was added to a four-necked flask beforehand. Then, under argon gas at 25 °C, solutions A and B were transferred to constant pressure dropping funnels respectively. The stirring speed of the four-necked flask was 700 rpm / min. After solutions A and B were added dropwise, stirring was continued for 1 hour. After stirring, the mixture was allowed to stand for 1 hour.

[0067] After standing, the supernatant was removed, and the remaining slurry was washed three times with deoxygenated water. The sample obtained by centrifugation was vacuum dried at 170°C for 12 hours to obtain the Prussian blue cathode material.

[0068] Example 5

[0069] A method for preparing a sodium-rich, low-water-content Prussian blue cathode material includes the following steps:

[0070] Take 0.04 mol sodium citrate, 0.024 mol sodium ferrocyanide decahydrate and 80 ml deoxygenated water in a round-bottom flask, and stir with argon gas until dissolved to obtain solution A.

[0071] Take 0.06 mol sodium citrate, 0.06 mol sodium chloride, 0.028 mol ascorbic acid, 0.016 mol ferrous sulfate and 80 ml deoxygenated water into a round-bottom flask, and purge with argon gas until dissolved to obtain solution B.

[0072] 40 ml of deoxygenated water was added to a four-necked flask beforehand. Then, under argon gas at 20 °C, solutions A and B were transferred to constant pressure dropping funnels respectively. The stirring speed of the four-necked flask was 500 rpm / min. After solutions A and B were added dropwise, stirring was continued for 2 hours. After stirring, the mixture was allowed to stand for 2 hours.

[0073] After standing, the supernatant was removed, and the remaining slurry was washed three times with deoxygenated water. The sample obtained by centrifugation was vacuum dried at 180°C for 12 hours to obtain the Prussian blue cathode material.

[0074] Example 6

[0075] A method for preparing a sodium-rich, low-water-content Prussian blue cathode material includes the following steps:

[0076] Take 0.04 mol sodium citrate, 0.05 mol sodium ferrocyanide decahydrate and 80 ml deoxygenated water in a round-bottom flask, and stir with argon gas until dissolved to obtain solution A.

[0077] Take 0.02 mol sodium chloride, 0.08 mol citric acid, 0.028 mol ascorbic acid, 0.016 mol ferrous sulfate and 80 ml deoxygenated water into a round-bottom flask, and purge with argon gas until dissolved to obtain solution B.

[0078] 40 ml of deoxygenated water was added to a four-necked flask beforehand. Then, under argon gas at 30 °C, solutions A and B were transferred to constant pressure dropping funnels respectively. The stirring speed of the four-necked flask was 1000 rpm / min. After solutions A and B were added dropwise, stirring was continued for 3 hours. After stirring, the mixture was allowed to stand for 3 hours.

[0079] After standing, the supernatant was removed, and the remaining slurry was washed three times with deoxygenated water. The sample obtained by centrifugation was vacuum dried at 180°C for 12 hours to obtain the Prussian blue cathode material.

[0080] Example 7

[0081] A method for preparing a sodium-rich, low-water-content Prussian blue cathode material includes the following steps:

[0082] Take 0.02 mol sodium citrate, 0.027 mol sodium ferrocyanide decahydrate and 80 ml deoxygenated water in a round-bottom flask, and stir with argon gas until dissolved to obtain solution A.

[0083] Take 0.04 mol sodium chloride, 0.08 mol disodium ethylenediaminetetraacetate, 0.028 mol ascorbic acid, 0.016 mol ferrous sulfate and 80 ml deoxygenated water into a round-bottom flask, and purge with argon gas until dissolved to obtain solution B.

[0084] 40 ml of deoxygenated water was added to a four-necked flask beforehand. Then, under argon gas at 30 °C, solutions A and B were transferred to constant pressure dropping funnels respectively. The stirring speed of the four-necked flask was 700 rpm / min. After solutions A and B were added dropwise, stirring was continued for 4 hours. After stirring, the mixture was allowed to stand for 4 hours.

[0085] After standing, the supernatant was removed, and the remaining slurry was washed three times with deoxygenated water. The sample obtained by centrifugation was vacuum dried at 180°C for 12 hours to obtain the Prussian blue cathode material.

[0086] Example 8

[0087] A method for preparing a sodium-rich, low-water-content Prussian blue cathode material includes the following steps:

[0088] Take 0.04 mol of tetrasodium ethylenediaminetetraacetate, 0.05 mol of sodium ferrocyanide decahydrate and 80 ml of deoxygenated water in a round-bottom flask, and stir with argon gas until dissolved to obtain solution A.

[0089] Take 0.08 mol sodium alginate, 0.02 mol citric acid, 0.05 mol ascorbic acid, 0.016 mol ferrous sulfate and 80 ml deoxygenated water into a round-bottom flask, and purge with argon gas until dissolved to obtain solution B.

[0090] 40 ml of deoxygenated water was added to a four-necked flask beforehand. Then, under argon gas at 30 °C, solutions A and B were transferred to constant pressure dropping funnels respectively. The stirring speed of the four-necked flask was 1000 rpm / min. After solutions A and B were added dropwise, stirring was continued for 8 hours. After stirring, the mixture was allowed to stand for 8 hours.

[0091] After standing, the supernatant was removed, and the remaining slurry was washed three times with deoxygenated water. The sample obtained by centrifugation was vacuum dried at 200°C for 12 hours to obtain the Prussian blue cathode material.

[0092] Comparative Example 1 (No complexing agent added to solution A)

[0093] A method for preparing a Prussian blue cathode material includes the following steps:

[0094] Take 0.027 mol of sodium ferrocyanide decahydrate and 80 ml of deoxygenated water in a round-bottom flask, and stir with argon gas until dissolved to obtain solution A.

[0095] Take 0.07 mol sodium citrate, 0.03 mol sodium chloride, 0.028 mol ascorbic acid, 0.016 mol ferrous sulfate and 80 ml deoxygenated water into a round-bottom flask, and purge with argon gas until dissolved to obtain solution B.

[0096] 40 ml of deoxygenated water was added to a four-necked flask beforehand. Then, under argon gas at 25 °C, solutions A and B were transferred to constant pressure dropping funnels respectively. The stirring speed of the four-necked flask was 700 rpm / min. After solutions A and B were added dropwise, stirring was continued for 1 hour. After stirring, the mixture was allowed to stand for 1 hour.

[0097] After standing, the supernatant was removed, and the remaining slurry was washed three times with deoxygenated water. The sample obtained by centrifugation was vacuum dried at 180°C for 12 hours to obtain the Prussian blue cathode material.

[0098] Comparative Example 2 (Solution B without sodium supplementation)

[0099] A method for preparing a Prussian blue cathode material includes the following steps:

[0100] Take 0.04 mol sodium citrate, 0.027 mol sodium ferrocyanide decahydrate and 80 ml deoxygenated water in a round-bottom flask, stir with argon gas until dissolved, and obtain solution A.

[0101] Take 0.08 mol citric acid, 0.016 mol ferrous sulfate, 0.028 mol ascorbic acid and 80 ml deoxygenated water into a round-bottom flask, and pass argon gas until dissolved to obtain solution B.

[0102] 40 ml of deoxygenated water was added to a four-necked flask beforehand. Then, under argon gas at 25 °C, solutions A and B were transferred to constant pressure dropping funnels respectively. The stirring speed of the four-necked flask was 700 rpm / min. After solutions A and B were added dropwise, stirring was continued for 1 hour. After stirring, the mixture was allowed to stand for 1 hour.

[0103] After standing, the supernatant was removed, and the remaining slurry was washed three times with deoxygenated water. The sample obtained by centrifugation was vacuum dried at 180°C for 12 hours to obtain the Prussian blue cathode material.

[0104] Comparative Example 3 (Sodium supplement concentration in Solution B is too high)

[0105] A method for preparing a Prussian blue cathode material includes the following steps:

[0106] Take 0.3 mol sodium citrate, 0.027 mol sodium ferrocyanide decahydrate and 80 ml deoxygenated water in a round-bottom flask, stir with argon gas until dissolved, and obtain solution A.

[0107] Take 0.08 mol citric acid, 0.4 mol sodium chloride, 0.028 mol ascorbic acid, 0.016 mol ferrous sulfate and 80 ml deoxygenated water into a round-bottom flask, and purge with argon gas until dissolved to obtain solution B.

[0108] 40 ml of deoxygenated water was added to a four-necked flask beforehand. Then, under argon gas at 25 °C, solutions A and B were transferred to constant pressure dropping funnels respectively. The stirring speed of the four-necked flask was 700 rpm / min. After solutions A and B were added dropwise, stirring was continued for 1 hour. After stirring, the mixture was allowed to stand for 1 hour.

[0109] After standing, the supernatant was removed, and the remaining slurry was washed three times with deoxygenated water. The sample obtained by centrifugation was vacuum dried at 180°C for 12 hours to obtain the Prussian blue cathode material.

[0110] Comparative Example 4 (Solution B without auxiliary complexing agent)

[0111] A method for preparing a Prussian blue cathode material includes the following steps:

[0112] Take 0.04 mol sodium citrate, 0.027 mol sodium ferrocyanide decahydrate and 80 ml deoxygenated water in a round-bottom flask, stir with argon gas until dissolved, and obtain solution A.

[0113] Take 0.08 mol sodium citrate, 0.02 mol sodium sulfate, 0.016 mol ferrous sulfate and 80 ml deoxygenated water into a round-bottom flask, and pass argon gas until dissolved to obtain solution B.

[0114] 40 ml of deoxygenated water was added to a four-necked flask beforehand. Then, under argon gas at 25 °C, solutions A and B were transferred to constant pressure dropping funnels respectively. The stirring speed of the four-necked flask was 700 rpm / min. After solutions A and B were added dropwise, stirring was continued for 1 hour. After stirring, the mixture was allowed to stand for 1 hour.

[0115] After standing, the supernatant was removed, and the remaining slurry was washed three times with deoxygenated water. The sample obtained by centrifugation was vacuum dried at 180°C for 12 hours to obtain the Prussian blue cathode material.

[0116] Comparative Example 5 (excessive concentration of auxiliary complexing agent)

[0117] A method for preparing a Prussian blue cathode material includes the following steps:

[0118] Take 0.04 mol sodium citrate, 0.027 mol sodium ferrocyanide decahydrate and 80 ml deoxygenated water in a round-bottom flask, stir with argon gas until dissolved, and obtain solution A.

[0119] Take 0.08 mol sodium citrate, 0.02 mol sodium sulfate, 0.1 mol ascorbic acid, 0.2 mol ferrous sulfate and 80 ml deoxygenated water into a round-bottom flask, and pass argon gas until dissolved to obtain solution B.

[0120] 40 ml of deoxygenated water was added to a four-necked flask beforehand. Then, under argon gas at 25 °C, solutions A and B were transferred to constant pressure dropping funnels respectively. The stirring speed of the four-necked flask was 700 rpm / min. After solutions A and B were added dropwise, stirring was continued for 1 hour. After stirring, the mixture was allowed to stand for 1 hour.

[0121] After standing, the supernatant was removed, and the remaining slurry was washed three times with deoxygenated water. The sample obtained by centrifugation was vacuum dried at 230°C for 12 hours to obtain the Prussian blue cathode material.

[0122] Comparative Example 6 (vacuum drying temperature too low)

[0123] A method for preparing a Prussian blue cathode material includes the following steps:

[0124] Take 0.04 mol sodium citrate, 0.027 mol sodium ferrocyanide decahydrate and 80 ml deoxygenated water in a round-bottom flask, stir with argon gas until dissolved, and obtain solution A.

[0125] Take 0.07 mol sodium citrate, 0.03 mol sodium chloride, 0.028 mol ascorbic acid, 0.016 mol ferrous sulfate and 80 ml deoxygenated water into a round-bottom flask, and purge with argon gas until dissolved to obtain solution B.

[0126] 40 ml of deoxygenated water was added to a four-necked flask beforehand. Then, under argon gas at 25 °C, solutions A and B were transferred to constant pressure dropping funnels respectively. The stirring speed of the four-necked flask was 700 rpm / min. After solutions A and B were added dropwise, stirring was continued for 1 hour. After stirring, the mixture was allowed to stand for 1 hour.

[0127] After standing, the supernatant was removed, and the remaining slurry was washed three times with deoxygenated water. The sample obtained by centrifugation was vacuum dried at 80°C for 12 hours to obtain the Prussian blue cathode material.

[0128] Comparative Example 7 (Vacuum drying temperature too high)

[0129] A method for preparing a Prussian blue cathode material includes the following steps:

[0130] Take 0.04 mol sodium citrate, 0.027 mol sodium ferrocyanide decahydrate and 80 ml deoxygenated water in a round-bottom flask, stir with argon gas until dissolved, and obtain solution A.

[0131] Take 0.07 mol sodium citrate, 0.03 mol sodium chloride, 0.028 mol ascorbic acid, 0.016 mol ferrous sulfate and 80 ml deoxygenated water into a round-bottom flask, and purge with argon gas until dissolved to obtain solution B.

[0132] 40 ml of deoxygenated water was added to a four-necked flask beforehand. Then, under argon gas at 25 °C, solutions A and B were transferred to constant pressure dropping funnels respectively. The stirring speed of the four-necked flask was 700 rpm / min. After solutions A and B were added dropwise, stirring was continued for 1 hour. After stirring, the mixture was allowed to stand for 1 hour.

[0133] After standing, the supernatant was removed, and the remaining slurry was washed three times with deoxygenated water. The sample obtained by centrifugation was vacuum dried at 350°C for 12 hours to obtain the Prussian blue cathode material.

[0134] Figure 1 This is the XRD pattern of the Prussian blue cathode material prepared in Example 1. Figure 1 As can be seen from the data, the Prussian blue cathode material prepared in Example 1 is a mixed phase of cubic and rhombic phases. Among them, 16.83° and 34.27° are diffraction peaks of cubic Prussian blue, and the remaining diffraction peaks are diffraction peaks of rhombic Prussian blue. Figure 2 This is a SEM image of the Prussian blue cathode material prepared in Example 1. From... Figure 2 As can be seen, uniform micron-sized Prussian blue cathode material can be prepared by the method of the present invention.

[0135] Figure 3 This is a thermogravimetric diagram of the Prussian blue cathode material prepared in Example 1, from... Figure 3 As can be seen, the material under nitrogen atmosphere under thermogravimetric analysis showed a weight loss of 3.44% up to 340℃. This corresponds to the weight loss of adsorbed water, interstitial water, and structural water. This water content is lower than the 8%-12% water content of cubic Prussian blue. Therefore, the preparation method of this invention can obtain Prussian blue cathode material with low water content.

[0136] Figure 4 These are the XRD patterns of the Prussian blue cathode materials prepared in Examples 2-4. Figure 4 It can be seen that the Prussian blue cathode materials obtained at the three temperatures of 150℃, 160℃ and 170℃ are all mixed phases of cubic and rhombic phases, and the diffraction peaks of cubic Prussian blue at 16.83° and 34.27° gradually weaken with increasing temperature.

[0137] Figure 5 This is the XRD pattern of the Prussian blue cathode material prepared in Comparative Example 1. Figure 6 This is a SEM image of the Prussian blue cathode material prepared in Comparative Example 1. From... Figure 5 and Figure 6 It can be seen that, due to the lack of a complexing agent in Comparative Example 1, the crystallization rate was too fast, resulting in the acquisition of nanoparticles in the range of 100-300 nm, and the inability to obtain uniform micron-sized particles. Furthermore, the Prussian blue cathode material obtained was a cubic phase structure with low sodium content. Figure 7 The thermogravimetric curve of the Prussian blue cathode material prepared in Comparative Example 1 is shown below. Figure 7 It can be seen that the Prussian blue cathode material obtained in Comparative Example 1, when subjected to thermogravimetric analysis under a nitrogen atmosphere, showed a weight loss of 8.32% up to 270°C. The water content of this comparative example is higher than that of the Prussian blue cathode material in Example 1.

[0138] Figure 8 The image shows an SEM image of the Prussian blue cathode material prepared in Comparative Example 3 with an excessively high sodium supplement concentration. As can be seen from the image, this comparative example cannot form uniform micron-sized particles.

[0139] Figure 9 This is the XRD pattern of the Prussian blue cathode material prepared in Comparative Example 6. Figure 9 It can be seen that the Prussian blue cathode material prepared in Comparative Example 6 has a cubic phase structure with high water content. Figure 10 This is a thermogravimetric diagram of the Prussian blue cathode material prepared in Comparative Example 6. Figure 10 It can be seen that the Prussian blue cathode material prepared in Comparative Example 6 underwent thermogravimetric analysis in a nitrogen atmosphere, and the weight loss up to 250°C was 10.08%. The water content of the Prussian blue cathode material prepared in Comparative Example 6 is higher than that of the Prussian blue cathode material prepared in Example 1 of this invention.

[0140] Preparation of sodium-ion batteries: The Prussian blue cathode material prepared in Examples 1-8 and Comparative Examples 1-7 was used to fabricate electrode sheets for CR2032 button batteries. The counter electrode was metallic sodium, the separator was Whatman glass fiber, and the electrolyte was an organic electrolyte. These were then assembled into button batteries and subjected to charge-discharge tests. The test results are as follows:

[0141] Figure 11 This is a charge-discharge curve of the battery assembled from the Prussian blue cathode material prepared in Example 1. Figure 12 This is a cycling graph of a battery assembled from the Prussian blue cathode material prepared in Example 1 under 1C (170 mA / g) conditions for 30 cycles. From... Figure 11 and Figure 12 It can be seen that the battery assembled using the Prussian blue cathode material prepared in Example 1 of this invention exhibits a first-cycle discharge specific capacity of 169.3 mAh / g and a coulombic efficiency of 98.94% at a current density of 0.1C. When switching from a current density of 0.1C to 1C after three cycles, the discharge specific capacity is 131.7 mAh / g, and the battery demonstrates good performance over 30 cycles. The high sodium content and low water content of the obtained Prussian blue cathode material enable it to release and insert more sodium during charge and discharge. + Therefore, the Prussian blue cathode material obtained by this invention has a discharge specific capacity at low current density that is much higher than that of cubic Prussian blue.

[0142] Figure 13 This is a charge-discharge curve of the battery assembled from the Prussian blue cathode material prepared in Comparative Example 1. Figure 14 This is a cycling graph showing the battery assembled from the Prussian blue cathode material prepared in Comparative Example 1 after 30 cycles at 1C (170 mA / g). Figure 13 and Figure 14 As can be seen, the battery assembled using the Prussian blue cathode material prepared in Comparative Example 1 of this invention exhibits a first-cycle discharge specific capacity of 119.7 mA h / g and a coulombic efficiency of 92.6% at a current density of 0.1C. When switching from a current density of 0.1C to 1C after three cycles, the discharge specific capacity is 93.1 mA h / g. Because no complexing agent was added during the preparation of solution A, the Prussian blue cathode material obtained in Comparative Example 1 has a low sodium content, a high water content, and consists of non-uniform nanoparticles. The ion diffusion coefficient of nanoparticles is much smaller than that of uniform micron-sized particles. Therefore, the electrochemical performance of the battery assembled using the Prussian blue cathode material prepared in Comparative Example 1 is inferior to that of the battery assembled using the Prussian blue cathode material prepared in the examples.

[0143] Figure 15The figure shows the 30-cycle performance of the battery assembled with the Prussian blue cathode material prepared in Comparative Example 6 under 1C (170 mA / g) conditions. As can be seen from the figure, the battery assembled with the Prussian blue cathode material prepared in Comparative Example 6 has a discharge specific capacity of 129.3 mA h / g and a coulombic efficiency of 99.03% at a current density of 0.1C. When switching from 0.1C to 1C after three cycles, the discharge specific capacity is 103.4 mA h / g. The electrochemical performance of the battery assembled with the Prussian blue cathode material prepared in Comparative Example 6 is worse than that of the battery assembled with the Prussian blue cathode material prepared in the examples. Due to the lower vacuum drying temperature, the obtained material was not completely dehydrated, resulting in a cubic phase structure with low sodium content and high water content in the Prussian blue cathode material obtained in Comparative Example 6. The low sodium content and high water content cause its discharge specific capacity to be much lower than that of the mixed-phase Prussian blue in the examples.

[0144] Table 1 shows the elemental analysis results of Na and Fe in Example 1 using ICP-OES. Table 2 shows the first-cycle discharge specific capacity and coulombic efficiency of batteries assembled using the Prussian blue cathode materials prepared in Examples 1-8 and Comparative Examples 1-7 of this invention under 0.1C (17mA / g) conditions.

[0145] Table 1

[0146] element quality score molar ratio Na 14.60％ 1.86 Fe 38.08％ 1

[0147] Table 2

[0148]

[0149]

[0150] As shown in Table 1, the molar ratio of Na to Fe in the Prussian blue cathode material prepared using Example 1 of this invention is 1.86:1, and the Na content is much higher than that of cubic Prussian blue materials prepared by the current co-precipitation method. Simultaneously, the higher initial Na content results in a higher specific capacity for the Prussian blue cathode material prepared by this invention. The results of the Prussian blue cathode materials prepared in Examples 2-8 are similar to those in Example 1, and will not be repeated here.

[0151] As shown in Table 2, the batteries assembled using the Prussian blue cathode materials prepared in Examples 1-8 of this invention exhibit significantly better first-cycle discharge specific capacity at 0.1C (17 mA / g) than those assembled using the Prussian blue cathode materials prepared in Comparative Examples 1-7. The batteries assembled using the Prussian blue cathode materials prepared in these examples show a first-cycle discharge specific capacity exceeding 149.5 mAh / g at a current density of 0.1C, with a maximum of 169.3 mAh / g. This is because the Prussian blue cathode material obtained using this invention has a high sodium content and low water content, enabling it to release and insert more sodium during charging and discharging. + Therefore, the Prussian blue cathode material obtained by this invention has a discharge specific capacity at low current densities that is much higher than that of cubic Prussian blue. The battery assembled using the Prussian blue cathode material prepared in Comparative Example 1 had a first-cycle discharge specific capacity of 119.7 mAh / g under 0.1C (17 mA / g) conditions. This was because no complexing agent was added during the preparation of solution A, resulting in excessively fast crystallization and low sodium content, high water content, and non-uniform nanoparticles in the Prussian blue cathode material obtained in Comparative Example 1. In Comparative Example 2, the initial Na content was low because no sodium supplement was added during the preparation of solution B. In Comparative Example 3, the sodium supplement concentration was too high, resulting in free Na... + Excessive amounts of pollutants cause the reaction to proceed too quickly, which is detrimental to the formation of uniform micron-sized particles. The resulting particles are larger and less uniform. Figure 8 As shown; in Comparative Example 4, the absence of an auxiliary complexing agent leads to excessive oxidation of ferrous ions, resulting in a decrease in the initial sodium content. Consequently, the first-cycle discharge specific capacity of Comparative Example 4 is only 105.7 mAh / g. In Comparative Example 5, an excessively high concentration of the auxiliary complexing agent causes significant changes in the pH value of the solution, which is not conducive to the co-precipitation reaction. In Comparative Example 6, an excessively low vacuum drying temperature prevents further removal of moisture from the Prussian blue material, resulting in the formation of a cubic phase structure with a high water content. In Comparative Example 7, an excessively high vacuum drying temperature reduces the moisture content of the material but also causes the structure of the Prussian blue to collapse, resulting in a first-cycle discharge specific capacity of only 92.8 mAh / g.

[0152] In summary, this invention provides a method for preparing a sodium-rich, low-water-content mixed-phase Prussian blue cathode material. This method involves introducing a complexing agent into a sodium ferrocyanide solution, and introducing a sodium-supplementing agent, a complexing agent, and an auxiliary complexing agent into a transition metal salt solution, followed by co-precipitation to obtain the sodium-rich, low-water-content mixed-phase Prussian blue cathode material. Compared to Prussian blue cathode materials prepared by existing co-precipitation methods, the Prussian blue cathode material prepared by this invention has the advantages of high sodium content and low water content. This is due to the fact that this invention simultaneously adds a complexing agent to both the sodium ferrocyanide solution and the transition metal salt solution, and adds a sodium-supplementing agent and an auxiliary complexing agent to the transition metal salt solution, thereby differentially controlling the crystallization rate. The complexing agent can react with Fe... 2+ The formation of complexes slows down the crystallization rate, increases the crystallinity of the material, and reduces the number of vacancies. The reduction in vacancies leads to a corresponding decrease in the structural water content of the material. Simultaneously, the addition of sodium supplementation agents allows for the maximization of Na... + Occupying corresponding spatial sites increases the initial sodium content of the material; simultaneously, the addition of auxiliary complexing agents can hydrolyze the solution to maintain a weakly acidic environment, which is beneficial for Fe 2+ The stable existence of Fe can also oxidize Fe. 2+ Reduction allows Fe to remain in a lower oxidation state, accommodating more Na. + This process also increases the Na content of the material; and under the final vacuum drying conditions of 150–300°C, interstitial water and structural water in the product are further removed, resulting in the sodium-rich, low-water-content mixed-phase Prussian blue cathode material. Compared to the cubic phase structure, the Prussian blue cathode material prepared by this invention has higher crystallinity and higher sodium content. The Prussian blue cathode material obtained by this invention comprises a mixed phase of rhombic and cubic phases, characterized by high sodium content, low vacancy rate, and low water content. Therefore, batteries assembled with the Prussian blue cathode material obtained by this invention have the advantage of high specific capacity. Furthermore, the preparation method proposed in this invention is simple, low-cost, and does not produce toxic substances, facilitating large-scale production.

[0153] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a sodium-rich, low-water-content Prussian blue cathode material, characterized in that, Includes the following steps: Step 1: Dissolve sodium ferrocyanide or its hydrate and complexing agent in deoxygenated water with a dissolved oxygen concentration of 0-3 mg / L under an inert gas atmosphere and at 0-80 °C to obtain mixed solution A; Step 2: Dissolve the transition metal salt or its hydrate, sodium supplement, complexing agent, and auxiliary complexing agent in deoxygenated water with a dissolved oxygen concentration of 0-3 mg / L under an inert gas atmosphere and at 0-80 °C to obtain mixed solution B; Step 3: Solution A obtained in Step 1 and Solution B obtained in Step 2 are added dropwise to a container with a positive pressure inert atmosphere and a temperature of 0–80 °C using dynamic rate control while continuously stirring. After the addition is complete, stirring continues for a period of time, followed by standing, centrifugation, and vacuum drying to obtain the Prussian blue cathode material. The dropwise rate is 0.1–2 ml / min; the stirring speed is 100–1000 rpm / min; the stirring time is 1–12 h; the standing time is 0–24 h; the centrifugation speed is 1000–10000 rpm / min; and the vacuum drying temperature is 150–300 °C. The molecular formula of the Prussian blue cathode material is Na. x M[Fe(CN)6] y •□ 1-y •nH2O, where M is the transition metal element in the transition metal salt, which is any one of Fe, Mn, Ni, Co, and Cu, □ represents a Fe(CN)6 vacancy, 1.8<x≤2, 0.95<y≤1, 0<n<1; The Prussian blue cathode material is a sodium-rich, low-water-content mixed phase, which includes a rhombic phase and a cubic phase. In steps one and two, the complexing agent is selected from one of sodium thiocyanate, diethanolamine, citric acid, sodium citrate, ammonia, ethylenediaminetetraacetic acid, disodium ethylenediaminetetraacetic acid, tetrasodium ethylenediaminetetraacetic acid, trisodium aminotriacetate, and sodium pyrophosphate; the concentration of the complexing agent in step one is 0.1 to 10 times the concentration of sodium ferrocyanide or its hydrate; the concentration of the complexing agent in step two is 0.1 to 10 times the concentration of the transition metal salt. In step two, the auxiliary complexing agent is selected from at least one of isoascorbic acid, sodium isoascorbate, ascorbic acid, and citric acid; the concentration of the auxiliary complexing agent is 0.1 to 10 times the concentration of the transition metal salt; and the concentration of the sodium supplement is 0.1 to 10 times the concentration of the transition metal salt.

2. The method for preparing a sodium-rich, low-water-content Prussian blue cathode material according to claim 1, characterized in that, In step one, the concentration of sodium ferrocyanide or its hydrate is 0.1–1 mol / L.

3. The method for preparing a sodium-rich, low-water-content Prussian blue cathode material according to claim 1, characterized in that, In step two, the transition metal salt is selected from any one of the sulfate, chloride, acetate, and nitrate salts of Fe, Mn, Ni, Co, and Cu; the concentration of the transition metal salt is 0.1–1 mol / L.

4. The method for preparing a sodium-rich, low-water-content Prussian blue cathode material according to claim 1, characterized in that, In step two, the sodium supplement is selected from any one of sodium tripolyphosphate, sodium acetate, sodium sulfate, sodium alginate, sodium chloride, and sodium carbonate.

5. A method for preparing a sodium-rich, low-water-content Prussian blue cathode material according to any one of claims 1-4, characterized in that, In steps one through three, the inert gas atmosphere is either N2 or Ar.

6. A sodium-rich, low-water-content Prussian blue cathode material, characterized in that, It is prepared using the method for preparing Prussian blue cathode material according to any one of claims 1-5.

7. A sodium-ion battery, characterized in that, Including the Prussian blue cathode material as described in claim 6.

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