Preparation method and application of high-compaction-density composite sodium iron phosphate

Abstract

The present application relates to a preparation method of high compact density composite sodium iron phosphate and its application. The preparation method comprises the following steps: a sodium source, an iron source and a phosphorus source are weighed according to the stoichiometric ratio of [xNa+(a-x)A]:[yFe+(b-y)B]:P=a:b:(c+2), A is a Na site dopant, and B is a Fe site dopant; the iron source, the sodium source, the phosphorus source and a chelating agent are mixed and dispersed into pure water, and then reacted at a certain temperature for a period of time; the reaction product is transferred into a sand mill for grinding; a dispersing agent and sucrose, dopants A and B are added; and after grinding, a slurry F is obtained. The slurry F is subjected to spray drying to obtain a powder K. The powder K is transferred into a tube furnace, calcined at a temperature of 450-600 DEG C under a N2 protective atmosphere with an oxygen content of less than 50 ppm, and then airflow pulverized to obtain a sodium iron phosphate pyrophosphate positive electrode material powder with a high compact density and a molecular formula of (Na x A a‑x )(Fe y B b‑y )(PO4) c P2O7. The product produced by the present application has high compact density, and the kinetic performance of the material is enhanced, the thermal stability and chemical stability of the material are improved, and the electrochemical performance is good.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of sodium-ion batteries, in particular to a high-pressing-density composite sodium iron phosphate and a preparation method and application thereof. BACKGROUND

[0002] With the promotion of global energy transformation and sustainable development, new energy storage technology is rapidly developing. Sodium-ion batteries are considered an important choice for large-scale energy storage systems due to their abundant resources, low cost, and environmental friendliness. The performance of the positive electrode material, which is a decisive factor for the performance of sodium-ion batteries, directly affects the energy density, cycle stability, and safety of the battery. Composite sodium iron phosphate (NFPP) materials are favored by researchers due to their good cycle stability, ultra-high safety performance, and excellent rate performance.

[0003] However, existing composite sodium iron phosphate (NFPP) materials still face many challenges in practical applications, and their technical problems mainly include:

[0004] 1. Low pressing density: The density of the material in its natural state is insufficient (generally, the pressing density is 2.0, and the true density is 3.2), which limits its energy density in battery applications.

[0005] 2. There are many pores in the material (higher porosity at the grain boundaries), which not only reduces the density of the material but also may affect its structural integrity and electrochemical performance.

[0006] 3. High processing cost: Traditional physical grinding methods are expensive and inefficient, leading to increased overall production costs.

[0007] 4. Damage to the structure caused by gas production during synthesis: During the synthesis of materials, chemical reactions may produce gas, which can cause damage to the structure of the material and affect its performance. For example, irregular and non-uniform micropores or mesopores may be formed due to fluctuations in local gas pressure or gas release during the synthesis process, further reducing the bulk density and pressing density of the material.

[0008] 5. Only physical grinding in the processing stage: Over-reliance on physical methods for material processing, lack of chemical method assistance, may result in substandard material performance.

[0009] 6. Low synthesis efficiency: Existing synthesis methods may have low efficiency due to low solid content, which is not conducive to large-scale production and cost reduction.

[0010] 7. Electrochemical performance is insufficient: Due to the above problems, the electrochemical performance of the material may not be optimal, such as the cycle stability and rate performance may not be ideal. In addition, the insufficient kinetics is also a key factor restricting its application in high-rate charging and discharging conditions, and will also cause rapid degradation of the charging and discharging cycle life in low temperature scenarios.

[0011] 8. Limited industrial application: Due to the limitations of cost and performance, the industrial application of NFPP materials in sodium-ion batteries is challenging. The preparation cost and the complexity of the process also become a bottleneck restricting its large-scale production and commercial application.

[0012] These technical problems need to be solved by innovative methods and processes to improve the performance of NFPP materials and reduce production costs, so as to promote their wide application in the field of sodium-ion batteries. Therefore, it is particularly urgent to develop a new high compaction density composite sodium iron phosphate material. SUMMARY

[0013] The purpose of the present application is to provide a preparation method of high compaction density composite sodium iron phosphate with high compaction density, enhanced kinetic performance, improved thermal stability and chemical stability, good electrochemical performance, and low production cost, and its application, in view of the above problems and deficiencies.

[0014] The present application aims to improve the compaction density of the material, reduce the porosity, improve the kinetic performance, and reduce the synthesis cost by innovative synthesis strategy, so as to realize the overall improvement of the performance of sodium-ion batteries and promote the development and application of sodium-ion battery technology.

[0015] The technical solution of the present application is as follows:

[0016] The preparation method of high compaction density composite sodium iron phosphate according to the present application comprises the following steps:

[0017] Step (1), the sodium source, iron source and phosphorus source are weighed according to the stoichiometric ratio of [xNa+(a-x)A]:[yFe+(b-y)B]:P=a:b:(c+2), wherein A is a Na site dopant, B is a Fe site dopant, A is selected from metal dopant elements with similar radius to Na ion, B is selected from metal dopant elements with radius equal to or less than Fe ion, and 3≤a≤4.3, 2≤b≤3, 1≤c≤2, a-b≤1, 0<a-x≤0.05, 0<b-y≤0.05; then the iron source, sodium source, phosphorus source and green degradable chelating agent are mixed and dispersed in pure water, reacted at a certain temperature for a period of time, then transferred to a sand mill (at a speed of 2800-3500 rpm / min) for grinding, then a proper amount of dispersant and sucrose are added, and the grinding is continued for a period of time, then the dopants A and B are added, and the slurry F is obtained after grinding.

[0018] Step (2), spray drying the slurry F with a spray drying tower (inlet temperature 220 DEG C, outlet temperature 110 DEG C), and the spray drying obtains powder K. The particle size of the powder K is controlled to be 2 um < D10 < 6 um, 10 um < D50 < 18 um, 25 um < D99 < 40 um;

[0019] Step (3), transferring the powder K to a tube furnace, calcining for 10-15 hours under the protection of N2 atmosphere, the oxygen content < 50 ppm, the temperature is between 450-600 DEG C, and finally airflow crushing to obtain the sodium iron phosphate positive electrode material powder with the molecular formula (Na x A a-x )(Fe y B b-y )(PO4) c P2O7.

[0020] In the present application, the ratio of the Na site dopant A and the Fe site dopant B used as the doping elements is controlled to be within 0.05, and too high ratio will cause significant performance reduction.

[0021] Further, a:b:(c+2) is 4:3:4 or 3.4:2.4, 3.4, and the preferred ratio is the latter.

[0022] In step (1), the adding amount of the chelating agent is 5%-15% of the total mass of the iron source, the sodium source and the phosphorus source, and the adding amount of the dispersing agent is 1%-3% of the total mass of the iron source, the sodium source, the phosphorus source and the chelating agent. When grinding, the grinding cavity temperature is controlled to be between 30-50 DEG C, and the viscosity of the slurry F is controlled to be between 300-2000 mpa*s. The particle size of the slurry F is controlled to be D10≤150 nm, D50≤300 nm, D90≤500 nm, Dmax<1 um, and the particle size distribution

defined as span=(D90-D10) / D50

[0023] In the raw materials of the present application, one or several green degradable chelating agents are used to prepare a solution containing no insoluble substances through chemical corrosion dissolution and chelation, and the physical action of mechanical sanding, so as to achieve uniform mixing at the atomic level, and then the NFPP electrode material with dense structure, high phase purity and high tap density is obtained through spray drying and calcination. This technology can be named as Ionic Chelation Preparation Technique.

[0024] In addition, the ratio of PO4 and P2O7 in the raw material ratio also needs to be considered, and the ratio is C:1.1≤c≤2. This can prevent the reaction process from generating impact on the structure and reducing the porosity of the structure, so as to improve the tap density of the electrode sheet made of the material. The raw material ratio considers PO4 2-, P2O7, because if the phosphate is compared with the design of the molecular formula, 2 PO4 2- loses an O to become P2O7 4- , will release additional oxygen gas to cause the appearance of mesoporous or microporous.

[0025] The present application is controlled by the ratio of phosphate (PO4 source) and pyrophosphate (P2O7 source) in the raw materials. Because PO4 is greater than 2, the PO4 ratio is too high, the negative effects are: 1. The electronic insulation of the material increases, thereby reducing the conductivity and ion diffusion rate, affecting the charge and discharge performance of the battery. 2. Excessive po4 3 - may cause the bond strength in the crystal structure to increase, improving the structural stability of the material, but at the expense of some electrochemical activity. 3. Higher po4 3 - ratio will improve the thermal stability of the material, because po4 3 - helps to form a more stable crystal structure. 4. In some cases, higher po4 3 - ratio may improve the chemical stability of the material, thereby improving the safety of the battery. PO4 is less than 1, the PO4 ratio is too low, the negative effects are: 1. Lower po4 3 - ratio may reduce the stability of the material; 2. Too little po4 3 - may cause the bond strength of the crystal structure to decrease, affecting the mechanical stability and cycle stability of the material. 3. Lower po4 3 - ratio may reduce the thermal stability of the material, increasing the risk of thermal runaway. 4. Lower po4 3 - ratio may make the material more susceptible to chemical attack, increasing the risk of side reactions, thereby affecting the safety of the battery.

[0026] In addition, ionic strength (I) is an indicator of the total amount of ions in a solution, often used to describe the concentration level of ions in a solution. Ionic strength affects many physical and chemical processes in solution, including solubility, reaction rate and equilibrium, etc. The formula for calculating ionic strength is as follows:

[0027]

[0028] Where:

[0029] · I is the ionic strength, usually in units of moles per liter (mol / L).

[0030] · c i is the concentration of the i-th ion, in units of moles per liter (mol / L).

[0031] · zi is the charge number of the i-th ion (for example, for Na +, zi = +1 for Cl - , zi = -1 for Ca 2+ , zi = +2, etc.

[0032] In the process of preparing complex sodium ferric phosphate by ion chelation technology, ion concentration can significantly affect the progress of chelation reaction, which is specifically manifested in solubility, coordination equilibrium, chelation constant, dissociation of ligand, charge distribution, reaction kinetics, etc. Therefore, the ionic strength needs to be controlled within a certain range, and the present application controls it at I = 0.03-0.1 mol / L to obtain the best reaction effect.

[0033] Here, the chelate (chelate formed by chelating agent and iron ion) equilibrium constant Kf (i.e. equilibrium condition) is calculated:

[0034]

[0035] Where [Fe]0 and [L]0 are the initial concentrations of iron ions and chelating agents at the beginning of the reaction, and [Fe] and [L] are the concentrations of iron ions and chelating agents in the solution at the equilibrium state.

[0036] Combined with the Debye-Hückel theory,

[0037] Where A is the Debye-Hückel constant, γ i is the activity coefficient (which can be considered as 1 under low concentration), and zi is the charge number of the ion. According to this formula, the final chelation reaction equilibrium relationship is obtained as

[0038]

[0039] According to this formula and combined with the actual situation, under the condition of a certain chelating agent concentration, it is found that lg(K f )>-2, indicating that the chelation reaction is very fast, and the chelating agent has good chelation effect on iron source, -6<lg(K f )<-2, indicating that the chelation reaction is slower, and the chelating agent has general effect, and lg(K f )<-6, indicating that the reaction almost does not occur, indicating that the chelating agent does not work.

[0040] In addition, the ion chelating agent also needs to be controlled within a certain concentration range, because too low or too high concentration can adversely affect the effect of chelation reaction.

[0041] When the chelating agent concentration is not enough, it may:

[0042] Insufficient chelation: too low concentration of chelating agent can lead to insufficient chelation of metal ions, which affects the chelation efficiency.

[0043] Slow reaction rate: Lower chelator concentrations can slow the rate of chelation reactions, extending the time required to reach equilibrium.

[0044] Decreased selectivity: In the presence of multiple metal ions, too low a chelator concentration can fail to effectively distinguish and selectively chelate specific ions.

[0045] While too high a concentration can:

[0046] Increased side reactions: Too high a chelator concentration can cause unwanted side reactions, such as over-chelation or reactions with other components in the solution.

[0047] Precipitate formation: Certain chelators can form precipitates at high concentrations, which reduces their effective concentration in solution and can clog equipment.

[0048] Increased cost: Using excess chelator increases raw material costs, which is not cost-effective from an economic perspective.

[0049] Toxicity issues: Certain chelators can be toxic to the environment or operating personnel at high concentrations.

[0050] In the present technical solution, the amount of chelator is limited by the ratio of the ion concentration of the chelator to the total ion concentration in the solution. In the present invention, the optimal range of this parameter is 0.005-0.03.

[0051] The sodium source in the application is from carbonate, sulfate, oxide, chloride, boride, hydroxide and the like compounds, including but not limited to sodium hydroxide (NaOH), sodium carbonate (Na2CO3), sodium chloride (NaCl), sodium pyrophosphate (Na4P2O7), sodium dihydrogen pyrophosphate (Na2H2P2O7), sodium nitrate (NaNO3), sodium bicarbonate (NaHCO3), sodium oxide (Na2O), sodium borohydride (NaBH4), sodium borate (Na2B4O7). The iron source is from carbonate, sulfate, oxide, chloride, phosphate, boride, hydroxide and the like compounds, including but not limited to ferrous sulfate (FeSO4), ferric sulfate (Fe2(SO4)3), ferrous chloride (FeCl2), ferric chloride (FeCl3), ferrous nitrite (Fe(NO2)2), ferric nitrite (Fe(NO2)3), ferrous nitrate (Fe(NO3)2), ferric nitrate (Fe(NO3)3), iron phosphate (FePO4), iron oxide (Fe2O3). The phosphorus source is from phosphate, chloride, oxide and the like compounds, including but not limited to sodium dihydrogen phosphate (NaH2PO4), sodium trihydrogen phosphate (Na3PO4), disodium hydrogen phosphate (Na2HPO4), trisodium hydrogen phosphate (Na2H2PO4), monosodium phosphate (NaH2PO3), disodium phosphate (Na2HPO3), trisodium phosphate (Na3PO3), phosphorus chloride (PCl3), phosphorus oxide (P2O5), sodium phosphite (NaH2PO2), ammonium dihydrogen phosphate (NH4H2PO4); the chelating agent is one or more of ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA), sodium hypophosphite, citric acid, oxalic acid, ascorbic acid, tartaric acid, ethylenediaminediphenyloxyacetic acid (EDDHA), hydroxyethylenediaminetriacetic acid (HEDTA), propylenediaminetetraacetic acid (PDTA), nitrilotriacetic acid (NTA), salicylic acid, phenolsulfonic acid, tetrasodium iminodisuccinate (IDS-Na4), tetrasodium glutamate diacetate (GLDA-Na4), tetrasodium aspartate diacetate (ASDA-Na4), trisodium methylglycine diacetate (MGDA-Na3);

[0052] The dispersant is one or more of polyvinylpyrrolidone (PVP), polyvinyl alcohol (PVA), sodium carboxymethyl cellulose (CMC), methylhydroxyethyl cellulose (MHEC), sodium tripolyphosphate (STPP), sodium dodecyl sulfonate (SDS).

[0053] The composite sodium iron phosphate described in the application can be widely applied to the positive electrode material of a sodium ion battery.

[0054] Compared with the prior art, the application has the following beneficial effects:

[0055] 1. Improved tap density: By using raw materials containing chelating agents, ensuring that the iron source is fully present in ionic form in the material, which helps to form a high-purity NFPP phase, increasing the intrinsic density of the material (intrinsic density, also known as true density, reaches 3.4-3.8), thereby increasing the tap density of the material (tap density reaches 2.2-2.5). And because the ion concentration significantly affects the progress of the chelation reaction in the preparation of composite sodium iron phosphate prepared by ion chelation technology, the ionic strength is controlled in the range of 0.03-0.1 mol / L to obtain the best reaction effect.

[0056] 2. Reduced porosity: By controlling the ratio of pyrophosphate to phosphate in the raw materials, the release of gases (such as oxygen, carbon dioxide, etc.) during the formation of the material is avoided, reducing the impact on the crystal structure of the material, thereby reducing the formation of pores and lowering the porosity.

[0057] 3. Improved crystal structure: By precisely controlling the ratio of pyrophosphate to phosphate, the damage to the crystal structure caused by gas release is avoided, resulting in complete and orderly secondary grains, improving the overall density of the material.

[0058] 4. Enhanced kinetic performance: Due to the presence of iron source in ionic form and the optimized crystal structure, the ion diffusion performance of the material is expected to be improved, thereby enhancing the kinetic performance of the material.

[0059] 5. Reduced synthesis cost: The optimization of raw material processing and synthesis process mentioned in this invention can simplify the production process, reduce energy consumption and the use of raw materials, and increase the solid content in the feed solution, thereby reducing the synthesis cost.

[0060] 6. Improved material stability: By reducing gas release, structural damage that may occur during the synthesis process of the material is avoided, improving the thermal and chemical stability of the material.

[0061] 7. Optimized electrochemical performance of the material: Through the above improvements, the electrochemical performance of the material, such as cycle stability, rate performance, and high-temperature performance, is expected to be significantly improved.

[0062] 8. Simplified process flow: If the material can be prepared through chemical etching dissolution and physical process assistance, the traditional sand milling and spray drying process can be simplified, reducing the dependence on equipment.

[0063] 9. Improved material safety: By reducing porosity and optimizing the crystal structure, the mechanical and thermal stability of the material during battery charging and discharging may be enhanced, thereby improving the safety of the battery.

[0064] These improvements directly address the shortcomings of existing composite sodium iron phosphate materials in terms of compaction density, porosity, and kinetic performance, significantly improving the performance of the material and meeting the needs of sodium-ion batteries in high-energy density and high-power density applications.

[0065] The application will be further described below with reference to the accompanying drawings. BRIEF DESCRIPTION OF DRAWINGS

[0066] Figure 1 SEM image of an embodiment of the application.

[0067] Figure 2 XRD image of an embodiment of the application. DETAILED DESCRIPTION

[0068] The application relates to a preparation method of high-compaction-density composite sodium iron phosphate, comprising the following steps:

[0069] Step (1), a sodium source, an iron source, and a phosphorus source are weighed according to the stoichiometric ratio [xNa+(a-x)A]:[yFe+(b-y)B]:P=a:b:(c+2), wherein A is a Na-site dopant, B is a Fe-site dopant, A is selected from metal dopant elements similar in radius to Na ions, B is selected from metal dopant elements equal in radius to Fe ions or smaller than Fe ions, and 3<=a<=4.3, 2<=b<=3, 1<=c<=2, a-b<=1, 0

[0070] Step (2), the slurry F is spray dried in a spray drying tower, the inlet temperature is 220 DEG C, the outlet temperature is 110 DEG C, and the spray drying obtains powder K. The particle size of the powder K is controlled to be 2um

[0071] Step (3), the powder K is transferred to a tube furnace, calcined at a temperature of 450-600 DEG C for 10-15 hours under a N2 protective atmosphere with an oxygen content of less than 50 ppm, and finally airflow pulverized to obtain a sodium iron pyrophosphate positive electrode material powder with high compaction density, the molecular formula of which is (Na x A a-x )(Fe y B b-y )(PO4) c P2O7.

[0072] In step (1) of the present application, sucrose is used as a carbon source and a reducing agent (reducing Fe3+ to Fe2+). The role of sucrose as a carbon source is similar to carbon-coated lithium iron phosphate. The controlled amount of sucrose is 0%-2% of the total mass.

[0073] Preferably, a:b:(c+2) is 4:3:4 or 3.4:2.4, 3.4, and the preferred ratio is the latter.

[0074] In the present application, the grinding cavity temperature is controlled to be between 30-50℃, the viscosity of slurry F is between 300-2000mpa*s; the addition amount of chelating agent is 5%-15% of the total mass of iron source, sodium source and phosphorus source, and the addition amount of dispersant is 1%-3% of the total mass of iron source, sodium source, phosphorus source and chelating agent.

[0075] In step (1), the grinding cavity temperature is controlled to be between 30-50℃, the particle size of slurry F is controlled, and the viscosity is controlled to be between 300-2000mpa*s. The particle size of slurry F is controlled to be D10≤150nm, D50≤300nm, D90≤500nm, Dmax<1um, and the particle size distribution

defined as span=(D90-D10) / D50

[0076] In step (1), the ionic strength of the chelating agent is controlled to be in the range of I=0.03-0.1mol / L.

[0077] The ratio of the ionic concentration of the chelating agent to the total ionic concentration in the solution is 0.005-0.03.

[0078] The present application will be further described in detail in conjunction with specific examples:

[0079] Example 1:

[0080] Step (1): According to the stoichiometric ratio, weigh sodium pyrophosphate: lithium hydroxide: ferric oxide: neodymium oxide: ammonium dihydrogen phosphate = 1.0:0.02:1.485:0.015:2, a total of 2000g. First, disperse ferric oxide, sodium pyrophosphate and ammonium dihydrogen phosphate into pure water, then add 145g of ethylenediaminetetraacetic acid, 25g of citric acid and 25g of ascorbic acid as chelating agents, react in a reaction kettle at 60℃ for 12h, then transfer to a sand mill and grind at a speed of 3000r / min. After 1h of grinding, add 20g of sodium dodecyl sulfate as a dispersant, 30g of sucrose, and then add the remaining materials lithium hydroxide and neodymium oxide after 3h of grinding. The grinding cavity temperature is controlled to be 40℃ during grinding. After grinding, the particle size of slurry F is controlled, and the particle size distribution is D10:100-150nm; D50:200-300nm; D90:300-500nm. At the same time, the viscosity of slurry F is <1000mpa*s;

[0081] Step (2): spray drying the slurry F with a spray drying tower, inlet temperature 220℃, outlet temperature 110℃, spray drying to obtain powder K, control the particle size of powder K D10: 5-6um, D50: 10-14um, D99: 25-30um;

[0082] Step (3): transfer the powder K to a tube furnace, calcine at 580℃ for 15 hours under N2 protective atmosphere, and finally airflow crushing to obtain the sodium iron pyrophosphate powder with the molecular formula (Na4Li 0.02 )(Fe 2.97 Nd 0.03 )(PO4)2P2O7.

[0083] Example 2:

[0084] Step (1): weigh 2000g of sodium pyrophosphate: lithium hydroxide: ferric oxide: neodymium oxide: ammonium dihydrogen phosphate according to the stoichiometric ratio of 0.85: 0.02: 1.2: 0.015: 1.4, first disperse ferric oxide, sodium pyrophosphate, ammonium dihydrogen phosphate into pure water, then add 145g of ethylenediaminetetraacetic acid, 25g of citric acid, 25g of ascorbic acid as chelating agent, react in a reaction kettle at 60℃ for 12h, then transfer into a sand mill to grind at a speed of 3000r / min, after grinding for 1h, add 20g of sodium dodecyl sulfate as dispersant, 30g of sucrose, then add the remaining materials after 3h grinding, control the grinding cavity temperature at 40℃ during grinding, after grinding, control the particle size of the slurry F, the particle size distribution is D10: 100-150nm; D50: 200-300nm; D90: 300-500nm, and the viscosity of the slurry is <1000mpa*s;

[0085] Step (2): spray drying the slurry F with a spray drying tower, inlet temperature 220℃, outlet temperature 110℃, spray drying to obtain powder K, control the particle size of powder K D10: 5-6um, D50≤14um(10-14), D99: 25-30um;

[0086] Step (3): transfer the powder K to a tube furnace, calcine at 580℃ for 15 hours under N2 protective atmosphere, and finally airflow crushing to obtain the sodium iron pyrophosphate powder with the molecular formula (Na 3.4 Li 0.02 )(Fe 2.4 Nd 0.03 )(PO4) 1.4 P2O7.

[0087] This example is a preferred example. The SEM image of this example is shown in Figure 1 , the XRD image is shown in Figure 2 . From Figure 1It can be seen that the material has less micropore and lower porosity. Figure 2 It is shown that the material prepared according to the method has no impurities and high purity.

[0088] Example 3

[0089] Step (1): 2000g of sodium pyrophosphate, lithium hydroxide, ferric sesquioxide, neodymium oxide and ammonium dihydrogen phosphate were weighed according to the stoichiometric ratio of 0.85:0.02:1.2:0.015:1.4. First, the ferric sesquioxide, sodium pyrophosphate, ammonium dihydrogen phosphate were dispersed into pure water, then 145g of hydroxyethyl ethylenediaminetriacetate, 25g of anhydrous oxalic acid and 25g of ascorbic acid were added as chelating agents, and the reaction was carried out at 60℃ for 12h in a reaction kettle. Then it was transferred to a sand mill and ground at a speed of 3000r / min. After 1h of grinding, 20g of sodium dodecyl sulfate was added as a dispersant, 30g of sucrose was added, and the remaining materials were added after 3h of grinding. The grinding cavity temperature was controlled at 40℃ during grinding. After grinding, the slurry F particle size was controlled, and the particle size distribution was D10: 100-150nm; D50: 200-300nm; D90: 300-500nm, and the viscosity of the slurry was <1000mpa*s;

[0090] Step (2): The slurry F was spray dried by a spray drying tower, with an inlet temperature of 220℃ and an outlet temperature of 110℃. The powder K was obtained by spray drying, and the particle size of the powder K was controlled as follows: D10≥5um, D50≤14um, and D99≤30um.

[0091] Step (3): The powder K was transferred to a tube furnace and calcined at 580℃ for 15h under N2 protection. Finally, the airflow was crushed to obtain a sodium pyrophosphate iron phosphate powder with a molecular formula of (Na 3.4 Li 0.02 )(Fe 2.4 Nd 0.03 )(PO4) 1.4 P2O7.

[0092] Example 2 was changed on the basis of the chelating agent.

[0093] Example 4

[0094] Step (1): 2000 g of sodium pyrophosphate: potassium hydroxide: ferrous oxalate: lanthanum oxide: phosphoric acid = 0.85: 0.02: 2.4: 0.015: 1.4 in stoichiometric ratio were weighed, the ferrous oxalate, sodium pyrophosphate and phosphoric acid were first dispersed into pure water, then 145 g of ethylenediaminetetraacetic acid, 25 g of citric acid and 25 g of ascorbic acid were added as chelating agents, and the mixture was reacted in a reaction kettle at 60°C for 12 h, and then was transferred into a sand mill for grinding at a speed of 3000 r / min. After 1 h of grinding, 20 g of sodium dodecyl sulfate was added as a dispersant, 30 g of sucrose was added, and the remaining materials were added after 3 h of grinding. The grinding cavity temperature was controlled at 30-50°C during grinding. After grinding, the particle size of the slurry F was controlled, and the particle size distribution was D10: 100-150 nm; D50: 200-300 nm; D90: 300-500 nm, and the viscosity of the slurry was <1000 mpa*s;

[0095] Step (2): The slurry F was spray dried in a spray drying tower, the inlet temperature was 220°C, and the outlet temperature was 110°C. The powder K was obtained by spray drying, and the particle size of the powder K was controlled to be D10≥5 um, D50≤14 um, and D99≤30 um.

[0096] Step (3): The powder K was transferred to a tube furnace, calcined at 580°C for 15 h under N2 protection, and finally airflow pulverized to obtain a sodium pyrophosphate iron phosphate powder. The molecular formula is (Na 3.4 K 0.02 )(Fe 2.4 La 0.03 )(PO4) 1.4 P2O7. The doping element is changed compared with Example 2.

[0097] Comparative Example 1

[0098] Step (1): 2000 g of sodium pyrophosphate: ferric trioxide: neodymium oxide: ammonium dihydrogen phosphate = 0.85: 1.2: 0.015: 1.4 in stoichiometric ratio were weighed, the ferric trioxide, sodium pyrophosphate and ammonium dihydrogen phosphate were first dispersed into pure water, then 145 g of ethylenediaminetetraacetic acid, 25 g of citric acid and 25 g of ascorbic acid were added as chelating agents, and the mixture was reacted in a reaction kettle at 60°C for 12 h, and then was transferred into a sand mill for grinding at a speed of 3000 r / min. After 1 h of grinding, 20 g of sodium dodecyl sulfate was added as a dispersant, 30 g of sucrose was added, and the remaining materials were added after 3 h of grinding. The grinding cavity temperature was controlled at less than 50°C during grinding. After grinding, the particle size of the slurry F was controlled, and the particle size distribution was D10: 100-150 nm; D50: 200-300 nm; D90: 300-500 nm, and the particle size distribution was D10: 0.1-0.15 um; D90: 0.3-0.5 nm, and the viscosity of the slurry was <1000 mpa*s;

[0099] Step (2): spray dry the slurry F with a spray drying tower, inlet temperature 220°C, outlet temperature 110°C, spray drying to obtain powder K, control the particle size of powder K D10≥5um, D50≤14um, D99≤30um;

[0100] Step (3): transfer the powder K to a tube furnace, calcine at 580°C for 15 hours under N2protective atmosphere, and finally airflow crush to obtain sodium iron phosphate powder with molecular formula Na 3.4 (Fe 2.4 Nd 0.03 )(PO4) 1.4 P2O7.

[0101] This example is compared with preferred example 2, without Na site doping, also using chelating agent preparation method.

[0102] Comparative example 2

[0103] Step (1): weigh sodium pyrophosphate, lithium hydroxide, ferric sesquioxide, and ammonium dihydrogen phosphate according to the stoichiometric ratio: 0.85:0.02:1.2:1.4, a total of 2000g, first disperse ferric sesquioxide, sodium pyrophosphate, and ammonium dihydrogen phosphate in pure water, then add 145g of ethylenediaminetetraacetic acid, 25g of citric acid, and 25g of ascorbic acid as chelating agents, react in a reaction kettle at 60°C for 12h, then transfer to a sand mill and grind at a speed of 3000r / min, after 1h of grinding, add 20g of sodium dodecyl sulfate as a dispersant, 30g of sucrose, and then add the remaining materials after 3h of grinding, control the grinding cavity temperature below 50°C during grinding, and control the particle size of the slurry F after grinding, the particle size distribution is D10: 100-150nm; D50: 200-300nm; D90: 300-500nm, and the viscosity of the slurry is <1000mpa*s;

[0104] Step (2): spray dry the slurry F with a spray drying tower, inlet temperature 220°C, outlet temperature 110°C, spray drying to obtain powder K, control the particle size of powder K D10≥5um, D50≤14um, D99≤30um;

[0105] Step (3): transfer the powder K to a tube furnace, calcine at 580°C for 15 hours under N2protective atmosphere, and finally airflow crush to obtain sodium iron phosphate powder with molecular formula Na 3.4 Li 0.02 )Fe 2.4 (PO4) 1.4 P2O7.

[0106] This example is compared with preferred example 2, without Fe site doping, also using chelating agent preparation method.

[0107] Comparative Example 3:

[0108] Step (1): 2000 g of sodium pyrophosphate, lithium hydroxide, ferric sesquioxide, neodymium oxide and ammonium dihydrogen phosphate were weighed according to the stoichiometric ratio of 1.0:0.02:1.485:0.015:2.1, and first, the ferric sesquioxide, sodium pyrophosphate and ammonium dihydrogen phosphate were dispersed into pure water, then 145 g of ethylenediaminetetraacetic acid, 25 g of citric acid and 25 g of ascorbic acid were added as chelating agents, and the reaction was carried out at 60°C for 12 h in a reaction kettle. Then it was transferred into a sand mill and ground at a speed of 3000 r / min. After 1 h of grinding, 20 g of sodium dodecyl sulfate was added as a dispersant, 30 g of sucrose was added, and the remaining materials lithium hydroxide and neodymium oxide were added after 3 h of grinding. The grinding cavity temperature was controlled below 50°C during grinding. After grinding, the particle size of the slurry F was controlled, and the particle size distribution was D10: 100-150 nm; D50: 200-300 nm; D90: 300-500 nm. At the same time, the viscosity of the slurry F was <1000 mpa*s;

[0109] Step (2): The slurry F was spray dried by a spray drying tower, with an inlet temperature of 220°C and an outlet temperature of 110°C. The powder K was obtained by spray drying, and the particle size of the powder K was controlled to be D10≥5 um, D50≤14 um and D99≤30 um;

[0110] Step (3): The powder K was transferred to a tube furnace and calcined at 580°C for 15 hours under N2 protection atmosphere. Finally, the gas flow was crushed to obtain a phosphoric acid pyrophosphate iron sodium powder with a molecular formula of (Na4Li 0.02 )(Fe 2.97 Nd 0.03 )(PO4) 2.1 P2O7.

[0111] Comparative Example 1, the example here is designed as PO4: P2O7=2.1:1.

[0112] Comparative Example 4:

[0113] Step (1): 2000 g of sodium pyrophosphate, lithium hydroxide, ferric sesquioxide, neodymium oxide and ammonium dihydrogen phosphate were weighed according to the stoichiometric ratio of 1.0:0.02:1.485:0.015:2.1, and first, the ferric sesquioxide, sodium pyrophosphate and ammonium dihydrogen phosphate were dispersed into pure water, then 145 g of ethylenediaminetetraacetic acid, 25 g of citric acid and 25 g of ascorbic acid were added as chelating agents, and the reaction was carried out at 60°C for 12 h in a reaction kettle. Then it was transferred into a sand mill and ground at a speed of 3000 r / min. After 1 h of grinding, 20 g of sodium dodecyl sulfate was added as a dispersant, 30 g of sucrose was added, and the remaining materials lithium hydroxide and neodymium oxide were added after 3 h of grinding. The grinding cavity temperature was controlled below 50°C during grinding. After grinding, the particle size of the slurry F was controlled, and the particle size distribution was D10: 100-150 nm; D50: 200-300 nm; D90: 300-500 nm. At the same time, the viscosity of the slurry F was <1000 mpa*s;

[0114] Step (2): spray drying the slurry F with a spray drying tower, inlet temperature 220℃, outlet temperature 110℃, spray drying to obtain powder K, control the particle size of powder K D10≥5um, D50≤14um, D99≤30um;

[0115] Step (3): transfer the powder K to a tube furnace, calcine at 580℃ for 15 hours under N2protective atmosphere, and finally airflow crushing to obtain the sodium iron pyrophosphate powder with the molecular formula (Na4Li 0.02 )(Fe 2.97 Nd 0.03 )(PO4)2P2O7.

[0116] Comparative Example 1, without using chelating agent method.

[0117] Comparative Example 5:

[0118] Step (1): weigh 2000g of sodium pyrophosphate: potassium hydroxide: ferrous oxalate: lanthanum oxide: phosphoric acid = 0.85: 0.02: 2.4: 0.015: 1.4 according to the stoichiometric ratio, first disperse ferrous oxalate, sodium pyrophosphate and phosphoric acid into pure water, then add, react in a reaction kettle at 60℃ for 12h, then transfer into a sand mill to grind at a speed of 3000r / min, after grinding for 1h, add 20g of sodium dodecyl sulfate as a dispersant, 30g of sucrose, and then add the remaining materials after 3h of grinding, control the grinding cavity temperature below 50℃ during grinding, and control the particle size of the slurry F after grinding, the particle size distribution is D10: 100-150nm; D50: 200-300nm; D90: 300-500nm, and the viscosity of the slurry is <1000mpa*s;

[0119] Step (2): spray drying the slurry F with a spray drying tower, inlet temperature 220℃, outlet temperature 110℃, spray drying to obtain powder K, control the particle size of powder K D10≥5um, D50≤14um, D99≤30um;

[0120] Step (3): transfer the powder K to a tube furnace, calcine at 580℃ for 15 hours under N2protective atmosphere, and finally airflow crushing to obtain the sodium iron pyrophosphate powder with the molecular formula (Na 3.4 K 0.02 )(Fe 2.4 La 0.03 )(PO4) 1.4 P2O7.

[0121] Comparative Example 4, without using chelating agent.

[0122] Comparative Example 6:

[0123] Step (1): 2000 g of sodium pyrophosphate, lithium hydroxide, ferric sesquioxide, neodymium oxide, and ammonium dihydrogen phosphate were weighed according to the stoichiometric ratio of 1.0:0.02:1.485:0.015:2.1, respectively. First, the ferric sesquioxide, sodium pyrophosphate, and ammonium dihydrogen phosphate were dispersed in pure water, and the reaction was carried out at 60°C for 12 h in a reaction kettle. Then, the mixture was transferred to a sand mill and ground at a speed of 3000 r / min. After 1 h of grinding, 20 g of sodium dodecyl sulfate was added as a dispersant, and 30 g of sucrose was added. After 3 h of grinding, the remaining materials, lithium hydroxide and neodymium oxide, were added. The grinding cavity temperature was controlled below 50°C during grinding. After grinding, the F slurry was controlled to have a particle size distribution of D10: 100-150 nm; D50: 200-300 nm; and D90: 300-500 nm. The viscosity of the F slurry was less than 1000 mPa·s.

[0124] Step (2): The F slurry was spray dried in a spray drying tower at an inlet temperature of 220°C and an outlet temperature of 110°C to obtain powder K. The particle size of the powder K was controlled to be D10≥5 um, D50≤14 um, and D99≤30 um.

[0125] Step (3): The powder K was transferred to a tube furnace and calcined at 580°C for 15 h under N2 protection. Finally, the product was air-milled to obtain a sodium iron pyrophosphate powder with a molecular formula of (Na4Li 0.02 )(Fe 2.97 Nd 0.03 )(PO4) 2.1 P2O7.

[0126] No chelating agent was used compared with Comparative Example 3.

[0127] Battery test part: The sodium iron pyrophosphate material prepared above was used to prepare a sodium ion battery electrode sheet, including the following steps:

[0128] (1) 150 g of polyvinylidene fluoride (PVDF) with a molecular weight of 80 to 100 million was slowly added to 2850 g of N-methyl pyrrolidone after being baked at 80°C under vacuum conditions for 10 h, and stirred at a speed of 700 rpm for 30 min under vacuum. After standing, a 5% (same as above) adhesive slurry was obtained. The amount of PVDF added after baking was 150 g.

[0129] (2) 681.5 g of CNT and 25 g of conductive carbon black (Super P) previously baked at 120°C under vacuum conditions for 12 h were added to the adhesive slurry, and stirred at a speed of 700 rpm for 2 h under vacuum. Then, 5000 g of sodium-based layered oxide material previously baked at 120°C under vacuum conditions for 12 h was added, and the stirring was continued at a speed of 700 rpm for 2 h under vacuum. Subsequently, an appropriate amount of NMP was added to adjust the viscosity to 5000 mPa·s.

[0130] (3) The main material prepared in step (2) is coated on an aluminum foil with a thickness of 12 μm, dried to remove the solvent, and then rolled and cut to obtain a composite positive electrode sheet.

[0131] Further, the composite positive electrode sheet obtained above is used to prepare a soft-pack battery, including the following steps:

[0132] (1) Hard carbon, styrene-butadiene rubber (SBR), and sodium carboxymethyl cellulose (CMC) are dissolved in deionized water at a mass ratio of 100:15:10, stirred uniformly, and then coated on a copper current collector to dry, and then rolled and cut to obtain a negative electrode sheet;

[0133] (2) The positive electrode sheet, the negative electrode sheet, and the separator are made into a core by a lamination process, and then a 15 Ah aluminum-plastic film soft-pack battery is obtained by packaging, liquid injection, formation, air extraction, and sealing.

[0134] The physicochemical properties of the sodium iron phosphate pyrophosphate powder obtained in the above examples are tested, and the test results are as follows:

[0135] Table 1 Physicochemical properties

[0136]

[0137]

[0138] From the results, Example 2 performs best overall, achieving the lowest resistivity and the highest true density, indicating that the chelating agent formula of Example 2 is the best. Comparative Examples 1 and 2 have significantly higher resistivity than Example 2 because there is no Na doping and Fe doping. The true density also decreases. Comparative Examples 4 and 5 have significantly higher resistivity and lower true density and tap density than Examples 1 and 4 because no chelating agent is used. Comparative Example 6 has lower specific capacity, lower true density and tap density, and larger resistivity than Comparative Example 3 because no chelating agent is used.

[0139] Table 2 Synthesis process parameters

[0140]

[0141] From the data in Table 2, it can be found that the type and ratio of the chelating agent used in Example 2 are suitable, achieving the largest equilibrium constant, indicating that the reaction efficiency of the chelating agent of Example 2 is best.

[0142] Table 3 Electrochemical indicators of button cells

[0143]

[0144] As can be seen from the table, Example 2 achieves the best overall half-cell test level, the highest specific capacity, the first efficiency and the compaction density. As can be seen from the data in the above table, Example 1 is compared with Comparative Example 3, because the ratio of PO4, P2O7 of Comparative Example 3 exceeds the range of 2:1, so the heterogeneous phase will be generated, and the intuitive effect is that the specific capacity is greatly reduced.

[0145] Table 4 Electrochemical indicators of 15 Ah soft pack battery

[0146]

[0147] As can be seen from Table 4, the soft pack battery prepared by the material of Example 2 with the best performance is the best, which is reflected in that the electrode compaction density reaches 2.63 g / cc, the first circle coulombic efficiency is 88.5%, the cycle process is very stable, and the energy density of the final soft pack battery reaches 128 Wh / kg, which is very beneficial to the application of the material in the energy storage scene and the two-wheeled vehicle scene.

[0148] Although the present application is described with reference to specific embodiments, such description is not meant to constitute a limitation of the present application. Other variations of the disclosed embodiments can be anticipated by those skilled in the art from the description provided herein, and such variations are intended to be within the scope of the claims.

Claims

1. A process for the preparation of a high compact density composite sodium iron phosphate, characterized in that The method comprises the following steps: Step (1), the sodium source, the iron source and the phosphorus source are weighed according to the stoichiometric ratio of [xNa+(a-x)A]: [yFe+(b-y)B]: P=a:b:(c+2), wherein A is a Na-site dopant, B is a Fe-site dopant, A is selected from lithium hydroxide or potassium hydroxide which has a similar radius to Na ion, B is selected from neodymium oxide or lanthanum oxide which has a radius equal to or smaller than that of Fe ion, and 3≤a≤4.3, 2≤b≤3, 1≤c≤2, a-b≤1, 0<a-x≤0.05, 0<b-y≤0.05; then the iron source, the sodium source, the phosphorus source and the green degradable chelating agent are mixed and dispersed in pure water, reacted at a certain temperature for a period of time, then transferred into a sand mill for grinding, an appropriate amount of dispersant and sucrose are added, and the grinding is continued for a period of time, then the dopants A and B are added, and the slurry F is obtained after grinding; Step (2), the slurry F is spray dried by a spray drying tower to obtain the powder K; Step (3), transfer the powder K to a tube furnace, calcine under N2protective atmosphere, oxygen content <50ppm, temperature 450-600℃, 10-15 hours, and finally airflow pulverization, to obtain a sodium iron phosphate positive electrode material powder with high tap density, molecular formula (Na x A a-x )(Fe y B b-y )(PO4) c P2O7. In step (1), the ionic strength of the chelating agent is controlled to be I = 0.03-0.1 mol / L. The chelating agent is one or more of ethylenediaminetetraacetic acid, diethylenetriamine pentaacetic acid, sodium hypophosphite, citric acid, oxalic acid, ascorbic acid, tartaric acid, ethylenediamine di-o-phenoxyacetic acid, hydroxyethyl ethylenediamine triacetic acid, propylene diamine tetraacetic acid, nitrilotriacetic acid, salicylic acid, phenol sulfonic acid, imino disuccinic acid tetrasodium, glutamic acid diacetic acid tetrasodium, aspartic acid diacetic acid tetrasodium, and methyl glycine diacetic acid trisodium.

2. The method for preparing high-density composite sodium iron phosphate according to claim 1, characterized in that: In step (1), the amount of the chelating agent added is 5%-15% of the total mass of the iron source, the sodium source and the phosphorus source, and the amount of the dispersant added is 1%-3% of the total mass of the iron source, the sodium source, the phosphorus source and the chelating agent.

3. The method for preparing high-density composite sodium iron phosphate according to claim 1, characterized in that: In step (1), the grinding is performed while the temperature of the grinding cavity is controlled to be between 30-50°C, the particle size of the slurry F is controlled, and the viscosity is controlled to be between 300-2000 mpa*s.

4. The method for preparing high-density composite sodium iron phosphate according to claim 1, characterized in that: The ratio of the ion concentration of the chelating agent to the total ion concentration in the solution is 0.005-0.

03.

5. The method for preparing high-compacted-density sodium iron phosphate according to claim 1, wherein: The particle size of the slurry F is controlled to be D10≤150 nm, D50≤300 nm, D90≤500 nm, Dmax<1 um, and the particle size distribution is <1.5; and the particle size of the powder K is controlled to be 2 um<D10<6 um, 10 um<D50<18 um, and 25 um<D99<40 um.

6. The method for preparing high-density composite sodium iron phosphate according to claim 1, characterized in that: The sodium source is from carbonates, sulfates, oxides, chlorides, borides and hydroxides, including sodium hydroxide, sodium carbonate, sodium chloride, sodium pyrophosphate, disodium hydrogen pyrophosphate, sodium nitrate, sodium bicarbonate, sodium oxide, sodium borohydride or sodium borate; The iron source is from carbonates, sulfates, oxides, chlorides, phosphates, borides and hydroxides, including ferrous sulfate, iron sulfate, ferrous chloride, ferric chloride, ferrous nitrite, ferric nitrite, ferrous nitrate, ferric nitrate, iron phosphate or iron oxide.

7. The method for preparing high-density composite sodium iron phosphate according to claim 1, characterized in that: The phosphorus source is from phosphates, chlorides and oxides, including sodium dihydrogen phosphate, sodium trihydrogen phosphate, sodium hydrogen phosphate, sodium hydrogen triphosphate, monosodium phosphate, disodium phosphate, trisodium phosphate, phosphorus chloride, phosphorus oxide, sodium hypophosphite or ammonium dihydrogen phosphate.

8. The method of claim 1, wherein the dispersant is one or more of polyvinylpyrrolidone, polyvinyl alcohol, sodium carboxymethylcellulose, methylhydroxyethylcellulose, sodium tripolyphosphate, and sodium dodecylsulfate. The application is applied to the positive electrode material of a sodium ion battery.

9. Use of a high-density compacted sodium iron phosphate prepared according to the process of any one of claims 1 to 8, characterized in that: ​

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