Iron-based phosphate composite cathode material for sodium-ion batteries and its preparation method

Abstract

This invention relates to an iron-based phosphate composite cathode material for sodium-ion batteries and its preparation method. The iron-based phosphate composite cathode material includes sodium phosphate-iron-sodium phosphate type NaFePO4 and olivine type LiFePO4; the general chemical formula is: xNaFePO4·LiFePO4, where 1≤x≤10. When this composite cathode material is used in a sodium-ion battery, during the charge and discharge process, lithium ions in the olivine type LiFePO4 undergo ion intercalation and deintercalation. As the number of charge and discharge cycles increases, the vacancies of the deintercalated lithium ions are gradually occupied by sodium ions in the electrolyte, and the olivine type LiFePO4 transforms into olivine type NaFePO4, forming a composite phase of olivine type NaFePO4 and sodium phosphate-iron-sodium phosphate type NaFePO4. During this process, the olivine type LiFePO4 provides a continuous sodium ion transport channel, improving the sodium ion transport rate and thus enhancing the electrochemical activity of sodium phosphate-iron-sodium phosphate type NaFePO4.

Description

Technical Field

[0001] This invention relates to the field of battery materials technology, and in particular to an iron-based phosphate composite cathode material for sodium-ion batteries and its preparation method. Background Technology

[0002] Among the reported sodium-ion battery cathode materials, polyanionic compound electrodes exhibit higher operating potentials for the same redox pair compared to layered transition metal oxides. More importantly, polyanionic compounds exhibit higher thermal stability due to the tight covalent bonding between oxygen and phosphorus ions. Phosphate-based cathode materials, in particular, offer superior safety, making them more suitable for large-scale applications.

[0003] Lithium iron phosphate (LiFePO4) boasts advantages such as high capacity, good stability, and high safety in lithium-ion batteries. Similarly, sodium iron phosphate has also attracted widespread attention. Sodium iron phosphate comprises two phase structures: olivine-type NaFePO4 and natronite-type NaFePO4. Olivine-type NaFePO4, as a cathode material in sodium-ion batteries, is a non-thermodynamically stable phase, requiring complex electrochemical ion exchange methods to synthesize from LiFePO4 precursors, which hinders the practical application of sodium-ion batteries. Thermodynamically stable natronite-type NaFePO4 has garnered more attention, and its preparation method is simple, making it highly suitable for industrial applications. However, due to the lack of sodium ion transport channels in its structure and its low intrinsic conductivity, natronite-type NaFePO4 is often considered to lack electrochemical activity.

[0004] The main methods for improving the electrochemical performance of sodium phosphate rock-type NaFePO4 include:

[0005] (1) The electrochemical performance of sodium phosphate rock type NaFePO4 was improved by charging it to 4.5V at a rate of 0.05C and then charging it at a constant voltage of 4.5V for 5 hours to induce its transformation to an amorphous phase structure. However, due to the long charging time and cumbersome process of this method, although the sodium phosphate rock type NaFePO4 modified by 4.5V has electrochemical activity, its cycle performance is poor.

[0006] (2) Electrospinning technology is used to uniformly embed sodium ferrophosphate (NaFePO4) nanoparticles into porous nitrogen-doped carbon nanofibers. Through the ultra-small nanosize effect and high-potential desodiuming process, the sodium ferrophosphate phase NaFePO4, which is generally considered electrochemically inactive, can be transformed into a highly active amorphous phase. However, electrospinning technology has some inherent disadvantages that are difficult to overcome, such as low spinning efficiency, high cost, and easy environmental pollution. These disadvantages limit the application of sodium ferrophosphate phase NaFePO4.

[0007] (3) The amorphous sodium phosphate-iron ore type cathode material NaFePO4 is prepared by low-temperature sintering combined with simple mechanical ball milling, which can eliminate the above-mentioned constant voltage charging process of 4.5V. Although this method simply and effectively activates the electrochemical activity of NaFePO4, when applied to batteries, there is a large electrochemical polarization between the positive and negative electrodes during the charging and discharging process, resulting in low energy conversion efficiency, which is still an important problem restricting its application. Summary of the Invention

[0008] The purpose of this invention is to address the problems of poor electrochemical activity, low specific capacity, and poor cycle performance in existing sodium-ion batteries that use sodium phosphate-type NaFePO4 as the cathode material, and to propose an iron-based phosphate composite cathode material for sodium-ion batteries and its preparation method.

[0009] To achieve the above objectives, in a first aspect, embodiments of the present invention provide an iron-based phosphate composite cathode material for sodium-ion batteries, wherein the iron-based phosphate composite cathode material includes sodium phosphate rock type NaFePO4 and olivine type LiFePO4.

[0010] The general chemical formula of the iron-based phosphate composite cathode material is: xNaFePO4·LiFePO4, where x is the molar ratio of NaFePO4 to LiFePO4, and 1≤x≤10.

[0011] Preferably, the iron-based phosphate composite cathode material is used in sodium-ion batteries;

[0012] During the charging and discharging process of the sodium-ion battery, the olivine-type LiFePO4 participates in the electrochemical reaction. Lithium ions in the olivine-type LiFePO4 undergo ion intercalation and deintercalation during the charging and discharging process. With the increase of the number of charging and discharging cycles, the vacancies of lithium ions extracted from the olivine-type LiFePO4 are occupied by sodium ions in the electrolyte, and the olivine-type LiFePO4 transforms into olivine-type NaFePO4, forming a composite phase of olivine-type NaFePO4 and sodium phosphate-iron ore-type NaFePO4. During this process, the olivine-type LiFePO4 provides a continuous sodium ion transport channel.

[0013] Secondly, embodiments of the present invention provide a method for preparing the iron-based phosphate composite cathode material described in the first aspect above, wherein the preparation method is a sol-gel method, comprising:

[0014] Sodium source material, iron source material, lithium source material and phosphate were weighed according to the stoichiometric ratio, placed in a stirring device, deionized water was added, and the mixture was stirred evenly to obtain a mixed solution.

[0015] The mixture is stirred at a certain temperature to form a precursor gel;

[0016] The precursor gel was placed in a grinding device and ground to obtain intermediate product powder.

[0017] The intermediate product powder is pressed into compact intermediate product discs and calcined under an argon atmosphere. After calcination, the mixture is cooled to room temperature to obtain an iron-based phosphate composite cathode material.

[0018] The general chemical formula of the iron-based phosphate composite cathode material is: xNaFePO4·LiFePO4, where x is the molar ratio of NaFePO4 to LiFePO4, and 1≤x≤10.

[0019] Preferably, the sodium source material includes one or more of sodium carbonate, sodium bicarbonate, sodium acetate, sodium oxalate, and sodium hydroxide;

[0020] The iron source material includes one or more of ferrous oxalate, ferrous chloride, ferrous phosphate, and ferrous acetate.

[0021] The lithium source material includes one or more of lithium carbonate, lithium acetate dihydrate, lithium hydroxide, and lithium bicarbonate.

[0022] The phosphate includes one or more of diammonium hydrogen phosphate, diammonium dihydrogen phosphate, phosphoric acid, sodium dihydrogen phosphate, disodium hydrogen phosphate, pyrophosphate, and sodium pyrophosphate.

[0023] The solid content of the mixture is 30wt%-70wt%;

[0024] The mixing equipment is a mixer; the speed of the mixer is between 20 r / min and 2000 r / min, and the mixing time is between 1 hour and 8 hours;

[0025] The specific meaning of continuing to stir at a certain temperature is: continuing to stir for 9 to 12 hours at a temperature of 90℃-100℃;

[0026] The grinding equipment is a grinding machine; the grinding machine's rotation speed is between 100 r / min and 4000 r / min, and the grinding time is between 0.1 hours and 48 hours;

[0027] The specific calcination conditions include: heating to 350℃-600℃ at a heating rate of 1℃ / min-3℃ / min and holding at that temperature for 10-20 hours.

[0028] Thirdly, embodiments of the present invention provide a method for preparing the iron-based phosphate composite cathode material described in the first aspect above, wherein the preparation method is a solid-state method, comprising:

[0029] Sodium source material, iron source material, lithium source material and phosphate were weighed according to stoichiometric ratio, placed in a ball mill jar, and solvent was added until it covered the grinding balls. After ball milling, a mixed liquid was obtained.

[0030] The ductile ink can containing the mixed liquid is placed in an oven and baked to obtain precursor powder.

[0031] The precursor powder was pressed into a compact disc and calcined in an argon atmosphere. After calcination, the sample was cooled to room temperature to obtain an iron-based phosphate composite cathode material.

[0032] Preferably, the sodium source material includes one or more of sodium carbonate, sodium bicarbonate, sodium acetate, sodium oxalate, and sodium hydroxide;

[0033] The iron source material includes one or more of ferrous oxalate, ferrous chloride, ferrous phosphate, and ferrous acetate.

[0034] The lithium source material includes one or more of lithium carbonate, lithium acetate dihydrate, lithium hydroxide, and lithium bicarbonate.

[0035] The phosphate includes one or more of diammonium hydrogen phosphate, diammonium dihydrogen phosphate, phosphoric acid, sodium dihydrogen phosphate, disodium hydrogen phosphate, pyrophosphate, and sodium pyrophosphate.

[0036] The solvent includes either ethanol or acetone.

[0037] The specific conditions for the ball milling process are as follows: the rotation speed of the ball mill jar is set between 400 r / min and 1000 r / min, and the ball milling time is between 6 hours and 12 hours.

[0038] The baking temperature is 60℃-80℃, and the baking time is 3 hours-10 hours.

[0039] The specific calcination conditions are as follows: heat to 350℃-600℃ at a heating rate of 1℃ / min-3℃ / min and hold for 10-20 hours.

[0040] Fourthly, embodiments of the present invention provide a positive electrode sheet, the positive electrode sheet comprising the iron-based phosphate composite positive electrode material described in the first aspect above.

[0041] Fifthly, embodiments of the present invention provide a method for preparing the positive electrode sheet described in the fourth aspect above, the method comprising:

[0042] The iron-based phosphate composite cathode material described in the first aspect above is ground and mixed with conductive additives until uniformly mixed to obtain a mixed powder.

[0043] Add the binder to the mixed powder, add the solvent, and continue grinding until homogeneous to obtain a mixed slurry;

[0044] The mixed slurry is coated onto the current collector, dried, and then placed in an oven for drying. After cutting, the positive electrode sheet is obtained.

[0045] Preferably, the conductive additive includes one or more of conductive carbon black Super P, acetylene black, carbon nanotubes, Ketjen black, and conductive graphite; the conductive additive accounts for 0-30 wt% of the iron-based phosphate composite cathode material.

[0046] The binder comprises one or more of the following: polyvinylidene fluoride (PVDF), sodium carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR), polyacrylonitrile (PAN), polyvinyl alcohol (PVA), polytetrafluoroethylene (PTFE), and polyimide (PI); the binder accounts for 0-30 wt% of the iron-based phosphate composite cathode material.

[0047] The solvent includes one or more of N-methylpyrrolidone, dimethyl carbonate, ethylene carbonate, or diethylene carbonate.

[0048] The drying conditions are: vacuum baking at 90℃-120℃ for 5-10 hours.

[0049] In a sixth aspect, embodiments of the present invention provide a sodium-ion battery, the sodium-ion battery comprising the positive electrode sheet described in the fourth aspect above;

[0050] The sodium-ion battery includes a liquid sodium-ion battery, a semi-solid sodium-ion battery, or an all-solid sodium-ion battery.

[0051] The sodium-ion battery operates within a voltage range of 1.5V to 4.5V.

[0052] This invention provides a method for preparing an iron-based phosphate composite cathode material for sodium-ion batteries. The method involves uniformly mixing raw materials according to a stoichiometric ratio, calcining the mixture under argon atmosphere, and then allowing it to cool naturally. After slow cooling, the iron-based phosphate composite cathode material is obtained, with the general chemical formula: xNaFePO4·LiFePO4 (where 1≤x≤10), where NaFePO4 is of the sodium phosphate type and LiFePO4 is of the olivine type. The iron-based phosphate composite cathode material xNaFePO4·LiFePO4… LiFePO4 is used to prepare positive electrode sheets and assemble them into sodium-ion batteries. During the charge-discharge cycle of sodium-ion batteries, lithium ions in the electrochemically active olivine-type LiFePO4 undergo ion intercalation and deintercalation. With the increase of charge-discharge cycles, the vacancies of lithium ions extracted from the olivine-type LiFePO4 are occupied by sodium ions in the electrolyte, and the olivine-type LiFePO4 transforms into olivine-type NaFePO4, forming a composite phase of olivine-type NaFePO4 and sodium phosphate-iron ore-type NaFePO4. In this process, the olivine-type LiFePO4 provides a continuous sodium ion transport channel, thereby improving the electrochemical activity of sodium phosphate-iron ore-type NaFePO4 and thus improving the cycle performance of sodium-ion batteries.

[0053] The iron-based phosphate composite cathode material prepared in this embodiment of the invention effectively shortens the Na0 oxidation time because olivine-type LiFePO4 is dispersed in the bulk phase of ferrophosphate-sodium ... + The transport pathways within sodium phosphate rock-type NaFePO4 enable the iron-based phosphate composite cathode material of this invention to possess excellent rate performance and high specific capacity, effectively solving the problems of low electrochemical activity and lack of Na in sodium phosphate rock-type NaFePO4 sodium-ion battery cathode materials. + The problem of transmission channels; the application of the iron-based phosphate composite cathode material of this invention to prepare cathode sheets and assemble them into sodium-ion batteries, because the iron-based phosphate composite cathode material has stable Na + The transmission channel enables sodium-ion batteries to have good cycle performance, especially at high voltages. The preferred voltage range is 1.5V-4.5V, which is wider than other phosphate compounds. It has good prospects for practical applications and can be widely used in solar energy, wind power generation, electric vehicles and other fields.

[0054] The method for preparing the iron-based phosphate composite cathode material xNaFePO4·LiFePO4 provided in this invention is simple to operate, uses inexpensive and readily available raw materials, and is suitable for commercial production. Attached Figure Description

[0055] Figure 1 This is a flowchart illustrating a method for preparing an iron-based phosphate composite cathode material according to an embodiment of the present invention.

[0056] Figure 2 A flowchart illustrating another preparation method of the iron-based phosphate composite cathode material provided in this embodiment of the invention.

[0057] Figure 3 The flowchart illustrates a method for preparing a positive electrode sheet using iron-based phosphate composite positive electrode material as the positive electrode active material, as provided in an embodiment of the present invention.

[0058] Figure 4 The images show the X-ray diffraction (XRD) patterns of the iron-based phosphate composite cathode materials prepared in Examples 1-6 of this invention.

[0059] Figure 5 The images show the XRD patterns of the iron-based phosphate composite cathode materials prepared in Examples 7-12 of this invention.

[0060] Figure 6 The images show the XRD patterns of the iron-based phosphate composite cathode materials prepared in Examples 13-20 of this invention.

[0061] Figure 7 The circuit capacity curves are for the sodium-ion batteries assembled in Examples 1, 7 and 13 of this invention.

[0062] Figure 8 The circuit capacity curves are for the sodium-ion batteries assembled in Examples 2, 8 and 14 of this invention.

[0063] Figure 9 The circuit capacity curves are for the sodium-ion batteries assembled in Examples 3, 9 and 15 of this invention.

[0064] Figure 10 The diagram shows the cycle capacity curves of the sodium-ion batteries assembled in Examples 4, 10, and 16 of this invention.

[0065] Figure 11 The diagram shows the cycle capacity curves of the sodium-ion batteries assembled in Examples 5, 11, and 17 of this invention.

[0066] Figure 12 The diagram shows the cycle capacity curves of the sodium-ion batteries assembled in Examples 6, 12, and 18 of this invention.

[0067] Figure 13 The diagram shows the coulombic efficiency and rate performance of the sodium-ion battery assembled in Example 4 of this invention.

[0068] Figure 14 The charge-discharge curves are of the sodium-ion batteries assembled in Examples 7, 8 and 9 of this invention.

[0069] Figure 15The charge-discharge curves are of the sodium-ion batteries assembled in Examples 10, 11 and 12 of this invention.

[0070] Figure 16 The above are charge-discharge curves of the sodium-ion batteries assembled in Example 10 and Comparative Example 1 of this invention. Detailed Implementation

[0071] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0072] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0073] This invention provides an iron-based phosphate composite cathode material for sodium-ion batteries, comprising sodium phosphate rock type NaFePO4 and olivine type LiFePO4.

[0074] The general chemical formula of the iron-based phosphate composite cathode material is: xNaFePO4·LiFePO4, where x is the molar ratio of NaFePO4 to LiFePO4, and 1≤x≤10, preferably 1≤x≤4.

[0075] Iron-based phosphate composite cathode materials are used in sodium-ion batteries;

[0076] During the charging and discharging process of sodium-ion batteries, olivine-type LiFePO4 participates in electrochemical reactions. Lithium ions in olivine-type LiFePO4 undergo ion intercalation and deintercalation during charging and discharging. With the increase of the number of charge and discharge cycles, the vacancies of lithium ions extracted from olivine-type LiFePO4 are occupied by sodium ions in the electrolyte, and olivine-type LiFePO4 transforms into olivine-type NaFePO4, forming a composite phase of olivine-type NaFePO4 and sodium phosphate-iron ore-type NaFePO4. In this process, the olivine-type LiFePO4 provides a continuous sodium ion transport channel.

[0077] This invention provides a method for preparing the above-mentioned iron-based phosphate composite cathode material, which is a sol-gel method, such as... Figure 1 As shown, the specific steps include:

[0078] Step 110: Weigh out sodium source material, iron source material, lithium source material and phosphate according to stoichiometric ratio, place them in a stirring device, add deionized water, and stir evenly to obtain a mixed solution;

[0079] The sodium source material includes one or more of the following: sodium carbonate (Na2CO3), sodium bicarbonate (NaHCO3), sodium acetate (CH3COONa), sodium oxalate (Na2C2O4), or sodium hydroxide (NaOH);

[0080] The iron source materials include one or more of the following: ferrous oxalate (FeC2O4), ferrous chloride (FeCl2), ferrous phosphate (Fe3(PO4)2), and ferrous acetate (C4H6FeO4);

[0081] The lithium source materials include one or more of the following: lithium carbonate (Li₂CO₃), lithium acetate dihydrate (CH₃COOL₃·2H₂O), lithium hydroxide (LiOH), and lithium bicarbonate (LiHCO₃);

[0082] Phosphates include one or more of the following: diammonium hydrogen phosphate ((NH4)2HPO4), diammonium dihydrogen phosphate (NH4H2PO4), phosphoric acid (H3PO4), sodium dihydrogen phosphate (NaH2PO4), disodium hydrogen phosphate (Na2HPO4), pyrophosphate (H4P2O7), and sodium pyrophosphate (Na4P2O7);

[0083] The solid content of the mixture is 30wt%-70wt%.

[0084] The mixing equipment is a mixer; the mixer speed is between 20r / min and 2000r / min, and the mixing time is 1 hour to 8 hours.

[0085] Step 120: Continue stirring the mixture at a certain temperature to form a precursor gel;

[0086] Specifically, continuing to stir at a certain temperature means stirring for 9 to 12 hours at a temperature of 90℃-100℃.

[0087] Step 130: Place the precursor gel in a grinding device and grind it to obtain intermediate product powder;

[0088] The grinding equipment is a grinding machine; the grinding machine's rotation speed is between 100 r / min and 4000 r / min, and the grinding time is between 0.1 hours and 48 hours.

[0089] Step 140: The intermediate product powder is pressed into a compact intermediate product disc and calcined under an argon atmosphere. After calcination, the mixture is cooled to room temperature to obtain an iron-based phosphate composite cathode material.

[0090] Specifically, the intermediate product wafers are placed in a tube furnace under an argon atmosphere and heated to 350℃-600℃ at a heating rate of 1℃ / min-3℃ / min, and held for 10-20 hours. The chemical formula of the resulting iron-based phosphate composite cathode material is: xNaFePO4·LiFePO4, where x is the molar ratio of NaFePO4 to LiFePO4, and 1≤x≤10, preferably 1≤x≤4.

[0091] The calcination temperature range for preparing iron-based phosphate composite cathode materials by the sol-gel method in this invention is between 350℃ and 600℃. It can be any temperature within the above range, such as 350℃, 400℃, 450℃, 500℃, 600℃, etc., but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0092] The preferred calcination temperature range for preparing iron-based phosphate composite cathode materials via the sol-gel method in this invention is 400℃-500℃. When the calcination temperature is below 400℃, although iron-based phosphate composite cathode materials containing sodium phosphate-iron ore type NaFePO4 and olivine-type LiFePO4 can be prepared, the olivine-type LiFePO4 formed during calcination has poor crystallinity due to the relatively low temperature, thus affecting the performance of the final iron-based phosphate composite cathode material and the cycle performance of the battery. When the calcination temperature is above 500℃, the sodium phosphate-iron ore type NaFePO4 has better crystallinity. However, since the crystal structure of the crystalline sodium phosphate-iron ore type NaFePO4 material lacks channels for sodium ion transport, calcination temperatures above 500℃ affect the first-cycle discharge specific capacity of the battery, causing a downward trend in the first-cycle discharge specific capacity.

[0093] This invention provides another method for preparing the above-mentioned iron-based phosphate composite cathode material, which is a solid-state method, such as... Figure 2 As shown, the specific steps include:

[0094] Step 210: Weigh out sodium source material, iron source material, lithium source material and phosphate according to stoichiometric ratio, place them in a ball mill jar, add solvent until it covers the grinding balls, and ball mill to obtain a mixed liquid;

[0095] The sodium source material includes one or more of the following: sodium carbonate (Na2CO3), sodium bicarbonate (NaHCO3), sodium acetate (CH3COONa), sodium oxalate (Na2C2O4), or sodium hydroxide (NaOH);

[0096] The iron source materials include one or more of the following: ferrous oxalate (FeC2O4), ferrous chloride (FeCl2), ferrous phosphate (Fe3(PO4)2), and ferrous acetate (C4H6FeO4);

[0097] The lithium source materials include one or more of the following: lithium carbonate (Li2CO3), lithium acetate dihydrate (CH3COOL i·2H2O), lithium hydroxide (LiOH), and lithium bicarbonate (Li HCO3);

[0098] Phosphates include one or more of the following: diammonium hydrogen phosphate ((NH4)2HPO4), diammonium dihydrogen phosphate (NH4H2PO4), phosphoric acid (H3PO4), sodium dihydrogen phosphate (NaH2PO4), disodium hydrogen phosphate (Na2HPO4), pyrophosphate (H4P2O7), and sodium pyrophosphate (Na4P2O7);

[0099] Solvents include either ethanol or acetone.

[0100] The grinding balls are made of agate.

[0101] Step 220: Place the ductile ink can containing the mixed liquid in an oven and bake it to obtain precursor powder.

[0102] The specific conditions for ball milling are as follows: the rotation speed of the ball mill jar is set between 400 r / min and 1000 r / min, and the ball milling time is between 6 hours and 12 hours.

[0103] The baking temperature is 60℃-80℃, and the baking time is 3-10 hours to remove the solvent.

[0104] Step 230: The precursor powder is pressed into a compact disc and calcined in an argon atmosphere. After calcination, the temperature is lowered to room temperature to obtain an iron-based phosphate composite cathode material.

[0105] Specifically, the precursor powder is placed in a tube furnace under an argon atmosphere and heated to 350℃-600℃ at a heating rate of 1℃ / min-3℃ / min, and held for 10-20 hours. The chemical formula of the resulting iron-based phosphate composite cathode material is: xNaFePO4·LiFePO4, where x is the molar ratio of NaFePO4 to LiFePO4, and 1≤x≤10, preferably 1≤x≤4.

[0106] The calcination temperature range for preparing iron-based phosphate composite cathode materials by solid-state method in this invention is between 350℃ and 600℃. It can be any temperature within the above range, such as 350℃, 400℃, 450℃, 500℃, 600℃, etc., but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0107] The preferred calcination temperature range for preparing iron-based phosphate composite cathode materials via solid-state method in this invention is 400℃-500℃. When the calcination temperature is below 400℃, although iron-based phosphate composite cathode materials containing sodium phosphate-iron ore type NaFePO4 and olivine-type LiFePO4 can be prepared, the olivine-type LiFePO4 formed during calcination has poor crystallinity due to the relatively low temperature, thus affecting the performance of the final iron-based phosphate composite cathode material and the cycle performance of the battery. When the calcination temperature is above 500℃, the sodium phosphate-iron ore type NaFePO4 has better crystallinity. However, since the crystal structure of the crystalline sodium phosphate-iron ore type NaFePO4 material lacks channels for sodium ion transport, calcination temperatures above 500℃ affect the first-cycle discharge specific capacity of the battery, causing a downward trend in the first-cycle discharge specific capacity.

[0108] This invention provides an application of the above-mentioned iron-based phosphate composite cathode material. The iron-based phosphate composite cathode material prepared by the sol-gel method or solid-phase method can be used to prepare cathode sheets.

[0109] This invention provides a method for preparing a positive electrode sheet containing the above-mentioned iron-based phosphate composite positive electrode material, such as... Figure 3 As shown, the preparation method includes:

[0110] Step 310: Grind and mix the above-mentioned iron-based phosphate composite cathode material with conductive additives until uniformly mixed to obtain a mixed powder;

[0111] The conductive additives include one or more of the following: conductive carbon black Super P, acetylene black, carbon nanotubes, Ketjen black, and conductive graphite; the conductive additives account for 0-30 wt% of the iron-based phosphate composite cathode material.

[0112] Step 320: Add the binder to the mixed powder, add the solvent, and continue grinding until uniform to obtain the mixed slurry;

[0113] The binder includes one or more of the following: polyvinylidene fluoride (PVDF), sodium carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR), polyacrylonitrile (PAN), polyvinyl alcohol (PVA), polytetrafluoroethylene (PTFE), and polyimide (PI); the binder accounts for 0-30 wt% of the iron-based phosphate composite cathode material.

[0114] The solvent is N-methylpyrrolidone (NMP).

[0115] Step 330: The mixed slurry is coated onto the current collector, dried, and then placed in an oven for drying. After cutting, the positive electrode sheet is obtained.

[0116] The drying conditions are as follows: vacuum baking at 90℃-120℃ for 5-10 hours, and the cutting size is 8×8mm. 2 Small square or 12mm diameter original sheets are used as positive electrode sheets, and then the positive electrode sheets are quickly transferred to a glove box filled with argon gas for later use.

[0117] The aforementioned positive electrode sheet containing iron-based phosphate composite positive electrode material can be used in sodium-ion batteries; sodium-ion batteries include liquid sodium-ion batteries, semi-solid sodium-ion batteries, or all-solid sodium-ion batteries; when positive electrode sheets prepared using the iron-based phosphate composite positive electrode material of this invention are assembled into sodium-ion batteries, the iron-based phosphate composite positive electrode material contains stable Na+. + The transmission channel enables sodium-ion batteries to have good cycle performance, especially at high voltages. The preferred operating voltage range of sodium-ion batteries containing the iron-based phosphate composite cathode material of this invention is 1.5V-4.5V, which is wider than that of other phosphate compounds, and has good prospects for practical application.

[0118] To better understand the technical solution provided by the present invention, the preparation process and characteristics of the iron-based phosphate composite cathode material of the present invention are illustrated below with several specific examples.

[0119] Example 1

[0120] This embodiment provides a preparation process and performance testing of an iron-based phosphate composite cathode material. The iron-based phosphate composite cathode material prepared using the solvent gelation method has the chemical formula 7NaFePO4·4·5.

[0121] The specific preparation process of LiFePO4 is as follows:

[0122] (1) Weigh out 500g of Na2CO3, FeC2O4, CH3COOL i·2H2O and NH4H2PO4 according to the stoichiometric ratio, place them in a mixer, add deionized water and stir at 1000r / min for 60 minutes. After stirring evenly, a mixture with a solid content of 30wt% is obtained.

[0123] (2) Continue stirring the mixture at 90°C for 10 hours to form a precursor gel.

[0124] (3) The precursor gel was placed in a grinder and ground at 2000 r / min for 12 hours to obtain intermediate product powder.

[0125] (4) The intermediate product powder is pressed into a compact intermediate product disc, placed in a tube furnace, and heated to 350°C at 1°C / min and held for 5 hours under an argon atmosphere to obtain the iron-based phosphate composite cathode material 7NaFePO4·LiFePO4.

[0126] The XRD pattern of the iron-based phosphate composite cathode material prepared in this embodiment is as follows: Figure 4 As shown in Example 1, although the iron-based phosphate composite cathode material has characteristic peaks of sodium phosphate rock type NaFePO4 and olivine type LiFePO4, the characteristic peaks are weak. This is because the crystallization of sodium phosphate rock type NaFePO4 and olivine type LiFePO4 is poor after calcination at 350℃.

[0127] The iron-based phosphate composite cathode material prepared in this embodiment is used to fabricate a cathode sheet, and a sodium-ion battery is assembled and tested using this cathode sheet. The specific process is as follows:

[0128] (1) Grind and mix 80g of the above iron-based phosphate composite cathode material with 10g of conductive additive super p conductive carbon black until uniformly mixed to obtain a mixed powder.

[0129] (2) Add 10g of PVDF binder to the mixed powder, add an appropriate amount of NMP solvent, and continue grinding until uniform to obtain a mixed slurry.

[0130] (3) Coat the mixed slurry onto the current collector aluminum foil, bake it under vacuum at 90°C for 5 hours, and cut it into 8×8mm pieces. 2 The small square positive electrode sheet is then quickly transferred to an argon-filled glove box for later use.

[0131] (4) The obtained positive electrode is assembled with the separator, electrolyte and negative electrode in a glove box filled with argon to form a sodium-ion battery; wherein, the electrolyte of the sodium-ion battery is preferably a NaPF6 solution with a molar mass of 1 mol / L (the solvent is 100 vol 1% polycarbonate PC and 5 vol 1% fluoroethylene carbonate FEC), and the negative electrode is a metallic sodium sheet.

[0132] The assembled sodium-ion batteries were tested using a constant current charge-discharge mode at a current density of 1C. The discharge cutoff voltage was 1.5V, and the charge cutoff voltage was 4.5V. The test results are detailed in Table 1.

[0133] The cycle capacity curve of the sodium-ion battery assembled in this embodiment is shown in the figure below. Figure 7 As shown.

[0134] Example 2

[0135] The difference between the preparation process of the iron-based phosphate composite cathode material provided in this embodiment and that in embodiment 2 is that the calcination in step (4) is to raise the temperature to 400℃ at 1℃ / min and hold it for 10 hours. The other preparation processes are exactly the same as in embodiment 2. The chemical formula of the iron-based phosphate composite cathode material prepared is 7NaFePO4·LiFePO4.

[0136] The XRD pattern of the iron-based phosphate composite cathode material prepared in this embodiment is as follows: Figure 4 As shown in Example 2.

[0137] The iron-based phosphate composite cathode material prepared in this embodiment was used to prepare a cathode sheet, and a sodium-ion battery was assembled and tested using this cathode sheet. The cathode sheet preparation process, battery assembly process, and testing process were all the same as in Example 1. The test results are detailed in Table 1.

[0138] The cycle capacity curve of the sodium-ion battery assembled in this embodiment is shown in the figure below. Figure 8 As shown.

[0139] Example 3

[0140] The difference between the preparation process of the iron-based phosphate composite cathode material provided in this embodiment and that in embodiment 2 is that the calcination in step (4) is to raise the temperature to 450℃ at 1℃ / min and hold it for 10 hours. The other preparation processes are exactly the same as in embodiment 2. The chemical formula of the iron-based phosphate composite cathode material prepared is 7NaFePO4·LiFePO4.

[0141] The XRD pattern of the iron-based phosphate composite cathode material prepared in this embodiment is as follows: Figure 4 As shown in Example 3.

[0142] The iron-based phosphate composite cathode material prepared in this embodiment was used to prepare a cathode sheet, and a sodium-ion battery was assembled and tested using this cathode sheet. The cathode sheet preparation process, battery assembly process, and testing process were all the same as in Example 1. The test results are detailed in Table 1.

[0143] The cycle capacity curve of the sodium-ion battery assembled in this embodiment is shown in the figure below. Figure 9 As shown.

[0144] Example 4

[0145] The difference between the preparation process of the iron-based phosphate composite cathode material provided in this embodiment and that in embodiment 2 is that the calcination in step (4) is to raise the temperature to 500℃ at 1℃ / min and hold it for 10 hours. The other preparation processes are exactly the same as in embodiment 2. The chemical formula of the iron-based phosphate composite cathode material prepared is 7NaFePO4·LiFePO4.

[0146] The XRD pattern of the iron-based phosphate composite cathode material prepared in this embodiment is as follows: Figure 4 As shown in Example 4.

[0147] The iron-based phosphate composite cathode material prepared in this embodiment was used to prepare a cathode sheet, and a sodium-ion battery was assembled and tested using this cathode sheet. The cathode sheet preparation process, battery assembly process, and testing process were all the same as in Example 1. The test results are detailed in Table 1.

[0148] The cycle capacity curve of the sodium-ion battery assembled in this embodiment is shown in the figure below. Figure 10 As shown.

[0149] The rate performance and coulombic efficiency diagrams of the sodium-ion battery assembled in this embodiment are shown below. Figure 13 As shown.

[0150] Example 5

[0151] The difference between the preparation process of the iron-based phosphate composite cathode material provided in this embodiment and that in embodiment 2 is that the calcination in step (4) is to raise the temperature to 550℃ at 1℃ / min and hold it for 10 hours. The other preparation processes are exactly the same as in embodiment 2. The chemical formula of the iron-based phosphate composite cathode material prepared is 7NaFePO4·LiFePO4.

[0152] The XRD pattern of the iron-based phosphate composite cathode material prepared in this embodiment is as follows: Figure 4 As shown in Example 5.

[0153] The iron-based phosphate composite cathode material prepared in this embodiment was used to prepare a cathode sheet, and a sodium-ion battery was assembled and tested using this cathode sheet. The cathode sheet preparation process, battery assembly process, and testing process were all the same as in Example 1. The test results are detailed in Table 1.

[0154] The cycle capacity curve of the sodium-ion battery assembled in this embodiment is shown in the figure below. Figure 11 As shown.

[0155] Example 6

[0156] The difference between the preparation process of the iron-based phosphate composite cathode material provided in this embodiment and that in embodiment 2 is that the calcination in step (4) is to raise the temperature to 600℃ at 1℃ / min and hold it for 10 hours. The other preparation processes are exactly the same as in embodiment 2. The chemical formula of the iron-based phosphate composite cathode material prepared is 7NaFePO4·LiFePO4.

[0157] The XRD pattern of the iron-based phosphate composite cathode material prepared in this embodiment is as follows: Figure 4 As shown in Example 6, through Figure 4As can be seen from the diffraction characteristic peaks of Examples 1-6, the intensity of the characteristic peaks of NaFePO4 of the phosphate iron ore type and LiFePO4 of the olivine type increases. This is because, during the preparation of iron-based phosphate composite cathode materials, as the calcination temperature of Examples 1 to 6 increases, the crystallinity of NaFePO4 of the phosphate iron ore type and LiFePO4 of the olivine type becomes better and better.

[0158] The iron-based phosphate composite cathode material prepared in this embodiment was used to prepare a cathode sheet, and a sodium-ion battery was assembled and tested using this cathode sheet. The cathode sheet preparation process, battery assembly process, and testing process were all the same as in Example 1. The test results are detailed in Table 1.

[0159] The cycle capacity curve of the sodium-ion battery assembled in this embodiment is shown in the figure below. Figure 12 As shown.

[0160] Example 7

[0161] This embodiment provides a preparation process and performance testing of an iron-based phosphate composite cathode material. The iron-based phosphate composite cathode material prepared using the solvent gelation method has the chemical formula 3NaFePO4·4·5.

[0162] The specific preparation process of LiFePO4 is as follows:

[0163] (1) Weigh out 500g of NaOH, FeCl2, LiOH and (NH4)2HPO4 according to the stoichiometric ratio, place them in a mixer, add deionized water and stir at 800r / min for 2 hours. After stirring evenly, a mixture with a solid content of 50wt% is obtained.

[0164] (2) Continue stirring the mixture at 100°C for 9 hours to form a precursor gel.

[0165] (3) The precursor gel was placed in a grinder and ground at 2000 r / min for 12 hours to obtain intermediate product powder.

[0166] (4) The intermediate product powder is pressed into a compact intermediate product disc, placed in a tube furnace, heated to 350°C at 1°C / min and held for 5 hours under an argon atmosphere, and then cooled to room temperature after calcination to obtain the iron-based phosphate composite cathode material 3NaFePO4·LiFePO4.

[0167] The XRD pattern of the iron-based phosphate composite cathode material prepared in this embodiment is as follows: Figure 5As shown in Example 7, although the iron-based phosphate composite cathode material has characteristic peaks of sodium phosphate rock type NaFePO4 and olivine type LiFePO4, the characteristic peaks are weak. This is because the crystallization of sodium phosphate rock type NaFePO4 and olivine type LiFePO4 is poor after calcination at 350℃.

[0168] The iron-based phosphate composite cathode material prepared in this embodiment was used to prepare a cathode sheet, and a sodium-ion battery was assembled and tested using this cathode sheet. The cathode sheet preparation process, battery assembly process, and testing process were all the same as in Example 1. The test results are detailed in Table 1.

[0169] The cycle capacity curve of the sodium-ion battery assembled in this embodiment is shown in the figure below. Figure 7 As shown.

[0170] The charge-discharge curve of the sodium-ion battery assembled in this embodiment is shown in the figure below. Figure 14 As shown.

[0171] Example 8

[0172] The difference between the preparation process of the iron-based phosphate composite cathode material provided in this embodiment and that in Example 8 is that the calcination in step (4) is to raise the temperature to 400℃ at 1℃ / min and hold it for 10 hours. The other preparation processes are exactly the same as in Example 8. The chemical formula of the iron-based phosphate composite cathode material prepared is 3NaFePO4·LiFePO4.

[0173] The XRD pattern of the iron-based phosphate composite cathode material prepared in this embodiment is as follows: Figure 5 As shown in Example 8.

[0174] The iron-based phosphate composite cathode material prepared in this embodiment was used to prepare a cathode sheet, and a sodium-ion battery was assembled and tested using this cathode sheet. The cathode sheet preparation process, battery assembly process, and testing process were all the same as in Example 1. The test results are detailed in Table 1.

[0175] The cycle capacity curve of the sodium-ion battery assembled in this embodiment is shown in the figure below. Figure 8 As shown.

[0176] The charge-discharge curve of the sodium-ion battery assembled in this embodiment is shown in the figure below. Figure 14 As shown.

[0177] Example 9

[0178] The difference between the preparation process of the iron-based phosphate composite cathode material provided in this embodiment and that in Example 8 is that the calcination in step (4) is to raise the temperature to 450℃ at 1℃ / min and hold for 10 hours. The other preparation processes are exactly the same as in Example 8. The chemical formula of the iron-based phosphate composite cathode material prepared is 3NaFePO4·LiFePO4.

[0179] The XRD pattern of the iron-based phosphate composite cathode material prepared in this embodiment is as follows: Figure 5 As shown in Example 8.

[0180] The iron-based phosphate composite cathode material prepared in this embodiment was used to prepare a cathode sheet, and a sodium-ion battery was assembled and tested using this cathode sheet. The cathode sheet preparation process, battery assembly process, and testing process were all the same as in Example 1. The test results are detailed in Table 1.

[0181] The cycle capacity curve of the sodium-ion battery assembled in this embodiment is shown in the figure below. Figure 9 As shown.

[0182] The charge-discharge curve of the sodium-ion battery assembled in this embodiment is shown in the figure below. Figure 14 As shown.

[0183] Example 10

[0184] The difference between the preparation process of the iron-based phosphate composite cathode material provided in this embodiment and that in Example 8 is that the calcination in step (4) is to raise the temperature to 500℃ at 1℃ / min and hold it for 10 hours. The other preparation processes are exactly the same as in Example 8. The chemical formula of the iron-based phosphate composite cathode material prepared is 3NaFePO4·LiFePO4.

[0185] The XRD pattern of the iron-based phosphate composite cathode material prepared in this embodiment is as follows: Figure 5 Example 10 is shown in the example.

[0186] The iron-based phosphate composite cathode material prepared in this embodiment was used to prepare a cathode sheet, and a sodium-ion battery was assembled and tested using this cathode sheet. The cathode sheet preparation process, battery assembly process, and testing process were all the same as in Example 1. The test results are detailed in Table 1.

[0187] The cycle capacity curve of the sodium-ion battery assembled in this embodiment is shown in the figure below. Figure 10 As shown.

[0188] The charge-discharge curve of the sodium-ion battery assembled in this embodiment is shown in the figure below. Figure 15 As shown.

[0189] Example 11

[0190] The difference between the preparation process of the iron-based phosphate composite cathode material provided in this embodiment and that in Example 8 is that the calcination in step (4) is to raise the temperature to 550℃ at 1℃ / min and hold it for 10 hours. The other preparation processes are exactly the same as in Example 8. The chemical formula of the iron-based phosphate composite cathode material prepared is 3NaFePO4·LiFePO4.

[0191] The XRD pattern of the iron-based phosphate composite cathode material prepared in this embodiment is as follows: Figure 5 As shown in Example 11.

[0192] The iron-based phosphate composite cathode material prepared in this embodiment was used to prepare a cathode sheet, and a sodium-ion battery was assembled and tested using this cathode sheet. The cathode sheet preparation process, battery assembly process, and testing process were all the same as in Example 1. The test results are detailed in Table 1.

[0193] The cycle capacity curve of the sodium-ion battery assembled in this embodiment is shown in the figure below. Figure 11 As shown.

[0194] The charge-discharge curve of the sodium-ion battery assembled in this embodiment is shown in the figure below. Figure 15 As shown.

[0195] Example 12

[0196] The difference between the preparation process of the iron-based phosphate composite cathode material provided in this embodiment and that in Example 8 is that the calcination in step (4) is to raise the temperature to 600℃ at 1℃ / min and hold it for 10 hours. The other preparation processes are exactly the same as in Example 8. The chemical formula of the iron-based phosphate composite cathode material prepared is 3NaFePO4·LiFePO4.

[0197] The XRD pattern of the iron-based phosphate composite cathode material prepared in this embodiment is as follows: Figure 5 As shown in Example 12, through Figure 5 As can be seen from the diffraction characteristic peaks of Examples 7-12, the intensity of the characteristic peaks of NaFePO4 of the phosphate iron ore type and LiFePO4 of the olivine type increases. This is because, during the preparation of iron-based phosphate composite cathode materials, as the calcination temperature of Examples 7 to 12 increases, the crystallinity of NaFePO4 of the phosphate iron ore type and LiFePO4 of the olivine type becomes better and better.

[0198] The iron-based phosphate composite cathode material prepared in this embodiment was used to prepare a cathode sheet, and a sodium-ion battery was assembled and tested using this cathode sheet. The cathode sheet preparation process, battery assembly process, and testing process were all the same as in Example 1. The test results are detailed in Table 1.

[0199] The cycle capacity curve of the sodium-ion battery assembled in this embodiment is shown in the figure below. Figure 12 As shown.

[0200] The charge-discharge curve of the sodium-ion battery assembled in this embodiment is shown in the figure below. Figure 15 As shown.

[0201] Example 13

[0202] This embodiment provides a preparation process and performance testing of an iron-based phosphate composite cathode material. The iron-based phosphate composite cathode material prepared using the solvent gelation method has the chemical formula 1.67NaFePO4·LiFePO4. The specific preparation process is as follows:

[0203] (1) Weigh out 500g of NaOH, FeC2O4, Li2CO3 and H4P2O7 according to the stoichiometric ratio, place them in a mixer, add deionized water and stir at 1000r / min for 2 hours. After stirring evenly, a mixture with a solid content of 50wt% is obtained.

[0204] (2) Continue stirring the mixture at 100°C for 10 hours to form a precursor gel.

[0205] (3) The precursor gel was placed in a grinder and ground at 2000 r / min for 12 hours to obtain intermediate product powder.

[0206] (4) The intermediate product powder is pressed into a compact intermediate product disc, placed in a tube furnace, heated to 350°C at 1°C / min and held for 5 hours under an argon atmosphere, and then cooled to room temperature after calcination to obtain the iron-based phosphate composite cathode material 1.67NaFePO4·LiFePO4.

[0207] The XRD pattern of the iron-based phosphate composite cathode material prepared in this embodiment is as follows: Figure 6 As shown in Example 13, although the iron-based phosphate composite cathode material has characteristic peaks of sodium phosphate rock type NaFePO4 and olivine type LiFePO4, the characteristic peaks are weak. This is because the crystallization of sodium phosphate rock type NaFePO4 and olivine type LiFePO4 is poor after calcination at 350℃.

[0208] The iron-based phosphate composite cathode material prepared in this embodiment was used to prepare a cathode sheet, and a sodium-ion battery was assembled and tested using this cathode sheet. The cathode sheet preparation process, battery assembly process, and testing process were all the same as in Example 1. The test results are detailed in Table 1.

[0209] The cycle capacity curve of the sodium-ion battery assembled in this embodiment is shown in the figure below. Figure 7 As shown.

[0210] Example 14

[0211] The difference between the preparation process of the iron-based phosphate composite cathode material provided in this embodiment and that in Example 8 is that the calcination in step (4) is to raise the temperature to 400℃ at 1℃ / min and hold for 10 hours. The other preparation processes are exactly the same as in Example 14. The chemical formula of the iron-based phosphate composite cathode material prepared is 1.67NaFePO4·LiFePO4.

[0212] The XRD pattern of the iron-based phosphate composite cathode material prepared in this embodiment is as follows: Figure 6 As shown in Example 14.

[0213] The iron-based phosphate composite cathode material prepared in this embodiment was used to prepare a cathode sheet, and a sodium-ion battery was assembled and tested using this cathode sheet. The cathode sheet preparation process, battery assembly process, and testing process were all the same as in Example 1. The test results are detailed in Table 1.

[0214] The cycle capacity curve of the sodium-ion battery assembled in this embodiment is shown in the figure below. Figure 8 As shown.

[0215] Example 15

[0216] The difference between the preparation process of the iron-based phosphate composite cathode material provided in this embodiment and that in Example 8 is that the calcination in step (4) is to raise the temperature to 450℃ at 1℃ / min and hold for 10 hours. The other preparation processes are exactly the same as in Example 14. The chemical formula of the iron-based phosphate composite cathode material prepared is 1.67NaFePO4·LiFePO4.

[0217] The XRD pattern of the iron-based phosphate composite cathode material prepared in this embodiment is as follows: Figure 6 Example 15 is shown in the text.

[0218] The iron-based phosphate composite cathode material prepared in this embodiment was used to prepare a cathode sheet, and a sodium-ion battery was assembled and tested using this cathode sheet. The cathode sheet preparation process, battery assembly process, and testing process were all the same as in Example 1. The test results are detailed in Table 1.

[0219] The cycle capacity curve of the sodium-ion battery assembled in this embodiment is shown in the figure below. Figure 9 As shown.

[0220] Example 16

[0221] The difference between the preparation process of the iron-based phosphate composite cathode material provided in this embodiment and that in Example 8 is that the calcination in step (4) is to raise the temperature to 500℃ at 1℃ / min and hold it for 10 hours. The other preparation processes are exactly the same as in Example 14. The chemical formula of the iron-based phosphate composite cathode material prepared is 1.67NaFePO4·LiFePO4.

[0222] The XRD pattern of the iron-based phosphate composite cathode material prepared in this embodiment is as follows: Figure 6 Example 16 is shown in the text.

[0223] The iron-based phosphate composite cathode material prepared in this embodiment was used to prepare a cathode sheet, and a sodium-ion battery was assembled and tested using this cathode sheet. The cathode sheet preparation process, battery assembly process, and testing process were all the same as in Example 1. The test results are detailed in Table 1.

[0224] The cycle capacity curve of the sodium-ion battery assembled in this embodiment is shown in the figure below. Figure 10 As shown.

[0225] Example 17

[0226] The difference between the preparation process of the iron-based phosphate composite cathode material provided in this embodiment and that in Example 8 is that the calcination in step (4) is to raise the temperature to 550℃ at 1℃ / min and hold for 10 hours. The other preparation processes are exactly the same as in Example 14. The chemical formula of the iron-based phosphate composite cathode material prepared is 1.67NaFePO4·LiFePO4.

[0227] The XRD pattern of the iron-based phosphate composite cathode material prepared in this embodiment is as follows: Figure 6 Example 17 is shown in the text.

[0228] The iron-based phosphate composite cathode material prepared in this embodiment was used to prepare a cathode sheet, and a sodium-ion battery was assembled and tested using this cathode sheet. The cathode sheet preparation process, battery assembly process, and testing process were all the same as in Example 1. The test results are detailed in Table 1.

[0229] The cycle capacity curve of the sodium-ion battery assembled in this embodiment is shown in the figure below. Figure 11 As shown.

[0230] Example 18

[0231] The difference between the preparation process of the iron-based phosphate composite cathode material provided in this embodiment and that in Example 8 is that the calcination in step (4) is to raise the temperature to 600℃ at 1℃ / min and hold it for 10 hours. The other preparation processes are exactly the same as in Example 14. The chemical formula of the iron-based phosphate composite cathode material prepared is 1.67NaFePO4·LiFePO4.

[0232] The XRD pattern of the iron-based phosphate composite cathode material prepared in this embodiment is as follows: Figure 6 As shown in Example 18, by Figure 6As can be seen from the diffraction characteristic peaks of Examples 13-18, the intensity of the characteristic peaks of NaFePO4 of the phosphate iron ore type and LiFePO4 of the olivine type increases. This is because, during the preparation of iron-based phosphate composite cathode materials, as the calcination temperature of Examples 13 to 18 increases, the crystallinity of NaFePO4 of the phosphate iron ore type and LiFePO4 of the olivine type becomes better and better.

[0233] The iron-based phosphate composite cathode material prepared in this embodiment was used to prepare a cathode sheet, and a sodium-ion battery was assembled and tested using this cathode sheet. The cathode sheet preparation process, battery assembly process, and testing process were all the same as in Example 1. The test results are detailed in Table 1.

[0234] The cycle capacity curve of the sodium-ion battery assembled in this embodiment is shown in the figure below. Figure 12 As shown.

[0235] Example 19

[0236] The iron-based phosphate composite cathode material prepared in this embodiment has the chemical formula 3NaFePO4·LiFePO4. The preparation process is exactly the same as in Example 10, and the calcination is also carried out by heating to 500℃ at 1℃ / min and holding for 10 hours.

[0237] The XRD pattern of the iron-based phosphate composite cathode material prepared in this embodiment is as follows: Figure 6 As shown in Example 19, we can see that it has strong characteristic diffraction peaks of NaFePO4 of the phosphate rock type and characteristic diffraction peaks of LiFePO4 of the olivine type.

[0238] In this embodiment, a positive electrode sheet is prepared using 3NaFePO4·LiFePO4 iron-based phosphate composite positive electrode material. The difference from Example 10 is in steps (1) and (2). No binder is added. 74g of iron-based phosphate composite positive electrode material and 26g of conductive carbon black super p (conductive agent ratio is 26wt%) are added with an appropriate amount of solvent NMP and ground in a high-speed planetary ball mill at a speed of 350rpm for 10 hours to obtain a mixed slurry. All other steps are the same as in Example 10.

[0239] Sodium-ion batteries were assembled using the positive electrode sheet prepared in this embodiment and tested. The testing process was the same as in Example 10, and the test results are detailed in Table 1.

[0240] Example 20

[0241] This embodiment provides a preparation process and performance testing of an iron-based phosphate composite cathode material. A solid-state method was used to prepare the iron-based phosphate composite cathode material with the chemical formula 3NaFePO4·LiFePO4. The specific preparation process is as follows:

[0242] (1) Weigh Na2CO3, FeC2O4, Li2CO3 and NH4H2PO4 according to the stoichiometric ratio, place them in a ball mill jar, add solvent to cover the agate grinding balls, and ball mill at 400 r / min for 6 hours to obtain a mixed liquid.

[0243] (2) Place the ductile ink can containing the mixed liquid in an oven and bake at 70°C for 5 hours to obtain precursor powder.

[0244] (3) The precursor powder is pressed into a compact disc and placed in a muffle furnace. Under an argon atmosphere, the temperature is increased to 500℃ at 1℃ / min and held for 10 hours. After calcination, the temperature is reduced to room temperature to obtain the iron-based phosphate composite cathode material 3NaFePO4·LiFePO4.

[0245] The XRD pattern of the iron-based phosphate composite cathode material prepared in this embodiment is as follows: Figure 6 As shown in Example 20, strong characteristic diffraction peaks of NaFePO4 of the phosphate rock type and characteristic diffraction peaks of LiFePO4 of the olivine type can be seen.

[0246] The iron-based phosphate composite cathode material prepared in this embodiment was used to prepare a cathode sheet, and a sodium-ion battery was assembled and tested using this cathode sheet. The cathode sheet preparation process, battery assembly process, and testing process were all the same as in Example 2. The test results are detailed in Table 1.

[0247] To better illustrate the effects of the embodiments of the present invention, Comparative Example 1 is compared with the above embodiments.

[0248] Comparative Example 1

[0249] This comparative example directly used 80g of sodium phosphate rock (NaFePO4) as the positive electrode active material to prepare the positive electrode sheet, and assembled a sodium-ion battery for testing. The preparation process of the positive electrode sheet, the battery assembly process, and the testing process were all the same as in Example 2. The test results are detailed in Table 1.

[0250] A comparison chart of the charge-discharge curves of the sodium-ion battery assembled in this comparative example and the sodium-ion battery assembled in Example 10, as shown below. Figure 16 As shown, the sodium-ion battery containing the iron-based phosphate composite cathode material prepared in Example 10 has better electrochemical performance.

[0251] Table 1 summarizes the test data of the first-cycle specific capacity and the cycle capacity retention rate after 100 cycles for the sodium-ion batteries assembled in Examples 1-20 and Comparative Examples 1-2.

[0252]

[0253]

[0254] Table 1

[0255] The test data in Table 1 shows that:

[0256] The sodium-ion batteries assembled in Examples 1, 7, and 13 all exhibited lower cycle capacity retention rates after 100 cycles compared to other examples. This is because Examples 1, 7, and 13 all used materials prepared by calcination at 350°C, resulting in poor crystallinity of the olivine-type LiFePO4 in the iron-based phosphate composite cathode material. Consequently, the iron-based phosphate composite cathode material had poor capacity, leading to lower cycle capacity retention rates for the assembled sodium-ion batteries after 100 cycles.

[0257] The sodium-ion batteries assembled in Examples 5, 6, 11, 12, 17, and 18 all had lower first-week discharge specific capacities than those in other examples. This is because the materials in Examples 5, 6, 11, 12, and 18 were calcined at temperatures above 500°C during preparation. When the calcination temperature is above 500°C, the diffraction peak intensity of the sodium phosphate-iron ore type NaFePO4 material increases significantly with increasing temperature, indicating better crystallinity. Since the crystal structure of the crystalline sodium phosphate-iron ore type NaFePO4 material lacks channels for sodium ion transport, the first-week discharge specific capacity shows a decreasing trend in the 500°C–600°C range. In contrast, the calcination temperatures of the other examples were 350°C–500°C. Within this temperature range, the diffraction peak intensity of the olivine-type LiFePO4 structure is higher than that of the sodium phosphate-iron ore type NaFePO4 structure. This is because the sodium phosphate-iron ore type NaFePO4 material has poorer crystallinity and the olivine-type LiFePO4 structure has lower crystallinity. FePO4 materials have good crystallinity, so the electrochemical activity of the material increases with the increase of sintering temperature. Therefore, the discharge specific capacity range of materials prepared at calcination temperatures of 350℃ to 500℃ shows an increasing trend in the first week.

[0258] The sodium-ion battery assembled in Comparative Example 1 had a low first-cycle discharge specific capacity and a low cycle capacity retention rate after 100 cycles. This is because Comparative Example 1 directly used sodium phosphate rock type NaFePO4 as the positive electrode material to prepare the sodium-ion battery. Since sodium phosphate rock type NaFePO4 has poor electrochemical activity, the assembled sodium-ion battery had a low first-cycle discharge specific capacity and poor cycle performance.

[0259] Furthermore, the test data in Table 1 also shows that the cycle capacity retention of the prepared iron-based phosphate composite cathode material with a molar ratio of sodium phosphate-iron-sodium phosphate (NaFePO4) to olivine (LiFePO4) of 3:1 and 1.67:1 is better than that of the sodium-ion battery assembled with an iron-based phosphate composite cathode material with a molar ratio of 7:1. This is because the molar ratios of sodium phosphate-iron-sodium phosphate (NaFePO4) to olivine (LiFePO4) of 3:1 and 1.67:1 are within the preferred molar ratio range of 1 to 4 in this invention. Within this range, olivine (LiFePO4) is more likely to provide a continuous sodium ion transport channel, thereby making the cycle performance of the sodium-ion battery better.

[0260] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. 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. An iron-based phosphate composite cathode material for sodium-ion batteries, characterized in that, The iron-based phosphate composite cathode material includes sodium phosphate rock type NaFePO4 and olivine type LiFePO4; The general chemical formula of the iron-based phosphate composite cathode material is: xNaFePO4·LiFePO4, where x is the molar ratio of NaFePO4 to LiFePO4, and 1≤x≤10.

2. The iron-based phosphate composite cathode material according to claim 1, characterized in that, The iron-based phosphate composite cathode material is used in sodium-ion batteries; During the charging and discharging process of the sodium-ion battery, the olivine-type LiFePO4 participates in the electrochemical reaction. Lithium ions in the olivine-type LiFePO4 undergo ion intercalation and deintercalation during the charging and discharging process. With the increase of the number of charging and discharging cycles, the vacancies of lithium ions extracted from the olivine-type LiFePO4 are occupied by sodium ions in the electrolyte, and the olivine-type LiFePO4 transforms into olivine-type NaFePO4, forming a composite phase of olivine-type NaFePO4 and sodium phosphate-iron-sodium phosphate-type NaFePO4. During this process, the olivine-type LiFePO4 provides a continuous sodium ion transport channel.

3. A method for preparing the iron-based phosphate composite cathode material according to any one of claims 1-2, characterized in that, The preparation method is a sol-gel method, including: Sodium source material, iron source material, lithium source material and phosphate were weighed according to the stoichiometric ratio, placed in a stirring device, deionized water was added, and the mixture was stirred evenly to obtain a mixed solution. The mixture is stirred at a certain temperature to form a precursor gel; The precursor gel was placed in a grinding device and ground to obtain intermediate product powder. The intermediate product powder is pressed into compact intermediate product discs and calcined under an argon atmosphere. After calcination, the mixture is cooled to room temperature to obtain an iron-based phosphate composite cathode material. The general chemical formula of the iron-based phosphate composite cathode material is: xNaFePO4·LiFePO4, where x is the molar ratio of NaFePO4 to LiFePO4, and 1≤x≤10.

4. The preparation method according to claim 3, characterized in that, The sodium source material includes one or more of the following: sodium carbonate, sodium bicarbonate, sodium acetate, sodium oxalate, and sodium hydroxide. The iron source material includes one or more of ferrous oxalate, ferrous chloride, ferrous phosphate, and ferrous acetate. The lithium source material includes one or more of lithium carbonate, lithium acetate dihydrate, lithium hydroxide, and lithium bicarbonate. The phosphate includes one or more of diammonium hydrogen phosphate, diammonium dihydrogen phosphate, phosphoric acid, sodium dihydrogen phosphate, disodium hydrogen phosphate, pyrophosphate, and sodium pyrophosphate. The solid content of the mixture is 30wt%-70wt%; The mixing equipment is a mixer; the speed of the mixer is between 20 r / min and 2000 r / min, and the mixing time is between 1 hour and 8 hours; The specific meaning of continuing to stir at a certain temperature is: continuing to stir for 9 to 12 hours at a temperature of 90℃-100℃; The grinding equipment is a grinding machine; the grinding machine's rotation speed is between 100 r / min and 4000 r / min, and the grinding time is between 0.1 hours and 48 hours; The specific calcination conditions include: heating to 350℃-600℃ at a heating rate of 1℃ / min-3℃ / min and holding at that temperature for 10-20 hours.

5. A method for preparing the iron-based phosphate composite cathode material according to any one of claims 1-2, characterized in that, The preparation method is a solid-state method, including: Sodium source material, iron source material, lithium source material and phosphate were weighed according to stoichiometric ratio, placed in a ball mill jar, and solvent was added until it covered the grinding balls. After ball milling, a mixed liquid was obtained. The ductile ink can containing the mixed liquid is placed in an oven and baked to obtain precursor powder. The precursor powder was pressed into a compact disc and calcined in an argon atmosphere. After calcination, the sample was cooled to room temperature to obtain an iron-based phosphate composite cathode material.

6. The preparation method according to claim 5, characterized in that, The sodium source material includes one or more of sodium carbonate, sodium bicarbonate, sodium acetate, sodium oxalate, and sodium hydroxide. The iron source material includes one or more of ferrous oxalate, ferrous chloride, ferrous phosphate, and ferrous acetate. The lithium source material includes one or more of lithium carbonate, lithium acetate dihydrate, lithium hydroxide, and lithium bicarbonate. The phosphate includes one or more of diammonium hydrogen phosphate, diammonium dihydrogen phosphate, phosphoric acid, sodium dihydrogen phosphate, disodium hydrogen phosphate, pyrophosphate, and sodium pyrophosphate. The solvent includes either ethanol or acetone; The specific conditions for the ball milling process are as follows: the rotational speed of the ball mill jar is set at... The ball milling speed is between 400 r / min and 1000 r / min, and the milling time is between 6 hours and 12 hours. The baking temperature is 60℃-80℃, and the baking time is 3 hours-10 hours. The specific calcination conditions are as follows: heat to 350℃-600℃ at a heating rate of 1℃ / min-3℃ / min and hold for 10-20 hours.

7. A positive electrode sheet, characterized in that, The positive electrode sheet comprises the iron-based phosphate composite positive electrode material as described in any one of claims 1-2.

8. A method for preparing the positive electrode sheet according to claim 7, characterized in that, The preparation method includes: The iron-based phosphate composite cathode material according to any one of claims 1-2 is ground and mixed with conductive additives until uniformly mixed to obtain a mixed powder. Add the binder to the mixed powder, add the solvent, and continue grinding until homogeneous to obtain a mixed slurry; The mixed slurry is coated onto the current collector, dried, and then placed in an oven for drying. After cutting, the positive electrode sheet is obtained.

9. The preparation method according to claim 8, characterized in that, The conductive additive includes one or more of the following: conductive carbon black Superp, acetylene black, carbon nanotubes, Ketjen black, and conductive graphite; the conductive additive accounts for 0-30 wt% of the iron-based phosphate composite cathode material. The binder comprises one or more of the following: polyvinylidene fluoride (PVDF), sodium carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR), polyacrylonitrile (PAN), polyvinyl alcohol (PVA), polytetrafluoroethylene (PTFE), and polyimide (PI); the binder accounts for 0-30 wt% of the iron-based phosphate composite cathode material. The solvent includes one or more of N-methylpyrrolidone, dimethyl carbonate, ethylene carbonate, or diethylene carbonate. The drying conditions are: vacuum baking at 90℃-120℃ for 5-10 hours.

10. A sodium-ion battery, characterized in that, The sodium-ion battery includes the positive electrode sheet as described in claim 7; The sodium-ion battery includes a liquid sodium-ion battery, a semi-solid sodium-ion battery, or an all-solid sodium-ion battery. The sodium-ion battery operates within a voltage range of 1.5V to 4.5V.

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