Lithium iron phosphate material and preparation method thereof, positive pole piece and lithium ion battery

Abstract

The invention provides a lithium iron phosphate material and a preparation method thereof, a positive pole piece and a lithium ion battery, the preparation method comprises the following steps: dissolving a soluble iron salt and a ligand in water to form an iron-based complex so as to obtain a first solution; placing the carboxylated cellulose material in a first solution to form an iron-cellulose complex, so as to obtain a first mixed material; carrying out first separation and drying on the first mixed material to obtain an iron-cellulose complex; and mixing a lithium source, an iron source, a phosphorus source, a carbon source and the iron-cellulose complex in a solvent, heating, carrying out second separation on the obtained reaction mixture, and sintering the obtained solid to obtain the lithium iron phosphate material. According to the method, an iron-cellulose complex is used as a one-dimensional structure template, the lithium iron phosphate material with a one-to-many electron transport mechanism is successfully constructed, the conductivity, the rate capability and the working voltage of the material are improved, and the problem of capacity fading of the lithium iron phosphate material under high current density is further solved.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, and more specifically, to a lithium iron phosphate material and its preparation method, a positive electrode sheet, and a lithium-ion battery. Background Technology

[0002] Against the backdrop of continuously growing global energy demand, a green and low-carbon transformation of energy production and consumption is urgently needed. As a key technology for renewable energy storage and conversion, secondary batteries play an irreplaceable role in promoting energy structure optimization and driving green and low-carbon development. Especially in the widespread application of electric vehicles and large-scale energy storage, advancements in secondary battery technology are directly related to the success of the energy transition. In this process, improving the performance of secondary batteries, particularly optimizing their energy density, safety, and cycle life, has become the core objective of technological innovation.

[0003] Cathode materials are one of the key factors determining the performance of secondary batteries, directly affecting their energy storage capacity and cycle stability. Among many cathode materials, lithium iron phosphate (LiFePO4, or LFP) has become a star material in the field of cathode materials due to its excellent cycle life, high thermal stability, environmental friendliness, and cost advantages, and is widely used in energy storage and power lithium batteries.

[0004] However, LFP materials generally have low electrical conductivity, mainly due to the presence of anionic groups (especially PO4) in their crystal structure. 3- The presence of carbon dioxide hinders rapid electron conduction. This slow electron transport dynamics leads to a significant decrease in battery performance at high rates, limiting the potential of LFP materials in high-power applications. To address this issue, researchers have explored various methods to improve the conductivity of materials, with carbon coating being one of the most common strategies. Carbon coating can provide an additional conductive network for LFP particles; however, traditional carbon coating methods often create random "point-to-point" contacts around the LFP material. This contact method not only results in low electron transport efficiency but also easily causes carbon layer breakage during high-rate charge-discharge processes, leading to minimal improvement in conductivity or even further reduction in conductivity.

[0005] In summary, overcoming the shortcomings of existing technologies, addressing the inherent defects in conductivity of LFP materials, and overcoming the limitations of carbon coating technology in improving electron transport efficiency, as well as improving the rate performance and operating voltage of LFP cathode materials, so that LFP materials can better meet the needs of high-power applications and provide more stable and higher-performance battery solutions for new energy vehicles, large-scale energy storage systems, etc., has become an urgent problem to be solved. Summary of the Invention

[0006] The main objective of this invention is to provide a lithium iron phosphate material and its preparation method, a positive electrode sheet, and a lithium-ion battery, in order to overcome the inherent defects in conductivity of lithium iron phosphate materials in the prior art, as well as the limitations of traditional carbon coating technology in improving the electron transport efficiency of lithium iron phosphate materials. The aim is to further improve the conductivity, rate performance, and operating voltage of lithium iron phosphate materials, so that lithium iron phosphate materials can better meet the needs of high-power battery applications.

[0007] This application provides a method for preparing lithium iron phosphate material, which includes the following steps: Step (1): Dissolving soluble iron salt and ligand in water to form an iron-based complex to obtain a first solution; placing carboxylated cellulose material in the first solution to form an iron-cellulose complex to obtain a first mixture; Step (2): After the first mixture is separated and dried, an iron-cellulose complex is obtained; Step (3): Mixing lithium source, iron source, phosphorus source, carbon source and iron-cellulose complex in a solvent and heating to obtain a reaction mixture; After the reaction mixture is separated a second time, the resulting solid is sintered to obtain lithium iron phosphate material.

[0008] Further, in the carboxylated cellulose material, the degree of substitution of carboxyl groups is 0.5~4.0 mmol / g; and / or, the ratio of the sum of the molar amounts of Li in the lithium source, Fe in the iron source, and Fe in the iron-cellulose complex to the molar amounts of P in the phosphorus source is (0.95~1.05):(0.95~1.00):(0.98~1.02); and / or, in step (1), after the carboxylated cellulose material is placed in the first solution, the ratio of the mass concentration of the carboxylated cellulose material in the resulting system to the molar concentration of the added ligand is (0.3~3.0) mg / L:(0.01~0.5) mol / L; and / or, the sum of the weights of the carbon source and the iron-cellulose complex is 5~20% of the dry weight of the reaction mixture.

[0009] Furthermore, the molar ratio of Fe ions to ligands in the soluble iron salt is 1:(0.5~3.0); and / or, the molar ratio of Fe ions to ligands in the soluble iron salt is 1:(1.0~2.0); and / or, the concentration of the ligands after dissolving in water is 0.01~0.5 mol / L.

[0010] Furthermore, the ligand is selected from one or more of gluconic acid, sodium gluconate, potassium gluconate, glucoheponic acid, sodium glucoheponicate, and potassium glucoheponicate.

[0011] Further, in step (1), when preparing the iron-based complex, a buffer solution is added to adjust the pH of the system to be alkaline; and / or, the pH is adjusted to 8~13; and / or, the buffer solution is a glycine-sodium hydroxide buffer solution; and / or, the temperature for forming the iron-cellulose complex is 40~100℃ and the time is 1~7 days.

[0012] Further, the heating temperature in step (3) is 40~200℃ and the time is 1~72h; and / or, the sintering process includes: first heating to the sintering temperature at a heating rate of 1~5℃ / min, and then holding the temperature to perform the sintering operation; and / or, the sintering process is carried out in a protective atmosphere; and / or, the drying temperature is 80~200℃ and the time is 18~36h; and / or, the first separation includes: first washing and filtration; and / or, the preparation method of carboxylated cellulose material includes the following steps: mixing cellulose nanofiber material and oxidant to convert the hydroxyl groups on the surface of cellulose nanofiber material into carboxyl groups to obtain a second mixture; the second mixture is then subjected to a second washing, filtration and drying to obtain carboxylated cellulose material.

[0013] Further, the sintering temperature is 500~800℃, and the time is 2~12h; and / or, the protective atmosphere is selected from at least one of nitrogen, argon, nitrogen / hydrogen mixture, or argon / hydrogen mixture; and / or, the first washing operation is performed using an aprotic solvent; the aprotic solvent is selected from one or more of dimethyl sulfoxide, N,N dimethylformamide, acetone, and diethyl ether; and / or, the mass ratio of cellulose nanofiber material to oxidant is 1:(0.1~0.2); and / or, the cellulose nanofiber material is selected from one or more of plant cellulose nanofiber material, bacterial cellulose nanofiber material, and seaweed cellulose nanofiber material.

[0014] Further, the soluble iron salt is selected from one or more of ferric chloride, ferric nitrate, and ferric sulfate; and / or, the lithium source is selected from one or more of lithium carbonate, lithium oxide, and lithium hydroxide; and / or, the phosphorus source is selected from one or more of ammonium dihydrogen phosphate, lithium dihydrogen phosphate, lithium phosphate, and phosphoric acid; and / or, the carbon source is selected from one or more of glucose, sucrose, citric acid, polyvinyl alcohol, and polyethylene glycol; and / or, the solvent is selected from one or more of ethanol, ethylene glycol, glycerol, and tert-butanol.

[0015] According to another aspect of the present invention, a lithium iron phosphate material is also provided, which is prepared by the above-described method for preparing lithium iron phosphate material.

[0016] According to a third aspect of the present invention, a positive electrode sheet is also provided, the positive electrode sheet comprising a positive electrode active material layer, the material of the positive electrode active material layer being the aforementioned lithium iron phosphate material.

[0017] According to a fourth aspect of the present invention, a lithium-ion battery is also provided, wherein the positive electrode of the lithium-ion battery is the aforementioned positive electrode.

[0018] This invention provides a method for preparing lithium iron phosphate (LFP) materials. The method includes: dissolving a soluble iron salt and ligand in water to form an iron-based complex, obtaining a first solution; placing carboxylated cellulose material in the first solution to form an iron-cellulose complex, obtaining a first mixture; subjecting the first mixture to a first separation and drying to obtain an iron-cellulose complex; mixing a lithium source, an iron source, a phosphorus source, a carbon source, and the iron-cellulose complex in a solvent and heating to obtain a reaction mixture; subjecting the reaction mixture to a second separation, and then sintering the resulting solid to obtain the LFP material. This preparation method successfully constructs LFP materials with a "one-to-many" electron transport mechanism by using the iron-cellulose complex as a one-dimensional structural template. The LFP material prepared by this method overcomes the limitations of traditional carbon coating technology in improving the electron transport efficiency of LFP materials, significantly improving the material's conductivity, rate performance, and operating voltage, thereby solving the capacity decay problem of LFP materials at high current densities. This provides a solid technical foundation for the production of next-generation high-performance LFP materials. Attached Figure Description

[0019] The accompanying drawings, which form part of this application, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings:

[0020] Figure 1 The diagram illustrates the mechanism of action of this invention in preparing lithium iron phosphate materials;

[0021] Figure 2 The SEM image of the lithium iron phosphate material prepared according to Example 1 of the present invention is shown. Detailed Implementation

[0022] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0023] As described in the background section, due to the presence of anionic groups (especially PO4) in the crystal structure of lithium iron phosphate (LFP) materials... 3-The presence of carbon dioxide hinders rapid electron conduction, resulting in generally low conductivity in lithium iron phosphate (LFP) materials. This slow electron transport kinetics leads to a significant performance degradation at high rates, limiting the potential of LFP materials in high-power battery applications. Traditional carbon coating strategies can provide an additional conductive network for LFP particles; however, the conductive network formed around the LFP material often results in random "point-to-point" contacts. This contact pattern not only has low electron transport efficiency but also easily causes carbon layer breakage during high-rate charge-discharge processes, leading to minimal improvement in conductivity or even further reduction, thus affecting the battery's electrochemical performance.

[0024] To address the aforementioned problems, this application provides a method for preparing lithium iron phosphate material, comprising the following steps: Step (1): dissolving soluble iron salt and ligand in water to form an iron-based complex, obtaining a first solution; placing carboxylated cellulose material in the first solution to form an iron-cellulose complex, obtaining a first mixture; Step (2): subjecting the first mixture to first separation and drying to obtain an iron-cellulose complex; Step (3): mixing lithium source, iron source, phosphorus source, carbon source and iron-cellulose complex in a solvent and heating to obtain a reaction mixture; subjecting the reaction mixture to second separation, and then sintering the resulting solid to obtain lithium iron phosphate material.

[0025] In the above method for preparing lithium iron phosphate materials, soluble iron salts and ligands are first reacted in water to form an iron-based complex. Then, carboxylated cellulose material and the iron-based complex undergo a comprehensive coordination reaction to obtain a first mixture containing an iron-cellulose complex. This process of first forming an iron-based complex with iron ions and ligands, and then forming an iron-cellulose complex with the carboxylated cellulose material, allows for better dispersion of iron on the surface of the carboxylated cellulose material, thus facilitating the iron-cellulose complex's better function as a one-dimensional structural template in subsequent processes. The first mixture containing the iron-cellulose complex is then sequentially separated and dried to obtain the iron-cellulose complex. This iron-cellulose complex is then mixed with a lithium source, an iron source, a phosphorus source, and a carbon source in a solvent and heated to react, yielding a lithium iron phosphate material precursor. In the above process, the iron-cellulose complex fully utilizes its "guiding" role as a one-dimensional structural template, allowing the lithium, iron, and phosphorus sources to nucleate on the surface of the iron-cellulose complex, using the highly dispersed iron elements as sites, along the parallel alignment of the iron-cellulose complex nanocrystals. This further optimizes the microstructure properties of the resulting lithium iron phosphate material. Then, after separating the lithium iron phosphate precursor formed in the above reaction process, further sintering is performed to fully carbonize the cellulose structure in the iron-cellulose complex under high temperature conditions. The carbonized cellulose network structure of the iron-cellulose complex comes into full contact with the lithium iron phosphate material. This carbonized cellulose network structure serves as an excellent three-dimensional ion network channel in the lithium iron phosphate material, aiding in electron and ion transport, thus constructing a lithium iron phosphate material with a "one-to-many" electron transport mechanism. Furthermore, the carbon source added during the preparation of lithium iron phosphate materials can effectively coat the nucleated lithium iron phosphate surface. Synergistically, with the cellulose network structure formed after the carbonization of the iron-cellulose complex, the prepared lithium iron phosphate material possesses better conductive network channels, which is beneficial for further improving the material's electron transport efficiency, rate performance, and operating voltage, enabling lithium iron phosphate materials to better meet the demands of high-power applications. Specifically:

[0026] In the preparation of lithium iron phosphate (LFP) materials, the carboxyl groups in carboxylated cellulose materials act as "rivets," effectively anchoring iron ions and ensuring a more uniform distribution of iron ions in the prepared iron-cellulose complex. This also contributes to better performance of the LFP material formed subsequently along the parallel alignment of the iron-cellulose complex nanocrystals. This is because carboxylated cellulose materials and cellulose materials have similar structures, consisting of parallel-aligned cellulose nanocrystals. In particular, the prepared iron-cellulose complex, while possessing excellent structural stability, can also serve as a one-dimensional hard template to assist in the construction of LFP nanofibers. Furthermore, after high-temperature carbonization, it can be transformed into oriented carbon nanofibers, resulting in a superior ion-conducting network structure in the prepared LFP material. The mechanism of action of the above process is described in [link to documentation]. Figure 1 First, the surface of cellulose nanofibers is modified with carboxyl functional groups, and the iron ions are anchored to the fiber surface by the coordination of the carboxyl groups and iron ions. Then, the cellulose molecular chains are swollen and depolymerized, providing ion channels for Fe ions to enter and forming a strong iron-cellulose complex. Next, the formed iron-cellulose complex is uniformly mixed with raw materials such as lithium, phosphorus, and iron sources. The iron-cellulose complex can serve as a one-dimensional structural template, allowing it to react on the surface of the iron-cellulose complex using iron ions as sites. A mixture including lithium iron phosphate precursors is then formed through a heat of solution reaction. The resulting mixture is then sintered at high temperature to convert the precursors into lithium iron phosphate nanofiber materials. Through the combination of nanofibers with nanofibers, nanofibers with nanoparticles, and nanoparticles with nanoparticles, a three-dimensional fast electron conductor network is formed, ultimately improving the electrochemical performance of lithium iron phosphate materials, especially their rate and voltage performance.

[0027] The lithium iron phosphate (LFP) material prepared according to the above method embeds the active material (i.e., LFP particles) into a continuous network composed of an electron transport phase, shortening the electron transport path and ensuring that electrons can be rapidly conducted to each reaction site, thereby achieving efficient redox reactions based on electron-ion coupling. By optimizing the electron-ion transport path, the performance of LFP materials under high-rate charge-discharge conditions can be significantly improved, achieving efficient "one-to-many" electron transport from the cellulose template to the LFP particle surface. Compared with the random "point-to-point" contact of the conductive network formed around LFP materials prepared by the traditional carbon coating strategy, this greatly shortens the electron transport path and improves the efficiency and rate of electron transport. This transport mechanism can promote the stability and capacity retention of LFP materials under high-rate charge-discharge conditions, achieving a significant improvement in material performance. Through the combination of cellulose nanofibers and LFP materials, a multi-level ordered composite structure is formed. This structure not only improves the electronic conductivity of the material but also maintains the original structural advantages and electrochemical properties of LFP materials, achieving significant performance gains and demonstrating a synergistic effect of "1+1>2".

[0028] In summary, the method for preparing lithium iron phosphate materials provided by this invention successfully constructs lithium iron phosphate materials with a "one-to-many" electron transport mechanism by using iron-cellulose complexes as one-dimensional structural templates. The lithium iron phosphate materials prepared by this method overcome the limitations of traditional carbon coating technology in improving the electron transport efficiency of lithium iron phosphate materials, significantly improve the conductivity, rate performance, and operating voltage of the materials, and thus solve the capacity decay problem of lithium iron phosphate materials at high current densities, providing a solid technical foundation for the production of next-generation high-performance lithium iron phosphate materials.

[0029] In a preferred embodiment, the degree of carboxyl substitution in the carboxylated cellulose material is 0.5–4.0 mmol / g; for example, 0.5 mmol / g, 1 mmol / g, 1.5 mmol / g, 2 mmol / g, 2.5 mmol / g, 3 mmol / g, 3.5 mmol / g, 4 mmol / g, or any ratio of any two of the above. Controlling the degree of carboxyl substitution in the carboxylated cellulose material within this range allows for better control of the distribution and coordination of iron in the iron-cellulose complex, resulting in a better "one-to-many" electron transport mechanism in the prepared lithium iron phosphate material, which is beneficial for further improving the overall electrochemical performance of the lithium iron phosphate material. Preferably, the degree of carboxyl substitution in the carboxylated cellulose material is 1–4.0 mmol / g. Controlling the degree of carboxyl substitution in the carboxylated cellulose material within this range results in even better performance of the prepared lithium iron phosphate material.

[0030] And / or, the ratio of the sum of the molar amounts of Li in the lithium source, Fe in the iron source, and Fe in the iron-cellulose complex to the molar amount of P in the phosphorus source is (0.95~1.05):(0.95~1.00):(0.98~1.02); and / or, the ratio of the sum of the molar amounts of Li in the lithium source, Fe in the iron source, and Fe in the iron-cellulose complex to the molar amount of P in the phosphorus source is (0.95~1.05):(0.95~1.00):(0.98~1.02); 95~1.05):(0.95~1.00):(0.98~1.02); and / or, in step (1), after placing the carboxylated cellulose material in the first solution, the ratio of the mass concentration of the carboxylated cellulose material to the molar concentration of the added ligand in the resulting system is (0.3~3.0) mg / L:(0.01~0.5) mol / L; and / or, the sum of the weights of the carbon source and the iron-cellulose complex is 5~20% of the dry weight of the reaction mixture. By controlling the addition ratio of lithium source, iron source, phosphorus source, carbon source and iron-cellulose complex within the above range during the preparation of lithium iron phosphate material, the performance of the prepared lithium iron phosphate material can be improved. In particular, controlling the relative addition ratio of the iron-based complex to the carboxylated cellulose material within the aforementioned range allows for a more uniform distribution of iron ions from the iron-based complex on the surface of the carboxylated cellulose material. This results in a superior conductive network structure in the prepared lithium iron phosphate material, which is beneficial for better addressing the technical problem of capacity decay at high current densities. Those skilled in the art will readily understand that if the molar amount of Fe in the iron-cellulose complex meets the iron requirements during the preparation of lithium iron phosphate material, then an iron source is unnecessary.

[0031] In a preferred embodiment, the molar ratio of Fe ions to ligands in the soluble iron salt is 1:(0.5~3.0); and / or, the concentration of the soluble iron salt dissolved in water is 0.005~0.1 mol / L. Controlling the molar ratio of Fe ions to ligands in the soluble iron salt and the concentration of the soluble iron salt within the above ranges is beneficial for more complete formation of stable iron-based complexes. And / or, the molar ratio of Fe ions to ligands in the soluble iron salt is 1:(1.0~2.0); the above limitations enable better formation of iron-based complexes.

[0032] In a preferred embodiment, the ligand is selected from one or more of gluconic acid, sodium gluconate, potassium gluconate, glucoheponic acid, sodium glucoheponicate, and potassium glucoheponicate; the above-mentioned types of ligands can better form iron-based complexes with iron ions in soluble iron salts, thereby enabling the prepared iron-cellulose complex to have better performance.

[0033] In a preferred embodiment, in step (1), during the preparation of the iron-based complex, a buffer solution is added to adjust the pH of the system to alkaline; and / or, the pH is adjusted to 8-13; and / or, the buffer solution is a glycine-sodium hydroxide buffer solution. Adding a buffer solution to adjust the pH of the system to alkaline during the preparation of the iron-based complex can better promote the swelling and depolymerization of cellulose molecular chains, giving the cellulose surface more open channels, thereby allowing iron ions to better enter and interact with the cellulose. Controlling the pH of the system within the above range can improve the above effects, and using a buffer solution to adjust the pH of the system can maintain the pH of the system in a more stable range. Preferably, the temperature for forming the iron-cellulose complex is 40-100°C, and the time is 1-7 days. Controlling the temperature and time during the formation of the iron-cellulose complex within the above range can improve the formation of the iron-cellulose complex.

[0034] In a preferred embodiment, the heating temperature in step (3) is 40~200℃, and the time is 1~72h. Heating the lithium source, iron source, phosphorus source, carbon source, and the iron-cellulose complex mixture within the above range allows for better "guiding" of the one-dimensional hard template of the iron-cellulose complex, enabling the lithium iron phosphate formed by the lithium source, iron source, and phosphorus source to nucleate on its surface using iron ions in the iron-cellulose complex as sites. This facilitates the final formation of lithium iron phosphate material with an excellent three-dimensional ion channel network structure. And / or, the sintering process includes: first heating to the sintering temperature at a heating rate of 1~5℃ / min, then holding at that temperature for sintering; and / or, the sintering process is carried out under a protective atmosphere; and / or, the drying temperature is 80~200℃, and the time is 18~36h; and / or, the first separation includes: first washing and filtration. High-temperature sintering facilitates the solid-phase transformation of lithium iron phosphate materials and promotes the carbonization of iron-cellulose complexes and carbon sources. This results in a more stable crystal structure in the formed lithium iron phosphate material, allowing the cellulose network structure to better perform its ion-electron conduction function. Gradual heating further enhances the crystallinity and stability of the lithium iron phosphate material.

[0035] In a preferred embodiment, the preparation method of carboxylated cellulose material includes the following steps: mixing cellulose nanofiber material and an oxidant to convert the hydroxyl groups on the surface of the cellulose nanofiber material into carboxyl groups, obtaining a second mixture; the second mixture is then subjected to a second washing, filtration, and drying process to obtain the carboxylated cellulose material. In the above preparation method, the hydroxyl groups on the surface of the cellulose nanofiber material are converted into carboxyl groups, thereby enabling better interaction with iron-based complexes formed by Fe ions and ligands to form iron-cellulose complexes, further improving the dispersibility of Fe ions on the cellulose surface.

[0036] Preferably, the mass ratio of cellulose nanofiber material to oxidant is 1:(0.1~0.2). By controlling the weight of cellulose nanofiber material and oxidant within the above range, the degree of carboxylation of cellulose nanofiber material can be optimized and controlled, thereby better controlling the "rivet" effect of carboxylated cellulose material on iron ions, and further controlling the distribution of lithium iron phosphate on the surface of the iron-cellulose complex, resulting in a better structure and better conductivity of the prepared lithium iron phosphate material. And / or, the oxidant is selected from one or more of tetramethylpiperidine oxide, sodium hypochlorite, sodium bromide, hydrogen peroxide, and potassium permanganate. Using the above oxidants can improve the performance of the prepared carboxylated cellulose material. Preferably, the cellulose nanofiber material is selected from one or more of plant cellulose nanofiber material, bacterial cellulose nanofiber material, and seaweed cellulose nanofiber material. Using the above types of cellulose nanofiber material to prepare carboxylated cellulose material can improve the performance of the prepared carboxylated cellulose material.

[0037] In a preferred embodiment, the sintering temperature is 500~800℃, and the time is 2~12h. Controlling the sintering temperature and time within the above range is beneficial to better improve the conductivity of lithium iron phosphate materials, thereby better improving the cycle stability and rate performance of the battery. And / or, the protective atmosphere is selected from at least one of nitrogen, argon, a nitrogen / hydrogen mixture, or an argon / hydrogen mixture; and / or, an aprotic solvent is used for the first washing operation; the aprotic solvent is selected from one or more of dimethyl sulfoxide, N,N-dimethylformamide, acetone, and diethyl ether. Sintering under the above atmosphere, or using the above-mentioned aprotic solvent for the first washing operation, both have excellent effects.

[0038] In a preferred embodiment, the soluble iron salt is selected from one or more of ferric chloride, ferric nitrate, and ferric sulfate. Using the aforementioned soluble iron salt in the preparation of the iron-cellulose complex improves the performance of the resulting iron-cellulose complex. For example, but not limitingly, the lithium source is selected from one or more of lithium carbonate, lithium oxide, and lithium hydroxide; and / or, the phosphorus source is selected from one or more of ammonium dihydrogen phosphate, lithium dihydrogen phosphate, lithium phosphate, and phosphoric acid; and / or, the carbon source is selected from one or more of glucose, sucrose, citric acid, polyvinyl alcohol, and polyethylene glycol; and / or, the solvent is selected from one or more of ethanol, ethylene glycol, glycerol, and tert-butanol. Using the specific lithium source, iron source, phosphorus source, carbon source, and solvent described above improves the performance of the prepared lithium iron phosphate material.

[0039] According to another aspect of the present invention, a lithium iron phosphate material is also provided, which is prepared by the above-described method for preparing lithium iron phosphate material.

[0040] It should be noted that, due to the limitations of existing testing and characterization methods, it is difficult to conduct a more comprehensive characterization and analysis of the complex microstructure of the lithium iron phosphate material prepared above. However, experiments show that the lithium iron phosphate material prepared by the preparation method provided in this application has better conductivity. When used in lithium-ion batteries, it can effectively improve the rate performance and operating voltage of lithium-ion batteries.

[0041] According to a third aspect of the present invention, a positive electrode sheet is also provided, the positive electrode sheet comprising a positive electrode active material layer, the material of the positive electrode active material layer comprising the above-mentioned lithium iron phosphate material.

[0042] According to a fourth aspect of the present invention, a lithium-ion battery is provided, wherein the positive electrode of the lithium-ion battery is the aforementioned positive electrode.

[0043] The present application will be further described in detail below with reference to specific embodiments, which should not be construed as limiting the scope of protection claimed in the present application.

[0044] Example 1

[0045] (1) Preparation of iron-cellulose complex

[0046] 10 g of bacterial cellulose nanofiber material (dry weight) was cut into small pieces and added to 2 L of deionized water. The mixture was mechanically stirred until completely dissolved. 100 mL of sodium hypochlorite was then added to the cellulose aqueous solution after adjusting the pH to 10 with hydrochloric acid. 0.16 g of 2,2,6,6-tetramethylpiperidine oxide and 1 g of sodium bromide were then added. The pH was maintained at 10.3 with sodium hydroxide solution, and the mixture was stirred continuously for 30 min to obtain a mixture containing carboxylated cellulose material. This mixture was washed with deionized water, filtered, frozen, and then dried in a freeze dryer (vacuum 10 Pa, cold trap temperature -50 °C, drying time 12 h) to obtain carboxylated cellulose material. The degree of carboxyl substitution in the obtained carboxylated cellulose material was 2 mmol / g.

[0047] Fe(NO3)3 and sodium gluconate were dissolved in 1 L of water (where the concentration of Fe(NO3)3 was 0.01 mol / L and the concentration of sodium gluconate was 0.01 mol / L), and the pH of the system was adjusted to 13.0 using a glycine-sodium hydroxide buffer solution (a mixture of 0.1 mol / L glycine and 0.1 mol / L sodium hydroxide) to form an iron-based complex, thus obtaining a first solution. The carboxylated cellulose material (0.5 mg) obtained above was placed in the first solution containing the iron-based complex and stirred at 80°C for 5 days to form an iron-cellulose complex, thus obtaining a first mixture. The first mixture obtained above was washed with DMF, then filtered and dried (100°C for 24 h) to obtain the iron-cellulose complex.

[0048] (2) Preparation of lithium iron phosphate materials

[0049] 1.6 g of the iron-cellulose complex obtained above, 15.4 g of FeCl3, and 4.28 g of Li(OH) were added. H2O, 11.53g of H3PO4, and 1.6g of citric acid were dissolved in 200mL of anhydrous ethanol, and then placed in a reaction vessel and heated at 160℃ for 24h to obtain a reaction mixture. The obtained reaction mixture was centrifuged, washed, and dried. The resulting solid was then heated to 700℃ at a heating rate of 3℃ / min under a nitrogen atmosphere, and then sintered at 700℃ for 6h to obtain lithium iron phosphate material. Figure 2 The SEM image of the obtained lithium iron phosphate material is shown. As can be seen from the figure, the prepared lithium iron phosphate material presents a nanofiber network with connection points between the nanofibers, forming an effective electron conduction network.

[0050] Example 2

[0051] (1) Preparation of iron-cellulose complex

[0052] 10g of plant cellulose nanofiber material (dry weight) was cut into small pieces and added to 2L of deionized water. The mixture was mechanically stirred until completely dissolved. 100mL of sodium hypochlorite was then added to the cellulose aqueous solution after adjusting the pH to 10 with hydrochloric acid. 0.16g of 2,2,6,6-tetramethylpiperidine oxide and 1g of sodium bromide were then added. The pH was maintained at 10.3 with sodium hydroxide solution, and the mixture was stirred continuously for 30 minutes to obtain a mixture containing carboxylated cellulose material. This mixture was washed with deionized water, filtered, frozen, and then dried in a freeze dryer (vacuum 10Pa, cold trap temperature -50℃, drying time 12h) to obtain carboxylated cellulose material. The degree of carboxyl substitution in the obtained carboxylated cellulose material was 2 mmol / g.

[0053] FeCl3 and potassium gluconate were dissolved in 1 L of water (where the concentration of FeCl3 was 0.01 mol / L and the concentration of potassium gluconate was 0.01 mol / L), and the pH of the system was adjusted to 13.0 using a glycine-sodium hydroxide buffer solution (a mixture of 0.1 mol / L glycine and 0.1 mol / L sodium hydroxide) to form an iron-based complex, thus obtaining a first solution. The carboxylated cellulose material (0.5 mg) obtained above was placed in the first solution containing the iron-based complex and stirred at 40 °C for 7 days to form an iron-cellulose complex, thus obtaining a first mixture. The first mixture obtained above was washed with DMF, then filtered and dried (200 °C, 18 h) to obtain the iron-cellulose complex.

[0054] (2) Preparation of lithium iron phosphate materials

[0055] 1.6 g of the iron-cellulose complex obtained above, 15.4 g of FeCl3, and 4.28 g of Li(OH) were added. H2O, 11.53g of H3PO4, and 1.6g of citric acid were dissolved in 200mL of anhydrous ethanol, and then placed in a reaction vessel and heated at 160℃ for 24h to obtain a reaction mixture. The obtained reaction mixture was centrifuged, washed, and dried. The resulting solid was then heated to 800℃ at a heating rate of 5℃ / min under a nitrogen atmosphere, and then sintered at 800℃ for 2h to obtain lithium iron phosphate material.

[0056] Example 3

[0057] (1) Preparation of iron-cellulose complex

[0058] 10g of seaweed cellulose nanofiber material (dry weight) was cut into small pieces and added to 2L of deionized water. The mixture was mechanically stirred until completely dissolved. 100mL of sodium hypochlorite was then added to the cellulose aqueous solution after adjusting the pH to 10 with hydrochloric acid. 0.16g of 2,2,6,6-tetramethylpiperidine oxide and 1g of sodium bromide were then added. The pH was maintained at 10.3 with sodium hydroxide solution, and the mixture was stirred continuously for 30 minutes to obtain a mixture containing carboxylated cellulose material. This mixture was washed with deionized water, filtered, frozen, and then dried in a freeze dryer (vacuum 10Pa, cold trap temperature -50℃, drying time 12h) to obtain carboxylated cellulose material. The degree of carboxyl substitution in the obtained carboxylated cellulose material was 2 mmol / g.

[0059] Fe(NO3)3 and sodium gluconate were dissolved in 1 L of water (wherein the concentration of Fe(NO3)3 was 0.01 mol / L and the concentration of sodium gluconate was 0.01 mol / L), and the pH of the system was adjusted to 13.0 using a glycine-sodium hydroxide buffer solution (a mixture of 0.1 mol / L glycine and 0.1 mol / L sodium hydroxide) to form an iron-based complex, thus obtaining a first solution. The carboxylated cellulose material (0.5 mg) obtained above was placed in the first solution containing the iron-based complex and stirred at 100°C for 1 day to form an iron-cellulose complex, thus obtaining a first mixture. The first mixture obtained above was washed with DMF, then filtered and dried (80°C, 36 h) to obtain the iron-cellulose complex.

[0060] (2) Preparation of lithium iron phosphate materials

[0061] 1.6 g of the iron-cellulose complex obtained above, 15.4 g of FeCl3, and 4.28 g of Li(OH) were added. H2O, 11.53g of H3PO4, and 1.6g of citric acid were dissolved in 200mL of anhydrous ethanol, and then placed in a reaction vessel and heated at 160℃ for 24h to obtain a reaction mixture. The obtained reaction mixture was centrifuged, washed, and dried. The resulting solid was then heated to 500℃ at a heating rate of 1℃ / min under a nitrogen atmosphere, and then sintered at 500℃ for 12h to obtain lithium iron phosphate material.

[0062] Example 4

[0063] The difference between Example 4 and Example 1 is that, in the process of preparing the iron-cellulose complex, a glycine-sodium hydroxide buffer solution was used to adjust the pH of the system to 8.0 in order to form an iron-based complex and obtain the first solution.

[0064] Example 5

[0065] The difference between Example 5 and Example 1 is that, in the process of preparing lithium iron phosphate material, the mass of iron-cellulose complex was adjusted to 3.2g and the mass of citric acid was adjusted to 0.2g.

[0066] Example 6

[0067] The difference between Example 6 and Example 1 is that in the preparation of lithium iron phosphate material, the reaction solvent is ethylene glycol and the reaction temperature is 180°C.

[0068] Example 7

[0069] The difference between Example 7 and Example 1 is that ethylene glycol is used as the reaction solvent in the preparation of lithium iron phosphate material.

[0070] Example 8

[0071] The difference between Example 8 and Example 1 is that the degree of carboxyl substitution in the prepared carboxylated cellulose material is 0.5 mmol / g.

[0072] Example 9

[0073] The difference between Example 9 and Example 1 is that the degree of carboxyl substitution in the prepared carboxylated cellulose material is 1 mmol / g.

[0074] Example 10

[0075] The difference between Example 10 and Example 1 is that the degree of carboxyl substitution in the prepared carboxylated cellulose material is 4 mmol / g.

[0076] Example 11

[0077] The difference between Example 11 and Example 1 is that the weight of the carboxylated cellulose material added to the first solution is 0.3 mg.

[0078] Example 12

[0079] The difference between Example 12 and Example 1 is that the weight of the carboxylated cellulose material added to the first solution is 3 mg.

[0080] Example 13

[0081] The difference between Example 13 and Example 1 is that when forming the iron-based complex, the concentration of Fe(NO3)3 is 0.5 mol / L and the concentration of sodium gluconate is 0.5 mol / L; the weight of carboxylated cellulose material added to the first solution is 0.3 mg.

[0082] Example 14

[0083] The difference between Example 14 and Example 1 is that when forming the iron-based complex, the concentration of Fe(NO3)3 is 0.5 mol / L and the concentration of sodium gluconate is 0.5 mol / L; the weight of carboxylated cellulose material added to the first solution is 3 mg.

[0084] Example 15

[0085] The difference between Example 15 and Example 1 is that the degree of carboxyl substitution in the prepared carboxylated cellulose material is 0.3 mmol / g.

[0086] Example 16

[0087] The difference between Example 16 and Example 1 is that the weight of the carboxylated cellulose material added to the first solution is 0.1 mg.

[0088] Comparative Example 1

[0089] The difference between this comparative example and Example 1 is that, in the process of preparing lithium iron phosphate materials, the carboxylated cellulose nanomaterials prepared in Example 1 are used instead of the iron-cellulose complex.

[0090] Comparative Example 2

[0091] 16.22g of FeCl3 and 4.28g of Li(OH)3 were added. H2O, 11.53 g of H3PO4, and 3.2 g of citric acid were dissolved in 100 mL of anhydrous ethanol, and then placed in a reaction vessel and heated at 160 °C for 24 h. The reaction product was then centrifuged, washed with deionized water, dried, and sintered at 700 °C under a nitrogen atmosphere for 6 h to obtain lithium iron phosphate material.

[0092] The lithium iron phosphate materials prepared in the above examples and comparative examples were used to fabricate coin cells according to the method in GB / T42161. The resulting coin cells were then subjected to relevant performance tests, and the results are shown in Table 1. The test conditions were as follows: Charging limit voltage: constant current charging to 4.0V at a 0.1C rate, followed by constant voltage charging, with a constant voltage charging cutoff current of 0.05C; Discharge termination voltage: constant current discharging to 2.0V at a 0.1C rate; 1C and 6C rate constant current charge-discharge tests were performed sequentially, and the discharge specific capacity was recorded, with 0.1C as the initial charge specific capacity.

[0093] Table 1

[0094]

[0095] As can be seen from the above description, the embodiments of the present invention achieve the following technical effects:

[0096] This application provides a method for preparing lithium iron phosphate (LFP) materials. This method successfully constructs LFP materials with a "one-to-many" electron transport mechanism by using an iron-cellulose complex as a one-dimensional structural template. The LFP materials in Examples 1 to 16 were prepared using the above method. The corresponding LFP materials were used in lithium-ion batteries, and their electrochemical performance was tested. The results are shown in Table 1. According to the results in Table 1, using the above LFP materials in lithium-ion batteries effectively improves the rate performance and operating voltage of the resulting lithium-ion batteries. In particular, controlling the parameters during the LFP material preparation process within the preferred range results in even better electrochemical performance of the corresponding lithium-ion batteries.

[0097] In Comparative Example 1, carboxylated cellulose nanomaterials were used instead of iron-cellulose complexes in the preparation of lithium iron phosphate materials. In Comparative Example 2, lithium iron phosphate materials were prepared using conventional carbon coating methods. The lithium iron phosphate materials prepared in the above comparative examples were used in lithium-ion batteries. According to the data in Table 1, the rate performance and operating voltage of the above lithium-ion batteries are significantly different from those of the lithium-ion batteries in the embodiments of this application, especially the capacity decay of the corresponding lithium-ion batteries at high current densities is obvious.

[0098] In summary, the method for preparing lithium iron phosphate material provided by this invention successfully constructs a lithium iron phosphate material with a "one-to-many" electron transport mechanism by using an iron-cellulose complex as a one-dimensional structural template. The lithium iron phosphate material prepared by this method overcomes the limitations of traditional carbon coating technology in improving the electron transport efficiency of lithium iron phosphate materials, significantly improves the conductivity, rate performance, and operating voltage of the material, and thus solves the capacity decay problem of lithium iron phosphate materials at high current densities, providing a solid technical foundation for the production of next-generation high-performance lithium iron phosphate materials.

[0099] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing lithium iron phosphate material, characterized in that, The preparation method includes the following steps: Step (1): Dissolve the soluble iron salt and ligand in water to form an iron-based complex, obtaining a first solution; place the carboxylated cellulose material in the first solution to form an iron-cellulose complex, obtaining a first mixture. Step (2): After the first mixture is separated and dried, an iron-cellulose complex is obtained; Step (3): The lithium source, iron source, phosphorus source, carbon source and the iron-cellulose complex are mixed in a solvent and heated to obtain a reaction mixture; the reaction mixture is then separated in a second step, and the resulting solid is sintered to obtain the lithium iron phosphate material.

2. The method for preparing lithium iron phosphate material according to claim 1, characterized in that, In the carboxylated cellulose material, the degree of carboxyl substitution is 0.5~4.0 mmol / g; And / or, the ratio of the sum of the molar amounts of Li in the lithium source, Fe in the iron source, and Fe in the iron-cellulose complex to the molar amount of P in the phosphorus source is (0.95~1.05):(0.95~1.00):(0.98~1.02). And / or, in step (1), after the carboxylated cellulose material is placed in the first solution, the ratio of the mass concentration of the carboxylated cellulose material in the resulting system to the molar concentration of the added ligand is (0.3~3.0) mg / L: (0.01~0.5) mol / L; And / or, the sum of the weights of the carbon source and the iron-cellulose complex is 5 to 20% of the dry weight of the reaction mixture.

3. The method for preparing lithium iron phosphate material according to claim 1, characterized in that, The molar ratio of Fe ions in the soluble iron salt to the ligand is 1:(0.5~3.0). And / or, the molar ratio of Fe ions in the soluble iron salt to the ligand is 1:(1.0~2.0). And / or, the concentration of the ligand dissolved in the water is 0.01~0.5 mol / L.

4. The method for preparing lithium iron phosphate material according to any one of claims 1 to 3, characterized in that, The ligand is selected from one or more of gluconic acid, sodium gluconate, potassium gluconate, glucoheponic acid, sodium glucoheponicate, and potassium glucoheponicate.

5. The method for preparing lithium iron phosphate material according to any one of claims 1 to 3, characterized in that, In step (1), when preparing the iron-based complex, a buffer solution is added to adjust the pH of the system to be alkaline; And / or, adjust the pH to 8-13; And / or, the buffer solution is a glycine-sodium hydroxide buffer solution; And / or, the temperature for forming the iron-cellulose complex is 40~100°C, and the time is 1~7 days.

6. The method for preparing lithium iron phosphate material according to any one of claims 1 to 3, characterized in that, The heating temperature in step (3) is 40~200℃ and the time is 1~72h; And / or, the sintering process includes: first heating to the sintering temperature at a heating rate of 1~5℃ / min, and then holding at the temperature to perform the sintering operation; And / or, the sintering process is carried out in a protective atmosphere; And / or, the drying temperature is 80~200℃, and the time is 18~36h; And / or, the first separation includes: a first washing and filtration; And / or, the method for preparing the carboxylated cellulose material includes the following steps: mixing cellulose nanofiber material and an oxidant to convert the hydroxyl groups on the surface of the cellulose nanofiber material into carboxyl groups, thereby obtaining a second mixture; the second mixture is then subjected to a second washing, filtration and drying process to obtain the carboxylated cellulose material.

7. The method for preparing lithium iron phosphate material according to claim 6, characterized in that, The sintering temperature is 500~800℃, and the time is 2~12h; And / or, the protective atmosphere is selected from at least one of nitrogen, argon, a nitrogen / hydrogen mixture, or an argon / hydrogen mixture; And / or, the first washing operation is performed using an aprotic solvent; the aprotic solvent is selected from one or more of dimethyl sulfoxide, N,N-dimethylformamide, acetone, and diethyl ether; And / or, the mass ratio of the cellulose nanofiber material to the oxidant is 1:(0.1~0.2); And / or, the cellulose nanofiber material is selected from one or more of plant cellulose nanofiber materials, bacterial cellulose nanofiber materials, and seaweed cellulose nanofiber materials.

8. The method for preparing lithium iron phosphate material according to any one of claims 1 to 3, characterized in that, The soluble iron salt is selected from one or more of ferric chloride, ferric nitrate, and ferric sulfate; And / or, the lithium source is selected from one or more of lithium carbonate, lithium oxide, and lithium hydroxide; And / or, the phosphorus source is selected from one or more of ammonium dihydrogen phosphate, lithium dihydrogen phosphate, lithium phosphate, and phosphoric acid; And / or, the carbon source is selected from one or more of glucose, sucrose, citric acid, polyvinyl alcohol, and polyethylene glycol; And / or, the solvent is selected from one or more of ethanol, ethylene glycol, glycerol and tert-butanol.

9. A lithium iron phosphate material, characterized in that, The lithium iron phosphate material is prepared by the method for preparing lithium iron phosphate material according to any one of claims 1 to 8.

10. A positive electrode sheet, comprising a positive electrode active material layer, characterized in that, The material of the positive electrode active material layer includes the lithium iron phosphate material as described in claim 9.

11. A lithium-ion battery, characterized in that, The positive electrode of the lithium-ion battery is the positive electrode as described in claim 10.

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