Lithium iron phosphate material with nanofiber structure, preparation method thereof, positive electrode and lithium ion battery

Abstract

The application provides a lithium iron phosphate material with a nanofiber structure, a preparation method of the lithium iron phosphate material, a positive electrode and a lithium ion battery. The lithium iron phosphate material comprises, from inside to outside, a core layer, an intermediate layer, a shell layer and a coating layer; the concentration of lithium in the core layer is equal to or lower than the concentration of lithium in the intermediate layer, and the concentration of lithium in the intermediate layer is lower than the total concentration of lithium in the shell layer and the coating layer; the concentration of iron in the core layer is higher than the concentration of iron in the intermediate layer, and the concentration of iron in the intermediate layer is equal to or higher than the total concentration of iron in the shell layer and the coating layer; and the coating layer is a lithium pyrophosphate coating layer. The lithium iron phosphate material has a concentration gradient of different elements in the radial direction, which can reduce the interface impedance between the lithium iron phosphate material and electrolyte while ensuring electrochemical activity, improve the lithium ion transmission efficiency and electronic conductivity, and thus improve the rate performance and cycle performance of the lithium ion battery.

Description

Technical Field

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

[0002] As a core representative of electrochemical energy storage technology, the performance improvement of lithium-ion batteries has always been a focus of attention for both the scientific research and industrial communities. Cathode materials, as a key determinant of lithium-ion battery performance, directly affect the battery's energy density, cycle life, and safety. Among numerous cathode materials, lithium iron phosphate (LiFePO4) has been widely used in power batteries and large-scale energy storage systems due to its excellent thermal stability, ultra-long cycle life, good cost-effectiveness, and environmental friendliness, demonstrating outstanding market potential.

[0003] However, traditional lithium iron phosphate materials suffer from low lithium-ion diffusion rates and low electronic conductivity. To address these issues, two commonly used improvement methods are carbon coating and nano-sizing. While these strategies can improve the electronic conductivity and lithium-ion diffusion of lithium iron phosphate materials to some extent, the uniform elemental distribution of the optimized cathode material makes it prone to iron ion dissolution and lithium loss during charge-discharge processes, leading to a decrease in the cycle life of the lithium iron phosphate material. Furthermore, existing improvement technologies struggle to simultaneously optimize bulk properties and interfacial characteristics.

[0004] Therefore, researching and developing a lithium iron phosphate material with high lithium-ion transport efficiency, high electronic conductivity, low interfacial impedance and nanofiber structure, and its preparation method, is of great significance for improving the rate performance and cycle performance of lithium-ion batteries. Summary of the Invention

[0005] The main objective of this invention is to provide a lithium iron phosphate material with a nanofiber structure, its preparation method, a cathode, and a lithium-ion battery, in order to solve the problems of low lithium-ion diffusion rate and low electronic conductivity of lithium iron phosphate cathode materials in the prior art, and the resulting poor rate performance and cycle performance of lithium-ion batteries.

[0006] To achieve the above objectives, the present invention provides a lithium iron phosphate material with a nanofiber structure, which comprises, from the inside out, a core layer, an intermediate layer, a shell layer, and a coating layer; wherein, the lithium concentration in the core layer is equal to or lower than the lithium concentration in the intermediate layer, and the lithium concentration in the intermediate layer is lower than the total lithium concentration in the shell layer and the coating layer; the iron concentration in the core layer is higher than the iron concentration in the intermediate layer, and the iron concentration in the intermediate layer is equal to or higher than the total iron concentration in the shell layer and the coating layer; the coating layer is a lithium pyrophosphate coating layer.

[0007] The lithium iron phosphate material with a nanofiber structure provided by this invention comprises, from the inside out, a core layer, an intermediate layer, a shell layer, and a coating layer. The lithium concentration in the core layer is equal to or lower than that in the intermediate layer, while the sum of the lithium concentrations in the shell and coating layers is higher than that in the intermediate layer. Similarly, the iron concentration in the core layer is higher than that in the intermediate layer, while the iron concentration in the intermediate layer is equal to or higher than the sum of the iron concentrations in the shell and coating layers. This elemental distribution in the lithium iron phosphate material results in a high total lithium concentration and a low total iron concentration in the shell and coating layers. The excess lithium can interact with phosphorus to form lithium pyrophosphate (Li4P2O7), forming the coating layer. This improves the structural stability of the lithium iron phosphate material and provides more migration paths for lithium ions, promoting rapid diffusion of lithium ions within the material and increasing the transport efficiency of lithium ions during charging and discharging. This, in turn, effectively improves the rate performance and cycle performance of lithium-ion batteries. The high iron concentration in the core layer increases the electron conduction path, improves the electronic conductivity of lithium iron phosphate (LFP) materials, reduces charge transfer resistance, lowers electrochemical polarization during charging and discharging, and reduces iron ion dissolution and lithium ion loss during charging and discharging. This improves the structural stability of LFP materials and enhances the rate performance and cycle stability of lithium-ion batteries. The intermediate layer allows for a gradual transition from a high iron concentration internally to a high lithium concentration externally, forming a continuous lithium-ion transport path. It also alleviates the stress caused by rapid changes in element concentration between the internal and external layers, improving the structural stability of LFP materials. Furthermore, it maintains the electrochemical activity of LFP materials during charging and discharging, thus enhancing the structural stability, rate performance, and cycle stability of lithium-ion batteries.

[0008] In summary, the lithium iron phosphate material provided by this invention exhibits a concentration gradient of different elements along the radial direction. This allows it to maintain the electrochemical activity of the lithium iron phosphate material while reducing the interfacial impedance between the material and the electrolyte, thereby improving the lithium-ion transport efficiency and electronic conductivity, and achieving synergistic optimization of bulk electron conduction and surface ion transport. Applying this lithium iron phosphate material to lithium-ion batteries can improve their rate performance and cycle performance, and extend their service life.

[0009] Furthermore, the lithium iron phosphate material has a one-dimensional nanofiber structure with an average diameter of 100–300 nm in its radial cross-section; preferably, the core layer has a diameter of 30–100 nm; the intermediate layer has a thickness of 15–40 nm; and the total thickness of the shell and coating layer is 20–60 nm.

[0010] Compared to other ranges, limiting the average diameter of the radial cross section and the thickness of each layer of lithium iron phosphate material within the above range is beneficial for the lithium iron phosphate material to gradually transition from the high iron element concentration inside to the high lithium element concentration outside more stably, which is beneficial for improving the lithium ion diffusion rate, electrical conductivity and mechanical strength of lithium iron phosphate material.

[0011] Furthermore, the specific surface area of ​​lithium iron phosphate materials is 30–60 cm². 2 / g; the ratio of the weight of the core layer, the weight of the intermediate layer, to the total weight of the shell and the covering layer is 1:(1.5~2.5):(2.5~4).

[0012] Compared to other ranges, limiting the specific surface area and weight ratio of each layer of lithium iron phosphate material within the above range is beneficial to improving the electrochemical activity of lithium iron phosphate material, and also beneficial to improving the lithium-ion diffusion rate, conductivity and mechanical strength of lithium iron phosphate material.

[0013] Furthermore, the ratio of the total molar concentration of lithium in the shell and coating layers, the molar concentration of lithium in the intermediate layer, and the molar concentration of lithium in the core layer is (1.05~1.2):(1~1.02):1; the ratio of the total molar concentration of iron in the shell and coating layers, the molar concentration of iron in the intermediate layer, and the molar concentration of iron in the core layer is 0.98:(0.98~1.02):(1.1~1.2).

[0014] Compared to other ranges, limiting the ratio of the molar concentration of lithium to the molar concentration of iron in each layer of lithium iron phosphate material to the above range is beneficial to forming a concentration gradient of lithium increasing from the inside to the outside in the radial direction, and a concentration gradient of iron decreasing from the inside to the outside in the radial direction. This is beneficial to improving the lithium-ion diffusion rate and electronic conductivity of lithium iron phosphate material, reducing charge transfer resistance, and thus improving the structural stability of lithium iron phosphate material, and improving the rate performance and cycle stability of lithium-ion batteries.

[0015] To achieve the above objectives, another aspect of the present invention provides a method for preparing the lithium iron phosphate material with the above-mentioned nanofiber structure provided by the present invention. The method includes: step S1, mixing a first lithium source, a first iron source, a first phosphorus source, a first polymer, and a first solvent to obtain a first spinning solution; mixing a second lithium source, a second iron source, a second phosphorus source, a second polymer, and a second solvent to obtain a second spinning solution; mixing a third lithium source, a third iron source, a third phosphorus source, a third polymer, and a third solvent to obtain a third spinning solution; wherein the concentration of lithium in the first spinning solution is equal to or lower than that in the second spinning solution. The concentration of lithium in the first spinning solution is lower than that in the third spinning solution; the concentration of iron in the first spinning solution is higher than that in the second spinning solution, and the concentration of iron in the second spinning solution is equal to or higher than that in the third spinning solution; Step S2: The first, second, and third spinning solutions are coaxially electrospun, and the precursor is obtained after drying; Step S3: The precursor is pre-oxidized to obtain a pre-oxidized product; Step S4: The pre-oxidized product is annealed under a protective gas atmosphere to obtain lithium iron phosphate material.

[0016] The lithium iron phosphate material prepared by the method described in this application exhibits a radial concentration gradient of different elements. This gradient can reduce the interfacial impedance between the lithium iron phosphate material and the electrolyte while maintaining the electrochemical activity of the lithium iron phosphate material, thereby improving the lithium-ion transport efficiency and electronic conductivity, and achieving synergistic optimization of bulk electron conduction and surface ion transport. Applying this lithium iron phosphate material to lithium-ion batteries can improve the rate performance and cycle performance of lithium-ion batteries, and extend their service life.

[0017] Furthermore, the first lithium source, the second lithium source, and the third lithium source are each independently selected from one or more of the group consisting of lithium hydroxide, lithium carbonate, lithium acetate, lithium nitrate, and lithium dihydrogen phosphate; the first iron source, the second iron source, and the third iron source are each independently selected from one or more of the group consisting of ferrous nitrate, ferric nitrate, ferrous chloride, ferric chloride, ferrous acetate, ferric acetate, ferric ammonium citrate, and iron oxide; and the first phosphorus source, the second phosphorus source, and the third phosphorus source are each independently selected from one or more of the group consisting of phosphoric acid, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, ammonium pyrophosphate, and ammonium hypophosphite.

[0018] Compared to other types, limiting the lithium, iron, and phosphorus sources to the above-mentioned ranges is beneficial to improving the crystallization performance of lithium iron phosphate materials, enhancing the electrochemical activity and structural stability of the prepared lithium iron phosphate materials, thereby improving the rate performance and cycle stability of lithium-ion batteries.

[0019] Furthermore, in the first spinning solution, the molar ratio of lithium in the first lithium source, iron in the first iron source, and phosphorus in the first phosphorus source is 1:(1.1~1.2):1; the weight ratio of the first polymer to the first solvent is (4~6):100; the weight ratio of the first polymer to the first lithium source is (0.95~1):1; the viscosity of the first polymer is 400~600mPa·s; preferably, the first polymer is selected from polyethylene oxide and / or polyvinyl alcohol; and the first solvent is selected from water and / or ethanol.

[0020] Compared to other ranges and types, limiting the molar ratio of lithium, iron and phosphorus, the weight ratio of the first polymer to the first solvent, the weight ratio of the first polymer to the first lithium source, the viscosity of the first polymer, and the types of the first polymer and the first solvent in the first spinning solution to the above ranges is beneficial to promoting the formation of a core layer with a higher iron concentration in the first spinning solution during subsequent preparation. This is beneficial to improving the conductivity and structural stability of lithium iron phosphate materials, and in turn, to improving the rate performance and cycle stability of lithium-ion batteries.

[0021] Further, in the second spinning solution, the molar ratio of lithium in the second lithium source, iron in the second iron source, and phosphorus in the second phosphorus source is 1:(0.98~1.02):1; the weight ratio of the second polymer to the second solvent in the second spinning solution is (6~10):100; the weight ratio of the second polymer to the second lithium source is (1.2~1.5):1; the viscosity of the second polymer is 1200~1800 mPa·s; preferably, the second polymer is selected from one or more of the group consisting of polyacrylonitrile and / or cellulose acetate; the second solvent is selected from N,N-dimethylformamide and / or dimethyl sulfoxide.

[0022] Compared to other ranges and types, limiting the molar ratio of lithium, iron and phosphorus, the weight ratio of the second polymer to the second solvent, the weight ratio of the second polymer to the second lithium source, the viscosity of the second polymer, and the types of the second polymer and the second solvent in the second spinning solution to the above ranges is beneficial to promoting the formation of an intermediate layer in the second spinning solution during subsequent preparation. This is beneficial to improving the ion diffusion rate, conductivity and structural stability of lithium iron phosphate materials, and thus beneficial to improving the rate performance and cycle stability of lithium-ion batteries.

[0023] Furthermore, in the third spinning solution, the molar ratio of lithium in the third lithium source, iron in the third iron source, and phosphorus in the third phosphorus source is (1.05–1.2):0.98:1; the weight ratio of the third polymer to the third solvent in the third spinning solution is (10–15):100; the weight ratio of the third polymer to the third lithium source is (1.5–2):1; the viscosity of the third polymer is 1800–2200 mPa·s; preferably, the third polymer is selected from polyvinylpyrrolidone and / or polymethyl methacrylate; the third solvent is selected from one or more of the group consisting of ethanol, water, and N,N-dimethylformamide.

[0024] Compared to other ranges and types, limiting the molar ratio of lithium, iron and phosphorus in the third spinning solution, the weight ratio of the third polymer to the third solvent, the weight ratio of the third polymer to the third lithium source, the viscosity of the third polymer, and the types of the third polymer and the third solvent to the above ranges is beneficial for forming a shell while promoting the formation of Li4P2O7 from the excess lithium and phosphorus in the third spinning solution, thus forming a coating layer. This is beneficial for improving the lithium-ion diffusion rate and structural stability of lithium iron phosphate materials, and consequently for improving the rate performance and cycle stability of lithium-ion batteries.

[0025] Further, in step S2, a triaxial coaxial electrospinning device is used for coaxial electrospinning; during the coaxial electrospinning process, the spinning voltage is 18-25kV, the receiving distance is 15-20cm, the consumption rate of the first spinning solution is 0.2-0.4mL / h, the consumption rate of the second spinning solution is 0.4-0.8mL / h, and the consumption rate of the third spinning solution is 0.8-1.2mL / h.

[0026] Compared to other ranges, limiting the spinning voltage, receiving distance, and consumption rates of the first, second, and third spinning solutions in the coaxial electrospinning process to the above ranges is beneficial to improving the stability of the coaxial electrospinning process, and to obtaining lithium iron phosphate materials with more suitable layer thicknesses. This facilitates a more stable transition from the high iron element concentration inside to the high lithium element concentration outside, thereby improving the lithium-ion diffusion rate, conductivity, and mechanical strength of the lithium iron phosphate material.

[0027] Furthermore, in step S2, the drying temperature is 70–90°C, and the drying time is 1–2 hours.

[0028] Compared to other ranges, limiting the drying temperature and time to the above range is beneficial for removing residual solvent components in the precursor and suppressing the deformation of lithium iron phosphate materials due to moisture caused by solvent residue.

[0029] Furthermore, in step S3, the pre-oxidation temperature is 200–250°C, and the time is 1–3 hours.

[0030] Compared to other ranges, limiting the pre-oxidation temperature and time to the above range is beneficial in two ways: firstly, it helps to remove some of the solvents and polymers remaining in the coaxial electrospinning process, and inhibits their volatilization during subsequent annealing, which would lead to the formation of pores inside the lithium iron phosphate material; secondly, it helps to maintain the original structure of one-dimensional nanofibers, and inhibits their collapse and fusion due to excessive temperature, thereby improving the mechanical strength of the prepared lithium iron phosphate material.

[0031] Further, in step S4, the heating rate of the annealing treatment is 2-5℃ / min, the temperature is 700-850℃, and the time is 10-20h; the protective gas is selected from one or more of the group consisting of nitrogen, inert gas and reducing gas, preferably one or more of the group consisting of nitrogen, helium and hydrogen.

[0032] Compared to other ranges, limiting the heating rate, temperature, time, and type of protective gas in the annealing process to the above ranges is beneficial to suppressing side reactions such as oxidation during the annealing process, improving the electrochemical activity of lithium iron phosphate materials, and also improving the structural stability of lithium iron phosphate materials, suppressing fiber structure collapse or fusion caused by excessively high temperatures, and abnormal growth of the lithium iron phosphate phase.

[0033] Another aspect of the present invention provides a positive electrode, which is obtained by molding the above-mentioned lithium iron phosphate material provided by the present invention.

[0034] The lithium iron phosphate material provided by this invention has high lithium-ion transport efficiency and electronic conductivity, as well as good mechanical properties. After molding, it can form a self-supporting positive electrode, thereby effectively improving the structural stability and electrochemical performance of the self-supporting positive electrode. Applying the above-mentioned self-supporting positive electrode to lithium-ion batteries can improve the rate performance and cycle performance of lithium-ion batteries and extend their service life.

[0035] A fourth aspect of the present invention also provides a lithium-ion battery, comprising a positive electrode, a negative electrode, an electrolyte, and a separator disposed between the positive electrode and the negative electrode, wherein the positive electrode is the aforementioned positive electrode provided by the present invention.

[0036] The positive electrode provided by the present invention is obtained by molding the lithium iron phosphate material with nanofiber structure provided by the present invention. The lithium iron phosphate material provided by the present invention has high lithium-ion transport efficiency and conductivity, low interface impedance and good structural stability. It is made into a self-supporting positive electrode by molding process, and the application of the self-supporting positive electrode in lithium-ion batteries can improve the rate performance and cycle performance of lithium-ion batteries and extend their service life. Attached Figure Description

[0037] 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:

[0038] Figure 1 A cross-sectional structural diagram of lithium iron phosphate material in a preferred embodiment of this application is shown;

[0039] Figure 2 The SEM image of the lithium iron phosphate material prepared in Example 1 of this application is shown;

[0040] Figure 3 The charge-discharge curve of the coin cell assembled in Embodiment 1 of this application is shown.

[0041] The above figures include the following reference numerals:

[0042] 100, core layer; 200, intermediate layer; 300, shell layer; 400, covering layer. Detailed Implementation

[0043] 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 embodiments.

[0044] As described in the background section, existing lithium iron phosphate cathode materials suffer from low lithium-ion diffusion rates and low electronic conductivity, resulting in poor rate performance and cycle performance of lithium-ion batteries. To address these technical problems, the first aspect of this application provides a lithium iron phosphate material with a nanofiber structure, such as... Figure 1 As shown, the lithium iron phosphate material comprises, from the inside out, a core layer 100, an intermediate layer 200, a shell layer 300, and a coating layer 400. The lithium concentration in the core layer 100 is equal to or lower than the lithium concentration in the intermediate layer 200, and the lithium concentration in the intermediate layer 200 is lower than the total lithium concentration in the shell layer 300 and the coating layer 400. The iron concentration in the core layer 100 is higher than the iron concentration in the intermediate layer 200, and the iron concentration in the intermediate layer 200 is equal to or higher than the total iron concentration in the shell layer 300 and the coating layer 400. The coating layer 400 is a lithium pyrophosphate coating layer.

[0045] like Figure 1As shown, the lithium iron phosphate material with a nanofiber structure provided in this application comprises, from the inside out, a core layer 100, an intermediate layer 200, a shell layer 300, and a coating layer 400. The lithium concentration in the core layer 100 is equal to or lower than the lithium concentration in the intermediate layer 200, while the sum of the lithium concentrations in the shell layer 300 and the coating layer 400 is higher than the lithium concentration in the intermediate layer 200. The iron concentration in the core layer 100 is higher than the iron concentration in the intermediate layer 200, while the iron concentration in the intermediate layer 200 is equal to or higher than the sum of the iron concentrations in the shell layer 300 and the coating layer 400. The elemental distribution in the aforementioned lithium iron phosphate material results in a high total lithium concentration and a low total iron concentration in the shell layer 300 and the coating layer 400. The excess lithium interacts with phosphorus to form lithium pyrophosphate (Li4P2O7), which forms the coating layer 400. This improves the structural stability of the lithium iron phosphate material and provides more migration pathways for lithium ions, promoting rapid diffusion within the material and increasing transport efficiency during charge and discharge. This, in turn, effectively enhances the rate performance and cycle stability of the lithium-ion battery. The high iron concentration in the core layer 100 increases electron conduction pathways, improves the electronic conductivity of the lithium iron phosphate material, reduces charge transfer resistance, lowers electrochemical polarization during charge and discharge, and reduces iron ion dissolution and lithium ion loss. This further enhances the structural stability of the lithium iron phosphate material and improves the rate performance and cycle stability of the lithium-ion battery. The presence of the intermediate layer 200 enables the lithium iron phosphate material to gradually transition from a high iron element concentration inside to a high lithium element concentration outside, forming a continuous lithium-ion transport path. At the same time, it alleviates the stress caused by the rapid change in element concentration inside and outside the lithium iron phosphate material, improving the structural stability of the lithium iron phosphate material. On the other hand, it can maintain the electrochemical activity of the lithium iron phosphate material during the charging and discharging process, thereby improving the structural stability, rate performance and cycle stability of the lithium-ion battery.

[0046] In summary, the lithium iron phosphate material provided in this application exhibits a concentration gradient of different elements along the radial direction. This allows for the reduction of interfacial impedance between the lithium iron phosphate material and the electrolyte while maintaining the electrochemical activity of the material. This, in turn, improves the lithium-ion transport efficiency and electronic conductivity of the lithium iron phosphate material, achieving synergistic optimization of bulk electron conduction and surface ion transport. Applying this lithium iron phosphate material to lithium-ion batteries can improve their rate performance and cycle performance, and extend their service life.

[0047] One-dimensional nanofiber structures refer to fibrous materials with nanoscale dimensions that are much larger in one dimension (length) than the other two dimensions. In a preferred embodiment, the lithium iron phosphate material has a one-dimensional nanofiber structure with an average radial cross-sectional diameter of 100–300 nm. The average radial cross-sectional diameter of the lithium iron phosphate material includes, but is not limited to, the above range. Limiting it to this range helps to shorten the diffusion path of lithium ions and the transport path of electrons, thereby improving the lithium ion diffusion rate and conductivity of the lithium iron phosphate material. It also helps to improve the mechanical strength of the lithium iron phosphate material, thus contributing to improved structural stability, rate performance, and cycle performance of lithium-ion batteries.

[0048] In order to make the lithium iron phosphate material more stably transition from the high iron element concentration inside to the high lithium element concentration outside, thereby further improving the lithium-ion diffusion rate, conductivity and mechanical strength of the lithium iron phosphate material, and further improving the structural stability, rate performance and cycle performance of lithium-ion batteries, preferably, the diameter of the core layer 100 is 30-100 nm; the thickness of the intermediate layer 200 is 15-40 nm; and the total thickness of the shell layer 300 and the coating layer 400 is 20-60 nm.

[0049] In a preferred embodiment, the weight ratio of the core layer 100, the intermediate layer 200, to the sum of the weights of the shell layer 300 and the covering layer 400 is 1:(1.5-2.5):(2.5-4). Compared to other ranges, limiting the weight ratio of the core layer 100, the intermediate layer 200, to the sum of the weights of the shell layer 300 and the covering layer 400 to the above range is beneficial for obtaining lithium iron phosphate materials with more suitable thicknesses for each layer. This promotes a more stable transition from a high iron element concentration inside to a high lithium element concentration outside, thereby improving the lithium-ion diffusion rate, conductivity, and mechanical strength of the lithium iron phosphate material, and consequently improving the structural stability, rate performance, and cycle performance of the lithium-ion battery.

[0050] In a preferred embodiment, the specific surface area of ​​the lithium iron phosphate material is 30–60 cm². 2 / g. The specific surface area of ​​lithium iron phosphate materials includes, but is not limited to, the range mentioned above. Limiting it to the range mentioned above is beneficial to improving the electrochemical activity of lithium iron phosphate materials, improving the lithium-ion diffusion rate and conductivity of lithium iron phosphate materials, thereby improving the rate performance and cycle performance of lithium-ion batteries.

[0051] In order to form a lithium concentration gradient that increases radially from the inside to the outside, and to promote the formation of Li4P2O7 from excess lithium and phosphorus to form the coating layer 400, thereby further improving the lithium-ion diffusion rate and structural stability of the lithium iron phosphate material, and further improving the rate performance and cycle stability of the lithium-ion battery, in a preferred embodiment, the ratio of the total molar concentration of lithium in the shell layer 300 and the coating layer 400, the molar concentration of lithium in the intermediate layer 200 and the molar concentration of lithium in the core layer 100 is (1.05~1.2):(1~1.02):1.

[0052] In a preferred embodiment, the ratio of the total molar concentration of iron in the shell layer 300 and the coating layer 400, the molar concentration of iron in the intermediate layer 200, and the molar concentration of iron in the core layer 100 is 0.98:(0.98~1.02):(1.1~1.2). Compared to other ranges, limiting the ratio of the total molar concentration of iron in the shell layer 300 and the coating layer 400, the molar concentration of iron in the intermediate layer 200, and the molar concentration of iron in the core layer 100 to the above range is beneficial to forming a radially decreasing concentration gradient of iron from the inside to the outside, resulting in a higher concentration of iron in the core layer 100. This increases the electron conduction path, improves the electronic conductivity of the lithium iron phosphate material, and reduces the charge transfer resistance, thereby improving the structural stability of the lithium iron phosphate material and enhancing the rate performance and cycle stability of the lithium-ion battery.

[0053] A second aspect of this application also provides a method for preparing the lithium iron phosphate material with the nanofiber structure described above. The method includes: step S1, mixing a first lithium source, a first iron source, a first phosphorus source, a first polymer, and a first solvent to obtain a first spinning solution; mixing a second lithium source, a second iron source, a second phosphorus source, a second polymer, and a second solvent to obtain a second spinning solution; and mixing a third lithium source, a third iron source, a third phosphorus source, a third polymer, and a third solvent to obtain a third spinning solution; wherein the concentration of lithium in the first spinning solution is equal to or lower than that in the second spinning solution. The lithium concentration in the second spinning solution is lower than that in the third spinning solution; the iron concentration in the first spinning solution is higher than that in the second spinning solution, and the iron concentration in the second spinning solution is equal to or higher than that in the third spinning solution; Step S2: The first, second, and third spinning solutions are coaxially electrospun, and the precursor is obtained after drying; Step S3: The precursor is pre-oxidized to obtain a pre-oxidized product; Step S4: The pre-oxidized product is annealed under a protective gas atmosphere to obtain lithium iron phosphate material.

[0054] The method for preparing the lithium iron phosphate material provided in this application can prepare a first spinning solution, a second spinning solution, and a third spinning solution with specific lithium and iron concentrations in step S1, thereby facilitating subsequent coaxial electrospinning. In step S2, the first spinning solution, the second spinning solution, and the third spinning solution are coaxially electrospun, with the first spinning solution in the innermost layer, the second spinning solution in the middle layer, and the third spinning solution in the outermost layer, thereby obtaining a fiber structure with the same axis and a specific structure. After drying, a precursor is obtained. In step S3, the precursor is pre-oxidized to remove residual first, second, and third polymers, providing a pre-oxidized product with higher purity for subsequent annealing. This also improves the structural stability of the pre-oxidized product, inhibiting the collapse of its fiber structure during subsequent annealing, thereby improving the mechanical strength of the obtained lithium iron phosphate material. In step S4, the pre-oxidized product is annealed under a protective gas atmosphere. This process can suppress side reactions such as oxidation, improve the crystallinity of the lithium iron phosphate phase, and obtain a purer and more stable lithium iron phosphate material. At the same time, it can improve the electrochemical activity of the lithium iron phosphate material.

[0055] In summary, the lithium iron phosphate material prepared by the method described in this application exhibits a radial concentration gradient of different elements. This gradient can reduce the interfacial impedance between the lithium iron phosphate material and the electrolyte while maintaining the electrochemical activity of the lithium iron phosphate material, thereby improving the lithium-ion transport efficiency and electronic conductivity, and achieving synergistic optimization of bulk electron conduction and surface ion transport. Applying this lithium iron phosphate material to lithium-ion batteries can improve the rate performance and cycle performance of lithium-ion batteries, and extend their service life.

[0056] In a preferred embodiment, the first, second, and third lithium sources each independently include, but are not limited to, one or more of the group consisting of lithium hydroxide, lithium carbonate, lithium acetate, lithium nitrate, and lithium dihydrogen phosphate. Compared to other types, using the above-mentioned lithium sources is beneficial to improving the electrochemical activity and structural stability of the prepared lithium iron phosphate material, thereby improving the rate performance and cycle stability of lithium-ion batteries.

[0057] In a preferred embodiment, the first, second, and third iron sources each independently include, but are not limited to, one or more of the group consisting of ferrous nitrate, ferric nitrate, ferrous chloride, ferric chloride, ferrous acetate, ferric acetate, ferric ammonium citrate, and iron oxide. Compared to other types, using the above-mentioned iron sources is beneficial for improving the crystallinity of lithium iron phosphate materials, obtaining lithium iron phosphate phases with higher purity, and improving the electrochemical activity and structural stability of lithium iron phosphate materials, thereby contributing to improved rate performance and cycle stability of lithium-ion batteries.

[0058] In order to obtain a lithium iron phosphate phase with higher crystallinity, further improve the electrochemical activity and structural stability of lithium iron phosphate materials, and further improve the rate performance and cycle stability of lithium-ion batteries, in a preferred embodiment, the first phosphorus source, the second phosphorus source and the third phosphorus source are each independently including but not limited to one or more of the group consisting of phosphoric acid, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, ammonium pyrophosphate and ammonium hypophosphite.

[0059] In a preferred embodiment, the molar ratio of lithium in the first lithium source, iron in the first iron source, and phosphorus in the first phosphorus source in the first spinning solution is 1:(1.1 to 1.2):1. Compared to other ranges, limiting the molar ratio of lithium in the first lithium source, iron in the first iron source, and phosphorus in the first phosphorus source to the above range is beneficial for promoting the formation of a core layer 100 with a higher iron concentration in the first spinning solution during subsequent preparation processes, thereby improving the conductivity and structural stability of the lithium iron phosphate material, and thus improving the rate performance and cycle stability of the lithium-ion battery.

[0060] In order to improve the viscosity and fluidity of the first spinning solution, thereby improving the stability of the subsequent coaxial electrospinning process and promoting the formation of a core layer 100 with a higher iron concentration in the first spinning solution during the subsequent preparation process, in a preferred embodiment, the weight ratio of the first polymer to the first solvent in the first spinning solution is (4-6):100.

[0061] In a preferred embodiment, the weight ratio of the first polymer to the first lithium source is (0.95 to 1):1. This weight ratio includes, but is not limited to, the range described above. Limiting it to this range helps improve the viscosity and flowability of the first spinning solution, enhances the stability of the subsequent coaxial electrospinning process, and yields a core layer 100 with a higher iron concentration, thereby improving the conductivity and structural stability of the lithium iron phosphate material.

[0062] In a preferred embodiment, the viscosity of the first polymer is 400–600 mPa·s. The viscosity of the first polymer includes, but is not limited to, the above range. Limiting it to this range is beneficial for improving the viscosity and flowability of the first spinning solution, enhancing the stability of the subsequent coaxial electrospinning process, forming a continuous and stable nanofiber structure, and also for improving the mechanical strength of the obtained lithium iron phosphate material.

[0063] To further improve the viscosity and flowability of the first spinning solution, further enhance the stability of the subsequent coaxial electrospinning process, form a more continuous and stable nanofiber structure, and further improve the mechanical strength of the obtained lithium iron phosphate material, preferably, the first polymer includes, but is not limited to, polyethylene oxide (PEO) and / or polyvinyl alcohol (PVA).

[0064] In order to improve the dispersion uniformity of the materials in the first spinning solution and obtain a more uniform and stable first spinning solution, thereby facilitating subsequent coaxial electrospinning, in a preferred embodiment, the first solvent includes, but is not limited to, water and / or ethanol.

[0065] In a preferred embodiment, the molar ratio of lithium in the second lithium source, iron in the second iron source, and phosphorus in the second phosphorus source in the second spinning solution is 1:(0.98–1.02):1. Compared to other ranges, limiting the molar ratio of lithium in the second lithium source, iron in the second iron source, and phosphorus in the second phosphorus source to the above range is beneficial for promoting the formation of an intermediate layer 200 in the second spinning solution during subsequent preparation, improving the ion diffusion rate, conductivity, and structural stability of the lithium iron phosphate material, and thus improving the rate performance and cycle stability of the lithium-ion battery.

[0066] In order to improve the viscosity and flowability of the second spinning solution, thereby improving the stability of the subsequent coaxial electrospinning process and promoting the formation of an intermediate layer 200 in the second spinning solution during the subsequent preparation process, in a preferred embodiment, the weight ratio of the second polymer to the second solvent in the second spinning solution is (6-10):100.

[0067] In a preferred embodiment, the weight ratio of the second polymer to the second lithium source is (1.2 to 1.5):1. The weight ratio of the second polymer to the second lithium source includes, but is not limited to, the above range. Limiting it to the above range is beneficial to improving the viscosity and flowability of the second spinning solution, improving the stability of the subsequent coaxial electrospinning process, and obtaining the intermediate layer 200, thereby improving the ion diffusion rate, conductivity, and structural stability of the lithium iron phosphate material.

[0068] In a preferred embodiment, the viscosity of the second polymer is 1200–1800 mPa·s. The viscosity of the second polymer includes, but is not limited to, the above range. Limiting it to this range is beneficial for improving the viscosity and flowability of the second spinning solution, enhancing the stability of the subsequent coaxial electrospinning process, forming a continuous and stable nanofiber structure, and also for improving the mechanical strength of the obtained lithium iron phosphate material.

[0069] To further improve the viscosity and flowability of the second spinning solution, further enhance the stability of the subsequent coaxial electrospinning process, form a more continuous and stable nanofiber structure, and further improve the mechanical strength of the obtained lithium iron phosphate material, preferably, the second polymer includes, but is not limited to, polyacrylonitrile (PAN) and / or cellulose acetate (CA).

[0070] In a preferred embodiment, the second solvent includes, but is not limited to, N,N-dimethylformamide (DMF) and / or dimethyl sulfoxide (DMSO). The types of second solvents include, but are not limited to, those described above. Using such second solvents helps improve the dispersion uniformity of materials in the second spinning solution, facilitating the obtaining of a more uniform and stable second spinning solution. This, in turn, improves the stability of subsequent coaxial electrospinning, resulting in a more continuous and stable nanofiber structure.

[0071] In a preferred embodiment, the molar ratio of lithium in the third lithium source, iron in the third iron source, and phosphorus in the third phosphorus source in the third spinning solution is (1.05–1.2):0.98:1. Compared to other ranges, limiting the molar ratio of lithium in the third lithium source, iron in the third iron source, and phosphorus in the third phosphorus source to the above range is beneficial for forming the shell layer 300 while simultaneously promoting the formation of Li4P2O7 from excess lithium and phosphorus in the third spinning solution, thus forming the coating layer 400. This is beneficial for improving the lithium-ion diffusion rate and structural stability of the lithium iron phosphate material, and consequently for improving the rate performance and cycle stability of the lithium-ion battery.

[0072] In order to improve the viscosity and flowability of the third spinning solution, thereby enhancing the stability of the subsequent coaxial electrospinning process and promoting the formation of a shell layer 300 and a coating layer 400 in the third spinning solution during the subsequent preparation process, in a preferred embodiment, the weight ratio of the third polymer to the third solvent in the third spinning solution is (10-15):100.

[0073] In a preferred embodiment, the weight ratio of the third polymer to the third lithium source is (1.5–2):1. This weight ratio includes, but is not limited to, the range described above. Limiting it to this range is beneficial for improving the viscosity and flowability of the third spinning solution, enhancing the stability of the subsequent coaxial electrospinning process, and thus improving the lithium-ion diffusion rate and structural stability of the lithium iron phosphate material.

[0074] In a preferred embodiment, the viscosity of the third polymer is 1800–2200 mPa·s. The viscosity of the third polymer includes, but is not limited to, the above range. Limiting it to this range is beneficial for improving the viscosity and flowability of the third spinning solution, enhancing the stability of the subsequent coaxial electrospinning process, forming a continuous and stable nanofiber structure, and also for improving the mechanical strength of the obtained lithium iron phosphate material.

[0075] To further improve the viscosity and flowability of the third spinning solution, further enhance the stability of the subsequent coaxial electrospinning process, form a more continuous and stable nanofiber structure, and further improve the mechanical strength of the obtained lithium iron phosphate material, preferably, the third polymer includes, but is not limited to, polyvinylpyrrolidone (PVP) and / or polymethyl methacrylate (PMMA).

[0076] In order to improve the dispersion uniformity of the materials in the third spinning solution and obtain a more uniform and stable third spinning solution, thereby facilitating subsequent coaxial electrospinning, in a preferred embodiment, the third solvent includes, but is not limited to, one or more of the group consisting of ethanol, water and N,N-dimethylformamide (DMF).

[0077] The triaxial coaxial electrospinning apparatus has three concentric nozzles, enabling it to process three different materials simultaneously, and is suitable for preparing multilayer fiber materials. In a preferred embodiment, in step S2, coaxial electrospinning is performed using the triaxial coaxial electrospinning apparatus. Compared to other equipment, coaxial electrospinning using the triaxial coaxial electrospinning apparatus can prepare lithium iron phosphate materials with the specific structure described above in this application.

[0078] To obtain lithium iron phosphate materials with more suitable layer thicknesses, and to facilitate a more stable transition from a high iron concentration internally to a high lithium concentration externally, thereby further improving the lithium-ion diffusion rate, conductivity, and mechanical strength of the lithium iron phosphate material, and further enhancing the structural stability, rate performance, and cycle performance of lithium-ion batteries, in a preferred embodiment, the consumption rate of the first spinning solution is 0.2–0.4 mL / h; the consumption rate of the second spinning solution is 0.4–0.8 mL / h; and the consumption rate of the third spinning solution is 0.8–1.2 mL / h.

[0079] In a preferred embodiment, during coaxial electrospinning, the spinning voltage is 18–25 kV and the receiving distance is 15–20 cm. The spinning voltage and receiving distance during coaxial electrospinning include, but are not limited to, the above ranges. Limiting them within these ranges helps improve the stability of the coaxial electrospinning process, resulting in a continuous and stable nanofiber structure.

[0080] In order to remove residual solvent components in the precursor, improve the stability of subsequent preparation processes, and suppress the deformation of lithium iron phosphate materials due to moisture caused by solvent residue, in a preferred embodiment, the drying temperature in step S2 is 70-90°C and the time is 1-2 hours.

[0081] In a preferred embodiment, in step S3, the pre-oxidation temperature is 200–250°C, and the time is 1–3 hours. The pre-oxidation temperature and time include, but are not limited to, the above ranges. Limiting them to these ranges helps to remove some of the solvents and polymers remaining from the coaxial electrospinning process, reduce volatiles in the subsequent annealing process, and suppress the formation of pores inside the lithium iron phosphate material. On the other hand, it helps to maintain the original structure of the one-dimensional nanofibers, suppressing their collapse and fusion due to excessively high temperatures, thereby improving the mechanical strength of the obtained lithium iron phosphate material.

[0082] In order to suppress the abnormal growth of lithium iron phosphate phase caused by excessive heating rate and to suppress the collapse of its nanofiber structure, in a preferred embodiment, the heating rate of the annealing treatment in step S4 is 2 to 5 °C / min.

[0083] In a preferred embodiment, the annealing temperature is 700–850°C, and the time is 10–20 hours. The annealing temperature and time include, but are not limited to, the above range. Limiting them to the above range is beneficial for improving the crystallinity of the lithium iron phosphate phase, improving the electrochemical activity of the lithium iron phosphate material, and also beneficial for improving the structural stability of the lithium iron phosphate material, suppressing the collapse or fusion of fiber structure and abnormal growth of lithium iron phosphate phase caused by excessively high temperature.

[0084] In order to suppress side reactions such as oxidation during the annealing process and further improve the purity of lithium iron phosphate materials, in a preferred embodiment, the protective gas includes, but is not limited to, one or more of the group consisting of nitrogen, inert gas and reducing gas; preferably, the protective gas includes, but is not limited to, one or more of the group consisting of nitrogen, helium and hydrogen.

[0085] A third aspect of this application also provides a positive electrode, which is formed from the lithium iron phosphate material with the nanofiber structure described above. The lithium iron phosphate material provided above has a different elemental concentration gradient along the radial direction, which can reduce the interfacial impedance between the lithium iron phosphate material and the electrolyte while ensuring the electrochemical activity of the lithium iron phosphate material, thereby improving the lithium-ion transport efficiency and electronic conductivity of the lithium iron phosphate material. This achieves synergistic optimization of bulk electron conduction and surface ion transport. Simultaneously, the lithium iron phosphate material also has good mechanical properties, and after forming, it can form a self-supporting positive electrode, which can improve the structural stability and electrochemical performance of the self-supporting positive electrode. Applying the self-supporting positive electrode to lithium-ion batteries can improve the rate performance and cycle performance of lithium-ion batteries and extend their service life.

[0086] In order to improve the mechanical properties and dimensional consistency of the obtained positive electrode, in a preferred embodiment, a stamping method is used for forming.

[0087] A fourth aspect of this application also provides a lithium-ion battery, including a positive electrode, a negative electrode, an electrolyte, and a separator disposed between the positive and negative electrodes, wherein the positive electrode is the aforementioned positive electrode provided in this application. The aforementioned positive electrode is formed from the aforementioned lithium iron phosphate material with a nanofiber structure provided in this application. The aforementioned lithium iron phosphate material provided in this application has high lithium-ion transport efficiency and conductivity, low interfacial impedance, and good structural stability. By forming it into a self-supporting positive electrode through a molding process, and applying this self-supporting positive electrode in a lithium-ion battery, the rate performance and cycle performance of the lithium-ion battery can be improved, and its service life can be extended.

[0088] 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.

[0089] Example 1

[0090] A method for preparing lithium iron phosphate material specifically includes the following steps:

[0091] (1) Mix LiOH, FeNO3, H3PO4, polyethylene oxide (PEO, with a weight-average molecular weight of 1 million and a viscosity of 500 mPa·s) with water to obtain the first spinning solution; wherein, the molar ratio of Li in LiOH, Fe in FeNO3 and P in H3PO4 is 1:1.15:1, the weight ratio of PEO to water is 5:100, and the weight ratio of PEO to LiOH is 1:1;

[0092] (2) LiOH, FeNO3, H3PO4, polyacrylonitrile (PAN, weight average molecular weight of 200,000, viscosity of 1500 mPa·s) and N,N-dimethylformamide (DMF) are mixed to obtain the second spinning solution; wherein, the molar ratio of Li element in LiOH, Fe element in FeNO3 and P element in H3PO4 is 1:1:1, the weight ratio of PAN to DMF is 8:100, and the weight ratio of PAN to LiOH is 1.4:1;

[0093] (3) Mix LiOH, FeNO3, H3PO4, polyvinylpyrrolidone (PVP, weight-average molecular weight of 1 million, viscosity of 2000 mPa·s) with ethanol to obtain the third spinning solution; wherein, the molar ratio of Li in LiOH, Fe in FeNO3 and P in H3PO4 is 1.1:0.98:1, the weight ratio of PVP to ethanol is 12:100, and the weight ratio of PVP to LiOH is 1.7:1;

[0094] (4) The first spinning solution obtained in step (1), the second spinning solution obtained in step (2), and the third spinning solution obtained in step (3) are respectively loaded into the injection pump of the triaxial coaxial electrospinning equipment. The consumption rate of the first spinning solution is set to 0.3 mL / h, the consumption rate of the second spinning solution is set to 0.5 mL / h, and the consumption rate of the third spinning solution is set to 1 mL / h. Coaxial electrospinning is performed under the conditions of 20 kV spinning voltage and receiving distance of 15 cm to obtain the precursor.

[0095] (5) The precursor obtained in step (4) was pre-oxidized for 2 hours in an air atmosphere at 200°C to obtain the pre-oxidized product.

[0096] (6) Under an inert atmosphere, the temperature is increased to 800°C at a heating rate of 5°C / min, and the pre-oxidized product obtained in step (5) is annealed at 800°C for 15 hours to obtain lithium iron phosphate material.

[0097] The lithium iron phosphate material prepared in Example 1 has a nanofiber structure, which includes, from the inside out, a core layer 100, an intermediate layer 200, a shell layer 300 and a coating layer 400.

[0098] The SEM image of the lithium iron phosphate material prepared in Example 1 is shown below. Figure 2 As shown, by Figure 2 It can be seen that lithium iron phosphate material has a nanofiber structure with uniform thickness, and its radial cross-section has an average diameter of about 200 nm.

[0099] Example 2

[0100] The difference from Example 1 is that in step (1), the molar ratio of Li element in LiOH, Fe element in FeNO3 and P element in H3PO4 is 1:1.2:1, and in step (3), the molar ratio of Li element in LiOH, Fe element in FeNO3 and P element in H3PO4 is 1.2:0.98:1; the remaining steps are the same as in Example 1.

[0101] Example 3

[0102] The difference from Example 1 is that in step (1), the molar ratio of Li in LiOH, Fe in FeNO3 and P in H3PO4 is 1:1.1:1, and in step (3), the molar ratio of Li in LiOH, Fe in FeNO3 and P in H3PO4 is 1.05:0.98:1; the remaining steps are the same as in Example 1.

[0103] Example 4

[0104] The difference from Example 1 is that in step (1), the molar ratio of Li element in LiOH, Fe element in FeNO3 and P element in H3PO4 is 1:1.3:1, and in step (3), the molar ratio of Li element in LiOH, Fe element in FeNO3 and P element in H3PO4 is 1.3:0.98:1; the remaining steps are the same as in Example 1.

[0105] Example 5

[0106] The difference from Example 1 is that in step (1), the molar ratio of Li in LiOH, Fe in FeNO3 and P in H3PO4 is 1:0.95:1, and in step (3), the molar ratio of Li in LiOH, Fe in FeNO3 and P in H3PO4 is 0.95:0.98:1; the remaining steps are the same as in Example 1.

[0107] Example 6

[0108] The difference from Example 1 is that in step (1), the weight ratio of PEO to water is 4:100, in step (2), the weight ratio of PAN to DMF is 6:100, and in step (3), the weight ratio of PVP to ethanol is 10:100; the remaining steps are the same as in Example 1.

[0109] Example 7

[0110] The difference from Example 1 is that in step (1), the weight ratio of PEO to water is 6:100, in step (2), the weight ratio of PAN to DMF is 10:100, and in step (3), the weight ratio of PVP to ethanol is 15:100; the remaining steps are the same as in Example 1.

[0111] Example 8

[0112] The difference from Example 1 is that in step (1), the weight ratio of PEO to water is 3:100, in step (2), the weight ratio of PAN to DMF is 4:100, and in step (3), the weight ratio of PVP to ethanol is 8:100; the remaining steps are the same as in Example 1.

[0113] Example 9

[0114] The difference from Example 1 is that in step (4), the consumption rates of the first spinning solution, the second spinning solution and the third spinning solution are 0.2 mL / h, 0.4 mL / h and 0.8 mL / h, respectively; the remaining steps are the same as in Example 1.

[0115] Example 10

[0116] The difference from Example 1 is that in step (4), the consumption rates of the first spinning solution, the second spinning solution and the third spinning solution are 0.4 mL / h, 0.8 mL / h and 1.2 mL / h, respectively; the remaining steps are the same as in Example 1.

[0117] Example 11

[0118] The difference from Example 1 is that in step (4), the consumption rates of the first spinning solution, the second spinning solution and the third spinning solution are 0.1 mL / h, 0.9 mL / h and 1.3 mL / h, respectively; the remaining steps are the same as in Example 1.

[0119] Example 12

[0120] The difference from Example 1 is that in step (5), the pre-oxidation temperature is 250°C and the time is 3 hours; the remaining steps are the same as in Example 1.

[0121] Example 13

[0122] The difference from Example 1 is that in step (5), the pre-oxidation temperature is 350°C and the time is 4 hours; the remaining steps are the same as in Example 1.

[0123] Example 14

[0124] The difference from Example 1 is that in step (6), the annealing temperature is 850°C and the time is 10h; the remaining steps are the same as in Example 1.

[0125] Example 15

[0126] The difference from Example 1 is that in step (6), the annealing temperature is 700°C and the time is 20h; the remaining steps are the same as in Example 1.

[0127] Example 16

[0128] The difference from Example 1 is that in step (6), the annealing temperature is 900°C and the time is 7 hours; the remaining steps are the same as in Example 1.

[0129] Example 17

[0130] The difference from Example 1 is that in step (6), the temperature is increased to 800°C at a heating rate of 10°C / min; the remaining steps are the same as in Example 1.

[0131] Comparative Example 1

[0132] The difference from Example 1 is that in step (1), the molar ratio of Li element in LiOH, Fe element in FeNO3 and P element in H3PO4 is 1:1:1, and in step (3), the molar ratio of Li element in LiOH, Fe element in FeNO3 and P element in H3PO4 is 1:1:1; the remaining steps are the same as in Example 1.

[0133] The lithium iron phosphate material prepared in Comparative Example 1 is a homogeneous lithium iron phosphate material, which does not have a concentration gradient in the radial direction.

[0134] Comparative Example 2

[0135] The difference from Example 1 is that step (5) is omitted, and the precursor obtained in step (4) is directly annealed; the remaining steps are the same as in Example 1.

[0136] The average radial cross-sectional diameter, core layer 100 diameter, intermediate layer 200 thickness, shell layer 300 thickness, and coating layer 400 thickness of the lithium iron phosphate materials prepared in all the embodiments and comparative examples of this application, as well as the specific surface area of ​​the lithium iron phosphate materials, are shown in Table 1. The specific surface area was measured using a specific surface area analyzer.

[0137] Table 1

[0138]

[0139] The lithium-ion battery was assembled as follows: (1) Preparation of positive electrode: The lithium iron phosphate material prepared in all the above examples and comparative examples was stamped. The pressure of the stamping process was 15 MPa and the holding time was 60 s. A circular piece with a diameter of 12 mm was made as the positive electrode; (2) Assembly of lithium-ion battery: The positive electrode prepared above was used as the positive electrode, and the lithium sheet was used as the counter electrode and reference electrode. A 1 mol / L lithium hexafluorophosphate electrolyte (the solvent is ethylene carbonate and diethyl carbonate, and the volume ratio is 1:1) was selected, and a polypropylene microporous membrane was used as the separator to assemble a button half cell.

[0140] Charge-discharge tests and cycle performance tests were conducted on the coin cells assembled in all the above embodiments and comparative examples. The specific test conditions are as follows:

[0141] (1) Discharge specific capacity: At 25℃, three cycles of constant current charge and discharge tests were performed sequentially at rates of 0.2C, 1C and 2C in the voltage range of 2.5 to 4.0V, and the discharge specific capacity at rates of 0.2C, 1C and 2C was recorded.

[0142] (2) Cyclic performance test: 500 cycles were performed at 50℃, 1C rate, and a voltage range of 2.5 to 4.0V. The capacity retention rate after 500 cycles was recorded.

[0143] The test results are shown in Table 2.

[0144] The charge-discharge curve of the coin half-cell assembled in Embodiment 1 of this application at a 1C rate is shown in the figure below. Figure 3 As shown, by Figure 3 It can be seen that the coin cell assembled in Example 1 has a stable charge and discharge platform, no side reaction phenomena, and no obvious polarization, indicating that the coin cell has good rate performance and cycle performance.

[0145] Table 2

[0146]

[0147] As can be seen from the above description, the embodiments of the present invention achieve the following technical effects:

[0148] Comparing Example 1 and Comparative Example 1, the lithium iron phosphate material prepared in Comparative Example 1 is a homogeneous lithium iron phosphate material, which has no concentration gradient along the radial direction. As shown in Table 2, the rate performance and cycle performance of the lithium-ion battery prepared in Comparative Example 1 are lower than those in Example 1. Therefore, compared to homogeneous lithium iron phosphate materials, the lithium iron phosphate material with a nanofiber structure provided in this application has a different elemental concentration gradient along the radial direction. This allows it to maintain the electrochemical activity of the lithium iron phosphate material while reducing the interfacial impedance between the lithium iron phosphate material and the electrolyte, improving the lithium-ion transport efficiency and electronic conductivity of the lithium iron phosphate material, and achieving synergistic optimization of bulk electron conduction and surface ion transport. This effectively improves the rate performance and cycle performance of lithium-ion batteries.

[0149] Comparing Examples 1, 12, and 13 with Comparative Example 2, where Comparative Example 2 did not undergo pre-oxidation during preparation, the pre-oxidation temperatures and times in Examples 1 and 12 were within the preferred ranges described above, while the pre-oxidation temperatures and times in Example 13 were outside the preferred ranges described above. As can be seen from Table 2, the rate performance and cycle performance of the lithium-ion battery prepared in Comparative Example 2 are significantly lower than those of Examples 1 and 12, especially its capacity retention after 500 cycles is only 34.3%. The rate performance of the lithium-ion battery prepared in Example 13 is slightly lower than that of Examples 1 and 12, but its cycle performance is significantly reduced, with a capacity retention of 77.3% after 500 cycles. Therefore, compared with other methods and scopes, pre-oxidizing the precursor and limiting the pre-oxidation temperature and time to the preferred range mentioned above can, on the one hand, remove some of the solvents and polymers remaining in the coaxial electrospinning process, reduce volatiles in the subsequent annealing process, and inhibit the formation of pores inside the lithium iron phosphate material; on the other hand, it can maintain the original structure of the one-dimensional nanofibers and inhibit their collapse and fusion due to excessive temperature, thereby improving the mechanical strength of the prepared lithium iron phosphate material, and thus significantly improving the cycle performance and rate performance of lithium-ion batteries.

[0150] Comparing Examples 1 to 5, the molar ratios of lithium, iron, and phosphorus in the first spinning solution of Examples 1 to 3 and the molar ratios of lithium, iron, and phosphorus in the second spinning solution are values ​​within the preferred ranges described above in this application, while Examples 4 and 5 are values ​​outside the limits. As can be seen from Table 2, the cycle performance and rate performance of the lithium-ion batteries prepared in Examples 4 and 5 are lower than those in Examples 1 to 3. Specifically, in Example 4, due to the imbalance in the molar ratio of lithium, iron, and phosphorus, when the iron content in the core layer 100 exceeds the standard and the lithium content in the shell layer 300 exceeds the standard, firstly, the excess iron in the core layer 100 easily generates Fe2P compounds. While these compounds promote conductivity in trace amounts, excessive amounts hinder lithium-ion transport, preventing lithium ions from freely entering and exiting. Secondly, the excessive lithium content in the shell layer 300 leads to the formation of an insulating Li3PO4 layer instead of a lithium pyrophosphate coating, thereby inhibiting electrical performance and significantly reducing the rate performance and cycle performance of Example 4. Therefore, compared to other ranges, limiting the molar ratios of lithium, iron and phosphorus in the first spinning solution and the molar ratios of lithium, iron and phosphorus in the second spinning solution to the aforementioned preferred ranges in this application is beneficial to improving the lithium-ion diffusion rate, conductivity and structural stability of lithium iron phosphate materials, thereby improving the rate performance and cycle stability of lithium-ion batteries.

[0151] Comparing Examples 1, 6 to 8, the concentrations of the first polymer in the first spinning solution, the second polymer in the second spinning solution, and the third polymer in the third spinning solution in Examples 1, 6, and 7 are all within the preferred range described above, while Example 8 is outside the range. As can be seen from Table 2, the cycling performance of Example 8 is poor. This is because the first, second, and third polymers act as carbon sources, and the concentrations of all three polymers in Example 8 are relatively low, resulting in a low carbon content after sintering. This exposes the lithium iron phosphate surface without carbon protection, leading to iron leaching and consequently reducing its cycling performance. Therefore, compared to other ranges, limiting the concentrations of the first polymer in the first spinning solution, the second polymer in the second spinning solution, and the third polymer in the third spinning solution to the aforementioned preferred ranges in this application is beneficial to improving the viscosity and flowability of the first, second, and third spinning solutions, enhancing the stability of the subsequent coaxial electrospinning process, thereby facilitating the acquisition of lithium iron phosphate materials with different elemental concentration gradients along the radial direction. This, in turn, helps to improve the conductivity and structural stability of the lithium iron phosphate material, and improves the rate performance of the resulting lithium-ion battery.

[0152] Comparing Examples 1, 9 to 11, the consumption rates of the first, second, and third spinning solutions in Examples 1, 9, and 10 are values ​​within the preferred range described above, while Example 10 is an out-of-range value. As shown in Table 2, the cycle performance and rate performance of Example 11 are lower than those of Examples 1, 9, and 10. Therefore, limiting the consumption rates of the first, second, and third spinning solutions to the preferred range described above, compared to other ranges, is beneficial for obtaining lithium iron phosphate materials with more suitable layer thicknesses. This facilitates a more stable transition from high iron element concentration internally to high lithium element concentration externally, thereby improving the lithium-ion diffusion rate, conductivity, and mechanical strength of the lithium iron phosphate material, and consequently improving the structural stability, rate performance, and cycle performance of the lithium-ion battery.

[0153] Comparing Examples 1, 14 to 17, the annealing temperature and time in Examples 1, 14 and 15 are values ​​within the preferred range described above, while the annealing temperature and time in Example 16 are values ​​outside the preferred range described above, and the heating rate in Example 17 is also outside the preferred range described above. As can be seen from Table 2, the cycle performance and rate performance of the lithium-ion batteries prepared in Examples 16 and 17 are lower than those in Examples 1, 14 and 15. In Example 16, the excessively high sintering temperature led to a significant increase in the individual size of the lithium iron phosphate particles, and the excessively high temperature caused the collapse of the lithium iron phosphate nanofiber structure, resulting in a more significant decrease in the cycle performance of the lithium-ion battery. Therefore, compared to other ranges, limiting the annealing temperature, time, and heating rate within the preferred ranges described above in this application is beneficial to improving the crystallinity of the lithium iron phosphate phase, enhancing the electrochemical activity of the lithium iron phosphate material, and also improving the structural stability of the lithium iron phosphate material. This also helps to suppress the collapse or fusion of the fiber structure and the abnormal growth of the lithium iron phosphate phase caused by excessively high temperatures, thereby improving the structural stability, rate performance, and cycle performance of the prepared lithium-ion battery.

[0154] It should be noted that the terms "first," "second," etc., used in the specification and claims of this application are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such terms can be used interchangeably where appropriate so that the embodiments of this application described herein can be implemented, for example, in a sequence other than those described herein.

[0155] 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 lithium iron phosphate material having a nanofiber structure, characterized in that, The lithium iron phosphate material comprises, from the inside out, a core layer (100), an intermediate layer (200), a shell layer (300), and a coating layer (400); wherein, the lithium concentration in the core layer (100) is equal to or lower than the lithium concentration in the intermediate layer (200), and the lithium concentration in the intermediate layer (200) is lower than the total lithium concentration in the shell layer (300) and the coating layer (400); the iron concentration in the core layer (100) is higher than the iron concentration in the intermediate layer (200), and the iron concentration in the intermediate layer (200) is equal to or higher than the total iron concentration in the shell layer (300) and the coating layer (400); the coating layer (400) is a lithium pyrophosphate coating layer.

2. The lithium iron phosphate material having a nanofibrous structure according to claim 1, wherein, The lithium iron phosphate material has a one-dimensional nanofiber structure with an average diameter of 100–300 nm in its radial cross section; The specific surface area of the lithium iron phosphate material is 30-60 m 2 / g; and / or, The ratio of the weight of the core layer (100), the weight of the intermediate layer (200), to the total weight of the shell layer (300) and the covering layer (400) is 1:(1.5-2.5):(2.5-4).

3. The lithium iron phosphate material with a nanofiber structure according to claim 1 or 2, characterized in that, The core layer (100) has a diameter of 30-100 nm; the intermediate layer (200) has a thickness of 15-40 nm; and the shell layer (300) and the covering layer (400) have a total thickness of 20-60 nm.

4. The lithium iron phosphate material with a nanofiber structure according to any one of claims 1 to 3, characterized in that, The ratio of the total molar concentration of lithium in the shell (300) and the covering layer (400), the molar concentration of lithium in the intermediate layer (200), and the molar concentration of lithium in the core layer (100) is (1.05–1.2):(1–1.02):1; and / or, The ratio of the total molar concentration of iron in the shell (300) and the covering layer (400), the molar concentration of iron in the intermediate layer (200), and the molar concentration of iron in the core layer (100) is 0.98:(0.98~1.02):(1.1~1.2).

5. A method for preparing a lithium iron phosphate material with a nanofiber structure as described in claim 1, characterized in that, The preparation method includes: Step S1: Mix a first lithium source, a first iron source, a first phosphorus source, a first polymer, and a first solvent to obtain a first spinning solution; mix a second lithium source, a second iron source, a second phosphorus source, a second polymer, and a second solvent to obtain a second spinning solution; mix a third lithium source, a third iron source, a third phosphorus source, a third polymer, and a third solvent to obtain a third spinning solution; wherein, the concentration of lithium in the first spinning solution is equal to or lower than the concentration of lithium in the second spinning solution, and the concentration of lithium in the second spinning solution is lower than the concentration of lithium in the third spinning solution; the concentration of iron in the first spinning solution is higher than the concentration of iron in the second spinning solution, and the concentration of iron in the second spinning solution is equal to or higher than the concentration of iron in the third spinning solution; in the third spinning solution, the molar ratio of lithium in the third lithium source, iron in the third iron source, and phosphorus in the third phosphorus source is (1.05~1.2):0.98:1; Step S2: Coaxial electrospinning is performed on the first spinning solution, the second spinning solution and the third spinning solution, and the precursor is obtained after drying. Step S3: Pre-oxidize the precursor to obtain a pre-oxidized product; Step S4: Under a protective gas atmosphere, the pre-oxidized product is annealed to obtain the lithium iron phosphate material.

6. The method for preparing lithium iron phosphate material with nanofiber structure according to claim 5, characterized in that, The first lithium source, the second lithium source, and the third lithium source are each independently selected from one or more of the group consisting of lithium hydroxide, lithium carbonate, lithium acetate, lithium nitrate, and lithium dihydrogen phosphate; and / or, The first iron source, the second iron source, and the third iron source are each independently selected from one or more of the group consisting of ferrous nitrate, ferric nitrate, ferrous chloride, ferric chloride, ferrous acetate, ferric acetate, ferric ammonium citrate, and ferric oxide; and / or, The first phosphorus source, the second phosphorus source, and the third phosphorus source are each independently selected from one or more of the group consisting of phosphoric acid, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, ammonium pyrophosphate, and ammonium hypophosphite.

7. The method for preparing lithium iron phosphate material with nanofiber structure according to claim 5, characterized in that, In the first spinning solution, the molar ratio of lithium in the first lithium source, iron in the first iron source, and phosphorus in the first phosphorus source is 1:(1.1~1.2):1; and / or, In the first spinning solution, the weight ratio of the first polymer to the first solvent is (4-6):100; and / or, The weight ratio of the first polymer to the first lithium source is (0.95–1):1; and / or, The viscosity of the first polymer is 400–600 mPa•s; the first solvent is selected from water and / or ethanol.

8. The method for preparing lithium iron phosphate material with nanofiber structure according to claim 7, characterized in that, The first polymer is selected from polyethylene oxide and / or polyvinyl alcohol.

9. The method for preparing lithium iron phosphate material with nanofiber structure according to claim 5, characterized in that, In the second spinning solution, the molar ratio of lithium in the second lithium source, iron in the second iron source, and phosphorus in the second phosphorus source is 1:(0.98~1.02):1; and / or, In the second spinning solution, the weight ratio of the second polymer to the second solvent is (6-10):100; and / or, The weight ratio of the second polymer to the second lithium source is (1.2–1.5):1; and / or, The viscosity of the second polymer is 1200–1800 mPa•s; the second solvent is selected from N,N-dimethylformamide and / or dimethyl sulfoxide.

10. The method for preparing lithium iron phosphate material with nanofiber structure according to claim 9, characterized in that, The second polymer is selected from one or more of the group consisting of polyacrylonitrile and / or cellulose acetate.

11. The method for preparing lithium iron phosphate material with nanofiber structure according to claim 5, characterized in that, In the third spinning solution, the weight ratio of the third polymer to the third solvent is (10-15):100; and / or, The weight ratio of the third polymer to the third lithium source is (1.5–2):1; and / or, The viscosity of the third polymer is 1800–2200 mPa•s; the third solvent is selected from one or more of the group consisting of ethanol, water, and N,N-dimethylformamide.

12. The method for preparing lithium iron phosphate material with nanofiber structure according to claim 11, characterized in that, The third polymer is selected from polyvinylpyrrolidone and / or polymethyl methacrylate.

13. The method for preparing lithium iron phosphate material with nanofiber structure according to any one of claims 5 to 11, characterized in that, In step S2, the coaxial electrospinning is performed using a triaxial coaxial electrospinning device; and / or, During the coaxial electrospinning process, the spinning voltage is 18-25kV and the receiving distance is 15-20cm.

14. The method for preparing lithium iron phosphate material with nanofiber structure according to claim 13, characterized in that, The consumption rate of the first spinning solution is 0.2–0.4 mL / h; the consumption rate of the second spinning solution is 0.4–0.8 mL / h; and the consumption rate of the third spinning solution is 0.8–1.2 mL / h.

15. The method for preparing lithium iron phosphate material with nanofiber structure according to claim 5, characterized in that, In step S2, the drying temperature is 70–90°C, and the time is 1–2 hours; and / or, In step S3, the pre-oxidation temperature is 200-250°C and the time is 1-3 hours.

16. The method for preparing lithium iron phosphate material with nanofiber structure according to any one of claims 5 to 11, characterized in that, In step S4, the annealing process involves a heating rate of 2–5 °C / min, a temperature of 700–850 °C, and a time of 10–20 h; and / or, The protective gas is selected from one or more of the group consisting of nitrogen, inert gases, and reducing gases.

17. The method for preparing lithium iron phosphate material with nanofiber structure according to claim 16, characterized in that, The protective gas is selected from one or more of the group consisting of nitrogen, helium, and hydrogen.

18. A positive electrode, characterized in that, The positive electrode is formed from lithium iron phosphate material with a nanofiber structure as described in any one of claims 1 to 4.

19. A lithium-ion battery, comprising a positive electrode, a negative electrode, an electrolyte, and a separator disposed between the positive electrode and the negative electrode, characterized in that, The positive electrode is the positive electrode as described in claim 18.

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