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

Abstract

The invention provides a lithium iron phosphate positive electrode material, a preparation method thereof and a lithium ion battery, and the preparation method comprises the following steps: mixing a phosphorus source, an iron source, a lithium source and a carbon source in a solvent to obtain slurry containing a magnetic component; performing first magnetic field induction treatment on the slurry to obtain a mixed solution, performing solid-liquid separation, and performing second magnetic field induction treatment and shaping treatment on a solid to obtain a precursor; and sintering the precursor to obtain the lithium iron phosphate positive electrode material. In the first magnetic field treatment and the second magnetic field treatment, magnetic components are driven by magnetic field force and are directionally arranged along magnetic lines of force to form a one-dimensional chain-shaped or layered structure, an ion rapid channel is constructed, grain boundary impedance is reduced, and the magnetic field can inhibit random growth of grains, promote preferred orientation of lithium iron phosphate along a specific crystal face and improve the ionic conductivity; the carbon coating is continuously distributed along the magnetic arrangement structure through sintering, and the ionic conductivity is further improved, so that the discharge performance and the rate capability of the lithium iron phosphate positive electrode material are improved.

Description

Technical Field

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

[0002] Lithium iron phosphate (LiFePO4) materials are gaining an increasingly important market share in the field of electrode materials due to their economic efficiency and safety. However, LiFePO4 materials exhibit poor discharge performance and rate performance at low temperatures. This is mainly due to the poor conductivity of the material itself, the slow lithium-ion diffusion rate, and the changes in the electrolyte and SEI film at low temperatures. In addition, LiFePO4 synthesized by the traditional hydrothermal method has problems such as insufficient exposure of the (010) crystal plane (<30%) and uneven carbon distribution.

[0003] In view of the above, this application is hereby submitted. Summary of the Invention

[0004] The main objective of this application is to provide a lithium iron phosphate cathode material, its preparation method, and a lithium-ion battery, in order to solve the problems of poor discharge performance and low rate performance of lithium iron phosphate materials synthesized by traditional hydrothermal methods in the prior art.

[0005] To achieve the above objectives, according to the first aspect of this application, this application provides a method for preparing a lithium iron phosphate cathode material, wherein the chemical formula of the lithium iron phosphate cathode material is Li. x Fe y The lithium iron phosphate cathode material has a PO4 / C ratio, where 1.0 ≤ x ≤ 1.05, 0.95 ≤ y ≤ 1.0, and a C mass content of 1.2%~1.6%. The preparation method includes:

[0006] Step S1: Mix phosphorus source, iron source, lithium source and carbon source in a solvent to obtain a slurry containing magnetic components;

[0007] Step S2: Place the slurry in the first magnetic field for first magnetic field induction treatment to obtain a mixed solution. Separate the mixed solution into solid and liquid to obtain a solid. Then, subject the solid to second magnetic field induction treatment and shaping treatment in the second magnetic field to obtain a precursor.

[0008] Step S3: Sinter the precursor to obtain the lithium iron phosphate cathode material.

[0009] Further, in step S1, the phosphorus source is selected from at least one of phosphoric acid, iron phosphate, and lithium dihydrogen phosphate.

[0010] Further, in step S1, the iron source is selected from at least one of ferric nitrate, ferric oxalate, ferric phosphate, and Fe3O4.

[0011] Further, in step S1, the lithium source is selected from at least one of lithium carbonate, lithium hydroxide, and lithium dihydrogen phosphate.

[0012] Furthermore, in step S1, the carbon source is selected from at least one of glucose, starch, and carbon nanotubes.

[0013] Furthermore, the lithium-phosphorus molar ratio of the lithium source to the phosphorus source is 1~1.05:1.

[0014] Furthermore, the iron-phosphorus molar ratio of the iron source to the phosphorus source is 0.95~1.0:1.

[0015] Furthermore, based on the mass of carbon elements, the mass of the carbon source is 1.2% to 1.6% of the mass of lithium iron phosphate.

[0016] Furthermore, in step S1, the particle size D50 of the solid particles in the slurry is 0.4μm~0.6μm.

[0017] Furthermore, phosphorus source, iron source, lithium source and carbon source are mixed in solvent and ground to obtain a slurry with a particle size D50 of 0.4μm~0.6μm.

[0018] Furthermore, in step S1, the solvent is water or an alcohol solvent.

[0019] Furthermore, in step S1, the solvent is water.

[0020] Furthermore, in step S2, the first magnetic field strength of the first magnetic field induction treatment is 1T~2T, the first magnetic field gradient is 100T / m~150T / m, and the first magnetic field treatment time is 1h~3h.

[0021] Furthermore, in step S2, the strength of the second magnetic field for the second magnetic field induction treatment is 1T~2T, and the gradient of the second magnetic field is 100T / m~150T / m.

[0022] Furthermore, the shaping process is laser heat treatment, with a laser power of 5kW~20kW and a laser heat treatment time of 5s~10s.

[0023] Furthermore, in step S3, the sintering is multi-stage sintering, which includes a first sintering, a second sintering, and a third annealing sintering performed sequentially.

[0024] Furthermore, the temperature of the first sintering is 300℃~350℃, and the time of the first sintering is 2h~4h.

[0025] Furthermore, the second sintering temperature is 600℃~650℃, and the second sintering time is 4h~6h.

[0026] Furthermore, the temperature of the third annealing sintering is 700℃~720℃, and the time of the third annealing sintering is 0.5h~1h.

[0027] Furthermore, the first sintering, the second sintering, and the third annealing sintering are each carried out independently under a protective gas atmosphere, wherein the protective gas is at least one of nitrogen, argon, or helium.

[0028] Further, in step S1, the iron source is a mixture of iron salt and Fe3O4, wherein the mass content of Fe3O4 is 0.05%~1.5% of the iron source, and the particle size D50 of Fe3O4 is 0.1~0.3μm.

[0029] To achieve the above objectives, according to a second aspect of this application, a lithium iron phosphate cathode material is provided, which is obtained according to the preparation method provided in the first aspect of this application.

[0030] According to a third aspect of this application, a lithium-ion battery is provided, the lithium-ion battery including a positive electrode material, the positive electrode material being a lithium iron phosphate positive electrode material obtained by the preparation method of the first aspect of this application or a lithium iron phosphate positive electrode material provided by the second aspect.

[0031] By applying the technical solution of this application, a precursor is obtained by subjecting a slurry containing magnetic components to a first magnetic field treatment, a second magnetic field treatment, and a shaping treatment. The precursor is then sintered to obtain a lithium iron phosphate cathode material. During the first and second magnetic field treatments, the magnetic components are driven by the magnetic field force to align along the magnetic field lines, forming a one-dimensional chain or layered structure, constructing a fast ion channel, reducing grain boundary impedance, and the magnetic field can suppress random grain growth, prompting lithium iron phosphate to preferentially align along a specific crystal plane (such as the

[010] crystal plane direction), thereby improving ionic conductivity. Subsequently, sintering allows the carbon coating to be continuously distributed along the magnetic alignment structure, further improving ionic conductivity, thereby enhancing the discharge performance and rate performance at low temperatures of the lithium iron phosphate cathode material. Detailed Implementation

[0032] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. The present application will now be described in detail with reference to the embodiments.

[0033] As described in the background section of this application, lithium iron phosphate materials synthesized using the conventional hydrothermal method exhibit poor discharge performance and rate performance at low temperatures. To address these issues, this application provides a lithium iron phosphate cathode material, its preparation method, and a lithium-ion battery.

[0034] In a first typical embodiment of this application, a method for preparing a lithium iron phosphate cathode material is provided, wherein the chemical formula of the lithium iron phosphate cathode material is Li. xFe y The lithium iron phosphate cathode material has a PO4 / C ratio, where 1.0 ≤ x ≤ 1.05, 0.95 ≤ y ≤ 1.0, and a C mass content of 1.2%~1.6%. The preparation method includes the following steps:

[0035] Step S1: Mix phosphorus source, iron source, lithium source and carbon source in a solvent to obtain a slurry containing magnetic components;

[0036] Step S2: Place the slurry in the first magnetic field for first magnetic field induction treatment to obtain a mixed solution. Separate the mixed solution into solid and liquid to obtain a solid. Then, subject the solid to second magnetic field induction treatment and shaping treatment in the second magnetic field to obtain a precursor.

[0037] Step S3: Sinter the precursor to obtain the lithium iron phosphate cathode material.

[0038] The method for preparing a lithium iron phosphate cathode material provided in this application involves subjecting a slurry containing magnetic components to a first magnetic field-induced treatment in a first magnetic field. This step enables the magnetic components to align in an orderly manner along the magnetic field direction, forming a one-dimensional chain or layered structure. This structure helps to construct channels for rapid lithium-ion transport, reduces grain boundary impedance, and thus optimizes the ionic conductivity of the material. A second magnetic field-induced treatment and shaping treatment are then performed in a second magnetic field. Utilizing the guiding effect of the second magnetic field, the lithium iron phosphate precursor is preferentially grown along a specific crystal plane (such as the

[010] direction), suppressing random grain growth and significantly improving the crystal orientation of the material. This, in turn, enhances the discharge performance and rate performance of the material under low-temperature conditions. The sintering process achieves a uniform distribution of carbon coating on the surface of the lithium iron phosphate, further improving the electronic conductivity of the material.

[0039] In summary, the method for preparing lithium iron phosphate cathode material provided in this application effectively solves the performance defects of lithium iron phosphate cathode material in low-temperature environments by combining magnetic field-induced sintering process, improves the conductivity and lithium-ion diffusion rate of the material, and finally obtains cathode material with excellent rate performance and discharge performance, which is suitable for high-performance battery fields such as long cycle and fast charging.

[0040] In some embodiments, in step S1 above, the phosphorus source is a phosphorus-containing substance commonly used in the art, including but not limited to any one or more mixtures of phosphoric acid, iron phosphate, and lithium dihydrogen phosphate; the lithium source is a lithium-containing substance commonly used in the art, including but not limited to any one or more mixtures of lithium carbonate, lithium hydroxide, and lithium dihydrogen phosphate; and the carbon source is a carbon-containing substance commonly used in the art, including but not limited to any one or more mixtures of glucose, starch, and carbon nanotubes.

[0041] The raw materials used to prepare lithium iron phosphate cathode materials are widely available and inexpensive. This not only helps reduce the cost of lithium iron phosphate cathode materials, but also facilitates the formation of a more stable lithium iron phosphate crystal structure, improves the electronic conductivity of the material, and thus enhances the cycle stability and rate performance of the battery.

[0042] In some specific embodiments, in step S1 above, the lithium-phosphorus molar ratio of the lithium source to the phosphorus source is 1.0~1.05:1, which is beneficial to improve the preparation efficiency of lithium iron phosphate cathode material and reduce energy waste. Too low lithium content may lead to a decrease in the initial charge capacity, which in turn leads to a decrease in the overall capacity.

[0043] Specifically, the lithium-phosphorus molar ratio of the lithium source to the phosphorus source is any value among 1.0:1, 1.01:1, 1.02:1, 1.03:1, 1.04:1, and 1.05:1, or any value between the two.

[0044] In some specific embodiments, in step S1 above, the iron-phosphorus molar ratio of the iron source to the phosphorus source is 0.95~1.0:1, which helps to improve the preparation efficiency of lithium iron phosphate cathode material while reducing energy waste. Excessive addition of phosphorus source may lead to a reduction in effective active sites, which may further lead to a decrease in capacity.

[0045] Specifically, the iron-phosphorus molar ratio of the iron source to the phosphorus source is any value among 0.95:1, 0.96:1, 0.97:1, 0.98:1, 0.99:1, and 1.0:1, or any value between the two.

[0046] In some specific embodiments, in step S1 above, the amount of carbon source used is 1.2% to 1.6% of the mass of lithium iron phosphate, based on the mass of carbon elements, so as to improve the conductivity of lithium iron phosphate cathode material while giving it a better energy density.

[0047] Specifically, the amount of carbon source used is based on the mass of carbon elements, and the mass of carbon elements in the carbon source accounts for 1.2%, 1.25%, 1.3%, 1.35%, 1.4%, 1.45%, 1.5%, 1.55%, and 1.6% of the mass of lithium iron phosphate. The mass of lithium iron phosphate is calculated based on the assumption that all iron elements in the iron source are converted into lithium iron phosphate.

[0048] Furthermore, it should be noted that in the lithium iron phosphate cathode material prepared in this application, lithium iron phosphate is the main component, with a mass content ≥97%. Therefore, by controlling the mass of carbon elements in the carbon source to account for 1%~3% of the mass of lithium iron phosphate during the feeding process, a lithium iron phosphate cathode material with a C mass content of 1%~3% can be prepared. Similarly, by controlling the mass of carbon elements in the carbon source to account for 1.2%~1.6% of the mass of lithium iron phosphate, a lithium iron phosphate cathode material with a C mass content of 1.2%~1.6% can be prepared.

[0049] In some embodiments, in step S1 above, the particle size D50 of the solid particles in the slurry is 0.4 μm to 0.6 μm. Specifically, the solid particles in the slurry are any value or a range between any two of 0.4 μm, 0.42 μm, 0.44 μm, 0.46 μm, 0.48 μm, 0.5 μm, 0.52 μm, 0.54 μm, 0.56 μm, and 0.58 μm. When the particle size D50 of the solid particles is controlled between 0.4 μm and 0.6 μm, particles within this size range can respond more effectively to the magnetic field force during the first magnetic field-induced treatment, promoting the directional alignment of the solid particles along the magnetic field lines and forming a one-dimensional chain or layered structure that facilitates rapid lithium-ion transport, thereby significantly improving the electrical conductivity and rate performance of the material.

[0050] In some embodiments, the specific type of solvent in step S1 is not limited, including but not limited to water or any one or more mixtures of alcohol solvents. From the perspective of environmental protection and cost reduction, water is preferred as the solvent. Using water as a solvent allows for better control of particle size and shape during the grinding process in an aqueous environment, ensuring consistency and effectiveness in the first magnetic field-induced treatment process. The aforementioned alcohol solvents include, but are not limited to, ethanol.

[0051] In some embodiments, the first magnetic field strength of the first magnetic field induced treatment is 1T~2T, the first magnetic field gradient is 100T / m~150T / m, and the first magnetic field treatment time is 1h~3h. The selection of the first magnetic field strength and gradient is more conducive to improving the directional arrangement efficiency of solid particles in the slurry and improving the structural stability of lithium iron phosphate cathode material. The appropriate treatment time is more conducive to the full arrangement of particles without excessive aggregation, thereby further reducing grain boundary impedance, promoting the preferred orientation growth of LiFePO4 along a specific crystal plane (

[010] direction), and further improving ionic conductivity.

[0052] Specifically, the first magnetic field strength for the first magnetic field induction treatment is any value or a range between any two of 1T, 1.1T, 1.2T, 1.3T, 1.4T, 1.5T, 1.6T, 1.7T, 1.8T, 1.9T, and 2T; the first magnetic field gradient is any value or a range between any two of 100T / m, 110T / m, 120T / m, 130T / m, 140T / m, and 150T / m; and the first magnetic field treatment time is any value or a range between any two of 1h, 1.5h, 2h, 2.5h, and 3h.

[0053] In some embodiments, the second magnetic field strength of the second magnetic field induced treatment is 1T to 2T, and the second magnetic field gradient is 100T / m to 150T / m. Specifically, the second magnetic field strength is any value or a range between 1T, 1.1T, 1.2T, 1.3T, 1.4T, 1.5T, 1.6T, 1.7T, 1.8T, 1.9T, and 2T; and the second magnetic field gradient is any value or a range between 100T / m, 110T / m, 120T / m, 130T / m, 140T / m, and 150T / m. The second magnetic field induced treatment enhances the orientation of the solid, promotes the growth of lithium iron phosphate materials along specific crystal plane orientations, thereby constructing a more efficient lithium-ion diffusion path, reducing ion transport resistance, and improving the rate performance of the material.

[0054] In some embodiments, to further improve the efficiency of the shaping process, laser heat treatment is preferred, with a laser power of 5kW to 20kW and a treatment time of 5 to 10 seconds. The laser heat treatment method enables efficient heating of the precursor in a short time, rapidly solidifying the fast ion channels formed around the precursor, stabilizing the magnetic arrangement structure of the material, further reducing resistivity, and enhancing the material's conductivity and lithium-ion diffusion capability. Furthermore, excessively high or low laser heat treatment power and time may lead to increased resistivity, unstable fast ion channel formation, or grain agglomeration and lattice distortion, further affecting the rate performance of the lithium iron phosphate cathode material.

[0055] Specifically, the laser power for the laser heat treatment is any value or a range between any two of 5kW, 6kW, 8kW, 10kW, 12kW, 15kW, 18kW, and 20kW; the laser heat treatment time is any value or a range between any two of 5s, 6s, 7s, 8s, 9s, and 10s.

[0056] In step S2 above, the mixed solution is separated into solid and liquid to obtain solid matter. The specific method of solid-liquid separation is not limited, including but not limited to filtration, centrifugation, etc.

[0057] In some embodiments, in step S3 above, to further improve the structural stability of the lithium iron phosphate cathode material, it is preferable that the sintering is multi-stage sintering, wherein the multi-stage sintering includes a first sintering, a second sintering, and a third annealing sintering performed sequentially. By adopting a multi-stage sintering strategy, the carbon coating is continuously distributed along the magnetic arrangement structure, which helps to gradually build the microstructure of the lithium iron phosphate material, promotes the uniform bonding between the carbon coating and the material body, further improves the ionic conductivity, and thus improves the rate performance of the material.

[0058] In some specific embodiments, the first sintering temperature is 300℃~350℃, the first sintering time is 2h~4h, low-temperature sintering is used in the initial stage of sintering, the low-temperature sintering stage is the carbon source conversion reaction stage, and lithium iron phosphate does not start to form phase during the first sintering process. The setting of the sintering temperature and sintering time of the first sintering is conducive to the formation of carbon layer to control the growth of lithium iron phosphate particles.

[0059] Specifically, the first sintering temperature is any value or a range between any two of 300℃, 305℃, 310℃, 315℃, 320℃, 325℃, 330℃, 335℃, 340℃, 345℃, and 350℃; the first sintering time is any value or a range between any two of 2h, 2.5h, 3h, 3.5h, and 4h.

[0060] In some specific embodiments, the second sintering temperature is 600℃~650℃, the second sintering time is 4h~6h, and medium-temperature sintering is used in the middle of the sintering. The sintering temperature and sintering time of the second sintering can further enable the lithium iron phosphate particles to grow completely and prevent the lithium iron phosphate particles from growing further, which is conducive to constructing a uniform structure of lithium iron phosphate.

[0061] Specifically, the second sintering temperature is any value or a range between any two of 600℃, 605℃, 610℃, 615℃, 620℃, 625℃, 630℃, 635℃, 640℃, 645℃, and 650℃; the second sintering time is any value or a range between any two of 4h, 4.5h, 5h, 5.5h, and 6h.

[0062] In some specific embodiments, the third annealing sintering temperature is 700℃~720℃, and the third annealing sintering time is 0.5h~1h. High-temperature short-time annealing sintering is used at the end of the sintering period, which is beneficial to eliminate lattice defects after lithium iron phosphate phase formation.

[0063] Specifically, the third annealing sintering temperature is any value among 700℃, 705℃, 710℃, 715℃, and 720℃, or a range between any two; the third annealing sintering time is any value among 0.5h, 0.6h, 0.7h, 0.8h, 0.9h, and 1h, or a range between any two.

[0064] In some specific embodiments, the first sintering, the second sintering, and the third annealing sintering are each carried out independently under a protective gas atmosphere. The protective gas is any one or any mixture of nitrogen, argon, or nitrogen, to avoid introducing impurities during the sintering process that could affect the performance of the lithium iron phosphate cathode material. The sintering process is carried out in an inert gas environment such as nitrogen, argon, or helium, which also effectively prevents the material from oxidizing at high temperatures, ensuring the purity and stability of the material.

[0065] In some embodiments, the iron source is a mixture of iron salt and Fe3O4, with the Fe3O4 content ranging from 0.05 wt% to 1.5 wt% of the iron source and the Fe3O4 particle size D50 being 0.1 μm to 0.3 μm. Using a mixture of iron salt and Fe3O4 as the iron source, and controlling the Fe3O4 content in the iron source to be between 0.05 wt% and 1.5 wt%, and limiting the particle size D50 to 0.1 μm to 0.3 μm, is more conducive to promoting better response of the solid particles in the slurry to the magnetic field guidance during the subsequent first magnetic field-induced treatment, and promoting the preferential growth of LiFePO4 grains along directions favorable for lithium-ion transport.

[0066] Specifically, the iron salt includes, but is not limited to, any one or a mixture of ferric nitrate, ferric oxalate, and ferric phosphate; the amount of Fe3O4 added is any value or a range between any two of 0.05wt%, 0.06wt%, 0.07wt%, 0.08wt%, 0.09wt%, 0.10wt%, 0.11wt%, 0.12wt%, 0.13wt%, 0.14wt%, and 0.15wt% of the iron source; and the particle size D50 of Fe3O4 is any value or a range between any two of 0.1μm, 0.11μm, 0.12μm, 0.15μm, 0.18μm, 0.2μm, 0.22μm, 0.25μm, 0.28μm, and 0.3μm.

[0067] In a second typical embodiment of this application, a lithium iron phosphate cathode material is provided, which is prepared according to the preparation method provided in the first typical embodiment of this application. This application significantly improves the overall performance of the lithium iron phosphate material through magnetic field control and sintering process optimization, exhibiting outstanding performance in high-rate charge / discharge capabilities, and is suitable for the production of lithium-ion battery cathode materials.

[0068] In a third typical embodiment of this application, a lithium-ion battery is provided, comprising a positive electrode material, which is a lithium iron phosphate positive electrode material prepared according to the preparation method provided in the second typical embodiment. The lithium iron phosphate positive electrode material of this application is suitable for lithium-ion batteries with high rate capability and long cycle life, expanding the application field of lithium iron phosphate materials in fast charging and low-temperature operating environments.

[0069] The beneficial effects of this application will be further illustrated below with reference to embodiments and comparative examples.

[0070] Example 1

[0071] This embodiment provides a method for preparing lithium iron phosphate cathode material, including the following steps:

[0072] (1) Weigh lithium carbonate, phosphoric acid and ferric nitrate in a molar ratio of lithium carbonate:phosphoric acid:ferric nitrate = 0.505:1:0.97. Weigh glucose with the mass of carbon element in glucose being 1.6% of the mass of lithium iron phosphate. Mix lithium carbonate, phosphoric acid, ferric nitrate and glucose in water and add Fe3O4 particles to the water. Grind to obtain a slurry containing magnetic components (the particle size D50 of the solid particles is 0.4μm). The amount of Fe3O4 particles added is 0.15wt% of the total mass of ferric nitrate and Fe3O4 particles, and the particle size D50 of Fe3O4 particles is 0.2μm.

[0073] (2) The above-mentioned slurry containing magnetic components is placed in a high-gradient electromagnetic slurry machine for first magnetic field induction treatment to obtain treated slurry. The treated slurry is then centrifuged and filtered to obtain solid matter. The obtained solid matter is placed under a second magnetic field for second magnetic field induction treatment, and simultaneously subjected to laser heat treatment for shaping treatment to obtain a precursor. The first magnetic field strength of the first magnetic field induction treatment is 2T, the first magnetic field gradient is 150T / m, and the first magnetic field treatment time is 3h. The second magnetic field strength is 2T, the second magnetic field gradient is 150T / m, the laser heat treatment power is 15kW, and the laser heat treatment time is 5s.

[0074] (3) The precursor was subjected to a first sintering, a second sintering, and a third annealing sintering in a nitrogen atmosphere to obtain a lithium iron phosphate cathode material Li with a C mass content of 1.6%. 1.01 Fe 0.97 PO4 / C. The first sintering temperature is 300℃ and the first sintering time is 2h. The second sintering temperature is 600℃ and the second sintering time is 4h. The third annealing sintering temperature is 700℃ and the third annealing sintering time is 0.5h.

[0075] Example 2

[0076] The only difference between this embodiment and Example 1 is that in step (1), lithium carbonate is replaced with lithium hydroxide, and the molar ratio of lithium hydroxide:phosphoric acid:ferric nitrate is 1.01:1:0.97, thus preparing a lithium iron phosphate cathode material with a C mass content of 1.6%. 1.01 Fe 0.97 PO4 / C.

[0077] Example 3

[0078] The only difference between this embodiment and Example 1 is that in step (1), iron phosphate is used to replace phosphoric acid and iron nitrate, and the molar ratio of lithium carbonate to iron phosphate is 0.505:1, thus preparing a lithium iron phosphate cathode material Li with a C mass content of 1.6%. 1.01 FePO4 / C.

[0079] Example 4

[0080] The only difference between this embodiment and Example 2 is that in step (1), the molar ratio of lithium hydroxide:phosphoric acid:ferric nitrate is adjusted to 1.0:1:0.98 to prepare a lithium iron phosphate cathode material with a C mass content of 1.6%. 1.0 Fe 0.98 PO4 / C.

[0081] Example 5

[0082] The only difference between this embodiment and Example 2 is that in step (1), the molar ratio of lithium hydroxide:phosphoric acid:ferric nitrate is adjusted to 1.05:1:0.95 to prepare a lithium iron phosphate cathode material with a C mass content of 1.6%. 1.05 Fe 0.95 PO4 / C.

[0083] Example 6

[0084] The only difference between this embodiment and embodiment 1 is that in step (1), the glucose is weighed to be 1.2% of the mass of lithium iron phosphate, based on the mass of carbon in the glucose.

[0085] Example 7

[0086] The only difference between this embodiment and embodiment 1 is that in step (1), the grinding process is adjusted so that the particle size D50 of the solid particles in the slurry is 0.6 μm.

[0087] Example 8

[0088] The only difference between this embodiment and embodiment 1 is that the first magnetic field strength is 1T, the first magnetic field gradient is 100T / m, and the first magnetic field induction treatment time is 1h in step (2).

[0089] Example 9

[0090] The only difference between this embodiment and Example 1 is that the amount of Fe3O4 particles added in step (1) is adjusted to 0.05 wt% of the total mass of ferric nitrate and Fe3O4 particles.

[0091] Example 10

[0092] The only difference between this embodiment and embodiment 1 is that the laser heat treatment power in step (2) is adjusted to 5kW and the laser heat treatment time is 8s.

[0093] Example 11

[0094] The only difference between this embodiment and embodiment 1 is that the laser heat treatment power in step (2) is 20kW and the laser heat treatment time is 10s.

[0095] Example 12

[0096] The only difference between this embodiment and embodiment 1 is that in step (3), the first sintering temperature is 350°C, the first sintering time is 4h, the second sintering temperature is 650°C, the second sintering time is 6h, the third annealing sintering temperature is 720°C, and the third annealing sintering time is 1h.

[0097] Example 13

[0098] The only difference between this embodiment and embodiment 1 is that in step (1), the glucose is weighed to be 1% of the mass of lithium iron phosphate, with the mass of carbon in the glucose being the same.

[0099] Example 14

[0100] The only difference between this embodiment and embodiment 1 is that in step (1), the glucose is weighed to be 3% of the mass of lithium iron phosphate, based on the mass of carbon in the glucose.

[0101] Example 15

[0102] The only difference between this embodiment and embodiment 1 is that the first magnetic field strength is 0.5T, the first magnetic field gradient is 80T / m, and the first magnetic field induction treatment time is 0.5h in step (2).

[0103] Example 16

[0104] The only difference between this embodiment and embodiment 1 is that the laser heat treatment power in step (2) is adjusted to 3kW and the laser heat treatment time is 3s.

[0105] Example 17

[0106] The only difference between this embodiment and embodiment 1 is that the laser heat treatment power in step (2) is 25kW and the laser heat treatment time is 12s.

[0107] Example 18

[0108] The only difference between this embodiment and embodiment 1 is that the amount of Fe3O4 particles added in step (1) is adjusted to 0.02% of the total mass of ferric nitrate and Fe3O4 particles.

[0109] Example 19

[0110] The only difference between this embodiment and embodiment 1 is that the amount of Fe3O4 particles added in step (1) is adjusted to 3% of the total mass of ferric nitrate and Fe3O4 particles.

[0111] Example 20

[0112] The only difference between this embodiment and Embodiment 1 is that the first sintering temperature is adjusted to 250°C, the first sintering time is 1 hour, the second sintering temperature is 550°C, the second sintering time is 7 hours, the third annealing sintering temperature is 650°C, and the third annealing sintering time is 0.2 hours.

[0113] Example 21

[0114] The only difference between this embodiment and Embodiment 1 is that the first sintering temperature is adjusted to 400℃, the first sintering time is 7h, the second sintering temperature is 700℃, the second sintering time is 3h, the third annealing sintering temperature is 750℃, and the third annealing sintering time is 2h.

[0115] Example 22

[0116] The only difference between this embodiment and Embodiment 1 is that the sintering is adjusted to a two-stage sintering process, wherein the first sintering temperature is 600℃ and the first sintering time is 8h, and the second sintering temperature is 700℃ and the second sintering time is 1h.

[0117] Example 23

[0118] The only difference between this embodiment and Embodiment 1 is that the sintering is adjusted to a two-stage sintering process, wherein the first sintering temperature is 300℃ and the first sintering time is 3h, and the second sintering temperature is 600℃ and the second sintering time is 6h.

[0119] Comparative Example 1

[0120] The only difference between this comparative example and Example 1 is that the first magnetic field induction treatment, the second magnetic field induction treatment, and the laser heat treatment were not applied. Instead, the solid was dried at 90°C to obtain the precursor.

[0121] Comparative Example 2

[0122] The only difference between this comparative example and Example 1 is that the second magnetic field induction treatment and laser heat treatment are not performed in step (2), and the solid is dried at 90°C to obtain the precursor.

[0123] Comparative Example 3

[0124] The only difference between this comparative example and Example 1 is that Fe3O4 particles are not added in step (1).

[0125] Comparative Example 4

[0126] The only difference between this comparative example and Example 1 is that in step (2), the gradient magnetic fields of the first magnetic field induction treatment and the second magnetic field induction treatment are replaced with a uniform magnetic field, and the magnetic field strength of the uniform magnetic field is 2T.

[0127] Experimental Example 1

[0128] 1. Button test

[0129] The above-mentioned lithium iron phosphate material, polyvinylidene fluoride (PVDF), and conductive carbon black (SP) were mixed and ground in a mass ratio of 8:1:1, then coated onto aluminum foil and dried to serve as the positive electrode of a lithium-ion battery. A CR2032 coin cell was assembled in an argon-filled glove box. The negative electrode was a lithium metal sheet, the separator was a polypropylene microporous membrane, and the electrolyte was a 1 mol / L LiPF6 solution. The electrolyte solvent was a 1:1:1 volume ratio of ethylene carbonate (EC): vinylene carbonate (DMC): ethyl methyl carbonate (EMC). The coin cell's measured voltage range was 2.5V~4V, and the specific capacity was tested at charge / discharge current rates of 0.2C, 1C, and 5C.

[0130] 2. Resistivity test

[0131] Weigh 0.7g of lithium iron phosphate cathode material powder, place it in a mold and vibrate it. After vibration, place it in a four-probe powder resistivity tester. Obtain the powder resistivity under different pressures through data feedback. Take the powder resistivity at 100MPa as the measured value.

[0132] 3. Low temperature test

[0133] After the battery has undergone the coin cell test, it is placed in a constant temperature test chamber and discharged at a 1C current rate with a voltage range of 2.5~4V after the temperature reaches -20℃. The discharge capacity is Q. b The specific capacitance at 1C current in the coin cell test is Q. a Capacity retention rate = Q b / Q a .

[0134] The test results for coin cell test, resistivity test, and rate performance test are shown in Table 1.

[0135] Table 1

[0136]

[0137]

[0138] As can be seen from Examples 1-23 and Comparative Examples 1-4, the lithium iron phosphate cathode materials prepared in Examples 1-23 have better rate performance, lower resistivity and higher capacity retention at low temperature compared with the lithium iron phosphate cathode materials prepared in Comparative Examples 1-3.

[0139] A comparison of Example 1 and Examples 13-14 shows that excessively high or low carbon content can affect resistivity and effective capacity, further leading to a reduction in rate performance.

[0140] A comparison between Example 1 and Example 15 shows that a magnetic field strength that is too low will affect the construction of fast ion channels, increase resistivity, and further reduce rate performance and capacity retention at low temperatures.

[0141] A comparison between Example 1 and Examples 18-19 shows that too low an amount of Fe3O4 particles will affect the construction of fast ion channels and increase resistivity, while too high an amount will affect the purity of the lithium iron phosphate cathode material and lead to a decrease in capacity.

[0142] A comparison of Example 1 and Examples 20-21 shows that a suitable sintering temperature is more conducive to reducing resistivity and improving rate performance.

[0143] A comparison of Example 1 and Examples 22-23 shows that low-temperature sintering and high-temperature annealing sintering affect the coating effect, resulting in reduced rate performance and increased resistivity.

[0144] A comparison of Example 1 and Comparative Examples 1-2 shows that the lithium iron phosphate cathode material treated with magnetic field induction and laser thermal treatment has better rate performance, lower resistivity, and higher capacity retention at low temperatures.

[0145] By comparing Example 1 and Comparative Example 3, it can be seen that Fe3O4 particles, as magnetic components, are driven by non-uniform magnetic field forces to align along magnetic field lines, forming a one-dimensional chain or layered structure, constructing a fast ion channel, reducing grain boundary impedance, and thus further improving ion conductivity and rate performance.

[0146] By comparing Example 1 and Comparative Example 4, it can be seen that the non-uniform magnetic field can drive the magnetic components to oriented along the magnetic field lines, forming a one-dimensional chain or layered structure, constructing a fast ion channel, and reducing grain boundary impedance; and the non-uniform magnetic field can suppress random grain growth, promote the preferential orientation of lithium iron phosphate along a specific crystal plane (such as the

[010] direction), and improve ionic conductivity.

[0147] From the above description, it can be seen that the above embodiments of this application achieve the following technical effects: a precursor is obtained by subjecting a slurry containing magnetic components to a first magnetic field treatment, a second magnetic field treatment, and a shaping treatment; the precursor is then sintered to obtain a lithium iron phosphate cathode material. During the first and second magnetic field treatments, the magnetic components are driven by the magnetic field force to align along the magnetic field lines, forming a one-dimensional chain or layered structure, constructing a fast ion channel, reducing grain boundary impedance, and the magnetic field can suppress random grain growth, prompting lithium iron phosphate to preferentially align along a specific crystal plane (such as the

[010] crystal plane direction), thereby improving ionic conductivity. Subsequently, sintering causes the carbon coating to be continuously distributed along the magnetic alignment structure, further improving ionic conductivity, thereby improving the discharge performance and rate performance at low temperatures of the lithium iron phosphate cathode material.

[0148] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A method for preparing a lithium iron phosphate cathode material, characterized in that, The chemical formula of the lithium iron phosphate positive electrode material is Li x Fe y PO4 / C, wherein 1.0<=x<=1.05, 0.95<=y<=1.0, the mass content of C is 1%-3%, and the preparation method of the lithium iron phosphate positive electrode material comprises the following steps: Step S1, mixing a phosphorus source, an iron source, a lithium source and a carbon source in a solvent to obtain a slurry containing a magnetic component; Step S2, placing the slurry in a first magnetic field to perform a first magnetic field induction treatment to obtain a mixed solution, and performing solid-liquid separation on the mixed solution to obtain a solid, and performing a second magnetic field induction treatment and a shaping treatment on the solid in a second magnetic field to obtain a precursor; Step S3, sintering the precursor to obtain the lithium iron phosphate positive electrode material.

2. The production method according to claim 1, characterized by, In the step S1, the phosphorus source is at least one selected from phosphoric acid, iron phosphate, lithium dihydrogen phosphate; and / or, the iron source is at least one selected from ferric nitrate, ferric oxalate, iron phosphate, Fe3O4; and / or, the lithium source is at least one selected from lithium carbonate, lithium hydroxide, lithium dihydrogen phosphate; and / or, the carbon source is at least one selected from glucose, starch, carbon nanotubes.

3. The method of claim 1, wherein, In the step S1, the lithium to phosphorus molar ratio of the lithium source to the phosphorus source is 1-1.05:1; and / or, the iron to phosphorus molar ratio of the iron source to the phosphorus source is 0.95-1.0:1; and / or, in the lithium iron phosphate positive electrode material, the mass content of C is 1.2%-1.6%, and the amount of the carbon source is 1.2%-1.6% of the mass of the lithium iron phosphate.

4. The method of claim 1, wherein, In the step S1, the particle size D50 of the solid particles in the slurry is 0.4-0.6 μm; and / or, the phosphorus source, the iron source, the lithium source and the carbon source are mixed in the solvent for grinding treatment to obtain the slurry with the particle size D50 of the solid particles being 0.4-0.6 μm; and / or, the solvent is water or an alcohol solvent, preferably water.

5. The preparation method according to claim 1, characterized in that, In the step S2, the first magnetic field induction treatment has a first magnetic field strength of 1-2 T, a first magnetic field gradient of 100-150 T / m, and a first magnetic field treatment time of 1-3 h.

6. The method of claim 1, wherein, In the step S2, the second magnetic field induction treatment has a second magnetic field strength of 1-2 T and a second magnetic field gradient of 100-150 T / m; and / or, the shaping treatment is a laser heat treatment, the laser heat treatment has a laser power of 5-20 kW, and the laser heat treatment has a time of 5-10 s.

7. The preparation method according to claim 1, characterized in that, In the step S3, the sintering is multi-stage sintering, wherein the multi-stage sintering comprises sequentially performed first sintering, second sintering and third annealing sintering; Preferably, the first sintering has a temperature of 300-350 °C and a time of 2-4 h; Preferably, the second sintering has a temperature of 600-650 °C and a time of 4-6 h; Preferably, the third annealing sintering has a temperature of 700-720 °C and a time of 0.5-1 h; Preferably, the first sintering, the second sintering and the third annealing sintering are each independently performed in a protective gas atmosphere, and the protective gas is at least one selected from nitrogen, argon and helium.

8. The production method according to any one of claims 1 to 7, characterized by, In the step S1, the iron source is a mixture of iron salt and Fe3O4, wherein the mass content of Fe3O4 is 0.05% to 1.5% of the iron source, and the particle size D50 of Fe3O4 is 0.1 to 0.3 microns.

9. A lithium iron phosphate cathode material, characterized in that, The lithium iron phosphate positive electrode material is obtained according to the preparation method in any one of claims 1-8.

10. A lithium-ion battery, characterized by, The lithium ion battery comprises a positive electrode material, and the positive electrode material is the lithium iron phosphate positive electrode material obtained according to the preparation method in any one of claims 1-8 or the lithium iron phosphate positive electrode material in claim 9.

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