Cathode material, preparation method thereof, cathode sheet and lithium ion battery

Abstract

The application provides a positive electrode material, a preparation method of the positive electrode material, a positive electrode sheet and a lithium ion battery. 1‑x FePO 4‑γ , 0 < x < 0.02, 0 < gamma < 0.02, the lithium iron phosphate core has anti-site defects in which iron occupies lithium sites, and the material of the phosphate coating layer comprises NASICON-type phosphate and lithium phosphate, the chemical general formula of the NASICON-type phosphate is Li a M b (PO4) c , 1 < a < 5, 1 < b < 2, and 1 < c < 3. The positive electrode material is applied to the lithium ion battery, which helps to improve the first charge capacity, the first charge-discharge efficiency and the rate performance of the lithium ion battery.

Description

Technical Field

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

[0002] Lithium iron phosphate (LFP) has become the mainstream cathode material for power batteries and energy storage batteries due to its advantages such as high safety performance, ultra-long cycle life, low cost, and environmental friendliness. However, traditional LFP has inherent defects: its internal structure consists of a one-dimensional lithium-ion diffusion channel, and the charging and discharging process is mainly a two-phase reaction. In high-rate fast charging scenarios, the ion-electron transport flux mismatch at the material interface leads to increased electrode polarization, resulting in problems such as low rate capacity and rapid capacity decay during long cycles, which seriously limits the application of LFP in the field of ultra-fast charging batteries.

[0003] Currently, the industry's modification methods for lithium iron phosphate mainly fall into three categories: lattice doping, carbon coating, and surface NASICON phosphate coating. Lattice doping can, to some extent, regulate lattice defects and improve bulk ion diffusion; carbon coating alone mainly improves the electronic conductivity of the material; and surface NASICON phosphate coating can optimize interfacial lithium-ion conduction. Most existing technologies employ single modification or a simple combination of two modification methods, without a systematic design considering bulk lattice defects, interfacial double-layer coating, and reaction mode regulation. Furthermore, the defect structures prepared by conventional sintering processes have poor thermodynamic stability, and defects are prone to annealing and annihilation during secondary heat treatment, making it impossible to maintain the modification effect in the long term. In addition, stepwise modification processes are prone to problems such as the coating component intruding into the matrix lattice, disrupting the original stoichiometry and defect structure of the matrix.

[0004] In summary, developing a lithium iron phosphate material that can stably maintain bulk defects, has a double-layer composite coating, is compatible with high-rate applications, and has a matching mass production process is a technical challenge that urgently needs to be solved in this field. Summary of the Invention

[0005] The main objective of this invention is to provide a cathode material and its preparation method, a cathode sheet, and a lithium-ion battery, in order to solve the problem of low rate performance of lithium iron phosphate materials in the prior art.

[0006] To achieve the above objectives, according to one aspect of the present invention, a cathode material is provided, comprising, from the inside out, a lithium iron phosphate core, a carbon coating layer, and a phosphate coating layer coated on the surface of the lithium iron phosphate core; wherein the general chemical formula of the lithium iron phosphate core is Li. 1-x FePO 4-γ, where 0<x≤0.02, 0<γ≤0.02, the lithium iron phosphate core has antisite defects where iron occupies lithium sites, the material of the phosphate coating layer includes NASICON-type phosphate and lithium phosphate, and the chemical general formula of the NASICON-type phosphate is Li a M b (PO4) c , the element M is selected from any one or more of Group IVB metal elements, Group IIIB metal elements, Group VA metal elements, Group IIIA metal elements, and Group VIIB metal elements, 1≤a≤5, 1≤b≤2, 1≤c≤3.

[0007] The lithium iron phosphate core has iron-lithium antisite defects and oxygen vacancies, which helps to improve the bulk lithium ion diffusion efficiency. The carbon coating layer is coated on the surface of the lithium iron phosphate core, which helps to improve the overall electronic conductivity of the positive electrode material. In the material of the phosphate coating layer, the NASICON-type phosphate has a three-dimensionally connected lithium ion channel with excellent ion conduction rate, the presence of lithium phosphate helps to supplement free lithium at the interface and optimize electrolyte wettability, and the synergistic interaction between the NASICON-type phosphate and lithium phosphate helps to reduce electrode polarization. The carbon coating layer is located between the phosphate coating layer and the lithium iron phosphate core, which helps to fundamentally prevent external lithium from penetrating into the matrix and destroying the lithium-deficient defect structure of the inner core. The combination of the lithium iron phosphate core with the inner carbon coating layer and the outer phosphate coating layer helps to reduce the polarization of the positive electrode material under high-rate charge and discharge. Applying the positive electrode material of the present application to a lithium ion battery helps to improve the first charge capacity, first charge-discharge efficiency and rate performance of the lithium ion battery.

[0008] Further, the concentration of the antisite defect is 1at%~4at%; and / or, the average particle size of the lithium iron phosphate core is 300~800nm; and / or, the mass ratio of the carbon coating layer to the lithium iron phosphate core is (0.0125~0.0375):1; and / or, the mass ratio of the phosphate coating layer to the lithium iron phosphate core is (0.0019~0.0076):1; and / or, the molar ratio of the NASICON-type phosphate to lithium phosphate is (1~5):1.

[0009] Controlling the concentration of the antisite defect within the above range helps to increase the bulk ion diffusion rate while maintaining the structural stability of the lithium iron phosphate core. Controlling the average particle size of the lithium iron phosphate core within the above range helps to shorten the lithium ion diffusion distance. Controlling the mass ratio of the carbon coating layer to the lithium iron phosphate core within the above range helps to increase the conductivity of the positive electrode material without affecting lithium ion transmission. Controlling the mass ratio of the phosphate coating layer to the lithium iron phosphate core within the above range helps to improve the lithium ion transmission efficiency. Controlling the molar ratio of the NASICON-type phosphate to lithium phosphate within the above range helps to further reduce electrode polarization.

[0010] Furthermore, the M element is selected from any one or more of the elements Ti, Zr, Sc, Bi, In, Sb, Mn, and Al.

[0011] Controlling the type of element M within the above range helps to improve the ionic conductivity, chemical stability, and mechanical strength of the phosphate coating, thereby contributing to further improvement in the rate performance of lithium-ion batteries.

[0012] Furthermore, the NASICON type phosphate is selected from any one or more of LiTi2(PO4)3, LiZr2(PO4)3, Li3Sc2(PO4)3, Li3Bi2(PO4)3, Li3In2(PO4)3, Li3Sb2(PO4)3 and Li5Mn2(PO4)3.

[0013] The aforementioned types of NASICON-type phosphates help to further improve the rate performance of lithium-ion batteries.

[0014] Furthermore, the NASICON type phosphate is a combination of LiTi2(PO4)3 and LiZr2(PO4)3, and the molar ratio of LiTi2(PO4)3 to LiZr2(PO4)3 is (1~5):1.

[0015] LiTi2(PO4)3 and LiZr2(PO4)3, with different crystal structures and conductivity properties, interpenetrate and coexist to construct a multi-level continuous lithium-ion transport network, effectively reducing charge transfer impedance and fast-charging concentration polarization. At the same time, the soft and hard crystals work together to buffer the lattice deformation caused by lithium insertion and extraction, preventing the coating layer from cracking and falling off. They can also improve the chemical stability of the interface and suppress electrolyte side reactions, resulting in high-rate discharge capability and superior overall performance.

[0016] According to another aspect of the present invention, a method for preparing the aforementioned cathode material is provided, the method comprising: step S1, mixing raw materials including a divalent iron source, a first lithium source, and a first phosphorus source, and then sequentially ball milling and performing a first calcination under a reducing atmosphere to obtain a lithium iron phosphate core, wherein the heating rate of the first calcination is 50~200℃ / min, the cooling rate of the first calcination is ≥20℃ / min, and the molar ratio of lithium element in the first lithium source to iron element in the divalent iron source is (0.98~1):1; step S2, first mixing raw materials including a lithium iron phosphate core and a carbon source, then adding raw materials including a second lithium source, a second phosphorus source, and an M salt and mixing, and then performing a second calcination to obtain a cathode material, wherein the M salt is selected from any one or more of group IVB metal salts, group IIIB metal salts, group VA metal salts, group IIIA metal salts, and group VIIB metal salts.

[0017] In step S1, a lithium-deficient stoichiometry is used to form controllable lithium iron phosphate antisite defects, constructing a three-dimensional diffusion channel, which helps improve lithium-ion transport efficiency. A reducing atmosphere introduces oxygen vacancies, reducing charge transfer impedance. A rapid heating and cooling calcination method freezes the lithium iron phosphate antisite defects and oxygen vacancies within the crystal lattice, resulting in a defect-stable lithium-deficient lithium iron phosphate core. The synergistic effect of the lithium iron phosphate antisite defects and oxygen vacancies helps improve the bulk lithium-ion diffusion coefficient. In step S2, a carbon source is first coated onto the surface of the lithium iron phosphate core, and then a phosphate coating layer is coated onto the surface of the carbon source. This helps form a dense and stable carbon coating layer and a phosphate coating layer including NASICON-type phosphate and lithium phosphate on the surface of the lithium iron phosphate core. Furthermore, this application does not require stepwise calcination; a single calcination is sufficient to form both the carbon coating layer and the phosphate coating layer on the surface of the lithium iron phosphate core, which helps reduce production energy consumption.

[0018] Furthermore, the temperature of the first calcination is 750~800℃, and the holding time is 1~20min; and / or, the reducing gas in the reducing atmosphere is a mixture of hydrogen and other gases, the volume fraction of hydrogen in the reducing gas is 3~10%, and the other gases are selected from any one or more of nitrogen, argon and helium.

[0019] Using the aforementioned first calcination conditions helps to control the concentration of antisite defects and the amount of oxygen vacancies within a suitable range, thereby further improving the bulk lithium-ion diffusion coefficient and thus improving the rate performance of lithium-ion batteries.

[0020] Furthermore, the second calcination includes: first raising the temperature to 380~520℃ and holding it for 1~3 hours, then continuing to raise the temperature to 550~700℃ and holding it for 1~6 hours.

[0021] Carbon source carbonization is completed by holding at 380~520℃ for 1~3h, forming a dense carbon coating layer. The temperature is then increased to 550~700℃ and held for 1~6h, causing an in-situ solid-state reaction of the precursors on the outer side of the carbon layer to generate a phosphate coating layer including NASICON-type phosphate and lithium phosphate. Controlling the temperature and time of the second calcination within the above range helps to avoid the annihilation of antisite defects and oxygen vacancies within the core, promoting full carbonization of the carbon source to form the inner carbon coating layer. The lithium, phosphorus, and metal precursors undergo solid-state reactions, generating an in-situ phosphate coating layer including NASICON-type phosphate and lithium phosphate on the outer side of the carbon layer.

[0022] Further, the mass of the carbon source is 5-15 wt% of the lithium iron phosphate core; and / or, the total mass ratio of the second lithium source, the second phosphorus source, and the M salt to the mass of the lithium iron phosphate core is (0.002-0.008):1; and / or, the divalent iron source is selected from ferrous oxalate and / or ferrous oxide; and / or, the first lithium source and the second lithium source are each independently selected from any one or more of lithium hydroxide, lithium acetate, and lithium nitrate; and / or, the first phosphorus source and the second phosphorus source are each independently selected from ammonium dihydrogen phosphate and / or diammonium hydrogen phosphate; and / or, the carbon source is selected from any one or more of citric acid, sucrose, phenolic resin, and glucose.

[0023] Controlling the proportion of carbon source usage within the aforementioned range helps control the thickness of the carbon coating layer within a suitable range, thereby improving the conductivity of the cathode material without affecting lithium-ion transport. Controlling the ratio of the total mass of the second lithium source, second phosphorus source, and M salt to the mass of the lithium iron phosphate core within the aforementioned range helps control the thickness of the phosphate coating layer within a suitable range, thereby improving lithium-ion transport efficiency. Controlling the type of divalent iron source within the aforementioned range facilitates the control of the formation of lithium iron phosphate antisite defects through a reducing atmosphere in lithium-deficient formulations. Controlling the types of the first and second lithium sources within the aforementioned range helps lower the temperatures of the first and second calcinations, thereby helping to prevent the annihilation of antisite defects and oxygen vacancies within the core. The aforementioned types of first and second phosphorus sources are easily decomposed, thereby promoting the formation of calcination products. Controlling the type of carbon source within the aforementioned range helps promote the formation of amorphous carbon, thereby improving the density of the carbon coating layer and further helping to prevent exogenous lithium from penetrating the matrix and damaging the lithium-deficient core structure.

[0024] According to another aspect of the present invention, a positive electrode sheet is provided, comprising a current collector and a positive electrode active layer disposed on at least one side of the current collector, wherein the material of the positive electrode active layer comprises the aforementioned positive electrode material or a positive electrode material prepared by the aforementioned method for preparing the positive electrode material.

[0025] Because the core of the cathode material in this application has lithium iron phosphate antisite defects and oxygen vacancies, a three-dimensional lithium-ion diffusion channel is constructed, which helps to reduce the bulk lithium-ion diffusion resistance. The carbon coating layer has excellent electrical conductivity, and the phosphate coating layer provides an excellent lithium-ion transport interface. Applying this cathode material to the cathode sheet helps to improve the capacity retention rate at high rates.

[0026] According to another aspect of the present invention, a lithium-ion battery is provided, comprising a positive electrode, an electrolyte, and a negative electrode, wherein the positive electrode is the aforementioned positive electrode.

[0027] Since the positive electrode of the above-mentioned lithium-ion battery contains the positive electrode material of this application, the lithium-ion battery has excellent first charge capacity, first charge-discharge efficiency and rate performance.

[0028] Applying the technical solution of this invention, the lithium iron phosphate core possesses lithium iron phosphate antisite defects and oxygen vacancies. The presence of oxygen vacancies helps reduce interfacial charge transfer resistance. The lithium iron phosphate antisite defects and oxygen vacancies synergistically construct multidimensional lithium-ion diffusion channels, thereby helping to improve the bulk lithium-ion diffusion efficiency. A carbon coating layer is coated on the surface of the lithium iron phosphate core, forming a continuous electron conduction network, which helps to improve the overall electronic conductivity of the cathode material. The NASICON-type phosphate in the phosphate coating layer has three-dimensional through-cell lithium-ion channels and excellent ion conduction rate. The presence of lithium phosphate helps to replenish free lithium at the interface and optimize electrolyte wettability. The synergistic effect between the NASICON-type phosphate and lithium phosphate helps to homogenize the lithium-ion flux, suppress the formation of sharp two-phase interfaces in traditional lithium iron phosphate, and transform the charge-discharge reaction into a quasi-solid solution reaction, thereby reducing electrode polarization. The carbon coating layer is located between the phosphate coating layer and the lithium iron phosphate core, which helps to fundamentally prevent exogenous lithium from penetrating into the matrix and damaging the lithium-deficient structure of the core. The lithium iron phosphate core, combined with an inner carbon coating layer and an outer phosphate coating layer, results in a high powder compaction density for the cathode material. This synergistically optimizes electron-ion interface transport and helps reduce polarization of the cathode material under high-rate charge-discharge conditions. Applying the cathode material of this application to lithium-ion batteries can help improve the initial charge capacity, initial charge-discharge efficiency, and rate performance of lithium-ion batteries. Attached Figure Description

[0029] 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:

[0030] Figure 1 SEM images of the cathode material in Embodiment 1 of this application are shown;

[0031] Figure 2 A TEM image of the cathode material in Embodiment 1 of this application is shown.

[0032] The above figures include the following reference numerals:

[0033] 1. Lithium iron phosphate core; 2. Coating layer. Detailed Implementation

[0034] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0035] The crystal structure framework of the NASICON-type phosphate in this application is constructed by cross-linking MO6 metal octahedra and PO4 phosphate tetrahedra through shared oxygen atoms to build a three-dimensional rigid through-framework, with three-dimensional ion channels in the gaps that allow for rapid lithium ion transition.

[0036] As analyzed in the background section of this application, the existing lithium iron phosphate materials have a low rate performance. To solve the above problem, this application provides a cathode material, a method for preparing the same, a cathode sheet, and a lithium-ion battery.

[0037] In a typical embodiment of this application, a cathode material is provided, which comprises, from the inside out, a lithium iron phosphate core, a carbon coating layer, and a phosphate coating layer covering the surface of the lithium iron phosphate core; wherein the general chemical formula of the lithium iron phosphate core is Li. 1-x FePO 4-γ Where 0 < x ≤ 0.02, 0 < γ ≤ 0.02, the lithium iron phosphate core has antisite defects where iron occupies lithium sites, and the phosphate coating material includes NASICON-type phosphates and lithium phosphate. The general chemical formula of NASICON-type phosphates is Li. a M b (PO4) c The element M is selected from any one or more of the metals in Group IVB, Group IIIB, Group VA, Group IIIA, and Group VIIB, where 1≤a≤5, 1≤b≤2, and 1≤c≤3.

[0038] The lithium iron phosphate (LFP) core possesses lithium iron phosphate antisite defects and oxygen vacancies. The presence of oxygen vacancies helps reduce interfacial charge transfer resistance. The LFP antisite defects and oxygen vacancies synergistically construct multidimensional lithium-ion diffusion channels, thereby improving bulk lithium-ion diffusion efficiency. A carbon coating layer covers the surface of the LFP core, forming a continuous electron conduction network, which helps improve the overall electronic conductivity of the cathode material. The NASICON-type phosphate in the phosphate coating material possesses three-dimensional interconnected lithium-ion channels with excellent ion conduction rates. The presence of lithium phosphate helps replenish free lithium at the interface and optimize electrolyte wettability. The synergistic effect between the NASICON-type phosphate and lithium phosphate helps homogenize lithium-ion flux, suppresses the formation of sharp two-phase interfaces in traditional LFP, and transforms the charge-discharge reaction into a quasi-solid solution reaction, thereby reducing electrode polarization. The carbon coating layer, located between the phosphate coating layer and the LFP core, helps to fundamentally prevent exogenous lithium from penetrating the matrix and damaging the lithium-deficient core structure. The lithium iron phosphate core, combined with an inner carbon coating layer and an outer phosphate coating layer, results in a high powder compaction density for the cathode material. This synergistically optimizes electron-ion interface transport and helps reduce polarization of the cathode material under high-rate charge-discharge conditions. Applying the cathode material of this application to lithium-ion batteries can help improve the initial charge capacity, initial charge-discharge efficiency, and rate performance of lithium-ion batteries.

[0039] In a preferred embodiment, the concentration of antisite defects is 1 at% to 4 at%, specifically 1 at%, 1.5 at%, 2 at%, 2.5 at%, 3 at%, 3.5 at%, 4 at%, and any range between two values; and / or, the average particle size of the lithium iron phosphate core is 300 to 800 nm; and / or, the mass ratio of the carbon coating layer to the lithium iron phosphate core is (0.0125 to 0.0375):1; and / or, the mass ratio of the phosphate coating layer to the lithium iron phosphate core is (0.0019 to 0.0076):1; and / or, the molar ratio of NASICON-type phosphate to lithium phosphate is (1 to 5):1, specifically 1:1, 2:1, 3:1, 4:1, 5:1, and any range between two ratios.

[0040] Controlling the concentration of antisite defects within the aforementioned range helps to improve the bulk ion diffusion rate while maintaining the structural stability of the lithium iron phosphate core. Controlling the average particle size of the lithium iron phosphate core within the aforementioned range helps to shorten the lithium-ion diffusion distance and increase the compaction density of the cathode material, thereby contributing to improved rate performance of the lithium-ion battery. Controlling the mass ratio of the carbon coating layer to the lithium iron phosphate core within the aforementioned range helps to improve the conductivity of the cathode material without affecting lithium-ion transport. Controlling the mass ratio of the phosphate coating layer to the lithium iron phosphate core within the aforementioned range helps to improve lithium-ion transport efficiency. Controlling the molar ratio of NASICON-type phosphate to lithium phosphate within the aforementioned range helps to further enhance the synergistic effect between the two, thereby further reducing electrode polarization and ultimately improving the rate performance of the lithium-ion battery.

[0041] In a preferred embodiment, the M element is selected from any one or more of the elements Ti, Zr, Sc, Bi, In, Sb, Mn, and Al.

[0042] Controlling the type of element M within the above range helps to improve the ionic conductivity, chemical stability, and mechanical strength of the phosphate coating, thereby contributing to further improvement in the rate performance of lithium-ion batteries.

[0043] To further improve the rate performance of lithium-ion batteries, in a preferred embodiment, the NASICON-type phosphate is selected from any one or more of LiTi2(PO4)3, LiZr2(PO4)3, Li3Sc2(PO4)3, Li3Bi2(PO4)3, Li3In2(PO4)3, Li3Sb2(PO4)3 and Li5Mn2(PO4)3.

[0044] In a preferred embodiment, the NASICON-type phosphate is a combination of LiTi2(PO4)3 and LiZr2(PO4)3, and the molar ratio of LiTi2(PO4)3 to LiZr2(PO4)3 is (1~5):1.

[0045] LiTi2(PO4)3 and LiZr2(PO4)3, with different crystal structures and conductivity properties, interpenetrate and coexist to construct a multi-level continuous lithium-ion transport network, effectively reducing charge transfer impedance and fast-charging concentration polarization. At the same time, the soft and hard crystals work together to buffer the lattice deformation caused by lithium insertion and extraction, preventing the coating layer from cracking and falling off. They can also improve the chemical stability of the interface and suppress electrolyte side reactions, resulting in high-rate discharge capability and superior overall performance.

[0046] In a preferred embodiment, the powder compaction density of the positive electrode material is 2.55~2.60 g / cc.

[0047] The cathode material with the above-mentioned powder compaction density has a better internal electron and ion transport network, which helps to improve the rate performance and cycle stability of lithium ions.

[0048] In another typical embodiment of this application, a method for preparing the aforementioned cathode material is provided. The method includes: step S1, mixing raw materials including a divalent iron source, a first lithium source, and a first phosphorus source, and then sequentially ball milling and performing a first calcination under a reducing atmosphere to obtain a lithium iron phosphate core. The heating rate of the first calcination is 50~200℃ / min, the cooling rate of the first calcination is ≥20℃ / min, and the molar ratio of lithium element in the first lithium source to iron element in the divalent iron source is (0.98~1):1; step S2, first mixing raw materials including a lithium iron phosphate core and a carbon source, and then adding raw materials including a second lithium source, a second phosphorus source, and an M salt and mixing them, and then performing a second calcination to obtain a cathode material. The M salt is selected from any one or more of group IVB metal salts, group IIIB metal salts, group VA metal salts, group IIIA metal salts, and group VIIB metal salts.

[0049] In step S1, a lithium-deficient stoichiometry is used. The lithium site vacancies force some iron atoms to occupy the lithium sites, forming a controllable lithium iron phosphate antisite defect. This breaks the one-dimensional ion diffusion limitation of lithium iron phosphate and constructs a three-dimensional diffusion channel, which helps to improve the lithium-ion transport efficiency. The reducing atmosphere can introduce oxygen vacancies and reduce charge transfer resistance. The rapid heating and cooling calcination method freezes the lithium iron phosphate antisite defects and oxygen vacancies in the lattice, resulting in a defect-stable lithium-deficient lithium iron phosphate core. The synergistic effect of the lithium iron phosphate antisite defects and oxygen vacancies helps to improve the bulk lithium-ion diffusion coefficient. In step S2, a carbon source is first coated on the surface of the lithium iron phosphate core, and then a phosphate coating layer is coated on the surface of the carbon source. This helps to form a dense and stable carbon coating layer and a phosphate coating layer including NASICON-type phosphate and lithium phosphate on the surface of the lithium iron phosphate core. Furthermore, this application does not require step-by-step calcination; a single calcination can form a carbon coating layer and a phosphate coating layer on the surface of the lithium iron phosphate core, which helps to reduce production energy consumption.

[0050] Including but not limited to, the above-mentioned M salt is selected from any one or more of tetrabutyl titanate, zirconium nitrate, scandium nitrate, bismuth nitrate, indium nitrate, antimony acetate, manganese nitrate, zirconium nitrate, scandium nitrate, and aluminum nitrate.

[0051] In a preferred embodiment, the heating rate of the first calcination is 120~200℃ / min, and the cooling rate of the first calcination is 50~60℃ / min.

[0052] The second calcination method can be selected from any one of microwave calcination, flash calcination, and Joule heating calcination.

[0053] In order to control the concentration of antisite defects and the amount of oxygen vacancies within a suitable range, thereby further improving the bulk lithium-ion diffusion coefficient and thus improving the rate performance of lithium-ion batteries, in a preferred embodiment, the first calcination temperature is 750~800℃ and the holding time is 1~20min; and / or, the reducing gas in the reducing atmosphere is a mixture of hydrogen and other gases, the volume fraction of hydrogen in the reducing gas is 3~10%, and the other gases are selected from any one or more of nitrogen, argon and helium.

[0054] In a preferred embodiment, the second calcination includes: first raising the temperature to 380~520℃ and holding it for 1~3 hours, then raising the temperature to 550~700℃ and holding it for 1~6 hours.

[0055] Carbon source carbonization is completed by holding at 380~520℃ for 1~3h, forming a dense carbon coating layer. The temperature is then increased to 550~700℃ and held for 1~6h, causing an in-situ solid-state reaction of the precursors on the outer side of the carbon layer to generate a phosphate coating layer including NASICON-type phosphate and lithium phosphate. Controlling the temperature and time of the second calcination within the above range helps to avoid the annihilation of antisite defects and oxygen vacancies within the core, promoting full carbonization of the carbon source to form the inner carbon coating layer. The lithium, phosphorus, and metal precursors undergo solid-state reactions, generating an in-situ phosphate coating layer including NASICON-type phosphate and lithium phosphate on the outer side of the carbon layer.

[0056] In a preferred embodiment, the mass of the carbon source is 5-15 wt% of the lithium iron phosphate core; and / or, the ratio of the total mass of the second lithium source, the second phosphorus source, and the M salt to the mass of the lithium iron phosphate core is (0.002-0.008):1; and / or, the ferrous iron source is selected from ferrous oxalate and / or ferrous oxide; and / or, the first lithium source and the second lithium source are each independently selected from any one or more of lithium hydroxide, lithium acetate, and lithium nitrate; and / or, the first phosphorus source and the second phosphorus source are each independently selected from ammonium dihydrogen phosphate and / or diammonium hydrogen phosphate; and / or, the carbon source is selected from any one or more of citric acid, sucrose, phenolic resin, and glucose.

[0057] Controlling the proportion of carbon source usage within the aforementioned range helps control the thickness of the carbon coating layer within a suitable range, thereby improving the conductivity of the cathode material without affecting lithium-ion transport. Controlling the ratio of the total mass of the second lithium source, second phosphorus source, and M salt to the mass of the lithium iron phosphate core within the aforementioned range helps control the thickness of the phosphate coating layer within a suitable range, thereby improving lithium-ion transport efficiency. Controlling the type of divalent iron source within the aforementioned range facilitates the control of the formation of lithium iron phosphate antisite defects through a reducing atmosphere in lithium-deficient formulations. Controlling the types of the first and second lithium sources within the aforementioned range helps lower the temperatures of the first and second calcinations, thereby helping to prevent the annihilation of antisite defects and oxygen vacancies within the core. The aforementioned types of first and second phosphorus sources are easily decomposed, thereby promoting the formation of calcination products. Controlling the type of carbon source within the aforementioned range helps promote the formation of amorphous carbon, thereby improving the density of the carbon coating layer and further helping to prevent exogenous lithium from penetrating the matrix and damaging the lithium-deficient core structure.

[0058] In a preferred embodiment, the carbon source is a combination of phenolic resin and citric acid, and the mass ratio of phenolic resin to citric acid is (2~6):1, and the number average molecular weight of phenolic resin is 2000~8000 g / mol.

[0059] Phenolic resin forms a dense film, providing a physical barrier effect, while small-molecule citric acid improves powder wetting and spreading, and alleviates agglomeration. The combination of the two works synergistically to improve coating uniformity while ensuring the continuous and dense carbon film, reduce particle agglomeration, and obtain a carbon coating layer with better overall performance.

[0060] In another typical embodiment of this application, a positive electrode sheet is provided, including a current collector and a positive electrode active layer disposed on at least one side of the current collector, wherein the material of the positive electrode active layer includes the aforementioned positive electrode material or a positive electrode material prepared by the aforementioned method for preparing positive electrode material.

[0061] Because the core of the cathode material in this application has lithium iron phosphate antisite defects and oxygen vacancies, a three-dimensional lithium-ion diffusion channel is constructed, which helps to reduce the bulk lithium-ion diffusion resistance. The carbon coating layer has excellent electrical conductivity, and the phosphate coating layer provides an excellent lithium-ion transport interface. Applying this cathode material to the cathode sheet helps to improve the capacity retention rate at high rates.

[0062] In another typical embodiment of this application, a lithium-ion battery is provided, including a positive electrode, an electrolyte, and a negative electrode, wherein the positive electrode is the aforementioned positive electrode.

[0063] Since the positive electrode of the above-mentioned lithium-ion battery contains the positive electrode material of this application, the lithium-ion battery has excellent first charge capacity, first charge-discharge efficiency and rate performance.

[0064] The beneficial effects of this application will be further illustrated below with reference to the embodiments.

[0065] Example 1

[0066] Ferrous oxalate, lithium hydroxide, ammonium dihydrogen phosphate, and deionized water were mixed and ball-milled for 4 hours. The molar ratio of lithium in lithium hydroxide to iron in ferrous oxalate was 0.99:1. Ammonium dihydrogen phosphate was added according to the molar ratio of iron to phosphorus in the final product being 1:1. The mixture was then dried to obtain a powder. The powder was placed in a 5% (v / v) hydrogen-nitrogen atmosphere for a first calcination. The temperature was increased to 780°C at a heating rate of 150°C / min, held for 10 min, and then cooled to room temperature at a cooling rate of 55°C / min to obtain the lithium iron phosphate core.

[0067] Lithium iron phosphate core, phenolic resin (purchased from Shanghai Yuanye Biotechnology Co., Ltd., number average molecular weight 5000 g / mol), and deionized water were mixed, with the phenolic resin weighing 7 wt% of the lithium iron phosphate core. The mixture was stirred at 100 rpm for 30 min. Then, a mixture of lithium hydroxide, ammonium dihydrogen phosphate, and tetrabutyl titanate was added, with a mass ratio of the mixture to the lithium iron phosphate core of 0.005:1. The addition ratio of the materials in the mixture was based on a final molar ratio of LiTi2(PO4)3 to lithium phosphate of 4:1. The mixture was then gently dispersed at low temperature and ultrasonically, and vacuum dried at 110℃ to obtain a precursor. The precursor was then subjected to a second calcination under a nitrogen atmosphere, heated to 490℃ and held for 2 h, then heated to 680℃ and held for 2 h, and finally cooled to room temperature to obtain the cathode material. The cathode material, from the inside out, consists of a lithium iron phosphate core, a carbon coating layer, and a phosphate coating layer covering the surface of the lithium iron phosphate core. The chemical formula of the lithium iron phosphate core is Li 0.989 FePO 3.992 The lithium iron phosphate core has antisite defects where iron occupies lithium sites, with an antisite defect concentration of 2.22 at%. The phosphate coating material is NASICON-type LiTi2(PO4)3 and lithium phosphate in a molar ratio of 4:1. The average particle size of the lithium iron phosphate core is 520 nm. The mass ratio of the carbon coating to the lithium iron phosphate core is 0.0175:1, and the mass ratio of the phosphate coating to the lithium iron phosphate core is 0.00475:1.

[0068] Example 2

[0069] The difference from Example 1 is that ferrous oxide, lithium hydroxide, diammonium hydrogen phosphate and deionized water were mixed and ball-milled for 4 hours. The molar ratio of lithium in lithium hydroxide to iron in ferrous oxide was 0.98:1. Diammonium hydrogen phosphate was added according to the molar ratio of iron to phosphorus in the final product of 1:1. Then the mixture was dried to obtain powder. The powder was placed in a hydrogen-nitrogen atmosphere with a volume fraction of 8% for the first calcination. The temperature was increased to 750°C at a heating rate of 120°C / min and held for 15 min. The temperature was then reduced to room temperature at a cooling rate of 52°C / min to obtain lithium iron phosphate core.

[0070] Lithium iron phosphate (LiFePO4) core, sucrose (6 wt% of LiFePO4 core), and deionized water were mixed and stirred at 120 rpm for 25 min. Then, a mixture of lithium nitrate, ammonium dihydrogen phosphate, and zirconium nitrate was added, with a mass ratio of the mixture to the LiFePO4 core of 0.004:1. The materials in the mixture were added according to a final molar ratio of LiZr2(PO4)3 to lithium phosphate of 4:1. The mixture was then gently dispersed at low temperature and ultrasonically, and vacuum dried at 110 °C to obtain a precursor. The precursor was then subjected to a second calcination under a nitrogen atmosphere, heated to 460 °C and held for 2.5 h, then heated to 650 °C and held for 3 h, and finally cooled to room temperature to obtain the cathode material. The cathode material, from the inside out, consists of a LiFePO4 core, a carbon coating layer, and a phosphate coating layer covering the surface of the LiFePO4 core. The chemical formula of the LiFePO4 core is Li... 0.980 FePO 3.987 The lithium iron phosphate core has antisite defects where iron occupies lithium sites, with an antisite defect concentration of 1.8 at%. The phosphate coating material is NASICON-type LiZr2(PO4)3 and lithium phosphate in a molar ratio of 4:1. The average particle size of the lithium iron phosphate core is 480 nm. The mass ratio of the carbon coating to the lithium iron phosphate core is 0.0150:1, and the mass ratio of the phosphate coating to the lithium iron phosphate core is 0.0038:1.

[0071] Example 3

[0072] The difference from Example 1 is that ferrous oxalate, lithium hydroxide, ammonium dihydrogen phosphate and deionized water were mixed and ball-milled for 4 hours. The molar ratio of lithium in lithium hydroxide to iron in ferrous oxalate was 1:1. Ammonium dihydrogen phosphate was added according to the molar ratio of iron to phosphorus in the final product of 1:1. Then the mixture was dried to obtain powder. The powder was placed in a hydrogen-nitrogen atmosphere with a volume fraction of 3% for the first calcination. The temperature was increased to 800°C at a heating rate of 200°C / min, held for 8 min, and then cooled to room temperature at a cooling rate of 55°C / min to obtain lithium iron phosphate core.

[0073] Lithium iron phosphate (LiFePO4) core, citric acid, and deionized water were mixed, with the citric acid comprising 9 wt% of the LiFePO4 core. The mixture was stirred at 80 rpm for 40 min. Then, a mixture of lithium acetate, ammonium dihydrogen phosphate, and scandium nitrate was added, with a mass ratio of the mixture to the LiFePO4 core of 0.006:1. The addition ratio of the materials in the mixture was based on a final molar ratio of Li3Sc2(PO4)3 to lithium phosphate of 4:1. The mixture was then gently dispersed at low temperature and ultrasonically, and vacuum dried at 110 °C to obtain a precursor. The precursor was then subjected to a second calcination under a nitrogen atmosphere, heated to 480 °C and held for 2 h, then heated to 670 °C and held for 1.5 h, and finally cooled to room temperature to obtain the cathode material. The cathode material, from the inside out, comprises a LiFePO4 core, a carbon coating layer, and a phosphate coating layer covering the surface of the LiFePO4 core. The chemical formula of the LiFePO4 core is Li... 0.996 FePO 3.982 The lithium iron phosphate core has antisite defects where iron occupies lithium sites, with an antisite defect concentration of 3.1 at%. The phosphate coating material is NASICON type Li3Sc2(PO4)3 and lithium phosphate, with a molar ratio of 4:1. The average particle size of the lithium iron phosphate core is 610 nm. The mass ratio of the carbon coating layer to the lithium iron phosphate core is 0.0225:1, and the mass ratio of the phosphate coating layer to the lithium iron phosphate core is 0.0057:1.

[0074] Example 4

[0075] The difference from Example 1 is that ferrous oxalate, lithium hydroxide, ammonium dihydrogen phosphate and deionized water were mixed and ball-milled for 4 hours. The molar ratio of lithium in lithium hydroxide to iron in ferrous oxalate was 0.99:1. Ammonium dihydrogen phosphate was added according to the molar ratio of iron to phosphorus in the final product of 1:1. Then the mixture was dried to obtain powder. The powder was placed in a hydrogen-nitrogen atmosphere with a volume fraction of 5% for the first calcination. The temperature was increased to 780°C at a heating rate of 180°C / min and held for 10 min. The temperature was then reduced to room temperature at a cooling rate of 55°C / min to obtain lithium iron phosphate core.

[0076] Lithium iron phosphate (LiFePO4) core, glucose, and deionized water were mixed, with the glucose content being 8 wt% of the LiFePO4 core. The mixture was stirred at 150 rpm for 20 min. Then, a mixture of lithium nitrate, ammonium dihydrogen phosphate, and bismuth nitrate was added, with a mass ratio of the mixture to the LiFePO4 core of 0.005:1. The addition ratio of the materials in the mixture was based on a final molar ratio of Li3Bi2(PO4)3 to lithium phosphate of 4:1. The mixture was then gently dispersed at low temperature and ultrasonically, and vacuum dried at 110 °C to obtain a precursor. The precursor was then subjected to a second calcination under a nitrogen atmosphere, heated to 500 °C and held for 2 h, then heated to 660 °C and held for 2 h, and finally cooled to room temperature to obtain the cathode material. The cathode material, from the inside out, consists of a LiFePO4 core, a carbon coating layer, and a phosphate coating layer covering the surface of the LiFePO4 core. The chemical formula of the LiFePO4 core is Li... 0.989 FePO 3.991 The lithium iron phosphate core has antisite defects where iron occupies lithium sites, with an antisite defect concentration of 2.0 at%. The phosphate coating material is NASICON type Li3Bi2(PO4)3 and lithium phosphate in a molar ratio of 4:1. The average particle size of the lithium iron phosphate core is 550 nm. The mass ratio of the carbon coating layer to the lithium iron phosphate core is 0.0200:1, and the mass ratio of the phosphate coating layer to the lithium iron phosphate core is 0.00475:1.

[0077] Example 5

[0078] The difference from Example 1 is that ferrous oxide, lithium hydroxide, ammonium dihydrogen phosphate and deionized water were mixed and ball-milled for 4 hours. The molar ratio of lithium in lithium hydroxide to iron in ferrous oxide was 0.995:1. Ammonium dihydrogen phosphate was added according to the molar ratio of iron to phosphorus in the final product of 1:1. Then the mixture was dried to obtain powder. The powder was placed in a hydrogen-nitrogen atmosphere with a volume fraction of 5% for the first calcination. The temperature was increased to 770°C at a heating rate of 150°C / min and held for 12 min. The temperature was then reduced to room temperature at a cooling rate of 53°C / min to obtain lithium iron phosphate core.

[0079] Lithium iron phosphate (LiFePO4) core, carbon source, and deionized water were mixed. The carbon source, consisting of a combination of citric acid and sucrose (7.5 wt% of the LiFePO4 core), was stirred at 100 rpm for 30 min. Then, a mixture of lithium hydroxide, ammonium dihydrogen phosphate, and indium nitrate was added, with a mass ratio of the mixture to the LiFePO4 core of 0.005:1. The materials were added according to a final molar ratio of Li3In2(PO4)3 to lithium phosphate of 4:1. The mixture was then gently dispersed at low temperature and ultrasonically, and vacuum dried at 110°C to obtain a precursor. The precursor was then subjected to a second calcination under a nitrogen atmosphere, heated to 470°C and held for 2 h, then heated to 690°C and held for 2 h, before being cooled to room temperature to obtain the cathode material. The cathode material, from the inside out, consisted of a LiFePO4 core, a carbon coating layer, and a phosphate coating layer covering the surface of the LiFePO4 core. The chemical formula of the LiFePO4 core was Li 0.993 FePO 3.990 The lithium iron phosphate core has antisite defects where iron occupies lithium sites, with an antisite defect concentration of 2.4 at%. The phosphate coating material is NASICON type Li3In2(PO4)3 and lithium phosphate in a molar ratio of 4:1. The average particle size of the lithium iron phosphate core is 500 nm. The mass ratio of the carbon coating layer to the lithium iron phosphate core is 0.01875:1, and the mass ratio of the phosphate coating layer to the lithium iron phosphate core is 0.00475:1.

[0080] Example 6

[0081] The difference from Example 1 is that ferrous oxide, lithium hydroxide, ammonium dihydrogen phosphate and deionized water were mixed and ball-milled for 4 hours. The molar ratio of lithium in lithium hydroxide to iron in ferrous oxide was 0.985:1. Ammonium dihydrogen phosphate was added according to the molar ratio of iron to phosphorus in the final product of 1:1. Then the mixture was dried to obtain powder. The powder was placed in a hydrogen-nitrogen atmosphere with a volume fraction of 6% for the first calcination. The temperature was increased to 760°C at a heating rate of 150°C / min and held for 12 min. The temperature was then reduced to room temperature at a cooling rate of 55°C / min to obtain lithium iron phosphate core.

[0082] Lithium iron phosphate (LiFePO4) core, glucose, and deionized water were mixed, with the glucose content being 7 wt% of the LiFePO4 core. The mixture was stirred at 120 rpm for 25 min. Then, a mixture of lithium acetate, ammonium dihydrogen phosphate, and antimony acetate was added, with a mass ratio of the mixture to the LiFePO4 core of 0.0045:1. The addition ratio of the materials in the mixture was based on a final molar ratio of Li3Sb2(PO4)3 to lithium phosphate of 4:1. The mixture was then gently dispersed at low temperature and ultrasonically, and vacuum-dried at 110°C to obtain a precursor. The precursor was then subjected to a second calcination under a nitrogen atmosphere, heated to 490°C and held for 2 h, then heated to 670°C and held for 2 h, and finally cooled to room temperature to obtain the cathode material. The cathode material, from the inside out, consists of a LiFePO4 core, a carbon coating layer, and a phosphate coating layer covering the surface of the LiFePO4 core. The chemical formula of the LiFePO4 core is Li... 0.985 FePO 3.989 The lithium iron phosphate core has antisite defects where iron occupies lithium sites, with an antisite defect concentration of 1.9 at%. The phosphate coating material is NASICON-type Li3Sb2(PO4)3 and lithium phosphate in a molar ratio of 4:1. The average particle size of the lithium iron phosphate core is 490 nm. The mass ratio of the carbon coating to the lithium iron phosphate core is 0.0175:1, and the mass ratio of the phosphate coating to the lithium iron phosphate core is 0.004275:1.

[0083] Example 7

[0084] The difference from Example 1 is that ferrous oxalate, lithium hydroxide, ammonium dihydrogen phosphate and deionized water were mixed and ball-milled for 4 hours. The molar ratio of lithium in lithium hydroxide to iron in ferrous oxalate was 0.99:1. Ammonium dihydrogen phosphate was added according to the molar ratio of iron to phosphorus in the final product of 1:1. Then the mixture was dried to obtain powder. The powder was placed in a hydrogen-nitrogen atmosphere with a volume fraction of 5% for the first calcination. The temperature was increased to 790°C at a heating rate of 180°C / min and held for 9 minutes. The temperature was then reduced to room temperature at a cooling rate of 55°C / min to obtain lithium iron phosphate core.

[0085] Lithium iron phosphate core, carbon source, and deionized water were mixed. The carbon source, comprising 8.5 wt% of the lithium iron phosphate core, was a mixture of phenolic resin (purchased from Shanghai Yuanye Biotechnology Co., Ltd., with a number average molecular weight of 5000 g / mol) and citric acid, with a mass ratio of phenolic resin to citric acid of 4:1. The mixture was stirred at 90 rpm for 35 min. Then, a mixture of lithium nitrate, ammonium dihydrogen phosphate, and manganese nitrate was added, with a mass ratio of the mixture to the lithium iron phosphate core of 0.006:1. The composition of the mixture... The additives were prepared according to a final molar ratio of Li5Mn2(PO4)3 to lithium phosphate of 4:1. The mixture was then gently dispersed at low temperature and ultrasonically, and vacuum dried at 110°C to obtain the precursor. The precursor was then subjected to a second calcination under a nitrogen atmosphere, heated to 480°C and held for 2 hours, then heated to 680°C and held for 2 hours, and finally cooled to room temperature to obtain the cathode material. The cathode material, from the inside out, comprises a lithium iron phosphate core, a carbon coating layer, and a phosphate coating layer covering the surface of the lithium iron phosphate core. The chemical formula of the lithium iron phosphate core is Li... 0.989 FePO 3.983 The lithium iron phosphate core has antisite defects where iron occupies lithium sites, with an antisite defect concentration of 3 at%. The phosphate coating material is NASICON type Li5Mn2(PO4)3 and lithium phosphate in a molar ratio of 4:1. The average particle size of the lithium iron phosphate core is 580 nm. The mass ratio of the carbon coating to the lithium iron phosphate core is 0.02125:1, and the mass ratio of the phosphate coating to the lithium iron phosphate core is 0.0057:1.

[0086] Example 8

[0087] The difference from Example 1 is that ferrous oxalate, lithium hydroxide, ammonium dihydrogen phosphate and deionized water were mixed and ball-milled for 4 hours. The molar ratio of lithium in lithium hydroxide to iron in ferrous oxalate was 1:1. Ammonium dihydrogen phosphate was added according to the molar ratio of iron to phosphorus in the final product of 1:1. Then the mixture was dried to obtain powder. The powder was placed in a hydrogen-nitrogen atmosphere with a volume fraction of 3% for the first calcination. The temperature was increased to 800°C at a heating rate of 200°C / min, held for 8 min, and then cooled to room temperature at a cooling rate of 55°C / min to obtain lithium iron phosphate core.

[0088] Lithium iron phosphate core, phenolic resin (purchased from Shanghai Yuanye Biotechnology Co., Ltd., number average molecular weight 5000 g / mol), and deionized water were mixed. The mass of phenolic resin was 6.5 wt% of the lithium iron phosphate core. The mixture was stirred at 130 rpm for 25 min. Then, a mixture of lithium acetate, ammonium dihydrogen phosphate, and tetrabutyl titanate was added. The mass ratio of the mixture to the lithium iron phosphate core was 0.0055:1. The addition ratio of the materials in the mixture was based on the final formation of LiT The i2(PO4)3 molar ratio of i2(PO4)3 to lithium phosphate was 4:1, and the mixture was dispersed at low temperature and with gentle ultrasonication. The precursor was then vacuum-dried at 110°C to obtain the precursor. The precursor was then subjected to a second calcination under a nitrogen atmosphere, heated to 450°C and held for 2.5 hours, then heated to 675°C and held for 2 hours, and finally cooled to room temperature to obtain the cathode material. The cathode material, from the inside out, consists of a lithium iron phosphate core, a carbon coating layer, and a phosphate coating layer covering the surface of the lithium iron phosphate core. The chemical formula of the lithium iron phosphate core is Li... 0.997 FePO 3.981 The lithium iron phosphate core has antisite defects where iron occupies lithium sites, with an antisite defect concentration of 2.1 at%. The phosphate coating material is NASICON-type LiTi2(PO4)3 and lithium phosphate in a molar ratio of 4:1. The average particle size of the lithium iron phosphate core is 630 nm. The mass ratio of the carbon coating to the lithium iron phosphate core is 0.0175:1, and the mass ratio of the phosphate coating to the lithium iron phosphate core is 0.003325:1.

[0089] Example 9

[0090] The difference from Example 1 is that ferrous oxide, lithium hydroxide, ammonium dihydrogen phosphate and deionized water were mixed and ball-milled for 4 hours. The molar ratio of lithium in lithium hydroxide to iron in ferrous oxide was 0.99:1. Ammonium dihydrogen phosphate was added according to the molar ratio of iron to phosphorus in the final product of 1:1. Then the mixture was dried to obtain powder. The powder was placed in a hydrogen-nitrogen atmosphere with a volume fraction of 5% for the first calcination. The temperature was increased to 775°C at a heating rate of 190°C / min and held for 11 min. The temperature was then reduced to room temperature at a cooling rate of 55°C / min to obtain lithium iron phosphate core.

[0091] Lithium iron phosphate (LiFePO4) core, glucose, and deionized water were mixed, with glucose accounting for 9 wt% of the LiFePO4 core. The mixture was stirred at 100 rpm for 30 min. Then, a mixture of lithium nitrate, ammonium dihydrogen phosphate, and zirconium nitrate was added, with a mass ratio of the mixture to the LiFePO4 core of 0.0052:1. The addition ratio of the materials in the mixture was based on a final molar ratio of LiZr2(PO4)3 to lithium phosphate of 4:1. The mixture was then gently dispersed at low temperature and ultrasonically, and vacuum dried at 110 °C to obtain a precursor. The precursor was then subjected to a second calcination under a nitrogen atmosphere, heated to 465 °C and held for 2 h, then heated to 660 °C and held for 2 h, and finally cooled to room temperature to obtain the cathode material. The cathode material, from the inside out, consists of a LiFePO4 core, a carbon coating layer, and a phosphate coating layer covering the surface of the LiFePO4 core. The chemical formula of the LiFePO4 core is Li... 0.990 FePO 3.991 The lithium iron phosphate core has antisite defects where iron occupies lithium sites, with an antisite defect concentration of 2.3 at%. The phosphate coating material is NASICON-type LiZr2(PO4)3 and lithium phosphate in a molar ratio of 4:1. The average particle size of the lithium iron phosphate core is 540 nm. The mass ratio of the carbon coating layer to the lithium iron phosphate core is 0.01625:1, and the mass ratio of the phosphate coating layer to the lithium iron phosphate core is 0.005225:1.

[0092] Example 10

[0093] The difference from Example 1 is that ferrous oxalate, lithium hydroxide, ammonium dihydrogen phosphate and deionized water were mixed and ball-milled for 4 hours. The molar ratio of lithium in lithium hydroxide to iron in ferrous oxalate was 0.988:1. Ammonium dihydrogen phosphate was added according to the molar ratio of iron to phosphorus in the final product of 1:1. Then the mixture was dried to obtain powder. The powder was placed in a hydrogen-argon atmosphere with a volume fraction of 6% for the first calcination. The temperature was increased to 765°C at a heating rate of 170°C / min and held for 14 min. The temperature was then reduced to room temperature at a cooling rate of 55°C / min to obtain lithium iron phosphate core.

[0094] Lithium iron phosphate (LiFePO4) core, citric acid, and deionized water were mixed, with the citric acid comprising 6.8 wt% of the LiFePO4 core. The mixture was stirred at 120 rpm for 25 min. Then, a mixture of lithium acetate, ammonium dihydrogen phosphate, and scandium nitrate was added, with a mass ratio of the mixture to the LiFePO4 core of 0.0048:1. The addition ratio of the materials in the mixture was based on a final molar ratio of Li3Sc2(PO4)3 to lithium phosphate of 4:1. The mixture was then gently dispersed at low temperature and ultrasonically, and vacuum-dried at 110 °C to obtain a precursor. The precursor was then subjected to a second calcination under a nitrogen atmosphere, heated to 485 °C and held for 2 h, then heated to 670 °C and held for 2 h, and finally cooled to room temperature to obtain the cathode material. The cathode material, from the inside out, comprises a LiFePO4 core, a carbon coating layer, and a phosphate coating layer covering the surface of the LiFePO4 core. The chemical formula of the LiFePO4 core is Li 0.988 FePO 3.990 The lithium iron phosphate core has antisite defects where iron occupies lithium sites, with an antisite defect concentration of 2 at%. The phosphate coating material is NASICON-type Li3Sc2(PO4)3 and lithium phosphate in a molar ratio of 4:1. The average particle size of the lithium iron phosphate core is 505 nm. The mass ratio of the carbon coating to the lithium iron phosphate core is 0.0225:1, and the mass ratio of the phosphate coating to the lithium iron phosphate core is 0.00494:1.

[0095] Example 11

[0096] The difference from Example 1 is that the heating rate of the first calcination is 50°C / min, and the cooling rate is 20°C / min, ultimately yielding the cathode material; the chemical formula of the lithium iron phosphate core is Li. 0.989 FePO 3.992 The concentration of inversion defects was 0.8 at.

[0097] Example 12

[0098] The difference from Example 1 is that a combination of NASICON-type LiTi2(PO4)3 and NASICON-type LiZr2(PO4)3 is used to replace NASICON-type LiTi2(PO4)3, and the molar ratio of NASICON-type LiTi2(PO4)3 to NASICON-type LiZr2(PO4)3 in the combination of NASICON-type LiTi2(PO4)3 and NASICON-type LiZr2(PO4)3 is 5:1, and finally the cathode material is obtained.

[0099] Example 13

[0100] The difference from Example 1 is that a combination of NASICON-type LiTi2(PO4)3 and NASICON-type LiZr2(PO4)3 is used to replace NASICON-type LiTi2(PO4)3, and the molar ratio of NASICON-type LiTi2(PO4)3 to NASICON-type LiZr2(PO4)3 in the combination of NASICON-type LiTi2(PO4)3 and NASICON-type LiZr2(PO4)3 is 1:1, and finally the cathode material is obtained.

[0101] Example 14

[0102] The difference from Example 1 is that the carbon source is a combination of phenolic resin (purchased from Shanghai Yuanye Biotechnology Co., Ltd., with a number average molecular weight of 5000 g / mol) and citric acid, and the mass ratio of phenolic resin to citric acid is 2:1, ultimately yielding the cathode material.

[0103] Example 15

[0104] The difference from Example 1 is that the carbon source is a combination of phenolic resin (purchased from Shanghai Yuanye Biotechnology Co., Ltd., with a number average molecular weight of 5000 g / mol) and citric acid, and the mass ratio of phenolic resin to citric acid is 6:1, ultimately yielding the cathode material.

[0105] Comparative Example 1

[0106] The difference from Example 1 is that the molar ratio of iron in ferrous oxalate, lithium in lithium hydroxide, and phosphorus in ammonium dihydrogen phosphate is 1:1.05:1, and the addition of the mixture of lithium hydroxide, ammonium dihydrogen phosphate, and tetrabutyl titanate is omitted, thus obtaining the cathode material.

[0107] Comparative Example 2

[0108] The difference from Example 1 is that the mixture of lithium hydroxide, ammonium dihydrogen phosphate and tetrabutyl titanate was omitted, and the positive electrode material was finally obtained.

[0109] Comparative Example 3

[0110] The difference from Example 1 is that the heating rate of the first calcination is 5°C / min and the cooling rate is 10°C / min, and finally the positive electrode material is obtained.

[0111] Comparative Example 4

[0112] The difference from Example 1 is that the coating order is reversed. First, the lithium iron phosphate core is mixed with a mixture of lithium hydroxide, ammonium dihydrogen phosphate and tetrabutyl titanate, and then phenolic resin is added and mixed to finally obtain the cathode material.

[0113] coin cell preparation

[0114] The positive electrode material, conductive carbon black, and polyvinylidene fluoride prepared in the examples and comparative examples were mixed in a mass ratio of 8:1:1 and fabricated into CR2032 coin cells in an argon protective atmosphere.

[0115] Performance testing

[0116] Antisite defect testing: Quantitative detection of antisite defects was performed using cyclic voltammetry (CV) calibration. The sample was assembled into a CR2032 coin cell, and CV curves were acquired at 25℃ and a scan rate of 0.1 mV / s, ranging from 2.5 to 4.0 V. The redox peak potential difference ΔV was read, and a calibration curve was established using pre-prepared standard samples with a 1–4 at% gradient defect. The concentration of antisite defects in lithium iron phosphate was then calculated. Average particle size testing of lithium iron phosphate cores: Lithium iron phosphate cores were prepared and observed using a scanning electron microscope (SEM). Images were acquired using an SEM with a built-in micrometer scale, and the equivalent circle diameter of the particles was measured manually.

[0117] Li 1-x FePO 4-γ x-value and γ-value testing: Rietveld refinement method of XRD was used. Li vacancies and oxygen vacancies will cause slight shifts in unit cell parameters. The O occupancy rate was fitted by full spectrum refinement. Li vacancies were measured by inductively coupled plasma optical emission spectroscopy (ICP-OES): Dry lithium iron phosphate powder was accurately weighed, digested by microwave with concentrated nitric acid and hydrofluoric acid, and then the volume was adjusted. The molar concentrations of Li, Fe and P elements in the solution were determined by ICP-OES. Based on the amount of P, the average bulk lithium deficiency coefficient was calculated according to the formula x=1-n(Li) / n(P).

[0118] Test of compacted density of cathode material powder: The compacted density was measured according to the method of GB / T24533-2019. The results of the powder compaction density are shown in Table 1.

[0119] Electrical performance testing of coin cells: The coin cells prepared in the examples and comparative examples were charged and discharged at 25°C with a voltage range of 2.5 to 4.0V. The specific capacity of the first charge at 0.2C, the efficiency of the first charge and discharge at 0.2C, the specific capacity of the discharge at 0.2C, the specific capacity of the discharge at 1C, and the specific capacity of the discharge at 2C were recorded. The results are shown in Table 1.

[0120] Table 1

[0121]

[0122] As can be seen from the test data in Table 1, all the sample examples prepared using the core defect-limited process, carbon coating followed by the addition of fast ion precursors, and single-furnace segmented sintering process described in this application, showed that all electrochemical indicators and powder compaction performance were superior to the four comparative sample groups. This is a substantial performance improvement brought about by the modified process, excluding interference from instrument testing errors. Comparative Example 1, due to its high lithium-iron ratio, lack of lattice defects, and absence of outer fast ion coating, suffers from dual limitations in bulk ion diffusion and interfacial conduction, resulting in the lowest initial charge capacity, discharge capacity at various rates, and compaction density among all samples. Comparative Example 2, retaining only carbon coating and eliminating phosphate outer layer modification, exhibits high resistance to lithium-ion transport at the particle interface, leading to significant capacity decay at high rates in 1C and 2C. Comparative Example 3, with its uncontrolled sintering heating and cooling rates, cannot lock lattice defects through rapid cooling, resulting in substantial annihilation of bulk defects and a simultaneous decline in discharge capacity across the entire range. Comparative Example 4, by reversing the order of carbon source and coating precursor feeding, allows exogenous lithium to directly contact the lithium iron phosphate matrix without carbon layer obstruction, causing some lithium components to penetrate the lattice and disrupt the pre-designed lithium-deficient defect structure, resulting in simultaneous deterioration of electrochemical and processing performance.

[0123] Figure 1 The above are SEM images of the cathode material in Embodiment 1 of this application. Figure 1 It can be seen that the primary particles of the cathode material are spherical, with a relatively concentrated particle size distribution and a dense overall particle structure, which is conducive to obtaining a high compaction density.

[0124] Figure 2 TEM images of the cathode material in Embodiment 1 of this application are shown. Figure 2 As can be seen, the cathode material includes a lithium iron phosphate core 1 and a coating layer 2. The lithium iron phosphate core 1 exhibits clear and regular lattice stripes and has good crystallinity. The coating layer 2 consists of a carbon coating layer and a phosphate coating layer. Since the carbon coating layer and the phosphate coating layer are prepared by a single calcination, the coating layers are tightly connected and have no obvious boundary lines.

[0125] As can be seen from the above description, the embodiments of the present invention achieve the following technical effects:

[0126] The lithium iron phosphate (LFP) core possesses lithium iron phosphate antisite defects and oxygen vacancies. The presence of oxygen vacancies helps reduce interfacial charge transfer resistance. The LFP antisite defects and oxygen vacancies synergistically construct multidimensional lithium-ion diffusion channels, thereby improving bulk lithium-ion diffusion efficiency. A carbon coating layer covers the surface of the LFP core, forming a continuous electron conduction network, which helps improve the overall electronic conductivity of the cathode material. The NASICON-type phosphate in the phosphate coating material possesses three-dimensional interconnected lithium-ion channels with excellent ion conduction rates. The presence of lithium phosphate helps replenish free lithium at the interface and optimize electrolyte wettability. The synergistic effect between the NASICON-type phosphate and lithium phosphate helps homogenize lithium-ion flux, suppresses the formation of sharp two-phase interfaces in traditional LFP, and transforms the charge-discharge reaction into a quasi-solid solution reaction, thereby reducing electrode polarization. The carbon coating layer, located between the phosphate coating layer and the LFP core, helps to fundamentally prevent exogenous lithium from penetrating the matrix and damaging the lithium-deficient core structure. The lithium iron phosphate core, combined with an inner carbon coating layer and an outer phosphate coating layer, results in a high powder compaction density for the cathode material. This synergistically optimizes electron-ion interface transport and helps reduce polarization of the cathode material under high-rate charge-discharge conditions. Applying the cathode material of this application to lithium-ion batteries can help improve the initial charge capacity, initial charge-discharge efficiency, and rate performance of lithium-ion batteries.

[0127] The above are merely embodiments of the present invention and are not intended to limit the invention. Those skilled in the art will recognize that the present invention can have various modifications and variations. 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 positive electrode material, characterized in that, The positive electrode material sequentially includes, from the inside to the outside, a lithium iron phosphate core, and a carbon coating layer and a phosphate coating layer coated on the surface of the lithium iron phosphate core; wherein the material of the lithium iron phosphate core has the general chemical formula Li 1-x FePO 4-γ , where 0<x≤0.02, 0<γ≤0.02, the lithium iron phosphate core has antisite defects where iron occupies lithium sites, the material of the phosphate coating layer includes NASICON-type phosphate and lithium phosphate, and the general chemical formula of the NASICON-type phosphate is Li a M b (PO4) c , the M element is selected from any one or more of Group IVB metal elements, Group IIIB metal elements, Group VA metal elements, Group IIIA metal elements, and Group VIIB metal elements, 1≤a≤5, 1≤b≤2, 1≤c≤3.

2. The cathode material according to claim 1, characterized in that, The concentration of the antisite defect is 1 at% to 4 at%; and / or, the average particle size of the lithium iron phosphate core is 300 to 800 nm; and / or, the mass ratio of the carbon coating layer to the lithium iron phosphate core is (0.0125 to 0.0375):1; and / or, the mass ratio of the phosphate coating layer to the lithium iron phosphate core is (0.0019 to 0.0076):1; and / or, the molar ratio of the NASICON type phosphate to the lithium phosphate is (1 to 5):

1.

3. The cathode material according to claim 1 or 2, characterized in that, The M element is selected from any one or more of the elements Ti, Zr, Sc, Bi, In, Sb, Mn, and Al.

4. The cathode material according to claim 3, characterized in that, The NASICON type phosphate is selected from any one or more of LiTi2(PO4)3, LiZr2(PO4)3, Li3Sc2(PO4)3, Li3Bi2(PO4)3, Li3In2(PO4)3, Li3Sb2(PO4)3 and Li5Mn2(PO4)3.

5. The positive electrode material according to claim 4, characterized in that, The NASICON type phosphate is a combination of LiTi2(PO4)3 and LiZr2(PO4)3, and the molar ratio of LiTi2(PO4)3 to LiZr2(PO4)3 is (1~5):

1.

6. A method for preparing a cathode material according to any one of claims 1 to 5, characterized in that, The preparation method includes: Step S1: After mixing the raw materials including the divalent iron source, the first lithium source, and the first phosphorus source, the mixture is ball-milled and then calcined in a reducing atmosphere to obtain the lithium iron phosphate core. The heating rate of the first calcination is 50~200℃ / min, the cooling rate of the first calcination is ≥20℃ / min, and the molar ratio of lithium in the first lithium source to iron in the divalent iron source is (0.98~1):

1. Step S2: First, the raw materials including the lithium iron phosphate core and the carbon source are mixed, and then the raw materials including the second lithium source, the second phosphorus source and the M salt are added and mixed. Then, a second calcination is performed to obtain the cathode material. The M salt is selected from any one or more of group IVB metal salts, group IIIB metal salts, group VA metal salts, group IIIA metal salts and group VIIB metal salts.

7. The preparation method according to claim 6, characterized in that, The first calcination temperature is 750~800℃, and the holding time is 1~20min; And / or, the reducing gas in the reducing atmosphere is a mixture of hydrogen and other gases, wherein the volume fraction of hydrogen in the reducing gas is 3-10%, and the other gases are selected from any one or more of nitrogen, argon, and helium.

8. The preparation method according to claim 6, characterized in that, The second calcination includes: first raising the temperature to 380~520℃ and holding it for 1~3 hours, then raising the temperature to 550~700℃ and holding it for 1~6 hours.

9. The preparation method according to claim 6, characterized in that, The mass of the carbon source is 5-15 wt% of the lithium iron phosphate core; and / or, the ratio of the total mass of the second lithium source, the second phosphorus source and the M salt to the mass of the lithium iron phosphate core is (0.002-0.008):1; And / or, the divalent iron source is selected from ferrous oxalate and / or ferrous oxide; And / or, the first lithium source and the second lithium source are each independently selected from any one or more of lithium hydroxide, lithium acetate and lithium nitrate; And / or, the first phosphorus source and the second phosphorus source are each independently selected from ammonium dihydrogen phosphate and / or diammonium hydrogen phosphate; And / or, the carbon source is selected from any one or more of citric acid, sucrose, phenolic resin, and glucose.

10. A positive electrode sheet, comprising a current collector and a positive electrode active layer disposed on at least one side of the current collector, characterized in that, The material of the positive electrode active layer includes the positive electrode material according to any one of claims 1 to 5 or the positive electrode material prepared by the preparation method of the positive electrode material according to any one of claims 6 to 9.

11. A lithium-ion battery, comprising a positive electrode, an electrolyte, and a negative electrode, characterized in that, The positive electrode is the positive electrode as described in claim 10.

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