Molten-salt-assisted valence-gradient-doped modified single-crystal positive electrode material, and preparation method therefor and use thereof

Abstract

A molten-salt-assisted valence-gradient-doped modified single-crystal positive electrode material, and a preparation method therefor and a use thereof. The preparation method comprises the following steps: mixing a transition metal source, a lithium source, a manganese source and a molten salt additive, sequentially performing first sintering and second sintering, and removing the molten salt additive to obtain a doped modified single-crystal positive electrode material precursor; performing tempering treatment on the doped modified single-crystal positive electrode material precursor to obtain a molten salt-assisted valence-gradient-doped modified single-crystal positive electrode material. The prepared doped modified single-crystal positive electrode material can effectively avoid capacity loss of the material, significantly reduce lattice mismatch during lithium deintercalation / intercalation, and alleviate volume strain during charging and discharging. Moreover, the formed M-O bond can enhance the structural stability of the material and inhibit the formation of micro-cracks in the material. The preparation method for the positive electrode material facilitates industrial application, and provides an idea for commercial modified positive electrode materials.

Description

A molten salt-assisted valence gradient doping modified single-crystal cathode material, its preparation method and application Technical Field

[0001] This invention relates to the field of new energy materials technology, and in particular to a molten salt-assisted valence gradient doping modified single-crystal cathode material, its preparation method, and its application. Background Technology

[0002] For a long time, the over-reliance on fossil fuel production and consumption has led to energy inconsistencies and insecurity, becoming a common problem faced by countries around the world. As concerns about energy depletion and environmental pollution in the fossil fuel-based global energy system intensify, there is an urgent need for renewable and clean energy sources, as well as corresponding energy conversion and storage systems.

[0003] Rechargeable lithium-ion batteries (LIBs) have advantages such as high output voltage, good safety performance, and long cycle life, and have become important power supply devices for portable electronic products, electric vehicles, and even large-scale energy storage systems. However, there are certain obstacles to the development of LIBs, such as the structural stability of cathode materials and surface side reactions. Among them, spinel LiMn2O4 cathode material has attracted much attention due to its advantages such as stable framework, three-dimensional diffusion channels of Li, low Coulomb repulsion and good thermal stability, as well as the outstanding advantages of abundant Mn reserves, non-toxicity, and environmental protection. Although the spinel structure has obvious advantages, the capacity decay of LiMn2O4 is a fatal defect, especially under high temperature environment, LiMn2O4 will undergo the following changes: (1) Mn 3+ Jahn-Taller distortion; (2) Mn 3+ disproportionation reaction (Mn 3+ →Mn 4+ +Mn 2+ (3) Electrolyte decomposition produces HF, which corrodes the electrode; (4) Oxygen defects caused by material synthesis process and / or electrolyte decomposition; (5) Irreversible phase transition triggers the formation of microcracks; (6) Deposition of manganese on the negative electrode leads to the destruction of the electrolyte interface (SEI) and an increase in impedance. Summary of the Invention

[0004] The purpose of this invention is to provide a molten salt-assisted valence gradient doping modified single-crystal cathode material, its preparation method, and its application, thereby solving the problem of Mn generation in traditional unmodified spinel LiMn2O4 cathode materials. 3+ Jahn-Taller distortion, Mn 3+ Disproportionation and other phenomena can lead to capacity decay.

[0005] To achieve the above-mentioned objectives, the present invention provides the following technical solution:

[0006] This invention provides a method for preparing a molten salt-assisted valence gradient doping modified single-crystal cathode material, comprising the following steps:

[0007] (1) A transition metal source, a lithium source, a manganese source and a molten salt additive are mixed and subjected to a first sintering and a second sintering in sequence. The molten salt additive is removed to obtain a doped and modified single crystal cathode material precursor.

[0008] (2) The doped modified single crystal cathode material precursor is tempered to obtain molten salt-assisted valence gradient doped modified single crystal cathode material.

[0009] Wherein, the transition metal source in step (1) is a source containing Mg 2+ 、Sm 3+ Ti 4+ 、Nb 5+ with Mo 6+ Transition metal oxides containing one or more doped elements.

[0010] Preferably, in the preparation method, in step (1), the lithium source includes LiOH·H2O or Li2CO3, the manganese source includes MnCO3, Mn3O4 or MnO2, and the molten salt additive is Li2SO4·H2O.

[0011] Preferably, in the preparation method, in step (1), the molar ratio of the transition metal source, the lithium source, the manganese source and the molten salt additive is 0.01:1 to 1.08:2:1 to 2.

[0012] Preferably, in the preparation method, in step (1), the mixing method is ball milling; the ball milling time is 3 to 6 hours, the rotation speed is 300 to 400 r / min, and the ball-to-material ratio is 10 to 20:1.

[0013] Preferably, in the preparation method, in step (1), the temperature of the first sintering is 400-500°C, the time is 3-6 hours, and the rate of heating to the temperature required for the first sintering is 1.5-3°C / min.

[0014] Preferably, in the preparation method, in step (1), the temperature of the second sintering is 750-850°C, the time is 10-14h, and the rate of heating to the required temperature of the second sintering is 1.5-3°C / min.

[0015] Preferably, in the preparation method, in step (1), the atmosphere of the first sintering and the second sintering is an oxygen atmosphere.

[0016] Preferably, in the preparation method, in step (2), the tempering temperature is 700-750°C, the time is 4-6 hours, and the rate of heating to the required tempering temperature is 1.5-3°C / min.

[0017] The present invention also provides a molten salt-assisted valence gradient doped modified single-crystal cathode material prepared by the above preparation method.

[0018] The present invention also provides an application of the molten salt-assisted valence gradient doping modified single-crystal cathode material in lithium-ion batteries.

[0019] As can be seen from the above technical solution, compared with the prior art, the present invention has the following beneficial effects:

[0020] (1) This invention employs molten salt to assist in the doping of transition metal elements into lithium manganese oxide cathode materials. The molten salt provides a molten liquid environment during sintering, allowing the transition metal dopant elements to be uniformly distributed within the material. In subsequent processing, the molten salt can be completely removed by washing, ultimately synthesizing single-crystal materials modified with different valence gradients. If the molten salt is not removed, pure lithium manganese oxide materials cannot be synthesized, and the final product contains impurity phases. The removal of molten salt will cause defects on the material surface. Tempering can repair the surface structure of the material and remove excess moisture.

[0021] (2) The doped modified single-crystal cathode material prepared by this invention can effectively avoid capacity loss, significantly reduce lattice mismatch during lithium insertion / extraction, and alleviate volume strain during charging and discharging. Simultaneously, the formed MO bonds (M refers to transition metal) enhance the structural stability of the material and suppress the formation of microcracks. Bulk doping can enhance the structural stability of the material without changing its basic framework, including adjustments to the bulk and interface structures. Furthermore, bulk doping can effectively improve lithium-ion diffusion kinetics, significantly reduce the Li transport barrier, and increase ion transport efficiency. The cathode material preparation method described in this invention is applicable to existing cathode material preparation methods, such as solid-state methods and wet chemical methods, facilitating industrial application and providing insights for commercially modified cathode materials.

[0022] (3) The lithium-ion battery prepared using the cathode material described in this invention has significantly increased cycle stability and suppressed side reactions on the electrode surface; in addition, the number of charge-discharge cycles of the resulting battery is significantly improved, and its energy density, safety, reliability and other properties are significantly better than those of the same type and model of unmodified lithium-ion batteries, which is conducive to commercial application.

[0023] (4) The molten salt-assisted valence gradient doping modified single crystal cathode material provided by the present invention will not increase the cost of lithium-ion battery preparation, and the production equipment involved is simple, the production cycle is short, the process is simple, and it is suitable for large-scale production.

[0024] (5) The doping modification method of the present invention has wide applicability and is applicable to lithium manganese oxide, lithium nickel manganese oxide, lithium-rich manganese-based cathode materials, lithium cobalt oxide, ternary layered materials or lithium iron phosphate cathode materials, etc. Attached Figure Description

[0025] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below.

[0026] Figure 1 is a schematic diagram of the preparation process of molten salt-assisted valence state gradient doping modified single-crystal cathode material;

[0027] Figure 2 shows the XRD patterns of the cathode materials obtained in Examples 1-5 and Comparative Example 1;

[0028] Figure 3 shows the cycle performance test results of the batteries obtained in Application Examples 1-5 and Comparative Application Example 1. Detailed Implementation

[0029] This invention provides a method for preparing a molten salt-assisted valence gradient doping modified single-crystal cathode material, comprising the following steps, as shown in Figure 1:

[0030] (1) A transition metal source, a lithium source, a manganese source and a molten salt additive are mixed and subjected to a first sintering and a second sintering in sequence. The molten salt additive is removed to obtain a doped and modified single crystal cathode material precursor.

[0031] (2) The doped modified single crystal cathode material precursor is tempered to obtain molten salt-assisted valence gradient doped modified single crystal cathode material.

[0032] Wherein, the transition metal source in step (1) is a source containing Mg 2+ 、Sm 3+ Ti 4+ 、Nb 5+ with Mo 6+ Transition metal oxides containing one or more doped elements.

[0033] In this invention, the transition metal source in step (1) is preferably Mg. 2+ 、Sm 3+ Ti 4+ 、Nb 5+ Or Mo 6+ Transition metal oxides doped with elements, more preferably containing Ti 4+ 、Nb 5+ Or Mo 6+ Transition metal oxides doped with elements, more preferably containing Mo 6+ Transition metal oxides doped with elements.

[0034] In this invention, the lithium source in step (1) preferably includes LiOH·H2O or Li2CO3.

[0035] In this invention, the manganese source in step (1) preferably includes MnCO3, Mn3O4 or MnO2, and more preferably includes MnCO3 or Mn3O4.

[0036] In this invention, the molten salt additive in step (1) is preferably Li2SO4·H2O.

[0037] In this invention, the molar ratio of the transition metal source, the lithium source, the manganese source and the molten salt additive in step (1) is preferably 0.01:1 to 1.08:2:1 to 2, more preferably 0.01:1.02 to 1.06:2:1.2 to 1.6, and even more preferably 0.01:1.05:2:1.4.

[0038] In this invention, the mixing method in step (1) is preferably ball milling.

[0039] In this invention, the ball milling equipment is preferably a planetary ball mill.

[0040] In this invention, the ball milling time is preferably 3 to 6 hours, more preferably 4 to 5 hours, and even more preferably 4 hours; the rotation speed is preferably 300 to 400 r / min, more preferably 350 to 400 r / min, and even more preferably 400 r / min; the ball-to-material ratio is preferably 10 to 20:1, more preferably 10 to 15:1, and even more preferably 10:1.

[0041] In this invention, the grinding balls used in the ball mill are preferably zirconia grinding balls.

[0042] In this invention, the equipment for the first sintering and the second sintering in step (1) is preferably a tube furnace.

[0043] In this invention, the temperature of the first sintering in step (1) is preferably 400-500°C, more preferably 450-500°C, and even more preferably 450°C; the time is preferably 3-6 hours, more preferably 4-5 hours, and even more preferably 4 hours; the rate of heating to the temperature required for the first sintering is preferably 1.5-3°C / min, more preferably 2-3°C / min, and even more preferably 2.5°C / min.

[0044] In this invention, the temperature of the second sintering in step (1) is preferably 750-850°C, more preferably 780-850°C, and even more preferably 800°C; the time is preferably 10-14h, more preferably 12-13h, and even more preferably 12h; the rate of heating to the temperature required for the second sintering is preferably 1.5-3°C / min, more preferably 2-3°C / min, and even more preferably 2.5°C / min.

[0045] In this invention, the atmosphere for the first sintering and the second sintering in step (1) is preferably an oxygen atmosphere.

[0046] In this invention, step (1) preferably includes cooling after the second sintering. The cooling conditions are not limited; any method well-known to those skilled in the art can be used.

[0047] In this invention, the method for removing the molten salt additive in step (1) is preferably washing.

[0048] In this invention, the washing agent is preferably water. The number of washes and the water temperature are not limited and can be adjusted as needed.

[0049] In this invention, the equipment for the tempering treatment in step (2) is preferably a tube furnace.

[0050] In this invention, the tempering temperature in step (2) is preferably 700-750°C, more preferably 725-750°C, and even more preferably 725°C; the time is preferably 4-6 hours, more preferably 5-6 hours, and even more preferably 6 hours; the rate of heating to the required tempering temperature is preferably 1.5-3°C / min, more preferably 2-3°C / min, and even more preferably 2.5°C / min.

[0051] In this invention, the atmosphere for the tempering treatment in step (2) is preferably an oxygen atmosphere.

[0052] In this invention, the tempering process in step (2) preferably includes cooling. The cooling conditions are not limited; any method well-known to those skilled in the art can be used.

[0053] The present invention also provides a molten salt-assisted valence gradient doped modified single-crystal cathode material prepared by the above preparation method.

[0054] The present invention also provides an application of the molten salt-assisted valence gradient doping modified single-crystal cathode material in lithium-ion batteries.

[0055] In this invention, the method of application is not limited, and any solution known to those skilled in the art can be used.

[0056] In this invention, the lithium-ion battery is preferably a CR2025 button cell battery.

[0057] In this invention, the lithium-ion battery preferably includes a positive electrode, a negative electrode, a separator, and an electrolyte.

[0058] In this invention, the method for preparing the positive electrode preferably includes the following steps:

[0059] Molten salt-assisted valence gradient doping modified single-crystal cathode material, conductive carbon black, binder and organic solvent are mixed to obtain cathode slurry; cathode slurry is coated onto aluminum foil, dried, dried in a vacuum drying oven, and cut to obtain cathode sheet.

[0060] In this invention, the adhesive is preferably polyvinylidene fluoride (PVDF).

[0061] In this invention, the organic solvent is preferably N-methylpyrrolidone (NMP).

[0062] In this invention, the mass ratio of the molten salt-assisted valence gradient doping modified single-crystal cathode material, the conductive carbon black, and the binder is not limited; any method well-known to those skilled in the art can be used. Specifically, in the embodiments of this invention, the preferred mass ratio of the molten salt-assisted valence gradient doping modified single-crystal cathode material, the conductive carbon black, and the binder is 8:1:1.

[0063] In this invention, the mixing method is preferably grinding. The grinding time is preferably 15-30 minutes, more preferably 20-30 minutes, and even more preferably 25 minutes.

[0064] In this invention, the drying temperature is preferably 80-120°C, more preferably 80-100°C, and even more preferably 90°C; the drying time is preferably 10-12 hours, more preferably 11-12 hours, and even more preferably 12 hours.

[0065] In this invention, the negative electrode sheet is preferably a lithium sheet.

[0066] In this invention, the diaphragm is preferably a Celgard 2400 diaphragm.

[0067] In this invention, the electrolyte is preferably 1M LiPF6 DMC:EC = 7:3wt%5%FEC.

[0068] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0069] Example 1

[0070] This embodiment provides a method for preparing a molten salt-assisted magnesium-doped modified single-crystal cathode material, including the following steps:

[0071] (1) MgO, LiOH·H2O, MnCO3 and Li2SO4·H2O were placed in a ball mill jar according to a molar ratio of 0.01:1.02:2:1.5 and milled for 3 hours in a planetary ball mill at 350 r / min and a ball-to-material ratio of 15:1 using zirconia grinding balls. The milled mixture was placed in a crucible and heated to 450℃ at 2.5℃ / min and held for 5 hours. Then it was heated to 780℃ at 2.5℃ / min and held for 12 hours. The mixture was then cooled in the furnace. The product was washed in water at 50℃ for 5 times to remove the molten salt and obtain the magnesium-doped modified single crystal cathode material precursor.

[0072] (2) The magnesium-doped modified single-crystal cathode material precursor was placed in a tube furnace, and an oxygen atmosphere was introduced. The temperature was raised to 725℃ at 2.5℃ / min and tempered for 6 hours. The furnace was then cooled to obtain molten salt-assisted magnesium-doped modified single-crystal cathode material, denoted as LMO-Mg.

[0073] Example 2

[0074] This embodiment provides a method for preparing a molten salt-assisted samarium-doped modified single-crystal cathode material, including the following steps:

[0075] (1) Sm2O3, Li2CO3, Mn3O4 and Li2SO4·H2O were placed in a ball mill jar according to a molar ratio of 0.01:1.05:2:1.2 and milled for 4 hours in a planetary ball mill at 400 r / min and a ball-to-material ratio of 10:1 using zirconia milling balls. The milled mixture was placed in a crucible and purged with an oxygen atmosphere in a tube furnace. The temperature was increased to 500℃ at 2℃ / min and held for 4 hours. Then the temperature was increased to 800℃ at 2℃ / min and held for 12 hours. The mixture was cooled with the furnace. The product was washed in water at 50℃ for 5 times to remove the molten salt and obtain the samarium-doped modified single crystal cathode material precursor.

[0076] (2) The samarium-doped modified single crystal cathode material precursor was placed in a tube furnace, oxygen atmosphere was introduced, and the temperature was raised to 700℃ at 2℃ / min for tempering treatment for 6h. The furnace was then cooled to obtain molten salt-assisted samarium-doped modified single crystal cathode material, denoted as LMO-Sm.

[0077] Example 3

[0078] This embodiment provides a method for preparing a molten salt-assisted titanium-doped modified single-crystal cathode material, including the following steps:

[0079] (1) TiO2, LiOH·H2O, MnO2 and Li2SO4·H2O were placed in a ball mill jar according to a molar ratio of 0.01:1.02:2:1.6 and milled for 6 hours in a planetary ball mill at 300 r / min and a ball-to-material ratio of 12:1 using zirconia milling balls. The milled mixture was placed in a crucible and purged with an oxygen atmosphere in a tube furnace. The temperature was increased to 500℃ at 2℃ / min and held for 5 hours. Then the temperature was increased to 850℃ at 2℃ / min and held for 10 hours. The mixture was cooled with the furnace. The product was washed in water at 50℃ for 5 times to remove the molten salt and obtain the titanium-doped modified single crystal cathode material precursor.

[0080] (2) The titanium-doped modified single-crystal cathode material precursor was placed in a tube furnace, and an oxygen atmosphere was introduced. The temperature was raised to 750℃ at 2℃ / min and tempered for 4h. The furnace was then cooled to obtain molten salt-assisted titanium-doped modified single-crystal cathode material, denoted as LMO-Ti.

[0081] Example 4

[0082] This embodiment provides a method for preparing a molten salt-assisted niobium-doped modified single-crystal cathode material, including the following steps:

[0083] (1) Nb2O5, LiOH·H2O, Mn3O4 and Li2SO4·H2O were placed in a ball mill jar according to a molar ratio of 0.01:1.06:2:1.4 and milled for 5 hours in a planetary ball mill at 300 r / min and a ball-to-material ratio of 15:1 using zirconia milling balls. The milled mixture was placed in a crucible and heated to 450℃ at 3℃ / min and held for 6 hours. Then it was heated to 800℃ at 3℃ / min and held for 13 hours. The mixture was cooled with the furnace. The product was washed in water at 50℃ for 5 times to remove the molten salt and obtain the niobium-doped modified single crystal cathode material precursor.

[0084] (2) The niobium-doped modified single crystal cathode material precursor was placed in a tube furnace, and an oxygen atmosphere was introduced. The temperature was raised to 750℃ at 3℃ / min and tempered for 5h. The furnace was then cooled to obtain molten salt-assisted niobium-doped modified single crystal cathode material, denoted as LMO-Nb.

[0085] Example 5

[0086] This embodiment provides a method for preparing a molten salt-assisted molybdenum-doped modified single-crystal cathode material, including the following steps:

[0087] (1) MoO3, LiOH·H2O, MnCO3 and Li2SO4·H2O were placed in a ball mill jar according to a molar ratio of 0.01:1.05:2:1 and milled for 4 hours in a planetary ball mill at 400 r / min and a ball-to-material ratio of 10:1 using zirconia grinding balls. The milled mixture was placed in a crucible and heated to 450℃ at 1.5℃ / min and held for 4 hours. Then it was heated to 800℃ at 1.5℃ / min and held for 12 hours. The mixture was then cooled in the furnace. The product was washed in water at 50℃ for 5 times to remove the molten salt and obtain the precursor of molybdenum-doped modified single crystal cathode material.

[0088] (2) The molybdenum-doped modified single crystal cathode material precursor was placed in a tube furnace, and an oxygen atmosphere was introduced. The temperature was raised to 725℃ at 1.5℃ / min and tempered for 5h. The furnace was then cooled to obtain molten salt-assisted molybdenum-doped modified single crystal cathode material, denoted as LMO-Mo.

[0089] Comparative Example 1

[0090] This comparative example provides a method for preparing a single-crystal cathode material, including the following steps:

[0091] (1) LiOH·H2O, MnCO3 and Li2SO4·H2O were placed in a ball mill jar at a molar ratio of 1.08:2:2 and milled for 5 hours in a planetary ball mill at 400 r / min and a ball-to-material ratio of 20:1 using zirconia grinding balls. The milled mixture was placed in a crucible and heated to 480℃ at 3℃ / min and held for 4 hours. Then it was heated to 780℃ at 3℃ / min and held for 13 hours. The mixture was then cooled in the furnace. The product was washed in water at 50℃ for 5 times to remove the molten salt and obtain a single crystal cathode material precursor.

[0092] (2) Place the single crystal cathode material precursor in a tube furnace, introduce an oxygen atmosphere, heat it to 700℃ at 3℃ / min and temper it for 5h, and cool it with the furnace to obtain the single crystal cathode material, denoted as LMO.

[0093] The cathode materials obtained in Examples 1-5 and Comparative Example 1 were subjected to XRD analysis, and the results are shown in Figure 2. As can be seen from Figure 2, the doped and modified lithium manganese oxide cathode material is the same as the lithium manganese oxide cathode material, without the formation of secondary phases and impurity peaks. All diffraction peaks point to the spinel LiMn2O4 standard card (JCPDS 35-0782), belonging to the Fd3m space group, which indicates it is a standard lithium manganese oxide cathode material.

[0094] Application Examples 1-5, Comparative Application Example 1

[0095] (1) The positive electrode materials obtained in Examples 1-5 and Comparative Example 1, conductive carbon black (manufacturer: Duoduo Chemical Reagent, model: Super C65) and PVDF (molecular weight 1 million) were mixed and ground for 30 min at a mass ratio of 8:1:1. NMP was added and the mixture was stirred to obtain a positive electrode slurry. The positive electrode slurry was uniformly coated onto aluminum foil with a loading of 4 mg·cm³. -2 Dry the material in a vacuum drying oven at 80°C for 12 hours, then cut it to obtain the positive electrode sheet.

[0096] (2) The positive electrode, lithium sheet, Celgard 2400 separator, and 1M LiPF6 DMC:EC = 7:3wt% 5% FEC electrolyte obtained in step (1) are respectively assembled into CR2025 button cells.

[0097] The batteries assembled in Application Examples 1-5 and Comparative Application Example 1 were placed in the Blue Electric Test System for cycle performance testing, and the results are shown in Figure 3. As can be seen from Figure 3, the doped and modified lithium manganese oxide cathode material has long-term cycle stability, which is much higher than the cycle retention rate of the undoped and unmodified lithium manganese oxide cathode material.

[0098] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

A method for preparing a molten salt-assisted valence state gradient doped modified single-crystal cathode material, characterized in that, Includes the following steps: (1) A transition metal source, a lithium source, a manganese source and a molten salt additive are mixed and subjected to a first sintering and a second sintering in sequence. The molten salt additive is removed to obtain a doped modified single crystal cathode material precursor. (2) The doped modified single crystal cathode material precursor is tempered to obtain molten salt-assisted valence gradient doped modified single crystal cathode material. Wherein, the transition metal source in step (1) is a source containing Mg 2+ 、Sm 3+ Ti 4+ 、Nb 5+ with Mo 6+ Transition metal oxides containing one or more doped elements. The preparation method according to claim 1 is characterized in that, In step (1), the lithium source includes LiOH·H2O or Li2CO3, the manganese source includes MnCO3, Mn3O4 or MnO2, and the molten salt additive is Li2SO4·H2O. The preparation method according to claim 1 or 2 is characterized in that, In step (1), the molar ratio of the transition metal source, the lithium source, the manganese source and the molten salt additive is 0.01:1 to 1.08:2:1 to 2. The preparation method according to claim 3 is characterized in that, In step (1), the mixing method is ball milling; the ball milling time is 3 to 6 hours, the rotation speed is 300 to 400 r / min, and the ball-to-material ratio is 10 to 20:

1. The preparation method according to claim 1 is characterized in that, In step (1), the temperature of the first sintering is 400-500℃, the time is 3-6h, and the rate of heating to the temperature required for the first sintering is 1.5-3℃ / min. The preparation method according to claim 1 or 5 is characterized in that, In step (1), the temperature of the second sintering is 750-850℃, the time is 10-14h, and the rate of heating to the required temperature of the second sintering is 1.5-3℃ / min. The preparation method according to claim 6 is characterized in that, In step (1), the atmosphere for the first sintering and the second sintering is an oxygen atmosphere. The preparation method according to claim 1 is characterized in that, In step (2), the tempering temperature is 700-750℃, the time is 4-6h, and the rate of heating to the required tempering temperature is 1.5-3℃ / min. The molten salt-assisted valence gradient doping modified single-crystal cathode material prepared by the preparation method according to any one of claims 1 to 8. Application of the molten salt-assisted valence gradient doping modified single-crystal cathode material according to claim 9 in lithium-ion batteries.

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