Lithium-rich cathode material, preparation method and application thereof

Abstract

The application provides a lithium-rich positive electrode material and a preparation method and application thereof. The lithium-rich positive electrode material comprises a lithium-rich core material and a coating layer. The coating layer is coated on the outer surface of the lithium-rich core material. The coating layer comprises a first coating material, and the first coating material is a metal selenide. In the application, the metal selenide is coated on the outer surface of the lithium-rich core material. The metal selenide has good electrical conductivity, improves the electrical conductivity of the lithium-rich positive electrode material, reduces the polarization of the lithium-rich positive electrode material, improves the homogenate process of the lithium-rich positive electrode material, and in addition, the metal selenide can inhibit the oxygen release of the lithium-rich positive electrode material at the interface, reduce the side reaction at the interface, and make the lithium-rich positive electrode material have good cycle performance and a high capacity retention rate.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of batteries, in particular to a lithium-rich cathode material and a preparation method and application thereof. BACKGROUND

[0002] With the further improvement of the requirements of the new energy vehicle market on the cruising range and charging time, it is imminent to study cathode materials with better rate performance and higher energy density. Developing new-type cathode materials with high voltage and high specific capacity is the key to further improve the energy density of lithium ion batteries. Lithium-rich layered cathode materials are considered as the ideal choice for the next generation of lithium ion battery cathode materials due to their high specific capacity, low cost, high safety and environmental friendliness. However, there are still some problems to be solved in the practical process, such as large irreversible capacity in the first cycle, poor rate performance and serious cycle capacity decay.

[0003] In addition, residual alkali may exist on the surface of the lithium-rich cathode material, which leads to the gelation of the cathode material, affects the coating of the slurry and increases the internal resistance of the battery, and further affects the various performances of the battery. In addition, the transition delithiation in the first charge and discharge of the lithium-rich cathode material destroys the crystal structure of the material surface, and the oxidation of the electrolyte and the generation of a small amount of hydrogen fluoride at high voltage will destroy the stability of the electrode / electrolyte interface, resulting in serious capacity decay and affecting the application of the battery. SUMMARY

[0004] The present application provides a lithium-rich cathode material and a preparation method and application thereof.

[0005] In a first aspect, the present application provides a lithium-rich cathode material, which comprises a lithium-rich core material and a coating layer, the coating layer is coated on the outer surface of the lithium-rich core material, and the coating layer comprises a first coating material, which is a metal selenide. The metal selenide has good conductivity, which improves the conductivity of the lithium-rich cathode material and reduces polarization. In addition, in the charging and discharging process, selenium ions will enter the oxygen ion vacancies in the lithium-rich cathode material to replace the positions of oxygen ions, thereby inhibiting the release of oxygen at the interface of the lithium-rich cathode material and reducing the side reactions at the interface.

[0006] In an embodiment, the coating layer further comprises a second coating material, which is a metal fluoride.

[0007] In the present application, the outer surface of the lithium-rich core material is coated with a metal selenide and a metal fluoride, so that the lithium-rich cathode material has a high initial coulombic efficiency, good cycle performance, a good capacity retention rate and can greatly improve the slurry process.

[0008] In one aspect, the metal selenide and the metal fluoride in the coating layer can form a synergistic relationship with the lithium-rich core material, ensuring a high charge-discharge capacity while improving the structural stability and slowing down the capacity decay.

[0009] On the other hand, the interaction between the metal selenide and the metal fluoride effectively improves the performance of the lithium-rich positive electrode material. The coating layer comprises both the metal selenide and the metal fluoride. The metal selenide has good electrical conductivity, and the metal fluoride is a good fast ion conductor, so that the lithium-rich positive electrode material provided by the present application can ensure the rapid back-and-forth deintercalation and transmission of lithium ions during the charge-discharge process of the lithium-rich positive electrode material, and can also ensure the electrical conductivity of the lithium-rich positive electrode material to reduce polarization. The metal fluoride can fill the hollows on the surface of the lithium-rich core material that are not completely coated, and the metal selenide and the metal fluoride together form a relatively complete and uniform coating layer, achieving a sufficient coating effect.

[0010] In another aspect, during the charge-discharge process, selenium ions in the metal selenide enter the oxygen ion vacancies in the lithium-rich positive electrode material to replace the positions of oxygen ions, thereby inhibiting the release of oxygen at the interface of the lithium-rich positive electrode material and reducing the side reactions at the interface. In addition, during the preparation process of the lithium-rich positive electrode material, a small amount of fluorine ions in the metal fluoride also enter the crystal lattice of the lithium-rich core material, effectively alleviating the dissolution of transition metals in the lithium-rich core material, increasing the interlayer spacing of the lithium-rich core material, and thereby expanding the transmission channel of lithium ions, facilitating the intercalation and deintercalation of lithium ions, and improving the initial discharge capacity and the initial coulombic efficiency of the battery.

[0011] In one embodiment, at least part of the metal fluoride is in contact with the lithium-rich core material. During the preparation process of the lithium-rich positive electrode material, at least part of the metal fluoride is in contact with the lithium-rich core material, which facilitates the entry of a small amount of fluorine ions in the metal fluoride into the crystal lattice of the lithium-rich core material, effectively alleviates the dissolution of transition metals in the lithium-rich core material, increases the interlayer spacing of the lithium-rich core material, and thereby expands the transmission channel of lithium ions, facilitates the intercalation and deintercalation of lithium ions, and improves the initial discharge capacity and the initial coulombic efficiency of the battery.

[0012] In one embodiment, the lithium-rich core material is doped with fluorine elements.

[0013] In one embodiment, the fluorine elements are located on the side of the lithium-rich core material close to the coating layer.

[0014] In one embodiment, at least part of the metal fluoride is dispersed in the metal selenide.

[0015] In one embodiment, the first coating material coats the outer surface of the lithium-rich core material, and the second coating material coats the outer surface of the lithium-rich core material and at least part of the outer surface of the first coating material.

[0016] In one embodiment, the first coating material coats the outer surface of the lithium-rich core material, and the second coating material coats the outer surface of the lithium-rich core material and part of the outer surface of the first coating material.

[0017] In one embodiment, the first coating material coats part of the outer surface of the lithium-rich core material, and the second coating material coats part of the outer surface of the lithium-rich core material and part of the outer surface of the first coating material.

[0018] In one embodiment, the first coating material coats part of the outer surface of the lithium-rich core material, and the second coating material coats part of the outer surface of the lithium-rich core material.

[0019] In one embodiment, a plurality of pores are formed between the first coating material, at least part of the pores are filled with the second coating material, and the metal fluoride contacts the lithium-rich core material through the pores.

[0020] In one embodiment, a plurality of pores are formed between the first coating material, part of the second coating material fills at least part of the pores, and another part of the second coating material coats part of the outer surface of the first coating material.

[0021] In one embodiment, at least part of the first coating material is not coated by the second coating material.

[0022] In one embodiment, at least part of the first coating material penetrates the second coating material along the radial direction of the lithium-rich core material.

[0023] In one embodiment, the metal selenide includes at least one of CoSe, NiSe2, and FeSe2.

[0024] In one embodiment, the metal fluoride includes at least one of LiF, AlF3, and CuF2.

[0025] In one embodiment, the mass ratio of the metal selenide to the metal fluoride is (2-10):1.

[0026] In one embodiment, the mass ratio of the coating layer to the lithium-rich core material is (0.025-0.10):1.

[0027] In one embodiment, the thickness of the coating layer is 50-300 nanometers.

[0028] In an embodiment, the structure general formula of the lithium-rich core material is Li 1+x A y O z wherein, 0≤x≤2, 0

[0029] In a second aspect, the application provides a preparation method of a lithium-rich positive electrode material, the preparation method of the lithium-rich positive electrode material comprising:

[0030] mixing a selenide with the lithium-rich core material to obtain a solid I;

[0031] heating the solid I under a protective gas atmosphere to obtain a solid II;

[0032] mixing and ball-milling the solid II with a fluoride to obtain a solid III;

[0033] heating the solid III under a protective gas atmosphere to obtain the lithium-rich positive electrode material.

[0034] In an embodiment, the heating temperature and heating time in the heating of the solid III under a protective gas atmosphere are greater than the heating temperature and heating time in the heating of the solid I under a protective gas atmosphere.

[0035] In a third aspect, the application provides a positive electrode tab, the positive electrode tab comprising the lithium-rich positive electrode material according to any one of the above or the lithium-rich positive electrode material prepared by the preparation method of the lithium-rich positive electrode material according to the above.

[0036] In a fourth aspect, the application provides a secondary battery, the secondary battery comprising a positive electrode tab, a separator and the positive electrode tab according to the above. BRIEF DESCRIPTION OF DRAWINGS

[0037] In order to more clearly illustrate the technical solutions in the embodiments of the application, the drawings needed to be used in the embodiments of the application will be described below.

[0038] Figure 1 is a schematic diagram of the lithium-rich positive electrode material provided by an embodiment of the application. DETAILED DESCRIPTION

[0039] The technical solutions in the embodiments of the application will be described below with reference to the drawings in the embodiments of the application. Obviously, the described embodiments are only a part of the embodiments of the application, not all the embodiments of the application.

[0040] The terms "first", "second", and the like in this context are used only to describe different instances and do not imply or suggest relative importance or a number of the indicated technical features. Thus, the features defined with "first", "second" can explicitly or implicitly include one or more of the features. In the description of the present application, the meaning of "a plurality of" is two or more, unless otherwise specified.

[0041] In addition, in this context, the orientation terms "upper", "lower", and the like are defined with respect to the orientation of the structure shown in the drawing, and it should be understood that these directional terms are relative concepts, which are used for relative description and clarification, and can change accordingly according to the change of the orientation of the structure placed.

[0042] The present application provides a lithium-rich positive electrode material 1, which includes a lithium-rich inner core material 10 and a coating layer 20 coated on the outer surface of the lithium-rich inner core material 10, and the coating layer 20 includes a first coating material, which is a metal selenide.

[0043] The lithium-rich positive electrode material 1 is a core-shell structure, the inner core of the core-shell structure is the lithium-rich inner core material 10, and the outer shell of the core-shell structure is the coating layer 20. The lithium-rich inner core material 10 is rich in lithium elements, and the lithium-rich inner core material 10 can be a single lithium compound or a mixture of multiple lithium compounds.

[0044] The coating layer 20 is coated on the outer surface of the lithium-rich inner core material 10, and the coating layer 20 can isolate the lithium-rich inner core material 10 from the external environment. For example, the coating layer 20 can isolate the lithium-rich inner core material 10 from the electrolyte. In an embodiment, the coating layer 20 can be continuously distributed to the outer surface of the lithium-rich inner core material 10. In an embodiment, the coating layer 20 can be discontinuously and intermittently distributed to the outer surface of the lithium-rich inner core material 10. In an embodiment, the coating layer 20 coats part of the outer surface of the lithium-rich inner core material 10. In an embodiment, the coating layer 20 fully coats the outer surface of the lithium-rich inner core material 10, i.e. the coating layer 20 coats the entire outer surface of the lithium-rich inner core material 10.

[0045] The outer surface of the lithium-rich core material 10 usually has residual alkali. If the outer surface of the lithium-rich core material 10 is not provided with the coating layer 20, on the one hand, when the lithium-rich core material 10 is used to prepare a battery positive electrode, the lithium-rich core material 10 is easy to cause the gelation of the slurry in the slurry preparation process, so that the slurry has poor flowability, which affects the coating of the slurry and increases the internal resistance of the battery, and further affects the various performances of the battery. On the other hand, if the outer surface of the lithium-rich core material 10 directly contacts with the electrolyte, the electrolyte will be oxidized and generate a small amount of HF under high voltage, which destroys the stability of the electrode / electrolyte interface and also causes the capacity attenuation. On the other hand, the lithium-rich core material 10 is transitionally delithiated during the first charge and discharge, which destroys the crystal structure of the surface of the lithium-rich core material 10 and causes serious capacity attenuation.

[0046] In the present application, the coating layer 20 is arranged on the outer surface of the lithium-rich core material 10, and the lithium-rich core material 10 is located in the coating layer 20. On the one hand, the residual alkali on the outer surface of the lithium-rich core material 10 does not contact with the external electrolyte, which reduces the gelation of the slurry in the slurry preparation process of the lithium-rich core material 10, greatly improves the slurry preparation process and improves the processing performance of the lithium-rich core material 10. On the other hand, the coating layer 20 isolates the lithium-rich core material 10 from the electrolyte, reduces the occurrence of side reactions on the surface of the lithium-rich core material 10 and improves the stability of the lithium-rich core material 10.

[0047] In the present application, the coating layer 20 includes a metal selenide. The metal selenide has good electrical conductivity and can improve the electrical conductivity of the lithium-rich positive electrode material 1. In the charge and discharge process, selenium ions enter the oxygen ion vacancy of the lithium-rich positive electrode material 1 to replace the position of oxygen ions, inhibit the oxygen release of the lithium-rich positive electrode material 1 at the interface and reduce the side reactions at the interface.

[0048] In an embodiment, the coating layer 20 further includes a second coating material, which is a metal fluoride.

[0049] In the present application, the coating layer 20 includes the metal selenide and the metal fluoride. The metal selenide and the metal fluoride in the coating layer 20 can form a synergistic relationship with the lithium-rich core material 10, which can ensure a high charge and discharge capacity and improve the structural stability and slow down the capacity attenuation.

[0050] In addition, the interaction of the metal selenide and the metal fluoride effectively improves the performance of the lithium-rich positive electrode material 1. First, when the lithium-rich positive electrode material 1 is prepared, the metal selenide can be coated first and then the metal fluoride is coated. Since the melting point of the metal fluoride is lower than that of the metal selenide, the metal selenide is difficult to be completely coated to the outer surface of the lithium-rich core material 10, and when the metal fluoride is coated to the outer surface of the lithium-rich core material 10, the molten metal fluoride can fill the holes of the metal selenide, the metal fluoride can greatly contact the metal selenide, and a relatively complete and uniform coating layer 20 is formed, so that the coating is sufficient, so that the coating layer 20 can better isolate the lithium-rich core material 10 from the outside, and better play the role of the coating layer 20.

[0051] Secondly, the metal selenide has good electrical conductivity, which can improve the electrical conductivity of the lithium-rich positive electrode material 1. In the charging and discharging process, selenium ions enter the oxygen ion vacancies in the lithium-rich positive electrode material 1 to replace the positions of oxygen ions, inhibit the oxygen release of the lithium-rich positive electrode material 1 at the interface, and reduce the side reaction at the interface.

[0052] The metal fluoride is a good fast ion conductor, which improves the ion conductivity of the lithium-rich positive electrode material 1, and the structure of the metal fluoride is relatively stable, which improves the stability of the coating layer 20. In particular, during the preparation of the lithium-rich positive electrode material 1, a small amount of fluoride ions in the metal fluoride will also enter the crystal lattice of the lithium-rich core material 10 in the core, effectively alleviating the dissolution of transition metals in the lithium-rich core material 10, increasing the interlayer spacing of the lithium-rich core material 10, and then expanding the transmission channel of lithium ions, facilitating the embedding and extraction of lithium ions, improving the first discharge capacity of the battery, and improving the first coulomb efficiency of the battery.

[0053] If the coating layer 20 on the outer surface of the lithium-rich core material 10 only includes the metal selenide, the coating layer 20 formed by the metal selenide is easy to have holes, so that the coating layer 20 cannot be completely coated to the outer surface of the lithium-rich core material 10. If the coating layer 20 on the outer surface of the lithium-rich core material 10 only includes the metal fluoride, the electrical conductivity of the metal fluoride is poor, so that the overall electrical conductivity of the lithium-rich positive electrode material 1 is poor.

[0054] In the present embodiment, the coating layer 20 simultaneously includes the metal selenide and the metal fluoride, the metal selenide has good electrical conductivity, and the metal fluoride is a good fast ion conductor, so that the lithium-rich positive electrode material 1 provided by the present application can ensure the rapid back-and-forth transmission of lithium ions during the charging and discharging process of the lithium-rich positive electrode material 1, and can also ensure the electrical conductivity of the lithium-rich positive electrode material 1, and reduce the polarization. The metal fluoride can fill the holes on the surface of the lithium-rich core material 10 which are not completely coated, and the metal selenide and the metal fluoride together form a relatively complete and uniform coating layer 20, so that the coating is sufficient.

[0055] The lithium-rich positive electrode material 1 provided in the present application is coated with metal selenide and metal fluoride on the outer surface of the lithium-rich inner core material 10, so that the lithium-rich positive electrode material 1 has a high initial coulombic efficiency, good cycle performance, a good capacity retention rate, and can greatly improve the homogenate process.

[0056] In an embodiment, at least part of the metal fluoride is in contact with the lithium-rich inner core material 10. The at least part of the metal fluoride is directly located on the outer surface of the lithium-rich inner core material 10, which increases the ionic conductivity of the lithium-rich positive electrode material 1 and realizes the rapid in-and-out deintercalation transmission of lithium ions during the charging and discharging process of the lithium-rich positive electrode material 1.

[0057] In an embodiment, the lithium-rich inner core material 10 is doped with fluorine elements. In the present application, the lithium-rich inner core material 10 is doped with fluorine elements, which effectively alleviates the dissolution of transition metals in the lithium-rich inner core material 10, increases the interlayer spacing of the lithium-rich inner core material 10, and further expands the transmission channel of lithium ions, facilitating the intercalation and deintercalation of lithium ions, improving the initial discharge capacity of the battery, and improving the initial coulombic efficiency of the battery.

[0058] In an embodiment, the fluorine elements are located on one side of the lithium-rich inner core material 10 close to the coating layer 20. The distribution position of the fluorine elements extends from the outer surface of the lithium-rich inner core material 10 to the core of the lithium-rich inner core material 10, that is, the fluorine elements are doped on the outside of the lithium-rich inner core material 10, and no fluorine elements are doped at the inner core position of the lithium-rich inner core material 10. Therefore, the lithium-rich positive electrode material 1 provided in the present application can improve the initial discharge capacity and the initial coulombic efficiency of the battery while saving raw materials and reducing costs.

[0059] In an embodiment, the fluorine elements are dispersed in the entire lithium-rich inner core material 10.

[0060] In an embodiment, the fluorine elements doped in the lithium-rich inner core material 10 are derived from the fact that when the lithium-rich positive electrode material 1 is coated with metal fluoride, a small amount of fluoride ions in the metal fluoride also enter the crystal lattice of the lithium-rich inner core material 10 in the inner core.

[0061] In an embodiment, the metal selenide includes at least one of CoSe, NiSe2, and FeSe2.

[0062] In an embodiment, the metal selenide includes one of CoSe, NiSe2, and FeSe2. That is, the metal selenide is CoSe, or the metal selenide is NiSe2, or the metal selenide is FeSe2.

[0063] In an embodiment, the metal selenide includes two of CoSe, NiSe2, FeSe2. Illustratively, in an embodiment, the metal selenide includes CoSe and NiSe2. In an embodiment, the metal selenide includes CoSe and FeSe2. In an embodiment, the metal selenide includes NiSe2and FeSe2.

[0064] In an embodiment, the metal selenide includes CoSe, NiSe2, and FeSe2.

[0065] In an embodiment, the metal fluoride includes at least one of LiF, AlF3, CuF2. In an embodiment, the metal selenide includes one of LiF, AlF3, CuF2. That is, the metal selenide is LiF, or the metal selenide is AlF3, or the metal selenide is CuF2.

[0066] In an embodiment, the metal selenide includes two of LiF, AlF3, CuF2. Illustratively, in an embodiment, the metal selenide includes LiF and AlF3. In an embodiment, the metal selenide includes AlF3and CuF2. In an embodiment, the metal selenide includes LiF and CuF2.

[0067] In an embodiment, the metal selenide includes LiF, AlF3, and CuF2.

[0068] In an embodiment, the number of types of metal selenide in the cladding layer 20 is the same as the number of types of metal fluoride. Illustratively, the cladding layer 20 includes one type of metal selenide and one type of metal fluoride.

[0069] In an embodiment, the cladding layer 20 includes CoSe and LiF.

[0070] In an embodiment, the cladding layer 20 includes CoSe and AlF3.

[0071] In an embodiment, the cladding layer 20 includes NiSe2and AlF3.

[0072] In an embodiment, the number of types of metal selenide in the cladding layer 20 is different than the number of types of metal fluoride. Illustratively, the cladding layer 20 includes one type of metal fluoride and two types of metal selenide.

[0073] In an embodiment, the cladding layer 20 includes CoSe, NiSe2, and CuF2.

[0074] In an embodiment, the cladding layer 20 includes CoSe, LiF, and CuF2.

[0075] In one embodiment, the first coating material coats the outer surface of the lithium-rich inner core material 10, and the second coating material coats the outer surface of the lithium-rich inner core material 10 and at least part of the outer surface of the first coating material. The metal selenide forms the first coating material, the metal fluoride forms the second coating material, the coating layer 20 is a multi-layer structure, and the metal fluoride is coated on the outermost side.

[0076] In one embodiment, the first coating material coats the outer surface of the lithium-rich inner core material 10, and the second coating material coats the outer surface of the lithium-rich inner core material 10 and part of the outer surface of the first coating material. That is, the first coating material coats part of the outer surface of the lithium-rich inner core material 10, part of the second coating material coats another part of the outer surface of the lithium-rich inner core material 10, and another part of the second coating material coats the outer surface of part of the first coating material. The outermost side of the lithium-rich positive electrode material 1 simultaneously includes metal fluoride and metal selenide. Since the melting point of the metal fluoride is lower than that of the metal selenide, the metal fluoride can be coated in a molten state on the outer surface of the lithium-rich inner core material 10 and part of the first coating layer during the preparation of the lithium-rich positive electrode material 1, so as to form a more complete and uniform thick coating layer 20. On the one hand, the coating layer 20 can better isolate the lithium-rich inner core material 10 from the outside, so as to better play the role of the coating layer 20. On the other hand, the second coating material only coats part of the outer surface of the first coating material, thereby improving the electrical conductivity of the lithium-rich positive electrode material 1.

[0077] In one embodiment, at least part of the first coating material is not coated by the second coating material.

[0078] In one embodiment, the first coating layer fully coats the outer surface of the lithium-rich inner core material 10, and the second coating layer fully coats the outer surface of the first coating layer. The metal fluoride is coated on the surface of the metal selenide away from the lithium-rich inner core material 10, that is, the metal selenide is located between the metal fluoride and the lithium-rich inner core material 10, and the outermost side of the lithium-rich positive electrode material 1 only has metal fluoride.

[0079] In an embodiment, the first coating material forms a plurality of pores, the second coating material fills at least part of the pores, and the metal fluoride contacts the lithium-rich core material 10 through the pores. The first coating material has pores, and the first coating layer only covers part of the outer surface of the lithium-rich core material 10. The pores of the first coating layer expose part of the outer surface of the lithium-rich core material 10, and the second coating layer fills at least part of the pores, i.e., the metal fluoride is located in at least part of the pores and contacts the lithium-rich core material 10. The metal fluoride fills at least part of the pores of the metal selenide, so that the coating layer 20 achieves the effect of sufficient coating as much as possible. By filling at least part of the pores with the second coating layer, the lithium-rich core material 10 coated and modified by the selenide and fluoride has more excellent performance, for example, the lithium-rich positive electrode material 1 has higher first coulombic efficiency, more excellent cycle performance and capacity retention. In particular, when the metal fluoride is coated, in a molten state, when the metal fluoride contacts the lithium-rich core material 10, part of the fluoride ions in the metal fluoride will enter the crystal lattice of the lithium-rich core material, effectively alleviate the dissolution of transition metals, increase the interlayer spacing, and thus expand the transmission channel of lithium ions.

[0080] In an embodiment, the second coating material fills all the pores.

[0081] In an embodiment, part of the second coating material coats the outer surface of the first coating layer, and another part of the second coating layer fills the pores and coats the outer surface of the lithium-rich core material 10.

[0082] Please refer to Figure 1 In an embodiment, all the second coating material is used to fill the pores.

[0083] In an embodiment, the second coating material is located in the pores between the first coating material, and the thickness of the first coating material is greater than or equal to the thickness of the second coating material.

[0084] In an embodiment, part of the second coating material coats part of the outer surface of the first coating layer, and another part of the second coating layer fills the pores.

[0085] In an embodiment, at least part of the metal fluoride is dispersed in the metal selenide. After the metal selenide coats the outer surface of the lithium-rich core material 10, the metal selenide has a porous structure, i.e. the metal selenide cannot completely coat the lithium-rich core material 10. At this time, at least part of the metal fluoride is dispersed in the porous structure of the metal selenide, and the metal fluoride fills at least part of the pores of the metal selenide, so that the coating layer 20 achieves the effect of complete coating as much as possible. By dispersing at least part of the metal fluoride into the metal selenide, the lithium-rich core material 10 coated and modified by the selenide and fluoride has more excellent performance, for example, the lithium-rich positive electrode material 1 has higher initial coulombic efficiency, more excellent cycle performance and capacity retention rate.

[0086] In an embodiment, all the metal fluoride is dispersed in the metal selenide. The metal fluoride is dispersed in the porous structure of the metal selenide, and the metal fluoride fills all the pores of the metal selenide, so that the coating layer 20 achieves the effect of complete coating. By dispersing the metal fluoride into the metal selenide, the lithium-rich positive electrode material 1 has higher initial coulombic efficiency, good cycle performance and better capacity retention rate.

[0087] In an embodiment, the metal fluoride is uniformly dispersed in the metal selenide. The metal fluoride and the metal selenide are uniformly mixed to form a coating layer 20 with uniform distribution of two components, so that the lithium-rich core material 10 coated and modified by the metal selenide and the metal fluoride has more excellent performance.

[0088] In an embodiment, the metal fluoride can also be non-uniformly distributed in the metal selenide.

[0089] In an embodiment, part of the metal fluoride is dispersed in the metal selenide, and the other part of the metal fluoride is located on the outer surface of the metal selenide.

[0090] In an embodiment, at least part of the first coating material penetrates the second coating material along the radial direction of the lithium-rich core material 10. Along the radial direction of the lithium-rich core material 10, one side of the first coating material is exposed and not coated by the second coating material, and the other side of the first coating material is in contact with the outer surface of the lithium-rich core material 10. The lithium-rich core material 10 can directly conduct electricity with the outside world through the first coating material, i.e. the metal selenide, thereby increasing the conductivity of the lithium-rich positive electrode material.

[0091] In an embodiment, the content of the metal selenide in the coating layer 20 is greater than the content of the metal fluoride.

[0092] In an embodiment, the mass ratio of the metal selenide to the metal fluoride is (2-10):1. In this case, the content of the selenide in the coating layer 20 is greater than the content of the fluoride. The relative content of the metal selenide to the metal fluoride affects the performance of the lithium-rich positive electrode material 1. If the content of the metal fluoride is too high, the conductivity of the lithium-rich positive electrode material 1 is reduced. If the content of the metal fluoride is too low, the metal fluoride has no obvious effect on improving the initial discharge capacity and the initial coulombic efficiency of the lithium-rich positive electrode material 1.

[0093] In the present embodiment, the mass ratio of the metal selenide to the metal fluoride is controlled within a suitable range, which can better the binding force between the metal selenide and the metal fluoride, and make the coating layer 20 in the present application have higher conductivity and ion conductivity, which can not only ensure the transmission of lithium ions in and out of the positive electrode material during the charging and discharging process, but also ensure the conductivity of the material and reduce polarization.

[0094] In an embodiment, the mass ratio of the metal selenide to the metal fluoride is (2.5-10):1.

[0095] In an embodiment, the mass ratio of the metal selenide to the metal fluoride is (3-10):1.

[0096] In an embodiment, the mass ratio of the metal selenide to the metal fluoride is (3.5-5):1.

[0097] In an embodiment, the mass ratio of the metal selenide to the metal fluoride is (4-5):1.

[0098] In an embodiment, the mass ratio of the metal selenide to the metal fluoride is (4.5-5):1.

[0099] In an embodiment, the mass ratio of the metal selenide to the metal fluoride is (2-9):1.

[0100] In an embodiment, the mass ratio of the metal selenide to the metal fluoride is (2-8):1.

[0101] In an embodiment, the mass ratio of the metal selenide to the metal fluoride is (2-7):1.

[0102] In an embodiment, the mass ratio of the metal selenide to the metal fluoride is (2-6):1.

[0103] In an embodiment, the mass ratio of the metal selenide to the metal fluoride is (2-5):1.

[0104] In an embodiment, the mass ratio of the metal selenide to the metal fluoride is (3-4.5):1.

[0105] In an embodiment, the mass ratio of the metal selenide to the metal fluoride is (3-4):1.

[0106] In an embodiment, the mass ratio of the metal selenide to the metal fluoride is (3-3.5):1.

[0107] In an embodiment, the mass ratio of the metal selenide to the metal fluoride is (3.5-4.5):1.

[0108] In an embodiment, the mass ratio of the metal selenide to the metal fluoride is 2:1, 2.5:1, 3:1, 3.2:1, 3.5:1, 3.8:1, 4:1, 4.2:1, 4.5:1, 4.8:1, 5:1, 5.5:1, 6:1, 6.5:1, 7:1, 7.5:1, 8:1, 8.5:1, 9:1, 9.5:1 or 10:1.

[0109] In an embodiment, the mass ratio of the coating layer 20 to the lithium-rich inner core material 10 is (0.025-0.10):1.

[0110] If the mass ratio of the coating layer 20 to the lithium-rich inner core material 10 is too small, the content of selenide and fluoride in the coating layer 20 is too small to improve the conductivity and ion conductivity of the lithium-rich positive electrode material 1, and there is no obvious promoting effect on the first coulomb efficiency, cycle performance and capacity retention of the lithium-rich positive electrode material 1. If the mass ratio of the coating layer 20 to the lithium-rich inner core material 10 is too large, the thicker coating layer 20 increases the processing difficulty of the lithium-rich positive electrode material 1, and affects the energy density of the battery and reduces the battery capacity.

[0111] In the present application, the thickness of the coating layer 20 is set in the range of (0.025-0.10):1. On the one hand, the lithium-rich positive electrode material 1 has good processing performance, and the adhesion between the coating layer 20 and the lithium-rich inner core material 10 is increased. On the other hand, the appropriate thickness of the coating layer 20 can better improve the first coulomb efficiency, cycle performance and capacity retention of the lithium-rich positive electrode material 1, and the lower mass ratio of the coating layer 20 will not increase the mass of the lithium-rich inner core material 10 too much, avoiding the influence of the large mass of the coating layer 20 on the energy density of the battery and reducing the capacity.

[0112] In an embodiment, the mass ratio of the coating layer 20 to the lithium-rich inner core material 10 is (0.025-0.07):1.

[0113] In an embodiment, the mass ratio of the coating layer 20 to the lithium-rich inner core material 10 is (0.03-0.07):1.

[0114] In an embodiment, the mass ratio of the coating layer 20 to the lithium-rich inner core material 10 is (0.04-0.07):1.

[0115] In an embodiment, the mass ratio of the coating layer 20 to the lithium-rich core material 10 is (0.05-0.07): 1.

[0116] In an embodiment, the mass ratio of the coating layer 20 to the lithium-rich core material 10 is (0.06-0.07): 1.

[0117] In an embodiment, the mass ratio of the coating layer 20 to the lithium-rich core material 10 is (0.025-0.08): 1.

[0118] In an embodiment, the mass ratio of the coating layer 20 to the lithium-rich core material 10 is (0.025-0.09): 1.

[0119] In an embodiment, the mass ratio of the coating layer 20 to the lithium-rich core material 10 is (0.025-0.06): 1.

[0120] In an embodiment, the mass ratio of the coating layer 20 to the lithium-rich core material 10 is (0.025-0.05): 1.

[0121] In an embodiment, the mass ratio of the coating layer 20 to the lithium-rich core material 10 is (0.025-0.04): 1.

[0122] In an embodiment, the mass ratio of the coating layer 20 to the lithium-rich core material 10 is (0.025-0.03): 1.

[0123] In an embodiment, the mass ratio of the coating layer 20 to the lithium-rich core material 10 is 0.025: 1, 0.03: 1, 0.035: 1, 0.04: 1, 0.045: 1, 0.05: 1, 0.055: 1, 0.06: 1, 0.065: 1, 0.07: 1, 0.075: 1, 0.08: 1, 0.085: 1, 0.09: 1, 0.095: 1, or 0.10: 1.

[0124] In an embodiment, the mass ratio of the metal selenide to the lithium-rich core material 10 is (0.02-0.05): 1. Within this range, the metal selenide can better improve the electrical conductivity of the lithium-rich positive electrode material 1.

[0125] In an embodiment, the mass ratio of the metal selenide to the lithium-rich core material 10 is (0.03-0.05): 1.

[0126] In an embodiment, the mass ratio of the metal selenide to the lithium-rich core material 10 is (0.04-0.05): 1.

[0127] In an embodiment, the mass ratio of the metal selenide to the lithium-rich core material 10 is (0.02-0.04): 1.

[0128] In an embodiment, the mass ratio of the metal selenide to the lithium-rich core material 10 is (0.02-0.03): 1.

[0129] In an embodiment, the mass ratio of the metal selenide to the lithium-rich core material 10 is (0.03-0.04): 1.

[0130] In an embodiment, the mass ratio of the metal selenide to the lithium-rich core material 10 is 0.02: 1, 0.025: 1, 0.03: 1, 0.035: 1, 0.04: 1, 0.045: 1 or 0.05: 1.

[0131] In an embodiment, the thickness of the coating layer 20 is 50-250 nm.

[0132] If the thickness of the coating layer 20 is too small, for example, the thickness of the coating layer 20 is less than 50 nm, the content of selenide and fluoride in the coating layer 20 is too small to improve the conductivity and ion conductivity of the lithium-rich positive electrode material 1, and there is no obvious promotion effect on the first coulomb efficiency, cycle performance and capacity retention of the lithium-rich positive electrode material 1. If the thickness of the coating layer 20 is too large, for example, the thickness of the coating layer 20 is greater than 250 nm, the thicker coating layer 20 increases the processing difficulty of the lithium-rich positive electrode material 1, and affects the energy density of the battery and reduces the capacity per gram of the battery.

[0133] In the present application, the thickness of the coating layer 20 is set in the range of 50-250 nm. On the one hand, the lithium-rich positive electrode material 1 has good processing performance, and the adhesion between the coating layer 20 and the lithium-rich core material 10 is increased. On the other hand, the appropriate thickness of the coating layer 20 can better improve the first coulomb efficiency, cycle performance and capacity retention of the lithium-rich positive electrode material 1, and the lower mass ratio of the coating layer 20 will not increase the mass of the lithium-rich core material 10 too much, avoiding the influence of the large mass of the coating layer 20 on the energy density of the battery and reducing the capacity.

[0134] In an embodiment, the thickness of the coating layer 20 is 50-200 nm.

[0135] In an embodiment, the thickness of the coating layer 20 is 50-150 nm.

[0136] In an embodiment, the thickness of the coating layer 20 is 50-100 nm.

[0137] In an embodiment, the thickness of the coating layer 20 is 100-300 nm.

[0138] In an embodiment, the thickness of the coating layer 20 is 150-300 nm.

[0139] In an embodiment, the thickness of the coating layer 20 is 200-300 nm.

[0140] In an embodiment, the thickness of the coating layer 20 is 250-300 nm.

[0141] In an embodiment, the thickness of the coating layer 20 is 100-250 nm.

[0142] In an embodiment, the thickness of the coating layer 20 is 150-200 nm.

[0143] In an embodiment, the thickness of the coating layer 20 is 50 nm, 80 nm, 100 nm, 120 nm, 150 nm, 180 nm, 200 nm, 220 nm, 250 nm, 280 nm or 300 nm.

[0144] In an embodiment, the structure of the lithium-rich core material 10 is Li 1+x A y O z wherein 0≤x≤2, 0

[0145] Li 1+x A y O z Such material has a high first cycle charge-discharge capacity, which can generally reach more than 200 mAh / g. Compared with other conventional positive electrode materials, such as lithium iron phosphate and lithium manganese iron phosphate, the lithium-rich core material 10 coated by the coating layer 20 has a higher first cycle discharge capacity, capacity retention rate and first cycle coulombic efficiency. 1+x A y O z After the lithium-rich core material 10 is coated by the coating layer 20, the lithium-rich positive electrode material 1 has a higher first cycle discharge capacity, capacity retention rate and first cycle coulombic efficiency.

[0146] In an embodiment, the lithium-rich core material 10 can be Li2NiO2, LiCoO2, Li2MnO2 or LiMn2O4.

[0147] The present application provides a preparation method of a lithium-rich positive electrode material 1, which comprises steps S1, S2, S3 and S4, and the specific steps are shown as follows:

[0148] Step S1, mixing and ball-milling a metal selenide and the lithium-rich core material 10 to obtain a solid I;

[0149] Step S2, heating the solid I in a protective gas atmosphere to obtain a solid II;

[0150] Step S3, mixing and ball-milling the solid II and a metal fluoride to obtain a solid III;

[0151] Step S4, heating the solid III under a protective gas atmosphere to obtain the lithium-rich cathode material 1.

[0152] In step S1, the metal selenide is mixed with the lithium-rich core material 10 and then ball-milled to reduce the particle size of the metal selenide and the lithium-rich core material 10, to make the metal selenide and the lithium-rich core material 10 mix uniformly, and to increase the contact area of the metal selenide and the lithium-rich core material 10, so as to facilitate the subsequent better coating of the metal selenide on the outer surface of the lithium-rich core material 10.

[0153] In step S2, the protective gas can prevent the lithium-rich core material 10 and the metal selenide from being oxidized during the heating process. The mixture of the metal selenide and the lithium-rich core material 10 is heated, so that the metal selenide is smoothly coated on the outer surface of the lithium-rich core material 10, and the solid III is the lithium-rich core material 10 coated with the metal selenide.

[0154] In step S3, the metal fluoride is mixed with the solid II and then ball-milled to reduce the particle size of the metal fluoride and the solid II, to make the metal fluoride and the solid II mix uniformly, and to increase the contact area of the metal fluoride and the solid II, so as to facilitate the subsequent better coating of the metal fluoride on the solid II.

[0155] In step S4, the protective gas can prevent the lithium-rich core material 10, the metal selenide and the metal fluoride from being oxidized during the heating process. The solid III is heated under a protective gas atmosphere, so that the metal fluoride is coated on the lithium-rich core material 10 to obtain the lithium-rich cathode material 1 with a coating layer 20 of the metal fluoride and the metal selenide.

[0156] In an embodiment, the protective gas can be at least one of nitrogen, helium, neon, argon, krypton, xenon, and radon.

[0157] In an embodiment, the protective gas in step S2 is the same as the protective gas in step S4. For example, the protective gas in step S2 and the protective gas in step S4 are both helium.

[0158] In an embodiment, the protective gas in step S2 is different from the protective gas in step S4. For example, the protective gas in step S2 is nitrogen, and the protective gas in step S4 is helium.

[0159] In this application, the preparation process of the lithium-rich cathode material 1 is simple, and the lithium-rich cathode material 1 is easy to popularize and apply.

[0160] In an embodiment, the heating temperature and the heating time in heating the solid III under the protective gas atmosphere are greater than the heating temperature and the heating time in heating the solid I under the protective gas atmosphere. That is, the temperature when the lithium-rich core material 10 is coated with the metal selenide is higher than the temperature when the lithium-rich core material 10 is coated with the metal fluoride.

[0161] The melting point of the metal selenide is higher than that of the metal fluoride. When the metal selenide is coated to the outer surface of the lithium-rich core material 10 (i.e. in step S2), due to the lower heating temperature, the metal selenide cannot reach a molten state, and the solid granular metal selenide cannot achieve complete coating on the outer surface of the lithium-rich core material 10. The metal selenide on the outer surface of the lithium-rich core material 10 has a porous structure, and part of the outer surface of the lithium-rich core material 10 is not coated with the metal selenide.

[0162] When the metal fluoride is coated to the outer surface of the lithium-rich core material 10 (i.e. in step S4), due to the lower melting point of the metal fluoride, under high temperature conditions, the metal fluoride melts into a liquid state, and the metal fluoride in a molten state can enter the porous structure left by the metal selenide in the solid III. The metal fluoride can greatly contact the metal selenide, so that the metal fluoride and the metal selenide can form a relatively complete and uniform thick coating layer 20, achieving the effect of double-component coating.

[0163] If an organic fluoride is used as the material of the coating layer 20, the organic fluoride is not a good lithium ion conductor material, which is not conducive to the embedding and extraction of lithium ions. At the same time, the organic fluoride will carbonize rather than melt under high temperature, and the organic fluoride is difficult to enter the porous structure of the metal selenide. The contact area between the organic fluoride and the metal selenide is small, and it is difficult to form a relatively dense coating layer 20 in which the fluorine element is dispersed into the selenium element.

[0164] If an organic selenide is used as the material of the coating layer 20, the conductivity of the organic selenide is lower than that of the metal selenide. The beneficial effects of using the metal selenide as the material of the coating layer 20 on the structure and performance of the lithium-rich positive electrode material 1 are not possessed by the organic selenide.

[0165] In the present application, the lithium-rich positive electrode material 1 coated and modified by metal selenide and metal fluoride has high initial coulombic efficiency, good cycle performance and good capacity retention rate. The interaction between the metal fluoride and the metal selenide in the preparation process effectively forms a relatively complete and uniform thick coating layer 20. The metal fluoride and the metal selenide complement each other to form the coating layer 20, which can ensure the transmission of lithium ions in and out of the lithium-rich positive electrode material 1 during the charging and discharging process, and can also ensure the conductivity of the lithium-rich positive electrode material 1 to reduce polarization. In addition, a small amount of fluoride ions will enter the lattice of the core lithium-rich positive electrode material 1 during the melting stage of the metal fluoride, effectively alleviating the dissolution of transition metals, increasing the interlayer spacing, and thus expanding the transmission channel of lithium ions. At the same time, the metal fluoride has a certain lithium ion extraction capacity, which can improve the initial discharge capacity and increase the initial coulombic efficiency. The selenium ions will enter the oxygen ion vacancies during the charging and discharging process to replace the positions of the oxygen ions, thereby inhibiting the oxygen release of the lithium-rich positive electrode material 1 at the interface and reducing the side reactions at the interface.

[0166] In an embodiment, the heating temperature of the solid I is 300-500°C. At this temperature, the metal selenide can be smoothly coated to the outer surface of the lithium-rich core material 10.

[0167] In an embodiment, the heating time of the solid I is 2-4 hours.

[0168] In an embodiment, the heating temperature of the solid III is 700-900°C. At this temperature, the metal fluoride is melted, and the molten metal fluoride is dispersed into the pore structure of the metal selenide, and forms a complete and complete coating layer 20 together with the metal selenide.

[0169] In an embodiment, the heating time of the solid III is 7-10 hours.

[0170] In an embodiment, the ball milling time after mixing the metal selenide with the lithium-rich core material 10 is longer than the ball milling time after mixing the metal fluoride with the solid II.

[0171] In an embodiment, the ball milling time after mixing the metal selenide with the lithium-rich core material 10 is 5-8 hours. The metal selenide and the lithium-rich core material 10 are ball milled for a long time, which reduces the particle size of the metal selenide and the lithium-rich core material 10, and makes the metal selenide and the lithium-rich core material 10 uniformly mixed.

[0172] In an embodiment, the ball milling time after mixing the metal fluoride with the solid II is 1-3 hours. The metal fluoride and the solid II are ball milled, which reduces the particle size of the metal fluoride and the solid II, and makes the metal fluoride and the solid II uniformly mixed.

[0173] In an embodiment, the heating rate of the solid I is greater than the heating rate when the solid III is heated.

[0174] In an embodiment, the preparation method of the lithium-rich positive electrode material 1 comprises the following steps:

[0175] The 2-5wt% cobalt selenide is mixed with the lithium-rich positive electrode material 1 and ball-milled for 5-8 hours to obtain a solid I;

[0176] The solid I is transferred to a gas atmosphere tube furnace, heated to 300-500℃ at a rate of 5℃ / min under an argon atmosphere, and kept for 2-4h to obtain a solid II;

[0177] The solid II is mixed with 0.5-2wt% lithium fluoride and ball-milled for 1-3 hours to obtain a solid III;

[0178] The solid III is transferred to a gas atmosphere tube furnace, heated to 700-900℃ at a rate of 2℃ / min under an argon atmosphere, and kept for 7-10h, and then ground and sieved to obtain the cobalt selenide and lithium fluoride coated lithium-rich positive electrode material 1.

[0179] The 2-5wt% cobalt selenide refers to the proportion of cobalt selenide in the lithium-rich core material 10, i.e. (the mass of cobalt selenide / the mass of lithium-rich core material 10) x 100% is 2-5%. Similarly, the 0.5-2wt% lithium fluoride also refers to the proportion of lithium fluoride in the lithium-rich core material 10, i.e. (the mass of lithium fluoride / the mass of lithium-rich core material 10) x 100% is 0.5-2%.

[0180] The present application provides a positive electrode tab, which comprises the lithium-rich positive electrode material 1 according to any one of the above, or the lithium-rich positive electrode material 1 prepared by the preparation method of the lithium-rich positive electrode material 1.

[0181] The present application provides a secondary battery, which comprises a negative electrode tab, a separator, and the positive electrode tab as described above.

[0182] In an embodiment, the secondary battery is a lithium ion secondary battery.

[0183] In order to illustrate the beneficial effects of the method of the present application, the following examples and comparative examples are also provided:

[0184] Example 1

[0185] Example 1 provides a lithium-rich positive electrode material, which comprises a lithium-rich core material and a coating layer coated on the outer surface of the lithium-rich core material, the coating layer comprises cobalt selenide and lithium fluoride, the lithium fluoride is dispersed in the cobalt selenide, the mass ratio of the cobalt selenide to the lithium fluoride is 2:1, and the mass ratio of the coating layer to the lithium-rich core material is 0.06:1.

[0186] The preparation method of the lithium-rich positive electrode material provided in Example 1 comprises the following steps:

[0187] The 4wt% cobalt selenide is mixed with the lithium-rich positive electrode material and ball-milled for 5-8 hours to obtain solid I;

[0188] The solid I is transferred into a tube furnace under argon atmosphere, heated to 300-500℃ at a rate of 5℃ / min and kept for 2-4h to obtain solid II;

[0189] The solid II is mixed with 2wt% lithium fluoride and ball-milled for 1-3 hours to obtain solid III;

[0190] The solid III is transferred into a tube furnace under argon atmosphere, heated to 700-900℃ at a rate of 2℃ / min and kept for 7-10h, and then ground and sieved to obtain the cobalt selenide and lithium fluoride coated lithium-rich positive electrode material.

[0191] Example 2

[0192] The lithium-rich positive electrode material provided in Example 2 is different from the lithium-rich positive electrode material in Example 1 in that the mass ratio of cobalt selenide to lithium fluoride in the lithium-rich positive electrode material provided in Example 2 is 5:1.

[0193] Example 3

[0194] The lithium-rich positive electrode material provided in Example 3 is different from the lithium-rich positive electrode material in Example 1 in that the mass ratio of cobalt selenide to lithium fluoride in the lithium-rich positive electrode material provided in Example 3 is 8:1.

[0195] Example 4

[0196] The lithium-rich positive electrode material provided in Example 4 is different from the lithium-rich positive electrode material in Example 2 in that the coating layer of the lithium-rich positive electrode material provided in Example 4 comprises nickel selenide and lithium fluoride, the lithium fluoride is dispersed in the nickel selenide, the mass ratio of the nickel selenide to the lithium fluoride is 5:1, and the mass ratio of the coating layer to the lithium-rich core material is 0.06:1.

[0197] Example 5

[0198] The lithium-rich positive electrode material provided in Example 5 is different from the lithium-rich positive electrode material in Example 1 in that the coating layer of the lithium-rich positive electrode material provided in Example 5 only comprises cobalt selenide and does not comprise lithium fluoride.

[0199] Comparative Example 1

[0200] The lithium-rich positive electrode material provided in Comparative Example 1 is different from the lithium-rich positive electrode material in Example 1 in that the surface of the lithium-rich positive electrode material provided in Comparative Example 1 is not coated and modified.

[0201] Comparative Example 2

[0202] The lithium-rich positive electrode material provided in Comparative Example 2 is different from the lithium-rich positive electrode material in Example 1 in that the surface of the lithium-rich positive electrode material provided in Comparative Example 2 is carbon-coated. The specific implementation steps are as follows: the prepared lithium-rich positive electrode material and organic carbon source are mixed (300 r / min) in anhydrous ethanol for 5 hours, then the mixture is placed in a forced air drying oven at 90°C for 10 hours, and then placed in a porcelain boat. Heat treatment is carried out in an argon atmosphere, sintering at 550-750°C for 15 hours, and grinding and sieving to obtain the desired carbon-coated lithium-rich positive electrode material.

[0203] Comparative Example 3

[0204] The lithium-rich positive electrode material provided in Comparative Example 3 is different from the lithium-rich positive electrode material in Example 1 in that the coating layer of the lithium-rich positive electrode material provided in Comparative Example 3 only includes lithium fluoride, but does not include cobalt selenide.

[0205] The lithium-rich positive electrode materials provided in Examples 1-5 and the lithium-rich positive electrode materials provided in Comparative Examples 1-3 are assembled into positive electrode sheets and lithium ion batteries according to the following methods, respectively:

[0206] Positive electrode: the lithium-rich positive electrode material is mixed with polyvinylidene fluoride and SP-Li at a mass ratio of 80:8:12 to obtain a positive electrode slurry by ball milling stirring, the positive electrode slurry is coated on the surface of an aluminum foil, vacuum dried at 110°C overnight, and rolled to obtain a positive electrode sheet;

[0207] Negative electrode: graphite is mixed with carboxymethyl cellulose (CMC), SBR and SP at a mass ratio of 95.8:1.2:2:1 to obtain a negative electrode slurry by ball milling stirring, the negative electrode slurry is coated on the surface of a copper foil, vacuum dried at 110°C overnight to obtain a negative electrode sheet;

[0208] Electrolyte: ethylene carbonate and methyl ethyl carbonate are mixed at a volume ratio of 3:7, and LiPF6 is added to form an electrolyte, the concentration of LiPF6 is 1 mol / L;

[0209] Separator: polypropylene microporous separator;

[0210] Lithium ion battery assembly: according to the assembly sequence of graphite negative electrode sheet-separator-electrolyte-positive electrode sheet, a button lithium ion full cell is assembled in an inert atmosphere glove box.

[0211] The electrochemical performance of each sodium ion battery assembled in the above sodium ion battery examples is tested according to the performance tests in Table 1, and all at a rate of 0.5C. The test results are shown in Table 1 below:

[0212] Table 1 Performance test results

[0213]

[0214] From the test results of Examples 1-5 and Comparative Examples 1-3 in Table 1, it can be seen that the first cycle specific capacity, the first cycle coulombic efficiency and the capacity retention rate of Examples 1-5 are all significantly improved compared with the pure sample in Comparative Example 1, indicating that the surface modification of the lithium-rich positive electrode material by cobalt selenide / lithium fluoride can effectively improve the internal structure of the lithium-rich positive electrode material and thus improve the discharge specific capacity of the material. At the same time, the surface modification of the lithium-rich positive electrode material by cobalt selenide / lithium fluoride can effectively inhibit the side reaction on the surface of the material and the collapse of the structure, thereby reducing the capacity loss of the material and improving the capacity retention rate of the material. For the core lithium-rich positive electrode material, the coating effect of the metal selenide / metal fluoride with a proper mass ratio is much better than that of the traditional carbon material, which also indicates that the metal selenide / metal fluoride coating layer not only has the properties of the traditional carbon material coating layer but also has some unique properties, for example, it has faster lithium ion transmission efficiency and more active lithium during the charging and discharging process.

[0215] At the same time, by comparing the performance data of Comparative Example 1, Example 2 and Example 3, it can be found that the m(cobalt selenide):m(lithium fluoride) should be within a suitable range to achieve the best surface modification effect. When m(cobalt selenide):m(lithium fluoride) = 2:1, too much molten lithium fluoride during high-temperature sintering may completely coat the cobalt selenide, which greatly reduces the conductivity of the overall material, resulting in a low discharge capacity. When m(cobalt selenide):m(lithium fluoride) = 8:1, too little molten lithium fluoride during high-temperature sintering fails to completely contact with the cobalt selenide and the core material, resulting in an insignificant improvement in the first cycle discharge capacity and the capacity retention rate. It is worth noting that by comparing the performance data of Examples 1, 2 and 3 with Comparative Example 5 and Comparative Example 3, it can be found that the double-component cobalt selenide / lithium fluoride coating is more excellent than the single cobalt selenide coating or the single lithium fluoride coating in releasing the performance of the material. This is mainly because the double-component cobalt selenide / lithium fluoride can achieve uniform coating of the core material during high-temperature sintering, and also endows the overall material with better conductivity and more extensive ion carrying channels, which the single cobalt selenide coating or the single lithium fluoride coating does not have. The single lithium fluoride coating in Comparative Example 3 reduces the conductivity of the overall material and increases the polarization.

[0216] In addition, the surface modification of the lithium-rich positive electrode material by nickel selenide / lithium fluoride in Example 5 also has a good coating effect, which is manifested in the excellent first cycle charging and discharging capacity, the first cycle coulombic efficiency and the capacity retention rate after 50 cycles of the overall material. This also proves the universality of the metal selenide / metal fluoride double-component coating for lithium-rich positive electrode materials.

[0217] The lithium-rich positive electrode material, the preparation method and the application thereof provided by the embodiments of the present application are described in detail above, and the principles and the embodiments of the present application are described by using specific examples; the above description of the embodiments is only used to help understand the method of the present application and the core idea thereof; meanwhile, for the general technical personnel in the art, the specific embodiments and the application range can be changed according to the idea of the present application; and in conclusion, the content of the specification should not be understood as the limitation of the present application.

Claims

1. A lithium-rich cathode material, characterized in that, The lithium-rich cathode material includes: Lithium-rich core materials; A coating layer is applied to the outer surface of the lithium-rich core material. The coating layer includes a first coating material, which is a metal selenide. The coating layer further includes a second coating material, which is a metal fluoride, and at least a portion of the metal fluoride is in contact with the lithium-rich core material; the first coating material coats the outer surface of the lithium-rich core material, and the second coating material coats the outer surface of the lithium-rich core material and a portion of the outer surface of the first coating material; The coating layer is continuously distributed on the outer surface of the lithium-rich core material; The lithium-rich core material is doped with fluorine, and the fluorine is located on the side of the lithium-rich core material near the coating layer. The metal selenide includes at least one of CoSe, NiSe2, and FeSe2; the metal fluoride includes at least one of LiF, AlF3, and CuF2. 2.The lithium-rich cathode material of claim 1, characterized in that, The first coating material has multiple pores between it, the second coating material fills at least part of the pores, and the metal fluoride contacts the lithium-rich core material through the pores; and / or, At least a portion of the first coating material penetrates the second coating material radially along the lithium-rich core material. 3.The lithium-rich cathode material of claim 1, wherein, The structure general formula of the lithium-rich core material is Li 1+x A y O z Wherein, 0≤x≤2, 0 y≤3, 0 4.The lithium-rich cathode material of claim 1, wherein, The mass ratio of the metal selenide to the metal fluoride is (2~10):1; and / or, The mass ratio of the coating layer to the lithium-rich core material is (0.025~0.10):1; and / or, The thickness of the coating layer is 50 nanometers to 300 nanometers.

5. A method for preparing a lithium-rich cathode material, characterized in that, The method for preparing the lithium-rich cathode material is used to prepare the lithium-rich cathode material as described in any one of claims 1-4, and the preparation method includes: Metal selenide was mixed with lithium-rich core material and ball-milled to obtain solid I; Solid I is heated under a protective gas atmosphere to obtain solid II; Solid II is mixed with a metal fluoride and ball-milled to obtain solid III; The solid III is heated under a protective gas atmosphere to obtain a lithium-rich cathode material.

6. The method for preparing the lithium-rich cathode material according to claim 5, characterized in that, The heating temperature in heating solid III under a protective gas atmosphere is greater than the heating temperature in heating solid I under a protective gas atmosphere.

7. A positive electrode sheet, characterized in that, The positive electrode sheet includes the lithium-rich positive electrode material as described in any one of claims 1-4, or the lithium-rich positive electrode material prepared by the method described in claim 5 or 6.

8. A secondary battery, characterized in that, The secondary battery includes a negative electrode, a separator, and a positive electrode as described in claim 7.

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