Lithium-rich phosphate positive electrode material and preparation method and application thereof

Abstract

The present application discloses a lithium-rich phosphate positive electrode material and its preparation method and application. The lithium-rich phosphate positive electrode material of the present application includes a lithium-rich phosphate positive electrode material, the lithium-rich phosphate positive electrode material includes a main phase and a lithium-rich phase for lithium supplementation, and the main phase and the lithium-rich phase are doped with each other to form a single crystal phase; wherein the main phase is LiM a PO 4 Phosphate-based cathode materials have high first coulombic efficiency, energy density and cycle performance. The preparation method of lithium-rich phosphate-based cathode materials can ensure that the structure and electrochemical performance of the prepared lithium-rich phosphate-based cathode materials are stable, and the efficiency is high, saving production costs.

Description

Technical Field

[0001] The present application belongs to the technical field of electrode materials, and specifically relates to a lithium-rich phosphate positive electrode material and a preparation method and application thereof. Background Art

[0002] The energy density of commercial lithium-ion batteries is approaching its theoretical limit. In order to further improve the energy density of lithium-ion batteries, the development of new electrode materials with high specific capacity is extremely important.

[0003] Although many new electrode materials have high specific capacity, their low first coulombic efficiency limits their practical application. In order to overcome this shortcoming, researchers are working hard to develop lithium supplement materials. For example, lithium phosphide is considered to be an excellent lithium supplement material because of its extremely high specific capacity (1550mAh / g). However, its lithium supplement application is still subject to several limitations. First, it is difficult to obtain lithium phosphide materials with high purity. Second, since metallic lithium is difficult to form particles, the prepared lithium phosphide is usually in block form and has a large size, which is not conducive to good contact between lithium phosphide and the conductive agent, making the problem of poor electronic conductivity of lithium phosphide more prominent. Third, lithium phosphide easily reacts with water vapor in the air, making it extremely difficult to further optimize lithium phosphide materials (such as reducing the scale, carbon coating, etc.) and apply them.

[0004] Phosphate-based cathode materials have quickly taken over the current market due to their safety, environmental friendliness, high capacity, and high cost-effectiveness of raw materials. However, due to their shortcomings such as low ion transmission rate and poor conductivity, their use has been restricted and has been subject to market controversy. Moreover, their low first coulomb efficiency has also limited their practical application. Summary of the invention

[0005] The purpose of this application is to overcome the above-mentioned deficiencies of the prior art and to provide a lithium-rich phosphate positive electrode material and a preparation method thereof, so as to solve the technical problems of the existing phosphate positive electrode materials such as unsatisfactory conductivity, ion diffusion rate, capacity retention rate and first coulombic efficiency.

[0006] Another object of the present application is to provide a positive electrode sheet and a secondary battery containing the positive electrode sheet to solve the technical problem that the electrochemical performance of existing secondary batteries, such as cyclability, is not ideal.

[0007] In order to achieve the above application objectives, the first aspect of the present application provides a lithium-rich phosphate positive electrode material. The lithium-rich phosphate positive electrode material of the present application includes a main phase and a lithium-rich phase for replenishing lithium, and the main phase and the lithium-rich phase are doped with each other to form a single crystalline phase; wherein the main phase is LiM a PO4, LiM a M in PO4 is a transition metal element, 0<a≤1.

[0008] The second aspect of the present application provides a method for preparing the lithium-rich phosphate positive electrode material of the present application. The method for preparing the lithium-rich phosphate positive electrode material of the present application comprises the following steps:

[0009] LiM a PO4 or its precursor and the lithium-rich precursor are mixed in proportion to obtain a mixture; LiM a M in PO4 is a transition metal element, 0<a≤1;

[0010] The mixture is sintered in a protective atmosphere to obtain a lithium-rich phosphate positive electrode material.

[0011] In a third aspect of the present application, a positive electrode sheet is provided. The positive electrode sheet of the present application comprises a positive electrode current collector and a positive electrode active layer bonded to the surface of the positive electrode current collector, wherein the positive electrode active layer contains the lithium-rich phosphate positive electrode material of the present application or the lithium-rich phosphate positive electrode material prepared by the method for preparing the lithium-rich phosphate positive electrode material of the present application.

[0012] In a fourth aspect of the present application, a secondary battery is provided, which comprises a positive electrode sheet and a negative electrode sheet, wherein the positive electrode sheet is the positive electrode sheet of the present application.

[0013] Compared with the prior art, this application has the following technical effects:

[0014] The lithium-rich phase contained in the lithium-rich phosphate positive electrode material of the present application can play the role of a lithium supplement additive, which can supplement the lithium ions consumed by the formation of the SEI film during the first charge and discharge process of the battery, thereby improving the first coulomb efficiency of the lithium-rich phosphate positive electrode material of the present application. a The PO4 main phase is compounded to form a single phase. On the one hand, the two phases can be in direct contact, which makes the interface impedance of the two phases smaller, thereby giving the lithium-rich phosphate positive electrode material of the present application a higher ion migration rate and conductivity; on the other hand, the main phase can protect the lithium-rich phase, avoiding direct contact between the lithium-rich phase and the air, thereby effectively ensuring the lithium-replenishing effect of the lithium-rich phase and improving the storage and processing performance of the lithium-rich phosphate positive electrode material of the present application.

[0015] The method for preparing the lithium-rich phosphate positive electrode material of the present application can effectively prepare a lithium-rich phosphate positive electrode material containing both a main phase and a lithium-rich phase, so that the prepared lithium-rich phosphate positive electrode material has both a lithium supplementation effect and phosphate positive electrode material activity, and has electrochemical properties such as high first coulomb efficiency, energy density, and cycle performance, and the electrochemical performance is stable and the storage performance is good. In addition, the method for preparing the lithium-rich phosphate positive electrode material can ensure that the prepared lithium-rich phosphate positive electrode material has stable electrochemical performance and high efficiency, saving production costs.

[0016] Since the positive electrode sheet of the present application contains the lithium-rich phosphate positive electrode material of the present application, the positive electrode sheet of the present application has a high energy density, has a lithium replenishing effect, and has a long cycle life.

[0017] Since the secondary battery of the present application contains the electrode sheet of the present application, the secondary battery of the present application has excellent first coulombic efficiency, high energy density and cycle stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] In order to more clearly illustrate the specific implementation methods of the present application or the technical solutions in the prior art, the drawings required for use in the specific implementation methods or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some implementation methods of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0019] Figure 1 This is a schematic diagram of a process for preparing a lithium-rich phosphate-based positive electrode material according to an embodiment of the present application;

[0020] Figure 2 This is an X-ray diffraction (XRD) diagram of the lithium-rich phosphate positive electrode material in Example 2 of the present application. DETAILED DESCRIPTION

[0021] In order to make the technical problems, technical solutions and beneficial effects to be solved by the present application more clearly understood, the present application is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and are not used to limit the present application.

[0022] In this application, the term "and / or" describes the association relationship of associated objects, indicating that there may be three relationships. For example, A and / or B can mean: A exists alone, A and B exist at the same time, and B exists alone. A and B can be singular or plural. The character " / " generally indicates that the associated objects are in an "or" relationship.

[0023] In this application, "at least one" means one or more, and "plurality" means two or more. "At least one of the following" or similar expressions refers to any combination of these items, including any combination of single items or plural items. For example, "at least one of a, b, or c", or "at least one of a, b, and c" can all mean: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, c can be single or multiple, respectively.

[0024] It should be understood that in the various embodiments of the present application, the size of the serial numbers of the above-mentioned processes does not mean the order of execution, some or all of the steps can be executed in parallel or sequentially, and the execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of the present application.

[0025] The terms used in the embodiments of the present application are only for the purpose of describing specific embodiments, and are not intended to limit the present application. The singular forms "a", "said" and "the" used in the embodiments of the present application and the appended claims are also intended to include plural forms, unless the context clearly indicates other meanings.

[0026] The weight of the relevant components mentioned in the embodiment description of the present application can not only refer to the specific content of each component, but also represent the proportional relationship between the weights of the components. Therefore, as long as the content of the relevant components is proportionally enlarged or reduced according to the embodiment description of the present application, it is within the scope disclosed in the embodiment description of the present application. Specifically, the mass described in the embodiment description of the present application can be a mass unit known in the chemical industry such as μg, mg, g, kg, etc.

[0027] The terms "first" and "second" are used only for descriptive purposes to distinguish objects such as substances from each other, and should not be understood as indicating or implying relative importance or implicitly indicating the number of indicated technical features. For example, without departing from the scope of the embodiments of the present application, the first XX may also be referred to as the second XX, and similarly, the second XX may also be referred to as the first XX. Thus, the features defined as "first" and "second" may explicitly or implicitly include one or more of the features.

[0028] In the first aspect, the embodiments of the present application provide a lithium-rich phosphate-based positive electrode material. The lithium-rich phosphate-based positive electrode material of the embodiments of the present application includes a lithium-rich phosphate positive electrode material. The lithium-rich phosphate positive electrode material includes a main phase and a lithium-rich phase for replenishing lithium, and the main phase and the lithium-rich phase are doped with each other to form a single crystalline phase, that is, doping on the lattice, specifically, the main phase and the lithium-rich phase are formed as one during sintering and lattice formation, thereby giving full play to the main phase and the lithium-rich phase, and improving the following synergistic effect between the two phases. The single crystalline phase is a mixture formed by simple physical mixing treatments that are different from the main phase and the lithium-rich phase.

[0029] Among them, the main phase of the lithium-rich phosphate positive electrode material is LiM a PO4. Therefore, the main phase gives the lithium-rich phosphate cathode material the electrochemical properties corresponding to phosphate-based cathode materials, such as high capacity and high cost performance. aM in PO4 is a transition metal element, 0<a≤1. In a specific embodiment, the transition metal element M shown therein includes at least one element of Fe, Mn, Ni, and Co. Based on these types of transition elements, the main phase can be LiFePO4, LiMnPO4, LiNiPO4, LiMn 0.3 Fe 0.7 PO4, etc. These transition metal elements can improve the energy density and electrode potential of lithium-rich phosphate positive electrode materials, that is, lithium-rich phosphate positive electrode materials.

[0030] The lithium-rich phase contained in the lithium-rich phosphate positive electrode material can act as a lithium supplement additive, which can replenish the lithium ions consumed by the formation of the SEI film during the initial charge and discharge process of the battery, thereby improving the initial coulombic efficiency of the lithium-rich phosphate positive electrode material of the present application embodiment. a The PO4 main phase is compounded to form a single phase, so that the two phases can directly contact each other, making the interface impedance of the two phases smaller, thus giving the lithium-rich phosphate positive electrode material of the embodiment of the present application a higher ion migration rate and conductivity. Moreover, the main phase can protect and isolate the lithium-rich phase, avoiding direct contact between the lithium-rich phase and the air, thereby effectively ensuring the lithium-replenishing effect of the lithium-rich phase, and having excellent storage and processing properties.

[0031] In the embodiment, the molar ratio of the lithium-rich phase to the main phase is x, and 0<x<1, further can be 0<x<0.5, and further can be 0.01<x<0.5. By controlling the content range of the lithium-rich phase in the lithium-rich phosphate positive electrode material, the lithium-replenishing effect of the lithium-rich phase can be fully exerted, providing abundant lithium ions to make up for the lithium ions consumed by the formation of the SEI film during the first charge and discharge process, and ensuring the energy density of the main phase, thereby improving the energy density of the lithium-rich phosphate positive electrode material. When the molar content of the lithium-rich phase gradually decreases, its lithium-replenishing effect gradually decreases; its content gradually increases, and its lithium-replenishing effect gradually increases. However, when the lithium-rich phase content x is higher than 1, that is, when the lithium-rich phase content is too much, the content of the main phase will be reduced, thereby reducing the energy density of the lithium-rich phosphate positive electrode material, and also reducing the protection and isolation effect of the main phase on the lithium-rich phase, so that the lithium-replenishing effect of the lithium-rich phase is correspondingly reduced, and the storage and processing performance of the lithium-rich phosphate positive electrode material of the embodiment of the present application is reduced.

[0032] In an embodiment, the lithium-rich phase includes Li3N b P 1-b , where 0≤b≤1. Therefore, according to the value range of b, the Li3N b P 1-b Can be Li3P, Li3N or Li3N b P 1-b(0<b<1) At least one. These lithium-rich phases have excellent lithium supplementation effects, and the single crystalline phase formed with the main phase and the secondary phase is stable, thereby ensuring the stability of the electrochemical performance of the lithium-rich phosphate positive electrode material of the embodiment of the present application, and the interface impedance with the main phase and the secondary phase is smaller, thereby improving the lithium-rich phosphate positive electrode material of the embodiment of the present application has a higher ion migration rate and conductivity. Based on the types of the above-mentioned lithium-rich phases, in a specific embodiment, when the lithium-rich phase is Li3P, the lithium-rich phase forms LiM a PO4·xLi3P single crystal phase. When Li3P is selected as the lithium-rich phase, the phosphorus in the Li3P is exactly the same as the phosphorus in the main phase phosphate material, and too many foreign elements will not be introduced, thereby making the structure of the lithium-rich phosphate positive electrode material more stable, and the cycle performance and rate performance are better.

[0033] Based on the main phase contained in the above-mentioned lithium-rich phosphate positive electrode material, it is ideal that it does not contain impurities or miscellaneous phases such as lithium phosphate. That is, in the lithium phosphate positive electrode material, the higher the purity of the lithium phosphate positive electrode material, the more desirable it is. However, in actual production, phosphate positive electrode materials often contain impurities or miscellaneous phases such as lithium phosphate, which can lead to unfavorable performances such as capacity, stability and processing of positive electrode materials such as phosphate-based positive electrode materials. In the embodiments of the present application, the inventors found in the actual production process that on the basis of the above-mentioned main phase and lithium-rich phase constituting a single crystalline phase, the presence of miscellaneous phases such as lithium phosphate is properly controlled, which has a certain positive effect on the electrochemical properties of lithium-rich phosphate positive electrode materials. Therefore, in the embodiment, the above-mentioned lithium-rich phosphate positive electrode material also includes a secondary phase. Based on the above-mentioned main phase, the secondary phase can be lithium phosphate (Li3PO4), specifically, the secondary phase is doped with the above-mentioned main phase and lithium-rich phase to form a single crystalline phase, which can play the role of lithium ion conductive material, and together with the lithium-rich phase, it plays a synergistic role in lithium ion conductivity, giving the lithium-rich phosphate-based positive electrode material a high ion migration rate and conductivity. The secondary phase may be distributed on the surface and / or surface layer of the primary phase, or may be doped in the primary phase.

[0034] Furthermore, the inventor has found that the content of the secondary phase in the lithium-rich phosphate positive electrode material can adjust the above-mentioned effect of the secondary phase. For example, in the embodiment, the weight ratio of the secondary phase to the main phase can be (0-15): (85-100), and further can be (2-15): (85-100). By adjusting the ratio between the main phase and the secondary phase, the content of the phosphate positive electrode material in the lithium-rich phosphate positive electrode material can be effectively guaranteed, and the lithium-rich phosphate positive electrode material can be given a high energy density. On this basis, the above-mentioned ion conductive effect of the secondary phase can be improved, thereby improving the high ion migration rate and conductivity of the lithium-rich phosphate positive electrode material.

[0035] In a further embodiment, the lithium-rich phosphate positive electrode material of the embodiment of the present application includes, in addition to the above-mentioned lithium-rich phosphate positive electrode material containing a main phase and a lithium-rich phase or further containing a secondary phase, an encapsulation functional layer. Then at this time, the lithium-rich phosphate positive electrode material of the embodiment of the present application is a core-shell structure, specifically, the above-mentioned lithium-rich phosphate positive electrode material containing a main phase, a secondary phase and a lithium-rich phase is used as a core body, and the encapsulation functional layer is used as a coating layer to coat the core body formed by the lithium-rich phosphate positive electrode material. In this way, the presence of the encapsulation functional layer can effectively coat the lithium-rich phosphate positive electrode material, ensure that the lithium-rich phosphate positive electrode material is in direct contact with the environment, such as isolating the adverse factors such as water vapor and carbon dioxide in the environment, thereby ensuring the stability of the lithium-rich phosphate positive electrode material, especially the lithium-rich phase, giving full play to the lithium-replenishing effect of the above-mentioned lithium-rich phase and the synergistic effect between the lithium-rich phase and the main phase and the secondary phase, and improving the electrode material activity of the lithium-rich phosphate positive electrode material. In the embodiment, the thickness of the encapsulation functional layer can be controlled to be 1-300nm; the weight of the encapsulation functional layer accounts for 0.01-10wt% of the total weight of the lithium-rich phosphate positive electrode material. By controlling and adjusting the thickness or content ratio of the encapsulation functional layer, the lithium replenishment effect, storage and processing performance, energy density and capacity retention rate, and cycle performance of the lithium-rich phosphate positive electrode material can be ensured.

[0036] In an embodiment, the encapsulation functional layer may be an ion conductor encapsulation layer or an electronic conductor encapsulation layer or a composite layer structure of an ion conductor encapsulation layer or an electronic conductor encapsulation layer. The ion conductor encapsulation layer can improve the insertion and extraction effect of lithium ions of the lithium-rich phosphate positive electrode material and the ionic conductivity of lithium ions.

[0037] When the encapsulation function layer includes an ion conductor encapsulation layer, the material of the ion conductor encapsulation layer may be a material that is conducive to improving ion conductivity, such as but not limited to at least one of perovskite type, NASICON type, and garnet type. The perovskite type may include Li 3x La 2 / 3-x TiO3(LLTO), specifically Li 0.5 La 0.5 TiO3、Li 0.33 La 0.57 TiO3、Li 0.29 La 0.57 TiO3、Li 0.33 Ba 0.25 La 0.39 TiO3, (Li 0.33 La 0.56 ) 1.005 Ti 0.99 Al 0.01 O3、Li 0.5 La 0.5 Ti 0.95 Zr 0.05O3, etc., NASICON type such as but not limited to Li1.4Al0.4Ti1.6(PO4)3(LATP), garnet type may include Li7La3Zr2O 12 (LLZO), Li 6.4 Ln3Z 1.4 Ta 0.6 O 12 , Li 6.5 Ln3Z 1.5 Ta 0.5 O 12 At least one of the above. By selecting the material of the ion conductor encapsulation layer, the ion conductivity of the ion conductor encapsulation layer can be further improved. The thickness of the ion conductor encapsulation layer can also be adjusted as needed, but it is beneficial to ion conduction on the basis of being beneficial to isolating the core body and being beneficial to the stability of the lithium-rich phosphate positive electrode material, that is, the core body.

[0038] When the encapsulation function layer includes an electronic conductor encapsulation layer, the material of the electronic conductor encapsulation layer may be a material that is conducive to improving electronic conductivity, such as at least one of a carbon material, a conductive oxide, and a conductive organic matter, but not limited thereto. Wherein, when the material of the electronic conductor encapsulation layer is a carbon material, the carbon material may include at least one of amorphous carbon, graphite, carbon black, graphene, etc. When the material of the electronic conductor encapsulation layer is a conductive oxide, the conductive oxide may include at least one of In2O3, ZnO, and SnO2. The conductive organic matter may be a conductive polymer, etc. By selecting the material of the electronic conductor encapsulation layer, the electronic conductivity of the electronic conductor encapsulation layer can be further improved. The thickness of the electronic conductor encapsulation layer may also be adjusted as needed, but it is conducive to electronic conduction on the basis of being conducive to isolating the core body and being conducive to the stability of the lithium-rich phosphate positive electrode material, that is, the core body.

[0039] In addition, the encapsulation functional layer may further include other functional layers as required, and the types of other functional layers may be flexibly selected as required.

[0040] After testing, the D50 particle size of the lithium-rich phosphate positive electrode material in the above embodiments satisfies: 0.5 μm ≤ D50 ≤ 20 μm, and its BET specific surface area can be 1-25 m 2 / g, and its tap density can be 1.0-1.9g / cm 3 , further can be 1.2-1.8g / cm 3 .

[0041] In the second aspect, the present application also provides a method for preparing the above-mentioned lithium-rich phosphate positive electrode material. The process flow of the method for preparing the lithium-rich phosphate positive electrode material in the present application is as follows: Figure 1 As shown, the following steps are included:

[0042] S01: LiM a PO4 or its precursor and a lithium-rich precursor are mixed in proportion to obtain a mixture;

[0043] S02: In a protective atmosphere, the mixture is sintered to obtain a lithium-rich phosphate positive electrode material.

[0044] Among them, in step S01, LiM a PO4 or its precursor is the main phase LiM contained in the above lithium-rich phosphate positive electrode material a PO4, therefore, LiM a M in PO4 is the transition metal element mentioned above, 0<a≤1. Therefore, when it is LiM a When the precursor is PO4, it contains a first lithium source, an M source and a phosphorus source. In a specific embodiment, the first lithium source may include at least one of lithium hydroxide, lithium carbonate, lithium oxide, lithium acetate, lithium oxalate, or other lithium sources, the M source may include at least one of an oxide (MO), a hydroxide (M(OH)2), a chloride (MCl2), a nitrate (M(NO3)2), and a carbonate (MCO3), and the phosphorus source may include at least one of potassium phosphate, sodium phosphate, ammonium phosphate, phosphoric acid, potassium phosphate, and sodium dihydrogen phosphate.

[0045] The lithium-rich precursor in step S01 is the lithium-rich phase contained in the above-mentioned lithium-rich phosphate positive electrode material. As in the embodiment, when the lithium-rich precursor includes a Li3P precursor, the Li3P precursor includes a second lithium source and elemental phosphorus. When the lithium-rich precursor includes a Li3N precursor, the Li3N precursor includes a second lithium source, and the second lithium source can be metallic lithium. When the lithium-rich precursor includes a mixture of Li3P and Li3N precursors, the mixture of Li3P and Li3N precursors includes a second lithium source and elemental phosphorus. Among them, the second lithium source can be metallic lithium, or the same as or different from the above-mentioned first lithium source. Elemental phosphorus can include at least one of white phosphorus, red phosphorus, black scale, etc.

[0046] Since LiM in step S01 a PO4 or its precursor and the lithium-rich precursor are sintered in step S02 to generate the main phase and lithium-rich phase contained in the above-mentioned lithium-rich phosphate positive electrode material, respectively. a The mixing ratio of PO4 or its precursor to the lithium-rich precursor should satisfy the requirement that the molar ratio of the lithium-rich phase to the main phase generated in step S02 is x, and 0<x<1.

[0047] In step S02, during the sintering process, when LiM a When PO4 is mixed with a lithium-rich precursor, the lithium-rich precursor is sintered to generate the lithium-rich phase contained in the lithium-rich phosphate positive electrode material mentioned above, and LiM aPO4 and lithium-rich phase form a single crystalline phase during sintering; when the mixture LiM a When the PO4 precursor is mixed with a lithium-rich precursor, the LiM a The PO4 precursor and the lithium-rich precursor are sintered to form the main phase and lithium-rich phase contained in the above-mentioned lithium-rich phosphate positive electrode material, and LiM a The PO4 main phase and the lithium-rich phase form a single crystalline phase during sintering. a When PO4 is used as raw material, the LiM a PO4 can be free of residual lithium phosphate or contain residual lithium phosphate; when LiM a When PO4 precursor is used as raw material, the LiM a The PO4 precursor may contain no residual lithium phosphate or may contain residual lithium phosphate after sintering. When residual lithium phosphate is contained, after sintering, the residual lithium phosphate, which is the secondary phase contained in the lithium-rich phosphate positive electrode material mentioned above, forms a single crystalline phase with the main phase and the lithium-rich phase during sintering. In addition, in the embodiment, the content of the lithium phosphate satisfies the secondary phase content contained in the lithium-rich phosphate positive electrode material mentioned above, such as the weight ratio of lithium phosphate to the main phase can be (0-15): (85-100), and further can be (2-15): (85-98).

[0048] In the embodiment, when the lithium-rich precursor in step S01 contains a Li3N precursor, the protective atmosphere during the sintering process contains nitrogen, and the nitrogen reacts with single lithium to generate Li3N.

[0049] In the embodiment, the sintering temperature can be 200-800° C., further 500-800° C. The sintering time should be sufficient, such as 3-24 hours. The sintering conditions are controlled so that the precursor components of the mixture generate a lithium-rich phosphate positive electrode material containing a main phase and a lithium-rich phase, or the content of the secondary phase lithium phosphate is further controlled.

[0050] In a further embodiment, after the above-mentioned step S02, it also includes the step of forming an encapsulation functional layer on the surface of the lithium-rich phosphate positive electrode material. Specifically, the layer structures can be formed by flexibly selecting appropriate methods according to the structure of the encapsulation functional layer and the type of material of the layer structure. For example, according to the characteristics of the layer structure material, in-situ coating followed by sintering treatment can be selected, and other methods can also be used to form other functional layers.

[0051] Therefore, the preparation method of the above-mentioned lithium-rich phosphate positive electrode material can effectively prepare a lithium-rich phosphate positive electrode material containing a secondary phase, a main phase and a rich phase at the same time, or further prepare a lithium-rich phosphate positive electrode material with a core-shell structure, thereby ensuring that the prepared lithium-rich phosphate positive electrode material has the excellent lithium supplementation effect, high energy density, cycle performance and high energy density and other electrochemical properties of the above-mentioned lithium-rich phosphate positive electrode material, and the electrochemical performance is stable and the storage performance is good. In addition, the preparation method of the lithium-rich phosphate positive electrode material can ensure that the prepared lithium-rich phosphate positive electrode material has stable electrochemical performance, high efficiency, and saves production costs.

[0052] In the third aspect, the embodiment of the present application also provides a positive electrode sheet. The positive electrode sheet of the embodiment of the present application includes a positive electrode current collector and a positive electrode active layer bonded to the surface of the positive electrode current collector, and the positive electrode active layer contains the lithium-rich phosphate positive electrode material of the embodiment of the present application. Since the positive electrode sheet of the embodiment of the present application contains the lithium-rich phosphate positive electrode material of the embodiment of the present application, the positive electrode sheet of the embodiment of the present application has a high energy density, has a lithium supplementation effect, and has high cycle performance and a long life.

[0053] In one embodiment, the mass content of the lithium-rich phosphate positive electrode material of the above-mentioned embodiment of the present application contained in the positive electrode active layer can be 80-95wt%. The positive electrode active layer includes a binder and a conductive agent in addition to the lithium-rich phosphate positive electrode material. Among them, the content of the binder can be 0.5-20wt%, and the binder can be a commonly used electrode binder, such as one or more of polyvinylidene chloride, soluble polytetrafluoroethylene, styrene-butadiene rubber, hydroxypropyl methylcellulose, methylcellulose, carboxymethyl cellulose, polyvinyl alcohol, acrylonitrile copolymer, sodium alginate, chitosan and chitosan derivatives. The content of the conductive agent can be 0.2-20wt%, and the conductive agent can also be a commonly used conductive agent, such as one or more of graphite, carbon black, acetylene black, graphene, carbon fiber, C60 and carbon nanotubes. When the lithium-rich phosphate positive electrode material is used as a positive electrode lithium supplement additive, it also contains a positive electrode active material. The positive electrode active material may include one or more of lithium cobalt oxide, lithium manganese oxide, lithium iron phosphate, lithium vanadium phosphate, lithium vanadium oxyphosphate, lithium fluorovanadium phosphate, lithium titanate, lithium nickel cobalt manganese oxide, and lithium nickel cobalt aluminum oxide.

[0054] In the embodiment, the positive electrode sheet preparation process can be: the positive electrode lithium supplement additive, the conductive agent and the binder are mixed or further mixed with the positive electrode active material to obtain an electrode slurry, the positive electrode slurry is coated on the positive electrode collector, and the positive electrode sheet is prepared through drying, rolling, die cutting and other steps.

[0055] In a fourth aspect, the present application also provides a secondary battery. The secondary battery of the present application includes necessary components such as a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte, and of course other necessary or auxiliary components. Among them, the positive electrode sheet is the positive electrode sheet of the present application embodiment, that is, the positive electrode active layer contained in the positive electrode sheet contains the lithium-rich phosphate positive electrode material of the present application embodiment.

[0056] Since the secondary battery of the embodiment of the present application contains the lithium-rich phosphate positive electrode material of the embodiment of the above text application, based on the high energy density and lithium replenishing effect of the lithium-rich phosphate positive electrode material of the embodiment of the above text application, the secondary battery of the present application has excellent first coulomb efficiency, high energy density and cycle performance, high capacity retention rate, long life, and stable electrochemical performance.

[0057] The following uses a number of specific embodiments to illustrate the lithium-rich phosphate positive electrode material, preparation method, and application of the present application.

[0058] 1. Lithium-rich phosphate cathode material and preparation method thereof:

[0059] Example 1

[0060] The present embodiment provides a lithium-rich phosphate positive electrode material and a preparation method thereof. The lithium-rich phosphate positive electrode material comprises a main phase, a lithium-rich phase and a secondary phase; wherein the main phase is LiFePO4, the secondary phase is Li3PO4, the lithium-rich phase is Li3P, and the molar ratio of the main phase to the lithium-rich phase is 1:0.02, that is, the main phase and the lithium-rich phase can be expressed as LiFePO4·0.02Li3P.

[0061] The preparation method of the lithium-rich phosphate positive electrode material in this embodiment includes the following steps:

[0062] S1. Preparation of lithium iron phosphate cathode material precursor:

[0063] According to Li:Fe:PO4 3- =1.1:1 molar ratio, weigh a certain mass of ferric chloride, ammonium dihydrogen phosphate, and lithium acetate, mix them evenly, and sinter the obtained mixture at 450° C. for 9 h under a nitrogen atmosphere to obtain LiFePO4 lithium iron phosphate positive electrode material;

[0064] S2. Preparation of lithium-rich lithium iron phosphate positive electrode material:

[0065] To the 5g precursor prepared in S1, add the corresponding metallic lithium and red phosphorus to generate 2% lithium phosphide. Under a nitrogen atmosphere, sinter at 650°C for 10h to obtain a lithium-rich phosphate positive electrode material containing LiFePO4 as the main phase, Li3P as the lithium-rich phase, and Li3PO4 as the secondary phase.

[0066] It was measured that the particle size of the lithium-rich lithium iron phosphate positive electrode material satisfies: D50 = 1.57 μm. The BET specific surface area is 5.0 m 2 / g, tap density 1.50g / cm 3 The purity of the main phase + lithium-rich phase LiFePO4·0.02Li3P is 97%, and the purity of the Li3PO4 secondary phase is 3%.

[0067] Example 2

[0068] This embodiment provides a lithium-rich phosphate positive electrode material and a preparation method thereof. The lithium-rich phosphate positive electrode material includes a main phase and a lithium-rich phase; wherein the main phase is LiFePO4, the lithium-rich phase is Li3P, and the molar ratio of the main phase to the lithium-rich phase is 1:0.05, that is, the main phase and the lithium-rich phase can be expressed as LiFePO4·0.05Li3P.

[0069] The preparation method of the lithium-rich lithium iron phosphate salt-based positive electrode material in this embodiment is the same as that in Embodiment 1, except that the content of the generated lithium phosphide is 5% of the mass of the lithium iron phosphate.

[0070] It was measured that the particle size of the lithium-rich lithium iron phosphate positive electrode material satisfies: D50 = 1.56 μm. The BET specific surface area is 5.3 m 2 / g, tap density 1.53g / cm 3 , the purity of LiFePO4·0.05Li3P of the main phase + lithium-rich phase is 100%.

[0071] Example 3

[0072] This embodiment provides a lithium-rich phosphate positive electrode material and a preparation method thereof. The lithium-rich phosphate positive electrode material includes a main phase and a lithium-rich phase; wherein the main phase is LiFePO4, the lithium-rich phase is Li3P, and the molar ratio of the main phase to the lithium-rich phase is 1:0.05, that is, the main phase and the lithium-rich phase can be expressed as LiFePO4·0.05Li3P.

[0073] The preparation method of the lithium-rich lithium iron phosphate salt positive electrode material in this embodiment is the same as that in Example 1, except that in step S2: to the 5g precursor prepared in S1, add the corresponding metal lithium, red phosphorus, and glucose to generate 5% lithium phosphide and 3% C.

[0074] It was measured that the particle size of the lithium-rich lithium iron phosphate positive electrode material satisfies: D50 = 1.52 μm. The BET specific surface area is 5.3 m 2 / g, tap density 1.55g / cm 3 The purity of the main phase + lithium-rich phase LiFePO4·0.05Li3P is 100%, and the thickness of the carbon coating layer is 5.1nm.

[0075] Example 4

[0076] This embodiment provides a lithium-rich phosphate positive electrode material and a preparation method thereof. The lithium-rich phosphate positive electrode material includes a main phase and a lithium-rich phase; wherein the main phase is LiFePO4, the lithium-rich phase is Li3N, and the molar ratio of the main phase to the lithium-rich phase is 1:0.05, that is, the main phase and the lithium-rich phase can be expressed as LiFePO4·0.05Li3N.

[0077] The preparation method of the lithium-rich lithium iron phosphate salt positive electrode material in this embodiment is the same as that in embodiment 1, except that in step S2: only corresponding metal lithium and glucose need to be added to generate 5% lithium nitride and 3% C. In addition, nitrogen is contained during the sintering process.

[0078] It was measured that the particle size of the lithium-rich lithium iron phosphate positive electrode material satisfies: D50 = 1.60 μm. The BET specific surface area is 5.4 m 2 / g, tap density 1.56g / cm 3 The purity of the main phase + lithium-rich phase LiFePO4·0.05Li3N is 100%, and the thickness of the carbon coating layer is 5.5nm.

[0079] Example 5

[0080] This embodiment provides a lithium-rich phosphate positive electrode material and a preparation method thereof. The lithium-rich phosphate positive electrode material comprises a main phase and a lithium-rich phase; wherein the main phase is LiMn 0.3 Fe 0.7 PO4, the lithium-rich phase is Li3P, and the molar ratio of the main phase to the lithium-rich phase is 1:0.05, that is, the main phase and the lithium-rich phase can be expressed as LiMn 0.3 Fe 0.7 PO4·0.05Li3P.

[0081] The preparation method of the lithium-rich manganese iron phosphate lithium salt positive electrode material of this embodiment includes the following steps:

[0082] S1. Preparation of lithium manganese iron phosphate cathode material precursor:

[0083] According to Li:Fe:Mn:PO4 3- :=1.1:0.7:0.3:1 molar ratio, weigh a certain amount of ferric chloride, manganese oxide, ammonium dihydrogen phosphate, and lithium acetate, mix them evenly, and sinter the obtained mixture at 550°C for 9h under a nitrogen atmosphere to obtain LiMn 0.3 Fe 0.7 PO4 lithium manganese iron phosphate positive electrode material;

[0084] S2. Preparation of lithium-rich lithium manganese iron phosphate positive electrode material:

[0085] To the 5g precursor prepared in S1, add the corresponding metal lithium, white phosphorus, and glucose to generate 5% lithium phosphide and 3.1% C. Sinter at 650℃ for 10h in a nitrogen atmosphere to obtain a LiMn 0.3 Fe 0.7 A lithium-rich lithium manganese iron phosphate positive electrode material with PO4 and Li3P lithium-rich phases as main phases, and a lithium-rich lithium manganese iron phosphate positive electrode material with a C coating layer structure coated on the surface of the lithium-rich lithium manganese iron phosphate positive electrode material.

[0086] It was measured that the particle size of the lithium-rich manganese iron phosphate positive electrode material meets the following requirements: D50 = 1.59 μm. The BET specific surface area is 5.2 m 2 / g, tap density 1.53g / cm 3 The purity of the main phase + lithium-rich phase LiFePO4·0.02Li3P is 100%, and the thickness of the carbon coating layer is 5.3nm.

[0087] Comparative Example 1

[0088] This comparative example provides an existing phosphate-based positive electrode material purchased from the market, and the phosphate-based positive electrode material is LiFePO4. That is, compared with Examples 1 and 4, the phosphate-based positive electrode material in this comparative example does not contain lithium-rich phases Li3P, Li3N and secondary phase Li3PO4.

[0089] Comparative Example 2

[0090] This comparative example provides a lithium-rich phosphate positive electrode material. The lithium-rich phosphate positive electrode material includes a mixture of LiFePO4, Li3PO4 and Li3P, which is different from Example 1 in that: the LiFePO4, Li3PO4 and Li3P are obtained by respectively obtaining LiFePO4, Li3PO4 and Li3P, and then directly mixing the three to form a physical mixture, that is, LiFePO4, Li3PO4 and Li3P are arranged in a single phase.

[0091] Comparative Example 3

[0092] This comparative example provides an existing phosphate-based positive electrode material purchased from the market. The phosphate-based positive electrode material is a carbon-coated LiMn 0.3 Fe 0.7 That is, compared with Example 5, the phosphate-based positive electrode material in this comparative example does not contain the lithium-rich phase Li3P.

[0093] 2. Lithium-ion battery example:

[0094] The lithium-rich phosphate-based positive electrode materials provided in Examples 1 to 5 and the phosphate-based positive electrode materials provided in the comparative example are respectively assembled into positive electrodes and lithium-ion batteries according to the following methods:

[0095] Positive electrode: under the same conditions, the positive electrode material, polyvinylidene fluoride and SP-Li were mixed and ball-milled in a mass ratio of 95:3:2 to obtain a positive electrode slurry, the positive electrode slurry was coated on the surface of aluminum foil, rolled, and vacuum dried overnight at 110°C to obtain a positive electrode sheet; wherein the positive electrode materials were the lithium-rich phosphate positive electrode materials provided in the above-mentioned Examples 1 to 5 and the phosphate positive electrode materials provided in Comparative Examples 1 to 3;

[0096] Negative electrode: lithium metal sheet;

[0097] Electrolyte: Ethylene carbonate and ethyl methyl carbonate were mixed in a volume ratio of 3:7, and LiPF6 was added to form an electrolyte, the concentration of LiPF6 was 1 mol / L;

[0098] Diaphragm: Polypropylene microporous separator;

[0099] Lithium-ion battery assembly: The structure of lithium metal sheet-diaphragm-electrolyte-positive electrode sheet is assembled into a lithium-ion battery in an inert atmosphere glove box.

[0100] 3. Related performance tests

[0101] 1. Related tests of lithium-rich phosphate cathode materials

[0102] The lithium-rich phosphate cathode materials in Examples 1 to 5 were subjected to X-ray diffraction (XRD) analysis, wherein the XRD pattern in Example 2 is as follows: Figure 2 As shown. Figure 2 It can be seen that the lithium-rich lithium iron phosphate positive electrode material contains characteristic diffraction peaks of lithium iron phosphate and lithium phosphide. Since some peaks of lithium iron phosphate overlap with those of lithium phosphide, it can be observed that some diffraction peaks of lithium phosphide are stronger, and the space groups of lithium iron phosphate and lithium phosphide are Pnma (62) and P63 / mmc (194), respectively.

[0103] The XRD patterns of other embodiments, such as Embodiment 1, 3, 4 and 5, also have corresponding characteristic diffraction peaks.

[0104] 2. Lithium-ion battery example:

[0105] The electrochemical performance of each lithium ion battery assembled in the above lithium ion battery embodiment was tested under the following test conditions:

[0106] The button battery was charged at a constant current and constant voltage of 0.1C to 4.3V, with a cut-off current of 0.01C. After standing for 5 minutes, it was discharged at a rate of 0.1C to 2.7V.

[0107] The electrochemical properties of the lithium secondary batteries of Examples 1 to 5 and Comparative Examples 1-3 are shown in Table 1.

[0108] Table 1

[0109]

[0110] As can be seen from Table 1, the lithium-rich phosphate-based positive electrode material in the embodiments of the present application improves the electrochemical performance of the lithium-ion battery due to the synergistic effect between the main phase, the secondary phase and the lithium-rich phase. For example, the first charge specific capacity in Example 1 is much higher than that in Comparative Example 1, and it can be seen from Comparative Example 2 that since Li3P and Li3PO4 in Comparative Example 2 are a simple physical mixture with LiFePO4, rather than forming a single crystalline phase like in Example 1, the improvement rate of its electrochemical performance is lower than that of Example 1; at the same time, it can be found that the first charge capacity of Example 2 is lower than that of Examples 3 to 5 because Example 4 contains a carbon source to form a C coating layer, which can further improve the conductivity of the material while preventing erosion by the external environment. Therefore, the capacity improvement rate of Example 4 is greater than that of Example 2.

[0111] The above embodiments only express several implementation methods of the present application, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the present application. It should be pointed out that, for a person of ordinary skill in the art, several modifications and improvements can be made without departing from the concept of the present application, and these all belong to the protection scope of the present application. Therefore, the protection scope of the present application shall be subject to the attached claims.

Claims

1. A lithium-rich phosphate positive electrode material, comprising a lithium-rich phosphate positive electrode material, wherein the lithium-rich phosphate positive electrode material comprises a main phase and a lithium-rich phase for replenishing lithium, and the main phase and the lithium-rich phase are doped with each other to form a single crystal phase; wherein, The main phase is LiM a PO4, LiM a M in PO4 is a transition metal element, 0<a≤1, and the lithium-rich phase includes Li3N b P 1-b ; where 0≤b<1.

2. The lithium-rich phosphate positive electrode material according to claim 1, characterized in that: The molar ratio of the lithium-rich phase to the main phase is x, and 0<x<1; and / or The M includes at least one element of Fe, Mn, Ni, and Co.

3. The lithium-rich phosphate positive electrode material according to claim 1, characterized in that: The lithium-rich phase is Li3P, and the lithium-rich phase and the main phase form a LiMaPO4·xLi3P single crystal phase.

4. The lithium-rich phosphate positive electrode material according to any one of claims 1 to 3, characterized in that: The lithium-rich phosphate positive electrode material further includes a secondary phase, and the secondary phase forms the single crystalline phase with the primary phase and the lithium-rich phase.

5. The lithium-rich phosphate positive electrode material according to claim 4, characterized in that: The secondary phase comprises Li3PO4; and / or The weight ratio of the secondary phase to the primary phase is (0-15):(85-100).

6. The lithium-rich phosphate positive electrode material according to any one of claims 1 to 3 and 5, characterized in that: It also includes an encapsulation functional layer, which covers and encapsulates the surface of the lithium-rich phosphate positive electrode material.

7. The lithium-rich phosphate positive electrode material according to claim 6, characterized in that: The thickness of the encapsulation functional layer is 1-300 nm; and / or The weight of the encapsulation functional layer accounts for 0.01-10 wt % of the total weight of the lithium-rich phosphate-based positive electrode material.

8. The lithium-rich phosphate positive electrode material according to any one of claims 1 to 3, 5 and 7, characterized in that: The D50 particle size of the lithium-rich phosphate positive electrode material satisfies: 0.5 μm≤D50≤20 μm; and / or The BET specific surface area of ​​the lithium-rich phosphate positive electrode material is 1-25 m 2 / g; and / or The tap density of the lithium-rich phosphate positive electrode material is 1.0-1.9 g / cm 3 .

9. The method for preparing the lithium-rich phosphate positive electrode material according to any one of claims 1 to 8, characterized in that: The steps include: LiM a PO4 or its precursor and the lithium-rich precursor are mixed in proportion to obtain a mixture; the LiM a M in PO4 is a transition metal element, 0<a≤1; The mixture is sintered in a protective atmosphere to obtain a lithium-rich phosphate positive electrode material.

10. The preparation method according to claim 9, characterized in that: When the lithium-rich precursor contains a Li3P precursor, the Li3P precursor includes a lithium source and elemental phosphorus; and / or When the lithium-rich precursor contains a Li3N precursor, the Li3N precursor includes single-substance lithium, and the protective atmosphere contains nitrogen; The following steps are also included: Forming an encapsulation functional layer on the surface of the lithium-rich phosphate positive electrode material, and coating and encapsulating the lithium-rich phosphate positive electrode material; and / or The sintering temperature is 200-800°C.

11. The preparation method according to claim 9 or 10, characterized in that: When the mixing process is to a When PO4 and the lithium-rich precursor are mixed in proportion, the temperature of the sintering treatment is 400-800°C.

12. A positive electrode sheet, comprising a positive electrode current collector and a positive electrode active layer bonded to the surface of the positive electrode current collector, characterized in that: The positive electrode active layer contains the lithium-rich phosphate positive electrode material according to any one of claims 1 to 8 or the lithium-rich phosphate positive electrode material prepared by the preparation method according to any one of claims 9 to 11.

13. A secondary battery, comprising a positive electrode sheet and a negative electrode sheet, characterized in that: The positive electrode sheet is the positive electrode sheet according to claim 12.

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