Composite cathode material and preparation method thereof

Abstract

The application discloses a composite positive electrode material and a preparation method thereof. The composite positive electrode material comprises a lithium-deficient phase as a positive electrode material and a lithium-rich phase for lithium supplement, and the lithium-rich phase and the lithium-deficient phase form a composite material, and the lithium-rich phase provides lithium ions to the lithium-deficient phase at least in the first charging. The lithium-rich phase contained in the composite positive electrode material can play a role of a lithium supplement additive in the first charging and discharging, releases lithium ions, forms a lithium-containing film layer on the negative electrode on one hand, and on the other hand, the lithium ions provided by the lithium-rich phase can also be reversible lithium ions, supplement lithium ions for the lithium-deficient phase, thereby improving the capacity of the positive electrode material and the energy density and cycle performance of the battery. The preparation method of the composite positive electrode material can ensure that the prepared composite positive electrode material has stable electrochemical performance, is high in efficiency and low in production cost.

Description

Technical Field

[0001] This application belongs to the field of lithium-ion battery technology, and particularly relates to a composite cathode material and its preparation method. Background Technology

[0002] A rechargeable battery, also known as a secondary battery or accumulator, is a battery that can be recharged after discharge to reactivate its active materials and continue to be used. Rechargeable batteries are small and lightweight, meeting the growing demand for miniaturized electronic devices. With the rapid development of rechargeable battery technology, lithium-ion batteries, using lithium metal as the active material, are widely used in electric vehicles, power tools, portable consumer electronics, and energy storage due to their high energy density, long cycle life, high safety, and environmental friendliness, gradually becoming the mainstream rechargeable battery. Lithium metal, with its low redox potential (relative to the standard hydrogen electrode, -3.045V) and high gravimetric energy density (3860mAh / g), shows promise as a negative electrode material for high-capacity rechargeable batteries.

[0003] Among them, the positive electrode material or negative electrode material is one of the important factors affecting the electrochemical performance of lithium-ion batteries. For example, during the charging and discharging process, the lithium ions contained in the positive electrode material will gradually form a solid electrolyte interphase (SEI) film on the negative electrode, resulting in a decrease in the lithium ion content of the positive electrode, which in turn affects the first-cycle capacity and cycle life of the positive electrode material.

[0004] To address the lithium consumption issue in the cathode material, current methods typically involve mixing the cathode material with a lithium replenishment additive to form an active cathode material layer. During the initial charging process, the lithium contained in the additive is sacrificed to maintain the content of lithium ions that can be intercalated or deintercalated in the cathode material. However, this method has unsatisfactory lithium replenishment effects and leads to a reduction in the cathode material content, thereby decreasing the battery's energy density.

[0005] Another approach uses lithium foil as the negative electrode to provide lithium ions and replenish those consumed during the initial charge. However, because lithium foil is elemental lithium metal with high chemical reactivity, it increases the difficulty of battery assembly. Furthermore, current manufacturing processes make it difficult to achieve extremely low thicknesses for lithium foil. Therefore, using lithium foil as both the negative electrode and the lithium source for replenishment results in two main problems: firstly, an excessive amount of lithium source material, increasing costs; and secondly, increased battery weight, reducing energy density. Summary of the Invention

[0006] The purpose of this application is to provide a composite cathode material and its preparation method to solve the technical problems of low lithium replenishment efficiency or low battery energy density caused by using lithium replenishment additives and lithium foil as a negative electrode for lithium replenishment.

[0007] To achieve the aforementioned objectives, a first aspect of this application provides a composite cathode material. This composite cathode material includes a lithium-poor phase serving as the cathode material and a lithium-rich phase for lithium replenishment, wherein the lithium-rich phase and the lithium-poor phase form a composite material, and during the first charge, the lithium-rich phase provides lithium ions to the lithium-poor phase at least once.

[0008] A second aspect of this application provides a method for preparing a composite cathode material. The method for preparing the composite cathode material of this application includes the following steps:

[0009] It provides a lithium-poor phase for use as a cathode material and a lithium-rich phase for providing lithium ions;

[0010] A composite cathode material is obtained by combining and heat-treating a lithium-poor phase and a lithium-rich phase in a certain proportion; wherein the heat treatment temperature is lower than the sintering temperature of the lithium-poor phase and the lithium-rich phase.

[0011] Compared with the prior art, this application has the following technical effects:

[0012] The composite cathode material of this application combines a lithium-rich phase and a lithium-poor phase. In this way, during the first charge and discharge process, the lithium-rich phase contained in the composite cathode material can act as a lithium supplement additive, releasing lithium ions. On the one hand, it forms a lithium-containing film layer on the negative electrode, which can supplement the lithium ions consumed by the SEI film and avoid the need to set up an additional lithium-rich negative electrode in the battery. On the other hand, the lithium ions provided by the lithium-rich phase can also act as reversible lithium ions to supplement the lithium-poor phase, thereby improving the capacity of the cathode material, the energy density of the battery, and the cycle performance.

[0013] The method for preparing composite cathode materials in this application involves combining lithium-poor and lithium-rich phases to form a composite cathode material. This composite cathode material possesses lithium replenishment capabilities and high capacity, exhibits stable electrochemical performance, and demonstrates good storage performance. Furthermore, the method ensures stable electrochemical performance and high efficiency of the prepared composite cathode material, while also saving production costs. Attached Figure Description

[0014] To more clearly illustrate the technical solutions in the specific embodiments of this application or the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this application. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0015] Figure 1 This is a schematic diagram of the structure of Example A, in which the lithium-poor phase and lithium-rich phase contained in the composite cathode material of this application form a composite.

[0016] Figure 2 This is a schematic diagram of the structure of Example B, in which the lithium-poor phase and lithium-rich phase contained in the composite cathode material of this application form a composite.

[0017] Figure 3 This is a schematic diagram of the structure of Example C, in which the lithium-poor phase and lithium-rich phase contained in the composite cathode material of this application form a composite.

[0018] Figure 4 Is Figure 3 The diagram shows a composite cathode material with a functional encapsulation layer formed on its surface.

[0019] Figure 5 This is a SEM (Scanning Electron Microscope) image of the composite cathode material in Example 2. Detailed Implementation

[0020] To make the technical problems, technical solutions, and beneficial effects of this application clearer, the following detailed description is provided in conjunction with embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0021] In this application, the term "and / or" describes the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, or B existing alone. A and B can be singular or plural. The character " / " generally indicates that the preceding and following related objects have an "or" relationship.

[0022] In this application, "at least one" means one or more, and "more than one" means two or more. "At least one of the following" or similar expressions refer to any combination of these items, including any combination of single or multiple items. For example, "at least one of a, b, or c", or "at least one of a, b, and c", can both mean: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, and c can be single or multiple.

[0023] It should be understood that in the various embodiments of this application, the order of the above processes does not imply the order of execution. Some or all steps may be executed in parallel or sequentially. 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 this application.

[0024] The terminology used in the embodiments of this application is for the purpose of describing particular embodiments only and is not intended to be limiting of this application. The singular forms “a,” “the,” and “the” used in the embodiments of this application and the appended claims are also intended to include the plural forms unless the context clearly indicates otherwise.

[0025] The weights of the relevant components mentioned in the embodiments of this application can refer not only to the specific content of each component, but also to the proportional relationship between the weights of the components. Therefore, any scaling up or down of the content of the relevant components according to the embodiments of this application is within the scope disclosed in the embodiments of this application. Specifically, the mass described in the embodiments of this application can be a mass unit known in the chemical industry, such as μg, mg, g, or kg.

[0026] During the first charge and discharge cycle of a battery, some lithium ions released from the positive electrode material are partially deposited on the negative electrode to form an SEI film, leading to a decrease in lithium ion concentration. This affects the cycle life and capacity of the positive electrode material, especially for lithium-poor positive electrode materials. To effectively address the problem of lithium ion consumption during the first charge and discharge cycle of lithium-poor positive electrode materials, the embodiments of this application propose the following solution:

[0027] In a first aspect, embodiments of this application provide a composite cathode material. The composite cathode material of this application includes a lithium-depleted phase serving as the cathode material and a lithium-rich phase for lithium replenishment, wherein the lithium-rich phase and the lithium-depleted phase form a composite material.

[0028] In this embodiment, the lithium-poor phase in the composite cathode material functions as the cathode material itself, thus playing a role in lithium ion insertion / extraction during charge / discharge. The lithium-rich phase, composite with the lithium-poor phase, refers to a material that releases lithium ions during charge / discharge, and these released lithium ions can be reversibly and abundantly added to the negative electrode current collector or the surface of the negative electrode material, thus serving as a lithium replenishment agent. Simultaneously, it can also reversibly replenish the lithium-poor phase with lithium ions. Therefore, during the initial charge / discharge process, the lithium-rich phase can effectively release lithium ions, forming a lithium-containing film layer on the negative electrode. This can replenish the lithium ions consumed by the SEI film and eliminate the need for an additional lithium-rich negative electrode. Furthermore, the lithium ions provided by the lithium-rich phase can also reversibly replenish the lithium-poor phase, thereby improving the cathode material's capacity, battery energy density, and cycle performance.

[0029] The composite cathode material of this application can be composed of either a lithium-poor phase or a lithium-rich phase in any way that allows both phases to exert a synergistic effect, such as at least one of the following composite embodiments A to C:

[0030] Composite Example A: As Figure 1 As shown, the lithium-poor phase has a porous structure 11, and the lithium-rich phase is filled in the porous structure 11 of the lithium-poor phase.

[0031] In the above-described composite embodiment A, the lithium-poor phase acts as a carrier for the positive electrode material, loading the lithium-rich phase and allowing the lithium-rich phase to be relatively uniformly embedded in the positive electrode material carrier, specifically in its porous structure 11. On the one hand, this enhances the uniformity of the dispersion of the lithium-rich phase among the lithium-poor phases, improving the synergistic effect between the two. This allows the lithium-rich phase to effectively improve its efficiency in providing reversible ions to the lithium-poor phase while simultaneously providing irreversible lithium ions to the negative electrode. On the other hand, embedding the lithium-rich phase into the porous structure 11 of the lithium-poor phase provides protection for the lithium-rich phase, such as isolating it from environmental moisture and harmful gases like carbon dioxide, thereby improving the stability of the lithium-rich phase and thus endowing the composite positive electrode material with processing and storage properties.

[0032] In the embodiments, Figure 1 The lithium-depleted phase in the lithium-depleted phase has a particle size of 500 nm-100 μm, more specifically 500 nm-30 μm, and even more specifically 500 nm-20 μm. In another embodiment, the pore diameter of the porous structure 11 in the lithium-depleted phase is 5 nm-10 μm, more specifically 5 nm-3 μm. In yet another embodiment, the pore distribution density of the porous structure 11 in the lithium-depleted phase is 1%-50%, more specifically 10%-50%, and even more specifically 20%-40%. By controlling and adjusting at least one of the particle size, pore diameter, and pore diameter distribution density of the lithium-depleted phase, the capacity and density of the lithium-depleted phase as a cathode material can be improved based on the effective loading of the lithium-rich phase, thereby improving the capacity and compaction density of the composite cathode material.

[0033] Composite Example B: As Figure 2 As shown, the lithium-rich phase is the lithium-rich core 21, and the lithium-poor phase forms a lithium-poor coating layer 22, which coats the lithium-rich core 21.

[0034] In the aforementioned composite embodiment B, the lithium-rich phase serves as the core, thus the lithium-poor phase acts as a protective layer, effectively isolating the lithium-rich phase from environmental moisture and harmful gases such as carbon dioxide. This improves the stability of the lithium-rich phase, thereby enhancing the processing and storage properties of the composite cathode material. Furthermore, as the core, the lithium-rich phase is positioned along the lithium-poor phase's path during delithiation. The lithium ions supplied by the lithium-rich phase first contact the lithium-poor phase coating layer 22, replenishing the lithium-poor phase with intercalation-deintercalation lithium ions. These lithium ions then migrate to the negative electrode to form a lithium-containing film. The lithium-poor phase, acting as a coating layer, shortens the reversible lithium ion intercalation-deintercalation efficiency and migration path, thereby improving the cycle performance of the composite cathode material.

[0035] In the embodiments, the particle size of the lithium-rich core 21 in the above-mentioned composite embodiment B is 50nm-100μm, further 500nm-30μm, and even further 500nm-20μm. It can be primary particles, secondary particles, or a mixture of primary and secondary particles. In another embodiment, the thickness of the lithium-poor coating layer 22 is 5nm-10μm, further 5nm-8μm, and even further 5nm-3μm. By controlling the particle size of the lithium-rich core 21 and / or the thickness of the lithium-poor coating layer 22, the synergistic effect between the lithium-poor phase and the lithium-rich phase can be improved, thereby improving the electrochemical performance of the composite cathode material, such as capacity, battery energy density, and cycle performance.

[0036] In this embodiment, the lithium-poor coating layer 22 is also doped with a lithium-rich phase, and the lithium-rich phase doped in the lithium-poor coating layer 22 exhibits a gradient decreasing distribution from the lithium-rich core 21 to the outer surface of the lithium-poor coating layer 22. In a further embodiment, the mass of the lithium-rich phase doped in the lithium-poor coating layer 22 is controlled to be 0-50% of the mass of the lithium-poor coating layer 22, more specifically 5-50%, and even more specifically 10-30%. Thus, the gradient decreasing doping of the lithium-rich phase in the lithium-poor coating layer 22 improves the contact between the lithium-rich and lithium-poor phases, increases the efficiency of lithium-ion extraction from the lithium-rich phase to the lithium-poor phase, and also avoids direct contact between the lithium-rich phase and the external environment, thereby improving the stability of the lithium-rich phase doped in the lithium-poor coating layer 22. The lithium-rich phase doped in the lithium-poor coating layer 22 is not limited to a single particle.

[0037] Composite Example C: As Figure 3 As shown, the lithium-poor phase is the lithium-poor core 31, and the lithium-rich phase forms a lithium-rich coating layer 32, which coats the lithium-poor core 31.

[0038] In the above-described composite embodiment C, a lithium-rich phase is used as the lithium-rich coating layer 32. During the initial charge-discharge process, the lithium-rich coating layer 32 preferentially delithiates and releases lithium ions, forming a lithium-containing film on the negative electrode, thus reducing or preventing lithium ion consumption from the lithium-poor phase. Simultaneously, the lithium-rich coating layer 32 can also replenish the lithium-poor phase in the lithium-poor core 31 with intercalable lithium ions, thereby improving the electrochemical performance of the composite cathode material, including capacity, battery energy density, and cycle performance.

[0039] In the embodiments, the particle size of the lithium-poor core 31 in the above-mentioned composite embodiment C is 50nm-100μm, further 500nm-30μm, and even further 500nm-20μm. It can be primary particles, secondary particles, or a mixture of primary and secondary particles. In another embodiment, the thickness of the lithium-rich coating layer 32 is 5nm-10μm, further 5nm-8μm, and even further 5nm-3μm. By controlling the particle size of the lithium-poor core 31 and / or the thickness of the lithium-rich coating layer 32, the synergistic effect between the lithium-poor phase and the lithium-rich phase can be improved, thereby improving the electrochemical performance of the composite cathode material, such as capacity, battery energy density, and cycle performance.

[0040] In this embodiment, the lithium-rich coating layer 32 is also doped with a lithium-poor phase, and the lithium-poor phase doped in the lithium-rich coating layer 32 exhibits a gradient decreasing distribution from the lithium-poor core 31 to the outer surface of the lithium-rich coating layer 32. In a further embodiment, the mass of the lithium-poor phase doped in the lithium-rich coating layer 32 is controlled to be 0-50% of the mass of the lithium-rich coating layer 32, more specifically 5-50%, and even more specifically 10-30%. Thus, the gradient decreasing doping of the lithium-poor phase in the lithium-rich coating layer 32 improves the contact between the lithium-rich and lithium-poor phases, thereby increasing the efficiency of lithium ion insertion / extraction from the lithium-rich phase to the lithium-poor phase.

[0041] In a further embodiment, such as Figure 4 As shown, a functional encapsulation layer 33 is also provided on the outer surface of the lithium-rich coating layer 32. The presence of this encapsulation layer 33 effectively encapsulates the lithium-rich coating layer 32, preventing the lithium-rich phase in the coating layer 32 from directly contacting the environment, such as isolating it from adverse factors like water vapor and carbon dioxide. This ensures the stability of the lithium-rich phase, fully leveraging its lithium-supplementing effect and the synergistic effect between the lithium-rich and lithium-poor phases, thereby improving the electrode material activity of the composite cathode material. In this embodiment, the thickness of the encapsulation layer 33 can be controlled to be 5-1000 nm, further 10-800 nm, and even further 50-500 nm. By controlling and adjusting the thickness of the encapsulation layer 33, the composite cathode material's storage and processing performance, energy density and capacity retention, and cycle performance can be guaranteed.

[0042] In this embodiment, the encapsulation functional layer 33 can be a composite layer structure of an ion conductor encapsulation layer, an electronic conductor encapsulation layer, or an ion conductor encapsulation layer or an electronic conductor encapsulation layer. The ion conductor encapsulation layer can improve the lithium-ion intercalation / deintercalation effect and the lithium-ion ionic conductivity of the lithium-rich phosphate cathode material.

[0043] When the encapsulation functional layer 33 includes an ion conductor encapsulation layer, the material of the ion conductor encapsulation layer can be a material that is beneficial to improving ion conductivity, such as, but not limited to, at least one of perovskite, NASICON, and garnet types. 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 0.05 At least one of O3, etc., NASICON type such as but not limited to Li 1.4 Al 0.4 Ti 1.6 (PO4)3(LATP), garnet type can include Li7La3Zr2O 12 (LLZO), Li 6.4 La3Zr 1.4 Ta 0.6 O 12 Li 6.5 La3Zr 1.5 Ta 0.5 O 12 At least one of the following. By selecting the appropriate material for the ion conductor encapsulation layer, the ionic 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 should be conducive to ion conduction while also being conducive to isolating the core and stabilizing the lithium-rich phosphate cathode material, i.e., the core.

[0044] When the encapsulation functional layer 33 includes an electronic conductor encapsulation layer, the material of this electronic conductor encapsulation layer can be a material that is beneficial to improving electronic conductivity, such as, but not limited to, at least one of carbon materials, conductive oxides, and conductive organic materials. Specifically, when the material of the electronic conductor encapsulation layer is a carbon material, the carbon material can include at least one of amorphous carbon, carbon nanotubes, graphite, carbon black, and graphene. When the material of the electronic conductor encapsulation layer is a conductive oxide, the conductive oxide can include at least one of In₂O₃, ZnO, and SnO₂. The conductive organic material can 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 can also be adjusted as needed, but it should be conducive to electron conduction while also being conducive to isolating the core and stabilizing the lithium-rich phosphate cathode material, i.e., the core.

[0045] In addition, the encapsulation functional layer 33 may include other functional layers as needed, and the types of other functional layers can be flexibly selected as required.

[0046] Regardless of the composite arrangement of the lithium-rich phase and the lithium-poor phase described above, the mass ratio of the lithium-rich phase to the lithium-poor phase can be controlled to be 1:(0.01-100). In the embodiments, the lithium-poor phase may include at least one of phosphate-based cathode materials, ternary cathode materials, and lithium cobalt oxide cathode materials, but is not limited thereto. When the lithium-poor phase is a phosphate-based cathode material, it may include Li... (1-x) A y B (1-y) PO4, wherein A is selected from at least one of the metallic elements Fe, Co, Ni, and Mn, and B is selected from at least one of the metallic elements Fe, Co, Ni, Mn, V, Mg, Ca, Cr, Cu, Zn, Ti, and Sn, and the values ​​of x and y satisfy: 0 <x≤1,0<y≤1。

[0047] Based on this Li (1-x) A y B (1-y) The elements and proportions contained in PO4 can vary depending on the value of x, resulting in materials with different lithium contents. In some specific embodiments, the value of x can be 1, 0.95, 0.9, 0.85, 0.8, 0.75, 0.7, 0.65, 0.6, 0.55, 0.5, 0.45, 0.4, 0.35, 0.3, 0.25, 0.2, 0.15, 0.1, 0.05, 0.03, 0.02, 0.01, 0.001, etc.

[0048] Specifically, when x approaches 0, the lithium-ion content of the phosphate-based cathode material is high; as the value of x increases, the lithium-ion content of the phosphate-based cathode material gradually decreases, and the phosphate-based cathode material becomes lithium-poor; when x = 1, Li...(1-x) A y B (1-y) The chemical formula of PO4 is A y B (1-y) PO4, at this time, the phosphate-based cathode material becomes a lithium-free material.

[0049] Correspondingly, the value of y satisfies: 0 < y ≤ 1. In some specific embodiments, the value of y can be 1, 0.95, 0.9, 0.85, 0.8, 0.75, 0.7, 0.65, 0.6, 0.55, 0.5, 0.45, 0.4, 0.35, 0.3, 0.25, 0.2, 0.15, 0.1, 0.05, 0.03, 0.02, 0.01, 0.001 and other specific values. When y=1, Li (1-x) A y B (1-y) PO4 does not contain element B, and the corresponding chemical formula at this time is Li (1-x) APO4.

[0050] In some embodiments of the present application, Li (1-x) A y B (1-y) PO4 is Li 1-x FePO4, Li 1-x MnPO4, Li 1-x CoPO4, Li 1-x NiPO4, Li 1-x Fe y Mn 1-y PO4, Li 1-x Fe y Co 1-y PO4, Li 1-x Fe y Ni 1-y PO4, Li 1-x Mn y Co 1-y PO4, Li 1- x Mn y Ni 1-y PO4, Li 1-x Co y Ni 1-y at least one of PO4.

[0051] In an embodiment, the lithium-rich phase described above can release lithium ions during charging and discharging to form a lithium supplement material on the surface of the negative current collector or the negative electrode material, and can also provide reversible lithium ions for the lithium-poor phase for lithium supplementation, such as the above-mentioned Li (1-x) A y B (1-y)The x-value in the lithium-lean phase of PO4 tends to be 1 to improve the capacity of the lithium-lean phase. In some embodiments, the lithium-rich phase includes lithium-rich metal oxides. In the embodiments, the lithium-rich metal oxide material includes at least one of Li5MO4, Li2MO2, Li6MO4, and Li8MO6, wherein M can be selected from at least one of the metal elements Fe, Co, Ni, Mn, Al, Cr, Ga, In, La, and Bi. As part of the composite cathode material, the above-mentioned lithium-rich metal oxide material can release irreversible lithium ions through an electrochemical reaction during charging, and the released lithium ions can be enriched on the surface of the negative electrode current collector or the negative electrode material to form an SEI film or a lithium-rich film layer. This lithium-rich film layer can directly form on the surface of the negative electrode current collector to play the role of a negative electrode active material, thereby avoiding the problems described in the background caused by using lithium foil as a negative electrode. Moreover, these lithium-rich metal oxide materials can also simultaneously replenish reversibly lithium ions to the lithium-lean phase, improving the capacity and cycle performance of the lithium-lean phase.

[0052] For example, when lithium-rich metal oxide materials include Li5MO4, Li5MO4 releases lithium ions during the first charge of the battery. Because the material after lithium release has better structural stability, Li5MO4 provides irreversible lithium-rich capacity. Li5MO4 can not only enrich lithium-poor or lithium-free phosphate systems... (1-x) A y B (1-y) PO4 provides lithium ions, and the excess irreversible lithium ions migrate to the negative electrode current collector to form a lithium metal layer, which functions as a negative electrode active material.

[0053] In some specific embodiments, Li5MO4 can be oxides such as Li5FeO4, Li5AlO4, Li5BiO4, Li5NiO4, Li5CoO4, Li5MnO4, Li5CrO4, Li5GaO4, Li5InO4, Li5LaO4, and Li5BiO4.

[0054] Based on the lithium-poor and lithium-rich phases contained in the aforementioned composite cathode material, in some embodiments, the composite cathode material contains a phosphate-based cathode material and Li5MO4 compounded with the phosphate-based cathode material, and the general structural formula of the composite cathode material is 5Li. (1-x) A y B (1-y) PO4·zxLi5MO4; where Li (1-x) A y B (1-y) For PO4, A, B, x, and y are as described above, and the value of z satisfies: 1 ≤ z. At this point, the lithium-rich phase Li5MO4 and the lithium-poor phase Li... (1-x) A y B (1-y)PO4 and PO4 each play their respective roles as described above, and the two can have a synergistic effect, improving the capacity and cycle performance of the composite cathode material.

[0055] As an example, when Li (1-x) A y B (1-y) In PO4, x is 1, and Li (1-x) A y B (1-y) The chemical formula of PO4 is A y B (1-y) PO4, at this point, the phosphate-based cathode material is in a lithium-free state. Under these conditions, during the first charge, the lithium-rich phase Li5MO4 can irreversibly provide lithium ions, some of which replenish the Li-2 phase. (1-x) A y B (1-y) In PO4, make A y B (1-y) PO4 is rich in reversible lithium ion insertion and extraction for lithium extraction and insertion during battery cycling. At the same time, Li5MO4 irreversibly provides lithium ions that can migrate to the negative electrode and form a lithium-rich film.

[0056] As another example, Li (1-x) A y B (1-y) In PO4, x is less than 1 but not 0. At this time, Li (1-x) A y B (1-y) PO4 is in a lithium-deficient state. In this condition, during the first charge, the lithium-rich Li5MO4 can irreversibly provide lithium ions, with some of these ions replenishing the Li4+. (1-x) A y B (1-y) In PO4, Li (1-x) A y B (1-y) PO4 is rich in reversible lithium-ion intercalation and deintercalation to facilitate lithium extraction and intercalation during battery cycling. Simultaneously, Li5MO4 irreversibly provides lithium ions that can migrate to the negative electrode and form a lithium-rich film.

[0057] In some specific embodiments, Li5MO4 can be Li5FeO4, corresponding to 5Li (1-x) A y B (1-y) PO4·zxLi5MO4 is Li 1-x FePO4·zxLi5FeO4、Li 1-x Fe y Mn 1-yat least one of PO4·zxLi5FeO4. Among them, Li5FeO4 has a suitable delithiation potential, high cost performance, the product is stable after delithiation during battery charging, and there is almost no reversible capacity within the working voltage range.

[0058] In the embodiments of the present application, taking Li (1-x) A y B (1-y) PO4 with a molar amount of 5 as the reference, the molar amount of Li5MO4 is zx. Wherein, zx represents the product of z and x, and the values of x and z satisfy: 0<x≤1, 1≤z. The embodiments of the present application can adjust the surplus of lithium ions in the positive electrode material by regulating the value of z, so as to supplement sufficient lithium ions for the lithium battery system.

[0059] In a possible implementation, the value of z satisfies: 1≤z≤100. In this case, after the assembled battery is charged for the first time, Li5MO4 releases lithium capacity, and the excess irreversible lithium ions migrate to the negative current collector to form a lithium-rich film layer with a thickness of 5nm-100μm. Exemplarily, the value of z can be 1, 3, 5, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, etc.

[0060] In a second aspect, embodiments of the present application provide a preparation method of the above composite positive electrode material. The preparation method in the embodiments of the present application comprises the following steps:

[0061] S01: providing a lithium-deficient phase as a positive electrode material and a lithium-rich phase for providing lithium ions;

[0062] S02: performing composite treatment and heat treatment on the lithium-deficient phase and the lithium-rich phase according to a certain ratio to obtain the composite positive electrode material.

[0063] In this way, the preparation method of the composite positive electrode material in the embodiments of the present application composites the lithium-deficient phase and the lithium-rich phase to form the composite positive electrode material, which can endow the prepared composite positive electrode material with lithium replenishment effect and high capacity characteristics, and has stable electrochemical performance and good storage performance. In addition, the preparation method of the composite positive electrode material can ensure stable electrochemical performance of the prepared composite positive electrode material, has high efficiency and saves production cost.

[0064] In step S01, both the lithium-poor phase and the lithium-rich phase are the lithium-poor phase and lithium-rich phase contained in the composite cathode material described above. Therefore, as in the embodiments, the lithium-rich phase may include lithium-rich metal oxides. In further embodiments, the lithium-rich metal oxide material includes at least one of Li5MO4, Li2MO2, Li6MO4, and Li8MO6; wherein M is selected from at least one of the metal elements Fe, Co, Ni, Mn, Al, Cr, Ga, In, La, and Bi. In the embodiments, the lithium-poor phase may include at least one of phosphate-based cathode materials, ternary cathode materials, lithium cobalt oxide cathode materials, lithium manganese oxide cathode materials, and lithium nickel manganese oxide cathode materials; wherein the phosphate-based cathode material may be Li (1-x) A y B (1-y) PO4.

[0065] In addition, the lithium-poor phase and the lithium-rich phase can be prepared according to their respective types and corresponding preparation methods, such as by existing methods.

[0066] In step S02, the composite treatment and heat treatment combine the lithium-poor phase and the lithium-rich phase to form a composite cathode material. Furthermore, the heat treatment temperature in step S02 should be lower than the sintering temperature of the lithium-poor and lithium-rich phases. This sintering temperature should be understood as the sintering temperature at which the lithium-poor phase precursor is sintered to prepare the lithium-poor phase, and the lithium-rich phase precursor is sintered to prepare the lithium-rich phase. Setting this heat treatment temperature lower than the sintering temperature ensures that the lithium-poor and lithium-rich phases do not melt or migrate during the heat treatment process.

[0067] In one embodiment, the method for combining and heat-treating a lithium-poor phase and a lithium-rich phase in a certain proportion includes the following steps:

[0068] The lithium-poor phase has a porous structure. The lithium-rich phase is filled into the porous structure of the lithium-poor phase and then heat-treated.

[0069] This method enables the preparation of the composite cathode material with the structure of the composite embodiment A described above. The heat treatment is used to stabilize the lithium-rich phase within the porous structure of the lithium-poor phase, thereby improving the stability of the prepared composite cathode material structure. The parameters of the lithium-poor phase and its contained porous structure, such as the lithium-poor phase particle size, the porosity of the porous structure, and the pore density, are the same as those of the lithium-poor phase in the composite embodiment A described above. The parameters of the lithium-rich phase are also the same as those of the lithium-rich phase in the composite embodiment A described above.

[0070] In one embodiment, the method for combining and heat-treating a lithium-poor phase and a lithium-rich phase in a certain proportion includes the following steps:

[0071] The lithium-rich phase is used as the lithium-rich core, and the lithium-poor phase is coated on the interface of the lithium-rich core and then subjected to heat treatment.

[0072] This method enables the preparation of composite cathode materials with the structure of Embodiment B in the above application. The heat treatment is performed to stabilize the lithium-poor phase coating layer structure formed at the lithium-rich core interface or surface, and to ensure strong bonding with the lithium-rich core interface, thereby improving the stability of the composite cathode material structure.

[0073] In a further embodiment, as described in the composite embodiment B of the above text application, when the lithium-poor phase contains a lithium-rich phase doped with it, during the process of coating the lithium-poor phase onto the lithium-rich core interface, a mixture containing a certain amount of the lithium-rich phase and the lithium-poor phase can be formed and coated together onto the lithium-rich core interface. Furthermore, during the formation of the coating layer, the coating process can be performed multiple times to control the content of the lithium-rich phase in the coating layer, such as controlling the lithium-rich phase to have a gradient decreasing distribution in the coating layer.

[0074] The parameters for the lithium-poor phase and the lithium-rich phase are the same as those for the lithium-poor phase and the lithium-rich phase in the composite embodiment C structure of the above application.

[0075] In one embodiment, the method for combining and heat-treating a lithium-poor phase and a lithium-rich phase in a certain proportion includes the following steps:

[0076] The lithium-poor phase is used as the lithium-poor core, and the lithium-rich phase is coated on the interface of the lithium-poor core before heat treatment.

[0077] This method enables the preparation of a composite cathode material with the structure of Embodiment C in the above application. The heat treatment is performed to stabilize the lithium-rich phase coating layer structure formed at the lithium-poor core interface or surface, and to ensure strong bonding with the lithium-poor core interface, thereby improving the stability of the composite cathode material structure.

[0078] In a further embodiment, as described in the composite embodiment C of the above text application, when the lithium-rich phase contains a lithium-poor phase, during the process of coating the lithium-rich phase onto the lithium-poor core interface, a mixture containing a certain amount of the lithium-poor phase and the lithium-rich phase can be formed and coated together onto the lithium-poor core interface. Moreover, during the formation of the coating layer, the coating process can be performed multiple times to control the content of the lithium-poor phase in the coating layer, such as controlling the lithium-poor phase to have a gradient decreasing distribution in the coating layer.

[0079] The parameters for the lithium-poor phase and the lithium-rich phase are the same as those for the lithium-poor phase and the lithium-rich phase in the composite embodiment C structure of the above application.

[0080] In addition, the heat treatment in the above embodiments is to improve the bonding strength between the lithium-rich phase and the lithium-poor phase and improve the electrochemical performance of the interface between the two. Therefore, in the embodiments, the temperature of the heat treatment should be lower than the melting temperature of the lithium-rich phase and the lithium-poor phase. For example, in the embodiments, the temperature range of the heat treatment in the above embodiments can be 200-800℃, and further can be 500-700℃.

[0081] The following examples illustrate the composite cathode material and its preparation method according to the embodiments of this application.

[0082] 1. Examples of composite cathode materials and their preparation methods:

[0083] Example 1

[0084] This embodiment provides a composite cathode material and its preparation method. The composite cathode material is FePO4·0.5Li5FeO4, wherein FePO4 is the lithium-poor phase and Li5FeO4 is the lithium-rich phase. The lithium-poor FePO4 phase consists of porous, large particles with a particle size D. 50 The particle size is approximately 6 μm, with an average pore size of approximately 300 nm; the lithium-rich Li5FeO4 phase consists of small particles with a particle size D. 50 Approximately 200 nm; Li5FeO4 fills the porous structure of FePO4 to form a garnet-like composite cathode material.

[0085] The preparation method of the composite cathode material in this embodiment includes the following steps:

[0086] S1. Preparation of lithium-poor FePO4 phase: Iron source, phosphate and pore-forming agent are mixed evenly according to stoichiometric ratio, precipitant is added for co-precipitation to obtain FePO4 precursor, pore-forming agent in precursor is washed and heat-treated to obtain porous lithium-poor FePO4 phase.

[0087] Preparation of S2. Li5FeO4 lithium-rich phase: Iron source and lithium source were mixed evenly in stoichiometric ratio, solvent was evaporated to obtain precursor, additional lithium source was added appropriately and ball-milled evenly with precursor, high-temperature solid-state sintering, and sand milling to nano-size to obtain Li5FeO4 lithium-rich phase.

[0088] S3. Composite treatment and heat treatment of lithium-poor and lithium-rich phases: The porous FePO4 lithium-poor phase and the nano Li5FeO4 lithium-rich phase are prepared according to the stoichiometric ratio, mixed uniformly by high-energy ball milling, and the structure is stabilized by medium-low temperature heat treatment at 600℃ to obtain garnet-shaped FePO4·0.5Li5FeO4 composite cathode material.

[0089] Example 2

[0090] This embodiment provides a composite cathode material and its preparation method. The composite cathode material includes a lithium-rich phase and a lithium-poor phase, wherein the lithium-rich phase is a lithium-rich core, and the lithium-rich phase is Li5FeO4 with a particle size D. 50 The thickness is approximately 3 μm; a lithium-poor phase forms a lithium-poor coating layer to coat the lithium-rich core. The lithium-poor phase is FePO4 with a thickness of approximately 3 μm; the composite cathode material is FePO4·0.25Li5FeO4.

[0091] The preparation method of the composite cathode material in this embodiment includes the following steps:

[0092] Preparation of Li5FeO4 lithium-rich phase: Iron source and lithium source were mixed evenly according to stoichiometric ratio, solvent was evaporated to obtain precursor, additional lithium source was added appropriately and ball-milled evenly with precursor, and high-temperature solid-state sintering was performed to obtain Li5FeO4 lithium-rich phase.

[0093] S2. Preparation of lithium-poor FePO4 precursor: Iron source and phosphate were mixed evenly according to stoichiometric ratio, and a precipitant was added for co-precipitation to obtain nano FePO4 precursor;

[0094] S3. Composite treatment and heat treatment of lithium-poor and lithium-rich phases: Large-particle Li5FeO4 lithium-rich phase and nano-FePO4 lithium-poor phase precursor are prepared according to stoichiometric ratio, mixed uniformly by high-energy ball milling, and then sintered at 650℃ to fuse the nano-FePO4 lithium-poor phase precursor structure and form a highly crystalline nano-FePO4 lithium-poor phase. At the same time, it is bonded to the interface of the Li5FeO4 lithium-rich phase, and finally forms a core-shell structure FePO4·0.25Li5FeO4 composite cathode material with lithium-poor phase encapsulating lithium-rich phase.

[0095] Example 3

[0096] This embodiment provides a composite cathode material and its preparation method. The composite cathode material includes a lithium-rich phase and a lithium-poor phase, wherein the lithium-poor phase is a lithium-poor core, and the lithium-poor phase is Fe... 0.4 Mn 0.6 PO4, particle size D 50 The thickness is approximately 1 μm; a lithium-rich phase forms a lithium-rich coating layer, which coats the lithium-poor core. The lithium-rich phase is Li5FeO4, with a thickness of approximately 500 nm; the composite cathode material is Fe. 0.4 Mn 0.6 PO4·0.25Li5FeO4.

[0097] The preparation method of the composite cathode material in this embodiment includes the following steps:

[0098] S1.Fe 0.4 Mn 0.6Preparation of lithium-poor phase of PO4: Iron source, manganese source and phosphate were prepared and mixed evenly according to stoichiometric ratio, and a precipitant was added for co-precipitation to obtain Fe. 0.4 Mn 0.6 PO4 precursor, heat treatment to obtain Fe 0.4 Mn 0.6 PO4 lithium-poor phase;

[0099] S2. Preparation of composite cathode material precursor: combining nanoscale iron source with Fe... 0.4 Mn 0.6 The PO4 lithium-depleted phase was prepared according to stoichiometric ratio and mixed by high-energy ball milling, so that the nano-iron source was uniformly attached to the Fe. 0.4 Mn 0.6 The lithium-poor phase surface of PO4 was then subjected to medium-low temperature heat treatment to stabilize the structure and obtain the precursor of composite cathode material.

[0100] S3. Composite treatment and heat treatment of lithium-poor and lithium-rich phases: The composite cathode material precursor and lithium source are prepared according to the stoichiometric ratio of the lithium-rich phase. The solid phase is mixed uniformly by high-energy ball milling, and then sintered at 500℃. The lithium source melts and fuses with the nano-iron source, and the reaction generates a dense lithium-rich phase that coats the interface of the lithium-poor phase, ultimately forming a core-shell structure of lithium-rich phase coating the lithium-poor phase. 0.4 Mn 0.6 PO4·0.25Li5FeO4 composite cathode material.

[0101] Example 4

[0102] This embodiment provides a composite cathode material and its preparation method. The composite cathode material includes a lithium-rich phase and a lithium-poor phase with a concentration gradient, wherein the lithium-rich phase is a lithium-rich core, and the lithium-rich phase is Li5FeO4 with a particle size D. 50 Approximately 3 μm; a lithium-poor phase forms a lithium-poor coating layer, which coats the lithium-rich core; the lithium-poor phase is Li. 0.5 FePO4, with a lithium concentration exhibiting a gradient, showing a relatively high lithium concentration in the inner layer and a relatively low lithium concentration in the outer layer, and a thickness of approximately 3 μm; the composite cathode material is Li. 0.5 FePO4·0.25Li5FeO4.

[0103] The preparation method of the composite cathode material in this embodiment includes the following steps:

[0104] Preparation of Li5FeO4 lithium-rich phase: Iron source and lithium source were mixed evenly according to stoichiometric ratio, solvent was evaporated to obtain precursor, additional lithium source was added appropriately and ball-milled evenly with precursor, and high-temperature solid-state sintering was performed to obtain Li5FeO4 lithium-rich phase.

[0105] S2. Preparation of lithium-poor FePO4 precursor: Iron source and phosphate were mixed evenly according to stoichiometric ratio, and a precipitant was added for co-precipitation to obtain nano FePO4 precursor;

[0106] S3. Composite treatment and heat treatment of lithium-poor and lithium-rich phases: Combining large-particle Li5FeO4 lithium-rich phase with nano-Li 0.5 The required lithium source in the lithium-poor FePO4 phase is prepared according to stoichiometric ratio and mixed uniformly by high-energy ball milling to form a mixed phase of lithium-rich Li5FeO4 and lithium source. Then, nano-FePO4 precursor is added according to stoichiometric ratio and mixed uniformly by high-energy ball milling to form a mixed structure of lithium-rich phase / lithium source / lithium-poor phase from the inside out. Finally, sintering at 700℃ fuses the nano-FePO4 lithium-poor phase precursor structure to form a highly crystalline nano-FePO4 lithium-poor phase. Simultaneously, due to concentration polarization, the lithium source permeates outward and partially intercalates with the nano-FePO4 lithium-poor phase, creating a gradient change in lithium concentration. Ultimately, a core-shell structure is formed where the lithium-poor phase with a lithium concentration gradient encapsulates the lithium-rich phase. 0.5 FePO4·0.25Li5FeO4 composite cathode material.

[0107] Comparative Example 1

[0108] This comparative example provides a lithium iron phosphate cathode material, LiFePO4.

[0109] Comparative Example 2

[0110] This comparative example provides a lithium-rich phase and a nano-lithium-poor phase in Example 2, and the lithium-poor phase and the lithium-rich phase are formed by room temperature stirring and mixing to form a blended FePO4·0.25Li5FeO4 composite cathode material.

[0111] 2. Example of a lithium-ion battery:

[0112] The composite cathode materials provided in Examples 1 to 4 and the cathode material provided in Comparative Example 2 were assembled into cathode electrodes and negative electrode-free lithium-ion batteries respectively according to the following methods:

[0113] The positive electrode material was mixed with SP and PVDF at a mass ratio of 95:2:3, and an appropriate amount of NMP solvent was added. The mixture was ball-milled and stirred for 30 min. After homogenization, coating, drying, rolling, and cutting, positive electrode sheets with a thickness of 110 μm were prepared. The positive electrode sheets were baked in a vacuum oven at 100℃ to remove trace amounts of water. Using carbon-coated copper foil as the negative electrode current collector, and a 1M LiPF6 solution as the electrolyte, with a 1:1 volume ratio of EC (ethylene carbonate) and DEC (diethyl carbonate) mixture as the solvent, a CR2032 type coin-type lithium-ion battery without a negative electrode was assembled.

[0114] The lithium iron phosphate cathode material provided in Comparative Example 1 was assembled into a cathode electrode and a lithium-ion battery as follows: LiFePO4 cathode material was mixed with SP and PVDF at a mass ratio of 95:2:3, and an appropriate amount of NMP solvent was added. The mixture was ball-milled and stirred for 30 min. After homogenization, coating, drying, rolling, and cutting, cathode sheets with a thickness of 110 μm were prepared. The cathode sheets were baked in a vacuum oven at 100 °C to remove trace amounts of water. A commercial graphite electrode with a thickness of 110 μm was used as the anode, and a 1M LiPF6 solution was used as the electrolyte. The solvent was a 1:1 volume ratio mixture of EC (ethylene carbonate) and DEC (diethyl carbonate) to assemble a CR2032 coin cell lithium-ion battery.

[0115] 3. Relevant performance tests

[0116] 1. Relevant Tests for Composite Cathode Materials

[0117] The composite cathode materials prepared in the above embodiments were analyzed by SEM. The SEM image of the composite cathode material in Example 2 is shown below. Figure 5 As shown. By Figure 5 It can be seen that the composite cathode material particles prepared in Example 2 are intact and uniform. SEM images of the composite cathode materials obtained in Examples 3 and 4 show that the composite cathode material particles in both examples are intact and uniform. However, the SEM image of the composite cathode material obtained in Example 1 shows that, from the particle surface, the particles contain abundant porous structures, and the pores are filled with a filling treatment, i.e., a lithium-rich phase.

[0118] 2. Example of a lithium-ion battery:

[0119] The electrochemical performance of each lithium-ion battery assembled in the above lithium-ion battery examples was tested under the following conditions:

[0120] (1) Electrochemical performance testing of lithium-ion batteries

[0121] The lithium batteries provided in Examples 1 to 4 and the lithium-ion batteries provided in the comparative examples were tested for their charge and discharge capacity performance according to the following method: constant current and constant voltage charging at a rate of 0.05C to 4.3V, with a cutoff current of 0.02C; after resting for 5 minutes, constant current discharge at a rate of 0.05C to 2.8V.

[0122] (2) Cell thickness test

[0123] The thickness of the lithium batteries provided in Examples 1 to 4 and the lithium-ion batteries provided in the comparative examples was tested from the positive electrode current collector to the negative electrode current collector.

[0124] The test results are shown in Table 1 below:

[0125] Table 1

[0126]

[0127] As shown in Table 1, the composite cathode material composed of lithium-poor and lithium-rich phases, due to the presence of sufficient surplus lithium ions, enables the assembly of electrodeless battery cells. This reduces the thickness of the negative electrode sheet in the cell system, resulting in a significant reduction in the overall cell thickness and corresponding cell volume. Although the introduction of the irreversible capacity of the lithium-rich phase leads to a decrease in the overall cycle reversible specific capacity of the composite cathode material, the emergence of this composite cathode material makes electrodeless batteries possible, thereby significantly improving the overall volumetric energy density of the cell. This enables the controllable and safe application of lithium metal batteries and facilitates the promotion of high-volume-density electrodeless lithium batteries.

[0128] Furthermore, it can be observed that the charge-discharge performance of the composite cathode materials prepared by the core-shell structure combined with heat treatment in Examples 2 to 4 is better than that of the composite cathode materials obtained by single physical-mechanical blending with the same component ratio in Comparative Example 2. This indicates that the material structure design and heat treatment can make the distribution of different crystal phases of the composite cathode material more reasonable and better combined, reduce the internal impedance and polarization of the composite material, and facilitate the solid-phase migration, conduction and insertion / extraction of lithium ions in the composite cathode material.

[0129] The above are merely preferred embodiments of this application and are not intended to limit this application. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A negative electrode-free lithium-ion battery, characterized in that, The invention includes a composite cathode material comprising a lithium-poor phase as the cathode material and a lithium-rich phase for lithium replenishment, wherein the lithium-rich phase and the lithium-poor phase form a composite material, and the lithium-rich phase comprises a lithium-rich metal oxide; the lithium-rich metal oxide comprises at least one of Li5MO4, Li2MO2, Li6MO4, and Li8MO6; wherein M is selected from at least one of the metal elements Fe, Co, Ni, Mn, Al, Cr, Ga, In, La, and Bi; and during the first charge, the lithium-rich phase provides lithium ions to the lithium-poor phase at least; the lithium ions released by the lithium-rich metal oxide during charging can accumulate on the surface of the negative electrode current collector to form a lithium-rich film layer; The lithium-poor phase includes a phosphate-based cathode material, which is Li. (1-x) A y B (1-y) PO4, wherein A is selected from at least one of the metallic elements Fe, Co, Ni, and Mn, and B is selected from at least one of the metallic elements Fe, Co, Ni, Mn, V, Mg, Ca, Cr, Cu, Zn, Ti, and Sn; the values ​​of x and y satisfy: x = 1, 0 <y≤1; The composite material formed by the lithium-rich phase and the lithium-poor phase is at least one of the following A to C: A: The lithium-poor phase has a porous structure, and the lithium-rich phase is filled in the porous structure of the lithium-poor phase, wherein the pore distribution density of the porous structure contained in the lithium-poor phase is 1%-50%; B: The lithium-rich phase is a lithium-rich core, and the lithium-poor phase forms a lithium-poor coating layer that coats the lithium-rich core. The lithium-poor coating layer is also doped with the lithium-rich phase, and the lithium-rich phase doped in the lithium-poor coating layer has a gradient decreasing distribution from the lithium-rich core to the outer surface of the lithium-poor coating layer. The mass of the lithium-rich phase doped in the lithium-poor coating layer is 5-50% of the mass of the lithium-poor coating layer. C: The lithium-poor phase is a lithium-poor core, and the lithium-rich phase forms a lithium-rich coating layer that coats the lithium-poor core. The lithium-rich coating layer is also doped with the lithium-poor phase. From the lithium-poor core to the outer surface of the lithium-rich coating layer, the lithium-poor phase doped in the lithium-rich coating layer has a gradient decreasing distribution. The mass of the lithium-poor phase doped in the lithium-rich coating layer is 5%-50% of the mass of the lithium-rich coating layer.

2. The composite cathode material as described in claim 1, characterized in that: When the composite material formed by the lithium-rich phase and the lithium-poor phase is A, the particle size of the lithium-poor phase is 500 nm-100 μm; and / or The pore diameter of the porous structure is 5nm-10μm.

3. The composite cathode material as described in claim 1, characterized in that: When the composite material formed by the lithium-rich phase and the lithium-poor phase is B, the particle size of the lithium-rich core is 50 nm-100 μm; and / or The thickness of the lithium-poor coating layer is 5nm-10μm.

4. The composite cathode material as described in claim 1, characterized in that: When the composite material formed by the lithium-rich phase and the lithium-poor phase is C, the particle size of the lithium-poor core is 50 nm-100 μm; and / or The thickness of the lithium-rich coating layer is 5 nm-10 μm; and / or An encapsulation functional layer is also provided on the outer surface of the lithium-rich coating layer. The encapsulation functional layer is an ion conductor encapsulation layer or an electronic conductor encapsulation layer, or a composite layer structure of an ion conductor encapsulation layer and an electronic conductor encapsulation layer.

5. The composite cathode material according to any one of claims 1-4, characterized in that: The mass ratio of the lithium-rich phase to the lithium-poor phase is 1:(0.01-100).

6. The composite cathode material as described in claim 5, characterized in that, The Li (1-x) A y B (1-y) PO4 includes Li 1- x FePO4, Li 1-x MnPO4, Li 1-x CoPO4, Li 1-x NiPO4, Li 1-x Fe y Mn 1-y PO4, Li 1-x Fe y Co 1-y PO4, Li 1-x Fe y Ni 1- y PO4, Li 1-x Mn y Co 1-y PO4, Li 1-x Mn y Ni 1-y PO4, Li 1-x Co y Ni 1-y At least one of PO4.

7. A method for preparing the composite cathode material according to any one of claims 1 to 6, characterized in that, Includes the following steps: It provides a lithium-poor phase for use as a cathode material and a lithium-rich phase for providing lithium ions; The lithium-poor phase and the lithium-rich phase are combined and heat-treated in a certain proportion to obtain a composite cathode material; wherein the heat treatment temperature is lower than the sintering temperature of the lithium-poor phase and the lithium-rich phase. The lithium-poor phase includes a phosphate-based cathode material, which is Li. (1-x) A y B (1-y) PO4, wherein A is selected from at least one of the metallic elements Fe, Co, Ni, and Mn, and B is selected from at least one of the metallic elements Fe, Co, Ni, Mn, V, Mg, Ca, Cr, Cu, Zn, Ti, and Sn; the values ​​of x and y satisfy: x = 1, 0 <y≤1; The method for combining and heat-treating the lithium-poor phase and the lithium-rich phase in a certain proportion includes the following steps: The lithium-poor phase has a porous structure, and the heat treatment is performed after the lithium-rich phase is filled into the porous structure of the lithium-poor phase. or The lithium-rich phase is used as a lithium-rich core, and the lithium-poor phase is coated on the interface of the lithium-rich core before the heat treatment is performed. or The lithium-poor phase is used as the lithium-poor core, and the lithium-rich phase is coated on the interface of the lithium-poor core before the heat treatment is performed.

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