Positive electrode material and preparation method therefor, positive electrode sheet, secondary battery and electric device

Abstract

A positive electrode material and a preparation method therefor, a positive electrode sheet, a secondary battery and an electric device. The positive electrode material comprises a conductive substrate material and an active material distributed on the conductive substrate material, wherein the active material comprises a nanoscale phosphate active material, and the conductive substrate material comprises doping-element-modified graphene; and based on the total weight of the positive electrode material, the weight content of the active material is 75-95%, and the weight content of the conductive substrate material is 5-25%. The positive electrode material is prepared by using the doping-element-modified graphene as a substrate material for loading nanoparticles of the phosphate active material; therefore, the agglomeration of the nanoscale phosphate positive electrode material can be reduced, the wettability of an electrolyte on the active material is improved, and the cycle performance of the positive electrode material is improved.

Description

Positive electrode material and preparation method thereof, positive electrode sheet, secondary battery, and electrical device

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to Chinese patent application No. 202410239215.2 filed on March 4, 2024, entitled “Positive electrode material and preparation method thereof, positive electrode sheet, secondary battery, and electrical device,” and the entire contents of that application are incorporated herein by reference. Technical Field

[0003] The present application relates to the field of battery technology, and specifically to a positive electrode material and a preparation method thereof, a positive electrode sheet, a secondary battery, and an electrical device. Background Art

[0004] Batteries are widely used in electronic devices such as mobile phones, laptops, electric vehicles, electric airplanes, electric boats, electric car toys, electric toy boats, electric toy airplanes and power tools.

[0005] In the process of battery development, how to improve the cycle performance of batteries is one of the problems that need to be solved urgently.

[0006] Summary of the Invention

[0007] The embodiments of the present application provide a positive electrode material and a preparation method thereof, a positive electrode plate, a secondary battery, and an electrical device, aiming to improve the cycle performance of the secondary battery.

[0008] An embodiment of the first aspect of the present application provides a positive electrode material, comprising a conductive matrix material and an active material distributed on the conductive matrix material; the active material comprises a nano-scale phosphate active material, and the conductive matrix material comprises graphene modified by doping elements; based on the total weight of the positive electrode material, the weight content of the active material is 75%-95%, and the weight content of the conductive matrix material is 5%-25%.

[0009] In the embodiment of the present application, the positive electrode material is prepared by using graphene modified by doping elements as the base material for loading nanoparticles of phosphate active materials. The graphene modified by doping elements is used as the base to anchor the phosphate active material particles with van der Waals force, which can reduce the agglomeration of nano-scale phosphate positive electrode materials and make the distribution of active materials more uniform, thereby increasing the wettability of the electrolyte to the active materials and improving the cycle performance of the positive electrode materials. Nano-scale phosphate active materials have a large specific surface area and a short lithium ion diffusion distance, which can improve the lithium ion diffusion kinetics of the positive electrode materials and improve the electrochemical performance of the positive electrode materials. The high conductivity of the graphene modified by doping elements can improve the electronic conductivity of the positive electrode materials, and the excellent mechanical properties of graphene can buffer the stress during the charge and discharge process of the positive electrode materials and improve the structural stability of the positive electrode materials. Through the synergistic effect of nano-scale phosphate active materials and doping element modified graphene, the high conductivity of graphene and the reduced lithium ion diffusion distance of nano-scale phosphate active materials can be fully utilized, showing excellent cycle stability.

[0010] In some embodiments, based on the total weight of the positive electrode material, the weight content of the active material is 80%-90%, and the weight content of the conductive matrix material is 10%-20%.

[0011] In some embodiments, the phosphate active material has the following chemical formula: Li x M y PO4, M includes one or more of Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Fe, Ni, Co, Ga, Sn, Sb, Nb, Ge, Mn, Cu, Cr, 0.9≤x≤1.1, 0.9≤y≤1.1.

[0012] In some embodiments, the volume distribution particle size Dv50 of the phosphate active material is 8 nm-300 nm, and the volume distribution particle size Dv90 is 20 nm-400 nm.

[0013] In some embodiments, the volume particle size distribution Dv50 of the phosphate-based active material is 20 nm-200 nm, and the volume distribution particle size Dv90 is 50 nm-350 nm.

[0014] In some embodiments, the specific surface area of ​​the phosphate active material is 30 m 2 / g-150m 2 / g.

[0015] In some embodiments, the specific surface area of ​​the phosphate active material is 50 m 2 / g-120m 2 / g.

[0016] In some embodiments, the doping element includes N, S, or a combination thereof.

[0017] In some embodiments, the doped element-modified graphene includes doped element-modified graphene nanosheets, and the thickness of the doped element-modified graphene nanosheets is 2 nm-20 nm.

[0018] In some embodiments, the weight content of the doping element is 5%-15% based on the total weight of the doping element-modified graphene.

[0019] The second embodiment of the present application provides a method for preparing a positive electrode material, comprising the following steps:

[0020] Provide nano-scale phosphate active materials and doped element modified graphene respectively;

[0021] The nano-scale phosphate active material and the doped element-modified graphene are dispersed in a solvent and ultrasonically mixed to obtain the positive electrode material.

[0022] In the examples of this application, a phosphate active material and doped element-modified graphene were prepared separately. The two materials were then dispersed in a solvent and ultrasonically assembled. Under ultrasound, the nanoparticles of the phosphate active material were "anchored" to the surface of the doped element-modified graphene via van der Waals forces, achieving a composite of the two materials under mild conditions. The phosphate active material nanoparticles "anchored" to the surface of the doped element-modified graphene acted as an isolation barrier, reducing agglomeration and overlap between the doped element-modified graphene sheets, thereby improving the uniformity of the phosphate active material dispersion.

[0023] In some embodiments, providing the nanoscale phosphate active material includes: taking a lithium source, a phosphorus source and an M source to perform a hydrothermal reaction to obtain a phosphate active material, wherein M includes one or more of Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Fe, Ni, Co, Ga, Sn, Sb, Nb, Ge, Mn, Cu, and Cr.

[0024] In some embodiments, the lithium source includes one or more of lithium hydroxide, lithium oxide, lithium oxalate, lithium phosphate, lithium carbonate, lithium dihydrogen phosphate, and lithium hydrogen phosphate.

[0025] In some embodiments, the phosphorus source includes one or more of ammonium dihydrogen phosphate, ammonium phosphate, sodium pyrophosphate, ammonium hypophosphite, ammonium polyphosphate, ammonium hexafluorophosphate, diammonium hydrogen phosphate, sodium phosphite, sodium metaphosphate, sodium tripolyphosphate, and sodium hexafluorophosphate.

[0026] In some embodiments, the M source includes one or more compounds containing an M element.

[0027] In some embodiments, providing the doping element-modified graphene includes: taking graphene oxide, a reducing agent and a doping element source to perform a solvent thermal reduction reaction to obtain the doping element-modified graphene, wherein the doping element source includes a nitrogen source, a sulfur source or a combination thereof.

[0028] In some embodiments, the reducing agent comprises one or more of hydrazine hydrate, sodium borohydride, and pure hydrazine.

[0029] In some embodiments, the nitrogen source includes one or more of ammonia, ethylenediamine, urea, hydrazine hydrate, acetonitrile, and pyrrole.

[0030] In some embodiments, the sulfur source includes one or more of elemental sulfur, hydrogen sulfide, and thiourea.

[0031] In some embodiments, the mass ratio of the reducing agent to the graphene oxide is 1:(0.01-0.5).

[0032] In some embodiments, the mass ratio of the doping element source to the graphene oxide is 1:(0.1-10).

[0033] In some embodiments, the step of dispersing the nano-scale phosphate active material and the doped element-modified graphene in a solvent and ultrasonically mixing them to obtain the positive electrode material includes: dispersing the nano-scale phosphate active material and the doped element-modified graphene in a solvent, ultrasonically treating them at a power of 100W-300W for 2h-12h to obtain the positive electrode material.

[0034] In some embodiments, the mass ratio of the nano-scale phosphate active material to the doping element-modified graphene is (3-19):1.

[0035] An embodiment of the third aspect of the present application provides a positive electrode plate, comprising the positive electrode material of the embodiment of the first aspect of the present application or the positive electrode material obtained according to the preparation method of the embodiment of the second aspect of the present application.

[0036] An embodiment of the fourth aspect of the present application provides a secondary battery, comprising the positive electrode sheet according to the embodiment of the third aspect of the present application.

[0037] An embodiment of the fifth aspect of the present application provides an electrical device, comprising a secondary battery according to the embodiment of the fourth aspect of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the following is a brief introduction to the drawings required for use in the embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0039] FIG1 is a scanning electron microscope (SEM) image of the positive electrode material in one embodiment of the present application.

[0040] FIG2 is a schematic diagram of a secondary battery according to an embodiment of the present application;

[0041] FIG. 3 is an exploded view of the secondary battery according to the embodiment of the present application shown in FIG. 2 .

[0042] FIG4 is a schematic diagram of a battery module according to an embodiment of the present application.

[0043] FIG5 is a schematic diagram of a battery pack according to an embodiment of the present application.

[0044] FIG. 6 is an exploded view of the battery pack shown in FIG. 5 according to an embodiment of the present application.

[0045] FIG. 7 is a schematic diagram of an electric device using a secondary battery as a power source according to an embodiment of the present application.

[0046] Explanation of reference numerals: 1 battery pack; 2 upper case; 3 lower case; 4 battery module; 5 secondary battery; 51 housing; 52 electrode assembly; 53 top cover assembly. DETAILED DESCRIPTION

[0047] Below, the embodiments of the battery cell, battery, and electrical device of the present application are described in detail with appropriate reference to the accompanying drawings. However, there may be cases where unnecessary detailed descriptions are omitted. For example, there may be cases where detailed descriptions of well-known matters and repeated descriptions of actually the same structure are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate the understanding of those skilled in the art. In addition, the drawings and the following description are provided for those skilled in the art to fully understand the present application and are not intended to limit the subject matter described in the claims.

[0048] " range " disclosed in the present application is limited in the form of lower limit and upper limit, and given range is limited by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundary of special range. The scope limited in this way can be to include end value or not include end value, and can be arbitrarily combined, that is, any lower limit can form a range with any upper limit combination. For example, if the scope of 60-120 and 80-110 is listed for specific parameters, it is understood that the scope of 60-110 and 80-120 is also expected. In addition, if the minimum range value 1 and 2 are listed, and if the maximum range value 3,4 and 5 are listed, then the following range can all be expected: 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5. In this application, unless otherwise specified, the numerical range " ab " represents the abbreviation of any real number combination between a and b, wherein a and b are all real numbers. For example, a numerical range of "0-5" indicates that all real numbers between "0-5" are listed herein, and "0-5" is simply an abbreviation for these numerical combinations. Furthermore, when a parameter is expressed as an integer ≥ 2, this is equivalent to disclosing that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0049] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form new technical solutions, and such technical solutions should be considered to be included in the disclosure of the present application.

[0050] Unless otherwise specified, all steps of the present application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or may include steps (b) and (a) performed sequentially. For example, the method may further include step (c), indicating that step (c) may be added to the method in any order, for example, the method may include steps (a), (b) and (c), or may include steps (a), (c) and (b), or may include steps (c), (a) and (b), etc.

[0051] Unless otherwise specified, the terms "connected" and "connection" in this application should be understood broadly. For example, they can refer to fixed connection, detachable connection, or integral connection; they can refer to direct connection, indirect connection through an intermediate medium, or internal communication between two components. Those skilled in the art will understand the specific meanings of the above terms in this application based on specific circumstances.

[0052] Unless otherwise specified, in this application, the term "active ions" refers to ions that can be intercalated and extracted between the positive and negative electrodes of a battery cell, including but not limited to lithium ions, sodium ions, etc.

[0053] As used herein, the term "multiple" refers to more than two, including two. As used herein, the term "multiple" refers to more than two, including two.

[0054] Phosphate-based active materials have relatively low electronic conductivity and lithium-ion diffusion coefficients, which result in poor rate performance. To improve the electron transport and lithium-ion diffusion kinetics of phosphate-based active materials, it is necessary to design phosphate-based active materials with more rational morphology and composition. Nanosizing is one effective means of improving the performance of phosphate-based active materials. When phosphate-based active materials are optimized to the nanoscale, the material's specific surface area increases, providing more active sites. Nanosizing also shortens the diffusion distance of lithium ions in phosphate-based active materials, effectively improving their lithium-ion diffusion performance.

[0055] However, nano-sized phosphate active materials have a large specific surface area and high specific surface energy of nanoparticles, which makes nano-scale phosphate active materials more likely to agglomerate, resulting in poor material cycle performance.

[0056] In view of the above problems, an embodiment of the present application provides a positive electrode material, which uses doped element-modified graphene as a loading substrate for nano-phosphate active materials, which can effectively improve the dispersion performance of nano-phosphate active materials, reduce agglomeration, and enhance the cycle performance of the positive electrode material.

[0057] cathode materials

[0058] An embodiment of the first aspect of the present application provides a positive electrode material, which includes a conductive matrix material and an active material distributed on the conductive matrix material; the active material includes a nano-scale phosphate active material, and the conductive matrix material includes graphene modified by doping elements; based on the total weight of the positive electrode material, the weight content of the active material is 75%-95%, and the weight content of the conductive matrix material is 5%-25%.

[0059] Nano-scale phosphate active materials refer to phosphate active material particles with a nanometer particle size. Nano-scale phosphate active materials have a large specific surface area and can provide more active sites. At the same time, nano-sizing shortens the diffusion distance of lithium ions in phosphate active materials, which can improve the lithium ion diffusion kinetics of the positive electrode material, thereby improving the electrochemical performance of the positive electrode material. Graphene has excellent electrical conductivity. By modifying it with doping elements, the electron cloud density of the graphene Pz orbital can be increased, thereby having a higher electronic conductivity, thereby improving the conductivity of the positive electrode material. At the same time, doped element-modified graphene can play the role of a conductive agent in the positive electrode material. It can cross-link with each other in the positive electrode material layer to form a continuous conductive network, providing a fast electron transmission channel, thereby improving high-rate cycle stability.

[0060] Using doped element-modified graphene as a carrier for nanoscale phosphate active materials, the nanoscale phosphate active materials are anchored to the graphene substrate via van der Waals forces, reducing the aggregation of phosphate active material nanoparticles and improving the cycling performance of the cathode material. Furthermore, the phosphate active material nanoparticles supported on the graphene can isolate the stacking of doped element-modified graphene sheets, reducing stacking and agglomeration between graphene sheets. Figure 1 shows a scanning electron microscope (SEM) image of the cathode material in one embodiment of the present application, showing that the phosphate active material is uniformly dispersed on the surface of the graphene nanosheets.

[0061] In the embodiment of the present application, a positive electrode material is prepared by using doped element-modified graphene as a base material for loading nanoparticles of phosphate active materials. The doped element-modified graphene is used as a base to anchor the phosphate active material particles with van der Waals force, which can reduce the agglomeration of nano-scale phosphate positive electrode materials and make the distribution of active materials more uniform, thereby increasing the wettability of the electrolyte to the active materials and improving the cycle performance of the positive electrode materials. Nano-scale phosphate active materials have a large specific surface area and a short lithium ion diffusion distance, which can improve the lithium ion diffusion kinetics of the positive electrode materials and improve the electrochemical performance of the positive electrode materials; the high conductivity of the doped element-modified graphene can improve the electronic conductivity of the positive electrode materials, and the excellent mechanical properties of graphene can buffer the stress during the charge and discharge process of the positive electrode materials and improve the structural stability of the positive electrode materials. Through the synergistic effect of nano-scale phosphate active materials and doped element-modified graphene, the high conductivity of graphene and the reduced lithium ion diffusion distance of nano-scale phosphate active materials can be fully utilized, showing excellent cycle stability.

[0062] In some embodiments, based on the total weight of the positive electrode material, the weight content of the material may be 80%-90%, and the weight content of the conductive matrix material may be 10%-20%.

[0063] By controlling the weight contents of the active material and the conductive matrix material in the positive electrode material within the above ranges, the positive electrode material can have better electrochemical properties while taking into account higher electronic conductivity, thereby further improving the cycle stability of the positive electrode material.

[0064] In some embodiments, the phosphate-based active material has the following chemical formula: Li x M y PO4, M includes one or more of Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Fe, Ni, Co, Ga, Sn, Sb, Nb, Ge, Mn, Cu, Cr, 0.9≤x≤1.1, 0.9≤y≤1.1.

[0065] In some embodiments, M may be selected from one or more of Fe, Mn, Co, Ni, and V, and may further be selected from Fe.

[0066] For example, the phosphate-based active material may include lithium iron phosphate (LiFePO4), lithium manganese phosphate (LiMnPO4), lithium iron manganese phosphate (LiMn 0.4 Fe 0.6 PO4), lithium vanadium phosphate (Li3V2(PO4)3), lithium cobalt phosphate (LiCoPO4), and lithium nickel phosphate (LiNiPO4).

[0067] In some embodiments, the volume distribution particle size Dv50 of the phosphate-based active material may be 8 nm to 300 nm, and the volume distribution particle size Dv90 may be 20 nm to 400 nm.

[0068] The volume distribution particle size Dv50 is the particle size corresponding to when the cumulative volume distribution percentage of the material reaches 50%, and the volume distribution particle size Dv90 is the particle size corresponding to when the cumulative volume distribution percentage of the material reaches 90%. Both can be measured by methods and instruments known in the art, for example, they can be measured using a laser diffractometer in accordance with GB / T 19077-2016 "Particle Size Analysis - Laser Diffraction Method".

[0069] The volume distribution particle sizes Dv50 and Dv90 of the phosphate active materials are both limited to the above ranges. The phosphate active materials have nanometer-scale particle sizes. Within this particle size range, the phosphate active materials have a larger specific surface area, which can provide more active sites for lithium ions. At the same time, the lithium ions have a shorter diffusion distance. The prepared positive electrode material has better lithium ion diffusion kinetics, thereby improving the electrochemical performance of the positive electrode material.

[0070] The volume distribution particle size Dv50 of the phosphate active material may be selected to be 20 nm-200 nm, and further selected to be 50 nm-150 nm.

[0071] The volume distribution particle size Dv90 of the phosphate active material may be selected to be 50 nm-350 nm, and further selected to be 80 nm-300 nm.

[0072] The volume distribution particle size Dv50 and Dv90 of the phosphate active material are limited to the above range, which has better performance and can further improve the electrochemical performance of the positive electrode material.

[0073] In some embodiments, the specific surface area of ​​the phosphate active material can be 30 m 2 / g-150m 2 / g. For example, the specific surface area of ​​phosphate active materials can be 30m 2 / g, 40m 2 / g, 50m 2 / g, 60m 2 / g、70m 2 / g、80m 2 / g、90m 2 / g、100m 2 / g、110m 2 / g, 120m 2 / g, 130m 2 / g, 140m 2 / g, 150m 2 / g, or a range consisting of any two of the above values.

[0074] Specific surface area refers to the total surface area per unit mass of a material. A larger specific surface area indicates more lithium ion adsorption sites on the active material, resulting in better electrochemical performance for the positive electrode material. Specific surface area can be measured using methods and instruments known in the art, for example, according to the methods and instruments specified in GB / T 19587-2017, "Determination of Specific Surface Area of ​​Solids by Gas Adsorption BET Method."

[0075] In some embodiments, the doping element may include N, S, or a combination thereof.

[0076] Nitrogen and sulfur have a good doping effect on graphene. Compared to carbon atoms, nitrogen and sulfur atoms have more extranuclear electrons. Doping graphene with them can increase the density of the pz orbital electron cloud, thereby giving graphene higher electronic conductivity. Furthermore, nitrogen and sulfur, as doping elements, can form "activated regions" on the graphene surface. These activated regions can improve the wettability of the electrolyte, increasing the electrolyte's contact area on the graphene surface, providing more active sites for electrochemical reactions, thereby improving the cycling performance of the cathode material.

[0077] In some embodiments, the doped element-modified graphene includes doped element-modified graphene nanosheets, and the thickness of the doped element-modified graphene nanosheets may be 2 nm-20 nm, optionally 5 nm-18 nm, and further optionally 8 nm-15 nm.

[0078] Doped element-modified graphene acts as a carrier for phosphate active materials, dispersing them to reduce agglomeration. Simultaneously, the incorporation of doped element-modified graphene can improve the electronic conductivity of the cathode material, thereby enhancing its electrochemical performance. Limiting the thickness of the doped element-modified graphene within the aforementioned range provides good electrical conductivity. Furthermore, the phosphate active material distributed on its surface has a high dispersion density, resulting in a higher proportion of active material in the resulting cathode material, which can improve its electrochemical performance.

[0079] In some embodiments, the weight content of the doping element can be 5% to 15% based on the total weight of the doping element-modified graphene. For example, the weight content of the doping element can be 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, or a range consisting of any two of the above values.

[0080] The weight content of the doping element within the above range can make graphene have higher conductivity while also maintaining good mechanical properties. When the content of the doping element is low, the modification effect on graphene is low, and the conductivity and wettability of graphene are slightly improved. When the content of the doping element is high, more doping elements will be introduced into the positive electrode material, and these doping elements will produce certain side reactions in the positive electrode material, affecting the capacity and cycle performance of the positive electrode material.

[0081] The weight content of the doping element in the doping element-modified graphene can be detected by methods known in the art, for example, by scanning electron microscopy (SEM mapping).

[0082] Preparation method of positive electrode material

[0083] The second embodiment of the present application provides a method for preparing a positive electrode material, comprising the following steps:

[0084] S100, respectively provides nano-scale phosphate active materials and doped element-modified graphene;

[0085] S200, dispersing the nano-scale phosphate active material and the doped element-modified graphene in a solvent and performing ultrasonic mixing to obtain a positive electrode material.

[0086] Doping element-modified graphene requires preparation via a high-temperature thermal reduction method. This method typically uses graphene oxide as the starting material, reducing it to graphene through a high-temperature thermal reduction reaction. The doping element then modifies the graphene. However, the precursors used to prepare phosphate-based active materials are highly reactive and readily react with groups on the graphene oxide during the reduction reaction, making it difficult to obtain a pure olivine-type phosphate-based active material, often resulting in the formation of impurities. Therefore, conventional in-situ synthesis methods are difficult to use for the composite preparation of phosphate-based active materials and doping element-modified graphene as described in the examples of this application.

[0087] To overcome the problem of different reaction conditions during the preparation of phosphate active materials and doped element-modified graphene, the present invention first prepares the phosphate active material and doped element-modified graphene separately. The two materials are then dispersed in a solvent and ultrasonically assembled. Under the action of ultrasound, the phosphate active material particles can be "anchored" to the surface of the doped element-modified graphene by van der Waals forces, achieving a composite of the two materials under mild conditions. The phosphate active material nanoparticles "anchored" to the surface of the doped element-modified graphene act as an isolation barrier, reducing agglomeration and overlap between the doped element-modified graphene sheets, thereby improving the uniformity of the dispersion of the phosphate active material.

[0088] The preparation method of the positive electrode material provided in the embodiment of the present application can load nanoparticles of phosphate active materials of different morphologies on the surface of doped element-modified graphene, such as lithium iron phosphate active material nanoneedles, nanorods, nanoparticles, nanocrystals, nanoflowers, etc., and the morphology of the obtained positive electrode material is controllable.

[0089] In some embodiments, in step S100, the nanoscale phosphate active material can be prepared by the following method:

[0090] A lithium source, a phosphorus source and an M source are subjected to a hydrothermal reaction to obtain a nanoscale phosphate active material, wherein M includes one or more of Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Fe, Ni, Co, Ga, Sn, Sb, Nb, Ge, Mn, Cu and Cr.

[0091] Phosphate active materials can be prepared by hydrothermal method. By regulating the order of addition of lithium source, phosphorus source and M source and the reaction conditions, the formation of phosphate active materials with different morphologies can be controlled.

[0092] In some embodiments, the reaction temperature of the hydrothermal reaction may be 170°C-190°C.

[0093] In some embodiments, the lithium source may include one or more of lithium hydroxide, lithium oxide, lithium oxalate, lithium phosphate, lithium carbonate, lithium dihydrogen phosphate, and lithium dihydrogen phosphate.

[0094] In some embodiments, the phosphorus source may include one or more of ammonium dihydrogen phosphate, ammonium phosphate, sodium pyrophosphate, ammonium hypophosphite, ammonium polyphosphate, ammonium hexafluorophosphate, diammonium hydrogen phosphate, sodium phosphite, sodium metaphosphate, sodium tripolyphosphate, and sodium hexafluorophosphate.

[0095] In some embodiments, the M source may include one or more compounds containing the element M. For example, the M source may be a compound containing the element Fe, such as one or more of ferrous sulfate, ferrous nitrate, ferrous phosphate, ferrous oxalate, and ferrous pyrophosphate; the M source may be a compound containing the element Mn, such as one or more of manganese sulfate, manganese nitrate, manganese carbonate, manganese oxalate, manganese acetate, and manganese chloride; the M source may be a compound containing the element Co, such as one or more of cobalt sulfate, cobalt nitrate, cobalt carbonate, cobalt acetate, and cobalt chloride; the M source may be a compound containing the element V, such as one or more of vanadyl sulfate and vanadyl oxalate; and the M source may be a compound containing the element Ni, such as one or more of nickel sulfate and nickel nitrate.

[0096] In some embodiments, in step S100, the doping element-modified graphene is prepared by the following method: taking graphene oxide, a reducing agent and a doping element source to perform a solvent thermal reduction reaction to obtain the doping element-modified graphene, wherein the doping element source includes a nitrogen source, a sulfur source or a combination thereof.

[0097] In some embodiments, the reducing agent may include one or more of hydrazine hydrate, sodium borohydride, and pure hydrazine.

[0098] In some embodiments, the nitrogen source may include one or more of ammonia, ethylenediamine, urea, hydrazine hydrate, acetonitrile, and pyrrole.

[0099] In some embodiments, the sulfur source may include one or more of elemental sulfur, hydrogen sulfide, and thiourea.

[0100] In some embodiments, the mass ratio of the reducing agent to the graphene oxide can be 1:(0.01-0.5), optionally 1:(0.05-0.45), and further optionally 1:(0.1-0.4).

[0101] The mass ratio of the reducing agent to graphene oxide is limited to the above range, so that the reducing agent and graphene oxide can fully react, reduce the oxygen-containing groups on the surface of the graphene oxide, and improve the conductivity of the graphene.

[0102] In some embodiments, the mass ratio of the doping element source to the graphene oxide may be 1:(0.1-10), optionally 1:(0.5-8), and further optionally 1:(1-5).

[0103] By adjusting the mass ratio of the doping element source to graphene oxide, doping element-modified graphene with different doping element contents can be obtained.

[0104] In some embodiments, the reaction temperature of the solvothermal reduction reaction may be 150°C-170°C.

[0105] In some embodiments, step S200 includes: dispersing the nano-scale phosphate active material and the doped element-modified graphene in a solvent, and ultrasonically treating the solvent at a power of 100W-300W for 2h-12h to obtain a positive electrode material.

[0106] In some embodiments, the ultrasonic power may be 100W-300W, optionally 120W-280W, further optionally 150W-250W.

[0107] In some embodiments, the ultrasonic treatment time may be 2 h to 12 h, optionally 5 h to 10 h.

[0108] In some embodiments, the solvent can include tetrahydrofuran.

[0109] In some embodiments, the mass ratio of the nano-scale phosphate active material to the doping element-modified graphene can be (3-19):1, and can be optionally (4-14):1.

[0110] By regulating the mass ratio of the phosphate active material and the doped element-modified graphene within the above range, the loading amount and distribution of the phosphate active material on the surface of the doped element-modified graphene can be adjusted to obtain a positive electrode material with good electrochemical performance and cycle stability.

[0111] Positive electrode

[0112] An embodiment of the third aspect of the present application provides a positive electrode plate, which includes the positive electrode material of the embodiment of the first aspect of the present application or the positive electrode material obtained according to the preparation method of the embodiment of the second aspect of the present application.

[0113] In some embodiments, the positive electrode sheet includes a positive electrode current collector and a positive electrode material layer disposed on at least one side of the positive electrode current collector. For example, the positive electrode current collector has two opposing surfaces in its thickness direction, and the positive electrode material layer is disposed on either or both of the two opposing surfaces of the positive electrode current collector.

[0114] In some embodiments, the positive electrode material layer comprises 80%-98% by mass of the positive electrode material based on the total weight of the positive electrode material layer.

[0115] In some embodiments, the positive electrode current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector may be made by forming a metal material, such as aluminum, aluminum alloy, copper, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy, on a polymer material substrate. The polymer material substrate may include a substrate such as polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.

[0116] In some embodiments, the thickness of the positive electrode current collector is 4 μm-20 μm, optionally 6 μm-18 μm, and further optionally 8 μm-16 μm.

[0117] In some embodiments, the positive electrode material layer may further optionally include a binder. As an example, the binder may include one or more of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), a vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, a vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, a tetrafluoroethylene-hexafluoropropylene copolymer, and a fluorine-containing acrylate resin.

[0118] In some implementations, the weight percentage of the binder in the positive electrode material layer is greater than or equal to 0.5%, which is conducive to obtaining good bonding performance.

[0119] In some embodiments, the positive electrode material layer further includes a conductive agent. As an example, the conductive agent may include one or more of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0120] In some embodiments, the positive electrode sheet can be prepared by the following method: the components for preparing the positive electrode sheet, such as the positive electrode active material, the conductive agent, the binder and any other components, are dispersed in a solvent (such as N-methylpyrrolidone) to form a positive electrode slurry; the positive electrode slurry is coated on the positive electrode collector, and after drying, cold pressing and other processes, the positive electrode sheet can be obtained.

[0121] secondary batteries

[0122] An embodiment of the fourth aspect of the present application provides a secondary battery, comprising the positive electrode sheet according to the embodiment of the third aspect of the present application.

[0123] Typically, a secondary battery consists of a positive electrode, a negative electrode, an electrolyte, and a separator. During the battery's charge and discharge processes, active ions are inserted and removed between the positive and negative electrodes. The electrolyte conducts ions between the positive and negative electrodes. The separator, located between the positive and negative electrodes, primarily prevents short circuits between the positive and negative electrodes while allowing ions to pass through.

[0124] [Positive electrode]

[0125] The positive electrode sheet includes a positive electrode current collector and a positive electrode material layer arranged on at least one surface of the positive electrode current collector, wherein the positive electrode material layer includes the positive electrode material of the first aspect of the present application.

[0126] [Negative electrode]

[0127] In some embodiments, the negative electrode sheet includes a negative electrode current collector and a negative electrode material layer disposed on at least one surface of the negative electrode current collector, wherein the negative electrode material layer includes a negative electrode active material.

[0128] Illustratively, the negative electrode current collector has two surfaces that face each other in its thickness direction, and the negative electrode material layer is disposed on either or both of the two facing surfaces of the negative electrode current collector. The negative electrode current collector can be made of a material such as metal foil, carbon-coated metal foil, or porous metal sheet, and copper foil is optionally used.

[0129] Illustratively, the negative electrode material layer includes a negative electrode active material, an optional conductive agent, and an optional binder. The conductive agent is used to improve the conductivity of the negative electrode material layer, and the binder is used to firmly bond the negative electrode active material and the binder to the negative electrode current collector. This application does not specifically limit the types of the conductive agent and binder, and they can be selected based on actual needs.

[0130] Illustratively, the negative electrode active material may include one or more of natural graphite, artificial graphite, mesophase microcarbon beads (MCMB), hard carbon, and soft carbon.

[0131] Exemplarily, the conductive agent may include one or more of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene and carbon nanofibers; the binder may include one or more of styrene-butadiene rubber (SBR), styrene-butadiene rubber (SBCs), water-based acrylic resin and carboxymethyl cellulose (CMC).

[0132] The negative electrode material layer may also optionally include a thickener, such as sodium carboxymethylcellulose (CMC).

[0133] [Electrolytes]

[0134] The electrolyte conducts ions between the positive and negative electrodes. This application does not specify the type of electrolyte, and the electrolyte can be selected based on the needs. For example, the electrolyte can be liquid, gel, or solid.

[0135] In some embodiments, the electrolyte is an electrolyte solution comprising an electrolyte salt and a solvent.

[0136] In some embodiments, the electrolyte may include an organic solvent and an electrolyte salt.

[0137] In some embodiments, the organic solvent may include at least one of ethylene carbonate, propylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methylpropyl carbonate, ethylpropyl carbonate, butylene carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, cyclopentane, dimethyl sulfone, methyl ethyl sulfone and diethyl sulfone.

[0138] In some embodiments, the electrolyte salt may include an anion, and the anion may include a bis(fluorosulfonyl)imide anion (FSI - ), bis(trifluoromethanesulfonyl)imide anion (TFSI - ), dioxalatoborate anion (BOB - ), difluorooxalatoborate anion (DFOB - ), difluorobis(oxaloyl)phosphate anion (DFOP - ), tetrafluorooxalophosphate anion (TFOP - ), difluorophosphate anion (PO2F2 - ), hexafluorophosphate anion (PF6 - ), tetrafluoroborate anion (BF4 - ), hexafluoroarsenate anion (AsF6 - ), trifluoromethanesulfonate anion (CF3SO3 - )

[0139] In some embodiments, the electrolyte salt may include cations, and the cations may include one or more of lithium ions and sodium ions.

[0140] In some embodiments, the concentration of the electrolyte salt may be greater than 0.3 mol / L, and may be greater than 0.7 mol / L. The concentration of the electrolyte salt may further be less than 4 mol / L, and may be less than 2.5 mol / L or less than 1.7 mol / L. When the concentration of the electrolyte salt is within the above range, the electrolyte solution can have suitable ionic conductivity.

[0141] In some embodiments, the electrolyte may further include additives. For example, the additives may include negative electrode film-forming additives, positive electrode film-forming additives, and additives that can improve certain battery properties, such as additives that improve battery overcharge performance, and additives that improve battery high or low temperature performance.

[0142] [Isolation film]

[0143] In some embodiments, the secondary battery further includes a separator. The present application has no particular limitation on the type of separator, and any known porous separator with good chemical and mechanical stability can be selected.

[0144] In some embodiments, the material of the separator may include at least one of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator may be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer may be the same or different, without particular limitation.

[0145] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator can be formed into an electrode assembly through a winding process or a lamination process.

[0146] In some embodiments, the secondary battery may include an outer packaging that can be used to encapsulate the electrode assembly and the electrolyte.

[0147] In some embodiments, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. Alternatively, the outer packaging of the secondary battery can be a soft shell, such as a pouch-type soft shell. The soft shell can be made of plastic, such as polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0148] The present application has no particular limitation on the shape of the secondary battery, which may be cylindrical, square, or any other shape. For example, FIG2 shows a secondary battery 5 having a square structure as an example.

[0149] In some embodiments, referring to Figure 3, the outer packaging may include a shell 51 and a top cover assembly 53. Among them, the shell 51 may include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate enclose a receiving cavity. The shell 51 has an opening connected to the receiving cavity, and the top cover assembly 53 can be covered on the opening to close the receiving cavity. The positive electrode sheet, the negative electrode sheet and the isolation membrane can form an electrode assembly 52 through a winding process or a lamination process. The electrode assembly 52 is encapsulated in the receiving cavity. The electrolyte is infiltrated in the electrode assembly 52. ​​The number of electrode assemblies 52 contained in the secondary battery 5 can be one or more, and those skilled in the art can select according to specific actual needs.

[0150] In some embodiments, secondary batteries can be assembled into a battery module. The number of secondary batteries contained in the battery module can be one or more. The specific number can be selected by those skilled in the art according to the application and capacity of the battery module.

[0151] Figure 4 shows an example battery module 4. Referring to Figure 4 , within the battery module 4, multiple secondary batteries 5 may be arranged sequentially along the length of the battery module 4. Of course, any other arrangement is also possible. Furthermore, the multiple secondary batteries 5 may be secured together using fasteners.

[0152] Optionally, the battery module 4 may further include a housing having a receiving space, and the plurality of secondary batteries 5 are received in the receiving space.

[0153] In some embodiments, the battery modules described above may also be assembled into a battery pack. The battery pack may contain one or more battery modules, and the specific number may be selected by those skilled in the art based on the application and capacity of the battery pack.

[0154] Figures 5 and 6 illustrate an example battery pack 1. Referring to Figures 5 and 6 , the battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box comprises an upper case 2 and a lower case 3. The upper case 2 can be positioned over the lower case 3 to form an enclosed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0155] Electrical devices

[0156] An embodiment of the fifth aspect of the present application provides an electric device, comprising at least one of the secondary battery, battery module, and battery pack provided in the present application. The secondary battery, battery module, or battery pack can be used as a power source for the electric device, or as an energy storage unit for the electric device. The electric device may include mobile devices (such as mobile phones, laptops, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc., but is not limited thereto.

[0157] As the electrical device, a secondary battery, a battery module or a battery pack can be selected according to its usage requirements.

[0158] Figure 7 shows an example of an electric device. This device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the high power and high energy density requirements of the secondary battery, a battery pack or battery module can be used.

[0159] Another example device may be a mobile phone, a tablet computer, a notebook computer, etc. Such a device is generally required to be lightweight and thin, and may use a secondary battery as a power source.

[0160] Example

[0161] Below, the embodiment of the present application is described. The embodiment described below is exemplary and is only used to explain the present application, and is not to be construed as limiting the present application. Where specific techniques or conditions are not specified in the embodiments, the techniques or conditions described in the literature in this area or the product specifications are used. Reagents or instruments used that do not specify the manufacturer are conventional products that can be obtained commercially.

[0162] Example 1

[0163] A positive electrode material is prepared by the following method:

[0164] Preparation of doped element-modified graphene

[0165] S10, dispersing graphene oxide in deionized water, and adjusting the concentration of the graphene oxide dispersion to 2 mg / mL;

[0166] S20, taking 25 mL of graphene oxide dispersion, adjusting the pH to 10 with 30% ammonia water, then adding 1.5 mL of hydrazine hydrate, and stirring the reaction for 10 minutes;

[0167] S30, transferring the reaction solution to a polytetrafluoroethylene reactor, reacting at 160° C. for 3 h, collecting the solid product by centrifugation, and then freeze-drying it for 48 h to obtain nitrogen-doped graphene nanosheets.

[0168] Preparation of phosphate active materials

[0169] S40, take 20 mL of ethylene glycol solution, add 4 mmol of FeSO4·7H2O and 4 mmol of H3PO4 respectively, and treat with ultrasound at 100 W power for 15 min. Then add 12 mmol of LiOH·H2O, stir thoroughly, transfer to the reactor, and keep at 180℃ for 10 h;

[0170] S50, filtering the reaction solution, washing with deionized water and anhydrous ethanol in sequence, and freeze-drying in vacuo for 24 hours to obtain lithium iron phosphate nanoneedles;

[0171] Preparation of positive electrode materials

[0172] S60, lithium iron phosphate nanoneedles and nitrogen-doped graphene nanosheets were dispersed in tetrahydrofuran at a mass ratio of 9:1, ultrasonically treated at a power of 160 W for 10 h, and then the solid product was collected by filtration and vacuum dried at 40 ° C for 12 h to obtain the positive electrode material.

[0173] Examples 2 to 14

[0174] The preparation method of the positive electrode material is consistent with that of Example 1, except that the raw material ratio of the positive electrode material is adjusted, as shown in Table 1.

[0175] Example 15

[0176] The difference from Example 1 is that the phosphate active material is lithium iron phosphate nanorods, and the specific preparation method is:

[0177] S40, take 20 mL of ethylene glycol solution, add 4 mmol of FeSO4·7H2O and 4 mmol of H3PO4 respectively, and treat with ultrasound at 100 W power for 15 min. Then add 10 mmol of LiOH·H2O, stir thoroughly, transfer to the reactor, and keep at 180℃ for 10 h;

[0178] S50, filtering the reaction solution, washing with deionized water and anhydrous ethanol in sequence, and freeze-drying in vacuo for 24 hours to obtain lithium iron phosphate nanorods.

[0179] Example 16

[0180] The difference from Example 1 is that the phosphate active material is lithium iron phosphate nanoparticles, and the specific preparation method is as follows:

[0181] S40, take 20 mL of ethylene glycol solution, add 4 mmol of LiOH·H2O and 4 mmol of H3PO4 respectively, and treat with ultrasound at 100 W power for 15 min. Then add 12 mmol of FeSO4·7H2O, stir thoroughly, transfer to the reactor, and keep at 180℃ for 10 h;

[0182] S50, filtering the reaction solution, washing with deionized water and anhydrous ethanol in sequence, and freeze-drying in vacuo for 24 hours to obtain lithium iron phosphate nanoparticles.

[0183] Example 17

[0184] The difference from Example 1 is that the phosphate active material is lithium iron phosphate nanocrystals, and the specific preparation method is as follows:

[0185] S40, take 20 mL of ethylene glycol solution, add 4 mmol of LiOH·H2O and 4 mmol of H3PO4 respectively, and treat with ultrasound at 100 W power for 15 min. Then add 10 mmol of FeSO4·7H2O, stir thoroughly, transfer to the reactor, and keep at 180℃ for 10 h;

[0186] S50, filtering the reaction solution, washing with deionized water and anhydrous ethanol in sequence, and freeze-drying in vacuum for 24 hours to obtain lithium iron phosphate nanocrystals.

[0187] Comparative Examples 1 to 3

[0188] The preparation method of the positive electrode material is consistent with that of Example 1, except that the raw material ratio of the positive electrode material is adjusted, as shown in Table 1.

[0189] Comparative Example 4

[0190] The positive electrode material was prepared by in-situ synthesis. The preparation method of nitrogen-doped graphene was the same as that in Example 1. The preparation method of the positive electrode material was as follows:

[0191] 20 mg of nitrogen-doped graphene nanosheets were ultrasonically dispersed in 100 mL of 0.3 mol / L LiOH·H2O aqueous solution. 100 mL of 0.1 mol / L H3PO4 was added under vigorous stirring, followed by 100 mL of 0.1 mol / L FeSO4·7H2O aqueous solution and 0.1 mol glucose. After thorough stirring, the mixture was transferred to a reactor, kept at 150°C for 10 h, cooled to room temperature, centrifuged, washed with deionized water, and vacuum dried to obtain lithium iron phosphate nitrogen-doped graphene composite materials.

[0192] Test section

[0193] The electronic conductivity of the positive electrode material is tested by the following method:

[0194] The test was carried out using the four-probe method with reference to GB / T 30835-2014 Carbon composite lithium iron phosphate cathode material for lithium-ion batteries.

[0195] The positive electrode material is used to prepare a secondary battery for cycle performance testing. The specific method is as follows:

[0196] Preparation of positive electrode sheet

[0197] The positive electrode material, binder polyvinylidene fluoride, and conductive agent Super P were mixed in a mass ratio of 80:10:10, and an appropriate amount of solvent N-methylpyrrolidone (NMP) was added and stirred evenly to obtain a positive electrode slurry. The positive electrode slurry was coated on both surfaces of the positive electrode current collector aluminum foil, dried, and cold pressed to form a positive electrode film layer, thereby obtaining a positive electrode sheet.

[0198] Preparation of negative electrode sheet

[0199] The negative electrode active material graphite, conductive agent Super P, thickener carboxymethyl cellulose (CMC), and binder styrene butadiene rubber (SBR) are mixed evenly in deionized water at a mass ratio of 80:15:3:2 to form a negative electrode slurry, and the solid content in the negative electrode slurry is 30wt%; the negative electrode slurry is evenly coated on the current collector copper foil and dried at 85°C, and then cold pressed, trimmed, cut into pieces, and slit, and then dried under vacuum conditions at 120°C for 12 hours to form a negative electrode sheet.

[0200] Preparation of isolation membrane

[0201] A 16 μm polyethylene film was used as the separator.

[0202] Preparation of secondary batteries

[0203] The positive electrode sheet, separator, and negative electrode sheet are stacked in order, with the separator positioned between the positive and negative electrode sheets. The electrode assembly is wound and the tabs are welded. The electrode assembly is placed in a housing and the electrolyte is injected. The battery is then packaged, allowed to stand, formed, shaped, and tested for capacity to complete the secondary battery. The electrolyte salt is LiFSI at a concentration of 1 mol / L, and the solvent is ethylene glycol dimethyl ether (DME).

[0204] The cycle performance of secondary batteries is tested by the following methods:

[0205] At 25°C, charge the secondary battery at a constant current of 1C to the cutoff voltage. Let it rest for 5 minutes, then discharge it at a constant current of 1C to 2.0V. Record the capacity at this point as the initial capacity, C0. Repeat step 1 above for the secondary battery, and record the discharge capacity, Cn, after 100 cycles. The capacity retention, Pn, = Cn / C0*100%. The cutoff voltage for constant current charging of secondary batteries assembled with lithium iron phosphate positive electrode materials is 3.65V, while that for secondary batteries assembled with other phosphate active materials is 4.1V.

[0206] The performance test data are detailed in Table 1.

[0207] It can be seen from the data in Table 1 that Example 1 and Comparative Example 4 respectively adopt different methods to prepare the positive electrode materials. Compared with the in situ synthesis preparation method in Comparative Example 1, the positive electrode material prepared by the distributed synthesis method in Example 1 has higher electronic conductivity, and the cycle performance of the assembled secondary battery is better.

[0208] Example 1 and Comparative Example 3 respectively use phosphate active materials with different particle sizes. It can be seen that the positive electrode material made of nano-scale phosphate active materials has higher electronic conductivity.

[0209] It can be seen from the data of Examples 1 to 4 and Comparative Examples 1 to 2 that the modified graphene by doping elements can significantly improve the electronic conductivity of the positive electrode material. By adjusting the ratio of phosphate active materials and modified graphene by doping elements in the positive electrode material, the conductivity of the positive electrode material and the cycle performance of the secondary battery can be further improved.

[0210] Although illustrative embodiments have been shown and described, those skilled in the art should understand that the above embodiments should not be construed as limitations on the present application, and that changes, substitutions, and modifications may be made to the embodiments without departing from the spirit, principles, and scope of the present application.

Claims

1. A positive electrode material comprising a conductive base material and an active material distributed on the conductive base material; The active material includes a nano-scale phosphate active material, and the conductive matrix material includes graphene modified by doping elements; Based on the total weight of the positive electrode material, the weight content of the active material is 75%-95%, and the weight content of the conductive matrix material is 5%-25%.

2. The positive electrode material according to claim 1, wherein Based on the total weight of the positive electrode material, the weight content of the active material is 80%-90%, and the weight content of the conductive matrix material is 10%-20%.

3. The positive electrode material according to claim 1 or 2, wherein The phosphate active material has the following chemical formula: Li x M y PO4, M includes one or more of Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Fe, Ni, Co, Ga, Sn, Sb, Nb, Ge, Mn, Cu, Cr, 0.9≤x≤1.1, 0.9≤y≤1.

1.

4. The positive electrode material according to any one of claims 1 to 3, wherein The volume distribution particle size Dv50 of the phosphate active material is 8 nm to 300 nm, and the volume distribution particle size Dv90 is 20 nm to 400 nm; and / or The specific surface area of ​​the phosphate active material is 30 m 2 / g-150m 2 / g.

5. The positive electrode material according to claim 4, wherein The volume particle size distribution Dv50 of the phosphate active material is 20nm-200nm, and the volume distribution particle size Dv90 is 50nm-350nm; and / or The specific surface area of ​​the phosphate active material is 50 m 2 / g-120m 2 / g.

6. The positive electrode material according to any one of claims 1 to 5, wherein The doping element includes N, S or a combination thereof.

7. The positive electrode material according to any one of claims 1 to 6, wherein The doped element-modified graphene includes a doped element-modified graphene nanosheet, and the thickness of the doped element-modified graphene nanosheet is 2nm-20nm; and / or Based on the total weight of the doping element-modified graphene, the weight content of the doping element is 5%-15%.

8. A method for preparing the positive electrode material according to any one of claims 1 to 7, comprising the following steps: Provide nano-scale phosphate active materials and doped element modified graphene respectively; The nano-scale phosphate active material and the doped element-modified graphene are dispersed in a solvent and ultrasonically mixed to obtain the positive electrode material.

9. The method for preparing the positive electrode material according to claim 8, wherein: The method of providing the nanoscale phosphate active material comprises: performing a hydrothermal reaction on a lithium source, a phosphorus source, and an M source to obtain the phosphate active material, wherein M comprises one or more of Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Fe, Ni, Co, Ga, Sn, Sb, Nb, Ge, Mn, Cu, and Cr.

10. The method for preparing a cathode material according to claim 9, wherein: The lithium source includes one or more of lithium hydroxide, lithium oxide, lithium oxalate, lithium phosphate, lithium carbonate, lithium dihydrogen phosphate, and dilithium hydrogen phosphate; and / or The phosphorus source includes one or more of ammonium dihydrogen phosphate, ammonium phosphate, sodium pyrophosphate, ammonium hypophosphite, ammonium polyphosphate, ammonium hexafluorophosphate, diammonium hydrogen phosphate, sodium phosphite, sodium metaphosphate, sodium tripolyphosphate, and sodium hexafluorophosphate; and / or The M source includes one or more compounds containing the M element.

11. The method for preparing a cathode material according to claim 8, wherein: The providing of doping element-modified graphene includes: taking graphene oxide, a reducing agent and a doping element source to perform a solvent thermal reduction reaction to obtain doping element-modified graphene, wherein the doping element source includes a nitrogen source, a sulfur source or a combination thereof.

12. The method for preparing a cathode material according to claim 11, wherein: The reducing agent includes one or more of hydrazine hydrate, sodium borohydride, and pure hydrazine; and / or The nitrogen source includes one or more of ammonia, ethylenediamine, urea, hydrazine hydrate, acetonitrile, and pyrrole; and / or The sulfur source includes one or more of elemental sulfur, hydrogen sulfide, and thiourea.

13. The method for preparing a cathode material according to claim 11, wherein: The mass ratio of the reducing agent to the graphene oxide is 1:(0.01-0.5); and / or The mass ratio of the doping element source to the graphene oxide is 1:(0.1-10).

14. The method for preparing a cathode material according to any one of claims 8 to 13, wherein: The step of dispersing the nano-scale phosphate active material and the doped element-modified graphene in a solvent and performing ultrasonic mixing to obtain the positive electrode material comprises: dispersing the nano-scale phosphate active material and the doped element-modified graphene in a solvent, and performing ultrasonic treatment at a power of 100W-300W for 2h-12h to obtain the positive electrode material.

15. The method for preparing a cathode material according to claim 14, wherein: The mass ratio of nano-scale phosphate active materials to doped element modified graphene is (3-19):

1.

16. A positive electrode sheet, comprising the positive electrode material according to any one of claims 1 to 7 or the positive electrode material obtained by the preparation method according to any one of claims 8 to 15. 17 . A secondary battery comprising the positive electrode sheet according to claim 16 .

18. An electric device comprising the secondary battery according to claim 17.

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