Positive electrode material, electrochemical device and preparation method of positive electrode material

Abstract

The invention provides a positive electrode material, an electrochemical device and a preparation method of the positive electrode material, and belongs to the technical field of secondary batteries, and the positive electrode material comprises a substrate and a coating layer. Wherein the matrix comprises an inner core and a shell layer, the inner core comprises a lithium iron manganese phosphate material, and the shell layer comprises a first MXene material; the surface of the matrix is coated with the coating layer, and the coating layer comprises a lithium iron phosphate material. According to the positive electrode material provided by the invention, the first MXene material and the lithium iron phosphate material particles are sequentially coated on the outer side of the lithium iron manganese phosphate core, so that the conductivity of the lithium iron manganese phosphate material is improved while dissolution of manganese ions in the lithium iron manganese phosphate material is inhibited by coating, and the capacity of the positive electrode material is ensured not to be attenuated due to coating.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of secondary batteries, in particular to a positive electrode material, an electrochemical device and a preparation method of the positive electrode material. BACKGROUND

[0002] Lithium ion batteries, as a kind of efficient energy storage devices, have been widely used in portable electronic digital products and electric vehicle fields, and play an increasingly important role in today's society.

[0003] The positive electrode material is one of the key materials that determine the performance of lithium ion batteries, and directly affects the energy density, cycle life, rate performance and safety performance of the battery. Lithium manganese iron phosphate as a battery positive electrode material has the remarkable advantages of high energy density, stability and safety, cost-effective and green environmental protection, and has received extensive attention in recent years. However, the lithium manganese iron phosphate material as a positive electrode material has the phenomenon of manganese ion dissolution during the battery cycle process. The dissolved manganese ions in the lithium manganese iron phosphate material into the electrolyte cause the structure of the lithium manganese iron phosphate material to collapse, resulting in capacity attenuation and conductivity reduction of the positive electrode material.

[0004] At present, in order to improve the manganese ion dissolution problem of the lithium manganese iron phosphate positive electrode material, the lithium manganese iron phosphate material is usually coated and modified. However, the conventional coating modification not only causes the capacity of the lithium manganese iron phosphate material to decrease, but also easily makes the already low ionic conductivity of the lithium manganese iron phosphate material further decrease.

[0005] Therefore, it is necessary to design a positive electrode material, an electrochemical device and a preparation method of the positive electrode material to solve the above problems. SUMMARY

[0006] In view of the above shortcomings of the prior art, the present application provides a positive electrode material, an electrochemical device and a preparation method of the positive electrode material, to solve the technical problems of capacity attenuation and conductivity reduction of the lithium manganese iron phosphate material after conventional coating modification in the prior art.

[0007] To achieve the above object and other related objects, the present application provides a positive electrode material, which comprises a substrate and a substrate coating layer.

[0008] The substrate comprises a core and a shell, the core comprises a lithium manganese iron phosphate material, and the shell comprises a first MXene material; the substrate coating layer is coated on the surface of the substrate, and the substrate coating layer comprises a lithium iron phosphate material.

[0009] In an example of the present application, the substrate coating layer further comprises a carbon material; and the carbon material is coated on at least part of the surface of the lithium iron phosphate material particles.

[0010] In an example of the present application, the mass ratio of the substrate to the substrate coating layer in the positive electrode material is (55-78):(22-45);

[0011] In an example of the present application, the mass ratio of the lithium iron manganese phosphate material to the first MXene material in the substrate is (87.6-93.2):(6.8-12.4);

[0012] In an example of the present application, the mass ratio of the lithium iron phosphate material to the carbon material in the substrate coating layer is (94.6-97.4):(2.6-5.6).

[0013] In an example of the present application, the substrate coating layer further comprises a second MXene material; in the substrate coating layer, the mass ratio of the lithium iron phosphate material, the carbon material, and the second MXene material is (88-94.2):(2.6-5.6):(3.2-6.4).

[0014] In an example of the present application, the substrate coating layer further comprises a second MXene material; in the substrate coating layer, the mass ratio of the lithium iron phosphate material and the second MXene material is (93.6-96.8):(3.2-6.4).

[0015] In an example of the present application, the D50 of the lithium iron manganese phosphate material particles is 0.4-0.8 μm, and the D90 is less than or equal to 4.8 μm.

[0016] The present application also provides a preparation method of the positive electrode material described in any of the above examples, which comprises:

[0017] The lithium iron manganese phosphate material is used as the inner core, and a shell layer is formed on the surface of the inner core to obtain a substrate; wherein the shell layer comprises a first MXene material;

[0018] A substrate coating layer raw material is provided, and the substrate is mixed and sintered with the substrate coating layer raw material under a protective atmosphere to form a substrate coating layer on the surface of the substrate, thereby obtaining a positive electrode material; wherein the substrate coating layer raw material comprises a lithium iron phosphate raw material.

[0019] In an example of the present application, the provision of the substrate coating layer raw material and the mixing and sintering of the substrate with the substrate coating layer raw material under a protective atmosphere comprises:

[0020] The substrate coating layer raw material is provided, and the substrate coating layer raw material comprises a mixture of a lithium iron phosphate raw material and a carbon source; after the substrate is mixed with the substrate coating layer raw material, the mixture is sintered under a protective atmosphere to obtain a positive electrode material; wherein the sintering temperature is 800-900°C, and the sintering time is 4-5 hours.

[0021] In an example of the present application, the step of providing the substrate coating layer raw material, mixing the substrate with the substrate coating layer raw material and sintering under a protective atmosphere comprises:

[0022] The substrate coating layer raw material comprises a mixture of lithium iron phosphate raw material, a carbon source and a second MXene material; after mixing the substrate with the substrate coating layer raw material, sintering under a protective atmosphere to obtain a positive electrode material; wherein the sintering temperature is 800-900 DEG C, and the sintering time is 4-5 hours.

[0023] In an example of the present application, the lithium iron phosphate raw material is a lithium iron phosphate material.

[0024] In an example of the present application, the lithium iron phosphate raw material comprises a lithium source, a phosphorus source and an iron source.

[0025] The present application also provides an electrochemical device, which comprises a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte, wherein the positive electrode sheet comprises a positive electrode current collector and a positive electrode active material layer, and the positive electrode active material layer comprises the positive electrode material of any one of the examples described above or the positive electrode material prepared by the preparation method of any one of the examples described above.

[0026] The present application provides a positive electrode material, which is prepared by coating a first MXene material and a lithium iron phosphate material particle on the outer side of a manganese iron phosphate lithium phosphate core in sequence, so as to inhibit the dissolution of manganese ions in the manganese iron phosphate lithium phosphate material while improving the electrical conductivity of the manganese iron phosphate lithium phosphate material and ensuring that the capacity of the positive electrode material will not be attenuated due to coating. In summary, the manganese iron phosphate lithium phosphate positive electrode material can significantly improve the material structure stability and kinetic performance without sacrificing the charge and discharge capacity, so that the positive electrode material can have good charge and discharge capacity, rate performance and cycle performance. BRIEF DESCRIPTION OF DRAWINGS

[0027] In order to more clearly illustrate the technical solutions of the embodiments of the present application or the prior art, the drawings needed in the embodiments or prior art description will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present application, and those skilled in the art can also obtain other embodiments according to these drawings without creative labor.

[0028] Figure 1 It is a scanning electron microscope test diagram of the positive electrode material in an embodiment of the present application;

[0029] Figure 2 It is a flowchart of the preparation method of the positive electrode material in an embodiment of the present application;

[0030] Figure 3 Flowchart for step S2 in one embodiment of the present application;

[0031] Figure 4 Flowchart for step S2 in another embodiment of the present application. DETAILED DESCRIPTION

[0032] The present application is herein described, by way of example only, with reference to certain embodiments thereof. It is to be understood that variations and modifications of the embodiments can be made while still remaining within the spirit and scope of the present application. The embodiments and features of the present application as described herein can be combined with one another as suitable to form further embodiments of the present application. It is also to be understood that the terminology used herein is for the purpose of describing the particular embodiments only and is not intended to limit the scope of the present application. Unless defined otherwise, all technical and scientific terms and any acronyms used herein have the same meanings as commonly understood by one of ordinary skill in the art to which this application belongs. Unless otherwise indicated, the methods and techniques of the present application can be carried out by any conventional method known to one of skill in the art. The practice of the present application will employ, unless otherwise indicated, conventional methods of chemistry, biochemistry, molecular biology, microbiology, recombinant DNA techniques, cell biology, and immunology, all within the skill of the art.

[0033] It should be understood that the use of relational terms such as "first", "second", "left", "right", "up", "down", "top" and "bottom" are used only to simplify the descriptions and are not intended to limit the scope of the application in any way. The terms "comprise", "comprising", "include", "including", "contain", "containing" or any other similar phrase are used in the above description and claims to mean "including, but not limited to", unless otherwise stated.

[0034] The positive electrode material is one of the key materials that determine the performance of the secondary battery, and directly affects the energy density, cycle life, rate performance and safety performance of the battery. In recent years, olivine-type lithium manganese iron phosphate material has been widely concerned due to its high energy density, low cost, environmental friendliness, safety and stability. However, when lithium manganese iron phosphate material is used as a positive electrode material, manganese ions are dissolved into the electrolyte during the charging and discharging process due to lattice distortion and side reactions, etc. On the one hand, the dissolved manganese ions can cause the structure of the positive electrode material to collapse, resulting in the attenuation of the charge and discharge capacity of the positive electrode material and the reduction of the conductivity of the positive electrode material. On the other hand, the dissolved manganese ions in the electrolyte increase the internal resistance of the battery, reduce the charging and discharging efficiency of the battery, and can form deposits on the negative electrode of the battery during long-term cycling, thereby destroying the SEI film (Solid Electrolyte Interface) on the surface of the negative electrode, consuming a large amount of active lithium for the continuous regeneration and repair of the SEI film, and further affecting the capacity and cycle performance of the battery.

[0035] Currently, in order to improve the manganese ion dissolution problem of the lithium manganese iron phosphate positive electrode material, the lithium manganese iron phosphate material is usually coated and modified. However, although the conventional coating modification can inhibit the dissolution of manganese ions by isolating the lithium manganese iron phosphate material from the electrolyte, the conventional coating structure on the outside of the lithium manganese iron phosphate material particles, such as the organic or inorganic metal conductive carbon layer coating (such as carbon layer coating), not only causes the capacity of the lithium manganese iron phosphate material to decrease, but also easily causes the already low ion conductivity of the lithium manganese iron phosphate material to further decrease, thereby affecting the charge and discharge capacity and cycle performance of the battery.

[0036] To solve the above problems, as shown in Figure 1 The present application provides a positive electrode material, which is coated with a first MXene material and a lithium iron phosphate material particle outside a lithium manganese iron phosphate core in sequence, thereby achieving significant improvement in the kinetic performance of the positive electrode material while maintaining the charge and discharge capacity of the positive electrode material.

[0037] The above positive electrode material includes a substrate and a substrate coating layer. The substrate includes a core and a shell, the core includes a lithium manganese iron phosphate material, the lithium manganese iron phosphate material is LiMn x Fe 1-x PO4, wherein 0 < x < 1; the shell is coated on the surface of the core, and the shell includes a first MXene material, the first MXene material is coated on the surface of the lithium manganese iron phosphate material particle in a laminated nanosheet morphology to form a shell structure outside the core. The substrate coating layer is coated on the surface of the substrate particle, and the substrate coating layer includes a lithium iron phosphate material, and the substrate coating layer is formed by accumulation of lithium iron phosphate material particles.

[0038] Since the MXene material is a transition metal carbide or nitride with a two-dimensional structure, the layered structure and abundant surface functional groups of the MXene material make it have excellent electrical conductivity and mechanical properties. The first MXene material in the shell can form a conductive network outside the core, thereby improving the electrical conductivity of the positive electrode material; and the functional groups (such as hydroxyl, oxyl and fluorine groups) on the surface of the first MXene material have strong affinity for manganese ions, which can capture and fix the manganese ions dissolved from the surface of the lithium manganese iron phosphate material through chemical adsorption, thereby effectively inhibiting the dissolution of manganese ions in the lithium manganese iron phosphate material, avoiding the collapse of the material structure due to the continuous dissolution of manganese ions in the positive electrode material, and preventing the charge and discharge capacity and electrical conductivity from decreasing during the charge and discharge process of the positive electrode material.

[0039] The lithium iron phosphate material particles as the active material in the substrate coating layer outside the substrate can not only improve the charge and discharge capacity of the positive electrode material, but also provide additional lithium ion deintercalation sites for the positive electrode material, improving the charge and discharge capacity and ion conductivity of the positive electrode material. Moreover, as positive electrode active materials with an olivine structure, the lithium iron phosphate material particles have better structural stability. Mixing the lithium iron phosphate material particles in the substrate coating layer can improve the structural stability of the positive electrode material while ensuring the discharge capacity, and improve the cycle and safety performance of the material.

[0040] In some embodiments, in the positive electrode material, the mass ratio of the substrate and the substrate coating layer is (55-78):(22-45); for example, the mass ratio of the substrate and the substrate coating layer can be 55:45, 60:40, 65:45, 70:30, 75:25, or 78:22.

[0041] In some embodiments, in the substrate, the mass ratio of the lithium iron manganese phosphate material and the first MXene material is (87.6-93.2):(6.8-12.4); for example, the mass ratio of the lithium iron manganese phosphate material and the first MXene material can be 87.6:12.4, 88:12, 89:11, 90:10, 91:9, 92:8, 93:7, or 93.2:6.8.

[0042] In some embodiments, the carbon material in the substrate coating layer is amorphous carbon, for example, hard carbon. The carbon material is coated on the surface of the lithium iron phosphate material particles, which can isolate the mutual contact between the lithium iron phosphate material particles in the substrate coating layer, thereby improving the coating effect of the substrate coating layer, such as avoiding the agglomeration of the lithium iron phosphate material particles in the substrate coating layer, and improving the conductivity of the substrate coating layer.

[0043] In some embodiments, in the substrate coating layer, the mass ratio of the lithium iron phosphate material and the carbon material is (94.6-97.4):(2.6-5.6); for example, the mass ratio of the lithium iron phosphate material and the carbon material can be 94.6:5.6, 95:5, 96:4, 97:3, or 97.4:2.6.

[0044] In some embodiments, the second MXene material is also included in the matrix coating layer, and the second MXene material is located between the lithium iron phosphate material particles, for example, the second MXene is mixed in the form of nanosheets in the matrix coating layer and is located between the lithium iron phosphate material particles. The second MXene material in the matrix coating layer can adsorb the manganese ions released from the matrix into the matrix coating layer, avoiding the manganese ions not adsorbed by the first MXene material in the shell of the matrix from dissolving into the electrolyte, thereby enhancing the inhibitory effect of the matrix coating layer on the dissolution of manganese ions in the positive electrode material. Moreover, the second MXene material mixed into the matrix coating layer as a high-conductivity material can further improve the conductivity of the positive electrode material.

[0045] In some embodiments, in the matrix coating layer, the mass ratio of the lithium iron phosphate material and the second MXene material is (93.6-96.8):(3.2-6.4), for example, the mass ratio of the lithium iron phosphate material and the second MXene material can be 93.6:6.4, 94:6, 94.5:5.5, 95:5, 95.5:4.5, 96:4, 96.5:3.5, or 96.8:3.2.

[0046] It should be noted that the first MXene material and the second MXene material described above can be selected from the same type of MXene material or different types of MXene material. The MXene material described above is MXene material M n+1 X n T y , n = 1-3; M is a transition metal element, such as Ti, Zr, V, Mo, etc., X is a C or N element, and T y is an end face group, usually -OH, -O, -F, and -Cl. The MXene material is obtained by acid corrosion and washing of a MAX phase powder material using a method well known to those skilled in the art, and the MXene material obtained by acid washing is overall negatively charged due to the negative ion groups carried by the end face; wherein the MAX phase material includes at least one of Ti3AlC2, Ti2AlC, Ti3AlCN, V2AlC, Nb2AlC, Nb4AlC3, and TiNbAlC.

[0047] In some embodiments, the first MXene material and the second MXene material are selected from at least one of Nb4C3T y , Nb2CT y , Ta4C3T y , Mo2CT y , Ti3C2T y , Ti3CNT y , V4C3T y , and V2CT y , wherein the end face group Ty any one or more combinations of -OH, -O, -F, and -Cl; that is, the first MXene material and the second MXene material can be any one of the above-listed material types, such as Nb4C3T y , Nb2CT y , Ta4C3T y , Mo2CT y , Ti3C2T y , Ti3CNT y , V4C3T y , or V2CT y , and so on; the first MXene material and the second MXene material can also be any two or more combinations of the above-listed material types, such as a combination of Nb4C3T y and Ti3C2T y , or a combination of V2CT y and Ti3C2T y , or a combination of Mo2CT y and Ti3C2T y , or a combination of V4C3T y and Nb2CT y , or a combination of Ta4C3T y and Ti3C2T y , or a combination of Ta4C3T y and Nb2CT y , or a combination of Ti3C2T y , Mo2CT y , and Nb2CT y , or a combination of Ta4C3T y , Mo2CT y , and Ti3C2T y , or a combination of Nb4C3T y , Ta4C3T y , Mo2CT y , and Ti3CNT y , or a combination of Nb2CT y , V2CT y , Ti3C2T y , and Ta4C3T y , and so on, which are not listed one by one here. In addition, when the first MXene material and the second MXene material are a combination of two or more materials, the ratio of each material in the combination is not limited. In other embodiments, the MXene material can also be a material type not listed above.

[0048] In addition, in some embodiments, in the positive electrode material, the structural form of the first MXene material and the second MXene material can be single-layer nanosheet morphology or few-layer nanosheet morphology, the few-layer nanosheet morphology refers to a nanomaterial with a number of layers of nanosheets less than 20, for example, can be 2 layers, 3 layers, 5 layers, 10 layers, 15 layers or 19 layers, etc.

[0049] In some embodiments, the matrix coating layer simultaneously comprises lithium iron phosphate material, carbon material and second MXene material. In the matrix coating layer, the carbon material is coated on the surface of the lithium iron phosphate material particles, and the second MXene material is located between the lithium iron phosphate material particles.

[0050] In some embodiments, in the matrix coating layer, the mass ratio of the lithium iron phosphate material, the carbon material and the second MXene material is (88-94.2):(2.6-5.6):(3.2-6.4). For example, the mass ratio of the lithium iron phosphate material, the carbon material and the second MXene material can be 88:5.6:6.4, 91.2:5.6:3.2, 91:2.6:6.4 or 94.2:2.6:3.2.

[0051] In some embodiments, the average particle size D50 of the lithium manganese iron phosphate material particles in the matrix is 0.4-0.8 μm, for example, the D50 of the lithium manganese iron phosphate material particles can be 0.4 μm, 0.45 μm, 0.5 μm, 0.55 μm, 0.6 μm, 0.65 μm, 0.7 μm, 0.75 μm or 0.8 μm; the D90 of the lithium manganese iron phosphate material particles in the matrix is less than or equal to 4.8 μm. It should be noted that in the present specification, the average particle size D50 can be defined as the particle size corresponding to 50% of the volume accumulation in the particle size distribution curve of the particles; D90 can be defined as the particle size corresponding to 90% of the volume accumulation in the particle size distribution curve of the particles. D50, D90 can be measured by, for example, a laser particle size analyzer (such as Malvern Mastersizer 3000).

[0052] See Figure 2 The present application also provides a preparation method of the positive electrode material of any one of the above embodiments, which comprises the following steps:

[0053] S1, taking lithium manganese iron phosphate material as the inner core, forming a shell layer on the surface of the inner core to obtain a matrix; wherein the shell layer comprises a first MXene material; the mass ratio of the lithium manganese iron phosphate material to the first MXene material is (87.6-93.2):(6.8-12.4);

[0054] S2, provide a substrate coating layer raw material, mix and sinter the substrate and the substrate coating layer raw material under a protective atmosphere, form a substrate coating layer on the surface of the substrate, and obtain a positive electrode material; wherein the substrate coating layer raw material comprises a lithium iron phosphate raw material.

[0055] Step S1 can use any coating process on the market to coat the first MXene material on the surface of the lithium manganese iron phosphate material particles. For example, in some embodiments, step S1 uses a ball milling process for coating, specifically including mixing and ball milling the lithium manganese iron phosphate material with the first MXene material to make the first MXene material adhere to the surface of the lithium manganese iron phosphate material particles, thereby forming a core-shell structure; wherein the mixing and ball milling time of the lithium manganese iron phosphate material and the first MXene material is 8 to 12 hours, for example, the ball milling time can be 8 hours, 9 hours, 10 hours or 12 hours.

[0056] As shown in some embodiments, step S2 includes the following steps: Figure 3

[0057] S211, provide a substrate coating layer raw material, the substrate coating layer raw material comprises a mixture of a lithium iron phosphate raw material and a carbon source; wherein the lithium iron phosphate raw material and the carbon source are matched in a ratio of (88-94.2):(2.6-5.6) according to the theoretical output mass ratio of the lithium iron phosphate material and the carbon material;

[0058] S212, mix the substrate and the substrate coating layer raw material, and then sinter under a protective atmosphere to obtain a positive electrode material; wherein the substrate and the substrate coating layer raw material are matched in a ratio of (55-78):(22-45) according to the theoretical output mass ratio of the substrate and the substrate coating layer.

[0059] After the sintering treatment of step S212, the lithium iron phosphate raw material reacts to form a lithium iron phosphate material, and the lithium iron phosphate material particles are stacked and adhered to the outside of the substrate particles to form a substrate coating layer; the carbon source is sintered to form a carbon material and is coated on the outside of the lithium iron phosphate material particles, thereby forming a conductive network between the lithium iron phosphate material particles while isolating the mutual contact of the lithium iron phosphate material particles, avoiding the agglomeration of the lithium iron phosphate material particles into large particle size particles during the sintering process, and thus the coating effect on the substrate is poor or even the coating fails.

[0060] As shown in some embodiments, step S2 includes the following steps: Figure 4

[0061] ​​S221, providing a substrate coating layer raw material, the substrate coating layer raw material comprising a mixture of lithium iron phosphate raw material, carbon source and second MXene material; wherein the lithium iron phosphate raw material, carbon source and second MXene material are proportioned according to the theoretical output mass ratio of lithium iron phosphate material, carbon material and second MXene material (88-94.2):(2.6-5.6):(3.2-6.4);

[0062] S222, after mixing the substrate with the substrate coating layer raw material, sintering under a protective atmosphere to make the lithium iron phosphate material particles stack and adhere to the outside of the substrate particles to form a substrate coating layer, obtaining a positive electrode material; wherein the substrate and the substrate coating layer raw material are proportioned according to the theoretical output mass ratio of the substrate and the substrate coating layer (55-78):(22-45).

[0063] After the sintering treatment of step S222, the lithium iron phosphate raw material reacts to form lithium iron phosphate material, and the lithium iron phosphate material particles stack and adhere to the outside of the substrate particles to form a substrate coating layer; the carbon source is sintered to form a carbon material and is coated on the outside of the lithium iron phosphate material particles, thereby forming a conductive network between the lithium iron phosphate material particles while isolating the mutual contact of the lithium iron phosphate material particles, avoiding the agglomeration of the lithium iron phosphate material particles into large particles during the sintering process, which would result in poor or even ineffective coating of the substrate; the second MXene material is mixed between the lithium iron phosphate material particles in the substrate coating layer to further improve the conductivity of the substrate coating layer, and also plays a role in adsorbing and fixing the dissolved manganese ions in the lithium manganese iron phosphate material, thereby enhancing the inhibition of the substrate coating layer on the dissolved manganese ions of the lithium manganese iron phosphate.

[0064] In the above embodiments, the protective gas during sintering in step S2 can be one or more of N2, Ar, Ne, He, and Kr.

[0065] In the above embodiments, steps S212 and S222 specifically include proportioning and mixing the substrate and the substrate coating layer raw material according to the theoretical output mass ratio of the substrate and the substrate coating layer (55-78):(22-45) to obtain a mixture; placing the uniformly mixed mixture in a furnace filled with a protective atmosphere, increasing the temperature in the furnace from room temperature to the sintering temperature at a rate of 5-10℃ / min, and then maintaining the sintering temperature for a sintering time to obtain a positive electrode material. The sintering temperature is 800-900℃, for example, it can be 800℃, 820℃, 840℃, 850℃, 860℃, 880℃ or 900℃; the sintering time is 4-5 hours, for example, it can be 4 hours, 4.5 hours or 5 hours.

[0066] In some embodiments, the MXene material used in the first MXene material and the second MXene material in steps S1 and S2 is selected from at least one of Nb4C3T y , Nb2CT y , Ta4C3T y , Mo2CT y , Ti3C2T y , Ti3CNT y , V4C3T y , and V2CT y , wherein the end face group T y is selected from any one or a combination of more than one of -OH, -O, -F, and -Cl. For example, the first MXene material and the second MXene material are selected from Ti3C2T y , Ti3CNT y , or Mo2CT y .

[0067] In some embodiments, the carbon source used in step S2 can be selected from at least one of graphite, sucrose, glucose, fructose, maltose, lactose, starch, formaldehyde, acetaldehyde, propyl aldehyde, phenol formaldehyde resin, epoxy resin, polyethylene glycol, cellulose, lignin, polyvinyl alcohol, polyvinyl chloride, polyethylene oxide, polyurethane, polyfurfural, citric acid, and cyclodextrin; optionally, the carbon source can be at least one of graphite, starch, fructose, glucose, and polyvinyl alcohol. The above-mentioned carbon source contains small organic molecules or polymers with elements such as oxygen and hydrogen, and after sintering and carbonization, a porous carbon layer can be formed on the surface of the lithium iron phosphate particles.

[0068] In some embodiments, the lithium iron phosphate raw material used in step S2 is a finished lithium iron phosphate material.

[0069] In some other embodiments, the lithium iron phosphate raw material used in step S2 includes a lithium source, a phosphorus source, and an iron source. In the lithium iron phosphate raw material, the lithium source, the phosphorus source, and the iron source are mixed in a molar ratio of Li:P:Fe of (1-1.2):1:1, and a small amount of lithium element component exceeding the stoichiometric ratio is used to compensate for the loss of Li during sintering.

[0070] In some embodiments, the lithium source includes, for example, a lithium-containing sulfate, nitrate, acetate, carbonate, oxalate, citrate, halide, hydroxide, or oxyhydroxide. Specifically, the lithium source can include at least one of Li2CO3, LiNO3, LiNO2, LiOH, LiOH H2O, Li2C2O4, LiH, LiF, LiCl, LiBr, LiI, CH3COOLi, Li2O, Li2SO4, CH3COOLi 2H2O, or Li3C6H5O7; alternatively, the lithium source can be selected from LiCl or Li2CO3.

[0071] In some embodiments, the iron source includes an iron-containing sulfate, nitrate, acetate, carbonate, oxalate, citrate, oxide, halide, hydroxide, or oxyhydroxide. Specifically, the iron source can be selected from at least one of FeCl2, Fe2O3, FeC2O4, Fe(NO3)2, Fe2(CO3)3, and FeSO4. Alternatively, the iron source is selected from Fe2O3 or Fe2(CO3)3.

[0072] In some embodiments, the phosphorus source is selected from at least one of H3PO4, FePO4, Fe3(PO4)2, Li3PO4, LiH2PO4, (NH4)2HPO4, NH4H2PO4, (NH4)3PO4. Alternatively, the phosphorus source is selected from Li3PO4 or (NH4)2HPO4.

[0073] The present application also provides an electrochemical device, which can be a liquid lithium-ion secondary battery. Taking the liquid lithium-ion secondary battery as an example, the electrochemical device includes a positive electrode sheet, a negative electrode sheet, a separator disposed between the positive electrode sheet and the negative electrode sheet, and an electrolyte. The positive electrode sheet includes a positive current collector and a positive active material layer disposed on the positive current collector, and the positive active material layer includes a conductive agent, a binder, and the positive electrode material described in any one of the above embodiments or prepared by the preparation method described in any one of the above embodiments. The negative electrode sheet includes a negative current collector and a negative active material layer disposed on the negative current collector, and the negative active material layer includes a negative electrode material, a conductive agent, a thickening agent, and a binder. The positive and negative electrode materials can store and release energy by deintercalating lithium ions, the electrolyte is a carrier for the transport of lithium ions between the positive and negative electrodes, and the separator is permeable to lithium ions but not conductive to prevent short circuiting between the positive and negative electrodes.

[0074] It should be noted that the positive electrode sheet, the negative electrode sheet, the separator, the electrolyte, and the assembly process of the electrochemical device can be prepared by conventional methods in the art. The preparation method of the electrochemical device taking the liquid lithium-ion secondary battery as an example is described below:

[0075] (1) Positive electrode tab preparation: mix the positive electrode material, conductive agent, and binder according to the weight ratio (90 to 99):(1 to 10):(1 to 10), which can be 98:1:1, add solvent N-methyl pyrrolidone (NMP), mix thoroughly, and obtain a positive electrode slurry. Stir the slurry under the action of a vacuum stirrer until the system is uniform and transparent, and obtain the positive electrode slurry. Uniformly coat the positive electrode slurry on the positive electrode current collector aluminum foil, transfer the positive electrode current collector aluminum foil to an oven after drying at room temperature, then cold-press and cut to obtain the positive electrode tab. The conductive agent can be at least one of carbon black (Super P), acetylene black, carbon nanotubes (CNT), graphene, and nanocarbon fibers (VGCF), for example, the conductive agent is SP and CNT, and the mass ratio of SP to CNT is 2:1. The binder can be at least one of polyvinylidene fluoride (PVDF) or polytetrafluoroethylene (PTFE), for example, the binder can be PVDF.

[0076] (2) Negative electrode tab preparation: mix the negative electrode material, conductive agent, thickening agent, and binder according to the mass ratio 96.6:0.4:1.2:1.8, add deionized water, adjust the solid content of the slurry to 55%, and then mix thoroughly under the action of a vacuum stirrer to obtain a negative electrode slurry. Uniformly coat the negative electrode slurry on both sides of the 8-μm negative electrode current collector copper foil. After drying at room temperature, transfer the negative electrode current collector copper foil to an oven for drying, and then perform cold-pressing, cutting, and other processes to obtain the negative electrode tab. The negative electrode material is selected from one or more of artificial graphite, natural graphite, soft carbon, hard carbon, pure silicon, silicon oxide compounds, and silicon carbon compounds. The conductive agent can be at least one of carbon black (Super P), acetylene black, carbon nanotubes (CNT), graphene, and nanocarbon fibers (VGCF). The binder is at least one of polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), and styrene butadiene rubber (SBR). The thickening agent is carboxymethyl cellulose, which can be sodium carboxymethyl cellulose (CMC-Na) or lithium carboxymethyl cellulose (CMC-Li).

[0077] (3) Electrolyte preparation: in an argon atmosphere glove box with a water content of <10 ppm, uniformly mix ethylene carbonate (DOL), propylene carbonate (DMC), ethyl acetate (EA), and vinylene carbonate (VC) according to the volume ratio 75:15:5:5 to obtain an organic solvent, and then dissolve the fully dried lithium salt LiPF6 in the mixed organic solvent to prepare an electrolyte with a concentration of 1 mol / L.

[0078] (4) Diaphragm preparation: The diaphragm is selected from a type commonly used in the art, for example, a PE porous membrane is selected as the diaphragm, the thickness of the diaphragm is 9 to 18 μm, the air permeability is 180 s / 100 mL to 380 s / 100 mL, and the porosity is 30% to 50%.

[0079] (5) Assemble the battery: The battery is assembled according to a conventional method, for example: after preparation, the negative electrode sheet, diaphragm, and positive electrode sheet are sequentially stacked in order and placed in an aluminum plastic film to obtain a dry battery (a soft-pack battery without liquid injection), and the dry battery is baked to remove water. The prepared electrolyte is injected into the dry battery to obtain a finished soft-pack lithium ion battery.

[0080] The technical solutions of the present application are described in detail below through several specific examples and comparative examples. Unless otherwise specified, the raw materials and reagents used in the following examples are commercially available or can be prepared by conventional methods in the art.

[0081] Example 1

[0082] The present embodiment provides a positive electrode material, which comprises a substrate and a substrate coating layer, and the mass ratio of the substrate to the substrate coating layer is 55:45; the substrate comprises a core and a shell layer, the core is LiMn 0.5 Fe 0.5 PO4, the shell layer comprises a first MXene material, and the mass ratio of the core to the shell layer is 87.6:12.4; the substrate coating layer comprises LiFePO4, a carbon material, and a second MXene material, and the mass ratio of LiFePO4, the carbon material, and the second MXene material is 91:2.6:6.4. The first MXene material and the second MXene material are Ti3C2, and the carbon material is hard carbon. The preparation method of the positive electrode material is as follows:

[0083] S1, LiMn 0.5 Fe 0.5 PO4 and the first MXene material are mixed in a mass ratio of 87.6:12.4 and ball milled for 10 hours to obtain a substrate;

[0084] S2, the lithium iron phosphate raw material, the carbon source and the second MXene material are proportioned according to the theoretical output mass ratio of the lithium iron phosphate material, the carbon material and the second MXene material of 91:2.6:6.4 to obtain a substrate coating layer raw material; the substrate and the substrate coating layer raw material are mixed according to the theoretical output mass ratio of the substrate and the substrate coating layer of 55:45, and after being uniformly mixed, are placed in a furnace filled with a nitrogen atmosphere, the temperature in the furnace is increased from room temperature to a sintering temperature of 800℃ at a rate of 10℃ / min, and the sintering temperature is maintained for 4 hours to obtain the positive electrode material; wherein the first MXene material and the second MXene material are Ti3C2; the carbon source is glucose; in the lithium iron phosphate raw material, the lithium source, the iron source and the phosphorus source are mixed according to the molar ratio of Li:Fe:P of 1.02:1:1, the lithium source is lithium carbonate, the iron source is iron oxide, and the phosphorus source is ammonium phosphate.

[0085] Example 2

[0086] In this embodiment, a positive electrode material of the same system as in Example 1 is prepared, which includes a substrate and a substrate coating layer, and the mass ratio of the substrate to the substrate coating layer is 78:22; the substrate includes a core and a shell layer, the core is LiMn 0.5 Fe 0.5 PO4, and the shell layer includes a first MXene material, and the mass ratio of the core to the shell layer is 93.2:6.8; the substrate coating layer includes LiFePO4, a carbon material and a second MXene material, and the mass ratio of LiFePO4, the carbon material and the second MXene material is 91.2:5.6:3.2. Wherein the first MXene material and the second MXene material are Ti3C2, and the carbon material is hard carbon. The difference between this embodiment and Example 1 is that in step S1, the LiMn 0.5 Fe 0.5 PO4 and the first MXene material are mixed according to the mass ratio of 93.2:6.8; in step S2, the lithium iron phosphate raw material, the carbon source and the second MXene material are proportioned according to the theoretical output mass ratio of the lithium iron phosphate material, the carbon material and the second MXene material of 91.2:5.6:3.2; and the substrate and the substrate coating layer raw material are mixed and sintered according to the theoretical output mass ratio of the substrate and the substrate coating layer of 78:22.

[0087] Example 3

[0088] In this embodiment, a positive electrode material of the same system as in Example 2 is prepared, which includes a substrate and a substrate coating layer, and the mass ratio of the substrate to the substrate coating layer is 78:22; the substrate includes a core and a shell layer, the core is LiMn 0.5 Fe 0.5LiFePO4, the shell layer comprises a first MXene material, and the substrate coating layer comprises LiFePO4, a carbon material and a second MXene material; wherein the first MXene material and the second MXene material are Ti3C2, and the carbon material is hard carbon. The difference between this embodiment and embodiment 2 is that in step S2, the sintering temperature of the substrate and the substrate coating layer raw material is 900 DEG C, and the sintering time is 5 hours.

[0089] Embodiment 4

[0090] This embodiment prepares a positive electrode material of the same system as that of embodiment 2, which comprises a substrate and a substrate coating layer; the substrate comprises a core and a shell layer, and the core is LiMn 0.5 Fe 0.5 PO4, the shell layer comprises a first MXene material, and the substrate coating layer comprises LiFePO4, a carbon material and a second MXene material; wherein the first MXene material and the second MXene material are Ti3C2, and the carbon material is hard carbon. The difference between this embodiment and embodiment 2 is that in step S2, the sintering temperature of the substrate and the substrate coating layer raw material is 900 DEG C, and the sintering time is 5 hours.

[0091] Embodiment 5

[0092] This embodiment prepares a positive electrode material of the same system as that of embodiment 2, which comprises a substrate and a substrate coating layer; the substrate comprises a core and a shell layer, and the core is LiMn 0.5 Fe 0.5 PO4, the shell layer comprises a first MXene material, and the substrate coating layer comprises LiFePO4 and a carbon material, without containing a second MXene material, and the mass ratio of LiFePO4 and the carbon material is 94.4:5.6; wherein the first MXene material is Ti3C2, and the carbon material is hard carbon. The difference between this embodiment and embodiment 2 is that in step S2, the substrate coating layer raw material provided only contains lithium iron phosphate raw material and a carbon source, and the lithium iron phosphate raw material and the carbon source are mixed in a ratio of 94.4:5.6 of the theoretical output mass ratio of lithium iron phosphate material and carbon material.

[0093] Embodiment 6

[0094] This embodiment prepares a positive electrode material of the same system as that of embodiment 4, which comprises a substrate and a substrate coating layer; the substrate comprises a core and a shell layer, and the core is LiMn 0.5 Fe 0.5LiMnFePO4, the shell layer comprises a first MXene material; the substrate coating layer comprises LiFePO4, a carbon material and a second MXene material; wherein the first MXene material and the second MXene material are Mo2C, and the carbon material is hard carbon. The difference between this embodiment and embodiment 4 is that Mo2C is used as the first MXene material and the second MXene material in steps S1 and S2.

[0095] Example 7

[0096] This embodiment prepares a positive electrode material of the same system as that of embodiment 4, which comprises a substrate and a substrate coating layer; the substrate comprises a core and a shell layer, and the core is LiMn 0.5 Fe 0.5 PO4, the shell layer comprises a first MXene material; the substrate coating layer comprises LiFePO4, a carbon material and a second MXene material; wherein the first MXene material and the second MXene material are Ti3CN, and the carbon material is hard carbon. The difference between this embodiment and embodiment 4 is that Ti3CN is used as the first MXene material and the second MXene material in steps S1 and S2.

[0097] Comparative Example 1

[0098] This comparative example provides a positive electrode material of the same system as that of embodiment 2, which comprises a substrate and a substrate coating layer; the positive electrode material does not perform coating treatment on the LiMnFePO4core of the substrate, and the substrate only comprises LiMn 0.5 Fe 0.5 PO4; the substrate coating layer comprises LiFePO4, a carbon material and a second MXene material; wherein the second MXene material is Ti3C2, and the carbon material is hard carbon. The difference between this comparative example and embodiment 2 is that the first MXene material is not coated outside the LiMn 0.5 Fe 0.5 PO4material particle in step S1.

[0099] Comparative Example 2

[0100] This comparative example provides a positive electrode material of the same system as that of embodiment 2, which only comprises the substrate with a core-shell structure in embodiment 2, and the substrate comprises a core and a shell layer, and the core is LiMn 0.5 Fe 0.5 PO4, the shell layer comprises a first MXene material, and the mass ratio of the core to the shell layer is 93.2:6.8; wherein the first MXene material is Ti3C2. The difference between this comparative example and embodiment 2 is that the preparation method of the positive electrode material does not perform step S2.

[0101] Comparative Example 3

[0102] The comparative example provides a positive electrode material, which is LiMn 0.5 Fe 0.5 PO4material.

[0103] Performance test:

[0104] The positive electrode materials prepared in Examples 1 to 7 and Comparative Examples 1 to 3 were respectively assembled into button half-cells (2025 button half-cells), and the positive electrode materials prepared in Examples 1 to 7 and Comparative Examples 1 to 3 were subjected to charge-discharge test and cycle capacity retention rate test by the button half-cells, and the test results are shown in Table 1, so as to verify the efficacy of the present application.

[0105] The preparation process of the button half-cell is as follows: the above-prepared positive electrode material, conductive agent Super P and binder polyvinylidene fluoride (PVDF) are mixed in a weight ratio of 98:1:1, a solvent N-methyl pyrrolidone (NMP) is added, and the mixture is fully stirred to obtain a positive electrode slurry; the positive electrode slurry is coated on a carbon aluminum foil, and a Φ14mm positive electrode sheet is prepared after drying; graphite is used as a negative electrode material, the negative electrode material, conductive Super P, thickening agent sodium carboxymethyl cellulose (CMC-Na) and binder styrene-butadiene rubber (SBR) are mixed in a mass ratio of 96.6:0.4:1.2:1.8, deionized water is added, the solid content of the slurry is adjusted to 55%, and then the negative electrode slurry is fully stirred and mixed under the action of a vacuum stirrer to obtain a negative electrode slurry; the negative electrode slurry is uniformly coated on a negative electrode current collector copper foil, and a Φ14mm negative electrode sheet is prepared after drying; a PP porous membrane with a thickness of 12μm is used as a separator, and the positive electrode shell, the positive electrode sheet, the separator, the negative electrode sheet and the negative electrode shell are sequentially assembled into a half-cell in a glove box. Among them, 25μL of electrolyte is added on both sides of the separator, and the electrolyte is fully soaked into the electrode sheet after being placed at room temperature for 24h, and then the subsequent battery test is carried out.

[0106] The button half-cell was subjected to charge-discharge test, so as to test the initial discharge capacity and cycle performance (1C / 1C cycle 1000cls capacity retention rate) of the positive electrode material, and the test method is as follows:

[0107] First, the half-cell was activated by charging and discharging at a current rate of 0.1C / 0.1C in a test voltage range of 2.8V (discharge cut-off voltage) to 4.3V (charge cut-off voltage) at room temperature for 3 cycles (the charge cut-off current rate was 0.05C); then, the half-cell was charged and discharged at a current rate of 1C / 1C in a test voltage range of 2.8V (discharge cut-off voltage) to 4.3V (charge cut-off voltage) at an ambient temperature of 45°C for 1000 cycles (the charge cut-off current rate was 0.05C), and the discharge capacity of the first cycle and the 1000th cycle of the half-cell at an ambient temperature of 45°C was recorded as the initial discharge capacity and the 1000th cycle discharge capacity, and the ratio of the 1000th cycle discharge capacity to the initial discharge capacity was calculated as the 1000th cycle capacity retention rate.

[0108] The material parameters of the positive electrode materials in Examples 1 to 7 and Comparative Examples 1 to 3 and the performance test results of the corresponding assembled half-cells are shown in Table 1.

[0109] Table 1: Material parameters of the positive electrode materials in Examples 1 to 7 and Comparative Examples 1 to 3.

[0110]

[0111]

[0112] In Table 1, the mass content of the first MXene is the mass percentage of the first MXene material relative to the substrate; the mass contents of lithium iron phosphate, carbon material and second MXene are the mass percentages of lithium iron phosphate, carbon material and second MXene material relative to the substrate coating layer, respectively.

[0113] Table 2: Performance test results of the positive electrode materials in Examples 1 to 7 and Comparative Examples 1 to 3.

[0114]

[0115] By comparing the test results of Examples 1 to 7, Comparative Example 1, Comparative Example 2 and Comparative Example 3, it can be seen that coating the MXene material on the surface of the lithium manganese iron phosphate material particles can inhibit the dissolution of manganese ions, improve the structural stability of the positive electrode material in the charging and discharging cycle, and increase the cycle capacity retention rate of the battery.

[0116] By comparing the test results of Examples 2, 5 and Comparative Example 1, it can be seen that in addition to coating the substrate coating layer composed of lithium iron phosphate particles on the outside of the lithium manganese iron phosphate material particles, further coating a layer of MXene material close to the surface of the lithium manganese iron phosphate material particles can more effectively inhibit the dissolution of manganese ions in the charging and discharging process of the positive electrode material, thereby effectively improving the initial discharge capacity and cycle capacity retention rate of the positive electrode material.

[0117] By comparing the test results of Example 5 and Comparative Example 2, it can be seen that, compared with the MXene material coating only on the outside of the lithium manganese phosphate material particles in Comparative Example 2, the lithium iron phosphate material continues to be coated outside the MXene material coating shell in Example 5, which can improve the coating effect, inhibit the dissolution of manganese ions, at the same time, effectively guarantee the specific capacity of the positive electrode material will not be attenuated due to coating by using the active material properties of lithium iron phosphate material, and further improve the initial discharge capacity and cycle capacity retention rate of the positive electrode material in battery application.

[0118] By comparing the test results of Example 2 and Example 5, it can be seen that the mixing of MXene material in the substrate coating layer of the positive electrode material can further improve the fixation of manganese ions dissolved from the lithium manganese phosphate material by the substrate coating layer, thereby further improving the structural stability of the positive electrode material in the charge-discharge cycle, and improving the discharge capacity and cycle capacity retention rate of the positive electrode material.

[0119] The present application provides a kind of positive electrode material, which is coated with first MXene material and lithium iron phosphate material particles outside lithium manganese phosphate core in turn, to inhibit the dissolution of manganese ions in lithium manganese phosphate material while improving the electrical conductivity of lithium manganese phosphate material, and guarantee the capacity of positive electrode material will not be attenuated due to coating. In summary, the lithium manganese phosphate positive electrode material can significantly improve the material structure stability and kinetic performance without sacrificing charge-discharge capacity, so that the positive electrode material can have better charge-discharge capacity, rate performance and cycle performance.

[0120] The above examples are only illustrative of the principles and effects of the present application, and are not intended to limit the present application. Any person skilled in the art can modify or change the above examples without departing from the spirit and scope of the present application. Therefore, all equivalent modifications or changes made by those skilled in the art without departing from the spirit and technical idea disclosed by the present application shall be covered by the claims of the present application.

Claims

1. A positive electrode material, characterized in that, include: The substrate includes a core and a shell, the core comprising lithium manganese iron phosphate material, and the shell comprising a first MXene material; A substrate coating layer, wherein the substrate coating layer covers the surface of the substrate, and the substrate coating layer comprises lithium iron phosphate material.

2. The cathode material according to claim 1, characterized in that, The matrix coating layer further includes carbon material; the carbon material coats at least a portion of the surface of the lithium iron phosphate material particles.

3. The cathode material according to claim 2, characterized in that, In the positive electrode material, the mass ratio of the substrate to the substrate coating layer is (55-78):(22-45); And / or, in the matrix, the mass ratio of the lithium manganese iron phosphate material to the first MXene material is (87.6-93.2):(6.8-12.4); And / or, in the matrix coating layer, the mass ratio of the lithium iron phosphate material to the carbon material is (94.6-97.4):(2.6-5.6).

4. The cathode material according to claim 2, characterized in that, The substrate coating layer further includes a second MXene material; in the substrate coating layer, the mass ratio of the lithium iron phosphate material, the carbon material and the second MXene material is (88-94.2):(2.6-5.6):(3.2-6.4).

5. The positive electrode material according to claim 1, characterized in that, The substrate coating layer further includes a second MXene material; in the substrate coating layer, the mass ratio of the lithium iron phosphate material to the second MXene material is (93.6-96.8):(3.2-6.4).

6. The cathode material according to claim 1, characterized in that, The D50 of the lithium manganese iron phosphate material particles is 0.4-0.8 μm, and the D90 is less than or equal to 4.8 μm.

7. A method for preparing a positive electrode material, characterized in that, include: A matrix is ​​obtained by forming a shell layer on the surface of a lithium manganese iron phosphate material as the core; wherein the shell layer includes a first MXene material. A substrate coating material is provided, and the substrate and the substrate coating material are mixed and sintered under a protective atmosphere to form a substrate coating on the surface of the substrate, thereby obtaining a positive electrode material; wherein, the substrate coating material includes lithium iron phosphate material.

8. The preparation method according to claim 7, characterized in that, The provision of the matrix coating material, and the mixing and sintering of the matrix and the matrix coating material under a protective atmosphere, include: The substrate coating material is provided, wherein the substrate coating material comprises a mixture of lithium iron phosphate material and carbon source; The substrate and the substrate coating material are mixed and sintered under a protective atmosphere to obtain a positive electrode material; wherein the sintering temperature is 800℃-900℃ and the sintering time is 4 to 5 hours.

9. The preparation method according to claim 7, characterized in that, The provision of the matrix coating material, and the mixing and sintering of the matrix and the matrix coating material under a protective atmosphere, includes: The substrate coating material is provided, and the substrate coating material includes a mixture of lithium iron phosphate material, carbon source and second MXene material; The substrate and the substrate coating material are mixed and sintered under a protective atmosphere to obtain a positive electrode material; wherein the sintering temperature is 800℃-900℃ and the sintering time is 4 to 5 hours.

10. An electrochemical device, characterized in that, It includes a positive electrode sheet, the positive electrode sheet having a positive electrode active material layer, the positive electrode active material layer comprising the positive electrode material according to any one of claims 1 to 6, or the positive electrode material prepared by the preparation method according to any one of claims 7 to 9.

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