Composite cathode material, preparation method thereof, cathode sheet and secondary battery

Abstract

The application provides a composite positive electrode material, a preparation method thereof, a positive electrode sheet and a secondary battery. The preparation method comprises the following steps: preparing a first precursor solution containing a sodium source, an iron source, a manganese source, a phosphorus source, a carbon source, a doping source and a solvent; performing first sanding treatment on the first precursor solution to obtain a second precursor solution; sequentially performing dispersion treatment and second sanding treatment on the second precursor solution to obtain a third precursor solution; performing drying treatment on the third precursor solution to obtain precursor particles; and performing calcination treatment on the precursor particles to obtain the composite positive electrode material. Through the first sanding treatment, the dispersion treatment and the second sanding treatment, the particle size of the solid particles in the precursor solution is synergistically optimized; meanwhile, based on the composite carbon source of the graphene quantum dot and the polydopamine composite carbon source, in-situ adsorption coating is realized, so that the purpose of improving the homogeneity of the precursor solution and the integrity of the carbon layer structure is achieved.

Description

Technical Field

[0001] This invention relates to the field of secondary batteries, and more specifically, to a composite cathode material and its preparation method, a cathode sheet, and a secondary battery. Background Technology

[0002] In recent years, Na4Fe3(PO4)2P2O7 (NFPP) has been widely studied due to its structural stability and low cost, but its low operating voltage (<3.1V) severely limits its energy density. A similar material, Na4Mn3(PO4)2P2O7 (NMPP), has a superior voltage plateau (3.8V), but suffers from Mn dissolution and Jahn-Teller (JT) distortion, leading to severe capacity decay. Meanwhile, the iron-manganese mixed phosphate solid solution Na4Fe... 1.5 Mn 1.5 (PO4)2P2O7 (NFMPP) effectively combines the advantages of NFPP and NMPP, exhibiting higher operating voltage, energy density and good stability, making it a promising candidate for the next generation of SIBs.

[0003] However, high-temperature calcination leads to the creation of Na vacancies in NFMPP, which are easily occupied by Mn ions with lower defect formation energy. The inherent Na / Mn antisite defects result in voltage hysteresis and capacity loss. Furthermore, high-spin Mn... 3+ The JT distortion further reduces Na ion mobility. Therefore, simultaneously mitigating the Na / Mn antisite defect and JT distortion in NFMPP to achieve high power and long lifetime remains a challenge. Meanwhile, the materials generally have poor electronic conductivity, requiring external carbon coating to improve charge transport efficiency. Currently, mainstream carbon coating methods are mostly physical mixing or carbon source co-precipitation, resulting in a loosely bonded coating layer that easily peels off during cycling, failing to effectively suppress particle aggregation and electrolyte erosion.

[0004] In terms of preparation processes, traditional solid-state methods suffer from defects such as uneven mixing, coarse particles, and uncontrollable morphology, making it difficult to meet the requirements of high-performance cathode materials for microstructure consistency. While wet synthesis can improve uniformity, problems such as agglomeration, sedimentation, or excessively wide particle size distribution in the precursor solution easily occur, directly affecting the crystallinity and secondary particle morphology of the material after subsequent heat treatment. Although spray drying can obtain spherical particles, if the precursor has poor dispersibility, hollow or broken spheres are easily formed, reducing compaction density and electrode processing performance. In addition, existing processes lack systematic research on the synergistic control of key parameters such as particle size distribution, slurry viscosity, and drying parameters.

[0005] Therefore, how to provide a sodium pyrophosphate manganese iron sodium composite cathode material with high energy density, excellent cycle stability and good rate performance, and significantly improve the energy density and cycle life of the sodium-ion battery by improving the uniformity of the precursor and the effectiveness of carbon coating and controlling the formation of crystal defects, is a technical problem that needs to be solved in this field. Summary of the Invention

[0006] The main objective of this invention is to provide a composite cathode material and its preparation method, cathode sheet, and secondary battery, in order to solve the problem that the sodium manganese pyrophosphate composite cathode material in the prior art is difficult to simultaneously possess high energy density, excellent cycle stability, and good rate performance.

[0007] To achieve the above objectives, a first aspect of the present invention provides a method for preparing a composite cathode material, comprising: step S1, preparing a first precursor solution comprising a sodium source, an iron source, a manganese source, a phosphorus source, a carbon source, a dopant source, and a solvent; the dopant source is selected from one or more of a vanadium source, a niobium source, and a titanium source, and the carbon source is a composite carbon source of graphene quantum dots and polydopamine; step S2, subjecting the first precursor solution to a first milling treatment to obtain a second precursor solution; in the second precursor solution, the particle size D50 of the solid particles is 0.5 μm to 1 μm; step S3, subjecting the second precursor solution to a dispersion treatment and a second milling treatment sequentially to obtain a third precursor solution; in the third precursor solution, the particle size D50 of the solid particles is 200 nm to 400 nm; step S4, subjecting the third precursor solution to a drying treatment to obtain precursor particles; and step S5, subjecting the precursor particles to a calcination treatment to obtain a composite cathode material.

[0008] Further, the amounts of iron source, manganese source, phosphorus source, sodium source, and dopant source added are in the stoichiometric ratio Fe:Mn:P:Na:M = (1.40~1.50):(1.40~1.50):(3.95~4.05):4.0:(0.05~0.1); and / or, based on the total weight of sodium source, iron source, manganese source, phosphorus source, carbon source, and dopant source being 100%, the amount of carbon source added is 7%~9%; and / or, the dopant source includes a first dopant source and a second dopant source; the first dopant source is a vanadium source, and the second dopant source is a niobium source and / or a titanium source; the molar ratio of the first dopant source to the second dopant source is (1~5):1; preferably, the doping amount of the second dopant source is 0.01mol%~0.02mol.

[0009] Furthermore, in the composite carbon source, the weight ratio of graphene quantum dots to polydopamine is 1:(3~5); and / or, the pH value of the first precursor solution is 7.5~9.0.

[0010] Further, the sodium source is selected from one or more of inorganic sodium salts, organic sodium salts, and sodium oxides; and / or, the iron source is selected from one or more of ferric phosphate, ferrous oxalate, ferric nitrate, and ferric sulfate; and / or, the manganese source is selected from one or more of manganese powder, manganese oxides, manganese carbonates, manganese oxalates, manganese acetates, and manganese nitrates; and / or, the phosphorus source is selected from one or more of sodium pyrophosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, phosphoric acid, triammonium phosphate, and sodium phosphate; and / or, the vanadium source is selected from one or more of orthovanadate, pyrovanadate, and metavanadate; and / Or, the niobium source is ammonium niobate and / or niobium pentoxide; and / or, the titanium source is selected from one or more of tetrabutyl titanate, titanium dioxide, and titanium sulfate; and / or, the solvent is water; preferably, the inorganic sodium salt is selected from at least one of trisodium phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium pyrophosphate, trisodium monohydrogen pyrophosphate, sodium dihydrogen pyrophosphate, sodium trihydrogen pyrophosphate, sodium carbonate, and sodium bicarbonate; and / or, the organic sodium salt is selected from at least one of sodium acetate, sodium oxalate, and sodium citrate; and / or, the sodium oxide is sodium oxide and / or sodium peroxide; preferably, the metavanadate is ammonium metavanadate; and the orthovanadate is ammonium vanadate.

[0011] Further, in step S2, the first sand milling process uses 0.3mm~0.4mm zirconia balls as abrasive, and the rotation speed of the first sand milling process is 2000rpm~3000rpm, and the time is 1h~2h; and / or, the viscosity of the second precursor solution is 300Pa·s~1000Pa·s.

[0012] Further, in step S3, the dispersion treatment is ultrasonic dispersion, and the ultrasonic power of ultrasonic dispersion is 200W~300W, and the ultrasonic time is 30min~60min; and / or, the second sand milling treatment uses 0.3mm~0.4mm zirconia balls as abrasive, and the rotation speed of the second sand milling treatment is 2000rpm~3000rpm, and the time is 30min~60min; and / or, the total solid content of the third precursor solution is 15%~40%.

[0013] Furthermore, in step S4, the drying process is spray drying, and the peristaltic pump used for spray drying has a rotation speed of 10 r / min to 35 r / min, an atomization pressure of 0.2 MPa to 0.5 MPa, an inlet air temperature of 180℃ to 260℃, and an outlet air temperature of 80℃ to 110℃.

[0014] Further, in step S5, the calcination treatment includes the following sequentially performed processes: a first stage with a holding temperature of 300℃~350℃ and a holding time of 2h~6h; and a second stage with a holding temperature of 600℃~660℃ and a holding time of 12h~24h; preferably, the heating rate of the first stage and the second stage is independently 2℃ / min~3℃ / min; preferably, the calcination treatment is carried out in a protective atmosphere; more preferably, the protective atmosphere is nitrogen and / or argon.

[0015] A second aspect of the present invention provides a composite cathode material prepared by the above-described method for preparing composite cathode materials; the composite cathode material comprises sodium iron pyrophosphate particles and a carbon layer coated on the sodium iron pyrophosphate particles; the chemical formula of the sodium iron pyrophosphate particles is Na. m Fe n Mn p M q (PO4)2P2O7, wherein m is 3.95~4.05, n is 1.40~1.50, p is 1.40~1.50, q is 0.05~0.1, and M is selected from one or more of Ti, Nb and V.

[0016] Furthermore, based on the total weight of the composite cathode material (100%), the carbon layer accounts for 7% to 9% of the total weight; preferably, the chemical formula of sodium iron pyrophosphate is Na. m Fe n Mn p V q1 Ti q2 (PO4)2P2O7 or Na m Fe n Mn p V q1 Nb q3 (PO4)2P2O7, where q1+q2=q, q1+q3=q, q1 / q2=(1~5):1, q1 / q3=(1~5):1; more preferably, q2 and q3 are each independently 0.01~0.05.

[0017] A third aspect of the present invention provides a positive electrode sheet comprising the aforementioned composite positive electrode material.

[0018] A fourth aspect of the present invention provides a secondary battery including a positive electrode sheet, wherein the positive electrode sheet is the aforementioned positive electrode sheet.

[0019] By applying the technical solution of this invention, the particle size of solid particles in the precursor solution is synergistically optimized through a first milling process, a dispersion process, and a second milling process. At the same time, based on the composite carbon source of graphene quantum dots and polydopamine composite carbon source, in-situ adsorption and coating are achieved, thereby improving the homogeneity of the precursor solution and the integrity of the carbon layer structure. This results in the technical effects of high solid density, excellent electronic conductivity, and structural stability of the composite cathode material. Attached Figure Description

[0020] The accompanying drawings, which form part of this application, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings:

[0021] Figure 1 The scanning electron microscope (SEM) characterization results of the composite cathode material prepared in Example 2 of the present invention are shown.

[0022] Figure 2 The XRD characterization results of the composite cathode material prepared in Example 2 of the present invention are shown.

[0023] Figure 3 The charge-discharge curve of the composite cathode material prepared in Example 2 of the present invention at 0.1C is shown. Detailed Implementation

[0024] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. The present invention will now be described in detail with reference to the embodiments.

[0025] As described in the background section, in existing technologies, the Na / Mn antisite defects and JT distortion in NFMPPs lead to difficulties in simultaneously achieving high energy density, excellent cycle stability, and good rate performance in sodium manganese iron pyrophosphate composite cathode materials. While constructing transition metal (TM) vacancies in the cathode has proven to be an effective method to mitigate TMO6 deformation, the presence of excessive TM vacancies results in severe Na / Mn antisite defects in manganese-based phosphates. Furthermore, research indicates that dopants (Ti, Nb, Mg, etc.) with strong metal-oxygen (MO) bonds in NFMPPs can effectively stabilize the MnO6 octahedron, thereby modulating the Na / Mn antisite defects and JT distortion. Based on this, in order to solve the above-mentioned technical problems, the first aspect of the present invention provides a method for preparing a composite cathode material, comprising: step S1, preparing a first precursor solution containing a sodium source, an iron source, a manganese source, a phosphorus source, a carbon source, a dopant source, and a solvent; the dopant source is selected from one or more of vanadium source, niobium source, and titanium source, and the carbon source is a composite carbon source of graphene quantum dots and polydopamine; step S2, subjecting the first precursor solution to a first milling treatment to obtain a second precursor solution; in the second precursor solution, the particle size D50 of the solid particles is 0.5 μm to 1 μm; step S3, subjecting the second precursor solution to a dispersion treatment and a second milling treatment in sequence to obtain a third precursor solution; in the third precursor solution, the particle size D50 of the solid particles is 200 nm to 400 nm; step S4, subjecting the third precursor solution to a drying treatment to obtain precursor particles; step S5, subjecting the precursor particles to a calcination treatment to obtain a composite cathode material.

[0026] The preparation method provided by this invention utilizes a composite carbon source of graphene quantum dots and polydopamine in the precursor solution. Polydopamine, with its strong adhesion and active phenolic hydroxyl / amino groups, selectively adsorbs onto the surface of tiny inorganic cores formed by metal ions. Graphene quantum dots, as highly conductive nanoparticles, are uniformly anchored by the polydopamine network, forming core-shell organic-inorganic hybrid precursor units. After obtaining the first precursor solution, a first milling process breaks down incompletely dissolved or agglomerated metal source microcrystals in the initial mixture into submicron-sized dispersed particles, narrowing the size distribution of solid particles in the solution and preventing large particles from forming unevenly distributed hard agglomerates during subsequent drying. Subsequently, a dispersion treatment effectively breaks down secondary agglomerations caused by localized high shear forces during milling (such as soft agglomerations induced by van der Waals forces or hydrogen bonds), restoring the particles to a monodisperse state. The secondary milling process further homogenizes the particle size distribution through controlled mechanical force, ensuring that the D50 is stably within the 200nm~400nm range, while preventing excessive particle size breakage. Furthermore, a drying process transforms the highly uniform suspension with controllable particle size into precursor particles with spherical profiles. The resulting cathode material retains its complete particle morphology after calcination, exhibiting continuous and strongly adhered carbon layer distribution, clear interparticle interfaces, and no severe agglomeration.

[0027] In summary, the above preparation method achieves high dispersibility, precise particle size control, and structural stability of the precursor solution through the interfacial adsorption characteristics of the composite carbon source, primary crushing and dispersion via first milling, and secondary homogenization via second milling. This results in precursor particles with uniform morphology, dense carbon coating, and no severe agglomeration, ultimately yielding a composite cathode material with high structural consistency and good carbon network connectivity. The obtained composite cathode material exhibits superior electrochemical activity and cycle stability.

[0028] Furthermore, the preferred amounts of iron, manganese, phosphorus, sodium, and dopant sources are added according to the stoichiometric ratio Fe:Mn:P:Na:M = (1.40~1.50):(1.40~1.50):(3.95~4.05):4.0:(0.05~0.1); and / or, based on the total weight of sodium, iron, manganese, phosphorus, carbon, and dopant sources as 100%, the amount of carbon source added is 7%~9%. Under the above preferred feeding ratios, the dissolution behavior of each precursor component in the solvent tends to be synchronized, which is more conducive to the formation of a homogeneous metal-phosphate complex system and reduces the tendency of heterogeneous nucleation. When the carbon source content is 7%~9%, it can form a more continuously distributed organic phase in the solution, providing a more stable suspension system for subsequent sand milling and drying. This reduces the phenomenon of excessive internal pores or excessive pyrolysis gas escape while ensuring continuous carbon layer coverage after calcination, ultimately significantly improving the structural stability of the obtained composite cathode material.

[0029] In several typical implementations, the doping source includes a first doping source and a second doping source; the first doping source is a vanadium source, and the second doping source is a niobium source and / or a titanium source; the molar ratio of the first doping source to the second doping source is (1~5):1. As mentioned above, introducing dopant ions with MO bonds can effectively stabilize the MnO6 octahedron, thereby regulating the Na / Mn antisite defect and JT distortion. However, although single-element doping can alleviate the Na / Mn antisite defect and JT distortion to a certain extent, the effect is limited, and it is difficult to simultaneously achieve both ion migration efficiency and structural stability. In the above preferred scheme, the vanadium source can preferentially form a stable coordination with phosphate, while the niobium / titanium source can act as an auxiliary dopant to regulate the local metal-O bond length and bond energy, making the metal ion distribution in the precursor more uniform and further optimizing the electrochemical performance of the resulting composite cathode material. Based on the above, the doping amount of the second doping source is further optimized to be 0.01mol%~0.02mol%, thereby reducing the local stress accumulation or uneven precipitation caused by high concentration doping, and better achieving the technical effect of uniform distribution of multi-metal doped components in the precursor, no local segregation, and good system stability, thus obtaining a composite cathode material with better electrochemical activity.

[0030] Regarding the composite carbon source, graphene quantum dots, due to their negatively charged surface, can be electrostatically adsorbed and coated by the positively charged groups of polydopamine. To promote more effective and complete encapsulation of graphene quantum dots by polydopamine, forming more stable organic-inorganic hybrid nanounits, and thus obtaining a more continuous and complete carbon layer, further improving the electrochemical performance of the resulting composite cathode material, the preferred weight ratio of graphene quantum dots to polydopamine is 1:(3~5). Simultaneously, the preferred pH value of the first precursor solution is 7.5~9.0 to facilitate high solubility of metal ions and to make them more closely approximate the carbon components at the molecular scale. Furthermore, the aforementioned preferred pH range also promotes the slow oxidative polymerization of polydopamine, forming an adhesive thin layer rich in phenolic hydroxyl and amino groups. Finally, after calcination, a composite cathode material with a more continuous carbon layer and better electrochemical activity and cycle stability is obtained.

[0031] In practical applications, the sodium source is selected from one or more of inorganic sodium salts (specifically, at least one of trisodium phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium pyrophosphate, trisodium monohydrogen pyrophosphate, sodium dihydrogen pyrophosphate, sodium trihydrogen pyrophosphate, sodium carbonate, and sodium bicarbonate), organic sodium salts (specifically, at least one of sodium acetate, sodium oxalate, and sodium citrate), and sodium oxides (specifically, sodium oxide and / or sodium peroxide); and / or, the iron source is selected from one or more of ferric phosphate, ferrous oxalate, ferric nitrate, and ferric sulfate; and / or, the manganese source is selected from manganese powder, manganese oxides, and manganese carbon. The components are selected from one or more of the following: salts, manganese oxalates, manganese acetates, and manganese nitrates; and / or, the phosphorus source is selected from one or more of sodium pyrophosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, phosphoric acid, triammonium phosphate, and sodium phosphate; and / or, the vanadium source is selected from one or more of orthovanadate (specifically ammonium vanadate), pyrovanadate, and metavanadate (specifically ammonium metavanadate); and / or, the niobium source is ammonium niobate and / or niobium pentoxide; and / or, the titanium source is selected from one or more of tetrabutyl titanate, titanium dioxide, and titanium sulfate; and / or, the solvent is water. All of the above components are water-soluble or dispersible compounds, exhibiting good solubility or colloidal dispersibility in water, thereby enabling the formation of a more stable homogeneous precursor system during stirring.

[0032] Furthermore, to achieve more efficient crushing of solid particles in the precursor solution, it is preferable that the first milling process in step S2 uses 0.3mm~0.4mm zirconia balls as abrasive, and the milling speed is 2000rpm~3000rpm for 1h~2h. Therefore, the viscosity of the resulting second precursor solution is preferably 300Pa·s~1000Pa·s, which facilitates a more suitable flow state, thereby better balancing efficient mass transfer and effective collision during subsequent dispersion and second milling processes.

[0033] In step S3, preferably: the dispersion treatment is ultrasonic dispersion, and the ultrasonic power of ultrasonic dispersion is 200W~300W, and the ultrasonic time is 30min~60min; and / or, the second sand milling treatment uses 0.3mm~0.4mm zirconia balls as abrasive, and the rotation speed of the second sand milling treatment is 2000rpm~3000rpm, and the time is 30min~60min. The above-mentioned preferred ultrasonic conditions can more effectively eliminate secondary agglomeration without causing solvent evaporation or structural damage; the preferred conditions of the second sand milling can more effectively homogenize the particle size. Furthermore, the total solid content of the obtained third precursor solution is preferably 15%~40%, so as to promote better atomization performance before spray drying, thereby further improving the dispersibility of the subsequently obtained precursor particles and narrowing their particle size distribution.

[0034] In step S4, the preferred drying process is spray drying, with the peristaltic pump speed being 10 r / min to 35 r / min, the atomization pressure being 0.2 MPa to 0.5 MPa, the inlet air temperature being 180℃ to 260℃, and the outlet air temperature being 80℃ to 110℃. These preferred drying conditions allow the precursor droplets to dehydrate more quickly in hot air, forming more complete, crack-free, and densely structured precursor spherical particles, further optimizing the various properties of the final composite cathode material.

[0035] In several typical embodiments, the preferred step S5 calcination treatment includes the following sequential processes: a first stage with a holding temperature of 300℃~350℃ and a holding time of 2h~6h; and a second stage with a holding temperature of 600℃~660℃ and a holding time of 12h~24h. This preferred approach can more effectively achieve the technical effects of orderly removal of organic components from the precursor, complete formation of the metal phosphate crystal structure, and continuous carbonization of the carbon layer without damaging the particle morphology. Specifically, in the first stage, this temperature range is sufficient to induce low-temperature pyrolysis and devolatileization of adsorbed water, free acid radicals, and the organic carbon source polydopamine in the precursor particles, while maintaining the integrity of the particle morphology. In the second stage, the 600℃~660℃ range provides sufficient thermodynamic driving force for the nucleation and grain growth of phosphate pyrophosphate crystals, enabling the amorphous precursor to more completely transform into a crystalline phase structure, while simultaneously promoting the carbonization of polydopamine to form a continuous carbon layer, which, together with graphene quantum dots, constructs a conductive network.

[0036] Furthermore, to more effectively control the thermal stress release rate during pyrolysis, reduce microcracks or structural collapse caused by uneven expansion of precursor particles due to excessive temperature gradients, and further optimize the structural stability of the resulting composite cathode, the heating rates of the first and second stages are preferably independently set at 2°C / min to 3°C / min. To prevent oxygen intervention, better protect the valence state of iron and manganese, and suppress the generation of oxygen vacancies and structural distortion in the phosphate framework, the calcination treatment is preferably carried out in a protective atmosphere, more preferably nitrogen and / or argon.

[0037] A second aspect of the present invention provides a composite cathode material prepared by the above-described method for preparing composite cathode materials; the composite cathode material comprises sodium iron pyrophosphate particles and a carbon layer coated on the sodium iron pyrophosphate particles; the chemical formula of the sodium iron pyrophosphate particles is Na. m Fe n Mn p M qThe composite cathode material prepared by the above method has a metal element ratio within the thermodynamically stable region, with dopants dispersed atomically in the crystal lattice. The carbon layer is a coexistence structure of amorphous carbon and graphene quantum dot-derived carbon, attached to the crystal surface. Therefore, the obtained cathode material exhibits high structural uniformity and good carbon network connectivity, thus demonstrating superior electrochemical activity and cycle stability. It should be noted that due to the complex structural formation and compositional changes during the preparation process, and the limitations of the electrode material field and existing testing and characterization methods, it is difficult to perform a comprehensive quantitative characterization of the complex microstructure of the obtained composite cathode material. However, performance test results have already shown that the cathode material obtained in this invention possesses higher electrochemical activity and better cycle stability.

[0038] Furthermore, taking the total weight of the composite cathode material as 100%, the carbon layer preferably accounts for 7% to 9% of the weight. Thus, its thickness is sufficient to form a continuous network on the particle surface, providing an electronic conduction pathway, while avoiding particle isolation, decreased compaction density, or hindered crystal growth due to excessive carbon content, thereby promoting higher electrochemical activity in the cathode material.

[0039] In several preferred embodiments, the chemical formula of sodium iron pyrophosphate is Na. m Fe n Mn p V q1 Ti q2 (PO4)2P2O7 or Na m Fe n Mn p V q1 Nb q3 (PO4)2P2O7, where q1+q2=q, q1+q3=q, q1 / q2=(1~5):1, q1 / q3=(1~5):1. V ions possess strong VO bonds, which can strengthen Mn-O bonds and reduce JT distortion; Nb or Ti ions have high ionic radii and electronegativity, which can further regulate Na / Mn antisite defects, while simultaneously constructing a more stable crystal structure and promoting Na ion migration. The synergistic effect of the two elements, compared to single vanadium doping, can further improve the specific capacity, cycle stability, and energy density of the material. Specifically, q2 and q3 can be preferably each independently set to 0.01~0.05 to ensure that the dopant elements enter the crystal lattice at the atomic level, without forming independent impurity phases and without exceeding the lattice capacity limit. This further reduces lattice distortion caused by excessive doping, improves the structural integrity of the cathode material, and maximizes the synergistic effect.

[0040] A third aspect of the present invention provides a positive electrode sheet comprising the aforementioned composite positive electrode material. The composite positive electrode material is prepared by controlled milling, drying, and two-stage calcination, exhibiting uniform carbon coating, dense particles, and a stable crystalline phase structure. When used as an active material in the positive electrode sheet, it provides a good processing foundation for electrode coating and rolling, while ensuring uniform contact with conductive agents and binders, resulting in a uniform distribution of active material, a continuous carbon network, a dense electrode structure, and good interfacial contact in the obtained positive electrode sheet.

[0041] A fourth aspect of the present invention provides a secondary battery including a positive electrode sheet, which is the aforementioned positive electrode sheet. After assembly to form a battery, the positive electrode sheet has a stable electrode interface and a continuous electron pathway. The composite positive electrode material has uniform particle size, continuous carbon layer, low structural expansion rate during cycling, and few interfacial side reactions. Therefore, the battery exhibits superior electrical and cycling performance.

[0042] The present application will be further described in detail below with reference to specific embodiments, which should not be construed as limiting the scope of protection claimed in the present application.

[0043] Unless otherwise defined, all technical terms used herein have the same meaning as commonly understood by those skilled in the art. The technical terms used herein are for the purpose of describing particular embodiments only and are not intended to limit the scope of the invention.

[0044] Example 1

[0045] A method for preparing a composite cathode material:

[0046] (1) According to the stoichiometric ratio of Fe:Mn:P:Na:V=1.45:1.45:4.0:4.0:0.05, weigh sodium dihydrogen phosphate (sodium source), ferrous oxalate, ammonium dihydrogen phosphate, manganese acetate and ammonium metavanadate, add deionized water, and then add graphene quantum dot-polydopamine composite carbon source (graphene quantum dot: polydopamine=1:3, weight ratio), the amount of carbon source added accounts for 7% of the total mass; adjust the pH of the system to 8.5 with Tris-HCl, stir at 2000 rpm for 1 h to prepare the precursor aqueous solution.

[0047] (2) The above precursor aqueous solution was added to a nano-mill (zirconia balls 0.3 mm) and milled for the first time at 2000 rpm for 1 h to obtain the second precursor solution. Its particle size was tested to be 0.8 μm and its viscosity was 300 Pa·s.

[0048] (3) Then, a second cycle of ultrasonic dispersion-sand milling was performed. The ultrasonic power was 200W and the ultrasonic time was 30min; the second sand milling speed was 2000rpm and the sand milling time was 30min. The nano-sand mill used was the same as in step (2). The particle size of the solid particles in the final solution was D50=300nm; the slurry was pumped out with a solid content of 15% and then spray-dried.

[0049] (4) The spray drying parameters are set as follows: peristaltic pump speed 10r / min, atomization pressure 0.2MPa, inlet air temperature 180℃, outlet air temperature 100℃, and drying and granulation to obtain vanadium-doped sodium manganese pyrophosphate precursor secondary spherical particles.

[0050] (5) The precursor particles were placed in a nitrogen atmosphere (purity 99.99%), heated to 300℃ at a rate of 2℃ / min and held for 2h, then heated to 650℃ at a rate of 2℃ / min and held for 12h, and then naturally cooled to room temperature to obtain V-NFMPP / C-rGO cathode material.

[0051] The resulting cathode material includes sodium iron pyrophosphate particles and a carbon layer coating the sodium iron pyrophosphate particles. The chemical formula of sodium iron pyrophosphate is Na₄Fe₂O₃. 1.45 Mn 1.45 V 0.05 (PO4)2P2O7, the carbon layer accounts for 7% of the weight of the composite cathode material.

[0052] Example 2

[0053] A method for preparing a composite cathode material:

[0054] (1) According to the stoichiometric ratio of Fe:Mn:P:Na:V:Nb=1.45:1.45:4.0:4.0:0.05:0.02, weigh sodium pyrophosphate (sodium source), iron phosphate, phosphoric acid, manganese carbonate, ammonium vanadate, and ammonium niobate (with a doping amount of 0.02 mol%), with a vanadium to niobium molar ratio of 2.5:1; add deionized water, and then add graphene quantum dot-polydopamine composite carbon source (graphene quantum dot: polydopamine = 1:4, weight ratio), with the carbon source accounting for 8% of the total mass; adjust the pH to 8.7 with Tris-HCl solution, and stir at 2400 rpm for 2 h to prepare the precursor aqueous solution.

[0055] (2) The above precursor aqueous solution was added to a nano-mill (zirconia balls 0.35 mm) and milled for the first time at 2500 rpm for 1.5 h to obtain the second precursor solution. Its particle size was tested to be 0.7 μm and its viscosity was 600 Pa·s.

[0056] (3) Then, a second cycle of ultrasonic dispersion-sand milling was performed. The ultrasonic power was 250W and the ultrasonic time was 45min; the second sand milling speed was 2500rpm and the sand milling time was 45min. The nano-sand mill used was the same as in step (2). The particle size of the solid particles in the final solution was D50=300nm; the slurry was pumped out with a solid content of 28% and was ready for spray drying.

[0057] (4) The spray drying parameters were set as follows: peristaltic pump speed 22r / min, atomization pressure 0.35MPa, air inlet temperature 220℃, air outlet temperature 105℃, and drying and granulation were used to obtain vanadium-doped sodium manganese pyrophosphate precursor secondary spherical particles.

[0058] (5) The precursor particles were placed in a nitrogen atmosphere (purity 99.99%), heated to 325℃ at a rate of 2.5℃ / min and held for 4h, then heated to 640℃ at a rate of 2.5℃ / min and held for 18h, and then naturally cooled to room temperature to obtain V-Nb-NFMPP / C-rGO cathode material.

[0059] The resulting cathode material includes sodium iron pyrophosphate particles and a carbon layer coating the sodium iron pyrophosphate particles. The chemical formula of sodium iron pyrophosphate is Na₄Fe₂O₃. 1.45 Mn 1.45 V 0.05 Nb 0.02 (PO4)2P2O7, the carbon layer accounts for 8% of the weight of the composite cathode material.

[0060] The SEM characterization results of the obtained cathode material are shown below. Figure 1 The XRD characterization results are shown in Figure 2 Furthermore, the charge-discharge curves of this cathode material at 0.1C are shown below. Figure 3 .

[0061] Example 3

[0062] A method for preparing a composite cathode material:

[0063] (1) According to the stoichiometric ratio of Fe:Mn:P:Na:V:Ti=1.45:1.45:4.0:4.0:0.05:0.02, the molar ratio of titanium to vanadium is 2:5; deionized water is added, and then graphene quantum dot-polydopamine composite carbon source (graphene quantum dot:polydopamine=1:5, weight ratio) is added, with the amount of carbon source added accounting for 9% of the total mass; the pH is adjusted to 8.8 with Tris-HCl, and the mixture is stirred at 2800 rpm for 3 h to prepare the precursor aqueous solution.

[0064] (2) The above precursor aqueous solution was added to a nano-mill (zirconia balls 0.4 mm) and milled for the first time at 3000 rpm for 2 h to obtain the second precursor solution. Its particle size was tested to be 0.6 μm and its viscosity was 1000 Pa·s.

[0065] (3) Then, a second cycle of ultrasonic dispersion-sand milling was performed. The ultrasonic power was 300W and the ultrasonic time was 60min; the second sand milling speed was 3000rpm and the sand milling time was 60min. The nano-sand mill used was the same as in step (2). The particle size of the solid particles in the final solution was D50=310nm; the slurry was pumped out with a solid content of 40% and was then spray-dried.

[0066] (4) The spray drying parameters are set as follows: peristaltic pump speed 35r / min, atomization pressure 0.5MPa, inlet air temperature 260℃, outlet air temperature 110℃, and drying and granulation are used to obtain vanadium-doped sodium manganese pyrophosphate precursor secondary spherical particles.

[0067] (5) The precursor particles were placed in a nitrogen atmosphere (purity 99.99%), heated to 350℃ at a rate of 3℃ / min and held for 6h, then heated to 650℃ at a rate of 3℃ / min and held for 24h, and then naturally cooled to room temperature to obtain V-Ti-NFMPP / C-rGO cathode material.

[0068] The resulting cathode material includes sodium iron pyrophosphate particles and a carbon layer coating the sodium iron pyrophosphate particles. The chemical formula of sodium iron pyrophosphate is Na₄Fe₂O₃. 1.45 Mn 1.45 V 0.05 Ti 0.02 (PO4)2P2O7, the carbon layer accounts for 9% of the weight of the composite cathode material.

[0069] Example 4

[0070] A method for preparing a composite cathode material:

[0071] The only difference between this embodiment and Embodiment 2 is that the total amount of carbon source added is changed to 6%.

[0072] Example 5

[0073] A method for preparing a composite cathode material:

[0074] The only difference between this embodiment and Embodiment 2 is that the total amount of carbon source added is changed to 10%.

[0075] Example 6

[0076] A method for preparing a composite cathode material:

[0077] The only difference between this embodiment and Embodiment 2 is that the weight ratio of graphene quantum dots to polydopamine in the composite carbon source is changed to 1:2.

[0078] Example 7

[0079] A method for preparing a composite cathode material:

[0080] The only difference between this embodiment and Embodiment 2 is that the weight ratio of graphene quantum dots to polydopamine in the composite carbon source is changed to 1:6.

[0081] Example 8

[0082] A method for preparing a composite cathode material:

[0083] The only difference between this embodiment and Embodiment 2 is that the pH value of the first precursor solution is adjusted to 7.0.

[0084] Example 9

[0085] A method for preparing a composite cathode material:

[0086] The only difference between this embodiment and Embodiment 2 is that the pH value of the first precursor solution is adjusted to 9.5.

[0087] Example 10

[0088] A method for preparing a composite cathode material:

[0089] The only difference between this embodiment and Embodiment 2 is that the stoichiometry Fe:Mn:P:Na:V:Nb is changed to 1.5:1.5:4.0:4.0:0.05:0.1, and the total metal content is greater than 3.

[0090] Example 11

[0091] A method for preparing a composite cathode material:

[0092] The only difference between this embodiment and Embodiment 3 is that the stoichiometry Fe:Mn:P:Na:V:Ti is changed to 1.40:1.40:4.0:4.0:0.05:0.1, and the total metal content is <2.9.

[0093] Example 12

[0094] A method for preparing a composite cathode material:

[0095] The difference between this embodiment and Example 2 lies only in the calcination conditions in step (5). Specifically, the precursor particles are placed in a nitrogen atmosphere (purity 99.99%), heated to 250°C at a rate of 2.5°C / min and held for 8 hours, then heated to 680°C at a rate of 2.5°C / min and held for 10 hours, and then naturally cooled to room temperature to obtain the V-Nb-NFMPP / C-rGO cathode material.

[0096] Example 13

[0097] A method for preparing a composite cathode material:

[0098] The difference between this embodiment and embodiment 2 lies only in the calcination conditions in step (5). Specifically, the precursor particles are placed in a nitrogen atmosphere (purity 99.99%), heated to 400°C at a rate of 2.5°C / min and held for 1 hour, then heated to 480°C at a rate of 2.5°C / min and held for 28 hours, and then naturally cooled to room temperature to obtain the V-Nb-NFMPP / C-rGO cathode material.

[0099] Comparative Example 1

[0100] A method for preparing a composite cathode material:

[0101] The only difference between this comparative example and Example 2 is that an equal weight of glucose is used instead of the graphene quantum dot-polydopamine composite carbon source in Example 2.

[0102] Comparative Example 2

[0103] A method for preparing a composite cathode material:

[0104] The only difference between this comparative example and Example 2 is that step (3) was not performed.

[0105] Comparative Example 3

[0106] A method for preparing a composite cathode material:

[0107] The only difference between this comparative example and Example 2 is that the grinding conditions in step (3) are changed so that the particle size D50 of the solid particles in the resulting third precursor solution is 600 nm.

[0108] Comparative Example 4

[0109] A method for preparing a composite cathode material:

[0110] The only difference between this comparative example and Example 2 is that graphene quantum dots of equal weight are used instead of the graphene quantum dot-polydopamine composite carbon source in Example 2.

[0111] Comparative Example 5

[0112] A method for preparing a composite cathode material:

[0113] The only difference between this comparative example and Example 2 is that an equal weight of polydopamine is used instead of the graphene quantum dot-polydopamine composite carbon source in Example 2.

[0114] Test methods

[0115] The compaction density of the cathode material sample was tested according to the national standard GB / T 44330-2024 "Determination of Compaction Density of Cathode Material Powder for Lithium-ion Batteries". A certain mass of cathode material powder was loaded into a cylindrical mold and held under a pressure of 220 MPa for no less than 30 s. The thickness difference of the sample before and after compaction was measured. The compaction density was calculated based on the mass and compaction volume, and the result was the average of multiple parallel tests.

[0116] Battery sample assembly and performance testing: The positive electrode material samples, conductive material (super P), and polyvinylidene fluoride binder (PVDF) obtained from each example and comparative example were mixed together at a certain mass ratio (9:5:5). N-methylpyrrolidone (NMP) was added to adjust the viscosity, and then the mixture was dispersed evenly using a high-speed disperser and coated onto aluminum foil. Subsequently, it was vacuum dried overnight at 60°C. The resulting active material loading of the working electrode was 1~1.3 mg / cm³. -2 Sodium sheet was used as the counter electrode, and the electrolyte system was 1M NaPF6 / DEC:EC (1:1), 5% FEC, with a microporous polypropylene membrane (Celgard 2400) as the separator.

[0117] Meanwhile, at 0.1C, the capacity retention of each battery sample was tested after 100 cycles in the range of 2V to 4.3V.

[0118] The test results are shown in Table 1.

[0119] Table 1

[0120]

[0121] As can be seen from the above description, the embodiments of the present invention optimize the particle size of solid particles in the precursor solution through a first milling process, a dispersion process, and a second milling process. At the same time, based on the composite carbon source of graphene quantum dots and polydopamine composite carbon source, in-situ adsorption and coating are achieved, thereby improving the homogeneity of the precursor solution and the integrity of the carbon layer structure. This results in the technical effects of high solid density, excellent electronic conductivity, and structural stability of the composite cathode material.

[0122] Specifically, in each embodiment:

[0123] Comparing Examples 4 and 5 with Example 2, it can be seen that by optimizing the amount of carbon source added, a more continuously distributed organic phase can be formed, providing a more stable suspension system for subsequent milling and drying. This reduces the phenomenon of excessive pores inside the particles or excessive escape of pyrolysis gases while ensuring continuous carbon layer coverage after calcination, ultimately significantly improving the structural stability of the obtained composite cathode material.

[0124] Comparing Examples 6 and 7 with Example 2, it can be seen that by optimizing the weight ratio of graphene quantum dots to polydopamine in the composite carbon source, polydopamine can more effectively and completely encapsulate the graphene quantum dots, forming a more stable organic-inorganic hybrid nanounit, thereby obtaining a more continuous and complete carbon layer and further improving the electrochemical performance of the obtained composite cathode material.

[0125] Comparing Examples 8 and 9 with Example 2, it can be seen that by optimizing the pH value of the first precursor solution, the metal ions are kept at a higher solubility, making them closer to the carbon components at the molecular scale. This also facilitates the slow oxidative polymerization of polydopamine, forming an adhesive thin layer rich in phenolic hydroxyl and amino groups. Ultimately, after calcination, a composite cathode material with a more continuous carbon layer and better electrochemical activity and cycle stability is obtained.

[0126] Comparing Examples 12 and 13 with Example 2, it can be seen that by optimizing the calcination conditions in step S5, the technical effects of orderly removal of organic components from the precursor, complete formation of metal phosphate crystal structure, and continuous carbonization of carbon layer without damaging particle morphology can be achieved more effectively.

[0127] It should be noted that the terms "first," "second," etc., used in the specification and claims of this application are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such terms can be used interchangeably where appropriate so that the embodiments of this application described herein can be implemented, for example, in a sequence other than those described herein.

[0128] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a composite cathode material, characterized in that, include: Step S1: Prepare a first precursor solution containing sodium source, iron source, manganese source, phosphorus source, carbon source, dopant source and solvent; The doping source is selected from one or more of vanadium source, niobium source and titanium source, and the carbon source is a composite carbon source of graphene quantum dots and polydopamine. Step S2: Perform a first sand milling process on the first precursor solution to obtain a second precursor solution; In the second precursor solution, the particle size D50 of the solid particles is 0.5 μm to 1 μm; Step S3: The second precursor solution is subjected to dispersion treatment and second milling treatment in sequence to obtain the third precursor solution; In the third precursor solution, the particle size D50 of the solid particles is 200nm~400nm; Step S4: The third precursor solution is dried to obtain precursor particles; Step S5: The precursor particles are calcined to obtain the composite cathode material.

2. The method for preparing the composite cathode material according to claim 1, characterized in that, The amounts of the iron source, manganese source, phosphorus source, sodium source, and dopant source added are in accordance with the stoichiometric ratio Fe:Mn:P:Na:M = (1.40~1.50):(1.40~1.50):(3.95~4.05):4.0:(0.05~0.1); and / or, Based on the total weight of the sodium source, the iron source, the manganese source, the phosphorus source, the carbon source, and the dopant source being 100%, the amount of carbon source added is 7% to 9%; and / or, The doping source includes a first doping source and a second doping source; the first doping source is a vanadium source, and the second doping source is a niobium source and / or a titanium source; the molar ratio of the first doping source to the second doping source is (1~5):1; Preferably, the doping amount of the second doping source is 0.01 mol% to 0.02 mol%.

3. The method for preparing the composite cathode material according to claim 1 or 2, characterized in that, In the composite carbon source, the weight ratio of the graphene quantum dots to the polydopamine is 1:(3~5); and / or, The pH value of the first precursor solution is 7.5~9.

0.

4. The method for preparing the composite cathode material according to any one of claims 1 to 3, characterized in that, The sodium source is selected from one or more of inorganic sodium salts, organic sodium salts, and sodium oxides; and / or, The iron source is selected from one or more of ferric phosphate, ferrous oxalate, ferric nitrate, and ferric sulfate; and / or, The manganese source is selected from one or more of manganese powder, manganese oxides, manganese carbonates, manganese oxalates, manganese acetates, and manganese nitrates; and / or, The phosphorus source is selected from one or more of sodium pyrophosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, phosphoric acid, triammonium phosphate, and sodium phosphate; and / or, The vanadium source is selected from one or more of orthovanadate, pyrovanadate, and metavanadate; and / or, The niobium source is ammonium niobate and / or niobium pentoxide; and / or, The titanium source is selected from one or more of tetrabutyl titanate, titanium dioxide, and titanium sulfate; and / or, The solvent is water; Preferably, the inorganic sodium salt is selected from at least one of trisodium phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium pyrophosphate, trisodium monohydrogen pyrophosphate, sodium dihydrogen pyrophosphate, sodium monohydrogen pyrophosphate, sodium carbonate, and sodium bicarbonate; and / or, the organic sodium salt is selected from at least one of sodium acetate, sodium oxalate, and sodium citrate; and / or, the sodium oxide is sodium oxide and / or sodium peroxide. Preferably, the metavanadate is ammonium metavanadate; the orthovanadate is ammonium vanadate.

5. The method for preparing the composite cathode material according to any one of claims 1 to 4, characterized in that, In step S2 The first grinding process uses 0.3mm~0.4mm zirconia balls as abrasive, and the grinding speed is 2000rpm~3000rpm for 1h~2h; and / or, The viscosity of the second precursor solution is 300 Pa·s to 1000 Pa·s.

6. The method for preparing the composite cathode material according to any one of claims 1 to 4, characterized in that, In step S3 The dispersion treatment is ultrasonic dispersion, and the ultrasonic power of the ultrasonic dispersion is 200W~300W, and the ultrasonic time is 30min~60min; and / or, The second grinding process uses 0.3mm~0.4mm zirconia balls as abrasive, and the grinding speed is 2000rpm~3000rpm for 30min~60min; and / or, The total solids content of the third precursor solution is 15% to 40%.

7. The method for preparing the composite cathode material according to any one of claims 1 to 6, characterized in that, In step S4, the drying process is spray drying, and the peristaltic pump used for spray drying has a rotation speed of 10 r / min to 35 r / min, an atomization pressure of 0.2 MPa to 0.5 MPa, an inlet air temperature of 180℃ to 260℃, and an outlet air temperature of 80℃ to 110℃.

8. The method for preparing the composite cathode material according to any one of claims 1 to 7, characterized in that, In step S5, the calcination treatment includes the following sequential processes: a first stage with a holding temperature of 300℃~350℃ and a holding time of 2h~6h; and a second stage with a holding temperature of 600℃~660℃ and a holding time of 12h~24h. Preferably, the heating rate of the first stage and the second stage are each independently 2℃ / min~3℃ / min; Preferably, the calcination treatment is carried out in a protective atmosphere; more preferably, the protective atmosphere is nitrogen and / or argon.

9. A composite cathode material, characterized in that, The composite cathode material is prepared by the method for preparing the composite cathode material according to any one of claims 1 to 8; the composite cathode material comprises sodium iron pyrophosphate particles and a carbon layer coated on the sodium iron pyrophosphate particles; the chemical formula of the sodium iron pyrophosphate particles is Na. m Fe n Mn p M q (PO4)2P2O7, wherein m is 3.95~4.05, n is 1.40~1.50, p is 1.40~1.50, q is 0.05~0.1, and M is selected from one or more of Ti, Nb and V.

10. The composite cathode material according to claim 9, characterized in that, Based on the total weight of the composite cathode material being 100%, the carbon layer accounts for 7% to 9% of the weight. Preferably, the chemical formula of the sodium ferric pyrophosphate is Na. m Fe n Mn p V q1 Ti q2 (PO4)2P2O7 or Na m Fe n Mn p V q1 Nb q3 (PO4)2P2O7, where q1+q2=q, q1+q3=q, q1 / q2=(1~5):1, q1 / q3=(1~5):1; More preferably, q2 and q3 are each independently 0.01 to 0.

05.

11. A positive electrode plate, characterized in that, The positive electrode sheet includes the composite positive electrode material as described in claim 9 or 10.

12. A secondary battery, comprising a positive electrode sheet, characterized in that, The positive electrode sheet is the positive electrode sheet as described in claim 11.

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