A manganese iron sodium pyrophosphate phosphate positive electrode material and a preparation method thereof

Abstract

The application discloses a kind of sodium manganese iron pyrophosphate phosphate positive electrode materials and preparation method thereof, belong to lithium battery technical field.The application is through the trinity collaborative modification strategy of " gradient doped element spatial distribution design + gradient carbon coating conductive network construction + in-situ lattice stress regulation", realize the gradient distribution of doped element from core to surface in material interior (the doped element in core area is mainly to stabilize lattice structure, and the doped element in surface layer is mainly to optimize electronic structure), construct efficient conductive network from bulk to surface by multilevel gradient carbon coating, release the structure stress generated by Jahn-Teller distortion by in-situ lattice stress regulation, finally obtain sodium manganese iron pyrophosphate phosphate positive electrode material with high specific capacity, excellent rate performance and super long cycle life.

Description

Technical Field

[0001] This invention relates to the field of lithium battery technology, specifically to a sodium manganese pyrophosphate cathode material and its preparation method. Background Technology

[0002] Sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7, abbreviated as NFPP), as a hybrid phosphate cathode material, combines the structural advantages of sodium iron phosphate and sodium iron pyrophosphate. It possesses a NASICON-type three-dimensional framework, a relatively small volume expansion rate (approximately 4%), and a high theoretical specific capacity (approximately 129 mAh / g), and is considered one of the most promising polyanionic cathode materials for industrialization. However, NFPP cathode materials have a low voltage plateau (approximately 3.0 V) and poor intrinsic electronic conductivity, which severely limits their energy density and rate performance.

[0003] By introducing Mn to partially replace Fe in NFMPP, sodium manganese ferric pyrophosphate (Na4MnFe2(PO4)2P2O7, abbreviated as NFMPP) is formed, which can effectively increase the working voltage of the material (introducing a high voltage plateau of approximately 3.8V), thereby achieving higher energy density. However, Mn-substituted NFMPP materials face a series of severe technical challenges: First, the Jahn-Teller structure exhibits severe distortion. Mn 3+ Jahn-Teller distortion is easily induced during charge and discharge, resulting in large structural strain and a sharp deterioration in the material's cycle stability, especially at high temperatures. Studies have shown that NFMPP has small volume changes, but Mn... 3+ The resulting local structural distortions remain a key bottleneck restricting its long cycle life.

[0004] Second, the intrinsic electronic conductivity is extremely low. The inherent insulating properties of polyanionic frameworks result in extremely poor electronic conductivity (typically below 10). -8 The ratio (S / cm) severely limits the utilization rate and rate performance of active materials.

[0005] Third, sodium ion diffusion kinetics are slow. The Na+ ions inside the material... + The high migration barrier and low ion diffusion coefficient lead to rapid capacity decay during high-current charging and discharging.

[0006] Fourth, the bonding strength between the carbon coating layer and the substrate is insufficient. In existing carbon coating methods, the bonding strength between the coating layer and the substrate material is insufficient, and the coating layer is prone to peeling off during long-term cycling, leading to performance degradation. Summary of the Invention

[0007] The technical problem to be solved by this invention is to overcome the shortcomings of the prior art and provide a sodium manganese ferrophosphate pyrophosphate cathode material and its preparation method. Through a three-in-one synergistic modification strategy of "gradient doping element spatial distribution design + gradient carbon coating conductive network construction + in-situ lattice stress regulation", the doping elements are distributed in a gradient from the core to the surface of the material (the core region is dominated by doping elements with stable lattice structure, and the surface region is dominated by doping elements with optimized electronic structure). A highly efficient conductive network from the bulk phase to the surface is constructed through multi-level gradient carbon coating. The structural stress generated by Jahn-Teller distortion is released through in-situ lattice stress regulation. Finally, a sodium manganese ferrophosphate pyrophosphate cathode material with high specific capacity, excellent rate performance and ultra-long cycle life is obtained.

[0008] The technical solution of this invention is as follows: On one hand, the present invention provides a method for preparing a sodium manganese ferrophosphate pyrophosphate cathode material, comprising the following steps: S1 dissolves manganese and iron sources in deionized water, adds precipitant and surfactant, and stirs the reaction at 50-90℃ for 2-8 hours, controlling the pH of the reaction system to 6.5-9.5 to obtain manganese-iron coprecipitation precursor slurry; then, after filtration, washing and vacuum drying, manganese-iron coprecipitation precursor powder is obtained. S2 combines sodium source, phosphorus source, a portion of the manganese-iron coprecipitate precursor powder obtained in step S1, and the first dopant element M. 1 The source is dissolved in deionized water, and a chelating agent and dispersant are added. The mixture is stirred at 60-95℃ for 2-5 hours to form a core-doped precursor sol A. The sodium source, phosphorus source, a portion of the manganese-iron co-precipitated precursor powder obtained in step S1, and the second dopant element M are then added. 2 The source is dissolved in deionized water, and chelating agent and dispersant are added. The mixture is stirred at 60-95℃ for 2-5 hours to form shell-doped precursor sol B. S3. Spray-dry the core-doped precursor sol A obtained in step S2 to obtain core-doped precursor powder. Mix it with a carbon source to obtain carbon-coated core precursor powder. Add the carbon-coated core precursor powder to the shell-doped precursor sol B obtained in step S2. Stir evenly and continue stirring for 2-4 hours to make the shell-doped precursor sol B uniformly coat the surface of the carbon-coated core precursor powder. Spray-dry the resulting slurry to obtain precursor powder with a gradient structure of "core doping-carbon transition layer-shell doping". S4 The precursor powder obtained in step S3 is subjected to stepwise gradient sintering under an inert atmosphere: (1) Low temperature pre-carbonization stage: heat up to 300-450℃ and keep warm for 2-4 hours to carbonize the carbon source in situ to form an internal carbon coating layer, while realizing the initial decomposition of the precursor; (2) Intermediate temperature nucleation stage: continue to heat to 480-560℃ and hold for 2-4 hours to promote uniform nucleation of the target crystal phase; (3) Medium and high temperature crystal growth stage: continue to raise the temperature to 580-660℃ and keep it at that temperature for 3-6 hours to complete the complete evolution and crystallization of the crystal phase; (4) Secondary carbon coating stage: After the heat preservation is completed, organic carbon source gas is introduced for chemical vapor deposition (CVD) treatment for 0.5-2h to form an outer carbon coating layer on the material surface; (5) Annealing stage: Cool to 300-400℃, keep warm for 1-2h for annealing to release lattice stress, and then cool naturally to room temperature to obtain sodium manganese pyrophosphate cathode material with gradient doping and gradient coating synergistic modification.

[0009] Preferably, in step S1, the manganese source is at least one of manganese acetate, manganese nitrate, manganese sulfate, and manganese chloride; and the iron source is at least one of ferric nitrate, ferric sulfate, and ferric chloride.

[0010] Preferably, in step S1, the precipitant is at least one of sodium hydroxide, sodium carbonate, sodium oxalate and ammonia water; the surfactant is at least one of polyvinylpyrrolidone (PVP), polyethylene glycol (PEG) and hexadecyltrimethylammonium bromide (CTAB), and the amount of surfactant added is 0.5-8% of the total mass of the manganese source and the iron source.

[0011] Preferably, in step S1, the amount of manganese source and iron source added satisfies the Mn:Fe molar ratio of 1:(1-2.5).

[0012] Preferably, in step S2, the sodium source is at least one selected from sodium carbonate, sodium bicarbonate, sodium hydroxide, sodium acetate, and sodium dihydrogen phosphate; the phosphorus source is at least one selected from ammonium dihydrogen phosphate, diammonium hydrogen phosphate, phosphoric acid, and sodium dihydrogen phosphate; and the first dopant element M... 1 It is at least one of Mg, Zr and Al, and its doping purpose is to stabilize the crystal structure and suppress Jahn-Teller distortion.

[0013] Preferably, in step S2, the sodium source, phosphorus source, a portion of the manganese-iron coprecipitate precursor powder obtained in step S1, and the first dopant element M are used. 1 Source according to Na:Mn:Fe:P:M 1 The molar ratio of 4:1:(1-2.5):4:(0.02-0.15) is added.

[0014] Preferably, in step S2, the second dopant element M 2 It is at least one of V, Nb, and Ti, and the purpose of doping is to optimize the electronic structure and reduce Na. +Diffusion barrier. Through differentiated doping design of the core and shell, a gradient synergy is achieved between "structurally stable doping" inside the material and "electronically structure-optimized doping" on the surface; sodium source, phosphorus source, partially manganese-iron co-precipitated precursor powder obtained in step S1, and the second doping element M... 2 Source according to Na:Mn:Fe:P:M 2 The molar ratio of the additives is 4:1:(1-2.5):4:(0.01-0.1); the mass of the manganese-iron coprecipitated precursor powders added to the core-doped precursor sol A and the shell-doped precursor sol B is equal; the chelating agent is at least one of citric acid, ethylenediaminetetraacetic acid (EDTA), aminotriacetic acid and tartaric acid, and the amount of chelating agent added is 1-2.5 times the total molar amount of metal ions; the dispersant is at least one of ethylene glycol, polyethylene glycol and ethanol.

[0015] Preferably, in step S3, the carbon source is at least one of glucose, sucrose, citric acid, starch and polyvinyl alcohol (PVA); the mass ratio of the core-doped precursor powder to the carbon source is 100:(2-8); the inlet temperature of the spray dryer is 180-250°C and the outlet temperature is 80-120°C.

[0016] Preferably, in step S4, the inert atmosphere is nitrogen or argon, and the gas flow rate is 50-500 sccm; the organic carbon source gas is methane, acetylene, ethylene or benzene, and the gas flow rate is 10-100 sccm.

[0017] On the other hand, the present invention provides a sodium manganese pyrophosphate cathode material, which is prepared by the above-described method for preparing sodium manganese pyrophosphate cathode material.

[0018] Compared with the prior art, the present invention has the following advantages: 1. This invention breaks away from the traditional mindset of uniform doping and proposes for the first time a gradient doping strategy that involves differentiated doping in different regions of the material. The material core is dominated by structurally stable doping elements such as Mg, Zr, or Al, effectively suppressing Mn. 3+ The Jahn-Teller distortion stabilizes the crystal lattice structure; the material surface is dominated by "electronically optimized" dopants such as V, Nb, or Ti, reducing Na... + A diffusion barrier optimizes the interface electronic structure. A gradient transition layer between the core and surface layers smoothly connects the two doping functions, avoiding interface mismatch issues caused by abrupt changes in dopant elements. Compared to uniform doping, this gradient doping design can more precisely meet the different doping function requirements of different regions of the material, achieving a multiplier effect in modification.

[0019] 2. This invention creatively prepares a precursor with a gradient structure of "core doping - carbon transition layer - shell doping" by using stepwise sol construction and spray drying techniques, based on the coprecipitation-sol-gel process. The carbon transition layer not only serves as the inner layer basis for subsequent gradient carbon coating, but also plays a role in preventing excessive interdiffusion of inner and outer dopants during sintering, ensuring the effective preservation of the gradient doping structure. This precursor structure design has not been reported in the prior art.

[0020] 3. This invention achieves in-situ carbonization of the carbon source through a low-temperature pre-carbonization stage, forming a tightly bonded inner carbon coating layer within the bulk phase and on the particle surface of the material. A uniform and dense outer carbon coating layer is then formed on the material surface through a high-temperature sintering CVD treatment. The inner carbon coating layer has a high chemical bond with the matrix material, while the outer carbon coating layer uniformly covers the material surface, repairing any defects that may exist in the inner coating. The two carbon coating layers work synergistically to construct an efficient electron transport channel from the bulk phase to the particle surface, improving the material's electronic conductivity. Compared to existing technologies with single carbon coating or multiple coatings lacking a gradient design, this invention's gradient carbon coating strategy achieves both division of labor and synergy among the coating layer functions.

[0021] 4. In the annealing stage of stepwise sintering, this invention effectively releases Mn by precisely controlling the cooling rate and annealing temperature. 3+ The lattice stress generated by Jahn-Teller distortion during charging and discharging reduces structural defects and dislocation density, further improving the structural stability and cycle life of the material. This stress regulation approach is innovative in the modification of NFMPP materials.

[0022] 5. This invention employs a multi-stage gradient heat treatment strategy, including low-temperature pre-carbonization, medium-temperature nucleation, medium-high temperature crystal phase growth, and annealing. Compared to traditional single-stage sintering or simple segmented sintering, the stepwise gradient sintering process of this invention can precisely control the reaction process at each stage, effectively avoiding the generation of impurity phases and abnormal grain growth, ensuring the synthesis of pure-phase, highly crystalline materials.

[0023] 6. Through the comprehensive effect of the three-in-one synergistic modification strategy of "gradient doping + gradient coating + in-situ stress regulation", the sodium manganese iron pyrophosphate cathode material prepared by this invention exhibits electrochemical performance that is significantly better than the prior art: the initial discharge specific capacity can reach more than 125 mAh / g (0.1C rate), the discharge specific capacity is still more than 86 mAh / g at a high rate of 10C, the capacity output is still more than 76 mAh / g at an ultra-high rate of 30C, and the capacity retention rate is more than 93% after 1000 cycles at a rate of 1C. The overall performance reaches the industry-leading level. Detailed Implementation

[0024] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0025] Example 1 The preparation method of the sodium manganese pyrophosphate positive electrode material in this embodiment includes the following steps: Preparation of manganese-iron-based precursors by S1 coprecipitation method 0.1 mol manganese acetate and 0.2 mol ferric nitrate were dissolved in 600 mL of deionized water, and 3 g of polyvinylpyrrolidone (PVP, K30) was added as a surfactant. The mixture was stirred until homogeneous. Under a water bath at 65 °C, 1 mol / L sodium carbonate solution was added dropwise at a rate of 2 mL / min, controlling the pH of the reaction system to 8 ± 0.2, and stirring was continued for 5 h. After the reaction was complete, the slurry was filtered, washed three times with deionized water, and dried in a vacuum drying oven at 80 °C for 15 h to obtain the manganese-iron coprecipitate precursor powder. Two equal amounts of the manganese-iron coprecipitate precursor powder were prepared according to the above method.

[0026] Stepwise preparation of S2 gradient doped precursor sol (1) Preparation of core-doped precursor sol A: 0.2 mol sodium carbonate, 0.4 mol ammonium dihydrogen phosphate, 1 part of the manganese-iron coprecipitate precursor powder obtained in step S1, and 0.003 mol magnesium acetate were dissolved in 400 mL deionized water. 0.8 mol citric acid was added as a chelating agent and 20 mL ethylene glycol was added as a dispersant. The mixture was stirred in an 85℃ water bath for 3 h to form a homogeneous sol system, which was denoted as core-doped precursor sol A.

[0027] (2) Preparation of shell-doped precursor sol B: 0.2 mol sodium carbonate, 0.4 mol ammonium dihydrogen phosphate, 1 part of the manganese-iron coprecipitate precursor powder obtained in step S1, and 0.003 mol ammonium metavanadate were dissolved in 400 mL of deionized water. 0.8 mol citric acid was added as a chelating agent and 20 mL ethylene glycol was added as a dispersant. The mixture was stirred in an 85℃ water bath for 3 h to form a homogeneous sol system, which was denoted as shell-doped precursor sol B.

[0028] Construction of S3 gradient-coated precursor Sol A was spray-dried (inlet temperature 220℃, outlet temperature 100℃) to obtain a Mg-doped core precursor powder. This powder was then mixed with glucose at a mass ratio of 100:5 and ground until homogeneous to obtain a carbon-coated core precursor powder. This powder was added to sol B, stirred until homogeneous, and stirred for another 3 hours to ensure that sol B uniformly coated the surface of the carbon-coated core precursor powder. The resulting slurry was then spray-dried (inlet temperature 220℃, outlet temperature 100℃) to obtain a precursor powder with a gradient structure of "Mg-doped core - carbon transition layer - V-doped outer shell".

[0029] In-situ transformation and stepwise sintering of S4 gradient carbon coating The gradient-structured precursor powder obtained in step S3 was placed in a tube furnace and subjected to stepwise gradient sintering under an argon atmosphere (flow rate 200 sccm). (1) Low-temperature pre-carbonization stage: Heat to 400℃ at a rate of 3℃ / min and hold for 3h; (2) Mesophilic nucleation stage: continue heating to 520℃ at a rate of 3℃ / min and hold for 3h; (3) Medium and high temperature crystal phase growth stage: continue to heat up to 630℃ at a rate of 3℃ / min and hold for 5h; (4) Secondary carbon coating stage: After the heat preservation is completed, acetylene gas (flow rate 30 sccm) is introduced for CVD treatment for 1 hour; (5) Annealing stage: Cool to 350℃ at a rate of 2℃ / min, hold for 1.5h for annealing, and then cool naturally to room temperature to obtain sodium manganese pyrophosphate cathode material with gradient doping and gradient coating synergistic modification.

[0030] Example 2 The preparation method of the sodium manganese pyrophosphate positive electrode material in this embodiment includes the following steps: Preparation of manganese-iron-based precursors by S1 coprecipitation method 0.1 mol manganese acetate and 0.2 mol ferric nitrate were dissolved in 600 mL of deionized water, and 3 g of polyvinylpyrrolidone (PVP, K30) was added as a surfactant. The mixture was stirred until homogeneous. Under a water bath at 65 °C, 1 mol / L sodium carbonate solution was added dropwise at a rate of 2 mL / min, controlling the pH of the reaction system to 8 ± 0.2, and stirring was continued for 5 h. After the reaction was complete, the slurry was filtered, washed three times with deionized water, and dried in a vacuum drying oven at 80 °C for 15 h to obtain the manganese-iron coprecipitate precursor powder. Two equal amounts of the manganese-iron coprecipitate precursor powder were prepared according to the above method.

[0031] Stepwise preparation of S2 gradient doped precursor sol (1) Preparation of core-doped precursor sol A: 0.2 mol sodium carbonate, 0.4 mol ammonium dihydrogen phosphate, 1 part of the manganese-iron coprecipitate precursor powder obtained in step S1, and 0.003 mol zirconium oxynitrate were dissolved in 400 mL of deionized water. 0.8 mol citric acid was added as a chelating agent and 20 mL ethylene glycol was added as a dispersant. The mixture was stirred in an 85℃ water bath for 3 h to form a homogeneous sol system, which was denoted as core-doped precursor sol A.

[0032] (2) Preparation of shell-doped precursor sol B: 0.2 mol sodium carbonate, 0.4 mol ammonium dihydrogen phosphate, 1 part of the manganese-iron coprecipitate precursor powder obtained in step S1, and 0.003 mol niobium oxalate were dissolved in 400 mL deionized water. 0.8 mol citric acid was added as a chelating agent and 20 mL ethylene glycol was added as a dispersant. The mixture was stirred in an 85℃ water bath for 3 h to form a homogeneous sol system, which was denoted as shell-doped precursor sol B.

[0033] Construction of S3 gradient-coated precursor Sol A was spray-dried (inlet temperature 220℃, outlet temperature 100℃) to obtain a Mg-doped core precursor powder. This powder was then mixed with glucose at a mass ratio of 100:5 and ground until homogeneous to obtain a carbon-coated core precursor powder. This powder was added to sol B, stirred until homogeneous, and stirred for another 3 hours to ensure that sol B uniformly coated the surface of the carbon-coated core precursor powder. The resulting slurry was then spray-dried (inlet temperature 220℃, outlet temperature 100℃) to obtain a precursor powder with a gradient structure of "Zr-doped core - carbon transition layer - Nb-doped outer shell".

[0034] In-situ transformation and stepwise sintering of S4 gradient carbon coating The gradient-structured precursor powder obtained in step S3 was placed in a tube furnace and subjected to stepwise gradient sintering under an argon atmosphere (flow rate 200 sccm). (1) Low-temperature pre-carbonization stage: Heat to 400℃ at a rate of 3℃ / min and hold for 3h; (2) Mesophilic nucleation stage: continue heating to 520℃ at a rate of 3℃ / min and hold for 3h; (3) Medium and high temperature crystal phase growth stage: continue to heat up to 630℃ at a rate of 3℃ / min and hold for 5h; (4) Secondary carbon coating stage: After the heat preservation is completed, methane gas (flow rate 40 sccm) is introduced for CVD treatment for 1.5 h; (5) Annealing stage: Cool to 350℃ at a rate of 2℃ / min, hold for 1.5h for annealing, and then cool naturally to room temperature to obtain sodium manganese pyrophosphate cathode material with gradient doping and gradient coating synergistic modification.

[0035] Example 3 The preparation method of the sodium manganese pyrophosphate positive electrode material in this embodiment includes the following steps: Preparation of manganese-iron-based precursors by S1 coprecipitation method 0.1 mol manganese acetate and 0.15 mol ferric nitrate were dissolved in 600 mL of deionized water, and 3 g of polyvinylpyrrolidone (PVP, K30) was added as a surfactant. The mixture was stirred until homogeneous. Under a water bath at 65 °C, 1 mol / L sodium carbonate solution was added dropwise at a rate of 2 mL / min, controlling the pH of the reaction system to 8 ± 0.2, and stirring was continued for 5 h. After the reaction was complete, the slurry was filtered, washed three times with deionized water, and dried in a vacuum drying oven at 80 °C for 15 h to obtain the manganese-iron coprecipitate precursor powder. Two equal amounts of the manganese-iron coprecipitate precursor powder were prepared according to the above method.

[0036] Stepwise preparation of S2 gradient doped precursor sol (1) Preparation of core-doped precursor sol A: 0.2 mol sodium carbonate, 0.4 mol ammonium dihydrogen phosphate, 1 part of the manganese-iron coprecipitate precursor powder obtained in step S1, and 0.004 mol aluminum nitrate were dissolved in 400 mL deionized water. 0.8 mol citric acid was added as a chelating agent and 20 mL ethylene glycol was added as a dispersant. The mixture was stirred in an 85℃ water bath for 3 h to form a homogeneous sol system, which was denoted as core-doped precursor sol A.

[0037] (2) Preparation of shell-doped precursor sol B: 0.2 mol sodium carbonate, 0.4 mol ammonium dihydrogen phosphate, 1 part of the manganese-iron coprecipitate precursor powder obtained in step S1, and 0.004 mol tetrabutyl titanate were dissolved in 400 mL deionized water. 0.8 mol citric acid was added as a chelating agent and 20 mL ethylene glycol was added as a dispersant. The mixture was stirred in an 85℃ water bath for 3 h to form a homogeneous sol system, which was denoted as shell-doped precursor sol B.

[0038] Construction of S3 gradient-coated precursor Sol A was spray-dried (inlet temperature 220℃, outlet temperature 100℃) to obtain a Mg-doped core precursor powder. This powder was then mixed with sucrose at a mass ratio of 100:8 and ground until homogeneous to obtain a carbon-coated core precursor powder. This powder was added to sol B, stirred until homogeneous, and stirred for another 3 hours to ensure that sol B uniformly coated the surface of the carbon-coated core precursor powder. The resulting slurry was then spray-dried (inlet temperature 220℃, outlet temperature 100℃) to obtain a precursor powder with a gradient structure of "Al-doped core - carbon transition layer - Ti-doped outer shell".

[0039] In-situ transformation and stepwise sintering of S4 gradient carbon coating The gradient-structured precursor powder obtained in step S3 was placed in a tube furnace and subjected to stepwise gradient sintering under an argon atmosphere (flow rate 200 sccm). (1) Low-temperature pre-carbonization stage: Heat to 400℃ at a rate of 3℃ / min and hold for 3h; (2) Mesophilic nucleation stage: continue heating to 520℃ at a rate of 3℃ / min and hold for 3h; (3) Medium and high temperature crystal phase growth stage: continue to heat up to 630℃ at a rate of 3℃ / min and hold for 5h; (4) Secondary carbon coating stage: After the heat preservation is completed, ethylene gas (flow rate 25 sccm) is introduced for CVD treatment for 1.2 h; (5) Annealing stage: Cool to 350℃ at a rate of 2℃ / min, hold for 1.5h for annealing, and then cool naturally to room temperature to obtain sodium manganese pyrophosphate cathode material with gradient doping and gradient coating synergistic modification.

[0040] Example 4 The preparation method of the sodium manganese pyrophosphate positive electrode material in this embodiment includes the following steps: Preparation of manganese-iron-based precursors by S1 coprecipitation method 0.1 mol manganese acetate and 0.2 mol ferric nitrate were dissolved in 600 mL of deionized water, and 3 g of polyvinylpyrrolidone (PVP, K30) was added as a surfactant. The mixture was stirred until homogeneous. Under a water bath at 65 °C, 1 mol / L sodium carbonate solution was added dropwise at a rate of 2 mL / min, controlling the pH of the reaction system to 8 ± 0.2, and stirring was continued for 5 h. After the reaction was complete, the slurry was filtered, washed three times with deionized water, and dried in a vacuum drying oven at 80 °C for 15 h to obtain the manganese-iron coprecipitate precursor powder. Two equal amounts of the manganese-iron coprecipitate precursor powder were prepared according to the above method.

[0041] Stepwise preparation of S2 gradient doped precursor sol (1) Preparation of core-doped precursor sol A: 0.2 mol sodium carbonate, 0.4 mol ammonium dihydrogen phosphate, 1 part of the manganese-iron coprecipitate precursor powder obtained in step S1, 0.002 mol magnesium acetate, and 0.0015 mol zirconium oxynitrate were dissolved in 400 mL deionized water. 0.8 mol citric acid was added as a chelating agent and 20 mL ethylene glycol was added as a dispersant. The mixture was stirred in an 85℃ water bath for 3 h to form a homogeneous sol system, which was denoted as core-doped precursor sol A.

[0042] (2) Preparation of shell-doped precursor sol B: 0.2 mol sodium carbonate, 0.4 mol ammonium dihydrogen phosphate, 1 part of the manganese-iron coprecipitate precursor powder obtained in step S1, 0.002 mol ammonium metavanadate, and 0.0015 mol niobium oxalate were dissolved in 400 mL deionized water. 0.8 mol citric acid was added as a chelating agent and 20 mL ethylene glycol was added as a dispersant. The mixture was stirred for 3 h in an 85℃ water bath to form a homogeneous sol system, which was denoted as shell-doped precursor sol B.

[0043] Construction of S3 gradient-coated precursor Sol A was spray-dried (inlet temperature 220℃, outlet temperature 100℃) to obtain a Mg-doped core precursor powder. This powder was then mixed with glucose at a mass ratio of 100:5 and ground until homogeneous to obtain a carbon-coated core precursor powder. This powder was added to sol B, stirred until homogeneous, and stirred for another 3 hours to ensure that sol B uniformly coated the surface of the carbon-coated core precursor powder. The resulting slurry was then spray-dried (inlet temperature 220℃, outlet temperature 100℃) to obtain a precursor powder with a gradient structure of "Mg / Zr doping in the core - carbon transition layer - V / Nb doping in the outer shell".

[0044] In-situ transformation and stepwise sintering of S4 gradient carbon coating The gradient-structured precursor powder obtained in step S3 was placed in a tube furnace and subjected to stepwise gradient sintering under an argon atmosphere (flow rate 200 sccm). (1) Low-temperature pre-carbonization stage: Heat to 400℃ at a rate of 3℃ / min and hold for 3h; (2) Mesophilic nucleation stage: continue heating to 520℃ at a rate of 3℃ / min and hold for 3h; (3) Medium and high temperature crystal phase growth stage: continue to heat up to 650℃ at a rate of 3℃ / min and hold for 4h; (4) Secondary carbon coating stage: After the heat preservation is completed, acetylene gas (flow rate 30 sccm) is introduced for CVD treatment for 0.8 h; (5) Annealing stage: Cool to 350℃ at a rate of 2℃ / min, hold for 1.5h for annealing, and then cool naturally to room temperature to obtain sodium manganese pyrophosphate cathode material with gradient doping and gradient coating synergistic modification.

[0045] Comparative Example 1 The difference from Example 1 is as follows: Step S2: 0.4 mol sodium carbonate, 0.8 mol ammonium dihydrogen phosphate, 2 portions of the manganese-iron coprecipitate precursor powder obtained in Step S1, 0.003 mol magnesium acetate, and 0.003 mol ammonium metavanadate are dissolved in 800 mL of deionized water. 1.6 mol citric acid is added as a chelating agent and 40 mL ethylene glycol as a dispersant. The mixture is stirred for 3 hours in an 85°C water bath to form a homogeneous sol system, denoted as the core-doped precursor sol. Step S3: The sol is divided into two equal portions. One portion is spray-dried (inlet temperature 220°C, outlet temperature 100°C) to obtain Mg and V doped precursor powder. This powder is mixed with glucose at a mass ratio of 100:5 and ground evenly to obtain carbon-coated core precursor powder. This powder is added to the other portion of the sol, stirred evenly, and stirred for another 3 hours to ensure that the sol uniformly coats the surface of the carbon-coated core precursor powder. The obtained slurry was spray-dried (inlet temperature 220℃, outlet temperature 100℃) to obtain precursor powder.

[0046] Comparative Example 2 The difference from Example 1 is that: in step S3, no glucose is added; in step S4, step (1) is not performed.

[0047] Comparative Example 3 The difference from Example 1 is that step (5) is not performed in step S4.

[0048] The positive electrode materials obtained in Examples 1-4 and Comparative Examples 1-3 were mixed with conductive carbon black and PVDF in a mass ratio of 8:1:1. An appropriate amount of N-methylpyrrolidone (NMP) was added and ground into a uniform slurry. This slurry was coated onto aluminum foil, vacuum dried, and then cut into electrode sheets with a diameter of 12 mm. Using metallic sodium as the counter electrode and 1 mol / L NaPF6 dissolved in ethylene carbonate (EC) / diethyl carbonate (DEC) (volume ratio 1:1) as the electrolyte, CR2032 coin cells were assembled in an argon glove box. Charge-discharge tests were conducted within a voltage range of 2-4.3 V.

[0049] The test results are shown in Table 1: Table 1. Electrochemical performance test results of the assembled batteries in Examples 1-4 and Comparative Examples 1-3 As shown in Table 1, the 0.1C capacity of Example 1 is increased by approximately 4.7% compared to Comparative Example 1, the 10C capacity is increased by approximately 8.3%, and the capacity retention rate after 1000 cycles is increased by approximately 5.6 percentage points. This indicates that the gradient doping strategy of Example 1 (mainly using structurally stable doping such as Mg in the core region and electronically optimized doping such as V in the surface layer) has significant advantages over traditional uniform doping. Gradient doping can more accurately meet the different doping functional requirements of different regions of the material, thereby achieving a multiplier effect of modification.

[0050] The 0.1C capacity of Example 1 was improved by approximately 3.5% compared to Comparative Example 2, the 10C capacity was improved by approximately 5.3%, and the capacity retention rate after 1000 cycles was improved by approximately 4.7 percentage points. This indicates that the gradient carbon coating strategy of "in-situ carbonization (inner layer) + CVD secondary coating (outer layer)" has significant advantages over single carbon coating. The inner carbon layer is chemically bonded to the matrix and has strong binding force, while the outer carbon layer is uniform and dense, repairing coating defects. Together, they construct an efficient and complete electron transport channel.

[0051] The capacity of Example 1 at 0.1C was increased by approximately 1.4% compared to Comparative Example 3, the capacity at 10C was increased by approximately 2.5%, and the capacity retention after 1000 cycles was increased by approximately 4 percentage points. While the capacity difference was relatively small, the difference in cycle stability was significant. This indicates that although annealing has a limited direct effect on capacity improvement, it plays a crucial role in releasing lattice stress caused by Jahn-Teller distortion, reducing structural defects, and improving long-term cycle stability.

[0052] In summary, the integrated synergistic modification strategy of "gradient doping + gradient coating + in-situ stress modulation" in this invention demonstrates significant comprehensive effects. Gradient doping ensures the stability of the core structure of the material while meeting the differentiated requirements of surface electron / ion transport dynamics; gradient carbon coating constructs a complete and efficient conductive network; and annealing effectively releases lattice stress. The synergistic effect of these three factors enables the final product to achieve industry-leading rate performance and long-cycle stability, verifying the scientific validity and advanced nature of the technical solution of this invention.

Claims

1. A method for preparing a sodium manganese ferrophosphate pyrophosphate cathode material, characterized in that, Includes the following steps: S1 dissolves manganese and iron sources in deionized water, adds precipitant and surfactant, and stirs the reaction at 50-90℃ for 2-8 hours, controlling the pH of the reaction system to 6.5-9.5 to obtain manganese-iron coprecipitation precursor slurry; then, after filtration, washing and vacuum drying, manganese-iron coprecipitation precursor powder is obtained. S2 combines sodium source, phosphorus source, a portion of the manganese-iron coprecipitate precursor powder obtained in step S1, and the first dopant element M. 1 The source is dissolved in deionized water, and a chelating agent and dispersant are added. The mixture is stirred at 60-95℃ for 2-5 hours to form a core-doped precursor sol A. The sodium source, phosphorus source, a portion of the manganese-iron co-precipitated precursor powder obtained in step S1, and the second dopant element M are then added. 2 The source is dissolved in deionized water, and chelating agent and dispersant are added. The mixture is stirred at 60-95℃ for 2-5 hours to form shell-doped precursor sol B. S3. Spray dry the core-doped precursor sol A obtained in step S2 to obtain core-doped precursor powder, mix it with a carbon source to obtain carbon-coated core precursor powder; add the carbon-coated core precursor powder to the shell-doped precursor sol B obtained in step S2, stir evenly and continue stirring for 2-4 hours, spray dry the resulting slurry to obtain precursor powder with a gradient structure of "core doping-carbon transition layer-shell doping"; S4. The precursor powder obtained in step S3 is subjected to stepwise gradient sintering under an inert atmosphere: the temperature is raised to 300-450℃ and held for 2-4 hours; the temperature is further raised to 480-560℃ and held for 2-4 hours; the temperature is further raised to 580-660℃ and held for 3-6 hours; after the holding period, an organic carbon source gas is introduced for chemical vapor deposition treatment for 0.5-2 hours to form an outer carbon coating layer on the material surface; the temperature is cooled to 300-400℃ and held for 1-2 hours for annealing treatment, and then naturally cooled to room temperature to obtain sodium manganese iron pyrophosphate cathode material.

2. The method for preparing the sodium manganese pyrophosphate iron sodium cathode material as described in claim 1, characterized in that, In step S1, the manganese source is at least one of manganese acetate, manganese nitrate, manganese sulfate, and manganese chloride; the iron source is at least one of ferric nitrate, ferric sulfate, and ferric chloride.

3. The method for preparing the sodium manganese pyrophosphate iron sodium cathode material as described in claim 1, characterized in that, In step S1, the precipitant is at least one of sodium hydroxide, sodium carbonate, sodium oxalate and ammonia water; the surfactant is at least one of polyvinylpyrrolidone, polyethylene glycol and hexadecyltrimethylammonium bromide, and the amount of surfactant added is 0.5-8% of the total mass of manganese source and iron source.

4. The method for preparing the sodium manganese pyrophosphate iron sodium cathode material as described in claim 1, characterized in that, In step S1, the amount of manganese source and iron source added satisfies the Mn:Fe molar ratio of 1:(1-2.5).

5. The method for preparing the sodium manganese pyrophosphate iron sodium cathode material as described in claim 1, characterized in that, In step S2, the sodium source is at least one of sodium carbonate, sodium bicarbonate, sodium hydroxide, sodium acetate, and sodium dihydrogen phosphate; the phosphorus source is at least one of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, phosphoric acid, and sodium dihydrogen phosphate; the first dopant element M... 1 It is at least one of Mg, Zr and Al.

6. The method for preparing the sodium manganese pyrophosphate iron sodium cathode material as described in claim 1, characterized in that, In step S2, the sodium source, phosphorus source, a portion of the manganese-iron coprecipitate precursor powder obtained in step S1, and the first dopant element M are used. 1 Source according to Na:Mn:Fe:P:M 1 The molar ratio of 4:1:(1-2.5):4:(0.02-0.15) is added.

7. The method for preparing the sodium manganese pyrophosphate iron sodium cathode material as described in claim 1, characterized in that, In step S2, the second dopant element M 2 It is at least one of V, Nb, and Ti; sodium source, phosphorus source, partially manganese-iron coprecipitate precursor powder obtained in step S1, and second dopant element M. 2 Source according to Na:Mn:Fe:P:M 2 The molar ratio of the additives is 4:1:(1-2.5):4:(0.01-0.1); the mass of manganese-iron coprecipitated precursor powder added to the core-doped precursor sol A and the shell-doped precursor sol B is equal; the chelating agent is at least one of citric acid, ethylenediaminetetraacetic acid, aminotriacetic acid and tartaric acid, and the amount of chelating agent added is 1-2.5 times the total molar amount of metal ions; the dispersant is at least one of ethylene glycol, polyethylene glycol and ethanol.

8. The method for preparing the sodium manganese pyrophosphate iron sodium cathode material as described in claim 1, characterized in that, In step S3, the carbon source is at least one of glucose, sucrose, citric acid, starch, and polyvinyl alcohol; the mass ratio of the core-doped precursor powder to the carbon source is 100:(2-8); the inlet temperature of the spray dryer is 180-250℃, and the outlet temperature is 80-120℃.

9. The method for preparing the sodium manganese pyrophosphate iron sodium cathode material as described in claim 1, characterized in that, In step S4, the inert atmosphere is nitrogen or argon, and the gas flow rate is 50-500 sccm; the organic carbon source gas is methane, acetylene, ethylene or benzene, and the gas flow rate is 10-100 sccm.

10. A sodium manganese ferrophosphate pyrophosphate cathode material, characterized in that, The cathode material was prepared by the method described in any one of claims 1-9, which is based on the method for preparing sodium manganese pyrophosphate.

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