Step-by-step coated lithium manganese iron phosphate material and preparation method thereof

Abstract

The invention provides a step-by-step coated lithium manganese iron phosphate material and a preparation method thereof. The method comprises the following steps: mixing a lithium source, an iron source, a manganese source and a phosphorus source with a first coating agent for morphology optimization and a carbon source, and carrying out wet processing and first sintering to obtain a precursor; and mixing the precursor with a second coating agent for interface strengthening and a carbon source, and carrying out secondary wet treatment and secondary sintering to obtain the final material. According to the method, the compaction density and the electrochemical capacity of the material are synergistically improved through a functional step-by-step coating strategy of morphology and interface in sequence, the technical contradiction that the compaction density and the capacity in the lithium manganese iron phosphate material are difficult to synergistically improve is effectively solved, and the method is controllable in process and suitable for preparation of the high-energy-density lithium ion battery positive electrode material.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery cathode material technology, specifically to a stepwise coated lithium manganese iron phosphate material and its preparation method. Background Technology

[0002] With the rapid development of electric vehicles and large-scale energy storage, higher demands are being placed on the energy density of lithium-ion batteries. As a key component determining battery energy density, the performance optimization of cathode materials is crucial. While lithium iron phosphate materials offer advantages such as high safety and long cycle life, their theoretical specific capacity and operating voltage plateau (approximately 3.4V) are relatively low, limiting further improvements in battery energy density.

[0003] Lithium manganese iron phosphate, as an upgraded material of lithium iron phosphate, significantly improves its theoretical energy density by introducing manganese, raising the operating voltage platform to approximately 4.1V. However, this material faces a long-standing technical contradiction in practical applications: its compaction density and electrochemical capacity (especially first discharge capacity) are difficult to improve in synergy, often resulting in a trade-off between the two.

[0004] Specifically, carbon coating technology, widely used to improve the conductivity of materials, can enhance electron transport efficiency, but excessively thick or excessive carbon layers can hinder the close packing of particles, leading to a decrease in compaction density. On the other hand, particle morphology control and densification processes used to improve compaction density often increase particle size or reduce surface activity, which in turn impairs the specific capacity and reaction kinetics of the material. Existing technologies, whether single carbon coating, ion doping, or particle size control, focus on improving a specific performance indicator, failing to systematically resolve the inherent conflict between compaction density and electrochemical capacity. For example, pursuing only high capacity may lead to difficulties in electrode pressing and low volumetric energy density; while simply pursuing high capacity may result in low utilization of active materials and poor rate performance.

[0005] Therefore, there is an urgent need in this field to develop an innovative material design and preparation method to significantly improve the compaction density of lithium manganese iron phosphate materials while maintaining high electrochemical capacity, thereby truly realizing its theoretical high volumetric energy density advantage and meeting the application requirements of high energy density lithium-ion batteries. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of the prior art and provide a stepwise coated lithium manganese iron phosphate material and its preparation method, thereby solving the technical problem that "it is difficult to synergistically improve the compaction density and electrochemical capacity of existing lithium manganese iron phosphate materials".

[0007] To achieve the above objectives, the present invention is implemented using the following technical solution: In a first aspect, the present invention provides a method for preparing lithium manganese iron phosphate material by stepwise coating, comprising the following steps: (1) A precursor is obtained by mixing lithium source, iron source, manganese source, phosphorus source with a first coating agent and carbon source, followed by wet grinding, drying and first sintering; (2) The above precursor is mixed with the second coating agent and carbon source, and then wet-milled, dried and sintered for a second time to obtain the lithium manganese iron phosphate material; The molar ratio of the lithium source, iron source, manganese source, and phosphorus source is 1:(0.2~0.3):(0.7~0.8):(1~1.1); The mass ratio of the first coating agent to the carbon source is 1:(1-3).

[0008] Specifically, the lithium source is one of lithium hydroxide, lithium peroxide, lithium oxide, lithium formate, lithium nitrate, and lithium carbonate.

[0009] Specifically, the iron source is one of ferrous oxide, ferric oxide, ferric oxalate, ferric phosphate, ferric nitrate, and ferrous acetate.

[0010] Specifically, the phosphorus source is one of lithium monohydrogen phosphate, lithium dihydrogen phosphate, lithium phosphate, ammonium dihydrogen phosphate, and phosphoric acid.

[0011] Specifically, the first coating agent is at least one of citric acid, ascorbic acid, polyvinylpyrrolidone, and sophorate.

[0012] Specifically, the second coating agent is at least one of tannic acid, chitosan, polyacrylonitrile, and polyacrylic acid.

[0013] Specifically, the carbon source is at least one of sucrose, glucose, fructose, polyvinyl alcohol, polyethylene glycol, and starch.

[0014] Specifically, in step (1), the molar ratio of lithium source, iron source, manganese source and phosphorus source is 1:(0.2~0.3):(0.7~0.8):(1~1.1), and the mass ratio of the first coating agent to the carbon source is 1:(1~3).

[0015] Specifically, in step (2), the amount of carbon source added is 1 to 5% of the precursor mass, and the amount of the second coating agent added is 1 to 3% of the precursor mass.

[0016] The present invention also provides a lithium manganese iron phosphate material prepared by the method described above.

[0017] Compared with the prior art, the beneficial effects achieved by the present invention are: (1) The present invention has successfully improved the compaction density and first discharge specific capacity of lithium manganese iron phosphate material through a step-by-step coating process, effectively solving the technical problem that the two are difficult to improve in synergy in this material system.

[0018] (2) The present invention adopts a specific sequence and component design of first morphology regulation and then interface strengthening to ensure the orderly construction and tight bonding of the functional coating layer, which significantly enhances the structural stability and lithium-ion transport performance of the material.

[0019] (3) While achieving high performance, the present invention precisely controls the residual carbon content in a low range of 1.2~1.5wt% and obtains excellent cycle stability. Attached Figure Description

[0020] Figure 1 This is a SEM image of the lithium manganese iron phosphate material of Example 2 of the present invention.

[0021] Figure 2 This is the XRD pattern of the lithium manganese iron phosphate material of Example 2 of the present invention. Detailed Implementation

[0022] The technical solutions 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 of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0023] The core of this invention lies in a multi-step coating strategy: First, a first coating agent and a carbon source are introduced during the precursor synthesis stage. The crystallization guiding effect of their functional groups is used to optimize the sphericity and uniformity of the primary particles, laying the morphological foundation for high capacity. Then, a second coating agent and a carbon source are introduced on the surface of the shaped particles. Through the interfacial cross-linking network formed during the sintering process, the bonding force between particles is enhanced, providing a guarantee for high compaction.

[0024] Through a rigorous step-by-step process prioritizing morphology and then interface, two coating agents with different functional orientations can function in an orderly manner at their respective appropriate stages. The core reason why this order cannot be reversed is that if the interface strengthening step with the second coating agent is performed first, the dense cross-linked carbon network formed in the first sintering will prematurely encapsulate and rigidly restrict the precursor particles, severely inhibiting the flexible guiding effect of the first coating agent on grain nucleation and growth, making it difficult to optimize the particle morphology and thus losing the structural basis for high capacity. Conversely, if the second coating agent is used first to shape the sphericity and uniformity of the particles, a regular and stable adhesion substrate is provided for the subsequent interface carbon network, allowing the strengthening layer to be constructed uniformly and firmly, thereby synergistically achieving a dual improvement in morphology and interface.

[0025] The first coating agent, such as polyvinylpyrrolidone (PVP), utilizes the highly polar pyrrolidone rings and long-chain structure in the PPVP molecule. During the precursor formation stage, it can effectively guide the uniform nucleation and growth of lithium manganese iron phosphate grains into spherical particles with high sphericity and uniform particle size through coordination and steric hindrance effects, thus completing the particle shaping first. The second coating agent, such as polyacrylonitrile (PAC), undergoes cyclization, cross-linking, and carbonization of its cyano groups during secondary sintering at higher temperatures. This forms a strong, dense, and firmly bonded nitrogen-doped carbon network at the particle interface, reinforcing the interface. This design fundamentally coordinates the intrinsic particle characteristics that affect capacity with the particle assembly behavior that affects compaction, thereby simultaneously and significantly improving the compaction density and electrochemical capacity of the material, achieving synergistic optimization of performance.

[0026] This invention provides a method for preparing a stepwise coated lithium manganese iron phosphate material, comprising: (1) Mix lithium carbonate, ferrous oxalate, manganese carbonate and ammonium dihydrogen phosphate in a molar ratio of 1:(0.2~0.3):(0.7~0.8):(1~1.1), first add polyvinylpyrrolidone and sucrose accounting for 0.1~3wt% of the total mass of raw materials, then add deionized water, stir and mix to prepare a first slurry with a solid content of 40~50%; (2) The first slurry is transferred to a sand mill and ground at a speed of 1000~2500r / min and a feed rate of 100~300mL / min. The ground slurry is centrifugally spray-dried at a feed rate of 8~12mL / min. The dried powder is placed in an atmosphere sintering furnace and heated at a heating rate of 2℃ / min under argon protection for the first sintering. Then it is cooled with the furnace to obtain the precursor. (3) The precursor is redispersed in deionized water at a solid content of 30-40%. Polyvinyl alcohol of 1-5% of the precursor mass and polyacrylonitrile of 1-3% of the precursor mass are added to the dispersion. At the same time, lithium hydroxide of 0.5-2% of the precursor mass is added. The mixture is stirred and mixed evenly to obtain the second slurry. (4) The second slurry is transferred to a sand mill and ground at a speed of 1000~2500r / min and a feed rate of 100~300mL / min. The ground slurry is then centrifugally spray-dried at a feed rate of 10~12mL / min. The dried powder is then sintered for the second time under argon protection and cooled to room temperature. The sintered material is then removed and subjected to air jet pulverization to obtain lithium manganese iron phosphate material.

[0027] Preferably, the mass ratio of polyvinylpyrrolidone to sucrose is 1:(1~3), more preferably 1:2. Preferably, the particle size of the slurry after grinding is 300~1000nm.

[0028] Preferably, the parameters of the spray dryer are: outlet temperature of 95~105℃ and spray pressure of 0.2~0.4MPa.

[0029] Preferably, the temperature of the first sintering is 250~550℃ and the time is 4~7h.

[0030] Preferably, the second sintering temperature is 600~800℃ and the time is 8~12h.

[0031] Example 1; (1) Weigh and mix lithium carbonate, ferrous oxalate, manganese carbonate and ammonium dihydrogen phosphate in a molar ratio of 1:0.2:0.7:1.0. First, add polyvinylpyrrolidone and sucrose accounting for 0.1 wt% of the total mass of raw materials. The mass ratio of polyvinylpyrrolidone and sucrose is 1:1. Then add deionized water, stir and mix to prepare a first slurry with a solid content of 40 wt%. (2) The first slurry was transferred to a sand mill and ground at a speed of 1000 r / min and a feed rate of 100 mL / min until the slurry particle size reached 1000 nm. The ground slurry was then centrifugally spray-dried at a feed rate of 8 mL / min, an outlet temperature of 95 °C, and a spray pressure of 0.2 MPa. The dried powder was placed in an atmosphere sintering furnace and heated to 250 °C at a heating rate of 2 °C / min under argon protection for the first sintering for 7 h. The powder was then cooled with the furnace to obtain the precursor. (3) The precursor is redispersed in deionized water at a solid content of 30wt%. Polyvinyl alcohol and polyacrylonitrile at a mass equivalent of 1% of the precursor are added to the dispersion, and lithium hydroxide at a mass equivalent of 0.5% of the precursor is added. The mixture is stirred and mixed evenly to obtain the second slurry. (4) The second slurry was transferred to a sand mill and ground at a speed of 1000 r / min and a feed rate of 100 mL / min until the slurry particle size reached 300 nm. The ground slurry was then centrifugally spray-dried at a feed rate of 10 mL / min, an outlet temperature of 95 °C, and a spray pressure of 0.2 MPa. The dried powder was then sintered for the second time under argon protection at a temperature of 600 °C for 8 h. After cooling to room temperature, the material was removed and subjected to air jet milling to obtain lithium manganese iron phosphate material.

[0032] Example 2; (1) Weigh and mix lithium carbonate, ferrous oxalate, manganese carbonate and ammonium dihydrogen phosphate in a molar ratio of 1:0.25:0.75:1.05. First, add polyvinylpyrrolidone and sucrose accounting for 1.55 wt% of the total raw material mass. The mass ratio of polyvinylpyrrolidone and sucrose is 1:2. Then add deionized water, stir and mix to prepare a first slurry with a solid content of 45 wt%. (2) The first slurry was transferred to a sand mill and ground at a speed of 2000 r / min and a feed rate of 200 mL / min until the slurry particle size reached 600 nm. The ground slurry was then centrifugally spray-dried at a feed rate of 10 mL / min, an outlet temperature of 100 °C, and a spray pressure of 0.3 MPa. The dried powder was placed in an atmosphere sintering furnace and heated to 400 °C at a heating rate of 2 °C / min under argon protection for the first sintering for 6 h. The powder was then cooled with the furnace to obtain the precursor. (3) The precursor is redispersed in deionized water at a solid content of 35wt%. Polyvinyl alcohol equivalent to 2.5% of the precursor mass and polyacrylonitrile equivalent to 1.6% of the precursor mass are added to the dispersion. At the same time, lithium hydroxide equivalent to 1.25% of the precursor mass is added. The mixture is stirred and mixed evenly to obtain the second slurry. (4) The second slurry was transferred to a sand mill and ground at a speed of 2000 r / min and a feed rate of 200 mL / min until the slurry particle size reached 400 nm. The ground slurry was then centrifugally spray-dried at a feed rate of 11 mL / min, an outlet temperature of 100 °C, and a spray pressure of 0.3 MPa. The dried powder was then sintered for the second time under argon protection at a temperature of 700 °C for 9 h. After cooling to room temperature, the material was removed and subjected to air jet milling to obtain lithium manganese iron phosphate material.

[0033] Example 3; (1) Weigh and mix lithium carbonate, ferrous oxalate, manganese carbonate and ammonium dihydrogen phosphate in a molar ratio of 1:0.22:0.72:1.02. First, add polyvinylpyrrolidone and sucrose, accounting for 0.66 wt% of the total raw material mass, with a mass ratio of 1:3 for polyvinylpyrrolidone and sucrose. Then add deionized water, stir and mix to prepare a first slurry with a solid content of 42 wt%. (2) The first slurry was transferred to a sand mill and ground at a speed of 1500 r / min and a feed rate of 150 mL / min until the slurry particle size reached 300 nm. The ground slurry was then centrifugally spray-dried at a feed rate of 9 mL / min, an outlet temperature of 97 °C, and a spray pressure of 0.25 MPa. The dried powder was placed in an atmosphere sintering furnace and heated to 300 °C at a heating rate of 2 °C / min under argon protection for the first sintering for 5 h. The powder was then cooled with the furnace to obtain the precursor. (3) The precursor is redispersed in deionized water at a solid content of 32wt%. Polyvinyl alcohol equivalent to 3% of the precursor mass and polyacrylonitrile equivalent to 2% of the precursor mass are added to the dispersion. At the same time, lithium hydroxide equivalent to 0.8% of the precursor mass is added. The mixture is stirred and mixed evenly to obtain the second slurry. (4) The second slurry was transferred to a sand mill and ground at a speed of 1500 r / min and a feed rate of 150 mL / min until the slurry particle size reached 500 nm. The ground slurry was then centrifugally spray-dried at a feed rate of 10.5 mL / min, an outlet temperature of 97 °C, and a spray pressure of 0.25 MPa. The dried powder was then sintered for the second time under argon protection at a temperature of 650 °C for 10 h. After cooling to room temperature, the material was removed and subjected to air jet milling to obtain lithium manganese iron phosphate material.

[0034] Example 4; (1) Weigh and mix lithium carbonate, ferrous oxalate, manganese carbonate and ammonium dihydrogen phosphate in a molar ratio of 1:0.29:0.79:1.09. First, add polyvinylpyrrolidone and sucrose, accounting for 2.3 wt% of the total raw material mass, with a mass ratio of 1:1 for polyvinylpyrrolidone and sucrose. Then add deionized water, stir and mix to prepare a first slurry with a solid content of 48 wt%. (2) The first slurry was transferred to a sand mill and ground at a speed of 2400 r / min and a feed rate of 280 mL / min until the slurry particle size reached 400 nm. The ground slurry was then centrifugally spray-dried at a feed rate of 11.5 mL / min, an outlet temperature of 104 °C, and a spray pressure of 0.38 MPa. The dried powder was placed in an atmosphere sintering furnace and heated to 500 °C at a heating rate of 2 °C / min under argon protection for the first sintering for 4 h. The powder was then cooled with the furnace to obtain the precursor. (3) The precursor is redispersed in deionized water at a solid content of 39wt%. Polyvinyl alcohol equivalent to 3.5% of the precursor mass and polyacrylonitrile equivalent to 2.5% of the precursor mass are added to the dispersion. At the same time, lithium hydroxide equivalent to 1.9% of the precursor mass is added. The mixture is stirred and mixed evenly to obtain the second slurry. (4) The second slurry was transferred to a sand mill and ground at a speed of 2400 r / min and a feed rate of 280 mL / min until the slurry particle size reached 600 nm. The ground slurry was then centrifugally spray-dried at a feed rate of 11.8 mL / min, an outlet temperature of 104 °C, and a spray pressure of 0.38 MPa. The dried powder was then sintered for the second time under argon protection at a temperature of 780 °C for 11 h. After cooling to room temperature, the material was removed and subjected to air jet milling to obtain lithium manganese iron phosphate material.

[0035] Example 5; (1) Weigh and mix lithium carbonate, ferrous oxalate, manganese carbonate and ammonium dihydrogen phosphate in a molar ratio of 1:0.29:0.79:1.09. First, add polyvinylpyrrolidone and sucrose, accounting for 2.75 wt% of the total raw material mass, with a mass ratio of 1:2.5 between polyvinylpyrrolidone and sucrose. Then add deionized water, stir and mix to prepare a first slurry with a solid content of 48 wt%. (2) The first slurry was transferred to a sand mill and ground at a speed of 2400 r / min and a feed rate of 280 mL / min until the slurry particle size reached 350 nm. The ground slurry was then centrifugally spray-dried at a feed rate of 11.5 mL / min, an outlet temperature of 104 °C, and a spray pressure of 0.38 MPa. The dried powder was placed in an atmosphere sintering furnace and heated to 500 °C at a heating rate of 2 °C / min under argon protection for the first sintering for 7 h. The powder was then cooled with the furnace to obtain the precursor. (3) The precursor is redispersed in deionized water at a solid content of 39 wt%. Polyvinyl alcohol equivalent to 4% of the precursor mass and polyacrylonitrile equivalent to 2.8% of the precursor mass are added to the dispersion. At the same time, lithium hydroxide equivalent to 1.9% of the precursor mass is added. The mixture is stirred and mixed evenly to obtain the second slurry. (4) The second slurry was transferred to a sand mill and ground at a speed of 2400 r / min and a feed rate of 280 mL / min until the slurry particle size reached 700 nm. The ground slurry was then centrifugally spray-dried at a feed rate of 11.8 mL / min, an outlet temperature of 104 °C, and a spray pressure of 0.38 MPa. The dried powder was then sintered for the second time under argon protection at a temperature of 780 °C for 12 h. After cooling to room temperature, the material was removed and subjected to air jet milling to obtain lithium manganese iron phosphate material.

[0036] Example 6; (1) Weigh and mix lithium carbonate, ferrous oxalate, manganese carbonate and ammonium dihydrogen phosphate in a molar ratio of 1:0.3:0.8:1.1. First, add polyvinylpyrrolidone and sucrose accounting for 3.0 wt% of the total raw material mass. The mass ratio of polyvinylpyrrolidone to sucrose is 1:1.5. Then add deionized water, stir and mix to prepare a first slurry with a solid content of 50 wt%. (2) The first slurry was transferred to a sand mill and ground at a speed of 2500 r / min and a feed rate of 300 mL / min until the slurry particle size reached 300 nm. The ground slurry was then centrifugally spray-dried at a feed rate of 12 mL / min, an outlet temperature of 105 °C, and a spray pressure of 0.4 MPa. The dried powder was placed in an atmosphere sintering furnace and heated to 550 °C at a heating rate of 2 °C / min under argon protection for the first sintering for 6 h. The powder was then cooled with the furnace to obtain the precursor. (3) The precursor is redispersed in deionized water at a solid content of 40wt%. Polyvinyl alcohol equivalent to 5% of the precursor mass and polyacrylonitrile equivalent to 3% of the precursor mass are added to the dispersion. At the same time, lithium hydroxide equivalent to 2% of the precursor mass is added. The mixture is stirred and mixed evenly to obtain the second slurry. (4) The second slurry was transferred to a sand mill and ground at a speed of 2500 r / min and a feed rate of 300 mL / min until the slurry particle size reached 1000 nm. The ground slurry was then centrifugally spray-dried at a feed rate of 12 mL / min, an outlet temperature of 105 °C, and a spray pressure of 0.4 MPa. The dried powder was then sintered for the second time under argon protection at a temperature of 800 °C for 9 h. After cooling to room temperature, the material was removed and subjected to air jet milling to obtain lithium manganese iron phosphate material.

[0037] Comparative Example 1; (1) Weigh and mix lithium carbonate, ferrous oxalate, manganese carbonate and ammonium dihydrogen phosphate in a molar ratio of 1:0.25:0.75:1.05, add sucrose accounting for 1.55 wt% of the total mass of raw materials, add deionized water, stir and mix, and prepare a first slurry with a solid content of 45 wt%. (2) The first slurry was transferred to a sand mill and ground at a speed of 2000 r / min and a feed rate of 200 mL / min until the slurry particle size reached 600 nm. The ground slurry was then centrifugally spray-dried at a feed rate of 10 mL / min, an outlet temperature of 100 °C, and a spray pressure of 0.3 MPa. The dried powder was placed in an atmosphere sintering furnace and heated to 400 °C at a heating rate of 2 °C / min under argon protection for the first sintering for 6 h. The powder was then cooled with the furnace to obtain the precursor. (3) The precursor is redispersed in deionized water at a solid content of 35wt%. Polyvinyl alcohol equivalent to 2.5% of the precursor mass is added to the dispersion, and lithium hydroxide equivalent to 1.25% of the precursor mass is added at the same time. The mixture is stirred and mixed evenly to obtain the second slurry. (4) The second slurry was transferred to a sand mill and ground at a speed of 2000 r / min and a feed rate of 200 mL / min until the slurry particle size reached 400 nm. The ground slurry was then centrifugally spray-dried at a feed rate of 11 mL / min, an outlet temperature of 100 °C, and a spray pressure of 0.3 MPa. The dried powder was then sintered for the second time under argon protection at a temperature of 700 °C for 9 h. After cooling to room temperature, the material was removed and subjected to air jet milling to obtain lithium manganese iron phosphate material.

[0038] Comparative Example 2; The difference between Comparative Example 2 and Example 2 lies in the difference in step (1). Step (1) is modified as follows: weigh and mix lithium carbonate, ferrous oxalate, manganese carbonate and ammonium dihydrogen phosphate in a molar ratio of 1:0.25:0.75:1.05, add sucrose accounting for 1.55 wt% of the total mass of raw materials, then add deionized water, stir and mix, and prepare a first slurry with a solid content of 45 wt%; the remaining steps are the same as in Example 2.

[0039] Comparative Example 3; The difference between Comparative Example 3 and Example 2 is the difference in step (3). Step (3) is modified as follows: the precursor is redispersed in deionized water at a solid content of 35wt%, and polyvinyl alcohol equivalent to 2.5% of the precursor mass is added to the dispersion, along with lithium hydroxide at 1.25% of the precursor mass. The mixture is stirred and mixed evenly to obtain the second slurry; the remaining steps are the same as in Example 2.

[0040] Comparative Example 4; (1) Weigh and mix lithium carbonate, ferrous oxalate, manganese carbonate and ammonium dihydrogen phosphate in a molar ratio of 1:0.25:0.75:1.05. First, add polyvinylpyrrolidone, polyacrylonitrile and sucrose accounting for 1.55 wt% of the total raw material mass. The mass ratio of polyvinylpyrrolidone, polyacrylonitrile and sucrose is 1:1:2. Then add deionized water, stir and mix to prepare the first slurry with a solid content of 45 wt%. (2) The first slurry was transferred to a sand mill and ground at a speed of 2000 r / min and a feed rate of 200 mL / min until the slurry particle size reached 400 nm. The ground slurry was then centrifugally spray-dried at a feed rate of 11 mL / min, an outlet temperature of 100 °C, and a spray pressure of 0.3 MPa. The dried powder was then sintered for the second time under argon protection at a temperature of 700 °C for 9 h. After cooling to room temperature, the material was removed and subjected to air jet milling to obtain lithium manganese iron phosphate material. The remaining steps were the same as in Example 2.

[0041] Comparative Example 5; (1) Weigh and mix lithium carbonate, ferrous oxalate, manganese carbonate and ammonium dihydrogen phosphate in a molar ratio of 1:0.25:0.75:1.05, add 2.5% polyvinyl alcohol and 1.6% polyacrylonitrile, then add deionized water, stir and mix to prepare a first slurry with a solid content of 45wt%; (2) The first slurry was transferred to a sand mill and ground at a speed of 2000 r / min and a feed rate of 200 mL / min until the slurry particle size reached 600 nm. The ground slurry was then centrifugally spray-dried at a feed rate of 10 mL / min, an outlet temperature of 100 °C, and a spray pressure of 0.3 MPa. The dried powder was placed in an atmosphere sintering furnace and heated to 400 °C at a heating rate of 2 °C / min under argon protection for the first sintering for 6 h. The powder was then cooled with the furnace to obtain the precursor. (3) The precursor is redispersed in deionized water at a solid content of 35wt%. Polyvinylpyrrolidone and sucrose at a mass ratio of 1.55wt% of the precursor are added to the dispersion. The mass ratio of polyvinylpyrrolidone to sucrose is 1:2. Then lithium hydroxide at a mass of 1.25% of the precursor is added. The mixture is stirred and mixed evenly to obtain the second slurry. (4) The second slurry was transferred to a sand mill and ground at a speed of 2000 r / min and a feed rate of 200 mL / min until the slurry particle size reached 400 nm. The ground slurry was then centrifugally spray-dried at a feed rate of 11 mL / min, an outlet temperature of 100 °C, and a spray pressure of 0.3 MPa. The dried powder was then sintered for the second time under argon protection at a temperature of 700 °C for 9 h. After cooling to room temperature, the material was removed and subjected to air jet milling to obtain lithium manganese iron phosphate material.

[0042] Test methods Compaction density test: 2.00g of lithium manganese iron phosphate material obtained from each example and comparative example was taken and mixed evenly with conductive carbon black and polyvinylidene fluoride (PVDF) binder in N-methylpyrrolidone (NMP) at a mass ratio of 92:4:4. The mixture was then coated onto aluminum foil and vacuum dried at 120℃ for 12h. After compaction, 2.00g of lithium manganese iron phosphate material obtained from each example and comparative example was taken and the electrode sheet was pressed under a pressure of 30MPa using a pressure density meter. The compaction density of the material was calculated by measuring the mass, area and thickness of the electrode sheet. The results are shown in Table 1.

[0043] Electrochemical performance testing: Using the electrode sheet made of the above-mentioned lithium manganese iron phosphate material as the positive electrode, the lithium metal sheet as the negative electrode, Celgard 2400 as the separator, and 1M LiPF6 / EC+DMC (volume ratio 1:1) as the electrolyte, CR2032 coin cells were assembled in an argon glove box. The Blue Battery testing system was used, and the test was conducted in a constant temperature environment of 25℃. The first cycle was charged at a constant current of 0.1C to 4.5V, then constant voltage was applied until the current dropped to 0.05C. After standing for 5 minutes, the cells were discharged at a constant current of 0.1C to 2.5V. The specific capacity of the first discharge was recorded. Subsequently, the cycling performance test at 1C rate was carried out. The initial discharge capacity at 0.1C and the capacity retention rate after 100 cycles at 1C were calculated. The results are shown in Table 1.

[0044] Residual carbon content test: Take 1.00g of lithium manganese iron phosphate sample, place it in an alumina crucible, heat it to 800℃ at 5℃ / min in air atmosphere and keep it at that temperature for 2h. Calculate the residual carbon content based on the mass difference before and after calcination. The results are shown in Table 1.

[0045] Table 1

[0046] As shown in Table 1, Examples 1-6 demonstrate that the lithium manganese iron phosphate material prepared by the stepwise coating method of the present invention exhibits a significant and synergistic improvement in both compaction density and 0.1C first discharge capacity. This proves that by first adding the first coating agent to optimize the morphology and then adding the second coating agent to achieve interface strengthening, the specific sequence resolves the contradiction between compacted lithium manganese iron phosphate and capacity. At the same time, the residual carbon content is controlled within a reasonable range (1.2-1.5%).

[0047] Comparative Example 1 shows that, without adding any coating agent of the present invention, the compaction density of the obtained material is only 2.253 g / cm³, and the first discharge capacity at 0.1C is only 150.5 mAh / g, with the lowest overall performance index. This proves that coating treatment is a necessary basis for improving the electrochemical performance and physical density of lithium manganese iron phosphate materials.

[0048] Comparative Example 2 shows that without the addition of the first coating agent for morphology optimization, the 0.1C first release capacity is improved but significantly lower than that of the complete example, and the compaction density is also only slightly improved. This proves that the first coating agent is crucial for controlling particle sphericity and uniformity, optimizing lithium-ion transport paths, and thus laying the foundation for high capacity.

[0049] Comparative Example 3 shows that without the addition of a second coating agent for interface strengthening, the increase in compaction density is very small, far lower than that of the complete embodiment. This demonstrates that the second coating agent is crucial for enhancing interparticle interaction forces, constructing a strong interface, and thus effectively converting excellent particle morphology into high compaction density.

[0050] Comparative Example 4 shows that even when the same raw materials as in the examples are used, the compaction density and capacity of the coating agent prepared by mixing all the coating agents with the carbon source at one time are much lower than those of the stepwise coating examples. This proves that the process sequence of stepwise coating is crucial for achieving the orderly action of functional components, avoiding mutual interference, and thus obtaining synergistic effects.

[0051] Comparative Example 5 shows that even when the coating agent is added in steps, if the order is reversed to add the second coating agent first and then the first coating agent, the overall performance is still significantly lower than that of Example 2. This further proves that optimizing the morphology first and then strengthening the interface is the key technology to achieve the synergistic maximization of compaction density and electrochemical capacity.

[0052] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from its spirit or essential characteristics. Therefore, the embodiments should be considered in all respects as exemplary and non-limiting, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, all variations falling within the meaning and scope of equivalents of the claims are intended to be included within the present invention. No markings in the claims should be construed as limiting the scope of the claims.

Claims

1. A method for stepwise coating preparation of lithium manganese iron phosphate materials, characterized in that, Includes the following steps: (1) A precursor is obtained by mixing lithium source, iron source, manganese source, phosphorus source with a first coating agent and carbon source, followed by wet grinding, drying and first sintering; (2) The above precursor is mixed with the second coating agent and carbon source, and then wet-milled, dried and sintered for a second time to obtain the lithium manganese iron phosphate material.

2. The method according to claim 1, characterized in that, The lithium source is one of lithium hydroxide, lithium peroxide, lithium oxide, lithium formate, lithium nitrate, and lithium carbonate.

3. The method according to claim 1, characterized in that, The iron source is one of ferrous oxide, ferric oxide, ferric oxalate, ferric phosphate, ferric nitrate, and ferrous acetate.

4. The method according to claim 1, characterized in that, The phosphorus source is one of lithium monohydrogen phosphate, lithium dihydrogen phosphate, lithium phosphate, ammonium dihydrogen phosphate, and phosphoric acid.

5. The method according to claim 1, characterized in that, The first coating agent is at least one of citric acid, ascorbic acid, polyvinylpyrrolidone, and sophorate.

6. The method according to claim 1, characterized in that, The second coating agent is at least one of tannic acid, chitosan, polyacrylonitrile, and polyacrylic acid.

7. The method according to claim 1, characterized in that, The carbon source is at least one of sucrose, glucose, fructose, polyvinyl alcohol, polyethylene glycol, and starch.

8. The method according to claim 1, characterized in that, In step (1), the molar ratio of lithium source, iron source, manganese source and phosphorus source is 1:(0.2~0.3):(0.7~0.8):(1~1.1); the mass ratio of the first coating agent to the carbon source is 1:(1~3).

9. The method according to claim 1, characterized in that, In step (2), the amount of carbon source added is 1 to 5% of the precursor mass, and the amount of the second coating agent added is 1 to 3% of the precursor mass.

10. A lithium manganese iron phosphate material prepared by the method according to any one of claims 1-9.

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