A lithium iron phosphate composite material, its preparation method and application

Abstract

This invention provides a lithium iron phosphate composite material, its preparation method, and its application. The lithium iron phosphate composite material includes a core and a shell covering the surface of the core. The core includes doped lithium iron phosphate, and the shell includes nitrogen-doped carbon material and two-dimensional nanosheets. The two-dimensional nanosheets are two-dimensional layered nanosheets with intercalated lithium supplements. This invention achieves high-speed and low-energy-consumption electron transport of lithium iron phosphate by coating the surface of doped lithium iron phosphate with carbon and two-dimensional layered nanosheets with intercalated lithium supplements. By intercalating the lithium supplement into the two-dimensional layered nanosheets, the chemical stability of the lithium supplement is improved, thereby enhancing the electrochemical performance of lithium iron phosphate.

Description

Technical Field

[0001] This invention belongs to the field of battery technology and relates to a lithium iron phosphate composite material, its preparation method and application. Background Technology

[0002] With its advantages of rechargeability and long cycle life, high-energy-density rechargeable lithium-ion batteries are considered the ideal power source for the next generation, especially in the field of electric vehicles.

[0003] LiFePO4 is one of the most important cathode materials for electric vehicles. Compared with other cathode materials, LiFePO4 does not contain precious metal elements, making it not only inexpensive, safe, and non-toxic, but also highly environmentally compatible. The stable olivine structure of lithium iron phosphate allows it to withstand long-term repeated charge-discharge cycles and possesses excellent cycle performance. However, the oxygen atoms in the PO4 tetrahedra separate the FeO6 octahedra, preventing the formation of a continuous octahedral structure. This disrupts the internal conductive network structure, limiting lithium ions to diffuse only through one-dimensional channels, resulting in poor electrical performance, especially rate performance.

[0004] Existing technologies have focused on improving the electron and lithium-ion transport rates of lithium iron phosphate. For example, CN117088348A discloses a method for carbon-coated modified lithium iron phosphate cathodes, which improves interfacial conductivity by coating a conductive carbon layer on the outside of lithium iron phosphate. However, this reduces its compaction density and thus affects its energy density. Alternatively, CN 104393291A discloses a doped and coated co-modified lithium vanadium phosphate cathode material and its preparation method. This method prepares lithium iron phosphate by doping and coating co-modification, using iron site doping to expand the lattice and improve the lithium-ion transport rate, and carbon coating to improve the electron transport rate. However, this method fails to change the fact that lithium iron phosphate has low intrinsic conductivity.

[0005] Based on the above research, there is a need for a lithium iron phosphate composite material that can improve the intrinsic conductivity and lithium-ion diffusion rate of lithium iron phosphate. Summary of the Invention

[0006] The purpose of this invention is to provide a lithium iron phosphate composite material, its preparation method and application. The lithium iron phosphate composite material, through two-dimensional layered nanosheets coated with carbon and intercalated with lithium supplement, not only achieves high-speed and low-energy electron transport of lithium iron phosphate, but also improves the chemical stability of the lithium supplement and enhances the electrochemical performance of lithium iron phosphate.

[0007] To achieve this objective, the present invention adopts the following technical solution:

[0008] In a first aspect, the present invention provides a lithium iron phosphate composite material, the lithium iron phosphate composite material comprising a core and a shell covering the surface of the core, the core comprising doped lithium iron phosphate, and the shell comprising nitrogen-doped carbon material and two-dimensional nanosheets;

[0009] The two-dimensional nanosheet is a two-dimensional layered nanosheet intercalated with a lithium supplement agent.

[0010] The present invention uses a two-dimensional layered nanosheet intercalated with a lithium supplement agent and carbon to hybridly coat a lithium iron phosphate cathode material. The two-dimensional layered nanosheet can achieve high-speed and low-energy-consuming electron transport, reduce heat loss, and moreover, carbon can fix the two-dimensional layered nanosheet intercalated with the lithium supplement agent on the inner core, while enhancing the electrical conductivity of the outer shell; for the two-dimensional layered nanosheet intercalated with the lithium supplement agent, the lithium supplement agent is dispersed between the layers of the two-dimensional layered nanosheet. On the one hand, it isolates the contact between the lithium supplement agent and the external environment, improves the chemical stability of the lithium supplement agent, so that during the first charging process, lithium ions in the lithium supplement agent migrate to the negative electrode to compensate for the lithium ions lost during the formation of the SEI; on the other hand, the insertion of the lithium supplement agent expands the layer spacing of the two-dimensional layered nanosheet, enabling the two-dimensional layered nanosheet to adapt to the volume change caused by the repeated charge and discharge of the cathode material, and improving the electrochemical performance of the doped lithium iron phosphate.

[0011] Preferably, the two-dimensional layered nanosheet includes any one or a combination of at least two of Bi2Se3 nanosheets, Sb2Te3 nanosheets, Bi2Se3 nanosheets or Sb2Se3 nanosheets.

[0012] The above-mentioned nanosheet adopted in the present invention has a unique layered topological structure, which can achieve high-speed and low-energy-consuming electron transport and reduce heat loss.

[0013] Preferably, the lithium supplement agent includes lithium oxide and / or lithium phosphide.

[0014] The lithium supplement agent adopted in the present invention is a material resistant to high temperatures above 550 °C, such as lithium oxide and / or lithium phosphide.

[0015] The carbon and nitrogen in the outer shell of the present invention are obtained by calcining dopamine, which can enhance the electrical conductivity of the outer shell and can also adhere the two-dimensional layered nanosheet intercalated with the lithium supplement agent on the surface of the inner core during the preparation process.

[0016] Preferably, the chemical general formula of the doped lithium iron phosphate is Li 3+ , M x FePO4, where 0.01 < x < 0.05, for example, it can be 0.011, 0.02, 0.03, 0.04 or 0.049, and M includes Mg 2+ , Al 3+ , Ti 4+ , Nb 5+ or W 6+ or a combination of any one or at least two of them.

[0017] The doped lithium iron phosphate of the present invention is also doped with high-valence metal ions, which can improve the intrinsic conductivity of the doped lithium iron phosphate. Furthermore, the doping with high-valence metal ions forms lithium iron phosphate with cation defects, which creates lithium vacancies in order to maintain electrical neutrality and improves the migration rate of lithium ions.

[0018] Preferably, in the lithium iron phosphate composite material, the mass ratio of doped lithium iron phosphate, two-dimensional nanosheets, and nitrogen-doped carbon material is 1:(0.01-0.2):(0.01-0.08), for example, it can be 1:0.1:0.05, 1:0.01:0.08, or 1:0.2:0.01, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0019] Preferably, in the outer shell, the mass ratio of lithium supplement to two-dimensional layered nanosheets is (0.05-0.25):1, for example, it can be 0.05:1, 0.1:1, 0.2:1 or 0.25:1, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0020] In a second aspect, the present invention provides a method for preparing the lithium iron phosphate composite material as described in the first aspect, the method comprising the following steps:

[0021] The lithium iron phosphate composite material is obtained by mixing and calcining doped lithium iron phosphate, a first carbon source, and two-dimensional nanosheets.

[0022] The first carbon source includes nitrogen; the two-dimensional nanosheet is a two-dimensional layered nanosheet with lithium supplementation intercalation.

[0023] The present invention obtains the lithium iron phosphate composite material by mixing and calcining doped lithium iron phosphate, a first carbon source, and two-dimensional layered nanosheets with lithium supplementation intercalation; wherein, the first carbon source can adhere the two-dimensional layered nanosheets with lithium supplementation intercalation to the surface of the doped lithium iron phosphate.

[0024] Preferably, the mass ratio of the doped lithium iron phosphate, the two-dimensional nanosheets, and the first carbon source is 1:(0.01-0.2):(0.01-0.12), for example, it can be 1:0.01:0.01, 1:0.05:0.05, 1:0.1:0.1, 1:0.15:0.12, or 1:0.2:0.12, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0025] The mass ratio of the doped lithium iron phosphate, the two-dimensional layered nanosheets of the lithium replenisher intercalation layer, and the first carbon source described in this invention affects the material performance. If too much of the first carbon source is added, it will reduce the proportion of lithium iron phosphate active material in the positive electrode composite material and reduce the capacity of the positive electrode material. If too little of the first carbon source is added, it will not play its role in bonding the two-dimensional layered nanosheets of the lithium replenisher intercalation layer to the core surface. If too much of the two-dimensional layered nanosheets of the lithium replenisher intercalation layer are added, the excess lithium ions will accumulate on the negative electrode interface after the first charge and discharge, hindering the diffusion and mass transfer of lithium ions, resulting in increased polarization of the battery and lithium plating. If too little is added, it will not play its due role in conductivity and lithium replenishment.

[0026] When mixing the doped lithium iron phosphate, the first carbon source, and the two-dimensional layered nanosheets intercalated with the lithium supplementer as described in this invention, the doped lithium iron phosphate is first sonicated in a dispersion containing the two-dimensional layered nanosheets intercalated with the lithium supplementer for 30-60 minutes, for example, 30 minutes, 40 minutes, 50 minutes, or 60 minutes. Then, the first carbon source is added and stirred for 10-60 minutes, for example, 10 minutes, 20 minutes, 30 minutes, 40 minutes, 50 minutes, or 60 minutes, but not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0027] Preferably, the calcination is carried out under a protective atmosphere.

[0028] Preferably, the calcination temperature is 400-550℃, for example, 400℃, 450℃, 500℃ or 550℃, and the time is 2-4h, for example, 2h, 3h or 4h, but not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0029] Preferably, the first carbon source includes dopamine.

[0030] Preferably, the method for preparing the two-dimensional nanosheets includes the following steps:

[0031] A lithium supplement, two-dimensional layered nanosheets, and a solvent are mixed to obtain a dispersion of two-dimensional layered nanosheets containing the lithium supplement intercalation.

[0032] Preferably, the mass ratio of the two-dimensional layered nanosheets to the lithium supplement is 1:(0.05-0.25), for example, it can be 1:0.05, 1:0.1, 1:0.15, 1:0.2 or 1:0.25, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0033] The mass ratio of the two-dimensional layered nanosheets to the lithium replenisher described in this invention affects the performance of the composite material. If there is too much lithium replenisher, some of it will not be able to be intercalated into the two-dimensional nanosheets and will not be able to effectively isolate the environment. If there is too little lithium replenisher, sufficient lithium replenishment will not be achieved during the first charge and discharge of the cathode material.

[0034] Preferably, the solvent includes anhydrous ethanol.

[0035] Preferably, the mixing of the lithium replenishing agent, the two-dimensional layered nanosheets and the solvent includes first dispersing the two-dimensional layered nanosheets in the solvent, and then adding the lithium replenishing agent and sonicating for 45-80 minutes, for example, 45 minutes, 60 minutes, 70 minutes or 80 minutes, but not limited to the listed values, and other unlisted values ​​within the range are also applicable.

[0036] Preferably, the method for preparing the two-dimensional layered nanosheets includes the following steps:

[0037] A polymer and an alcohol solvent are mixed to obtain a polymer solution. Then, the polymer solution, a first element source, a second element source, and an acid are mixed to obtain a mixture. The mixture is heated, and a reducing agent is added to the mixture to carry out a reaction. Then, post-processing is performed to obtain the two-dimensional layered nanosheets.

[0038] Preferably, the concentration of the polymer solution is 2wt%-6wt%, for example, it can be 2wt%, 3wt%, 4wt%, 5wt% or 6wt%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0039] Preferably, in the mixture, the acid concentration is 5wt%-15wt%, for example, it can be 5wt%, 8wt%, 10wt%, 12wt% or 15wt%, and the concentration of the first element source is 0.5-3mol / L, for example, it can be 0.5mol / L, 1mol / L, 2mol / L or 3mol / L, but is not limited to the listed values, and other unlisted values ​​within the range are also applicable.

[0040] Preferably, the molar ratio of the first element source and the second element source is 2:(3-3.2), for example, it can be 2:3, 2:3.1 or 2:3.2, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0041] Preferably, the mixture is heated to 150-190°C, for example, 150°C, 170°C or 190°C, but not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0042] Preferably, the reaction time for adding the reducing agent to the mixture is 10-30 minutes, for example, 10 minutes, 20 minutes or 30 minutes, but not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0043] Preferably, the post-treatment includes washing and drying, wherein the washing solvent used includes acetone.

[0044] Preferably, the first element source includes a bismuth source and / or an antimony source, more preferably a bismuth source, and the second element source includes a selenium source and / or a tellurium source, more preferably a selenium source.

[0045] Preferably, the bismuth source includes any one or a combination of at least two of bismuth acetate, bismuth oxide, bismuth nitrate pentahydrate, or bismuth chloride, and the selenium source includes any one or a combination of at least two of selenium powder, sodium selenite, or selenium dioxide.

[0046] Preferably, the polymer comprises PVP (polyvinylpyrrolidone).

[0047] Preferably, the alcohol solvent includes ethylene glycol.

[0048] Preferably, the acid includes acetic acid.

[0049] Preferably, the reducing agent comprises a solution containing hydroxylamine, wherein the solution containing hydroxylamine comprises hydroxylamine, water, and alcohol.

[0050] Preferably, the concentration of hydroxylamine in the reaction solution obtained by adding the reducing agent is 5wt%-12wt%, for example, it can be 5wt%, 8wt%, 10wt% or 12wt%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0051] Preferably, the method for preparing the doped lithium iron phosphate includes the following steps:

[0052] The lithium source, phosphorus source, iron source, second carbon source and doped metal source are mixed and sintered to obtain the doped lithium iron phosphate.

[0053] Preferably, the lithium source, phosphorus source, iron source, second carbon source and doped metal source are mixed 2-3 times, and the mixing time for each mixing is 1-1.5 hours, for example, 1 hour, 1.25 hours or 1.5 hours. The mixing method is ball milling, and the dispersant for ball milling includes ethanol and / or isopropanol.

[0054] Preferably, the sintering includes first holding at 200-350℃, for example, 200℃, 250℃, 300℃ or 350℃ for 3-6 hours, for example, 3 hours, 4 hours, 5 hours or 6 hours, and then holding at 500-900℃, for example, 500℃, 600℃, 700℃, 800℃ or 900℃ for 6-12 hours, for example, 6 hours, 8 hours, 10 hours or 12 hours, but is not limited to the listed values, and other unlisted values ​​within the range are also applicable.

[0055] Preferably, the sintering is carried out in a protective gas.

[0056] Preferably, the second carbon source includes any one or a combination of at least two of glucose, sucrose, phenolic resin, starch, dextrin, citric acid, oxalic acid, cellulose, or vitamins.

[0057] Preferably, the doped metal source includes any one or a combination of at least two of magnesium, aluminum, titanium, niobium, or tungsten sources.

[0058] Preferably, the lithium source includes any one or a combination of at least two of lithium carbonate, lithium hydroxide, or lithium acetate; the phosphorus source includes any one or a combination of at least two of ammonium dihydrogen phosphate, ammonium phosphate, or lithium dihydrogen phosphate; and the iron source includes any one or a combination of at least two of ferrous oxalate, ferrous sulfate, ferrous acetate, ferrous chloride, or ferrous oxide.

[0059] As a preferred embodiment of the preparation method of the present invention, the preparation method includes the following steps:

[0060] (1) Mix lithium source, phosphorus source, iron source, second carbon source and doped metal source, then keep at 200-350℃ for 3-6h, and then keep at 500-900℃ for 6-12h to obtain the doped lithium iron phosphate.

[0061] (2) A polymer and an alcohol solvent are mixed to obtain a polymer solution with a concentration of 2wt%-6wt%. Then, the polymer solution, a first element source, a second element source and an acid are mixed to obtain a mixture with an acid concentration of 5wt%-15wt% and a first element source concentration of 0.5-3mol / L. The mixture is heated to 150-190℃, and a reducing agent is added to the mixture to react for 10-30min. Then, the mixture is washed and dried to obtain the two-dimensional layered nanosheets.

[0062] The molar ratio of the first element source and the second element source is 2:(3-3.2); the reducing agent includes a solution containing hydroxylamine, and the concentration of hydroxylamine in the reaction solution obtained by adding the reducing agent is 5wt%-12wt%;

[0063] The lithium supplement, the two-dimensional layered nanosheets, and the solvent are mixed to obtain a dispersion of two-dimensional layered nanosheets containing lithium supplement intercalation.

[0064] The mass ratio of the two-dimensional layered nanosheets to the lithium supplement is 1:(0.05-0.25);

[0065] (3) Under a protective atmosphere, the dispersion of the doped lithium iron phosphate, dopamine and the two-dimensional layered nanosheets containing lithium supplementation intercalation in step (1) is mixed and calcined at 400-550°C for 2-4 hours to obtain the lithium iron phosphate composite material.

[0066] The mass ratio of the doped lithium iron phosphate, the lithium supplement intercalated two-dimensional layered nanosheets, and dopamine is 1:(0.01-0.2):(0.01-0.12);

[0067] Steps (1) and (2) are not in any particular order.

[0068] Thirdly, the present invention provides a lithium-ion battery, wherein the lithium-ion battery is a lithium iron phosphate composite material as described in the first aspect.

[0069] Compared with the prior art, the present invention has the following beneficial effects:

[0070] This invention utilizes two-dimensional layered nanosheets with lithium supplementation intercalation and carbon to coat lithium iron phosphate cathode materials. Carbon enhances the conductivity of the material and the stability of the two-dimensional layered nanosheets on the core surface. The two-dimensional layered nanosheets enable high-speed and low-energy electron transport, reducing heat loss, protecting the lithium supplementation, preventing the lithium supplementation from contacting the external environment, and improving the chemical stability of the lithium supplementation. This allows the lithium supplementation to effectively replenish lithium ions. At the same time, the two-dimensional layered nanosheets alleviate the volume changes caused by repeated charging and discharging of the cathode material, thereby improving the electrochemical performance of the doped lithium iron phosphate. Attached Figure Description

[0071] Figure 1 This is a SEM image of the lithium iron phosphate composite material described in Example 5 of the present invention;

[0072] Figure 2 The images show the XRD patterns of the lithium iron phosphate composite material described in Example 5 of this invention, as well as the standard XRD pattern of lithium iron phosphate. Detailed Implementation

[0073] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention in any way.

[0074] Example 1

[0075] This embodiment provides a lithium iron phosphate composite material, which includes a core and a shell covering the surface of the core. The core includes doped lithium iron phosphate, and the shell includes nitrogen-doped carbon material and two-dimensional nanosheets. The two-dimensional nanosheets are Bi2Se3 nanosheets with lithium supplementation intercalation.

[0076] The doped lithium iron phosphate is Li 0.97 Nb 0.03 FePO4; the lithium supplement is lithium oxide; in the lithium iron phosphate composite material, the mass ratio of doped lithium iron phosphate, lithium supplement intercalated Bi2Se3 nanosheets and nitrogen-doped carbon material is 1:0.15:0.06, and the mass ratio of lithium supplement to Bi2Se3 nanosheets is 0.15:1;

[0077] The preparation method includes the following steps:

[0078] (1) Lithium carbonate, ferrous oxalate, niobium nitrate and phosphoric acid were mixed evenly in a molar ratio of Li, Nb, Fe and P of 0.97:0.03:1:1. Then, 2 wt% of glucose of the total mass of the doped lithium iron phosphate was added and placed in a ball mill. Dispersant ethanol was added to the ball mill and the mixture was ball-milled twice, 1 h each time. The ball-milled product was then kept at 300 °C for 3 h in an argon atmosphere and then at 750 °C for 10 h to obtain the doped lithium iron phosphate.

[0079] (2) Dissolve 1g of PVP in 30mL of ethylene glycol solution to obtain a PVP solution with a concentration of 3.3wt%. Then add bismuth nitrate pentahydrate and sodium selenite in a molar ratio of 2:3. The concentration of bismuth nitrate pentahydrate is 0.8mol / L. Add 3mL of glacial acetic acid and continue stirring for 2h until homogeneous. Heat to 170℃ and add a mixture containing hydroxylamine to react for 20min until the final concentration of hydroxylamine in the reaction solution is 8wt%. After naturally cooling to room temperature, wash with acetone by centrifugation and dry under vacuum overnight at 60℃ to obtain the Bi2Se3 nanosheets.

[0080] The hydroxylamine-containing mixture has a hydroxylamine concentration of 50 wt% and a water to ethylene glycol volume ratio of 1:1.

[0081] Bi2Se3 nanosheets were dispersed in anhydrous ethanol, and lithium oxide was added and sonicated for 60 min to obtain a dispersion of Bi2Se3 nanosheets with lithium supplementation intercalation.

[0082] The mass ratio of the Bi2Se3 nanosheets to the lithium supplement is 1:0.15;

[0083] (3) In an argon atmosphere, the doped lithium iron phosphate described in step (1) is sonicated in the dispersion of Bi2Se3 nanosheets containing lithium supplementation intercalation described in step (2) for 40 min, then dopamine is added and stirred for 30 min, and then calcined at 450°C for 2 h under nitrogen protection to obtain the lithium iron phosphate composite material.

[0084] The mass ratio of the doped lithium iron phosphate, the lithium supplement intercalated Bi2Se3 nanosheets, and the dopamine is 1:0.15:0.08.

[0085] Steps (1) and (2) are not in any particular order.

[0086] Example 2

[0087] This embodiment provides a lithium iron phosphate composite material, which includes a core and a shell covering the surface of the core. The core includes doped lithium iron phosphate, and the shell includes nitrogen-doped carbon material and two-dimensional nanosheets. The two-dimensional nanosheets are Bi2Se3 nanosheets with lithium supplementation intercalation.

[0088] The doped lithium iron phosphate is Li 0.97 Nb 0.03 FePO4; the lithium supplement is lithium oxide; in the lithium iron phosphate composite material, the mass ratio of doped lithium iron phosphate, lithium supplement intercalated Bi2Se3 nanosheets and nitrogen-doped carbon material is 1:0.01:0.06, and the mass ratio of lithium supplement to Bi2Se3 nanosheets is 0.15:1;

[0089] The preparation method includes the following steps:

[0090] (1) Lithium carbonate, ferrous oxalate, niobium nitrate and phosphoric acid were mixed evenly in a molar ratio of Li, Nb, Fe and P of 0.97:0.03:1:1. Then, 2% of glucose by mass of the doped lithium iron phosphate was added and placed in a ball mill. Dispersant ethanol was added to the ball mill and the mixture was ball-milled three times for 1.5 hours each time. The ball-milled product was then kept at 200°C for 6 hours and then at 500°C for 12 hours in an argon atmosphere to obtain the doped lithium iron phosphate.

[0091] (2) Dissolve 0.6g of PVP in 30mL of ethylene glycol solution to obtain a PVP solution with a concentration of 2wt%. Then add bismuth nitrate pentahydrate and sodium selenite in a molar ratio of 2:3. The concentration of bismuth nitrate pentahydrate is 2mol / L. Add 5mL of glacial acetic acid and continue stirring for 2h until homogeneous. Heat to 190℃ and add a mixture containing hydroxylamine. React for 10min until the final concentration of hydroxylamine in the reaction solution is 12wt%. After naturally cooling to room temperature, wash with acetone by centrifugation and dry under vacuum overnight at 60℃ to obtain the Bi2Se3 nanosheets.

[0092] The hydroxylamine-containing mixture has a hydroxylamine concentration of 50 wt% and a water to ethylene glycol volume ratio of 1:1.

[0093] Bi2Se3 nanosheets were dispersed in anhydrous ethanol, and lithium oxide was added and sonicated for 80 min to obtain a dispersion of Bi2Se3 nanosheets with lithium supplementation intercalation.

[0094] The mass ratio of the Bi2Se3 nanosheets to the lithium supplement is 1:0.15;

[0095] (3) In an argon atmosphere, the doped lithium iron phosphate described in step (1) is sonicated in the dispersion of Bi2Se3 nanosheets containing lithium supplementation intercalation described in step (2) for 60 min, then dopamine is added and stirred for 10 min, and then calcined at 550°C for 2 h under nitrogen protection to obtain the lithium iron phosphate composite material.

[0096] The mass ratio of the doped lithium iron phosphate, the lithium supplement intercalated Bi2Se3 nanosheets, and the dopamine is 1:0.01:0.08.

[0097] Steps (1) and (2) are not in any particular order.

[0098] Example 3

[0099] This embodiment provides a lithium iron phosphate composite material, which includes a core and a shell covering the surface of the core. The core includes doped lithium iron phosphate, and the shell includes nitrogen-doped carbon material and two-dimensional nanosheets. The two-dimensional nanosheets are Bi2Se3 nanosheets with lithium supplementation intercalation.

[0100] The doped lithium iron phosphate is Li 0.98 Nb 0.02 FePO4; the lithium supplement is lithium oxide; in the lithium iron phosphate composite material, the mass ratio of doped lithium iron phosphate, lithium supplement intercalated Bi2Se3 nanosheets and nitrogen-doped carbon material is 1:0.2:0.06, and the mass ratio of lithium supplement to Bi2Se3 nanosheets is 0.15:1;

[0101] The preparation method includes the following steps:

[0102] (1) Lithium carbonate, ferrous oxalate, niobium nitrate and phosphoric acid were mixed evenly in a molar ratio of Li, Nb, Fe and P of 0.98:0.02:1:1. Then, 2% of glucose by mass of the doped lithium iron phosphate was added and placed in a ball mill. Dispersant ethanol was added to the ball mill and the mixture was ball-milled twice, 1 hour each time. The ball-milled product was then kept at 350°C for 3 hours and then at 900°C for 6 hours in an argon atmosphere to obtain the doped lithium iron phosphate.

[0103] (2) Dissolve 1.8g of PVP in 30mL of ethylene glycol solution to obtain a PVP solution with a concentration of 6wt%. Then add bismuth nitrate pentahydrate and sodium selenite in a molar ratio of 2:3. The concentration of bismuth nitrate pentahydrate is 1.5mol / L. Add 3mL of glacial acetic acid and continue stirring for 2h until homogeneous. Heat to 150℃ and add a mixture containing hydroxylamine to react for 30min until the final concentration of hydroxylamine in the reaction solution is 5wt%. After naturally cooling to room temperature, wash with acetone by centrifugation and dry under vacuum overnight at 60℃ to obtain the Bi2Se3 nanosheets.

[0104] The hydroxylamine-containing mixture has a hydroxylamine concentration of 50 wt% and a water to ethylene glycol volume ratio of 1:1.

[0105] Bi2Se3 nanosheets were dispersed in anhydrous ethanol, and lithium oxide was added and sonicated for 45 min to obtain a dispersion of Bi2Se3 nanosheets with lithium supplementation intercalation.

[0106] The mass ratio of the Bi2Se3 nanosheets to the lithium supplement is 1:0.15;

[0107] (3) In an argon atmosphere, the doped lithium iron phosphate described in step (1) is sonicated in the dispersion of Bi2Se3 nanosheets containing lithium supplementation intercalation described in step (2) for 30 min, then dopamine is added and stirred for 60 min, and then calcined at 400°C for 4 h under nitrogen protection to obtain the lithium iron phosphate composite material.

[0108] The mass ratio of the doped lithium iron phosphate, the lithium supplement intercalated Bi2Se3 nanosheets, and the dopamine is 1:0.2:0.08.

[0109] Steps (1) and (2) are not in any particular order.

[0110] Example 4

[0111] This embodiment provides a lithium iron phosphate composite material. Except for the preparation method, in step (2), the mass ratio of Bi2Se3 nanosheets to lithium supplementer is 1:0.06, so that the mass ratio of Bi2Se3 nanosheets to lithium supplementer in the lithium iron phosphate composite material is 1:0.06, the rest is the same as in Example 1.

[0112] Example 5

[0113] This embodiment provides a lithium iron phosphate composite material. Except for the preparation method, in step (2), the mass ratio of Bi2Se3 nanosheets to lithium supplementer is 1:0.24, so that the mass ratio of Bi2Se3 nanosheets to lithium supplementer in the lithium iron phosphate composite material is 1:0.24, the rest is the same as in Example 1.

[0114] The SEM image of the lithium iron phosphate composite material is shown below. Figure 1 As shown, the XRD patterns of the lithium iron phosphate composite material and the standard XRD pattern of lithium iron phosphate are as follows. Figure 2 As shown, by Figure 2 It can be seen that the lithium iron phosphate composite material contains characteristic peaks of Bi2Se3 and lithium oxide, a lithium supplement.

[0115] Example 6

[0116] This embodiment provides a lithium iron phosphate composite material. Except for the preparation method, in step (2), the mass ratio of Bi2Se3 nanosheets to lithium supplementer is 1:0.02, so that the mass ratio of Bi2Se3 nanosheets to lithium supplementer in the lithium iron phosphate composite material is 1:0.02, the rest is the same as in Example 1.

[0117] Example 7

[0118] This embodiment provides a lithium iron phosphate composite material. Except for the preparation method, in step (2), the mass ratio of Bi2Se3 nanosheets to lithium supplementer is 1:0.3, so that the mass ratio of Bi2Se3 nanosheets to lithium supplementer in the lithium iron phosphate composite material is 1:0.3, the rest is the same as in Example 1.

[0119] Example 8

[0120] This embodiment provides a lithium iron phosphate composite material. Except for the preparation method, in step (3), the mass ratio of doped lithium iron phosphate, lithium supplementation intercalated Bi2Se3 nanosheets and dopamine is 1:0.15:0.02, so that the mass ratio of doped lithium iron phosphate, lithium supplementation intercalated Bi2Se3 nanosheets and nitrogen-doped carbon material in the lithium iron phosphate composite material is adapted to 1:0.15:0.01. Otherwise, it is the same as in Example 1.

[0121] Example 9

[0122] This embodiment provides a lithium iron phosphate composite material. Except for the preparation method, in step (3), the mass ratio of doped lithium iron phosphate, lithium supplementation intercalated Bi2Se3 nanosheets and dopamine is 1:0.15:0.11, so that the mass ratio of doped lithium iron phosphate, lithium supplementation intercalated Bi2Se3 nanosheets and nitrogen-doped carbon material in the lithium iron phosphate composite material is adapted to 1:0.15:0.08. The rest is the same as in Example 1.

[0123] Example 10

[0124] This embodiment provides a lithium iron phosphate composite material. Except for the preparation method, in step (3), the mass ratio of doped lithium iron phosphate, lithium supplementation intercalated Bi2Se3 nanosheets and dopamine is 1:0.15:0.005, so that the mass ratio of doped lithium iron phosphate, lithium supplementation intercalated Bi2Se3 nanosheets and nitrogen-doped carbon material in the lithium iron phosphate composite material is adapted to 1:0.15:0.001. Otherwise, it is the same as in Example 1.

[0125] Example 11

[0126] This embodiment provides a lithium iron phosphate composite material. Except for the preparation method, in step (3), the mass ratio of doped lithium iron phosphate, lithium supplementation intercalated Bi2Se3 nanosheets and dopamine is 1:0.15:0.15, so that the mass ratio of doped lithium iron phosphate, lithium supplementation intercalated Bi2Se3 nanosheets and nitrogen-doped carbon material in the lithium iron phosphate composite material is adapted to 1:0.15:0.10. The rest is the same as in Example 1.

[0127] Example 12

[0128] This embodiment provides a lithium iron phosphate composite material. Except for the preparation method, in step (3), the mass ratio of doped lithium iron phosphate, lithium supplementation intercalated Bi2Se3 nanosheets and dopamine is 1:0.005:0.08, so that the mass ratio of doped lithium iron phosphate, lithium supplementation intercalated Bi2Se3 nanosheets and nitrogen-doped carbon material in the lithium iron phosphate composite material is adapted to 1:0.005:0.06. The rest is the same as in Example 1.

[0129] Example 13

[0130] This embodiment provides a lithium iron phosphate composite material. Except for the preparation method, in step (3), the mass ratio of doped lithium iron phosphate, lithium supplementation intercalated Bi2Se3 nanosheets and dopamine is 1:0.25:0.08, so that the mass ratio of doped lithium iron phosphate, lithium supplementation intercalated Bi2Se3 nanosheets and nitrogen-doped carbon material in the lithium iron phosphate composite material is adapted to 1:0.25:0.06. The rest is the same as in Example 1.

[0131] Comparative Example 1

[0132] This comparative example provides a lithium iron phosphate composite material, which is the same as that in Example 1 except that dopamine is not added in step (3) of its preparation method, so that the shell of the lithium iron phosphate composite material does not contain carbon and nitrogen.

[0133] Comparative Example 2

[0134] This comparative example provides a lithium iron phosphate composite material. Except for the fact that the Bi2Se3 nanosheets in step (3) are replaced with Bi2Se3 powder particles in the preparation method to change the adaptability of the lithium iron phosphate composite material, the rest of the lithium iron phosphate composite material is the same as that in Example 1.

[0135] Comparative Example 3

[0136] This comparative example provides a lithium iron phosphate composite material. Except for the fact that in its preparation method, step (2) does not use a lithium supplement agent to intercalate Bi2Se3 nanosheets to change the adaptability of the lithium iron phosphate composite material, the rest of the lithium iron phosphate composite material is the same as that in Example 1.

[0137] The lithium iron phosphate composite materials described in the above embodiments and comparative examples were used to formulate button batteries, and their electrochemical performance was tested. The specific steps were as follows: The lithium iron phosphate composite materials described in the above embodiments and comparative examples, the conductive agent acetylene black, and the adhesive polyvinylidene fluoride were uniformly mixed in N-methylpyrrolidone in a ratio of 90:5:5, and then coated onto the battery electrode aluminum foil. The mixture was then dried in a vacuum drying oven at 120°C. After drying, the batteries were assembled in an argon glove box and pressed into positive electrode sheets with a diameter of 12 mm using a tablet press. The negative electrode was a lithium metal sheet, the electrolyte was 1M LiPF6-EC:DMC (volume ratio 1:1), and a polypropylene porous membrane was used as the separator. The charge and discharge voltage was controlled between 2.5-4.5V, and its electrochemical performance was tested. The test results are shown in Table 1.

[0138] Table 1

[0139]

[0140] As can be seen from Table 1:

[0141] (1) As can be seen from Example 1 and Comparative Example 1, whether or not dopamine is coated on the positive electrode material affects the overall electrical performance of the material. Without the introduction of dopamine self-polymerization coating, the lithium supplement intercalation Bi2Se3 nanosheets cannot be firmly fixed to the outside of lithium iron phosphate as a shell, resulting in a low discharge capacity after long-term cycling. Similarly, without the dopamine carbonized nitrogen-carbon shell to provide conductivity, its conductivity is low, affecting the charge and discharge capacity of the material.

[0142] (2) As can be seen from Example 1 and Comparative Examples 2-3, Bi2Se3 does not exist in the form of nanosheets or does not introduce a lithium replenishing agent, and its first charge-discharge capacity efficiency of the cathode material is low. Among them, in Comparative Example 2, because Bi2Se3 does not exist in the form of nanosheets, the lithium replenishing agent and Bi2Se3 exist in the form of a mixture, which cannot isolate the lithium replenishing agent from the external environment, and its lithium replenishing agent has poor chemical stability and cannot play its due role. In Comparative Example 3, there is no introduction of lithium replenishing agent, and its charging capacity is the lowest, only 157.8 mAh / g at 0.1C and the discharge capacity is 151.7 mAh / g. As can be seen from Example 1, Example 6-7, and Example 10-13, the amount of lithium replenishing agent added, the amount of Bi2Se3 nanosheets added for lithium replenishing agent intercalation, and the amount of dopamine added will affect the material performance, thereby affecting the electrochemical performance of the battery.

[0143] In summary, this invention provides a lithium iron phosphate composite material, its preparation method, and its application. The lithium iron phosphate composite material, through two-dimensional layered nanosheets coated with carbon and intercalated with lithium supplement, not only achieves high-speed and low-energy electron transport of lithium iron phosphate, but also improves the chemical stability of the lithium supplement and enhances the electrochemical performance of lithium iron phosphate.

[0144] The above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A lithium iron phosphate composite material, characterized by, The lithium iron phosphate composite material comprises an inner core and a shell coated on the surface of the inner core, the inner core comprises doped lithium iron phosphate, and the shell comprises nitrogen-doped carbon material and two-dimensional nanosheet; The two-dimensional nanosheet is a two-dimensional layered nanosheet intercalated with a lithium supplementing agent; The two-dimensional layered nanosheet comprises any one of Bi2Se3 nanosheet, Sb2Te3 nanosheet, Bi2Se3 nanosheet or Sb2Se3 nanosheet or a combination of at least two thereof; The chemical formula of the doped lithium iron phosphate is Li 1-x M x FePO4, wherein 0.01 < x < 0.05, M includes any one or a combination of at least two of Mg 2+ , Al 3+ , Ti 4+ , Nb 5+ , or W 6+ . In the lithium iron phosphate composite material, the mass ratio of the doped lithium iron phosphate, the two-dimensional nanosheet and the nitrogen-doped carbon material is 1:(0.01-0.2):(0.01-0.08).

2. The lithium iron phosphate composite material of claim 1, wherein, The lithium supplementing agent comprises lithium oxide and / or lithium phosphide.

3. The lithium iron phosphate composite material of claim 1, wherein, In the shell, the mass ratio of the lithium supplementing agent and the two-dimensional layered nanosheet is (0.05-0.25):

1.

4. A method of producing the lithium iron phosphate composite material according to any one of claims 1 to 3, characterized by, The preparation method comprises the following steps: mixing and calcining the doped lithium iron phosphate, a first carbon source and a two-dimensional nanosheet to obtain the lithium iron phosphate composite material; The first carbon source comprises nitrogen; and the two-dimensional nanosheet is a two-dimensional layered nanosheet intercalated with a lithium supplementing agent.

5. The preparation method according to claim 4, characterized in that, The mass ratio of the doped lithium iron phosphate, the two-dimensional nanosheet and the first carbon source is 1:(0.01-0.2):(0.01-0.12).

6. The preparation method according to claim 4, characterized in that, The calcination is performed under a protective atmosphere.

7. The preparation method according to claim 4, characterized in that, The calcination is performed at a temperature of 400-550℃ for 2-4h.

8. The preparation method according to claim 4, characterized in that, The first carbon source comprises dopamine.

9. The preparation method according to claim 4, characterized in that, The method for preparing the two-dimensional nanosheet comprises the following steps: mixing a lithium supplementing agent, a two-dimensional layered nanosheet and a solvent to obtain a dispersion liquid containing the two-dimensional layered nanosheet intercalated with the lithium supplementing agent.

10. The method of claim 9, wherein, The mass ratio of the two-dimensional layered nanosheet and the lithium supplementing agent is 1:(0.05-0.25).

11. The method of claim 4, wherein, The method for preparing the two-dimensional layered nanosheet comprises the following steps: mixing a high molecular polymer and an alcohol solvent to obtain a high molecular polymer solution, then mixing the high molecular polymer solution, a first element source, a second element source and an acid to obtain a mixture, heating the mixture, adding a reducing agent to the mixture to react, and then performing post-treatment to obtain the two-dimensional layered nanosheet.

12. The method of claim 11, wherein, The concentration of the high molecular polymer solution is 2wt%-6wt%.

13. The preparation method according to claim 11, characterized in that, In the mixture, the concentration of the acid is 5wt%-15wt%, and the concentration of the first element source is 0.5-3mol / L.

14. The method of claim 11, wherein, The molar ratio of the first element source and the second element source is 2:(3-3.2).

15. The preparation method according to claim 11, characterized in that, The mixture is heated to 150-190℃.

16. The method of claim 11, wherein, The reaction of adding the reducing agent to the mixture is performed for 10-30min.

17. The method of claim 11, wherein, The post-treatment comprises washing and drying.

18. The method of claim 11, wherein, The first element source comprises a bismuth source and / or an antimony source, and the second element source comprises a selenium source and / or a tellurium source.

19. The method of claim 11, wherein, The high molecular polymer comprises PVP.

20. The method of claim 11, wherein, The alcohol solvent comprises ethylene glycol.

21. The method of claim 11, wherein, The reducing agent comprises a solution containing hydroxylamine.

22. The method of claim 11, wherein, In the reaction liquid obtained by adding the reducing agent, the concentration of hydroxylamine is 5wt%-12wt%.

23. The preparation method according to claim 4, characterized in that, The method for preparing the doped lithium iron phosphate comprises the following steps: mixing a lithium source, a phosphorus source, an iron source, a second carbon source and a doping metal source and sintering to obtain the doped lithium iron phosphate.

24. The method of claim 23, wherein, The sintering comprises first holding at 200-350 DEG C for 3-6 hours, and then holding at 500-900 DEG C for 6-12 hours.

25. The preparation method according to claim 23, characterized in that, The second carbon source comprises any one or a combination of at least two of glucose, sucrose, phenol formaldehyde resin, starch, dextrin, citric acid, oxalic acid, cellulose or vitamin.

26. The preparation method according to claim 23, characterized in that, The doping metal source comprises any one or a combination of at least two of a magnesium source, an aluminum source, a titanium source, a niobium source or a tungsten source.

27. A lithium-ion battery, characterized by, The lithium ion battery comprises the lithium iron phosphate composite material according to any one of claims 1-3.

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