阅读说明：本技术 一种纳米球形磷酸铁锂的制备方法及磷酸铁锂材料 (Preparation method of nano spherical lithium iron phosphate and lithium iron phosphate material ) 是由 陈迎迎 肖益帆 于 2021-09-07 设计创作，主要内容包括：本发明提供了一种纳米球形磷酸铁锂的制备方法及磷酸铁锂材料,包括以下步骤：预混浆料：采用钒掺杂多孔型无水磷酸亚铁、草酸亚铁作为混合铁源,采用磷酸锂作为锂源,采用乙酸钴作为钴源,采用蔗糖和柠檬酸作为混合碳源,加入预混罐中,各种物料逐一加入,并在预混过程中分阶段加入适量纯水；砂磨：研磨至浆料粒径D50≤0.15um,D99≤1.0um；喷雾干燥：对研磨后的浆料进行喷雾干燥；烧结：烧结制备得钒钴联合掺杂的磷酸铁锂材料；筛分除铁：对烧结后的磷酸铁锂材料进行筛分除铁至磁性物质含量<0.3ppm,得到纳米球形磷酸铁锂成品。本发明采用中空多孔的磷酸亚铁为前驱体,将钒和钴掺杂到铁锂颗粒内部,掺杂元素均匀分布,提高了材料的电子导电率,降低了材料内阻。(The invention provides a preparation method of nano spherical lithium iron phosphate and a lithium iron phosphate material, and the preparation method comprises the following steps: premixing slurry: vanadium-doped porous anhydrous ferrous phosphate and ferrous oxalate are used as mixed iron sources, lithium phosphate is used as a lithium source, cobalt acetate is used as a cobalt source, sucrose and citric acid are used as mixed carbon sources, the materials are added into a premixing tank one by one, and a proper amount of pure water is added in stages in the premixing process; sanding: grinding until the particle size of the slurry D50 is less than or equal to 0.15um and the particle size of D99 is less than or equal to 1.0 um; spray drying: spray drying the ground slurry; and (3) sintering: sintering to prepare a vanadium-cobalt co-doped lithium iron phosphate material; screening and deironing: and screening and deironing the sintered lithium iron phosphate material until the content of magnetic substances is less than 0.3ppm, thus obtaining the finished product of the nano spherical lithium iron phosphate. According to the invention, the hollow porous ferrous phosphate is used as a precursor, vanadium and cobalt are doped into the lithium iron particles, and doping elements are uniformly distributed, so that the electronic conductivity of the material is improved, and the internal resistance of the material is reduced.)

1. A preparation method of nano spherical lithium iron phosphate is characterized by comprising the following steps:

premixing slurry: vanadium-doped porous anhydrous ferrous phosphate and ferrous oxalate are used as mixed iron sources, lithium phosphate is used as a lithium source, cobalt acetate is used as a cobalt source, sucrose and citric acid are used as mixed carbon sources, the materials are added into a premixing tank one by one, a proper amount of pure water is added in stages in the premixing process, and the solid content of the slurry is controlled to be 30-35%;

sanding: grinding until the particle size of the slurry D50 is less than or equal to 0.15um and the particle size of D99 is less than or equal to 1.0 um;

spray drying: spray drying the ground slurry, and controlling the particle size of the dried material D50: 5um to 10um, and the water content is less than 1.0 percent;

and (3) sintering: sintering to prepare a vanadium-cobalt co-doped lithium iron phosphate material;

screening and deironing: and screening and deironing the sintered lithium iron phosphate material until the content of magnetic substances is less than 0.3ppm, thus obtaining the finished product of the nano spherical lithium iron phosphate.

2. The preparation method of nano spherical lithium iron phosphate according to claim 1, wherein the preparation method of the vanadium-doped porous anhydrous ferrous phosphate comprises the following steps:

mixing ammonium metavanadate crystals with pure water at the temperature of 15-25 ℃, and then grinding to obtain a vanadium salt feed liquid;

adding a vanadium salt feed liquid into an ammonium phosphate solution to prepare a phosphate solution, and controlling the phosphorus content in the phosphate solution to be 4-6 wt% and the temperature to be 20 +/-5 ℃;

taking the refined ferrous sulfate solution as a base solution, adding pure water for dilution to prepare a ferrous sulfate reaction solution, wherein the ferrous sulfate content of the ferrous sulfate reaction solution is controlled to be 220g/kg, the pH value is 3-4, and the temperature is 20 +/-5 ℃;

dropwise adding the phosphate solution into the ferrous sulfate reaction solution, simultaneously controlling the adding flow of ammonia water to stabilize the pH value of the reaction system at 4.5-5.5, controlling the temperature of the system to be less than or equal to 30 ℃ in the reaction process, controlling the dropwise adding time of the ammonia water and the phosphate solution to be 40 +/-5 min, and after dropwise adding is finished, continuously stirring and reacting for 50min to prepare ferrous phosphate slurry;

performing solid-liquid separation on the ferrous phosphate slurry, washing a filter cake by using pure water until the washing water conductance is less than or equal to 200us/cm, drying by using an oven protected by inert atmosphere, and drying the filter cake until the water content is less than 1% at the temperature of 120-160 ℃ to obtain ferrous phosphate octahydrate powder;

sintering the ferrous phosphate octahydrate powder in a rotary furnace at the temperature of 450-600 ℃ under the inert gas atmosphere for 2-4 h to prepare the vanadium-doped porous anhydrous ferrous phosphate.

3. The method for preparing nano spherical lithium iron phosphate according to claim 2, wherein the method comprises the following steps:

the vanadium-doped porous anhydrous ferrous phosphate takes refined ferrous sulfate solution as an iron source, ammonium phosphate solution as a phosphorus source, ammonia water as an acid-base regulator, ammonium metavanadate crystal as a vanadium source, and the addition amount of the iron source, the vanadium source and the phosphorus source is determined according to the ratio of the amount of substances of iron element, vanadium element and phosphorus element, i.e. n (Fe), (V), n (P) 1 (0.06-0.1): (0.68-0.75).

4. The preparation method of the nano spherical lithium iron phosphate according to claim 2, wherein the specific preparation process of the vanadium salt solution is as follows:

adding ammonium metavanadate crystals into cold water at 15-25 ℃, and grinding the ammonium metavanadate crystals to a particle size D50100-150 nm by using a sand mill to obtain a vanadium salt feed liquid.

5. The method for preparing nano spherical lithium iron phosphate according to any one of claims 1 to 4, wherein:

the adding amount of the mixed iron source and the lithium source is that according to the amount ratio n (Fe) of each element substance: n (li) ═ 1, (1.05-1.08), and the ratio of the amount of the vanadium-doped porous anhydrous ferrous phosphate and the amount of iron provided by ferrous oxalate in the mixed iron source is 1: (0.02-0.04), the adding amount of the cobalt source is determined according to the cobalt element content of 700ppm-1000ppm in the finished lithium iron phosphate product, the adding amount of the carbon source is determined according to the carbon element content of 2.2 wt% -2.6 wt% in the finished lithium iron phosphate product, and the mass ratio of the citric acid to the sucrose is (0.05-0.08): 1.

6. The method for preparing nano spherical lithium iron phosphate according to any one of claims 1 to 4, wherein the pre-mixed slurry specifically comprises the following steps:

adding half of pure water into the premixing tank in advance to prime;

and adding the materials into the premixing tank one by one, flushing the tank wall of the premixing tank with 5-20kg of pure water after each material is added, and finally adding the rest pure water at one time.

7. The method for preparing nano spherical lithium iron phosphate according to any one of claims 1 to 4, wherein the spray drying process conditions are as follows:

a centrifugal spray dryer is adopted, the rotating speed of an atomizing wheel is 15000rpm-17000rpm, hot nitrogen is adopted as a heat source, the temperature of the nitrogen is 240 ℃ to 250 ℃, and the discharging temperature is 80 ℃ to 90 ℃.

8. The method for preparing nano spherical lithium iron phosphate according to any one of claims 1 to 4, wherein the sintering process conditions are as follows:

sintering by adopting an atmosphere roller furnace, preserving heat for 6-9 h at 700-750 ℃ by adopting nitrogen or argon as inert gas, controlling the oxygen content in the atmosphere roller furnace to be less than 3ppm by volume fraction, and controlling the pressure in the atmosphere roller furnace to be 10-15 Pa.

9. The method for preparing nano spherical lithium iron phosphate according to any one of claims 1 to 4, wherein the screening for removing iron specifically comprises the following steps:

sieving the sintered lithium iron phosphate material by an ultrasonic vibration sieve, wherein the mesh number of the sieve is 60-80 meshes;

and then a secondary electromagnetic iron remover is adopted for removing iron, the magnetic strength of the magnetic conduction net is more than or equal to 15000GS, and the iron is removed until the content of magnetic substances is less than 0.3 ppm.

10. The lithium iron phosphate material is characterized by being prepared by the preparation method of the nano spherical lithium iron phosphate of any one of claims 1 to 9, wherein primary particles of the lithium iron phosphate material are nano-scale spheroidal particles, and the particle size is 40nm to 80 nm.

Technical Field

The invention relates to the technical field of lithium ion battery materials, and particularly relates to a preparation method of nano spherical lithium iron phosphate and a lithium iron phosphate material.

Background

With the development of lithium ion batteries, the advantages of high voltage platform, large energy density, no memory effect, long cycle life and the like are gradually shown, and the lithium ion batteries are gradually widely applied to various fields of daily life, industry, military and the like. The lithium iron phosphate serving as the lithium ion battery anode material has excellent safety performance and cycle performance, does not pollute the environment, is considered to be a power lithium ion battery material with great potential, and becomes a hot spot of development and research in recent years.

At present, the lithium iron phosphate material is generally prepared by a carbothermic method, iron phosphate is used as the most common precursor at present, an iron source and a phosphorus source are provided at the same time, the particle size and the morphology of the iron phosphate directly influence the properties of the lithium iron phosphate, and the properties of the lithium iron phosphate are generally controlled by controlling the features of the morphology, the particles and the like of the iron phosphate. Patent application CN107522188A provides a method for preparing nano spherical iron phosphate and nano iron phosphate, lithium iron phosphate and lithium battery prepared by the method, wherein a mixed solution is formed by dripping a phosphorus source compound solution and an oxidant solution into a soluble ferrous compound solution, and simultaneously, a nano spherical control agent is added and stirred and mixed, under the condition of reflux, stirring reaction is carried out, the appearance of an iron phosphate product is controlled, and the iron phosphate product is filtered and calcined to form a spherical or similar spherical iron phosphate product, thereby taking the iron phosphate product as a basis and improving the performance of the lithium iron phosphate.

However, the preparation method of lithium iron phosphate in the prior art has the following defects:

(1) the iron phosphate is used as an iron source, an oxidant is needed in the technical process, the cost is high, the iron phosphate is generally in a nano rod shape, the specific surface area is small, although a few nano spheres exist, the iron phosphate depends on a nano sphere control agent more, the technology is complex, and the cost is high.

(2) The lithium iron phosphate in the prior art has poor rate performance, low electronic conductivity and high material internal resistance.

Disclosure of Invention

Aiming at the technical problems in the prior art, the invention provides a preparation method of nano spherical lithium iron phosphate and a lithium iron phosphate material, vanadium-doped porous anhydrous ferrous phosphate and ferrous oxalate are used as mixed iron sources, an oxidant is not needed in the process, and the production cost is reduced. The hollow porous ferrous phosphate is used as a precursor, vanadium and cobalt are doped into the lithium iron particles, and doping elements are uniformly distributed, so that the electronic conductivity of the material is improved, and the internal resistance of the material is reduced. Meanwhile, the electrolyte can enter the inner holes, so that the migration rate of lithium ions can be effectively improved, and the rate performance is improved.

The invention provides a preparation method of nano spherical lithium iron phosphate, which comprises the following steps:

premixing slurry: vanadium-doped porous anhydrous ferrous phosphate and ferrous oxalate are used as mixed iron sources, lithium phosphate is used as a lithium source, cobalt acetate is used as a cobalt source, sucrose and citric acid are used as mixed carbon sources, the materials are added into a premixing tank one by one, a proper amount of pure water is added in stages in the premixing process, and the solid content of the slurry is controlled to be 30-35%;

sanding: grinding the slurry by a sand mill until the particle size of the slurry D50 is less than or equal to 0.15um and the particle size of the slurry D99 is less than or equal to 1.0 um;

spray drying: spray drying the ground slurry, and controlling the particle size of the dried material D50: 5um to 10um, and the water content is less than 1.0 percent;

and (3) sintering: sintering in a roller furnace under the atmosphere of inert gas to prepare the vanadium-cobalt co-doped lithium iron phosphate material;

screening and deironing: and screening and deironing the sintered lithium iron phosphate material until the content of magnetic substances is less than 0.3ppm, thus obtaining the finished product of the nano spherical lithium iron phosphate.

According to the invention, vanadium-doped porous anhydrous ferrous phosphate and ferrous oxalate are used as the mixed iron source, and an oxidant is not needed in the process, so that the production cost is reduced. Using lithium phosphate asThe lithium source adopts cobalt acetate as a cobalt source, vanadium and cobalt are doped into the lithium iron particles, and doping elements are uniformly distributed, so that the electronic conductivity of the material is improved, and the internal resistance of the material is reduced. Sucrose and citric acid are used as a mixed carbon source, wherein the citric acid can also be used as a dispersing agent while being used as a carbon source, so that the particle size is uniform, and the material agglomeration is prevented. By carrying out superfine grinding on the slurry, the particle size D50 of the slurry is controlled to be less than or equal to 0.15um, and the particle size D99 of the slurry is controlled to be less than or equal to 1.0um, so that the primary particle size of the material is reduced, the lithium ion diffusion path is favorably shortened, and the rate capability of the material is improved. According to the preparation method of the nano spherical lithium iron phosphate, provided by the invention, the hollow porous ferrous phosphate is used as a precursor, vanadium and cobalt are doped into lithium iron particles, doping elements are uniformly distributed, the electronic conductivity of the material can be improved, the internal resistance of the material is reduced, meanwhile, an electrolyte can enter into internal holes, the migration rate of lithium ions can be effectively improved, and the rate capability is improved. The primary particles of the prepared finished lithium iron phosphate product are nano-scale spheroidal particles with uniform particle size between 40nm and 80nm, excellent rate capability, 10C discharge capacity of more than 145mAh/g, and specific surface area BET of more than or equal to 20m²(g), the tap density TP is more than or equal to 1.35 g/cc.

Further, in the above technical solution, the preparation method of the vanadium-doped porous anhydrous ferrous phosphate includes the following steps:

mixing ammonium metavanadate crystals with pure water at the temperature of 15-25 ℃, and then grinding to obtain a vanadium salt feed liquid;

adding the vanadium salt feed liquid into an ammonium phosphate solution to prepare a phosphate solution, wherein the phosphorus content of the phosphate solution is controlled to be 4-6 wt%, and the temperature is controlled to be 20 +/-5 ℃;

taking a refined ferrous sulfate solution as a base solution, adding pure water for dilution, and preparing a ferrous sulfate reaction solution, wherein the ferrous sulfate content of the ferrous sulfate reaction solution is controlled to be 160-220g/kg, the pH value is 3-4, and the temperature is 20 +/-5 ℃;

adding a pH regulator into the solution obtained after the ferrous sulfate crystals as the titanium dioxide byproduct are dissolved, regulating the pH value to 4-4.5, and then filtering impurities; the pH regulator is one or more of iron powder, ammonia water, sodium (hydrogen) carbonate, caustic soda flakes, ammonium (hydrogen) carbonate and potassium (hydrogen) carbonate. Dropwise adding the phosphate solution into the ferrous sulfate reaction solution, simultaneously controlling the adding flow of ammonia water to stabilize the pH value of the reaction system at 4.5-5.5, controlling the temperature of the system to be less than or equal to 30 ℃ in the reaction process, controlling the dropwise adding time of the ammonia water and the phosphate solution to be 40 +/-5 min, and after dropwise adding is finished, continuously stirring and reacting for 50min to prepare ferrous phosphate slurry;

performing solid-liquid separation on the ferrous phosphate slurry, washing a filter cake by using pure water until the washing water conductance is less than or equal to 200us/cm, drying by using an oven protected by inert atmosphere, and drying the filter cake until the water content is less than 1% at the temperature of 120-160 ℃ to obtain ferrous phosphate octahydrate powder;

sintering the ferrous phosphate octahydrate powder in a rotary furnace at the temperature of 450-600 ℃ under the inert gas atmosphere for 2-4 h to prepare the vanadium-doped porous anhydrous ferrous phosphate.

Specifically, in the above technical scheme, the vanadium-doped porous anhydrous ferrous phosphate uses a refined ferrous sulfate solution as an iron source, an ammonium phosphate solution as a phosphorus source, ammonia water as an acid-base regulator, an ammonium metavanadate crystal as a vanadium source, and the amounts of the iron source, the vanadium source and the phosphorus source are determined according to the ratio of the amounts of iron element, vanadium element and phosphorus element (n) (v) to n (p) (1) (0.06-0.1): (0.68-0.75).

The preparation process of the vanadium salt feed liquid comprises the following steps: adding ammonium metavanadate crystals into cold water at 15-25 ℃, and grinding the ammonium metavanadate crystals to a particle size D50100-150 nm by using a sand mill to obtain a vanadium salt feed liquid.

The invention adopts ammonium metavanadate as a vanadium source, and because the ammonium metavanadate is slightly soluble in cold water and insoluble in ammonium phosphate solution, in the reaction process, the temperature of the system is controlled, so that the ground nano-scale ammonium metavanadate exists in the reaction system in the form of crystals, can be used as a crystal nucleus for ferrous phosphate precipitation reaction, and can be mutually doped with newly generated ferrous phosphate precipitate to form a coprecipitate of ferrous phosphate and ammonium metavanadate, and in the high-temperature sintering process, the ammonium metavanadate is decomposed to generate a coprecipitate of the ferrous phosphate and the ammonium metavanadateTo form V₂O₅Water vapor and ammonia gas to form internal holes, and V₂O₅The porous vanadium-doped anhydrous ferrous phosphate is uniformly distributed in the holes to obtain porous vanadium-doped anhydrous ferrous phosphate, namely a hollow porous ferrous phosphate precursor in the preparation method of the nano spherical lithium iron phosphate.

It should be noted that the ammonium phosphate solution as the phosphorus source may be a monoammonium phosphate solution and/or a diammonium phosphate solution. The content of ferrous sulfate in the ferrous sulfate reaction solution is controlled to be 160g/kg-220g/kg, the pH value is 3-4, and the temperature is 20 +/-5 ℃.

Further, in the above technical solution, the addition amount of the mixed iron source and lithium source is in accordance with the ratio n (fe) of the amount of the lithium iron material: n (li) ═ 1, (1.05-1.08), and the ratio of the amount of the vanadium-doped porous anhydrous ferrous phosphate and the amount of iron provided by ferrous oxalate in the mixed iron source is 1: (0.02-0.04), the adding amount of the cobalt source is determined according to the cobalt element content of 700ppm-1000ppm in the finished lithium iron phosphate product, the adding amount of the carbon source is determined according to the carbon element content of 2.2 wt% -2.6 wt% in the finished lithium iron phosphate product, and the mass ratio of the citric acid to the sucrose is (0.05-0.08): 1.

According to the invention, the further control of the shape and the particle size of the lithium iron phosphate is realized by limiting the proportion of the vanadium-doped porous anhydrous ferrous phosphate and the ferrous oxalate in the mixed iron source, and the improvement of the specific surface area and the uniformity of the particle size are facilitated. According to the invention, the content of the carbon source and the dispersion effect are controlled by limiting the adding amount of the sucrose and the citric acid, so that the prepared lithium iron phosphate has uniform particle size, and the material agglomeration can be effectively prevented.

Preferably, in the above technical solution, the premixed slurry specifically includes the following steps:

adding half of pure water into the premixing tank in advance to prime;

and adding the materials into the premixing tank one by one, flushing the tank wall of the premixing tank with 5kg-20kg of pure water after each material is added, and finally adding the rest pure water at one time.

According to the invention, half of pure water is added in advance to prime, and then the wall of the tank is washed by pure water after each material is added, so that the materials are effectively prevented from being adhered to the wall of the premixing tank, and the accuracy of material proportioning is ensured.

It is noted that the total amount of pure water added needs to be controlled, and the solid content of the slurry is ensured to be controlled at 30-35%.

Zirconia beads with the diameter of 0.3mm are used as grinding media for sanding, the grinding efficiency is high, the grinding effect is good, and the slurry is ground until the particle size of D50 is less than or equal to 0.15um and D99 is less than or equal to 1.0 um.

Preferably, in the above technical solution, the process conditions of the spray drying are as follows: a centrifugal spray dryer is adopted, the rotating speed of an atomizing wheel is 15000rpm-17000rpm, hot nitrogen is adopted as a heat source, the temperature of the nitrogen is 240 ℃ to 250 ℃, and the discharging temperature is 80 ℃ to 90 ℃. And the nitrogen at the air outlet is heated and recycled after being collected and treated.

The invention adopts a centrifugal spray dryer to carry out spray drying, water on the surface of the hollow porous ferrous phosphate precursor and in the inner holes can volatilize in the spraying process, and cobalt acetate can be uniformly attached to the inner holes and the outer surface, thereby being beneficial to doping cobalt into the lithium iron phosphate material.

In a specific embodiment of the present invention, the sintering process conditions are as follows: sintering by adopting an atmosphere roller furnace, preserving heat for 6-9 h at 700-750 ℃ by adopting nitrogen or argon as inert gas, controlling the oxygen content in the atmosphere roller furnace to be less than 3ppm by volume fraction, and controlling the pressure in the atmosphere roller furnace to be 10-15 Pa.

According to the invention, an atmosphere roller hearth furnace is adopted for sintering, cobalt acetate can be decomposed in the sintering process to generate steam, carbon and cobalt oxide, the vanadium-cobalt combined doped lithium iron phosphate material is prepared, vanadium and cobalt are doped into lithium iron particles, doping elements are uniformly distributed, the electronic conductivity of the material is favorably improved, the internal resistance of the material is reduced, the multiplying power performance is improved, the prepared lithium iron phosphate material has excellent multiplying power performance, and the 10C discharge capacity is more than 145 mAh/g.

In a preferred embodiment of the present invention, the screening for iron removal specifically comprises the following steps:

sieving the sintered lithium iron phosphate material by an ultrasonic vibration sieve, wherein the mesh number of the sieve is 60-80 meshes;

and then a secondary electromagnetic iron remover is adopted for removing iron, the magnetic strength of the magnetic conduction net is more than or equal to 15000GS, and the iron is removed until the content of magnetic substances is less than 0.3 ppm.

The invention is screened by the ultrasonic vibration screen, deironing is carried out by adopting the secondary electromagnetic deironing device, and the finished product lithium iron phosphate can be obtained by packaging after the deironing is qualified.

The invention also provides a lithium iron phosphate material which is prepared by the preparation method of the nano spherical lithium iron phosphate, and the primary particles of the lithium iron phosphate material are nano-scale spheroidal particles with the particle size of 40nm-80 nm.

The lithium iron phosphate material provided by the invention has excellent rate capability, 10C is more than 145mAh/g, and the specific surface area BET is more than or equal to 20m²(g), the tap density TP is more than or equal to 1.35 g/cc.

Compared with the prior art, the invention has the beneficial effects that:

(1) according to the preparation method of the nano spherical lithium iron phosphate, the hollow porous ferrous phosphate is used as a precursor, vanadium and cobalt are doped into lithium iron particles, and doping elements are uniformly distributed, so that the electronic conductivity of the lithium iron phosphate material is improved, the internal resistance of the material is reduced, and meanwhile, when the nano spherical lithium iron phosphate is used, an electrolyte can enter into internal holes, so that the migration rate of lithium ions can be effectively improved, and the rate capability is improved;

(2) according to the preparation method of the nano spherical lithium iron phosphate, the vanadium-doped porous anhydrous ferrous phosphate and ferrous oxalate are used as the mixed iron source, and compared with the iron phosphate process in the prior art, an oxidant is not needed in the process; meanwhile, in the sintering process, because a ferrous iron source is adopted, ferric iron does not need to be reduced into ferrous iron, and the carbon source only plays a role in coating, the use amount of sucrose can be reduced, and the production cost is reduced;

(3) according to the preparation method of the nano spherical lithium iron phosphate, provided by the invention, sucrose and citric acid are used as a mixed carbon source, and the citric acid plays a role as a carbon source and a dispersing agent, so that the particles are uniform in size, and the agglomeration of materials is effectively prevented; meanwhile, the higher carbon content is also beneficial to enhancing the conductivity of the material and improving the conductivity of the material;

(4) the preparation method of the nano spherical lithium iron phosphate provided by the invention adopts superfine grinding, the particle size D50 of the slurry reaches below 150nm, the lithium ion diffusion path can be effectively shortened, and the rate capability of the material is improved;

(5) in the preparation method of the nano spherical lithium iron phosphate, the adopted hollow porous ferrous phosphate precursor, namely the vanadium-doped porous anhydrous ferrous phosphate, does not need a special nano spherical control agent for shape control and an oxidant in the preparation process, ammonium metavanadate is used as a vanadium source, and in the reaction process, the temperature of a system is controlled to ensure that the ground nano ammonium metavanadate exists in the reaction system in a crystal form, can serve as a crystal nucleus of ferrous phosphate precipitation reaction and can be mutually doped with a newly generated ferrous phosphate precipitate to form a coprecipitate of the ferrous phosphate and the ammonium metavanadate, and in the high-temperature sintering process, the ammonium metavanadate is decomposed to generate V₂O₅Water vapor and ammonia gas to form internal holes, and V₂O₅The porous silicon material is uniformly distributed in the holes, and the preparation method is simple and reliable.

(6) The primary particles of the lithium iron phosphate material prepared by the preparation method of the nano spherical lithium iron phosphate provided by the invention are nano-scale spheroidal particles, the particle size is 40nm-80nm, the rate capability is excellent, 10DC is more than 145mAh/g, 0.5DC is more than 150mAh/g, FCC is more than 155mAh/g, the specific surface area is larger thanBET≥20m²(g), the tap density TP is more than or equal to 1.35 g/cc.

Drawings

Fig. 1 is a schematic flow chart of a method for preparing nano spherical lithium iron phosphate provided in embodiment 1 of the present invention;

fig. 2 is an SEM image of a lithium iron phosphate material prepared by the method for preparing nano spherical lithium iron phosphate according to embodiment 1 of the present invention;

fig. 3 is an SEM image of a lithium iron phosphate material prepared by the method for preparing nano spherical lithium iron phosphate according to embodiment 1 of the present invention;

fig. 4 is an XRD chart of the lithium iron phosphate material prepared by the method for preparing nano spherical lithium iron phosphate provided in embodiment 1 of the present invention;

fig. 5 is an SEM image of vanadium-doped porous anhydrous ferrous phosphate prepared in the preparation process of the method for preparing nano spherical lithium iron phosphate according to embodiment 1 of the present invention;

fig. 6 is an SEM image of a lithium iron phosphate material prepared according to the prior art route of iron phosphate provided in comparative example 4.

Detailed Description

The present invention is further described in detail below with reference to specific examples so that those skilled in the art can more clearly understand the present invention.

The following examples are provided only for illustrating the present invention and are not intended to limit the scope of the present invention. All other embodiments obtained by a person skilled in the art based on the specific embodiments of the present invention without any inventive step are within the scope of the present invention.

In the examples of the present invention, all the raw material components are commercially available products well known to those skilled in the art, unless otherwise specified; in the examples of the present invention, unless otherwise specified, all technical means used are conventional means well known to those skilled in the art.

In the examples of the present invention, the raw materials used were all conventional commercially available products.

Example 1

As shown in fig. 1, an embodiment of the present invention provides a method for preparing nano spherical lithium iron phosphate, including the following steps:

(1) preparing ferrous phosphate, taking refined ferrous sulfate solution as an iron source, ammonium phosphate solution as a phosphorus source, ammonia water as an acid-base regulator, and ammonium metavanadate crystal as a vanadium source, wherein the adding amount of the iron source, the vanadium source and the phosphorus source is determined according to the ratio of the amount of substances of iron element, vanadium element and phosphorus element, namely n (Fe), (V), n (P), 1:0.06: 0.70:

s101, preparing a phosphate solution, namely adding ammonium metavanadate crystals into cold water at the temperature of 20 +/-5 ℃, grinding the particle size of the ammonium metavanadate crystals to D50100 nm-150nm by using a sand mill to obtain a vanadium salt feed liquid, adding the vanadium salt feed liquid into an ammonium phosphate solution to prepare the phosphate solution, and controlling the content of phosphorus in the phosphate solution to be 5 wt% and the temperature to be 20 +/-5 ℃;

s102, preparing a ferrous sulfate reaction solution, adopting a refined ferrous sulfate solution obtained by adjusting the pH value to 4-4.5 through ammonia water, removing impurities, filtering and refining as a base solution, adding pure water for dilution, and preparing the ferrous sulfate reaction solution, wherein the ferrous sulfate content in the ferrous sulfate reaction solution is controlled at 200g/kg, the pH value is 3-4, and the temperature is 20 +/-5 ℃;

s103, performing synthetic reaction, namely priming with a ferrous sulfate reaction solution, dropwise adding a phosphate solution into the ferrous sulfate reaction solution, controlling the adding flow of ammonia water to stabilize the pH value of a reaction system at 4.5-5.5, controlling the dropwise adding time of the ammonia water and the phosphate solution to be 40 +/-5 min, continuously stirring and reacting for 50min after the dropwise adding is finished, and preparing ferrous phosphate, wherein the temperature of the system is controlled to be less than or equal to 30 ℃ in the reaction process;

s104, performing filter pressing and washing, namely performing solid-liquid separation on the prepared ferrous phosphate slurry, and then washing a filter cake by using pure water until the washing conductance is less than or equal to 200 us/cm;

s105, drying by using an oven protected by inert atmosphere at 150 ℃ until the moisture content is less than 1% to obtain ferrous phosphate octahydrate powder;

and S106, sintering in an atmosphere, and sintering the ferrous phosphate octahydrate powder in a rotary furnace at 550 ℃ in an inert gas atmosphere for 3 hours to obtain the vanadium-doped porous anhydrous ferrous phosphate, wherein an SEM image of the vanadium-doped porous anhydrous ferrous phosphate is shown in FIG. 5.

(2) Preparing lithium iron phosphate, wherein vanadium-doped porous anhydrous ferrous phosphate and ferrous oxalate are used as mixed iron sources, lithium phosphate is used as a lithium source, cobalt acetate is used as a cobalt source, sucrose and citric acid are used as mixed carbon sources, and the adding amount of the mixed iron sources and the lithium sources is as follows according to the ratio n (Fe) of the amount of each element substance: n (li) 1:1.06, wherein the ratio of the amount of the vanadium-doped porous anhydrous ferrous phosphate to the amount of the iron provided by the ferrous oxalate in the mixed iron source is 1: 0.03, the adding amount of the cobalt source is determined according to the cobalt element content of 700ppm in the finished lithium iron phosphate product, the adding amount of the carbon source is determined according to the carbon element content of 2.2 wt% in the finished lithium iron phosphate product, the mass ratio of the citric acid to the sucrose is 0.06:1,

s107, premixing, namely adding half of pure water into a premixing tank in advance to bottom, adding various materials into the premixing tank one by one, flushing the tank wall of the premixing tank by 5% of the total amount of the pure water after each material is added, and finally adding all the residual pure water, wherein the solid content of the slurry is controlled to be 30% -35% in the whole premixing process; .

And S108, performing superfine grinding, namely performing superfine grinding by using a sand mill, wherein a grinding medium is zirconia beads with the diameter of 0.3mm, and grinding until the particle size D50 of the slurry is less than or equal to 0.15um and D99 is less than or equal to 1.0 um.

S109, spray drying, namely, adopting a centrifugal spray dryer, controlling the rotating speed of an atomizing wheel to be 17000rpm, adopting hot nitrogen as a heat source in the drying process, controlling the temperature of the nitrogen to be 245 +/-5 ℃, controlling the discharging temperature to be 85 +/-5 ℃, and controlling the particle size D50 of the dried material to be: 5-10 μm, water content less than 1.0%, and the nitrogen gas at the air outlet can be heated for reuse after collection and regeneration.

S110, sintering in an atmosphere, namely sintering in an atmosphere roller furnace, preserving heat for 8 hours at the temperature of 700 +/-5 ℃ by using nitrogen as inert gas, and controlling the oxygen content (volume fraction) in the furnace to be less than 3ppm and the pressure in the furnace to be 10-15 Pa.

S111, screening and deironing, namely screening the sintered lithium iron phosphate material by using an ultrasonic vibration screen, wherein the mesh number of the screen is 60 meshes, deironing by using a secondary electromagnetic deironing device, and stopping until the content of magnetic substances is less than 0.3 ppm;

and S112, packaging to obtain a finished product of lithium iron phosphate, namely an LFP finished product, wherein an SEM picture is shown in figures 2-3, and an XRD picture is shown in figure 4.

Example 2

The embodiment of the invention provides a preparation method of nano spherical lithium iron phosphate, which is different from the embodiment 1 in that:

when preparing the ferrous phosphate, the adding amount of the iron source, the vanadium source and the phosphorus source is determined according to the ratio of the amount of the iron element, the vanadium element and the phosphorus element, namely n (Fe), n (V), n (P), 1:0.1:0.75, when preparing the lithium iron phosphate, the adding amount of the carbon source is determined according to the content of the carbon element in a finished lithium iron phosphate product, namely 2.6 wt%, and the adding amount of the cobalt source is determined according to the content of the cobalt element in the finished lithium iron phosphate product, namely 1000 ppm.

Example 3

The embodiment of the invention provides a preparation method of nano spherical lithium iron phosphate, which is different from the embodiment 1 and the embodiment 2 in that:

when preparing the iron phosphate, the adding amount of the iron source, the vanadium source and the phosphorus source is determined according to the ratio of the amount of the iron element, the vanadium element and the phosphorus element, namely n (Fe), n (V), n (P), 1:0.08:0.73, when preparing the lithium iron phosphate, the adding amount of the carbon source is determined according to the content of the carbon element in a finished lithium iron phosphate product, namely 2.4 wt%, and the adding amount of the cobalt source is determined according to the content of the cobalt element in the finished lithium iron phosphate product, namely 850 ppm.

Comparative example 1

The difference from example 3 is that:

adopting a ferrous sulfate solution and a phosphate solution as an iron source and a phosphorus source, adopting ammonia water as a pH regulator, and not doping vanadium element in the process of preparing ferrous phosphate to obtain conventional ferrous phosphate (without holes inside); when preparing lithium iron phosphate, V, Co elements are doped, the doping amount is the same as that of the embodiment 3, and other preparation processes are the same as that of the embodiment 3.

Comparative example 2

The difference from example 3 is that:

when the lithium iron phosphate is prepared, the sand grinding particle size D50 is controlled to be 400nm-500nm, the carbon content is controlled to be 1.5 wt% -1.8 wt%, and other preparation processes are the same as those in the embodiment 3.

Comparative example 3

The difference from example 3 is that:

when preparing the ferrous phosphate, vanadium is not doped, and the conventional ferrous phosphate (without holes inside) is obtained; when preparing lithium iron phosphate, cobalt is not doped; the other preparation processes are the same as in example 3.

Comparative example 4

The differences from the above examples and comparative examples are:

the preparation method adopts a conventional carbothermic method for preparation, namely, ferric phosphate is used as a phosphorus source and an iron source, lithium carbonate is used as a lithium source, glucose is used as a carbon source to prepare lithium iron phosphate, and the carbon content of a finished product is controlled to be 1.5-1.8 wt%. The SEM image is shown in FIG. 6.

The performance data of the lithium iron phosphate materials prepared in the above examples 1 to 3 and comparative examples 1 to 4 are shown in the following table 1:

TABLE 1

And (4) analyzing results:

analysis table 1 shows that the specific surface area BET of the lithium iron phosphate material prepared in the embodiment of the present invention is not less than 20m²(iv)/g, tap density TP is more than or equal to 1.35 g/cc; compared with the electrochemical performance detection result, the discharge performance of the lithium iron phosphate material prepared in the embodiment and the comparative example is basically similar under the 0.1C low-rate discharge condition, the discharge performance of the comparative example is slightly lower than that of the embodiment under the 0.5C discharge condition, the electric performance of the embodiment is larger than 145mAh/g under the 10C high-rate discharge condition, and the discharge performance of the comparative example is obviously lower than that of the embodiment. The preparation method of the nano spherical lithium iron phosphate provided by the invention is simple and reliable, and the rate capability of the prepared lithium iron phosphate material is obviously improved.

By analyzing fig. 2 and fig. 3, it can be seen that the primary particles of the lithium iron phosphate material prepared by the embodiment of the present invention are nano-sized spheroidal particles, and the particle size of the primary particles is between 40nm and 80 nm.

By analyzing fig. 5, it can be seen that the anhydrous ferrous phosphate prepared by the example of the present invention is of a hollow porous type.

Comparing fig. 2, fig. 3, and fig. 6, it can be seen that the lithium iron phosphate material prepared in the example has a uniform particle size, while the lithium iron phosphate material prepared in the comparative example 4 has some larger particles.

It should be noted that the above examples are only for further illustration and description of the technical solution of the present invention, and are not intended to further limit the technical solution of the present invention, and the method of the present invention is only a preferred embodiment, and is not intended to limit the protection scope of the present invention. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.