CN110683527B

CN110683527B - Preparation method of carbon-doped high-compaction iron phosphate

Info

Publication number: CN110683527B
Application number: CN201910974279.6A
Authority: CN
Inventors: 王敏
Original assignee: Individual
Current assignee: Individual
Priority date: 2019-10-14
Filing date: 2019-10-14
Publication date: 2021-01-12
Anticipated expiration: 2039-10-14
Also published as: CN110683527A

Abstract

The invention discloses a preparation method of carbon-doped high-compaction iron phosphate. Dissolving disodium EDTA in water, preparing a solution with the concentration of 0.25-0.3mol/L at the temperature of 30-35 ℃, adding a ferric nitrate solution into the disodium EDTA solution, then adding an acid-base regulator until the pH value of the mixed solution is 1-1.5, stirring and mixing uniformly, then adding a mixed solution of phosphoric acid and ammonium dihydrogen phosphate, adding for 1-2h, and then stirring and reacting for 15-30min to obtain a reaction slurry; adding alkali into the reaction slurry, adjusting the pH value of the solution, heating, stirring for reaction, cooling, filtering, washing and drying to obtain a dried material; and calcining the dried material in a rotary furnace under the nitrogen atmosphere to obtain the carbon-doped high-compaction iron phosphate. The preparation method is simple, the obtained iron phosphate doped with carbon has compact particle growth, small porosity and large primary particle size.

Description

Preparation method of carbon-doped high-compaction iron phosphate

Technical Field

The invention relates to a preparation method of carbon-doped high-compaction iron phosphate, belonging to the technical field of lithium batteries.

Background

Lithium batteries are gradually developed along with the development of new energy automobiles, and currently, mainstream lithium batteries are divided into lithium iron phosphate batteries, lithium manganate batteries and ternary batteries.

The total yield of 123 thousands of new energy automobiles in 2018 accounts for 60% of the total global sales, and the power battery installation 56Gwh is increased by 68% on a same scale. In 2018, under the background that the whole automobile city slides down, the new energy automobile still gives out a perfect answer sheet under the support of subsidies.

At present, the price of lithium iron phosphate of mainstream manufacturers is about 5.3 ten thousand yuan/ton, and compared with the price of 9-9.5 ten thousand yuan/ton in the last year, the price is reduced by over 61 percent. The price of the main stream of lithium iron phosphate falls to 5 ten thousand in the future, the price falls to bring about a great increase in demand, and the situation is different from the situation of market demand atrophy in the last year, the lithium iron phosphate shows a vigorous growth trend in the beginning of 2019, the installed electric quantity of the lithium iron phosphate battery is about 1.40GWH in 1 month in 2019, and the installed electric quantity is increased by 174% on a same scale.

As is known, the main bottleneck affecting the application of lithium iron phosphate is the low energy density, mainly due to the following reasons: the mainstream technology of the existing lithium iron phosphate is still carbon-coated lithium iron phosphate, but the technology has the following problems: the conductivity between particles of the carbon-coated lithium iron phosphate particles is increased, but the conductivity of a single particle of the lithium iron phosphate is not obviously improved; meanwhile, the carbon coated is flocculent amorphous carbon, and the carbon affects the compaction density of the lithium iron phosphate, because if the particles of the lithium iron phosphate are made to be larger, although the compaction density is high, the size of the particles is too large, the migration distance of lithium ions is far, the capacity is poor, and in order to ensure the capacity, the size of the particles can only be reduced, so that the compaction of the lithium iron phosphate is affected, and the capacity and the compaction are the same as those of a seesaw and cannot be achieved at the same time.

Therefore, there is an urgent need for highly compacted iron phosphate doped with carbon, which has a dense growth, a small porosity and a large primary particle size.

Disclosure of Invention

In view of the above, the invention provides a preparation method of carbon-doped high-compaction iron phosphate, which is simple, and the obtained carbon-doped iron phosphate has compact particle growth, small porosity and large primary particle size.

The invention solves the technical problems by the following technical means:

the invention relates to a preparation method of carbon-doped high-compaction iron phosphate, which comprises the following steps:

1) dissolving disodium EDTA in water, preparing a solution with the concentration of 0.25-0.3mol/L at the temperature of 30-35 ℃, adding a ferric nitrate solution into the disodium EDTA solution, then adding an acid-base regulator until the pH value of the mixed solution is 1-1.5, stirring and mixing uniformly, then adding a mixed solution of phosphoric acid and ammonium dihydrogen phosphate, wherein the molar ratio of the phosphoric acid to the ammonium dihydrogen phosphate is 1:0.05-0.5, the adding time is 1-2h, and then stirring and reacting for 15-30min to obtain a reaction slurry;

2) adding alkali into the reaction slurry obtained in the step (1), adjusting the pH value of the solution to 2.2-3, heating to 90-95 ℃, then stirring for reaction for 15-30min, cooling to 50-60 ℃, then filtering, washing and drying to obtain a dried material;

3) and calcining the dried material in a rotary furnace under the nitrogen atmosphere at the calcining temperature of 620-650 ℃ for 2-3h to obtain the carbon-doped high-compaction iron phosphate.

In the step (1), in the process of adding the ferric nitrate solution into the EDTA disodium solution and in the process of adding the acid-base regulator, the temperature of the solution is maintained at 30-35 ℃, the concentration of the ferric nitrate solution is 1-3mol/L, the pH value is less than or equal to 1, the molar ratio of the ferric nitrate to the EDTA disodium is 1:0.1-0.15, and the acid-base regulator is ammonia water or nitric acid.

The molar ratio of phosphoric acid to ammonium dihydrogen phosphate in the mixed solution of phosphoric acid and ammonium dihydrogen phosphate in the step (1) is 2-2.5, the molar ratio of ferric nitrate to phosphate radical in the mixed solution of phosphoric acid and ammonium dihydrogen phosphate is 1:1.15-1.3, and the ferric nitrate solution and the acid-base regulator are added into the EDTA disodium solution at a stirring speed of 300-400 r/min.

And (3) adding alkali into the reaction slurry in the step (2), wherein the adding time is 30-60min, the concentration of the alkali is 2-5mol/L, and the alkali is at least one of sodium hydroxide, lithium hydroxide and potassium hydroxide.

Mixing the mother liquor filtered in the step (2) with washing water, adding alkali to adjust the pH value of the solution to 10-12, continuously blowing off through 6-8 stages, spraying and absorbing blown-off gas through a phosphoric acid solution, adjusting the pH value absorbed by the solution to 3-5 to obtain an ammonium dihydrogen phosphate solution, concentrating and crystallizing the blown-off solution until the Baume degree is 45-50, cooling to the temperature of 20-30 ℃ to obtain mixed crystals of phosphate and nitrate, filtering, adding the mother liquor into the phosphoric acid solution, adjusting the pH value of the solution to 3-4, filtering to obtain EDTA crystals, and returning and mixing the rest solution into the mother liquor and the washing water filtered in the step (2).

And (3) in the calcining process of the dried material in the rotary furnace in the nitrogen atmosphere, maintaining the nitrogen content in the gas in the rotary furnace to be more than 85 percent and the oxygen content to be 0.5 to 1 percent.

And (4) after the calcined material in the step (3) is subjected to airflow crushing, screening by an ultrasonic vibration screen, wherein the mesh number of the screen is 100-200 meshes, removing iron by using a 2-stage battery iron remover, and stopping removing iron until the magnetic substance is less than 0.5ppm, so as to obtain the carbon-doped high-compaction iron phosphate.

And (3) carrying out water spraying absorption on the gas obtained by calcining in the step (3), and mixing the obtained absorption liquid with the mother liquor and the washing water filtered in the step (2) for treatment.

According to the invention, the solubility difference between EDTA disodium salt and EDTA is skillfully large, namely the solubility of EDTA disodium salt is large and the solubility of EDTA is small, the EDTA disodium salt solution is added as a base solution, then ferric nitrate is added, the ferric nitrate is acidic, the pH of the whole solution can be reduced, and then the pH of the whole solution is reduced, and finally the EDTA disodium salt and the ferric nitrate are addedAdding an acid-base regulator until the pH value of the mixed solution is 1-1.5, stirring and mixing uniformly to form an iron complex solution, crystallizing to separate out EDTA crystals, and adding a mixed solution of phosphoric acid and ammonium dihydrogen phosphate, wherein Fe is used as the Fe³⁺The ferric iron is slowly released, EDTA crystals are separated out by depending on crystallization, the EDTA crystals grow by taking as crystal nuclei, then alkali is added, the pH of the solution is adjusted to be 2.2-3, the temperature is raised to 90-95 ℃, iron can be dissociated from an EDTA complexing agent under higher pH, the recovery rate of iron is improved, the temperature is raised, the crystallinity of the iron phosphate can be improved, the EDTA crystals are obtained as nuclei, the iron phosphate is in a shell structure, then the iron phosphate is calcined at high temperature, EDTA decomposition is divided into two stages under inert atmosphere, the first stage is that the EDTA is boiled at high temperature and is distributed on iron phosphate particles, the second step is that the EDTA is decomposed, oxygen, nitrogen, hydrogen and partial carbon react to obtain nitrogen oxygen gas, carbon oxygen gas and water vapor, and residual carbon is doped in the iron phosphate particles, and a small amount of carbon will be distributed on the surface of the iron phosphate particles.

Meanwhile, in the reaction system of the invention, because EDTA solution exists, complex ions can be obtained, and the complexing ability is very strong, iron ions can be slowly released in a phosphate system, thereby obtaining the iron phosphate particles with high compactness and good sphericity.

Meanwhile, the invention skillfully utilizes the mode that the pH of the mother liquor after concentration and crystallization is adjusted back to be acidic, EDTA salt can be concentrated in the concentration and crystallization process, then the pH is adjusted back, the EDTA salt is converted into EDTA, thereby reducing the solubility of the EDTA, obtaining EDTA crystals, and recycling part of EDTA in the EDTA crystals.

The invention has the beneficial effects that: the preparation method is simple, the obtained iron phosphate doped with carbon has compact particle growth, small porosity and large primary particle size.

Drawings

FIG. 1 is an SEM of the product obtained in example 1 of the present invention.

FIG. 2 shows XRD of the product obtained in example 1 of the present invention.

FIG. 3 is an SEM of the product obtained in example 2 of the present invention.

FIG. 4 is an SEM of the product obtained in example 3 of the present invention.

Detailed Description

The present invention will be described in detail with reference to specific examples, wherein the method for preparing carbon-doped highly compacted iron phosphate of this example comprises the following steps:

Example 1

A preparation method of carbon-doped high-compaction iron phosphate comprises the following steps:

1) dissolving disodium EDTA in water, preparing a solution with the concentration of 0.26mol/L at 30 ℃, adding a ferric nitrate solution into the disodium EDTA solution, then adding an acid-base regulator until the pH of the mixed solution is 1.25, stirring and mixing uniformly, then adding the mixed solution of phosphoric acid and ammonium dihydrogen phosphate, adding for 1.5h, and then stirring and reacting for 20min to obtain a reaction slurry;

2) adding alkali into the reaction slurry obtained in the step (1), adjusting the pH value of the solution to 2.5, heating to 93 ℃, stirring for reaction for 25min, cooling to 55 ℃, filtering, washing and drying to obtain a dried material;

3) and calcining the dried material in a rotary furnace under the nitrogen atmosphere at the temperature of 625 ℃ for 3h to obtain the carbon-doped high-compaction iron phosphate.

In the step (1), in the process of adding the ferric nitrate solution into the EDTA disodium solution and in the process of adding the acid-base regulator, the temperature of the solution is maintained at 32 ℃, the concentration of the ferric nitrate solution is 2mol/L, the pH value is less than or equal to 1, the molar ratio of the ferric nitrate to the EDTA disodium is 1:0.13, and the acid-base regulator is ammonia water.

In the step (1), the molar ratio of phosphoric acid to ammonium dihydrogen phosphate in the mixed solution of phosphoric acid and ammonium dihydrogen phosphate is 2.2, the molar ratio of ferric nitrate to phosphate radicals in the mixed solution of phosphoric acid and ammonium dihydrogen phosphate is 1:1.25, and the ferric nitrate solution and the acid-base regulator are added into the EDTA disodium solution at a stirring speed of 350 r/min.

And (3) adding alkali into the reaction slurry in the step (2), wherein the adding time is 40min, the concentration of the alkali is 4mol/L, and the alkali is sodium hydroxide.

Mixing the mother liquor filtered in the step (2) with washing water, adding alkali to adjust the pH value of the solution to be 11, continuously blowing off the solution by 7 stages, spraying and absorbing the blown-off gas by a phosphoric acid solution to absorb the pH value of the solution to be 3.5 to obtain an ammonium dihydrogen phosphate solution, concentrating and crystallizing the blown-off solution until the Baume degree is 46, cooling the solution to the temperature of 25 ℃ to obtain mixed crystals of phosphate and nitrate, filtering the mixed crystals, adding the mother liquor into the phosphoric acid solution to adjust the pH value of the solution to be 3.5, filtering the mixed crystals to obtain EDTA crystals, and returning and mixing the rest solution into the mother liquor and the washing water filtered in the step (2).

And (3) in the calcining process of the dried material in the rotary furnace in the nitrogen atmosphere, maintaining the nitrogen content in the gas in the rotary furnace to be more than 85 percent and the oxygen content to be 0.8 percent.

And (3) after the calcined material is subjected to jet milling, sieving by an ultrasonic vibration sieve with the mesh number of 150 meshes, removing iron by using a 2-grade battery iron remover, and stopping removing iron until the magnetic substance is less than 0.5ppm to obtain the carbon-doped high-compaction iron phosphate.

The indices of the iron phosphate obtained are as follows:

the method for measuring the compaction density comprises the steps of putting iron phosphate powder into a die of powder compaction measuring equipment, pressing the iron phosphate powder under the pressure of 3 tons until the thickness of the powder is not changed any more, and dividing the powder by the volume to obtain powder compaction data.

As shown in fig. 1 and 2, the iron phosphate obtained in this example was spherical particles, had good dispersibility, had a high sphericity as seen from SEM pictures, and had a small amount of flocculent carbon layer on the surface of some of the particles. As seen from fig. 2, the obtained iron phosphate was a high-crystallinity and impurity-free iron phosphate.

Example 2

1) dissolving disodium EDTA in water, preparing a solution with the concentration of 0.28mol/L at 32 ℃, adding a ferric nitrate solution into the disodium EDTA solution, then adding an acid-base regulator until the pH of the mixed solution is 1.2, stirring and mixing uniformly, then adding the mixed solution of phosphoric acid and ammonium dihydrogen phosphate, adding for 1.5h, and then stirring and reacting for 20min to obtain a reaction slurry;

2) adding alkali into the reaction slurry obtained in the step (1), adjusting the pH value of the solution to 2.5, heating to 90 ℃, stirring for reaction for 25min, cooling to 55 ℃, filtering, washing and drying to obtain a dried material;

3) and calcining the dried material in a rotary furnace under the nitrogen atmosphere at 645 ℃ for 2.5 hours to obtain the carbon-doped high-compaction iron phosphate.

In the step (1), in the process of adding the ferric nitrate solution into the EDTA disodium solution and in the process of adding the acid-base regulator, the temperature of the solution is maintained at 30 ℃, the concentration of the ferric nitrate solution is 2mol/L, the pH value is less than or equal to 1, the molar ratio of the ferric nitrate to the EDTA disodium is 1:0.12, and the acid-base regulator is ammonia water.

In the step (1), the molar ratio of phosphoric acid to ammonium dihydrogen phosphate in the mixed solution of phosphoric acid and ammonium dihydrogen phosphate is 2.3, the molar ratio of ferric nitrate to phosphate radicals in the mixed solution of phosphoric acid and ammonium dihydrogen phosphate is 1:1.20, and the ferric nitrate solution and the acid-base regulator are added into the EDTA disodium solution at a stirring speed of 350 r/min.

And (3) adding alkali into the reaction slurry in the step (2), wherein the adding time is 50min, the concentration of the alkali is 5mol/L, and the alkali is lithium hydroxide.

Mixing the mother liquor filtered in the step (2) with washing water, adding alkali to adjust the pH value of the solution to be 11, continuously blowing off the solution by 7 stages, spraying and absorbing the blown-off gas by a phosphoric acid solution to absorb the pH value of the solution to be 4.5 to obtain an ammonium dihydrogen phosphate solution, concentrating and crystallizing the blown-off solution until the Baume degree is 48, cooling the solution to the temperature of 25 ℃ to obtain mixed crystals of phosphate and nitrate, filtering the mixed crystals, adding the mother liquor into the phosphoric acid solution, adjusting the pH value of the solution to be 3, filtering the mixed crystals to obtain EDTA crystals, and returning and mixing the rest solution into the mother liquor and the washing water filtered in the step (2).

And (3) in the calcining process of the dried material in the rotary furnace in the nitrogen atmosphere, maintaining the nitrogen content in the gas in the rotary furnace to be more than 85 percent and the oxygen content to be 0.6 percent.

And (3) after the calcined material is subjected to jet milling, sieving by an ultrasonic vibration sieve with the mesh number of 120 meshes, removing iron by using a 2-grade battery iron remover, and stopping removing iron until the magnetic substance is less than 0.5ppm to obtain the carbon-doped high-compaction iron phosphate.

The indices of the iron phosphate obtained are as follows:

index (I)	Fe	P	Iron to phosphorus molar ratio	C
					Numerical value	36.41％	20.18％	1.002	0.67％
Index (I)	BET	Loose-pack	Tap density	Co
					Numerical value	6.3m2/g	0.72g/mL	1.18g/mL	1.5ppm
Index (I)	Ni	Ca	Mn	Zn
					Numerical value	5.1ppm	21.6ppm	26.6ppm	5.8ppm
Index (I)	Na	Cd	Mg	Moisture content
					Numerical value	32.6ppm	1.4ppm	12.7ppm	1564ppm
Index (I)	K	pH	Sulfur	Magnetic substance
					Numerical value	13.6ppm	3.11	46ppm	0.43ppm
Index (I)	D10	D50	D90	Density of compaction
					Numerical value	1.6μm	4.5μm	9.1μm	2.12g/mL

As shown in fig. 3, the iron phosphate obtained in this example is spherical particles, and has partially agglomerated particles, and the density of single crystal particles is higher as seen from the SEM picture.

Example 3

1) dissolving disodium EDTA in water, preparing a solution with the concentration of 0.3mol/L at 35 ℃, adding a ferric nitrate solution into the disodium EDTA solution, then adding an acid-base regulator until the pH value of the mixed solution is 1, stirring and mixing uniformly, then adding a mixed solution of phosphoric acid and ammonium dihydrogen phosphate, adding for 1.5h, and then stirring and reacting for 20min to obtain a reaction slurry;

2) adding alkali into the reaction slurry obtained in the step (1), adjusting the pH value of the solution to 2.8, heating to 90 ℃, stirring for reaction for 25min, cooling to 55 ℃, filtering, washing and drying to obtain a dried material;

3) and calcining the dried material in a rotary furnace under the nitrogen atmosphere at 635 ℃ for 2.5 hours to obtain the carbon-doped high-compaction iron phosphate.

In the step (1), in the process of adding the ferric nitrate solution into the EDTA disodium solution and in the process of adding the acid-base regulator, the temperature of the solution is maintained at 35 ℃, the concentration of the ferric nitrate solution is 2mol/L, the pH value is less than or equal to 1, the molar ratio of the ferric nitrate to the EDTA disodium is 1:0.14, and the acid-base regulator is ammonia water.

In the step (1), the molar ratio of phosphoric acid to ammonium dihydrogen phosphate in the mixed solution of phosphoric acid and ammonium dihydrogen phosphate is 2.3, the molar ratio of ferric nitrate to phosphate radicals in the mixed solution of phosphoric acid and ammonium dihydrogen phosphate is 1:1.25, and the ferric nitrate solution and the acid-base regulator are added into the EDTA disodium solution at a stirring speed of 350 r/min.

And (3) adding alkali into the reaction slurry in the step (2), wherein the adding time is 60min, the concentration of the alkali is 4mol/L, and the alkali is potassium hydroxide.

Mixing the mother liquor filtered in the step (2) with washing water, adding alkali to adjust the pH value of the solution to be 11, continuously blowing off the solution by 7 stages, spraying and absorbing the blown-off gas by a phosphoric acid solution to absorb the pH value of the solution to be 4 to obtain an ammonium dihydrogen phosphate solution, concentrating and crystallizing the blown-off solution until the Baume degree is 48, cooling the solution to the temperature of 25 ℃ to obtain mixed crystals of phosphate and nitrate, filtering the mixed crystals, adding the mother liquor into the phosphoric acid solution to adjust the pH value of the solution to be 4, filtering the mixed crystals to obtain EDTA crystals, and returning and mixing the rest solution into the mother liquor and the washing water filtered in the step (2).

And (3) after the calcined material is subjected to jet milling, screening by an ultrasonic vibration screen, wherein the mesh number of the screen is 100 meshes, removing iron by using a 2-grade battery iron remover, and stopping removing iron until the magnetic substance is less than 0.5ppm to obtain the carbon-doped high-compaction iron phosphate.

The indices of the iron phosphate obtained are as follows:

as shown in fig. 4, the iron phosphate obtained in this example is spherical-like particles, and has good dispersibility, and from an SEM picture, the density of single crystal particles is high, and flocculent carbon exists on the surfaces of some of the particles.

The iron phosphate materials of examples 1 to 3 and commercially available iron phosphate not doped with titanium were mixed with lithium carbonate and glucose, and then milled, dried, calcined, and pulverized to obtain a lithium iron phosphate material, and finally the carbon content of the lithium iron phosphate was 1.5 ± 0.15%, and then a power-on test was performed, with the results as follows:

the electricity deduction test method comprises the following steps: SP: the mass ratio of PVDF is 90: 5: and 5, assembling to obtain a buckle for testing, wherein the cut-off voltage is 2.0V, and the compaction density measurement method is the same as the above.

Finally, the above embodiments are only for illustrating the technical solutions of the present invention and not for limiting, although the present invention has been described in detail with reference to the preferred embodiments, it should be understood by those skilled in the art that modifications or equivalent substitutions may be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all of them should be covered in the claims of the present invention.

Claims

1. A preparation method of carbon-doped high-compaction iron phosphate is characterized by comprising the following steps:

2. The method for preparing carbon-doped highly compacted iron phosphate according to claim 1, wherein the method comprises the following steps: in the step (1), in the process of adding the ferric nitrate solution into the EDTA disodium solution and in the process of adding the acid-base regulator, the temperature of the solution is maintained at 30-35 ℃, the concentration of the ferric nitrate solution is 1-3mol/L, the pH value is less than or equal to 1, the molar ratio of the ferric nitrate to the EDTA disodium is 1:0.1-0.15, and the acid-base regulator is ammonia water or nitric acid.

3. The method for preparing carbon-doped highly compacted iron phosphate according to claim 1, wherein the method comprises the following steps: the molar ratio of phosphoric acid to ammonium dihydrogen phosphate in the mixed solution of phosphoric acid and ammonium dihydrogen phosphate in the step (1) is 2-2.5, the molar ratio of ferric nitrate to phosphate radical in the mixed solution of phosphoric acid and ammonium dihydrogen phosphate is 1:1.15-1.3, and the ferric nitrate solution and the acid-base regulator are added into the EDTA disodium solution at a stirring speed of 300-400 r/min.

4. The method for preparing carbon-doped highly compacted iron phosphate according to claim 1, wherein the method comprises the following steps: and (3) adding alkali into the reaction slurry in the step (2), wherein the adding time is 30-60min, the concentration of the alkali is 2-5mol/L, and the alkali is at least one of sodium hydroxide, lithium hydroxide and potassium hydroxide.

5. The method for preparing carbon-doped highly compacted iron phosphate according to claim 1, wherein the method comprises the following steps: mixing the mother liquor filtered in the step (2) with washing water, adding alkali to adjust the pH value of the solution to 10-12, continuously blowing off through 6-8 stages, spraying and absorbing blown-off gas through a phosphoric acid solution, adjusting the pH value absorbed by the solution to 3-5 to obtain an ammonium dihydrogen phosphate solution, concentrating and crystallizing the blown-off solution until the Baume degree is 45-50, cooling to the temperature of 20-30 ℃ to obtain mixed crystals of phosphate and nitrate, filtering, adding the mother liquor into the phosphoric acid solution, adjusting the pH value of the solution to 3-4, filtering to obtain EDTA crystals, and returning and mixing the rest solution into the mother liquor and the washing water filtered in the step (2).

6. The method for preparing carbon-doped highly compacted iron phosphate according to claim 1, wherein the method comprises the following steps: and (3) in the calcining process of the dried material in the rotary furnace in the nitrogen atmosphere, maintaining the nitrogen content in the gas in the rotary furnace to be more than 85 percent and the oxygen content to be 0.5 to 1 percent.

7. The method for preparing carbon-doped highly compacted iron phosphate according to claim 1, wherein the method comprises the following steps: and (4) after the calcined material in the step (3) is subjected to airflow crushing, screening by an ultrasonic vibration screen, wherein the mesh number of the screen is 100-200 meshes, removing iron by using a 2-stage battery iron remover, and stopping removing iron until the magnetic substance is less than 0.5ppm, so as to obtain the carbon-doped high-compaction iron phosphate.

8. The method for preparing carbon-doped highly compacted iron phosphate according to claim 1, wherein the method comprises the following steps: and (3) carrying out water spraying absorption on the gas obtained by calcining in the step (3), and mixing the obtained absorption liquid with the mother liquor and the washing water filtered in the step (2) for treatment.