A method for low-cost synthesis of lithium iron phosphate by using lithium phosphate as lithium source

Abstract

The application discloses a method for synthesizing lithium iron phosphate with low cost by taking lithium phosphate as a lithium source, and belongs to the technical field of lithium iron phosphate preparation. The application solves the problem of how to improve the specific capacity and the compaction density of lithium iron phosphate, takes lithium phosphate instead of lithium carbonate as the lithium source, reduces the raw material cost of lithium iron phosphate, adds a chelating agent to the lithium salt solution to remove impurities, then uses a resin to adsorb the residual sodium ions, mixes the lithium salt solution with a phosphate solution, adjusts the pH to obtain lithium phosphate precipitation, mixes the phosphate solution with a ferrous salt solution, then adds an antioxidant, adjusts the pH, and obtains a slurry; the slurry is filtered and washed to obtain a filter cake containing a large amount of free water eight ferrous phosphate, which is ground with lithium phosphate, glucose, a surfactant, pure water and a small amount of phytic acid to obtain a fine particle size slurry with a D50 of less than or equal to 400 nm; sintering and crushing are carried out under an inert atmosphere to obtain a lithium iron phosphate material, which has the advantages of high capacity, high compaction density and high iron-phosphorus ratio.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of lithium iron phosphate preparation, and particularly relates to a method for synthesizing lithium iron phosphate at low cost by taking lithium phosphate as a lithium source. BACKGROUND

[0002] Lithium iron phosphate is one of the most popular positive electrode materials in the lithium battery industry at present, has an orthorhombic olivine structure, and P and O are combined by a covalent bond, so that the structure is extremely stable. Therefore, the lithium iron phosphate material has good cycle performance, and the cycle number can reach more than 2000 times; meanwhile, the lithium iron phosphate material has good thermal stability and excellent safety performance. In addition, the lithium iron phosphate material does not contain precious metal elements, iron and phosphorus are both conventional industrial raw materials, are low in price, are rich in reserves, and are low in production cost. Moreover, the lithium iron phosphate material does not contain any heavy metal elements harmful to human bodies, is green and environmentally friendly, is pollution-free, and is a truly green new energy material.

[0003] The theoretical specific capacity of lithium iron phosphate is 170 mAh / g, the voltage platform is about 3.2 V, and the true density is 3.6 g / cm 3 . Through years of optimization by technical personnel in the industry, the specific capacity can reach more than 150 mAh / g, the compaction density can reach 2.4 g / cm 3 , and the system energy density of the lithium iron phosphate battery can reach 160 Wh / kg at most. However, compared with the system energy density of more than 180 Wh / kg of the ternary material, there is still a not small gap. Moreover, whether the specific capacity or the compaction density, the lithium iron phosphate material has certain room for improvement. Therefore, how to develop a preparation method of nano lithium iron phosphate with high capacity and high iron-phosphorus ratio becomes a technical problem to be solved urgently. SUMMARY

[0004] In view of the problem of how to develop a preparation method of nano lithium iron phosphate with high capacity and high iron-phosphorus ratio in the prior art, the application provides a method for synthesizing lithium iron phosphate at low cost by taking lithium phosphate as a lithium source.

[0005] The technical scheme adopted by the application is as follows.

[0006] A method for synthesizing lithium iron phosphate at low cost by taking lithium phosphate as a lithium source, comprising the following steps:

[0007] Step A: removing soluble impurities in a lithium salt solution to obtain a lithium salt solution after impurities are removed;

[0008] Step B: obtaining lithium phosphate precipitation from the lithium salt solution obtained in step A;

[0009] Step C: obtaining ferrous phosphate octahydrate by reacting a phosphorus salt solution with a ferrous salt solution;

[0010] Step D: grinding the ferrous phosphate octahydrate obtained in step C with the lithium phosphate obtained in step B, glucose, a surfactant, pure water, and phytic acid to obtain a fine particle size slurry;

[0011] Step E: drying the fine particle size slurry obtained in step D to obtain a lithium iron phosphate precursor, and calcining, sintering, and crushing the lithium iron phosphate precursor to obtain lithium iron phosphate.

[0012] After the technical scheme is adopted, the lithium resource reserves in salt lakes are much greater than the lithium resource reserves in ores, and the cost of preparing lithium salt by extracting lithium from salt lakes is low, which has great advantages. At present, the market of lithium phosphate is small, and most of the lithium phosphate is prepared by the salt lake lithium precipitation process, which has a low cost. The disadvantage is that there are many impurities in the salt lake, especially magnesium and lithium, which are difficult to separate due to their similar chemical properties. Therefore, a lithium salt purification process is also developed to improve the purity of lithium salt and the performance of lithium iron phosphate products. The present technology uses lithium phosphate instead of lithium carbonate as a lithium source to reduce the cost of lithium iron phosphate raw materials. The preparation of ferrous phosphate process needs to be added to the reaction, beating, aging, washing, drying, and calcining sections to prepare a batch of products, which takes at least 24 hours. However, the ferrous phosphate process only needs a reaction section without drying, and it only takes 2 hours to prepare a batch. Therefore, the present process greatly shortens the reaction time in the ferrous phosphate section compared to the ferrous phosphate process, and does not need to add hydrogen peroxide and phosphoric acid in the process, which reduces the safety hazards of the experiment and saves time and raw material costs. The iron loss rate is low during the synthesis of ferrous phosphate, which is as low as 0.4% or less, and the iron-phosphorus ratio is between 1.49 and 1.51. When combined with lithium phosphate at a ratio of 1:1 to obtain lithium iron phosphate products, the iron-phosphorus ratio is basically between 0.98 and 0.99. In the present process, phytic acid is used as an additive to effectively improve the lithium ion diffusion capacity of lithium iron phosphate. The layered structure of the carbon film space formed by phytic acid on the surface of ferrous phosphate can store more capacity, and high-capacity, high-density, and high-iron-phosphorus-ratio lithium iron phosphate is obtained.

[0013] As a preferred embodiment, the specific steps of step A are as follows:

[0014] Step A1: adding a chelating agent to the lithium salt solution to form a soluble compound from the soluble impurities in the lithium salt solution, the soluble impurities including calcium ions, magnesium ions, sodium ions, and potassium ions;

[0015] Step A2: filtering the solution obtained in step A1 to obtain a filtrate and a filter cake;

[0016] Step A3: redissolving the filter cake obtained in step A2 and adding a resin to adsorb the remaining sodium ions to obtain a lithium salt solution after impurity removal.

[0017] By adding the chelating agent, calcium, magnesium, sodium and potassium in the lithium salt form soluble compounds, and then filtering, and then dissolving the filter cake and adding the resin to adsorb the remaining sodium ions, a relatively pure lithium salt solution is obtained, and the impurity removal rate can reach 99%.

[0018] Preferably, the chelating agent is EDTA, and the mass ratio of the added chelating agent to lithium is (1.002-1.005):1.

[0019] Preferably, in step B, the lithium salt solution obtained in step A is mixed with a phosphate solution, and a pH adjusting agent is added to adjust the pH value, and then lithium phosphate precipitate is obtained, and the pH adjusting agent is one or more of ammonia, ammonium carbonate, and ammonium bicarbonate.

[0020] Preferably, the specific steps of step C are as follows:

[0021] Step C1: The phosphate salt solution is mixed with the ferrous salt solution, and the molar ratio of the ferrous salt solution to the phosphate salt solution is (1.48-1.50):1, and an antioxidant is added for reaction;

[0022] Step C2: The pH value of the solution obtained in step C1 is adjusted to 5-8;

[0023] Step C3: The solution obtained in step C2 is filtered and washed to obtain ferrous phosphate octahydrate filter cake.

[0024] If the molar ratio is less than 1.48, other high-phosphorus impurities will appear in the ferrous phosphate solution, and if the molar ratio is greater than 1.50, the ferrous reaction will not be complete. In the reaction, the phosphate salt solution is mixed with the ferrous salt solution and a small amount of antioxidant is added for reaction, which is used to prevent oxidation of the ferrous salt solution, affect the conversion rate and productivity, if no antioxidant is added, other compounds are easily formed, affecting the uniformity of the product. The pH of the reaction is adjusted to 5-8 to obtain a slurry, which is beneficial to the formation of ferrous phosphate octahydrate. If the pH is less than 5, the crystal structure of the formed ferrous phosphate will not fully grow, affecting the compaction density of the lithium iron phosphate formed later; if the pH is greater than 8, the formed ferrous phosphate will bring out a large amount of metal impurities.

[0025] Preferably, the molar ratio of the ferrous salt to the antioxidant added in step C1 is 1:(0.005-0.02).

[0026] Preferably, the antioxidant is one or more of inositol hexaphosphate, isohydroxy acid, gallic acid, citric acid, ascorbic acid, and sodium salt thereof.

[0027] By using the technical scheme, the antioxidant prevents oxidation of ferrous ions while becoming a carbon source for lithium iron phosphate coating.

[0028] Preferably, the molar ratio of ferrous phosphate octahydrate to lithium phosphate in step D is 1:(1.00-1.10).

[0029] After the technical scheme is adopted, lithium ions are slightly excessive in the reaction of generating lithium iron phosphate, so that the ferrous phosphate is fully reacted.

[0030] Preferably, in step D, the glucose, anhydrous ethanol, pure water, the ferrous phosphate obtained in step C and the lithium phosphate obtained in step B are mixed first, the mass of the added glucose is 6.5-9.0 wt% of the mass of the ferrous phosphate octahydrate, then the phytic acid is added, the mass of the added phytic acid is 0.6-1.0 wt% of the mass of the ferrous phosphate octahydrate, and the pure water, the anhydrous ethanol and the phosphoric acid are added in the process of grinding to control the pH of the fine particle size slurry to be 1.0-2.2 and the solid content to be 30-40 wt%.

[0031] After the technical scheme is adopted, the mass of the glucose is 6.5-9.0 wt% of the mass of the ferrous phosphate, the glucose is carbonized to amorphous carbon under the inert atmosphere, a part of the amorphous carbon is used as a reducing agent to reduce the trivalent iron to divalent iron, a part of the sulfate is reduced to SO2 and discharged, and the remaining carbon is used as a conductive agent to coat the surface of the lithium iron phosphate particles; if the added amount of the glucose is less than 6.5%, the amount of the coated carbon is too low to affect the conductivity of the product, and if the added amount of the glucose is greater than 8.0%, the amount of the coated carbon is too high to affect the tap density and the specific surface area of the product.

[0032] The added amount of the pure water is 30-40 wt% of the solid content; if the solid content is less than 30 wt%, the equipment capacity is wasted, and if the solid content is greater than 40 wt%, the grinding difficulty is increased to affect the grinding effect.

[0033] The added amount of the phytic acid is 0.6-1.0 wt% of the mass of the ferrous phosphate, and the added amount of the phytic acid is too small to improve the performance of the material to a limited extent, and the added amount of the phytic acid is too large to damage the original structure of the lithium iron phosphate, reduce the stability of the lithium iron phosphate and increase the cost.

[0034] The anhydrous ethanol is added as a dispersant in the mixed slurry reaction, which can effectively prevent the secondary agglomerates of the electrode active material in the slurry, does not introduce impurity ions which are not contained in the precursor, so as to ensure the uniformity of the precursor mixture, and further prepare the lithium iron phosphate product with superior performance.

[0035] The composite carbon source is added in the mixed slurry reaction, and the phytic acid is added to improve the capacity, so that the traditional carbon chain coating structure is changed to a carbon layer inside the lithium iron phosphate, which increases the conductivity of the lithium iron phosphate, the carbon layer can store energy, and thus the specific capacity of the lithium iron phosphate is improved.

[0036] Preferably, the temperature for drying in step E is 120-180℃, and the calcination, sintering and crushing of the lithium ferrous phosphate precursor are carried out in an inert gas atmosphere, the calcination temperature is 350-450℃, the calcination time is 2-6h, the sintering temperature is 700-800℃, and the sintering time is 4-8h, so that the obtained lithium iron phosphate is a particle with a particle size of 300-500nm.

[0037] After the above technical scheme is adopted, the free water and crystal water are removed by drying at 120-180℃, then the nano-sized lithium iron phosphate particles are formed by calcination at 350-450℃ for 2-6h, and finally the final particles of lithium iron phosphate with a particle size of about 300nm are obtained by sintering at 700-800℃ for 4-8h.

[0038] In summary, due to the adoption of the above technical scheme, the present application has the following beneficial effects:

[0039] 1. The lithium iron phosphate powder obtained by the present technology has a compaction density of 2.55g / cm 3 , a first discharge capacity of 162mAh / g, and a 1C capacity of 155mAh / g.

[0040] 2. The present technology uses lithium phosphate instead of lithium carbonate as a lithium source, thereby reducing the cost of lithium iron phosphate raw materials. The present technology uses lithium phosphate and ferrous phosphate as raw materials, and uses ferrous phosphate as a precursor raw material, so that hydrogen peroxide does not need to be added in the reaction process, and the cost of lithium source and iron source is reduced. It is estimated that the cost can be saved by 3000 yuan / ton (lithium source 4000, hydrogen peroxide 500 yuan, operating cost-1500), and the cost can be saved by 6000 million yuan according to 20,000 tons.

[0041] 3. Compared with the phosphoric acid iron process, the present technology has a shorter reaction time in the ferrous phosphate section, and does not need to add hydrogen peroxide and phosphoric acid in the process, thereby reducing the safety hazards of the experiment, and saving time cost and raw material cost.

[0042] 4. Compared with the existing phosphoric acid iron process, the present technology can reduce the emission of 149kg of carbon dioxide per ton of lithium iron phosphate, and can reduce the emission of 50kg of carbon dioxide from glucose decomposition. If the cost of treating carbon dioxide is 500 yuan / ton, the cost of producing lithium iron phosphate can be saved by 100 yuan / ton of carbon dioxide treatment cost, and the emission of 199kg of carbon dioxide per ton can be reduced, thereby achieving the effect of energy saving and emission reduction, and realizing environmental protection. BRIEF DESCRIPTION OF DRAWINGS

[0043] Figure 1 The xrd diffraction pattern of Example 1 is shown in the following table:

[0044] Figure 2 The SEM image of Example 1 is shown in the following table:

[0045] Figure 3Electrochemical performance chart for Example 1;

[0046] Figure 4 XRD diffraction chart for Example 2;

[0047] Figure 5 SEM chart for Example 2;

[0048] Figure 6 Electrochemical performance chart for Example 2. DETAILED DESCRIPTION

[0049] In order to make the purposes, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only some of the embodiments of the present application, but not all the embodiments of the present application.

[0050] Example 1

[0051] A method for synthesizing lithium iron phosphate at low cost by taking lithium phosphate as a lithium source, comprising the following steps:

[0052] Step A: removing soluble impurities in the lithium salt solution to obtain a lithium salt solution after impurities removal;

[0053] Step A1: heating the lithium salt solution to 80℃, adding EDTA into the lithium salt solution at a mass ratio of 1:0.005, adjusting the pH to 2, and waiting for 30 min, so that the soluble impurity ions in the lithium salt solution form soluble compounds, the soluble impurity ions including calcium ions, magnesium ions, sodium ions and potassium ions;

[0054] Step A2: filtering the solution obtained in step A1 to obtain a filtrate and a filter cake;

[0055] Step A3: resolubilizing the filter cake obtained in step A2 and adding resin to adsorb the remaining sodium ions, the resin used in the embodiment being T-42 cation exchange resin with a filling height of 50 cm, to obtain a lithium salt after impurities removal, dissolving the purified lithium salt to obtain a lithium salt solution, the concentration of Li + in the lithium salt solution being 0.75 mol / L;

[0056] Step B: dissolving MAP with pure water to obtain a phosphorus salt solution, the content of P in the phosphorus salt solution being 0.65 mol / L, adjusting the pH of the phosphorus salt solution to 8 with ammonia water, adding the phosphorus salt solution dropwise into the lithium salt solution obtained in step A, the molar ratio of lithium to phosphorus being 3.1:1, the stirring speed being 360 rpm, the reaction time being 1.5 h, and the pH being adjusted to 10 to generate a lithium phosphate slurry;

[0057] Step C: The ferrous phosphate octahydrate is obtained by reacting the phosphorus salt solution with the ferrous salt solution;

[0058] Step C1: Dissolve FeSO4 in pure water to obtain a ferrous salt solution, the concentration of FeSO4 in the solution is 0.6 mol / L, dissolve MAP in pure water to obtain a phosphorus salt solution, the content of P in the solution is 0.5-1.0 mol / L, adjust the pH of the phosphorus salt solution to 6 with ammonia water, mix the obtained ferrous salt solution with the phosphorus salt solution, the molar ratio of the ferrous salt solution to the phosphorus salt solution is 1.48:1, add 0.02% (based on ferrous sulfate) gallic acid to react, the stirring speed is 200 rpm, the reaction time is 0.5 h, and the ferrous phosphate slurry is generated;

[0059] Step C2: Adjust the pH of the solution obtained in step C1 to 8;

[0060] Step C3: Filter the slurry obtained in step C2 with a filter press and wash with pure water, the washing endpoint is that the conductivity of the washing water is ≤50 μS / cm, and the ferrous phosphate octahydrate filter cake is obtained.

[0061] Step D: Mix the ferrous phosphate octahydrate obtained in step C with lithium phosphate obtained in step B (the molar ratio is 1:1.05), glucose (6.35% of the mass of the ferrous phosphate octahydrate), pure water and anhydrous ethanol, the solid content of the mixed solution is 40%, add phytic acid doping (the phytic acid is 1.0% of the weight of the ferrous phosphate), grind in a sand mill, grind to a slurry particle size of <0.4 nm;

[0062] Step E: Spray dry the fine particle size slurry obtained in step D, the drying temperature is 150°C, and the lithium ferrous phosphate precursor is calcined, sintered and crushed in an inert gas atmosphere, the calcination temperature is 400°C, the calcination time is 5 h, the sintering temperature is 800°C, and the sintering time is 8 h, and the obtained lithium iron phosphate is a particle with a particle size of 300 nm.

[0063] It can be seen from Figure 1 that the characteristic peak intensity of the obtained lithium iron phosphate is consistent with the corresponding pdf card, the crystallinity is good, and it presents a pnma type olivine structure; it can be seen from Figure 2 that the micro-morphology of the prepared lithium iron phosphate is spherical and the particle size distribution is uniform, about 300 nm; according to the standard test scheme, a 2032 button cell is assembled and tested, and it can be seen from Figure 3 that under the condition of charge-discharge rate of 0.1C, the initial charge-discharge capacity of the material can reach 162 mAh / g, the powder compaction density reaches 2.55 g / cm 3 , and the pole piece compaction density is 2.93 g / cm 3 .

[0064] The iron loss rate in the process of synthesizing ferrous phosphate is shown in Table 1:

[0065] Table 1

[0066] Number Synthetic solution Fe % Mother liquor Fe / ppm Fe loss rate % 1 3.3 37.89 0.23 2 2.78 52.82 0.38 3 2.86 31.88 0.22

[0067] As shown in Table 1, the iron loss rate in the process of synthesizing ferrous phosphate by the method is low, which can be as low as below 0.4%.

[0068] Example 2

[0069] A method for synthesizing lithium iron phosphate at low cost by using lithium phosphate as a lithium source, comprising the following steps:

[0070] Step A: removing soluble impurities in the lithium salt solution to obtain a lithium salt solution after impurities removal;

[0071] Step A1: heating the lithium salt solution to 80℃, adding EDTA into the lithium salt solution at a mass ratio of 1:0.005, adjusting the pH to 2.9, and waiting for 30 min, so that the soluble impurity ions in the lithium salt solution, including calcium ions, magnesium ions, sodium ions and potassium ions, form soluble compounds;

[0072] Step A2: filtering the solution obtained in step A1 to obtain a filtrate and a filter cake;

[0073] Step A3: resolubilizing the filter cake obtained in step A2, and adding resin to adsorb the remaining sodium ions to obtain a lithium salt after impurities removal, wherein the sodium ions in the obtained lithium salt are less than 50 ppm, and the purified lithium salt is dissolved to obtain a lithium salt solution, wherein the concentration of Li + in the lithium salt solution is 0.74 mol / L;

[0074] Step B: dissolving MAP with pure water to obtain a phosphorus salt solution, wherein the P content in the phosphorus salt solution is 0.65 mol / L, and the pH of the phosphorus salt solution is adjusted to 6 by using ammonia water, and the phosphorus salt solution is added dropwise into the lithium salt solution obtained in step A, the molar ratio of lithium to phosphorus is 3.02:1, the stirring speed is 300 rpm, the reaction time is 1.5 h, the pH value is adjusted to 10, and a lithium phosphate slurry is generated;

[0075] Step C: obtaining ferrous phosphate octahydrate by reacting the phosphorus salt solution with a ferrous salt solution;

[0076] Step C1: FeSO4 is dissolved in pure water to obtain a ferrous salt solution with a concentration of 0.5 mol / L, and MAP is dissolved in pure water to obtain a phosphorus salt solution with a P content of 0.32 mol / L, the pH of the phosphorus salt solution is adjusted to 6 with ammonia water, the ferrous salt solution and the phosphorus salt solution are mixed in a molar ratio of 1.49:1, and ascorbic acid 0.02% (based on ferrous sulfate) is added for reaction, the stirring speed is 200 rpm, the reaction time is 0.5 h, and a ferrous phosphate slurry is generated;

[0077] Step C2: The pH value of the solution obtained in step C1 is adjusted to 8;

[0078] Step C3: The slurry obtained in step C2 is filtered with a filter press and washed with pure water, and the washing endpoint is a washing water conductivity of ≤50 μS / cm, to obtain a ferrous phosphate octahydrate filter cake.

[0079] Step D: The ferrous phosphate octahydrate obtained in step C is mixed with lithium phosphate obtained in step B (molar ratio = 1:1.03), glucose (6.75% of the mass of ferrous phosphate octahydrate), and pure water and anhydrous ethanol, the solid content of the mixture is 35%, and phytic acid is added for doping (phytic acid is 0.7% of the weight of ferrous phosphate), and a sand mill is used for grinding until the slurry particle size is <0.4 nm.

[0080] Step E: The fine particle size slurry obtained in step D is spray dried at a temperature of 150°C, and the lithium ferrous phosphate precursor is calcined, sintered and crushed in an inert gas atmosphere, the calcination temperature is 400°C, the calcination time is 5 h, the sintering temperature is 800°C, and the sintering time is 8 h, and the obtained lithium iron phosphate is a particle with a particle size of 300 nm.

[0081] As can be seen from Figure 4 , the characteristic peak intensity of the obtained lithium iron phosphate is consistent with the pdf card, the crystallinity is good, and it presents a pnma type olivine structure; as can be seen from Figure 5 , the micro-morphology of the prepared lithium iron phosphate is spherical and the particle size distribution is uniform, about 300 nm; according to the standard test scheme, a 2032 button cell is assembled and tested, and as can be seen from Figure 6 , under the condition of charge-discharge rate of 0.1C, the initial charge-discharge capacity of the material can reach 157 mAh / g, the powder compaction density reaches 2.64 g / cm 3 , and the pole piece compaction density is 3.036 g / cm 3 .

[0082] The above embodiments only express the specific implementation of the present application, which is described in more detail and specifically, but cannot be understood as a limitation to the protection scope of the present application. It should be noted that for those skilled in the art, without departing from the technical concept of the present application, a number of modifications and improvements can be made, which are all within the protection scope of the present application.

Claims

1. A method for low cost synthesis of lithium iron phosphate using lithium phosphate as lithium source, characterized in that: The method comprises the following steps: Step A: removing soluble impurities in the lithium salt solution to obtain a lithium salt solution after impurity removal; the specific steps of step A are as follows: Step A1: adding a chelating agent to the lithium salt solution to form soluble compounds from the soluble impurities in the lithium salt solution, wherein the soluble impurities include calcium ions, magnesium ions, sodium ions and potassium ions; the chelating agent is EDTA, and the mass ratio of the added chelating agent to lithium is (1.002-1.005):1; Step A2: filtering the solution obtained in step A1 to obtain a filtrate and a filter cake; Step A3: resolubilizing the filter cake obtained in step A2 and adding a resin to adsorb the remaining sodium ions to obtain a lithium salt solution after impurity removal; Step B: obtaining a lithium phosphate precipitate from the lithium salt solution obtained in step A; Step C: obtaining ferrous phosphate octahydrate by reacting a phosphate salt solution with a ferrous salt solution; the specific steps of step C are as follows: Step C1: mixing the phosphate salt solution with the ferrous salt solution, wherein the molar ratio of the ferrous salt solution to the phosphate salt solution is (1.48-1.50):1, and adding an antioxidant for reaction; the molar ratio of the added ferrous salt to the antioxidant is 1:(0.005-0.02); Step C2: adjusting the pH value of the solution obtained in step C1 to 5-8; Step C3: filtering and washing the solution obtained in step C2 to obtain a ferrous phosphate octahydrate filter cake; Step D: grinding the ferrous phosphate octahydrate obtained in step C, the lithium phosphate precipitate obtained in step B, glucose, anhydrous ethanol, pure water and phytic acid to obtain a fine particle size slurry; in step D, the glucose, anhydrous ethanol, pure water, the ferrous phosphate octahydrate obtained in step C and the lithium phosphate precipitate obtained in step B are first mixed, the mass of the added glucose is 6.5-9.0 wt% of the mass of the ferrous phosphate octahydrate, then the phytic acid is added, the mass of the added phytic acid is 0.6-1.0 wt% of the mass of the ferrous phosphate octahydrate, and pure water, anhydrous ethanol and phosphoric acid are added during the grinding process to control the pH of the fine particle size slurry to be 1.0-2.2 and the solid content to be 30-40 wt%; Step E: drying the fine particle size slurry obtained in step D to obtain a lithium iron phosphate precursor, and calcining, sintering and crushing the lithium iron phosphate precursor to obtain lithium iron phosphate.

2. The method for synthesizing lithium iron phosphate with low cost by taking lithium phosphate as lithium source according to claim 1, characterized in that: In step B, the lithium salt solution obtained in step A is mixed with a phosphate salt solution, and a pH adjusting agent is added to adjust the pH value to 6-8 to obtain a lithium phosphate precipitate, wherein the pH adjusting agent is one or more of ammonia, ammonium carbonate and ammonium bicarbonate.

3. The method for synthesizing lithium iron phosphate at low cost by using lithium phosphate as lithium source according to claim 1, characterized in that: The antioxidant is one or more of inositol hexaphosphate, isovaleric acid, gallic acid, citric acid, ascorbic acid and sodium salt thereof.

4. The method for synthesizing lithium iron phosphate at low cost with lithium phosphate as lithium source according to claim 1, characterized in that: In step D, the molar ratio of the ferrous phosphate octahydrate to the lithium phosphate is 1:(1.00-1.10).

5. The method for low cost synthesis of lithium iron phosphate using lithium phosphate as lithium source as claimed in claim 1 wherein: The temperature in step E is 120-180 DEG C, and the calcination, sintering and crushing of the lithium ferrous phosphate precursor are carried out in an inert gas atmosphere, the calcination temperature is 350-450 DEG C, the calcination time is 2-6 h, the sintering temperature is 700-800 DEG C, and the sintering time is 4-8 h, to obtain the lithium iron phosphate with a particle size of 300-500 nm.

Images