Lithium secondary battery, positive electrode of lithium secondary battery, lithium iron phosphate active material and preparation of ferrous phosphate precursor

Abstract

The invention belongs to the field of battery positive electrode materials, and particularly relates to a preparation method of a ferrous phosphate precursor, a Fe < 3 + > solution and a phosphate radical source solution with the Fe / P element ratio of 3: (1.8-2.1) are mixed and subjected to a first-stage precipitation reaction in advance, and the initial pH A of the first-stage precipitation reaction is 0.5-2; then supplementing an alkaline substance to carry out a second-stage precipitation reaction under the condition that the pH value is 2.5-5, and then carrying out solid-liquid separation to obtain an iron phosphate-Fe (OH) 3 composite material; and compounding the iron phosphate Fe (OH) 3 composite material with a carbon source, and carrying out reduction roasting treatment to obtain the ferrous phosphate precursor. The invention also comprises a preparation method and application of the material. According to the method, the performance of the prepared material can be improved, and the low-temperature and batch quality stability of the material is particularly facilitated.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-ion batteries, and particularly relates to the field of lithium iron phosphate cathode materials. Technical Background

[0002] Lithium iron phosphate batteries are continuously expanding their market share due to their excellent electrochemical performance and cost advantages. Facing the growing market demand, low-cost production is of great significance for cost reduction and efficiency improvement in the lithium iron phosphate industry.

[0003] Currently, the process routes for producing lithium iron phosphate include the ferrous oxalate process, the iron oxide red process, the hydrothermal process, and the iron phosphate process. Among them, the ferrous oxalate process generates a large amount of waste gas during sintering, resulting in high environmental protection pressure and low yield; the iron oxide red process has high requirements for the purity and particle size of the raw material iron oxide red, leading to general comprehensive performance of the lithium iron phosphate produced by this process; the hydrothermal process has certain safety hazards and requires a phosphoric acid purification process, resulting in high costs; the iron phosphate process produces lithium iron phosphate with good comprehensive performance, but also has the disadvantage of high costs.

[0004] In summary, the existing preparation methods of lithium iron phosphate generally have problems such as unsatisfactory utilization rate of Fe / P elements, difficulty in precise control, unsatisfactory product quality stability, and electrochemical performance, especially low-temperature performance. Summary of the Invention

[0005] Aiming at the problems of difficult precise control and utilization of Fe / P in the existing lithium iron phosphate materials, and unsatisfactory product quality stability and performance, the first object of the present invention is to provide a preparation method of a ferrous phosphate precursor (also simply referred to as a precursor in the present invention), aiming to provide a precursor that can precisely control the Fe / P ratio, achieve efficient utilization of elements, and is conducive to preparing an electrochemically active material with excellent performance.

[0006] The second object of the present invention is to provide the precursor prepared by the above preparation method and its application in the preparation of lithium iron phosphate.

[0007] The third object of the present invention is to provide a preparation method of a lithium iron phosphate active material (also referred to as an active material in the present invention), aiming to prepare an active material with excellent electrochemical performance and quality stability by using the above precursor.

[0008] The fourth object of the present invention is to provide the lithium iron phosphate active material prepared by the above preparation method and its application in lithium secondary batteries.

[0009] The fifth object of the present invention is to provide a lithium secondary battery containing the active material prepared by the preparation method of the present invention and its positive electrode.

[0010] For the preparation of lithium iron phosphate active materials, the existing technology mainly uses the precipitation method to prepare iron phosphate, and further composites it with lithium by roasting to obtain lithium iron phosphate. However, this process generally has deficiencies such as difficult complete utilization of Fe and P, difficult precise control of the Fe / P ratio, unsatisfactory quality stability of the product, and especially unsatisfactory electrochemical performance, especially low-temperature performance. In response to this problem, through in-depth research, the present invention provides the following solutions:

[0011] A method for preparing an iron phosphate precursor, which mixes an Fe 3+ solution with an Fe / P element ratio of 3:1.8 - 2.1 and a phosphoric acid root source solution and pre-performs a first-stage precipitation reaction. Among them, the starting pH of the first-stage precipitation reaction A is 0.5 - 2; then an alkaline substance is added to carry out a second-stage precipitation reaction under the condition of pH B of 2.5 - 5, and then solid-liquid separation is carried out to obtain an iron phosphate@Fe(OH) 3 composite material;

[0012] The iron phosphate@Fe(OH) 3 composite material and a carbon source are compounded and subjected to a reduction roasting treatment to obtain the iron phosphate precursor.

[0013] In the present invention, trivalent iron is innovatively pre-treated with phosphate radical by a first-stage precipitation, and then further treated with an alkaline substance by a second-stage precipitation. Further, in combination with the combined control of the Fe / P molar ratio and pH of the two-stage precipitation reaction, an iron phosphate@Fe(OH) 3 composite material can be obtained. Then, innovatively, the material with this special phase and structural characteristics is subjected to a reduction roasting in the form of carbon addition, which can realize the complete utilization of Fe / P, contribute to the precise control of Fe / P, and can prepare iron phosphate with high phase purity, high crystallinity, and taking into account special microphysical and chemical structural characteristics; innovatively, it is roasted with lithium addition, which helps to improve the quality stability of the prepared active material and is beneficial to improving its low-temperature performance.

[0014] In the present invention, the two-stage precipitation method, the combined idea of carbon addition reduction, and the combination of the parameters of the treatment process are the keys to synergistically improving the Fe / P utilization rate, improving its precise control degree, and further facilitating the improvement of the quality stability and low-temperature performance of the subsequent prepared active material.

[0015] In the present invention, the Fe 3+ solution is an aqueous solution of any Fe 3+ water-soluble salt.

[0016] The Fe 3+ solution can be prepared based on analytical pure materials, or can be oxidized from a solution containing ferrous iron. In addition, considering the treatment economy, it can also be obtained by acid leaching and oxidation of iron-containing solids;

[0017] In the present invention, the phosphorus source is at least one of phosphoric acid and its water-soluble salts;

[0018] Preferably, the Fe / P element ratio is 3:1.95 - 2.05, and more preferably 3:2. It has been found that at this ratio, the phase and crystal grains can be further improved, which helps to further improve the low-temperature performance and consistency of the prepared material.

[0019] In the present invention, the pH A is 0.6 - 1.5, and further 0.8 - 1.2;

[0020] Preferably, the temperature of the first precipitation reaction is 70 - 95 °C;

[0021] Preferably, the time of the first precipitation reaction is 1 - 3 h;

[0022] Preferably, the basic substance is at least one of sodium hydroxide, potassium hydroxide, and ammonia water;

[0023] Preferably, the pH B is 3 - 4.5, and more preferably 3.5 - 4;

[0024] Preferably, the temperature of the second precipitation reaction is 70 - 95 °C;

[0025] Preferably, the time of the second precipitation reaction is 2 - 5 h;

[0026] Preferably, after the second precipitation reaction, solid-liquid separation is carried out to obtain a FePO₄@Fe(OH) 3 composite material, which includes a FePO₄ core and Fe(OH) 3 .

[0027] In the present invention, innovatively, the material with a special composite phase structure obtained by the two-stage treatment is subjected to a reduction roasting treatment in the form of carbon addition, which can unexpectedly and significantly solve the problem of impurity phases, and high-phase-purity iron phosphate can be obtained. In addition, it can also control the crystal grain characteristics, improve the element utilization rate, and improve the quality stability.

[0028] In the present invention, the carbon source can be at least one of any small molecule organic substances and polymers; preferably at least one of starch, glucose, PEG, and cyclodextrin;

[0029] Preferably, the carbon source is 5 wt% - 30 wt% of the weight of the FePO₄@Fe(OH) 3 composite material, preferably 10 - 20 wt%, and further can be 15 - 20 wt%;

[0030] Preferably, the atmosphere in the reduction roasting stage is a protective atmosphere;

[0031] Preferably, the temperature in the reduction roasting stage is 500 - 750 °C, more preferably 600 - 700 °C;

[0032] Preferably, the time for reduction roasting is 2 - 10 h, preferably 3 - 7 h.

[0033] The present invention also provides a ferrous phosphate precursor prepared by the described preparation method.

[0034] In the present invention, the special two-stage precipitation method of phosphate first and then hydroxide, further combined with the Fe / P molar ratio, pH during the formation process, and the combination of carbon-containing reduction roasting, can unexpectedly prepare a precursor with a more precise Fe / P ratio and taking into account the special physical and chemical structure characteristics given by the method. Innovatively, the precursor prepared by the described preparation method is subjected to lithium-containing roasting, and an active material with high electrochemical performance, especially low-temperature performance, and excellent mass stability can be obtained.

[0035] The present invention also provides a preparation method of a lithium iron phosphate active material, which performs heat treatment on a solid mixture containing the ferrous phosphate precursor prepared by the preparation method of the present invention, a lithium source, and a carbon source A;

[0036] Alternatively, a slurry containing the ferrous phosphate precursor, a lithium source, and a carbon source A of the present invention is subjected to spray pyrolysis to obtain the product.

[0037] In the present invention, the lithium source is at least one of lithium oxide, lithium hydroxide, lithium carbonate, and lithium phosphate;

[0038] In the present invention, the carbon source A is at least one of small molecule organic substances and polymers; preferably at least one of starch, glucose, PEG, and cyclodextrin;

[0039] In the present invention, the carbon source A is 5 wt% - 30 wt% of the weight of the ferrous phosphate precursor, preferably 10 - 20 wt%;

[0040] In the present invention, the slurry is obtained by liquid-phase ball milling and sand milling of the ferrous phosphate precursor, a lithium source, and a carbon source A;

[0041] In the present invention, the solid mixture is obtained by solid-phase mixing and solid-phase ball milling of the ferrous phosphate precursor, a lithium source, and a carbon source A; or the slurry is spray-dried to obtain the solid mixture;

[0042] In the present invention, the heat treatment is carried out in a protective atmosphere;

[0043] In the present invention, the temperature of the heat treatment is 650 - 800 °C, further 650 - 700 °C;

[0044] In the present invention, the heat treatment time is 4 to 24 hours, and further can be 5 to 10 hours;

[0045] In the present invention, the inlet temperature in the spray drying stage is 200 to 300 °C, and the outlet temperature is 70 to 120 °C.

[0046] The present invention also provides a lithium iron phosphate active material prepared by the described preparation method.

[0047] In the present invention, the described special precursor construction method can endow the material with an accurate Fe / P ratio and a microscopic physical and chemical structure. Innovatively performing lithium-doping heat treatment on it can obtain an active material with excellent electrochemical performance, especially low-temperature performance, and excellent quality stability.

[0048] The present invention also provides a positive electrode of a lithium secondary battery, including a current collector and a positive electrode material compounded on the current collector. The active material contained in the positive electrode material contains the lithium iron phosphate active material prepared by the present invention.

[0049] For the positive electrode of the present invention, except that the active material contains the active material prepared by the preparation method of the present invention, other materials and their component structures can be well-known.

[0050] For example, the positive electrode material further contains a binder and a conductive agent;

[0051] Again, in the positive electrode material, the contents of the binder and the conductive agent are 1 to 10 wt%, and the balance is the active material.

[0052] The present invention also provides a lithium secondary battery, including a positive electrode containing the active material prepared by the preparation method of the present invention.

[0053] For the lithium secondary battery of the present invention, except that the positive electrode contains the active material of the present invention, other battery structures and materials can be well-known in the industry.

[0054] Advantages of the present invention:

[0055] The present invention innovatively performs a first-stage precipitation treatment on ferric iron and phosphate radicals in advance, and then performs a second-stage precipitation treatment with an alkaline substance. Further, in combination with the joint control of the Fe / P molar ratio and pH of the two-stage precipitation reaction, iron phosphate @Fe(OH) can be obtained 3The composite material is innovatively subjected to reduction roasting treatment in the form of carbon addition, which can improve the utilization rate of Fe / P and the precise control of the ratio, and can also obtain ferrous phosphate with high phase purity, high crystallinity and special microphysical and chemical structure characteristics; innovatively subjecting it to lithium addition roasting helps to improve the quality stability of the prepared active material and is beneficial to improving its low-temperature performance. Brief Description of the Drawings

[0056] Figure 1 SEM image of the precursor A obtained in step 5 of Example 1.

[0057] Figure 2 XRD pattern of the ferrous phosphate obtained in step 6 of Example 1.

[0058] Figure 3 XRD pattern of the lithium iron phosphate obtained in step 7 of Example 1.

[0059] Figure 4 Electrochemical performance diagram of the lithium iron phosphate obtained in Example 1.

[0060] Figure 5 Cyclic Coulomb efficiency diagram of the lithium iron phosphate obtained in Example 1. Detailed Description of the Invention

[0061] In the present invention, the trivalent iron source solution can be obtained based on known methods. For example, in the present invention, considering cost, it can be obtained by acid leaching and oxidation of a solid iron source. For example, its steps are as follows:

[0062] Step (1): Dissolve the iron source with acid at a certain temperature, stir for a period of time, filter to obtain an iron-containing solution and test the iron ion content therein; in step (1), the dissolution temperature is 50-90 °C and the stirring time is 4-8 h;

[0063] Step (2): Add an oxidant to the iron solution in step (1) to oxidize the divalent iron ions present therein into trivalent iron ions.

[0064] In the present invention, the trivalent iron solution is innovatively subjected to a first-stage precipitation treatment with phosphate radicals first, and then a second-stage precipitation treatment with an alkali to obtain a ferric phosphate composite material loaded with iron hydroxide on the surface, and then subjected to carbon addition reduction to obtain the ferrous phosphate precursor. Its typical steps are, for example:

[0065] Step (a): At a certain temperature, add a solution containing phosphate radicals in a certain proportion to the trivalent iron solution, control a certain iron-to-phosphorus ratio, raise the temperature to a certain temperature and maintain it for a period of time to obtain a slurry containing ferric phosphate; the reaction temperature is 30-60 °C, the iron-to-phosphorus ratio is 3:1.8-2.1; the temperature for the temperature-raising reaction is 70-95 °C, and the heat preservation time is 1-3 h;

[0066] Step (b): Add an alkali solution to the slurry in step (a), adjust the pH value of the reaction system, and keep it warm for a certain period of time; the pH value is 2.5 - 4.5, and the heat preservation time is 2 - 5 h.

[0067] Step (c): Wash the slurry in step (b) multiple times, take the filter cake for drying and sintering dehydration to obtain the precursor A containing iron hydroxide @ iron phosphate, mix the precursor A with a certain proportion of carbon source, and sinter it under certain atmosphere conditions to obtain the iron phosphate precursor.

[0068] In the present invention, based on the known process, the precursor can be subjected to lithium - doping roasting to obtain lithium iron phosphate. For example, mix the obtained iron phosphate precursor with a certain proportion of lithium source and carbon source, and use the combined process of ball milling - sand milling - spraying - sintering to obtain the lithium iron phosphate material.

[0069] Example 1

[0070] Step (1): Dissolve ferric oxide with hydrochloric acid at 60 °C, stir for 3 h and then filter to obtain an iron - containing solution and test the iron ion content therein.

[0071] Step (2): Add hydrogen peroxide to the iron solution in step (1) to oxidize the existing divalent iron ions into trivalent iron ions.

[0072] Step (3):

[0073] At 60 °C, add a certain proportion of ammonium dihydrogen phosphate solution to the iron solution in step (2), control the iron - phosphorus ratio to be 3:2. After the ammonium dihydrogen phosphate solution is added (the initial pH is 0.8), heat the slurry to 88 °C and keep it warm for 2 h (the first - stage reaction) to obtain a slurry containing iron phosphate.

[0074] Step (4):

[0075] Add ammonia water to the slurry in step (3) to adjust the pH of the reaction system to 3.5, and keep it warm for 3 h (the second - stage reaction, the temperature is 80 - 85 °C).

[0076] Step (5):

[0077] After step (4), through solid - liquid separation, washing with water, and drying (the temperature is, for example, 60 - 80 °C), the precursor A containing iron phosphate @ iron hydroxide can be obtained (the SEM image is shown in Figure 1 )

[0078] Step (6):

[0079] Precursor A and 20 wt% glucose (based on the weight of precursor A) are mixed and sintered under an argon atmosphere (sintered at 700 °C for 4 h under a nitrogen atmosphere), and the iron phosphate precursor can be obtained (XRD see Figure 2 ).

[0080] Step (7):

[0081] The iron phosphate precursor obtained in step 6, lithium phosphate (Li / P molar ratio of 1.03:1), and starch (12 wt.% of the weight of the iron phosphate precursor) are dispersed and mixed in water, and a combined process of ball milling - sand milling - spraying - sintering is carried out (the inlet temperature of spray drying is 280 °C, and the outlet temperature is 100 °C; the sintering process is carried out under a nitrogen atmosphere at 700 °C for 6 h), and the lithium iron phosphate material can be obtained (XRD see Figure 3 ).

[0082] Electrochemical performance test: The lithium iron phosphate cathode material prepared in Example 1, acetylene black, and binder are mixed evenly according to a mass ratio of 8:1:1, and NMP is used as a solvent, and the mixture is manually ground to obtain a uniformly mixed slurry. The obtained slurry is coated on an aluminum foil sheet and dried in a vacuum oven at 120 °C for 12 h, and then it is punched into a disc-shaped electrode sheet with a diameter of 12 mm, and this electrode sheet is assembled into a CR2025 button cell in a pure argon glove box. At 25 °C, the charge-discharge cut-off voltage is set to 2.5 - 4.1 V for 1C constant current charge-discharge test. As Figure 4 shown, after 100 cycles, the lithium iron phosphate still maintains a specific capacity of 142.4 mAh / g, and the Coulomb efficiency graph is as Figure 5 shown.

[0083] Example 2

[0084] Compared with Example 1, the difference is only that step 1 is omitted, and in step 2, an aqueous solution of ferrous sulfate is directly used as the iron solution (the iron element content is the same as that in Example 1) for subsequent treatment, and other operations and parameters are the same as those in Example 1.

[0085] Example 3

[0086] Compared with Example 1, the difference is only that in step (3), at 50 °C, a certain proportion of diammonium hydrogen phosphate solution is added to the iron solution in step (2), and the iron to phosphorus ratio is controlled to be 3:1.9. After the addition of the diammonium hydrogen phosphate solution is completed (the initial pH is controlled to be 1.2), the slurry is heated to 90 °C and kept warm for 1 h (the first-stage reaction), and other operations and parameters are the same as those in Example 1.

[0087] Example 4

[0088] Compared with Example 1, the difference is only that in step (4), sodium hydroxide is added to the slurry to adjust the pH of the reaction system to 4 and keep it warm for 3 h, and other operations and parameters are the same as those in Example 1.

[0089] Example 5

[0090] Compared with Example 1, the difference is only that in step (6), precursor A and 18 wt% cyclodextrin (based on the weight of precursor A) are mixed and sintered under an argon atmosphere (sintered at 600 °C for 6 h under a nitrogen atmosphere). In step 7, starch is replaced with glucose, and its dosage is 15 wt.% of the weight of the iron phosphate precursor, the Li / P molar ratio is 1.02:1, the sintering temperature is 650 °C, and the time is 8 h. Other operations and parameters are the same as those in Example 1.

[0091] Comparative Example 1

[0092] Compared with Example 1, the difference is only that steps 1, 2, and 6 are omitted, and a ferrous sulfate solution with an equal Fe molar amount is directly used as the iron solution to perform the treatments of steps 3, 4, and 5 to obtain an iron phosphate precursor, and then the treatment of step 7 is directly performed to obtain lithium iron phosphate. The step parameters of steps 3, 4, 5, and 7 are the same as those in Example 1.

[0093] Comparative Example 2

[0094] Compared with Example 1, the difference is only that the oxidation reaction in step 2 is missing (the Fe valence state in the system is not raised to Fe 2+ ), and other operations and parameters are the same as those in Example 1. 3+ )

[0095] Comparative Example 3

[0096] Compared with Example 1, the difference is only that in step (3), the iron to phosphorus ratio is controlled to be 3:1.7, and other operations and parameters are the same as those in Example 1.

[0097] Comparative Example 4

[0098] Compared with Example 1, the difference is only that in step (3), the iron to phosphorus ratio is controlled to be 0.95:1, and other operations and parameters are the same as those in Example 1. This comparative case did not obtain the pure-phase iron phosphate precursor of the present invention.

[0099] Comparative Example 5

[0100] Compared with Example 1, the difference is only that the second-stage reaction in step 4 is missing, that is, the slurry in step 3 is directly subjected to step 5, and other operations and parameters are the same as those in Example 1.

[0101] Comparative Example 6

[0102] Compared with Example 1, the difference is only that the pH in Step 4 is 5.5, and other operations and parameters are the same as those in Example 1.

[0103] Comparative Example 7

[0104] Compared with Example 1, the difference is only that in Step 6, instead of adding glucose, 10v% H 2 / Ar atmosphere was used for thermal reduction (sintered at 700°C for 4 h), and other operations and parameters are the same as those in Example 1.

[0105] Comparative Example 8

[0106] Steps 1 to 5 are missing, and in Step 6, ferrous phosphate was directly used to replace Precursor A, and Step 7 was carried out after Step 6.

[0107] Comparative Example 9

[0108] Compared with Example 1, the difference is only that the slurry in Step 3 was subjected to solid-liquid separation, washed with water and dried to obtain a ferric phosphate precursor, which was then used as a precursor and mixed with lithium carbonate and a carbon source for the treatment in Step 7.

[0109] The obtained lithium iron phosphate material was tested for BET of the material by a specific surface area analyzer; the particle size and morphology distribution of the material were tested by SEM.

[0110] Electrochemical performance test: The material was mixed with PVDF and carbon black using NMP as a solvent to prepare a positive electrode sheet, and a button cell was assembled according to the standard of GB 31241-2014. Its electrochemical performance was tested, and the test conditions were as follows: The electrochemical performance of the button cell was tested using a Neware / BlueTester test system (range I: 10 mA; range U: 5 V). The test voltage range was 2.5 - 4.1 V, the charge and discharge current for the capacity test was set at 0.1C rate, the test temperature was room temperature 25°C, and the low temperature test temperature was -40°C.

[0111] Table 1 shows the test data of the finally prepared lithium iron phosphate products in each example and each comparative example

[0112]

[0113] Note: (a) The low temperature performance refers to the low temperature discharge performance: Discharge capacity at -40°C at 0.1C / Discharge capacity at room temperature (25°C) at 0.1C * 100%;

[0114] (b) The material consistency refers to the variance D(X) = E[X - E(X)]^ 2 ;

[0115] It can be seen from Table 1 that by using the process described in the present invention, a material with better low temperature stability can be unexpectedly obtained.

Claims

1. A method for preparing a ferrous phosphate precursor, characterized in that: The Fe / P ratio is 3:1.8~2.1 3+ The solution and the phosphate source solution are mixed and the first stage precipitation reaction is carried out in advance, wherein the first stage precipitation reaction starts at pH A 0.5~2; then add alkaline substances to make the pH B The second stage precipitation reaction was carried out under the condition of 2.5 to 5, and then the solid-liquid separation was carried out to obtain the iron phosphate@Fe(OH)3 composite material; The iron phosphate@Fe(OH)3 composite material and the carbon source are combined and subjected to reduction roasting treatment to obtain a ferrous phosphate precursor.

2. The method for preparing a ferrous phosphate precursor according to claim 1, wherein: The Fe 3+ The solution is Fe 3+ Aqueous solutions of water-soluble salts; Preferably, the Fe 3+ The solution is obtained by acid leaching and oxidation treatment of iron-containing solids; Preferably, the phosphate source is at least one of phosphoric acid and its water-soluble salts; Preferably, the Fe / P element ratio is 3:1.95-2.05, and more preferably 3:

2.

3. The method for preparing a ferrous phosphate precursor according to claim 1, wherein: The pH A 0.6~1.5; Preferably, the temperature of the first stage precipitation reaction is 70-95°C; Preferably, the first precipitation reaction time is 1 to 3 hours; Preferably, the pH B 3 to 4.5; Preferably, the temperature of the second stage precipitation reaction is 70-95°C; Preferably, the second precipitation reaction time is 2 to 5 hours; Preferably, the alkaline substance is at least one of sodium hydroxide, potassium hydroxide and ammonia water; Preferably, after the second stage precipitation reaction is completed, solid-liquid separation is performed to obtain an iron phosphate@Fe(OH)3 composite material, which includes an iron phosphate core and Fe(OH)3 embedded on its surface.

4. The method for preparing a ferrous phosphate precursor according to claim 1, wherein: The carbon source is at least one of small molecule organic matter and polymer; preferably at least one of starch, glucose, PEG, and cyclodextrin; Preferably, the carbon source is 5 wt% to 30 wt% of the weight of the iron phosphate@Fe(OH)3 composite material, preferably 10 to 20 wt%; Preferably, the atmosphere in the reduction roasting stage is a protective atmosphere; Preferably, the temperature of the reduction roasting stage is 500-750°C; Preferably, the reduction roasting time is 2 to 10 hours.

5. A ferrous phosphate precursor obtained by the preparation method according to any one of claims 1 to 4.

6. A method for preparing a lithium iron phosphate active material, characterized in that: Heat treating a solid mixture comprising the ferrous phosphate precursor according to claim 5 and a lithium source and a carbon source A; Alternatively, the slurry comprising the ferrous phosphate precursor as claimed in claim 5 and the lithium source and carbon source A is subjected to spray pyrolysis to obtain the product.

7. The method for preparing the lithium iron phosphate active material according to claim 6, characterized in that: The lithium source is at least one of lithium oxide, lithium hydroxide, lithium carbonate and lithium phosphate; Preferably, the carbon source A is at least one of a small molecule organic substance and a polymer; preferably at least one of starch, glucose, PEG, and cyclodextrin; Preferably, the carbon source A is 5wt% to 30wt% of the weight of the ferrous phosphate precursor, preferably 10 to 20wt%; Preferably, the ferrous phosphate precursor, the lithium source, and the carbon source A are liquid-phase ball-milled and sand-milled to obtain the slurry; Preferably, the ferrous phosphate precursor and the lithium source and the carbon source A are solid-phase mixed and solid-phase ball-milled to obtain the solid-phase mixture; or the slurry is spray-dried to obtain the solid-phase mixture; Preferably, the heat treatment is carried out under a protective atmosphere; Preferably, the temperature of the heat treatment is 650-800°C; Preferably, the heat treatment time is 4 to 24 hours; Preferably, the inlet temperature of the spray drying stage is 200-300°C, and the outlet temperature is 70-120°C.

8. A lithium iron phosphate active material obtained by the preparation method according to claim 6 or 7.

9. A positive electrode of a lithium secondary battery, comprising a current collector and a positive electrode material composited on the current collector, characterized in that: The active material contained in the positive electrode material contains the lithium iron phosphate active material according to claim 8; Preferably, the positive electrode material further comprises a binder and a conductive agent; Preferably, in the positive electrode material, the content of the binder and the conductive agent is 1-10wt%, and the balance is active material.

10. A lithium secondary battery, characterized in that: Comprising the positive electrode as claimed in claim 9.

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