Preparation method and application of lithium iron phosphate precursor

Abstract

The application relates to the field of lithium ion battery positive electrode materials, in particular to a preparation method and application of a lithium iron phosphate precursor. The method comprises the following steps: mixing an iron source, a phosphorus source and an oxidizing agent and then performing a precipitation reaction; performing first washing on a precipitation slurry; mixing first clear liquid and second clear liquid; adding the oxidizing agent and the iron source into the mixed clear liquid; mixing the precipitation slurry and slurry A and then performing second washing; performing a first aging reaction on slurry C, and adding phosphoric acid into the slurry C during the first aging reaction; performing a second aging reaction on slurry D, and adding phosphoric acid into the slurry D during the second aging reaction; mixing slurry E and slurry F, and then performing heat preservation and second washing. The prepared lithium iron phosphate precursor can improve the compaction density and capacity of lithium iron phosphate, and has the advantages of environmental friendliness and low cost.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of lithium ion battery cathode material, in particular to a preparation method and application of lithium iron phosphate precursor. BACKGROUND

[0002] Lithium iron phosphate is considered as one of the competitive lithium ion battery electrode materials due to its excellent electrochemical performance, good safety and low price. However, lithium iron phosphate battery also has inevitable shortcomings, such as low energy density. The problem of low energy density can be improved by increasing the gram capacity and the compaction density; at present, the gram capacity of lithium iron phosphate material has exceeded 160 mAh / g, close to its theoretical capacity of 170 mAh / g, and the space for improvement is small; the compaction density of lithium iron phosphate cathode material is improved, and at present, most of the researches focus on only using physical and mechanical method to mix solid lithium iron phosphate in the preparation process of lithium iron phosphate, but the compaction effect is not obvious in general, which can lead to uneven mixing, small particles cannot be effectively filled, and the compaction density of the product can be greatly different in different batches.

[0003] Therefore, the present application is proposed. SUMMARY

[0004] The present application aims to provide a preparation method and application of lithium iron phosphate precursor, the preparation method of lithium iron phosphate precursor, the prepared lithium iron phosphate has moderate particle size, and the lithium iron phosphate with mixed particle size can be directly prepared, the compaction density of lithium iron phosphate is high, and the capacity is high.

[0005] In order to achieve the above-mentioned purpose of the present application, the following technical scheme is adopted:

[0006] In one aspect of the present application, a preparation method of lithium iron phosphate precursor is provided, which comprises the following steps:

[0007] (a) mixing iron source, phosphorus source and oxidizing agent, and then performing precipitation reaction to obtain a precipitation slurry; the precipitation slurry is subjected to first washing to obtain a first clear liquid;

[0008] (b) mixing the first clear liquid and the second clear liquid to obtain a mixed clear liquid; adding the oxidizing agent and the iron source into the mixed clear liquid to obtain slurry A; mixing the precipitation slurry and the slurry A, and then performing second washing to obtain a one-washing filter cake;

[0009] (c) respectively adding water into the one-washing filter cake to prepare slurry C with a concentration of 15wt%-20wt% and slurry D with a concentration of 5wt%-10wt%;

[0010] (d) the slurry C is subjected to a first aging reaction, and phosphoric acid is added to the slurry C during the first aging reaction to obtain a slurry E; the slurry D is subjected to a second aging reaction, and phosphoric acid is added to the slurry D during the second aging reaction to obtain a slurry F;

[0011] The content of phosphoric acid in the slurry E is 3wt%-5wt%.

[0012] The content of phosphoric acid in the slurry F is 0.1wt%-0.3wt%.

[0013] (e) the slurry E and the slurry F are mixed, and then subjected to heat preservation and a second washing to obtain the second clear liquid and a secondary filter cake; the secondary filter cake is calcined to obtain the lithium iron phosphate precursor.

[0014] The preparation method of the lithium iron phosphate precursor and the lithium iron phosphate precursor obtained by the method are used for the preparation of lithium iron phosphate, which can improve the compaction density and capacity of the lithium iron phosphate and has the advantages of environmental friendliness and low cost.

[0015] Another aspect of the present application also relates to a lithium iron phosphate positive material mainly made of the lithium iron phosphate precursor obtained by the preparation method.

[0016] Another aspect of the present application also relates to a lithium ion battery comprising the lithium iron phosphate positive material.

[0017] Compared with the prior art, the present application has the following beneficial effects:

[0018] (1) The preparation method of the lithium iron phosphate precursor provided by the present application, the precipitating section has low phosphorus content in the washing water and the filtrate, and the mixing of the normal precipitating slurry can produce phosphorus iron particles with different sizes; the aging section controls the different amounts of phosphoric acid, so that the supersaturation of amorphous phosphorus iron in the crystal type conversion process is different, further increasing the size difference of the particles, and the preparation of lithium iron phosphate can improve the compaction density and capacity of the lithium iron phosphate.

[0019] (2) The preparation method of the lithium iron phosphate precursor provided by the present application can reuse the washing water and the filtrate generated in the precipitating and aging sections, thereby reducing the preparation cost of the lithium iron phosphate precursor and being environmentally friendly.

[0020] (3) The lithium iron phosphate positive material provided by the present application has reasonable combination of lithium iron phosphate with different particle sizes, and has the advantages of high compaction density and high capacity. BRIEF DESCRIPTION OF DRAWINGS

[0021] In order to more clearly illustrate the technical solutions in the specific embodiments or prior art of the present application, the following will briefly introduce the drawings needed to be used in the description of the specific embodiments or prior art. Obviously, the drawings described below are some embodiments of the present application, and other drawings can also be obtained by those skilled in the art without any creative effort based on these drawings.

[0022] Figure 1 The preparation method flow chart of the lithium iron phosphate precursor provided for the embodiments of the present application is shown in the following figure:

[0023] Figure 2 The scanning electron microscope images of the iron phosphate and lithium iron phosphate in the precipitation section and the aging section of Example 1 are shown in the following figure:

[0024] Figure 3 The scanning electron microscope images of the iron phosphate and lithium iron phosphate in the precipitation section and the aging section of Comparative Example 1 are shown in the following figure. DETAILED DESCRIPTION

[0025] The technical solutions of the present application will be described clearly and completely in the following with reference to the drawings and specific embodiments, but those skilled in the art will understand that the following described embodiments are part of the embodiments of the present application, not all the embodiments, and should not be regarded as limiting the scope of the present application. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without any creative effort are within the scope of protection of the present application. The specific conditions not mentioned in the embodiments are carried out according to the conventional conditions or the conditions recommended by the manufacturer. The reagents or instruments not mentioned by the manufacturer are all conventional products that can be purchased in the market.

[0026] In one aspect of the present application, a preparation method of a lithium iron phosphate precursor is provided, which comprises the following steps:

[0027] (a) mixing an iron source, a phosphorus source and an oxidizing agent to carry out a precipitation reaction to obtain a precipitation slurry; the precipitation slurry is subjected to a first washing to obtain a first clear liquid;

[0028] (b) mixing the first clear liquid and a second clear liquid to obtain a mixed clear liquid; adding the oxidizing agent and the iron source to the mixed clear liquid to obtain a slurry A; mixing the precipitation slurry and the slurry A and then carrying out a second washing to obtain a one-washing filter cake;

[0029] (c) adding water to the one-washing filter cake to prepare a slurry C with a concentration of 15wt%-20wt% (for example, 15wt%, 16wt%, 17wt%, 18wt%, 19wt% or 20wt%) and a slurry D with a concentration of 5wt%-10wt% (for example, 5wt%, 6wt%, 7wt%, 8wt%, 9wt% or 10wt%) respectively;

[0030] (d) the slurry C is subjected to a first aging reaction, and phosphoric acid is added to the slurry C during the first aging reaction to obtain a slurry E; the slurry D is subjected to a second aging reaction, and phosphoric acid is added to the slurry D during the second aging reaction to obtain a slurry F;

[0031] The content of phosphoric acid in the slurry E is 3wt% to 5wt% (for example, 3wt%, 4wt% or 5wt%).

[0032] The content of phosphoric acid in the slurry F is 0.1wt% to 0.3wt% (for example, 0.1wt%, 0.2wt% or 0.3wt%).

[0033] (e) the slurry E and the slurry F are mixed, and then subjected to heat preservation and a second washing to obtain the second clear liquid and a secondary filter cake; the secondary filter cake is calcined to obtain the lithium iron phosphate precursor.

[0034] The preparation method of the lithium iron phosphate precursor mixes the washing water and the filtrate obtained in the precipitation section with the precipitation slurry to perform primary pressure filtration washing, and the content of phosphorus in the filtrate and the washing water is low. In the case of low solution supersaturation, the growth driving force of the crystal nucleus after nucleation is small, and the growth of the crystal is mainly affected by thermodynamics. Therefore, the same temperature is used for the reaction to perform the precipitation reaction, and the particles are small. After being mixed with the normal precipitation slurry, the amorphous iron phosphate with different particle sizes is obtained.

[0035] Since the purpose of the aging section is to convert the crystal form, the amorphous iron phosphate will recombine to form iron phosphate with an orthorhombic system in this process. Therefore, the size advantage of the amorphous iron phosphate will be weakened in the aging section. Therefore, the amount of phosphoric acid added in the aging section is changed, the supersaturation degree during the crystal form conversion is different, the nucleation and growth speed of the crystal are different, and the slurry concentration is changed to improve the difference, so that the particle size difference of the mixed sample after the crystal form conversion is more obvious. The particle size of the small particles is 50 to 100 nm, and the particle size of the large particles is 200 to 300 nm.

[0036] The size of the lithium iron phosphate material depends largely on the size of the iron phosphate. The size of the iron phosphate prepared by the preparation method of the lithium iron phosphate precursor is moderate, and can be directly used to prepare lithium iron phosphate. The lithium iron phosphate with mixed particle sizes can be prepared, and has high compaction density and high capacity.

[0037] Further, the content of phosphorus in the mixed clear liquid is 0.1wt% to 0.5wt% (for example, 0.1wt%, 0.2wt%, 0.3wt%, 0.4wt% or 0.5wt%).

[0038] Further, the one-washed filter cake comprises: first particles having a particle size of 200-500 nm (e.g., 200 nm, 250 nm, 300 nm, 350 nm, 400 nm, 450 nm, or 500 nm) and second particles having a particle size of 10-50 nm (e.g., 10 nm, 20 nm, 30 nm, 40 nm, or 50 nm).

[0039] Further, in step (a), the iron source, the phosphorus source, and the hydrogen peroxide are mixed in a molar ratio of (1-1.3):1:(0.5-1) (e.g., 1:1:1, 1.1:1:0.8, 1.2:1:0.6, or 1.3:1:0.5).

[0040] Further, in step (a), the iron source is added in the form of an aqueous iron source solution.

[0041] Further, the aqueous iron source solution has a concentration of 40-90 g / L (e.g., 40 g / L, 50 g / L, 60 g / L, 70 g / L, 80 g / L, or 90 g / L).

[0042] Further, the iron source comprises at least one of ferrous sulfate, ferrous oxalate, or ferrous chloride.

[0043] Further, in step (a), the phosphorus source is added in the form of an aqueous phosphorus source solution.

[0044] Further, the aqueous phosphorus source solution has a concentration of 2 wt%-5 wt% (e.g., 2 wt%, 3 wt%, 4 wt%, or 5 wt%).

[0045] Further, the phosphorus source comprises at least one of phosphoric acid, ammonium dihydrogen phosphate, or di-ammonium hydrogen phosphate.

[0046] Further, the oxidizing agent comprises hydrogen peroxide.

[0047] Further, in step (b), the iron source, the phosphorus in the mixed supernatant, and the oxidizing agent are added in a molar ratio of (1-1.3):1:(0.5-1) (e.g., 1:1:0.5, 1.1:1:0.7, 1.2:1:0.8, or 1.3:1:1).

[0048] Further, in step (b), the precipitate slurry and the slurry A are mixed in a volume ratio of 100:(5-15) (e.g., 100:5, 100:8, 100:10, 100:13, or 100:15).

[0049] Further, in step (e), the slurry E and the slurry F are mixed in a volume ratio of (0.1-0.3):1 (e.g., 0.1:1, 0.2:1, or 0.3:1).

[0050] Further, the solid content of the precipitated slurry is 7wt% to 10wt% (for example, 7wt%, 8wt%, 9wt% or 10wt%).

[0051] Further, the solid content of the slurry A is 7wt% to 10wt% (for example, 7wt%, 8wt%, 9wt% or 10wt%).

[0052] Further, the temperature of the first aging reaction is 30 to 70℃ (for example, 30℃, 35℃, 40℃, 45℃, 50℃, 55℃, 60℃, 65℃ or 70℃).

[0053] Further, the time of the first aging reaction is 30 to 120min (for example, 30min, 40min, 50min, 60min, 70min, 80min, 90min, 100min, 110min or 120min).

[0054] Further, the temperature of the second aging reaction is 30 to 70℃ (for example, 30℃, 35℃, 40℃, 45℃, 50℃, 55℃, 60℃, 65℃ or 70℃).

[0055] Further, the time of the second aging reaction is 30 to 120min (for example, 30min, 40min, 50min, 60min, 70min, 80min, 90min, 100min, 110min or 120min).

[0056] Further, the temperature of the holding is 85 to 95℃ (for example, 85℃, 87℃, 89℃, 91℃, 93℃ or 95℃).

[0057] Further, the time of the holding is 0.5 to 4h (for example, 0.5h, 1h, 1.5h, 2h, 2.5h, 3h, 3.5h or 4h).

[0058] Another aspect of the present application also relates to a lithium iron phosphate cathode material, which is mainly made of the lithium iron phosphate precursor prepared by the preparation method of the lithium iron phosphate precursor.

[0059] Further, the preparation method of the lithium iron phosphate cathode material comprises the following steps:

[0060] The prepared lithium iron phosphate is mixed with a lithium source, a carbon source and an additive, and then subjected to sand milling, spray drying and sintering.

[0061] Further, the lithium source is at least one of lithium carbonate, lithium dihydrogen phosphate or lithium hydroxide.

[0062] Further, the carbon source is at least one of glucose, white sugar or PEG6000.

[0063] Further, the additive is at least one of titanium dioxide or vanadium pentoxide.

[0064] Further, the molar ratio of Li in the lithium source and P in the phosphorus source is 0.99-1.01, and the addition amount of the carbon source and the additive is 10wt%-20wt% and 0.05wt%-0.3wt% of the iron phosphate.

[0065] Further, the particle size after sand milling is 0.35-0.5μm.

[0066] Further, the sintering temperature is 700-800℃, and the sintering time is 4-12h.

[0067] Further, the compaction density of the lithium iron phosphate positive electrode material is 2.45-2.55g / cm 3 .

[0068] Another aspect of the present application also relates to a lithium ion battery comprising the lithium iron phosphate positive electrode material.

[0069] The embodiments of the present application will be described in detail below with specific examples and comparative examples.

[0070] Example 1

[0071] The preparation method of the lithium iron phosphate precursor provided in the present embodiment comprises the following steps as shown in the formula: Figure 1

[0072] (1) 500mL of ferrous sulfate, phosphoric acid and hydrogen peroxide are mixed to perform a precipitation reaction to obtain a precipitation slurry, the molar ratio of ferrous sulfate, phosphoric acid and hydrogen peroxide is 1:1:0.75; the content of ferrous iron in ferrous sulfate is 60g / L, the content of phosphorus in phosphoric acid is 3%, and the solid content of the precipitation slurry is 8wt%;

[0073] (2) The precipitation slurry is subjected to pressure filtration and washing to obtain a clear liquid 1;

[0074] (3) The clear liquid 1 and the clear liquid 2 are mixed to obtain a mixed clear liquid with a phosphorus content of 0.2wt%, and ferrous sulfate and hydrogen peroxide are added to the mixed clear liquid, the phosphorus, ferrous sulfate and hydrogen peroxide in the mixed clear liquid are added according to the molar ratio of 1:1:0.75 to obtain a slurry A; the solid content of the slurry A is 8%;

[0075] (4) The precipitation slurry and the slurry A are mixed in a volume ratio of 100:8 to obtain a mixed slurry B;

[0076] ​(5) the mixed slurry B is washed by pressure filtration to obtain a first washing filter cake;

[0077] (6) the first washing filter cake is respectively added with water to prepare a slurry C with a concentration of 18wt% and a slurry D with a concentration of 8wt%;

[0078] (7) 4wt% of phosphoric acid with a concentration of 85% is added to the slurry C at 60°C to perform a first aging reaction, the first aging reaction time is 2h, to obtain a slurry E; 0.2wt% of phosphoric acid with a concentration of 85% is added to the slurry D to perform a second aging reaction, the second aging reaction time is 30min, to obtain a slurry F;

[0079] (8) after the slurry E and the slurry F are both whitened, the slurry E is mixed with the slurry F at a volume ratio of 0.2:1, and after being kept at 90°C for 1h, the slurry is washed by pressure filtration to obtain a clear liquid 2, and the filter cake is calcined to obtain anhydrous iron phosphate.

[0080] Example 2

[0081] The preparation method of the lithium iron phosphate precursor provided in the embodiment, as shown in Figure 1 , comprises the following steps:

[0082] (1) 500mL of ferrous sulfate, phosphoric acid and hydrogen peroxide are mixed to perform a precipitation reaction to obtain a precipitation slurry, the molar ratio of the ferrous sulfate, the phosphoric acid and the hydrogen peroxide is 1.3:1:1; the content of ferrous iron in the ferrous sulfate is 60g / L, the content of phosphorus in the phosphoric acid is 3%, and the solid content of the precipitation slurry is 10wt%;

[0083] (2) the precipitation slurry is washed by pressure filtration to obtain a clear liquid 1;

[0084] (3) the clear liquid 1 and the clear liquid 2 are mixed to obtain a mixed clear liquid with a content of phosphorus of 0.5wt%, and ferrous sulfate and hydrogen peroxide are added to the mixed clear liquid, the phosphorus, the ferrous sulfate and the hydrogen peroxide in the mixed clear liquid are added according to a molar ratio of 1:1.1:0.75 to obtain a slurry A; the solid content of the slurry A is 7%;

[0085] (4) the precipitation slurry and the slurry A are mixed at a volume ratio of 100:1 to obtain a mixed slurry B;

[0086] (5) the mixed slurry B is washed by pressure filtration to obtain a first washing filter cake;

[0087] (6) the first washing filter cake is respectively added with water to prepare a slurry C with a concentration of 20wt% and a slurry D with a concentration of 10wt%;

[0088] (7) 65℃, 5wt% phosphoric acid with 85% concentration was added to slurry C to carry out first aging reaction, the first aging reaction time was 1.5h, slurry E was obtained; 0.1wt% phosphoric acid with 85% concentration was added to slurry D to carry out second aging reaction, the second aging reaction time was 1h, slurry F was obtained;

[0089] (8) After slurry E and slurry F were both whitened, slurry E and slurry F were mixed according to the volume ratio of 0.1:1, 85℃, 4h, then through pressure filtration washing, clear liquid 2 was obtained, the filter cake was calcined to obtain anhydrous iron phosphate.

[0090] Example 3

[0091] The preparation method of the lithium iron phosphate precursor provided in the embodiment, as shown in the following scheme, comprises the following steps: Figure 1

[0092] (1) 500mL of ferrous sulfate, phosphoric acid and hydrogen peroxide were mixed to carry out precipitation reaction, and slurry was obtained, the molar ratio of ferrous sulfate, phosphoric acid and hydrogen peroxide was 1:1:0.5; the content of ferrous iron in ferrous sulfate was 60g / L, the content of phosphorus in phosphoric acid was 3%, and the solid content of the slurry was 7wt%;

[0093] (2) The slurry was washed by pressure filtration to obtain clear liquid 1;

[0094] (3) The clear liquid 1 and the clear liquid 2 were mixed to obtain a mixed clear liquid, the content of phosphorus in the mixed clear liquid was 0.1wt%, ferrous sulfate and hydrogen peroxide were added to the mixed clear liquid, the molar ratio of phosphorus, ferrous sulfate and hydrogen peroxide in the mixed clear liquid was 1:1.2:0.75, and slurry A was obtained; the solid content of slurry A was 10%;

[0095] (4) The slurry was mixed with slurry A according to the volume ratio of 100:15 to obtain mixed slurry B;

[0096] (5) The mixed slurry B was washed by pressure filtration to obtain a washing filter cake;

[0097] (6) The washing filter cake was respectively prepared into slurry C with a concentration of 15wt% and slurry D with a concentration of 5wt%;

[0098] (7) 55℃, 3wt% phosphoric acid with 85% concentration was added to slurry C to carry out first aging reaction, the first aging reaction time was 1h, slurry E was obtained; 0.3wt% phosphoric acid with 85% concentration was added to slurry D to carry out second aging, the second aging reaction time was 1h, slurry F was obtained;

[0099] ​(8) After both slurry E and slurry F become white, mix slurry E and slurry F at a volume ratio of 0.3:1, and after 0.5 h of incubation at 95℃, wash by pressure filtration to obtain clear liquid 2, and calcine the filter cake to obtain anhydrous iron phosphate.

[0100] Comparative Example 1

[0101] This comparative example differs from Example 1 in that the filtrate is not reused to wash the water, and slurry E and slurry F are not mixed, and specifically includes the following steps:

[0102] (1) After mixing 500 mL of ferrous sulfate, phosphoric acid, and hydrogen peroxide, perform a precipitation reaction to obtain a precipitate slurry, with a molar ratio of ferrous sulfate:phosphoric acid:hydrogen peroxide of 1:1:0.75; the ferrous content in the ferrous sulfate is 60 g / L, the phosphorus content in the phosphoric acid is 3%, and the solid content of the precipitate slurry is 8 wt%;

[0103] (2) Wash the precipitate slurry by pressure filtration to obtain a first washing filter cake;

[0104] (3) Prepare slurry C with a concentration of 18 wt% and slurry D with a concentration of 8 wt% by adding water to the first washing filter cake, respectively;

[0105] (4) Perform a first aging reaction by adding 4 wt% of phosphoric acid with a concentration of 85% to slurry C at 60℃, with a first aging reaction time of 2 h, to obtain slurry E; perform a second aging reaction by adding 0.2 wt% of phosphoric acid with a concentration of 85% to slurry D, with a second aging reaction time of 30 min, to obtain slurry F;

[0106] (5) After both slurry E and slurry F become white, perform an incubation reaction, respectively, after 1 h of incubation at 90℃, wash by pressure filtration, and mix the filter cakes after calcination to obtain anhydrous iron phosphate.

[0107] Comparative Example 2

[0108] This comparative example differs from Example 1 only in step (7), where 4 wt% of phosphoric acid with a concentration of 85% is added to slurry D to perform a second aging reaction.

[0109] Comparative Example 3

[0110] The preparation method of the lithium iron phosphate precursor provided in this comparative example includes the following steps:

[0111] (1) Same as step (1) of Example 1;

[0112] (2) Wash the precipitate slurry by pressure filtration to obtain a first washing filter cake;

[0113] (3) Same as step (6) of Example 1;

[0114] (4) Same as step (7) of Example 1;

[0115] (5) Same as step (8) of Example 1.

[0116] Comparative Example 4

[0117] The preparation method of the lithium iron phosphate precursor provided by the present comparative example comprises the following steps:

[0118] (1)-(5) Same as Example 1;

[0119] (6) The filter cake was added with water to prepare a slurry C with a concentration of 18 wt%;

[0120] (7) 4 wt% of phosphoric acid with a concentration of 85% was added to the slurry C at 60°C to perform a first aging reaction, and the first aging reaction time was 30 min, to obtain a slurry E;

[0121] (8) After the slurry E turned white, it was kept at 90°C for 1 h, and then washed by pressure filtration to obtain a clear liquid 2, and the filter cake was calcined to obtain anhydrous iron phosphate.

[0122] Experimental Example

[0123] The lithium iron phosphate precursor prepared in each example and comparative example was prepared into lithium iron phosphate by the following method: phosphorus in the prepared iron phosphate and lithium in lithium carbonate were added in a molar ratio of 1:1.005, 7 wt% of white sugar and 3 wt% of PEG6000 were added, and sand milling was performed to 0.42 μm, and sintering was performed at 770°C for 9 h to prepare. The tap density of the lithium iron phosphate was detected, and the FT-100F powder automatic tap density instrument was used for testing, and the results are shown in Table 1.

[0124] Table 1

[0125] Compacted density Example 1 2.55 g / cm 3 ]] Example 2 2.51 g / cm 3 ]] Example 3 2.46 g / cm 3 ]] Comparative Example 1 2.15 g / cm 3 ]] Comparative Example 2 2.21 g / cm 3 ]]> Comparative Example 3 2.28 g / cm 3 ]]> Comparative Example 4 2.08 g / cm 3 ]]

[0126] Figure 2 With Figure 3 It can be seen from the comparison that, as for the precipitation section, in Comparative Example 1, the filter liquid and washing water are not recycled in the precipitation section, and the particle size is relatively uniform, but in Example 1, there are large and small particles, that is, the amorphous iron phosphate with different particle sizes is obtained;

[0127] As for the aging section, in Comparative Example 1, the amount of phosphoric acid added in the slurry E is large, the slurry concentration is high, the pH is low, the crystal type conversion is accelerated, the supersaturation is large, and the crystal nucleation is prior to the crystal growth, so that the particle size is small; the amount of phosphoric acid added in the slurry F is small, the slurry concentration is low, and the particle size is larger; and in Example 1, there are still large and small particles after aging, that is, this process is easy to form iron phosphate with large and small particles;

[0128] Using the above iron phosphate, lithium iron phosphate was prepared by the same process, and in Comparative Example 1, the particle size was relatively uniform, and the powder compaction was small, which was 2.15 g / cm 3whereas in Example 1 the size of the particles is clearly visible and the powder compaction is high, 2.55 g / cm 3 .

[0129] Although the present application has been illustrated and described with reference to specific embodiments, it should be recognized that the above examples are merely illustrative of the present application and are not intended to limit the scope of the present application. It will be apparent to those of ordinary skill in the art that modifications, including significant modifications, can be made to the techniques described herein and some other steps described herein can be used instead of others and certain features described herein can be used instead of or in combination with other features described herein without departing from the spirit and scope of the present application. Accordingly, the disclosure is intended to embrace all such alterations, modifications and variations which fall within the scope of the present application.

Claims

1. A method for preparing a lithium iron phosphate precursor, characterized in that: The following steps are involved: (a) mixing an iron source, a phosphorus source, and an oxidant and subjecting the mixture to a precipitation reaction to obtain a precipitated slurry; and subjecting the precipitated slurry to a first washing to obtain a first clear liquid; (b) mixing the first clear liquid and the second clear liquid to obtain a mixed clear liquid; adding the oxidant and the iron source to the mixed clear liquid to obtain slurry A; The precipitation slurry and the slurry A are mixed and then subjected to a second washing to obtain a washed filter cake; (c) adding water to the washed filter cake to prepare slurry C with a concentration of 15 wt % to 20 wt % and slurry D with a concentration of 5 wt % to 10 wt %; (d) the slurry C is subjected to a first aging reaction, during which phosphoric acid is added to the slurry C to obtain slurry E; the slurry D is subjected to a second aging reaction, during which phosphoric acid is added to the slurry D to obtain slurry F; The content of phosphoric acid in the slurry E is 3 wt% to 5 wt%; The content of phosphoric acid in the slurry F is 0.1 wt% to 0.3 wt%; (e) mixing the slurry E and the slurry F, performing heat preservation and a second washing, to obtain a second clear liquid and a secondary filter cake; The secondary filter cake is calcined to obtain the lithium iron phosphate precursor.

2. The method for preparing a lithium iron phosphate precursor according to claim 1, wherein: The content of phosphorus in the mixed clear liquid is 0.1 wt % to 0.5 wt %.

3. The method for preparing a lithium iron phosphate precursor according to claim 1, wherein: The first-wash filter cake includes: first particles with a particle size of 200-500 nm and second particles with a particle size of 10-50 nm.

4. The method for preparing a lithium iron phosphate precursor according to claim 1, wherein: In step (a), the iron source, the phosphorus source and the oxidant are mixed in a molar ratio of (1-1.3):1:(0.5-1).

5. The method for preparing a lithium iron phosphate precursor according to claim 1, wherein: In step (b), the iron source, the phosphorus in the mixed clear solution, and the oxidant are added in a molar ratio of (1-1.3):1:(0.5-1).

6. The method for preparing a lithium iron phosphate precursor according to claim 1, wherein: In step (b), the precipitation slurry and the slurry A are mixed in a volume ratio of 100:(5-15).

7. The method for preparing a lithium iron phosphate precursor according to claim 1, wherein: In step (e), the mixing volume ratio of the slurry E and the slurry F is (0.1-0.3):

1.

8. The method for preparing a lithium iron phosphate precursor according to claim 1, wherein: The solid content of the precipitation slurry is 7 wt % to 10 wt %.

9. The method for preparing a lithium iron phosphate precursor according to claim 1, wherein: The solid content of the slurry A is 7 wt % to 10 wt %.

10. The method for preparing a lithium iron phosphate precursor according to claim 1, wherein: The temperature of the first aging reaction is 30-70°C.

11. The method for preparing a lithium iron phosphate precursor according to claim 1, wherein: The first aging reaction time is 30 to 120 minutes.

12. The method for preparing a lithium iron phosphate precursor according to claim 1, wherein: The temperature of the second aging reaction is 30-70°C.

13. The method for preparing a lithium iron phosphate precursor according to claim 1, wherein: The second aging reaction time is 30 to 120 minutes.

14. The method for preparing a lithium iron phosphate precursor according to claim 1, wherein: The insulation temperature is 85-95°C.

15. The method for preparing a lithium iron phosphate precursor according to claim 1, wherein: The insulation time is 0.5 to 4 hours.

16. A lithium iron phosphate positive electrode material, characterized in that: The lithium iron phosphate precursor is mainly prepared by the preparation method of the lithium iron phosphate precursor according to any one of claims 1 to 15.

17. A lithium ion battery, characterized in that: Including the lithium iron phosphate positive electrode material according to claim 16.

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