Preparation method of battery-grade ferrous oxalate

Abstract

The invention provides a preparation method of battery-grade ferrous oxalate. The preparation method comprises the following steps: respectively preparing an iron source and oxalic acid into an iron source solution and an oxalic acid solution according to a certain proportion; carrying out high-energy ball milling on ferrous oxalate to obtain an active ferrous oxalate seed crystal; reacting the iron source solution with an oxalic acid solution to obtain ferrous oxalate slurry; and drying, calcining and crushing the ferrous oxalate to obtain the battery-grade ferrous oxalate. According to the method, seed crystals are activated through ball milling and surface functionalization in the preparation process of ferrous oxalate, and the porous iron phosphate with the three-dimensional nanoflower structure is obtained through induction of the active seed crystals and a surfactant. The ferrous oxalate material prepared by the method has a regular three-dimensional net nanocrystallization structure, high specific surface area and high porosity. The lithium iron phosphate material prepared from the precursor material is relatively high in compaction and can be adapted to a high-compaction lithium iron phosphate process.

Description

Technical Field

[0001] This technical solution mainly relates to the technical field of preparing precursors for lithium-ion battery cathode materials, and specifically relates to a method for preparing battery-grade ferrous oxalate. Background Art

[0002] As one of the current mainstream secondary batteries, lithium-ion batteries are widely used in fields such as portable electronic devices and electric vehicles. Lithium iron phosphate cathode materials are considered to be one of the most promising lithium-ion battery cathode materials due to their advantages such as rich resources, environmental friendliness, and good thermal stability. Ferrous oxalate is an excellent precursor for preparing lithium iron phosphate, and it shows obvious advantages in physical and chemical properties such as purity, morphology, and particle size, making it have a higher tap density and better rate performance compared to lithium iron phosphate prepared by the iron phosphate method. However, existing ferrous oxalate materials still have problems such as large particles, irregular morphology, and poor consistency, corresponding to problems of unstable tap density and electrochemical performance in lithium iron phosphate materials. Relevant research shows that the performance of lithium iron phosphate materials is directly related to the physical and chemical properties of ferrous oxalate precursors. Therefore, it is necessary to effectively control the particle size, morphology, and purity of ferrous oxalate precursors. In the existing technology, the preparation of battery-grade ferrous oxalate mainly uses the liquid-phase precipitation method, which has problems such as complex process, high cost, low product purity, uneven morphology, and wide particle size distribution. Although some technologies can control the morphology and structure of ferrous oxalate, the reaction time is long, the reaction temperature is high, and the preparation method is too complex. Therefore, there is an urgent need to develop a preparation method with simple process, low cost, high purity of obtained ferrous oxalate, uniform morphology, and narrow particle size distribution to meet the requirements of lithium-ion battery cathode materials. Summary of the Invention

[0003] In order to achieve the above object, the present invention provides a method for preparing battery-grade ferrous oxalate. In this method, ferrous oxalate seeds with a narrow particle size distribution range are surface-functionalized to exhibit special reaction activity. The synthesized ferrous oxalate has a three-dimensional nanoflower structure, and the synthesis method is simple and efficient, and can be scaled up industrially. The preparation method includes the following steps: S1. Raw material preparation: Prepare solution A and solution B by separately dissolving an iron source and oxalic acid. S2. Seed preparation: Weigh ferrous oxalate seeds and place them in a high-energy ball milling tank for dry milling. Then add an activator and stir well at 80 - 90 °C. Then filter to obtain active seeds, and the filtrate can be reused for seed activation.

[0004] S3. Precipitation reaction: Add the active seeds obtained in S2 to solution B and mix evenly. Weigh a surfactant according to the proportion of 0.01 - 0.02 of the mass of oxalic acid and add it to solution A and mix evenly. Then slowly add solution B to solution A and stir at high speed to obtain a ferrous oxalate slurry; S4. Dehydration reaction: The obtained ferrous oxalate in S3 is washed, dried, pulverized and calcined to obtain a battery-grade ferrous oxalate material; In step S1, the iron source is one of ferrous sulfate, ferrous phosphate and ferrous nitrate.

[0005] The mass ratio of iron to oxalic acid in the iron source is 1 - 1.5:1.

[0006] The mass ratio of the ferrous oxalate crystal seed to oxalic acid is 0.03 - 0.05:1.

[0007] In step S2, the rotation speed of the ball mill tank is 1300 - 2100 r / min, the ball milling time is 1 - 2 h, and the D50 of the ferrous oxalate fine powder is 5 - 25 μm.

[0008] Under the ball milling conditions at the above-mentioned ball milling speed, it can ensure that the ferrous oxalate crystal seed is in the best active state. If the ball milling is too fast, such as when ball milling at 2200 r / min, the crystal form and structure will change abnormally. If the ball milling speed is too slow, such as when ball milling at 500 r / min, the activation effect is not obvious.

[0009] In step S2, the activator is one of sulfuric acid, phosphoric acid and nitric acid. This activator helps the surface functionalization of ferrous oxalate and is beneficial to growing ferrous oxalate with a specific morphology.

[0010] The content of sulfuric acid, phosphoric acid and nitric acid is 40 - 50 wt%, and the mass ratio of the acid to the ferrous oxalate crystal seed is 1:0.1 - 0.5.

[0011] The activation step is to first perform dry milling treatment, and then perform high-temperature activation treatment with the activator. This step can effectively ensure that the crystal seed does not decompose during the activation process.

[0012] Under this reaction temperature condition, the reaction system has a relatively high surface potential, and a small amount of acidolysis will occur on the surface of the ferrous oxalate crystal seed, resulting in the exposure of a small amount of Fe 2+ , taking sulfuric acid ball milling as an example, the free SO4 2- and Fe 2+ undergo electrostatic attraction, resulting in a non-uniform charge distribution on the surface of the ferrous oxalate crystal seed, and a nano-flower-like structure is obtained by oriented growth during the subsequent synthesis of ferrous oxalate.

[0013] In step S3, the addition rate of solution B is 50 - 100 mL / min.

[0014] In step S3, the stirring speed is 40 - 50 Hz.

[0015] In step S4, the washing conductivity is 0.3 - 0.4 μS / cm.

[0016] In step S4, the drying temperature is 100-130 °C and the drying time is 1-2 h.

[0017] In step S4, the sintering temperature is 300-400 °C and the sintering time is 2-3 h.

[0018] By using this method to prepare battery-grade ferrous oxalate, the present invention has the following beneficial effects: (1) In the present invention, active ferrous oxalate seeds are obtained through high-energy ball milling and high-temperature activation. The particle size distribution of these seeds is relatively narrow, the particle sizes are uniform, the average particle size D50 ≤ 8 μm, the maximum particle size D90 ≤ 12 μm, and the surface contains specific active functional groups, which can induce the directional growth of ferrous oxalate, thus forming a flower-like nanostructure. The material synthesized by this method has a three-dimensional nanostructured shape, a high specific surface area (BET is 40-50 m 2 / g), and a relatively narrow particle size range.

[0019] (2) By using a surfactant to induce the growth of specific crystal planes, the effect of poisoning crystal planes is achieved, and at the same time, it promotes the uniformity of the material particle size; (3) The raw materials of this reaction are cheap and easily available, the reaction process is stable and controllable. The tap density of the lithium iron phosphate prepared using this iron phosphate material is relatively high (2.56-2.60), and it can be used to prepare high-tap-density lithium iron phosphate, having good practical and economic values. Description of the Drawings

[0020] Figure 1 SEM images of the iron phosphate prepared in Example 1 (a) and Comparative Example 1 (b) in the present invention.

[0021] Figure 2 XRD patterns of the ferrous oxalate prepared in Example 1 and Comparative Example 1 in the present invention.

[0022] Figure 3 Process flow diagram of the present invention. Detailed Embodiments

[0023] In the ranges disclosed in the present invention, the endpoints and any values are not limited to the exact ranges or values. These ranges or values should be understood to include values close to these ranges or values. For numerical ranges, between the endpoint values of each range, between the endpoint values of each range and individual point values, and between individual point values, they can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed in the present invention.

[0024] A method for preparing battery-grade ferrous oxalate provided by the present invention includes the following steps: S1. Raw material preparation: Prepare solution A and solution B by respectively formulating an iron source and oxalic acid in a ratio of 1-1.5:1; S2. Seed preparation: Weigh ferrous oxalate seeds according to the ratio of 0.03 - 0.05 of the mass of oxalic acid and place them in a high - energy ball - milling tank. Control the rotation speed of the ball - milling tank at 1300 - 2100 r / min. After ball - milling for 1 - 2 h, take out the ferrous oxalate seeds. Then add an activator and stir thoroughly at 80 - 90 °C. Subsequently, filter to obtain active seeds, and the filtrate can be reused for seed activation; S3. Precipitation reaction: Add the active seeds obtained in S2 to solution B and mix evenly. Weigh a surfactant of 0.01 - 0.02 of the mass of oxalic acid and add it to solution A. Then slowly add solution B to solution A and stir at high speed to obtain ferrous oxalate slurry; S4. Dehydration reaction: Wash the ferrous oxalate in S3 until the conductivity of the washing water is 0.3 - 0.4 μS / cm. Then dry at 100 - 130 °C for 1 - 2 h to obtain a dried filter cake. Finally, calcine at 300 - 400 °C for 2 - 3 h to obtain battery - grade ferrous oxalate.

[0025] The preparation method of high - tap - density lithium iron phosphate provided by the present invention includes the following steps: S5. Mix the ferrous oxalate prepared in steps S1 - S4 with a lithium source and a carbon source, and perform spray drying to obtain a lithium iron phosphate precursor; S6. Sinter the lithium iron phosphate precursor in an inert atmosphere to obtain high - tap - density lithium iron phosphate.

[0026] To better understand the present invention, the content of the present invention will be further elaborated below in conjunction with embodiments and related drawings. However, the content of the present invention is not limited to the following embodiments.

[0027] Example 1 A1: Dissolve 480 g of ferrous sulfate heptahydrate and 270 g of oxalic acid dihydrate in 2000 g of deionized water respectively to obtain solution A and solution B; A2: Weigh 9 g of ferrous oxalate into a ball - milling tank, grind it at 1500 r / min for 1.5 h to obtain ferrous oxalate fine powder. Then add it to 200 g of 1 + 1 sulfuric acid and stir at 90 °C for 2 h. After filtration, obtain ferrous oxalate active seeds; A3: Add the active seeds obtained in A2 to solution A and stir evenly. Weigh 3 g of PVP, mix it evenly with solution B and add it to the reaction kettle. Then slowly add solution A to solution B, control the stirring frequency at 30 Hz, and discharge after reacting for 3 - 5 h to obtain ferrous oxalate slurry; A4: Feed the ferrous oxalate slurry in A3 into a filter press for solid-liquid separation, then wash it with water until the conductivity of the washing water is < 0.3 μS / cm. Subsequently, dry the filter cake at 100 °C for 2 h, crush it, and calcine it at 350 °C for 3 h to obtain battery-grade ferrous oxalate; A5: Add the ferrous oxalate prepared in the above steps A1 - A4 into a sand mill, and add lithium dihydrogen phosphate, graphite carbon black, and glucose. The molar ratio of FePO4:Li2CO3 is 1:0.6, and the graphite carbon black and glucose are added at 5% and 10% of the mass of iron phosphate respectively; Use methanol as the dispersion medium, stir for 1 h, and then spray-dry to obtain the lithium iron phosphate precursor; A5: Calcinate the lithium iron phosphate precursor at 800 °C for 12 h under a nitrogen atmosphere, and naturally cool it to room temperature to obtain lithium iron phosphate.

[0028] As Figure 1 Shown in (a), in Example 1, the particle size of ferrous oxalate is relatively small (100 - 200 nm), the secondary particle aggregates are in a three-dimensional nanoflower-like structure, and the product particle size distribution is uniform.

[0029] Examples 2 - 4 Examples 2 - 3 are examples in which the ball milling speed in step A2 is changed, and the other conditions are the same as those in Example 1. The ball milling speed in Example 2 is 1300 rpm; the ball milling speed in Example 3 is 1800 rpm; the ball milling speed in Example 4 is 2100 rpm. Under the same conditions (same as Example 1), the ferrous oxalate slurry is obtained by reaction, and battery-grade ferrous oxalate is obtained through subsequent steps.

[0030] Examples 5 - 6 Examples 5 - 6 are examples in which the activation temperature in step A2 is changed, and the other conditions are the same as those in Example 1. The activation temperature in Example 5 is 80 °C; the activation temperature in Example 6 is 90 °C. Under the same conditions (same as Example 1), the ferrous oxalate slurry is obtained by reaction, and battery-grade ferrous oxalate is obtained through subsequent steps.

[0031] Examples 7 - 9 Examples 7 - 9 are examples in which the type of activator in step A2 is changed, and the other conditions are the same as those in Example 1. Sulfuric acid is used in Example 7; phosphoric acid is used in Example 8; nitric acid is used in Example 9. Under the same conditions (same as Example 1), the ferrous oxalate slurry is obtained by reaction, and battery-grade ferrous oxalate is obtained through subsequent steps.

[0032] Comparative Examples 1 - 2 Comparative Examples 1 - 2 are comparative examples in which the ball milling speed in step A2 is changed. The ball milling speed in Comparative Example 1 is 500 rpm; the ball milling speed in Comparative Example 2 is 3000 rpm. Other reaction conditions are the same as those in Example 1, and battery-grade ferrous oxalate is obtained through subsequent steps.

[0033] Comparative Example 3 Comparative Examples 3-4 are comparative examples in which the activation temperature in Step A2 is changed. The activation temperature in Comparative Example 3 is 100°C. Other reaction conditions are the same as in Example 1, and battery-grade ferrous oxalate is obtained through subsequent steps.

[0034] Comparative Examples 4-5 Comparative Examples 4-5 are comparative examples in which the type of activator in Step A2 is changed. The activator in Comparative Example 4 is hydrochloric acid; the activator in Comparative Example 5 is acetic acid. Other reaction conditions are the same as in Example 1, and battery-grade ferrous oxalate is obtained through subsequent steps.

[0035]

[0036] The above embodiments are only for clearly illustrating the examples made, rather than limiting the implementation manners. For those of ordinary skill in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to enumerate all the implementation manners here. Therefore, the obvious changes or modifications derived therefrom are still within the protection scope of the present invention.

Claims

1. A preparation method of battery-grade ferrous oxalate, characterized in that, The method includes the following steps: S1. Raw material preparation: Mix an iron source and oxalic acid with water respectively to obtain an iron source solution and an oxalic acid solution; S2. Seed preparation: Grind ferrous oxalate with a high-speed planetary ball mill to obtain ferrous oxalate fine powder, add an activator and activate it under high temperature conditions to obtain active seeds; S3. Precipitation reaction: Add the active seeds in S2 to the iron source solution and disperse evenly, then add it to the oxalic acid solution to obtain a ferrous oxalate slurry; S4. Dehydration reaction: Perform solid-liquid separation on the ferrous oxalate slurry in S3, wash with water and filter to obtain a ferrous oxalate filter cake. The ferrous oxalate filter cake is dried, calcined and pulverized to obtain battery-grade ferrous oxalate.

2. The preparation method of battery-grade ferrous oxalate according to claim 1, wherein In step S1, the iron source includes any one or a combination of more than one of ferrous sulfate, ferrous phosphate, ferrous nitrate, and ferrous chloride.

3. The preparation method of battery-grade ferrous oxalate according to claim 1, wherein In step S2, the rotation speed of the ball mill is 1300 - 2100 r / min, the ball milling time is 1 - 2 h, and the particle size of the ferrous oxalate fine powder is 5 - 25 μm.

4. The preparation method of battery-grade ferrous oxalate according to claim 1, characterized in that, In step S2, the activator is one or more of sulfuric acid, phosphoric acid, and nitric acid; the activation temperature is 80 - 90 °C.

5. The preparation method of battery-grade ferrous oxalate according to claim 1, wherein In step S3, a surfactant is also added to the oxalic acid solution; The surfactant is one or more of PVP, CTAB, and CTDB.

6. The preparation method of battery-grade ferrous oxalate according to claim 1, characterized in that In step S3, the mass ratio of ferrous sulfate:oxalic acid:seed:surfactant is 1 - 1.5:1:0.03 - 0.05:0.01 - 0.02; The stirring rotation speed is 40 - 50 Hz.

7. A method for preparing battery-grade ferrous oxalate according to claim 1, characterized in that, In step S4, the conductivity of the water wash is 0.3 - 0.4 μS / cm.

8. The preparation method of battery-grade ferrous oxalate according to claim 1, characterized in that, In step S4, the ferrous oxalate filter cake can be used as seeds for recycling to prepare battery-grade ferrous oxalate; in step S4, the drying temperature is 100 - 130 °C, the drying time is 1 - 2 h, the sintering temperature is 300 - 400 °C, and the sintering time is 2 - 3 h.

9. Ferrous oxalate for battery grade, characterized in that, Prepared by using the preparation method described in any one of claims 1 - 8.

10. A lithium iron phosphate battery material, characterized in that, Including the battery-grade ferrous oxalate described in claim 9.

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