Polycrystalline battery-grade ferrous oxalate, preparation method thereof and lithium battery positive electrode material

Abstract

Polycrystalline battery-grade ferrous oxalate, a preparation method thereof and a lithium battery positive electrode material relate to the technical field of materials.The preparation method comprises the steps that a ferrous sulfate solution and excessive oxalic acid are mixed to form a C2 / c seed crystal, then the ferrous sulfate solution and a dispersing agent are supplemented, and an I2 / a new phase is formed on the (001) crystal face of a C2 / c crystal phase through induction of the dispersing agent. The ferrous oxalate C2 / c and I2 / a crystal phases are metastable phases, so that the ferrous oxalate C2 / c and I2 / a crystal phases can be gradually converted to an orthorhombic phase Cccm structure under the action of high-temperature aging; by adjusting the proportion and the time, the polycrystalline battery-grade ferrous oxalate with three mixed crystal phases can be obtained. The preparation method is simple and convenient to operate, the polycrystalline mixed battery grade ferrous oxalate can be efficiently obtained, the stress concentration problem of single crystal form ferrous oxalate during sintering is reduced, and the high-quality lithium battery positive electrode material is obtained.

Description

Technical Field

[0001] The present invention relates to the technical field of materials, and in particular, to a polymorphic battery-grade ferrous oxalate, a preparation method thereof, and a cathode material for a lithium battery. Background Art

[0002] As a green secondary battery with high energy density, high voltage, long cycle life and no memory effect, a lithium battery is an important basis for current power batteries. The excellent performance of lithium iron phosphate batteries in terms of safety performance and cycle life has made them one of the mainstream power batteries for electric vehicles.

[0003] The current mainstream production processes of lithium iron phosphate include the ferric phosphate process, the iron red process, the liquid phase method and the ferrous oxalate process. As an important precursor of lithium iron phosphate, ferrous oxalate is currently mainly prepared by mixing high-purity ferrous salts and oxalate solutions, and the pH needs to be adjusted during the process. The preparation method has high costs, a large amount of water treatment, and a large amount of by-products and pollutant emissions. The lithium iron phosphate battery material prepared by the ferrous oxalate process has excellent cycle performance and high rate performance, and can quickly respond to rapid charging and discharging under large currents, having unique advantages compared with other processes. With the rapid development of economy and technology, the demand for power batteries with fast charging response for power batteries will further increase. Therefore, it is of great significance to develop a technology for preparing low-cost ferrous oxalate.

[0004] Ferrous oxalate exists in multiple crystal structures, and its main synthesis method is to react oxalate ions with divalent iron ions. In the synthesis of lithium iron phosphate by the ferrous oxalate process, the single crystal form of ferrous oxalate mainly has the following problems: the single crystal form of ferrous oxalate is prone to stress concentration and defects when sintered with a lithium source. The mixed crystal form of ferrous oxalate can produce a good synergistic effect due to the difference in bond energy decomposition, making the sintered product lithium iron phosphate more stable and uniform. Therefore, it is of great significance to prepare ferrous oxalate with multiple crystal forms and controllable proportions. Summary of the Invention

[0005] The purpose of the present invention is to provide a polymorphic battery-grade ferrous oxalate and a preparation method thereof, which are simple and convenient to operate, can efficiently obtain a polymorphic mixed battery-grade ferrous oxalate, and reduce the stress concentration problem of single crystal form ferrous oxalate during sintering.

[0006] Another purpose of the present invention is to provide a cathode material for a lithium battery, which uses polymorphic battery-grade ferrous oxalate as an iron source, can effectively solve the stress concentration problem during sintering, reduce product defects, and improve the quality.

[0007] The embodiments of the present invention are implemented as follows: A preparation method of a polymorphic battery-grade ferrous oxalate, which includes: S1. Mix the solution of ferrous sulfate with excessive oxalic acid to obtain the first slurry; S2. Add the solution of ferrous sulfate and a dispersant to the first slurry to obtain the second slurry; S3. Age the second slurry.

[0008] A polycrystalline battery-grade ferrous oxalate is prepared by the preparation method of the polycrystalline battery-grade ferrous oxalate as described above.

[0009] A lithium battery cathode material is prepared by using the above polycrystalline battery-grade ferrous oxalate as an iron source.

[0010] The beneficial effects of the embodiments of the present invention are as follows: The embodiments of the present invention provide a polycrystalline battery-grade ferrous oxalate and its preparation method. In this preparation method, the solution of ferrous sulfate is first mixed with excessive oxalic acid to form C2 / c crystal seeds, and then the solution of ferrous sulfate and a dispersant are added. The dispersant is used to induce the formation of a new I2 / a phase on the (001) crystal plane of the C2 / c crystal phase. Subsequently, aging is carried out. Since both the C2 / c and I2 / a crystal phases of ferrous oxalate are metastable phases, they will gradually transform into the orthorhombic Cccm structure under the action of high-temperature aging. By adjusting the ratio and time, a polycrystalline battery-grade ferrous oxalate mixed with three crystal phases can be obtained. This preparation method is simple and convenient to operate, can efficiently obtain a polycrystalline mixed battery-grade ferrous oxalate, reduce the stress concentration problem of single-crystal ferrous oxalate during sintering, and obtain a high-quality lithium battery cathode material. Description of the Drawings

[0011] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for use in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention, and therefore should not be regarded as limiting the scope. For those of ordinary skill in the art, other related drawings can also be obtained based on these drawings without creative efforts.

[0012] Figure 1 It is a flowchart of a preparation method of a polycrystalline battery-grade ferrous oxalate provided in Embodiment 1 of the present invention; Figure 2 It is a scanning electron microscope image of the polycrystalline battery-grade ferrous oxalate provided in Embodiment 1 of the present invention; Figure 3 It is an XRD pattern of the polycrystalline battery-grade ferrous oxalate provided in Embodiment 1 of the present invention. Detailed Embodiments

[0013] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. For those not specified in the embodiments, the conventional conditions or the conditions recommended by the manufacturer are followed. For the reagents or instruments without indicating the manufacturer, they are all conventional products that can be obtained through commercial purchase.

[0014] The following specifically describes a coated and modified phosphate cathode composite material and its preparation method and application in the embodiments of the present invention.

[0015] A preparation method of polycrystalline battery-grade ferrous oxalate, which includes: S1. Mix a solution of ferrous sulfate with an excessive amount of oxalic acid to obtain a first slurry; S2. Add a solution of ferrous sulfate and a dispersant to the first slurry to obtain a second slurry; S3. Age the second slurry.

[0016] Among them, in step S1, the molar ratio of ferrous sulfate to oxalic acid is 1:(0.8~1.0). Oxalic acid, as the simplest dibasic carboxylic acid, the structural formula of the oxalate ion presents a straight line, forming a linear molecular structure, and the two carbon-oxygen double bonds and single bonds in the molecule are arranged alternately; at the same time, the oxalate ion also has a strong complexing effect and can chelate with transition alkali metal ions. Therefore, in the reaction with ferrous sulfate, different crystal forms of ferrous oxalate will be generated according to the existence form, proportion, and reaction temperature of the oxalate ion. In step S1, oxalic acid is in an excessive state, and the strongly coordinating oxalate ion forms a C2 / c crystal seed with the special deformed octahedral structure composed of Fe 2+ , two water molecules, and its structural formula is as follows .

[0017] Furthermore, the solution of ferrous sulfate can be directly purchased as a commercial sulfuric acid liquid and configured into a solution, or it can be obtained by reacting an iron source with sulfuric acid. Optionally, the iron source is selected from industrial iron products such as reduced iron powder and iron blocks with an iron content of more than 99%, and the sulfuric acid concentration is 15%~20%. The molar ratio of sulfuric acid to iron element is 0.9~1.2:1. The obtained iron liquid concentration is 80~85 g / L (the mass fraction of Fe% is 7.0wt%~8.0wt%).

[0018] In step S1, oxalic acid directly participates in the reaction in the form of solid oxalic acid without the need to be configured into a solution, which can reduce water consumption. At the same time, after the aging is completed, the mother liquor can be further recycled by filtering out the solid product.

[0019] Optionally, in steps S1 and S2, the dosage ratio of ferrous sulfate is 1:1 to 4. By adjusting the ratio of ferrous sulfate in steps S1 and S2, the proportion of monoclinic I2 / a and monoclinic C2 / c in the polycrystalline battery-grade ferrous oxalate can be adjusted. Increasing the dosage of ferrous sulfate in step S2 can obtain more monoclinic I2 / a.

[0020] In step S1, the reaction temperature of ferrous sulfate and oxalic acid is 40 to 90 °C. At this temperature, the reaction can be promoted, and C2 / c crystal seeds are rapidly formed.

[0021] Furthermore, in step S2, first mix the added ferrous sulfate solution and the dispersant to obtain a mixed solution, and then mix the mixed solution with the first slurry. In the mixed solution, the concentration of the dispersant is 1 vol% to 20 vol%. The dispersant includes at least one of glycerol, propylene glycol, and ethylene glycol. Preferably, the dispersant is glycerol. In step S2, excessive Fe 2+ forms an infinite chain structure with the oxalate groups, and its structural formula is as follows .

[0022] After this chain structure reaches saturation, a new I2 / a phase will be formed on the (001) crystal plane of the C2 / c crystal phase under the induction of the dispersant.

[0023] Optionally, in step S2, add the first slurry to the mixed solution at a rate of 200 to 500 L / h and maintain the reaction temperature at 40 to 90 °C. By controlling the reaction rate, the ferrous oxalate particles in the slurry can better adhere to the surface of the crystal seeds and further grow, which is beneficial to the growth of the new I2 / a phase.

[0024] The aging time of the second slurry is 30 to 240 min, and the aging temperature is 40 to 90 °C. Both the C2 / c and I2 / a crystal phases of ferrous oxalate are metastable phases, and they will gradually transform into the orthorhombic Cccm structure under the action of high-temperature aging. By controlling the time of the aging reaction, ferrous oxalate with a mixture of three crystal forms can be obtained.

[0025] After aging is completed, filter and separate the mixed slurry. The solid part is washed and then dried to obtain the polycrystalline battery-grade ferrous oxalate product, and the separated mother liquor is recycled and reused.

[0026] The embodiment of the present invention also provides a polycrystalline battery-grade ferrous oxalate, which is prepared by the above preparation method of polycrystalline battery-grade ferrous oxalate. It includes three crystal phases, namely orthorhombic Cccm, monoclinic I2 / a, and monoclinic C2 / c, and the unit cell parameters of the three crystal phases are shown in Table 1.

[0027] Table 1: Unit cell parameters of orthorhombic Cccm, monoclinic I2 / a, and monoclinic C2 / c crystal structures Crystal structure a / nm b / nm c / nm α / ° β / ° γ / ° Orthorhombic Cccm 1.221 0.555 1.547 90 90 90 Monoclinic I2 / a 1.220 0.552 0.992 90 128 90 Monoclinic C2 / c 0.991 0.555 0.970 90 104 90 And by adjusting the preparation parameters, such as the proportion of ferrous sulfate, pumping rate, reaction temperature, etc. in steps S1 and S2, the proportion of the three crystal phases can be further adjusted.

[0028] An embodiment of the present invention also provides a cathode material for a lithium battery, which is prepared from the above-mentioned polymorphic battery-grade ferrous oxalate as an iron source. When the polymorphic battery-grade ferrous oxalate is sintered and synthesized into a lithium iron phosphate material, a synergistic effect is generated, making the synthesized lithium iron phosphate have good rate performance and cycling performance.

[0029] The features and properties of the present invention will be further described in detail below in conjunction with embodiments. Example 1

[0030] This example provides a polymorphic battery-grade ferrous oxalate, and its preparation method is as Figure 1 shown, including: S1. Using 20 kg of industrial pure iron products with a purity above 99% as iron powder, putting it into 250 L of dilute sulfuric acid solution with a concentration of 15.01%, starting stirring at 40 Hz, reacting at 70 °C for 5 h, and filtering to obtain a ferrous sulfate solution with a Fe% concentration of 7.21%.

[0031] S2. Dividing the ferrous sulfate solution into two equal parts and flowing them into reaction kettles 1# and 2#, introducing nitrogen protection and starting stirring, with the reaction temperature being 60 °C. Adding 45 kg of oxalic acid to reaction kettle 1# to fully react to form a first slurry, and adding glycerol with a volume fraction of 15% to reaction kettle 2# to form a mixed solution.

[0032] S3. Pumping the first slurry into the mixed solution in reaction kettle 2# at a pump speed of 500 L / h, and reacting to form a second slurry, with the reaction temperature being 60 °C.

[0033] S4. Continuing to age at 60 °C for 180 min to obtain a ferrous oxalate slurry with a particle size D50 = 26.3 um; centrifuging and filtering, titrating the separated mother liquor for acidity and then reconfiguring it to return to leaching, and the filtered ferrous oxalate is washed, dried, pulverized, and batch-mixed to obtain a light yellow finished battery-grade ferrous oxalate with a purity of 99.9%.

[0034] The electron micrograph of this battery-grade ferrous oxalate is as Figure 2 shown. It can be seen from the figure that ferrous oxalate with different morphological structures exists, and at the same time, the prepared ferrous oxalate particles have regular morphology and no dust on the particle surface, indicating that the ferrous oxalate crystals have good crystallinity.

[0035] The XRD pattern of this battery-grade ferrous oxalate is asFigure 3 As shown in the figure, it can be seen that the peak positions correspond to the three crystal phases of orthorhombic Cccm, monoclinic I2 / a, and monoclinic C2 / c in the database, indicating that the battery-grade ferrous oxalate obtained in this example simultaneously exists in three crystal phases. Example 2

[0036] This example provides a polymorphic battery-grade ferrous oxalate, and its preparation method is as follows: S1. Take 20 kg of industrial pure iron products with a purity of over 99% as iron powder, put it into 250 L of dilute sulfuric acid solution with a concentration of 15.01%, start stirring at 45 Hz, react at 80 °C for 3 h, and filter to obtain a ferrous sulfate solution with a Fe% concentration of 7.69%.

[0037] S2. Divide the 40% and 60% ferrous sulfate solutions into the 1# and 2# reactors respectively, introduce nitrogen protection and start stirring, and the reaction temperature is 70 °C. Add 40 kg of oxalic acid to the 1# reactor among them to fully react to form the first slurry, and add glycerol with a volume fraction of 10% to the 2# reactor to form a mixed solution.

[0038] S3. Pump the first slurry into the mixed solution in the 2# reactor at a pump speed of 300 L / h to react to form the second slurry, and the reaction temperature is 70 °C.

[0039] S4. Continue to age at 70 °C for 60 min to obtain a ferrous oxalate slurry with a particle size D50 = 26.8 um; centrifuge and filter, titrate the separated mother liquor for acidity and then reconfigure it to return to leaching. The filtered ferrous oxalate is washed, dried, crushed, and batch-mixed to obtain a light yellow finished product battery-grade ferrous oxalate with a purity of 99.8%.

[0040] S5. Supplement the filtered mother liquor with concentrated sulfuric acid to a predetermined acidity value, then return the mother liquor to leach 17 kg of iron powder, start stirring at 45 Hz, react at 80 °C for 3 h, and filter to obtain a ferrous sulfate solution with a Fe% concentration of 7.4%.

[0041] S6. For the ferrous sulfate solution in step S5, divide the 40% ferrous sulfate solution into the 1# reactor, and pump the 60% ferrous sulfate solution into the 2# reactor, introduce nitrogen protection and start stirring, and the reaction temperature is 70 °C. Add 40 kg of oxalic acid to the 1# reactor among them to fully react to form the first slurry, and add glycerol with a volume fraction of 10% to the 2# reactor to form a mixed solution.

[0042] S7. Pump the first slurry into the mixed solution in the 2# reactor at a pump speed of 400 L / h to form the second slurry.

[0043] S8. The second slurry is further aged at 70 °C for 120 min to obtain ferrous oxalate slurry with a particle size D50 = 32.5 um; centrifugation, filtration, and separation are carried out to obtain the secondary recycled mother liquor and ferrous oxalate. The filtered ferrous oxalate is washed, dried, pulverized, and batch-mixed to obtain a light yellow finished battery-grade ferrous oxalate with a purity of 99.4%. Example 3

[0044] This example provides a polymorphic battery-grade ferrous oxalate, and its preparation method is as follows: S1. On the basis of Example 2, the secondary recycled mother liquor is acid-supplemented and adjusted to a predetermined acidity, and the mother liquor is recycled and returned to leach 19 kg of iron powder. Stirring is started at 40 Hz, and the reaction is carried out at 80 °C for 5 h. Filtration is carried out to obtain a ferrous sulfate solution with an Fe% concentration of 8.21%.

[0045] S2. The 30% ferrous sulfate solution is split into the 1# reactor, and the remaining is split into the 2# reactor. Nitrogen protection is introduced and stirring is started, and the reaction temperature is 80 °C. 44 kg of oxalic acid is added to the 1# reactor, and a first slurry is fully reacted. A 10% volume fraction of glycerol is added to the 2# reactor to form a mixed solution.

[0046] S3. The first slurry is pumped into the mixed solution in the 2# reactor at a pump speed of 300 L / h, and a second slurry is formed by reaction, and the reaction temperature is 80 °C.

[0047] S4. It is further aged at 80 °C for 30 min to obtain ferrous oxalate slurry with a particle size D50 = 46.8 um; centrifugation and filtration are carried out, and the separated mother liquor is titrated for acidity and then reconfigured and returned to leaching. The filtered ferrous oxalate is washed, dried, pulverized, and batch-mixed to obtain a light yellow finished battery-grade ferrous oxalate with a purity of 99.8%.

[0048] Comparative Example 1 This comparative example provides a battery-grade ferrous oxalate, and its preparation method is basically the same as that of Example 1, except that in step S2, glycerol is not added to the 2# reactor.

[0049] Comparative Example 2 This comparative example provides a battery-grade ferrous oxalate, and its preparation method is basically the same as that of Example 1, except that in step S3, the first slurry is pumped into the 2# reactor at a rate of 2000 L / h.

[0050] Comparative Example 3 This comparative example provides a battery-grade ferrous oxalate, and its preparation method is basically the same as that of Example 1, except that in step S4, aging is not carried out.

[0051] Comparative Example 4 This comparative example provides a battery-grade ferrous oxalate, and its preparation method is basically the same as that of Example 1, except that in step S4, the aging time is extended to 24 h.

[0052] Test Example 1 Using the battery-grade ferrous oxalate provided in Examples 1 to 3 and Comparative Examples 1 to 4, the proportion of three crystal phases in it was detected by XRD, and the detection results are shown in Table 2.

[0053] Table 2: Proportion of each crystal phase in the examples Example Temperature / °C Ratio of seed crystal / % Aging time / h Orthorhombic Cccm Monoclinic C2 / c Monoclinic I2 / a Example 1 60 50% 3 35.8% 51.7% 12.5% Example 2, first cycle 70 40% 1 33.9% 43.5% 22.6% Example 2, second cycle 70 40% 2 34.2% 45.0% 20.8% Example 3 85 30% 0.5 32.3% 38.0% 29.7% Comparative example 1 60 50% 3 40.7% 59.3% 0 Comparative example 2 60 50% 3 39.5% 56.8% 3.7% Comparative example 3 60 50% 0 9.7% 68.9% 21.4% Comparative example 4 60 50% 24 99.8% 0.2% 0 As can be seen from Table 2, by using the preparation methods of Examples 1 to 3, ferrous oxalate mixed with three crystal phases can be obtained. Among them, in Example 2, the mother liquor was recycled, and polycrystalline battery-grade ferrous oxalate could be obtained in both cycles, and the proportion of each crystal phase changed little, indicating that the method of the embodiment of the present invention can achieve continuous cyclic production. In contrast, in Comparative Example 1, glycerol was not used for induction, and it can be seen that there is no monoclinic I2 / a crystal phase in the ferrous oxalate product. In Comparative Example 2, the pumping rate of the first slurry was increased, and too fast a pumping rate is not conducive to the growth of the monoclinic I2 / a crystal phase, and the product also has only a small amount of the monoclinic I2 / a crystal phase. In Comparative Example 3, the second slurry was not aged, and only a small amount of orthorhombic Cccm crystal phase was generated in the product. In Comparative Example 4, the aging time was extended, and under long-term aging, the metastable monoclinic I2 / a and monoclinic C2 / c crystal phases were almost completely converted into the orthorhombic Cccm crystal phase. It can be seen that by adjusting the preparation condition parameters, the adjustment of the proportion of the three crystal phases can be achieved.

[0054] Test Example 2 Using the battery-grade ferrous oxalate prepared in Examples 1 to 3 and Comparative Examples 1 to 4, and commercially available ferrous oxalate (control example) as the iron source, diammonium hydrogen phosphate as the phosphorus source, lithium carbonate as the lithium source, and glucose as the carbon source, the cathode material for lithium iron phosphate batteries was prepared. The performance of each group of cathode materials for lithium iron phosphate batteries was compared, and the comparison results are shown in Table 3.

[0055] Table 3. Performance comparison of cathode materials for lithium iron phosphate batteries Experiment serial number D50 / μm Discharge specific capacity at 0.1C (mAh / g) Discharge specific capacity at 1C (mAh / g) Example 1 1.28 161.0 142.0 Example 2, first cycle 1.16 159.3 140.7 Example 2, second cycle 1.32 160.7 141.5 Example 3 1.21 159.9 140.0 Comparative example 1 1.41 156.4 135.8 Comparative example 2 1.38 157.8 137.6 Comparative example 3 1.27 158.6 138.8 Comparative example 4 1.39 156.1 134.0 Control example 1.32 156.6 133.6 As can be seen from Table 3, for the cathode materials of the battery prepared using the ferrous oxalate provided in Embodiments 1 to 3 of the present invention as the iron source, their tap density is in the range of 1.21 - 1.32 μm, the discharge specific capacity at 0.1C reaches above 159.3 mAh / g, and the discharge specific capacity at 1C reaches above 140 mAh / g, showing excellent rate performance. In contrast, when using the ferrous oxalate in the prior art as the iron source, its rate performance is significantly reduced. The discharge specific capacity at 0.1C is only 156.6 mAh / g, and as the rate increases, the gap in the discharge specific capacity becomes more obvious. The discharge specific capacity at 1C is only 133.6 mAh / g. Although Comparative Examples 1 - 4 also contain more than one crystal phase, due to the unbalanced proportion of each crystal phase, their rate performance is poor and they fail to fully exert their synergistic effect. Compared with the embodiments, the discharge specific capacity at both 0.1C and 1C is reduced to varying degrees, slightly higher than that of the control example. Especially for Comparative Example 4, its ferrous oxalate is basically composed of a single orthorhombic Cccm crystal phase, and the performance of the cathode material of the battery prepared with it is basically the same as that of the control example. It can be seen that the polycrystalline battery-grade ferrous oxalate prepared in the embodiments of the present invention indeed has a positive promoting effect on the performance of the cathode material of the battery.

[0056] In summary, the embodiments of the present invention provide a polycrystalline battery-grade ferrous oxalate and its preparation method. In this preparation method, a solution of ferrous sulfate is first mixed with an excessive amount of oxalic acid to form C2 / c seeds, and then a solution of ferrous sulfate and a dispersant are added. The dispersant is used to induce the formation of a new I2 / a phase on the (001) crystal plane of the C2 / c crystal phase. Subsequently, aging is carried out. Since both the C2 / c and I2 / a crystal phases of ferrous oxalate are metastable phases, they will gradually transform into the orthorhombic Cccm structure under the action of high-temperature aging. By adjusting the ratio and time, polycrystalline battery-grade ferrous oxalate with a mixture of three crystal phases can be obtained. This preparation method is simple and convenient to operate, can efficiently obtain polycrystalline battery-grade ferrous oxalate, reduce the stress concentration problem of single-crystal ferrous oxalate during sintering, and obtain high-quality cathode materials for lithium batteries.

[0057] The above are only the preferred embodiments of the present invention and are not used to limit the present invention. For those skilled in the art, the present invention can have various modifications and changes. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A preparation method of polycrystalline battery-grade ferrous oxalate, characterized in that, Comprising: S1. Mix a solution of ferrous sulfate with an excessive amount of oxalic acid to obtain a first slurry; S2. Add a solution of ferrous sulfate and a dispersant to the first slurry to obtain a second slurry; S3. Age the second slurry.

2. The preparation method according to claim 1, characterized in that, In step S1, the molar ratio of ferrous sulfate to oxalic acid is 1:(0.8 - 1.0).

3. The preparation method according to claim 1, characterized in that, In steps S1 and S2, the dosage ratio of ferrous sulfate is 1:1 - 4, and the iron concentration in the ferrous sulfate solution is 7.0wt% - 8.0wt%.

4. The preparation method according to claim 1, characterized in that, In step S1, the reaction temperature of ferrous sulfate and oxalic acid is 40 - 90°C.

5. The preparation method according to claim 1, characterized in that, In step S2, the dispersant includes at least one of glycerol, propylene glycol, and ethylene glycol.

6. The preparation method according to claim 1, wherein In step S2, first mix the added ferrous sulfate solution and the dispersant to obtain a mixed solution, and then mix the mixed solution with the first slurry. In the mixed solution, the concentration of the dispersant is 1vol% - 20vol%.

7. The preparation method according to claim 6, characterized in that, In step S2, add the first slurry to the mixed solution at a rate of 200 - 500L / h and maintain the reaction temperature at 40 - 90°C.

8. The preparation method according to claim 1, wherein, The aging time of the second slurry is 30 - 240min, and the aging temperature is 40 - 90°C.

9. A polymorphic battery-grade ferrous oxalate, characterized in that, Prepared by the preparation method of polymorphic battery-grade ferrous oxalate according to any one of claims 1 - 8.

10. A cathode material for a lithium battery, characterized in that, It is prepared using the polymorphic battery-grade ferrous oxalate according to claim 9 as an iron source.

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