Preparation method of carbon source precursor for preparing lithium iron phosphate

Abstract

The invention discloses a preparation method of a carbon source precursor for lithium iron phosphate. The preparation method comprises the following steps: firstly, carrying out auto-polymerization on glucose, fructose or cane sugar to obtain a polysaccharide aqueous solution, then adding ammonium citrate to adjust the pH value to 4-6, and finally adding vitamin C to obtain a macromolecular polysaccharide precursor; the precursor prepared by the invention can uniformly coat the surface of lithium iron phosphate in an airflow drying process, so that a uniform and compact carbon coating layer is obtained, and a lithium iron phosphate positive electrode with higher conductivity and higher compaction density can be obtained; the 1C charge-discharge capacity is improved by more than 7mAh / g, and the trailing phenomenon is also improved.

Description

Technical Field

[0001] The invention relates to a method for preparing a carbon source precursor for lithium iron phosphate, in particular to a high molecular weight polysaccharide precursor mixture. Background Art

[0002] Lithium iron phosphate cathode materials have stable structure, good safety performance, abundant resources and low preparation cost. They are one of the mainstream cathode materials for lithium-ion batteries. However, the conductivity of pure lithium iron phosphate is very low, and the diffusion coefficient of lithium ions is also very low. Only by providing lithium iron phosphate with both electronic and ionic conductivity can the good electrochemical performance of lithium iron phosphate be guaranteed. Carbon coating is one of the common methods for industrial preparation of lithium iron phosphate. Generally, glucose is used as the carbon source, which has a wide source and low cost. However, glucose has poor compatibility with lithium iron phosphate precursors (iron phosphate, lithium carbonate). The viscosity of the grinding added during the precursor grinding process is high and the grinding time is long. On the other hand, during the precursor spray drying process, due to the low molecular weight of glucose, it will drift with the drying airflow, resulting in uneven distribution on the surface of the precursor particles, which ultimately leads to uneven carbon coating on the surface of lithium iron phosphate and poor electrochemical performance of lithium iron phosphate particles. Summary of the invention

[0003] In view of the shortcomings of the prior art, the present invention provides a high molecular weight polysaccharide precursor mixture, which can be uniformly coated on the surface of lithium iron phosphate during airflow drying to obtain a uniform and compact carbon coating layer, and a lithium iron phosphate positive electrode with higher conductivity and higher compaction density can be obtained. On the other hand, the prepared polysaccharide precursor has surface activity, which can improve the dispersion of the precursor, increase the solid content of the precursor grinding, and improve the preparation efficiency.

[0004] A method for preparing a carbon source precursor for preparing lithium iron phosphate, the steps are as follows:

[0005] (1) dissolving a sugar source in deionized water in proportion, and allowing the sugar source to undergo a self-polymerization reaction at 50-100° C. for 6-24 hours under nitrogen protection to obtain a caramel-colored aqueous solution;

[0006] (2) adding ammonium citrate to the caramel color aqueous solution obtained in step (1) to adjust the pH to 4-6;

[0007] (3) Add vitamin C and dissolve it in the above solution to obtain a high molecular weight polysaccharide precursor, i.e., a carbon source precursor for lithium iron phosphate.

[0008] In step (1), the sugar source is glucose, fructose or sucrose;

[0009] Preferably, the temperature for the self-polymerization of the sugar source is 60-80° C. and the time is 12-18 hours.

[0010] In step (2), ammonium citrate is used as a pH adjuster to keep the solution weakly acidic.

[0011] In steps (1) to (3), the ratio of the sugar source, deionized water and vitamins is 40-50 g: 80-100 g: 15-20 g;

[0012] Preferably, the usage ratio of sugar source, deionized water and vitamins is 48g:80g:18g.

[0013] The carbon source precursor prepared by the present invention is used for preparing a lithium iron phosphate positive electrode. The steps are: sequentially adding water, carbon source precursor, iron phosphate, and lithium carbonate into a batching tank, emulsifying, grinding, spray drying, and sintering at a high temperature of 700-800° C. to obtain a lithium iron phosphate positive electrode.

[0014] The mass ratio of iron phosphate to carbon source precursor is 100:2.5, wherein the carbon source precursor needs to be converted into solid, and the solid content is 36-40%;

[0015] The molar ratio of iron phosphate to lithium carbonate is 1:1;

[0016] The emulsification time was 60 minutes, the grinding speed was 1800 rpm, and the time was 6 hours.

[0017] The beneficial effects of the present invention are:

[0018] (1) The present invention uses glucose to self-polymerize to obtain a polysaccharide aqueous solution. The obtained polymer can be uniformly coated on the surface of lithium iron phosphate during airflow drying, thereby obtaining a uniform and compact carbon coating layer, and a lithium iron phosphate positive electrode with higher conductivity and higher compaction density can be obtained; the glucose small molecule undergoes a condensation reaction of carbonyl and amino groups at high temperature, and after cyclization, dehydration, rearrangement, isomerization, alcohol aldehyde condensation and other reactions, unsaturated hybrid chemicals such as imidazole, pyrrole, pyridine and pyrazine are obtained. This structure has a larger molecular weight than glucose. The above-mentioned imidazole, pyrrole, pyridine and pyrazine have stronger polarity and better dispersibility, and can be adsorbed on the surface of lithium iron phosphate precursor to improve the dispersion of precursor particles and avoid secondary agglomeration of the precursor. In addition, the formed imidazole, pyrrole, pyridine and pyrazine heterocyclic polymers are more likely to form SP2 hybridized cyclic carbon chains during the carbonization process, thereby obtaining a coating layer with better conductivity and more uniformity.

[0019] (2) Ammonium citrate is added to the polysaccharide aqueous solution. Ammonium citrate can not only adjust the pH, but also generate gas during the spray drying and carbonization heating process. The generated gas can make the formed carbon film have a larger specific surface area and a more uniform coverage, thereby forming a more uniform coating on the surface of the lithium iron phosphate precursor.

[0020] (3) Vitamin C is added to the polysaccharide aqueous solution as a composite carbon source. Vitamin C has a strong reducing property and can adjust the ratio of divalent iron to trivalent iron in the lithium iron phosphate precursor, thereby stabilizing the ferrous ions in the lithium iron phosphate.

[0021] (4) The polysaccharide precursor prepared by the present invention has surface activity, which can improve the dispersion of the precursor, increase the solid content of the precursor during grinding, and improve the preparation efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 The polysaccharide precursor obtained in Example 1 can form a stable sugar film after spray drying and calcination. DETAILED DESCRIPTION

[0023] The present invention is further described below in conjunction with the accompanying drawings and specific embodiments, but the protection scope of the present invention is not limited thereto.

[0024] Example 1

[0025] (1) Dissolve 48 g of glucose in 80 g of deionized water, introduce nitrogen after the mixture is uniformly dissolved, and react at 60° C. for 12 hours under nitrogen protection to obtain a caramel-colored aqueous solution;

[0026] (2) adding 60 g of ammonium citrate to the caramel color aqueous solution obtained in step (1) to adjust the pH to 5-6;

[0027] (3) Finally, 18 g of vitamin C was added to the solution obtained in step (1) to obtain a polysaccharide precursor solution.

[0028] Furthermore, the polysaccharide precursor obtained in Example 1 can form a stable sugar film after spray drying and calcination, such as Figure 1 As shown, the performance parameters are tested as shown in Table 1:

[0029] Table 1

[0030] Solid content (%) Viscosity(cPs) <![CDATA[Density (g / cm 3 )]]> pH Ash content at 770℃ under nitrogen protection 36.3 / 36.7 19.2 1.17 5.07 13.73％

[0031] Application Example 1

[0032] The polysaccharide precursor obtained in Example 1 was used to prepare a lithium iron phosphate positive electrode, and the specific synthesis process was as follows:

[0033] The polysaccharide precursor was added during the grinding process of the lithium iron phosphate precursor according to the mass ratio of iron phosphate to polysaccharide precursor (converted into solid, the solid content is 36-40%) of 100:2.5.

[0034] The addition order is to add water, polysaccharide precursor, iron phosphate and lithium carbonate to the batching tank, emulsify for 60 minutes, grind at 1800 rpm for 6 hours, spray dry, and sinter at 700°C to obtain a lithium iron phosphate positive electrode, and the electrochemical performance of the lithium iron phosphate positive electrode is tested.

[0035] Comparative Example 1

[0036] The polysaccharide precursor was added during the grinding process of the lithium iron phosphate precursor at a mass ratio of iron phosphate to glucose of 100:2.5.

[0037] The addition order is to add water, glucose, iron phosphate and lithium carbonate to the batching tank in turn, emulsify for 60 minutes, grind at 1800 rpm for 6 hours, spray dry, and sinter at 700°C to obtain a lithium iron phosphate positive electrode, and the electrochemical performance of the lithium iron phosphate positive electrode is tested.

[0038] The lithium iron phosphate preparation process parameters of Application Example 1 and Comparative Example 1 are shown in Table 2 below:

[0039] Table 2

[0040]

[0041]

[0042] From the slurry data in Table 2, it can be seen that by adding the polysaccharide precursor described in Example 1, the solid content of the slurry during the lithium iron phosphate grinding process is increased by 17%, the grinding efficiency is improved, the carbon content is reduced, and the obtained lithium iron phosphate has a higher compaction density.

[0043] Table 3

[0044]

[0045] The electrochemical performance of the lithium iron phosphate positive electrode obtained in Application Example 1 and Comparative Example 1 was tested, and the test results are shown in Table 3. It can be seen from Table 3 that after using the polysaccharide precursor obtained in Example 1 in Application Example 1, the carbon source polarization is improved, the 1C charge and discharge capacity is increased from 148.28 to more than 156, an increase of more than 7mAh / g, and the tailing phenomenon is also improved.

[0046] The embodiments are preferred implementations of the present invention, but the present invention is not limited to the above-mentioned implementations. Any obvious improvements, substitutions or modifications that can be made by those skilled in the art without departing from the essential content of the present invention belong to the protection scope of the present invention.

Claims

1. A method for preparing a carbon source precursor for preparing lithium iron phosphate, characterized in that: The steps include: (1) dissolving a sugar source in deionized water in proportion, and allowing the sugar source to undergo a self-polymerization reaction at 50-100° C. for 6-24 hours under nitrogen protection to obtain a caramel-colored aqueous solution; (2) adding ammonium citrate to the caramel color aqueous solution obtained in step (1) to adjust the pH to 4-6; (3) Add vitamin C and dissolve it in the above solution to obtain a high molecular weight polysaccharide precursor, i.e., a carbon source precursor for lithium iron phosphate.

2. The preparation method according to claim 1, characterized in that In step (1), the sugar source is glucose, fructose or sucrose.

3. The preparation method according to claim 1, characterized in that: In step (1), the temperature for the self-polymerization reaction of the sugar source is 60-80° C. and the time is 12-18 hours.

4. The preparation method according to claim 1, characterized in that: In steps (1) to (3), the usage ratio of sugar source, deionized water and vitamin is 40-50 g: 80-100 g: 15-20 g.

5. The preparation method according to claim 4, characterized in that: The usage ratio of sugar source, deionized water and vitamin is 48g:80g:18g.

6. Use of the carbon source precursor prepared by the preparation method according to any one of claims 1 to 5 for preparing a lithium iron phosphate positive electrode.

7. The use according to claim 6, characterized in that The steps are: Water, carbon source precursor, iron phosphate and lithium carbonate are sequentially added to a batching tank, emulsified, ground, spray dried, and sintered at a high temperature of 700-800°C to obtain a lithium iron phosphate positive electrode.

8. The use according to claim 7, characterized in that The mass ratio of iron phosphate to carbon source precursor is 100:2.5, wherein the carbon source precursor needs to be converted into solid, and the solid content is 36-40%; The molar ratio of iron phosphate to lithium carbonate is 1:1; The emulsification time was 60 minutes, the grinding speed was 1800 rpm, and the time was 6 hours.

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