阅读说明：本技术 一种原位生长磷酸铁锂薄膜的制备方法及应用 (Preparation method and application of in-situ grown lithium iron phosphate film ) 是由 黄勇平 胡振宇 金磊 陈佳敏 邵国祥 于 2021-09-30 设计创作，主要内容包括：本发明提供了一种原位生长磷酸铁锂薄膜的制备方法,包括如下步骤:(1)向磷酸锂中加入柠檬酸溶液,完全溶解后制得磷酸锂的柠檬酸稀释液；(2)将球形磷酸铁锂粉末浸入磷酸锂的柠檬酸稀释液中,搅拌至均匀,(3)接着边搅拌边倒入硫酸亚铁,继续搅拌一段时间；(4)固液分离、洗涤、干燥,得到前驱体；(5)前驱体在惰性气氛条件下进行烧结得到成品。本发明在常压下水相条件下,包覆薄膜后的球形磷酸铁锂晶体表面变得光滑、整体形貌趋于规则球型,压实密度和循环性均得到提升。(The invention provides a preparation method of an in-situ grown lithium iron phosphate film, which comprises the following steps of (1) adding a citric acid solution into lithium phosphate, and preparing a citric acid diluent of the lithium phosphate after complete dissolution; (2) soaking spherical lithium iron phosphate powder into citric acid diluent of lithium phosphate, stirring until the mixture is uniform, (3) pouring ferrous sulfate while stirring, and continuously stirring for a period of time; (4) carrying out solid-liquid separation, washing and drying to obtain a precursor; (5) and sintering the precursor under the inert atmosphere condition to obtain a finished product. Under the aqueous phase condition under normal pressure, the surface of the spherical lithium iron phosphate crystal coated with the film becomes smooth, the overall appearance of the spherical lithium iron phosphate crystal tends to be regular spherical, and the compaction density and the cyclicity are both improved.)

1. A preparation method of an in-situ grown lithium iron phosphate film is characterized by comprising the following steps: the method comprises the following steps:

(1) adding a citric acid solution into lithium phosphate, and completely dissolving to obtain a citric acid diluent of the lithium phosphate;

(2) soaking spherical lithium iron phosphate powder into citric acid diluent of lithium phosphate, stirring to be uniform,

(3) then pouring ferrous sulfate while stirring, and continuously stirring for a period of time;

(4) carrying out solid-liquid separation, washing and drying to obtain a precursor;

(5) and sintering the precursor under the inert atmosphere condition to obtain a finished product.

2. The method for preparing an in-situ grown lithium iron phosphate thin film according to claim 1, characterized in that:

the finished product is a rough lithium iron phosphate crystal grain surface on which a new lithium iron phosphate film grows, the thickness is 0.1-0.5 mu m, and the lithium iron phosphate crystal grain surface coated with the film becomes smooth and the overall appearance tends to be ellipsoidal.

3. The method for preparing an in-situ grown lithium iron phosphate thin film according to claim 1, characterized in that:

in the citric acid diluent of the lithium phosphate prepared in the step (1), the molar ratio of the lithium phosphate to the citric acid is 1: (1-1.5), and the concentration of the citric acid is 0.1-0.5 mol/L.

4. The method for preparing an in-situ grown lithium iron phosphate thin film according to claim 1, characterized in that:

the molar ratio of the spherical lithium iron phosphate to the lithium phosphate in the step (2) is 1: (0.05-0.2) and stirring for 1-3 h.

5. The method for preparing an in-situ grown lithium iron phosphate thin film according to claim 1, characterized in that:

in the mixed solution after the ferrous sulfate is added in the step (3), the molar ratio of the ferrous sulfate to the lithium phosphate is 1: (0.96-1.0) and stirring for 3-6 h.

6. The method for preparing an in-situ grown lithium iron phosphate thin film according to claim 1, characterized in that:

the solid-liquid separation and washing in the step (4) are any one of suction filtration, filter pressing and centrifugation;

the drying is any one of spraying and drying.

7. The method for preparing an in-situ grown lithium iron phosphate thin film according to claim 6, characterized in that:

the spray drying temperature is 120-300 ℃;

the drying temperature is 40-200 ℃, and the drying time is 2-12 hours.

8. The method for preparing an in-situ grown lithium iron phosphate thin film according to claim 1, characterized in that:

the inert atmosphere in the step (5) refers to nitrogen or argon atmosphere, the sintering temperature is 400-600 ℃, and the high-temperature sintering time is 3-8 h.

9. The application of the in-situ grown lithium iron phosphate film prepared by the preparation method according to any one of claims 1 to 7 in a lithium ion battery.

Technical Field

The invention belongs to the technical field of energy storage materials, and particularly relates to a preparation method and application of an in-situ grown lithium iron phosphate film.

Background

At present, the high-temperature solid phase method is mainly used for synthesizing spherical lithium iron phosphate in industry, but secondary recrystallization is easily generated in the sintering process. The normal spherical lithium iron phosphate is of a uniformly dispersed smooth sphere type, but small lithium iron phosphate grains scattered at high temperature can be suddenly and rapidly gathered on the surface of a spherical crystal for recrystallization, so that secondary recrystallization with rough surface and even irregular morphology is caused.

The large amount of secondary recrystallization crystals leads to rough crystal surface and irregular appearance in the spherical lithium iron phosphate product, thus leading to low compaction density and poor cycle performance of the product.

Upon search, the following similar patents were found:

(1) patent CN110931787B provides a method for preparing in-situ growth lithium iron phosphate whiskers, the length range of the whiskers is 0.1-10um, and the diameter range is 10-100 nm. The original irregular shape of the lithium iron phosphate crystal grain is not changed, and only a cluster of lithium iron phosphate whiskers grows on the surface of the lithium iron phosphate crystal grain.

(2) Patent 201110425266.7 discloses a method for in-situ synthesizing lithium iron phosphate and carbon nanotube composite material, wherein the in-situ growth of the composite material is not lithium iron phosphate, but rather a uniform coating layer of carbon nanotubes is grown on the surface of lithium iron phosphate particles in situ, without changing the overall irregular morphology of the lithium iron phosphate crystal particles.

Through careful comparison and analysis, the invention is fundamentally different from the technical scheme.

Disclosure of Invention

In view of the above, under the water phase condition under normal pressure, the present invention synthesizes a uniform lithium iron phosphate precursor film on the surface of the rough spherical lithium iron phosphate crystal through the action of a chelating agent (citric acid), and then, after densification sintering in a high temperature inert atmosphere, a lithium iron phosphate film grows in situ on the surface of the rough spherical lithium iron phosphate crystal. The surface of the spherical lithium iron phosphate crystal coated with the film becomes smooth, the overall appearance of the spherical lithium iron phosphate crystal tends to be regular spherical, and the compaction density and the cyclicity are improved.

In order to achieve the purpose, the technical scheme of the invention is realized as follows:

a preparation method of an in-situ grown lithium iron phosphate film comprises the following steps:

(1) adding a citric acid solution into lithium phosphate, and completely dissolving to obtain a citric acid diluent of the lithium phosphate;

(2) soaking spherical lithium iron phosphate powder into citric acid diluent of lithium phosphate, stirring to be uniform,

(3) then pouring ferrous sulfate while stirring, and continuously stirring for a period of time;

(4) carrying out solid-liquid separation, washing and drying to obtain a precursor;

(5) and sintering the precursor under the inert atmosphere condition to obtain a finished product.

Preferably, in the citric acid diluted solution of lithium phosphate prepared in the step (1), the molar ratio of lithium phosphate to citric acid is 1: (1-1.5), wherein the concentration of the citric acid is 0.1-0.5 mol/L; a large number of experimental studies have found that: if the addition amount of the citric acid is insufficient or excessive, the carbon coating effect in the product is not increased, and the electrical property is low.

Preferably, the molar ratio of the spherical lithium iron phosphate to the lithium phosphate in the step (2) is 1: (0.05-0.2), stirring for 1-3 h; a large number of experimental studies have found that: the proportion of lithium phosphate is too small, so that the film is too thin, and the performance is not remarkably improved; too large proportion of lithium phosphate will result in too thick film, poor lithium ion deintercalation effect and deviation of electrical properties.

Preferably, in the mixed solution after the ferrous sulfate is added in the step (3), the molar ratio of the ferrous sulfate to the lithium phosphate is 1: (0.96-1.0), stirring for 3-6 h; a large number of experimental studies have found that: the maladjustment of the proportion of the ferrous sulfate and the lithium phosphate can cause that a pure-phase lithium iron phosphate film cannot be synthesized, and the electrical property is deteriorated.

Preferably, the solid-liquid separation and washing in the step (4) are any one of suction filtration, filter pressing and centrifugation;

the drying is any one of spraying and drying.

Preferably, the spray drying temperature is 120-300 ℃; the drying temperature is 40-200 ℃, and the drying time is 2-12 hours.

Preferably, the inert atmosphere in the step (5) refers to nitrogen or argon atmosphere, the sintering temperature is 400-600 ℃, and the high-temperature sintering time is 3-8 h.

The invention also provides the film-coated lithium iron phosphate material obtained by the preparation method, the material is a new lithium iron phosphate film grown on the surface of a rough lithium iron phosphate crystal grain, the thickness of the new lithium iron phosphate film is 10-50nm, and the surface of the film-coated lithium iron phosphate crystal grain becomes smooth and the overall appearance of the film-coated lithium iron phosphate crystal grain tends to be ellipsoidal.

The invention also provides application of the in-situ grown lithium iron phosphate film in a lithium ion battery.

Compared with the prior art, the invention has the following advantages:

1. the spherical lithium iron phosphate which is not coated with the film has a rough surface and is in an irregular spherical shape as a whole, so that the compaction density is low and the cycle performance is poor; the spherical lithium iron phosphate coated with the film has a smooth surface, is in a regular spherical shape as a whole, and has excellent compaction density and cycle performance.

2. The preparation process is simple, and the first discharge specific capacity, the 2000 th cycle discharge specific capacity and the 2000 th cycle capacity retention rate of the material prepared by the method are respectively and averagely improved by 20-23%, 9.2-9.6% and 9.0-10.6% compared with the material prepared by the method before film coating; the product of the invention has obvious advantages of electrical properties and extremely high application and popularization values.

Drawings

Fig. 1 is an SEM image of a finished product obtained in example 1 of the present invention, and shows that the spherical lithium iron phosphate coated with the thin film has a smooth surface and is a regular sphere as a whole.

Fig. 2 is an SEM image of a finished product obtained in comparative example 1 of the present invention, and shows that spherical lithium iron phosphate without a thin film is rough in surface and irregular spherical as a whole.

Detailed Description

In order to better illustrate the present invention and facilitate the understanding of the technical solutions of the present invention, the present invention is further described in detail below. The following examples are merely illustrative of the present invention and do not represent or limit the scope of the claims, which are defined by the claims.

Unless defined otherwise, technical terms used in the following examples have the same meanings as commonly understood by one of ordinary skill in the art to which the present invention belongs. The test reagents used in the following examples, unless otherwise specified, are all conventional biochemical reagents; the experimental methods are conventional methods unless otherwise specified.

The following are typical, but non-limiting, examples of the invention.

Example 1

1) Adding a citric acid solution into lithium phosphate, and completely dissolving to obtain a citric acid diluent of the lithium phosphate, wherein the molar ratio of the lithium phosphate to the citric acid is 1: 1, the concentration of citric acid is 0.1 mol/L;

2) soaking spherical lithium iron phosphate powder to be treated into a citric acid diluent of lithium phosphate, and stirring for 1h until the mixture is uniform, wherein the molar ratio of the spherical lithium iron phosphate to the lithium phosphate is 1: 0.05;

3) continuously stirring while pouring ferrous sulfate, and continuously stirring for 3 hours after the ferrous sulfate is completely added, wherein the molar ratio of the ferrous sulfate to the lithium phosphate is 1: 0.96;

4) carrying out suction filtration on the uniformly stirred solution, and then carrying out spray drying to obtain a precursor, wherein the spray drying temperature is 120 ℃;

5) and sintering the precursor at 400 ℃ under the nitrogen atmosphere, and naturally cooling after 3h to obtain a finished product.

Example 2

This example is carried out except that in step (1) the molar ratio of lithium phosphate to citric acid is 1: 1.5 the other raw materials and operations were the same as in example 1.

Example 3

In this embodiment, except that the molar ratio of spherical lithium iron phosphate to lithium phosphate in step (2) is 1: the raw materials and operations other than 0.2 were the same as in example 1.

Example 4

This example is carried out except that in step (3), the molar ratio of ferrous sulfate to lithium phosphate is 1: the raw materials and operations other than 1.0 were the same as in example 1.

Example 5

1) Adding a citric acid solution into lithium phosphate, and completely dissolving to obtain a citric acid diluent of the lithium phosphate, wherein the molar ratio of the lithium phosphate to the citric acid is 1: 1.2, the concentration of the citric acid is 0.3 mol/L; (ii) a

2) Soaking spherical lithium iron phosphate powder to be treated into a citric acid diluent of lithium phosphate, and stirring for 3 hours until the mixture is uniform, wherein the molar ratio of the spherical lithium iron phosphate to the lithium phosphate is 1: 0.1;

3) continuously stirring while pouring ferrous sulfate, and continuously stirring for 5 hours after the ferrous sulfate is completely added, wherein the molar ratio of the ferrous sulfate to the lithium phosphate is 1: 0.96;

4) carrying out suction filtration on the uniformly stirred solution, and then carrying out spray drying to obtain a precursor, wherein the spray drying temperature is 240 ℃;

5) and sintering the precursor at 500 ℃ under the nitrogen atmosphere, and naturally cooling after 6 hours to obtain a finished product.

Example 6

1) Adding a citric acid solution into lithium phosphate, and completely dissolving to obtain a citric acid diluent of the lithium phosphate, wherein the molar ratio of the lithium phosphate to the citric acid is 1: 1.4, the concentration of the citric acid is 0.2 mol/L; (ii) a

2) Soaking spherical lithium iron phosphate powder to be treated into a citric acid diluent of lithium phosphate, and stirring for 2 hours until the mixture is uniform, wherein the molar ratio of the spherical lithium iron phosphate to the lithium phosphate is 1: 0.2;

3) continuously stirring while pouring ferrous sulfate, and continuously stirring for 5 hours after the ferrous sulfate is completely added, wherein the molar ratio of the ferrous sulfate to the lithium phosphate is 1: 0.98 of;

4) carrying out suction filtration on the uniformly stirred solution, and then drying in an oven at the drying temperature of 200 ℃ for 12 hours to obtain a precursor;

5) and sintering the precursor at 600 ℃ under the argon atmosphere condition, and naturally cooling after 8 hours to obtain a finished product.

Comparative example 1

And sintering the spherical lithium iron phosphate powder to be treated at 600 ℃ under the nitrogen atmosphere, and naturally cooling after 8 hours to obtain a finished product.

The comparative example is a blank control group, and the product is spherical lithium iron phosphate without a coating measure.

Comparative example 2

This example is carried out except that in step (1) the molar ratio of lithium phosphate to citric acid is 1: 1.8, the other raw materials and operations were the same as in example 1.

Comparative example 3

In this embodiment, except that the molar ratio of spherical lithium iron phosphate to lithium phosphate in step (2) is 1: the raw materials and operations other than 0.25 were the same as in example 1.

Comparative example 4

This example is carried out except that in step (3), the molar ratio of ferrous sulfate to lithium phosphate is 1: the raw materials and operations other than 0.8 were the same as in example 1.

Please supplement the experimental data in Table 1

TABLE 1 test results for examples 1-6 and comparative examples 1-4

Item	1C specific first discharge capacity (mAh/g)	Specific discharge capacity (mAh/g) of 2000 th cycle	Capacity retention at 2000 cycles (%)	Average particle size (. mu.m)	Average thickness of film (μm)
						Example 1	155.6	146.73	94.30%	5.57	0.275
Example 2	154.9	145.92	94.20%	6.03	0.405
						Example 3	157.3	148.96	94.70%	6.09	0.485
Example 4	156.2	146.98	94.10%	5.66	0.32
						Example 5	157.1	148.30	94.40%	5.64	0.31
Example 6	155.4	146.08	94.00%	5.36	0.17
						Comparative example 1	142.1	120.96	85.12%	5.02	0
Comparative example 2	139.7	111.20	79.60%	6.74	1.36
						Comparative example 3	141.3	118.72	84.02%	6.84	0.91
Comparative example 4	137.2	118.41	86.30%	5.33	0.155

As can be seen from table 1, the coated spherical lithium iron phosphate materials prepared in embodiments 1 to 6 of the present invention have good electrochemical properties, because the preparation method in the above embodiments synthesizes a uniform lithium iron phosphate precursor film on the surface of the rough spherical lithium iron phosphate crystal through the action of the chelating agent (citric acid), and then performs densification sintering in a high temperature inert atmosphere to grow a lithium iron phosphate film on the surface of the rough spherical lithium iron phosphate crystal in situ. The surface of the spherical lithium iron phosphate crystal coated with the film becomes smooth, the overall appearance of the spherical lithium iron phosphate crystal tends to be regular spherical (as shown in figure 1), and the compaction density and the cyclicity are both improved.

As can be seen from table 1, the first discharge specific capacity, the 2000 th cycle discharge specific capacity and the 2000 th cycle capacity retention ratio of comparative example 1 are lower than those of example 1 because the spherical lithium iron phosphate is not film-coated in comparative example 1, as shown in fig. 2; the specific discharge capacity at the first time, the specific discharge capacity at the 2000 th cycle and the retention rate of the specific discharge capacity at the 2000 th cycle of the material prepared by the method are respectively and averagely improved by 20-23%, 9.2-9.6% and 9.0-10.6% compared with the retention rate before film coating.

In comparative example 2, excessive addition of citric acid results in excessive carbon coating thickness, increased internal resistance, difficult lithium deintercalation, and low electrical properties. In comparative example 3, the ratio of lithium phosphate was too small, resulting in a thin film and insignificant performance improvement. In comparative example 4, the ratio of ferrous sulfate to lithium phosphate is not adjusted, so that a pure-phase lithium iron phosphate film cannot be synthesized, and the electrical property is deteriorated.

From the experimental results of comparative examples 2 to 4, it can be seen that even in the case where the process steps are completely the same, changing any one of the parameters will directly result in a decrease in the electrical properties of the product. Therefore, the process parameters of the invention are strictly required to be implemented in the process of manufacturing the product.

The above description is only for the purpose of illustrating the preferred embodiments of the present invention and is not to be construed as limiting the invention, and any modifications, equivalents, improvements and the like that fall within the spirit and principle of the present invention are intended to be included therein.