CN114314552B

CN114314552B - Lithium iron phosphate, preparation method thereof, lithium ion battery and electric driving device

Info

Publication number: CN114314552B
Application number: CN202111676898.0A
Authority: CN
Inventors: 陈巍; 张科; 褚春波; 张耀
Original assignee: Sunwoda Electric Vehicle Battery Co Ltd
Current assignee: Xinwangda Power Technology Co ltd
Priority date: 2021-12-31
Filing date: 2021-12-31
Publication date: 2023-07-14
Anticipated expiration: 2041-12-31
Also published as: CN114314552A

Abstract

The invention provides lithium iron phosphate, a preparation method thereof, a lithium ion battery and an electric driving device. The method comprises the following steps: under a first inert atmosphere and an acidic condition, carrying out a first hydrothermal synthesis reaction on a soluble lithium source compound, a soluble ferrous iron source compound and a soluble phosphorus source compound and water to obtain a first solid-liquid mixture; forcibly dispersing the first solid-liquid mixture to obtain a dispersion liquid; and (3) under a second inert atmosphere, carrying out a second hydrothermal synthesis reaction on the dispersion liquid to obtain lithium iron phosphate, wherein the temperature of the second hydrothermal synthesis reaction is higher than that of the first hydrothermal synthesis reaction, and supplementing raw materials in the second hydrothermal synthesis reaction process. The lithium iron phosphate material prepared by the method has the advantages of less impurity phase, high purity and uniform particle size distribution, and is favorable for shortening the diffusion path of lithium ions in the material, so that the dynamic performance of the material can be greatly improved. As a positive electrode material for lithium ion batteries, excellent electrochemical performance can be exhibited.

Description

Lithium iron phosphate, preparation method thereof, lithium ion battery and electric driving device

Technical Field

The invention relates to the field of lithium ion battery production, in particular to lithium iron phosphate, a preparation method thereof, a lithium ion battery and an electric driving device.

Background

In order for a pure electric vehicle to actually have competitiveness in the whole vehicle market, the power battery is required to have great advantages in the aspects of quick charge, high endurance mileage, low manufacturing cost and the like. The endurance mileage of a pure electric vehicle is greatly limited by the energy of a single lithium ion battery, and depends on the chemical system of an electrode of the pure electric vehicle and the process parameters of battery charging and discharging, such as the selection of materials of a positive electrode and a negative electrode, the multiplying power charging and discharging current density and the upper limit cut-off voltage. In commercial applications, the negative electrode material is typically designed to the capacity achieved by the positive electrode material, which typically determines the capacity of the overall battery, plus a suitable safety buffer. The mileage limitations of a pure power battery are therefore mainly ascribed to "positive limitation".

Among the numerous lithium ion battery cathode materials, the olivine-type lithium iron phosphate cathode material with an orthogonal structure is greatly emphasized and is widely researched and rapidly developed due to wide raw material sources, low cost and no environmental pollution. The current lithium iron phosphate anode material has the following advantages: 1. the safety is excellent, and the thermal stability of the material is good; 2. the environment is friendly, and no heavy metal elements harmful to human bodies are contained; 3. the actual capacity exceeds 150mAh/g;4. the working voltage is moderate, and is 3.45V relative to the metal lithium; 5. the voltage platform has good characteristics and is very stable; 6. stable structure, long cycle life, etc. However, the main problems of this material are also low electron conductivity and low lithium ion diffusion rate, resulting in its lower room temperature discharge capacity and poor cycle performance.

Currently, methods for preparing lithium iron phosphate materials mainly include a solid-phase method and a hydrothermal method. The solid phase method has simple synthesis equipment and preparation process, easy control of preparation adjustment and earliest realization of industrialization. The lithium iron phosphate material synthesized by the solid phase method has higher synthesis purity, good crystallinity and small product particle size, but the solid phase method mechanically grinds the raw materials, so that equipment is generally inevitably worn, fe impurities are inevitably introduced in the grinding process, and the product phase is uneven. The hydrothermal method is to use aqueous solution as reaction medium in a high-pressure reaction kettle, and to create a high-temperature and high-pressure reaction environment by heating the reaction container, so that the substances which are generally difficult or insoluble are dissolved and recrystallized to obtain a uniform phase. Currently, liFePO is prepared by hydrothermal method ₄ The reaction temperature of the catalyst is reported from 120 ℃ to 390 ℃, and although the lower reaction temperature can reduce the energy consumption of material preparation, the hydrothermal product is usually accompanied by hetero-phase lithium phosphate, ferrous phosphate and the like, the temperature is lower, li/Fe inversion defects exist in the product, and the lattice constant is unstable; the synthesis temperature of more than 200 ℃ has extremely high requirements on the safety performance of equipment, and the cost naturally rises. The particle size distribution of the product particles also varies greatly from one hydrothermal reaction temperature to another.

The current patents on preparing lithium iron phosphate anode materials by a hydrothermal method are as follows:

the prior literature provides a lithium iron phosphate and a preparation method and application thereof. The method prepares lithium iron phosphate with lower impurity phase and smaller particle size by adjusting the PH of twice hydrothermal reactions and twice rapid temperature rise, and shows excellent electrochemical performance.

Another prior document provides a preparation method of lithium iron phosphate material, which comprises the steps of adding alcohols as a solvent for reaction, inhibiting tertiary ionization of ferric phosphate in the alcohols solution, reducing the progress of side reaction, ensuring that the polarity of the alcohols solution is smaller than that of water, ensuring that solid particles have better dispersibility and no agglomeration phenomenon, and greatly improving the performance of the material, wherein the prepared lithium iron phosphate has higher specific capacity and better capacity retention rate.

Still another prior document provides a synthesis method for preparing a lithium iron phosphate precursor with high cost performance by controlling crystallization, which comprises the steps of preparing a lithium salt solution, a ferrous salt solution and a phosphate solution into a mixed solution, adjusting the pH of the solution, preparing the lithium iron phosphate precursor at a low temperature, and obtaining a byproduct lithium phosphate.

The existing hydrothermal reaction has the following defects:

(1) The low hydrothermal reaction temperature leads to incomplete hydrothermal reaction, and lithium phosphate, ferrous phosphate and other impurities exist in the product of the hydrothermal reaction process, so that the electrochemical performance of the material is seriously affected.

(2) The product of the hydrothermal reaction process has Li/Fe inversion defect, which is unfavorable for rapid deintercalation of lithium ions from the structure.

(3) The particle size distribution of the product particles in the hydrothermal process is not uniform, which is not beneficial to improving the compaction density of the material.

In order to solve the above problems, it is necessary to provide a new preparation method of lithium iron phosphate materials.

Disclosure of Invention

The invention mainly aims to provide lithium iron phosphate, a preparation method thereof, a lithium ion battery and an electric driving device, so as to solve the problems that the existing lithium iron phosphate positive electrode material prepared by hydrothermal synthesis has more impurity phases, has Li/Fe inversion defects, and has uneven particle size distribution, so that the electrochemical performance is poor and the compaction density is low.

In order to achieve the above object, according to an aspect of the present invention, there is provided a method for preparing lithium iron phosphate, comprising: under a first inert atmosphere and an acidic condition, carrying out a first hydrothermal synthesis reaction on a soluble lithium source compound, a soluble ferrous iron source compound and a soluble phosphorus source compound and water to obtain a first solid-liquid mixture; and (3) forcibly dispersing the first solid-liquid mixture to obtain a dispersion liquid, and carrying out a second hydrothermal synthesis reaction on the dispersion liquid in a second inert atmosphere to obtain the lithium iron phosphate, wherein the temperature of the second hydrothermal synthesis reaction is higher than that of the first hydrothermal synthesis reaction, and meanwhile, raw materials are supplemented in the second hydrothermal synthesis reaction process, and comprise a soluble lithium source compound, a soluble ferrous iron source compound and a soluble phosphorus source compound.

Further, the temperature of the first hydrothermal synthesis reaction is 110-120 ℃, the reaction time is 2-5 h, and the pH of the reaction system is 4-6.

Further, the forced dispersion process includes: and (3) stirring the first solid-liquid mixture at a high speed by adopting a mechanical stirring device, wherein the stirring speed is 40-60 r/min, and the stirring time is 0.5-2 h.

Further, the reaction temperature of the second hydrothermal synthesis reaction is 140-200 ℃, the reaction time is 0.5-1 h, and the heating rate is-10 ℃/min.

Further, in the first hydrothermal synthesis reaction, the molar ratio of the soluble lithium source compound to the soluble ferrous iron source compound to the soluble phosphorus source compound is 3 (0.9-1.5): 0.9-1.5; in the raw material supplementing step, the mole ratio of the soluble lithium source compound to the soluble ferrous source compound to the soluble phosphorus source compound is (0-1.5): 1-2.5, and in the raw material supplementing step, the solid content in the whole reaction liquid is controlled to be 20% -50%.

Further, the soluble lithium source compound is selected from one or more of the group consisting of lithium hydroxide, lithium chloride, lithium carbonate, lithium acetate, lithium fluoride, lithium bromide, lithium nitrate, lithium oxalate, and lithium trifluoromethane sulfonate; the soluble ferrous source compound is selected from one or more of ferrous sulfate, ferrous chloride, ferrous acetate, ferrous nitrate and ferrous oxalate; the soluble phosphorus source compound is selected from one or more of the group consisting of phosphoric acid, monoammonium phosphate, diammonium phosphate, and lithium dihydrogen phosphate.

The second aspect of the present application also provides a method for preparing lithium iron phosphate, the method for preparing lithium iron phosphate comprising: preparing a solid-phase product through the second hydrothermal synthesis reaction; and calcining the solid-phase product and a carbon coating agent to obtain the carbon-coated lithium iron phosphate.

The third aspect of the application also provides lithium iron phosphate, which is prepared by the preparation method provided by the application.

The fourth aspect of the present application also provides a lithium ion battery comprising a positive electrode material comprising the lithium iron phosphate provided herein.

The fifth aspect of the present application also provides an electric drive device, which includes the lithium ion battery provided by the present application.

By applying the technical scheme of the invention, the lithium iron phosphate crystal nucleus can be formed through the first hydrothermal synthesis reaction, the granularity of the lithium iron phosphate crystal nucleus can be thinned through the forced dispersion process, and meanwhile, the dispersibility and uniformity of the lithium iron phosphate crystal nucleus are improved. The temperature of the second hydrothermal synthesis reaction is higher than that of the first hydrothermal synthesis reaction, so that lithium phosphate and a ferrous phosphate hetero-phase in lithium iron phosphate crystal nucleus are promoted to generate a lithium iron phosphate crystalline phase, the hetero-phase in a finally obtained product is reduced, and a high-purity-phase lithium iron phosphate product is obtained. Meanwhile, in the process of growing the lithium iron phosphate crystal, the dislocation phenomenon of iron can be reduced by increasing the temperature of the hydrothermal reaction, and a stable lithium iron phosphate crystal product is obtained. In addition, in the second hydrothermal synthesis reaction process, the temperature rise and the raw material supplementation are favorable for regulating and controlling the nucleation distribution mode and the growth rate of the crystal nucleus, so that a product with uniform particle distribution is obtained, and the compaction density of the anode material is further improved. On the basis, the lithium iron phosphate material prepared by the method has the advantages of less impurity phase, high purity and uniform particle size distribution, and is favorable for shortening the diffusion path of lithium ions in the material, so that the dynamic performance of the material can be greatly improved. As a positive electrode material for lithium ion batteries, excellent electrochemical performance can be exhibited.

Drawings

The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the invention. In the drawings:

FIG. 1 is an XRD pattern of lithium iron phosphate materials prepared in examples 1 to 3;

fig. 2 is a graph showing the comparison of particle size distribution of lithium iron phosphate materials prepared in examples 1 and 4 and comparative examples 1 and 2.

Detailed Description

It should be noted that, in the case of no conflict, the embodiments and features in the embodiments may be combined with each other. The present invention will be described in detail with reference to examples.

As described in the background art, the existing lithium iron phosphate positive electrode material prepared by hydrothermal synthesis has many mixed phases, has Li/Fe inversion defects and uneven particle size distribution, so that the problem of poor electrochemical performance exists. In order to solve the technical problems, the application provides a preparation method of lithium iron phosphate, which comprises the following steps: under a first inert atmosphere and an acidic condition, carrying out a first hydrothermal synthesis reaction on a soluble lithium source compound, a soluble ferrous iron source compound and a soluble phosphorus source compound and water to obtain a first solid-liquid mixture; forcibly dispersing the first solid-liquid mixture to obtain a dispersion liquid; and (3) carrying out a second hydrothermal synthesis reaction on the dispersion liquid in a second inert atmosphere to obtain lithium iron phosphate, wherein the temperature of the second hydrothermal synthesis reaction is higher than that of the first hydrothermal synthesis reaction, and meanwhile, raw material supplementation is carried out in the second hydrothermal synthesis reaction process, wherein the raw materials comprise a soluble lithium source compound, a soluble ferrous iron source compound and a soluble phosphorus source compound.

Lithium iron phosphate nuclei can be formed through the first hydrothermal synthesis reaction, and particle sizes of the lithium iron phosphate nuclei can be refined through the forced dispersion process, and meanwhile, dispersibility and uniformity of the lithium iron phosphate nuclei are improved. The temperature of the second hydrothermal synthesis reaction is higher than that of the first hydrothermal synthesis reaction, so that lithium phosphate and a ferrous phosphate hetero-phase in lithium iron phosphate crystal nucleus are promoted to generate a lithium iron phosphate crystalline phase, the hetero-phase in a finally obtained product is reduced, and a high-purity-phase lithium iron phosphate product is obtained. Meanwhile, in the process of growing the lithium iron phosphate crystal, the dislocation phenomenon of iron can be reduced by increasing the temperature of the hydrothermal synthesis reaction, and a stable lithium iron phosphate crystal product is obtained. In addition, in the second hydrothermal synthesis reaction process, the temperature rise and the raw material supplementation are favorable for regulating and controlling the nucleation distribution mode and the growth rate of the crystal nucleus, so that a product with uniform particle distribution is obtained, and the compaction density of the anode material is further improved. On the basis, the lithium iron phosphate material prepared by the method has the advantages of less impurity phase, high purity and uniform particle size distribution, and is favorable for shortening the diffusion path of lithium ions in the material, so that the dynamic performance of the material can be greatly improved. As a positive electrode material for lithium ion batteries, excellent electrochemical performance can be exhibited.

The temperature and reaction time of the first hydrothermal synthesis may be in the range used in the art. In a preferred embodiment, the temperature of the first hydrothermal synthesis reaction is 110-120 ℃, the reaction time is 2-5 h, and the pH of the reaction system is 4-6. The temperature and the reaction time of the first hydrothermal synthesis reaction are limited in the above ranges, so that the reaction rate of the first hydrothermal synthesis reaction is improved, and the reaction is favorable for inhibiting the hydrolysis of ferrous ions in an acidic environment, so that the impurity phase is reduced, and the purity of the lithium iron phosphate is improved.

In a preferred embodiment, the forced dispersion process comprises: and (3) stirring the first solid-liquid mixture at a high speed by adopting a mechanical stirring device, wherein the stirring speed is 40-60 r/min, and the stirring time is 0.5-2 h.

The temperature of the second hydrothermal synthesis reaction is higher than that of the first hydrothermal synthesis reaction, so that the impurity phases are reduced, grains are refined, and the distribution uniformity of the grains is improved. In a preferred embodiment, the reaction temperature of the second hydrothermal synthesis reaction is 140-200 ℃, the reaction time is 0.5-1 h, and the temperature rising rate is-10 ℃/min. The temperature, reaction time and heating rate of the second hydrothermal synthesis reaction include, but are not limited to, the above ranges, and limiting the above ranges is beneficial to further improving the purity of the prepared lithium iron phosphate, and further beneficial to further improving the dynamic performance in the application process.

The temperature rising process in the second hydrothermal synthesis reaction may be a process of always rising the temperature, or rising and falling, or repeatedly rising and falling the temperature. For example, (a) a hydrothermal synthesis reaction is performed by raising the temperature from 140 ℃ to a higher temperature; (b) Heating from 140 ℃ to a certain higher temperature, preserving heat for a period of time, and then continuously heating to the higher temperature to carry out hydrothermal synthesis reaction, wherein the process can be repeated for several times, and the heating range, the heating rate and the heating time of each time can be the same or different; (c) Heating from 140 ℃ to 200 ℃, preserving heat for a period of time, and cooling to a certain temperature to perform hydrothermal synthesis reaction; (d) After the temperature is increased to 200 ℃ from 140 ℃, the temperature is kept for a period of time after the temperature is reduced to a certain temperature, the temperature is continuously reduced to a lower temperature for carrying out hydrothermal synthesis reaction, the process can be repeated for several times, the temperature is reduced and the temperature is kept, and the temperature reduction amplitude, the temperature speed and the temperature keeping time can be the same or different.

In the second hydrothermal synthesis reaction, the time points of the supplementary raw materials include, but are not limited to: (a) replenishing raw materials during the temperature rising process; (b) replenishing raw materials in the heat preservation process; (c) supplementing the raw materials in the cooling process. The supplementary material may be continuous supplementary material or sectional supplementary material. The raw material replenishment can be one-time replenishment or multiple replenishment, and the amount of the raw material can be the same or different for each replenishment, and the rate of replenishment of the raw material can be the same or different. In order to further improve the uniformity of the particle size of the lithium iron phosphate and thus the electrochemical performance in the subsequent application process thereof, preferably, the second hydrothermal synthesis reaction process includes the following schemes: (1) After the first hydrothermal synthesis reaction is finished, adopting a heating rate of 10 ℃/min to quickly heat to 140 ℃ in the first stage hydrothermal synthesis reaction process, preserving heat for 0.5-1 h, supplementing raw materials, and keeping the solid content in the hydrothermal synthesis reaction solution to be 20-50%; rapidly heating to 200 ℃ at 10 ℃/min, and preserving heat for 1-2h to perform a second stage hydrothermal synthesis reaction; then cooling to 175 ℃ at a cooling rate of 10 ℃/min, and preserving heat for 1h; finally, the temperature is reduced to 140 ℃ at a temperature reduction rate of 5 ℃/min, and the temperature is kept for 2 hours. (2) After the first hydro-thermal synthesis reaction is finished, raw material supplementation is carried out, and the solid content in the hydro-thermal synthesis reaction solution is kept to be 20% -50%; quickly heating to 140 ℃ at 10 ℃/min, and preserving heat for 0.5-1 h; rapidly heating to 175 ℃ at 10 ℃/min, and preserving heat for 1h; rapidly heating to 200 ℃ at 10 ℃/min, and preserving heat for 1h; naturally cooling; (3) Supplementing raw materials after the first hydrothermal synthesis reaction is finished, keeping the solid content in the hydrothermal synthesis reaction solution to be 20-50%, adopting the heating rate of 10 ℃/min to quickly heat up to 140 ℃, preserving heat for 0.5-1 h, and adopting the heating rate of 10 ℃/min to quickly heat up to 200 ℃ and preserving heat for 1h; cooling to 175 ℃ at a cooling rate of 10 ℃/min, and preserving heat for 1h; cooling to 140 ℃ at a cooling rate of 5 ℃/min, and preserving heat for 2h.

In a preferred embodiment, the ratio of the number of moles of the soluble lithium source compound, the soluble ferrous source compound and the soluble phosphorus source compound in the first hydrothermal synthesis reaction is 3 (0.9 to 1.5): 0.9 to 1.5. The molar ratio of the lithium source compound, the iron source compound and the phosphorus source compound comprises but is not limited to the above range, and the ratio is limited to the above range, so that the generation of impurity phases is reduced, the growth of crystal grains is better controlled, and the energy density and the specific discharge capacity of the lithium iron phosphate material in the application process are improved.

In a preferred embodiment, the ratio of the number of moles of the soluble lithium source compound, the soluble ferrous source compound and the soluble phosphorus source compound in the raw material supplementing step is (0 to 1.5): (1 to 2.5), and the solid content in the whole reaction liquid in the raw material supplementing step is controlled to be 20 to 50%. And in the raw material supplementing process, the mole ratio of the soluble lithium source compound, the soluble ferrous source compound and the soluble phosphorus source compound and the supplementing amount of the raw material liquid are limited in the range, so that the uniformity of particle size distribution is improved, and meanwhile, the agglomeration of particles is inhibited, thereby being beneficial to shortening the diffusion path of lithium ions in the material and improving the dynamic performance of the material.

The iron source compound, phosphorus source compound and lithium source compound used in the present application may be of the types commonly used in the art. In a preferred embodiment, the soluble lithium source compound includes, but is not limited to, one or more of the group consisting of lithium hydroxide, lithium chloride, lithium carbonate, lithium acetate, lithium fluoride, lithium bromide, lithium nitrate, lithium oxalate, and lithium trifluoromethane sulfonate; soluble ferrous source compounds include, but are not limited to, one or more of the group consisting of ferrous sulfate, ferrous chloride, ferrous acetate, ferrous nitrate, and ferrous oxalate; the soluble phosphorus source compound is selected from one or more of the group consisting of phosphoric acid, monoammonium phosphate, diammonium phosphate, and lithium dihydrogen phosphate.

In order to further improve the conductivity of the lithium iron phosphate, the second aspect of the present application also provides another preparation method of the lithium iron phosphate, which includes: under a first inert atmosphere and an acidic condition, carrying out a first hydrothermal synthesis reaction on a soluble lithium source compound, a soluble ferrous iron source compound and a soluble lithium source compound to obtain a first solid-liquid mixture; forcibly dispersing the first solid-liquid mixture to obtain a dispersion liquid; under a second inert atmosphere, carrying out a second hydrothermal synthesis reaction on the dispersion liquid to obtain a solid-phase product; and calcining the solid-phase product and a carbon coating agent to obtain the carbon-coated lithium iron phosphate.

The third aspect of the present application also provides a lithium iron phosphate, which is prepared by the preparation method provided by the present application. The lithium iron phosphate material prepared by the method has the advantages of less impurity phase, high purity and uniform particle size distribution, and is favorable for shortening the diffusion path of lithium ions in the material, so that the dynamic performance of the material can be greatly improved. Accordingly, as a positive electrode material for lithium ion batteries, excellent electrochemical performance can also be exhibited.

The fourth aspect of the present application also provides an electric drive device comprising the lithium ion battery provided by the present application. The lithium ion battery containing the lithium iron phosphate is used as an energy module of the electric driving device, so that the advantages of the lithium ion battery in energy storage and endurance can be greatly improved.

The present application is described in further detail below in conjunction with specific embodiments, which should not be construed as limiting the scope of the claims.

Example 1

A method for preparing lithium iron phosphate, comprising:

(1) First hydrothermal synthesis reaction:

sequentially dissolving a certain amount of lithium hydroxide, ferrous sulfate and phosphoric acid in deionized water according to the molar ratio of 3:1:1 to form a solution A, mechanically stirring at room temperature, controlling the pH value of the solution A to be 4-6, and transferring the solution A into a high-pressure hydrothermal reaction kettle. Carrying out a first hydrothermal synthesis reaction at the reaction temperature of 120 ℃ for 3 hours, and naturally cooling to room temperature after the hydrothermal synthesis reaction is finished to obtain a hydrothermal synthesis reaction solution B.

(2) Forced dispersion: solution B was mechanically stirred for 2 hours at high intensity until the precipitate in solution B was well dispersed, with a stirring rate of 40r/min.

(3) The second hydrothermal synthesis reaction is a multistage temperature changing process, and is specifically as follows:

transferring the solution B into a high-pressure reaction kettle, quickly heating to 140 ℃, and carrying out the first-stage hydrothermal synthesis reaction at a heating rate of 10 ℃/min for 1h.

After the first-stage hydrothermal synthesis reaction is finished, a certain amount of lithium hydroxide, ferrous sulfate and phosphoric acid are sequentially added into a hydrothermal reaction kettle according to the corresponding molar ratio of 1:2:2, the solid content in a hydrothermal synthesis reaction solution is kept to be 35%, the temperature of the hydrothermal reaction kettle is quickly increased to 200 ℃ (the heating rate is 10 ℃/min), and the second-stage hydrothermal synthesis reaction is carried out for 1h. After the second-stage hydrothermal synthesis reaction is finished, the temperature of the hydrothermal reaction kettle is reduced to 175 ℃ (the temperature reduction rate is 10 ℃/min), the reaction is kept for 1h, and finally the temperature of the hydrothermal synthesis reaction is reduced to 140 ℃ (the temperature reduction rate is 5 ℃/min), and the third-stage hydrothermal synthesis reaction is carried out for 2h. And after the hydrothermal synthesis reaction in the third stage is finished, washing, filtering, drying and the like are carried out on the obtained hydrothermal product, so that the high-purity-phase high-compaction-density lithium iron phosphate material is obtained.

Example 2

The difference from example 1 is that: in the second stage hydrothermal synthesis reaction in the step (3), the fed raw materials do not contain lithium hydroxide, and are added in a ratio of 1:1 of mole number of phosphoric acid to mole number of ferrous sulfate. The other steps were the same as in example 1.

Example 3

The difference from example 1 is that: in the second stage hydrothermal synthesis reaction in the step (3), the fed raw materials are added in a molar ratio of 1:1:1 of lithium hydroxide, ferrous sulfate and phosphoric acid. The other steps were the same as in example 1.

Example 4

The difference from example 1 is that: the reaction temperature of the second stage hydrothermal synthesis reaction in the step (3) is 175 ℃, the reaction time is 2 hours, and the third stage hydrothermal synthesis reaction is not performed. The other steps were the same as in example 1.

Example 5

The difference from example 1 is that: the reaction temperature of the hydrothermal synthesis reaction in the second stage in the step (3) is 160 ℃, and the reaction time is 1h. After the second stage hydrothermal synthesis reaction is finished, the temperature of a product system is directly reduced to 140 ℃ from 160 ℃ to carry out the third stage hydrothermal synthesis reaction. The other steps were the same as in example 1.

Example 6

The difference from example 1 is that: in the raw material supplementing process, the mole ratio of the soluble lithium source compound to the soluble ferrous source compound to the soluble phosphorus source compound is 0:2.5:2.5. The other steps were the same as in example 1.

Example 7

The difference from example 1 is that: in the raw material supplementing process, the mole ratio of the soluble lithium source compound to the soluble ferrous source compound to the soluble phosphorus source compound is 1.5:1:1. The other steps were the same as in example 1.

Example 8

The difference from example 1 is that: in the raw material supplementing process, the mole ratio of the soluble lithium source compound to the soluble ferrous source compound to the soluble phosphorus source compound is 0:3:3. The other steps were the same as in example 1.

Example 9

The difference from example 1 is that: the process of the second hydro-thermal synthesis reaction is that the first hydro-thermal synthesis reaction is finished, raw material supplementation is carried out, and the solid content in the hydro-thermal synthesis reaction solution is kept to be 20% -50%; quickly heating to 140 ℃ at 10 ℃/min, and preserving heat for 0.5-1 h; rapidly heating to 175 ℃ at 10 ℃/min, and preserving heat for 1h; rapidly heating to 200 ℃ at 10 ℃/min, and preserving heat for 1h; naturally cooling. The other steps were the same as in example 1.

Example 10

The difference from example 1 is that: the process of the second hydrothermal synthesis reaction is to supplement raw materials after the first hydrothermal synthesis reaction is finished, keep the solid content in the hydrothermal synthesis reaction solution to be 20-50%, rapidly heat up to 140 ℃ at a heating rate of 10 ℃/min, keep the temperature for 0.5-1 h, rapidly heat up to 200 ℃ at a heating rate of 10 ℃/min, and keep the temperature for 1h; cooling to 175 ℃ at a cooling rate of 10 ℃/min, and preserving heat for 1h; cooling to 140 ℃ at a cooling rate of 5 ℃/min, and preserving heat for 2h. The other steps were the same as in example 1.

Comparative example 1

The difference from example 1 is that: the second hydrothermal synthesis reaction temperature in step (3) was constant at 140℃and the other steps were the same as in example 1.

Comparative example 2

The difference from example 1 is that: the procedure of example 1 was repeated except that the material replenishment was not performed in the second hydrothermal synthesis reaction in step (3).

Performance test:

FIG. 1 is an XRD pattern of lithium iron phosphate materials prepared in examples 1 to 3; fig. 2 is a graph showing the comparison of particle size distribution of lithium iron phosphate materials prepared in examples 1 and 4 and comparative examples 1 and 2.

The materials prepared in examples 1 to 10 and comparative examples 1 and 2 were tested for electrochemical performance using a CR2032 button cell, one of which was extremely highThe core-shell carbon-coated nanoscale lithium iron phosphate anode material, the acetylene black and polyvinylidene fluoride mixture (weight ratio of 97:1.5:1.5) prepared in the embodiment of the application is a metal lithium sheet, and the electrolyte is LiPF of 1mol/L ₆ Dissolved in a solvent of EC/DMC/EMC (volume ratio 1:1:1). The constant current charge-discharge voltage range is 2.0-3.7V.

1) Gram capacity for first discharge and coulombic efficiency test for first time:

after the button cell is assembled, (1) charging: constant current charging is carried out to 3.7V at 0.1C, and the specific charge capacity is recorded as Q1; (2) discharging: constant current of 0.1C is discharged to 2V, and the specific discharge capacity is recorded as Q2; first coulombic efficiency is abbreviated as ICE, ice=q2/Q1.

2) And (3) testing the cycle performance:

(1) charging: constant current charging is carried out on 1C until the voltage reaches 3.7V, and the interval is 10min; (2) discharging: constant current of 1C is put to 2V for 10min; (3) repeating (1) and (2) 3000 circles; the capacity retention ratio is abbreviated CR.

3) And (3) multiplying power performance test:

(1) charging 0.1C constant current to 3.7V, and discharging 0.1C constant current to 2V after 10min interval; (2) repeating the process (1) for 10 circles; (3) the current density in "(1), (2)" was raised to 1C, 3C, 5C and 10C, wherein the discharge capacities corresponding to 1C, 3C, 5C and 10C were Q3, Q4, Q5 and Q6, respectively.

The performance test of the positive electrode materials of the lithium ion batteries of the examples and the comparative examples is shown in table 1.

TABLE 1

From the data in table 1, it can be seen from the data in the comparative examples and comparative examples that the distribution mode of the nucleation nuclei of lithium iron phosphate and the growth rate of the nuclei can be controlled by the multistage hydrothermal synthesis reaction performed at different temperature gradients and the replenishment of the raw materials in the hydrothermal process, so that the size distribution of the grown lithium iron phosphate particles is more uniform, the compaction density of the materials is improved, in addition, the Li/Fe inversion defect of the lithium iron phosphate crystals can be reduced at the higher hydrothermal synthesis reaction temperature, and the rapid deintercalation of lithium ions from crystal lattices is facilitated, thereby remarkably improving the gram capacity, the cycle stability and the rate capability of the electrode material.

From examples 1, 2, 3 and 6 to 8, the proportions of the raw materials of the components to be supplemented in the subsequent hydrothermal process affect the purity of the product, and the poor electrochemical properties of the electrodes of examples 2 and 3 are mainly attributed to the partial impurity phases contained in the lithium iron phosphate product. Example 8 the very poor electrochemical performance of the electrode illustrates that the impurity phase in the product lithium iron phosphate has become more pronounced.

From examples 1 and 4, the poor electrochemical performance of example 4 can be explained by its uneven particle size distribution of lithium iron phosphate.

From examples 1 and 5, the relatively poor cycle performance and rate performance of example 5 is attributed to the possible presence of more Li/Fe inversion defects in its lithium iron phosphate crystals.

From examples 1 to 10, the selection of the preferred temperature change mode in the second hydrothermal synthesis process is beneficial to further improving the electrochemical comprehensive performance of the lithium iron phosphate.

From example 1 and comparative examples 1 and 2, it is apparent that the electrochemical properties of the lithium iron phosphate material can be improved by stepwise adjusting the temperature of the hydrothermal synthesis reaction, which suggests that the rate of growth of lithium iron phosphate nuclei can be controlled by adjusting the temperature of the hydrothermal synthesis reaction. The supplementary raw materials are carried out in the hydrothermal synthesis reaction process, so that the distribution of the lithium iron phosphate nucleation crystal nucleus is more uniform. By controlling the distribution mode of the crystal nucleus and the growth rate of the crystal nucleus, the size distribution of lithium iron phosphate particles is more uniform, and the compaction density of the material is improved. The lithium iron phosphate prepared in comparative example 1 has a discharge capacity comparable to that of the product prepared in the present application at a low current, but its discharge capacity is significantly reduced under a high current (10C) condition. Comparative example 2 in the preparation of lithium iron phosphate, the replenishment of the raw material was not performed, resulting in an excessive amount of the lithium source compound; although the obtained lithium iron phosphate has better electrochemical performance in the application process, the lithium iron phosphate has higher synthesis cost because the lithium source compound is not fully utilized.

It should be noted that the terms "first," "second," and the like in the description and in the claims of the present application are used for distinguishing between similar objects and not necessarily for describing a particular sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances such that the embodiments of the application described herein are, for example, capable of operation in sequences other than those described herein.

The above description is only of the preferred embodiments of the present invention and is not intended to limit the present invention, but various modifications and variations can be made to the present invention by those skilled in the art. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. The preparation method of the lithium iron phosphate is characterized by comprising the following steps of:

under a first inert atmosphere and an acidic condition, carrying out a first hydrothermal synthesis reaction on a soluble lithium source compound, a soluble ferrous iron source compound and a soluble phosphorus source compound and water to obtain a first solid-liquid mixture;

forcibly dispersing the first solid-liquid mixture to obtain a dispersion liquid;

carrying out a second hydrothermal synthesis reaction on the dispersion liquid in a second inert atmosphere to obtain the lithium iron phosphate, wherein the temperature of the second hydrothermal synthesis reaction is higher than that of the first hydrothermal synthesis reaction, and raw material supplementation is carried out in the second hydrothermal synthesis reaction process, and the raw materials comprise the soluble lithium source compound, the soluble ferrous iron source compound and the soluble phosphorus source compound;

the temperature of the first hydrothermal synthesis reaction is 110-120 ℃, the reaction temperature of the second hydrothermal synthesis reaction is 140-200 ℃, and the mole number ratio of the soluble lithium source compound, the soluble ferrous iron source compound and the soluble phosphorus source compound in the first hydrothermal synthesis reaction is 3 (0.9-1.5): 0.9-1.5;

in the raw material supplementing step, the mole ratio of the soluble lithium source compound, the soluble ferrous iron source compound and the soluble phosphorus source compound is (0-1.5): 1-2.5.

2. The method for producing lithium iron phosphate according to claim 1, wherein the reaction time of the first hydrothermal synthesis reaction is 2 to 5 hours, and the pH of the reaction system is 4 to 6.

3. The method for preparing lithium iron phosphate according to claim 1, wherein the forced dispersion process comprises: and (3) stirring the first solid-liquid mixture at a high speed by adopting a mechanical stirring device, wherein the stirring speed is 40-60 r/min, and the stirring time is 0.5-2 h.

4. The method for preparing lithium iron phosphate according to claim 1, wherein the reaction time of the second hydrothermal synthesis reaction is 0.5-1 h, and the heating rate is-10 ℃/min.

5. The method for producing lithium iron phosphate according to claim 1, wherein in the raw material replenishing step, the solid content in the whole reaction liquid is controlled to be 20% to 50%.

6. The method for producing lithium iron phosphate according to claim 1, wherein,

the soluble lithium source compound is selected from one or more of the group consisting of lithium hydroxide, lithium chloride, lithium carbonate, lithium acetate, lithium fluoride, lithium bromide, lithium nitrate, lithium oxalate and lithium trifluoromethane sulfonate;

the soluble ferrous source compound is selected from one or more of ferrous sulfate, ferrous chloride, ferrous acetate, ferrous nitrate and ferrous oxalate;

the soluble phosphorus source compound is selected from one or more of the group consisting of phosphoric acid, monoammonium phosphate, diammonium phosphate, and lithium dihydrogen phosphate.

7. The preparation method of the lithium iron phosphate is characterized by comprising the following steps of:

preparing a solid phase product by a second hydrothermal synthesis reaction according to any one of claims 1 to 6;

and calcining the solid-phase product and a carbon coating agent to obtain carbon-coated lithium iron phosphate.

8. Lithium iron phosphate, characterized in that it is produced by the production method according to any one of claims 1 to 7.

9. A lithium ion battery comprising a positive electrode material, wherein the positive electrode material comprises the lithium iron phosphate of claim 8.

10. An electric drive, characterized in that it comprises the lithium ion battery of claim 9.