CN114314552B - Lithium iron phosphate, preparation method thereof, lithium ion battery and electric drive device - Google Patents

Abstract

The invention provides a lithium iron phosphate, a preparation method thereof, a lithium ion battery and an electric drive device. The method includes: under the first inert atmosphere and acidic conditions, the soluble lithium source compound, the soluble ferrous iron source compound and the soluble phosphorus source compound are subjected to a first hydrothermal synthesis reaction with water to obtain a first solid-liquid mixture; A solid-liquid mixture is forcibly dispersed to obtain a dispersion; under a second inert atmosphere, the dispersion is subjected to a second hydrothermal synthesis reaction to obtain lithium iron phosphate, and the temperature of the second hydrothermal synthesis reaction is higher than that of the first hydrothermal synthesis The temperature of the reaction is supplemented with raw materials during the second hydrothermal synthesis reaction process. The lithium iron phosphate material prepared by the above method has few impurity phases, high purity, and uniform particle size distribution, which is conducive to shortening the diffusion path of lithium ions in the material, thereby greatly improving the dynamic performance of the material. As a cathode material for lithium-ion batteries, it can exhibit excellent electrochemical performance.

Description

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

technical field

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

Background technique

If pure electric vehicles want to be truly competitive in the entire automotive market, this requires that the power battery must have great advantages in fast charging, high cruising range, and low manufacturing costs. The cruising range of pure electric vehicles is largely limited by the energy of a single lithium-ion battery, which in turn depends on the chemical system of its electrodes and the process parameters of battery charge and discharge, such as: the selection of positive and negative electrode materials, the rate of charge and discharge current density and upper cut-off voltage. In commercial applications, the negative electrode material is usually designed according to the capacity achieved by the positive electrode material, plus a suitable safety buffer. Usually, the positive electrode material determines the capacity of the entire battery. Therefore, the mileage limitation of pure power batteries is mainly attributed to "positive electrode limitation".

Among many lithium-ion battery cathode materials, due to its wide source of raw materials, low price and no environmental pollution, the orthogonal structure olivine-type lithium iron phosphate cathode material has received great attention and has caused extensive research and rapid development. The current lithium iron phosphate cathode material has the following advantages: 1. Excellent safety, good thermal stability of the material; 2. Environmentally friendly, does not contain any heavy metal elements harmful to human body; 3. High reversible capacity, the actual capacity has exceeded 150mAh /g; 4. The working voltage is moderate, 3.45V relative to metal lithium; 5. The voltage platform has good characteristics and is very stable; 6. The structure is stable and the cycle life is long. However, the main problems of this material are the low electronic conductivity and low lithium ion diffusion rate, resulting in its low room temperature discharge capacity and poor cycle performance.

At present, the methods for preparing lithium iron phosphate materials mainly include solid-phase method and hydrothermal method. The solid-phase synthesis method has simple equipment and preparation process, easy control of preparation adjustment, and the earliest industrialization. The lithium iron phosphate material synthesized by solid phase has high synthesis purity, good crystallinity, and small product particle size, but the raw material is mechanically ground by the solid phase method, which usually inevitably wears out the equipment and inevitably causes damage during the grinding process. Fe impurities are introduced, and the phase of the product is not uniform. The hydrothermal method is to use aqueous solution as the reaction medium in a high-pressure reactor, and create a high-temperature, high-pressure reaction environment by heating the reaction vessel, so that usually insoluble or insoluble substances are dissolved and recrystallized to obtain a uniform phase. At present, the reaction temperature of LiFePO ₄ prepared by hydrothermal method has been reported from 120°C to 390°C. Although the lower reaction temperature can reduce the energy consumption of material preparation, the hydrothermal product is usually accompanied by heterogeneous lithium phosphate. , ferrous phosphate, etc., the temperature is low, there will be Li/Fe antisite defects in the product, and the lattice constant will be unstable; and the synthesis temperature above 200°C will have extremely high requirements on the safety performance of the equipment, and the cost will naturally increase . The particle size distribution of product particles also varies greatly with the difference of hydrothermal reaction temperature.

The current patents on the preparation of lithium iron phosphate cathode materials by hydrothermal method are as follows:

The existing literature provides a lithium iron phosphate and its preparation method and application. The method prepares lithium iron phosphate with lower impurity phase and smaller particle size by adjusting the pH of two hydrothermal reactions and two rapid temperature rises, and exhibits excellent electrochemical performance.

Another existing document provides a preparation method of lithium, a kind of lithium iron phosphate material, including adding alcohols as a solvent for the reaction, which suppresses the tertiary ionization of iron phosphate in the alcohol solution and reduces the occurrence of side reactions. and the polarity of the alcoholic solution is smaller than that of water, the solid particles are better dispersed, and there will be no agglomeration phenomenon. The prepared lithium iron phosphate has a higher specific capacity and a better capacity retention rate. The material performance has been greatly improved.

Another existing document provides a synthetic method of controlling crystallization to prepare a cost-effective lithium iron phosphate precursor, including making a mixed solution of lithium salt solution, ferrous salt solution and phosphate solution, and adjusting the pH of the solution. The precursor of lithium iron phosphate is prepared at low temperature and the by-product lithium phosphate is obtained. This synthesis method is simple and fast, and its reaction conditions are easy to control, which is convenient for popularization and application, and is conducive to industrial implementation.

The defective that existing hydrothermal reaction exists is:

(1) The low hydrothermal reaction temperature leads to incomplete hydrothermal reaction, and the products of the hydrothermal reaction process will contain impurities such as lithium phosphate and ferrous phosphate, which will seriously affect the electrochemical performance of the material.

(2) The product of the hydrothermal reaction process has Li/Fe antisite defects, which is not conducive to the rapid deintercalation of lithium ions from its structure.

(3) The particle size distribution of the product particles in the hydrothermal process is not uniform, which is not conducive 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 material.

Contents of the invention

The main purpose of the present invention is to provide a kind of lithium iron phosphate, its preparation method, lithium ion battery and electric driving device, to solve the problem that the lithium iron phosphate cathode material obtained by the existing hydrothermal synthesis method has many impurity phases, and there are Li/Fe Antisite defects and uneven particle size distribution lead to poor electrochemical performance and low compaction density.

In order to achieve the above object, the present invention provides a preparation method of lithium iron phosphate on the one hand, the preparation method of the lithium iron phosphate comprises: under the first inert atmosphere and acidic conditions, make the soluble lithium source compound, the soluble ferrous source The compound and the soluble phosphorus source compound are subjected to the first hydrothermal synthesis reaction with water to obtain the first solid-liquid mixture; the first solid-liquid mixture is forcibly dispersed to obtain the dispersion liquid. Under the second inert atmosphere, the dispersion liquid is subjected to the second hydrothermal synthesis reaction. Thermal synthesis reaction to obtain lithium iron phosphate, wherein the temperature of the second hydrothermal synthesis reaction is higher than the temperature of the first hydrothermal synthesis reaction, and at the same time, the raw materials are supplemented during the second hydrothermal synthesis reaction process. The raw materials include soluble lithium source compounds, Soluble ferrous iron source compound and soluble phosphorus source compound.

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

Further, the forced dispersion process includes: using a mechanical stirring device to stir the first solid-liquid mixture at a high speed, and the stirring speed is 40-60 r/min, and the stirring time is 0.5-2 hours.

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

Further, in the first hydrothermal synthesis reaction, the molar ratio of the soluble lithium source compound, the soluble ferrous source compound and the soluble phosphorus source compound is 3:(0.9～1.5):(0.9～1.5); raw material replenishment step Among them, the molar ratio of the soluble lithium source compound, the soluble ferrous source compound and the soluble phosphorus source compound is (0～1.5):(1～2.5):(1～2.5), and in the raw material replenishment step, the control The solid content in the whole reaction liquid is 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 trifluoromethanesulfonate species; soluble ferrous iron source compound is selected from one or more of the group consisting of ferrous sulfate, ferrous chloride, ferrous acetate, ferrous nitrate and ferrous oxalate; soluble phosphorus source compound is selected from phosphoric acid, phosphoric acid One or more of the group consisting of ammonium dihydrogen phosphate, diammonium hydrogen 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 includes: obtaining a solid phase product through the above-mentioned second hydrothermal synthesis reaction; making the solid phase product and the carbon coating agent Calcination is carried out to obtain 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 in the present application.

The fourth aspect of the present application also provides a lithium ion battery, including a positive electrode material, and the positive electrode material includes the lithium iron phosphate provided in the present application.

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 solution of the present invention, the lithium iron phosphate crystal nucleus can be formed through the first hydrothermal synthesis reaction, and the particle size of the lithium iron phosphate crystal nucleus can be refined through the forced dispersion process, while improving its dispersion and uniformity. The temperature of the second hydrothermal synthesis reaction is higher than that of the first hydrothermal synthesis reaction, which is conducive to promoting the lithium iron phosphate and ferrous phosphate heterophase in the lithium iron phosphate crystal nucleus to form a lithium iron phosphate crystal phase, thereby reducing the amount of lithium iron phosphate in the final product. The impurity phase, obtains the high-purity lithium iron phosphate product. At the same time, during the growth of lithium iron phosphate crystals, increasing the temperature of the hydrothermal reaction can reduce the dislocation of iron and obtain a stable lithium iron phosphate crystal product. In addition, during the second hydrothermal synthesis reaction, the increase of temperature and the supplement of raw materials are beneficial to control the nucleation distribution and growth rate of crystal nuclei, so as to obtain products with more uniform particle distribution, thereby increasing the compaction density of cathode materials. On this basis, the lithium iron phosphate material prepared by the above method has less impurity phases, high purity, and uniform particle size distribution, which is conducive to shortening the diffusion path of lithium ions in the material, thereby greatly improving the dynamic performance of the material. As a cathode material for lithium-ion batteries, it can exhibit excellent electrochemical performance.

Description of drawings

The accompanying drawings constituting a part of the present application are used to provide a further understanding of the present invention, and the schematic embodiments and descriptions of the present invention are used to explain the present invention, and do not constitute an improper limitation of the present invention. In the attached picture:

Fig. 1 is the XRD figure of the lithium iron phosphate material that embodiment 1 to 3 makes;

FIG. 2 is a comparison chart of particle size distribution of lithium iron phosphate materials prepared in Examples 1 and 4 and Comparative Examples 1 and 2.

Detailed ways

It should be noted that, in the case of no conflict, the embodiments in the present application and the features in the embodiments can be combined with each other. The present invention will be described in detail below in conjunction with examples.

As described in the background technology, the lithium iron phosphate positive electrode material produced by the existing hydrothermal synthesis method has many impurity phases, Li/Fe anti-site defects, and uneven particle size distribution, resulting in poor electrochemical performance. . In order to solve the above technical problems, the application provides a preparation method of lithium iron phosphate, the preparation method of the lithium iron phosphate comprises: under the first inert atmosphere and acidic conditions, make the soluble lithium source compound, the soluble ferrous iron source compound performing a first hydrothermal synthesis reaction with 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; under a second inert atmosphere, subjecting the dispersion to a second water Thermal synthesis reaction to obtain lithium iron phosphate, wherein the temperature of the second hydrothermal synthesis reaction is higher than the temperature of the first hydrothermal synthesis reaction, and at the same time, the raw materials are supplemented during the second hydrothermal synthesis reaction process. The raw materials include soluble lithium source compounds, Soluble ferrous iron source compound and soluble phosphorus source compound.

The lithium iron phosphate crystal nucleus can be formed through the first hydrothermal synthesis reaction, and the particle size of the lithium iron phosphate crystal nucleus can be refined through the forced dispersion process, while improving its dispersion and uniformity. The temperature of the second hydrothermal synthesis reaction is higher than that of the first hydrothermal synthesis reaction, which is conducive to promoting the lithium iron phosphate and ferrous phosphate heterophase in the lithium iron phosphate crystal nucleus to form a lithium iron phosphate crystal phase, thereby reducing the amount of lithium iron phosphate in the final product. The impurity phase, obtains the high-purity lithium iron phosphate product. At the same time, in the process of growing lithium iron phosphate crystals, increasing the temperature of the hydrothermal synthesis reaction can reduce the dislocation of iron and obtain a stable lithium iron phosphate crystal product. In addition, during the second hydrothermal synthesis reaction, the increase of temperature and the supplement of raw materials are beneficial to control the nucleation distribution and growth rate of crystal nuclei, so as to obtain products with more uniform particle distribution, thereby increasing the compaction density of cathode materials. On this basis, the lithium iron phosphate material prepared by the above method has less impurity phases, high purity, and uniform particle size distribution, which is conducive to shortening the diffusion path of lithium ions in the material, thereby greatly improving the dynamic performance of the material. As a cathode material for lithium-ion batteries, it can exhibit excellent electrochemical performance.

The temperature and reaction time of the first hydrothermal synthesis can adopt the ranges used in the art. In a preferred embodiment, the temperature of the first hydrothermal synthesis reaction is 110-120° C., the reaction time is 2-5 hours, and the pH of the reaction system is 4-6. Limiting the temperature and reaction time of the first hydrothermal synthesis reaction within the above-mentioned range helps to improve the reaction rate of the first hydrothermal synthesis reaction, while making it react in an acidic environment is conducive to suppressing the hydrolysis of ferrous ions, thereby reducing impurities. Phase, improve the purity of lithium iron phosphate.

In a preferred embodiment, the forced dispersion process includes: using a mechanical stirring device to stir the first solid-liquid mixture at a high speed, and the stirring speed is 40-60 r/min, and the stirring time is 0.5-2 hours.

Making the temperature of the second hydrothermal synthesis reaction higher than that of the first hydrothermal synthesis reaction is beneficial to reduce impurity phases, refine crystal grains, and improve the uniformity of their distribution. In a preferred embodiment, the reaction temperature of the second hydrothermal synthesis reaction is 140-200° C., the reaction time is 0.5-1 h, and the heating rate is -10-10° C./min. The temperature, reaction time and heating rate of the second hydrothermal synthesis reaction include but are not limited to the above-mentioned range, and being limited to the above-mentioned range helps to further improve the purity of the obtained lithium iron phosphate, and then helps to further improve its application process in the dynamic performance.

The temperature raising process in the second hydrothermal synthesis reaction can be a process of increasing the temperature all the time, or raising the temperature first and then lowering it, or repeating the process of raising and lowering the temperature. For example, (a) heat up from 140°C to a higher temperature for hydrothermal synthesis reaction; (b) heat up from 140°C to a higher temperature for a period of time, then continue to heat up to a higher temperature for hydrothermal synthesis reaction , this process can be repeated several times of heating and holding, and the amplitude, rate, and holding time of each heating can be the same or different; (c) after heating from 140°C to 200°C, hold for a period of time, and then cool down to a certain temperature Carry out hydrothermal synthesis reaction; (d) after heating up from 140°C to 200°C, keep it warm for a period of time, then keep it warm for a period of time after cooling down to a certain temperature, and continue to cool down to a lower temperature to carry out hydrothermal synthesis reaction. This process can be repeated several times For the cooling and heat preservation process, the magnitude, speed and heat preservation time of each time cooling can be the same or different.

In the second hydrothermal synthesis reaction, the time points for supplementing raw materials include but are not limited to: (a) supplementing raw materials during the heating process; (b) supplementing raw materials during the heat preservation process; (c) supplementing raw materials during the cooling process. The way of replenishing raw materials can be continuous replenishment of raw materials, or segmented. The replenishment of raw materials can be one-time replenishment or multi-time replenishment, and the amount of raw materials replenished each time can be the same or different, and the rate of raw material replenishment can be the same or different. In order to further improve the uniformity of lithium iron phosphate particle size, thereby improving its electrochemical performance in the subsequent application process, preferably, the second hydrothermal synthesis reaction process includes the following schemes: (1) the first hydrothermal synthesis reaction ends Finally, in the first stage of the hydrothermal synthesis reaction process, the temperature is raised rapidly to 140°C at a heating rate of 10°C/min, kept for 0.5-1h, and the raw materials are replenished to keep the solid content in the hydrothermal synthesis reaction solution at 20-50%; Rapidly raise the temperature to 200°C at 10°C/min, hold for 1-2h for the second-stage hydrothermal synthesis reaction; then cool down to 175°C at a cooling rate of 10°C/min, hold for 1h; finally use a cooling rate of 5°C/min Cool down to 140°C and keep warm for 2h. (2) After the first hydrothermal synthesis reaction is completed, supplement raw materials to keep the solid content in the hydrothermal synthesis reaction solution at 20% to 50%; rapidly raise the temperature to 140°C at 10°C/min, and keep it warm for 0.5 to 1h; 10°C/min rapid heating to 175°C, heat preservation for 1h; rapid heating at 10°C/min to 200°C, heat preservation for 1h; natural cooling to cool down; (3) Replenish raw materials after the first hydrothermal synthesis reaction to maintain the hydrothermal synthesis reaction The solid content in the solution is 20-50%, and the temperature is raised to 140°C at a heating rate of 10°C/min, and the temperature is kept for 0.5-1h. Cool down to 175°C/min and keep warm for 1h; cool down to 140°C at a cooling rate of 5°C/min and keep warm for 2h.

In a preferred embodiment, in the first hydrothermal synthesis reaction, the molar ratio of the soluble lithium source compound, the soluble ferrous iron source compound and the soluble phosphorus source compound is 3:(0.9～1.5):(0.9～ 1.5). The molar ratio of the lithium source compound, the iron source compound and the phosphorus source compound includes but is not limited to the above-mentioned range, and limiting it within the above-mentioned range is beneficial to reduce the formation of impurity phases, better control the growth of crystal grains, and increase the concentration of ferric phosphate. Energy density and discharge specific capacity during the application of lithium materials.

In a preferred embodiment, in the raw material replenishment step, the molar 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):(1 ~2.5), and in the raw material replenishment step, the solid content in the entire reaction solution is controlled to be 20% to 50%. In the raw material replenishment process, limiting the molar ratio of the soluble lithium source compound, the soluble ferrous source compound, and the soluble phosphorus source compound and the replenishment amount of the raw material solution within the above range is conducive to further improving the uniformity of the particle size distribution, while inhibiting The particles are agglomerated, which is beneficial to further shorten the diffusion path of lithium ions in the material and improve the dynamic performance of the material.

The iron source compound, phosphorus source compound and lithium source compound used in the present application can adopt common types in this field. In a preferred embodiment, the soluble lithium source compound includes but not limited to lithium hydroxide, lithium chloride, lithium carbonate, lithium acetate, lithium fluoride, lithium bromide, lithium nitrate, lithium oxalate and lithium trifluoromethanesulfonate One or more of the group; soluble ferrous iron source compounds include but 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 one or more selected from the group consisting of phosphoric acid, ammonium dihydrogen phosphate, diammonium hydrogen phosphate and lithium dihydrogen phosphate.

In order to further improve the conductivity of lithium iron phosphate, the second aspect of the present application also provides another preparation method of lithium iron phosphate, the preparation method of the lithium iron phosphate includes: under the first inert atmosphere and acidic conditions, make the soluble The lithium source compound, the soluble ferrous iron source compound and the soluble lithium source compound are subjected to a first hydrothermal synthesis reaction to obtain a first solid-liquid mixture; the first solid-liquid mixture is forcibly dispersed to obtain a dispersion liquid; under a second inert atmosphere , making the dispersion liquid undergo a second hydrothermal synthesis reaction to obtain a solid-phase product; calcining the solid-phase product and a carbon coating agent to obtain carbon-coated lithium iron phosphate.

The third aspect of the present application also provides lithium iron phosphate, which is prepared by the above-mentioned preparation method provided in the present application. The lithium iron phosphate material prepared by the above method has few impurity phases, high purity, and uniform particle size distribution, which is conducive to shortening the diffusion path of lithium ions in the material, thereby greatly improving the dynamic performance of the material. Correspondingly, as a lithium-ion battery cathode material, it can also exhibit excellent electrochemical performance.

The fourth aspect of the present application also provides an electric drive device, which includes the lithium-ion battery provided by the present application. Using the lithium-ion battery containing the above-mentioned lithium iron phosphate as the energy module of the electric drive device can greatly improve its advantages in energy storage and battery life.

The present application will be described in further detail below in conjunction with specific examples, and these examples should not be construed as limiting the scope of protection claimed in the present application.

Example 1

A preparation method for lithium iron phosphate, comprising:

(1) The first hydrothermal synthesis reaction:

Dissolve a certain amount of lithium hydroxide, ferrous sulfate and phosphoric acid in deionized water in sequence according to their molar ratio of 3:1:1 to form solution A, mechanically stir at room temperature, and control the pH value of solution A to 4 ~6, transferred to a high-pressure hydrothermal reactor. The first hydrothermal synthesis reaction was carried out at a reaction temperature of 120° C., and the reaction time was 3 h. After the hydrothermal synthesis reaction was completed, it was naturally cooled to room temperature to obtain a hydrothermal synthesis reaction solution B.

(2) Forced dispersion: The solution B was subjected to high-intensity mechanical stirring for 2 hours until the precipitate in the solution B was fully dispersed, and the stirring rate was 40r/min.

(3) The second hydrothermal synthesis reaction, the second hydrothermal synthesis reaction is a multi-stage variable temperature process, as follows:

The solution B was transferred to an autoclave and the temperature was rapidly raised to 140°C at a heating rate of 10°C/min to carry out the first-stage hydrothermal synthesis reaction with a reaction time of 1 h.

After the first stage of hydrothermal synthesis reaction, a certain amount of lithium hydroxide, ferrous sulfate and phosphoric acid are added to the hydrothermal reaction kettle in sequence according to the corresponding molar ratio of 1:2:2 to maintain the hydrothermal synthesis reaction. The solid content in the reaction solution was 35%, and the temperature of the hydrothermal reaction vessel was rapidly raised to 200° C. (the heating rate was 10° C./min), and the second-stage hydrothermal synthesis reaction was carried out, and the reaction time was 1 h. After the second-stage hydrothermal synthesis reaction, the temperature of the hydrothermal reaction kettle was lowered to 175°C (the cooling rate was 10°C/min), and kept for 1h, and finally the hydrothermal synthesis reaction temperature was lowered to 140°C (the cooling rate was 5 °C/min), the third stage hydrothermal synthesis reaction was carried out for 2h. After the hydrothermal synthesis reaction in the third stage is completed, the obtained hydrothermal product is washed, filtered and dried to obtain a high-purity phase and high compacted density lithium iron phosphate material.

Example 2

The difference with Example 1 is: in the second-stage hydrothermal synthesis reaction in the step (3), lithium hydroxide is not contained in the raw material replenished, and be 1 with the molar ratio of phosphoric acid and ferrous sulfate: 1 ratio was added. Other steps are the same as in Example 1.

Example 3

The difference with Example 1 is: in the second-stage hydrothermal synthesis reaction in the step (3), the raw material added is 1:1:1 with the molar ratio of lithium hydroxide, ferrous sulfate and phosphoric acid Ratio joins. Other steps are 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 step (3) is 175° C., the reaction time is 2 h, and the third-stage hydrothermal synthesis reaction is not performed. Other steps are the same as in Example 1.

Example 5

The difference from Example 1 lies in that the reaction temperature of the second-stage hydrothermal synthesis reaction in step (3) is 160° C., and the reaction time is 1 h. After the second-stage hydrothermal synthesis reaction is completed, the temperature of the product system is directly lowered from 160° C. to 140° C. to carry out the third-stage hydrothermal synthesis reaction. Other steps are the same as in Example 1.

Example 6

The difference from Example 1 is that in the raw material replenishment process, the molar ratio of the soluble lithium source compound, the soluble ferrous iron source compound and the soluble phosphorus source compound is 0:2.5:2.5. Other steps are the same as in Example 1.

Example 7

The difference from Example 1 is that in the raw material replenishment process, the molar ratio of the soluble lithium source compound, the soluble ferrous iron source compound and the soluble phosphorus source compound is 1.5:1:1. Other steps are the same as in Example 1.

Example 8

The difference from Example 1 is that in the raw material replenishment process, the molar ratio of the soluble lithium source compound, the soluble ferrous iron source compound and the soluble phosphorus source compound is 0:3:3. Other steps are the same as in Example 1.

Example 9

The difference from Example 1 is that the process of the second hydrothermal synthesis reaction is that the first hydrothermal synthesis reaction ends, and the raw materials are supplemented to keep the solid content in the hydrothermal synthesis reaction solution at 20% to 50%; ℃/min rapid heating to 140 ℃, heat preservation 0.5 ~ 1h; 10 ℃/min rapid heating to 175 ℃, heat preservation 1h; 10 ℃/min rapid heating to 200 ℃, heat preservation 1h; natural cooling down. Other steps are 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 replenish the raw materials after the first hydrothermal synthesis reaction is completed, keep the solid content in the hydrothermal synthesis reaction solution at 20% to 50%, and adopt a heating rate of 10°C/min rapid heating to 140°C, heat preservation 0.5～1h, rapid heating at 10°C/min to 200°C, heat preservation 1h; cooling rate 10°C/min to 175°C, heat preservation 1h; cooling rate 5 ℃/min to cool down to 140 ℃, keep warm for 2h. Other steps are 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) is constant at 140° C., and other steps are the same as in Example 1.

Comparative example 2

The difference from Example 1 is that no raw material replenishment treatment is performed in the second hydrothermal synthesis reaction in step (3), and the other steps are the same as in Example 1.

Performance Testing:

FIG. 1 is an XRD diagram of the lithium iron phosphate material prepared in Examples 1 to 3; FIG. 2 is a comparison diagram of the particle size distribution of the lithium iron phosphate material prepared in Examples 1 and 4 and Comparative Examples 1 and 2.

The materials prepared in Examples 1-10 and Comparative Examples 1 and 2 were tested for their electrochemical properties using CR2032 button cells, one of which was the core-shell carbon-coated nanoscale lithium iron phosphate prepared in the examples of this application A mixture of positive electrode material, acetylene black, polyvinylidene fluoride (weight ratio 97:1.5:1.5), the other extremely metal lithium sheet, the electrolyte is 1mol/L LiPF ₆ dissolved in EC/DMC/EMC (volume ratio 1: 1:1) in the solvent. Constant current charging and discharging voltage range is 2.0-3.7V.

1) First discharge gram capacity and first coulombic efficiency test:

After the button battery is assembled, ①charging: 0.1C constant current charge to 3.7V, the charging specific capacity is recorded as Q1; ②discharging: 0.1C constant current is discharged to 2V, the discharging specific capacity is recorded as Q2; the first coulombic efficiency is abbreviated as ICE, ICE=Q2/Q1.

2) Cycle performance test:

①Charging: 1C constant current charging to 3.7V, 10min interval; ②Discharging: 1C constant current discharge to 2V, 10min interval; ③Repeat "①,②" 3000 times; the capacity retention rate is abbreviated as CR.

3) Rate performance test:

①0.1C constant current charge to 3.7V, after 10 minutes, then 0.1C constant current discharge to 2V; ②Repeat "①" 10 times; ③Increase the current density in "①, ②" to 1C, 3C, 5C and 10C, The discharge capacities corresponding to 1C, 3C, 5C and 10C are Q3, Q4, Q5 and Q6 respectively.

See Table 1 for the performance tests of the lithium-ion battery anode materials of the examples and comparative examples.

Table 1

From the data in Table 1, comparing the data in Examples and Comparative Examples, it can be seen that the multi-stage hydrothermal synthesis reaction carried out under different temperature gradients and the supplementation of raw materials in the hydrothermal process can regulate the distribution and distribution of lithium iron phosphate nucleation nuclei. The growth rate of the crystal nucleus makes the size distribution of the grown lithium iron phosphate particles more uniform, which is conducive to improving the compaction density of the material. In addition, the higher hydrothermal synthesis reaction temperature can reduce the Li/Fe antisite defect of the lithium iron phosphate crystal , which is conducive to the rapid deintercalation of lithium ions from the lattice, so that the gram capacity, cycle stability and rate performance of the electrode material will be significantly improved.

From examples 1, 2, 3 and 6 to 8, the ratio of supplementing the raw materials of each component in the subsequent hydrothermal process will affect the purity of the product, and the poor electrochemical performance of the electrodes of example 2 and example 3 is mainly It is attributed to the fact that the product lithium iron phosphate contains part of the impurity phase. The extremely poor electrochemical performance of the electrode in Example 8 shows that the impurity phase in the product lithium iron phosphate has become more obvious.

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

From the perspective of Examples 1 and 5, the relatively poor cycle performance and rate performance of Example 5 can be attributed to the fact that there may be more Li/Fe antisite defects in the lithium iron phosphate crystal.

From Examples 1 to 10, choosing the preferred temperature-changing method of the present application in the second hydrothermal synthesis process is conducive to further improving the comprehensive electrochemical performance of lithium iron phosphate.

From Example 1 and Comparative Examples 1 and 2, the stepwise adjustment of the temperature of the hydrothermal synthesis reaction can obviously improve the electrochemical performance of the lithium iron phosphate material, which shows that the phosphoric acid can be controlled by adjusting the temperature of the hydrothermal synthesis reaction. The growth rate of iron-lithium nuclei. The supplementary raw materials in the hydrothermal synthesis reaction process make the lithium iron phosphate nucleation nuclei distribute more uniformly. By controlling the distribution mode of crystal nuclei and the growth rate of crystal nuclei, the particle size distribution of lithium iron phosphate is more uniform, which is beneficial to improve the compaction density of the material. Although the lithium iron phosphate prepared in comparative example 1 has a discharge capacity equivalent to that of the product prepared by the present application at a relatively low current, its discharge capacity will obviously decrease under high current (10C) conditions. In the process of preparing lithium iron phosphate in comparative example 2, there was no supplement of raw materials, resulting in an excessive amount of lithium source compound; although the electrochemical performance of lithium iron phosphate obtained by it is also good in the application process, but due to the lithium source compound It has not been fully utilized, so the synthesis cost of lithium iron phosphate is relatively high.

It should be noted that the terms "first" and "second" in the specification and claims of the present application are used to distinguish similar objects, but not necessarily used to describe a specific order or sequence. 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 descriptions are only preferred embodiments of the present invention, and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and changes. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention shall be included within the protection scope of the present invention.

Claims (10)

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.

Images